Investigation of Whitening Mechanism on Cultural Relic Surfaces Treated with Paraloid B72

Abstract

In the conservation of cultural relics, the application of Paraloid B72 in humid environments frequently results in the surface whitening of artifacts, which impairs their appearance and conceals important details. This study investigates the mechanisms underlying this phenomenon by examining the effect of ambient humidity, Paraloid B72 mass concentration, solution addition volume, and solvent type. To evaluate the microstructure, transmittance, and composition of the films, a range of analytical techniques were employed, including optical microscopy, scanning electron microscopy, a UV-Visible Spectrophotometer, and Fourier transform infrared spectroscopy. The findings indicate that higher ambient humidity, lower Paraloid B72 mass concentration, smaller solution addition volume, and solvents with higher volatility and water miscibility increase water content during curing, intensifying the whitening effect. These factors modify the interaction between water and solvent, altering the surface structure of Paraloid B72. The whitening mechanism is linked to the cooling effect of solvent volatility, which lowers the dew point temperature at the air–solution interface, causing moisture condensation. Moisture forms droplets that leave irregular pores upon volatility, resulting in surface roughness, optical heterogeneity, and a reduced refractive index, resulting in whitening. This study provides a theoretical basis for understanding and preventing the whitening of Paraloid B72.

1. Introduction

Protective materials form the material foundation of cultural heritage conservation, playing a crucial role in cleaning, consolidating, bonding, and coating artifacts. Among these materials, acrylic resins are among the most widely used, with Paraloid B72 being particularly prominent [1]. Paraloid B72 is a binary hydrophilic copolymer of ethyl methacrylate (70%) and methyl acrylate (30%) [2,3], appearing as transparent granules soluble in organic solvents such as acetone, ethyl acetate, and toluene. The glass transition temperature is 40 °C, and its density is 8.3 lbs/gal (US). Due to its transparency, durability, and non-yellowing properties [4], Paraloid B72 is extensively used as a consolidant, adhesive, and surface coating agent in the conservation of various materials, including ceramics [5], murals [6], metals [7], and wood [8,9]. Despite its significant role in the preservation of cultural heritage and artifacts, it has been observed that Paraloid B72 can cause whitening on the surfaces of artifacts in humid environments [10,11] (Figure 1).

According to a previous literature search, the majority of current research on the whitening of acrylic materials is focused on emulsion-type acrylic protective materials and acrylic-based latex blend films [12,13], with relatively little attention given to solvent-based acrylic materials. Studies on Paraloid B72 materials mainly address their modification [14,15], but there is a lack of research on the whitening mechanisms of artifacts treated with Paraloid B72. This study investigated the whitening phenomenon of Paraloid B72 films by examining factors involved in the recuring system of Paraloid B72, including ambient humidity, Paraloid B72 mass concentration, solution amount, and solvent type. The effects of these factors on the whitening phenomenon were studied using optical microscopy, scanning electron microscopy, a UV-Visible (UV-VIS) Spectrophotometer, and Fourier transform infrared spectroscopy to analyze the microstructure, transmittance, and composition of the films.

The objective of the present study was to contribute to a more comprehensive understanding of solvent-based coating whitening issues by enriching the research on the whitening mechanism of artifact surfaces following the application of Paraloid B72. Furthermore, this research establishes a foundation for future endeavors aimed at elucidating the prevention and restoration of whitening induced by Paraloid B72.

2. Materials and Methods

2.1. Materials and Instruments

Materials: Paraloid B72 (solid, Rohm and Haas, Midland, MI, USA), ethyl acetate (Analytical Reagent, Tianjin Fuyu Fine Chemical Co., Ltd., Tianjin, China), butyl acetate (Analytical Reagent, Macklin Biochemical Co., Ltd., Shanghai, China), acetone (Analytical Reagent, Lianlong Bohua Pharmaceutical Chemistry Co., Ltd., Tianjin, China), sodium dodecyl benzene sulfonate (SDBS, Analytical Reagent, Aladdin Chemical Reagent Co., Ltd., Shanghai, China), and silicone paper.

Instruments: Fourier transform infrared spectrometer (TENSOR 27, Bruker, Billerica, MA, USA), UV-VIS spectrophotometer (Hitachi U2001, Tokyo, Japan), optical microscope (CX40M, Sunny Optical Technology (Group) Co., Ltd., Yuyao, China), and tungsten filament scanning electron microscope (VEGA 3XMU, TESCAN, Brno, Czech Republic).

2.2. Experimental Methods

(1)

Sample Preparation: Prior to the formal experiment, our research group conducted a series of preliminary experiments and reached the conclusion that the whitening of Paraloid B72 was related to the environmental humidity during the curing process. Based on this conclusion, we took the participating factors in the curing process as variables to study whether the participating factors would affect the degree of whitening. According to the pre-experiment results, the factors in the curing process of Paraloid B72 were designed as experimental variables to explore the whitening mechanism of Paraloid B72, as shown in

Table 1. Specific parameters of the samples.

Sample	Humidity (%)	Paraloid B72 Concentration (%)	Solution Amount (mL)	Solvent	Additive
1	60	10	8	Ethyl acetate	/
2	70	10	8	Ethyl acetate	/
3	80	10	8	Ethyl acetate	/
4	90	10	8	Ethyl acetate	/
5	90	1	8	Ethyl acetate	/
6	90	5	8	Ethyl acetate	/
7	90	15	8	Ethyl acetate	/
8	90	20	8	Ethyl acetate	/
9	90	10	4	Ethyl acetate	/
10	90	10	12	Ethyl acetate	/
11	90	10	8	Ethyl acetate	SDBS
12	90	10	8	Butyl acetate	/
13	90	10	8	Butyl acetate	SDBS
14	60	1	8	Acetone	/

Sodium dodecyl benzene sulfonate (SDBS).

. A certain amount of Paraloid B72 solution was placed in a silicone paper box with a bottom area of 7.6 × 2.5 cm² and cured in a constant temperature and humidity chamber at different ambient humidities, with the temperature set at 25 °C. In order to increase the rationality of the experiment, three parallel samples were set for each group of samples, and the properties of the films were subsequently measured after curing. In order to guarantee the stipulated humidity conditions and to minimize the impact of the chamber’s air circulation system on the curing of Paraloid B72 films, an open acrylic box was positioned within the chamber, with the opening positioned away from the strongest airflow. The samples were then subjected to curing within the aforementioned acrylic box.

(2)

Transmittance Testing: The transmittance of the films was measured using a UV-VIS spectrophotometer. Samples were cut into 0.5 × 0.5 cm pieces and numbered, and then random sampling was conducted for testing the transmittance at 580 nm, with a scanning rate of 100 nm/min. The measurements were taken five times, and the average value was recorded.

(3)

Optical Microscopy Testing: The cured Paraloid B72 films were placed on the stage of an optical microscope with the side in contact with silicone paper facing downward. The samples were illuminated with transmitted light, and their morphology was observed at different magnifications.

(4)

Infrared Absorption Spectroscopy Testing: The infrared absorption spectra of the films were measured using a Fourier transform infrared spectrometer with an attenuated total reflectance (ATR) attachment. A small amount of film was placed on the ATR sample stage, and the spectra were recorded in the wavenumber range of 4000–600 cm⁻¹ with a resolution of 4 cm⁻¹. A total of 64 background and sample scans were conducted.

(5)

Scanning Electron Microscopy Testing: The microstructure of the cross-sections of the film samples, fractured in liquid nitrogen, was observed using a scanning electron microscope. The testing conditions were as follows: no gold coating, direct observation, scanning mode: secondary electrons (SEs), accelerating voltage: 10 kV, working distance: 19.95 mm, beam intensity: 10.00.

3. Results and Discussion

3.1. Physicochemical Characteristics of Whitening

To elucidate the underlying causes of the whitening phenomenon in Paraloid B72 films, both the transparent films during the curing process and the whitened films were subjected to microscopic observation and infrared absorption spectroscopy. The microscopic examination of the film’s optical micrographs (Figure 2a) revealed a bubble-free surface on the transparent films, with the only imperfections being scratches at the bottom due to contact with silicone release paper. In contrast, the whitened films exhibited irregularly sized pores at the whitened areas, with smaller pores accumulating around larger ones (Figure 2b). The scanning electron microscopy of the cross-sections of the whitened films indicated that pores were present on the side of the film exposed to air and the film surface was rough (Figure 2c). Infrared absorption spectroscopic analysis showed identical absorption spectra for both transparent and whitened films, with no new absorption peaks emerging (Figure 2a), and no new functional group formation showed that the whitening process is a physical change in the phase transition. These findings indicate that the whitening of the films was associated with the formation of pores, and no chemical transformation occurred during the whitening process.

3.2. The Effects of Ambient Humidity on Whitening Phenomenon

In practical applications, it has been observed that the Paraloid B72 commonly displays whitening under humid environments. To enhance the differentiation in whitening phenomenon among samples, this study focused on the role of ambient humidity as a variable in conditions of consistent humidity to high humidity. Samples 1–4 were utilized to investigate the effects of varying ambient humidity levels—60%, 70%, 80%, and 90%—on the whitening of Paraloid B72 films. These experiments were conducted with the solvent being ethyl acetate, Paraloid B72 mass concentration at 10%, and a solution volume of 8 mL. Transmittance, a fundamental property of a coating’s reflecting appearance, was measured to characterize the degree of whitening and allow for visual comparison. This was accomplished using a UV-VIS Spectrophotometer, which measures film transmittance at the median wavelength of the visible spectrum, specifically at 580 nm [16,17,18]. The experimental results show that there were variations in the whitening regions among different samples. In this study, we utilized the software ImageJ (1.54j) to calculate the area of the whitening regions. The extent of whitening was quantified by the proportion of the whitening area relative to the total area.

The results indicate that as ambient humidity increased, the area of the whitening region of the film expanded and its transmittance decreased, resulting in a more pronounced degree of whitening. At 60% humidity, the Paraloid B72 film exhibited transparency with a transmittance of 85.30% and a smooth surface devoid of pores. At 70% humidity, the film exhibited a noticeable whitening effect, accompanied by a significant reduction in transmittance to 68.46%. The whitened surface displayed a profusion of dense micro-pores with an average diameter of approximately 8 μm, distributed in a clustered pattern. At 80% humidity, the film exhibited severe whitening, with a further decrease in transmittance to 57.94%. The average pore diameter was approximately 12 μm, and the pores appeared in aggregations resembling islands. At the highest humidity tested (90%), the film exhibited the most pronounced whitening, with a corresponding decrease in sample transmittance to 52.90% (Figure 3). The surface pore density exhibited a notable increase, accompanied by considerable variations in pore size, spanning from approximately 25 μm to 250 μm. The pores exhibited a high degree of independence, with smaller pores coalescing toward larger ones.

The data indicate that the hypothesis that the occurrence of whitening could be closely related to the moisture content in the air, with the severity of whitening attributed to the increased number of surface pores on the film.

3.3. Effect of Paraloid B72 Mass Concentration and Solution Amount on Whitening Phenomenon

The selection of the Paraloid B72 mass concentration in conservation work is contingent upon the underlying material. It is generally used at higher concentrations of 5% and above for consolidation purposes, and at lower concentrations ranging from 1% to 5% for protective coatings [8,19,20,21,22]. To reasonably represent the impact of concentration on the properties of Paraloid B72 films, Samples 4 and 5–8 used ethyl acetate as the solvent, with 8 mL of the solution added, to study the effects of Paraloid B72 mass concentrations of 1%, 5%, 10%, 15%, and 20%. In order to enhance the differences in whitening degree among samples, an experimental condition of 90% ambient humidity was selected.

The results indicate that as the mass concentration of the Paraloid B72 increased, the area of the whitening region of the film decreased, while its transmittance increased. At a 1% Paraloid B72 mass concentration, the film’s transmittance at 580 nm was 29.44%, with pore diameters between 8 and 1250 µm, composed of densely packed small pores. At a 5% Paraloid B72 mass concentration, the film’s transmittance was 18.92%, with pore diameters approximately 50–500 µm, featuring large pores formed by the aggregation of smaller, more uniformly sized and tightly connected pores. However, the whiteness of the films with 5%–20% Paraloid B72 mass concentration decreased with the increase of the concentration. This was likely due to the fact that the transmittance in UV-VIS spectroscopy was also influenced by the thickness of the film, a 1% Paraloid B72 film was too thin, resulting in a higher transmittance compared to a 5% Paraloid B72 film (Figure 4). Thus, an analysis of the relationship between pore diameter and whitening degree revealed that the 1% Paraloid B72 film exhibited greater pore size variability, while the 5% Paraloid B72 film had more uniform pores, indicating a more severe whitening effect from a microscopic perspective.

The transmittance of Paraloid B72 films decreased with increasing Paraloid B72 mass concentration, illustrating a correlation between Paraloid B72 concentration and the whitening phenomenon observed in films.

To further explore whether the amount of solution affects the whitening phenomenon, Samples 4, 9, and 10 investigated the influence of 90% ambient humidity, using ethyl acetate as the solvent, with a 10% Paraloid B72 mass concentration and solution volumes of 4 mL, 8 mL, and 12 mL. The experiments demonstrated that, with other variables constant, different solution volumes resulted in distinct whitening degrees after curing. Less solution led to more pronounced whitening, indicating that the degree of whitening was contingent upon the solvent content (Figure 5). Microscopic analysis revealed that the surface pores of film V4 were tightly connected and exhibited a relatively uniform diameter, with an average of approximately 30 µm. In contrast, the surface pores of film V8 exhibited significant variation in diameter, ranging from 25 µm to 250 µm. Additionally, the pores of film V8 were relatively independent, with a tendency for small pores to coalesce into larger ones. In contrast, the pores of film V12 exhibited greater uniformity in size, with an average diameter of approximately 16 µm to 83 µm. Moreover, the pores of film V12 were completely independent. This indicated that the microscopic state of whitening films involved an increase in pore numbers. In this context, the degree of whitening was correlated with either the presence of unevenly sized pores or the presence of uniformly distributed, closely packed pores.

The principle of solvent evaporation posits that a reduction in solution volume will result in a more rapid overall solvent evaporation rate, thereby facilitating the formation of a film in a shorter time frame. This led to the conclusion that the whitening phenomenon was influenced by the solvent.

3.4. The Effect of Solvent on the Whitening Phenomenon

Since the solvent was the main component of the solution, to investigate its impact on whitening, Samples 4 and 11–13 explored the whitening phenomenon under 90% ambient humidity, with 8 mL of solution added and 10% Paraloid B72 mass concentration, using ethyl acetate and butyl acetate as solvents.

Table 2. Solvent-related physical coefficients.

Solvent	Relative Evaporation Rate	Miscibility with Wate (15 °C)
Butyl acetate	100	7.83%
Ethyl acetate	615	0.50%

shows the physical parameters of the ethyl acetate and butyl acetate. To investigate the effect of solvent miscibility with water and relative evaporation rate on Paraloid B72 film whitening under similar water solubility conditions, 1 g of surfactant was added to the Paraloid B72 ethyl acetate and Paraloid B72 butyl acetate solutions, shaken uniformly, and the upper homogeneous solution was collected for film formation. The experiments showed that, under the same water solubility conditions, the Paraloid B72 ethyl acetate film, with a higher relative evaporation rate, exhibited more severe whitening and had a transmittance of 11.30%, while the Paraloid B72 butyl acetate film with a slower relative evaporation rate had a transmittance of 37.06%. However, contradicting the previous conclusion, the percentage of whitened area of the Paraloid B72 butyl acetate film with higher light transmittance was rather larger. It can be inferred that this phenomenon occurred due to the fact that the low relative volatility of butyl acetate is less affected by machine wind than that of ethyl acetate, resulting in a more uniform whitening effect.

In the same solvent, the Paraloid B72 ethyl acetate film exhibited a transmittance of 52.90%, which then sharply declined to 11.30% following the addition of the surfactant. This part was again the same as the previous conclusion—the lower the transmittance of the film, the greater the percentage of whitened area—and the enhanced whitening observed following the addition of the surfactant provided further evidence that the source of whitening involved the participation of atmospheric moisture. From the optical micrographs, it can be seen that the surface of the Paraloid B72 butyl acetate film showed roughness but no holes appeared, and after the addition of SDBS, dense holes appeared, but the diameter of the holes was smaller than that of the Paraloid B72 ethyl acetate +SDBS film. (Figure 6a).

These experiments demonstrated that a higher solvent water solubility and higher relative evaporation rate led to greater moisture involvement, resulting in the formation of more pores and a more pronounced whitening effect. To verify this result, acetone with a relative evaporation rate of 1120 and infinitely miscible with water was chosen. When a 1% Paraloid B72 acetone solution was cured under 60% humidity, the Paraloid B72 film exhibited whitening even under normal humidity conditions (Figure 6b). This indicated that the whitening phenomenon in Paraloid B72 materials did not have a single critical humidity value, but rather, it was influenced by a complex interplay of factors, including the mass concentration of Paraloid B72, the ambient humidity, the amount of solution, and the solvent.

3.5. Mechanism of Whitening in Coating Films

In this paper, the white degree of the film is characterized by the proportion of the white area and the transmittance. However, it is important to note that film thickness affects the transmittance of the film by the UV-VIS spectrophotometer. This study did not directly investigate the relationship between film thickness and transmittance. Instead, it investigated the factors affecting film thickness, the mass concentration of Paraloid B72, and the volume of solution added separately to explore the influence of each single factor. In accordance with Lamberbier’s law, thickness is one of the factors affecting the transmittance, exhibiting a negative correlation. In Samples 1–4, the experimental variables were the different ambient humidity. Theoretically, the thickness of the samples was held constant; however, the experimental results indicate that the higher the humidity, the more pronounced the whitening effect. This suggests that the thickness of the samples did not exert a significant influence on transmittance. In Samples 4, 5–8, the mass concentration of Paraloid B72 was the variable, and the sample thickness increased with the concentration. However, the experimental results indicate that the higher the concentration, the higher the transmittance. The thickness was found to be positively correlated with transmittance, indicating that the thickness had an effect on transmittance, albeit to a lesser degree than that caused by the difference in the solvent relative evaporation rate. In Samples 4, 9, and 10, the variable was the volume of solution added. It was observed that as the volume increased, the film thickness increased, and the transmittance decreased. However, the results also show that as the volume increased, the transmittance increased, indicating that the thickness itself has an influence on the transmittance. However, the degree of influence is considerably less than that caused by the different rates of solvent relative evaporation rate. Similarly, Samples 4 and 12 examined the impact of solvent type on transmittance. The thicker Paraloid B72 butyl acetate film exhibited higher transmittance than the Paraloid B72 ethyl acetate film, suggesting that film thickness may influence transmittance. However, the observed effect was found to be considerably less pronounced than that attributed to differences in the solvent relative evaporation rate (Figure 7).

During the drying and film formation process of coatings, the internal solvent evaporates, causing the particles to closely pack and deposit onto the substrate, resulting in the formation of a solid-state film [23]. The evaporation of the solvent from the surface leads to the shrinkage of the film, creating a porous skeleton. As drying progresses, the gas–liquid interface recedes into the solute particle skeleton, forming menisci within the pores between solute particles. The menisci generate capillary forces, driving the solvent upward to the film surface, thereby emptying most of the solvent from the lower part. The remaining solvent clusters within the skeletal structure evaporate from the inter-particle pores and diffuse to the film surface. As the drying process continues, the particles become increasingly bonded, ultimately forming a pure solid-state dry film [24].

The drying dynamics significantly influence the morphology of the film during the deposition process [25]. The drying kinetics of polymer films can be divided into two stages. In the first stage, the solvent mass diminishes rapidly due to the high solvent content, facilitating rapid solvent transport within the film. The drying behavior is controlled by solvent transport in the gas phase, with the solvent mass fraction at the air/film interface remaining high (>0.25), resulting in significant diffusion and evaporation. As the solvent evaporates and its load in the film reduces, solvent transport slows, and drying becomes increasingly governed by solvent diffusion within the film and gas–liquid phase equilibrium. In the second stage, although the solvent concentration at the air/film interface remains lower (<0.25), diffusion slows, preventing adequate regeneration of the solvent near the interface. Consequently, the solvent content at the gas–liquid interface approaches zero, significantly reducing the drying rate. The diffusion coefficient is dependent on the polymer/solvent ratio, and as the solvent evaporation flux weakens, the concentration gradient is reduced [26,27,28,29,30,31].

In the gaseous environment of the film drying and formation system, in the absence of alterations in water vapor content and air pressure, a reduction in temperature results in the air reach saturation with water vapor. If the temperature continues to decline below the dew point, the water vapor will condense into liquid water droplets [32]. During the condensation process, water vapor molecules randomly aggregate on the condensation surface, forming micron-sized clusters that grow until initial nucleation occurs. The initial liquid droplets absorb surrounding water vapor in order to grow. Initially, adjacent small droplets appear isolated, but they subsequently couple in temperature fields, with significant fluid field interference between droplets. This leads to the formation of observable liquid bridges. These bridges extend under the influence of surface tension to form larger, more stable droplets during the late stage of coalescence (Figure 8) [33].

In materials science, it is observed that polymer composites experience stress whitening under tensile, compressive, and bending stresses. Researchers have found that whitening in different materials is due to cavities formed under stress, leading to changes in the refractive index, presenting as white [34,35,36]. This study demonstrates that the appearance of holes causes a reduction in the refractive index, resulting in whitening.

The experimental results indicate that during the curing process of Paraloid B72 films, rapid solvent evaporation in the first stage absorbs heat, lowering the air/film interface temperature. As the temperature continues to decline below the dew point, the water in the surrounding air becomes saturated and begins to condense on the film surface. As the surface viscosity of Paraloid B72 increases and solvent evaporation slows into the second stage, the Paraloid B72 films cure, water starts to evaporate, and the cavities left by the evaporating water droplets appear as observed holes (Figure 9). Infrared absorption spectroscopy analysis of Paraloid B72 film samples indicates that the observed whitening is caused by physical changes due to alterations in surface roughness, which manifest as holes.

The extent of film surface whitening during curing is influenced by a number of factors, including ambient humidity, the concentration of Paraloid B72, the quantity of solution added, and the type of solvent used. These factors exert a direct or indirect influence on the interaction between water and the film surface, which in turn leads to an increased involvement of water in condensation. Sufficient water leads to closer distances between droplets, causing smaller droplets to merge into larger ones. A lower Paraloid B72 mass concentration results in higher solvent content, prolonging the first stage of solvent evaporation, making nearby air more prone to condensation. As the Paraloid B72 mass concentration increases, particle packing on the Paraloid B72 surface facilitates reaching the second stage of solvent evaporation, extending the overall drying time, causing droplets to invade the hydrophilic Paraloid B72 structure. However, higher concentrations shorten the first stage of solvent evaporation, restricting the solvent evaporation rate and reducing condensation. Lower solution addition amounts significantly shorten the drying time with the same gas–liquid interface area, increasing water content in the air, causing droplets to contact. With increased solution addition, small droplets aggregate, and new small droplets form due to surface tension. Further increases in solution addition result in greater Paraloid B72 surface deposition, affecting the solvent evaporation rate and reducing the first stage drying time, thereby impacting condensation. The type of solvent directly reflects the solvent’s miscibility with water and its relative evaporation rate. A higher miscibility and evaporation rate facilitate whitening; under the same experimental conditions, the surface of the Paraloid B72 butyl acetate film showed a rough state, but no holes appeared (Figure 10). The solvent’s evaporation absorbs heat, causing water in the air to condense on the curing film surface, participating in film curing and forming irregular cavities on the surface. These cavities lead to optical inhomogeneity and a reduction in the refractive index, resulting in whitening.

4. Conclusions

This study investigated the whitening mechanism of Paraloid B72 films by examining the effects of ambient humidity, Paraloid B72 mass concentration, solution addition volume, and solvent type during the curing process. The key findings are as follows:

(1)

Moisture content is a crucial factor in determining the extent of whitening. Variations in influencing factors lead to increased moisture content in the air, a decrease in the dew point temperature at the gas–liquid interface, moisture condensation on the film surface, and pore formation upon evaporation. This surface roughness causes optical heterogeneity, resulting in whitening.

(2)

During the Paraloid B72 curing process, factors such as ambient humidity, Paraloid B72 mass concentration, solution addition volume, and solvent type are interrelated. There is no fixed threshold for ambient humidity. Under normal humidity conditions, managing other influencing factors can still produce a whitening effect on the film.

(3)

In routine cultural heritage conservation, controlling factors such as ambient humidity (it is recommended to use under 50% ambient humidity) and solvent type can minimize the moisture involved in condensation or prevent the microenvironment from reaching the dew point temperature, thereby preventing whitening. According to the conclusion that the whitening is due to the change in physical structure on the surface of Paraloid B72, a further study could investigate whether the Paraloid B72 film can revert to the transparent state after the physical structure is changed again by placing the white film in an environment with a temperature higher than the glass transition temperature.