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DOI: http://dx.doi.org/10.1590/1516-1439.250313Materials Research. 2014; 17(5): 1180-1200 © 2014

*e-mail: [email protected]

1. Introduction

1.1. High performance fibersHigh-tech fiber is an expressive way to indicate that

advanced science and technology have been used to produce this fiber. They are high-modulus and high-tenacity (HM-HT) fibers1. Although the main difference between natural and synthetic fibers is in structure also natural fibers and synthetic fibers can be classified as high-function fibers and high performance fibers, respectively. Synthetic fibers were developed initially as copies of silk, wool, or cotton2.

The first high performance fiber, Kevlar was developed in the 1965’s at DuPont - a company with a long history in fiber synthesis - by discovering a new method of producing a polymer chain coupled to form a liquid crystalline solution3.

In the 1980s, polyethylene, first synthesized in the 1930s, was processed into a high-performance commercial fiber (with the trade names Spectra and Dyneema), based on gel-spinning technology invented at DSM in the Netherlands.

The high-strength polymeric fiber Zylon, based on rigid-rod polymer research that began in the 1970s at the U.S. Air Force Research Laboratory, was commercialized in 1998 by Toyobo in Japan. High-strength polyacrylonitrile - based carbon fibers were developed by optimizing some parameters such as the polyacrylonitrile-copolymer composition, fiber spinning, and polyacrylonitrile stabilization and carbonization. Other high-strength and high-modulus fibers have relatively high densities, are

inorganic fibers which include silicon carbide, alumina, glass and alumina borosilicate fibers4.

1.2. Aramid storyThe twentieth century witnessed huge researches and

innovations for fibers. After Nylon revolution DuPont Co.’s researchers struggled to develop stiffer and tougher nylon-related fiber. In 1965 Stephanie Louise Kwolek developed the first liquid crystal poly (p-phenzamide) (PBA) polymer, which provided the basis for poly (p-phenylene terephthalamide) under trade name of Kevlar fiber in 1971[1,5-7]. The wholly aromatic structure with all para-substitutions creates stiff macromolecular rod-like structures.

The first commercial material, named Kevlar, were made by DuPont in USA, Ireland and Japan. Another type of PPTA fibers, named Twaron, was produced in 1978 by Teijin Aramid B.V. in Netherland1,5,8-12.

Poly (p-phenylene terephthalamide) abbreviated PPTA, are organic fibers acronym originating from aromatic poly (amide) with a distinct chemical composition of wholly aromatic polyamides which at least 85% of amide groups (—CO—NH—) are bond directly to two aromatic rings2,12-15. Burchill16 reported that it is largely poly (p-phenylene terephthalamide), Figure 1.

Aramid is an acronym originating from aromatic poly (amide). Alternative names were PPTA, poly (p-phenylene terephthalamide), aramid, aramide, polyaramid and polyaramide. The International Union of Pure and Applied Chemistry (IUPAC) nomenclature for a Kevlar fiber is Poly (imino-1,4-phenyleneiminocarbonyl-1,4- phenylenecarbonyl)17.

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

Microstructural Developments of Poly (p-phenylene terephthalamide) Fibers During Heat Treatment Process: A Review2014; 17(5) 1181

2. PPTA Fibers FabricationThe processes steps of PPTA fibers fabrications begin

by synthesis of the polymer, preparation of the spinning dope (with suitable solvent and concentration) and, finally, formation of the fiber by spinning18.

2.1. Polymer synthesisThe 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 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 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 preparationSynthesized poly (p- phenylene terephthalate) (PPTA) is

dissolved in concentrated sulphuric acid (H2SO4) to prepare a dope for fiber spinning. The H2SO4 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 viscosity of the anisotropic solution of PPTA is hinh = 4.9 in concentrated H2SO4. Figure 4 explains the relationship between the concentration and the solution viscosity for poly (p-benzamide) (PBA) (hinh= 2.41) by Kwolek5. The Oriented domains are formed at concentrations of more than 12 wt. %25.

Figure 1. Chemical structure of PPTA fiber.

Figure 2. Synthesis of poly (p-phenylene terephthalamide).

Figure 3. Structure of poly (p-phenylene terephthalamide).

Figure 4. Relationship between concentration of polymer and solution viscosity for PBA5,15.

Dawelbeit et al.1182 Materials Research

2.3. Spinning processPPTA 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.

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.

Figure 5. Liquid crystalline structure of PPTA/H2SO4 solution13.

Figure 6. Flexible and rigid polymers, differences in behaviour during spinning process3.

In 1970, Blades27 innovated that the high-strength and high-modulus fibers can be spun from anisotropic solutions (spinning dope) of aramid polymers by using dry jet-wet spinning processes. Dry jet-wet spinning gives a higher strength, a higher modulus and a lower elongation - Figure 7.

Figure 7. Dry-jet wet-spinning process by Blades24,27,28.

Figure 8. PPTA fibers formation13,18,24,31.

The crucial fiber microstructure formation takes place during the coagulation stage of this spinning process. The 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 oriented and has the necessary high crystallinity and the chain-extended structure to give it high modulus and high strength greater than those of the flexible semi-crystalline polymer fibers. The elongation of the resulting fiber here is low17,24,33. The modulus of the PPTA fibers is essentially independent of the inherent viscosity and the air gap. The


high performance, PPTA fibers are known to have very high modulus and very high strength and good thermal stability as a result of their chemical structure17.

The relationships between the processing parameters - which are the conditions, the fiber structure and the properties of the PPTA fiber - are crucial to understand the behaviour of the fibers, the yarns, and the fabrics for any desired application. Table 1 shows the difference between the dry jet-wet spinning and the wet spinning of PPTA fibers. The modulus is a function of the total spinning strain tends to increase with the increasing of the spin stretch factor (SSF) of the ratio between the velocity of the fiber leaving the coagulating bath and the jet velocity (This stretch factor should be in the range 1 to 14)21.

3. Microstructure of PPTA Fibers

3.1. Crystal structurePPTA fiber’s structure was early studied by Northolt on

the basis of X-ray diffraction. Northolt investigated the fiber structure. He reached the conclusion that the supermolecular structure instead of having a two-phase structure consisting of amorphous and crystalline domains, it- probably- has a paracrystalline structure34. The crystal molecular structure of PPTA fibers is monoclinic (pseudo-orthorhombic). The unit cell has a = 7.87 Å, b = 5.18 Å, c (fiber axis) =12.9 Å – other alternative measured values are tabulated in Table 2. The angle γ = 90° and the crystal structure possesses Pn or P21/n space-group35,36. The layer of the hydrogen-bonded 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 spinningConcentration High concentration anisotropic dope Low concentration dopeSpinning High downdown and solidification in air gap Low downdownCoagulation Low temperature (slow) High temperature (fast)Mechanical properties High strength 15-30 gpd Modulus (controlled) 200-

1000 gpd Low elongation < 5%Low strength < 12 gpd Intermediate modulus 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 5.23b, Å 5.18 5.1 5.19 4.5-5.2 5.28±0.005 7.76c, 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.


and b: (a) the projection of the crystal structure parallel to the a-axis of a layer of hydrogen–bonded chains and (b) is the projection parallel to the c-axis. The unit cell contains two chains one points up and the other points down25.

The flexibility of the polymer chains is low due to the absence of free rotations around the Ph-C bond and the 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

reflection-planes, respectively. While Hindeleh and Abdo41 reported that the angular range of the diffraction angle 2θ of the peaks of the (110) and the (200) reflection-planes is 12-35°, and after 35° the scan gave flat intensity curves. Figure 11 illustrates the WAXD patterns of the equatorial and meridional scan of Kevlar 29 and Kevlar 49 fibers.

Figure 10. The equatorial and meridional scan of Kevlar fibers41,43.

– reported by Northolt and van Aartsen34, Panar et al.40, Yang13,31, Hindeleh and Abdo41 and Hindeleh and Obaid42.

Yang13,31 reported that the equatorial scan gave peaks at diffraction angle 2θ (Bragg angle) of 20.5° for the (110), 23° for the (200) and 28.2° for the (211) reflection-planes, respectively. On the other hand, the meridional scan gave peaks at diffraction angle 2θ of 6.9° for the (001), 13.7° for the (002), 27.1° for the (004) and 42° for the (006)

Figure 11. WAXD patterns of Kevlar fibers, (up) Kevlar 2944, and (down) Kevlar 49[41].

3.2. Morphological structure

3.2.1. Level-scales structure (cross-section)

Compared with other fibers, PPTA fibers have several levels of microscopic and macroscopic morphology-see Figure 12. These levels of structures consist of microfibrils whose proposed structures are nearly of the perfect crystal type. The morphological behaviours of PPTA fibers contain pleat structures, defect layer structures, extended chains structures, fibrillar structures, skin-core structures and


microvoids structures. The skin-core structures are found to be roughly circular cylinders with 30-50 nm diameters40,45-47.

3.2.2. Fibrillar structure

Panar et al. suggested that PPTA fibers, in fact, consist of fibrils40. An optical micrograph of a broken end of a PPTA fiber – illustrated in Figure 13 – showed bundles of fibrils

which support Panar et al.’s suggestions. These fibrils are oriented along the fiber axis and they are 600 nm wide and several centimetres long – as illustrated in Figure 14. These fibrils had been also observed by the transmission electron microscope (TEM) on the surface of a PPTA fiber as shown in Figure 1540.

Figure 12. Micro and macro fibrillar structures of PET, HMPE, Kevlar and UHMWPE(Spectra) fibers48.

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.


3.2.3. Skin-core structure

During the coagulation stage in the spinning process, the skin of the formed fibers strongly coagulates at the initial instant the fibers are in contact with the path, while the core relaxes rapidly7. Figure 16 illustrates the skin-core structure of etched PPTA fibers – shown by the scanning electron microscope (SEM). Panar et al.40 used the plasma technique to cut the ends of the fiber so as to study the skin-core structure of PPTA fibers. They found that, the surface fibrils are uniformly axially oriented, while the core ones are imperfectly packed and are ordered (so, voids will be

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 no definite skin-core structure, so, it can be concluded that it has a low tensile strength and a high modulus and that its surface is much more permeable to the moisture than its inside51-53.

3.2.4. Pleat sheet structureA combination of electron diffraction and electron

microscope dark-field image technique is used by Dobb et al.46 to study Kevlar 49, concluded that the (200) planes form two alternate systems near the edges of the fiber at a small angle to each other along the fiber axis- known as a pleat sheet, see Figure 20. This pleat sheet is found to have an axial banding at 250 and 500 nm periodicities in the longitudinal fiber sections. It is concluded that the 250 nm spacing is just one-half of the 500 nm periodicity

Figure 16. Skin-core structure of PPTA fibers40.

Figure 17. Structural model of the skin and core structure by Panar et al.40.

Figure 18. Structural model of core structure by Li et al.35,50.


resulting from changes in the crystalline orientation. There is good evidence that the pleated sheet consists of parallel oriented crystals which are oriented such that the b-axis (the axis parallel to the intermolecular hydrogen bonds) is perpendicular to the fibre surface. The pleat sheets mentioned here are formed during the coagulation of the PPTA fibers. The periodicity of a pleat sheet is not easy to be varied by changing the spinning conditions. The formation of the mentioned pleat sheet structure gives the fiber an inherent elongation (elasticity). The angle between the two planar surfaces forming the pleated sheet structure is about 170° - Figure 21. Each pleat sheet has a misorientation angle of about 5°-7° to the fiber axis-this misorientation also applies to the crystallites forming a pleated sheet13,45,52,54.

3.2.5. Chain-end defectsThe chain-end distribution is determined by the

solidification process (PPTA-H2SO4) which should not be changeable by the subsequent fiber fabrication (neutralization, washing and drying). These fiber fabrication processes, might only, affect the radial direction of the swollen fibers so they will not alter the chain-end distribution. Under high stress fields the crystals of a PPTA fiber will form columns parallel to the stress field – termed shish kebab – which consist of a central fibrillar core of aligned macromolecular extended-chain and the crystal lamellae grow perpendicularly on these fibrillar nuclei

(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 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 fiber occurs in the shape of a 60 nm in diameter cylindrical crystallites. Within each of these cylindrical crystallites a number of the chain ends are assumed to cluster.

The chain-end concentrations and distributions along a PPTA fiber axis (Figure 23) in both the skin and the core of the fiber are the main structural factors affecting deformation, failure processes and the strength of PPTA fibers. Northolt and Sikkema55 postulated a simple model based on x-ray and electron diffractions. He considered the fiber as being built up of a parallel array of identical fibrils. Each of the fibrils consists of a series of crystallites parallel to their symmetry axes which follow the orientation distribution with respect to the fiber axis, Figure 24.

Comparing the three models of the PPTA fiber structures given by Panar et al.40, Morgan et al.52 and Northolt and Sikkema55 we can observe that the 600 nm fibril diameter deduced by Panar et al. is ten times larger than the 60 nm fibril diameter deduced by Morgan et al.

Grujicic et al.56 explanation is that the chain-end defect resulted in the chain scission (brake) and the side group defect causes the chain side functionalization.

3.2.6. Deformation defectsCrack propagation can readily occur parallel to the rods

because of the rupture of the hydrogen bonds. The difference in orientation and alignment of the skin chains versus the core microfibrils, which are sub-structured by crystallites,

Figure 19. Cross-polarized microphotograph of Kevlar 14951.

Figure 20. The pleat structure model, Dobb et al.46.

Figure 21. Microfibrillar structure diagram54.

Figure 22. Model of the chain-end distribution by Morgan et al.52.


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 bandsThe 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 23. Chain-end distribution in the PPTA fiber skin (- - -) and core (ــــــ) along the fiber axis52.

Figure 24. The model of PPTA fiber, by Northolt and Sikkema55.

Figure 25. Crack propagation path of PPTA fibers52.

Figure 26. Schematic of defect band structure, by Panar et al.35,40.


etching techniques in refluxing hydrochloric acid solutions plus small angle X-ray scattering (SAXS).

The defected layer and its relationship to chain extension are crucial to understanding the structural features of PPTA fibers which are most closely related to the high strength of these fibers. Figure 26 is a schematic drawing of a defected band structure – individual lines represent molecular backbones40.

As mentioned above, the 80 nm crystal size (axial correlation length) is more than (twice) the 35 nm long period (defect spacing).

The increased crystal size may be due to a high proportion of chains being largely extended as they pass through the defected bands. The crystal size and the long period of the PPTA fiber are the seriously affected structural parts in heat treatment processes as a consequence of the high-modulus property40.

3.2.8. MicrovoidsThe electron density discontinuities in the small angle

x-ray scattering (SAXS) intensity on the equator pattern 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 are found to be circular in cross-section and have a ratio of length to width (L/W) of about four, and aligned approximately parallel to the c-axis of the crystallite. The dark regions shown in Figure 28 are locations of microvoids in Kevlar 29 stained by silver sulphide (Ag2S). The reduction

in the SAXS intensity notified the presence of moisture in the voids57-60.

Grujicic et al.56,61 reported that the insertion of molecular nitrogen will cause voids. Furthermore, the insertion of sulphuric acid will cause interstitials which can be the main factor of point defects (voids) increments.

3.3. PPTA computational modellingLaboratory research time benefit and cost minimization

can appropriately be optimize by the use of Computer-Aided Engineering (Computer-Aided Design/Machine CAD/CAM). Nowadays, computational approach to material properties, design and its behaviours is greatly developed and accurately designed.

An outstanding analysis of PPTA fibers at multi scale levels is investigated by Grujicic et al.56,62-65.

The computational investigations properly described the material models which are used to achieve the material (PPTA) behaviour at the length-scale levels from chemical structure (atoms) to composite structure (laminate) with brief description in eight levels. Grujicic et al.’s length-

Figure 27. Small angle x-ray scattering pattern of Kevlar 49 fibers62.

Figure 28. Micrographs of longitudinal sections through stained Kevlar 29[60].

Table 3. Various length-scales level of PPTA fibers.

Scale level (Length-scale)

Description

Laminate -The laminate are fully homogenized. -No discernible (microstructural / architectural 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.

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 molecular topology (e.g. the formation of helical structure) is considered explicitly.


scale levels are summarized in Table 3. Moreover, the molecular-level model involves investigating the influences of the molecular structures and defects on the properties. Furthermore, these computational series were extended and established to investigate the effects of prior mechanical stresses on the residual mechanical properties of such materials (Kevlar KM2).

4. Properties of PPTA FibersPPTA fibers classified as highly oriented polymeric fibers

have high strength and have low extensibility26. Also, they

have high tenacity (HT) (tenacity=tensile strength), high stability-temperature (HT), high modulus (HM), low creep and low density (1.44-1.45 dtex), see Table 4. Moreover, PPTA fibers have high glass transition temperatures (about 360 °C), high decomposition temperatures (about 530 °C) and high melting temperatures (about 560 °C). They, also, have low combustibility (they are difficult to be ignited and do not feed the flame) and have good thermal insulating capabilities (they are good dielectrics and are almost non-conductive). Their environmental stability in sea water, oils 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 149

Kevlar 68 Kevlar 119

Kevlar 129

Twaron 1000

Twaron 1055/6

Twaron 2000

Product type standard modulus

High modulus

Ultra-high modulus

Intermediate modulus

High Elongation

High strength

standard modulus

High modulus

High strength

Density (dtex) 1.44 1.45 1.47 1.44 1.44 1.45 1.44 1.45 1.44Tensile modulus (GPa)

70 135 143 99 55 99 66 125 90

Tensile strength (GPa)

2.9 2.9 2.3 3.0 3.1 3.4 2.7 2.8 3.8

Elongation (%) 3.6 2.8 1.5 3.3 4.4 3.3 3.4 2.5 3.5Moisture regain (%)

5-7 3-4 1-1.2 4-6 5-7 4-6 7 3.5 5.5

Figure 29. The stress–strain curve of PPTA fibers under heat treatment69.

saturated steam at pH of 4 to 8. They have good resistance to high energy radiation with exception to the ultraviolet radiation for which they have poor resistance. Figure 29 illustrates a comparison between stress–strain curves for a PPTA fiber and other types of fibers. The PPTA fiber curve bends upwards as compared to the other curves. This means that a PPTA fiber has a modulus at high stress which is greater than those of the other types of fibers9,11,13,21,28,59,66-74.

5. Applications of PPTA FibersNowadays 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 strength. Kevlar 49 fibers are used extensively in ballistic 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.

6. Heat Treatments of PPTA Fibers

6.1. DefinitionThe International Federation For The Heat Treatment

Of Materials (IFHT) has defined the heat treatment term as “A process in which the entire object, or a portion of it, is intentionally subjected to thermal cycles and, if required, to chemical or additional physical actions in order to achieve


specific desired structures and properties or changes in them.”

The term thermal cycles implicitly involves the change of temperature with time during a heat treatment process. These changes may be essential to improve processing (e.g., machinability) or to meet proper service conditions “(e.g., increased (or controlled) surface and core hardnesses, wear, fatigue resistance, dimensional accuracy, desired microstructure and residual stress profile and improved tensile and yield strength, toughness, ductility, impact strength, weld integrity and magnetic and electrical properties”82.

6.2. Heat treatment zoneFujiwara et al.83 deduced mathematical equations

describing the appropriate heating temperatures for the case of drying treatment and for heating treatment under tension (post-heating):

6.2.1. Drying treatmentHere a wet PPTA fiber is to be dried by heating. It was

found that the value x of the drying temperature in °C must be such that its product with the drying time t in seconds raised to the power 0.08 be approximately in the range 150 to 300.

Mathematically (approximately):

150 ≤ xt0.08≤ 300

6.2.2. Heat treatment under tension (post-heating)Here a dry or wet PPTA fiber is to be heated under

tension for the sake of structure and property improvements. It was found that the value y of the heating temperature in °C must be such that its product with the treating time τ in seconds raised to the power 0.08 be approximately in the range 250 to 550.

Mathematically (approximately):

250 ≤ yτ0.08≤ 550

{Vlodek et al.24 reported a range of 250 to 550 °C for the treating temperature under 5-50% tension for time less than 10 minutes}.

6.2.3. Pre-heat treatment (Drying)The spinning of PPTA solutions start when the spinneret

is placed in a coagulation bath of water7. Then, the resulting fiber is dried under tension on rollers (to draw the fiber) at a temperature lying below its glass transition temperature

Tg and satisfying Fujiwara et al’s deduction - this process is known as cold drawing84. Morgan et al. reported a drying temperature of about 65 °C52.

6.2.4. Post-heat treatment (subsequent heat treatment)The concept of a fiber heating process is always done

with adding tensions to draw the fiber. Thus, the issue of heating is that of the drawing needed to orient the molecular chains (orientation phase) in the direction of the fiber axis and at the same time to increase the ordered arrangement of the intermolecular structure (crystallization phase)85. Moreover, drawing with applying heat (post-heat treatment) should be done at temperatures lying between the glass transition temperature Tg and the melting temperature Tm and satisfying Fujiwara et al.’s deduction72 so as to produce a fiber of a higher modulus52.

6.3. Thermal ageingMany researchers studied the environmental exposure

effects on PPTA fibers. They examined the structural mechanism and the mechanical properties of heat treated fibers by means of the thermal and oxidative methods15,75.

7. Heat Treatment Devices

7.1. Heating tubes and ovensHeat transfer takes place in these tubes and ovens by

convection. Blades86 post-heat treated a pre-heat treated fiber by using a heating tube containing a flow of a nitrogen gas. It is noticed that in Blades’s method the fiber is heat treated in its solid state. In such methods, of heat treatments, the presence of an atmosphere of an inert gas (such as nitrogen) is essential for the heat transfer by convection. In Blades’s method the fiber is passed over the tension guide around the fiber spool which is controlled by a magnetic brake, then, it is passed over the idler roll after which it’s passed over a pulley equipped with a force gauge and then passed through the heating tube and pulled by pairs of driven rolls to pass it to the constant tension windup roll-as illustrated in Figure 30. Chern and Trump, also, used this process to manufacture a high modulus and a high strength PPTA fiber. They processed the never-dried fiber swollen with water of controlled acidity. They reported that the fiber, preferably, should have about 20-100% water (based on the weight of a dry fiber). They used heating temperatures in the range 500-660 °C with time exposure of 0.25 to 12 seconds and tension of about 1.5 to 4 gpd[87].

Table 5. Some Applications Of PPTA Fibers78.

Applications ExamplesFriction materials and gasket Prosthetics, brake linings, clutch facings, gaskets, thixotropic additiveMedical applications Prosthetics, fibrous bone cementOptical applications Optical fiber cablesProtective applications Bulletproof vests, helmets, vehicle protection, fire-fighting, cut protection, ballistics, heat

resistant work wear , flame-retardant textilesComposites Tires, pipes, pressure vessels, plastics additive, civil engineeringRopes and cables Ignition cables, aerial optical fiber cable, electrocable, mooring ropesSport equipments Sail cloths, tennis strings


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 rollersIn 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 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 °C. The heating gas used here is – generally – a supper saturated steam.

For the case of the production of Twaron fibers, the heat treatment is done by running the yarn on a series of hot driven rollers. The rollers are running at the same speed as the fibers. But, differences in velocity between the fiber and the rollers occur due to the fiber elongation by the force gradients on the rollers. The temperature and the rotation speed of the rollers can be varied separately, so as, to apply a force and temperature profile on the running fibers93.

Figure 30. Heating tube used by Blades86.

Figure 31. Heat treatment device used by Lee et al.53,88.

Figure 32. Tension control device used by Lee et al.53,88.

Then, K. G. Lee et al.53 used a heating device in which he controlled the treatment time by controlling the velocity of the fibers that are passed through a constant heating temperature zone by applying a controllable tension throughout the process.

The Lee device for the treating of the fibers has five basic parts, which are: The take-in device (feeder), the


The main function of heat, here, is the removal of moisture and the molecular the orientation of the molecular of the fiber. The fiber filament force and the temperature profile are the main factors that determine the final mechanical properties of every individual fiber94.

Cochran and Yang95, also, investigated an on-line system by treating the dried PPTA fibers, containing at least 10% moisture, over hot rollers under 3-7 gpd tension at a drying temperature not more than 175 °C and a treating temperature, optionally, in the range 300-600 °C under a tension of at least 0.5gpd. The drying roller was heated by saturated steam. Such heat treated filaments have more improved tenacity as compared with similar filaments dried under low tension and heat treated under the same conditions.

Further modifications can be done by heat settings on hot drums at 420-450 °C. Figure 35 illustrates a full Schematic diagram of the spinning of PPTA fibers in a production plant producing up to 124 parallel running filaments (in a wrap)96.

8. Heat Treatment ProcessHeat treatment can be used to produce changes in both

the morphology and the mechanical behaviour of the fibers that are needed to improve the properties and the structural characteristics of PPTA fibers97. As-spun PPTA fiber can be subsequently heat treated at a high temperature and a high tension for several seconds to increase its crystallinity and degree of crystalline orientation. In this method the fiber is heat treated in its solid state. The fiber modulus is increased by this short term of heating under tension.

The responses of as-spun dry jet-wet and wet spun PPTA fibers to heat treatment are studied by Jaffe and Jones21. Figure 36 shows the difference in response of the fibers produced by the two types of mentioned spinning methods.

Many other researchers worked in the field of heat treatment of as-spun fibers. The heat treatment conditions for as-spun PPTA fibers used by Blades86, Morgan et al.52, Cochran and Strahorn91, Cochran and Yang95, Chatzi and Koenig35, Wu et al.38, Chern and Trump87, 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 improvementsThe 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.

9.2. Structural defectsThe 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 propertiesThe 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 modulusThe 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 size (ACS) is accompanied by an increase of the fiber

Figure 33. Heat treatment apparatus used by Cochran and Strahorn91.

Figure 34. On-line heat treatment device used by Chern92.


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.


modulus - provided that the tension during the treatment is high enough. On the other hand, if the tension does not exceed a certain critical level, then, no increase in the modulus will take place - even if the temperature is elevated.

The tensile moduli of the Kevlar fibers are such that the modulus for Kevlar 149 > for Kevlar 49 > for Kevlar 29 > for Kevlar 1197,89,102.

9.3.2. Tensile strengthThe tensile strength of PPTA fibers decreases largely

under heat treatment, although – as mentioned above – the tensile modules increases. In fact, the decrease in the tensile strength is due to exposing the fiber to high temperatures103. The reasons for this decrease are:

First, the pleats are straightened-out during the heat treatment under tension. This process may create microvoids around these pleats104. Voids tend to reduce the tensile strengths60.

Second, although the fiber tensile strength is controlled by the overall molecular orientation, but it may also be reduced with the increasing of the skin region as a result of a high orientation105.

Third, the heated fiber strength decreases with the increase of the apparent crystal size (ACS)89.

Fourth, the effect of the weak Van der Waal’s interactions in the a-axis direction in changing the unit cell dimensions is weakened by the heat exposures75.

Fifth, the increase of the surface helix angle by the twisting of the PPTA fibers106.

Sixth, the strength is adversely affected by overall gross morphological defects58.

The closing-up of the peak positions for the (110) and the (200) diffraction planes, is a sign that a sharp decrease in the tensile strength took place76.

The tensile strengths of Kevlar fibers are such that the tensile strength for Kevlar 149 < for Kevlar 49 < for Kevlar 29 < for Kevlar 119 7,89,102.

9.3.3. Compressive propertiesThe compressive properties of the PPTA fibers have

been studied in many literatures15,90,107-113. Tensile tests of compressively kinked poly(p-phenylene terephthalamide) (PPTA) fibers reveal only a 10% loss in tensile strength after application of compressive strains much greater than the critical strain required for kink band initiation114. A new route of synthesis, based on PPTA compatible reactive oligomers added to the standard PPTA dope prior to the fiber spinning process, done by Knijnenberg et al.115, resulted in an increase in the compressive strength properties when these new PPTA fibers are heat treated at temperatures of 340 to 510 °C for a duration of 5 to 300 seconds.

9.4. Crystallite structure

9.4.1. Apparent Crystal Size (ACS)The apparent crystal sizes of the (110) and (200)

reflection planes – which are determined by the equatorial scan using Scherrer formula - are found to be sensitive to

Chatzi Cochran IIb

Chern

Lee Rao KnijnenbergDry fibers Never-dried fibers (water-

swollen)Temperature, °C 250 300-600 200-660 500-660 400 180-370 340-510Time, seconds 1-6 0.2-0.6 0.25-5 0.006-9 180 30-60Tension, gpd 6 0.5 1.5-4 0.1-6 VariedSpeed, m/s 2.5-6Stretch % 5Moisture contain% Control the

moisture contain in

drying stage 7-10

aCochran and Strahorn91; bCochran and Yang95.

Table 6. Heat treatment process of PPTA fibers.

Blades MorganCochran Ia

Wu1st array 2ed array Cooling

Temperature, °C 450-580 550 175-300 175-300 0-100 Preferred 25

380-480

Time, seconds 0.5-5 5Tension, gpd 1-8 1-6 0.6-4.0 0.6-4.0 0.5-1.5

Speed, m/s 4.7 4.7Stretch % 5 0.1-1.6 0.2-1.16 Between

the two arraysMoisture contain%


the applied tensions and the heating temperatures. They are found to increase monotonically with increasing the heating temperatures. However, the apparent crystal size of the (110) plane increases faster than that of the (200) plane. The apparent crystal sizes of the (110) and the (200) reflection planes are found to have the respective values of 52 and 46 Å for Kevlar 29, the values 66 and 41 Å for Kevlar 49, the values 123 and 76 Å for Kevlar 149 and the values 52.1 and 49.9 Å for Kevlar 129[39,43]

9.4.2. Axial crystal size (L)The axial crystal sizes of the (00l) reflection planes of

the PPTA fibers – which are determined by the meridional scan using Scherrer formula – are affected by the applied tensions and heating temperatures. Rao et al.43 and Hindeleh and Abdo41 reported that the reflection order of the (002) plane decreases with increasing the heating temperatures when the fiber is heated to temperatures above 400 °C. The values 656.14 nm, 737.15 nm, and 1547.79 nm are found for the reflection order of the (002) plane for Kevlar 29, Kevlar 49, and Kevlar 149 fibers, respectively. It is noticed that these values are greater than the corresponding apparent crystal sizes13,89

Chang-Sheng Li et al. reported the value 94.7 Å for Kevlar 129 in (004) plane39.

(N.B. Rao’s et al. nm units are surprising!)

9.4.3. Lattice distortion (crystalline perfection)The paracrystalline lattice structure analysis developed

by Barton116 is founded by Hosemann117-121. The defects in the crystal lattice can be improved by the possibility of lateral crystal growth (Axial crystal size) particularly along the hydrogen bonding direction occurring during the heat treatment of the PPTA fiber. The level of the distortion parameter of the paracrystallinity and the constant α* can be modified by the heat treatment as had been concluded by Hindeleh and Hosemann122 and Hindeleh et al.44,123. Lee reported that the distortion of the lattice decreases as the modulus increases during the fiber heat treatment process under tension. Kevlar fibers have high molecular crystalline perfections (the crystalline perfection for Kevlar 149 > for Kevlar 49 > for Kevlar 29)124.

9.4.4. Crystal orientationThe decrease in the orientation angle for heat treated

PPTA fibers is an indication for improvement in the crystallites alignment43,53. This improvement is produced

by the increasing of both the temperature and the applied tension. Kevlar fibers have high molecular orientation (the molecular orientation for Kevlar 149 > for Kevlar 49 > for Kevlar 29).

The misorientation of the fibrils in heat treated fibers decreases with increasing the stretch ratio as a result of an increase in the total orientation43,89. However, the fiber modulus, the crystal perfection and the crystallinity increase as a result of the increase in the total orientation due to the mentioned stretching (as illustrated by Jaffe and Jones in Figure 37)21. On the other hand, the total crystal orientation decreases with increasing the compressive strain125.

The orientation angle of a PPTA (Kevlar) fiber is about 12° and it decreases to about 9° after heat treatment99,126 – an indication that the heat treatment of a PPTA fiber increases its total orientation.

10. SummaryThe spinning process is the fiber forming step and the

subsequent fiber-heat treatment is the fiber re-structuring step to improve its properties26,127.

The following observations relate to PPTA fibres:The benefits which can be got from the heat treatment

processes are improving the molecular orientation and the crystal perfection, and increasing the crystallinity and the modulus of PPTA fibers – as a result of structural improvements.

The improvement of the molecular orientation at a given chain length is the most important factor correlating with the fiber strength128.

The relationship between the tensile modulus and the orientation are illustrated in Figure 37. On the other hand, the tensile strength of the spinning of PPTA fibers is a function of the total spinning stress and it tends to increase with increasing the spin stretch factor, increasing the inherent viscosity and with decreasing the air gap. The crystal structural characterizations show that the wet spun fiber is less crystalline and has less efficiency for hydrogen bonding in the unit cell than the dry jet-spun fiber which gives low elongation21.

The stress–strain curve for heat treated PPTA fibers bends upwards to give greater moduli at higher stresses as the pleats are pulled out, Figure 29[69,94,124].

The fiber tensile strengths and failure strains decrease 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 temperature, however, the orientation is more sensitive to the applied tension89.

Figure 37. The relationship between the tensile modulus and orientation angle21.


In heat treatment processes the voids formed for prolonged periods and the void contents increase with the increase in crystallite size. Voids affect the mechanical properties of the fibers, namely, they tend to decrease the tensile strength58,59,131.

The PPTA fabrics capacity to absorb water and ability to take up dyestuff are very low compared to other fabrics130.

11. ConclusionThe benefits which can be got from the studies of the

heat treatment of PPTA fibers are:1. Improving the molecular orientation & improving the

crystal perfection and increasing the crystallinity & increasing the modulus of PPTA fibers - as a result of structural improvements due to the heat treatment of the PPTA fibers.

2. The improvement of the molecular orientation at a given chain length is the most important factor correlating with the fiber strength.

3. The reduction of the tensile strength value is -essentially- influenced by the molecular-level defects.

AcknowledgementsI would like to take this opportunity to extent my

acknowledgements to the National Basic Research Program of China (973 Project) - under Project 2011CB606101 - for financing this piece of research.

Also, my acknowledgements are extended to the China Scholarship Council (CSC) for giving me a scholarship to do post-graduates studies in China.

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