Hardening by chemical reaction

The adhesives which harden primarily by chemical reaction are usually applied to the substrates as monomers or oligomers (sometimes referred to as resins ) and are then polymerized in situ in the joint between the substrates. The 

It is convenient to divide the chemical reactions by which this class of adhesives harden into two groups  

In other formulations the chemical reactions will only proceed at elevated temperatures. Most of the chemical types of adhesive mentioned above may be formulated to harden in this manner and are usually one-part formulations containing an hardener which is only activated when heat is applied. However, some chemical rection may occur slowly at room temperature and hence the limited storage life of this type of adhesive and the common practice of storage in a freezer to prolong the shelf-life. 

The relative slowness of hardening the adhesive by chemical reaction, coupled with the fact that it occurs in situ, means that some mechanical design 

Epoxy adhesives represent the most common structural adhesives and have gained wide acceptance in many diverse industries. They essentially consist of an epoxy resin, often based upon the diglycidyl ether of bisphenol A, and harden to give a thermosetting polymer by step-growth polymerization or addition polymerization. 

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Thermodynamics A rigorously mathematical analysis of energy relationships involving heat, work, temperature and equilibrium. It describes systems whose states are determined by thermal parmeters, such as temperature, in addition to mechanical and electromagnetic parameters. Thermosetting A material which hardens by chemical reaction and is not remeltable. 

This merging of particles will not take place unless the polymer molecules in the particles have freedom of movement. Either the glass temperature of the particle must be below room temperature, or heat must be applied. In the former event, this will mean that the final film will be relatively soft, unless the droplets contain a solvent that evaporates later, or unless the polymer can harden by chemical reaction, e.g. by oxidative drying. In practice then, there are three possible types of paint ... 

A chemical bond with hardening by chemical reaction at ambient temperature or at a temperature lower than that of a ceramic bond... 

Thermoplastic films harden by solvent evtqmration, whereas thermoset films harden by chemical reaction. 

Considerable development has occurred on sintered ceramics as bone substitutes. Sintered ceramics, such as alumina-based ones, are uru eactive materials as compared to CBPCs. CBPCs, because they are chemically synthesized, should perform much better as biomaterials. Sintered ceramics are fabricated by heat treatment, which makes it difficult to manipulate their microstructure, size, and shape as compared to CBPCs. Sintered ceramics may be implanted in place but cannot be used as an adhesive that will set in situ and form a joint, or as a material to fill cavities of complicated shapes. CBPCs, on the other hand, are formed out of a paste by chemical reaction and thus have distinct advantages, such as easy delivery of the CBPC paste that fills cavities. Because CBPCs expand during hardening, albeit slightly, they take the shape of those cavities. Furthermore, some CBPCs may be resorbed by the body, due to their high solubility in the biological environment, which can be useful in some applications. CBPCs are more easily manufactured and have a relatively low cost compared to sintered ceramics such as alumina and zirconia. Of the dental cements reviewed in Chapter 2 and Ref. [1], plaster of paris and zinc phosphate... 

Sinter bridges are formed when particles are partially melted, due to heat, and then resolidify as agglomerates. A second method is by chemical reaction or by the use of a viscous or hardening binder. 

Curing The irreversible hardening of a material by chemical reaction or solvent evaporation. 

The formation of solid bridges by chemical reactions or hardening binders depends only on the participating materials, their chemical reactivity, and their tendency to harden. Sometimes elevated temperatures and/or pressure improve the binding characteristics and may cause a modified, potentially stronger, bridge structure. Chemical reactions are often activated by the presence of moisture. 

The formation of solid bridges by chemical reaction or hardening binders de-... 

The foaming methods vary widely. One is to whip air into suspension or a solution of the plastic, which is then hardened by heat curing. A second is to dissolve a gas in a mix, then expand it when the pressure is reduced. Another is to let a liquid component of a mix be volatilized by heat. Similarly, water produced in an exothermic chemical reaction can be volatilized within the mass by the heat of reaction. A different technique lets carbon-dioxide gas be produced within the mass by chemical reaction. A related way is for a gas such as nitrogen to be liberated within the mass by the thermal decomposition of a chemical blowing agent [12]. Finally, tiny beads of resin or even glass microballoons can be put into a plastic mix or syntactic foam or the like. 

The convertible binders undergo chemical reaction in the film. The drying and hardening of the film is catalyzed by driers with oxygen absorption. Hence, there is a limit to the thickness of the film, which will dry. The two-pack materials, notably epoxies and polyurethanes, cure by chemical reaction between two components in the film with no oxygen absorption. 

Epoxy Contain epoxide resin cures by chemical reaction two-part system (hardener and resin) excellent adhesion to most substrates, tough and durable films good resistance to solvents, water, chemicals, and abrasion poor resistance to sunlight (chaUdng breakdown process) which limits outdoor use to primers and intermediate coats most commonly used are polyamide- and amine-cured modified epoxies can be used in steel concrete, flooring and lining applications. 

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