Adhesion processes vary widely across different industries that incorporate bonding, sealing, printing, coating, painting or cleaning into their manufacturing operations. Every adhesion process is constructed differently based upon what the product being manufactured requires. However, all successful adhesion processes rely on a contamination-free bond surface.
That principle bears repeating: Every adhesion process, regardless of the application or industry, hinges upon creating a bond surface that is free of chemical contamination.
Achieving this chemical state on a surface is a multi-step process that involves more than just the cleaning, treatment and bonding, coating, printing or painting steps.
In order to know how to best control the surface quality as it relates it adhesion-readiness, manufacturers have to ask themselves several questions.
How is the Bond Failing?
Not all bond failures are created equal. Deciding what course of action to take once failure occurs depends greatly on where in the material system (the combination of the materials being bonded which includes the adhesive or coating as well) the bond is severing.
All adhesion failure can be classified by lumping them into one of three buckets: adhesive failure, substrate failure or cohesive failure. Determining which kind of failure your material system is experiencing is the first step in knowing how to remediate the issue.
- Adhesive failure - a failure that occurs at the interface of the adhesive or coating and the material being adhered to.
- Substrate failure - a failure with the bulk material of the metal, polymer, etc. being bonded or coated.
- Cohesive failure - a failure within the bulk material of the adhesive or coating itself.
If you analyze the nature of the failure and determine it to be cohesive or substrate, then you need to look at the mechanics of the bonding, coating or printing operation because there is possibly something inherently flawed in the constituent parts. There could also be an issue with the curing steps not fully finishing the job. For instance, the adhesive may not be strong enough to hold to the extent mandated by the performance requirements or it is not being cured properly and therefore is causing inconsistencies in a coating or ink.
One method of testing bond strength limits is with an Instron Universal Testing Machine. This video demonstrates how the machine breaks a bond in order for the failure to be examined further. Depending on how much glue remains on either side of the material surface you can determine which one of the three types of failure modes you're witnessing.
If the failure is adhesive, then the root cause of the problem lies somewhere upstream.
Adhesive failure indicates an inability for the two things being bonded together to properly adhere at the top few molecular layers of the material. This kind of failure can be identified by observing the way the adhesive remains on only one side of the material (image of aluminum with black adhesive failing with adhesive only on one coupon). The adhesion was never able to occur on a chemical level, which is where adhesion happens.
When bonds are experiencing adhesive failure you must work backwards through the production process to explore what has not been controlled and optimized to meet the performance standards.
How Often is the Failure Occurring?
It’s important to get a handle on the frequency of the failure. Does it happen in batches? Is it strictly intermittent? Can the failures be narrowed down to happening during specific time periods? The answers to these questions could offer clues as to where the failure is originating.
The nature of the adhesion failures and the times it is occurring offers insight into where to look when conducting root cause analysis. Look for changes in the process that are only occurring during the intervals the adhesion problems are determined to be happening.
- Is the wash fluid being changed promptly enough?
- Is there an intermittent delay between treatment and bonding?
- Are some materials exposed to potential contaminants while others are not?
Finding these inconsistencies in the adhesion process and building certainty back into the process can go a long way to creating predictability.
Are the Steps Optimized?
Not having the equipment and techniques used in an adhesion process properly dialed in and set up to produce the desired surface quality can spell disaster. These steps need to be fully controlled and the amount of change they are incurring needs to be measured and understood.
The surface quality (how ready for adhesion the surface is) needs to be quantitatively evaluated at before every step that alters the surface. Then, after the operation, it is crucial to evaluate the surface again in order to get a measurable value of the change that is happening. If the change to the surface meets the requirements set forth then the step is at its optimal settings. If this isn’t the case then the parameters need to be configured to produce the necessary outcome.
Are the Steps in the Right Order?
In order to get the most out of an adhesion process all of the constituent steps need to be fully optimized, executed with consistency and in the proper sequence.
Before printing, coating, sealing, painting or any interfacial bonding can be successful, a chemically clean and consistent surface needs to be created. This is fundamental, but to achieve this, chemical cleanliness needs to be understood.
Cleaning operations in an adhesion process are removing something from the surface of a material in preparation for the final step. This may not be all that is required to ensure the correct molecular chemistry is present on that material.
Activation in the form of plasma treatment, corona treatment, flame treatment, chemical etching and some abrading operations have become widely used in manufacturing as advanced materials are becoming common substrates in the designs of new products. In order for these operations to properly activate the surfaces they are treating, a cleaning step may be required beforehand to remove oils or other debris so that the true bond surface can be revealed and accept the treatment.
The purpose of an adhesion process is to create surfaces that are chemically ideal for adhesion success. In order to achieve this, the failure needs to be understood and the process has to be designed with changes to the chemical nature of the surface in mind. This chemical state is altered throughout the entirety of the adhesion process and needs to be controlled every step of the way. To learn more about looking at manufacturing holistically as a means to eliminating adhesion failure, download the “Checklist: Adhesion Failure Root-Cause Analysis for Manufacturers” eBook.