Polyurethane Rubbery epoxy

Flexural strain reflects the flexibility of a material. Highly flexible resins like rubbery epoxy, polyurethane, and modified resins do not break even after a large deflection determination of ultimate flexural strength is impractical for them. The relevance of mechanical tests depends on the form in which the material is going to be used. If a material is to be used as a beam, then a flexural test is more relevant than a tensile test. 

Two-pack polyurethanes Also called urethanes, these materials are similar to two-pack epoxies in that they can be formulated to provide different properties. They can be made into foams or soft, rubbery materials, as well as very hard, tough, abrasion-resistant coatings. 

The IPNs prepared were composed of a rubbery polyurethane and a glassy epoxy component. For the polyurethane portion, a carbodiimide-modified diphenyl-methane diisocyanate (Isonate 143L) was used with a polycaprolactone glycol (TONE polyol 0230) and a dibutyltin dilaurate catalyst (T-12). For the epoxy, a bisphenol-A epichlorohydrin (DER 330) was used with a Lewis acid catalyst system (BF -etherate). The catalysts crosslink via a ring-opening mechanism and were intentionally selected to provide minimum grafting with any of the polyurethane components. The urethane/epoxy ratio was maintained constant at 50/50. A number of fillers were included in the IPN formulations. The materials used are shown in Table I. 

While thermoplastic polymers soften at Tg, and if semicrystalline, melt at T, cross-linked polymers do not melt and flow (Fig. 5.2c ). Lightly cross-linked polymers soften as the temperature exceeds Tg, but they remain rubbery solids until the polymer decomposes. Highly cross-linked polymers often do not even soften and retain a high modulus until reaching the decomposition temperature. Thermoset resins, like unsaturated polyester, epoxy, and polyurethanes, have varied levels of cross-linking. However, thermoplastic resins can be modified to contain few cross-links lightly cross-linked polyethylene (XLPE) often improves the mechanical properties of rotomolded parts. 

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