As indicated in Figure 10.1, PU adhesives are commercialized in numerous product forms. Each form exists because it caters to the needs of particular industries and customers. The particular technology used bespeaks the mode of adhesive action and the requirements of the application. The global use of formulated PU adhesives according to product form is shown in Figure 10.5.
FIGURE 10.5 Global consumption of polyurethane adhesives by total formulated product.
FIGURE 10.6 Global consumption of polyurethane in polyurethane adhesives.
Of course, the fully formulated product is weighted by the accompanying formulation components that, in the case of solvent-borne adhesives for instance, can actually outweigh the PU polymer component. The distribution representing only the binder component looks quite different (Fig. 10.6).
The large volume of solvent-borne PU adhesive reflects the very large volumes applied for shoe sole construction. This is particularly evident in PU adhesive consumption in China where footwear manufacturing is dominant and estimated 80% of solvent-borne PU adhesive is consumed . The geographical distribution of adhesive consumption by format is complex and changing based on downstream manufacturing. Due to the specialty and customer-specific nature of the applications, the adhesive producers are located close to the assembly operations that utilize the particular adhesive. As manufacturers of finished goods move to more favorable geographies, the adhesive supply and portfolio of adhesive solutions tend to follow.
The large chemical producers supply the adhesive formulators with the typical building blocks used for other PU applications. Due to economic advantage brought by their back integration, most of these large chemical producers have a significant adhesive product offering, either as fully formulated systems, or via component sales to small and large adhesive producers. This situation can result in these companies being in direct competition with their customers in some markets.
While many small formulators have minimal technical capabilities beyond blending and packaging, other formulators are technically sophisticated, highly capable, and well known. Consumers of building blocks for adhesives face the same commercial challenges and must choose from similar building blocks to develop adhesive solutions. The result of this constant evolution has resulted in methylene diphenyl diisocyanate (MDI) and polymeric MDI (pMDI) enjoying approximately three-fourth of the overall PU market for adhesives, and polyethers dominating polyester soft segment volumes by 4:1 (Fig. 10.7). The dominance of MDI/pMDI for
FIGURE 10.7 Global consumption of (a) isocyanates and (b) polyols for polyurethane adhesives formulation.
adhesives reflects the uniform reactivity of the isocyanate groups, low volatility resulting in easier workplace handling, and the fact that most adhesives do not require specific inhibition from light-stimulated degradation such as required for coatings. Coatings exposed to degrading light require an aliphatic isocyanate rather than less-expensive aromatic isocyanate. The dominance of polyether polyols reflects that they are room temperature liquids, offer superior low-temperature flexibility, and are much less sensitive to hydrolysis than polyesters. Polyesters are more common in applications requiring faster set times or initial green strength such as required for some hotmelt adhesives, and highly automated high unit volume applications.
Table 10.1 lists the various PU adhesive formats and representative associated applications. Some of the different formats may be used for the same application and may find favor with specific manufacturers based on customer requirements, environmental-health-safety goals of the manufacturer, and preference of the manufacturer. There has been a general move toward waterborne formulations as manufacturers attempt to reduce the volatile organic compounds (VOCs) in their workplace and customer venues [10-12]. Another characteristic of these categories is that two-part adhesives are higher performing than a corresponding one-part adhesives in terms of thermal and solvent resistance because of the ability to build hard segment volume . The growth of reactive PU hotmelts is recognition of their ability to generate lower viscosity at elevated temperatures, obtain excellent cohesive and adhesive strength as full cure is achieved, and the attractive properties of PU backbones are fully realized [14, 15]. Thus, the bookbinding application has particularly been able to generate customers for reactive hotmelts due to the long-term durability and flexibility of the product.
The large number of adhesive formats, substrates, applications, and application techniques has resulted in high reliance on standardized test methods to facilitate
TABLE 10.1 Major applications for polyurethane adhesives as a function of format
comparisons of adhesives and adhesion. A representative list of standardized test methods is provided in Table 10.2 . Along with the standardized tests, it is also commonplace for industrial adhesive consumers to perform nonstandardized application testing, or to demand the same of their suppliers. Many of the tests lay emphasis on sample preparation, test geometry, and conditions, due to difficulty separating factors associated with overall adhesive performance. The large number of controls can help differentiate confusing variables that can mislead the designer from determining true performance.
The measure of adhesion using standards proscribed in Table 10.2 are the final result of interfacial and rheological forces PUs offer in building a joined system. PUs function well due to their high internal cohesive strength within the adhesive layer, as a result of its block copolymer structure and internal hydrogen bonding. The same polymer material properties contribute to strong interfacial adhesives resulting from the same strong polar and hydrogen bonding interactions. In addition, reactive PU adhesives have the potential to chemically bond across the interface with substrates that offer active hydrogens on the surface. Detailed discussions of the specifics of adhesion and adhesives is available from numerous excellent source books devoted to the subject [17, 18].