Thermosets fracture toughness

Fig. 5. Interlaminar fracture toughness, for a number of thermosetting and thermoplastic composites (36,37). Open white bars represent glass-fiber composites shaded bars are for carbon fibers. The materials are A, polyester (unidirectional) B, vinyl ester (CSM = chopped strand mat) C, epoxy (R/BR1424) D, epoxy (T300/914) E, PPS F, PES and G, PEEK. To convert J/m to fdbf/in. multiply by 2100.

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

Fig. 8.2. Composite mode I interlaminar fracture toughness, G[, as a function of respective neat resin toughness, G] ( ) Kim et al. (1992) (O) from Russell and Street (1987) ( ) toughened thermosets and ( ) thermoplasties from Hunston et al. (1987) (A) from Bradley (1989a) ( ) from Jordan and Bradley...

Within the past several years, improvements in the toughening of high-temperature epoxies and other reactive thermosets, such as cyanate esters and bismaleimides, have been accomplished through the incorporation of engineering thermoplastics. Additions of poly(arylene ether ketone) or PEK and poly(aryl ether sulfone) or PES have been found to improve fracture toughness. Direct addition of these thermoplastics generally improves fracture toughness but results in decreased tensile properties and reduced chemical resistance. 

If a material exhibits linear-elastic stress-strain behavior prior to rupture (an ideal behavior approximated by many thermosets), then a simple relationship exists between the material s fracture toughness and its fracture surface energy, J (or G),... 

Fig. 10 a and b. Fracture toughness versus rate (a) and temperature (b) showing typical thermoset fracture behavior. I = initiation A = arrest E = stable crack growth... 

The properties of thermosetting and thermoplastic resin systems are continually improved to meet increasing performance requirements of end users. One way to enhance material properties is to incorporate nano-modifiers, based on elastomeric silicone particles, which are optionally grafted with other (acrylic) polymers to control dispersibility, viscosity, and other parameters. As an example, epoxy resin formulations have been modified with silicone nanospheres to improve low-stress behavior. Table 1 shows the outstanding fracture toughness improvement of silicone coreshell nanospheres, even at very low particle loading levels. 

The use of rubbers (particularly epoxy-terminated butadiene nitrile, ETBN, rubber or carboxy-termi-nated butadiene acrylonitrile, CTBN, rubber) to toughen thermoset polymers is perhaps the most widely explored method and has been applied with some measure of success in epoxy resins. Phase separation of the second rubbery phase occurs during cure and its incorporation in the epoxy matrix can significantly enhance the fracture toughness of the thermoset. Although the rubber has a low shear modulus, its bulk modulus is comparable to the value measured for the epoxy, ensuring that the rubber inclusions introduced... 

Kim et al (1999) examined the morphology and cure of semi-IPN epoxy resin or dicyanate-polyimide/polysulfone-carbon-flbre Aims. Polyimide or polysulfone Aims were inserted into the curing epoxy-dicyanate monomers to form semi-IPNs with sea-island morphology at the thermoset-thermoplastic interface. The final carbon-flbre-thermoplastic-dicyanate Aims had fracture toughness three to five times higher than that of unmodified carbon-flbre-dicyanate composites. 

Developing biphasic materials in order to improve the fracture toughness of thermoset resins is now a common practice. Thermoplastics that have a high glass-transition temperature (Tg) are used as tougheners in preference to low-Tg elastomers because of their insignificant effect on the thermal and modulus properties. 

Epoxy thermosets are typical densely cross-linked polymer materials. They are used in a wide variety of practical applications and thus have been studied extensively. However, the quantitative dependence of physical properties, such as strength, stiffness, and fracture toughness, on network microstructure are largely undetermined. This can be attributed, in part, to the lack of adequate techniques for characterizing densely cross-linked network structure. Several microstructiu e variables that have been studied with some success are (1) cross-link density,... 

Fracture toughness data (room temperature measurements) for various continuous fiber composites under different loading modes arc presented in Table 4. The data from different sources are reasonably consistent for a given material system with the same fiber content. Small differences may result from the variations in the degree of cross-linking for thermosets and the degree of crystallinity for the thermoplastic matrices. 

Other variables investigated with respect to fracture toughness include processing parameters, e.g.. production schedules and postcure cycles [166.171] for thermosets, and cooling rates for thermoplastics [171] matrix composites, and moisture content. A study [171] on composites based on epoxy and vinyl ester matrices has shown that whilst matrix plasticization improves mode I fracture toughness, mode II fracture toughnc.ss deteriorates due to interface degradation. The sensitivity of (Jut to water absorption has been demonstrated even for matrices that absorb very small amounts of water such as pol propy lene [168]. 

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