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
4.1. Material Analysis (THM-GCMS) ... 43 4.2 Colour Measurements ... 43 4.3. Discussion ... 49 5. Conclusions ... 54 Summary ... 57 Bibliography ... 59 Appendices ... 64
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.
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.
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 ofcalcium carbonate (CaCO3) inpolyvinyl 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.
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.
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.
1.2. 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.
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
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.
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,
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
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.
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.
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
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.
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.
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
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.
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.
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.
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.
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.
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
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
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.
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
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.
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.
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. 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.
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.
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
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
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.
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.
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
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.
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
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.
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.
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.
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
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.
[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.
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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
69 O. Chiantore et al. “Photooxidative degradation of acrylic and methacrylic polymers”. Polymer. 41. (2000). p.
1657-1658, 1658-1663.
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.
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.
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.
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.
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.
