Claudia Vallo

Photobleaching of camphorquinone during polymerization of dimethacrylate-based resins

OBJECTIVE The aim of this study was to compare the photobleaching rate of CQ in different dental resins. METHODS The photodecomposition rate of CQ/amine system in bis-GMA/TEGDMA, bis-EMA and UDMA polymerizing monomers was evaluated at different light intensities. The photobleaching of the CQ was studied by monitoring the decrease in light absorption as a function of continuous irradiation time. The absorption changes were assessed by recording the transmitted light that passed through samples of monomers containing CQ/amine. RESULTS Complete photobleaching of CQ was observed in all the monomer tested and the rate constant for the photobleaching was proportional to the radiation intensity. Hydrogen abstraction from amines by the excited CQ state via electron transfer and direct hydrogen abstraction from monomer structures were involved in the CQ photoreduction. CQ was photobleached in the absence of coinitiator in a dimethacrylate monomer containing a carbamate functional group (UDMA). This behavior was attributed to the presence of labile hydrogen atoms in the UDMA monomer. The CQ photobleaching rate constant in UDMA containing CQ/amine was similar to that in UDMA in the absence of amine. Moreover, the efficiency of CQ to photoinitiate the polymerization of UDMA in the absence of amine demonstrated that the radicals derived from the UDMA monomer via hydrogen abstraction are highly reactive toward double bonds. SIGNIFICANCE CQ photoinitiates the polymerization of the UDMA monomer in the absence of amine and the efficiency of this process is comparable to that of traditional bis-GMA and bis-EMA monomers activated with CQ/amine.

Influence of filler content on static properties of glass‐reinforced bone cement

A commercial acrylic bone cement was modified by the incorporation of different weight fractions of glass spheres. The influence of the filler proportion on the mechanical behavior was assessed. Composite cements were prepared by replacing part of the powder phase of the cement by an equivalent weight of glass particles, which resulted in an increase in the liquid-to-powder (L/P) ratio of the polymeric matrix. Dynamic mechanical analysis revealed an increase in residual monomer content with increasing filler proportion as a consequence of the increase in L/P. Flexural, compressive, and fracture properties of the cement with varying amounts of glass particles were measured. It was found that up to 50 wt% glass particles could be added with significant increases in flexural modulus and fracture toughness. The mechanical behavior was explained in terms of both the reinforcing effect of the filler and the plasticizing effect of the monomer. Glass-filled bone cements displayed superior workability compared with the standard cement, which was attributed to a decrease in the viscosity of the initial mix and the surface characteristics of the glass particles. The observed increase in fracture toughness could be rationalized through the application of proposed mechanisms for toughening of particle-reinforced polymers.

Role of intrinsic flaws upon flexural behaviour of a thermoplastic modified epoxy resin

Abstract A bisphenol A-based epoxy resin (DGEBA) was modified with 15 weight percent polysulphone (PSU) and thermally cured using 4-4′diaminodiphenylsulphone (DDS). Starting from a homogeneous DGEBA/DDS/PSU mixture, the system developed a two-phase morphology upon network formation. Dynamic mechanical analysis (DMA), transmission optical microscopy (TOM) and scanning electron microscopy (SEM) studies showed that the system developed a co-continuous morphology consisting of two distinct domains. One of the domains was an epoxy rich matrix containing PSU particles while the other consisted of a dispersion of epoxy particles within a PSU rich phase. Flexural strength distributions of unmodified and thermoplastic modified epoxy resin were obtained by testing the materials in three-point bending according to the ASTM D790 protocol. The flexural behaviour of the epoxy resin was not improved by the presence of thermoplastic. In addition, the thermoplastic modified epoxy resin displayed a higher data scatter compared with the neat resin. The fracture mechanism of unmodified and thermoplastic modified epoxy resins was demonstrated to be sensitive to the intrinsic flaw distribution. The two-parameter Weibull model, which was used to analyse the experimental data, gave a good representation of the fracture loads distribution with regression coefficients of 0.99.

Influence of thermal expansion on shrinkage during photopolymerization of dental resins based on bis-GMA/TEGDMA.

OBJECTIVE The aim of this study was to assess volume changes that occur during photopolymerization of unfilled dental resins based on bis-GMA-TEGDMA. METHODS The resins were activated for visible light polymerization by the addition of camphorquinone (CQ) in combination with dimethylamino ethylmethacrylate (DMAEMA) or ethyl-4-dimethyl aminobenzoate (EDMAB). A fibre-optic sensing method based on a Fizeau-type interferometric scheme was employed for monitoring contraction during photopolymerization. Measurements were carried out on 10mm diameter specimens of different thicknesses (1 and 2mm). RESULTS The high exothermic nature of the polymerization resulted in volume expansion during the heating, and this effect was more pronounced when the sample thickness increased. Two approaches to assess volume changes due to thermal effects are presented. Due to the difference in thermal expansion coefficients between the rubbery and glassy resins, the increase of volume due to thermal expansion was greater than the decrease in volume due to thermal contraction. As a result, the volume of the vitrified resins was greater than that calculated from polymerization contraction. The observed trends of shrinkage versus sample thickness are explained in terms of light attenuation across the path length during photopolymerization. SIGNIFICANCE Results obtained in this research highlight the inherent interlinking of non-isothermal photopolymerization and volumetric changes in bulk polymerizing systems.

Mechanical and fracture behaviour evaluation of commercial acrylic bone cements

The deformation and fracture behaviour of some commercial acrylic bone cements have been investigated. Cements were characterized by gel permeation chromatography, dynamic mechanical analysis and scanning electron microscopy. The influence of liquid to powder ratio, curing temperature, strain rate and non-reacted monomer was analysed for one radiolucent cement. Results showed that the β transition activation process influences both deformation and fracture behaviour. Fracture surface stress whiteness revealed the presence of crazes as the main plastic deformation mechanism. Non-reacted monomer acted as a plasticizer leading to materials with lower yield strength, σy, that induces crack tip blunting and improves toughness. It appears that the presence of radiopacifier fillers also improves fracture toughness by promoting interactions between the crack and the second phase dispersion.

Residual monomer content in bone cements based on poly(methyl methacrylate)

Acrylic bone cements are widely used in orthopaedics, and it is generally accepted that due to the vitrification phenomenon the monomer does not reach complete conversion after the cure of the resin. The degree of polymerization attainable in a commercial acrylic bone cement based on poly(methylmethacrylate) (PMMA) has been investigated by differential scanning calorimetry (DSC) using isothermal and dynamic modes. Because DSC tends to be less sensitive at high conversions, especially if there exists a permanent residue, gas chromatography (GC) was also used. The residual monomer has also been determined in samples cured under adiabatic conditions. The autocatalytic model developed by Kamal is used to analyse the curing kinetics. The final kinetic model is satisfactorily applied to dynamic and isothermal curing reactions. © 2000 Society of Chemical Industry

Influence of load type on flexural strength of a bone cement based on PMMA

The Weibull cumulative distribution function, which is commonly used in the characterisation of brittle materials, was used to describe the wide data scatter observed in the strength of a PMMA-based bone cement. A commercial additive-free bone cement was tested in flexure and the degree of fit of the flexural strength data with the Weibull distribution was assessed. The Weibull model gave a good representation of the results with regression coefficients of at least 0.98. Cements tested in three-point bending resulted in a higher flexural strength than cement tested in four-point bending. The ratio of mean strength measured in the different load arrangements, as well as the specimen size effect, were satisfactorily predicted by the Weibull model. The failure strengths of bone cements based on PMMA are governed by the severest flaw and may hence be treated by the Weibull statistics.

Rejuvenation of epoxy glasses subjected to uniaxial compression

Abstract Two epoxy networks, differing significantly in their glass transition temperatures (Tgs), were subjected to uniaxial compression tests in the glassy state (20°C). One of the systems was based on the diglycidyl ether of bisphenol-A (DGEBA) cured with ethylenediamine (EDA). The other one was based on an epoxidized novolac (EPN) cured with 4,4′-diaminodiphenyl sulfone (DDS). Both systems were physically aged by specific thermal treatments. The endothermic enthalpy relaxation peak, characteristic of aged glasses, could be erased at temperatures well below Tg by large mechanical deformations, i.e. close to the incipient strain hardening level in uniaxial compression tests. This phenomenon, which was accompanied by an increase in specific volume, constitutes a clear manifestation of rejuvenation produced by large mechanical stimuli. The validity of statements presenting an opposite point of view is also discussed.

Toughened-hybrid epoxies: influence of the rubber-phase morphology on mechanical properties

A toughened-epoxy polymer based on diglycidylether of bisphenol A (DGEBA) cured with ethylenediamine (EDA) and modified with a rubber based on an epoxy-terminated acrylonitrile-butadiene random copolymer (ETBN), exhibited different morphologies depending on the rubber content. Up to 10% rubber, the morphology consisted of a random dispersion of spherical domains rich in rubber, while at 15% rubber, large and irregular domains were present, turning into a co-continuous structure at 20% rubber. Mechanical properties (elastic modulus, uniaxial compression yield stress, critical stress intensity factor, KIC and strain energy release rate, GIC) of the toughened epoxies and hybrid-particulate materials containing glass beads, were analysed. The best mechanical properties were exhibited by hybrids with a random dispersion of rubbery domains in the epoxy matrix. The presence of large and irregular domains or co-continuous structures led to materials with poor mechanical properties.

Elastic modulus and yield stress of epoxy networks in the glassy state

Abstract Elastic modulus (E) and yield stress in uniaxial compression (σy) were determined for a series of epoxy-amine networks prepared with different stoichiometric ratios. Tests were performed at room temperature, i.e. in the glassy state of the polymeric networks. Observed trends were analysed and compared with literature results. The following conclusions were obtained. (i) E depends both on packing density (ϱ∗) and sub-vitreous relaxations (Tβ) that are active at the testing conditions, but the (ϱ∗) is the predominant factor when it varies significantly in the particular series, i.e. Δϱ ∗ ∼ 10 −2 . (ii) The sstress at the post-yield plateau (in uniaxial compression) may be correlated with the crosslink density of the epoxy network. (iii) There is no general relationship between E and σy as is frequently stated (Browns rule).