Epoxy adhesive formulations

Tetrahydrofurfuryl alcohol is used in elastomer production. As a solvent for the polymerization initiator, it finds appHcation in the manufacture of chlorohydrin mbber. Additionally, tetrahydrofurfuryl alcohol is used as a catalyst solvent-activator and reactive diluent in epoxy formulations for a variety of apphcations. Where exceptional moisture resistance is needed, as for outdoor appHcations, furfuryl alcohol is used jointly with tetrahydrofurfuryl alcohol in epoxy adhesive formulations. 

Materials and additives that are chemically basic in nature have a detrimental effect on the curing of cationic-initiated epoxy systems. These substances can either stop the curing mechanism completely or produce under-cured polymers. Therefore such additives as amines or imides that are known to be adhesion promoters cannot be used in the EB-curable epoxy adhesive formulations. 

Other effects were more selective. While recycle usually lowered impact strength and heat deflection temperat ire, pre-soaking polyamine recycle surprisingly improved both of these properties. While recycle usually lowered volume resistivity, polyamide recycle improved it. Finally, adhesion of epoxy formulations to the aluminum mold, in spi/ e of wax and silicone mold release agents, was dramatically increased by the use of presoaked recycle, especially in the ai ydrlde system, suggesting unexpected usefulness in epoxy adhesive formulations. 

Such modification is commonly described as formulating or compounding. Formulating is necessary to achieve an adhesive that will yield the desired application characteristics and end-use properties at an acceptable cost. As a result, an enormous number of epoxy adhesive formulations are possible. 

Single-component epoxy adhesive formulations are the largest type of epoxy adhesives sold, with about 55 percent of the consumption, while two-component formulations account for another 44 percent of the volume. Radiation cure formulations represent the remainder of the market. Epoxy adhesives can also take many forms including solids, solvent-free liquids, solvent-borne systems, and waterborne systems. 

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. 

The job of the adhesive formulator has been made particularly difficult by the lack of practical information on the topic. There are only several forums that provide an introduction to adhesive formulating. One such forum is the Adhesive and Sealant Council, which offers short courses on adhesive formulating. There are also a few books that offer information and guides on adhesive formulations.23-27 Information specific to epoxy adhesives is usually found in a chapter or section within the work. There has been no book devoted solely to epoxy adhesive formulations, although several have focused on the more general topics of epoxy resins and their applications.28,29... 

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. 

Two types of epoxy resins are formed by this process (1) cycloaliphatic resins and (2) aliphatic resins. Of the many structures that can be synthesized by this process, the cycloaliphatic diepoxies offer the most interesting combination of properties. However, the aliphatic epoxy resins have the greatest utilization in epoxy adhesive formulation. 

TABLE 3.3 Surface Tension of Several Liquids Including Epoxy Adhesive Formulations (Top) and Critical Surface Tension of Various Substrate Materials (Bottom)... 

The major problem is that the attainment of properties such as peel, flexibility, and toughness is generally accompanied by the reduction in properties such as heat resistance, chemical resistance, and shear strength. Future chapters discuss how the epoxy adhesive formulator can merge these properties. 

Perhaps the most significant property that is controlled by the degree of crosslinking is the glass transition temperature Tg. The importance of Tf in epoxy adhesive formulations is discussed next. 

The epoxy resin is a primary component in any epoxy adhesive formulation, and it is often referred to as the base polymer. However, it is certainly not the only or even not always the predominant component in influencing desirable end properties. Epoxy resins by themselves are often too rigid to provide the required properties such as flexibility, peel and impact strength, and thermal cycling resistance. As a result, they are often modified with other components or hybridized with other types of polymeric resins to provide these functions. 

Future chapters describe the other raw materials that contribute to the epoxy adhesive formulation (curing agents and catalysts, Chap. 5 solvents and diluents, Chap. 6 hybrid resins, Chap. 7 flexibilizers and tougheners, Chap. 8 fillers, Chap. 9 and adhesion promoters, Chap. 10). Complete adhesive formulations are then discussed in subsequent chapters. 

Their use in adhesive systems is minimal because they are relatively brittle and higher in cost than aromatic resins. However, cycloaliphatic epoxy resins are used in cationi-cally cured epoxy adhesive formulations. These are cured via uv or electron beam (EB) radiation. 

DEAPA was used in several early commercial epoxy adhesive formulations. Schonhom and Sharpe9 have shown that this amine is surprisingly effective in reducing the surface tension of epoxy resins (Table 5.4). It is speculated that the utility of DEAPA adhesives is in part due to better wetting than other epoxy formulations. 

Two curing agents that have found their way into many epoxy adhesive formulations are the polyamides and amidoamines. These are commonly used in the hardware store variety two-part epoxy resins that cure at room temperature. Both are reaction products of aliphatic amines, such as diethylenetriamine, and should be included under the subclassification of modified amines. However, these products have such widespread and popular use, they are addressed here as a separate classification. 

Anhydrides are sometimes used in epoxy adhesives to provide specific properties or to provide improvements in handling strengths. The most important anhydride in epoxy adhesive formulations is pyromellitic dianhydride (PMDA), which provides very high temperature properties. 

Benzyldimethylamine (BDMA) is another tertiary amine that can be used as either a sole catalyst or an accelerator with other curing agents. It is used with DGEBA epoxy resins at 6 to 10 pph. The pot life is generally 1 to 4 h, and the cure will be complete in about 6 days at room temperature. When used by itself, BDMA can provide epoxy adhesive formulations with high-temperature resistance (Chap. 15). However, BDMA is mostly used as an accelerator for anhydride and dicyandiamide cured epoxy resins. 

EMI cured epoxy adhesive formulations are claimed to have outstanding adhesion to metals, and for this reason it is added as a co-curing agent in many compositions. It is an excellent anhydride accelerator providing higher thermal resistance than typical tertiary amine accelerators. 

Epoxy adhesive formulations demand a great variety of solvents and diluents with a wide range of evaporation rates, solvent strengths, and dispersion powers. These variations are required due to (1) the many types of epoxy resins, curing agents, and possible organic additives that can be used within a formulation and (2) the many different possible methods that can be used to apply the epoxy to the substrate (brush, spray, trowel, etc.). 

The choice of solvent or diluent is made with regard to the solubility of individual components and to the viscosity, drying times, and wetting characteristics required of the final product. All these properties affect the bond performance of the resulting epoxy adhesive formulation. 

The choice of solvent or solvent blend for a particular epoxy adhesive formulation will primarily depend on... 

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. 

Reactive diluents enter into the polymerization reaction of the epoxy resin and the curing agent. In this way the final adhesive characteristics are determined by the reaction product of the binder and the diluent. The most common reactive diluents used for epoxy adhesive formulations are shown in Table 6.2. 

Since many of the difunctional epoxy diluents are added to epoxy adhesive formulations as flexibilizers as well as viscosity reducers, they are discussed in greater detail in Chap. 8 with other flexibilizing additives. 

TABLE 7.3 Comparison of Bond Strengths of Epoxy Adhesive Formulations with and without Nitrile Rubber Addition1... 

Epoxy adhesive formulators have generally addressed the problem of improving flexibility in several ways ... 

FIGURE 8.2 Impact resistance of varying concentrations of polysulfide resin in an epoxy adhesive formulation.6... 

Diluents are primarily used in epoxy adhesive formulations to reduce viscosity. However, diluents also provide a degree of flexibility by two mechanisms ... 

CTBN and ATBN are the most commonly used in structural epoxy adhesive formulations. CTBN (Fig. 8.6) is generally the elastomer of choice because of its miscibility in many epoxy resins. These tougheners were originally developed by BF Goodrich (now Noveon, Inc.) under the tradename Fly car. 

Fillers and extenders are used in epoxy adhesive formulations to improve properties and to lower cost. Properties that can be selectively improved include both the processing properties of the adhesive as well as its performance properties in a cured joint. However, the use of fillers can also impair certain properties. Typically, the formulator has to balance the improvements against property decline. The advantages and disadvantages of filler addition in epoxy formulations are listed in Table 9.1. Common fillers used in epoxy formulations and the properties that they are used to modify are shown in Table 9.2. 

The filler concentration in any epoxy adhesive formulation will depend primarily on the following three factors ... 

Solvent addition or blending the epoxy resin with low-molecular-weight diluents is another method of lowering the viscosity so that fillers can be efficiently added to the epoxy adhesive formulation. However, in these cases the formulator must address the high vapor pressures of the solvent or diluent (as well as various health, safety, and environmental issues). In the case of diluent addition, the reduction in crosslinking density and thermal or... 

This section considers only the more common types of fillers for epoxy adhesive formulations. They are categorized by the specific contributions that they provide to adhesive properties. Modifications to the following properties are discussed ... 

Inorganic Extenders. There are many mineral extenders that are commonly used in epoxy adhesive formulations. The more important ones are summarized below. 

Nonconductive fillers are employed with electrical-grade epoxy adhesive formulations to provide assembled components with specific electrical properties. Metallic fillers generally degrade electrical resistance values, although they could be used to provide a degree of conductivity as discussed above. 

Properties of an epoxy system filled with glass microballoons are shown in Table 9.11. Other low-density microballoons, based on phenolic and other materials, have been developed for use in epoxy adhesive formulations. 

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