Tensile and transverse strength of novel copolymers for denture base.

: Objectives: Compare tensile and transverse strength of new copolymers for denture base. Materials and methods: The specimens were prepared from heat cured acrylic resin with three types of additives: Acryester B, Ethoxycarbonylethylene, and Propenoic acid at a percentage of 5% and 10%. The tensile and transverse strains were tested, recorded and compared. Results: The analysis of variance display statistically significant difference. The p -value was 0.001 for each of tensile and transverse strain tests. Conclusions: The tensile strength of the novel copolymers increased. The transverse strength of some of the novel copolymers increased.


INTRODUCTION.
Poly methyl methacrylate is considered the most common material used to fabricate denture base. However, it has some disadvantages like low strength and low toughness. 1 Among the desired mechanical properties of denture base material, high tensile and transverse strength are particularly important to meet the required performance during function. 2 Numerous attempts have been made to enhance durability and strength of dental acrylic resin by metal wires or plates. The chief disadvantage of metal wire is the weak adhesion between the resin and wire. In case of metal inserts, stress concentration around the embedded insert lead to failure. The inclusion of fibers has various problems such as tissue irritation, difficult placement and orientation, and adhesion between resin and fibers. Some attempts have been made to increase the strength of the denture acrylic by chemical modification through copolymerization and cross-linking. 3, 4 The aims of this research are to evaluate tensile and transverse strength of new copolymers of poly methyl methacrylate to be used as materials for denture base.

MATERIALS AND METHODS.
Specimens preparation were accomplished by heat-cured acrylic resin (VDBV, Holland) and liquid additives. The liquid additives used are: Acryester B, Ethoxycarbonylethylene, and Propenoic acid. The liquid additives were used at 5% and 10%. The control specimens preparation were achieved by heat-cured acrylic resin (VDBV, Holland) with no additives.
For each group, each liquid additive was added to the acrylic monomer, then mixed together until a homogeneous liquid mixture was achieved. Then, the powder was added to the liquid. Mixing, curing, and finishing were done according to the manufacturer instructions.
The specimens for tensile strength (90x10x3mm) were constructed according to ANSI/ADA standardization. For each group, the number of specimens was five. A universal tensile testing machine (KYOTO, JAPAN) Al-Ali AAS, Al-Noori AK & Taqa AA. Tensile and transverse strength of novel copolymers for denture base. J Oral Res Special Issue. 2019;S1:7-10. doi:10.17126/joralres.2019.001 was used. A tensile load of 100 newtons (which resembles the chewing and swallowing forces) was applied to the specimen. The resultant tensile strain was recorded. 5 The specimens for transverse strength (65x10x2.5mm) were constructed according to ANSI/ADA standardization. Testing was carried on as described above, and the resultant transverse strain was recorded. 6 The data were analyzed using SPSS software to obtain the means, one way ANOVA, and Duncan multiple range test.

RESULTS.
One-way ANOVA for the tensile strain showed statistically significant differences (p-value=0.001).
Means and Duncan multiple range test illustrate the comparison of tensile strain among all groups. (Figure 1) As the tensile strain decreased, the tensile strength increased. The tensile strain of the control group is statistically significantly increased compared to the other group, which means that the tensile strength of control group is significantly lower than the other groups.
The tensile strain of the 5% Propenoic acid is significantly higher than that of 10% Ethoxycarbonylethylene. There is no significant difference among the remaining groups. The transverse strain of control group showed a statistically significant increase compared to 10% Acryester B, 5% Propenoicacid, and 10% Propenoicacid; this mean that the transverse strength of the control group is significantly lower than that of these groups.
There are no significant differences in transverse strain between control group and 5% Ethoxycarbonylethylene, 10% Ethoxycarbonylethylene, and 5% Acryester B.
The transverse strain of 5% Ethoxycarbonylethylene is significantly higher than that of 10% Acryester B, 5% Propenoicacid, and 10% Propenoicacid. The transverse strain of each of 5% Acryester B and 10% Ethoxycarbonylethylene is significantly higher than that of 10% Propenoicacid.

DISCUSSION.
Microscopically, a cured acrylic resin has two phases in its internal section. The first is poly methyl methacrylate particles based upon powder and the second is poly methyl methacrylate matrix that results from the polymerization of the monomer. In internal sections, molecular weight distribution is different.
Another problem is that acrylic resin has macro interface within its internal section. The presence of flaws, interfaces, or cracks introduce concentration of stress in the inner-section of the denture or on the surface, and denture fracture is propagated. 7,8 In this research an attempt was made to dissolve the micro-structural behavior to prevent fracture initiation.