Adhesive Bonds and Failures: What’s Going On?
Adhesives are foundational to airtight and watertight enclosures. It is essential that the adhesives we use bond for as much as 50 years, 100 years, or longer. How do we know when we have the right kind of adhesion? What are the different ways a tape's adhesion will be able to provide us with long-term performance? To understand we first need to know the science behind how tapes adhere and therefore what kind of failures can occur.
Understanding Adhesion vs Cohesion
For a complete understanding, we recommend you watch this 13 minute video (well worth the time, if you have it), which is a segment of a webinar from November 2019 with Pro Clima’s Lueder Herms. It provides the background needed to separate the concept of adhesion from cohesion, and differences between SOLID acrylic glue and acrylic dispersion glue. Lueder is a world leader in building airtightness and vapor control, and is active in the research and development of new solutions to achieve durable airtightness in mass timber and masonry buildings.
There are two forces at play in adhesive bonding: adhesion and cohesion - each also, at the molecular level, described as Van der Waals force.
|Adhesion is the tendency of dissimilar particles or surfaces to cling to one another. With Pro Clima SOLID acrylic adhesive tapes, the adhesion is pressure activated, the adhesive is "wetted" providing "the flow" to form strong clinging at the molecular level -not dissimilar to what we see in nature with geckos. In taped connections the adhesive bond occurs at the backing of the tape as well as to the substrate. What the adhesive is adhering to is critically important.||Cohesive force is the action or chemical property of molecules resisting separation, being mutually (intermolecular) attractive - like water forming in droplets. The chemical property of the Pro Clima SOLID acrylic forms a very strong bond between the adhesive molecules within itself - preventing creep (more on this later).|
Types of Failure: Substrate, Adhesion, and Cohesion
1. Substrate or backing failure
The substrate is weaker than the forces transferred to it by adhesive and cohesive bonds. The material breaks apart and the substrate is separated into pieces.
Shown below is Pro Clima TESCON VANA tape on the right, which adhesion is so strong (and water resistant) that the substrate is forced to failure.
Below: The SOLID acrylic Pro Clima tape adheres so well even in humid conditions that test forced the substrate to failure. A conventional acrylic dispersion adhesive (left) in the same condition shows and adhesion failure to the substrate.
The SOLID acrylic Pro Clima tape (right) adheres so well, even in humid conditions, that tests force the substrate to failure. A conventional acrylic dispersion adhesive (left) in the same condition shows adhesion failure to the substrate.
OSB's rough side and aged wood can have this issue as well. The tape will adhere to the first layer of wood, but when force is applied to the tape the adhesion will pull off, showing splinters attached to the adhesive. A good way to improve adhesion to OSB and older wood, is to make sure it is clean and use TESCON Primer RP. The recommended tape for taping OSB joints is TESCON VANA - typically this will work without primer. If there is concern due to specific quality of OSB, test the tape by adhering/pressurizing one day and placing in shear stress the following day (see more on Substrate Testing below). Another option is to sand it down, but that is labor intensive and creates a lot of dust which has to be cleaned off before adhering the tape.
2. Adhesion failure:
The bond between the substrate and the adhesive is broken when the forces exerted on the connection is greater than the force created between the substrate and the adhesive. This is a very common form of failure when the substrate you are taping to has one of the following characteristics:
Substrate is sandy or unstable - This would be the case with brickwork, mortars and (old) concrete. To not adhere to this sand / dust - use TESCON Primer RP to stabilize / solidify the substate, enhance the adhesion, and assure a durable bond.
Surface tension of substrate is too low - For a good bond the surface tension should be greater than 40 mN/m. If it is less than 30 mN/m, the material becomes so smooth that the substrate is too similar to the release paper for the tape. A large number of PE membranes available on the market are coated with a wax that makes production easier and static resistant - but this makes the surface tension very low and thus less suitable for adhering to.
It is recommended to do an adhesion test to verify performance of the adhesion. Having a clean and dry substrate will result in best adhesion results, although the Pro Clima tapes can be used in damp conditions, unlike acrylic dispersion tapes. In some cases you could be able to remove the tape from the PE with some force, but the shear strength of the adhesion would be very high, as explained later, this is one of the most important aspects of a airsealing tape.
3. Cohesion failure
The bond between the molecules within the adhesive is forced to failure because the external force exceeds the cohesive bond. There are tapes that do not really "set" solidly, these tapes create a bond that float on a layer of adhesive and can more readily move or creep. If acrylic dispersion adhesives dry out over time, cohension failure could occur. With Pro Clima SOLID acrylic tapes, the adhesion sets by pressure in the first 24 hours and forms a permanent solid bond, greatly reducing the chance of cohesion failure. This is one of the reasons that when you try to remove Pro Clima tapes, you most of the time end up destroying the substrate or the backing, not the adhesive bond.
TESCON VANA: SOLID acrylic adhesive improves with time.
Cohesion Failure: red arrows show extent of creep, causing air leakage.
Substrate Suitability Testing
An adhesion test is always recommended when working with a new or unknown substrate. No matter how good the tape is, if the substrate fails, the result is compromised airtightness. This is a reason why wood fiber boards, concrete, brickwork and certain types of OSB require a high quality primer. Not because the material itself isn't strong, but because the surfaces of these materials can delaminate or fray when they are exposed to tension that rips their surface apart when the adhesive tape transfers that force to them. The result is a material break and a piece of tape with wood fibers, cement dust, or splinters stuck to it. The tape held, but the substrate failed.
Testing Adhesive Bonds
How do you realistically test tapes to evaluate both the long term bond and which forces will act on that bond? The first one is easy to solve: you just tape and wait. But who has the time to wait 3 weeks, much less 50 years? Fortunately the Pro Clima tapes have been rapid age tested and proven to not show degradation of the adhesion strength after 100 years. The most important reason for the adhesive capability to withstand the test of time, is that the SOLID acrylic adhesive does not contain any solvents. It sets when pressurized and creates a solid, non-slipping bond - assuring a long-lasting, durable connection between the tape and the substrate.
But to what forces should we expose the tape to see if it will maintain an airseal over a joint for the long term? The options are generally two types of tests:
- Short Term Peeling Stresses: pull straight up (90 degrees) or pulling backwards on itself (180 degrees) ASTM D3330.
- Long Term Shear Stresses: ASTM D3654 - method A
If we know that buildings move very slowly and we are concerned about tapes covering joints (between membranes or plywood/osb), the forces they will be exposed to are slow, but sustained forces caused by different rates of expansion and contraction on both sides of the joint. The tape covers this joint to create an airseal and both the adhesive should keep the tape's backing attached to both sides of the joint, while allowing the backing to move with the slight movements. These conditions are unlike the short term pull tests noted above first, and can give an unreliable indication of job site bond strength - a false signal. Besides peeling adhesion at 180° and peeling adhesion at 90°, it is above all a high shearing strength that is required. This expresses how well the adhesive tape ‘welds’ to the base surface.
Shear Forces are more indicative of job site conditions.
Pro Clima's SOLID acrylic tapes perform very well with suitable or primed substrates as our ASTM tests have shown that when TECSON VANA was tested. It either caused failures of taped membranes at 87.5N/100mm or peeled from OSB at very high forces >57.25 N/100mm. Additionally TESCON VANA’s adhesion performs very well compared to other competing European air-sealing tapes, as is shown in the images below in which 2 square inch of adhesion of tape weighed down by a 8 oz block of wood.
|After 48 hours||After 2 years – of the 47 tapes, 7 are still hanging, 3 of which are Pro Clima tapes|
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