polymers

Article Characterization and Long-Term Stability of Historical PMMA: Impact of Additives and Acrylic Sheet Industrial Production Processes

Sara Babo 1,* , Joana Lia Ferreira 1,* , Ana Maria Ramos 2, Anna Micheluz 3, Marisa Pamplona 3, Maria Helena Casimiro 4 , Luís M. Ferreira 4,5 and Maria João Melo 1

1 Department of Conservation and Restoration and Research Unit LAQV-REQUIMTE, NOVA School of Sciences and Technology (FCT NOVA), 2829-516 Caparica, Portugal; [email protected] 2 Department of Chemistry and Research Unit LAQV-REQUIMTE, NOVA School of Sciences and Technology (FCT NOVA), 2829-516 Caparica, Portugal; [email protected] 3 Conservation Science Department, Deutsches Museum, Museumsinsel 1, 80538 Munich, Germany; [email protected] (A.M.); [email protected] (M.P.) 4 Center for Nuclear Sciences and Technologies (C2TN), Instituto Superior Técnico (IST), Universidade de Lisboa, 2695-066 Bobadela LRS, Portugal; [email protected] (M.H.C.); [email protected] (L.M.F.) 5 Department of Nuclear Sciences and Engineering (DECN), Instituto Superior Técnico (IST), Universidade de Lisboa, 2695-066 Bobadela LRS, Portugal * Correspondence: [email protected] (S.B.); [email protected] (J.L.F.)



Received: 3 September 2020; Accepted: 22 September 2020; Published: 25 September 2020


Abstract: This work aims at understanding the influence of the production processes and materials in the properties and long term behavior of acrylic sheet, i.e., poly(methyl methacrylate) (PMMA), a material generally considered very stable in museum collections. A comparative study was conducted in samples from cast acrylic sheets produced in the early 2000s, from which manufacturing details were known, and samples provided by the artist Lourdes Castro from acrylic sheets she had bought in the 1960s. Transparent and red opaque cast acrylic samples, containing cadmium red pigment, were used. All samples were artificially aged in a solarbox with irradiation λ > 300 nm for a total period of 8000 h, and alterations were followed by a multi-analytical approach which included Raman, infrared (FTIR-ATR) and UV-Vis spectroscopies; gravimetry; size exclusion chromatography (SEC); thermogravimetry (TGA); micro-indentation; colorimetry; and optical microscopy. Not all cast PMMA sheets presented similar stabilities. We have concluded that the production processes (which may include the polymerization conditions, the organic additives and the origin of the monomer) play a more important role in the properties and long-term behavior of these acrylic sheets than the presence of cadmium red and/or the age of the material.

Keywords: poly(methyl methacrylate); PMMA; photodegradation; accelerated aging; production processes; additives; cadmium red; conservation

1. Introduction Acrylic sheet, which consists of almost pure poly(methyl methacrylate) (PMMA), was developed industrially in the 1930s as the first “organic glass” [1,2]. Its remarkable optical properties, stiffness, light weight, mechanical and weathering resistance, ability to be easily thermoformed, and its capacity to be produced in different colors and transparency/opacity levels, made this material suitable for countless applications, such as airplane cockpits, architectural roofing systems, illuminated signs, sanitary furniture, or sophisticated design objects, just to name a few examples [3,4].

Polymers 2020, 12, 2198; doi:10.3390/polym12102198 www.mdpi.com/journal/polymers Polymers 2020, 12, 2198 2 of 24

The properties of PMMA sheets were also attractive to artists, who began to incorporate this material in their artistic production, acquiring it from international recognized industrial companies or from micro-size enterprises, especially from the 1960s onwards when plastics in general became more available and widely spread in society [5,6]. In the conservation field, PMMA is considered a very stable plastic in opposition to other more instable plastics such as cellulose acetate, cellulose nitrate, plasticized poly(vinyl chloride) (PVC), and polyurethane [7–10]. However, there is still much to be known on how intrinsic factors, related with the materials and methods used by different producers, may impact on PMMA stability and lifetime. Following our previous research [11] our aim was to understand which intrinsic factors—e.g., the temperature and completeness of the polymerization process; the origin of the monomer; and the presence of additives, including an inorganic colorant (cadmium red)—may have a significant influence on the properties and susceptibility of PMMA sheets to photooxidation. The main aging mechanism of PMMA, photooxidation, has been extensively studied [12–23]. The photodegradation mechanism of methacrylic and acrylic polymers has been followed by size exclusion chromatography and a distinction in behavior has been observed [22,24,25]. In both, scission and crosslinking are present, with scission being the dominant mechanism in acrylics, whereas in methacrylics crosslinking may compete depending on the volume of the side group, and it is therefore residual in PMMA (the methacrylic with the smallest side group, a methyl). Films and sheets of PMMA should absorb bellow 260 nm [13,15–20]; however, chromophores such as hydroperoxide groups are usually present in the polymer matrix as consequence of synthesis and processing, which may absorb radiation at wavelengths higher than the polymer and initiate photodegradation reactions [26]. The singularity of the stability of PMMA may be further explained by the fact that, to obtain the excellent optical properties typical of an acrylic sheet, the level of impurities must be extremely low [27]. Moreover, these impurities, or other chromophore species responsible for the initiation of photooxidation processes, do not have a catalytic effect. According to Sirampiguer and co-workers [19], alcoholic groups are formed as PMMA photooxidation products. These authors also suggested that in PMMA cast sheets of 3–6 mm thickness, the photooxidation would take place only in a superficial layer of ca 500 µm, because of the low permeability of oxygen in PMMA [19]. Most of the photodegradation studies on PMMA have been performed on pure polymer films prepared in laboratory, sometimes with controlled addition of other compounds/additives, and typically with radiation at λ < 300 nm to accelerate degradation. Fewer studies [19,28–30] have been conducted with commercial PMMA sheets and with radiations more similar to what an artwork may be exposed in real life. It is also known that pigments can play a major role in the stability of a polymer matrix. Regarding photostability, pigments can either induce a protective effect by absorbing and/or screening UV light, or catalyze/accelerate the photochemical breakdown of the polymer by being photoactive [9,31]. Cadmium reds consist of cadmium sulfide co-precipitated with selenium, (CdS1-xSex)[32,33]. In the coloring of plastics, cadmium pigments are commonly used together with BaSO4 in the form of lithopones, allowing the reduction of costs while maintaining good color values and stability [32,34]. In a recent work, the identification of this family of pigments in historical PMMA samples has been investigated [35], but to the best of our knowledge, studies that correlate their presence with the photostability of PMMA are still missing.

Industrial Production Processes of Cast Sheets PMMA sheets may be cast or extruded. Extruded sheets only became available in the late 1970s [3,36] and are of poorer quality than cast sheets [36]. The process of producing cast sheets is well described in several references [4,37,38] and consists basically in introducing monomer (or a pre-polymerized syrup of monomer), initiator, and other additives into glass molds. The filled molds are heated at controlled temperatures for polymerization of the methyl methacrylate (MMA). Polymers 2020, 12, 2198 3 of 24

The duration of the process may vary from few hours to days depending on the sheet thickness and the technology used. In our previous work [11], detailed information was collected about the production process of acrylic sheet in two different companies operating in Portugal between 1960s–2000s, Plásticos do Sado and Paraglas. These two companies produced exclusively cast PMMA sheets through radical initiated bulk polymerization. Even so, different methods were followed at each plant, which may have influenced the properties of the final material. The main aspects are summarized in Table1.

Table 1. Main characteristics of the production of poly(methyl methacrylate) (PMMA) sheets in the two different plants (adapted from [11]).

Plásticos do Sado Paraglas Produced by the company through chemical Pure monomer acquired from Degussa Monomer recycling (depolymerization by pyrolysis) of (Germany) or Repsol (Spain) acrylic scrap 1 Pre-polymerized syrup (degree of Solution poured into polymerization empirically tested, prepared Monomer + initiator (AIBN) the molds by heating monomer with initiator AIBN) + colorants and additives + colorants and additives Rostero process [39–41]. Polymerization Polymerization in water tanks. Molds placed in a chamber with pressure and vertically in water tanks and heated to temperature control. Molds held Polymerization 50–60 ◦C vertically between metallic plates and heated to 75–80 ◦C Polymerization completed in water tanks at Polymerization completed in the chamber higher temperature (but <100 ◦C) at 120 ◦C (post-polymerization) 1 From cutting remains or end of life material. Several of the recycling steps were controlled visually, based on the empirical knowledge of the workers.

Comparing the processes used by the two companies, the one used by Paraglas most probably presented advantages concerning the final quality of the material produced, namely:

1. Use of pure monomer, assuring less contaminants in the final product. Even though feedstock recycling of PMMA through pyrolysis is a well-established method [42,43], the purity of the material obtained may be affected by the presence of water and the composition of the scrap used [44]. Therefore, acrylic produced from it may present inferior properties compared to that prepared from neat MMA [45,46]. 2. Better control of the uniformity of the sheets thickness, as the glass molds were hold against rigid metal surfaces.

3. Post-polymerization at 120 ◦C. In the first step of polymerization, conversion only reaches 80% to 90% because glasslike solidification of the reaction mixture occurs. To assure full transformation of the monomer into PMMA, it is necessary to raise the temperature above its glass transition temperature (Tg)[38].

Differences in the molecular and physical properties between PMMA sheets produced by these two companies have been revealed for the first time by our preliminary studies [11]. In the present work, this research is continued by further exploring the characterization of the samples, which includes identification of the additives, and by conducting an accelerated aging experiment in order to investigate the impact of the differences found on their long term stability. Samples from transparent and red acrylic sheets produced by the two Portuguese companies (no longer operating but from which manufacturing details are known) and samples from PMMA sheets provided by the artist Lourdes Castro (Plexiglas® and Altuglas®) were used. Lourdes Castro (b. 1930) is a Portuguese artist, who explored the possibilities given by acrylic sheet in her artworks between 1964 and 1968, while working in Paris [5,47]. The artist refers to the material she used as “plexiglas” (one of the most famous names by which cast PMMA sheet was commercialized) even though she has used both Plexiglas® and Polymers 2020, 12, 2198 4 of 24

Altuglas®, which she believed were the best quality acrylic sheets she could buy [12]. Plexiglas® was the name of the material produced by the German company Röhm und Haas, but it was also used for the material produced in France by Alshtom/Altulor until 1962, through an agreement with the German company. It was only in 1962 that PMMA sheets produced by Altulor started to be named Altuglas® and to compete in the market with the German Plexiglas® [48], which may explain the preponderance of the name. Even though production details about Lourdes Castro’s PMMA samples were unknown and these samples had already more than 50 years of natural aging, they were closer to real cases in Museums and we expected to gain more knowledge about them by comparison with the Portuguese ones. Therefore, all samples were subjected to irradiation λ > 300 nm during 8000 h. Long periods of artificial aging are especially important in the conservation field, since artworks are expected to last for centuries. Alterations on the samples were followed by a multi-analyical approach, which included gravimetry; Raman, infrared (FTIR-ATR) and ultraviolet and visible (UV-Vis) spectroscopies; size exclusion chromatography (SEC); thermogravimetry (TGA); micro-indentation; colorimetry; and optical microscopy. Results are presented and discussed, attempting to correlate the different behaviors observed with the differences in their compositions and industrial production processes used. As it will be shown in this work, not all the cast acrylic sheets are alike and present the same stability.

2. Materials and Methods

2.1. Samples Samples from six different cast acrylic sheets were used in this study (Table2). Pieces of transparent and red acrylic were kindly provided by Lourdes Castro and, according to the artist, both are dated from the 1960s and identified as Plexiglas and Altuglas, respectively. Transparent sheets and red samples from color swatches, dated from 2000s and produced by Plásticos do Sado and Paraglas, were selected from the material archive of the Conservation and Restoration Department FCTNOVA and used also in this study for comparison. Selection of the two red samples from the color swatches was based on similarity of chemical composition with the one provided by Lourdes Castro [11]. The Plásticos do Sado and Paraglas samples were fundamental for this research because production details were known based on our previous work as described in the Introduction above. Samples of 15 15 mm2 and 5 5 mm2 were cut from the different PMMA sheets by a 3D carving × × machine (Carvey® by Inventables, Chicago, IL, USA) with a 300 W DC spindle of 12,000 rpm and 1/16” solid carbide bit, using the following cut settings: 1219.2 mm/min feed rate, 228.6 mm/min, and 0.5 mm of depth per pass. All samples were cleaned from dust and grease from handling in an agitated bath at room temperature of a mixture of 1% neutral detergent (Neutracon®) in distilled water; followed by rinsing with at least 3 agitated baths of distilled water. Excess water was removed with absorbent paper and samples were left to dry in a desiccator with silica gel. Polymers 2020, 12, x FOR PEER REVIEW 5 of 24 Polymers 2020, 12, x FOR PEER REVIEW 5 of 24 Polymers 2020, 12, x FOR PEER REVIEW 5 of 24 Table 2. Identification codes of the test samples and corresponding details. PolymersPolymers2020 2020, 12, ,12 2198, x FORTable PEER 2. Identification REVIEW codes of the test samples and corresponding details. 5 of 24 5 of 24 Polymers 2020, 12, x FORTable PEER 2. Identification REVIEW Date codes of of the test samples and correspondingThickness details. 5 of 24 Code Image a Producer Date of Description Colorants c Thickness Notes Code Image aTable Producer 2. Identification Production codes of theDescription test samples andColorants corresponding c (mm) details. Notes ProductionDate of Thickness(mm) Code ImageTable aTable 2.Producer 2.Identification Identification codes codes of of the theDescription test test samples samples andColorants and corresponding corresponding c details. details. Notes ProductionDate of Thickness(mm) From a sheet Code Image a Producer DescriptionColorless Colorants c FromNotes a sheet TPS Colorless n.a. 3.12 ± 0.12 fragment with a ProductionDateDate of of c Thickness(mm) CodeTPS Image a Producer TransparentDescription Colorantsn.a. c 3.12 ± 0.12 fragmentFromNotes a sheet with Code Image Producer Production DescriptionTransparentColorless Colorants (mm) protectionNotes film. TPS Plásticos do Production n.a. 3.12(mm) ± 0.12 protectionfragmentFrom a sheet with film. Plásticos do TransparentColorless TPS Sado(PT) n.a. 3.12 ± 0.12 protectionfragmentFromFrom a sheetcolor withfilm. a sheet PlásticosSado(PT) do Colorless From a color Plásticos ColorlessTransparent From a color TPSRPSTPS Plásticos do Red Opaque Cdn.a.n.a. (S,Se) 3.122.88 ± 0.010.12 swatch.protectionfragmentfragment One with film. small with Sado(PT) TransparentTransparent ± From a color RPS do Sado Red Opaque Cd (S,Se) 2.88 ± 0.01 swatch.protectionprotectionpiece. One film. small film. PlásticosSado(PT) do 2000s piece. RPS (PT) 2000s Red Opaque Cd (S,Se) 2.88 ± 0.01 swatch.Frompiece. One a color small Sado(PT) 2000s Red From a color swatch. RPSRPS 2000s Red Opaque CdCd (S,Se) (S,Se) 2.88 ± 0.01 swatch.Frompiece. Onea sheetcolor small OpaqueColorless ± FromOne a smallsheet piece. TPARPS 2000s Red Opaque Cdn.a. (S,Se) 3.572.88 ± 0.100.01 swatch.fragmentpiece. One with small TransparentColorless TPA 2000s Transparent n.a. 3.57 ± 0.10 fragmentFrompiece.From a sheet with a sheet Paraglas ColorlessTransparentColorless protection film. TPATPA ParaglasParaglas 2000s n.a.n.a. 3.57 ± 0.10 protectionfragmentFromfragment a sheet with film. with Paraglas TransparentTransparentColorless ± TPA (PT)(PT) n.a. 3.57 ± 0.10 protectionfragmentFromprotection a sheetcolor withfilm. film. Paraglas(PT) TransparentColorless From a color RPATPA Red Opaque Cdn.a. (S,Se) 2.873.57 ± 0.010.10 swatch.protectionfragmentFrom One a color with film. small RPA Paraglas(PT) RedTransparentRed Opaque Cd (S,Se) 2.87 ± 0.01 swatch.From One a color small swatch. RPARPA Red Opaque CdCd (S,Se) (S,Se) 2.87 ± 0.01 swatch.From Onea color small Paraglas(PT) Opaque ± protectionOnepiece. small film. piece. RPA Red Opaque Cd (S,Se) 2.87 ± 0.01 swatch.Frompiece. One a color small (PT) RPA PlexiglasPlexiglas ColorlessRedColorless Opaque Cd (S,Se) 2.87 ± 0.01 swatch.Frompiece. Onea color small TPL n.a. 4.20 ± 0.12 From a sheet TPLRPA Plexiglasbb RedColorless Opaque Cdn.a. (S,Se) 4.202.87 ± 0.010.12 swatch.Frompiece. Onea sheet small TPL (DE?)(DE?) 1960s b TransparentTransparent n.a. 4.20 ± 0.12 From a sheet Plexiglas(DE?) b b TransparentColorless Fromfragment a sheet with TPL 1960s b n.a. 4.20 ± 0.12 fragmentpiece. with Plexiglasb 1960s b Colorless fragmentFromprotection a sheet with paper. AltuglasAltuglas(DE?) TransparentRed protection paper. RALRALTPL b b Red Opaque CdCd (S,Se)n.a. (S,Se) 2.804.20 ± 0.020.12 RAL PlexiglasAltuglas(DE?)bb 1960s RedTransparentColorless Opaque Cd (S,Se) 2.80 ± 0.02 protectionfragmentFrom a sheet paper.with RALTPL (FR)(FR) RedOpaque Opaque Cdn.a. (S,Se) 4.202.80 ± 0.120.02 Altuglas(FR) bb 1960s b fragmentFrom a sheet with RAL (DE?) RedTransparent Opaque Cd (S,Se) 2.80 ± 0.02 protection paper. a Photographsa Photographs taken taken onAltuglas top(FR)on top of b a of black a black paper;1960s paper;b b Samples b Samples provided provided by Lourdes by Lourdes Castro; Castro; date and dateprotection brandfragment and werebrand paper.with indicated RALa Photographs taken on top of a black paper; b SamplesRed Opaque provided Cd by (S,Se) Lourdes 2.80 Castro; ± 0.02 date and brand by thewere artist; indicatedc Identification by theAltuglas(FR) artist; byb XRF,c Identification SEM-EDS andby XRF, Raman SEM-EDS [11]. and Raman [11]. protection paper. RALa Photographs taken on top ofc a black paper; b SamplesRed Opaque provided Cd by (S,Se) Lourdes 2.80 Castro; ± 0.02 date and brand were indicated by the(FR) artist; b Identification by XRF, SEM-EDS and Raman [11]. a b werePhotographs indicated taken by the on artist; top of c Identification a black paper; by SamplesXRF, SEM-EDS provided and by Raman Lourdes [11]. Castro; date and brand 2.2. Artificiala Aging and Characterization Procedureb 2.2.2.2. Artificial ArtificialwerePhotographs indicated Aging Aging taken and byand the Characterization Characterizationon artist; top of c Identification a black Procedurepaper; Procedure by SamplesXRF, SEM-EDS provided and by Raman Lourdes [11]. Castro; date and brand 2.2. ArtificialTowere compare indicated Aging the byand behavior the Characterization artist; ofc Identification the different Procedure by PMMAs XRF, SEM-EDS under study,and Raman samples [11]. were artificially aged in ToTo compare compare the the behavior behavior ofof thethe different different PMMAs PMMAs un underder study, study, samples samples were were artificially artificially aged agedin in a a2.2. Solarbox Artificial 3000e Aging accelerated and Characterization aging apparatus Procedure (CO.FO.ME.GRA) equipped with a Xenon-arc light Solarbox2.2.a Solarbox ArtificialTo 3000e compare 3000e Aging accelerated the accelerated and behavior Characterization aging agingof the apparatus differentapparatus Procedure (CO.FO.ME.GRA) PMMAs (CO.FO.ME.GRA) under study, equipped equipped samples withwerewith aartificiallya Xenon-arcXenon-arc aged lightlight in source source filtered to λ > 300 nm, with constant irradiation of 800 W/m2 and black standard temperature filteredasource SolarboxTo to filtered λcompare> 3000e300 to nm, theλaccelerated > behavior300 with nm, constant with agingof the constant differentapparatus irradiation irradiation PMMAs (CO.FO.ME.GRA) of 800 un of W der800/m study, W/m2 and equipped2 samplesand black black standard werewith standard artificiallya Xenon-arc temperature temperature aged light in varying varying between 60 to 100 °C (air temperature measured inside the chamber <40 °C), for a maximum sourcea SolarboxTo filtered compare 3000e to the λaccelerated > behavior300 nm, with agingof the constant differentapparatus irradiation PMMAs (CO.FO.ME.GRA) un of der800 study, W/m equipped2 samplesand black werewith standard artificiallya Xenon-arc temperature aged light in betweenvarying 60 between to 100 60C (airto 100 temperature °C (air temperature measured measured2 inside theinside chamber the chamber<40 <40C), °C), for afor maximum a maximum period of perioda Solarbox of 8000 3000e h (total◦ accelerated irradiance aging = 22,943 apparatus MJ/m2 ).The(CO.FO.ME.GRA) UV-Vis transmission equipped2 sp◦ ectrumwith a ofXenon-arc the filter usedlight varyingsourceperiod offiltered between 8000 hto (total 60λ > to 300 irradiance100 nm, °C (airwith = temperature 22,943constant2 MJ/m irradiation measured2).The UV-Vis of inside 800 transmission W/m the chamber and black sp <40ectrum standard °C), of for the temperaturea maximumfilter used 8000insource the h (total solarbox filtered irradiance tois presentedλ > 300= nm,22,943 in withSupplementary MJ constant/m ).The irradiation Info UV-Visrmation. of transmission 800 Samples W/m2 wereand spectrum black analyzed standard ofbefore the temperature filterirradiation used in the periodvaryingin the solarbox of between 8000 h is (total presented60 to irradiance 100 °C in (airSupplementary = temperature 22,943 MJ/m Info measured2).Thermation. UV-Vis inside Samples transmission the chamber were analyzed sp ectrum<40 °C), before of for the a irradiation maximumfilter used solarboxandvarying after is between presented250, 500, 60 1000, to in 100 Supplementary 2000, °C (air 4000, temperature 6000 and Information. 8000 measured2 h. The Samplesinside 15 mm the side werechamber samples analyzed <40 (in °C), triplicates before for a maximum irradiation of each and inperiodand the after solarbox of 8000250, 500, his (total presented 1000, irradiance 2000, in Supplementary4000, = 22,943 6000 MJ/mand Info8000).Thermation. h. UV-VisThe 15 Samples transmissionmm side were samples analyzed spectrum (in triplicatesbefore of the irradiation filter of usedeach afterPMMAperiod 250, of 500,typology, 8000 1000, h (total 2000,except irradiance 4000, RPS 6000 and= 22,943 andRPA) 8000MJ/m were h.2).The Theus edUV-Vis 15 for mm thetransmission side non-destructive samples spectrum (in triplicates analysis of the filter methods of each used PMMA andPMMAin the after solarbox typology, 250, 500, is presented 1000,except 2000, RPSin Supplementary4000, and 6000 RPA) and were 8000Info rmation. ush.ed The for 15 Samples themm non-destructiveside were samples analyzed (in analysis triplicatesbefore irradiation methods of each (gravimetry,in the solarbox optical is presented microscopy, in Supplementary colorimetry, Info infrared,rmation. Raman Samples and were UV-Vis analyzed spectroscopies) before irradiation and typology,PMMA(gravimetry,and after excepttypology, 250, optical 500, RPS 1000,except and microscopy, RPA)2000, RPS 4000, were and colorimetry, 6000 usedRPA) and for were the8000 infrared, non-destructive ush.ed The for 15Raman themm non-destructive sideand analysis samplesUV-Vis methods (inspectroscopies) analysistriplicates (gravimetry, methods of eachand optical returnedand after to250, the 500, solarbox 1000, 2000, after 4000,analysis. 6000 The and seri 8000es h.of The5 mm 15 mmsamples side weresamples used (in for triplicates the destructive of each microscopy,(gravimetry,returnedPMMA typology, to colorimetry, the optical solarbox except microscopy, infrared,after RPS analysis. and colorimetry, Raman RPA) The and wereseri infrared,es UV-Visus ofed 5 mmfor spectroscopies)Raman thesamples non-destructive and were UV-Vis andused spectroscopies) returned foranalysis the destructive tomethods the and solarbox methodsPMMA typology, (micro-indentation, except RPS size and exclusion RPA) werechroma ustographyed for the and non-destructive thermogravimetry), analysis therefore, methods at afterreturnedmethods(gravimetry, analysis. to(micro-indentation, the Theoptical solarbox series microscopy, of after 5 mmsize analysis. samplesexclusioncolorimetry, The were chromaseri infrared,es used oftography 5 formm Raman the samples and destructive andthermogravimetry), were UV-Vis used methods spectroscopies) for the(micro-indentation, therefore, destructive and at each(gravimetry, time, two optical samples microscopy, of each PMMA colorimetry, type we reinfrared, removed Raman from the and solarbox UV-Vis for spectroscopies) analyses. and sizemethodseachreturned exclusion time, (micro-indentation,to two the chromatography samples solarbox of after each size analysis.PMMA and exclusion thermogravimetry), type The wechromaserirees removed oftography 5 mm therefore,from samples and the thermogravimetry), solarbox were at each used for time, analyses.for the two therefore, destructive samples at of each eachreturnedmethods time, to(micro-indentation, two the samples solarbox of after each size analysis.PMMA exclusion type The wechromaserirees removed oftography 5 mm from samples and the thermogravimetry), solarbox were used for analyses.for the therefore, destructive at PMMA2.3. Analytical type were Methods removed from the solarbox for analyses. methods2.3.each Analytical time, (micro-indentation, two Methods samples of each size PMMA exclusion type wechromare removedtography from and the thermogravimetry), solarbox for analyses. therefore, at 2.3.each Analytical time, two Methods samples of each PMMA type were removed from the solarbox for analyses. 2.3.2.3.1. Analytical Optical Methods Microscopy (OM) 2.3.1.2.3. Analytical Optical Microscopy Methods (OM) 2.3.1.2.3. AnalyticalOptical Optical microscopyMicroscopy Methods (OM)(OM) was used to characterize visually the surfaces of the samples and 2.3.1. OpticalOptical Microscopy microscopy (OM) was used to characterize visually the surfaces of the samples and detect2.3.1. Optical eventual Microscopy alterations. (OM) Images were acquired using a Zeiss Axioplan 2 Imaging system 2.3.1.detectOptical Optical eventual microscopyMicroscopy alterations. (OM)(OM) Images was used were to acqucharacteriired usingze visually a Zeiss the Axioplansurfaces of2 theImaging samples system and (Oberkochen,Optical microscopy Germany), (OM) equipped was usedwith halogen to characterize (HAL 100) visually and mercury the surfaces (N HBO of 103) the samplesilluminators, and detect detect(Oberkochen,Optical eventual microscopy Germany), alterations. equipped(OM) Images was with used were halogen to acqu characteri ired(HAL using ze100) visually anda Zeiss mercury the Axioplan surfaces (N HBO of2 103)theImaging samplesilluminators, system and eventualcoupledOptical alterations. to Nikonmicroscopy DXM1200F Images (OM) were wasmicroscope acquired used to cameracharacteri using aand Zeissze ACT-1visually Axioplan control the surfaces 2 Imagingsoftware of the(Tokyo, system samples (Oberkochen,Japan). and (Oberkochen,detectcoupled eventual to Nikon Germany), alterations. DXM1200F equipped Images microscope with were halogen acqucamera (HiredAL andusing 100) ACT-1 anda Zeiss mercury control Axioplan (Nsoftware HBO 2 103) Imaging(Tokyo, illuminators, Japan).system Germany),Microphotographsdetect eventual equipped alterations. were with acquired halogen Images (HALat allwere aging 100) acqu andintervired mercuryals using using (N areflected HBOZeiss 103)Axioplan light illuminators, in bright 2 Imaging field coupled mode, system at to Nikon coupledMicrophotographs(Oberkochen, to Nikon Germany), wereDXM1200F acquiredequipped microscope at with all aging halogen camera interv (HAL alsand 100)using ACT-1 and reflected mercury control light (Nsoftware inHBO bright 103) (Tokyo, field illuminators, mode, Japan). at 50×(Oberkochen, and 500× magnifications.Germany), equipped When with necessary halogen to exam(HALine 100) some and alterations,mercury (N di HBOfferent 103) magnifications illuminators, DXM1200FMicrophotographs50×coupled and 500×to microscope Nikon magnifications. wereDXM1200F camera acquired When andmicroscope at necessary ACT-1all aging controlcamera intervto exam als softwareandine using ACT-1some reflected (Tokyo, alterations, control light Japan). software diinfferent bright Microphotographs (Tokyo, magnifications field mode, Japan). at were andcoupled illumination to Nikon modes DXM1200F were also microscope used, including camera tr ansmittedand ACT-1 light, control dark softwarefield and cross(Tokyo, polarizing Japan). acquired50×andMicrophotographs andillumination at 500× all aging magnifications. modes intervalswere wereacquired usingWhen also atused, reflectednecessary all aging including light intervto exam intralsansmitted brightine using some field reflected light, alterations, mode, dark light atfi dield 50infferent brightandand cross magnifications 500field polarizing mode,magnifications. at filters.Microphotographs Fluorescence were microscopy acquired images at all agingwere intervacquiralsed usingwith blue-violet reflected light light, in using×bright the field Zeiss× mode, Filter at Whenand50×filters. necessaryandillumination Fluorescence 500× magnifications. to modes examine microscopy were some Whenalso images alterations,used, necessary includingwere acquirto di examff trerentansmittededine with magnificationssome blue-violet light, alterations, dark light, fi and elddifferent using and illumination cross themagnifications Zeiss polarizing modesFilter were set50× 05 and (excitation 500× magnifications. BP 395–440 nm,When beamsplitter necessary FTto exam 460 nm,ine emissionsome alterations, LP 470 nm). different magnifications alsofilters.andset used, 05 illumination (excitation Fluorescence including BPmodes transmitted 395–440microscopy were nm, also light, imagesbeamsplitter used, dark wereincluding field acquirFT 460 and transmitteded nm, cross with emission polarizingblue-violet light, LP dark 470 light, filters. finm).eld using and Fluorescence cross the Zeiss polarizing Filter microscopy and illumination modes were also used, including transmitted light, dark field and cross polarizing imagessetfilters. 05 were (excitation Fluorescence acquired BP 395–440microscopy with blue-violet nm, imagesbeamsplitter light,were acquirFT using 460ed nm, the with Zeissemission blue-violet Filter LP 470 set light, 05nm). (excitationusing the Zeiss BP 395–440Filter nm, filters.set 05 (excitation Fluorescence BP 395–440microscopy nm, imagesbeamsplitter were acquirFT 460ed nm, with emission blue-violet LP 470 light, nm). using the Zeiss Filter beamsplitter FT 460 nm, emission LP 470 nm). set 05 (excitation BP 395–440 nm, beamsplitter FT 460 nm, emission LP 470 nm).

2.3.2. Color Measurements

Color determinations were made using a Datacolor International colorimeter (Microflash). The optical system of the measuring head uses diffuse illumination from a pulsed Xenon-arc lamp over an 8 mm-diameter measuring area. The color measurements were performed using the standard Polymers 2020, 12, 2198 6 of 24

illuminant D65 and the CIE 1964 standard colorimetric observer (10◦) geometry. Before color determinations, calibration was performed with bright white and black standard plates. Both the equipment measuring head and the sample were positioned in a costume made positioning mask, which allowed future measurements on exactly the same area. A white base composed of several layers of lab filter paper was used for all measurements and stored in the dark while not in use. For each determination of L*, a* and b* color coordinates, the mean value and standard deviation of three independent measurements over the same measurement area on the three sample replicates were calculated (nine readings), except for RPS and RPA which do not have replicates (three readings). Color variation in samples during artificial aging was calculated according to the CIE 1976 (CIELAB) color difference (∆E*) expression [49]:

∆E* = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2, (1) in which ∆L* is the variation in Lightness, ∆a* the variation in the red/green color coordinate, and ∆b* the variation in the blue/yellow color coordinate.

2.3.3. Gravimetry Weight measurements were performed in a Sartorious CP225 D micro analytical balance (Göttingen, Germany). At each irradiation time, the mean value of three independent weight measurements was calculated and mass loss determined by subtracting the corresponding value obtained at t = 0 h. Final mass loss corresponds to the mean of the mass loss values of the three replicates for each typology (includes nine readings) converted to percentage of total initial weight. Samples were kept in a desiccator after irradiation until weight measurements were performed.

2.3.4. UV-Vis Spectroscopy Ultra-violet and Visible spectroscopy was carried out both in transmittance and reflectance modes, given that both transparent and opaque samples were under study. Spectra obtained in transmittance mode were collected using an Agilent Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies, Santa Clara, CA, USA), version 1.12, in a spectral range from 250–800 nm, at a scan rate of 600 nm/min, data interval of 1 nm and acquisition average time of 0.1 s. PMMA samples were placed in a specially designed holder (to ensure analysis was always performed in the same area) and analyzed in 0◦ angle (perpendicular to the light beam). Spectra obtained in reflectance mode were collected in a Shimadzu UV2501PC spectrophotometer (Kyoto, Japan) equipped with an integrating sphere, in a spectral range from 240–900 nm, at very slow option scan rate, and data interval of 1 nm. BaSO4 was used as reference sample. All spectra were collected as absorption (Abs) spectra.

2.3.5. Infrared Spectroscopy in Attenuated Total Reflectance Mode (ATR-FTIR) ATR-FTIR spectroscopy was carried out using an Agilent Handheld 4300 FTIR spectrophotometer, equipped with a ZnSe beam splitter, a Michelson interferometer and a thermoelectrically cooled DTGS 1 detector. All spectra were acquired with a diamond ATR module, 64 scans and 4 cm− resolution. Three independent spectra were collected for each sample. All spectra are presented as acquired, without baseline corrections or other treatments except normalization to the carbonyl peak intensity, allowing a direct comparison of relative intensities to be made.

2.3.6. Raman Spectroscopy (µ-Raman) Raman spectroscopy was carried out using a Labram 300 Horiba JobinYvon spectrometer (Kyoto, Japan), equipped with a He–Ne 17 mW laser operating at 632.8 nm. The system was calibrated using a silicon standard. The laser beam was focused with a 50 Olympus objective lens. The laser power × at the surface of the samples was controlled with neutral density filters. Raman data analysis was Polymers 2020, 12, 2198 7 of 24 performed using LabSpec 5 software. All spectra are presented as acquired without any baseline correction or other treatment except normalization.

2.3.7. Size Exclusion Chromatography (SEC) Molecular weight distributions were determined with a Knauer Smartline system composed by a model 3800 autosampler (Berlin, Germany), a 1000 pump, and a 2300 refractive index detector. Data was collected with DataApex Clarity software, version 5.0.5.98. Separation was performed by two Waters Styragel HR 5E columns (Prague, Czech Republic), with 7.8 mm 300 mm, and × 5 µm particle size, after a Waters Styragel pre-column. Sample solutions in tetrahydrofuran (THF anhydrous >99.9%, Sigma-Aldrich, St. Louis, MO, USA) at approximately 0.15% (w/v) concentration were prepared at room temperature and filtered with 0.45 µm pore filters. Distilled THF stabilized with butyl-hidroxytoluene (BHT) was used as eluent at a flow rate of 1ml/min; the operating temperature was 30 ◦C. Column calibration was performed with monodispersed PMMA standards 3 6 from Polymer Laboratories (Mp: 1.14 10 to 1.25 10 ). Molecular weight distribution values, × × including weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular-weight dispersity (ÐM), were determined with the software DataApex Clarity, version 7.4, using the GPC analysis extension. Material for analysis was collected with a scalpel from the surface of the PMMA samples. At least two analyses, with independent sample collection and preparation, were performed to verify the consistency of the results.

2.3.8. Micro-Indentation Vickers micro-hardness measurements were performed on an Indentec (Zwick/Roell, Ulm, Germany) ZHµ Micro Hardness Tester. Vickers hardness (HV) is the quotient obtained by dividing the load applied by the square area of indentation, according to:

F HV = 1.854 , (2) × d2 where F is the load in kgf and d the arithmetic mean of the two diagonals in mm. In this study, the load used was 0.3 kgf and the application time was 15 s. For each sample, the values presented result from the arithmetic mean of five different HV measurements, performed at a distance >5d from each other. Due to the influence of relative humidity in the HV, samples collected at each aging time were kept in a desiccator and measured in a row in consecutive days at the end of the experiment.

2.3.9. Thermal Analysis Thermal analysis was performed by thermogravimetry (TGA). Measurements were carried out in a TGA Q500 (TA Instruments, New Castle, DE, USA), in nitrogen atmosphere, from 25 to 500 ◦C, at a rate of 10 ◦C/min. All PMMA typologies under study were analyzed before accelerated aging; analysis during aging was only conducted in the transparent samples (TPS, TPA, and TPL) after 0, 500, 2000, 8000 h of irradiation. At least three TGA measurements were performed for each sample tested.

2.3.10. Thermodesorption-Gas Chromatography/Mass Spectrometry (TD-GC/MS) Additive characterization was performed using a Multi shot Pyrolyzer EGA/PY-3030D (Frontier Lab.) coupled with a 7890 B gas chromatograph and a 5977 B MSD mass spectrometer (both Agilent Technologies, Santa Clara, CA, USA). Around 200 µg of sample was added directly into a stainless steel Eco-cup sample holder (Frontier Lab). The thermodesorption was carried out from 50 (hold for 30 s) to 250 ◦C (hold for 3 min) with an increasing ratio of 20 ◦C/min and cryo-trap for the volatile focalization. GC separations were performed using a Frontier UA5 capillary column (30 m–0.25 F, 30 m 250 µm × 0.25 µm, Frontier Lab.), Helium as a carrier gas at the flow rate of 1.2 mL/min, and a split ratio of × 30:1. The injector temperature was set to 300 ◦C. The column temperature program was set from 40 ◦C Polymers 2020, 12, 2198 8 of 24

Polymers 2020, 12, x FOR PEER REVIEW 8 of 24 (hold for 2 min), increasing rate of 20 C/min until 280 C (hold for 15 min). The ionization mode was source temperature at 230 °C and a scanning◦ mass range◦ of 29 to 550 m/z. MSD ChemStation (Agilent electron impact at 70 eV in positive mode, the transfer line at 280 C, the ion source temperature at Technologies) software was used for data analysis and the ◦compounds were identified by 230 PolymersC and a2020 scanning, 12, x FORmass PEER REVIEW range of 29 to 550 m/z. MSD ChemStation (Agilent Technologies)8 of software 24 interpretation◦ of their EI mass spectra and comparison to NIST MS Search 2.2 and F-Search 3.5.0 was used for data analysis and the compounds were identified by interpretation of their EI mass (Frontiersource Lab) temperature databases. at 230 °C and a scanning mass range of 29 to 550 m/z. MSD ChemStation (Agilent spectra and comparison to NIST MS Search 2.2 and F-Search 3.5.0 (Frontier Lab) databases. Technologies) software was used for data analysis and the compounds were identified by interpretation of their EI mass spectra and comparison to NIST MS Search 2.2 and F-Search 3.5.0 3. Results and Discussion (Frontier Lab) databases. 3.1. Characterization of the Test Samples 3. Results and Discussion 3.1.1. Optical Microscopy and Colorimetry 3.1. Characterization of the Test Samples Observation of the transparent samples through op opticaltical microscope (OM) showed no significant significant features,3.1.1. and Opticaland the the surfacesMicroscopy surfaces from and from the Colorimetry three the dithreeff erent diff sheetserent seemed sheets identical. seemed Inidentical. turn, some In particularities turn, some wereparticularities observedObservation were in the observedof red the transparent samples. in the Figure redsamples samples.1 presentsthrough Figure op OMtical 1 images microscopepresents of OM cross-sections (OM) images showed of no cross-sections of significant the three red of samples.the threefeatures, Itred is samples. visibleand the that surfacesIt is RPS visible sample from that the presents RPS three samp biggerdiffleerent presents pigment sheets bigger agglomeratesseemed pigment identical. thanagglomerates RPAIn turn, or RAL, somethan which RPA mightor RAL,particularities indicate which the might were use observed ofindicate less finely in the the groundedusered samples.of less pigment finelyFigure 1grounded by presents Plásticos OM pigment doimages Sado. byof cross-sections RegardingPlásticos do the of Sado. RAL sheet,Regardingthe one three the of red theRAL samples. surfaces sheet, It one ofis visible theof the samples that surfaces RPS presents samp of thlee presents samples more accumulationbigger presents pigment more ofagglomerates accumulation pigment than than of theRPA pigment other, or RAL, which might indicate the use of less finely grounded pigment by Plásticos do Sado. Figurethan the2, whichother, mightFigure indicate 2, which that might polymerization indicate that of polymerization the PMMA was of conducted the PMMA in horizontal was conducted position in Regarding the RAL sheet, one of the surfaces of the samples presents more accumulation of pigment horizontal position allowing the pigment to sediment. Although in both industrial methods studied allowingthan the the other, pigment Figure to 2, sediment. which might Although indicate th inat both polymerization industrial of methods the PMMA studied was conducted in Portugal in the polymerizationin Portugalhorizontal the position waspolymerization mentioned allowing the towas havepigment mentioned been to conductedsediment. to have Although in been vertical conducted in both orientation, industrial in vertical the methods horizontal orientation, studied position the ® washorizontal alsoin Portugal common position the [polymerization 36was–38 also] and common agrees was mentioned with [36–38] the description andto have agrees been ofwith conducted the the process description in vertical for Altuglas orientation,of the productionprocess the for inAltuglas [4,horizontal50].® production position in was [4,50]. also common [36–38] and agrees with the description of the process for Altuglas® production in [4,50].

FigureFigure 1. OpticalOptical 1. Optical microscopy microscopy microscopy (OM) (OM) (OM) images imagesimages of of cross-sections cross-sections of of ofthe the different different different red redPMMA PMMA samples samples beforebefore artificialartificial artificial aging. aging. Images Images Images acquired acquiredacquired under underunder reflected reflected light light and and cross-polarizing cross-polarizing filters filtersfilters at 200× atat 200200× × originaloriginal magnification.magnification. magnification.

FigureFigure 2. OM 2. OM images images of of the the two two surfaces surfaces ofof RALRAL acquir acquireded under under reflected reflected light, light, bright bright field, field, at 500× at 500 × originaloriginal magnification. magnification. Images Images show show that that the the two two surfaces surfaces of of the the sheet sheet are are not not similar, similar, one one havinghaving more Figureaccumulationmore 2. OMaccumulation ofimages pigment of of thepigment agglomerates two agglomeratessurfaces than of RAL the than other. acquirthe other. Theed sedimentationTheunder sedimentation reflected of light, theof the pigment bright pigment field, might might at be500× an originalindicationbe an magnification. indication that polymerization that polymerizationImages ofshow the PMMA thatof the the PMMA sheets two sheetssurfaces was conductedwas of conducted the sheet in horizontal in arehorizontal not position.similar, position. one having more accumulation of pigment agglomerates than the other. The sedimentation of the pigment might be an indication that polymerization of the PMMA sheets was conducted in horizontal position.

Polymers 2020, 12, 2198 9 of 24

Table3 presents the average values and respective standard deviation for colorimetric measurements (L*, a*, b*) on the test samples. The values obtained for the transparent samples result from the white paper used as base during measurements, with the goal of detecting and following an eventual yellowing during the artificial aging experience. Therefore, the lower L* value of TPL when compared to the other transparent samples before aging, may be attributed to the higher thickness of these samples, which results in a greater barrier for light reflection on the white paper. In turn, the lower L* value obtained for the RPA sample reflects its visible darker color when compared with the other red samples. The higher standard deviation associated with b* coordinate obtained for RAL samples is most probably related to the differences between top and bottom surfaces (Figure2).

Table 3. Average values and standard deviation for color coordinates (L*, a*, b*) of the test samples, before and after 8000 h artificial aging, and respective variation.

0 h 8000 h Variation L* a* b* L* a* b* ∆L* ∆a* ∆b* ∆E* 91.16 0.25 4.78 89.81 0.21 5.37 1.35 0.04 0.60 1.48 TPS − − − ( 0.12) ( 0.02) ( 0.03) ( 0.17) ( 0.02) ( 0.03) ( 0.05) ( 0.03) ( 0.05) ( 0.03) ± ± ± ± ± ± ± ± ± ± 38.73 61.33 52.81 40.15 56.92 42.19 1.43 4.41 10.62 11.59 RPS − − ( 0.01) ( 0.02) ( 0.05) ( 0.01) ( 0.02) ( 0.07) ( 0.01) ( 0.04) ( 0.11) ( 0.11) ± ± ± ± ± ± ± ± ± ± 91.32 0.04 4.14 90.07 0.07 4.98 1.25 0.03 0.83 1.51 TPA − − − − ( 0.00) ( 0.01) ( 0.01) ( 0.04) ( 0.01) ( 0.01) ( 0.05) ( 0.02) ( 0.01) ( 0.04) ± ± ± ± ± ± ± ± ± ± 32.78 56.72 41.64 31.93 54.04 40.18 0.85 2.68 1.46 3.17 RPA − − − ( 0.01) ( 0.01) ( 0.18) ( 0.01) ( 0.03) ( 0.17) ( 0.02) ( 0.04) ( 0.12) ( 0.05) ± ± ± ± ± ± ± ± ± ± 89.75 0.30 4.73 87.26 0.04 5.39 2.50 0.26 0.66 2.60 TPL − − − ( 0.20) ( 0.01) ( 0.02) ( 0.21) ( 0.02) ( 0.01) ( 0.27) ( 0.02) ( 0.02) ( 0.25) ± ± ± ± ± ± ± ± ± ± 36.33 57.74 51.26 39.34 56.17 47.28 3.01 1.57 3.98 5.28 RAL − − ( 0.18) ( 0.25) ( 1.09) ( 0.04) ( 0.13) ( 0.31) ( 0.22) ( 0.12) ( 1.07) ( 0.77) ± ± ± ± ± ± ± ± ± ±

3.1.2. Molecular Characterization Polymer matrixes were characterized by FTIR-ATR and Raman spectroscopies, Figure3. PMMA 1 homopolymer is identified in all samples by the strong C=O stretching absorption peak at 1723 cm− and the characteristic profiles and relative intensities of the other diagnostic peaks, namely the C–C–O 1 1 stretching at 1269 and 1239 cm− , C–O–C stretch at 1190 and 1143 cm− , and C–H stretching at 2995, 1 2951, and 2843 cm− in the FTIR-ATR spectra. In the Raman spectra, the correspondent C–H stretching 1 vibration peaks are visible around 2996, 2949, and 2842 cm− , as well as the carbonyl stretching at 1 1 1727 cm− . The other dominant bands in the Raman spectra are the C–H bend at 1450 cm− , the 1 1 C–O–C stretching at 811 cm− and C–C–O stretching at 599 cm− [51,52]. No other peaks that could be attributed to additives have been detected. This was expected since additives in PMMA sheet production are normally used in small amounts [53,54] and therefore may be present below the detection limits of these techniques. Alterations in the typical absorptions that could be related with degradation of the polymer in the artist’s samples (TPL and RAL) were also not visible. Figure4 presents the UV-Vis spectra of the three types of transparent samples before artificial aging. It is known that pure PMMA absorbs mainly wavelengths <260 nm [15–20,55], but thick samples may present absorption until 300 nm due to the accumulation of chromophores, present in the polymer matrix as consequence of processing [19]. This is probably the case of TPL; however, the additional absorption at 300–400 nm observed in the UV-Vis spectra of TPS and TPA samples is most probably associated with the presence of additives. PMMA for standard use is usually modified with additives, namely UV absorbers to protect from aggressive UV radiation [56]. The different profiles reflect different sheet formulations, which were studied further by mass spectrometry. Polymers 2020, 12, x FOR PEER REVIEW 10 of 24 Polymers 2020, 12, x FOR PEER REVIEW 10 of 24 additives, namely UV absorbers to protect from aggressive UV radiation [56]. The different profiles additives,reflect different namely sheet UV absorbersformulations, to protect which fromwere agstudiedgressive further UV radiation by mass spectrometry.[56]. The different profiles reflectPolymers different2020, 12, 2198 sheet formulations, which were studied further by mass spectrometry. 10 of 24

Figure 3. Infrared (FTIR-ATR) (a) and Raman (b) spectra of all the transparent and red samples, before Figure 3. Infrared (FTIR-ATR) (a) and Raman (b) spectra of all the transparent and red samples, before Figureartificial 3. Infrared aging; all (FTIR-ATR) spectra present (a) and a similar Raman profile, (b) spectra which of all corresponds the transpar toent PMMA and red homopolymer. samples, before artificial aging; all spectra present a similar profile, which corresponds to PMMA homopolymer. artificial aging; all spectra present a similar profile, which corresponds to PMMA homopolymer.

Figure 4. UV-Vis spectra of transparent PMMA samples; TPS (dashed line), TPA (solid line) and TPL (dash-pointedFigure 4. UV-Vis line) spectra before of artificial transparent aging. PMMA samples; TPS (dashed line), TPA (solid line) and TPL Figure(dash-pointed 4. UV-Vis line) spectra before of artificialtransparent aging. PMMA samples; TPS (dashed line), TPA (solid line) and TPL (dash-pointedTD-GC/MS line) analysis before allowed artificial aging. the identification of several additives such as plasticizers, UV stabilizers,TD-GC/MS and analysis release allowed agents the on theidentification PMMA samples; of several a summary additives of such the resultsas plasticizers, obtained UV is shownstabilizers,TD-GC/MS in Table and4 release.analysis agents allowed on the the PMMA identification samples; of a summaryseveral additives of the results such obtainedas plasticizers, is shown UV in stabilizers,Table 4. and release agents on the PMMA samples; a summary of the results obtained is shown in TableTable 4. 4. Summary of additives identified by TD-GC/MS in the PMMA samples. For the abundance calculation:Table 4. Summary Peak area of MMAadditives/peak identified area1 < 5: by xxx; TD-GC/MS 5–15: xx; 15–100:in the PMMA x; > 100: samples. (x) (peak For area the1 = abundancepeak area Tableofcalculation: the 4. compound Summary Peak ofarea of interest). additives MMA/peak identified area1 < by5: xxx; TD-GC/MS 5–15: xx; in 15–100: the PMMA x; > 100: samples. (x) (peak For area the 1abundance = peak area

calculation:of the compound Peak area ofCompounds interest). MMA/peak area1 < 5: xxx; 5–15: xx; TPS 15–100: RPS x; > 100: TPA (x) (peak RPA area1 = TPL peak area RAL of the compound of interest). InitiatorsCompounds TPS RPS TPA RPA TPL RAL Azobisisobutyronitrile (AIBN) x x x x x x CompoundsInitiators TPS RPS TPA RPA TPL RAL Plasticizers AzobisisobutyronitrileInitiators (AIBN) x x x x x x Diethyl phthalate (DEP) (x) (x) ((x)) ((x)) (x) (x) DibutylAzobisisobutyronitrile phthalatePlasticizers (DBP) (AIBN) / (x) x / x x x x / x xx x Bis(2-ethylhexyl) phthalate (DEHP)DiethylPlasticizers= phthalatedioctyl phthalate (DEP) (DOP) xxx xxx (x (x)) (x) /((x)) ((x))xxx (x) / (x) Diisononyl phthalate (DINP) // x /// DiethylDibutyl phthalate phthalate (DEP) (DBP) (x) / (x) (x) ((x)) / ((x)) x (x) / (x) xx Bis(2-ethylhexyl)UV phthalateDibutyl stabilizers phthalate (DEHP) (DBP)= dioctyl phthalate (DOP) xxx/ (x) xxx /(x) x / xxx/ xx / Ethyl 2-cyano-3,3-diphenylacrylate (Etocrylene) x ///// Bis(2-ethylhexyl) phthalateDiisononyl (DEHP) phthalate = dioctyl (DINP) phthalate (DOP) xxx / xxx / (x) x / / xxx / / / 2-(2-hydroxy-5-methylphenyl)benzotriazole (Drometrizole) x // x // DiisononylUV phthalate stabilizers (DINP) / / x / / / Ethyl 2-cyano-3,3-diphenylacrylateRelease agents (Etocrylene) x / / / / / Palmitic acid, methylUV stabilizers ester (x) (x) / (x) (x) / 2-(2-hydroxy-5-methylphenyl)benzotriazoleEthylStearic 2-cyano-3,3-diphenylacrylate acid, methyl ester (Etocrylene) (Drometrizole) (x) (x) x x/ / / (x) / / / (x) x / / ((x)) / / Release agents xxx: High2-(2-hydroxy-5-methylphenyl)benzotriazole abundance; xx: Medium abundance; x: Positive (Drometrizole) presence; (x): Trace; ((x)): x Ultra-trace; / / /: Not x detected. / / Release agents

Polymers 2020, 12, 2198 11 of 24

The initiator azobisisobutyronitrile (AIBN) was detected in all samples, which confirms the information collected regarding Plásticos do Sado and Paraglas during the historical research [11], presented in Table1. Di fferent plasticizers were detected in the samples; all were identified as phthalates, the most widely used plasticizers worldwide [57]. Bis(2-ethylhexyl) phthalate (DEHP), a structural (or constitutional) isomer of dioctyl phthalate (DOP), was detected in Plásticos do Sado samples (both TPS and RPS) and also in the artist’s sample TPL. Its identification in Plásticos do Sado also agrees with the information gathered in the historical research [11]. In the other artist’s sample, RAL, the plasticizer identified was dibutyl phthalate (DBP). In Paraglas samples, two different plasticizers were detected: Diisononyl phthalate (DINP) in TPA, and DBP in RPA. Additionally, traces of diethyl phthalate (DEP) were detected in all samples. Although a quantitative analysis of the additives was not performed in this study, it is possible to infer from the chromatograms the relative amount of plasticizer in the different PMMA typologies by comparing the ratio between plasticizer and MMA peaks (the main volatile). When evaluating the reference samples, it is clear that more plasticizer was used in PMMAs by Plásticos do Sado than by Paraglas. In the artist’s samples, higher amount of plasticizer was detected in TPL than in RAL but this difference is not so marked. UV-stabilizers were found only in two reference PMMAs: Etocrylene (ethyl 2-cyano-3,3-diphenylacrylate) in TPS and drometrizole (2-(2-hydroxy-5-methylphenyl)benzotriazole) in TPS and RPA. The reason for using two different UV-stabilizers in the same PMMA sheet, TPS, was not clarified. Palmitic and stearic acids were detected in trace in all samples except TPA; these compounds are normally used as lubricants or release agents, facilitating the removal of PMMA sheets from the molds. The use of stearic acid as release agent in Plásticos do Sado agrees with the data collected during historical research. Molecular weight data obtained by SEC are presented in Table5. In what concerns the results for the test samples before artificial aging (0 h), all weight-average molecular weight (Mw) values fall in the order of millions (106) confirming that all the sheets were cast [38]. Comparing the samples produced by Plásticos do Sado to the ones produced by Paraglas, it is visible that the first have somewhat lower molecular weights and higher molecular-weight dispersity (ÐM) values. This might be explained by two factors: The use of recycled monomer (that may introduce impurities that act as “chain-transfer agents” which short-stop the polymer growth locally and create a more irregular structure [58] and the lack of a post-polymerization step (residual monomer may also contribute to a more irregular structure). Interesting is also the difference between the two artist’s samples (TPL and RAL). TPL presents similar Mw and ÐM values to the ones obtained for TPS, while RAL presents the highest molecular weight and the lower dispersity of all samples. This difference may be explained by different productions processes (unknown to the authors) as it seems to be the case with the Portuguese samples. Even so, considering that both samples have already more than 50 years of natural aging and that PMMA degrades mainly by chain scission [21], these values are an indication of the great stability of both PMMA sheets.

Table 5. Evolution of weight-average molecular weight (Mw), number-average molecular weight (Mn), and molecular-weight dispersity (ÐM) of the test samples during artificial aging.

0 h 500 h 2000 h 4000 h 8000 h M M M M M M M M M M w n Ð w n Ð w n Ð w n Ð w n Ð ( 105) ( 105) M ( 105) ( 105) M ( 105) ( 105) M ( 105) ( 105) M ( 105) ( 105) M × × × × × × × × × × TPS 12.25 4.32 2.8 4.08 1.62 2.6 1.66 0.65 2.6 1.48 0.49 3.1 1.15 0.43 2.6 RPS 12.31 3.46 3.6 6.50 1.58 3.9 2.94 0.59 4.4 2.17 0.41 5.3 1.78 0.30 6.0 TPA 13.30 6.20 2.1 5.12 1.89 2.7 2.31 1.05 2.2 1.98 0.88 2.3 1.73 0.69 2.5 RPA 12.58 4.99 2.5 9.04 3.19 2.8 4.70 1.84 2.6 3.00 1.13 2.6 1.55 0.59 2.6 TPL 12.12 4.61 2.6 3.19 1.39 2.3 1.25 0.57 2.2 1.39 0.56 2.5 0.95 0.39 2.4 RAL 19.08 9.39 2.0 12.13 5.27 2.3 5.19 2.05 2.5 2.29 0.83 2.8 1.99 0.80 2.5

3.1.3. Mechanical Characterization (Via Vickers Hardness) Vickers Hardness (HV) values obtained for the test samples before artificial aging are reported in Figure5. The presence of pigments and eventual fillers in the sheets does not seem to be a determinant factor for their superficial hardness, since no tendency that separates transparent from red opaque Polymers 2020, 12, x FOR PEER REVIEW 12 of 24

TPL 12.12 4.61 2.6 3.19 1.39 2.3 1.25 0.57 2.2 1.39 0.56 2.5 0.95 0.39 2.4 RAL 19.08 9.39 2.0 12.13 5.27 2.3 5.19 2.05 2.5 2.29 0.83 2.8 1.99 0.80 2.5

3.1.3. Mechanical Characterization (Via Vickers Hardness) Vickers Hardness (HV) values obtained for the test samples before artificial aging are reported in Figure 5. The presence of pigments and eventual fillers in the sheets does not seem to be a determinant factor for their superficial hardness, since no tendency that separates transparent from Polymers 2020, 12, 2198 12 of 24 red opaque samples is observed. The same applies for the age of the sample, as the two artist’s samples, both with 50–60 years old, present the highest and one of the lowest HV values, 26 for RAL samplesand 22.6 is for observed. TPL, respectively. The same applies Again, forthe the production age of the method sample, seems as the to two be artist’sthe main samples, factor influencing both with 50–60the surface years old,hardness present of thePMMA highest sheets, and as one both of thesamples lowest (transparent HV values, and 26 forred) RAL from and each 22.6 Portuguese for TPL, respectively.factory present Again, similar the production values, being method the material seems to by be Paraglas the main clearly factor influencing harder than the the surface one by hardness Plásticos ofdo PMMA Sado. sheets,What are as boththe aspects samples in (transparent the production and that red) influence from each the Portuguese hardness? factory One important present similar aspect values,seems beingto be the the plasticizer. material by As Paraglas expected, clearly samples harder with than more the plasticizer one by Plá (Tablesticos do4) are Sado. softer What (lower are theHV) aspectsthan the in theothers. production However, that Altuglas influence sample the hardness? (RAL) apparently One important has more aspect plasticizer seems to in be its the composition plasticizer. Asthan expected, the sheets samples produced with moreby Paraglas plasticizer (TPA (Table and RPA)4) are but softer is harder. (lower HV)This thandifference the others. is most However, probably Altuglasdue to the sample higher (RAL) molecular apparently weight has of more RAL plasticizer sample (Table in its 5). composition Samples with than lower the sheets molecular produced weights by Paraglaspresent (TPAalso the and lower RPA) butHV isvalues. harder. The This data diff erenceobtained is mostillustrates probably that duethe tosurface the higher hardness molecular of the weightPMMA of sheets RAL sample results (Tablefrom 5the). Samplescombination with of lower these molecular two factors, weights amount present of added also theplasticizer lower HV and values.molecular The weight. data obtained illustrates that the surface hardness of the PMMA sheets results from the combination of these two factors, amount of added plasticizer and molecular weight.

Figure 5. Average Vickers hardness values obtained for all PMMA samples before artificial aging and respectiveFigure 5. errorAverage bars. Vickers hardness values obtained for all PMMA samples before artificial aging and respective error bars. 3.1.4. Thermal Stability 3.1.4.The Thermal thermogravimetric Stability weight loss curves (TG) and the weight loss derivative curves (DTG) of both redThe and thermogravimetric transparent PMMA weight samples loss arecurves presented (TG) an ind Figure the weight6; the respectiveloss derivative thermal curves parameters (DTG) of areboth presented red and in transparent Table6. All samplesPMMA showsamples a severe are pres decreaseented inin weight Figure between 6; the 320–400respective◦ C,thermal with aparameters maximum decompositionare presented in rate Table at 362–3686. All samples◦C, as sh seenow bya severe the sharp decrease peak in on weight the DTG between curves. 320–400 This corresponds°C, with a maximum to PMMA decomposition thermal degradation rate at 362–368 via chain °C, end as seen initiation by the or sharp a mixture peak on of chainthe DTG end curves. with randomThis corresponds chain scission to PMMA initiation thermal [59], leading degradation to complete via chain unzipping end initiation of the polymer or a mixture chain of into chain MMA. end Becausewith random MMA chain volatizes, scission the weightinitiation drops [59], o ffleadingto 0% afterto complete this step unzipping for all the of samples. the polymer Samples chain TPA, into RPA,MMA. and Because RAL degrade MMA volatizes, in this single the weight step, drops while off samples to 0% after TPS, this RPS, step and for TPL all the present samples. a previous Samples degradationTPA, RPA, and step RAL at around degrade 200 in◦C. this single step, while samples TPS, RPS, and TPL present a previous degradation step at around 200 °C.

PolymersPolymers2020 2020,,12 12,, 2198x FOR PEER REVIEW 1313 of 2424

Figure 6. TG and DTG curves of PMMA samples before artificial aging. Curves obtained at a heating Figure 6. TG and DTG curves of PMMA samples before artificial aging. Curves obtained at a heating rate of 10 ◦C/min, under a N2 atmosphere. rate of 10 °C/min, under a N2 atmosphere. Table 6. Thermal parameters obtained from the TG and DTG curves of the different PMMA samples, includingTable 6. Thermal onset temperature parameters (T obtained0) and maximum from the processTG and rateDTG temperature curves of the (T differentmax) in ◦C, PMMA and respective samples,

massincluding losses onset (∆m )temperature in % of original (T0) mass,and maximum for the two process thermo rate degradation temperature steps (Tmax observed.) in °C, and respective

mass losses (∆m) in % of original mass, for the two thermo degradation steps observed. Initial Step Main Step

T0/∆m InitialT maxStep/∆ m T0/∆m Main StepTmax /∆m

TPS 206T/20/∆m 207Tmax/2/∆m 325T/018/∆m T 368max/70/∆m RPSTPS 206/206/22 208 207/2/3 324 325/18/17 364368/70/66 TPARPS -206/2 - 208/3 328 324/17/14 362364/66/60 RPATPA -- - - 325 328/14/12 367362/60/69 TPL 215/3 224/4 333/20 367/68 RPA - - 325/12 367/69 RAL - - 324/11 367/73 TPL 215/3 224/4 333/20 367/68 RAL - - 324/11 367/73 Thermal degradation of PMMA with more than one step has been reported by several authorsThermal [43,60 –degradation64] and the lowerof PMMA temperature with more degradation than one step steps has are been attributed reported to internalby several structural authors defects[43,60–64] created and the during lower polymerization temperature degradation such as unsaturated-end steps are attributed groups to (vinylidene internal structural ends produced defects bycreated disproportionation during polymerization termination), such head-to-head as unsaturated-end links (produced groups by(vinylidene recombination ends termination),produced by anddisproportionation/or peroxides (formed termination) when, polymerization head-to-head occurslinks (produced in the presence by recombination of oxygen), which termination), all have thermallyand/or peroxides labile links. (formed Hirata when and polymerization co-workers [60 ]occurs have concludedin the presence that volatilizationof oxygen), which of impurities, all have suchthermally as unreacted labile links. initiator Hirata or residual and co-workers monomer, [60] may have be responsible concluded forthat weight volatilization losses at of temperatures impurities, belowsuch as 210 unreacted◦C by comparing initiator the or TG residual and DTG monomer, curves of amay commercial be responsible PMMA sample for weight before andlosses after at purification.temperatures In below addition, 210 °C it isby known comparing that thethe presenceTG and DTG of additives curves of may a commercial also influence PMMA the thermal sample behaviorbefore and of after the samples purification. [64,65 In]. addition, it is known that the presence of additives may also influence the thermalSeveral behavior aspects may of the contribute samples [64,65]. to the di fferences in the thermal behavior of Plásticos do Sado samplesSeveral (TPS aspects and RPS) may in contribute what relates to tothe the differences first degradation in the thermal step (around behavior 206 ◦ofC). Plásticos The presence do Sado of impuritiessamples (TPS may and be relatedRPS) in with what the relates use of to recycled the first monomer. degradation In turn, step residual(around monomer 206 °C). The (if still presence present) of isimpurities due to the may lack ofbe the related post-polymerization with the use of step. recycl Consideringed monomer. the compositionIn turn, residual of the monomer samples (Table (if still4), thepresent) presence is due of more to the plasticizer lack of (alsothe post-polymer observed in TPL)ization may step. also playConsidering a role. However, the composition in these PMMA of the sheets,samples plasticizers (Table 4), are the present presence in small of more quantities plasticizer (not detectable (also observed by FTIR-ATR) in TPL) and may since also the play plasticizer a role. DEHPHowever, decomposes in these PMMA at temperatures sheets, plasticizers close to PMMA, are present around in small 323 quantities◦C[65], the (not first detectable degradation by FTIR- step cannotATR) and be directly since the related plasticizer with theDEHP volatilization decomposes of plasticizer. at temperatures However, close if to the PMMA, presence around of plasticizer 323 °C in[65], the the polymer first degradation matrix reduces step the cannot inter-chain be directly interactions related [57 ]with it may the influence volatilization its thermal of plasticizer. behavior However, if the presence of plasticizer in the polymer matrix reduces the inter-chain interactions [57]

Polymers 2020, 12, 2198 14 of 24 originating a less resistant structure. The fact that the same plasticizer, DEHP, was identified in Plásticos do Sado samples and in the artist sample TPL raises the question if not only the quantity used but also the nature of the plasticizer may have an influence on thermal degradation, but with the present data it is impossible to reach a conclusion. Nevertheless, TG curves of Plásticos do Sado reveal the presence of more structural defects in the polymer chains of these samples, which are most probably associated with the use of recycled monomer and the lower control of the polymerization. On the contrary, the use of a chain transfer/termination agent [66] and the performance of a post-polymerization step by Paraglas result in fewer thermally instable links in the PMMA structure, and therefore the first stage of thermal degradation of the polymer is not present. Interestingly, the presence of Cd(S,Se) pigment does not seem to have an influence in the thermal degradation of the polymer in the reference samples. Regarding the artist’s samples, Plexiglas sample (TPL) shows a similar behavior to Plásticos do Sado ones, while Altuglas sample (RAL) is closer to the Paraglas ones. It seems that 50 years of natural aging did not affect the thermal stability of the samples, confirming the incredible stability of PMMA. Once more, differences observed are probably related to the presence of additives and the control of the polymerization. It is also worth notice that higher molecular weight implies less end groups potentially labile in which the chain scission can initiate; this might be contributing to the superior thermal stability of RAL compared to TPL.

3.1.5. Short Note Regarding the Characterization of the Artist’s Samples The differences observed between the two artist samples, TPL and RAL, were surprising. Given the common origin of Plexiglas and Altuglas in France [48,50], it was expected that the technologies used by the two companies would be similar, and consequently, the materials produced. Several aspects should be considered at this point which may contribute to these differences: (1) The two sheets have different thicknesses which may influence the properties of the material; (2) both Rohm & Haas and Altulor produced different types of PMMA cast sheets for different applications, therefore, we may not be comparing materials of similar grade; (3) the origin of the sheets is based exclusively in the notes of the artist, and thus subject to errors.

3.2. Assessment of Aging Behavior

3.2.1. Molecular Alterations As expected, molecular alterations in the polymer were clearly observed by SEC analysis. The evolution of Mw, Mn, and ÐM along the artificial aging for the test samples is presented in Table5. Regarding the reference samples, the first aspect that can be noted is that all four PMMA sheet typologies have experienced a severe decrease in their Mw (one order of magnitude), visible already after 500 h of irradiation even though radiation with λ 300 nm (just in the absorption limit of ≥ PMMA) was used for artificial aging. Furthermore, during sample preparation for SEC analysis, no gel formation was observed, which confirmed that no crosslinking took place during irradiation, in accordance with the findings of other authors [12,23,25]. These results have also confirmed that, as expected [14,18,21], chain scission was the main photodegradation mechanism for all PMMA samples. Regarding dispersity, no significant variations were observed along the aging, except in RPS that presents a strong increase in the ÐM value. For an easier comparison of the different PMMAs photo-stabilities, the number of scissions per chain was calculated and plotted against irradiation time. A linear fit was performed until 4000 h and the respective parameters are presented in Figure7. The values obtained are higher than in previous studies [12,30] which may be explained by the different conditions used during the SEC analysis and in the artificial aging, namely the higher temperature inside the aging chamber. With the exception of RPA, degradation appears to happen in two phases with two different rates, with a shift around 4000 h, being the degradation rate higher in the first phase than in the second. Siampiringue and co-authors [19] have followed the photooxidation of PMMA sheets at long Polymers 2020, 12, 2198 15 of 24 Polymers 2020, 12, x FOR PEER REVIEW 15 of 24

λ PMMAwavelengths is controlled ( > 300 nm)by byexternal FTIR andchromophores UV spectroscopies (contaminants) and concluded which that do photooxidation not form new of chromophoresPMMA is controlled on the by polymer external structure chromophores during (contaminants) photooxidation which and, do therefore, not form newthe photo-reaction chromophores stopson the when polymer the structure external during photo-inductor photooxidation is consumed. and, therefore, Although the photo-reaction in the present stops study when the photooxidationexternal photo-inductor did not isstop, consumed. as we Althoughcontinue into theobserve present chain study scissions, the photooxidation the slowdown did not of stop,the degradationas we continue rate to might observe be chain explained scissions, by the the reduct slowdownion/consumption of the degradation of these rate external might bechromophores. explained by Anotherthe reduction explanation/consumption would ofbe these that externalthe new chromophores.oxidized groups Another formed explanation by the chain would scission be that would the participatenew oxidized in other groups reactions formed and by the therefore chain scission not contribute would participatesignificantly in to other new reactions chain scissions. and therefore not contributeParaglas samples significantly appear to newto be chain more scissions. resistant to photodegradation (less chain scission) than PlásticosParaglas do Sado samples samples. appear If to the be morephotostability resistant toof photodegradationPMMA is controlled (less by chain the scission) presence than of contaminants,Plásticos do Sado this samples. difference If the between photostability the two of samp PMMAles ismay controlled be explained by the presenceby the presence of contaminants, of more additivesthis difference (Table between 4) or by the the two use samples of recycl mayed monomer be explained (Table by 1) the by presence Plásticos of do more Sado. additives (Table4) or byWhen the use comparing of recycled samples monomer from (Table the same1) by company, Pl ásticos doit is Sado. possible to observe that, with exception of RPSWhen at 8000 comparing h of irradiation, samples fromred opaque the same samp company,les performed it is possible better to than observe the transparent that, with exception ones. This of RPS at 8000 h of irradiation, red opaque samples performed better than the transparent ones. This may be an indication of some protection/stabilization effect from the Cd (Se,S) pigment and/or BaSO4. Cadmiummay be an red indication is considered of some a protectionvery stable/stabilization pigment [32] e ffbutect to from the thebest Cd of (Se,S)our knowledge, pigment and there/or BaSOare no4. studiesCadmium that red have is consideredspecifically a looked very stable at the pigment influence [32 of] butthis topigment the best on of the our photostability knowledge, there of PMMA. are no Pintusstudies et that al. have[67] have specifically studied looked the influence at the influence of several of inorganic this pigment pigments, on the photostabilityincluding cadmium of PMMA. red, onPintus the etphotooxidative al. [67] have studied stability the of influence an acrylic of severalemulsion inorganic binding pigments, medium, including p(nBA/MMA), cadmium after red, UV on aging.the photooxidative Their thermal stability analysis of results an acrylic seem emulsion to show that binding paint medium, samples p(withnBA cadmium/MMA), afterred exposed UV aging. to UVTheir radiation thermal were analysis less resultsprone to seem chain to scission show that than paint the other samples pigmented with cadmium paints tested, red exposed but they to did UV notradiation compare were it lesswith prone the binding to chain medium scission thanalone. the It otheris known pigmented that the paints photostability tested, but of they a polymer did not systemcompare depends it with thenot bindingonly on mediumthe chemical alone. and It phys is knownical natures that the of photostability the polymer and of a the polymer pigment, system but alsodepends on the not additives only on present the chemical and the and possible physical intera naturesctions of between the polymer them and [31] the and pigment, therefore but the also same on pigmentthe additives may present have different and the possibleeffects in interactions different polymeric between them systems [31] and[68]. therefore The change the same of tendency pigment observedmay have for diff erentthe RPS effects sample in diff aterent 4000–8000 polymeric h of systems irradiation [68]. Themust change result of from tendency some observed not identified for the alterationRPS sample that at occurred 4000–8000 in h the of irradiationpolymer system must resultduring from that some period. not identified alteration that occurred in the polymer system during that period.

Figure 7. Scissions per chain of reference samples as a function of irradiation time and respective linear Figurefit for 0–4000 7. Scissions h; gray per lines chain indicate of reference an inflection samples on the as evolutiona function with of irradiation time. time and respective linear fit for 0–4000 h; gray lines indicate an inflection on the evolution with time. Regarding the artist’s samples, several observations can be made. First, chain scission was also the degradationRegarding the mechanism artist’s samples, observed. several Second, observatio the twons samples can be made. do not First, present chain a behavior scission thatwas mayalso thebe considereddegradation significantly mechanism distinctive observed. fromSecond, the the reference two samples sheets. do This not means present that a behavior the fact thatthat thesemay besamples considered have alreadysignificantly 50 years distinctive of natural from aging, the does reference not seem sheets. to be This a significant means that factor the infact the that way these they samples have already 50 years of natural aging, does not seem to be a significant factor in the way they have responded to artificial aging through photodegradation. Third, in these samples, as it was

Polymers 2020, 12, 2198 16 of 24 have responded to artificial aging through photodegradation. Third, in these samples, as it was also observed for the reference ones, the decrease in molecular weight was not linear, being faster in an initialPolymers phase. 2020 In, 12 RAL, x FOR the PEER shift REVIEW happens around 4000 h, but in TPL it happens before, at 200016 h,of when24 it seems to stabilize until 4000 h, then continuing again. Even so, after 8000 h, the observed number of also observed for the reference ones, the decrease in molecular weight was not linear, being faster in scissions per chain was similar for the two samples, c.10.8, and very close to the highest value observed an initial phase. In RAL the shift happens around 4000 h, but in TPL it happens before, at 2000 h, for the reference samples, 10.6 (for RPS). when it seems to stabilize until 4000 h, then continuing again. Even so, after 8000 h, the observed numberGravimetric of scissions analysis per did chain not was show similar significant for the variationstwo samples, that c.10.8, could and be very clearly close related to the withhighest the loss of volatilesvalue observed as result for of the photooxidative reference samples, degradation. 10.6 (for RPS). After 8000 h of artificial aging, both reference and artist’s samplesGravimetric had lostanalysis weight, did not but show alterations significant were variations under 0.4% that ofcould original be clearly weight related (maximum with the value measuredloss of forvolatiles the RPS as result sample). of photooxidative degradation. After 8000 h of artificial aging, both reference andAlterations artist’s samples were also had not lost detected weight, but by FTIR-ATRalterations were and Ramanunder 0.4% spectroscopies of original weight in any (maximum of the samples. The shapevalue measured of the peaks for the and RPS their sample). relative intensities remained unchanged, without formation of new shouldersAlterations and/or bands were bothalso innot infrared detected and by RamanFTIR-ATR spectra and Raman of the spectroscopies aged samples. in In any the of one the hand, samples. The shape of the peaks and their relative intensities remained unchanged, without since PMMA degraded through chain scission, the degradation products maintain basically the same formation of new shoulders and/or bands both in infrared and Raman spectra of the aged samples. molecular structure except additional termination groups. With the high molecular weights of cast In the one hand, since PMMA degraded through chain scission, the degradation products maintain sheet,basically the proportion the same ofmolecular the additional structure termination except additional groups termination to the main groups. structure With the would high molecular be insuffi cient to produceweights visible of cast changessheet, the in proportion the spectra. of the On addition the otheral termination hand, because groups additives to the main were structure used in would quantities underbe the insufficient detection to limitproduce of these visible two changes techniques, in the spectra. it was impossibleOn the other to hand, follow because their additives eventual were alteration or lossused during in quantities aging. under the detection limit of these two techniques, it was impossible to follow their eventualBy UV-Vis alteration spectroscopy, or loss during however, aging. it was possible to detect some alterations. Regarding the referenceBy samples, UV-Vis alterationsspectroscopy, in however, the polymer it was matrix possible were to detect not detected some alterations. because Regarding the absorption the by additivesreference in thesamples, 250–400 alterations nm interval in the maskspolymer the matrix region were where not detected alterations because on the the absorption PMMA could by be additives in the 250–400 nm interval masks the region where alterations on the PMMA could be expected [13,16,19,23]. However, in TPA it was possible to follow the decrease of the band at 340 nm expected [13,16,19,23]. However, in TPA it was possible to follow the decrease of the band at 340 nm between 2000 and 8000 h of artificial aging (Figure8a), which indicates that the additive responsible between 2000 and 8000 h of artificial aging (Figure 8a), which indicates that the additive responsible for thisfor absorptionthis absorption is beingis being consumed consumed/released/released duringduring irradiation irradiation exposure. exposure. In TPS, In TPS, the absorption the absorption in thesein these wavelengths wavelengths is sois so high high that that saturates saturates the signal signal and and eventual eventual alterations alterations cannot cannot be detected. be detected. AlterationsAlterations in the in the spectra spectra of of the the red red samples samples (RPA(RPA and RPS) RPS) are are related related with with color color alteration alteration and are and are presentedpresented in Supplementary in Supplementary Materials. Materials. RegardingRegarding the artist’sthe artist’s samples, samples, a decrease a decrease in absorption in absorption in thein tailthe attail 280–400 at 280–400 nm innm TPL in TPL is observed is (Figureobserved8b). This (Figure is interesting 8b). This is because interesting it is because the opposite it is the of opposite what was of observedwhat was observed by other by authors other [ 19], whichauthors have [19], correlated which have an increase correlated in an this increase region in withthis region the formation with the formation of photoproducts of photoproducts during the during the photooxidation of the extrinsic chromophores. Would the decrease observed in our case photooxidation of the extrinsic chromophores. Would the decrease observed in our case be related be related with the consumption/volatilization of those photoproducts, which were maybe formed with the consumption/volatilization of those photoproducts, which were maybe formed during the during the natural aging? Another possible explanation would be the loss of additives, which also naturalabsorb aging? in this Another region. Interestingly, possible explanation this alteration would only happens be the loss in the of first additives, 2000 h, after which that, also between absorb in this region.4000 and Interestingly, 8000 h, the absorption this alteration is stabilized, only happens which means in the that first whatever 2000 h, afterwas being that, consumed between 4000has and 8000disappeared. h, the absorption is stabilized, which means that whatever was being consumed has disappeared.

FigureFigure 8. 8. Absorption spectra spectra of: (a of:) Reference (a) Reference sample TPA sample and (b) artist TPA sample and (TPL,b)artist during sampleartificial TPL, duringaging. artificial aging.

Polymers 2020, 12, 2198 17 of 24

Polymers 2020, 12, x FOR PEER REVIEW 17 of 24 3.2.2. Visual Alterations 3.2.2. Visual Alterations Alterations on the test samples were also followed by colorimetry and results are presented in Table3.Alterations For the reference on the test transparent samples were samples, also followed∆E* values by colorimetry after 8000 h and of artificial results are aging presented are around in 1.5Table for both 3. For TPS the andreference TPA, transparent which is a samples, variation ∆E under* values the after JND 8000 (just h of noticeable artificial aging difference) are around limit of 2.31.5 [69 for]. However,both TPS and for TPA, artist which sample is a TPL, variation a ∆E* unde valuer the of 2.6JND was (just calculated, noticeable whichdifference) is above limit theof 2.3 JND and[69]. reflects However, mainly for a artist decrease sample in the TPL,L* a coordinate. ∆E* value of This 2.6 higherwas calculated,∆L* value which is probably is above due the to JND its greaterand thicknessreflects mainly and its a aging. decrease Variations in the L* incoordinate. red samples This arehigher more ∆L significant,* value is probably especially due in to RPSits greater samples withthickness a ∆E* =and11.59. its aging. This value Variations reflects inmainly red samples a variation are more on the significant,b* axis towards especially a less in RPS saturated samples color. with a ∆E* = 11.59. This value reflects mainly a variation on the b* axis towards a less saturated color. Even so, it is possible to confirm the excellent lightfastness of the cadmium red pigment after such Even so, it is possible to confirm the excellent lightfastness of the cadmium red pigment after such a a long period of irradiation. The red artist sample, RAL, presents a variation higher than what was long period of irradiation. The red artist sample, RAL, presents a variation higher than what was observed for the reference sample RPA but half of the alteration on RPS. Apparently, the 50 years of observed for the reference sample RPA but half of the alteration on RPS. Apparently, the 50 years of natural aging of the artist’s sample did not have a negative effect on its aging performance, in what natural aging of the artist’s sample did not have a negative effect on its aging performance, in what concerns the visual parameters observed. More data regarding color alteration in red samples is concerns the visual parameters observed. More data regarding color alteration in red samples is reportedreported in in Supplementary Supplementary Materials. Material. InIn addition, addition, alterations alterations on on the the samples samples were were followed followed byby OMOM byby photographingphotographing the same areas areas of theof samples the samples along along the agingthe aging experience. experience. AsAs observed observed by by the the other other techniques, techniques, results results show sh diowfferences differences in the in photostability the photostability of the referenceof the samplesreference from samples Plásticos from do Plásticos Sado and do Paraglas.Sado and Paraglas. InIn RPS, RPS, the the formation formation of of several several bubblebubble like featur featureses in in the the interior interior of of the the sample sample became became evident evident afterafter 2000 2000 h h of of irradiation. irradiation. TheseThese features,features, probably formed formed by by volatile volatile photodegradation photodegradation products, products, havehave enlarged enlarged and and/or/or have have migratedmigrated toto thethe surfacesurface afterwards, as as visible visible in in Figure Figure 9.9 .The The same same phenomenaphenomena seemed seemed toto have started started in in the the TPS TPS sample sampless after after 6000 6000 h, but h, in but a much in a minor much degree. minor degree.This Thiswas was not not observed observed in the in theParaglas Paraglas samples. samples.

FigureFigure 9. 9.OM OM images images of of thethe surfacesurface of an RPS sample sample duri duringng artificial artificial aging; aging; ac acquiredquired under under reflected reflected lightlight and and in in bright-field bright-field mode. mode.

AnotherAnother important important aspect aspect waswas thethe development of of auto-fluorescence auto-fluorescence in in the the transparent transparent samples samples duringduring artificial artificial aging.aging. Auto-fluorescence Auto-fluorescence was was observed observed when when samples samples were illuminated were illuminated with blue- with blue-violetviolet light light (λ = (395–440λ = 395–440 nm), Figure nm), Figure10. Studies 10. that Studies use fluorescence that use fluorescence microscopy microscopy as a tool to follow as a tool topolymer follow polymer degradation degradation are scarce are scarce[70] and [70 ]none and nonewas wasfound found that that specifically specifically follows follows PMMA PMMA photodegradation.photodegradation. The The development development ofof fluorescencefluorescence in in artificially artificially aged aged PMMA PMMA samples samples was was detected detected byby fluorescence fluorescence spectroscopy spectroscopy by by Dickens Dickens andand co-workers [28], [28], with with a amaximum maximum excitation excitation at at~375 ~375 nm nm andand emission emission at at 480 480 nm, nm, attributed attributed byby thethe authorsauthors to to the the formation formation of of carbonyl carbonyl groups groups in inα-diketonesα-diketones andandα ,βα,βunsaturated unsaturated aldehydes. aldehydes. Other authors authors [19] [19 ]have have observed observed an an increase increase in inthe the absorption absorption spectraspectra between between 280–500 280–500 nm nm in in commercial commercial acrylic acrylic sheets sheets artificially artificially aged; aged; the the absorption absorption at atλ λ> >400 400 nm wasnm related was related with with the photoproductsthe photoproducts that that resulted resulted from from photooxidationphotooxidation of of extrinsic extrinsic chromophore chromophore contaminantscontaminants in in the the polymer polymer matrix. matrix. Probably simi similarlar photoproducts photoproducts are are responsible responsible for for the the observed observed fluorescencefluorescence in in the the samples samples within within the the present present study. study. This This phenomenon phenomenon is is more more pronounced pronounced in in the the TPS samples,TPS samples, which which indicates indicates that this that sheet this issheet more is pronemore toprone photooxidation, to photooxidation, even ifeven only if by only the by extrinsic the extrinsic chromophores. This agrees with the results obtained with the other techniques. chromophores. This agrees with the results obtained with the other techniques.

Polymers 2020, 12, 2198 18 of 24 Polymers 2020, 12, x FOR PEER REVIEW 18 of 24

FigureFigure 10. 10.Fluorescence Fluorescence microscopy microscopy images images under under blue-violet blue-violet light light of of TPS TPS and and TPA TPA surfaces. surfaces. A A non-aged non- sampleaged sample (0 h) and (0 h) an and artificial an artifici agedal aged sample sample (8000 (8000 h) h) of of each each PMMAPMMA typology typology were were photographed photographed side-by-side.side-by-side. Both Both PMMA PMMA sheets sheets presented presented no no fluorescence fluorescence beforebefore artificialartificial agingaging (dark(dark surfaces at 0 h); TPSh); developedTPS developed more more fluorescence fluorescence (lighter (lighter blue bl underue under radiation) radiation) than than TPA TPA during during aging. aging.

ObservationObservation by by OM OM of of the the artist’s artist’s PMMA PMMA samples sample coulds could only only detect detect alterations alterations on TPL.on TPL. After After 6000 h of6000 irradiation, h of irradiation, small features small features seem to seem have to started have started to form to in form the in interior the interior of the of sample the sample with with a similar a appearancesimilar appearance and degree and to degree what was to what observed was onobserved TPS after on theTPS same after period the same of irradiation. period of irradiation. Additionally, a micro-crackAdditionally, wasa micro-crack observed was on theobserved top TPL on the surface top TPL and surface several and on several the bottom. on the Therebottom. was There not a significantwas not a developmentsignificant development of auto-fluorescence. of auto-fluorescen In RALce. no In alterationsRAL no alterations were observed were observed after 8000 after h of artificial8000 h of aging. artificial aging.

3.2.3.3.2.3. Mechanical Mechanical Alterations Alterations (Via(Via VickersVickers Hardness) FigureFigure 11 11aa shows shows the the variation variation ofof HV in percenta percentagege versus versus irradiation irradiation time, time, for for both both transparent transparent andand red red reference reference samples. samples. TheThe firstfirst aspect worth worth no noticetice is is the the increase increase inin surface surface hardness hardness with with artificialartificial aging aging in allin fourall four samples. samples. This This is curious is curious since since SEC analysisSEC analysis has shown has shown a decrease a decrease in molecular in weightmolecular as result weight of as chain result scission, of chain expected scission, toexpe correspondcted to correspond to a decrease to a decrease of the mechanical of the mechanical strength. Astrength. similar increase A similar in increase surface hardnessin surface(in hardness nano-scale) (in nano-scale) was observed was observed by other authorsby other [authors71] for PMMA[71] for PMMA exposed to UV radiation, who have related this behavior with a relaxation process leading exposed to UV radiation, who have related this behavior with a relaxation process leading to a more to a more compact reorganization of the macromolecules of the top layer. Another factor that may be compact reorganization of the macromolecules of the top layer. Another factor that may be responsible responsible for the increase in hardness in the samples under study, could be the loss (or further for the increase in hardness in the samples under study, could be the loss (or further reactions) of reactions) of plasticizers at the surface level. This would explain why Plásticos do Sado samples, plasticizers at the surface level. This would explain why Plásticos do Sado samples, which have more which have more plasticizer (see Table 4), present a stronger increase in surface hardness than the plasticizerones by Paraglas. (see Table Another4), present interesting a stronger aspect increase is that apparently, in surface in hardness both brands, than red the samples ones by seem Paraglas. to Anotherhave suffered interesting less superficial aspect is that alte apparently,rations than inthe both transparent brands, ones. red samples This tendency seem towas have also su partiallyffered less superficialobserved alterationsfor the molecular than the weight transparent alteration, ones. rein Thisforcing tendency that the was presence also partially of the observedred cadmium for the molecularpigment weightand/or the alteration, filler might reinforcing be having that a theprotec presencetion role of in the the red system cadmium against pigment photooxidation. and/or the filler might beIn Figure having 11b a protection is possible role to follow in the systemthe variation against of photooxidation.the surface hardness of the artist’s samples alongIn Figurethe artificial 11b is aging. possible As can to followbe observed, the variation the two samples of the surface show very hardness different of thebehaviors. artist’s While samples alongin RAL the the artificial surface aging. hardness As can is kept be observed, almost stable the twoduring samples artificial show aging very (variation different is behaviors. less than 2%), While in in RALTPL the the surface surface hardnesshardness is increasing kept almost continuously. stable during It seems artificial that aging after 8000 (variation h of aging, is less this than already 2%), in TPLnaturally the surface aged hardnesssample is still is increasing in a process continuously. of surface reorganization. It seems that Several after 8000 tentative h of aging, explanations this already of naturallythis phenomenon aged sample could is be still discussed; in a process one could of surface be related reorganization. with the loss Several of plasticizers, tentative which explanations in this ofthicker this phenomenon sample may could take belonger. discussed; Both behaviors one could are be relateddifferent with from the what loss was of plasticizers, observed for which the in thisreference thicker samples, sample in may which take after longer. an initial Both period behaviors of increase are di inff erentsurface from hardness, what the was samples observed seemed for the referenceto stabilize. samples, in which after an initial period of increase in surface hardness, the samples seemed to stabilize.

Polymers 2020, 12, x FOR PEER REVIEW 19 of 24 Polymers 2020, 12, 2198 19 of 24 Polymers 2020, 12, x FOR PEER REVIEW 19 of 24

Figure 11. Vickers hardness values variation versus irradiation time for: (a) Reference samples (TPS, Figure 11. Vickers hardness values variation versus irradiation time for: (a) Reference samples (TPS, FigureRPS, TPA, 11. Vickers and RPA); hardness and (b values) artist’s variation samples versus (TPL and irradiation RAL). time for: (a) Reference samples (TPS, RPS, TPA, and RPA); and (b) artist’s samples (TPL and RAL). RPS, TPA, and RPA); and (b) artist’s samples (TPL and RAL). 3.2.4. Alterations on Thermal Stability 3.2.4. Alterations on Thermal Stability 3.2.4. InAlterations Figure 12 on are Thermal presented Stability the TG and DTG curves of the transparent samples after different In Figure 12 are presented the TG and DTG curves of the transparent samples after different irradiation times. A common feature in both reference samples (TPS and TPA) is that, with aging, irradiationIn Figure times. 12 are A commonpresented feature the TG in and both DTG reference curves samples of the transparent (TPS and TPA) samples is that, after with different aging, degradation starts at lower temperatures, even though the temperature of maximum decomposition degradationirradiation times. starts A at common lower temperatures, feature in both even reference thoughthe samples temperature (TPS and of maximumTPA) is that, decomposition with aging, rate remains the same. This shift for lower temperatures reflects the presence of more thermally labile ratedegradation remains starts the same. at lower This temperatures, shift for lower even temperatures though the reflects temperature the presence of maximum of more thermallydecomposition labile links, which are formed during the photooxidation reactions, in particular as result of chain scission. links,rate remains which arethe formedsame. This during shift the for photooxidation lower temperatur reactions,es reflects in particularthe presence as of result more of thermally chain scission. labile links, which are formed during the photooxidation reactions, in particular as result of chain scission.

FigureFigure 12. TG 12. and TG DTG and curvesDTG curves of the transparentof the transparent samples samples after 0, after 500, 0, 2000, 500, and 2000, 8000 and h 8000 of irradiation. h of irradiation. As observedFigure 12. for TG the and reference DTG curves samples, of the alsotransparent the artist’s samples sample afterTPL 0, 500, shows 2000, anand increase 8000 h of of volatiles at lower temperatures with artificial aging. Butirradiation. even though the thermogram of the sample before As observed for the reference samples, also the artist’s sample TPL shows an increase of volatiles aging was similar to the reference sample TPS, in this case, a shift to temperatures lower than 200 ◦C at lowerAs observed temperatures for the with reference artificial samples, aging. also But the ev artien thoughst’s sample the TPLthermogram shows an of increase the sample of volatiles before was not observed. ataging lower was temperatures similar to the with reference artificial sample aging. TPS, But in ev thenis thoughcase, a shiftthe thermogramto temperatures of the lower sample than before200 °C agingwas not was observed. similar to the reference sample TPS, in this case, a shift to temperatures lower than 200 °C was not observed.

Polymers 2020, 12, 2198 20 of 24

4. Conclusions In this work we have studied the impact of specific intrinsic factors, as additives and industrial production processes, on the properties and susceptibility of PMMA cast sheets to photooxidtion. The data collected showed that equivalent PMMA cast sheets produced by different companies in the same period (2000s) presented different properties such as molecular weight distribution, surface hardness, or thermal behavior. Differences were also observed in the behavior of the samples during artificial aging. Namely, Plásticos do Sado samples showed a higher degree of scissions per chain, higher increase in surface hardness, thermal degradation at lower temperatures, appearance of bubble like features inside the sample, and a higher development of UV fluorescence, when compared to Paraglas samples. These differences were correlated with the presence of more plasticizer in the sample composition (Table4) and more defects in the polymeric structure due to use of recycled monomer and the lack of a post polymerization step (Table1). The presence of the inorganic pigment Cd(S,Se) did not seem to influence the initial properties of the PMMA sheets but might have a protective effect during photodegradation, as seen by SEC analysis and micro-hardness measurements. To the best of the authors’ knowledge, it was the first time that the influence of cadmium red pigment in PMMA photodegradation was accessed. Regarding PMMA artist’s samples produced in the 1960s, it was expected that the technologies used by the two companies would be similar, and consequently, also the quality of the materials produced. However, the behavior of the Plexiglas sheet was closer to the Plásticos do Sado reference sheet than to the Altuglas one. This is most probably related with their similar compositions, particularly in what refers to plasticizers, as shown by TD-GC/MS analysis. Nevertheless, unknown differences between the production processes of the two artist’s samples cannot be ruled out either. Surprisingly, we did not detect major differences between references from 2000s and historical artist’s samples from 1960s. Therefore, with these results, we can conclude that the global production process (which may include the polymerization conditions, the monomer origin and/or the organic additives added) plays a more important role in the properties and aging behavior of the PMMA cast sheets than the presence of cadmium red, or the production decade of the acrylic sheets. This confirms our original hypothesis and the concerns of Lourdes Castro in the 1960s, by consciously choosing to buy what she thought were the best brands of PMMA sheet to use in her artworks. Relevant characterization methods for assessing intrinsic properties of PMMA to help estimating their photostability were size exclusion chromatography, micro-indentation, thermogravimetry, and thermodesorption-gas chromatography/mass spectrometry. Further research is still necessary for deeper understanding mechanisms behind the differences observed, but the results obtained show the importance of considering that not all the PMMA cast sheets are alike. This is a significant aspect when selecting testing samples for research in conservation of PMMA, and especially when considering conservation strategies for artworks made of this material. In Museum collections, the origin of the materials is most of the times unknown, but our work has proved that historical and material research may give important clues to evaluate their stability.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/12/10/2198/s1, Figure S1: UV-Vis transmission spectrum of the filter used in the solarbox during the artificial aging experience; Figure S2: Absorption spectra of RPS and RPA during artificial aging; Figure S3: Evolution of the a* and b* coordinates of the red reference samples along the artificial aging. Author Contributions: Conceptualization, J.L.F.; methodology, S.B., J.L.F., A.M.R., and M.J.M.; investigation, S.B., A.M., and M.H.C.; resources, J.L.F., M.J.M., A.M.R., M.P., and L.M.F.; writing—original draft preparation, S.B.; writing—review and editing, J.L.F., M.J.M., A.M.R., A.M., and M.P., M.H.C., L.M.F.; supervision, J.L.F., M.J.M., A.M.R., M.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Fundação para a Ciência e Tecnologia, Ministério da Ciência, Tecnologia e Ensino Superior (FCT/MCTES), Portugal, through CORES Ph.D. Doctoral Programme in the Conservation and Restoration of Cultural Heritage with the SFRH/BD/52318/2013 doctoral grant, through IF/00653/2015 project grant, through the Associate Laboratory for Green Chemistry—LAQV (UIDB/50006/2020) and through C2TN (UIDB/04349/2020). Polymers 2020, 12, 2198 21 of 24

Acknowledgments: The authors are thankful to Lourdes Castro for providing the PMMA historical samples; to Nuno Costa and Laboratório de Análises REQUIMTE—Rede de Química e Tecnologia, FCT NOVA, for performing the SEC analysis; to Vitor Rosa from the Chemistry Department of FCT NOVA for distillation of THF for the SEC analysis; and to Rui Silva from DCM and CENIMAT|I3N, FCTNOVA for giving access to the micro-hardness tester. We would also like to thank Elke Cwiertnia for the initial Py-GC/MS analysis that supported the TD-GC/MS results presented in this paper. Conflicts of Interest: The authors declare no conflict of interest.

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