Adhesives are used to bind materials together. Adhesives are widely applicable since their properties can be tailored to the materials that need to be adhered. More and more they become a suitable alternative for rivets and bolts.
Adhesives are popular products used in a diverse range of industries: automotive, construction, graphical industry and bookbinding, furniture industry, paper, packaging and textile industry.
Adhesives must yield good wetting of the surfaces to adhere (good spreading) and subsequently solidify to join the two surfaces together. Solidification may happen through a chemical reaction, evaporation of a solvent or through cooling. Pressure sensitive adhesives are an exception. They stay sticky and are used in, for example, tape and stickers.
Surface pre-treatments prevent the formation of weak boundary layers. Pre-treatment means surface cleaning, surface activating or effective surface enlargement. Pre-treatments for metallic surfaces could mean degreasing (by means of alcohols, esters and ketones), etching using acids or sodium hydroxide, anodisation or mechanical coarsing. For plastics this could mean removing migrated components from the surface, plasma treatment (argon, oxygen, ammonia), etching using acids or sodium hydroxide, oxidizing (chromic acid, flame or corona) and /or mechanical coarsing. The effectiveness of the surface treatment can be determined by contact angle measurements.
Using primers and coupling agents, which adhere chemically to the substrate surface, may also promote adhesion. Roughness enhances mechanical anchorage of an adhesive. Solvents and/or temperature may promote adhesion between plastic surfaces through interchange of entanglements.
Adhesives can be divided in four categories:
a. Chemically curing adhesives
b. Adhesives that dry due to solvent evaporation
c. Hot melts that solidify upon cooling
d. Pressure sensitive adhesives
The polymers are either present in the formulation or are formed during drying after application of the adhesive.
Chemically curing adhesives
The polymers used in these kinds of adhesives are similar to thermosetting polymers used in coatings.
Epoxy resins are widely used. A limited number of epoxy resins are commercially available, but a great variety of properties can be obtained through mixing with numerous different curing agents. Curing can either take place at room temperature or at elevated temperatures. An advantage of epoxy resins is that during curing no volatile components are formed, leading to very limited shrinkage. Epoxy resins can withstand high shear stress, are rigid and are dimensionally stable.
Phenolic resins are the reaction product of phenol and formaldehyde. A cross-linked structure is formed by means of thermosetting. These resins are heat resistant and are therefore used in high-temperature appliances like ovens and toasters.
Acrylates are based on acrylic monomers and are cured by a radical polymerization performed at room temperature. An example is methylmethacrylate (MMA) used by dentists to mount ceramic caps on teeth. Here, curing is achieved by UV-light.
Cyanoacrylate resins react through anionic polymerization initiated by water present in the air and/or at the surfaces to adhere. Oxygen blocks the reaction. Curing only takes place when surfaces are pressed together, thereby excluding oxygen.
Silicones are often used for seals in bathrooms. They consist of polydimethyl siloxane (PDMS) with molecular weights between 300 and 1600 g/mol and carrying reactive end-groups. The silicone chains are interconnected due to reactions between these end-groups, thereby forming a soft and flexible network with good chemical resistance. Silicones are applicable at temperatures ranging from -60 to 2000C.
A two-component polyurethane adhesive is based on the reaction between low molecular weight polymers and isocyanates that are mixed instantly prior to application. A one-component adhesive consists of low molecular weight polymers bearing isocyanate groups. Atmospheric water facilitates the chemical reaction.
In adhesives based on solvent evaporation (e.g. contact adhesives) no chemical reaction takes place. A polymer solution is applied on both surfaces, the solvent is allowed to evaporate for a while and subsequently the surfaces are pressed against each other.
Starch and milk proteins can be utilized to prepare biodegradable adhesives. In these cases, drying is based on the evaporation of water. Polyvinyl acetate (PVAc) adhesives are polymeric dispersions in water and are widely used for wood products.
Hot melt adhesives are applied as hot viscous liquids that solidify upon cooling (no chemical reaction). Examples of hot melts are polyethylene vinyl acetate (EVA), polyamide and polyester.
Pressure sensitive adhesives
Pressure sensitive adhesives remain sticky after application and are used in tape, stickers or band-aids. Adhesion is promoted by applying a slight pressure. Generally, these adhesives are based on natural rubber, styrene, butadiene copolymers or acrylates. Tape is mostly prepared from plasticized PVC of PE. In case of stickers, the sticker counterpart (substrate) is usually coated with polydimethyl siloxane (PDMS). Due to the low surface tension of this material, the sticker easily releases from the substrate