Heat deflection temperature defined

From the practical point of view, the glass transition is a key property since it corresponds to the short-term ceiling temperature above which there is a catastrophic softening of the material. For amorphous polymers in general, and thus for thermosets, one can consider that the glass transition temperature, Tg, is related to the conventional heat deflection temperature (HDT) (usually, HDT is 10-15°C below Tg, depending on the applied stress and the criterion selected to define Tg). 

At elevated temperatures al polymers soften, dependent on their glass-rubber transition points, Tg, and/of their melting points, Tm. These temperatures limit the practical use of plastics. To characterize the softening behaviour, in practice various types of standard tests are being carried out, resulting in values for the softening temperature , defined in different ways. The values mostly used are the ISO Heat Deflection Temperature (HDT) and the Vicat Softening Temperature (VST or... 

Define the temperature failure criteria for the material. The producer determines experimentally the maximum service temperature his material can achieve and still provide acceptable performance. The producer determines the heat deflection temperature (ASTM D 648), Vicat Softening Temperature (ASTM D 1525), Coefficient of Thermal Expansion (ASTM D 696) or other appropriate quantitative measure of material s performance under heat. The producer then adds a suitable safety factor to the temperature determined to cause failure. 

Define the temperature failure criteria for the material. The experimental formulas were blended and extruded. The extruded products were measured for heat deflection temperature using ASTM D 648 as a guideline. Multiple measurements at various heating rates were conducted. An appropriate engineering safety factor was applied to the data. A critical temperature failure criteria was defined as 70°C for these particular experimental formulas. 70°C was considered the maximum sustained temperature the extrusions could withstand and still provide acceptable engineering performance. 

Heat Deflection Temperature The heat deflection temperature is defined as the temperature at which a specimen deflects a specified amount under specific heat and load conditions. 

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). 

Heat deflection temperature is defined as the temperature at which a standard test bar deflects by a standard amount under a standard load. Generally loads of 0.45 and 1.80 MPa are used. The values of heat deflection temperature of various plastics are compared at different loads in Table 14. It can be seen Ifom the table that the heat deflection temperature of PP is higher than the PE but, it is outranked by more expensive engineering thermoplastics. 

Heat Deflection Temperature n The temperature at which a material specimen (standard bar) is deflected by a certain degree under specified load. At this temperature, a material achieves a specific modulus which is defined by the applied stress and the sample geometry. Also called heat distortion temperature, heat distortion point, heat deflection point, deflection temperature under load, DTUL, tensile heat distortion temperature, HDT. See also ISO 75. 

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... 

The dimensional stability of a plastic when exposed to heat is another crucial property. The Vicat method of determining a plastic s softening point and the heat deflection temperature (HDT) method are two ways of rapidly determining which polymers have suitable characteristic values. In both methods the test specimens are heated under defined load with a certain heating rate while deformation is measured. Vicat softening temperature and HDT are each defined as the temperatures at which deformation reaches a given value [8]. 

A good applications-oriented measure of the use temperature for a ma-teral is the heat distortion or heat deflection test (HDT). The HDT is described by ASTM-D648 as the temperature at which a sample of defined dimensions (5 X Vi X Vs (or Va) in.) deflects under a flexural load of 66 or 264 psi placed at its center. In case of a largely amorphous polymer, the HDT temperature is typically slightly (10 to 20 °C) lower than the Tg as determined by DSC or DTA, whereas with more-crystalline polymers, it more closely correlates with the Tm. The HDT temperature is a useful indicator of the temperature limits for structural (load-supporting) applications. A loaded cantilever beam is used in another heat deflection test called the Martens method. 

One of the more important of the practical tests is the heat distortion temperature (HDT). The HDT is defined as the temperature at which a 100 mm length, 3 mm thick specimen bar at 1.82 MPa in a three-point bending mode deflects 0.25 mm. Young s modulus at the HDT is 0.75 Gpa (133,134). For unfllled polymers, both the Vicat and the HDT tests usually record a temperature just above the glass transition temperature, or for melting conditions, just below the temperature of flnal disappearance of crystallinity. For polymer blends, both the Vicat and the HDT will tend to reflect the properties of the continuous phase. If the polymer contains filler which raises the modulus, the HDT will be somewhat increased. 

Transition temperatures that characterize the stmcture and behavior of polymers have already been dealt with some length. From a practical point of view, limiting temperatures for use is also of interest. One should differentiate between a statistical value derived from use data without material damage and a standard test under prescribed conditions, namely, heat distortion or deflection. In the latter, the temperature is measured wherein the samples undergo a definite deformation under a defined load (usually 264 psi). This temperature is taken to be an upper limit for use of the material without the danger of warping. This value obviously depends on the load (inversely affected). Thermal endurance can also be expressed by time and temperature data that affect mechanical and electrical properties. Data verify that for most polymers, the upper limiting useful temperature is rather low (60 -85 C),... 

So-called deflection elements (Figure 2.44) or fountain cooling systems are used if the aforementioned elements cannot be used due to insufficient space. The temperature control medium is introduced into the mold core using a transverse baffle or tube, and the heat of the injection is removed over a defined sequence. 

One commonly referenced material property is known as the HOT This is a standard test where a specimen of a material is subjected to a defined load and then slowly heated while measuring the deflection. As the material gets warmer, it becomes less stiff and the deflection will increase. Once a defined amount of deflection is achieved, the test is complete and the temperature is recorded. This temperature is the HDT for that material. Occasionally, the HDT will be measured using different loads (most data sheets will reference the load along with the measured HDT). 

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