Glass transition temperature is a fundemental polymer property that can have drastic effects on the high temperature performance of a polymer adhesive. The high temperature performance of epoxy adesives in particular is generally determined by first measuring the glass transition temperature of the polymer system.
Other polymer properties worth checking out:
- Comparing Shore A And Shore D Hardness Values
- Comparing Lap Shear Strengths Of Polymer Adhesives
- High Vs. Low Modulus Of Elasticity
What Is The Glass Transition Temperature?
Although the glass transition temperature (Tg) of a polymer adhesive is normally presented as a single temperature, it is actually a range of temperatures during which the polymer changes from a brittle state to a more rubbery one. This is not a phase transition (like ice melting) but rather a point where the solid material begins to act quantifiably different.
Essentially at the Tg and above, the polymer chains of the solid material become significantly more mobile having an effect on a handful of physical performance properties. As a general rule, performance properties are worse above the Tg (there are some serious excpetions to this rule which we will explain soon!).
One important note to make is that the physical changes experienced at and above the Tg are generally reversible. Short durations above the Tg will not have drastic effects on the polymer once it returns to temperatures below the Tg. Long durations above the Tg may have long term effects, however.
Why Does Tg Matter?
Short answer: because the changes in physical properties can be substantial. The two most important physical changes that occur are:
- Changes in CTE: above the Tg the coefficient of thermal expansion dramatically increases (sometimes 3-5x). Basically, the amount the solid material will expand or contract as a result of temperature changes increases. This can result in more thermally induced stress on bonded components.
- Bond strength: because the polymer chains become more mobile above the glass transition temperature, the polymers ability to hold a strong polymer bond is reduced above the Tg. This has a drastic effect on the lap shear strength of the adhesive system.
However, we should note that there are some exceptions to be considered. For some types of chemistries, its actually expected that the polymer will always be used above the Tg. Silicones for example commonly have Tg values between -40C and -100C, and silicone products tend to have exceptional high temperature capabilities.
Epoxies, on the other hand, are more sensitive to temperature above their Tg. However, they can also be occasionally used above their Tg, as long as its not for long periods.