The effects of finish coatings on ultraviolet and visible light stability of inkjet prints MA thesis
Student: Ella Solomon Student number: 11596139 Supervisor: Katrin Pietsch Programme: Conservation and Restoration of Cultural Heritage, [Photography] Date: July 2020
Acknowledgments
I would like to express my appreciation to the many professionals who have helped me along the way. First, I would like to thank Katrin Pietsch, my course coordinator and thesis supervisor at the University of Amsterdam (UvA), for her constructive guidance and patience through the entire process. Next, I would like to sincerely thank Dr René Peschar at UvA who kindly supported the challenge of understanding the characteristics and photodegradation of polymers in light of the experimental results. A special thanks goes out to Clara von Waldthausen, Prof. Dr Maarten van Bommel, Prof. Dr Ella Hendriks and Dr Maartje Stols-Witlox at UvA for overseeing the general research process, providing guidance and sharing their insights. Colour measurements would not have been possible without the patient explanations by Dr Emilie Froment and Kate van Lookeren Campagne at UvA on performing the measurements. This research study was made possible by Ryan Boatright, the co-founder of the printing and conservation studio Atelier Boba, who brought up the idea for this research study and created the samples for the experiment in his Paris studio. The experimental part of the study would not have been possible without the generosity of Drs Agnes Brokerhof, senior scientist at the Rijksdienst voor het Cultureel Erfgoed (RCE), who helped me through the entire process and introduced me to the concept of artificial weathering. A special thanks goes out to ing. Saskia Smulders, MA, PD res. and Henk van Keulen, RCE, who performed and analysed the coatings materials during the turbulent times of the COVID-19 pandemic. I am also thankful to Dr Bill Wei, Drs Frank Ligterink and Dr Han Neevel who shared their knowledge and offered input on the solar spectrum and colour measurements. Lastly, I would like to thank my classmates for sharing their experiences, especially Tessa Maillette De Buy Wenniger who was a great help in the practical experiment in the days of the COVID-19 outbreak and was willing to answer any chemistry-related questions. A final thanks is due my family and friends in my home country for their long-distance support and to my friends from the Netherlands who offered their kind expert assistance.
The effects of finish coatings on ultraviolet and visible light stability of inkjet prints Master’s Thesis
Ella Solomon (11596139) University of Amsterdam, July 2020
Light stability of finish coatings and their effect on inkjet prints in terms of colour change is discussed. The coatings’ material content was analysed using gas chromatography mass spectrometry (GC-MS). The coatings were applied on unprinted and printed samples of Fine Art paper and put in Xenontest for overall 121 mega lux hours. The samples were then compared using L*a*b colour space. The chemical reactions causing colour change are complex due to the various materials involved.
Dit werk bespreekt de lichtstabiliteit van verschillende coatings en hun effect op inkjetprints wat betreft kleurverandering. De bestanddelen van de zes coatings zijn geanalyseerd met gaschromatografie massaspectrometrie (GC-MS). De coatings zijn toegepast op onbedrukte en bedrukte monsters kunstenaarspapier en gedurende in totaal 121 megalux uur aan een Xenontest onderworpen. De kleurverandering van de monsters is gemeten aan de hand van het L*a*b kleurenmodel. De chemische reacties die kleurverandering veroorzaken zijn complex vanwege de diverse betrokken materialen.
Table of Contents
Introduction ...... 1 1. Fundamentals ...... 2 1.1. History and technology of inkjet ...... 2 1.1.1 The inkjet print ...... 2 1.1.2. Component materials of the inkjet print: inks and media ...... 3 1.1.3. Finish coatings for inkjet prints ...... 4 1.2. Research Material Focus ...... 5 1.2.1. Epson UltraChrome Pro inks and Hahnemühle Photo Rag® fine art paper...... 5 1.2.2. Hahnemühle, Rauch and Breathing Color finish coatings ...... 6 1.3. Research Objective ...... 6 1.4. Current Scientific Knowledge ...... 7 2. Polymer Binders and Light Stabilisers in Finish Coatings ...... 11 2.1. Characteristics ...... 11 2.1.1. Polyethylene glycol-polyvinyl alcohol ...... 11 2.1.2. Polyurethanes...... 14 2.1.3. Polyacrylates ...... 16 2.1.4. Photostabilising additives in coatings...... 17 2.2. Photodegradation pathways of polymer binders ...... 20 2.2.1. Polyethylene glycol-polyvinyl alcohol ...... 22 2.2.2. Polyester urethanes ...... 24 2.2.3. Polyacrylates ...... 27 2.2.4. Incorporation of light stabilisers and their effect on polymer binders ...... 30 2.3. Discussion: the ageing behaviour of weathered finish coatings on inkjet prints ...... 31 3. Methodology ...... 34 3.1. Questionnaires and correspondences: manufacturers, conservators and scientists ...... 34 3.2. Experimental ...... 36 3.2.1. Sample creation ...... 36 3.2.2. Material analysis using gas chromatography mass spectrometry ...... 39 3.2.3. Artificial UV and light ageing using Xenontest ...... 39 3.2.4. Colour measurements ...... 41 4. Results and Discussion ...... 43
4.1. Material Analysis (THM-GCMS) ...... 43 4.2 Colour Measurements ...... 43 4.3. Discussion ...... 49 5. Conclusions ...... 54 Summary ...... 57 Bibliography ...... 59 Appendices ...... 64
The effects of finish coatings on ultraviolet and visible light stability of inkjet prints
Introduction Inkjet printing is a common practice in digital photography and is employed by contemporary artists and by the general public in day-to-day life in the office or at home. In addition to the technological developments of inkjet printing, products such as finish coatings were marketed to achieve long-lasting durability. This document will discuss the photostability qualities of six inkjet finish coatings and their effect on Epson UltraChrome Pro inks and Hahnemühle Photo Rag® fine art paper typically employed by printing studios and photographers worldwide for their aesthetic and durable characteristics. Despite the inert lightfastness provide by the inks and paper, inkjet prints are still prone to deterioration and discolouration when exposed to light and ultraviolet (UV) radiation. Although finish coatings have been created to improve the prints’ life expectancy, little is known of their long-term performance. For this research study, six organic finish coatings for inkjet prints were chosen for their material content and light fastness. The finish coatings were applied on eighteen samples: twelve directly on Hahnemühle unprinted Photo Rag® fine art paper and six on printed paper with Epson UltraChrome Pro inks. Three of the coatings were analysed using gas chromatography mass spectrometry (GC-MS). The material analysis provided further understanding of the polymer matrices and the UV and light resistance additives. The samples were then aged in a Xenontest weathering instrument for approximately 121 megalux hours measured for colour changes using the chromatic coordinates L*a*b* of the CIELAB colour calculating system. A literature research on the ageing behaviour of polymers and light stabilising additives encountered during analysis offered an explanation for the samples’ post weathering colour changes.
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1. Fundamentals 1.1. History and technology of inkjet 1.1.1 The inkjet print An inkjet print is an image created by ink droplets consisting of dyes or pigments, which are deposited on a substrate (e.g., paper) by a printing device based on commands sent to it from a computer. The 1990s and 2000s saw rapid developments in inkjet technology, with prominent companies competing for the best print quality and developing related products such as custom printer papers and inks. The Iris printer, first built in 1984, employed a printing technology known as continuous inkjet, which emits a continuous stream of charged ink droplets that can be electronically deflected into a recycling system or allowed to pass and make contact with the paper.1 Despite their high-resolution output, Iris printers required high maintenance, and their prints faded when exposed to light. Their usage declined rapidly with the introduction of drop-on-demand (DOD) inkjet technology, which ejects ink droplets as required.2 In the middle of the 1990s, DOD printers started using pigment-based inks, which proved more durable than dyes when exposed to UV and visible light radiation. Around the same time, matte-coated paper, resin-coated paper and glossy white film with microporous ink-receptor layers (IRL) were introduced to provide quality similar to analogue photographic prints. Towards the end of the 1990s, large-format DOD printers were developed for fine art inkjet printing.3 New systems were developed for the digital image printing market such as processes involving electrostatics and thermal transfer of ink to substrates. Nevertheless, the DOD inkjet is still the most common and popular printing process employed by artists and the general public in daily use at home and in the office. This document therefore refers to inkjet prints as those made by DOD printers with pigment-based ink.
1 “Inkjet”. DP3. Image Permanence Institute. Accessed February 27, 2020. http://www.dp3project.org/technologies/digital-printing/inkjet. 2 M.C. Jürgens. The Digital Print. (UK: Thames and Hudson, 2009). p. 25, 74. 3 Ibid. p. 77-82.
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1.1.2. Component materials of the inkjet print: inks and media Inks
Inks for inkjet prints are composed of a colourant (dye or pigment), liquid vehicle (water or solvent that evaporates after drying, oils, waxes or UV-curable polymers) and other additives. The inks can contain a single colourant or mixtures of several colourants to create a particular hue. Dye-based inks dissolved in water have been typically employed in continuous inkjet and early DOD printers but are prone to fading and bleeding when coming into contact with moisture. The use of dyes in inkjet printing therefore decreased considerably, while pigments became common.4,5 The vehicle for dyes and pigments is also crucial when designing an ink system. Aqueous inks are more sensitive to moisture, and colourants can bleed in the media, while other solvent-based inks can prevent moisture-related deterioration. Additives such as surfactants and biocides are incorporated in the aqueous ink systems to improve viscosity, slow evaporation, improve lightfastness and protect against abrasion.6
Media
Inkjet printing technology offers a large range of media on which to print, with plastics and natural and synthetic paper being a small part. Each type of media can also be coated or uncoated. This study will review on type of fine art paper substrate. The location of the ink within the paper determines its appearance and sensitivity to mechanical and chemical degradation. In terms of appearance, the image will show higher saturation and brilliance if the ink’s drop remains on the media surface because the drop will have sharp edges. If the ink saturates into the paper, it will bleed and expand between the fibres, resulting in lower image resolution. Several developments in inkjet paper have been introduced to solve this problem, one of which is the IRL, a surface coating for plain paper that acts as a barrier between the paper substrate and the ink, keeping the drops above the surface. Over the past decades, IRLs have undergone development, improving their characteristics for printed images. Fine art paper made of cotton fibre with matte IRLs are the most popular choice among photographers (Fig. 1.1.2.1).
The matte IRL is composed of fine particles of calcium carbonate (CaCO3) in polyvinyl alcohol (PVAl) and/or starch binder and silica or alumina as matting agents.7
4 Ibid. p. 76, 85. 5 M. Bale. “Inkjet Ink and Its Important Additives.” Inkjet Insight. October 19, 2018. Accessed May 2, 2020. https://inkjetinsight.com/knowledge-base/inkjet-ink-important-additives/. 6 Jürgens. The digital. p. 86-87. 7 Ibid. p. 91-92.
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Figure 1.1.2.1. M.C. Jürgens. Cross section of ink on Epson Smooth Fine Art Paper with matte IRL. M.C. Jürgens. The Digital Print. (UK: Thames and
Hudson, 2009). p. 91.
1.1.3. Finish coatings for inkjet prints Finish coatings for inkjet prints are a type of material designed to improve the life expectancy of inkjet prints. According to their manufacturers, finish coatings for inkjet prints provide various protective properties such as abrasion resistance, smudge and fingerprint repellence and UV protection. During the 1990s, a number of new fixative sprays were developed and marketed for Iris inkjet prints due to the inks’ sensitivity to light and moisture and later for pigment-based aqueous inks. Fixatives are typically solvent-based and contain acrylics, urethanes or vinyl with added plasticisers and matting agents. Liquid finish coatings were also developed and consist of synthesised water-based or solvent-based polymers and additives, although the latter are less common due to health hazards.8 Spray and liquid finish coatings (water-based or solvent-based) are complex solutions that usually contain several polymers blended and synthesised together. Additives, such as silicones, biocides and light stabilisers, are incorporated into the coating matrix to achieve the desired properties. Detailing the interaction between the materials in different matrices is beyond the scope of this research study, which instead focuses on three types of polymer binders found during the material analysis of six selected inkjet finish coatings and their ability to protect inkjet prints from UV and visible light radiation (see chapter 2).
8 Ibid. p. 52-53, 76.
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1.2. Research Material Focus The materials discussed in this document are the materials chosen for the experimental part of the study. Six finish coatings by three companies (Hahnemühle, Rauch and Breathing Color) were chosen to be applied on Hahnemühle Photo Rag® fine art paper samples. Some of the samples were printed by the Epson SureColor P20000 printer with UltraChrome Pro ink and were then coated, as were the unprinted samples. The overall material content and further details on these particular materials will be explained.
1.2.1. Epson UltraChrome Pro inks and Hahnemühle Photo Rag® fine art paper The Epson SureColor P20000 is a DOD printer for large scale prints. The printhead consists of ten aqueous pigment-based inks named “UltraChrome Pro”: cyan, light cyan, vivid magenta, vivid light magenta, yellow, black, dark grey, grey and light grey. Each ink’s material safety data sheet (MSDS) revealed no additional information except for the addition of colourants and glycerol additives, tritanol amine and proprietary organic materials; other information remains a trade secret.9 The inks were tested by Wilhelm Imaging Research institute for light fastness on various Epson paper substrates. The results showed noticeable changes after 107–116 years when exposed to 5400 lux hours a day (approximately 211 megalux hours) on paper that is close in nature to Hahnemhüle fine art paper under conditions of being displayed behind glass.10,11,12 At the time of writing, the materials present in the paper are not known. However, in his book, Jürgens states that Hahnemhüle is fine art paper made of cotton fibre with matte IRL consisting of finely ground CaCO3 in PVAL and/or starch binder and silica or alumina as matting agents.13 According to the MSDS, the IRL contains a small amount of optical brighteners.14
9 “Epson SureColor P20000”. Epson. Accessed April 7, 2020. https://epson.com/Support/Printers/Single- Function-Inkjet-Printers/SureColor-Series/Epson-SureColor-P20000/s/SPT_SCP20000SE#manuals. 10 “Epson SureColor P10000 and P20000-Print Permanance Ratings”, Wilhelm Imageing Research, Inc. Accessed February 15, 2019. http://www.wilhelm- research.com/epson/WIR_Epson_SureColor_P10000_and_P20000_Printers_2019-02-15.pdf. 11 “Epson Legacy Papers”, Epson, Accessed April 7, 2020. https://epson.com/pro-photo-legacy-papers. 12 Epson legacy papers offer 100% cotton fiber papers with little to no optical brighteners, as similar to Hahnemhüle fine art paper. More information on material content is missing. 13 Jürgens. The digital. p. 91-92. It is not known if Hahnemhüle made changes since the writing of the book. 14 Photo Rag® Matt FineArt – smooth”. Data Sheet. Rev. 03. Hahnemhüle. Accessed. April 7, 2020. https://www.hahnemuehle.com/fileadmin/user_upload/pdf/dfa/datenblaetter_dfa/PhotoRag308-Rev03.pdf
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1.2.2. Hahnemühle, Rauch and Breathing Color finish coatings This document will discuss six coatings by three manufactures and distributors: Hahnemühle, Rauch and Breathing Color. Hahnemühle manufactures the Hahnemühle Protective Spray (coating #01) for inkjet prints on paper and the Hahnemühle Varnish (coating #04) for inkjet prints on canvas. At the time of writing, Rauch does not manufacture inkjet finish coatings but only sells them using their own brand name.15 It is therefore important to clarify that the manufacturer of the spray coating Rauch Schutzlack Firnis für Fine Art Papiere for paper (coating #02) is unknown and is applied by spraying. Rauch ClearShield™ Type C matte Seidenglänzender UV-Schutzlack” (coating #05) is manufactured by Marabu (a US company) and is a liquid coating suitable for inkjet prints on canvas substrate. Breathing Color manufactures Glamour II (coating #03) for both prints on canvas and paper and Timeless Varnish (coating #06) for inkjet prints on paper or canvas, both of which are liquid finish coatings. The six finish coatings guarantee, among other things, protection for inkjet prints from UV and light radiation. Each finish coating has different polymers as binders, different additives and different UV absorbers (UVAs) and/or Hindered Amine Light Stabilisers (HALS), which some were identified by GC-MS material analysis that was conducted for this research (for overall details on the coatings, see appendix I).
1.3. Research Objective The conservation of contemporary photographs entails numerous challenges, which include identifying and understanding the behaviour of new products sold on the art market. Given the difficulties in maintaining the pace of production while conducting comprehensive learning on these new products, it is important to use the opportunity for research to add knowledge to the field of art and conservation of photographic prints. The subject of this thesis, light stability of inkjet finish coatings and their influence on inkjet prints, originated from Ryan Boatright, the co-founder of Atelier Boba, a printing and conservation studio. Boatright employs these materials for prints made for artists and was interested to know more on the light stability of his prints when combined with finish coatings. After discussing the presence of finish coatings on works of art with several leading conservators, it was then realised that there was insufficient awareness that these materials might be present on inkjets.
15 “Purchasing Schutzlack Firnis für Fine Art Papiere in Paris”. Email to T. Wöhrstein. Export Manager at Rauch. April 2nd, 2020.
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This study’s objective is to provide a basic understanding of the material characteristics of finish coatings for inkjets, their ageing behaviour and their effect on colour perception of inkjet prints after being exposed to solar radiation. In addition, the study will: 1. Help conservators and collection managers identify the presence of finish coatings on inkjet prints. 2. Enable conservators to implement suitable preventive measures, conservation treatment and exhibition protocols for inkjet prints with finish coatings. 3. Provide a base of knowledge for artists and printers who use these products, thereby enabling them to choose the optimal product. 4. Offer a basis for further research into the chemical characteristics and degradation behaviour of contemporary finish coatings on photographs.
These objectives led to the following question: What are the effects of different inkjet finish coatings on the UV and light stability of inkjet prints? To answer this, the following subquestions needs to be answered: 1. What is the material content of the various finish coatings? 2. How does UV and visible light affect the variooue finish coatings over time? 3. Which product when applied to an inkjet print is least affected by UV and visible light exposure?
Another important question is, “how does UV and visible light affect the various finish coatings on the inkjet’s ink and paper substrate over time?” This question will be partially answered by using colour measurements. Explaining the mechanisms behind the changes involving both inks and substrate is beyond the scope of this study because it involves a detailed understanding of the chemical reactions of materials that are not currently known but could be a subject for future study.
1.4. Current Scientific Knowledge The current scientific knowledge on coatings can be divided into three areas: 1) the role of polymers as binders and their ageing behaviour, 2) the protective mechanism of light stabilisers and 3) the lightfastness of inkjet prints. There has been significant research on each of these topics but no specific research on finish coatings for inkjets. Journals such as Polymer Degradation and Stability and Journal of Coatings Technology and Research often publish research on the stability of polymer binders and additives under specific weathering systems.
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A review of those journals showed that most research on the life expectancy of coatings in general and UV and light resistance in particular has been conducted on coatings for metals, plastics and wood but not on paper or inkjet prints.
1) The role of polymers as binders in finish coatings and their ageing behaviour
Two books provide an overview of the characteristics of polymers and binders and their degradation mechanisms when absorbing solar energy. The first, The Synthetic Resins Used in the Treatment of the Polychrome Works of Art (Borgioli and Cremonesi, translated edition, 2019) and Materials for Conservation (Horie, 2010). Borgioli and Cremonesi start by introducing polymer degradation mechanisms and then delves into each synthetic polymer’s characteristics. Unfortunately, the book covers mostly specific products that are sold for the conservation field, and these products do not necessarily consist of the polymer binders in the six finish coatings. The second book offers introduction to polymers characteristics, and used to complete missing information that was not obtained from other articles. This was done especially in the case of the polymer binder polyethylene glycol-polyvinyl alcohol (PEG-PVAL). In their article Introduction to Chemistry and Biological Applications of Poly(ethylene glycol) (1997), Zalipsky and Harris provide a brief introduction to the characteristics of PEG. Since it was difficult to find more information on the polymer, Materials for Conservation by Horie (2010) provided complementary information on the degradation mechanism of PEG and PVAL separately. Szycher’s Handbook of Polyurethanes (Szycher, 2012) provides a general overview of polyurethanes (PU), addresses their basic concepts, briefly explains their synthesis from various fragments (isocyanates and polyols) and then goes into detail about each fragment’s behaviour and the many varieties of fragments that change the binder’s characteristics. The handbook, however, does not deal with degradation mechanisms. Therefore, a couple of articles were chosen on specific polyester urethanes as examples from which to deduce their degradation mechanism. In their study Infrared analysis of the photochemical behavior of segmented polyurethanes: Aliphatic poly(ester-urethane) (1997), Wilhelm and Gardette suggested several degradation pathways for aliphatic polyester urethane that are caused by UV absorption, resulting in breaking chemical bonds. In their study Quantitative spectroscopic analysis of weathering of polyester-urethane coatings (2015), Makki et al suggest several degradation pathways for aromatic polyester urethane. By combining the information from the two articles,
8 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints it is possible to understand the mechanisms by which the various PU fragments affect the characteristics and degradation patterns of a polyester PU. The general characteristics of acrylate and methacrylate groups were covered by Borgioli and Cremonesi and Horie. The degradation pathways were mainly covered in Photooxidative degradation of acrylic and methacrylic polymers by Chiantore and Lazzari (2000) who studied the difference in degradation patterns associated with each group, thereby discovering that each can go through bond breakage and cross-linking, unlike PUs and PEG- PVAl that do not cross-link. These reactions depend on the structure, length of side chains and monomer characteristics.
2) The protective mechanism of light stabilisers in finish coatings
The most important article covering different polymer binders and light stabilising additives is Photostabilization of Coatings. Mechanism and Performance by J. Jospíšil and S. Nešpurek (2000), which explains the chemical protective mechanism of UVAs and HALS and comprehensively details their reactions within various polymer compositions. The article also explains the general photooxidation mechanism of a polymer binder and addresses the performance of photostabilisers and their role in protecting coatings from cracking, gloss loss, colour change and other outcomes of photodegradation. Organic vs Inorganic Light Stabilisers for Waterborne Clear Coats: a Fair Comparison, an essential article on UVAs by C. Schaller et al (2011), discusses the long-term weathering performance of various organic (benzotriazoles [BTZ] and hydroxyphenyl triazines [HTP]) and inorganic UVAs (titanium dioxide, zinc oxide, ceric oxide). The UVAs were tested with various concentrations in a polymer binder to better determine the effects of concentration as a function of the protective qualities. This information helps better to assess the protective qualities of each UVA. However, the coatings were tested on wood and white and black cardboard, and therefore the results cannot be compared fully with the results of the weathering of inkjet finish coatings on paper. In addition, the tested coating was in fact waterborne acrylic paint and not a finish coating. The article’s contribution lies in determining the difference in performance between different UVAs while suggesting that combining photostabilising additives can yield better protective values. In their study Long-Term Weathering Behavior of UV-Curable Clearcoats: Depth Profiling of Photooxidation, UVA, and HALS Distributions (2005), C.M. Seubert et al tested the photostabilisation of acrylic urethane clearcoats with both UVA and HALS as a function of the coating thickness. Although the study tested using UV-curable coatings designed for the
9 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints automotive industry, it provided a number of relevant finding. First, the study explains the coating’s degradation mechanism and the mechanism by which UVAs and HALS slow the degradation. Second, the study describes the effect of thickness on the photostabilising additives’ consumption: UVA molecules embedded deeper within the coating experience a much lower light intensity than those at the top of the layer due to UV absorption by the UVA molecules near the surface of the coating.
3) Lightfastness of inkjet prints
To gain insight into the mechanisms by which prints react without finish coatings, general information regarding the UV resistance of inkjet prints is needed. Stability issues and Test Methods for Ink Jet Materials (2001), a thorough study by B. Vogt as part of her dissertation, presents the question of permanence of inkjet prints and tests their fading behaviour and colour change in both dye and pigment-based inks on two different paper substrates. The author found that each dye and pigment showed different behaviours when exposed to UV and visible light. The article provides valuable insight into the colour changes of pure cyan, magenta, yellow and black (CMYK) as well as red, green and blue (RGB). The samples were made as printed targets, and the colour measurements showed changes after exposure to light. Wilhelm Imaging Research, Inc. conducted the Epson SureColor P1000 and P2000- Print Permanence Ratings tests, showing the UV and light stability of UltraChrome Pro inks on various Epson paper substrates. Depending on the substrate, the tests resulted in no noticeable change in the inks’ colour after a range of 107 and 208 years, when displayed under glass. The samples were exposed to 5400 lux hours a day. These findings can help us compare the ink’s behaviour on Hahnemühle Photo Rag® paper substrate. However, the test was performed on a different substrate, and therefore conclusions are difficult to draw. The abovementioned studies add value to the understanding of the effects of UV and light exposure on finish coatings for inkjets prints. Despite the dissimilarity between the polymers and additives on one hand and the material content of inkjet finish coatings on the other, the articles provide an abundance of information on polymers such as vinyl, urethane and acrylic, where the basic degradation pathways are the same. The literature also contributes to the knowledge of the effects of incorporating UVAs and HALS and their reaction in polymer matrices in achieving photostabilisation.
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2. Polymer Binders and Light Stabilisers in Finish Coatings
Polymer binders in finish coatings are the main component responsible for the coating’s properties. This section discusses the monomers’ chemistry of three polymer binders present in six of the inkjet finish coatings, beginning with a review of the characteristics to provide a background for understanding their photodegradation and then delving into the photodegradation pathways. This section will also discuss the protective mechanism of light stabilisers, incorporated additives that used to slow photodegradation. It is important to stress that the following review discuss binders and light stabilisers that were found in material analysis of three coatings. Unfortunately, the analysis was not finalised due to time limitations and the materials discussed in this section are the outcome of logical deduction followed by interpretation of inconclusive results. It is most likely that the polymers reviewed in this part are part of a more complex system of copolymers that has yet to be defined.
2.1. Characteristics Finish coating binders are synthesised polymers, which are large molecules consisting of chemical units called monomers. Most polymers are copolymers, consisting of monomers with differing chemical composition.16 Organic finish coatings differ in the variety of the binders’ chemical structures (e.g., acrylate, urethane, vinyl), the polymer’s molecular weight (MW), the manufacturing processes and the additives that alter the polymer’s behaviour (e.g., solubility, elasticity). 17 Covering all of the variables that affect polymer behaviour is beyond the scope of this study.
2.1.1. Polyethylene glycol-polyvinyl alcohol PEG-PVA was found during material analysis that was done in coating #02. The PVA is most likely referring to PVAL and not polyvinyl acetate (PVAC) since the latter is less workable in room temperature and has a tendency to yellow and degrade fast.18 Polyethylene glycol-polyvinyl alcohol (PEG-PVAL) is a synthetic copolymer mainly employed in producing spray coatings. In its commercial form, PEG-PVAL consists of approximately 25% PEG and approximately 75% PVAL.19 The PEG part forms the backbone for the grafting of PVAL side
16 The Editors of Encyclopedia Britannica. “Polymer”. Encyclopedia Britannica. Feb 26, 2019. Accessed. May 17th, 2020. https://www.britannica.com/science/polymer. 17 R.B. Gilleo. “Rheology and Surface Chemistry”. Coatings Technology Handbook. ed. A. A. Tracton. (US: Taylor & Francis Group, LLC, 2006). 3rd ed. p. (1_10). 18 L. Borgioli P. Cremonesi. The Synthetic Resins Used in The Treatment of The Polychrome Works of Art. trans dr. R. Peschar and E. van der Veer-Curpan. (Netherlands: Drukkerij Wilco, 2019). 3rd ed. p.93. 19 PEG-PVAL is commercially available under brand name Kollicoat®.
11 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints chains (Fig. 2.1.1.1).20 The PEG-PVAL grafted copolymer is water soluble, and the PVAL segment provides the film-forming properties, while the PEG part acts as an internal plasticiser. The grafted copolymer offers functional advantages over the individual components, such as high flexibility and low viscosity in aqueous solutions enabling easier application.21
Figure 2.1.1.1. Structure of polyethylene glycol-polyvinyl alcohol copolymer. The PEG backbone is marked in blue, and PVAl is circled in yellow. Image credit: F.F. Heuschmid et al., Polyethylene glycol-polyvinyl alcohol grafted copolymer: Study of the bioavailability after oral administration to rats. 51. no.1. (2013) p.1.
Polyethylene glycol Ethylene glycol is a polymer produced by the reaction of ethylene oxide and water. PEG is a linear or branched polyether that is soluble in water and most organic solvents. However, PEG loses its solubility at high temperatures,22 and its solubility changes with MW: at MWs less than 1000 amu, PEG is a viscous, colourless liquid, while at higher MWs, it is a waxy, white solid.23 The most interesting property of PEG is its ability to attach to molecules and surfaces with differing polarity, having little effect on their chemistry while controlling their solubility. The monomer unit of PEG is constructed from a polar oxygen atom and a larger non-polar
(CH2)2 group. When in contact with a hydrophobic surface, the (CH2)2 molecule can orient towards this surface (with the oxygen atom pointing away) thereby con bond with PVAL. The terminal hydroxyl groups of the PEG molecule provide a site for covalent bonding to other polar molecules and surfaces, such as water.24
20 Graft polymers are segmented copolymers with a linear backbone of one composite and randomly distributed branches of another composite. 21 F. F. Heuschmid et al. “Polyethylene glycol-polyvinyl alcohol grafted copolymer: Study of the bioavailability after oral administration to rats”. Food and Chemical Toxicology. 51. no. 1 (2013). p. S3. 22 J. M. Harris and S. Zalipsky. eds. Poly(ethylene glycol): Chemistry and Biological Applications. (Washington: American Chemical Society, 1997). p. 16. 23 J. M. Harris. Poly(Ethylene Glycol) Chemistry Biotechnical and Biomedical Applications. (New York: Springer Science+Business Media, LLC. 1992). p.2. 24 Harris. Poly(Ethylene Glycol) Chemistry Biotechnical. p.1,6.
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Polyvinyl alcohol
PVAL is one of the many derivatives of a vinyl group, which itself is derived from ethylene (Fig. 2.1.1.2). To create PVAL, an acetyl group is removed from PVAC by a process called deacetylation, which is a specific form of hydrolysis (Fig. 2.1.1.3).25 Given the deacetylation can be partial, a certain number of acetyl groups can remain in the chain. The characteristics of PVAL (e.g., polarity, opacity, viscosity, resistance to weathering, tendency to acidify) will depend on the degree of deacetylation.26
Figure 2.1.1.2. Creation of polyvinyl derivative from ethylene. Image credit: L. Borgioli P. Cremonesi. The Synthetic Resins Used in The Treatment of The Polychrome Works of Art. trans dr. R. Peschar and E. van der Veer-Curpan. (Netherlands: Drukkerij Wilco, 2019). 3rd ed. p. 88.
Figure 2.1.1.3. Creation of polyvinyl alcohol from hydrolysis of polyvinyl acetate. Image credit L. Borgioli P. Cremonesi. The Synthetic Resins Used in The Treatment of The Polychrome Works of Art. trans dr. R. Peschar and E. van der Veer-Curpan. (Netherlands: Drukkerij Wilco, 2019). 3rd ed. p. 88. PVAL is highly polar when hydroxyl groups are present in large numbers, to the point where the polymer becomes soluble in water and other polar solvents and insoluble in other less polar organic solvents.27 The films produced from partially hydrolysed PVAL solutions are less polar and therefore have higher resistance to water and do not mix well in it, while those made from fully hydrolysed PVAL are more polar and therefore more soluble. When PVAL is
25 Hydrolysis is a chemical reaction in which a molecule of water ruptures one or more chemical bonds. 26 Borgioli and Cremonesi. The Synthetic Resins. p.88-96. 27 Ibid. p.92.
13 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints employed as an adhesive, low levels of hydrolysis provide it adhesion to hydrophobic surfaces, while higher levels of hydrolysis lead to adhesion to hydrophilic surfaces.28 To summarise, PEG with low MW is a liquid that bonds to other molecules by intermolecular interactions (London dispersion forces, dipole-dipole interactions and hydrogen bonds). The monomer unit has polar and non-polar sides and thus has the ability to bond with various hydrophobic and hydrophilic substances. Given that PVAL is highly or fully hydrolysed, it can bond well to low MW molecules to create grafted PEG-PVAL copolymers in the presence of polar solvents such as water. PEG therefore functions as an additive in PEG- PVAL for increasing a coating’s utility.
2.1.2. Polyurethanes
PUs are made from isocyanates and polyhydroxy compounds, known as polyols29 (Fig. 2.1.2.1). PU chemistry produces a broad spectrum of polymer structures. Isocyanates and polyols have differing chemical structures, and combining them achieves certain properties that entail numerous characteristics. Making generalisations about PU characteristics is therefore difficult. This part will discuss basic concepts for the various components that are most likely to be part of an inkjet finish coating.
Figure 2.1.2.1. Synthesis of one type of polyurethane. Image credit: J.O. Akindoyo et al., “Polyurethane types, synthesis and applications –a review”. RSC Adv. 6. (2016). p. 114461.
Isocyanates are derivatives of isocyanic acid (H–N=C=O), in which aliphatic or aromatic groups are directly linked to nitrogen.30,31 Compounds containing aromatic structures
28 E. Ogur. “Polyvinyl Alcohol: Materials, Processing and Applications”. Rapra Review Reports. 16. no. 12. (2005). p.7,10. 29 A polyol is an organic compound containing multiple hydroxyl groups (OH). 30 Aromatic compounds have benzene rings (a typical chemical structure that contains six carbon atoms, cyclically bonded with alternating double bonds), whereas the aliphatic does not have benzene rings. 31 M. Szycher. Szycher’s Handbook of Polyurethanes. (US: Taylor & Francis Group, 2013). p. 90, 63.
14 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints degrade faster than aliphatic compounds when absorbing UV radiation, and therefore most manufacturers replace aromatic isocyanates with the more resistant aliphatic isocyanates. Hexamethylene diisocyanate (HMDI) is an aliphatic isocyanate produced from MDI aromatic isocyanate (Fig. 2.1.2.2). The polymer morphology of HMDI systems yields amorphous or semicrystalline segments in the polymer. The semicrystalline segments provide low reactivity, light stability and hydrolytic stability.32
Figure 2.1.2.2. MDI aromatic isocyanate and the aliphatic isocyanates made of it. Image credit: V.R. Sastri. ed. Plastics in Medical Devices, (UK: Elsevier, 2014). 2nd ed. p.141.
Polyols have the same variety, and their structure has a direct effect on the processing of PU and its final properties. There are four classes of polyols: polyether polyols, amine- terminated polyethers, polyester polyols and polycarbonate polyols. An explanation of the synthetization of polyols from these polymers is beyond the scope of this article. However, it is important to note that each PU will have differing characteristics, depending on the polyol entity.33 For instance, polycarbonate-derived polyols have high gloss and clarity and improved resistance to heat, hydrolysis, solvents and other chemicals when compared with the equivalent polyester polyol. The aliphatic and aromatic structures of polyols affect UV resistance.34 High concentrations of polar functional groups that can disperse in water enables the PU to be waterborne. Waterborne PUs are an increasingly important and highly versatile group of binders for inks, adhesives and various protective and decorative coatings. An almost endless variety of PU properties is achievable merely by altering the type and relative proportions or nature of the monomers.
32 Ibid. p.123. 33 Ibid. p.135. 34 Ibid. p.434-435
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2.1.3. Polyacrylates Polyacrylates are a family of thermoplastic polymers, synthesised from the esters of acrylic acid, and are widely employed for formulating consolidating and protective coatings due to their good adhesion, film-forming properties, transparency, hydrophobic character when dried and relative weathering stability. As with all polymers, the polyacrylate’s properties are determined in large part by the monomer’s chemistry.35 Acrylates containing a methyl group in the ester moiety are methyl acrylates, and those with an additional methyl group attached to an α-carbon are methyl methacrylates (Fig. 2.1.3.1). This addition results in a considerable difference in behaviour: poly(methyl acrylate) (PMA) is a white rubber at room temperature, while poly(methyl methacrylate) (PMMA) is a strong, hard, clear plastic known as plexiglass.36
Figure 2.1.3.1. Acrylate structures. Left, poly(methyl acrylate); Right, poly(methyl methacrylate). The addition of the CH3 group (methyl group) changes the polymer’s properties. Image credit: “Polyacrylate Basics” Polymer science learning center. Accessed May 20, 2020. https://pslc.ws/macrog/acrylate.htm.
Another example is replacing the methyl group in the acrylate with a butyl group, creating poly(butyl acrylate) (PBA), which completely changes the properties, resulting in PBA being liquid at room temperature. Adding a methyl group to PBA creates poly(butyl methacrylate) (PBMA), giving the polymer a rubbery consistency at room temperature (Fig. 2.1.3.2).
35 E. Princi. Handbook of Polymers in Stone Conservation. (Shrewsbury: Smithers Rapra, 2014). p. 211. 36 “Polyacrylate Basics”. Polymer science learning center. Accessed May 20, 2020. https://pslc.ws/macrog/acrylate.htm.
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Figure 2.1.3.2. Acrylate structures. Left, poly(butyl acrylate); Right, poly(butyl methacrylate). Image credit: “Polymers”. Polymer Processing. Accessed May 20, 2020. http://www.polymerprocessing.com/polymers.
PMMA, PMA, PBMA and PBA are employed for coatings, paints, inks and pressure- sensitive adhesives and are often copolymerised (to varying degrees) with other monomers such as styrene and acrylamide to modify their properties.37 Adhesive strength, for example, is increased by using monomers with low glass transition temperatures (Tg) such as butyl acrylate.38 Cohesive strength is usually imparted by harder acrylic monomers such as methyl methacrylate and methyl acrylate.39
2.1.4. Photostabilising additives in coatings The long-term performance of polymers in commercial products depends on the inherent resistance of the particular polymer binder and incorporated stabilising additives, the latter of which have differing chemical structures that interfere with the completion of the polymer binders’ degradation pathways.40 Photostabilising additives are divided into two groups: UVAs and HALS. The UVA additive group is divided into subcategories of inorganic and organic products. Since inorganic UVAs introduce colour to the coating, they are less likely to be employed in clear coats such as inkjet finish coatings and will therefore not be discussed.41,42
37 “Polyacrylates”. Polymer database. Accessed May 20, 2020. http://polymerdatabase.com/polymer%20chemistry/Polyacrylates.html 38 The glass transition temperature of a polymer is the average value in degrees Celsius representing a range of temperatures through which the polymer changes from a hard and often brittle material into soft, rubberlike properties 39 Princi. Handbook of Polymers. p. 46_1. 40 J. Jospíšil and S. Nešpurek. “Photostabilization of Coatings. Mechanism and performance”. Progress in Polymer science. 25. no.9. (2000). p. 1262, 1287. 41 Ibid. p. 1279. 42 Inorganic UVAs were not mentioned by the manufacturers of the six finish coatings, and testing for their presence is beyond the scope of the current study. Their presence in the researched coatings is optional.
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Organic UV absorbers Organic UVAs are colourless or nearly colourless compounds with high absorption coefficients in the UV region of the solar spectrum. These compounds protect coatings against light-induced damage by absorbing harmful radiation, thereby preventing bond breakage that can result in the creation of chromophores.43 Effective UVAs should have absorption coefficients in the wavelength range for which the entire coating system is most susceptible to photodegradation. UVAs therefore need to absorb radiation in the 290–400 nm region, considering the protection of binders, pigments and dyes.44 The most common choices of UVAs for finish coatings are BTZ and HTP, both of which consist of phenols (benzene rings bonded to a hydroxy group) bonded to triazole and triazine, respectively (Figs. 2.1.4.1 and 2.1.4.2). The protection mechanism of a UVA basically consists of transferring the absorbed radiation into less harmful thermal (or vibrational) energy through an excited-state intramolecular proton transfer, a photophysical process that involves ground-state and excited- state molecules. When it absorbs UV radiation, a UVA molecule becomes excited and transfers a proton from the benzene part to the nitrogen atom within the molecule through intramolecular hydrogen bonds. The proton then returns to its original position, thereby enabling the molecule to release thermal energy.45
Figure 2.1.4.1. Benzotriazole derivative functioning as a UV absorber. Also known as Tinuvin 1130. Composed of different benzene derivatives. Image credit: “PI-38052 Tinuvin-1130”. PICHEMICALS. Accessed June 7, 2020. http://internal.pipharm.com/catalog/PI-38052.html.
43 A chromophore is a region in a molecule where UV and visible light is absorbed, thereby exciting an electron from its ground state to an excited state, followed by a covalent bond break. Chromophores can already exist in a molecule or can be created after a bond cleavage. 44 Jospíšil and Nešpurek. Photostabilization of Coatings. p. 1279. 45 Ibid, 1285; For technical information on various UVA types, see: G. Wypych. ed. Handbook of UV Degradation and Stabilization. (Toronto: Chemtech publishing, 2011).
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Figure 2.1.4.2. HTP derivative functioning as a UV absorber. Composed of different benzene derivatives. Image credit: “Hydroxyphenyl-triazine”. ANC Chemicals. Accessed June 7, 2020. http://www.anc-chem.com/products- detail.php?product_id=5.
BTZ derivatives have absorption spectra in the 300–400 nm range, while HTP derivatives can absorb light in the 300–340 nm range. In general, the absorption profile depends on the molecule’s chemistry, MW and structure, which will determine the chemical reactions. The efficiency of UVAs to screen out UV radiation will also depend on the UVA concentration, its homogenous distribution in the binder and the film thickness. UVAs are less effective in protecting thin coating films and coating surfaces. To enhance the stabilising effect, the concentration or the film thickness needs to be increased. The former approach is mostly employed when the commercial coatings need high UVA levels (0.25–3%). UVAs from various groups are frequently blended due to their differing absorption ranges, thereby achieving better coverage for absorbing a wider spectrum.46,47
Hindered amine light stabilisers HALS are mainly derivatives of 2,2,6,6-tetramethylpiperidine (an example of which is shown in Fig. 2.1.4.3) and protect polymer coatings against photooxidative damage, mainly through the formation of nitroxide radicals, which subsequently consume damageing radical species in a process known as the Denisov Cycle. Although HALS have been employed commercially for many years, the mechanism by which nitroxides protect polymer coatings from photooxidative damage has been the subject of ongoing debate. The Denisov Cycle involves reactions that scavenge both polymeric and peroxyl radicals by nitroxides and their
46 C. Schaller et al. “Hydroxyphenyl-s-triazines: advanced multipurpose UV-absorbers for coatings”. Journal of Coatings Technology and Research. 5. (2008), p. 25-27. 47 Jospíšil and Nešpurek. Photostabilization of coatings. p. 1282-1284.
19 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints conversion to nonradical products, with subsequent regeneration of the nitroxide. The performance of HALS is measured by the amount of nitroxides consumed.48 Unlike UVAs, HALS activity is independent of polymer film thickness because it does not react to UV absorption but rather to the radicals formed in the binder when it degrades due to UV absorption, heat, pollutants and other agents of deterioration. However, HALS efficiency can be greatly affected by their MW, structure, side groups and surface conditions.49
Figure 2.1.4.3. Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, also known as Tinuvin 292. The compound is a HALS based on 2,2,6,6-tetramethylpiperidine (blue), ester (yellow) and a methyl backbone (orange). Image credit: “Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate”. Pubchem . Accessed June 10, 2020. https://pubchem.ncbi.nlm.nih.gov/compound/586744.
2.2. Photodegradation pathways of polymer binders Throughout their service life, coatings are affected by a variety of environmental influences, such as solar radiation, pollutants and temperature and humidity fluctuations. In addition to colour changes, these effects can lead to surface defects, such as cracking, delamination and loss of gloss. UV radiation constitutes approximately 1–5% of the solar
48 J. L. Hodgson and M. L. Coote. “Clarifying the Mechanism of the Denisov Cycle: How do Hindered Amine Light Stabilizers Protect Polymer Coatings from Photo-oxidative Degradation?”. Macromolecules.43. no. 10. (2010), p. 4574. 49 Jospíšil and Nešpurek. Photostabilization of coatings. p. 1304, 1307.
20 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints radiation; however, it is the most harmful type for most materials. Visible light (39–53% of solar radiation) photosensitizes chromophores and accelerates polymer degradation but over a longer period. The solar radiation that reaches the earth’s surface is characterised by wavelengths of approximately 295–2500 nm (Fig. 2.2.1). Solar radiation that reaches the earth is classified as UV-B (285–315 nm, with an energy of 380–426 kJ/mol),50 UV-A (315–400 nm, with an energy of 300–380 kJ/mol and less damageing for organic materials than UV-B), visible light (400–760 nm, with an energy of 300–170 kJ/mol) and infrared (760–2500 nm).51
Figure 2.2.1. Electromagnetic spectrum and the photon energy of each range. Image credit: V.V. Tuchin. “Tissue Optics and Photonics: Biological Tissue Structures”. Journal of Biomedical Photonics & Engineering. 1. no. 1. (2015). p. 9.
The energy of the radiation in the 285–760 nm wavelength range is transferred to electrons in a molecule, changing the molecule’s configuration by exciting the electrons from a ground state to an excited state, potentially splitting the molecule into two radical fragments. This process called chain scission and depends on the bond dissociation energy (BDE), i.e., the energy required to homolytically break a covalent bond in a molecule into two fragments. For instance, an energy of 350 kJ/mol is required to start a radical chain reaction in a certain C-C bond in a molecule, which is the amount of energy that UV-A radiation can introduce. When it comes to radiation absorbance, some functional groups act as chromophores and are more prone to absorb radiation, such as carbonyl groups (ketone, aldehyde, carboxylic acid) and hydrogen
50 Radiation can be expressed as kJ/mol, which expresses the amount of energy (kJ) imparted by a quantity of photons (mol) to a surface. Short-wavelength radiation (low number of nm) has high energy (large number of kJ/mol). 51 Jospíšil and Nešpurek. Photostabilization of coatings. p. 1264.
21 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints in an allylic position (hydrogen bonded to a carbon atom adjacent to C=C double bonds, to C=O carbonyl groups or bonded to tertiary carbon atoms).52,53 The reaction of the radicals created from a chain scission can continue through several pathways until stabilisation of the molecules occurs, also known as the termination phase.54 In one pathway, the single electron of a radical can form a double bond with a neighbouring radical. Over time, this can result in the formation of a conjugated system, a system of alternating single (C-C) and double (C=C) carbon bonds. Like carbonyls and hydrogens in allylic positions, conjugated systems are chromophores, which absorb UV and visible light radiation and cause further chain scission of the polymer until the termination phase is reached. The absorption of solar radiation by chromophores results in yellowing.55 Instead of creating a conjugated system, the molecule fragments can bond with other materials in the polymer matrix, creating a cross-linked system that results in a rigid insoluble substance.56 The main photodegradation pathway is photooxidation, where oxygen bonds with radical fragments and creates peroxides or hydroperoxides until the molecules are stabilised, leading to the breakup of the polymer into smaller molecules. All degradation pathways lead to physical changes in the polymer, and can be observed as colour deviation, embrittlement and changes in gloss.57 This section will explain the mechanisms by which UV and visible light energy is absorbed and affects polymer binders, using three examples for the six researched inkjet finish coatings: PEG-PVAL, polyester-urethane and acrylates and methacrylates. Along with the degradation paths of chain scission, oxidation, chromophore creation and cross-linking, this section will discuss the rate of polymer degradation as a function of its structure and wavelength absorbance. The contributions of photostabilising additives and their behaviour in polymer binders would be addressed, as well as the significance of these materials for inkjet prints. 2.2.1. Polyethylene glycol-polyvinyl alcohol PEG-PVAL is a copolymer present in inkjet finish coating #02 and most likely consists of PEG backbone and grafted PVAL moieties with an aliphatic structure. When the copolymer absorbs UV radiation, it begins to break apart and form radicals. In the PEG backbone, primary
52 E.R. de la Rie. “Polymer Stabilizers. A Survey with Reference to Possible Applications in The Conservation Field”. Studies in Conservation. 33. no. 1. (1988). p. 10. 53 R. J. Ouellette and J. D. Rawn. Organic chemistry: structure, mechanism, and synthesis. (US: Elsevier, 2014). p. 89. 54 Chain termination occurs when two free radical species react with each other to form a stable, non-radical adduct. 55 Ouellette and Rawn. Organic chemistry. p. 385-386. 56 Cross linking can occur independently of chain scission. 57 V. Horie. Materials for Conservation. (UK: Elsevier, 2010). 2nd ed. p. 40-41.
22 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints radicals can form from the abstraction of hydrogen from a C-H bond.58 Once the hydrogen leaves the molecule, the carbon atom can bond with oxygen (O2). In the PVAL part, a specific C-H bond can break, and oxidation can occur (Fig. 2.2.1.1). However, there is a difference in the ability of each C-H bond to break, where the C-H bonds in the PEG will break relatively easily.59 Another type of bond that can break over time is the C-C in either the PEG or PVAL, depending on the bonds BDE (Fig. 2.2.1.2).60,61
[1]
[2]
Figure 2.2.1.1. PEG-PVAL possible degradation path: Oxidation. The C-H bond in the PEG backbone will break before [1] the C-H bond in the PVAL [2]. Image credit: Author.
Figure 2.2.1.2. PEG-PVAL possible degradation path: Oxidation. The chain scission in the C-C bond creates smaller molecules. Image credit: Author.
58 Jospíšil and Nešpurek. Photostabilization of coatings. p. 1272 59 Y. Lou. Comprehensive Handbook of Chemical Bond Energies. (US: Taylor and Francis group, 2007). p. 71, 74. 60 Borgioli and Cremonesi. The Synthetic Resins. p. 96. 61 Horie. Materials for Conservation. p. 144.
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The degradation path of PEG-PVAL due to UV radiation exposure will involve only one degradation path: chain scission followed by oxidation. According to the literature, PVAL is stable with respect to radiation62, and therefore chain scission of the PEG backbone is the most likely process. This degradation results in a decrease in the coating’s protective and aesthetic qualities. Aliphatic structures are relatively stable, and it is hard to predict the energy and time required to break the polymer into smaller molecules until reaching a failure of the polymer binder. This failure will depend on the polymer’s structure, the location of the carbon atoms (which influence the BDE) and the MW (which is reflected by the chain length).
2.2.2. Polyester urethanes PU resistance to solar radiation, especially at UV-A wavelengths, is relatively low, due mainly to the fact that most urethanes contain benzene rings that are, in fact, conjugated systems that act as chromophores and cause yellowing. Changes in the coating formulation can therefore be performed, such as the use of aliphatic isocyanates and aliphatic polyols.63 Polyester urethane, which might be present in inkjet finish coating #05, is a large family of polymers. The two polyester urethanes (aromatic and aliphatic) discussed in this section might not be the same as in inkjet finish coating #05. Scientists from the Eindhoven University of Technology and the Dutch Polymer Institute investigated the photodegradation of one type of aromatic polyester urethane coating. Several articles were published on the various aspects of degradation, such as coating thickness as a function of degradation and on the degradation rate as a function of the amount of energy.64 The researched coating is composed of polyester poly(neopentyl isophthalate), which is the polyester moiety, and hexamethylene diisocyanurate trimer, which is the isocyanate moiety, also referred to as the urethane moiety. This specific polymer has two benzene rings and several carbonyl groups that act as chromophores and excite when absorbing photons (Fig. 2.2.2.1).
62 Horie. Material for Conservation. p. 144. 63 S. Rossi et al. “Accelerated weathering and chemical resistance of polyurethane powder coatings”. Journal of Coatings Technology and Research. 13. (2016). p. 427-428. 64 For further information, see the articles by Koen N. S. Adema.
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Figure 2.2.2.1. A type of polyester urethane clear coat consisting of hexamethylene diisocyanurate trimer isocyanate (yellow) polyester moiety (blue) Image credit: H. Makki et al. “Quantitative spectroscopic analysis of weathering of polyester- urethane coatings”. Polymer Degradation and Stability. 121. (2015). p. 281.
Polyester moiety photodegradation
According to Quantitative spectroscopic analysis of weathering of polyester-urethane coatings (H. Makki et al, 2015) a bond can break in three different areas: C-O bonds (Fig. 2.2.2.2, reactions a and b) and C-C-bonds (Fig. 2.2.2, reaction c).65
Figure 2.2.2.2. Possible covalent bond cleavage of polyester moiety in polyester urethane clear coat. Image credit H. Makki et al. “Quantitative spectroscopic analysis of weathering of polyester-urethane coatings”. Polymer Degradation and Stability. 121. (2015). p. 282.
The radicals resulting from chain scission can undergo hydrogen abstraction and oxidation, as can occur in the PEG backbone. Oxidation results in peroxide radicals (Fig. 2.2.2.3, reaction a), which can undergo further hydrogen abstraction reactions, giving rise to
65 H. Makki et al. “Quantitative spectroscopic analysis of weathering of polyester-urethane coatings”. Polymer Degradation and Stability. 121. (2015). p. 281.
25 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints polymer hydroperoxides (Fig. 2.2.2.3 reaction b). However, in the case of aromatic structures, the chromophores add another level of degradation. An excited chromophore can transfer energy to a hydroperoxide group that is eventually decomposed. This radical can undergo another hydrogen abstraction reaction or form a carbonyl group (Fig. 2.2.2.3, reaction c). In the termination step, two radicals recombine to create a different molecule.66
Figure 2.2.2.3. Creation of radicals in the polyester moiety of polyester urethane clear coat. Image credit: H. Makki et al. “Quantitative spectroscopic analysis of weathering of polyester- urethane coatings”. Polymer Degradation and Stability. 121. (2015). p. 282.
Urethane moiety photodegradation
According to H. Makki et al, the polyester moiety hardly absorbs photons at wavelengths shorter than 300 nm. The chemical moiety most sensitive to weathering in aerobic conditions is the urethane group. The oxidation likely starts with hydrogen abstraction near the nitrogen atom by other radicals, followed by hydroperoxide formation. After the urethane concentration is depleted, ester bond scission is accelerated by the increase in the coating’s optical absorptivity, which is most likely due to the formation of new chromophores.67 The urethane moiety’s sensitivity over that of the polyester moiety was confirmed in studies on aliphatic structures of polyester urethanes. In Infrared analysis of the photochemical behaviour of segmented polyurethanes: Aliphatic poly(ester-urethane) (Wilhelm and Gardette, 1997), the authors found that photon absorption leads to the formation of radicals at the carbon
66 Ibid. p. 282. 67 Ibid. p. 290-291.
26 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints atom that is bonded to the nitrogen atom in the molecule, followed by oxidation (Fig. 2.2.2.4).68 The only difference between this aliphatic urethane and the aromatic is that degradation is not induced by chromophoric groups and is therefore slower.
Figure 2.2.2.4. Photooxidation of the aliphatic urethane moiety in polyester urethane coating. Image credit: C. Wilhelm and J-L. Gardette. “Infrared analysis of the photochemical behaviour of segmented polyurethanes: 1. Aliphatic poly(ester-urethane)”. Polymer. 38. no. 16. (1997). p. 4026.
In this example of polymer degradation pathways, the aromatic and aliphatic structures of polyester urethane are represented. Thus, polymers containing chromophoric sites (benzene rings and carbonyls) can affect the rate and reactions that lead to degradation differently. Here, the presence of chromophores leads to faster degradation, causing more excited sites in the polymer. However, it is important to stress that this process would not necessarily happen in the inkjet finish coatings researched for this study.
2.2.3. Polyacrylates Polyacrylates were present in three of the six inkjet finish coatings investigated in this study. According to the material analysis, finish coating #03 contain either PMA, PMMA, PBA or PBMA copolymer. Coating #05 contain styrene and PMMA-BA copolymer. According to the manufacturer’s information, coating #06 contain acrylic copolymers. Since the manufacturers of coatings #03 and #06 provided vague information on the exact acrylates present, this section will discuss the different degradation mechanisms of the two groups: acrylates (PMA, PBA) and methacrylates (PMMA and PBMA) (Fig. 2.2.3.1). Acrylic polymers can be made from a mixture of a wide variety of monomers. An explanation of the degradation mechanism of copolymers with different types of monomers is beyond the scope of this study. Given that styrene- acrylate was detected in coating #05, however, its degradation will be discussed later in this section.
68 C. Wilhelm and J-L. Gardette. “Infrared analysis of the photochemical behaviour of segmented polyurethanes: 1. Aliphatic poly(ester-urethane)”. Polymer. 38. no. 16. (1997). p. 4026.
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The photostability of acrylate and methacrylate polymers is generally high because they have an aliphatic structure, and the carbonyl ester groups in the polymers are relatively insensitive to photodegradation, with most of the photodegradation occurring in the UV-A spectrum. Both groups can show either chain scission and oxidation or chain scission and cross- linking. Chiantore et al. (2000) studied the degradation pathways of acrylates and methacrylates and showed that, in general, both groups are prone to undergo photooxidation (Fig. 2.2.3.2). As with all polymers, the photooxidation rate depends on the wavelength absorbed. At wavelengths longer than 300 nm, methacrylates appear to be more stable than acrylates, and chain scission takes longer. Acrylates also undergo random chain scission in visible light (longer than 400 nm), forming carbonyls that act as chromophores that subsequently cause further chain scission. In certain circumstances, cross-linking is more evident in methacrylates. When side chains are long, there is a tendency for cross-linking. When absorbing photons below their Tg, methacrylates are more prone to undergo photooxidation. When absorbing photons above their Tg, methacrylates will undergo cross-linking. The presence of a butyl group (as in PBMA) can also cause cross-linking (Fig. 2.2.3.3). The side chains in the polymer are oxidised and produce radicals, resulting in unstable side chains that are highly mobile and that can bond with other materials in the polymer matrix.69
Figure 2.2.3.1. Different acrylates with aliphatic structures. Image credit: “Poly(methyl acrylate) solution“. Sigma- Aldrich. Accessed June 26, 2020. sigmaaldrich.com/catalog/product/aldrich/182214?lang=en®ion=US; “Poly(methyl methacrylate)”. Sigma-Aldrich. Accessed June 26, 2020. sigmaaldrich.com/catalog/substance/polymethylmethacrylate12345901114711?lang=en®ion=US; “Poly(butyl acrylate) solution”. Sigma-Aldrich. Accessed June 26, 2020. sigmaaldrich.com/catalog/substance/polybutylacrylatesolution12345900349011?lang=en®ion=US; “Poly(butyl methacrylate)”. Sigma-Aldrich. Accessed June 26, 2020. sigmaaldrich.com/catalog/substance/polybutylmethacrylate12345900363811?lang=en®ion=US
69 O. Chiantore et al. “Photooxidative degradation of acrylic and methacrylic polymers”. Polymer. 41. (2000). p. 1657-1658, 1658-1663.
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Figure 2.2.3.2. Photooxidation pathway of poly(methyl acrylate). Image credit: O. Chiantore et al. “Photooxidative degradation of acrylic and methacrylic polymers”. Polymer. 41. (2000). p. 1661.
Figure 2.2.3.3. Cross linking of poly(butyl methacrylate). Image credit: O. Chiantore et al. “Photooxidative degradation of acrylic and methacrylic polymers”. Polymer. 41. (2000). p. 1667.
Styrene-acrylate
Copolymers are linear chains of several alternating types of polymers. In the case of styrene-acrylate, a styrene containing a benzene ring bonds with acrylate in the same polymer chain (Fig. 2.2.3.4). The benzene ring, acting as a conjugated system and therefore a chromophore, causes faster photodegradation than the aliphatic acrylates and copolymers made from just acrylates.
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Figure 2.2.3.4. Structure of styrene-butyl acrylate copolymer. Image credit: B.V. Hassas and F. Karakaş. The Usage of Sodium Bentonite in Styrene Butyl Acrylate Composites. (2013). Accessed June 26, 2020. https://www.researchgate.net/figure/Structure-of-styrene-butyl-acrylate-copolymer_fig1_258048351.
2.2.4. Incorporation of light stabilisers and their effect on polymer binders The incorporation of UVAs or HALS can improve the polymer binder’s life expectancy. Despite the advantages of UVAs and HALS, their protection involves self-sacrifice, and they too undergo degradation. The filtering effect of UVAs is expressed by the amount of light that is not stopped by the UVA but instead reaches the substrate. The percentage of transmitted light is related to the consumption coefficient and concentration of the UVA in the coating and to the coating thickness. UVA molecules deeper within the coating experience lower light intensity than those higher up due to UV radiation absorption by the UVA molecules near the coating’s surface.70 The UVA is more readily consumed at the coating’s surface where light intensity is highest. The loss of UVA during the coatings’ service life is the result of chemical and physical mechanisms. The chemical-related loss is a function of the UVA’s inherent photostability in which the photochemical degradation is caused by the chromophores. The physical-related loss is due to evaporation, migration and leaching and is mainly influenced by the molecule’s MW, structure and polarity, which affect solubility.71,72 UVAs alone are inefficient in protecting the coating's surface against surface defects under exterior weathering conditions. Studies have shown that after artificial ageing, colour deviation is possible in specific BZT and HTP derivatives. In the study Long-Term Weathering
70 Jospíšil and Nešpurek. Photostabilization of coatings. p. 1283. 71 C.M Suebert et al. “Long-Term Weathering Behavior of UV-Curable Clearcoats: Depth Profiling of Photooxidation, UVA, and HALS Distributions”. Journal of Coatings Technology and Research. 2. no. 7. (2005). p. 535. 72 C. Schaller et al. “Organic vs inorganic light stabilizers for waterborne clear coats: a fair comparison”. Journal of Coatings Technology and Research. 9. no. 4. (2012). p. 434.
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Behavior of UV-Curable Clearcoats: Depth Profiling of Photooxidation, UVA, and HALS Distributions (C.M. Suebert et al, 2005), it was shown that the surface of samples containing only UVA were photooxidised more rapidly than the surfaces with only HALS. UVAs are therefore combined with HALS that effectively scavenge free radicals at the coating’s surface, thereby preventing surface defects.73,74 HALS are also prone to physical loss due to evaporation, migration and leaching. By creating radicals that scavenge photooxidation products from the polymer binder, HALS can introduce certain yellowing.75 In the chemical loss of HALS, the derived nitroxides absorb light in the 300–320 nm range and form excited states that cause chain scission. Another form of loss of HALS is when the nitroxides going through hydrogen abstraction and cause the formation of different smaller molecules.76 The MW, structure, chemistry, compatibility with the binder and spectral absorption influence the life expectancy of light stabilisers. When more radicals are created, more light stabilisers are consumed, leading to faster degradation of the polymer binder. For example, light stabilisers in aromatic urethane will be consumed faster than in aliphatic acrylates. As stated earlier, combining stabilisers is highly beneficial for polymers and stabilisers: the effect of both stabiliser types is complementary. HALS lowers the degradation rate of UVAs by the same mechanism that is activated for polymer binders, and UVAs prevent chain scission of HALS caused by UV and light absorption.77
2.3. Discussion: the ageing behaviour of weathered finish coatings on inkjet prints When exposed to UV and visible light radiation, polymer binders (the basic components of coatings) change the function and appearance of the coating through cracking, gloss loss, colour change, delamination and other forms of damage. The light stability of polymer binders varies widely and depends on the binders’ structure and composition. In general, polymers without inherent chromophores do not easily absorb light in the 295–400 nm range,78,79 whereas
73 Suebert. Long-Term Weathering. p. 534. 74 Schaller. Organic vs inorganic. p. 434. 438. 75 Y. Zhang et al. “Study on photostabilization in situ of reactive hindered amine light stabilizers applied to UV- curable coatings”. Journal of Coatings Technology and Research. 9. (2012). p. 462-463, 465. 76 Jospíšil and Nešpurek. Photostabilization of coatings. p. 1306-1308. 77 Ibid. p. 1296-1297, 1317. 78 Ibid. p. 1270. 79 “Photodegradation of Polymers”. Polymer database. Accessed June 5, 2020. https://polymerdatabase.com/polymer%20chemistry/Photo%20Oxidation.html
31 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints those with weak bonds and high concentrations of chromophoric groups are highly sensitive to photodegradation. This chapter introduced three polymer binders and their degradation pathways. PEG- PVAL is a relatively stable polymer due to its aliphatic structure, absence of chromophores and the strong bonds of the PVAL grafted chains. The PEG backbone is more prone to undergo the photooxidation pathway. The same mechanism was also found to be dominant in aliphatic polyurethanes, acrylates and methacrylates. Polyester urethane shows degradation of either aromatic or aliphatic structures. Aromatic groups act as chromophores and increase the coating’s sensitivity to photooxidation, a similar result found in styrene-acrylate copolymers. Various methacrylates show not only chain scission and oxidation pathways but also the possibility of cross-linking such as in PMMA and PBMA. The degradation rate of polymer binders can vary and depends on the monomers’ structure, MW, absorbed spectrum and time irradiated. To slow the degradation rate, two classes of photostabilisers may be employed: UVAs (e.g., benzotriazoles and hydroxyphenyl triazines) and HALS. UVAs protect the coatings from photooxidation by filtering the harmful radiation that starts the formation of free radicals, while HALS reduce gloss loss, yellowing and cracking by scavenging radical peroxides, hydroperoxides and carbonyl groups. A proper selection of stabilisers and combinations thereof that fit the type of binder ensures the highest possible long-term performance. However, the stabilising effect depends on the stabilisers’ self- consumption. The ability to protect the binders is due to the sacrificing of stabilisers while undergoing chain scission and oxidation instead of damageing the polymer binders. These additives can also disappear from the coating due to evaporation. The additives’ performance therefore depends on their concentration (to compensate for high loss rates), inherent resistance, compatibility with components of the polymer matrix and the environmental conditions. When discussing the six researched finish coatings for inkjet prints, it should be considered that even at stable temperatures and relative humidity with only UV and visible light radiation, it is difficult to predict and compare degradation rates. For instance, PUs with aromatic structures are logically most likely to rapidly degrade, unless the amount of light stabilisers is sufficient to slow chain scission, as compared with methacrylate polymers that are aliphatic and less prone to degradation but might contain an insufficient amount of light stabilisers. Another example is the type of light stabiliser employed. HTP derivatives are known to be more durable than BZT derivatives, and a combination of UVAs and HALS provides optimal protection against radiation. However, each coating can have different blends of light
32 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints stabilisers. Differing solar absorption ranges and durations can therefore cause different stabiliser consumption rates, resulting in differences in the degradation rate for polymer binders. The finish coating’s performance will have a direct effect on an inkjet print. The degradation pathways and rate can interfere with colour saturation, contrast and other aesthetic parameters. The degradation products can also react with other materials in the paper and ink, causing further changes in appearance. Inkjet finish coatings will eventually break and lose their protective qualities, causing cracking, gloss loss, colour change, chalking, blistering and delamination. The issue is therefore whether the coating prolongs the life expectancy of inkjet prints, thereby providing a significant advantage, or causing more harm than good in the long term.
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3. Methodology The study’s methodology is divided into the theoretical and the practical. The theoretical portion of the study involved a literature search and is presented in the second chapter. To better understand the use, material content and light stability of coatings and since no research has yet been conducted on inkjet finish coatings, the study began by consulting several conservation and research professionals and sending questionnaires to manufacturers. The practical portion of the study was conducted simultaneously and included material analysis using GC-MS, artificial weathering using Xenontest and, lastly, colour measurements.
3.1. Questionnaires and correspondences: manufacturers, conservators and scientists Questionnaires for manufacturers: Hahnemühle, Rauch and Breathing Colors
Questionnaires were submitted to each of the three manufacturers80 who produce inkjet finish coatings, either for paper or canvas. The questionnaires and their responses are available in appendix II. The manufacturers were asked for further material content information not available to customers on light fastness tests and the typical use of their products. Julien Lagaye, a specialist in Hahnemühle Digital FineArt media, represented Hahnemühle France. Unfortunately, he was unable to provide further information on material content other than that stated on the products’ label. Regarding the testing of light fastness, Lagaye stated that both finish coatings (for paper and canvas inkjet prints) were tested but did not provide information on the test method. Additional communication with Stefan Neumann, Manager Technical Support in Hahnemühle Germany stated that the tests were performed by Wilhelm Research Institute, Laboratoire national de métrologie et d'essais in France and inhouse. The coating for paper prints (coating #01) was tested on Hahnemühle FineArt paper. With regard to the frequency of use of the finish coatings, Lagaye stated that Hahnemühle Varnish (coating #03) for canvas is rarely used.81,82
80 Rauch is not a manufacturer but rather a distributor. The manufacturer of Rauch spray for paper is (coating #02) unknown, while Marabu manufactures the liquid varnish for canvas (coating #05). The connection with Marabu was unsuccessful due to unavailable contact person. Nevertheless, Rauch’s representative attempted to answer the questionnaire as much as possible. 81 "Hahnemühle Coating Products." Questionnaire. J. Lagaye. Digital FineArt Specialist at Hahnemühle France. January 28, 2020. 82 “Questions regarding Hahnemühle products”. Email to S. Neumann Manager Technical Support at Hahnemühle Germany. July 7, 2020.
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Thomas Wöhrstein, an export manager and trained photographer, represented Rauch in Germany83 but unfortunately was unable to provide information on the materials’ content other than that stated on the product’s label. Regarding the testing of light fastness, Wöhrstein stated that Schutzlack Firnis für Fine Art Papiere (coating #02) was tested by the Wilhelm Research Institute and that ClearShield™ Type C (coating #05) was also tested but did not indicate by whom. With regard to the frequency of use of Rauch finish coatings, Wöhrstein stated that the Fine Art paper spray is not used much, while ClearShield™ Type C is more often employed. Ashley N. Love, a customer success representative for Breathing Color in Austin, Texas, answered the questionnaire on behalf of the company.84 With regard to the materials’ content, Love referred to the MSDS and technical data sheet posted on the company’s website; however, the information makes no mention of light stabilisers. In an email correspondence, Love stated that the company does not share this information. With regard to the testing of light fastness, Love stated that the Timeless Varnish (coating #06) was tested for light fastness according to the Blue Wool standard and is certified by the Fine Art Trade Guild for lightfastness. Glamour II (coating #06), however, was not tested for light fastness. In terms of the frequency of use, Love stated that the Timeless Varnish has twice the sales of Glamour II.
Correspondence with scientists and conservators in the field of photograph conservation
To determine the usage patterns of finish coatings for inkjets and their relevancy in the conservation field, specialists were interviewed on these topics via email. In a correspondence with Daniel Burge,85 a senior research scientist at the Image Permanence Institute (Rochester, NY) and College of Art and Design (Rochester, NY), regarding areas in the inkjet print industry that lacked research, he suggested investigating the degradation of binders in inkjet coatings.
Katrin Pietsch, a lecturer on photograph conservation at the University of Amsterdam and the former head of the conservation lab at the Nederlands Fotomuseum in Rotterdam, mentioned that she had encountered few coated inkjet prints. However, a number of photograph conservators such as Clara von Waldthausen (a lecturer on photograph conservation at the
83 “Information zu Ihren Finishing Produkten”. Questionnaire. T. Wöhrstein. Export Manager at Rauch. January 15, 2020. 84 “Questionnaire from an inkjet researcher”. E-mail to A.N. Love. Customer Success Representative. April 15, 2020. 85 “Master thesis on inkjets”. E-mail to D. Burge. Senior Research Scientist at the Image Permanence Institute and College of Art and Design. November 8, 2019.
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University of Amsterdam and a private photograph conservator) and other leading conservators in renown museums stated that they rarely or never encountered coated inkjet prints.86
3.2. Experimental Given the lack of research specifically on the ageing behaviour of inkjet finish coatings, practical research would provide value and complimentary knowledge to the fields of photography and conservation science. The experiment was designed in several steps: 1) Samples were created by coating unprinted and printed fine art paper with six different inkjet finish coatings. 2) Prior to ageing, a material analysis using GC-MS was performed to determine the materials present in each coating. 3) the samples were inserted into a Xenontest for artificial UV and visible light ageing. 4) Colour measurements were performed to determine the degree to which the photodegradation affected each sample’s colour change.
3.2.1. Sample creation Four sets of samples were created by Ryan Boatright at the Atelier Boba printing studio in Paris. The chosen substrate was Hahnemühle Photo Rag® 308 gsm, cut to a page size of 13 cm by 4.5 cm.
Set A: Unprinted coated samples for step ageing (Fig. 3.2.1.1)
Each specimen was fully coated with the following finish coating for inkjet prints: 1. Protective Spray by Hahnemühle (for inkjets on paper). 2. Schutzlack Firnis für Fine Art Papiere by Rauch (for inkjets on paper). 3. Glamour II matte by Breathing Color (for inkjets on paper). 4. Protective Varnish by Hahnemühle (for inkjets on canvas). 5. ClearShield™ Type C matte by Rauch (for inkjets on canvas). 6. Timeless by Breathing Color (for inkjets on canvas). 7. Control: unprinted and uncoated piece.
86 C. von Waldthausen. Photograph conservation lecturer at the University of Amsterdam, Interview by author. November 12, 2019; “Request for help from a photo conservation student”. E-mail to L-A. Daffner. Conservator of Photographs at MoMA and Andrew W. Mellon Foundation. November 13, 2019; “Request for help from a photo conservation student”. E-mail to Nora Kennedy. Photograph Conservator at the Met Museum November 13, 2019.
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Set B: Unprinted coated samples as controls (not aged)
This set was identical to Set A.
Figure 3.2.1.1. Illustration of sets A and B. Set A was employed for step weathering, and Set B was employed for comparison with Set A. Image credit: Author.
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Set C: Printed samples fully coated for ageing (Fig. 3.2.1.2)
Each specimen was coated fully with the six finish coatings for inkjet prints, with the addition of a control (printed and uncoated inkjet).
Figure 3.2.1.2. Illustration of Set C. Image credit: Author.
Figure 3.2.1.2. Illustration of Set C. Image credit: Author.
Set D: Half-coated printed samples for ageing (Fig. 3.2.1.3)
Each specimen was half coated with the six finish coatings for inkjet prints. This set was only used for easier comparison by the naked eye and colour measurements were not conducted on it after ageing.
Figure 3.2.1.3. Illustration of Set D. Image credit: Author.
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The coatings were applied systematically to avoid different thicknesses as much as possible. All samples of each coating were applied at once, e.g., two unprinted and two printed samples were sprayed at once using coating #01. The same procedure was performed with coating #02. Two unprinted and two printed samples were coated by roller using liquid coating #03. The same procedure was performed with liquid coatings #04, #05 and #06. The samples for Set D were half covered during the application of the coating.
3.2.2. Material analysis using gas chromatography mass spectrometry Ing. Saskia Smulders, MA, PD res. and Henk van Keulen, scientists from the Cultural Heritage Agency of the Netherlands (Rijksdienst voor het Cultureel Erfgoed, RCE) performed the THM-GCMS, an analytical method in which a gas phase component mixture is separated into individual components that are detected and identified. Appendix V provides a research report with further information on the technique and conditions used during the analysis. Due to time limitations in the wake of the COVID-19 pandemic, only coatings #02, #03 and #05 were analysed. There is no further information on the material content for coatings #01, #4 and #06 other than that mentioned on the products’ label. It was also not possible to fully identify the three analysed coatings and further analysis needs to be conducted to get better defined results.
3.2.3. Artificial UV and light ageing using Xenontest The Xenontest device provides the best available simulation of full-spectrum sunlight (UV, visible, and infrared). In this study, the artificial UV and visible light weathering test was conducted by exposing samples in an Atlas Xenontest 440 weathering instrument under the supervision of Drs Agnes Brokerhof, a senior scientist at RCE. The samples were exposed to a radiation range of 320–800 nm, with a test chamber temperature of 40 °C and a relative humidity of 40%. The irradiance intensity of the xenon lamp was 50 W/m2 using a window glass filter. The unprinted samples were exposed and covered in five steps representing the increasing dose of UV and visible light (Fig. 3.2.3.1). The fully coated printed samples were half covered with cardboard wrapped with aluminium foil before ageing and withdrawn after the final step of 121 megalux hours and 181,414 kJ/m2 UV light. The half-coated printed samples were not covered and were withdrawn after the final step (Fig. 3.2.3.2). The spectral distribution contains a significant amount of UV radiation (300-400 nm). The UV content is 0.4 mW/lumen and can be expressed as kJ/m2, i.e., the total amount of UV energy on a surface. Photography test standards specify the use of lux (the total amount of
39 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints visible light energy on a surface) combined with hours as a means for quantifying dosage. Given that significant photodegradation resulting from exposure to the short-wave UV region can go undetected when only lux hours are employed, both spectrum components will be mentioned.87
SET (A) Unprinted samples: step ageing
Figure 3.2.3.1. Set A samples aged in the Xenontest aged in five steps with building up dosage of UV and visible light radiation. Image credit: Author.
87 F. Ligterink, RCE. Telephone conversation with author. July 1, 2020.
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SET (C) Printed samples: fully coated, half covered (left)
Figure 3.2.3.2. Set C samples aged in the Xenontest. Half of each sample was covered to compare colour changes before and after ageing. Image credit: Author.
3.2.4. Colour measurements Characterisation of colour change is subjective and often results in poor colour judgement because individuals can interpret colour differently, and the words used to describe colour can vary. A more accurate and consistent approach for evaluating colour differences is to use a colour measurement instrument, such as a spectrophotometer that separates the visible spectrum into intervals that mathematically simulate the eye’s colour response. For the colour measurements, this study employed the CIELAB colour space model, which employs three chromatic coordinates (L*, a* and b*), with L* representing lightness from dark (0) to light (100), a* representing colour from green (-60) to red (+60) and b* representing colour from blue (-60) to yellow (+60). The colour measurements were performed using a Konica Minolta CM-2600d spectrophotometer with standard illuminant D65 and 10 circular illumination, with specular component excluded to eliminate gloss. The measuring area was 0.3 cm2, and each measurement was performed at a different position for each sample to avoid defects and dirt and ensure comparable results as much as possible.
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L*a*b* values were taken from overall 94 areas. The colour variations in Set A before and after every weathering step are expressed as the overall colour difference (ΔE*) given in the CIE L*a*b* system by the following equation: ΔE*= √ (ΔL*2) +(Δa*2) + (Δb*2) Gradual colour changes were measured in each sample of Set A using the ΔE* values for each step. Overall colour differences were also employed for set C. Measurements were taken in the saturated yellow, magenta, cyan and black of the coated samples of coated covered areas (represent ‘before ageing’) and coated exposed areas (represent ‘after ageing’ of approximately 121 mlx h) and compared with the uncoated printed and exposed area of the control to determine the colour change in the aged coated printed samples and to estimate the protective performance of the aged coating compared with an uncoated print (see appendix III for the colour measurements plan). Colour changes noticeable by the naked eye were determined when ΔE* is ≥1.5.
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4. Results and Discussion
The section covers the results of the THM-GCMS material analysis of coatings #02, #03 and #05 and the colour measurements of sets A, B and C. The results of the colour measurements were combined with the findings in the material analysis and the theoretical research that was conducted and shown on the second chapter.
4.1. Material Analysis (THM-GCMS) See appendix V for the material analysis results and the interpretation by Ing. Saskia Smulders, MA, PD res. and Henk van Keulen (RCE). See appendix I for the overall material content results and the manufacturers’ statements and remarks.
4.2 Colour Measurements Colour measurements were performed on Set A, which included samples with six inkjet finish coatings on Hahnemühle Photo Rag® fine art paper (coating-paper system) and one uncoated plain paper (control). Set A was aged in five steps, and the results after each step were compared with its matching sample in Set B, which included coated unaged samples. The results are presented in Figure 4.2.1 and Table 4.2.2, and the noticeable outcome is shown in Figure 4.2.3. Colour measurements were performed on Set C, which included samples with six inkjet finish coatings on printed Hahnemühle Photo Rag® fine art paper (coating-ink-paper system) and one uncoated printed paper (control). These samples were half exposed to maximum ageing of approximately 121 mlx h and 181,414 kJ/m2, and each ink was compared with the respective uncoated half-exposed ink on paper. The results are presented in Figure 4.2.4 and Table 4.2.5, and the noticeable outcome is shown in Figure 4.2.6.
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Results Sets A and B: Aged and unaged coatings on plain paper
10 9 Control 8 Sample #1 7 Sample #2 6
5 Sample #3 E*
Δ 4 Sample #4 3 2 Sample #5 1 Sample #6 0 0 ~0.8 Mlx h and ~3 Mlx h and ~17.5 Mlx h and ~57.5 Mlx h and ~121 Mlx h and 122kJ/m2 4705 kJ/m2 26,616 kJ/m2 86,860 kJ/m2 181,414 kJ/m2
Visible light and UV dose
Figure 4.2.1. Comparison of colour change rates of six coated papers and one uncoated aged paper. The image above shows the trend of colour build-up over time. Image credit: Author.
Ageing step 1 2 3 4 5 6 Visible light 0 ~0.8 ~3 ~17.5 ~57.5 ~121 dose, Mlx h UV dose, kJ/m2 0 122 4705 26,616 86,860 181,414 Samples set (A) Uncoated paper 0 0.35 0.77 1.49 2.77 3.45 (Control) ΔE* 0 0.24 1.56 1.90 2.39 3.34 Sample #1 ΔE* 0 0.07 1.37 1.60 2.50 2.86 Sample #2 ΔE* 0 0.75 1.87 4.16 4.26 5.67 Sample #3 ΔE* 0 0.84 2.62 8.92 8.19 8.74 Sample #4 ΔE* 0 0.16 0.99 0.94 1.05 0.73 Sample #5 ΔE* 0 0.30 3.62 3.87 6.11 9.47 Sample #6
Table 4.2.2. Comparison of colour change rates of six coated papers and one uncoated aged paper. The table represents the numerical change in ΔE* values. Colour changes noticeable to the naked eye were determined when ΔE* >1.5.
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Figure 4.2.3. Six coated papers and one uncoated paper that were step aged (top) compared with their respective unaged coated papers and uncoated paper (bottom). Image credit: Author.
• Control (Uncoated paper): There is a noticeable colour change after the fourth step.
• Sample #01: There was a noticeable colour change after the third step, signifying a chemical reaction causing the coating-paper system to change colour faster than if there were no coating. However, at the maximum dose, there was not much change in the ΔE* of the control (3.45) and the ΔE* of Sample #1 (3.34), which could mean that, at some point, the chemical reaction that causes the colour change slows or that the control’s colour change accelerates.
• Sample #02: There was a noticeable colour change after the fourth step, as in the control. From this point onwards, however, the sample appeared to experience slower colour change. In the sixth step, there was less build-up of change (2.86 vs 3.34 in sample #01 and 3.44 in the control uncoated paper).
• Sample #03: There was a noticeable colour change after the third step. The change built up quickly towards the fourth step, then slowed toward the fifth step and then rapidly increased. At the maximum dose, the result was 5.67 compared to 3.44 in the uncoated paper.
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• Sample #04: There was a noticeable colour change after the third step. This sample apparently reacted with the aluminium foil used to cover the samples, because the sample adhered to the foil when the sample was withdrawn from the Xenontest, and there was a noticeable heterogenous darkening and yellowing in those areas. The part which is the most comparable is the sixth step that was not covered by the foil. Compared with the other coatings so far, this sample experienced a severe colour change at the maximum dose, with an ΔE* of 8.74, a remarkable result considering that the ΔE* of the uncoated paper was 3.44.
• Sample #05: There was no noticeable change, although there was a mild colour change after the fourth step, followed surprisingly by remission. Sample #05 shows less build-up than the other samples, and the ΔE* values were very low. When the sample was compared with the uncoated paper after the last step (for both), the ΔE* showed a major difference, with 0.73 for Sample #05 and 3.44 for the uncoated paper (control).
• Sample #06: There was a noticeable colour change after the third step. The curve became steeper and increased significantly in the fifth step. At the maximum dose, the sample showed the most significant colour change of all samples, with ΔE* of 9.47 as compared with 3.45 for the uncoated paper after the last step and a higher ΔE* than the 8.74 of Sample #04 at the maximum dose.
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Results Set C: coatings on printed paper (inkjet)
90 80 70
60 Yellow
50 Magenta 40 Cyan 30 Black 20 White 10 (paper) 0 Uncoated #01 #02 #03 #04 #05 #06 control
Figure 4.2.4. Comparison of the colour changes for the six coated printed papers before and after the maximum dose of ~121 Mlx h and 181,414 kJ/m2. The chart provides a comparison of colour changes in each saturated ink (yellow, magenta, cyan, black or white-plain paper) and each ink’s behaviour for the six coatings.
Samples set Yellow Magenta Cyan Black White (C) (paper) Uncoated 79.88 19.46 33.82 2.25 3.45 Control ΔE* 81.85 6.82 14.45 2.64 3.34 Sample #1 ΔE* 80.58 7.84 18.21 2.86 2.86 Sample #2 ΔE* 56.17 5.08 7.37 0.57 5.67 Sample #3 ΔE* 68.60 6.61 8.99 0.32 8.74 Sample #4 ΔE* 27.01 5.24 5.41 1.23 0.73 Sample #5 ΔE* 79.22 7.96 12.02 2.14 9.47 Sample #6
Table 4.2.5. Comparison of the colour changes of six coated printed papers before and after the maximum dose of ~121 Mlx h and 181,414 kJ/m2. The table shows the change in numerical ΔE* values. Changes noticeable by the naked eye occurred at ΔE* >1.5.
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Figure 4.2.6. Six coated half-exposed inkjet prints on paper compared with uncoated prints are shown above to demonstrate the noticeable colour changes between the aged coated print and aged uncoated print. The coatings demonstrated a definitive deceleration in the loss of saturation, and each ink reacted differently with the combination of coatings.
Most of the colour changes in Set C are reflected in the L* value, where the colour becomes lighter, which is perceived as loss of saturation. Saturated yellow was severely affected by colour changes when compared with the other inks in each sample. The least affected was the yellow in Sample #05 followed by Sample #03. When saturated yellow was compared with the uncoated paper before and after ageing, samples #01, #02, #04 and #06 did not appear to prevent the colour from changing. After yellow, saturated cyan experienced the greatest colour change among the other inks and white (paper substrate). In Sample #04, the difference in the cyan after ageing was almost identical to the difference in the white. Sample #02 showed the greatest difference in the cyan area when compared with the control, but this was due to uneven flaking of the surface layer, and therefore this measurement might have yielded false data. When comparing the saturated cyan in each of the samples, there is clearly less colour change than in the uncoated control, with the cyan in sample #05 experiencing the least change.
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Saturated magenta was the third ink to be affected by the ageing of the coating-ink- paper systems. In Sample #03, the magenta experienced the same amount of colour change as the white; In sample #05, the magenta experienced the same amount of colour change as the cyan. When comparing the saturated magenta in each of the samples, there is clearly less colour change than in the uncoated control. Black was the least affected ink, showing no significant differences between the aged coated samples #01, #02 and #03 and the control. Nevertheless, when coatings #03, #04 and #05 were present, the black ink appeared to undergo less change than without the coatings. The effects of maximal ageing of the white (paper substrate) have been previously presented. To summarise, samples #01 and #02 experienced almost the same trend as the uncoated paper. Samples #04 and #06 presented a significant colour change after ageing, while sample #05 showed no noticeable colour change. When compared with other inks, the paper substrate appears to experience less change than the yellow, magenta and cyan. It is interesting that the cyan and magenta in samples #01 and #02 experienced greater colour changes than the white paper and less changed than samples #03, #04 and #06, while the white paper experienced the opposite: a large difference in colour change (towards yellowing) in samples #03, #04 and #06 and less change in samples #01 and #02.
4.3. Discussion The main results from the experiment involving sets A, B and C are as follows: • The unprinted uncoated paper changed colour after approximately 16.5 mega lux hours and 26,616 kJ/m2 UV content. • Colour changes in the coating-paper system were mostly evident in the b* value, where the colour shifted towards yellow. Colour changes in coating-ink-paper system were mostly evident in the L* value, where the colour became lighter. • Most of the coating-paper samples yellowed over time, except for Sample #05 (Type C matte) by Rauch, liquid coating for canvas) in which the colour change is entirely unnoticeable and even shifting toward less yellowing at the maximum weathering step of approximately 121 mega lux hours and 181,414 kJ/m2 UV content. • At the maximum dose, the magenta and cyan inks on uncoated printed paper changed colour more than when covered by any of the coatings, indicating that the coatings in the coating- ink-paper system slow down colour changes of the prints.
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• Various chemical reactions occur due to the inks’ chemical properties. The results of the colour measurements show that most of the coated yellow (at the maximum exposure of any coating-ink-paper sample) had faded, as if the coating were not present. In contrast, the colour change in the coated magenta and cyan was slowed down by the presence of the coatings. • Most of the coating-ink-paper system samples experienced significant colour change, except Sample #05. It is important to stress that each coating introduced its own colour before ageing, and the initial starting point for the weathering and colour change was different for each coating (see appendix IV). Another point to consider is that the measurements were taken from the most comparable area (with fewer defects, better coating coverage and absence of dirt) and this can influence the numerical results, but not the overall trends.
Interpretation of the colour changes as a reflection on the photodegradation mechanism SET A In Sample #01, the build-up in colour change was relatively slow. The sample appeared to yellow a bit more than the uncoated control, until the two samples reached a similar point at the last step. These results are difficult to interpret, because it is not possible to determine the association between the yellowing and the particular material in the coating-paper system, and a material analysis could not be performed due to time limitations. It is only possible to deduce that coating #01 combined with the paper substrate does not necessarily prevent colour changes of the coated paper over time.
The curve for Sample #02 shows a trend similar to that of Sample #01 in terms of colour change build-up, which could be due to their similar spray application method or material content. Although the chemical reactions involved in this colour change cannot be determined due to the lack of information regarding the sample’s materials, a few explanations can be suggested. As previously mentioned, the polymer binder in coating #02 is PEG-PVAL, which is very unlikely to yellow. To prevent colour changes, a hydroxyl-phenyl triazine derivative (HTP UVA) was incorporated into the binder. The possible explanations for why the sample yellowed include:
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1. GC-MS found several aromatics (benzene, styrene, toluene and naphthalene), which could act as chromophoric sites contributing to the yellowing. 2. The coating was applied using a spray and therefore introduced a very thin layer, likely resulting in a low HTP concentration and/or large consumption. 3. The HTP spectrum coverage was narrow. 4. There were no HALS and therefore no protection against the formation of radicals. 5. There were unknown reactions between the paper and coating that caused yellowing. 6. There were unknown reactions between the materials in the coating itself that caused yellowing.
Sample #03 experienced a higher build-up of colour changes than samples #01 and #02. GC-MS found acrylic copolymers (MA/MMA/BA/BMA). There are a number of possible explanations for the colour change in the coating itself, in addition to the optional chemical reactions between the paper and coating: 1. The various types of petroleum listed in the manufacturer’s material content often contain aromatic structures that act as chromophores and can lead to yellowing. 2. GC-MS found no organic light stabilisers. The manufacturer of the coating provided no further details regarding stabiliser additives and mentioned that the sample’s coating was not tested for light fastness. In the absence of light stabilisers, colour changes can occur much faster. Nevertheless, the build-up in colour change of Sample #03 was less steep than that of samples #04 and #06 (Fig. 4.2.1). The reason for the lack of severe yellowing in Sample #03 likely depends on the other materials present in the polymer matrix that interact with each other or with the paper. It is therefore possible to deduce that Sample #03 does not prevent colour change and adds more colour (by yellowing) than if the paper were uncoated.
The curves for samples #04 and #06 show a sharp rise in colour build-up after the third step. Unfortunately, information on the content of samples #04 and #06 is not available. Due to time limitations, the material content of the two coatings could not be tested with GC-MS. Furthermore, during the ageing process, Sample #04 reacted with the aluminium foil used to cover it, and the only measurable part of Sample #04 was not covered during the sixth step. It is difficult to deduce the causes of such a profound colour change in both samples without
51 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints further information on the material content. However, it is an important finding that both samples experienced the severest colour change.
The curve for Sample #05 showed no noticeable colour change (Fig. 4.2.1), and even showing remission during the last ageing step. GC-MS yielded interesting results that might explain this behaviour. The binder is a made of acrylic copolymer, MMA-BA, polyester urethane and styrene. Without considering a reaction with the paper and other materials in the coating, the colour change could have been caused by the styrene, which has an aromatic structure, and urethane which might have an aromatic structure. Up until the fifth step, there was a very slight build-up in colour change, in contrast to the other coatings that contained aromatic groups, which could reflect the positive influence of the combination of UVAs (BZT derivatives) and HALS. The low values of colour change could also be explained by the thickness of the coating, which can hold a large concentration of light stabilisers. Coatings #01 and #02 are fixative sprays that apply a very thin layer that can hold lower concentrations of light stabilisers, while the other coatings are liquids that were applied with a roller and are therefore thicker but might not contain any light stabilisers, except for coating #05. The remission during the last step could be the outcome of more activated protective additives or a different chemical reaction with the paper or other materials in the coating.
Although some reasons were provided for the colour change, it is not possible to isolate the yellowing of the coating from the paper merely based on the photodegradation mechanism of each coating. The samples are built of layers from wide variety of materials that could interact. In addition, information on the exact content of the polymer binders is not available, and it is therefore unrealistic to generalise as to which polymer binder is more protective. The only possible outcome of this research is to analyse the results from an observational viewpoint. In other words, colour measurements can provide information on the final appearance of the coating-paper samples. Based on the results of this study, Coating #05 causes the least colour change, followed by coatings #01 and #02. Coatings #3, #4 and #6 experienced the most severe yellowing on Hahnemühle fine art paper.
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SET C The mechanisms behind the colour change in set C (coating-ink-paper system) are difficult to explain due to the lack of information on the ink materials and behaviour, alone, and when combined with Hahnemühle Photo Rag® substrate. Therefore, observation alone is employed to assess the coatings’ appearance on inkjets. The results can only suggest which inks change less compared with others in a sample and how each coating affects a certain ink’s colour ager ageing. The results show differing trends in colour change when ink is present, which is not surprising considering the results from the dissertation by B. Vogt (Stability Issues and Test Methods for Ink Jet Materials; 2001). In general, UV and visible light absorbance appears to cause severe colour changes in uncoated inkjet compared with any coated inkjet. Magenta, cyan and black inks and white paper substrate appear to be relatively less affected by weathering of the coating-ink-paper system compared with yellow ink. Of the inkjet finish coatings, inks in sample #05 appears to have experienced the least colour change, which might offer insights for the future appearance of prints. As was shown with Set A, the white substrate tends to yellow, while the yellow ink becomes lighter and less yellow. Another interesting result was observed with samples #01 and #02 where the inks experienced greater colour change than the white in their respective sample, which could be explained by the BZT (UVA) capability to absorb a certain spectrum range. The results of the colour measurements of Set C can be further investigated, and only a brief discussion has been included in this study. These results should be further analysed once the material content of the ink and paper is made known. It is also important to keep in mind that changes in gloss can affect the naked eye’s perception of the colour, a factor that was not measured in this study.
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5. Conclusions A preliminary inquiry into the relevance of researching the light fastness of finish coatings for inkjet prints yielded interesting results. There is a lack of awareness among conservators regarding the presence of such materials on prints. Communication with the manufacturers revealed that coatings are still in use by artists and printers, while researchers noted that detailed information on these contemporary materials would benefit the conservation science field. This research study showed how the colours of a coated inkjet print change over time as a function of exposure to UV and visible light radiation. To quantify the change in colour, the difference between unexposed and exposed areas was compared in each sample using the ΔE* value. The analysed samples were exposed to a spectrum of 320–800 nm, which simulates the exposure conditions of prints behind glass, which blocks a portion of the terrestrial UV-A spectrum. The results showed that uncoated paper yellows over time but that coating-paper systems yellow even more, except for sample #05 where the change was unnoticeable. The colour measurements in the coating-ink-paper system showed that most of the coated yellow ink had faded, as if the coatings were not present, except for sample #05. As opposed to that, the colour change in the coated magenta and cyan was slowed down by the presence of the coatings. The results indicate that different chemical reactions occur due to the paper substrate and the inks’ chemical properties. The material analysis using GS-MS found polymer binders and other materials, discovering PEG-PVA and BZT (UVA) in Coating #02; MA/MMA/BA/MBA acrylates and possibly urethane in coating #03 but no organic light stabilisers; and polyester-urethane and acrylates (MMA-BA), BZT (UVA) and Tinuvin 292 (HALS) in coating #05. The theoretical study might offer explanations for the colour changes. Studies have shown that chromophores are the main cause of colour changes that are reflected as yellowing. Studies have shown that most of the polymer binders discussed mostly degrade within the UV-A range but very little within the visible light range. With the addition of light stabilisers, the rate of photodegradation slows considerably. Due to the different protective mechanisms, the combination of UVAs and HALS in a polymer offers a major advantage in preventing colour changes. The information accumulated from the theoretical study is difficult to interpret in terms of the reasons for the colour changes in the samples, given the numerous variables that might influence the colour changes. First and foremost, the colour change was measured as a
54 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints combination of a coating and paper and a combination of a coating, ink and paper. It is difficult to evaluate colour changes when there are unknown materials in the ink, when the paper can react with the coating and when the materials within the coating can react with each other. For example, this is evident by the significant colour change in the yellow ink when compared with cyan and by the different changes in each cyan paper sample, depending on the coating employed. Deducing the reasons that one coating- paper system has a greater tendency to yellow over another is therefore not possible within the confines of this study. The current research can only show how the colour changes in certain weathering conditions while helping evaluate the appearance of the samples. Nevertheless, the results yield some important information regarding chemical compositions. As mentioned before, the combination of UVAs and HALS provide coatings with better light fastness. Indeed, the printed and unprinted samples with coating #05, which had this combination, show less colour change. When no UVAs and HALS were found in the coatings, the colour change was much more significant. It is important to bear in mind that the weathering conditions were extreme and demonstrated a scenario of coated prints exposed to solar radiation behind glass at 40 C for maximum of 120,096,068 lux hours, which is equivalent to 100 lux for 137 years. These conditions were employed to raise awareness of the possible presence of a coating that might otherwise go unnoticed. Photodegradation usually occurs within the UV-A range, with not much occurring within the visible light range. Thus, in stable environmental conditions where exposure to UV radiation is minimised, the coatings provide protection, and a significant amount of time would be required for photodegradation leading to colour changes to occur as compared with uncoated prints. Inkjet finish coatings are chosen by artists and printers not just for the coatings’ photostability performance but also for aesthetic reasons, such as changes in surface texture and saturation. In contrast, conservators strive to prevent colour changes, developing conservation guidelines for exhibitions that recommend specific light exposure dosages when exhibiting inkjet prints. In the event of noticeable colour changes, it must be considered that colour perception is subjective, and a colour change that is acceptable to a conservator or curator might not be acceptable to the artist. This study therefore offers an objective method for evaluating the appearance of prints, to choose the suitable coating-ink-paper combination and establish the initial point for an identification method based on the yellowing of the white areas.
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Further research
Certain information that was unavailable at the time of this study could shed more light on the photostability of coated inkjet prints. Further material analysis GC-MS could be performed to find more information on the coatings’ material content and offer more defined results to assess better the degradation behaviour of the materials within each coating. Fourier- transform infrared spectroscopy and a material analysis could be performed after artificial weathering to determine the degradation products and deduce the possible reactions of the combination of coatings, inks and paper. Due to time limitations and the design of the research, it was not possible to perform step weathering to the printed samples. It would be beneficial to continue the research with understanding more on the rate of colour change of each ink in the coating-ink-paper samples. Further research could be conducted on the layers separately: the coatings could be applied to glass to isolate their reactions, Hahnemühle Photo Rag® fine art paper degradation could be further studied, as could that of Epson UltraChrome Pro inks. Another optional research can focus on the thickness of a coating and/or application method in order to test possible behavioural change of each coating. Despite the difference in thickness between the coatings that stemmed of application method (coatings #01 and #02 are fixative sprays that introduce a thinner layer then the rest liquid coatings) these parameters could not be covered in the current research and might have an effect on colour change. During this study, a number of unexpected noticeable reactions occurred that could not be addressed, such as tackiness of samples with coatings #03, #04 and #06 after taken out of the Xenontest. Therefore, a complimentary research on the dark/thermal degradation of coated inkjet prints could be beneficial. Flaking of the coated and uncoated ink layer in exposed printed samples was also noticed during the research and could not be addressed, as the reaction of coating #04 with aluminium foil and some texture surface changes manifesting as blistering and uneven changes in gloss.
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Summary
The effects of finish coatings on the UV and visible light stability of inkjet prints Master’s Thesis
Ella Solomon (11596139) University of Amsterdam, July 2020
Inkjet printing is a common practice in digital photography and is widely employed by contemporary photographers. An inkjet print is an image created by ink droplets consisting of dyes or pigments deposited on a substrate. This article discusses the photostability of six inkjet finish coatings and their effect on Epson UltraChrome Pro inks and Hahnemühle Photo Rag® fine art paper commonly employed by printing studios and photographers. The finish coatings were applied directly on fine art paper samples and on printed fine art paper samples. Three of the coatings were analysed using gas chromatography mass spectrometry. The material analysis provided further understanding on the types of polymer binders and light stabilisers. The samples were then aged in a Xenontest weathering instrument at 40 C, 40% relative humidity and a light spectrum range that simulates the display behind glass. Coating-paper samples were aged in five steps to monitor each coating’s rate of colour change. Coating-ink-paper samples were aged to the maximum of approximately 121 megalux hours to determine whether each coating reacts differently with the inks when compared with the coating-paper samples. Colour measurements were employed to detect and quantify colour changes using the chromatic coordinates L*a*b* of the CIELAB colour space. The results showed that most of the coating-paper system samples yellowed over time, except for Sample #5 (Rauch Type C matte for inkjets on canvas). At the maximum step of ageing, most of the coated yellow ink in the coating-ink-paper samples had faded, as if the coating was not present, while the colour change of the coated magenta and cyan had slowed. Sample #05 experienced the least colour change compared with the other coating-ink-paper samples. The results show an obvious different behaviour when the inks are present, due to their differing chemical properties. Understanding the various chemical reactions that occur in coating-paper and coating- ink-paper systems is complex. However, a literature research on the ageing behaviour of binders and light stabilisers offered some explanation for the colour changes. The yellowing exhibited by the coating-paper samples is most likely related to the chromophores in the binders that act as sites that absorb solar radiation. The combination of various types of light
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stabilisers significantly slows the photodegradation. The only coating known to contain this combination was Coating #05, which might explain why the samples with this coating show unnoticeable colour change. When colour change is noticeable, it needs to be considered that colour perception is subjective and that a colour change that is acceptable to a conservator or curator might not be acceptable to an artist or owner. This study offers methods for quantifying colour changes and determining whether they are acceptable, for choosing suitable coating-ink-paper combinations and raising awareness regarding the possibility of the presence of coatings on inkjet prints.
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Appendices
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Appendix I: overall material content
Manufacturer and No. Stated materials by manufacturer/distributer Material analysis result Polymer binder Method of application Remarks finish coating and Light stabiliser brand 1 A mixture of alpha-3-(3-(2H-benzotriazol-2-yl)-5-tert-butyl-4- Not analysed Binder: unknown Spray Tested for light fastness hydroxyphenyl)propionyl-omega-hydroxypoly (oxyethylene) and alpha- Light stabilizer: Suitable for paper. Hahnemühle 3-(3-(2H-benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl)propionyl- BZT (UV absorber) Solvent borne. omega-3-(3-(2Hbenzotriazol-2-yl)-5-tert-butyl-4- Protective Spray hydroxyphenyl)propionyloxypoly(oxyethylene)
2 Polyvinyl resin (vinyl co polymer which is synthetic), Dimethyl ether, Vinylic compounds and several aromatics (such as Binder: PEG-PVA Spray Tested for light fastness Rauch Butyl acetate, Ethanol, Diacetone alkohol benzene, styrene, toluene and naphthalene). In Light stabilizer: Suitable for paper.
combination with the vinylic compounds it is likely a HTP (UV absorber) Solvent borne. Schutzlack Firnis polyethyleneglycol-pva coating. The mass spectrum für Fine Art indicates HTP UV absorber. There is also a trace of the Papiere additive trimethyl phosphate.
3 Liquid Resin XK-190, Liquid Resin BT-26, Dipropylene Glycol Acrylic compounds (MA/MMA/BA/BMA) and some Binder: MA /MMA Liquid material, applied Not tested for light Monomethyl Ether, 2,4-Pentanediol, 2-Methyl, Distillates, Petroleum, aromatics. There are traces of urethane but these are not /BA /BMA and with roller fastness. Solvent-Dewaxed Heavy Paraffinic, 2-(Dimethylamino)ethanol, convincing enough. Furthermore, there are traces of maybe urethane Suitable for paper. Distillates, Petroleum, Hydrotreated Heavy Paraffinic, Alkylaryl additives (propanoic and butanoic acid). Light stabilizer: Water borne. Breathing Color Polyether Alcohol, Polyethylene Glycol Distearate, water, Methyl unknown Alcohol, Distillates, Petroleum, Solvent-Dewaxed Light Parraffinic,
Glamour II Nonionic Polyethylene Emulsion, Dimethyl Silicone Polymer with Silica, Distillates, Patroleum, Solvent-Refined Light Paraffinic, 2- Butoxyethanol, Silica, Amorphous, Hydrophobic Silica, 2-Methoxy-1- Propanol, Glutaraldehyde, Octamethylcyclotetrasiloxane
4 water, co-polymer-dispersion, additive, 1,2- benzisothiazl-3(2H)-one, 2- Not analysed Not analysed. Liquid material, applied Tested for light fastness. Hahnemühle methyle-2H-isothiazol-3-one, Reaction mass of: 5- chloro-2-methyl-4- Unknown. with roller Suitable for canvas.
isothiazolin-3-one , 2-methyl-4-isothiazolin-3-one , biocides Water borne. Varnish 5 Water containing liquid laminate based on polyurethane: Alpha -3(3-(2h- Tadipic acid (unsaturated fatty acid 2C6) and Binder: polyester- Liquid material, applied Tested for light fastness. benzotriazol-2-yl)-5tert-butyl-4-hydroxyphenyl)propionyl-omega- neopentylglycol. polyester urethane polyester. There are urethane = and with roller Suitable for canvas. Rauch hydroxypoly (oxyethylene), Alpha-3-(3-(2h-benzotriazol-2-yl)-, also styrenated-acrylates (MMA-BA), Tinuvin 292 (a styreneated acrylates Water borne.
Reaction mass of bis (1,2,2,6,6-pentamethyl-4-piperidyl), Sebacate and HALS stabilizer) and traces of additives (propanoic and (MMA-BA) ClearShield™ methyl 1,2,2,6,6- pentamethyl-4-piprydyl sebacate. [3-(2,3- butanoic acid, triphenyl phosphate) present. Light stabilizers: Typ C matte epoxypropoxy)propyl]diethoxymethylsilane Triethylamine benzotriazole Seidenglänzender derivative (UV UV-Schutzlack absorber) and Tinuvin 292 (HALS) 6 Water, Acrylic Copolymer Not analysed Binder: Acrylic Liquid material, applied Tested for light fastness. Breathing Color copolymer with roller Suitable for both canvas
Light stabilizers: and paper. Timeless Varnish unknown Water borne.
Table. I.1 Overall material content by the manufacturers and GC-MS material analysis, including further functional details on each of the researched coatings.
65 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints
Appendix II: questionnaires for manufacturers
Hahnemühle France Dear Sir/ Madam, My name is Ella Solomon and I am a master’s student for photography conservation at the University of Amsterdam. In the following months I will write my thesis on over coatings of inkjet prints on paper and canvas. I would like to analyze the long term effects of some of your coatings and it would mean a lot if you can help me with answering the following questions:
1. I am interested to know more about “Hahnemühle Protective Spray”: a) Can you provide the full material content? Nothing more than what is written on the back of the spray : “ Polyvinyl resin, UV absorber (triazole derivates) benzotriazole derivate b) Was it tested for light fastness (and UV)? - If so, was it tested when applied on fine art paper? I don’t know what was the process to test the spray, but it had been tested on FineArt papers - If so, what is the maximum lux hours before noticeable change of color (if any)? I don’t know what was the process to test the spray, but it had been tested on FineArt papers I will ask to try to get an accurate answer. We rarely have the year/hour number of the durability of a product 2. I am interested to know more about “Protective varnish for canvas inkjet prints” a) Can you provide the full material content? There either I do not have the full material content but only what is written on the bottle : “Enthalt; 5-Chlore-2-Methyl-2H-isothiazol-3-on, 2-methyl-2H-isothiazol-3-on, 1,2 Benzisothiazol-3(2H)- on b) Was it tested for light fastness (and UV)? I know it was tested on all our canvas, but I do not know the process exactly. - If so, was it tested when applied also on fine art paper? It is only for Canvas as it fit with the poly-cotton fibers, on fineart paper (mostly cellulose and cotton fibers the varnish will not be fixed very well) - If so, what is the maximum lux hours before noticeable change of color (if any)? I will ask to try to get an accurate answer. We rarely have the year/hour number of the durability of a product c) What is the different between “Hahnemühle Protective Spray “and “Protective varnish for canvas inkjet prints” in terms of: • reaction with the print (ink and substrate) • material content • long term stability
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d) Do you think it matters if Protective varnish for canvas inkjet prints will be applied on paper? please elaborate. As canvas is a very rare use I don’t have a lot of talking about this varnish. What I have been told is that it won’t change anything about the color but “mechanically” it won’t be fixed and it will took off or disappear 3. Do you have other products that can be used as coatings for inkjet prints, especially for paper? If so: Protective spray do not change the color print or make it more glossy for exemple. But few hours of drying before using it is recommended. It is for papers. Talking about protective spray I tell to use it on Matt papers mostly. To protect a print that I will manipulate many times (portfolios book for exemple). It is not a protection against hits but against papers friction. VarnisheS (one is Satin, one is Glossy, the last one turns paper Matt) are made for canvas only. We do not have anything else chemical, or to apply on coatings. a) Can you provide the full material content? b) Are you willing to provide their MSDS? c) Was it tested for light fastness? - If so, were they tested when applied on fine art paper? - If so, what is the maximum lux hours before noticeable change of color (if any)? d) Which type of paper do you recommend to use with them? e) What is the difference between these products to the two stated above?
4. I would be more than happy to collaborate with you during the process. Would you consider helping me in the near future by answering more questions? Yes of course, and I am sorry that I don’t have many answers. I would like to express my full appreciation for taking the time to answer the above questions. Kind regards, Ella Solomon
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Hahnemühle Germany
Hello Ella, I’ll do my very best.
1. Regarding Hahnemuhle fine art paper photo rag 308gsm: a. What mill is used for producing the paper? Where? Our Mill at the Headquaters in Dassel Gemany. It is made on Foundrinier Mashine. b. What type of IRL does it have? It is a Microporous Coating c. What is the material content (as far as you can share)? Production Secret. d. Was it tested for light fastness, and if so, what were the conditions and the results? Who tested it? Yes, several times. At Whilhelms Research, LNE in France and inhouse. https://www.hahnemuehle.com/de/digital-fineart/alterung-und-bestaendigkeit.html
2. Regarding Hahnemuhle protective spray: a. Is it true that it is the same product as "Rauch Schutzlack Firnis für Fine Art Papiere"? No it is not the same b. I would like to know more about the popularity of this product. How much is being sold annually (in Germany or worldwide)? This are internal figures not for sharing. c. Did you test this product for light fastness and if so, what were the conditions and the results? Who tested it? Wilhelm’s Research, LNE in France and inhouse.
3. Regarding Hahnemuhle varnish: a. I would like to know more about the popularity of this product. How much is being sold annually (in Germany or worldwide)? This are internal figures not for charing. b. Did you test this product for light fastness and if so, what were the conditions and the results? Who tested it? Wilhelm’s Research, LNE in France and inhouse.
I hope that helps a little. Mit freundlichen Grüßen / Best regards Stefan Neumann - Manager Technical Support -
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Rauch Germany Dear Sir/ Madam, My name is Ella Solomon and I am a master’s student for photography conservation at the University of Amsterdam. In the following months I will write my thesis on over coatings of inkjet prints on paper. I would like to analyze the long term effect of some of your coatings and it would mean a lot if you can help me with answering the following questions:
1. I am interested to know more about Schutzlack Firnis für Fine Art Papiere 5241004004 c) Are you willing to provide the full material content? Polyvinylharz, UV_Absorber, Alkohole. d) Was it tested for light fastness? If so, was it tested when applied on fine art paper? It is UV resistant, applied in 2-3 layers, each with 50% overlap. Checked by the Wilhelm Research Institute e) Which brand of paper do you recommend to use it? Currently all our fine art papers from the mediaJET series, as well as Hahnemühle and Awagami 2. 1. I am interested to know more about ClearShield™ Typ C matte Seidenglänzender UV-Schutzlack 5243120100 a) Are you willing to provide the full material content? Benzotriazol-2yl)-5-tert-butyl-4-hydroxphnyl)propionyl-omega-hydroxypoly(oxyethylen) und alpha-3-(3-(2h-Benzotriazol-2-yl)-5-tert-butyl-4-hydroyphenyl)propionyl-omega-3-(3-(2h- benzotriazol-2-yl)-5-tert-buryl-4-hhydroxyph, Reaktionsmasse von bis (1,2,2,6,6-pentamethyl- 4-piperidyl) sebacat und methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacat. b) Was it tested for light fastness? If so, was it tested when applied on fine art paper? UV stabiler Schutzlack, ja. c) Which brand of paper do you recommend to use it? Only canvas products, e.g. from our series: Bergamo, Roma, Como, Como BW, Venezia, Pop Art, Siena Nuvo, Florence, as well as other products from our price list 153. d) What is the difference between Schutzlack Firnis für Fine Art Papiere to ClearShield™ Typ C matte Seidenglänzender UV-Schutzlack in terms of: • reaction with the print (ink and paper) clearshield is only for canvas products, while the Artist Line protective varnish is especially for papers. • material content see above • long term stability see above
e) Do you think it matters if ClearShield™ Typ C matte Seidenglänzender UV-Schutzlack will be applied on paper? please elaborate.
69 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints
3. Do you have other products that can be used as coatings for inkjets prints, especially for paper? If so: f) Are you willing to provide the full material content? There are a total of 3 different products for the canvas / canvas products, but they only differ in terms of their appearance on the material surface: i. ClearShield Gloss = glossy surface impression ii. ClearShield Type C semi-gloss = silk matt surface impression iii. ClearShield Type C matt = matt surface impression g) Was it tested for light fastness? If so, was it tested when applied on fine art paper? As with the other products, they are UV protective varnishes h) Which type of paper do you recommend to use it? i) What is the difference between this product to the others stated above?
4. I would be more than happy to collaborate with you during the process. Would you consider helping me in the near future with a contact person? That would be mine: Thomas Wöhrstein Export Manager and trained photographer: [email protected] You are welcome to contact me with any questions: +49 7424 9485 31 I would like to express my full appreciation for taking the time to answer the above questions. Kind regards, Ella Solomon
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Breathing Color, Texas, US
Hi Ella,
Most of the information about specific chemistry is proprietary information that only our chemistry team has access to. I've attached what information we are able to share for both Timeless and Glamour 2 as PDF files. This includes technical data sheets, material safety data sheets, and Archival certifications.
Timeless is tested for light fastness by the Fine Art Trade Guild. They have their standards listed on their website as for the specifics of their testing methods I'm not sure. https://www.fineart.co.uk/stqs/Fine_ArtTrade_Guild_Standards.aspx
Timeless is designed to dry quickly and is best used on smaller prints. Glamour 2 takes longer to dry and is best used on larger prints.
Prints must be properly out gassed to avoid ink pickup or smearing from either varnish. Timeless works on both canvas and fine art papers. Glamour 2 is best used for canvas as it has a denser water content that canvas is in general more attuned to absorb while it tends to over saturate fine art papers.
Glamour 2 is not tested for light fastness.
As for your questions, I've been told that Timeless sells more than Glamour around 2:1. As for approximate sales numbers, I would not be able to get that information.
I hope this answers your questions, but please let me know if you need anymore help.
Ashley Customer Success Rep II Breathing Color, Inc.
71 Solomon | UvA | 2020 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints
Appendix III: colour measurement plan
Figure III.1. Measurements of color difference between non aged sample (0) from set (B) and each step of the sample (1-5) from set (A) using ΔE*. This was done in each of the six coated samples and the uncoated control Hahnemühle fine art paper. The results show rate of degradation of each coating-paper system and compared with the rate of the control and other equivalent samples. For example, in sample #06 step 0 was compared with step 1, step 2 and so on. The ΔE* yielded a certain curve for samples #06 that is different than the curve of the control or other samples and present the buildup of colour change compared to uncoated weathered paper and other coated weathered samples.
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Figure III.2. Measurements of difference between unexposed coated saturated yellow (1), exposed coated yellow (2) and exposed uncoated yellow in the control (3) using ΔE*. The same was done in the saturated magenta, saturated cyan and black. This comparison was done in each of the six printed coated samples to examine if the inks react differently. For example, if yellow experience more colour change than cyan in the same sample. The comparison was also done to see if each coating affects the same ink differently. For example, if coatings #02 and #05 applied on saturated cyan show different colour change.
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Appendix IV: coatings’ inherent colour
5 4.5
4 3.51
3 2.77 2.84
E* Δ 2 1.39 1.17 1
0 #01 #02 #03 #04 #05 #06
Figure V.1. The chart shows the colour change that each coating introduces to the paper (0,0) after application and before weathering. Coating #05 added the most colour to the paper after application and before ageing, as compared with the other coatings.
Solomon | UvA | 2020 74 The effects of finish coatings on ultraviolet and visible light stability of inkjet prints
Appendix V: GC-MS Research report
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Identification of coatings for inkjet prints on paper
Research report
Saskia Smulders Henk van Keulen
Cultural Heritage Laboratory
Requested by: University of Amsterdam 73RCE Amsterdam: 2020-023 Date: July 2020
Cultural Heritage Agency of the Netherlands (RCE) Cultural Heritage Laboratory Hobbemastraat 22 1071 CZ AmsterdamError! No document variable supplied. Error! No document variable supplied.Error! No document variable supplied.www.cultureelerfgoed.nl
RCE Contact person S. Smulders 0031 611422951 [email protected]
RCE project No. 2020-023
Accessibility of this document
Information provided by the Cultural Heritage Agency of the Netherlands (RCE) is subject to the ‘Wet openbaarheid van bestuur’ (Wob). Reports are accessible through the library of the RCE. Exceptions can be requested in written form.
© 2020 Cultural Heritage Agency of the Netherlands. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the Cultural Heritage Agency of the Netherlands (RCE).
All other photographs, if not indicated otherwise: © Cultural Heritage Agency of the Netherlands
How to cite this document
Smulders, S., Van Keulen , H, 2020, Identification of coatings for inkjet prints on paper., RCE Research Report No. 2020-023, Amsterdam: Cultural Heritage Agency of the Netherlands, Cultural Heritage Laboratory.
Gas Chromatography Mass Spectrometry (GC/MS) With gas chromatography mass spectrometry, a gas phase component mixture is separated into individual components which are subsequently detected and identified. Separation is based on the boiling point of the components and its interaction with the stationary phase of a column. A GC column is a thin capillary tube with a stationary phase on the inner wall. In this phase components dissolve to different degrees while the inert gas helium flows through the column. An unknown component mixture is dissolved in a suitable solvent and applied at the beginning of the column. After the solvent has evaporated, the oven is gradually heated. When the temperature of the oven becomes so high that one of the components in the mixture evaporates, the helium gas stream passes it through the column to the detector, the mass spectrometer. The mass spectrometer ionizes and fragments the component into loose mass fragments of different weight and intensity. This process is specific to each individual substance. The entirety of these mass fragments is called a mass spectrum. Due to the standardized setting of the mass spectrometer, these mass spectra are reproducible and components can be identified.
Thermally assisted hydrolysis and methylation Py-GC/MS Traditional and modern binders, resins and waxes have been analyzed with thermally assisted hydrolysis and methylation gas chromatography-mass spectrometry (THM-GCMS), used in combination with pyrolysis as a sample introduction technique. To form a suspension sample material is triturated with a few drops of a solution of tetra-methyl ammonium hydroxide in methanol (5%). The suspension is transferred to a metal pyrolysis cup and analyzed. Pyrolysis, hydrolysis and/or methylation takes place of the fatty acids, the resin acids and the polymeric fraction of the sample. The total components mixture is separated with the aid of gas chromatography and the separated components are detected and identified by mass spectrometry. A Frontier Lab 3030D pyrolyser was used in combination with a Thermo Scientific Trace 1310 gas chromatograph and a Thermo Scientific ISQ mass spectrometer. The pyrolysis technique used was a rapidly rising temperature range from 350°C to 700°C; the temperature of the pyrolysis interface was 290°C. The pyrolysis unit is directly linked to a SLB5 ms Supelco column (with a length of 20m, an internal diameter of 0.18 mm and a film thickness of 0.18 microns) by a split connector. Helium with a programmed flow (0,5 to 1,2 ml/min) is used as carrier gas in combination with a temperature program of 35°C (1) – 60°C/min – 110°C – 14°C/min – 240°C – 5°C/min – 315°C (2). The column is directly coupled to the ion source of the mass spectrometer. The temperature of the interface was 250˚C, the temperature of the ion source 220˚C. Mass spectra were recorded from 29 to 600 AMU at a speed of 7 scans per second. Xcalibur software 4.1 was used for recording and processing the spectral data.
Py-GC/MS To optimize analysis of polymers Py-GC/MS is performed without any sample treatment A Frontier Lab 3030D pyrolyser was used in combination with a Thermo Scientific Trace 1310 gas chromatograph and a Thermo Scientific ISQ mass spectrometer. The pyrolysis technique used was at a fixed temperature of 650°C; the temperature of the pyrolysis interface was 290°C. The pyrolysis unit is directly linked to a SLB5 ms Supelco column (with a length of 20m, an internal diameter of 0.18 mm and a film thickness of 0.18 microns) by a split connector. Helium with a constant flow (0,9 ml/min) is used as carrier gas in combination with a temperature program of 35°C (1) – 16°C/min – 2200°C – 10°C/min – 315°C (1). The column is directly coupled to the ion source of the mass spectrometer. The temperature of the interface was 250˚C, the temperature of the ion source 220˚C. Mass spectra were recorded from 10 to 600 AMU at a speed of 7 scans per second. Xcalibur software 4.1 was used for recording and processing the spectral data.
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Results
Sample Coating_#02
▼Chromatogram sample #02-on glass
glycol dimethyl ether HPT UV absorber
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► distribution of materials ► distribution of vinylic markers
This sample consists of vinylic compounds and several aromatics (such as benzene, styrene, toluene and naphthalene). The chromatogram shows a series of peaks with a curve pattern, maximizing at RT 13min, pointing to the presence of glycol dimethyl ether.88 In combination with the vinylic compounds it is likely a polyethyleneglycol-pva coating. The chromatogram also shows a significant peak at RT 15.78 min, the mass spectrum indicates a HPT UV absorber.89 There is also a trace of the additive trimethyl phosphate.
88 These components are not shown in the pie-chart 89 HTP = hydroxyl phenyl triazine , this compound is not shown in the pie-chart
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Sample Coating_#03
▼Chromatogram sample #03-on paper
► distribution of materials ► distribution of acrylic markers
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This sample consists clearly of acrylic compounds (MA/MMA/BA/BMA)90 and some aromatics. There are some vinylic compounds but the most specific components for a vinyl are missing. There are traces of urethane but these are not convincing enough. Furthermore there are traces of additives (propanoic and butanoic acid).
90 MA = Methyl Acrylate; MMA = Methyl Methacrylate; BA = Butyl Acrylate; BMA = Butyl Methacrylate
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Sample Coating_#05
▼Chromatogram sample #05-on paper (analysis with TMAH)
neopenthylglycol
adipic acid
► distribution of materials ► distribution of acrylic markers
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This sample contains a striking amount of adipic acid (unsaturated fatty acid 2C6) and neopentylglycol.91 These two components are pointed out in the chromatogram. According to the provided details of the coatings this particular coating is a liquid laminate based on polyurethane. Adipic acid is known for its use in an urethane polyester. Possible adipic-neopentylglycol is used as a flexible part in an urethane polyester coating. However, there is no indication of the presence of an urethane compound in this analysis and adipic acid is also used in combination with other polymers. In order to confirm or exclude the presence of urethane other pyrolysis analyses must be performed.
There are also styrenated-acrylates (MA/MMA/BA/BMA), Tinuvin 292 (a HALS stabilizer) and traces of additives (propanoic and butanoic acid, triphenyl phosphate) present.
91 Both compounds are not shown in the pie-chart
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▼Chromatogram sample #05-on paper (Py@ 600°C, no TMAH)
MMA
styrene BA
PUR- ester BA
Py@600°C analysis without TMAH confirmed the presence of styrene, MMA and BA. The peak at RT 2.74min confirms the presence of an urethane compound, a PUR-ester component is indentified.
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