Three-layer knitted materials for protective clothing

The results of investigating multifunctional 3D knitted materials dedicated for protective clothing were presented. The 3D design structures were made on a circular knitting machine using yarns with flame retardant or electrostatic properties. The functionality imparted to each of the assortments developed was verified during the tests in accredited laboratories as well as by assessing their biophysical properties. Based on the analysis of the test results, a beneficial effect of the raw materials and the 3D structure of knitted fabrics were demonstrated. Designed garments could be useful as individual protection clothing for workers exposed to harmful occupational environment factors, such as heat and static electricity. The study was conducted within the project EUREKA E! 5799 BATAN “Multifunctional knitted fabrics with barrier properties for clothing”.


1.
Introduction The problems of human health and life protection in work environment are a current subject of studies in many research centers. One of them is the development of protective clothing and search for new technological and structural solutions in the processes of their manufacture. Textiles designed for protective clothing gain their desired properties by using proper textile raw materials, designed structures and special finishing treatments.
for the producers of textiles, whose production is connected with fulfilling many specified requirements [6]. A significant position within a wide range of protective clothing assortments is occupied by textiles protecting against heat at elevated temperature and static electricity as well as those that meet the requirements of flame-retardant fabrics. An expected feature is also their antibacterial capacity. In this connection, consumers, in relation to the protective clothing offered, point out that their functional properties should comprise multi-functional capabilities including the physiological comport of use. The issue of multi-functionality of fabrics designed for protective clothing has been a subject of many research works at the Textile Research Institute that pays much attention to fabrics made by the technique of knitting [7][8][9].
In recent years, studies have been carried out on multi-functional knitted fabrics with a multi-layer structure designed for protective clothing or elements of protective, flame-retardant and antistatic clothing. This paper presents selected results of these studies concerning weft-knitted fabrics with a three-layer structure.

Technological tests
The aim of the research undertaken was to develop new assortments of knitted fabrics designed for clothing with barrier properties against elevated temperature and static electricity, showing also biological activity concerning antibacterial action. Moreover, the studies were aimed at the multifunctionality of knitted fabrics, including features of improved physiological comfort and an aesthetic appearance expected by users. In order to obtain the multi-functionality of knitted fabrics consisting in imparting to them particular properties, the following tasks were performed:  Flame-retardant and antistatic properties of fabric were obtained by using for their production yarns of raw materials with the required features.  To obtain antibacterial features, a chemical agent containing nanoparticles of metallic silver was use in the finishing processes of fabrics.  To obtain the physiological comfort required with the given barrier properties, there were designed three-layer, weft knitted structures as fabrics with LR stitches (two layers) combined by a middle layer formed of a flame-retardant monofilament yarn. Designing the structures of knitted fabrics, there were used, in particular layers, such types of yarns to obtain the internal layers (in contact with user's skin) of clothing with good properties of physiological comfort [9]. A rib knitting machine, type Ovja from Mayer Company, modernized at the Textile Research Institute, was selected to produce the knitted fabrics to be tested. Its technical parameters were adapted for the production of three-layer knitted fabrics with a set zone of distance. The modernization of the knitting machine constitutes the subject of an improvement application (submitted in 2012) and a part of Patent Application No. P 406 791, 2014, to the Patent Office of the Polish Republic; "Distance 3D weftknitted fabric and a process and system of its making". The invention consists in that the stitch system designed uses the technique of weft-knitted fabrics of three-layer structures, in which the layers are combined by means of tucking links of monofilament yarn, located between the needles of both beds forming the distance space [10]. In this connection, there were designed three-layer 3D distance knitted fabrics, including a technological program of producing samples of these knitted fabric with the following compositions: Sample 1. Metaaramid + metaaramid/viscose FR + monofil PES Sample 2. Metaaramid + viscose FR + monofil PES Sample 3. Metaaramid + Rezistat* + metaaramid/viscose FR + monofil PES */Rezistattrade name of blended yarn: Vsc Lenzing + PES/carbon compounds Sample 4. Metaaramid + modakryl Protex/cotton + monofil PES Sample 5. Metaaramid + viscose FR + monofil PES Sample 6. Metaaramid /antistatic + metaaramid + wool/viscose FR + monofil PES. The knitted fabrics were made with the use of combined stitch that resulted in a spatially developed structure owing to the application of the stitch system in the following way: the top layer is formed by means of even RL stitch or RL plated stitch and the under surface by means of RL stitch. Both layers were combined with a monofilament yarn using a half cardigan stitches. The use of monofilament yarn to combine the layers caused the formation of spacing between the fabric layersa distance of about 3 mm, while the linksyarn sections between the layers formed the middle layer. The preservation of distance in knitted fabrics depends, among others, on the thickness of monofilament yarn and the way of its inserting between the layers. This is essential due to the need for providing fabric elasticity fabrics. In the preparation of the sample, two methods of setting the tucking links were used: in samples 1, 2, 3 and 4, a skew roof setting of the monofilament yarn was used, while in Samples 5 and 6an alternating skew setting in particular fabric courses was applied. Each of the three layers of particular fabric sample was made of different raw material. The top layers were made of aramid (meta-aramid) yarn and antistatic yarn (Rezistat, meta-aramid/antistatic), while the bottom layerswith a predominance of natural and man-made yarns (meta-aramid/viscose FR, viscose FR, modacryl Protex/cotton, wool/viscose FR). In all the samples, the distance layer was made of PES monofilament yarn. It was assumed that the three-layer knitted fabrics designed in the way mentioned above will show particular biophysical properties creating the effect of micro-climate around the user's body wearing the clothing made their use. In view of expected good thermo-regulating properties, the three-layers knitted fabrics have been planned to use for warming outerwear, such as waistcoats and warming elements in outerwear. The linear density of the most multi-filament yarns used ranged from 20 to 25 tex, while that of monofilament yarn was 100 dtex. Table 2 presents the percentage content of particular raw materials in the fabric samples made. The finishing treatment of the three-layer knitted fabrics was carried out according to the technological parameters of finishing knitted fabrics made of synthetic (aramid yarns) and natural and man-made yarns (cotton, wool, viscose yarns) in the following finishing stages: washing, neutralization, rinsing, centrifuging, strip reversing, fabric splitting along needle set-out, drying and stabilization.  Preliminary stabilization was added to the finishing process as the first phase.  In the finishing of knitted fabrics no softening agents were used. In order to impart antibacterial properties to the fabrics, their selected assortments were treated with biologically active agents in the finishing process, using an antibacterial agent Ruco Bac AGP (Sample 4) or an agent containing nanoparticles of metallic silver (Sample 5).  As a result of the assessment of the knitting and finishing processes followed by the organoleptic evaluation of the fabric samples, especially with respect to the fabric spatial structure, Samples 3, 4, 5 and 6 were selected for the tests confirming the expected functional properties.

Assessment of the functionality of three-layer knitted fabrics
The assessment of functionality of the knitted fabrics under investigation was carried out with respect to their structural and physico-mechanical properties as well as their physiological comfort and functional barrier features concerning flammability, static electricity and biological activity [9]. The following parameters characterizing the fabric physical and functional properties were tested ( For the knitted fabrics, whose raw material system was designed to include antistatic yarns (Samples 3 and 6), electrostatic properties were determined, assessing the following parameters (Table 8)     Assessment: the knitted fabrics tested fulfill the requirements for the resistance to the action of thermal radiation at an effectiveness level of C2. Assessment: the knitted fabrics tested fulfill the required resistance to heat penetration with the action of flame at an effectiveness level of B2.   Assessment: sample 4 shows bacteriostatic and bactericidal properties against bacteria Staphylococcus aureus, while against bacteria Escherichia coli this fabric has no biologically active properties. In the case of sample 5, its bacteriostatic and bactericidal properties were confirmed against both microorganisms.