Temperature Under Load

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.). 

This test is only a guide. It represents a method that could be correlated to product designs (see Chapters 4—5), but, as with most other tests conducted on test specimens and not on a finished product, it is just a guide. In this test, if the specimen contains internal stresses the value will be lower than a specimen with no stresses. In fact, the test can be used to determine the degree of stress. Since a stress and the deflection for a certain depth of test bar are specified, this test may be thought of as establishing the temperature at which the flexural modulus decreases to particular values 35,000 psi (240 MPa) at 66 psi load stress, and 140,800 psi (971 MPa) at 264 psi. 

For applications having only moderate thermal requirements, thermal decomposition may not be an important consideration. However, if the product requires dimensional stability at high temperatures, it is possible that its service temperature or processing temperature may approach its temperature of decomposition (Table 2-20). A plastic s decomposition temperature is largely determined by the elements and their bonding within the molecular stmctures as well as the characteristics of additives, fillers, and reinforcements that may be in the compounds (see Table 2-13). 

Aging at elevated temperatures typically involves exposing test specimens or products at different temperatures for different extended times (see chapters 3-6). Tests are performed at room or die respective testing temperatures for whatever mechanical, physical, or electrical property is of interest. These tests of aging can be used as a measure of thermal stability in design as is done with other materials. 

The Underwriters Laboratories (UL) tests are recognized by various industries to provide continuous temperature ratings, particularly in electrical applications. These ratings include separate listings for electrical properties, mechanical properties (including impact), and mechanical properties without impact. The temperature index is important if the final product has to receive UL recognition (see Chapters 3-5). 

In this test, if the specimen contains internal stresses the value will be lower than a specimen with no stresses. In fact, the test can 

Polyphenyls Polyphenylene oxide Decompose at 530°C (986°F) infusible, insoluble polymers. Decomposes close to 500°C (932°F) heat cures above 150° C (302°F) to elastomer usable heat range —135-185 C (—211-365°F). 

Polyphenylene sulfide Melts at 270-315°C (578-599°F) crosslinked polymer stable to 450°C (842°F) in air adhesive and laminating applications. 

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TetrabromobisphenoIA. Tetrabromobisphenol A [79-94-7] (TBBPA) is the largest volume bromiaated flame retardant. TBBPA is prepared by bromination of bisphenol A under a variety of conditions. When the bromination is carried out ia methanol, methyl bromide [74-80-9] is produced as a coproduct (37). If hydrogen peroxide is used to oxidize the hydrogen bromide [10035-10-6] HBr, produced back to bromine, methyl bromide is not coproduced (38). TBBPA is used both as an additive and as a reactive flame retardant. It is used as an additive primarily ia ABS systems, la ABS, TBBPA is probably the largest volume flame retardant used, and because of its relatively low cost is the most cost-effective flame retardant. In ABS it provides high flow and good impact properties. These benefits come at the expense of distortion temperature under load (DTUL) (39). DTUL is a measure of the use temperature of a polymer. TBBPA is more uv stable than decabrom and uv stable ABS resias based oa TBBPA are produced commercially. 

To convert J/m to ftlbf/in., divide by 53.38. Deflection temperature under load. 

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.)... 

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 ). 

Notable among the alternative materials are the MBS polymers, in which methyl methacrylate and styrene are grafted on to the polybutadiene backbone. This has resulted in two clear-cut advantages over ABS. The polymers could be made with high clarity and they had better resistance to discolouration in the presence of ultraviolet light. Disadvantages of MBS systems are that they have lower tensile strength and heat deflection temperature under load. 

Such reinforcement leads to a substantial increase in tensile strength, modulus, hardness, creep resistance, ASTM deflection temperature under load and a sharply reduced coefficient of expansion. Typical figures are shown in Table 18.7. 

As with the aliphatic polyamides, the heat deflection temperature (under 1.82 MPa load) of about 96°C is similar to the figure for the Tg. As a result there is little demand for unfilled polymer, and commercial polymers are normally filled. The inclusion of 30-50% glass fibre brings the heat deflection temperature under load into the range 217-231°C, which is very close to the crystalline melting point. This is in accord with the common observation that with many crystalline polymers the deflection temperature (1.82 MPa load) of unfilled material is close to the Tg and that of glass-filled material is close to the T. ... 

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. 

Polycarbonates with superior notched impact strength, made by reacting bisphenol A, bis-phenol S and phosgene, were introduced in 1980 (Merlon T). These copolymers have a better impact strength at low temperatures than conventional polycarbonate, with little or no sacrifice in transparency. These co-carbonate polymers are also less notch sensitive and, unlike for the standard bis-phenol A polymer, the notched impact strength is almost independent of specimen thickness. Impact resistance increases with increase in the bis-phenol S component in the polymer feed. Whilst tensile and flexural properties are similar to those of the bis-phenol A polycarbonate, the polyco-carbonates have a slightly lower deflection temperature under load of about 126°C at 1.81 MPa loading. 

Being irregular in structure the polymer is amorphous and gives products of high clarity. In spite of the presence of the heterocyclic ring the deflection temperature under load is as low as that of the poly(butylene terephthalates) and is also slightly softer. Some typical properties are given in Table 25.9. 

The heat deflection temperature under load is equal to that of a polysulphone. 

Deflection temperature under load (1.8 MPa) Coefficient of linear expansion (-30°C to +30°C Specific heat 20-300°C >300°C... 

Heat distortion temperature (deflection temperature under load) of cured... 

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. 

Dyes can also have an undesirable effect on properties. Because they dissolve in the matrix, they can sometimes have a plasticizing effect. This will reduce the material s tensile and flexural strength as well as the HDT (heat deflection temperature under load). The plasticizing effect of dyes can also influence the way they process... 

The most commonly used are the measurement of the Vicat softening point (ASTM D1525, DIN 53460, ISO 306) and the deflection temperature under load (DTUL, ASTM D648). Both measurements monitor the modulus change with temperature, and determine an endpoint when a macroscopic change can... 

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