S. Parker

Water absorption of methacrylate soft lining materials

The water absorption of soft methacrylate prosthetic materials is very high compared with rigid methacrylates and is not at an equilibrium value even after 6 yr. This very high and prolonged uptake can be explained qualitatively in terms of the presence of water soluble impurities which form sites for the formation of water droplets. These droplets grow until the osmotic and elastic forces balance; this mechanism can be accompanied by creep or even rupture under the osmotic pressure. Desorption on the other hand, is rapid and obeys diffusion laws, giving diffusion coefficients of 5-7 X 10(-8) cm2 s-1.

The relationship between Shore hardness of elastomeric dental materials and Young's modulus

OBJECTIVES Hardness of elastomers can be directly related to Youngs modulus, a relationship that was investigated in detail by Gent in a paper in 1958. The aim of this study was to test this relationship for 13 dental elastomers (12 silicone and 1 polyether) using the equation derived by Gent and one from BS 903 (1950) that accounts for departures at low values. METHODS The dental elastomers were subjected to tensile testing and Shore A scale hardness measurements. Youngs moduli were calculated from the hardness values using the Gent equation and the BS 903 equation. These calculated values were then compared with values derived experimentally from the tensile tests. RESULTS Hardness values were in the range 30.2 (+/-0.5)-62.9 (+/-0.8) with the corresponding calculated modulus values in the range 1.1-4.1MPa and 0.9-4.3MPa for the Gent and modified equations, respectively. Youngs modulus values derived from the tensile data were in the range 0.8 (+/-0.3)-4.1 (+/-0.3)MPa, showing good agreement with those calculated from the hardness values. Providing viscoelastic creep is minimal during the duration of the test, there is a reasonably well-defined relationship between Shore hardness and Youngs modulus in the hardness range studied. SIGNIFICANCE Simple, non-destructive hardness measurements can be used to determine Youngs modulus values. Such values are needed in any calculations of stress distributions in soft lining materials, e.g. by FEA.

Formulation of tissue conditioners

Dental tissue conditioners are compliant gels, formed in situ under a denture from a polymer powder and a plasticizer system. Hitherto, the powder has been poly(ethyl methacrylate) and the liquid a phthalate/ethanol mixture. These materials are temporary, because they harden from plasticizer leaching. The current work has been aimed at producing material with extended oral lifetimes. A range of n-butyl/ethyl methacrylate copolymers have been studied, together with various ester-ethanol liquid systems, with respect to gelation behaviour with various alcohols and esters, viscoelastic behaviour of the set gels, and water extraction. Materials based on poly(ethyl methacrylate) show that gelation speed depends upon particle size, and the degree of ball milling of the powder, and the molar volume of the plasticizer. Systems based on n-butyl/ethyl methacrylate copolymers need less or no ethanol in the liquid system, although gelation speed is very temperature dependent. The gels from these last materials are more compliant, and retain their compliance longer in aqueous media than those based on poly(ethyl methacrylate).

Isoprene}styrene copolymer elastomer and tetrahydrofurfuryl methacrylate mixtures for soft prosthetic applications

Novel elastomer/methacrylate systems have been developed for potential soft prosthetic applications. Mixtures of varying compositions of an isoprene-styrene copolymer elastomer and tetrahydrofurfuryl methacrylate (SIS/THFMA) formed one-gel systems and were heat cured with a peroxide initiator. The blends were characterised in terms of sorption in deionised water and simulated body fluids (SBF), tensile properties and viscoelastic parameters of storage modulus and tan delta, as well as glass transition temperatures using dynamic mechanical analysis (DMA). DMA data gave two distinct peaks in tan delta, a lower temperature transition due to the isoprene phase in SIS and one at high temperature thought to be a combination of THFMA and the styrene phase in SIS. The tensile data showed a clear phase inversion within the mid range compositions changing from plastic to elastomeric behaviour. The sorption studies in deionised water showed a two stage uptake with an initial Fickian region that was linear to t 1/2 followed by a droplet growth/clustering system. The slope of the linear region was dependent on the composition ratio. The extent of overall uptake was osmotically dependent as all materials equilibrated at a much lower uptake in SBF. The diffusion coefficients were found to be concentration dependent.

Water uptake of soft lining materials from osmotic solutions.

OBJECTIVES The water uptake characteristics of soft lining materials are of obvious importance in that they are expected to function in the oral environment. Results for Novus (Hygenic Corp., Akron, OH, USA) show a very high uptake from distilled water. Despite this high uptake, Novus appears to function satisfactorily in the mouth. High water uptake of soft lining materials has been attributed to the presence of water soluble impurities that, on immersion, form solution droplets; the driving force for the uptake being the osmotic gradient between the droplets and the external solution. Uptake should therefore be less from ionic solutions. The object of this study was to test the applicability of this theory to Novus and two experimental soft lining materials. METHODS Water uptake of two experimental materials and Novus has been determined from distilled water and two saline solutions (0.45 and 0.9 M). After 196 days specimens were desorbed to constant weight and then subjected to a second sorption cycle. RESULTS Novus had the highest uptake from distilled water at approximately 18%, the experimental materials having an uptake approximately 7%. Desorptions were all rapid, minimum weight being reached within 1-2 days. Uptakes of the second sorptions from water were all higher. Uptake from saline solutions was approximately 12% for all materials, uptake from 0.9 M saline being the lowest. Second sorption results from solution were similar to the first. CONCLUSION The results obtained support the theory that the high water uptake of elastomeric materials is osmotically driven.

Soft prosthesis materials based on powdered elastomers

A new class of soft prosthesis material has been developed, based on the combination of a powdered elastomer and a methacrylate monomer that polymerizes to an elastomer. Such systems are processable by conventional dental technology. This principle avoids the use of plasticizers. Natural rubber, butadiene styrene and butadiene acrylonitrile elastomers have been used, together with a number of higher alkyl methacrylates (C8-C13) and 2-ethoxyethyl methacrylate. Such systems have been evaluated with respect to mechanical properties, including tear strength, adhesion to denture base poly(methyl methacrylate), water sorption and visco-elastic properties. A number of potentially viable systems have emerged, which may be useful in external prostheses. Many have high long-term water absorption, which makes questionable their long-term usefulness interorally. Nevertheless, there is still considerable scope for development in this area.

An experimental and theoretical study of the effect of sample thickness on the Shore hardness of elastomers

OBJECTIVE The Shore hardness test is a quick and convenient way to measure hardness of elastomers. The test specifies that the specimen should be at least 6mm thick; however, published work in the literature indicates that workers often use much thinner samples. The aim of this study was to investigate the effect of thickness on the measured hardness of a range of dental elastomers and develop a theoretical relationship to predict the effect. METHODS 7 dental silicone elastomers were selected to give a range of hardness values. Shore A scale hardness measurements were made on standard thickness samples, and those of lower thicknesses. A theoretical analysis was derived, whereby the effect of thickness of the sample on its measured hardness can be predicted and was tested using the results obtained. RESULTS The results show reasonable agreement between theoretical and experimental values. The measured Shore hardness increased inversely with thickness, the effect being most pronounced with thin samples. Results ranged from 66.3+/-1.6-80.9+/-0.4 for 1mm thick to 35.0+/-1.7-69.1+/-0.6 for the 6mm thickness. Ratio of the measured hardness of 1mm/6mm ranged from 1.97 for the softest material to 1.17 for the hardest. SIGNIFICANCE Measurements obtained with a Shore A hardness tester on samples of less than the specified 6mm thickness will give erroneously high values, the error being greater as thickness decreases. The theoretical treatment proposed gives a basis on which data obtained from studies that used thinner specimens can be better analysed.

The effect of particle size on the gelation of tissue conditioners

Tissue conditioner materials comprise a methacrylate polymer powder, usually poly(ethyl methacrylate) (PEM), and a plasticiser that also contains ethanol. The aim of this study was to investigate the effect of ball milling the PEM on gelation time. PEM was used as received and after ball milling for 2, 4, 8 and 16 h. Particle size was measured in each case and gelation time at 37 degrees C when mixed with butyl phthalyl butyl glycollate (BPBG). Gelation time reduced with increasing milling time of the powder, however it was found that increase in the fines initially had a more significant affect than overall reduction in average particle size. The gelation time of unmilled and 16 h milled PEM were also measured when mixed with BPBG and various levels of ethanol. Unmilled PEM with 4% ethanol had a gelation time of 13.5 +/- 2.0 min compared to 5.5 +/- 1.0 min for the 16h milled PEM which was similar to that for the commercial material, Viscogel (6.0 +/- 1.0 min). Ball milling the PEM reduces the level of ethanol required to produce a clinically acceptable gelation time.

Reinforcement of polymers of 2,2 bis-4(2-hydroxy-3-methacryloyloxy propoxy) phenyl propane by ultra-high modulus polyethylene fibres

A study has been made of the reinforcement of 2,2 bis-4(2 hydroxy-3-methacryloyloxy propoxy) phenyl propane/tetra-hydrofurfuryl methacrylate copolymers with ultra-high modulus polyethylene fibres. The fibres were orientated longitudinally, in loadings up to 50% w/w, and both untreated and surface treated fibres were studied. Modulii up to approximately 35 GPa were achieved in the axial direction, and the specimens could not be broken in the simple flexure test employed. Electron microscopy of fractured specimens showed extremely good contact between resin and fibre. No deterioration in properties was observed over 6 months in water.

Influence of additives on the water uptake of hydrosilanized silicone rubbers

Water uptake characteristics of a silicone polymer containing additives of varying solubility have been investigated. A hydrophobic silica, a sparingly soluble agent and a soluble catalyst were added to a stoichiometrically balanced hydrosilanized silicone polymer (the inherent absorption of which was 0.09 wt%). The diffusion coefficients for desorption of all materials were of the order 10(-6) cm2s-1, whereas the diffusion coefficients for absorption showed a decrease with increasing hydrophilicity and solubility of the additive. The greater the solubility of the additive, the more prolonged the uptake and the greater the deviation from classic diffusion theory.