An innovative spacecraft manufacturer used composite materials for a structural component of their ship. These composite components were manually prepared and bonded together. The company lacked an accurate, quantifiable, objective and in-line method to monitor the surface treatment process.
The manufacturer utilized a surface preparation method that included a solvent wipe, hand sanding, and a final solvent wipe. This manual process left plenty of room for human error and lacked the ability to measure and monitor consistency, allowing for uncertainty and potential for failure and re-work.
The spacecraft manufacturer sent a sample of their material component to BTG’s Materials and Processing Lab where the company’s M&P Engineers used the Surface Analyst to examine their process step-by-step.
Three contact angle measurements were taken on four areas of the part. The first measurement was on the as-received sample; the second, after the first solvent wipe; and the third was taken after the sanding and final solvent wipe. While the contact angle varied slightly at each measurement point, the trend was a decrease in contact angle throughout the process. The Surface Analyst showed the sample’s increase in surface energy through the preparation process.
BTG Labs was then able to help the manufacturer determine contact angle pass/fail parameters that could be loaded into the Surface Analyst, allowing the surface preparation process to seamlessly flow on the manufacturing floor.
When manufacturers supply products to customers, they want to have the highest confidence in the quality of those products. But, when proper measures aren’t accessible or available to guarantee quality, a major need arises.
A recent customer, a manufacturer of automotive oil filters, had finally reached a critical point in their business as a result of problems with product quality, specifically regarding adhesion.
Adhesion failures in the field damage brand reputation, the user’s productivity, and in today’s lightning fast social media blogosphere, word of these failures can quickly spread. After a year of product failures, the manufacturer decided upon a proactive approach and brought in BTG Labs.
The hydraulic filters, which comprised of nylon polymer caps and a mesh filter were coming apart upon removal out of the hydraulic system.
When the customer needed to change the filter, the cap would come apart from the rest of the piece due to poor adhesion. Leaving the rest of the filter stuck in the system caused more delays as it was difficult to remove and created a mess.
When looking into an adhesion issue, BTG Labs first asks the question: what is the in-place surface preparation process?
The Materials & Process
The manufacturer was using a 2-step process which included plasma treatment on their end caps followed by a heat cure.
Plasma treatment is a very effective way to treat a surface. However, there existed a potential for leaving contaminants on the surface because the technicians did not have a way to verify the amount of plasma to utilize. Thus, treatment levels varied.
The second step–heat cure process–has the ability to remove some of the contaminants left behind, and in theory could make the adhesive more tolerant of contaminants, but this didn’t solve the problem as failures still ran rampant.
The problem came through the varying amount of plasma treatment levels. Because there was no consistency, no definitive process, and no way to verify the characteristics of the surface, no one knew the appropriate treatment level to ensure adhesion. Furthermore, the caps to these hydraulic filters came from several different suppliers which created even more inconsistency.
Knowing the nature of these contaminants would be an initial step in modifying the process. All of these suppliers were asked to refrain from using a silicone based mold release. Silicone, a highly hydrophobic substance, is a huge contaminant and will prevent a bond from properly adhering. Knowing that there existed no silicone mold release on the surface of these caps, BTG Labs ran some analysis tests.
The Measurements & Solution
BTG Labs’ engineers used the Surface Analyst™ to measure surface energy, or surface cleanliness of the caps.
They obtained two caps from each supplier, one treated, one untreated. When the drop of water contacted the surface of the cap, the drop reacted rather strangely. What BTG Labs saw was dynamic wetting: the behavior of a drop of water that lands on a hydrophilic surface and spreads out over time as it absorbs the substance. The testing revealed an inconsistent contact angle, which is not ideal.
BTG Labs then treated a different section of the cap with a solvent wipe, followed by another inspection using the Surface Analyst. The measurement remained the same.
This indicated that the surface held a contaminant that the solvent wipe was removing. Furthermore, because of the behavior of the water droplet, the contaminant was assumed to be of a hydrophilic nature. So, while the suppliers were adhering to the non-hydrophobic solution, they were using a hydrophilic soap. This soap was leaving a residue on the caps.
BTG Labs went one step further and using their fully equipped Materials & Process Laboratory, tested the caps with their XPS Mass Spectrometer which supported this claim. There was, in fact, a hydrophilic substance on the surface of the caps. Furthermore, this substance showed high levels of potassium which is common in soap mold releases. When the specs were originally designated, the manufacturer only wanted to avoid hydrophobic mold releases. However, these tests prove that the hydrophilic mold release also caused contamination on the cap. And depending on the level of plasma, the soap remained on the surface as a contaminant even after treatment. This meant the specs for suppliers would have to change.
Using the Surface Analyst, the company was able to discover the presence of an unwanted substance on the surface of some of their incoming products.
The next steps would be to reevaluate their assembly process. New specs would need to be in place, a determination of proper surface cleanliness must be decided, and a new surface treatment process must be constructed.
This included a requirement for the supplier to wipe the part with a solvent prior to shipping. Then, the manufacturer would also use a solvent wipe before plasma treatment. This ensured that the soap did not remain as a contaminant on the surface. Consistency and coherence, we created. Consequently, all of the parts could now confidently arrive from suppliers. The technicians could then clean and treat to a specification number to pass on to customers for successful implementation.
The surface preparation processes used to assemble or repair an aircraft include multiple steps to achieve a final bond. For a strong, safe, and effective bond, all steps in the process must be consistent and validated prior to moving forward in the the process.
So, how can aircraft manufacturers know that each step in the process is complete to a satisfactory degree, when there previously has been only outdated, ineffective or objective methods?
The effects of this crucial question can be summed up in saying, when the desired result is not produced, every surface preparation step must be reexamined.
The Material & Process
An engineer from an aircraft manufacturer specializing in major defense applications was teaching mechanics how to use the Surface Analyst to verify clean composite surfaces during their assembly process.
Their surface preparation process included three steps:
• solvent wipe
• hand abrasion
• varied solvent wipe
After these steps, the engineer measured the composite panel surface using the Surface Analyst. The contact angle was incredibly high, at around 70°.
• How could this surface display such poor quality when the “proper” steps were being taken to prepare it appropriately for bonding?
In a typical preparation process situation, the higher the contact angle, the less reactive, or “dirty” the surface is. The lower the contact angle, the more reactive, or “clean” the surface is. These measurements indicate a surfaces’ preparedness for bonding.
Needless to say, this measurement tilted a few heads. Their surface showed more contamination after the specified preparation process than before! When and where in the process was this happening?
The original contact angle taken prior to any surface treatment read 70°.
• After the first solvent wipe, the contact angle read 50°
• After the hand abrasion, 30°
• After the final step of the solvent wipe, the contact angle read higher than the original angle of 70°.
These findings led them to investigate the solvent itself, which was stored in an opaque container. The team poured the solvent into a clear beaker.
Surprisingly, the cleaner was a dark charcoal color, rather than clear. That’s when the team explained to the engineer that they first had been using the solvent to soak other parts and then poured it into a container to be reused in the next step of the process.
The Surface Analyst revealed that the manufacturer was using an extremely contaminated solvent to “clean” their surfaces. While the coloring of this solvent is obvious contamination, a solvent can quickly obtain contamination and may lose its integrity well before showing physical proof. But, with in-place verification procedures with the Surface Analyst, the quality of a solvent would never be ambiguous.
Based on these results, the company refined their surface preparation process and use of solvent protocol.
These new procedures were made possible due to the Surface Analyst. The instrument detected and identified the Achilles heel of the surface preparation process.
As a result, the company’s process became stronger and more efficient. For technicians, mechanics, engineers, and anyone involved within the manufacturing process, the Surface Analyst verifies surface’s proper preparation within every step of the process, and can troubleshoot reliably when issues arise. These uses and several others, demonstrate how the Surface Analyst can improve and monitor the surface preparation process for reliable bonds.
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