Effects of process parameters on the properties of wet-spun solid PVDF fibers

Abstract

A PVDF polymer has very good strength, toughness and piezoelectric properties and is nowadays used as the film in strain sensors, mechanical actuators, energy harvesters and artificial muscles. Furthermore, PVDF polymer is used as the fiber in hollow fiber membranes for filtration applications. This article introduces the manufacturing of solid PVDF fibers by a wet spinning process and investigates the effects of the process parameters (i.e. drawing temperatures and drawing ratios on the first and second bath) on macroscopic properties (i.e. fiber linear density, fiber shape, density, porosity, strength and elongation) of solid wet-spun PVDF fibers. Increasing the drawing ratio at the first region of the wet spinning of the PVDF fiber increases the porosity. However, if the drawing is performed at the second drawing bath, the porosity of the PVDF fibers remains almost same. The slopes of the strength vs. drawing ratio and elongation vs. drawing ratio curves increase if the drawing is performed at the second drawing bath. The drawing ratio at the first bath does not affect the tensile properties of fibers, such as tensile strength and elongation. Information about the relationships between the process parameters and macro properties of PVDF fibers is very important so that PVDF fibers with the required properties can be produced with a wet spinning process by setting the correct process parameters.

Keywords

spinning fiber properties strength PVDF porosity

Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) is a highly non-reactive and pure thermoplastic fluoropolymer with good piezoelectric properties, hardness, durability, flexibility, and biocompatibility, and a relatively low melting point. PVDF polymer is able to convert mechanical energy into electrical energy when it is deformed mechanically. It generates a potential difference between both its sides perpendicular to the pulling direction. Owing to their piezo property, PVDF polymers are mostly used in applications such as strain sensors, mechanical actuators, energy harvesters, and artificial muscles.^1–7

PVDF polymer has α-, β-, and γ-phases in its molecules. The placement of carbon, hydrogen, and fluorine atoms in a three-dimensional molecule determines the existence of these three phases. Figure 1 shows the molecular differences between the α- and β-phases of PVDF molecules.

Figure 1.

(a) α-phase, (b) β-phases of PVDF.

The α-phase is the most stabilized phase formation of PVDF polymer. The reason for the preferred formation of the α-phase is its higher crystallization rate at higher temperatures (110–150℃). The β-phase PVDF crystallizes at temperatures below 80℃. Therefore the process temperature of a PVDF polymer affects the phase formation. The process temperature should be less than 80℃ if the β-phase PVDF is to be produced. However, even at a process temperatures lower than 80℃, the produced PVDF will be a mixture of β- and α-phase PVDF polymers.⁸

A PVDF polymer should be in the β-phase in order to have piezo properties, because the fluorine (−) and hydrogen (+) atoms that create the dipole moments are on the two different sides t. When the load is applied in the chain direction, the hydrogen atoms respond with a positive potential and fluorine atoms respond with a negative potential so that a potential difference takes place (see Figure 2). This potential difference could be captured by taking all the potential differences from both sides of the polymer.

Figure 2.

Piezoelectricity of PVDF fiber.

PVDF films are used in strain sensors, mechanical actuators, energy harvesters, and artificial muscles. Hackworth et al.⁹ produced an α-crystalline phase PVDF film and sandwiched this film into two fabric layers to produce a smart fabric. Then this composite material was drawn 5% for around 90 minutes with a 25 V/µm potential difference at 80℃ to change the crystalline phase of PVDF from the α-phase to the β-phase. After this process, the sandwiched PVDF film has a piezo property. As seen, changing the crystalline structure of the PVDF needs very delicate and sensitive conditions. If the PVDF is produced directly in a β crystalline phase, a voltage is applied to force the positive and negative parts of the molecules to move in opposite directions.

Figure 5.

The shape of a non-circular cross-section of PVDF fiber.

PVDF is also produced in fiber form. In recent years, the research on piezoelectricity properties of PVDF fibers has become more popular. In the literature, four main methods are reported for PVDF fiber production. These methods are: (1) melt spinning of PVDF fiber, (2) wet spinning of hollow PVDF fiber, (3) electrospinning, and (4) melt spinning of the bicomponent PVDF-PP fiber.

One of the most-used methods for PVDF fiber production is melt spinning.^10–13 Vatansever et al.¹³ produced polarized PVDF fibers by using a melt spinning process. They added an extra process step into the classical melt spinning process, where the PVDF fiber is wound to the bobbin with piezo characteristics. Then the β-phase PVDF polymer could be produced if the process temperature during the crystallization of the PVDF is less than 70℃.^10,11 This is the main reason why additional drawing under high voltage should be performed in order to change the phase of the PVDF from the α-phase to the β-phase during melt spinning. The α-phase PVDF polymer does not have the ability to generate electricity under mechanical deformation.

In addition to its piezo properties, PVDF polymer is also produced as hollow fiber for mesh applications. PVDF polymer is selected for this purpose because of its dimensional stability and very high durability.^6,7,14,15 In this method, the PVDF polymer is dissolved using dimethylacetamide (DMA), dimethylformamide (DMF), polysulfone, or N-methyl-pyrrolidone (NMP) solvents.

Over the past decade, production of PVDF fiber by using electrospinning has been reported in several research papers.^16,24 The advantage of the electrospinning process is that the voltage and drawing can be applied together to the fiber during electrospinning. As a result, the PVDF fiber web has piezo properties because of the nature of the process. The main disadvantage of electrospinning is that the produced web has a very low thickness since the fiber diameters are at nanometer levels. Furthermore, the produced web has an uncontrolled density and fiber orientation, which is why research on the yarn level has been impossible during these years.

Glauss et al.²⁵ produced PVDF/PP bicomponent fibers. They used PVDF polymer on the outside of the fiber (sheath) and PP polymer at the inside of the fiber (core). The minimum bicomponent fiber thickness was found to be around 100 microns.

Since the production of the β-phase PVDF polymer is difficult and an additional process is needed for melt spinning, a wet spinning process appears to be a strong alternative way for the production of PVDF fibers. Unlike melt spinning, temperature is not a big issue in the wet spinning process. Another difference between the melt-spun fiber and wet-spun fiber is that wet-spun fibers have pores inside the fibers. These pores in the fiber will affect the piezoelectricity so the piezoelectricity behavior of these two different types of fibers will also be different. Therefore, wet spinning process can be an alternative to melt spinning process for the production of PVDF fibers for piezoelectrical properties rather than mesh applications. In this article, the effect of process parameters (i.e. drawing ratio and drawing temperature) in a wet spinning process on the macroproperties of the PVDF fibers will be investigated.

Materials and methods

The PVDF polymer selected was Kynar Type 740 obtained from Arkema Chemistry, since this type is suitable for fiber production using a wet spinning process.

Wet spinning process

Wet spinning is a well-known fiber-spinning process that uses dissolved polymer to produce wet-spun fiber.^26,27 The number and the shape of the holes in the spinneret decide the number of fibers in a yarn and the cross-sectional shape of the fiber. Figure 3 shows the PVDF wet spinning unit used in this research.

Figure 3.

Wet spinning unit used for the production of PVDF fibers.

The steps followed in the wet-spun PVDF fiber production are as follows. PVDF polymer dissolved in solvent is fed through a feeding pump and then the solution (PVDF + DMA) is pushed through a one-hole spinneret to produce a PVDF monofilament. This process is called wet-jet wet spinning as the spinneret is in the water, which removes the solvent from solution and solidifies the PVDF polymer. After the PVDF fiber passes through the first bath, pre-solidified monofilament is wound to the first winder around three to four times to make sure that no sliding of the fiber occurs. PVDF is first drawn into the first bath between the feeding pump and the first winder because of the speed ratio between them. This drawing step affects the crystallization of the PVDF fiber. After the first winder, fiber passes through the second bath filled with water for further solidification and more drafting. PVDF fiber is then wound into the second winder for three to four times. Finally, the fiber is wound into the bobbin winder. The second drafting region is between the first winder and the second winder and the drafting ratio is defined as the speed ratio between them. This second drafting changes the molecular orientation since the total PVDF fiber is almost solidified.

In this research, DMA is used to dissolve the solid PVDF polymer chips. The PVDF/DMA mixture is mixed for three hours at around 55℃. The PVDF polymer percentage in the DMA solvent is selected as 20% w/w.

The wet spinning unit at “Zirve University Fiber Production Center” was used for the wet-spun PVDF fiber production. As shown in Table 1, different PVDF fiber samples (S1–S10) were produced with different wet drawing temperatures (20℃ and 40℃) and drawing ratios (1.0, 1.5, 2.0) at the first and second baths. Air drawing and hot air drying were not used during the production of all the samples.

Table 1.

Produced PVDF fiber samples and the fiber production parameters

	Wet drawing temperature		Drawing ratio
Sample	1st region	2nd region	1st region	2nd region
S1	40℃	40℃	1	1
S2	40℃	20℃	1.5	1
S3	40℃	20℃	2	1
S4	20℃	20℃	1	1
S5	20℃	20℃	1.5	1
S6	20℃	20℃	2	1
S7	20℃	20℃	1	1.5
S8	20℃	20℃	1	2
S9	20℃	40℃	1	1.5
S10	20℃	40℃	1	2

Drawing ratios are taken more than one on only one bath in order to investigate the effect of drawing ratio on the PVDF fiber properties for each bath separately. In addition, when the drawing ratios of both baths are taken more than one, then the total drawing ratio of the fiber would increase as the multiplication of these ratios. During the PVDF fiber spinning, the total drawing ratio is found to be less than 2.5 for all the samples.

Wet drawing temperature is the temperature of the water in the bath that the PVDF fiber passes through. In this research, the PVDF fiber is drafted either in the first bath or in the second bath. No drawing was performed for the PVDF fiber samples S1 and S4. For the rest of the samples, the drawing ratio was set to 1.5 or 2 at the first or at the second drafting region. When the drawing is performed in the first bath, the temperature at the second bath is kept constant at 20℃. Only for the sample S1 were the first and second region temperatures set to 40℃ for a better comparison for the drawing temperatures.

After production of all the samples, PVDF fibers are put into fresh water at 20℃ for a day to make sure that all the solvent is taken out from the fiber. After that, fibers are dried for a day at 20℃ room temperature and 50% humidity. The linear density (tex values) of the fibers is measured by weighing one meter of fibers with 10 measurements on each sample. The averages of these 10 measurements are taken as the fiber linear density of each sample.

Fiber shape calculations

The shape of the fiber is important in order to determine the effect of drawing ratio and drawing temperature on the concavity of the PVDF fiber. In order to determine the rate of concavity, the fibers S1–S4 and S7–S10 were assumed to have a cross-section as given in Figure 4.

Figure 4.

Representative non-circular cross-section with concavity of PVDF fiber.

The concavity of the fibers is calculated by using the following equations:

D_{avg} = (\frac{D_{1} + D_{2}}{2})

(1)

Concavity = (\frac{D_{avg} - H_{1}}{D_{avg}}) * 100

(2)

Here D_avg is the average diameter of the fiber, D₁ is the fiber diameter at one side, D₂ is the fiber diameter at the other side, and H₁ is the lowest thickness of the fiber.

Tensile tests and SEM measurements

Fiber tensile strength and elongation at break values were measured according to the ASTM D-3822²⁸ with an INSTRON 5440 device with a load cell of 5 N. The 10 cm-fiber length was used for strength and elongation measurements. From each type of PVDF fiber (S1–S10), 30 samples were tested in order to provide a reliable mean and standard deviation.

The scanning electron microscopy (SEM) image were obtained in the University–Industry–Government Association of Improvement, Application and Research Center in Kahramanmaras Sutcu Imam University using a ZEISSEVO LS10 model SEM device. Before the SEM measurements, samples were coated with gold using a Cressington Sputter Coater 108 Auto coater.

Density and porosity calculations

For the density and porosity measurements of the PVDF fiber samples, first the cross-sectional areas of the fibers were calculated with the help of SEM images. Five fiber samples from each type were used for density and porosity calculations. Cross-sectional shapes of some PVDF fibers are either almost circular or non-circular in shape. For the almost circular shapes, the area of the cross-section was taken as the area of the circle. For non-circular-shape fibers, the longitudinal and transverse diameters of each fiber were used for the area calculation and the average of these values was taken as an appropriate average cross-section area (see Figure 5).

Here D₁ and D₂ are the diameters of left and right sections of the fiber cross-section, L is the length and A is the cross-sectional area. The fiber cross-sectional area is then calculated by using equation (3).

A = \frac{π D_{1}^{2}}{8} + \frac{π D_{2}^{2}}{8} + (\frac{D_{1} + D_{2}}{2}) \cdot (L - D_{1} - D_{2})

(3)

After the area is calculated, the real density ρ_real of the PVDF fiber can be calculated by using equation (4). For the following calculations, the mean of the real density values are used.

ρ_{real} = \frac{tex [g / 1000 m]}{A [{cm}^{2}]} \times 10^{- 5}

(4)

The density ρ_sol of solid PVDF is taken as 1.78 g/cm³. Then the porosity (η) of a PVDF fiber is calculated as follows:

η = \frac{1.78 - ρ_{^{real}}}{1.78}

(5)

Results and discussion

The results and discussion are reported based on the fiber properties, including fiber linear density, fiber shape, fiber density and porosity, and fiber strength and elongation.

Fiber linear density

Linear densities of the produced PVDF fibers are fairly large compared to the fibers used in industrial textiles. Since the fibers investigated in this study will be used for the measurements of piezo responses, the fiber thickness should be thick enough to provide high-voltage responses. If the fibers produced have a low linear density then the measurement of the piezo response would be more difficult. The average linear density and the average diameter values for each sample group are given in Table 2.

Table 2.

Average linear density values of the PVDF fiber samples

Sample	Tex (g/1000 m)	Average diameter (mm)
S1	206.4	0.439
S2	142.2	0.331
S3	116.8	0.325
S4	257.2	0.524
S5	133.8	0.623
S6	101.2	0.602
S7	140.8	0.425
S8	104.2	0.293
S9	152.8	0.367
S10	114.4	0.289

Theoretically, when the drawing ratio increases, the fiber linear density value should decrease since the drawing process decreases the thickness (cross-sectional area) and results in a higher fiber density. As given in Table 2, fiber linear densities take smaller values for high drawing ratios. For higher drawing ratios, the preformed fiber is drawn at higher rates in the first and the second drawing regions and the fiber is pulled and gets longer. Since linear density is the mass per unit length, the same length of fiber becomes lighter after the drawing process. Comparing fiber linear density of sample S1 (40℃ drawing temperature) and sample S4 (20℃ drawing temperature) with no drawings reveals that a higher drawing temperature results in a lower fiber linear density. This result is expected because higher drawing temperatures decrease the viscosity of the polymer solution so that the fiber solution leaves the spinneret more quickly and PVDF fiber becomes thinner. Therefore linear density decreases at higher bath temperatures.

However, for the samples S3, S6, S8, and S10 with two draftings at the 1st and 2nd baths with 20℃ and 40℃ drawing temperatures, the fiber linear densities do not change dramatically. Therefore, as the drawing ratio increases, the effect of drawing temperature on average fiber linear density disappears. The reason is that temperature is less effective than drawing in the drawing baths so the effect of temperature is overtaken by the effect of drawing. This explanation is verified in Figure 6.

Figure 6.

Average fiber linear density for different PVDF fiber samples.

In Figure 7, the linear densities for specimens are shown, where error bars show the maximum and minimum values of 10 measurements. The variation of linear density values of 10 measurements for all groups is very small. Therefore, 10 measurements are enough for the determination of mean linear density value. This shows that the PVDF fiber samples are very consistent throughout their length.

Figure 7.

Deviation of fiber linear density measurements for each specimen type.

Fiber shape

The cross-section shapes of the fibers produced by different drawing ratios at 20℃ and 40℃ were investigated. The drawing ratio and the drawing temperature of PVDF fiber samples affect the shape of the fiber. In Figure 8, the cross-sectional shapes of the fibers of S1 to S10 are shown.

Figure 8.

Cross-sectional SEM pictures of the fiber samples S1–S10.

It is clear to see that for S5 and S6, the cross-section approaches a circular cross-section. This means that when the drawing is applied at 20℃ at the first region, the non-circularity of the fiber cross-section decreases. The concavities of samples S5 and S6 are so small that they are assumed to have circular cross-sections. However, for the rest of the fiber samples, the cross-sections are not circular. For samples S1, S2, S9, and S10, the concavity appears and the middle of the fiber becomes thinner. Table 3 shows the concavity of the cross-section of the fibers calculated using equations (1) and (2).

Table 3.

Concavities of the PVDF fiber samples

Sample	Concavity (%)
S1	19.6
S2	9.8
S3	8.6
S4	0.3
S7	7.3
S8	11.5
S9	14.8
S10	13.2

As shown in Table 3, the concavity of the PVDF fiber is almost zero for sample S4 and the cross-sectional shape is elliptic. The concavities of the fibers decrease from S1 to S3 where only the drawing ratio changes. As the fiber goes out from spinneret, it has a high fluidity. At the first drafting region, the fiber core is still in a liquid phase and fiber is solidifying from the surface so that fiber shape changes more easily. At a drafting ratio of two, the fiber moves faster in the first bath compared to that in no drafting and does not have enough time to splay. Therefore, fiber leaves the first bath as more circular and less concave. When fiber is not drafted at the first bath, the liquid core of fiber splays and the concavity becomes higher.

The drawing is performed at 40℃ at the first region for samples S1–S3. Higher temperatures of the bath during wet spinning increase the shape-change ability of the PVDF fiber because the molecules in the fiber move faster at higher temperatures. Additionally, fiber can also be drawn and reshaped more easily. Therefore, at the drawing ratios of 1.5 and 2.0, fiber changes its shape more in the drafting direction so the resulting fiber becomes more circular in shape with less concavity.

When drawing is performed at the second region at 40℃ (samples S1, S9, and S10), the concavity decreases but not as much as by the decrease in drawing at the first region because the PVDF fiber is solidified in the second bath more than in the first bath and the fiber surface is thicker and more resistant for the fiber core to splay. At the end of the second bath, the fiber is mostly solidified.

S5 and S6 were assumed to be almost circular and the concavity at S4 is almost zero. This means that drawing at the first region at 20℃ does not introduce any concavity because the low bath temperature solidifies the fiber faster and the fiber core cannot splay. At the drawing ratios of 1.5 and 2.0 compared to no drafting, the cross-sectional shape of the PVDF fiber becomes more circular for the same reason.

Fiber density and porosity

The calculated densities and porosities of the fibers, using circular and non-circular cross-section calculations of fiber (see Figure 9) are shown in Table 4.

Figure 9.

SEM images of (a) S5 and (b) S9 PVDF fiber samples.

Table 4.

Fiber density and porosity values of the produced fiber specimens

Sample	Density (g/cm³)	Porosity
S1	0.956	0.463
S2	1.030	0.421
S3	1.017	0.428
S4	0.919	0.481
S5	0.447	0.747
S6	0.362	0.795
S7	1.061	0.400
S8	0.995	0.438
S9	0.763	0.551
S10	0.796	0.551

Increasing the drawing ratio at the first drawing region at 20℃ decreases the density and increases the porosity of the fibers (S4–S5–S6). Since the PVDF polymer solution leaves the spinneret in liquid phase and starts to solidify in the first bath, drawing at the first region affects the fiber crystallinity. During the drawing in the first bath, the cross-sectional diameter of the preformed PVDF fiber increases while fiber denier decreases (see Table 2), because at higher drawing rates the amount of DMA solvent removed by water increases and, therefore, the porosity increases at higher drawing ratios.

However, if the drawing is performed at the second drawing bath (S4, S7, and S8), the porosity of the PVDF fibers remains same. As the fiber reaches the second bath, it becomes solid. As the solid fiber is drawn in the second bath, only the orientation of PVDF molecules in the amorphous part of the fiber changes. Since no change occurs in the pores, the porosity does not change. Another reason for stable porosity is that the temperature of the bath is taken low (≤40℃) and this temperature is not enough for the porosity change in the PVDF fiber.

Comparing the densities of S7 and S8 samples with S9 and S10 samples indicates that the temperature increase at the second the drawing bath decreases the density of the produced PVDF fiber, as shown in Figure 10. Because a higher temperature makes the fibers more oriented, and more orientation in the fiber results in the compaction of the PVDF parts in the fiber. Therefore, more pores are generated and, in total, fiber density decreases.

Figure 10.

SEM images of (a) S7 and (b) S9 PVDF fibers.

Fiber strength and elongation

In Figure 11, the measured strength values of different PVDF fibers are given with error bars that show the maximum and minimum of 30 samples for each type of PVDF. Investigated PVDF fiber samples have lower strength values than traditional fibers such as cotton and polyester, since PVDF fibers produced for this research have higher porosities, higher linear densities, and lower drawing ratios than traditional cotton and polyester fibers used in textiles.

Figure 11.

Strength values of PVDF fibers.

When Figure 11(a) and (b) are compared, it is clearly seen that if the drawing is performed at the second region, the slope of the strength vs. drawing ratio curve increases. The same behavior can also be observed in Figure 11(c) and (d). This represents that drawing at the second region makes the fiber stronger. This result is feasible because the PVDF fiber solidifies at the first drafting region so the fiber orientation does not change much. At the second drafting region, PVDF molecules become more oriented in the fiber and make the fiber stronger.^29–31

The effect of the drawing temperature on strength can also be seen in Figure 11. As the drawing temperature increases, the slope of the tensile strength vs. drawing ratio of PVDF fiber decreases. The reason is that higher bath temperatures delay the solidification of the PVDF fiber so when the fiber is not solidified enough, the increase in the tensile strength will be less. Another possible explanation is that at a higher temperature the drawing baths will increase the porosity and therefore weaker points in the fiber increase. An increase in weaker points results in weaker fibers.

Figure 12 shows the elongation at break values for the PVDF samples produced. Elongation results have a high deviation. For drawing at the 2nd bath [Figure 12(b) and (d)], elongation at break increases with an increase of the drawing ratio. At higher drawing ratios, PVDF molecules in the fiber are oriented parallel to the drawing axis, which results in an anisotropy in the fiber. PVDF molecules are long molecules. If they are randomly oriented, they prevent each other from elongating in the drawing direction and they break at a lower elongation. If the molecules are oriented parallel to the drawing direction, they will slide on each other and failure will occur when the shear stresses reach to intermolecular friction between molecules at high elongations. When different drawing ratios at the first bath are investigated, the elongation at break does not change with the drawing ratios at the first bath.

Figure 12.

Elongation at break values of PVDF fibers.

In Figure 12, it is clear that the elongation of fibers with 20℃ and 40℃ follows similar paths for both the first and second region drawing. This means the morphology of the PVDF fiber is not affected much with the different drawing temperatures. According to strength and elongation measurements, both strength and elongation of PVDF fibers increase if the fiber is drawn at the second region. This means that if a higher toughness is of interest, the PVDF fiber should be drawn at the second region.

Conclusions

In this article, the effects of wet spinning process parameters on macroscale material properties of wet-spun solid PVDF fiber are investigated. According to the results, the following conclusions are reported;

Fiber linear densities have smaller values for high drawing ratios and higher drawing temperatures result in lower fiber linear density values. Fiber linear densities are not affected by two draftings at the 1st and 2nd baths with 20℃ and 40℃ drawing temperatures.

When the drawing is performed at 20℃ at the first region, the non-circularity of the fiber cross-section decreases. At 40℃ drawing temperature, the concavity decreases when the drawing is performed at the 1st bath. When the drawing is performed at the second region at 40℃, the concavity also decreases but not as much as in drawing at the first region since the PVDF fiber is more solid than at the first bath and more resistant to shape changes. Drawing at the first region at 20℃ does not introduce any concavity because a low bath temperature increases the solidification and the fiber core cannot move easily.

Increasing the drawing ratio at the first region of the wet spinning of PVDF fiber increases the porosity. However if the drawing is performed at the second drawing bath, the porosity of the PVDF fibers remains the same.

When the drawing is performed at the second region, the slopes of the strength vs. drawing ratio and elongation vs. drawing ratio curves increase. The drawing ratio at the first bath does not change the tensile properties of fibers, such as tensile strength and elongation. Moreover, as the drawing temperature increases, the slope of the tensile strength vs. drawing ratio of PVDF fiber decreases.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Acknowledgments

The author thanks the Arkema Chemical Company for providing the Kynar PVDF polymer chips. Moreover, the University–Industry–Government Association of Improvement, Application and Research Center in Kahramanmaras Sutcu Imam University is acknowledged for providing the SEM pictures.

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