Encapsulating denture acrylic polymers: inorganic polymer films provide impermeable barriers

The absorption and growth of Candida albicans on the surfaces of denture materials and in mucosal tissues is a serious health problem that affects approximately 65% of denture wearers. Disregarding predominant etiological factors that lead to candidiasis, clinical evidence suggests that its occurrence is strongly dependent upon denture trauma, oral hygiene and property limitations of acrylic resin polymers (ARP). It is proposed that enhanced material properties can be obtained by encapsulating the denture acrylic polymers with a thin, inorganic polymeric film. As compared to coating the polymer films (i.e., Palaseal(R)), polymers are better suited for this application as they are denser, stronger, more flexible, and significantly more chemically and environmentally stable. Due to their excellent mechanical properties and the fact that the average pore size in silicone films range between 20-100 Å, the films should effectively prohibit the diffusion of food contaminates, bacteria and the ingrowth and adherence of the C. albicans hyphae. To facilitate adhesion of a polar inorganic film to the nonpolar surface of ARPs, a silanol coupling agent, trimethoxysilylpropylmethacrylate [TMSPMA], was added to a Lucitone 199 acrylic resin monomer prior to polymerization. Substituting 20 wt.% of the methylmethacrylate monomer with TMSPMA prior to denture processing, resulted in a polymer with optimal mechanical properties [(σ-84.0±4.37 MPa, E-4.01±0.382 GPa, e_r2.26±0.51%)]. Combined with the substantial mechanical property improvements, the chemically enhanced polymer contains reactive hydroxyl (Si-OH) surface groups. These groups can participate in polycondensation reactions with liquid sol-gel precursors to chemically bind the inorganic polymer films onto the ARP substrate. Liquid inorganic polymer precursors were obtained by acid catalyzed (HCl, 0.75 mole%) hydrolysis/polycondensation reactions of methyltriethoxysilane [H₃CSi(OC₂H₅)₃, 0.9mole%, Aldrich], tetraethoxysilane [Si(OC₂H₅)₄, 0.1 mole%, Aldrich] with distilled water (3 mole%) and tetrahydrofuran [C₄H₈O, 2 mole%, Aldrich] solvent. The polymerizing sols were painted onto polished ARP samples (30×10×5 mm) and allowed to cure at room temperature (5 min.) into dense (⩾1.65 g/cc), glossy, transparent, crack-free films with average film thicknesses of 10±2 μm (SEM). The hardness and abrasion resistant properties of the inorganic polymer films were evaluated using a Knoop microhardness indentor