Gelation, rubber-modified epoxy

The adhesive properties of epoxy resins coupled with their dielectric behavior have made them attractive to the electronic industry. The evaluation of thermally cured rubber modified epoxy thermosets has been the subject of recent studies (1, 2), which dealt with the dependence of morphology on the curing parameters, e.g., catalyst, cure schedule, time of gelation, etc. This work utilizes one of the new series of photocationic initiators (PCI) developed by Crivello, et al (3) which are presently commercially available. These onium salts initiate the reaction by absorbing the actinic radiation, generating radicals and producing a protonic acid. The radicals can lead to polymerization of olefinic moieties (4) while the acid initiates the polymerization of the epoxy groups (3). 

In the thermally initiated cure of rubber modified epoxy, the rubber may be present within in the epoxy matrix as distinct domains. The morphology of the cured resin has been shown to be dependent on (1) the cure temperature and accelerator concentration, since the extent of particle (domain) size growth appears to be limited by gelation and (2) the nature (percent acrylonitrile) of the rubber used, since mixture compatibility increases with the acrylonitrile content of the rubbers (1,2). 

Fig. 13 is a TTT cure diagram of three systems a neat epoxy resin and the same epoxy modified with two reactive rubbers at the same concentration level. The times to the cloud point, gelation and vitrification are shown for each system. The cloud point is the point of incipient phase separation, as detected by light transmission. The modified system with the longer times to the cloud point and gelation, and the greater depression of Tg, contains the more compatible of the two rubbers. The difference in compatibility could then be used to account for differences in the volume fractions of the phase separated rubber-rich domains and in the mechanical properties of the neat and the two rubber-modified systems. 

Fig. 13. TXT cure diagram temperature of cure vs. the times to phase separation (doud point), gelation and vitrification for a neat and two rubber-modified systems. of the neat system is also included. The systems studied were DER331/TMAB O, gelation , vitrificaticm modified with IS parts rubber per hundred parts epoxy 1) pr eacted carboxyl-terminated butadiene-acrylonitrile (CTBN) copolymer containing 17% acrylonitrile (K-293, Spencer Kellog Co.) A, phase separation , gelation , vitrification, and 2) polytetramethylene oxide terminated with anmiatic amine (ODA2000, Polaroid Corp.) A. phase separation O, gelation O, vitrification. (DER331/TMAB/ K-293 data from Ref. )...

It has previously been shown that the morphology of a rubber modified resin is determined at gelation (1). In these photoinitiated systems over 80% of the film has gelled within the first 11 seconds. This would imply that an epoxy matrix has been formed during the irradiation which controls the morphology of the film. The irradiation is followed by a thermal cure during which the unreacted species within the matrix react. 

Elastomer-modified epoxy resin systems with more complexity to their preparation scheme have been demonstrated. Two examples suffice. Shelley and Clarke (9 ) instruct that a vulcanization procedure can be successfully employed to Improve elevated temperature properties in the cured resin mass. This step occurs subsequent to the esterification regime. It can be practiced with impunity at low rubber contents (7.5-10%) without gelation or Indeed very much viscosity Increase. Peroxides appear to be preferred over sulfur/sulfur donor systems. Table VII displays an example of this procedure with a solid DGEBA resin. 

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)...

From the steady shear sweeps, the gel time of blends from epoxy crosslinldng was identified as a crossover of either G and G", or the maximum peak in tanb Jyo-tishkumar et al. [67] calculated the apparent activation energy ( ) of gelation based on Eq. (4.3). The values of neat epoxy and thermoplastic-modified systems were all close to 65 kj mol , which was quite similar to that of ordinary epoxy resin and rubber-modified systems. Clearly, in these systems the phase separation had been completed before gelation of the epoxy resin, and so had a limited effect on the chain mobility of epoxy during gelation. 

Fig. 26. Integrated SAXS intensity (arbitrary units) vs polymerization time at 50 °C for an epoxy-diamine system (DGEBA-3DCM) modified by 15 wt% of two different rubbers (ETBN and NFBN). Morphology of the particles (p-phase), as observed by transmission electron microscopy (TEM), is plotted qualitatively. Cloud-point times (tcp), determined by light transmission, and gelation times are indicated...

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