**3. Thermal polymerization processing of PMMA**

The denture bases made up of acrylic PMMA resin, which is in contact with the oral mucosa of the patient is a critical aspect for biocompatibility in contact with tissues. The PMMA resin was chosen due to this important adequate processing technique [14]. Polymerization of PMMA by water bath and microwaves are the most commonly used processing techniques for making denture bases [12]. The water bath and microwave polymerization techniques produce a material with reduced porosity and irregularities on the PMMA surface. Independent of the processing method, the PMMA surface exhibits some defects (pores, cracks, and irregularities) that are produced at the time of its elaboration [30, 31]. These defects can be excellent reservoirs for fungi and opportunistic bacteria, besides decreasing the elastic modulus and flexural strength [12, 13, 32].

Over the years, the water bath processing technique has been the most widely used due to its ease of handling and cost effectiveness. But, the residual monomer content and porosity have been suggested as the most significant reasons for the reduced flexural strength [33]. It has been accounted the unfavorable thermal gradient produced during the processing technique. In the water bath processing technique, the benzoyl peroxide (initiator) was activated by heating the water to a very high temperature, which leads the polymerization reaction by crosslinking methyl methacrylate moieties. At this point, the methyl methacrylate particles begin to boil by creating porosities in the denture base resin [34]. As the reaction progresses, heat is liberated and cannot escape easily as the water surrounding the flasks is being heated as well. Thus, an unfavorable thermal gradient was created [35].The residual monomer inside the polymeric mass can negatively influence the physical and mechanical properties of the materials due to its plasticizing action [36]. On the other hand, during the microwave polymerization, monomer molecules move in a high-frequency electromagnetic field [37]. The microwaves cause the methyl methacrylate molecules within the acrylic resin to orient themselves in the electromagnetic field at a frequency of 2450 MHz [38], and numerous polarized molecules are flipped over rapidly and generate heat due to molecular friction [39]. Numerous intermolecular collisions are promoted, causing a rapid internal heating in which energy was immediately absorbed by the resin regardless of the thermal conductivity of the materials involved in the processing of the prosthesis [40]. This warming occurs rapidly and homogeneously and thereby transfer of heat from the water bath to the resin inside the flask occurs faster in this method [41].

There are several studies to compare the flexural strength and elastic modulus values of PMMA using water bath and microwave polymerization [13, 35, 36, 41–43]. In most cases, the results of microwave polymerization did not differ from those obtained with water bath, independent of the acrylic resins used [41, 43]. However, in some studies, water bath technique showed higher flexural strength than microwave processing technique [44]. On the contrary, in other studies, a statistically higher flexural strength was found for microwave-processed denture resins [45, 46]. Other researchers did not find a significant difference in porosity between microwave polymerization and conventional water bath cycles [39, 47, 48]. In contrast, the other work reported that heat polymerization technique presents lower mean porosity values than microwave-polymerization method [31]. Both processing techniques produced PMMA material with divergent properties. Therefore, new processing techniques for PMMA are needed to reduce the amount of residual monomer and porosity and to increase its physical strength.

#### **3.1. Thermal polymerization assisted with ultrasound**

The most widely used heat-curable acrylic material to make dental bases and temporary restorations is PMMA. A disadvantage of this acrylic resin is the residual monomer which remains in the polymer even after its polymerization is finished [49]. Several attempts were made to find a better strategy in order to prevent the presence of residual monomer. For example, the effects of temperature, time, initiator concentration, curing environment, water bath or microwave oven, pressure, and mixing ratio (polymer:monomer) have been investigated [32].

The first effort to employ ultrasound for the acceleration of conventional chemical reactions [50] by Richards and Loomis was reported in 1927. A lot of interest has been attracted for the use of ultrasound toward the development of synthetic routes in a variety of areas of chemistry, chemical production, and materials science [27, 51]. It is possible to generate chemical changes in consequence of acoustic cavitation while more powerful ultrasound at a lower frequency is applied to a system. During cavitation, bubble collapse produces intense local heating, high pressures, and very short lifetimes. These transient and localized hot spots drive high energy toward completing chemical reactions faster [52]. Besides, the physical effect of the medium on the wave was referred to low power or high frequency ultrasound [53].

In previous studies, Charasseangpaisarn and Wiwatwarrapan [49, 54] found that the use of an ultrasonic treatment at several frequencies reduced the presence of residual monomer in acrylic resins. For example, heat-polymerized MMA by the immersion in water at 50°C for 10 min at 40 kHz reduced the residual monomer. They have concluded that sonication could reduce the amount of residual monomer in acrylic resins. According to the authors, the ultrasonic treatment could enhance the extraction rate of the residual monomer from the resin and could cause postpolymerization of the residual monomer.

## *3.1.1. Influence of frequency and power of ultrasonic waves on the flexural strength and elastic modulus*

polarized molecules are flipped over rapidly and generate heat due to molecular friction [39]. Numerous intermolecular collisions are promoted, causing a rapid internal heating in which energy was immediately absorbed by the resin regardless of the thermal conductivity of the materials involved in the processing of the prosthesis [40]. This warming occurs rapidly and homogeneously and thereby transfer of heat from the water bath to the resin inside the flask

There are several studies to compare the flexural strength and elastic modulus values of PMMA using water bath and microwave polymerization [13, 35, 36, 41–43]. In most cases, the results of microwave polymerization did not differ from those obtained with water bath, independent of the acrylic resins used [41, 43]. However, in some studies, water bath technique showed higher flexural strength than microwave processing technique [44]. On the contrary, in other studies, a statistically higher flexural strength was found for microwave-processed denture resins [45, 46]. Other researchers did not find a significant difference in porosity between microwave polymerization and conventional water bath cycles [39, 47, 48]. In contrast, the other work reported that heat polymerization technique presents lower mean porosity values than microwave-polymerization method [31]. Both processing techniques produced PMMA material with divergent properties. Therefore, new processing techniques for PMMA are needed to reduce the amount of residual monomer and porosity and to increase its physical strength.

The most widely used heat-curable acrylic material to make dental bases and temporary restorations is PMMA. A disadvantage of this acrylic resin is the residual monomer which remains in the polymer even after its polymerization is finished [49]. Several attempts were made to find a better strategy in order to prevent the presence of residual monomer. For example, the effects of temperature, time, initiator concentration, curing environment, water bath or microwave oven, pressure, and mixing ratio (polymer:monomer) have been investigated [32]. The first effort to employ ultrasound for the acceleration of conventional chemical reactions [50] by Richards and Loomis was reported in 1927. A lot of interest has been attracted for the use of ultrasound toward the development of synthetic routes in a variety of areas of chemistry, chemical production, and materials science [27, 51]. It is possible to generate chemical changes in consequence of acoustic cavitation while more powerful ultrasound at a lower frequency is applied to a system. During cavitation, bubble collapse produces intense local heating, high pressures, and very short lifetimes. These transient and localized hot spots drive high energy toward completing chemical reactions faster [52]. Besides, the physical effect of the medium on the wave was referred to low power or high frequency ultrasound [53].

In previous studies, Charasseangpaisarn and Wiwatwarrapan [49, 54] found that the use of an ultrasonic treatment at several frequencies reduced the presence of residual monomer in acrylic resins. For example, heat-polymerized MMA by the immersion in water at 50°C for 10 min at 40 kHz reduced the residual monomer. They have concluded that sonication could reduce the amount of residual monomer in acrylic resins. According to the authors, the ultrasonic treatment could enhance the extraction rate of the residual monomer from the resin and

occurs faster in this method [41].

48 Acrylic Polymers in Healthcare

**3.1. Thermal polymerization assisted with ultrasound**

could cause postpolymerization of the residual monomer.

The method of denture processing is directly related to the physical properties of the acrylic resins. One of those properties is Young's modulus, also known as elastic modulus. That is defined as capacity of a body to deform to the application of stress and strain after removing the body recovers its original shape. It can be assumed that the relationship between the increased effort and increased deformation is constant [55]. Flexural failure of denture base of PMMA is considered to be the main form of clinical failure [56]. The dental prostheses are subjected to various conditions such as forces during chewing, drastic changes of temperature and humidity, and acidic environment in the oral cavity. Therefore, it is important that prosthetic materials possess an adequate elastic modulus [42]. The elastic modulus can be determined by indentation techniques. However, the correct use of these techniques requires knowing their limitations in order to avoid misinterpretation.

Experimental results about the elastic modulus and flexural strength (ISO20795-1:2008 *Part 1: Denture base polymers)* of commercial acrylic resin (Opticryl®) indicate that the thermopolymerization assisted with ultrasound is a good option for the processing of PMMA. Commercial acrylic resin (Opticryl®) specimens (*n* = 25) were prepared according to the technical sheet with a volume ratio of monomer to polymer (1:6). For the processing condition by ultrasound waves, two frequencies and powers were used at 80°C of water bath for 1 hour: 37 or 80 kHz and 50 or 100%, respectively, in order to obtain four experimental groups (**Table 2**). Water bath and microwave technical processing were considered to be the control groups. The results of the elastic modulus and flexural strengths are given in **Figure 3** and **Table 2**.

For statistic comparison among the groups, Kruskal-Wallis test and Mann-Whitney *U*-test were used for analyzing the data. These tests were used because not all groups had a normal distribution as shown by the Shapiro-Wilk normality test (see **Table 3**).

Kruskal-Wallis test showed that there are significant differences among the groups (for elastic modulus, *p* = 0.006 and for flexural strength, *p* = 0.018). Mann-Whitney *U* test was conducted


Specimens processed by water bath and microwaves are considered the control groups.

**Table 2.** Elastic modulus and flexural strength of specimens processed by ultrasound at 80°C: 37 or 80 kHz and 50 or 100% of power under constant temperature of water (80°C).

**Figure 3.** Results of elastic modulus and flexural strength of commercial Opticryl resin polymerized by ultrasound. Control groups were processed by water bath and microwave energy.


**Table 3.** Results of Shapiro-Wilk normality test.

among groups in all possible combinations to determine the differences among groups. The results are shown in **Table 4**.

The specimens processed at 80 kHz and 100% of power (group 4) exhibited the highest values with an elastic modulus of 1744.40 ± 441.85 MPa and a flexural strength of 60.57 ± 14.91 MPa. However, the flexural strength values were not statistically significant compared to those processed by the water bath and microwave, respectively (**Figure 3**). But, with regards to New Trends for the Processing of Poly(Methyl Methacrylate) Biomaterial for Dental Prosthodontics http://dx.doi.org/10.5772/intechopen.69066 51


**Table 4.** Comparison among groups using Mann-Whitney *U*-test.

among groups in all possible combinations to determine the differences among groups. The

**Figure 3.** Results of elastic modulus and flexural strength of commercial Opticryl resin polymerized by ultrasound.

Elastic modulus 37/50 0.940 32 **0.076**\*

Flexural strength 37/50 0.893 32 0.004

**Shapiro-Wilk statistic analysis**

37/100 0964 38 **0.261**\* 80/50 0.844 26 0.001 80/100 0.922 35 0.017 Water Bath 0.967 19 **0.711**\* Microwave 0888 21 0.020

37/100 0.962 38 **0.225**\* 80/50 0.924 26 0.056\* 80/100 0.932 35 0.032 Water bath 0.955 19 **0.472**\* Microwave 0.919 21 **0.083**\*

**Statistic value Degrees of freedom** *p-***Value**

Control groups were processed by water bath and microwave energy.

**power (%)**

**Mechanical properties Frequency (kHz)/**

50 Acrylic Polymers in Healthcare

The specimens processed at 80 kHz and 100% of power (group 4) exhibited the highest values with an elastic modulus of 1744.40 ± 441.85 MPa and a flexural strength of 60.57 ± 14.91 MPa. However, the flexural strength values were not statistically significant compared to those processed by the water bath and microwave, respectively (**Figure 3**). But, with regards to

results are shown in **Table 4**.

The data follow a normal distribution if *p* ≥ 0.05.

**Table 3.** Results of Shapiro-Wilk normality test.

\*

the elastic modulus, a highly significant difference among the specimens of group 4 and the specimens processed with microwave (group 6) was found. Hence, these results indicate a better performance of the PMMA processed by ultrasound in 80 kHz and 100% of power in comparison to that by microwave processing.

In addition, it is further noted that the acrylic resins processed at 80 kHz and 50% of power (group 3) had significantly lower values compared to the other experimental groups, for both the elastic modulus and the flexural strength. No statistically significant differences in the elastic modulus and flexural strength among groups (water bath and microwave) were found. Therefore, it was concluded that these two methods have similar results. From these results, it seems that the power is more important than the frequency of ultrasound for better results in the processing of PMMA.

Spearman correlation test was performed in order to determine if the elastic modulus values and flexural strength values are correlated. It was found that a weak correlation existed since the correlation coefficient between the elastic modulus and flexural strength was 0.618 (*p* ≤ 0.001). Since the coefficient is a positive value, the increasing elasticity modulus value also increases the flexural strength.

In summary, the best conditions for higher values of both elastic modulus and flexural strength correspond to the specimens processed at 80 kHz and 100% of power (group 4). The processing of PMMA with water bath or microwave processing generated similar values for elastic modulus and flexural strength. Ultrasound can be used to process the acrylic resin (Opticryl®) as an alternative technique for PMMA processing with similar results to those obtained using water bath or microwave processing (control groups). The correlation coefficient between the elastic modulus and the flexural strength indicates a weak correlation but statistically significant association between these two variables. The sign of the coefficient is positive, this means that as the values of the elastic modulus increase, those of the flexural strength also increase.
