by Shari Loushin, senior technical service specialist – Industrial Adhesives and Tapes, 3M Company
Strength, durability, longevity and versatility. These are all words that describe the functionality of adhesives and bonding tapes in assembly, yet some manufacturers still doubt their effectiveness in design.
Manufacturers face plenty of challenges in assembly, many of which can be solved by using adhesive or bonding tape solutions. These challenges include bonding small parts, lightweighting, overcoming design constraints presented by mechanical fasteners and considering environmental impacts while increasing product life cycle. Despite lingering reluctance, more people are turning to adhesives and bonding tapes as ideal alternatives to traditional joining methods to solve challenges for all sorts of applications, particularly plastic bonding.
Manufacturers have found that adhesives and bonding tapes provide better durability, improved aesthetic value and a wider variety of options in design than more traditional mechanical fasteners or attachments, especially when working with plastic materials that may have low surface energy.
Some plastic materials are intrinsically difficult to bond with a structural adhesive or high-performance bonding tape, but using the right adhesive or bonding tape can provide a solution for joining these tough materials, while reducing or eliminating tedious surface preparation steps. Adhesives and bonding tapes can interact in unique ways with various plastic materials to create a robust, lasting bond.
Mechanical fasteners vs. adhesives and bonding tapes in plastics
In using mechanical fasteners, such as rivets and bolts, the manufacturer risks damaging the plastic because all of the load-bearing stress is held at one small point where the fastener is located. Mechanical attachments, such as clips, screws, etc., can be used with almost any surface, but they require additional steps to mold or create features for the attachment. Additionally, using these attachments can lead to stress concentrations, which may cause the plastic to crack and/or fail prematurely or result in unsightly surfaces.
Adhesives and bonding tapes distribute the stresses across the entire bonded joint area, evenly distributing the load. This distribution of stress ultimately leads to higher-strength joints, especially with thinner substrates that are more prone to damage and fatigue failure induced by force-concentrating joining methods, such as rivets or bolts.
End users often consider the appearance of products when making a purchase decision. Mechanical fasteners or attachments generally are very visible when used to connect two substrates. The obvious presence of fasteners interrupts the aesthetic of a product by calling attention to the joining points. Adhesives and bonding tapes, on the other hand, are virtually invisible and provide a cohesive, polished finish.
To avoid mechanical fasteners and their limitations, some manufacturers use techniques such as friction or ultrasound welding to join plastics, but these methods can incur extra costs and may require additional tooling. These issues can be inconvenient for products that are updated frequently or have short runs. Furthermore, these methods may not work with all combinations of plastics or composites. Solvent welding also is an option; however, that method typically relies on hazardous or noxious solvents. It also can lead to weakening of the substrates or unsightly effects, such as crazing.
Conversely, adhesives and bonding tapes are simple and easy to apply. They also can be pre-applied in areas that would be inaccessible to mechanical fasteners or with geometries not suitable to friction or ultrasonic welding, opening new options for designing with plastic.
Adhering to plastic
A few different mechanisms allow adhesives and bonding tapes to bond plastic materials. First, adhesives and bonding tapes can interlock mechanically with a substrate’s surface when the adhesive or bonding tape flows into the microscopic texture of the surface. After dwell or cure, it is difficult to remove the adhesive or tape because it’s caught in the topography of the surface.
For some adhesives, even higher strength plastic bonds can be achieved by creating a network of cured adhesive within the plastic. Some types of liquid adhesives are capable of swelling the plastic, allowing diffusion of uncured monomer into the plastic surface (similar to the action of solvent welding adhesives, but with reduced volatile content). The adhesive then can cure to form a robust bond with the plastic, because the adhesive polymer chains actually are entangled in the plastic substrate itself.
In addition to these factors, various chemical interactions between the adhesive and the surfaces being bonded also contribute to adhesive strength – from weaker interactions such as Van der Waals forces to covalent bonds.
All of these bonding mechanisms are dependent upon forming intimate contact between the adhesive and the surface of the bulk plastic or other substrate being bonded. This can be an issue when very low surface energy substrates resist the flow of adhesives or tapes over the surfaces.
Bonding hard-to-bond plastic
Adhesives and bonding tapes are proven solutions to problems that mechanical fasteners can present; however, some plastic materials are naturally more difficult to bond due to their chemical composition. These plastics are described as having low surface energy (LSE).
For optimum adhesion, the adhesive or bonding tape must flow, or “wet out,” on the bonding surface, providing close contact and maximization of the attractive forces between the adhesive or bonding tape and the substrate. These low surface energy plastics typically are difficult to wet out, so standard bond mechanisms reviewed previously cannot function optimally.
Examples of LSE materials are polyolefin (polypropylene, polyethylene, TPO), silicone, acetyl, polyester (PBT), polystyrene and PTFE, as well as some paints and coatings (PVDF). To bond these materials, extensive surface preparation steps such as priming, corona, flame or plasma treatments, acid etching or solvent-based adhesion promoters typically are required.
The goal of these treatments is to create a higher surface energy layer that allows better wet out of adhesives and tapes for additional possibility for surface interactions – hence better bonding. However, these surface treatments can incur additional costs and complexity, and also may present environmental concerns or safety issues.
Adhesive technology has advanced to the point where high-performance adhesives and bonding tapes are available that can bond low surface energy (LSE) plastics and minimize tedious surface preparation steps.
Bonding low surface energy plastics
Some specially developed two-part structural adhesives are able to bond LSE plastics without priming or surface treatments in numerous applications – from decorative panels to protective equipment. These adhesives also create bonds that can withstand weathering. These products are designed to have higher load-bearing capabilities, which is especially useful for smaller joints that will experience high stress. Structural adhesives allow for positioning activities, such as aligning or inserting parts prior to cure. Their chief disadvantage is that, like other two-part adhesives, they require some dwell to build initial strength.
Another solution for bonding LSE plastics is using specialized pressure-sensitive bonding tapes. Unlike adhesives, pressure-sensitive adhesives do not cure or undergo a chemical change when applied. These bonding tapes adhere immediately when pressure is applied, but continue to flow onto the surface, achieving a higher level of strength over time. These bonding tapes typically are formulated to be a layer of pressure-sensitive adhesive, sometimes with a reinforcing scrim internal to the tape. They are supplied on liners for ease of handling.
Another advantage of bonding tapes is that users do not have to bond the adhesive to both substrates at the same time. The bonding tape can be applied to the first substrate without removal of the liner. At a later time, the liner is removed and the part is applied to the second substrate. This is especially helpful when supplying parts to an end customer or end user.
New acrylic pressure-sensitive adhesive technology allows manufacturers to bond to a wide variety of LSE plastics without priming or surface energization. These acrylics are ideal for light- to medium-weight bonding applications with low static loads, especially when easy, quick assembly and excellent aesthetics are desired. They do require fairly thin and consistent bond lines, however, and do not provide structural load-bearing strength.
Adhesives and bonding tapes provide undeniable strength
Not only do adhesives and bonding tapes provide solutions to the aesthetic and functional disadvantages of some traditional joining methods, but they also now can be used to join hard-to-bond materials and even eliminate surface preparation steps, such as priming or surface activation.
Though some manufacturers may be skeptical of adhesives and tapes, their strength often is drastically underestimated. With the huge amount of adhesives technology that has been developed, something is available for every application imaginable. From thin, aesthetic mounting of trims and labels to high load bearing insertions in parts that will see high stresses, adhesive or tape options are available for virtually every application.
Shari Loushin is a lead technical service specialist in 3M’s Industrial Adhesives and Tapes Laboratory in St. Paul, Minnesota. Her primary focus is on structural adhesives, a position she has held for seven years. Prior to that, she worked with attachment tapes for automotive applications. She holds a master’s degree in physical chemistry and an MBA. At 3M, science is applied in collaborative ways to improve lives daily. With $30 billion in sales, its 90,000 employees connect with customers all around the world. For more information, visit www.3M.com/assemblysolutions.