Journal of Interdisciplinary Dentistry

: 2018  |  Volume : 8  |  Issue : 3  |  Page : 87--91

Color stability of heat-cured polymethyl methacrylate denture base resin coated with titanium dioxide upon storage in different beverages

Rio S U. Kashyap1, M Nalinakshamma2, Shilpa Shetty2, Savitha P Rao2,  
1 Department of Prosthodontics, Crown and Bridge, VS Dental College and Hospital, Bengaluru, Karnataka, India
2 Department of Prosthodontics, VS Dental College and Hospital, Bengaluru, Karnataka, India

Correspondence Address:
Rio S U. Kashyap
VS Dental College and Hospital, Bengaluru, Karnataka


Context: The purpose of this study is to evaluate the change in color of titanium dioxide (Sigma-Aldrich)-coated heat-cured polymethyl methacrylate resin samples (DPI Heat Cure) and compare it with the color change of conventionally polished samples upon storage in different beverages. Materials and Methods: Fifty disc-shaped samples were made from the DPI Heat Cure. Twenty-five samples were coated with siloxane primer (Ultradent Products) followed by titanium dioxide using an airbrush. They were then subjected to heat treatment. Remaining twenty-five samples were routinely polished. Initial color values were recorded using a spectrophotometer (Datacolor SPECTRUM 650). The samples were then immersed in artificial saliva, coffee, cola, alcohol, and turmeric solution for 10 min, followed by storage in distilled water daily for 56 days. The color change values (ΔE) were calculated using a mathematical formula. Statistical Analysis: The data were subjected to one-way analysis of variance and Tukey's honest significant difference test. Results: Uncoated samples showed higher value of color change than the coated ones in all groups. Maximum color change was seen with the turmeric solution in both groups, which was highly unacceptable clinically (ΔE≈20). Conclusion: TiO2 coating slows down the process of color change of heat-cured acrylic resin.

How to cite this article:
U. Kashyap RS, Nalinakshamma M, Shetty S, Rao SP. Color stability of heat-cured polymethyl methacrylate denture base resin coated with titanium dioxide upon storage in different beverages.J Interdiscip Dentistry 2018;8:87-91

How to cite this URL:
U. Kashyap RS, Nalinakshamma M, Shetty S, Rao SP. Color stability of heat-cured polymethyl methacrylate denture base resin coated with titanium dioxide upon storage in different beverages. J Interdiscip Dentistry [serial online] 2018 [cited 2020 Dec 2 ];8:87-91
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Full Text

 Clinical Relevance to Interdisciplinary Dentistry

The application of improvements in dental materials greatly influences the success of any dental prosthesisMaterials such as titanium dioxide have been tested for various parameters when being used with denture base resins. Denture base resins are very susceptible to color change due to extrinsic factors. The color stability, being an important parameter, is being tested in the current studyThe results of this study demonstrate that a thin coating of titanium dioxide spray on the denture base resins may help to limit the effects of extrinsic factors on the color stability of denture base resinsThis study demonstrates that further research is required to devise different methods of coating TiO2 on the properly polished surface of denture base resins.


Since the 20th century, acrylic resin has been successfully used for denture fabrication owing to its various advantages such as its ease of manipulation, biocompatibility, adequate physical and mechanical properties, low cost, and satisfactory appearance.[1],[2] However, unwanted characteristics such as loss of elasticity, abrasion, porosity, and color change might occur with time. One of the indicators of aging or damaging of materials is change in the color. It is one of the most important clinical properties of dental materials.[3],[4]

Regarding the self-cleansing action and esthetic features of denture base materials, depending on the methods used for polishing and clinical experience, there is a difference between the recontouring and polishing work regardless of the instrument used. It is difficult to retain the surface glossiness during long-term use. With the use of surface glazing products, the long hours of polishing and laboratory work can be reduced. The surface glazes have the properties of lubrication and surface durability. TiO2 may also be able to be used as the surface glazing agent because it can afford the high levels of glossiness to dentures.[5]

The color change of a polymeric material may be caused by various intrinsic and extrinsic factors. Intrinsic factors occur during the aging process of the material due to many physical and chemical conditions resulting in resin discoloration and matrix changes. Furthermore, various extrinsic factors can also cause discoloration such as thermal change, artificial dyes used in food, cleaning procedures, stain accumulation, and mishandling by the patient.[2]

Color change is the result of changes in the matrix and aging process of the material occurring due to intrinsic factors. This cannot be easily prevented because it is not in the control of the dentist. However, changes due to extrinsic factors can be avoided with the help of an impervious, self-cleaning, and scratch-resistant surface coating of titanium dioxide nanoparticles.[6],[7],[8],[9]

The aim of this study is to evaluate the change in color of titanium dioxide-coated heat-cured polymethyl methacrylate (PMMA) resin samples and compare it to the color change of conventionally polished samples upon storage in different beverages.

 Materials and Methods

Polymethyl methacrylate samples

A total number of 50 standardized circular disc samples of PMMA were prepared from heat-cured acrylic resin (DPI, Mumbai, India). The powder-to-liquid ratio was 2:1 by volume.[8] The resulting mixture was packed into a plaster mold (20-mm diameter and 2.0-mm depth) and immersed in water at 60°C for 60 min (for primary polymerization) and then in boiling water for 60 min (for secondary polymerization). The cured samples were smoothened up to the same grit[1],[4] [Figure 1].{Figure 1}

Titanium dioxide coating

The PMMA samples were pretreated with a siloxane primer (Ultradent Products, Utah), of which the main component is methoxysilylpropyl methacrylate 5–15 wt% in 92% isopropanol. One layer of this primer was painted onto the samples with a brush tip provided with the system and then air dried for 10 min. TiO2-coating agent was prepared using TiO2 nanopowder (637254, Sigma-Aldrich, Bengaluru, India), containing 2.0% anatase TiO2 (<25 nm particle size) in water and ethanol. This solution was then sprayed onto the samples using an airbrush gun (Aerograph, UK) connected with 0.6 MPa compressor, followed by drying in a hot air oven for 10 min at 70°C.[5]


Solutions used in this study (10 ml per sample) were artificial saliva (Wet Mouth, ICPA Health Products, Ankleshwar, India), coffee (Nescafe Classic, New Delhi, India), cola (Coca-Cola, India), alcohol (Rizo Port Wine, Eagle Wines, Nashik, India), and turmeric solution (0.5 g in 100 ml, DS spice Co., Noida, India).[1],[2],[10],[11]


The Commission Internationale de l'Eclairage L*a*b* values were obtained for all the samples using a spectrophotometer (Datacolor SPECTRUM 650). The light was blocked by a black background, and all measurements were performed in triplicate. The means of L*, a*, and b * were calculated. These were the initial color values.

The samples were stored in different containers containing 50 ml (10 ml per sample) of the test solutions [Table 1] for 10 min (average time for which a beverage is consumed during the day),[10] followed by storage in distilled water daily for 56 days. It is stated that exposure of acrylic to different beverages for a minimum of 56 days is required to bring about a clinically perceptible color change[10] [Figure 2].{Table 1}{Figure 2}

The samples were blotted dry using tissue paper. The final color values were obtained using the same spectrophotometer. The difference in color value for each sample was calculated using the formula: ΔE = ([ΔL]2 + [Δa2 + [Δb]2)½.

Adding titanium dioxide to the acrylic powder causes significant changes in the physical properties.[12] However, adding a coating of TiO2 does not have any effect except the change in color.

According to a study, values of ΔE ≥3.3 are visually perceptible and clinically unacceptable to 50% of the trained observers.[13] Between PMMA and TiO2-coated specimens, the ΔE was recorded to be 1.02, which is clinically imperceptible.[5]

Statistical analysis

Mean values of color change were compared by one-way analysis of variance. The difference between the two groups was highly statistically significant as concluded by Tukey's honest significant difference test (P > 0.0001).


The mean differences in color values are shown in [Table 2], and [Figure 3] and [Figure 4] show the samples after storage of 56 days.{Table 2}{Figure 3}{Figure 4}

Uncoated samples showed higher value of color change than the coated ones in all groups. Comparison of coated and uncoated samples is depicted in [Figure 3]. This demonstrates that the TiO2 coating prevents color change to some extent.

Clinically unacceptable (ΔE >6) change was seen with all solutions in both groups except artificial saliva. This shows the susceptibility of the heat-cured PMMA resin to color change by daily consumption of beverages.

Least color change was seen with artificial saliva. Maximum color change was seen with the turmeric solution in both groups, which was highly unacceptable clinically (ΔE ≈ 20). It is a common ingredient in an average Indian diet. The solution with maximum staining potential was turmeric solution followed by alcohol, cola, coffee, and artificial saliva.


Discoloration of a dental material can be evaluated subjectively and using instruments such as spectrophotometer and colorimeter.[14] There are inconsistencies in color perception specifications among observers. Therefore, evaluation of color by visual comparison is unreliable. Instrumental measurements eliminate the subjective interpretation of color comparison. In case of dental materials, colorimeters and spectrophotometers have been used to measure color change.[11],[15] Spectrophotometers have been shown to be more accurate in measuring the change in color than colorimeters.[16]

There are different thresholds of color difference values above which the color change is perceptible to the human eye. These values range from ΔE = 1,[17] between 2 and 3,[18] ≥3.3,[18] and ≥3.7.[14] ΔE values between 0 and 2 are imperceptible; values of ΔE in the range of 2–3 are mildly perceptible, values from 3–8 are moderately perceptible to the human eye, and the values above 8 are markedly perceptible.[18] Clinically acceptable value is considered to be a ΔE value of 3.7 or less.[14],[19]

In the present study, ΔE values ranged from 5.08 to 20.6, which is clinically perceptible.

For effective bonding of the TiO2 nanoparticle coating on the acrylic surface, a primer coating agent was used. Chen and Yakovlev investigated interactions of silane-coupling agents with commercially available TiO2 nanoparticles. They concluded that the silane-coupling agents covalently bond onto the surface of TiO2 nanoparticles.[20]

For bonding to TiO2, one end of silane-coupling agent contains a methacrylate (or similar groups) for resin bonding, whereas in the Si group, the other end of the primers is for bonding to metal alloys and oxides.[21]

Two methods have been proposed in the application of titanium dioxide on denture materials: one, in which titanium dioxide is added in the mixture, and in the other method, it is coated on the surface of the material. Several studies have shown that the addition of TiO2 nanoparticles to heat-cured acrylic resin can adversely affect the mechanical properties of the material.[22],[23],[24] In a study where the TiO2 coating was studied, ΔE was <3, and thus, the color difference value was considered to be less than the level of clinical perceptibility. This was considered to be due to the coating being very thin and the small amount of titanium dioxide employed. Furthermore, spraying method for coating using airbrushes produces extremely thin coatings. The results in a study revealed that even thin layers give high degrees of glossiness.[5] This suggests that the work relating to polishing can be decreased and surface of the dentures can be standardized.

Furthermore, it has been reported that this coating suppresses microbial adhesion onto the surface.[7] As determined from scanning electron microscope analysis,[5] the coating material was seen in the fissured areas of the acrylic resin, suggesting that adhesion of external deposits and bacteria can be avoided from a microscopic perspective.

In another study, the titanium dioxide coating was not disrupted, even under the application of 100,000 brushes which is equal to brushing a maxillary denture at 300 gf for 90 s/day for 1 year.[8]


TiO2 coating when applied in a thin layer hardly causes any color change of the surface. It is antimicrobial and renders the surface self-cleansing. It also promotes a high degree of glossiness. This coating is durable and scratch resistant. The results of this study demonstrate that on storage in different beverages, titanium dioxide-coated PMMA samples also showed a color change on immersion in different beverages, but it was significantly lesser than the change observed in uncoated samples (P > 0.05).

Therefore, TiO2 coating slows down the process of color change of heat-cured acrylic resin, thus increasing the life of the prosthesis.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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