The molten state

The molten state of polymers is more dependent on the molar mass than any of their other physical states. Flexible-chain polymer molecules possess essentially random conformations in the molten state. The coiled molecules entangle in high molar mass polymers. These chain entanglements are very important for the rheological properties of the melt. The next part of this chapter (section 6.4) deals with the rheology of flexible-chain polymer melts. A discussion of the deformation mechanisms, including theoretical aspects, is also presented. 

A relatively new class of polymers, the liquid-crystalline polymers, exhibits orientational order, i.e. alignment of molecules along a common director in the molten state. Liquid-crystalline polymers are used, after solidification, as strong and stiff engineering plastics and fibres. Functional liquid-crystalline polymers with unique electrical and optical properties are currently under development. The fundamental physical and rheological aspects of liquid-crystalline polymers are the third subject of this chapter (section 6.5). 

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Reduction of the aromatic nuclei contained in catalytic C-9 resins has also been accomplished in the molten state (66). Continuous downward concurrent feeding of molten resin (120°C softening point) and hydrogen to a fixed bed of an alumina supported platinum—mthenium (1.75% Pt—0.25% Ru) catalyst has been shown to reduce approximately 100% of the aromatic nuclei present in the resin. The temperature and pressure required for this process are 295—300°C and 9.8 MPa (lOO kg/cni2), respectively. The extent of hydrogenation was monitored by the percent reduction in the uv absorbance at 274.5 nm. 

Lead—antimony or lead—arsenic ahoys must not be mixed with lead—calcium (aluminum) ahoys in the molten state. Addition of lead—calcium—aluminum ahoys to lead—antimony ahoys results in reaction of calcium or aluminum with the antimony and arsenic to form arsenides and antimonides. The dross containing the arsenides and antimonides floats to the surface of the molten lead ahoy and may generate poisonous arsine or stibine if it becomes wet. Care must be taken to prevent mixing of calcium and antimony ahoys and to ensure proper handling of drosses. 

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. 

Pseudocumene is shipped ia barges, tank cars, tank tmcks, isocontainers, and dmms. Mesitylene is shipped ia tank tmcks, isocontainers, and dmms, whereas durene is shipped molten ia heated tank tmcks, isocontainers, and occasionally as a cast soHd in dmms. Mesitylene, pseudocumene, and hemimellitene are classified as flammable Hquids the higher homologues are classified as combustible. The higher melting PMBs requite additional precautions when handled in the molten state to avoid thermal bums. Detailed shipping and handling procedures are described in manufacturers material... 

Pig iron and iron and steel scrap are the sources of iron for steelmaking in basic-oxygen furnaces. Electric furnaces have rehed on iron and steel scrap, although newer iron sources such as direct-reduced iron (DRI), iron carbide, and even pig iron are becoming both desirable and available (see Iron bydirectreduction). In basic-oxygen furnaces, the pig iron is used in the molten state as obtained from the blast furnace in this form, pig iron is referred to as hot metal. 

In the CIS pitch coke is made by carbonizing a hard coke-oven pitch in modified coke ovens. The hard pitch has an R-and-B softening point of 140—150°C and is made by air-blowing a mixture of medium-soft pitch and recycled coking oils. This feedstock is charged in the molten state over a period of 5 h and coked for 17—18 h at 1250—1300°C. The coke yield is 70%. Oils, which are recycled, amount to 20% by weight of the pitch fed. The gas yield... 

By-product biphenyl is usually sold as a dye carrier in the molten state in tank tmck or tank car lots. Grades of higher purity are also sold in the molten state or as flakes in 22.7 kg bags. 

Because biphenyl is often transported in the molten state, a moderate fire ha2ard does exist under these circumstances. Biphenyl, with a flash point of 113°C, has a lower flammability limit of about 0.6% (by volume) at the flash point to an upper limit of 5.8% at 166°C (42). Dust explosions are a ha2ard when vapors from a hot Hquid surface condense in air in a confined space. 

The washed slime is dried and melted to produce slag and metal. The slag is usually purified by selective reduction and smelted to produce antimonial lead. The metal is treated ia the molten state by selective oxidation for the removal of arsenic, antimony, and some of the lead. It is then transferred to a cupel furnace, where the oxidation is continued until only the silver—gold alloy (dorn) remains. The bismuth-rich cupel slags are cmshed, mixed with a small amount of sulfur, and reduced with carbon to a copper matte and impure bismuth metal the latter is transferred to the bismuth refining plant. 

Whereas finely divided cobalt is pyrophoric, the metal in massive form is not readily attacked by air or water or temperatures below approximately 300°C. Above 300°C, cobalt is oxidized by air. Cobalt combines readily with the halogens to form haUdes and with most of the other nonmetals when heated or in the molten state. Although it does not combine direcdy with nitrogen, cobalt decomposes ammonia at elevated temperatures to form a nitride, and reacts with carbon monoxide above 225°C to form the carbide C02C. Cobalt forms intermetallic compounds with many metals, such as Al, Cr, Mo,... 

The temperature at which decarboxylation occurs is of particular interest in manufacturing processes based on polymerisation in the molten state where reaction temperatures may be near the point at which decomposition of the diacid occurs. Decarboxylation temperatures are tabulated in Table 2 along with molar heats of combustion. The diacids become more heat stable at carbon number four with even-numbered acids always more stable. Thermal decomposition is strongly influenced by trace constituents, surface effects, and other environmental factors actual stabiUties in reaction systems may therefore be lower. 

Of course, aH materials that are processed in the molten state can cause bums if the hot material comes in contact with the skin. Care must be taken to avoid this, and it should be noted that molten material left in the barrel of an extmder or injection mol ding machine can "spit" unexpectedly. In aH cases, it is recommended that the manufacturer s Material Safety Data Sheet be consulted before working with any of these materials. 

Some materials that are prepared in the molten state are converted advantageously to flake form by cooling a thin layer continuously on the surface of a rotating drum. Another way is to spray cool from the melt, using a spray diyer with cold air. Thus, massive cooling and subsequent pulverizing are avoided. See the Index for details of these other methods. 

Very high molecular weight polyethylenes (A/ in the range 1-6 X 10 ) prepared by the Ziegler process have also become available. As might be expected from consideration of Figure 3.1 these polymers cannot be processed easily in the molten state without decomposition and it is therefore often necessary to process in the rubbery phase. 

There are a number of general points to be borne in mind when processing the polymer in the molten state which may be summarised as follows ... 

Hot melts are 100% solid thermoplastic compounds. They are compounded and applied in the molten state at elevated temperatures. The resultant properties are obtained by cooling. Due to the quick cooling, bonds can be established in a very short time. 

Chemical Reactivity - Reactivity with Water Hot water may cause frothing. Reaction with cold water is slow and non-hazardous Reactivity with Common Materials No reaction Stability During Transport Stable Neutralizing AgerUs for Acids and Caustics Solid spills can usually be recovered before any significant reaction with water occurs. Flush area of spill with water Polymerization Very unlikely at ordinary temperatures, even in the molten state Inhibitor of Polymerization None. 

Vanadium pentoxide, VjOj, is used as a eatalyst in the oxidation of sulfur dioxide. The meehanism involves oxidation-reduetion of V2O5 that exists on the support at operating eonditions in the molten state. The meehanism of reaetion is ... 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

The Group 1 elements are soft, low-melting metals which crystallize with bee lattices. All are silvery-white except caesium which is golden yellow "- in fact, caesium is one of only three metallic elements which are intensely coloured, the other two being copper and gold (see also pp. 112, 1177, 1232). Lithium is harder than sodium but softer than lead. Atomic properties are summarized in Table 4.1 and general physical properties are in Table 4.2. Further physical properties of the alkali metals, together with a review of the chemical properties and industrial applications of the metals in the molten state are in ref. 11. 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

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