Heat deflection temperature under load test

Two particular test methods have become very widely used. They are the Vicat softening point test (VSP test) and the heat deflection temperature under load test (HDT test) (which is also widely known by the earlier name of heat distortion temperature test). In the Vicat test a sample of the plastics material is heated at a specified rate of temperature increase and the temperature is noted at which a needle of specified dimensions indents into the material a specified distance under a specified load. In the most common method (method A) a load of ION is used, the needle indentor has a cross-sectional area of 1 mm, the specified penetration distance is 1 mm and the rate of temperature rise is 50°C per hour. For details see the relevant standards (ISO 306 BS 2782 method 120 ASTM D1525 and DIN 53460). (ISO 306 describes two methods, method A with a load of ION and method B with a load of SON, each with two possible rates of temperature rise, 50°C/h and 120°C/h. This results in ISO values quoted as A50, A120, B50 or B120. Many of the results quoted in this book predate the ISO standard and unless otherwise stated may be assumed to correspond to A50.)... 

Two particular test methods have become very widely used. They are the Vicat softening point test (VSP test) and the heat deflection temperature under load test (HDT test) (which is also widely known by the earlier name of heat distortion temperature test). These two tests were discussed earlier in Sections 1.3.2.1 and 1.3.2.2. 

In the deflection temperature under load test (heat distortion temperature test) the temperature is noted at which a bar of material subjected to a three-point bending stress is deformed a specified amount. The load (F) applied to the sample will vary with the thickness (t) and width (tv) of the samples and is determined by the maximum stress specified at the mid-point of the beam (P) which may be either 0.45 MPa (661bf/in ) or 1.82 MPa (264Ibf/in ). 

The ASTM heat distortion temperature (deflection temperature under load) test may be used to characterise a resin. Resins must, however, be compared using identical hardeners and curing conditions. 

The strength properties more often specified for plastics materials are (1) tensile strength and elongation, (2) flexural strength, (3) Izod and Gardner impact, and (4) heat deflection temperature under load. Our purpose here is not to describe each test in detail but to point out some of the known effects that colorants and other formulation ingredients can have on these properties. Table 22.1 lists the ISO and ASTM test methods for most of the physical properties, and ref. 1 (pp. 7-112) describes each of the methods in detail. Table 22.2 lists typical values of the above cite four properties for selected thermoplastics. 

To enhance the resistance to heat softening his-phenol A is substituted by a stiffer molecule. Conventional bis-phenol A polycarbonates have lower heat distortion temperatures (deflection temperatures under load) than some of the somewhat newer aromatic thermoplastics described in the next chapter, such as the polysulphones. In 1979 a polycarbonate in which the bis-phenol A was replaced by tetramethylbis-phenol A was test marketed. This material had a Vicat softening point of 196 C, excellent resistance to hydrolysis, excellent resistance to tracking and a low density of about l.lg/cm-. Such improvements were obtained at the expense of impact strength and resistance to stress cracking. 

HDT heat deflection temperature under flexural load (1820 kPa) Heat distortion test (HDT) ASTM D-648-72 CEAST 6005... 

The sample is deflected to produce a defined surface stress and then placed in a temperature bath at 23°C. The force is allowed to act for 5 min, and then the temperature is raised at a uniform rate of 2°C/min. The deflection of the test bar is continuously observed the temperature at which the deflection reaches 0.010 in. is reported as the heat deflection temperature. This is also known as the heat distortion temperature or the deflection temperature under load (DTUL). 

The deflection temperature under load (DTUL), also called the heat distortion temperature (HDT) of a plastic is a method to guide or assess its load-bearing capacity at an elevated temperature. Details on the method of testing are given in ASTM D648. Basically, a 1.27-cm (i-in.)-deep plastic test bar is mounted on supports 10.16 cm (4 in.) apart and loaded as a beam (see Fig. 2-21). A bending stress of either 66 psi or 264 psi (455 g Pa or 1,820 g Pa) is applied at the center of the span. The test is conducted in a bath of oil, with the temperature increased at a constant rate of 2 C per minute. The DTUL is the temperature at which the sample attains a deflection of 0.0254 cm (0.010 in.). 

ISO 75 An International Organization for Standardization (ISO) standard test method for determination of heat deflection temperature (HDT) and deflection temperature under load (DTUL). HDT is a relative measure of a materials ability to perform for a short time at elevated temperatures while supporting a load. The test measures the effect of temperature on stiffiiess a standard test specimen is given a defined surface stress and the temperature is raised at a uniform rate. Alternate test methods for HDT and DTUL are DIN 53461 and ASTM D648. 

ASTM D 648 describes a method for determining the heat deflection temperature (HDT) or deflection temperature under load (DTUL). With the trend toward globalization, this method is reflected in and refined by ISO 75. Both tests seek to define the temperature at which a given degree of bending is achieved in a sample placed under a fixed flexural stress. The apparatus used to conduct the test is shown (Figure 1). The working portion of the instrument is immersed in an oil-based fluid, which is used as the heat transfer medium. A specimen is placed in a 3-point bend fixture and the desired stress... 

We use a variant of flexural testing to measure a sample s heat distortion temperature. In this test, we place the sample in a three point bending fixture, as shown in Fig. 8.6 b), and apply a load sufficient to generate a standard stress within it. We then ramp the temperature of the sample at a fixed rate and note the temperature at which the beam deflects by a specified amount. This test is very useful when selecting polymers for engineering applications that are used under severe conditions, such as under the hoods of automobiles or as gears in many small appliances or inside power tools where heat tends to accumulate. 

Heat deflection temperature The temperature at which a standard test bar (ASTM D648) deflects 0.010 in., under a stated load of either 66 or 264 psi. 

A very popular test method for measuring the softening point of plastics is the temperature of deflection under load test, also known as the heat distortion or HDT test, standardized in ISO 75 [138]. The standard is now published in three parts. Part 1 covering general principles. Part 2 for plastics and ebonite, and Part 3 for reinforced plastics. Part 3 will not be considered further in this chapter. 

---