UV curing adhesives use ultraviolet (UV) light or other radiation sources to initiate curing and form a permanent bond without heating. They are compatible with substrates made of ceramic, composite material, concrete, fabric, glass, metal, paper, plastic, rubber, or wood. UV curing adhesives use various chemical systems, most of which are polymer-based. Choices include acrylic, epoxy (EP) polybutadiene, polyester, silicone, styrene copolymer, and vinyl. Some UV curing adhesives contain additional modifiers such as fillers, pigments, or chopped fiber reinforcements. Others contain solvent-based adhesive resins which use a volatile organic compound (VOC) to thin or alter viscosity. Typically, solvent-based UV curing adhesives pose greater environmental or regulatory issues.
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Selecting UV Curing Adhesives
Selecting UV curing adhesives requires an analysis of processing and physical properties, thermal properties, and mechanical properties. Processing and physical properties include gap fill, the spacing or gap between the substrate and adhesive or sealant; and viscosity, a measurement of a fluid’s resistance to flow. Use temperature, thermal conductivity, and coefficient of thermal expansion (CTE) are important thermal properties to consider. Use temperature is the range of temperatures to which products can be exposed without the degradation of structural or other required end-use properties. Thermal conductivity is the linear heat transfer per unit area through a material for a given applied temperature gradient. CFE is the amount of linear expansion or shrinkage that occurs in UV curing adhesives with a change in temperature.
Specifications for Electrical and Optical Properties
UV curing adhesives also carry specifications for electrical and optical properties. These include electrical conductivity, dielectric strength, dielectric constant or relative permittivity, index of refraction, and transmission. The inverse of conductivity, resistivity is the longitudinal electrical resistance (ohm-cm) of a uniform rod of unit length and unit cross-sectional area. Dielectric strength is the maximum voltage field that UV curing adhesives can withstand before electrical breakdown occurs. Dielectric constant is the relative permittivity of a material compared to a vacuum or free space. The index of refraction is a measure of the speed of light in a material. This is the amount of light transmitted through a material.
With UV curing adhesives, setting or curing time is the time required for fully curing or setting a bond system. In thermosetting, hydraulically or other chemically-setting systems, the time will vary depending on the actual curing temperature. In UV curing adhesives, however, cure time can be quite rapid. Some styles of UV curing adhesives bond instantly upon the application of UV light. Others, which are more common, begin tacking when contacted by UV, but still require a given length of time to fully set. Longer cure times will be required for lower-curing temperatures.
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Conductive compounds provide an electrically and/or thermally conductive path between components.
Conformal coatings encapsulate circuit boards and their electronic components in order to prevent the ingress of moisture, fungus, dust and other environmental contaminants.
Epoxy adhesives are chemical compounds for joining components. They require clean surfaces and are valued for their toughness and resistance to chemical and environmental damage.
Gel elastomers are highly viscoelastic polymer gel materials that have excellent shock absorption and damping characteristics. They are available in a variety of material types and grades.
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 Materials
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.