It is resistant to moisture and most of the chemicals. It is flexible at room temperature and low temperature and can be heat sealed. Since it is an inexpensive plastic it is made in large amounts to cater demand. For example:. Polyphenylene oxide PPO , which is obtained from the free-radical, step-growth oxidative coupling polymerization of 2,6-xylenol, has many attractive properties such as high heat distortion and impact strength, chemical stability to mineral and organic acids, and low water absorption.
Polyphenylene sulfide PPS obtained by the condensation polymerization of p-dichlorobenzene and sodium sulfide, has outstanding chemical resistance, good electrical properties, excellent flame retardance, low coefficient of friction and high transparency to microwave radiation. PPS is principally used in coating applications. Principal applications of injection and compression molded PPS include cookware, bearings, and pump parts for service in various corrosive environments.
Polypropylene PP is useful for such diverse products as reusable plastic food containers, microwave- and dishwasher-safe plastic containers , diaper lining, sanitary pad lining and casing, ropes, carpets, plastic moldings, piping systems, car batteries , insulation for electrical cables and filters for gases and liquids. In medicine, it is used in hernia treatment and to make heat-resistant medical equipment.
Polypropylene sheets are used for stationery folders and packaging and clear storage bins. Polypropylene is defined by the recyclable plastic number 5. Although relatively inert, it is vulnerable to ultraviolet radiation and can degrade considerably in direct sunlight. It is also somewhat permeable to highly volatile gases and liquids.
Polystyrene is manufactured in various forms that have different applications. Extruded polystyrene PS is used in the manufacture of disposable cutlery, CD and DVD cases, plastic models of cars and boats, and smoke detector housings. Expanded polystyrene foam EPS is used in making insulation and packaging materials, such as the "peanuts" and molded foam used to cushion fragile products.
Polystyrene copolymers are used in the manufacture of toys and product casings. Polyvinyl chloride PVC is a tough, lightweight material that is durable, fairly rigid and versatile, and is resistant to acids and bases. Much of it is used by the construction industry, such as for vinyl siding, drainpipes, gutters and roofing sheets. It is also converted to flexible forms with the addition of plasticizers, thereby making it useful for items such as hoses, tubing, electrical insulation, coats, jackets and upholstery.
Flexible PVC is also used in inflatable products, such as water beds and pool toys. PVC is also a common material in vinyl action figures , especially in countries such as Japan , where the material is used extensively in so-called Sofubi figures. As PVC bends easily and has a tendency to be bent during transit, a method to mitigate this deformation is to heat the plastic until it becomes mobile, then reform the material into the desired shape.
PVC is produced in many specific modifications to affect its chemical and physical properties. In plasticized polyvinyl chloride pPVC , plasticizers are added to the raw material before molding to make it more flexible or pliable. Early on, the health and environmental aspects of this were poorly understood and replacements and product bans resulted after studies.
The original form is often referred to as unplasticized polyvinyl chloride uPVC , which is the more commonly used type for installations such as water, waste, and sewer conveyance plumbing. Chemical modification often produces more drastic changes in properties. This increase in elemental chlorine content contributes to CPVC's increased expression of chlorine-based characteristics, such as chemical durability, resistance to acids, bases, and salts; susceptibility to ammonia-based compounds, aromatics, esters, ketones  ; chemical stability; heat energy transfer resistance.
CPVC is commonly used in water, chemical, hot and cold, delivery systems for residential, commercial, and industrial applications. Polyvinylidene fluoride, PVDF , belongs to the fluoropolymer class of thermoplastics and is known for its high chemical inertness and resistance.
PVDF is obtained through the polymerization of the vinylidene fluoride monomer. These features translate into poor mechanical properties and low-value products, that is, incompatibility. When this is the case, strategies for achieving compatibility are sought, generally employing block or graft copolymers to be located at the interface, much like surfactants. These copolymers can be formed separately and added to the blend or formed in situ by reactive coupling at the interface during processing.
The former route has, for example, made it possible to make blends of polyethylene and polystyrene useful for certain packaging applications by addition of block copolymers formed via anionic synthesis. However, viable synthetic routes to block copolymers needed for most commercially interesting combinations of polymer pairs are not available.
For this reason, the route of reactive compatibilization is especially attractive and is receiving a great deal of attention for development of commercial products.
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It involves forming block or graft copolymers in situ during melt processing by reaction of functional groups. Extensive opportunities exist for developing schemes for compatibilization and for fundamental understanding of their mechanisms. A better understanding of polymer-polymer interactions and interfaces e.
Especially important is the development of experimental techniques and better theories for exploring the physics of block and graft copolymers at such interfaces. This knowledge must be integrated with a better understanding of the rheology and processing of multiphase polymeric materials so that the morphology and interfacial behavior of these materials can be controlled. A wide variety of compatibilized polymer alloys have been commercialized, and the area is experiencing a high rate of growth. A product based on poly phenylene oxide , a polyamide, and an elastomer has been introduced for use in forming injection-molded automobile fenders and is currently being placed on several models of U.
The polyamide confers toughness and chemical resistance, the poly phenylene oxide contributes resistance to the harsh thermal environment of automotive paint ovens, while the elastomer provides toughening. Another automotive application is the formation of plastic bumpers by injection molding of ternary blends of polycarbonate, poly butylene terephthalate , and a core shell emulsion-made elastomeric impact modifier Figure 3.
In this blend, the polycarbonate brings toughness, which is augmented at low temperatures by the impact modifier, while the poly butylene terephthalate brings the needed chemical resistance to survive contact with gasoline, oils, and greases. In the first example, the poly phenylene oxide and polyamide are very incompatible, and reactive coupling of the phases is required for morphology control and for interfacial strengthening. In the second example, the polycarbonate and polyester apparently interact well enough that no compatibilizer is needed.
They are chosen for their class-A surface, dimensional stability, impact strength, and corrosion and chemical resistance. The side claddings on these vehicles are molded of a resin that is a polyester-polycarbonate alloy, chosen for its cold temperature impact strength, chemical resistance, and quality surface. More than 60 pounds of engineering thermoplastics can be found on many of the vehicles. Toughening by the addition of rubber was first practiced for commodity polymers, such as polystyrene, poly vinyl chloride , polypropylene, and poly methyl methacrylate PMMA.
Widely different processes and product designs were required to achieve optimal products. Now this approach is being applied to engineering thermoplastics and thermosets in order to move these materials into applications that require stringent mechanical performance under demanding conditions. This ensures an excellent growth opportunity for a variety of toughening agents. Elastomers with low glass transition temperatures are needed to impart toughness at low use temperatures, while thermal and oxidative.
In addition, these elastomers must be dispersed within the matrix to an appropriate morphology or size scale and adequately coupled to the matrix. These two issues are often interrelated and specific to the particular matrix material.
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Continued efforts will be required to produce a better understanding of the various toughening mechanisms that are applicable to engineering polymers. Numerous opportunities exist to achieve better understanding that would shorten the time to develop new blends and alloys. There is an interesting parallel between this field and alloying in metallurgy, and the polymer community may be able to learn from the long experience of metallurgists. Both fields involve a broad spectrum of issues including synthesis, processing, physical structure, interfaces, fracture mechanics, and lifetime prediction.
The United States is currently in a position of technical leadership; however, companies and universities around the world are also aggressively pursuing research and development in this field. Polymer composites can provide the greatest strength-to-weight and stiffness-to-weight ratios available in any material, even the lightest, strongest metals. Hence, high-performance and fuel-economy-driven applications are prime uses of such composites.
One of the most important attributes is the opportunity to design various critical properties to suit the intended application. Indeed, performance may be controlled by altering the constituents, their geometries and arrangement, and the interfaces between them in the composite systems. This makes it possible to "create" materials tailored to applications, the single greatest advantage and future promise of these material systems.
Structural composites are of interest in aerospace applications and in numerous industrial and consumer uses in which light weight, high strength, long fatigue life, and enhanced corrosion resistance are critical. Much needs to be done to advance processibility and durability, to provide a more comprehensive database, and to improve the economics of these systems. A wide range of future needs encompasses synthesis, characterization, processing, testing, and modeling of important polymer matrix composite systems. In general, the future of polymer matrix composites is bright.
The engineering community is now in the second generation of applications of composites, and primary structures are now being designed with these materials. There is a growing confidence in the reliability and durability of polymer composites and a growing realization that they hold the promise of economic as well as engineering gain. Commercial programs such as high-speed civil transport will not succeed without the use of polymer composites. Integrated synthesis, processing, characterization, and modeling will allow the use of molecular concepts for the.
A more precise understanding of the manufacturing, processing, and component design steps will greatly accelerate the acceptance of these advanced materials. New horizons for properties and performance, for example, in smart and intelligent materials, actuators, sensors, high-temperature organic materials, and multicomponent hybrid systems, will involve the potential of introducing a new age of economic success and technical excellence. Advanced polymer matrix composites have been used for more than 20 years, for example, on the B-1 bomber and for many top-of-the-line Navy and Air Force jet fighters.
For military purposes, the high performance and stealthiness of composites have often outweighed issues of durability and even safety. Building lighter, more maneuverable tanks, trucks, and armored vehicles might be an area for future military growth. However, as the Pentagon's budget shrinks, efforts to transform these materials into civilian uses are under way Pasztor, Problems include the need to identify significant nondefense companies that will use advanced composites.
For nearly 30 years, it has been suggested that aircraft designers around the world would rapidly utilize these new materials. Unfortunately, those predictions have not been realized, and U. For a number of reasons, there is continued reticence to employ these advanced materials in many areas, particularly in commercial aviation.
Costs, processibility, and durability appear to be the major issues. To this point, this area has been considered a technical success but not a financial success. Nevertheless, aircraft in various stages of development have composites as some fraction of their structural weight. For example, 15 percent of the Boeing , 6 percent of the MD Trijet, and 15 percent of the MD are estimated to be composites. European aviation firms have begun flight-testing an all-composite tail rotor for a helicopter, and Japanese efforts are under way to develop a military helicopter that has a very high composite content.
It has been predicted that in the future, fiber-reinforced composites FRCs will partially replace conventional materials in civil engineering applications. These could include buildings, bridges, sewage and water treatment facilities, marine structures, parking garages, and many other examples of infrastructure components. Composite materials are also expected to help replace conventional materials such as steel and concrete in many future projects.
The polymer matrix resin composites discussed above have already made inroads in areas such as antenna coverage and water treatment plants. Less expensive fiber-reinforced. Sheet molding compounds, which are used extensively in automobiles and housing, are not considered by many structural engineers to be suitable for infrastructure replacement owing to their relatively low strength.
Advanced polymer composites, on the other hand, which often consist of continuously reinforced fiber materials, have superior strength and stiffness. The liquid crystalline nature of stiff polymer molecules in solution was predicted by Onsager in , further refined by Flory in , and experimentally verified through aramid investigations at the Du Pont Company in the s.
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Flory suggested that as the molecular chain becomes more rodlike, a critical aspect ratio is reached, above which the molecules necessarily line up to pack efficiently in three dimensions. Liquid crystal polymer concepts have been extended to encompass a vast number of homopolymer and copolymer compositions that exhibit either lyotropic or thermotropic behavior. Industrially, most of the effort has been focused on the main-chain nematic polymers.
These polymers combine inherently high thermal and mechanical properties with processing ease and versatility. Processing ease originates from the facile way that molecular rods can slide by one another, the very high mechanical properties come from the "extended chain" morphology present in the solid state, and the thermal stability derives from the highly aromatic chain chemistry.
Inherent in this structure is a high level of structural, and hence property, anisotropy for example, the axial modulus is 1 to 2 orders of magnitude higher than the transverse modulus. The direction of molecular chain orientation is coincident with the direction of covalent bonding in the chain; normal to the orientation direction the bonding is secondary van der Waals, hydrogen bonding, and so on.
Low orientation in these materials means global but not local randomness, and properties within "domains" are highly anisotropic. A useful spin-off of the study of liquid crystal polymers was the recognition of the importance of mesophases in the development of structure in conventional polymers. Examples of this include the stiffening of polyimide backbones to reduce the expansion coefficient and improve processibility and the recognition of the importance of a pseudo-hexagonal rotator, transient nematic phase in the crystallization of oriented polymer melts. Increasing the end-to-end distance of conventional polymers through the application of either mechanical or electromagnetic fields can lead to the formation of structure equivalent to that achieved by the manipulation of molecularly stiff molecules.
Fibers from lyotropic para-aramid polymers Figure 3. The fibers are dry-jet wet spun from percent sulfuric acid solution with sufficient. An annealing step may be performed to improve structural perfection, resulting in an increase of fiber modulus. These fibers have very high modulus and tensile strengths as well as excellent thermal and environmental stability. Weaknesses include low compressive properties endemic with all highly uniaxially oriented polymers and a significant moisture regain. Worldwide fiber production capacity is about 70 million pounds Selling prices vary according to grade i.
Consumption worldwide in was about 50 million pounds, somewhat trailing capacity. Major markets include reinforcement for rubber and composites, protective apparel, ropes and cable, and asbestos replacement. The use of para-aramid fiber is projected to grow at greater than 10 percent per year worldwide over the next 5 years.
The environmental issues involved in the handling and disposal of large quantities of sulfuric acid or other solvents may make thermotropic approaches more attractive in the future. During the s, thermotropic copolyesters were commercialized world-wide. More versatile than the lyotropic polymers, these nematic copolyesters Figure 3. While fiber products exist, most of the commercial thermotropic copolyester is sold as glass-or mineral-filled molding resins, the majority into electrical and electronic markets.
As in the case of the aramids, thermal and environmental stability is excellent.
Advantages of these molding resins are the extremely low viscosity, allowing the filling of complex, thin-walled molds, excellent mold reproduction because of the low change in volume between liquid and solid, and fast cycle times. Weaknesses include property anisotropy and high cost.
The future growth of the main-chain nematogenic polymers will be dominated by two factors:. Processing technology allowing cost-effective exploitation of properties, including orientation control in finished parts, as well as new forms e. Two particularly intriguing properties of nematogenic polymers not yet important commercially are ductility under cryogenic conditions and very low permeabilities of small molecules through the solid-state structure high barrier properties.
A potentially attractive route to both lower price and improved property control is the blending of liquid crystal polymers with conventional polymers. An extensive literature exists, and interesting concepts such as self-reinforcing composites and molecular composites have been developed to describe immiscible and miscible liquid crystal polymer-containing blends. Major problems encountered in this technology include:. Inherent immiscibility of mesogenic and conventional polymers, leading to large-scale phase separation;.
Strong dependence of blend morphology properties on processing and polymer variables; and. To date, commercial success for such blends has proved elusive. A related approach is the use of liquid crystal polymers in conventional composites, either as reinforcing fiber, matrix, or both. Penetration into conventional composite markets has been slow, the major problems being poor adhesion, poor compression fibers , and the lack of design criteria for composite parts where both matrix and ply are anisotropic.
The potential of polymeric liquid crystals in device rather than structural applications has been recognized in both industry and academia, but no commercially viable products have yet emerged.
The combination of inherent order, environmental stability, and ease of processing has led to interest in the use of polymeric liquid crystalline textures in applications as diverse as nonlinear optics, optical data storage, and "orienting carriers" for conducting polymers. With structural parameters of secondary importance, all textures are under active investigation. Both main-chain and side-chain approaches are of interest, the goal. Emerging problems include achieving sufficient density of active species to produce materials with competitive figures of merit i.
Clearly, the introduction of mesogenicity into polymers opens vast possibilities for molecular design, which may ultimately lead to the creation of materials with highly specific and unique property sets. Polymers are used in many applications in which their main function is to regulate the migration of small molecules or ions from one region to another. Examples include containers whose walls must keep oxygen outside or carbon dioxide and water inside; coatings that protect substrates from water, oxygen, and salts; packaging films to protect foodstuffs from contamination, oxidation, or dehydration; so-called "smart packages," which allow vegetables to respire by balancing both oxygen and carbon dioxide transmission so that they remain fresh for long storage or shipping times; thin films for controlled delivery of drugs, fertilizers, herbicides, and so on; and ultrathin membranes for separation of fluid mixtures.
These diverse functions can be achieved partly because the permeability to small molecules via a solution-diffusion mechanism can be varied over enormous ranges by manipulation of the molecular and physical structure of the polymer. The polymer that has the lowest known permeability to gases is bonedry poly vinyl alcohol , while the recently discovered poly trimethylsilyl propyne is the most permeable polymer known to date.
The span between these limits for oxygen gas is a factor of 10 A variety of factors, including free volume, intermolecular forces, chain stiffness, and mobility, act together to cause this enormous range of transport behavior. Recent experimental work has provided a great deal of insight, while attempts to simulate the diffusional process using molecular mechanics are at a very primitive stage. There is clearly a need for guidance in molecular design of polymers for each of the types of applications described in more detail below.
In addition, innovations in processing are needed. As shown earlier, packaging applications currently consume roughly one-third of the production of thermoplastic polymers for fabrication of a wide array of rigid and flexible package designs see Figure 3. These packages must have a variety of attributes, but one of the most important is to keep contaminants, especially oxygen, out, while critical contents such as carbon dioxide, flavors, and moisture are kept inside.
Metals and glass are usually almost perfect barriers, whereas polymers always have a finite permeability, which can limit. In spite of this deficiency, the light weight, low cost, ease of fabrication, toughness, and clarity of polymers have driven producers to convert from metal and glass to polymeric packaging. Polymers often provide considerable savings in raw materials, fabrication, and transportation, as well as improved safety for the consumer relative to glass; however, these advantages must be weighed against complex life-cycle issues now being addressed.
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.
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.
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. 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.
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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. 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.
The U. 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 to 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. Based on different water regulations in different countries, manufacturers in the building and construction industries need to offer an alternative material to metal in parts they produce. When designing with this material, it offers thinner wall thickness, dimensional stability, better hydrolysis resistance, and less variation in part assembly, thus, reducing scrap rates compared to other polyamide-based materials on the market.
The flow behavior and wide processing window means better over mold stability and bonding performance, and less internal stress, giving designers more freedom. EcoPaXX Q-DWX10 absorbs 30 percent less water and offers superior chemical and hydrolysis resistance compared to other polyamide-based materials, which is especially important when in contact with chlorinated water. It passed more than 1 million lifetime cycles testing in varying water temperatures, and fully complies with all major drinking water certifications see certifications listed below.
Manufacturers can therefore supply a global plumbing market—in the U. Besides offering a product portfolio that meets purity requirements for water contact applications, DSM offers in-house expertise on applications that need to meet the necessary drinking water certifications. A number of e-scooter components require specialized materials to ensure safe and reliable performance. DSM's Arnitel family of thermoplastic elastomers is helping tire manufacturers "reinvent the wheel" by developing non-pneumatic tires. Specialty polymers meet the need for lead-free water contact solutions.