Uncured epoxy resins: properties

The general properties of cured and uncured epoxy resins are reviewed in Chap. 3. The chemical structures of the resin and curing agent will determine these physical properties. They will also determine, to a great extent, the surface chemistry and adhesion properties of the final product. 

Proper formulation of epoxy adhesives requires knowledge of the chemical reactions that lead to polymerization as well as the chemical and physical properties of both the uncured mixture and the cured material. This chapter reviews the general principles of epoxy resin chemistry including synthesis of the epoxy monomer itself and its possible polymerization reactions. 

Properties often used by epoxy resin manufacturers to specify particular grades of resin include the epoxy content, viscosity or softening point, and color. In addition, properties such as density, vapor pressure, flash point, refractive index, solubility characteristics, and hydroxyl content are often reported. The important properties of uncured epoxy formulations with regard to most adhesive applications are... 

A wide range of epoxy resins as well as a wide range of curing agents and catalysts are available for formulating solid epoxy adhesives. Resins with different viscosities, amounts of reactive groups, and structures are available. Additives that change the uncured resin viscosity, reduce brittleness, or impart some other property are also available. 

Typically tape or film epoxy adhesives are modified with synthetic thermoplastic polymers to improve flexibility in the uncured film and toughness in the cured adhesive. Epoxy resins can also be blended with phenolic resins for higher heat resistance. The most common hybrid systems include epoxy-phenolics, epoxy-nylon, epoxy-nitrile, and epoxy-vinyl hybrids. These hybrid film adhesives are summarized in Table 13.2, and structural properties are shown in Table 13.3. 

Epoxy resins are amenable to modification in many ways and commercial epoxy formulations often contain a number of materials whose presence serves to modify the properties and characteristics of both uncured and cured epoxy. These additives fall into the following general categories ... 

Consequent to documentation surrounding methods of employing reactive nitrile elastomers to modify epoxy resins is a growing body of literature which serves to characterize and elucidate these systems. Such topics as morphology in the cured and uncured state, transitions from toughening to flexlbilization, viscoelastic effects, equilibrium physical properties and phase structure are available to the investigator (12-17). 

The rate of cure is temperature dependent and many formulations stop curing altogether below a temperature of about 5 °C. If carefully formulated the change in volume between the uncured resin-hardener system and the fully cured polymer can be very low. This property, together with their relatively high strength and claimed resistance to moisture and chemical attack, forms the basis of the use of epoxy resins as structural adhesives. 

The most commonly used tests for properties of the materials used in epoxy adhesive formulation and uncured mixed adhesives are viscosity, shelf life, percent of solids, and moisture content. These are generally applied to the resinous components. 

In the literature, there exist many papers on the properties of epoxy glasses, but most of them are difficult to generalize due to incomplete characterization of the chemical and topological structure of the investigated samples. Sometimes, the difficulties of polymer characterization arise from the industrial origin of uncured resins and curing agents. 

To be useful, an antioxidant must be soluble in the uncured resin components, must have sufficient UV stability to survive the curing process, and must not adversely affect the efficiency of the photoinitiator. We have studied a number of commercial stabilizers to determine if they have the requisite properties, and have examined their effectiveness in suppressing the oxidation of an epoxy diacrylate. 

Uses Aliphatic trifunctlonal epoxy diluent Improving surf, wetting props, of uncured resins/hardeners In severe thennal cycling conditions Regulatory Not regulated for transport Canada DSL Europe EINECS Japan MITI China SEPA Australia AlCS SARA 311/312 Immediate Health Hazard, 313 nonreportable WHMIS D2B symbol T Properties Cl. liq. si. odor negligible sol. in water sp.gr. 1.02 vapor pressure < 0.1 mm Hg flash pt. (CC) > 93 F... 

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