Advantages of Adhesive Bonding for Composite Materials

The interplay between joining technology and materials technology is an engineering story as old as time: Genghis Khan used bows made from wood, horn, sinew and animal glue to conquer nearly a fifth of the world, but the adhesive’s vulnerability to moisture might explain why the bow’s design failed to spread to rainier climates.

Nowadays, manufacturers need to take more than just a product’s environment into account when selecting a joining technology, especially if they’re bonding composite materials. Plastics and structural composites can be found in many industries; these materials often replace traditional metals, particularly in aerospace and automotive applications.

Bonding composite materials is a complex problem, since lighter composite materials, similar to thinner gauges of sheet metal, will often suffer unacceptable levels of damage when subjected to welding or through-part fastening techniques. These processes also tend to be labor intensive and can easily be overkill in terms of the bonding strength that’s actually required.

As a result, adhesive bonding of composites is rapidly finding its way into many new niches—it solves the old problems presented by mechanical fastening, such as uneven load distribution. However, to understand the advantages of composite adhesive bonding, it’s worth taking a step back to look at bonding for composites more generally.

Bonding Composite Materials

Composites are used to reduce a product’s weight, improve its aesthetics or environmental resistance, allow for greater design freedom and increase its stiffness-to-weight ratio. The category of composite materials is rather broad; for our purposes, let’s focus on polymer matrix composites specifically.

Polymer matrix composites—also known as fiber-reinforced plastics—are finding increasingly frequent use in place of metals for casings, retaining structures, body armor and vehicle bodies. These materials consist of a polymer matrix reinforced with fibers (typically glass, basalt, carbon or aramid). Their primary advantage over metals is that they weigh less without compromising material strength, often resulting in a considerable savings in manufacturing costs.

Examples of polymer matrix composites include thermoset composites, glass-filled polyester resins (usually used in permanently molded configurations) and thermoplastic composites, which are also glass-filled with a matrix composed of polyethylene, polypropylene, nylon or other similar materials.

The trouble with bonding these types of composites is that they can suffer performance limitations when paired with traditional joining techniques, such as mechanical fastening. The necessity of drilling holes or otherwise puncturing the surface will damage the continuity of the matrix or reinforcing fibers, eliminating many of the benefits of using the materials in the first place.

As a result, composite adhesive bonding is becoming increasingly more common, particularly as advanced adhesives and tapes find their way into a variety of manufacturing applications. Technological advances mean that these substances are able to meet the same strength requirements as traditional rivets or mechanical attachments, while offering a plethora of other specific, often tailored advantages, such as sealing, preventing galvanic corrosion between dissimilar metals and offering a cleaner final look without costly finishing work.

The automotive industry has made considerable use of composite adhesive bonding as a result of the demand for lighter weight, more fuel-efficient vehicles. For example, 3M’s Scotch-Weld Structural Plastic Adhesive DP8010NS Blue finds use in the production of large, light-weight molded interior panels for automobiles because it’s able to bond the thermoplastic bulkhead to an aluminum frame securely.

Weight saving matters in other industries, too. In the oil & gas industry, every pound added to a platform comes at the cost of oil that can be pumped and stored. With that in mind, companies are using 3M Scotch-Weld Structural Epoxy Adhesive DP490  to achieve lighter shale shaker builds, bonding in both carbon fiber-carbon fibre and carbon fibre-stainless steel configurations. The adhesive is able to meet the challenge of both the movement and presence of crude oil and hydrocarbon mud at high temperatures.

Adhesives Used in Composites

For many adhesives used in composites, there’s no issue in terms of material specificity. Numerous adhesives can be used to bond a variety of plastics, including ABS, polycarbonate, PVC and acrylic, as well as traditional composites like SMC or carbon-fiber reinforced epoxy composites. There are, for example, adhesive and tape bonding options like 3M SMC/Fiberglass Repair Adhesive.

For heavier-duty applications, such as highly stressed vehicle components, a structural adhesive can ensure both high interfacial strength and high load-bearing performance. For lower load applications, such as plastic trim and molding, a specialty hot-melt adhesive is a good option. As another example, 3M VHB Acrylic Foam tapes or thin bonding tapes enable small parts to be bonded quickly and securely.

A particularly tricky bonding application is one involving low surface energy (LSE) plastics or composites. These materials come in many forms, including polyolefin (TPO), HDPE, Polypropylene (including glass-filled composites) and some powder coats. Although friction or ultrasonic welding work well with these materials, these methods can be costly and may not work when bonding to dissimilar materials.

For this reason, specialty adhesives like 3M Scotch-Weld Structural Plastic Adhesive DP8010 Blue make for a good alternative. These adhesives are used in a wide range of applications, from medical equipment to small engine-based tools. Polypropylene can be similarly difficult to bond, but pressure-sensitive adhesives, such as 3M Acrylic Adhesive 300LSE, are specifically designed for these materials.

Tips for Composite Adhesive Bonding

If you’re looking to maximize joint durability in the context of adhesive bonding of composites, remember that it will depend on your adhesive’s capacity for evenly distributing the load to the matrix of the composite, in order to prevent premature failure due to stress localization. Consequently, the performance of the joint also depends on the modulus of the substrate in addition to that of the adhesive.

Generally speaking, it is important to ensure that the stresses in the joint will not exceed the adhesive’s capabilities, allowing for an appropriate safety factor. The stress-strain curve can help you understand how your adhesive responds to stress.

Initially, a polymer (i.e., adhesive) will respond elastically to the application of a load, with stress increasing at a constant rate. At a certain point, the polymeric backbone will begin to deform plastically, resulting in a permanent deformation. Usually, this means it will elongate until it finally breaks.

The area under the curve represents the energy the polymer absorbs during this process; the larger the area, the more energy absorbed before the material yields or breaks.

Lap shear value is another important consideration, representing adhesion to a substrate surface combined with the cohesive strength of the adhesive as a cured polymer. Adhesion depends on chemical compatibility between adhesive and substrate, which in turn determines the adhesive’s ability to fully wet the substrate’s surface.

As a general rule, you should try to balance an adhesive’s capability for wetting a substrate’s surface to generate sufficient adhesion (in terms of lap shear) with your requirements for cohesive strength (as related to the stress/strain curve). It’s worth noting that the first criterion depends on the chemical nature of the adhesive before it’s cured, while the second depends on the nature and composition of the adhesive after it’s cured.

If you’re uncertain about the best adhesive option for your composite material application, the following table can help by laying out a sample portfolio of products for bonding composite materials:

Bonding for Composites

The Mongolian bow design enabled Genghis Khan to build one of the largest empires the world has ever seen, but it fell victim to a flawed bonding solution. Don’t make the same mistake when it comes to adhesive bonding of composites. Talk to an adhesives expert to find out your best option for composite adhesive bonding.

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This post is sponsored by 3M.  All opinions are mine.  –James Anderton