Polyurethane adhesives classification

Owing to the various original compounds to which the reactive isocyanate and hydroxide groups are chemically bound, a variety of polyurethane adhesives are available, which will be briefly described below. A classification is to be found in Figure 4.3. 

The second major classification of common polyurethane adhesives is the two-component system. Two-component polyurethane adhesives are widely used where fast cure speeds are critical, as on OEM (original equipment manufacturers) assembly lines that require quick fixture of parts, especially at ambient or low bake temperatures. Two-component urethanes are required in laminating applications where no substrate moisture is available or where moisture cannot penetrate through to the adhesive bond. Two-component urethanes are also useful where CO2 (generated by a one-component moisture cure) or a volatile blocking agent would interfere with the adhesive properties. 

Lamination (sticking together of layers) of fabric, foam and film, in various combinations, can be performed using latex- and solvent-based or fusible adhesives (see Adhesive classification). In addition, adhesives are sometimes applied from solution or latex for subsequent reactivation by heat during lamination. Acrylic dispersions are the main type of latex-based adhesives used in the laminating industry butadiene rubbers and polyurethanes are also utilized in smaller quantities. The latex-based adhesives are used for aesthetic reasons and also on heat-sensitive or open-structured fabrics. The solvent-based adhesives, for example rubbers and polyurethanes, are used in applications similar to those for latex adhesives. 

The thermoplastic polyurethane elastomers (Chen et al 1992) exhibit very similar morphologies to that of the SBS or SIS and, similarly to them, allow the characteristics of both phases, crystalline and glassy, to be combined. Polyurethane adhesives can also be classified in the Adhesives implemented by Chemical Process category that can induce certain confusion. To be consistent with our classification one must consider that two types of polyurethane-based adhesives can be commercially available the ones that exhibit semicrystalline properties that can be implemented via a physical process, and the seconds with dormant reactive functions that can react via a chemical process to lead to a network. 

Polyurethanes are produced by the chemical action of di-isocyanate and polyol. The properties can be varied by the type of isocyanate used and the proportion of the two monomers. There are four main groups of classification for the thermoplastic groups of polyurethane, i.e. rigid foam, flexible foam, non-cellular and cellular polymers. Two main isocyanates used are toluene di-isocyanate (TDI) and diphenylmethane diisocyanate (MDI). Polyurethanes have limited application in the pharmaceutical or medical industries. Polyurethane is used as an adhesive for laminations (thermosetting material). Like thermosetting polyurethane, thermoplastic polyurethanes can be found as esters and ethers. 

EN 15425 2008, Adhesives, one-component polyurethane, for load bearing timber structures — Classification and performance requirements. 

Additionally, the classification of a product that has both adhesion and sealing capabilities is obscure. For this reason, the classification does not have a well-defined demarcation line. In some respects, the physical properties of urethane adhesives differ from those of urethane sealants. In line with that, this section presents the main types of polyurethane sealants and adhesives, their characteristics, advantages and disadvantages according to the form in which they are found, i.e. 100% solids, solventbome, waterborne, and the cure characteristics whether one or two-component material. 

As previously stated, classification of polyurethanes does not have a well-defined demarcation line. However, in order to include the principal urethane adhesive types in this section, they will be explained as depicted in Table 15. Following... 

Due to its richness and diversity, this class of compounds, that results from the polycondensation of di- or multi-isocyanates and of di- or polyols (of di- or polyamines the case of the polyureas), is one of the hardest to place in a classification. Indeed, polyurethanes are just as much elastomers, presented in solution, possibly in emulsion or in the form of structures that are self cross-linking (through atmospheric humidity), as they are structural adhesives, where the polymer networks are obtained from two-part and one-part formulations (with hidden isocyanate functions). 

A wide variety of polyurethanes exist and many of these have been used or contemplated for use in medical applications. Urethane, urea, ester and ether groups may be present and different classifications based on the dominant groupings are available. In biomedical applications, polyurethanes are usually described as either poly(ester urethane)s or poly(ether urethane)s. The former group found the earliest use but in vivo degradation and disintegration became apparent in applications such as bone adhesives and arterial prostheses and generally little use has been made of them in recent years. 

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