The Effect of Age and Recycling on Paper Quality

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

4-1996

Zhuang Wu

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Recommended Citation Wu, Zhuang, "The Effect of Age and Recycling on Paper Quality" (1996). Master's Theses. 4921. https://scholarworks.wmich.edu/masters_theses/4921

This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. THE EFFECT OF AGE AND RECYCLING ON PAPER QUALITY

by Zhuang Wu

A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Paper and Printing Science and Engineering

Western Michigan University Kalamazoo, Michigan April 1996 ACKNOWLEDGEMENTS

I extend my sincere appreciation to the members of my committee, Dr. Raja Aravamuthan, Dr. David Peterson and Dr. Ellsworth Shriver for their guidance and support throughout the course of this work. I wish to thank Mr. Rick Reames for his suggestions and help. Many thanks are due to my friends, colleagues and others who contributed in different ways. Finally, I am also pleased to acknowledge my family members for their financial and other support in finishing my study.

Zhuang Wu

ii THE EFFECT OF AGE AND RECYCLING ON PAPER QUALITY

Zhuang Wu, M.S. Western Michigan University, 1996

The effects of recycling paper that has undergone aging for different periods on paper and fiber proper ties like strength, weighted average fiber length, and water retention value were investigated in this study. Old Newspapers (ONP) with different periods of natural aging (between 1 to 10 months) were recycled repeatedly for up to six times. Another goal was the study of aging effects on paper properties using accelerated aging at high temperatures (105±2) °C. Four different aging times (24, 48, 72, and 144 hours) were employed. The results showed that naturally aged ONP loses little in strength properties up to 7 months; later, there is a significant decrease. The accelerated aging tests showed strength loses for all aging periods. The water retention value decreases with increasing aging time and number of repeated recycling. The age of the recycled paper is found to be a more important variable than the number of times that the paper has been recy cled. It is also found that 10 months of natural aging is equivalent to less than 24 hours of accelerated aging under the experimental conditions. TABLE OF CONTENTS

ACKNOWLEDGEMENTS...... ii

LIST OF TABLES ...... vi

LIST OF FIGURES ...... vii

CHAPTER

I. INTRODUCTION ...... 1

II. REVIEW OF LITERATURE...... 6

Properties of Secondary Fibers ...... 6

The Effects of Recycling on Paper Quality .....7

General Effects of Recycling ...... 7

The Recycle Potential of Pulp ...... 9

Recycling Methods ...... 14

Strength Properties ...... 15

Wood Species ...... 18

Deinking and Bleaching ...... 19

Improving the Strength of Recycled Fiber ...... 21

Analysis of the Literature ...... 22

III. STATEMENT OF THE PROBLEM ...... 24

IV. OBJECTIVES OF THIS STUDY .•...... 26

V. EXPERIMENTAL DESIGN AND METHODOLOGY ...... 28

Experimental Design...... 27

iii Table of Contents-Continued CHAPTER Experimental Methods ...... 30 The Recycling Procedure ...... 30 Properties of Handsheets and Fibers ...... 32 VI. RESULTS AND DISCUSSION ...... 34 Experimental Data Analysis...... 34 Discussion ...... 35 Effect of Recycling on Fines...... 35 Effect of Recycling on Fiber Length ...... 37 Effect of Aging and Recycling on Zero-span Strength...... 39 Effect of Aging and Recycling on Tensile Strength...... 42 Effect of Aging and Recycling on Burst Strength...... 47 Effect of Aging and Recycling on Tear Strength ...... 51 Effect of Aging and Recycling on Water Retention Value ...... 54 Effect of Aging and Recycling on Scattering Coefficient ...... 58 Comparison of Natural Aging versus Accelerated Aging ...... 61 VII . CONCLUSIONS...... 63 VIII. SUGGESTIONS FOR FURTHER STUDY ...... 66

iv Table of Contents-Continued

APPENDIX

A. Regression Model Parameters...... 67 REFERENCES •.•....••••••••..•••.•••••••...••...... • 7 0

V LIST OF TABLES

1. Newsprint Properties Before and After Calendering ...... 14 2. Handsheet Testing Methods ...... 33 3 . Fines Percentage...... 3 6 4. Weighted Averaged Fiber Length...... 38 5. Zero Span Fiber Strength ...... 40

6. Tensile Strength...... 43

7. Burst Strength...... 48

8. Tear Strength...... 52 9. Water Retention Value ...... � ..55

vi LIST OF FIGURES

1. u. s. Paper Recovery Rate...... 2 2. Effect of Recycling on Fibre Saturation Point...... 8 3. Effect of Recycling on Fibre Flexibility...... 10 4. Effect of Recycling on Breaking Length ...... 16 5. Effect of Recycling on Tear Index...... 18 6. Effect of Recycling on Burst Index...... 18 7. Schematic for Natural Aging Experiments...... 28 8. Schematic for Accelerated Aging Experiments...... 29 9. Effect of Recycling on Fines...... 36 10. Effect of Recycling on Weighted Average Fiber Length ...... 3 8 11. Effect of Recycling and Natural Aging on Fiber Strength ...... 41 12. Effect of Recycling and Accelerated Aging on Fiber Strength...... , ...... 41 13. Effect of Recycling and Natural Aging on Tensile Index...... 4 4 14. Effect of Recycling and Accelerated Aging on Tensile Index...... 44 15. Effect of Recycling and Natural Aging on Burst Index ...... 49 16. Effect of Recycling and Accelerated Aging on Burst Index...... 49

vii List of Figures-Continued

17. Effect of Recycling and Natural Aging on Tear Index ...... 53 18. Effect of Recycling and Accelerated Aging on Tear Index ...... 53 19. Effect of Recycling and Natural Aging on Water Retention Value...... 56 20. Effect of Recycling and Accelerated Aging on Water Retention Value ...... 56 21. Effect of Recycling and Natural Aging on Scattering Coefficient...... 59 22. Effect of Recycling on Density...... 60 23. Effect of Recycling and Accelerated Aging Scattering Coefficient...... 60 24. Comparison of Natural Aging Versus Accelerated Aging...... 62

viii ;

CHAPTER I

INTRODUCTION

Recycled fibers (secondary fibers) are defined as fi bers that have been through at least one papermaking cycle.

Recycled fibers have lower strength and higher drainage re sistance than virgin fibers. The mechanical properties of fibers as well their ability to swell are diminished after they are exposed to the pulping and drying conditions im posed during the papermaking cycle.

Recycling is not a new technology. It became a commer cial proposition when Matthias Koops established the Neck inger Mill, UK, in 1800 [1]. Since 1968, when the use of waste paper as a fiber source for papermaking was at a low point at 19.5% of total fiber sources and 10.2 million tons, waste paper has become progressively more important as a fiber resource in papermaking. The recovery rate for waste paper has increased steadily throughout the 1970s and

1980s. This trend is expected to continue at least to the end of the century (Figure 1). Paper recycling is unusual among the technology of the pulp and paper industry in that its practice is perceived to be both economically and envi-

1 /

2 ronrnentally beneficial. The incentive for recycling appears

50 45 40 � 35 GI a:: 30

GI 25 20 GI a:: 15 10 5 0 1985 1990 1993 2000goal

*Note: Actual data for 1985 and 1990; projected data for 1993 and 2000. *Sources: American Forest and Paper Association/Franklin Associates ,Ltd.

Figure 1. U.S. Paper Recovery Rate[2]. to be related in an inverse way to the availability of vir gin fiber resources, although various other factors are in volved. As forest area per person is being decreased, the recovery of fiber is more and more important. For 1990-1991 the utilization rate (the amount of recovered fiber used in the manufacture of paper, expressed as a percentage of the total fiber used) is a respectable 44.6%[3].

Annual consumption of recycled fiber has been pre dicted to grow at a rate of 3-4% per year, from 20.5 mil lion tons in 1989 to 26-30 million tons per year by the /

3 year 2000[3]. Two of the main areas of predicted growth in

clude the use of secondary fiber as a pulp substitute in

container board and as a deinked chemical-fiber substitute in tissue grades. Environmental pressure, and in many cases, legislation is also stimulating very strong efforts to recycle old newsprint to supply 20-40% of the 16-million

-ton/year newsprint market[3].

With the increased use of secondary fibers significant

development has been made in recent years in stock prepara

tion, including deinking. Also, improvements in technology

have brought with them an improved finished product. Al

though recycling originated in 1800, until the late 1960s

there were very few investigations into the effects of re

cycling on sheet properties. From then until the late 1970s

a considerable amount of work was carried out to identify

the effects of recycling on pulp properties and the causes

of those effects. Establishing the cause and effect rela

tionships has naturally led to procedures for ameliorating

some of the less desirable results of recycling. The lit

erature can most usefully be summarized under the following

headings: (a) the general effect of recycling, (b) factors

controlling the recycle potential, and (c) the recovery of

papermaking potential.

The effects of recycling on chemical pulp fibers are 4 quite well documented in the literature. Until recently, there has been little work on the effect of recycling on mechanical pulp fibers. Loreen D. Ferguson[4] presents a review of the recent work reported in the literature to as sess the effects that recycling has on the physical proper ties of mechanical pulp based paper. Researchers have exam ined the effects of the original pulping process, for exam ple TMP and CTMP; papermaking processes like repulping and calendaring; the effects of the chemicals, and the effects of mixed ONP(Old Newspaper)/OMG(Old Magazine) type furnish. During the last thirty years, the effect of recycling on pulp properties has been investigated in many different institutions and countries. It is, perhaps, surprising that there should be a need for another study. And yet it is the very variety of previous studies which has given rise to this latest one. On reviewing the publications that report on the physical properties resulting from mechanical pulp, it is seen that most of them are focused more on the effects of including deinked pulp into regular production than on the recycling process itself. Michaud and Harvey (5), Chlaydon (6) and Patterson [7] compared the physical and optical properties of deinked pulp with virgin fiber. Helmling[8] and Pfalzer[9] compared the effect on strength properties between wash and flotation deinking. Scallan[l0] 5 examined the role pulping and recycling have on the wet fi ber wall. Recent works by Chatterjee and co-worker [11], and Feguson [12] have taken a systematic look at the ef fects from repeated recycling of the same furnish. However, the age of the paper being recycled on strength properties has been studied by none of the researchers. The age of ONP is the time that elapses between the date of issue and the date on which the paper is recycled in a paper machine. This period involves usage and stor age under different conditions of moisture, heat, and light. This will introduce varying amounts of deteriora tion in the properties of the recycled paper. In other words, all recycled papers may not possess equal poten tial for recycling. Hence, it seems necessary to investi gate the change in the properties of paper made from ONP with different aging time. CHAPTER II

REVIEW OF LITERATURE

Properties of Secondary Fibers

Waste paper contains cellulosic fibers which form the major component of paper products. Fibers are mainly influ enced by the characteristics of the original pulping and paper making systems. Furthermore, the properties are also subject to variation from alternative pulping processes given to a specific choice or blend of wood species. Fiber recovered from waste paper is also incorporated in numerous grades of paper products which again becomes part of the waste paper source. Generally, as the recycle rate in creases within a given grade, the quality-principally the fiber length and bonding-decreases. Waste papers also con tain a multitude of noncellulosic materials making up from 1 to 50% of the overall weight. The various additives, chemicals, and materials that are placed in or on the paper during manufacture or applied to paper products in convert ing and other manufacturing operations to enhance its spe cific user purpose, become the major contaminants in recy cling.

6 7 The Effects of Recycling on Paper Quality

General Effects of Recycling

A considerable number of researchers [6,7,10,12,13] have examined what might be termed_the fundamental problem in recycling, ie how fibers are affected by recycling pro cedures, and what resulting effects are seen in paper made from those fibers.

Investigations into the effect of recycling have been many and varied. Furnishes range from unbleached chemical pulps to mechanical pulps, including blends. Recycling pro cedures have used the British Standard handsheet machine, other sheetmaking procedures, pilot machines, or combina tions of these. The first recycle causes the greatest change in any properties, and this appears to be true re gardless of whether the virgin pulp was originally dry or moist. Some of the impact on recyclability will depend on how the fibers were initially turned into paper. Oye and co-workers[14] report that during the drying process, the water leaving the lamellae of the cell wall, draws them closer together and in some instances the lamellae adhere to one another. Rewetting may not entirely reverse the situation. This phenomenon is predominant with chemical pulp fibers. The strength reduction in chemical pulps is 8 due to the reduction in fiber bonding potential, and not due to any physical reduction in the length or strength of the individual fibers[l0,14).

Scallan and Tigerstrom[lO] have shown that high yield pulps(TMP, CTMP etc) can recover the ability to take up wa ter following a drying treatment, while lower yield pulps cannot. It can be seen in Figure 2 that TMP was essentially unaffected by the recycling process, whereas the (ultra high yield sulphite) and kraft saw a 20% and 35% reduction, respectively, after four recycles. It is agreed that the

20 -6-Tt.t=> 0 -t----;---1---1-----1 -x-UHS 3 5 4 -x-KRAFT Nmber of Rcycles

Figure 2. Effects of Recycling on Fiber Saturation Point. loss of flexibility and plasticity is due to this reduced swelling capacity of the fiber once it has been made into paper. 9 Scallan and Tigerstrom [10] report that lignin is be lieved to be a cross-liking agent within the cell wall; hemicellulose is believed to coat the pores and act as a coupling agent between the lignin and cellulose. When the fiber is rewetted the water is tak�n up through the pores and turns the ligno-hemicellulose into a gel. This gel may contribute to covalent and hydrogen bonding within the cell wall. Changes in the degree of bonding within the cell wall will affect the elastic modulus. The loss in water absorb ing ability in chemical pulps is due to the reduction in the amount of ligno-hemicellulose gel due to the pulping process. A steadily decreasing flexibility for Kraft fibers can be noted, whereas the TMP fibers gains a slight degree of flexibility(Figure 3) [11]. A distinction can be made be tween the collapse and the int�rlammellaer bonding that oc curs with chemical fibers and the flattening that occurs with mechanical fibers, which is simply a mechanical defor mation [15].

The Recycle Potential of Pulp

Not all pulps have the same recycle potential, and any individual pulp can have a different recycle potential de pending on its manufacturing history. Certain factors in- 10 fluencing the recycle potential of pulp have been identi fied in the literature and possible explanations advanced for that influence.

l x

t: .x_ � 0.5 x--I ,: t� -x--� x-x-x 0 I I I I I 3 4 5 Nmber of Rcyles

Figure 3. Effect of Recycling on Fiber Flexibility.

1. Effect of Furnish: In so far as the recycle poten tial of different pulps can be assessed from existing work, no systematic study exists. Bovin [16] noted that by com parison with chemical pulps a mechanical pulp lost little of its properties with recycling. Bovin also determined that unbleached sulphite pulp lost less of its properties than bleached sulphite. Unbleached Kraft fell between these two. Yamagishi [17] and Oye et. al [14] found that the loss of recycle potential of a softwood unbleached kraft was very much the same as that of a hardwood unbleached kraft.

The ability to take up water again is reflected in the 11 elastic modulus for both high and low yield pulps. 2. Initial Beating of Virgin Pulp: There is general agreement that the greater the initial degree of beating, the greater the loss of pulp quality on recycling, and the lower the recovery potential of sheet properties that are a direct function of fiber bonding, e.g. burst strength and tensile strength [13]. Recycling under minimal conditions of refining results in fiber network strength losses that are weakly related to species fiber morphology and the num ber of times the fiber is recycled. The decrease in strength is positively correlated to the extent of beating that the fibers undergo during their first use, particu larly, for softwoods [18]. The current understanding of the lost recycle potential is that it corresponds to the inter nal swelling or lack thereof that has been observed by Scallan [18]. 3. Effect of Drying and Wet Pressing: When a fiber is dried, physical discontinuities in the cell wall are col lapsed by high surface-tension forces that pull the sur faces together. These surfaces become hydrogen bonded, which reduces swelling in the next cycle. In subsequent beating stages, the recycled fiber will not be able to de laminate and swell as well as virgin fiber. This mechanism is confirmed by experimental work where hydroxyl groups on 12 the cellulose fiber were blocked by derivatization, thus reducing the amount of irreversible shrinkage during dry ing. With irreversible shrinkage reduced, there were fewer differences between the properties of sheets made from once-dried and never-dried pulp [19]. In subsequent beating, this part of the cell wall stays resistant to delamination, and indeed the whole fiber becomes stiffer and perhaps more brittle. An increase in cellulose crystallinity has been noted by Yamagishi et. al [17] during the recycling of commercial hardwood and soft wood bleached pulp, though the increase was small. These various observations are parts of the "irreversible horni fication" phenomenon described earlier. The possible mecha nisms of this crack healing and hornification process have been discussed in more detail by Back [20]. He suggested that three different factors are involved. The first is an auto-crosslinking reaction that is promoted by higher dry ing temperatures. The second is a re-hydrophobation of cel lulosic surfaces due to the redistribution of olefinic ma terials such as fatty acids- present either in the pulp or from added rosin size. The third is the hydrolytic breaking of covalent bonds in the cellulose chains. Nissan [21] sug gested that the "irreversible honification'' can indeed be a two-component phenomenon, leading to both an overall stiff- 13 ening of the fiber and a change to the fiber surface chem istry. Little has been published concerning the possible ef fect of the degree of wet pressing on the recycle potential of paper made at different levels of _pressing. Carlsson[22] measured the WRV (water retention value) of an unbleached kraft pulp after wet pressing and subsequent redisintegra tion. At moderate levels of PFI mill beating, there was only a slight fall in WRV (approximately 5%) up to a solids content after pressing of 60%. For heavily beaten pulp, the fall was more significant, around 15%. 4. Effect of Calendering: Calendering has quite an im pact on the initial strength properties of the sheet. Grat ton [23] has shown that the method used to calender the pa per changes the physical properties of the sheet. Table 1 shows the results for TMP, calendered by three methods: conventional(CONV), temperature gradient(TG), and extreme calendering(XT) using slow speed, high·temperature ( 200°c) and high nip pressures. In the temperature gradient and ex treme type of calendering the fibers were permanently de formed and flattened, as confirmed by scanning electron mi crographs [23]. Gratton reports that damage done in calen dering is not reversible by reslushing and recycling and that calendaring significantly reduces the elastic modulus 14 of all handsheets, most likely due to bond breakage and fi ber damage.

Table 1 Newsprint Properties Before and After Calendering

PARTICULARS PROPERTIES OF UNCAL. CONV. TG XT 2 Burst Index, KPa.m /g 1. 89 1.02 1.10 0.86 2 Tear Index (MD), mN.m /g 5.1 2.7 3.6 1. 9 Tensile Index (MD), Nm/g 59.3 32.9 40.1 34.7 Young's Modulus, Nm/g 9389 4594 5168 3921 Zero-Span Brk. Lgth. (MD), Km 14.7 13.7 14.4 13.2 Thickness, microns 15.2 85 84 57

*UNCAL-Uncalendered.

Recycling Methods

Chatterjee and co-workers reported in a series of three papers [11,27,28,] the effects from four recycles on three different pulps: unbleached unbeaten kraft, ultra high yield sulphite and thermomechanical pulp. Their tech nique for recycling involved repeated sheet making, wet pressing, high temperature restrained drying and gentle reslushing. No chemicals were added, and the loss of fines 15 was restricted to the recycling loop. This method was used for four recycles. Howard and Bichard [15] used the recycling method for a total of five recycles on five pulp samples: CTMP, SGW, TMP, unbleached and bleached kraft. The handsheet machine was adapted to collect the white water and return it back to the deckle for subsequent handsheets. The handsheets were pressed in the conventional manner. No chemicals were used. Ferguson [12] was primarily concerned with the effect of deinking chemicals-sodium hydroxide, sodium silicate, DTPA, hydrogen peroxide and fatty acid soap on the strength properties. The furnish was 70/30 ONP/OMG. Ten cycles were conducted, with the results reported for 1-8 and 10th cy cles. The cycle was: repulping, flotation deinking, pH re duction, dewatering and then return to the pulper. No pressing and drying was used. The backwater from the thick ening stage was collected and used for pulper dilution.

Strength Properties

Tensile Strength: Figure 4 [11] shows the results for tensile strength as reported in terms of breaking length. It can be seen in Figure 4 that TMP does not change and UHYS undergoes a minor loss in breaking length. The 16

kraft pulp drops to 50% of the original strength after four

recycles.

1�..------, 100 ��-/Jx /J . 4 i -: -x-x c 80 "--... � 'iii� 60 --x I � o 40 20 �TM 0 +------x-UHS 2 3 4 -x-KRF

Nmber of Rcycles

Figure 4. Effect of Recycling on Breaking Length.

Scallan [10) has reported that additional swelling can

result if chemical groups attached to the cell wall are

ionized. This creates an osmotic imbalance and extra water

is taken up into the wall. However, there is a limit, as

increasing resistance to further expansion is provided by

the elastic nature of the cell wall. The collapse and la

mellar bonding of the kraft fibers significantly reduced

the degree of expansion.

Tear Strength and Burst Strength: Tear strength has

frequently been abused for not simulating paper machine or

pressroom real-life [4]. However, tear does give an indica

tion of the force required to delaminate the sheet and pull 17 out or break fibers. Chatterjee (24] reports that TMP, and UHYS are unaffected, while the kraft samples lose 26% of tear strength after four recycles. Howard et. al studied the tear strength for SGW, CTMP, Kraft and a 50/50 blend of CTMP and _kraft. The stone ground wood is unaffected by five recycles, and the CTMP shows a small reduction. The kraft fibers show a significant gain (28%) in tear strength. Ferguson's results (Figure 5) show an initial increase in tear followed by a slight reduction after fourth recycle. The difference in the results of various research groups might be due to the difference in freeness and or in fiber morphology. Figure 6 (15] shows the effect of recycling on burst strength after five recycles, for a blend of ONP and kraft. From the figure, it can be seen that there is a steady increase in burst strength. The reason for the gain in burst strength may possibly be due to the flattening of the mechanical pulp and increased strain to failure. Howard [15] reports that the mechanical pulp fibers gain(5-7%) in burst strength, whereas the kraft fibers lose burst strength(16-18%). The 50/50 blend of CTMP/kraft shows initial reduction(8%) followed by a gain in burst strength (8%) after four recycles. 18

11!:l � t£:. Ill!: ... �- I. .,,._ V - E Ill!: 6 V - - - - E 5 V - - - - )( V ,,GI 4 - - - - .5 3 - - - .. 2 1--' - - - GI I- 1 V --- 0 .. .. - .. 2 3 4 5 6 7 8 10

Nmber of Rcycles

Figure 5. Effect of Recycling on Tear Index.

2.5 ,I!.· ,11:i L-': � t-- 2 ,..a ,..a ,o;,;· - E itl!l ,I:. t-- t-- - - - t-- � 1.5 � - )( ,, t-- ,__ - .... - - t-- .5 t-- - - t-- 0.5 V - -

0 ------2 3 4 5 6 7 8 10

Nmber of Rcyles

Figure 6. Effect of Recycling on Burst Strength.

Wood Species

Bobalek et. al [25) reported the effects from three recycles on commercial bleached kraft that came from dif- 19 ferent wood species. The surface properties of the hand sheets changed with recycling, although they differed dis tinctly and uniformly from one another. Under the mild beating and recycling conditions used, the optical and roughness properties of the handsheets depended entirely on the invariance of the network dimensions, which in turn was directly related to the differences in fiber morphology among the species. In accordance with earlier studies [13,14,26], the characteristic decrease in fiber network strength was ob served with recycling. In Bobalek's study, however, he used four different measures of strength(tensile energy absorp tion, Z-direction tensile strength, Scott bond, and zero span tensile strength) instead of tensile strength alone. Since there was no change in fiber morphology within a fur nish, the changes in strength can be attributed entirely to the effects of fiber drying and the loss in bonding area [25]. With minimal refining, optical and dimensional char acteristics of the sheets were constant. Losses in strength were related to the fiber morphology of the species and the number of times the fiber was recycled [25].

Deinking and Bleaching

Inks are formulated to provide specific properties in 20 terms of printability, drying, color, and end use. Although the composition of ink may vary greatly depending on its use, the three common components are: colorant, modifier, and vehicle. How easily and effectively the dried ink film can be removed from a printed waste_furnish (ie its

"deinkability") is primarily determined by the type of ve hicle and drying method used. Typically, the colorants and modifiers do not pose deinking problems. But, many vehicles form tough films resistant to both mechanical and chemical treatment upon drying. In general, chemically cross-linked ink films are more difficult to break up and remove from the fiber surface.

In general, deinking wastepaper results in upgrading the finished product. Within the deinking process there are both mechanical and chemical operations. These operations encompass three basic process steps. Initially, there is defibration which is repulping. Secondly, large impurities are removed by mechanical action of screening and cleaning.

Finally, ink is removed by chemical and mechanical action

in flotation and washing.

Ink removal is obviously the primary function of a

deinking stage. However, ink removal does not relate easily

to any finished-product attribute. 21 Improving the Strength of Recycled Fiber

Strength loss generally can be regained by refining

[28]. Unfortunately, this usually reduces drainage and pro duction capacity. Increased refining also limits the amount of strength that can be regained by refining in future cy cles. The use of chemical additives, which improves the strength properties without changing the repulping require ment, can provide an alternative method to refining [29].

Two resins often used are an anionic polymer [30], which is capable of facilitating hydrogen bonding, and a cationic polymer, which is capable of forming strong electrostatic bonds between fibers and fines. These resins improve the dry strength of paper by increasing both the strength and the area of the interfiber bonds [31].

Treatment of wastepaper with sodium hydroxide in creases the freeness and the strength properties of recy cled fiber [32]. Sodium hydroxide treatment promotes fiber

swelling, thereby increasing fiber flexibility and surface

conformability. Both alkaline treatment and delignification

can improve the papermaking potential of recycled fibers.

Oxygen-alkali delignification has recently been studied as

a means of improving strength properties in old corrugated

container recycled pulp [33]. The delignification treatment 22 was found to improve bonding and strength characteristics, probably because of softening, swelling, and lignin re moval. The strength improvement in the fiber is especially noticeable in the higher burst value and strain-to-failure value at a given drainage rate[33]. _

Analysis of the Literature

On reviewing the publications reporting on the physi cal properties resulting from mechanical pulp, most are fo cused more on the effects of including recycled pulp into regular production than on the recycling process itself

[5,6,8,9). Recently, Chatterjee et. al [11,24), Howard

[15), and Ferguson [12) have taken a systematic look at the effects from repeated recycling of the same furnish for

TMP, CTMP, KRAFT and UHYS (ultra high yield sulphite).

Strength loss of recycled fiber can be regained by refin ing, using chemical additives, sodium hydroxide, and oxy gen-alkali [30, 32, 33,34). Researchers have shown that me chanical pulp furnishes suffer minor effects from repeated recycling. Scattering coefficient is reduced, but physical properties such as burst, tensile and fold are increased.

The question which arises when reviewing this varied previous work is: to what extent are the observed effects of recycling due to the raw materials, and to what extent 23 are they due to the recycling procedures adopted? In all other areas of paper science the methods used for slushing, sheetmaking, drying etc. affect the properties of the prod uct, and for recycling it must be the same. It seemed that no two researchers used the same methods. Because the observations relating to different pulp types do not form part of any one experiment, no reasons have been advanced for the relative performance of the pulp. However, Loss of swelling has been confirmed as the principal cause of changes to beaten chemical pulps. Curl removal was the principal cause for unbeaten pulp, and flattening of fibers was the major cause of changes to the mechanical pulps. Based on the above studies and differences observed in experimental conditions between each study, there is enough scope to investigate the effects of recycling on fiber and paper properties. Further, no study has been found on the effects of the age of the paper between successive recy cles. When both the factors of aging and recycling is con sidered, how does the recycling potential change? The pre sent study offers help in that direction. CHAPTER III

STATEMENT OF THE PROBLEM

Traditionally, newsprint was made from a mixture of stone groundwood and chemical pulps. Over the last 20 years, this has been replaced by thermomechanical pulp (TMP) or chemi-thermomechanical pulp (CTMP) and most of the chemical pulps has been eliminated. Newspaper from the date of issue to recycling takes about 6 to 10 months. As time goes on, the newspaper loses its strength and brightness due to the action of light, temperature, and moisture of surrounding air. That is, the properties of old newspaper are different from the fresh newspaper. Most of the waste paper is collected by merchants who then supply the paper to manufacturers, or export it abroad. So waste paper un dergoes an aging process. None of the researchers have considered the age of the paper being recycled. Old newspaper is generated by all sectors, but households are by far the primary source. An earlier study by the Forest Products Laboratory U.S.A. in dicated that newspaper comprised about 40% of the paper in household trash and that the 78 pounds discarded per person

24 25 per year corresponded to about 80% of the total amount of newspaper printed. Old newspaper is a primary part of waste paper for recycling. Studying effects of age of paper being recycled on paper properties is hence necessary. These ef fects will more practically reflect.papermaking experi- ences. CHAPTER IV

OBJECTIVES OF THIS STUDY

The six properties of paper and fiber considered in this study are tensile index, tear index, burst index, scattering coefficient, zero-span strength, and water re tention value. The objectives are:

1. To estimate the effect of aging on fiber character istics.

2. To evaluate the effects of the age of paper being recycled between cycles on strength properties of hand sheets.

3. To investigate the strength differences of ONP with different degrees of aging (less than 1 month, 6 months, and 9-10 months).

4. To compare the effects of natural aging against ac celerated aging at high temperature.

26 CHAPTER V

EXPERIMENTAL DESIGN AND METHODOLOGY

Experimental Design

Old newspaper was used in the experiments. Initially, a sufficiently large number of handsheets were prepared so that handsheets can be used for testing strength properties and be disintegrated for subsequent cycles. The redisinte grated pulp was used to prepare handsheets all over again. This procedure was repeated until a total of six recycles was achieved. The technique involved repeated sheet making, drying, and reslushing without the use of any chemicals be tween recycles. Experiments were conducted to measure the change in specific fiber characteristics such as fines per centage and weighted average fiber length due to recycling as well as fiber strength and fiber water retention value due to both recycling and aging. The schematic for recy cling is shown in Figure 7 for naturally aged ONP and in Figure 8 for ONP with accelerated·aging. It is intended not to re-beat between recycles. Re beating may be thought to be more representative of commer cial operations, but it immediately raises the question of

27 28

ONP

Less Than 1 Month ONP 6 Month ONP 9-10 Month ONP

Recycle 1 Recycle 1 Recycle 1

Recycle 2 Recycle 2 Recycle 2

Recycle 3 Recycle 3 Recycle 3

Recycle 4 Recycle 4 Recycle 4

Recycle 5 Recycle 5 Recycle 5

Recycle 6 Recycle.6 Recycle 6

Figure 7. Schematic For Natural Aging Experiments. 29

ONP

Recycle 1

Aging (24 hr.) Aging(48 hr.) Aging (72 hr.) Aging(144 hr.)

Recycle 2 Recycle 2 Recycle 2 Recycle 2

Aging(24 hr.) Aging(48 hr.) Aging (72 hr.) Aging( 14 4 hr. )

Recycle 3 Recycle 3 Recycle 3 Recycle 3

Aging(24 hr.) Aging (4 8 hr.) Aging(72 hr.) Aging( 144 hr.)

Recycle 4 Recycle 4 Recycle 4 Recycle 4

Aging(24 hr.) Aging(48 hr.) Aging(72 hr.) Aging(144 hr.)

Recycle 5 11 Recycle 5 11 Recycle 5 1 1 Recycle 5

Aging(24 hr.) Aging(48 hr.) Aging (72 hr.) Aging( 14 4 hr .)

IRecyL 61 IRecyL 61 IRecyL 61 IRecylle 61

Figure 8. Schematic for Accelerated Aging Experiments. 30 the degree of re-beating. Should it be to a freeness, to a strength, or to a sheet density? All these different ap proaches have been tried by other workers. In this study, Since I wish to evaluate the effects of the age of paper being recycled between cycles on th� strength properties, no re-beating was done.

Experimental Methods

The Recycling Procedure

A recycling procedure involving deinking of old news paper (ONP), the manufacture of pulp handsheets, and the aging of handsheets followed by reslushing was used to de termine the basic effects of age of paper being recycled on the quality of paper from recycled fiber. ONP for the first recycle and handsheets for subsequent recycles were soaked overnight in water before repulping. 1. Experimental Material: The investigation was car ried out on old newspaper collected from Kalamazoo Gazette which was issued on October 3, 1994. The newspaper made from 20% chemical pulp and 80% mechanical pulp was kept stored at a constant temperature (20-25°C) for a period ex tending up to 10 months. 2. Deinking Process: In the first recycle, chemicals 31 (3% NaoH, 1% H2O2 , and 2% Na2SiO4} were added in the disinte grator for deinking. Peroxide (H2O2) was added in the disin tegrator to prevent brightness reversion due to alkali darkening. In subsequent recycles, no chemicals were added. The conditions of recycles were 60 �inute repulping time, 5% repulping consistency, and 75-80°C repulping tempera ture. In each recycle, latency was removed from the pulp by diluting and stirring it in hot water at 90 degree Celsius for 5 minutes before testing water retention value and also before making handsheets for testing and for further recy cling. 3. Handsheet Formation: Handsheets were prepared ac cording to TAPPI Standard T205. In every cycle, a certain number of handsheets was retained for testing their strength properties, while remaining handsheets were used for next cycle. 4. Fiber Characteristics: Classification of fibers was carried out on a Clark classifier. Weight percentages re tained on four different mesh sizes were used to calculate the fines percentage lost through 200 mesh screen. The screens used were +28, +48, +100, and +200. The arithmetic average length of the fractions in the compartments were measured by the Kajaani analyzer FS-100. 32 5. Accelerated Aging Experiments: Handsheets were aged using TAPPI method T453. This method specifies the proce dure for dry heat treatment of handsheets in order to ob tain inferences regarding the aging qualities of the paper. An oven with forced circulation that can maintain a uniform temperature where the handsheets are located with means of shielding them from direct radiation from the heating elements was used. The variable involved on the ag ing experiment is heating time at a uniform temperature (105�2 °c). The variable values were 24�0.25 hours, 48�0.5 hours, 72+0.7 hours, and 144+1.5 hours, respectively.

Properties of Handsheets and Fibers

The properties tested include the following: (a) Tear Strength, (b) Tensile Strength, (c) Burst Strength, (d) Light Scattering Coefficient, (e) Zero-Span Breaking Length, and (f) Water Retention Value. Testing of hand sheets was carried out in accordance with TAPPI test meth ods shown in Table 2. 33 Table 2 Handsheet Testing Methods

Test TAPPI Test Method

Fines Percentage T-233 cm-82 Fiber Lengrh T-271 pm-91

Water Retention Value T-256 (U) Tear Strength T-414 Tensile Strength T-494 Scattering Coefficient T-425 om-91 Zero-Span Strength T-231 Burst Strength T-403 CHAPTER VI

RESULTS AND DISCUSSION

There is no evidence of other published results on variations of paper properties across aging and recycling factors. These results cannot hence be compared with those of other workers.

Experimental Data Analysis

Regression analysis is an applied subject describing methods for handling data and a statistical tool that pre dicts the relation between one or more independent vari ables and a response variable. Regression analysis estab lishes functional as well as statistical relations. The re lationship explained is between a response variable and the number of independent variables considered. According to the experimental data from this study, polynomial regres sion was adopted to find fitting curves for describing the effects of aging and recycling on paper quality. The poly nomial regression equation in question is:

2 4 5 6 Yk=a+bXk+cXk +dX/+eXk +fXk +gXk Xr Number of recycle ( 1 to 6) ,

34 35

= Yk handsheet properties in the k recycle a, b, c, d, e, and f are parameters determined by the equa tion. The values of these parameters as calculated from the power regression model are presented in Appendix. In the discussion below, all c�rves in figures are plotted by power regression curves.

Discussion

Effect of Recycling on Fines

Fines percentage was calculated as the moisture-free weight of the pulp lost through 200 mesh screen based on the feed to the Clark classifier. Table 3 shows the average value of four duplicate runs with standard deviations for the six recycles. Figure 9 is a comparative graph for the effect of re cycling on fines percentage. Fines percentage decreases with increasing number of recycle. The major loss in fines occurs in the first recycle and evens out in later recy cles. 36 Table 3

Fines percentage

Pulp Type ONP Standard Deviation

Recycle 1 32.58 1. 42

Recycle 2 26.06 0.80

Recycle 3 24.71 1.04

Recycle 4 24.89 2.41

Recycle 5 24.01 2.99

Recycle 6 22.32 1.21

0 28 26 24 22

20 3 4 5 6 Number of Recycles

Figure 9. Effect of Recycling on Fines. 37 Effect of Recycling on Fiber Length

Weighted average fiber length for six recycles with standard deviation is presented in Table 4. These fiber lengths were calculated from Clark classification and Ka jaani fiber analysis of each fraction. The number average

(arithmetic) fiber length from Kajaani analysis was applied in calculating the weight average fiber length shown by the following equation.

wk = the moisture free weight of the pad from k th compartment in Clark classifier(k=l,2,3,and 4). w5 the moisture free weight of the pulp lost through the finest screen. w = the weight of the pulp added to the classifier. lk = the arithmetic mean fiber length of k th fraction in the classifier(k=l,2,3,and 4).

15(-200 mesh) is assumed to be 0.05 mm in average length.

Figure 10 shows a reduction in weighted fiber length during the second recycle. Fiber length changes are small and they are probably accounted for in the weighted average by any slight change in fines content. No major reduction in fiber length is evident after the second recycle. This is attributed to the continuing loss of fines during recy cles owing to the absence of a closed white water system.

The loss of fines, which contributes to a higher weighted 38 Table 4

Weighted Average Fiber Length of ONP(l0 month age)

Recycle No. Weighted Average Sta. Deviation of Weighted Fiber length mm Fiber Average Length mm

1 0.85 0.02

2 0.76 0.03

3 0.73 0.01

4 0.70 0.02

5 0.71 0.02

6 0.72 0.02

0.85 x------

..QI .Cl

QI E l!Cl E QI .c > '5, 0.75

0.65 +------,------1------1------!----� 3 4 5 6 Number of Recycles

Figure 10. Effect of Recycling on Weighted Average Fiber Length. 39 average fiber length, could compensate for the adverse effect of recycling on fiber length due to repeated pulp ing, handsheet-making, and drying. However, it should be remembered that the handsheets were neither aged nor beaten between recycles.

Effect of Aging and Recycling on Zero Span Strength

Fiber strength was assessed by measuring the zero-span tensile strength using the Pulmac instrument. Average zero span tensile index values with standard deviations are re ported in Table 5. Comparative graphs for the effect of ag ing and recycling on zero-span tensile index are shown in Figures 11 and 12. Figures 11 and 12 show the zero-span index of hand sheets with different aging periods expressed as a function of the number of times that fibers have been recycled. In Figure 11, it can be seen that in no case does the fiber strength change significantly due to the number of recycles for 1 month ONP, while the zero-span strength has some re duction after 6 month aging compared to 1 month aging. These results show that there is some change in intrinsic fiber strength due to aging. In Figure 12, while the fiber strength for 24 hour and 48 hour accelerated aging at 105°c has a small reduction 40 Table 5

Zero-Span Fiber Strength

PARTICULARS Recycle NO. #1 #2 #3 #4 #5 #6

Zero-Span Tensile Index Nrn/g

ONP / 1 Month 72.03 71.74 72.13 72.03 71.05 71.34

ONP/ 6 Months 72.91 72.54 71.64 70.74 69.97 70.74

ONP/ 9-10 Months 71. 34 70.95 71.34 70.56 70.17 69.69

Sheet/ 24 hr.A.A. 71.54 71. 74 71.54 69.29 70.76

Sheet/ 48 hr.A.A. 70.76 71.25 69.78 67.52 69.18

Sheet/ 72 hr.A.A. 71.15 70.36 70.17 61.74 54.78

Sheet/144 hr.A.A. 70.54 67. 62 68.89 57.92 49.98

Std. Deviation

ONP/ 1 month 2.91 1.18 1.35 1. 84 2.30 1. 66

ONP/ 6 Months 2.32 1.04 1.21 1.73 0.69 0.52

ONP/9-10 Months 0.76 0.66 0.31 0.76 0.80 1.21

Sheet/ 24 hr.A.A. 2.22 2.15 4.26 1.66 1.32

Sheet/ 48 hr.A.A. 1.32 1.32 1.21 1.11 1. 77

Sheet/ 72 hr.A.A. 1. 35 1. 84 1.35 1.73 1.07

Sheet/144 hr.A.A. 2.08 0.95 2.60 1.59 2.39

*A.A-acclerated aging. 41

en x-I - 75 X e, E + + + � " z + + + >< 1 GI 70 -='C .!! 65 ·;;; C GI 60 I- <> X 1 roonth ONP C + I ca 55 I 6 roonth ONP C. 9-10 roonth ONP Cl/ 0 50 ...GI

45 3 2 4 5 6 Number of Recycles

Figure 11. Effect of Recycling and Natural Aging on Fiber Strength (Zero-Span Tensile Index).

en - 75 E � z >< GI 70 'C -= <> 65 X NoA.A. � /j, CII 24 hr. A.A C X 48 hr. A.A GI 60 0 72 hr. A.A I- 144 hr. A.A. C X ca 55 C. Cl/ 0 50 GI... N 45 3 2 4 5 6 Number of Recycles

Figure 12. Effect of Recycling and Accelerated Aging on Fiber Strength (Zero-Span Tensile Index). 42 of about 2-3%, the fiber strength has a large reduction of about 25-30% for 72 hour and 144 hour accelerated aging af ter three recycles. The loss of the fiber strength might be due to the fact that the molecular chain of cellulose in the fiber was degraded due to the long aging time the fiber had been exposed. On the other hand, because the fibers un derwent a repeated aging and recycling process, they might have become fragile and brittle and lost their length dur ing recycling. These results indicate that the loss in fi ber strength was mainly caused by aging. However, recycling may promote aged fiber to be shortened by mechanical forces produced in the recycling process.

Effect of Aging and Recycling on Tensile Strength

Average tensile index values with their standard de viations are reported in Table 6. It can been seen in Figure 13 that maximum tensile strength is lost in the first two recycles. The handsheets made from ONP with 9-10 month natural aging time have lower tensile strength than those from ONP with 1 month natural aging time. For all handsheets made from 1 month to 9-10 month age ONP, no significant change in tensile strength was observed after three recycles. Figure 14 shows the results for the handsheets made in 43 Table 6

Tensile Strength

PARTICULARS Recycle No. #1 #2 #3 #4 #5 #6

Tensile Index Nm/g

ONP / 1 Month 32.00 27.57 26.48 26.06 25.71 24.24

ONP/ 6 Months 30.91 28.32 25.99 26.36 25.09 23.78

ONP/ 9-10 Months 28.82 26.12 26.07 25.48 24.99 22.03

Sheet/ 24 hr. A.A. 26.25 21.38 20.48 16.89 16.12

Sheet/ 48 hr. A.A. 24.34 17.65 13.28 13.89 12.91

Sheet/ 72 hr. A.A. 22.74 14.59 15.27 12.03 11.29

Sheet/144 hr. A.A. 22.99 12.74 12.88 8.82 7.33

Std. Deviation

ONP/ 1 Month 0.76 1.09 1.12 0.22 0.48 0.64

ONP/ 6 Months 0.98 0.34 1.23 0.58 0.60 0.17

ONP/ 9-10 Months 0.19 0.52 0.62 0.52 0.23 0.17

Sheet/ 24 hr. A.A. 0.67 1.45 0.91 0.59 0.16

Sheet/ 48 hr. A.A. 1.32 0.79 0.37 0.52 0.24

Sheet/ 72 hr. A.A. 0.84 0.30 0.38 0.54 0.36

Sheet/144 hr. A.A. 0.93 0.75 0.74 0.32 0.65

*A.A-accelerated aging. 44

35 1 X 0 : en 3 +-+ 25 + �- -� + )( "' GI ]+ "Cl 20 .E <> 1 rronth ONP J! 6 rronth ONP 'iii 15 X C + 9-10 rronth ONP GI I I 10

5 2 3 4 5 6 Number of Recycles

Figure 13. Effect of Recycling and Natural Aging on Tensile Index.

D NoAA. <> 24 hr. AA. t:. 48 hr. AA X 72 hr. AA. z 25 llC 144 hr. AA. Q) 20 15 .!! X Ill 10 C I Q) X I- 5 0 2 3 4 5 6 Numbr o Recycls

Figure 14. Effect of Recycling and Accelerated Aging on Tensile Index. 45 the accelerated aging. In these cases the tensile strength underwent a rapid reduction in the first three recycles.

The longer the accelerated aging time of handsheets is, the greater is the loss in tensile strength of handsheets made from them.

TMP does not change in breaking length and the chemi cal pulp drops to 50% of the original strength after four recycles[ll]. It is believed that this is due to an in crease in bonding from the flattening of the mechanical fi bers and the collapse and lamellar bonding of the chemical fibers during each recycle. In this study, the ONP for this experiment was made from 80% mechanical pulp and 20% chemi cal pulp. So both the properties of mechanical and chemical fibers can affect the tensile strength of paper made from these two fibers. In addition, the natural aging and high temperature treatment (accelerated aging) of the paper be ing recycled have their adverse effects on strength proper ties.

The great importance of the interfiber bonding in pa per has led to the belief that this is the predominant fac tor in tensile strength, with fiber strength playing a sec ondary role. Four different factors in the loss of tensile strength might be involved:

1. The loss of strength by chemical pulp fibers causes 46 some of the strength reduction of the blend and the fiber flexibility is not improved due to no chemicals being used between recycles.

2. Handsheet production did not include white water recirculation and rebeating between ·cycles. After drying and disintegration, the fines content of the pulp decreased for each cycle. This means the loss in the tensile strength because the function of fines is to contribute to increased bonding for mechanical pulp [34].

3. The natural and accelerated aging of fibers in crease the resistance to further expansion provided by the

elastic nature of the cell wall. The collapse and lamellar

bonding of chemical pulp fibers means that the degree of

expansion is significantly reduced. On the other hand, ag

ing is a light and heat process. Under light and heat ac

tion, the fibers became stiffer and perhaps more brittle

and the fiber surface chemistry could be changed. Many of

the fibrils and microfibrils might be dried down onto the

fiber surface due to aging. Rewetting dried fibers does not

raise a satisfactory number of fibrils and microfibrils

[35]. Poorer interfiber bonding may result. The loss in in

terfiber boding was more pronounced than the loss in fiber

strength as indicated by the fact that recycling and aging

for 24 and 48 hours had almost no effect on zero-span 47 strength but had a significant loss in tensile strength. 4. The multiple recycling of chemical pulp fibers leads to an increase in cellulose crystallinity [1]. The increased crystallinity might give rise to a reduction in swelling and interfiber bonding. At _the same time, the fi bers from mechanical pulp get flattened, which is simple mechanical deformation; they hence become progressively more flexible during successive recycles. These results in crease the surface and bonding area of the fiber and pre vent further reduction in tensile strength after the third recycle.

Effect of Aging and Recycling on Burst Strength

Table 7 shows average burst index values and the stan dard deviations. Figure 15 and Figure 16 show the results from this study on burst index. It can be seen in Figure 15 that the handsheets made from ONP with 1-6 month aging time gain an increase in burst index whereas the handsheets from ONP with a 9-10 month age have no significant change in burst strength for six recycles. There is a significant decrease in the burst strength of paper made from handsheets beyond 48 hour accelerated aging time and after the third recycle, as shown in Figure 16. 48 Table 7

Burst Strength

PARTICULARS Recycle No. #1 #2 #3 #4 #5 #6 2 Burst index KPa.m /g

ONP/ 1 Month 1. 64 1. 75 1.78 1. 84 1.96 2.02

ONP/ 6 Months 1.61 1.77 1.81 1.74 1.88 1.98

ONP/ 9-10 Months 1.59 1.71 1.75 1.72 1.75 1.83

Sheet/ 24 hr. A.A. 1. 68 1.58 1.72 1. 77 1.80

Sheet/ 48 hr. A.A. 1. 61 1.71 1. 63 1. 68 1. 50

Sheet/ 72 hr. A.A. 1. 64 1.70 1.59 1.43 1.45

Sheet/ 144 hr. A.A. 1.72 1.74 1.48 1.42 1.3

Std. Deviation

ONP/ 1 Month 0.03 0.02 0.05 0.04 0.04 0.20

ONP/ 6 Months 0.02 0.02 0.07 0.06 0.06 0.04

ONP/ 9-10 Months 0.04 0.22 0.04 0.01 0.01 0.03

Sheet/ 24 hr. A.A. 0.08 0.09 0.08 0.04 0.02

Sheet/ 48 hr. A.A. 0.08 0.06 0.08 0.04 0.04

Sheet/ 72 hr. A.A. 0.06 0.07 0.02 0.07 0.03

Sheet/ 144 hr. A.A. 0.06 0.06 0.01 0.03 0.11

*A.A-acceleraed aging. 49

2.2

en 2 X N'" �x E; -I+ ftl 1.8 X + 0. + + � - >< 1.6 + Q,I "C <> 1 month ONP .E 1.4 X 6 month ONP I!! + 9-10 month ONP ::s m 1.2

1 2 3 4 5 6 Number of Recycles

Figure 15. Effect of Recycling and Natural Aging on Burst Index.

2.2

en 2 N'" E; 0 ftl 1.8 0. ..x 0 NoAA. ')( 24 hr. AA ::ii:: 6 X + 6 48 hr. AA. >< 1.6 ,)IC Q,I X 72 hr. AA. "C +- 144 hr. AA. .E -d + 1.4 -+---._J+ ::s m 1.2

2 3 4 5 6 Number of Recycles

Figure 16. Effect of Recycling and Accelerated Aging on Burst Index. 50 In this study, the handsheets were pressed and dried on a hot cylinder. The flattening of the mechanical pulp fibers increased the potential bonding area, but the aging of fiber decreased the flexibility and swelling of fiber. Figure 16 shows that longer the aging period and higher the number of recycles, the greater is the loss in the burst index. When accelerated aging time was beyond 48 hours, no significant change in burst index was initially observed but reduced during later recycles. The rapid reduction in burst strength can be attributed to the fact that the aging effect outweighs the flattening of the mechanical pulp fi bers. The handsheets made from ONP with 1-6 month shows a significant gain in burst strength after the fourth cycle. It may be due, as was discussed in the tensile strength re sults, to the repeated flattening of the fibers and the re sulant increase in fiber flexibiliy. However, the hand sheets made from ONP in the 9-10 month category shows no increase in burst strength during recycling. It is possible that the degree of flattening of the fiber was reduced due to the fiber aging. These results show that ONP over different aging peri ods has different recycling potential and the recycling po tential of ONP depends on the pulp type and aging history. It is also probable that the increase in burst values may 51 be due to an increase in strain to failure [36]; however this can not be ascertained, as elongation values were not measured in this study.

Effect of Aging and Recycling on Tear Strength

Average tear index values with the standard deviations are reported in Table 8. Tear strength can give an indication of the force re quired to delaminate the handsheet and break the fiber. Figure 17 shows that the tear index of handsheets made from ONP with 1 to 10 month natural aging time initially de creased in the first two recycles, and then suffered no significant change during subsequent recycles. The explana tion for these trends is that: Initially, hornification of the chemical pulp fibers reduces the bonding potential giv ing a weaker sheet; Next, flattening and flexibilizing of the mechanical pulp fibers progressively takes over along with some loss of fines causing some tear strength recov ery. Figure 18 shows that the handsheets with no acceler ated aging have higher tear index as compared with the handsheets with accelerated aging. For papers with no ac celerated aging, the reduction of tear strength occurred in the first two recycles; but, for accelerated aging, the 52 Table 8

Tear Strength

PARTICULARS Recycle No. #1 #2 #3 #4 #5 #6

Tear Index rnNm� / g

ONP/ 1 Month 8.47 7.89 7.81 7.61 7.63 6.59

ONP/ 6 Months 8.30 7.81 7.77 7.80 7.36 6.88

ONP/ 9-10 Months 8.01 7.71 7.56 7.44 7.41 6.24

Sheet/ 24 hr. A.A. 7.34 7.01 6.83 6.28 5.49

Sheet/ 48 hr. A.A. 7.22 6.87 6.09 6.21 5.29

Sheet/ 72 hr. A.A. 6.99 5.41 5.54 4.77 4.12

Sheet/144 hr. A.A. 6.63 5.23 5.05 3.36 2.71

Std. Deviation

ONP/ 1 Month 0.10 0.11 0.18 0.28 0.14 0.18

ONP/ 6 Months 0.22 0.40 0.06 0.11 0.04 0.13

ONP/ 9-10 Months 0.08 0.08 0.01 0.05 0.08 0.07

Sheet/ 24 hr. A.A. 0.39 0.16 0.17 0.09 0.20

Sheet/ 48 hr. A.A. 0.14 0.42 0.24 0.25 0.12

Sheet/ 72 hr. A.A. 0.11 0.36 0.20 0.12 0.03

Sheet/144 hr. A.A. 0.24 0.37 0.14 0.09 0.05

*A.A-accelerated aging. 53

X en �- � 8 ;:.- lX zE 6.5 E )( 5.5 QI D 1 month ONP .E 4.5 X 6 nonth ONP 9-10 month ONP ..IV I!,. QI 3.5

2.5 1 2 3 4 5 6

Number of Recycles

Figure 17. Effect of Recycling and Natural Aging on Tear Index.

en 7.5 ;:.- o f'«>AA. E 6.5 x 24 hr. AA. z 6 48 hr. AA. llC 72 hr. AA. )( 5.5 QI o 144 hr. AA.

.E 4.5 IV QI I- 3.5

2.5 2 3 4 5 6

Number of Recycles

Figure 18. Effect of Recycling and Accelerated Aging on Tear Index. 54 tear index continued to decrease during with each recycle.

This shows that the effect of aging on the tear index of handsheets is greater than the effect of recycling. A sig nificant reduction in tear index may possibly be due, as was discussed in the zero-span and tensile strength re sults, to the loss of bonding potential and the shorting of the fiber length under the action of light and heat on the fibers.

Effect of Aging and Recycling on Water Retention Value

Average water retention values with the standard de viations are shown in Table 9.

Water retention value can be used as a measure of in ternal fiber swelling capacity and is bound to vary with the change in fiber flexibility and plasticity. For natural aging, there is a similar decreasing trend in water reten tion values with increasing recycles for all aging peri ods(Figure 19). At the same time, water retention values decreased with increasing age of ONP. For accelerated ag ing, the most rapid decrease took place during the first four recycles, as shown in Figure 20. The longer the period of accelerated aging, the lower is the water retention value. 55 Table 9

Water Retention Value

PARTICULARS Recycle No. #1 #2 #3 #4 #5 #6

Water Retention Value

ONP/ 1 Month 2.18 1. 85 1.71 1. 65 1.44 1.02

ONP/ 6 Months 2.07 1.79 1. 77 1. 59 1.36 0.96

ONP/ 9-10 Months 1.92 1. 72 1.69 1.61 1.28 1.01

Sheet/ 24 hr. A.A. 1.87 1. 66 1.50 1.23 0.81

Sheet/ 48 hr. A.A. 1.76 1.54 1.10 0.97 0.73

Sheet/ 72 hr. A.A. 1. 60 1.42 1.03 0.81 0.59

Sheet/ 144 hr. A.A. 1.52 1.16 0.78 0.65 0.48

Std. Deviation

ONP/ 1 Month 0.08 0.16 0.05 0.08 0.05 0.05

ONP/ 6 Months 0.12 0.09 0.34 0.14 0.11 0.03

ONP/ 9-10 Months 0.07 0.06 0.07 0.04 0.08 0.06

Sheet/ 24 hr. A.A. 0.12 0.06 0.11 0.10 0.07

Sheet/ 48 hr. A.A. 0.05 0.08 0.08 0.09 0.07

Sheet/ 72 hr. A.A. 0.09 0.08 0.07 0.09 0.07

Sheet/ 144 hr. A.A. 0.14 0.09 0.10 0.09 0.05

*A.A-accelerated aging. 56

2.5 ,------�

X ]�2 � >ca · .c c LL .2 ci1. 5 0 t:CII en -CII 1::: o 1 monthONP o:::_CII x 6 monthONP ... ca t:. 9-10 monthONP CII � '; en0.5 == - 0 ------+------2 3 4 5 6 Number of Recycles

Figure 19. Effect of Recycling and Natural Aging on Water Retention Value.

2.5 ,------,

CII - C 2 -=ca � .c > ·- o NoAA. C LL • 5 o C 1. x 24 hr. AA. �oCII en - 1::: t:. 48 hr. AA. CII CII 1 D::: - ... ca X 72hr. AA. CII == ' e0.5 o 144hr. == - 0+-----+-----+------+------+---� 2 3 4 5 6 Number of Recycles

Figure 20. Effect of Recycling and Accelerated Agin on Water Retention Value. 57 A reduction in water retention value corresponds to the loss of internal swelling, resulting in the loss of re cycle potential. As Figure 19 and Figure 20 show, water re tention values decrease with aging times whether natural or accelcerated. This might be due tq the fact that the larger pores in a fiber which preferentially close up dur ing aging may not be able to re-swell when subjected to re cycling [35]. The aging process is also a drying process since the temperature is around 20-22°C for natural aging and 105°c for accelerated aging. So the aging process in troduces irreversible changes in the fiber structure. These might promote the auto-crosslinking reaction and the hydro lytic breaking of covalent bonds in the cellulose chains due to drying [23]. Bonding takes place within the fiber wall, probably by hydrogen bonding, and the lack of com plete reversal of this internal bonding prevents complete reswelling when the fibers are recycled. The reslushed fi bers are thus stiffer and less conformable. For the accelerated aging process, the fibers under went a repeated aging between recycles. Accelerated aging produced a large overall reduction in water retention value (Figure 20). At the sixth recycle, water retention values were reduced to 72%(natural aging) and 56%(accelerated ag ing). For the natural aging, it is quite evident that there 58 is a greater difference in water retention value between 1 month ONP and 9-10 month ONP during the first recycle. Af ter the second recycle, the difference in water retention values between 1 month and 9-10 month ONP was decreased

(Figure 19) due to the absence of any additional aging process between recycles.

These results indicate that while the two factors of aging and recycling affected water retention value, aging is the predominant factor in the loss of water retention value.

Effect of Aging and Recycling on Scattering Coefficient

Scattering coefficient decreased with increasing aging period and number of recycles as shown in Figure 21 and 23.

Scattering coefficient decreases with increasing density.

The recycling of chemical pulp fibers causes the sheet to become bulkier and mechanical pulp fibers to become more dense [1,6,7,12). The density of handsheets made from ONP might be increased due to two opposing trends in this ex periment as shown in Figure 22. The tendency towards re duced sheet density brought about by chemical pulp hornifi cation is countered by the gain in density of mechanical pulp fraction. In subsequent cycles, the mechanical pulp fibers continue to change resulting in overall increase in 59 sheet density. Similar explanations account for the reduced scattering coefficient because a higher density means fewer voids in the structure that will provide light scattering surfaces.

60 Cl

GI 50 Cl o 1 month ONP ·;: GI 45 x 6 month ONP t:. 9-10 month

40 2 3 4 5 6 Number of Recycles

Figure 21. Effect of Recycling and Natural Aging on Scattering Coefficient.

When the aging factor between recycles was considered, scattering coefficient rapidly decreased as shown in Figure

23. It might be due to (a) the loss of the fines due to the absence of a closed white water system during recycling; and (b) many of the fibrils getting dried down onto the fi ber surface during aging(drying), which did not fully re cover on rewetting. Since fines and fibrils have an ex- 60

0.45 �------�

0.44 E en o.43 >, ell C o.42 <> 1 monthONP QI C A x 6 monthONP X // + o.41 T + 9-10 month

0.4 ______, 2 3 4 5 6 Number of Recycles

Figure 22. Effect of Recycling on Density.

Cl 55 50 E 45 D !"«>AA. 24hr. AA. QI X = 40 t:. 48 hr. AA. X 72hr. AA. 35 0 144 hr. AA. ·;:: 30 QI 25 "' 20 15 2 3 4 5 6 Number of Recycles

Figure 23. Effect of Recycling and Accelerated Aging on Scattering Coefficient. 61 tremely high specific surface area, They can form addi tional scattering surfaces. There is reduction of scatter ing surface area owing to the lost fines and the fibrils collapsed onto the fiber during recycling and aging. On the other hand, the swelling ability of _the fiber reduced by aging, resulting in decreasing light scattering from the fiber since the surface area of fibers is reduced. These indicate that the aging of the fibers affects the internal scattering power of individual fibers and reduces light scattering interfaces.

Comparison of Natural Aging versus Accelearted Aging

Figure 23 shows the comparison between the effect of natural aging(l0 months) and accelerated aging(24 hours) on fiber and paper properties. It can be seen that tensile index, burst index, and scattering coefficient for natural aging(l0 months) are ei ther equal or slightly greater than those for accelerated aging(24 hours). These indicate that effects of natural aging on vari ous properties can be predicted by conducting accelerated aging trials. Accelerated aging for a period of 24 hours or slightly less may be sufficient for this purpose. However, in the case of zero-span tensile index, natural aging 62 (10 months) shows a larger decrease than 24 hours of accel

erated aging. It is seen that zero-span tensile index for

natural aging(lO months) is between 24 and 48 hours of ac

celerated aging.

� E z ,,•)C

• �:2 3 :4 :5 t I- 3 4 5 6 6 i;f::-2 � : i Number of Recycles Number of Recycles

a b

iz E ,,)C•

I- ff2 :3 :4 :5 6 2 3 4 5 6 : t H(!t:Number of Recycles : : l Number of Recycles

C d

It: u � 75 ------'t I � f73 £ l � 'E R i 1[ :�;� 67 •t R AX i 3 4 :�c:2 :15 6 � 65 .,______....,. i : 1 2 3 4 5 6 Number of Recycles Number of Recycles

e f

� 24 hr. A.A. X 48 hr. A. A. +10 month ONP

a) Burst index. b) Tensile index. C) water retention Value d) Tear index. e) Scattering coefficient. f) Zero-span tensile Index

Figure 24. Comparison of Natural Aging Versus accelerated Aging. CHAPTER VII

CONCLUSIONS

The effect of age and recycling on paper quality and fiber properties has been evaluated by both accelerated ag ing and natural aging for six recycles in this study. Fiber strength (zero-span strength) data did diminish due to recycling old newspaper. However, the fiber strength shows a reduction by increasing aging time at high tempera ture. The effects caused by aging occur at different rates for different periods of aging. For natural aging, the loss in tensile and tear strength occurred mainly in the first two recycles. However, for accelerated aging due to addi tional aging between recycles, the tensile strength got re duced during each of the recycles. The loss in tensile strength is attributed to reduced bonding ability and flexibility of fibers. The adverse effect of aging on bond ing ability and flexibility of fi�ers is greater than that of recycling. The loss in bonding potential caused by aging is more pronounced than the loss in fiber strength as indi cated by the fact that aging for 24 and 48 hours had almost

63 64 no effect on zero-span strength but had a significant loss in tensile and tear strength during recycling. The effects of recycling depend on the pulp type. For chemical pulps there will be loss of swelling, while for mechanical pulps, the result will be fiber flattening [17]. In this study, ONP was made from 20% chemical pulp and 80% mechanical pulp. The gain in burst strength for natural ag ing can represent the net result of two different pulps during recycles. For accelerated aging, the swelling reduc tion and strain hardening of the fibers caused by aging outweighs the flattening of mechanical pulp fibers caused by recycling, resulting in a reduction in burst strength of handsheets. With increased aging time of paper whether natural or accelerated, water retention values decreased. A large overall reduction in water retention value was produced when the fibers underwent a repeated aging between recy cles. Both the factors of aging and recycling affected wa ter retention value of fiber, but aging was the predominant factor. The recycle potential of ONP with 10 month age is nearly equivalent to that of paper aged for 24 hours at high temperature. The effects of recycling depend greatly on the aging 65 period of the paper being recycled. When the paper under goes repeated aging and recycling, its recycling potential will decrease. CHAPTER VIII

SUGGESTIONS FOR FURTHER STUDY

It is recommended to study whether the properties of aged and recycled paper can be improved by re-beating. The effect of aging on chemical and mechanical pulps needs to be studied independently.

Identification of the change in the surface character istics and the molecular chain of fibers before and after aging is a good area for study utilizing electron micros copy and X-ray diffraction.

Also an investigation of the swelling ability of aged fibers is a potential subject for future study to help in determining the need for chemicals in recycles.

66 Appendix A

Regression Model Parameter Data

67 68 Appendix

Regression Model Parameter Data

a b C d e f

Tensile Index

ONP (lM) 31.3000 -9.1883 3.8625 -10.0420 .1357 -.0075

OMP (6M) 35.6000 -5.6000 1.1167 -.0417 -.0167 .0017

ONP (10M) 41.5650 -23.9538 15.8130 -5.1838 .7790 -.0464

24 hr. AA 40.1000 -9.9800 .9750 .2750 -.0750 .0050

48 hr. AA 50.6000 -27.2950 9.7875 -1.9958 .2125 -.0092

72 hr. AA 53.5000 -31. 6567 11.5853 -2.3333 .2417 -.0100

144 hr. AA 55.1000 -32.8476 10.8583 -1. 9958 .1917 -.0075 Burst Index

ONP (lM) 1. 5200 .1157 .0124 -.0096 .0016 -.0001

ONP (6M) 1. 4150 .2780 -.0931 .0204 -.0024 .0001

ONP (l0M) 1. 4660 .1955 -.0677 .0149 -.0017 .0001

24 hr. AA 1. 7030 -.1501 .1165 -.0366 .0045 -.0002

48 hr. AA 1. 6620 -.0662 .0695 -.0309 .0060 -.0004

72 hr. AA 1. 5180 .2511 -.1680 .0437 -.0050 .0002

144 hr. AA 1.3830 .4947 -.2995 .0677 -.0060 .0001 Tear Index

ONP (lM) 9 .1100 -.6990 .0442 .0188 -.0042 .0003

ONP (6M) 9.4600 -1. 4170 .4724 -.0975 .0108 -.0005

ONP (10M) 9.3000 -1.4683 .4850 -.0962 .0100 -.0004

24 hr. AA 10.5300 -2.9083 1.5300 -.2304 .0275 -.0012

48 hr. AA 10.8600 -3.3333 1.1625 -.2454 .0275 -.0012

72 hr. AA 12.0270 -4.8897 1.6163 -.3149 .0327 -.0014

144 hr. AA 12.0400 -4.3465 .8262 -.0442 -.0063 .0007 69 Appendix-Continued

a b C d e f

Water Retention Value

ONP (lM} 3.4100 -1.5747 .5767 -.1221 .0133 -.0006

ONP (6M} 3.0700 -1.0689 .2654 -.0267 -.0004 .0002

ONP (l0M} 2.5100 -.5728 .1038 -.0121 .0013 -.0001

24 hr. AA. 3.3800 -1.4357 .4733 -.0988 .0117 -.0006

48 hr. AA. 3.8800 -2.2298 .8436 -.1842 .0213 -.0010

72 hr. AA. 4.1800 -2.7012 1. 0621 -.2375 .0279 -.0013

144 hr. AA. 4. 7200 -3.5723 1.4808 -.3358 .0392 -.0018 Zero-Span Strength

ONP (lM} 7.4700 -.1840 . 0967 -.0258 .0033 -.0002

ONP (6M} 7.7700 -.4972 .2279 -.0575 .0071 -.0003

ONP (10M} 7.5300 -.3803 .2158 -.0642 .0092 -.0005

24 hr. AA. 7.4800 -.1748 . 0721 -.0200 .0029 -.0002

48 hr. AA. 7.5900 -.3145 .1029 -.0204 .0021 -.0001

72 hr. AA. 6.7900 1.3922 -1.1137 .3354 -.0462 .0024

144 hr. AA. 6.7000 1.6767 -1.3592 .3908 -.0508 .0025 Scattering Coefficient

ONP (lM} 63.5000 -7.5833 1.2500 .0792 -.0500 .0042

ONP ( 6M} 74.4000 -20.3717 3.1225 .0167 -.0625 .0050

ONP (l0M} 68.6000 -15.4830 5.8830 -1. 3583 .1667 -.0083

24 hr. AA. 74.4000 -20.3717 3.1225 .0167 -.0625 .0050

48 hr. AA. 123.100 -113.398 62.7875 -17.529 2.3625 -.1225

72 hr. AA. 109.635 -82.4832 37.9731 -9.2896 1.1399 -.0552

144 hr. AA. 113.900 -85.8783 36.7024 -8.4750 .0996 -.0467 REFERENCES

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