Hard to Bond Plastics?

Designer Tip - Surface Treatment!

Have you ever selected  a pliable, chemical resistant, low cost plastic that seemed to meet all of your medical device needs only to discover it won’t adhere to anything when scaling up to large performance and qualification builds?   As a contract manufacturer, have you ever wondered how certain materials were selected on drawings, but more importantly, how you will ever bond them? If you’ve answered yes to one of these questions, you’re not alone.

These scenarios are commonplace in the ever-changing world of plastics but fortunately there are ways to combat the challenges of bonding certain plastics, even in the 9th inning when it seems too late to turn things around.  The solution lies in Surface Treatment!

There are four main types of surface treatment that rely on a layer of ionized gas at the polymer surface. They are: Corona, Flame, Plasma in Vacuum and Plasma in Air.

Other Surface Treatments

Primers - Commonly used with Acetals, Fluoropolymers, Polybutylene, Terephthalate, Polyolefins, Polyurethanes, Silicones

Primers typically consist of a reactive chemical species dispersed in a solvent.  To use the primer, the solution is brushed or sprayed onto the substrate surface.  The carrier solvent is then allowed to flash off, leaving the active species behind.  Depending on the type of primer, the surface may be ready to bond immediately, as in the case of polyolefin primers for cyanoacrylates. In other cases, the surface may require time to react with atmospheric moisture before the application of the adhesive.  Primers that must react with atmospheric moisture include silane and isocyanate-based primers which are typically used for silicone and polyurethane-based adhesives respectively.  Surface primers generally improve substrate bondability by acting  as a chemical bridge between the substrate and the adhesive.  Typically, the reactive species in a primer will be multifunctional, with one set of reactive groups that will preferentially react with the substrate surface, and additional groups that will have a high affinity for the adhesive. 

Surface Roughening - effective on many plastics

Surface roughening is a simple, low cost method of increasing the bondability of many plastics by dramatically increasing the number of mechanical interlocking sites.  To determine if it will be effective on a specific plastic material, refer to the Loctite® Design Guide for Bonding Plastics.

 

LOCTITE® Design Guide for Bonding Plastics

This comprehensive Design Guide contains a description of twelve surface treatments in the Surface Treatments section.  It also details adhesive shear strengths with commonly used plastics paired with select Henkel adhesives. The data can be used to make correlations to products and understand how surface treatments will affect the bond.  

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