Rubber-toughened Epoxy Resins

Amine-cured epoxy resins are extensively used as structural matrices in composite manufacture and as such the improvement of their mechanical properties is very desirable. As in the case of polystyrene the addition of a rubber phase provides an energy dissipation mechanism below and this increases the toughness of the overall material. Carboxy-terminated butadiene acrylonitrile (CTBN) is initially soluble in the reaction mixture of digylcidyl ether of bisphenol A (DGEBA) and the low-temperature cure system triethylenetetra-amine (TFTA)  

Carboxy-terminated butadiene acrylonitrile random copolymer (CTBN) 

As in the case of the polystyrene system, increasing the molar mass of the epoxy resin polymer dispersed in the CTBN-rich phase will lead to phase separation. 

Two chemically dissimilar polymers will naturally attempt to phase separate and the structure that is formed will reflect the way in which this process occurs and the driving forces associated with the process. Phase separation is used to achieve rubber toughening in thermoset resin systems. Low molar mass CTBN copolymer is soluble in the simple mixtures of monomers used to create amine-cured epoxy resins systems. However, as the molecular mass of the epoxy resin increases so the balance of entropy and enthalpy of mixing of these components changes and a driving force for phase separation is created. 

Morphology of TETA-DGEBA-CTBN systems (a) low-resolution image showing the distribution of CTBN phases in the matrix (b) enlargement of one of the CTBN phases showing the included epoxy nodules within the CTBN phase. 

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Pearson and Lee (1991) examined the effects of particle-size and particle-distribution effects on rubber-toughened epoxy resins. They examined a variety of CTBN liquid rubbers and a methacrylated butadiene styrene core-shell particle in a DGEBA-piperidine system. They found that the toughening mechanism for small particles was internal cavitation of the... 

Some linear-elastic results were obtained for direct comparison with experimental values of mechanical properties available in the literature. The materials were rubber-toughened epoxy resin (5) and epoxy resin filled with glass beads (22). The material properties used for these analyses are shown in Table II. 

Rubber toughened epoxy resins are the well known examples of impact modified thermosets utilizing reactive rubbery prepolymers. Epoxy resins can be toughened or flexibilized by any one of the following types of oligomeric reactive elastomers ... 

Rubber-toughened epoxy resins have been characterized by their resin and rubber glass transition temperatures (57) using torsional braid analysis (TEA). These are reported in the literature as eTg and rTg, respectively. The Torsional Braid Analyzer has also been used to characterize ATBN-modifled coatings regarding transition temperatures. 

Siebert, A. R. Riew, C. K., "The Chemistry of Rubber-Toughened Epoxy Resins", 161st National Meeting, ACS, ORPL, March, 1971. 

Deformation mechanisms in rubber-toughened epoxy resins... 

In rubber-toughened epoxy resin materials, the particles act in the usual way as stress concentrators which initiate shear yielding of the matrix and give rise to increases in the critical size of the deformation zone. The particles first cavitate and then dilate during further deformation of the material. There is clear experimental evidence that cavitation of the rubber occurs first and is followed by shear yielding of the epoxy matrix [84,85,102,122], this being interpreted in terms of the need to relieve local constraint in the matrix before shear yielding and plastic deformation of the matrix can occur. The necessity for particle... 

In a contribution from B. F. Goodrich, Drake and Siebert extensively review the journal and patent literature since 1975 on reactive butadiene/acrylonitrile liquid and solid elastomers used in formulating epoxy structural adhesives. Areas reviewed include the preparation of elastomer-modified epoxy resins, the characterization of rubber-toughened epoxy resins, fracture mechanics and adhesive formulation and testing. 

An intense interest, currently without abatement, in the characterization of rubber-toughened thermosets in general, and rubber-toughened epoxy resins in particular, has surfaced over the last several years. Riew (18) summarized work leading to the optimization of toughness in an epoxy/CTBN/piperidine model. Concurrently, Kaelble (19) reported on such systems from a block copolymer orientation and examined properties related to adhesive... 

C.B. Bucknall and T. Yoshii, "Relationship Between Structure and Mechanical Properties in Rubber-Toughened Epoxy Resins," British Polymer J., (3/1978). 

Figure 2 shows fracture surfaces for bonds between zinc and rubber-toughened epoxy resin (see Toughened adhesives). In Fig. 2(a), there is a region of cohesive failure within the resin the sites of bubbles, which may have initiated the fracture, can be seen. In Fig. 2(b), a piece of resin is seen adhering to what appears to be the bare zinc substrate. 

Fig. 22. Raman image at 1665 cm (minus 1720 cm backgroimd) of a thin film of rubber-toughened epoxy resin (187).

Lazzeri and Bucknall [131] have proposed that the pressure dependence of yield behaviour caused by the presence of microvoids can explain the observation of dilatation bands in rubber-toughened epoxy resins [132], rubber-toughened polycarbonate [133] and styrene butadiene diblock copolymers [134]. These dilatation bands combine in-plane shear with dilatation normal to the shear plane. Whereas true crazes contain interconnecting strands, as described in Section 12.5.1 above, dilatation bands contain discrete voids that, for rubber-toughened polymers, are confined to the rubber phase. 

Fig. 5.51 An SEM image of a fractured rubber toughened epoxy resin exhibits brittle fracture. Holes from the dispersed phase particles show the rubber is incompatible with the matrix resin and there is poor adhesion resulting in rubber particles being pulled out during fracture.

Fig. 5.52 SEM of a rubber toughened epoxy resin shows that brittle fracture occurs through both the matrix and the dispersed phases. Voids (arrows) are observed within the dispersed phase and also within the matrix. Small subinclusions are seen within the dispersed phases.

The fracture morphology of a rubber toughened epoxy resin is shown in the SEM micrograph in Fig. 5.66 that is typical of glassy or brittle fracture with failure occurring across the well adhered dispersed phase particles. Subinclusions of the resin are observed vidthin the dispersed phase particles, likely due to the high... 

Fig. 5.67 Dependence of upon rubber phase volume fraction for a rubber-toughened epoxy resin (after Bucknall and Yoshi (1978) Brit. Polym. J. 10, 53).

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