The water uptake of experimental soft lining materials
In order to develop a successful soft lining material various factors have to be considered; physical strength, adhesion to the denture base ( or prosthetic) and the durability of the material's properties when in the mouth. It has been recognised that in order to fulfil these criteria the material must be stable and have a low water uptake from the aqueous environments of the mouth. In the dental field comparatively little work has focused on how soft lining materials behave in water where as water in polymers has received a considerable amount of interest, with many different types of behaviour being observed and explained. It has been realised by previous authors that the water uptake of elastomers is primarily driven by soluble impurities, these form solution droplets within the material. The nature of the growth is somewhat more debatable, with both Fickian and dual sorption kinetics being reported. Two basic types of materials were used in the study; silicone polymers and elastomer / methacrylate materials. Silicone polymers are characterised by a low water uptake and form the basis of perhaps the most successful soft lining material ('Molloplast B'). The elastomer / methacrylate materials were based on those developed by Parker (1982), Parker and Braden (1990) which showed considerable promise but suffered from an extensive protracted uptake. Water uptake at 370C in conjunction with the tensile strength were used to evaluate the materials produced as these simple tests enabled the behaviour of the material in service to estimated. Three different types of silicone polymers were used during the study classified by the curing mechanism (condensation, peroxide and hydrosilanised), various fillers and additives (such as calcium stearate) were incorporated into the materials and different uptakes observed. The condensation silicones demonstrated large weight losses (up to 20 wt%) in water which is attributed to hydrolytic instability of the siloxane bridge in the presence of an organo tin compound leading to a leaching of siloxane. The pure peroxide and hydrosilanised materials both demonstrated a low water uptake but when doped they form solution droplets in a similar way to that described in the literature. Other additives showed different behaviour with the formation of cracks within the silicone due to failure of the material around the droplets, the action of hydrophilic but insoluble fillers also promotes the uptake. The hydrosilanised silicone polymers showed considerable promise as soft lining materials with low water uptake and good tensile strength. 11 The elastomer / methacrylate materials were based initially on butadiene styrene copolymer and a higher methacrylate monomer which formed a gel this was then free radically cured. The water uptake of these materials was attributed to soluble separating agent added to the butadiene styrene (to prevent particle agglomeration) during the production of the powdered elastomer. The extent of the uptake could be controlled by improving the strength of the material but the overall uptake remained too high. When the material was placed in an osmotic solution (Na CI or glucose) the water uptake was significantly reduced and the behaviour could be described by a modified (for small strains) version of the Thomas and Muniandy (1987) theory for the growth of water droplets in a elastomer. In order to reduce the water uptake of the elastomer / methacrylate materials butadiene styrene copolymers without separating agent was used. The emulsion polymerised material contained soluble impurities from the polymerisation (i.e. soap)which acted to drive the water uptake. Solution polymerised butadiene styrene also demonstrated a high uptake but this is attributed to a clustering behaviour of carboxylic and hydroxyl groups which formed post production. Similar behaviour is also seen for a solution polymerised isoprene styrene elastomer. The role of crosslinking the material in restraining the growth of the droplets is also investigated with dramatic reductions in the uptake being observed as the crosslink density increased. The employment of a reinforcing silica filler proved more effective than simply using a dimethacrylate. Oxidation is another problem (characterised by an upturn in the absorption), although not observed in every case it was a problem for all of the unsaturated elastomers and was found to be promoted by ions present within saliva. Saturated butyl based (including chloro and bromo butyl) elastomers were used instead and did not show any tendency for oxidation but they still showed an uptake of approximately 3 to 4 wt%. Their stability however and reasonable strength makes them suitable for further development as soft lining materials. Theoretical considerations were investigated by HI NMR imaging with the formation of droplets being observed, the profiles seen indicating the absorption to be two stage rather than Fickian. The role of creep or stress relaxation is also identified as a mechanism for extending the uptake by reducing the restraining force. Further reasoning on all the data presented here concluded the role of chemical potential change associated with the water into the matrix or the droplets will determine the nature of the uptake observed.