Combining materials of choice is important for the appliance industry where “form follows function” is archetypal. Appearance, performance, cost, and assembly ease constantly dog the engineering community in this industry. This is particularly true when joining dissimilar materials, such as metals and plastics, into one unit or subassembly.

Combining materials of choice is important for the appliance industry where “form follows function” is archetypal. Appearance, performance, cost, and assembly ease constantly dog the engineering community in this industry. This is particularly true when joining dissimilar materials, such as metals and plastics, into one unit or subassembly.

The options for joining dissimilar materials are mechanical fastening and adhesive bonding. Mechanical fasteners require pass-through holes, producing stress concentrations and the need for thicker material and included bosses. Adhesive bonding produces a continuous joint with better stress distribution and frequently allows material savings with fewer molded inserts.

Plastics in appliances, especially thermoplastics, offer advantages in cost, weight reduction, corrosion mitigation, and parts consolidation. But thermoplastics are often difficult to bond. Why?

Adhesives only bond to surfaces. Bonding first requires wetting, which combines the surface receptivity for an applied fluid with the physical ability of the fluid to spread over the surface. Receptivity combines polar surface chemistry and molecular packing ability, while the fluid must have low enough viscosity and surface tension to film-form on the surface under the attractive pull of the surface polarity. Wetting occurs when the surface energy of the adhesive overcomes the surface tension of the receiving surface

Naturally polar surfaces, including metals and many paints, wet readily with many adhesives. Non-polar surfaces, especially polyolefins and elastomers, do not wet easily. Solvent-borne adhesives enhance wetting, but carry the undesirable need for solvent abatement. Surface treatments for thermoplastics, using flame or corona treatment, increase surface polarity and improve wetting. Primers improve bonding by forming an interactive tie layer with the surface. Any surface should be free of mold release and surface waxes before bonding.

Including polar modifications in a copolymer increases wetting for adhesives (and paints). Inclusion of acrylic acid groups, addition of ethylene vinyl acetate, or vinyl alcohols in the copolymer improves wetting. Polar modifications may also improve temperature resistance or stiffness of the polymer.

Engineering thermoplastics such as filled-nylons, polysulfides, and polysulfones are more demanding, and, therefore, often require priming. A reinforced thermoplastic can be treated as would the parent resin to bond them. A glass-filled or mica-filled polypropylene can use the same primer or flame treatment as would the unfilled polymer.

Elastomers such as natural rubber (polyisoprene), SBRs, butyl rubber, and silicones bond with difficulty. These require oxidative treatments or priming. Polyurethane rubber often bonds with relative ease, especially when using a urethane adhesive. Similarly, silicone rubber bonds best with a silicone sealant or adhesive. Elastomers offer the separate challenge that they are designed to stretch or conform. The adhesive must follow that mechanical shift, otherwise disbonding or stress cracking may occur. Fig. 1 lists several polymers by their ease of bonding.

Reinforced thermoset polymers like polyester-fiberglass (SMCs and BMCs), vinylester-fiberglass, and epoxy-fiberglass bond well and typically require little preparation other than removal of mold release or other contaminants. Mechanical ablation of a resin-rich surface is beneficial in stubborn cases. Destructive fiber tear is the benchmark of good bonding, indicating the adhesive bond exceeds the strength of the resin-reinforcement interface.

What about metals? Metals have natural oxide layers that typically bond well if they are free of contaminating forming oils. Weak oxide layers, such as rust or aluminum scale, must be removed, or the weak oxide will fail mechanically. Metal treatment by etching, anodizing, or phosphatizing provides a strong, adherent surface which bonds well with adhesives.

An attractive approach with appliances is to bond to a painted surface. Polyester, epoxy, and urethane paints bonded with adhesives frequently show the weak link as the paint-metal interface. Bonding to a pre-painted surface is a good way to take advantage of the surface preparation already employed for the paint as an anchor for adhesive bonding.

When bonding thermoplastics to a painted surface, failure may occur at the thermoplastic interface. If failure occurs at the paint-metal interface, that is the best that can be achieved. Failure will occur at the weakest link in any bonded system. It must be determined if the overall strength is high enough for the intended purpose.

What about joint design? Adhesives work best in compressive loading, or shear loading, but perform poorly in peel (see Fig. 2). Reacting out-of-plane (peel) loads into a compressive load by joint design is a good practice. For shear joints, longer and wider bonds are preferred over short, stubby bonds, because they mitigate peel and promote shear loading. Most joints are simple lap joints, and there are many modeling guides specific to such joints. 

Bonding of dissimilar materials generates another possible difficulty from thermal expansion of metals and plastics. Resulting induced strain can be reacted into a flexibilized or toughened adhesive. Combined with good joint design, the adhesive will dissipate the stress induced by thermal mismatch.

What are the requirements and options in adhesive selection? The adhesive must bond to both prepared surfaces and must provide the required bond strength over the temperature and environmental range. Most adhesives perform well up to 150 DegF and several will operate up to >250 DegF. This range accommodates most thermoplastics, many thermosets, and all metals in combination. In some cases, the adhesive must also perform at temperatures near
–40 DegF. Adhesives that favor low-temperature use sacrifice upper-temperature limit. The more exotic the temperature excursions required, the fewer the choices and the higher the cost.

For many appliances, chemical resistance is important along with environmental resistance. Laundry chemicals especially might attack adhesives over time. The acid-base pH range of foods is rarely a challenge for adhesives. The concern with food contact is the possible leaching of volatiles from the adhesive. Many adhesives carry FDA ratings for food contact and storage.

Acrylic and epoxy adhesives have good chemical resistance and good thermal resistance. Acrylic adhesives offer rapid cures, while epoxy adhesives have slower cures and sometimes require heating to give maximum performance. Epoxy-based adhesives are common in the automotive industry. They are usually applied during forming and fabrication, followed by cure in the primer or paint bake cycle. There are hybrid adhesives that will bond directly to polyolefins with minimal treatment.

Urethane adhesives bond well to many plastics and metals and feature a good temperature range. They are available in a range of cure times. Their chemical resistance is not as robust as epoxies or acrylics. Unfortunately, most adhesives do not fair well against bleach, strong oxidizers, and some cleaners or sterilizing agents.

The cyanoacrylate adhesives offer rapid fixturing. They wet most surfaces well, by virtue of available low viscosity, and many are tailored to be surface-insensitive, allowing them to bond metals-to-plastics. Their toughness varies by type and most have temperature resistance <180 DegF and low peel resistance. New formulations provide claimed temperature resistance >350 DegF after a post-bake such as a paint cycle. Cyanoacrylate adhesives work best with narrow-gap bondlines (<0.01 in.), which requires good part fit-up. Good practice for any adhesive is to keep bondlines under 0.025 in.

Tapes and hot-melt adhesives work well with difficult-to-bond thermoplastics. Tapes are frequently produced from coated thermoplastics giving high surface compatibility. Many suppliers offer two-sided structural tapes with good environmental and chemical resistance. Some tapes and hotmelts can operate at 250 DegF, offering good thermal compatibility with plastics. Most have minimal disposal or shelf-life issues.

Tapes are easy to apply in roll form or as die-cut appliqués. Surfaces must be clean, dry, and dust free. Bonding to painted surfaces is an ideal use. Hot-melts have an open time, which is the allowed time after application of the hot liquid and the closure of the joint. Hot-melts do not work well on un-heated metal because of freeze-off. However, they work well in wetting and bonding many plastics and films.

Another option in joining metals is to employ “weldbonding.” This is the combination of a welding method and adhesives bonding in one process step. The advantages are reduced stress concentrations, reduced number of welds, improved fatigue performance, and potentially improved noise performance.

Weldbonding provides automatic fixturing of the adhesive reducing the need to allow for cure time during assembly. This broadens the choice of adhesives to include structural sealants since the welds carry some of the structural load and greatly improve peel performance, reducing the brute-force demand on adhesive performance. 

There are many welding methods available for weldbonding. The most common is resistance spot weldbonding, which is universal worldwide for steels in the automotive industry. However, ultrasonic weldbonding, laser weldbonding, and friction-stir weldbonding are effective on aluminum. Ultrasonic and laser weldbonding can be used on plastics also.

Difficulties in bonding dissimilar materials can be overcome by judicious attention to materials selection, surface preparation, joint design, adhesive selection, and use of a good reliable process. Adhesive bonding offers many opportunities to design and manufacturing engineers as they develop cost-effective designs with high customer appeal. Materials choice need not be limited by the joining method. Adhesive bonding and combined welding methods can produce many innovative, reliable structures at competitive cost. 

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