We’re all thinking a lot more about the surfaces we touch and the invisible things that might be on those surfaces. Manufacturers have been thinking about how surfaces interact with our skin and the environment long before COVID-19 made us all much more cognizant of what we can get from touching something and what we leave behind.
Coatings that are “self-cleaning” in various ways have been formulated for years. Most of these are variations on hydrophobic coatings that just want to get debris off a surface as quickly as possible by repelling fluid so it runs off, ostensibly taking dirt, dust and other undesirables with it.
Oleophobic coatings work in much the same way, although they primarily repel oils. Grease and oils from hands are deposited on absolutely everything we touch, especially highly chemically reactive surfaces that have been cleaned or treated during manufacturing processes. One of the many reasons we stress the importance of wearing gloves and testing the cleanliness of surfaces throughout the production process.
The biggest applications for these coatings are for medical devices that need low-friction surfaces and electronics with touchscreens. Oils are repelled by these coatings in much the same way that oil is repelled by water. So, surfaces with these coatings do not prevent fingerprints or face oils from getting on them but make it so you can just wipe away the oils with a dry cloth. The oils don’t have a chance to bond to the surface so when you wipe them, they don’t just smear around, but are actually removed.
These coatings are not necessarily anti-viral or even antimicrobial in that they are not designed to kill bacteria or microbes. But, since the coatings are extremely difficult for oils to cling to, surfaces coated with oleophobic films are much easier to clean.
There is research being done that uses the principles behind oleophobic coatings to formulate coatings that discourage viruses like COVID-19 from attaching themselves to surfaces, even with the spike glycoprotein that surrounds the edges of this Coronavirus which is one of the reasons it stays on surfaces for as long as it has been reported to. There are antimicrobial fabrics and materials that have naturally antibacterial elements, like silver, embedded in them and there are coatings that have a variety of repellent properties called omniphobic coatings.
One of the biggest challenges for manufacturers when it comes to using oleophobic coatings is not necessarily how to get them to adhere to the surface, although that is a big concern. What consumers want is for these coatings to last. It is extremely advantageous for manufacturers to be able to say, with as much confidence as possible, how long components of their products will last. If an oleophobic coating is designed to last the two years that the electronic components are created to remain viable, then there needs to be direct evidence that the coatings will do their job that entire length of time.
But there are a few things that make this less straightforward than you might expect.
Oleophobic coatings come in a few varieties but the key characteristic is the inclusion of a fluorocarbon-base or another very non-polar collection of molecules. Non-polar molecules have no charge to them and therefore are not attracted to or by other molecules. Silicon, fluorine, and chlorine atoms are often formed into polymeric films to create a barrier on the surface of materials that manufacturers don’t want anything to stick to.
These coatings have many applications because they are so lubricious and have extremely low surface energy (or chemical reactivity). A popular fluoropolymer, PTFE, is used to make medical devices that do not transmit bacteria but also make the business of inserting metal components less traumatic to the patient’s body.
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Fluoropolymers are also used in the automotive and aerospace industries since oil is such a present element in these fields. These oleophobic coatings help keep oil from getting where it shouldn’t and adds a low-friction surface that can help stave off scratches and wear that would lead to moisture damage and corrosion.
Oleophobic coatings can be in the form of nanoparticle coatings, which are extremely thin films deposited through plasma chemical vapor deposition (CVD). This process takes place in a chamber where the coatings are introduced in a gaseous state and bonded to the surface of the glass, metal, plastic or other material that needs to be coated. These coatings are invisibly shallow and transparent so they’re perfect for optics and photonic applications, and were originally developed within the ophthalmology field.
If the surface beneath the coatings weren’t properly prepared, the chance of a long life for the coating is greatly diminished. Since these coatings that are made to be repellent, getting them to adhere to surfaces can be difficult, yet crucial. Using an abrasive and a chemical etch can create a chemically compatible surface that will bond well with the coating.
Any residues, especially oily ones, on the surface before the CVD operation are extremely detrimental contaminants to this process. Cutting fluids, greases and other common oils found in manufacturing environments can cause inconsistencies in coating thickness due to dewetting - when a coating pulls back due to incompatibility with a substance on a surface.
Also, due to the chemical needs of oleophobic coatings, a primer might be needed to promote adhesion, but even then, the cleanliness of the underlying material is crucial for the primer to stick properly.
To ensure the surface is clean enough for the coating to adhere to, using a simple and quick contact angle measurement before and after each cleaning and treatment step will allow you to see how close the surface is to being ready for coating. Contact angles are a direct measurement of surface energy and can detect microscopic residues that will cause the coating to dewet.
Contact angles work by looking at the behavior of a drop of liquid on a prepared surface. If the drop beads up (like oil on an oleophobic coating) it indicates the surface has low energy and is not chemically reactive in the way that will promote adhesion to the coating. You can measure the angles made by how large or low the drop is on the surface because the liquid has a high resolution when it comes to sensitivity to the top few molecular layers of the material. These angles can offer great insight into what might not have been cleaned off a surface or if a treatment was strong enough to create a reactive surface, perfect for adhesion.
It’s also extremely important to control oleophobic substances being applied to certain components of a larger assembly because these coatings can be contaminants in their own right if they are found on a surface they are not meant to be on. Measuring contact angles on all surfaces at strategic points throughout production processes is the best defense against surface quality drifting out of spec for any reason.
For manufacturers who are applying these coatings, it is vitally important to be able to say, with authority, that the coatings will endure for the length of time the rest of the device remains operational.
Manufacturers are looking for a reliable way to monitor coating durability and to be able to reasonably, and verifiably, assure their customers that they will not have to resort to aftermarket oleophobic coatings due to wear through typical use. Also, we all know that consumers who are not satisfied with their new tech are among the most vocal on the internet.
Many, many smart devices these days are equipped with touchscreens, from watches to thermostats, but none come into contact with more skin more frequently (as well as other environmental oils and residues) than the phones we hold in our pockets.
Again, it’s important to realize that these oleophobic coatings do not keep oils like fingerprints from getting on surfaces, they keep them from staying on the surfaces. They make them “easy-clean”, “smudge-free”, and “streak-free” since they can be easily wiped without a cleaning solution or other cleansing agent.
There are not many good ways to test and monitor the life-cycle of an oleophobic coating available to manufacturers. Even though most screens are mostly flat, there are curved edges that can be difficult to test on.
That’s why the contact angle measurement technique that is useful when assessing the cleanliness of a surface prior to coating is an elegant and simple solution to test the coating for uniform application and durability under various conditions. There are contact angle measurement systems that are able to inspect real components directly within the production process so samples don’t need to be wasted and manufacturers can conduct 100% verification on all products. There are automated systems available for industries that require high-rate inspection as they produce hundreds or thousands of units at a time.
With the right deposition technique and measurement vantage point (whether the drop is looked at from the top or from the side when its angle is being measured), the liquid drop can easily be deployed onto curved surfaces. It’s also possible to map out the entirety of the coated surface to get a sense of the uniformity of the application. With the right contact angle system, all of this can be done easily and quickly.
The contact angle should be quite high (sometimes >90°) on an oleophobic surface. As the coated device is used -- rubbed with fingers swiping back and forth, held next to a person’s cheek for hours, being taken in and out of pants pockets -- the coating will inevitably degrade to some degree. But how much? This is what manufacturers need to know and be able to convey to their customers.
Tracking the lifecycle of the coating through the changing behavior of the liquid drop on the surface offers a quantitative, repeatable quality assurance process that can accurately predict how long these coatings will remain intact.
To learn more about how to implement an inspection process that can ensure the life of coatings will keep up with your products, download our eBook about using verification techniques to increase your production intelligence. Build more accuracy into your surface preparation and coating process. Get your copy today: Predictable Adhesion in Manufacturing Through Process Veriﬁcation