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ORIGINAL RESEARCH Table of Contents   
Year : 2009  |  Volume : 20  |  Issue : 3  |  Page : 293-297
Effect of microwave postpolymerization treatment on residual monomer content and the flexural strength of autopolymerizing reline resin

Department of Maxillofacial Prosthodontics and Implantology, HKE'S Nijalingappa Institute of Dental Sciences and Research, Gulbarga - 585102, Karnataka, India

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Date of Submission16-Jan-2008
Date of Decision03-Apr-2008
Date of Acceptance25-Aug-2008
Date of Web Publication30-Oct-2009


Background : Microwave postpolymerization has been suggested as a method to improve the flexural strength of an autopolymerizing denture reline resin. However, the effect of microwave postpolymerization on the residual monomer content and its influence on flexural strength have not been investigated.
Objectives : This study analyzed the effect of microwave postpolymerization on the residual monomer content and its influence on the flexural strength of an autopolymerizing reline resin (Denture Liner).
Materials and Methods : A total of 70 specimens (64 Χ 10 Χ 3.3 mm) were polymerized according to the manufacturer's instructions and divided into 7 groups (n = 10). Control group specimens were not subjectedto any further processing. Before testing, the specimens were subjected to postpolymerization in a microwave oven using different power (550 and 650 W) and time (3, 4, and 5 min) settings. Two specimens of each group were then manually ground into fine powder and samples extracted from the specimens using reflux method. The samples were then subjected to gas chromatography for residual monomer determination in area%. Eight specimens were subjected to a three-point bending device with a span of 50 mm and crosshead speed of 5 mm/min, and the flexural strength was determined in MPa. Data analyses included Student's t-test and one-way analysis of variance.
Results : For the Denture Liner reline resin, the residual monomer content decreased and the flexural strength increased significantly with the application of microwave irradiation using different time/power combinations. The specimens with the lowest residual monomer content were the similar specimens which presented with the highest flexural strength.
Conclusion : Microwave postpolymerization irradiation can be an effective method for increasing the flexural strength of denture liner (at 650 W for 5 min) by reducing the residual monomer content by further polymerization at free radical sites.

Keywords: Autopolymerizing reline resin, flexural strength, residual monomer

How to cite this article:
Patil PS, Chowdhary R, Mandokar RB. Effect of microwave postpolymerization treatment on residual monomer content and the flexural strength of autopolymerizing reline resin. Indian J Dent Res 2009;20:293-7

How to cite this URL:
Patil PS, Chowdhary R, Mandokar RB. Effect of microwave postpolymerization treatment on residual monomer content and the flexural strength of autopolymerizing reline resin. Indian J Dent Res [serial online] 2009 [cited 2019 Sep 15];20:293-7. Available from:
Complete denture adaptation to the oral tissue is one of the important factors to achieve good support, stability, and retention. However, alveolar resorption is a continuous process, with resulting loss of fit in local areas of the denture base, [1],[2],[3] requiring reline of the complete denture to re-establish the fit and improve the supportive capability of the denture base.

Many commercially available hard autopolymerizing reline resins [4],[5],[6] allow the dentist to reline removable prostheses directly, intraorally, and also have the advantage of reproducing the morphological features of oral soft tissue directly on the denture base when compared to the heat cure reline resin. The heat cure reline resins have the added disadvantage of extra patient visits, as well as laboratory fee, and the patient must be without dentures for a period of time. [7] A complete denture relined with autopolymerizing hard reline resins has a decreased mechanical strength due to the presence of a higher residual monomer content, [8],[9] which can also elicit irritation, inflammation, and sometimes allergic response in oral mucosa. [10],[11],[12]

It has been demonstrated that the residual monomer in an autopolymerizing acrylic resin may be reduced by further polymerization at the free radical sites which could be achieved following a period of immersion in hot water. [13],[16] Microwave energy has been used for polymerization, having the advantage of reduced time for curing, a smaller time for obtaining the plastic phase, a bigger homogeneity of the mixture, and the achievement of a prosthetic material with an excellent mechanical strength. [17] These microwaves are electromagnetic waves generated by a magnetron. It has been reported that microwave polymerization involves heating the acrylic resin monomer only, and not the polymer. This allows a relatively low processing temperature around the material, resulting in little residual monomer and a good dimensional accuracy. And also that microwave irradiation of an autopolymerizing acrylic resin soon after polymerization decreases the residual monomer content by 25%, with an increase in the impact strength and glass transition temperature. [18] Similarly, it has been demonstrated that microwave postpolymerization resulted in a high degree of conversion and higher flexural strength of an autopolymerizing denture reline resin repair material. [19] With an appropriate combination of power and time for the polymerization of a heat-polymerized denture base acrylic resin using microwave irradiation, it is possible to minimize the level of residual monomer and reduce toxicity. [17]

The influence of different power and time settings of microwave postpolymerization on the flexural strength has been investigated. But the same on the residual monomer content and also the influence of a decrease in the residual monomer content on the flexural strength of an autopolymerzing denture reline resin have not been investigated. Therefore, the purpose of the investigation was to determine, quantitatively, the effect of different microwave oven power/time combinations on the residual monomer content and its influence on the flexural strength of an autopolymerizing, hard, denture reline material.

   Materials and Methods Top

One commercially available autopolymerizing hard denture reline resin was selected for this study [Table 1]. A total of 70 samples were fabricated in a stainless steel mold of dimension 64 Χ 10 Χ3.3 mm as per ISO/FDI 1567 standards. [20] The material was proportioned and manipulated following the manufacturer's instructions [Table 1] and packed within the mold in a temperature-maintained chamber. The samples were finished with a 400-grit silicon carbide paper to remove the irregularities. The accuracy of the dimensions was verified with a digital vernier caliper, at three locations of each dimension to within 0.2 mm tolerance. The final sample dimensions were measured as follows: Length 64 ± 0.2 mm; width 10 ± 0.2 mm; height 3.3 ± 0.2 mm. All samples were stored in a thermostatically controlled water bath at 35°C for 48 h before testing. [20]

The 70 samples were divided into 7 groups (n = 10). Control Group C were not subjected to any further processing. Six experimental groups (Group 3A to Group 5B) of samples were subjected to different post polymerization irradiation procedures in a domestic wattage adjustable microwave oven by altering the power (550 and 650 W) and the time settings (3, 4, and 5 min) [Table 2]. The samples were placed in the microwave oven and exposed to microwave energy directly.

Gas chromatography test

The specimens of each group were then manually ground into fine powder and samples extracted from the specimens using the reflux method. Gas chromatography was employed to know the residual monomer content, using a glass column of 2 mm length and 3.5 mm internal diameters, filled with 10% carbowax 20 M (polyethylene glycol 20,000). The oven temperature was 85°C, the injection temperature 200°C, and the flame ionization detector temperature was 150°C. Nitrogen was used as a carrier gas, 30 ml/min. The detector output was linked to a chart recorder, with the chart speed set at 2 min/ cm. An amount of 2 ΅l of the methanolic extract was injected into the gas-liquid chromatography and the calibration of the instrument was checked using a known amount of monomer in methanol before and after each set of monomer determinations.

Flexural strength test

All the samples were subjected to the flexural strength test in a servo-hydraulic Instron testing machine (ITM), using three-point loading. A crosshead speed of 5 mm/min was used and the distance between the supports was 50 mm. Load was applied until fracture, and the fracture load was recorded in Newtons (N) by reading the peak of the graph provided by the ITM software. The flexural strength (MPa) was calculated using the formula: FS = 3 WL/2 (bd) 2 .

Statistical analysis

The data were evaluated statistically using Student's t-test and one-way analysis of variance. The data of the material were analyzed using Student's t-test to study the significance of flexural strength differences resulting from the various power and time combinations within the group. One-way analysis of variance was used to evaluate the significance of flexural strength keeping the three different time factors as constant and varying the power for each.

   Results Top

Statistically analyzing [Figure 1] and [Figure 2], a comparison between the control group and the other samples subjected to microwave postpolymerization treatment showed that the control group had the least flexural strength and highest residual monomer content. The 650/5 combination of Denture Liner reline resin had the least amount of residual monomer content and highest flexural strength.

The statistical analysis was performed with Student's t-test, and the values are tabulated in [Table 3]. The best combination was 650/5 for Denture Liner.

The analysis of variance (ANOVA) was performed to compare the groups with the same time variable, and the "f"- value is recorded and tabulated in [Table 4] for Denture Liner as f calculated values. Significant f-values are designated as they are greater than f tabulated values, i.e., 4.41 at the 5% level. Results showed that there is a significant difference with 650/5, being the best combination for maximal flexural output and minimal residual monomer level for the Denture Liner reline resin (DLRR).

   Discussion Top

During the last few years, curing processes have been modified in order to improve the physical and mechanical properties of the materials. Among the several curing methods, microwave energy [15],[16],[21],[22] has the advantage of reduced time for curing, and requires a smaller time to obtain the plastic phase and a bigger homogeneity of the mixture and for the achievement of a prosthetic material with an excellent adaptation.

The flexural strength testing conducted in this investigation is relevant since it reflects the loading arrangement in the clinical situation. [23] The samples were subjected to microwave energy under dry conditions because the uptake of water by the acrylic resin would lead to plasticization of the resin, making it more flexible and resilient. [24]

For the microwave-cured acrylic autopolymerized reline resins, it has been demonstrated that the temperature developed during the reaction is not constant. It increases quickly at the beginning, goes through a maximum, and then decays, being able to reach peaks of the order of 150-200°C, depending on the working conditions. [25],[26] Hence, both the power of the microwave and time of exposure can be regulated to control, in these systems, the rate of polymerization and the degree of conversion.

On evaluation, the 650/5 Denture Liner samples revealed the least amount of residual monomer and highest flexural strength (P < 0.001) which was in accordance with the study conducted by Vergani et al. [27] No further beneficial effect appeared to be gained on extending the polymerization time beyond this as the residual monomer levels remained constant thereafter. In fact, too long an irradiation time caused wrapping, distortion, and eventually discoloration, as evident by the specimens irradiated for 9 min or more than that. [17]

Dogan et al. [28] showed that a longer curing period at 100°C decreased the level of residual monomer. Harrison and Huggett [19] reported that a terminal boiling stage assured an optimum in terms of minimal residual monomer. The results from this study collaborate their reports, when comparing the curing process of hot water bath heat and microwave energy used in this study. These findings are in agreement with those reported by Blagojevich and Murphy, [18] who observed that microwave irradiation of an autopolymerizing acrylic resin increased the degree of conversion, the impact strength, and the glass transition of an autopolymerizing resin. Similar results have been found by Neisser and Novaes Olivieri [29] when contrasting impact resistance and Knoop hardness of several resins with different polymerization cycles and microwave-cured resins. The results of the present study were in contrast with the findings of a study conducted by Azzarri et al. [17] to evaluate the effect of the different conditions of curing on the residual monomer levels and the hardness and impact strength of a microwave-polymerized acrylic resin in which no difference was found among the different groups. The difference in results may be due to the method of grouping the samples, where low-power samples were coupled with longer duration of time and vice versa.

The highest flexural strength of the microwave-polymerized DLRR could be related to the lowest residual monomer level obtained as a result of a higher degree of conversion in accordance with the findings of Harrison et al. [30] who showed that the highest level of residual monomer was found when the material was at its weakest.

Thus, the present study supports the statement that the highest flexural strength of the microwave-polymerized specimens could be related to the lowest residual monomer obtained as a result of a higher degree of conversion.

The reason for the variation of flexural strength with power and time combination among the samples could be supported by the following points:

  1. Since microwave heating is independent of thermal conductivity, this method of dielectric heating raises the temperature rapidly and the inside and outside of the material are equally heated. Hence, additional heat could not prove to be beneficial as seen in the case of heat cure resins. [19]
  2. The presence of cross-linking agents may be an important factor in the flexural strength observed in the present study. The cross-linking agents of denture base polymers may also affect the residual monomer content of the polymer. The final conversion of methylmethacrylate (MMA) with an ethyleneglycol dimethacrylate (EGDMA) cross-linking agent decreases with the increasing content of the cross-linking agent. [31] This is due to the cross-linked main chain segments which are bound together via the cross-linking agent. A rigid polymer structure thus hinders the conversion of MMA monomers especially at a curing temperature lower than the glass transition temperature. Hence, once this glass transition temperature is reached, further conversion of MMA does not take place, thereby limiting the content of the residual monomer with no effect of heat thereafter.

Limitations of the study

  1. The methodology used in this study such as microwave curing of the samples with an increase in power and time has given the values of flexural strength and residual monomer in an exponential range but if the procedure would have been in the fashion of an increase in power and a decrease in the time unit and vice versa, then the results could have proved a much better interpretation with respect to importance of curing with microwave energy.
  2. Masticatory load applies to the denture reline material and to the denture base as a whole, rather than to the reline material alone. The present study did not simulate the intraoral environment to evaluate the mechanical response of the samples and properties at the reline resin/ denture base resin interface.

   Conclusion Top

The mechanical property of the reline resins that are microwave postpolymerized depends on the exposition time and microwave power. These two parameters play a pivotal role in achieving the best mechanical performance of the material. The polymerization reaction never reaches 100% conversion and the monomer remains free within the material. Modifying one or both parameters could reduce the undesirable effect of the free residual monomer.

Within the limitations of the study, following conclusion can be drawn:

  1. For the DLRR, maximum flexural strength and minimal residual monomer content were offered with the application of microwave irradiation at 650-W power for 5 min.
  2. The least amount of residual monomer achieved with microwave postpolymerization is related to the highest flexural strength which is inversely proportional.

       References Top

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    Correspondence Address:
    Padmakar S Patil
    Department of Maxillofacial Prosthodontics and Implantology, HKE'S Nijalingappa Institute of Dental Sciences and Research, Gulbarga - 585102, Karnataka
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    Source of Support: None, Conflict of Interest: None

    DOI: 10.4103/0970-9290.57363

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      [Figure 1], [Figure 2]

      [Table 1], [Table 2], [Table 3], [Table 4]

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