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The Structural Formation and Physical Behaviour of Cross-Linked Epoxy Resins

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Network structures and physical properties of products obtained either by crosslinking polyepoxides with polyphenols, and by dicyandiamide or by catalytic polymerization are discussed and compared with those obtained by amine or anhydride curing. The highest crosslinking density is achieved by the polymerization of epoxy compounds. In polymerization, the glass transition temperature may rise by more than Delta T sub gv 100 K. Amine and phenol curing result in similarly structured networks with mobile aliphatic segments and comparatively low crosslinking densities. Impact resistance based on dissipation of mechanical energy increases as network density decreases, a maximum being achieved with a medium chain length of 25-35 atom intervals between crosslinking points. The mechanical stability of polymers is limited by the cohesive strength KF. This latter corresponds to the maximum shear strength of bonds TKF sub max, which was measured within the temperature range of 77 K to 450 K, in accordance with the equation TKF sub max KF B - C . T T T sub g. This equation was derived from Eyrings model of viscosity, correlating B and C with activation volume, activation energy, T sug g and strain rate. B equals the cohesive strength at 0 K. It is determined by intermolecular forces but does not depend on the density of crosslinking. An increase of T sub g due to crosslinks or bulky segments causes a decrease of C and therefore a reduction of the temperature dependence of KF. Hence, cohesive strength at room temperature is improved. Author

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  • Plastics
  • Mechanics

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