A green method to fabricate porous polypropylene fibers: development toward textile products and mechanical evaluation

Abstract

In this study, a series of immiscible polymer blend fibers with polypropylene (PP) and polyvinyl alcohol (PVA) was obtained by a melt spinning process, and two different draw ratios were attempted. Efforts were made to obtain the porous PP fibers by removing the water-soluble PVA phase. The thermal properties of the blends were tested by thermogravimetric analysis and differential scanning calorimetry. The blends showed excellent thermal stability and differentiated fractionated crystallization behaviors of PP. The melt flow indexes of the blends were evaluated, exhibiting a higher fluidity than that of the neat polymers. Among the possible candidates for the spinning process, only the PP₇₀–PVA₃₀ had suitable spinnability, for which the draw ratio reached 3. The morphology of the fibers was investigated by selective extraction experiment and scanning electron microscopy, as well as wide-angle X-ray diffraction. The biphasic morphology and the crystallization behaviors varied according to the PVA content. Furthermore, the mechanical properties of the multifilament fibers were studied via tensile testing and dynamical mechanical analysis. The 70/30 weight ratio (PP/PVA) was the most suitable for producing biphasic fibers with a high degree of accessibility in PVA and mechanical properties that increase with the increase in the drawing ratio. The feasibility of fabric knitting was checked, and the mechanical properties and air permeability of the obtained textile structure were also evaluated.

Keywords

porous fibers immiscible blends melt spinning mechanical properties

Immiscible polymer blending is a common strategy for giving materials additional or even combined properties, and their most common structures contain co-continuous, lamellar and matrix-droplet elements.¹ Morphological control is of great importance for the performance of materials. The blending strategy also offers the possibility of manufacturing porous materials, if one of the phases can be removed. The most studied structure is the co-continuous morphology, which offers complete accessibility of the sacrificial phase with a solvent. It provides a scaffold in many applications, such as tissue engineering,^2,3 the biomedical fields,^4,5 etc. Polymeric synthetic fibers are an essential part of today's textile industry, due to the possibility to obtain functional or multifunctional materials having a high specific surface area. Melt spinning is one of the main routes to produce upstream polymeric fibers for textile use,² with cost-effective, rapid and large-scale production.

Much research has been dedicated to the formation of mixed fibers from polymer blends.^6–8 The melt spinning of polymer blends endows the fibers with the matrix-fibril morphology,⁹ and it has become a practical approach for obtaining micro-/nanofibers after the matrix etching. Tran et al.^10–12 discussed the influence of the process conditions and made mathematical simulations about the melt spinning of blend fibers from poly(lactic acid) (PLA) and polyvinyl alcohol (PVA) with the weight ratio of 30/70. It was revealed that the spherical or ellipsoidal dispersed PLA droplets become elongated and coalesced into rod-like or ellipsoidal shapes, passing through the capillary die. The final morphology of the PLA phase is altered by the spinning conditions, controlled not only by the deformation of the initial sizes but also by the combination of deformation, coalescence and breaking up. Fakirov et al.^13–16 have also studied the spinning of biphasic polymer pairs. The idea of the “concept of converting instead of adding” was proposed, as dispersion problems of nanomaterials can be solved by making the polymer nanofibrillar during manufacturing. Aimed at the selection of components, water-soluble polymers were strongly recommended. The contribution of coalescence to the fibril formation mechanism was discussed as well.

Polypropylene (PP) has an excellent chemical and mechanical resistance against a harsh and aggressive environment. Also, the PP melt processing temperature span is relatively wide, which suggests ease of use with another polymer in the melt spinning process. PP is one of the cheapest synthetic polymers and finds various applications under its fibrous form.¹⁷ The low density of PP allows it to float as nets, ropes or other textile products.¹⁸ PVA is biodegradable and water-soluble, and has important fiber-forming properties.¹⁹ In addition to traditional wet spinning,^20–22 melt spinning technology^23–25 is employed to produce PVA fibers. PP–PVA is a combination of apolar/polar thermoplastic polymers, which have significant immiscibility. PVA has excellent properties for its oxygen barrier, dyeing and biodegradability. Thus, the research on polyolefin–PVA blends focuses on improving the associated properties.^26–28 Some researchers^29–31 have made efforts to expand the application of polyolefin–PVA by the melt spinning process, among which Ku et al. carried out an in-depth study about the spinning parameters during the manufacturing process.³⁰³¹ It has been found that the viscosities of the blends show a negative deviation, which is lower than that of both of the components. The draw ratio (DR) is a key factor to impact the elongational flow, and the convergence of blending ratio leads to higher elongation rates as well as lower elongational resistance, which is negative toward the melt spinning process. Compared with their research, our study aims to reveal the regularity of the morphology evolution of the biphasic fibers and accompanied changes in the mechanical behaviors, which is still under-reported to our knowledge. In addition, no study reports the feasibility of manufacturing porous fibers and textiles, taking advantage of the water solubility of PVA. Due to its water solubility, PVA can be even reused during the manufacturing process, and the whole process can be free of organic solvents, which demonstrates a green method. Our preliminary work focused on analyzing the morphological evolution of extrudates of binary PP–PVA and ternary PP–PVA–silica.³² It has been found that the micromorphology of PP–PVA extrudates varies significantly with component fractions or even a low fraction of silica nanoparticles.

The purpose of this research is to study the morphological evolution from extrudates to multifilament yarns of PP–PVA, through different tests along the whole textile production process. Different PP/PVA ratios have been studied to optimize the appropriate formulation to obtain porous PP–PVA blend fibers by melt spinning followed by phase extraction. Thus, PP and PVA were selected as the dominant and sacrificial phases, respectively. To obtain the PP-matrix/PVA-dispersed structure, the PVA content varied from 30% to 50% by weight. The thermal properties of the blends were examined, as well as the biphasic morphology and mechanical properties of fibers. The feasibility of further processing into knitted fabrics was also checked.

Materials and methods

Materials

The materials used are summarized in Table 1.

Table 1.

The raw polymeric materials used in the experiment

Raw materials	Model	Enterprise	Melting point (℃)	Melt flow index (MFI) (g/10 min) (200℃, 2.16 kg)
PP	PPH 9069	Total Enterprise, France	168	15
PVA	OKS-8112P	Nichigo Gohsei, Japan	179	30

PP: polypropylene; PVA: polyvinyl alcohol.

Extrusion of polymer blends

All the materials were melt-mixed into blends by means of a co-rotating intermeshing twin-screw extruder (Thermo Haake, USA, screw diameter = 16 mm, L/D = 25). Different ratios (50/50, 60/40, 70/30 (wt.%/wt.%)) of PP and PVA were dried in an oven at 80℃ overnight to eliminate the residual moisture. The extruder possesses five heating zones, of which the temperatures were 160℃/170℃/180℃/190℃/200℃, respectively. The screw rotational speed was 100 rpm during the extrusion process. The material, in the form of rods, was obtained and rapidly cooled down by an airstream, followed by cutting pelletization for further characterization and the melt spinning step. Depending on the weight ratio differences, the pellets are denoted as PP_x–PVA_y, where the weight ratio of PP and PVA is x wt.%/y wt.%. For instance, the label PP₇₀–PVA₃₀ corresponds to a sample with a weight ratio of PP to PVA of 70 wt.%/30 wt.%.

Melt spinning of polymer blends

Multifilament yarns were made via a melt spinning process using a Busschaert Engineering Spinboy I spinning device (Deerlijk, Belgium). PP/PVA pellets were first introduced into the feeding zone of a single-screw extruder and heated gradually from 180℃ to 215℃ using five heating zones. A volumetric pump at a flow rate of 52.5 cm³/min forced the molten polymer blends toward the dies with 80 holes. Therefore, yarns of at most 80 monofilaments were obtained and immediately cooled by air, and afterward drawn by two rolls with the temperatures of 60℃ and 80℃. The two rolls exhibited two distinct speeds and realized the further stretch. The first and second roll speeds were set as 100 and 200/300 rpm, respectively. The DR is the value of the second roll speed divided by the first roll speed. Thus, the DR was adapted to 2 and 3. The samples are denoted as PP_x–PVA_y–DR_z, where the weight ratio of PP to PVA is x wt.%/y wt.%, and the DR is z. In addition, neat PP and PVA were also spun with two DRs for comparison, denoted as PP–DR₂, PP–DR₃, PVA–DR₂ and PVA–DR₃.

Preparation of the knitted fabrics

Prior to being knitted on a flat knitting machine (Dubied, Couvet, Switzerland) with a gauge of 7 (needles/inch), multifilament yarns were twisted (25 twists per meter in the Z direction) by using a twisting machine (Twistec, Barcelona, Spain). The texture used was 1 × 1 rib stitch.³³

Selective phase extraction experiment

The porous fiber was obtained via a selective phase extraction experiment. Water was selected as the solvent for removing PVA from the polymer blends. The fraction of removable sacrificial PVA phase (PVA accessibility degree) reflects the continuity extent.³⁴ Approximately 4.0 g of polyblend fibers and filter papers were placed in an oven at 50℃ to remove the moisture, and then weighed. The dried samples were immersed in 100 ml distilled water to remove the PVA. The extraction process was conducted at 80℃ for 5 h under constant magnetic stirring. Afterward, the suspension was filtered by weighed filter paper with a glass funnel. The residuals were rinsed with warm water in order to dilute the PVA solution. The sample with the filter paper was put into the oven at 50℃ overnight, and dried porous samples were obtained. The PVA accessibility degree was determined from Equation (1). The same experiments were conducted at least two times, and the average values with deviations were carried out

PVAaccessibilitydegree (%) = \frac{W_{i} - (W_{r + f} - W_{f})}{W_{i} \times ω_{PVA}} \times 100

(1)

where W_i represents the initial weight of the blend before extraction; W_r + f is the total weight of the dried residual polymer with the filter paper after selective extraction; W_f represents the weight of the unused dried filter paper; and ω_PVA is the weight fraction of PVA in the blend.

The suffix –Ex is utilized for the labels of the porous fibers after the selective extraction. For instance, PP₇₀–PVA₃₀–DR₂–Ex refers to the fibers after extraction from PP₇₀–PVA₃₀–DR₂.

In addition, the knitted fabrics were extracted by water using larger experimental devices by similar approaches.

Thermal analyses

To test the thermal stability of the samples, thermogravimetric analysis (TGA) was carried out on a TGA/differential scanning calorimetry (DSC) 3 + (Mettler Toledo, USA) under nitrogen atmosphere at a purge rate of 50 ml/min. For each experiment, a sample of approximately 10 mg was used. A heating rate of 10℃/min was selected, and the temperature was increased from 20℃ to 600℃. The initial decomposition temperature (T_5%) that is measured for 5 wt.% loss during degradation was determined, as well as the decomposition temperatures at two maximal degradation points (T_dmax1, T_dmax2) and the residual rate at 600℃. The maximal degradation points were searched with the aid of Origin software from the derivative thermogravimetry (DTG) curves.

The thermal behavior of all the samples was also examined with DSC with TGA/DSC 3 + (Mettler Toledo, USA), under a constant nitrogen flow (50 ml/min). The sample was placed in a hermetically sealed aluminum pan. In order to erase any previous thermal history in the material, the scanning procedure involved an initial heating from 20℃ to 220℃ at a rate of 10℃/min, followed by an isothermal step at 220℃ for 10 min. The sample was then cooled at a rate of 10℃/min to 20℃, before repeating the temperature scan in the heating and cooling range between 20℃ and 220℃ at 10℃/min as the second cycle, during which the melting endotherm and the crystallization exotherm were investigated. The parameters, including melting temperatures (T_m) and enthalpies (ΔH_m), crystallization temperatures (T_c1, T_c2, T_c3) and corresponding enthalpies (ΔH_c1, ΔH_c2, ΔH_c3), as well as the total number of crystallization enthalpies (ΔH_cT), were determined.

Melt flow index

The melt flow index (MFI) value was tightly related to the spinnability of polymers, and it was determined by a Melt Flow Tester (Thermo Haake, USA). Based on the standard ASTM D1238,³⁵ the piston as well as the tested sample were pre-heated for 4 and 3 min, respectively. The load was chosen as 2.16 kg and the temperature was set at 190℃. The same experiments were conducted twice for accuracy.

Morphological investigation

Images of the fibrous residuals after the selective extraction experiment and the knitted fabrics were acquired by using a digital camera (Huawei P9 EVA-AL00, Shenzhen, China).

The morphology of the fibers before and after selective extraction was observed by field emission scanning electron microscopy (SEM) on an Apreo S (ThermoFisher Scientific, USA) with accelerating voltages of 5.0 kV. In addition, the fibers were cut along the direction of the cross-sections, after treatment by liquid nitrogen. The cross-sections were observed by the SEM machine (Inspect F, FEI, USA). The accelerating voltages were 20.0 kV. Prior to the SEM experiment, all the samples were sputter-coated with gold.

X-ray diffraction analysis

The wide-angle X-ray diffraction (WAXD) of the multifilament yarns was taken on an X-ray diffractometer Ultima IV (Rigaku, Japan). The angle 2θ ranged from 5° to 60°. The radiation was Cu Kα with the wavelength of 1.54 Å. Fibers were manually fixed onto a sample holder. The crystal size was estimated from Scherrer's equation³⁶ in Equation (2)

L = \frac{0.9 λ}{cos θ}

(2)

where L represents the crystallite dimension, β stands for the breadth at half maximum intensity, θ refers to the Bragg angle and λ is the wavelength.

Mechanical properties

In order to compare the mechanical properties of the yarns, tensile tests were performed on single filaments extracted from the yarns, using a Zwick 1456 (Germany) machine, based on the ISO 5079 standard. The load cell was 10 N, the test length was set at 20 mm and the deformation rate was controlled at 20 mm/min. Tensile tests were conducted in a standard atmosphere (relative humidity: 65 ± 5%; temperature: 20 ± 2℃). The same measurements were conducted for 10 different single fibers from the same yarn to ensure accuracy. In order to determine the fineness of each single fiber before extraction prior to tensile tests, a vibroscope machine (Vibroskop, Lenzing Instruments, Germany) was used according to the NF G 07-306 standard. In addition, the fineness of the filaments after selective extraction was confirmed by weighing. The fiber fineness is used to express the tensile strength and to calculate Young's modulus in MPa, as well as tenacity in cN/Tex.

Dynamic mechanical analyses (DMAs) of the neat and blend fibers composed of PP and PVA were carried out using a dynamic mechanical analyzer (Q800, TA instrument, USA) with a frequency of 3.5 Hz and a strain amplitude of 0.8%. A bundle of multifilaments was fixed onto the clamp, and the temperature was increased from room temperature to 120℃ at a heating rate of 3℃/min.

The mechanical properties of the knitted fabrics were tested via an electromechanical universal test system (MTS Criterion Model 43, Eden Prairie, USA) with a load cell of 10 kN. The width of the selected fabrics was near 50 mm, the gauge length of the sample was fixed at 120 mm and the deformation rate was 60 mm/min. The thicknesses of the fabrics were determined by a fabric thickness tester (Sodemat, France).

Air permeability

The air permeability of textile fabrics is determined by the rate of flow of air passing perpendicularly through a given area of fabric by measuring at a given pressure its difference across the fabric test area over a given time period. Transverse air permeability was measured with a “Permeabilimetre à l'air” (Emi Développement, Bréviandes, France) with a pressure applied of 100 Pa and a test area of 20 cm², according to ISO 9237 (ISO, 1995). Air permeability tests were conducted in a standard atmosphere (relative humidity: 65 ± 5%; temperature: 20 ± 2℃).

Results and discussion

Thermal properties of the blends

In order to demonstrate how the PVA influences the thermal stability of PP–PVA blends, TGA and DSC analyses were conducted for the polymer blend extrudates in an N₂ atmosphere. TGA curves of the neat and blend polymers of PP and PVA are shown in Figure 1 and the results are summarized in Table 2. PP has one weight-loss peak in the DTG curve, and shows superior thermal stability with T_5% of 393.1℃. Its temperature at the maximum decomposition rate is 454.4℃. PP is thermally decomposed into volatile products through a radical chain process due to carbon–carbon bond scission.³⁷ PP decomposes completely at approximately 480℃. PVA displays two weight-loss peaks, and the initial decomposition temperature of PVA at 5% weight loss is 295.1℃, which is far higher than the melting temperature of 179℃. This illustrates that there is a broad thermal range for PVA processing. Typically, the elimination of volatile products often occurs from the side groups (e.g., OH groups) approximately below 260℃.³⁸ However, the elimination peak is postponed toward 320.6℃, attributed to the modification of the multi-hydroxyl structure.³⁹ The second degradation stage containing T_dmax2 (437.1℃) is ascribed to the chemical degradation from C-C bond scission.

Figure 1.

Thermogravimetric analysis (TGA) results of neat and blend polymers of polypropylene (PP) and polyvinyl alcohol (PVA) in N₂ atmosphere: (a) TGA curves; (b) derivative thermogravimetry (DTG) curves.

Table 2.

Thermogravimetric data of polypropylene (PP) and polyvinyl alcohol (PVA) in N₂ atmosphere

Pellets	T_5% (℃)	T_dmax1 (℃)	T_dmax2 (℃)	Residue at 600℃ (%)
PP	393.1	–	454.4	0.0
PP₇₀–PVA₃₀	310.4	329.3	470.1	0.65
PP₆₀–PVA₄₀	309.2	342.4	469.5	1.34
PP₅₀–PVA₅₀	304.4	321.3	470.0	1.53
PVA	295.1	320.6	437.1	4.39

For the PP–PVA blends, the DTG curves are split into two distinct peaks, one close to PVA and the other approaching PP, which is a feature of immiscible polymers.⁴⁰ Note that the T_dmax2 is significantly increased, which originates from the breakage of carbon–carbon, compared to the raw polymers.

The melting and crystallization behaviors in the DSC measurement are illustrated in Figure 2, and the related thermal parameters are listed in Table 3. Figure 2(a) demonstrates that the polymers exhibit only one single melting peak, regardless of the components of PP and PVA. This results from the fact that the melting temperatures of PP (170.5℃) and PVA (179.3℃) are too close. The crystallization peaks of the samples are shown in Figure 2(b). The crystallization peak of PVA is located at 124.9℃, and that of PP appears at the lower position of 107.0℃, from which differences can be distinguished. The binary blends have three peaks, assigned to the crystallization peak of the PVA component (T_c1, near 130℃), the bulk crystallization peak of PP (T_c2, near 110℃) and the fractionated crystallization peak of PP (T_c3, near 80℃) from high to low temperature, respectively. The bulk crystallization is ascribed to the heterogeneous nucleation, and the fractionated crystallization is due to homogenous nucleation induced by the confined PP phase.^41,42 The bulk crystallization temperature of PP in the blends is near that of pure PP (107.0℃). For PP₇₀–PVA₃₀, the bulk crystallization peak predominates compared with the fractionated peak. With the increment of the PVA fraction, fractionated crystallization behaviors are strengthened, which indicates that the dispersion of PP is enhanced.⁴⁰ The fractionated peak of PP₆₀–PVA₄₀ is the most compelling, which corresponds well with the phase inversion of the PP and PVA phases.

Figure 2.

Differential scanning calorimetry analyses of neat and blend polymers of polypropylene (PP) and polyvinyl alcohol (PVA): (a) melting process in the second cycle; (b) crystallization process in the second cycle.

Table 3.

Thermal parameters of neat and blend polypropylene (PP)–polyvinyl alcohol (PVA) materials

Pellets	T_m (℃)	ΔH_m (J/g)	T_c (℃)			ΔH_c (J/g)			ΔH_cT (J/g)
Pellets	T_m (℃)	ΔH_m (J/g)	1	2	3	1	2	3	ΔH_cT (J/g)
PP	170.5	39.05	–	107.0	–	–	46.87	–	46.87
PP₇₀–PVA₃₀	170.3	34.10	123.1	110.5	80.2	0.13	23.19	9.25	32.57
PP₆₀–PVA₄₀	170.3	36.13	136.9	111.6	81.7	4.88	0.80	30.34	36.02
PP₅₀–PVA₅₀	168.2	33.44	124.3	110.0	86.8	1.47	5.81	17.57	24.85
PVA	179.3	13.45	124.9	–	–	18.13	–	–	18.13

Melt flow index

In order to investigate the fluidity of the polymer blends, MFI measurements were carried out. The MFI spreads direct information about the viscosity at specific temperatures, which is a critical factor for melt spinning process. The MFIs of PP, PVA and PP–PVA blends with the three different ratios at 190℃ were tested, and the corresponding results are shown in Figure 3. The MFIs of PP and PVA are respectively 11.8 ± 0.6 and 21.6 ± 0.0 g/10 min, and PVA shows a higher fluidity than PP. For PP with the addition of PVA, the MFI values are significantly increased, even higher than that of PVA. PP–PVA blends show a positive deviation in MFI, and negative deviation in viscosity, which is ascribed to the weak interactions between the two components. The MFI values of PP₇₀–PVA₃₀, PP₆₀–PVA₄₀ and PP₅₀–PVA₅₀ are 27.6 ± 0.3, 48.2 ± 4.8 and 62.8 ± 2.3 g/10 min, respectively, exhibiting decreasing viscosity, which implies an increasing area of incompatible interphases of polymer blends.³¹ The MFI value of 15–30 g/10 min is the most suitable area for the melt spinning device that was used. It indicates that for PP₇₀–PVA₃₀, the temperature for melt spinning is suggested to be set at 190℃. The spinning temperature for PP₆₀–PVA₄₀ and PP₅₀–PVA₅₀ may be adapted at a lower temperature.

Figure 3.

The melt flow index of neat and blend polymers of polypropylene (PP) and polyvinyl alcohol (PVA).

Spinnability and morphology

The spinning condition was described in detail in the Melt spinning of polymer blends section, under which multifilament fibers were successfully obtained for each sample. However, the spinning process could not realize the stable production of the PP₅₀–PVA₅₀ and PP₆₀–PVA₄₀ fibers, often concomitant with broken threads, and the maximum DR only reached 2. For PP₇₀–PVA₃₀ blends, the spinning process was stable and ceaseless, even if the DR was further increased to 3.

The melt-spun fibers were selectively extracted with warm water to obtain the porous fibers. The PVA accessibility degree was also calculated according to Equation (1), and the related results are displayed in Figure 4. A higher PVA accessibility signifies that more PVA can be removed and even reused. In our previous research, the extrudates obtained from melt extrusion, which were the raw materials for melt spinning, were also selectively extracted to remove the PVA phase in order to obtain its accessibility.³² It has been found that with the increase of the PVA fraction, PVA accessibility increases significantly, approaching a co-continuous structure. However, the microstructure will be changed a great deal after melt spinning. PP₇₀–PVA₃₀–DR₂ possesses a high PVA accessibility degree of 64.9% ± 0.6%, and PVA accessibility is diminished with the increase of the PVA fraction, which is not beneficial for obtaining porous materials. Also, increasing the DR to 3 increases the PVA accessibility degree of the fibers from PP₇₀–PVA₃₀ to 78.7% ± 1.1%. For PP₇₀–PVA₃₀ polymer blends, the increase of the DR elevates the PVA accessibility obviously, due to the enhanced interconnectivity of PVA nodules and the enlarged specific areas of the biphasic fibers under the elongational flow.

Figure 4.

The polyvinyl alcohol (PVA) accessibility degree of polymer blends and digital photos of extracted fibers from (a) PP₇₀–PVA₃₀–DR₂, (b) PP₆₀–PVA₄₀–DR₂ and (c) PP₅₀–PVA₅₀–DR₂. PP: polypropylene; DR: draw ratio.

Figure 4 shows the residual morphology of the fibers with DR = 2 after selective extraction. Unfortunately, for PP₆₀–PVA₄₀–DR₂ and PP₅₀–PVA₅₀–DR₂, the fiber morphology was damaged and collapsed in a heap. In contrast, the fiber shape of PP₇₀–PVA₃₀–DR₂ was maintained, and the fibers with DR = 3 also behaved well. Mechanically robust material is the goal of our research, and PP₇₀–PVA₃₀ is a suitable candidate.

The cross-section of all the blend fibers is circular, based on the optical microscopy observation, which is decided by the spinneret hole shape in melt spinning. In order to further confirm the morphology of the fibers, the SEM observation is shown in Figure 5. There are different degrees of wrinkles on the surface of the fibers, among which that of PP₆₀–PVA₄₀–DR₂ is the most significant, even accompanied with some detached fibrous phases. It hints that the unsatisfactory spinnability of PP₆₀–PVA₄₀ originates from the immiscibility of the polymer blends. After the selective extraction, there emerge long and narrow holes in parallel on the fiber surface from PP₇₀–PVA₃₀–DR₂–Ex, which offers solvent access to the inner part of the fibers. With the increase of the DR, the holes are further elongated with a higher density due to the transformation of the extracted PVA phase. The cross-sections of the PP₇₀–PVA₃₀ fibers are also revealed in Figures 5(i) and (j). The diameters of the fibers are not uniform, which often occurs for the polyblend fibers. Furthermore, it demonstrates that there are numerous submicron-scale holes in the inner part of the fibers, which proves the porous structures. With the increment of the DR, the diameters of the fibers as well as those of the pores are decreased. In contrast, the fibrous matrix is evolved toward bundle-like microfibers for PP₆₀–PVA₄₀ and PP₅₀–PVA₅₀ with some short, detached and large-size nodules. With a higher magnification, it is found that the microfibers are not completely isolated, and instead they are partially interconnected. This further proves that the phase inversion lies near the fractions,³² which gives rise to the difficulty in melt spinning.

Figure 5.

The scanning electron microscopy observation of polypropylene (PP)–polyvinyl alcohol (PVA) blend fibers before and after extraction: (a) PP₅₀–PVA₅₀–DR₂; (b) PP₅₀–PVA₅₀–DR₂–Ex; (c) PP₆₀–PVA₄₀–DR₂; (d) PP₆₀–PVA₄₀–DR₂–Ex; (e) PP₇₀–PVA₃₀–DR₂; (f) PP₇₀–PVA₃₀–DR₂–Ex; (g) PP₇₀–PVA₃₀–DR₃; (h) PP₇₀–PVA₃₀–DR₃–Ex; (i) the cross-sections of PP₇₀–PVA₃₀–DR₂ fibers; (j) the cross-sections of PP₇₀–PVA₃₀–DR₃ fibers. DR: draw ratio; Ex: porous fibers after the selective extraction.

Apart from the surface topography, X-ray diffraction (XRD) patterns of the neat PP and PVA fibers and the related blend fibers are illustrated in Figures 6(a) and (b), respectively. As for PP, the α-crystalline form dominates, in which the peaks of 14.3°, 17.1°, 18.7° and 25.7° belong to the (110), (040), (130) and (060) planes of the α phase,⁴³ respectively, and 21.8° refers to the (131) and (041) planes of the α phase.⁴⁴ The XRD peak data are illustrated in Table 4, with parameter specifics at about the (110) and (040) planes of PP to investigate its crystallite size. In contrast, the XRD pattern of PVA shows a single peak at 19.6°, corresponding to the PVA crystalline phase.⁴⁵ With the DR increment of PP and PVA fibers, the XRD pattern is not changed significantly, indicating that the crystallinity is not altered strongly, resulting from the restriction on the stretched chains at high DRs.⁴⁶

Figure 6.

X-ray diffraction spectra of (a) polypropylene (PP) and polyvinyl alcohol (PVA) fibers with different draw ratios. (b) PP–PVA blend fibers with different fractions and draw ratios (DRs).

Table 4.

X-ray diffraction peak data of polypropylene (PP)-containing fibers

Fibers	2θ₍₁₁₀₎	FWHM₁₁₀	D₁₁₀ (Å)	2θ ₍₀₄₀₎	FWHM₀₄₀	D₀₄₀ (Å)	2θ₍₁₃₀₎	2θ_(131)/(041)	2θ₍₀₆₀₎
PP–DR₂	14.3°	0.763	104.9	17.1°	0.695	115.5	18.7°	21.8°	25.7°
PP–DR₃	14.2°	0.824	97.1	17.0°	0.750	107.1	18.6°	21.7°	25.6°
PP₇₀–PVA₃₀–DR₂	14.2°	0.785	101.9	17.1°	0.694	115.7	18.7°	21.6°	–
PP₇₀–PVA₃₀–DR₃	14.4°	0.850	94.2	17.3°	0.765	105.0	18.8°	21.8°	–
PP₆₀–PVA₄₀–DR₂	14.4°	0.750	106.7	17.1°	0.646	124.3	19.0°	21.8°	–
PP₅₀–PVA₅₀–DR₂	14.4°	0.751	106.6	17.2°	0.691	116.2	18.9°	–	–

FWHM: full width at half maximum; PVA: polyvinyl alcohol; DR: draw ratio.

For the blends of PP and PVA, the patterns tend to be a combination of the XRD patterns of PP and PVA, including the positions of the related peaks. This means that the crystal form of PP is not influenced by the addition of PVA. The peak of PVA is overlapped with that of the (130) planes of PP. With the increment of DRs of PP and PP₇₀–PVA₃₀ fibers, the (110), (040) crystallite sizes are decreased, as the crystal was shortened in the direction perpendicular to the drawing direction.⁷ The improvement of the DR with PP₇₀–PVA₃₀ increases the crystallinity degree. Although the PVA fraction is low in PP₅₀–PVA₅₀–DR₂, the intensity of the peaks is strong, indicating that the crystallization of the PP phase in PP₅₀–PVA₅₀ is improved. It is suggested that the PVA promotes the crystallization of the PP phase. Furthermore, the crystal grain size of PP (perpendicular to the drawing direction) is slightly increased due to the introduction of PVA.

Mechanical properties

Tensile test

The tensile test for a single fiber from multifilament yarns was conducted, and the corresponding results of the mechanical properties of PP–PVA blends with different ratios are displayed in Table 5. In order to compare the mechanical performance with that of the neat fibers, the related information of the PP and PVA fibers is also illustrated in the same table. Meanwhile, the stress–strain curves of PP–PVA fibers before and after selective extraction are shown in Figures 7(b) and (c). Similarly, the stress–strain curves of PP and PVA fibers are shown in Figure 7(a). From the stress–strain curves, PP fibers show ductile behaviors with high tensile toughness (breaking elongation >300%), while PVA fibers show brittle behaviors with low tensile toughness (breaking elongation <50%). The ductile behaviors are still maintained after the addition of a small quantity of PVA. However, when the fraction of PVA is increased to 50 wt.%, brittle behaviors emerge within the polyblend fibers. This behavior is related to the phase inversion of PP and PVA. It illustrates that the mechanical properties of PP₆₀–PVA₄₀–DR₂ are the weakest, which shows the worst spinnability as well. It implies that the morphological approach of a co-continuous structure leads to difficulties in melt spinning¹⁷ due to the lack of adhesion between the two polymers. Among the three polyblend fibers with DR = 2, the breaking elongation of PP₇₀–PVA₃₀ is the highest, which signifies that the excellent tensile toughness of pure PP fibers is not dominantly influenced. They also possess a relatively high Young's modulus and tenacity, slightly weaker than those of the pure PP fibers.

Figure 7.

Stress–strain curves of (a) neat fibers from polypropylene (PP) or polyvinyl alcohol (PVA) with a draw ratio (DR) value of 2 or 3. (b) PP–PVA fibers of different fractions with a DR value of 2. (c) PP₇₀–PVA₃₀ with different DR values before and after selective extraction.

Table 5.

Mechanical properties of blend fibers of polypropylene (PP) and polyvinyl alcohol (PVA) with different ratios

Fibers	Before extraction			After extraction
Fibers	Young's modulus (MPa)	Tenacity (cN/Tex)	Breaking elongation (%)	Young's modulus (MPa)	Tenacity (cN/Tex)	Breaking elongation (%)
PP–DR₂	2174 ± 238	17.9 ± 2.2	516 ± 66	–	–	–
PP–DR₃	2951 ± 322	24.2 ± 2.1	302 ± 100	–	–	–
PVA–DR₂	3379 ± 507	7.1 ± 1.0	45 ± 11	–	–	–
PVA–DR₃	3792 ± 1224	11.2 ± 2.0	28 ± 7	–	–	–
PP₇₀–PVA₃₀–DR₂	1853 ± 249	12.3 ± 1.7	449 ± 51	1570 ± 389	13.1 ± 2.9	547 ± 65
PP₇₀–PVA₃₀–DR₃	2871 ± 617	16.7 ± 2.6	256 ± 108	2012 ± 220	20.9 ± 2.5	371 ± 74
PP₆₀–PVA₄₀–DR₂	1426 ± 486	6.5 ± 0.7	229 ± 176	–	–	–
PP₅₀–PVA₅₀–DR₂	1962 ± 394	13.2 ± 2.4	24 ± 9	–	–	–

DR: draw ratio.

The DR can be adjusted to a higher value toward 3 for PP₇₀–PVA₃₀, and it can also retain a robust scaffold after the selective extraction. The impact of the DR on the mechanical properties was examined. With the DR increase, the Young's modulus and tenacity are enhanced, and the breaking elongation as well as tensile toughness are decreased, for neat PP and PVA fibers as well as PP₇₀–PVA₃₀ fibers. Only PP₇₀–PVA₃₀ fibers provide complete porous scaffolds, of which the mechanical properties were also tested. After the selective extraction experiment, the stress–strain curves indicate that the porous PP fibers exhibit ductile behaviors similar to those of neat PP fibers. In addition, the Young's modulus was decreased for the removal of PVA. Meanwhile, the tenacity and breaking elongation (tensile toughness) were increased. Hence, the ratio of PP to PVA can be adjusted to 70 wt.%/30 wt.%, of which the polymer blend manufactures into robust multifilaments. The DR is suggested be a higher value if better mechanical performance is required.

Dynamic mechanical analyses

The storage modulus versus temperature plot of neat and blend fibers from PP and PVA are shown in Figure 8(a). The storage modulus conveys the elastic response of the viscoelastic material, indicating its stiffness and the energy conserved under an applied load.⁴⁷ It shows that the storage modulus of all the polymers decreases with the increment of the temperature, resulting from the increased segmental mobility.⁴⁸ The change of PVA is the most dramatic, which evolved from a glassy toward a rubbery state. At a lower temperature, the storage modulus of PVA is significantly higher than that of the other fibers, showing a distinct difference. The results of tan δ of PVA and PP–PVA fibers are presented in Figure 8(b). The glass transition temperature of neat PVA is 80.1℃ and, after the addition of PP, it shows no distinct variation, related to the immiscibility between the PP and PVA phases.

Figure 8.

(a) Storage moduli of polypropylene (PP)–polyvinyl alcohol (PVA) fibers with different fractions. (b) Tan δ of PP–PVA fibers with different fractions. DR: draw ratio.

DMA is also a useful method to determine the PVA continuity.⁴⁹ For the PP–PVA, the storage moduli are affected by the crystallinity and orientation of the polymers.⁵⁰ The DR of the multifilament is low, and it does not experience a post-drawing process.⁵¹ Irrespective of the effects, the storage modulus of the components is regarded as invariable under the same spinning conditions (e.g., DR). Therefore, it can be regarded as a semiquantitative measurement.

There are two extreme modes of the combination of biphasic polymers: the parallel mode as the upper bound and the series mode as the lower bound. For the parallel mode, the stress applies directly to the two components; for the series mode, there exists indirect stress. When the case is converted into the biphasic microstructure, the upper bound can be regarded as the co-continuity structure and the lower bound can be seen as the matrix-dispersed structure.

The parallel and series modes are indicated in Equations (3) and (4) respectively⁵²

M = M_{1} Φ_{1} + M_{2} Φ_{2}

(3)

1 / M = Φ_{1} / M_{1} + Φ_{2} / M_{2}

(4)

where M is the mechanical property of binary blend; M₁ and M₂ refer to the property of the two components; and ϕ₁ and ϕ₂ refer to the volume fraction. Herein, the mechanical property refers to the storage modulus.

After examining the storage modulus of the two components, PP and PVA, the lower and upper bounds are simulated by Equations (3) and (4). The storage modulus of the blends varies with the increment of PVA content. The experimental and theoretical results of PP–PVA fiber as a function of PVA weight fraction near room temperature (35℃) are illustrated in Figure 9. It is indicated that the experimental storage moduli are more relevant to the curve of the lower bound series model (solid line, from Equation (4)). Therefore, these fibrous blends are not highly co-continuous, and the interconnectivity of PVA nodules has not been drastically constructed.

Figure 9.

The experimental values and simulated curves of the storage moduli of polypropylene–polyvinyl alcohol (PVA) fibers as a function of the PVA weight fraction near room temperature.

Textile creation by knitting technology

In order to check the feasibility of fabric knitting from the blend fibers, the knitting technology was attempted for the materials. It was found that the thickness of the yarns is required. Unfortunately, the fibers with a DR of 2 were too difficult to knit. The only candidate is PP₇₀–PVA₃₀, of which the DR can be further increased to 3. The mechanical properties and air permeability of the knitted fabrics from PP₇₀–PVA₃₀–DR₃ fibers are displayed in Table 6, denoted as PP₇₀–PVA₃₀–DR₃–Knit and PP₇₀–PVA₃₀–DR₃–Knit–Ex (after selective extraction). The average stress–strain curves of the two kinds of fabrics are also displayed in Figure 10, accompanied with digital photos, which indicates that the shrinkage of the fabric is not obvious after the selective extraction. The air permeability of PP₇₀–PVA₃₀–DR₃–Knit is 1906 ± 44 mm/s, and after the removal of PVA, its air permeability shows almost no change at 1951 ± 149 mm/s. Because of the selective extraction, the thickness and areal density are decreased. The mechanical properties of the fabrics behave well, with high values of maximum stress δ_max of 9.1 ± 0.9 MPa and related elongation of 113.9 ± 19.8%, performing as a brittle behavior in the stress–strain curve. After the selective extraction process, the maximum stress becomes slightly higher at 10.2 ± 0.2 MPa, and the elongation at δ_max is prolonged to 224.2% ± 14.6%, exhibiting a more ductile behavior. This is caused by the original mechanical properties of the PP materials, due to the extraction of the PVA phase.

Figure 10.

Stress–strain curves of the knitted fabrics from PP₇₀–PVA₃₀–DR₃ before and after selective extraction. PP: polypropylene; PVA: polyvinyl alcohol; DR: draw ratio.

Table 6.

Mechanical properties and air permeability of the fabrics knitted from PP₇₀–PVA₃₀–DR₃ fibers

Fabrics	Thickness (mm)	Areal density (g/m²)	σ_max (MPa)	Elongation at σ_max (%)	Air permeability (mm/s)
PP₇₀–PVA₃₀–DR₃–Knit	3.129	711.5	9.1 ± 0.9	113.9 ± 19.8	1906 ± 44
PP₇₀–PVA₃₀–DR₃–Knit–Ex	2.725	552.8	10.2 ± 0.2	224.2 ± 14.6	1951 ± 149

PP: polypropylene; PVA: polyvinyl alcohol; DR: draw ratio; Ex: porous fibers after the selective extraction.

Conclusion

This study reports on a simple method to fabricate porous PP fibers from PP–PVA blends. The motivation is to explore the ideal ratio of PP and PVA to spin biphasic multifilament fibers with good performance as porous fibers. Three different ratios were employed for melt spinning, and the fraction of PVA was increased from 30 wt.% to 50 wt.%. For the extrudates, the PP–PVA blend shows excellent thermal stability, which favors stable melt processing, and the crystallization behavior of PP is influenced by the state of its dispersion. The MFI result indicates that there is a negative deviation in viscosity, due to the weak interactions between the two polymers. The main factors, including PVA accessibility and mechanical properties of the fibers, were investigated.

Only PP₇₀–PVA₃₀ performs an excellent spinnability, along with good mechanical properties, with a Young's modulus value of 1853 MPa, tenacity of 12.3 cN/tex and breaking elongation of 449% for PP₇₀–PVA₃₀–DR₂. Furthermore, it is inspiring that although PP₇₀–PVA₃₀ extrudates possess a large quantity of isolated PVA, the drawing during melt spinning makes the PVA interconnected, exhibiting an outstanding value of PVA accessibility. The extraction of dominant PVA leaves complete porous fibrous shapes, with good mechanical performance. In addition, a higher DR is suggested to be adopted, which contributes to higher PVA accessibility and mechanical properties. Furthermore, thanks to the higher draw ability, the PP₇₀–PVA₃₀–DR₃ fibers were successfully knitted into fabrics for extending the potential textile use, and porous PP fabric was also obtained.

Conversely, a high PVA fraction offers a worse spinnability with more fragile mechanical behavior, although the PP crystallization degree is not changed distinctly by the increment of PVA content. What is more, it cannot preserve a high PVA accessibility and cannot provide a porous fibrous scaffold after selective extraction, and instead it offers microfibers with detached nodules. The biphasic fibers with DR = 2 exhibit a series model of two polymers from the DMA results. The results verify the feasibility and establish the most suitable ratios of PP and PVA to fabricate porous PP fibers and related fabrics with dominant PVA removal and ideal mechanical properties. The next step will concentrate on introducing nanofillers to manufacture further the surface-modified porous PP fiber products.

Footnotes

Acknowledgements

The devices for measurement and characterizations were mainly from the Laboratoire de Génie et Matériaux Textiles (GEMTEX, ENSAIT, France) and the State Key Laboratory of Polymer Materials and Engineering (Sichuan University, China). The authors wish to thank all the collaborators for their help. The authors would also like to thank Yanni Xu for fruitful discussion and Prof Christopher Fuhrman for polishing the language.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

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