Adhesives

In terms of materials, there are three main types of industrial adhesives: silicone adhesive, acrylic adhesive and polyurethane adhesive. These are classified according to their adhesive properties, composition and curing mechanism. The curing mechanism is the method employed to harden the adhesive. Adhesives manufacturers fabricate products for the automotive, woodworking, appliance assembly, packaging, labeling and plumbing industries. They are also used in book binding, shoe making, aquarium and tank assembly and furniture making. The construction industry utilizes adhesive sealing and bonding to replace fasteners during fabrication of doors, staircases, window frames, cabinets and to install wood floors and laminate on floors or counters. Adhesives are sometimes defined by their method of curing, or hardening. Hot melt adhesives require heat to cure while ultraviolet adhesives require sunlight and pressure sensitive adhesives are tactile, so they attach themselves when pressure is applied. Other types include conductive adhesives, which conduct heat and electricity, and laminating adhesives, which use a backing material to mount objects. Epoxy adhesives, shaped from the dual chemical base epoxy, are the strongest and most chemically resistant type of adhesive available on the market today.

Adhesives can come as one-part or two-part formulas. Multi-part adhesives consist of two or more separate components that need to be mixed. They can also be applied to separate adherents and pressed together to create a chemical reaction. This reaction leads to their properties bonding together, which is what happens with epoxies, acrylics and urethanes. Some require homogenous mixing to activate, and are therefore mixed moments before application. One-part adhesives have latent bonding properties that are activated by various energy sources, which source depending on the type of adhesive. A couple possibilities are moisture, radiation and heat. Ultraviolet adhesives are developed through a version of this one-part curing system. Most adhesives have different cure time methods. For example, some set and dry almost instantly, while others need a full night to finish bonding to the surface. They may come with backing, like laminate adhesives, and apply to other objects with the use of pressure. These are called pressure sensitive adhesives, and can often be removed or repositioned if necessary without damaging the surface or losing tactility. Adhesive material is also found on the back of stickers and all kinds of tape.

Adhesives can have differing setting temperatures ranging from above 212º F to below 68º F, depending on how much they need to be cooled or dried to harden. For example, hot melt adhesives are activated to viscous liquid states at elevated temperatures and set when cooled. Adhesives have various setting speeds, some remaining tacky for a certain amount of time, which allows more time for parts to be assembled. Adhesives that have residual tack offer repositioning options. Once hardened, some adhesives cannot be softened with heat; these are thermoset adhesives. However, some can be softened due to temperature change or moisture contact, so care must be taken when selecting an adhesive for more demanding applications. Adhesives also vary as far as their bond strengths. They are measured by tensile strength, wet strength, impact strength, fatigue strength, cleavage strength, peel strength and shear strength. Acrylic adhesives are a particularly impressive two-part epoxy, being that it bonds two objects together so well it is the adhesive used by doctors to attach implants to bone. Another impressive adhesive type is the polyurethane adhesive, which is incredibly strong but equally flexible. This allows the parts it has bound together to bend without cracking.

Adhesive manufacturers and adhesive suppliers can offer products in the form of solids, liquids, pastes, foams, films and aerosol sprays. Solid adhesive supplies include hot melt sticks, powder, granules, pellets, and chips, just to name a few. Film adhesives offer a uniform glue line, which are activated by heat and/or pressure and come with or without release paper. Laminating adhesives are common film adhesives. A gap filling adhesive can be utilized as a sealant because it does not shrink much when set. Adhesive manufacturers and adhesive suppliers often sell application products such as hot melt glue guns and dispensers. Application methods include spreading with a tool or brush, spraying, roll coating, transfer printing, screen printing, curtain coating and dispensing through a nozzle like a hand-held squeeze bottle, caulking gun or complex air-actuated nozzle. All of the products and processes above are synthetic adhesives. However, there are natural adhesives as well. Animals, natural resin and starch are all sources of natural adhesives, although they are rarely used in modern products because synthetic adhesives are easier to produce and often cheaper to supply to the public. Essential to keeping machinery and systems together in industrial, commercial and domestic settings, adhesives are an important part of modern life.

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Adhesives Types

• Acrylic adhesives and acrylate adhesives offer fast bonding at room temperature and are highly resistant to environmental conditions. They are able to stick to oily surfaces and many types of materials, including most metals, plastics, glass, ceramics and wood.
  
• Adhesives Manufacturers are companies that produce adhesive products and adhesive accessories.
• Aerosol adhesives are industrial spray adhesives that offer convenience and effectiveness for general purposes, such as foam and fabric, upholstery, screen printing, labeling, palletizing, trim and laminating, high bond high strength, high strength fast tack, pressure sensitive repositionable and temporary or permanent bond applications.
  
• Anaerobic adhesives cure in the absence of oxygen. Curing is catalyzed when bonding with surfaces where metallic ions are present.
  
• Conductive adhesives, or electrically conductive adhesives, offer electrical and/or thermal conductivity between components.
  
• Cyanoacrylate adhesives are fast setting adhesives commonly referred to as "crazy glue." Only a small amount of these one-component adhesives are necessary to form a rigid plastic layer that has high strength.
  
• Epoxies, or epoxy resins, are raw materials that can be formulated to make paints, coatings or adhesives.
  
• Epoxy adhesives are very strong and highly resistant to heat and chemicals. They can be formulated to be either flexible or rigid, transparent or opaque, fast setting or slow setting. All these characteristics make them appropriate for nearly all uses.
  
• Hot melt adhesives, or thermal adhesives, are viscous liquids at elevated temperatures that generally set quickly when cooled. Types include fast set, delayed set and pressure sensitive. Common uses include bookbinding, product assembly and box and carton heat sealing.
  
• Industrial adhesives are bonding products specifically designed for manufacturing environments.
  
• Laminating adhesives are substances that are used for bonding in thin layers. They come on sheets of release paper that are wound in rolls to be used in lamination presses or applied by hand with a plastic squeegee or hand roller. The liner is then removed from the adhesive.
  
• Methacrylates are a newer form of adhesive technology developed to offer superior performance compared to acrylic adhesive, which can be brittle and less reliable. Methacrylates provide good gap fill, excellent impact resistance, flexibility and peel and shear strengths, medium to fast curing, and tolerance of dirty surfaces.
  
• Membrane press adhesives are used in membrane press operation. They are heated to the proper temperature for lamination in the press and then quickly set for the unloading and trimming of the piece.
  
• Moisture cure adhesives react with moisture in the air or the bonding substrate to form a cured polymer layer with high strength. Silicone and polyurethane are the most common.
  
• Polyurethane adhesives come as two-part formulas or pre-mixed, which need to be mixed very well to give the best quality tough yet flexible bonds that they can. They can form strong bonds to most materials and are more flexible than epoxies.
  
• Pressure sensitive adhesives are pressure induced tacky materials that bond two flat surfaces together. This adhesive can be coated onto fabric, plastic or metal, and then stuck to another flat surface of metal, plastic, wood and paper.
  
• A silicone adhesive can create a permanent seal between two surfaces, also working as a watertight sealant and surface lever when necessary.
• Thermoset adhesives can not be softened with heat once they are set. Thermoset materials include epoxies, polyesters, silicones, rubbers and polyurethanes.
  
• Two-part adhesives consist of two or more components that react to become chemically cross-linked. Their higher costs are related to their extremely high bond strengths and exceptional performance, such as epoxies, polyurethanes, acrylics, and silicones.
  
• Ultraviolet adhesives are used to seal or bond objects together through a curing process. Fiber optics and dentistry both use this adhesive.
• Urethane adhesives bond with a wide range of materials and are tough and flexible at low temperatures but weaken due to high temperatures and contact with moisture.
• Water-based adhesives, or aqueous adhesives, use water as a carrier or diluting medium. They set when the water evaporates or is absorbed by the substrate.

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Adhesives Terms

Abhesive - A substance that is resistant to adhesion and can be used as a non-sticky surface coating for baking tins, frying pans, metal pots, etc. Examples are Teflon and silicone.

Adherend - Something bonded to something else through the use of an adhesive.

Cleavage Strength - How crack resistant a bonded adhesive is when stretched and strained.

Curing - Hardening or solidifying by cooling, drying or crystallization. Also referred to as setting.

Curtain Coating - Covers large areas with a relatively heavy coating of adhesive. Parts are passed through a "waterfall" of coating in an automated conveyor line.

Encapsulant - Can be an adhesive coating that hardens to form a protective layer to prevent degradation of whatever it encapsulates, such as electronic components.

Fatigue Strength - The maximum load an adhesive bond will sustain when subjected to repeated stress.

Impact Strength - An adhesive's ability to resist shock from a direct perpendicular physical blow.

Peel Strength - A measurement of the bond strength of an adhesive determined by the force per unit width required to separate bonded materials by applying stress in a "peeling" motion.

Release Paper - An easily removable protecting and/or carrier sheet for certain adhesives, commonly film and laminating adhesives.

Resins - Thick, sticky hydrocarbon plant secretions great for varnishes and adhesives.

Roll Coating - A method for applying adhesive, the simplest form of which is using a paint roller, but usually the coating rolls are part of a roll coating machine that precisely controls layer thickness, does not allow waste and is good for large surfaces at high speeds.

Screen Printing - A method of applying adhesive in specific patterns by way of forcing it through a screen using a squeegee. The size of the screen openings determines the coating thickness.

Shear Strength - How resilient a material, such as a cured adhesive, is to a parallel stress acting upon it, which can cause an irreversible continuous, non-fracturing deformation.

Substrate - The material surface upon which an adhesive is spread for bonding or coating. More specifically adherend.

Tensile Strength - A measurement of an adhesive's bond strength based on how resistant it is to tension, being stretched and strained.

Transfer Printing - A fast method of applying a thin layer of adhesive in a precise pattern, such as on envelope flaps. Usually done using rollers; flat plates can also be used.

Wet Strength - An adhesive's bond strength immediately after it has been immersed in a liquid under specified conditions of time, temperature and pressure.