Preparation and filtration performance of the circular weft-knitted seamless weft-insertion fabric materials

Abstract

Air particulate matter pollution has become a severe environment concern calling for filtration materials with great filtration performance. As the development of seamless forming technology, knitted filtration materials gradually show great potential. This study aimed to develop a novel kind of knitted seamless structure for filtration materials of filter bags with high production efficiency and excellent filtration performance. A new type of the circular weft-knitted seamless weft-insertion fabric (CKSW) filtration materials were developed on the modified circular knitting machine. This CKSW filtration materials consisting of the ground yarns, connection yarns and weft-insertion yarns, polyester full drawn yarns, and polyester draw texturing yarns with different yarn configurations were employed to realize series of CKSW samples. The polytetrafluoroethylene filaments with tourmaline particles were used to verify whether the static electric material produced an adsorption filtration effect on the CKSW filtration materials or not. After pretreatment, the filtration performance of the CKSW filtration materials was evaluated by analyzing its pore size, porosity, and filtration efficiency. Ultimately, the CKSW filtration materials with ground yarns and weft insertion yarns of draw texturing yarn and the connection yarns of full drawn yarn exhibited the most excellent filtration performance. The CKSW filtration materials show a high porosity of 87.14%, the pore size of 67.55 µm, and good filtration efficiency of 91.57% with the particles size of ≥ 5.0 µm. The successful fabrication of such knitted filtration materials may provide ideas for the development of filtration materials with new architecture mainly used as filter bags for baghouse.

Keywords

Weft knitting weft insertion fabric filtration materials seamless filter bag filtration performance

Introduction

With the gradual development of the industry, it is worth noting that 90% of air pollution in the environment comes from industrial pollution. The pollution of industrial waste gas threatens human health all the time. Studies have shown that air pollution, mostly by particulate matter (PM) 2.5, leads to 3.3 million premature deaths per year worldwide, and the air pollution is receiving more attention from the whole world [1 –3]. Therefore, the industrial waste gas pollution has become an urgent problem to be solved currently. The baghouse is usually installed to control the emission of air pollutants in industry. The filter bag has the advantages of a wide range of filtration particle size and high filtration efficiency [4]. Furthermore, on the equipment for metal production, baghouse is also used to filter dust emissions containing metal particles, so the baghouse is the most widely used dust collector to filter the particulate pollutant [5,6]. With the increasing requirements of environmental protection, the filtration materials of baghouse have to feed the needs of higher filtration efficiency and longer service life, which depend on the fiber material, structure, and fabrication method [7].

The conventional filter bag is fabricated with non-woven, woven or composite structures by stitching or hot-melting process to form a cylindrical shape. Filter bags of various materials and different processes are applied in different working environments. The area of sewing thread with the width of 10–20 mm on the side and bottom of filter bags always exist. The filter bag using the hot-melting process will be stitched as well in the post finishing for its strength and fastness. However, the needle pores of the suture are much larger than the pores of the filtration materials. Thus, there is a filtration blind zone in the suture area. In addition, the dust attaching onto the outer wall of the filter bag will be cleaned by vibration, backflush or pulse blowing during the process of usage. For example, the dust will be cleaned under a frequency of every 120 s–180 s and a pressure of 2 MPa – 2.5 MPa by pulse blowing [8]. Thus, placing higher demands on the strength and stiffness of the filter bag are necessary for the practical production. Since still a large improvement potential existing in the performance of filter bag and the new type of filter materials with novel architecture and property is required, filter bag constituted by the circular weft-knitted seamless weft-insertion fabric (CKSW) is significant to be developed and investigated. Compared with woven fabric, CKSW filtration materials have a large number of circuitous pore channels that can block small particles. Moreover, coupled with the unique advantages of weft-knitted seamless technology, knitted structural materials have great potential in the usage and application of filtration materials.

The CKSW filtration materials with a novel structure will be developed, and the performance of these materials has been researched in the present study. The weft-insertion yarns involved do not participate in knitting, which improve the instability and strength of the filtration materials [9 –11]. The pore size, porosity, and filtration efficiency of the CKSW filtration materials under the different yarn configurations were thoroughly investigated. These seamless knitted filtration materials with a weft-enhancing structure have a good filtering effect and eliminate dust leakage caused by the suture hole of the filter bag. Moreover, the CKSW filtration materials can greatly reduce the processing procedure and provide a new idea for the development and application of the filtration materials.

Experimental

Materials

The materials used for the CKSW should have certain knitting feasibility. The polyester full drawn yarns (FDYs) in the specifications of 150 D/44F and 500 D/155F, 150 D/44F and 500 D/155F polyester draw texturing yarns (DTYs; density = 1.38 g/cm³), and 150 D polytetrafluoroethylene (PTFE) filaments containing 5 wt% tourmaline particles (density = 2.24 g/cm³, Suzhou Net New Material Technology Co., Ltd., China) were utilized in this research. PTFE and tourmaline are both the typical static electric materials which are widely used in the field of filtration [12 –14]. Filter bag was obtained from Jiangsu Hengsheng Environmental Protection Technology Co., Ltd. China.

Preparation of the CKSW filtration materials

Fabric structure

In this paper, it is highlighted that the circular weft-knitted weft-insertion stitch was used as the structure for researching and developing the CKSW filtration materials. This weft insertion stitch has three system yarns, namely ground yarns, connection yarns and weft insertion yarns. As shown in Figure 1(a), in the stitch, a course is formed by four yarns from four feeders separately. It can be seen from Figure 1(a) and (b) that the ground yarns form the front and back layers of the yarns of the fabric after looped on the cylinder and the dial needle beds, respectively, and the connection between the front and back layers is realized by the connection yarns through the knitting method of tuck. The weft insertion yarns feed after the connection yarns in the third feeder, as illustrated in Figure 1(c) and (d), the weft insertion yarns can be non-crimped between the front and back two yarn layers and are tied together by the ground yarns and the connection yarns.

Figure 1.

The weft-insertion stitch: (a) weft knitting notation, (b) cam arrangement notation, (c) weft loop pattern draft, and (d) 3D simulation sketch.

Knitting equipment

The weft-insertion stitch was knitted by modified double-sided seamless circular knitting machine, and the circular knitting machine before modification is shown in Figure 2(a). The equipment adopts rib arrangement, total of 12 feeders, four of which forms a course. Each course is inserted with a weft insertion yarn, so a weft insertion feeder is added to every four feeder yarns on the equipment. Corresponding to Figure 1(a) the weft knitting notation, the weft insertion feeder is installed next to the connection yarns in the third feeder, the weft insertion feeding device is shown in Figure 2(b). Table 1 shows the parameters of this type seamless circular knitting machine.

Figure 2.

Images of the circular knitting machine: (a) Before modification. (b) The feeding device of weft insertion after modification.

Table 1.

Machine specification parameters.

Machine specification	Specific value
Bundling structure	Weft-insertion stitch
Gauge	E18
Diameter (inch)	8
Needle type	Latch needle
Machine configuration	Rib configuration
Feeds	12
Number of the ground yarns feeders	6
Number of the connection yarns feeders	6

Sample preparation

The CKSW filtration materials were made from polyester yarns of both FDY and DTY yarns. The single-variable comparison tests towards the ground yarns, connection yarns and weft insertion yarns of the fabric were carried out to obtain the specimen with the best filtration performance. At the same time, to investigate the effect of tourmaline and PTFE on the filtration efficiency of the CKSW filtration materials, the PTFE yarns containing tourmaline were employed to participate in the knitting as the ground yarns. The fabric specification parameters are shown in Table 2. Among them, the F/F sample is the blank sample without yarn insertion, and filter sample is the contrast sample. Furthermore, the morphological characteristics of all samples are presented in Figure 3.

Figure 3.

Figure of samples and its corresponding cross-sectional images: (a) Sample No. 1 (F/F), (b) Sample No. 2 (F/F/F), (c) Sample No. 3 (F/F/D), (d) Sample No. 4 (D/F/D), (e) Sample No. 5 (PTFE/F/D) and (f) Sample No. 6 (Filter sample).

Table 2.

Fabric specification parameters.

Type of yarns	Sample No. 1 (F/F)	Sample No. 2 (F/F/F)	Sample No. 3 (F/F/D)	Sample No. 4 (D/F/D)	Sample No. 5 (PTFE/F/D)	Sample No. 6 (Filter sample)
Ground yarns	150D PET (FDY)	150D PET (FDY)	150D PET (FDY)	150D PET (DTY)	150D PTFE (5% tourmaline)	Non-woven filter bag (base fabric is Woven)
Connection yarns	150D PET (FDY)	150D PET (FDY)	150D PET (FDY)	150D PET (FDY)	150D PET (FDY)
Weft insertion yarns	None	500D PET*2 (FDY)	500D PET*2 (DTY)	500D PET *2 (DTY)	500D PET *2 (DTY)

FDY: full drawn yarn; DTY: draw texturing yarn.

Test Methods

Morphological properties

The appearance and cross-sectional images of the CKSW filtration materials were obtained by electron microscope (SK2700U, SAIKEDIGITAL), and the mesoscopic morphology of the intercepting particles was measured by scanning electron microscope (SU1510, Hitachi) at 1000× magnifications.

Pretreatment

Due to its good elasticity, knitted fabrics are prone to deformation during usage. In order to eliminate the inner-stress in the yarn and the stretching of the fabric during the knitting process, the knitted fabric was pretreated with a heat setting process to obtain the stable knitted structure [15]. Therefore, the grey fabric directly from the seamless circular knitting machine needed preshrink finish, and the samples were processed in oven under 130℃ for 60 s by the dry heat shrinkage setting method commonly used in the actual production process in the factory.

Thickness

In the calculation of the porosity of the CKSW filtration materials, thickness is one of the factors affecting the porosity and filtration pressure drop, so the thickness of the samples are tested in this paper. The thickness of the CKSW filtration materials was measured using fabric thickness meter (YG141D, Wenzhou) according to the standard of GB/T 3820-1997. In this test, the presser foot area was 100 mm². The pressure weight was 50 CN, and the pressure time was 10 s. Each sample was randomly selected 10 positions for tests, and the results were average.

Air permeability

Industrial filtration materials have specific ventilation requirements, and the air permeability is usually proportional to the filtration efficiency of the material. The air permeability test of the CKSW filtration materials was conducted using air permeability tester (YG461E-III, Wenzhou) according to the standard of GB/T 5453-1997. The sample was cut into 20 × 20 cm² and fixed into the automatic air permeability tester by using 200 Pa air pressure through the knitted fabric filtration materials, and then the rate of air flow was used to determine the air permeability of the tested sample (five samples per specification and averaged).

Pore size

The pore size of the filtration material reflects the ability to prevent passage of filtered particles. The pore size and pore size distribution of the CKSW filtration materials were characterized by Capillary Flow Porometer (CFP-1100A, Porous Materials Inc). The average value was obtained after five determinations.

Porosity

Porosity is the ratio of the pore volume of a material to its total volume, and it is an indicator of the pore volume. At present, the porosity of fabrics and fiber materials can be theoretically calculated by the formula, but has certain limitations. Therefore, image method has been utilized to assist in the measurement and analysis of the porosity of the CKSW filtration materials [16].

Formula calculation method

At present, the porosity of fabrics and fiber materials can be obtained by the formula calculation method. The porosity of the filtration materials is calculated according to the following formula (1) [17].

η = (1 - \frac{w}{δ \times ρ \times 10^{3}}) \times 100 %

(1)

where η is the porosity of the filtration materials, w is the mass per unit area of the filtration materials, ρ is the fiber density, and δ is the thickness of the filtration materials.

Image method

With the advancement of information technology, computer image processing technology can digitally convert intuitive image signals, and it is widely used in various industries [18,19]. In textiles, this technology is mainly used in textile testing, covering the characterization test of raw materials, semi-products and finished products, such as fiber fineness, wool curl [20,21], fabric density, porosity, and mechanics performance [22]. Computer image processing technology can reduce the influence of human subjective factors, objectively and accurately assess the appearance and internal quality of textiles, and provide intelligent real-time monitoring such as online detection. A large number of studies have used computer image processing technology to analyze the pore structure, size and distribution of fabrics. Turan and Okur [23], Abou-Ana et al. [24], and Aydilek et al. [25], used image method to obtain the porosity of the woven fabrics, knitted fabrics and non-woven fabric.

In the present paper, based on image processing technology, the CKSW filtration materials were processed by image preprocessing, threshold segmentation, removal of impurities and binarization. Firstly, the samples imaged using a metallographic microscope (SK2700D, SAIKEDIGITAL). The metallographic microscope is an optical microscope, which combined with the photoelectric conversion technology and computer image processing technology, and the metallographic image can be conveniently observed on a computer to analyze the metallographic image. The obtained image is a 24-bit color image. Since the image acquisition process of the samples is affected by various factors such as illumination, the collected image may have noise and signal distortion. So the image must be image-enhanced, corrected for distortion, and noise-eliminated before analysis, while retaining useful information about the target to be tested. The preprocessed image is subjected to threshold segmentation binarization processing by Adobe Photoshop CS according to the Otsu method proposed and named by Japanese Otsu. According to the gray information of the image, the image f(x, y) is composed of the object and the background two parts, and the different gray values of the image are divided into two types of regions with the threshold (S) as a boundary, and the image can be converted into a binary image according to the formula (2) [26].

\begin{matrix} g (x, y) = {\begin{matrix} 0 & IFf (x, y) < S \\ 1 & IFf (x, y) \geq S \end{matrix}} \end{matrix}

(2)

The binary image is divided into two parts: black and white. The image processing software recognizes the pixels of the target (black part - pore) and obtains the porosity of the sample according to the percentage of the pixel area of the pore part to the whole pixel area of the image.

Filtration performance

The filtration performance of the CKSW filtration materials evaluated by measuring the filtration efficiency for different sizes of particles according to the standard of GB/T 6719-2009, and the test was operated in the multi-purpose test machine for filtration materials (LZC-H, Suzhou).The test bench was evaluated for filtration efficiency by NaCl aerosol generator. The particle size distribution of the system test was between 0.3 and 10 µm, and each sample was tested five times and averaged. The filtration efficiency was tested by counting method, the number of dust particles contained before and after filtration is recorded, and the filtration efficiency is calculated by a formula. The percentage of the captured particles and the number of particles originally contained is the filtration efficiency, as shown in the following formula (3) [27].

η = \frac{n_{1}}{n_{2}} \times 100 %

(3)

where η is the filtration efficiency, n₁ is the number of captured particles, and n₂ is the number of particles contained in the upstream. The average value was obtained after five determinations.

Results and discussion

Treatment and dimensional stability of the CKSW filtration materials

In order to obtain the CKSW filtration materials with higher density and stability, the treatment was employed with the method of dry heat shrinkage. The main morphological properties of the filtration materials for filter bag are characterized by three indexes: mass per unit area, thickness, and width. Therefore, as exhibited in Table 3, the morphological properties of the CKSW fabric are analyzed from these three aspects. For thickness, comparing the F/F, F/F/F, and F/F/D samples, the thickness of the samples are getting larger with the insertion of the weft insertion yarns and the type change of the yarns. It is worth noting that the thickness of the F/F/D, PTFE/F/D, and D/F/D samples with the low-elastic weft insertion yarns are generally large, and the thickness of these samples are basically as the same as the contrast sample. So the low-elastic polyester yarn is suitable as the weft insertion yarn of The CKSW filtration materials to enhance the thickness of the structure.

Table 3.

Parameter list of the filtration materials.

Type of sample	Thickness (mm)	Mass per unit area (g·m⁻²)	Width (cm)
F/F	1.15	217	12.23
F/F/F	1.35	295	14.52
F/F/D	1.70	315	13.87
D/F/D	1.73	307	13.70
PTFE/F/D	1.79	797	15.00
Filter sample (contrast sample)	1.75	356	13.00

The mass per unit area of the PTFE/F/D sample knitted with the PEFT yarn containing 5 wt% tourmaline particles was increased because of the addition of tourmaline. In addition, the remaining developed CKSW filtration materials samples all have a lower weight than the contrast sample, which is of great significance for the development of lightweight filter bags.

After the dry heat shrinkage setting pretreatment, the dimensional change of the CKSW filtration materials is shown in Figure 4. It can be seen from that after the pre-treatment, compared to F/F sample without weft insertion yarn, the mass per unit area of the CKSW filtration materials samples are almost unchanged from 0.61% to 0.86%. This is because the weft insertion yarns in the inter-layer of the CKSW stitch is under the state of no elasticity and plays a role of reinforcement. Thus, the structure shows tiny shrinkage. Obviously, the change rate of the density of the CKSW filtration materials is largest which is up to 15.38%, and the change rate of course per unit length is larger than that of wale per unit length. It is notably that dimensional change rate of D/F/D sample is generally greater than that of other the CKSW filtration materials samples, this is because the D/F/D sample incorporates polyester low-elastic yarn DTY in the ground yarns, and the polyester low-elastic filament has the better heat-setting performance. From the above analysis, the CKSW filtration materials researched and developed in this paper has stable heat setting properties, and it will have a flat appearance and good morphological stability during usage. The appropriate dimensional stability of the filtration materials would play a very important role in the life of the filter bag, which is one of the strong advantages of knitted materials.

Figure 4.

Dimensional change rate of the samples.

The pore structure of the CKSW filtration materials

The pore structure characterizations of textile materials mainly include pore size and porosity, which have a great influence on the filtration efficiency and air permeability of filtration materials. The results of the pore size test for different samples as shown in Table 4. The pore size and porosity of the contrast sample in the chart show that the ideal filtration materials have the characteristics of small pore size and high porosity.

Table 4.

Pore size test results of different samples.

Type of sample	Mean flow pore diameter	Smallest flow pore diameter	Largest flow pore diameter	Standard deviation of average pore diameter
F/F	318.58	125.42	319.66	99.83
F/F/F	264.73	35.76	268.48	124.44
F/F/D	108.11	46.66	319.57	80.55
D/F/D	67.55	45.14	128.12	36.79
Filter sample (contrast sample)	47.80	42.03	77.19	23.81

Among the CKSW filtration materials samples, the sample of F/F with no weft insertion had the largest pore size was about 318.58 µm. The pore size of the F/F/F, F/F/D, and D/F/D sample was gradually decreased, and the main impact factor was yarns configuration. Compared with F/F/F sample, the F/F/D sample whose weft insertion yarns were replaced with fluffier polyester low-elastic DTY exhibited smaller pore sizes. Furthermore, the weft insertion yarns and the ground yarns of the D/F/D sample were both replaced with polyester DTY yarns that led to tinier pore size. The pore sizes of F/F/F, F/F/D, and D/F/D samples are sequentially 264.73 µm, 108.11 µm, and 67.55 µm. The fluffiness of polyester DTY of weft insertion yarns built a fiber layer between two layers of ground stitches. Thus, the pore size of CKSW materials is smaller than that of the ordinary fabric. It can greatly reduce the pore size of the filtration materials. Therefore, the polyester DTY yarn is the ideal choice for the CKSW filtration materials.

From the comparison of the porosity parameters in Figure 5, it can be seen that the F/F sample has a minimum value of approximately 65.75%, and the porosity of the CKSW samples is increased by around 20% due to the insertion of weft insertion yarns, and which is close to the porosity of the filter standard sample. This indicates that the insertion of the weft insertion yarns can greatly increase the porosity of the CKSW filtration materials.

Figure 5.

The porosity of samples based on formula calculation method.

As shown in Table 5, the porosity measurement of samples based on image method had converted 24 bitmap into a binary image. The binary image was divided into two parts: black and white; here the black part indicates the pore of the samples. Then the image processing software could count the pixel area of different parts in the binary image. The percentage of the pixel area of the pore part to the whole image pixel area of is the porosity of the samples. It should be noticed that the porosity measurement of image method is an intuitive testing method, and the algorithm of measuring porosity by image method has been constantly optimized. Here, the image method could not identify and detect the circuitous pores in the filtration materials. Thus, there is a certain deviation between the results measured by the calculation method and the image method. It is impressive that the knitted fabric filtration materials have a great pore tortuosity, and the greater the pore tortuosity of the filtration materials, the stronger the ability to intercept the particles, and the better filtration performance as well.

Table 5.

The porosity of samples based on image method.

	F/F/F	F/F/D	D/F/D
Light microscope image
binary image
porosity	80.33%	85.30%	87.4%

When the samples were tested in the multi-purpose test machine for the filtration efficiency, the aerogel passed through the fabric, and the tested fabric was placed under the scanning electron microscope to observe the filtered state. Figure 6 shows the appearance of the D/F/D sample's different parts. It can be seen from Figure 6 that there are fine particles with a diameter of 0.3–10 µm distributed on the fabric. Although the surface of the yarn will have certain impurities under normal conditions, it will not be so much and evenly distributed. Therefore, it can be determined that CKSW filtration materials can intercept small particles in the air to achieve the purpose of filtration. This cylindrical seamless structure enables it to be widely used in filtration materials for filter bags.

Figure 6.

SEM images of D/F/D sample for different parts.

Filtration performance of the CKSW filtration materials

Figure 7 shows the filtration performance comparison of the samples. It can be seen from Figure 7(a) that the F/F sample has the lowest filtration efficiency, and the filtration efficiency of the CKSW samples were gradually increased with the addition of low-elastic yarn; on the contrary, the air permeability gradually decreased in Figure 7(b). The filtration efficiency with the particle size ≥ 5.0 µm of F/F/F, F/F/D, and D/F/D sample reached 70.33%, 85.07%, and 91.57%, while the filtration efficiency of the filter sample with the particle size ≥ 5.0 µm was 94.09%.

Figure 7.

Filtration performance of the samples as (a) filtration efficiency and (b) air permeability.

While the D/F/D sample of the CKSW filtration materials researched and developed in this paper has the best filtration efficiency, under the same particle size, the filtration efficiency of D/F/D is 3% lower than the one of contrast filter sample. The filtration efficiency was carried out on the static filtration performance test bench, and the test method was based on two-dimensional (The three-dimensional filtration performance test method based on the cylindrical filter bag has not been issued yet). As shown in Figure 8(a), the traditional filter bag is a three-dimensional cylindrical shape. However, there are 10–20 mm width area of sewing thread at the bottom and the side of the filter bag in order to ensure the strength and fastness of the filter bag. Figure 8(b) is the microscope image of the hole of sewing thread. It is obvious that the hole of sewing thread is much bigger than the pore of filter bag. Therefore, the filtration blind zone at the suture on the conventional filter bag did no contribution to the filtration test data. The CKSW filtration materials developed in this research has the advantage of seamless technology. Thus, the effective filtration area is average distribution on the filtration materials and the filtration performance was enhanced effectively.

Figure 8.

The sewing characteristic of the traditional filter bag: (a) The area of sewing thread in the filter bag, and (b) the microscope image of the hole of sewing thread.

Effect of the tourmaline and PTFE for the CKSW filtration materials

PTFE and tourmaline are both the novel materials widely used in the field of filtration, which have the electrostatic field effect. Tourmaline has the characteristics of self-polarization, which can use electrostatic adsorption to purify the gas and achieve the function of improving the filtration efficiency, while the PTFE filament has an electrostatic charge on the surface [28,29]. A previous study has shown that as the tourmaline particles contents in the PTFE base material increases, the electrostatic effect also increased correspondingly [12]. On this basis, the PTFE filaments containing tourmaline particles were employed in the CKSW filtration materials, which aimed to explore the effect of tourmaline and PTFE as the yarns knitting into CKSW filtration materials on the filtration performance. Figure 9 is the microscope image of PTFE filaments containing tourmaline particles and PTFE yarns containing the tourmaline knitting in the CKSW filtration materials.

Figure 9.

The microscope image of PTFE containing the tourmaline particles: (a) the PTFE filaments containing tourmaline particles. (b) PTFE yarns containing the tourmaline knitting into the CKSW filtration materials.

It can be seen in Figure 10(a) that the filtration efficiency to the particle size ≥5.0 µm of the PTFE/F/D sample added with PTFE yarn containing tourmaline is the highest, up to 93.25%. This is because the tourmaline and PTFE has the performance and ability to capture the static electricity, and these characteristics indicate that it can adsorb suspended particles and improve the filtration effect of filtration materials. Furthermore, it is interesting to note that in Figure 10(b) the pore size of the PTFE/F/D sample added with tourmaline modified PTFE yarn is increased to 78.16 µm compared to the 67.55 µm of the D/F/D sample. Because the tourmaline adheres to the surface of the PTFE yarn, increasing the viscosity of the yarn surface, the rheological behavior is better. It can be seen from Figure 10(c) that the porosity of the sample is also reduced after the ground yarns are replaced by the PTFE yarn. This is because that the PTFE yarn containing the tourmaline is a monofilament and the yarn structure is not so fluffy. Therefore, the porosity of the fabric is also smaller than that of the D/F/D sample. In the case of ensuring the filtration effect, the higher the porosity of the filter material, the more particles are filtered by the interception or inertial collision. In general, the PTFE yarn containing tourmaline knitting into the CKSW filtration materials can increase the filtration efficiency slightly, but at the same time, it will reduce the porosity to a certain extent thus increase the pressure drop.

Figure 10.

The performances of the samples: (a) filtration efficiency, (b) pore size, and (c) porosity.

Comprehensive evaluation of filtration performance of the CKSW filtration materials

The so-called fuzzy comprehensive evaluation is employed to make a general evaluation of the things or phenomena affected by various factors. The fuzzy comprehensive evaluation in first-level comprehensive evaluation is a single-level evaluation method for multiple factors. The general steps of fuzzy comprehensive evaluation are as follows: Firstly, the fuzzy comprehensive evaluation index is constructed, and the weight vector is constructed by expert experience method or AHP level analysis method. Then the appropriate membership function is established to construct the evaluation matrix. Finally, the matrix is synthesized by the appropriate synthesis factor, and the result vector is explained.

Five samples of the CKSW filtration materials involved in this work were analyzed. According to the performance characteristics of the CKSW filtration materials, the thickness, air permeability, pore size, porosity, and filtration efficiency of the fabric were selected as the index for comprehensive evaluation of the filtration performance of the CKSW filtration materials. The test results of the various indicators of the samples are shown in Table 6.

Table 6.

The test results of the various indicators of the samples.

Type of sample	Filtration efficiency (%) (particle size ≥5.0 µm)	Air permeability (mm/s)	Pore size (µm)	Porosity (%)	Thickness (µm)
F/F	70.33	1846.8	318.58	65.75	1.15
F/F/F	79.89	1084.6	264.73	84.17	1.35
F/F/D	85.07	974.2	108.11	86.53	1.70
D/F/D	91.57	772.64	67.55	87.14	1.73
PTFE/F/D	93.25	770.58.	78.16	84.39	1.79

The number of samples is 5 as the evaluation set $U = (\begin{matrix} μ_{1}, μ_{2}, \dots, μ_{m} \end{matrix})$ , where m = 5, and the index affecting the filtration performance of the product is taken as the factor set $V = (\begin{matrix} ν_{1}, ν_{2}, \dots, ν_{m} \end{matrix})$ , where n = 5.

According to the sample condition, the following model is used to calculate the membership degree [30].

r_{ij} = \frac{X_{ij} - X_{i min}}{X_{i max} - X_{i min}}

(4)

r_{ij} = \frac{X_{i max} - X_{ij}}{X_{i max} - X_{i min}}

(5)

where r_ij is the element of the judgment matrix R, r_ij∈[0,1]; X_ij is the test value of the i index of the j sample; X_imax is the maximum value of the i index; and X_imin is the minimum value of the ith index. Among them, the larger the index, the better the filtering performance, the formula (4) is selected, otherwise, the formula (5) is selected. The judgment matrix R can be obtained by using formulae (4) and (5) as follows

\begin{matrix} R = [\begin{matrix} 0 & 0.417 & 0.643 & 1 & 0.927 \\ 1 & 0.292 & 0.189 & 0.002 & 0 \\ 0 & 0.215 & 0.838 & 1 & 0.958 \\ 0 & 0.861 & 0.972 & 1 & 0.872 \\ 0 & 0.315 & 0.859 & 0.906 & 1 \end{matrix}] \end{matrix}

Weight coefficient is the degree to which each factor affects the performance of the product's filtration performance. In this work, the subjective method is selected, and according to the role of each factor in the product filtration performance and its status, the weight coefficient of each indicator is determined through investigation and review by multiple experts and related technicians. After the results are summarized and the weights of the five factors are assigned as,

A = (\begin{matrix} 0.45, & 0.25, & 0.05, & 0.25, & 0.05 \end{matrix})

The comprehensive evaluation result vector is calculated as follows

B = A \cdot R = (\begin{matrix} b_{1}, & b_{2}, & \dots, & b_{m} \end{matrix}) = (\begin{matrix} 0.250, & 0.481, & 0.632, & 0.746, & 0.686 \end{matrix})

(6)

Normalize the results and the comprehensive evaluation result vector is given as follows

B = (\begin{matrix} 0.089, & 0.172, & 0.226, & 0.267, & 0.245 \end{matrix})

According to the above comprehensive evaluation results, the filtration effect of sample D/F/D is the best, followed by PTFE/F/D, F/F/D, F/F/F, and F/F samples. Through the analysis and collation of experimental data, it can be basically determined that the CKSW samples had the polyester DTY ground yarn the polyester DTY weft insertion yarn, and the polyester FDY connection yarn involved has the best filtration performance.

Conclusion

The CKSW filtration materials developed on the modified circular seamless machine were researched, which had the characteristics of high production efficiency and good filtration performance. Polyester FDY, low-elastic yarns DTY, and PTFE filaments with tourmaline particles were used as yarns materials. The yarn configuration of the CKSW was multifarious. Through fuzzy comprehensive evaluation analysis, it could be found that the CKSW samples had the polyester DTY ground yarn the polyester DTY weft insertion yarn, and the polyester FDY connection yarn involved exhibited the smallest pore diameter and the highest porosity, named D/F/D sample. It had the 84.46% filtration efficiency towards the particles on size of ≥2.0 µm, and up to 91.57% of the filtration efficiency towards the particle on size of ≥ 5.0 µm. More importantly, it was also verified that the novel static electric materials such as PTFE modified by tourmaline had the effect of improving the filtration efficiency of CKSW filtration materials. The experimental results would provide a reference for the development of filtration materials, and at the same time develop and expand the industrial application of weft-knitted weft-insertion structure products.

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 authors would like to give the acknowledgement to the following financial support for the research: Provincial and Ministry Joint Open Project (M2-201805) National First-class Discipline Program of Light Industry Technology and Engineering (2018-28) and Fundamental Research Funds for the Central Universities (JUSRP11807).

References

Dockery

Stone

. Cardiovascular risks from fine particulate air pollution. New Engl J Med 2007; 356: 511–513.

Kadam

Wang

Padhye

. Electrospun nanofibre materials to filter air pollutants – a review. J Ind Text 2016; 47: 2253–2280.

Lelieveld

Evans

Fnais

, et al. The contribution of outdoor air pollution sources to premature mortality on a global scale. Nature 2015; 525: 367–371.

Bao

Musadiq

Kijima

, et al. Influence of fibers on the dust dislodgement efficiency of bag filters. Text Res J 2013; 84: 764–771.

Cao

Cheng

C-M

Chen

C-W

, et al. Abatement of mercury emissions in the coal combustion process equipped with a fabric filter baghouse. Fuel 2008; 87: 3322–3330.

Pelyk

Vasylechko

Kutsyk

, et al. Innovative filters made of polyoxadiazole fibers for industrial gas and dust emissions treatment. Adsorpt Sci Technol 2017; 35: 817–824.

Yeo

Kim

Lim

, et al. Effects of processing condition on the filtration performances of nonwovens for bag filter media. J Mater Sci 2005; 40: 5393–5398.

Shim

Joe

Y-H

Park

H-S

. Influence of air injection nozzles on filter cleaning performance of pulse-jet bag filter. Powder Technol 2017; 322: 250–257.

Hasani

Hassanzadeh

Abghary

, et al. Biaxial weft-knitted fabrics as composite reinforcements: a review. J Ind Text 2016; 46: 1439–1473.

10.

Mao

Jiang

. Design and manufacturing of warp-knitted filterable fabric with PET/PVDF fiber. J Text Inst 2017; 108: 2090–2095.

11.

Dabiryan

Hasanalizade

Sadighi

. Low-velocity impact behavior of composites reinforced with weft-knitted spacer glass fabrics. J Ind Text. 2019; 49: 465–483.. DOI: 10.1177/1528083718787533.

12.

Mao

Jiang

. Filtration efficiency investigation of mesh fabrics by polytetrafluoroethylene filament with surface static electricity. J Text Inst 2019; 110: 451–459.

13.

Zhang

Sun

, et al. Large-sized graphene oxidemodified tourmaline nanoparticle aerogel with stable honeycomb-like structure for high-efficiency PM2.5 capture. J Mater Chem A 2018; 6: 16139–16148.

14.

Wang

Chen

. Self-assembly of polytetrafluoroethylene nanoparticle films using repulsive electrostatic interactions. Langmuir 2014; 30: 976–983.

15.

Rajan

Souza

Ramakrishnan

, et al. Comfort properties of functional warp-knitted polyester spacer fabrics for shoe insole applications. J Ind Text 2016; 45: 1239–1251.

16.

Cuifang

Xia

Ting

, et al. Inspection method of lattice apron porosity based on image processing. J Text Res 2014; 35: 49–54.

17.

Shen

Deng

Liu

, et al. Preparation and properties of gradient filter materials with different packing density. J Text Res 2017; 38: 23–27.

18.

Merzban

Elbayoumi

. Efficient solution of Otsu multilevel image thresholding – a comparative study Otsu. Expert Syst Appl 2019; 116: 299–309.

19.

Hammouche

Diaf

Siarry

. A multilevel automatic thresholding method based on a genetic algorithm for a fast image segmentation. Comput Vis Image Und 2008; 109: 163–175.

20.

Xing

Xin

Deng

, et al. A novel digital analysis method for measuring and identifying of wool and cashmere fibers. Measurement 2019; 132: 11–21.

21.

Wang

Xin

Deng

, et al. Single vision based identification of yarn hairiness using adaptive threshold and image enhancement method. Measurement 2018; 128: 220–230.

22.

Rodrigues

Figueiredo

Diogo

, et al. Mechanical behavior of PET fibers and textiles for stent-grafts using video extensometry and image analysis. Sci Technol Mater 2018; 30: 23–33.

23.

Turan

Okur

. Investigation of pore parameters of woven fabrics by theoretical and image analysis methods. J Text Inst 2012; 103: 875–884.

24.

Abou-Ana

Youssef

Pastore

, et al. Assessing structural changes in knits during processing. Text Res J 2003; 73: 535–540.

25.

Aydilek

Oguz

Edil

. Digital image analysis to determine pore opening size distribution of nonwoven geotextiles. J Comput Civil Eng 2002; 16: 280–290.

26.

Gribble

Matthews

Laudone

, et al. Porometry, porosimetry, image analysis and void network modelling in the study of the pore-level properties of filters. Chem Eng Sci 2011; 66: 3701–3709.

27.

, et al. Experimental investigation of air pressure affecting filtration performance of fibrous filter sheet. Environ Technol 2017; 38: 558–565.

28.

Safak

Karaca

. Production and characterization of poly(ethylene terephthalate) nanofibrous mat including tourmaline additive. Text Res J 2016; 86: 1651–1658.

29.

Greason

Oltean

Kucerovsky

. Triboelectric charging between polytetrafluoroethylene and metals. IEEE Trans Ind Appl 2004; 40: 442–450.

30.

Wang

, et al. Fuzzy evaluation of the mechanical properties of bamboo/flax composites. J Text Res 2007; 28: 34–37.