Quantification and Prediction for Aging of Printing & Writing Papers

Quantification and Prediction for Aging of Printing & Writing Papers Exposed to Light

ASTM Institute for Standards Research Program Effect of Aging on Printing & Writing Papers

USDA Forest Service Forest Products Laboratory Madison, Wisconsin

Final Report

August 2000

FPL 1 Table of Contents Section ...... Page Program Overview...... 3 Introduction to Light Aging Studies ...... 7 Executive Summary...... 9 Natural Aging of ISR Papers ...... 12 Exposure Conditions...... 12 Testing Frequency and Protocols...... 14 Initial Paper Characteristics...... 15 Optical Properties ...... 15 Physical Properties...... 23 Chemical Changes ...... 30 Accelerated Aging of ISR Papers ...... 44 Exposure Conditions...... 44 Optical Properties ...... 45 Mechanical Properties...... 54 Chemical Changes ...... 56 Procedures and Equipment ...... 68 Rayonet Photoreactor Exposure...... 68 Solar Simulator Exposure ...... 68 Natural Chamber Exposures ...... 71 Photo-Exposure Measurement and Calibration ...... 73 Measurement of Optical Properties ...... 75 Mechanical Testing Methods...... 76 Chemical Analysis Methods ...... 77 Conclusions...... 80 Natural Aging Studies...... 80 Accelerated Aging Studies...... 82 Protocol Issues ...... 84 Illumination Spectrum ...... 85 Intensity and Duration of Illumination ...... 85 Uniformity of Illumination ...... 86 Temperature ...... 86 Relative Humidity...... 87 Testing of Aged Samples...... 87 Existing Standards ...... 88 Joint FPL/KCL Accelerated Light Aging Test Recommendation...... 89 References...... 94 List of Tables ...... 96 List of Figures...... 97 Appendixes ...... 99

FPL 2 Program Overview Papers exist that are more than 500 years old and are still in very good condition. On the other hand, papers made less than 100 years ago have deteriorated so badly they are no longer usable. Until now, there has been no good means for predicting which papers would perform well for extended times compared with those that would fail at an early date.

The Paper and Paper Products Committee of the American Society for Testing and Materials (ASTM) has been confronted with this issue for a number of years. The remedy for the last half-century has been to specify a prescribed composition for those papers that must have long life expectancy. In 1992, discussion within the Committee identified a need to create standards that are based only on paper performance. To do so, it would be necessary to test papers to see how they would perform when new and to further test them to estimate the time for which they would continue to meet the end-use performance requirements. The purpose of the research program was to develop the needed test methods. Development of specific paper performance standards required by end users was left to others. To guage the life expectancy of paper, it was deemed necessary to accelerate aging of the paper over a short period. The accelerated aging process would have to create aging in a way that produced essentially the same physical and chemical changes as would occur in a natural aging process under normal conditions of storage and handling.

Because the interest in developing scientifically sound test methods was substantial, a Task Group was formed by ASTM to define the research necessary to create such methods. The Task Group quickly became a new Subcommittee of the Paper Group. The Paper & Paper Composition Subcommittee (ASTM D6.50) guided the effort thereafter.

In 1994, a 3-day workshop was held in Philadelphia, PA. Its objective was to develop a program of research that would answer key scientific questions that at that time prevented movement to acceptable accelerated aging test methods. It was agreed that to be successful, the test methods would have to be so scientifically sound that all with a stake in paper aging issues could endorse the methods. More than a hundred people from 12 countries attended the workshop; they included leading scientists in the field of paper aging and representatives from the pulp and paper industry, academia, government, libraries, archive and paper conservation, and environmental organizations.

Under the guidance of skilled facilitators, the workshop participants reached consensus on the purpose and proposed scope of the research: • Development of credible, reliable accelerated aging test methods • Identification of high probability that chosen methods would predict life expectancy for any reasonable paper composition • Study and definition of important chemical and physical mechanisms of aging

FPL 3 A request for research proposals to accomplish this program was sent to 25 distinguished institutions worldwide. Fifteen institutions responded with firm proposals, and five laboratories were chosen by a panel accountable for scientific and administrative direction to the program. The laboratories conduct research on three means of accelerating aging: temperature, light, and atmospheric pollutant gases:

Temperature: • Canadian Conservation Institute, Ottawa, Ontario, Canada—Dr. David Grattan, overall leadership; Mr. Paul Bégin, principal investigator • U.S. Library of Congress Preservation and Research Testing Division, Washington, DC—Dr. Chandru Shahani

Light: • Finnish Pulp & Paper Research Institute, Espoo, Finland—Dr. Ingegerd Forsskåhl, principal investigator • USDA Forest Products Laboratory, Madison, WI—Dr. Rajai Atalla and Dr. James Bond as principal investigators, and Dr. Umesh Agarwal and Mr. Chris Hunt as members of the team.

Atmospheric pollutant gases: • Image Permanence Institute, Rochester Institute of Technology, New York State— Mr. James Reilly, overall direction; Dr. Peter Adelstein and Mr. Edward Zinn, principal investigators

The sponsors were aware of the importance of being able to repeat the scientific studies in other laboratories. This would be done, only if needed; to fully validate the scientific information generated. Therefore, great care was taken to fully document all steps of the program. Instead of producing laboratory handsheets, it was decided to make the paper on a small “pilot” paper machine. The Herty Foundation in Savannah, Georgia, was chosen for this task. Machine-made paper was felt to more closely approximate the type of paper that will be sought by end users.

Fifteen types of paper were chosen for study. They included a cross section of acid and alkaline papers (Table 1). The fiber composition covered a full range of currently used paper pulps. This included mechanically produced stone groundwood (SGW), the fiber that has been a prominent ingredient of newspapers. The SGW fiber contains lignin, which in wood serves as cement between fibers and a stiffening agent within fibers. Lignin is known to darken and yellow when exposed to light or heat. At the other end of the spectrum, paper was made with pure textile cotton fiber. In the middle, papers were made with chemically produced bleached kraft wood pulp and bleached chemithermomechanical pulp (BCTMP), another fiber that contains substantial quantities of lignin. Kraft pulps are very similar to cotton in that they contain very high levels of cellulose.

FPL 4 Because of difficulty in meeting the cotton paper specifications on the pilot machine at the Herty Foundation, Crane & Co. Inc. (the makers of U.S. currency paper) generously donated the cotton papers to the program. These papers were made on one of Crane’s production paper machines in Dalton, Massachusetts.

Immediately following production, the papers were thoroughly tested to characterize their properties. The large rolls of paper were then cut to standard office “cut sized” sheets. The papers were subsequently randomized thoroughly so that all parts of each production run were included in each 500-sheet carton of any given paper sent to a test laboratory.

Approximately 30,000 sheets of paper were made of each of the fifteen paper types. The paper that was not in use at the research laboratories was kept in cold, dark storage at a temperature just above freezing to minimize the amount of aging that would occur before research studies were launched.

In addition to the investigations by the five participating laboratories, a need was recognized for a thoroughly documented and scientifically designed study of the long- term natural aging of a variety of papers. Accordingly, ASTM launched such a study. Fifteen sets of books, each containing 350 sheets of a given type of paper, were produced. The books were shipped to 10 libraries in North America, which have agreed to hold and document them for the next 100 years. At ten intervals, samples will be removed and tested at the participating research laboratories. Plots of the physical, chemical, and optical changes that occur will be made. This information will serve as a scientific database for future generations for improving the correlation of accelerated aging and natural aging data. This should improve the accuracy of life-expectancy predictions when future work is conducted.

Thirty-three organizations contributed to the success of this program. They included producers of pulp and paper, paper conservation organizations, government agencies, and suppliers to paper producers. A full listing of these organizations is included in Appendix N

Including both direct contributions of money and in-kind contributions of time and materials, the total value of the research effort is approximately $4 million U.S. dollars.

FPL 5 Table 1. Paper production parameters as provided by ASTM Paper BN- BN- SW- HW- SW- TEXTILE pH CaCO3 Starch Additives Number SWK HWK BCTMP BCTMP SGW COTTON % 1 100 5.0 0 No Alum, Hercules Nuphor 635 rosin size 2 100 8.1 5 No 3 100 5.0 0 No Alum, Hercules Nuphor 635 rosin size 4 100 8.1 5 No 5 100 5.0 0 No Alum, Hercules Nuphor 635 rosin size 6 100 8.1 5 No 7 20 80 5.0 0 No Alum, Hercules Nuphor 635 rosin size 8 20 80 7.0 5 No SMI process 9 20 80 8.1 0 No 10 20 80 8.1 5 No 11 50 50 8.1 0 No 12 50 50 8.1 5 No 13 50 50 8.1 5 No 14 50 50 5.0 0 No Alum, Hercules Nuphor 635 rosin size 15 50 50 8.1 5 Yes AKD internal, Penford Gum size

Code: BN = Bleached Northern SW = Softwood SGW = Stone Groundwood BS = Bleached Southern HW = Hardwood S(H)WK = Soft(Hard)wood Kraft BCTMP = Bleached Chemithermomechanical Pulp

FPL 6 Introduction to Light Aging Studies The body of this portion of the ASTM paper aging research program covers work undertaken in a collaboration of two laboratories, each of which has been active and internationally acknowledged for its studies of the effects of light exposure on wood- derived fibers. This collaboration was sought to insure the full measure of scientific inquiry necessary to establish the possibility of establishing an accelerated aging test in a scientifically rigorous method. The participating laboratories were the USDA Forest Products Laboratory (FPL) in Madison, Wisconsin, USA and the Finnish Pulp & Paper Research Institute (KCL) in Espoo, Finland. At FPL, the research team primarily consisted of Drs. Rajai Atalla, James Bond, and Umesh Agarwal and Mr. Christopher Hunt. At KCL, the research team included Dr. Ingegerd Forsskåhl and Ms. Ursula Suppanen in collaboration with Dr. Henrik Tylli of the Department of Chemistry of the University of Helsinki. Both teams had additional technical support in their laboratories.

The research programs at the two laboratories were developed in the following manner: • Portions of the research were undertaken at both laboratories; these portions involved evaluation of highly important phenomena by identical or similar means for cross- validation of important scientific findings. • The research relied on utilizing, for the most part, similar commercially available instrumentation to analyze the optical, physical, and chemical changes in paper upon prolonged exposure to light. • At each laboratory, the researchers pursued several distinct research avenues for which they were uniquely qualified and equipped. • These complementary paths offered opportunities to pursue different approaches to validating mechanisms that were thought to be at the heart of the light aging process.

Common research avenues included • natural aging of the selected 15 papers in both daylight and fluorescent light, albeit at different conditions in the laboratories, and • accelerated aging through the use of xenon arc lamps with appropriate filters designed to simulate the solar spectrum, but at higher photon flux levels than would be the case in ambient environments.

While these avenues for exploring aging were generally common to both laboratories, they differed in the following ways: • At FPL, natural daylight at the 47o North latitude of Madison was presented to the test papers through a North-facing window. Therefore, only diffuse light impinged the papers because the sun never travelled far enough into the northern sky to shine directly onto them. The papers were exposed continuously for over 4 years. • At KCL, because of rapid changes in the hours of daylight at the latitude of Espoo (60o North), the exposure was applied only from June through September during two sequential summers. This natural daylight exposure was made through a South–

FPL 7 Southeast-facing window to ensure substantial light aging during this short summer period. Thus, the papers received direct sunlight. • At FPL, the fluorescent exposure was applied around the clock for the entire year (365 days). The papers were exposed for over 4 years in a room especially designed to provide continuous exposure at the same conditions as those for a well-illuminated office desk. The room was sufficiently large that all 15 papers could be exposed continuously. The chamber was designed to also allow simultaneous exposure to halogen lamp-based illumination, but in a separate compartment. • At KCL, the fluorescent exposures were made under a common fluorescent desk lamp at a distance of 30 cm between the lamp and the samples. • At FPL, the accelerated exposure was carried out with an Oriel solar simulator with a “short arc” xenon light source. • At KCL, researchers used an Hereaus “Suntest” xenon-arc solar simulator, which uses a “long arc” xenon light source. The filters were somewhat different from those used at FPL and produced moderately different spectra.

This report primarily describes the findings made by FPL. Where appropriate, comparisons will be made between the findings of the FPL and KCL findings.

The overall intent of the combined work of FPL and KCL was twofold: (1) to conduct the scientific inquiries necessary to guide the design of a suitable accelerated test method to assess the effects of the exposure of paper to light and (2) to carry out the scientific inquiry necessary to validate such a method. The results of these complementary inquiries will provide the basis for design of an accelerated test method that can be used with confidence to assess the relative stability of the vast majority of papers that are likely to be used for archival purposes

FPL 8 Executive Summary The Forest Products Laboratory (FPL) of the USDA Forest Service participated in the part of the ISR/ASTM program directed at assessing the stability of properties upon exposure to light. This report sets forth the findings of the program undertaken at FPL. The goals of the program were to carry out in-depth studies of photo-aging effects sufficient to establish the guidelines for developing a credible accelerated aging protocol. The approach adopted was to carry out studies of photo-aging of paper in two different environments. The first was intended to simulate normal aging under typical conditions, with exposure to both artificial and daylight illumination; these conditions are referred to as natural aging. The second environment provided higher photon flux levels in order to accelerate the photo-aging process. Studies carried out in both of these environments included monitoring the changes in optical and mechanical properties that resulted from the aging process. The studies also included spectral and chemical characterization of the changes brought about by photo-exposure. Through comparison of the effects of aging in the different environments, and characterization of the changes that were observed, guidelines for accelerated aging protocols have been developed.

Findings Regarding Optical Properties After Natural Exposure

Perhaps the most significant finding was that none of the measures of the dose of radiation received by the paper could provide a basis for correlating the degree of property loss for all papers. It was clear from the observed property changes that the chemistry of photo-decay differed when lignin was present in the paper. It was equally clear that in lignin-free papers, damage was caused by photo-exposure to a degree that was not anticipated when the program of investigations was undertaken. The decay in properties with natural aging was such that all papers suffered significant losses in one or another of their key properties when they were exposed for an extended period of time at ambient intensities to light of any type.

For lignin-containing papers all environments eventually led to a similar level of decay of optical properties. The finding that was most surprising was related to the response of lignin-free papers to photo-exposure. The optical properties of lignin-free papers were more severely degraded by halogen and fluorescent lighting than by north window illumination. This suggests that visible light plays a more significant role than ultraviolet radiation in the photo-yellowing of lignin free papers when they are were exposed for extended periods of time.

Two other observations apply generally to both lignin-free and lignin-containing papers. These are: 1) When two papers were made of the same furnish except that only one included calcium carbonate, the calcium carbonate-loaded sheet was brighter both before and after exposure; 2) The decay of optical properties in both lignin-containing and lignin-free papers was affected by the relative humidity and/or temperature variations in the aging chamber.

FPL 9 Findings Regarding Optical Properties After Accelerated Aging

Just as in the natural aging studies, the presence of lignin altered the pattern of response to accelerated photoexposure. Thus, under accelerated aging conditions, the responses of the lignin-free and lignin-containing papers fell into two groups. The solar simulator- based accelerated aging studies could reliably rank the relative stabilities of the lignin- containing papers in a manner that paralleled the stability in the natural environments. The protocol was not adequate for ranking the optical stability of lignin-free papers.

For lignin-containing papers, there was good agreement in the ranking of optical properties for papers exposed in the north window exposure and papers exposed in the solar simulator. This was consistent with the similarity in the spectral distribution of the radiation under the two different sets of conditions. It was generally the case that lignin- containing papers that were susceptible to direct photochemical reactions were also susceptible to photo-initiated dark reactions. This effect was not investigated in lignin- free papers.

Findings from Chemical Analyses after Accelerated Aging

The analyses of chemical change were focused on the lignin-containing papers, as it was not anticipated that changes in the lignin-free papers could be detected chemically after the relatively short exposure times. All chemical and spectral analyses indicated that both photo-aging protocols (that is, natural aging and aging in the solar simulator) produced similar changes in the papers, and that the photochemistry involved in the natural and accelerated aging were similar as well.

Preliminary Recommendations for an Accelerated Aging Test

It is clear that one cannot anticipate the condition of a particular sample of paper 50 years into the future because of the possibilities of wide variation in environmental conditions to which particular samples are exposed. It is likely, however, that distinctions can be made between what may be anticipated for different classes of papers. With this premise in mind, it is possible to contemplate test protocols that will allow comparisons to be made within the range of variables associated with the fabrication of the experimental papers that have been the basis of the studies described in this report.

On the basis of the results outlined above, it is suggested that full spectrum irradiation be the basis of the primary screening protocol, and that the exposure should be at a temperature in the 20o to 30oC range. This is likely to initiate the full range of photochemically induced reactions that are likely to arise from exposure to light, and to cause

FPL 10 them to occur at an accelerated rate. These effects are desirable in an accelerated test in order to simulate the full range of chemical reactions that can occur under natural conditions. Different papers will have chemical constituents that react differently to different wavelengths of light. Therefore, the test that includes all wavelengths of light encountered in natural exposure will be the one that is more broadly reliable. The temperature limitation is important to limit departures from phenomena that would be expected to occur under ambient conditions.

With the above observations and considerations in mind, it is suggested that a two-level standard be considered by ASTM. The primary one would be based on broadband illumination to determine general levels of stability of different classes of papers. This could be complemented with a test based on illumination with ultraviolet radiation alone to make possible development of the distinctions between the most stable papers. Both would be at near ambient temperatures.

The analyses outlined above indicate that any accelerated photo-exposure protocol will not reproduce exactly the chemical changes that would occur in paper under natural aging conditions. However, the protocols that are suggested do reproduce a considerable majority of the chemical and physical changes observed. Within the boundaries of the manufacturing variables assessed, papers that are stable to photo-exposure in these accelerated tests are stable under the wide range of natural aging conditions explored. It is expected, therefore, that the protocols recommended in this work will provide a reliable basis for accelerated assessments of the stability of papers when exposed to electromagnetic radiation in the near ultraviolet and visible wavelength region.

FPL 11 Natural Aging of ISR Papers

Exposure Conditions For natural aging of Institute for Standards Research (ISR) papers, a north-facing room with large windows was modified to create three natural aging chambers: one each for exposure to northern daylight (North), halogen lamps (Hal), and fluorescent lamps (Fluor). Figure 1 shows the layout of these chambers; 160- to 8.5-inch by 11-inch sheets of paper were mounted in each chamber. The area of paper placement was 7 ft high and 14 ft long. The halogen and fluorescent chambers were lit continuously, while the north- facing chamber followed the normal diurnal cycle.

Figure 1. Floor plan of natural aging chamber.

Because of the great variability in lighting conditions in the north window chamber, light flux was monitored continuously. The halogen and fluorescent chambers were continuously illuminated for the duration of the program. Air circulated freely between the three natural chambers, but since ventilation was provided by the building and not further modified, there were wide variations in temperature and humidity. Temperature and relative humidity (RH) were continuously monitored in the fluorescent chamber, but good air circulation ensured uniform conditions in all three chambers. Typical RH readings ranged from 10% in winter to 60% in summer, while temperature ranged from 22o to 32oC. In winter, papers in the north window chamber were typically 2 oC to 4oC cooler than papers in other chambers as a result of radiant heat loss through the windows. More detailed information regarding chamber conditions can be found in the Procedures section and Appendix A.

FPL 12 Table 2 shows the average illumination intensity in the aging chambers; Table 3 is included as a benchmark for comparison. Table 2 shows that (1) UV intensity was far higher in the north window chamber than in the halogen or fluorescent chambers, (2) each natural aging chamber had a distinct spectrum, and (3) the Oriel solar simulator used for accelerated aging provided a fairly good reproduction of the north chamber but at approximately 300 times the intensity.

Table 2. Measured light intensity in aging chambers, as measured through filtersa

Filter Region Intensity (W/m 2 ) Wavelength (nm) Name North* Halogen Fluorescent Oriel/100 395-728 Visible 3.2 6.4 5.9 8.2 347-389 UV 0.25 0.0 0.0 0.7 374-464 Blue 0.79 0.3 0.7 2.1 392-602 Aqua 2.1 1.7 4.7 6.0 300-1000 NoFilter 6.4 28.5 9.5 11.6 aNorth window values are year-long average intensity from datalogger.

Table 3. Measured intensity in normal offices (for comparison)a

Filter Region Intensity (W/m2 ) Wavelength (nm) Name Interior Cloudy Sunny N Dec N June E892 395-728 Visible 1.8 2.5 4.0 6.2 8.7 445 347-389 UV 0.01 0.09 0.17 0.68 0.54 25 374-464 Blue 0.3 0.5 0.8 2.2 1.9 98 392-602 Aqua 1.5 1.8 2.6 4.9 5.6 286 300-1000 NoFilter 1.7 3.9 7.9 8.8 21.9 703 aInterior: fluorescent lights, no windows. Cloudy: beside south windows at 2 p.m. on overcast November day. Sunny: Same as cloudy setup but on sunny day. N Dec: facing same as that of papers in north window chamber, December. N June: Same configuration as December, but June. E892: ASTM E892, noon direct solar illumination.

FPL 13 To provide a more precise measure of lighting conditions, intensity was measured at 10-nm intervals from 200 to 800 nm for each chamber. This data is shown in Figure 2.

1.4 North Window Chamber

1.2 Halogen

/10nm) Chamber 2

1 Fluorescent Chamber

0.8

0.6

0.4

0.2 Illumination Intensity (Watts/M

0 300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm)

Figure 2. North window spectrum from mid-day (sunny, snow-covered ground, winter). The exact position of UV cutoff changed through the day and season. Halogen and fluorescent spectra are constant over time. Spectra corrected for instrument response.

Blue wool standards are cloth specimens used to calibrate the UV intensity of xenon arc instruments by the rate of fading. The lighting in the north, halogen, and fluorescent chambers faded the blue wool standard L2 at the same rate as a xenon arc operated with 0.22, 0.11, and 0.08 W/m2, respectively, of 300–400 nm irradiation. The change in plots of blue wool color over time, as well as further information on lighting conditions, can be found in the section on chamber illumination as well as in Appendix A. Testing Frequency and Protocols Each natural aging chamber contained space for 10 sheets of each ISR paper. Table 4 shows how those 10 spaces were used during the course of the project. Six spaces were filled with sheets that were exposed for the entire study. During the first year of the study, 2 places were used to age papers for 1 year, and 2 places were used to age samples for 43 days, 3 months, and 6 months, sequentially. After the first year, these 4 spaces were filled with 8 half-sheets, which were exposed for the remainder of the study. Half- sheets of paper provided an opportunity to compare control and aged results from both halves of the same sheet. This reduced statistical variation and resulted in more precise measurement of changes in paper properties. The optical properties of two half-sheets of each lignin-containing paper were measured monthly and provided the data for optical property vs. time plots

FPL 14 Table 4. Sequence of sample placement in natural aging chambers

Spaces 1996 1997 1998 1999 2000 (no.) April July Sept Jan April July Sept Jan April July Sept Jan April July Sept Jan 2 1.5 mo 3 mo 6 mo 4-15-97 - Present Test portion, return to chamber 2 1 year 4-15-97 - Present Test portion, return to chamber 6 4/10/96 - Present Test portion, return to chamber

All optical and mechanical properties were measured in a room conditioned to 23°C/50% RH. ASTM and TAPPI test standards were followed except when noted. Initial Paper Characteristics Table 1 (see Overview) contains data on the production specifications of the papers used in this program. These tables are also reproduced in Appendix B. Klason lignin content, acid insoluble lignin, surface pH, moisture content, and the content of copper, iron, manganese, nickel, and cobalt were measured at FPL and included in Appendix B. Optical Properties

Lignin-Containing Papers Changes in optical properties of lignin-containing papers are presented for the subgroup of papers whose properties were recorded monthly starting in April 1997. CIELAB brightness, yellowness (b*), and redness (a*) for lignin-containing papers in the fluorescent chamber are shown in Figures 3, 4, and 5, respectively. Appendix C contains curves for all papers in all chambers. The acid and alkaline bleached northern softwood kraft (BNSWK) papers are included in these figures to provide a reference condition.

As is typical for lignin-containing papers (Spinner 1962), optical properties (brightness, b*, and a*) changed very quickly when the paper was first exposed to light, but approached limiting values after prolonged exposure. Pulping types had some effect, as unbleached stone groundwood (SGW) papers 7 and 8 appeared to be more stable than bleached chemithermomechanical (BCTMP) papers 3 and 4 with similar lignin content. However, since these SGW papers were unbleached and started at much lower brightness and at greater yellowness levels, they had proportionately less distance to travel to reach their limiting values. At the end, the SGW papers reached limiting values very similar to their BCTMP counterparts.

Among BCTMP papers, those with higher lignin content yellowed to a greater extent. When two papers were made from the same fiber but only one contained calcium carbonate (CaCO3) buffer, the carbonate-containing sheet was always brighter before and after exposure. The carbonate may have prevented enough light from reaching the fiber that it effectively reduced the light exposure. Certainly the role of calcium carbonate as a white pigment had a bearing in improving optical properties.

FPL 15 90 3 100% SW-BCTMP + Alum 4 100% SW-BCTMP 80 +CaCO3 7 80% SW-SGW + Alum 70 8 80% SW-SGW + CaCO3 60 9 80% HW-BCTMP

50 10 80% HW-BCTMP + CaCO3 14 50/50 HW- 40 BCTMP/BNSWK +Alum

Directional Brightness 13 50/50 HW- 30 BCTMP/BNHWK +CaCO3 1 100% BNSWK + Alum 20 2 100% BNSWK +CaCO3 0 200 400 600 800 1000 1200 Days Exposure

Figure 3. Directional brightness of paper in fluorescent chamber with time. Filled symbols and solid lines denote alkaline papers filled with calcium carbonate. Open symbols and dashed lines denote acid papers without calcium carbonate. Kraft papers 1 and 2 included for comparison.

FPL 16 35 3 100% SW-BCTMP + Alum 4 100% SW-BCTMP 30 +CaCO3 7 80% SW-SGW + Alum

25 8 80% SW-SGW + CaCO3

20 9 80% HW-BCTMP

10 80% HW-BCTMP + 15 CaCO3 14 50/50 HW- BCTMP/BNSWK +Alum b* (Yellowness) 10 13 50/50 HW- BCTMP/BNHWK +CaCO3 5 1 100% BNSWK +Alum 2 100% BNSWK +CaCO3 0 0 200 400 600 800 1000 1200 Temp/RH Trendline Days Exposure

Figure 4. Yellowing (b*) of paper in fluorescent chamber with time. Temp/RH line shows environmental trends. Fitted line max: 60%RH/30oC; minimum: 0%RH/23oC.

8 3 100% SW-BCTMP + Alum 7 4 100% SW-BCTMP 6 +CaCO3

5 7 80% SW-SGW + Alum

4 8 80% SW-SGW + CaCO3 3 9 80% HW-BCTMP 2 a* (Redness) 1 10 80% HW-BCTMP + CaCO3 0 14 50/50 HW- 0 200 400 600 800 1000 1200 BCTMP/BNSWK +Alum -1 13 50/50 HW- -2 BCTMP/BNHWK +CaCO3 Days Exposure

Figure 5. Redness (a*) of paper in fluorescent aging chamber with time.

FPL 17 During the initial stages of aging, optical properties decayed most quickly in the north window chamber (see Figure 6). Halogen and fluorescent exposures produced results very similar to each other, both showing less dramatic initial change than the north window. After prolonged exposure, however, the optical property changes in north window papers slowed dramatically while the properties of papers in other chambers continued to decline. By the end of 3 years, the optical properties of halogen- and fluorescent-aged sheets were similar to those of sheets aged in the north window. This suggests that while UV strongly promoted initial decay, the UV dose was not the predominant factor in determining optical properties after prolonged exposure.

It appears that the rate of decay of optical properties correlated with environmental conditions in the aging chambers. Optical properties appeared to stabilize or improve after the peak of winter when the aging chambers were cooler and drier than they were in summer. Since this effect was seen in all the natural aging chambers, it could not have been caused by fluctuations in natural light levels. The only factor we identified that changed in such a manner was environmental RH and temperature. Figure 4 contains a sine curve fitted to the temperature and humidity data in the chambers. Optical properties were more severely degraded after the peak in temperature and RH. In summer, conditions in the aging chambers were approximately 10oC warmer and RH was 40% higher than in winter. (See Appendix A for data on environmental conditions in the chambers.)

There was concern that this difference in measured properties could also be a result of deviations from standard test methods. Papers used in developing these curves were not conditioned before testing, so the moisture content during testing varied between summer and winter. The effect of this deviation was determined by measuring the difference in optical properties of the same sheet after 1 day at high humidity (30o/80%RH) vs. 1 day at low humidity (32o/20%RH). The difference in brightness was uniform across samples and less than one-half point, far less than the brightness variations observed in figure 3.. Results were similar for b* and a*. Therefore, the variations in optical properties were real and not an artifact of the testing protocol.

A possible mechanism for these variations in optical properties with change in moisture levels is that photoyellowing was a dynamic equilibrium process and moisture content affected the equilibrium. In this scenario, high moisture content favored the species that absorbed light, resulting in a lower brightness sheet. Another possibility is that if the phenolate centers, which are the centers of photo-oxidation in lignin-containing papers (Leary 1994, Forsskåhl and Maunier 1993), were not as easily dissociated in the dry state, the photoyellowing could have been slowed.

FPL 18 80

North 60 Halogen Fluorescent 50 Temp/RH Trend Directional Brightness 40

30 0 200 400 600 800 1000 Days in Aging Chambers

Figure 6. Directional brightness of paper 14 vs. time in aging chambers.

The fact that papers brightened in winter, rather than just yellowed more slowly, suggests that the photo-yellowing process had a dynamic equilibrium component that was subject to perturbation by temperature and moisture conditions.

Ideally, optical properties could have been plotted against some axis such that the slope was the same no matter what illumination spectrum was used. This would allow a user to predict how long the optical properties of a paper would stay above a given level based on knowledge of accelerated test performance and use conditions. Developing such a decay scale requires knowing the action spectrum—the relative efficiency by which light at different wavelengths promotes a chemical change. However, research has shown that different bulbs have different action spectra (Forsskåhl and Tylli 1993), so that a scale does not exist for paper in general. In photo-exposure studies, radiation is often measured by the 300–400 nm (UV) dose. Figure 7 shows how poorly the results of this practice were in cases where UV dose was small compared to visible dose, as it was in the halogen and fluorescent chambers.

FPL 19 Paper 9 80 North Window Halogen 70 Fluorescent Accelerated 60

50 Directional Brightness 40

30 00.511.522.53 Cumulative UV Dose (MJ/m2)

Figure 7. Brightness of paper 9 vs. UV dose, illustrating poor correlation between UV dose and optical properties. North, halogen, and fluorescent chambers received 21, 1.8, and 1.0 MJ/m2 of UV (347–389 nm), respectively, in 970 days.

We attributed the dramatic loss in properties with UV dose in halogen and fluorescent chambers to the yellowing (possibly thorough photo-initiated dark reactions) caused by visible wavelengths. The yellowing due to visible light was very weak and so is often ignored. In the case of realistic indoor exposures such as these, however, the UV component of illumination was so small that yellowing due to visible light was significant. Lignin-Free Papers Optical properties of lignin-free papers decayed in natural aging but not as severely as the optical properties of lignin-containing papers (p. 17). The most stable kraft sheets were all buffered alkaline sheets. Paper 11 (unbuffered alkaline) was somewhat less stable than the other lignin-free samples; paper 1 (unbuffered acid) was the most unstable. The cotton papers bleached more than did other papers during the first 3 months, then stabilized or slowly darkened over the course of the study. Cotton papers were best at maintaining or increasing brightness, though the decay of acid cotton #5 seemed to proceed more quickly than the decay of buffered kraft papers after 720 days of exposure.

It appears from Figure 8 that the smooth, gradual decay of optical properties in lignin- free papers displayed the same dependence on temperature and/or humidity as the decay of the lignin-containing papers, though the reason is unclear. Lignin-free papers appeared

FPL 20 to have better optical properties after periods of dry, cool exposure than after warm, moist periods. Brightness, yellowness (b*) and redness (a*) decay curves are provided in Appendix C.

1 100% BNSWK

2 100% BNSWK +CaCO3 85 5 100% Cotton

6 100% Cotton +CaCO3 80 11 50/50 BNSWK/BNHWK

12 50/50 BNSWK/BNHWK + CaCO3 Directional Brightness 75 15 50/50 BNSWK/BNHWK + CaCO3, AKD, Gum Temp/RH Trend

70 0 200 400 600 800 1000 Days in Aging Chamber

Figure 8. Directional brightness with fluorescent photo-exposure of lignin-free papers.

As seen in Figure 9, in most cases north window illumination had less effect on the optical properties of lignin-free papers than did artificial light, even bleached cotton papers 5 and 6. Halogen and fluorescent exposure yellowed all papers.

That the north window exposure caused the least amount of darkening and yellowing was the opposite of the expected result. It is generally accepted that UV darkens paper while visible light tends to bleach it (Forsskåhl and Maunier 1993). Yet the north window chamber, with 10 to 20× more UV and 2.5× less visible intensity than the halogen or fluorescent chambers, caused less darkening. One reason why this may not have been noted previously is the lack of long-term well-documented natural photo-exposure studies in the literature.

FPL 21 92

80 Unaged 78 North Directional Brightness Halogen 76 Fluorescent 74

72 1 11 2 15 12 5 6 Paper Number

Figure 9. Directional brightness of lignin-free papers after 17 months of exposure.

Changes After Natural Exposure None of the measures of the radiation dose received by the paper could provide a basis for correlating the degree of property loss for all papers. It was clear from the observed property changes that the chemistry of photo-decay differed when lignin was present in the paper. It was equally clear that in lignin-free papers, damage was caused by photo- exposure to a degree that was not anticipated when the program of investigations was undertaken. The decay in properties with natural aging was such that all papers suffered significant losses in one or another of their key properties when they were exposed for an extended time at ambient intensities to light of any type.

For lignin-containing papers, all environments eventually led to the same level of decay of optical properties. Exposure to northern daylight caused the onset of discoloration to occur earlier than when the same papers were exposed to halogen or fluorescent illumination. However, after 3 years of exposure, the optical properties of lignin- containing papers aged in the halogen and fluorescent chambers, which have lower levels of UV radiation and higher levels of visible radiation, were similar to the optical properties of the samples aged in the north window.

The finding that was most surprising was related to the response of lignin-free papers to photo-exposure. The optical properties of lignin-free papers were more severely degraded

FPL 22 by halogen and fluorescent lighting than by north window illumination. This occurred despite the fact that the halogen and fluorescent chambers delivered approximately 2.5× the visible radiation dose and 0.1× to 0.05× the ultraviolet dose received in the north window chamber. This suggests that visible light played a significant role in the photo- yellowing of lignin-free papers in this study.

Two other observations generally apply to both lignin-free and lignin-containing papers: (1) when two papers were made of the same furnish except that only one included calcium carbonate, the calcium carbonate-loaded sheet was brighter both before and after exposure; (2) the decay of optical properties in both lignin-containing and lignin-free papers was affected by relative humidity and/or temperature variations in the aging chamber. Physical Properties After 29 months exposure, the highest lignin content papers (3, 4, 7, and 8) were quite brittle—often creasing and then cracking before bending to a 0.5-cm radius, although fibers bridged the crack and prevented the pieces from parting. All the lignin-containing sheets were somewhat stiffer after long-term exposure, which indicated that photo- exposure may have produced cross-linking of the lignin units. Further evidence for lignin cross-linking was observed in the companion KCL study. After 100 h exposure in the KCL xenon arc apparatus, lignin-rich papers 3 and 7 did not disperse when stirred in water. Brittleness in old library volumes is usually due to acidic depolymerization of cellulose, a different mechanism. Old brittle papers typically disperse easily in water (Shahani, personal communication) and may contain no lignin at all. Sensitivity to Light-Induced Mechanical Damage In the early stages of this research program, mechanical properties were tested in an effort to determine which tests were most sensitive to the effects of photo-exposure. Sensitivity was defined as the ability to take a given amount of paper, test it, and determine whether it had changed from the baseline value using Student’s t-test for statistical significance. The log of MIT fold was found to be more sensitive than stretch, tensile strength, TEA, brittleness index, zero span tensile strength, or tear. Appendix D contains a summary of the data and describes how this determination was made.

Advantages of the MIT fold test are that (1) the values change dramatically with aging, (2) the test uses only small amounts of sample, and (3) the test is accepted by the community of conservators as a method of determining paper degradation. MD stretch and MD TEA were also fairly sensitive to mechanical property changes with photo- exposure.

The log of the fold number was used, rather than the fold number itself, because section 11 of the TAPPI fold standard (T 511 om-88) states “the log 10 of the fold number provides a much more realistic and less misleading result” then does the raw data. In addition, the log of fold data has a much more Gaussian distribution than the fold itself, and standard statistical tests assume a Gaussian (normal) distribution.

FPL 23 MIT Fold There was a statistically significant loss in MIT for all but one ISR paper after long-term natural exposure (Figures 10 and 11, Appendix E). The change in fold of the buffered alkaline cotton sheet declined consistently, but not enough to be significant in most cases. The decline in all other samples indicated that even the most stable papers could be damaged by prolonged exposure to typical indoor lighting conditions. Figure 11 shows change in the log of fold with exposure; Figure 10 is included to emphasize the magnitude of the change. On the log of fold plots, a decrease of 1 unit corresponds to a 10-fold reduction in the fold number. Some individual samples that typically withstood 50 folds in control survived only 2 folds after extended photo-exposure.

100%

90%

80%

70%

60%

50%

40%

30% Loss of Fold Number (%) 20%

10%

0% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Paper ID

Figure 10. Percent loss in MIT fold number after 29 months of north window exposure, shown to emphasize the magnitude of change in folding endurance.

FPL 24 1.4

1.2

1.0

0.8

0.6

Loss in Log of Fold 0.4

0.2

0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Paper Type

Figure 11. Loss in MIT fold after 29 months of north window exposure. Same data as Figure 10, but shown in logarithmic units. Error bars are 96% confidence intervals.

Compositional variables had significant effects on the fold performance. Of those considered, pH had the most clear and consistent impact on fold endurance. On average, pH 5 papers lost 0.18 log of fold units more than papers with the same composition at pH 8.

Papers 5 and 6, made from long staple (textile) cotton, were significantly more stable (average 0.18 log of fold units) than lignin-free wood-based papers 1, 2, 11, 12, and 15. The stability of cotton compared with kraft paper may be due to the complex morphology of the cotton. Without testing modern paper made with cotton linters, we do not know whether such papers would be more stable than kraft papers. Because cotton papers performed so much better than did wood-based lignin-free papers, the following comparisons were made using data from wood-based fibers only.

Lignin had a significant effect on retention of fold endurance but the interactions were complex. Papers with klason lignin of 24%–25% (papers 3, 4, 7, and 8) lost on average 0.14 log of fold units more than did lignin-free papers 1, 2, 11, 12, and 15. The loss in properties due to lignin and acid together were roughly additive.

Papers with klason lignin levels of 7%–14% were compared to kraft (lignin-free) papers with similar pH and carbonate levels. Buffered alkaline lignin-containing sheets 10 and 13 were an average of 0.13 logfold units more stable than lignin-free papers 2, 12, and

FPL 25 15. Acidic lignin-containing paper 14 was 0.19 logfold units more stable than lignin-free paper 1. There was no consistent difference between unbuffered alkaline sheets 11 and 9.

For lignin-containing papers, there was a clear trend (average 0.21 logfold units) toward more damage in the north window chamber than in the halogen or fluorescent chambers. For lignin-free (kraft) papers, there was no consistent difference between chambers.

No significant difference in permanence was seen between hardwood and softwood kraft papers, as measured by comparing papers 2 and 12. The role of calcium carbonate was addressed by comparing papers 10 vs. 9 and 12 vs. 11. The only significant difference was that paper 9 lost significantly more fold endurance than did paper 10 after 29 months of exposure. One possible explanation for this observation is that the organic acids generated after long-term exposure in lignin-containing papers 9 and 10 were neutralized by the buffer in paper 10 but not in paper 9. Future research by FPL is planned to investigate this question.

Moisture Content Moisture content of paper after photo-exposure was determined to help explain the loss in folding endurance. Although there was a general loss of moisture content with exposure, it could not account for all the mechanical property changes observed. On average, moisture content of lignin-containing and lignin-free papers declined by 0.7% and 0.3%, respectively, after 29 months of natural exposure, as shown in Table 5. Data on the effect of moisture on fold endurance (Crook and Bennett 1962) showed that a reduction in moisture content by 0.3% would reduce fold in a kraft paper by less than 0.1 logfold unit. This was less than the change observed for all the kraft papers included in this study (Figure 11). We would expect that the fold change in lignin-containing papers would also be smaller if caused only by a change in moisture content.

FPL 26 Table 5. Change in equilibrium moisture content for papers aged 29 months in natural aging chamber vs. cold storage (90% confidence interval for individual paper = ±0.8) Reduction in Reduction in Paper ID Paper ID Moisture Content Moisture Content 1 0.3 3 0.2 2 0.1 4 1.5 5 0.6 7 0.3 6 0.4 8 0.6 11 -0.2 9 0.6 12 0.5 10 1.0 15 0.3 13 0.6 Lignin-free 14 0.8 0.3 Average Lignin- Containing 0.7 Average

Viscosity We found that that even lignin-free, calcium-carbonate-loaded sheets suffered mechanical properties damage after prolonged photo-exposure. To understand this, viscosity was measured on papers 5, 6, 11, and 12. Aged sheets were exposed for 29 months in the north window chamber, followed by 10 months at 4o C sealed in the dark in polyethylene bags. In Tables 7 and 8, viscosity is given in centipoise (cps) and the 95% confidence interval for values in the range of 5–20 cps is ±0.8. From the loss of viscosity shown in Table 7, we conclude that even the most stable papers are subject to chemical degradation over time, when exposed to natural lighting through windows. This reduction in viscosity could be directly correlated with a reduction in degree of polymerization (DP) of the cellulose. Since the decay of paper through acid hydrolysis also constituted loss of DP, the results of photo-exposure and acid exposure were in some ways the same. Another parallel between acid and photo-induced degradation was that photo-oxidation reactions in cellulose produced carbonyl or carboxyl groups (Daruwalla et al. 1967). These groups are acidic and so they catalyzed acid degradation of the paper.

Cellulose and hemicellulose do not absorb light at wavelengths used in this study, so they would be expected to be inert. However, photosensitizers such as transition metal ions, titanium dioxide, dyes or pigments, and lignin have been proposed as the absorption- center that creates a free radical, which then damages the cellulose. Though these compounds usually occur at very low concentrations, Buschle–Diller and Zeronian (1993) ,among others, have documented the depolymerization of cellulose by light filtered through glass.

FPL 27 Further testing was done to determine the relative effectiveness of different lighting conditions toward depolymerizing cellulose. From the results in Table 8, we conclude that the three chambers are not statistically distinguishable. Because UV intensity was an order of magnitude higher in the north window chamber and UV was more efficient at initiating photo-degradation, the north window was expected to cause a greater drop in viscosity. Apparently the 2.4× higher average intensity of visible light in the halogen and fluorescent chambers was enough to offset the lower efficiency of the visible light in initiating photochemical reactions.

The results reported by Daruwalla et al. (1967) are especially relevant in that they showed depolymerization of cotton (cellulose) after irradiation with UV, blue, green, yellow, red, and infrared radiation. This supported the theory that visible wavelengths are damaging to cellulose and that the mechanism probably depends on products available in only trace amounts.

Table 6. Viscosity measurements of lignin-free papers with and without 29 months of photo-exposurea

Paper Exposure pH CaCO3 Pulp Viscosity Control 14.8 11 80%Kraft Aged 5.9 Control 11.5 12 85%Kraft Aged 6.2 Control 73.2 5 5 0% Cotton Aged 6.4 Control 83.8 6 8 5% Cotton Aged 10.2 aLow values for controls 5 and 6 are due to incomplete dissolution of cellulose

FPL 28 Table 7. Viscosity of cotton papers after 44 months of natural aginga

Paper Exposure pH CaCO3 Pulp Viscosity Unaged 119.3b North 5.3 5 5 0% Cotton Halogen 4.6 Fluor 3.7 Unaged 89.8 North 5.4 6 8 5% Cotton Halogen 4.5 Fluor 5.1

aModern cotton papers usually have lower viscosity values because they are made from linters rather than textile-grade cotton. bControl 5 clogged the viscometer.

The magnitude of change in viscosity numbers relative to the error suggests that viscosity is an excellent means of monitoring photo-induced decay of papers. Especially in cases of very stable papers like buffered cotton, changes in viscosity were evident before any mechanical tests indicated a change in properties. It also confirms the fear that even visible light with little UV, over a long time, can significantly harm even the best archival documents. Findings from Physical Tests We observed that the mechanical property most sensitive to photo-aging effects was the response to multiple folding, as measured by the logarithm of the number of folding cycles in the MIT method. Almost all papers used in this study were observed to experience decline in MIT folding endurance following photo-exposure. Other tests of mechanical properties that were quite sensitive to photo-exposure included machine direction (MD) stretch prior to failure and MD tensile energy absorption (TEA).

After 29 months exposure in the natural aging chambers, the equilibrium moisture content of all the papers declined. The extent of the decline was approximately 0.3% for the lignin-free papers and approximately 0.7% for the lignin-containing papers. In both instances, this effect pointed to a certain amount of cross-linking occurring within the fibers as the result of photo-oxidatively initiated free radical reactions.

For lignin-free papers, the degree of polymerization (DP), as reflected in viscosity measurements, was found to be the most sensitive measure of photo-degradation resulting from photo-exposure. These changes pointed to the occurrence of a significant level of chain cleavage reactions.

In mechanistic terms, it would appear that for lignin-free papers the loss in MIT fold strength after 29 months of exposure was primarily associated with the decline in the DP of cellulose in the fibers. In the lignin-containing papers, an extra factor may have been

FPL 29 embrittlement of the fibers due to photo-induced cross-linking reactions associated with the lignin. Chemical Changes Multiple analyses were performed to characterize the chemical changes in naturally aged papers. The primary goal of this testing was to compare papers aged with the accelerated protocol to those aged under natural conditions. This section describes observations made on the naturally aged papers. Extensive chemical testing was necessary to compare the results of natural and accelerated aging, and it will also help us understand the mechanism of degradation. Constituent Sugars Cellulose is a very long linear polymer of glucose. It makes up 45%–50% of the weight of the tree and is remarkably insoluble in water, even at low molecular weight. Hemicelluloses in softwood such as loblolly pine are 20%–25% of the wood weight and are composed primarily of xylan and mannan with small amounts of other sugars (Tylli et al. 1997). Hardwood hemicelluloses are dominated by xylan. Hemicelluloses have much shorter chain length than does cellulose, typically containing 100 to 200 sugar units. Hemicelluloses are also likely to be branched and are far more water-soluble than lignin. They are often removed along with lignin in pulping processes and are more likely than cellulose to be in physical contact with lignin.

Analysis of water-soluble extracts for constituent sugars of photo-exposed (35 months) paper was carried out using ion chromatography (IC). Papers were soaked in water and removed. The water was then treated with acid to reduce carbohydrate polymers to individual sugars. All 15 ISR papers in the north chamber, as well as papers 3, 4, 9, and 10 in the halogen and fluorescent chambers, were investigated. Almost 8% of the mass of some sheets was extracted with water after photo-exposure, whereas less than 1% of the same sheet was extracted when unexposed. The factors correlating with a large amount of extracted sugars were low pH and high quantities of lignin. North window, fluorescent, and halogen exposure released the most sugars in that order, as shown in Figure 12. Xylan was prominent in all wood-based pulp and mannan was observed in softwoods. Glucose at 0.19% and 0.09% by weight was extracted from cotton papers 5 and 6, respectively vs. 0.02% in control sheets, indicating a small amount of cellulose hydrolysis; thus, free radical cellulose chain scission probably did occur. Glucose residues in other papers were similar in quantity to arabinose and galactose, indicating that hemicellulose was probably their primary source. A complete set of data can be found in Appendix F. Standard deviations ranged from 0.02% to 0.05%.

FPL 30 3.5%

3.0%

Fucose 2.5% Arabinose

2.0% Galactose

Rhamnose 1.5% Glucose

Xylose 1.0% Mannose

Sugar Extracted (% Total Mass) 0.5%

0.0% Control North Fluorescent Halogen

Figure 12. Carbohydrates extracted with water from paper 3 exposed for 35 months. Fucose was not detected in any samples of paper 3.

If cellulose was depolymerized, significant levels of glucose could have been expected to appear in the water-soluble extracts. Note that cellulose fragments must have 5 or fewer glucose units to dissolve, and the fraction of such fragments arising from random breakage of cellulose chains is very small. On the other hand, hemicelluloses are soluble at up to hundreds of sugar units, and the parent chains are much shorter. Therefore, it is reasonable that the extracted carbohydrates were dominated by hemicelluloses.

There are many paths by which the carbohydrates could be released upon photo- exposure. The lignin network was clearly damaged by photo-oxidation. This could free carbohydrates in several ways. Carbohydrates that were physically trapped in the lignin matrix could be released. Reactive lignin fragments produced in photoyellowing could attack carbohydrates. Also, metal ions (and possibly lignin) adsorbed onto the carbohydrates could act as photo-sensitizers, allowing photo-oxidation of carbohydrates either directly or through formation of free radicals.

The total carbohydrate released from calcium-carbonate-filled sheets numbers 4, 8, 10, and 13 was on average 0.71% less (based on total paper mass) than that of their unbuffered companion papers 3, 7, 9, and 14. Despite this very strong trend in BCTMP (filled) papers, there appeared to be no difference between SGW papers 8 (filled) and 7 (unfilled). The cause of this difference was unclear.

FPL 31 Surface pH Surface pH was measured on all 15 ISR papers, control and aged 29 months. Surface pH was recorded 3 min after a flat electrode was placed on a drop of deionized water on the paper surface. The goal was to determine the extent of the acidification of the surface of the paper through photo-oxidation. Sheets with a nominal pH of 5.0 dropped an average of 1.2 pH units, while the alkaline sheets dropped only 0.1 pH units on average. This could also be interpreted to mean that calcium-carbonate-buffered sheets lost 0.1 pH units while those without CaCO3 lost 0.9. Because of the high correlation between calcium carbonate loading and initial pH, we could not determine which one was responsible for the pH change, but we suspect the calcium carbonate buffering prevented pH change. Acidic groups were probably formed in all sheets, but were neutralized in the buffered sheets. Alkaline papers containing BCTMP or SGW pulp (lignin-containing papers) lost an average of 0.7 pH units on exposure, compared to a loss of 0.2 units in lignin-free papers, indicating that lignin was a significant factor in paper degradation. Photo-oxidation of lignin to organic acids and photo-oxidation of cellulose to form carbonyl and carboxyl groups are well-documented methods of light-induced acidification. HPLC High pressure liquid chromatography (HPLC) was used to separate and quantify lignin fragments extracted with methanol from papers 3, 9, and 10 (pH 5 SW, pH 5 HW, and pH 8 HW) after 35 months of photo-exposure. Compounds were identified by their retention time on the separation column and by their UV/VIS spectra. Chromatograms for all three papers are included in appendix G.

Figure 13 illustrates the following observations: Lignin was clearly depolymerized or fragmented by photo-exposure—much more material was released in aged samples vs. the control. Hardwood and softwood lignin showed clear structural differences, evident by comparing papers 3 and 9. Natural exposure with different light sources produced the same compounds with variation in the relative quantities of

Some products that were evident in the control (unexposed) sheet were unexpected. It is proposed that some small amount of photo-exposure did occur during handling, sample preparation, and extraction.

FPL 32 6000 10 Vanillic Acid 3 Fluorescent 9 Syringic Acid 13 Syringaldehyde 8 4-Hydroxy Benzoic Acid 9 Fluorescent + Vanillin 5000 9 North 9 Halogen 3 9 control 4000

6 1 11 3000 4 5

7 2000 2 11 12

Absorbance (Arbitrary Units) 1000

0 4 5 6 7 8 9 10 11 12 13 Time (min)

Figure 13. HPLC chromatograms of extracts from papers 3 and 9 after 35 months of exposure. Chromatograms are offset for clarity. Absorbance at 220 nm. Data labels show peak numbers. Top-bottom order is preserved in legend.

We specifically looked for acetosyringone, acetoguiacone, veratric acid, 4-OH- acetophenone, and ferulic acid, as well as the compounds named in Figure 13. These compounds have all been reported from lignin photo-degradation. All of these compounds eluted between 10 and 16 min, and it was not possible to match the UV absorbance of the small peaks with those of standards. Therefore, we believe that those compounds were not present at levels above 6 ppm in the papers. Fluorescence Measurements Fluorescence measurements were made by irradiating samples with a specific wavelength of light. The wavelength and intensity of light emitted from the sample were observed. This technique was useful because it focused on the chemicals likely to be photo-active. The only chemicals that fluoresce are those compounds that absorb incoming light and hold on to the energy long enough to re-emit it. Steady State Steady-state fluorescent measurements quantify the wavelength and intensity of emissions. The most obvious change upon exposure was that overall fluorescence intensity was greatly reduced upon photo-exposure. This suggested that fluorescent compounds were photochemically reactive and tended to be consumed during irradiation. This reduction in intensity was not uniform, however, as some fluorescence signals were enhanced and some diminished during photo-exposure. Generally, the photo-active species that were excited in the UV light (<400 nm) and emitted at wavelengths below

FPL 33 500 nm were greatly diminished by photo-exposure. Photo-active species excited at wavelengths above 400 nm and all emissions at wavelengths greater than 500 nm tended to increase with exposure. The increase in long wavelength emissions agreed with and complemented the scope of the observations by Tylli et al. (1997), which suggests a general similarity in the evolution of populations of chromophores derived from lignin. It also indicated that the yellow compounds absorbing at wavelengths above 400 nm were fluorescent and emitted light at wavelengths longer than 500 nm.

The photo-active species contributing to the SW–SGW fluorescence appeared to be distinctly different from those active in the BCTMP spectra. The emission peaks of unaged samples did not coincide; similarly, there were significant differences in the spectra of the chromophores generated upon photo-exposure. This reflected the sensitivity of the spectrofluorometric method to differences between the different chemical structures that dominated the response of lignin in the two different papers.

Fluorescence emission intensity declined 3%–8% per 10o C increase in temperature for kraft, BCTMP, and SGW papers. Intensity decreased slightly when high temperature samples were humidified so that moisture content in the paper approached normal room conditions. The most likely cause for this was that the energy in photo-excited states was converted to heat more efficiently at higher temperature, supporting the argument for lower temperature irradiation. The decrease in fluorescence intensity with RH suggested that moisture might have been stabilizing a photochemical intermediate state, thereby diverting the energy of the excited-state molecule toward that intermediate state and away from fluorescence. This was also supported by observations that papers yellow more quickly in moist environments, whether in accelerated or natural aging. The loss of fluorescence with higher RH could also have been a result of more efficient thermalization of the excited state energy, which would result in less photochemical reactions. In either case, fluorescence observations indicated that the effect of RH was small, which was consistent with all other observations. Fluorescence Lifetime Fluorescence lifetime measurements determine the average time between absorption and re-emission of a photon. Such measurements showed that photo-exposure reduced or eliminated any long-lived fluorophores (on the microsecond time scale) that may have been present in the paper originally, and that the lifetimes of fluorophores remaining after photo-exposure were predominantly 0.5 ns. Two lifetime categories, 0.5 and 2 ns, accounted for at least 70% of the signal in every high-lignin ISR paper, before or after exposure. These observations, as well as the decrease in overall fluorescence, indicated that virtually all the long lifetime as well as many short lifetime fluorophores were chemically changed by photo-exposure.

The distribution of lifetimes in paper 1 (kraft) was only slightly affected by photo- exposure, supporting the hypothesis that there are different excited state reactions in lignin-containing and lignin-free papers. Some fluorophores were not captured in lifetime studies because they decayed too quickly for measurement.

FPL 34 UV/VIS Spectroscopy Diffuse reflectance UV/VIS spectroscopy was used to analyze all 15 ASTM papers. This technique measures the reflectance of paper at all wavelengths of UV and visible light. It has been found to be very sensitive to chromophores that are produced upon photo- exposure (Schmidt and Heitner 1993). The spectra shown are the difference in reflectance of the unaged and aged sheets. The figures show increased absorption with light exposure as a positive peak. Lignin-Containing Papers After 3 months of natural exposure, paper 3 developed a peak and shoulder in absorbance at 424 and 370 nm, respectively (Figure 14). The same peaks were present for paper 4, but the 370-nm peak was larger. For papers 3 and 4 after 3 years, the 424-nm peak had grown stronger and had shifted to 435 nm. The 370-nm peak was still evident in fluorescent and halogen exposures, but was small compared to the 435-nm absorption. After 4 years exposure, an increased absorbance at 500 nm and a small absorbance at 260 nm were evident in all three exposures.

45 North 4 years Hal 4 years 40 Fluor 4 years 35 North 3mo 30 Hal 3mo Fluor 3mo 25

Loss in Reflectance (%) 10

0 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 14. Loss in reflectance in paper 3 after different natural exposure times (3 months and 4 years).

Loss-in-reflectance curves for papers 9 and 10 were clearly different from those of softwood-based papers 3 and 4. After 3 months of north window exposure, the band at 370 nm was much stronger than the small shoulder at 413 nm. After 3 years, the shoulder at 370 nm was not as prominent, which could have been a consequence of the fact that the main peak was shifted to 413 nm (for North, to 404 nm). This shift caused an overlap with the 370-nm peak and therefore the 370-nm peak could no longer be discerned. After 4 years of exposure, there was not as much absorption in the 500-nm region (compared to that in papers 3 and 4), but the absorbance at 260 nm was present in all three aging conditions. Such changes in spectra were felt to have been due to structural differences in lignin between hardwoods (papers 9 and 10) and softwoods (papers 3 and 4).

FPL 35 45 North 4 years Hal 4 years Fluor 4 years 35 North 3mo Hal 3mo 25 Fluor 3mo

Loss in Reflectance (%) 5

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 15. Loss in reflectance of paper 9 after 3 months and 4 years of natural exposure.

For papers 13 and 14, UV/VIS changes in reflectance were very similar to those obtained for papers 9 and 10. The only difference was that the wavelength positions for the papers naturally aged under three different conditions were slightly different. Whereas fluorescence exposure produced a peak at 419 nm, halogen and north window aging generated peaks at 405 and 400 nm, respectively. The similarity between the photo-aging behavior of papers 9,10, 13, and 14 was expected because all of these papers contained the same fiber in different proportions.

In contrast to the pulps already mentioned, papers 7 and 8 contained unbleached fiber. This meant that coniferaldehyde was present in these papers and would have a large impact on the reflectance curves. After 3 months exposure, an absorption minimum was evident at 355 nm due to degradation of coniferaldehyde. There was no absorption evident at 370 nm, which may have been due to a cancellation of effects between the diminishing 355-nm band and an increasing 370-nm band. The main peak was at 420 nm and was similar to that of paper 3 at 3 months. After 3 and 4 years, however, the main peak was shifted to 450 nm and its shape on the lower wavelength side was somewhat modified. As with papers 3 and 4, papers 7 and 8 developed the band at about 260 nm.

FPL 36 35 North 4 years Hal 4 years

25 Fluor 4 years North 3mo Hal 3mo Fluor 3mo 15

5 Loss in Reflectance (%)

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 16. Loss in reflectance of paper 7 after 3 months and 4 years of natural exposure.

Lignin-Free Papers After 4 years of natural aging, both papers 1 and 2 yellowed under all natural aging conditions. However, there were some differences in the reflectance loss curves. For paper 1 yellowed under fluorescence and halogen conditions, bands were detected at 268, 303 (weak), and 350 nm. For paper 2, by comparison, the band at 303 nm was much more prominent and instead of appearing at 350 nm, the band was present at 378 nm. When the reflectance loss curves of north window yellowed papers 1 and 2 were contrasted, we found that although both curves showed the 268-nm band, the 303-nm peak was more pronounced in paper 1. In addition, paper 2 had higher absorbance in the visible region and showed a peak at 378 nm. Such differences may have been related to the differences in the pH of the papers. When a comparison was made between the curves of the papers that were exposed to light for 3 months, both papers 1 and 2 showed negative absorption at 246 and 325 nm, indicating that species capable of absorbing at these wavelengths were degraded. Comparing the 3-month and 4-year data, we concluded that the paper degradation behavior was initially similar but differences were likely to arise later.

FPL 37 25

North 4 years 20 Hal 4 years Fluor 4 years 15 North 3mo Hal 3mo

10 Fluor 3mo

5 Loss in Reflectance (%) 0

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 17. Loss in reflectance of paper 1 after 3 months and 4 years of natural exposure.

Naturally aged papers 11 and 12 showed changes that to a great extent were similar to those observed in papers 1 and 2. Fluorescence aging caused bands to appear at 268, 303, and 350 nm. Halogen light, on the other hand, was the cause of the peaks at 268, 303, and 378 nm in the reflectance loss spectra of both papers. In contrast, under North exposure, the peaks were detected at 268 and 303 nm and were not well separated. Additionally, the long wavelength band seemed to appear at an even longer wavelength (414 nm). In the case of 3-months photoexposed papers 11 and 12, the results obtained were very similar to those obtained for papers 1 and 2. In both sets, the paper reflectance increased at 245 and 314 nm.

FPL 38 30

North 4 years 25 Hal 4 years 20 Fluor 4 years North 3mo 15 Hal 3mo

10 Fluor 3mo

0 Loss in Reflectance (%)

-5

-10 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 18. Loss in reflectance of paper 12 after 3 months and 4 years of natural exposure.

Paper 15 was similar in composition to paper 11 with the exception that the latter had 5% CaCO3 along with starch and additives. The reflectance loss curves indicated that upon 4 years of aging, the spectral changes were analogous to that of paper 1 except that under halogen exposure the long wavelength band maximum was at 350 nm as opposed to 378 nm (paper 1). Other features in the reflectance loss curves for paper 15 closely resembled those for paper 11. This was also the case when paper 15 was photo-exposed for 3 months (in the three chambers). Increased reflectance in the reflectance loss curves was detected at 245 and 314 nm. Infrared Spectroscopy Infrared (IR) spectroscopy measures the vibrations of polar molecular bonds, while Raman spectroscopy measures the vibrations of non-polar molecular bonds. Both techniques were used to analyze some of the lignin-containing naturally aged papers and their methanol extracts. Methanol extraction tends to concentrate the photo-degraded material, making structural information more apparent.

IR spectroscopy has produced useful information from photo-exposed pulps/papers (Forsskåhl and Janson 1992, Agarwal and McSweeny 1999). For instance, it is well established that an IR band due to carbonyl groups can be easily detected in the spectra of photo-exposed papers. In certain published reports, additional IR information has been interpreted in terms of various chemical changes. However, in our opinion, such interpretations may not necessarily be correct.

In our work, IR spectroscopy was used to analyze selected lignin-containing naturally aged papers and their methanol extracts. Material extracted in methanol was studied because better spectral information could be obtained (Agarwal and McSweeny 1999).

FPL 39 Upon sunlight exposure, IR spectra of lignin-containing papers 3 and 9 (which were selected for in situ analysis) did not change dramatically. The only easily notable change was the appearance of a band at about 1727 cm-1. The latter band has been well documented in the literature and is thought to arise as a result of the formation of carbonyl groups. Considering the results of a previous study (Agarwal and McSweeny 1999) where IR analysis of the methanol-extracted material from photo-exposed pulp provided additional useful information, North window exposed papers were extracted using methanol. For this, papers 3, 4, 7, and 8 were chosen.

IR spectra of the extracted materials are shown in Appendix I. These spectra resembled each other quite closely and were significantly different from the spectra of extracted materials from control papers. Especially noteworthy was the fact that in some IR spectra of the photo-degraded materials an IR band at about 1677 cm-1 was detected. This IR band has been assigned to p-quinones (Agarwal 1998). In certain other spectra, the 1677- cm-1 band was not well resolved and appeared to have merged with the higher wavenumber band at 1727 cm-1. Depending upon the extent of this overlap, either both original band positions were modified or only one band was detected at 1700 cm-1.

The band present at 1386 cm-1 in some spectra was an artifact and could be traced to contamination in the potassium bromide that was used for making the pellets.

The IR spectra of the extracted materials from controls were likely to be due to extractives because the spectra differed significantly from those of photo-degraded materials. The two additional characteristics of the control-extracted materials were that these materials were not strongly colored and their amounts were very small. The latter explains why their IR spectra had low signal-to-noise ratios.

The other lignin-containing papers were not analyzed because information similar to that obtained from papers 3, 4, 7 and 8 was expected. Lignin-free papers were also not analyzed since, based on prior experience, their spectra were not significantly changed by photo-exposure. Raman Spectroscopy Previously, Raman spectroscopy has been applied in the study of lignin-containing and lignin-free pulps/papers (Agarwal 1999). In addition, lignin, cellulose, and their model compounds have been studied. Based on such research, Raman spectra of pulps and papers have been assigned in terms of contributions of various components, functional groups and/or sub-structures.

Earlier Raman studies of photo-aging indicated that only certain regions of a spectrum were influenced by changes when a pulp/paper sample was exposed to light (Agarwal and McSweeny 1999). Such changes were detected at 1600, 1620, 1654, 1660 and 1690 cm-1. A study was also conducted on the material that was extracted (from photo-exposed papers) using methanol. The main advantage of the extracted material analysis was that the contribution at about 1675 cm-1 was clearly detected. From the literature,

FPL 40 contributions at this and other wavenumbers where changes occurred, were assigned to the groups/units of lignin shown in Table 8.

Table 8. Assignment of IR bands in photo-exposed papers Wavenumber/region (cm-1) Assignment 1600 Benzene ring 1620 Coniferaldehyde (C=C) 1654 Coniferyl Alcohol (C=C) and coniferaldehyde (C=O) 1660–1690 p-Quinone

In the work reported here, papers 3 and 9 were selected for in situ analysis whereas methanol extracts of papers 3, 4, 7, and 8 were analyzed. The main difference between papers 3 and 7 was that paper 3 was made of bleached pulp whereas paper 7 was composed of unbleached pulp (the same difference existed between papers 4 and 8). The chemical differences between the bleached and unbleached pulps were mostly due to differences in the chromophoric nature of lignins. Compared to a bleached pulp lignin, the unbleached pulp lignin contained coniferaldehyde and p-quinone units.

The decision to analyze methanol-extracted material was based on the rationale that since photo-degradation was mainly confined to the paper surface and because both modified and unmodified areas were being sampled during the in situ analysis, the spectral contribution due to modified entities would be limited. If removed from the surface and concentrated, the degraded material would give a Raman spectrum that better represented the photo-exposed material. The choice of methanol was based on prior work that had indicated that of the solvents investigated, methanol was the best suited for extracting the colored material.

When Raman spectra of sunlight-exposed and control papers (both papers 3 and 9) were compared, spectral changes indicated a slight broadening of the 1600-cm-1 band, a decline in the intensity of the 1654-cm-1 peak, and increased intensity in the 1660–1690 cm-1 region. Because both papers 3 and 9 showed similar changes, the results implied that the photochemical processes occurring in softwood- and hardwood-based papers were similar. Spectral similarity was also seen between the spectra of the materials that were extracted in methanol from the sunlight-exposed papers. Especially noticeable was the fact that extracted material from both papers gave a Raman peak at 1672 cm-1 (indicative of p-quinones). Therefore, p-quinones were produced upon sunlight exposure and their presence was likely why the photo-exposed papers turned yellow.

Extracted materials of photo-exposed papers 3, 4, 7, and 8 produced spectra that were very similar (Figure 19). Once again, the feature at about 1672 cm-1 was quite prominent, indicating that p-quinone entities were present in significant amount. This suggested that the photo-induced changes were likely to have been similar among papers 3, 4, 7, and 8. Papers 3 and 4 differed with respect to pH (5 and 8.1, respectively), the amount of CaCO3 (paper 4 had 5%), and additives (paper 3 had alum and rosin), and still showed Raman changes that were similar. Therefore, it was likely that these differences did not play a significant role in photo-degradation. This conclusion was further supported by the

FPL 41 results of Raman analysis of papers 7 and 8 (like papers 3 and 4). These papers also had similar differences with respect to pH, CaCO3, and alum, and both papers showed similar changes in Raman spectra.

0.3 8 Nat

0.25 4 Nat 3 Nat 7 Nat 0.2

0.15

0.1

0.05 Raman Intensity (Arbitrary Units)

0 250 750 1250 1750 2250 2750 3250 Wavenumber (cm-1)

Figure 19. Raman spectra of papers 3, 4, 7 and 8 after north window exposure. Vertical order of spectra is conserved in legend.

Extracts of control papers were also obtained and analyzed to ensure that indeed the spectra of the photo-exposed materials were different and provided information that was related to photo-aging. When these spectra were compared to the spectra of the extracted materials from photo-degraded papers, we found that the spectra of control paper extracted materials differed significantly. The latter were likely to have represented extractives in the papers.

Papers that were photo-aged in fluorescent and halogen chambers were not analyzed using Raman technique because previous experience had indicated that such analysis was likely to provide information similar to that obtained for sunlight-exposed papers. Findings from Chemical Analyses Considerable evidence showed that photo-exposure resulted in depolymerization of the constituents of the papermaking fibers. This change was manifested in increased solubility of polysaccharide and lignin fragments in water and methanol, respectively. Up to 7% of the mass of some papers became water soluble upon exposure. The soluble polysaccharide fragments were primarily derived from hemicelluloses, though there were some indications that a low level of fragmentation of the cellulose also occurred.

Absence of calcium carbonate, low pH at formation, and presence of lignin were correlated with a decline in surface pH upon photo-exposure. Because of the interactions between calcium carbonate content and pH levels, it was not possible to separate the effects of these two variables in a systematic manner.

FPL 42 Analyses of the solubilized products by HPLC showed that the three natural aging environments (north window, halogen, and fluorescent exposure) produced many of the same products in differing quantities. The products that were quantified included 4- hydroxybenzoic acid, vanillic acid, syringic acid, and syringaldehyde/vanillin. Seven unknown peaks were also quantified.

Fluorescence measurements suggested that, as anticipated at the outset of the program, higher temperatures could result in the quenching of photo-excited species. Thus, some photo-excited molecules were removed before they could initiate any of the photochemically induced changes that were observed at lower temperatures.

Ultraviolet/visible spectra pointed to the development of chromophoric groups with broad absorption bands centered at 435 nm in softwood pulps and 412 nm in hardwood pulps; these observations were typical for lignin-containing pulps. The spectra also indicated substantial decay of coniferaldehyde (absorbing at 355 nm) with photo- exposure of stone groundwood papers.

The IR and Raman spectra of naturally aged lignin-containing papers and their extracts indicated that photo-exposure resulted in reduced levels of coniferaldehyde and coniferyl alcohol. They also suggest the formation of p-quinone groups from some of the lignin substructures.

FPL 43 Accelerated Aging of ISR Papers The goal of this research project was to recommend conditions for a sound accelerated photo-aging procedure. After initiating the natural aging program, we focused on delineating a set of aging conditions for such a test that would be scientifically valid. Exposure Conditions The initial work with accelerated aging used a Rayonet photoreactor (RPR). This apparatus is very inexpensive, small, safe, and simple compared to broadband light sources. Several types of lamps are available for the apparatus, and the RPR was able to provide a very high (90-W/m2) UV flux. However, the RPR was unable to deliver a broadband spectrum. Since virtually all visible and UV wavelengths of light are capable of initiating photochemical changes, broadband illumination was believed to be necessary for an accelerated test. For this reason, an Oriel solar simulator, delivering a high intensity beam with wavelength distribution similar to actual sunlight illumination, was used for most accelerated aging experiments. Rayonet Photoreactor The RPR consisted of a box containing 16 fluorescent tubes arranged in a circle inside a cylindrical reflector. The paper was mounted on a vertical stainless steel cylinder 9.5 inches high and 11 inches in circumference. The cylinder, with papers attached, rotated in the center of the circle of fluorescent tubes. Tubes had peak emissions at 350, 419, and 575 nm, each with a width at half-maximum of approximately 50 nm. Solar Simulator The recommended accelerated aging protocol was developed using an Oriel brand solar simulator incorporating a xenon arc lamp. A xenon arc was chosen because it provided a continuous spectrum over the entire range of wavelengths likely to be involved in the natural photo-aging of papers. As used in this study, the Oriel simulator delivered a light dose approximately equal to 300 days of north window exposure each day. Therefore, the standard 4-day solar simulator exposure delivered a light dose similar to 3.3 years in the north window chamber. The Oriel solar simulator instrument, with appropriate filtration, produced a spectrum that mimicked global solar radiation through 1.5 atm (ASTM E892). Window glass was used to attenuate wavelengths below 340 nm, which were unlikely to occur indoors. These wavelengths are especially energetic and could have easily led to chemistry very different from that observed under natural aging conditions.

FPL 44 45

40 /10nm)

2 35

30 Rayonet 350

25 Solar Sim

20 ASTM E892

5 Illumination Intensity (Watts/M

0 300 400 500 600 700 800 Wavelength (nm)

Figure 20. Illumination spectra of Rayonet photoreactor (RPR) with standard 350-nm lamps, solar simulator (configuration used for FPL experiments = 1.5 global + glass), and ASTM solar standard.

The instrument was operated in a conditioned room (23°C/50% RH) and air was blown over the paper surface during exposure to minimize environmental variations. Bulb power was adjusted to maintain constant intensity during normal operation. This was important because our experience with natural aging had shown that the rate of decay of optical properties was dependent on temperature and/or humidity conditions during exposure. Optical Properties

Lignin-Containing Papers The solar simulator was most similar to north window exposure in both spectrum and in the results produced. As seen in Figure 21, the decay curves for north window exposure and accelerated exposure (with x axis expanded by a factor of 300) overlapped fairly well in the initial phase of aging, reflecting the strong similarity of the two light sources. The factor of 300 was used because the solar simulator had 300× the average north window intensity.

The halogen and fluorescent aging chambers produced very similar decay curves. We speculated that this could have been due to the fact that the halogen and fluorescent chambers had similar UV and visible light levels. The decay of brightness for paper 9 is

FPL 45 shown in Figure 21, while Appendix K shows a similar pattern in all the other lignin- containing papers.

Paper 9 80 North Window Halogen 70 Fluorescent Accelerated x 300 60 Accelerated x 3500

40 Directional Brightness

20 0 200 400 600 800 1000 1200 1400 Days in Aging Chamber

Figure 21. Comparison of paper 9 brightness under various photo-exposure conditions. 90% confidence interval for accelerated exposure = ±1.5 brightness units.

Optical Stability Ranking of Papers The recommended accelerated aging protocol could not perfectly predict the final optical properties of a paper, but it could rate how strongly the properties would respond to light. The brightness and color ranking of sheets exposed to different light sources was generally preserved (Figure 22). This was true whether sheets were exposed in the north window, the halogen or fluorescent chambers, or the solar simulator. The sheets that yellowed substantially in one exposure condition, yellowed substantially in all.

FPL 46 80 Unaged 70 SSim North 60 Halogen Fluorescent 50

40 Directional Brightness

20 7 8 3 4 9 101413 ISR Paper ID

Figure 22. Brightness of paper exposed in 4-day solar simulator and 29-month natural exposure. Solar simulator exposure gave a good indication of stability of lignin-containing papers. Papers ordered by initial brightness.

Figure 23 shows the post-exposure brightness of papers after different types of aging. This graph highlights different interactions between paper types and lighting conditions. The order of papers (ranked from top to bottom) were very similar in the halogen and fluorescent chambers whereas those in the north window chamber are ordered differently, especially papers 7 and 8. Unbleached SGW papers 7 and 8 were brighter after north window exposure than after halogen, while all other papers (made from BCTMP) were darker. Figure 23 also shows how the solar simulator and RPR produced similar results with lignin-containing papers. The ranking order and relative spacing between papers were very similar. The only obvious difference was that the RPR produced a greater change in optical properties.

FPL 47 90 3 4 80 7

70 8 9 60 10 14 50 13

40 Directional Brightness

20 Unaged Oriel 4 RPR 4 F 17 H 17 N 17 F 29 H 29 N 29 Day Day

Figure 23. Directional brightness of lignin-containing papers after various exposure conditions. Key: N, H, and F = north, halogen, and fluorescent exposures; numbers 17 and 29 indicate 17 and 29 months of exposure. Numbers on the figure indicate papers. Lines between points show trends within each group of exposures.

Photochemical and Photo-Initiated Dark Reactions As seen in Figure 21, the brightness decay curve from accelerated and north window exposures overlapped only during the initial stage of decay. The fact that the x axis represented an equal radiation dose and the two curves overlapped suggest that during this initial stage of aging, photochemical reactions were the dominant factor in the yellowing process. In this stage of yellowing, the same chemical reactions likely took place in both chambers, with rates determined by the photon flux. As time progressed, the yellowing reactions in the accelerated chambers were clearly hindered relative to those in the naturally aged sheet.

Figure 24 gives some indication of the source of this photo-yellowing hindrance. Figure 24 shows the decay of two papers in the solar simulator, followed by storage in a cold room, then further illumination. Photo-initiated dark reactions clearly occurred— reactions that caused yellowing and occurred only after light exposure. These samples lost 6 to 8 points of brightness in 6 weeks of storage at 4oC. Storage overnight and over a weekend also resulted in notable changes in optical properties. We assumed that papers in the natural aging chambers experienced the same phenomenon. Therefore, it appears that while accelerated aging may well have reproduced the chemistry of natural aging during the initial stage, it probably could not reproduce all aspects of long-term natural exposure because dark reactions did not have enough time to progress to equilibrium. We

FPL 48 suspect that the lack of photo-initiated dark reactions was responsible for the higher leveling off of value of optical properties in solar simulator exposures. The sheets aged in the solar simulator experienced direct photochemical reactions, but the sheets aged in the north window experienced photo-initiated dark reactions as well and so darkened more.

43 Sits 6 weeks 41

39 Sits overnight

33 Sits over weekend 31 Paper 10 Directional Brightness 29 Paper 3 27

25 012345678 Illumination Time, Days

Figure 24. Color change during 3 different dark-storage periods at 4°C following solar simulator irradiation. After 6 weeks in a cold room, paper brightness was similar to that of paper exposed in the north window with same radiation dose. (This exposure was conducted at half normal intensity.)

It appeared that post-irradiation darkening was at least partially reversible. In the case of the data shown in Figure 24, papers were irradiated and then stored at 4°C for 6 weeks. They were then returned to the solar simulator. Corresponding data for papers 7 and 9 are included in Appendix L. The photo-initiated dark reactions appeared to have been reversed when radiation was resumed after 6 weeks of storage. This suggests that chromophores in photo-yellowed papers were apparently involved in a dynamic equilibrium. The chromophores generated in the dark were apparently bleached in the presence of light. This was also seen (to a lesser extent) when 3½-year-old, naturally aged papers were placed in the solar simulator. These very dark papers were bleached to higher brightness as well as to lower b* and a* values when exposed in the solar simulator.

A small study was undertaken to understand the magnitude of such changes by measuring brightness following the end of illumination. After short RPR illumination (to 10 points of brightness loss), brightness rose approximately 3 points during the next day. After

FPL 49 severe illumination (to 30 points of brightness loss), brightness rose only 1 point during the following day. In both cases, the rise was logarithmic, with more than half the rise occurring in the first 4 h. After 24 h, brightness began to decline and continued to do so for as long as it was measured. Although this trial was done with paper 3 in the RPR, observations made of all the lignin-containing papers exposed in the Oriel solar simulator and in the RPR were consistent with these results. The peaks in optical properties after overnight and weekend storage as seen in Figure 24, as well as the discontinuities in all accelerated aging decay curves, were examples of this phenomenon. Brightness changes were observed when papers were stored at 4°C, but storage in a freezer at –90°C appeared to have stopped this phenomenon. Papers exposed in the natural chamber also darkened and yellowed after several months of storage in the cold room at 4°C. The trends seen in brightness were mirrored in b* and a* values. Illumination Spectrum A continuum of wavelengths of light interacted with lignin-containing papers to influence their optical properties. Investigations of 350, 419, and 575 nm light sources were made in the Rayonet Photoreactor (RPR). Although all wavelengths changed the optical properties of lignin-containing papers, the 350 nm wavelength was the most damaging. The 419 and 575 nm wavelengths might bleach or darken the paper, depending on the its chemical composition and past light exposure. The literature on photo-yellowing has documented the wavelength dependent photo-cycling of chromophores in lignin-containing pulp (Forsskåhl and Maunier 1993, Ek et al. 1993).

The yellowing effects of visible wavelengths were discussed with reference to Figure 7, page 15. The more typical bleaching behavior of visible light was observed when sheets that had been exposed for 3 years in the north window became brighter and whiter (lower b*) upon placement in the solar simulator. In this case, it was suspected that products formed from photo-initiated dark reactions were bleached much faster by the intense Oriel illumination than by north window exposure. It was very likely that this bleaching was caused by visible wavelengths. More evidence of this phenomenon was the fact that when papers were exposed for a given UV dose in the RPR and in the solar simulator, the RPR sheets were subsequently several points lower in brightness. This ability of visible wavelengths to initiate several photochemical reactions that influenced color convinced the research team that a robust accelerated test should include visible as well as UV illumination. Intensity There was a concern that at high enough intensity, photo-excited molecules would be dense enough to interact with each other rather than with ground-state molecules, as in natural aging. Ideally, accelerated aging procedures should be designed to avoid this situation. To determine whether this threshold existed and where it was located, a series of paper samples were exposed to a standard dose of light at different rates. Figure 24 shows the result of this experiment. The lines are fitted exponential functions. It appears that higher intensity for a short time did less damage than did lower intensity for longer periods. In this case, delivering the same radiation dose at 18× higher intensity than the weakest light made a difference of more than 5 brightness units. All papers received the same UV dose (9.0 kWH/m2) at different rates. These data show that the final optical

FPL 50 properties depended on the time required to carry out an exposure, as well as the total dose of light. Because of this, we believe that the intensity of exposure (or time required to deliver the dose) as well as the total dose were important parameters to specify for a standard photo-exposure procedure.

50 Paper 10

Paper 9 45 Paper 7

Paper 3

Final Directional Brightness 40

35 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Illumination Intensity, KW/M2

Figure 25. Directional brightness after 9.0 kWh/m2 of irradiation at 250–1000 nm as applied at different rates in solar simulator. Direct solar illumination is defined as 0.70 kW/m2 in ASTM E892. Curves represent fitted logarithmic functions.

Stability Based on Initial Decay We recognized that some people are interested in using accelerated aging to predict how long a paper will last above a certain threshold. Therefore, an attempt was made to determine stability from the initial stages of aging. As with full 4-day solar simulator exposures, papers that were unstable in accelerated aging tended to be unstable in natural aging. Short-term solar simulator exposure separated lignin-containing papers into various categories of stability. When trying to quantify this relationship, we found that the time axis multiplier needed to overlay an accelerated aging plot onto the plot of north window aging differed by more than a factor of 2 between different papers. The difference in multipliers needed to overlay curves was increased as aging progressed. Despite this weakness, a general idea of stability can be determined with short exposure. In Figure 26, 100 days corresponds to 8 hours in the solar simulator (100/300).

A test based on very short exposure time could have been very useful, but it was believed to be far less robust than an extended exposure. After short-term exposure, optical

FPL 51 properties decayed quickly when removed from the exposure chamber and were very unstable after removal. Small variations in intensity, spectrum, and irradiation time resulted in variation in the radiation dose. During a very short exposure, these small differences affected the measured optical properties. A long-term exposure circumvented this problem by irradiating until optical properties were no longer responsive to small changes in dose. The instability of samples after short-term exposure could result in significant variation in measured optical properties due to differences in time following the completion of the exposure.

80 1 SSim 75 1 North 14 SSim 70 14 North 65 3 SSim 3 North 60

50 Directional Brightness 45

35 0 20406080100 Days in North Window (Solar Simulator x 300)

Figure 26. Papers are easily separated into general classes of photostability by solar simulator exposure.

Lignin-Free Papers Lignin-free papers generally had stable optical properties compared to those of lignin- containing papers, but some papers, particularly acidic and/or unbuffered papers, did suffer some decay upon natural photo-exposure. The solar simulator-based accelerated protocol used for the lignin-containing pulps did not determine the relative stability of lignin-free papers. In the solar simulator, all lignin-free papers darkened in the first hour and then bleached upon continued exposure. In contrast, the naturally aged cotton papers were bleached and paper 1 (acid-containing BNSWK) lost more than 10 points of brightness after 3 years of exposure in the north window.

FPL 52 Table 9. Loss in directional brightness (control minus exposed) of lignin-free papers after various exposuresa Loss in brightness for various papers Condition 1256111215 Oriel 4 Day -0.2 -0.6 -3.5 -1.2 0.2 0.4 -0.6 North 17 Mo. 4.3 1.6 -1.7 -1.7 3.0 0.9 1.1 RPR 4 Day 6.3 2.4 -1.9 -1.8 4.4 1.5 2.0 Fluorescent 17 Mo. 7.2 2.7 0.5 0.3 3.7 2.1 2.0 Halogen 17 Mo. 9.6 3.3 2.9 1.0 7.4 2.4 2.1

aNote: The solar simulator exposure tended to bleach papers. The north window 17-month and the RPR 4-day exposures produced the same relative stability ranking for lignin-free papers.

The RPR with 350-nm illumination was able to reproduce the rank order of lignin-free papers observed in the 17-month north window exposure. More important, data from the RPR exposure made it possible to make accurate assessments of the relative stability of various lignin-free papers.

Although the solar simulator and RPR had similar UV intensity, the Oriel also delivered a large dose of visible radiation. We believe that this visible radiation dominated the color reactions in lignin-free papers and was responsible for bleaching. Changes in Optical Properties Just as in the natural aging studies, the presence of lignin altered the pattern of response to accelerated photo-exposure. Thus, under accelerated aging conditions, the responses of the lignin-free and lignin-containing papers fell into two groups. The solar simulator- based accelerated aging studies could reliably rank the relative stability values of the lignin-containing papers in a manner that paralleled that of papers in the natural environments. The protocol was not adequate for ranking the optical stability of lignin- free papers. In contrast, use of the 350-nm radiation alone in the Rayonet photo-reactor provided a basis for accurately ranking the optical stability of all papers.

For lignin-containing papers, there was very good agreement in the ranking of optical properties for papers exposed in the north window and those exposed in the solar simulator. This was consistent with the similarity in the spectral distribution of the radiation under the two different sets of conditions.

Our observations further suggest that two classes of processes occurred during photo- exposure. The first was processes that are the direct results of photo-excitation. These processes were able to unfold along a number of reaction pathways depending on the nature of the absorbing center. They resulted in some decline in optical properties of the papers. The second class of reactions included those that were initiated by photo- excitation but that continued to unfold whether or not exposure to light continued; that is, over time, these photo-initiated dark reactions appeared to continue to affect optical properties regardless of whether light continued to be present. This effect was usually

FPL 53 observed in lignin-containing papers. Because of the longer time scale needed to complete these photo-initiated dark reactions, we believe that they were under- represented in the proposed accelerated aging protocol.

In general, lignin-containing papers that were susceptible to direct photochemical reactions were also susceptible to photo-initiated dark reactions. This effect was not investigated in lignin-free papers.

Another important observation was that the total irradiation dose was not sufficient to describe the illumination conditions experienced by the samples. Light intensity and spectral distribution were also important factors that affected the final optical properties. Mechanical Properties The mechanical properties of papers were generally degraded after exposure in either the solar simulator or the Rayonet photo-reactor (RPR). Figure 27 compares the change in log of fold of papers aged under standard solar simulator conditions with 29-month north window exposure. Error bars represent 96% confidence interval. Appendix E contains a complete set of data for change in fold upon photo-exposure.

1.4

1.2 SSim 1 North 29 Mo

0.8

0.6

0.4 Loss in Log of Fold 0.2

0 123456789101112131415 -0.2

Figure 27. Loss in log of fold with exposure. Solar simulator and 29 months in north window. Error bars are 96% confidence intervals.

It is interesting to note that the solar simulator exposure always produced less damage than north window aging, even though the solar simulator delivered a photon dose equal to more than 3 years of north window aging with very similar spectra. Again, this may

FPL 54 have been due to the time difference between natural and accelerated aging. Research has shown that light exposure produces long-lived free radicals (Buschle–Diller and Zeronian 1993,Wan et al. 1993). These photo-induced free radicals have been shown to degrade the optical properties of paper when the photo-exposed sheet is subjected to accelerated thermal aging after light exposure (Forsskåhl 1991).

The correlation coefficients between fold values obtained between solar simulator and 17-month exposure, as well as solar simulator and 29-month exposures, were both 0.54. The source of this poor correlation was in part due to the variability of the fold test, since the correlation coefficient between 17 and 29 months of exposure was only 0.73. Another factor contributing to poor correlation between accelerated and north window exposure was that after solar simulator exposure, the loss of log fold was highly correlated with lignin content. In natural aging, pH was most important. In the case of accelerated aging, the damage mechanism was probably photo-oxidation. In the case of natural aging, the longer time frame provided more opportunity for acid hydrolysis, including hydrolysis by acids formed from photo-oxidation.

Exposure in the solar simulator caused losses in mechanical properties when monitored by the MIT fold test. This was consistent with observations from the natural aging experiments. However, for the accelerated aging samples, the loss in fold endurance was more highly correlated with lignin content than with pH and buffering. This was in contrast with the results from natural aging conditions, where the reverse was true. This difference in dependence, as well as variability of the fold test, prevented the use of mechanical tests to rank stability after accelerated exposure to light.

After 4 days of solar simulator exposure, the viscosity of a mixture of papers 11 and 12 dropped from 19 to 10 cps. This indicates that even though the fold test is not sensitive to the chemical change, viscosity can be used in lignin-free samples to evaluate stability. Probably a better measure for stability would be degree of polymerization or probability of chain scission, both derived from the viscosity measurement. Probability of chain scission is calculated by (DPi/DPf –1) / DPi where DPi and DPo refer to initial and final degree of polymerization, respectively. Of course such a measurement carries all the limitations of the viscosity measurement, as well as the conversion from viscosity to DP.

FPL 55 Chemical Changes

Multiple Analyses on Subset of Samples One of the sample sets used for many different chemical analyses involved the lignin- containing papers aged to similar brightness, both naturally and in the solar simulator. This was achieved by taping papers to a clean, single-pane window until their brightness was similar to results obtained from 4 days of accelerated (solar simulator) aging. The natural aging required 3 months. These papers, referred to as “equal brightness,” were analyzed by UV/VIS and IR. Their methanol extracts were analyzed with Raman, ion chromatography, and HPLC techniques.

Many researchers who study photo-yellowing believe that 350 nm of light alone is sufficient for producing the majority of photo-yellowing products in paper. If this is true, then the chemical information derived from naturally and RPR-aged papers should be very similar. North window and solar simulator exposure should produce an even closer match due to their greater spectral similarity. To verify that natural and accelerated aging produce very similar products, chemical analyses were conducted on papers 3 and 9 after north window and RPR exposures. These papers were analyzed using IR, Raman, and fluorescence spectroscopy; the methanol-extracted materials were examined using Raman and GC/MS.

UV/VIS Spectroscopy

Lignin Containing Papers When equal brightness samples were compared, we observed that only small differences occurred between sheets aged naturally and those aged in the solar simulator. All the patterns described for 4 years natural exposure (as discussed on page 35) were valid for paper aged in the solar simulator, except that the changes in the solar simulator were not as intense. This can be seen in figure 28. Because of the great similarity, we conclude that UV/VIS spectra revealed no significant differences between naturally and solar simulator exposed sheets at similar brightness. The reflectance at 457 nm was higher after accelerated aging (lower loss in reflectance peak) as well, which agrees with the brightness measurements. Close inspection of the data revealed some differences between the UV/VIS reflectance at 457 nm and the brightness measurements, which was expected due to the different methods of measurement. The important fact was that the trends observed between different aged papers in the two data sets were the same

FPL 56 50

North 4 years 40 Hal 4 years Fluor 4 years S Sim 30

0 Loss in Reflectance (Delta R)

-10 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 28. Loss in reflectance of paper 10, comparing solar simulator to 4 years natural exposure.

FPL 57 Lignin-Free Papers

20 North 4 years Hal 4 years 15 Fluor 4 years 10 S Sim

-5

Loss in Reflectance (Delta R) -10

-15 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 29. Loss of reflectance of paper 2 with natural 4 year and solar simulator exposures.

Figure 29 shows how the solar simulator exposure reproduces some but not all features of papers aged 4 years in the north window. In all the lignin-free papers, solar simulator 4 day exposures mimic the features of 4 years north exposure, but all contain a significant offset below 450nm. Infrared Spectroscopy Papers 3, 4, 7, and 8 of the “equal brightness” series were extracted in methanol and the extracted materials were analyzed using IR spectroscopy. A representative spectrum is shown in Figure 30; Appendix I contains the complete set of spectra.

Although spectra were not quantitative, a comparison of IR features indicated that both photo-aging protocols produced similar changes in paper; hence, the photo-chemistry involved in the natural and accelerated aging was expected to be similar. In some cases where small shifts between peak positions were present, the shifts were due to the significant overlapping of the neighboring bands. This was especially important for bands present at 1720 and 1677 cm-1. Due to this overlap, the peak position of the higher wavenumber band moved to lower frequency and that of lower band increased in papers 3 and 4. In papers 7 and 8, the overlap was so strong that only one band could be visually discerned.

FPL 58 0.6

3 Nat 0.5 3 SSim

0.4

0.3

Absorbance 0.2

0.1

0 400 900 1400 1900 2400 2900 3400 3900 Wavenumber

Figure 30. IR absorbance of paper 3.

Raman Spectroscopy Methanol-extracted material of papers 3, 4, 7, and 8 from the equal brightness sample set were analyzed using Raman spectroscopy. The goal was to determine whether significant differences existed between naturally and artificially exposed samples. Obtained spectra are compared in Figure 31 and Appendix J.

Although spectra were not quantitative, a comparison of various spectral features indicated that solar simulator and natural aging produced Raman changes that were similar. This suggests that the photo-chemistry involved in natural and accelerated aging is similar.

FPL 59 0.18

0.16 3 SSim

0.14 3 Nat

0.12

0.1

0.08

0.06

0.04

Raman Intensity (Arbitrary Units) 0.02

0 250 750 1250 1750 2250 2750 3250 Wavenumber (cm-1)

Figure 31. Raman spectra of paper 3 after natural and accelerated aging

Fluorescence Measurements In comparing papers aged naturally and in the accelerated apparatus, we found that a large degree of similarity was in both the steady-state fluorescence signal and the lifetime distribution. Even when a paper was irradiated in the RPR with 350-nm radiation only, there was good agreement. As seen in other analyses (IR, Raman, GC/MS) of RPR- exposed papers, illumination with only UV produced results similar to natural aging. This suggests that accelerated photo-exposure produces many of the same chemical reactions as natural aging. Ion Chromatography In the analysis of water-extractable carbohydrates, all trends observed in naturally aged sheets were also observed after accelerated aging. The quantity of sugars removed from solar simulator exposed samples was 60%–80% of similar north window exposed samples for lignin-containing papers and 15%–40% for lignin-free papers. There was good correlation (R2 = 0.96) between the amount of material extracted from natural and accelerated exposure samples of lignin-containing samples, as seen in Figure 32. Paper 15 is not shown in this figure because the Penford gum (carbohydrate) size washed out of control samples as well as out of the aged sheets. Appendix F contains data from both natural and accelerated exposure.

FPL 60 As in natural exposures (see page 30), carbonate-buffered sheets released less material upon aging than did unbuffered samples. The average difference in solubilized carbohydrate between buffered and unbuffered lignin-containing sheets after accelerated aging was 0.30%, compared to 0.71% in naturally aged sheets. The observation that buffer was less important in accelerated as opposed to natural exposures was consistent with the theory that acid hydrolysis is relatively less important as a mechanism of paper degradation in light-accelerated aging.

4 3 7 4% 9 8

10 Lignin-Containing Papers 3% 13 14

1% Lignin-Free Papers Solar Simulator (% Mass Extracted)

0% 0% 1% 2% 3% 4% 5% 6% 7% 8% 9% North Window (% Mass Extracted)

Figure 32. Mass (% by dry weight) of carbohydrates extracted after 35 months of north window vs. solar simulator photo-exposures.

A portion of each extract was not hydrolyzed to its constituent sugars. These extracts were analyzed under slightly different conditions to observe the mixture of polysaccharides of various sizes (oligosaccharides). Many peaks were observed at a range of elution times, indicating a wide variety of products. Figure 33 shows the fairly good correspondence between the products produced in both natural and accelerated aging. No attempt was made to identify the peaks, but likely candidates were carbohydrate oligomers, methylglucuronic acids, and hexenuronic acids. It appeared that many of the oligomers produced in natural and accelerated aging were the same, though relative quantities may have been different.

FPL 61 35000

30000

25000

20000 7 Sol Sim 7 North 29 Mo 15000 7 Control

10000

5000

0 4 9 14 19 24 29 34 39 Minutes

Figure 33. Carbohydrate oligomers extracted from paper 7 after solar simulator and north window exposures.

High Pressure Liquid Chromatography Extracts of papers that had been aged 3 years in the natural chambers were compared to papers aged in the solar simulator. The relative heights of many peaks were different in chromatograms of sheets aged naturally vs. accelerated, but almost every peak observed in natural aging was also present in accelerated aging, and vice versa. Though the relative amounts of the compounds may have been different, the same compounds appeared to be produced in both natural and accelerated aging.

Some of the extracted compounds were more prominent in naturally aged than in solar simulator samples. For instance, in paper 3 (Figure 34), peaks 1, 3, 11, and 12, and the leading shoulder of 3 were all smaller in accelerated aged sheets than in naturally aged papers. Peak 8, as well as the peaks around 11 min, appeared somewhat larger in the solar simulator exposed sample. In paper 9, peaks 1, 3, 10, and 11 were smaller, while in the paired paper 10, peaks 2, 10, and 11 were notably smaller in accelerated aging samples. Figure 34 shows chromatograms for paper 3. Appendix G contains chromatograms for papers 3, 9, and 10. Also included are absolute concentrations of 4-hydroxyBenzoic acid, syringic acid, vanillic acid, and the combination syringaldehyde/vanillin peak.

FPL 62 6000 10: Vanillic Acid 3 North 9: Syringic Acid 13: Syringaldehyde + Vanillin 8: 4-Hydroxy Benzoic Acid 5000 3 S Sim

3 Control 4000

3000

3 2000 Detector Response 6 1 7 2 5 11 1000

0 45678910111213 Time (min)

Figure 34. HPLC chromatograms of methanol extracts of paper 3 after exposures. Traces are offset for clarity. Absorbance was measured at 220 nm.

Effect of Temperature

Optical Properties Optical properties generally degraded faster with higher temperature accelerated exposure. For 100% BCTMP, a 40o increase in temperature during the 4-day solar simulator irradiation resulted in a drop of 2 to 3 points of brightness. It was interesting to note that papers irradiated at higher temperatures appeared to be more stable in the days after completion of the accelerated aging. Both these trends are evident in Tables 10, and 11.

Table 10. Brightness immediately after solar simulator exposure at various temperatures Brightness of various papers Temp 3 4 7 8 9 10 13 14 20oC 32.6 38.3 39.2 43.1 38.6 43.2 49.7 45.2 40oC 31.5 37.7 38.6 43.9 38.3 44.2 50.5 46.4 60oC 30.4 34.5 37.2 43.8 36.6 41.9 48.1 42.6

FPL 63 Table 11. Loss of brightness following accelerated exposure. Papers aged at higher temperatures appeared more stable after exposure. Samples stored at 4oC. Loss of brightness for various papers Temp and time 3 4789101314 20o + 7 Days 3.2 2.3 3.2 2.8 4.0 3.1 2.3 4.5 40o + 10 Days 2.3 1.9 3.3 2.5 2.3 3.0 2.5 3.2 60o + 5 Days 1.2 0.7 1.3 1.2 0.9 0.9 0.9 1.4

During exposure, dark samples tended to get warmer than light-colored samples. In other polymeric systems, this temperature difference has been shown to produce a significant effect on sample color, as shown in Table 12. In this experiment with a polystyrene sample, the black panel temperature in an accelerated apparatus was held constant at 70o for two accelerated tests, one with half the radiation intensity of the other. The white panel (similar to sample) temperatures were 58oC and 42oC for low and high intensity, respectively. Though both samples were aged for the same time, the polystyrene aged at low intensity/high temperature yellowed more than did the samples at twice the intensity and 16oC lower temperature. This result has two implications for an accelerated aging protocol. First, whether a sample is irradiated at 40 o C or 60o C during exposure can make a significant impact on final optical properties in some chemical systems. Second, there should be significant airflow over samples during exposure to minimize the temperature variation between light and dark samples.

Table 12. Effect of increased temperature during exposure on optical propertiesa Irradiance ∆b* of polystyrene Test Black standard White panel (W/m2) at after 7 days no. temperature temperature 300–400 nm exposure 1 70C 42.5 58C 3.95 2 70C 85 42C 3.23 aKetola and Grossman 1994.

UV/VIS UV/VIS spectroscopy was used to study differences in papers aged in the solar simulator at 20oC, 40oC, and 60oC to similar brightness (see Table 9). The UV/VIS reflectance loss spectra from papers at all temperatures, as well as that of the naturally aged “equal brightness” sample, are compared in Figure 35.

FPL 64 45 9@20 40

35 9@40

30 9@60

25 9 Nat

Loss in Reflectance 10

0 190 290 390 490 590 690 -5 Wavelength (nm)

Figure 35. Paper 9 loss in reflectance upon irradiation. Sample temperature during 4-day solar simulator exposure = 20oC, 40oC, 60oC.

The loss-in-reflectance curves show that 20oC, 40oC, and 60oC solar simulator exposures produced very similar results. At shorter wavelengths, the curves could almost be superimposed. Above 500 nm, there was some divergence with no clear pattern. The cause of this difference was unclear, but it again reinforced the hypothesis that a reproducible test must be temperature controlled in order to get the most reproducible results. HPLC High performance liquid chromatography (HPLC) was used to investigate the effects of temperature during illumination. Papers aged in the solar simulator at 58oC versus 33oC generally produced the same reaction products in slightly higher quantities. This data suggested that for these papers and temperatures, there was little change in the lignin fragmentation detectable with HPLC. Effect of Storage after Exposure The effect of 3 weeks storage after photo-exposure was also investigated. We hypothesized that HPLC or UV/VIS might detect changes between papers subjected or not subjected to photo-initiated dark reactions. Dark reactions were allowed to progress by placing samples in a dark desk drawer at ambient temperature for 3 weeks. Storing samples at –90oC (cold) immediately after dark photo-exposure stopped these additional photo-initiated reactions. We observed that optical properties of (cold) papers were unchanged after 3 weeks of –90oC storage, while papers stored in the desk (ambient)

FPL 65 darkened by approximately three brightness points (see Table 11). This indicated that the cold storage significantly slowed the photo-initiated dark reactions. Further analysis with HPLC (Appendix G) provides further evidence that small chemical changes do occur during ambient temperature storage.

Though the three points of brightness change attributed to dark reactions in the (ambient) sample represented a significant increase in the number of chromophores, (change in PC# from 66 to 79), the UV/VIS curves (Figure 36) do not reveal much about the source of this difference. The only notable features were an obvious change in peak intensity reflected in brightness measurements and increased absorption above 500 nm for ambient aged papers. This same kind of feature developed after long periods of natural aging.

North 40 -90C Storage 20C Storage 30

Change in Reflectanace (%) 10

0 190 290 390 490 590 690 Wavelength (nm)

Figure 36. UV/VIS loss in reflectance for paper 3 stored at 21oC (ambient) or –90oC (cold) for 3 weeks.

Findings From Chemical Analyses The analyses of chemical change were focused on the lignin-containing papers because we did not anticipate that changes in the lignin-free papers could be detected chemically after the relatively short exposure times.

All chemical and spectral analyses indicated that both photo-aging protocols (that is, natural aging and aging in the solar simulator) produced similar changes in the papers and that the photochemistry involved in the natural and accelerated aging were similar as

FPL 66 well. The IR and Raman spectral analyses were consistent with the occurrence of p- quinone in paper after both natural and accelerated aging.

The photo-induced depolymerization of paper constituents observed for naturally aged papers was also observed when the papers were exposed under accelerated conditions. Solar simulator exposure solubilized relatively fewer carbohydrates in lignin-free papers. This was consistent with the finding, on the basis of mechanical properties, that depolymerization was more sensitive to acid in natural aging and to lignin in accelerated aging.

Accelerated exposure at higher temperatures (60°C vs. 40oC vs. 20oC) produced slightly more degradation. This was observed in HPLC analyses as increased quantities of the same fragmentation products.

After 3 weeks of dark storage at room temperature following solar simulator exposure, the effects of photo-initiated dark reactions were observable, but for the most part, small.

FPL 67 Procedures and Equipment All optical properties and exposures were made on the felt side of the sheet. The felt side was facing up in the packing boxes as received. All paper types tended to curl away from the felt side. Papers were always handled with gloves to avoid contamination.

Samples were stored in a walk-in refrigerator maintained at 4oC. The room was dark except when occupied by people placing or retrieving samples. Papers were left in their original boxes, and the boxes were placed in large plastic bags that were sealed by folding the bag opening under the contents. After irradiation for 3, 6, 12, 17, and 29 months, samples were separated by paper type and stored in polyethylene Ziplock-type bags that were placed in corrugated boxes in the same room. Samples from each accelerated exposure were also placed in Ziplock bags in similar boxes in the cold room. Rayonet Photoreactor Exposure Unless otherwise specified, the Rayonet photoreactor (RPR–100, Southern New England Ultraviolet Co., Branford, CT) was operated with 16 lamps emitting 350-nm light. These are also known as UV–A lamps. ASTM G 53 states that UV–A lamps provide a good simulation for sunlight filtered through typical window glass. Sample temperature was kept at 20oC by operation of the aging tests within the walk-in refrigerator where samples were stored. The UV intensity (300–400 nm) is given as 92 W/m2 in the product literature, although we measured 67 W/m2 with new bulbs. After bulbs were broken in, 60 W/m2 was more typical, although some decay occurred as bulbs aged. By comparison, the solar simulator delivered 76 W/m2 of UV light as well as a large dose of visible radiation.

The paper was mounted on a vertical stainless steel cylinder 9.5 inches high and 11 inches in circumference. Wire clips and thin steel rods were used to hold papers in place during exposure. The cylinder, with papers attached, was rotated in the center of the circle of fluorescent tubes. Tubes had peak emissions at 350, 419, and 575 nm each, with a width at half-maximum of approximately 50 nm. The 419- and 575-nm lamps were used only when studying the effect of visible wavelengths on optical properties.

While the RPR was a very useful instrument for initial studies, we concluded that for a robust accelerated aging test, broadband irradiation was necessary to reproduce natural aging more faithfully. The RPR work with 419- and 575-nm bulbs had indicated that photochemical reactions in lignin-containing paper occur at virtually every wavelength throughout the UV and visible spectrum. Solar Simulator Exposure An Oriel Solar Simulator (Model #81192, Oriel Electric Devices Corp., Branford, CT) was used with their Air Mass 1.5 Global Filter series (catalog numbers 81090, 81091, and 81094). It was operated in a 23oC and 50% RH controlled room with a fan directed at the sample surface to provide cooling. Wavelengths below 320 nm very rarely occur indoors, since artificial light sources do not produce these wavelengths and window glass

FPL 68 absorbs such short wavelength light. To ensure the exclusion of these short wavelengths and to mimic the action of sunlight passing through a glass window to reach the surface of the paper, a sheet of window glass was placed in the light beam from the solar simulator. This glass was used in all exposures discussed in this report. Sample Mounting The sample platform was a 6-inch-wide metal plate. Samples were cut 5¾ inches long and of various widths. To ensure a representative environment in case of chemical migration, strips of the identical paper type were placed under each aging sample. The two ends of each sample were fastened with binder clips to the metal plate. Illumination Spectrum The illumination spectrum observed in the solar simulator apparatus is presented in Table 14, along with the ASTM standard for light passing through window glass. The FPL Experiments spectrum was calculated by multiplying the manufacturer’s published spectrum by the transmittance of the window glass placed in the light beam.

Table 14. Illumination spectrum observed in solar simulator— Data from ASTM G 155–97 (table 3) and FPL measurements Irradiance (% total 300–400 nm) ASTM G 155 ASTM G 155 estimated Bandpass, nm Xenon arca Window glassb FPL experiments 290 to 300 <0.1 0 0 301 to 320 0.0 to 3.3 0.1 to 1.5 0.0 321 to 340 1.9 to 14.3 9.4 to 14.8 1.9 341 to 360 18.8 to 23.0 23.2 to 23.5 19.5 361 to 380 27.5 to 34.1 29.6 to 32.5 33.5 381 to 400 31.8 to 46.7 30.9 to 34.5 45.1 aRanges based on spectral power distribution measurements made for water- and air-cooled devices with various lots and ages of window glass filters. Ranges based on three sigma (σ) limits from averages of these data. Xenon arc filters that provided a spectral power distribution closer to window glass filtered sunlight than those shown in these tables were considered to meet the requirements of this standard. bCalculated sunlight through window glass.

Table 15. Illumination spectrum observed in solar simulator— Data from ASTM G155–97 (table 4) and FPL calculated value Irradiance (% total 300–800 nm) ASTM G 155 ASTM G 155 Bandpass, nm Xenon arc Estimated window glass FPL experiments 300 to 400 8.6 to 10.4 9.0 to 11.1 6.3 401 to 700 64.2 to 80.8 71.3 to 73.1 75

Inspection of the data in Tables 14 and 15 revealed that the FPL apparatus emitted a UV distribution that was within the boundaries of the standard but generally low at the shortest wavelengths. The ratio of UV to visible irradiance was also lower than desired for the FPL apparatus. Despite these shortcomings, trials using other filtration that gave

FPL 69 significantly higher UV levels (relative to visible) produced very similar trends in optical properties. Illumination Uniformity Xenon lamps darken over time, requiring increased power to maintain constant output. At the beginning of each run, the intensity of illumination was checked in the center of the illumination area with the IL1400 meter and the lamp wattage was adjusted if necessary. Adjustments were rarely necessary during the last 90% of bulb life. During the first 10% of bulb life, measurements and adjustments were made more often, as the output was less stable. Illumination uniformity was specified as 10% by the manufacturer, but averaged 7% in these investigations. Uniformity was usually recorded at the same time as intensity. Uniformity was adjusted by following manufacturer specifications upon installation of new bulbs and adjustment was rarely necessary thereafter.

Illumination uniformity in the Oriel unit was measured according to manufacturer’s specifications. Intensity was sampled in each cell of a 9-cell grid covering the entire 4- by-4-inch illumination area. Uniformity was defined as (maximum reading − minimum reading)/average reading x 100%. The central cell was always the brightest. The eight perimeter cells had a typical variation of less than 2%, but had typically 7% lower intensity than the central spot. Other solar simulators of similar design were available for less cost, but they do not have as uniform an illumination area. Sample Temperature Paper temperatures were recorded with an optical pyrometer (infrared-sensing thermometer, Cole Parmer Model #39650-12) aimed at the sample surface during irradiation. Measurements were taken with the sample illuminated since this did not affect the temperature reading. Sample surface temperature was 40±4oC unless otherwise stated.

The temperature of samples during photo-exposure had been a concern from the inception of the project. Many strategies were used to regulate sample temperature during photo-exposure. RPR exposures were conducted in a walk-in cold room in which the sample temperature was maintained at 20oC. In the Oriel studies, a concern about relative humidity dictated that the apparatus should be operated in an RH-controlled chamber; thus, a 23oC/50% RH conditioned room was used. Consideration was given to drawing air through the paper or to placing the paper on a cooled metal plate as alternate methods to control the paper temperature. The variation in paper porosity and conductivity, however, indicated that these methods would not cool all specimens equally; instead, a large volume of air was forced over the specimen surface. A 14-W fan (as obtained from a desktop computer) was used to cool the samples, after the first two fans contaminated the samples with carbon from brushes and then oil from supposedly sealed bearings.

FPL 70 Natural Chamber Exposures

Chamber Construction The aging chambers were constructed using nominal 2-by-4 studs and drywall. Sections measuring 22 by 28 cm (8.5 by 11 inches) were marked and numbered with pencil on the drywall. Papers for exposure were attached to the wall with pushpins. A second sheet of the same paper type was always placed behind the exposed sheet to minimize the possibility of chemical migration out of the drywall. Sheets were placed so that the right and top portion of a sheet was always exposed. Chamber Illumination

North Window Chamber In the north window chamber, there were four windows 2 m (6.5 ft) high and 1.2 m (4 ft) wide in front of the bank of papers, which was 2.2 m (7 ft) high. The papers at the bottom were much more exposed to the sky (and therefore degraded much faster) than those at the top, which were much more exposed to the ground. The difference in illumination between sheets at the top and bottom depended on wavelength. The short wavelength components were scattered by the sky but were not reflected well by solid objects on the ground. The result was that papers at the bottom received somewhat more intensity and much more UV radiation than those at the top. Fluorescent Chamber Sylvania FO32/735 bulbs were used in the fluorescent chamber. These bulbs are very commonly used in the United States for commercial lighting and so were selected as a representative sample. Average bulb life is 4 years of continuous service, but the technical support staff noted that the UV portion of the spectrum decays the most quickly. Therefore, to avoid any UV fluctuations over time, the fluorescent lamps were replaced every 4 months. A standard feature in fluorescent fixtures is a diffuser, which was included in the chamber. This diffuser effectively eliminates the 315-nm emission from the lamps. Halogen Chamber The halogen chamber used Sylvania Capsylite 75W floodlights and 100W diffuse bulbs, which both provided the same spectrum. These were blackbody radiators with a target color temperature of 3000 K. According to the Sylvania literature, 70% of the energy consumed is emitted as infrared, 10%–12% visible, 0.2% UV, and the rest converted directly to heat. Color temperature only drops 50oC over the life of the lamp. Lamps were replaced every 6 months; only one bulb burned out before replacement during the entire study.

The lighting in the halogen and fluorescent chambers was arranged by an electrician, who was directed to establish uniform intensity at bright office conditions. He used a standard silicon photodiode-based intensity meter and chose 1000 lux (1.5 W/m2 of visible light) as the target intensity. Later measurements with the IL1400 discussed in the first section showed that his meter was not well calibrated and the intensity was higher than planned. To provide uniform target intensity in the halogen chamber, the electrician placed the

FPL 71 lights on a rheostat at 83 V. The measured illumination spectrum from all three aging chambers is given in the section on Exposure Conditions (p. 12).

While constructing the chambers, we observed that all areas could not be uniformly illuminated. To accommodate this problem, each chamber was divided into high, medium, and low intensity regions, and test samples were distributed evenly among these regions. The regions were mapped by placing 160 samples of unprinted newspaper stock throughout each of the halogen and fluorescent exposure chambers. After 12 days, the samples were removed and brightness was measured. Average diffuse brightness loss was found to be 1.5 points. Based on the brightness loss from the newsprint, positions 1– 53 were designated as low, 54–107 as medium, and 108–160 as high intensity locations. In the north window chamber, the regions were determined simply by the height on the wall, so the light intensity of the north chamber increased with number, rather than decreased.

The uniformity of illumination was measured and expressed according the formula: [(Maximum Intensity – Minimum Intensity)/Average Intensity] × 100%. According to this formula, perfectly uniform illumination is 0%. Uniformity was 100% in the halogen chamber and 60% in the fluorescent chamber. When one particularly bright spot in the halogen chamber was taken out of the data set, the uniformity in that chamber dropped to 65%. These variations were due to our desire to obtain a large number of samples illuminated and the limited space available. The effect of these variations on the decay of the papers was relatively small. Appendix M contains plots of optical properties for papers where one sheet was in a high intensity zone and one in a low intensity zone. All other plots of optical properties with time in this report are the average of one high and one low intensity curve.

Table 16. Uniformity of illumination in natural aging chambers Wavelength region Uniformity (%) Short λ Long λ Filter North Halogen Fluorescent 374 464 Blue 90% 120% 55% 392 602 Aqua 50% 110% 100% 300 1000 No Filter 40% 80% 55%

AATCC Blue Wool UV Intensity Measurement The American Association of Textile Colorists and Chemists (AATCC) produces blue wool standards for use as a standard method of measuring UV exposure in an accelerated aging apparatus. The reference materials (AATCC Technical Manual and shipping insert) state that a specified amount of color change in the fabric corresponds to a specific radiation dose from a standard Xenon arc. This cloth was used as one method of monitoring UV intensity in the natural aging chambers.

Sections of blue wool were cut into strips and folded to make a 3-ply, 7.5- by 5.5-cm (3- by 3-inch) patch. The patches were then sewn onto index cards. The fabric was slightly stretched on the card and did not move, shift, or bulge during measurements and

FPL 72 handling. The index cards containing the patches were labeled and placed on the wall where ISR papers were mounted. The blue wool patches were placed in high, medium, and low intensity regions of the halogen, fluorescent, and north window chambers. When placed in the aging chamber, half the patch was covered with an index card and the other half was exposed to the light. The two sides of each patch were compared to calculate the color change. Samples were initially placed on April 17, 1998.

Color change was measured in units of delta E (δE). Color change is correlated with radiation dose in Table 17. It is interesting to note that for standard L2, the fourfold difference in exposure needed to change from 5 to 20 fading units was generally upheld. However, the twofold difference in time needed to change from 20 fading units on wool L2 to 5 units on wool L5 was not reflected in the actual data for the north window chamber.

A Technidyne S–5 directional brightness meter was used to measure the CIELAB values of brightness and the blue/yellow and red/green components in the light reflected from the cloth (45° incident/0° reflected). The formula used for calculating δE was

2 2 2 1/2 δE = [(Lc* − Le*) + (ac* − ae*) + (bc* − be*) ]

Where subscripts c and e referred to control and exposed, respectively. The control was that half of the patch covered with an index card at all times. The fading over time of blue wool samples is shown in Appendix A.

Table 17. Fading of AATCC blue wool samples in natural aging chambers Total exposure AATCC Fading (MJ/m2 radiation) Average time blue wool units δE 300–400 nm from for color change standard (no.) xenon arc (days) North Halogen Fluores window -cence

L2 5 2.51 +-.28 0.864 40 100 85 L2 20 5.96 +-.60 3.456 160 500 375 L5 5 2.51 +-.28 6.912 >600 >600 >600

Table 17 provides information on UV intensity in the natural aging chambers. This information can be used to compare the intensity of illumination in other locations to the intensity used in the FPL aging studies. Photo-Exposure Measurement and Calibration Temperature, RH, and light intensity in several spectral bands in the aging chamber were continuously monitored and recorded. An Analog to Digital (A/D) data acquisition board received voltage inputs and fed the digitized signal to a “Labtech Notebook” operating on a PC. RH and temperature were measured with a Vaisala HMI 32

FPL 73 psychrometer/thermometer and the analog output was sent to the A/D board. Light intensity was measured at three photodiodes with various filters. The filters had transmittance windows of 374–464 nm (blue), 392–602 nm (aqua), and 395–728 nm (visible). The diode output was converted to voltage and was also fed into the A/D board. Measurements were recorded every 30 min and the entire data file was retrieved monthly. Some data were lost as a result of power interruptions.

Calibration of temperature and RH was done with a wet bulb/dry bulb apparatus. The thermometers in this apparatus were NIST traceable and were accurate to ±0.25oC. ASTM E337 (Standard Test Method for Measuring Humidity with a Psychrometer) was used to calculate RH, based on wet bulb and dry bulb temperatures. After correction, standard deviation of the difference in measured values between the two instruments was ±0.5oC and ±2% RH. One exception may have occurred during December 1998. At very low RH, the data logger read an RH of 10% below actual; thus, the 10% RH shown in appendix A was actually taken from off-scale readings. Electronic RH sensors are generally not reliable above 90% and below 10% RH.

Light intensity readings were made on two different instruments: The first was an International Light IL1400 photometer. This detector consists of a photodiode covered with a quartz diffuser and fitted with a handheld display. The instrument was factory calibrated from 300–1000 nm in 10-nm increments with the diffuser. The second detector was the bank of three photodiodes mentioned previously. These diodes were placed in the north window chamber adjacent the paper samples, halfway between the top and bottom samples.

Several glass filters were used in combination with these detectors to sample sections of the spectrum. The transmittance of the filters was measured in a Genesis 5 UV/VIS spectrometer (Spectronic Instruments, Rochester, NY). The filters were replicas of those used in the data logger, as well as a UV filter transmitting 347–389 nm.

To measure light intensity with the IL1400, these filters were placed in front of the sensor head and output recorded (the effect of no filter was also recorded). This was done in six different positions in each of the natural aging chambers. The positions were chosen to represent the extreme high and low intensity found in each chamber. Intensity in the solar simulator was also monitored, although a 1% neutral density filter was necessary to keep the measurements within scale.

The photodiodes in the north window chamber were calibrated by correlation with numbers from the IL1400. Two photodiodes were covered with blue or aqua filters and one had no filter. The IL1400 detector was placed adjacent to the photodiodes with identical filters. The output of the photodiodes was scaled to match the actual intensity recorded on the IL1400.

The IL1400 and blue wool provided two independent measures of intensity in the natural aging chambers. The results of those two UV monitors are given in Table 18.

FPL 74 Table 18. Time required to deliver radiation in natural aging chambers, as recorded by different methods Time (years) to deliver 3.5 MJ/m2 of 300–400 nm radiation Method North window Halogen Fluorescent AATCC blue wool 0.44 1.0 1.3 Room monitor 0.31 3.6 6.5

Since the blue wool was calibrated for a xenon arc instrument, which was similar to the north window in spectrum, these numbers were expected to agree and to do fairly well. The blue wool did not correlate as well with the room monitor when illuminated with different spectra in the other chambers. Since the room monitor and IL 1400 measured 347–389 nm and the blue wool was calibrated at 300–400 nm, the room monitor data were increased by a factor of 1.5 to reflect the larger bandpass. This value was determined by ratio of integrals of the two regions in the recorded north window illumination spectrum.

We verified that the measured intensity value in the north window chamber was consistent with other published intensity values. This was validated by checking data from the Atlas Electric Company, who report an average of 8004 MJ/m2 total annual illumination, with 333 MJ/m2 in UV (<385 nm) for a south-facing outdoor exposure plate in Phoenix, AZ. The measured values in the north window chamber of 201 and 8.5 MJ/m2 represented 2.5% of the outdoor, south-facing plate values. We observed that measurements from the north window chamber were between 1% and 6% of the intensity of outdoor, south-facing measurements in Madison, WI, thus confirming that the reported numbers were the appropriate order of magnitude. Measurement of Optical Properties CIE a* and b* were measured according to TAPPI T524, although only two or three measurements were made per sample. Directional brightness was measured simultaneously using TAPPI T442. Directional brightness and CIELAB values were measured on a Brightmeter Micro S–5 (Technidyne Instruments, New Albany, IN). All measurements were made with the light incident along the MD direction only and on the felt side of the sheet only.

Diffuse brightness was measured on a Technidyne Technibrite TB–1 using TAPPI T525. Two to five measurements were made per sample.

UV/VIS diffuse reflectance measurements were made on a Hitachi 3010 with a Hitachi 150mm BaSO4 integrating sphere. All measurements were made vs. BaSO4.

Optical properties of samples from solar simulator exposures were not preconditioned according to standards. Optical properties were measured within 15 min of removal from the solar simulator because they were generally very unstable after photo-exposure. Had the samples been allowed to rest before each measurement, the brightness decay vs. time curves would not have reflected the true decay of a continuous exposure. Trials indicated that the color of single sheets of paper stabilized within 2 min of arrival in the

FPL 75 conditioned room. Color changes beyond this time showed no trend and were within the boundaries of normal measurement error. Observed standard deviations for directional brightness a* and b* were 0.5, 0.6, and 0.2, respectively, based on brightness of all papers as measured before exposure.

Because of the large range of measurements, exposures, and paper types, it was difficult to state error estimates for properties under all conditions. The variation in brightness after solar simulator exposure was estimated by placing two strips of paper 9 in three consecutive 4-day runs. The papers were measured upon completion of aging. One standard deviation for an individual determination was found to be 1.0 brightness units. The reproducibility after shorter exposure times was approximately the same.

To study the effect of intensity on final optical properties, neutral density filters (6 inch square, metallic film on quartz) were placed in the light beam (see Intensity Effects, p. 50). Actual transmission at any wavelength between 300 and 800 nm was within 1% of nominal values of 48.5%, 32%, and 11%. Runs were performed in a randomized order. With no neutral density filters in place, the intensity was that given in the Exposure Conditions section (p. 12) and the illumination time was 7.8 h.

Because the optical properties were so unstable after illumination, we felt the results would be skewed if properties measured immediately after an 8-h exposure were compared to those of a 6-day exposure. Therefore, for this experiment, optical properties were recorded 2 weeks after illumination started. Mechanical Testing Methods Preconditioning for all chemical and physical testing was carried out by placing the samples in a room at 33o/20% RH for 24 h, followed by 24 h at 23o/50% RH. This was considered more acceptable than oven drying because photo-aged papers are more sensitive to thermal degradation than are normal sheets.

After conditioning, full sheets were trimmed so that the right side was perpendicular to the top. Samples were then cut and ID labels copied by the test lab staff. All tests were conducted according to ASTM and/or TAPPI standards except where noted. Even in cases where exceptions were made, the control samples were tested in exactly the same way as aged samples to prevent the method from biasing results. Samples were transported and stored in sealed Ziploc-type polyethylene bags. Each paper type was kept in a separate bag during long-term storage.

The MIT fold procedure (ASTM D 2176–97A or TAPPI T511 om-88) was modified to reflect the need to conserve sample. Strips for testing were cut as long as possible. Multiple tests were run from a single strip. Care was taken to avoid testing a section of sheet that had been clamped in a previous test. When results of this method were directly compared in the same trial with the ASTM method, there was no difference in results. Specimens were cut so that the fold was across the primary fiber direction, which was called the MD fold.

FPL 76 Tear testing was performed on single sheets to conserve sample, but otherwise conformed to TAPPI T414 om-88. Zero-span tensile specimens were also smaller than standard, but otherwise complied with TAPPI T231.

Tensile and TEA were conducted according to ASTM D828–97 and TAPPI T494 except that the length of the specimen was reduced by a factor of 2. Chemical Analysis Methods

Moisture Content TAPPI T550 om-90 was used, with the exception that samples sizes ranged from 0.6 to 4 g wet weight. Two replicates were run of each paper, but at different times. North window exposure samples were used in the first run and halogen samples in the second because the first determination had used the entire north window sample. Viscosity The procedure of TAPPI T230 om-89 was followed, with the exception that cotton samples were stored in cupriethylenediamine under nitrogen in a refrigerator for 2 days to complete dissolution. Errors were based on 15 identical blind samples of 15-cps viscosity, run over several months. All viscosity samples were taken from the high light intensity section of the respective chambers. Ion Chromatography Carbohydrates released upon photo-exposure were determined for samples aged in the solar simulator as well as for those aged for 35 months in the three natural chambers. First, samples of all 15 papers were extracted with water and 0.02% sodium azide (biocide) for 3 days. The liquid was then removed and filtered to 0.45 µm. This sample was used for analysis of oligosaccharides. To determine neutral sugars, a portion of this o sample was acidified to 4% w/w H2SO4, hydrolyzed at 120 C for 1 h in a steam autoclave, and filtered again. Samples were refrigerated at 5oC when not in use.

Analysis of neutral sugars was conducted using ion chromatography and pulsed amperometric detection (IC–PAD), as described by Davis (1998). Sugar separation was achieved within 11 min using a Carbo–Pac PA1 guard and analytical columns (Dionex Corp, Sunnyvale, CA) connected in series with water eluent. The flow rate was 1.2 mL/min at room temperature. The column was washed with 170 mM NaC2H3O2 in 200 mM NaOH, and re-equilibrated between each run. Sugars were analyzed by their oxidation at a gold electrode surface, which was facilitated by post-column addition of 0.5 mL/min of 300 mM NaOH. Fucose, arabinose, galactose, rhamnose, glucose, xylose, and mannose were all well resolved. Both retention times and quantitative results were very stable, resulting in a standard deviation of only 0.02% to 0.05% of paper mass.

Analysis of oligosaccharides was done on the same system as the neutral sugars with a slightly different elution protocol. Elution occurred during a 30-min ramp from 80:15:5 mixture to 55:15:30 ratio of H2O, 1 M NaOH, and 1 M NaC2H3O2 respectively. The

FPL 77 column was washed with a 15:5:80 mixture followed by a 50:45:5 ratio of the same eluents and was equilibrated between every run. Surface pH Surface pH was measured in August 1999 according to TAPPI T529 om-88. A new Beckman #511066 flat glass electrode was used with a Φ11 pH meter. Aged samples were specimens that had been photo-exposed for 29 months in the north window chamber, tested in fold in the fall of 1998, and then stored in the cold room. Their pH values were recorded 3 min after the water was placed on the surface of the paper. In most papers, one measurement was taken on each of three different sheets, although the total determinations ranged from two to six. Based on three measurements of pH per sample and a pooled variance, the 95% confidence interval for the change in pH with exposure was 1.0 pH units. HPLC To extract samples for HPLC analysis, approximately 0.1 g of paper was weighed, rolled into a loose tube, and placed in a 1.5-mL plastic vial. The vial received 1.2 mL of methanol (verified by weighing after extraction) and was then sealed and gently tumbled in the dark for 40 h. The liquid was then removed, passed through a 0.45-µm nylon/polypropylene filter, and placed in HPLC injection vials. Extracts were stored in the dark.

The HPLC eluent was 80%, pH 2.5, H2O/20% acetonitrile at ambient temperature (21°C to 23°C). After 15 min, the column was washed with 100% ACN for 10 min, followed by 15 min of re-equilibration. The column was C18 coated silica (Phenomenex Luna 5µ). Solvent flow rate was 1 mL/min with a 10-µL injection volume. Reference standards were all at 98% purity or better, as supplied by Aldrich (Milwaukee, WI). UV/VIS Spectroscopy UV/VIS measurements were made on single-thickness samples on a Hitachi U3010 spectrometer and a 150-mm BaSO4 integrating sphere. All measurements were made relative to BaSO4. No special sample preparation was followed, since the instrument was not kept in a humidity-controlled environment. Loss in reflectance curves was calculated by subtracting reflectance of the aged sheet from the control. Infrared Spectroscopy The IR spectra were recorded using the Galaxy 5000 spectrometer (Mattson Instruments, Middleton, WI). Paper samples were studied directly using the diffuse reflectance accessory. The methanol-extracted materials from papers were analyzed using the KBr pellet sampling technique and their transmission spectra were obtained.

FPL 78 Raman Spectroscopy Papers and extracted materials were analyzed using a Bruker RFS 100 instrument (Bruker Instrument Inc., Billerica, MA). The Raman system was equipped with a 1000- W 1064-nm diode Nd:YAG laser. While sampling papers, a front surface mirror was kept behind the samples to enhance the Raman signal. The extracted materials were sampled in a sampling device called an “Aluminum well.” Laser power used for the sample excitation was about 300 mW, and, depending upon the signal-to-noise ratio, either 600 or 1200 scans were accumulated. Fluorescence Measurements Papers 7, 3, and 9 as aged for 6 months in the north window and RPR accelerated aging samples were studied most extensively with fluorescence. All experiments were conducted on a Spex Fluorolog Tau-2 system with a double grating monochrometer mounted on the emission port to minimize Rayleigh scatter. Lifetime measurements were acquired using Shott KV series filters and 350-nm excitation. Lifetime data were analyzed using Globals Unlimited software (Laboratory for Fluorescence Dynamics, University of Illinois, Urbana–Champaign).

FPL 79 Conclusions

Natural Aging Studies

Optical Properties Perhaps the most significant finding was that none of the measures of the dose of radiation received by the paper could provide a basis for correlating the degree of property loss for all papers. It was clear from the observed property changes that the chemistry of photo-decay differed when lignin was present in the paper. It was equally clear that in lignin-free papers, damage was caused by photo-exposure to a degree that was not anticipated when the program of investigations was undertaken. The decay in properties with natural aging was such that all papers suffered significant losses in one or another of their key properties when exposed for an extended period at ambient intensities to light of any type.

For lignin-containing papers, all environments eventually led a similar level of decay of optical properties. Exposure to northern daylight caused the onset of discoloration to occur earlier than when the same papers were exposed to halogen or fluorescent illumination. However, after 3 years of exposure, the optical properties of lignin- containing papers aged in the halogen and fluorescent chambers, which have lower levels of ultraviolet and higher levels of visible radiation, were similar to the optical properties of the samples aged in the north window.

The finding that was most surprising was related to the response of lignin-free papers to photo-exposure. The optical properties of lignin-free papers were more severely degraded by halogen and fluorescent lighting than by north window illumination. This occurred despite the fact that the halogen and fluorescent chambers delivered approximately 2.5 times the visible radiation dose and 0.1 to 0.05 times the ultraviolet dose received in the north window chamber. This suggested that visible light played a more significant role than did ultraviolet radiation in the photo-yellowing of lignin-free papers when they were exposed for extended periods.

FPL 80 were quite sensitive to photo-exposure included machine direction (MD) stretch prior to failure and MD tensile energy absorption (TEA).

After papers were subjected to 29 months of exposure in the natural aging chambers, their equilibrium moisture content decreased. The extent of the decline was approximately 0.3% for the lignin-free papers and approximately 0.7% for the lignin- containing papers. In both instances, this effect pointed to a certain amount of crosslinking within the fibers as a result of photo-oxidatively initiated free radical reactions.

For lignin-free papers, viscosity measurements were found to be the most sensitive measure of photo-degradation resulting from photo-exposure. These changes pointed to the occurrence of a significant level of cellulose chain cleavage reactions.

In mechanistic terms, it would appear that for lignin-free papers, the loss in MIT fold strength after 29 months of exposure was primarily associated with the decline in the DP of cellulose in the fibers. In the lignin-containing papers, an extra factor may have been embrittlement of the fibers due to photo-induced crosslinking reactions associated with the lignin. Chemical Analyses There was considerable evidence to the effect that photo-exposure resulted in depolymerization of the constituents of the papermaking fibers. This change was manifested in increased solubility of polysaccharide and lignin fragments in water and methanol, respectively. Up to 7% of the mass of some papers became water soluble upon exposure. The soluble polysaccharide fragments were primarily derived from hemicelluloses, though there were some indications that a low level of fragmentation of the cellulose also occurred.

The absence of calcium carbonate, low pH at formation, and the presence of lignin were correlated with a decline in surface pH upon photo-exposure. However, because most papers with calcium carbonate also had high pH levels, it was not possible to separate the effects of these two variables in a systematic manner.

Analyses of the solubilized products by HPLC showed that the three natural aging environments (north window, halogen, and fluorescent exposure) produced many of the same products in differing quantities. The products that were quantified included 4- hydroxybenzoic acid, vanillic acid, syringic acid, and syringaldehyde/vanillin. Seven unknown peaks were also quantified.

Fluorescence measurements suggested that, as anticipated at the outset of the program, higher temperatures could result in the quenching of photo-excited species. Thus, some of these temperatures were removed before they could initiate any of the photochemically induced changes that were observed at lower temperatures.

FPL 81 Ultraviolet/visible spectra pointed to the development of chromophoric groups with broad absorption bands centered at 435 nm in softwood pulps and 412 nm in hardwood pulps; these observations were typical for lignin-containing pulps. The spectra also indicated substantial decay of coniferaldehyde (absorbing at 355 nm) with photo- exposure of stone groundwood papers.

The infrared (IR) and Raman spectra of naturally aged lignin-containing papers and their extracts indicated that photo-exposure resulted in reduced levels of coniferaldehyde and coniferyl alcohol. They also suggest the formation of p-quinone groups from some of the lignin substructures.

Accelerated Aging Studies

Optical Properties Just as in the natural aging studies, the presence of lignin altered the pattern of response to accelerated photo-exposure. Thus, under accelerated aging conditions, the responses of the lignin-free and lignin-containing papers fell into two groups. The solar simulator- based accelerated aging studies could reliably rank the relative stability values of the lignin-containing papers in a manner that paralleled their stability in the natural aging environments. The protocol was not adequate for ranking the optical stability of lignin- free papers. In contrast, use of the 350-nm radiation alone in the Rayonet photo-reactor provided a basis for accurately ranking the optical stability of lignin-free papers.

The observations further suggest that two classes of processes occurred during photo- exposure. The first processes were the direct results of photo-excitation. These processes were able to unfold along a number of reaction pathways depending on the nature of the absorbing center, and they resulted in some decline of optical properties. The second class of reactions included those that were initiated by photo-excitation but which continued to unfold whether or not exposure to light continued. Over time, these photo- initiated dark reactions appeared to continue to affect optical properties regardless of whether light continued to be present. This effect was usually observed in lignin- containing papers. Because of the longer time framework needed to complete these photo-initiated dark reactions, it is believed that they were under-represented in the proposed accelerated aging protocol.

In general, the lignin-containing papers susceptible to direct photochemical reactions were also susceptible to photo-initiated dark reactions. This effect was not investigated in lignin-free papers.

FPL 82 Another important observation was that the total irradiation dose was not sufficient to describe the illumination conditions experienced by the samples. Light intensity and spectral distribution were also important factors that affected the final optical properties. Mechanical Properties Exposure in the solar simulator caused losses in mechanical properties when monitored by the MIT fold test. This was consistent with observations from the natural aging experiments. However, for the accelerated aging samples, the loss in fold endurance was more highly correlated with lignin content than with pH and buffering. This was in contrast with the results from natural aging conditions, where the reverse was true. This difference in dependence, as well as variability of the fold test, prevented the use of mechanical tests to rank stability after accelerated exposure to light. Chemical Analyses The analyses of chemical change were focused on the lignin-containing papers because we did not anticipate that changes in the lignin-free papers could be detected chemically after the relatively short exposure times.

All chemical and spectral analyses indicated that both photo-aging protocols (natural aging and aging in the solar simulator) produced similar changes in the papers and that the photochemistry involved in the natural and accelerated aging were similar as well. The IR and Raman spectral analyses were consistent with the occurrence of p-quinone in paper after both natural and accelerated aging.

The photo-induced depolymerization of paper constituents observed for naturally aged papers was also observed when the papers were exposed under accelerated aging conditions. Solar simulator exposure solubilized relatively fewer carbohydrates in lignin- free papers. This was consistent with the finding, on the basis of mechanical properties, that depolymerization was more sensitive to acid in natural aging and to lignin in accelerated aging.

Accelerated exposure at higher temperatures (60° vs. 40o vs. 20o) produced slightly more degradation. This was observed in HPLC analyses as increased quantities of the same fragmentation products.

After papers were stored in the dark for 3 weeks at room temperature following solar simulator exposure, the effects of photo-initiated dark reactions were observable but, for the most part, small.

FPL 83 Protocol Issues The results of this research clearly show that the condition of a particular sample of paper can not be anticipated 50 years into the future because of the possibilities of wide variation in environmental conditions to which particular samples are exposed. It is likely, however, that distinctions can be made between what may be anticipated for different classes of papers. With this premise in mind, it is possible to contemplate test protocols that will allow comparisons to be made within the range of variables associated with the fabrication of the experimental papers that have been the basis of the studies described in this report.

On the basis of the results of this study, we suggest that full-spectrum irradiation be the basis of the primary screening protocol and that the exposure be at a temperature in the 20o to 30oC range. This is likely to initiate the full range of photochemically induced reactions that are likely to arise from exposure to light, and to cause them to occur at an accelerated rate. These effects are desirable in an accelerated test in order to simulate the full range of chemical reactions that can occur under natural conditions. Different papers will have chemical constituents that react differently to different wavelengths of light. Therefore, the test that includes all wavelengths of light encountered in natural exposure will be the one that is more broadly reliable. The temperature limitation is important to limit departures from phenomena that would be expected to occur under ambient conditions.

The disadvantage of including a broad-spectrum source is that it includes visible light, which, in some instances, will bleach the papers allowing the false illusion that the change is more limited than might be detected through chemical analysis. Thus, the distinctions between relatively stable papers would be masked. These distinctions emerge more clearly when ultraviolet illumination at 350 nm is used as the only source of radiation. When 350 nm illumination is used, the gradations in the relative stability of lignin-free papers emerge more clearly.

The analyses outlined in this report indicate that any accelerated photo-exposure protocol will not reproduce exactly the chemical changes that would occur in paper under natural aging conditions. However, the suggested protocol does reproduce a considerable majority of the chemical and physical changes observed. Within the boundaries of the manufacturing variables assessed, papers that are stable to photo-exposure in these accelerated tests are generally stable under the wide range of natural aging conditions explored. Acid kraft paper number 1 is an exception; it was classified as stable in accelerated aging and moderately stable in natural aging. It is expected, therefore, that the protocol recommended in this work will provide a reliable basis for accelerated assessments of the stability of papers when exposed to electromagnetic radiation in the near ultraviolet and visible wavelength region.

FPL 84 Illumination Spectrum The natural exposure results have shown that no precise answer can be given to the question “How well will this paper perform in natural aging” because natural conditions are so variable. The fact that the same paper responds differently in different natural aging conditions suggests that there is not a single correct light source for accelerated aging. Rather, there are many good choices of light sources.

The features of natural exposure situations that should be represented in an accelerated test are broadband illumination and limiting the intensity of emissions below 340 nm. Since many portions of the visible spectrum are photochemically active with paper, the entire visible spectrum should be included in an accelerated aging protocol. Second, because window glass begins to absorb wavelengths below 360 nm, the amount of radiation with wavelength below 320 nm inside a building is minute. These short wavelengths can cause photochemical reactions that are not possible with longer wavelengths. Therefore, they should be controlled in an accelerated testing protocol. These principles are reflected in ASTM G155–97 (see Tables 14 and 15 in the work presented here).

In our view, IR is unnecessary for paper aging and eliminating it is desirable to reduce the heating of samples. Therefore dichroic mirrors, long arc Xenon lamps, or other devices which reduce IR may be considered as long as they allow the UV and visible spectrum to reach the sample.

The disadvantage of including a broad-spectrum source is that it includes visible light, which in some instances will bleach the papers allowing the false illusion that the change is more limited than might be detected through chemical analyses. Thus, the distinctions between relatively stable papers would be masked. These distinctions emerge more clearly when ultraviolet illumination at 350 nm is used as the only source of radiation. When 350-nm illumination is used, the gradations in the relative stability of lignin-free papers emerge more clearly. Intensity and Duration of Illumination The recommended radiation dose is 24 hours at 800W/m2 radiation intensity (290- 800nm). This dose is equivalent to 5.5 days of continuous noontime direct solar exposure or at least two weeks in the most demanding natural environment possible. This dose was chosen for two reasons: first, it makes the protocol robust in that after this much exposure, the optical properties of papers change very little in response to deviations from the standard radiation dose. Second, such a large radiation dose provides assurance that any chemical component of the paper that is consumed by photochemical reactions during initial stages of natural illumination will more likely be consumed during an accelerated test of this exposure. The amount of additive needed to stabilize a paper during the accelerated test would be enough to protect the paper for a significant period of natural exposure. Illuminating for a much shorter time would provide information about the initial stability of the paper, but conducting such a test reproducibly would be a

FPL 85 much more difficult technical challenge, as discussed in the section “Stability Based on Initial Decay,” page 51. Uniformity of Illumination Uniformity of illumination inside an accelerated aging apparatus is a problematic issue. Instruments that attempt to collimate light, such as the Oriel, tend to have a bright spot in the center of the sample plane that usually limits uniformity. Instruments with samples rotating around the bulb (Atlas weatherometer, for example) have been reported to have uniformity problems in the vertical direction. Consequently, it is very difficult to deliver uniform irradiation to all samples.

Standard practice, already adopted in many ASTM standards, is to rotate samples at intervals during the exposure to ensure uniform dosage. Temperature Until all the products of the aging reactions and the reasons for differences in amounts of products at different temperatures are known, there is no basis for recommending any temperature other than ambient (20oC–30oC) for accelerated aging. The key for the standards writing committee will be how precisely they wish to reproduce the natural aging conditions.

The working premise throughout this study has been that allowing samples to rise above 40o during illumination would produce chemical reactions that were not represented in natural aging and therefore would compromise the protocol. To demonstrate this, FPL researchers tried to find evidence that the chemical species present in papers exposed at 60oC are less similar to naturally aged sheets than are papers exposed at 40oC or 20oC. We were unable to find any such evidence, although our search was by no means exhaustive. It appears that irradiating at 60oC actually makes the testing protocol more robust in some ways. Due to the large uncertainty associated with irradiation at 60oC, however, we feel that it would be wiser to specify 30oC or less in the aging protocol.

The concern over high temperature irradiation is grounded in several principles. High temperatures alone are well known to induce degradation. Higher temperature is also known to reduce the efficiency of photochemical reactions by reducing the population of chemical species in photo-excited states. The moisture content of samples during irradiation will depend to a great extent on the difference in temperature between ambient air temperature and sample temperature, so high temperature irradiation will likely result in very dry samples. Lastly, maintaining a high temperature will result in larger differences in temperature between dark and lightly colored sheets exposed at the same time.

On the other hand, if samples are allowed to reach 60oC during exposure, their optical properties degrade somewhat faster and are more stable after the samples are removed from the photoreactor. Chemical analysis of the photo-exposed papers did not indicate that any exposure temperature (20o, 40o, or 60o) was better at reproducing the chemical products observed in naturally aged sheets.

FPL 86 The discussion so far and all the work has centered on the temperature of samples, but irradiation instruments usually uses a black panel or black standard thermometer to control temperature. Black samples are expected to have a temperature similar to that of a black panel thermometer, but light samples will be cooler depending on their absorbance.

Relative Humidity Observations of natural aging indicated that moisture was involved in the photoyellowing phenomenon and should therefore be controlled in an accelerated aging protocol. In the FPL study, the instrument was placed in a TAPPI controlled room. A paper sample kept warmer than the surrounding air will tend to lose moisture to the air, so it is desirable to keep the difference between coolant air temperature and paper temperature as small as possible and/or to operate with high RH in the coolant air. To make results consistent, we recommend that the apparatus be operated within some temperature/humidity boundaries. Testing of Aged Samples We recommend that some measure of brightness (either directional or diffuse) and b* be taken. Results of optical property measurements can be significantly affected by the time lag between exposure and measurement, as discussed in“Photochemical and Photo- Initiated Dark Reactions,” p. 48. A narrow window for optical property measurements should be provided, such as 24 h after completion of exposure. Papers should be moved from the aging chamber directly to a 23oC/50% RH controlled room for equilibration before measurement. It can be assumed that papers will be at a low moisture content after exposure because of the elevated sample temperature during irradiation, so this protocol should ensure that all papers approach standard conditions from a dry condition.

As discussed earlier as discussed in the sections “Accelerated Aging of ISR papers, Mechanical Properties,” although log of fold is the most sensitive test, there is poor correlation between data from naturally aged and accelerated aged sheets. Accelerated aging caused very little change in fold of lignin-free papers, even if there was significant decay in those papers with natural aging. If mechanical properties are tested after photo- exposure, the rank ordering of papers will be flawed: lignin-containing papers will be ranked more poorly (and conversely unstable lignin-free papers will be ranked better) in the accelerated test than in natural exposure.

When it can be measured, viscosity (TAPPI T 230) is useful in determining the extent of photochemical damage in paper. The standard test is limited to lignin-free papers, but it is the test most sensitive to depolymerization of cellulose in the initial stages of decay. Measuring the probability of chain scission per cellulose bond upon standard exposure would be a way to compare the resistance to chain scissions in various papers.

FPL 87 Existing Standards

Many photo-exposure standards already exist, but the following were thought to be especially pertinent to the current project.

ASTM G 26 “Standard Practice for Operating Light-Exposure Apparatus (Xenon-Arc Type) With and Without Water for Exposure of Nonmetallic Materials. This document describes the current standard for xenon arc operation. Most important are 4.1.2.1c describing existing filter systems, and 4.1.5.1b, describing the current standard for monitoring the irradiance of a xenon arc attempting to reproduce light through window glass exposure.

ASTM D 3424 “Standard Test Methods for Evaluating the Relative Lightfastness and Weatherability of Printed Matter”. This standard gives an overview of current methods of evaluating light sensitivity of materials. Sections 10-1-7 through 10-1-12 specify a daylight through window glass exposure and refers to ASTM G 26. Sections 10-1-22 through 10-1-23 describe operation of a fluorescent lamp apparatus which provides broad spectrum irradiation as well as extra UV. This is described in more detail in ASTM D 4674. Irradiation with UV-A lamps only to gain more precise optical stability for lignin-free papers, could be done with the apparatus described in 4674 using only the UV-A lamps. A rayonet photoreactor was used for this work in our laboratory but we do not know of any standards written for this instrument.

ASTM G 155 “Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Nonmetallic Materials. This is very similar to G 46 but defines the allowed irradiance more precisely.

FPL 88 Joint FPL/KCL Accelerated Light Aging Test Recommendation

This joint recommendation for an accelerated light aging test was developed from the combined research efforts of FPL and KCL. It utilizes both the very long-term natural aging findings at FPL (Madison, WI) and KCL (Espoo, Finland) and the various accelerated aging research studies as the basis for which to judge the efficacy of the recommended protocol.

Following is the specific proposed method.

1. Apparatus

A test chamber that utilizes a “long-arc” xenon lamp is to be used to illuminate the test samples. The lamp spectrum is to be according to ASTM Standard G-155, as per Tables 3 and 4 of that document. A glass filtration system is to be used to simulate natural daylight that has passed through window glass. This is to cut off the very short wavelength light (nominally that which is below 320 nm) as occurs when daylight passes through window glass. The glass filtration system is to be provided as defined in ASTM Standard G-155.

A cooling system is to be provided such that temperature at the paper surface is maintained at ≥ 20°C ≤ 30°C for all paper types. If air is used as a cooling medium and is blown over the surface of the paper sample, it is to be provided at 23°C and 50% relative humidity.

The test chamber is to be so designed as to assure that it is free of ozone gas.

2. Calibration

The intensity [irradiance (E)] of the xenon arc lamp is to be controlled to 800 W/m2 ±20% as measured in the 290-800 nm wavelength range. Recalibration of the instrument is to be sufficiently frequent to ensure continual preservation of both the light spectrum and intensity.

The arrangement of the test chamber is to be constructed so as to assure uniformity of light intensity (irradiance) across the paper sample area in a way that is ≤10% deviation from the target intensity.

Assurance that paper surface temperature is at ≤ 30°C is to be frequently checked by taking measurements with a properly calibrated optical pyrometer.

3. Conditioning

All samples are to be conditioned in the dark prior to the light aging test according to ASTM Standard Practice D 685.

FPL 89 4. Procedure

The initial optical properties of the paper samples are to be measured after preconditioning and just prior to insertion in the test chamber.

The test is to be conducted in a temperature and humidity controlled room that is maintained at 23°C and 50% relative humidity.

Samples are to be mounted on the appropriate surface of the test chamber with clamps provided with the device.

The exposure is to be for 48-hours. Samples are not to be removed from the chamber during the period of exposure. Samples are to be removed from the test chamber at the end of the exposure at the same time of day at which the test was initiated.

Immediately upon removal from the test chamber, the paper samples are to be post- conditioned in the dark for 24-hours according to ASTM Standard Practice D 685.

Immediately upon removal from the post-conditioning process, the optical properties of the samples are to be once again measured.

The optical properties to be measured both before and after the light exposures are found in TAPPI Standard T452, “Brightness of pulp, paper and paperboard (directional reflectance at 457 nm)” and T 524, “Color of paper and paperboard (45°/0° geometry).” Directional Reflectance at 457 nm, (R), and yellowness (b*) are the properties to be measured.

5. Calculation and Interpretation of Results

The percentage change in reflectance at 457 nm (brightness) is to be calculated according to the following formula:

% Change = Ri – Rf x 100 Ri

Where Ri = Initial reflectance (brightness) R f = Final reflectance (brightness)

The absolute change in yellowness is to be calculated according to the following formula:

Change of yellowness, ∆b* = b*f – b*i where b*f = Final yellowness b* i = Initial yellowness

FPL 90 With regard to brightness (reflectance at 457 nm) stability, the following classes are recommended:

Stable: ≤ 5% brightness change Moderately Stable: >5% ≤ 20% brightness change Unstable: >20% brightness change

Note: Papers in the “Moderately Stable” range may be fully stable for some users. However, if a very high level of optical stability is required, papers should be selected that meet the “Stable” criteria above.

With regard to change of yellowness, the following classes are recommended:

Stable: ≤ 3 points of b* increase Moderately Stable: >3 ≤ 8 points of b* increase Unstable: >8 points of b* increase

Note: If all that is desired is legibility of a printed text, paper can become significantly yellowed and still meet the requirements of the end user, even though the changes in optical properties may position it in the “unstable” category.

6. Report

Report the average reflectance at 457 nm (brightness) and yellowness values both before and after the light exposure.

Report the range or standard deviation of the reflectance at 457 nm and yellowness values for each paper specimen evaluated. Annex

1. Strength Testing

Very long-term continuous exposure to natural daylight and to common artificial light has been shown to cause to cause loss of strength in uncoated papers regardless of their fiber composition. This was a very unexpected finding.

The most sensitive test by which to measure physical property loss is cellulose degree of polymerization (DP). This method has problems for use in a standard accelerated light aging test. DP (viscosity) can be measured for lignin-free papers using the well- established CED (cupriethylene diamine) test. However, for lignin-containing papers, a special test that uses a process developed by the Canadian Conservation Institute is required. This procedure calls for partial removal of the lignin with a mild acid chlorite treatment and then uses cadoxen instead of CED. This procedure is required to provide a reliable measure of loss of DP in lignin-containing papers. The cadoxen method is not currently a standard test in most laboratories.

FPL 91 2. Additional Useful Information

Post Color Number (PC) change may be useful to track. PC is calculated from Kubelka- Munk theory according to the following equations:

2 k/s = (1-R∞) /2 R∞

and PC = 100 [(k/s)final – (k/s)initial]

where k is an absorption coefficient, s is a scattering coefficient and R∞ is the reflectance at 457 nm for an “infinitely” thick pad of the material. (The pad must be thick enough so that light does not reach the back surface.)

For calculation of PC Number, the reflectance values at 457 nm are most often used. PC calculations based on reflectance measurements in the reflectance spectrum from 250 to 750 nm wavelength are also valid.

The absorption coefficient (k) is a linear parameter with respect to chromophore concentration, whereas the reflectance (brightness) is not. To calculate the values of k and s, the basis weight (W) of the paper and the results of two reflectance measurements are used: the reflectance of a sheet of paper over a black (zero reflectance) backing (R0) and the reflectance of an “infinitely” thick pad of paper sheets (R∞). If the reflectance measurements are made in the wavelength region 250-750 nm, informative absorption coefficient spectra (k-spectra) can be obtained which more closely characterize chromophores causing yellowing.

Note: In the calculations of PC and k, R can be measured at 457 nm for an “infinitely” thick pad (R∞) or alternatively for one sheet backed with a white background (Rw). In both of these calculations, R0 is obtained from the reflectance of one sheet of paper over a black backing of zero reflectance (R0). Either calculation is an approximation and involves an element of error because photochemical yellowing is a surface phenomenon and the irradiated sheet is not homogeneously yellowed, which is a requirement for applying the Kubelka-Munk theory. The most accurate way of measurement would be to irradiate thin sheets (10-20 g/m2) and to calculate the absorption coefficient, k, according to Kubelka-Munk theory. The absorption coefficient is a linear parameter with respect to chromophore concentration.

3. Correlation between Natural and Accelerated Photoaging

Within the framework of this research program, solar simulator-based (xenon) accelerated photoaging studies could reliably rank the relative stabilities of lignin- containing and, for the most part, lignin-free papers in a manner that paralleled photostability in natural environments. It is expected, therefore, that the recommended protocol will provide a reliable basis for accelerated assessments of the stability of papers when exposed to electromagnetic radiation in the near ultraviolet and visible wavelength region.

FPL 92 (Note – Lignin-containing papers showed a significant loss in brightness after 2.5 years of natural aging. In addition, all lignin-free papers showed some loss in brightness after this period of natural aging. Acid kraft paper showed the greatest loss in brightness of the lignin-free papers. With continued exposure, it is likely that additional lignin-free papers would be found unstable.)

It should be mentioned that natural aging is variously the result of the action of light, heat, and chemicals (e.g. pH), including pollutants from the air that become entrained into the paper. This protocol is intended to characterize only photo-induced reactions. In different conditions of natural aging, one can find an infinite range of conditions where these elements are differently “mixed”. Therefore, for the greatest understanding of possible future aging effects, the investigator may wish to accelerate paper aging separately by elevated light flux, by elevated temperature, and by increased concentration of common pollutant gases.

4. Classes of Stability

It is very important to note that what is a stable paper for one user may be unstable for another. Therefore, the limits of acceptability (the points at which a paper is no longer useful for its intended purpose) must be defined by end-users. It is only with such information in hand that accurate definitions of the optical stability of paper can be made.

FPL 93 References Agarwal, U.P. Assignment of the photoyellowing related 1675 cm-1 Raman/IR band to p- quinones and its implication to the mechanism of color reversion in mechanical pulps. J. Wood Chem. Technol. 18(4), 381–402, 1998.

Agarwal, U.P. An overview of Raman spectroscopy as applied to lignocellulosic materials. Advances in Lignocellulosics Characterization. D.S. Argyropoulous, ed. Ch. 9 and references cited therein. Tappi Press, Atlanta, GA, 1999.

Agarwal, U.P. and McSweeny, J.D. Photoyellowing of thermomechanical pulps: Looking beyond α-carbonyl and ethylenic groups as the initiating structures. J. Wood Chem. Tech. 17, 1, 1999.

Buschle–Diller, G. and Zeronian, S.H. Weathering and photodegradation of cellulose. In Photochemistry of Lignocellulosic Materials. C. Heitner and J.C. Scaiano, eds. ACS Symp. Ser. #531, ch. 14. American Chemical Society: Washington, DC, 177–189, 1993.

Crook, D.M. and Bennett, W.E. The effect of humidity and temperature on the physical properties of paper. The British Paper and Board Industry Research Association, Kenley, Surrey, UK, 1962.

Daruwalla, E.H., D’Silva, A.P., and Mehta A.C. Photochemistry of cotton and chemically modified cotton. Part I. Behavior during exposure to carbon arc and solar irradiations. Textile Res. J., 37(3), 1967.

Davis, M.W. A rapid modified method for the compositional carbohydrate analysis of lignocellulosics by high pH anion-exchange chromatography with pulsed amperometric detection. J. Wood Chem. Technol. 18(2), 235–252, 1998.

Ek, M., Lennholm, Lindblad, G., Iversen, T., and Gray, D.G. Photochemical behavior of UV-irradiated mechanical pulps. Photochemistry of Lignocellulosic Materials. C. Heitner and J.C. Scaiano, eds. ACS Symp. Series #531, ch. 11. American Chemical Society, Washington DC, 1993.

Forsskåhl, I. Sequential treatment of mechanical and chemimechanical pulps with light and heat. Part 1. UV-VIS Reflectance Spectroscopy. Nord. Pulp Paper Res.. 3, 118, 1991.

Forsskåhl, I. and Janson, J. Sequential treatment of mechanical and chemimechanical pulps with light and heat. Part 2. FT–IR and UV–Vis absorption-scattering spectra. Nord. Pulp Paper Res.. 2, 48, 1992.

Forsskåhl, I. and Maunier, C. Photocycling of chromophoric structures during irradiation of high-yield pulps. The Photochemistry of Lignocellulosic Materials. C. Heitner and J.C.

FPL 94 Scaiano, eds. ACS Symp. Series #531, ch. 12. American Chemical Society, Washington DC, 1993.

Forsskåhl, I. and Tylli, H. Action spectra in the UV and visible region of light-induced changes of various refiner pulps. The Photochemistry of Lignocellulosic Materials. American Chemical Society. C. Heitner and J.C. Scaiano, eds. ACS Symp. Series #531, ch. 3. American Chemical Society, Washington DC, 1993.

Ketola, W.D and Grossman, D. eds. Accelerated and outdoor durability testing of organic materials—STP 1202. American Society for Testing and Materials, Philadelphia, PA, 1994.

Leary, G. Recent progress in understanding and inhibiting the light-induced yellowing of mechanical pulps. J. Pulp. Paper Sci. 20, J154, 1994.

Schmidt, J.A. and Heitner, C Use of UV-visible diffuse reflectance spectroscopy for chromophore research on wood fibers: A review. Tappi J. 76, 117, 1993.

Shahani, Chandru. Director, U.S. Library of Congress, Preservation research office. Personal communication.

Sjöstrom, E. Wood Chemistry: Fundamentals and Applications. Academic Press, NY, 1981.

Spinner, I.H. Brightness reversion: A critical review with suggestions for further research. Tappi J., 45, 495, 1962.

TAPPI, T 511 om-88 Folding endurance of paper, TAPPI test methods TAPPI, Atlanta, GA, 1992.

Tylli, H., Forsskåhl, I., and Olkkonen, C., Effect of heat, light, and infrared radiation on chemimechanical pulp studied by fluorescence spectroscopy. In Proc. 9th ISWPC, June 9–12, 1997, Montreal, Canada, paper 116-1.

Wan, J.K.S, Tse, M.Y., and Heitner, C., An ESR and time-resolved CIDEP study of the light-induced yellowing processes of TMP paper. J. Wood Chem. Tech. 13, 327, 1993.

FPL 95 List of Tables Table Number Title 1 Paper production parameters 2 Measured light intensity in aging chambers 3 Measured light intensity in normal offices 4 Sequence of sample placement in natural aging chambers 5 Change in equilibrium moisture content for papers aged 29 months in natural aging chamber vs. cold storage 6 Viscosity measurements of lignin-free papers with and without 29 months of photo-exposure 7 Viscosity of cotton papers after 44 months of natural aging 8 Assignment of IR bands in photo-exposed papers 9 Loss in directional brightness (control minus exposed) for lignin-free papers after various exposures 10 Brightness immediately after solar simulator exposure at given temperatures 11 Loss of brightness after accelerated exposure over time 12 Effect of increased temperature during exposure on optical properties 13 Change in peak heights between papers stored at 21oC vs. –90oC for 3 weeks 14 Data from ASTM G 155–97 (table 3) and FPL tests 15 Data from ASTM G 155–97 (table 4, and FPL calculated values 16 Uniformity of illumination in natural aging chambers 17 Fading of AATCC blue wool samples in natural aging chambers 18 Time to deliver 3.5 MJ/m2 of 300–400 nm radiation in natural aging chambers

FPL 96 List of Figures

Figure Number Title

1 Floor plan of natural aging chambers 2 North window spectrum from mid-day 3 Directional brightness of paper in fluorescent chamber 4 Yellowing (b*) of paper in fluorescent chamber 5 Redness (a*) of paper in fluorescent chamber 6 Directional brightness of paper 14 vs. time 7 Brightness of paper 9 vs. UV dose 8 Directional brightness with fluorescent photo-exposure of lignin-free papers 9 Directional brightness of lignin-free papers after 17-months exposure 10 Loss in MIT fold number after 29 months of north window exposure 11 Loss in MIT fold after 29-month north window exposure (logfold units) 12 Carbohydrates extracted with water from paper 3 exposed for 35 months 13 HPLC chromatograms of extracts from papers 3 and 9 after 35 months of exposure 14 Loss in reflectance in paper 3 after 3 months and after 4 years of natural exposure 15 Loss in reflectance of paper 9 after 3 months and after 4 years of natural exposure 16 Loss in reflectance of paper 7 after 3 months and after 4 years of natural exposure 17 Loss in reflectance of paper 1 after 3 months and after 4 years of natural exposure 18 Loss in reflectance of paper 12 after 3 months and after 4 years of natural exposure 19 Raman spectra of papers 3, 4, 7 and 8 after north window exposure 20 Illumination spectra of Rayonet photoreactor, solar simulator and ASTM solar standard 21 Comparison of paper 9 brightness under various photo-exposure conditions 22 Brightness of paper exposed in 4-day solar simulator vs. 29-month natural exposure 23 Directional brightness of lignin-containing papers after various exposure conditions

FPL 97 24 Color change during 3 different dark-storage periods at 4oC following solar simulator irradiation 25 Directional brightness after 9.0 KWh irradiation at 250–1000 nm as applied at different rates in solar simulator 26 Separation of papers by solar simulator into general classes of photostability 27 Loss in log fold with exposure. Solar simulator and 29 months of north window exposure 28 Loss in reflectance of paper 10, comparing solar simulator to 4 years natural exposure 29 Loss of reflectance of paper 11 with natural 3-month and solar simulator exposures 30 IR absorbance of paper 3 31 Raman spectra of paper 3 after natural and accelerated aging 32 Mass of carbohydrates extracted after 35 months of north window exposure vs. solar simulator 33 Carbohydrate oligomers extracted from paper 7 after solar simulator and north window exposures 34 HPLC chromatograms of methanol extracts of exposed papers 35 Paper 9 loss in reflectance upon irradiation 36 UV/VIS loss in reflectance for paper 3 stored at 21oC or –90oC for 3-weeks

FPL 98 Appendixes

Quantification and Prediction for Aging of Printing & Writing Papers Exposed to Light

ASTM Institute for Standards Research Program Effect of Aging on Printing & Writing Papers

USDA Forest Service Forest Products Laboratory Madison, Wisconsin

Final Report

August 2000

FPL 99 Table of Contents Appendix Page Number

A Conditions in Natural Aging Chambers...... 101 Temperature and Humidity...... 101 UV Illumination Intensity Measured by AATCC Blue Wool ...... 102 B Paper Properties ...... 105 C Optical Properties - Decay Curves from Natural Aging Chambers...... 107 Lignin-Containing Papers...... 107 Lignin-Free Papers...... 113 D Comparison of Sensitivity of Mechanical Tests to Photo-Degradation...... 118 E Change in Log of Fold Upon Photo-Exposure...... 119 F Carbohydrates in Water Extracts...... 120 G HPLC of Photo-Exposed Papers...... 122 H UV/VIS Spectra After Natural (3 Months and 4 Years) and Solar Simulator Exposure ...... 124 Lignin-Containing Papers...... 125 Lignin-Free Papers...... 129 I Infrared Spectra ...... 133 J Raman Spectra...... 135 K Optical Properties Decay After Natural and Accelerated Exposure...... 137 Lignin-Containing Papers...... 138 Lignin-Free Papers...... 142 L Photo-Initiated Dark Reactions During Storage at 40C...... 145 M Effect of Non-Uniform Illumination on Optical Properties Decay...... 146 N Sponsors and Supporting Cooperators...... 148 O Lists of Figures and Tables in Appendixes...... 149 List of Figures...... 149 List of Tables ...... 151

FPL 100 A Conditions in Natural Aging Chambers

Temperature and Humidity

80 Daily Ave RH, %

70 Daily Average Temp, C

Sine Curve Fit to RH 60 Sine Curve Fit to Temp 50

20 Average Temp, RH Values

0 Jul-98 Nov-98 Feb-99 May-99 Aug-99 Dec-99 Mar-00

Figure 1: Temperature and RH data from natural aging chamber, and fitted sinusoidal curves, with period of 365 days.

FPL 101 UV Illumination Intensity Measured by AATCC Blue Wool

12 North Ave 10

8 Halogen Ave

6 Fluor Ave

4 5 AATCC Fading Units Color Change (Delta E) 2 20 AATCC 0 Fading Units 0 100 200 300 400 500 600 Days in Chamber

Figure 2: Average color change in AATCC blue wool L2 with time

2.5 North Ave 2

Halogen 1.5 Ave

1 Fluor Ave

Color Change, Delta E 0.5 5 AATCC Fading 0 Units 0 100 200 300 400 500 600 Days in Aging Chamber

Figure 3: Average color change in AATCC blue wool L5 with time

FPL 102 20 18 16 North Low 14

12 North Med 10 8 North Hi 6 5 AATCC 4 Fading Units Color Change (Delta E) 2 20 AATCC 0 Fading Units 0 100 200 300 400 500 600 Days in Chamber

Figure 4: Blue wool L2 color change: high, medium, and low intensity zones of north window .

9 8

7 Halogen Low 6 Halogen Med 5 4 Halogen Hi 3 2 5 AATCC Fading Units Color Change (Delta E) 1 20 AATCC 0 Fading Units 0 100 200 300 400 500 600 Days in Chamber

Figure 5: Blue wool L2 color change: high, medium, and low intensity zones of halogen chamber.

FPL 103 12

10 Fluor Low 8 Fluor Med 6 Fluor Hi 4

5 AATCC Fading Units Color Change (Delta E) 2

20 AATCC 0 Fading Units 0 100 200 300 400 500 600 Days in Chamber

Figure 6: Blue wool L2 color change: high, medium, and low intensity zones of fluorescent chamber.

FPL 104 B Paper Properties Table 1: Selected paper properties as specified by ASTM/ISR Paper BN- BN- SW- HW- SW- TEXTILE pH CaCO3 Starch Additives Number SWK HWK BCTMP BCTMP SGW COTTON % 1 100 5.0 0 No Alum, Hercules Nuphor 635 rosin size 2 100 8.1 5 No 3 100 5.0 0 No Alum, Hercules Nuphor 635 rosin size 4 100 8.1 5 No 5 100 5.0 0 No Alum, Hercules Nuphor 635 rosin size 6 100 8.1 5 No 7 20 80 5.0 0 No Alum, Hercules Nuphor 635 rosin size 8 20 80 7.0 5 No SMI process 9 20 80 8.1 0 No 10 20 80 8.1 5 No 11 50 50 8.1 0 No 12 50 50 8.1 5 No 13 50 50 8.1 5 No 14 50 50 5.0 0 No Alum, Hercules Nuphor 635 rosin size 15 50 50 8.1 5 Yes AKD internal, Penford Gum size Code: BN = Bleached Northern SW = Softwood SGW = Stone Groundwood BS = Bleached Southern HW = Hardwood S(H)WK = Soft(Hard)wood Kraft BCTMP = Bleached Chemithermomechanical Pulp

FPL 105 Table 2: Paper properties as measured by FPL staff

ASTM FPL Cu Fe Mn Ni Klason Acid Sol Moisture Surface Number Code Wt ppm Wt ppm Wt ppm Wt ppm Lignin Lignin Wt % pH 1 W 74 29 0.6 >1 0.7% 0.5% 6.3 5.5 2 X 69 78 3.1 9 0.2% 0.4% 6.3 5.2 3 B 68 23 1.1 1 24.9% 0.9% 7.5 6.0 4 A 64 62 2.2 17 23.6% 0.9% 7.8 6.1 5 Y 71 46 2.1 6 1.7% 0.3% 6.0 6.3 6 Z 118 94 4.4 13 1.6% 0.3% 5.5 5.7 7 C 85 33 21.2 6 23.8% 0.5% 8.3 5.7 8 D 72 85 55.7 37 22.3% 0.4% 7.3 6.3 9 E 85 22 8.0 4 11.7% 3.0% 7.5 5.0 10 F 71 65 2.8 7 10.9% 3.0% 7.0 6.6 11 U 80 28 2.4 3 0.2% 0.4% 6.1 5.5 12 T 84 67 2.8 10 0.2% 0.4% 5.5 6.4 13 H 72 57 2.9 5 7.4% 2.3% 7.3 5.5 14 G 94 29 2.6 2 7.1% 2.3% 6.7 6.4 15 V 84 78 3.0 10 0.4% 0.4% 6.5 6.3 Metals measured by graphite furnace atomic absorption Klason Lignin by TAPPI T236 Acid Soluble lignin by TAPPI T 222 Surface pH by TAPPI T 529 FPL code uses only 1 character, progresses from high to low lignin content

FPL 106 C Optical Properties - Decay Curves from Natural Aging Chambers

Lignin-Containing Papers

Results presented here are the average of measurements from two different sheets. After measurement, each sheet was returned to the same place on the wall for further exposure, measurements were made in the same position every month. Variability in data is addressed in Appendix N.

Papers from the same fiber furnish are presented on the same page. The acid paper is always presented first. This convention is followed throughout the appendixes.

Lower brightness indicates a darker sheet, positive b* (b star) indicates yellow hue, positive a* (a star) indicates red hue.

FPL 107 90 3 100% SW-BCTMP + Alum 4 100% SW-BCTMP 80 +CaCO3 7 80% SW-SGW + Alum

70 8 80% SW-SGW + CaCO3

60 9 80% HW-BCTMP 10 80% HW-BCTMP + 50 CaCO3 14 50/50 HW- BCTMP/BNSWK +Alum 40 13 50/50 HW- BCTMP/BNHWK +CaCO3 Directional Brightness 1 100% BNSWK + Alum 30 2 100% BNSWK +CaCO3 20 Temp/RH Trendline 0 200 400 600 800 1000 1200 Days Exposure

Figure 7: Directional brightness in north window chamber

90 3 100% SW-BCTMP + Alum 4 100% SW-BCTMP 80 +CaCO3 7 80% SW-SGW + Alum 70 8 80% SW-SGW + CaCO3

60 9 80% HW-BCTMP

50 10 80% HW-BCTMP + CaCO3 14 50/50 HW- 40 BCTMP/BNSWK +Alum Directional Brightness 13 50/50 HW- BCTMP/BNHWK +CaCO3 30 1 100% BNSWK + Alum

20 2 100% BNSWK +CaCO3 0 200 400 600 800 1000 1200 Days Exposure

Figure 8: Directional brightness in halogen chamber

FPL 108 90 3 100% SW-BCTMP + Alum 4 100% SW-BCTMP 80 +CaCO3 7 80% SW-SGW + Alum 70 8 80% SW-SGW + CaCO3

60 9 80% HW-BCTMP

50 10 80% HW-BCTMP + CaCO3 14 50/50 HW- 40 BCTMP/BNSWK +Alum Directional Brightness 13 50/50 HW- BCTMP/BNHWK +CaCO3 30 1 100% BNSWK + Alum

20 2 100% BNSWK +CaCO3 0 200 400 600 800 1000 1200 Days Exposure

Figure 9: Directional brightness in fluorescent chamber

40 3 100% SW-BCTMP + Alum 35 4 100% SW-BCTMP +CaCO3 30 7 80% SW-SGW + Alum 25 8 80% SW-SGW + CaCO3 20 9 80% HW-BCTMP

b* (Redness) 15 10 80% HW-BCTMP + 10 CaCO3

14 50/50 HW- 5 BCTMP/BNSWK +Alum

0 13 50/50 HW- BCTMP/BNHWK +CaCO3 0 200 400 600 800 1000 1200 Days Exposure

Figure 10: b* in north window chamber

FPL 109 40 3 100% SW-BCTMP + Alum 35 4 100% SW-BCTMP +CaCO3 30 7 80% SW-SGW + Alum 25 8 80% SW-SGW + CaCO3 20

9 80% HW-BCTMP 15 b* (Yellowness)

10 10 80% HW-BCTMP + CaCO3

5 14 50/50 HW- BCTMP/BNSWK +Alum

0 13 50/50 HW- 0 200 400 600 800 1000 1200 BCTMP/BNHWK +CaCO3 Days Exposure

Figure 11: b* in halogen chamber

35 3 100% SW-BCTMP + Alum 4 100% SW-BCTMP 30 +CaCO3 7 80% SW-SGW + Alum 25 8 80% SW-SGW + CaCO3

20 9 80% HW-BCTMP

10 80% HW-BCTMP + 15 CaCO3 14 50/50 HW-

b* (Yellowness) BCTMP/BNSWK +Alum 10 13 50/50 HW- BCTMP/BNHWK +CaCO3 1 100% BNSWK +Alum 5 2 100% BNSWK +CaCO3 0 Temp/RH Trendline 0 200 400 600 800 1000 1200 Days Exposure

Figure 12:b* in fluorescent chamber

FPL 110 8 3 100% SW-BCTMP + Alum 7 4 100% SW-BCTMP 6 +CaCO3

5 7 80% SW-SGW + Alum

4 8 80% SW-SGW + CaCO3 3 9 80% HW-BCTMP 2 a* (Redness) 1 10 80% HW-BCTMP + CaCO3

0 14 50/50 HW- 0 200 400 600 800 1000 1200 BCTMP/BNSWK +Alum -1 13 50/50 HW- -2 BCTMP/BNHWK +CaCO3 Days Exposure

Figure 13: a* in north window chamber

8 3 100% SW-BCTMP + Alum 7 4 100% SW-BCTMP 6 +CaCO3

5 7 80% SW-SGW + Alum

4 8 80% SW-SGW + CaCO3 3 9 80% HW-BCTMP 2 a* (Redness) 1 10 80% HW-BCTMP + CaCO3

0 14 50/50 HW- 0 200 400 600 800 1000 1200 BCTMP/BNSWK +Alum -1 13 50/50 HW- -2 BCTMP/BNHWK +CaCO3 Days Exposure

Figure 14: a* in halogen chamber

FPL 111 8 3 100% SW-BCTMP + Alum

7 4 100% SW-BCTMP 6 +CaCO3

5 7 80% SW-SGW + Alum

4 8 80% SW-SGW + CaCO3 3 9 80% HW-BCTMP 2 a* (Redness) 1 10 80% HW-BCTMP + CaCO3 0 14 50/50 HW- 0 200 400 600 800 1000 1200 BCTMP/BNSWK +Alum -1 13 50/50 HW- -2 BCTMP/BNHWK +CaCO3 Days Exposure

Figure 15: a* in fluorescent chamber

FPL 112 Lignin-Free Papers Paper type indicated in legend.

90 1 2 85 5 6 11 80 12 15

75 Directional Brightness (MD)

70 0 100 200 300 400 500 600 700 800 900 Days of North Window Exposure

Figure 16: Directional Brightness with North window exposure.

90 1 2 85 5 6 11 80 12 15

75 Directional Brightness (MD)

70 0 100 200 300 400 500 600 700 800 900 Days of Halogen Light Exposure

Figure 17: Directional brightness with halogen exposure

FPL 113 95

90 1 2 85 5 6 11 80 12 15

75 Directional Brightness (MD)

70 0 100 200 300 400 500 600 700 800 900 Days of Fluorescent Light Exposure

Figure 18: Directional brightness with fluorescent exposure

7 1 2 6 5 5 6 11 4 12 3 15 b star (yellowness) 2

0 0 100 200 300 400 500 600 700 800 900 Days of North Window Light Exposure

Figure 19: b* with north window exposure

FPL 114 10

7 1 2 6 5 5 6 11 4 12 3 15 b star (yellowness) 2

0 0 100 200 300 400 500 600 700 800 900 Days of Halogen Light Exposure

Figure 20: b* with halogen exposure

7 1 2 6 5 5 6 11 4 12 3 15 b star (yellowness) 2

0 0 100 200 300 400 500 600 700 800 900 Days of Fluorescent Light Exposure

Figure 21: b* with fluorescent exposure

FPL 115 1

0.5

1 0 2 5 -0.5 6 11 12 -1

a star (redness) 15

-1.5

-2 0 100 200 300 400 500 600 700 800 900 Days of North Window Light Exposure

Figure 22: a* with north window exposure

0.5

1 0 2 5 -0.5 6 11 12 -1

a star (redness) 15

-1.5

-2 0 100 200 300 400 500 600 700 800 900 Days of Halogen Light Exposure

Figure 23: a* with halogen exposure

FPL 116 1

0.5

1 0 2 5 -0.5 6 11 12 -1

a star (redness) 15

-1.5

-2 0 100 200 300 400 500 600 700 800 900 Days of Fluorescent Light Exposure

Figure 24: a* with fluorescent exposure

FPL 117 D Comparison of Sensitivity of Mechanical Tests to Photo- Degradation

Table 3: Probability that two samples of papers were from the same group.

Sensitivity Analysis: Comparison of the Ability of Mechanical Tests to Distinguish Papers Aged Under Different Conditions. Values displayed are probability that two data sets are from the same pool. (Student T-Tests) Compar- Stretch Ten- Brittle- Log Zero TEA Fold Tear ison # % sile ness Fold Span 1 1.1E-01 3.0E-02 5.2E-02 4.5E-01 1.4E-09 4.0E-12 1.4E-01 2 3.4E-05 1.5E-02 7.8E-04 7.0E-04 2.3E-10 2.6E-23 8.7E-10 3 9.9E-02 1.5E-01 1.3E-01 3.9E-02 2.3E-18 9.4E-22 1.2E-02 4.5E-04 4 2.9E-06 1.5E-03 1.5E-06 1.3E-04 5.2E-24 4.7E-32 2.8E-11 1.2E-03 5 2.0E-04 9.3E-01 3.6E-03 1.5E-01 2.2E-11 4.3E-11 6 6.8E-03 4.1E-04 9.8E-04 4.9E-02 3.0E-14 2.2E-12 7 2.0E-04 4.7E-01 2.6E-03 1.6E-03 5.1E-05 7.1E-06 8 3.5E-01 7.4E-01 3.7E-01 1.9E-01 3.3E-03 3.3E-03 9 4.9E-01 4.4E-01 9.6E-01 6.4E-01 4.1E-04 3.7E-05 10 2.9E-06 5.5E-02 4.8E-05 7.4E-04 4.3E-11 1.1E-11 11 1.2E-05 3.5E-04 6.2E-06 2.4E-01 8.0E-13 2.8E-17 12 3.6E-07 2.5E-07 4.3E-06 1.4E-06 4.7E-11 6.9E-19 13 3.1E-06 2.0E-05 1.9E-05 5.8E-04 3.7E-13 7.9E-21 14 8.2E-03 2.5E-03 1.0E-02 3.4E-01 6.6E-01 7.6E-01 15 8.3E-01 8.9E-01 5.2E-01 7.3E-01 6.8E-02 5.0E-02 16 4.2E-01 1.2E-01 3.0E-01 1.9E-01 5.2E-01 3.5E-01 17 8.6E-01 7.6E-01 9.2E-01 6.5E-01 5.4E-03 2.6E-03 18 3.9E-02 4.8E-01 2.9E-01 4.2E-02 7.7E-01 8.5E-01 19 5.2E-01 3.2E-01 4.5E-01 7.2E-01 6.7E-04 8.4E-05 20 1.2E-01 7.6E-01 1.5E-01 2.7E-02 3.8E-04 1.4E-04 Success 10/20 6/20 9/20 6/20 16/20 16/20 2/4 2/2 # “Best”** 220161300 Difference determined according to student’s t-test. The 5.0E-2 or less is significant at 95% confidence level. “Success” – 2nd last row, = number of times the test was able to distinguish the groups with 95% confidence or better. Last row, “# Best” refers to the number of times that test discriminated groups with the greatest confidence.

FPL 118 E Change in Log of Fold Upon Photo-Exposure

Table 4: Loss in Log of Fold with exposure

Paper # Solar 17 North 29 North 17 Hal 29 Hal 17 Fluor 29 Fluor Sim 1 -0.07 0.33 0.60 0.33 0.61 0.27 0.53 2 0.10 0.33 0.32 0.27 0.29 0.11 0.54 3 0.90 1.12 0.97 0.50 0.66 0.34 0.44 4 0.49 0.34 0.51 0.32 0.58 0.15 0.12 5 0.03 0.09 0.36 0.18 0.39 0.29 0.41 6 -0.06 0.04 0.04 0.00 0.45 -0.02 0.09 7 0.38 0.61 1.15 0.46 1.01 0.42 0.92 8 0.34 0.45 0.56 0.27 0.61 0.40 1.25 9 0.42 0.48 0.58 -0.01 0.28 0.00 0.64 10 0.17 0.37 0.28 0.12 0.10 0.07 0.19 11 0.09 0.02 0.33 0.42 0.46 0.13 0.40 12 0.07 0.33 0.46 0.29 0.46 0.22 0.21 13 0.07 0.31 0.36 0.05 0.19 0.06 0.32 14 0.24 0.24 0.59 0.13 0.31 0.12 0.16 15 0.07 0.24 0.50 0.28 0.34 0.22 0.58

Table 5: 95% confidence interval for each value of loss of fold given in Table 4

Paper # Solar 17 North 29 North 17 Hal 29 Hal 17 Fluor 29 Fluor Sim 1 0.08 0.06 0.12 0.07 0.09 0.07 0.09 2 0.10 0.04 0.06 0.04 0.05 0.09 0.05 3 0.13 0.08 0.10 0.07 0.08 0.07 0.09 4 0.14 0.09 0.08 0.06 0.06 0.07 0.07 5 0.18 0.09 0.08 0.07 0.07 0.08 0.07 6 0.17 0.11 0.10 0.08 0.09 0.09 0.08 7 0.20 0.10 0.13 0.09 0.12 0.10 0.12 8 0.17 0.06 0.09 0.07 0.09 0.11 0.12 9 0.08 0.08 0.09 0.08 0.09 0.08 0.09 10 0.12 0.07 0.08 0.09 0.09 0.11 0.08 11 0.15 0.09 0.10 0.12 0.11 0.11 0.13 12 0.20 0.12 0.11 0.13 0.11 0.11 0.10 13 0.16 0.10 0.11 0.12 0.13 0.09 0.11 14 0.13 0.04 0.08 0.04 0.06 0.06 0.05 15 0.09 0.11 0.10 0.08 0.09 0.08 0.08 For any fold value in Table 4, the corresponding 95% confidence interval (+/-) is given in the corresponding position of Table 5.

FPL 119 F Carbohydrates in Water Extracts

Table 6: Percent by total mass of various sugars in water extract of photo-exposed papers

Pape Total Exposur Fucos Arabanos Galactos Rhamnos Glucos r Xylose Mannose e e e e e e Type Carbo 1 Control 0.10% nd 0.02% 0.00% nd 0.01% 0.06% 0.01% 1 SSim 0.31% nd 0.03% 0.01% nd 0.04% 0.18% 0.05% 1 North 0.91% nd 0.09% 0.03% 0.00% 0.10% 0.53% 0.15%

2 Control 0.06% nd 0.00% 0.00% nd 0.01% 0.04% 0.01% 2 SSim 0.22% nd 0.01% 0.01% nd 0.03% 0.12% 0.03% 2 North 0.54% nd 0.04% 0.02% nd 0.08% 0.31% 0.10%

3 Control 0.49% nd 0.09% 0.05% 0.01% 0.01% 0.30% 0.02% 3 SSim 4.84% nd 0.47% 0.52% 0.08% 0.40% 2.24% 1.14% 3 North 7.84% nd 0.65% 0.75% 0.11% 0.86% 3.32% 2.15% 3 Fluorescent 6.17% nd 0.65% 0.67% 0.09% 0.47% 2.82% 1.46% 3 Halogen 4.06% nd 0.48% 0.44% 0.06% 0.26% 1.99% 0.82%

4 Control 0.22% nd 0.02% 0.02% 0.00% 0.01% 0.16% 0.01% 4 SSim 4.72% nd 0.44% 0.52% 0.07% 0.39% 2.22% 1.08% 4 North 7.10% nd 0.57% 0.69% 0.09% 0.75% 3.08% 1.93%

5 Control 0.02% nd 0.00% 0.00% nd 0.01% 0.00% nd 5 SSim 0.07% nd 0.01% 0.01% 0.00% 0.03% 0.02% nd 5 North 0.30% nd 0.03% 0.01% 0.00% 0.19% 0.07% 0.00%

6 Control 0.02% nd 0.00% 0.00% 0.00% 0.01% 0.00% nd 6 SSim 0.05% nd 0.00% 0.01% 0.00% 0.03% 0.01% nd 6 North 0.15% nd 0.01% 0.01% 0.00% 0.09% 0.04% 0.00%

7 Control 0.23% nd 0.05% 0.05% 0.00% 0.03% 0.04% 0.06% 7 SSim 4.17% nd 0.26% 0.45% 0.04% 0.60% 0.93% 1.89% 7 North 6.41% nd 0.42% 0.63% 0.05% 0.96% 1.61% 2.74%

8 Control 0.24% nd 0.03% 0.05% 0.01% 0.04% 0.05% 0.06% 8 SSim 3.85% nd 0.23% 0.41% 0.03% 0.55% 0.98% 1.65% 8 North 6.41% nd 0.37% 0.59% 0.04% 0.97% 1.72% 2.71%

9 Control 0.74% nd 0.01% 0.01% 0.01% 0.01% 0.71% 0.00% 9 SSim 4.09% 0.01% 0.19% 0.15% 0.12% 0.08% 3.47% 0.07% 9 North 5.97% 0.01% 0.23% 0.20% 0.17% 0.21% 4.99% 0.16% 9 Fluorescent 3.17% 0.01% 0.20% 0.15% 0.11% 0.06% 2.61% 0.05% 9 Halogen 3.05% 0.00% 0.17% 0.12% 0.09% 0.05% 2.56% 0.05%

FPL 120 Table 6 – continued on next page

Table 6 Continued –

Percent by total mass of various sugars in water extract of photo-exposed papers

Paper Exposur Total Fucos Arabanos Galactos Rhamnos Mannos Glucose Xylose Type e Carbo e e e e e 10 Control 0.38% 0.00% 0.01% 0.01% 0.01% 0.01% 0.34% 0.00% 10 SSim 3.61% 0.01% 0.18% 0.15% 0.11% 0.09% 3.00% 0.07% 10 North 4.52% 0.01% 0.20% 0.17% 0.13% 0.15% 3.75% 0.11% 10 Fluorescent 3.48% 0.01% 0.23% 0.17% 0.12% 0.08% 2.81% 0.06% 10 Halogen 2.87% 0.00% 0.18% 0.13% 0.09% 0.07% 2.34% 0.06%

11 Control 0.05% nd 0.00% 0.00% nd 0.00% 0.04% 0.00% 11 SSim 0.18% nd 0.01% 0.01% nd 0.02% 0.13% 0.01% 11 North 1.14% 0.00% 0.06% 0.02% 0.00% 0.12% 0.83% 0.11%

12 Control 0.04% nd 0.00% 0.00% nd 0.00% 0.03% 0.00% 12 SSim 0.14% nd 0.01% 0.00% nd 0.02% 0.09% 0.01% 12 North 0.83% 0.00% 0.04% 0.02% 0.00% 0.11% 0.55% 0.10%

13 Control 0.22% nd 0.01% 0.01% 0.00% 0.01% 0.19% 0.00% 13 SSim 2.61% 0.01% 0.14% 0.11% 0.08% 0.08% 2.14% 0.06% 13 North 3.53% 0.01% 0.16% 0.13% 0.09% 0.14% 2.87% 0.12%

14 Control 0.33% nd 0.02% 0.01% 0.01% 0.01% 0.28% 0.00% 14 SSim 2.86% 0.00% 0.13% 0.10% 0.08% 0.08% 2.40% 0.06% 14 North 4.18% 0.01% 0.19% 0.14% 0.10% 0.18% 3.39% 0.16%

15 Control 3.02% nd 0.00% 0.00% 0.00% 2.98% 0.03% 0.00% 15 SSim 2.52% nd 0.01% 0.01% 0.00% 2.39% 0.10% 0.01% 15 North 3.73% nd 0.05% 0.02% 0.00% 3.07% 0.50% 0.09% nd = not detected

FPL 121 G HPLC of Photo-Exposed Papers Chromatograms recorded at 220nm. Methanol extracts of exposed papers. Chromatograms for control samples are often very close to baseline. 6000

3 North 5000 3 Halogen 4000 3 Fluor 3000 3 30C Cold

2000

Detector Response 3 60C Cold

1000 3 60C Amb

0 3 Control 4681012 Time (min)

Figure 25: HPLC chromatograms of paper 3 extracts

Note: top-bottom order of chromatograms is preserved in legend. 6000 9 North 5000 9 Halogen

4000 9 Fluor

3000 9 30C Cold

9 30C Amb 2000 Detector Response 9 60C Cold 1000 9 60C Amb 0 9 Control 4681012 Time (min)

Figure 26: HPLC chromatograms of paper 9 extracts

FPL 122 6000

10 Halogen 5000 10 Fluor 4000 10 30C Cold 3000 10 30C Amb

2000

Detector Response 10 60C Cold

1000 10 60C Amb

0 10 Control 4681012 Time (min)

Figure 27: HPLC chromatograms of paper 10 extracts

FPL 123 H UV/VIS Spectra After Natural (3 Months and 4 Years) and Solar Simulator Exposure

Change in reflectance upon exposure: Control – Aged. Less reflectance upon exposure results in positive value in these figures.

FPL 124 Lignin-Containing Papers

50 North 4 years 45 Hal 4 years 40 Fluor 4 years 35 North 3mo 30 Hal 3mo 25 Fluor 3mo 20 S Sim 15

Loss in Reflectance (%) 10

0 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 28: Reflectance decline of paper 3

North 4 years 50 Hal 4 years

40 Fluor 4 years North 3mo

30 Hal 3mo Fluor 3mo 20 S Sim

Loss in Reflectance (%) 10

0 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 29: Reflectance decline of paper 4

FPL 125 30 North 4 years 25 Hal 4 years Fluor 4 years 20 North 3mo 15 Hal 3mo Fluor 3mo 10 S Sim 5 Loss in Reflectance (%) 0

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 30: Reflectance decline of paper 7

30 North 4 years 25 Hal 4 years

20 Fluor 4 years North 3mo 15 Hal 3mo

10 Fluor 3mo S Sim 5 Loss in Reflectance (%) 0

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 31: Reflectance decline of paper 8

FPL 126 55 North 4 years

45 Hal 4 years Fluor 4 years 35 North 3mo Hal 3mo 25 Fluor 3mo S Sim 15 Loss in Reflectance (%) 5

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 32: Reflectance decline of paper 9

55 North 4 years

45 Hal 4 years Fluor 4 years 35 North 3mo Hal 3mo 25 Fluor 3mo S Sim 15 Loss in Reflectance (%) 5

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 33: Reflectance decline of paper 10

FPL 127 45 North 4 years 40 Hal 4 years 35 Fluor 4 years 30 North 3mo 25 Hal 3mo 20 Fluor 3mo 15 S Sim 10

Loss in Reflectance (%) 5

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 34: Reflectance decline of paper 14

55 North 4 years

45 Hal 4 years Fluor 4 years 35 North 3mo Hal 3mo 25 Fluor 3mo S Sim 15 Loss in Reflectance (%) 5

-5 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 35: Reflectance decline of paper 13

FPL 128 Lignin-Free Papers

20 North 4 years Hal 4 years 15 Fluor 4 years North 3mo Hal 3mo 10 Fluor 3mo S Sim 5

-5

-10

Loss in Reflectance (Delta R) -15

-20 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 36: Reflectance decline of paper 1

20 North 4 years Hal 4 years 15 Fluor 4 years North 3mo Hal 3mo 10 Fluor 3mo S Sim 5

-5

-10

Loss in Reflectance (Delta R) -15

-20 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 37: Reflectance decline of paper 2

FPL 129 50 North 4 years 40 Hal 4 years Fluor 4 years North 3mo Hal 3mo 30 Fluor 3mo S Sim 20

0 Loss in Reflectance (Delta R)

-10 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 38: Reflectance decline of paper 5

50 North 4 years 40 Hal 4 years Fluor 4 years North 3mo Hal 3mo 30 Fluor 3mo S Sim 20

0 Loss in Reflectance (Delta R)

-10 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 39: Reflectance decline of paper 6

FPL 130 30

25 North 4 years Hal 4 years 20 Fluor 4 years North 3mo 15 Hal 3mo Fluor 3mo 10 S Sim 5

-5

-10 Loss in Reflectance (Delta R) -15

-20 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 40: Reflectance decline of paper 11

25 North 4 years Hal 4 years 20 Fluor 4 years North 3mo 15 Hal 3mo Fluor 3mo 10 S Sim 5

-5

-10 Loss in Reflectance (Delta R) -15

-20 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 41: Reflectance decline of paper 12

FPL 131 30

25 North 4 years Hal 4 years 20 Fluor 4 years 15 North 3mo Hal 3mo 10 Fluor 3mo 5

-5

-10 Loss in Reflectance (Delta R) -15

-20 200 250 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Figure 42: Reflectance decline of paper 15

FPL 132 I Infrared Spectra Infrared spectra of papers 3,4,7,8. Spectra offset for clarity.

0.6

3 Nat 0.5 3 SSim

0.4

0.3

Absorbance 0.2

0.1

0 400 900 1400 1900 2400 2900 3400 3900 Wavenumber

Figure 43: IR spectra of paper 3

Vertical order of spectra is preserved in legend.

0.5

0.45 4 SSim 0.4 4 Nat 0.35

0.3

0.25

0.2 Absorbance 0.15

0.1

0.05

0 400 900 1400 1900 2400 2900 3400 3900 Wavenumber

Figure 44: IR spectra of paper 4

FPL 133 0.4

0.35 7 Nat

0.3 7 SSim

0.25

0.2

Absorbance 0.15

0.1

0.05

0 400 900 1400 1900 2400 2900 3400 3900 Wavenumber

Figure 45: IR spectra of paper 7

0.5

0.45 8 Nat 0.4 8 SSim 0.35

0.3

0.25

0.2 Absorbance

0.15

0.1

0.05

0 400 900 1400 1900 2400 2900 3400 3900 Wavenumber

Figure 46: IR spectra of paper 8

FPL 134 J Raman Spectra Raman spectra of papers 3, 4, 7 and 8. Spectra offset for clarity. Vertical order of spectra is preserved in legend.

0.2

0.18 3 SSim

0.16 3 Nat 0.14

0.12

0.1

0.08

0.06

0.04 Raman Intensity (Arbitrary Units) 0.02

0 250 750 1250 1750 2250 2750 3250 Wavenumber (cm-1)

Figure 47: Raman spectra of paper 3

0.3

4 SSim 0.25 4 Nat 0.2

0.15

0.1

0.05 Raman Intensity (Arbitrary Units)

0 250 750 1250 1750 2250 2750 3250 Wavenumber (cm-1)

Figure 48: Raman spectra of paper 4

FPL 135 0.14

7 SSim 0.12

7 Nat 0.1

0.08

0.06

0.04

Raman Intensity (Arbitrary Units) 0.02

0 250 750 1250 1750 2250 2750 3250 Wavenumber (cm-1)

Figure 49: Raman spectra of paper 7

0.25

8 Nat

0.2 8 SSim

0.15

0.1

0.05 Raman Intensity (Arbitrary Units)

0 250 750 1250 1750 2250 2750 3250 Wavenumber (cm-1)

Figure 50: Raman spectra of paper 8.

FPL 136 K Optical Properties Decay After Natural and Accelerated Exposure

Papers from the same fiber furnish are presented on the same page. The alkaline/carbonate buffered paper is always presented second.

Apparent discontinuities in accelerated exposures are due to the papers being removed from the solar simulator and placed in cold storage overnight or over a weekend.

FPL 137 Lignin-Containing Papers Time axis is multiplied by 300 for accelerated exposures.

60 Paper 3 North Window 50 Halogen Fluorescent 40 Accelerated Directional Brightness

20 0 200 400 600 800 1000 Days in Aging Chamber Figure 51: Directional brightness of paper 3 over time

60 Paper 4 North Window 50 Halogen Fluorescent 40 Accelerated Directional Brightness

20 0 200 400 600 800 1000 Days in Aging Chamber

Figure 52: Directional brightness of paper 4 over time

FPL 138 60

45 Paper 7 North Window 40 Halogen 35 Fluorescent Accelerated

Directional Brightness 30

20 0 200 400 600 800 1000 Days in Aging Chamber

Figure 53: Directional brightness of paper 7 over time

45 Paper 8 North Window 40 Halogen 35 Fluorescent Accelerated

Directional Brightness 30

20 0 200 400 600 800 1000 Days in Aging Chamber

Figure 54: Directional brightness of paper 8 over time

FPL 139 90

70 Paper 9 60 North Window Halogen 50 Fluorescent 40 Accelerated Directional Brightness

20 0 200 400 600 800 1000 Days in Aging Chamber

Figure 56: Directional brightness of paper 9 over time

70 Paper 10 60 North Window Halogen 50 Fluorescent 40 Accelerated Directional Brightness

20 0 200 400 600 800 1000 Days in Aging Chamber

Figure 55: Directional brightness of paper 10 over time

FPL 140 90

70 Paper 14 North Window 60 Halogen Fluorescent 50 Accelerated Directional Brightness

30 0 200 400 600 800 1000 Days in Aging Chamber

Figure 58: Directional brightness of paper 14 over time

70 Paper 13 North Window 60 Halogen Fluorescent 50 Accelerated Directional Brightness

30 0 200 400 600 800 1000 Days in Aging Chamber

Figure 57: Directional brightness of paper 13 over time

FPL 141 Lignin-Free Papers Measurements made after accelerated exposure had different backing/measurement technique than after natural aging, resulting in an offset and different initial values.

80 Paper 1 Accelerated 78 North Window 76 Halogen Fluorescent

Directional Brightness 74

70 0 200 400 600 800 1000 Days in Aging Chamber

Figure 59: Directional brightness of paper 2 over time over time

85 Paper 2 84 Accelerated

83 North Window Halogen 82 Fluorescent Directional Brightness 81

79 0 200 400 600 800 1000 Days in Aging Chamber

Figure 60: Directional brightness of paper 1 over time

FPL 142 89

86 Paper 5 85 Accelerated 84 North Window 83 Halogen 82 Fluorescent Directional Brightness 81

79 0 200 400 600 800 1000 Days in Aging Chamber

Figure 61: Directional brightness of paper 5 over time

Paper 6 89 Accelerated North Window 88 Halogen Fluorescent Directional Brightness 87

86 0 200 400 600 800 1000 Days in Aging Chamber

Figure 62: Directional brightness of paper 6 over time

FPL 143 88

84 Paper 11 83 Accelerated

82 North Window

81 Halogen

80 Fluorescent Directional Brightness 79

77 0 200 400 600 800 1000 Days in Aging Chamber

Figure 63: Directional brightness of paper 11 over time

86 Paper 12 Accelerated 85 North Window 84 Halogen Fluorescent

Directional Brightness 83

81 0 200 400 600 800 1000 Days in Aging Chamber

Figure 64: Directional brightness of paper 12 over time

FPL 144 L Photo-Initiated Dark Reactions During Storage at 40C

33 Paper 3 31

Directional Brightness Paper 10 29 Paper 9 Paper 7 27

25 012345678 Illumination Time, Days

Figure 65: Papers placed in cold storage after 4 days solar simulator exposure

Light exposure continued after 6 weeks in dark cold storage. Irregularities at 4.5 and 5.75 days coincide with storage overnight and over weekend, respectively.

FPL 145 M Effect of Non-Uniform Illumination on Optical Properties Decay

This appendix is included to indicate the amount of variability in optical properties due to the difference in intensity between different sections of the same chamber. In each chamber, papers from the high intensity zone were compared to papers in the low intensity zone. In each case, the decay curves from papers in the two zones were different. The two papers chosen from each chamber are the papers with the worst and the best overlap between the high and low intensity curve.

90 10 Low 80 10 High

70 14 Low

60 14 High

40 Directional Brightness

20 0 100 200 300 400 500 600 700 800 900 1000 Days Exposure

Figure 66: Papers 10 and 14 in north window. Low and high intensity region of chamber

FPL 146 90 9 High 80 9 Low

70 13 High

60 13 Low

40 Directional Brightness

20 0 100 200 300 400 500 600 700 800 900 1000 Days Exposure

Figure 67: Papers 9 and 13 in halogen chamber. Low and high intensity region of chamber

90 4 High 80 4 Low

70 7 High

60 7 Low

40 Directional Brightness

20 0 100 200 300 400 500 600 700 800 900 1000 Days Exposure

Figure 68: Papers 4 and 7 in fluorescent chamber. Low and high intensity region of chamber

FPL 147 N Sponsors and Supporting Cooperators

Pulp & Paper Companies Abitibi-Consolidated Inc. Appleton Papers Inc. Boise Cascade Corp. Bowater Pulp & Paper Canada Inc. Crane & Co., Inc. Domtar, Inc. Donohue, Inc. Fibreco Pulp Inc. FICAB (A consortium of four Nordic pulp producers) Fletcher Challenge Canada Ltd. Fraser Paper Inc. Millar Western Forest Products Ltd. Simpson Paper Company Stone Container (Canada) Tembec Inc. West Fraser Pulp Sales

Government Conservation Organizations Australian Archives National Archives & Records Administration (USA) National Gallery of Art (USA) National Library of Medicine (USA) US Library of Congress

Non-Government Conservation Organizations Canadian Cooperative Heritage FACTS Institute National Information Standards Organization (NISO) - USA National Institute for Conservation of Cultural Property (USA)

Government Agencies Alberta Economic Development & Tourism Industry Canada National Institute for Standards and Technology (NIST) - USA USDA Forest Products Laboratory

Suppliers to the Pulp & Paper Industry Andritz Sprout-Bauer Ciba Specialty Chemicals Corp. Degussa Canada Inc. (Formerly DuPont Canada) Specialty Minerals Inc.

FPL 148 O Lists of Figures and Tables in Appendixes

List of Figures

Figure 1: Temperature and RH data from natural aging chamber, and fitted sinusoidal curves, with period of 365 days...... 101 Figure 2: Average color change in AATCC blue wool L2 with time...... 102 Figure 3: Average color change in AATCC blue wool L5 with time...... 102 Figure 4: Blue wool L2 color change: high, medium, and low intensity zones of north window ...... 103 Figure 5: Blue wool L2 color change: high, medium, and low intensity zones of halogen chamber...... 103 Figure 6: Blue wool L2 color change: high, medium, and low intensity zones of fluorescent chamber...... 104 Figure 7: Directional brightness in north window chamber ...... 108 Figure 8: Directional brightness in halogen chamber...... 108 Figure 9: Directional brightness in fluorescent chamber...... 109 Figure 10: b* in north window chamber...... 109 Figure 11: b* in halogen chamber ...... 110 Figure 13: a* in north window chamber...... 111 Figure 14: a* in halogen chamber...... 111 Figure 15: a* in fluorescent chamber ...... 112 Figure 16: Directional Brightness with North window exposure...... 113 Figure 17: Directional brightness with halogen exposure ...... 113 Figure 18: Directional brightness with fluorescent exposure ...... 114 Figure 19: b* with north window exposure...... 114 Figure 20: b* with halogen exposure...... 115 Figure 21: b* with fluorescent exposure...... 115 Figure 22: a* with north window exposure ...... 116 Figure 23: a* with halogen exposure...... 116 Figure 24: a* with fluorescent exposure...... 117 Figure 25: HPLC chromatograms of paper 3 extracts ...... 122 Figure 26: HPLC chromatograms of paper 9 extracts ...... 122 Figure 27: HPLC chromatograms of paper 10 extracts ...... 123 Figure 28: Reflectance decline of paper 3 ...... 125 Figure 29: Reflectance decline of paper 4 ...... 125 Figure 30: Reflectance decline of paper 7 ...... 126 Figure 31: Reflectance decline of paper 8 ...... 126 Figure 32: Reflectance decline of paper 9 ...... 127 Figure 33: Reflectance decline of paper 10 ...... 127 Figure 34: Reflectance decline of paper 14 ...... 128 Figure 35: Reflectance decline of paper 13 ...... 128 Figure 36: Reflectance decline of paper 1 ...... 129

FPL 149 Figure 37: Reflectance decline of paper 2 ...... 129 Figure 38: Reflectance decline of paper 5 ...... 130 Figure 39: Reflectance decline of paper 6 ...... 130 Figure 40: Reflectance decline of paper 11 ...... 131 Figure 41: Reflectance decline of paper 12 ...... 131 Figure 42: Reflectance decline of paper 15 ...... 132 Figure 43: IR spectra of paper 3 ...... 133 Figure 45: IR spectra of paper 7 ...... 134 Figure 47: Raman spectra of paper 3 ...... 135 Figure 48: Raman spectra of paper 4 ...... 135 Figure 49: Raman spectra of paper 7 ...... 136 Figure 50: Raman spectra of paper 8...... 136 Figure 51: Directional brightness of paper 3 over time...... 138 Figure 52: Directional brightness of paper 4 over time...... 138 Figure 53: Directional brightness of paper 7 over time...... 139 Figure 55: Directional brightness of paper 10 over time...... 140 Figure 56: Directional brightness of paper 9 over time...... 140 Figure 57: Directional brightness of paper 13 over time...... 141 Figure 58: Directional brightness of paper 14 over time...... 141 Figure 59: Directional brightness of paper 1 over time...... 142 Figure 60: Directional brightness of paper 2 over time over time ...... 142 Figure 61: Directional brightness of paper 5 over time...... 143 Figure 62: Directional brightness of paper 6 over time...... 143 Figure 63: Directional brightness of paper 11 over time...... 144 Figure 64: Directional brightness of paper 12 over time...... 144 Figure 65: Papers placed in cold storage after 4 days solar simulator exposure ...... 145 Figure 66: Papers 10 and 14 in north window. Low and high intensity region of chamber ...... 146 Figure 67: Papers 9 and 13 in halogen chamber. Low and high intensity region of chamber...... 147 Figure 68: Papers 4 and 7 in fluorescent chamber. Low and high intensity region of chamber...... 147

FPL 150 List of Tables

Table 1: Selected paper properties as specified by ASTM/ISR...... 105 Table 2: Paper properties as measured by FPL staff ...... 106 Table 3: Probability that two samples of papers were from the same group...... 118 Table 4: Loss in Log of Fold with exposure...... 119 Table 5: 95% confidence interval for each value of loss of fold given in Table 4...... 119 Table 6: Percent by total mass of various sugars in water extract of photo-exposed papers ...... 120

FPL 151 In: Quantification and Prediction for Aging of Printing & Writing Papers Exposed to Light , Final report, August 2000 / by USDA Forest Service, Forest Products Laboratory. [Paper no.1], ASTM Paper Aging Research Program [CD-ROM]. [West Conshohocken, PA] : ASTM International, c2003.