Epoxy adhesives general characteristics

The adhesive base or binder is the principal component of an adhesive. The binder provides many of the main characteristics of the adhesive such as wettability, curing properties, strength, and environmental resistance. The binder is often by weight the largest component in the adhesive formulation, but this is not always the case, especially with highly filled adhesives or sealant systems. The binder is generally the component from which the name of the adhesive is derived. For example, an epoxy adhesive may have many components, but the primary material or base is an epoxy resin. Once the binder is chosen, the other necessary ingredients can be determined. Chapter 4 describes in detail the various polymeric resins that are commonly used as bases or binders in epoxy adhesive formulations. 

These hybrid epoxy adhesives are generally used for demanding structural applications such as in the aerospace industry where the optimal properties from each component are desired. For example, epoxy is generally used to provide good adhesion and processing characteristics. They are blended with the following resins to provide additional improvements in the properties noted. 

In two-part epoxy adhesive systems, the filler can generally be incorporated into either the epoxy resin or the curing agent component. Prevailing factors will be the characteristics mentioned above (viscosity of each component, dispersion characteristics, etc.). However, the effect on the mix ratio and on the ease of mix-... 

Another characteristic difference between a two-component, room temperature curing epoxy adhesive system and a one-component, heat curing system is the shelf life. The shelf life of a one-component epoxy can be 3 to 6 months when stored at room temperature and may even require refrigeration or freezing for a practical shelf life. The shelf life of each component in a two-component system is generally 6 months to 1 year. 

Tape and film are the most common forms of solid epoxy adhesives. Tape and film are terms that are used rather loosely for adhesives in a thin sheet form. The term tape generally refers to an adhesive that is supported on a web of paper or nonwoven fabric, or on an open-weave scrim of glass, cotton, or nylon. Films, on the other hand, are free of supporting material and consist only of the adhesive. Both tape and film products may be accompanied with a release liner depending on the tack or blocking characteristics of the adhesive. 

The sections below describe the characteristics of various metal substrates with regard to epoxy adhesives. Only the more common substrates and those that are most reactive (to either the adhesive or the environment) are discussed. Metal substrates not covered are generally easy to bond with epoxy adhesives, and there is little change in properties with environmental aging. 

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 degree of chaos is the general characteristic of the structure and therefore it should influence both structural parameters and properties of epoxy polymers. It has been shown earlier that epoxy polymers can be considered as natural nanocomposites, for which the interfacial (intercomponent) adhesion level is one of the most important characteristics [34]. For polymer composites (nanocomposites) this property can be characterised by the parameter b, estimated according to the equation [35] ... 

Diethylenetriamine and Triethylenetetramine. Diethylenetriamine (DETA) and trieth-ylenetetramine (TETA) are very reactive, low-viscosity liquids that are widely used with DGEBA epoxy resins. The application characteristics and cured properties of adhesive formulations prepared with these two curing agents are very similar. The lower vapor pressure of TETA generally favors its use. 

These blends can take a number of different forms. The added resin may be reacted with the epoxy resin, or it may be included as an unreacted modifier. The modifier may be blended into a continuous phase with the epoxy resin (epoxy alloys) or precipitated out as a discrete phase within the epoxy resin matrix (as is generally done in the case of toughening modifiers). Epoxy hybrid adhesives are often used as film (supported and unsupported) or tape because of the ease with which formulated systems can be dissolved into solvent and applied to a carrier or deposited as a freestanding film. Some systems, notably epoxyurethanes and epoxy-poly sulfides, can be employed as a liquid or paste formulation because of the low-viscosity characteristics of the components. 

Fillers generally represent one of the major components by weight in an adhesive formulation. However, their concentration is quite often limited by viscosity constraints, cost, and negative effects on certain properties. The degree of improvement provided by a filler in an epoxy formulation will heavily depend on the type of filler and its properties (particle size, shape, size distribution, and concentration), surface chemistry, dispersion characteristics, dryness, and compatibility with the other components in the formulation. Table 9.3 summarizes the properties of selected fillers. 

The effect of electrical-grade fillers (e.g., silica) on the electrical properties of the adhesive is usually marginal. Generally fillers are not used to improve electrical resistance characteristics such as dielectric strength. The unfilled epoxy is usually optimal as an insulator. Also under conditions of high humidity, fillers may tend to wick moisture and considerably degrade the electrical resistance properties of the adhesive. 

Characteristics of typical film adhesives vary widely depending on the type of adhesive used. Epoxy film adhesives are made in both unsupported and supported types. The carrier for supported films is generally fibrous fabric or mat. 

Generally these compositions contain an epoxy-novolac, a hardener, a catalyst, silica fillers, and an internal lubricant/mold release compound. Brom-inated epoxies and antimony trioxide are included to provide the required flame retardant characteristics. Other, unspecified additives are used to promote adhesion or to reduce corrosion rates. Because of their superior thermal capabilities and electrical properties, epoxidized novolacs are preferred over epoxy homopolymers. Near stoichiometric amounts of hardeners such as novolacs (Equation 1), anhydrides, and primary amines can be used to cure the resins in the presence of a catalyst. The linkages which are formed include ethers, esters, or secondary amines, respectively. 

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. 

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