There are a variety of high performance adhesives for the assembly of composite products available in the market today. The key reason for this is the increase in demand for composites that are lightweight with superior strength and low density properties.
Various industries such as automotive, aerospace, domestic appliance, construction, biomedical/dental, general industrial, industrial-machine, marine, consumer electronics, and sports equipment, are favouring these composites rather than heavier and weaker materials that were used in the past. Thus adhesives have become an integral part of many advanced composites projects.
Modern adhesives are designed with mechanical properties that match those of the materials they are bonding, thereby adding to the strength of the finished part.
Classification of Composites Adhesives
There are two main categories of adhesives used in composites:
Structural or engineering adhesives - Structural adhesives include epoxy, polyurethane and acrylic adhesives. For example, Master Bond’s epoxy resin formulations bond to a large variety of surfaces including metals, glass, ceramics, rubbers and plastics, Kevlar, fiberglass, carbon/graphite, boron, polyimide, etc. Epoxy resin adhesives offer mechanical shock resistance, resistance to thermal cycling, cryogenic shock resistance, electrical insulation, resistance to chemicals, serviceability in extreme temperatures, and low shrinkage.
Structural adhesives are known to reduce component weight, increase durability, provide better design latitude, handle high levels of stress, and require less machining. They are cheaper than other fixing media. They are also preferred for their aesthetic advantages such as cleaner finishes without any protruding nails or weld marks.
Synthetic adhesives - Synthetic adhesives are basically based on thermoplastics, emulsions, elastomers, and thermosets. These have good adhesion to a variety of substrates, and can be applied quickly. They have several excellent properties, and are cost-effective. They include epoxy, contact cement, cyanoacrylate, hot melt, polyurethane, resorcinol, polyvinyl acetate (PVA), silicone, spray and urea formaldehyde.
Materials in Composite Adhesives
Adhesives for composites are formed using a number of materials. These materials can be in the form of film, foam, hot melt, paste, pellets, solvent- based, and water-based. The various types of adhesives include pressure sensitive, thermosetting, hot melt, reactive hot melt, and contact. For bonding to take place perfectly and be durable for a long period, the surfaces need to be cleaned thoroughly, and specific pre-treatment methods need to be carried out.
The different adhesive base materials include:
- Acrylic –a type of resin that is used in adhesives to provide substrates with better stain protection, adhesion and blocking, water resistance, better cracking and blistering resistance, and resistance to alkali cleaners.
- Urethane – This adhesive is known for its strength and versatility. It is useful for bonds that require flexibility for expansion, contraction, or other types of movement.
- Epoxy - Epoxy resins are also known as polyepoxides and belong to a class of reactive prepolymers and polymers.
- Bismaleimide – These adhesives provide high temperature cure/service up to 315°C (600°F)
- Cyanoacrylate – These are strong fast-acting adhesives referred to as Super Glue or Crazy Glue in the market. For the cyanoacrylate glue to function, water works as the hardener.
- Methyl methacrylate (MMA) - MMA adhesives are basically less sensitive to differences in mix ratio and degree of mixing, thus allowing the ratio of resin to hardener to vary from 100:3 to 1:1, therefore providing more time for adhesive application and assembly of parts.
- Phenolics - Phenolics have been one of the initial adhesives used for bonding metal to metal and metal to wood. They require heat and pressure for the curing process.
- Polyimide – These are mainly used in applications where the temperature can reach 350°C (662°F) and are available as liquids or films.
- Silicone – This adhesive is not very strong; however it has very good flexibility, durability and high temperature resistance.
- Polyester – Polyester adhesives are less expensive than epoxy and are widely used in marine and other industrial applications; however polyester is a chemically weak adhesive when compared to epoxy, and also possess a high degree of shrinkage.
Composite Adhesives Applications
The following are the various application areas of adhesives for composites:
- Launch vehicles
- Racing cars and automobiles - engine-block flanges and body shell
- Orthopedic devices
- Aerospace components
- Fishing rods
- Storage tanks
- Metal to metal sandwich structures
- Co-curing sandwich structures
- In-situ pipeline repair such as cavities and holes in the pipe, replace corroded sections with patches
- Perforated panel assembly
- Lightning strike protection
- Rail -carriage panels to frame
- Core splice and edge inserts
- Consumer goods packaging
- Seams of envelopes and stamps
- Medical - bone cement etc.
Sources and Further Reading