September 19th, 2014

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Bonding Substrates with Different Thermal Expansion Coefficients

A major challenge many engineers and/or designers face in a bonding or potting application is overcoming different coefficients of thermal expansion (CTE). What is the coefficient of thermal expansion? The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure. Why is a mismatch in CTE an issue? During thermal cycles or just over time, when materials expand and contrast at different rates and the adhesive cannot compensate for the difference there can be devastating results such as cracks in the adhesive, cracks or breaks of the substrate, delamination, etc.

Very common substrates challenges are:
• rubbers to metals
• ceramic to plastics
• aluminum to glass
• glass to plastics

A much overlooked mismatch however is the adhesive (epoxy, silicone, urethane, etc.) to air. Air expands and contrasts due to temperature changes just like any other substance. This is how we get thunder, the rapid expansion of air. Expansion and contraction of air (entrapped air in the polymer or an air bubble) can cause severe cracks in the bond line or potted component which can lead to parts failures.

How does one overcome this challenge?

There are few avenues one can go down to solve the issue of CTE mismatches. The simplest way (however maybe not the easiest) is to design the substrates to have a similar CTE as the adhesive. This is will result in similar expansion contraction rates decreasing stress on the adhesive and the substrates.
Choosing the right adhesive to bond dissimilar substrates is crucial for preventing component failures. One such type of adhesive used for this application is known as a toughened adhesive. In particular, toughened epoxy adhesives have in them a rubberized polymer. This rubberized polymer, a toughener, increases bond strength and shock resistance and decreases cracking.

We have a variety of 1 and 2 component toughened epoxy and urethane systems designed to help this common application issue. Unlike most toughened epoxies such as 3M DP-460 or Loctite E-60HP where there is difficulty in maintaining consistency when incorporating the rubber polymers due to manufacturing process and raw material selection, Epoxyset toughened systems uses novel toughening rubbers and dispersions to always deliver a consistent and quality adhesive. Some of our well known toughened epoxy systems are EB-260, EB-127TC, and EB-316M.
The second adhesive solution to bond or encapsulate substrates with dissimilar CTE values is to use a more flexible adhesive. Flexible adhesives can be epoxies, polyurethanes, silicones, and polysulfide. Flexible adhesives maintain its bond without cracking and flexes while the substrates expand. While the bond strength of these adhesives tend to be lower than toughened adhesives, they are still very effective as cracking is significantly reduces. Using flexible potting compounds may not only reduce cracking they also increase impact resistance.

Our flexible epoxy adhesives used for such applications include EB-109M-4, EB-316M, and EB-116. Our UB-21, UB-30, and UB-70 urethane adhesives are highly used in the aerospace and medical device industries as flexible adhesives for bonding plastics to metals. Our urethane potting compounds are also used for applications needing high impact resistance.

Choosing the right Adhesive:

How do we choose the right adhesive? Choosing the right adhesive is an integral part of the design of any type of component needing bonding or potting. What type of adhesive should be used? Our immediate response would be to contact Epoxyset and speak to our technical team to help choose the best option. Below is helpful information to help you decide which type of adhesives is best for your application.
Epoxy adhesives and potting compounds are abundantly used in a variety of industries. Advantages of epoxy adhesives are its versatility, bond strength, and chemical resistance. Epoxies, in conjunction with different catalysts offer quick 5 minute cure times, to highly cross-linked chemically resistant systems. Epoxy has temperature limitations both high and low. Most epoxies can also be highly rigid and more prone to cracking. Engineered epoxies, including the above mentioned toughened epoxies however can be highly effective for crack resistance.
Polyurethane based adhesives can set solid and relatively inflexible, or can remain rubbery and flexible. It adheres to most things and can bond different materials together. It is successful in bonding non porous materials such as metal to wood, plastics, rubbers, glass, and ceramics. Urethane systems also tend to be more flexible than epoxy making it a great first look when trying to bond different substrates. Many urethanes also have a very low Glass Transition Temperature (Tg). This unique property makes urethane very useful for low temperature applications because it retains its flexible properties.
Silicone adhesives and potting compounds have the highest heat resistance out of the 3 chemistries. Most silicones can withstand temperatures of up 205°C continuously and some can withstand over 250°C. Silicone systems are softer and more flexible but also do not have the same bond strength as epoxies and urethanes.

What are the key properties of an adhesive system under extreme thermal stress?

To compensate for mismatches of thermal expansion rates, the adhesive system must be able to withstand temperature cycles. This is one of the main properties to look at when choosing systems. It is also important to consider specifics in your application such as pressure, moisture, and chemical environment.
For bonding dissimilar substrates, low modulus (flexible) stress absorbers are preferred as they take up the deflections of the adjoining materials, allowing the bonded entities to move more freely with little constraint. These types of adhesive are more forgiving when pushed and pulled (expand or contract) and will not crack under extreme temperature cycles.
For further technical information, visit our website at www.epoxyset.com. Our product pages are easy to use and categorized by properties and suggested applications. Our website product catalog contains widely used products but we have hundreds of more products not on display on our website. Please contact us for your application so we may suggest the absolute best product.