Testing, 326: heat distortion temperature

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 ). 

Four modes of characterization are of interest chemical analyses, ie, quaUtative and quantitative analyses of all components mechanical characterization, ie, tensile and impact testing morphology of the mbber phase and rheology at a range of shear rates. Other properties measured are stress crack resistance, heat distortion temperatures, flammabiUty, creep, etc, depending on the particular appHcation (239). 

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.)... 

Whilst the Vicat test usually gives the higher values the differences are quite modest with many polymers (e.g. those of types A, B and C). For example, in the case of the polycarbonate of bis-phenol A (Chapter 20) the heat distortion temperatures are 135-140°C and 140-146°C for the high and low stress levels respectively and the Vicat softening point is about 165°C. In the case of an acetal homopolymer the temperatures are 100, 170 and 185°C respectively. With nylon 66 the two ASTM heat distortion tests give values as different as 75 and 200°C. A low-density polyethylene may have a Vicat temperature of 90°C but a heat distortion temperature below normal ambient temperatures. 

At the risk of oversimplification it might be said that the Vicat test gives a measure of the temperature at which a material loses its form stability whilst the higher stress level heat distortion temperature (1.82 MPa) test provides a measure of the temperature at which a material loses its load-bearing capacity. The lower stress (0.45 MPa) heat distortion temperature test gives some rather intermediate figures and it is perhaps not surprising that it is today less often quoted than the other two tests. 

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. 

Another approach to increase the heat distortion temperature is to produce cocondensates of bisphenol A with bishydroxyphenyl fluorene. Some variations of this copolymer had heat distortion temperatures in excess of 200°C and with the potential to be produced at lower cost than such temperature-resistant thermoplastics as polysulphones and polyetherimides. Plans to develop this material were however abandoned when it was found, during trials of test materials, that workers developed skin rashes said to be similar to those encountered on contact with poison ivy. 

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 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 (V2 in.) deep plastic test bar is mounted on supports 10.16 cm (4 in.) apart and loaded as a beam. A bending stress of either 66 psi or 264 psi (455 gPa or 1,820 gPa) is applied at the center of the span. 

TDI isomers, 210 Tear strength tests, 242-243 TEDA. See Triethylene diamine (TEDA) Telechelic oligomers, 456, 457 copolymerization of, 453-454 Telechelics, from polybutadiene, 456-459 TEM technique, 163-164 Temperature, polyamide shear modulus and, 138. See also /3-transition temperature (7)>) Brill temperature Deblocking temperatures //-transition temperature (Ty) Glass transition temperature (7) ) Heat deflection temperature (HDT) Heat distortion temperature (HDT) High-temperature entries Low-temperature entries Melting temperature (Fm) Modulu s - temperature relationship Thermal entries Tensile strength, 3, 242 TEOS. See Tetraethoxysilane (TEOS)... 

Nylon-6-clay nanocomposites were also prepared by melt intercalation process [49]. Mechanical and thermal testing revealed that the properties of Nylon-6-clay nanocomposites are superior to Nylon. The tensile strength, flexural strength, and notched Izod impact strength are similar for both melt intercalation and in sim polymerization methods. However, the heat distortion temperature is low (112°C) for melt intercalated Nylon-6-nanocomposite, compared to 152°C for nanocomposite prepared via in situ polymerization [33]. 

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. 

Except for a lew thermoset materials, most plastics soften at some temperatures, At the softening or heat distortion temperature, plastics become easily deformahle and tend to lose their shape and deform quickly under a Load. Above the heat distortion temperature, rigid amorphous plastics become useless as structural materials. Thus the heat distortion test, which defines The approximate upper temperature at which the material can be Safely used, is an important test (4,5.7.24). As expected, lor amorphous materials the heat distortion temperature is closely related to the glass transition temperature, hut tor highly crystalline polymers the heat distortion temperature is generally considerably higher than the glass transition temperature. Fillers also often raise the heat distortion test well above... 

Finally, most formulations successful in meeting other criteria have been tested for heat distortion temperature as a final criterion for judgment. A commercial device (Tinius-Olsen) has been employed for these tests. 

Analysts. It has been our objective to determine criteria for resin, curative or formulation which would permit prediction of sucess prior to potting tests. Many tests, both chemical and physical in nature, have been executed on commercial resin systems. These have included high pressure liquid chromatography (HPLC), Fourier Transform infrared spectrometry (FTIR), gel permeation chromatography, compressive tensile tests by Instron on resin plaques in air and under various aqueous solutions and heat distortion temperature. 

FTIR has shown the close similarity of most resins based on Bis-Phenol A and has helped narrow the focus of development on the curative as the principal contributor to successful formulation. For present applications oligomeric polyamide amines appear successful in meeting present criteria. However, the only objective analysis of cured resin to date exhibiting a correlation of measured value with success in creep resistance as well as adhesion is heat distortion temperature. The following presents a correlation of heat distortion temperatures and adhesion for several formulations tested. In most cases, pass/fail criteria was based on the majority of six samples tested. 

Conversely, ASA itself may serve as a heat distortion improver additive for poly(vinyl chloride) (PVC) (36). The increase of the heat distortion temperature is linearly dependent on the amount of ASA added. Therefore, it is easy to add just the amount needed without doing a lot of preliminary testing with various formulations. ASA can be used in a blend with PVC. Another approach is the coextrusion of the ASA with PVC in such a way that only ASA is exposed to high temperatures. 

It was also found that the tensile heat distortion temperatures of films containing only a few mole per cent of these units were considerably higher than those found for bisphenol A polycarbonate. X-ray diffraction studies made on the test samples used in the tensile heat distortion apparatus could not demonstrate an increase in crystallinity of the samples. Only a slight indication of increase of orientation was apparent. Glass transition temperatures measured by the refracto-metric method were considerably lower than the heat distortion temperatures. 

Heat Distortion Temperature. Heat distortion temperatures were obtained in flexure, using a load of 264 psig on molded bars 5 X % X % inches. The heat distortion temperatures were taken as the temperatures at which test specimens had deformed 0.010 inch, where a heating rate of 2°C per minute was applied. 

The heat distortion test is similar to a creep test, except that the temperature is increased at a uniform rate rather than being kept constant. At the softening or heat distortion temperature the polymer begins to deform at a rapid rate over a narrow temperature interval. In the heat distortion test ISO-HDT-A or ISO-HDT-B a standard bar is loaded... 

Heat Distortion Test. Two remaining bars from the cleavage tests are used to determine heat distortion temperature in accordance with ASTM D-621 test methods. These thermal-mechanical tests are necessary to determine whether a brittle resin has been truly toughened or whether it has been merely flexibilized. The morphology of the resin also effectively describes a true toughening situation and can aid immeasurably in explaining departures from true toughening. 

Heat distortion temperatures (HDTs) are widely used as design criteria for polymeric articles. These are temperatures at which specimens with particular dimensions distort a given amount under specified loads loads and deformations. Various test methods, such as ASTM D648, are described in standards compilations. Because of the stress applied during the test, the HDT of a polymer is invariably higher than its Tg. 

ASTM D1637-61, "Test for Tensile Heat Distortion Temperature of Plastic Sheeting f Philadelphia (1970 discontinued in 1990). 

Two particular test methods have become very widely used. These are the Vicat softening point test and the test widely known as the heat distortion temperature test (also called the deflection temperature under load test). In the Vicat softening point test a sample of polymer is heated at a specified rate temperature increase and the temperature is noted at which a needle of specified dimensions indents into the polymer a specified distance under a specified load. 

The different polymers maybe classified into several groups according to the element present as shown in Table 3.9. The focus of identification may be further narrowed down on the basis of other preliminary observations, e.g., fusibifity or otherwise, melting point or range, heat distortion temperature, flame tests... 

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