**1.1. History of polyurethane**

The discovery of polyurethane [PU] dates back to the year 1937 by Otto Bayer and his coworkers at the laboratories of I.G. Farben in Leverkusen, Germany. The initial works focussed on PU products obtained from aliphatic diisocyanate and diamine forming polyurea, till the interesting properties of PU obtained from an aliphatic diisocyanate and glycol, were realized. Polyisocyanates became commercially available in the year 1952, soon after the commercial scale production of PU was witnessed (after World War II) from toluene diisocyanate (TDI) and polyester polyols. In the years that followed (1952-1954), different polyester-polyisocyanate systems were developed by Bayer.

Polyester polyols were gradually replaced by polyether polyols owing to their several advantages such as low cost, ease of handling, and improved hydrolytic stability over the former. Poly(tetramethylene ether) glycol (PTMG), was introduced by DuPont in 1956 by polymerizing tetrahydrofuran, as the first commercially available polyether polyol. Later, in 1957, BASF and Dow Chemical produced polyalkylene glycols. Based on PTMG and 4,4'-diphenylmethane diisocyanate (MDI), and ethylene diamine, a Spandex fibre called Lycra was produced by Dupont. With the decades, PU graduated from flexible PU foams (1960) to rigid PU foams (polyisocyanurate foams-1967) as several blowing agents, polyether polyols, and polymeric isocyanate such as poly methylene diphenyl diisocyanate (PMDI) became available. These PMDI based PU foams showed good thermal resistance and flame retardance.

In 1969, PU Reaction Injection Moulding [PU RIM] technology was introduced which further advanced into Reinforced Reaction Injection Moulding [RRIM] producing high performance PU material that in 1983 yielded the first plastic-body automobile in the United States. In 1990s, due to the rising awareness towards the hazards of using chloro-

alkanes as blowing agents (Montreal protocol, 1987), several other blowing agents outpoured in the market (e.g., carbon dioxide, pentane, 1,1,1,2-tetrafluoroethane, 1,1,1,3,3 pentafluoropropane). At the same time, two-pack PU, PU- polyurea spray coating technology came into foreplay, which bore significant advantages of being moisture insensitive with fast reactivity. Then blossomed the strategy of the utilization of vegetable oil based polyols for the development of PU. Today, the world of PU has come a long way from PU hybrids, PU composites, non-isocyanate PU, with versatile applications in several diverse fields. Interests in PU arose due to their simple synthesis and application protocol, simple (few) basic reactants and superior properties of the final product. The proceeding sections provide a brief description of raw materials required in PU synthesis as well as the general chemistry involved in the production of PU.

Polyurethane: An Introduction 5

NCO

NCO

NCO

**NDI**

OCN NCO

**HDI**

isocyanate; para substituted aromatic diisocyanates are more reactive that their ortho analogs primarily attributed to the steric hindrance conferred by the second –NCO functionality. The reactivities of the two-NCO groups in isocyanates also differ with respect to each other, based on the position of –NCO groups. For example, the two-NCO groups in IPDI differ in their reactivity due to the difference in the point of location of –NCO groups. TMXDI serves as an aliphatic isocyanate since the two isocyanate groups are not in conjugation with the aromatic ring. Another isocyanate of increasing interests is vinyl terminated isocyanate since along with the –NCO group, the extra vinyl group provides

OCN NCO

OCN

**IPDI H12MDI**

R N C O R N C O

<sup>R</sup> <sup>N</sup> <sup>C</sup> <sup>O</sup> <sup>R</sup> <sup>N</sup> <sup>C</sup> <sup>O</sup>

sites for crosslinking (Figure 2).

OCN NCO CH3

**2,4-TDI 2,6-TDI**

OCN NCO

**4, 4'-MDI**

OCN NCO

or

**Figure 1.** Common isocyanates

**Scheme 1.** Resonance in isocyanate

H3C
