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Abstract

The aim of this work was to investigate the barrier and comfort properties of protective uncoated and coated-impregnated three-layered knitted fabrics with different arrangements of special yarns, such as conductive yarns and yarns with different filament cross sections. Depending on content (7.5–30%) of conductive PES yarns with carbon core filaments and PES/stainless steel spun yarns, fabrics were grouped into A and B. In order to achieve multifunctional barrier and comfort properties, high porosity polyurethane and fluorocarbon resin coatings were applied. At the beginning of the research, the fabrics of groups A and B were coated with commercially available micro-porous polyurethane foam Tubicoat® MB according to the crushed foams method and impregnated with fluorocarbon water-repellent agent Tubiguard® 270. The achieved functional, barrier (water and oil repellency, resistance to water penetration, and electrostatic shielding), and comfort (air permeability, water vapor permeability, water retentivity, and drying intensiveness) properties were determined.

Keywords

Multifunctional textiles comfort properties electrostatic properties water proofness

Introduction

New industrial sectors and expanding use of new technologies (microelectronic, IT, biotech) and focus on the quality and safety of life improvement opens a wide space for new high quality added value products. Barrier textiles are often exploited in harsh conditions (thermal, airstream, and liquid and moisture management behavior). The market intended for protective and workwear clothing focuses on elevated requirements for personal safety and security in industrial, health care, and civil sphere (medical, hospital, microelectronics, automotive) as to the improvers of the safety and comfort of end-users. Multifunctional textiles are widely used in industrial and health care sectors and improve the safety and comfort of end-users. Knitted fabrics are known for their excellent comfort properties (resistance to wrinkling, easy care, moisture management etc.) and attractive price which make them popular for many end uses including casual-wear, active-wear, sportswear, underwear, and so on [1].

The dynamical interactions of fabric and human skin while wearing are important factors which have to be considered in terms of product (clothing) development. Every friction and extra moisture or heat management decreases level of comfort and can lead to inconveniences such as sensitive skin and irritation or discomfort when temperature of the skin increases [2,3]. The most efficient sweating occurs when all generated sweat evaporates and the vapor passes through the enough permeable garment system into the relatively dry air outside the body. The thermal discomfort sensation starts at around 33℃, and becomes very high at 35℃ [4]. However, under less favorable conditions, the sweat accumulates in the clothing and evaporates from the clothing surface [5]. Generally, moisture transfer and evaporation in the textile materials depend mainly on the moisture absorbency of fibers and wicking capability through pore distribution, pathways, and surface tension, whereas the drying rate of a material is related to the molecular structure and geometrical parameters of fibers [6]. Moisture vapor can pass through openings between fibers or yarns [7]. Liquid transportation and drying rate of fabrics are two vital factors affecting the physiological comfort of garments [8 –10]. The moisture transfer and quick dry behaviors of textiles depend mainly on the capillary capability and moisture absorbency of their fibers [11]. Profiled fibers are very useful for applications that need to have good water transferring instead of water absorption. The profiled fibers have a bigger surface area which increases wickability [12,13]. Fabrics, composed of fibers having a cruciform cross section are more hydrophobic than round ones [13]. Three mechanisms can be considered for water vapor transfer through fabric: the first is through fabric pores, the second is through absorption by fabric and then evaporation from fabric surfaces, and the third is transferring the vapor (liquid) though the fibers constituting the fabric [14].

Manufacturing textiles for protection products often requires the use of synthetic fibers, which are characterized as fibers having very low electrical conductivity and a high ability to accumulate electric charges [15]. Wicking for most synthetic fabrics, are not taking place due to their high contact angles [11]. The efficiency of leaking electrostatic charges from textiles is achieved with electroinductive fibers and it depends on the kind of raw materials used and on the materials' structure, which have to ensure good contact between fibers and the rubbing surface [16].

Waterproof-breathable fabrics are of significance in the fields of hygiene, agriculture, protective clothing, sportswear, and construction industries. Waterproof breathable fabrics balance two contradicting properties: waterproof and yet water vapor permeable. Hence, producing a material which has both of these properties has proved to be a major challenge for manufacturers of waterproof performance fabrics [17]. Different types of breathable fabrics can be classified as: (a) closely woven fabrics, (b) microporous membranes and coatings, (c) hydrophilic membranes and coatings, (d) combination of microporous and hydrophilic membranes and coatings, (e) retroreflective microbeads, (f) smart breathable fabrics, (g) fabrics based on biomimetics [18,19]. Water-repellent fabrics have open pores and are permeable to air and water vapor. Water-repellent fabrics will permit the passage of liquid water once hydrostatic pressure is high. Waterproof fabrics are resistant to the penetration of water under much higher hydrostatic pressure than are water-repellent fabrics. These fabrics have fewer open pores and are less permeable to the passage of air and water vapor. The more waterproof the fabric is, the lesser is its ability to permit the passage of air or water vapor. Waterproof is an overstatement; a more descriptive term is impermeable to water. A fabric is made water-repellent by depositing a hydrophobic material on the fibre's surface. However, waterproofing requires filling the pores as well [20]. But, still there are a number of commercial breathable fabrics available in the market. For example, widely known Gore-Tex and Sympatex fabrics possess breathability owing to microporous and hydrophilic membrane respectively laminated with fabric; whereas Microft fabric is composed of polyester microfiber of 0.7 denier/filament [21].

The aim of the research was to evaluate the impact of microporous PU coating and fluorocarbon (FC) impregnation on the knitted fabrics, and hence to investigate its influence on comfort and barrier properties of three-layered conductive knitted fabrics.

The novelty of the investigation is that there are not much data about investigations of multifunctional barrier knitted fabrics with microporous coating and fluorcarbon impregnation. Furthermore, uncoated and coated-impregnated three-layered knitted fabrics were invetigated in this research, where special yarns, such as conductive yarns and yarns consisting of filaments with special cross-section shape, were combined in one fabric. We have not found any literature where such kind of fabrics have been investigated.

Experimental

Two groups (A and B) of three-layered weft-knitted fabrics differing in fiber content were manufactured for this research work. All fabrics were knitted on the circular interlock knitting machine Metin Nov of gage 20E. The combined pattern was chosen for these fabrics, where number of courses and wales per cm were 20 and 12, respectively and yarns in courses C3, C6, C9, and C12 forms inner layer; C1, C4, C7, and C10 forms middle layer; and C2, C5, C8, and C11 forms outer layer. The number of stitches was calculated according to EN 14971 standard.

The schematic illustration of knitted fabrics structure is presented in Figure 1. The detailed description of investigated fabrics is given in Table 1.

Figure 1.

Schematic illustration of combined pattern knitted fabrics structure.

Table 1.

The main characteristics of the three-layered weft-knitted fabrics.

Code of fabric	Type of separate layer	Type of yarn	Linear density (tex)	Yarn content in the fabric (%)
1A	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with carbon core	15.6	30
	III – outer	PP	20.0	36
2A	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with round hollow core	7.8 × 2	7.5
	II – middle	PES with carbon core	15.6	22.5
	III – outer	PP	20.0	36
3A	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with round hollow core	7.8 × 2	15
	II – middle	PES with carbon core	15.6	15
	III – outer	PP	20.0	36
4A	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with round hollow core	7.8 × 2	22.5
	II – middle	PES with carbon core	15.6	7.5
	III – outer	PP	20.0	36
1B	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with carbon core	15.6	30
	III – outer	PES/stainless steel (80%/20%)	20.0	36
2B	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with round hollow core	7.8 × 2	7.5
	II – middle	PES with carbon core	15.6	22.5
	III – outer	PES/stainless steel (80%/20%)	20.0	36
3B	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with round hollow core	7.8 × 2	15
	II – middle	PES with carbon core	15.6	15
	III – outer	PES/stainless steel (80%/20%)	20.0	36
4B	I – inner	Cotton/PES (67%/33%)	20.0	34
	II – middle	PES with round hollow core	7.8 × 2	22.5
	II – middle	PES with carbon core	15.6	7.5
	III – outer	PES/stainless steel (80%/20%)	20.0	36

For achieving additional functional properties such as waterproofing and water and oil repellency of investigated knits, an aqueous synthetic dispersion-based polyurethane (PU) Tubicoat MP, melamine resine Tubicoat fixing agent HT, fluorcarbon resin Tubiguard 270, and wetting agent Kollasol CDO were used. All untreated knits presented in Table 1 were scoured with nonionic detergent 2 g/L Felosan RG-N at 40℃ in the washing machine WASCATOR FOM71MP – Lab, then dried in the laboratory oven TFO/S IM 350 at 100℃. After washing and drying, fabrics were treated with readymade microporous PU coating according to the crushed foam method as follows: 100 g of a readymade PUcompound Tubicoat® MP and 5 g (5%) of a crosslinker Tubicoat® fixing agent HT were mixed and foamed in undiluted conditions by means of a Heidolpf RZR-50 laboratory mixer until a weight of foam 210 g/L was achieved. The foamed PU Tubicoat® MP was applied on the outer layer of knitted fabrics with a help of a knife. Then coated fabrics were dried at 100 ºC in the laboratory oven TFO/S IM 350 and crushed at 4.7 bar in the laboratory padder EVP-350. After this, the coated fabrics were impregnated with 50 g/L of Tubiguard® 270 and 1 g/L of Kollasol® CDO aqueous solution (pH=5) in the padder, condensed for 2 min at 170 ºC in the oven and cold-calendered at 20 bar on a Universal Calender 350. Additional impregnation with a FC resin Tubiguard 270 was used to ensure the oil repellency for knits. This type of impregnation is enviromentally friendly as no organic solvents are used. The microscopic view of PU coating on knitted fabric is presented in Figure 2. It is seen from the figure that the size of the foam pores varies from 4 µm to 70 µm. Using the software ImageJ it was determined that the mean diameter of coating pores was approximately 40 µm.

Figure 2.

The microscopic image of polyurethane coating on the tested knitted fabric.

The mass per unit area of conditioned fabrics was determined out according to EN 12127 standard. The arithmetic mean of five specimens is presented in this paper. The coefficient of variation is approximately 3%. The thickness of fabrics was measured according to EN ISO 5084 standard. The conditioning of samples and testing was carried out at standard atmospheric conditions: temperature (20 ± 2)℃, relative humidity (65 ± 4)%.

The bending rigidity of both adjusting systems was determined using “FAST method”. The bending rigidity B_fast (Figure 3) was calculated by the following formula

B_{FAST} = W \times c^{3} \times 9.807 \times 10^{- 6} (μ N \cdot m)

(1)

where W is the mass per unit area of specimen (g/m²); c is the bending length—half length (l/2) of bended specimen (mm).

Figure 3.

Method for determination of fabrics bending rigidity in FAST system.

The KES-FB (Kawabata Evaluation System for Fabrics) system was used for the determination of the fabric shear properties (G – shear modulus, 2HG – force hysteresis at shear angle 0.5°, 2HG5 – force hysteresis at shear angle 5°). The shear force–shear angle curve (see Figure 4) obtained during this deformation cycle, exhibits a typical hysteresis effect. Shear stiffness was determined by the slope of the curve between angles 0.5° and 2.50°. The shear hysteresis was calculated by the difference in shear force at the loading and unloading cycle at small angles (2HG, at 0.50) and at large angles (2HG5, at 50). The shear force–shear angle curve was obtained by applying a shear deformation to the sample, at constant rate from 8° to −8°. The width of the sample was 20 cm and the distance between clamps was 5 cm. The clamps moved parallel to each other, resulting in a shear deformation. Samples were conditioned and test was carried out under standard laboratory conditions of (20 ± 2)℃ and (65 ± 4)% relative humidity.

Figure 4.

Typical shear force–shear angle curve.

The thermal comfort property such as air permeability was determined according to EN ISO 9237 standard. The air permeability was measured by using the Frazier 2000TM differential pressure air permeability tester, at pressure drop of 100 Pa. The test surface area of 20 cm² was used and 10 places for each fabric were measured in order to have better accuracy. The coefficients of variation of measurements were approximately 7%. The conditioning of samples and testing was carried out at standard atmospheric conditions: temperature (20 ± 2)℃, relative humidity (65 ± 4)%.

The water vapor permeability was measured with a cup method: the test cup was filled with the distilled water, then the circular specimen was tight covered the cup and the cup prepared for testing was located in the controlled environment (temperature in the cup 38℃, room temperature 19℃, relative humidity in the cup 100%, relative humidity in the room 55%). An equal initial air gap between the water level and the fabric was always maintained in the cup. The water vapour diffusion through a textile to the environment occurs due to the pressure difference between the vapour inside and outside the cup. For the experiments conducted under the same testing conditions, the obtained results allowed us to compare the water vapor permeability values of investigated samples. The investigation stand is presented in Figure 5. The arithmetic mean of five specimens for each fabric was counted and the coefficient of variation was not more than 5%.

Figure 5.

The investigation stand of water vapor permeability (cup method).

Water retentivity was carried out by immersing specimens in the reservoir of distilled water for 1 min. Then wet specimens were taken from the reservoir, slightly squeezed between filter paper, and then the water retentivity was calculated using equation (2). The air conditions during test were: temperature 19 ºC and relative humidity 55%.

W_{r} = ((m_{w} - m) / (m_{w} - m) m m) 100

(2)

where W_r is the water retentivity (%); m_w is the mass of wet specimens (g); and m is the mass of dry specimen (g).

It is well known that the amount of absorbed water of individual fabrics is different. That is why the drying intensiveness (Di) was calculated as the ratio between quantities of evaporated water and all absorbed water, expressed in percentage

Di = (m_{w} - m_{d}) / (m_{w} - m_{d}) (m_{w} - m)) (m_{w} - m)) 100

(3)

where m_w is the mass of wet specimen (g); m is the mass of dry specimen (g); m_d is the mass of damp specimen after specified period of drying (g).

The “drying intensiveness” is a relative value, when it reaches 100%, the sample is completely dried-up. The quantities of all absorbed water were determined by immersing five specimens of each fabric in the water and then recording their mass instantly after taking them out from the reservoir with water and after 30, 90, 150, 210, 270, 330, 390, 510, and 540 min of drying. The specimens taken from water reservoir at first were slightly squeezed between two filter papers and lay flat to dry. Drying kinetics experiments were carried out at 19℃ and 55% relative humidity.

Along with the comfort properties of fabrics, parameters of electrostatic properties such as surface and vertical resistances, shielding factor, and half decay time were measured. Vertical and surface resistances were measured according to EN 1149-2 and EN 1149-1 standards respectively with Terra-Ohm-Meter 6206. The range of measured values of ohmmeter was 10³–10¹⁴ Ω. The conditioning and testing were carried out in dry conditions, i.e. relative humidity (25 ± 5)%, temperature (23 ± 1)℃. The mean values of five specimens are presented in this work. The coefficient of variation of resistance values was less than 5%.

The shielding factor and half decay time were determined according to EN 1149-3, 2^nd method (induction charging). The conditioning and testing was carried out in dry conditions. The arithmetical mean of three specimens is presented. The charge decay parameters were determined with the electric charge meter ICM-1. The instrument was controlled by a microprocessor which carried out measurements, provided automatic calculations, and displayed measured data. The coefficient of variations of measured parameters was not more than 1.5%. The values of the shielding factor (S) were obtained using the following equation

S = 1 - \frac{E_{R}}{E_{max}}

(4)

where E_R is the maximum electric field strength indicated by the recording device with the test specimen in the measuring position, and E_max is the electric field strength indicated by the recording device without test specimen in the measuring position.

The special property of coated knitted fabrics—resistance to surface wetting was defined according to EN ISO 4920. The evaluation scale of wetting grade is presented in Table 2. The mean value of three specimens for each knit was presented. The resistance to water penetration of fabrics was investigated according to EN 20811 standard, when rising speed of water pressure was 60 cm of water column per minute and water pressure was applied from the bottom of the fabric. The arithmetic mean of five specimens is presented in this paper. The coefficient of variation of investigated parameters is less than 5%.

Table 2.

The evaluation scale of wetting grade, according to EN ISO 4920 standard.

Wetting grade	Description
5	No sticking or wetting of the specimen
4	Slight random sticking or wetting of the specimen face
3	Wetting of the specimen face at spray points
2	Partial wetting of the specimen face beyond the spray points
1	Complete wetting of the entire specimen face beyond the spray points
0	Complete wetting of the entire face of the specimen

The classes of oil repellency of knitted fabrics were determined according to EN ISO 14419 standard. The oil-repellency grade is the highest numbered test liquid which is not absorbed by the fabric surface. The list of test liquids is presented in Table 3.

Table 3.

The standard test liquids, according to EN ISO 14419 standard.

Composition	Oil test liquid number
None (fails white mineral oil)	0
White mineral oil	1
65:35 white mineral oil: n-hexadecane by volume	2
n-Hexadecane	3
n-Tetradecane	4
n-Dodecane	5
n-Decane	6
n-Octane	7
n-Heptane	8

Results and discussions

The manufactured and investigated PU, coated and impregnated with FC fabrics are flexible and can be easily used for outer garment designs. Fabrics are not intended for underwear garments, but are intended to be used as barrier textiles exploited in harsh conditions or for multifunctional protective clothing and work-wear, for personal safety and security in industrial, health care and civil sphere (medical, hospital, microelectronics, automotive, food industry). It is very important that fabrics, which are used for manufacturing of garments with special properties, in this case for garments intended to prevent electrostatic discharge and to protect from surface wetting, should have excellent thermal comfort properties.

Two groups of three-layered knitted fabrics were manufactured and later foamed with PU microporous foam coating and impregnated with FC resin for this investigation. It was chosen to coat the outer layer of knits, i.e. for fabrics of group A, polypropylene (PP) side was coated and for group B, PES/stainless steel side was coated. The results of mass per unit area have showed that all fabrics became heavier after the coating and impregnation (see Figure 6). The applied microporous coating was approximately 0.2 mm thick for knits of group A and approximately 0.1 mm thick for knits of group B. It is seen from data presented that thickness of the coating influenced the total mass of fabrics distinctly. The results of fabrics thickness is presented in Table 4. It is seen from Figure 6 that the mass of knits of group A became much heavier after coating and impregnation comparing to fabrics of group B, because microporous coating attached to the PP much easier than to PES/stainless steel yarns. This can be explained by the density of PP fiber—it is only 0.91 g/cm³ [22]. Meanwhile, densities of PES and stainless steel fibers is 1.39 g/cm³ and 7.95 g/cm³ respectively [22,23]. The PP fibers with smaller density have a bigger surface area and cavities in the structure of PP yarns are greater, the overall diameter bigger, than the PES/stainless steel yarns. That is why PU foam easily intrude into the middle of PP yarns than into the PES/stainless steel yarn.

Figure 6.

Mass per unit area of knitted fabrics.

Table 4.

Thickness of the three-layered uncoated and coated-impregnated weft-knitted fabrics.

Code of fabric	Thickness (mm)
Code of fabric	Uncoated	Coated-impregnated
1A	1.84	2.06
2A	1.84	2.06
3A	1.85	2.07
4A	1.83	2.04
1B	1.84	1.91
2B	1.85	1.90
3B	1.84	1.91
4B	1.85	1.9

The mass of group A fabrics after coating approximately increased by 39% and that of group B by only 18%. So, fabrics of group A are approximately two times easier than of group B. This was influenced by the difference of mass per unit area data between uncoated and coated-impregnated fabrics.

Although it looks like fabrics with PU coating should be very stiff and unflexible, but special aqueous, solvent-free synthetic dispersion based on PU Tubicoat® is a readymade stable foam compound, which do not compose inflexible fabrics as ordinary well-known PU coatings. As it was mentioned earlier, the coatings were carried out by foam application by so-called “crushed-foam” method. It is known that by choosing the right initial fabric for coating, very soft effects can be achieved [24,25]. The finished product usually exhibits a softer handle fabric than a paste coat, as the chemical tends to sit on top of the fabric and there is very little penetration.

In order to prove that investigated fabrics are quite flexible, compared to ordinary PU coated fabrics, bending (flexural) rigidity was determined of investigated fabrics in wale and course directions (see Table 5).

Table 5.

Bending (flexural) rigidity of uncoated and coated-impregnated knitted fabrics.

Code of fabric	B_FAST (μNm)
	Uncoated fabrics		Coated-impregnated fabrics
	Wale direction	Course direction	Wale direction	Course direction
1A	111.9	12.1	223.8	114.4
2A	131.6	15.0	245.9	114.8
3A	135.6	15.5	243.0	107.7
4A	121.7	12.5	225.8	110.0
1B	151.6	22.5	247.4	83.6
2B	138.2	21.4	238.1	86.8
3B	128.1	20.0	224.2	82.4
4B	152.6	22.1	251.0	83.6

According to the FAST “Fingerprint” recommendations [26] for thin woven fabrics, woven fabrics with bending rigidity of more than 14 μNm are considered as stiff fabrics. Taking into account that our investigated fabrics are coated-impregnated and uncoated three-layered knitted fabrics, we cannot compare our results with FAST “Fingerprint”, but we can notice that bending rigidity of some investigated uncoated knitted fabrics of group A in course direction varies between 12 and 15 μNm. Also it can be seen from data presented in Table 5 that fabrics of group A are more flexible compared to fabrics of group B, because stainless steel fibers, present in fabrics of group B, make textiles tighter.

As it is seen from Table 5, bending rigidity of all investigated fabrics are higher for fabrics in wale direction compared to course direction, i.e. it means that fabrics are more flexible in course direction than in wale direction. After impregnation and coating, the bending rigidity of investigated fabrics obviously increases: for fabrics of group A in wale direction it increases approximately 2 times (very similar to fabrics of group B), and in course direction increases approximately 6 times; as for fabrics of group B in course direction bending rigidity increases only approximately 4 times. So, it can be concluded that bending rigidity is very sensitive to the thickness of coating used (see Tables 4 and 5).

Yip and Ng [27] investigated bending rigidity of uncoated three-dimensional spacer fabrics. They showed that bending rigidity in wale direction varies from 146 μNm till 296 μNm and in course direction—from 96 μNm to 400 μNm; while for bending rigidity of coated-impregnated fabrics of A and B groups, investigated in this paper, in wale direction it varies from 223 μNm to 251 μNm and in course direction—from 83 μNm to 114 μNm.

Saceviciene and Masteikaite [28] investigated bending rigidity of plain weave cotton fabric coated with PU, PVC, and polyester weft-knitted fabric laminated with PVC, used for manufacturing of protective clothings. They determined that bending rigidity of cotton fabric, coated with PU, PVC in wale direction was more that 300 μNm, while in course direction it was approximately 150 μNm; and for polyester-knitted fabrics laminated with PVC, bending rigidity was 250 μNm in wale direction and 120 μNm in course direction. So, the bending rigidity of three-layered weft-knitted investigated fabrics is better than the fabrics investigated by other authors.

In order to examine the influence of the mechanical properties of investigated fabric with and without coating-impregnation KES-FB (shearing rigidity), measurements were performed. The results are presented in Tables 6 to 8.

Table 6.

Shear stiffness G (gf/cm×degree).

Code of fabric	Uncoated knitted fabrics		Knitted fabrics with coating
Code of fabric	Wale direction	Course direction	Wale direction	Course direction
1A	1.50	1.06	9.17	8.99
2A	1.50	1.22	9.51	9.47
3A	1.38	2.17	10.07	11.94
4A	1.33	2.00	9.21	11.11
1B	1.44	1.89	10.01	13.33
2B	1.61	2.00	12.14	12.61
3B	1.00	1.35	8.77	9.13
4B	1.94	0.83	10.34	8.79

Table 7.

Force hysteresis at shear angle 0.5° (2HG, gf/cm).

Code of fabric	Uncoated knitted fabrics		Knitted fabrics with coating
Code of fabric	Wale direction	Course direction	Wale direction	Course direction
1A	8.74	8.50	6.29	6.14
2A	10.11	9.07	7.08	6.59
3A	12.54	12.49	8.40	8.94
4A	12.48	11.25	6.98	9.01
1B	12.21	13.54	8.75	9.50
2B	11.98	14.89	8.86	9.42
3B	14.91	12.06	7.45	8.44
4B	19.25	16.24	10.51	9.71

Table 8.

Force hysteresis at shear angle 5° (2HG5, gf/cm).

Code of fabric	Uncoated knitted fabrics		Knitted fabrics with coating
Code of fabric	Wale direction	Course direction	Wale direction	Course direction
1A	8.75	2.27	40.13	15.40
2A	5.52	2.51	27.18	15.35
3A	5.03	7.48	24.64	35.71
4A	4.48	6.06	27.12	34.87
1B	5.55	6.43	25.07	35.02
2B	4.50	6.45	24.50	32.22
3B	4.53	4.25	22.63	25.01
4B	8.47	4.07	39.58	23.14

It can be concluded from the data presented in Tables 6 to 8 that the shearing characteristics of noncoated knitted fabrics of groups A and B are very close to the results of different knitted fabrics investigated by other authors [29 –32]. Higher values of shearing rigidity are appropriate for coated fabrics [33]. The shearing rigidity of fabrics after coating increases approximately 6–8 times. Although, in terms of the article by Ancutiene et al., [29] where the object of research was woven fabrics, investigated fabrics of group A and B can be attributed to the high stiffness fabrics, but compared to other materials used for technical purposes, the shearing rigidity of even coated fabrics is not so high. For example, Lomov et al. [34] received significantly higher values of shearing rigidity for uncoated fabrics. Overall coating increases the hysteresis of shearing at 0.5° – 2HG5, but values of 2HG are smaller. This proves that the coating applied on the investigated fabrics of groups A and B is soft and elastic; although the fabric is quite stiff, but after deformation it returns to the initial state much quicker compared to fabrics coated with other types of coatings.

Comfort properties

Although the PP yarn is specified as hydrophilic, which distinguishes such a unique property as transfer the moisture and stay dry at the same time, but our investigation have showed that the air permeability of uncoated and coated-impregnated fabrics of group B is two times higher than that of PP (fabrics of group A) (see Figure 7). Also it is seen from Table 1 and Figure 8 that the presence of PES yarns with round hollow core filaments in the fabrics decreases air permeability of knits more than 10 times. This means that filaments of special shape trap the air inside and the air permeability of fabrics decreases. The same conclusion was made in Varnaitė-Žuravliova et al. [35]. It is also obvious that PES with round hollow core filaments do not have such a big influence on air permeability of coated-impregnated fabrics when compared to uncoated fabrics, because values of tested parameter of all coated-impregnated fabrics in the group is in the same range. Hence, coating plays a crucial role here, which traps the air flow through fabrics and imparts better isolation; coating reduces air permeability very visibly, but not fully. Some part of the air still passes through microporous coating. Air permeability of group A fabrics decreases almost 10 times after coating and that of group B decreases three times.

Figure 7.

Air permeability of knitted fabrics.

Figure 8.

Air permeability versus quantity of PES yarns with round hollow filaments in knitted fabrics.

Water vapor permeability is one of characteristics, which defines breathing property of fabrics. According to some authors [36,37] breathable fabrics are those whose water vapor permeability is more than 5000 g/m²×24 h. It is seen from Figure 9 that all uncoated fabrics satisfy this requirement and this parameter is approximately the same for all fabrics, if we take error intervals in to consideration. Although Wang and Yasuda [38] say that the type of finish applied (hydrophobic or hydrophilic) to the fabric has no great effect on the diffusion process of water vapor through textiles, water vapor permeability notably decreases after fabrics coating, i.e. the parameter of water vapor permeability decreases approximately 500 g/m²×24h, because water vapor can pass the fabric only through microporous coating (see Figure 2). So, the distribution of yarns with special characteristics, such as PP or PES with round hollow core filaments do not have any significant influence on investigated parameter. Coating only reduces breathability of textiles.

Figure 9.

Water vapor permeability of knitted fabrics.

Water retentivity indicates an absorbed water quantity of submerged and dragged fabric out from the water. It is seen from Figures 10 and 11 that the water retentivity of fabrics of group A is significantly lower than that of fabrics of group B. This can be explained with dependence on fiber content. Fabrics with PP yarns (fabrics of group A) in their structure absorb water more than fabrics without this special yarn (fabrics of group B), so fabrics of group B do not provide a property to absorb water better. It is also noticed that the same as for air permeability parameter, quantity of PES yarns with round hollow core filaments also influences water retentivity characteristic of fabrics. Greater the presence of PES yarns with round hollow core filaments in the fabrics, higher is the water retentivity of fabrics (see Figure 11). This special yarn caps not only the air inside the fabric but water fills empty channels of the hollow PES filaments and fabric absorbs more water. However, coating-impregnation of textiles decreases water retentivity very distinctly, i.e. coated-impregnated fabrics do not soak. The water retentivity of group A fabrics after coating decreases approximately by 70%, while that of group B fabrics by 90% (see Figure 10). Also it seen from Figure 11 that parameters of water retentivity of both groups coated fabrics are uniform.

Figure 10.

Water retentivity of knitted fabrics.

Figure 11.

Water retentivity versus quantity of PES yarns with round hollow filaments in knitted fabrics.

Data presented in Figure 12(a) and (b) show that fabrics with PP yarn dry faster than fabrics with PES/stainless steel yarns. This is due to special property of PP yarns to transport moisture and remain dry. Uncoated fabrics of group A become dry after less than 300 min, while fabrics of group B become dry only after 500 min. Coating-impregnation of fabrics increases their drying intensiveness and equalizes drying time of both groups of fabrics: fabrics of group A and of group B become dry almost at the same time, i.e. after 270 min all tested coated-impregnated fabrics became completely dry.

Figure 12.

Drying intensiveness of knitted fabrics.

Electrostatic properties

In order to determine the electrostatic properties of fabrics two types of resistances were measured. The results have showed that these parameters depend on the quantity of conductive yarns in the fabrics. It is known from the literature that fabrics with more than one conductive additive usually have high electromagnetic effectiveness. Conductive materials have different absorption and reflection behaviors: metals are known for having high reflectivity whereas carbon fibers and conductive polymers are known for having high absorption behavior [39]. That is why two types of conductive yarns were used in the manufacture of fabrics. It was noticed that greater the conductive PES with carbon core filament yarns and PES/stainless steel yarns knitted in the fabrics, lesser will be the values of surface and vertical resistances. Values of surface resistance of uncoated fabrics of group A is in the order of magnitude 10¹⁰ Ω, and vertical resistance of 10¹¹ Ω (see Table 9). This is due to the presence of carbon fibers in the fabrics. Surface and vertical resistances of fabrics of group B, where PES/stainless steel yarns are knitted in, are less than 2 × 10³ Ω. Such difference in resistance values of fabrics is because of the resistance of inox, which is in the PES/stainless steel yarns, it is only 0.12 × 10⁻⁶ Ω·m, while of carbon it is 0.5 × 10⁻⁶ Ω·m. So, the electric conductivity of inox is much greater than that of carbon. This can be seen from obtained values of resistance of fabrics. According to the source [40], fabrics applicable for electromagnetic shielding and heating have relatively low resistivity (less than 10³ Ω), while for that of antistatic fabrics 10¹⁰ Ω is sufficient. So, fabrics of group A can be used for antistatic purposes, whereas fabrics of group B can be used for protection against electromagnetic waves. Coating-impregnation increases values of both resistances of fabrics of group A, but the electrostatic property of fabrics is still retained. Although values of resistances of group B fabrics remain lesser than 2 × 10³ Ω, but referring to the results of group A fabrics, we can conclude that values of resistances changes the same as for group A.

Table 9.

Electrostatic properties of fabrics.

Code of fabric	Surface resistance (Ω)		Vertical resistance (Ω)		Shielding factor		Half decay time (s)
Code of fabric	Knitted fabrics	Knitted fabrics with coating- impregnation	Knitted fabrics	Knitted fabrics with coating- impregnation	Knitted fabrics	Knitted fabrics with coating- impregnation	Knitted fabrics	Knitted fabrics with coating- impregnation
A1	8.05 × 10¹⁰	2.86 × 10¹²	1.04 × 10¹²	1.60 × 10¹¹	0.98	0.96	<0.01	<0.01
A2	7.80 × 10¹⁰	2.83 × 10¹²	7.42 × 10¹¹	1.63 × 10¹¹	0.97	0.96	<0.01	<0.01
A3	5.96 × 10¹⁰	1.93 × 10¹²	4.66 × 10¹¹	2.13 × 10¹¹	0.97	0.95	<0.01	<0.01
A4	6.07 × 10¹⁰	2.12 × 10¹²	6.22 × 10¹¹	3.33 × 10¹¹	0.96	0.92	<0.01	<0.01
B1	<2 × 10³	<2 × 10³	<2 × 10³	<2 × 10³	1	1	<0.01	<0.01
B2	<2 × 10³	<2 × 10³	<2 × 10³	<2 × 10³	1	1	<0.01	<0.01
B3	<2 × 10³	<2 × 10³	<2 × 10³	<2 × 10³	1	1	<0.01	<0.01
B4	<2 × 10³	<2 × 10³	<2 × 10³	<2 × 10³	1	1	<0.01	<0.01

The values of shielding factor of all fabrics with and without coating-impregnation are very close or equal to unity and half decay time is less than 0.01 s (see Table 9). This means that all fabrics can be used for manufacturing of protective clothing against electrostatic charge decay. So, electric charges do not accumulate on the surfaces of such fabrics and decays immediately.

Water-proofness and oleofobic properties

All investigated fabrics were coated with microporous foam, which is generally used for open or stretchable fabrics. The finished product is air permeable and, therefore, there is some data which shows that it is not a good technique for waterproofing of fabrics, but is suitable for filtration products. That is why investigated fabrics were impregnated with a FC chemical which helps to repel water and oil, as well as act as a protective barrier for stains and the accumulation of dirt particles. As the foam pores are relatively large (see Figure 2), an impregnation with FC polymer, which is known as water- and oil-repellent finish, was applied to improve the water- and oil-resistant properties.

The waterproofness of fabrics at first was characterized with the parameter of resistance to water, which resulted in a surface wetting grade. It was determined that wetting of the whole of the sprayed surface is seen in all investigated uncoated fabrics. This is because outer and middle layers of all fabrics were composed of the same hydrophobic yarns, which do not repel water. The surface wetting grade of coated-impregnated fabrics increased till grade 3. Coating-impregnation provides better hydrophilic properties to the surface of investigated fabrics, which wets only at small discrete areas of the sprayed surface. So, PU foam coating and especially FC impregnation helped to repel water from surfaces of fabrics. The numerical assessment of surface wetting grade is presented in Table 10.

Table 10.

Properties of resistance to water of investigated fabrics.

Code of fabric	Surface wetting, grade		Resistance to water penetration
	Surface wetting, grade		Hydrostatic pressure, cm of water column
	Knitted fabrics	Knitted fabrics with coating- impregnation	Knitted fabrics	Knitted fabrics with coating- impregnation
A1	1	3	25	32
A2	1	4	26	31
A3	1	3	26	32
A4	1	3	25	32
B1	1	3	25	30
B2	1	3	27	29
B3	1	3	26	29
B4	1	3	26	30

The resistance to water penetration was evaluated by measuring a hydrostatic pressure. It was determined that water penetrates through uncoated fabrics when hydrostatic pressure reaches approximately 26 cm of water column. According to references [17] and [20], these fabrics can be classified more as water repellent or impermeable to water than waterproof, because they permit the passage of water. Coating-impregnation increases resistance to water penetration of knitted fabrics very slightly. It is seen from the data presented in Table 10 that water drops penetrate through the coated-impregnated knitted fabrics, when hydrostatic pressure reached approximately 30 cm of water column. It means that such fabrics will stand only weak rain without moistening. The difference between resistances to water penetration is very inconspicuous. This is due to the microporous PU foam coating (see Figure 2). Although it is generally known that water drop is more than 100 µm in diameter, but the coating is too fine (approximately 0.1–0.2 mm) and pores of coatings are quite big (mean diameter 40 µm), and the diameter increases even more, due to the pressure used in hydrostatic head tester, because knitted fabrics are very stretchable. That is why coated-impregnated fabrics were not completely able to shed the water and do not let it to penetrate through the fabrics.

Apart from waterproofness, the oil repellency of knitted fabrics was also investigated. The data presented in Table 11 shows that all uncoated knitted fabrics do not repel any oil—oil repellency class is equal to 0. But, high porosity PU coating and especially FC resin impregnation give excellent oleofobic properties to knitted fabrics of both A and B groups.

Table 11.

Oil repellency of investigated fabrics.

Code of fabric	Oil repellency, class
Code of fabric	Knitted fabrics	Knitted fabrics with coating-impregnation
A1	0	7
A2	0	5
A3	0	6
A4	0	6
B1	0	6
B2	0	7
B3	0	6
B4	0	6

Conclusions

The barrier and comfort properties of specially designed and muanufactured multifunctional three-layered knitted coated-impregnated and uncoated fabrics were investigated in this work. These fabrics were characterized as having good barrier (antistatic, oil repellency, resistance to surface wetting) and good comfort (water vapor and air permeability) properties.

It was determined that PU microporous coating attaches to the PP much easier than to PES/stainless steel yarns, thus increasing the mass per unit area of knitted fabrics by 39% and 18%, respectively. Coating increased bending rigidity of fabric obviously and fabrics became stiffer, but results of shearing rigidity of investigated fabrics proves that the coating applied is soft and elastic and after deformation fabrics quite easily returns to the initial state.

As it was expected, coating-impregnation decreased parameters of air permeability and water retentivity, but has not showed a significant effect on water vapor permeability. The parameters of water vapor permeability of knitted fabrics after coating-impregnation decreased only approximately by 10%, while water retentivity decreased more obviously by 70% for group A fabrics and 90% for group B fabrics. Coated-impregnated fabrics have not been soaked. At the same time, air permeability decreased 10 and 3 times of knitted fabric for group A and group B, respectively. This was influenced by microporous coating, which partially trapped the path through the coated knits to water, water vapor, and air molecules. The sublime impact on air permeability and water retentivity characteristics showed the quantity of PES yarns with round hollow filaments, because water filled the empty hollow channels and increased absorption of knits, while air molecules where trapped in these channels and increased isolation properties of fabrics.

It was also determined that fabrics with PP yarns dry faster than those with PES/stainless steel, this was affected by PP property to transport moisture and remain dry at the same time. While coating-impregnation of knits increases the drying intensiveness of fabrics by equalized drying time of fabrics.

The electrostatic properties of fabrics depend on the composition of conductive yarns. The resistances are smaller for fabrics with metal particles inside, compared to resistances of fabrics with carbon additives. It was also determined, that coating-impregnation of fabrics decreases values of resistances, but fabrics with metal particles still remains with the values less than 2 × 10³ Ω, which means that they can be used not only for protection from incendiary discharges, but also for electromagnetic interference. Values of shielding factor and half decay time confirm that.

It was also determined that the PU coating and especially FC impregnation improved hydrophobic and oleophobic properties of knitted fabrics very dramatically. Coated fabrics became water and oil repellent, i.e. surface wetting increased from grade 1 till grade 3 and oil repellency increased till class 6, while it was class 0 for uncoated fabrics. Unfortunately, investigated fabrics cannot be considered as waterproof fabrics, because after coating-impregnation of fabrics resistance to water penetration increased only till 30 cm of water column, which is not enough. But, the aim of this research was to develop good quality comfortable multifunctional coated-impregnated knitted fabrics with good barrier properties and this purpose was attained.

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: This work was supported by EUREKA project E!5799 BATAN – Barrier Textile and Nanomaterials.

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The investigation of barrier and comfort properties of multifunctional coated conductive knitted fabrics