|Year : 2014 | Volume
| Issue : 2 | Page : 65-70
Physical and mechanical properties of pressure-molded and injection-molded denture base acrylics in different conditions
Yousef A Shibat Al Hamd1, Veerendra B Dhuru2
1 Consultant Prosthodontist and Associate Clinical Professor, Department of Dentistry, Prince Sultan Military Medical City, Riyadh, Kingdom of Saudi Arabia
2 Adjunct Professor General Dental Sciences, Marquette University School of Dentistry, Milwaukee, USA
|Date of Web Publication||12-Aug-2014|
Dr. Yousef A Shibat Al Hamd
Department of Dentistry, Prince Sultan Military Medical City, P.O. Box 88135, Riyadh - 11662
Kingdom of Saudi Arabia
Source of Support: None, Conflict of Interest: None
Background and Aim: The aim of the study was to compare the physical (processing shrinkage) and mechanical properties (bending deflection, flexure strength, and flexure modulus) of pressure-molded (Lucitone 199 and ProBase Hot) and injection-molded (SR-Ivocap) denture base materials in different conditions.
Materials and Methods: Two denture base materials for pressure molding, Lucitone 199 and ProBase Hot, and one for injection molding (SR-Ivocap) were tested. Polymerization shrinkage (PS) was determined by measuring the linear distances between the reference points on the wax patterns (65 mm × 55 mm × 6 mm) and the corresponding cured acrylic plates and calculating the difference. Rectangular specimens (50 mm × 10 mm × 2.5 mm) were machined from these acrylic plates and used to measure bending deflection (BD) at various load levels, flexure strength (FS), and modulus of elasticity using a three-point bend test on an Instron Universal Testing Machine. A minimum of seven specimens were tested for each variable category. For each material, the percentage of specimens that failed within the load range of 9-10 kg was noted. Statistical analysis involved calculation of mean and standard deviations followed by group comparison of properties of different materials by using analysis of variance (ANOVA) and Tukey's multiple range tests. Level of significance was set at P < 0.05.
Result: There was no significant difference between the PS values. ProBase Hot exhibited significantly less BD and greater FS values than the other two materials (P < 0.05). Within the load range of 9-10 kg., 5% of the lucitone specimens, 25% of the SR-Ivocap specimens, and all of the ProBase Hot specimens fractured.
Conclusion: The three tested denture base acrylics did not show any significant differences in processing shrinkage. However, ProBase Hot showed significantly lower bending deflection values than Lucitone 199 and SR-Ivocap. Compared to Lucitone 199 and SR-Ivocap, ProBase Hot is a tougher and a stiffer material. Hence, it is more likely to fracture readily if subjected to extreme loading conditions due to the accidental fall of the denture on a hard surface or if the denture wearer inadvertently bites on a particularly hard particle.
Keywords: Denture base material, injection molding, mechanical property, physical property pressure molding
|How to cite this article:|
Shibat Al Hamd YA, Dhuru VB. Physical and mechanical properties of pressure-molded and injection-molded denture base acrylics in different conditions. Saudi J Oral Sci 2014;1:65-70
|How to cite this URL:|
Shibat Al Hamd YA, Dhuru VB. Physical and mechanical properties of pressure-molded and injection-molded denture base acrylics in different conditions. Saudi J Oral Sci [serial online] 2014 [cited 2019 Mar 18];1:65-70. Available from: http://www.saudijos.org/text.asp?2014/1/2/65/138463
| Introduction|| |
Polymethyl methacrylate (PMMA) resins are commonly used for the fabrication of denture bases, due to their good esthetics, simple processing and ease of repair. , There are two possible reasons why acrylic denture bases fracture intraorally in clinical situations, processing shrinkage, and lack of adequate mechanical strength. Processing of acrylic denture bases involves curing reaction of the monomer-polymer mixture (dough) filled in a specially fabricated gypsum mold. The mold is then subjected to a thermal curing cycle to complete the polymerization reaction. During this reaction, acrylic bases undergo polymerization shrinkage as well as shrinkage due to cooling down to room temperature. Such processing shrinkage, which is unavoidable for these materials, may cause dimensional changes in the base during fabrication. These changes would lead to warpage and may result in improper fitting of the base over the oral tissues. This, in turn, may cause build-up of non-uniform stresses in the base resulting in the fracture of dentures under various loading conditions encountered in the mouth.
Traditionally, processing shrinkage of denture base acrylics fabricated by different methods has been investigated using dentures constructed on gypsum or metal models. [3-12] The topics of these investigations have included, technique of filling the mold with pressure packing, injection molding, and pouring types of resins , and employed either heat cure, auto cure methods. , The dimensional changes were measured by using various instruments such as calipers, micrometer, tool maker's microscope, pantograph, optical comparator.  The difficulty in making the measurements of extremely small magnitude clinically  and the inevitable nature of polymerization shrinkage  have been noted.
American Dental Association's Specification No. 12 includes tests for measuring the mechanical properties of bending deflection, transverse strength and modulus of elasticity.  The last two properties were alternately known as flexural strength and flexural modulus. Such tests have been employed by investigators in their original form or with some modifications to study different types of denture base materials. [14-23] Traditionally, the acrylic dough is packed in the mold space under pressure or using the technique of injection molding. In this study, processing shrinkage and mechanical properties of bending deflection at various load levels, flexural strength, and flexural modulus were measured for ProBase Hot and the two widely used materials - Lucitone 199 and SR-Ivocap. Lucitone 199 and ProBase Hot were processed by the standard, pressure packing, and heat-cure technique while SR-Ivocap was processed by injection molding and heat-cure technique. The hypothesis of this study is that ProBase Hot material is superior in physical and mechanical properties to Lucitone 199 and SR-Ivocap. Therefore, the aim of this study was to compare some of the physical and mechanical properties of two pressure molding (Lucitone 199 and Probase) and one injection molding (SR-Ivocap) denture base materials.
| Materials and Methods|| |
Two pressure-molded acrylic denture base materials, Lucitone 199 and ProBase Hot, and an injection-molded material, SR-Ivocap, were used in this study. The first two materials were supplied in the form of a liquid-methylmethacrylate (MMA) monomer and polymethylmethacrylate (PMMA) polymer powder. SR-Ivocap was supplied in premeasured capsules containing powder and liquid in two separate chambers [Table 1].
Specimens for the processing shrinkage test
Sheets of 2 mm thick base plate wax were heated and sandwiched together to prepare rectangular patterns (60 mm × 55 mm × 6 mm). Two reference marks made on the surface of the pattern by pressing the two square ends of a caliper set 40 mm apart. The distance between the two reference marks was measured using a traveling microscope to the accuracy of 0.005 mm. Five wax patterns for Lucitone 199 and five for ProBase Hot were invested in the standard denture processing flasks. An additional five patterns were invested in the special flasks provided for SR-Ivocap the injection molding material. The respective manufacturer's instructions were followed for processing the acrylic plates. The pressure molded materials were processed using standard laboratory technique. The injection-molded material capsule (SR-Ivocap) was mixed in the apparatus supplied by the manufacturer for 5 minutes and the mix was allowed to cool on the bench for 25 minutes. The mixture was injected into the flask under pressure over a period of 10 minutes. The flasks were placed in the curing baths and were subjected to curing cycle as recommended by the manufacturer. Upon deflasking, the acrylic plates were removed finished and polished using the standard laboratory procedures employed for acrylic removable dentures. The plates were identified with appropriate code numbers.
Specimens for testing mechanical properties
Four specimens (50 mm × 10 mm × 2.5 mm) were machined from each acrylic plate. A total of 20 specimens for each material were fabricated [Figure 1] and [Figure 2]. They were dried in air for 24 hours and stored in a jar containing a desiccating material Silica Gel until the time of testing.
Measurement of processing shrinkage
The acrylic plates were dried at room temperature for 1 week. The distance between the two marks on the plate was measured using a travelling microscope. The difference between the two measurements, the first one on the wax pattern and the second one on the processed acrylic plate indicated the amount of processing shrinkage.
Measurement of the flexure strength and flexure modulus and other mechanicalproperties
Seven of the 20 dried specimens for each material were subjected to three-point bending test. Each specimen was positioned in a special fixture and was supported at the ends on two, 2 mm diameter steel rods positioned 45 mm apart [Figure 3] and [Figure 4]. The fixture permitted placement of the load via a third rod located above the center of the specimen. Incremental load was applied on to the specimen at a cross-head speed of 5.1 mm/min (0.2 in/min) until it fractured. 
The values of deflection were recorded to the accuracy of 0.01 mm at various increments of load levels as designated in the American Dental Association (ADA) Specification No. 12.  These levels ranged from 1.0 kg (9.81 N) to 10.0 kg (98.10 N) with increments of 0.5 kg (4.90 N).
Flexure strength and flexure modulus under dry conditions were calculated using the peak values of load sustained by the specimens and applying the following two equations. 
Where FS = Flexure strength
P = Load at fracture
b = Width of specimen
d = Thickness of specimen
Where FM = Flexure modulus
P = Peak load
I = Distance between the two supports
y = Deflection
b = Width of specimen
d = Thickness of specimen
The weights of the remaining 13 specimens were recorded to the nearest 0.001 g. The specimens were stored in distilled water at 37°C for 4 weeks and their weights were monitored weekly to ensure that they reached constant weight. At the end of this period, 7 out of the 13 specimens were subjected to the measurement of flexure strength and flexure modulus under water sorped condition. The remaining 6 specimens were stored in desiccators at room temperature for 4 weeks and their weights were monitored weekly to ensure they reached constant weight. At the end of this period, the specimens were measured for flexure strength and flexure modulus under desorped condition. [Figure 5] shows the summary of mechanical tests procedure. All the data were subjected to one-way analysis of variance and Tukey's multiple range tests.
| Results|| |
Lucitone 199 showed the greatest processing shrinkage 0.55 ± 0.21% followed by ProBase Hot 0.49 ± 0.09% and SR-Ivocap0.34 ± 0.06%. No statistically significant difference between the three materials was observed [Table 2].
Bending deflection, flexure strength and flexure modulus
The mean deflection values for the three materials under dry, sorped, and desorped conditions are shown in [Table 3]. ProBase Hot displayed significantly less deflection values than those for the other two materials (P < 0.05).
|Table 3: The mean and standard deviation of the maximum deflection of the three denture base materials (mm)|
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The mean values of flexure strength in Mega Pascal were 90.84 for Lucitone 199, 97.54 for SR-Ivocap, and 97.06 for ProBase Hot [Table 4].
|Table 4: Mean and standard deviation values for flexural strength (in MPa)|
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The number of specimens that fractured at the maximum load for the three materials is presented in [Table 5]. Five percent specimens of Lucitone 199, 100% of ProBase Hot, and 25% of SR-Ivocap were fractured. The rest of the specimens remained intact at the maximum load level of 98.10 N. Since the percentage of specimens that fractured at maximum load for Lucitone and SR-Ivocap were low, no analysis of variance was performed on the data for flexure strength. The mean values of flexure modulus are presented in [Table 6]. ProBase Hot displayed significantly greater values than those for the other two materials (P < 0.05). [Figure 6] and [Figure 7] shows the differences in mean weight change in milligrams after water desorption and water sorption on acrylic denture material.
|Table 5: Number of specimens fractured at the maximum load for the three Denture base materials at different conditions|
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|Table 6: The mean and standard deviation values of flexure modulus for three denture base materials under different conditions (in MPa)|
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| Discussion|| |
The mean linear processing shrinkage of 0.55% for Lucitone 199 measured in this study was comparable with the previous findings of 0.5%  , 0.18-0.58%  0.16-0.49%  , and 0.5-2.0%.  We found no significant difference between the three materials tested in the present study. The processing shrinkage is a net result of dimensional changes occurring in the material due to thermal changes and polymerization shrinkage. The distance between the reference marks in the wax pattern reproduced in gypsum investment was likely to increase slightly due to the setting expansion of the gypsum. During the peak temperature of the curing of acrylic, further expansion of the mold and the polymer could have caused an increase in the dimension. Concurrently, the polymerization shrinkage would diminish reference distance. Finally, cooling of the flask to room temperature would further reduce the reference distance causing processing shrinkage. Thus, the shrinkage observed from the wax pattern to the processed denture is a net result of a series of expansion and contraction phenomena related to temperature and polymerization.
For deflection and flexural strength, results showed a significant difference between the three materials. The Tukey's multiple range tests showed that ProBase Hot exhibited a significantly smaller mean deflection value than the other two materials. This indicates that the ProBase Hot is a relatively stiffer material than the other two denture base materials considered in the study as shown by the greater number of specimens fractured with the load range used in the study. The deflection values obtained in the present study for Lucitone 199 were in agreement with the findings of other studies. , Meng and Latta reported 8.6 mm deflection at breakage.  For SR-Ivocap, deflection values were in agreement with the 8.48 mm reported by Murphy et al.  The mean flexure strength value for Lucitone 199 was similar to the findings of other studies, , whereas the value for SR-Ivocap was similar to the findings reported by Uzun and Hersek.  However, these values differed from those reported by Memon et al. 83.6 ± 7.2 MPa  for a different brands of acrylic denture base material processed by compression technique. The flexure strength values obtained in the present study were in contrast with that reported by other studies of 69.4 ± 4.9 MPa,  165.4 ± 13.7 MPa,  and 111 ± 35 MPa.  Although Lucitone 199 was also used in some of these studies, the variation in specimen configuration curing cycle and rate of load application may account for the reported differences in these studies. However, Lucitone 199 exhibited less stiffness and greater deformation compared to the traditional acrylic resin. 
The limitations of the study involved, only three types of materials were included and secondly, only the selected properties were studied. Both the limitations were due to the constraints of time and funding to conduct the investigation. The tested properties were selected for their relative significance in fabrication and clinical usage of dentures.
The clinical significance of investigation was that compared to Lucitone 199 and SR-Ivocap, ProBase Hot was tough and stiff material. Therefore, it is more likely to fracture readily if subjected to extreme loading conditions. Further investigations are needed which will include different types of denture based materials and test impact strength and changes in color due to aging.
| Conclusions|| |
The three tested denture base acrylics did not show any significant differences in processing shrinkage. However, ProBase Hot showed significantly lower bending deflection values than Lucitone 199 and SR-Ivocap.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]