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What is the difference between Glass Transition Temperature and Heat Deflection Temperature?

Both processes measure the same thing, the point where the cured resin goes through a change in its molecular structure.  At this point mechanical properties decrease at an increasing rate and the coefficient of thermal expansion increases.

Heat Deflection Temperature (HDT) is a mechanical method of measuring this point.  A cast bar measuring X X 5 is suspended between two points 4 apart.  A load is applied halfway between the two suspension points.  The entire apparatus is submerged in an oil bath, with the temperature of the oil raised at a set rate.  When the cast bar deflects 0.010-inches under the load, the oil temperature is recorded as the HDT.  This number is commonly reported at two load levels, 64 psi and 264 psi.  Measuring HDT is a time consuming procedure that can tie up a lab technician for hours.

Note: Some companies report high HDT numbers by testing laminates instead of cast bars.  Fiber reinforcement keeps the bar from deflecting even after the resin system has passed its HDT.  These numbers do not accurately represent the capacity of a resin system.

Glass Transition Temperature (Tg) is a computerized method of measuring the molecular change.  There are three types of equipment used to determine Tg, by different measuring methods: Differential Scanning Calorimeter (DSC), Dynamic Mechanical Analyzer (DMA) and Thermal Mechanical Analyzer (TMA).  The chemist decides which equipment is the most accurate for a particular product.  Any of these methods are much faster than measuring HDT.

Epoxy HDT and Tg measurements are highly correlated.  Flexible urethane elastomers exhibit no meaningful HDT or Tg numbers.  Rigid urethanes are measured by both methods, with HDT giving more consistent and relevant results.