Secondary Polyurethane Dispersions

Another important class of materials used for OEM coatings are aqueous polyurethanes due to their versatility in their properties [74-80]. Since the introduction of polyurethane dispersions in 1960s, they have enjoyed considerable interest and commercial acceptance. Polyurethane dispersions can be classified into three main groups  

In the first step of the synthesis, a conventional polyether- or polyester-based isocyanate-terminated prepolymer is obtained by condensation polymerization of a diol and a diol containing a carboxyl function, preferably reacting the hydroxyl groups of dimethylol propionic acid with isocyanate groups. In the next step, the carboxyl groups are neutralized with an amine which is subsequently dispersed in water and 

The blending of resins is a simple and useful technique for improving paint properties. While water-borne acrylic resins and polyurethanes have been widely used as polymers for automotive coatings, both water-borne resins are inferior to corresponding solvent based counterparts because of hydrophilic functional groups or surfactants which are introduced to impart dispersion stability to these resins. Table 7-6 shows some of the advantages and disadvantages of polyurethane and acryhc resins. 

The most widely utiHzed technique for making hybrids is to free radically polymerize a combination of monomers in the presence of a pre-formed polymer which may or may not be intrinsically dispersible. If the preformed polymer is water dispersible, it can be used directly as a seed for subsequent free radical polymerization. 

Polyurethanes Mar resistance Elongation Softness and adhesion Cost 

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Cationic polyurethane dispersions can be prepared from these prepolymers after quaternization or neutralization. 2-Alkyl-2-dialkylamino-methyl-1,3-propanediols are readily available by nucleophilic ring opening of corresponding oxetones with aliphatic secondary amines (Fig. 10.5). 

Polyurethane dispersions are secondary dispersions typically produced by polymerization of isocyanates and diols in organic solvent After polymerization water is added followed by solvent removal. The polyester-polyol component can be designed to form crystalline structures (Fig. 8-25), which make a significant contribution to the internal strength. 

The recycling of contaminated polyurethane waste is described with reference to a glycolysis process developed by Bayer and Daimler-Benz in which the wastes from three-layer composite instrument panels are treated. The reuse of contaminated PU is achieved by dispersely integrating the impurities in the secondary polyol during the glycolytic dissociation. The impurities are modified in such a way that they act as a filler in the secondary polyol. 3 refs. 

In contrast to the process of creating a secondary dispersion as was used for the preparation of, e.g., polyurethanes and epoxide resins, it was shown that the miniemulsion polymerization process allows one to mix monomeric components together, and polyaddition and polycondensation reactions can be performed after miniemulsification in the miniemulsified state [125]. 

One of the ways of improving wear resistance and reliability of sealing elements is filling of porous semis by components able to form a secondary porous system [146]. For example, a blank of a polyurethane foam is impregnated with a mixture of dispersed PE with an inhibited lubricant. The formation of a gel under certain temperature regimes is accompanied by phase distribution within the material. The contact surface of the blank is cleaned by acetone to remove the lubricant. As the acetone evaporates, the prepared zones are zinc-plated and pores freed from the lubricant become filled with zinc. This makes the coating adhere strongly to the surface and allows zinc to penetrate to a depth acceptable for wear limits. 

Glue Polyurethanes polyester polyols Secondary dispersions in water Crystalline parts give high strength... 

An approach to this problem is by the appropriate modification of acrylic primary dispersions to suit automotive coating requirements or solvent-free secondary dispersions of conventional resin types Hke epoxies, polyesters, polyurethanes. Today, the latest water-borne coatings are much more robust in terms of usage or appHca-tion friendhness and require significantly less heating or air-conditioning than two decades ago. They still require some additional dehydration or special kinds of flashes before going into ovens to help remove the water. 

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