How to Bond Plastics to Metals and Alloys

A common problem for design engineers is bonding plastic to any sort of metal or alloy.

The type of plastic is one of the major issues. Plastics like polyethylene and polypropylene can be hard work without preparation of surfaces. Most plastics are more cooperative and have sufficient surface energy to bond well.

More exotic plastics, notably hybrids like GFN, can have major issues with their adhesion as

a result of moisture levels. In these cases, a combination of treatment of the material and selection of adhesive is required.

Metals, however, can present their own problems. Metals, particularly alloys, have their own idiosyncrasies. Galvanized zinc, for interest, regularly makes a nuisance of itself because of its weak surface, which simply loses the peel. This type of alloy responds better to a flexible adhesive with more peel strength and good impact resistance.

Metals, in fact, have unique properties in terms of creating adhesion problems.  Their surfaces can be seriously affected by particulates, chemicals, and oxidized layers, as well as almost invisible area scarring and pitting that creates a veritable smorgasbord of different adhesion problems on the surface. Some alloys need treatments like abrasion and cleaning for better bonding.

Size really does matter when bonding plastics and metals. In larger areas, different expansion and contraction characteristics (every type of metal and plastic expands and contracts at different rates) can mean major stresses and very different behaviours between the bonded elements. In cases where the environmental temperature is expected to vary considerably, a more supple and adaptable adhesive and a thick application will provide a reliable response to the different expansion and contraction rates.

(This isn’t as difficult as it sounds. Modern adhesives have excellent and highly consistent, measurable levels of flexibility. The differentials between materials are a good indication of the type of adhesive required. The trick is to adequately cover the expansion and contraction rates of both materials.)

The common knee jerk response to these issues, and it’s often exactly the wrong approach from a design perspective, is to assume that things like fasteners are simpler than adhesives. Fasteners can create more problems than they solve, and quickly. In any design where weight is an issue, fasteners add weight, and sometimes a lot of it. Fasteners can also affect the materials, and can create and aggravate stresses and fractures. They can be very much an own goal.

Adhesives are a far safer choice in many ways. Adhesives actually prevent stresses, and protect substrate surfaces. They also eliminate the need to drill holes in materials, reducing trauma on them. Aesthetically, they’re significantly more flexible from a design presentation perspective. Using adhesives also allows a much wider choice of possible substrates as design elements because of these factors.

If you’re looking for a suitable adhesive or need some assistance with substrate surface preparation please feel free to contact Permabond for free technical assistance.

E-mail help.europe@permabond.com or telephone +44 (0)1962 711661.

Article written by R. Wilmot, Permabond Engineering Adhesives Ltd.

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