The Water Uptake of Experimental Soft Lining Materials
Abstract
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.
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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.
Authors
Riggs, Paul DavidCollections
- Theses [4352]