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Cyanoacrylate Adhesives General Description Cyanoacrylates are one-part, room-temperature-curing adhesives that are available in viscosities ranging from water-thin liquids to thixotropic gels. When pressed into a thin film between two surfaces, cyanoacrylates cure rapidly to form rigid thermoplastics with excellent adhesion to most substrates. One of the benefits cyanoacrylates offer is the availability of a wide variety of specialty formulations with properties tailored to meet particularly challenging applications. For example, rubber-toughened cyanoacrylates offer high peel strength and impact resistance to complement the high shear and tensile strengths characteristic of cyanoacrylates. Thermally resistant cyanoacrylates are available which offer excellent bond strength retention after exposure to temperatures as high as 250°F for thousands of hours. Moreover, “Surface- insensitive” cyanoacrylates offer rapid fixture times and cure speeds on acidic surfaces, such as wood or dichromated metals, which could slow the cure of a cyanoacrylate. In some cases, the use of a general purpose cyanoacrylate adhesive was hampered by the appearance of a white haze around the bond line. This phenomenon is known as “blooming” or “frosting” and occurs when cyanoacrylate monomer volatizes, reacts with moisture in the air, and settles on the part. To eliminate this problem, “Low Odor/Low Bloom” cyanoacrylates were developed. They have a lower vapor pressure than standard cyanoacrylates and therefore are less likely to volatize. Ultraviolet curing (UV) cyanoacrylates are the latest advancement in cyanoacrylate technology. UV cyanoacrylates utilize proprietary photoinitiators to allow cyanoacrylates to surface cure in seconds when exposed to ultraviolet or visible light of the appropriate wavelength. Light Cure Technology makes cyanoacrylates cure even faster, overcome blooming, and limit or eliminate stress cracking. While advances in cyanoacrylate formulating technology have played a key role in offering additional benefits to the end user, there have also been important developments in cyanoacrylate primer and accelerator technology. Accelerators speed the cure of cyanoacrylate adhesives and are primarily used to reduce cure/fixture times, to cure fillets on bond lines and/or excess adhesive. Accelerators consist of an active ingredient dispersed in a solvent. The accelerator is typically applied to a substrate surface prior to the application of the adhesive. Once the carrier solvent has evaporated, the cyanoacrylate can immediately be applied and its cure initiated by the active species that the accelerator has left behind. Depending on the particular solvent and active species present in the accelerator, the solvent can require 10 to 60 seconds to evaporate, and the active species can have an on-part life ranging from 1 minute to 72 hours. Accelerator can also be sprayed over a drop of free cyanoacrylate to rapidly cure it. This technique has been widely used for wire tacking in the electronics industry. Another benefit offered by cyanoacrylates is the availability of primers which enable them to form strong bonds with polyolefins and other difficult-to-bond plastics such as fluoropolymers and acetal resins. Like the accelerators, polyolefin primers consist of an active ingredient dispersed in a solvent. Once the carrier solvent has evaporated, the surface is immediately ready for bonding, and the primer will have an on-part life ranging from minutes to hours. Depending on the plastic, bond strengths of up to 20 times the unprimed bond strength can be achieved. Chemistry Cyanoacrylate adhesives are cyanoacrylate esters, of which methyl and ethyl cyanoacrylates are the most common. Cyanoacrylates undergo anionic polymerization in the presence of a weak base, such as water, and are stabilized through the addition of a weak acid. When the adhesive contacts a surface, the water present on the surface neutralizes the acidic stabilizer in the adhesive, resulting in the rapid polymerization of the cyanoacrylate. Advantages • One-part system • Solvent-free • Rapid room temperature cure • Excellent adhesion to many substrates • Easy to dispense in automated systems • Wide range of viscosities available • Excellent bond strength in shear and tensile mode • Primers available for polyolefins and difficult-to-bond plastics • UV/Visible cure formulas available Disadvantages • Poor peel strength • Limited gap cure • Poor durability on glass • Poor solvent resistance • Low temperature resistance • Bonds skin rapidly • May stress crack some plastics 6 The Loctite® Design Guide for Bonding Plastics, Volume 6PDF Image | Design Guide for Bonding Plastics Volume 6 LT-2197
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