Team Bonding, or: How MicroTau’s engineering team ensures effective aircraft surface adhesion

The durability of our drag-reducing product is something we take extremely seriously at MicroTau. The aviation industry operates on a foundation of regulatory compliance, ensuring that the uptake of novel technologies is mediated by a thorough certification process that evaluates the safety of these technologies before and after they are introduced. Beyond the implications for aircraft safety and airworthiness, we are also cognisant of the impact of product durability on long-term product performance, with product design and material formulation optimising for long-term aerodynamic performance despite exposure to environmental stressors.

One category of stressors we optimise for is surface adhesion. Let’s get stuck in.

MicroTau’s Riblet Modification Package is an adhesive-backed drag-reducing film product, meaning it is adhered to the outer-mold-line (exterior) of aircraft in much the same way as graphic decals commonly used in commercial aviation. During manufacturing, an adhesive layer and ‘release liner,’ protecting the adhesive, are applied to the underside of the riblets and an ‘application tape,’ for protection during transportation and installation, applied over the top.

Image: Riblet Modification Package schematic (not to scale)

What is adhesion? Let’s define it. 

First of all, it is important to understand the concept of interfaces, which are defined as the surface separating two substances and that has different properties to the other materials. As seen in the schematic above, our product has 5 different material layers, creating 4 interfaces.

Adhesion, on the other hand, refers to the mechanical behavior of the interface between two materials. Whenever a product has an interface between two materials, adhesion is a design factor. 

The third property of concern is cohesion, which refers to how a single substance sticks together.  Cohesion is related to the forces of attraction between molecules within the material. Both adhesion and cohesion play an important role in adhesive systems. 

Material Importance

Let’s talk about why adhesion is important for our product.

As mentioned above, our riblet product comprises 5 different materials, creating 4 interfaces, each with different adhesion requirements:

• The application tape must adhere sufficiently to our microstructured coating, but not so strongly that it rips it off upon removal

• The riblet microstructured coating must adhere strongly to the carrier film - the stronger the better

• The adhesive needs to adhere strongly to both carrier film and substrate, but leave a clean surface behind when peeled off and completely stay on the carrier film

• The release liner needs to have a very low adhesion to the adhesive, so it can be peeled off easily

Understanding how adhesion works and how it is measured is an important step for designing a product that meets these specific and varied adhesion requirements. 

Adhesion measurement: a sticky subject

Adhesion measurement is a sticky subject because an absolute value of adhesion “strength” does not exist for any practical purpose in the same way that absolute values for, say, pressure or force exist. 

Instead, the type of adhesion test used determines the adhesion value, because each test type creates a unique combination of stresses and strains on the adhesive. Ultimately, the product requirements determine the most appropriate method to measure adhesion strength and the necessary target values. The product requirements themselves are determined by the use case for each adhesive/interface as outlined above. 

Image: Types of adhesive strength tests. Credit: tesa.com

Here are a few common ways to measure adhesion:

Peel Tests: These are a go-to for many applications. They measure the force required to peel a tape or film from the surface at a specific angle, like 90° or 180°.

Shear Tests: For applications where the adhesive needs to resist forces parallel to the bond, you'd use a shear test like a Lap Shear or Static Shear test.

Tensile Pull-Off Test: This test measures the force needed to pull a bonded material straight off the surface.

The results of these tests are not just about the adhesive - adhesion is a property of the entire system. That means every variable - from the peel angle (α) and peel speed (ν) of the test to the properties of the backing film, adhesive, and substrate - affects the outcome. Environmental factors like temperature and humidity can also change the results. For example, the peel force often increases with the adhesive thickness. It's a complicated relationship, but one that's vital to get right for the highly regulated aerospace sector. 

Image: Adhesive peel testing of MicroTau Riblet Modification Package

Failure modes

When designing and testing an adhesive it is important to understand the potential failure modes as they influence the system optimisation strategy. A failure mode is the manner or way in which a system or product could fail to meet its intended function. 

In materials science, potential failure modes typically arise from deformation of the material, fracturing of the material, or some loss of functionality. The below are some common types of failure modes when the material is subjected to mechanical loading:

• Adhesive or interfacial failure is failure at the interface, where the two materials come apart and separate, like a band-aid coming off your skin

• Cohesive failure occurs when the cohesive force holding the adhesive together is weaker than the adhesion at the interfaces, resulting in the adhesive itself failing. Imagine glue becoming too hot and ‘melting’ - this is cohesive failure

• Substrate failure occurs when the substrate itself breaks. 

Multiple failure modes can occur at the same time within a specimen, depending on how homogeneous the system is and the type of test employed. A common thread between these failure modes is a propagating crack, which, in materials under load, is a stress concentrator that will amplify an applied stress. When an increasing stress is applied to a material containing a crack, a force is reached at which the crack will propagate leading to failure (fracture). 

Public Service Announcement (PSA): Pressure Sensitive Adhesives (PSAs) 

Pressure Sensitive Adhesive (PSA). The name says it all: they bond near-instantly with the application of pressure. Heat, solvents, or a long curing process are not required to activate the adhesive. PSAs are also great for our product because their bond is reversible, which means they can be removed relatively easily. 

The most common chemical compositions for PSAs are viscoelastic polymers, meaning that they behave like both a viscous fluid and an elastic solid, and this dual nature is highly dependent on time (or speed) and temperature.

This viscoelasticity is the secret sauce behind our product’s adhesive performance. Viscoelasticity allows the adhesive to absorb and dissipate a large amount of energy when it's being peeled off an aircraft surface. This "dissipative" adhesion is a key characteristic of PSAs, and it explains why they're so effective. When you peel a tape with a high-dissipation PSA, the energy of the peel is spread out over a broader area, which requires more force to fully peel away from the substrate. We’ve been very selective about our choice of adhesive to ensure that the product stays firmly affixed to the aircraft surface except when removed for aircraft maintenance, inspection, or patch repair. 

Putting it all together

MicroTau’s Engineering team’s rigorous testing of adhesives and deep understanding of the chemical mechanisms behind them has resulted in a product that nails the balance of delicate adhesive requirements for each interface layer. For our customers, this means a product that can be installed quickly and easily and that remains on the aircraft reliably despite the extreme conditions.