The Physics of Tape Adhesion: Understanding Surface Energy and Wet Out
For pressure-sensitive adhesive (PSA) tapes, effective adhesion determines the durability, reliability, and performance in a wide range of applications, from automotive assembly to electronic component mounting. However, successful adhesion depends heavily on two related concepts: surface energy and wet out. These two factors help determine how well a PSA interacts with a substrate and help determine whether a PSA tape will adhere successfully to a given material. Achieving a strong, lasting bond is critical no matter the application, so understanding these two concepts is critical to help understand whether a specific PSA tape will work with your application.
What is surface energy and wet out? How do they impact the adhesion process?
What Is Surface Energy?
Surface energy is a measure of the molecular attraction at the surface of a material. It essentially reflects how receptive a surface is to adhesives, including PSA tapes. There is a wide range of material surface energies from high to low.
High Surface Energy (HSE)
Materials with high surface energy, including metals, glass, and ceramics, tend to have strong molecular attractions. This means that these surfaces allow adhesives to spread out (a process known as “wet out”) more effectively. This creates a stronger bond over time. HSE substrates enable adhesives to spread thinly and make maximum contact, enhancing bond strength.
Low Surface Energy (LSE)
Conversely, materials with low surface energy, such as polyethylene, polypropylene, Teflon, and powder coated surfaces, have weaker molecular attractions. These surfaces resist wet out, making it harder for adhesives to form a strong bond. LSE substrates often require specially formulated adhesives or surface treatments to improve adhesion. Find out more about adhesion to low surface energy substrates here.
Medium Surface Energy (MSE)
There are a wide range of materials that fall within these two extremes. Many MSE materials are common paints and plastics like polyester, ABS, and polycarbonate for example. While many PSA tapes can bond well due to the receptive material attraction, not all do. Also, some formulations of the plastic material may be more difficult to adhere to than others.
What Is Wet Out and How Does it Work?
Wet out describes the ability of an adhesive to flow and make intimate contact with a substrate’s surface. Effective wet out ensures a larger adhesive contact area, leading to a stronger bond.
Wet out is influenced by two key factors:
Surface Energy: higher surface energy promotes better wet out by allowing the adhesive to flow easily across the surface.
Adhesive Properties: the viscosity and rheology of the adhesive also affect its ability to wet out on a surface.
For example, a firm adhesive may struggle to spread across a surface, reducing its bonding area and overall adhesion strength.
The Relationship Between Surface Energy and Wet Out
Surface energy and wet out are linked, as materials with higher surface energy facilitate better wet out, creating stronger adhesive bonds. The key threshold for achieving sufficient wet out is known as the critical surface tension—the minimum surface energy a material must possess for effective bonding.
Adhesive Compatibility for Different Surface Energies
Adhesives must be carefully selected to match the surface energy of the substrate:
For HSE materials like metals or glass, most PSA tapes work well, as these surfaces naturally promote wet out.
For LSE materials like plastics or silicone, specialized PSA tapes with tailored formulations are necessary to ensure proper adhesion.
For MSE materials, you will have a wider range of PSA Tape options, but should be careful generalizing across PSA tapes and materials.
How to Test for Surface Energy and Wet Out
With the variety of materials and plastic blends it can be difficult to know the surface energy of a material. Several tools and techniques can help engineers assess the surface energy and ability for an adhesive to wet out on a surface, these include:
Using Dyne Pens: dyne pens are used to measure a material’s surface energy. By applying a liquid with a known surface tension, engineers can determine whether a substrate meets the critical threshold for adhesion.
Contact Angle Testing: this method measures the angle at which a liquid adhesive interacts with a surface. Smaller contact angles indicate better wet out and higher adhesion potential.
Adhesion Strength Tests: tests like peel, shear, and tensile assessments evaluate the strength and durability of adhesive bonds under different conditions. You can find out more about these types of forces that impact adhesion and their respective tests here.
The Bonding Challenges with Low Surface Energy (LSE) Materials
The primary challenge with LSE substrates is their low surface energy, which hinders an adhesive's ability to wet out effectively. Wet out is critical for creating a strong bond, as it allows the adhesive to spread across the surface and maximize contact. On LSE materials adhesive wet out can fail to occur:
The adhesive may not flow into the substrate, leaving microscopic gaps in the bond line.
Limited contact area reduces the adhesive's overall strength and durability.
Even with pressure, the adhesive may fail to form a continuous bond.
Optimal adhesion also relies on intermolecular forces, such as Van der Waals forces, to create a bond between the adhesive and the substrate. LSE materials are often chemically inert, meaning they resist these interactions.
In many cases, standard adhesives applied to LSE substrates simply fail to bond, leading to:
Delamination: the adhesive layer separates entirely from the substrate.
Reduced Load Capacity: the bond cannot bear the intended loads, reducing the overall functionality of the product.
Fortunately, there are specialized PSA tapes (like the 3M™ VHB™ LSE Series and the 3M 300 LSE Adhesive Transfer tapes) for LSE materials. These types of PSA tapes can be grouped into:
Acrylic Foam Tapes: these tapes offer excellent adhesion to LSE surfaces, such as plastics and composites, by balancing flexibility, strength, and environmental resistance.
Silicone-Based Adhesives: silicone adhesives are particularly effective on silicone rubber and other low-energy surfaces, providing superior bonding performance even in challenging environments.
Modified Rubber Adhesives: these adhesives are cost-effective and deliver strong initial tack and peel adhesion on LSE materials.
Outside of specialized PSA tapes, there are also surface treatments—techniques like plasma or corona treatments temporarily increase surface energy, allowing adhesives to wet out more effectively—and primers that can create an intermediary layer that enhances adhesion on difficult surfaces.