Assembly Considerations
Adhesives have been a subject of considerable research due to the advantages they offer in joining applications. Improvements are continuously being made in the technology. A designer today has many choices and many families of adhesives at his or her disposal. Adhesives are polymeric materials. Common adhesives are based on elastomeric, thermoplastic and thermosetting resins. Neoprene, nitrile, urethanes, and styrene butadiene-based adhesives are elastomer-based adhesives. Acrylic, cellulose nitrite, cyanoacrylate, EVA, polyamides, polyvinyl acetate, and thermoplastic polyester are examples of thermoplastics-based adhesives. Anaerobic, butyral phenolic, reactive acrylate monomer, silicones, thermosetting polyester, and urea formaldehyde are some of the thermosetting resin-based adhesives. Adhesives are also available as water-based or solvent-based formulations. Given below are short introductory remarks on some of the different families of adhesives. Adhesives are constantly evolving. Formulations are available in consistencies ranging from water-like to gels, can be cured in different ways (e.g. one-part, two-part, light curing etc.), and have different property profiles (e.g. flexible, rigid, conductive, nonconductive, impact modified etc.). Consult adhesive suppliers or their literature for more information. Acrylic adhesives are structural adhesives. They have good low and high (to 350°F) temperature resistance and can develop joint strengths up to 3000 psi. One-part acrylics can be cured with heat or UV light. Two-part acrylics are catalyst activated and can be cured at room temperature. Anaerobic adhesives only cure when in contact with active metal ions and in the absence of oxygen. They are aggressive with polycarbonates and are, therefore, not recommended for use with them. They are used as thread lockers for preventing screws from loosening from vibration. Cyanoacrylate adhesives, also known as instant adhesives, are fast curing adhesives that polymerize in the presence of moisture. These solvent-free, one-part adhesives can develop bond strengths up to 2000 psi. They are available in viscosities ranging from water-thin to thixotropic gel. They have poor peel strength and are aggressive to some plastics. They are sometimes used as fixturing adhesives for slower curing epoxies. Epoxies are typically rigid, two-part adhesives. They cure at room temperature and can attain very high joint strength (2000 to 6000 psi). They can fill gaps and have high temperature resistance to 350°F. They can be used with a wide variety of thermoplastics, have high cohesive strength, and offer good environmental resistance. Two-part epoxies are sensitive to mix ratio and they have typically long cure times. Hot melt adhesives are usually polypropylene, EVA, polyesters, or polyamides resins. They have strength limitations, having strength only in the range of 100 to 500 psi. They are generally not used in plastic to plastic assembly but are used in plastic to wood assemblies. Phenolic adhesives are structural adhesives developing joint strength up to 3000 psi. They have good chemical resistance and high temperature resistance to 300°F. Polyurethane adhesives are structural adhesives that can attain strengths to 2500 psi. They can seal as well as bond, have good gap-filling capabilities, and have high temperature resistance to 250°F. The newer, reactive polyurethanes are like hot-melt adhesives that are dispensed hot. After cooling, they react with moisture and crosslink into a tough thermosetting Urethane adhesive. Pressure Sensitive adhesives (PSAs) are usually rubber-based adhesives that are used for their flexibility and their ability to be removed without leaving a residue. They are cured by solvent evaporation. A light pressure is used for bonding. They have low strength (10 psi only), and poor chemical resistance. Silicone adhesives are typically cured via reaction with ambient humidity. They cure to soft thermoset elastomers with moderate strength but excellent property retention over an extensive temperature range, from a low of 100°F to a high of 600°F. Silicones have good adhesion to many substrates, but are limited in their utility as structural adhesives by their low cohesive strength. For a more detailed list of adhesive suitable for Bayer engineering thermoplastics, consult Bayer's publication "Joining Techniques—A Design Guide". |