|Year : 2013 | Volume
| Issue : 3 | Page : 356-362
|An in vitro study to compare the transverse strength of thermopressed and conventional compression-molded polymethylmethacrylate polymers
Anjana Raut1, Polsani Laxman Rao2, BVJ Vikas2, T Ravindranath3, Archana Paradkar4, G Malakondaiah4
1 Department of Prosthodontics, Kalinga Institute of Dental Sciences, Bhubaneswar, Odisha, India
2 Department of Prosthodontics, Army College of Dental Sciences, Hyderabad, India
3 Ex-Principal, Army College of Dental Sciences, Hyderabad, India
4 Scientist, Department of Mettalurgy, DMRL, Hyderabad, India
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|Date of Submission||25-Feb-2012|
|Date of Decision||09-Aug-2012|
|Date of Acceptance||25-Nov-2012|
|Date of Web Publication||12-Sep-2013|
| Abstract|| |
Statement of Problem: Acrylic resins have been in the center stage of Prosthodontics for more than half a century. The flexural fatigue failure of denture base materials is the primary mode of clinical failure. Hence there is a need for superior physical and mechanical properties.
Purpose: This in vitro study compared the transverse strength of specimens of thermopressed injection-molded and conventional compression-molded polymethylmethacrylate polymers and examined the morphology and microstructure of fractured acrylic specimens.
Materials and Methods: The following denture base resins were examined: Brecrystal (Thermopressed injection-molded, modified polymethylmethacrylate) and Pyrax (compression molded, control group). Specimens of each material were tested according to the American Society for Testing and Materials standard D790-03 for flexural strength testing of reinforced plastics and subsequently examined under SEM. The data was analyzed with Student unpaired t test.
Results: Flexural strength of Brecrystal (82.08 ± 1.27 MPa) was significantly higher than Pyrax (72.76 ± 0.97 MPa). The tested denture base materials fulfilled the requirements regarding flexural strength (>65 MPa). The scanning electron microscopy image of Brecrystal revealed a ductile fracture with crazing. The fracture pattern of control group specimens exhibited poorly defined crystallographic planes with a high degree of disorganization.
Conclusion: Flexural strength of Brecrystal was significantly higher than the control group. Brecrystal showed a higher mean transverse strength value of 82.08 ± 1.27 MPa and a more homogenous pattern at microscopic level. Based on flexural strength properties and handling characteristics, Brecrystal may prove to be an useful alternative to conventional denture base resins.
Keywords: Crazing, ductile fracture, flexural strength, injection molding
|How to cite this article:|
Raut A, Rao PL, Vikas B, Ravindranath T, Paradkar A, Malakondaiah G. An in vitro study to compare the transverse strength of thermopressed and conventional compression-molded polymethylmethacrylate polymers. Indian J Dent Res 2013;24:356-62
Polymethylmethacrylate resin has many favourable properties, and is thus well established as a reliable material for use in removable dentures. Its remarkable working characteristics include ease of manipulation, polishability, seamless use with inexpensive equipment, stability in the oral environment and optimum esthetic results. By virtue of these characteristics, this material has been extensively used as a denture base polymer. Conventional acrylic resin denture base polymer does not fulfill all the requirements in terms of optimal mechanical properties due to its brittle nature.  Over the years, curing procedures have been modified with a view to improve the physical and mechanical properties of resin materials. These advances led to the introduction of injection molding techniques and polymers exhibiting improved impact resistance.
|How to cite this URL:|
Raut A, Rao PL, Vikas B, Ravindranath T, Paradkar A, Malakondaiah G. An in vitro study to compare the transverse strength of thermopressed and conventional compression-molded polymethylmethacrylate polymers. Indian J Dent Res [serial online] 2013 [cited 2018 Jul 16];24:356-62. Available from: http://www.ijdr.in/text.asp?2013/24/3/356/118006
Fracture of the dentures occurs quite frequently because of the fatigue and chemical degradation of the base material. Combined use of resin with reinforcing materials like glass fibers, carbon fibers, nylon fibers and metal wires has also been tried in the past with comparable results. Vallittu et al. demonstrated significant increase in transverse strength of denture base resins with fiber reinforcement, but improper impregnation of polymer matrix with the fiber bundle caused reduction in transverse strength of denture base polymer.  Generally, the most common causes of fracture are faults in denture fabrication like poor fit, lack of balanced occlusion, limitations of denture base material, improperly contoured occlusal plane, thickness of denture base etc., Results of a survey showed that 33% of the repairs carried out were due to debonded or detached teeth. Around 29% were repairs to midline fractures, more commonly seen in maxillary complete denture in a ratio of 2:1.  There is adequate evidence available in dental literature wherein researchers have demonstrated the various advantages of injection molding system over conventional compression molding. ,,,,,,
The purpose of this in vitro study was to evaluate the transverse strength of the newly introduced Brecrystal (Bredent, Germany) denture base material with conventionally used resin, namely, Pyrax (Pyraxpolymers, Uttarakhand, India) based on the null hypothesis that there would be no statistically significant difference in the transverse strength of these materials.
Transverse strength was selected as the unit of comparison because it is the value that has been reported most commonly in dental literature. The transverse strength is important because it reflects the rigidity of the material, which in turn is important for the integrity of the supporting ridge and tissues, along with the fitting accuracy of the denture. Denture base resin should not deform under loading to permit proper load distribution to the underlying structures. The prosthesis may fracture accidentally due to an impact while outside the mouth, or it may crack while in service in the mouth. The latter is generally the result of fatigue failure caused by repeated flexure over a period of time. Transverse strength is also known as flexural strength or modulus of rupture. Transverse strength is a collective measurement of tensile, compressive and shear stresses.
| Aims|| |
The aims of the present study were:
- To evaluate and compare the transverse strength of specimens of thermopressed injection-molded (Brecrystal) and conventional compression-molded polymethylmethacrylate polymers (Pyrax) and
- To examine the morphology and microstructure of fractured acrylic specimens.
| Materials and Methods|| |
Advances in polymer science have developed new molding and activation techniques, and high strength polymers. The question arises as to whether these resins are also resistant to flexural fatigue. Therefore there is a need for further investigation.
Two commercial denture base resins, available in the market were selected for this study. The study was carried out in the Department of Prosthodontics, Army College of Dental Sciences, Secunderabad.
The denture base resins examined were Brecrystal (Thermopressed injection-molded, modified polymethylmethacrylate) and Pyrax (conventional heat-polymerized methacrylate, control group). Each denture base material was tested for flexural strength according to ISO 1567 Standards.  Metal specimens of dimension 64 × 10.5 × 3.5 mm were prepared by a tool manufacturer.
Preparation of experimental samples
Preparation of conventional heat cured resin samples
Type III dental stone was used to invest metal dies. Before investing, the metal dies were coated with a thin film of petroleum jelly (Bioline ® ) for easy removal of the die once the dental stone sets. A powder: Liquid ratio of 100 g of stone to 30 ml of water was used to prepare the moulds for all the specimens. The mix of polymethylmethacrylate was prepared in a mixing jar according to the manufacturer's instructions (10 g of polymer in 4.5 ml of monomer). Once the acrylic resin reached the dough stage, it was packed in the flask initially at 1500 psi and finally at 3500 psi maintained for 60 min in a hydraulic press. After removing the flash, it was bench cured for 30 min. Then the flask was immersed in a thermostatically controlled Acrylizer (Confident) at room temperature. The temperature was slowly raised to 74°C and held for 90 min followed by boiling at 100°C for 30 min.
Preparation of thermopressed injection-molded resin samples
Metal dies were invested into elastomeric impression material and wax specimens were fabricated. Sprues were attached to the wax specimens and invested in dental stone. After dewaxing, resin cartridge was introduced into the mold that was pre-heated to 260°C for 15 min using injection pressure of 9.2 bars. The injection process was carried out in the injection unit (Thermopress 400) at a temperature of 260°C. The dental flask was bench cooled before deflasking.
The width and the thickness of the specimens were measured using digital vernier calipers of +0.1 mm accuracy (Bakers, India). All specimens were trimmed and finished to a final dimension of 64 × 10 × 3.3 mm.
The accuracy of the dimensions was verified at three locations of each dimension. Since the width and thickness are factors assessed for determining transverse strength, the resin specimens only with slight variations in size (up to 0.2 mm) were included in the study. Those with greater variation in size (>0.2 mm) were discarded from the sample so as not to interfere with transverse strength calculation. The accuracy of the dimensions was verified at three locations for each dimension. The final sample dimensions were measured as follows: Length 64 ± 0.2 mm; width 10 ± 0.2 mm; height 3.3 ± 0.2 mm.
The samples were stored in distilled water at 37°C for 48 h prior to testing to simulate oral conditions. They were dried thoroughly with an absorbent cloth before testing. A total of 60 specimens were fabricated from two different denture base materials. The specimens were divided into two groups:
Group I: 30 samples of thermopressed injection-molded acrylic resin.
Group II: 30 samples of compression-molded heat-cured acrylic resin.
Test samples were labeled on each end before testing as B1, B2, B3, …, B30 for Brecrystal injection molded samples and as C1, C2, C3, …, C30 for Pyrax, compression-molded heat cure samples, so that the fractured pieces could be reunited.
Measurement of transverse strength
Transverse strength of the experimental specimens were measured using 3-point bending testing device in a Universal testing machine (5500 R Instron Ltd, England) according to the American Society for Testing and Materials standard D790-03 for flexural testing of reinforced plastics  [Figure 1].
The testing device consists of a central loading plunger and two polished cylindrical supports 3.2 mm in diameter and 10.5 mm long. The distance between the centers of the support is 50 mm. The specimens were centered on the device, the loading wedge was brought down at a crosshead speed of 5 mm/min and the loading continued until fracture occurred. The compressive force was applied perpendicular to the center of the specimens until a deviation of the load-deflection curve and the fracture of specimen occurred.
|Figure 1: Specimen in place for the three‑point loading transverse strength test|
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Transverse strength is computed using the following equation:
S = transverse strength
W = load at fracture (N)
L = distance between supporting wedges (50.00 mm)
b = width of the specimen (mm)
d = thickness of the specimen (mm)
According to ISO 1567, the minimum flexural strength of denture base materials of Type 1, Type 3, Type 4 and Type 5 (heat polymerized polymers, thermoplastic blank or powder, light polymerized materials and microwave polymerized materials, respectively) should not be less than 65 MPa.
Preparation of samples for SEM examination
Fractured segments from two randomly selected specimens of each group were prepared by sputter-coating with gold-palladium alloy, and examined with a Scanning electron microscope at ×250 magnification. As the specimens of Brecrystal showed plastic deformation, it was sectioned 5 mm from the edge of the fracture under water cooling to prevent overheating, using a diamond-coated disc at 200 rpm in a high speed cutter.
| Results|| |
[Table 1] summarizes the results of the determined transverse strength. The mean transverse strength values and standard deviation were calculated for each group, and the data was analyzed by means of student unpaired t test at a 95% confidence level to determine the mean difference for each study group. The student t test indicated there was significant difference [Table 2] in the resin transverse strength between each of the two groups (P < 0.001). Brecrystal specimens deflected beyond the capacity of the transverse test machine and came off the rollers of the jig. Flexural yield strength value was taken because the specimens did not break during the test [Figure 2]. All specimens of the control group fractured during the test. During visual inspection as the fractured segments of control group could be repositioned at the fractured line and presented a smooth surface, the fractures were classified as brittle. In contrast, specimens of Brecrystal presented plastic deformation and exhibited rough and jagged surface; they were recorded as ductile fractures. The representative load displacement curves [Graph 1] [Additional file 1] of the control and Brecrystal suggest that the absorbed energy at peak load for Brecrystal (350 J) is 2.6 times higher than the control group (135 J). Brecrystal resin showed remarkable plastic deformation even after the peak load and did not break till ~25% of strain, and tests were interrupted at that point. It showed a total deflection of ~30 mm. Beyond 6 mm of deflection, the graph started falling down until the sides of the specimens started touching the support rollers resulting in a rise in the graph. The highest point of the first peak has been taken as the yield point. The control group samples fractured under a load of approximately 70 N and showed a maximum displacement of ~6 mm. The Brecrystal specimen showed a definite yield point and elongation with evidence of plastic flow rather than a brittle catastrophic failure. The graph clearly indicates that the area under the load displacement curve for Brecrystal is more than Pyrax, indicating higher ability to absorb energy.
|Figure 2: Sample of Pyrax (left) and Brecrystal (right) after three‑point loading test|
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Microscopic fracture analysis was made from two specimens after 3-point loading test. All fragments were subsequently submitted to SEM (FEI QUANTA 400, Netherland) to verify microstructural behavior and fracture morphology. The SEM image of the bent surface for Brecrystal material revealed a consistent lamellar appearance and multiple microcracks/cavitations distributed perpendicular to the tensile stresses indicating greater ability of the material to sustain cracks under continuous loading without failure [Figure 3]. Material shows crazing lines (white in color) in the direction of tensile stresses demonstrating area of greater stress concentration. The figure shows linkage of cracks in the Brecrystal material used in the present study.
|Figure 3: Scanning electron micrograph (magnification ×250) of bent Brecrystal resin surface|
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[Figure 4] is suggestive of more dense and compact matrix. Voids and porosities are not appreciable. The polymeric crystallographic planes are more definite. Arrow shows presence of crazing, revealing high plastic deformation and energy absorption.
|Figure 4: Scanning electron micrograph (magnification ×250) of fractured Brecrystal resin surface|
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The fractured surface of the Pyrax material, however, showed a dimpled/crater-like pattern resulting from the large numbers of porosities and pits of variable dimensions [Figure 5]. These patterns of the specimens were consistent for each of the materials tested. Conventional resin showed poorly defined crystallographic planes with a certain degree of disorganization. Irregularities could be seen in each acrylic resin fracture, suggestive of a granular microstructure, demonstrating that acrylic resin fails by transgranular or transcrystalline fracture.
|Figure 5: Scanning electron micrograph (magnification ×250) of fractured Pyrax resin surface|
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| Discussion|| |
The present study compared mean transverse strength recorded for thermopressed injection-molded and heat-polymerized compression-molded resin specimens. The null hypothesis that there would be no statistically significant difference in the transverse strength of the two tested denture base materials was rejected [Graph 2] [Additional file 2].
To reduce the incidence of midline fracture of denture bases, a good processing technique that reduces or eliminates residual stress within the denture and avoids surface defects and inclusions is essential. Using higher strength polymers, such as modified polymethylmethacrylate, will reduce the tendency to fracture.  Some authors stated that the mechanical properties of denture base materials varied depending on the polymerization method. ,,
This present study demonstrated significant difference between the mean transverse strengths of the two groups. The Brecrystal material showed a definite yield point and elongation with evidence of plastic flow rather than a brittle catastrophic failure. The control specimens showed negligible elongation and brittle fracture caused by periodic accelerations of the crack front. This is perhaps because they suffer little plastic deformation and absorb less energy. As the control resin material was manually mixed and packed, it was difficult to obtain dense specimens. For convenience in clinical processing, polymers are made from mixtures of polymeric powders and acrylic monomers. Generally the grains of polymer originally present in the powder persist in the final product and, along with a matrix provided mainly by the polymerized monomer. This significantly influences the mechanical properties.
The SEM image of the bent surface for the Brecrystal material simulates a bundle of extended chains suggestive of a consistent lamellar appearance. This is probably because of the continuous application of pressure to the system and the subsequent layered processing of the material. The SEM image of the core looks like blocks of segments situated close to one another. The lamellae thickness increases towards the core due to the fast cooling and freezing of the oriented structure on the surface, and increased temperature and relaxation time of molecular chains towards the core. There are also multiple microcracks/cavitations present perpendicular to the direction of tensile stresses. The cavitation process is usually initiated at the stress level close to the yield point.  Crazing lines (white in color, [Figure 4]) are present in the direction of tensile stresses.
A previous study has concluded that multiple crazing can lead to general yielding and may act as a toughening mechanism in polymers.  Ductile fracture is usually associated with extensive shear yielding mechanism. The fracture pattern of control group specimens exhibited poorly defined crystallographic planes with a high degree of disorganization. Irregularities could be seen in each acrylic resin fracture that was suggestive of a granular microstructure, demonstrating transgranular or transcrystalline fracture. Comparison of these results directly with results obtained by other workers is not possible for there is no such published result available on transverse tests in relation to Brecrystal. According to manufacturers, Brecrystal is a descendent of earlier thermoplastic resin material Polyan. A previous study reported that the flexural strength and modulus for Polyan was slightly higher to heat-polymerized resin.  The result of the present study is not in agreement with this study where highly significant difference was found in the mean transverse strength between conventional resin and modified polymethylmethacrylate denture base material processed by injection molding technique. This finding might be explained by the presence of the large number of porosities and voids in conventional heat cure resins.  The differences in chain formation and composition may also explain the different transverse strength values recorded for the materials. Therefore, the transverse strength of acrylic resins depends on several factors, such as polymer molecular weight, polymer bead size, residual monomer level, plasticizer composition, internal porosity of the polymer matrix, processing method, patient factors, type of polishing etc. 
Representative SEM micrographs of Brecrystal material shows significant crazing at microscopic level. Crazing is a microscopically localized phenomenon that initiates with the nucleation of microvoids in areas of stress concentration. Interestingly, the process of craze initiation, widening and breakdown in the initiation region involves a significant amount of plastic deformation and energy absorption, which would have otherwise been used for crack propagation leading to early fracture. Crazing is a relatively high energy consuming process and, thus, in fact, plays a decisive role in improving the fracture toughness of polymers as in the present study. Crazing generally occurs in the regions, where the stress on the sample has become highly concentrated owing to, for example, surface defects such as flaws, scratches, or inclusions within the material. Crazes can also occur in homogenous polymers, i.e., those without any contaminants or additives and which are flaw-free as in the present study. In such a case, the craze originates at a particular point because of an intrinsic local heterogeneity in the molecular structure, such as local density variation. The crazes that result from any of such localized process can be looked upon as load-bearing cracks, where the load-bearing capacity is provided by highly drawn fibrils that can support the crack and help prevent or delay the failure. The final fracture then occurs by linkage of cracks so formed. [Figure 4] shows linkage of cracks (encircled) in the Brecrystal material used in the present study. It may be recalled here that the crazing is absent in control material.
The results from transverse test and scanning electron microscopy indicated that differences observed can be attributed to polymer constituents and to the method of polymerization. This is in agreement with a previous study that evaluated fracture surfaces of acrylic resin.  Since the composition of Brecrystal was not provided by the manufacturer, therefore the influence of the composition could not be considered. Polymer crystallization is a kinetic event, variations in processing conditions (e.g., deformation rate, strain, temperature) and in molecular parameters (e.g., molecular weight, molecular weight distribution, chain branching and molecular architecture) of samples have great influence on final morphology and final mechanical properties. Considering that the dimensional stability of removable prosthesis can be affected by factors such as polymerization cycle, crack propagation and fracture, identifying the ability of stress absorption in the denture base acrylic resin and its implications in microstructural morphology may contribute to the understanding of material failure.
Limitations of the study
Although in vitro tests may not always reflect intraoral conditions and be predictive of clinical performance, they are valuable and can be applicable to clinical situations. The limitation of this investigation included the use of three-point flexural strength test in air. The use of a simple rectangular-shaped specimen rather than a complex denture design, as well as the absence of longer periods of water storage or thermal cycling, is a limitation of the present study. Since the present study did not aim to investigate the presence of residual monomer in the acrylic resin, it is not possible to affirm whether or how these factors affected the transverse strength results. Also, the alignment of molecular chains in the segment is out of the resolution of SEM, so it is not discussed here in detail, but is worth to be further investigated. Further studies are needed to evaluate other mechanical properties as well as to determine the potential benefits that methylmethacrylate-free materials can offer to prosthodontics treatment.
| Conclusion|| |
This investigation evaluated the transverse strength of two different denture base materials, Brecrystal and Pyrax. Within the limitations of this study, it was concluded that:
Based on flexural strength properties and handling characteristics, Brecrystal may prove to be a useful alternative to conventional denture base resins.
- When comparing the transverse strength of the two denture base resins, a statistically significant difference was found between Brecrystal and Pyrax.
- Brecrystal showed a higher mean transverse strength value of 82.08 ± 1.27 MPa compared to Pyrax (72.76 ± 0.97 MPa) and a more homogenous pattern at microscopic level.
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Department of Prosthodontics, Kalinga Institute of Dental Sciences, Bhubaneswar, Odisha
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]
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