A bewildering variety of adhesives are available from a range of adhesive manufacturers. However, it is possible to simplify the choice by classifying the adhesive, and this can be done either by the way they are used or by their chemical type. The strongest adhesives solidify by a chemical reaction, Weaker varieties harden by some physical change. The major classifications are described in the following sections. Anaerobic adhesives cure when in contact with metal, and the air is excluded, e.g. when a bolt is home in a thread. They are often known as "locking compounds", being used to secure, seal and retain turned, threaded, or similarly close fitting parts. They are based on synthetic acrylic resins. Cyanoacrylate adhesives cure through reaction with moisture held on the surface to be bonded. They need close fitting joints and usually solidify in seconds. Cyanoacrylates are suited to small plastic parts and to rubber. They are a special type of acrylic resin. Toughened acrylics are fast curing and offer high strength and toughness. Both one and two part systems are available. In two part systems, no mixing is required because the adhesive is applied to one substrate, the activator to the second substrate, and the substrates joined. They tolerate minimal surface preparation and bond well to a wide range of materials. Epoxy adhesives consist of an epoxy resin plus a hardener. They allow great versatility in formulation since there are many resins and many different hardeners. Epoxy adhesives can be used to join most materials. These materials have good strength, do not produce volatiles during curing, and have low shrinkage. However, epoxies can have low peel strength and flexibility and can be brittle. Epoxy adhesives are available in one part, two part and film form and produce extremely strong durable bonds with most materials. Polyurethane adhesives are chemically reactive formulations that may be one or two part systems and are usually fast curing. They provide strong resilient joints which are impact resistant and have better low temperature strength than any other adhesive. Polyurethanes are useful for bonding glass fibre reinforced plastics (GRP). The fast cure usually necessitates applying the adhesives by machine. They are often used with primers. Silicones are not very strong adhesives, but are known for their flexibility and high temperature resistance. They are available in single or two part forms. The latter function like the two part epoxies, the former like the single part polyurethanes. When the single part adhesives cure they liberate either alcohol or acetic acid (the familiar smell of vinegar). They are often used as bath and shower sealants. Their adhesion to surfaces is only fair but like their flexibility, their durability is excellent. The two part versions need a hardening agent to be mixed into the resin. Two forms are available, those which liberate acid on curing and those that do not. As might be anticipated the two part adhesive systems give a better cure in thick sections than do the single part types. Phenolics were the first adhesives for metals and have a long history of successful use for joining metal to metal and metal to wood. They require heat and pressure for the curing process. Polyimide adhesives are based on synthetic organic chains. They are available as liquids or films, but are expensive and difficult to handle. Polyimides are superior to most other adhesive types with regard to long term strength retention at elevated temperatures. The following adhesives undergo a physical change and are less effective at forming the adhesive bond. Hot melts are based on modern thermoplastics and are used for fast assembly of structures designed to be only lightly loaded. Plastisols are modified PVC dispersions that require heat to harden. The resultant joints are often resilient and tough. Rubber Adhesives: Rubber adhesives are based on solutions or latexes and solidify through the loss of the medium. They are not suitable for sustained loadings. Polyvinyl Acetate (PVA’s)Vinyl acetate is the principal constituent of the PVA emulsion adhesive. They are suited to the bonding of porous materials, such as paper or wood, and to general packaging work. Pressure sensitive adhesives are suited for use as tapes and labels and although they do not solidify they are often able to withstand adverse environments. This type of adhesive is not suitable for sustained loadings.