Introduction

Since 1973 acrylic resins is the  substance used  for  denture base due it is  affordability, esthetics and ease of application (1). However, there are few clinical challenges including fracture resistance of resin bases that are subject to further research. Clinical research, it indicate that common problem face is middle fracture related to  maxillary  denture (2,3). Concentration of stress and increased flexing usually lead to denture fracture. Denture break  happened as result of one of these ; weakining of the base substances during chewing of something or happens when denture is dropped on a hard surface. Further, water absorption by denture base material in the oral cavity make it more susceptible to fracture (4).Mechanical properties of resin bases upgrading consume much time. Mechanical improvement of PMMA has been discussed under three different topics: First PMMA subtitute substance improvement; second, modification of PMMA formula by adding a rubber graft copolymer; and third  other substances like ; carbon fibers, glass fibers and ultra-high modulus polyethylene has been  used  to supplemented PMMA (5). High-impact resin such as; co-polymers, cross-linking agents and a rubber phase in the bead polymer have been used to  bolstered the acrylic. The rubber-reinforced acrylic contains butadien styrene rubber substances grafted with methyl methacrylate, which is dispersed in a polymethyl methacrylate matrix. The involvement of rubber serve as crack progress protection  (6). Different type of materials known as high strength denture base resin are available in the market. Conventional heat-cured acrylic resin  can afford  these  subtitute substances (7). Beside efforts in improvement of resistance fracture by the addition of a  stregthen sbustances, various polymerization procedures were investigated in a effort to reduce polymerization complications. Heat-cured polymerizarion, also known as a conventional method, is the most popular method. The conventional technique entailes monomer-polymer dough moulding and polymerization in a water bath (8). Different curing protocols such as pressure or hot water curing have been proposed to increase PMMA strength (9). Unfortunately, mechanical property challenges including denture fractures have not completely resolved and are still considered common incidents in patients wearing dentures (10). There is modification approach  over years in polymerazation  approach underwent  certain process  in an endeavor  to upgrade physical  mechanic properties of acrylic resins. Polymerization in an autoclave with higher temperature have shown to increase the transverse strength compare to conventional heat cured acrylic resin (11). PMMA producers of denture resins products  mentioned  that their products are of high quality,upgraded and even lay claim to have novel thou their is no any scientific evidence to  prove any strength differences between contemporary versions of PMMA products. Since there is no information about the effect of high polymerization temperature in acrylic resins, further research is required to investigate the mechanical characteristics of high impact acrylic resin. The purpose of this in vitro research is to analyse different effect of temperatures and length of time in polymerization on transverse strength and elastic modulus of a high strength acrylic denture base resin. The null hypothesis was that temperature and length of time in polymerization would effect the transverse strength and elastic modulus of the resin.

Materials and Methods

 High strength denture base resin (Rodex, Povere, Milano, Italy) was used in the research (In Table I). Chemical composition of the acrylic resin is been presented. From each group seven samples were drew up for the mechanical test. Ninety rectangular samples of acrylic resin, measuring 65 mm×10 mm×3.3 mm teflon rectangular matrix (ISO/FDSI 1567) were prepared for transverse strength test. Metal master model was individually supported with waxes patterns to copy the sample. Samples  were supported in Type III dental stone (Gyproc, Prevest Denpro, Jammu, India) in metal dental flasks.

For control group acrylic resin was mixed base on manufacturers directives as control group. The polymerized sample that was processed in hot water at  hundred degree for thirty minutes were observed as control. Test group details are shown in Table II. Then specimens were deflasked and finished using an automatic polishing machine with 200-400-600 grit waterproof carbide papers (Grin PO 2V grinder-polisher, Metkon A.Ş, Bursa, Turkey). Later, samples at period of 20 minutes were cleansed by ultrasound and then stored in distilled water at 37ºC for 48±2 hours before test. Three point bending procedure were conducted in the transverse strength test these are: Universal testing machine (Lloyd Instruments, LRX, Fareham Hant, United Kingdom) calibrated with a 500 kg load cell and a cross-head speed of 5 mm/min. The instruments use for transverse testing are two polished cylindrical support  which  is  3.2 mm in diameter and 10.5 mm long  and a central loading plunger. The two supports has average interval of fifty millimeter which constitute maxillary molars average distance in a complete denture.  To get the occurance of fracture and deviation of load deflection a penpendicular compressive force use to apply at the center of the specimen. Regulating of the transverse strength (σ) was done by calibrating the machine and computing the values automatically using the following equation:

σ=3Fl / 2bh²

The latter F represent the maximum load applied (N), l represent  the length between the supports (span length= 50 mm), b  represent  the width (10 mm) and h (3.3 mm) is the thickness of the sample. The minimum flexural strength of denture base materials (Type 1 heat-polymerized polymers) should not be below 65 MPa (ISO 1567). If the sample of result four out of five goes accordance with the requirement, the substance is consider pass. If only one or two of the sample goes accordance with the requirements the substance is considered failed. In a situation were three of the samples go accordance with the demand test of six new samples will be organised for repeatation. The substance is considered pass if five out of the second serries go accordance with the demand. Elastic modulus (E) was calculated from transverse strength test. Additionally, regulating of samples deflection in (mm) and corresponding forces (N) were done. Equation below was used to calculate.  The elastic modulus :

E= Fl³ / 4bh³d where d is the deflection (mm).

The elastic modulus of denture base materials (Type 1 heat polymerized resin)  should not be below 2000 MPa (ISO 1567).  In a situation were three of the sample did not tally with the demand the sample failed. In a condition that three of the transverse modulus results  go with  with the requirement,  preparation of  six new sample must be done for retest. And in situation were minimum five sample results of the second series go according with the requirement, the substance is regarded passes.

Table I. Chemical composition of the denture base resin used in this study.

Table II. Test groups used in this study

Statistical analysis

SPSS software (SPSS Institute Inc., version 9.0, Cary, NC) was used for statistical assessement with significance level at 5%.

One-way ANOVA was used in analyzing transverse strength and the elastic modulus data, while for comparing the  group Tukey HSD test was  used.

Results

The Table III was used for presenting results of the transverse strength. Mean values and standard deviations of transverse strength and modulus of elasticity test of each group are summarized in Figures 1-2. Groups XII and XIII showed significant differences compared to the other groups in transverse strength values (p<0.05). Group XIII showed the highest transverse strength value (124,53±16,38), followed by group XII (112,20±8,48). Group II and III showed the lowest values (90,42±7,59 and 90,82±4,21).

Table III. Mean values and standard deviations of transverse strength test.

* The mean difference is significant at the .05 level.

Figure 1. Transverse strength of resin in different polymerizing conditions

Figure 2. Modulus of elasticity results of resin in different polymerizing conditions

Table IV was used to present  results of the elastic modulus test. Groups XII and XIII showed significant differences regarding the elastic modulus values (P<0.05) when it contrast with other group. Group XIII showed the highest elastic modulus value (4,75±0,08), followed by group XII (3,91±0,39). Group XI showed the lowest value (2,36±0,65).

Table IV. Mean values and standard deviations of elastic modulus test