低分子量聚丙烯熔融纺丝高强度纤维制备方法

Mao Qianchao，Wyatt Tom P.，Chien An-Ting，Chen Jinnan，Yao Donggang

1 INTRODUCTION

Melt spinning is a common process to produce hightenacity polypropylene (PP) fiber，one of the leading commercial fibers in the field of industrial textiles. Efforts to make strong PP fibers began as early as the 1960s and were mostly conducted three decades ago. Previous studies demonstrated that high-strength PP fibers can be produced from PP resins with relatively low melt flow index (MFI) by using a combined jet stretching and drawing process. This remains to be the standard process in the industry for producing high-strength PP fiber.

However，PP resin with low MFI has high molecular weight and high viscosity，resulting in certain processing disadvantages in melt spinning. To successfully extrude high-viscosity resins，it is necessary to raise the extrusion temperature，increase the extrusion pressure，and decrease the output rate，leading to slow production with increased costs. The high molecular weight further renders an elastic extrudate，resulting in considerable die swell. To produce a fiber with a typical fiber diameter (~20-50μm)，one has to rely on an extensive jet stretching process to reduce the extrudate diameter. It was reported，however，that increased molecular orientation of the extrudate during jet stretching would reduce the elongation at break and consequently decrease drawability. Furthermore，it was reported that a high degree of preferential c-axis orientation parallel to the fiber axis introduced during spinline stretching resulted in defects and fibers with poor ultimate mechanical properties. Nevertheless，in the case of high-molecular-weight PP，combined jet stretching and solid-state drawing is an adequate process for producing useful PP fiber with sufficiently high strength exceeding 400MPa.

Melt spinning PP resins with low molecular weight (high MFI) addresses several disadvantages associated with spinning highmolecular-weight resins.Owing to the lower viscosity of high-MFI PP，the extrusion speed can be faster with reduced die swell and extrusion stress，leading to improved process efficiency.Several studies have been reported involving melt spinning PP fibers from resins with high MFI；however，relatively low tenacities were reported:2.3g/day (~184MPa) for resin with MFI of 35g/10min；170MPa for resin with MFI of 74~78g/10min and 250MPa for MFI of 300g/10min.In these studies，the same processing strategy as used in spinning high-MFI PP was used，i.e.，applying a large amount of jet stretching in the melt state.Although jetstretching is efficient for production，this seems to be not suitable for producing high-strength PP fiber from high MFI resins.Due to rapid relaxation of the shorter chain length，jetstretching is not an ideal process to orientate low-molecularweight PP and obtain high-strength fiber.

In this study，a different processing strategy has been explored to produce high-strength PP fiber from high-MFI resins. In contrast to the standard process，we extrude high-MFI PP with minimal jet stretch and orientate the fiber only in the solidstate hot drawing stage. Since minimal stretching of the extrudate occurs in the melt state，the extruded filaments exhibit high drawability in the solid state，leading to good molecular orienta tion and tensile strength. Hot drawing in the solid state is emphasized since it offers the possibility to produce high-strength fibers from high-MFI resins due to the significantly slower molecular relaxation in the solid state compared to the molten state.

2 EXPERIMENTAL

2.1 Material

PP pellets used in this study were Marlex HGZ-1200，provided by Phillips Sumika Polypropylene Company (The Woodlands，TX)，with a reported density of 0.907g/cm³and an MFI of 115g/10min at 230℃

2.2 Fiber Spinning

Fiber spinning was carried out using an Alex James and Associates piston extruder (Alex James and Associates Inc.，Greer，SC) with a 2.54cm bore diameter and 150mL capacity.The PP pellets were fed into the bore and equilibrated at the set temperature for 1h.The melt in the bore was extruded through a 0.5mm die orifice at a speed of about 5m/min.The apparent shear ratecan be calculated by

whereD is the diameter of the spinneret andq_Vis the volumetric flow rate of the melt，which can be estimated by

whereR is the radius of the spinneret andu is the extrusion speed of the melt.

The molten filament was quenched in ambient air o ver a distance of about 30cm and collected onto bobbins at a speed of 5m/min.The collection and extrusion speeds in the spinning stage were set the same in order tominimize the stretching of the extrudate and consequently avoid stress-induced crystallization and orientation.Two stages of hot drawing were performed through a heated polyethylene glycol (PEG) bath.The molecular weight of PEG was 400g/mol.The total length through the hot bath was 0.6m.The first stage was performed at temperatures ranging from 120 to 150℃ with a feeding speed of 1 m/min and collection speeds from 15 to 19 m/min to obtain desired draw ratios.A second heat-setting stage was performed between 140 and 160℃ in PEG bath with a feeding speed of 0.5m/min and collection speeds from 0.6 to 0.8m/min to keep filaments under a certain amount of tension.The heat-setting time was about 1min.Heat-set fibers were quenched in ambient air.Slow speeds were chosen in this study to demonstrate the feasibility on the lab scale.Draw ratio is defined as the ratio of the collection roller speed to the feed roller speed.The total draw ratio of the final filament is calculated by multiplying the draw ratios of two stages.Fig.1 shows the schematic diagram of the PP melt spinning process.

Fig.1 Schematic diagram of the PP melt-spinning process

2.3 Characterization

Diameter measurements were obtained by weighing a known length of fiber and calculating the cross-sectional area assuming a density of 0.907g/cm³.Before being weighed，the hot-drawn fibers were briefly rinsed with water to remove residual PEG from hot drawing stage and dried.

Viscosity measurements were performed on an LCR7000 capillary rheometer (Dynisco Co.，Franklin，MA).PP pellets were fed in the barrel and maintained at 180℃ for 15min.The orifice diameter was 0.5mm.Viscosity of the melt under different shear rates was collected.

Tensile properties for single filaments were measured using an Instron 5566 universal testing machine (instron，Norwood，MA). Fiber samples were wound onto wooden rods approximately 2mm in diameter and superglued over the wound fiber ends. The prepared single-filament samples were clamped using Instron 2711 Series Lever Action Grips rated for 5N. Crosshead speed was 50mm/min with a gauge length of~10cm. All ten-sile tests were performed under air-conditioned room conditions. Six samples from each condition were tested and averaged. Experimental error was estimated using the standard error of the mean，defined as the standard deviation divided by the square root of the sample number.

X-ray with a beam size <0.3mm was generated using aRigaku Micro Max 002 generator(CuK_αradiation，λ＝0.154nm) operating at 45kV and 0.65 mA.Diffraction patterns were recorded by a detection system(Rigaku R-axis IV++) and analyzed by MDI Jade(version 9.0).The detector has a pixel readout resolution of 100×100μm.The system as well as the distance between samples and the detector was calibrated by corundum and silver behenate.Exposure time was 30min for each sample.All diffraction patterns were corrected by removing the background diffraction patterns taken under the same ambient conditions.The WAXD system was also purged by helium gas to reduce the influence from air diffraction.The crystalline orientation factor was computed using the method developed by Wilchinsky.The 110 and 040 diffractions were used to determine the orientation factor based on the monoclinic PP unit cell with dimensionsa＝0.665nm，b＝2.096nm，c＝0.65nm，andβ－99°8′.

The melting process of PP granules and PP fibers was studied on a TA Q200 differential scanning calorim-eter unit (DSC，TA Instruments，New Castle，DE). The PP granules were subjected to a heat-cool-heat cycle in the range from 40 to 200℃ at 10℃/min. Data from the second heating cycle was collected for the PP granules. In a different procedure from the PP granules，the PP fiber was heated from 40 to 200℃ at 10℃/min，and data from the first heating cycle was collected for the PP fiber. The DSC unit was purged with nitrogen during all experiments. From the heat of fusion，an apparent crystallinity of PP fiber was determined by the following equation:

where ΔH_fis the measured melting enthalpy of PP fibers andis the melting enthalpy of 100% crystalline PP which is 190 J/g.

Scanning electric microscopy (SEM) images of single filament were collected on an LEO 1550 (LEO Co.，Germany). Fiber samples were mounted onto carbon tape and gold sputtered.

3 RESULTS AND DISCUSSION

3.1 Extrusion

Choosing proper conditions is essential to obtain a uniform fiber extrudate，among which the extrusion temperature is one of the most important factors since it affects the viscosity of the melt. Relatively high extrusion temperature above 200℃ was usually applied in previous works for low MFI PP resin to decrease the melt viscosity and the extrusion stress. For high-MFI PP resin in this study，frequent breakage occurred when the barrel and die temperatures were above 200℃ due to the low melt viscosity. An extrusion temperature of 180℃ was found to be the minimum for generating a uniform extrudate. To minimize the potential for thermal degradation of the melt at higher extrusion temperature，180℃ was chosen as the barrel and die temperature in this study.

Fig.2 Viscosity versus apparent shear rateof high-MFI PP resin at 180℃

The viscosity versus apparent shear rate of high-MFI PP resin at 180℃ is shown in Fig.2.Since the melt extrusion speed is 5m/min and the diameter of the die is 0.5mm，the volumetric flow rate of the melt calculated usingEq.2 is 0.98cm³/min.The apparent shear rate calculated usingEq.1 equals 1333.3s^－1，corresponding to the viscosity of about 120Pa-s as shown in Fig.2.Due to the low melt viscosity and shorter molecular chains of the high-MFI PP resin，rapid relaxation is likely to occur during stretching in the melt state.Therefore，jet stretching is not optimal for producing strong fibers from highMFI resins.Furthermore，it was reported that the melt fracture occurs when the shear stress exceeds the critical stress during extrusion and the critical stress is likely to occur at relatively high shear rate.According to capillary rheometer data presented in Fig.2，the onset of melt fracture of the high-MFI PP resin occurs when the apparent shear rate exceeds about 10⁵s^－1at 180℃.Assuming a critical apparent shear rate of 10⁵s^－1，the volumetric flow rate equals 73.8cm³/min and the mass output rate equals 54.5g/min with the density of 0.739g/cm³for the PP melt in this study.The relatively high theoretical mass output rate demonstrates the feasibility of improving the production efficiency using high-MFI resin to spin fibers.Nadella et al.mentioned that the mass output rate was 2.1g/min for PP resin with MFI higher than 2.55g/10min；however，for PP resin with MFI of 0.45g/10min，the mass output rate had to be reduced to 0.5g/min to eliminate the extrudate distortion due to the high extrusion shear stress.It further indicates the relatively good spinability and flow stability of PP resins with high MFI.

In this study，slight jet stretch due to gravity occurred in the air gap region during the spinning process. Sheehan et al. pointed out that the more the filaments were oriented during the spinning process，the lower the ultimate tenacities due to lower drawability during subsequent hot drawing. Therefore，minimizing the orientation of the precursor fibers during the spinning process should be beneficial to the ultimate mechanical properties of the fibers. To avoid significant jet stretch，the extrusion and collection speeds were set the same and consequently stress-induced crystallization and orientation during the spinning process were minimized.(www.xing528.com)

Fig.3 DSC thermograms of PP pellets and PPfibers after different processing stages

Fig.3 shows the DSC thermograms of PP pellets and precursor fibers.The melting temperature and crystallinity of the precursor fiber was 166.3℃ and 44.2%，respectively，both slightly higher than the original PP resin which was 165.8℃ and 43.0%，suggesting that slight crystallization occurred during the extrusion and take-up process.Concentric rings in the WAXD pattern shown in Fig.4A implied no significant orientation in the undrawn precursor fiber.Four strong diffractions corresponding to the(110)，(040)，(130)，(131/041) crystal planes are shown in the total integration of Fig.5.These peaks correspond to theα-monoclinic structure.

Slight orientation was demonstrated in the azimuthalintegration of the undrawn precursor fiber based on the intensity distribution at approximately 90° for (040) crystal plane as shown in Fig.6.Accordingly，crystalline orientation factor was calculated to be 0.24 suggesting slight orientation was imparted during the fiber extrusion stage，probably due to the shear in the die and stretch from the weight of the fiber.Avci et al.reported that the crystalline orientation factor of as-spun fiber was 0.92 for PP resin with an MFI of 4.1g/10min when the take-up speed was fixed around 3000m/min.Nadella et al.stated that the crystalline orientation factor of the as-spun fiber was 0.64 for PP resin with an MFI of 12g/10min when the takeup speed was 500m/min.The crystal-line orientation factor for asspun fiber in this experiment is lower than the studies which apply significant jet stretch，suggesting that the shear and jet stretch in the present spinning process was not sufficient to impart significant orientation in the precursor fibers.

Fig.4 WAXD pattern of spun fibers：(A) undrawn fiber；(B) fiber drawn in the first stage；(C) fiber after the sec-ond stage of drawing. [Color figure can be viewed in the online issue，which is available at wileyonlinelibrary.com.]

Fig.5 Total integration of precursor and drawn fibers afterthe first and second stages of hot drawing

Fig.6 Azimuthal integration of precursor and drawn fibers after the first and second stages of hot drawing：(A) 040equatorial diffractions；(B) 110 equatorial diffractions

3.2 First-Stage Hot Drawing

For melt spinning PP fibers，several studies have focused on the drawing process. Since the mechanical properties of drawn fibers are primarily influenced by the drawing temperature and draw ratio，it is essential to study the two parameters.

The maximum draw ratio is limited by the fracture of the specimen.The PP fiber was drawn to a maximum ratio of 19×at 130，140，and 150℃ although the drawing is not stable at this draw ratio with frequent breakage occurring before sufficient sample could be collected.In order to attain a stable drawing process，a draw ratio of 18×was chosen to determine the optimal drawing temperature.The melting temperatures and crystallinities of the fibers drawn to 18×at temperatures from 120 to 150℃ are shown in Fig.7A.No considerable changes were observed for melting temperature and crystallinity with increasing drawing temperature.Fig.7B shows the tensile properties of fibers drawn to 18×at various temperatures.There is no observable dependence on temperature for tensile modulus.This is expected because the same draw ratios of 18×should lead to similar crystalline orientation induced by drawing.Furthermore，the tensile strength peaked at 530 ± 20.3MPa at 130℃.At higher drawing temperature，the polymer molecules should have more mobility to crystallize and orient，consequently reducing defects and contributing to increased tensile strength.The molecular relaxation effects axe stronger when the drawing temperature was too high (e.g.，150℃) leading to the loss of the fiber strength because the molecules have a higher probability to assume a random coil.Therefore，130℃ was chosen as the optimal temperature for the first drawing stage based on the maximum tensile strength.

With respect to the draw ratio，it was well studied that the higher draw ratio is beneficial to the fiber strength from highmolecular-weight PP resins. However，no literature reported the relationship between the draw ratios and the fiber properties from low-molecularweight PP resins. In order to verify the influence of draw ratio，the properties of fibers drawn at 130℃ to various draw ratios ranging from 16×to 19×were investigated. Significant whitening of the filaments occurred when the draw ratio was above 16×. This whitening may be caused by the increased crystallinity. The melting temperatures and crystallinities of fibers drawn to different draw ratios at 130℃ are also shown in Fig.8A. No significant changes were observed for melting temperature and crystallinity with increasing draw ratio.

Fig.7 Properties of PP fibers drawn to 18×at variousemperatures：(A) melting temperatures and crystallinities；(B) tensile strengths and modulus；the solid markers correspond to the optimal condition

Fig.8B demonstrates the tensile properties of fibers drawn at different ratios at 130℃.The results show that the fiber modulus increased from 7.42 ± 0.65 to 10.33 ± 0.47GPa as the draw ratio increased from 16×to 17×.A slight fluctuation in modulus occurs at draw ratios above 17×with the highest value of 10.65 ± 0.39 GPa at 18×.Alignment of amorphous polymer chains and rearrangement of the crystalline regions may occur during drawing，which likely contributes to the higher modulus.The tensile strengths of the fibers show an upward trend as the draw ratio increased.Bigg et al.claimed that drawing could create more tie molecules through additional chain unfolding and an increase in amorphous orientation during drawing also may contribute to the fiber strength.In this study，the strength of the fiber drawn to 18×and 19×achieved 530 ± 20.3 and 550 ± 32.9MPa，respectively.However，drawing to 19×was not stable enough for continuous production，so the draw ratio of 18×was chosen as the optimal ratio for the first stage.

The melting temperature and crystallinity of fiber drawn to 18×at 130℃ reached 168.8℃ and 63.0%，respectively，which was 2.5℃ and 18.8% higher than the precursor fiber，respectively (Fig.3).The WAXD pattern shows a highly oriented crystalline structure de-veloped during the drawing process (Fig.4B).The intense diffractions at 2θof 14.8° and 17.1°correspond to the 110 and 040 planes，respectively，and are indicative of the typical PP monoclinic unit cell (Fig.5).The azimuthal integration of the fibers drawn in the first stage (Fig.6) shows strong intensity along the fiber axis (azimuthal angle of 90°) indicating good crystalline alignment along the fiber axis which is further supported by the crystalline orientation factor of 0.82.The SEM figure shown in Fig.9B depicts a relatively uniform and oriented fibrillar structure.

Fig.8 Properties of PP fibers drawn to various ratios at 130℃：(A) melting temperatures and crystallinities；(B) tensile strengths and modulus；the solid markerscorrespond to the optimal condition

3.3 Heat Setting

It is known that the oriented semicrystalline fibers are rarely in their equilibrium state leading to the dimensional instability and shrinkage caused by temperature，moisture，and load. Heat setting with tension at an elevated temperature but below the melting temperature for a specific amount of time，can reduce the residual stresses that persist from the melt-spinning stage. Therefore，heat setting under appropriate conditions can stabilize the fibers against shrinkage，dimensional changes，and reduce of structural defects. Consequently，heat setting under appropriate conditions may improve the mechanical and thermal properties of fibers. In this context，it becomes necessary to heat set the fibers that have undergone a first stage of hot drawing.

In this study，heat setting was conducted at slow speed on the lab scale to demonstrate the feasibility of heat setting to improve the properties of the fibers meltspun from high-MFI resins. The scale-up of this process to high-speed production will be reserved for future investigations.

It is known that heat setting conditions are dependent on the temperature，tension，and environment.To determine the optimal conditions for heat setting in this study，the fibers were drawn to ratios of 1.0×，1.2×，and 1.4×at temperatures of 140，150，and 160℃.Table 1 shows the tensile strengths and modulus of the fibers heat-set under different conditions.Regardless of heat setting temperature，tensile strength and modulus decreased for fibers with a draw ratio of 1.0×.This is probably because the tension applied at 1.0×is not sufficient to overcome molecular relaxation effects at the relatively high temperatures.At a higher ratio of 1.2×，the tensile strength of fibers heat set at 140℃ main tained the same value with the first stage while the modulus slightly dropped.Heat setting at the lower temperatures may not sufficiently reduce defects or recrystallize because of the limited molecular mobility.At the maximum temperature of 160℃，both the tensile strength and modulus were lower than that of the first stage.The corresponding orientation factor was calculated to be 0.81，slightly lower than 0.82 of the fibers drawn in the first stage.At higher temperatures，the molecular relaxation effects become significant resulting in lower strength and modulus.For the highest draw ratio of 1.4×，only fibers heat-set at 140 and 150℃ could achieve the desired ratio.However，the heat setting becomes unstable at draw ratio of 1.4×causing frequent filament breakage.

Fig.9 SEM images of melt-spun PP fiber：(A) undrawn precursor；(B) fiber after the first stage；(C) fiber after heat setting；(D) fiber after heat setting，higher magnification

Table 1 Tensile strength and modulus of PP fibers after heat setting

A maximum tensile strength and modulus of 580 ± 34.2MPa and 11.6 ± 0.583 GPa，respectively，were achieved at a draw ratio of 1.2×and a heat-setting temperature of 150℃，and thus were chosen as the optimal condition for heat setting.The melting temperatures of the drawn fibers shift to higher values compared to the original resin and the precursor (Fig.3).It is noteworthy that the melting temperature of the heat-set fiber was 170.8℃，approximately 2℃ higher than that of the fibers drawn in the first stage.Crystallization of oriented a-morphous phases that axe well parallelized may occur during the heatsetting process by gaining sufficient mobility of molecules with the addition of thermal energy.This effect may contribute to the increased melting temperature of the heat-set fibers.

The intense diffraction along the equator shown in Fig.4C is visible for heat-set fibers suggesting a highly oriented crystalline structure.Strong diffractions(110)，(040)，(130)，(131/041)，(022) shown in Fig.5 correspond well to theα-monoclinic structure.Based on the peaks of the total integration，the α-monoclinic structure remained throughout the spinning and drawing process due to the stability ofα-monoclinic structure.The strong intensity in the azimuthal integration shown in Fig.6 suggests good fiber axis crystalline orientation with a crystalline orientation factor of 0.84，slightly higher than that of fiber drawn in the first stage.The total draw ratio of the fibers after heat-setting increased from 18×to 21.6×.The additional extension of molecules in the solid state caused by the draw ratio applied during heat-setting to maintain fiber tension contributes to the increased crystalline orientation.The SEM photographs in Fig.9C show a uniform and smooth fiber surface morphology.

The diameter of the final fiber was about 78.2μm.Since the die diameter is 0.5mm and assuming minimal die swell，the theoretical total draw ratio can be estimated to be approximately 41.1×based on the ratio of the cross-sectional areas.Therefore，the stretch occurring in the air-gap region due to gravity during spinning is estimated to be approximately 1.9×.This small amount of stretch is much lower than the total draw hotdraw ratio of 21.6×in the solid state.The small draw down ratio provides the optimal precursor fiber morphology for good hotdrawability of the fibers and hence the improved ultimate mechanical properties.The fiber strength after first drawing stage achieved around 530MPa which is relatively high for industrial production even without the heat setting stage.A faster heat setting process would be necessary and interesting to be investigated in the future study to further demonstrate the production efficiency.In addition，the extrusion and hot drawing in the solid state can be continuously conducted on a single fiber production line.Therefore，it should be feasible to apply this processing strategy to industrial production.

4 CONCLUSIONS

In this study，a processing route to produce highstrength PP fibers from low-molecular-weight PP resin with an ultrahigh MFI of 115g/10min was explored.A two-stage hot drawing procedure in the solid state withminimal stretching in the melt state was developed which is distinguished from the typical jetstretching process for low-MFI resin in the existing literature.Most of the molecular orientation and crystallization occurred in the first drawing stage yielding a tensile strength of 530 ± 20.3MPa and an orientation factor of 0.82 when fibers were drawn to 18×at 130℃.Fibers drawn in the second stage (heat setting stage) to 1.2×at 150℃ achieved a tensile strength of 580 ± 34.2MPa and a tensile modulus of 11.6 ± 0.583GPa.The melting temperature of the final fiber reached 170.8℃，approximately 5℃ higher than that of the original resin.A WAXD study showed that the stableα-monoclinic crystalline structure was developed during the drawing process.A welloriented crystalline structure along the fiber axis was generated with a crystalline orientation factor as high as 0.84.A uniform fiber surface morphology and a fibrillar structure oriented along the fiber axis were observed in SEM photographs.

5 ACKNOWLEDGMENTS

One of the authors (Q. C. Mao) was supported by the Chinese Scholarship Council. The authors acknowledge Xudong Fang at Georgia Institute of Technology for assistance with SEM observation.