Materials Research. 2014; 17(5): 1180-1200 © 2014 DOI: http://dx.doi.org/10.1590/1516-1439.250313
Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review
Dawelbeit Ahmed, Zhong Hongpeng, Kong Haijuan, Liu Jing, Ma Yua, Yu Muhuo*
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, 201620, Shanghai, China
Received: October 31, 2013; Revised: April 6, 2014
Poly (p-phenylene terephthalamide) fibers prepared by wet or dry-jet wet spinning processes have a notable response to very brief heat treatment (seconds) under tension. The modulus of the as-spun fiber can be greatly affected by the heat treatment conditions (temperature, tension and duration). The crystallite orientation and the fiber modulus will increase by this short-term heating under tension. Poly (p-phenylene terephthalamide) fibers have a very high molecular orientation (orientation angle 12‑20°). Kevlar and Twaron fibers are poly (p-phenylene terephthalamide) fibers. This review reports for PPTA fibers the heat treatment techniques, devices and its process conditions. It reports in details the structural relationships between the fiber properties which are influenced by the heat treatment process. In particular, focused deeply on the effect of the crystal structure and the morphology of the fibers on the mechanical properties of PPTA fibers.
Keywords: heat treatment, PPTA, crystal structure, mechanical properties, microstructure
1. Introduction inorganic fibers which include silicon carbide, alumina, glass and alumina borosilicate fibers4. 1.1. High performance fibers 1.2. Aramid story High-tech fiber is an expressive way to indicate that The twentieth century witnessed huge researches and advanced science and technology have been used to produce innovations for fibers. After Nylon revolution DuPont this fiber. They are high-modulus and high-tenacity (HM- Co.’s researchers struggled to develop stiffer and tougher 1 HT) fibers . Although the main difference between natural nylon-related fiber. In 1965 Stephanie Louise Kwolek and synthetic fibers is in structure also natural fibers and developed the first liquid crystal poly (p-phenzamide) synthetic fibers can be classified as high-function fibers (PBA) polymer, which provided the basis for poly and high performance fibers, respectively. Synthetic fibers (p-phenylene terephthalamide) under trade name of Kevlar were developed initially as copies of silk, wool, or cotton2. fiber in 1971[1,5-7]. The wholly aromatic structure with all The first high performance fiber, Kevlar was developed para-substitutions creates stiff macromolecular rod-like in the 1965’s at DuPont - a company with a long history in structures. fiber synthesis - by discovering a new method of producing a The first commercial material, named Kevlar, were polymer chain coupled to form a liquid crystalline solution3. made by DuPont in USA, Ireland and Japan. Another type In the 1980s, polyethylene, first synthesized in the of PPTA fibers, named Twaron, was produced in 1978 by 1930s, was processed into a high-performance commercial Teijin Aramid B.V. in Netherland1,5,8-12. fiber (with the trade names Spectra and Dyneema), Poly (p-phenylene terephthalamide) abbreviated PPTA, based on gel-spinning technology invented at DSM in the are organic fibers acronym originating from aromatic poly Netherlands. (amide) with a distinct chemical composition of wholly The high-strength polymeric fiber Zylon, based on aromatic polyamides which at least 85% of amide groups 2,12-15 rigid-rod polymer research that began in the 1970s at the (—CO—NH—) are bond directly to two aromatic rings . 16 U.S. Air Force Research Laboratory, was commercialized Burchill reported that it is largely poly (p-phenylene in 1998 by Toyobo in Japan. High-strength polyacrylonitrile terephthalamide), Figure 1. - based carbon fibers were developed by optimizing Aramid is an acronym originating from aromatic poly some parameters such as the polyacrylonitrile-copolymer (amide). Alternative names were PPTA, poly (p-phenylene terephthalamide), aramid, aramide, polyaramid and composition, fiber spinning, and polyacrylonitrile polyaramide. The International Union of Pure and stabilization and carbonization. Other high-strength and Applied Chemistry (IUPAC) nomenclature for a Kevlar high-modulus fibers have relatively high densities, are fiber is Poly (imino-1,4-phenyleneiminocarbonyl-1,4- *e-mail: [email protected] phenylenecarbonyl)17. 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1181
2. PPTA Fibers Fabrication The processes steps of PPTA fibers fabrications begin by synthesis of the polymer, preparation of the spinning dope (with suitable solvent and concentration) and, finally, 18 formation of the fiber by spinning . Figure 1. Chemical structure of PPTA fiber. 2.1. Polymer synthesis The low temperature polycondensation process found by Paul W. Morgan19 led to the preparation of unmeltable high molecular weight polymers. The low temperature solution polymerization methods employ reactions which proceed at high rates. This method is designated broadly for polycondensation solutions - although the process encompasses systems in which the reactants and polymer stay dissolved as well as systems in which the reactants are not all dissolved at the start and great many from which the polymer precipitates. Solution polycondensation method can be used to Figure 2. Synthesis of poly (p-phenylene terephthalamide). prepare many classes of polymers such as polyamide polyureas, poly urethanes, and poly phenyl esters e.g. Poly (m-phenylene isophthalamide) (MPD-I) “commercially is Nomex and Teijincontex” and poly (p- phenylene terephthalate) (PPTA) “commercially is Kevlar and Twaron”. The reaction here proceeds under dry nitrogen conditions at temperatures in the range –10 ~ 60 °C for several minutes to several seconds. In this method the reaction mixture will become increasingly viscous and the polymer may precipitate at a certain point. Kwolek and Morgan investigated the method of preparing an aromatic polyamide (Poly aramid) of poly (p-phenylene terephthalate) by low temperature polymerization which involve the reaction of an aromatic Figure 3. Structure of poly (p-phenylene terephthalamide). diamine (p-phenylene diamine ‘PPD’) and an aromatic diacid chloride (terephthaloyl chloride ‘TCl’ in an amide solvent, as shown in Figure 2[10,19-21]. The PPTA chain is fully extended and consists of an alternation of rigid aromatic rings and amide groups. The chain conformation, may be assumed as a kind of planar- zigzag form consisting of short and long linear bonds which correspond to the amide C(O)-N bond and the para substituted phenylene ring, respectively22. The greater degree of conjugation and more linear geometry of the para linkages shown in Figure 3, combined with the greater chain orientation derived from this linearity, are primarily responsible for the increasing strength23. The hydrogen bonding and the chain rigidity of these polymers lead to very high glass transition temperatures24. 2.2. Dope preparation Synthesized poly (p- phenylene terephthalate) (PPTA) is dissolved in concentrated sulphuric acid (H2SO4) to prepare a dope for fiber spinning. The 2H SO4 solution of PPTA shows a typical lyotropic solution property. The viscosity of the solution reaches its minimum point at a concentration of around 20 wt. % and the temperature is about 65-70 °C. The h viscosity of the anisotropic solution of PPTA is inh = 4.9 in concentrated H2SO4. Figure 4 explains the relationship between the concentration and the solution viscosity for h 5 poly (p-benzamide) (PBA) ( inh= 2.41) by Kwolek . The Oriented domains are formed at concentrations of more Figure 4. Relationship between concentration of polymer and than 12 wt. %25. solution viscosity for PBA5,15. 1182 Dawelbeit et al. Materials Research
2.3. Spinning process PPTA fibers could be spun from amide and salt solutions using a conventional wet-spinning process. These solutions are typically of low concentration and have anisotropic nematic states, as shown in Figure 5[26]. Figure 6 shows the differences in behaviour during spinning process of flexible and rigid polymers.
Figure 7. Dry-jet wet-spinning process by Blades24,27,28.
The anisotropic solution (liquid crystal domains) pass (extruded) through the spinneret and being stretched in about 0.5 or 1 cm of the air gap and the coagulation bath have 0 or 5 °C. The fiber comes out with high speed up to 200 m/min[18,28-30]. The dope viscosity should be low so as to allow processibility and the polymer concentration should be high so as to minimize the cost. The polymer molecular weight should be high to maximize the mechanical properties21. In the model of polymer molecular orientation in a dry-jet wet-spinning process emerges a single strand from the spinneret, as shown in Figure 8. 13 Figure 5. Liquid crystalline structure of PPTA/H2SO4 solution .
Figure 8. PPTA fibers formation13,18,24,31.
The crucial fiber microstructure formation takes place Figure 6. Flexible and rigid polymers, differences in behaviour during the coagulation stage of this spinning process. The during spinning process3. final stages of the spinning process for the coagulated filaments consist of washing, neutralizing and drying. During the drying process a light tension is applied. This tension does not make significant stretching32. The resulting fiber has a fibrillar texture, but it is highly In 1970, Blades27 innovated that the high-strength oriented and has the necessary high crystallinity and the and high-modulus fibers can be spun from anisotropic chain-extended structure to give it high modulus and high solutions (spinning dope) of aramid polymers by using strength greater than those of the flexible semi-crystalline dry jet-wet spinning processes. Dry jet-wet spinning gives polymer fibers. The elongation of the resulting fiber here a higher strength, a higher modulus and a lower elongation is low17,24,33. The modulus of the PPTA fibers is essentially - Figure 7. independent of the inherent viscosity and the air gap. The 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1183 high performance, PPTA fibers are known to have very high 3. Microstructure of PPTA Fibers modulus and very high strength and good thermal stability as a result of their chemical structure17. 3.1. Crystal structure The relationships between the processing parameters PPTA fiber’s structure was early studied by Northolt on - which are the conditions, the fiber structure and the the basis of X-ray diffraction. Northolt investigated the fiber properties of the PPTA fiber - are crucial to understand the structure. He reached the conclusion that the supermolecular behaviour of the fibers, the yarns, and the fabrics for any structure instead of having a two-phase structure consisting desired application. Table 1 shows the difference between of amorphous and crystalline domains, it- probably- has a 34 the dry jet-wet spinning and the wet spinning of PPTA fibers. paracrystalline structure . The crystal molecular structure of PPTA fibers is monoclinic (pseudo-orthorhombic). The The modulus is a function of the total spinning strain tends unit cell has a = 7.87 Å, b = 5.18 Å, c (fiber axis) =12.9 Å to increase with the increasing of the spin stretch factor – other alternative measured values are tabulated in Table 2. (SSF) of the ratio between the velocity of the fiber leaving The angle γ = 90° and the crystal structure possesses Pn or the coagulating bath and the jet velocity (This stretch factor P21/n space-group35,36. The layer of the hydrogen-bonded should be in the range 1 to 14)21. chains and the contents of the cell are shown in Figures 9a
Table 1. Dry jet-wet spinning versus wet spinning process of PPTA fibers21.
Dry jet-wet spinning Wet spinning Concentration High concentration anisotropic dope Low concentration dope Spinning High downdown and solidification in air gap Low downdown Coagulation Low temperature (slow) High temperature (fast) Mechanical properties High strength 15-30 gpd Modulus (controlled) 200- Low strength < 12 gpd Intermediate modulus 1000 gpd Low elongation < 5% 100‑500 gpd High elongation up to 10% Heat treatment Not needed (inherent high properties) Properties are developed through drawing under heat treatment
Table 2. Lattice Constants of PPTA13,34,37-39.
Lattice constant Northolt Haraguchi* Yang Rebouillat Wu Z.quan Chang-Sheng Li** a, Å 7.87 8.0 7.80 7.9 7.88±0.06 5.23 b, Å 5.18 5.1 5.19 4.5-5.2 5.28±0.005 7.76 c, Å 12.9 12.9 12.9 12.8 12.9±0.2 12.86 * PPTA film; ** It seems Chang-Sheng Li’s a and b constants are swapped.
Figure 9. The PPTA crystal lattice with pseudo-orthorhombic unit cell33,34. 1184 Dawelbeit et al. Materials Research
and b: (a) the projection of the crystal structure parallel to reflection-planes, respectively. While Hindeleh and Abdo41 the a-axis of a layer of hydrogen–bonded chains and (b) is reported that the angular range of the diffraction angle 2θ the projection parallel to the c-axis. The unit cell contains of the peaks of the (110) and the (200) reflection-planes is two chains one points up and the other points down25. 12-35°, and after 35° the scan gave flat intensity curves. The flexibility of the polymer chains is low due to the Figure 11 illustrates the WAXD patterns of the equatorial absence of free rotations around the Ph-C bond and the and meridional scan of Kevlar 29 and Kevlar 49 fibers. Ph-N bond and, so, no regular chain folding or kinks and jogs can be envisaged in the solid polymer. The tensile crystallite modulus has been calculated for the all-trans conformation and the final result is that Kevlar fibers have two separate amorphous and crystalline phases, but they have an extended chain structure or oriented amorphous structure and they may be reasonably considered to be of a more paracrystalline structure35. In the wide angle X-ray diffraction (WAXD) of PPTA materials is observed intensity peaks of diffraction in the (hk0), the (h00) and the (00l) planes at the equatorial and meridional scan as shown in the curves of Figure 10
Figure 11. WAXD patterns of Kevlar fibers, (up) Kevlar 2944, and (down) Kevlar 49[41].
3.2. Morphological structure Figure 10. The equatorial and meridional scan of Kevlar fibers41,43. 3.2.1. Level-scales structure (cross-section) – reported by Northolt and van Aartsen34, Panar et al.40, Compared with other fibers, PPTA fibers have several 13,31 41 42 Yang , Hindeleh and Abdo and Hindeleh and Obaid . levels of microscopic and macroscopic morphology-see 13,31 Yang reported that the equatorial scan gave peaks Figure 12. These levels of structures consist of microfibrils at diffraction angle 2θ (Bragg angle) of 20.5° for the (110), whose proposed structures are nearly of the perfect crystal 23° for the (200) and 28.2° for the (211) reflection-planes, respectively. On the other hand, the meridional scan gave type. The morphological behaviours of PPTA fibers contain peaks at diffraction angle 2θ of 6.9° for the (001), 13.7° pleat structures, defect layer structures, extended chains for the (002), 27.1° for the (004) and 42° for the (006) structures, fibrillar structures, skin-core structures and 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1185
Figure 12. Micro and macro fibrillar structures of PET, HMPE, Kevlar and UHMWPE(Spectra) fibers48. microvoids structures. The skin-core structures are found to which support Panar et al.’s suggestions. These fibrils are be roughly circular cylinders with 30-50 nm diameters40,45-47. oriented along the fiber axis and they are 600 nm wide and 3.2.2. Fibrillar structure several centimetres long – as illustrated in Figure 14. These Panar et al. suggested that PPTA fibers, in fact, consist fibrils had been also observed by the transmission electron of fibrils40. An optical micrograph of a broken end of a PPTA microscope (TEM) on the surface of a PPTA fiber as shown fiber – illustrated in Figure 13 – showed bundles of fibrils in Figure 1540.
Figure 13. Bundles of PPTA fibrils40.
Figure 14. Fibril Model by Panar et al.40. Figure 15. The etched surface of PPTA fiber by TEM40. 1186 Dawelbeit et al. Materials Research
Figure 16. Skin-core structure of PPTA fibers40. Figure 18. Structural model of core structure by Li et al.35,50.
formed in the core): Figures 17 and 18. They, also, found that the difference in skin-core structure is more clear in the less crystalline regions on the surfaces of Kevlar fibers49, and that this difference in skin-core structure is related to the fiber orientation and to the perfection of the chain packing a cross the fibers40. Yang13,28 reported that the skin-core structure difference in density, voids and fibrillar orientation results from the fiber coagulation process. M. Fukuda and H. Kawai51 found that the skin thickness is 1 mm. The orientation of the polymer chains in the skin of PPTA fibers is more perfect in Kevlar 49 than in Kevlar 29. This happens because the passage of the water molecules in the case of Kevlar 29 is larger than in the case of Kevlar 49. This result is due to the fact that Kevlar 49 is made by the heat treating process of dried Kevlar 29 under tension at a high temperature. On the other hand, in the case of Kevlar 149 the skin thickness is larger than both of that in the cases of Kevlar 29 and Kevlar 49. Figure 19 shows the cross-polarized microphotograph of Kevlar 149. The Kevlar 149 fiber has Figure 17. Structural model of the skin and core structure by no definite skin-core structure, so, it can be concluded that Panar et al.40. it has a low tensile strength and a high modulus and that its surface is much more permeable to the moisture than 3.2.3. Skin-core structure its inside51-53. During the coagulation stage in the spinning process, 3.2.4. Pleat sheet structure the skin of the formed fibers strongly coagulates at the A combination of electron diffraction and electron initial instant the fibers are in contact with the path, while microscope dark-field image technique is used by 7 the core relaxes rapidly . Figure 16 illustrates the skin-core Dobb et al.46 to study Kevlar 49, concluded that the (200) structure of etched PPTA fibers – shown by the scanning planes form two alternate systems near the edges of the fiber electron microscope (SEM). Panar et al.40 used the plasma at a small angle to each other along the fiber axis- known technique to cut the ends of the fiber so as to study the skin- as a pleat sheet, see Figure 20. This pleat sheet is found core structure of PPTA fibers. They found that, the surface to have an axial banding at 250 and 500 nm periodicities fibrils are uniformly axially oriented, while the core ones in the longitudinal fiber sections. It is concluded that the are imperfectly packed and are ordered (so, voids will be 250 nm spacing is just one-half of the 500 nm periodicity 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1187
Figure 21. Microfibrillar structure diagram54. Figure 19. Cross-polarized microphotograph of Kevlar 14951.
Figure 22. Model of the chain-end distribution by Morgan et al.52.
(stress direction)52. The chain-end distribution model is shown in Figure 22. The chain ends in the skin are essentially arranged randomly relative to one another but they become Figure 20. The pleat structure model, Dobb et al.46. progressively more clustered in the fiber interior resulting in periodic transverse weak planes separated by about 200 nm along the fiber axis. The crystal growth of a PPTA resulting from changes in the crystalline orientation. There fiber occurs in the shape of a 60 nm in diameter cylindrical is good evidence that the pleated sheet consists of parallel crystallites. Within each of these cylindrical crystallites a oriented crystals which are oriented such that the b-axis number of the chain ends are assumed to cluster. (the axis parallel to the intermolecular hydrogen bonds) The chain-end concentrations and distributions along is perpendicular to the fibre surface. The pleat sheets a PPTA fiber axis (Figure 23) in both the skin and the mentioned here are formed during the coagulation of the core of the fiber are the main structural factors affecting PPTA fibers. The periodicity of a pleat sheet is not easy to be deformation, failure processes and the strength of PPTA varied by changing the spinning conditions. The formation fibers. Northolt and Sikkema55 postulated a simple model of the mentioned pleat sheet structure gives the fiber an based on x-ray and electron diffractions. He considered inherent elongation (elasticity). The angle between the two the fiber as being built up of a parallel array of identical planar surfaces forming the pleated sheet structure is about fibrils. Each of the fibrils consists of a series of crystallites 170° - Figure 21. Each pleat sheet has a misorientation parallel to their symmetry axes which follow the orientation angle of about 5°-7° to the fiber axis-this misorientation also distribution with respect to the fiber axis, Figure 24. applies to the crystallites forming a pleated sheet13,45,52,54. Comparing the three models of the PPTA fiber structures given by Panar et al.40, Morgan et al.52 and Northolt and 3.2.5. Chain-end defects Sikkema55 we can observe that the 600 nm fibril diameter The chain-end distribution is determined by the deduced by Panar et al. is ten times larger than the 60 nm solidification process (PPTA-H2SO4) which should fibril diameter deduced by Morgan et al. not be changeable by the subsequent fiber fabrication Grujicic et al.56 explanation is that the chain-end defect (neutralization, washing and drying). These fiber fabrication resulted in the chain scission (brake) and the side group processes, might only, affect the radial direction of defect causes the chain side functionalization. the swollen fibers so they will not alter the chain-end distribution. Under high stress fields the crystals of a PPTA 3.2.6. Deformation defects fiber will form columns parallel to the stress field – termed Crack propagation can readily occur parallel to the rods shish kebab – which consist of a central fibrillar core of because of the rupture of the hydrogen bonds. The difference aligned macromolecular extended-chain and the crystal in orientation and alignment of the skin chains versus the lamellae grow perpendicularly on these fibrillar nuclei core microfibrils, which are sub-structured by crystallites, 1188 Dawelbeit et al. Materials Research
Figure 23. Chain-end distribution in the PPTA fiber skin (- - -) and .along the fiber axis52 (ــــــ) core
Figure 25. Crack propagation path of PPTA fibers52.
Figure 24. The model of PPTA fiber, by Northolt and Sikkema55. has been often used to support the hypothetical fracture model (crack propagation path)1. Figure 25 shows the crack propagation path (fracture model). Morgan et al.52 explained the result of the deformation and failure of Kevlar 49, they stated that the skin will exhibit a more continuous structural integrity in the fiber direction than the core, and the core will fail more readily by transverse crack propagation as a result of chain-end model. 3.2.7. Defect bands The structure of chain extension – in defected layers – in PPTA fibers have a fibrillar morphology in which the individual fibrils having a high proportion of extended chains passing through periodic defected layers. These extended chains distinguish PPTA fibers from conventional ones. In conventional fibers the defect spacing (referred to as long period) is longer than the correlation length (referred to as crystal size). In PPTA fibers, in contrast, the defect spacing (about 35 nm) is smaller than the correlation length, which is over 80 nm. The value 35 nm of the defect spacing in the banding of the PPTA fibers is observed by Panar et al.40 by using Figure 26. Schematic of defect band structure, by Panar et al.35,40. 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1189
etching techniques in refluxing hydrochloric acid solutions in the SAXS intensity notified the presence of moisture in plus small angle X-ray scattering (SAXS). the voids57-60. The defected layer and its relationship to chain extension Grujicic et al.56,61 reported that the insertion of molecular are crucial to understanding the structural features of PPTA nitrogen will cause voids. Furthermore, the insertion of fibers which are most closely related to the high strength of sulphuric acid will cause interstitials which can be the main these fibers. Figure 26 is a schematic drawing of a defected factor of point defects (voids) increments. band structure – individual lines represent molecular backbones40. 3.3. PPTA computational modelling As mentioned above, the 80 nm crystal size (axial Laboratory research time benefit and cost minimization correlation length) is more than (twice) the 35 nm long can appropriately be optimize by the use of Computer-Aided period (defect spacing). Engineering (Computer-Aided Design/Machine CAD/ The increased crystal size may be due to a high CAM). Nowadays, computational approach to material proportion of chains being largely extended as they pass properties, design and its behaviours is greatly developed through the defected bands. The crystal size and the long and accurately designed. period of the PPTA fiber are the seriously affected structural An outstanding analysis of PPTA fibers at multi scale parts in heat treatment processes as a consequence of the levels is investigated by Grujicic et al.56,62-65. 40 high-modulus property . The computational investigations properly described 3.2.8. Microvoids the material models which are used to achieve the material (PPTA) behaviour at the length-scale levels from chemical The electron density discontinuities in the small angle structure (atoms) to composite structure (laminate) with x-ray scattering (SAXS) intensity on the equator pattern brief description in eight levels. Grujicic et al.’s length- of dried fiber indicate the presence of microvoids in PPTA fibers, Figure 27. This phenomenon was studied by Dobb et al.57 and Dobb and Robson58. These microvoids Table 3. Various length-scales level of PPTA fibers. are found to be circular in cross-section and have a ratio Scale level Description of length to width (L/W) of about four, and aligned (Length-scale) approximately parallel to the c-axis of the crystallite. The Laminate -The laminate are fully homogenized. -No dark regions shown in Figure 28 are locations of microvoids discernible (microstructural / architectural in Kevlar 29 stained by silver sulphide (Ag2S). The reduction features). Stacked-lamina -Each lamina is fully homogenized, i.e. no microstructural features are resolved within the lamina. -Laminate are stacked and interconnected via lamina/lamina interfaces. Single lamina -Two-phase microstructure of each lamina is recognized Each phase is fully homogenized. -The architecture of the reinforcing phase is highly simplified. Figure 27. Small angle x-ray scattering pattern of Kevlar 49 fibers62. Fabric unit-cell -Distinction is made between the warp- and weft-yarns in the reinforcing phase. -Yarns and the matrix are fully homogenized. -Yarn- yarn contact and sliding are accounted for explicitly. Yarn -Each yarn is considered as an assembly of fully-homogenized mechanically engaged discrete fibers. -Fiber-fiber contact and sliding are modelled explicitly. Fiber -Fibers are considered as an assembly of nearly-parallel fibrils held together by weak van der Waals forces. -Adjacent fibrils differ with respect to the orientation of the stacked sheets. Fibril -Fibrils are considered as crystalline thread- like entities containing parallel sheets of aligned and hydrogen bonded molecular chains. -Inter-sheet bonding is of a van de Waals character. Molecular-level -Individual molecules are represented as assemblies of covalently bonded atoms. -The effect of hydrogen bonding on the Figure 28. Micrographs of longitudinal sections through stained molecular topology (e.g. the formation of helical structure) is considered explicitly. Kevlar 29[60]. 1190 Dawelbeit et al. Materials Research
scale levels are summarized in Table 3. Moreover, the have high tenacity (HT) (tenacity=tensile strength), high molecular-level model involves investigating the influences stability-temperature (HT), high modulus (HM), low creep of the molecular structures and defects on the properties. and low density (1.44-1.45 dtex), see Table 4. Moreover, Furthermore, these computational series were extended and PPTA fibers have high glass transition temperatures (about established to investigate the effects of prior mechanical 360 °C), high decomposition temperatures (about 530 °C) stresses on the residual mechanical properties of such and high melting temperatures (about 560 °C). They, also, materials (Kevlar KM2). have low combustibility (they are difficult to be ignited and do not feed the flame) and have good thermal insulating 4. Properties of PPTA Fibers capabilities (they are good dielectrics and are almost non- PPTA fibers classified as highly oriented polymeric fibers conductive). Their environmental stability in sea water, oils have high strength and have low extensibility26. Also, they and solvents is good but, they have moderate stability in a
Table 4. Mechanical properties of PPTA fibers9,13,28,72,75.
Product name Kevlar 29 Kevlar 49 Kevlar Kevlar 68 Kevlar Kevlar Twaron Twaron Twaron 149 119 129 1000 1055/6 2000 Product type standard High Ultra-high Intermediate High High standard High High modulus modulus modulus modulus Elongation strength modulus modulus strength Density (dtex) 1.44 1.45 1.47 1.44 1.44 1.45 1.44 1.45 1.44 Tensile modulus 70 135 143 99 55 99 66 125 90 (GPa) Tensile strength 2.9 2.9 2.3 3.0 3.1 3.4 2.7 2.8 3.8 (GPa) Elongation (%) 3.6 2.8 1.5 3.3 4.4 3.3 3.4 2.5 3.5 Moisture regain 5-7 3-4 1-1.2 4-6 5-7 4-6 7 3.5 5.5 (%)
5. Applications of PPTA Fibers Nowadays PPTA fibers have more advantages than high strength bulk materials such as steel. They are becoming increasingly used in cement composites. The PPTA fiber properties that facilitate their usage in cement composites are: Low density, high specific strength (tensile strength per unit linear density) and that they can be easily processed into different complex shapes. Also, the PPTA fibers are used in ropes, cables, protective apparel, industrial fabrics that require high cut and puncture resistant properties, ballistic protections, aerospace, telecommunications (to avoid the damage of the optical transmission which emerges when the optical fiber is stretched), permselective use and other applications that need high modulus and high tensile Figure 29. The stress–strain curve of PPTA fibers under heat strength. Kevlar 49 fibers are used extensively in ballistic treatment69. armour, asbestos replacements and certain composites that require greater damage tolerance. Some specific applications of PPTA fibers are summarized in Table 52,4,11,72,76-81. saturated steam at pH of 4 to 8. They have good resistance to high energy radiation with exception to the ultraviolet 6. Heat Treatments of PPTA Fibers radiation for which they have poor resistance. Figure 29 6.1. Definition illustrates a comparison between stress–strain curves for a The International Federation For The Heat Treatment PPTA fiber and other types of fibers. The PPTA fiber curve Of Materials (IFHT) has defined theheat treatment term as bends upwards as compared to the other curves. This means “A process in which the entire object, or a portion of it, is that a PPTA fiber has a modulus at high stress which is intentionally subjected to thermal cycles and, if required, to greater than those of the other types of fibers9,11,13,21,28,59,66-74. chemical or additional physical actions in order to achieve 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1191
Table 5. Some Applications Of PPTA Fibers78. Applications Examples Friction materials and gasket Prosthetics, brake linings, clutch facings, gaskets, thixotropic additive Medical applications Prosthetics, fibrous bone cement Optical applications Optical fiber cables Protective applications Bulletproof vests, helmets, vehicle protection, fire-fighting, cut protection, ballistics, heat resistant work wear , flame-retardant textiles Composites Tires, pipes, pressure vessels, plastics additive, civil engineering Ropes and cables Ignition cables, aerial optical fiber cable, electrocable, mooring ropes Sport equipments Sail cloths, tennis strings
specific desired structures and properties or changes in Tg and satisfying Fujiwara et al’s deduction - this process them.” is known as cold drawing84. Morgan et al. reported a drying The term thermal cycles implicitly involves the change temperature of about 65 °C52. of temperature with time during a heat treatment process. These changes may be essential to improve processing 6.2.4. Post-heat treatment (subsequent heat treatment) (e.g., machinability) or to meet proper service conditions The concept of a fiber heating process is always done “(e.g., increased (or controlled) surface and core hardnesses, with adding tensions to draw the fiber. Thus, the issue of wear, fatigue resistance, dimensional accuracy, desired heating is that of the drawing needed to orient the molecular microstructure and residual stress profile and improved chains (orientation phase) in the direction of the fiber axis tensile and yield strength, toughness, ductility, impact and at the same time to increase the ordered arrangement strength, weld integrity and magnetic and electrical of the intermolecular structure (crystallization phase)85. properties”82. Moreover, drawing with applying heat (post-heat treatment) should be done at temperatures lying between the glass 6.2. Heat treatment zone transition temperature Tg and the melting temperature Tm Fujiwara et al.83 deduced mathematical equations and satisfying Fujiwara et al.’s deduction72 so as to produce describing the appropriate heating temperatures for the a fiber of a higher modulus52. case of drying treatment and for heating treatment under tension (post-heating): 6.3. Thermal ageing Many researchers studied the environmental exposure 6.2.1. Drying treatment effects on PPTA fibers. They examined the structural Here a wet PPTA fiber is to be dried by heating. It was mechanism and the mechanical properties of heat treated found that the value x of the drying temperature in °C must fibers by means of the thermal and oxidative methods15,75. be such that its product with the drying time t in seconds raised to the power 0.08 be approximately in the range 150 7. Heat Treatment Devices to 300. Mathematically (approximately): 7.1. Heating tubes and ovens 0.08 150 ≤ xt ≤ 300 Heat transfer takes place in these tubes and ovens by convection. Blades86 post-heat treated a pre-heat treated fiber 6.2.2. Heat treatment under tension (post-heating) by using a heating tube containing a flow of a nitrogen gas. Here a dry or wet PPTA fiber is to be heated under It is noticed that in Blades’s method the fiber is heat treated tension for the sake of structure and property improvements. in its solid state. In such methods, of heat treatments, the It was found that the value y of the heating temperature in presence of an atmosphere of an inert gas (such as nitrogen) °C must be such that its product with the treating time τ in is essential for the heat transfer by convection. In Blades’s seconds raised to the power 0.08 be approximately in the method the fiber is passed over the tension guide around range 250 to 550. the fiber spool which is controlled by a magnetic brake, Mathematically (approximately): then, it is passed over the idler roll after which it’s passed 250 ≤ yτ0.08≤ 550 over a pulley equipped with a force gauge and then passed through the heating tube and pulled by pairs of driven rolls {Vlodek et al.24 reported a range of 250 to 550 °C for to pass it to the constant tension windup roll-as illustrated the treating temperature under 5-50% tension for time less in Figure 30. Chern and Trump, also, used this process to than 10 minutes}. manufacture a high modulus and a high strength PPTA fiber. They processed the never-dried fiber swollen with water of 6.2.3. Pre-heat treatment (Drying) controlled acidity. They reported that the fiber, preferably, The spinning of PPTA solutions start when the spinneret should have about 20-100% water (based on the weight of is placed in a coagulation bath of water7. Then, the resulting a dry fiber). They used heating temperatures in the range fiber is dried under tension on rollers (to draw the fiber) at 500-660 °C with time exposure of 0.25 to 12 seconds and a temperature lying below its glass transition temperature tension of about 1.5 to 4 gpd[87]. 1192 Dawelbeit et al. Materials Research
Figure 30. Heating tube used by Blades86.
tension control device with Teflon fiber guide, the heating (annealing) device which is 12 mm long, the quenching device (at a 0.1 mm distance from the heating device) and the take-up device. The take-up device (roller) is driven by a motor and is connected by a timing belt with the take-in device (roller), see Figures 31 and 32. The Lee device steps of the fiber treatment are as follows: The fiber is passed from the spool by the take-in device to the take-up device under controlled tension through the tension control device then it is passed to the heating device after which it is passed to the quenching device and finally to the take-up device53,88. Other researchers worked in the field of heat treatment under tension by using heating tubes and ovens are Rao et al.43,89 and Knijnenberg90. 7.2. Hot rollers In the heat treatment of the PPTA fibers by hot rollers the heat transfer of the heat used to treat the fibers takes place by conduction. Cochran and Strahorn91 heat treated the PPTA fibers at high speed under tension by passing them over twenty rollers arranged in two sets of arrays of rollers Figure 31. Heat treatment device used by Lee et al.53,88. close to each other (Figure 33). The first set is the heating rollers (1 to 12) and the second set is the cooling rollers (13 to 20). The steps of the heat treatment in this method are as follows: First the fiber is passed over the first subset of the heating rollers 1 to 5, then it is passed over the second subset of the heating rollers 6 to 12 and finally it is passed over the set of the cooling rollers 13 to 20. Another an on-line fiber heat treatment (shown in Figure 34) is innovated by Chern92 for the simultaneous drying and heat treating. In this innovation the wet spun fibers to be treated should have greater than 20 percent water (based on weight of dry fiber). These fibers are supplied continuously in multiple wraps around a pair of rolls in a heating zone having temperatures of 200 to 660 Figure 32. Tension control device used by Lee et al.53,88. °C. The heating gas used here is – generally – a supper saturated steam. For the case of the production of Twaron fibers, the Then, K. G. Lee et al.53 used a heating device in which heat treatment is done by running the yarn on a series of he controlled the treatment time by controlling the velocity hot driven rollers. The rollers are running at the same speed of the fibers that are passed through a constant heating as the fibers. But, differences in velocity between the fiber temperature zone by applying a controllable tension and the rollers occur due to the fiber elongation by the force throughout the process. gradients on the rollers. The temperature and the rotation The Lee device for the treating of the fibers has five speed of the rollers can be varied separately, so as, to apply basic parts, which are: The take-in device (feeder), the a force and temperature profile on the running fibers93. 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1193
The main function of heat, here, is the removal of 8. Heat Treatment Process moisture and the molecular the orientation of the molecular Heat treatment can be used to produce changes in both of the fiber. The fiber filament force and the temperature the morphology and the mechanical behaviour of the fibers profile are the main factors that determine the final that are needed to improve the properties and the structural mechanical properties of every individual fiber94. characteristics of PPTA fibers97. As-spun PPTA fiber can be Cochran and Yang95, also, investigated an on-line system subsequently heat treated at a high temperature and a high by treating the dried PPTA fibers, containing at least 10% tension for several seconds to increase its crystallinity and moisture, over hot rollers under 3-7 gpd tension at a drying degree of crystalline orientation. In this method the fiber is temperature not more than 175 °C and a treating temperature, heat treated in its solid state. The fiber modulus is increased by this short term of heating under tension. optionally, in the range 300-600 °C under a tension of at least The responses of as-spun dry jet-wet and wet spun PPTA 0.5gpd. The drying roller was heated by saturated steam. fibers to heat treatment are studied by Jaffe and Jones21. Such heat treated filaments have more improved tenacity Figure 36 shows the difference in response of the fibers as compared with similar filaments dried under low tension produced by the two types of mentioned spinning methods. and heat treated under the same conditions. Many other researchers worked in the field of Further modifications can be done by heat settings on hot heat treatment of as-spun fibers. The heat treatment drums at 420-450 °C. Figure 35 illustrates a full Schematic conditions for as-spun PPTA fibers used by Blades86, 52 91 diagram of the spinning of PPTA fibers in a production plant Morgan et al. , Cochran and Strahorn , Cochran and 95 35 38 87 producing up to 124 parallel running filaments (in a wrap)96. Yang , Chatzi and Koenig , Wu et al. , Chern and Trump , Chern92, Chern et al.98, Lee et al.53, Rao et al.43,89, and Knijnenberg et al.90 are summarized in Table 6.
9. Microstructural Developments and Property Relationships of PPTA Fibers 9.1. Structure improvements The heat treatment of PPTA fibers under different conditions can make the fibers to rearrange their structures for further improvements in their crystalline perfection, crystalline orientation and their fiber’s moduli99. Ran et al.61 and Chu and Hsiao100,101 applied a new technique by using hot-pins for heating process to study the structural changes and the deformations occurring during the drawing process of PPTA (Kevlar 49) fiber via small and wide angle x-ray scattering techniques. Figure 33. Heat treatment apparatus used by Cochran and Strahorn91. 9.2. Structural defects The over-all properties of PPTA fibers are affected by the type and the concentration of the various structural defects (crystallographic and morphological). Grujicic et al.56,62-65 reported that defect’s type, size and concentration are sensitive functions of the PPTA fiber fabrication (polymer synthesis, dope preparation and spinning process) – which are summarized in the present review in section 2. 9.3. Mechanical properties The tensile modulus and the tensile strength of heat treated PPTA fibers are much higher compared with those of the other conventional fibers (Nylon, Polyester, etc.).While the elongation at break is relatively lower for PPTA fibers13,28,70. 9.3.1. Tensile modulus The rearrangement of the fibers’ structures by means of heating under tension corresponds to producing a higher tensile modulus. Analysis of equatorial X-ray diffraction patterns reveals that the increase of the apparent crystalline Figure 34. On-line heat treatment device used by Chern92. size (ACS) is accompanied by an increase of the fiber 1194 Dawelbeit et al. Materials Research
Figure 35. Full Schematic diagram of dry-jet wet spinning of PPTA fibers, Fourne96.
Figure 36. Effect of heat treatment on dry jet-wet and wet spun PPTA fibers21. 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1195
Table 6. Heat treatment process of PPTA fibers.
Cochran Ia Blades Morgan Wu 1st array 2ed array Cooling Temperature, °C 450-580 550 175-300 175-300 0-100 Preferred 380-480 25 Time, seconds 0.5-5 5 Tension, gpd 1-8 1-6 0.6-4.0 0.6-4.0 0.5-1.5
Speed, m/s 4.7 4.7 Stretch % 5 0.1-1.6 0.2-1.16 Between the two arrays Moisture contain% Chern
Chatzi Cochran IIb Dry fibers Never-dried Lee Rao Knijnenberg fibers (water- swollen) Temperature, °C 250 300-600 200-660 500-660 400 180-370 340-510 Time, seconds 1-6 0.2-0.6 0.25-5 0.006-9 180 30-60 Tension, gpd 6 0.5 1.5-4 0.1-6 Varied Speed, m/s 2.5-6 Stretch % 5 Moisture contain% Control the moisture contain in drying stage 7-10 aCochran and Strahorn91; bCochran and Yang95. modulus - provided that the tension during the treatment Sixth, the strength is adversely affected by overall gross is high enough. On the other hand, if the tension does morphological defects58. not exceed a certain critical level, then, no increase in the The closing-up of the peak positions for the (110) and modulus will take place - even if the temperature is elevated. the (200) diffraction planes, is a sign that a sharp decrease The tensile moduli of the Kevlar fibers are such that the in the tensile strength took place76. modulus for Kevlar 149 > for Kevlar 49 > for Kevlar 29 > The tensile strengths of Kevlar fibers are such that the for Kevlar 1197,89,102. tensile strength for Kevlar 149 < for Kevlar 49 < for Kevlar 29 < for Kevlar 119 7,89,102. 9.3.2. Tensile strength The tensile strength of PPTA fibers decreases largely 9.3.3. Compressive properties under heat treatment, although – as mentioned above – the The compressive properties of the PPTA fibers have tensile modules increases. In fact, the decrease in the tensile been studied in many literatures15,90,107-113. Tensile tests of strength is due to exposing the fiber to high temperatures103. compressively kinked poly(p-phenylene terephthalamide) The reasons for this decrease are: (PPTA) fibers reveal only a 10% loss in tensile strength after First, the pleats are straightened-out during the heat application of compressive strains much greater than the treatment under tension. This process may create microvoids critical strain required for kink band initiation114. A new route around these pleats104. Voids tend to reduce the tensile of synthesis, based on PPTA compatible reactive oligomers strengths60. added to the standard PPTA dope prior to the fiber spinning Second, although the fiber tensile strength is controlled process, done by Knijnenberg et al.115, resulted in an increase by the overall molecular orientation, but it may also be in the compressive strength properties when these new PPTA reduced with the increasing of the skin region as a result of fibers are heat treated at temperatures of 340 to 510 °C for a high orientation105. a duration of 5 to 300 seconds. Third, the heated fiber strength decreases with the increase of the apparent crystal size (ACS)89. 9.4. Crystallite structure Fourth, the effect of the weak Van der Waal’s interactions in the a-axis direction in changing the unit cell dimensions 9.4.1. Apparent Crystal Size (ACS) is weakened by the heat exposures75. The apparent crystal sizes of the (110) and (200) Fifth, the increase of the surface helix angle by the reflection planes – which are determined by the equatorial twisting of the PPTA fibers106. scan using Scherrer formula - are found to be sensitive to 1196 Dawelbeit et al. Materials Research
the applied tensions and the heating temperatures. They by the increasing of both the temperature and the applied are found to increase monotonically with increasing the tension. Kevlar fibers have high molecular orientation (the heating temperatures. However, the apparent crystal size molecular orientation for Kevlar 149 > for Kevlar 49 > for of the (110) plane increases faster than that of the (200) Kevlar 29). plane. The apparent crystal sizes of the (110) and the (200) The misorientation of the fibrils in heat treated fibers reflection planes are found to have the respective values decreases with increasing the stretch ratio as a result of of 52 and 46 Å for Kevlar 29, the values 66 and 41 Å for an increase in the total orientation43,89. However, the fiber Kevlar 49, the values 123 and 76 Å for Kevlar 149 and the modulus, the crystal perfection and the crystallinity increase values 52.1 and 49.9 Å for Kevlar 129[39,43] as a result of the increase in the total orientation due to the mentioned stretching (as illustrated by Jaffe and Jones in 9.4.2. Axial crystal size (L) Figure 37)21. On the other hand, the total crystal orientation The axial crystal sizes of the (00l) reflection planes of decreases with increasing the compressive strain125. the PPTA fibers – which are determined by the meridional The orientation angle of a PPTA (Kevlar) fiber is about scan using Scherrer formula – are affected by the applied 12° and it decreases to about 9° after heat treatment99,126 – an tensions and heating temperatures. Rao et al.43 and Hindeleh indication that the heat treatment of a PPTA fiber increases and Abdo41 reported that the reflection order of the (002) its total orientation. plane decreases with increasing the heating temperatures when the fiber is heated to temperatures above 400 °C. The 10. Summary values 656.14 nm, 737.15 nm, and 1547.79 nm are found for the reflection order of the (002) plane for Kevlar 29, The spinning process is the fiber forming step and the Kevlar 49, and Kevlar 149 fibers, respectively. It is noticed subsequent fiber-heat treatment is the fiber re-structuring that these values are greater than the corresponding apparent step to improve its properties26,127. crystal sizes13,89 The following observations relate to PPTA fibres: Chang-Sheng Li et al. reported the value 94.7 Å for The benefits which can be got from the heat treatment Kevlar 129 in (004) plane39. processes are improving the molecular orientation and (N.B. Rao’s et al. nm units are surprising!) the crystal perfection, and increasing the crystallinity and the modulus of PPTA fibers – as a result of structural 9.4.3. Lattice distortion (crystalline perfection) improvements. The paracrystalline lattice structure analysis developed The improvement of the molecular orientation at a given by Barton116 is founded by Hosemann117-121. The defects in chain length is the most important factor correlating with the crystal lattice can be improved by the possibility of lateral the fiber strength128. crystal growth (Axial crystal size) particularly along the The relationship between the tensile modulus and hydrogen bonding direction occurring during the heat treatment the orientation are illustrated in Figure 37. On the other of the PPTA fiber. The level of the distortion parameter of the hand, the tensile strength of the spinning of PPTA fibers paracrystallinity and the constant α* can be modified by the heat is a function of the total spinning stress and it tends to treatment as had been concluded by Hindeleh and Hosemann122 increase with increasing the spin stretch factor, increasing 44,123 and Hindeleh et al. . Lee reported that the distortion of the the inherent viscosity and with decreasing the air gap. The lattice decreases as the modulus increases during the fiber crystal structural characterizations show that the wet spun heat treatment process under tension. Kevlar fibers have high fiber is less crystalline and has less efficiency for hydrogen molecular crystalline perfections (the crystalline perfection for bonding in the unit cell than the dry jet-spun fiber which 124 Kevlar 149 > for Kevlar 49 > for Kevlar 29) . gives low elongation21. 9.4.4. Crystal orientation The stress–strain curve for heat treated PPTA fibers bends upwards to give greater moduli at higher stresses as The decrease in the orientation angle for heat treated the pleats are pulled out, Figure 29[69,94,124]. PPTA fibers is an indication for improvement in the The fiber tensile strengths and failure strains decrease crystallites alignment43,53. This improvement is produced with the increase of the heat treatment temperature129. It is important for every post-drawing process (heat treatment process) at elevated temperatures to know the lifetime of the fiber at that particular drawing temperature84. The spun yet undrawn fiber has a very high strength96. High temperature resistance and high modulus are joint properties of PPTA fibers because they both arise from the strong intermolecular forces in the fiber structure130. The hydrolysis of amide linkages happens between the microfibrils in the core rather than in the skin of the PPTA fiber39. During heat treatment processes the lattice distortion (crystalline perfection) is sensitive to the heating Figure 37. The relationship between the tensile modulus and temperature, however, the orientation is more sensitive to orientation angle21. the applied tension89. 2014; 17(5) Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review 1197
In heat treatment processes the voids formed for 2. The improvement of the molecular orientation at prolonged periods and the void contents increase with the a given chain length is the most important factor increase in crystallite size. Voids affect the mechanical correlating with the fiber strength. properties of the fibers, namely, they tend to decrease the 3. The reduction of the tensile strength value is tensile strength58,59,131. -essentially- influenced by the molecular-level The PPTA fabrics capacity to absorb water and ability to defects. take up dyestuff are very low compared to other fabrics130.
11. Conclusion Acknowledgements The benefits which can be got from the studies of the I would like to take this opportunity to extent my heat treatment of PPTA fibers are: acknowledgements to the National Basic Research Program 1. Improving the molecular orientation & improving the of China (973 Project) - under Project 2011CB606101 - for crystal perfection and increasing the crystallinity & financing this piece of research. increasing the modulus of PPTA fibers - as a result Also, my acknowledgements are extended to the China of structural improvements due to the heat treatment Scholarship Council (CSC) for giving me a scholarship to of the PPTA fibers. do post-graduates studies in China.
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