Study of Elastic Warp Knitted Bands: Production and Properties Študija elastičnih snutkovnih pletenih trakov: izdelava in lastnosti

Elastic ﬁ tted goods are identiﬁ ed as a separate group of medical textiles. This group includes elastic bandages, abdominal binders, posture correctors, corsets, recliners, etc. Elastic knitted bands are widely used in rehabilitation and prophylactic goods. This research studied the properties of elastic warp knitted bands that were made on an 18E gauge crochet machine. In order to reduce the product weight and to increase its comfort, a partial set (2-in/1-out) of elastomeric threads is used. This yarn is the main component of elastic fabric that aﬀ ects stretch properties and end-use. In the warp knitted band, the polyurethane thread is usually used as longitudinal inlay yarn, which is located between the loop’s overlap and underlap. In order to study the eﬀ ect of polyurethane pre-elongation before knitting on band properties, seven pairs of gears were used and elongation was varied from 280% to 395%. The weft ﬁ lling yarn connects the separate wales into the band. To prevent contact between polyurethane threads and the human body, the weft yarns were laid on both sides of the inlay yarn. The movements of weft guides were in opposite directions. In order to study the eﬀ ect of weft yarn diameter on cover factor and bands properties, 2 ends, 4 ends or 6 ends of 16.7 tex polyester yarn were used to achieve corresponding overall linear densities of 33.4 tex, 66.8 tex and 100.2 tex. It was concluded that the partial drawing-in of the guide bar with polyurethane thread facilitated a reduction of up to 20% in the basis weight of the elastic band, while ensuring suﬃ cient stretch properties. The impact of technological factors on the structural parameters and properties of the elastic band was established.


Introduction
Compression garments are designed to provide fi xed pressure to the human body. Such products are eff ective functional means in both therapy and the prevention of a number of diseases: varicose veins, the consequences of burns, post-surgery and post-traumatic edema, etc [1]. Th ere are a number of requirements for compression garments and for materials for their production [2]. Th e two main requirements are: the stability of the product and the specifi ed level of compression during use, as well as the guarantee of the product's comfort for consumers for the duration of use. Th e necessary pressure on the human body is provided by fabric properties such as stretchability and elasticity, and by the product's construction: size and shape [3]. Th e elasticity of a knitted fabric is ensured by the incorporation of elastomeric thread [4] or corespun yarn with elastane core into the knitted structure as the fi lling yarn that is laid in the stretching direction [5]. High residual deformation and a signifi cant change in linear dimensions aft er washing aff ect a product's size and the fabric structure. Th is also negatively aff ects the compression properties of products. Elastic material contours to the human body and accumulates residual deformation in the most curved parts when a compression garment is used. Th us, unlike static loading, there is an increase in the part of residual deformations in certain areas of compression clothing and a change in the fabric structure with a corresponding increase in the stretchability of the material. Th is leads to a change in the properties and the deterioration of the product's appearance. Th e main factor in the changing shape and size of clothing, including compression garments, is thus the accumulation of cyclic residual deformation, as well as a change in the stitch density due to a change in the fabric thickness [6]. Scientists around the world are studying the structural parameters [7,8] and properties [9] of elastic knitted fabric, in particular mechanical characteristics such as deformations [10], stretchability [11] and elasticity [12]. Th is indicate great interest in the problem and its relevance. Th e results of such studies can be used in the development and manufacture of new materials with improved properties [13]. When manufacturing clothes from elastic materials that fi t tightly to the body, patterns are usually made smaller than ones from ordinary materials. At the same time, there is an important requirement to maintain conditions for normal blood circulation and other physiological processes in the human body. Th e maximum permissible pressure on the human body should not exceed 1330−2000 Pa [6]. At the section where clothes are tightly fi tted to the body, the pressure level is directly proportional to the stress (σ) in the stretched fabric and inversely proportional to the radius of cross-section curvature (R) [14]. Th us, under the same load, the pressure of the fabrics with diff erent elongation is diff erent [15]. Garment pressure on the human body depends on the stresses that arise during fabric stretching. Th us, the study of the pressure of elastic fabric revealed its dependence on the knitting parameters and conditions of the product's use [16]. As a result of the two-factor experiment, it was determined that fabric pressure on the human body depends on the pre-elongation of the elastomer fi lament, and on the fabric elongation and surface curvature. Another study [17] attempted to investigate the infl uence of the inlay-yarn insertion density into a knitted structure and the area of a rigid element integrated into a knitted orthopaedic support on a compression generated by that support. It was concluded that the lower inlay-yarn insertion density and its total amount can be used for orthopaedic supports of lower compression class. Th e design of compression products is usually based on an analysis of the experimental dependence of the distributed load (or voltage) on the relative deformation obtained, usually, at a constant rate of the premera votkovne preje na faktor kritja in lastnosti trakov je bil preučen z združevanjem 2, 4 ali 6 poliestrskih prej dolžinske mase 16 [13]. Th erefore, the majority of studies on elastic materials involve the determination of their deformation properties using stretching diagrams, and using tests based on the load-unloading-relaxation cycle. An indirect approach for measuring pressure from a set of compression bandages and hosiery was developed by Cassandra Kwon et al. [18], from which rigidity (EI) values were determined, and tension-elongation curves and pressure-elongation data were calculated. Th e calculated pressure values were compared with PicoPress sensor readings measured on 10 participants. Results showed that the correlation between both approaches varied among bandage and hosiery samples. However, during the use of compression products, the pressure on the body is not constant and decreases gradually to some equilibrium value. Th e authors [19] predicted the deformation properties of knitted fabric on the basis of a generalised Maxwell model. It was characterised by two average terms of relaxation and allowed the stress relaxation processes to be reliably simulated. Moreover, it allowed the dependence of the equilibrium stress component on the deformation to be predicted. Th e proposed method only needs the stress relaxation curve, which signifi cantly reduces the test time. Ferdinand Tamoue et al. [20] concluded that the prediction of an applied pressure according to the modifi ed Young-Laplace equation is realistic for both cotton-based and elastomer-based bandages. Th e main new fi ndings were the utilisation of the specimen's stretched length for the prediction of the interface pressure in the modifi ed equation, in contrast to the equation commonly found in literature, which uses the circumference of a randomly picked human subject's ankle to predict the pressure. From the above-mentioned literature, the deformation properties of elastic textile materials are mostly determined on the one-cycle study by loading-unloading-relaxation [21]. As a result, the full deformation of the fabric and its components can be obtained, as well as the contents of elastic, plastic and residual deformations. Th ere are several methods for determining the deformation characteristics of textile materials, which diff er by the duration and conditions of the studies. Th e analysis of test methods of stretch properties of elastic fabric [22] allowed us to formulate recommendations on the efficiency of each.
Th e comfort characteristics of fabrics (particularly thermal insulation and permeability properties) are closely associated with changes in their structural parameters [23][24][25][26]. Th e evaluation of the air permeability of knitted fabrics containing elastane fi bre applies both the standard method and a new approach based on fabric thickness measurement at diff erent pressures [27]. Test results have shown that the air permeability of textile depends on their structure, fi bre composition and porosity evaluated with regards to fabric thickness diff erence measured at diff erent pressures [28]. Th e compressive behaviour of knitted elastic fabric aff ects excellence in comfort. It was found [29] that the stitch density (loop size) has a signifi cant eff ect on the compressive load. On the other hand, almost all research work examines elastic weft knitted fabrics, while only a few of them present study results of warp knitted fabric. Th e crocheting technique is widely used for elastic band production, but it is not suffi ciently represented in scientifi c literature. Knitted fabric with elastomeric thread in each wale is usually used for rehabilitation products. It provides a high level of elasticity of the material and increases its density at the same time. Th e permeability of elastic knitted fabric can be increased by reducing the number of elastomeric yarns in its structure by not laying it in every course of weft knit and in every wale of a warp knit. However, this can lead to a decrease in elasticity and resilience. Th us, the purpose of this work was to study the structure and properties of elastic warp knitted bands with the partial threading of the guide bar by elastomeric threads.

Sample production
All samples were produced on an 18E gauge LB-5000A crochet knitting machine made by Taiwan Giu Chun Ind. Co. Ltd. It was equipped with a heddle bar and 3-roller feeder for elastic threads or rubber. Th e pillar stitch with closed loops (G1) from 16.7 tex polyester yarn was the ground interlooping of the studied fabrics ( Figure 1). An elastomeric thread with a diameter of 0.8 mm was introduced into the knit structure as a longitudinal inlay yarn (G3). In order to reduce the product weight and to increase its permeability, the elastomeric fi laments were drawn according to the repeat: 2 in + 1 out, while 16.7 tex polyester yarn was also used as weft inlay yarn (G2 and G4). It was laid on both sides of fabric to ensure the connection of chains in the fabric and to cover the elastomer. To determine the eff ect of the weft in laid yarn thickness on the properties of the fabric and reliable covering of the elastomeric fi lament in the structure, 2 ends, 4 ends or 6 ends of the polyester yarn were used to achieve the resultant 33.4 tex, 66.8 tex and 100.2 tex weft yarn respectively (X1). Th e parameters of the knitted structure and properties of the knitted fabric with elastomeric threads typically depend on its content. Th e elastomeric content can be limited by both the laying repeat and the degree of pre-elongation before entering the knitting zone [11,30]. Th e preliminary elongation of the elastomeric fi laments on the crochet machine is ensured by the ratio of the speed of the shaft s' rotation in the feeding zone ( Figure 2). In this study, it was varied by the number of gear teeth: leading z 1 -27, 29, 31 and driven z 2 -21, 23, 25, resulting in seven levels of the pre-elongation (X2) of the elastomeric fi laments ( Table 1) Figure 3) showed a good convergence, which confi rms their accuracy.
Th e studies of the coverage degree of the elastomeric threads by transverse weft threads were carried out by taking a photo of knitted samples at diff erent elongation levels. A specimen was fi xed in the clamps of a tensile testing machine; the camera was located to fi x the middle part of the specimen. Stretching of the samples to a certain elongation (10%, 20%, 30% ... 100% was carried out at a constant speed (50 mm/min) of the lower clamp. Th e machine was stopped and a photo was taken.

Results and discussion
Th e structural parameters of elastic warp knitted bands are presented in Table 2 and in the graphs in Figures 4 and 5. It was observed that all studied knitted fabrics had two similar interdependent parameters: the number of wales per 100 mm, which was 74, and the length of the weft in-laying yarn per stitch of fabric, whose average value was 1.39 mm. For warp knitted fabric, these parameters mainly depend on the distance between needles, i.e. on the knitting machine gauge. Since all the samples were made on the same equipment, the values were unchanged. Th e thickness of the knitted fabric was a function of the interlooping, as well as the number and diameter of the threads that were used for its production. Th us, in this study, it only depended on the linear density (33.4 tex, 66.8 tex or 100.2 tex) of the weft inlay yarn. Th e band thickness yields a mean value of 1.35 mm, 1.40 mm and 1.43 mm respectively. Th e results also showed that the preliminary elongation of the elastomeric fi lament (X2) signifi cantly affected its length per stitch (Figure 4.a). When preelongation was increased from 280% to 395%, the length of the elastomer thread per stitch decreased by 10%, regardless of the linear density of the transverse weft yarn. It should be noted that the parameter's value for warp knitted bands with a 33.4 tex weft yarn was 10% less than for the corresponding fabrics with 66.8 tex and 100.2 tex weft yarns. Th e X2 increase also led to some reduction of the length of the ground pillar stitch (Figure 4.b). Th is can be explained by the change in the stresses in the draw-off zone because the pulling load is the determining parameter of the loop length on a warp knitting machine. In this case, the trend was more pronounced for knitted fabric with 33.4 tex weft yarn where the observed value decreased by 10%, while the value decreased by only 5% for warp knitted bands with 66.8 tex weft yarn and was practically constant for bands with 100.2 tex weft yarn. Th ese observations were infl uenced by the increasing contact area between the weft and the elastomer yarns arising from the increase in the frictional forces, which aff ected the degree of elastomer relaxation in the knitted structure. Th e number of courses per 100 mm is an indicator that determines the fabric density vertically and depends on the loop height inversely, and therefore on the elastomeric thread length per stitch. Th e eff ects of the structural parameters are shown in Figure 5a. Th e index increased by increasing elastomeric thread pre-elongation and was larger for bands with 33.4 tex polyester as weft inlay yarn. Th is means that reducing the linear density of the weft threads reduced the number and size of its contact zones with the elastomeric fi laments, which contributed to the elastomer shrinkage in the knitted structure and the increase in the stitch density.   Th e mass per unit area of the fabric determines material consumption and the weight of the fi nished product. Th e developed warp knitted bands contain elastomeric threads that were laid according to repeat, which facilitated a reduction in their basic weight by 20% compared to the fabric with a full set of elastomeric threads [35].  showed a good convergence, which confi rms their accuracy. Th e results of full deformation and its components calculations are presented in Table 3. As a result, it was established that the full deformation of the elastic warp knitted band (Figure 6a) was from 115% to 140%, which facilitates their use in medical binders and other support products. Th e full deformation of the investigated variants was directly proportional to the pre-elongation of the elastomeric fi laments. Increasing the linear density of the weft a) Figure 6: Eff ect of pre-elongation ε of elastomeric threads on stretch properties: a) full deformation and b) elastic deformation contribution b) Its value increased with the pre-elongation level of the elastomeric fi lament (Figure 6b), which confi rmed the conclusions made by the authors in a previous study [22]: increasing the pre-elongation of elastomeric yarn leads to an increase in the yarn strain. As a result, the relaxation processes in the fabric structure are faster. Th e knitted band with 33,4 tex weft threads demonstrated the smallest level of elastic deformation. It should be noted that the residual deformation of the elastic warp knitted bands was insignifi cant (did not exceed 1.7%) and therefore will not aff ect the quality of the medical and prophylactic products for which this elastic fabric is designed. It is obvious that the residual component of full deformation was near zero for the elastic band with 100.2 tex weft threads. When using elastomeric yarn without any wrapping, the comfort of the fabric may be degraded. An elastomeric yarn should not be placed at the surface of the knitted structure as in the initial state as well as in a stretched state. Studies of the coverage degree of the elastomeric threads by transverse weft threads were carried out by taking a photo of knitted samples at diff erent elongation levels (Table 4).
A specimen was fi xed in the clamps of the tensile testing machine; the camera was located to fi x the middle part of the specimen. Samples were stretched to a certain elongation (10%, 20%, 30% ... 100%) at a constant speed (50 mm/minute) of the lower clamp. Th e machine was stopped and a photo was taken. Obviously, at the initial state (elongation 0%), the transverse weft threads completely covered the elastomer, preventing it from reaching the surface in all samples. For samples with a 33.4 tex transverse weft thread, the elastomer was visible even at 20% elongation. For samples with a 100.2 tex, the transverse weft thread visibility of the elastomer was observed at 60% or higher elongation.

Conclusion
An elastic warp knitted band for use as a fi xing element in rehabilitation and prophylactic products has been developed. It is proposed that the elastic thread should not be inlaid in every wale and the guide bar threaded according to repeat 2: 1 in order to reduce the material consumption and product's weight. Th is results in a 20% reduction in the mass per unit area of the warp knitted band, while maintaining relaxation characteristics within the relevant requirements for rehabilitation and prophylactic products. Based on the two-factor experiment planned and conducted in the work, the following was concluded: the linear density of the weft yarn (X1) aff ected the thickness, vertical density and surface density of the knitted material, and, to a lesser extent, the content of the elastic component in full deformation; and the pre-elongation of the elastomeric threads be-fore the knitting zone had a signifi cant eff ect on the vast majority of the investigated properties: an increase in pre-elongation from 280% to 395% which led to an increase in the number of courses per 100 mm by 15-27%, mass per unit area by 7-10% and full deformation and its elastic component by 15%, and a decrease in the length of the elastomer fi lament per stitch by 10%, as well as the residual component of the full deformation. From the result of our studies, it was found that the use of 100.2 tex transverse weft threads guarantees full coverage of the elastomer within the elastic band's elongation of up to 60%. Despite the fact that there is 15-20% saving in material consumption and that fabrics satisfy the elasticity indices when using 33.4 tex transverse weft thread, their use is not recommended since even with an elongation of 20% there is the possibility of elastomeric threads making contact with the human body.