High-Tech Fibrous Materials - American Chemical Society

Chapter 1

High-Tech Textiles Evolution or Revolution 1

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Albin F. Turbak and Tyrone L. Vigo 1

Southern College of Technology,1110South Marietta Parkway, Marietta, GA 30060 Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 19687, New Orleans, LA 70179

Downloaded by 80.82.77.83 on May 9, 2018 | https://pubs.acs.org Publication Date: April 3, 1991 | doi: 10.1021/bk-1991-0457.ch001

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The new world of textile technology now encompasses realms of products that were not even dreamed of ten years ago. Hightech textiles are prevalent as major components in such diverse products as artificial organs, structural bone replacements, rocket motors and nozzles, space shuttle shields, the Stealth bomber and most of the new design military, commercial and civilian aircraft, boats, bicycles, high-performance sporting goods, road beds, food/water removal systems, pollution control devices, translucent roofs for stadiums, airports and shopping malls, components for converting sea water into potable water, optical fibers for transmission of laser-pulsed communication and extended, temperature-controlled garments. Fibers that are five to ten times as strong as steel on a weight basis now provide the lead technology for making automobiles that are 50% lighter, safer and twice as fuel-efficient as current models. These examples and the history of the development of this technology will be reviewed and discussed in depth in this book.

The textile industry is one of American's major industries and employs 2.3 million people for producing yarn and converting it into finished textile products. The American consumer spends $110 billion annually for apparel and an additional $120 billion for other textiles such as upholstery, furnishings, carpets, etc. (1). Over two-thirds of all carpets made in the United States and one-third of all carpets made in the world are produced in the Dalton, Georgia area. This constitutes about 1.3 billion pounds of this textile product annually (2). Major U. S. automobile manufacturers (such as General Motors, Ford and Chrysler) use approximately 18 million yards of fabric in their vehicles. The textile industry purchases $2 billion worth of new foreign machinery annually on which they pay 0097-6156/91/0457-0001$06.00/0 © 1991 American Chemical Society

Vigo and Turbak; High-Tech Fibrous Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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HIGH-TECH FIBROUS MATERIALS

$110 million in import taxes (3). In spite of this massive machinery purchase and fantastic domestic consumption, the United States still produces only 13 billion pounds of fiber while it consumes 16 billion pounds - a 3 billion pound shortfall (4). With such impressive statistics, one would assume that textiles should be in a very strong economic position. Yet, exactly the opposite is true! In 1988, foreign imports of textiles accounted for a $20 billion deficit second only to the monstrous $55 billion auto import deficit (5). New developments by U . S. textile companies are few because of two main factors:


1. Leveraged buy outs of textile companies usually leave the new owners with such massive debt and interest payments that research and development are immediately sacrificed to improve cash flow. 2. While the federal government collects $110 million in import taxes each year on the textile machinery purchases, not one cent of these revenues is returned to universities engaged in textile education and research. A return of such revenues for R & D could help the domestic industry remain healthy and globally competitive. Moreover, there is an urgent need for the establishment of a National Textile Institute patterned along the lines of NSF to coordinate such R & D (6). This could initially be achieved by allocating at least $10 million of the $110 million import taxes now being collected for the foreign machinery purchases. While the staid textile industry is working out its dubious future in areas of conventional uses, forward-looking companies and entrepreneurs have begun to capitalize on advanced materials technology by utilizing modified natural and specially constructed synthetic fibers to produce textile products with exciting and high tech features. This high tech evolution or revolution is worldwide. However, the contributions in this book are all from American scientists and companies, indicating that substantial progress has already been achieved and more innovative developments are on the horizon. Although the tenacity of these new textile fibers has not yet reached the theoretically predicted 100 gram-perdenier level and the new textile products are not yet in the realm of the apparel in the classic movie "The Man in the White Suit", there are many exciting innovations in diverse areas of technology. These include, but are not limited to: 1. Biomedical uses for artificial hearts, heart valves, artificial kidneys and arteries (Figures 1 and 2), improved prosthetic devices, knee and hip joints, antimicrobial sutures, and optical fibers to guide lasers for pulverizing kidney stones and for vaporizing arterial-plaque blockage. 2. Transportation uses for making lighter weight and more fuel-efficient airplanes (Figure 3), vehicles, faster boats, rocket nozzles and ablative heat shield tiles for the aerospace program.



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High-Tech Textiles: Evolution or Revolution

Figure 1. Artificial heart - 50% textile fiber content

Figure 2. Knit polyester artificial arteries


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3. Protective clothing that can stop bullets (Figure 4) and resist flame, gloves that can't be readily cut with a razor blade and can catch white rivets with impunity, lumberjack trousers with built-in pads that can stop a chain saw, total body suits for protection against the newest skincontact nerve gases while allowing for moisture transport from within, and comfort clothing that simulates natural down or that literally and reversibly absorbs and releases heat based on incorporation of phase change materials. 4. Geotextiles used in civil engineering applications such as road-bed liners to minimize pothole formation, prevent retainer-wall movement and collapse as well as shifting of earth embankments, strengthen the dikes in Holland, minimize soil erosion and gully formation, prevent undesirable water seepage into subterranean aquifers or commercial structures, pond liners to preserve valuable water resources, and reverse osmosis units that supply 300 hundred cities worldwide with up to 3.5 million gallons of drinking water daily for each city. 5. Architectural and construction uses where flexible composites of glass and silicone are used for fabricating roofs for the Hubert Humphrey Dome in Minneapolis (Figure 5), the airport terminal in Saudia Arabia (Figure 6a and 6b), translucent roofs for modern shopping centers, and silos for storage of grain and agricultural products. These also include horticultural uses of pigmented polyolefin fabrics for shade covers and fiber composite roofs for greenhouses. 6. Recreational uses in which carbon-fiber reinforced composites make possible lightweight and strong tennis rackets, golf clubs, snowmobiles, spas, bicycles, speedboats, and other recreational equipment and accessories. 7. Specially constructed nonwoven fabric assemblies for filtration of gases and liquids in a variety of industries and applications such as dairy, pollution control, chemical and water purification, for reduction of dust/ particles and as clean room garments in the computer and biomedical industries. For example, "Intersept"-treated fabrics, filters and carpets (7) which prevent bacterial, mold and growth on fabrics used in hospitals and provide a cleaner and healthier office and living environment. Also, high-temperature resistant fibers which are used to filter hot exhaust gases up to 2000°C. While "High Tech " Textiles are impacting our daily lives in significantly new ways, commodity fibers have also been employed to extend their horizons of utility. These changes include bicomponent, carbon-loaded conductive nylon and polyester fibers for elimination of static charge, "Flecton" metal-coated fibers and fabrics for electromagnetic shielding (8), melt-blown polypropylene structures for diaper liners to wick away moisture more effectively, acrylic PAN fibers for



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Figure 3. Lear Jet - 98% carbon fiber composite - world's most fuel-efficient airplane

Figure 4. High tech fiber body protection -- bullet-proof vest and new army helmet




Figure 5. Hubert Humphrey Fabric-Domed Sports Arena, Minneapolis, M N




TURBAK & VIGO

Figure 6. Fabric roof of Saudi Arabian Airport Terminal: (a) inside and (b) outside


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concrete reinforcement and modified nylons for stain-resistant carpets. Such modifications are obviously important, but the inherent properties of standard fibers will limit their upper performance levels for consideration in a number of high tech areas. With these kinds of advances taking place, it is difficult to contain an enthusiasm for the future of what had been dubbed in the early 1980's by A . F. Turbak as Hi Tech Textiles (9). In each of the above-mentioned areas, new types of structured fibers with outstanding properties resulted in their advantageous utilization for exciting new areas of use. In considering these advances, it is important to realize that new fibers and textile products have resulted from fundamental and applied discoveries of a dual chemical and physical nature. High-Tech-Fiber Evolution Some excellent examples of high-tech-fiber evolution are found in the historical development of nylon, polyethylene and carbon fibers. In the case of nylon, Dr. Wallace Carothers at duPont first discovered in the early 1930's that aliphatic dibasic acids could be linked with aliphatic diamines or aliphatic glycols to form respectively polyamides and polyesters. The physical form of these polymers varied from oils and greases to meltable solids capable of being drawn into fibers by removing water as the reaction proceeded from 90 to 99.999+ % completion. His equation (Equation 1) relating degree of polymerization (DP) to the extent of completion of water removal was basic to giving direction to the new science of polymer formation via condensation technology: DP

=

1/(1 - B)

(1)

where DP = degree of polymerization and B = decimal % of reaction completion. These then new polymers exhibited tenacities of 6-10 grams per denier (gpd) whereas cotton, wool, rayon and acetate fibers at that time had only 1.5-3.5 gpd. It took 40 more years before Dr. Stephanie Kwolek (also of duPont) utilized "aromatic polyamides" having a length to width (or aspect) ratio of at least 10/1 to produce "liquid crystal" solutions of nylon. Upon dissolving this type of polymer, she discovered that as the concentration was increased, there came a point where the viscosity began to drop rather than increase. At this point, it was also observed that the solution became iridescent, and she reasoned that these concurrent phenomena were due to the close association of these more linear polymer chains into clusters or liquid crystals. Such "pre-organized" chain bundles ultimately led to the spinning of a nylon having a strength of 22 gpd - 10 times the strength of cotton and other natural fibers (10-14). This type of material was finally commercialized as "Kevlar" in a wide variety of products requiring high tenacity and high modulus such as bullet-proof vests, truck tires and cut-proof gloves. Kevlar also has a melting temperature of about 700°F compared to about 480°F for regular aliphatic nylon. This led to use of aromatic polyamides for a wide variety of high


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temperature applications such as nonwoven batting covers for airplane seats to retard combustion of the polyurethane foam in the seats and as an asbestos replacement in vehicular brakes. A similar story exists for polyethylene which for years was nothing more than a refinery by-product that housewives used to melt seal the tops of their jams and preserves. While some preliminary efforts were made to produce highmolecular-weight polyethylene by use of high pressures, it wasn't until 1953 that Dr. Carl Ziegler discovered his special Al(Et)/TiCl heterogeneous catalyst system used to produce highly crystalline, high-density, linear, high-molecularweight polyethylene at room temperature and atmospheric pressure. Shortly thereafter, he showed his invention to his friend Dr. Julio Natta who promptly returned to his native Italy and used the catalyst system to produce stereoregular isotactic polypropylene and other polyolefins that resulted in a totally new array of textile and related products. For his discovery, Ziegler ultimately shared a Nobel Prize in Chemistry with Natta in 1963. But now as Paul Harvey says, you need to know "the rest of the story!" Up through the mid 1970's, everyone involved in polymers had reasoned that the high strength of nylon over cotton or wool was due to the high degree of hydrogen bonding that occurred between chains. No one really took much time to note that Dr. Kwolek's Kevlar discovery, in fact, might have led to less hydrogen bonding energy because aromatic nitrogens were less basic and the rigid-chain structures had less tortuosity, thus limiting atomic proximities while the product still gave significantly higher strengths. This observation, along with Ziegler's discovery of stereoregularity, were the first concrete suggestions that structure perhaps played as great as, or an even greater role than, chemical composition in determining ultimate fiber and film properties and performance. In the 1970's the effects of structural considerations were finally brought into focus as researchers such as Dr. A. J. Pennings and his associates in Holland made another startling discovery. They found that by spinning high molecular weight, high-density polyethylene from a hot solution into a nonsolvent, they could affect what they called "Gel Spinning." This process gave them control of extremely high orientation and continuity between crystalline and amorphous regions (minimizing chain folding into a lamellar structure) thereby producing fiber strengths of 44 gpd — twice the strength previously achieved with Kevlar (15). Thus, the strongest fiber in the world is now being made from a hydrocarbon nonpolar monomer (ethylene) that simply cannot have a high degree of hydrogen bonding. This fiber has been recently commercialized by the Allied-Signal Company as "Spectra 1000" with the commercial version having strengths in the 32-35 gpd range. It is now obvious that we are finally beginning to understand how to approach the truly available fiber strengths of about 100 gpd calculated on a theoretical bond-strength basis. It is also important to demonstrate that old technology is always worth examining more closely, and no better example of this can be found than in the history of carbon-fiber production. When rayon was first produced in 1880, Count Hilaire de Chardonnet of France took some rayon samples to Switzerland and left them with his friends. They tried to combust them in an atmosphere of limited oxygen and succeeded in making the first carbon fibers. Their driving 4


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force for this work was that Swan in England and Thomas Edison in America had both just invented the light bulb and were urgently seeking new filament materials that would last longer than two hours; moreover, Edison was willing to pay the unheard price of $2.00 per pound for such a material. The carbon fibers were a great improvement and gave illumination for several weeks compared to the other materials, but they were ultimately replaced by the special metal alloy filaments still in use today. Carbon-fiber technology lay in abeyance for nearly 80 years until 1963, when the new space program required fibers that had exceptional heat resistance, high modulus, reasonable textile processability and excellent ablative performance to protect space vehicles which were experiencing extreme heat generation on returning to earth. The best fiber combining all these properties is carbon fiber which is finally graphitized at temperatures of up to 3000°C (5432°F). Today, the rocket nozzles made of graphite-fiber composites are used to contain the flames from the lift-off, propulsion-rocket-motor combustion and also are the best ablative tile materials applied to the front and bottom surfaces of the shuttle vehicle itself for re-entry into the earth's atmosphere (Figure 7). Carbon fibers can be made from a number of different precursor materials, the three most common raw materials being rayon, polyacrylonitrile (PAN) and meso pitch. The two prime contenders at present are rayon and PAN fibers. Of these two, rayon currently has a performance advantage for aerospace in two important areas: density and thermal protection (as illustrated in Table I). Table I. Carbon Fiber Properties from Various Precursors (16) Precursor Rayon Polyacrylonitrile Pitch

Density 1.44-1.52 1.55-1.78 1.88-2.00

Thermal Conductivity (W/mK) 3.7-4.1 8.5-43 22-120

The main disadvantage of rayon as a carbon-fiber precursor has been its unpredictability as a raw material since the sole supplier of the proper type of rayon, Avtex, Inc., was temporarily closed in 1988 and finally closed in 1989 by the state of Virginia for pollution; thus, the government agencies had to scramble to try to develop a second source of supply to keep various space and rocket programs intact. This situation seems presently to be temporarily under control. As future research improves the performance of PAN-based materials, this precursor may well replace rayon in these critical applications. However, gaining acceptance as a "qualified fiber" for NASA uses will take several years of testing, and thus rayon can be expected to maintain its dominant position for space use for the foreseeable feature. For less critical uses not involving the specific needs of the space program, P A N fibers presently offer sufficient advantages over rayon in that they are the materials of choice for ease of


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High-Tech Textiles: Evolution or Revolution 11

handling, ease of carbonization and have superior specific fiber properties. Pitchbased carbon fibers offer cost advantages compared to the other two precursor materials. Some experimental work has also been undertaken using Kevlar as a graphite-fiber precursor material, but the fiber produced did not exhibit any spectacular improvement in its final fiber properties. The main research areas currently requiring the most attention are: (a) how to greatly increase the speed of carbonization and (b) how to improve carbon fiber adhesion to polymeric matrices.


High-Tech-Fiber Utilization In many instances, fibers are preferred over films or other structural shapes because of the high-surface area available for a given weight of material. A good example of this is found in artificial kidneys where such devices were first made from cellophane tubing having a flat width of about an inch (Kolf dialyzers) or from cellophane sheets arranged as a parallel plate filter (Kiil dialyzers). Each of these designs presented significantly less surface and resulted in significantly more blood retention than the present artificial kidneys which employ bundles of 7,000 hollow fibers. These fiber bundles are about 12 inches long and have fibers of 10 microns outer diameter with a hole about half that size. These newer units allow for much more rapid patient dialysis rates with a drastically reduced serum retention and clotting. A recent extension of the application of the artificial kidney is the use of extracorporeal dialysis to cleanse the blood of individuals who have taken an overdose of sleeping pills, accidentally consumed toxic chemical or overdosed with drugs. This rapid means of removing excess harmful material is helping to save many more lives. An opposite situation from dialysis occurs in the case of reverse osmosis where similar types of modules with different fibers are used to filter salt from sea water or to concentrate dilute feed streams. It is possible to employ the proper polymer and copolymers to achieve either Ultrafiltration or Hyperfiltration. Such units have found extensive commercial use in water desalination and purification, effluent pollution abatement and concentration of materials that cannot endure heat without undergoing significant alteration or decomposition. Over 300 cities depend on hyperfiltration to supply their daily potable water of up to 3.5 million gallons per day (Figure 8). The typical cost for water purification by reverse osmosis is about 80 cents per 1000 gallons which includes investment depreciation and operating expenses. By comparison, regular purification of river water costs about 60 cents per 1000 gallons. "Hybrinet" polysulfone hollow fiber modules are available for use in bioreactors to grow mammalian and plant cell cultures (17). Since reverse osmosis media have pores small enough to reject salt, they are also capable of rejecting bacteria and viruses. In Japan, small reverse osmosis units are installed under sinks in the operating rooms and deliver sterile water when a physician steps on a pump-starting pedal. In space, the astronauts use reverse osmosis units to recycle their water to minimize the total weight of water that must be carried on each trip. Recently, a retired engineer and his wife



HIGHTECH FIBROUS MATERIALS

Figure 7. NASA Space Shuttle with carbon fiber heat shield

Figure 8. Commercial reverse osmosis fiber, potable-water installation



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High-Tech Textiles: Evolution or Revolution 13

sailing in the Pacific had their craft sunk by a school of whales, but they survived for 44 days because they had included a hand-operated reverse osmosis unit on their raft. In the medical area, optical fibers are finding increased uses in directing laser beams. Optical fibers and lasers are now being used to locate and disintegrate kidney stones by well-established procedures. Recent extensions of this technique utilize high-intensity, ultraviolet-excimer laser beams to destroy arterial plaque. This technique may ultimately replace the now standard angioplasty which uses inflatable balloons to compress such life-threatening cholesterol and low-density, lipoprotein plaque material against the arterial wall and usually requires repeated treatments. While this technique is still experimental, the initial results have been very promising with a very high level of success and low side effects. Excimer lasers differ from regular Yag and similar infrared ray lasers in that they have a significantly smaller focus diameter and they leave essentially no scar tissue since they do not generate heat. In the aerospace, aircraft and transportation areas, high performance fibers are rapidly becoming indispensable materials for advanced structural use. The new "Stealth Bomber" is an obvious example of such use. The high- specific tensile and modulus values coupled with the high-temperature stability achievable by use of these fibers have led to composites that greatly outperform metals. This High-Tech aspect of textile composites is currently being actively pursued by the three largest American automobile manufacturers who have now formed a "Composites Consortium" to jointly fabricate the composite car of the future. The exceptional weight savings that can be achieved with composites over metals coupled with the much simpler fabrication of complex shapes easily justifies these efforts. Data in Table II indicate some of the weight advantages of composites over metals in vehicles. Table II. Automobile Weight-Saving with Carbon Fiber Composites Component Part Hood Door, right rear Hinge, upper left front Hinge, lower left front Door, guard beam Suspension arm, upper Suspension arm, lower Transmission support Drive shaft Air conditioner (lateral brace) Air conditioner (compressor bracket)

Steel Ob)

Graphite Ob)

Reduction 0b) % 25.00 17.60 1.78 1.90 . 1.45 2.17 1.63 1.80 15.40

62 58 79 71 37 56 56 77 56

40.00 30.25 2.25 2.67 3.85 3.85 2.90 2.35 27.40

15.00 12.65 0.47 0.77 2.40 1.68 1.27 0.55 12.00

9.50

3.25

6.25

66

5.63

1.35

4.28

76



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The major weight savings, however, will come from use of composites in the engines where such engines will be designed with ceramic sleeves for combustion chambers. The gasoline automobile of the future, therefore, will weigh about half of its present weight, be stronger and easier to manufacture and repair and will have significantly improved gasoline mileage per gallon. Figure 9 compares the strength and stiffness of composites made from different high-tech fiber materials. In the area of geotextiles, a great deal has already been accomplished through the use of High-Tech Textiles. Nonwoven mats such as the heavy nylon denier "ENKAMAT" have been extensively used to retain soil on the sides of new highways for preventing erosion, and this same type of material is now used in Holland to help reinforce dikes and prevent leakage. A modified version of this concept is now also being used by construction engineers to lay along the sides of new high-rise building foundations to prevent water seepage into subterranean levels. This type of construction also has vastly improved mildew resistance. Other geotextile products are being used as liners to control polluted water run-off from land fills, while polypropylene "TYVEK" nonwovens are being used as sublayers or liners in the construction of road beds, particularly where a soft sub-base is involved (Figure 10). In the area of protective clothing, High-Tech Textiles are playing a key role. The use of "KEVLAR" for bullet-proof vests and cut-proof gloves is well known. The new "SPECTRA" fibers, however, can now provide the same type of protection at a much lighter weight. Thus, today's bullet-proof vests for police work weigh about half that of earlier vests because of the much lower density of the "SPECTRA" polyethylene vs. the "KEVLAR" aromatic polyamide. However, "SPECTRA" does not have the high-temperature performance characteristics of "KEVLAR." High-temperature fibers are rapidly becoming very important industrially. The current F A A rulings require that airplane seats have improved flame resistance, and now a nonwoven batting comprised of "NOMEX" and "KEVLAR" (both aromatic polyamides) is being placed over the polyurethane cushions of all new airplane seats to meet these new specifications. It is entirely conceivable that similar standards will ultimately be required for school-bus seats and even for automobile seats. High-temperature fibers are also very important to the space program where polybenzimidazole (PBI) yarns produced by Hoechst-Celanese Corporation are being used for the astronauts' suits worn during blast-off of the shuttle (Figure 11). The polyphenylene sulfide (PPS) fibers from Phillips Petroleum Co. are also rapidly establishing a niche in related areas. What is still missing is a lighter weight fireman's suit. Even with all our current technology, the fireman's suit still weighs about 75 to 85 pounds, and thus represents a major detriment to fire-fighting efficiency, especially when they have to carry out victims and climb ladders with this heavy clothing. Similarly absent are improved quality cold-water suits for airmen downed into icy water where the present expected time of survival is only five minutes.


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High-Tech Fibrous Materials - American Chemical Society

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