Acrylic adhesives environmental resistance

In addition to the methacrylate monomers that are responsible for adhesive hardening, and the initiators and stabilizers already mentioned, most acrylic adhesives also contain polymers of various types and molecular weight. These additional components are included by formulating chemists to control handling properties, cure rates, flexibility, strength, adhesion, and environmental resistance. The exact nature of specific components is held as a trade secret by companies. Some insight into the types of polymers used and their combinations and levels can be found by searching patent literature and various technical journals. 

UV-curing aerobic acrylic adhesives, while less expensive on a per pound basis than cyanoacrylates, provide a structural wire-tack that resists environmental conditions. A useful property is that the aerobic acrylics fluoresce to optimum brightness only upon completion of cure, providing a visual means of end-point determination. An elastomer formulation is recommended for permanent wire-tacking in a high vibration environment. Removable wire-tacking formulations are also available (Fig. 1). 

Modification of properties in acryUc adhesives is more often conducted at the chemical level, through changes in formulation or combination with other base resins. Wide ranging properties can include impact resistance, surface insensitivity, environmental resistance, etc. The emergence of urethane acrylate adhesives, as well as acrylated epoxies, begins to make simplistic adhesive classifications more challenging. 

Although development of toughened adhesives has been limited to the various acrylic (both anaerobic and non-anaerobic) and epoxy types, overall performance has been improved spectacularly. Both environmental resistance and peel and impact performance have been raised substantially without sacrificing shear strength. 

Unmodified materials are brittle, so when the phenolics high strength and environmental performance are required and there is any possibility of flexing or impact damage, then the modified versions must be used. With the development of the toughened epoxy and acrylic adhesives (see Section 5.1.12), the phenolics-even the modified versions - should only be used where their excellent durability and heat resistance is vital. 

In a study of nine commercially available acrylic adhesives, Wilkinson and Tyler found a considerable variation in the resistance to environmental exposure. The tests employed were 1,000-h water soak, 30 days at 125°F/100% RH, and 1,000-h 5% salt spray at 95°F aluminum adherends were used. The nature of the alloy was also a factor in determining joint durability. The authors concluded that excellent durability can be obtained with the proper selection of adhesive and alloy. A similar study by Zalucha using recently developed high-performance acrylic adhesives also indicates excellent performance for many acrylic-bonded assemblies. Minford has conducted weathering tests on both the early formulations and the more recently introduced second generation acrylics." He found that the durability of the earlier products was generally poor in both water soak and... 

The development of high strength thermoplastic adhesive copolymers of the EAA type containing reactive carboxyl groups has made it practical to bond insulation such as polyethylene to metal conductors to provide adequate bonding and long term environmental resistance. Frequently, the EAA copolymer is extruded around the cable. Peacock reports that when a layer of acrylic acid-ethylene copolymer (7 43) was extruded at 140°C over a flexible aluminum conductor followed by a layer of polyethylene insulation, the bond strength between the insulation and the conductor was 72 N/cm compared with 1.4 N/cm for a control assembly without the copolymer adhesive. There are numerous similar patent claims for insulated conductor cable. 

This task represents a continuation of efforts to maximize the hydrophobicity of acrylic, epoxy, and other polymeric systems for resistance to water penetration and environmental degradation, and to minimize the dielectric constant and improve the processability for adhesives and coatings, without compromising the necessary structural characteristics for materials used for, e.g., structural elements, liners, paints, and microelectronic devices. 

The top-coats are responsible for the durability and resistance to water and chemical impacts from the environment. Mainly acrylic and fiuori-nated polymers are used for top-coats on various base coats (PVC, PUR, PTFE, silicone rubber, etc.) to enhance resistance to soiling and ageing. Certain additives for UV protection can be added. Due to its self-cleaning non-adhesive surface the top-coat is often hard and brittle. On PVC coatings the top-coat gives not only protection against environmental infiuences but also a barrier against emission of plasticizer out of the PVC. 

Manufacturers have begun using UV adhesives because of the very large differences in the cost of energy used for cure and the increase in productivity associated with faster cure cycles. Specially designed UV-aerobic acrylics cure rapidly, adhere to borosilicate glass, and withstand the severe environmental and vibration resistance required of automotive parts. 

Water-based acrylic paints provide no harmful solvents. They are easy to apply and a wide variety of colors is available. Lacquers contain solvents that evaporate allowing the paint coating to dry without chemical reaction. The lacquer must be carefully chosen so the solvent is compatible with the plastic being painted. Enamels are based on a chemical reaction that causes the coating film to form and harden. The effect of this reaction on the parent plastic part must be carefiilly noted. Certain plastics may require a primer to help improve the adhesion of the paint to the plastic. After coating, a top coat may also be required to improve the environmental or wear resistance of the paint. Two part paints, such as epoxy or urethane coatings, provide excellent adhesion qualities and do not require a top coat. 

Because of their wide property range, clarity, and resistance to degradation by environmental forces, acrylic polymers are used in an astounding variety of applications that span the range from very soft adhesive materials to rigid non-filmforming products (Table 11). 

Blends of ethylene copolymers (such as ethylene-vinyl acetate, ethylene-ethyl acrylate, ethylene-acrylic acid copolymers) have been typically added to LDPE, HDPE and LLDPE to improve filler and additive acceptance, balance film properties, improve environmental stress crack resistance, tear resistance, toughness and surface properties. These blends are particularly prevalent in film formulations and as such are rarely disclosed by the manufacturer. Equistar has introduced functionalized polyolefins (Integrate ) to improve dispersion and adhesion in wood fiber filled and mineral filled polyolefin composites. Arkema Group offers Lotryl ethylene-acrylate (methyl, butyl and 2-ethyl hexyl) copolymers and notes the wide range of compatibility with other polyolefins as well as polyamides and polyesters. 

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