Polyurethane

Chemical Properties

Polyurethane foams are resistant to a wide range of solvents. In this respect, polyester foams are generally superior to polyether foams, particularly in resistance to dry cleaning solvents. Polyurethane foams are subject to degradation by aqueous acids and alkalis and steam. Ester, amide and urethane groups represent sites for hydrolytic attack. Since the ether group is not readily attacked, polyether foams are generally more resistant to hydrolysis than polyester foams.

Chemical Properties

clear solid, white powder or milky suspension

History

Polyurethanes are an immensely versatile class of polymers used in insulators, foams, elastomers, synthetic skins, coatings, adhesives, and so forth. Polyurethane was first developed through essential diisocyanate polyaddition reactions by Dr. Otto Bayer and partners. In 1937, it reached industrial-scale synthesis and was established in the market in the 1950s[1].

Uses

Prosthetic aid (internal bone splint).
Polyurethane resins and foams are two important industrial polymers. They can be produced as rigid, semirigid, or elastic foams or resins, which give PUR many versatile commercial uses. Polyurethane can be found in furniture, bedding material, automotive sealing material, adhesives, carpet, packaging material and coatings, and many other products. It is favored industrially because of its resistance to oil, light, and solvents, in addition to its strength and flexibility. These polymers are formed by polyaddition reactions between a diisocyanate and a polyhydroxyl compound, such as a polyol.

Uses

Several isocyanates (tolylene diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, etc.) are used in preparing polyurethanes. All are lowviscosity liquids at room temperature with the exception of 4, 4
- diphenylmethane diisocyanate (MDI), which is a crystalline solid. The aromatic isocyanates are more reactive than the aliphatic isocyanates and are widely used in urethane foams, coatings, and elastomers. The cyclic structure of aromatic and alicyclic isocyanates contributes to molecular stiffness in polyurethanes.

Uses

Flexible polyurethane foams are open-cell structures which are usually produced with densities in the range 24-48 kg/m3 (1.5-3Ib/ft3). The major interest in flexible foams is for upholstery applications and thus the loadcompression characteristics are of importance.

Preparation

Polyurethane foams are produced by forming a polyurethane polymer concurrently with a gas evolution process. Provided these two processes are balanced, bubbles of gas are trapped in the polymer matrix as it is formed and a cellular product results. The matching of the two reactions is essential for the formation of satisfactory foams. If the evolution of gas is too rapid, the foam initially rises well but then collapses because polymerization has not proceeded sufficiently to give a matrix strong enough to retain the gas. If polymerization is too fast, the foam does not rise adequately.
By selection of appropriate reactants, it is possible to prepare foams of varying degrees of cross-linking. Slightly cross-linked products are flexible whilst highly cross-linked products are rigid. Both flexible and rigid polyurethane foams are of commercial importance.

Production Methods

Castable polyurethane elastomers are fabricated from polyurethane prepolymers, which are obtained by reacting an excess of diisocyanate with high-molecular-weight diols. These NCO-terminated oligomers (prepolymers) are commercially available under a variety of trade names and in numerous types depending upon the types of diisocyanates and polyols that are used to synthesize them. The NCO content of these prepolymers can vary from less than 3% to as much as 20%. A cast polyurethane fabricator mixes these liquid prepolymers with approximately stoichiometric quantities of a curing agent (or a blend of curing agents) such as an appropriate low-molecular- weight diol or diamine.
Generally a prepolymer is heated to reduce its viscosity before mixing it with a liquid or a molten curing agent. The prepolymer curing agent blends have a limited working time (pot life) during which they are still liquid and can be poured into molds. The liquid prepolymer/curing agent blend is degassed and then poured into molds, which are often heated to expedite curing. After curing, the solid polyurethane elastomer articles are removed from the mold and are sometimes finished by keeping them at elevated temperature to complete the cure and to maximize mechanical properties.

brand name

Ostamer (Marion Merrell Dow).

Hazard

Evolves toxic fumes on ignition.

Safety Profile

Questionable carcinogen with experimental tumorigenic data. When heated to decomposition it emits acrid toxic fumes of CNand NOx.

References

[1] Sonnenschein, M. “INTRODUCTION TO POLYURETHANE CHEMISTRY.” 2014. 0.