Secrets of Bond Strength

David Bieniak
Huntsman Advanced Materials Australia
Victoria, Australia

Edited by Jean M. Hoffman

Plastic replacements for metal parts and assemblies are a hot topic these days. Automotive and aerospace manufacturers use plastics to cut weight and boost fuel efficiency. Similarly, plastics are used to make durable, easy-to-mold, lightweight housings and internal components for other industries as well.

The preferred way to assemble plastic parts is through adhesive bonding rather than mechanical fastening. Adhesive joints are aesthetically pleasing and free from deformations due to welded joints or any mechanical attachments. Adhesive bondlines are continuous and “leakproof.” And they resist corrosion better than other assembly methods. Continuous bondlines also withstand concentrated stresses better than spot-welded or mechanically fastened joints. In addition, adhesive bonding turns out parts with good vibration damping and simplifies assembly when a single bond replaces several mechanical fasteners.

However, there are some tricks of the trade for bonding plastics. All substrates should be degreased and, typically, lightly abraded before bonding. But the way to get the maximum strength and long-term resistance to deterioration is through a chemical or electrolytic pretreatment, particularly on thermoset and thermoplastic surfaces.

Polymeric substrates

Molded, cast, and laminated parts made f rom thermos et plastics such as glass-reinforced epoxy (GRE), glass and carbon-reinforced plastic (GRP and CFRP), and sheet-molding compound (SMC) usually bond without difficulty. Of course joint surfaces must be free of all dirt and residual release agent before adhesive is applied. Abrasion with emery cloth, grit-blasting, and cleaning with a solvent such as acetone are all good practices.

Thermoplastics, by contrast, are more difficult to bond. Each type of plastic — ABS, polycarbonate (PC), polyvinyl chloride (PVC), polypropylene (PP), polyethylene (PE), polymethyl methacrylate (PMMA), polyamide and polyetheretherketone (PEEK) — may vary considerably in properties that determine bond strength.

Bonds are stronger with properly prepared surfaces. Pretreatment removes low-surface-energy contaminants such as waxes, oils, greases, plasticizers, and release agents. Proper preparation also reduces dust, dirt, and loose particles resulting from abrasion. Substrate pretreatment expands the surface area available for bonding by creating textured surfaces.

In addition, surface prep promotes the thorough adhesive wetting needed for strong bonds. It does so by giving the material being bonded higher surface energy than that of the adhesive.

Metal substrates exhibit much higher surface energies than polymers. As a result, they are easier to wet-out and bond with adhesivelike epoxies that have surface energies of around 40 mJ/m2. Conversely, polymers don’t wet-out as well because their surface energies are slightly lower than those of epoxy adhesives. That’s why their surface energy must be raised to get a strong bond. A variety of techniques can accomplish this. These methods also help produce joints that better withstand moisture and aggressive chemicals without disbonding.

In most cases, abrading surfaces and wiping them with a solvent such as acetone, ethanol, or isopropanol will be enough. But certain plastics may need flame, corona, or other chemical pretreatments to change their surface textures.

Solvent wipe is the simplest form of surface preparation and is ideal for removing waxes, oils, and other low-molecular-weight contaminants from substrates. The technique relies on the contaminants being soluble in the solvent and the solvent itself being free of dissolved contaminants. But some solvents aren’t compatible with particular polymeric substrates. Some solvents dissolve thermoplastics or will create stress cracking or crazing on surfaces. Common solvents are acetone, MEK, MIBK, Xylene, TCE, ethanol, and IPA.

Apply solvents with clean, lint-free cloths or paper towels. Take care to prevent cross-contamination from sample to sample by not reusing cloths or dipping a contaminated cloth dipped into the solvent.

It’s also important to monitor for hazardous and toxic-vapor buildup when using solvents. Solvent wiping is not suitable for large-scale bonding projects. Instead, vapor degreasing or ultrasonic vapor degreasing are more appropriate.

Abrasion can be accomplished several ways. Manual abrasion includes wire brushing, paper sanding, and filing. Automated techniques include highspeed sanding, grinding, and shot/grit blasting. Mechanical/automated abrasion processes are relatively quick and economical. They are also reproducible and depend less on skilled labor than manual methods.

Flame treatment partially oxidizes surfaces, producing polar groups that raise the polymer’s surface energy. This technique uses a gas or gas/ oxygen flame. It works well for uneven profiles eand thick substrates. It is easy to adjust and control the gas/oxygen ratio, flow rate, exposure time, and proximity of flame to the substrate. It is an effective method for use on PE and PP. Thinner substrates are better suited to corona pretreatment as described below.

Plasma treatment, as the name implies, uses a plasma created by charging a gas with a high amount of energy. The free ions and electrons in the plasma clean the surfaces of any material it touches. On organic surfaces, plasmas, like flames, create polar groups/active radicals that boost surface energy and aid adhesion. Low-pressure plasma is a technique that involves exciting a gas with the high frequency and high voltage between two electrodes in a lowpressure chamber. The process uses various plasmas of argon, ammonia, nitrogen, or oxygen, which make it suitable for a wide range of substrates.

With low-pressure plasmas, air can serve as the plasma source. In this case, oxygen in air produces the greatest results because it reacts with carbohydrate- based contaminants and breaks up large-chain molecules. The process readily removes smaller molecules. Handheld and automated devices are available for the method. Some devices can be fit to a robotic head so adhesives can go on immediately after pre-treatment. The process creates surfaces with energies greater than 70 mJ/m2 in many cases.

Low-pressure plasma has an active bandwidth of 0.5 in. It can be used at up to 3,000 fpm and is a candidate for large industrial applications. Among the plastics suitable for treatment with low-pressure plasma are PP, PE, polyamide, PET, ABS, PC, rubber and composites.

Corona discharge works like low-pressure plasma in principle, but events take place in air at atmospheric pressure. A corona is generated by applying high voltage (up to 30 kV) at frequencies ranging from 9 to 50 kHz to an electrode separated from a grounded table by an air gap. At 3 to 5 kV/mm, a current passes through the air gap to generate free electrons which move toward the positive electrode with great energy. The free electrons displace electrons from molecules in the air gap and, in turn, create more free electrons. The corresponding ions contribute to current that flows across the gap. As ionization currents rise, the corona discharge rate also increases (i.e., the particles move faster). The resulting plasma then activates the surface of the plastic part onto which the discharge is directed. Corona discharge is especially well suited for thin films and composite laminates.

Chemical treatment works well for bonding polymeric substrates. These treatments are usually applied by polymer manufacturers or companies specializing in chemical pretreating. Treatments used for polymers include etchants for PTFE, caustic soda etching for polyesters, proprietary primers for PP, sulfuric acid for PS, and the Sicor process for thermoplastics. Sicor was developed by Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia, primarily for the automotive industry.)

Finally, the choice of adhesive also makes a difference. For example, adhesives based on methacrylate often produce durable joints even on poorly prepared substrates.

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