Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2019  |  Volume : 30  |  Issue : 4  |  Page : 583--589

Evaluation and comparison of different polymerization techniques, curing cycles, and thicknesses of two denture base materials


Jayant N Palaskar1, Sunint Singh2, Sanjeev Mittal3,  
1 Department of Prosthodontics, Sinhgad Dental College and Hospital, Pune, Maharashtra, India
2 Department of Prosthodontics, Dr. Harvansh Singh Judge Institute of Dental Sciences and Hospital, Punjab University, Sector 23, Chandigarh, Punjab, India
3 Department of Prosthodontics, M.M. College of Dental Sciences and Research, Mullana, Ambala, Haryana, India

Correspondence Address:
Dr. Jayant N Palaskar
Department of Prosthodontics, Sinhgad Dental College and Hospital, Pune - 411 041, Maharashtra
India

Abstract

Purpose: The study aimed to compare the effect of different materials, thicknesses, and polymerization methods and cycles, on the surface porosity of acrylic denture base resins. Materials and Methods: Conventional heat-polymerized polymethyl methacrylate (PMMA) and specially designed acrylic resin (Acron MC) were used to make 84 rectangular samples. They were divided into three groups to evaluate and compare the polymerization techniques, curing cycles, and thicknesses of the two denture base materials (28 samples each). Group A contained PMMA samples polymerized using water bath method (control group); Group B contained Acron MC samples polymerized by microwave method, and Group C contained PMMA samples polymerized by microwave method. Each group was further divided based on sample thickness and polymerization cycles. Each sample was scanned for surface porosity and area of each pore was measured using optical microscope. Data was analyzed using ANOVA, Bonferroni, and student t-tests. Unpaired student t-test was performed to compare the means of surface porosity with polymerization cycles and thicknesses among the groups. The power of study was kept at 80%. Results: Group C showed highest mean % of porosity depending on method of polymerization, different polymerization cycles (short and long) and polymerization cycles within the group. Group B showed the highest mean % of porosity depending on thickness and thickness within the groups. Conclusions: Microwavable acrylic resin polymerized by microwave energy exhibited statistically insignificant increase in porosity when compared to conventional heat cured acrylic resin by water bath method. Conventional acrylic resin polymerized by microwave energy exhibited high statistically significant porosity irrespective of sample thickness. There was statistically insignificant increase in porosity depending on sample thickness irrespective of material and method of polymerization.



How to cite this article:
Palaskar JN, Singh S, Mittal S. Evaluation and comparison of different polymerization techniques, curing cycles, and thicknesses of two denture base materials.Indian J Dent Res 2019;30:583-589


How to cite this URL:
Palaskar JN, Singh S, Mittal S. Evaluation and comparison of different polymerization techniques, curing cycles, and thicknesses of two denture base materials. Indian J Dent Res [serial online] 2019 [cited 2020 Dec 4 ];30:583-589
Available from: https://www.ijdr.in/text.asp?2019/30/4/583/271051


Full Text



 Introduction



Porosity in acrylic denture base resins is an undesirable property of this material. In spite of this flaw, it has been the most commonly used material for dental fixtures in the recent decades. Occurrence of porosity is a complex phenomenon with multifactorial origins.[1] Since the 1930s, conventional water bath has been the method of choice for polymerization. Although Nishi suggested the use of microwave for polymerization in 1968, microwave polymerization method is still not in common practice all over the globe which may be due to the occurrence of porosities and the lack of sufficient promotion activities by the manufacturers.[2] Although microwave polymerization has many advantages such as greatly reduced polymerization time, less cumbersome equipment, and clean processing, its huge cost is a major hindrance in adopting this method of polymerization in common practice.[3] In 1983, Kimura et al.[3] found that polymerization by microwave energy resulted in good adaptation of the resin to the stone cast at the posterior border of the denture base. Other advantages, claimed but not substantiated, include short dough forming time, more homogeneous resin dough, and minimal color changes in the resin base.[4]

Although porosities in specially designed resins for microwave polymerization have been studied, not much research has been done on conventional PMMA polymerized by microwave energy. Surface porosity can be highly unaesthetic and unhygienic as it can harbor microorganisms and food debris which may lead to serious complications.[5] According to the Academy of Denture Prosthetics, for a denture to be hygienically acceptable it should be nonporous because porosity will detrimentally affect the resistance of the material to staining, calculus deposition, and adherent substances.[6] Porosity in PMMA also results in high internal stress and vulnerability to distortion and warpage.[7]

Thickness and polymerization cycle have a direct effect on the occurrence of porosity. Reitz et al.[8] found no significant difference in thin strips polymerized by water bath and microwave polymerization; however, specimens that were more than 10 mm in thickness and had been polymerized by microwave energy had visible porosity in contrast to the water bath method. Pero et al.[9] concluded that microwave polymerization cycles and specimen thickness influence porosity.

To evaluate porosity, microscopic observation, water absorption, photography, mercury porosimetry etc., have been used. In the present study, optical microscope is used for evaluating the porosity as it provides a detailed analysis with precise measurements and even the size of a single pore through its software.

Since porosity reflects the quality of polymerization, studies for better polymerization techniques should be conducted. In this context, microwave technology is examined for its effect on porosity. All the available literature are with regard to investigations of an acrylic resin material tailor-made for microwave use for which availability, cost, and its associated technology are of concern. However, if existing materials like conventional PMMA were to be amalgamated with newer yet widely available technology like microwave, it would be of great benefits to the patients and professionals in terms of time and use of manpower and resources (water and heating resources). With this background, this study was conducted to evaluate and compare the surface porosity of heat-cure acrylic resin polymerized by microwave energy and conventional water bath using different curing cycles and thicknesses.

The null hypothesis for this study was that there is no effect of the polymerization method, polymerization cycle, material, and thickness on the occurrence of surface porosity in denture base resins.

 Materials and Methods



Molten wax was poured into metal moulds with three squares (2 mm × 2 mm) engraved one at the centre and two at the corners 1 mm away from the border. Half of the samples were 25 × 12 × 3 mm and the other half were 25 × 12 × 5 mm in dimensions.

Two heat-activated denture base resins, one conventional (DPI Heat Cure - Denture Base Polymer Resin - Dental Products of India, Mumbai, India) and one specially designed acrylic resin for microwave polymerization (Acron MC, GC Corporation, Japan) were used to prepare a total of 84 test samples. Resin samples were divided into three groups. Group A— conventional heat cure PMMA polymerized by water bath; Group B—microwavable acrylic resin (Acron MC) polymerized by microwave and Group C—conventional heat cure PMMA polymerized by microwave. Each group contained 28 samples which were further divided into four subgroups containing seven samples each [Figure 1].{Figure 1}

The acrylic was mixed as per the manufacturer instructions. For the microwave polymerization method, a domestic microwave oven with a rotating table was used (Onida, India). After polymerization treatment, the flasks were allowed to bench cool for 0.5 hour and then kept under tap water; each specimen was polished and the three engraved portions of each specimen were observed under optical microscope (Zeiss Imager, model no.Z1, Carl Zeiss, Germany). The perimeter of each surface pore, evident in the engraved square was outlined and the area of each pore was measured with the help of the software attached to the microscope.

Data was collected and compiled on an MS excel sheet and properly tabulated. Means, standard deviations, and percentage porosity of surface areas were calculated. Data was analyzed using SPSS (version 17.0 software; Chicago Il1). For intergroup comparison of means, one-way ANOVA test was used followed by post hoc Bonferroni test for pairwise comparison. For comparison of means of surface porosity with polymerization cycles and thicknesses within the groups, unpaired student t-test was used. P value <0.05 was considered to be statistically significant and P value <0.001 was considered as highly significant. Power of the study was kept at 80%.

 Results



Results were obtained and analyzed on the basis of material, method of polymerization, polymerization cycle, specimen thickness, and all interactions between them. Area of surface pores was expressed in percentage (%).

First, the percentage porosities of different groups were compared using one-way ANOVA test taking into account only the material and thickness and not the polymerization method. The P values indicated that there was a significant difference between all the groups (P < 0.001). Post hoc test suggested that Groups 5CL and 5CS were statistically significant than all the other groups. There was no statistical significant difference among the other remaining groups [Table 1].{Table 1}

Second, the percentage porosities of different groups based on the material and the method of polymerization and not considering its sample thickness were compared using one-way ANOVA test; P values showed that there was a significantly high statistical difference in the percentage porosity of all the groups (P < 0.001). There was a large difference in the means of the group C [Figure 2] as compared with groups A and B. The post hoc test confirmed the significantly high statistical difference among these groups. Means of group A [Figure 3] and B [Figure 4] were comparable [Table 2].{Figure 2}{Figure 3}{Figure 4}{Table 2}

Third, the percentage porosities of different groups were compared based on the material and the polymerization cycles one way ANOVA test without considering sample thickness; P values were obtained for both short and long polymerization cycles. There was a significantly high statistical difference in the percentage porosity of all the groups (P < 0.001). There was a large difference in the means of the Groups C and A, B. This was confirmed by post hoc test that showed highly significant statistical difference in Groups A and C. Means of Groups A and B were comparable [Table 3].{Table 3}

Fourth, intragroup comparison was performed between short and long polymerization cycles within the same groups irrespective of thickness using unpaired t test. The results showed a statistically significant difference between Groups A and B when the samples were polymerized by both short and long polymerization cycles. However, long cycles showed statistically significantly less porosity. In Group C, both the polymerization cycles showed significantly high porosity [Figure 5],[Figure 6] and [Figure 7], [Table 4].{Figure 5}{Figure 6}{Figure 7}{Table 4}

Finally, intragroup comparison with regards to percentage porosity was performed with specimens of the same group with varying thicknesses and irrespective of polymerization cycles. The results showed no statistically significant difference between the groups even in varied thicknesses in both the materials irrespective of the method of polymerization [Table 4].

 Discussion



Porosity in acrylic denture base resins is highly undesirable as it can adversely affect its physical properties. However, through current scientific literature, the causes of and the ways to minimize porosities in conventional heat cure acrylic resins is known. Although microwave polymerization has been studied for the last three decades, it is still not in common practice. Therefore, it is necessary to study and analyze the occurrence of surface porosities in acrylic resins polymerized by microwave energy. Surface porosity, in particular, can be objectionably unaesthetic and unhygienic.

Acrylic resin is an immensely popular denture base material and is easily available. In this study, conventional heat-cure PMMA was polymerized using microwave energy. The hypothesis was that if properties of PMMA polymerized by microwave energy remain comparable with conventional water bath polymerization, it can be a boon to the field of dentistry, as PMMA is easily available and microwave polymerization is time saving, neat, and clean procedure.

Conventional acrylic resin (PMMA) polymerized by water bath method (Group–A) exhibited least porosities in both 3 mm and 5 mm thicknesses. Short polymerization cycle samples showed marginally higher percentage of surface porosity than the samples polymerized by long cycle within Group A samples. Group (A) was taken as a control group and findings from Groups B and C were compared with it.

Surface porosity in samples obtained from Acron MC—Group B (specially designed resin for microwave polymerization) that were subjected to short microwave polymerization and were 3 mm and 5 mm in thickness showed marginally high porosity within the same group. When percentage surface porosity was compared with samples from the control group, the surface porosity was less in control group samples in both the thicknesses and polymerization cycles. Yannikakis et al.[10] conducted a similar study and concluded that heat cure resin samples polymerized by water bath method showed no porosity, heat cure samples polymerized by microwave energy showed maximum porosity and microwavable resin samples polymerized by microwave energy showed low porosity. These findings are not in accordance with the results of the present study where results show porosity in all the samples irrespective of their thickness and method of polymerization. Samples from Group A, B, and C showed least, moderate, and maximum porosities, respectively.

When conventional PMMA samples were polymerized by microwave energy (Group-C), they exhibited maximum percentage surface porosity with regard to both thicknesses and polymerization cycles as compared to Groups A and B. These findings are in accordance with studies conducted by Reitz et al.,[10] Sanders et al.,[1] Bafile et al.[11] and Al Doori et al.[12] who had concluded that PMMA polymerized by microwave energy showed maximum porosity.

When samples from Group A were compared within the group depending on its polymerization cycle irrespective of the sample thickness, it showed statistically significant difference in surface porosity between the samples. Samples polymerized by long polymerization cycle showed less percentage surface porosity (0.0174) than the samples polymerized by short polymerization cycle (0.3824). These findings are contradictory to the findings of Bonatti et al.[13] who concluded that porosity was directly affected by water adsorption, that is, the longer the contact with water, the more pores in the acrylic resin. It was further explained that long cycles expose the acrylic resin to water for a long time (8 hours), allowing water adsorption and increasing the number of pores. Gay et al.[14] and Truong et al.[15] found that conventional resin specimens of thickness ≤10 mm and polymerized in a water bath were free of pores. In contrast, Bafile et al.[11] found pores of 4 or 5 mm diameter, in relatively thick conventional resin specimens that were polymerized with microwave energy. A study conducted by Singh et al.[16] concluded that clinically acceptable surface porosity was exhibited in conventional resin (PMMA) samples polymerized by microwave energy.

Excessive heating can cause surface discoloration, increase the number of surface pores, and hence, decrease the mechanical resistance of the material.[17] Ogawa et al.[18] showed that the increase in dry-heat temperature activates a chemical reaction between the monomers and polymers, producing a more complete polymerization, with small number of pores. In accordance to this study, samples from Group B showed statistically significant difference in percentage porosity in samples polymerized by short polymerization cycle (0.8439) than long polymerization cycle (0.2579). The reason for the similarities of the results to those of Ogawa et al. might be the higher temperature generated by microwave energy in the long polymerization cycle. Microwave oven generates an electromagnetic field where polarized molecules of any material can be heated by changing the directions of the molecules many billion times per second, which results in rapid heating due to numerous intermolecular collisions and frictions. Monomers used in conventional heat cure acrylic resin have polarized molecules and high vapor pressure, which when exposed to microwave polymerization generate heat much beyond 100.3°C and vaporize resulting in generation of porosity. The microwave monomer material contains either a triethylene or a tetraethylene glycol, both dimethylacrylates with a reactive group on each end. Dimethylacrylates have low vapor pressures even at a temperature of 100°C to 150°C; therefore, it has a high boiling point. The low vapor pressure would allow processing at elevated temperature without the danger of porosity.[11] This was confirmed in this study where samples from Group C showed maximum porosity. In Group C, both polymerization cycles showed high percentage surface porosity (short = 2.1989 and long = 2.2355) which was not statistically significant within the group. A study by Kartika et al. concluded that conventional acrylic resin polymerized in a microwave did not exhibit significant porosity and was thus clinically acceptable; Therfore, it can be used for polymerization dentures without any effect on its mechanical property.[19] Sanders et al.[1] reported that porosity in denture base acrylic resins is evident regardless of the method of polymerization (water bath or microwave) or the material used (conventional or specially designed for microwave use).

The polymerization reaction is exothermic. Thus, if a large or thick mass of uncured material is placed in boiling water or under high watt in the microwave, the temperature of the resin may rise well above 100.3°C and vaporize the monomer. The vaporized monomer causes gaseous porosity, which appears as fine uniform bubbles, particularly in the thicker sections of the denture. From this study, it can be concluded that to minimize porosity in thick portions of acrylic resin processed by microwave energy, selection of an appropriate resin and polymerization cycle is very important.

Limitations of this study:

As it is an in vitro study, these results may not necessarily be extrapolated to in vivo conditionsFurther in vitro evaluation of physical and mechanical properties such as color stability, impact strength, flexural strength, water sorption, wear, residual monomer, dimensional stability, and marginal adaptation of conventional PMMA polymerized by conventional water bath and microwave energy need to be studied.

 Conclusions



Based on the observations and results of this study, the following conclusions were drawn:

Conventional acrylic resins polymerized in water bath exhibited least surface porosity, regardless of specimen thicknessMicrowavable acrylic resins polymerized by microwave exhibited statistically insignificant increase in porosity as compared to conventional heat-cure acrylic resins polymerized by water bath methodConventional acrylic resins polymerized by microwave method exhibited high statistically significant porosity irrespective of sample thicknessPorosity was statistically significant and higher in microwavable acrylic resin samples subjected to short polymerization than those subjected to long polymerization cyclesPorosity of conventional acrylic resin polymerized by short cycle was more than long cycle which was statistically significantThere was statistically insignificant increase in porosity depending on the sample thickness irrespective of the material and the method of polymerization.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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