Prepolymer contamination

We have described a process by which small quantities of foam can be made by the prepolymer method. A number of methods are available to bring the variables involved in prepolymer making under control. We assume that the starting point for such processes is the acquisition of commercial isocyanates, polyols, and additives. Other than for audit purposes, we will assume they arrive with certification that they are of the so-called urethane grade. In the case of polyols, this typically means they contain less than 0.01% water and have good color. Free acids and low metal and chloride contents are important considerations for isocyanates. Manufacturers are well aware of the problems that will arise if these contaminants are not controlled and the materials cannot be used with confidence. 

In contrast to those block copolymers synthesised from styrene in bulk, those synthesised from isoprene and butyl acrylate in emulsion or solution were contaminated by only small amounts of homopolymer. Furthermore, it should be noted that Piirma et al. 74 7S) have turned to the reverse reaction order for preparing poly(styrene-b-MMA), i.e. they synthesised the prepolymer using an azo initiation and the subsequent block copolymer via a peroxide redox initiation. 

The manufacturer of the prepolymer endeavors to supply a low-moisture, low-free isocyanate, gas-free material. Moisture may be introduced with repeated opening of the prepolymer container. Contaminants will form bubbles in the final material. Any moisture or free isocyanates will have an effect on the reaction ratios as well as change the chemistry to a degree. 

Dermatitis can result from using a heat glove (nonimpervious) that has been contaminated with either a solvent or polyurethane prepolymer. The same applies to gloves used in the manufacture of prepolymers. 

Compressed air should have any free water removed to prevent contamination of prepolymers. 

The advantages of the Durham route are (1) contaminating catalyst residues can be removed because the precursor polymers are soluble and can be purified by dissolution and reprecipitation and (2) the precursors can be cast as films or drawn and oriented prior to conversion to the all-trans form of polyacetylene. This allows a degree of control over the morphology of the final product which in the pristine state appears to be fibrous and disordered. Because conductivity increases by alignment of the polymer chains, stretching the film or fiber assists this process and this can be performed using the prepolymer. 

Hq evolution from water as electron source. Chloroplasts entrapped in Hypol 5000 were as active, if not better, in catalysing photo H production as were free chloroplasts. The polyurethane foam-entrapped chloroplasts lost 56% of their photosynthetic electron transport capacity (measured as ferricyanide reduction) whereas the free chloroplasts lost 49% of their activity when stored in light at 25°C. In previous studies with Hypol 3000 prepolymer (Rao et al 19821 the rates of Hp production with foam-immobilized chloroplasts were only 10% of the control. Hypol 5000 is completely free of toluene diisocyanate and toluene diamine - possibly these contaminants in Hypol 3000 may have caused inhibition of chloroplast electron transport. 

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