Chemists Corner: The Mechanisms of Primer Adhesion
To get waterproofing right, it helps to take a view of waterproofing as a system rather than just a single product. The substrate could be considered the first layer in the waterproofing system. All coatings that go on to the substrate must essentially work with the substrate by either balancing out or enhancing the substrate properties. The better the condition of the substrate the better the foundations of the system.
Primers, the second layer in the system and the first coating to go onto a substrate, may be used to condition the substrate to prevent moisture rising through the substrate through reverse hydrostatic pressure, or they may be used to enhance the adhesion between the substrate and the subsequent waterproofing coatings, or both. While the primer is coated over it is imperative that this first product is selected and applied effectively.
Primers are solution providers. When used on porous and uneven substrates they act as a bonding agent between the waterproofing membrane and the substrate and provide an even surface to build up the paint film on. Likewise, on damp substrates, or green concrete, (less than 28 days old) the correctly selected primer can act as a moisture barrier thus providing a solution by allowing faster scheduling of waterproofing installations.
When we are talking about primers promoting adhesion, it is helpful to look at how adhesion works. Adhesion occurs through various mechanisms, mainly; adsorption, chemical and mechanical bonding. When a primer is adsorbed onto a substrate as a continuous film, it is referred to as good wetting. The more porous a substrate is, or the greater the wetting power of the primer, the better the adhesion. Poor wetting is where the coating fails to form a continuous film in full contact with the substrate surface. Good wetting is a universal requirement for all primers.
Chemical bonding occurs through the primer forming chemical bonds to the substrate surface followed by adsorption onto the substrate. These types of primers work by forming a chemical bridge between the substrate and the primer and are most commonly used for non-porous substrates particularly metals or glass. Mechanical adhesion occurs through physical interlocking of the primer and the substrate, much like puzzle pieces locking together. This makes it difficult for a clean path of separation between the coating and the substrate. This type of adhesion is assisted by roughening of the substrate by sanding or grinding.
Internal cohesion of the primer is another property essential to good adhesion of the entire system. Cohesion is the inner strength of the material and its ability to resist tensile force. The internal adhesion of a primer is important when considering the application rate of primers. Some primers are required to be applied thinly with no ponding or thick layers to ensure the best adhesion and cohesion is achieved. This is an often-misunderstood property of primers, as can be seen by DIYers unnecessarily applying thick coats of primer.
For a successful build it is best to ensure the substrate is in the best condition ready for priming. A roughened, dust free, dry, stable, clean substrate is the ideal starting point for the rest of the waterproofing system. Selecting and applying the right system is the next step. Make sure the correct primers are used for the different conditions found on builds. Wet-seal Independent Franchisees are expertly trained to determine the requirements for the best waterproofing installation using CSIRO certified and BRANZ appraised Wet-seal Waterproofing Systems.
Elisabeth Brazier, MSc. BSc(Chemistry) – Wet-seal Product Manager
For any other advice of a technical nature, Wet-seal’s highly experienced technical expert, Robert Rath, will be happy to take your call. Robert can be contacted on 0413 008 303 or via email email@example.com
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