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Surface Energy vs Amorphisation

OAS interviews adhesion expert Steven Abbott 

It’s widely understood in industry that flame, plasma and corona treatments change the surface energy of materials. And it is also understood that they are effective at promoting adhesion to difficult-to-bond substrates like plastics. A common supposition is that any improvement in bonding resulting from these techniques is caused solely by the change in surface energy. To adhesion expert Steven Abbott, this is a frustrating topic, so I asked him to explain.

Why does this misconception arise?
“I think it is because people find comfort in simple surface energy tests like water contact angle or Dyne pens. However what people often fail to understand is that correlation does not mean causation; bad wetting leads to poor adhesion, but good wetting does not guarantee good adhesion”.

So surface energy doesn’t play as big a role in adhesion promotion as people think?
“It can be shown that surface energy effects are around one thousand times too small to be of any relevance [to bonding]”.

Why does plasma improve bonding then?
“People use plasma for semi-crystalline polymers like PET because it makes the plastic surface more amorphous. It’s a gnomic comment, but amorphisation results in good adhesion”.

Semi-crystalline vs amorphous material definition
In semi-crystalline materials the poam vs SClymer chains are packed together in ordered crystalline regions. Amorphous materials have less order and contain free polymer regions which are able to intermingle and entangle.

 


But why does making a surface amorphous lead to stronger bonding?

“Coatings, adhesives and other polymer systems are able to intermingle and entangle with the polymer chains of an amorphous surface, and it can readily be shown that polymer-polymer adhesion becomes strong with full entanglement. (In the world of PE heat sealing, people make use of the entanglement mechanism routinely as a method of bonding). Furthermore, when you try to pull the joint apart the entangled polymers absorb the fracture energy, causing it to dissipate.”

Whilst the theory behind these treatments is often misinterpreted they can certainly be effective for improving bonding to advanced materials. However they are not practical for all manufacturing environments and are not effective for all materials. Onto™ treatments from OAS are a great alternative for effective surface treatment when plasma, corona or flame treatments are not practical for your manufacturing process.

Steven Abbott is an expert in adhesion, coating and formulation science. He sits on the OAS Technology Advisory Board, bringing a wealth of industrial, scientific and commercial knowledge from both academia and industry. 

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