A Complete Guide to Adhesives

Modern adhesive is made from a variety of carefully-engineered chemical substances. Adhesives are characteristically organised by adhesion methods. These are then further separated to define reactive and non-reactive adhesives, referring to if the adhesive undergoes a chemical reaction to harden. Common types of adhesives include epoxy, silicone, and acrylic.

Advantages of Bonding Using Adhesive

Using adhesive is advantageous when compared to alternative joining techniques like sewing, thermal bonding, and mechanical fastening.

The advantages of bonding two surfaces together using an adhesive include:

• Distributing stress evenly across joints

• The flexibility to bond a variety of surfaces together

• Cost-effectiveness of a process that is easy to mechanise

• Improved aesthetics and design

• Greater design freedom

Conversely, the disadvantages of using adhesives include:

• Increased difficulty separating surfaces when testing

• Reduced stability in heat and extreme temperatures

• Comparative lack of strength when bonding larger objects with small surface areas

Adhesive Uses

Although typically associated with repairs when used in a domestic setting, adhesive is widely used in manufacturing and construction to bind components and parts together.

The use of adhesives also extends beyond binding. As sealants, they are used to form a protective coating across cavities and porous surfaces in engineering. When this cures (hardens), it reinforces delicate components, protecting them from air and liquid to prevent damage from rust, contaminants, and corrosion. Silicone is a common ingredient of sealants.

How Do Adhesives Work?

The science behind glue is not fully understood even today. An effective bind involves the interplay of two natural forces – cohesion (the tendency of a substance to stick together) and adhesion (the tendency of a substance to stick to another substance). In other words, it is not enough for glue to stick to a surface. It must also stick to itself otherwise it will simply tear down the middle and shear off under pressure or weight.

The precise mechanism behind adhesion is complex and varies between different substances. Essentially, however, it involves two chemical forces – adsorption and chemisorption. Adsorption is an electrostatic attraction between the molecules making up the adhesive substance and those of the surface to which it has been applied. Chemisorption, meanwhile, is the formation of a deep molecular bond between the glue and the substance to which it has been applied.

What Makes Glue Sticky?

The answer to this common question involves more complex chemistry. On a chemical level, most glues contain polymers. These are large molecule chains which bind easily to each other (cohesion) and other substances (adhesion). Water is added to most adhesives during the manufacturing process, to prevent it from hardening immediately. When it is later opened and exposed to air, the water begins to evaporate, leaving behind polymers ready to start sticking onto things.

Modern Adhesive Categories

Modern adhesives fall into three broad categories. Let’s take a look:

Solvent-Based Adhesives

Solvents are solutes – substances which dissolve other substances, creating a liquid solution. Solvent-based adhesives contain polymers in a solution form. Once applied, the solvent evaporates and as it does so, the glue hardens in place to create a bond.

Water-Based Adhesives

Also known as dispersion adhesives, water-based adhesives are supplied either in solution form or as dry powders for mixing with water. When the water evaporates or is absorbed by the surface to which it has been applied, the remaining water-soluble polymers, dispersed by the liquid, coagulate into an adhesive bond.

Common water-based adhesives include:

• White school glue

• Latex glue

• Starch glue

Polymer-Based Adhesives

Polymer glue is also known as emulsion adhesive. Typically milky-white in colour, it is made from powerful bonding polymers such as polyvinyl acetate (PVA). Polymer-based adhesives are sensitive to heat, melting at certain temperatures and setting again once they have cooled. They are ideal for use when glueing wood.

Types of Adhesives

Multiple types of adhesive are available, ranging from familiar household brands through to highly specialist products for use in manufacturing. Below are some of the different types of adhesive you may encounter:

As well as those mentioned above, additional types of adhesive include:

Anaerobic Adhesive

Anaerobic glue is distinctive because it will only harden in the absence of air and when in the presence of metallic ions. It is fast-acting but approximately 24-hours is typically needed for it to fully cure. Given its characteristics, anaerobic adhesive is normally used for metal-to-metal joins.

Typical applications include:

• Nuts and bolts

• Gaskets

• Sealing in screw threads

Phenolic Adhesive

Phenolic adhesives are made from formaldehyde. These liquid glues can withstand high temperatures and heavy loads without suffering deformation or breakage.

Polyimide Adhesive

Polyimide adhesive is available as both liquid glue and in solid form such as glue sticks. These adhesives have varying viscosities and stick well to leather, wood, ceramics, metals, and plastics.

FAQs

What Temperature Does Super Glue Melt at?

Once set, the melting point of superglue is 187°C (368°F).

How Strong is Epoxy Resin?

Epoxy resin is a very strong substance. There are varieties available with a flexural strength of as high as 17,500 pounds per square inch (psi). Flexural strength is the resistance of a substance to bending or flexing.

What are Hot Glue Sticks Made of?

Several different polymers are used to make hot glue sticks, with varying strengths and qualities. These include:

• Polyesters

• Ethylene-methyl acrylate (EMA)

• Polyethylene

• Ethylene-vinyl acetate (EVA)

Is Hot Glue Waterproof?

No. Not glue will only provide an effective bond for porous materials because the glue hardens and binds within those pores. However, porous materials are, by definition, not waterproof and the hot glue bond will weaken once the material is swollen by water. Water also typically lowers the temperatures of materials and this will also encourage the glue to shrink and weaken. Exposure to low temperatures is an effective method for removing hot glue.

What Temperature Does Hot Glue Melt at?

Hot glue typically melts at 79.4°C but is normally applied at significantly higher temperatures between 149°C and as high as 288°C.

How Do You Use a Glue Gun on Plastic?

The following steps show how to use a glue gun on plastic:

• Choose a gun that will take 11-12mm glue sticks with an operating temperature of up to 194°C

• Choose a glue specially-formulated for plastics, such as polyolefin

• Inspect the gun, ensuring the nozzle is free from glue residue

• Insert a fresh glue stick into the opening at the back of the gun

• Plug in the gun and wait for it to heat up

• Apply glue sparingly, using light pressure to the trigger, and press the target objects into place

• Wait for the glue to fully dry