Introduction to Polymer Science and Technology Thermal properties

7.5 Determination of softening temperature

Designers and engineers need to know the maximum-use temperature, i.e., the extent of thermal stability, when selecting polymers for engineering applications. This enables them to establish the maximum temperature at which a polymer can be used as a rigid material. It is already covered that Youngs modulus is a material property that indicates rigidity or stiffness and its measurement over a temperature range becomes a guide for material thermal stability by enabling the identification of the glass-transition temperature.

The softening temperature for a polymer is closely related to T for amorphous thermoplastics and thermosetting polymers and to T_m for crystalline polymers. Many polymers exhibit softening at a temperature between T and T_m, depending on the degree of crystallinity, presence of additional intermolecular force and also the method used to determine the softening temperature. The methods popularly used to establish the upper limit of safe operating temperatures for products fabricated from a given polymeric material include heat distortion temperature (HDT) or the deflection temperature under load (DTUL) and the Vicat softening temperature. The HDT/DTUL is identified as the temperature at which a standard test bar deflects a specified amount when loaded in 3-point bending, and therefore, the property can be deemed to be a mechanical-thermal property. The Vicat softening point is the temperature at which a flat-ended needle, under load, penetrates a heated sample of polymer to a certain depth. Maximum continuous use temperatureis another parameter that is assigned to polymers as an indicator of softening temperature. This is based upon the Underwriters Laboratories (UL) rating for long-term continuous use, and is defined as the temperature at which the room temperature tensile strength of the material reduces to half its value, in the absence of any applied external stresses, as a result of high-temperature exposure for 100,000 hours. As a rule of thumb, a 10 °C increase in temperature is equivalent to a decade increase in time (Tripathy 2002, p27). For example, the maximum continuous use temperature for PP is 100 °C, which would be equivalent to 10,000 h at 110 °C or 10⁶ h at 90 °C. Therefore, certain grades of PP are suitable for short-term exposure to high temperatures at 140 °C and are used as steam-sterilisable hospital ware.

7.5.1 Heat distortion temperature

Heat distortion or deflection temperature is used to determine short-term heat resistance. It distinguishes between materials that are able to support light loads at high temperatures and those that cannot and lose their rigidity over a narrow temperature range. The standard methods for conducting HDT include:

ISO 75-1:2004 Plastics - Determination of temperature of deflection under load - Part 1: General test method ISO 75-2:2004 Plastics - Determination of temperature of deflection under load - Part 2: Plastics and ebonite

ISO 75-3:2004 Plastics - Determination of temperature of deflection under load - Part 3: High-strength thermosetting laminates and long-fibre-reinforced plastics

ASTM D648 - 07 Standard test method for deflection temperature of plastics under flexural load in the edgewise position

The test procedure consists of positioning the specimen under the deflection measuring device for 3-point bending, see Figure 7.29. A load of 0.45 MPa or 1.80 MPa or 8.00 MPa is placed on the specimen depending on the test method

Introduction to Polymer Science and Technology

Thermal properties

followed. The specimen is then lowered into a silicone oil bath and heated at 2 °C/ min until a certain deflection is reached at the centre of the support span. The level of deflection is 0.25 mm for ASTM, 0.34 mm for ISO flatwise and 0.32 mm for ISO edgewise testing.

Figure 7.29 HDT test set up

The temperature at which the prescribed deflection occurs is taken to be the HDT. The high load of 8 MPa is suitable for testing TPs with very high softening temperatures, otherwise under the lower load of 1.8 MPa the test will last much longer and the HDT can be so high as to cause the decomposition of the heating oil (the silicone oil, normally used in these tests, is prone to decomposing rapidly).

In ISO standard methods, the edgewise testing (where the loading nose is placed across the thickness) uses a test bar of 120 x 10 x 4 mm with a 100 mm support span length and the flatwise testing (where the load is applied across the width) uses a test bar of 80 x 10 x 4 mm with a 64 mm support span length, similar to the normal flexural test. The ASTM method only describes an edgewise test for 5" x Vi" x V4" specimens. The HDT test is essentially a flexural test, and therefore the equations for bending (see Section 6.3) apply.

The HDT test is used to determine short-term heat resistance and should not be used alone for product design to establish the upper limit of safe operating temperatures for products for long term performance. Other factors such as the time of exposure to an elevated temperature, the rate of temperature increase, and the part geometry all affect the performance. Other factors include polymer type and structure, filler/reinforcement type and loading levels, fibre orientation, oxidative stability and moulded-in residual stresses; all of which may cause the actual maximum-use temperature to be significantly different than the HDT. Accordingly it should be used together with other thermal behaviour indicators for guidance.

The techniques such as the DMTA, in comparison, produce a greater insight to the material behaviour. For instance, the HDT values for short and long glass-fibre reinforced polybutylene terephthalate (PBT) polyester are quite similar, approximately 210 °C at 1.8 MPa, however, this does not reveal the actual material rigidity profile across the temperature

Introduction to Polymer Science and Technology

Thermal properties

range as shown by the DMTA traces in Figure 7.30: as can be seen, the long-fibre reinforced PBT alternative shows a greater elastic modulus in the glassy region, which is maintained above the T up to the HDT.

100 150200

temperature, °C

Figure 7.30Storage modulus vs. temperature for glass-fibre reinforced PBT:
( ) long fibre; (____) short fibre

It is, however, the case that, if the differences in the elastic moduli of the materials are known, then HDT is a good indicator of the temperature where to expect a collapse in material stiffness irrespective of the stiffness characteristics at temperatures below HDT.

Date: 2015-12-11; view: 911