Effect of Nano Titanium Oxide with Different Surface Treatments on Color Stability of Red-Tinted Silicone Rubber

Abstract

To improve the color stability of facial prosthesis silicone rubber, this paper studied the effect of nano titanium oxide with different surface treatments on the color stability of red pigment-colored silicone rubber. Under the simulated sunlight aging condition, this paper takes MDX4-4210 silicone rubber as the matrix, silicon aluminum-coated nano TiO₂ as the shading agent, and cadmium red oil paint as the colorant, and it observes the values of silicon aluminum-coated nano-TiO₂ silicone rubber film with 1 mm thickness and different concentrations (0, 0.05%, 0.10%, and 0.15%) before and after aging. The experimental results showed that in the four concentrations of silicon aluminum-coated nano-TiO₂ film, the $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\hspace{0.17em}\mathrm{\Delta }{a}^{\ast },\hspace{0.17em}\mathrm{\Delta }{b}^{\ast }$ values gradually decreased with the increase of the concentration of silicon aluminum-coated nano-TiO₂. The lowest was in the 0.10% group; however, it increased in the 0.15% group. There was a significant difference among the concentration groups $P<0.05$. The method of covering nano-TiO₂ silicone rubber film with different concentrations of silicon aluminum has a certain effect on delaying the discoloration of prosthetic silicone rubber, and it provides a new idea for improving the color stability of the prosthetic silicone rubber.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

The maxillofacial region is an important area of personal appearance, and also, it is the location of many important organs, such as those for vision, hearing, and smell. It bears the important functions of human feeling, breathing, eating, emotion, and so on. The defects of maxillofacial organs, such as eyes, ears, nose, cheeks, and jaws, caused by tumors, trauma, and some congenital factors, will lead to not only serious maxillofacial deformities but also a series of functional losses, cause serious psychological trauma to patients, make social and normal work difficult, and even make one lose confidence in life [1]. However, most maxillofacial organs are special and have complex anatomical structures. For example, the defects of eyeball, orbit, ear, nose, and other parts are difficult to repair with autologous tissues. They still need to be repaired in the form of prostheses to restore the patient’s face, maximize the loss of chewing, language, and swallowing functions, and help the patients rebuild their confidence in life. Therefore, the reconstruction of maxillofacial defects has important physiological and psychological significance [2].

At present, the colorimetric research of facial defect repair mainly focuses on the color stability of the prosthesis and the accurate color matching of the prosthesis. The color of prosthesis for repairing maxillofacial soft tissue defects should match the color of surrounding skin. For a long time, the precise color matching of prosthesis has been concerned. However, the disadvantage of silicone rubber and pigments is that they will fade over time [3]. Some data show that in clinical use, the time for patients to request the replacement of prostheses because of color change is months to a year, and color mismatch has become the main reason for the end of the service life of prostheses. Such a short service cycle obviously cannot meet the needs of patients. Therefore, scholars at home and abroad have done a lot of research on the color stability of silicone rubber. Figure 1 shows the preparation of nano materials and their photocatalytic properties.

[figure(s) omitted; refer to PDF]

2. Literature Review

In response to this research problem, Zhou and others introduced silicone rubber as a prosthetic material into maxillofacial prosthesis. Silicone rubber has become the preferred material for maxillofacial prosthesis because of its large inertia, high strength, stable physical and chemical properties, excellent aging performance, and simple fabrication. However, in the process of clinical use, silicone rubber is often subject to the aging effects of light, heat, oxygen, ozone, mechanical fatigue, and other factors, resulting in the gradual reduction of the physical and mechanical properties of the material, and the appearance of discoloration, fading, and other phenomena, which eventually lose its practical value [4]. Petrovi et al. and others matched five colorants (dry soil pigment, viscose fiber flocking, artistic oil painting pigment, kaolin, and liquid cosmetics), with three kinds of silicone rubber (A-type viscous medical silicone rubber, MDX4-4210 silicone rubber, and A-2186 silicone rubber), respectively, and the uncolored silicone rubber was used as the control group. After 6 months of natural aging and storage in dark, the results show that not only the colored silicone rubber changes color after natural aging but also the uncolored silicone rubber changes color after storage in dark for months [5]. Giacomini et al. and others believe that the fading of pigments exposed to sunlight is not a process of gradual disappearance of color substances but a chemical change caused by ultraviolet rays. Ultraviolet rays react with pigments or stimulate the chemical reaction of pigments. With the participation of air and water vapor, the pigments change into colorless or light-colored compounds, resulting in the macroscopic fading of the prosthesis [6]. After testing the color stability of five pigments in a silicone rubber, Cao et al. and others concluded that the early color change of the prosthesis may be caused by the fading of pigments sensitive to ultraviolet light, while the color change after the long-term use may be caused by the color change in the silicone rubber [7]. Zhang et al. and others tested the influence of three common exogenous colorants (tea, coffee, and red wine) on the dyeing of SY-1 silicone rubber. The experimental results showed that the exogenous colorants, namely coffee, tea, and red wine, were visible to the naked eye, and the dyeing was strengthened with the increase of soaking time. Among them, red wine has the greatest impact on silicone rubber. Tea has the effect of dyeing silicone rubber yellow. The dyeing effect of coffee on silicone rubber is mainly manifested in the decrease of lightness [8].

The prosthesis is often affected by the aging of light, heat, oxygen, ozone, mechanical fatigue, and other factors in the process of use, which leads to the gradual reduction of the physical and mechanical properties of the material, discoloration and fading occur, and finally, it losses its use value [9]. Scholars at home and abroad have done a lot of research and tried various methods to improve the color stability of the prosthesis. Ultraviolet absorbers, light stabilizers, color blocking agents, and other additives are added to the silicone rubber of the prosthesis to improve the color stability of the prosthesis, however, no significant results have been achieved. Therefore, this study is the first attempt to cover MDX4-4210 prosthesis silicone rubber body with nano-TiO₂ silicone rubber film coated with different concentrations of silicon aluminum, observe its fading process, and explore the effectiveness of this method to improve the color stability of prosthesis silicone rubber to provide a new idea for improving the color stability of artificial silicone rubber.

3. Research Methods

3.1. Experimental Materials

MDX4－4210RTV silicone rubber, nano-TiO₂ (average particle size ≤150 nm), sodium silicate (analytically pure), sodium sulfate (analytically pure), sodium hydroxide (analytically pure), and oil paint cadmium red.

3.2. Experimental Equipment

FA2004S & B electronic balance, SF-2500 constant temperature electric oven, B-220 constant temperature water bath, SPT spectrophotometer, computer color-measuring and matching system (light source D65), pH meter, centrifuge, SK-1 quick mixer, and simulated daylight bulb (300W, wavelength 365∼400 nm).

3.3. Specimen Fabrication

3.3.1. Material Preparation

After budget, there are 5 test pieces in each group and 20 test pieces in 4 groups. As the amount of nano-TiO₂ and pigment coated with silicon aluminum as the color-masking agent added in each group is very small and because of the special physical properties of silicone rubber, it is difficult to be accurate in the weighing on the balance, the mixing of pigments, and the final introduction into the mold to reduce the impact of errors on the experimental results. The experiment adopts the method of unified configuration of all color-masking agents or pigments [10]. Firstly, prepare MDX4-4210 silicone rubber with 0.2% silicon aluminum-coated nano-TiO₂ by mass ratio, add 0.08 g silicon aluminum-coated nano-TiO₂ to 40 g silicone rubber, and fully mix it. Then, add 0.16 g cadmium red oil paint, and mix it with a quick mixer and manual stirring for 30 min until the paint and nano-TiO₂ are fully dispersed. Prepare a colored silicone rubber with 0.4% cadmium red oil paint for standby. Weigh 0, 0.0025, 0.0050, and 0.0075 g silicon aluminum-coated nano-TiO₂, respectively, and add them to 5 g of silicon rubber. Prepare silicon rubber with the mass ratios of 0, 0.05%, 0.10%, and 0.15%. Mix them with a quick mixer and manual mixing for 30 min and fully disperse them for standby.

3.3.2. Mold Making and Pouring

Based on the 24-hole plate, the height of the 24-hole plate was reduced to 10 mm, and the hole bottom was removed. After polishing, the 24-hole plate was placed on the glass plate lined with a layer of tin paper. The 24-hole plate and the glass plate were tightened to prevent leakage before the silicone rubber was cured, and then the template with a thickness of 1 mm was made. Put the silicone rubber with the mass ratio of 0.4% cadmium red oil paint into the syringe and inject it into the mold to complete the main part of the test piece [11, 12]. Put the mold into the vacuum box, maintain it at −0.1 mpa for 20 min, and remove the negative pressure at midway intervals for exhaust. Then, cover the 1 mm thick template, inject the silicone rubber film with the mass ratio of 0, 0.05%, 0.10%, 0.15% silicon aluminum-coated nano-TiO₂, put the mold into a vacuum box, keep it under −0.1mpa for 20 min, remove the intermediate negative pressure exhaust. Then, put it into a 37°C constant temperature water bath for curing for 24 h, remove the phenanthrene edge, and repair and store it in the dark for standby.

3.4. Simulated Sunlight Aging Method and Color Measurement

3.4.1. Simulated Solar Aging Method

After completing the test piece, measure the original spectral data of the test piece and derive the $L^{\ast }\hspace{0.17em}{a}^{\ast }\hspace{0.17em}{b}^{\ast }$ value recommended by CIE1976. Under the conditions of room temperature (23 ± 1)°C and relative humidity of 50% ± 5%, place the test piece under the simulated solar aging bulb, with a distance of 50 cm, exposure for 68.56 h, which is equivalent to 800H of noon sunshine (3 min of simulated sunlight is equivalent to 35 min of noon sunshine). Then, measure the spectral data of the test piece, derive the $L^{\ast }{a}^{\ast }{b}^{\ast }$ value recommended by CIE1976, and calculate the $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\mathrm{\Delta }{a}^{\ast },\mathrm{\Delta }{b}^{\ast }$ value [13].

3.4.2. Color Measurement and Calculation

The measurement and calculation of color shall be calculated with formulas (1)–(4).

Lightness difference is calculated as follows:$$\begin{array}{}\text{(1)}& \Delta L^{\ast }=L_1^{\ast }-L_2^{\ast }.\end{array}$$

Chromaticity difference is calculated as follows:$$\begin{array}{}\text{(2)}& \Delta a^{\ast }=a_1^{\ast }-a_2^{\ast },\text{(3)}& \Delta b^{\ast }=b_1^{\ast }-b_2^{\ast }.\end{array}$$

Total color difference is calculated as follows:$$\begin{array}{}\text{(4)}& \Delta E_{ab}={{\Delta L^{\ast }}^2+{\Delta a^{\ast }}^2+{\Delta b^{\ast }}^2}^{1/2}.\end{array}$$

3.5. Statistical Analysis

The SPSS11.5 software was used for data processing, analysis of variance was used for comparison between groups, S–N–K test was used for comparison between groups, and $p<0.05$ was the significance test level [14].

4. Result Analysis

See Figures 2–4 and Tables 1–4 for $L^{\ast }\hspace{0.17em}{a}^{\ast }\hspace{0.17em}{b}^{\ast }$ and $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\mathrm{\Delta }{a}^{\ast },\mathrm{\Delta }{b}^{\ast }$ values of silicone rubber specimens with 1 mm thickness and different concentrations of silicon aluminum-coated nano-TiO₂ film before and after aging.

[figure(s) omitted; refer to PDF]

Table 1

$L^{\ast }$ value and differences before and after the aging of silicon aluminum-coated nano-TiO₂ film-covered silicone rubber specimens with different concentrations (F = 7486.332, $P<0.05$).

Chromatic aberration	Concentration
	0%	0.05%	0.10%	0.15%
Front	39.16 ± 0.50	53.01 ± 0.70	56.83 ± 0.79	65.58 ± 0.29
After	42.13 ± 0.50	51.68 ± 0.72	56.03 ± 0.79	62.01 ± 0.28
$\mathrm{\Delta }{L}^{\ast }$	2.98 ± 0.12−	−1.34 ± 0.04	0.80 ± 0.03	−3.57 ± 0.05

Table 2

$a^{\ast }$ value and difference before and after the aging of silicone rubber specimens coated with nano-TiO₂ with different concentrations of silicon and aluminum (F = 20779.00, $P<0.05$).

Chromatic aberration	Concentration
	0%	0.05%	0.10%	0.15%
Front	37.96 ± 0.30	29.30 ± 0.36	26.13 ± 0.26	16.94 ± 0.52
After	23.10 ± 0.23	26.62 ± 0.44	25.14 ± 0.24	15.69 ± 0.57
$\mathrm{\Delta }{a}^{\ast }$	−14.86 ± 0.14	−2.69 ± 0.09	−0.99 ± 0.04	−1.25 ± 0.11

Table 3

$b^{\ast }$ value and difference before and after the aging of silicone rubber specimens coated with nano-TiO₂ with different concentrations of silicon and aluminum (F = 1951.636, $P<0.05$).

Chromatic aberration	Concentration
	0%	0.05%	0.10%	0.15%
Front	15.22 ± 0.50	7.57 ± 0.48	5.15 ± 0.21	1.09 ± 0.09
After	23.20 ± 0.47	13.40 ± 0.52	9.72 ± 0.21	10.00 ± 0.15
$\mathrm{\Delta }{b}^{\ast }$	7.98 ± 0.10	5.83 ± 0.10	4.57 ± 0.10	8.91 ± 0.12

Table 4

Difference before and after the aging of silicone rubber specimens coated with nano-TiO₂ with different concentrations of silicon and aluminum (F = 15623.69, $P<0.05$).

Chromatic aberration	Concentration
	0%	0.05%	0.10%	0.15%
$\mathrm{\Delta }E$	17.15 ± 0.11	6.56 ± 0.09	4.75 ± 0.08	9.68 ± 0.10

The statistical results from Table 1 to Table 4 show that in the test pieces with the same thickness of 1 mm and four concentrations (0, 0.05%, 0.10%, and 0.15%) of silicon aluminum-coated nano-TiO₂ coating, $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\mathrm{\Delta }{a}^{\ast },\mathrm{\Delta }{b}^{\ast }$ gradually decrease with the increase of the concentration of silicon aluminum-coated nano-TiO₂, reach the lowest in the 0.1% group, and then increase in the 0.15% group [15], i.e., $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\mathrm{\Delta }{a}^{\ast },\mathrm{\Delta }{b}^{\ast }$ decreased first and then increased with the increase of the concentration of silicon aluminum-coated nano-TiO₂ film. There was a significant difference in the total color difference $\mathrm{\Delta }E$ among the samples with different silicon aluminum-coated nano-TiO₂ concentration silicone rubber cover film of 1 mm thickness $P<0.05$. The color difference $\mathrm{\Delta }E$ of the sample with 0.1% silicon aluminum-coated nano-TiO₂ silicone rubber cover film was the smallest, and the color difference of the sample without silicon aluminum-coated nano-TiO₂ silicone rubber cover film $\mathrm{\Delta }E$ was the largest. Except for the positive value of $\mathrm{\Delta }{L}^{\ast }$ in the specimen without silicon aluminum-coated nano-TiO₂ silicone rubber cover film, the other concentrations were negative, and there were significant differences among the concentrations $P<0.05$, of which the 0.10% group was the smallest and the 0.15% group was the largest. The $\mathrm{\Delta }{a}^{\ast }$ of the silicone rubber cover film with nano-TiO₂ coated with silicon aluminum at all concentrations was negative, and there was significant difference among the concentrations $P<0.05$. The group without nano-TiO₂ coated with silicon aluminum was the largest, and the group with 0.05% was the smallest. The $\mathrm{\Delta }{b}^{\ast }$ of nano-TiO₂ silicone rubber cover film coated with silicon aluminum at various concentrations is positive, of which 0.10% group is the smallest and 0.15% group is the largest.

4.1. Application of Silicon Aluminum Coated Nano-TiO₂ in Prosthesis

Nano-TiO₂, as a shading agent, mainly has antiultraviolet, antidiscoloration, and antichalking abilities. The modification of nano-TiO₂ includes organic modification and inorganic modification. Organic modification is mainly to make nano-TiO₂ have better compatibility and dispersion in the organic system. Inorganic modification is mainly to form a barrier on the surface of nano-TiO₂, improve its dispersion and surface activity, enhance the antiultraviolet ability of the device, and improve the antipulverization, color retention, and weather resistance abilities and photochemical stability. Considering the advantages and disadvantages of organic modification and inorganic modification, the inorganic modification of nano-TiO₂ with double mixed coating of silicon and aluminum was carried out in this experiment. Silicon coating can enhance the aging resistance and durability of nano-TiO₂, and aluminum coating can enhance the UV resistance and dispersion of nano-TiO₂ in organic media [16].

4.2. Application of Colorimetry in Color Measurement of Prosthesis

Colorimetry is an interdisciplinary subject involving physical optics, visual physiology, visual psychology, and psychophysics. The main color measurement methods used in the field of prosthodontics are visual measurement, spectrophotometry, and spectrophotometry. The spectral reflectance of the object is measured by spectral photometry [17]. The instrument that obtains the tristimulus value and chromaticity coordinates of the object color is called the color measuring spectrophotometer. For the accurate measurement of the object surface color, the spectral spectrophotometer method should be used.

4.3. Effect of Silicon Aluminum-Coated Nano-TiO₂ Coating on Color Stability of Prosthetic Silicone Rubber

4.3.1. Color Difference $\mathrm{\Delta }E$ of Nano-TiO₂ Silicone Rubber Film Coated with 1 mm Thick Silicon Aluminum with Different Concentrations after Simulated Sunlight Aging

The effect of adding cadmium yellow oil paint to SY-1 silicone rubber on the mechanical properties of silicone rubber was studied. It was found that the mechanical properties of silicone rubber changed significantly when the amount of pigment was greater than 0.2%. The main performance is that when the pigment concentration is 0.2%. The tensile strength and tear strength of the experimental group are significantly different from those of the control group [18]. As silicon aluminum-coated nano-TiO₂ can be used as a color masking agent and a white pigment, the concentration higher than 0.2% is not used in this experiment, and the maximum concentration is 0.15%. The results of this experiment show that after 68.56 h of simulated solar aging, there is a significant difference between the color difference of 1 mm thick silicon aluminum-coated nano-TiO₂ silicone rubber cover film specimens with different concentrations $P<0.05$, in which the total color difference of 0.10% silicon aluminum-coated nano-TiO₂ silicone rubber cover film specimen is the smallest, and the total color difference $\mathrm{\Delta }E$ of nonsilicon aluminum-coated nano-TiO₂ silicone rubber cover film specimen is the largest. It can be inferred from Table 1 that after coating, when the concentration of silicon aluminum-coated nano TiO₂ is 0.10%, it is the best to weaken the total color difference $\mathrm{\Delta }E$ of the test piece. At the same time, when the concentration is lower than 0.10% or higher than 0.1%, the total color difference $\mathrm{\Delta }E$ increases, and the ability to weaken the total color difference $\mathrm{\Delta }E$ of the test piece decreases [19].

4.3.2. Lightness $\mathrm{\Delta }{L}^{\ast }$ of Nano-TiO₂ Silicone Rubber Film Coated with 1 mm Thick Silicon Aluminum of Different Concentrations after Simulated Sunlight Aging

Except for the positive value of $\mathrm{\Delta }{L}^{\ast }$ in the specimen without silicon aluminum-coated nano-TiO₂ silicone rubber cover film, the other concentrations were negative, and there were significant differences among the concentrations $P<0.05$, of which the 0.10% group was the smallest and the 0.15% group was the largest. It can be seen that after simulated solar aging, the brightness value of the samples with nano-TiO₂ silicone rubber coating without silicon aluminum coating becomes brighter. However, the lightness values of the test pieces with other concentrations darken after aging, of which the darkening degree of 0.10% group is the lowest. The darkening degree of 0.15% group is the highest, and the darkening degree of 0.10% group is the second [20]. Therefore, it is reasonable to think that 0.10% silicon aluminum-coated nano-TiO₂ silicone rubber cover film has a good effect on protecting the lightness value of the test piece.

4.3.3. Red Green Quality Difference $\mathrm{\Delta }{a}^{\ast }$ of Nano-TiO₂ Silicone Rubber Film Coated with 1 mm Thick Silicon Aluminum with Different Concentrations after Simulated Sunlight Aging

The $\mathrm{\Delta }{a}^{\ast }$ of the samples with nano-TiO₂ silicone rubber cover film coated with silicon aluminum at all concentrations was negative, and there was significant difference between the concentration groups $P<0.05$. The $\mathrm{\Delta }{a}^{\ast }$ of the group without nano-TiO₂ silicone rubber cover film coated with silicon aluminum was the largest, i.e., the red color faded the most, and the protection of the red color was the worst. The $\mathrm{\Delta }{a}^{\ast }$ of the 0.10% silicon aluminum-coated nano-TiO₂ silicone rubber cover film group is the smallest, i.e., the red color fading is the smallest. Therefore, it can be inferred that the covering of 0.10% silicon aluminum-coated nano-TiO₂ silicone rubber film has better red protection for the whole sample, while the sample without silicon aluminum-coated nano-TiO₂ silicone rubber film has more serious discoloration because it has no protection of silicon aluminum-coated nano-TiO₂.

4.3.4. After Simulated Sunlight Aging, Cover the Test Piece with 1 mm Thick Silicon Aluminum-Coated Nano-TiO₂ Silicone Rubber Film with Different Concentrations and Yellow Blue Quality Difference $\mathrm{\Delta }{b}^{\ast }$

The $\mathrm{\Delta }{b}^{\ast }$ of the samples coated with nano-TiO₂ silicone rubber cover film by silicon aluminum at all concentrations is positive, and the 0.10% group is the smallest, i.e., the degree of yellowing is the smallest. The 0.15% group had the largest yellowing degree, followed by the group without silicon aluminum-coated nano-TiO₂ silicone rubber cover film, and the 0.05% group followed. It is speculated that the reason may be that the concentration of silicon aluminum-coated nano-TiO₂ in the 0.15% group cover film is too high, the shading ability is strong, the red color is smaller, and the yellowing degree is very high. However, the concentration of silicon aluminum-coated nano-TiO₂ without silicon aluminum-coated nano-TiO₂ group and 0.05% group is too low, resulting in insufficient antiaging ability of silicone rubber. Hence, the yellowing degree is greater than that of 0.1% [21, 22].

5. Conclusion

From the overall trend, in the test pieces with the same thickness of 1 mm and four silicon aluminum-coated nano-TiO₂ concentrations (0, 0.05%, 0.1%, 0.15%), $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\mathrm{\Delta }{a}^{\ast },\mathrm{\Delta }{b}^{\ast }$ gradually decreased with the increase of silicon aluminum-coated nano-TiO₂ concentration, reached the lowest in the 0.10% group, and then increased in the 0.15% group, i.e., $\mathrm{\Delta }E,\mathrm{\Delta }{L}^{\ast },\mathrm{\Delta }{a}^{\ast },\mathrm{\Delta }{b}^{\ast }$ decreased first and then increased with the increase of the concentration of silicon aluminum-coated nano-TiO₂ film. Under the simulated sunlight condition, the main body surface with the same pigment concentration is covered with 1 mm thick silicon aluminum-coated nano-TiO₂ silicone rubber film with different concentrations to protect the color, and the effect of 0.1% concentration is the best.

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