Epoxy-based resin adhesive

One problem with early epoxy formulations is that they cured to a relatively brittle material. By using reactive flexibilizers, such as polysulfides, epoxy adhesive formulators have obtained the flexibility required for many applications in this industry. Polyamides and even coal tars have also been used to provide flexibility to epoxy base resins. 

A variety of toughening agents have been used to modify epoxy adhesives to improve peel strength and fracture toughness without significantly affecting other properties of the epoxy base resin. Generally these modifiers can be classified into three types ... 

Vinyl ether monomers, like DVE-3, proved to be very effective reactive diluents in UV-curable epoxy-based resins, not only to lower the formulation viscosity and make it suitable for spray or roller applications, but also to achieve a faster and more complete curing, as VE and epoxy monomers were shown to undergo copolymerisation by a cation-driven mechanism [50]. Moreover, the addition of vinyl ether may impart some flexibility to the UV-cured epoxide, thus improving its adhesion on flexible substrates. 

Figure 7. Paint adhesion loss in salt spray exposure (ASTM B117) as a function of ester content for chain-extended epoxy-amine and epoxy-ester resin based coatings. All coatings applied at 20-25 urn film thickness to SAE 1010 steel test panels, baked, scribed and exposed for 24 hours to salt spray conditions.

The use of epoxy resins in industry extends back over fifty years since their introduction commercially and they have found an extremely wide range of applications as diverse as coatings and adhesives for electronics to use as matrix materials in aerospace composites. It is the use of advanced composites that is pressuring the advancement of epoxy science and the improvement of epoxy based... 

Over the past several decades, significant advances have been made in developing epoxy-based adhesives having improved performance over these early adhesive systems. These improvements were made possible by (1) the incorporation of toughening additives into epoxy resin formulations and (2) the use of multifunctional epoxy resins primarily for high-temperature applications. These innovations are discussed in later chapters. 

An ingredient added to an adhesive to reduce the concentration of base resin or binder is called a diluent. Diluents are principally used to lower the viscosity and modify the processing conditions of some adhesives. The degree of viscosity reduction caused by various diluent additions to a conventional epoxy adhesive is shown in Fig. 1.5. Diluents do not evaporate as does a solvent, but they become part of the final adhesive. Reactive diluents react with the resin base during cure, so that the final adhesive characteristics are determined by the reaction product of the binder and diluent. Nonreactive diluents do not react with the resin or curing agent and, therefore, more seriously weaken the final properties. Coal and pine tar are common nonreactive diluents. 

Most of the epoxy resins that are used in the formulation of adhesives have EEWs in the range of 180 to 3200, corresponding to a molecular weight range from 250 to 3750. This applies equally to epoxy resins that have undergone chemical reaction or modification to be used as a base resin in an adhesive formulation. 

For example, using the following materials in the formulation of an epoxy adhesive base resin ... 

It should also be emphasized that the epoxy resins are only building blocks in the development of epoxy-based adhesives. The formulation will ultimately involve one or more epoxy resins curing agents modifiers, such as plasticizers, flexibilizers, accelerators, stabilizers, and flow control agents and fillers. In general, flexibility can be controlled by the parameters shown in Table 3.8. 

Fumed silica is often used in 100 percent solids, liquid polymers. With epoxy adhesives and sealants only a few percent by weight of the additive will eliminate common problems such as slumping and separation. The fumed silica also raises the effective viscosity of the base resin to prevent other components from settling while the extrudability or spreadabil-ity is unaffected. Also note that fumed silica provides a surface that is free of texture. This is important in architectural-grade paints and sealants. 

Epoxy-based primers are commonly used in the aerospace and automotive industries. These primers have good chemical resistance and provide corrosion resistance to aluminum and other common metals. Primer base resins, curing agents, and additives are much like adhesive or sealant formulations except for the addition of solvents or low-viscosity resins to provide a high degree of flow. 

Table 11.12 shows two rapid-setting, room temperature cure epoxy adhesives based on epoxy acrylate resins with aliphatic amine curing agents. These adhesives have gel times of less than 5 min for a 100-g mass. The bond strength development is rapid with handling strength occurring in about 1 h at room temperature. 

Pyrolysis is simple thermal destruction of the molecular chain of the base polymer in the adhesive or sealant formulation. Pyrolysis causes chain scission and decreased molecular weight of the bulk polymer. This results in reduced cohesive strength and increased brittleness. Resistance to pyrolysis is predominantly a function of the intrinsic heat resistance of the polymers used in the adhesive formulation. As a result, many of the aromatic and multifunctional epoxy resins that are used as base resins in high-temperature adhesives are rigidly crosslinked or are made of a molecular backbone referred to as a ladder structure, as shown in Fig. 15.4. 

A waste is an ignitable hazardous waste if it has a flash point of less than 60 °C, readily causes fires and burns so vigorously as to create a hazard. A simple method of determining the flash point of a waste is to review the material safety data sheet, which can be obtained from the manufacturer or distributor of the material. Naphtha, lacquer thinner, epoxy resins, adhesives, and oil based paints are all examples of ignitable hazardous wastes. 

Solvent-based adhesives, two-component epoxy resin adhesives, two-component methacrylates for assembly purposes (priming, filling, smoothing, sealing, etc.) For these applications, a multitude of so-called building/ construction adhesives is commercially available. 

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