Elastomers cross-linking efficiency

Studies performed on elastomers have shown that the cross-linking efficiency is relatively low. G(X) values of 1 and approximately 3 were found for natural rubber and polybutadiene, respectively, when irradiated at... 

The occurrence of these reactions is restricted to the amorphous phase. Therefore, the photo-cross-linking process has to be performed at temperatures exceeding the crystalline melting point in the case of highly crystalline polymers such as polyethylene. The cross-linking efficiency can be strongly enhanced by the addition of small amounts of multifunctional compounds such as triallyl cyanurate, TAG (see Chart 7.9), or by the incorporation of special diene moieties into copolymers such as ethylene propylene diene copolymers (EPDM elastomers) [33]. 

Few monomeric radicals are lost by coupling with polymeric radicals when diaUcyl peroxides are used as the curative. Also, if the elastomer is properly chosen, the scission reaction is not excessive. For dicumyl peroxide in natural rubber, the cross-linking efficiency has been estimated at about 1.0. One mole of cross-links is formed for each mole of peroxide cross-linking is mainly by the coupling of two polymeric radicals. One peroxide moiety gives two monomeric free radicals that react with rubber to give two polymeric radicals, which couple to form one cross-hnk. 

Peroxides are probably the most common materials used after sulfur because of their ability to cross-link a variety of diene- and nondiene-containing elastomers, and their ability to produce thermally stable carbon-carbon crosslinks. Carbon-carbon bonds are inherently stronger than the carbon-sulfur bonds developed with sulfur vulcanization (49). Peroxides decompose when heated to produce active free radicals, which in turn react with the rubber to produce crosslinks. The rate of peroxide cure is controlled by temperature and selection of the specific peroxide, based on half-life considerations (see Initiators, Free-Radical). Although some chemicals, such as bismaleimides, triallyl isocyanurate, and diallyl phthalate, act as coagents in peroxide cures, they are not vulcanization accelerators. Instead, they act to improve cross-link efficiency (cross-linking vs scission), but not rate of cross-link formation. 

The new co-agents revealed high tendency to agglomeration in elastomer matrix what had a beneficial effect on vulcanizates mechanical properties. ApplicatitMi of unsaturated acids and metal oxides as the new co-agents in peroxide vulcanization of HNBR caused mostly a decrease of vulcanization time, considerable improvement of vulcanizates mechanical properties, as well as the increase of cross-link efficiency. Thus, they may be used successfully in rubber technology. 

The interest of that type of material has been largely demonstrated, f.i. by the Hytrel-type of products (i.e. polyether-ester multiblocks), particularly in the field of thermoplastic elastomers thanks to the efficient cross-linking action of the harder blocks. Due again to the swift development of increasingly sophisticated catalytic techniques, more and more original achievements... 

Although radical cations are generated in some electron-irradiated monomers (e.g., vinyl ethers or epoxies), efficient cationic polymerization is not observed. Under certain conditions (addition of iodonium, sulfonium, or sulfoxonium salts, cationic polymerization with the use of electron beam irradiation can be induced. Several studies on radiation cross-linking of elastomers support the concept of ionic mechanism. ... 

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),... 

Peroxides are highly efficient vulcanizing agents for elastomers and plastics. In addition they are used as polymerization catalysts in the coatings industry. Tliese materials are commercially available in several physical forms and container types to suit the needs of users. Sulfur vulcanization of rubber was discovered in the early 1800s. The cross-linking of natural rubber with a peroxide was discovered in 1914 by a Russian, who used benzoyl peroxide. However, benzoyl peroxide vulcanizates have lower strength and poorer heat resistance than do sulfur vulcanizates. In 1950... 

Thermoplastic elastomers (TPEs) exhibit properties characteristic of chemically cross-linked elastomers (vulcanized rubbers) at room temperature but, at elevated temperatures, behave as thermoplastics (thermo from the Greek word 0ep x6i , meaning hot). Consequently, they can soften and flow and, thus, can be processed with high speed, efficiency, and economy on conventional thermoplastic equipment. Furthermore, unlike vulcanized mbber, TPEs can be reused. 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

Peroxides are probably the most common materials used instead of sulfur for rubber vulcanization, because of their ability to cross-link a variety of unsaturated and saturated elastomers and also their ability to produce thermally stable carbon-carbon cross-links. There are also some disadvantages of peroxide cross-linking. The most important are the poor mechanical properties of vulcanizates and low crosslink efficiency [1,2]. The use of co-agents coupled with peroxides to cure elastomers has been a common practice... 

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