Viscosity epoxy adhesives

Epoxies can cure in deep sections and are useful in potting and deep-section sealing applications. They adhere well to different substrates and therefore are used in the general assembly of many medical devices. A clear, medical-grade, low-viscosity epoxy adhesive has proved useful in the fabrication of access ports that are implanted beneath the skin of patients who require multiple infusions.21... 

Secondary ingredients in epoxy adhesives include reactive diluents to adjust viscosity mineral fillers to lower cost, adjust viscosity, or modify the coefficient of thermal expansion and fibrous fillers to improve thixotropy and cohesive strength. Epoxy resins are often modified with other resins to enhance certain properties that are necessary for the application. Often these modifications take the form of additions of elastomeric resins to improve toughness or peel strength. 

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. 

The viscosity increase in a epoxy resin-curing agent system could result in poor wetting of the substrate surface, resulting in suboptimal adhesion. Several reaction mechanisms can also occur to an epoxy adhesive once it is mixed and applied to a substrate but before the substrates are mated. These mechanisms can result in a weak boundary layer, which will prevent optimal wetting and reduce the strength of the adhesive. 

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. 

When used in epoxy adhesive systems, solvents are generally employed for reducing the viscosity for formulation or application purposes. Once the adhesive is applied to the substrate, the solvent must evaporate prior to cure. Otherwise, bubbles or vapor pockets could form in the bond line, causing a physically weak joint with poor adhesion. The solvent in the adhesive formulation must not adversely affect the substrate to which it is applied. Plastics, elastomers, and polymeric foams are especially sensitive to certain solvents used in epoxy adhesives. 

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. 

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. 

Functionalized, liquid polybutadiene derivatives have also been developed as hybrid flexiblizers for epoxy resins. Carboxyl-terminated butadiene/acrylonitrile polymers, butadiene homopolymers, and maleic anhydride-amino acid grafted butadiene homopolymers have been used as flexibilizers to impart good low-temperature strength and water resistance to DGEBA-based epoxy adhesives. An epoxy system toughened by polybutadiene with maleic anhydride is claimed to provide a hydrophobic backbone, low viscosity, softness, and high tensile strength and adhesion (Table 7.10). 

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

One of the disadvantages of CTBN-epoxy adhesives has been their high viscosity, which limits additional formulation options. Recently new adducts, such as EPON 58003 and RSM-2577 from Resolution Performance Products LLC, have been introduced which have significantly lower viscosities.19 In addition, lower concentrations of these new CBTN-epoxy adducts are generally required to achieve equivalent adhesive performance. 

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... 

In two-part epoxy adhesive systems, the filler can generally be incorporated into either the epoxy resin or the curing agent component. Prevailing factors will be the characteristics mentioned above (viscosity of each component, dispersion characteristics, etc.). However, the effect on the mix ratio and on the ease of mix-... 

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. 

Talc is also often used as an extender in epoxy adhesives and sealants, but it also has flow control properties. Talc is used in higher-solids, high-viscosity applications such as caulking compounds, automotive putties, mastics, and sealants. Talc is a hydrophobic and organophilic material. 

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. 

As a family of curing agents for epoxy resins, the amidoamines are lower in viscosity than the polyamides. They exhibit very good adhesive properties due to their chemical structure and easy penetration. Amidoamine cured epoxy adhesives have shown very good properties on concrete and other porous substrates. They cure extremely well under humid conditions. In fact amidoamine cured epoxy formulations have been used to cure underwater in certain applications. A typical general-purpose room temperature curing epoxy-amidoamine system is described in Table 11.7. This adhesive is used as a general-purpose metal-to-metal adhesive and body solder in the automotive industry. 

Regardless of the heating source, elevated-temperature curing epoxy adhesives necessitate that the parts be held with clamps, pressure pads, or fixturing methods to apply constant and uniform pressure throughout the curing cycle. Reduction in viscosity due to elevated temperatures could result in excessive adhesive flow during cure. This could result in joints that are starved of adhesive or movement of the parts, if not firmly fixtured. 

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. 

The applications and performance characteristics of waterborne epoxy adhesives can be significantly improved by the incorporation of additives and modifiers into the adhesive formulation. Fillers such as calcium carbonate, talc, and silicas are often used to adjust the viscosity of the liquid adhesive and the thermal expansion, modulus, and strength characteristics of the cured adhesive film. 

The usual approach to good bonding practice is to prepare the aluminum surface as thoroughly as possible, then wet it with the adhesive as soon afterward as practical. In any event, aluminum parts should ordinarily be bonded within 48 h after surface preparation. However, in certain applications this may not be practical, and primers are used to protect the surface between the time of treatment and the time of bonding. Primers are also applied as a low-viscosity solution which wets a metal surface more effectively than more viscous, higher-solids-content adhesives. Corrosion-resistant epoxy primers are often used to protect the etched surface during assembly operations. Primers for epoxy adhesive systems are described in Chap. 10. 

Adhesives commonly used on concrete must be formulated so that they cure well in the presence of moisture. The adhesive should have a viscosity that allows it to penetrate the concrete for mechanical bonding. Epoxy and epoxy-polysulfide adhesives are especially effective adhesives for concrete. The starting formulations given in Table 16.16 are for an ambient cure 100 percent solids epoxy and a waterborne epoxy adhesive. 

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. 

Next to viscosity, epoxy content and hydroxyl content are the prime properties of importance in formulating epoxy-based adhesive systems. Epoxy content allows one to determine the correct amount of curing agent to use. With certain curing agents it may also be important to know the hydroxyl content. These values determine the reactivity of the system. 

Although a major advantage of epoxy adhesives is their ability to be formulated without solvents, under certain circumstances solvents may be included. On porous substrates solvents may be added to reduce viscosity and assist penetration. On certain nonporous substrates, particularly some plastics, addition of a small percentage (1 to 3%) of a suitable solvent will improve adhesion. Common solvents are low-boiling aromatic solvents, ketones, or esters. 

Some of the first commercial conductive epoxy adhesives were simply based on silver powder dispersed in a liquid epoxy resin [e.g., diglycidyl ether of bisphenol A (DGEBA)] with an aliphatic amine [e.g., triethylene tetramine (TETA)] as a curing agent. Although capable of room-temperature cure, commercialization of this type of system was hampered by severe mix ratio disparity (typically, 50 1 by weight), problematic mixing due to viscosity differences, short pot life, and safety concerns. See Table 3 for typical properties of this type of formulation. 

The viscosity of liquid adhesives decreases with increasing temperature until polymerization begins to occur, then increases. The viscosities of epoxies are reported to decrease 20% to as much as 80% for a 10°C rise in temperature above room temperature.Figure 2.4 shows this viscosity reduction as a function of temperature for two anhydride-cured epoxy adhesives. For low-molecular-weight polymers, the temperature dependence of viscosity follows the Arrhenius equation ... 

Surface treatments for Cab-O-Sil also influence the viscosity and thixotropic properties of an adhesive as shown in Fig. 3.18 where MS-5 is untreated Cab-O-Sil, TS-720 is treated with polydimethylsiloxane, and TS-610 and TS-530 are treated with dimethyldichlorosilanes. The silica filler treated with polydimethylsiloxane (TS-720) produces greater van der Waals forces of attraction and thus higher viscosity compared to TS-610 and TS-530.1 The surface area of the filler has a pronounced effect on viscosity. As the surface area of silver-flake filler increases, the frictional forces between particles increase resulting in higher viscosity. Epoxy formulations filled with constant-size silver flakes of various surfaces areas show this effect when viscosity is plotted as a function of the filler surface areas (Fig. 

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