Bond classification, adhesion

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. 

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. 

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-... 

Definition of Adhesives and Adhesive Bonding Functions of Adhesives Classification of Adhesives Advantages and Disadvantages of Joining Using Adhesives... 

Table V. Classification of Surface Pretreatments for Adhesive Bonding of Titanium"...

J. L. Rose, Aspects of the adhesive bond strength classification problem, in Durability of Adhesive Bonded Structures, pp. 389-398, Proc. Applied Polymer Symposium 32, Pitcatinny Arsenal, NJ (October 27-29, 1976). 

Specific substrates are described under each classification. Of course, adhesive applications are not limited to joints having only one type of substrate. Metal-to-plastic, aluminum-to-steel, metal-to-wood, glass-to-metal, and an infinite variety of other joint configurations are all possible. In these applications the nature of each substrate needs to be understood and considered in the overall selection of an adhesive formulation and bonding process. 

Adhesives (a term generally synonymous with glues, mastics, mucilages, and pastes) produce relatively permanent surface bonds between two substances (paper, glass, wood, metal, etc.) that develop by chemical reaction or as the adhesive cools or dries. The enormous variety of adhesives limits any system of nomenclature, but classification by binder, the major active ingredient, yields the following ... 

Classification Nonaromatic ether Definition Glycidyl ether of Cl 2-14 alcohols Properties Dens. 0.890 ref. index 1.4470 Toxicology Skin irritant sensitizer possible mutagen Uses Reactive epoxy diluent modifier for flooring, aggregate bonding, potting, encapsulation, adhesives Manuf./Distrib. Aldrich... 

The 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 could argue that such an adhesive may be placed in either classification. However, the superglues (cyanoacrylates)... 

The wetting and spreading action necessarily demands that the adhesive is applied in the liquid phase, whereas the final bond depends on the adhesive being a solid with appropriate mechanical characteristics. The two most widely used and nsefnl taxonomic schemes for adhesives address either the way in which this phase change takes place or the chemical nature of the adhesive itself, although other schemes such as a classification by suitability for a particular type of adherend or end-use industry are also current and sometimes more applicable. 

The final bond produced by the vast majority of adhesives consists largely of a solid organic polymer. The fluidity necessary at the time of application and the subsequent means of transition to solidity thns form the basis of the most frequently used system of classification. 

These types of adhesives can be broadly classified by their physical form of application and/or the way they are activated to form a bond with the material to which they are applied, for example, as emulsion polymers, solution polymers, hot melts and reactive polymers (see Adhesive classification). Each of these will now be considered in turn. 

The classification of epoxide adhesives refers to those that contain a predominance of epoxy resins in their formulation, the epoxy resin being characterized by a thiee-membered ring (epoxy or oxirane ring) in which two carbon atoms are singly bonded to an oxygen atom. 

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