Crosslinking oxygen

The inhibition time reflects the generation of protons in the system used, see Eq. 5. Usually in cationic crosslinking oxygen has no irtfluence on the crosslinking kinetics Nevertheless, our results show that the inhibition time was influenced by the air pressure (Fig. 7). Moreover, this influence is stronger at higher... 

Detailed studies on radiation chemistry of PEO have been performed [74-77]. Upon y-irradiation, the gel-dose drops abruptly along with an increase in the concentration and molecular weight of the polymer, thus reaching values of 0.15-0.25 Mrad in the range of practical interest [75]. Oxygen is a strong inhibitor and when it is carefully removed from the solution, crosslinking of PEO occurs at doses as low as 0.01 Mrad [76]. 

In die presence of oxygen, more complex thermo-oxidative processes occur in polyesters containing aliphatic moieties. They result in crosslinked products and in the formation of compounds such as aldehydes, carboxylic acids and vinyl esters, as reported in the case of PET.93,94 On the other hand, the presence of oxygen has little effect on the thermal resistance of wholly aromatic polyesters below 550°C. Above this temperature a char combustion process takes place.85... 

Polyester-based networks are typically prepared from polyester prepolymers bearing unsaturations which can be crosslinked. The crosslinking process is either an autoxidation in the presence of air oxygen (alkyd resins) or a copolymerization with unsaturated comonomers in the presence of radical initiators (unsaturated polyester resins). It should also be mentioned that hydroxy-terminated saturated polyesters are one of the basis prepolymers used in polyurethane network preparation (see Chapter 5). 

Urethane alkyds and urethane oils are oil and alkyd resin-modified polyurethanes dissolved in a volatile solvent. Upon application and solvent evaporation, the coating is crosslinked and cured via oxidation by atmospheric oxygen. 

The properties and applications are very variable according to the nature and the ratio of the substituents, the crosslinking reaction and the formulation but one always finds a good fire resistance resulting in a high oxygen index, a ... 

Aryloxyphosphazene copolymers can also confer fireproof properties to flammable materials when blended. Dieck [591] have used the copolymers III, and IV containing small amounts of reactive unsaturated groups to prepare blends with compatible organic polymers crosslinkable by the same mechanism which crosslinks the polyphosphazene, e.g. ethylene-propylene and butadiene-acrylonitrile copolymers, poly(vinyl chloride), unsaturated urethane rubber. These blends were used to prepare foams exhibiting excellent fire retardance and producing low smoke levels or no smoke when heated in an open flame. Oxygen index values of 27-56 were obtained. 

There are over 400 different commercial alkyd resin formulations based on phthalic anhydride used in the coatings business. Alkyd resins for paints are made by reacting phthalic anhydride with a poly-alcohol (usually from naturally occurring sources rather than synthetic) that contains unreacted double bonds. The paint dries by the resin crosslinking through reaction of the double bonds under the influence of oxygen in the air. 

There are examples in which base radicals undergo reaction with adjacent base residues. The 5-(2 -deoxyuridinyl)methyl radical (63, Scheme 8.30) can forge an intrastrand cross-link with adjacent purine residues. Cross-link formation is favored with a guanine residue on the 5 -side of the pyrimidine radical and occurs under low-oxygen conditions. A mechanism was not proposed for this process, but presumably the reaction involves addition of the nucleobase alkyl radical to the C8-position of the adjacent purine residue. Molecular oxygen likely inhibits crosslink formation by trapping the radical 63, as shown in Scheme 8.24. The radical intermediate 89 must undergo oxidation to yield the final cross-linked product 90,... 

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