Prepolymer dispersal

Emulsion—Suspension Polymerized Pigment Ink. Polymerization of a polar prepolymer as the internal phase in an oil-based external phase (24) gives a fluorescent ink base in which spherical fluorescent particles are dispersed. This base is suitable for Htho and letterpress inks (qv). An... 

Most general-purpose release agents have been developed for this market in part because of their low toxicity and chemical inertness and do not usually present health and safety problems. Some of the solvent dispersions require appropriate care in handling volatile solvents, and many supphers are offering water-based alternatives. Some of the sohds, particularly finely divided hydrophobic sohds, can also present inhalation problems. Some of the metallic soaps are toxic, although there is a trend away from the heavier, more toxic metals such as lead. The reactive type of release coating with monomers, prepolymers, and catalysts often presents specific handling difficulties. The potential user with health and safety questions is advised to consult the manufacturer directly. 

Two basic steps are needed in order to make a waterborne urethane dispersion a prepolymer step and a chain extension step. 

The waterborne prepolymer process is similar to the prepolymer synthesis described earlier, except that most of the waterborne prepolymers are based on aliphatic isocyanates and contain an internal emulsifier. There are several types of internal emulsifiers, both anionic and cationic. A good summary of these stabilizers is found elsewhere [56], The majority of the waterborne urethanes are anionic dispersions. An internal surfactant, such as dimethylolpropionic acid, is often incorporated into the prepolymer ... 

A tertiary amine such as triethylamine is then added to the isocyanate-terminated prepolymer (containing carboxylic acid groups). The tertiary amine reacts with the pendant carboxylic acid groups, forming a carboxylic acid salt. The presence of this salt, together with adequate stirring, allows the dispersion of the prepolymer in water by the so-called melt dispersion process [57]. 

The block lengths and the final polymer molecular weight are again determined by the details of the prepolymer synthesis and its subsequent polymerization. An often-used variation of the one-prepolymer method is to react the macrodiol with excess diisocyanate to form an isocyanate-terminated prepolymer. The latter is then chain-extended (i.e., increased in molecular weight) by reaction with a diol. The one- and two-prepolymer methods can in principle yield exactly the same final block copolymer. However, the dispersity of the polyurethane block length (m is an average value as are n and p) is usually narrower when the two-prepolymer method is used. 

In the entrapment studies, a concentrated dispersion of the cells was mixed with an equal mass of prepolymer. For adsorption, the foams were prepared, washed, sterilized in an autoclave, then placed in suspensions of the algae for 14 days. The entrapped algae showed no respiration for the majority of the prepolymers including the TDI-based hydrophilic Hypol 2002. Only five of the prepolymers were thought to be acceptable they included the two MDI-based prepolymers (Hypol 4000 and 5000). 

In this regard, polyurethane technology offers a product designer a particular advantage. An aqueous solution or dispersion can be emulsified conveniently with a hydrophilic prepolymer and thus incorporated into the polyurethane matrix. The incorporation is accomplished by covalently bonding within the polyurethane backbone and by entrapping it within the matrix. Both methods are evident in foams produced by this technique. 

The prepolymer is dispersed by weight into the containers. The scales used must be trade certified and filled to the specified weight. This may be carried out either manually or automatically with controlling output from the scales. 

If required, pigment and any other additive may be added and predispersed. The pigments used must be in a dry, nonreactive medium. If the pigments are dispersed in a polyol, the paste may absorb moisture and will react with some of the isocyanate groups. Some yellow pigments need to be added to the prepolymer prior to its final heating, as they disperse poorly. 

Recently, hot melt PSA systems have been introduced and radiation curable PSA systems are at the commercial development stage. High solids (50%-70% by wt.) nonaqueous dispersion acrylic PSA systems have also been reported(1). Unlike the hot melt and radiation cured systems which require new capital outlay in coating head and/or curing (drying) equipment, BFG has developed PSA systems, based on Hycar 2100R reactive acrylic liquid polymers and isocyanate terminated prepolymer, which can be processed at 80% solids (by wt.) with equipment presently used in the PSA industry, namely, the reverse roll and knife-over-roll coater. 

Core-shell polystyrene-polyimide high performance particles have been successfully prepared by the dispersion copolymerization of styrene with vinyl-benzyltrimethyl ammonium chloride (VBAC) in an ethanol-water medium using an aromatic poly(amic acid) as stabilizer, followed by imidization with acetic anhydride [63]. Micron-sized monodisperse polystyrene spheres impregnated with polyimide prepolymer have also been prepared by the conventional dispersion polymerization of styrene in a mixed solvent of isopropanol/2-methoxyethanol in the presence of L-ascorbic acid as an antioxidant [64]. 

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