Basic Chemistry of Polyurethanes

The high reactivity of the isocyanate group with hydrogen active compounds can be explained by the following resonance structures [1-3]  

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In spite of polyurethane is a hazardous polymer, it can be modified through the basic chemistry of polyurethanes, which can modify a wide variety of soft and hard segments, morphological features, thermic and mechanical properties of structures, just by changing several conditions, such as the ratio NCO/OH, the aliphatic or aromatic isocyanate, the molecular weight, and the ester or ether form of the polyol, but especially the nature of the monomer, whether synthetic or natural. Among the natural options than can be used for synthesis are oil, polysaccharides, and amino acids. 

As noted, most commercial polyurethanes are useful because of their physical properties. Except in the field of hydrophilic polyurethanes, little work has been done on the chemistry of polyurethanes. We hope this book will change that to a degree. Until then, however, basic research in this area will require the production of your own polymers. 

This section of the chapter will present the basic reactions found in the chemistry of polyurethane compounds, such as the reaction of isocyanates with polyols, water, and amines. The reactions of isocyanates with urethanes, ureas, and amides are also of significant importance in poljrurethane chemistry as they will lead to an increase in materials choice. 

This area of adhesives technology is obviously where the polymer chemist plays a crucial role but a review of the basic chemistry of adhesives and of the formulation of adhesives is well beyond the scope of the present book. For such information the reader should consult references, such as Skeist s excellent Handbook of Adhesives [1] where these aspects are reviewed in detail for every imaginable chemical type of adhesive. There are also many specialized articles and chapters devoted to particular adhesive types such as those available on natural materials [2], solutions and latices [3], hot-melts [4], polyurethanes [5,6], phenolics [7], epoxies [8,9], acrylics [10,11], imide resins [12,13] and pressure-sensitive adhesives [14,15]. To translate from a particular chemical type of adhesive into a commercially available product reference may be made to the technical data sheets provided by the adhesives companies and to adhesive product directories, such as that produced by Shields [16]. 

Synthesis and Properties. Several polymers containing HFIP-O groups have been investigated, the most common beeing epoxies and polyurethanes. The development of fluorinated epoxy resins and the basic understanding of their chemistry has been reviewed (127). 

The transition metal catalysed addition of HCN to alkenes is potentially a very useful reaction in organic synthesis and it certainly would have been more widely applied in the laboratory if its attraction were not largely offset by the toxicity of HCN. Industrially the difficulties can be minimised to an acceptable level and we are not aware of major accidents. DuPont has commercialised the addition of HCN to butadiene for the production of adiponitrile [ADN, NC(CH2)4CN], a precursor to 1,6-hexanediamine, one of the components of 6,6-nylon and polyurethanes (after reaction with diisocyanates). The details of the hydrocyanation process have not been released, but a substantial amount of related basic chemistry has been published. The development of the ligand parameters % and 0 by Tolman formed part of the basic studies carried out in the Du Pont labs related to the ADN process [1],... 

The preparation of prepolymers and quasiprepolymers allows the production of polyurethane parts by component manufacturers without the large capital outlay required to produce materials from the basic raw materials. The production of any prepolymer requires a good understanding of the chemistry involved. The final quality of the polyurethane product is dependent on the initial control of the chemistry of the system and would be expensive for small operators to carry. 

All industrial polyurethane chemistry is based on only a few types of basic isocyanates. The most significant aromatic diisocyanates are TDI and MD. TDI is derived from toluene. This is initially nitrated to dinitrotoluene, then hydrogenated to diamine, and finally phosgenated to diisocyanate. A defined mixture of isomers comprising toluene-2,4-and 2,6-diisocyanate is obtained. Approximately 1.3 million tons/year of TDI are produced world-wide, most of which is used in the production of polyurethane flexible foam materials. 

Basic polyurethane chemistry was discovered by Otto Bayer in 1937, but polyurethane polymers were first developed as replacements for rubber at the start of World War II. Numerous applications followed including fibres, rigid and flexible foams, mouldings and elastomers (Brydson, 1999). The preparation of polyurethane polymers occurs via a reaction process intermediate between those of addition and condensation (Brydson, 1999). Like addition polymerization, there is no splitting off of small molecules, but the kinetics are otherwise similar to condensation polymerization. 

The basic chemical structure of polyurethane prepolymers is illustrated in Fig. 19. A good overview on polyurethane adhesive chemistry is given by Habenicht [8]. 

The properties of polyurethanes can be varied by changing the type or amount of the three basic building blocks of polyurethane chemistry diisocyanate, short-chain diol, and long-chain diol. Polyurethanes are generally classified by the type of polyol used, for example, polyester polyurethane or polyether polyurethane. 

A pilot plant was opened in the UK by ICI in 1998 to look at the feasibility of chemically recycling polyurethane, their method being a process called split-phase glycolysis [8]. Like hydrolysis, the method is complicated by the presence of by-products. Commercial depolymerisation units using the glycolysis of polyurethanes operate in Germany, Austria and Denmark [4]. The requirement for separation can be avoided in this case by further chemical reactions. The basic chemistry involved in these reactions is presented by Ehrig [9] for those who require further details. 

Polyurethane in the rubber industry can be used in (1) the thermoplastic elastomer form (TPE), discussed earlier, (2) a two-part liquid system in reaction injection molding (RIM), (3) the cast molding of rubber parts, or (4) as a millable gum that can be processed on a two-roll mill and cured with agents such as peroxides or sulfur, just as with conventional rubber. Although different versions of polyurethane elastomers must be tailor-made for each of these four common applications, the basic chemistry used is very similar in all. 

FIGURE 6.7 Basic scheme of the Vertifoam vertical foaming process to obtain flexible PU foams. (Adapted from Ashida, K., 2007, In Polyurethane and Related Foams. Chemistry and Technology, ed. Ashida K., 67-82. New York CRC Press/Taylor Francis.)... 

This chapter introduces readers to the fundamentals of polyurethane chemistry, the basics of polyurethane adhesives, their applications and test standards. The chapter will also highlight relevant developments that have led formulators to create and tailor-make many polyurethane (PUR) adhesives and sealant products. 

The basic chemistry is the reaction of polyol and diisocyanate to give a crosslinked polyurethane (see Sections IV.A and IV.B). 

Basically, the chemistry of making flexible foams is similar to that used to prepare polyurethane elastomers and rigid foams. However, the chemistry has some added features and requirements for control over the reaction processes that are particularly severe (20). 

The real challenge in polyurethane foam formation is to control the chemical and physiochemical processes up to the point where the material finally sets. The sequence and the rate of the chemical reactions are predominately a function of the catalyst and the reactivity of the basic raw materials, polyol and isocyanate. The physiochemical contribution to the overall stability and processability of a system is provided by the silicone surfactants. Optimum foaming results will be achieved only if the correct relationship between chemistry and physics exists [4]. 

The basic chemical reactions, used to synthesize monomer and polymer resins and the chemistry involved in the use of curing agents to polymerize the resins, have been extensively studied and are well documented. This section serves as a summary only of those polymers that are primarily used in adhesives formulations for electronic applications. Among these polymers are the epoxies, silicones, polyurethanes, polyimides, acrylates, cyanate esters, and cyclo-olefins. Further technical detail for these polymers maybe acquired through literature searches in the transactions of the American Chemical Society (Polymer Group), Society of Plastics Engineers (SPE), and the Society for the Advancement of Materials and Process Engineers (SAMPE). 

A full account of basic polyurethane science will be found in PST 3. The process used is reaction injection moulding, and in the next section we shall see how the chemistry given in PST 3 is adapted to the process for the manufacture of shoe soles. 

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