Effects of alkali solution on the durability of sewing thread made of modified polyphenylene sulfide and polytetrafluoroethylene

Abstract

The alkali resistance of sewing thread made of modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) has a crucial influence when used in the field of filtering high-temperature dusty gas. Therefore, the effects of alkali (NaOH) solution on the properties of MPPS/PTFE sewing thread at different temperatures, different concentrations and different times were studied. The results showed that white particulate matter and bump materials appeared on the surface of MPPS fibers in the MPPS/PTFE sewing thread. The maximum strength loss of MPPS/PTFE sewing thread was around 12.9% and the maximum deviation of elongation at break was about 4.5% after treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h. By analysis, it could be concluded that the structure of the benzene ring skeleton of the macromolecular chain in the MPPS/PTFE sewing thread did not change after treatment with NaOH solution, but the C-S bonds attached to the benzene ring in the MPPS/PTFE sewing thread had rotated, or even broken, which could be confirmed by the curves of Fourier transform infrared spectroscopy. The thermal stability of MPPS/PTFE sewing thread was decreased after treatment with NaOH solution, which was caused by surface damage of MPPS fibers in the MPPS/PTFE sewing thread.

Keywords

modified polyphenylene sulfide/polytetrafluoroethylene sewing thread alkali solution durability maximum strength loss corrosion mechanism

In the past, factories such as coal-fired power plants, garbage incinerators, steel mills and cement plants produced a large amount of high-temperature soot and gas in the actual production process, which would have serious impacts on people's living environment once discharged into the atmosphere.^1,2 Therefore, it was necessary to filter the high-temperature fume using a bag dust collector, usually used in high-temperature dust removal, which that had high efficiency of dust removal,^3–5 low influence by the combustion of boiler and dust characteristics, high-temperature resistance and stable operation.^6–8 The filter bag was the core component of the bag dust collector, which was mainly composed of high-temperature resistant filter material and sewing thread.^9,10 Therefore, the filtration efficiency of the bag dust collector mainly depended on the type, diameter and molding method of fibers in the filter materials and sewing threads, and the size of gaps and holes in the filter bags.^11–14 At present, most studies have focused on filtration velocity,^15,16 filtration efficiency,¹⁷ air permeability and filtration resistance of filter bags.¹⁸ Park et al.¹⁹ numerically investigated flow characteristics along the bag filter in detail, and made the new finding that the filtration velocity was non-uniform along the axial direction of a long filter bag when the height of the filter was greater than 10 m. Sun et al.²⁰ investigated the filtration and loading performance of two kinds of composite filter media with different nanostructures against NaCl particles and soot agglomerates. Maduna et al.²¹ investigated the effects of fiber type, area weight and water jet pressure on the air permeability of spunlaced fabrics using the Box–Behnken experimental design.

However, in actual use, the high-temperature dust bag would be corroded by acidic gas^22,23 and alkaline oxide dust in the flue gas.²⁴ The corrosion of these substances was not strong above the acid dew point, but after encountering abnormal high or low temperatures, acid gas and alkaline oxide dust in the flue gas would be condensed, and then would be dissolved in water, which would form an acid or alkali solution of low concentration. With the increase of time, acid or alkali solution of high concentration would form, which makes the high-temperature dust bag fail due to corrosion. Therefore, the alkali resistance of sewing thread had an important impact on the service life of high-temperature dust removal bags.²⁵

There have been many studies that have focused on the effects of internal factors,^26–29 such as twist, twist direction,²⁷ linear density and rheological characteristics, on the performance of sewing threads,²⁹ and the influence of external factors,^30–35 such as sewing speed, temperature, humidity, acid and alkali solution, on its service performance. Tang et al.²⁵ studied the influence of alkali solution on the mechanical properties of Nomex, polytetrafluoroethylene (PTFE) and E-glass fiber sewing threads to simulate the impact of corrosive gas and dust in high-temperature flue gas on sewing thread strength. Rodionov et al.³⁴ studied the influence of moisture and temperature on properties of the composite sewing thread with identification codes 120 KShN KR-KA and 120 KShN KR-PrA. It was concluded that the composite sewing threads were strong enough to meet the needs of the finished product even if it was exposed to water or high temperature for a long time. Liu and Shu³⁵ studied the influence of temperature on the mechanical properties of PTFE, polyphenylene sulfide (PPS), Nomex, polyacrylonitrile (PAN) and polyester (PET) sewing thread with a powerful machine equipped with a heating box, and concluded that PTFE sewing thread was significantly affected by temperature. In contrast, PPS sewing thread had good thermal stability at 240℃. However, most of these studies did not take into account the effects of alkali solutions on the sewing threads. A few researches on the alkali resistance of the sewing thread have been made of modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE).

In this work, MPPS/PTFE sewing thread was taken as research object to explore the influence of alkali solution on its properties and the mechanism of alkali solution on this sewing thread. The outcomes from this research are believed to provide an essential reference for the application of MPPS/PTFE sewing thread in high-temperature dust removal.

Experimental details

Materials and reagent

In this study, three kinds of sewing thread were used: MPPS/PTFE sewing thread (linear density: 247.2 tex); PTFE sewing thread (linear density: 167.9 tex); and MPPS sewing thread (linear density: 65.2 tex), respectively, which were provided by Suzhou Naide New Material Technology Co. Ltd. Figure 1 demonstrates the preparation process of MPPS/PTFE composite sewing thread. MPPS fibers were obtained by partial oxidation of PPS fibers. The indexes of basic performance are shown in Table 1 and Figure 2. The average diameter of MPPS fiber was measured at 13.9 µm, and varied between 12.8 and 15.2 µm. The average diameter of PTFE fiber was measured at 190.7 µm, and varied between 170.1 and 216.8 µm. Sodium hydroxide (NaOH) used as reagent was purchased from Sinopsin Group Chemical Reagent Co. Ltd.

Figure 1.

Production process flow chart of modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) composite sewing thread.

Figure 2.

(a) Scanning electron microscopy images, (b) Fourier transform infrared spectroscopy curves, (c) stress–strain curves and (d) initial modulus of modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) composite sewing thread.

Table 1.

Tensile properties of experimental sewing threads

Index	Breaking force (cN)	Breaking strength (cN/tex)	Breaking elongation (mm)	Elongation at break (%)	Work of fracture (N*mm)	Breaking time (s)
MPPS/PTFE	4343.8	17.6	47.1	9.4	1529.9	11.3
PTFE	4165.3	24.8	27.1	5.4	739.9	6.5
MPPS	1344.3	20.6	86.7	17.3	628.7	10.4

MPPS/PTFE: modified polyphenylene sulfide and polytetrafluoroethylene.

Alkali treatment

Alkali treatment for MPPS/PTFE sewing thread was conducted according to GB/T 35748-2017 “Teflon filament yarn,” while the other two sewing threads were used as contrast samples. The treatment procedure for tested samples with NaOH solution mainly included four steps. Firstly, the samples of MPPS/PTFE, PTFE and MPPS sewing thread were prepared via balancing them for 24 h under standard temperature and humidity conditions. Secondly, the prepared samples from the first step were immersed in NaOH solution with concentrations of 2, 4, 6, 8 and 10 mol/L, respectively. Thirdly, the beakers containing the samples were placed into a thermostat water bath that had reached the set temperature with a value of 25℃ (or 85℃). After processing for 24, 48, 72, 96 and 120 h (or 2, 4, 6, 8 and 10 h), respectively, the beakers containing the samples were taken out. Finally, all samples were rinsed thoroughly with distilled water and then tested with pH test paper until neutral. After that, all samples were dried and put into standard temperature and humidity environment to balance for 24 h.

Characterization

Surface damage

In order to characterize surface damage of MPPS/PTFE sewing thread after treatment with NaOH solution, scanning electron microscopy (SEM) (FLEX SEM1000, Hitachi, Japan) was used in this experiment. For SEM observation, the samples were placed on an electron microscope holder as required and then coated with gold film to eliminate the discharge.

Tensile property

According to the standard GB/T 3916-2013 “Determination of breaking strength and elongation at break of single yarn for textile winding (CRE method),” tensile properties of MPPS/PTFE sewing thread were established using an electronic single yarn strength tester (YG061F, China) in which the gage length was 500 mm, the cross-head speed of the tester was 250 mm/min or 500 mm/min and the pre-tension was 0.5 cN/tex for the conditioning sample. The breaking strength retention rate (B) was used to characterize the loss of tensile properties of sewing thread before and after treatment with NaOH solution, which was the ratio of the breaking strength of sewing thread after treatment with NaOH solution (P₁) and the breaking strength of untreated sewing thread (P₂), which is shown in as follows

B = \frac{P_{1}}{P_{2}} \times 100 %

(1)

Mechanism of force loss

To investigate the corrosion mechanism of NaOH solution on MPPS/PTFE sewing thread, changes of functional groups and macromolecular chains inside the thread were characterized by an attenuated total reflection Fourier transform infrared spectrometer (ATR-FTIR) (Nicolet 6700, Thermo Fisher, USA) in the region of 4000–600 cm⁻¹ at room temperature. Meanwhile, thermal stability of MPPS/PTFE sewing thread was characterized by a thermal gravimetric (TG) analyzer (TG 209 F1 Libra, Netzsch, Germany) and a differential scanning calorimeter (DSCQ20, TA, USA). For TG measurement, the sample in the form of powder (3–5 mg) was charged in an aluminum oxide crucible and heated in the range of 50–800℃ at a heating rate of 20℃/min in a nitrogen atmosphere. For differential scanning calorimetry (DSC) measurement, the samples (3–5 mg) were charged in an aluminum crucible with lids of the DSC, and heated from 30℃ to 380℃ at a heating rate of 20℃/min in a nitrogen atmosphere; in order to eliminate the effect of thermal history, the sample was held for 5 min at 380℃, and then cooled to 30℃ at 20℃/min to obtain the recrystallization curve. After that, the sample was reheated to 380℃ at 20℃/min to get the second melting curve. The nitrogen velocity was 20 mL/min.

Results and discussion

Surface damage

After treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h, the surface morphology of MPPS/PTFE, PTFE and MPPS sewing thread was as shown in Figure 3. It was observed that there were no significant changes on the surface of MPPS/PTFE, PTFE and MPPS sewing thread after treatment with NaOH solution, but some substances attached to the surface of MPPS fibers had been almost removed. These substances may be some additives introduced by the pretreatment process. Because these substances had poor alkali resistance, some of them had been removed by the NaOH solution. However, there was no observed damage on the surface of MPPS and PTFE fibers, which could be attributed to their good alkali resistance.^36–38 So, it could be concluded that the surfaces of MPPS/PTFE, PTFE and MPPS sewing threads were almost undamaged after treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h.

Figure 3.

Surface morphology of (a1)(b1)(c1) untreated sewing thread and (a2)(b2)(c2) alkali-treated sewing thread; (a1)(a2) modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) sewing thread; (b1)(b2) PTFE sewing thread; (c1)(c2) MPPS sewing thread; when the temperature was 25℃.

The impact of treatment time on the surface morphology of MPPS/PTFE, PTFE and MPPS sewing thread at a temperature of 85℃ and the concentration of 2 mol/L through using NaOH solution is displayed in Figure 4. It was observed from Figure 4 that the NaOH solution treatment time had little effect on the surface morphology of PTFE sewing thread, but it had great influence on the surface morphology of MPPS/PTFE and MPPS sewing thread. It was noticed that with increase of treatment time of NaOH solution, the surface damage of MPPS/PTFE and MPPS sewing thread became worse. With increasing the treatment time of NaOH solution to 4 h, the white particulate matter would appear on the surface of MPPS/PTFE sewing thread, while cracks and holes would appear on the surface of MPPS sewing thread. Moreover, bump materials on the surface of MPPS fibers in the MPPS/PTFE sewing thread would appear with the treatment time of 6 h. However, the integrity of MPPS fiber was good, and there were no large cracks on the surface. These above phenomena suggested that the surface of MPPS fibers in the MPPS/PTFE sewing thread were slightly damaged after treatment with NaOH solution in this condition.

Figure 4.

Effect of NaOH solution treatment time on the surface morphology of (a1)(a2) (a3) modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) sewing thread; (b1)(b2)(b3) PTFE sewing thread; (c1)(c2) (c3) MPPS sewing thread with the time of 2, 4 and 6 h; when the temperature was 85℃.

Figure 5 shows the effects of NaOH solution concentration on the surface morphology of MPPS/PTFE, PTFE and MPPS sewing thread at a temperature of 85℃ and treatment time of 2 h. As can be seen from Figure 5, when the temperature was 85℃ and the treatment time was 2 h, the surface of MPPS/PTFE, PTFE and MPPS sewing thread was almost unchanged when the concentration of NaOH solution did not exceed 10 mol/L, except that the substances on the surface of MPPS fibers had been almost removed. So, it could be concluded that the addition of PTFE fibers could improve the alkali resistance of MPPS/PTFE sewing thread.³⁹ In addition, the alkali resistance of MPPS/PTFE sewing thread was good when the temperature was 85℃ and the treatment time was 2 h.

Figure 5.

Effect of NaOH solution concentration on the surface morphology of (a1)(a2) (a3) modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) sewing thread; (b1) (b2)(b3) PTFE sewing thread; (c1)(c2)(c3) MPPS sewing thread with the concentration of 4, 6 and 10 mol/L; when the temperature was 85℃.

Effect of alkali treatment on the tensile properties of MPPS/PTFE sewing thread

After treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 24, 48, 72, 96 and 120 h, respectively, the tensile properties of MPPS/PTFE, PTFE and MPPS sewing thread were as shown in Figure 6.

Figure 6.

Effect of NaOH solution treatment time on the tensile properties of sewing thread when the temperature was 25℃: (a) breaking strength retention rate; (b) elongation at break. MPPS: modified polyphenylene sulfide; PTFE: polytetrafluoroethylene.

As shown in Figure 6, when the temperature was 25℃ and the concentration was 2 mol/L, the tensile properties of MPPS/PTFE sewing thread showed worse behavior compared to that of PTFE and MPPS sewing thread after treatment with NaOH solution. For MPPS/PTFE, PTFE and MPPS sewing thread, the maximum strength loss was around 12.9%, 2.3% and 4.6%, respectively, while the maximum deviation of elongation at break for MPPS/PTFE, PTFE and MPPS sewing thread was about 4.5%, 1.7% and 2.6%, respectively. By comparison, it could be found that the alkali resistance of PTFE sewing thread was the best³⁹ within 120 h at the temperature of 25℃ and the concentration of 2 mol/L.

After treatment with NaOH solution at a temperature of 85℃ and a concentration of 2 mol/L for 2, 4, 6, 8 and 10 h, respectively, the tensile properties of MPPS/PTFE, PTFE and MPPS sewing thread were as shown in Figure 7.

Figure 7.

Effect of NaOH solution treatment time on the tensile properties of sewing thread when the temperature was 85℃: (a) breaking strength retention rate; (b) elongation at break. MPPS: modified polyphenylene sulfide; PTFE: polytetrafluoroethylene.

As can be seen from Figure 7, when the temperature was 85℃ and the concentration was 2 mol/L, the alkali resistance of MPPS/PTFE sewing thread and PTFE sewing thread was better than that of MPPS sewing thread. For MPPS/PTFE, PTFE and MPPS sewing thread, the maximum strength loss was around 3.8%, 3.9% and 12.2%, respectively, while the maximum deviation of elongation at break was about 4.4%, 1.9% and 2.0%, respectively. From the above analysis, it could be concluded that the surfaces of MPPS fibers in the MPPS/PTFE sewing thread were damaged when the temperature was 85℃ and the concentration was 2 mol/L.

The comparison between tested samples with different temperatures and the same concentration showed that MPPS/PTFE sewing thread had better alkali resistance compared to that of MPPS sewing thread. At the same time, PTFE sewing thread showed the best alkali resistance, whether it was treated with alkali solution for a short time or for a long time. PTFE fiber was a kind of high-performance fiber that is made of Teflon and cut or fibrillated after spinning or making thin films. In its molecular structure, the volume of the fluorine atom was larger than that of the hydrogen atom, and the fluorocarbon bond was strong, which could protect the entire carbon–carbon backbone of the PTFE fiber. Therefore, PTFE fiber has the advantage of excellent chemical stability.⁴⁰ Therefore, the addition of PTFE fibers could improve the alkali resistance of MPPS/PTFE sewing thread.³⁹

After treatment with NaOH solution at a temperature of 85℃ and a concentration of 2, 4, 6, 8 and 10 mol/L for 2 h, the tensile properties of MPPS/PTFE, PTFE and MPPS sewing thread were as shown in Figure 8.

Figure 8.

Effect of NaOH solution concentration on the tensile properties of sewing thread when the temperature was 85℃: (a) breaking strength retention rate; (b) elongation at break. MPPS: modified polyphenylene sulfide; PTFE: polytetrafluoroethylene.

As can be seen from Figure 8, when the temperature was 85℃ and the treatment time was 2 h, the concentration of alkali solution had little effect on the breaking strength retention rate and elongation at break of MPPS/PTFE and PTFE sewing thread, but it had great influence on the tensile properties of MPPS sewing thread. In addition, the tensile properties of MPPS/PTFE and PTFE sewing thread were better than that of MPPS sewing thread when the concentration of NaOH solution did not exceed 10 mol/L. For MPPS/PTFE, PTFE and MPPS sewing thread, the maximum strength loss was around 2.0%, 4.4% and11.3%, respectively, while the maximum deviation of elongation at break was about 4.4%, 1.7% and 1.9%, respectively.

Corrosion mechanism of NaOH solution on MPPS/PTFE sewing thread

Infrared spectra of alkali-treated MPPS/PTFE sewing thread

The infrared spectrum of untreated and alkali-treated MPPS/PTFE sewing threads is shown in Figure 9. After treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h, it could be found that the stretching vibration peaks of the benzene ring skeleton of MPPS/PTFE sewing thread^41–43 had changed little, which were the bands at 3086, 1470 and 1386 cm^–l, respectively. It was observed that the main structure of MPPS/PTFE sewing thread was not damaged after treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h.

Figure 9.

Fourier transform infrared spectrum of (a) modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) sewing thread before and after treatment with NaOH solution when the temperature was 25℃: (b), (c), (d) locally enlarged views.

However, MPPS/PTFE sewing thread showed that the peak at 1636 cm^–l was increased and shifted to 1640 cm^–l. In the meantime, the peak at 1567 cm⁻¹ was decreased and shifted to 1571 cm^–l (Figure 9(b)). Besides, it was clearly noticed that the peak at 1202 cm^–l disappeared, and the height and area of the peak at 1155 cm^–l was slightly decreased. Meanwhile, the peak at 1089 cm⁻¹ was split into two small peaks, which caused a new peak at 1068 cm⁻¹ to appear, while the intensity and area of the characteristic peak at 1007 cm^–l was increased. As shown in Figure 9(d), the intensity and area of the characteristic peak at 805 cm^–l were slightly reduced, and the vibration frequency of this peak was slightly decreased.

The peak at 1636 cm^–l was produced by the expansion and vibration of the absorption peak of the benzene ring double bond inside MPPS/PTFE sewing thread, while the peak at 1567 cm⁻¹ and that at 805 cm⁻¹ were characteristic peaks of the para-substituted proton on the benzene ring, which further proved that the structure of the benzene ring skeleton of the macromolecular chain inside the sewing thread did not change after treatment with NaOH solution, but the C-S bonds attached to the benzene ring had rotated, or even broken. Meanwhile, the peak at 1202 cm^–l came from PTFE fibers in MPPS/PTFE sewing thread,⁴⁴ but it was unchanged for PTFE sewing thread after treatment with NaOH solution in this condition, so the disappearance of the peak at 1202 cm^–l was attributed to the rotating or breaking of the C-S bond. This conclusion could be further confirmed by the change of the peak at 1089 cm⁻¹, which was the vibration peak of C-S bonds.

After treatment with NaOH solution at a temperature of 85℃, the infrared spectrum of MPPS/PTFE sewing thread was as shown in Figure 10. The stretching vibration peaks of the benzene ring skeleton of MPPS/PTFE sewing thread^41–43 had changed little, which were the bands at 3086, 1636 and 1391 cm^–l, respectively. So, it could be concluded that the main structure of MPPS/PTFE sewing thread was not damaged after treatment with NaOH solution at a temperature of 85℃. The FTIR spectrum of alkali-treated MPPS/PTFE sewing thread was almost the same as that of untreated MPPS/PTFE sewing thread at the concentration of 2 mol/L, temperature of 85℃ and treatment time of 2 h. This indicated that MPPS/PTFE sewing thread was almost undamaged in this condition.

Figure 10.

Figure 10(a) shows that the height and area of the peak at 1567 cm⁻¹ were increased and shifted. At the same time, the height and area of the peak at 1464 cm⁻¹ were also increased and shifted after treatment with NaOH solution. When the treatment time was increased to 10 h, the peak at 1202 cm^–l was disappeared, which is shown in Figure 10(c), and the peak at 1089 cm⁻¹ was shifted to two small peaks. Meanwhile, the height and area of the peak at 1005 cm⁻¹ were increased and shifted to 1007 cm⁻¹. When the concentration of NaOH solution was 10 mol/L, the temperature was 85℃ and the treatment time was 2 h, the peak at 1202 cm^–l disappeared, the peak at 1089 cm⁻¹ was shifted to two small peaks and the height and area of the peak at 1005 cm⁻¹ were increased and shifted.

Because the peak at 1567 cm⁻¹ and the peak at 1464 cm⁻¹ were the characteristic peaks of the para-substituted proton on the benzene ring, it could be concluded that the C-S bonds attached to the benzene ring in the MPPS/PTFE sewing thread had rotated, or even broken, which could be further confirmed by the change of the peak at 1089 cm⁻¹. In the same way (from the above analysis), it could be concluded that the disappearance of the peak at 1202 cm^–l was attributed to the rotating or breaking of the C-S bond.

Thermal properties of alkali-treated MPPS/PTFE sewing thread

TG curves of untreated and alkali-treated MPPS/PTFE sewing thread are shown in Figure 11. The decomposition temperature (T_d) was defined as the temperature at the corresponding point when the weight loss was 5%.⁴⁵ For untreated MPPS/PTFE sewing thread, it could be seen that the decomposition temperature (T_d) was 527.7℃, and the residual carbon rate was 30.8%. After treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h, it could be seen that the decomposition temperature (T_d) was decreased to 524.0℃ and the residual carbon rate was decreased to 30.7%. After treatment with NaOH solution at a temperature of 85℃ and a concentration of 10 mol/L for 2 h, it could be seen that the decomposition temperature (T_d) and the residual carbon rate were decreased to 515.2℃ and 18.8%, respectively. It was noticed after treatment with NaOH solution that the thermal stability of MPPS/PTFE sewing thread was decreased.

Figure 11.

Thermal gravimetric curves of modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) sewing thread before and after treatment with NaOH solution.

The DSC curves of untreated and alkali-treated MPPS/PTFE sewing thread are shown in Figure 12. For untreated MPPS/PTFE sewing thread, it could be found that there were two melting temperatures (T_m), namely 287.9℃ and 342.2℃, which were consistent with that of MPPS fibers and PTFE fibers, respectively. The crystallization temperature (T_c) also had two values, 169.3℃ for MPPS fibers and 317.3℃ for PTFE fibers. The above differences between MPPS fibers and PTFE fibers indicated that the thermal stability of PTFE fibers was better than that of MPPS fibers. So, the addition of PTFE fibers could improve the thermal stability of MPPS/PTFE sewing thread.

Figure 12.

Differential scanning calorimetry curves of modified polyphenylene sulfide and polytetrafluoroethylene (MPPS/PTFE) sewing thread before and after treatment with NaOH solution: (a) absorption of heat; (b) heat release.

After treatment with NaOH solution at a temperature of 25℃ and a concentration of 2 mol/L for 120 h, it could be found that the melting temperatures for MPPS fibers and PTFE fibers were 288.4℃ and 342.5℃, respectively, which were almost same as that of untreated MPPS/PTFE sewing thread. However, the crystallization temperatures for MPPS fibers and PTFE fibers were 174.0℃, and 317.0℃, respectively. By comparison, it was observed that the crystallization temperature of MPPS fibers was about 5℃ higher than that of untreated MPPS/PTFE sewing thread, while the crystallization temperature of PTFE fibers was slightly changed. After treatment with NaOH solution at a temperature of 85℃ and a concentration of 10 mol/L for 2 h, it could be found that the melting temperatures of MPPS/PTFE sewing thread changed little, while the crystallization temperature of MPPS fibers was about 20℃ higher than that of untreated MPPS/PTFE sewing thread for MPPS fibers. It could be concluded that the thermal stability of MPPS/PTFE sewing thread was decreased after treatment with NaOH solution, which was caused by the damage of MPPS fibers.

Conclusions

This article studied the effects of NaOH solution on the durability of MPPS/PTFE sewing thread, and the corrosion mechanism of NaOH solution on MPPS/PTFE sewing thread was brought up. SEM showed that, after treatment with NaOH solution, there were no significant changes on the surface of PTFE sewing thread, but white particulate matter appeared on the surface of MPPS/PTFE sewing thread, and cracks and holes appeared on the surface of MPPS sewing thread. The results for tensile properties showed that the addition of PTFE fibers could improve the alkali resistance of MPPS/PTFE sewing thread. Meanwhile, after treatment with NaOH solution, the maximum strength loss of MPPS/PTFE sewing thread was around 12.9% and the maximum deviation of elongation at break was about 4.5%. The curves of FTIR displayed that the C-S bonds attached to the benzene ring of MPPS/PTFE sewing thread had rotated, or even broken. The curves of TG and DSC showed that after treatment with NaOH solution, the thermal stability of MPPS/PTFE sewing thread was decreased.

Footnotes

Acknowledgements

The authors are also thankful to Suzhou Naide New Material Technology Co. Ltd for providing the materials.

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 disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported by the Fundamental Research Funds for the Central Universities and Graduate Student Innovation Found of Donghua University (Grant No. CUSF-DH-D-2020006).

ORCID iD

Bin Zhang

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