Saturated Hydrocarbon Polymers

The chlorination of polyethylene, poly(vinyl chloride), and other saturated polymers has been studied [Favre et al., 1978 Lukas et al., 1978 McGuchan and McNeil, 1968 Stoeva and Vlaev, 2000]. The reaction is a free-radical chain process catalyzed by radical initiators. 

Chlorinated poly (vinyl chloride) (CPVC) has increased Tg compared to PVC, and this increases its upper use temperature. Applications include hot- and cold-water pipe as well as pipe for the handling of industrial chemical liquids. Chlorinated polyethylene (CPE) finds use as roofing and other vapor barrier membranes, as pond liners, and as an additive to improve the impact strength of PVC. 

The reaction of polyethylene with chlorine in the presence of sulfur dioxide yields an elastomer containing both chloro and chlorosulfonyl groups 

The commercial products contain one chlorine atom per 2-3 repeat units and about one chlorosulfonyl group per 70 repeat units. The chlorosulfonyl groups allow the elastomer to be vulcanized with metal oxides such as lead or magnesium oxide by the formation of metal sulfonate linkages  

Additional curing is often achieved with sulfur, peroxide, or maleimide formulations. Chloro-sulfonated polyethylene has improved resistance to oil, ozone, and heat compared to many other elastomers. Applications include harrier membranes and liners, surface coatings on fabrics, automobile air-conditioner hose, electrical cable insulation, and spark-plug boots [Andrews and Dawson, 1986], 

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Materials that promote the decomposition of organic hydroperoxide to form stable products rather than chain-initiating free radicals are known as peroxide decomposers. Amongst the materials that function in this way may be included a number of mercaptans, sulphonic acids, zinc dialkylthiophosphate and zinc dimethyldithiocarbamate. There is also evidence that some of the phenol and aryl amine chain-breaking antioxidants may function in addition by this mechanism. In saturated hydrocarbon polymers diauryl thiodipropionate has achieved a preeminent position as a peroxide decomposer. 

Manganese, copper, iron, cobalt and nickel ions can all initiate oxidation. Untinned copper wire can have a catastrophic effect on natural rubber compounds with which it comes into contact. Inert fillers for use in rubbers are usually tested for traces of such metal ions, particularly copper and manganese. The problem is perhaps less serious in saturated hydrocarbon polymers but still exists. 

Another approach was developed by Scott in the 1970 s (7.8) which utilises the same mechanochemistry used previously by Watson to initiate the Kharacsh-type addition of substituted alkyl mercaptans and disulphides to olefinic double bonds in unsaturated polymers. More recently, this approach was used to react a variety of additives (both antioxidants and modifiers) other than sulphur-containing compounds with saturated hydrocarbon polymers in the melt. In this method, mechanochemically formed alkyl radicals during the processing operation are utilised to produce polymer-bound functions which can either improve the additive performance and/or modify polymer properties (Al-Malaika, S., Quinn, N., and Scott, 6 Al-Malaika, S., Ibrahim, A., and Scott, 6., Aston University, Birmingham, unpublished work). This has provided a potential solution to the problem of loss of antioxidants by volatilisation or extraction since such antioxidants can only be removed by breaking chemical bonds. It can also provide substantial improvement to polymer properties, for example, in composites, under aggresive environments. 

The polymerization products of propylene have been observed to be saturated hydrocarbon polymers and terpenelike unsaturated hydrocarbons (Kuhn, 64). The condensation of formaldehyde with phenols and cyclohexanols by means of aqueous hydrogen fluoride has also been observed (Badertscher el al., 65). 

Absorption due to main intermediates such as polymer cation radicals and excited states, electrons, and alkyl radicals of saturated hydrocarbon polymers had not been observed for a long time by pulse radiolysis [39]. In 1989, absorption due to the main intermediates was observed clearly in pulse radiolysis of saturated hydrocarbon polymer model compounds except for electrons [39,48]. In 1989, the broad absorption bands due to polymer excited states in the visible region and the tail parts of radical cation and electrons were observed in pulse radiolysis of ethylene-propylene copolymers and the decay of the polymer radical cations were clearly observed [49]. Recently, absorption band due to electrons in saturated hydrocarbon polymer model compounds was observed clearly by pulse radiolysis [49] as shown in Fig. 2. In addition, very broad absorption bands in the infrared region were observed clearly in the pulse radiolysis of ethylene-propylene copolymers [50] as shown in Fig. 3. Radiation protection effects [51] and detailed geminate ion recombination processes [52] of model compounds were studied by nano-, pico-, and subpicosecond pulse radiolyses. 

H., Satoru, J., Naoki, T., and Tatsuo, T. (1976) Hydroxylated saturated hydrocarbon polymers. Japan Kokai 7661593 Chem. Abstr., (1976) 85,... 

In addition to the vulcanization of diene hydrocarbon polymers using sulfur, other methods of crosslinking hydrocarbon polymers, which do not require a double bond and which do not use sulfur have been developed. Thus, saturated hydrocarbon polymers and, in particular, PE, are crosslinked by reactions resulting from the addition of a peroxide to the polymer at elevated temperatures (6). 

Therefore, the improvement obtained in the solid state properties of a hydrocarbon polymer by crosslinking have been highly appreciated. However, crosslinking causes a tolerable loss in fabricabil-ity. Crosslinking reduces the crystallinity of saturated hydrocarbon polymers, thereby decreasing the stiffness and rigidity of the product. 

The radiation effects on saturated hydrocarbon polymers such as polyethylene and ethylene-propylene co-polymers are almost independent of LET. 

Halogenation of saturated hydrocarbon polymers can hardly be controlled and is frequently assodated with chain degradation phenomena In contrast, the presence of randomly distributed olefinic unsaturations, allows selective halogenation reactions by adopting appropriate conditions. For instance, butyl rubber can be chiorinated or brominated in allylic positions and chloro-butyl or bromo-butyl rubber results The latter polymers are very interesting since they exhibit fast curing rates when sulfur and ZnO are introduced in the formulations. 

Secondary aromatic amines are effective antioxidants in the protection of saturated hydrocarbon polymers (polyolefins) against autooxidation. Their role in the stabilization of unsaturated hydrocarbon polymers (rubbers) is more complex depending on their structure, they impart protection against autooxidation, metal catalyzed oxidation, flex-cracking, and ozonation. The understanding of antioxidant, antiflex-cracking and antiozonant processes together with involved mechanistic relations are of both scientific and economic interest. 

Finally, it is highly desirable to improve the ability to calculate the properties of surfaces and interfaces involving polymers by means of fully atomistic simulations. Such simulations can, potentially, account for much finer details of the chemical structure of a surface than can be expected from simulations on a coarser scale. It is, currently, difficult to obtain quantitatively accurate surface tensions and interfacial tensions for polymers (perhaps with the exception of flexible, saturated hydrocarbon polymers) from atomistic simulations, because of the limitations on the accessible time and length scales [49-51]. It is already possible, however, to obtain very useful qualitative insights as well as predictions of relative trends for problems as complex as the strength and the molecular mechanisms of adhesion of crosslinked epoxy resins [52], Gradual improvements towards quantitative accuracy can also be anticipated in the future. 

The effect of structure on the mechanism of thermal decomposition of saturated hydrocarbon polymers has been studied more recently by Wall and Straus [40]. Linear and branched polyethylene, polypropylene and various copolymers have been investigated and the rates of volatilization compared. 

Acid chloride groups have been introduced that are then treated with amine antioxidant (5). However, all of these techniques require some type of functionality on the polymer chain and will fail with saturated hydrocarbon polymers such as polyethylene or polypropylene. 

Saturated hydrocarbon polymers are also crosslinked by the action of organic peroxides, though branching reduces the efficiency. Polyethylene is crosslinked by dicumyl peroxide at an efficiency of about 1.0, saturated EPR gives an efficiency of about 0.4, while butyl rubber cannot be cured at all. For polyethylene, the reaction scheme is similar to that of the unsaturated elastomers. 

In the process, the unsaturated hydrocarbon monomer, ethylene, is changed to a saturated hydrocarbon polymer, polyethylene. 

Another saturated hydrocarbon polymer that has been studied is polypropylene . The isotactic form (with the methyl groups on one side of the chain) crystallizes the atactic form has insufficient configurational order for crystallization. It has been shown that in relatively highly crystalline isotactic material G(sc)/G(cl) 1.5 —1.8, while in material with low crystallinity (atactic as well as isotactic) a value of ca 0.8 is found. 

W.W. Graessley, R. Krishnamoorti, G.C. Reichart, N.P. Balsara, L.J. Fetters, D.J. Lohse, Regular and irregular mixing in blends of saturated hydrocarbon polymers. Macromolecules 28(4), 1260-1270 (1995)... 

J.K. Maranas, M. Mondello, G.S. Grest, S.K. Kiunar, P.G. Debenedetti, W.W. Graessley, Liquid structure, thermodynamics, and mixing behavior of saturated hydrocarbon polymers. 

It is clear that blends of most polymers with PP are highly incompatible, due to the large phase sizes exhibited by such materials. However, on the basis of chemical similarity, one might suspect that PP would be miscible with other saturated hydrocarbon polymers, such as the other polyolefins, at least under certain conditions. This has been an area of much investigation in the last fifteen years, and this is summarized in this section. 

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