- Pressure sensitive adhesives
Pressure sensitive adhesives (PSA) form a bond by the application of light pressure to marry the adhesive with the adherend. They are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow (i.e. "wet") the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in close proximity, molecular interactions, such as Van der Waals forces, become involved in the bond, contributing significantly to its ultimate strength.
PSAs are designed for either permanent or removable applications. Examples of permanent applications include safety labels for power equipment, foil tape for HVAC duct work, automotive interior trim assembly, and sound/vibration damping films.
Removable adhesives are designed to form a temporary bond, and ideally can be removed after months or years without leaving residue on the adherend. Removable adhesives are used in applications such as surface protection films, masking tapes, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact (wound care dressings, EKG electrodes, athletic tape, analgesics and transdermal drug patches, etc.). Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally can not support much weight.
Pressure sensitive adhesives are manufactured with either a liquid carrier or in 100% solid form. Articles are made from liquid PSAs by coating the adhesive and drying off the solvent or water carrier. They may be further heated to initiate a cross-linking reaction and increase molecular weight. 100% solid PSAs may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive; or they may be high viscosity materials that are heated to reduce viscosity enough to allow coating, and then cooled to their final form. Major raw material for PSA´s is acrylate based polymers.
- Contact adhesives
Contact adhesives are used in strong bonds with high shear-resistance like laminates, such as bonding Formica to a wooden counter, and in footwear, as in attaching outsoles to uppers.
Natural rubber and polychloroprene (Neoprene) are commonly used contact adhesives. Both of these elastomers undergo strain crystallization.
Contact adhesives must be applied to both surfaces and allowed some time to dry before the two surfaces are pushed together. Some contact adhesives require as long as 24 hours to dry before the surfaces are to be held together. Once the surfaces are pushed together, the bonds are formed very quickly. It is usually not necessary to apply pressure for a long time, so there is less need for clamps.
- Hot adhesives
A glue gun, an example of a hot adhesive
Hot adhesives, also known as hot melt adhesives, are simply thermoplastics applied in molten form (in the 65-180 C range) which solidify on cooling to form strong bonds between wide ranges of materials. These adhesives are popular for crafts because of their ease of use and the wide range of common materials they can join. A glue gun is one method of applying hot adhesives. The glue gun melts the solid adhesive then allows the liquid to pass through its barrel onto the material, where it solidifies.
Thermoplastic glue may have been invented around 1940 by Procter & Gamble as a solution to water-based adhesives commonly used in packaging at that time failing in humid climates, causing packages to open and become damaged.
- Drying adhesives
There are two types of adhesives that harden by drying: solvent based adhesives and polymer dispersion adhesives, also known as emulsion adhesives.
Solvent based adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent. White glue, contact adhesives and rubber cements are members of the drying adhesive family. As the solvent evaporates, the adhesive hardens. Depending on the chemical composition of the adhesive, they will adhere to different materials to greater or lesser degrees.
Polymer dispersion adhesives are milky-white dispersions often based on polyvinyl acetate (PVA). Used extensively in the woodworking and packaging industries. Also used with fabrics and fabric-based components, and in engineered products such as loudspeaker cones.
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