The impact of zirconium dioxide nanoparticles on the color stability of artificially aged heat-polymerized maxillofacial silicone elastomer

Abstract

The limited service life of craniofacial prostheses due to degradation and color instability is a significant challenge. This in vitro study aimed to determine how zirconium dioxide (ZrO₂) nanoparticles affect the color stability of M511 heat temperature vulcanizing (HTV) maxillofacial silicone elastomers after artificial aging. ZrO₂ nanoparticles were added at concentrations of 1, 2, and 3 wt% to M511 HTV silicone elastomer. Two intrinsic silicone pigments were used (red and mocha). Silicone with pigment and without ZrO₂ nanoparticles were used as the control. Eighty disk-shaped specimens were fabricated and divided into eight experimental groups, each containing ten specimens (n = 10). All specimens were subjected to artificial aging, and color changes were recorded at 252, 504, and 1008 h intervals. The L*a *b * values were measured using a colorimeter and the CIE-Lab system. To interpret the recorded color differences, a 50:50 percent perceptibility threshold (ΔE* = 1.1) and acceptability threshold (ΔE* = 3.0) were implemented. A one-way analysis of variance and Tukey's post hoc test at a significance level of 0.05 were used for the statistical analysis. We found that every evaluated specimen group exhibited a chromatic change (ΔE* > 0). The ΔE* values for the mocha pigments with and without ZrO₂ nanoparticles were below the perceptible threshold (1.1 units). The ΔE* values of the red pigment with and without ZrO₂ nanoparticles were significantly higher than the acceptable threshold (P < 0.000). According to the findings of this in vitro study, all the specimens underwent color changes (ΔE* > 0). The red pigment exhibited highly significant chromatic alterations. In contrast, mocha pigments with and without ZrO₂ nanoparticles exhibited the least color change and were below the perceptible threshold. ZrO₂ nanoparticles provided important protection and showed a reduction in color change.

Keywords

artificial aging color stability color change elastomers maxillofacial silicone nanoparticles

Introduction

Maxillofacial prosthetics is a branch of prosthodontics that focuses on the treatment and management of maxillofacial abnormalities. Extraoral maxillofacial prostheses commonly replace facial components lost due to congenital defects, trauma, or resection. These prostheses restore function and aesthetics, significantly increasing a patient's quality of life.^1–5 Extraoral sections of maxillofacial prostheses are often made of silicone, commonly known as polydimethylsiloxane. Owing to its durability, resistance, biocompatibility, and simplicity of manipulation, it has been used for more than 50 years to fabricate facial prosthetics for patients.^6–12

However, concerns have been raised about the longevity and upkeep of silicone prostheses.^13–16 Prostheses made with silicone elastomers are only color-stable for 6 to 12 months and must then be redone.^5,17–19 The most frequent cause of facial prosthesis replacement is discoloration of the silicone elastomers.^16,20–22

The color change of facial prostheses is caused by exposure to ultraviolet (UV) radiation, air pollution, pigments included in the material, and the use of cleaning solvents.^5,23–25 UV light constitutes a small portion of the total solar radiation but significantly affects the color stability of facial prostheses. Radiation in the UV spectrum includes wavelengths between 100 and 400 nm. Various researchers have utilized artificial weathering or aging chambers, which help assess the overall deterioration of materials, to simulate an outdoor environment.^23,24,26

UV photons are absorbed by polymers, such as silicone, which can lead to the destruction of polymeric chains and the release of new radicals that can further damage the polymer networks. This process may cause surface cracking, loss of gloss, discoloration, and fading, as well as a decrease in molecular weight and reduced flexibility.²⁷ The incorporation of nano-oxide particles into polymers provides UV protection and acts as a strengthening agent. Their strengthening mechanism is derived from the increased chemical reactivity of the particles, which enables them to react with and stabilize the polymer chains. It has been postulated that nanosized oxides contribute to improved UV absorbance capacity and promote the dispersion of UV photons, resulting in decreased overall UV energy absorption by the polymer network.^28,29

The CIE-Lab system, developed by the Commission Internationale de L'Éclairage (CIE), is often used to define color notations. In this system, the overall color difference caused by all the changes in the color coordinates is denoted by ΔE*. The perceptibility threshold for light-skinned tone-silicone maxillofacial prosthetics is 1.1, while the acceptability threshold is 3.0.^30,31 The CIE-Lab system is a nearly uniform color space containing L*, which represents the lightness axis (where black is equal to 0 and white to 100); b* shows the color varying from blue (negative axis) to yellow (positive axis); and a* represents the color varying from green (negative axis) to red (positive axis).³²

Researchers have discovered that the addition of nanoparticles to polymers improves their physical and optical properties. Nanoscale ZrO₂ has a small size, active functions, a large specific surface area, and powerful interactions with organic polymers. Consequently, it can enhance the optical and physical properties of the polymers and their resistance to environmental stress-induced fractures and aging. ZrO₂ nanoparticles are compatible, abrasion-resistant, and possess high flexural strength, modulus of elasticity, dielectric constant, broad bandgap, and good thermal stability.^33–35 A previous study reported encouraging improvements in mechanical properties when ZrO₂ nanofiller was added to maxillofacial silicone and found that the Zirconia nano-powder was nontoxic and did not affect the cytotoxicity of maxillofacial silicone.³⁶

This study aims to evaluate the effect of ZrO₂ nanoparticles on the color stability of M511 heat-vulcanized maxillofacial silicone elastomers after artificial aging. The null hypothesis was that the addition of ZrO₂ nanoparticles to M511 HTV maxillofacial silicone and artificial aging would not affect the color of the silicone.

Methods

Experimental materials

Parts A and B maxillofacial silicone elastomer were sourced from M511 HTV; Technovent Ltd, UK. ZrO₂ nanoparticles with a purity of 99.9%, 20 to 30 nm, SSA of >35 m²/g, and density of 5.89 g/cm³ in nearly spherical, white nano-powder were sourced from SkySpring Nanomaterials Inc., USA. Dry pigment (red and mocha) was sourced from Technovent Ltd, UK.

Experimental design and specimen preparation

A total of 80 specimens in the form of disks (2 mm thick, 20 mm diameter)^37,38 were fabricated and divided into eight experimental groups, each with ten specimens (n = 10). Control specimens were fabricated without ZrO₂ nanoparticles (0% ZrO₂) and contained only pigments. The study specimens were fabricated by mixing varying concentrations of ZrO₂ nanoparticles (1%, 2%, and 3% by weight) with silicone and pigments (red and mocha). Figure 1 shows the distribution of the specimens.

Figure 1.

Flow chart of the specimen distribution and preparation.

The metal molds were fabricated using laser-cut cast iron sheets. Each mold contained 16 specimen holes, and the iron sheet had a thickness of 2 mm. Two stainless-steel plates with exact outer dimensions were cut for each mold to sandwich the mold between them and withstand the clamping force.

The M511 silicone elastomer was supplied as a base (part A) and catalyst (part B), which were combined in a weight ratio of 10:1, according to the manufacturer's instructions. The weight of the pigment was equivalent to 0.2% of the total weight of the silicone.^39–42

Silicone specimens with each pigment were prepared by weighing and mixing the pigment with M511 silicone part A. The pigment and part A were first measured using a digital electronic weight balance (Nimbus® Analytical, Adam Equipment, USA) and then mixed according to the manufacturer's instructions in a vacuum mixer (AX-2000C; Aixin Medical Equipment Co., Ltd, China) for 10 min at a speed of 360 rpm and vacuum of −0.09 MP. However, vacuuming was not performed for the first 2 min to prevent pigment suctioning.⁴³ Pigment and ZrO₂ nanoparticles were weighed and added to M511 silicone part A to fabricate the specimens for the study group. They were combined in a vacuum mixer for 10 min. To prevent the suction of the pigment and ZrO₂ nanoparticles, the vacuum was turned off for the first 2 min, similar to earlier silicon specimen preparations containing only the pigment. The mixing bowl was then set aside to cool to room temperature as the rotation of the mixer generated heat, thereby reducing the working duration of the material. A vacuum mixer was used to combine part B for an additional 5 min. The mixture was then poured into molds using a metal spatula and placed in a vacuum chamber for 2 min to eliminate air bubbles that may have formed during the loading procedure. After that, the molds were placed in a pressure pot (Pentola A pressione typodont; leone s.p.a., Italy) at 0.2 MPa for 2 min to smooth the surface of the mixture and break any air bubbles on its surface. The mold was then sealed and subjected to a 0.03 MPa hydraulic press for 5 min. After sealing and clamping the molds using G-clamps, the material was polymerized in a hot-air oven for 1 h (Memmert; Memmert GmbH + Co. KG, Germany).⁴⁴

The specimens were removed from the molds, washed with water and liquid soap, and dried using tissue paper. The specimens were then cut using scissors to remove the excess material. Before testing, the specimens with visible defects were discarded. All specimens were stored in a lightproof black box to prevent color changes. Color measurements were performed using a digital colorimeter (WR10QC colorimeter; FRU, China). The specimens were subjected to artificial aging in an aging chamber (Weather-Ometer device QUV) in accordance with ASTM G154 Cycle 1 (approximate wavelength: 340 nm; energy irradiance: 0.89 W/m²/nm. The exposure cycle includes 8 h UV at 60 (± 3) °C black panel temperature; 4 h condensation at 50 (± 3) °C black panel temperature).⁴⁵ Specimens were removed for color reading in intervals 252, 504, and 1008 h of artificial aging.^5,41,46 The L, a, and b values of each specimen were measured at baseline and after 252, 504, and 1008 h of artificial aging. The ΔE* was calculated using the formula below⁴⁷:

Δ E * = [{(Δ L)}^{2} + {(Δ a)}^{2} + {(Δ b)}^{2}]^{1 / 2}

where L is the lightness, a is the green–red coordinate, and b is the blue–yellow coordinate.

Statistical analysis

Multiple one-way analyses of variance (ANOVAs) were performed on the ΔE* values using statistical software (IBM SPSS Statistics, v24; IBM Corp, USA) to determine whether there were differences in color between the groups. Post hoc Tukey tests were performed for multiple comparisons when the one-way ANOVA was significant (P = 0.05).

Results

The means and standard deviations of ΔE* with Tukey's HSD statistical significance in the control and ZrO₂ nanoparticle groups for both pigments (red and mocha) subjected to 252, 504, and 1008 h of artificial aging are presented in Table 1. Table 2 shows the ANOVA results for the color alterations (ΔE*). All evaluated specimens of the control and ZrO₂ nanoparticle groups experienced a chromatic alteration (ΔE* > 0). The ΔE* values for the red pigment groups with and without ZrO₂ nanoparticles were above the perceptible and acceptable thresholds. However, ΔE* of the mocha pigment groups with and without ZrO₂ nanoparticles were determined to be below the perceptible threshold of 1.1 units.

Table 1.

Means and standard deviations of color change (ΔE*) with Tukey’s HSD statistical significance for the control and ZrO₂ nanoparticles groups after artificial aging.

Pigments	Control	ZrO₂ nanoparticles
Pigments	Control	1% ZrO₂	2% ZrO₂	3% ZrO₂
After 252 h of artificial aging
Red	25.79 ± 1.31^AA	33.67 ± 0.83^AA	36.31 ± 0.57^Aa	37.23 ± 0.57^Aa
Mocha	0.64 ± 0.40^(A)A	0.37 ± 0.25^(A)A	0.32 ± 0.14^(A)A	0.42 ± 0.21^(A)A
After 504 h of artificial aging
Red	36.98 ± 1.37^Aa	37.13 ± 0.64^Aa	39.63 ± 1.02^Aa	39.55 ± 0.67^Aa
Mocha	0.72 ± 0.45^(A)A	0.51 ± 0.22^(A)A	0.38 ± 0.20^(A)A	0.45 ± 0.18^(A)A
After 1008 h of artificial aging
Red	45.40 ± 1.17^AA	41.42 ± 0.66^Aa	42.16 ± 0.72^Aa	42.31 ± 1.00^Aa
Mocha	0.96 ± 0.44^(A)A	0.53 ± 0.12^(A)A	0.42 ± 0.14^(A)A	0.46 ± 0.20^(A)A

ΔE*, color change of the specimens were evaluated using the following formula ΔE* = [(ΔL)² + (Δa)² + (Δb)²]^1/2; SD, standard deviation

^AA

statistically significant from the perceptible and acceptable thresholds, and all other groups (P < .05), ^(A)A statistically significant from the acceptable threshold and other groups but not from the perceptible threshold and each other; ^Aa statistically significant from the perceptible and acceptable thresholds, and other groups but not from each other.

Table 2.

ANOVA result for color changes (ΔE*) after artificial aging.

	SS	DF	MS	F	p-value
Between Groups	90264.45	24	3761.02	8537.70	0.000*
Within Groups	99.12	225	0.44
Total	90363.56	249

SS, sum of squares; DF, degree of freedom; MS, mean square; F, F-statistic, the ratio of two mean squares that forms the basis of a hypothesis test.; p-value, probability; *, Significant at 5% level of significance (p < 0.05).

It is crucial to note that there were relevant variations in the ΔE* values among the groups. For the red pigment specimens, all groups, including the control and 1%, 2%, and 3% ZrO₂ nanoparticles, showed highly significant color changes (P < 0.000) after 252, 504, and 1008 h of artificial aging. The ΔE* values of all groups were above the acceptable threshold of 3.0 units. The groups treated with ZrO₂ nanoparticles at concentrations of 1%, 2%, and 3% showed a significant reduction in color change after 1008 h of artificial aging (P < 0.000) compared with the control group. The ΔE* value of the control group after 1008 h of aging was 45.40, while values of ΔE* of the 1%, 2%, and 3% ZrO₂ groups were 41.42, 42.16, and 42.31, respectively. The mocha pigment showed good color stability compared to the red pigment, and ΔE* of all groups was below the perceptible threshold of 1.1 units. The color change in the mocha pigment groups was significantly below the acceptable threshold (P < 0.000). The groups of mocha pigment containing different concentrations of ZrO₂ nanoparticles showed a statistically insignificant reduction in color change (P > 0.05).

Discussion

The results of this in vitro study provide evidence for rejecting the null hypothesis because artificial aging altered the color of the M511 maxillofacial silicone elastomer, and the ZrO₂ nanoparticles protected and significantly reduced the color change in the red pigment in all groups. All groups demonstrated a color change (ΔE* > 0). The red pigment exhibited the most substantial color change (ΔE* > 3.0).

In addition, red and mocha pigment changes were observed in all groups, with and without ZrO₂ nanoparticles. Even though several of these color changes were well below the perceptibility threshold of 1.1 for the mocha pigment, the red pigment was more substantial than the acceptability threshold of 3.0.⁴⁸ This indicates that the color change of the silicone specimens in the present study was acceptable for the mocha pigment but not for the red pigment. The perceptibility threshold is defined as the color difference perceived by the human eye. In contrast, the acceptability threshold is an acceptable color difference from an aesthetic standpoint.⁴⁹ In a clinical setting, the material changing color is permissible if the change is below the acceptable threshold or above the detectable threshold. This indicates that the color change of a material can be noticed clinically but remains aesthetically appealing.

When exposed to artificial aging, all specimen groups, irrespective of the ZrO₂ nano-oxide, exhibited color instability (ΔE* > 0). Both intrinsic (material self-discoloration) and extrinsic (adsorption or absorption of other substances) causes may contribute to color deterioration.⁵ UV radiation was observed to significantly influence the color deterioration of facial prostheses among environmental conditions such as solar radiation, temperature, and humidity.¹³

The color stability of maxillofacial materials can be assessed quickly, efficiently, and effectively using artificial aging. It has been proposed that artificially aged silicone, with or without nanoparticles, produces color instability.^46,50 The accelerated aging duration of 1008 h equals one year of clinical prosthesis usage.

UV light, an electromagnetic wave covering a small fraction of the visible spectrum, most frequently harms colorants and polymers. The electrons in the nanoparticles of a medium vibrate when they are irradiated by UV light. Because nanoparticles have smaller sizes than UV light wavelengths, some UV light is dispersed, and some is simultaneously absorbed.⁵¹ In accordance with these basic principles, UV protection, emerges from the absorption and scattering of nanoparticles. As ZrO₂ is a nanoparticle that can scatter and absorb UV radiation, it may provide comparable UV protection. In this study, a vacuum mixer was used to equally disperse ZrO₂ nanoparticles and silicone pigments in a silicone elastomer matrix, which considerably reduced the silicone color change. The smaller the oxide particles, the greater the UV shielding that can be achieved. Therefore, it can be hypothesized that ZrO₂ nanoparticles could help materials retain their color and that the color shift was minimized in the groups with ZrO₂ nanoparticles.

The reduction in the color change of the groups containing ZrO₂ nanoparticles could be because ZrO₂ nanoparticles are heat resistant and considerably increase the cross-linking reaction temperature of the polysiloxane side groups. This, in turn, enhances the heat-aging properties of the elastomers.²⁹

The red pigment exhibited a substantial color shift in terms of both perceptibility and acceptability thresholds (P < 0.000). Based on the findings of Kiat-Amnuay et al.²⁶ the red pigment was chosen as the intrinsic color for this study. In their study, it was determined that the red pigment had the most adverse effect on the color stability of silicone elastomers. Beatty et al.¹ studied the effect of UV light on the color of dry-pigmented maxillofacial elastomers. They observed that red cosmetic dry earth pigments exhibited significant color changes after 400 h of exposure.

Another study discovered that the significant color change observed is primarily due to the loss of red pigment caused by irradiation lighting.⁵²

Groups containing ZrO₂ nanoparticles had lighter hues than the control group. This is because ZrO₂ nano-oxides function as opacifiers,⁵³ and might explain the increase in ΔE* of red pigment after 252 and 504 h of aging.

Zarrati et al.⁵⁴ found that mixing an opacifier and pigment could reduce the degree of color change. They found that adding 1% TiO₂ as an opacifier led to acceptable protection of silicone specimens against UV radiation.

A prior study by Haug et al.⁵⁵ who assessed the color stability of a frequently used colorant-elastomer combination after exposure to aging, found that many colorant-elastomer combinations exhibited color changes due to coloring.

This in vitro study had several limitations. In this study, the effect of artificial aging on color stability was examined. Only one type of maxillofacial silicone was tested in this study. Two pigments and a specific concentration of ZrO₂ nanoparticles were tested, which could be considered a limitation. Future research should examine the effects of other variables, such as outdoor weathering, different types of nanoparticles, silicone, and pigments.

Conclusions

Based on the results of this in vitro study, artificial aging resulted in color alterations in each specimen. The red pigment with and without ZrO₂ nanoparticles exhibited a highly significant color change to the perceptible and acceptable thresholds. The mocha pigments with and without ZrO₂ nanoparticles were below the perceptible threshold. The ZrO₂ nanoparticles played an essential role in protecting the silicone specimens and decreasing the amount of color change.

Footnotes

Acknowledgment

The authors are grateful to the Nanotechnology Research Laboratory, Department of Physics, University of Sulaimani, for laboratory support. We would like to thank Editage () for English language editing.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Bruska Azhdar

Author biographies

Mohammed Abdalqadir is an assistant lecturer in Prosthodontics/College of Dentistry. He holds a master's degree in Prosthodontics (MSc). His area of research is maxillofacial prosthetic silicone elastomer, nanomaterial, and nanocomposite.

Kaml Mohammed is a lecturer at the College of Dentistry/Department of Prosthodontics. He holds a master's degree in Prosthodontics (MSc). His area of research includes maxillofacial prosthetic material, nanomaterial, nanocomposite, and polymethylmethacrylate denture base material.

Bruska Azhdar is an assistant professor in Physics. His area of research is nanotechnology, synthesis of nanomaterial, and nanocomposites.

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