High performance fibres-2

High performance fibres-2

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

Limited, 2001

• Most meta-oriented aramid 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 even more rigid molecular structure.

• Para-aramids, such as Kevlar, belong to the family of liquid-crystalline polymers (LCP).

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Liquid crystalline polymers (LCP)

• Modern fibres based on liquid crystalline polymers manifest outstanding tensile mechanical properties. They can reach a tensile modulus of up to 300 GPa and tensile strength of up to about 6 GPa.

• Remember that m-aramid have tensile modulus <18 Gpa and tensile strength around 0.6 GPa.

• A common situation in polymer solutions is that of randomly coiled polymer chains.

• However, if the chains are relatively stiff and are linked to extend the chain in one direction, then they are ideally described in terms of a random distribution of rods.

• Crystalline solids are ordered in three dimensions, while liquids are entirely disordered: liquid crystals lies between these two extreme cases, i.e. they exhibit long-range order in one or two dimensions, but not in all three.


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• The molecular asymmetry is the most important requirement for a macromolecule in order to originate the various possible LC phases (called mesophases).

• These asymmetry can be manifested either as rods of axial ratio grater than about 3, or thin platelets of biaxial order.

• Another fundamental requirement is a sufficiently high chain stiffness.

• Liquid crystals can be divided into thermotropic and lyotropic. In fact, the liquid crystalline behaviour may occur either in the diluted state (lyotropic liquid crystal) in a critical concentration range, or in the molten state (thermotropicliquid crystals) in the proper temperature range.

• Lyotropic and thermotropic LCPs are probably the ideal precursors for preparing fibres. In the diluted or molten states the degree of uniaxial orientation is typically very high and the extensional flow that is associated with the extrusion process orients the mesophases in the flow direction.


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• As the concentration of rod-like macromolecules is increased and the saturation level for a random array of rods is attained, the system will simply become a saturated solution with excess non-solved polymer;

• or more interestingly, if the solvent/polymer relationships are right, additional polymer may be dissolved by forming regions in which the solvated polymer chains approach a parallel arrangement. These ordered regions define a mesomorphic or liquid crystalline state.

• Continued addition and dissolution of polymer forces more polymer into the ordered state. If the rod-like chains are arranged in an approximately parallel array but are not otherwise organised, then the ordered phase is termed ‘nematic’.


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Lyotropic Liquid Crystal

• To retain a minimal volume, and indeed a minimal free energy, above a certain critical concentration, orientationalorder of the rods appears.

• In this case the solution becomes anisotropic. The degree of this anisotropy will be less than the strict three-dimensional ordering typical of a crystalline system, but at the same time it will differ significantly from an isotropic state characteristic of amorphous systems.

• The concentration threshold defining the transition to the liquid crystalline state will depend on the degree of shape asymmetry of the macromolecules, which will be determined as the ratio of their equilibrium length to their diameter, termed the ‘axial ratio’.


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• If concentrations above a critical limit are used, spinnability is affected due to undissolved material; therefore the resulting fibrehas inferior mechanical properties.

• Because these rod-like polymers are rigid, they orientate themselves with respect to each other, forming a nematic phase as illustrated in figure, which shows the orientation angle b with respect to the director n. This phase is dominated by liquid crystalline domains that contain aligned polymer chains. The degree of orientation of these polymer chains depends on solution temperature and polymer concentration.


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Dry-jet wet spinning

• Polymer spinning solutions are extruded through spinning holes and are subjected to elongationalstretch across a small air gap (of 10-15 mm).

• Under shear, the crystal domains become elongated and orientated in the direction of the deformation. Once in the air gap, elongationalstretching takes place. This is effected by making the velocity of the fibre as it leaves the coagulating bath higher than the velocity of the polymer as it emerges from the spinning holes.


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• The precipitation in the quench water bath freezes the structure in a highly oriented state.

• In comparison to the wet spinning process used for conventional organic fibres, where the spinning nozzle is immersed in the coagulation liquid, the air gap of the dry-wet spinning process induces a higher degree of molecular orientation and hence an improvement of the mechanical properties.

• The resulting stretch in the air gap furtherperfects the respective alignment of the liquidcrystal domains. Overall, a higher polymerorientation in the coagulation mediumcorresponds to higher mechanical properties ofthe fibre.

• Because of the slow relaxation time of theseliquid crystal systems, the high as-spun fibreorientation can be attained and retained viacoagulation with cold water.


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• The importance of the orientation induced during the spinning process clearly emerges when the effects of the solution concentration on the mechanical properties are considered.

• In fact, the mechanical properties are significantly improved only if the solution concentration is significantly higher than the critical concentration so that a LC is obtained.

• Fibres prepared by a dry-jet wet-spun process have a noteworthy response to very brief heat treatment (seconds between 150-550°C) under tension. These fibres will not undergo drawing in the conventional sense, showing an extension of less than 5% even at temperatures above 500°C, but the crystalline orientation and fibre modulus is increased by this short-term heating under tension.

• X-ray diffraction analysis shows that the heat treatment induces an increase of the apparent crystallite size and a reduction of the axial crystal orientation angle from about 15-20° to 10° or less.


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• Fibres can exhibit three possible lateral or transverse crystalline arrangements and these are illustrated in the Figure.


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• Poly(p-phenylene-terephthalamide (PPTA), a typical para-oriented aramid, is formed by the reaction:


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Para-oriented aramid fiber

• PPTA forms anisotropic solution in strong acids such as sulphuric acid, chloro- and fluorosulphuric acids, and hydrogen fluoride.

• At low concentration (below 8%), rod-like PPTA molecules are randomly oriented in an isotropic dilute solution.

• When the concentration approaches a critical value (around 12%), the molecules pack close together and rearrange is small domains, which remain randomly oriented.

• When the solution is under flow, shear and elongational stresses induce an orientation of the LC domains in the flow direction.

• PPTA provides an example of a polymer that has very limited solubility in suitable solvents. In the laboratory, para-phenylene diamide –PPD- is dissolved in an amide solvent in concentrations up to 0.5 mol/L. The solution is cooled to near 0◦C, stirring vigorously.

• Solid terephthaloyl chloride-TCL- in an equal stoichiometricquantity is added and within a few minutes the solution becomes opalescent. Vigorous stirring is continued as the polymerizing mixture solidifies and then breaks into particles with the consistency of wet sawdust. This crumb can then be neutralized with dilute caustic, washed with water, and dried.


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Laboratory synthesis

• Solvents using mixtures of hexamethylphosphoramide -HMPA - and Methylpyrrolidone –NMP- or HMPA and DMA produce polymers with higher molecular weight than any of the three solvents alone.

• Similarly, salts can be added to amide solvents to increase the solubility of PPTA and thereby increase the molecular weight. The combination of NMP and CaCl2 is especially useful for providing high molecular weight PPTA.

• Another approach for increasing the level of molecular weight that can be attained is the use of an acid acceptor, such as tertiary amines and tributyl amine.

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DMANMP

Commercial polymerization process

U.S. Pat. 3850888 (Nov. 26, 1974), J. A. Fitzgerald and K. K. Likhyani (to E. I. du Pontde Nemours & Co., Inc.)

TCL teraphthaloyl chlorideICL isophthaloyl chloride


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Spinning of PPTA• Unlike MPDI, high molecular weight PPTA is not soluble in

amide solvents, with or without the addition of inorganic salts. Formation of fibers from PPTA became possible when it was discovered that concentrated solutions of the polymer in 100% sulfuric acid had relatively low viscosity, could be spun at moderate temperatures.

• The patent literature U.S. Pat. 3767756 (Oct. 23, 1973), H. Blades (to E. I. du Pont de Nemours & Co.,Inc.) describes a spinning process in which PPTA is dissolved in 98–100% sulfuric acid at a concentration of greater than 18%. The solution is pumped through a spinneret into an aqueous coagulating/quenching bath, with an air gap separating the spinnerets from the bath.

• The fiber is then washed thoroughly with water and dried.


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• Fibers formed by this spinning process are highly oriented even without the type of high temperature drawing common for other polyamide fibers, and have high stiffness (tensile moduli of 50–75 GPa).

• Even higher moduli (Kevlar 149 has a modulus of 180 GPa) can be obtained by subjecting the fibers to a stretching process at high temperature.

• This would appear to be the basis for the high modulus versions of Kevlar and Twaron.


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• Conventional synthetic fibers cannot attain sufficient strength without drawing, and would not find practical use. However, when a para-aramid is spun from this concentrated sulphuricacid solution, the fiber shows high tenacity-high Young’s modulus without the need for drawing and additionally exhibits high heat resistance.

• The amorphous phase is virtually absent and a very small fraction (few per cent) of unoriented crystalline component is present.

• The crystalline structure of aramid fibres is arranged to form ordered lamellae, stacks of platelets with approximately 3 nm spacing perpendicular to the fibre axis.


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The lamellae are loosely connected as microfibrils (about 600 nm wide) with random tie points between fibrils. The fibrillar structure is superimposed on the crystalline structure.

Skin-core structure

• Aramid fibres also present a skin-core structure. The surface fibrils are uniformilyoriented in the axial direction, while the fibrils in the inner core are imperfectly packed.

• Skin and core regions

differ also in terms

of density and void content.

Properties


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Effect of temperature on modulus

Applications of PPDA-1• Protective Clothing. PPTA fibers have become the standard

from which body armor for the protection of police and military personnel is made. These protective fabrics must be thicker than their fire protection cousins, but they still must be designed to be comfortable.

• PPTA is also used to make cut resistant fabrics for use in gloves and chain saw chaps.

• Composites. High strength and low weight provides the basis for the use of PPTA reinforced composites for the strength members in aircraft, boats, the transportation industry, and sports equipment. Related applications would include providing the protective shielding in lightweight helmets for the military and as rigid armor in military vehicles, police cars, helicopters, and banks.


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Health and safety• The safety requirements in the production of aramid fibers are substantial. The

monomers used in these processes are highly toxic and are sometimes handled at high temperatures. They require protective clothing and great care to avoid any contact.

• Sulfuric acid is used in the fiber formation of PPTA, and protective clothing is a requirement for all personnel who enter the spinning area. The high strength of PPTA fibers makes handling the yarn at high speed a hazardous enterprise, and special training of production workers is especially important.

• PPTA and MPDI are relatively safe products that present minimal risk to human health and the environment. MPDI fabrics have been worn for over 30 years without significant effect on the skin, and PPTA products have a similar history.

• The Food and Drug Administration now provides that many forms of PPTA fiber may be safely used as components of articles that come in repeated contact with food.

• During the processing of these fibers some respirable fibrous particles are always produced, and inhalation of these particles should be minimized. Adherence to good industrial hygiene practices for ventilation and cleanup will protect against significant exposure.


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• PPTA is also used in pulp form in a variety of automotive and industrial applications such as brake and clutch linings, gaskets, and nonwoven felts, where it has replaced asbestos.

• Ropes and Cables. Here, again, high specific strength and stiffness are important. Cables based on PPTA fibers anchor oil rigs and provide ship to shore mooring lines. PPTA fibers also provide tension reinforcement for fiber optic cables, where high stiffness and dielectric properties are key advantages.

• PPTA fibers were developed originally to replace polyester and steel as the reinforcing fiber in tires. That market continues to be important today, although it never reached the level envisioned in the 1960s. Key market segments today include high performance automobile tires, heavy-duty machinery and aircraft tires and, on the other end of the spectrum, puncture resistant, high performance bicycle tires.

Tensile properties of high-performance fibers


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High performance fibres-2