Fundamentals of Adhesion & Why 3 Molecular Layers Matter

Pierce Geary

Adhesion is a powerful phenomenon that can hold massive structures together and fuse microscopic compounds. It is also extremely dependent upon delicate and fragile processes to succeed.

In manufacturing, adhesive bonding takes many forms but the principles of adhesion are always the same. Even if the application is metal joint welding, gasket sealing with glue, implantable medical device coating, inkjet printing on plastic film or automotive painting, proper adhesion relies on the understanding and control of three distinct yet interrelated elements.

The three things that must be controlled for adhesion to be successful are:

  1. The composition of the adhesive, coating, ink or paint
  2. The application of the adhesive, coating, ink or paint
  3. The quality of the bond surface

All of these present their own challenges to control, but the first two have been well understood and defined by the suppliers of the adhesives and dispensing equipment. The third element is influenced by the widest array of variables and goes through the most changes without detection. 

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The intricate process the material surface goes through and the delicacy of the surface quality makes it the most difficult to control and the hardest to understand. However, it is possible to gain control of this process and understand the fundamentals in order to build more predictability into the adhesion process.

What is a surface?

It seems like a simple enough question, but under closer examination there’s more to a surface than what meets the eye. As it relates to adhesion, a surface is an infinitesimally thin layer made up of molecules sitting on top of the substrate. These molecules are what every operation, every piece of equipment, every person who handles the material, and what the adhesives, coatings, paints and inks are interacting with. The very air itself in the production facility can have a dramatic effect on the molecules making up the material surface.

A molecular layer can be likened to the way a stack of LEGO blocks. All materials are made up of structures consisting of stacked molecules (individual LEGO blocks) and when something comes into contact with the material it is only interacting with the top blocks. Like LEGO blocks with the small pegs, molecules need to have the proper characteristics to connect with the molecules of the substance it is trying to bond with. A LEGO block cannot properly connect to a cinder block and similarly certain molecules cannot bond with other molecules, making proper adhesion impossible. 

All of the preparation steps of an adhesion process are used to make the characteristics of the top few layers of molecules align with the ideal criteria for bonding with the adhesive, coating, paint or ink.

The characteristics of the surface molecules involves their chemistry (i.e. if they are carbon molecules or oxygen molecules or silicone molecules, etc.) and their composition (i.e. how much of each molecule exists on the surface and if it’s concentrated in certain areas).

How do you know what’s on your surface?

Characterizing the molecular layers based on chemistry and composition allows manufacturers to truly understand the aspects that make adhesion work. So, it’s vitally important to have this information or at least be advised by someone who does. Specialized equipment using sophisticated techniques like x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy and needed to map out what the surface truly looks like. This kind of equipment is really only available in specialized Materials Science laboratories but it’s possible to correlate other techniques to the sort of data XPS and FTIR analysis offers. 

In order to evaluate the surface quality based on these chemical attributes is to use equipment sensitive to the top few molecular layers. There is accessible lab equipment available, like benchtop goniometers, that many companies utilize when XPS and FTIR are out of the question. Goniometers measure water contact angle to understand the surface energy of a material. Surface energy refers to how reactive a surface is and how ready to bond it is. Contact angle measurements are sensitive to the microscopic layers that the sophisticated lab equipment analyzes. 

But most companies are looking for something that can measure surface quality with high accuracy directly on the production line. Until recently there were no options for this. But there have been major advances in handheld and automated devices that verify surface quality using non-destructive contact angle measurements.

As the surface is altered throughout the adhesion process, these tools monitor and track those changes so manufacturers can respond immediately and appropriately to changes in surface chemistry.

What is a surface

Who do you look to for surface expertise?

So, it’s clear now that in order to create an adhesion process that produces predictable, consistent results, three elements need to be fully controlled and understood. Two of those are well understood and have been well managed by suppliers of adhesives, coatings, inks and paints, as well as the manufacturers of deposition equipment like sprayers, dip-coaters and the like. 

The one element that most manufacturers typically do not have expertise to fall back on is the quality of the material surface. Surface quality is difficult to measure and understand without knowing what actually makes a surface ready for adhesion. The attributes that indicate readiness can be different from application to application, product to product, and production line to production line. 

Since one of the most crucial elements of adhesion is invisible to the eye, help is needed to detect subtle, yet massively influential, changes. Just like how you can’t truly know if a shelf you just installed is level without a level tool, you can’t tell if your surface is ready for adhesion without help.

Read this blog article to learn what to look for when choosing a Materials Science expert to help with your adhesion process.

When you start talking about the things that affect the chemical characteristics of your surface you begin to see how many variables there are that come into play and why what seems like a simple problem (i.e. my glue, paint, ink or coating didn’t stick properly) is actually due to a complex web of uncontrolled factors. So, chemistry and manufacturing expertise matters a great deal. Solving this issues is possible when you know where to look.

Read our eBook about what steps to take to actually get to the root cause of adhesion and cleanliness issues. The Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers”  eBook examines what it takes to truly eradicate adhesion failure from the ground up.

Adhesion Failure Root-Cause Analysis for Manufacturers

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