Epoxy thermosets phase separation

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. 

Several basic morphologies are observed in thermoplastic-modified epoxies and, indeed, other thermosets. Homogeneous [Fig. 6(A), in which no phase separation is observed] and particulate [Fig. 6(B), in which the modifier phase separates to produce small domains] morphologies occur at low concentrations of modifier. In these cases, the thermoplastic modifier is encapsulated within a thermoset matrix, whereas in the phase-inverted morphology, [Fig. 6(C)] the minor thermoplastic component is the continuous phase surrounding large, discontinuous domains of the major... 

It was noted in the previous section that the carboxyl end groups on the CTBN elastomer affected the final performance of the material as a toughener since these groups would co-react with the epoxy resin and facilitate stress transfer from the brittle matrix to the phase-separated elastomer. Without this adhesion the particles could debond prematurely, which would lead to poor dissipation of the energy of the growing crack. It has also been noted that excessive adhesion between an epoxy resin and a thermoplastic could be deleterious to the performance (Williams et al, 1997). The process of toughening of a thermoset... 

Poncet et al. (1999) monitored frequency-dependent dielectric measurements to examine the phase-separation process in poly(2,6-dimethyl-1,4-phenylene ether) (PPE) in a DGEBA-MCDEA resin. Dielectric measurements measured the build up in Tg both in the PPE-rich continuous phase and in the epoxy-rich occluded phases for 30-60-wt.% PPE mixtures. In the 30% PPE mixmre, the rate of reaction of the thermoset phase is equivalent to that of the neat system due to two opposing effects, namely a slower reaction rate due to dilution and a low level of conversion at vitrification due to the presence of high-Tg PPE. In the 60-wt.% mixture the dilution effect of the PPE has a large effect of decreasing the reaction rate. The continuous thermoplastic-rich phase vitrifies first, followed by the thermoset occluded phase. The final morphology (size of occluded particles and composition of continuous phase) is affected by kinetics, diffusion and viscosity during phase separation. 

Phase Separation of Two-Phase Epoxy Thermosets That Contain Epoxidized Triglyceride Oils... 

Preparation of Two-Phase Epoxy Thermosets. Two-phase epoxy thermosets were prepared from homogeneous stoichiometric mixtures of DGEBA and diamine (DDM or DDS), which contained varying amounts of liquid rubber (ESR or VR). The rubber was dissolved first in DGEBA at 70 °C. Then the diamine (DDM or DDS) was added and the mixture was stirred at 70 °C until the diamine dissolved (about 15 min). The homogeneous (one-phase) transparent mixture was then degassed before it reached its cloud point (phase separation). The formulations cross-linked with DDS were cured at 150 °C for 2 h, whereas the formulations cross-linked with DDM were cured first at 75 °C for 4 h and then at 150 °C for 2 h. 

Dirlikov, S. I. Frischinger Z. Chen. Phase separation of two-phase epoxy thermosets that contain epoxidized triglyceride oils. Chem. Ser. 1996, 252, 95-108. 

Thermosetting polymers, like phenolics, epoxies, unsaturated polyesters, etc, are frequently used in formulations containing a low or high-molar-mass rubber, a thermoplastic polymer, an oil, etc, in an amount of the order of 2-50 wt% with respect to the thermoset. This extra component, called the modifier, may initially be immiscible or may phase-separate during cure (reaction-induced phase separation). 

Fig. 7. TTT diagram representing times for phase separation (doud point), gelation and vitrification for a castor-oil-modified epoxy system (4ho = 0.176) at different temperatures (Reprinted from Polymer International, 30, R.A. Ruseckaite, L. Hu, CC. Riccardi, R.JJ. Williams, Castor-oil-modified epoxy resins as model systems of rubber-modified thermosets. 2 Influence of cure conditions on morphologies generated, 287-295, Copyright (1993), with kind permission from the Society of Chemic Industry, London, UK)...

Fig. 23. Ratios (r) of amine epoxy equivalents in both phases as a function of the overall conversion of epoxide groups in the CO-modified DGEBA-EDA system (Reprinted from Polymer, 35, C.C. Ric-cardi, J. Borrajo, R.J J. Williams, Thermodynamic analysis of phase separation in rubber-modified thermosetting polymers influence of the reactive polymer polydis-persity, 5541-5550, Copyright (1994), with kind permission from Butterworth-Heinemann journals, Elsevier Science Ltd, The Boulevard, Langford Lane, Kidling-ton 0X5 1GB, UK)...

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