Comparison of hardening systems

Comparison of hardening systems Miscellaneous Epoxide Resins... 

Microabsorption and extinction, if present, can seriously decrease the accuracy of the direct comparison method, because this is an absolute method. Fortunately, both effects are negligible in the case of hardened steel. Inasmuch as both the austenite and martensite have the same composition and only a 4 percent difference in density, their linear absorption coefficients are practically identical. Their average particle sizes are also roughly the same. Therefore, microabsorption does not occur. Extinction is absent because of the very nature of hardened steel. The change in specific volume accompanying the transformation of austenite to martensite sets up nonuniform strains in both phases so severe that both kinds of crystals can be considered highly imperfect. If these fortunate circumstances do not exist, and they do not in most other alloy systems, the direct comparison method should be used with caution and checked by some independent method. 

The products of the chemical degradation of PETP with triethylene tetramine and triethaneolamine can be used as epoxy resin hardeners, it is demonstrated. Products of PETP aminolysis with triethylene tetramine and aminoglycolysis with triethanolamine, were characterised using NMR and rheometric measurements. Characteristics of the crosslinking process for the system epoxy resin/ PETP/amine degradation product, and epoxy resin/TETA for comparison were investigated by DSC. Three classes of liquid epoxy resins based on bisphenol A, bisphenol F and epoxy novolak resins were used in the experiments. 16 refs. 

Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state NMR spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by CP/MAS solid-state NMR spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-<(>, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). 

In order to demonstrate the predictability of the above model, the macroscopic behavior of the SMPF is estimated and the prediction is compared with the test result. In the following, three such predictions and/or comparisons are made. The first one investigates the effect of the heating rate on fully constrained stress recovery. The second one evaluates the effect of the amount of the amorphous phase and the crystalline phase on the stress-strain behavior under cyclic tension. The last one examines the growth of the crystalline phase due to stress induced crystallization. In all cases, the SMPF has a diameter of 0.04 mm. The material parameters for the stress recovery and strain hardening modeling as well as for the amorphous and crystalline subphase modules and for crystalline phase shp systems are summarized in Tables 5.2 to 5.4, respectively. 

The application of the metal-containing admixtures in the epoxy compositions consisting of an epoxy oligomer and a hardener is most often presented in the literature in comparison with the other methods of introducing metals into the epoxy matrices. The function of these admixtures is to remove the defects peculiar to the systems of the epoxy oligomers with the hardeners. 

In contrast to such one component systems, it is very difficult to get adhesion on oily surfaces with 2-C adhesives. Due to the immediate start of the curing after mixing of the two components, the time to replace the oil on the surface is too short in comparison with the curing speed, because the monomer or prepolymer components of the adhesive resin and hardener react faster with each other than they are able to wet the surface. Only with very long open time adhesives there is some chance to get adhesion. Significant efforts are made to use 2-C adhesives for bonding of hoods to reduce the thickness of the outer sheet without surface marking. 

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