Hexamethyl diisocyanate

As for the end-linking, an experimental work has already been published for the viscoelastic relaxation of poly(propylene sulfide) star by Nicol et al., they discussed the relation of p and viscoelasticity in detail. (Nicol et al, 2001) It is a 3-arm star polymer and can be end-linked by the reaction with hexamethyl diisocyanate(HMDI) in the presence of a small amount of catalyst of dibutyl tin dilaurate. Also, it is regarded as a network formation of RA3 + R B2 type reaction.(The values are functionalities.) The characteristic features to remark for poly(propylene sulfide) star, abbreviated as PPS star, is low Tg( -37°C). 

In Refs. 125, 126 the viscosity is measured with a magnetic sphere rheometer at decreasing shear rate (its lowest value is 10" s ) in different polymeric systems free-radical copolymerization of styrene-meta-divinylbenzene with solvent and polycondensation of hexamethyl diisocyanate with polyoxypropylene with and without solvent. These experiments yield k = 0.79 in both systems. The standard deviation of experiments performed at different shear rates at the gel points is 0.07 in k. Since t and E are measured in Ref. 126 using the same apparatus and the same sample, T is determined as the time vriiere 9 and E are equal to zero, the precision in Tc being higher than 10" %. 

Materials and Purification. Chemicals were purchased from Aldrich chemical company and used as received unless otherwise noted 1,1,1,3,3,3-hexamethyl disilazane, ethylene glycol, triphosgene, poly(ethylene oxide) (MW = 600), poly(tetramethylene oxide) (MW = 1000), poly(caprolactonediol) (MW = 530), toluene diisocyanate (TDI), anhydrous ethanol (Barker Analyzed), L-lysine monohydride (Sigma) and methylene bis-4-phenyl isocyanate (MDI) (Kodak). Ethyl ether (Barker Analyzer), triethylamine and dimethyl acetamide were respectively dried with sodium, calcium hydride and barium oxide overnight, and then distilled. Thionyl chloride and diethylphosphite were distilled before use. 

Von Burg R Toxicology update hexamethyl-ene diisocyanate. J Appl Toxicol 13 435-439, 1993... 

Isocyanates. The most common isocyanates encountered in polyurethane propellants are 2,4-tolylene diisocyanate, TDI, and hexamethyl-ene diisocyanate, HDI. 3-Nitrazapentane diisocyanate has also been used in high density propellants. In some polyurethane liners Adiprene L (duPont), a NCO-terminated condensation product of poly (1,4-oxybutyl-ene)diol and TDI is used, as well as some of the more highly functional isocyanates, such as polymethylenepolyphenyl isocyanate (Papi, Carwin Chemical Co.). 

Where color and light stability of the adhesive are important, and cure speed or cost is less important, aliphatic isocyanates are frequently used. Adhesives derived from isophorone diisocyanate (IPDI), hexamethyl-ene diisocyanate (HDI), or 4,4 -dicyclo-hexylmethane diisocyanate (H12MDI) are available. 

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