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What makes epoxy resins good adhesives? Why do they bond so strongly to surfaces?

The excellent adhesive properties of epoxy resins are due to the attractive forces between the epoxy resin and the surface of the substrate.  These forces are usually polar forces or direct bonds that can form between reactive sites in the resin and reactive or polar sites on the surface of the substrate.  Typical epoxy resins have pendant hydroxyl (- OH) groups along their chain which can form bonds or strong polar attractions to oxide or hydroxyl surfaces.  Most inorganic surfaces, i.e. metals, minerals, glasses, ceramics, etc. have polarity so they have high surface energy.  Organic polymer surfaces are generally less polar (more covalent) so they have lower surface energy.

Some typical surface energies are:

The important factor in determining good adhesive strength is whether the surface energy of the substrate is close to or higher than the surface energy of the cured adhesive.  On most surfaces the forces between the surface and the epoxy resin are stronger than the forces between the epoxy resin itself.  This results in the substrate surface and the adhesive having strong force between them.  If a substrate has low surface energy compared to the adhesive, then the adhesive will be attracted to itself rather than to the substrate.  This is characterized by having the adhesive bead up on the surface rather than spreading out and wetting the surface. 

If an adhesive material beads up on a surface, the surface will not bond well due to its surface energy being much lower than the surface energy of the adhesive material.  Basically, the beading-up is due to the adhesive being attracted to itself rather than to the surface.  If the adhesive spreads on a surface, this surface will bond well because the surface has high surface energy which can overcome the surface energy of the adhesive material. 

The easy wetting of the surface by the adhesive means there are good attractive forces between the two materials so the adhesive is essentially attracted to the surface rather than to itself.  It also means the adhesive can fill the microscopic voids on the surface, resulting in higher adhesive forces over the entire surface.

A substrate must have surface energy that is close to or higher than the epoxy’s surface energy in order to bond well with epoxy resins.  Typical cured epoxies have surface energy around 45 dyne/cm.  If the substrate’s surface energy is 30 dyne/cm or lower, epoxy adhesives do not adhere well.  Some polymers, e.g. PTFE (Teflon) with surface energy = 19 dyne/cm, are difficult to bond with epoxy.  The low surface energy results in poor wetting which means the adhesive does not spread easily over the surface.  The surface energy of the substrate may be changed (raised) by careful surface preparation, e.g. roughing, chemical etching, solvent cleaning, plasma treating.

An important advantage of epoxy resins that also makes them good adhesives is that they do not need anything other than the chemicals themselves to cause the cure.  Some adhesives only cure in the absence of air, some only cure in the presence of moisture or humidity.  Epoxy glues cure by themselves without any other material being needed.  The versatility of epoxies is also an advantage in considering them for adhesive applications. 

• they can cure at room temperature
• they can be heat-cured to provide a high service temperature for the adhesive
• they can be supplied as a one-part adhesive that does not require any mixing
• they can be supplied in flexible or rigid forms to match their cured state properties to the stresses they will be exposed to
• they can be cured underwater