|Year : 2017 | Volume
| Issue : 2 | Page : 67-72
Evaluation of the mechanical properties of high impact denture base resin with different polymer to monomer ratios: An In vitro study
Sheen Juneja Arora, Aman Arora, Viram Upadhyaya, Aditi Goyal
Department of Prosthodontics, DAV© Dental College and Hospital, Yamunanagar, Haryana, India
|Date of Web Publication||26-May-2017|
Department of Prosthodontics, DAV© Dental College, Room No. 5, Model Town, Yamunanagar - 135 001, Haryana
Source of Support: None, Conflict of Interest: None
Aim: This study aims to evaluate the flexural strength, hardness, and impact strength of heat-cured high-impact denture base resins with different polymer/monomer ratios. Materials and Methods: A total of 150 acrylic samples were prepared using high impact denture base resin (Travelon Hi). The samples were divided into five groups based on different powder/liquid ratios (g/ml) with 30 samples in each group. The P/L ratio in Group 1 (Ratio - 2.2:1) was the manufacturer's recommended ratio and was used as control. In Group 2, the ratio was 2.7:1, in Group 3, the ratio was 3.2:1, in Group 4, the ratio was 1.9:1, and Group 5 the ratio was 1.6:1. Each group with 30 samples was further subdivided into three different subgroups comprising 10 samples each, based on the properties to be evaluated, i.e., for flexural strength, hardness, and impact strength evaluation. The samples were tested for flexural strength, Vicker's hardness number (VHN) and impact strength. One-way ANOVA including post hoc-Tukey's tests was used to calculate the difference of means for quantitative variables and for intergroup comparison as well. Results: The results showed that the flexural strength values and VHN values showed a similar trend. The values decreased significantly as the ratio was increased or decreased from the control group. The results also showed that there was nonsignificant difference between the mean impact strength values for all the groups. Conclusion: For reinforced resins or high impact resins, the manufacturer's recommended polymer/monomer mixing ratio should be used to obtain the appropriate strength of the material.
Keywords: Flexural strength, hardness and impact strength, high impact denture base resin (Travelon Hi)
|How to cite this article:|
Arora SJ, Arora A, Upadhyaya V, Goyal A. Evaluation of the mechanical properties of high impact denture base resin with different polymer to monomer ratios: An In vitro study. Indian J Dent Sci 2017;9:67-72
|How to cite this URL:|
Arora SJ, Arora A, Upadhyaya V, Goyal A. Evaluation of the mechanical properties of high impact denture base resin with different polymer to monomer ratios: An In vitro study. Indian J Dent Sci [serial online] 2017 [cited 2018 May 20];9:67-72. Available from: http://www.ijds.in/text.asp?2017/9/2/67/207106
| Introduction|| |
Polymethyl methacrylate (PMMA) has been the most popular material for construction of dentures since the 1940s due to many advantages, including good esthetics, accurate fit, stability in the oral environment, easy laboratory and clinical manipulation, and inexpensive equipment. However, there are certain shortcomings of acrylic resin particularly in terms of strength, which leads to denture fracture.
Acrylic resins have shown to flex in function to a much greater degree than would be expected. Therefore, to overcome such disastrous eventualities, many modifications in the conventional denture base resin to improve its strength were introduced. One such attempt led to the chemical modification of acrylic resin through the incorporation of rubber in the form of butadiene styrene has been successful in terms of improving the impact strength.
Denture fracture results from two types of forces, namely, flexural fatigue and impact forces. Studies have shown that processing variables such as curing environment, mixing ratio, temperature, and time of curing regimes and postprocessing water storage can all have effects on the mechanical properties of denture bases. Of all the properties of denture base resins, the flexural strength, hardness, and impact strength are the important properties which are used to evaluate the strength of the material.
Various studies have been done in the past evaluating the effect of P/L ratio. Previous studies demonstrated that the mixing ratio has an effect on the strength of unreinforced polymerized material. However, little is known about its effect on reinforced PMMA resin. Several studies have been done on evaluating the flexural strength, hardness, and impact strength of different denture base resins using various methods. However, very few studies have been done in the past on heat cure denture base resins, evaluating the effect of varying polymer/monomer ratios on mechanical properties.
Thus, the null hypothesis to be tested was that a lower or a higher than the recommended P/L ratio decreases the strength properties of high-impact denture base resin.
Hence, the aim of this study was to evaluate the flexural strength, hardness, and impact strength of heat-cured high-impact denture base resins with different polymer/monomer ratios.
| Materials and Methods|| |
This study was undertaken to evaluate the effect of five different polymer/monomer ratios on the flexural strength, hardness, and impact strength of heat-polymerized high-impact (reinforced) PMMA resin – Travelon Hi. To fabricate the acrylic samples, a silicone mold was fabricated with five compartments, each measuring 65.5 mm × 10.5 mm × 3.5 mm in length, breadth, and thickness, respectively. Molten modeling wax was flowed in each compartment to obtain rectangular wax patterns. These wax patterns were retrieved, flasked, and dewaxing was done to create mold space for acrylic samples. The appropriate amount of powder and liquid were mixed according to the five ratios to be used in the study for each particular group and packed into the mold space obtained after dewaxing. Curing was done using short curing cycle to achieve rectangular acrylic samples.
The acrylic samples thus obtained were then trimmed, finished, and polished to obtain rectangular acrylic samples of the dimensions 65 mm × 10 mm × 3 mm in length, breadth, and thickness, respectively (according to ISO 1567 specifications).
The samples were then stored in distilled water for 28 days at 37°C to simulate the oral conditions. This was done because of the influence of water on the flexural strength, which causes a significant decrease in the flexural strength values. The maximum influence occurs during the first 4 weeks of immersion., Hence, immersion for 28 days in distilled water at 37°C in an incubator was used in this study. The incubator was used to maintain the temperature which simulates the oral conditions.
A total of 150 acrylic samples were prepared using high impact denture base resin (Travelon Hi). The samples were divided into five groups based on different powder/liquid ratios (g/ml) with 30 samples in each group. The P/L ratio in Group 1 (Ratio - 2.2:1 or 25 g/11 ml) was the manufacturer's recommended ratio and was used as control. In Group 2, the ratio was 2.7:1 or 30 m/11 ml, in Group 3, the ratio was 3.2:1 or 35 m/11 ml, in Group 4, the ratio was 1.9:1 or 25 m/13 ml, and in Group 5, the ratio was 1.6:1 or 25 m/15 ml. Each group with 30 samples was further subdivided into three different subgroups comprising 10 samples each, based on the properties to be evaluated, i.e., subgroup a, for evaluation of flexural strength; subgroup b, for evaluation of hardness; and subgroup c, for evaluation of impact strength. Thus, Group 1 was divided into three subgroups, i.e., 1a, 1b, and 1c each comprising 10 samples for evaluation of flexural strength, hardness, and impact strength, respectively. Similarly, other groups were also subdivided into three different subgroups as Group 2 was divided into 2a, 2b, and 2c; Group 3 was divided into 3a, 3b, and 3c; Group 4 was divided into 4a, 4b, and 4c; and Group 5 was divided into 5a, 5b, and 5c, respectively. The samples were tested for flexural strength using Universal Testing Machine, for Vicker's hardness number (VHN) using Microhardness Tester and for impact strength using Izod Impact Tester.
Collected data were entered into the MS Excel spreadsheet, coded appropriately. The analysis was carried out using Statistical Package for Social Studies for Windows version 21.0 and online GraphPad software (Prism 5 for Windows) version 5.01 (IMB Co.). One-way ANOVA including post hoc-Tukey's tests was used to calculate the difference of means for quantitative variables and for intergroup comparison as well.
| Results|| |
[Table 1] and [Graph 1] show the comparison of flexural strength values (N/mm 2) within five groups with different powder/liquid ratios using ANOVA test.
|Table 1: Comparison of mean values of flexural strength (Subgroup a) for each group using ANOVA test|
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[Table 2] shows the intergroup comparison of flexural strength (N/mm 2) values using post hoc– Tukey's test.
|Table 2: Post hoc - Tukey's test of significance for flexural strength(N/mm2) (Subgroup a)|
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[Table 3] and [Graph 2] show the comparison of hardness (VHN) within five groups with different powder/liquid ratios using ANOVA test.
|Table 3: Comparison of mean values of Vicker's hardness number (Subgroup b) for each group using ANOVA test|
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[Table 4] shows the intergroup comparison of hardness (VHN) values using post hoc– Tukey's test.
|Table 4: Post hoc - Tukey's test of significance for Vicker's hardness number (Subgroup b)|
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[Table 5] and [Graph 3] show the comparison of impact strengths (kJ/m 2) within five groups with different powder/liquid ratios using ANOVA test.
|Table 5: Comparison of mean values of impact strength (Subgroup c) for each group using ANOVA test|
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[Table 6] shows the intergroup comparison of impact strength (kJ/m 2) values using post hoc– Tukey's test.
|Table 6: Post hoc - Tukey's test of significance for impact strength (kJ/m2) (Subgroup c)|
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| Discussion|| |
Despite excellent properties of acrylic resins, there are certain shortcomings of acrylic resins in terms of strength properties. Due to these shortcomings, there is a need for improvement. Over the years, there have been various modifications attempted to improve the mechanical properties of PMMA. The chemical modification of PMMA is through the incorporation of butadiene styrene to produce graft copolymer (high-impact denture base resins). In the present study, the high-impact material used was Trevalon-Hi.
It was observed that the results obtained for flexural strength values and VHN values showed a similar trend. It was found that with increase in powder (i.e., P/L ratio 2.7:1; Group 2) there was a decrease in flexural strength and hardness values and with additional increase in powder content (i.e., P/L ratio 3.2:1; Group 3), there was a further decrease in the flexural strength and hardness values. This could be explained on the basis that proportions with higher powder (polymer) content can promote a dry mixture due to lack of liquid (monomer), resulting in a material mass with disabilities to convert monomer into polymer as stated by Lopes et al.
Similarly, with increase in liquid (monomer) (i.e., P/L ratio 1.9:1; Group 4), there was a decrease in the flexural strength and hardness values which further declined significantly as the liquid (monomer) content was further increased (i.e., P/L ratio 1.6:1, Group 5). This could be explained on the basis that higher monomer content in the mixture results in the greater the amount of residual monomer as found in a study done by Kedjarune et al. The residual monomer degrades the mechanical properties of the material as stated by Okuyama et al. The most probable reason for this is that the residual monomer acts as plasticizer, which reduces the strength by a decrease in interchange forces, making deformation to occur more easily under load as suggested by Nisar et al. and Al-Kadi et al. This decrease in strength could be due to the release of residual monomer which makes the material more brittle as stated by Dandekeri et al. The unreacted monomer also has the potential for cytotoxicity as stated by Nisar et al.
On comparing the flexural strength and hardness values of groups with higher P/L ratio, i.e., Groups 2 and 3 with that of the Groups with lower P/L ratio, i.e., Groups 4 and 5, it was found that the groups with higher P/L exhibited higher values. This could be explained on the basis that the higher P/L ratio may be associated with a greater entanglement of polymers and produces a closer three-dimensional (3-D) network structure and also leads to decreased quantities of the unreacted monomers as suggested by Okuyama et al.
On observation of results for impact strength values, it was found that the impact strength values increased (nonsignificantly) with the increase in powder (polymer) content (i.e., P/L ratio 2.7:1; Group 2), which further increased (nonsignificantly) with the additional increase in powder (polymer) (i.e., P/L ratio 3.2:1; Group 3). This could be explained on the basis that the higher P/L ratio may be associated with the formation of a greater number of macromolecules which produces a closer 3-D network structure as stated by Okuyama et al. The increase in impact strength values is also attributed to internal plasticization by copolymerization with rubber. The addition of rubbers to PMMA produces a resin that consists of a matrix of PMMA, within which is dispersed an interpenetrating network of rubber and PMMA. So, if a crack develops, it will propagate through the PMMA but will decelerate at the rubber interface. Hence, the rubber reinforced or “high impact” resins absorb greater amounts of energy at a higher strain rate before getting fractured as stated by Jagger et al.
Similarly, within limits, increase in liquid (monomer) content (i.e., P/L ratio 1.9:1; Group 4) results in an increase (nonsignificant) in impact strength values. This could be explained on the basis that the resulting residual monomer acts as an efficient plasticizer as suggested by Smith, which increases the impact strength of a polymer by lowering the glass transition temperature. Thus, as the temperature is increased to the glass transition temperature or higher, molecular motion in the backbone of the polymer chains is increased. This increases the energy dissipation per unit volume which is enough to relieve stress concentrations as stated by Kim and Watts. This results in an increase in the impact strength of amorphous polymers and most crystalline polymers. These plasticizers also decrease notch sensitivity and impede crack propagation as suggested by Kim and Watts.
However, with further increase in liquid (monomer) (i.e., P/L ratio 1.6:1; Group 5), decrease (nonsignificant) in the impact strength values was observed. The most probable reason for this could be that the increase in monomer beyond limits, results in higher residual monomer content as suggested by Kedjarune et al. Thus, the higher levels of residual monomer eventually result in excessive leaching of residual monomer which in turn results in higher number of void formation in the resin. Also, since there is a parallel relation between the level of residual monomer and the percentage of water sorption as suggested by Dogan et al., therefore it results in greater water intake. These water molecules can easily penetrate the polymer network allowing the diffusion of residual monomer from the polymer chain network as stated by Bettencourt et al. Thus, the water molecules diffused between the macromolecules of the material, forces them apart, as stated by Abdulla  leading to dimensional instability, thus subjecting the material to internal stresses. This may result in crack formation and eventually, fracture of the material, and may have resulted in decreased impact strength.
Furthermore, increased liquid (monomer) content (i.e., P/L ratio 1.6:1; Group 5) beyond limits, results in greater amounts of residual monomer, and when subjected to curing change into gas form as the boiling temperature of methyl methacrylate is close to the boiling temperature of water. This produces bubbles in the polymer matrix. This type of gas formation will also be enhanced by exothermic heat production during polymerization. These gas bubbles in the polymer matrix also form voids, thereby increasing the porosity and developing high internal stresses as suggested by Dogan et al.
On analyzing the results, it was concluded that the polymer/monomer ratio had no significant effect on the impact strength which is in accordance with the study done by Jerolimov et al. However, the impact strength values in both the studies were different which may be due to the different parameters used.
Through the previous studies done by Williams et al., Syme et al., Geerts and du Rand  it is known that change in the P/L ratio influences the mechanical properties of unreinforced autopolymerizing resin materials. However, little is known about its effect on reinforced heat polymerized PMMA resin.
Hence, the present study was done to evaluate the effect of different polymer/monomer ratios on the flexural strength, hardness, and impact strength of high-impact heat-cure resin.
In our study, the dynamic intraoral conditions were not simulated, and in vitro static load tests were taken into account. Thus, the microcracks and defects that develop during mastication could not be simulated.
Moreover, the present study utilized only a specific shape of samples and that the complex shapes like complete dentures were not used. As the shapes of the complete dentures were not incorporated, the mechanism of fracture associated with them in the oral environment could not be simulated and can be considered as a study limitation.
Since the samples were stored in water, they tend to absorb water which leads to increase in weight of the samples and causes dimensional changes. However, in the present study, weight of the samples and dimensional changes were not taken into account. Hence, there is a scope for further studies keeping these things into consideration.
Furthermore, the effect of different ratios on the mechanical properties of high-impact denture base resin samples of the complex shape of complete denture after cyclic loading could be investigated in the future.
The null hypothesis to be tested was that a lower or a higher than the recommended P/L ratio decreases the strength properties of high impact denture base resin. This hypothesis can be accepted.
| Conclusion|| |
From the observation of results, it was thus concluded that for reinforced resins or for high impact resins, the manufacturer's recommended polymer/monomer mixing ratio should be used to obtain the appropriate strength of the material and proper care should be taken for accurate proportioning of the polymer and monomer. Varying the polymer/monomer ratios can deteriorate the strength of the material leading to its failure, which ultimately causes hindrance in its success rate.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]