Adhesives and Sealants

Adhesives are used in most industries and come in many forms.

A look at the website of 3m adhesives and they break their latest products down by industry...
original equipment manufacturers OEM adhesives, MRO adhesives, automotive adhesives, electronics industry adhesives and general industrial adhesives.

They talk about cyanoacrylate adhesives, anaerobic adhesives, ultra violet curing adhesives, structural adhesives and silicone sealants.

Hot melt adhesives

Hot melt adhesives are used for packaging adhesives, labels, woodworking adhesives, bookbinding.

Electronics industry adhesives

Electronics industry adhesives are also a full range of adhesive types.

Epoxy resins offer rapid and slow cure, twin and single pack, low viscosity epoxies with high and low temperature resistance. These are ideal for coil winding, electronic control and component encapsulation.

Polyurethane adhesives for high impact applications with the benefit that they are fire retardant, moisture resistant, very flexible with good temperature and climatic resistance.

Hot Melt adhesives are low temperature, free flowing thermoplastic and thermosetting for quick fill, fast cure applications.

Silicone adhesives are good when high temperature resistance and flexible finish is required ie for electronics use and the sealing, damping and bonding of components, wire tacking and staking.

Cyanoacrylates and Acrylics are for the general bonding of components where fast assembly, high strength and durability are required, for example for sticking magnets, glass fixtures, rubber and plastic assemblies.

Solvented adhesives can be used for bonding flexible films, tamper-proof fixings and other general applications.

Primers promote adhesives, Activators are for pre and post curing applications and Debonders are for removal.

Building adhesives.

The buildings in which we live and work are constructed using a whole variety of adhesives.
flooring adhesives, tile adhesives

How adhesives work

The experts don't agree on this one, except that they all know the mechanism of bonding is affected by the surface preparation of the adherent surfaces and the materials being attached.

Joints bonded with adhesives are generally stronger in compression, shear and tension than in peeling/tearing, ie it is much easier to break an adhesive joint by accessing an edge and peeling it away. It is also apparent that it is relatively difficult to ensure that an adhesive joint is in pure tension and if the tension load is off centre or is not normal to the joint there is a tendency for peeling. The best adhesive joints are designed for shear stresses. In other words "pull straight on either end of two items bonded by adhesive and that will be the hardest way to separate them".

Everyone agrees that there are conditions which result in higher adhesive bond strengths...

The bond surface should ideally be clean...cleaned of loose matter and also cleaned of surface oxides.
The choice of adhesive should be such that it wets the adherent surface and also solidifies under an acceptable regime of time, temperature and pressure.
The adhesive should be selected to suit the conditions of environment and temperature. The difference in coefficient of thermal expansion between the adhesive and adherent can affect the joint design.

There are many explanations often given of how adhesives work: 
 
Is there a mechanical interlock, based on the fact that, under a microscope all surfaces consist of crevices, cracks and pores? The adhesive penetrates these features and hardens, keying into the surfaces and forming a strong surface bond. The adhesive is able to bond two surfaces together and ideally the only weak part of the bonded joint is the adhesive strength.

Wet adhesives. The adhesive works because it "wets" the surface of the adherent surface (meaning that the adhesive applied to the adherent spreads spontaneously when the join is formed ). Adhesives have been developed which have a lower surface tension than the adherent surfaces. Epoxy resins wet steel and result in a good bond, but these resins do not wet plastics and don't work with other materials. The theory here is that in the event of intimate contact between the adhesive and the adherent, the adhesive strength comes from secondary intermolecular forces at the interface.

Chemical absorption is a variation of this where stronger chemical bonds (ionic, covalent metallic ) form across the joint interface. Molecular bonding between the adhesive and the adherent will obviously improve the adhesive bond strength. This can be attained by reactions at the surfaces, using proper surface treatments, or by using coupling agents.

Electrostatic bonding could be the answer if an electrostatically charged double bond develops at the bond interface as a result of the interaction of the adhesive and and adherent which contributes significantly to the bond strength. Many theorists doubt the significance of the forces involved, although there are examples where improved adhesion strength results with lowering of temperature.

Molecule diffusion could be happening, ie When an adhesive contains an adherent solvent, the adhesive can diffuse into the adherent surface (substrate) with an interchange of molecules. This is only really applicable to polymers where a movement and entanglement of long molecules can occur.
While this diffusion theory applies well for cases of self-adhesion or auto-adhesion, it does not really explain polymer to polymer adhesion. High molecular weight thermoplastic polymers often display very high melt viscosity and will not diffuse easily within the time scale of most bonding operations.

When bonding metals there is a surface oxide layer. For a successful bond this layer is usually removed by surface treatment before a strong adhesive bond can be achieved. Aluminium, on the other hand, has a strong coherent oxide layer which is suitable for bonding. Perhaps this is what determines effective bonding.