Nonreactive Hotmelt Adhesive Hotmelt adhesives are thermoplastic materials that (i) are heated and applied to a substrate in the molten state, (ii) the bonding substrates are assembled, and (iii) the thermoplastic resolidifies back to its thermoplastic state. If the substrate is wet by the adhesive, if the polar interactions across the adhesive-substrate interface are of sufficient number and strength, and if the adhesive has adequate cohesive energy, a durable bond can be established. TPUs have been employed for this application and have a market appeal, particularly for wood bonding and shoe sole operations. Polyester polyols are often used in these applications since crystallization of the soft segment can speed up TPU resolidification and therefore increase the rate of adhesive strength build. Simple TPU hotmelt adhesives usually suffer in performance comparison to high-performance polyamide and polyester hotmelt adhesives. This is ascribed to the higher viscosity of TPU at standard application gun temperatures that limit substrate wetting. It is possible to reduce viscosity and thereby improve hotmelt adhesive properties by addition of a monol (e.g., replacing some amount of butanediol with 1-butanol). Substitution of even 5% of the butanediol chain extender with 1-butanol can reduce molecular weight of the chain limited polymer by a factor of 2-3 and reduce the viscosity of the polymer melt by over an order of magnitude based on the a Mn35 rule . This also can slightly reduce the overall polymer tensile performance by reducing hard segment formation and phase separation. An example of a PU hotmelt based on this design is provided in Table 10.4. The measures of quality for a hotmelt adhesive are unique to the technology reflecting viscosity of a polymer melt, and creep properties under a shear load. In addition, it is common to characterize the strength build of a hotmelt adhesive referred to as its "green strength" and its glass transition temperature.
Reactive Hotmelt Adhesive Reactive PU hotmelt adhesives are a technology introduced in the 1990s [26, 27]. It is essentially a low-percentage isocyanate prepolymer that is applied to a substrate and allowed to cure by reaction via adventitious moisture, or by direct reaction with substrates possessing active hydrogen on their surface. Reactive hotmelts possess a highly desirable grouping of properties including low viscosity at application temperatures, rapid generation of green strength when using
TABLE 10.4 Formulation and properties of a nonreactive polyurethane hotmelt adhesive
TABLE 10.5 Formulation and properties of a reactive polyurethane hotmelt adhesive
crystalizable soft segments, gradual development of very strong adhesive bonding, no VOCs, and the excellent material properties of PU elastomers. These properties and performance have made PU reactive hotmelts very competitive with polyamide and polyester hotmelts, and allowed this PU adhesive class to grow at twice the rate of PU s as a whole . Prepolymers and their preparation are covered in Chapters 2 and 9. Table 10.5 provides the composition and properties of a 4000g/mol polycap-rolactone4)ased MDI prepolymer with 2.3% isocyanate useable as a reactive PU hotmelt adhesive. Polyether polyols can also be used to make very good reactive hotmelt adhesives, especially when the prepolymers are made of blended molecular weights to provide the mix of low viscosity and rapid green strength build.