Cross-link density of cure

In addition to the above techniques, inverse gas chromatography, swelling experiments, tensile tests, mechanical analyses, and small-angle neutron scattering have been used to determine the cross-link density of cured networks (240—245). Si soHd-state nmr and chemical degradation methods have been used to characterize cured networks stmcturaHy (246). H- and H-nmr and spin echo experiments have been used to study the dynamics of cured sihcone networks (247—250). 

In addition, the two latter epoxides lower the cross-link density of cured polyepoxydes. 

The compression set of sihcone mbber is similar to organic types of mbber at low (0—50°C) temperatures, ranging from 5 to 15% (380). Above 50°C, sihcone mbber is superior, but compression set increases with time and temperature. Sihcone mbber is more tear-sensitive than butyl mbber, and the degree of sensitivity is a function of filler size and dispersion, cross-link density, and curing conditions. The electrical properties of sihcone mbber are generally superior to organic mbbers and are retained over a temperature range from —50 to 250°C (51). Typical electrical values for a heat-cured sihcone mbber are shown in Table 9. 

Figure 3. Cross-link density versus curing temperature. Straight lines are drawn through data points of nature rubber (NR). ...

Table 11. Cross-linking density of some EPTMs, EPDMs and EPM cured with dicumyl peroxide (DCP)... 

Including 4-bromophenol in the phenol-formaldehyde resol system impacts the cross-link density of the cured product. In a systematic study of this copolymer, a comparison was made among the polymers obtained using phenol only, a 9 1 mole ratio of phenol to 4-bromophenol and a 1 1 mole ratio of phenol to 4-bromophenol. Comparisons included measurement of interlaminar shear strength and cone calorimetry tests of composites prepared using these phenolic resins and S2-glass fiber plain weave. 

Nonetheless, for the more than 50 years since the first publication in this field, NIPUs still do not have sufficiently broad application. This can be explained by certain features of these materials. Cyclic carbonate (CC) groups interact with aliphatic and cycloaliphatic polyamines at ambient temperatures more slowly than isocyanates with hydroxyl groups. The rate of this reaction is comparable to the rate of curing epoxy resins (ER) with amines. At the same time, the CCs react only with primary amino groups, in contrast to the ERs, which react with primary and with secondary amino groups. This results in a decrease in cross-linking density of the polymer network. 

The properties of epoxy resin was not seriously affected by the addition of 5% RVP grafted AC 5120. However, the properties decreased as the concentration of modifier increased. But the decrease was due to the lower cross-link density of the cured blend since lesser amount of hardener HHPA (70 parts) was used in the blends instead of 80 parts in the neat resin. 

Unlike UF, AESO resin cannot be sprayed on straw particles because of its high viscosity. In order to reduce AESO viscosity, styrene, which also increases the strength, stiffness and glass transition temperature due to an increase in cross-link density of the cured resin, was blended with AESO in the amount of 33 wt% [7,12]. Since the polymerization process of AESO is addition type, a free radical initiator should be used for the curing. Benzoyl peroxide was used as free radical initiator in the amount of 5 wt% of the AESO + styrene mixture. 

The mechanism of anhydride cure is complex and controversial because of the possibility of several competing reactions. The imcatalyzed reaction of epoxy resins with acid anhydrides proceeds slowly even at 200°C both esterification and etherification occur. Secondary alcohols from the epoxy backbone react with the anhydride to give a half ester, which in turn reacts with an epoxy group to give the diester. A competing reaction is etherification of an epoxy with a secondary alcohol, either on the resin backbone or that formed during the esterification, resulting in a -hydroxy ether. It has been reported that etherification is a probable reaction since only 0.85 eqiuvalents of anhydrides are required to obtain optimiun cross-linked density and cured properties (103). 

Rings. In 0-ring applications, the primary consideration is resistance to compression set. The choice of a fluorocarbon elastomer gum is based on gum viscosity, cross-link density, and cure system. Formulations are given in Table 5 (compounds 1 and 2). 

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