Glass Transition Temperature (Tg)

Glass transition temperature (Tg) is the critical temperature at which a polymer material (such as plastics, rubber, resins) transitions from a glassy state to a rubbery state (or vice versa). In foreign trade, this term is commonly found in specifications and contract technical annexes for products such as chemicals, plastic raw materials, adhesives, and coatings. Usage scenarios include: when customers request Tg values in inquiries to assess material heat resistance, flexibility, or processing conditions; and in quality inspection clauses where Tg is stipulated as an acceptance criterion. Note: Tg is not the melting point (Tm), nor the heat deflection temperature (HDT), and should not be confused; different test methods (DSC, DMA, TMA) yield different results, so the test standard (e.g., ASTM E1356 or ISO 11357) must be specified in the contract. Difference from 'softening point': softening point is more related to Vicat or ring-and-ball methods, while Tg is a thermodynamic transition. Foreign trade practitioners should ensure that the Tg data provided by suppliers matches customer requirements to avoid claims caused by different test methods.