Temperature brittle

At low temperatures, all plastics tend to become rigid and brittle. This happens mainly because the mobility of polymer chains is greatly reduced. Brittle temperature is closely related to the second-order glass transition temperature (Section 4.2.1). Brittle temperature is defined as the temperature at which 50% of the specimens tested exhibit brittle failure under specified impact conditions. The brittle temperature of different grades of PP are given in Table 15. 

Due to the comparatively higher brittle temperature of PP, its use in low-temperature environments should be carefully considered and compared with other available thermoplastics. Low-temperature brittleness of some common thermoplastics is compared in Table 4. It can be seen that LDPE, HOPE, ABS and PVC offer lower brittle temperature. Impact strength at lower temperatures, e.g., -40 °C, should also be considered as a useful criterion for material selection for use under arctic conditions. 

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Tetralluoroethylene polymer has the lowest coefficient of friction of any solid. It has remarkable chemical resistance and a very low brittleness temperature ( — 100°C). Its dielectric constant and loss factor are low and stable across a broad temperature and frequency range. Its impact strength is high. 

Polycarbonates are the toughest of all thermoplastics. They are window-clear, amazingly strong and rigid, autoclavable, and nontoxic. They have a brittleness temperature of — 135°C. 

The isopropylidene linkage imparts chemical resistance, the ether linkage imparts temperature resistance, and the sulfone linkage imparts impact strength. The brittleness temperature of polysulfones is — 100°C. Polysulfones are clear, strong, nontoxic, and virtually unbreakable. They do not hydrolyze during autoclaving and are resistant to acids, bases, aqueous solutions, aliphatic hydrocarbons, and alcohols. 

Low Temperature Brittleness. Brittleness temperature is the temperature at which polyethylene becomes sufficiently brittle to break when subjected to a sudden blow. Because some polyethylene end products are used under particularly cold climates, they must be made of a polymer that has good impact resistance at low temperatures namely, polymers with high viscosity, lower density, and narrow molecular weight distribution. ASTM D746 is used for this test. 

It may be seen from Fig. 2.80 that some plastics experience the change from ductile to brittle behaviour over a relatively narrow temperature range. This permits a tough/brittle transition temperature to be quoted. In other plastics this transition is much more gradual so that it is not possible to attribute it to a single value of temperature. In these circumstances it is common to quote a Brittleness Temperature, rg(l/4). This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4 mm tip radius) equals 10 kJ/m. This temperature, when quoted, gives an indication of the temperature above which there should be no problems with impact failures. It does not mean that the material should never be used below Tb(1/4) because by definition it refers only to the sharp notch case. When the material has a blunt notch or is un-notched its behaviour may still be satisfactory well below Tb(1/4). 

The brittleness temperature does not put any lower limit on service temperature for end use products. The brittleness temperature is sometimes used in specifications. 

The replacement of 5-10% of the methyl groups on the silicon atom with phenyl groups gives polymers which exhibit superior low temperature properties. Brittleness temperatures of approximately -117 °C can be achieved compared to the approximately -70 °C for the VMQ types. The ISO designation for the phenyl modified silicones is either PMQ or PVMQ depending on whether the grade is vinyl modified. 

ISO 812 1991 Rubber, vulcanized-Determination of low-temperature brittleness ISO 974 2000 Plastics - Determination of the brittleness temperature by impact ISO 1432 1988 Rubber, vulcanized or thermoplastic - Determination of low temperature stiffening (Gehman test)... 

With an amorphous thermoplastic the brittleness temperature is about Tg, unless a secondary transition temperature Tgec occurs in the glassy region in that case the brittleness temperature may be in the neighbourhood of Tstc-... 

Gloss Temperature. Same as Brittle Point or Brittleness Temperature described in Vol 2 of Encycl, pp B302-L to B303-L... 

DYNH. ASTM brittle temperature test No. D-46-44T was used in making these measurements. Statex 125 carbon black is manufactured by Columbian Carbon Co., New York, N. Y. 

Internal Plasticizers. These are actually a part of the polymer molecule-e.g., a second monomer is copolymerized into the polymer structure, thereby making it less ordered, and therefore, more difficult for the chains to fit closely together. This softens the polymer-i.e., lowers the glass temperature (T0) or the modulus. Usually the internal plasticizer is a monomer whose polymer has good low temperature properties. A few representative values of Tg or 7 (brittle temperature) for homopolymers and copolymers are shown in Table I ... 

Perhaps the most simple approach to measuring the low temperature behaviour of rubbers is to find the temperature at which it has become so stiff as to be glassy and brittle. The main disadvantage of this approach is that only one facet of low temperature behaviour is measured and that is at a point where, for many purposes, the rubber has long since become inadequate for its job. Nevertheless, brittleness temperature has been found to be a useful measure and innumerable ad hoc brittleness tests have been devised. These tests usually take the form of quickly bending a cooled strip of rubber and are almost inevitably very operator dependent and do not define the strain rate or the degree of strain precisely. Hence, they show poor between-laboratory variability. 

By impacting test pieces at a series of temperatures the brittleness temperature can be found as the lowest temperature at which none of the specimens tested failed. Failure is defined as the occurrence of any crack, fissure, hole or complete breakage visible to the naked eye. For specification purposes, it is usual to test at a given temperature and record a pass or fail judgment. 

A revision of ISO 812 is being developed in which an alternative measure termed 50% brittleness temperature is included. This is the temperature at which 50% of test pieces fail and is claimed to be more... 

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