The dictionary defines a composite as something made up of various parts or elements.
When talking about composites in the manufacturing or construction industries, two questions immediately arise:
- What, exactly, are the elements that make up the composite material?
- What physical properties do we need to accommodate when working with them?
Most composites have a matrix—that is, the surrounding medium, also known as the continuous phase—combined with reinforcement, which consists of fibers or particles embedded in the medium, also known as the discontinuous phase. When it comes to identifying composites, some are defined by matrix and others by reinforcement. Matrices can be chosen from broad categories of materials, including metal, ceramic and polymers. Likewise, the choice of reinforcement materials is vast. Glass fibers, carbon fibers, inorganic fillers and Kevlar® are just a few. And there’s yet another complication: laminates, or sandwich panels, are called composite panels even though they are constructed by layering—not intermixing—materials.
Let’s look at some common composites used by automotive and building product manufacturers. Based on the various properties of the composites, how should adhesives be selected?
FRP and SMC
Fiber-Reinforced Plastic (FRP) typically uses glass fibers (woven or chopped or both). The fibers are placed in a polyester, vinyl ester or epoxy resin matrix. FRP is often produced in an open or one-sided mold. An example of FRP use in the automotive industry is heavy-duty truck hoods.
Sheet Molded Composite (SMC) uses a polyester resin matrix. It is produced as sheets of both the resin and matrix of material, most often chopped glass. SMC sheets are added to a heated mold (2-sided) and compression molded with heat added to cure the SMC. Typical applications include automotive liftgate skins and personal watercraft decks and hulls.
Urethane adhesives are an excellent choice for bonding FRP and SMC. Urethanes adhere well to the polyester resin and provide a good amount of elongation. These adhesives typically exceed the strength of the SMC and produce fiber-tearing bonds which can withstand vibration and impact due to their elongation and nature of the adhesive polymer.
Epoxies can also be used for these composites. Most often, epoxies are used for their ability to be heat cured rapidly using heated tooling. This affords faster cycle time in the bonding operation of the production. Some urethanes may also be heat cured for faster production.
Epoxy Resin and Woven Carbon Fiber
Compared to polyester resins, epoxy resins are more durable and have higher strength to weight ration. These composites, also known as CFRP, are good choices where high performance or lightweighting are deciding factors in composite selection. As well, CFRP has greater temperature and chemical resistance than older FRP or SMC. Composite parts made with epoxy resin matrices are increasingly used in automotive structural components.
Epoxies or newer, 4th generation urethanes are good choices for bonding CFRP. Both can be accelerated with heat during the curing operation leading to faster production times. Epoxies are favored when the assembly will be subjected to high heat as part of the duty cycle, whereas 4th generation urethanes are ideal choices when the high heat isn’t a consideration of the duty cycle and rapid manufacturing cycle times are needed.
Thermoplastic composites are becoming a popular choice for automotive applications due to the demand for lightweighting vehicles and the ever-present need for cost reduction. These composites use a thermoplastic resin such as polypropylene or polyethylene along with a chopped glass fiber matrix. Most often the glass is long glass fiber for the increase in strength afforded by this discontinuous phase material. Areas where these materials are gaining increased acceptance are the inner members of liftgates and spoilers.
Due to the nature of the resin matrix (PP, PE or Nylon) the parts produced in this injection molding process exhibit low surface energies and require some form of surface treatment – flame or plasma treatment to make them bondable. Once treated, thermoplastic composites can be bonded using acrylics, epoxies or urethanes. Rapid cycle time due to the large volume of parts needed for automotive components is often the deciding factor in adhesive selection.
Composite panels used as truck sidewalls, usually consist of a polypropylene or polyethylene polymer core sandwiched between painted steel or aluminum. Honeycomb sandwich panels have metal or plastic interiors, with the interior layer constructed in a honeycomb geometry and having surfaces laid perpendicular to outer layers. They are considered laminate composites.
Acrylic adhesives are recommended for metal-to-metal or metal-to-composite applications. They can withstand the internal stresses caused by materials with differing coefficients of thermal expansion (CTE).
Composites are constantly evolving to meet consumer and industry needs, whether those needs are for better performance of manufactured parts or greater efficiency on the plant floor. And changes to composites can mean changes when it comes to adhesive choice. Let’s collaborate to find the best adhesive for your application.