Bryan Ellis

The effects of process variables on levels of residual monomer in autopolymerizing dental acrylic resin.

Abstract Relationships have been established between process variables and the concentration of residual monomer in an autopolymerizing dental acrylic resin. It has been shown that long-lived polymer radicals are present in greater concentration after an initial cure at 22° than after cure at 55°. Assay of monomer concentrations by infrared spectroscopy showed residual monomer to be reduced by higher powder/liquid ratios, low temperature of initial cure, storage at elevated temperature and the exclusion of air (oxygen).

Loss into water of residual monomer from autopolymerizing dental acrylic resin

Abstract A study has been made of the loss of residual monomer from samples of autopolymerizing acrylic resin immersed in water. Diffusion of monomer out of the resin into the water could account for only part of the loss. It was shown that free radicals continue to be present in the acrylic after completion of cure, and it is postulated that the residual monomer in an autopolymerized acrylic resin may be removed by further polymerization at these sites, as well as by diffusion out of the resin. The implications of this hypothesis are discussed.

The glass transition temperatures of highly crosslinked networks: Cured epoxy resins

Abstract The glass transition temperatures of a series of cured epoxy resins have been determined, using broad line proton n.m.r. and stress-strain measurements. The networks were prepared by curing bis-phenol-A epichlorohydrin epoxy resins, I, with the stoichiometric amount of diaminodiphenyl methane (DDM), II. The length of the network chains was varied by a factor of 17. The glass transition temperatures ranged from 367 to 410K and were found to be linearly related to the reciprocal of the number average molecular weight, M n , of the pre-polymer molecules. A theory is presented in which each component of the network has an associated thermodynamic transition in the pure state. Development of this theory predicts a limiting linear relationship between glass transition temperature and the reciprocal of the number average molecular weight of the network chains between the DDM crosslinks.

The glass transition temperature of an epoxy resin and the effect of absorbed water

SummaryThe glass transition temperature, Tg, of a diamino-dipenyl methane (DDM, II) cured diglycidyl ether of bispenol-A epoxy resin (I, n=0) has been determined by proton NMR line width studies to be 410°K. In the presence of 2.5±0.1 w/o absorbed water Tg is reduced to 378°K. Such a large reduction of Tg by such a small amount of water requires specific interactions. It is postulated that segmental motion is increased as a result of the disruption of hydroxyl group hydrogen bonds by the absorbed water molecules.

Epoxy based coatings on glass: strengthening mechanisms

Glass may be strengthened by epoxy coatings although the strengthening mechanisms remain unclear. Possible strengthening mechanisms are reviewed and are used to analyse strength data for both a solvent based and a water based coating system. The coatings either fill (solvent based coatings), or partially fill (water based coatings) surface cracks and it is shown that closure stresses arising from the thermal expansion mismatch of the coating within these cracks can account for the observed degrees of strengthening. It is also demonstrated that other suggested mechanisms such as flaw healing cannot fully account for the observed degree of strengthening.

The characterization of anomalous adsorption sites on silica surfaces

Abstract A study of a typical particulate pyrogenic silica using measurements of vapor adsorption and T.G.A. is reported which allows a detailed characterization of the surface. The present estimated concentration of “isolated” surface hydroxyl sites agrees with the characteristic concentration proposed elsewhere, and an unconventional silica surface site having a concentration of approximately 0.34 per 100 A2 has been found. The reversible adsorption of benzene, ethylene oxide and epichlorhydrin is found to be associated with isolated surface hydroxyl sites at low surface coverages, whereas methyl alcohol is adsorbed onto both hydroxyl groups and the anomalous sites.

The composition and rheology of denture adhesives

Abstract Ten brands of denture adhesive on sale in the United Kingdom have had their active constituents identified by infrared spectroscopy. Six contained Karaya gum, an acidic substance which in aqueous solution could cause decalcification of enamel. The viscosity of the denture adhesives studied was of the order of 106 poise. After mixing with water all showed an increase in viscosity, some showing a subsequent fall as further dilution with water took place.

A study of the composition and diffusion characteristics of a soft liner

Abstract The composition of Coe-Soft, a soft liner, has been determined. The polymer powder is essentially polyethyl methacrylate and the liquid is a mixture of dibutyl phthalate, benzyl salicylate and ethanol. The diffusion of ethanol out of Coe-Soft into air proceeds by two processes and their rates have been measured. On immersion in water the ethanol diffuses out by the faster process at the same rate as into air. Subsequently water is absorbed without limit and equilibrium is not attained even after 131 days (4·68 months).

The structure and surface topography of acrylic denture base materials

Abstract The structure of acrylic-type denture base materials has been studied using scanning electron microscopy (SEM) and determination of their sol : gel ratios. From the sol : gel ratios it is established that there is direct chemical bonding between the polymer beads and the cross-linked matrix. From optical microscopic studies of mixtures of polymer beads and liquid monomer it is shown that all beads swell very rapidly and that small beads dissolve. The bead structure is not observed on a polished surface but can be revealed by treatment with suitable etchants such as acetone. The bead structure can be observed in SEM photographs of unpolished surfaces.

Fibrous reinforcement of glass-ionomer cements

Abstract Glass-ionomer cements (GICs) are used very successfully as dental restorative materials. However, their mechanical properties can be improved by the incorporation of fibrous reinforcement, allowing other, more stringent applications of these materials to be possible. Such a potential use for materials with higher elastic moduli and strength is as a bone cement for hip prothesis. In this paper methods of incorporating carbon and Safil fibres into GICs are discussed. The optimum compositions for these reinforced cements has been determined. Data is presented which shows that it is at least possible to double the Youngs modulus and strength of GICs by the incorporation of a modest volume fraction of reinforcing fibres. Extensive fibre pull-out occurs with carbon fibres, with the fibres bridging the crack. However, with Safil fibres brittle fracture occurs and there is little pull-out.