Microstructural developments of poly (p-phenylene terephthalamide) fibers during heat treatment process: a review

Microstructural developments of poly (p-phenylene terephthalamide) fibers during heat treatment process: a review

Dawelbeit Ahmed; Zhong Hongpeng; Kong Haijuan; Liu Jing; Ma Yu; Yu Muhuo^*

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, 201620, Shanghai, China

ABSTRACT

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^ï and Twaron^ï 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

1.1. High performance fibers

High-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^ï and Dyneema^ï), based on gel-spinning technology invented at DSM in the Netherlands.

The high-strength polymeric fiber Zylon^ï, 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 story

The twentieth century witnessed huge researches and innovations for fibers. After Nylon^ï 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^ï 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^ï, were made by DuPont in USA, Ireland and Japan. Another type of PPTA fibers, named Twaron^ï, was produced in 1978 by Teijin Aramid B.V. in Netherland^1,5,8-12.

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 rings^2,12-15. 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.

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, formation of the fiber by spinning¹⁸.

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 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 preparation

Synthesized 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 η_inh = 4.9 in concentrated H₂SO₄. Figure 4 explains the relationship between the concentration and the solution viscosity for poly (p-benzamide) (PBA) (η_inh= 2.41) by Kwolek⁵. The Oriented domains are formed at concentrations of more than 12 wt. %²⁵.

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.

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.

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.

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 low^17,24,33. 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 structure

PPTA 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-group^35,36. The layer of the hydrogen-bonded chains and the contents of the cell are shown in Figures 9a 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 - reported by Northolt and van Aartsen³⁴, Panar et al.⁴⁰, Yang^13,31, Hindeleh and Abdo⁴¹ and Hindeleh and Obaid⁴².

Yang^13,31 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) 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.

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 diameters^40,45-47.

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.

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.

Yang^13,28 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 inside^51-53.

3.2.4. Pleat sheet structure

A 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 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 sheet^13,45,52,54.

3.2.5. Chain-end defects

The 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 defects

Crack 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, 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 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. Microvoids

The 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 voids^57-60.

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 modelling

Laboratory 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-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 FIBERS

PPTA 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 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 fibers^{9,11,13,21,28,59,66-74}.

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 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 5^{2,4,11,72,76-81}.

6. HEAT TREATMENTS OF PPTA FIBERS

6.1. Definition

The 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 zone

Fujiwara 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 treatment

Here 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 < xt^0.08< 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 < yτ^0.08< 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 T_G 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 T_G and the melting temperature T_M and satisfying Fujiwara et al.'s deduction⁷² so as to produce a fiber of a higher modulus⁵².

6.3. Thermal ageing

Many 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 methods^15,75.

7. HEAT TREATMENT DEVICES

7.1. Heating tubes and ovens

Heat 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].

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 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 device^53,88.

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 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.

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 PROCESS

Heat 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 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 Hsiao^100,101 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 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 fibers^13,28,70.

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 size (ACS) is accompanied by an increase of the fiber 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 119^7,89,102.

9.3.2. Tensile strength

The 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 properties

The compressive properties of the PPTA fibers have been studied in many literatures^{15,90,107-113}. 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 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 sizes^13,89

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 Hosemann^117-121. 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 orientation

The decrease in the orientation angle for heat treated PPTA fibers is an indication for improvement in the crystallites alignment^43,53. 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 orientation^43,89. 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 treatment^99,126 - an indication that the heat treatment of a PPTA fiber increases its total orientation.

10. SUMMARY

The spinning process is the fiber forming step and the subsequent fiber-heat treatment is the fiber re-structuring step to improve its properties^26,127.

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.

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 strength^58,59,131.

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

11. CONCLUSION

The 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.

ACKNOWLEDGEMENTS

I 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.

Received: October 31, 2013

Revised: April 6, 2014

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Publication Dates

• Publication in this collection
  09 Sept 2014
• Date of issue
  Oct 2014

History

• Accepted
  06 Apr 2014
• Received
  31 Oct 2013