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Categorising Surface Energy
The surface energy of a material can be measured in dynes per centimetre, but what does that mean in terms of bond strength? Classified into three groups - high, medium and low - the surface energy of a material affects how readily it bonds with an adhesive. Learn about the types here, and the considerations to make when securing different materials like metal, wood or plastic with adhesives.
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High Surface Energy
Molecules on the surface are so strongly attracted to each other that they will very happily be attracted to liquid molecules as well. As a result, these materials are relatively easy to wet out and, in turn, to bond. High surface energy materials have surface energies on the order of 100s or 1000s of dynes/cm and include many metals and glass.
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Mechanical Interlocking Adhesion
Everything is relative. Somewhere between perfectly wet-out films and perfectly spherical droplets, we define medium surface energy. These are materials typically 36 dynes/cm up to about 300 dynes/cm. Many engineered plastics have surface energies in this range, as do natural materials such as wood, stone or concrete.
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Low Surface Energy
Molecules on the surface of low-surface-energy materials are quite happy the way they are. There is very little attraction to any molecule, especially adhesive molecules. Materials with a surface energy below 36 dynes/cm are considered low surface energy and are very difficult to bond. These include polyolefin plastics such as polypropylene and polyethylene as well as “non-stick” surfaces such as polytetrafluorethylene (PTFE).
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Surface Energy of Different Materials
Recall the material bonding categories and the range of surface energies for each (pictured right). For each of these categories we will explore surface energies for common materials types.
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Low Surface Energy Plastics
LSE Plastics are generally soft, low-density and have a low melting point. These low surface energy materials stick together and repel liquids, making them difficult to bond. 3M has designed adhesives and tapes that work, but there are far fewer options to choose from.
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Engineered Plastics
Engineered Plastics are very common in manufacturing: they’re low-cost, strong and sturdy, yet they’re also lightweight and easy to mold into complicated shapes to suit specific needs. Engineered Plastics have higher surface energies than LSE Plastics, so they’re easier for tapes and adhesives to wet out and bond.
This is a broad and varied group: basically it includes a range of substrates that have higher surface energies than plastics but lower surface energies than metals. This category includes old technologies like glass, ceramics and concrete as well as natural materials like wood, leather and fabric. Traditional materials are relatively easy to bond, but each one has its own considerations.
Metals are strong and perform well across a wide range of temperatures and environments. They have very high surface energies and they’re open to contact with liquids and thus very easy to wet out. Most adhesives and tapes will adhere to most metals, so any time you’re bonding metal you have a wide range of adhesive options.
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The Science of Adhesion Educational Series is designed to be a comprehensive introduction to Adhesion Science and the use of tapes and adhesives in design applications. Visit the Science of Adhesion to view all articles or select from the featured topics below.
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