Epoxy adhesives are suitable for such a wide range of applications that they are sometimes mistakenly rejected on the grounds that they are too generalist.

In fact there are so many grades available today that an epoxy’s properties can usually be matched extremely closely to the application requirements, whether the need is for an adhesive, sealant, encapsulation compound or repair material (Fig.1). Furthermore, manufacturers of epoxies are continually launching new products that improve existing applications and present new possibilities.

Industries that use epoxy adhesives include automotive, aerospace, marine, medical, electronics, optical, photonic, sports goods, furniture and general assembly. Both low-volume and high-volume production can benefit from epoxies as a result of the choice of curing regimes and packaging options available. In terms of materials that can be bonded, the scope is vast: metals, plastics, elastomers, ceramics, glass, wood – and any combination of these. Even the ‘difficult’ polymers can be bonded, including PTFE, polyethylene and polypropylene.

If particular material properties are required from the cured epoxy, suitable grades are usually available as standard products from one of the many manufacturers. For example, Tra-Bond2248 from Tra-Con complies with specification ASTME-595-90 for NASA outgassing acceptability, and Master Bond’s two-component EP45HTMED has full USP ClassVI certification for biocompatibility in the assembly of disposable and reusable medical devices. When cured, this tough, durable compound is resistant to thermal shock and many chemicals. It also withstands exposure to radiation, steam and chemical sterilants, plus it is an outstanding electrical insulator.

Other epoxy variants include flame retardant grades, optically clear grades, and specialist grades for applications where thermal aging might otherwise present a problem. While epoxy resins are normally electrical insulators, alternative products are available if electrical conductivity is required, and metal-filled epoxies can be used where thermal conduction is needed. One epoxy, MasterBondEP24AN, has the advantage of possessing an exceptionally high thermal conductivity as well as superior electrical insulation properties.

Packaging options

In addition to the wide range of material properties offered by the epoxy family, the varied form of supply further helps with their versatility. Bulk users are able to purchase drums – in a range of sizes – or the application may be better suited to epoxies supplied in cartridges for use with static mixers. Where smaller quantities are used in manual assembly, a particularly convenient option is the Bipax packaging from Tra-Con for its two-part epoxies. Package sizes are available from 2g to 200g, with the resin and catalyst supplied in a clear plastic sachet, separated by a leak-proof clip. For even smaller quantities, the Minipax is a six-pack of 1g tubes. Mixing is achieved by removing the clip and massaging the sachet until the two components – typically different colours – are thoroughly mixed. This packaging design minimises mess, reduces the risk of the user coming into contact with the chemicals, and ensures that the components are mixed in the correct ratio.

Nevertheless, some users still perceive that multi-component epoxies are inconvenient to use and carry a risk that the components will not be mixed in the correct ratios, so several manufacturers have now developed single-part epoxies. Devcon’s new SC2002NS one-part epoxy adhesive, for example, bonds to a remarkable variety of substrates, including fabrics, wood, steel, aluminium, ceramics, glass, and concrete. This non-sagging, self-levelling gel is easy to use, has excellent gap-filling properties and produces flexible bonds with long-term durability and excellent thermal stability. 100percent reactive, SC2002NS requires no mixing, provides unlimited working time at room temperature, and is heat-activated; at 135°C it has a fixture time of approximately 1minute. Fully cured bonds withstand service temperatures from -51 to 149°C.

Another highly convenient option is offered by Tra-Con for its products aimed at the micro-electronics industry. Pre-mixed and frozen adhesives are, as the name suggests, premixed in the correct ratio by Tra-Con, degassed, packaged in syringes or cartridges and frozen at -40°C to prevent curing. These can then be packed in dry ice and delivered overnight to almost anywhere in the world.

Ultraviolet (UV) light can be used to cure some single-part grades of epoxy adhesive, giving an unlimited shelf life at ambient temperatures and a rapid cure. However, these need light-tight packaging to keep the adhesive in good condition. Master Bond’s system, for instance, includes a ‘double light barrier’ in which the UV-activated compound is packaged in a black applicator and the syringe itself is sealed in a black plastic pouch until ready for use. All packaging processes are carried out in total darkness.

The same company is also able to supply epoxy adhesives in the form of thin films. Furthermore, Master Bond can supply laser or die cut performs to meet specific application requirements. Preforms require no mixing and are therefore easy to use. Using performs can enhance productivity yield, save time and reduce waste. They are available in thicknesses of 0.05 to 0.2mm and up to 406mm square. Cured preforms offer excellent bond strength plus high chemical and moisture resistance.

Various formulations are available, delivering electrically conductive, thermally conductive and electrically insulating properties. Preforms do not need to be refrigerated and they are formulated to cure rapidly at moderately elevated temperatures. Custom formulations can be designed for applications requiring special temperature resistance, chemical resistance, flexibility or other material properties.

Cure regimes

Generally speaking, cure regimes can be selected by customers to suit the application. In some cases a long pot life will be beneficial, perhaps because large components take a long time to position, or maybe the adhesive is applied at one stage in the production and the components are not mated until further along the production line. Elsewhere, a short cure time (or ‘snap’ cure) is preferred, as the assembly needs to be handled without the parts becoming misaligned; five minutes or less is typical for snap cure epoxy adhesives. Some epoxy adhesives, such as MasterBond’sEP19HTFL, are single-component, no-mix epoxy resin systems with an unlimited working life at room temperature (Fig.2). However, EP19HTFL cures in one to two hours at 150°C, or faster at higher temperatures.

Two interesting developments from Tra-Con are intended to help users establish when the cure process is complete. Tra-Bond F230 is a low-viscosity epoxy adhesive with a very high glass transition temperature and a pot life of one hour. It can be cured in as little as one minute at 150°C and is intended for applications such as the termination of single- and multi-mode connectors where bonding to optical materials is required. The resin and catalyst are both light yellow, the mixture is light green, and the fully cured adhesive is reddish-amber. Tra-Bond2230 changes colour the same way but is a low-viscosity two-part epoxy that provides excellent wicking characteristics and develops strong bonds with most metals, ceramics, glass and many plastics. Tra-Bond2230 has a one-hour pot life and requires a high-temperature heat cure.

A new epoxy product from Cotronics Corporation is both versatile and flexible. Duralco4538N is a super-flexible, rubber-like epoxy that provides the chemical stability of an epoxy and the flexibility of a silicone, as well as having excellent shock resistance and sound absorption properties (Fig.3). Furthermore, by adjusting the ratio of resin and catalyst, it is possible to vary the cured epoxy’s hardness from 100 to 30 ShoreA. This unusual material adheres to most plastics, metals, ceramics, glass, rubber and even treated PTFE, curing in 16 to 24hours at room temperature. Its operating temperature range is -73 to 232°C and it is suitable for bonding, sealing and encapsulating.

For cryogenic applications at temperatures down to -269¢ªC, Master Bond offers a nickel-filled, flexible, two-component epoxy adhesive called EP21TDCNFL. The cured material has a low volume resistivity together with a tensile lap shear strength in the order of 13.8MPa. Master BondEP21TDCNFL is also recommended for applications where prolonged exposure to unfavourable environmental conditions could compromise the assembly's performance. EP21TDCNFL is available in containers of various sizes or, alternatively, as a one-part product in pre-mixed, frozen syringes.

At the other extreme, Cotronics’ Duralco4700HT is capable of operating at temperatures as high as 343°C. This low-shrinkage epoxy provides excellent adhesion to metals, glass, ceramics and most plastics, has high electrical resistance and high temperature stability, and is highly resistant to chemicals, solvents and radiation.

Today it seems that almost whatever the bonding requirement, it is likely that an epoxy adhesive will meet the performance criteria. And if there is no standard product available, many of the suppliers will develop custom formulations. Come what may, be sure that you do not dismiss epoxy adhesives simply because nothing suitable was available last time you looked; this is a fast-moving technology.