Clive B. Bucknall

Toughening tetrafunctional epoxy resins using polyetherimide

Abstract Toughened epoxy resins have been prepared by dissolving polyetherimide (PEI) in a tetraglycidyl-4,4′-diaminodiphenyl methane (TGDDM) based resin with 30 p.h.r. diaminodiphenyl sulphone (DDS) as curing agent. The polyetherimide forms a separate phase, with a dynamic loss peak which varies between 200 and 212°C. The loss peak of the resin occurs at about 265°C. Three-point bend tests show a linear increase in KIC with PEI content, from 0.5MPa√m in the parent resin to 1.42MPa√m at 25 p.h.r. of PEI. Youngs modulus at 23°C shows a modest reduction from 3.6 to 3.5 GPa over the same composition range.

Phase separation in epoxy resins containing polyethersulphone

Abstract Scanning electron microscopy and dynamic mechanical spectroscopy have been used to study phase separation of dissolved polyethersulphone (PES) from trifunctional and tetrafunctional epoxy resins during curing. No phase separation was observed at high concentrations of the tetrafunctional resin. Observations of nodules on fracture surfaces, and of multiple peaks in the dynamic mechanical spectra provided evidence for a separate, crosslinked, PES-rich phase in the remaining materials. Despite the variety of morphologies obtained in mixtures of PES with different hardeners and resins, modulus and fracture toughness showed little dependence upon composition.

Dilatational bands in rubber-toughened polymers

A theory is advanced to explain the effects of rubber particle cavitation upon the deformation and fracture of rubber-modified plastics. The criteria for cavitation in triaxially-stressed particles are first analysed using an energy-balance approach. It is shown that the volume strain in a rubber particle, its diameter and the shear modulus of the rubber are all important in determining whether void formation occurs. The effects of rubber particle cavitation on shear yielding are then discussed in the light of earlier theories of dilatational band formation in metals. A model proposed by Berg, and later developed by Gurson, is adapted to include the effects of mean stress on yielding and applied to toughened plastics. The model predicts the formation of cavitated shear bands (dilatational bands) at angles to the tensile axis that are determined by the current effective void content of the material. Band angles are calculated on the assumption that all of the rubber particles in a band undergo cavitation and the effective void content is equal to the particle volume fraction. The results are in satisfactory agreement with observations recorded in the literature on toughened plastics. The theory accounts for observed changes in the kinetics of tensile deformation in toughened nylon following cavitation and explains the effects of particle size and rubber modulus on the brittle-tough transition temperature.

A model for particle cavitation in rubber-toughened plastics

An energy-balance criterion for cavitation of rubber particles, which was proposed in an earlier paper [A. Lazzeri and C. B. Bucknall, J. Mater. Sci.28 (1993) 6799], is developed by including a term for the energy stored in the matrix and released during expansion of the voids. The model relates the critical volume strain at cavitation to the radius of the rubber particle, and to the shear modulus, surface energy and failure strain of the rubber. The effects of temperature, strain rate and type of stress field upon cavitation behaviour and the resulting toughness of the two-phase polymer are discussed in terms of the model.

Applications of a dilatational yielding model to rubber-toughened polymers

A model for dilatational yielding in rubber-toughened polymers, which was proposed in an earlier paper, is developed further. According to the model, deformation begins with cavitation of the rubber particles, and progresses through the growth of dilatational bands, which are cavitated planar yield zones combining inplane shear with extension normal to the band. The model is used to predict band angles and other characteristics of yielding in toughened plastics, including the conditions under which polymers yield without cavitating. Electron micrograph evidence for the formation of dilatation bands is presented, and the influence of these bands on deformation and fracture behaviour is discussed, using a novel method of presentation based on cavitation diagrams.

Blends of poly(methyl methacrylate) with epoxy resin and an aliphatic amine hardener

Abstract Miscibility, phase separation and curing behaviour have been studied in blends of poly(methyl methacrylate) (PMMA) with diglycidyl ether of bisphenol A (DGEBA) resin and 4,4′-diamino-3,3′-dimethyldicyclohexyl-methane (3DCM) hardener. Quasi-binary mixtures of PMMA with the resin monomer show complete miscibility over the whole composition range, at all temperatures studied (−25 to 200°C). However, PMMA is much less miscible with the hardener, and cast films show two Tg values. Several techniques have been used to monitor phase separation and curing characteristics in PMMA blends, including viscometry, size exclusion chromatography, turbidity and optical microscopy. The morphologies and the thermal and dynamic mechanical properties of the cured blends, including non-stoichiometric compositions, are related to these observations.

Phase separation from solutions of poly(ether sulfone) in epoxy resins

Abstract Cloud-point measurements were made on quasi-binary mixtures of diglycidyl ether of bisphenol A (DGEBA) epoxy resin with poly(ether sulfone) (PES). The relative molecular masses of both resin and polymer were varied, and temperature-dependent Flory-Huggins interaction parameters were estimated on the assumption that both components are monodisperse. The data were used to calculate lower critical solution temperatures (LCST) and compositions, and to construct coexistence curves. The evolution of two-phase morphology during curing of epoxy-PES blends containing 4,4′-diaminodiphenylmethane (DDM) hardener is related to these results.

Rubber toughening of plastics: Part 9Effects of rubber particle volume faction on deformation and fracture in HIPS

Electron microscopy of a high-impact polystyrene (HIPS) polymer, containing 8.5 wt% polybutadiene, shows that the volume fraction, ϕ, of composite rubber particles is 35%. The rubber particle size distribution has 8 median diameter of 1.6 µm. By making a series of blends between this HIPS and polystyrene, it is shown that Youngs modulus decreases linearly with ϕ. Dilution with polystyrene results in a sharp drop in notched Charpy impact strength. The relevance of these data to the interpretation of structure-property relationships for a wide range of HIPS morphologies is discussed.

Phase separation in styrenated polyester resin containing a poly(vinyl acetate) low-profile additive

Abstract Scanning electron microscopy has been used to observe morphology in styrenated polyester resins containing poly(vinyl acetate) (PVA). Resins containing 8% PVA form composite spherical particles which occupy 35 vol% of the total material. It is concluded that these particles consist of resin sub-inclusions embedded in the continuous matrix of polyester resin. Increasing the PVA content to 16% results in a phase inversion: PVA forms the matrix, and the resin is present as spherical particles. These observations are interpreted with the aid of a ternary diagram.

Mechanism of shrinkage control in polyester resins containing low-profile additives

Abstract A commercial low-profile additive containing acid-terminated poly(vinyl acetate) (PVAc) was added, at concentrations of 0–16 wt%, to a solution of unsaturated polyester resin in styrene. The blends were cured under standard conditions. In all cases, the linear shrinkage during cure was 3.2 ± 0.2%, independent of PVAc content. In the absence of PVAc, addition of up to 60 wt% of CaCO 3 simply reduced the shrinkage of the resin in proportion to the volume fraction of filler. However, a combination of CaCO 3 with 16% PVAc gave a synergistic effect: the resulting shrinkage was substantially smaller than with CaCO 3 alone. On the basis of microscopy and other evidence, it is concluded that low-profile modifiers work by providing weak co-continuous regions in the resin, which can cavitate in response to tensile stresses arising from thermal- and cure-contraction in the presence of mechanical constraints. These constraints may be imposed internally, by mineral fillers or glass fibres, or externally, by forces acting on the surface of the resin. Optical microscopy provides evidence to support this interpretation.