Strong, lightweight composites can solve many product design problems, but bonding them to other materials using traditional methods is often impossible. Most composites won’t stand up to welding, and holes to accommodate traditional fasteners will also weaken them. Nevertheless, we see composites being adopted in the aerospace and automotive industries, and adhesives are making this materials revolution possible.
Polymer matrix composites, also called fiber-reinforced plastics, are taking the place of metal in casings and retaining structures. The fibers used include glass, basalt, carbon, and aramid, and since they’re both strong and lightweight, they fulfil the needs of designers in terms of increasing stiffness-to-weight ratio while allowing for improved aesthetics and resistance to environmental stress. The materials include glass-filled polyester resins and thermoplastic composites. Drilling holes in these materials breaks the reinforcing fibers, compromising their strength.
Composite adhesive bonding, on the other hand, is made possible with the help of advanced adhesives and tapes which are as strong (and sometimes stronger) than rivets or welds. But adhesives offer more than just bond strength. Manufacturers can choose adhesives to seal joins, or to separate dissimilar substrates that would otherwise be subject to galvanic corrosion, and the advantages of a clean, almost seamless bond is attractive from an aesthetic perspective.
The automotive industry has followed aerospace’s example in adopting composites for the many benefits it provides including lightweighting, structural integrity, aesthetics and other key reasons.
The oil and gas industry is also using adhesives so that it can benefit from lightweighting as well. In this case, weight reduction increases the amount of oil a platform can pump and store.
Many of today’s adhesives are able to bond a wide variety of materials, but for heavy-duty use, specific interfacial strengths and load-bearing properties will be required. When the load-bearing requirement is light, foam tapes or bonding tapes will often be sufficient to the task.
When bonding composites, manufacturers should take the following criteria into account:
(1)The modulus of the substrate and the adhesive.
(2)The lap shear value, which depends on the compatibility of adhesive and substrate.
(3)Requirements for cohesive strength.
Broadly speaking, end users of adhesives have a variety of requirements and the adhesive and sealant industry has the products for the job including:
l Urethanes and two-part epoxies for general composite bonding
l Toughened epoxies for high strength and durability
l Flexible epoxies and urethanes
l Acrylics for bonding plastics and metals
l Low Surface Energy (LSE) acrylics for bonding polyolefins
l Combination adhesives and sealants used for sealing, bonding, or large component lamination.
In short, the secret to successful composite bonding is the choice of the right adhesive and with adhesive companies being best acquainted with their products’ capabilities, end users should always ask for advice for their specific applications.