Low-temperature properties

Polyethylene-derived diesel is susceptible to waxing problems. It is not just the molecular weight (i.e. chain length) that will cause waxing, it is the presence of linear (unbranched) n-aUcane chains (i.e. paraffins) and these are more likely to be formed from PE feedstocks. Such straight chains can readily fold and pack tightly to form crystals. 

The solubility of linear alkanes present in diesel quickly decreases with temperature. The temperature at which the first crystals appear is called the cloud point and is one of the most important specifications for fuels concerning their low temperature behaviour. The utilization of a fuel is restricted to temperatures above their cloud point. Cloud point measures the temperature at which the wax crystals form a haze. Below the cloud point, the presence of crystals in suspension impairs the flow, plugging fuel filters. Immediately below the cloud point the fluid stops flowing, reaching what is known as pom point. The CFPP test measmes the highest temperature at which wax separating out of a sample 

The exact temperature at which the cloud point is reached depends on the total n-alkane content of the fuel, the average size of the n-alkane molecules, their size distribution and chain structure (e.g. degree of branching). Conventional diesels contain as much as 20% of long-chain n-alkanes of limited solubility in the fuel. Pyrolysis-diesels from PE feedstocks can contain more than 40% long-chain n-alkanes. Paraffins crystallize at low temperature into very thin rhombic plates which can clog filters, transfer lines, and pumps, and can lead to engine failure at low temperatures. 

Two processes are commonly used to produce fuels within a given specification. The use of additives, such as cloud point depressants, and, fuel blending where a fuel with an excessively high cloud point is mixed with a low-cloud-point fluid to produce a fuel that meets the specifications. Most often both approaches are used simultaneously, since cloud point depressants have a limited action. 

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Using one or another type depends on the desired properties for the grease, in particular the high or low temperature properties. 

In the sheeting market, the low density polyethylenes are less important than the high density resins. The high density resins have excellent chemical resistance, stress-crack resistance, durabiUty, and low temperature properties which make them ideal for pond liners, waste treatment faciUties, and landfills. In thicker section, HMW-HDPE sheet makes good containers, trays, tmck-bed liners, disposable items, and concrete molds. The good durabiUty, abrasion resistance, and light weight are critical elements for its selection. 

Properties. Sulfur hexafluoride is a good dielectric because a high gas density can be maintained at low temperatures. Properties are given in... 

Materials similar ia high temperature properties to the Du Pont material with better low temperature properties have been synthesized usiag direct fluofination. The first was produced by reaction of fluorine with iaexpensive hydrocarbon polyethers such as poly(ethylene oxide). In the simplest case, poly(ethylene oxide) is converted to the perfluoroethylene oxide polymer ... 

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. 

Water—Glycol Solutions. These materials are transparent solutions of water and glycol having good low temperature properties. They frequently contain water-soluble additives to improve performance in corrosion resistance, anti-wear, etc. A water-soluble polymer is commonly utilized to boost viscosity. As solutions their advantage over emulsions is their inherent stabdity. 

Plasticizers. Monomeric (mol wt 250—450) plasticizers (qv) are predominantiy phthalate, adipate, sebacate, phosphate, or trimeUitate esters. Organic phthalate esters like dioctyl phthalate (DOP) are by far the most common plasticizers in flexible PVC. Phthalates are good general-purpose plasticizers which impart good physical and low temperature properties but lack permanence in hot or extractive service conditions and are therefore sometimes called migratory plasticizers. Polymeric plasticizers (mol wt up to 5000 or more) offer an improvement in nonmigratory permanence at a sacrifice in cost, low temperature properties, and processibiHty examples are ethylene vinyl acetate or nitrile polymers. 

Wide range of viscosity ia commercial petroleum oils is illustrated by the representative types listed ia Table 3. Despite this range, the largest proportion of oils are ia the 25-75 mm /s at 40°C viscosity range. Oils ia this range combine generally adequate hydrodynamic load capacity with low power loss, low volatiUty, and satisfactory low temperature properties. 

Film. By far the largest appHcation for LLDPE resins (over 60% in the United States) is film. Because LLDPE film has high tensile strength and puncture resistance, it is able to compete with HDPE film for many uses. The toughness and low temperature properties of LLDPE film also exceed those of conventional LDPE. Furthermore, because LLDPE resins exhibit relatively low strain hardening in the molten state and lower extensional viscosity, it can be produced at high rates with Httle risk of bubble breaks. 

Nitrile Rubber. Nitrile mbbers are made by the emulsion copolymerization of acrylonitrile (9—50%) and butadiene (6) and designated NBR. The ratio of acrylonitrile (ACN) to butadiene has a direct effect on the properties on the nature of the polymers. As the ACN content increases, the oil resistance of the polymer increases (7). As the butadiene content increases, the low temperature properties of the polymer are improved (see Elastomers, SYNTHETIC-NITRILE RUBBER). 

Nitrile mbber compounds have good abrasion and water resistance. They can have compression set properties as low as 25% with the selection of a proper cure system. The temperature range for the elastomers is from —30 to 125°C. The compounds are also plasticized using polar ester plasticizers. The main dilemma is the selection of a heat-stable, nonfugitive plasticizer that also gives good low temperature properties. 

Fluoroelastomers excel compared to aU other elastomers in heat, chemical, flame, weathering, fuel, and o2one resistance. In addition oU, oxygen, and water resistance ate very good. The fluoroelastomers, however, ate attacked by amines and some highly polar solvents. The abrasion resistance and low temperature properties ate adequate for most appHcations. 

The increase in fuel viscosity with temperature decrease is shown for several fuels in Figure 9. The departure from linearity as temperatures approach the pour point illustrates the non-Newtonian behavior created by wax matrices. The freezing point appears before the curves depart from linearity. It is apparent that the low temperature properties of fuel are closely related to its distillation range as well as to hydrocarbon composition. Wide-cut fuels have lower viscosities and freezing points than kerosenes, whereas heavier fuels used in ground turbines exhibit much higher viscosities and freezing points. 

However, the final properties of the tire show some poiats of superiority over natural mbber, ie, iu higher abrasion resistance of cold SBR. Siace the polymer has a T of —45° C compared with the —72 " C of aatural mbber, it shows poorer low temperature properties. Also, siace the resdieace of SBR is only about 50%, compared to at least 70% for the aatural mbber, there is more heat budd-up with SBR. Ia fact, although it is eatirely possible to produce an ad-synthetic automobde tire, this is not the case for tmck tires because their greater mass leads to an unacceptable degree of heat budd-up (13). 

Processing ndProperties. Neoprene has a variety of uses, both in latex and dry mbber form. The uses of the latex for dipping and coating have already been indicated. The dry mbber can be handled in the usual equipment, ie, mbber mills and Banbury mixers, to prepare various compounds. In addition to its excellent solvent resistance, polychloroprene is also much more resistant to oxidation or ozone attack than natural mbber. It is also more resistant to chemicals and has the additional property of flame resistance from the chlorine atoms. It exhibits good resiUence at room temperature, but has poor low temperature properties (crystallization). An interesting feature is its high density (1.23) resulting from the presence of chlorine in the chain this increases the price on a volume basis. 

The fluorine atom confers chemical inertness, but some hydrogen atoms must be in the chain to maintain mbbery properties. Some fluorinated siHcones are also available where superior low temperature properties are requked (see Elastomers, synttietic— fluorocarbonelastomers). 

Properties. Polyurethane elastomers generally exhibit good resiHence and low temperature properties, excellent abrasion resistance, moderate solvent resistance, and poor hydrolytic stabiHty and poor high temperature resistance. As castable mbber, polyurethanes enjoy a variety of uses, eg, footwear, toys, soHd tires, and foam mbber. 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

The process yields a random, completely soluble polymer that shows no evidence of crystallinity of the polyethylene type down to —60°C. The polymer backbone is fully saturated, making it highly resistant to ozone attack even in the absence of antiozonant additives. The fluid resistance and low temperature properties of ethylene—acryUc elastomers are largely a function of the methyl acrylate to ethylene ratio. At higher methyl acrylate levels, the increased polarity augments resistance to hydrocarbon oils. However, the decreased chain mobiUty associated with this change results in less fiexibihty at low temperatures. 

Low Temperature Properties. Medium hardness compounds of average methyl acrylate, ie, VAMAC G, without a plasticizer typically survive 180° flex tests at —40° C. Such performance is good for a heat-resistant polymer. Low temperature properties can be greatly enhanced by the use of ester plasticizers (10). Careful selection of the plasticizer is necessary to preserve the heat resistance performance of the polymer. Plasticized high methyl acrylate grades lose only a few °C in flexibiUty, compared to grades with average methyl acrylate levels. 

A.C7ylonitnk Content. Standard grades available ia the market contain between 15 to 50% acrylonitrile. The acrylonitrile content of nitrile mbber has a significant effect on two properties chemical resistance and low temperature performance. As the acrylonitrile content of the polymer is iacreased, the chemical resistance is improved whereas the low temperature properties are diminished. 

The excellent low temperature properties of FZ have been iadicated ia Table 1. Modulus curves were obtained usiag dynamic mechanical spectroscopy to compare several elastomer types at a constant 75 durometer hardness. These curves iadicate the low temperature flexibiUty of FZ is similar to fluorosihcone and ia great contrast to that of a fluorocarbon elastomer (vinyUdene fluoride copolymer) (Fig. 3) (15). 

Since polychloroprene crystallizes under stress, fine particle size carbon black is not ordinarily needed or used to enhance tensile strength. More frequently, mineral fillers, for example clay, can be added to reduce cost. A light process oil, free of polycycHc aromatics, can be used to improve the flexibihty or hand of films. On the other hand, an ester plasticizer can be used to improve low temperature properties (161). 

A large volume usage of S—B—S-based compounds is ia footwear. Canvas footwear, such as sneakers and unit soles, can be made by injection mol ding. Frictional properties resemble those of conventionally vulcanised mbbers and are superior to those of the flexible thermoplastics, such as plasticized poly(vinyl chloride). The products remain flexible under cold conditions because of the good low temperature properties of the polybutadiene segment. 

Nickel increases toughness and improves low-temperature properties and corrosion resistance. Chromium and silicon improve hardness, abrasion resistance, corrosion resistance, and resistance to oxidation. Molybdenum provides strength at elevated temperatures. 

The low-temperature properties of metals have created some unusual problems in fabricating ciyogenic equipment. 

Tetrafluoroetbyleue, Teflon) exceptionally good low-temperature properties. Relatively low compressibility and resilience. 

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