Coatings prepolymer technique

This prepolymer technique is frequently used in the manufacture of polyurethane elastomers, coatings, sealants, flexible foams, monocomponent polyurethanes, etc. 

For example, in practice, polyethers with an alkaline ion content of 50-400 ppm are used successfully. This is possible because in rigid PU foam production the one shot technique is used predominantly. The prepolymer technique is used to a small extent for one component rigid PU foams, used as sealants or in coatings. In this case the polyol needs less than 2 ppm potassium ion (for example propoxylated glycerol), in order to avoid the gellification of the prepolymers, due to the trimerisation of -NCO groups catalysed by K+ ions. 

The basic poly(phenylene vinylene) (PPV) polymer is commonly prepared by the sufonium prepolymer route developed by WessUng and Zimmerman in 1968 but much modified by subsequent workers. The synthesis starts from 1,4-bis(chloromethyl)benzene, via the bis-sulfonium salt formed by reaction with tetrahydrothiophene, and then polymerisation is effected to give the prepolymer by reaction with lithium hydroxide (Figure 3.39). Because of the inherent insolubility of PPV it is this prepolymer that is used to form the film coating on the substrate, for example by using a doctor blade technique. The prepolymer is converted into PPV on the substrate by heating in an oven under vacuum at 200 °C for 8-10 h. 

Once the fiber is sufficiently cool, e.g. below 80 C (0, it can be coated, generally with one or two layers of organic polymers. Because the application method must not damage the glass surface, the polymers are applied in the liquid state, commonly as reactive prepolymer or hot melt formulations. The coating diameter and concentricity are monitored and controlled via suitable techniques (5 ). Once applied the coating must solidify very rapidly, before the fiber... 

On this substructure a thin dense layer (in the range of 0.5 to 10 pm thick) is coated that has a very high separation capability. Different coating techniques are in use, most commonly a solution of the respective polymer in an appropriate solvent is spread onto the porous substructure. The solvent is evaporated, followed by further treatment to effect crosslinking of the polymer. Photosensitive, solvent-free prepolymers may be used for coatings that are later crosslinked by irradiation, e.g. with UV-light or electrons. 

Prepolymers are soluble in several solvents, including acetone, methyl ethyl ketone, ethyl acetate and toluene, where with their curatives they react much more slowly in solvent than as solventless simple mixtures, and the solutions can be used for spraying, brushing, knife- or roller-coating and dipping. Whichever technique is used, an oven cure for 1 h at 100°C is... 

This technique can be used to determine the unreacted or free TDl present in urethane prepolymer as a result of incomplete reaction or the use of non-stoichiometric quantities. The free TDI in the urethane prepolymer is thermally expelled within the vaporizing chamber of a gas chromatograph, but below the decomposition temperature of the parent or matrix material. Subsequently, on a column coated with silicone gum rubber, it is... 

In contrast, the preparation of hydrogels from prepolymers via cross-linking condensation reaction offers a simple method for the preparation of films/mem-branes and coated surfaces. Here, in addition to the aforementioned laboratory equipment, a spin coater is necessary as well as a cabinet for drying and curing. The use of inert gas techniques is not required, since the underlying reaction is not affected by oxygen. 

As mentioned above, the technique of minimizing the variability of isocyanate-polyol systems by using an excess of the isocyanate has limitations. An alternative approach is to employ isocyanate-terminated pre-polymers (of the type described in section 16.7.1(b)) to cross-link the polyol. Since these prepolymers are themselves film-forming materials, the use of a stoichiometric excess in a two-component system is not detrimental to the final film. This technique is applicable to the production of both flexible and hard coatings the former are suitable finishes for leather and rubber whilst the latter find use as sanding sealers. 

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