Preparation and properties of melt-spinning fluorinated ethylene propylene fibres

Abstract

In this work, a novel fibre, fluorinated ethylene propylene (FEP) filament, was fabricated by melt-spinning. In addition, the properties of FEP fibres were investigated through apparent morphological observation, tensile testing, dry-hot shrinkage, differential scanning calorimetry, thermogramitric analysis (TGA) and Fourier transform infrared spectroscopy. Apparent morphological observation shows that FEP fibres present circular cross-sections and smooth surfaces. Just like the conventional fibres prepared by melt-spinning, such as poly (ethylene terephthalate) (PET) and polyamide 6, the stress–strain curves of FEP fibres show obvious stress yield points. The elastic recovery ratio of FEP fibres (80%) is much greater than that of polytetrafluoroethylene (PTFE) fibres (40%), endowing FEP textiles good wrinkle resistance. When FEP fibres are deposited in flame, smoke and droplets do not appear but shrinkage does occur. After treatment in hot air, the breaking strength of FEP fibres decreases, but the elongation at break increases. The melting temperature of FEP fibres is as closely high as that of the PET fibres, while the crystallization temperature is much higher, which puts forward greater challenges for FEP spinning. Compared with PTFE fibres, the crystallinity and the melting temperature of FEP fibres are much lower. TGA results demonstrate that FEP fibres present excellent thermal stability that is as stable as that of the PTFE fibres.

Keywords

Fluorinated ethylene propylene fibre melt-spinning properties

Introduction

Similar to polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP) is another perfluoroalkyl-terminated polymer with a series of outstanding properties such as chemical stability, flame retardance, thermal stability, electrical insulation, radiation resistance, and so on. It can be considered as the modified PTFE material in which some F atoms are substituted by CF₃ group. The chemical structure of FEP is shown in Figure 1. To be precised, FEP is randomly copolymerized from tetrafluoroethylene and hexafluoropropylene in a certain proportion (weight ratio 84:16). FEP and PTFE mainly consist of F and C elements, and the C chains are completely surrounded by F atoms. Compared with PTFE, the main chains of FEP are connected with branches and side chains. Moreover, the diameter of CF₃ groups is larger than that of F atoms. These result in larger gaps among macromolecules of FEP. The differences in the structure are responsible for lower crystallinity and narrower range of melting temperature.¹ Therefore, compared with hard processability of PTFE, FEP can be manufactured by conventional molding method.

Figure 1.

Chemical structure of FEP. FEP: fluorinated ethylene propylene.

In molecular structure of FEP, the C–F bonds with high bond energy (approximately 485 kJ mol⁻¹) are formed, and C chains are closely surrounded by F atoms, endowing FEP macromolecules good thermal stability and high melting temperature. The substitution of CF₃ for F atom increases the non-regularity of the macromolecules, reducing the melting and thermal decomposition temperature of FEP.² As a kind of modified PTFE, the workability of FEP is obviously improved.³ Furthermore, FEP, whose limiting oxygen index is greater than 95, presents excellent flame retardance. FEP is mainly composed of C and F elements and they connect each other in the form of C–F groups with high bond energy, which supplies FEP excellent resistance to chemical corrosion. Moreover, it can almost resist all kinds of chemical reagents, even concentrated nitric acid, fuming sulfuric acid and aqua regia. Generally speaking, the specific resistance of FEP is generally greater than 10¹⁵ Ω m, and it is not affected by temperature, water and moisture. Nowadays, FEP is widely used in chemical industry, petroleum, textile, paper making, food, machinery, medicine, electronics, electric cable wire and other technical fields in virtue of its hydrophobicity, heat resistance, corrosion resistance, flame retardance, low friction, electrical insulation, and so on.

As is known to all, PTFE fibres become more and more popular due to their excellent chemical corrosion resistance, flame resistance, heat resistance and self-lubrication. However, PTFE fibres cannot be fabricated by solution or traditional melt-spinning method because of infusibility and insolubility^,4 which imposes great limitations to their applications and development. Fortunately, FEP can be processed by using conventional molding, and it can be considered as an important modified material of PTFE, which not only presents physical and chemical properties that are similar to PTFE but also shows some advantages. As a result, FEP fibres have drawn more and more attention in academic and industrial fields. To date, the preparation of FEP fibres have been reported in few scientific papers. In this work, the as-spun fibres of FEP (Figure 2) were fabricated by melt-spinning method. Subsequently, the apparent morphology, mechanical property, thermal property and chemical structure were also investigated. This paper provides theoretical guidance for the application of FEP fibres.

Figure 2.

Digital photo of FEP fibres. FEP: fluorinated ethylene propylene.

Experimental

Materials

FEP chips (melt flow indexer (MFI) = 1.5 g/10 min) were friendly provided by Shandong Huaxia Shenzhou New Material Co. Ltd., Shandong Province, China. PTFE fibres were commercially available from Shanghai Linflon Film Technology Co. Ltd., Shanghai, China. Poly (ethylene terephthalate) (PET) chips (η = 0.67 dl g⁻¹) were provided by Jiangsu Hengli Chemical Fiber Co., Ltd., Jiangsu Province, China.

Preparation of FEP fibres

A special twin screw extruder (Research Group of Professor Shulin An, Tianjin Polytechnic University) was employed to carry out the experiment. The barrel temperatures were set to be 260, 270, 285, 285, 295 and 295°C from hopper to die. Melt-spinning was performed at the extrusion temperature of 295°C using a spinneret plate with 36 holes, each having a diameter of 0.3 mm. The extruded fine streams were air-cooled and then spun into as-spun filaments at the spinning speed of 300 m min⁻¹.

Characterization

The surface morphologies were observed on an optical microscope (OM) stage (Laborlux 12, Ernst Leitz GmbH, Wetzlar, Germany). A bunch of fibres with length of 1 cm were deposited on the slides and they were separated into a number of single fibres with the aid of glycerin. Subsequently, the diameters of fibres were measured using the software of fibre fineness analyzer (HD002C, China Textile Academy, Beijing, China). Tensile testing was performed using a universal testing machine (Model 2343, Instron, Norwood, Massachusetts, USA). With a gauge length of 200 mm, individual filaments were tested at the crosshead speed of 200 mm min⁻¹. Furthermore, the measurement of elastic recovery ratio was conducted by means of constant elongation cycle. Shrinkage was calculated according to percentage change in length when the fibres (100 mm) were exposed to hot air in a vacuum oven at 130°C for 1 h, 2 h and 3 h. The melting and crystallization properties were measured using a differential scanning calorimeter thermal analyzer (DSC, Pyris1, Perkin-Elmer, USA). Each sample of 5–10 mg was weighed before being put in DSC pan. Under nitrogen atmosphere, it was heated from room temperature to 290°C at the heating rate of 20°C min⁻¹. At 290°C, the samples were kept for 5 min to eliminate heat history, and they were immediately cooled to room temperature at the cooling rate of 20°C min⁻¹. Afterwards, the samples underwent the second heating course at the rate of 20°C min⁻¹. Crystallinity (X_c) of the fibres was determined by the following equation: ^5
–7

X_{c} = \frac{Δ H_{c}}{Δ H_{c}^{0}} \times 100 %,

where ΔH_c is the crystallization enthalpy, $Δ H_{c}^{0}$ refers to the enthalpy of 100% crystalline FEP, considered as 87.9 J g⁻¹.⁸ A thermogravimetric analyzer (TG, STA 449 C, NETZSCH, Germany) was used to determine the thermal stability of FEP fibres under nitrogen at the scan rate of 10°C min⁻¹, from 25°C to 800°C Fourier transform infrared spectra (FTIR, 5700, Nicolet, Minnesota, USA) of FEP fibres were recorded over a frequency range of 400 to 4000 cm⁻¹. The fibres were directly tested by attenuated total reflection mode.

Results and discussion

Observation surface morphologies and fineness

The OM images of cross-section and longitudinal morphologies for FEP fibres are shown in Figure 3. As shown in the images, the FEP fibres present regularly circular cross-sections and smooth longitudinal morphogy. In detail, some spots and vertical stripes appear on the surface. In view of this phenomenon, the extractions of the fibres in ethanol and acetone were carried out. After treatment, a slight curl occurs in the fibres, which may be caused by the addition of plasticizers in FEP spinning process.

Figure 3.

OM images of FEP fibres (400×). ((a) cross-section (b) longitudinal direction). OM: optical microscope; FEP: fluorinated ethylene propylene.

The linear density of FEP fibres (T_t) was determined by weighing method in accordance with GB/T 14343-2008 (Standard of the Peoples’ Republic of China). It was calculated on the basis of the following equation:

T_{t} = \frac{1000 \times G}{L n},

where G is the weight of samples (g), L refers to the length of fibres (m) and n is the number of filaments per bundle.

The linear density and the diameter of fibres (D) abide by the following relationship:

D = 0.03568 \times \sqrt{\frac{T_{t}}{δ}},

where δ is apparent density of the fibres, considered as 2.15 g cm⁻³ for FEP fibres.

The calculated value of the diameter is about 73 μm according to equation (3), and the measured values are listed in Table 1. It shows that the calculated values are close to the measured ones. Moreover, the coefficient of variation of the measured values is extremely small (1.8%), which also informs a uniform fineness along longitudinal direction of FEP fibres.

Table 1.

The measured values of fibre diameter.

Serial number	1	2	3	4	5	6	7	8	9	10
Diameter (μm)	72.5	72.5	72.3	72.1	70	69.9	71.9	70.5	70.4	70.2
Mean value (μm)	71.6
CV value (%)	1.8

CV: coefficient of variation.

Tensile properties

The strength-elongation curves of FEP fibres, which are similar to those of the conventional melt-spinning fibres such as PET and polyamide 6 (PA6),^9,10 are illustrated in Figure 4. The curves show obvious stress yield points. It is very favourable for us to understand the effect of mechanical properties on textile processing and wearing properties.

Figure 4.

The strength-elongation curves of FEP fibres. FEP: fluorinated ethylene propylene.

As is known to us all, the tensile properties are not only determined by the fibre itself but also related to experimental conditions. Other conditions remaining unchanged, the influences of experimental conditions on tensile properties of FEP fibres were investigated by changing the gauge and cross head speed. The corresponding curves are illustrated in Figure 5. As shown, the tensile strength of FEP fibres decreases with the increasing of gauge, while it increases with the enhancing of cross head speed. It can be concluded that the effects of experimental conditions on mechanical properties of FEP fibres are similar to those of common fibres.

Figure 5.

The effects of gauge and cross head speed on strength of FEP fibres. ((a) Gauge (b) cross head speed). FEP: fluorinated ethylene propylene.

FEP fibres can be considered as an ideally modified material of PTFE, so a comparative study of the elastic recovery properties between FEP and PTFE fibres was conducted. The elasticity of fibres refers to the recovery capability after deformation, and it is commonly characterized by elastic recovery ratio, plastic deformation ratio and stress relaxation ratio. The elastic recovery properties of FEP and PTFE fibres were tested in tensile mode by means of constant elongation cycle. The results are summarized in Table 2. Under the same conditions, the elastic recovery ratio of FEP fibres is about 80%, while that of PTFE fibres are only 40%, which indicates that the tensile elasticity of FEP fibres is superior to that of PTFE fibres. The cause of this is that F atoms are substituted by CF₃ groups in a FEP chain unit, and it destroys the original spiral structure of PTFE and increases the non-regularity of the macromolecules, improving the elasticity of FEP fibres. As a result, FEP textiles are able to undergo several stretches and still remain good wrinkle resistance. This provides a theoretical guidance to develop wearing textiles using perfluoroalkyl-terminated polymer fibres.

Table 2.

The elastic recovery properties of FEP and PTFE fibres.

Serial number	Elastic recovery ratio (%)		Plastic deformation ratio (%)		Stress relaxation ratio (%)
Serial number	FEP	PTFE	FEP	PTFE	FEP	PTFE
1	87.2	44.1	0.7	3.3	20.1	51.1
2	91.0	44.9	0.5	3.2	20.2	49.8
3	89.0	33.1	0.6	3.9	21.1	44.6
4	87.8	28.6	0.7	4.2	18.5	45.7
5	87.6	42.2	0.7	3.3	18.7	49.2
6	83.6	37.1	0.9	3.6	21.0	47.9
7	84.5	40.8	0.9	3.6	21.3	50.2
8	76.3	47.0	1.3	3.1	25.6	50.7
9	86.2	38.1	1.0	3.6	20.2	56.4
10	89.7	21.0	0.6	4.7	19.9	52.2
Mean value (%)	86.3	37.7	0.8	3.7	20.7	49.8

FEP: fluorinated ethylene propylene; PTFE: polytetrafluoroethylene.

Flame retardance

It is well-known that PTFE fibres with the limiting oxygen index over 95 show non-combustibility.¹¹ Similar to PTFE, FEP is another perfluoroalkyl-terminated polymer, which is supposed to present excellent flame retardance. Figure 6 illustrates combustion state of FEP fibres. When FEP fibres are deposited in flame, smoke and droplets do not appear, but shrinkage does occur. In this process, there is no enough heat to feed back to other parts of the fibre, making it unable to produce cracking, burning and carbonization, which effectively prevents the further combustion of fibres. Therefore, FEP fibres are also incombustible.

Figure 6.

The combustion state of FEP fibres. ((a) before combustion, (b) in combustion). FEP: fluorinated ethylene propylene.

Thermal shrinkage

PTFE fibres, with application temperature range of −160°C–260°C,⁴ become an ideal choice for high temperature resistant materials in complex chemical environment. The dimensional stability of fibres is an important factor that affects the structural stability of industrial textiles. Fortunately, the thermal shrinkage of fibres is an important controllable index during production and industrial application process. The relationship between temperature and shrinkage ratio in hot air is presented in Figure 7. As is shown, with rising of temperature, the movement of macromolecule chain in fibres intensifies and the internal structure becomes irregular, increasing the disorientation and thermal shrinkage.¹² Furthermore, at the same temperature, the structure of FEP fibres tends to be stable and the shrinkage ratio is basically unchanged with extending the treatment time.

Figure 7.

Shrinkage ration as a function of temperature of the air.

Figure 8 illustrates the strength-elongation curves of FEP fibres after treatment at 140°C. As depicted, the elongation at break of treated FEP fibres increases. This is due to the shrinkage caused by the motion of macromolecular chains in loose structure of as-spun FEP fibres. In the process of tensile test, reorientation of molecular chains in the fibres occurs. After treatment in hot air, FEP fibres possess enough expanding space for bond length and bond angle, endowing the fibres greater elongation at break. Nevertheless, the strength of FEP fibres decreases. During the process of heat treatment, crystallization in FEP fibres occurs. The resultant crystals act as physical cross linking points that hinder the reorientation of molecular chains during the process of tensile test, resulting in the lower orientation of moleculars and smaller strength. After treatment at 140°C, the fibres show high preservation rates of strength and elongation at break. Moreover, the shrinkage ratio in boiling water is only 3.5%. Therefore, FEP fibres present good dimensional stability.

Figure 8.

The strength-elongation curves of FEP fibres after treatment at 140°C. FEP: fluorinated ethylene propylene.

Melting and crystallization

Figure 9 shows the DSC curve of FEP fibres during the first heating course. From the curve, the glass transition temperature (112°C) and a melting endotherm peak at about 250°C are observed, which infers that FEP fibres show obvious melting characteristics. In addition, the glass transition temperature is less than the abovementioned temperature (140°C), cold crystallization may occur in process of heat treatment, which supports the speculation that resultant crystals act as physical crosslinking points.

Figure 9.

The first heating DSC curves of FEP fibres. DSC: differential scanning calorimetrY; FEP: fluorinated ethylene propylene.

Figure 10 illustrates the comparison of melting and crystallization curves between FEP and PET fibres. As shown in the curves, the melting temperature of FEP fibres is close to that of PET fibres (about 250°C), but the crystallization temperature of FEP fibres (240°C) is much higher than that of PET fibres (187°C). Accordingly, the degree of supercooling of FEP fibres is significantly smaller than that of PET fibres, thus the crystallization rate of FEP fibres is relatively fast. This means that the melt-spinning of FEP fibres can draw on the successful experience of PET spinning, while very fast crystallization rate of FEP proposes higher requirements for FEP spinning. The melting enthalpy and crystallization enthalpy of FEP fibres are very low, and the enthalpy of 100% crystalline FEP fibres is only 87.9 J g⁻¹, which facilitates the processing of FEP fibres. The thermal transformation parameters of FEP, PET and PTFE fibres are summarized in Table 3. Compared with PTFE fibres, the crystallinity and melting temperature of FEP fibres are much lower. Besides, the temperature range of PTFE crystallization is only about 20°C,¹³ while the width of crystallization peak of FEP fibre is nearly 50°C. In general, the spinnability of FEP is not as good as that of PET, but it is obviously superior to that of PTFE.

Figure 10.

The comparison of the melting and crystallization curves between FEP and PET fibres. ((a) FEP (b) PET). PET:poly (ethylene terephthalate); FEP: fluorinated ethylene propylene.

Table 3.

The thermal transformation parameters of FEP, PET and PTFE fibres.

Fibres	Crystallization temperature (°C)	Crystallization enthalpy (J g⁻¹)	Melting temperature (°C)	Melting enthalpy (J g⁻¹)	Crystallinity (%)
FEP	239.5	17.0	250.2	19.0	19.3
PET	187.6	38.8	246.2	37.1	15.7
PTFE	313.3	32.8	327.8	28.5	30.7

FEP: fluorinated ethylene propylene; PTFE: polytetrafluoroethylene; PET: poly (ethylene terephthalate).

Thermal stability

The original and derivative curves obtained from thermogramitric analysis are presented in Figure 11. As can be seen, the initial decomposition temperature (T_onset) of FEP fibres is 533.6°C, which is slightly lower than that of PTFE fibres (544.9°C).¹⁴ The residual weight fraction of FEP fibres (1.47%) is close to that of PTFE fibres (2.97%). In addition, the temperature (T_p) at maximum decomposition rate of FEP fibres is 578.4°C and that of PTFE fibres is 581.1°C, which shows a similar trend. In conclusion, FEP fibres present excellent thermal stability, which is as stable as that of PTFE fibres.

Figure 11.

Weight changes as a function of temperature for FEP fibres. ((a)TG (b) DTG). FEP: fluorinated ethylene propylene.

FTIR spectra of FEP fibres

Figure 12 shows the FTIR spectra of FEP fibres. From the curve, the bands in vicinity of 1205 cm⁻¹ and 1150 cm⁻¹ are very strong, which are attributed to antisymmetric and symmetric stretching vibration of the group CF₂. The absorption peak of the group CF₃ appears at 982 cm⁻¹.¹⁵ Moreover, the bands in the vicinity of 1150 cm⁻¹ correspond to amorphous structure of FEP that is copolymerized from tetrafluoroethylene and hexafluoropropylene in a certain proportion (weight ratio 84:16). Thus, the absorption peaks of group CF₂ are stronger than those of group CF₃. Furthermore, FTIR spectra demonstrate that FEP is a kind of modified PTFE.

Figure 12.

The FTIR spectra of FEP fibres. FTIR:Fourier transform infrared spectra; FEP: fluorinated ethylene propylene.

Conclusions

FEP fibres were fabricated by melt-spinning. With an OM, it can be observed that the cross-section of FEP fibres is regularly circular and the longitudinal morphology is uniform and smooth. The measured diameters of FEP fibres are consistent with the calculated ones, which implies that the fibres present good uniformity in longitudinal direction. Much like conventional fibres prepared by melt-spinning such as PET and PA6, the stress–strain curves of FEP fibres present obvious stress yield point. The elastic recovery ratio of FEP fibres (80%) is superior to PTFE fibres (40%), endowing FEP textiles good wrinkle resistance. When FEP fibres are deposited in flame, smoke and droplets do not appear, but shrinkage does occur. After treatment at 120–150°C in hot air, the breaking strength of the fibres decreases and elongation at break increases. The melting temperature of FEP fibres is close to that of PET fibres (about 250°C). However, the crystallization temperature of FEP fibres (240°C) is much higher compared with PET fibres (187°C). Accordingly, the crystallization rate of FEP fibres is relatively fast, increasing the difficulty of FEP spinning. Compared with PTFE fibres, the crystallinity and melting temperature of FEP fibres are both lower. As a whole, the spinnability of FEP is not as good as that of PET, but it is obviously superior to that of PTFE. The initial decomposition temperature (T_onset), the temperature at maximum decomposition rate (T_p) and the residual weight fraction of FEP fibres are comparable to those of PTFE fibres. In FTIR spectra of FEP fibres, the absorption peak of CF₂ and CF₃ group simultaneously appear, demonstrating that FEP is a kind of modified PTFE.

Footnotes

Declaration of Conflicting Interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The current work was financially supported by the Doctoral Scientific Research Funds of Shandong University of Technology (4041-413047), the open project program of key laboratory of eco-textiles, ministry of education, Jiangnan University (No. KLET1401), Shandong Province Higher Educational Science and Technology Program (J14LA56) and Shandong Provincial Natural Science Foundation, China (ZR2014EMP004).

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