Brittle point temperature

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. 

The addition of filler typically increases brittle point temperature and low-temperarnre stiffness. At high levels of plasticizer and with low-temperature plasticizers such as DOA or DOS, this effect is minimal with the usual loadings (10-50 phr) of calcium carbonate. Thus, in compounds formulated for low-temperature service, modest levels of calcium carbonate have little or no effect on performance. 

Low Temperature Properties. The property of solvent resistance makes fluorosihcone elastomers usefiil where alternative fluorocarbon elastomers cannot function. The abiHty to retract to 10% of their original extension after a 100% elongation at low temperature is an important test result. Eluorosihcones can typically pass this test down to —59°C. The brittle point is approximately —68°C. 

Determination of the glass-transition temperature, T, for HDPE is not straightforward due to its high crystallinity (16—18). The glass point is usually associated with one of the relaxation processes in HDPE, the y-relaxation, which occurs at a temperature between —100 and —140° C. The brittle point of HDPE is also close to its y-transition. 

In cases where the copolymers have substantially lower glass-transition temperatures, the modulus decreases with increasing comonomer content. This results from a drop in crystallinity and in glass-transition temperature. The loss in modulus in these systems is therefore accompanied by an improvement in low temperature performance. However, at low acrylate levels (< 10 wt %), T increases with comonomer content. The brittle points in this range may therefore be higher than that of PVDC. 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

Face-seal materials can be chosen from filled, molded or reinforced resins with which water, hydraulic fluids, mineral oils or synthetic oils are all compatible. Their maximum temperature in service depends on the brittle point of the resin but, generally, the range is from —50°C to 100°C (122°F to 212°F). Abrasion resistance is generally good but, as far as possible, resins are not used in the presence of foreign solids. 

The majority of TPEs function as mbber at temperature as low as —40°C or even lower as measured by their brittle point. The upper temperature limit is determined by the maximum temperature at which it can give satisfactory retention of tensile stress-strain and hardness properties. Table 5.14a... 

When an elastomer sample is subjected to low temperatures, the brittle point is the highest temperature at which the sample breaks when subjected to a sharp blow. The brittle point is one indication of low temperature flexibility and is usually somewhat higher than the glass transition temperature. 

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 . 

The glass transition temperatures of polybutene by DSC measurements are generally about -18°C. Some copolymers have lower brittle points down to -34°C, for example. 

At low temperatures, general-purpose grades generally have a brittle point of -65°C down to -80°C. 

The brittle points are generally in the range from -70°C up to -65°C, with a few grades with higher temperatures, -50°C for example. Low service temperatures are often about -40°C. These results relate to some grades only and cannot be generalized. 

The brittle points generally range from -90°C up to -30°C. Clash-Berg stiffness temperatures are higher, from -50°Cup to -17°C. 

The flexibility of amorphous polymers is reduced drastically when they are cooled below a characteristic transition temperature called the glass transition temperature (Tg). At temperatures below Tg there is no ready segmental motion and any dimensional changes in the polymer chain are the result of temporary distortions of the primary covalent bonds. Amorphous plastics perform best below Tg but elastomers must be used above the brittle point, or they will act as a glass and be brittle and break when bent. 

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,... 

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

The so-called brittle point, associated with sample failure in impact tests, may be determined qualitatively using a penetrometer or a Shore durometer (an instrument used to measure resistance of a sample to penetration by a blunt needle) to measure the change in penetration hardness with temperature. Also, a thin film of a polymer may be readily folded at temperatures above T but may crack when folded at temperatures below Tr... 

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. 

For many applications low-temperature flexibility of the plasticized composition is also important. Plasticizers of low viscosity and low viscosity-temperature gradient are usually effective at low temperature. There is also a close relationship betv/een rate of oil extraction and low-temperature flexibility plasticizers effective at low temperature are usually rather readily extracted from the resin. Plasticizers containing linear alkyl chains are generally more effective at low temperature than those containing rings. Low-temperature performance is evaluated by measuremen t of stiffness in flexure or torsion or by measurement of second-order transition point, brittle point or peak dielectric loss factor. 

At higher temperatures, the failure occurs with yielding, which is now the predominant deformation mechanism. From an experimental point of view, domains define what is called the ductile-brittle transition temperature, TB, which is a very important characteristic for polymers. The ductile-brittle transition is also associated with a stiffness-toughness balance. Note that it is also possible to determine a ductile-brittle strain rate transition varying k at a given temperature. 

---