Structural adhesives classification

L17-500 1991 Aerospace series. Structural adhesives. Classification and qualification. 

From the chemists has come the classification of adhesives and sealants by chemical type, and from the mechanical engineers the classification as either structural or non-structural. Neither is a pure system, since many adhesives and sealants have more than a single chemical base resin, and many structural -based resin systems are used in non-structural applications. In a chapter entitled Structural Adhesives we are told that the term structural adhesive came into general use in the 1960 1970 period, but to this day all definitions are inadequate [9, Chap. 7]. Adhesive manufacturers and their advertising... 

The sample specification, or compliance spectrum which follows was introduced by the authors in 1988 and published in the Proceedings of the Institution of Civil Engineers(l). It addresses the requirements of adhesives, bonding procedures and test methods for structural steel-to-concrete bonding. The test methods recommended were based on the experience gained from extensive programmes of research into the control, classification and durability of structural adhesives for application in civil engineering undertaken at Dundee University. 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

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. 

Function. The functional classification defines adhesives as being structural or nonstructiu-al. Structural adhesives are materials of high strength and permanence. They are generally represented by ther ... 

Structural adhesives may be classified in a number of ways, such as those shown in Table I. The criterion chosen as a basis for the classification is usually the one that emphasizes the characteristic of most interest. In many cases the application and processing conditions are of most concern these depend on the physical form and cure conditions of the adhesive. A classification of structural adhesives that takes these properties into consideration has been suggested. 

Adhesives can be broadly classified as being thermoplastic, thermosetting, elastomeric, or alloy blend. These four adhesive classifications can be further subdivided by specific chemical composition as described in Tables 7.20 through 7.23. The types of resins that go into the thermosetting and alloy adhesive classes are noted for high strength, creep resistance, and resistance to environments such as heat, moisture, solvents, and oils. Then-physical properties are weU suited for structural-adhesive applications. 

BS EN 204 1991 Classification of non-structural adhesives for joining of wood and derived timber products. 

EN 204 Classification of Non-structural Adhesives for Joining of Wood and Derived Timber Products. 

In O Sect. 12.4, the physical form of adhesives is used for classification purposes and specific adhesives are discussed for each class of adhesives to provide more insight on adhesive categories including chemical families. This is because, many adhesives have cross-category characteristics, as already illustrated with some of the above listed adhesive examples. Additional examples are epoxies, which are typically listed as structural adhesives, but are also available in elastomer-epoxy forms high-temperature thermoplastic polyimide adhesives. 

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). 

Tlie distinction between structural and nonstructural bonds is not always clear. For example, is a hot melt adhesive used in retaining a fabric s plies structural or nonstructural One may argue that such an adhesive can be placed in either classification. However, the superglues (cyanoacrylates) are classified as structural adhesives even though they have poor resistance to moisture and heat. 

A 45-kDa mucin, expressed only in invasive trypomastigotes and shed to the medium, is involved in adhesion to mammalian cells.126 It was recognized by the anU-T-cruzi mucin monoclonal antibody MabC20, although no structural data are included to support its classification as a mucin. 

Tumor that is adherent to other organs or structures macroscopically is classified T4. However, if no tumor is present in the adhesion microscopically, the classification should be pTs. The V and L substaging should be used to identify the presence or absence of vascular or lymphatic invasion. 

Nonwoven geotextiles. Nonwoven fabrics are defined as a sheet, web, or batt of directionally or randomly oriented fibres/filaments, bonded either by friction, and/or cohesion, and/or adhesion. In general, nonwoven fabric formation can be considered as a two-step process web formation (aligning the fibres with certain orientation characteristics) and bonding these fibres by mechanical, thermal, or chemical means. This two-step process has formed the classification of nonwoven structures, i.e. carded, airlaid, spunlaid, meltblown, needlepunched, hydroentangled, adhesive bonded, thermal bonded, stitch bonded, etc. Some of the important processes that are used for the manufacture of nonwoven geotextiles are discussed below. 

CEN/TC 193 Adhesives , DIN EN 204 2001 Classification of thermoplastic wood adhesives for non-structural applications, Beuth Verlag GmbH, Berlin, Germany (2001). 

BS EN 301 2006 Adhesives, phenolic and aminoplastic, for load-bearing timber structures. Classification and performance requirements. 

EN 204 (2002). Classification of thermoplastic wood adhesives for non-structural applications. 

---