Polyisobutylene degradation

Some polymers like PE and NR get cross-linked on exposure to radiation while others like those based on vinylidene polymers, e.g., polymethylmethacrylate (PMMA), polyisobutylene, degrade. Certain other types of polymer stmctures (high aromatic content or thermoset) resist degradation by high-energy radiation. Coating polymers usually contain acrylic, methacryUc, or fumaric vinyl unsaturation along or attached to the backbone. 

Although it is well established that polyisobutylene degrades under radiation, the main-chain scission radicals were never observed as primary radicals in any of these ESR studies. A possible explanation is that the two free radicals formed by chain scission are unable to migrate from the reaction site. The two end-group radicals are then likely to react with each other by either recombination or disproportionation. 

Porter, R. S. and Johnson, J. F., Polyisobutylene degradation in laminar flow Composition and shear variables, J. Appl. Phys. 35 3149 (1964). 

Wiindrich, K. (1974) The dependence of radiation-induced polyisobutylene degradation on molecular mobilities. Eur. 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

Antony, P., Puskas, J.E., and Kontopoulou, M. The Rheological and Mechanical Properties of Blends Based on Polystyrene-Polyisobutylene-Polystyrene Triblock Copolymer and Polystyrene. Proceedings of MODEST, International Symposium on Polymer Modification, Degradation and Stabilization, Budapest, Hungary, 2002. 

Polyisobutylene and IIR have chemical resistance expected of saturated hydrocarbons. Oxidative degradation is slow and the material may be further protected by antioxidants, for example, hindered phenols. 

The photo-cross-linkability of a polymer depends not only on its chemical structure, but also on its molecular weight and the ordering of the polymer segments. Vinyl polymers, such as PE, PP, polystyrene, polyacrylates, and PVC, predominantly cross-link, whereas vinylidene polymers (polyisobutylene, poly-2-methylstyrene, polymethacrylates, and poly vinylidene chloride) tend to degrade. Likewise, polymers formed from diene monomers and linear condensation products, such as polyesters and polyamides, cross-link easily, whereas cellulose and cellulose derivatives degrade easily. ... 

Upon irradiation, polyisobutylene and its copolymers tend to degrade. There are a large number of studies determining the nature of the process and mechanisms, and it was established that it involves formation and reaction of free radicals. The radical concentration increases linearly with a dose up to 100 Mrad (1,000 kGy). ... 

Degradation is the predominant process in irradiation of polyisobutylene. Generally, the scission yield increases with temperature in accord with the temperature dependence of the formation of unsaturation. For... 

Nitrous oxide reduces the amount of radiation degradation of polyisobutylene (5), though this is a typical polymer which degrades under radiation. 

Polyisobutylene. The solution viscosity of an irradiated polyisobutylene block was measured in CC14 at 30 °C. to determine the degree of degradation (5). The variation of viscosity-average molecular weight, Mv, with the dose, r, is shown in Figure 3. Nitrous oxide reduced the... 

Polyisobutylene. The viscosity of irradiated polyisobutylene is plotted against N20 pressure in Figure 5. The curve increases monotonically with increasing pressure. If nitrous oxide is effective in reducing the amount of degradation, the behavior of the curve seems to be reasonable since the gas concentration in the polymer solid should increase with the gas pressure. 

From industry s viewpoint, the action of nitrous oxide should be useful to cut down the dose required for the crosslinking of polyethylene or polypropylene and to keep polyisobutylene from degradation. However, it is a question whether the complexity of processing caused by using nitrous oxide would pay economically. 

Polyisobutylene. The polymers which we have dealt with until now are of the type which mainly crosslink under the influence of radiation in vacuo. Polyisobutylene with one tetrasubstituted carbon in each repeat unit can be considered as the simplest hydrocarbon polymer of the other type—i.e., the polymers degraded by radiation. 

M.R. Grimbley and R.S. Lehrle, The degradation mechanism of polyisobutylene Part 2. characterisation of the products and the dependence of their yields on sample thickness provides detailed mechanistic information, Polym. Degrad. Stab., 48(3) 441-455,1995. 

T. Sawaguchi and M. Seno, Thermal degradation of polyisobutylene effect of end initiation from terminal double bonds, Polym. Degrad. Stab., 54(l) 33-48, October 1996. 

T. Sawaguchi and M. Seno, Detailed mechanism and molecular weight dependence of thermal degradation of polyisobutylene, Polymer, 37 (25) 5607-5617,1996. 

T. Sawaguchi, T. Ikemura, and M. Seno, Effect of molecular weight on formation of non-volatile oligomers by thermal degradation of polyisobutylene and its kinetic analysis, Polymer, 37(24) 5411-5420, November 1996. 

PB. Sulekha, R. Joseph, and K.E. George, Studies on polyisobutylene bound paraphenylene diamine antioxidant in natural rubber, Polym. Degrad. Stab., 63(2) 225-230, February 1999. 

Somewhat similar conclusions were drawn by Ayrey, and Turner (104) from their work with the effect of scavengers and other additives on the radiation degradation of polyisobutylene. When the polymer is irradiated in monomer the side chain macroradicals can terminate by... 

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