By: Albin F Turbak
The pursuit of a better fiber and a better fabric is yielding products used in medicine, aeronautics, astronautics, seawater desalination, and construction of buildings and roads.
Today a typical man's shirt contains about 900 miles of filament-enough to stretch from Atlanta to New York city. Filament fiber extruders spin this fine yarn at a speed of 250 miles an hour. This ability to spin cobweb-size filaments at jet airplane speeds with sufficient precision to maintain constant elongation, molecular orientation, fiber diameter, and tenacity (fiber strength per unit weight) dramatically illustrates the sophistication of the new textile technology.
The new kinds of textiles possess characteristics that make them useful in numerous formerly unexpected applications. Although textiles are still the major component of the clothes we wear and of many furnishings in our homes and offices, they are also used widely in medicine, aeronautics, astronautics, pollution abatement, and numerous other fields. Some new textiles possess qualities that make them stain-resistant, flameproof, and even stiff. Some are "'nonwoven" matrices of overlapping fibers. Innovation in textile technology continues and more unusual products will almost surely emerge.
Replacing body parts
Certain fibers and textile materials are especially suitable for use in building synthetic body parts and medical scientists are steadily expanding the types of body parts whose function can be mimicked.
The artificial kidney is made from 7,000 hollow fibers, each of which is about the size of a human hair. Patients whose kidneys no longer function normally must have their blood freed by dialysis of metabolic wastes and excess water about every three days. This is accomplished by pumping the blood through a textile, hollow-fiber module while clean-sing solution rinses the blood free of urea, creatinine, and other impurities. Hospitals also use such blood dialysis on patients who have taken poisons or overdoses of medicines or drugs. This technique is more likely than time-consuming normal body elimination to save their lives.
Without the especially prepared cuprammonium rayon hollow textile fibers, there would be no artificial kidneys and thousands of people would die each year. These rayon fibers have exactly the right pore size to allow poisons and waste products to pass through while retaining the blood for return to the body after cleaning.
Artificial arteries made of knitted polyester textile tubes are used for many patients whose natural arteries leading to their legs are blocked. Patients with diabetes have a tendency to suffer from cholesterol blockage of arteries leading to their feet. If not corrected, poor circulation can lead to gangrene and loss of limbs. Artificial arteries that look like pencil diameter corrugated vacuum cleaner hoses are surgically inserted to bypass the blockages, thus restoring circulation and saving limb functions. These implants require crucial textile technology to prevent clotting and rejection. It is estimated that more than 150,000 people in the United States have now had these artificial arteries for over five years.
The. Jarvik-7 artificial heart is composed of over 50 percent textile construction including a polyurethane inner structure and Velcro junction fittings for greater comfort.
Sutures made from textile fibers of all types (silk, collagen, polyester, or nylon) for closing incisions after surgery are among the most expensive textiles, selling for more than $2,000 a pound. Bone replacements of carbon fiber composites are nonantigenic (not rejected by the body). By proper control of the composite porosity, it is becoming possible to have injured bone tissue accept and grow into the inserted replacement units.
Disposable, sterile, nonwoven coverings for operating tables, surgeon's masks, and disposable gowns are other examples of textile technology at work in medicine. To keep the operating covers from ripping when they are wet with blood, special nonstretched nylon fibers are co-blended into the nonwoven.
In the fields of aerospace engineering, construction, water purification and pollution abatement, a similar proliferation of new applications is emerging from research laboratories.
The first synthetic fibers
Originally all textiles were made from natural fiber such as cotton. wool, mohair, linen, ramie, and vicuna. All of these were available only as staple fibers that had to be spun into yarns before they were converted to cloth.
Silk was the first monofilament material, and for years scientists were obsessed with trying to make and 'artificial' silk. This was achieved in the mid1800s when rayon was produced by dissolving cellulose and regenerating it into the shiny filaments resembling silk in appearance. This was rapidly followed by acetate, which was prepared by modifying cellulose with acetic anhydride and then "dry spinning" the syrupy solution to recover the organic acetone solvent.
These developments laid the foundation for the scientific principles that led Wallace Carothers at the Du Pont company to invent nylon and polyester fibers and yarns. Under the competitive industrial pressures of the times.
Carothers committed suicide thinking that his efforts were failures because his managers were not satisfied with his rate of progress. Yet, nylon was, and still is, a tremendous commercial success. It offered greater strength, abrasion resistance. resilience, wrinkle recovery, and was easier to take care of than previous fibrous materials. The sheerness of knitted nylon stockings captured the public's aesthetic sense.
During World War II, it was possible to barter and trade almost anything of value for a pair of ladies' nylon stockings.
The high strength of nylon led to its use in tires to replace most rayon cord. This was perhaps due more to clever marketing than to any superiority of nylon over rayon. Subsequent problems, however, with the flat-spotting syndrome of nylon-belted tires left standing overnight led manufacturers to try successively glass-belted, steelbelted. and polyester-belted tire bodies.
Although great advances in fiber technology were made during the period from 1930 to 1950, it was innovations by Karl Ziegler in Germany in the mid1950s that set the stage for the next era of textile-fiber advancement. Ziegler discovered a process for producing polymers of the ethylene molecule, which at that time was a waste product of petroleum refineries. A polymer is any substance composed of giant molecules formed of many smaller molecules of the same substance. The molecular weight of a polymer describes the number of building block units that are linked together. For example, the basic unite (called a monomer) of ethylene gas (CH2=CH2) has a molecular weight of 28, but 10 such units linked together end to end as polymer (CH2=CH2) 10 have a molecular weight of 280 and a degree of polymerization of 10.
Ziegler discovered a substance that would catalyze the formation of high-molecular weight polyethylene. By bubbling ethylene gas into such catalyst dispersions, he produced a polymer of ethylene, polyethylene that had excellent melt-extrusion and melt-forming properties in comparison with previously known low-molecular weight waxes used for sealing jars of homemade preserves.
Ziegler's trusting nature led him into trouble. He shared his catalyst secret with Julio Natta who went back to Italy and claimed this same catalyst system for polymerizing propylene gas into polypropylene. Both ethylene and propylene up to this time had been waste products of petroleum refining. The usable polymer forms of both ethylene and propylene are called stereoregular polyolefins. The ability to upgrade these cheap gases into useful plastics and fibers immediately received the support of oil companies who could now turn waste into profit. They entered the textile industry as fiber producers.
Ziegler and Natta, who had been close friends, ended up not speaking to each other. Natta received a Nobel Prize for the invention of stereoregular polypropylene, and Phillips Petroleum Company acquired the patent for controlled-regularity stereo regular polyolefins based on an entirely different method of previous laboratory preparation. The use of polypropylene is now common for indoor/outdoor carpets and for furniture from which spills and stains can be easily removed with soap and water.
The textile industry has further learned that polypropylene nonwettable, waxy-based fibers can be spun into fine-diameter. Fibers and so constructed that they absorb as much moisture as highly absorbent natural fibers. Such fine fibers retain as much fluid by capillary action between fibers as natural fibers do by absorption within fibers. Thus today, cold-weather thermal underwear is worn next to the skin to keep the body dry, carrying perspiration away by capillary action to amore absorbent outer layer.
While the commercial development of stereoregular polyolefins was taking place, a new chapter in textile-fiber history was concurrently being written by Stephanie Kowalek at Due Pont. In reexamining the chemistry of nylon in 1957, she investigated polyamides made from aromatic (closed carbon ring) acids and aromatic amines rather than the previously employed aliphatic (open carbon chain) systems used in making regular nylon. Aromatic nylons have a much stiffer backbone structure since the cyclic rings have fewer degree of freedom of motion because of their inherent structure and bulkiness. Such aromatic nylons de not melt readily and it is necessary to dissolve them in rather exotic solvent systems to produce thick syrupy dopes. In working with such systems, Kowalek noted that, as she tried to dissolve ever more polymer, there came a point at which adding more polymer produced a more fluid rather than a more viscous solution.
At this point, the solution also displayed an iridescence due to the formation of a liquid-crystal phase. Spinning fibers from liquid-crystal systems led to the development of significant increases in tensile strength and stiffness, and the product so made was ultimately marketed as "Kevlar. Fibers made from spinning a liquid-crystal solution generally exhibit high tensile strength and have a high stiffness or modulus of elasticity. One can imagine the liquid-crystal phase as one in which the linear polymer rods align into domains much like matchsticks' or toothpicks' aligning together. Stretching during or after spinning is then more effective in obtaining good molecular alignment and resultant high fiber strength per unite weight (tenacity) and modulus of elasticity. As a result of the high degree of alignment, such fibers are excellent under tension but poor under compression and have low elongation and poor knot strength. Most materials display reduced strength if pulled after being tied in a knot, with stiffer materials usually exhibiting greater losses. In every case where fibers are made from liquid crystal solutions, such fibers exhibit pronounced increases in strength and stiffness.
Furthermore, Kevlar has excellent resistance to fire and will not burn even from direct contact with a propane torch.
The flameproof nature of Kevlar coupled with its high tenacity and high stiffness have made it the material of choice for bulletproof vests, fire fighter coats, and other protective equipment. Realizing the potential for new uses of such high-performance materials, the fiber industry moved rapidly to make other exotic fibers. For example, Celanese Corporation makes PBI (polybenzimidazole) fibers that have superb resistance of flame exposure and are used for gloves for working near molten metal and for astronauts' launch suits.
Pure carbon fibers
Another high-performance fiber now reaching commercial status is carbon fiber. This material has exceptional high-temperature stability and excellent tensile strength and stiffness. It is made by decomposing modified acrylic fibers of the type normally used to make sweaters or socks. By first treating such acrylic copolymers at about 400C in the absence of oxygen, it is possible to cyclize or join the adjacent nitride groups, which then release any residual nitrogen atoms upon further heating to temperatures up to 3,000C to give pure carbon or graphite fibers.
These are four times lighter and five times stronger than steel. They can be used in a wide variety of products.
One of the most recent developments is a super strength polyethylene fiber that was invented by Albert Pennings at Dutch States Mines in Holland. It is now commercially produced by the Allied Signal Corporation in the United States under the trade name "Spectra 900." For years, fiber scientists accepted the hypothesis that strong fibers could only be produced by using highly polar monomers containing atoms other than carbon in which large amounts of hydrogen-bonding could be present. Pennings discredited these theories by suing what is perhaps the most nonpolar material-polyethylene.
Through his innovative gel-spinning technique, he prepared a polyethylene The flameproof nature of Kevlar, fiber five times as strong as nylon and coupled with its high tenacity and high twice as strong as Kevlar. Gel-spinning is a procedure whereby the polymer is spun from a gel state rather than from a solution state.
It took scientists several hundred years to make the first synthetic cellulose fibers (rayon) having tensile strengths of about 3 grams/denier. [A one-denier fiber contains 9,000 meters per gram weight of fiber. Tensile strength measures the breaking strength in grams per denier. It took about fifty years more to develop polyamides (nylon) having tensile strengths of 6 grams/denier. It took about ten years longer to learn how to make polyamides (Kevlar) of 20 grams/denier, and less than ten additional years to discover how to make polyethylene fibers (Spectra 900) at 40 grams/denier. Each of these developments has brought with it a whole array of new products for which textiles were previously not considered materials of choice. Today it is difficult to define the term 'textile' because structured-fiber assemblies are so ubiquitous.
The nonwoven materials
Any attempt to describe the new textile industry would be incomplete without mentioning nonwovens and composites. These two areas have experienced phenomenal growth in technology and in commercial product importance. The nonwovens have probably undergone as much technological changes as textile fibers and fabrics.
Initially, most nonwoven materials were produced by using a wet-laid process where short-length (short-staple) fibers of less than half-inch lengths were added to modified paper machines in places of or in addition to, beaten pulps and were held together by use of resin binders. Among commercial products made with these new nonwoven textiles are products as diverse as baby diapers and automobile oil filters.
In fiber-extrusion technology where melted polymer is forced under pressure to make nonwovens directly out of a spinnerette, two techniques have been developed. One of these - called "spunbonded"- makes use of air currents that blow perpendicularly to the melted filaments as they exit from the extruder. These air currents force the tacky fibers to stick to each other at random points to produce a continuous filament nonwoven finished structure directly from the extruder.
The second method-referred to as "melt blown"-employs a machine similar to the one that makes cotton candy at the amusement park but uses melted resins instead of melted sugar. The fibers exit from a centrifugal head and are a laid onto moving belts as a rather densely packed nonwoven web. The webs produced by melt-blown technology can be controlled to give high capillary structures capable of excellent water pickup or can be so densely packed that they can be used as waterproof liners.
All nonwovens obviously have large cost advantages over wovens if they can provide adequate performance characteristics. Usually, however, nonowovens are much stiffer than wovens.
Conventional textile structures and nonwoven webs can be combined with a wide range of elastomeric (rubber-like) and plastic materials to make composites. In most composites, the textiles comprise less than 50 percent of the weight but account for more than 75 percent of the strength. This is particularly true when the matrix material has a high degree of elasticity or deformation as in tires, hoses, or conveyor belts. Similarly, the use of high-stiffness, high-performance fibers leads to exceptionally high-modulus composites when more rigid matrices are employed. Some examples of these are the new high-performance graphite fishing roads, gold clubs, and tennis racquets. By the proper combination of high-performance fiber and matrix it is possible to make products that will not burn when exposed to open blowtorches. Almost all products of high-performance composites are far stronger and far lighter than similar products made from metals. At present, the composite market in the United States totals about $ 1.1 billion and is growing at a rate of 30 percent a year.
Lighter, stronger, and more tough
Light-weight aircraft can be constructed by taking advantage of the stiffness and strength of lightweight, nonflammable carbon fiber and other high-performance fibers. Such lighter planes give vastly improved fuel economy.
Carbon, Kevlar. and boronnitride fibers are now used as composite reinforcements in making wings and other body parts for a variety of commercial and military aircraft. The new Boeing 767 commercial jet contains 46 percent textile composite in its body segments.
The McDonnell Douglas F-18 attack fighter uses some 56 percent textile composite in its body, and the Grumman A-6 and F-14 military fighters contain significant quantities of composites. The Air Force's most advanced forward-wing-design X-29 experimental airplane uses textile composites in its wings since no other material can withstand the stresses and would twist off. The Lear Fan has a body of 100 percent textile carbon-fiber composite. Its structural ribs and the drive shafts are also made of carbon-fiber composite. Many helicopter manufacturers use textile composites.
NASA space suits for launch and for space walks require zero-defect performance. The launch suits are made from PBI nonflammable high-performance fibers. The space-walk suits have different requirements. They require air-purifying, cooling, and pressurizing systems. Each suit is tailor-made for a particular astronaut and costs $1-1.5 million. Since the astronaut is under an oxygen pressure of eight pounds per square inch in this suit, special flexibility is needed to allow him or her to bend an elbow or grasp an article. Circular-woven fabrics together with layered structures of polyester, urethane, and other materials are employed.
Airplane parts other than body construction are also made of textiles. All U.S. commercial jets have brakes made from carbon composites. These are the only materials that can withstand the extremely high temperatures generated if takeoff is aborted. Stopping a plane weighing many tons in a short distance generates temperatures high enough to melt metals, making carbon brakes indispensable for heavy jets. Kevlar nonwoven felt liners are now used as fire barriers to cover the urethane foam seats on all aircraft to prevent the production of highly toxic cyanide gases when such foams burn during airplane accidents.
Rocket exhausts and nose-cone covers for space shuttles are made of carbon and other high-performance fibers. These protect the vehicles from heat from air friction during launch and reentry. The flames generated on the launchpad do not ignite the rocket because of the flame-resisting properties of graphite carbon-fiber-textile exhaust shields. Similarly on reentry, the white-hot temperatures from atmospheric friction do not consume the shuttle because high-performance fiber and ceramic structures provide protection.
Purification of water and air
Whole-body gas suits are required to protect soldiers from the chemicals used in gas warfare today. These chemicals can kill by absorption into the bloodstream through skin. The suits allow for transport of perspiration moisture to prevent soldiers from being overcome internally as would occur from a non permeable film covering. It is well known to chemists that a cube of activated charcoal powder measuring one inch on each side has an adsorptive capacity equal to a football field.
It was therefore believed that properly constructed porous carbon fibers could exhibit superior gas-adsorption capability. This has been achieved by scientists at the School of Textile Engineering at the Georgia Institute of Technology in Atlanta, Georgia. Specially prepared carbon fibers developed there have one acre of adsorption surface per ten grams of fibers. Wholebody gas suits can be constructed that give soldiers up to one hour of protection while allowing for personal comfort. After each exposure, a suit can be decontaminated for reuse.
Drinkable sea water is now available through properly prepared hollow fiber reverse osmosis modules. Sea water is forced through these modules under a pressure of 400 pounds per square inch. Pure drinking water passes through the hollow fiber wall while concentrated salt water exudes out of the end. About 300 cities obtain their daily water supplies from reverse osmosis stations that deliver upto 3.5 million gallons each day at a cost of about 70 cents for 1,000 gallons. This is only slightly higher than most other municipal water supply costs and includes both operating and depreciation costs. Jidda in Saudi Arabia; Sarasota, Florida: Elath, Israel, and thirty cities in Japan use reverse osmosis technology.
Concentration of liquids that normally deteriorate from heating is possible with reverse osmosis fibers and membrane systems. Many liquids, including orange juice and tomato juice, can be concentrated by pressure without heat to preserve the thermally unstable flavor ingredients. Most orange-juice concentrate on the market today is prepared this way. Similarly concentration of gases can be achieved by proper use of membrane and fiber composition. Gas-separation systems are currently in use at most U.S. petroleum refineries.
Pollution abatement through reverse osmosis technology is used in several US. paper mills, in food plants (to keep down biological oxygen demand), and in the metal-plating industry to purify toxic-component effluent streams. An unusual use of reverse osmosis is the purification of astronauts waste liquids for reuse. Since water is the heaviest material on a space mission, recycling is mandatory and today's expended collected liquid is tomorrow's coffee.
Water passed through the reverse osmosis process is sterile. Several hospitals use reverse osmosis membranes to filter out viruses and supply sterile water to operating rooms.
Superdomes and stadiums are being constructed with roofs of silicone-coated fiberglass. This is particularly important in northern areas where heavy snowfall has caused the collapse of heavy concrete roofs. At the Hubert Humphrey dome in Minnesota, 800,000 square yards of such material in two layers allow warm air to be circulated to melt the snow. At an air terminal in Saudi Arabia, a multi-funnel design allows circulation by convection of the air up through hollow top ports. Many shopping malls are turning to these flexible composites to allow for more freedom in architectural design, improved aesthetics, better air circulation and better light transmission.
A large number of reservoirs such as farm ponds and city water supplies depend on rubber-coated textiles for bottom liners to prevent seepage of the stored water.
Road beds undergoing upheaval and potholes from severe weather conditions can be lined with nonwovens that significantly improve the stability and durability of the road.
Stiff fiber-reinforced, composites have improved golf clubs, tennis rackets, fishing rods, and vaulting poles, some of which have been used to set world records. Glass fiber-reinforced water skis and new styles of very durable fishing boats would not be possible without modern textile fibers.
The GM Corvette super sports car has a glass-fiber-reinforced molded body. Many textile fiber composites are four or five times stronger than steel, yet much lighter. Automobile manufacturers are developing textile composites as substitutes for a variety of structural parts.
Rapid technological advances in the textile industry have opened now opportunities for many technical disciplines, but have resulted in a shortage of textile chemists, textile engineers, and textile-management executives.
Some U.S. colleges with textile-engineering programs report that they could place nearly three times as many students in well paying jobs as they are currently graduating. Together with a bright outlook for further rapid advances and new products with unusual but useful properties, this shows that textiles remain among the most dynamic contemporary sectors in technology.
About the Author:
Albin F Turbak is the Director of the School of Textile Engineering at the Georgia Institute of Technology, Atlanta, Georgia.