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Publicly Available Published by De Gruyter June 8, 2017

The effect of TiO2 component on the properties of acrylic and urea-aldehyde resins under accelerated ageing conditions

Helen Veronika Farmakalidis, Stamatis Boyatzis, Antonios M. Douvas, Ioannis Karatasios, Sophia Sotiropoulou, Panagiotis Argitis, Yannis Chryssoulakis and Vassilis Kilikoglou

Abstract

Synthetic resins were introduced in paintings conservation during the 1930s, as an alternative to natural resins, due to their superior resistance to degradation. Their composition usually includes a small amount of additives, such as titanium dioxide. The objective of this work is to study the effect of TiO2 additive on the durability of Paraloid B72 (acrylic resin) and Laropal A81 (urea-aldehyde condensation polymer), both used in art conservation, against photochemical degradation. A methodology involving separating particulate TiO2 from the organic fraction of the resins has been applied, followed by accelerated ageing of the resins in their commercial (C) and modified (M, i.e. after TiO2 removal) has been implemented. The morphological characteristics of resin films were examined through scanning electron microscopy (SEM). Chemical changes, colour properties and photo-chemical stability of the resins were studied with FTIR, UV-Vis absorption spectroscopy and spectro-colorimetry. The results showed a considerably different behaviour between the C and M states of both materials. In particular, C-Paraloid B72 collapses after prolonged irradiation, but within a certain time frame it appears to be relatively stable; on the other hand, C-Laropal A81 is considerably destabilized in comparison to its M state. It can be suggested that TiO2 acts as a UV-blocker for the underlying pigment layers, at the expense of resins’ stability.

Introduction

Synthetic resins are used as surface protection materials, varnishes or consolidants in paintings’ conservation due to their superior resistance to degradation and higher service time as compared to natural resins, which were very popular in the past [1], [2]. However, colour change, development of microcracks, reduced tranparency and overall deterioration of performance have also been routinely observed in synthetic resins, after exposure to various museum, or other natural environmental conditions. All these changes have always been connected to chemical degradation [3], [4]. Paraloid B72 (an acrylic resin) and Laropal A81 (a urea-aldehyde resin) are considered among the synthetic resins most commonly used in paintings conservation.

Paraloid B72 has been used for more than 50 years in conservation interventions. It is produced by Rohm and Haas and is indicated as a methyl acrylate/ethyl methacrylate (MA/EMA, 30/70%w/w) copolymer (Table 1) [5], [6]. The composition of Paraloid B72 slightly varies over time [7], while it has been used as a consolidant and varnish on a wide range of archaeological materials and art objects [3], [8].

Table 1:

Paraloid B72.

Laropal A81 (urea-aldehyde) is produced by BASF and has been introduced in conservation science since the late 90s. It is a low molecular weight resin with chemical composition based on urea-aldehyde compounds. It is formed through a condensation reaction between urea, formaldehyde and another aliphatic aldehyde (isobutyraldehyde) (Table 2) [1], [9], [11]. Laropal A81 is mainly used as varnish on easel paintings and panel paintings, with excellent optical properties very similar to traditional varnishes.

Table 2:

Laropal A81 [9], [10].

Polymers industry has formulated synthetic resins as protective coatings incorporating various additives, among which, TiO2, an efficient UV absorder, known for its photo-catalytic properties is probably the most frequently employed. Although this specific additive has been scarcely reported and discussed in the literature [12], [13], titanium is typically detected in synthetic resins used in conservation [14] (see also experimental section in this paper). In commercial paint systems hindered amine light stabilizers are also used to control degradation, not as UV absobers but as free radical scavengers [15], [16].

Titanium dioxide (anatase, but also rutile) shows a variable photochemical behaviour depending on its actual form and preparation procedures [17]. It has been extensively applied as additive in polymers and polymer composites due to its potential protective properties for the underlying paint layers [18], [19]. Research groups specialising in conservation material studies, have recently focused on the use of TiO2 in industrial paints formulations, where its catastrophic impact on the paint medium has been emphasized [20], [21], [22]. Furthermore, accelerating degradation effects of TiO2 in different polymeric media, have also been investigated by a number of workers [23], [24].

The main objective of the present work was to study titanium dioxide in synthetic resins and its effect on their durability after photochemical ageing. More specifically, the actual effect of TiO2 on the performance characteristics of synthetic resins was studied by comparing the TiO2-impregnated industrial products with their “pure” (or TiO2-deprived) counterparts. The microstructure and chemical parameters of specimens based on the above mentioned synthetic resins were studied, during accelerated photochemical ageing at preset time periods, while the results were compared to those of the reference specimens. Finally, useful conclusions are drawn regarding the service life of these materials, especially at the level that these address their usage in conservation of heritage assets.

Experimental

Film preparation

Paraloid B72 and Laropal A81 films were applied by spin coating (3000 rpm for 120 s, Headway Research INC model No CR15) on silicon wafers and quartz slides to be used for FTIR and UV-Vis measurements respectively. The thickness of the produced films determined with a XP2 AMBIO micro-profilometer ranged between 450 and 1500 nm.

According to the experimental protocol, two groups of specimens were prepared, namely commercial (C) and modified (M). In the first group the commercially available products were utilised without any treatment, while in the second group, modified, TiO2 particles were previously separated from polymers by centrifugation. The concentration of the solutions in both cases was 10% w/w. The solvent used for the dilution of the commercial products was tetrahydrofuran (THF) and acetone for the modified ones, after centrifugation.

A third group of films were prepared through air/hair brushing the commercial and modified materials on glass slides, for examination in SEM, colorimetry measurements and solubility tests.

Photochemical ageing

Accelerated ageing of the specimens was carried out in a SUNTEST chamber equipped with a Xenon light source (300–800 nm) filtered for λ<295 nm, that provides constant irradiation at 765 W/m2. The maximum temperature on the samples during irradiation was 23±2°C and 47% of relative humidity (RH), for 1338 h [4], [5], [25]. The above procedure was used for studying the degradation mechanisms of the two resins and comparing their durability on photochemical stability. The microstructure and chemical parameters of the polymers were studied during light ageing at pre-set time periods, while the results were compared to those of the reference specimens (not aged).

Methods of analysis

FTIR was used for providing detailed information on the chemical structure of resins, of monitoring changes in chemical bonding during accelerated ageing and for evaluating their preservation condition [26]. Spectra were collected on a Bruker-Tensor 27 spectrometer, in transmission mode, in the range of 4000–400 cm−1 (resolution 4 cm−1) [27]. Due to the relatively low film thicknesses (<1 μm) which were cast either on silicon wafers or quartz plates, oxygen was diffused throughout the material allowing for efficient oxidation rates [28]. Transmission mode allowed collection of average information across films, representing changes in the entire body of each material. All measurements were carried out at the same film area of the specimens, so that the results can be comparable. FTIR spectra were processed on Bruker OPUS software; baseline correction was applied with special care not to affect any particular absorption, without smoothing or normalization.

Since the degradation of resins strongly affects their colour and optical properties [27], [28], [29], the determination of the colour parameters during accelerated ageing of the resins was used as an indicator for their preservation during ageing. Colour parameters were measured with a Minolta CM-2002 portable reflectance spectrophotometer, including a xenon lamp and UV radiation cutting filter and an integrating sphere which allows diffuse reflectance measurements on the samples’ surface, with spot size 4 mm.

UV-Vis spectroscopy was used in order to detect the formation of chromophore groups during the accelerated ageing of resins [30]. The UV-Vis spectra were collected in a Perkin-Elmer UV-Vis Lambda 40 spectrophotometer, at wavelength range 1100–190 nm.

Microstructure and texture of the resins films before and after accelerated ageing were examined in SEM (FEI – Quanta Inspect), coupled with an energy dispersive X-ray spectrometer (EDS). Aiming to eliminate the damage of the films by the electrons beam [31], the specimens were coated with gold.

For the solubility test, cyclohexane, toluene and acetone mixtures were used, providing a series of solvents of increasing polarity, as proposed by Feller [28]. For each resin, the dispersion force contribution (fd), the dipolar contribution (fp) and the hydrogen bonding (fh) parameters were determined and the solubility ability was evaluated based on Teas triangular diagram [32]. The above solvents were applied by using a cotton swab; each resin was categorized as soluble or not soluble in a solvent mixture, depending on whether or not it was removed in less than a minute by normal rubbing [6], [28].

Contact angle measurements were performed using Contact Angle Model EWS DIGIDROP GBX instrument equipped with high speed camera, monitored with time step, 21 frames per second. The measurements were used as a rapid method for determining surface alteration on resin films [33]. To investigate the changes in surface energy, the measurements were performed on resins before and after accelerating ageing.

Results and discussion

The investigation of all resin samples by scanning electron microscopy coupled with energy dispersive X-ray analyser (SEM/EDS) revealed the presence of titanium dioxide (TiO2) particles of varying sizes (50 nm−1 μm) and shapes (Fig. 1), depending on the deposition process and drying conditions. The concentration of TiO2 particles was determined up to 7% wt, similarly to the concentrations which were reported by the manufacturer (Paraloid B72 Technical Data Sheet, Rohm and Haas) [34]. TiO2 is typically used in polymeric films in order to absorb light and protect the surface from undesirable photochemical and oxidation reactions [12], [13]. Some investigators suggest that TiO2 provides UV protection by reflecting and/or scattering the UV-rays through its high refractive index; while others that it absorbs UV radiation because of its semiconductive properties [21].

Fig. 1: 
          SEM micrographs of films coated by hair brush on glass slide showing particles with significant TiO2 concentration.

Fig. 1:

SEM micrographs of films coated by hair brush on glass slide showing particles with significant TiO2 concentration.

The role of TiO2 particles in the materials under study is further investigated throughout this paper. As mentioned in the introduction, the scope of the present work was the study of degradation processes and the evaluation of service life of the materials which are routinely used in heritage conservation, and more particularly, in what level TiO2 can influence the photostability of resins. The microstructure and chemical behaviour of the resins were studied during accelerated photochemical ageing at pre-set time periods, while the results were compared to those of the reference specimens (not aged). The above ageing procedure was applied for studying the degradation mechanisms of Paraloid B72 and Laropal A81 with and without titanium dioxide concentration for comparing their durability.

Paraloid B72

The thicknesses of Paraloid B72 films were measured before and after ageing by micro-profilometry and in some cases the results were confirmed by SEM. The thickness of C-Paraloid B72 film after ageing was found to be reduced by 51%, due to solvent removal and subsequent decrease of the film free-volume which is associated with material loss and finally resins’ degradation. The decrease of solubility by 16% is a significant ageing consequence of the polymer. The effect of ageing is also evidenced by contact angle measurements; after 1338 h of photo-exposure, a decrease of 19% in the water-film surface contact angle was also observed, indicating an increase of surface hydrophilicity and consequent change of surface polarity (Table 3).

Table 3:

Physical properties of C-Paraloid B72 films after accelerated ageing.

Accelerated ageing, C-Paraloid B72a
Examination method Reference material Aged materialb Notes
Film thickness 1.170 nm 570 nm Thickness reduction 51%
Feller test (solubility test) Teas parameter (fd) Ν 91 (Cyclohexane 75% – Toluene 25%) Teas parameter (fd) Ν 76 (Acetone 12.5% – Toluene 87.5%) Solubility reduction 16%
Contact angle 72 58 Contact angle reduction 19%

Hydrophilic behaviour

  1. See experimental section for measurements in all examination methodologies.

  2. aMaterial as supplied by the manufacturer; contains TiO2.

  3. b1338 h of photo-ageing (see experimental section).

M-Paraloid B72 samples also exhibited 46% thickness reduction. However, the resin preserved the initial solubility, besides the significant reduction (48%) of the contact angle, indicating increase polymers hydrophilicity (Table 4).

Table 4:

Physical properties of M-Paraloid B72, after accelerated ageing.

Accelerated ageing, M-Paraloid B72a
Examination method Reference material Aged materialb Notes
Film thickness 665 nm 357 nm Thickness reduction 46%
Feller test (solubility test) Teas parameter (fd) Ν 91 (Cyclohexane 75% – Toluene 25%) Teas parameter (fd) Ν 91 (Cyclohexane 75% – Toluene 25%) No change
Contact angle 77 40 Contact angle reduction 48%

Hydrophilic behaviour

  1. See experimental section for measurements in all examination methodologies.

  2. aMaterial after removing TiO2 (see experimental section).

  3. b1350 h of photo-ageing (see experimental section).

Colorimetry measurements reveal significant colour change of C-Paraloid B72 indicating a shift to yellow colour, which is also visible by naked eye. More specifically, there was a significant change on b* parameter (from 3.93 to 4.44 which, corresponds to a colour change of ΔΕ=3.23), related to a yellowing effect (Fig. 2). In contrast, M-Paraloid B72 presents colour stability recording minor change on b* parameter (from 3.87 to 4.15 which corresponds to a minor colour difference of ΔΕ=0.67), not visible by naked eye [26].

Fig. 2: 
            Colorimetric measurements La*b* (a) C-Paraloid B72 (with TiO2), Visible spectrum measurements (b). Measurements reveal significant colour change, indicating a shift to yellow colour.

Fig. 2:

Colorimetric measurements La*b* (a) C-Paraloid B72 (with TiO2), Visible spectrum measurements (b). Measurements reveal significant colour change, indicating a shift to yellow colour.

UV-Vis measurements were carried out at different ageing periods. The spectra of the C-Paraloid B72 exhibited a characteristic intense peak at λ<202 nm. After 1000 h exposure in sun-light, a reduction and broadening of the absorption peak on the UV spectrum was recorded, attributable to degradation of the material in relation to the chromophore groups, to the extent that peak broadening indicates discoloration and crosslinking of the resin [35].

After ageing for 1338 h, a significant decrease (from 0.26 to 0.14) of peak intensity at λ<212 nm was recorded, along with broadening of the ~285 nm absorption peak, assigned to first stages of degradation and appears to be related to the initial presence of chromophores [8], [35] (Fig. 3a).

Fig. 3: 
            UV-Vis absorption spectra of C-Paraloid B72 (a) and M-Paraloid B72 (b) during different times (0–1350 h). UV-Vis spectrum indicates M-Paraloid B72 stability during ageing.

Fig. 3:

UV-Vis absorption spectra of C-Paraloid B72 (a) and M-Paraloid B72 (b) during different times (0–1350 h). UV-Vis spectrum indicates M-Paraloid B72 stability during ageing.

Comparing those findings with the UV-Vis spectrum of M-Paraloid B72, a minor decrease (from 0.12 to 0.10) during the initial time period, which can be attributed to solvent evaporation, indicates stability of the material during ageing (Fig. 3b).

Analysis of C-Paraloid B72 at the initial state through FTIR spectroscopy showed the following characteristic peaks: C–H stretching (2900–3000 cm−1), ester carbonyl stretching at 1733 cm−1, C–H bending at 1448 and 1388 cm−1, and finally the multiple absorptions due to stretching of C–O ester bonds from 1238 to 1161 cm−1 [8], [36] (Table 1, Fig. 4). Although this material has been confirmed by SEM microanalysis to contain TiO2, this is not detected through FTIR due to its rather low amounts.

Fig. 4: 
            FTIR spectra of the C-Paraloid B72 (a). Detail of FTIR spectra with significant change of intensity after 1338 h ageing (b). The overall intensity reduction of the absorption on FTIR spectra indicates depolymerization of the resin.

Fig. 4:

FTIR spectra of the C-Paraloid B72 (a). Detail of FTIR spectra with significant change of intensity after 1338 h ageing (b). The overall intensity reduction of the absorption on FTIR spectra indicates depolymerization of the resin.

The overall intensity reduction of the absorption on FTIR spectrum after 1000 h ageing of C-Paraloid B72 indicates depolymerization of the resin as a result of the material loss associated with the photo-oxidation progress (Fig. 4). By comparing this spectrum with that of M-Paraloid B72 it is evident that the latter presents superior stability, with slower development of the photochemical degradation phenomena (Figs. 4 and 5).

Fig. 5: 
            FTIR spectra of M-Paraloid B72 (a). Detail of FTIR spectra with significant change of intensity after 1350 h ageing (b). Significant reduction during the final ageing hours indicating overall deterioration of resin ester groups.

Fig. 5:

FTIR spectra of M-Paraloid B72 (a). Detail of FTIR spectra with significant change of intensity after 1350 h ageing (b). Significant reduction during the final ageing hours indicating overall deterioration of resin ester groups.

During the last ageing hours, C-Paraloid B72 exhibits an intense reduction of C–H peak at 2900–3000 cm−1 and 1448–1388 cm−1. Moreover, the carbonyl peak 1733 cm−1 related to ester group is reduced and broadened towards lower wavenumbers, which can be attributed to the formation of carboxylic acids and possibly ketones, while the broadening of the carbonyl peaks at 1785 cm−1 is attributed to the formation of low amounts of γ-lactones. Furthermore gradual decrease of intensity of the ester links at 1237 and 1146 cm−1 suggests deterioration of the ester groups at prolonged ageing (Fig. 4). Importantly, a significant broadening and decrease of all absorptions is marked at the highest ageing time (1338 h), which in correlation with the formation of peaks at 1785 and 1105 cm−1 (possibly, lactones) can be attributed to complete destruction of linear esters and lactonization of the acrylic polymer material [4], [25], [35].

In M-Paraloid B72 films, the deterioration process was significantly slower as compared to that of the commercial (C) formula. The spectra collected at different ageing periods indicated that the polymer remained almost unaffected throughout 1026 h of ageing. Extensive UV exposure (1350 h of ageing), M-Paraloid B72 is less affected as compared to the C- formula, while a small decrease in intensity of the ester band (C=O) at 1733 cm−1 is detected. The broadening of the carbonyl peaks at 1780 cm−1 is attributed to the formation of γ-lactones, while the absorption band attributed to hydroxyl groups can be assigned to the formation of carboxylic acids. Also, significant reduction of the intensity at 1238, 1161 cm−1 of the ester links and the broadening of the peak at 1024 cm−1 was recorded during the final ageing hours indicating overall deterioration of resin ester groups (Fig. 5). Comparison of the commercial C-Paraloid B72 formulation with that of the modified material shows collapse of the former at prolonged exposure, while the latter exhibits gradual and less severe changes throughout the photochemical ageing process.

Laropal A81

Laropal A81 polymer in the commercially available formula (containing TiO2) exhibited 49% thickness reduction during photochemical accelerated ageing. After ageing, C-Laropal A81 maintained solubility in most mixtures of polar solvents, exhibiting a significant solubility reduction (27.6%), as a consequence of chemical changes in resin. In addition, a decrease of contact angle (57%) was observed, indicating increase of hydrophilicity (Table 5).

Table 5:

Physical properties of C-Laropal A81 films, after accelerated ageing.

Accelerated ageing, C-Laropal A81
Examination method Reference materiala Agedb Notes
Film Thickness 1.175 nm 600 nm Thickness reduction 49%
Feller test (solubility test) Teas parameter (fd) Ν 91 (Cyclohexane 75% – Toluene 25%) Teas parameter (fd) Ν 80 (Toluene 100%) Solubility reduction 27.6%
Contact angle 70 30 Contact angle reduction 57%; hydrophilic behaviour

  1. See experimental section for measurements in all examination methodologies.

  2. aMaterial as supplied by the manufacturer; contains TiO2.

  3. b1350 h of photo-ageing.

The modified specimens without titanium dioxide exhibited higher thickness reduction (86%) after ageing. However, although the material preserved its initial solubility, film surface hydrophilicity was significantly increased; as a consequence, contact angle was not measurable in the aged sample (Table 6). In addition, non-uniform extensive swelling of the resin film was recorded (Fig. 6).

Table 6:

Physical properties of M-Laropal A81 films, after accelerated ageing.

Accelerated ageing, M-Laropal A81a
Examination method Reference material Agedb Notes
Film Thickness 439 nm 60 nm Thickness reduction 86%
Feller test (solubility test) Teas parameter (fd) Ν 94 (Cyclohexane 100%) Teas parameter (fd) Ν 91 (Cyclohexane 75% Toluene 25%) Solubility reduction 3%
Contact angle 80 Not measurable Contact angle reduction 100%; hydrophilic behaviour

  1. See experimental section for measurements in all examination methodologies.

  2. aMaterial after removing TiO2.

  3. b1350 h of photo-ageing.

Fig. 6: 
            M-Laropal A81, film exhibiting non-uniform extensive swelling after receiving a water drop.

Fig. 6:

M-Laropal A81, film exhibiting non-uniform extensive swelling after receiving a water drop.

Colorimetric measurements of C-Laropal A81 presented colour stability during ageing, with minor differences on b* parameter (from 4.29 to 5.54, ΔΕ=0.45). M-Laropal A81 exhibited lower colour stability however, again with minor differences on b* parameter (from 3.89 to 4.52, ΔΕ=1.51), indicating slight differences in yellow hue, not visible by naked eye (Fig. 7).

Fig. 7: 
            Colorimetric measurements La*b* (a) M-Laropal A81, Visible spectrum measurements (b). M-Laropal A81 exhibited lower colour stability, not visible by naked eye.

Fig. 7:

Colorimetric measurements La*b* (a) M-Laropal A81, Visible spectrum measurements (b). M-Laropal A81 exhibited lower colour stability, not visible by naked eye.

The UV-Vis absorption spectra of both materials (commercial and modified) were very similar. C-Laropal A81 exhibits a peak at 225 nm with no other features in the range 257–800 nm in its initial state; upon ageing, no significant changes were recorded. On both materials, the spectra exhibited a small increase in film scattering at λ>240 nm (Fig. 8), which is a consequence of the various chemical changes occurring within the films (see below).

Fig. 8: 
            UV-Vis absorption spectra of C-Laropal A81 (a) and M-Laropal A81 (b) during different times (0–1350 h). Both materials, exhibited a small increase at λ>240 nm, as result of chemical changes occurring within the films.

Fig. 8:

UV-Vis absorption spectra of C-Laropal A81 (a) and M-Laropal A81 (b) during different times (0–1350 h). Both materials, exhibited a small increase at λ>240 nm, as result of chemical changes occurring within the films.

FTIR spectrum of C-Laropal A81 in its initial state (Table 2, Fig. 9) showed peaks at 2963 and 2874 cm−1 due to asymmetric and symmetric stretching vibrations, respectively, of C–H bonds in the CH3 and CH2 backbone groups; ester and tertiary amide carbonyl absorptions at 1734 cm−1 and 1653 cm−1, respectively. In addition, methylene CH2 vibrations at 1489 and 1457 cm−1, symmetric stretching absorptions of CH3 at 1390–1369 cm−1, the C–N stretching vibrations in the amide groups at 1311 cm−1, ester C–O bonds at 1264 cm−1, deformation of geminal methyl groups at C(CH3)2 at 1218 and 1154 cm−1 are also observed [2], [10], [11].

Fig. 9: 
            FTIR spectra of the C-Laropal A81 (a). Detail of FTIR spectra with significant change of intensity on 1338 h ageing (b). The FTIR spectra showed significant photochemical alterations for the aged material.

Fig. 9:

FTIR spectra of the C-Laropal A81 (a). Detail of FTIR spectra with significant change of intensity on 1338 h ageing (b). The FTIR spectra showed significant photochemical alterations for the aged material.

The FTIR spectra showed significant photochemical alterations for the aged material. More specifically, a dramatic decrease of the tertiary amide carbonyl (1653 cm−1) was marked in parallel with decrease of the alkyl C–H bonds (stretching and bending vibrations) [2], [37]. Gradual decrease and broadening was also observed for the ester absorption (1734 cm−1), indicating the formation of various carbonyl compounds as a result of photochemical ageing. The above correspond to drastic removal of the amide and methylene groups suggesting significant destruction of the material structure. On the other hand, the methyl groups appear unchanged as evidenced by their stretching and bending absorptions at 2877 and 1372 cm−1, respectively.

Additionally, new features are observed, such as the broad OH stretching band (3300–3200 cm−1), and the broad absorption ranging at 1300–1150 cm−1 (various C–O bonds) (Fig. 9). Also, the formation of carboxylic acids, as indicated by the intense new shoulder at 1694 and the peak at 1105 cm−1, is an additional evidence for oxidative degradation, which can be related to the observed significant polarity changes of the resin upon ageing as confirmed by contact angle measurements (see above) [1], [4], [9].

The infrared spectrum of M-Laropal A81 showed better stability for the amide group as compared to the commercial formulation, which however suffers significant change at the highest photochemical exposure time. Additionally, it presented gradual decrease of absorption intensity at 2963 and 2874 and also at 1489 cm−1 (C–H bonds in methylene groups). At the same ageing time, gradual decrease of amide (1654 cm−1) in parallel with broadening and increase of the ester region at 1734 cm−1 was recorded. The above indicate a gradual removal of the amide–containing structure of the M material at moderate ageing which is in sharp contrast to the steep destruction of the same group in C-Laropal A81. These are followed by alterations that favour the formation of esters, which occur at similar degrees for the two materials. The bond changes through FTIR spectra define the degradation process that occurred on polymer backbone structure (Fig. 10).

Fig. 10: 
            FTIR spectra of the M-Laropal A81 (a). Detail of FTIR spectra with significant change of intensity on 1350 h ageing (b). The bond changes through FTIR spectra define the degradation process that occurred on polymer backbone structure.

Fig. 10:

FTIR spectra of the M-Laropal A81 (a). Detail of FTIR spectra with significant change of intensity on 1350 h ageing (b). The bond changes through FTIR spectra define the degradation process that occurred on polymer backbone structure.

The experimental results of the present work provide some evidence on the actual role of TiO2 in synthetic resins and its effect on the materials as protective agents for cultural heritage objects. Due to the fact that polymer/TiO2 composites have been documented to act as both photostabilizers and photodegradation accelerating agents, depending on the base polymer in which they are added; therefore, the comparative study of an acrylic resin and of a urea-aldehyde condensation polymer, which are both used in art conservation and both carry titanium dioxide as additive, is of particular interest. In the case of applications to paintings, an additional function of a TiO2-bearing synthetic resin is its effect on pigments. On the one hand, acrylic materials (such as Paraloid B72) are more benefitted within a certain exposure time, but on the other hand, the urea-aldehyde condensation resin appears to be destabilized, as compared to its behaviour in the modified form (i.e. TiO2 removed); the commercial formula of this material appears to be unsuitable for exposure under ultraviolet light. In the case of applications to paintings, an additional function of a TiO2-bearing synthetic resin is its possible effect on pigments. Presumably, protection to the underlying pigment layer(s) may be offered. A common feature for both commercial resins, however, is that when this limit is exceeded (for instance, the highest exposure time, 1350 h), collapse of both materials’ chemical structure eventually occurs. In addition, in the case that TiO2-loaded resins are used for protecting underlying pigments in paintings, the commercially formulated resin behaves as sacrificial after specific time periods and their replacement within due time periods is needed [20], [21].

Conclusion

This work presents the application of combined physico-chemical methodologies for the characterization of Paraloid B72 and Laropal A81, as examples of acrylic and urea-aldehyde resins during photochemical accelerated ageing. Titanium oxide (TiO2), was detected by SEM (concentration up to 6.5% w/w) in both materials, the particles of which, significantly vary on size and shape. Titanium oxide has been introduced in resin formulations during production process and functions as a stabilizing agent towards UV photocatalytic reactions; however significant destabilizing effect has been detected in certain cases.

The methodology followed, which was based on separating particulate TiO2 from the organic fraction of the resins, in combination with the photochemical ageing process, allowed for valuable comparisons among the commercial and modified states of both materials. The destabilizing effect of titanium dioxide on the commercial Laropal A81 formulation against structural degradation, dictates cautious use of this material concerning its application in art objects, especially in direct or diffuse sunlight. On the other hand, the commercial Paraloid B72 formulation was found to be slightly affected by the presence of TiO2 within approximately 1000 h of photo-ageing; however, it collapsed at longer exposure times, an effect that in the light of the comparatively carried study of the modified material, can be attributed to interactions of photo-excited titanium dioxide with the acrylic material at elevated UV light dosage.

The main outcome of this study is that the use of titanium dioxide as photo-stabilizer in synthetic commercial resins is questionable, as it can significantly vary, depending on the actual organic resin structure. Even more, the various uses of TiO2-containing materials in art objects need specific caution, involving prevention of sunlight, or strict periods for removal and replacement of the material. In addition, when this additive speeds up deterioration processes, the time span of “service life” of the commercial resins may be controlled. Most probably TiO2-containing formulations act as UV protection of underlying pigment layers as far as they remain intact, since they eventually deteriorate in a sacrificial fashion.


Article note

A collection of invited papers based on presentations at the 16th International Conference on Polymers and Organics Chemistry (POC-16), Hersonissos (near Heraklion), Crete, Greece, 13–16 June 2016.


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Published Online: 2017-06-08
Published in Print: 2017-10-26

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