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.
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.
The chart below compares the relative surface energies of commonly used substrates:
(Image courtesy of 3M.)
You need to take the surface energy of your substrate into account in order to choose the best adhesive for your application. For example, 3M High Performance Acrylic Adhesive 200MP will not readily adhere to substrates with low surface energy, whereas 3M Adhesives 300, 320, 350 and 300LSE modified acrylics or 700 synthetic rubber adhesives are specifically designed for such substrates.
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 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 preparation goes a long way toward ensuring adhesion, which is why your surface should be dry and free of contaminates. Additionally, if your surface sees regular contact with oils, it may be necessary to use a more specialized adhesive, such as 3M Laminating Adhesive 300LSE.
If you’re using a high-strength, acrylic foam tape, such as 3M’s VHB Tapes, 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.
Surfaces that have been in contact with heavy oils may require a degreaser or solvent-based cleaner—such as 3M Prep Solvent-70, 3M Citrus Base Cleaner, mineral spirits or naphtha—prior to applying a water/IPA mixture, which can help ensure that any residue or film has been completely removed.
3M 4941 VHB Double-Sided Acrylic Foam Tape. (Image courtesy of 3M.)
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. Common sealants include paint, varnish and other hard surface coatings, such as 3M Rubber and Vinyl Spray 80 Adhesive.
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. For this reason, there are several 3M VHB Tapes specially formulated with this issue in mind. Alternatively, 3M Scotch-Weld Plastic Adhesive 2262—suitably thinned—can serve as a barrier between your tape and the PVC.
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.
(Image courtesy of 3M.)
Using a bonding primer promotes adhesion on difficult-to-stick surfaces, from paint and metal to polyethylene, polypropylene, ABS and PET/PBT blends. The advantage of using an isopropyl alcohol-based primer, such as 3M Adhesion Promoter 111
, is that it can act as both cleaner and primer in a single step when bonding metals and some plastics. Isopropyl alcohol also dries very quickly, which helps eliminate downtime.
For more challenging adhesion applications, such as films and vinyl graphics in automotive detailing, a dedicated primer, such as 3M Tape Primer 94, may be necessary. 3M Tape Primer 94 can be used to improve tape adhesion on a variety of surfaces, including concrete, wood, glass and metal.
However, these surfaces must still be clean before the primer is applied using a brush, swab, pressurized flow gun or knurled roller. Porous surfaces may require an additional application of the primer to ensure uniform coverage and good adhesion. If necessary, the primer can be removed with isopropyl alcohol.
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.
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.
If you’re unsure of the best adhesive or bonding primer for your application, consider reaching out to an expert who can help you identify the best design solutions.
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This post is sponsored by 3M. All opinions are mine. – Ian Wright
3M, Scotch-Weld and VHB are trademarks of 3M.