The fate of colors in the 20th - 21st centuries: preserving the organic colorants in plastic artifacts

Objectos modernos e contemporâneos feitos de plástico são amplamente encontrados no património cultural. Presentemente, a sua preservação levanta questões críticas aos conservadores e cientistas uma vez que estes objectos podem facilmente sofrer degradação num curto espaço de tempo. Um dos fenómenos que pode alterar significativamente a aparência de objectos em plástico é a alteração de cor (descoloração). De um modo geral, a descoloração é habitualmente associada à degradação dos polímeros, contudo, os pigmentos, que são parte integrante das formulações do plástico, também podem desvanecer devido à exposição à luz. A identificação de objetos de plástico com pigmentos sensíveis à luz é um exercício bastante exigente devido à sensibilidade dos mesmos a alterações na cor. A caracterização dos corantes nos plásticos é normalmente realizada através de amostragem, métodos de extração e testagem destrutiva. Como alternativa, esta tese apresenta uma abordagem inovadora e multi- analítica baseada em espectroscopias que foi desenvolvida para a identificação in situ dos pigmentos em plásticos históricos. Esta metodologia compreendeu a utilização de microscopia ótica (MO), microespectrometria por fluorescência de raios X dispersiva de energias (μ-EDXRF), espectroscopia UV-Vis-NIR de reflectância, fotoluminescência (PL) e micro-espectroscopia de Raman (μ-Raman) na análise de obras de arte, objetos industriais e de uso diário, datados de 1950-2000s e pertencentes a coleções Portuguesas. Deste estudo resultou a identificação dos pigmentos comumente presentes na paleta de cor dos coloristas da indústria dos plásticos portuguesa: óxido de ferro (PR 101, α-Fe2O3), molibdato de cromato de chumbo (PR 104, Pb(Cr,Mo,S)O4), vermelho de cádmio (PR 108, Cd(S,Se); PR 113, (Cd,Hg)S), amarelo de cádmio (PY 37, CdS; PY 35; (Cd,Zn)S), branco de titânio (PW 6, TiO2 ambos rutilo e anátase), oxicloreto de bismuto (PW 14, BiOCl) e lacas do pigmento orgânico β-naftol (PR 48, PR 49, PR 53). Adicionalmente, foi também identificado um pigmento fora do comum, o pigmento perlascente plumbonacrite Pb5(CO3)3O(OH)2. Para todos os casos de estudo, μ-Raman foi a ferramenta chave para a caracterização dos pigmentos nos objetos de plástico, aportando dados conclusivos para a identificação dos mesmos. A impressão digital vibracional dos pigmentos orgânicos e inorgânicos foi adquirida com sucesso recorrendo à focagem do laser na superfície das partículas. A aquisição de dados espectrais de pigmentos com concentrações muito baixas (0.1 % a 5%, aproximadamente) à escala micro foi possível através de microscopia confocal, que faz parte do sistema do equipamento de μ-Raman. Adicionalmente, foi também possível obter informação sobre o polímero base (principalmente termoplásticos) e cargas. Os métodos analíticos desenvolvidos neste estudo deverão, em trabalhos futuros, facilitar a obtenção de informação complementar sobre estes objetos de plástico e permitir uma melhor identificação e avaliação do seu estado de conservação. Esta tese foca particularmente objectos de plástico vermelhos visto que estes foram identificados como os mais severamente afetados por alterações de cor. O estado avançado de desvanecimento identificado no pigmento β-naftol PR 53 mostrou a sua fraca estabilidade à luz em formulações de plástico. Esta situação, junto com as alterações de cor descritas em literatura para o pigmento PR 48 em objetos de plástico, sugere uma sensibilidade dos pigmentos vermelhos da família dos β-naftol ao desvanecimento. O PR 53 e os pigmentos vermelhos da família dos β-naftóis são pigmentos históricos facilmente encontrados em objetos do património cultural. No entanto, o conhecimento acerca da sua estabilidade a longo prazo e resistência à foto-degradação é limitado, especialmente para os casos onde os mesmos se encontram em polímeros, sendo que este conhecimento é essencial para a sua preservação. Neste estudo, a quantificação da foto-estabilidade para uma série de pigmentos vermelhos da família dos β-naftol foi realizada pela primeira vez, através do cálculo do rendimento quântico de fotodegradação (ΦR). Os valores obtidos variaram entre 3x10-6 e 4x10-5, indicativo de uma estabilidade relativamente boa à luz por parte das moléculas. Tendo em consideração que a estabilidade dos pigmentos não se limita exclusivamente ao pigmento em si, mas também à sua interação com o meio envolvente, foram realizados ensaios de envelhecimento por exposição à luz (λ>300 nm) do pigmento em solução, em pó e incorporado em polímeros de modo a avaliar o papel do meio na estabilidade à luz dos pigmentos e as vias pelas quais estes se degradam. Verificou-se que o ligante tem um impacto significativo na estabilidade do pigmento uma vez que se foi detetada uma maior sensibilidade à luz dos pigmentos PR 48 e PR 53 quando incorporados nos plásticos, comparativamente ao ensaio do pigmento em pó. Este novo conhecimento irá contribuir para o desenvolvimento de novas estratégias na conservação dos plásticos com estes pigmentos vermelhos fotossensíveis através da previsão do desvanecimento. Espectrometrias de massa (MS) por cromatografia em fase líquida e gasosa foram utilizadas na caracterização dos principais subprodutos da degradação. Observou-se uma fotodegradação significativa e a formação de compostos ftálicos e ftalatos nos pigmentos em solução e em pó.Modern and contemporary objects made of plastics are widely found in cultural heritage. Today, their preser- vation poses critical issues to conservators and scientists, as they can suffer from extensive degradation in a short time period. Color change (discoloration) is one of the alteration phenomena that can significantly affect their appearance. Discoloration is commonly associated with the degradation of polymers. However, pigments within plastics can also fade due to exposure to light. The identification of objects that contain light-sensitive pigments is demanding because of the sensitivity of plastics to color change. Normally sampling, extraction methods and destructive testing are required for the characterization of colorants in plastics. In this work, an innovative multi-analytical spectroscopic approach for the in situ identification of pigments in historical plastics was developed. Optical microscopy (MO), micro-energy dispersive X-ray fluo- rescence (μ-EDXRF), UV-Vis-NIR reflectance, photoluminescence (PL) and Raman microscopy (μ-Raman), were used for the analysis of artworks, industrial and daily objects dated from 1950s-2000s from Portuguese collections. A common colorists’ palette within the Portuguese plastics industry was identified: iron oxide (PR 101, α-Fe2O3), lead chromate molybdate (PR 104, Pb(Cr,Mo,S)O4), cadmium red (PR 108, Cd(S,Se); PR 113, (Cd,Hg)S) and cadmium yellow (PY 37, CdS; PY 35; (Cd,Zn)S) pigments, titanium whites (PW 6, TiO2 both rutile and anatase), bismuth oxychloride (PW 14, BiOCl) and organic β-naphthol lakes (PR 48, PR 49, PR 53). An exceptional pigment found was the pearlescent plumbonacrite pigment Pb5(CO3)3O(OH)2. In all the case studies, μ-Raman was the key analytical tool for pigment characterization in the plastic objects, providing conclusive data for their identification. The vibrational fingerprint of both inorganic and organic pigments was successfully recorded by focusing the laser beam on particle surfaces. The confocal microscopy system used in μ-Raman enabled the collection of spectral data from low concentrations of pigments (ap- proximately 0.1%-5%) on the micro-scale. In addition to pigments, information on the base polymer (mainly thermoplastics) and fillers was obtained. The analytical methods developed will facilitate the acquisition of complementary data from plastics allowing material identification and condition assessment in the future. This thesis focused on red pigmented plastic artifacts, as they were found to be severely faded among the studied objects. The identification of β-naphthol pigment lake PR 53 as a faded pigment highlighted its poor fastness in plastics, that together with the color change of PR 48 in plastic objects, reported in literature, suggests the particular susceptibility of β-naphthol red lakes to fading. PR 53, and the other β-naphthol reds, are historical pigments widely found in cultural heritage. However, little is known about their photodegradation and stability, especially when they are found in polymer media, and this knowledge is essential for their long- term preservation. For the first time, photodegradation quantum yields (ΦR) were calculated for a series of red pigments based on β-naphthol in order to quantify their photo-stability. ΦR values ranging from 3x10-6 to 4x10-5 were obtained, indicating relatively light-stable molecules. Bearing in mind that pigment fastness is not only related to the pigment itself, but also to its interaction with the confined environment, light-aging experiments (λ>300 nm) were conducted in solution, on powders, and in polymers to assess the role of the medium on the lightfastness of the pigments and their degradation pathways. A significant impact of the binder on their stability was found. Indeed, a higher sensitivity to light of PR 48 and PR 53 pigments, when incorporated in plastics than in powder, was observed. This new knowledge will contribute to the prediction of plastic fading and inform effective preventive conservation strategies for objects containing light- sensitive β-naphthol red pigments. Liquid- and gas-chromatography mass spectrometry (MS) were used for the characterization of the main degradation products. Extensive photodegradation was observed with the formation of phthalic compounds and phthalates in both solution and powder phases

a degradation study of a humanoid skin mask made of soft urethane elastomer

Funding Information: The authors would like to thank Dr. Clarimma Sessa, of the Chair of Conservation-Restoration, Art Technology and Conservation Science at the TUM for her support with SEM-EDX investigations; Prof. Takuya Hashimoto of the Tokyo University of Science for sharing information about the robot SAYA; Dr. Frank Dittmann, Nicolas Lange and Susanne Grießbach, curators and conservator of the Deutsches Museum respectively, for the fruitful discussions on the decision-making-process regarding the preservation of SAYA. Publisher Copyright: © 2022, The Author(s).Understanding the degradation of plastic materials is a big challenge for curators, conservators and conservation scientists in museums worldwide aiming to preserve their collections due to the variety of formulations of synthetic polymers and pigments. The conservation of polyurethane (PUR) based objects is challenging because they can suffer from extensive degradation. Particularly PUR elastomers can degrade shortly after their production, as occurred to the mask of the Japanese robot SAYA, which within 8 years suffered from two large tears, discoloration and stickiness. This research aims at studying the degradation phenomena of the androids’ synthetic skin. Better knowledge of the chemical composition of the mask and the chemical and physical decay will contribute to planning a suitable stabilization treatment. Within a multi-analytical approach, colorimetric and microscopic investigations highlighted discolored areas, which showed further color changes within a five months monitoring campaign, confirming the instability of the material likely due to ongoing degradation. Raman microscopy allowed the identification of Pigment White 6 (titanium dioxide TiO2) in the anatase form, known to promote the photosensitivity of PUR substrates towards ultraviolet (UV) light. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy identified the PUR composition of the mask, the presence of phthalates as plasticizers and suggested the formation of quinone chromophores in the polymer structure as a result of photo-oxidation, possibly responsible for the mask yellowing. Evolved gas analysis-mass spectrometry (EGA-MS) and double-shot-gas chromatography/mass spectrometry (TD/Py–GC/MS) analyses support the characterization of the formulation of the mask as being made of methylene diphenyl diisocyanate (MDI) PUR ether elastomer. Plasticizers in high concentration, mainly diisononyl phthalate (DINP) and bis(2-ethylhexyl)phthalate (DEHP), and the UV stabilizer Tinuvin 328 were also detected. In addition, the presence of styrene-acrylonitrile (SAN) could also contribute to the mask’s chemical instability. More amount of UV stabilizer and phthalates were detected at the surface (contributing to its stickiness) than in the inner core. The degradation of the mask results from the light susceptibility of MDI PUR ether and SAN, as well as the higher photochemical activity of anatase. The mask was transferred on to a mannequin and placed in the storage area to prevent light exposure and photo-oxidation. As loose edges had to be stabilized, tests were conducted and adhesive stripes glued with a PUR dispersion were selected for keeping the head’s shape. The novelty of this study is the implementation of conservation science on the study of androids with PUR elastomeric components in robotic collections, which are becoming increasingly popular in technical museums, however still seldomly studied.publishersversionpublishe

Shedding Light on Degradation Gradients in Celluloid: An ATR-FTIR Study of Artificially and Naturally Aged Specimens

: Celluloid artifacts are known by conservation professionals to be prone to degradation, threatening their own integrity and that of nearby heritage collections. Celluloid alteration can have a heterogeneous nature, and this research topic is still in its infancy for heritage science. This article investigates degradation gradients, both along depth and width, of artificially aged celluloid sheets, and compares them to three-dimensional (3D) historical objects with the aim of gaining a better insight into the nature and evolution of their decay. ATR-FTIR was used to systematically study different sampling points of the artificially and naturally aged specimens and allowed us to recognize better-preserved surfaces and more deteriorated cores. ATR-FTIR was found suitable for assessing the molecular changes induced by degradation, particularly denitration and formation of carbonyl-containing degradation products in severely aged specimens. Even though the severely artificially aged sheets displayed unusual alteration phenomena, they present a degradation gradient similar to the one observed for the naturally aged 3D objects under study. This research underlines that sampling at different depths and/or widths is relevant for characterizing the heterogeneity of degraded celluloid, and further investigation with chromatographic techniques would greatly benefit the understanding of the complex degradation of celluloid artifacts

Discoloration of historical plastic objects: New insight into the degradation of β-naphthol pigment lakes

Funding Information: This research was funded by Funda??o para a Ci?ncia e a Tecnologia, Minist?rio da Ci?ncia Tecnologia e Ensino Superior (FCT/MCTES), Portugal, through doctoral programme CORESPD/00253/2012, PB/BD/114412/2016 doctoral grant. Associate Laboratory for Green Chemistry? LAQV,(PTDC/IVC-HFC/5174/2014). Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Light is a determining factor in the discoloration of plastics, and photodegradation processes can affect the molecular structures of both the polymer and colorants. Limited studies focused on the discoloration of heritage plastics in conservation science. This work investigated the discoloration of red historical polyethylene (PE) objects colored with PR 48:2 and PR 53:1. High-density and low-density PE reference polymers, neat pigment powders, and historical samples were assessed before and after accelerated photoaging. The applied methodology provided insight into the individual light-susceptibility of polyethylenes, organic pigment lakes, and their combined effect in the photoaging of historical plastic formulations. After light exposure, both PE references and historical samples yellowed, PR53:1 faded, and PR 48:2 darkened; however, both organic pigments faded severely in the historical samples. This highlights the role played by the plastic binder likely facilitating the pigment photofading. Fourier transform infrared spectroscopy and mass spectrometry techniques—EGA-MS, PY-GC/MS, and TD-GC/MS—were successfully employed for characterizing the plastic formulations and degradation. The identification of phthalic compounds in both aged β-naphthol powders opens new venues for studies on their degradation. This work’s approach and analytical methods in studying the discoloration of historical plastics are novel, proving their efficacy, reliability, and potentiality.publishersversionpublishe

Pretreatment of plastic waste: Removal of colorants from hdpe using biosolvents

UIDB/50011/2020 UIDP/50011/2020 CA18220, 2020.00647.CEECIND UIDB/50006/2020 UIDP/50006/2020Plastics recycling remains a challenge due to the relatively low quality of the recycled material, since most of the developed recycling processes cannot deal with the additives present in the plastic matrix, so the recycled products end up in lower-grade applications. The application of volatile organic solvents for additives removal is the preferred choice. In this study, pretreatment of plastic packaging waste to remove additives using biosolvents was investigated. The plastic waste used was high-density polyethylene (HDPE) with blue and orange colorants (pigment and/or dye). The first step was to identify the type of colorants present in the HDPE, and we found that both plastics presented only one colorant that was actually a pigment. Then, limonene, a renewable solvent, was used to solubilize HDPE. After HDPE dissolution, a wide range of alcohols (mono-, di-, and tri-alcohols) was evaluated as antisolvents in order to selectively precipitate the polymer and maximize its purity. The use of limonene as solvent for plastic dissolution, in combination with poly-alcohols with an intermediate alkyl chain length and a large number of hydroxyl (OH) groups, was found to work best as an antisolvent (1,2,3-propanetriol and 1,2,4-butanetriol), leading to a removal of up to 94% and 100% of the blue and orange pigments, respectively. Finally, three cycles of extraction were carried out, proving the capability of the solvent and antisolvent to be recovered and reused, ensuring the economic viability and sustainability of the process. This pretreatment provides a secondary source of raw materials and revenue for the recycling process, which may lead to an increase in the quality of recycled polymers, contributing to the development of an economical and sustainable recycling process.publishersversionpublishe

Applicability of single-shot and double-shot Py-GC/MS for the detection of components in vinyl acetate-based emulsions used in modern-contemporary art

Funding Information: This work was co-funded by the research project ‘Plastic Paints in Art: the impact of manufacturing processes on their long-term stability’, financed by national funds from FCT/MCTES ( IF/00653/2015/CP1293/CT0005 as well as UIDB/50006/2020 , UIDB/00286/2020 , and UIDP/00286/2020 ). We would like to thank Anthony J. Baragona (Institute of Conservation and Restoration, University of Applied Arts, Vienna, Austria) for helping with the English corrections. Publisher Copyright: © 2022This research explores the use of different pyrolysis-based techniques for studying the chemical composition of vinyl acetate (VAc) based emulsions widely used in modern-contemporary art and contributes to the understanding of their composition by the detection of different monomers and additives. For this purpose, six different commercial VAc-based emulsions from the early 21st century were analyzed for the first time by single-shot (SS) and double-shot (DS) pyrolysis – gas chromatography / mass spectrometry (Py-GC/MS). The results obtained were compared, and highlight the advantages of DS in comparison to SS for the analysis of VAc-based emulsions. Several different components assigned to plasticizers, surfactants, intermediates, and antioxidant families, were detected, mainly by the thermal desorption step of DS. The information gained by this research enriches the knowledge of the formulation of VAc-based emulsions, by considering for the first time a large variety of them. Moreover, it also points to the importance and the potential of DS Py-GC/MS for investigating these materials, which have been rarely explored with this technique.publishersversionpublishe

Organic red colorants in Islamic manuscripts (12th-15th c.) produced in al-Andalus, part 1

The conservation of Islamic manuscripts in the Fondo Ka'ti created the opportunity to study the organic red colourants applied in five manuscripts, which include a Koran (1198), a theology treatise (14th c.), a book of poems from Al-Sarishi (15th c.), a biography of the Prophet (1468) and manuscript 19 (1485). These dark red colours were characterized using fibre optic reflectance spectroscopy (FORS-VIS), microspectrofluorimetry and infrared spectroscopy (microFTIR). Microspectrofluorimetry detected the presence of a lac dye chromophore in all the manuscripts studied and ascribed it to specific medieval recipes for three of the manuscripts. This was based on the very good matches obtained with our database of paint reconstructions that were prepared according to medieval technical sources; the dark reds found in the Koran compared very well with the recipe ‘to make red ruby from lukk’ from Ibn Bādīs text (11th c.); the brighter reds applied in the book of poems and in the biography of the Prophet, with recipe 113 from the Paduan manuscript (16th c.). MicroFTIR completed the characterization of the paint formulation, identifying the proteinaceous nature of the binding media as well as the fillers. It also showed the presence of oxalate compounds, possibly, resulting from the binding media degradation, a mark of the recent and dramatic history of these books. Finally, these red dyes were successfully compared to lac dye colours previously characterized in 12th-13th c. Portuguese manuscript illuminations. From Mali to Iberia, tracing the rich diversity of a precious heritage legated by medieval Arabic culture

Shedding Light on Degradation Gradients in Celluloid: An ATR-FTIR Study of Artificially and Naturally Aged Specimens

Celluloid artifacts are known by conservation professionals to be prone to degradation, threatening their own integrity and that of nearby heritage collections. Celluloid alteration can have a heterogeneous nature, and this research topic is still in its infancy for heritage science. This article investigates degradation gradients, both along depth and width, of artificially aged celluloid sheets, and compares them to three-dimensional (3D) historical objects with the aim of gaining a better insight into the nature and evolution of their decay. ATR-FTIR was used to systematically study different sampling points of the artificially and naturally aged specimens and allowed us to recognize better-preserved surfaces and more deteriorated cores. ATR-FTIR was found suitable for assessing the molecular changes induced by degradation, particularly denitration and formation of carbonyl-containing degradation products in severely aged specimens. Even though the severely artificially aged sheets displayed unusual alteration phenomena, they present a degradation gradient similar to the one observed for the naturally aged 3D objects under study. This research underlines that sampling at different depths and/or widths is relevant for characterizing the heterogeneity of degraded celluloid, and further investigation with chromatographic techniques would greatly benefit the understanding of the complex degradation of celluloid artifacts

Mock-Ups in Plastic Conservation Research: Processing and Aging of 3D Celluloid Specimens Simulating Historical Objects

The preparation of mock-ups in heritage science studies represents a valid alternative for investigation purposes, avoiding extensive sampling of cultural heritage objects. This work presents for the first time the successful preparation of three dimensional (3D) mock-ups made of celluloid, considering a combination of historical industrial production strategies and small-scale lab facilities. Prefabricated transparent celluloid sheets were acquired and then shaped through compression molding for creating mock-ups with 3D geometries. These reflected common and representative shapes encountered in the collection of the Deutsches Museum. Visual inspection of the mock-ups allowed determining the best compression molding conditions. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) confirmed the absence of molecular heterogeneity due to the processing method. Artificial aging of the mock-ups was conducted to reach degradation states comparable with naturally aged objects. ATR-FTIR investigation offered first insights into the induced artificial degradation. Ion chromatography (IC) and gel permeation chromatography (GPC) analyses allowed to assess the extent of the artificial aging of the celluloid mock-ups and confirmed the occurrence of loss of camphor, denitration, and main chain polymer scission, the latter being the predominant decay path. The comparison with historical objects highlighted that the mock-ups are representative of moderately aged artifacts. As such, this study paves the way for implementing moderately aged celluloid 3D mock-ups in heritage science research, enabling in-depth testing for the scope of conservation

Mock-Ups in Plastic Conservation Research: Processing and Aging of 3D Celluloid Specimens Simulating Historical Objects

The preparation of mock-ups in heritage science studies represents a valid alternative for investigation purposes, avoiding extensive sampling of cultural heritage objects. This work presents for the first time the successful preparation of three dimensional (3D) mock-ups made of celluloid, considering a combination of historical industrial production strategies and small-scale lab facilities. Prefabricated transparent celluloid sheets were acquired and then shaped through compression molding for creating mock-ups with 3D geometries. These reflected common and representative shapes encountered in the collection of the Deutsches Museum. Visual inspection of the mock-ups allowed determining the best compression molding conditions. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) confirmed the absence of molecular heterogeneity due to the processing method. Artificial aging of the mock-ups was conducted to reach degradation states comparable with naturally aged objects. ATR-FTIR investigation offered first insights into the induced artificial degradation. Ion chromatography (IC) and gel permeation chromatography (GPC) analyses allowed to assess the extent of the artificial aging of the celluloid mock-ups and confirmed the occurrence of loss of camphor, denitration, and main chain polymer scission, the latter being the predominant decay path. The comparison with historical objects highlighted that the mock-ups are representative of moderately aged artifacts. As such, this study paves the way for implementing moderately aged celluloid 3D mock-ups in heritage science research, enabling in-depth testing for the scope of conservation