Structural adhesives offer many advantages over traditional bonding methods such as riveting, welding, or taping.
Improved Manufacturing Process:Talking to an expert about your requirements is the safest bet, but it is always good to educate yourself on the topic before you do.
We can break down the structural adhesive selection into a four-step process: 1 – understand your application performance requirements; 2 – select the best category (chemistry) of adhesive based on the application; 3 – narrow down the choice based on the exact performance requirements and manufacturing process considerations; and 4 – test the limited number of alternatives before going to production.
The first step is to identify the performance needs of your finished products and the requirements and limitations of your manufacturing process.
For the product requirements, consider the following:
For the manufacturing process, consider the following:
Specific adhesives can be somewhat tailored to the application. For example, add thickeners or thinners to increase or decrease viscosity, add accelerators to decrease cure time, or add a toughening agent to increase fatigue resistance.
However, the chemistry of the adhesive (typically classified as acrylic, epoxy, urethane, or cyanoacrylate – although they are hybrids) is the most significant determinant of the fitness for your application. The table below compares the properties of the epoxies, acrylics, urethanes, and cyanoacrylates.
Use the information to narrow down which chemistry is best suited for your product.
This is where more detailed analysis comes into play, and it is an excellent time to involve your vendor in your selection process.
First, you will need to dive into spec sheets to compile a list of suitable adhesives based on your specific bonding performance requirements. Once you have a list based on performance criteria, consider your manufacturing process, handling/storage requirements, and shelf life of adhesives.
Depending on your application, you may need to consider the trade-off between the ease of application and handling and processing.
Two-part epoxies are most stable, followed by acrylics. Urethanes have the shortest shelf life at room temperature and are sensitive to moisture in the air.
Two-part adhesives require time to cure after mating the parts – you need to clamp them until the adhesive sets. This slows the manufacturing process, but the extra time allows for positioning. Consult the manufacturers’ spec sheets for set times.
The two-part adhesive cure times are also temperature-dependent. They will cure faster in warmer temperatures and slower in cooler temperatures. Manufacturers typically list cure times at room temperature (about 70°F). The rule of the thumb is that the adhesives will cure twice as fast at 88°F, and take twice as long to cure at 52°F.
Adhesives typically thin in warmer temperatures and thicken in cooler temperatures, but you can also select viscosity to match your assembly needs. Two-part epoxies and urethanes come in the broadest viscosity range options.
Some adhesives contain volatile organic chemical components or other components that can emit odors. Some can also cause skin or respiratory irritation. Before using any adhesive, consult the manufacturer-provided Safety Data Sheet and follow all precautions.
If you decided in Step 2 that epoxies best suit your need, you may need to look at the differences between one-part and two-part epoxies.
One-part epoxies generally provide the highest shear strengths on metal and resistance to high temperatures and solvents. Because they come pre-mixed, there is no need to measure and mix curative and base before application. The trade-off is that one-part epoxies are prone to premature aging and have cold temperature storage requirements. They also require high-temp curing at temperatures up to 350°F.
Contrast this with two-part epoxies, which must be carefully measured and mixed but are easy to dispense and do not require temperature-controlled storage.
Cyanoacrylates do not require mixing and have long pot lives. However, they can be temperature sensitive, and they cannot be repositioned once bonded, which may be an issue in some situations. They are also prone to blooming, so the whitish or rainbow haze along the bond line may not be acceptable where aesthetics are essential.
Following the first three selection steps should produce a list of a few adhesives to test.
For a final selection, however, conduct testing. You cannot rely solely on manufacturers’ spec sheets to make your final determination.
Typically, peel tests and overlap shear tests determine the strength of the adhesive on particular substrates or under specific environmental conditions. Design your test to reflect the conditions of your project. If you are supplying parts to your customer, consider testing your parts in their final assembly.
Our experts will work with you to choose and test the solution for your product. We can also involve LORD structural adhesive engineers to help you design or adjust your manufacturing process for the best results and efficiency.