Important Reactive Adhesives

As already described in Section 3.2 there are two different kinds of reactive adhesives, that is, 

In the following description of the most important reactive adhesives it will be particularly referred to their allocation to one of these two groups. Furthermore, additional advice will be given regarding the specific properties of the adhesives, their application conditions and essential chemical formulas. 

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Figure 4.8 Classification of important reactive adhesives according to their way of formation.

Phenolic and other formaldehyde condensation polymers are also important reactive adhesives. Powdered phenolic resin is mixed with abrasive grains and the mixture is compression molded to form grinding wheels. A B-stage phenolic (Chapter XX) in a solvent is used to impregnate tissue paper. The solvent is evaporated, and the dry sheets are placed between layers of wood in a heated press, where the resin first melts and then cures, bonding the wood to form plywood. Similarly, sheets of paper impregnated with a B-stage melamine-formaldehyde resin are laminated and cured to form the familiar Formica counter tops. 

In addition to poly(methyl methacrylate) plastics and polyacrylonitrile fibres, acrylic polymers find widespread use. First introduced in 1946, acrylic rubbers have become established as important special purpose rubbers with a useful combination of oil and heat resistance. Acrylic paints have become widely accepted particularly in the car industry whilst very interesting reactive adhesives, including the well-known super-glues are also made from acrylic polymers. 

As described above, adhesives on polymer basis have to be transferred into a processible state by using adequate solvents or water. Thus, the important group of solvent-based adhesives develops, in contrast to reactive adhesives processed in the form of monomers, which, due to their mostly liquid or pastelike state, do not require solvents. In the common language use, only products containing organic and, in the majority of cases, combustible solvents in their formulations are called solvent-based adhesives. 

For single applications in the repair sector, reactive adhesives in tubes or booklets separated by sealed seams are available. Here, the components are squeezed out in equal strand lengths and mixed by an spatula. At any rate, it is important not to exceed the pot life indicated on the package. After squeezing out the components from tubes don t change their closures by mistake, otherwise they will be adhesively bonded to the tubes. 

To avoid inner tensions through thermal stress, it is recommended to use only adhesives that cure at room temperature. The adhesive selection is limited by the fact that many applications require an invisible glueline. In such cases, adhesives with fillers are excluded, cyanoacrylates and in particular radiationcuring products (Section 9.3.3) are the suitable choice. If the visual appearance of the bonded joint is not important, two-component reactive adhesives based on expoxides, polyurethanes, methacrylates, contact adhesives and, if required, adhesive tapes are recommended. 

Broad acceptance of reactive adhesives will be contingent upon the development of systems with acceptable toxicity limits. Until then, applications will be limited to those in selected, high performance adhesive markets. However, their technology has important implications for adhesives of the future. 

Polymers are used in much less obvious but often very important engineering applications. A major hidden use of polymers is as adhesives. The traditional glues were of course polymers derived from natural products. Some of the best of these still survive but synthetic polymer chemistry had led to very significant advances, particularly in the development of hot-melt adhesives that do not require solvents for their application. Another important development is the reactive adhesives based on ethyl cyanoacrylate ... 

The fiber properties of primary importance to adhesion are reactivity, surface characteristics, and finish. Rayon has many reactive hydroxyl groups. Nylon is less reactive but contains highly polar amide linkages. Polyester is relatively inert. Thus an adhesive system must be designed for each type of fiber. 

Acrylic adhesives are a versatile class of reactive adhesives that have in recent years gained wide acceptance in industry as bonding agents in the assembly of components. They are also to be found in the consumer market, where the industrially important benefits of rapid bonding, strength and ease of use can also be appreciated. 

Polyesters. Unsaturated polyesters dissolved in styrene or methacrylates are used as reactive adhesives. Hot-melt adhesives can be made from saturated and fusible polyesters based on aliphatic and/or aromatic carboxylic acids, in some cases by using dimerized fatty acids. High molecular mass amorphous polyesters and low molecular mass crystalline polyesters are important raw materials for moisture cross-linking polyurethane adhesives in solution or solvent-free systems. 

Phenolic Resins [9003-35-4], Curable phenol-formaldehyde resins of the resol type are of considerable importance as hot-setting reactive adhesives for bonding wood and metals. Suitable types are used also in combination with poly(vinyl acetals) and epoxy resins for special, in some cases filmlike, reactive adhesives for bonding metals. 

Urea Resins [9011-05-6], In quantitative terms, urea - formaldehyde resins are one of the most important groups of raw materials for adhesives. They are used in the form of aqueous solutions or spray-dried powders, generally containing add hardeners, as reactive adhesives for bonding wood, particularly for the production of plywood and chipboard. 

Caiboxyl reactive liquid polymers (CRLP) are chemically reactive with epoxy resins and capable of chain extension and crosslinking through the carboxyl groups. The reactions most important in adhesive applications are those with epoxide groups and with aliphatic hydroxyl groups. A list of several commercially available CRLP products is shown in Table 2. 

Important cure related considerations for evaporative and chemieally reactive adhesive and sealant types inelude ... 

The most important types of two-component adhesives are moisture-sensitive polyurethane, comprising polyol resin and isocyanate catalyst, and heat-sensitive epoxy, comprising bisphenol A or F resins and polyamine catalyst. Both single- and two-component adhesive systems are available in the market. Single-component reactive adhesives, which include cyanoacrylate and silicone, are also widely known, with the latter often being used as a sealant. 

For raw materials of reactive adhesives, the chemical equivalent values are very important to specify the reactivity, and often tested. Epoxy equivalents for epoxy resins, and functional group equivalents for hardeners are experimentally determined. Methods to measure the epoxy equivalents are described by industrial standards as follows International organization for standardization (ISO) 3001, Japan industrial standards (JIS) K 7236, and American society for testing and materials (ASTM) D 1652. For instance, JIS K 7236 indicates the methods using titration. In this process, an epoxy sample is solved with chloroform, mixed with acetic acid, tetraethylammonium bromide, and crystal violet solution. The epoxy sample solution is finally titrated with perchloric acid—acetic acid solution. If automatic titrators can be used, the epoxy... 

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