Physical and Chemical Properties

Polyurethanes were first produced and investigated by Dr. Otto Bayer in 1937. Polyurethane is a polymer in which the repeating unit contains a urethane moiety. Urethanes are derivatives of carbamic acids which exist only in the form of their esters (Dombrow 1957). This structure can be represented by the following, generalized amide-ester of carbonic acid:

Variations in the R group and substitutions of the amide hydrogen produce multiple urethanes. Although PU may contain urethane groups, other moieties such as urea, ester, ether or an aromatic may also be included (Saunders and Frisch 1964). The addition of these functional groups may result in fewer urethane moieties in the polymer than functional groups.

The urethane linkage results most readily through the reaction of an isocyanate with an alcohol (Dombrow 1957; Kaplan et al. 1968). The hydrogen atom of the

Table 14.1 Raw materials for synthesis of polyurethane

Polyisocyanate

Polyol

Polyester-type

Polyether-type

Chain extension/ crosslinking agent

2.4-Tolylene diisocyanate 4,4'-Diphenylmethane diisocyanate 1,3-Xylylene diisocyanate Hexamethylene diisocyanate

1.5-Naphthalene diisocyanate

Poly(butylene adipate) Poly(ethylene butylene adipate) Poly(ethylene adipate) Polycaprolactone Poly(propylene adipate) Poly(ethylene propylene adipate)

Poly(oxytetramethylene) glycol Poly(oxypropylene) glycol Poly(oxypropylene)-poly(oxyethylene) glycol

1,4-Butanediol Ethylene glycol 1,3-Butanediol

2,2-Dimethyl-1,3-propanediol Trimethylopropane Glycerol 1,2,6-Hexanetriol hydroxyl group is transferred to the nitrogen atom of the isocyanate (Bayer 1947). The major advantage of PU is that the chain is not composed exclusively of carbon atoms but rather of heteroatoms, oxygen, carbon and nitrogen (Bayer 1947). The simplest formula for PU is linear and represented by:

II II

(-R-O-C-NH-R2-NH-C-O-)n R represents a hydrocarbon containing the alcohol group, R2 is a hydrocarbon chain and n is the number of repetitions. Diisocyanates are employed in PU production reactions because they will react with any compound containing active hydrogen (Dombrow 1957).

For industrial applications, a polyhydroxyl compound can be used. Similarly, polyfunctional nitrogen compounds can be used at the amide linkages. By changing polyhydroxyl and polyfunctional nitrogen compounds, different PU can be synthesized (Dombrow 1957). Polyester or polyether resins containing hydroxyl groups are used to produce polyester- or polyether-PU, respectively (Urbanski et al. 1977). Examples of the raw materials used in the synthesis of PU are summarized in Table 14.1.

Variations in the number of substitutions and the spacing between and within branch chains produce PU ranging from linear to branched and flexible to rigid. Linear PU is used for the manufacture of fibers and molding (Urbanski et al. 1977). Flexible PU is used in the production of binding agents and coatings (Saunders and Frisch 1964). Flexible and rigid foamed plastics, which make up the majority of PU produced, can be found in various forms in the industry (Fried 1995). Using low molecular mass pre-polymers, various block copolymers can be produced. The terminal hydroxyl group allows alternating blocks, called segments, to be inserted into the PU chain. Variation in these segments results in varying degrees of tensile strength and elasticity. Blocks providing rigid crystalline phase and containing the chain extender are referred to as hard segments (Fried 1995). Those yielding an amorphous rubbery phase and containing the polyester/ polyether are called soft segments. Commercially, these block polymers are known as segmented PU (Young and Lovell 1994).

0 0

Post a comment