Adhesives

Adhesives or glues are a permanent joining method that use a substance to bond two surfaces together so that they resist separation. There are different types of adhesives that fall into these main categories;

Contact - must be applied first to both surfaces and given time to dry before the two are held together bonding rapidly together
Drying - bond hardens through the drying of the substance over time as the chemical solvent evaporates
Hot - also known as melt adhesives, use a thermoplastic heated to 65–180°C and cool to bond between two surfaces
Pressure sensitive - designed to be a temporary bond between two surfaces and removable as found on labels/tape
Reactive - rely on a chemical reaction between often two substances that form a cross-link thermoset polymer when mixed after a curing

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Epoxy Resin (ER), is an example of a reactive adhesive. It is combined chemically with a catalyst, known as a hardener, which cures the resin creating cross-linked molecular chains. The epoxide content of the resin determines the quantity of hardener required to cure the resin. Chemically, the reaction is 'exothermic' as heat is released when the two elements are mixed. This reaction can occur at room temperature using products such a Araldite. The resin forms a incredibly strong bond between a diverse range of materials but must be applied quickly to prevent it curing early. It is used often for joining mixed materials

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PVA or polyvinyl acetates is an example of a drying adhesive that is commonly found in school workshops. It is a thick white substance that dries colourless and odourless creating a permanent bond. It is only suitable for porous surfaces that allow the absorption of the chemical such as timber and therefore cannot be used for most polymers such as acrylic. Often it is used as a surface primer for porous materials to seal surfaces to be painted and prevent absorption to improve the finish
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Contact adhesives adhere to themselves and are typically rubber based. Evostik is a popular choice which when applied to a each surface rapidly dries, removing the solvent within a few hours. Unlike PVA, it does not require a porous surface as it is not absorbed by the material. Once dried on each of the two surfaces, they can be rapidly bonded together simply by applying pressure to the materials. The speed of bonding requires a high level of accuracy as once joined they will not separate easily. Often it is used for large surface areas such as kitchen worktop laminates

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Superglue, known as Cyanoacrylates, are a group of incredibly fast acting adhesives which are acrylic based and cure almost instantly. The chemical exothermic reaction is triggered by ions in water found in most materials and in the air as humidity. Once reacted a durable plastic mesh is formed between the materials through polymerisation. Originally, it was designed to seal wounds on the battle field in place of stitches and has industrial and medical applications. Although a strong bond is formed, Superglue has a short shelf life, quickly bonds to skin and requires high accuracy

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Hot-melt adhesive is a thermoplastic adhesive sold in cylindrical tubes. They are passed through a heating element under pressure using a glue gun at around 120°C. As the glue cools it hardens and forms a bond between the two materials when compressed. The glue requires no absorption and therefore is ideal for non-porous materials and mixed materials. As a result, bonds lack structural strength and thermal resistance which can be an advantage if a temporary bond is required as boiling water will soften the thermoplastic bond

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Acrylic cement or Tensol® is one of the most effective ways to join two pieces of acrylic together permanently. It is a watery clear liquid that is applied to the materials, typically using a brush or syringe, and they are left clamped together for 24 hours to bond. As a drying adhesive, the solvent quickly evaporates and the chemical reacts with the surface of the acrylic to melt and weld the surfaces together without discoloration. Although a strong adhesive, the chemical emits harmful fumes and must only be used in a well ventilated place

Joining Methods - Adhesives

Mr BILLINGTON