Chemically hardened adhesives examples

Diluents. These are generally incorporated to reduce the viseosity of the freshly mixed adhesive to offset the effect of the filler. This may be required to improve handling and spreading characteristics or to allow filler additions which tend to reduce cost. Other properties of the fresh and hardened adhesive can be affected by the use of diluents, for example pot life, flexibility and glass transition temperature. If the diluent is non-reactive, such as solvents which remain in the cured system, the net result is a deterioration of chemical and mechanical properties such as increased shrinkage and reduced adhesion. Reactive diluents containing epoxy compounds are capable of combining chemically with the resin/hardener system. 

Some adhesives solidify simply by the evaporation of a carrier liquid (water or solvent). Others harden as a result of going from a molten liquid to a cooled solid. Still others solidify by means of a chemical reaction. Some adhesive systems may require several mechanisms to harden. For example, thermosetting waterborne adhesives require the water carrier to first evaporate and then chemical cross-linking occurs before a final bond is achieved. 

Thus, for using chemically curing adhesives only a few systems are applicable such as two-part systems consisting of a resin and a hardener, for example, epoxy resins. Another appropriate approach is the application of, for example, urethanes and silicones that cure by humidity and do not need complex hardware. However, the bi est disadvantage of using these systems is the expanded necessary curing time. 

The presence of catechols and more complex, oxidizable polyphenols in nature is widespread, and their functions are not limited to chemical defense. However, biological control of their oxidation is usually a feature of their function, as it is (1) in melanin synthesis,3 (2) in immunologically mediated delayed-type hypersensitivity responses,4 (3) in the hardening or curing of arthropod secretions (for example, as in the surface attachment adhesives of the barnacle and in tanning of the cuticle in insects),5 as well as (4) in defensive mechanisms in higher plants, particularly in the unleashing of immediate necrotrophic responses.6... 

Previous investigators have drawn attention to the beneficial effect of lime when added in small quantities to asphaltic bitumen. The lime helps retard oxidative hardening (13) and reduces the tendency towards water-stripping (4,11,12). Most asphalts are slightly acidic because of the presence of phenolic or carboxylic substituents and would therefore react with basic oxides to form insoluble salts. For example, Fromm (10) has described the use of iron salts of naphthenic acids as adhesion promoters to improve the water resistance of asphalt concretes. This promising approach is now undergoing commercial trials. The literature also describes methods of chemically modifying asphalt with maleic anhydride or acrylic acid (14), sulfur trioxide (15), sulfur dioxide (16), acetyl sulfate (17-21), and sulfuric acid (20). (For a recent review of the interfacial phenomena in asphaltic compositions see Ref. 4.)... 

An adhesive may be defined as a material which when applied to surfaces of materials can join them together and resist separation. Adhesive is the general term and includes cement, glue, paste, etc. and these terms are all used essentially interchangeably. Various descriptive adjectives are often applied to indicate certain characteristics. For example, to indicate the physical form of the adhesive, e.g. liquid adhesive, liquid two-part adhesive, film adhesive its chemical form, e.g. epoxy adhesive, cyanoacrylate adhesive, polychloroprene adhesive to indicate the type of materials bonded, e.g. metahto-metal adhesive, paper adhesive, wood adhesive or to show the conditions of use, e.g. solvent based adhesive, cold-hardening, or -curing, adhesive, hot-melt adhesive. 

The period over which a packaged adhesive retains satisfactory properties is known as the shelf life. Adhesives are based on organic chemicals, and provided they are kept in containers which exclude oxygen and UV-light, they have very long, perhaps indefinite, shelf lives. Examples are epoxides where resin and hardener are packed separately, hot-melts, and adhesives which are solutions of polymers in organic solvents. 

Structural acrylic adhesives contain monomer, and cure is by a redox system which generates free radicals which cause cure by addition polymerization. Redox systems have two components, one being dissolved in the adhesive and the other in the hardener or catalyst. Once mixed cure is rapid. Examples are cumene hydroperoxide and N,N-dimethyl aniline. Should the peroxide be in the adhesive, then it will slowly decompose to give free radicals, which will cause cure and eventually terminate the shelf life, which is typically 12 months. Chemical reactions obey the Arrhenius relation (O Eq. 18.1). Here k is the rate constant. 

The hardening of the adhesives happens either through a chemical reaction or through simply physically binding to the surface. The chemical reaction in case of a one-part adhesive depends on the chemical nature of the adhesive (Habenicht 1990). So do polyurethane- or silicone-based adhesives cure in the presence of moisture, whereas cyan-acrylic-based adhesives are inhibited by the presence of oxygen and cure under nonaerobic conditions. Other adhesives like epoxy one-part adhesives cure with heat. Two-part adhesives contain beside the resin part, a hardener part which consists of reactive molecules or polymers which react together with the resin part. Typical examples are epoxy- or acrylic-based adhesive formulations. 

There are two classifications of binder systems, thermoplastic and thennoset. Thermoplastic polymeric materials are those that do not undergo any chemical change during film formation. The film is formed by the evaporation of the solvent (or water). The properties of the film must reside in the properties of the polymer used in the formulation. The only change that occurs over time is a continued loss of volatile material, which will cause the film to continue to harden and become more resistant to damage. Examples of thermoplastic coatings are acrylic lacquers or vinyls. The adhesion promoter for TPO substrates, chlorinated polyolefin, is an example of a thermoplastic polymer. 

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