Thermosets structural adhesives

The following sections discuss the chemistry of some major classes of thermosetting, structural adhesives. 

Thermosetting resins have been used extensively in industry for applications such as structural adhesives, composites, RIM, coatings and sealants which need to resist severe service conditions. Thermosetting resins are generally composed of low molecular weight oligomers which allow fabrication convenience. 

Structural Adhesives A structural adhesive is a resin system, usually a thermoset, that is used to bond high strength materials in such a way that... 

Epoxy adhesives represent a significant part of the overall structural adhesives market (about 1.8 bihion). The main competitors to epoxy adhesives are polyurethanes however, thermosetting acrylics and cyanoacrylate adhesives are also strong challengers in certain market segments. 

The crosslink density ultimately defines the rheological and mechanical properties of the polymer. Polymers that have a high crosslink density are thermosets and are infusible, insoluble, and dimensionally stable under load. These properties make epoxy resin systems useful as structural adhesives as well as important materials in other applications. Polymers that have a low crosslink density are more flexible and show greater resistance to stress concentration, impact, and cold. 

Epoxy adhesives are most commonly used as liquids or pastes. However, certain types of epoxies can be employed in the form of a solid. The components in these adhesives are mixed and processed to a stage where the resulting adhesive product is in a solid but still fusible (uncrosslinked) state. When the applied solid adhesive is heated, it melts, flows, and wets the substrate. Additional heating time then causes the adhesive to cure completely into a strong, thermosetting structure. 

Solid epoxy adhesives may also be fused or compacted into various shapes or preforms (see Fig. 13.2) such as sticks, rings, and beads.5 They can then be easily applied to a specifically shaped part. Similar to thermoplastic hot-melt adhesives, these thermosetting epoxy adhesives flow on the substrate by heating. However, the product will cure to an infusible thermoset stage with continued heating. Once cured, the product demonstrates properties similar to those of other structural epoxy adhesives. These solid epoxy adhesives are sometimes referred to as thermosetting hot melts. 

The bonding materials are generally (1) in the form of thermoplastic preforms that bond or seal as a hot-melt type of adhesive or (2) thermosetting paste which cures to a structural adhesive. 

Alkyd parts are generally very rigid, and the surfaces are hard and stiff. Surface preparation for alkyd parts consists of simple solvent cleaning and mechanical abrasion. Epoxies, urethanes, cyanoacrylates, and thermosetting acrylics are commonly used as structural adhesives. 

Practical uses of epoxies include load bearing applications such as structural adhesives and composite matrices. In these applications, their most detrimental feature is a characteristic low resistance to brittle fracture. The desire to improve this property has motivated studies on thermoset fracture behavior for the last two decades. Of particular interest is the relationship between the molecular structure and the failure properties of thermosetting epoxies, the subject of this chapter. 

Acetal Copolymer (CELCON ) Two types of non-solvent adhesives are used, structural and non-structural. Most structural adhesives are based on thermoset resins and require the use of a catalyst and/or heat to cure. This type of adhesive is normally used in applications which require maximum bond strength and minimum creep of the adhesive joint under sustained loading. Many structural adhesives can be used continuously at temperatures up to 350 F, which is higher than the... 

Rubber-Toughened Thermosetting Polymers in Structural Adhesives—Developments in Resins and Primers Kinloch, A. J., Ed. Applied Science Publishers London, 1980 Chapter 5, pp. 127-162. 

Epoxy resins are among the most important of the high performance thermosetting polymers and have been widely used as structural adhesives and matrix for fiber composites. Epoxy resins are characterized by the presence of epoxide groups before cure, and they may also contain aliphatic, aromatic, or heterocyclic structures in the backbone. The epoxy group can react with amines, phenols, mercaptans. 

Young RJ (1986) Rigid-particulate reinforced thermosetting polymers in Structural Adhesives - Developments in Resins and Primers. Kinloch AJ (ed) Elsevier Applied Science Publishers London... 

Structural adhesives may be loosely defined as thermosetting resin systems which are used to bond high strength materials, usually metals, in such a way that the bonded structure is able to bear a permanent load. Commonly, structural adhesives are designed to withstand... 

This term is usually used to apply to all adhesives other than natmal adhesives (i.e., elastomeric, thermoplastic, thermosetting, and alloys). All structural adhesives are synthetic. 

These adhesives are important as structural adhesives especially in metal bonding. They comprise a thermosetting and a thermoplastic polymer, including certain elastomers. Although each component has adhesive... 

Of all the thermosetting plastics, epoxies are more widely used than any other plastic, in a variety of applications. There are resin/hardener systems (two-part) that cure at room temperature, as well as one-part systems that require extreme heat cures to develop optimum properties (e.g., 121°C and 177°C). Proper selection of various hardeners, resins, modifiers, and fillers allows the development of desired properties for a particular application. Because of the wide versatility and basic adhesive qualities, epoxies make excellent structural adhesives that can be engineered to widely different specifications. Essentially no shrinkage occurs during polymerization because epoxies are completely reactive producing no volatiles during cure. Epoxy adhesives can be formulated to meet a wide variety of bonding... 

Polyvinyl butyral is commonly used in safety-glass laminates. Polyvinyl acetals are used in making thermoset resins more flexible to obtain structural adhesives for metals. 

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