And how does stress cracking occur? – When force is applied to plastic, it travels along the polymer chains causing molecules to strain to remain in contact. The polymer chains eventually creep toward the pull and deform. This stage is called creep. With force and time, crazing or stress cracking occurs. Adding chemicals or temperature can speed up the process.
Some plastics, such as ABS, acrylic, polycarbonate, and polystyrene, are more susceptible to stress cracking. These are amorphous plastics, and they bond readily. Other plastics, such as polyethylene, polypropylene, and polytetrafluoroethylene (Teflon) are semi-crystalline thermoplastics and are more difficult to bond.
I expect most of you will agree with me that there is little point in creating a super strong bond to plastic if that bond causes the plastic to break.
The flip side is if you use a difficult-to-bond plastic – we need to ensure that the bond doesn’t break.
(again, examples include ABS, Acrylic, Polycarbonate, and Polystyrene).
There are many UV curable adhesives which bond amorphous plastics readily. Some are designed for adhesion to specific plastics. The keys to reducing the effects of stress cracking when bonding with UV’s include:
Cyanoacrylates are also a good choice for bonding amorphous plastics. To avoid stress cracking with cyanoacrylates:
For both UV and Cyanoacrylate bonds, check for other stress-inducing chemicals. What does that mean? Some chemicals can travel (leach) from one substrate to another. The adhesive is not always a barrier to that leaching. When bonding PVC to polystyrene, the plasticizers in the PVC can leach straight through the adhesive and attack the polystyrene. Add heat, and it will attack even faster!
Similarly, for both bonding methods, reduce stress, either by creative joint design or by distributing the stress over a larger bond area. Many UV adhesives are flexible, allowing more stress absorption than cyanoacrylate.
Since semi-crystalline plastics are less apt to stress crack, the issue is more often achieving high enough bond strength rather than reducing stress cracking.
Although some UV curable adhesives will provide some bond strength, consider using a mechanical lock in addition to the bond should you require a durable bond.
Cyanoacrylates alone will generally not provide sufficient strength. However, when used in conjunction with a primer such as Permabond POP (polyolefin primer), excellent bond strength can be achieved. Although these plastics are less prone to stress cracking, care should still be taken to ensure:
Glass or other reinforcing materials are often added to plastics. Although many adhesives bond glass, this glass is surrounded by plastic – so best bond strengths are generally achieved when using a plastic bonding adhesive as opposed to a glass bonding adhesive.
New structural acrylics are surpassing the durability of the bond strength of cyanoacrylates on many semi-crystalline plastics. Benefits they provide over cyanoacrylates include:
Please tell us about your application, and we’ll have a technical specialist provide insight on reducing or eliminating stress cracking in your application.