Surface Energy and Adhesive Selection

When bonding together two substrates, it is important to consider the surface energy of the material.

Pressure-sensitive adhesives (PSA) have made it possible to permanently adhere two dissimilar substrates together. While bonding two surfaces together, there are several factors that need to be considered including surface tension and texture of the substrate, bond strength, surface area, environmental conditions, design, and product application. However, one crucial factor that influences the selection of adhesive is the surface energy of a substrate. Surface energy is the excess energy that flows on the surface of the substrate and is measured in dynes/cm. The dyne level is the actual reading of the critical surface tension.

How Does the Surface Energy Influence Adhesive Selection?

Based on the surface energy, substrates can be broadly categorized into three groups - high surface energy (HSE), medium surface energy (MSE), and low surface energy (LSE). With high surface energy ranging from 250-1103 dynes/cm, metals like copper, aluminum, zinc, and stainless steel are some of the most popular HSE substrates. The surface energy takes a big dip to 38-50 dynes/cm for MSE substrates such as polycarbonate, polyester, nylon, acrylonitrile butadiene styrene (ABS), and acrylic. Finally, materials with surface energy below 37 dynes/cm fall into the category of LSE. The widely employed LSE substrates include polyethylene, polyvinyl alcohol (PVA), ethylene vinyl acetate (EVA), and polypropylene.

To understand the importance of surface energy in bonding substrates, let's consider "wetting". Wettability is the ability of an adhesive to spread on the surface, thereby increasing the contact area and creating a stronger bond. In most cases, when you pour water on a HSE metal such as copper, the water will quickly spread across the surface and form puddles. On the other hand, when you pour water on ABS, it will form small beads, thus preventing the surface from wetting. Simply put, HSE substrates aid the wetting of the adhesive, while LSE substrates avert the wetting. The surface energy of the substrate dictates the strength of attraction and therefore remains as one of the most critical aspects in bonding.

There is a wide variety of PSA solutions available, each of them offering a unique set of advantages and disadvantages. For example: 3M offers approximately 25 different PSA solutions, with 100MP, 200MP, 300MP and the 300LSE being the most dominantly utilized adhesives. While the high-performance acrylic adhesive family such as the 100MP and 200MP, strongly adheres to most HSE substrates, the 200MP is frequently preferred for MSE substrates, and 300LSE is usually chosen for both MSE and LSE substrates. It is the interplay of several factors that ultimately dictate the selection of the adhesive, thus the optimal solution varies from one application to the other.

With years of process innovation and material science experience, Boyd has the capabilities to not only tailor adhesives to match your unique application, but also address any other bonding needs and challenges. To learn more, schedule a consultation with our experts.