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How to Choose the Right Lightweight Adhesive

    March 23, 2017

Finding the right adhesive for your lightweight component is dependent on the right chemical mix. Some adhesives are rigid. Others have high degrees of elasticity. Some offer increased energy absorption. How do you know which is right for the job?

It starts with considering some basic questions. What are the materials you need to join? What are your goals? Are you aiming for improved acoustic and vibration design? Are you pursuing a significant increase in energy absorption in the body structures for increased safety? What are the assembly process limitations the adhesive will be required to meet?

adhesive-roof

In many ways, it’s like ordering the perfect cup of coffee. Light or French roast? Macchiato or Cappuccino? Whip or no whip? Each decision makes a difference in the taste, texture and ultimate enjoyment for the coffee drinker.

While one person may be looking for a beverage to sip and savor, another may only be interested in a quick caffeine jolt to provide an energy boost. For true coffee aficionados, it’s all in the mix. Ingredients and their proportions can make all the difference in the perfect cup versus one that’s destined for the drain.

The same is true for adhesives, and finding the perfect mix is dependent upon several factors:

1. The properties of adhesives:

Kinetics: This dictates the time-to-cure as well as the appropriate temperature for curing. Do speed and temperature matter?

Viscoelasticity: This describes the strength of the adhesive itself as well as its ability to handle the residual stresses where the lightweight materials are joined. Is the adhesive strong enough for what it will face?

Plasticity: This refers to the mechanical behavior of an adhesive and enables it to undergo load deformation when compressed without failure. Will the adhesive hold up under deformation?

2. The chemistry of adhesive types and the properties they provide:

Different adhesives have different properties.

Epoxies are high-strength adhesives for structural applications and torsional stiffness, and are a solid choice for crash-resistance applications.

Polyurethanes (PU) are flexible structural adhesives that have an optimized combination of stiffness, strength, elasticity, fast cycle times, more elongation and occasionally do not require pretreatments. They also have an accelerated cure time and can cure at room temperature.

Acrylates provide strength and durability, have fast setting times and high adhesion with substrates that are difficult to bond.

The substrate used in your application matters, too. Certain adhesive chemistries are better suited for certain substrates. Combinations commonly used in automotive lightweighting include:

  • Aluminum-steel
  • Aluminum CRFP (carbon fiber reinforced plastic)
  • Steel-CRFP
  • Ultra-high strength steel (UHS)

Choosing the right adhesive to bond these disparate materials in a variety of specifications requires exploration prior to implementation.

3. Methodology and testing

Methodology and testing are important steps in selecting the right adhesive for specific automotive lightweighting applications. Joint design and FE analysis are highly recommended because they can predict probable points of failure and failure modes. The assembly processes prior to and after application of adhesive can also play a role in defining the appropriate chemistry.

In addition, the different thermal expansion properties of substrates used in multi-material designs need to be considered when choosing an adhesive. Otherwise, different thermal expansion rates may lead to destructive mechanical loads.

methodology-and-testing

For example, the thermal sensitivity of composites prevents standard oven processes and drives bonding processes into the assembly line. In these circumstances, it is essential that the assembly process is designed to accommodate the application of bonding materials, including their respective curing times, so that they can perform at an optimum level within the lightweight vehicle structure.

4. Adhesive types

Henkel has developed several adhesive types that have been tested and proven beneficial in a number of automotive lightweighting applications. These specific products are primarily used for automotive components/systems outside of the body and paint shop assembly areas.

Epoxy
TEROSON® EP 5065, an epoxy, offers a unique combination of strength and elongation and is able to fit in a white space to provide a new level of bonding support. It has several process advantages, including the capability to bond to a wide range of substrates, and offers reliable strength and flexibility for multi-substrate bonding. It cures at room temperature or can speed cure with the application of heat.

Due to its elasticity, this high strength adhesive for structural applications and torsional stiffness provides excellent corrosion resistance and very high crash resistance. As a result, it is especially suited for use as a structural adhesive in highly stressed vehicle components, especially liftgates.

Elastic-structural

Elastic-structural adhesives offer bonding flexibility such as long open time and no moisture sensitivity. They can also bond to multiple substrates including plastics, aluminum and carbon fiber.

Henkel’s TEROSON® PU 1711 elastic structural adhesive is unique in that it “snap cures” in 30 seconds at 85 C, requires no mixing, and has fast initial strength. Its glass transition temperature is less than -40 C for consistent mechanical properties over the service temperature, and it has high elasticity and strength, excellent adhesion properties and elastic response in CTE mismatch bondlines.

Polyurethane
These flexible adhesives are designed for use on composites and multi-material bonding. They provide stiffness, strength and elasticity for bonding flexibility.

LOCTITE® UK 2015 is one of the available polyurethane adhesives. It offers 100 percent elongation, requires no pre-treatment and is optimized for fast cycle times of under five minutes. Curing can also be accelerated by heat.

Want to learn more?

Adhesives are often the most versatile option in lightweighting. We can help you find the right adhesive to use in the right place for automotive lightweighting materials, especially with multi-material body designs.

Check out the Smart Chemistry Hub to learn more about the relevant parameters for choosing adhesive selection for the successful bonding of dissimilar substrates used in vehicle lightweighting. It could actually be easier than choosing what goes into your favorite coffee beverage!

About the Author, Greg Ryba

Greg Ryba leads the North America Adhesives Steering Unit at Henkel Corporation and is responsible for engineering, business development and laboratory application technicians. He has over 25 years of sales, marketing and operations management experience. Mr. Ryba holds a bachelor's degree in marketing from Central Michigan University.

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