Smart Surface Preparation Makes Perfect Adhesive Bonds

1. Smart Surface Preparation Makes Perfect Adhesive Bonds In manufacturing, adhesive bonding offers significant advantages over mechanical fasteners or welding for many applications. Strength, low cost and adaptability are major drivers of the bonding revolution, but for strong, high reliability bonds, it’s more complicated than simply sticking two parts together, particularly when it comes to surface preparation. Whether you’re using liquid adhesives or bonding tapes, there are a host of important questions to consider:  What are your substrates?  How should you clean your surfaces?  Do you need a bonding primer?  What surface preparation solutions will work best for your application? As is usually the case in manufacturing, the answers to these questions are highly dependent on your specific application. Nevertheless, there are some general tips, tricks and techniques that can be applied to most situations involving industrial adhesive bonding. Adhesive Fundamentals Chemical adhesion refers to the tendency of unlike materials to cling to one another through molecular attraction. The attraction’s strength is determined, in part, by the surface energy of the materials: the higher the surface energy, the greater the molecular attraction. Materials with a high surface energy enable increased contact between the adhesive and the substrate, since the adhesive is able to flow or “wet-out” across the substrate’s surface and create a stronger bond. For this reason, modified acrylic and synthetic adhesives with better flow or “wet-out” characteristics can help with adhesion to substrates with low surface energy. You need to take the surface energy of your substrate into account in order to choose the best adhesive for your application. In addition to surface energy, you’ll need to take several other factors into account in order to select the best adhesive for your application, including assembly type, substrate type, manufacturing process and environmental conditions. These variables notwithstanding, there are also some general considerations that apply to the performance of any adhesive. Ultimately, it comes down to maximizing adhesive contact with the surface. That’s why it’s important to

2. apply firm, consistent pressure when applying the adhesive. It should also be noted that surface contact and adhesion values will generally increase over time and with elevated temperature, though only up to a point of course. Surface PreparationIf you’re using a high-strength, acrylic foam tape it’s sufficient to prepare most substrates using a 50:50 mixture of water and isopropyl alcohol (IPA). However, there are some exceptional cases that will require different surface preparation solutions. Other surfaces will require an abrasive cleaning to remove heavy dirt or oxidation and increase surface area for improved adhesion. Tapes require a finely abraded surface, ideally one with very small scratches generated through circular motions rather than straight lines. Numerous micro-scratches can yield up to 40 percent additional surface area, which can considerably improve adhesion. In some instances, abrasion can chemically alter the surface to which you’re bonding—for example, aluminum oxidizes rapidly after abrasion, which means the actual bonding surface for two aluminum components is aluminum oxide. Porous materials, such as wood, particleboard or concrete, may need to be sealed in order to create a unified surface. Materials with unique properties—glass, copper or flexible plastics, for example—may require more specific surface preparation solutions. Some materials such as glass, stone and ceramic are hydrophilic by nature, which makes pressure- sensitive adhesive bonds susceptible to high humidity or exposure to moisture. Adding a silane coupling agent to your water/IPA mixture as part of your surface preparation can help reduce this tendency. Copper, brass and bronze are prone to oxidize even after you’ve applied your bonding agent, so these materials should be lacquered or varnished. Flexible PVC contains plasticizers that could potentially migrate into your tape and affect its performance. Finally, rubber materials, such as EPDM and neoprene, may also contain plasticizers and oils in addition to having low surface energy. These materials require an adhesion promoter to ensure stable bond strength. You can test the compatibility between these materials and your adhesive system with accelerated aging at elevated temperatures, checking for softening of the adhesive, discoloration or reduction in bond strength. Choosing the Right Industrial Adhesive the complexities of surface preparation are belied by the apparent simplicity of the adhesion process. If you’re considering a high-strength tape for your application, be sure to know whether your substrate has high or low surface energy and handle your surface preparation accordingly.

3. Isopropyl alcohol is a necessary surface preparation solution in many cases, but it may not be sufficient if your surface is contaminated with oil, grease or lubricants. Those surfaces may require more aggressive degreasers or abrasion. If you need a bonding primer, several options exist to promote strong bonds to a wide variety of substrates.