Brittleness, low-temperature

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. 

Polypropylene. PP is a versatile polymer, use of which continues to grow rapidly because of its excellent performance characteristics and improvements in its production economics, eg, through new high efficiency catalysts for gas-phase processes. New PP-blend formulations exhibit improved toughness, particularly at low temperatures. PP has been blended mechanically with various elastomers from a time early in its commercialisation to reduce low temperature brittleness. 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

The carboxylated types (XNBR) contain one, or more, acrylic type of acid as a terpolymer, the resultant chain being similar to nitrile except for the presence of carboxyl groups which occur about every 100 to 200 carbon atoms. This modification gives the polymer vastly improved abrasion resistance, higher hardness, higher tensile and tear strength, better low temperature brittleness, and better retention of physical properties after hot-oil and air ageing when compared to ordinary nitrile rubber. 

Low-temperature brittleness or toughness the samples are cooled to a temperature far lower than the supposed temperature of brittleness, and then gradually warmed up. At each selected step temperature, the test specimens are subjected to a specified impact. The temperature at which specimens deteriorate or fail is the brittle point . In some other tests, the lowest temperature to which specimens can be cooled without deterioration is regarded as the limiting temperature of toughness or no brittleness . 

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)... 

Rigid PVC natural UV sensitivity, but special grades benefit from long-time outdoor-exposure guarantees sensitivity to heat, creep, aromatic or chlorinated hydrocarbons, esters and ketones low-temperature brittleness high density toxicity and corrosivity of smoke in fires less easy injection. 

Molecular weight. The molecular weight influences the melt viscosity, tensile strength, the low temperature brittleness, and the resistance to tearing. 

Epichlorohydrin (ECO) has excellent resistance to fuel and oil swell. The ECOs show a volume swell of 35% at room temperature compared to 70% for a medium ACN—nitrile rubber in ASTM Reference Fuel C. The copolymer has a low temperature brittle point of —40°C and the homopolymer,... 

Some properties are not significantly dependent on cross-linking and remain neady invariant as cure progresses. These include thermal conductivity, electrical properties, and low temperature brittleness. 

A relatively new development which promises to gain in importance in the future is the modification of asphalt by butadiene-styrene block polymers.44 The block polymers help reduce the low temperature brittleness and impart resistance to flow at elevated temperatures. Applications in mastics, automobile body undercoatings and waterproofing materials such as high quality roofing membranes are envisaged. 

Figure 15-4. Low temperature brittleness test broken lines show position of striker and test piece (assumed unbroken) after impact...

Known as EPR. this material is of limited use hecausc it cannot be vulcanized in readily available systems. However, (he rubbers arc made from low -cost monomers, have good mechanical and elastic properties, and outstanding resistance to ozone, heal, and chemical attack. They remain flexible to very low temperatures (brittle point about -95 C), They are superior to butyl rubber in dynamic resilience. 

The addition of molybdenum to the alloy, as in type 316, increases the corrosion resistance and high-temperature strength. If nickel is not included, the low-temperature brittleness of the material is increased and the ductility and pit-type corrosion resistance are reduced. The presence of chromium in the alloy gives resistance to oxidizing agents. Thus, type 430, which contains chromium but no nickel or molybdenum, exhibits excellent corrosion resistance to nitric acid and other oxidizing agents. 

It seems unlikely that long-fiber ceramic matrix composites with strong bonds will find application because of their low temperature brittleness. However, for completeness, a model which applies to the creep of such materials can be stated. It is that due to Kelly and Street.21 It is possible also that the model applies to aligned whisker-reinforced composites since they may have strong bonds. In addition, the model has a wide currency since it is believed to apply to weakly bonded composites as well. However, the Mileiko18 model predicts a lower creep strength for weakly bonded or unbonded composites and therefore is considered to apply in that case. 

Ethylene copolymers were compared with liquid plasticisers for use as additives to improve the flexibility of poly(vinyl chloride) (PVC) for electrical cable insulation applications. The PVCs were assessed by determining smoke generation, flammability, tensile properties and the low temperature brittle point. The ethylene copolymers gave similar peak heat release rates, but the peak smoke and the total smoke generation were lower. They also gave similar or increased strength, similar elongation and flexural modulus, and lower brittle point temperatures. 4 refs. 

Evaluation of mechanical damage due to photo-oxldatlon was monitored by changes in low-temperature brittleness per ASTM D746 and by decrease In percent elongation as determined in a standard tensile strength test. Selected samples were examined for changes In bulk oxidative stability by thermal analysis where a specimen Is heated In an oxygen atmosphere and the time to onset of the exotherm due to oxidation measured. 

Low-Density Polyethylene. The data from outdoor weathering are summarized In Figure 1. The low-temperature brittleness (LTB) measurements show that a well-dispersed carbon black of > 1% concentration and < 35 millimicrons (mu) particle size Is necessary for maximum resistance to photo-oxldatlon. The results from elongation measurements agree with those found for LTB, as would be expected, since both measurements reflect the notch sensitivity of polyethylene to micro cracks caused by photo-oxldatlon. Accelerated weathering studies on these compounds have been reported previously (2) and Indicate the same ranking as found in outdoor exposure. 

Stabilized with 0.2% amine antioxidant High Density Polyethylene. Extrusion grades of hlgh-denslty polyethylene protected by 2.5% channel type black of 19 my particle size were exposed In a Weather-O-Meter for 10,000 hours without any evidence of serious mechanical damage, as measured by low temperature brittleness and elongation tests. This represents the useful limit In our accelerated weathering test, since as described In an earlier paper,(2) fllm-like deposits form on... 

Figure 7. The typical low-temperature brittle fracture of 6% asphalt mixtures is clearly evident from stress-strain measurements using 3-point...

A white metal, malleable and ductile at low temperature, brittle when heated which accompanies Zn in certain of its ores. It resembles zinc in its physical as well as its chemical characters. It is used in certain fusible aUoye, and its iodide is used in photography. 

Low temperature brittleness to pulverize, e.g., paints off bumpers surfaces or PVC from metal trim at the cryogenic temperatures. The powders are then removed by sieving. 

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