Thermoplastic Modifier Research Articles

Environmental effects on thermoplastic and elastomer toughened cyanate ester composite systems

The effects of temperature and moisture on thermal and mechanical properties of high-temperature cyanate ester composite materials were investigated. A resin transfer molding process was used to impregnate glass fiber fabrics with matrices that underwent thermoplastic or elastomeric toughness modifications. The elastomer-modified material obtained the highest mode I fracture toughness values primarily because the toughener did not phase separate. Extended exposure to 200°C, however, deteriorated initial toughness improvements regardless of the modifier utilized. Although the thermal stability was increased by using thermoplastic modifiers in comparison to the elastomer-modified material, the degradation was mainly governed by the cyanate ester network. Gaseous degradation products caused delaminations and therefore reduced strength when the materials were exposed to 200°C for 1000 h. Also, upon immersion in water at 95°C, the matrices absorbed up to 3.3 wt % more than previous values reported in the literature. Fiber/matrix interfacial phenomena were responsible for this behavior because fiber/matrix adhesion also was reduced drastically as shown by the strong reduction in flexural strength. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 556–567, 2000

Investigation of readily processable thermoplastic‐toughened thermosets. V. Epoxy resin toughened with hyperbranched polyester

This article describes the use of hyperbranched polyester oligomers (HBPs) as modifiers for epoxy thermosets. The effect of HBP molar mass, end group, and loading on prepolymer viscosity, thermoset fracture toughness, Tg, and high-temperature dynamic storage modulus (E′) were measured. The HBP molar mass was systematically increased from nominal values of ∼ 1750 g mol (Generation 2, or G2) up to ∼ 14,000 g mol (Generation 5, or G5), which corresponds from a low of two layers of monomer up to a maximum of five layers of monomer around the central core. Toughness increased only modestly with the molar mass of the HBP. At 7% loading in the epoxy thermoset, the G5 HBP increased toughness by ∼ 60% over the untoughened control. Toughness increased to 82% above the untoughened control at a loading of 19% G5 HBP, but the toughness decreased at 28% HBP loading. The Tg and E′ were influenced by the HBP modifier, but the effect was not systematic and may have been due to competing effects of HBP molar mass and end group. The effect of the architecture of the thermoplastic modifier was investigated by introducing a linear aliphatic polyester (∼ 5400 g mol) with a repeat unit structure, which was similar to that of the HBP. At the molecular weight range investigated, neither the prepolymer viscosity nor the thermoset toughness of the HBP–epoxy was significantly different from that of the linear polyester in epoxy. Preliminary results are presented showing the effect of thermoplastic molecular weight and architecture on morphology. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 151–163, 1999

Investigation of readily processable thermoplastic-toughened thermosets. V. Epoxy resin toughened with hyperbranched polyester

This article describes the use of hyperbranched polyester oligomers (HBPs) as modifiers for epoxy thermosets. The effect of HBP molar mass, end group, and loading on prepolymer viscosity, thermoset fracture toughness, Tg, and high-temperature dynamic storage modulus (E′) were measured. The HBP molar mass was systematically increased from nominal values of ∼ 1750 g mol (Generation 2, or G2) up to ∼ 14,000 g mol (Generation 5, or G5), which corresponds from a low of two layers of monomer up to a maximum of five layers of monomer around the central core. Toughness increased only modestly with the molar mass of the HBP. At 7% loading in the epoxy thermoset, the G5 HBP increased toughness by ∼ 60% over the untoughened control. Toughness increased to 82% above the untoughened control at a loading of 19% G5 HBP, but the toughness decreased at 28% HBP loading. The Tg and E′ were influenced by the HBP modifier, but the effect was not systematic and may have been due to competing effects of HBP molar mass and end group. The effect of the architecture of the thermoplastic modifier was investigated by introducing a linear aliphatic polyester (∼ 5400 g mol) with a repeat unit structure, which was similar to that of the HBP. At the molecular weight range investigated, neither the prepolymer viscosity nor the thermoset toughness of the HBP–epoxy was significantly different from that of the linear polyester in epoxy. Preliminary results are presented showing the effect of thermoplastic molecular weight and architecture on morphology. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 151–163, 1999

Toughening of epoxies through thermoplastic crack bridging

The fracture toughness and toughening mechanism of two epoxy matrices containing varying concentrations of pre-formed polyamide-12 particles was investigated. The pre-formed thermoplastic modifier was used to keep the physical and morphological characteristics of the second phase constant while varying the matrix intrinsic toughness to simplify the interpretation of toughening results. We observed that these particles toughened the epoxies through a crack bridging mechanism involving large plastic deformation of the second phase.This mechanism was found to be effective independent of the potential of the matrix for plastic deformation since the increasing fracture toughness was accomplished without significant amounts of plastic deformation in the epoxy matrix. A quantitative model was adapted to account for the increase in toughness due to the crack bridging mechanism. From this model, it was possible to determine the factors which are most important when attempting to toughen a material through thermoplastic crack bridging. A better understanding of the specific factors which influence the efficiency of the crack bridging mechanism enables the fracture properties of brittle materials to be further improved with thermoplastic addition. This was shown to be very important when attempting to enhance the toughness of materials which are believed to be “un-toughenable” by conventional rubber modification, or materials whose other mechanical properties suffer from the addition of elastomeric materials.

Amorphous phenolphthalein-basedpoly(arylene ether) modified cyanate ester networks: 1. Effect of molecular weight and backbone chemistry on morphology and toughenability

Low toughness is a major drawback with most cross-linked thermosetting materials, including the cyanate ester networks. Reactive functional thermoplastic toughness modifiers not only enhance toughness but also permit highly desirable stability to solvent stress racking without seriously affecting the moderately high modulus. Careful control of the heterophase morphological structure is necessary to achieve significant toughening. In the present work, hydroxyl functional phenolphthalein-based amorphous poly(arylene ether ketone)s, poly(arylene ether sulfone)s and poly(arylene ether phosphine oxide)s were investigated as potential toughness modifiers for the bisphenol-A based cyanate ester networks. In particular, the use of poly(arylene ether sulfone)s resulted in remarkable improvements in toughness. Multiphase networks were generated without compromising either T g or the moderately high modulus of the unmodified cyanate ester networks. It was demonstrated that the toughenability of these systems is governed by microphase-separated morphologies and the sizes of the phase-separated domains. The formation of these heterophase morphological structures is strongly influenced by the backbone chemistry and the molecular weight of the thermoplastic modifier, probably via control of the polymer-polymer interaction parameter.

Shear ductility and toughenability study of highly cross-linked epoxy/polyethersulphone

The objective of the present study was to determine whether the ductility and toughenability of a highly cross-linked epoxy resin, which has a high glass transition temperature, T g, can be enhanced by the incorporation of a ductile thermoplastic resin. Diglycidyl ether of bisphenol-A (DGEBA) cured by diamino diphenyl sulphone (DDS) was used as the base resin. Polyethersulphone (PES) was used as the thermoplastic modifier. Fracture toughness and shear ductility tests were performed to characterize the materials. The fracture toughness of the DDS-cured epoxy was not enhanced by simply adding PES. However, in the presence of rubber particles as a third component, the toughness of the PES–rubber-modified epoxy was found to improve with increasing PES content. The toughening mechanisms were determined to be rubber cavitation, followed by plastic deformation of the matrix resin. It was also determined, through uniaxial compression tests, that the shear ductility of the DDS-cured epoxy was enhanced by the incorporation of PES. These results imply that the intrinsic ductility, which had been enhanced by the PES addition, was only activated under the stress state change due to the cavitation of the rubber particles. The availability of increasing matrix ductility seems to be responsible for the increase in toughness.

Heterogeneous phase separation around fibres in epoxy/PEI blends and its effect on composite delamination resistance

Blends of difunctional epoxy with poly(ether imide) modifier were prepared and combined with unidirectional (UD) glass-fibre tape to produce prepregs. These prepregs were laid-up, autoclave cured into UD fibre-reinforced composites and tested under mode I loading.The phase separation in the curing PEI/epoxy blends was studied by microscopic techniques and was compared to that developing in the presence of individual glass, carbon and aramid fibres. During cure, a discrete epoxy-rich layer formed around glass fibres, caused by preferential wetting of the fibre by the epoxy phase. The extent of the epoxy-rich layer was found to be dependent on the PEI concentration in the blend and on the proximity of other fibres. No such layer was detected in the presence of carbon or aramid fibres.The presence of this discrete epoxy-rich layer in the glass-fibre composite resulted in premature failure upon loading and hence limited the benefit of the thermoplastic modification to the fracture resistance of the composite.

On the effects of modifiers on the viscoelastic behavior of composites

AbstractSome of the details concerning the reasons for the difference in behavior between untoughened and toughened composites are presented. This work demonstrates the influence of localized plastic deformation on the viscoelastic behavior of polymer composites. As an example, the presence of a thermoplastic modifier and its potential influence on the microscale deformation is presented by using two micromechanical models to determine the conditions under which the observed behavior may occur. Results suggest that the effect of thermoplastic tougheners is to increase the amount of time‐dependent plastic deformation in loadings that produce sufficiently large local stress concentrations. The tougheners, however, do not seem to change the functional form of the time‐dependent response, rather they change the magnitude of this response. The two models discussed are used to qualitatively characterize these phenomena by relating model parameters to constituent properties. Differences in observed behavior with respect to plastic deformation in creep tests vs. tests such as dynamic mechanical analysis are attributed to the small deformations in the latter test types, which do create insufficient localized zones of plastic deformation.

Amorphous Bisphenol A Based Poly(Arylene Ether) Modified Cyanate Ester Networks

Cyanate ester or triazine networks are receiving considerable attention as potential candidates for high-temperature adhesives and composite matrices. Low toughness is a major drawback with most cross linked thermosetting materials, including the cyanate ester networks. Considerable attention has been devoted to the aspect of toughening such brittle networks in our laboratories. Reactive functional thermoplastic toughness modifiers not only enhance toughness but also permit highly desirable stability to solvent stress cracking without seriously affecting the moderately high modulus. We have earlier reported on various aspects of this technology as applied to epoxy and bismaleimide systems. Careful control of the hetetophase morphological structure is necessary to achieve significant toughening. In the present work, we have focused on modifications of a specific cyanate ester network system based on bisphenol-A with thermoplastic modifiers of varying backbone molecular weight and chemistry. In particular, hydroxyl or cyanato functional bispheno]-A based amorphous poly(arylene ether sulfone)s and poly(arylene ether ketone)s have been successfully utilized. Blends of reactive and non-reactive polysulfones were also demonstrated to be useful tougheners, apparently by allowing phase size control. The use of polyarylene ether ketones (which are of lower polarity than the polyarylene ether sulfones) resulted in larger, well defined morphologies, which in turn afforded tougher networks. It was demonstrated that either hydroxyl or cyanato reactive end groups could be effectively utilized. Both were superior to non-reactive systems in terms of both mechanical performance and solvent stability..

New functionalized poly(arylene-ether ketone)s and their use as modifiers for bismaleimide resin

A series of new functionalized poly(arylene-ether ketone) copolymers has been prepared by the nucleophilic aromatic displacement reaction from 4,4'-difluorobenzophenone and mixtures of bisphenol-A and 2,2'-bis(3-allyl,4-hydroxyphenyl)-hexat Luoropropane (6F-DABA) in the presence of potassium carbonate. The propenyl functional groups are statistically distributed along the polymer backbone and their concentration was tailored by adjusting the concentration of the starting monomers. These functionalized poly(aryleneether ketone)s (FPAEK) were used as thermoplastic modifiers for a bismaleimide (BMI) resin. The influence of the propenyl group concentration on the toughness of BMI,'FPAEK blend systems was studied.

Synthesis and characterization of novel toughened thermosets derived from pendent amines on the backbone of poly(arylene ether sulphone)s

Random incorporation of pendent aryl amines on the backbone of poly(arylene ether sulphone)s was achieved by the copolymerization of a minor amount of a second activated aromatic dihalide, bis(4-fluorophenyl)-3-aminophenylphosphine oxide (amino DFTPPO), with 4,4′-dichlorodiphenylsulphone (DCDPS) and bisphenol A via aromatic nucleophilic substitution polymerization using N-methylpyrrolidone (NMP) as the solvent, toluene as the azeotroping agent and potassium carbonate as the base. The pendent amines were optionally chemically modified by conversion to maleimides and thermally cured to afford tough insoluble networks. Epoxy networks were also obtained by reacting the pendent amines with liquid epoxy resin and 4,4′-diaminodiphenylsulphone (DDS). The synthesis and characterization of these novel thermosetting materials were investigated, and it was demonstrated that selected compositions could significantly improve the fracture toughness of the epoxy networks. Two important variables identified were the weight or volume fraction of the thermoplastic modifier and, importantly, the concentration of the pendent aminophenyl ‘cure’ sites. In this research, about 2.5 mol% cure sites was the optimum concentration utilized, which produced tough multiphase systems. Higher aminophenyl concentrations resulted in more homogeneous morphologies and fracture toughness was about the same as the control.

Effect of thermoplastic modifier variables on toughening a bismaleimide matrix resin for high-performance composite materials

A series of engineering thermoplastic toughness modifiers were used to modify the fracture toughness properties of a bismaleimide resin. The effects of thermoplastic loading, molecular weight and functionality were examined. Substantial improvements in K Ic stress intensity values were found when modifiers possessing reactive end-groups were used. Increases in modifier molecular weight and weight per cent loading produced steady increases in K Ic values and a limiting value at high concentrations and molecular weights. These observations were attributed to the thermoset network being the limiting material with respect to fracture toughness. By controlling molecular weight and adding reactive end-groups to the toughener, the fracture toughness properties of the commercial bismaleimide resin were successfully improved without sacrificing its desirable hot-melt processing characteristics. A unidirectional carbon-fibre prepreg was prepared and mode I and II fracture toughness tests were performed using double cantilever beam and end notch flexure specimens. Thermoplastic loadings of 15 and 20% of maleimide-terminated poly(ether sulphone) ( M n = 12 800 g mol −1) yielded composite G Ic values of 489 ± 25 and 734 ± 1 J m −2, respectively—a substantial improvement over the unmodified composite value of 359 ± 17 J m −2.

Recent developments of thermosetting polymers for advanced composites

The aircraft/aerospace industry is the fastest growing market sector for advanced composites. To use fully the potential of advanced composites for demanding structural applications it requires improved properties of its constituents, the carbon fibres and polymer matrix resins. This article emphasizes new developments of thermosetting polymers such as epoxy resins and bismaleimides. The concepts to achieve improved thermosetting polymer matrix resins are new monomers (resins and curing agents), thermoplastic modification, liquid crystalline monomers and impact tolerant interleafed systems. The paper highlights thermoset resins and systems which provide improvements in the areas of moisture absorption, fracture toughness and high temperature stability.

Toughening of AT-Resins with a Polyphenylquinoxaline

Abstract Acetylene terminated (AT) resins are addition-curable thermoset materials which do not generate volatiles during cure and therefore can be fabricated into void-free structures. They retain good thermal and mechanical properties even after exposure to high humidity environments. Their use as composite matrix resins and adhesives has shown promise. These resins, however, are brittle. Molecular structure modifications and blending with thermoplastic modifiers have been used to improve their toughness. In this work, improvement in toughness has been sought through the use of a polyphenylquinoxaline (PPQ) modifier. The blended systems showed improvements in toughness, thermooxidative stability, and lap shear strength over the original AT-resins.