Jimee Patel, MSDS Author at Master Bond Inc., talks to AZoM about the thermal properties of adhesives and how to best select the right product for the right temperature environment.
What are the main points an engineer should consider when selecting an adhesive for high temperature applications?
Of course, the temperatures to which the application will expose the adhesive need to be realistic. If an adhesive with more temperature resistance than is actually required is chosen, thermal cycling capabilities can suffer.
Another factor to be considered is the duration of exposure to high temperatures. Many adhesives can withstand even 300°C for a few seconds, but if that exposure time is increased to hours, days or even months, the choice of suitable adhesive products is heavily reduced.
It is important to focus on applications that must withstand sustained temperatures of more than 200°C. Normally, adhesives with high heat resistance also have high Tg values meaning they are rigid across their operating temperature range. These adhesives typically offer less give in thermal cycling applications.
Can you provide an explanation of Tg?
The temperature at which the majority of temperature resistant adhesives change from a rigid glassy state to a more pliable form is known as Tg.
This thermal property acts as a good guideline of the ability of an adhesive product to stand up to sustained exposure to high temperature.
The adhesive should remain in a rubbery, ductile state above the Tg, with more strength (but with more brittleness) below the Tg.
What are the exceptions to the relationship between heat resistance and a high Tg?
Certain silicones and B-staged flexible epoxies do not necessarily follow these rules.
Due to the unique nature of the molecular backbone of silicones and the flexible cure state of B-stage epoxies, these adhesives have both considerably low Tg and good heat resistance, whilst still maintaining their flexibility in extremely high temperatures.
For example, B-staged epoxies are offered by Master Bond that has a Tg of 35°C and service temperature up to 260°C.
Some real world applications use adhesives with operating temperatures that exceed Tg for short periods of time, or by small margins, but this will not cause mechanical properties to degrade enough to matter.
Tg gives a good indication of good design practice in high temperature applications. When an adhesive is selected, design engineers should consider the expected service temperature, Tg, mechanical properties requirements, and thermal stress concerns.
What products does Master Bond formulate to match the extremities from a thermal standpoint?
Master Bond has a line of several high and low temperature resistance epoxies and silicones. They also have the ability to custom blend for a specific need. A few examples would be:
- EP17HT—this one-component epoxy has an exceptionally high Tg.
- EP36AO, a low Tg, B-staged system, integrates toughness and flexibility, and provides excellent thermal cycling resistance.
- MS800 is a one-component silicone with excellent high temperature resistance.
- EP21TCHT-1 is a two-component epoxy with a high Tg and cryogenic serviceability, providing for a very wide serviceable temperature range.
- Supreme 46HT-2 is a two component epoxy with a high Tg.
Lets talk about cold temperature extremes. What role does Tg play in these situations?
In cold temperature applications the Tg does not provide the same clear window into adhesive performance as it does in high temperature applications. The dip in temperatures below the Tg, causes adhesives to become highly brittle and more susceptible to low failure stresses. Theoretically, that reasoning would seem to favor flexible adhesives with low Tg values for the coldest applications.
Practically the opposite is often true. For example, epoxy adhesives do not experience a considerable loss of properties even at cryogenic temperatures, implying they work best in a rigid state that extends from their Tg into far colder territory.
Master Bond makes certain one- and two-component epoxy adhesives that provide structural bonds, serviceable from a cryogenic 4K to a blistering 205°C. Other high temperature epoxies likewise perform well at cold temperatures above cryogenic levels.
What about severe environments that mix extreme heat and cold?
This is often the case in aerospace applications. Adhesives with a low Tg that function well in a cold environment tend to exhibit large coefficients of thermal expansion (CTEs) as they heat up which can lead to thermal stress problems.
Comparatively, adhesives with a high Tg have lower, more manageable CTEs across their entire operating temperature range, and may be more suitable for these mixed extreme environments.
What should one be mindful of regarding the curing process for temperature resistant adhesives?
The grades with the very best temperature resistance tend to be products which need an oven cure, possibly also with the need for fixturing. Systems that require only a room temperature cure may lose out on some temperature resistance. One-component, heat curing systems eliminate the need for mixing and offer fast elevated temperature cures.
It is of course important to be aware of the manufacturer’s cure recommendations. While that advice applies to most adhesive applications, it’s all the more crucial with temperature resistant products since the Tg can be affected by improper curing.
About Jimme Patel
Jimee Patel is currently employed as a Material Safety Data Sheets (MS).
Author at Master Bond Inc. For the past five years, she has been working in the field of Environmental Health and Safety. She uses her technical expertise to write MSDSs and hazard warning labels, and ensures site compliance with applicable federal and state laws. She also assists with the technical sales department.
Ms. Patel earned a Masters in Environmental Health and Safety Management from Rochester Institute of Technology, and a Masters in Environmental Science from SP University in India.
She received a Bachelors in Environmental Science from MS University, India.
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