He received a Ph. We use cookies to help provide and enhance our service and tailor content and ads. By continuing you agree to the use of cookies. Polymer Volume 44, Issue 16 , July , Pages Author links open overlay panel Bobby G. Sumpter a Donald W. Noid a Michael D. McCord and Pease [62] have demonstrated that an STM tip can be used as the source of the electron beam to expose thin tens of nanometers films of poly methyl-methacrylate , PMMA.
The STM has the advantages of relatively low-voltage, high-current density, and very high spatial resolution. Fault Injection Techniques and Tools for Embedded Systems This can be a entire consultant to fault injection recommendations used to judge the dependability of a electronic procedure.
Principles, Concepts and Techniques PDF The objective of stock administration should be to give an explanation for the dynamics of stock management's ideas, recommendations, and methods as they relate to the total offer chain customer call for, distribution, and product transformation processes. Characterization of Solid Polymers: New techniques and developments by D. New techniques and - download pdf or read online.
The following discussion illustrates the current state of this technology, its problems, and future opportunities. There are certain polymer molecular structures that provide good barrier properties; however, these structural features seem invariably to lead to other problems. For example, the polar structures of poly vinyl alcohol , polyacrylonitrile, and poly vinylidene chloride make these materials extremely good barriers to oxygen or carbon dioxide under certain conditions, but each material is very difficult to melt fabricate for the same reason.
The good barrier properties stem from the strong interchain forces caused by polarity that make diffusional jumps of penetrant molecules very difficult. To overcome these same forces by heating, so that the polymer chains can move in relation to one another in a melt, requires temperatures that cause these reactive materials to degrade chemically by various mechanisms. Thus neither poly vinyl alcohol nor polyacrylonitrile can be melt processed in its pure form. Resorting to solvent processing of these materials or using them to make copolymers compromises their value. Poly vinyl alcohol , by virtue of its hydrogen bonding capability, is very hygroscopic, to the point of being water soluble, and this property prevents its use as a barrier material in the pure form even if it could be melt processed.
In general, polarity favors good oxygen barrier properties but leads to poor water barrier properties. This is true for aliphatic polyamides nylon. On the other hand, very nonpolar materials, such as polyethylene and polypropylene, are excellent barriers to water but not oxygen. This property-processibility trade-off has led to an interest in composite structures. The ''composites" can be at the molecular level copolymers , microlevel blends , or macrolevel multilayers.
The attractive barrier characteristics of poly vinyl alcohol have been captured via copolymers, and this achievement has led to some important commercial products using clever molecular engineering and processes that minimize its shortcomings of water uptake and lack of melt processibility. Copolymers containing units of ethylene and vinyl alcohol are made commercially by starting with ethylene and vinyl acetate copolymers and then hydrolyzing them.
By critically balancing the structure of these materials, melt processible products that are relatively good barriers with reduced moisture sensitivity can be achieved. These copolymers are incorporated into multilayer structures by coextrusion processes. For example, blow-molded bottles with five to seven layers in the side wall are in commercial use for marketing very sensitive foodstuffs.
Lightweight, squeezable, fracture-resistant bottles for ketchup are now on the market. Interlayers are often needed to adhere the functional layers to one another when the two differ greatly in chemical structure. Sometimes a mixed layer is included to accommodate recycled material from the process. The barrier function can also be provided by metal foil or by coatings of other polymers or inorganic layers onto containers. Of course, composite structures are inherently more difficult to recycle. Layers based on halogen-based polymers generate acid gases upon incineration. Reconciling these issues will be a major preoccupation during the next decade.
One of the major developments over the past two decades has been the replacement of glass with plastics in bottles for soft drink merchandising. The driving forces for this conversion were issues of cost, weight, safety, and total energy considerations. The commercialization of this technology using poly ethylene terephthalate , or PET, involved innovative developments in processing for increasing molecular weight solid-state reaction and for fabrication injection-blow molding to achieve a highly oriented and transparent bottle.
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The carbon dioxide permeability of PET provides just enough shelf-life for very successful marketing of large 2-liter products; however, smaller bottles, such as the half liter, with a higher surface-to-volume ratio, have a shorter shelf-life. PET is also easily recycled, and considerable progress is being made in this area. PET, however, has not been able to succeed so far in the beer packaging market, owing to marginal oxygen barrier characteristics among other issues.
Polyesters with much better properties are known, such as poly ethylene naphthalene-2, 6-dicarboxylate , but these have not yet become commercial because economical processes for raw material production have not been developed. Current areas of focus include the development of packages that can be directly microwaved, such as packages for soups in single-serving sizes, and controlled atmosphere packaging, which is capable of keeping fruits and vegetables fresh for weeks.
Successes in the latter area could revolutionize the agriculture and food industries of the world in terms of where produce is grown, how it is distributed, and who has access to it. There are some clear fundamental challenges for development of new barrier materials that are economical, melt processible, and environmentally friendly, but significantly better than current ones in terms of permeability to oxygen, water, and oil.
Membrane-based processes that provide many useful functions for society, usually at lower cost, particularly in terms of energy, have achieved substantial commercial importance relatively recently. The majority of the membranes used are made from polymers. The United States is clearly in the lead position, but Europe and Japan are gaining rapidly. There is interest in other materials, such as ceramics, but it is clear that polymers will dominate in most uses.
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For the most part, the major limitation of membrane technology is the performance of the membrane itself; hence, sustained growth demands new developments in membrane materials and membrane fabrication. Membranes are used to produce potable water from the sea and brackish waters, to treat industrial effluents, to recover hydrogen from off-gases, to produce nitrogen and oxygen-enriched air from air, to upgrade fuel gases, and to purify molecular solutions in the chemical and pharmaceutical industries.
They are the key elements in artificial kidneys and controlled drug delivery systems. Basically, membranes may function in one of two general ways, depending on the separation to be performed and the structure of the membrane. Some membranes act as passive filters, albeit usually on a very small scale. These membranes have pores through which fluid flows, but the pores retain larger particles, colloids, or macromolecules e. Depending on the scale of the pores and the solute or particles, the operations are subdivided into ultrafiltration, microfiltration, and macrofiltration. The material dictates the manner in which the membrane can be formed and especially the size and distribution of the pores.
Porous polymer-based membranes are made by solution processes, mechanical stretching, extraction, or ion bombardment processes. The nature of the membrane material is a key factor in resistance to damage and fouling and in compatibility with the fluid phase e. When the membrane is nonporous, the polymer is a more direct participant in the transport process.
Permeation across the membrane involves dissolution of the penetrant into the polymer and then its diffusion to the other surface, that is, a solution-diffusion mechanism.
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The thermodynamic solubility and kinetic diffusion coefficients of penetrants in polymers depend critically on the molecular structures of the penetrant and the polymer and their interactions. This is the mechanism by which reverse osmosis, gas separation, and pervaporation membranes function. In order to have usefully high rates of production in membrane processes, it is generally necessary to have membranes that are very thin and to have a very large membrane area packaged in small volumes.
Ingenious approaches have been developed to achieve both. Membranes may be in the form of a flat sheet wrapped into a spiral for packaging into modules or in the form of very fine hollow fibers. In either case the membrane has a dense skin that is very thin 0. The skin and the substructure may be integral, made of the same material. The method to fabricate such asymmetric membranes was discovered in the s and was first applied to make reverse osmosis membranes and later to make gas separation membranes.
A variety of composite membrane concepts were developed later that have the advantage that the skin and porous support are not integral and can, in fact, be made of different materials. This is especially useful when the active skin material is very expensive. Reverse osmosis and gas separation membranes of both types are in current use. There is growth in almost all sectors of the membrane industry; however, the opportunities for future impact by new polymer technology appear somewhat uneven.
For example, one of the major limitations to the use of ultrafiltration-type processes in the growing biotechnology arena is the tendency for surface fouling by protein and related macromolecules. The discovery of new membrane materials or surface treatments that solve this problem would be of major importance. Intense polymer research related to reverse osmosis during the s and s led to commercial installation of desalinization plants around the world. Membranes in use are made of cellulose acetate and polyamides. Future demands for fresh water from the sea could stimulate renewed research interest in this area.
Currently most of the efforts are devoted to developing reverse osmosis membranes and processes for removal of organic pollutants, rather than salt, from water. Gas separation is clearly one of the most active and promising areas of membrane technology for polymer science and engineering Figure 3. The first commercial membranes introduced in the late s were hollow fibers formed from polysulfone by using a unique technology to remove minute surface defects.
Since then, other polymers have been introduced in the United States, including cellulose acetate, polydimethylsiloxane PDMS , ethyl cellulose, brominated polycarbonate, and polyimides. The first materials selected for this purpose were simply available commercial polymers that had adequate properties. New generations of materials especially tailored for gas separation are being sought to open new business opportunities. The key issues involve certain trade-offs. The polymer must be soluble enough to be fabricated into a membrane, but it needs resistance to chemicals that may be in the feed streams to be separated.
The membrane should have a high intrinsic permeability to gases in order to achieve high productivity, but the permeation should be selective; that is, one gas, for example, O 2 , must permeate much faster than another, for example, N 2. New polymers whose permeability and selectivity are higher than those of current membrane materials are being developed via synthesis of novel structures that prevent dense molecular packing, thus yielding high permeability, while restraining chain motions that decrease selectivity. Pervaporation is a process in which a liquid is fed to a membrane process and a vapor is removed.
The difference in composition between the two streams is governed by permeation kinetics rather than by vapor-liquid equilibrium as in simple evaporation. Thus, pervaporation is useful for breaking azeotropes and is. Also shown bottom is a cut-off section of a bundle of thousands of tiny hollow fibers made of polysulfone embedded in an epoxy tube sheet that fits into each tubular module shown.
Photographs courtesy of Permea Inc. Europe and Japan seems to be the leaders of research in this field. Major breakthroughs in membrane materials and fabrication are needed and appear to be possible. Until the early s most coatings contained only 15 to 30 percent paint solids, the remaining 70 to 85 percent being organic solvents, which were released as air pollutants when the films dried. Since then, reduction of solvent emissions has been the most important single driving force for technology change.
Two kinds of percent solids coatings are now being sold for specialized applications. Powder coatings are electrostatically applied and subsequently heat fused. Radiation-cured coatings are based on solventless liquid oligomers responding to ultraviolet or electronbeam cure. Solvent-borne high-solids coatings play an ever-increasing role. These are often based on oligomers containing hydroxyl groups as cross-linkable sites.
Cross-linking is accomplished by formaldehyde-based methylolated or alkoxy-methylolated nitrogenous amino compounds. Acid-catalyzed heat cure causes the formation of multiple ether-based cross-links. Alternately, hydroxyl functional oligomers are cross-linked with isocyanates to form urethanes. Other common high-solids coatings have drying oil functionality and are cross-linked by air oxidation. Such high-solids coatings now contain about 20 to 50 percent volatile organic compounds VOCs.
The VOCs include solvents, by-products of the cross-linking reaction, and amines used to block catalysts. The role of aqueous coatings in reducing emissions of VOCs has increased greatly in the past few years. These coatings are based mostly on high-molecular-weight latex polymers and still contain some organic solvents to help film formation and wetting. Alternately, aqueous coatings are based on lower-molecular-weight hydrophilic polymers, which, unlike latexes, are synthesized not in water but in organic solvents and are subsequently dispersed into water.
The ratio of VOCs to paint solids in aqueous coatings is now about 1: The environmental concerns leading to a reduction in VOCs were met by newly developed coatings based on new polymer chemistry that often provide better performance than the traditional coatings. In particular, resistance to weathering and to corrosive environments including acid rain was improved, not only for heat-cured but also for ambient-cured coatings. Lowering the cure temperature has allowed the application of. The structure-property relationships for the great variety of new polymers are poorly understood.
In fact, the fast empirical development of the new coatings has outstripped our scientific understanding. A few examples illustrate this. The most sophisticated coating resin system cannot be described solely by the overall monomer composition. Latexes are now being synthesized so that the monomer composition varies stepwise or gradually from the center to the surface of the particles. For example, a high- T g glass transition temperature core and a low- T g skin in the latex particles provide relatively hard coatings related to high T g with ease of film formation related to low T g. Because the latex particles are very small, typically 50 to nanometers nm , it is practical to blend into them thermodynamically incompatible polymers.
Layering, as described above, is one example. Acrylic monomers can be polymerized into urethane latexes, leading to separation into microphases within the latex particles or to formation of interpenetrating polymer networks. Alternately, latexes of differing compositions are synthesized separately and then blended. Also, latexes can be blended with separately synthesized low-molecular-weight water-soluble polymers. A single coating can be based on several polymers varying greatly in composition: This way, the rheology, wetting properties, reactivity, and chemical resistance of the coatings can be optimized beyond what can be obtained by single polymer systems.
Apart from reduction of VOCs, another driving force for change is elimination of toxic ingredients from coatings. Some of the several sources of toxicity are cross-linkers for hydroxy-functional polymers: New cross-linking mechanisms are being sought. Promising early results involve reactions of carboxyl groups with epoxies or with nitrogenous heterocycles.
The Michael reaction of acrylic double bonds with amines or activated methylene groups looks promising. Silane-functional polymers cross-link when exposed to atmospheric moisture. Some of these new systems provide chemically resistant cross-links even at ambient temperature, a great advantage for applications where high-temperature cure is not possible, such as car refinishes, aviation, implements, plastics, wood furniture, and architecture. New, very inert cross-linked resins in the coatings often provide good corrosion protection, allowing the use of nontoxic chrome-free corrosion inhibitors.
Also, some new polymers provide improved interaction with difficult-to-disperse pigments so that toxic lead-or chrome-based pigments can be conveniently replaced by all-organic pigments. The greater variety of resins now available for coatings allows the elimination of toxic aromatic and ethylene-oxide-based solvents from high-solids or aqueous coatings. Because the United States began to impose legislation to control the environment.
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These new technologies are now internationally available. However, the potential for further reducing VOCs and toxic ingredients and for further improving coating performance is huge. For further progress, better understanding of solid-and liquid-state rheology and thermodynamics of complex resin systems is required.
New chemical approaches for precision synthesis and for cross-linking of polymers and oligomers are also needed. Aqueous polymer systems are particularly fertile ground for new research. The preparation of complex acrylic latexes, urethane latexes, epoxy dispersions, solubilized polyesters, alkyds, and vinyl resins is still an empirical art.
The materials described above are made up primarily of polymer molecules based on covalently bonded chains in which carbon is the principal element. Polyethylene, polystyrene, poly methyl methacrylate , and poly vinyl chloride are all based on carbon chains with differing side groups. Nitrogen and oxygen can also be incorporated into polymer chains, as in polyamides nylons and polyesters, but carbon atoms separate the hetero-atoms.
Considering the importance and variety of organic polymers, natural and synthetic, it is remarkable that the chemistry of carbon is so unique and so dominant. Other polymers exist in which carbon is less dominant. Polypeptides and proteins have one nitrogen for every two carbons, and the great variety is derived entirely from differences in groups attached to the alpha carbon atom.
All of these materials are, however, entirely "organic" in character. Polyoxymethylene is a polymer with alternating oxygen and carbon along the main chain. It is a partially crystalline molding compound. Poly ethylene oxide , with two carbons for each oxygen in the chain, has been of particular interest because of its water solubility. Chains containing no carbon C exist, and virtually limitless compositional and structural diversity is accessible through utilization of more of the periodic table.
Silicon, which is in the same group as carbon in the periodic table, is one such example. The vignette "Silicones" explores the properties and uses. At this time, however, the polysiloxane family is by far the most important. These materials have been available for several decades in the form of liquids, gels, greases, and elastomers that exhibit good stability and properties. They are the most thoroughly studied and highly commercialized class of inorganic polymers.
Although the chain is entirely inorganic—with alternating single silicon Si and oxygen O atoms—organic side groups usually methyl or phenyl are attached to the silicon atoms. Many applications of polysiloxanes derive from the extraordinary flexibility of the siloxane backbone. The Si—O bond is significantly. Beginning in , B bombers were flown from factories in the United States to airbases in Great Britain.
But before the Flying Fortresses could cross the Atlantic—much less raid German factories—a critical problem had to be solved. The thin air at cruising altitudes can be ionized by the high-voltage electricity of an aircraft ignition system. The effect is not large in the relatively dry air over land, but the water vapor in damp oceanic air is highly susceptible.
The moist air would work its way into tiny pores in the rubber wires insulating the ignition system. Soon the pilot would notice a blue glow around the leading edge of an engine nacelle—a corona of electricity arcing from spark plug to cylinder, shorting out the plug.
The engine would start misfiring, eventually dying altogether as more pistons quit. If more than one engine went south, the crew might have to ditch, and the "Fort" would be lost at sea. A silicone polymer developed for waterproofing electrical equipment aboard submarines proved to be the answer. Applied liberally to the spark plug wires and boots, the silicone grease kept the wires dry and the bombers airborne—it was, in fact applied to all U.
Silicones are polymers whose backbones are long, flexible chains of alternating silicon and oxygen atoms. Dangling from the backbone like charms from a bracelet are side chains, usually small, carbon-based units, and the choice of these side chains gives silicones a remarkable range of properties. The water-repellent grease has oily, nonpolar side chains such as methyl groups. The nonpolar side chains and the polar water molecules do not mix, repelling the water from the silicone.
Another application of silicones depends on a careful balance between polar and nonpolar side chains. Small amounts of silicone foaming agents control the bubble size in polyurethane foams. A high proportion of polar side chains makes the foam foamier. The bubbles become bigger, forming open pores and producing the soft foams found in car seats and furniture cushions. Reduce the number of polar side chains, and the bubbles remain small.
These tiny bubbles do not open up to form pores, and the foam is a much stiffer solid used for insulation. Silicones have other, seemingly contradictory properties. A silicone resin coating the bread pans in a bakery keeps fresh-baked bread from sticking in the pan, and a liquid silicone polymer on the molds in tire factories does the same thing for newly made tires. But adding a "tackifying resin" makes the silicone sticky and produces the drug-permeable contact adhesive used on those skin patches containing nicotine for smokers who are trying to quit or scopolamine for seasickness sufferers who are trying not to lose it.
Silicon and oxygen are the two most abundant elements on Earth, and they combine naturally to form silicates, including glass and such minerals as quartz and granite. These two elements were first combined synthetically—as silicones—in the United States in the s. They were originally expensive and unhandy to make, but the discovery of a cheaper, easier method of producing them, coinciding with the interest in their novel properties sparked by World War II, started an avalanche of research into new uses for these versatile polymers that continues unabated today.
Important applications include high-performance elastomers, membranes, electrical insulators, water-repellent sealants, adhesives, protective coatings, and hydraulic, heat-transfer, and dielectric fluids. The polysiloxanes also exhibit high oxygen permeability and good chemical inertness, which lead to a number of medical applications, such as soft contact lenses, artificial skin, drug delivery systems, and various prostheses. Another family of inorganic polymers—the polyphosphazenes—is based on a chain of alternating phosphorus P and nitrogen N atoms.
Over different polymers have been synthesized, mainly by variation of the pendant groups. The pendant groups may be organic, inorganic, or organometallic ligands. The nature of the pendant groups affects the skeletal flexibility, solubility, refractive index, chemical stability, hydrophobicity, electronic conductivity, nonlinear optical activity, and biological behavior. Thus by choice of the appropriate side group, polyphosphazenes can be tailored for a variety of applications.
These materials are prepared by methods that give little control over stereoregularity and, hence, mixed-substituent polyphosphazenes are amorphous. Fluoroalkoxy substituents yield hydrocarbon-resistant materials that could be useful as fuel lines, o-rings, and gaskets in demanding environments.
Ether side groups coordinate lithiumions, which leads to possible applications as polymeric electrolytes for high-technology batteries. The ease of side group substitution has also led to new applications in biomedical materials. Hydrophobic polymers such as poly[bis trifluoroethoxy phosphazene] minimize the "foreign body" interactions that normally occur when nonliving materials are implanted in contact with living tissues, such as blood.
Hydrophobic polyphosphazenes are therefore good candidates for use in cardiovascular replacements or as coatings for pacemakers and other implantable devices. Hydrophilic or mixtures of hydrophilic and hydrophobic groups can be substituted to produce hydrophilic or amphiphilic polymers deliberately designed to stimulate tissue adhesion or infiltration or to generate a biochemical response. Unfortunately, while polyphosphazenes are an interesting class of materials that have physical and chemical characteristics that suggest many applications, they are costly to produce, and commercial success has consequently been modest.
Polysilanes also called polysilylenes have been the subject of research interest within the last decade. These all-silicon chains, with alkyl or phenyl side groups, are analogous to vinyl polymers, but they are made from silyldichlorides rather than from the analogue of ethylene. The chains of silicon atoms are flexible. Applications that have been proposed for polysilanes include ceramic precursors for silicon carbide , photoresists, photoinitiators, and nonlinear optical materials.
The use of these inorganic polymers as ceramic precursors is important because the precursor can be spun into a fiber that yields fibrous ceramics following processing.
characterization-of-solid-polymers-new-techniques-and-developments
The fibers are heated first in air to create a silica skin that prevents melting of the fiber during subsequent pyrolysis in N 2 to produce silicon carbide SiC. The polyphosphates are inorganic polymers of interest in their own right, but their most important role is that they serve as part of the repeat unit in polynucleotides. The phosphate sequence is, therefore, being extensively studied with regard to biopolymer functions, for example in replication.
Polysilazanes, with alternating silicon and nitrogen in the main chain, have also been prepared. Although these materials have not been as extensively studied as the polysilanes, it has been demonstrated that they can be used as precursors for silicon nitride ceramics. The organic substituents can be designed as pendant groups that modify the inorganic polymer or network as in poly organo siloxanes , as oligomers or polymers covalently bonded to the inorganic network, as interpenetrating networks, or as combinations of two or more of these functions. Hybrid systems are most often synthesized by low-temperature sol-gel hydrolysis using combinations of metal alkoxides and alkyl substituted and organofunctional alkoxysilanes: Hybrid systems can potentially combine the advantageous properties of both organic and inorganic polymers.
The organic network provides resiliency and toughness, while the inorganic network provides hardness. The incorporation of elastomers such as polydimethylsiloxane and poly tetraethylene oxide into inorganic networks imparts flexibility to otherwise brittle materials, allowing large complex shapes to be processed rapidly without cracking. Independent free-radical and hydrolysis. The isolation of organic dye molecules, liquid crystals, or biologically active species in inorganic or hybrid matrices has led to a vast array of composite optical materials currently being developed as lasers, sensors, displays, photo-chromic switches, and nonlinear optical devices.
These materials are superior to organic matrix composites because the inorganic matrix normally silica exhibits greater transmittance and is less susceptible to photodegradation. Organic molecules embedded in inorganic matrices can also serve as templates for the creation of porosity. Removal of the templates by thermolysis, photolysis, or hydrolysis creates pores with well-defined sizes and shapes. Inorganic materials with tailored porosities are currently of interest for membranes, sensors, catalysts, and chromatography. Inorganic, organometallic, and hybrid polymers and networks represent a potentially huge class of materials with virtually unlimited synthesis and processing challenges.
It is envisioned that future research will continue to explore the periodic table in search of new combinations of materials, new molecular structures, and improved properties. Hybrid systems appear especially rich for research in the area of multifunctional materials, that is, smart materials that perform several optical, chemical, electronic, or physical functions simultaneously.
The development of hybrid materials that exhibit some of the extraordinary strengths and fracture toughness of natural materials such as shell and bone is also anticipated. The remarkable versatility of polyphosphazenes and polysiloxanes will continue to be exploited for biomedical applications such as drug delivery and organ and soft tissue replacement as well as advanced elastomers, coatings, and membranes. The future of preceramic polymers and sol-gel systems appears bright.
A major challenge is to develop synthetic routes to pure, stoichiometric nonoxide ceramics, especially SiC, that exhibit spinnability and high ceramic yields. New synthetic routes such as ''molecular building block" approaches to multicomponent ceramics will be explored to prepare superconductor, ferroelectric, nonlinear optical, and ionic conducting phases, primarily in thin film form.
The use of sol-gel processing to prepare "tailored" porous materials for applications in sensors, membranes, catalysts, adsorbents, and chromatography is an especially attractive area of research and development. The growth of polymers both in volume and in number of uses, as described above, is in part related to their ease in processing. Contrary to popular perception, plastics are often more expensive than steel—that is, on a per-pound basis—but they are also much lighter than steel, glass, or aluminum.
The great advantage to polymers lies in the many ways that they can be processed for. Melt processing is the most widely used and generally the preferred processing method. It is used for polymers that become liquid at elevated temperatures so that they can be extruded into fibers, films, tubes, or other linear shapes or molded into parts of complex shape.
Such processes involve much more than simply changing the physical shape of the polymer; they also influence phase morphology, molecular conformations, and so on and ultimately have an important role in the performance of the product. A mold is a hollow form that imparts to the material its final shape in the finished article. The term "molding" is employed for processes involving thermosets and thermoplastics and includes injection, transfer, compression, and blow molding.
The injection molding process is the most common method of making plastic parts. In that process, thermoplastic pellets are melted and pumped toward a melt reservoir by a rotating screw. When enough molten plastic has accumulated, the screw plunges forward to push the melt into a steel mold. The plastic solidifies on cooling, and the mold is opened for removal of the part.
Injection molding cycle times vary from a few seconds to minutes, depending on the plastic and the part size. Molding machines have become very sophisticated, and they are capable of turning out large numbers of molded articles with little or no operator attention. The heated plastic conforms intimately to the polished mold surface, which may be of complex shape, and the part produced usually requires little or no further machining or polishing. The mold and the machine that delivers plastic to the mold can be quite costly; therefore, the technology is suited only to parts needed in large numbers.
Even so, injection molding is a process capable of exceptionally low cost in comparison with production processes for metal or ceramic parts. Some of the current challenges in polymer processing include developing new materials, achieving greater precision, pursuing process modeling and development, and recycling.