Heat Distortion Temperature HDT

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Heat distortion temperature HDT

General

The objective of the heat distortion temperature HDT method is to quantitatively characterize the heat resistance of a plastic. For this purpose, the heat distortion temperature HDT is determined, which, for methodological reasons, does not have to be quantitatively the same as the heat distortion temperatures determined by other experimental methods, such as the Vicat softening temperature [1].

Methodological principles of HDT testing

Heat distortion temperature HDT testing is performed according to ISO 75-1 to 3 [2]. In this test (see Fig. 1), the specimen is loaded according to the three-point bending principle, whereby the bending moment is not constant over the loaded specimen length, but increases from the support points to the point of application of the individual load. In the case of plastics and hard rubber (for example Ebonite) according to ISO 75-2, the concentrated load is dimensioned so that a maximum bending stress of 1.80 MPa (method A), 0.45 MPa (method B) or 8.0 MPa (method C) is present in the test specimen. Heating is carried out at a heating rate of 2 K min^-1, with air or silicone oil usually being used as the heat transfer medium.

Fig. 1:

Measuring set-up for determining the heat distortion temperature HDT

Thermoplastics can be tested in flatwise or edgewise arrangement. Different specimen geometries and arrangements are used for this purpose. For flatwise testing, test specimens with dimensions of 80 x 10 x 4 mm³ are used. In this case, the support span is 64 mm. The edgewise test is performed with a support span of 100 mm using specimens of 120 mm length, 9.8 mm to 15 mm width and 3 to 4.2 mm thickness.

The total test force F to be applied can be calculated for the corresponding method according to the equation

\sigma _{f}\,=\,{\frac {3\cdot F\cdot l}{2\cdot b\cdot h^{2}}}

mit

$\sigma$ _{_f}		flexural strength at tree-point-bend test
L		support span
b		specimen with
h		specimen thickness

whereby the arrangement of the test specimen on the support (flatwise or edgewise) must be taken into account in each case. To calculate the mass of additional weights to achieve the total test force, device-specific factors must be taken into account. The HDT value to be recorded is the temperature at which the test specimen reaches a standard deflection specified in tabular form in the standard. This standard deflection corresponds to a peripheral fibre strain of 0.2 %.

Examples of HDT temperatures

**Table**: Heat distortion temperature HDT in °C for several unreinforced and reinforced plastics and thermosets [3, 4]
Materials	HDT (°C)
	A	B	C
Thermoplastics unreinforced
PE-HD	45
PP	55	85
POM	100
PA 6	70	170	65
PET	70	75
PBT	60	150
PEEK	152
PC	128	136
PMMA	95	100
PS	68	80
SAN	98	103
ABS	90	93
PUR	47	86
Thermoplastics reinforced
PET + 15 M.-% GF	192	231
PET + 30 M.-% GF	210	240
PET + 40 M.-% GF	220	242
PBT + 15 M.-% GF	205	220
PBT + 30 M.-% GF	210	220
PP + 20 M.-% Talcum	70	120
PP + 40 M.-% Talcum	75	125
Thermosets
Phenolic resin	165	215	145
Melamine resin	160	200	125
UP resin (Standard type)	55
Epoxy resin	100

The testing of high-strength thermoset laminates and long-fibre-reinforced plastics according to ISO 75-3 is performed exclusively flatwise with a support span s of 60 to 210 mm (s = 30 x b). The specimen length is derived from the support span with L ≥ s + 10 mm. The thickness is 2 to 7 mm and the specimen width 9.8 to 12.8 mm. In contrast to the plastics and hard rubber, the load on the high-strength thermoset laminates and long-fibre-reinforced plastics is not fixed, but is one-tenth of the flexural strength. This makes it possible to apply the method to materials with a wide range of strength and flexural moduli. The test specimens made of thermoset laminates or long-fibre-reinforced plastics are subjected to a bending stress corresponding to 10 % of a fixed or measured flexural strength. The HDT value shall be recorded as the temperature at which the specimen reaches a standard deflection calculated from the specimen height and corresponding to a peripheral fibre strain of 0.1%.

A comprehensive literature analysis on heat distortion temperatures HDT for different plastics is given in [5].

[1]	Grellmann, W., Seidler, S. (Hrsg.): Polymer Testing. Carl Hanser Munich (2022) 3. Edition, p. 570 – 573 (ISBN 978-1-56990-806-8; see AMK-Library under A 22)
[2]	ISO 75: Plastics – Determination of Temperature of Deflection under Load
	Part 1 (2020-02): General Test Method
	Part 2 (2013-04): Plastics and Ebonite
	Part 3 (2025-05): High-strength Thermosetting Laminates and Long-fibre-reinforced Plastics (Draft)
[3]	Carlowitz, B.: Tabellarische Übersicht über die Prüfung von Kunststoffen. Giesel Verlag für Publizität, Isernhagen (1992), ISBN 978-3980294201 (see AMK-Library under C 9)
[4]	Campus® Datenbank: http://www.campusplastics.com (access: 09.04.2025)
[5]	Kotter, I., Grellmann, W.: Vicat Softening Temperature and Heat Distortion Temperature. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume VIII/6A3, Springer Verlag, Berlin (2014) S. 62–75 (ISBN 978-3-642-55165-9; see AMK-Library under A 16)

General

Methodological principles of HDT testing

Examples of HDT temperatures

See also