Epoxy adhesives Amine

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] ... 

In primer formulations for adhesive bonding of metals, the coupling agents that are most frequently used are those based on epoxy and amine functionalities. Aqueous solutions of aminosilanes have been successfully used for obtaining stable adhesive bonds between epoxy and steel [10] and epoxy and titanium [11,12], while epoxy functional silanes are preferable for applications involving aluminum substrates [13,14], A simple solution of % epoxy functional silane in water is currently used for field repairs of military aircraft [15] where phosphoric acid anodization would be extremely difficult to carry out, and performance is deemed quite acceptable. 

A typical comonomer could be a polydimethylsilane to increase the toughness of the polymerized film, or an amine funetional silane for ehemieal eoupling with an epoxy adhesive. 

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. 

The compatibility of the quaternary amlne-contalnlng NTMP with the nltrlle-modlfled epoxy adhesive, which leads to a bond that falls only within the adhesive (l.e., cohesive failure) can be explained. The FM 123-2 adhesive contains the storage-stable curing agent, dlcyanodlamlde, which does not release low-molecular weight amines until It becomes soluble In the resin above 90°C. (Equation 2) (25)... 

Another indication of hydrolysis is shown by the KR. spectrum Figure II. The change in -CH3 absorption at 2980 cm" at room temperature was followed with a F.T.I.R. employing time lapse technique(7). 3) Functional groups that will interact with acid must be present to obtain maximum adhesion. This conclusion is supported by the fact that epoxy and amine containing silanes are all excellent adhesion promotors as shown in Table V. 

Abstract—The structure of films formed by a multicomponent silane primer applied to an aluminum adherend and the interactions of this primer with an amine-cured epoxy adhesive were studied using X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and attenuated total reflectance infrared spectroscopy. The failure in joints prepared from primed adherends occurred extremely close to the adherend surface in a region that contained much interpenetrated primer and epoxy. IR spectra showed evidence of oxidation in the primer. Fracture occurred in a region of interpenetrated primer and adhesive with higher than normal crosslink density. The primer films have a stratified structure that is retained even after curing of the adhesive. 

Diethylaminopropylamine. Diethylaminopropylamine (DEAPA) is an aliphatic polyamine that is used for curing epoxy adhesives where extended pot lives and low heat exotherms are required. This is a reactive primary amine with two active hydrogens as well as a tertiary amine with catalytic activity. 

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. 

There are several reasons why unmodified aliphatic amines, such as those described above, are less widely used than other curing agents in epoxy adhesive systems. These include... 

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. 

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. 

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. 

Table 11.8 presents a typical triethylenetetramine cured epoxy adhesive formulated with selected fillers. In this formulation the use of aluminum powder and alumina increases substantially the resistance of the adhesive to boiling water.7 This is also true when DETA is used as the curing agent.8 A typical room temperature cured aliphatic amine cured epoxy adhesive for general-purpose use is shown in Table 11.9. This shows the difference that is achieved in shear strength by curing at elevated temperatures versus room temperature.

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. 

Figures 11.3 and 11.4 show the tensile shear strength development at 23 and -18°C, respectively, of mercaptan cured epoxy formulations relative to accelerated amine and ami-doamine curing systems. The practical cure times at -18°C illustrate why mercaptan cured epoxy adhesive systems are popular in the building and construction industries where the adhesive can be applied at cold, outdoor temperatures.

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. 

Epoxy-polysulfide systems can be formulated either as a liquid DGEBA epoxy mixed with liquid poly sulfide polymer or as an epoxy-terminated polysulfide polymer either may be cured with a tertiary amine such as DMP-30. Table 11.19 describes the formulation and shows the physical properties of these epoxy-polysulfide adhesives compared to an unmodified epoxy adhesive. 

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. 

Temperature-resistant two-part, elevated-temperature curing epoxy adhesives can be formulated with aromatic amines, such as metaphenylenediamine (MPDA), methylene dianiline (MDA), or a eutectic blend of the two. These adhesives will provide relatively high temperature strength, but they are generally brittle. When mixed with epoxy resin at concentrations of about 15 pph for MPDA and 26 pph for MDA, they provide complete cure in about 30 min at 175°C. The aromatic amines also provide a working life of several hours at room temperature. Starting formulations for aromatic amine cured epoxy adhesives are shown in Table 12.4. 

Accelerators for dicyandiamide cured epoxy adhesive formulations include tertiary amines, modified aliphatic amines, imidiazoles, and substituted ureas. All except the substituted ureas can cure epoxy resins by themselves. All these materials provide good latency and excellent adhesive applications. 

Table 12.9 shows a formulation for an accelerated general-purpose one-component, dicyandiamide cured epoxy adhesive compared to one with a modified aliphatic amine curing agent. Notice that the dicyandiamide cured system provides a higher glass transition tem-... 

TABLE 12.10 Starting Formulation for an Epoxy Adhesive Cured with BF3 Amine Catalyst12... 

Amines have also been microencapsulated within small cellulosic or polyelectrolyte capsules. This is a method for keeping the amine separate from the epoxy resin during storage. When the user decides to initiate cure, the capsules are broken, usually in the application process, and the amine is free to react with the epoxy resin. A successful example of this type of product is an epoxy adhesive that can be preapplied to machine screw threads. When the screw is ultimately threaded into place, the shearing action causes the capsule to break. Bond strengths are generally low for this type of adhesive, but this may not be important in certain applications. 

Although they are more difficult to formulate into epoxy adhesive systems, anhydride cured epoxies have somewhat better thermal stability than amine cured systems. Aromatic and cyclic anhydrides, such as phthalic anhydride, pyromellitic dianhydride, and chlorendic anhydride, provide the most stable structures. 

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