Single-pack polyurethane

Polyurethanes are frequently used in anticorrosion coatings. The R D drive of most coatings and resin producers is the reduction of the solvent content of the paint and resin systems. One method of achieving the environmental aims demanded by legislation in the PU coatings industry is to use oxazolidines which are suitable as latent hardeners to enable production of single-pack moisture activated PUs. The isocyanate chosen for this research is tetramethylxylene diisocyanate, which has been available commercially only since 1988. 19 refs. 

Single component systems rely in most cases for the cure to take place by the reaction of moisture in the air with the prepolymer to form a solid polymer. Carbon dioxide gas is given off during this reaction and either escapes into the air or is trapped by fillers in the system. These systems are used to make waterproof barriers and single pack polyurethane paints. 

Aqueous polyurethane systems have been developed which are considered to be safe from a health and safety point of view. They do not contain harmful isocyanates. The major application of this type of system is for coatings and floor finishes. Aqueous systems are based on single-pack, clear-water-based polyurethane. Two-pack and pigmented systems are also available. 

Polyurethane coatings are used as a single-pack system or a two-pack system. There are three basic types of single-pack systems and two of the two-pack system. More recently water-borne urethanes have been developed to reduce solvent emmission. 

Aircraft coatings normally consist of two coats. The topcoat used for civil aircraft is a high-gloss two-pack polyurethane product, cured with aliphatic isocyanates. Single-coat systems based on thermoplastic acrylic resins have been used for military aircraft, but are being replaced by polyurethane topcoats [11.10]. 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

A model for the chain conformation and packing of diphenyl methane 4,4 -di-isocyanate butane diol (M) hard segments in polyurethane elastomers" has been made based on the single crystal structure of the monomer unit, Me-M-Me in which... 

---