Polyurethane Chemistry

Polyurethane was invented in 1937 by the German company Bayer, prompted at least in part as an alternative to Nylon, which was effectively protected by patents. It came into general use in the 1950's where it was either used in an expanded form (as polyurethane foam) for mattresses and bedding etc., or used to produce speciality fibres such as Lycra and Spandex (vide supra).

The chemistry underlying all polyurethane synthetic pathways is the reactivity of the isocyanate group. This functionality imparts sufficient high reactivity to the appropriate monomers to polymerise in an acceptable time-scale, whilst remaining sufficiently kinetically inert towards reactions such as nucleophilic attack to allow its use without requiring special handling (Burkus and Eckert, 1958). The two most common routes to polyurethanes are the reaction between a diamine and a bischloroformate1 or, more usually, the reaction between an isocyanate and an alcohol as shown in Fig 3.2. The alcohol may be a simple diol, as in the example in Fig. 3.2 or it may be an oligiomeric hydroxyl-terminated polyester or polyether. The reaction can be catalyzed by a wide range of catalysts, most commonly tertiary amines and certain tin compounds. Although there are a range of kinetic profiles for the reaction and certain monomers react more rapidly than others (e.g. aliphatic diols react more rapidly than phenols) in general the reactions occur on a timescale that allows the synthesis and processing to occur in a single shot in a process known as reaction injection molding.

1 Though not commonly performed on an industrial scale, bischloroformates are accessible through the reaction of alcohols with phosgene as in the following example.

Fig. 3.2 The reaction between a linear diol and a hexamethylene diisocyanate yields a simple linear polymer with some similarities to Nylon a CH, CH,

Isocyanates can be formed in a number of ways in the laboratory; possibly the most well known examples being through rearrangements such as the Curtius and Hoffmann Rearrangements (Harwood, 1992). Industrially however, the major route to such materials is via the reaction of an amide with an isocyanate, Common monomers are shown in Fig. 3.3; these include tolylene diisocyanate (TDI, [1]) prepared via the nitration and subsequent reduction of toluene(either found as the pure 2,4-isomer, or as a mixture of the 2,4 and 26 diisocyanate); and methylene-4,4'-diphenyldiisocyanate (MDI[2]) which is produced from the reaction of aniline with formaldehyde (Saunders, 1973). Aliphatic isocyanides include 1,6- hexamethylene-diisocyanate (HDI, [3]), 3-Isocyanatomethyl- 3,5,5- trimethyl- cycohexylisocyanate commonly known as isophorone Diisocyanate (IPDI, [4]) and the hydrogenated form of MDI known as 4,4'- Dicyclohexylmethane-diisocyanate (or H12-MDI, [5]). Such monomers when combined with the array of available diols offer considerable scope for tailoring properties, for example the aromatic systems impart rigidity to the polymer.

There are concerns that, during in vivo use of polyurethanes based on the some of the isocyanides shown in Fig. 3.3, degradation will occur to form amides, particularly aromatic amides, which may be toxic and/or carcinogenic. This there is considerable interest in the use of monomers such as lysine diiso-cyanate ethyl ester [6], amongst others, since the likely degradation product is lysine, which of course is present naturally. The use of such alternatives however may involve substantial adjustments to the chemistry, as we shall see later.

Though a polyurethane fibre based on 1,4-butandiol was considered for commercial production as a material in the early 1940's, the fibres produced are generally felt to be inferior for commercial reasons to the Nylons2 and their use is restricted to mouldings for small mechanical components such as

2 Polyurethanes are more expensive, generally have a lower melting point, are difficult to dye, and have a rather rough surface texture.



Fig. 3.3 [1]-[6] show the chemical structures of some typical isocyanates used in polyurethane production bearings or gears. However, polyurethanes have found a multitude of other uses, amongst these coatings as in, for example, paints and of course polyurethane foams are important commercial materials.3 These and other applications tend to use hydroxyl-terminated polymers in particular polyesters and polyethers rather than simple glycols. In fact most commercial foams are based on polyethers (Saunders, 1973). Polyurethanes based on biologically compatible monomers such as polycaprolactone [7] have attracted considerable interest (Guan et al., 2005; Ping et al., 2005) for biomedical applications. In terms of fibre production, the success of poly-urethanes lies in the ability to form elastomeric fibres. Polyurethane elastomers are effectively block copolymers; containing alternating "hard" and "soft" segments, a well-known example of this is Spandex, which is manufactured by Du Pont. If the two segments are incompatible then there is the possibility of microphase separation; thus the hard segment in effect acts as

3 A full description of the production of polyurethane foams is outside the scope of this work. However, the procedure involves the formation of cross-linked polymer (by using a trifunc-tional monomer) whilst simultaneously generating gas; this gas may be CO2 generated from the reaction of excess diisocyanate with water, or formed by the use of a blowing agent, a process which may in the past have had unfortunate environmental consequences.







Fig. 3.4 The synthesis of a polyurethane elastomer using a diamine chain extender (Stevens, 1990)

a cross-linker leading to a thermoplastic elastomers. The synthetic route to this type of material involves the reaction of a hydroxyl-terminated poly-ether or polyester with an excess of diisocyanate to form a high molecular weight diisocyanate. This macromonomer is then reacted with a so-called chain extender, which can be a diol, an amine or a hydrazine, to form a high molecular weight system. The soft section is the polyurethane based on the diol while the hard section is actually a polyurea. This process is described in Fig. 3.4.

Polyurethane is a term which can be used to describe many different materials derived in part from isocyanates. It is essential in any discussion to identify the key components. By adapting the fraction and distribution of these components many different materials can be produced.

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook

Post a comment