Adhesives

• Physically setting adhesives
• Chemically curing adhesives.

• Physically setting adhesives: With these adhesives, the finished product is already present when applied. They can be divided as follows:
• Solvent-containing wet adhesives: Here the polymer is dissolved in organic solvents and is thus applied. By evaporation of the solvents, the adhesive sets and finally solidifies through the formation of physical interactions between the polymer chains. Wet adhesives can be used for so-called cold welding of thermoplastics. This type of adhesive is colloquially referred to as "all-purpose adhesive".
• Dispersion adhesives: Water is usually used as the mobile phase (dispersion agent). After application, the dispersion breaks due to the escape of the dispersion agent. They are widely used as a substitute for solvent-based adhesives (not fire and explosion hazardous).
• Hot-melt adhesives ("hot glues"): They are solid at room temperature and can be processed by melting. For hobby and small users, hot-melt adhesives in the form of adhesive candles (glue sticks) are available on the market, which can be processed with hot-melt glue guns.
• Contact adhesives: Solvent adhesives or dispersion adhesives, which are processed by contact bonding. For this purpose, both adhesive surfaces are first evenly coated with adhesive and dried at room temperature. Afterwards, the adhesive surfaces must be joined together exactly within the open processing time.
• Plastisols: Small polymer beads are distributed in a liquid phase. After application, the plastisol is gelled by applying heat. During this process, the polymer beads absorb the liquid (usually a plasticizer), swell and thus grow into a homogeneous layer. For complete hardening after gelation, a further increase in heat must be applied. PVC plastisols, for example, are frequently used in automotive engineering as seam sealing or underbody protection.
• Chemically curing adhesives (reaction adhesives; multi-component adhesives): Here, the individual chemical building blocks for the adhesive are brought together in the correct ratio. The adhesive is then consolidated by chemical reaction of the components with each other. A basic distinction is made between two-component (or multi-component) and one-component systems.
• Cyanoacrylate adhesives: Known as "instant adhesives". These are thin-bodied or deliberately thickened esters of cyanoacrylic acid which are marketed as monomers and react by polymerisation reaction in the joining gap to form the actual adhesive polymer. Special esters of cyanoacrylic acid are used in medicine for wound closure.
• Methyl methacrylate adhesives: Typical two-component reactive adhesives in which the monomer used (the methyl ester of methacrylic acid) is polymerised by a radical chain reaction. A reactive radical is needed to start the polymerisation reaction. This radical is usually formed from a peroxide when an accelerator is added to it.
• Anaerobically curing adhesives: This group of acrylic ester adhesives also cure according to a radical chain mechanism similar to methyl methacrylates. The special feature is that the curing reaction only starts in the absence of oxygen (anaerobic) and in the presence of metal ions when the adhesive is sealed from the ambient air in a narrow metallic adhesive joint.
• Radiation-curing adhesives. Adhesives that also cure by radical polymerisation, whereby the formation of the starting radicals is caused by irradiation with UV light (or other radiation sources such as electrons). The wavelength of the UV light must be precisely matched to the adhesive system used. Well-known examples are the gluing of artificial fingernails or the gluing of plastic tooth fillings; in each case, curing is achieved by irradiation with UV light.
• Phenol-formaldehyde resin adhesives: They consist of phenol derivatives and formaldehyde which are condensed to a polymer (first plastic bakelite). In adhesive technology, a mixture of phenol-formaldehyde resins is used, which is characterised by good temperature resistance.
• Silicones: Systems used less as adhesives but more as sealants (e.g. sanitary silicones) with various additives for cross-linking, e.g. vinegar compounds.
• Epoxy resin adhesives: Two-component adhesives consisting of resin and hardener. The epoxy resin is made up of polymeric building blocks which carry so-called epoxy groups at the end. The hardening reaction can be carried out at room temperature or at higher temperatures. Since the cured adhesive has a very high strength, this class of adhesives is often used for structural bonding, for example in vehicle and aircraft construction.
• Polyurethane adhesives: widely used in road/rail/air/water transportation, footwear, glass and bookbinding industries They are available as 1 or 2 component adhesives. They contain: Diphenylmethane-4,-4',-diisocyanate (MDI), Isophorone diisocyanate (IPDI), Toluene diisocyanate (TDI), Hexamethylene diisocyanate (HDI). The isocyanates can pose considerable health risks. Free, unbound isocyanates are acutely toxic or harmful to health (carcinogenic potential).

1. Finally W(1998): adhesives and sealants in modern technology. A practical handbook of adhesives and sealants application. Vulkan-Verlag, Essen
2. Hillen U et al. (2007) Contact sensitisation to components of adhesives with consideration of professional contact sensitisation. Dermatology at work and in the environment 55: 10-19

Epicutaneous test series Adhesives