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What is Tg (Glass Transition Temperature)?

Time to exercise the old grey matter! Permabond’s technical advisors and development chemists are often asked by customers for data regarding Tg (glass transition temperature) measurements of our engineering adhesive materials.

What is Tg, how is it measured?

Glass Transition Temperature (Tg) is the point at which a material alters state – going from a glass-like rigid solid to a more flexible, rubbery compound.  Tg is normally measured on a Differential Scanning Calorimeter (DSC) piece of whizzy equipment.  The DSC automatically plots a chart which you can calculate the approximate Tg from. The glass transition takes place over a range on the graph so doesn’t automatically come out with an exact figure, but a skilled eye can interpret a good graph and may be able to make a qualified estimate if a specific figure is required.

Why bother?!

Good question, but it is an important measurement if a conscientious design engineer wants to assess how his adhesive bonded joint is going to operate within a service temperature range and how it could have an impact on other materials he or she is bonding.  For example, an automotive application bonding dissimilar materials – say some type of metal to a composite, the application is likely to see temperatures of below freezing during wintery conditions and then in summertime temperatures , a sealed cabin can reach in excess of 60°C, not forgetting to mention much higher temperatures in the engine compartment.   It is important automotive adhesive can withstand these extremes whilst still offering high strength, impact and vibration resistance.

Will an adhesive work above its Tg temperature? The glass transition temperature of epoxy adhesive looks fairly low?

Adhesives such as epoxies and other structural adhesives continue to work well above the Tg temperature. They become slightly more flexible which actually improves certain features such as impact and vibration resistance as well as slightly higher peel strength.  Epoxies do not actually melt, they are thermosetting resins (as opposed to thermoplastic which melt at high temperatures).  Low temperatures and freezing conditions make adhesives more brittle. It is important to remember this if you are bonding dissimilar materials with different coefficients of thermal expansion – in this instance a toughened, more flexible adhesive is recommended.  Interestingly, the coefficient of thermal expansion (CTE) of the adhesive is greater above the Tg and lower below the Tg. The other interesting point to mention (if you’re into this) is that you can increase the Tg of a room temperature curing two part epoxy adhesive by carrying out a heat cure on the product instead (although this may not be suitable for fast-setting epoxies or if curing in bulk due to exothermic reaction – in this instance a secondary heat cure could achieve a higher Tg).

Why aren’t the figures on all the datasheets?

Most engineers need to run their own tests matching their individual requirements and on their specific substrate materials to see whether or not the adhesive is strong enough and if it can survive accelerated ageing tests, thermal shock and thermal cycling tests. However, some engineers do request this data from us – in which case we have the equipment to perform the tests if necessary.

For further assistance, please feel free to contact the Permabond technical team.