Epoxy resins rubber modification

In epoxy resins rubber modification imparts changes in the cured polymer(l2). These changes, reflected by the solvent uptake data, can be attributed to a decrease in the crosslinked density and/or the presence of domains which allow a high degree of solvent uptake. 

This section focuses on the modification of epoxy resins by blending with acrylonitrile butadiene (nitrile) resins. These are true alloyed blends since the nitrile rubber usually contains no groups that are normally reactive with epoxy groups. The nitrile molecules and the epoxy molecules intermingle as a blend to provide a single-phase alloy. If a large elastomer concentration is used, no phase separation will occur to form precipitates. 

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. 

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. 

It is interesting that, upon rubber modification, the CET resin matrix can no longer form dilatation bands (18). Only rubber-particle cavitation and matrix shear yielding are detected. This observation implies that a dilatational stress component is required to trigger the formation of dilatation bands. In other words, upon rubber-particle cavitation, the dilatational stress component in the matrix is reduced. This suppresses the formation of dilatation bands. This conjecture finds support in the work of Glad (27), who investigated thin-film deformation of epoxy resins with various cross-link densities and could not find any signs of dilatation bands in his study. 

It is these solid carboxylic nitrile elastomers which began to show utility in the modification of epoxy resins. Processing needs for solid elastomer Inclusion, particularly in liquid epoxy resins, have not always been advantageous. Associated problems include gel, viscosity threshold limitations and achieving desired rubber levels in excess of 5-6 phr. Sometimes processing must be carried out in selected solvents, not always a desirable or tolerable step. 

Lewis and co-workers (42) developed improved powder coatings with nitrile rubber-modification of an appropriate epoxy base (solid resin admixture) cured with an imidazoline-accelerated modified phenolic type hardener. Model coatings ground to 55 pm particle size, electrostatically applied to metals, cured 10 170°C, gave excellent therraocycling results as well as retained resistance to solvent attack. Elastomer-modified epoxy powder coatings have been covered extensively by Gelbel, Romanchick and Sohn in Chapter 5 of this volume. 

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. 

Static and impact fracture data (room temperature measurements) mainly for epoxy resin systems is presented in Table 2. These relatively brittle materials require modification by, for instance, blending with a suitable rubber or thermoplastic to improve fracture toughness. Such improvements, however, depend not only on the types of materials but also on the composition and therefore the type of blend structure (e.g.. continuous-discrete phases. 

Modification of epoxy resins by such modifiers is usually achieved in two ways by integrating the rubber into the epoxy pol5rmer or by copolymerizing the epoxy and rubber obgomers simultaneously with the process of epoxy resin curing at 393 or 433 K. 

For modification of epoxy resins with rubbers containing double bonds, the principal difficulty lies in the formation of hydroperoxide groups. These in turn can fragment into radicals, which cause undesirable and uncontrolled side-reactions. 

Thus, from the above data one could conclude that to achieve the maximal values of Kjc the method of modification with PER is most effective. Additionally, this method is more technologicsLlly apphcable because it can be used in formation of cold-cure compounds. Special attention must be paid to the correlation of the Kjc characteristic with X2,3 foi mixtures such as epoxy resin—modifier and curing agent-modifier. The presented characteristics of the ERG cracking resistance indicate the strong dependence of Kic on the structure of these materials, which enables the use of this characteristic in optimizing the compositions of new and already known epoxy rubber materials. 

These findings allow control of the modification of epoxy resins by rubbers. For example, to obtain ERC with improved adhesion strength it is necessary to use a quantity of the rubber at which thermod3mamic incompatibility with the epoxy oligomer is observed. 

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