Best practices for bonding low surface energy plastics.
When choosing a plastic adhesive, it can be challenging to find the strongest adhesive. There are many types of plastic. Each type has it’s own strengths and weaknesses. Many are easy to bond; others, like polyolefins, can be difficult. Think of the reasons design engineers want to use polyolefin plastics: they are slick to the point of lubricating, scratch resistant, and chemically resistant. Well there you have it in a nutshell – all the reasons that make polyolefins difficult for many adhesives to bond them.
Specialty adhesives have been formulated which provide very high strength adhesion to low surface energy plastics such as polypropylene and polyethylene, and good strength on PTFE. Following are a review of some plastic adhesives as well as tricks of the trade to bond virtually anything.
Understanding why some plastics are so difficult to bond will help you optimize the surface and joint design.
The surface energy of the plastic either allows or prevents wetting the surface.
Different forms of surface preparation can increase the surface energy which increases wetting.
Historically, low surface energy plastics required some type of surface treatment such as abrasion, flame, corona, plasma, chemical treatment or primers prior to bonding.
New specialty plastic adhesives are eliminating the need to pre-treat the plastic. It is always a good idea to start with a clean, dry, surface – and ideally follow plastic surface preparation guidelines.
One form of surface preparation is a primer. Permabond’s polyolefin primer is for use ONLY with cyanoacrylates. It will not increase adhesion of other types of adhesives. Permabond cyanoacrylate and POP form strong bonds to polyolefin and silicone.
For many applications Permabond Polyolefin Primer and a cyanoacrylate bond these plastics very well. But are they the strongest adhesive? In shear strength these plastic adhesives perform very well and in many cases the bond strength exceeds the strength of the plastic. However, cyanoacrylates aren’t a fit for every application. If more strength, impact resistance, or water resistance is needed, two component 1:1 mixable structural acrylic polyolefin adhesive may be preferred.
Permabond’s structural acrylics for polyolefins are 1:1 mixable plastic adhesives that are applied either manually or via automated equipment. They do not require surface treatment to form strong bonds to polyolefins. The polypropylene lap shears to the left were bonded with Permabond TA4610. The adhesive bond is so strong that when pulled, the plastic stretches, deforms, and fails before the bond.
If you want even more strength – especially on PTFE, consider maximizing the joint design. Following are some options.
Even more strength needed? Ok, let’s get into the tricks.
Adhesive Trick #1
If bonding PTFE to another substrate – let’s say steel – you can configure a joint such that the adhesive bonds and provides a mechanical lock.
If you have two pieces of steel and want a piece of PTFE between them, consider a hole in the PTFE. This allows the adhesive to bond steel to steel while also bonding and holding the PTFE.
This trick works best when the PTFE is as thin as possible. Note a round hole may allow some twisting, so it’s best to elongate or if possible add some edges.
Adhesive Trick #2
What if you only need one piece of steel bonded to PTFE? The hole trick may still work providing enough adhesive comes through the hole and mushroom caps over the top of the PTFE. For very thick sections of PTFE ensure a high shear strength adhesive is used and consider a counter bore which can help to contain the adhesive neatly. This is a bit like using a liquid rivet which cures and bonds both surfaces as well as mechanically holding them. For joints that are only in shear consider a hook effect. For example, a notch in the PTFE will provide a good lock to the metal.
Adhesive Trick #3
Permabond structural acrylics for polyolefins bond well to a variety of other surfaces. If you are unlucky enough to find a substrate they don’t bond well to – don’t give up. Try my next trick. Use the structural acrylic adhesive for polyolefin on that surface and change the other. Joking guys! If you picked that substrate for a reason and don’t want to change it, no trouble! If you can’t change the substrate simply change the surface of that substrate – apply and cure a coating of an adhesive that bonds well to that substrate then bond it with the structural acrylic for polyolefin.
When in search of the strongest adhesive for plastic, consider not only the plastic adhesive, but also the joint design, surface energy, type of stresses, and environmental resistance required.