Epoxy-polyamide adhesives

The two-part epoxy adhesive, readily available in hardware stores or other consumer outlets, comes in two tubes. One tube contains the epoxy resin, the other contains an amine hardener. Common diamine room temperature epoxy curing agents are materials such as the polyamides, available under the trade name Versamid. These polyamides are the reaction products of dimer acids and aUphatic diamines such as diethylenetriamine [111-40-0] ... 

Fig. 5. Examples of ihe correlation between measured adhesive strength and (l+cos6). (a) Plot of data from Raraty and Tabor [171J for adhesion of ice to various solids, (b) Plot of data of Barbaris [172] for adhesion of a mixture of epoxy and polyamide resin to low density poly(ethylene) treated in various ways. Both figures from ref. [31], by permission.

Condensation polymers, which are also known as step growth polymers, are historically the oldest class of common synthetic polymers. Although superseded in terms of gross output by addition polymers, condensation polymers are still commonly used in a wide variety of applications examples include polyamides (nylons), polycarbonates, polyurethanes, and epoxy adhesives. Figure 1.9 outlines the basic reaction scheme for condensation polymerization. One or more different monomers can be incorporated into a condensation polymer. 

Primer Epoxy vs. Nitrile-Modified Epoxy. The compatibility of the epoxy-polyamide primer with the nitrile-modified epoxy adhesive facsimile and the aluminum oxide surface was also evaluated by the wedge test, since earlier tests using the primer as the adhesive had failed immediately. As shown in Fig. 8, the addition of the primer directly to the prepared... 

The corrosion resistance and polymer-bonding compatibilities of the lonizable organophosphonates and the neutral organo-silanes are directly related to their inherent chemical properties. Specifically, NTMP inhibits the hydration of AI2O2 and maintains or Improves bond durability with a nitrile-modified epoxy adhesive which is cured at an elevated temperature. The mercaptopropyl silane, in addition to these properties, is compatible with a room temperature-cured epoxy-polyamide primer and also exhibits resistance to localized environmental corrosion. These results, in conjunction with the adsorbed Inhibitor films and the metal substrate surfaces, are subsequently discussed. 

Adsorbed NTMP exhibits a pH-dependent surface coverage on anodized aluminum, which Includes a region characterized by a multilayer of hydrogen-bonded phosphonate molecules. These thick layers are weak and fall to provide good bond durability in a humid environment. NTMP monolayers are protective against hydration and are compatible with a nitrile-modified epoxy adhesive, but not with an epoxy-polyamide primer topcoat. 

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. 

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. 

FIGURE 3.8 Journal bearing application. Outer cylinder (stainless steel) is bonded to inner cylinder (polyamide-imide) with an epoxy adhesive. Exposure to low temperatures causes significant stress on bond due to differences in coefficient of thermal expansion. 

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. 

The pot life of polyamide or amidoamine cured epoxy adhesives is generally on the order of hours at room temperature. Full cure is achieved in 5 to 7 days at room temperature, and handling strength is achieved in about 16 to 24 h. A faster cure can be achieved in 20 min to 4 h by heating to 60 to 150°C. When room temperature cures are required, an accelerator such as an amine is often added to the formulation. 

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. 

As with amidoamine and polyamide cured adhesives, epoxy resins cured with aliphatic amines exhibit tensile shear strength that is dependent on the type of filler and concentration. Table 11.10 shows the effect of filler loading on strength of a simple general-purpose, room temperature curing epoxy adhesive composed of liquid DGEBA epoxy mixed with 10 pph of a tertiary amine. 

TABLE 11.13 Starting Formulation for a Flexible Epoxy Adhesive Containing Polyamide Curing Agent and Reactive Diluent18... 

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

When the epoxy adhesive cannot be made flexible enough, the thermal conductivity and thermal expansion coefficient are controlled by appropriate fillers. General-purpose room temperature cured epoxy-polyamide adhesive systems can be made serviceable at low temperatures by the addition of appropriate fillers to control thermal expansion. 

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

General-purpose epoxy adhesive with amidoamine curing agents Effect of fillers on tensile shear strength of polyamide cured epoxy... 

Starting formulation for a flexible epoxy adhesive containing polyamide curing agent and reactive diluent Starting formulation for a high-peel-strength adhesive Epoxy-polysulfide adhesive formulation Formulation for a flexible epoxy-polysulfide adhesive Epoxy-polysulfide adhesive formulations... 

Polyamide (nylon) Acetone, methyl ethyl ketone 1. Abrasion. Grit or vapor blast or abrade with 100-grit emery cloth followed by solvent degreasing. 2. Prime with a spreading dough based on the type of rubber to be bonded in an admixture with isocyanate. 3. Prime with resorcinol formaldehyde adhesives. Sand or steel shot is suitable abrasive Suitable for bonding polyamide textiles to natural and synthetic rubbers Good adhesion to primer coat with epoxy adhesives in metal-to-plastic joints... 

These polyamidoamines are available from several suppliers worldwide (Cognis (Veramide ), Arizona Chemical (Uni-Rez ), Air Products (Ancamide ), and others) and are among the most common curatives in the general-purpose, do-it-yourself two-pack-age epoxy adhesives. They have a distinctive odor somewhat like popcorn and are easily recognized in adhesive formulations. The polyamide backbone does contribute to the overall good mechanical properties of the polyamide amine cured adhesives. 

Polystyrene Although polystyrene is usually bonded by solvent cementing, it can be bonded with vinyl acetate/vinyl chloride solution adhesives, acrylics, polyurethanes, unsaturated polyesters, epoxies, urea-formaldehyde, rubber-base adhesives, polyamide (Versamid-base), polymethylmethacrylate, and cyanoacrylates. The adhesives should be medium-to-heavy viscosity and room-temperature and contact-pressure curing. An excellent source is a Monsanto Company technical information bulletin which recommends particular commercial adhesives for bonding polystyrene to a number of different surfaces. Adhesives are recommended in the fast-, medium-, and slow-setting ranges (10). 

In addition to FRP and adhesives, a third important use of epoxy resins in aerospace is in surface coatings. Epoxy resin-polyamide combinations are used as primers under urethane top coats. The most... 

In many adhesives formulations, the resin portion is the same or similar and what determines its properties is the hardener or catalyst that is used to cure the resin. Thus, depending on the hardener, epoxy adhesives may be referred to as amine-cured, anhydride-cured, polyamide-cured, or novolac-cured. Polyurethanes may be polyol-cured or hydroxypolyester-cured. 

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