A Complete Guide to Adhesives

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 the glue to stick to a surface (adhesive forces). It must also be self-adhesive (cohesive forces) otherwise it will simply tear down the middle and shear off under pressure or weight.

Mechanical bonding is the most common form of adhesion due to its simplicity. The bond formed between adhesives and surfaces happens when the adhesive gets into the tiny pores on the material, holding the two together.

Chemical bonding occurs when adhesives bond with surfaces at a molecular level. This is similar to the process that takes place when atoms bond together to create molecular structures like H2O (water).

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.

When adhesive is spread, it dampens the surface it is being applied on. Many weak electrostatic forces (known as Van der Waals forces) push between the molecules of the glue and the surface to bind them. Adhesives must spread thinly and cover the surface well to work successfully. The bond between the glue and the surface is not a chemical one; it is simply a large amount of microscopic, attractive forces. The surface and glue molecules stick to one another in an almost magnetic way.

Sometimes, an adhesive is able to create a far stronger bond with a surface. As an example, if you bond certain glues and plastics together, the materials will merge to form a highly robust chemical bond. In essence, a new chemical compound is created at the adhesive joint – a process referred to as chemisorption.

The answer to this common question involves more complex adhesive chemistry. On a chemical level, most glues contain polymers. These are large molecule chains that bind easily to each other (cohesion) and other substances (adhesion). Water is added to most adhesives during the manufacturing process, to prevent them 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.

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

Hot glue adhesive is supplied as a solid plastic in tubes known as sticks. These sticks are then inserted into a hot melt glue gun which will apply a high level of heat to the stick, melting the plastic. This is applied via a nozzle in the applicator gun to the objects or surfaces requiring adhesion. The glue gathers in the pores across the surfaces, cools and solidifies.

Hot melt glue becomes liquid when heated, moistening the surface before cooling into a solid polymer. This adhesive is found in a variety of engineering applications where professional variants of the standard glue gun are used.

This highly potent adhesive is sold under the more familiar brand names Loctite Super Glue or Gorilla Glue. It's also commonly known as superglue. Cyanoacrylate (CA) will form a strong, permanent bond in only thirty seconds. It has hygroscopic properties so is not water-resistant, however, this should not be problematic provided that the correct grade is used and the joint is properly designed. Thanks to its strength, cyanoacrylate is widely used in engineering and manufacturing. Bonds formed by this adhesive resist pressure well.

Polyvinyl acetate (PVA) is a widely-used thermoplastic (heat-sensitive) adhesive, also known as white glue, school glue, craft glue, or wood glue. Sticking wood is the primary use of PVA glue, which is available in liquid form and applicator pens and sticks. It is normally water-resistant (type I) but a fully waterproof variety for outdoor use (type II) is also available. The latter is typically yellow. PVA is a versatile, multi-purpose adhesive that can also be used as a sealant for plastering and painting.

Pressure-sensitive adhesive (PSA) is solvent-free and requires no water or other substance to activate. Pressure is all that is required. PSA is a familiar form of adhesive. It can be found on the back of sticky notes, masking tape, insulation tape, scotch tape, and duct tape. Typically, pressure-sensitive adhesive tape is made from various plastics, to which a film or backing of PSA has been applied. Double-sided tape features adhesive on both sides.

Epoxy adhesive is a derivative of petroleum. It forms a strong bond and is resistant to moisture, chemicals and corrosion, forming an effective sealant. Epoxy resin works by combining two parts – a reactive agent (epoxide) and a curing agent, which hardens the adhesive once applied. It is also a good conductor, making it an ideal choice for electrical components and circuit boards. Epoxies are one of the few types of glue that are effective when applied to metal and glass. Resin is also noted for its long shelf life and multiple varieties of epoxy adhesive glue are available.

Uses include:

• As an all-purpose adhesive
• As a coating for industrial applications
• For construction and manufacturing
• Engineering
• Vehicle construction

Acrylic adhesives are versatile glues that can be used on a wide range of materials including acrylic, plaster, wood, and stone. Acrylic-based adhesives are resistant to ultraviolet (UV) exposure, making them a good choice for outdoor use. Two-part acrylic adhesive combines a spray or aerosol with a paste for strong bonding. Always use personal protective equipment, including gloves and a face mask, and ensure that the area is well-ventilated when using acrylic adhesive.

Contact adhesives are so-called dry film glues. When applied to a surface, they will auto-grip, forming a strong bond. They are widely used with carpets, rubber, cardboard, and wooden surfaces (for example, when attaching laminate to plywood). Unlike some alternative glues, they will adhere well to non-porous surfaces.

Also known as impact or grab adhesives, contact adhesives can be applied relatively simply via a brush or as spray adhesives. They are available in two primary forms – solvent-based, and in water. The former is thicker and dries more quickly, while the water-based alternative will only bond once the moisture has fully evaporated. Contact adhesives are noted for their resistance to cracking pressures and voltage surges. Instant contact adhesive varieties are also available which bond immediately on contact.

Silicone adhesives are another strong and versatile glue, used not only for general adhesion but widely within industry. In particular, silicone glue is used within the electronics, vehicle engineering, construction, and aeronautics sectors.

They are noted for:

• The relative ease of application
• Resistance to UV light exposure, moisture, varying temperatures, and general weathering
• Even transmission of weight, minimising metal fatigue and general wear
• Versatility – silicone adhesive can be used as a substitute for rivets and screws and is equally effective with fibreglass, rubber, wood, metal, and plastics
• The flexible, bend-resistant bond produced

Polyurethane glue is used with both porous and non-porous materials. For example, plastics, foam, wood, metal, and stone. It is an ideal choice for use with wood and can be used both indoors and outdoors. Polyurethane is a high strength adhesive noted for its elastic properties, absence of toxicity, and the fact that it will retain its cohesion over time. It requires moisture to harden and will expand as it hardens, making it an efficient cavity-filler.

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.

Threadlocker is an example of anaerobic adhesive. It is used to prevent bolts, screws, and threaded parts from loosening, typically added to a thread to safeguard against loosening through vibration. Threadlockers work by completely filling the gaps between mating threads to seal the threads to the adherend.

Typical applications include:

• Thread sealing
• Nuts and bolts
• Retaining and gasketing
• Sealing in screw threads

Hybrid adhesives contain a combined formula as they are made up of multiple different types of adhesive. They are beneficial as they offer favourable properties from both adhesive types. As an example, a hybrid cyanoacrylate/epoxy adhesive could provide the speed of CA as well as the strength and chemical resistance of the epoxy. This means that hybrid adhesive offers an innovative, versatile solution for various sticking and sealing applications.

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

Phenolic Adhesive

Phenolic adhesives are made from formaldehyde. These high-performance liquid glues can withstand extreme temperatures and heavy-duty 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.