Bisphenol epoxy resin, properties

Selecting a flame retardant for an adhesive system has many ramifications, depending on the formulation being modified, the end use, how it will be processed, and the cost/performance ratio. When one is choosing a flame retardant, characteristics such as water extraction, particle size, viscosity, toxicity, dusting, uniformity, as well as economics must be considered. The materials chosen to perform the function of flame retardation must not interfere with the final product s performance. The major problem with incorporating flame retardants in adhesives is that very often a significant amount is required, and they interfere with the other properties of the adhesive and contribute to the cost. This is why bromo bisphenol epoxy resins are often employed in flame-retardant epoxy adhesives. 

Other quinoxaline-type polymers can have higher Tg values, e.g. 400°C. DGEBA stands for diglycidyl ether of bisphenol A. Epoxy resin properties can depend considerably on the hardener used. 

Commercial PBDs are available (e.g., from Sartomer) that bear epoxy entities on the polymer backbone and also have terminal hydroxyl groups The hydroxyl groups can cure a formulation containing reactive monomers such as cycloaliphatic or siloxane epoxides, or the epoxidized PBD itself [237]. Bisphenol epoxy resins show an increased impact strength when the formulation contains less than 10 wt% of an epoxidized HTPB. Higher contents of HTPBD result in a resin that is softer and has a lower Tg and generally unfavorable mechanical properties [115]. 

Bisphenol A diglycidyl ether [1675-54-3] reacts readily with methacrylic acid [71-49-4] in the presence of benzyl dimethyl amine catalyst to produce bisphenol epoxy dimethacrylate resins known commercially as vinyl esters. The resins display beneficial tensile properties that provide enhanced stmctural performance, especially in filament-wound glass-reinforced composites. The resins can be modified extensively to alter properties by extending the diepoxide with bisphenol A, phenol novolak, or carboxyl-terrninated mbbers. 

The bisphenol A-derived epoxy resins are most frequendy cured with anhydrides, aUphatic amines, or polyamides, depending on desired end properties. Some of the outstanding properties are superior electrical properties, chemical resistance, heat resistance, and adhesion. Conventional epoxy resins range from low viscosity Hquids to soHd resins. 

This low viscosity resin permits cure at low (70°C) temperatures and rapidly develops excellent elevated temperature properties. Used to increase heat resistance and cure speed of bisphenol A epoxy resins, it has utihty in such diverse appHcations as adhesives, tooling compounds, and laminating systems. A moleculady distilled version is used as a binder for soHd propellants (see Explosives and propellants) and for military flares (see Pyrotechnics). Its chief uses depend on properties of low viscosity and low temperature reactivity, particularly with carboxy-terminated mbbers. 

Table 6. Comparative Properties of Bisphenol A—Epoxy Resins Cured with Different Hardeners ...

Epoxy resins are really polyethers but are named epoxies because of the presence of epoxide groups in the starting material. They were initially synthesized from epichlorohydrin and bisphenol A in the 1940s. General properties are listed in Table 4.9. 

In addition to the Bisphenol-A backbone epoxy resins, epoxies with substituted aromatic backbones and in the tri- and tetra- functional forms have been produced. Structure-property relationships exist so that an epoxy backbone chemistry can be selected for the desired end product property. Properties such as oxygen permeability, moisture vapor transmission and glass transition temperature have been related to the backbone structure of epoxy resins5). Whatever the backbone structure, resins containing only the pure monomeric form can be produced but usually a mixture of different molecular weight species are present with their distribution being dictated by the end-use of the resin. 

Resin content Epoxy resin The bisphenol A type is most common. However, there are many other types of epoxy resins having differing properties. 

Bisphenol F-based epoxy resins (Fig. 2.7) are analogous to DGEB A-based epoxy resins in most respects. They use the same curing agents and reaction mechanisms. Bisphenol F epoxies are often used in blends with DGEBA resins to lower the viscosity or to modify certain properties. 

Diluents are higher-MW components than solvents that are also added to the epoxy adhesive formulation to lower the viscosity and modify processing conditions. The primary function of a diluent in an epoxy resin formulation is to reduce its viscosity to make it easier to compound with fillers, to improve filler loading capacity, or to improve application properties. Solvents, certain curing agents, and flexibilized epoxy resins can also lower the viscosity of epoxy adhesive formulations, but this is not their primary function. The effect of various diluents on the initial viscosity of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin is illustrated in Fig. 6.3. 

For adhesive systems, the liquid epoxy resins most widely used with LP-3 polymers are liquid unmodified and diluent-modified bisphenol A resins and liquid blends of bisphenol A and bisphenol F resins. Solid bisphenol A, multifunctional, and aliphatic diepoxy resins have also been used. Ratios of liquid polysulfide polymer to epoxy are in the range of 1 2 to 2 1. The effect of various degrees of polysulfide on cure properties of a DGEB A epoxy is shown in Table 7.7. An increase in elongation and impact strength is the result of increased amounts of the liquid polysulfide polymer. 

For maximum heat resistance, pyromellitic dianhydride is often used. The composition and properties of metal-to-metal adhesives based on a combination of epoxy novolac and a bisphenol A epoxy resin have been described in Sec. 12.5.1. Depending on the cure temperature and the choice of amine curing agent, strength as high as 3000 psi at room temperature and over 1000 psi in the range of-55 to + 150°C is possible. 

Effect of Bisphenol A Addition Two-Particle Size Systems. If we modify the Hycar CTBN-epoxy resin system so as to produce a two-particle size system, we can produce the toughest epoxy resins. We modified the CTBN-epoxy resin system by adding bisphenol A. A many-fold improvement in fracture energy is obtained when Hycar CTBN is used with bisphenol A and epoxy resin. The maximum improvement, when 5 phr Hycar CTBN is used, is at a bisphenol A concentration of 24 parts per hundred parts of epoxy resin. Typical data describing the properties of these systems are shown in Table VI. 

---