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What is Adhesive Bonding and How Does It Work?
Image header courtesy of DuPont
Components that are bonded with adhesives are everywhere around us. Common examples include cellphone screens, insulation panels, computer heat sinks, cooling components in EV batteries, and electric motor housings.
To apply adhesives, manufacturers use a method called adhesive bonding. Due to its many benefits, it is widely used to replace welds and mechanical fasteners (screws, bolts, and rivets).
In this article, you will learn the essentials of this process and the role of surface preparation to achieve strong bonds.
Table of Contents
- What Is Adhesive Bonding?
- What is Adhesive Bonding Used For?
- How Does Adhesive Bonding Work?
- What are the Advantages of Adhesive Bonding?
- How Does Surface Preparation Improve Bonding?
- What Are the Surface Preparation Methods?
- How Do You Evaluate the Adhesive Bond Strength?
- Laser Surface Preparation to Improve Bond Performance
What Is Adhesive Bonding?
Adhesive bonding is the process of joining surfaces using adhesive materials like liquid adhesives, pastes, tapes, and sheets. During bonding, adhesives mechanically interlock with and chemically bond to the substrates (i.e., to the surface to which they are bonded).
Dispensers are used to apply liquid adhesives and pastes. They precisely control the amount and placement of the adhesive on the surfaces being bonded. Other types of adhesives can be applied without dispensers, such as by applying pressure.
What is Adhesive Bonding Used For?
Adhesive bonding is very common in manufacturing. You can find it in sectors such as automotive, aerospace, medical, electronics, and food packaging. Manufacturers use it to join all types of surfaces and dissimilar materials, including metals, ceramics, glass, plastics, rubbers, and composites.
In the automotive industry, adhesive bonding plays an increasingly important role due to the electrification of transportation. Electric vehicles have requirements that are easier to meet with adhesive bonding than with alternatives like welding and mechanical fastening.
The process helps EV manufacturers minimize weight, seal off against environmental conditions, manage temperature through thermal conduction and insulation, provide electrical insulation, and improve structural integrity.
Here are examples of how adhesive bonding is used in EV manufacturing:
- Structural adhesives are used in structural batteries so that batteries play an important role in the load-bearing structure in addition to being the source of energy.
- Thermally conductive adhesives are used on cooling plates and tubes to help conduct heat away from the batteries and toward heat sinks.
- Liquid gaskets are used as sealants for e-motor housings, improving protection against factors such as water, dust, road salt, and fluids.
How Does Adhesive Bonding Work?
Depending on the type of adhesive, the bonding process varies slightly.
Curing adhesives undergo a chemical reaction to form a bond. The reaction can be triggered by a range of factors.
- Heat-curing adhesives, for example, undergo that chemical reaction when exposed to high temperatures.
- Similarly, UV-curing adhesives rely on exposure to ultraviolet light to trigger the curing process.
- Most epoxy adhesives are two-part curing adhesives because a resin and a hardener need to be mixed to trigger a chemical reaction and start the curing process.
Other types of adhesives do not need curing to form a bond. These adhesives rely on other mechanisms.
Solvent-based adhesives, for example, rely on the evaporation of a solvent to go from a liquid to a solid. The adhesive material adheres to the surfaces as it reaches the solid form.
Another example is pressure-sensitive adhesives, which only need pressure for application. These can include pressure-sensitive tapes like masking tapes or duct tapes.
What are the Advantages of Adhesive Bonding?
There are good reasons why adhesives are everywhere around us. They offer many advantages compared to other joining methods.
| Advantage | Explanation |
|---|---|
| Can bond dissimilar materials | Adhesives can bond different types of materials together. This versatility allows for innovative designs and assemblies. |
| Add less weight to assemblies | Adhesives typically contribute minimal additional weight to assemblies compared to mechanical fasteners or welding, which can be crucial in applications where weight is a concern, such as in the aerospace and automotive industries. |
| Provide environmental protection | Adhesive bonds can seal against external factors like water and dust, providing environmental protection to the bonded materials. This helps enhance durability and longevity, particularly in outdoor or harsh environments. |
| Assemblies gain strong mechanical properties | Adhesive bonds can distribute stress more evenly across bonded joints, leading to strong and durable bonds. This can result in assemblies with superior mechanical properties. |
| Can serve as thermal conductors or insulators | Depending on the adhesive material used, adhesives can act as either thermal conductors or insulators. This property allows for precise temperature management in various applications, from electronics to automotive components. |
| Can serve as electrical conductors or insulators | Adhesives can also be designed for electrical conductivity or insulation, making them suitable for use in electronic devices where electrical connections need to be made or protected. |
How Does Surface Preparation Improve Bonding?
We’ve already explained in full how to prepare surfaces for adhesive bonding in another post, so we’ll explain the short version here.
Before adhesive bonding, it’s essential to prepare the surface to obtain a strong bond.
The substrates need to be clean of contaminants, slightly rough, and have a chemical composition that favors bonding. Here’s how each of these aspects contributes to a stronger bond.
- Remove contaminants: Contaminants like oxides and greases can interfere with the bonding process. By removing them, you eliminate the risk of unwanted chemical reactions between the adhesive and the contaminants. You also free up surface energy, making room for more bonds between the substrate and the adhesive.
- Roughen the surface: Rough surfaces have more surface area to which adhesives can adhere. They improve mechanical interlocking and create more opportunities for chemical bonds.
- Modify the chemical composition: Chemical bonds can be weaker or stronger depending on the chemical composition of the surface. By adding new molecules or rearranging how atoms are configured on the surface to form new molecules, it is possible to create a more favorable surface for chemical bonding.
What Are the Surface Preparation Methods?
Different surface treatments can be used to prepare surfaces for adhesive bonding. Here are the most common ones used in manufacturing:
- Chemical treatments: Chemical can do it all. They can dissolve contaminants, etch the surface to increase roughness, and introduce new elements on the surface to create a favorable chemical composition. Examples include chemical baths in which parts are immersed, or primers which are directly applied on the surface. It’s important to note that chemicals have enormous downsides in terms of safety, environment management, and operating costs.
- Laser cleaning: Like chemicals, lasers can do it all. They can remove oxides and surface contaminants, texture the surface, and introduce beneficial oxides that promote strong bonds. While lasers come at a high initial investment, they require minimal maintenance and operating costs. Due to their high precision, they are the best solution to treat localized areas without damaging surrounding components.
- Mechanical abrading: One of the strengths of methods like sanding, grinding, and blasting is their ability to roughen a surface. In terms of cleaning however, these methods can only remove oxides and inorganic contaminants. They need to be complemented by chemical pretreatments to ensure that all contaminants are removed. Abrasives also generate a lot of dusts that are not always easy to manage. Dusts can recontaminate the substrate, leading to poor adhesive quality.
How Do You Evaluate the Adhesive Bond Strength?
In manufacturing, adhesive bonds need to meet certain specifications based on how the products will be used during their useful life. Depending on the needs, it is possible to test for specific types of mechanical stress.
In EV batteries for example, it’s very common to perform lap-shear tests and pull-off tests to make sure that the batteries will be able to withstand heavy loads (as part of the load-bearing structure), constant vibrations, and dynamic forces experienced during accelerations and stops.
Force testers can provide the strength in megapascals that an adhesive joint can withstand before it fails. You can see results from these tests here.
If during these tests there is adhesive failure (i.e., the bond between the substrate and the adhesive breaks), it indicates that there is still room to improve the bond strength. The goal is to have such a strong bond that when there is a failure, it occurs within the adhesive. This type of failure is called a cohesive failure.
Laser Surface Preparation to Improve Bond Performance
Surface preparation plays an important role in reaching the high bond strength needed to prevent adhesive failure. If you want to optimize your bonding process, we can help you with laser surface preparation.
Catherine Veilleux
Catherine holds a bachelor’s degree in Engineering Physics and a master's degree in Physics. She completed her master’s in partnership with Laserax to develop industrial solutions for the laser texturing of metallic surfaces. She is now the Applications Lab Supervisor at Laserax, where she oversees the team that tests and optimizes laser processes for clients.
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