Coating Adhesion: The Bonds That Bind
By Jason Smith, The Garland Company, Inc.
Whether it’s dew clinging to a blade of grass, a spider web swaying in the morning breeze, stain being applied to a deck, or a highly waxed car in a dealer’s showroom — what you’re seeing is adhesion at work. Although there is no single theory on the property known as adhesion, there are five basic mechanisms commonly used to describe it.
Chemical adhesion is what happens when two substrates form a chemical compound at their interface, either by swapping or sharing atoms (which creates a very strong bond) or through hydrogen bonding (which is a weaker interaction that applies to substrates containing oxygen, nitrogen, or fluorine atoms.) Hydrogen bonding is also sometimes referred to as dispersive adhesion, which is described below.
Mechanical adhesion is best evidenced by the grip of Velcro®, and can also be seen in rough or etched substrates. Coatings that use mechanical adhesion actually penetrate into the pores or voids of a rough surface, then interlock onto the substrate. Applying a coating to polyurethane foam roofs is one example of mechanical adhesion. In addition to some chemical interaction that occurs at the coating/foam interface, the coating flows (or “wets out”) into the nooks and crannies of the foam, then locks into place once the coating has dried or cured.
Electrostatic adhesion occurs when an electrostatic charge is applied over one substrate, creating an attractive force that draws in the other substrate. One example of this kind of adhesion is what happens when you forget to put a softener sheet into the clothes dryer. When you try to pull the clothes apart, a static charge is dissipated, creating a crackling or snapping noise.
Diffusive adhesion occurs when one substrate’s molecules move and intermingle (diffuse) with molecules of the other substrate. The adhesion used for building plastic models, which are usually made of polystyrene, is a good example of diffusive adhesion. Since the model pieces do not adhere to one another by themselves, a toluene-based clear adhesive is used, enabling a portion of Part 105 to intermingle or diffuse into a portion of Part 32, thereby creating a single-welded piece. PVC pipe bonded with solvent-based pipe adhesive is another example of a substrate that is bonded using diffusive adhesion.
Dispersive adhesion occurs between atoms or molecules of two substrates in close proximity of each other. Taken on an individual basis, these molecular interactions are not very strong. However, when these interactions occur along the entire surface of the substrate, their combined bond strength can become significant. It is dispersive adhesion that enables the bonding of polar and non-polar surfaces and coatings.
Factors Influencing Adhesion
Each of these five mechanisms of adhesion is affected by common factors. Regardless of the application, properly specifying an appropriate adhesive requires thoughtful consideration of these factors:
• Surface energy and substrate polarity
• Surface area covered and contact points achieved
• Surface contamination
• Surface texture
• Coating cohesive strength
Surface Energy And Substrate Polarity
The type of coating chosen will depend greatly on the surface energy of the substrate (and ultimately, whether the surface is polar or non-polar). Polar substrates carry a positive or negative charge and adhere best to other polar coatings. Non-polar substrates are charge-neutral and have to rely on other adhesion mechanisms, such as diffusive or mechanical bonds, for bonding.
For example, thermoplastic olefin (TPO) single-ply roofs are non-polar because they are made of polyethylene and polypropylene. TPO’s are very hard to coat, especially with polar coatings like polyurethanes or acrylic, because these coatings have no charged surface with which to interact. As a result, the cured or dried film can be removed easily from the surface. In order to coat a TPO, a diffusive bond must be formed with a solvent-based primer. The suitable primer then becomes a more compatible substrate onto which a topcoat is applied.
In contrast, polar substrates, such as polyurethane foams, can be coated easily with acrylic or polyurethane coatings, which are also polar. In such cases, the polar coating interacts with the polar surface to create a dispersive force which, when taken over the whole area, is very strong. In addition, other factors such as surface contamination and surface texture further strengthen the bond. (These factors will be discussed in greater detail further in this article.)
Surface Area Covered And Contact Points Achieved
The polarity of the substrate also affects its surface energy. Surface energy is what enables a coating to wet out or bead on a substrate. “Wet out,” as the phrase implies, is the ability of the coating or adhesive to spread onto the surface to which it is applied. In contrast, beading is what occurs when the adhesive or coating is not wetting out well (think of water on a waxed car or a newly stained wood deck). Simply put, the more the coating is able to wet out, the more surface area is covered. The more surface area covered, the more contact points achieved. The more contact points achieved, the better the adhesion to the substrate.
Polar Surface ⇒ High Surface Energy ⇒ Good Wet Out ⇒ More Surface Area Covered ⇒ More Contact Points Achieved ⇒ Strong Adhesive Bond
One can observe the phenomenon of beading and wet out when water is applied on a freshly waxed car. Prior to waxing, a car’s paint is dusty and dirty. Chalky paint residue is blended into the dust and dirt as well. Dust carries a static charge, making the surface that it coats polar (charged) with a high surface energy. The charged surface attracts more dust, dirt, and chalky paint residue onto itself. When water is applied to this surface, the beads of water spread out nearly flat, exhibiting wet out. In this case, each water droplet adheres strongly to the surface.
In contrast, when the car is waxed, a non-polar (uncharged) hydrocarbon-based compound is deposited onto the paint surface. The once polarized surface is now replaced with a layer of non-polar hydrocarbons that have a low surface energy. As a result, dust or dirt does not stick well to the surface. Each drop of water (which is polar) is repelled by the low-energy surface, forming a bead. The lower the surface energy, the tighter, more spherical the bead; the higher the surface energy, the flatter the bead. Within a short time, the rain and sun dissipate the wax and make it necessary to wax the car once more.
When a substrate is contaminated with dust, oil, debris, spores, pollen, etc., coating adhesion is impaired. Areas with high concentrations of contaminates provide an alternate substrate over the intended substrate. Because the coating adheres so well to these polar contaminate particles, it has only partial or no contact with the intended substrate. An inadequate coating bond to the substrate can lead to blistering, peeling, or film delamination. This is why coating manufacturers and distributors stress a clean surface prior to coating application. The surface also must be dry, because even water can act as a contaminate.
For example, a roof coating that covers a water droplet is a prime candidate for cracking and blistering. When the water droplet dries and diffuses through the coating, the void left underneath the coating will expand and contract with changing atmosphere and, depending on the strength of the coating, will crack. If this occurs on a roof, this will lead to water absorption and eventual leakage. In addition, if the water droplet is trapped between two non-porous substrates, the droplet itself could expand within its confines during a hot day and actually cause localized delamination or blistering.
The texture of a substrate also can impact the adhesive bond. You will recall that the greater the surface area covered by a coating or adhesive, the better the adhesion. Well, a substrate has more surface area when its surface is rough than when it is smooth. The simple act of etching or abrading a surface even a few microns deep, prior to coating, will increase the surface area the coating will “see”. The greater the area covered, the greater the contact points that improve adhesion. There is a catch, however. If the substrate has a low surface energy, the coating will not wet out and will not cover as much area. This reduced surface coverage means less contact points, especially if the surface is etched. A non-polar, low-energy substrate will not allow a coating to fill all of the voids.
An etched, polar high-energy surface will wet out a polar coating and adhere at an infinite number of contact points along its bond line, resulting in very strong adhesion.
Porous substrates, such as wood or foam, provide the additional possibility of forming a mechanical bond with a coating. A wood deck coating or stain is able to flow into the cellulose pores of the aged wood and, upon drying, form an anchored mechanical bond. In addition to being an aesthetic improvement to the deck, the coating itself becomes a barrier to further attack from moisture and the elements.
Coating Cohesive Strength
The factors described to this point have dealt mainly with the substrate that is going to be coated. Such factors affect adhesive strength between the substrate and the coating, due to the molecular differences between them. The final factor is the cohesive strength of the coating itself.
Cohesive strength, or rather, how well the coating “sticks” to itself, comes from the Latin cohaerere, which means to "stick or stay together.” Cohesion is a chemical attraction of like molecules within the coating, which holds the molecules together.
Water is very polar, and by virtue of its wide “V” molecular shape, has very high cohesive strength that enables it to bead without much difficulty on a surface. When the surface energy of the substrate becomes high enough to exceed that of the cohesive strength of the water droplet, the water bead can no longer hold itself together and the bead flattens out (as with a dusty dirty car).
Likewise, an acrylic coating on a low-energy surface such as polyethylene will bead because the cohesive strength of the coating is much higher than the surface energy of the polyethylene. If that same coating is applied to, say, polyurethane foam, which has a higher surface energy than the coating, the coating will not be able to bead and will wet out. In such cases, it is the thickness of the film that provides the coating its internal strength.
The key to specifying a coating that provides reliable, predictable adhesion is understanding the mechanisms by which adhesion is achieved, and the factors that affect them. Identifying the physical properties of the coatings you are considering and the substrates for which they are intended, and verifying their compatibilities through a thoughtful analysis of the factors reviewed in this article, should ensure lasting solutions for a full range of coating applications.
Jennings, C.W.; J. Adhes. 1972, 4, 25-4.
Wake, W.C.; Polymer. 1978, 19, 291-308.
John Comyn, Adhesion Science, Royal Society of Chemistry Paperbacks, 1997.
A.J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, 1987.
The author, Jason Smith, is the Sr. Research and Development Chemist for The Garland Company, Inc., a Cleveland-based manufacturer of high-performance solutions for the commercial building envelope. Prior to joining Garland, Smith was a senior development chemist for an international manufacturer and distributor specializing in adhesives for the industrial and consumer markets. He has an M.S. in polymer chemistry and coatings technology from DePaul University, Chicago.
The Garland Company Inc. is one of the worldwide leaders of quality, high-performance roofing and building envelope solutions for the commercial, industrial and institutional markets. For over 120 years, Garland has continually developed unique product and service offerings that have raised the bar of performance while exceeding the individual needs of customers throughout the world. Today, Garland's network of over 200 local building envelope professionals is strategically positioned throughout the United States, Canada and the United Kingdom to provide quality building envelope solutions for single and multi-property facilities. The Garland Company Inc., headquartered in Cleveland, Ohio, is an ISO 9001:2008 certified company. For more information about Garland, visit www.garlandco.com or call 800-321-9336.
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