Struktureller aufbau und physikalisches verhalten vernetzter epoxidharze

Abstract

Network structures and physical properties of products obtained either by crosslinking polyepoxides with polyphenols, and by dicyanodiamide 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 ATgv = 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,,, which was measured within the temperature range of 77 K to 450 K, in accordance with the equation T K F , , = K F = B -C . T ; T < T gThis equation was derived from Eyring's model of viscosity, correlating B and C with activation volume, activation energy, Tg, 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 Tg 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.*) Systematischer IUPAC Name: 1,3,5-Tris(2,3-epoxypropyl)-l,3,5-perhydrotriazin-2,4,6-trion.

Epoxy Resins

“…In amine curing agents, each hydrogen on an amine nitrogen is reactive and can open one epoxide ring to form a covalent bond [1][2][3][4][5] …”

Properties and failure of polymes with tailored distances between crosslinks

“…[100] and Lohse et al [101] pointed out the dominating effect of molecular strand length on the ultimate properties and the toughness of crosslinked polymers. Later, Batzer et al [46], Schmid [44], and Fischer et al [45] compared the behavior of various networks composed of epoxy resins. An interesting and elegant complementary approach to study the fracture of crosslinked polymers was chosen by Nguyen, Kausch et al [102] and Kausch et al [103] who explored the healing of cracks in crosslinked styrene-co-acrylonitrile and other polymer alloys.…”

“…The yielding of polymers can be understood analogue to the transition-state theory [15,45,60,[66][67][68][69][70][71]. In viscous liquids the molecules pass each other with some The applied shear stress x deforms the energy barrier analogous to Eyring's theory of viscosity; v: activation volume hindrance when they are squeezed through the narrow gap between adjacent molecules.…”

“…The activation energy u is determined from the temperature dependence of the yield stress [45]. Fischer et al [45] found u/v = 248 MPa for Phenoxy resins.…”

“…Fischer et al [45] found u/v = 248 MPa for Phenoxy resins. Therefore, the activation energy is about u = 0.5 x i0-18 j, if the activation volume v = 2 nm a.…”