Polyurethanes. Mark F. Sonnenschein

structures of the listed commodity polymers are relatively simple repeating units (Figure 1.2). Their simplicity is in part responsible for their high level of utility and low‐cost positions. The plastics industry has generated variants of the structures shown in Figure 1.2 by, for instance, introducing branches, but these complexities do not fundamentally alter the basic polymer structure.

FIGURE 1.2 Illustrative structures of high‐volume commodity polymers.

Polyurethane is the largest volume commodity polymer that cannot be characterized by a simple structure such as that shown in Figure 1.2. Instead, polyurethane represents a class of polymers, and any polymer with a urethane repeat unit is classified as a polyurethane regardless of the other functional or polymer structures incorporated (Figure 1.3).

FIGURE 1.3 The urethane unit within a polyurethane polymer chain.

Specific polyurethane structures used for making mattress foam, insulation foam, or shoe foam can be significantly different from one another and cannot be neatly represented like the structures in Figure 1.2. In fact, even structures of different insulation foams can vary so widely that they also cannot be easily represented by a single structure. Another difference with other commodity polymers is that large‐volume polyurethane applications require the mixing of two reactive liquid components rather than the processing of a pellet into a molded or extruded object. Given these complexities it is remarkable that polyurethanes have developed into a commodity plastic category, and it is testament to the versatility and performance of polyurethanes that they are so difficult to replace in their favored applications.

Polyurethane polymers as a class are made from commodity building block reagents and short‐chain polymers (or oligomers). These building blocks include, for example, the following categories: polyisocyanates, polyethers, polyesters, water, and amines (Figure 1.4). As building block categories they also cannot be represented by unique structures and are denoted by “R” to allow designers to insert any conceivable chemically allowable unit.

FIGURE 1.4 Chemical structures of isocyanate, polyester, and polyether. To make a polyurethane the Rʹ of the isocyanate structure must also have an isocyanate function [1].

The polyurethane unit is easily mistaken for the related polyester, polyurea, or polyamide (nylon) structures (Figure 1.5). In fact, polyureas, polyesters, and polyurethanes are often joined into polyurethane materials and still broadly classified as polyurethane. (Polyamides were not previously a part of polyurethane chemistry because of their vastly different processing characteristics. However, recent literature indicates nascent explorations of urethane–amide hybrids; see Chapter 13.)

FIGURE 1.5 Structures of urea, ester, amide, and urethane functionalities.

As commodity products, polyurethanes have achieved a certain establishment status in academic science. However, activity in polyurethane science shows no sign of abating owing to its high potential for design and innovation [1–18]. Figure 1.6 shows total global publication activity, including patents, journal articles, reviews, meeting abstracts, governmental documents, etc., for the years 1954–2019 and for the period 2013–2019 for all of the commodity plastics named in Figure 1.1. While many plastics exhibit publication activity approximately in proportion to their production, polyurethane publication activity is more than double its production. Figure 1.7 shows polyurethane publishing activity by language for the same periods, demonstrating the explosive growth in materials research in China. The steady growth of activity appears independent of general global economic activity. Figure 1.8 quantifies the kinds of publications over this time period, showing that patent publications predominate but open literature activity is nearly as prevalent. In the first edition of this book it was found that open literature and patent literature were produced at very similar levels.

FIGURE 1.6 Publication activity focused on commodity plastics for (a) 1954–2019 and (b) 2013–2019. The total includes all public literature, patent filings, conference proceedings, and books where the subject focus is the plastic. The total number of publications for all listed plastics is 2.5 million.

FIGURE 1.7 Publication activity in polyurethane science by language from (a) 1954–2013 and (b) 2014–2019. Data demonstrate the recent explosive growth in materials research in China.

FIGURE 1.8 Types of publication (all languages) where the focus of the work is polyurethane polymer properties for 2013–2019. The high level of open literature and patent activity demonstrates the continuing intellectual and commercial interest in these materials. The logarithmic scale may exaggerate the importance of items at the low end of the distribution.

Further exploration of these trends shows (Figure 1.9) that patent activity is broad across technologies and focused on specific applications in the areas of coatings and fibers, and is related to performance in construction applications and fire resistance. Exploration of the coatings applications shows broad use of polyurethanes for fiber sizings, hair colorants, wound dressings, traffic paints, golf ball coatings, and numerous other unrelated fields. In contrast, categorization of topics studied in the open literature related to polyurethane properties tends more toward fundamental physical materials science issues.

FIGURE 1.9 Analysis of (a) patented polyurethane topics during 2013–2019 and (b) open literature topics during 2013–2019 by prevalence. The patents analysis is for patents in English, whereas the open literature analysis reflects activity in all languages.

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