Cured epoxy resins, homogeneity

In)Homogeneity of Cured Epoxy Resins and Control of the Curing Reaction... 

Solvent absorption measurement has been shown to be a sensitive and useful test method in the manufacture of epoxy powder coatings. A test method was defined and the effects of time and temperature of immersion described. It was shown that solvent absorption is a measure of raw material properties (EEW of the epoxy resin, and CTBN elastomer type and concentration), the homogeneity of the extrudate, as well as the state of cure. The information obtained from solvent absorption measurements has proven to be extremely important not only in quality control analysis but also in providing an insight into the structure function relationships in epoxy resin chemistry. 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

Zeiss [48] describes a method for measuring sections of milled ferrite powder. The powder was mixed in 40 60 volume ratios with epoxy resin using a homogenizing head rotating at 25,000 rpm. The mixture was then poured into a 0.5 in diameter mold and cured at 60°C and 1000 psi to eliminate air bubbles. The casting was then polished in a vibratory polisher using 0.3 and 0.5 pm alumina in water. A photomicrograph of the polished section was used for subsequent analysis. 

Epoxy resins may be cured in the manner of polyadditions, i. e., homogeneously catalyzed by multifunctional amines and isocyanates, or cyclic anhydride, dicyan-diamide, or biguanide derivatives. On the other hand epoxy resins are also subject to homopolymerization. The catalysts represent Lewis bases, preferably tertiary amines, imidazoles, or ureas (the latter exclusively for the dicyandiamide curing)... 

Malkin and Kulichikin (1991) initially reviewed the rheokinetics of cured polymers and highlighted the first empirical chemorheological models. They showed that for a simple homogeneous reaction with no diffusion limitations or gel effects for reacting epoxy-resin systems the chemoviscosity could be described by... 

The binders for cathodic electrodeposition are epoxy resin combinations dispersed in water (see Section 3.8). Advantages of anticorrosive electrocoatings include excellent corrosion resistance at a dry film thickness of ca. 20-30 pm. Electrocoats are stoved at 165 -185 °C to obtain films with the desired properties. The paint industry is now developing electrocoats that can be cured at lower temperatures (140 150 "C). Electrocoating produces a homogeneous film that covers the entire car body surface, including recesses and cavities. 

In studies of the ERC heterogeneous structure obtained in copolymerization between the epoxy and rubber oligomers that occurs along with the epoxy resin cure (i.e., hot cure compoimds), the average size of the rubber particles was found to be dependent on its polarity and on the level of compatibility with the epoxy resin as well as on the chemical composition of the modifier [132]. The author also showed that the size of the rubber inclusions decreases and the structure of the composition becomes more homogeneous with increase of the polarity of the elastomers and the curing temperature of the ERC. 

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