Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2017  |  Volume : 28  |  Issue : 4  |  Page : 442--449

A comparative study to determine strength of autopolymerizing acrylic resin and autopolymerizing composite resin influenced by temperature during polymerization: An In Vitro study


Anuj Chhabra, IV Rudraprasad, DB Nandeeshwar, C Nidhi 
 Department of Prosthodontics, Bapuji Dental College and Hospital, Davangere, Karnataka, India

Correspondence Address:
Anuj Chhabra
Department of Dental Surgery, Hindu Rao Hospital, North DMC Medical College, New Delhi
India

Abstract

Aim: Temporary coverage of a prepared tooth is an important step during various stages of the fixed dental prosthesis. Provisional restorations should satisfy proper mechanical requirements to resist functional and nonfunctional loads. A few studies are carried out regarding the comparison of the effect of curing environment, air and water, on mechanical properties of autopolymerizing acrylic and composite resin. Hence, the aim of this study was to compare the transverse strength of autopolymerizing acrylic resin and autopolymerizing composite resin as influenced by the temperature of air and water during polymerization. Materials and Methods: Samples of autopolymerizing acrylic resin and composite resin were prepared by mixing as per manufacturer's instructions and were placed in a preformed stainless steel mold. The mold containing the material was placed under different controlled conditions of water temperature and air at room temperature. Polymerized samples were then tested for transverse strength using an Instron universal testing machine. Results: Alteration of curing condition during polymerization revealed a significant effect on the transverse strength. The transverse strength of acrylic resin specimens cured at 60°C and composite resin specimens cured at 80°C was highest. Polymerizing the resin in cold water at 10°C reduced the mechanical strength. Conclusions: Polymerization of the resin in hot water greatly increased its mechanical properties. The method of placing resin restoration in hot water during polymerization may be useful for improving the mechanical requirements and obtaining long-lasting performance.



How to cite this article:
Chhabra A, Rudraprasad I V, Nandeeshwar D B, Nidhi C. A comparative study to determine strength of autopolymerizing acrylic resin and autopolymerizing composite resin influenced by temperature during polymerization: An In Vitro study.Indian J Dent Res 2017;28:442-449


How to cite this URL:
Chhabra A, Rudraprasad I V, Nandeeshwar D B, Nidhi C. A comparative study to determine strength of autopolymerizing acrylic resin and autopolymerizing composite resin influenced by temperature during polymerization: An In Vitro study. Indian J Dent Res [serial online] 2017 [cited 2019 Sep 15 ];28:442-449
Available from: http://www.ijdr.in/text.asp?2017/28/4/442/213046


Full Text

 Introduction



Temporary coverage of prepared tooth is an important step during various stages of prosthodontic treatment. Temporary or provisional restorations are often neglected in fixed prosthodontics as sufficient time is not allowed for the fabrication of the restoration.[1] A provisional restoration should meet the following requirements such as pulpal protection, positional stability, occlusal function, easy to clean, nonimpinging margins, strength, retention, and esthetics.[2] Autopolymerizing acrylic resin and composite resins are most commonly used biomaterial for provisional restoration. Since the introduction of acrylic resin in 1936, polymethyl methacrylate resins have been successfully used for various applications in dentistry including all conventional removable partial and complete dentures for many years.[3],[4],[5] It also continues to be the material of choice for fabricating provisional fixed partial denture due to its ease of processing and generally favorable physical properties. Bis-acryl composites have gained popularity due to their convenience, accurate, and consistent mix, and easy cartridge delivery system.

The provisional fixed partial dentures are subjected to various compressive, tensile, and shear stresses during function. Regardless of its undoubtedly excellent qualities and properties as a provisional restorative material, polymethyl methacrylate acrylic resin and bis-acryl composites have exhibited a high incidence of fracture as a result of flexural fatigue failure. Flexural strength is an important mechanical property that has been used as possible predictors of the ability of materials to function in the oral environment. Flexural strength also known as transverse strength is a measurement of the strength of bar (supported at each end) under a static load.

Several techniques have been proposed to increase the strength and life of provisional restorations. They can be reinforced with metals; nylon; cross-linking agents; rubber polymers; corpuscles of ceramic, silica and sapphire; aluminum oxide, glass, polyethylene, aramid, and carbon fibers.[6],[7],[8],[9] Hazelton and Brudvik [10] suggested reinforcement of autopolymerizing provisional restorations with stainless steel orthodontic band material.

Heat processed acrylic resin has greater strength, wear resistance, color stability and resistance to fracture than autopolymerizing resins. Provisional prosthesis made from heat processed acrylic resin can function satisfactorily for extended periods. However, in long-span edentulous situations, fractures may still occur mainly due to flexural fatigue or failures resulting from impact.[6],[11] Thus breakage is still a potential problem of provisional restorations, especially when a long term or long-span provisional restorations are required.

A method that effectively increases the strength of the provisional restorative material is desirable. Therefore, the purpose of this study was to examine the effect of curing environment, air and water, on mechanical properties of auto polymerizing acrylic and composite resins.

 Materials and Methods



The provisional restorative materials, self-cure tooth molding (DPI, Mumbai, India) autopolymerizing methyl methacrylate resin for crown and bridge temporary material, and integrity (Dentsply Caulk) bisacryl composite resin temporary crown and bridge material were used in the study [Figure 1]. The method employed in this study have been divided into following steps: (1) Fabrication of split machined steel mold (2) fabrication of testing jig (3) preparation of specimen under different curing conditions (4) load application on the provisional restorative material specimens mounted on testing jig through Instron universal testing machine and (5) measuring the transverse strength.{Figure 1}

Fabrication of split machined steel mold

Precisely machined steel mold was designed to obtain definite size predetermined specimens. The dimension of the specimen mold cavity was 25 mm × 6 mm × 2 mm. The machined mold was double split between for easy retrieval of specimens. The steel mold was fabricated in Jupiter Tools and Laboratory, New Delhi, India [Figure 2].{Figure 2}

Fabrication of testing jig

A testing jig was fabricated to position the specimen on a flexural strength 3-point bend test in Instron universal testing machine (Zwick GmbH and Co, Ulm-Eingsingen, Hounsfield, England). Specimens were positioned with 20 mm support separation.

Preparation of the specimen under different curing environment

The provisional restoration test materials used were autopolymerizing methylmethacrylate (Dental Products India, Mumbai), supplied in powder and liquid form and autopolymerizing bis-acryl composite material (Integrity, Dentsply Caulk, USA), supplied in their cartridge delivery system. Both materials were mixed according to manufacturer's directions. The schedule was designed to simulate a direct technique commonly used in fabricating crowns [Figure 3] and [Figure 4].{Figure 3}{Figure 4}

Manufacturer's directions for autopolymerizing acrylic resin are:

0 s - Mixing the resin10 s - Placing the resin into the mold1 min 50 s - Incubating the resin specimen under controlled conditions6 min - Removing the resin specimen from the mold and dimensional measurement8 min - Mechanical testing.

For the cartridge dispensed materials, a small amount was extruded and discarded before the application of the material into the mold. The bisacryl composite resin was dispensed through a cartridge system and allowed to auto-polymerize under controlled various curing conditions and as per the manufacture's instruction.

Time diagram of bisacryl composite resin specimen experimental protocol:

0 s - Material dispensed into the specimen mold30 s - Incubating the resin specimen under controlled curing conditions6 min - Removing the resin specimen from the mold and dimensional measurements8 min - Mechanical testing.

Controlled curing conditions to which specimens were subjected were following:

In air at room temperature (Control)In water at 10°CIn water at 20°CIn water at 30°CIn water at 40°CIn water at 60°CIn water at 80°C.

Polymerization under water with different controlled temperature conditions was achieved by placing the mold loaded with specimen in a thermostat unit [Figure 5] and [Figure 6]. Ten specimens for each and every water temperature/curing condition were fabricated. After removing the excessive if any using an abrasive paper, dimensional measurements of specimens were performed [Figure 7].{Figure 5}{Figure 6}{Figure 7}

Load application on the provisional restorative material specimen mounted on testing jig through Instron universal testing machine

Mechanical properties of the test resin were assessed using transverse strength. Eight min after the start of mixing, the transverse test (3-point flexural test) was carried out at a crosshead speed of 1 mm/min using an Instron universal testing machine (Zwick, GmbH and Co., Ulm-Eingsigen, Hounsfield, England) at room temperature (29°C). The span of 3 point loading was 20 mm [Figure 8] and [Figure 9].{Figure 8}{Figure 9}

Measuring the transverse strength

Transverse strength (T) were expressed by the equation

T = 3 × 20 L/2 wt 2

Respectively, maximum load (L), width (w), thickness (t).

The values were calculated and the differences in transverse strength of the specimens polymerized under different curing environment were compared using statistical analysis.

 Results



Results revealed that the alteration of polymerization conditions had a significant effect on the transverse strength of the autopolymerizing acrylic and composite resin (P < 0. 01). [Table 1] shows the transverse strength (MPa) of autopolymerizing acrylic resin samples being cured in air at room temperature and water at various temperatures. They indicate as follows: AS1 - Acrylic resin specimen cured in water at 10°C, AS2 - Acrylic resin specimen cured in water at 20°C, AS3 - Acrylic resin specimen cured in water at 30°C, AS4 - Acrylic resin specimen cured in water at 40°C, AS5 - Acrylic resin specimen cured in water at 60°C, AS6 - Acrylic resin specimen cured in water at 80°C, AS7 - Acrylic resin specimen cured in air at 29°C (room temperature). Acrylic resin specimens cured at 10°C in water showed the least transverse strength (47.91 MPa) and the specimens cured at 60°C in water showed the highest transverse strength (65.78 MPa).{Table 1}

[Table 2] depicts the transverse strength (MPa) of autopolymerizing composite resin samples being cured in air at room temperature and water at various temperatures. They indicate as follows: CS1 - Composite resin specimen cured in water at 10°C, CS2 - Composite resin specimen cured in water at 20°C, CS3 - Composite resin specimen cured in water at 30°C, CS4 - Composite resin specimen cured in water at 40°C, CS5 - Composite resin specimen cured in water at 60°C, CS6 - Composite resin specimen cured in water at 80°C, CS7 - Composite resin specimen cured in air at 29°C (room temperature). Composite resin specimens cured at 10°C in water showed the least transverse strength (56.08 MPa) and the specimens cured at 80°C in water showed the highest transverse strength (81.65 MPa).{Table 2}

Descriptive data are presented as mean and standard deviations and were used for analysis [Table 3] shows mean and standard deviation of transverse strength (MPa) and P values to compare the mean transverse strength (MPa) of autopolymerizing acrylic resin samples cured in air at room temperature and water at various temperatures.{Table 3}

One-way analysis of variance was used for multiple group (temperature) comparisons followed by Newman–Keul's range test for pairwise comparisons. Unpaired t-test was used for testing the difference between two groups. [Table 4] shows mean and standard deviation of transverse strength (MPa) and P values to compare the mean transverse strength (MPa) of autopolymerizing composite resin samples cured in air at room temperature and water at various temperatures. [Table 5] shows the P values to compare the mean transverse strength (MPa) of the autopolymerizing acrylic and composite resin specimens cured in different polymerizing conditions.{Table 4}{Table 5}

 Discussion



Providing the most appropriate provisional restoration for a given patient is both challenging and rewarding. Provisional restorations must satisfy proper mechanical requirements to resist functional and nonfunctional loads. At present, there is no provisional restorative material that meets the optimal requirements for all situations. Clinicians typically choose the product based on ease of manipulation, cost, and esthetics.

The provisional fixed partial dentures are subjected to various compressive, tensile and shear stresses during function, which can result in fracture of the restorations. Mechanical properties of provisional restorations were assessed using flexural strength or transverse strength. The flexural strength is a combination of tensile and compressive strength tests and includes elements of proportional limit.[12]

Attempts have been made to strengthen acrylic resin materials by reinforcement with either chemical modification with grafted copolymers and stronger cross-linkage or by the inclusion of various organic and inorganic reinforcing fibers. Materials used for fiber reinforcement include metal, glass, carbon graphite, sapphire, kevlar, polyester, and rigid polyethylene.[3]

The inclusion of metal fibers resulted in poor esthetics, and these should be used only in locations where esthetics is least important. Polyethylene fibers are more esthetic, but the process of etching may not be practical in the dental office. Whisker fibers and similar materials appear to have the greatest potential in strengthening poly-methyl-methacarylate (PMMA) for esthetic provisional restorations. Untreated fibers act as inclusion bodies in the acrylic resin mixtures and instead of strengthening, actually weaken the resin. Most of these materials have had little or no success in increasing resin strength.[13]

Some investigators have attempted to increase the fracture strength of the materials by decreasing the size and number of the internal porosities through pressure curing and microwave energy.[14],[15] The increase in fracture strength was noted with this method, but not to a significant degree. Nevertheless, autopolymerizing polymethyl methacrylate resin remains the material most frequently used by clinicians for the direct and indirect fabrication of provisional fixed restorations. Recently, bis-acryl resin composites, although more expensive than PMMA, have become a popular choice for clinicians in the direct fabrication of provisional restorations. Research has shown bis-acryl-type resin composites to provide several fabrication and physical property advantages compared to PMMA as a provisional crown restorative material. Advantages reported include low polymerization shrinkage, low exothermic reaction, acceptable surface hardness, and ease of manipulation.[2]

The most important determinant of resin strength is the degree of polymerization by the material. Heat activates the polymerization reaction. Placing the provisional resin restorations in hot water is an accepted procedure and often, is recommended in the manufacturer's directions. Specific water temperature that hastens polymerization has not been defined. This study demonstrated that the strength of autopolymerizing acrylic and composite resin is influenced by different polymerizing conditions during polymerization.

Acrylic resin specimens cured at 10°C in water (47.91 MPa) showed the least transverse strength. This may be due to delayed resin polymerization and high water sorption in specimens polymerized at this temperature thereby increasing the distance between the molecular chains which may lower the transverse strength, corroborating the findings of Davenport.[16] Other rationalization could be related to the presence of porosity in specimens cured at this temperature. This agreed with studies by Davenport;[16] Harcourt et al.;[17] El-Hadary and Drummond JL;[18] Gjerdet [19] who too concluded that if porosity reaches the surface, the transverse strength will be lowered. It also may be related to the presence of higher residual monomer resulted from lower temperature of polymerization which acts as plasticizer responsible for lowering the transverse strength. This explanation corresponds with the findings of Lamb et al.[20] and Yau et al.[14]

Specimens cured in water at the temperatures above the room temperature showed increase in transverse strength with the highest transverse strength at 60°C. The transverse strength at 60°C in water (65.78 MPa) was approximately 1.2 times more than the transverse strength of specimens cured at room temperature in air (56.95 MPa). The increase in transverse strength of specimens polymerized at the temperatures above the room temperature in comparison with that polymerized in air could be related to the high molecular weight and long polymer chain length resulted from more complete polymerization at these temperatures that would increase the transverse strength. This explanation is in accordance with the study conducted by Ray.[21]

Increasing the temperature above 60°C of water did not show any increase in transverse strength, but still found to be more than the transverse strength found at room temperature in air. This is because, heat activates chemical reaction between the monomer and the polymer components and at 60°C it allows maximum utilization of monomer and nearly complete polymerization. Increased transverse strength likely indicates a greater resistance to fracture of the resin.

A study reported by Ogawa et al. showed that transverse strength and transverse modulus both increased with increase in water temperature.[22] Water temperature of 60°C–80°C produced two times greater transverse strength and modulus or resin compared with polymerization in 23°C in air. Results of this study were supported by the findings of this study.

However, studies have also showed that water temperature during polymerization was a significant factor that affected the marginal fit of provisional crowns.[23] Provisional crowns polymerized in 20°C and 30°C water had much better marginal adaptation than those fabricated in water at a higher temperature than or in 20°C air.

Use of temperature above room temperature may result in poor marginal adaptation. This may be the limitation of this technique. From this point of view, some clinician may be hesitant to place the resin in hot water during polymerization. In clinical situation, because polymerization shrinkage of the resin is still unavoidable to some extent, it is usual to readjust and reline a provisional restoration a few times until an acceptable, marginal fit is obtained.

Composite resin specimens cured at 10°C in water (56.08 MPa) showed the least transverse strength. This may be again due to delayed and inhibited resin polymerization. In a study conducted by Bausch et al., it was concluded that at low temperature, when the material is still in the viscous state, the diffusion of the reactive groups is a dominant factor in the polymerization reaction and can take place for a relatively long time. The formation of long chains will consume the peroxide groups available, reducing the chance of the high energy cross-linking. A rubbery material will be formed with a relatively high internal mobility. The material will also have a nonhomogenous structure.[24] Specimens cured in water at the temperatures above 40°C showed an increase in transverse strength with the highest transverse strength at 80°C. The transverse strength at 80°C in water (81.65 MPa) was approximately 1.26 times more than the transverse strength of specimens cured at room temperature in air (64.31 MPa).

Increasing the temperature till 80°C of water resulted in a maximum increase in transverse strength and found to be more than the transverse strength found at room temperature in air. This is due to occurrence of cross-linking, probably due to the thermal activation of the remaining peroxides at approximately 60°C–70°C.[24] With the surplus of energy available, the reaction product will have a homogenous structure and becomes hard and brittle. The higher the temperature rises, the earlier the ultimate properties reached.

Comparing the autopolymerizing acrylic resin with composite resin specimens, composite resin specimens showed increased transverse strength in relation to acrylic resin specimens at all respective polymerizing conditions in air or water.

Regardless of direct or indirect technique in fabricating provisional restorations use of hot water provides dentist most economical method to obtain improved mechanical properties of provisional restorations, decreases the pulpal irritation due to reduced monomer content, reduces the discrepancy between provisional and the final restoration felt by the patient during biting, also reduces chair time, and may extend their clinical performance in the relatively long span provisionals with greater clinical convenience, fracture resistance and durability promising the success of the definitive prosthesis.

On the basis of the present data regarding mechanical properties of the resin and prior studies indicating resin polymerization shrinkage, placing acrylic resin provisional restoration in hot water of 60°C and composite resin provisional restorations in hot water at 80°C is recommended. When fabricating a provisional crown, hot water should be used to reinforce the outer surface of the crown. The inner surface or margin should be readjusted or relined without placing the resin in hot water.

 Conclusions



With the limitations of the laboratory testing conditions of this study, the following conclusions are drawn:

Polymerizing the resin in hot water greatly increased its mechanical strengthThe transverse strength of acrylic resin specimens cured at 60°C in water was the highest while transverse strength was highest for composite resin specimens when cured at 80°C in waterThe transverse strength of acrylic resin specimens cured at 60°C in water was 1.2 times more than that of specimens cured in room temperature airThe transverse strength of composite resin specimens cured at 80°C in water was 1.26 times more than that of specimens cured in room temperature airPolymerizing both the resin in cold water (at 10°C) reduced the mechanical strengthThe increase in temperature above 60°C water conditions produced a decrease in transverse strength for acrylic resin but continued to improve the strength for composite resin specimen till 80°C testedThe transverse strength of composite resin provisional restorations is more than that of acrylic resin provisional restorations at all respective polymerizing conditions in air or water.

The method of placing resin restoration in hot water provides the most economical method to obtain improved mechanical properties of provisional restorations, reduces the pulpal irritation by the residual monomer, shortens the chair time, and extends their clinical performance.

It can be used in patients with long-span provisionals, cantilever provisionals, complete arch provisionals, or implant supported provisionals.

Acknowledgment

I would like to take this opportunity to express my gratitude to my guide, co-guides and my colleagues for their continuous support and encouragement throughout the course of my study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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