Manufacturers are using more and more adhesives, and the applications where they’re being used are more critical every year. Consequently, the cost of failure of adhesively bonded products is growing. Users of adhesives are aware that bond failures can occur, but they may not know why. Adhesive manufacturers often provide only simplistic instructions on preparing surfaces before bonding, and this is a leading cause of bond failures. Why don’t adhesives manufacturers provide more detailed instructions? Partly because they may think that it’s in their best interest to position products as needing little preparation: apply the adhesive to a clean, dry surface, and wait for it to cure. Voila! A bonded structure!
These sorts of simplistic instructions neglect the fact that the quality of the bond depends intimately on the chemical composition of the surface being bonded. And the “surface” we’re talking about here is not what you see, it’s what you don’t see: the submicroscopic stuff on the surface that we call contaminants which frequently can’t be removed without well-thought-out and controlled cleaning processes.
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To ensure a successful bond, it is essential to understand what the adhesive's technical data sheets don't tell you about bonding. By learning about the composition of surfaces and how to prepare them properly, users can avoid failures and create reliable and lasting bonds. Cleaning and surface preparation details are critical and typically specific to a particular substrate. They can also be specific to the supplier because materials from different suppliers will generally have different amounts and types of contaminants. These specifications will require different surface preparation to obtain a good bond surface.
For example, a cleaning process that works well for material from “Supplier A," which uses a wax-based rolling lubricant in forming their aluminum will likely fail when used on aluminum from "Supplier B," which uses silicone-based lubricants. This is because cleaning processes that frequently remove waxes or oils don't always work as well to remove silicone-based lubricants. This is just one example of how the instructions on technical data sheets can be overly simplistic when they fail to consider the wide variety of variables involved in surface preparation.
Contamination Affects Bond Performance
The type and amount of contamination on the bond surface can significantly impact the bonding process's success or failure. But even if the surface has been cleaned well, unintended contamination events can occur in the elapsed time between surface preparation and adhesive application. A cleaned part that sits for a few hours in the manufacturing facility can become unintentionally contaminated by things as seemingly innocuous as a passing forklift or an HVAC system circulating air from other parts of the plant where mold releases are being applied. These contamination events cannot be detected by the naked eye or through “white glove” tests.
The probability of an unintentional contamination event increases with each passing minute between surface preparation and bonding. Therefore, it is crucial to be aware of the potential for contamination and take steps to minimize the risk.
Roughening the Surface Can Help
Most adhesive data sheets include instructions to 'roughen the surface' to create a successful bond, and the instructions frequently imply that roughness is an important characteristic of a bond surface.
While roughness can certainly help make a good bond better, the requirements for strong and reliable adhesion are more subtle than simply ensuring a rough surface.
Imagine a nicely roughened surface ready for bonding that gets accidentally exposed to a bit of mold release mist that wafted over from another manufacturing cell. Although the roughness hasn’t measurably changed, the surface is now unbondable without a cleaning remediation step.
More important than roughening is to ensure that the surface is extremely clean. Furthermore, the cleaning needs to be performed prior to any roughening step. This is because abrading a contaminated surface grinds contaminants into the nooks and crannies created by abrasion. These contaminants are now much harder to remove.
The best abrasion processes are ones that include three steps: precleaning, abrasion, and post-cleaning. The precleaning step, either aqueous cleaning with a high-quality rinse or solvent-based, removes the bulk of the contaminants. This following abrasion step can remove any weak oxides and create a fresh, mechanically strong surface. (A rough surface can also re-direct crack growth away from the interface, making crack initiation more difficult and improving fracture toughness). The final post-abrasion cleaning removes debris generated by the abrasion process. These three-step processes can produce excellent surfaces for reliable bonding and coating.
In summary, while roughness can enhance a clean bond surface, without ensuring molecular-level cleanliness, abrasion will only produce a mediocre bond surface at best.
The Details of Surface Preparation Are Crucial
The amount of contamination that can interfere with adhesive bonding is so small that it is not visible to the eye or to a ‘white glove test’. This means that preparing the surface well and consistently is highly dependent on the details of the preparation process. This can become a big problem for manual surface preparation processes when there are slight variances in how different operators implement cleaning methods. Slight differences in technique, like how often cloth or abrasives are rotated, how much pressure is used during wiping, is the wiping process linear or circular, can make a big difference in the performance of the final bonded structure. How can the results of a surface preparation process be evaluated and compared, besides by destructive testing of the final bonded object?
Control Of Surface Preparation Process Requires Measurement
The only way to ensure that a surface is adequately and consistently prepared for adhesive bonding is to use a measurement technique that is sensitive to the same low levels of contaminants that affect adhesion. One particularly convenient technique is to measure the contact angle formed by a drop of water on the surface. Contact angles provide a precise and accurate way to sense the surface energy of a material and immediately flag the presence of contaminants and sub-standard surface preparation. Modern instrumentation (like the Brighton Science Surface Analyst™) provides fast and quantitative contact angle measurements on the manufacturing floor with a single button push, along with software that integrates the measurements into overall quality and process control systems.
How Adhesives Manufacturers Are Reducing Bond Failure Through Improved Technical Data Sheets
Leading adhesives manufacturers who recognize the subtleties of surface preparation and its effect on product performance put great value on quantitative control of bond surfaces. An example is Henkel, who now includes contact angle specifications in its Technical Data Sheets (TDS). This detailed and quantifiable product application guideline improves customer success and fosters stronger customer relationships.
Read how Henkel and Brighton Science partnered to establish surface measurement standards for their adhesive technical datasheets
Filling in the Gaps in Technical Data Sheets for Improved Bond Performance
Surface preparation for reliable, high-performance adhesive bonding and coating is a subtle and sensitive process. One size usually doesn’t fit all, meaning that, in reality, adhesives manufacturers would need a different TDS for every application scenario. To avoid this, most adhesives manufacturers err on the side of too little information and provide TDSs that are so general regarding surface preparation as to have almost no value.
An appreciation for the sensitivity of adhesive bond performance to the molecular-level cleanliness and composition of the bond surface, and an awareness of tools like contact angle measurement for measuring and controlling this surface composition, allows manufacturers whose product performance depends on the adhesion to ensure quality and improve the customer experience.
To learn how to leverage contact angle measurements in new product development or bonding, coating, and cleaning operations, download the eBook - The Advanced Guide to Transforming Product Development Through Surface Intelligence Data & Technology.
