One-part epoxy adhesives

Figure 15.22 shows the long-term effect of a heat cured one-part epoxy adhesive to various chemical environments. As can be seen, the temperature of the immersion medium is a significant factor in the aging properties of the adhesive. As the temperature increases, the adhesive generally adsorbs more fluid, and the degradation rate increases. 

The type of pumps used to dispense epoxy adhesives depends on whether it is a one-part or two-part adhesive. One-part epoxy adhesives are dispensed using direct metering extrusion pumps. An electric motor pushes a follower plate into a drum of adhesive, which is then extruded through a hose to the dispensing valve. This technique is used for medium-and high-volume production rates. 

Figure 1. Lap-shear strength (top) and T-peel strength (bottom) of one-part epoxy adhesives (Adhesive A formulations). Key CRS, oily cold-rolled steel EGS, electrogalvanized steel CTy control (unmodified resin) T-Ay T-By and T-C refer to particle sizes of MEAc tougheners 5 to 45 pm (T-A)y 17.8 pm (T-B), and 2.3 pm (T-C) and L13y Hycar CTBN (1300x13).

One-part epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-part epoxy adhesives are usually comprised of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed. In general, two-part systems are mixed, applied within the recommended pot life (a few minutes to several hours), and cured at room temperature for up to 24 hours, or at elevated temperatures to reduce the cure time. Typical cure conditions range from 3 hours at 60°C to 20 minutes at 100 C. ... 

One-part epoxy adhesives generally need to be kept frozen and allowed to come to room temperature before use. In this case, the pot life is the time where the adhesive is workable at the ambient. With two-part systems, the epoxy resin and hardeners are mixed just before use and the pot life is taken from that time. Any change in parameters such as viscosity, skin-over, or loss of bond strength may be... 

Other crosslinkers such as aromatic amines, dicyandiamide, dihydrazides, and boron trifluoride amine complexes give stable formulations at room temperature and only eine when heated at elevated temperatures. These cme systems are the basis for one part epoxy adhesives. Crosslinkers such as polysulfides and polyamides can be used to increase the flexibility of the adhesive bonds. Other eomponents that may be added to modify the properties of the adhesives are diluents, flexibilizers, modifiers, fillers, and reinforcing agents. 

O Figure 37.9 shows the viscosity stability over time at 23°C storage for two one-part epoxy adhesive formulation. Adhesive B increases only very little in viscosity over time and therefore is significantly more stable than adhesive A. Adhesive A would need to be stored in a cool area, for example, in a fridge or a air conditioned warehouse. 

O Figure 37.10 shows the decrease of the lap shear strength of shear specimens which are bonded with one-part epoxy adhesives and stored under 80% relative humidity at 23°C before being cured. Adhesive B shows a much better resistance to water absorption than adhesive A. If using adhesive A for bonding, the joint should be cured quickly, at least before about 1 week after the application of the adhesive. Adhesive B could stay uncured for about 6-8 weeks. 

The preparation for bonding is different, whether one-part heat-curing adhesives or two-part formulations are used. In the case of one-part epoxy adhesives, the surface just needs to be cleaned from any contamination like dust. Oil is tolerated in an amount lower than about 3 g/m. Higher viscous oils like dry lubes are tolerated up to about 2 g/m. In case of applying two-part formulations, with the exception of acrylic-epoxy hybrids, oil and any other contamination need to be carefully removed. 

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] ... 

ATBN liquid polymers cannot be mixed directly into the epoxy resin component of a two-part epoxy adhesive or in a one-part adhesive since crosslinking and shortened shelf life will result. ATBN adducts are, therefore, mixed with the curing agent component of two-component epoxy adhesives.30... 

Visible light cured epoxy adhesives and coatings have been developed for architectural, industrial, and maintenance applications and for products difficult to heat or uv/EB cure because of their size. These are clear, one-part epoxy resins that cure by exposure to visible light for a few hours. They are formulated with cycloaliphatic epoxy compounds and a cationic photoinitiator that generates a strong acid when exposed to sunlight. 

Induction Curing Adhesives. Typically one-component epoxy adhesive systems are used for induction bonding of structural parts. However, two-component adhesives have also been used. The properly designed induction cured adhesive should provide the following application and performance properties ... 

FIGURE 15.17 Effect of adhesive on the durability of etched 6061-T6 aluminum alloy joints exposed to a marine environment. (1) Two-part epoxy, (2) one-part epoxy, (3) nitrile-phenolic, (4) vinyl-phenolic, (5) vinyl-phenolic.33... 

Powder adhesives are generally one-part, epoxy-based systems that do not require metering and mixing but often must be refrigerated for an extended shelf life. Elastomeric... 

Emcast [Bectronic Materials] One-part epoxy resins uv-curable flexible resin for decttonic encqpsulaticni, d n g, and adhesion applies. 

Uniset . [Emerson Cuming] One-part epoxies and uv-cured systems circuit assembly materials, dielectric and con-ducdve adhesives. 

Other adhesives recommended hy ICI are 3M Company s Scotch Weld 2216 two-part epoxy, Amicon s Uniset A-359 one-part aluminum-filled epoxy, American Cyanamid s BR-89 one-part epoxy, Bostik s 7026 synthetic ruhher and 598-45 two-part adhesive, GE s Silgrip SR-573, and Goodyear s Vitel polyester with isocyanate curing agent. 

Application and utilisation of these film adhesives Structural thermosetting film adhesives The film is cut to the desired size and laid on one of the substrates (refer to Fig. 92). Parts are assembled and hot pressed in an autoclave or a hot press, for instance for a one component epoxy adhesive for 30 min at 120°C, under a pressure of 100-300 kN/m for REDUX 312, or 60 min at 170°C with REDUX 308 or REDUX 322. 

They are placed between parts to be bonded, they melt during the oven cycle and bond the parts together. They may also act as a potting and encapsulating product. The epoxy formulation may also contain conductive particles in order to allow cure by induction or high frequency. Curing conditions are those of the one-component epoxy adhesives, i.e. from 15 to 60 min at temperatures ranging from 120 to 160°C. 

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