Polydienes ozonization

Table 2 Summary of studies focused on ozonization of polydienes... 

As a result of its saturated polymer backbone, EPDM is more resistant to oxygen, ozone, UV and heat than the low-cost commodity polydiene rubbers, such as natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR). Therefore, the main use of EPD(M) is in outdoor applications, such as automotive sealing systems, window seals and roof sheeting, and in under-the-hood applications, such as coolant hoses. The main drawback of EPDM is its poor resistance to swelling in apolar fluids such as oil, making it inferior to high-performance elastomers, such as fluoro, acrylate and silicone elastomers in that respect. Over the last decade thermoplastic vulcanisates, produced via dynamic vulcanisation of blends of polypropylene (PP) and EPDM, have been commercialised, combining thermoplastic processability with rubber elasticity [8, 9]. 

Reactive intermediates may be captured when working with very high dilution of a reactant, in nonaqueous solvents and at a low temperature. It should be emphasized that at the reaction of ozone with polydienes, the main chain of a macromolecule is broken down ultimately at the site of the original double bond which leads to the deterioration of mechanical properties of rubbers. 

Since 1971 our department is Involved in study of photodegradation, sensitized photooxidation and photostabilization of polymers. A big effort is made in the study of singlet oxygen oxidation of several important polymers, such as polydienes, poly(vinyl chloride), polystyrene and polyesters, reactions of atomic oxygen and ozone with polydienes and photostabilization of these polymers. 

The interest in the reaction of ozone with polydienes is mainly due to the problems of ozone degradation of mbber materials [1-4] and the application of this reaction to the elucidation of the stmctures of elastomers [5-8], It is also associated with the possibilities of preparing bifunctional oligomers by partial ozonolysis of some unsaturated pol5miers [9-12], Usually, the interpretation of experimental results are based on a simplified scheme of Criegee s mechanism of C=C-double bond ozonolysis, explaining only the formation of the basic product, ozonides [13, 14],... 

It was found out that the relative viscosity decreases exponentially upon ozonation of SKD solutions (Figiue 10.1). As the viscosity is proportional to the molecular weight it follows that the polydiene consumption should be described by first or pseudo-first order kinetics. 

Ivan, G. and Giurginca, M. Ozone destruction of some trans-polydienes. Polym. Deg. Stab. 1998, 62, 441. 

EPDM rubbers are modified by 1,2-addition of iV-chlorothiosulfonamides to their olefinic sites (Hopper, 1975,1976 Hopper et al., 1984). Such additions may be carried out in solution or without solvent in an internal mixer or extmder. The solventless reactions are facilitated by added carboxylic acids (Hopper, 1989, 1990) or by certain metal salts of weak acids (White et al., 1990). The modified EPDMs are of interest because of their ability to covulcanize in ozone-resistant blends with polydiene rubbers (Hopper, 1975, 1976 Hopper et al., 1984). Although less fiilly explored, A-chlorothiocarboxamides and imides also react with EPDM to produce modified products that covulcanize in blends with polydienes (Hopper, 1977). 

Most companies now use ethylidene norbornene as termonomer. EPT rubbers contain 15 double bonds per 1000 carbon atoms, whereas 1,4-cw-poly (butadiene) and l,4-cw-poly(isoprene) contain about 250 and 200 double bonds per 1000 carbons atoms, respectively. Consequently, EPT rubbers are much more stable to ozone than are the polydienes. Because of this, 20%-25% of EPR is added to NR or SBR to make tire walls. While 100% EPR tires can be used for automobiles, the work of deformation is too large for 100% EPR tires for trucks Too much heat is developed and the elasticity decreases. 

Degradation by the Action of Ozone. The degradation of polydiene rubbers by the action of atmospheric ozone is characterized by the appearance of cracks on the surface of a finished rubber product. This degradation is caused by direct ozone attack and reaction with the double bond sites of unsaturation in a polydiene rubber. 

Antiozonants are important for rubber products, especially when they are subjected to periodic deformations. Ozone directly destroys the surface of rubber. The attack of ozone on polydienes seems to involve reactions such as below. 

Synthesis of Graft Copolymers from the Ozone Activation of the Polymer Ozone, commonly written as O3, is an inexpensive gas (quite soluble in fluorinated solvents), but it is well known for environmental concerns. The ozonization (or ozonation) of polymers has been investigated by many authors and was recently reviewed [122]. It allows the activation of a wide range of polymers, mainly polyolefins (polyethylene, polypropylene, and PVC) but also PS, polydienes, PDMS, polyurethanes, and finally copolymers [122]. 

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