Brittle points

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. 

Figure 11.6. Variation of ASTM D746 brittle point with melt flow index and isotactic index (After...

Many monomers have been copolymerised with ethylene using a variety of polymerisation systems, in some cases leading to commercial products. Copolymerisation of ethylene with other olefins leads to hydrocarbon polymers with reduced regularity and hence lower density, inferior mechanical properties, lower softening point and lower brittle point. 

All PVC plasticisers have a solubility parameter similar to that of PVC. It appears that differences between liquids in their plasticising behaviour is due to differences in the degree of interaction between polymer and plasticiser. Thus such phosphates as tritolyl phosphate, which have a high degree of interaction, gel rapidly with polymer, are more difficult to extract with solvents and give compounds with the highest brittle point. Liquids such as dioctyl adipate, with the lowest interaction with polymer, have the converse effect whilst the phthalates, which are intermediate in their degree of interaction, are the best allround materials. 

Figure 15.12. Brittle points of n-alkyl acryl acrylate and methacrylate ester polymers. (After Rehberg and Fisher, copyright 1948 by The American Chemical Society and reprinted by permission of the...

A large number of organic acrylic ester polymer have been prepared in the laboratory. Poly (methyl acrylate) is tough, leathery and flexible. With increase in chain length there is a drop in the brittle point but this reaches a minimum with poly-(n-octyl acrylate) (see Figure 15.12.). The increase in brittle point with the higher acrylates, which is similar to that observed with the poly-a-olefins and the poly(alkyl methacrylate)s, is due to side-chain crystallisation. 

The polymers are amorphous with brittle points (quite closely related to the Tg) of about -15°C and - 0°C respectively. 

Vulcanisation can be effected by diamines, polyamines and lead compounds such as lead oxides and basic lead phosphite. The homopolymer vulcanisate is similar to butyl rubber in such characteristics as low air permeability, low resilience, excellent ozone resistance, good heat resistance and good weathering resistance. In addition the polyepichlorohydrins have good flame resistance. The copolymers have more resilience and lower brittle points but air impermeability and oil resistance are not so good. The inclusion of allyl glycidyl ether in the polymerisation recipe produces a sulphur-curable elastomer primarily of interest because of its better resistance to sour gas than conventional epichlorhydrin rubbers. 

The use of mixtures of dihalides, Thiokol FA, which has less odour and a lower brittle point than Thiokol A, is produced from a mixture of ethylene dichloride and di-2-chloroethyl formal. 

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... 

It has been shown that inclusion of fine mbber particles in asphalt reduces the cracking of pavement in adverse weather conditions [60,61]. There are two methods for introducing ground waste mbber into asphalt, namely, wet and dry processes. Wet process is carried out at 170°C-220°C for 45-120 min. Rubber particles absorb components with similar value of solubility parameter (5) from the asphalt, causing them to swell. The interaction between mbber and asphalt is mainly of physical nature. In the dry process, mbber is used as a replacement for part of the aggregate and is added to the mineral material before the latter is mixed with the asphalt binder. Addition of mbber greatly improves the elasticity of the binder and generally lowers its brittle point. Incorporation of GRT... 

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. 

The - very fuzzy - definition of the brittle point is based on a more or less sudden reduction in the impact resistance or the flexibility. ITie indicated values must be carefully considered. 

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 . 

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