Influence of composition and powder/liquid ratio on setting characteristics and mechanical properties of autopolymerized hard direct denture reline resins

Reline of ill-fitting complete and removable partial dentures enhances their stability and retention by improving the adaptation between the denture intaglio surface and denture-bearing mucosa1). In clinical situations, that reline procedure is usually performed using a direct (chair-side reline system) rather than indirect (laboratory-processed reline system) method, because the former is more convenient, inexpensive, and less time consuming2,3). Although many kinds of autopolymerized hard direct denture reline resins are available for use with the direct method, they have some disadvantages such as unpleasant taste and odor, irritation to the oral mucosa by the monomer, and generation of heat during polymerization1-6). Furthermore, the stiffness of autopolymerized hard direct denture reline resins is lower than that of heatpolymerized denture base resins7-9). The mechanical properties of reline resins influence the longevity of the relined denture as well as the denture base resins. Autopolymerized hard direct denture reline resins for chair-side use are generally supplied in powder and liquid forms. The powder mainly consists of poly (ethyl methacrylate), while poly(methyl methacrylate) and poly(methyl methacrylate/ethyl methacrylate) are also available2). A peroxide initiator and pigment are also included. Commercially available liquids are classified into 3 groups; a monofunctional methacrylate (monomethacrylate) monomer, monofunctional methacrylate monomer and plasticizer, and monofunctional methacrylate monomer and crosslinking agents2). A chemically activated accelerator such as tertiary amine is also added to the liquid6). The differences in composition have great influence on mechanical properties and setting behavior, which are important factors in clinical situations. Several studies have been conducted to seek improvement in the mechanical properties of autopolymerized hard direct denture reline resins. In our previous study, we evaluated the effects of addition of fluorinated monomer on the properties of reline resins based on various monomers10). It has been found that the effects of an added fluorinated monomer on stiffness vary among the base monomers used, with changes in the mechanical properties of reline resins containing the fluorinated monomer over time smaller as compared to materials containing no fluorinated monomer. Thus, a reline resin based on a specific monomer may be stiffer, while that based on another may be more viscous with addition of a fluorinated monomer. In addition, the effects of cross-linking agents on the properties of autopolymerized hard direct denture reline resins have also been evaluated. Cross-linking agents have been added to liquids in denture base resins to improve the mechanical properties such as flexural strength, and resistance to temperature and solvent attack11,12), with ethylene glycol dimethacrylate (EGDMA) widely used11, 13). It has also been shown that the solubility of the liquid components of autopolymerized hard direct denture reline resins drops with an increase in crosslinking agents over the whole range of concentrations in the liquids, though the influence on water absorption varies among types of cross-linking agents11), as some decrease and others increase water absorption with increasing concentration. Water absorbed by denture Influence of composition and powder/liquid ratio on setting characteristics and mechanical properties of autopolymerized hard direct denture reline resins based on methyl methacrylate and ethylene glycol dimethacrylate