High performance fibres

High performance fibres

High performance fibres, edited by J W S Hearle, Woodhead Publishing

Limited, 2001

• Although a strict definition of high-performance fibers does not yet exist, the term generally denotes fibers that give higher values in use in a range of applications.

• It commonly refers to fibers with some unique characteristics that differentiate them from commodity fibers such as nylon, polyester, and acrylic fibers.

• Synonyms are specialty fibers and high-functional fibers.

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• Commodity fibers are typically used in a highly competitive price environment which translates into large scale high volume programs in order to compensate for the (often) low margins.

• Conversely, high performance fibers are driven by special technical functions that require specific physical properties unique to these fibers.

• Some of the most prominent of these properties are:

• tensile properties

• operating temperature

• chemical resistance

• Each fiber has a unique combination of the above properties which allows it to fill a niche in the high performance fiber spectrum.


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Commodity vs. High Performance Fibres


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Basic properties- an overview

High-performance fibers can be classified broadly into three categories according to their applications:

1. High-Modulus and High-Strength fibers (HM-HS);

2. Heat-resistant fibers, including flame-retardant ones;

3. Chemically resistant


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• Tensile strength is often the determining factor in choosing a fiber for a specific need. A major advantage of high strength fibers over steel, for example, is the superior strength-to-weight ratio that such fibers can offer.

• Para-aramid fiber (e.g. Kevlar) offers 6-8 times higher tensile strength and over twice the modulus of steel, at only one-fifth the weight.


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High Tenacity- High Modulus fibres

• Two methods have proved effective in preparing high modulus, high tenacity (HM-HT) polymer fibres:

(a) perfecting the drawing technique of precursor fibres to attain draw ratios far above ten, as in the polyethylene Dyneema® and Spectra®.

(b) manipulating rigid rod-like molecules into fibresthat are already very highly oriented in the as-spun state, as in PPTA fibres (Kevlar ®).


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• Temperature resistance often plays an integral role in the selection of a fiber. Heat degrades fibers at different rates depending on the fiber type, atmospheric conditions and time of exposure. The key property for high temperature resistant fibers is their continuous operating temperature.

• Fibers can survive exposure to temperatures above their continuous operating temperatures, but the high heat will begin to degrade the fiber. This degradation has the effect of reducing the tensile properties of the fiber and ultimately destroying its integrity.


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• A common mistake is to confuse temperature resistance with flame retardant ability. Flameretardant ability is generally measured by the Limiting Oxygen Index.

• LOI is the amount of oxygen needed in the atmosphere to support combustion. Fibers with a Limiting Oxygen Index (LOI) greater than 25 are said to be flame retardant, that is there must be at least 25% oxygen present in order for them to burn.


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• Just as heat can degrade a fiber, chemical exposure, such as contact with acids or alkalis, can have a similar effect. Some fibers, such as PTFE (i.e. DuPont’s Teflon), are extremely resistant to chemicals.

• Others lose strength and integrity quite rapidly depending on the type of chemical and the degree of concentration of the chemical or compound.


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Examples

m-aramidsp-aramids

UHMW-PE


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Aramids• Aromatic polyamides first appeared in the

patent literature in the late 1950s and early 1960s, when a number of compositions were disclosed by researchers at DuPont.

• These polymers were made by the reaction of aromatic diamines with aromatic diacidchlorides in an amide solvent. Over 100 examples of aromatic polymers and copolymers described in patents.


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Para-phenylenediamine PPD Terephthaloyl chloride

• Sixty years later, after the expenditure of much time and money, the number of commercially important aromatic polyamide polymers has been reduced to three:

• two homopolymers, poly(m-phenyleneisophthalamide) (MPDI- commercial name Nomex) and poly(p-phenylene terephthalamide) (PPTA- commercial name Kevlar),

• and one copolymer, copoly(p-phenylene/3,4-diphenyl ether terephthalamide) (ODA/PPTA-commercial name Technora).


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• Simple AB homopolymers may be synthesisedaccording to the scheme below:

A is the amine group

B is the carboxylic group


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• AABB aromatic polyamides are prepared from various aromatic diamines and diacids or diacid chlorides. The early AABB polymers contained predominantly meta-orientated linkages.

• The earliest representative of this class is m-phenyleneisophthalamide, which was commercialised by DuPont in 1967 as Nomex® aramid fibre.


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• Kevlar® aramid fibre was launched by DuPont in 1971; the corresponding chemical formula is given below:

Kevlar® fibres are poly (p-phenylene terephthalamide) (PPTA), the simplest form of AABB para-orientated polyamide.


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• Because fibers from these aromatic polyamides have properties that differ significantly from the class of fibers known as polyamides, the United States Federal Trade Commission adopted the following definition to describe aromatic polyamide-based fibres : ‘a manufactured fibre in which the fibre-forming substance is a long chain synthetic polyamide in which at least 85% of the amide (—CO—NH—) linkages are attached directly to two aromatic rings’.


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• The important properties of this class of polymers include thermal and chemical stability and the potential for high strength and modulus.

• Aliphatic polyamides melt at temperatures below 300◦C, whereas most aromatic polyamides do not melt or melt above 350◦C.

• Aramids also exhibit greater chemical resistance and low flammability. These properties derive from the aromatic character of the polymer backbone that can provide high chain rigidity.


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• Aromatic polyamide fibers can have very high strength and modulus, and these properties persist at elevated temperatures.

• Because of their low density, aromatic polyamides have higher specific strength and modulus than steel or glass.

• In recent years, design engineers have been able to utilize these unique properties to create products which protect personnel from fire, bullets, and cuts, reduce the weight of aircraft and automobiles, and hold oil drilling platforms in place.


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Caution: Capacity dates back to 1974.

Meta-oriented aramid fibre

• The polymer is prepared by low-temperature solution polymerization or interfacial polymerization according to the following reaction:


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Solution polymerisation• In this technique, the aromatic diamine is dissolved in an amide

solvent (N,N-dimethylacetamide DMA or N-Methyl-2-pyrrolidone NMP ) and a stoichiometric quantity of the aromatic diacid chloride is added to the diamine solution while stirring vigorously.

• Since the reaction between an acid chloride and an amine is highly exothermic, the heat generated can significantly increase the temperature of the polymerizing solution. The extent of the temperature rise will depend on the diamine concentration and the degree of cooling.

• High temperature can lead to side reactions that produce unreactive ends, thus the reaction must be carried out at low temperature (0°C).

• The resulting polymer remains in solution.

NMPCH3

CH3

CH3

DMA15/10/2012 23

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Interfacial polymerisation• In interfacial polycondensation, the two fast-reacting intermediates

are dissolved in a pair of immiscible liquids, one of which is preferably water.

• In this technique the isophthaloyl chloride – ICL- is dissolved in a solvent with limited water solubility (i.e. tetrahydrofuran) and then added to an aqueous solution of the m-phenylene diamine –MPD-with vigorous stirring.

• The aqueous solution contains a base that neutralizes the HCl that is generated.

• Polymer formation takes place at or near the liquid–liquid interface when the two solutions are brought in contact or stirred together.


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THF

• Interfacial polymerization gives higher molecular weight than the solution polymerization technique and therefore, improved fiber properties. This is the process that Teijin uses to produce the polymer for their MPDI product, Teijinconex®.


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• In both the solution and the interfacial methods, the following factors are important for the preparation of a high molecular mass polymer:

• Use of high-purity monomers

• Stoichiometric balance of the two parent monomers.


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Commercial Polymerisation process of m-aramid

• U.S. Pat. 3287324 (Nov. 22, 1966), W. Sweeny (to E. I. du Pont de Nemours & Co.,

• Inc.)


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Solution polymerization

• The diamine noted in the figure is actually a 9% solution of m-phenylene diamine (MPD) in DMA in the patent example, and the diacid chloride is molten isophthaloyl chloride (ICL)- melting temperature 45°C.

• The MPD solution is cooled to −15◦C, while the molten ICL is supplied at 60◦C. The heat of reaction brings the temperature of the effluent from the mixer to 74◦C.

• This effluent is then cooled before Ca(OH)2 is added to neutralize the HCl formed in the polymerization reaction. Finally, the polymer solution is blended, deaerated, and filtered before being pumped to storage for use in spinning.


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Interfacial polymerization

• H. Fujie, NikkakyoGeppo40, 8 (1987)


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Spinning of MPDI

• Most of the aromatic polyamides have high melting points that prevent the type of melt processing common to aliphatic polyamides, polyolefins, and other polymers.

• Thus, most applications are based on forms of the polymer that can be prepared from solutions of the polymers. These would include fiber, films, and pulp.

• Techniques for processing polymer solutions are well known and include wet spinning and dry spinning of fibers.


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Wet spinning of m-aramid

• H. Fujie, NikkakyoGeppo40, 8 (1987)


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• The process involves dissolving the dry, salt-free polymer in an organic solvent at low temperature and then heating the dispersion to near 100◦C to form a clear solution.

• This solution is wet spun into an aqueous solution containing a high concentration of an inorganic salt.

• The coagulated fiber is washed, and then drawn and post-treated.


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• The dry spinning of MPDI from a DMF solution into an air column maintained at 225◦C has also been described - U.S. Pat. 3063966 (Nov. 13, 1962), S. L. Kwolek, P. W. Morgan, and W. R. Sorenson (to E. I. du Pont de Nemours & Co., Inc.)

• After the fibers thus formed are drawn 4.75× and the remaining solvent and salt removed by extraction in hot water, they exhibit a tenacity of 0.6 GPa and an elongation of 30%.


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Post-Spinning Processes.

• Significant portion of the aramid fiber sold in recent years has been in the form of staple, floc, or pulp products that are produced by the fiber manufacturer.

• Short fiber products are produced by cutting continuous filament yarn into lengths that range from about 1 mm to over 100 mm. Products in the 1- to 6-mm length range are referred to as floc, while the longer products are called staple.


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To convert strength and modulus values to cN/dTex, multiply GPa by 10.0 and divide by density (g/cm3)


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• Most of the mechanical properties of PMIA fibers are about the same as those of commodity fibers.

• Fibers from wholly aromatic polymers are, in general, highly sensitive to light exposure and poorly dyeable.

• Intrinsic color is a defect of wholly aromatic polymers, particularly when they are used for textile materials. Only PMIA is colorless and can thus be employed in dyed textiles.


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• Because PMIA exhibits high crystallinity and strong intermolecular cohesion due to hydrogen bonding, it has a high melting point and a high decomposition temperature. Accordingly, PMIA fibers have better thermal properties than commodity fibers.

• At elevated temperature, PMIA fibers offer better long-term retention of mechanical properties than commodity fibers; they also have good dimensional stability.


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The physical propertiesof Teijinconex, a typical PMIA fiber


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• MPDI fibers have found a substantial market as the fiber used to produce garments designed to protect workers from the hazard of fire. Obviously this includes firefighters and race car drivers, but it is also becoming standard clothing for workers who operate in chemical factories and other places where the danger of flash fires is real.

• Key properties include its inherent flame resistance; abrasion, wear, and chemical resistance which allows clothing to be washed and worn many times; and high elongation and low modulus which allows the design of comfortable clothing.

• Antistatic properties and other characteristics can be incorporated by use of fiber blends.

• MPDI fabrics have also been used as a fire blocking material in aircraft seat upholstery, where regulations require such functionality. They are finding increasing use as a fire block in hospitals and as upholstery where fire resistance is important.

• Another major market for MPDI fabrics is as bag filters for a number of industries, such as power plants, cement factories, and steel factories, where the ability to withstand hot, corrosive gases is critical.


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• Because of the excellent properties of PMIA fibers (e.g., high thermal and chemical resistance, as well as radiation resistance), their end uses are growing.

• Typical applications follow:• Clothing. Meta-oriented aramid fibers do not ignite, flare, or melt

and stick to the skin. This makes them suitable for heat-resistant clothing material in the following areas: – 1.Heated furnaces: work uniforms, aluminized coats and pants, capes

and sleeves, gloves and mitts, leggings, and spats; – 2. Emergency services: aluminized proximity suits, turnout coats and

jumpsuits, station uniforms, rescue uniforms, fire-fighting and aviation garments, riot police uniforms, ranger uniforms, gloves, underwear, leggings, and spats;

– 3. Fuel handling: work uniforms, rubber coats, gloves, socks, underwear, etc.


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• Interior Fittings. Materials from PMIA are used in aircraft interiors (for increased safety and enhanced flame retardance).

• Industrial Materials. Uses here include filtration fabrics (especially filter bags for hot stack gases); high-temperature heat insulants (especially replacing asbestos); reinforcement in fire hoses, V-belts, and conveyor belts; threads for high-speed sewing; and cut-fiber reinforcement for rubber composites.

• Electrical Insulation. High-temperature paper insulation for electric motors, dynamos, transformers, and cables; braided tubing for wire insulation; and dryer belts for papermaking are among the uses of PMIA fibers.

• Miscellaneous Uses. Home ironing-board covers and kitchen gloves are also made from PMIA fibers.


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• Most meta-oriented fibers are heat-resistant; they are regarded as the first generation of high performance fibers. Para-oriented fibers are considered the second generation of high-performance fibers; they are composed mainly of para-substituted residues, instead of the meta-substituted residues of the first generation.

• Du Pont initiated the second generation with Kevlar, a successor to the first-generation Nomex. Compared to meta-oriented fibers, highly sophisticated polymerization and production techniques are needed for the para-oriented type to overcome difficulties caused by their rigid molecular structure.

• Para-aramids, such as Kevlar, belong to the family of rigid-rod polymers.


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High performance fibres