Epoxy-matrix phase

The solvent-etched fracture surface of folly cured PEI modified epoxy with different composition is shown in Figure 3.8. For PEI content smaller than 10wt%, the PEI-rich phase is dispersed in a continuous epoxy-rich matrix [i.e., sea-island morphology is observed (a and b)]. Above 25 wt% PEI content, nodular structure was observed (e and f) where the epoxy-rich phase forms spherical nodules and the PEI rich phase forms the matrix. With PEI content between 15 wt% and 20 wt%, dual phase morphology, where sea-island morphology and epoxy nodular structure coexist, is present (c and d). Similar morphology was observed in PEI/BPACY blend [47],... 

On the other hand, if the cure rate is much faster than the phase separation, then the morphology is controlled by the cure rate through a chemical pinning process. In this system, phase separation is mainly controlled by the cure rate of the epoxy matrix. Faster curing rates and shorter gel times lead to smaller PEI-rich particles with an increasing cure temperature. The temperature effect on the viscosity of reaction mixture is relatively small (i.e., the complex viscosities measured by Physica are 7 and 4 Pa.s at curing temperatures of 150 and 190°C, respectively). 

In summary of these points, it is seen that the isolation of particles from the epoxy matrix, the effective volume fraction of the elastomeric phase, and strength of the interface interact to control modulus. The morphology which a particular siloxane modifier promotes determines the contribution of any or all of these three factors to the modulus of the modified resin. 

For epoxy networks modified by liquid reactive rubbers, it is not so easy to discuss these parameters separately, because they are interdependent. For example, an increase in the acrylonitrile content of the carboxy-termi-nated butadiene acrylonitrile rubber (CTBN) induces a size reduction of the rubbery domains but also a higher miscibility with the epoxy-rich phase, leading to a higher amount remaining dissolved in the matrix at the end of cure (Chapter 8). It is not possible to separate the influence of these two effects on toughness. 

The main reason for the greater flexibility is due to long-chain difunctional materials that upon cure become part of the epoxy matrix. The result is a single-phase, flexible system. The disadvantage of this approach is a reduction in the crosslink density and consequently reductions in glass transition temperatures as well as the heat and chemical resistance of the system. 

Formulations have been developed where small rubber domains of a definite size and shape are formed in situ during cure of the epoxy matrix. The domains cease growing at gelation. After cure is complete, the adhesive consists of an epoxy matrix with embedded rubber particles. The formation of a fully dispersed phase depends on a delicate balance between the miscibility of the elastomer, or its adduct with the resin, with the resin-hardener mixture and appropriate precipitation during the crosslinking reaction. 

The degree of toughness is determined by the crosslink density of the matrix, the elastomer particle size and size distribution, the volume fraction of the elastomeric phase, and the degree of adhesion between the epoxy matrix and the particle. The formulating procedure was found to have as strong an effect on the fracture toughness as the materials themselves.16... 

Electron Microscopy. We replicated the surfaces developed from the double cantilever cleavage test and the surface developed from the tensile test specimen. Replicas are first taken with a gelatin solution from the fracture surface (9). A typical electron micrograph of a CTBN-epoxy system is shown in Figure 1. This micrograph is from a surface of a fractured cantilever cleavage specimen. It shows a uniformly dispersed rubber phase in a brittle epoxy matrix. 

We believe that the Hycar 1312 is not cured. In these systems there exist only the van der Waals attractive and/or hydrogen bonding type forces between the epoxy matrix and the liquid rubber particles. In such two-phase systems the liquid rubber merely acts as a diluent and will show physical properties analogous to mechanical mixtures of two different polymers. 

Table II shows that the chemical bonding between the epoxy matrix and the rubbery phase is important. The terminal reactive groups are more effective than the pendant groups in toughening epoxy resins.

High temperature epoxy resins are brittle materials, and one method of improving their fracture properties is to incorporate reactive liquid rubbers in the formulations In situ phase separation occurs during cure the cured rubber-modified epoxy resins consist of finely dispersed rubber-rich domains ( 0.1-S pm) bonded to the epoxy matrix. TTT diagrams can be used to compare different rubber-modified systems. 

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... 

The approach taken by Sefton et al. for the synthesis of novel thermoplastics designed to undergo phase separation from the thermoset epoxy matrix is complementary to the method employed by Bucknall and Patridge, in which the nature of the thermoset is varied to achieve a similar result. 

Figure 10. Variation of the maximum von Mises stress in the epoxy matrix with the volume fraction of the rubber phase, for different values of the bulk modulus, K, of the rubber particle, and a void. The applied pure hydrostatic tension is 100 MPa.

In a photoinitiated system, in whTcF the reaction proceedes rapidly during exposure (resulting in relatively short gel times), it is thought that the epoxy matrix is set up during the initial irradiation thus influences the final morphology. The final particle size and distribution would then be dictated by the compatibility of the components in the uncured state since the rapid formation of the gel would be expected to preclude the redistribution of the phases. 

Figure 8.20 also shows the phase morphology of the cured material. The SEM micrograph was taken from a fractured and etched surface so that remaining material is cured epoxy. The connected-globule structure can be explained as a two-phase morphology of interconnected spherical domains of the epoxy-rich phase dispersed in a PES matrix. 

The polypropylene continuous phase was transparent and epoxy phase appeared dark in the optical micrograph. However, the cause of elongated structure was not fully understood. The formation of elongated structure could be due to the friction between carbon black coated dispersed particles and the matrix (61), which decreased the interfacial tension between the immiscible polymers (63) or both (64). Based on extraction studies and thermogravimetric analysis, it was concluded that nearly all the carbon black particles were located in the epoxy matrix. 

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