Processing melts

Melt processing is a good alternative technique which is particularly useful for dealing with thermoplastic polymers. This approach makes use of the fact that thermoplastic polymers soften when heated. Amorphous polymers can be processed above their glass transition temperature, whereas 

For many applications, fibres are more suitable than bulk materials. In addition, fibre production techniques tend to be suited to the alignment of nanotubes with in the fibre. A number of studies have focused on the production of composite fibres by melt processing. Fibre processing is generally similar to melt processing, but usually involves a process such as extrusion to produce an elongated sample which can then be drawn into a 

Melt processing is a common and simple method, particularly useful for thermoplastic polymers. This technique consists of blending the nanoparticles with the polymer matrix in the molten state. The nanoparticles are mechanically dispersed into a polymer matrix using a high-temperature and high-shear-force mixer or compounder [192]. 

Melt processing has been successfully applied for the preparation of different polymer-CNT composites such as PP/CNT [211-213], PE/CNT [212-216], polycarbonate (PC)/CNT [217-219], PMMA/CNT [220-223], poly-oxymethylene/CNT [224], polyimide/CNT [225], PA6/CNT [226,227], and so on. The shear forces from the mix-ers/extruders should help break nanoparticle aggregates or prevent their formation. Unfortunately, the dispersion of CNTs in a polymer matrix is quite poor compared to the dispersion that may be achieved through solution mixing. In addition, the CNTs must be lower due to the high viscosities of the composites at higher loading of CNTs. 

Melt processing has been used also for polymer and polymer blend/silica nanocomposites. In particular, extensive studies are reported for PP [228], PP-based copolymer [229], PE [230], PE-based copolymer [231,232], PS [233-235], PMMA [234,235], PC [234], PC-based copolymer [236], polyethylene naphthalate (PEN) [237], perfluoropolymer [238], PET [239-241], PA [242], polyvinyl acetate (PVAc) [243], co-polyetherester [244], styrene-butadiene rubber [245,246], ethylene vinyl acetate (EVA) [247], PET/PS [248], PLEA [249], and many others. 

Jin et al. (26) used melt blending to fabricate MWCNT-PMMA composites with different CNT loadings varying from 0 to 26 wt%. They used a laboratory mixing molder to disperse MWCNT in PMMA at 200°C followed by compression molding at 210°C. Their TEM studies revealed good dispersion even at high MWCNT concentration. The composites showed enhanced mechanical and thermal properties. 

Combination of solvent casting followed by compression molding is also one of the approaches to fabricate CNT-PMMA composites (21,24,57). Slobodian et al. (57) fabricated MWCNT-PMMA composites by this technique and studied electrical conductivity of composites obtained after two-time compression molding. Different percolation thresholds were found for different solvent systems. Mathur et al. (24) also reported improvement in the mechanical properties of the CNT-PMMA composite over the one prepared by simple solvent casting. The reason attributed is the removal of any solvent trapped in the cast films. 

CaCOj-polylactide nanocomposites can also be prepared via melt compounding technique using twin screw extruder [64]. The temperature of the mixing zone varied from 150°C at entry to 190° at exit at a mixing speed of 150 rpm. These samples were then injection molded to the desired shape. 

The SiO -polylactic acid (PLA) and SiO -polyCe-capraloctone) (PCL) nanocomposite films were fabricated using melt blending technique followed by compression molding [65], Ihe molding temperatme was 120°C for PCL and 210°C for PLA. 

TiOj-polyethylene nanocomposites were fabricated via melt blending technique [37,66]. Similarly, TiO -PP nanocomposite was prepared via melt blending technique [67]. ZnO-polyoxymethylene nanocomposites were prepared by melt blending technique [35]. 

TiO -PP nanocomposite was prepared in-situ with the assistance of corotating twin screw extmder [68], Composite was prepared by the injection of 30 wt% of titanium n-butoxide precursor to achieve 9.3 wt% of Ti02, after hydrolysis-condensation reaction. The titanium n-butoxide-PP mixture was treated in hot water at 80°C for 72 h. During this time period, the following hydrolysis and condensation reactions occur in precursor, which leads to the formation of in-situ TiO -PP nanocomposites. 

ZnO-Acrylic nanocomposites are prepared by adding various amounts of 3-(trimethoxysilyl) propyl methacrylate (TPMA)-modified ZnO nanoparticles in 2 ml ethanol blended with methyl methacrylate (0.90 g), hydroxyl ethyl methacrylate (1.95 g), and trimethylolpropane triacrylate (1.77 

Rheology is defined as the science of the deformation and flow of matter. To enable polyethylene to be shaped into useful articles, the polymer must be melted and is typically heated to temperatures of -190 °C. Even at such temperatures, the molten polymer is very viscous. Hence, rheological properties of molten polyethylene are crucial to its end use and much study has been devoted to this subject. Strict mathematical treatment of polymer rheology can become quite complex and is outside the scope of this text. However, general discussions of polymer rheology (12) and specifically for polyolefins (13-15) are available. 

If a fluid (e.g., water) flows in direct proportion to the force applied, it is said to exhibit Newtonian flow. However, the flow of molten polyethylene is not directly proportional to force applied, and polyethylene is said to exhibit non-Newtonian flow. Polyethylene becomes less viscous at higher stress ( shear ). This is called shear thinning and is typical of molten polyethylene. 

As previously discussed in Chapter 1, melt index (MI) is a standard method for measuring flow of molten polyethylene and is indicative of molecular weight of the polymer. However, melt index is a limited indicator of rheological properties. 

Techniques used for processing molten polyethylene are many. Some of the more important fabrication methods are listed below  

Flgitro 24 Mlcrostructure of 1203 - 2r02 composite obtained using eutectic melt processingj see text. 

The second method of melt processing composites is to infiltrate compacts consisting of the second phase. This may be the most limited melt approach in that it is most likely to be successful mainly with particulate composites. However, infiltration of B C C compacts with molten Si to make B C + SiC composites is one practical example. Similarly, infiltration of comp2K ts of carbon fibers and powder with molten Si has been used by Hillig 

The third method of melt processing of composites is solidification of the composite from the melt. The extensive work on fusion-cast refractories, typically using multiphase compositions, with many of these involving eutectic structures, some for very refractory non-oxide systems are a guide to some of the possibilities. More recently. Rice and colleagues  

A major method of controlling solidification porosity is directional solidification. Such solidification of eutectics results in unidirectional leunellar or rod reinforced composites, wherein the thickness of the lamellae or rod diameters are inversely proportional to the solidification 

Another example of directionally solidified oxide eutectics is 

The product properties and its market value depend on the composition of the material, mostly affected by the purity of the feed stock. Most carpet waste contains two immiscible plastics, nylon and polypropylene. The immiscibility of these two components leads to poor mechanical properties. When carpet is recycled using melt blending, compatibilizers could be used to improve the properties of the blends. 

United Recycling Inc. (URI), which was in operation from the early 1990s to 

introduced two extruded blends (URl 20-001 and URl 10-001) from postconsumer carpet waste for injection molding in 1993. These were the first commercial recycled carpet compounds. The process used both polypropylene and nylon carpet The products were desalbed as proprietary blends containing nylon, polypropylene and other polymers and inorganic fillers. 

Polymers from carpet waste produced by melt processing may be used to make products in a molding process, either alone or blended with virgin polymers. The recycled polymers may also be used as matrices in glass fiber reinforced composites. For such applications, the properties of the composites are dominated by the reinforcement (glass fibers), and therefore even recycled polymers without compatibilization could provide the composites with satisfactory mechanical properties.  

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Acrylonitrile copolymeri2es readily with many electron-donor monomers other than styrene. Hundreds of acrylonitrile copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymeri2ations is readily available (34,102). Copolymeri2ation mitigates the undesirable properties of acrylonitrile homopolymer, such as poor thermal stabiUty and poor processabiUty. At the same time, desirable attributes such as rigidity, chemical resistance, and excellent barrier properties are iacorporated iato melt-processable resias. 

In the late 1980s, new fully aromatic polyester fibers were iatroduced for use ia composites and stmctural materials (18,19). In general, these materials are thermotropic Hquid crystal polymers that are melt-processible to give fibers with tensile properties and temperature resistance considerably higher than conventional polyester textile fibers. Vectran (Hoechst-Celanese and Kuraray) is a thermotropic Hquid crystal aromatic copolyester fiber composed of -hydroxyben2oic acid [99-96-7] and 6-hydroxy-2-naphthoic acid. Other fully aromatic polyester fiber composites have been iatroduced under various tradenames (19). 

Solution Casting. The production of unsupported film and sheet by solution casting has generally passed from favor and is used only for special polymers not amenable to melt processes. The use of solvents was generally very hazardous because of their flammabiUty or toxic nature. The cost of recovery and disposal of solvents became prohibitive for many lower price film appHcations. The nature of the drying operations leads to problems with solvent migration and retention that are not problems with melt-processed polymers. 

Rhenium hexafluoride is a cosdy (ca 3000/kg) material and is often used as a small percentage composite with tungsten or molybdenum. The addition of rhenium to tungsten metal improves the ductility and high temperature properties of metal films or parts (11). Tungsten—rhenium alloys produced by CVD processes exhibit higher superconducting transition temperatures than those alloys produced by arc-melt processes (12). 

Peifluorinated ethylene—piopjiene (FEP) lesin [25067-11-2] is a copolymer of tetiafluoioethylene [116-14-3] (TFE) and hexafluoiopiopylene [116-15-4] (HEP) thus its blanched stmctuie contains units of —CF2—CF2— and units of —CF2—CF(CF2)—. It retains most of the desirable characteristics of polytetrafluoroethylene (PTFE) but with a melt viscosity low enough for conventional melt processing. The introduction of hexafluoropropylene lowers the melting point of PTFE from 325°C to about 260°C. 

As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

Hexafluoiopiopylene and tetiafluoioethylene aie copolymerized, with trichloiacetyl peroxide as the catalyst, at low temperature (43). Newer catalytic methods, including irradiation, achieve copolymerization at different temperatures (44,45). Aqueous and nonaqueous dispersion polymerizations appear to be the most convenient routes to commercial production (1,46—50). The polymerization conditions are similar to those of TFE homopolymer dispersion polymerization. The copolymer of HFP—TFE is a random copolymer that is, HFP units add to the growing chains at random intervals. The optimal composition of the copolymer requires that the mechanical properties are retained in the usable range and that the melt viscosity is low enough for easy melt processing. 

Gases and vapors permeate FEP resin at a rate that is considerably lower than that of most plastics. Because FEP resins are melt processed, they are void-free and permeation occurs only by molecular diffusion. Variation in crystallinity and density is limited, except in unusual melt-processing conditions. 

Peifluoioalkoxy (PFA) fluoiocaibon lesins aie designed to meet industry s needs in chemical, electrical, and mechanical appHcations. These melt processible copolymers contain a fluorocarbon backbone in the main chain and randomly distributed perfluorinated ether side chains ... 

The polymer is separated from the medium and converted to usehil forms such as melt-extmded cubes for melt processible appHcations. Teflon PEA is also available as a dispersion, a fine powder, or in unmelted bead form. 

This article focuses on the commercial, ethylene-based ionomers and includes information on industrial uses and manufacture. The fluorinated polymers used as membranes are frequently included in ionomer reviews. Owing to the high concentration of polar groups, these polymers are generally not melt processible and are specially designed for specific membrane uses (see Fluorine compounds, organic—perfluoroalkane sulfonic acids Membrane technology). 

A process based on saponification of ethylene—acrylate ester copolymers has been practiced commercially in Japan (29). The saponification naturally produces fully neutralized polymer, and it is then necessary to acidify in order to obtain a pardy neutralized, melt-processible product. Technology is described to convert the sodium ionomer produced by this process to the zinc type by soaking pellets in zinc acetate solution, followed by drying (29). 

Ethylene—Dicarboxylic Acid Copolymers. Partial neutralization of copolymers containing carboxyls in pairs on adjacent carbons, eg, ethylene—maleic acid, has been described (11). Surprisingly, there is no increase in stiffness related to neutralization. Salts with divalent metal cations are not melt processible. The close spacing of the paired carboxyl groups has resulted in ionic cluster morphology which is distinct from that of the commercial ionomer family. 

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

The volume of hulls generated is nominally 62 m /1 of fuel, which is about 10 times the actual volume of metal. Whereas they are not yet in commercial use, both compaction and melting processes are being developed to improve waste handling economics (41). 

The packaging (qv) requirements for shipping and storage of thermoplastic resins depend on the moisture that can be absorbed by the resin and its effect when the material is heated to processing temperatures. Excess moisture may result in undesirable degradation during melt processing and inferior properties. Condensation polymers such as nylons and polyesters need to be specially predried to very low moisture levels (3,4), ie, less than 0.2% for nylon-6,6 and as low as 0.005% for poly(ethylene terephthalate) which hydrolyzes faster. 

Polyamides, often also lefeiied to as nylons, are liigli polymers which contain the amide repeat linkage in the polymer backbone. They are generally characterized as tough, translucent, semicrystalline polymers that ate moderately low cost and easily manipulated commercially by melt processing. 

Transesterification. There has been renewed interest in the transesterification process for preparation of polycarbonate because of the desire to transition technology to environmentally friendly processes. The transesterification process utilizes no solvent during polymerization, producing neat polymer direcdy and thus chlorinated solvents may be entirely eliminated. General Electric operates a polycarbonate plant in Chiba, Japan which produces BPA polycarbonate via this melt process. 

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