Rapid cross-linking

The main producers of organic accelerators for mbber vulcanization are shown in Table 3. This table is not meant to be completely comprehensive, but rather to indicate the principal historical suppHers to the mbber industry. Most producers offer chemical equivalents in the largest-volume products. Within the range of smaHer-volume specialty accelerators, chemical equivalents become less common. Each producer may offer different products to achieve the same purpose of rapid cross-linking, resistance to thermal degradation, or other performance characteristics. Many offer proprietary blends of accelerators. 

Smith condensed phthalic anhydride with glycerol in 1901 to prepare a glassy brittle material. Kienle in 1924 started investigating these resins and called them alkyds, He modified the polyesters with drying oils. By this modification they become soluble, on exposure to oxygen rapid cross-linking occurs and... 

Irradiation of polychloroprene latexes of two different structures, one containing some sulfur and having a lower degree of branching, and another highly branched polymer made by mercaptane modification, showed a more rapid cross-linking of... 

Under UV-laser irradiation, photosensitive multifunctional acrylate resins become rapidly cross-linked and completely insoluble. The extent of the reaction was followed continuously by both UV and IR spectroscopy in order to evaluate the rate and quantum yield of the laser-induced polymerization of these photoresist systems. Two basic types of lasers emitting in the UV range were employed, either a continuous wave (C.W.) argon-ion laser, or a pulsed nitrogen laser. 

The coating must also be solidified very rapidly once it is applied to the fiber so that it will offer protection when the capstan is reached. This requirement rules out solvent-containing formulations for all but the thinnest coatings, because solvent removal is a slow process. Solvent-free coating formulations that are rapidly cross-linked by thermal activation or by UV radiation can be used very successfully. Thermoplastic hot-melt systems that solidify quickly upon cooling are also viable. The material systems of choice for this application depend upon both coating-application and coating-performance considerations. 

As mentioned earlier, UV-curable resin formulations are very attractive for fiber coating because of the rapid cross-linking rates that are achievable. Most commonly, epoxy- or urethane-acrylate resins are employed (18-22), and viscosity and cross-link density are controlled through the addition of reactive diluents. With these systems work has focused on producing low modulus, low T properties (20-22) through the incorporation of appropriate chemical constituents to enhance higher chain flexibility, for example, ether linkages. 

During the last few years we have developed methods for rapid cross-linking of polyethylenes, ethylene-propylene-diene elastomers (EPDM)2 and linear unsaturated polyesters using UV light for initiation. Efficient crosslinking to high gel content has been obtained for these polymer systems, which contain small amounts of UV-absorbing initiator and a multifimctional monomer (a crosslinker),... 

Accelerated-sulfur vulcanization is the most widely used method. For many applications, it is the only rapid cross-linking technique that can, in a practical manner, give the delayed action required for processing, shaping, and forming before the formation of the intractable vulcanized network. It can be used to vulcanize natural rubber (NR), synthetic isoprene rubber (IR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), butyl rubber (HR), chlorobutyl rubber (CIIR), bromobutyl rubber (BUR), and ethylene-propylene-di-ene-monomer rubber (EPDM). The reactive moiety for all of these elastomers can be represented by... 

Lim DW, Nettles DL, Setton LA, Chilkoti, A (2007) Rapid cross-linking of elastin-like polypeptides with (hydroxymethyl)phosphines in aqueous solution. Biomacromolecules 8 1463-1470... 

The most important initiators used at elevated temperatures are peroxides, which liberate free radicals as a result of thermal decomposition. A peroxide which is widely used in this way is benzoyl peroxide (XII), the decomposition of which is discussed in Section 1.4.2.1. Other peroxides which find application in the cross-linking of unsaturated polyesters are 2,4-dichlorobenzoyl peroxide (XIII), di-tert-butyl peroxide (XIV) and lauroyl peroxide (XV). Mixtures of polyester resin and this type of peroxide are comparatively stable at room temperature but rapidly cross-link at temperatures ranging from about 70 to 150°C, depending on the choice of peroxide. Such peroxides are used principally in processes employing matched metal moulds, when short curing times are required. 

Polyfunctional alcohols rapidly cross-link chlorosulphonated polyethylene rubbers in the presence of a strong base. 

The base polymers are mainly high-molecular-weight linear polydimethyl-siloxanes prepared by the base- or acid-catalyzed polymerization of cyclic dimethylsiloxanes [(CH3)2SiO—] (see Section 2,A). The copolymerization of methylvinylcyclosiloxanes with dimethylsiloxanes afford polymers with methylvinylsiloxy groups that can be rapidly cross-linked by peroxides such as 2,4-dichlorobenzoyl peroxide or other systems as described in Preparation 2-7. 

Ozone addition to EPDM-like polymers can cause rapid cross-linking and chain scission [27]. Depending on conditions and the nature of the epoxy material, one of the two mechanisms can become dominant. If ozone reacts with a different kind of polymer such as polyvinyl alcohol (PVA), chain scissions remain dominant, but lead to different products due to disproportionation and other side reactions [28]. There is also evidence that the concentration of ozone present in the environment does not have a significant impact on the degradation of the polymer, and is always a slow process [29]. 

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