Stable crosslinks

Figure 4.22 The Mannich reaction occurs between an active-hydrogen-containing compound (phenol) and an amine-containing molecule in the presence of an aldehyde (formaldehyde). The condensation reaction forms stable crosslinks.

Different behaviors and mechanisms were clearly recognized between these resins. Epoxy resin cured with amine showed no degradation during immersion because of its stable crosslinks. Epoxy resin cured with anhydride showed the uniform corrosion with the softening and dissolution of the surface and also behaved similar to the oxidation corrosion of the metal at high temperature obeying linear law. 

Mixed aziridine or mixed epoxide-MAPO cures have been the practical solution to problems encountered with single curing agents. The mechanical properties of mixed aziridine or mixed epoxide—aziridine-cured propellants show less change on aging than those of propellants cured with MAPO or BITA alone. It appears that the BITA or the multifunctional epoxide (ERLA-0510 or Epon X-801) in the mixed curing systems provides stable crosslinks and a slight amount of postcure, which results... 

Crosslinkages reducible with borohydride are characteristic of newly formed collagen but these disappear and are replaced by more stable crosslinks as collagen matures. For example, a 3-hydroxypyridine that joins three triple helices may be formed from the reaction of two ketoamine groupings (with elimination of one glycosylated hydroxylysine residue).636 639 Similar chemistry can also produce pyrrole crosslinks.639... 

Polymer structures that hold silanols at the interface. Good examples of hydrolytically stable crosslinked structures are silica and silicate rocks. Although every oxane bond in these structures is hydrolyzable, a silicate rock is quite resistant to water. Each silicon is bonded to four oxygens under equilibrium conditions with a favorable equilibrium constant for bond retention. The probability that all four bonds to silicon can hydrolyze simultaneously to release soluble silicic acid is extremely remote. With sensitive enough analytical techniques it is possible to identify soluble silica as it -leaches from rocks, but an individual rock will survive in water for thousands of years. 

There is still no way of determining whether or not a given desmosine crosslinks 1, 2, 3, or 4 polypeptide chains of elastin. Based on model studies, however, the most favorable arrangement would be expected if only two chains are crosslinked together by a desmosine (19). This extends from observations that polyalanyl-rich peptides typically favor a-helical conformations and that it is difficult to interconnect more than two polypeptide chains around any given desmosine. With regard to the other amino acids that could potentialy crosslink elastin, the exact number of dehydrolysinonorleucine, dehydromerodesmosine and allysine aldol residues that are involved as intra- or intermolecular crosslinks, and the extent to which these residues may be reduced to form stable crosslinks is not known. 

Matsuda et al. have also reported the s)mthesis of a Sialyl Lewis -RGD conjugate [199]. In this example the authors placed a flexible linker between the two binding epitopes and examined their ability to bind to both integrin and P-selectin. Not only did this conjugate bind tightly to the selectin and integrin molecules, it was found to form stable crosslinks between the two... 

Tensile strengths of the above products are equal to those of their chemically crossllnked and reinforced non-grafted counterparts. Compression set values of the grafted products, especially after annealing, are unusually low. This Indicates the presence of thermally and mechanically stable crosslinks and little tendency to form a second network under pressure. 

Fig. 2.3 Reaction of IsoK/LG with primary amines to form stable adducts. Primary amines including lysine react with IsoK/LGs to form a hemiaminal adduct. Unlike most aldehydes which can only form the highly reversible Schiff base adduct, the hemiaminal adduct of y-ketoaldehydes can undergo a second nucleophilic attack to form a pyrrolidine adduct which dehydrates to form an irreversible pyrrole adduct. In the presence of oxygen, the pyrrole is converted to lactam and hydroxylactam adducts. Oxidation of the pyrrole leads to formation of stable crosslinked species...

Unreacted melamine can form additional stable crosslinks when sunlight is absent. 

The nature of the side-group also has a considerable effect on the thermal stability of polyorganosiloxanes polymers containing a methyl group are the most thermally stable when the p-dimethylaminophenyl groups are stable. Crosslinked and ladder organosilicon polymers are characterised by high thermal stability [1]. 

A promising approach toward reducing flammability of polymer systems is to alter the condensed-phase chemistry at elevated temperatures. Structure modification can alter the decomposition chemistry to favour transformation of the polymer to a char residue. Such an increase occurs when thermally induced stable crosslinking structures and/or aromatic rings are produced. Work by the Brooklyn Polytechnic in New York has examined systems such as polystyrene, aramids, PF resins and nitrile polymers such as SAN. 

When the polymer chains are connected to each other by stable crosslinks, either chemical or physical ones, the whole system is stationary, although the chain elements between the crosslinks are in rapid motion. On a macroscopic scale, the systan is solid-like, whereas on a microscopic scale, it behaves as a liquid. Such a system is called a gel. Thus, a gel is a system in which colloidal particles are interconnected and form a coherent macroscopic structure that is permeable for the solvent... 

The difference with sulfur vulcanized blends was almost 26%, 25% and 22.9% of the blends still remaining, with respect to 0%, 10% and 30% TPS content. A similar finding was observed in the HVA-2 system, where 18.4%, 22% and 31.1% of the blends still remained undissolved in xylene. Since all the unvulcanized samples were being dissolved, this study proposed that the suitable time for extraction of sulfur and HVA-2 vulcanized HDPE/NR/TPS was around 8 h. Furthermore, as can be seen at 30% TPS content, the gel content of HVA-2 vulcanized blends was observed to be higher than the NR component. This could be due to the vulcanizing fraction, which may hinder complete leaching of the TPS particles. The results obtained can be explained by the fact that a stable crosslink structure had been formed after the blends were subjected to HVA-2 crosslinker. The formation of a crosslink structure implies that the blends were resistant to chemical penetration and cannot be easily removed. 

The effect of heat on the performance of a polymeric system can be studied by thermal ageing. During ageing, scission of the main chain, crosslink formation or crosslink breakage can take place. Sometimes, existing crosslinks may be replaced by more stable crosslinks. All these factors contribute to the properties of the aged samples. 

Organic peroxides like dicumyl peroxide split under irradiation or heat to produce oxy radicals as in Reaction 1. These radicals are very reactive and will abstract hydrogen from the natural rubber molecule according to Reaction 2. The resultant polymer radical is also very reactive and joins with another such radical to form a stable crosslinked product (Reaction 3). 

Two adduct radicals can also react with each other, forming a stable, crosslinked-but oxidized-product, with water as a byproduct. 

Storage stable crosslinkable resins Japan 43,938 1979 Shikoku Ka Ken Kogyo... 

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