A study of structure-property relationships in certain polymeric dental materials
Acrylic resin artificial teeth may be produced by Transfer or Pseudo compression moulding. Both of these moulding techniques were investigated to formulate the best conditions for use in a mass production environment and evaluated on a commercial basis. It was found that the transfer technique, despite higher material costs, was the most commercially viable. The effects of seven process variables on the mechanical properties of polymer systems produced by the transfer method were evaluated by comparing their stiffness, strength, viscoelastic behaviour, and hardness. The variables studied were: temperature, pressure, material condition, crosslinking agent concentration, concentration of BisGMA/THFMA, monomer: polymer ratio, and the initiator concentration. It was found that some of these variables and their interactions affected the properties of the polymer produced to varying degrees. Using the above study as a screening technique to identify the variables which improved the performance of the polymer system, the effects of BisGMA/THFMA and benzoyl peroxide initiator were studied in greater depth. Mathematical modules have been constructed which describe the effect of BisGMA/THFMA and benzoyl peroxide on the stiffness and viscoelasticity of the polymer. Consideration of these models and the effects the two variables have on the strength of the polymer produced indicated that 40% BisGMA/THFMA (by volume of liquid) and 2.16% benzoyl peroxide (by weight of mix) are the optimum levels of the variables. The effect of reinforcing this optimum formulation with different volume fractions of 2m silica was evaluated. Stiffness, surface hardness and viscoelastic properties increase but strength is reduced. An organozirconate coupling agent was added to untreated silica (particle size of 50 nM). It is found that the coupling agent modifies both the rheology of the unpolymerised composite resin and the mechanical properties of the resin itself. The synthesis of the diketone derivatives of BisGMA, termed BisGMAO, was investigated. Several oxidations of BisGMA were attempted in order to achieve high yield of BisGMAO. Polymerisation of both monomers and the evaluation of their mechanical properties of the derived polymers showed BisGMA to be superior.