Polyurethane adhesives and sealants provide excellent flexibility, impact resistance, and durability. They are available in one-part or two-part systems. Polyurethanes, which are also referred to as urethanes, are formed through the reaction of an isocyanate component with amines, polyols or other active hydrogen compounds. They bond well to plastic surfaces and make an excellent flexible potting compound. Polyurethane adhesives and sealants require a catalyst, heat, or air evaporation to initiate and complete curing. Some disadvantages of polyurethane adhesives and sealants are their short shelf life due to hydroscopic (water absorption) tendencies, and their generally slower cure times. More complex handling and curing procedures are also required.
There are a wide variety of polyurethane adhesives and sealants available. They differ in terms of features such as bulk flexibility, resistance to oil or chemicals, heat resistance, gap filling and leveling, retaining, sealing, and threadlocking. Polyurethane adhesives and sealants with bulk flexibility form a layer that can bend or flex without cracking or delaminating. They provide additional support in applications that deal with raised levels of sound, vibration or shock. Polyurethane adhesives and sealants that are resistant to oils and chemicals are not damaged by exposure to acids, alkalis, oils or fuel. They are used in applications such as sealing fuel or oil tanks, and are used in chemical process vessels, piping, and fittings that are exposed to oil and corrosive mixtures. Heat resistant polyurethane adhesives and sealants are not damaged by prolonged exposure to raised temperatures. Many are flame resistant, meaning that they resist ignition when exposed to high temperatures and can insulate the substrate and delay damage to it.
In addition to providing bonding and protection, many polyurethane adhesives and sealants are used to level, fill gaps, and seal holes. They vary based on the composition of the adhesive and its style (i.e., flowing liquid, viscous liquid, or compound paste). A common filling application is gap filling, which involves simply closing up gaps between two bonded substrates. There are also leveling mastics, highly viscous polyurethane adhesives that are applied by trowel to give thick glue lines with gap-sealing properties. Typically, mastics are used to bond tiles to sub-flooring. They fill in gaps or irregularities in a surface before tile or other materials are applied.
Polyurethane adhesives and sealants with retaining properties are used to reliably retain cylindrical mating parts such as bearings, pulleys, couplings, rotors and gears. These products eliminate the need for press or shrink fits and provide up to twice the shear strength of these methods of assembly on slip-fit parts. Threadlocking or threadlocker polyurethane adhesives and sealants lock all kinds of threads that are subject to transverse and axial loads against vibrational loosening. They reduce production costs and improve reliability by eliminating lock washers and other expensive locking devices. These polyurethane adhesives and sealants also preserve on-torque and distribute the load over the entire engagement length of the fastener, effectively eliminating premature material fatigue and fastener failure.
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Pressure Sensitive Adhesives (PSA) and Contact Adhesives
Pressure sensitive adhesives (PSA) and contact adhesives adhere to most surfaces with very slight pressure. They are available in solvent and latex or water-based forms.
Silicone Adhesives and Sealants
Silicone adhesives and sealants have a high degree of flexibility and a very high temperature resistance (up to 600° F), but lack the strength of other epoxy or acrylic resins.
Thermoplastics and Thermoplastic Resins
Thermoplastics and thermoplastic materials soften when heated and harden when cooled. They can withstand many heating and cooling cycles and are often suitable for recycling.
Thermosets and Thermoset Resins
Thermosets and thermoset materials are crosslinked polymeric resins that are cured or set using heat or heat and pressure. They generally have a higher resistance to heat than thermoplastics.