Morphology rubber-modified epoxy

Studies of the particle—epoxy interface and particle composition have been helpful in understanding the rubber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in rubber-modified epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and tf-butyl acrylate—acrylic acid copolymers (309), have been used for toughening epoxy resins. 

Mechanical properties of rubber-modified epoxy resins depend on the extent of mbber-phase separation and on the morphological features of the mbber phase. Dissolved mbber causes plastic deformation and necking at low strains, but does not result in impact toughening. The presence of mbber particles is a necessary but not sufficient condition for achieving impact resistance. Optimum properties are obtained with materials comprising both dissolved and phase-separated mbber (305). 

Woo et al. (1994) studied a DGEBA/DDS system with both polysul-fone and CTBN. The thermoplastic/rubber-modified epoxy showed a complex phase-in-phase morphology, with a continuous epoxy phase surrounding a discrete thermoplastic/epoxy phase domain. These discrete domains exhibited a phase-inverted morphology, consisting of a continuous thermoplastic and dispersed epoxy particles. The reactive rubber seemed to enhance the interfacial adhesive bonding between the thermoplastic and thermosetting domains. With 5 phr CTBN in addition to 20 phr polysul-fone, Glc of the ternary system showed a 300% improvement (700 Jm-2 compared with 230 J m 2 for the neat matrix). 

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). 

Sayre, J. A. Assink, R. A. Lagasse, R. R. Ibid. 87-9A Sohn, J. E. "Morphology of Solid Uncured Rubber-Modified Epoxy Resins", 181st National Meeting, ACS, ORPL, (March, 1981). 

All of the above solid rubber-modified epoxy resins visually displayed clearly biphasic morphological properties (i.e., discreet rubber domains in a continuous epoxy matrix). If there is a reaction between a rubber moiety and an epoxide, it would best be studied in a homogeneous reaction mixture. Lower molecular weight epoxy resins are more compatible with CTBN elastomers and will form homogeneous solutions at elevated temperatures. Reaction of an epoxide with a reactive moiety contained in the elastomer, R, will most likely obey the following rate law ... 

The successful scale-up of advancement and modification of rubber-modified epoxy resins is discussed. Mechanisms are proposed for both advancement and esterification reactions as catalyzed by triphenylphosphine which are consistent with experimental results. A plausible mechanism for the destruction of the catalyst is also presented. The morphology of these materials is determined to be core-shell structures, dependent upon composition and reaction and processing conditions. Model studies have been performed to determine the effects of thermal history on the kinetics of reaction. These efforts have resulted in the successful scale-up and use of rubber-modified epoxy resins as functional coatings in the electronics industry. 

Fig. 32. Viscosity at the cloud point vs average diameter of dispersed phases particules for particular rubber (ETBN) modified epoxies (based on DGEBA) cured at different temperatures a) and b) differ in the nature of the rubber while c) corresponds to a different diamine (Reprinted from Journal of Applied Polymer Science, 42, D. Verchere, J.P. Pascault, H. Sautereau, S.M. Moschiar, C.C. Riccardi, R.J.J. Williams, Rubber-modified epoxies. If. Influence of the cure schedule and rubber concentration on the generated morphology, 701-716, Copyright (1991), with kind permission from John Wiley Sons, Inc., New York, USA)...

Thermoset epoxy resins were toughened by small elastomeric inclusions of a carboxy terminated butadiene-acrylonitrile (CTBN) random copolymer by Visconti and Marchessault [198], who showed the variation in size as a function of CTBN content by TEM and light scattering. A major study of rubber modified epoxy resins has been reported by Manzione et al. [202,203], who showed a range of morphologies which result in a range of mechanical properties, even for a single polymer. An amine cured rubber modified epoxy... 

The second noteworthy morphological feature is presented in Fig. 12b. This micrograph depicts the pre-crack front of 15-1500-70F, which had a value significantly above that of the control, as shown in Fig. 11 a. The holes may be examples of the dilatation effect observed in CTBN-modified epoxies l9,22> in which rubber particles dilate in mutually perpendicular directions under the application of a triaxial stress and then collapse into spherical cavities following fracture. Dilatation requires a mismatch in coefficients of thermal expansion of resin and rubber 11. This effect will therefore be most striking when the elastomeric phase is homogeneous, as is apparently the case here. 

The morphology of ruber modified epoxy photopolymers was found to depend on the cure conditions as well as the nature and concentration of rubber. The commercially available acrylonitrile-butadiene copolymer rubber modifiers with varying percentages of acrylonitrile content were used. They were polymerized using a photocationic initiator involving a UV exposure followed by a thermal cure. Transmission electron micrographs of osmium tetroxide stained specimens, coupled with dynamic mechanical measurements indicated that phase separation and particle size distribution depended not only on rubber concentration and compatibility, but also on the cure conditions. 

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. 

It is believed that this particular two phase morphology is the key to the toughening mechanism of the host matrix (4-9). The success of the epoxy toughening principle by RLP depends vitally on the interaction of the rubber with the epoxy matrix, and on the phase separation mechanism. Control of these two major variables is complex and only partially understood (8,9). Experimental results of the mechanical properties of RLP modified epoxy resins are frequently conflicting, and depend on the preparation techniques of the polymer composites (9,10,11). 

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)...

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