Epoxy resins attributes

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

While freshly cleaved surfaces clearly contain some steps and other defects, the surfaces are remarkably stable to corrosion. Dramatic evidence of the stability of MoSe2 surfaces was provided by Stickney et al. who obtained LEED and Auger spectra of surfaces that had been exposed to the atmosphere and a variety of oxidizing solutions (35). Except for the presence of a ubiquitous carbon which was attributed to the epoxy resin or cleaving tape, no evidence for surface reactions was found. Long term stability tests of a photoelectrochemical cell made with a WSe2 electrode, where over 400,000 coulombs/cm2 were passed through the cell with no detectable photocorrosion, also attest to the durability of these surfaces (36). 

In our initial studies ( ), we examined several series of cured resins using ortho and meta isomers of phenylene diamine. p-Phenylenediamlne weis not considered for stu< due to its known ceurclnog lc nature. As shown in Table I, o-phenylenediamine (OPD) used edone was found to impart inferior properties to the crosslinked epoxy resin. We subject the poor performance of OPD is attributable to intramolecular hydrogai bonding, as shown below ... 

Since no difference was found between FR and non-FR formulations in device aging studies, some other cause must account for the relatively early failures observed for devices molded in the electrical grade epoxy material and aged under bias at 200°C. These failures are attributed to chloride contamination present in the non-semiconductor grade epoxy resin. The extractable Cl concentration is a factor of four higher than Br, and this is correlated with a much higher concentration of CHoCl than CH Br in the EGA data below 200°C. The high Br concentration is also attributed to the... 

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

The gluability of the lignin-epoxy resin adhesives was found to be improved by the addition of calcium carbonate (50% by weight) to the liquid resin. This must be attributed to the nature of the weak alkali in calcium carbonate as a cure accelerator, and to the reinforcement effect of fillers. Since wood surfaces are acidic, the addition of alkaline fillers effectively alters the pH of the glue line. 

Superior toughness and mechanical properties The strength of properly formulated epoxy resins usually surpasses other types of casting resins. The cured epoxy resins [Structure (4.35)] are seven times tougher than the cured phenolic resins [Structure (4.36)] as is evident from Figure 4.15. The relative toughness is attributed to the distance between the crosslinking sites and presence of internal aliphatic chains. 

Pure epoxy resins, so-called basic resins, are unsuited to building applications because of their high viscosity. Modifications are necessary to achieve the required viscosity, wettability, carbonate resistance, curing rate, cost reduction and numerous other properties. However, the modifiers must be chosen so as not to impair the other valuable attributes of the epoxy resins. For example volatile solvents are unsuitable for thick coatings, because any solvent retained in the cured system will reduce the mechanical and thermal properties and the corrosion resistance. The specific property needs for a particular application may be tailored to each system to maximise the remarkable potential of epoxy resins (Dow Chemical Company, undated c). 

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