Curing rubber-modified epoxy

High resolution l3C NMR is also used in the determination of the composition of the dispersed phase in cured rubber modified epoxies in order to analyze the chemical structure of the mobile segments 152). In this case quantitative analysis is possible because the areas under each peak are approximately equal to the number of carbons contributing to the peak, and the intensities of the broad lines from the rigid phase are very low, almost indistinguishable from the baseline noise. The structure of crosslinked networks based on poly(3,4-pyrrolidinediethylene), synthesized by different methods, was determined from gels swollen in water and chloroform 153). 

Figure 8.7 SEM photograph of a fully cured rubber-modified epoxy network. The rubber CTBN (26 wt% AN) is first pre-reacted with a large excess of diglycidyl ether of bisphenol A (DGEBA) to obtain an epoxy-terminated rubber. Then an equivalent of 15 wt% initial CTBN is introduced in DGEBA-4,4 -diamino diphenyl sulfone, DDS, system precured at 135°C (time > tgei) and then postcured at 230°C. Rubber-rich particles are spherical, D 2.8 0.5 gm, and well dispersed. (From LMM Library.)...

All of the commercial epoxy adhesives presented in App. B bond well to aluminum and to a wide variety of other materials. Sell22 has ranked a number of aluminum adhesives in order of decreasing durability as follows nitrile-phenolics, high-temperature epoxies, elevated-temperature curing epoxies, elevated-temperature curing rubber-modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes. 

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. 

Hedreul et al. (1998) examined a model of the cure kinetics of a thermally and microwave-cured rubber-modified epoxy-resin formulation. The phenomenological cure kinetic model used was... 

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). 

N.C. Paul, et al., "An Aliphatic Amine Cured Rubber Modified Epoxy Adhesive...," Polymer, 1, 945 (1977). 

Structure of an Amine Cured Rubber Modified Epoxy," Polymer, 87 (1981). ... 

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... 

With the increased usage of 120°C cured, rubber modified epoxy structural adhesives for aluminum airframes, certain service problems have been observed which have been attributed to environmental factors. The problems associated with the combined effects of sustained load, elevated temperature and high humidity upon the aluminum substrate, corrosion inhibiting primers, and the structural epoxy adhesive matrix are discussed. A particular type adhesive matrix, based on acrylonitrile/butadiene rubber modified bisphenol type epoxy systems is discussed in detail, and important advances in the preparation of more moisture resistant aluminum (oxide) surfaces are reviewed. 

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

In essence, the durability of metal/adhesive joints is governed primarily by the combination of substrate, surface preparation, environmental exposure and choice of adhesive. As stated earlier, the choice of the two-part nitrile rubber modified epoxy system (Hughes Chem - PPG) was a fixed variable, meeting the requirement of initial joint strength and cure cycle and was not, at this time, examined as a reason for joint failure. Durability, as influenced by substrate, surface preparation, and environmental exposure were examined in this study using results obtained from accelerated exposure of single lap shear adhesive joints. 

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. 

In this Section, an experimental approach for constructing isothermal TTT cure diagrams has been described, TTT diagrams of representative epoxy systems including high Tg and rubber-modified epoxy resins have been discussed, and perturbations to the TTT cure diagram due to thermal degradation and rubber modification have been illustrated. 

For high temperature and rubber-modified epoxy resins, thermal degradation events and the cloud point curve are included on the diagrams, respectively. Two degradation events have been assigned devitrification, or a glass-to-rubber event and revitrification, which is associated with char formation. The cloud points and depressions of Tg for different rubber-modified epoxies can be compared and related to volume fractions of the second phase and to the mechanical properties of the cured materials. 

The material used was a diglycidyl ether of bisphenol A (DGEBA) based epoxy resin (Ciba-Geigy, GY250) cured using stoichiometric amounts of 4,4 -diamin-odiphenyl sulfone (DDS). The rubber used for the modifications was Hycar car-boxy-teminated butadiene-acrylonitrile (CTBN) rubber (1300 x 13). The curing schedule for all the rubber-modified epoxy-DDS systems was as follows first the rubber and then DDS were mixed with the epoxy resin and stirred at 135 °C until the DDS was dissolved the systems were cured for 24 h at 120 °C and then postured for 4 h at 180 °C. The control epoxies were cured according to the same schedule. 

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. 28. SAXS intensity as a function of the scattering angle, for a rubber (ETBN) modified qmxy-amine (DGEBA-3DCM) system reacted at 50°C, at different cure tunes. (1) 80 min, (2) 105 min, (3) 130 min, (4) 180 min (Reprinted from Polymer Internationa], 32, D. Chen, J.P. Pascault, H. Sautereau, G. Vigier, Rubber-modified epoxies. II. A reaction-induced phase separation observed in-situ and a posteriori with different methods, 369-379, Copyright (15 3), with kind permission from the Society of Chemical Industry, London, UK)...

Figure 32 [103] shows the viscosity at the cloud point as a function of the average diameter D of dispersed phase particles for particular rubber-modified epoxies cured at different temperatures. As the viscosity at the cloud point, ricp> decreases an increase in the average size of dispersed phase particles is observed. Correspondingly, as Vq remains constant, there is a decrease in the concentration of dispersed phase particles, Cp., i-e- D and Cp , follow opposite trends as shown in Fig. 33. 

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)...

Adhesive systems based on free-radical curing epoxymethacrylates are cited (16). And Dudgeon (17) has covered cationic, heat-curable rubber-modified epoxy resin systems which very likely have latent-cure, structural adhesive capability. 

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