Epoxide Resins and their Formulation

The epoxide resins are a class of materials that possess all the properties required to make them useful for a wide range of applications throughout the electronics industry. They have good electrical properties, low shrinkage, good adhesion and resistance to thermal and mechanical shock, whilst also possessing resistance to moisture, solvents and general chemical attack. 

Epoxide resins are all based on the epoxide group, a strained three-membered carbon, carbon, oxygen ring structure (also known as the oxirane group) 

One of the most widely used series of epoxide resins is made by reacting epichlorohydrin with bisphenol A, in the presence of an alkali, to give the diglycidyl ether of bisphenol A (DGEBA) 

In order to be useful, a cured epoxide compound should possess as many of the following properties as possible  

In order to impart these properties to the final product, very careful selection of the epoxide resin and hardener system, as well as all the other components of the formulation, is vitally important. As the cured material is crosslinked, the structure and properties of the original resin will be completely changed. The material will no longer be soluble without decomposition and its chemical resistance will depend as much upon the hardener selected as upon the epoxy resin. As an illustration of the dependency of the cured system upon the hardener, acid anhydride based hardeners offer excellent resistance to acid environments, whilst amine-type hardeners offer maximum resistance to alkaline environments. 

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Flexibilisation of a formulation can also be achieved by using flexi-bilising curing agents or other additives. These materials are similar in their basic structures to the flexible epoxide resins and usually have reactive end groups separated by flexible long chain molecular segments. A typical example is dodecenyl succinic anhydride which has the structure... 

Vinyl esters are thermosetting resins that consist of a polymer backbone with an acrylate or methacrylate termination. The backbone component of vinyl ester resins can be derived from epoxide, polyester or urethane but those based on epoxide resins have most commercial significance. Bisphenol A epoxy formed vinyl esters were designed for chemical resistance and commonly formulated for viscosity for use in filament winding of chemical containers. Typically styrene is used as a reactive dilutent to modify viscosity. Phenolic novolac epoxies are used to produce vinyl esters with higher temperature capability and good solvent resistance, particularly in corrosive environments, and their FRP composites have demonstrated initial economy and better life cycle costs compared with metals. 

Structural adhesives are normally categorized or subdivided into subclasses based on the resin chemistries used in their formulation, which can be in the form of solids, liquids, pastes or films. The most important categories of structural adhesives are the thermoset cured adhesives based on phenolic, epoxy and polyurethane or acrylic resins and include phenolic adhesives. Epoxide adhesives, Toughened epoxide adhesives, Polyurethane adhesives, Acrylic adhesives and Toughened acrylic adhesives. Several of the categories can be further subdivided into both one- and two-component adhesives. The one-component structural adhesives, which can be liquids, pastes or solids (films), usually require the inclusion of added energy for activation or to effect cure in the... 

It is generally true that nearly all conmiercial epoxide resins have extenders incorporated in their formulation, the final selection being determined by the properties to be altered, the application and the other components of the system. Some of the properties affected by the addition of extenders are listed in Table 4.4. (For some specialised applications, certain of the listed disadvantages may in fact be advantages.)... 

The machinability of epoxides varies considerably from filler to filler, and, if ease of machinability is required, the use of talc may provide this property. Talc is also of use in epoxide resin formulations when the thermal expansion coefficient has to be lowered by a large amount, since loadings up to 80 parts per hundred of resin can be tolerated. Loading epoxides with talc has also been shown to increase their lap shear strengths when they are used as adhesives. 

Early efforts to impart flame retardancy upon epoxide resin formulations involved blending co-reactive halogenated compounds with liquid epoxides. These merely served as diluents and quite often resulted in products with markedly reduced physical properties such as Tg, thermal stability and compressive strengths. In order to avoid these problems, flame retardant epoxide resins have been synthesised containing halogen atoms within their molecular structures and these are now commonly used in epoxide resin based formulations. Their structures... 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

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