Epoxy mechanical constants

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Closely related to the problem of the structure of the effective rate constant is the above-mentioned problem of the compensation mechanism. Without a knowledge of this mechanism, it would be impossible to understand why in such a complicated epoxyamine system one can frequently observe relatively simple kinetic principles, viz., a weak dependence of the effective rate constant on conversion, simple dependences of the initial rate on reagent concentrations, a linear dependence of the total heat release on conversion and almost equal values of the heat release and enthalpy of the epoxy ring opening. The latter two aspects have been discussed above, whereas the first two problems can be understood, say, from a consideration of a noncatalytic reaction. 

Table 11 presents one more result important for the chemistry of epoxy compounds, namely within the experimental error the rate constant of the free ion is the same for all counterions. This means that such strong nucleophilic particles as carbanions (and evidently alkoxy anions) are capable of opening the epoxy ring without additional electrophilic activation. This result explains the apparently contradictory results that, depending on the reaction conditions, either tri-140 144,166-I71) or bimolecular kinetics 175-I79> is observed. The bimolecular kinetics also can be explained in terms of the trimolecular mechanism, since proton-donor additives play a dual role. 

Kannebley 30) and Sorokin et al. 31,32) obtained second-order rate constants for the relevant model bimolecular reactions whereas Arnold 19) and Doszlop et al.37) consider curing of epoxy resin with anhydride or reactions of monoepoxides with various proton-donor compounds to be first order (Table 1). Considering the mechanism of individual reactions which proceed during curing or in the non-catalyzed... 

Grillet et al. (1991) studied mechanical properties of epoxy networks with various aromatic hardeners. It is possible to compare experimental results obtained for networks exhibiting similar Tg values (this eliminates the influence of the factor Tg — T). For instance, epoxy networks based on flexible BAPP (2-2 - bis 4,4-aminophenoxy phenyl propane) show similar Tg values ( 170°C) to networks based on 3-3 DDS (diamino diphenyl sulfone). However, fracture energies are nine times larger for the former. These results constitute a clear indication that the network structure does affect the proportionality constant between ay and Tg — T. Although no general conclusions may be obtained, it may be expected that the constant is affected by crosslink density, average functionality of crosslinks and chain... 

Experimentally, the glass transition has also manifested itself by a sharp increase in relative rigidity (measured by dynamic-mechanical methods) and a simultaneous drastic decrease in the rate constant of the autocatalytic epoxy-amine reaction The mobility or rigidity of the system is a function of reaction conversion a in the pre-gel region it can be characterized by dynamic viscosity which is proportional to M of the reacting system. Beyond the gel point, still in the rubbery region but not close to the gel point, the dynamic modulus, G, is at low frequencies proportional to " (m a 1)... 

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. 

Composite piezoelectric transducers made from poled Pb-Ti-Zr (PZT) ceramics and epoxy polymers form an interesting family of materials which highlight the advantages of composite structures in improving coupled properties in soilds for transduction applications A number of different connection patterns have been fabricated with the piezoelectric ceramic in the form of spheres, fibers, layered, or three-dimensional skeletons Adding a polymer phase lowers the density, the dielectric constant, and the mechanical stiffness of the composite, thereby altering electric field and concentrating mechanical stresses on the piezoelectric ceramic phase. 

Note Polymerization, like its handmaiden, catalysis, has long been one of the most complex and productive areas of chemical research from year to year new materials and reaction mechanisms are constantly being explored, sometimes with only marginal success. But it need only be recalled that such now commonplace materials as polyethylene, polycarbonate, nylon, neoprene, epoxies, acrylics, to mention only a few, as well as block, graft, and stereospecific polymers, have resulted from continuous and intensive research by many brilliant chemists over the last 60 years, and this research continues undiminished. 

---