What are Surfactants and How Do They Impact Surface Tension?
In recent articles we’ve discussed what surface tension and surface energy are. Manufacturers need to acquaint themselves with these concepts because controlling surface quality through surface energy measurement of solid materials is the most predictive method of ensuring high performance bonds and coatings. An important tool in our toolboxes for controlling surface tension is the class of chemical compounds known as surfactants. They are pretty much universally found in cleaning agents, coatings, inks, lubricants, adhesives, and cosmetics: one important characteristic of surfactants is that they can help dissimilar substances mix (like oil and water). Surfactants are incredibly useful and beneficial...until they appear in places where you may not want them to be!
Creating and maintaining chemically clean surfaces is a vital part of building reliable products in many instances. For example, automobile engine blocks are assembled with silicone sealants. If the surfaces are not chemically clean before applying the sealant, the engine will leak oil onto your driveway. Chemical cleanliness of sealing surfaces can be achieved through carefully monitored and controlled aqueous cleaning processes. These processes depend on surfactants to make contaminants soluble in the cleaning solution. However, if the surfactants aren't completely removed via a rinsing step, the end effect is that we have just replaced the original soils and contaminants with a new one that will also inhibit sealant adhesion: residual surfactant.
Sometimes a chemically clean surface is all we need for manufacturing success. However, with many applications, we need to further engineer the surface through surface treatment to obtain desirable properties like corrosion resistance or strong and durable adhesion of coatings or adhesives. The presence of residual surfactants on these surfaces can prevent successful surface treatment.
For inks, paints and adhesives to work properly they have to spread over the surface that they're applied to. This requires that the surface tension of the ink, paint or adhesive is lower than the surface energy of the substrate. (This is just another way of saying that the molecules in the paint, adhesive or ink need to be attracted more strongly to the surface than they are to each other). We can control the surface tension of a coating or adhesive by the addition of surfactants.
Surfactants are directly related to surface tension and play a very important role in adhesion processes. But, how do they work and how can manufacturers make sure surfactants are doing their job without disrupting the work of creating chemically clean surfaces?
By looking at what surfactants are intended for, their effect on surface tension and how to recognize when they are on our material surfaces, we can make use of these powerful manufacturing tools to create clean surfaces that can guarantee high-performing products.
A surfactant, at its most basic, is a substance that is designed to reduce the surface tension of a liquid. For many operations in manufacturing processes, it is necessary for a liquid to spread out and wet a surface. Adding a surfactant to a coating or detergent lowers the surface tension of the liquid so it will flow more, covering the entirety of the surface. For instance, surfactants are often added to insecticides to ensure the substance fully spreads out over the entire surface of leaves instead of just a small portion of the plant. This allows the insecticide to be maximally effective.
As a reminder, surface tension is the attractive force of the molecules present at the surface of a liquid towards each other. Modifications to the surface tension of a liquid is one factor that helps determine the performance of a bond between a solid surface and a liquid.
It’s important to remember that the constituent parts of a liquid affect the surface tension of that liquid and the surface energy of the material it’s being applied to must be controlled in such a way that adds surfactant into the equation. To ensure a strong bond, the liquid and the solid surface must be chemically compatible.
There are two major properties that we can characterize surfactants by in order to use them appropriately and to the proper extent.
The single most important characteristic of a surfactant is the amount at which it will reduce surface tension to the desired level. This ideal amount of surfactant is called critical micelle concentration (CMC). Adhesive, coating and detergent formulators need to be aware of exactly how much surfactant is necessary to get the desired wetting once the liquid comes into contact with a solid surface.
The second characteristic encompasses all the properties of the surfactant related to how it fits the needs of the application. Any time a chemical compound is introduced into a manufacturing process it has consequences that manufacturers must consider. The surfactant can do its job and reduce the surface tension, but what other effects is it having on the process? Considerations outside of its direct effect on surface tension, such as the possibility of its harmfulness to humans or the environment, need to be understood before using any surfactant.
Surface tension is high or low based on how attracted the molecules in a given liquid are to each other. Liquids, like water, have high surface tension because the molecules attract each other very strongly. Surfactant molecules have a weak attraction to one another. When a surfactant is introduced to a liquid like water, some of the surfactant molecules migrate to the surface of the water. This creates a layer of weakly attracted molecules on the surface of this water/surfactant compound. The surface tension of this liquid is lower than if it were just water.
The surfactant molecules that remain in the bulk of the liquid form micelles, which are little aggregated bundles of surfactant molecules. These make it possible for certain substances to be soluble in the liquid when they otherwise wouldn’t be. This is essentially how detergents work. The surfactant molecules on the surface of the water the detergent is in makes it spread out and then the micelles make it possible to remove oils and waxes that can’t be handled by water alone. The point at which the surfactant molecules in the bulk of the liquid no longer form micelles is the CMC, which was mentioned earlier as the point where the ideal amount of surfactant has been introduced to the liquid.
In manufacturing processes, any step that presents an opportunity to change the surface of a material positively or negatively, even if we’re just talking about at the molecular-level, this step is called a Critical Control Point (CCP). Any substance that comes into contact with a material surface at one of these CCP’s is potentially a contaminant.
When we clean a surface with a detergent to prepare it for adhesive bonding, painting or printing, we need to finish the wash process with a thorough rinse cycle. If we do not remove all of the surfactant from the surface, we leave behind a thin layer of low surface tension contaminants that can interfere with adhesion of paint or a coating on the surface.
The surfactant that helped clean the surface of our material now becomes detrimental to our bonding process.
Many single stage washers, or washers with a poorly maintained or insufficient rinse stage, leave a thin film of surfactant on the surface of the parts. The parts appear clean but the thin film of the substance can interfere with the performance of the part when attempting to bond to it with an adhesive or coating.
A common cleanliness test in manufacturing is the water break test, which is done by pouring water over a surface and watching whether it sheets off the surface uniformly, as it presumably would on a clean surface, or if the water breaks and doesn’t fully coat the surface due to a contaminant on the surface. In our experience at BTG Labs, we’ve seen this extremely subjective test used in many industries. We’ve even witnessed technicians, who use this test as their primary surface quality inspection, intentionally spraying the material surface with surfactants so the water would sheet off and “pass” the water break test.
They probably didn’t realize they were sabotaging the adhesion performance, but contaminating their surfaces like that could have very real effects on the reliability of their final products.
To bring this point home, consider what happens when you add a rinse aid to your dishwasher’s rinse cycle. The tap water in the dishwasher isn't pure. Water spots are the non-water 'stuff' left behind when the water forms droplets on the dishes before they dry. Rinse aids are mostly surfactants that are released during the rinse cycle. These surfactants reduce the surface tension of the water so it will 'sheet off' the plates and cutlery to form a film, not discrete droplets. When the film dries, there is still non-water 'stuff' there, but it's distributed as a thin uniform film and no longer visible as pesky, unattractive water spots. But now, there may be an additional non-water component left on the surface of your dishes: the surfactant. If you use a mug washed using this process and attempt to whip up some foamy milk for your morning latte, you may have a disappointing time trying to get it to froth. Any residual surfactant on the inside of the mug will reduce the surface tension of the milk and no matter how hard you try, it isn’t going to foam up.
Well, if your milk doesn’t foam, it’s a pretty good indicator. But if you need to determine if a surface in a manufacturing process is contaminated with a surfactant you use water.
When you put a drop of water on a clean surface the drop quickly spreads out, making a low contact angle, and then stops. When you put a drop of water that has a thin film of surfactant on it, the water will continue to spread for several seconds. The water droplet absorbs the surfactant on the surface and the water’s surface tension decreases as it spreads, causing it to continue spreading after it would normally stop.
On a clean surface without surfactant, a droplet of water will simply make contact, wet out and then cease moving. The amount of spreading the water droplet does after it would normally stop depends on the amount of surfactant left on the surface.
Surfactants play an important role in manufacturing to help get surfaces chemically clean and prepared for bonding or coating, but when they are left behind after a cleaning process they become a major liability for manufacturers. Surfactants are great at lowering surface tension by changing the molecular composition of the surface of liquids, but they also can alter the composition of surfaces that need adhesives or paints to bond with them strongly and not slide off or cause inconsistencies that damage bond strength.
The Surface Analyst has a patented Wetting Analytics software feature that equips manufacturers with the ability to instantly detect the presence and amount of surfactant on a surface. To ensure each CCP isn’t at risk of contributing to adhesion failure or compromised product performance. The Surface Analyst provides a clear, decisive and fast way to mitigate risk that comes from using necessary surfactants within your manufacturing process.
To learn more about identifying and monitoring all of your Critical Control Points, download our free eBook: The Manufacturer’s Roadmap to Eliminate Adhesion Issues in Production.