Resin toughening effect

This substantial toughening effect of CTBN on the cycloaliphatic epoxide ERL-4221, coupled with significant increase of the strength of the resin without seriously lowering the heat distortion temperature, is quite unusual for thermosetting systems. 

In general, the elastomer must be prereacted (adducted) with the epoxy for the toughening effect to take place. Adducts reduce the likelihood of early phase separation and maintain the solubility of the elastomer in the uncured resin system. For CTBN the reaction is carried out at high temperatures (150 to 160°C) and usually in the presence of a catalyst, such as tris-dimethylamino phenol or piperidine. The resulting epoxy-CTBN adducts are available from several suppliers, and they can be easily formulated into epoxy adhesives. 

Heat Distortion Test. Two remaining bars from the cleavage tests are used to determine heat distortion temperature in accordance with ASTM D-621 test methods. These thermal-mechanical tests are necessary to determine whether a brittle resin has been truly toughened or whether it has been merely flexibilized. The morphology of the resin also effectively describes a true toughening situation and can aid immeasurably in explaining departures from true toughening. 

In rubber-modified polymers like high impact polystyrene or acrylonitrile-butadiene-styrene (ABS) resins, the toughening effect of the dispersed rubber particles appears only in the presence of block or graft copolymers. These copolymers regulate the particle size of the rubber dispersion and achieve adhesion of the two phases. Hence, graft copolymers are of practical importance in polymer alloys. 

The characteristics of fracture surfaces of F-185 neat resin and those of F-185 matrix In the composites are similar to those reported In the literature ( ). The fact that the fracture surfaces of F-185 neat resin and F-185 matrix In the composites show typical ductile fracture behavior, while the unmodified HX-205 shows brittle fracture behavior, seems to Indicate the toughening effect of F-185 as a result of Incorporation of CTBN rubber. The fracture energies of these materials are being... 

In Fig. 13.41, results for the fracture energies of epoxy resins modified with rubber particles and glass spheres to a combined volume fraction of/ = 0.18 are given. We evaluate this toughening effect whereby cavitation occurs at the particles by means of the ratio of eq. (13.56) and eq. (13.57) for the quantities given above, obtaining... 

Lu, J. and Wool, R.P. (2008) Additive toughening effects on new bio-based thermosetting resins from plant oils. Composites Science and Technology, 68(3-4), 1025-1033. 

A commercial grade of high-impact (notched Izod > 900 J/m) POM resin (Delrin 100 ST, DuPont) is believed to be a blend of POM with >30 wt% of a thermoplastic poly(ester-urethane) elastomer derived from poly(l, 4-butane adipate) diol and methylene-bis-(4,4 -diphenyl diisocyanate) (MDl) (Hexman 1989). This blend is reported to have a cocontinuous or semi-interpenetrating network of the elastomer in a matrix of the polyacetal (Flexman et al. 1990). The toughening effect in such a blend of IPN-type morphology was interpreted to occur partly through a rubber band mechanism by which the fracture energy is absorbed. The bands of rubbery domains were believed to span the crack and participate in the deformation process. 

In this method, a liqnid rnbber is blended with a thermoset resin before curing [108-200]. Various liquid rubbers used for toughening thermoset resins are presented Table 4.1. The toughening effect of a particular modifier is largely dependent on the... 

However, this theory has several drawbacks. It can only explain the modest increase in toughness and cannot explain the dramatic increase in fracture energy reported in many rubber-toughened epoxy systems. There are many discrepancies between reported data for improvement in toughness for thermoset resins due to the sensitivity of the toughening effect to the curing condition and inherent matrix ductility [119, 120]. The effect of G on test temperature and strain rate cannot be explained using... 

Blending of rubber has been tried out for toughening phenolic resin. Phenolic-nitrile rubber (NBR) was developed in the early 1950s, which has found wide applications [242] such as structural adhesives, O-rings, and gaskets. However, it was reported that when NBR is mixed with resole and cnred, they phase separate into undesirably large domains with low adhesion between the phases [243]. A discussed earlier, for an effective toughening effect the domain size and interfacial adhesion are important. 

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