Experimental study of structural characteristics, dimensional change in washing, non-creasing properties and air permeability of Swiss double piqué flax knit fabrics

The structural characteristics, dimensional change after four washing cycles, non-creasing properties and air permeability of Swiss double piqué knitted fabrics from cotton/flax (70% cotton, 30% flax), PAN/flax (70% PAN, 30% flax) and 100% flax yarns have been investigated. These knitted fabric samples of varying densities were produced on a 10 -gauge circular knitting machine. The results of investigation showed that the structural characteristics, dimensional change in washing, non-creasing properties and air permeability are highly dependent on the type of yarn used. Besides the type of yarn, the structural characteristics, such as the number of wales W and courses C per centimeter, fabric stitch density S, weight Ws, and fabric tightness K, are also influenced by the average loop length la. The weight Ws and thickness t of knitted fabrics are determined by the diameter of the yarn used. Usually, primary dimensional alterations of knitted fabrics of all yarn types occur after the first and second washing cycles. The non-creasing properties of the examined knitted fabrics are mainly affected by the type of yarn, rather than the average loop length la. Lastly, the air permeability AP of knitted fabrics is affected by the type of yarn, the average loop length la, and fabric density S.


Introduction
The growing demand for eco-friendly and natural fibers in the textile industry provides significant opportunities for the increase of flax fiber usage.Flax is a great fiber for producing textile materials for warm weather as it has numerous advantages, such as: high hygroscopicity and absorbency, specific handles, high protection against UV radiation, perfect conditions for skin comfort, good thermal and optimal electrostatic properties, lack of an allergenic effect and high strength. 1,2However, flax fiber has disadvantages, such as low elasticity, which can be avoided by the right choice of the combination of fabric structures. 2 Some research into flax has focused on blending flax with other fibers.The production of blended yarn made from flax and other textile fibers is the largest potential market for using flax fibers in the textile industry.The stiffness of flax fibers influences the blend ratio of flax and other fibers in the textile materials.The suitable blend ratio of flax with other fibers in yarn is 20:80.Increasing the proportions of flax fibers in blended yarn creates a tendency to crease and feel prickly against the skin. 3The qualitative and statistical analysis of the cotton-flax blended yarn with different blend ratios (100:0, 80:20, 70:30, 45:45, 0:100) showed that 100% cotton yarn possessed the highest evenness quality, in contrast to the poorest result of 100% flax yarn.The 100% flax yarn has the highest strength and the lowest elongation.Evenness properties and elongation deteriorated with the increase of the flax content in the yarn, and at the same time the strength and tenacity of the yarn decreased.The blended cotton/flax yarn with the ratio of 80:20 proved to have optimal yarn properties. 4The cotton/flax yarn with the ratio of 70:30 and 80:20 has better yarn properties compared to cotton/flax yarn with the ratio of 60:40. 5The analysis of the effects of different blending ratios of flax and cotton fibers and production parameters of spinning of the yarn showed that physical yarn properties were affected negatively with the increase of the ratio of flax fibers and the effects were found statistically significant. 6The investigation of the technology of flax-containing yarn production showed that for the reduction of yarn unevenness and better stability of the spinning process, it is recommended to use flax fiber with a low linear density and limiting the flax percentage in the blend to 30%. 7The cotton/flax blends improve some of the properties, such as moisture wicking, air permeability and durability of the textile materials.Pure cotton sport clothes can make the skin feel clammy during exercise, since it does dry slowly.The use of cotton/flax blends for production of sport clothes improve moisture wicking and air permeability, which allows them to dry more quickly. 8,9nit fabrics made from flax yarn are a great choice for warm-weather clothing.They feel cool in the summer and appear crisp and fresh even in hot weather.The investigation of the quality of four plain single jersey weft knitted fabrics of different structural characteristics (stitch density, weight, and thickness) made from flax yarn revealed that the fabric with the lowest structural characteristic values has the highest compressibility, thickness loss, and air permeability.However, it exhibited the least compressive resilience, water retention, bursting strength, and ball traverse elongation, both before and after pilling. 10he structural characteristics, dimensional stability and air permeability of the knitted fabrics produced from different kinds of yarn in general have been the subject of investigations by many research workers.The results of these investigations showed the dependence of structural characteristics, dimensional stability and air permeability of the knitted fabrics on the type of the knitted structure, type and diameter of yarn, loop length, stages of relaxation and other factors.Early investigations analyze the effects of changes in knit structures and density of knitted fabrics, mainly concentrated on the influence of different kind of fabric structures (in general plain, 1 × 1 rib, half Milano and Milano rib, miss and tuck stitches made mostly from cotton and wool yarns), loop length, knitting parameters, type of relaxation and other factors, on the dimensional properties of knitted fabrics.Some of them have been concentrated on the main properties of knit fabrics made by combining 1 × 1 rib and plain stitches (e.g.half Milano and Milano rib).
The geometry and dimensional properties of plain knitted fabrics made from wool and cotton yarns in the relaxed state are determined by the loop length of yarn. 11The physical and mechanical properties of single jersey knitted fabrics made from cotton yarn are dependent on the loop length of the yarn. 12Experimental studies of the dimensional properties of half Milano and Milano rib fabrics with wool and acrylic yarns found that with an increase in loop length, there was a linear regression with a high correlation coefficient for both course-spacing and wale-spacing. 13The results of geometrical models for presser-foot knitted 1 × 1 ribs, interlock and half Milano rib knitted fabrics showed that the run-in ratio between the plain course and the rib course of half Milano rib fabrics is an important parameter for obtaining a stress-free structure. 14he effect of tightness on the main parameters was discussed through the geometrical model to predict the main properties of conventionally knitted 1 × 1 rib wool fabrics. 15It is worth pointing out that the model can be applied to 1 × 1 rib fabrics of different yarn types.However, some researchers suggest that the model for prediction of main properties is not universally applicable since complete relaxation is practically difficult to achieve.Knits with miss stitches (half Milano rib and single piqué) have better dimensional stability than fabrics with only knit stitches. 16he effect of the type of knit structure (1 × 1 and 2 × 2 rib, Milano rib, Lacoste, half cardigan and others) from 80% lambswool and 20% wool yarn on the dimensional properties inspected is highly significant. 17The shape of loop of the wool, silk, cotton/viscose/rayon, regular acrylic, and double-ply wool plain knitted structures are dependent on the yarn's physical properties, mechanical processing and knitting variables. 18The knit density constant and fabric thickness are proved to be independent from loop length and dependent only on yarn diameter in the fully relaxed state. 19Geometric characteristics such as fabric thickness and bulk density are significantly dependent on fabric tightness. 20The dimensional properties of the single jersey, 1 × 1 rib, 1 × 1 interlock, single piqué, and two-thread fleece knitted fabrics are significantly influenced by the yarn types used during knitting.
The investigations showed that the dimensional properties of knitted fabrics made from 100% cotton and cotton/ elastane yarns (95% cotton, 5% elastane), such as the loop length, wales and courses per centimeter, stitch density, tightness factor, loop shape factor and take-up rate, are significantly influenced by the presence of an elastane yarn.The loop length of single jersey, 1 × 1 rib, and interlock knitted fabrics made from elastane yarns was reduced, while in single pique and fleece it was increased. 21The investigation of the geometric and dimensional properties of double piqué knit fabrics from cotton elastomeric core spun yarn showed that the changes in loop length had a significant impact. 224][25][26] The type of knit structure, such as knit loop, tuck loop and float loop stitches in single-jersey cotton knitted fabrics significantly influences the fabric's drape ability, width-wise extensibility, shrinkage, thickness, areal density and low-stress mechanical properties. 27he dimensional properties of the single-jersey weft jacquard structure are determined by the width repeat of the single float stitch. 280][31] The knit structures with tuck loop stitches have a higher degree of shrinkage compared to knit structures with float or miss loop stitches. 32,33he use of tuck stitches resulted in increased areal density, pilling resistance, drape coefficient, and dimensional stability.However, it did not have any impact on the color fastness. 34The number of tuck knit stitches in the repeat of the double knit structures has a significant impact on the structural parameters and deformation properties of the knitted fabrics from half wool yarn 35 and elastomeric yarn. 36The studies of the influence of the type of double stitches on the shielding ability of knitted fabrics made from cotton yarn showed that the half Milano rib knitted fabric has a higher shielding ability than other types of structures. 37The lengthwise dimensional stability of knitted fabrics is determined by the frequency of washing, treatment methods and the width repeat of single float stitches. 38The structural characteristics of piqué knitted fabrics have an influence on the dimensional changes that occur during washing. 39he dimensional stability of knitted fabrics during wear and washing is a crucial end-use characteristic that significantly impacts their quality.Many investigations have concluded that the dimensional properties of knitted fabrics depend on a variety of factors, including the kind and characteristics of yarn, the type of knit structure and its structural features, the type of relaxation and its schedule, the type of drying etc. Investigation into the influence of yarn type on the dimensional changes of knitted fabrics revealed that hydrophilic yarns, such as cotton, silk and rayon, experienced the greatest dimensional changes after their first wetting.Subsequent wettings also caused additional changes in dimension.In contrast, the dimensional changes of woolen knitted fabrics exhibited a different trend. 40Yarn type and type of fiber blending have a significant impact on the dimensional change of cotton and cotton/polyester weft-knitted fabrics. 41imensional and physical properties of single knit fabrics depend on yarn count. 42In addition to yarn count, the stitch length of the yarn is also one of the most important factors that significantly influences the tightness factor, dimensional stability (shrinkage), pilling, and abrasion of 1 × 1 rib knit fabrics made from cotton 12,43 and wool plain knitted fabrics. 44Fabric dimensions and yarn properties influence the states of relaxation of the plain weft-knitted structures. 457][48][49] Investigation of the influence of the laundering procedure and the composition of knitted fabrics showed that knitted fabrics from blended ramie-cellulosic yarn have better dimensional stability than knitted fabrics from pure cotton and rayon yarns. 50The principal structural features of single-jersey knitted fabrics are dependent on the full wet-processing cycle, which comprises of scouring, bleaching, enzyme treatment, dyeing, washing off and finishing with a softener.Upon completion of the full wet-processing process, the number of wales and courses per centimeter, fabric stitch density and fabric weight increase, whereas the length of the yarn decreases. 51he physical and mechanical properties of 1x1 rib, 2x2 rib, half-cardigan, and full-cardigan knitted fabrics made from multifilament glass yarn and aramid yarn depend on the knit structure and type of yarn used. 52The dimensions of interlock and Swiss double-piqué knitted fabrics made from wool yarn in a relaxed state depend on type of yarn, fabric and machine variables.However, their dimensional properties in the fully-relaxed state are largely independent of these variables and can be predicted by constants that are specific for each structure.Additionally, the interlock structure exhibits anisotropic behavior, meaning a length shrinkage is usually accompanied by a width expansion, while the Swiss double-piqué structure behaves isotropically, meaning it experiences length and width shrinkages concurrently.It is hypothesized that the difference in dimensional behavior between these structures is caused by the distinct, non-relaxed geometric shapes of the structural units that form these two double-knit structures. 53ir permeability of knitted fabrics depends on yarn type, yarn linear density 54 and geometrical parameters of knitted fabrics. 55The thermal comfort properties, such as thermal resistance, thermal conductivity, air permeability and water-vapor transmission of single jersey knitted fabrics are determined by the type of yarn used.Fabrics made from 100% cotton and blended cotton/polyester (80/20 and 60/40) yarns exhibit varying degrees of thermal comfort properties. 56The air permeability of cotton single jersey, 1 × 1 and 2 × 1 rib knitted fabrics decreased disproportionally to the mass of fabrics.Rib fabrics had higher air permeability values compared to single jersey fabrics. 57espite this, few papers had been submitted on the topic of dimensional properties of combined knit structures, which are made as a combination of four courses (e.g.different kinds of double piqué) knitted from pure and blended flax yarn.The flax fibers have unique hygienical, physic-mechanical and physic-chemical properties which allowed them to be used in a wide range of uses for millennia. 58But the use of pure flax yarn in knitting production has not reached a wide application because of the irregularity of the yarn diameter, impurity, small elongation etc. Blended flax with high linear density must be mainly processed on flat knitting machines of a low gage number.Knitting blended flax yarn on circular machines of a medium gage is limited by the linear density of yarn as well as by the type of the fabric's structure.
It is known that by using knit structures that include areas of single and double stitches, it is possible to achieve significant savings in raw materials thanks to the lower density of the fabric and the presence of float loops instead of some knit loops.At the same time, the thermal properties which characterize products produced using double stitches can be achieved. 23The analysis of the properties of four-row double piqué showed that the presence of rows of incomplete single stitches reduces the stretchability of the fabric in width and increases its dimensional stability.Compared to French pique, Swiss piqué knit fabrics exhibits a less pronounced diagonal effect, as noted in the publication. 23

The subject of investigation
In this study, the structural characteristics, dimensional stability, non-creasing properties and air permeability of Swiss double piqué knit fabrics from cotton/flax, PAN/flax and pure flax yarns have been investigated experimentally.
The investigation of the previously mentioned properties of combined knitted structures manufactured on a circular knitting machine, from blended flax and pure flax yarn, contribute to widen the range of flax yarn application as well as to expand the assortments of the knitting goods for end-uses.Additionally, using combined knitting structures may provide an advantage in cost reduction.The geometrical and main properties of combined knitted fabrics are dependent on the combination of the different types of knit stitches in the structures repeat in the height. 23,59It is well-known that using knitted fabric combined from single and double knitted stitches leads to the reduction of stich density, owing the reduction to the presence of floats instead of loops, and reduction of raw material consumption during production, without losing the thermal properties which characterized fabrics with double knitted stitches.For these reasons we chose to investigate dimensional behavior of double Swiss piqué knitted fabrics based on flax yarn, despite combined knitted fabrics with ribs loops having less elasticity compared to the rib knitting structure. 60

Knit fabrics
Swiss double piqué knitted structures (Figure 1) are made by combining four courses: rib with a 2 × 1 repeat on the first and third feeders and single float stitch on the second and fourth feeders.As such, the technical face and back sides of Swiss double piqué knitted fabric are different.As it can be seen from the Figure 1, single float stitch loops on the second feeder are produced by the needle of the dial (D), that is by the needle producing 2 × 1 rib stitch loops on the first feeders.The 2 × 1 rib stitch loops and single float stitch loops on the third and fourth feeders are produced by the needles of the dial (D) which were out of work on the first and the second feeders.That way, the 2 × 1 rib loops and single float loops, produced by the first and second feeders, would fill up by 2 × 1 rib loops and single float loops which had been produced by the third and fourth feeders.
The samples of Swiss double piqué knit fabrics were knitted on a 10 -gauge circular knitting machine from three types of yarn: blended cotton/flax (70% cotton, 30% flax), blended PAN/flax (70% PAN, 30% flax) and pure flax (100% flax).The machine was set to knit at a low speed and only four feeders were used.The loop-forming positions of the cylinder and dial needles are measured as the distance in needles between the two stitch cam knock-over points.In this study, the distance between the cylinder and the dial stitch cams, the yarn tension and take-down were kept constant.For the first and third feeders that knit 2 × 1 rib, the cylinder stitch cam position was aligned with the position of the dial stitch in those same feeders.Similarly, the position of the dial stitch cam in the second and fourth feeders, which knit single float stitch, was set to the same position as the dial stitch cams in the first and third feeders.In the final step, the position of the cylinder and dial stitch cams in all feeders was adjusted individually and simultaneously to ensure they were aligned.This was accomplished by turning the stitch cam regulator clockwise by the same amount, causing the cams to descend uniformly.For feeders 1-4, the starting position of the stitch cams was set to the upward position, which guarantees problem-free knitting for all yarn types.The stitch cam accuracy after the change was checked using the same runner-length.Тhe stitch cams for the cylinder and dial needles in all feeders were set to the same position, in order to prevent yarn breakage in the knitting process, especially when using the blended and pure flax yarn. 61

Determination of the dimensional and mechanical properties of the yarn
After being under a standard atmosphere according to ISO 139:2005 62 the following parameters were measured:

•
• The linear density according to the ISO 2060:2012. 63he presented results represent an average of 20 measurements per each type of yarn.• • Experimental diameter of yarn.The technique for determining the experimental diameter of the yarn is as follows: at least 3 m of yarn is wound from the package before testing.The yarn is wound onto a cardboard A4 paper with a small interval in between each turn, for a total of 10 turns.Afterward, 5 points at equal distances are marked on each turn of the yarn.The cardboard with wound yarn is then placed under a microscope, and while magnifying it (seven times), the diameter of the yarn is measured. 64The presented results represent an average of 50 measurements per each type of yarn.• • The twist in yarns according to the ISO 2061:2016. 65he presented results represent an average of 20 measurements per each type of yarn.• • The strength (breaking force and tenacity) and breaking elongation according to the ISO 2062:2012 66 and ISO 6939:1988. 67The presented results represent an average of 50 measurements per each type of yarn.

Determination of the structural characteristics of the knit fabrics
The structural characteristics of the knitted fabrics after washing were analyzed through: 1.The number of wales W and courses C per centimeter, which were determined according to the EN 14971:2006 and GOST 8846-87. 68,69The measurements were taken over a span of 10 cm and subsequently extrapolated to represent the count per 1 cm.The presented results represent an average of 10 different places per sample for the technical face and back of fabrics.2. The loop length for the single float stitch l 1 and 2 × 1 rib l 2 was measured in millimeters for each feeder's knit stitches.For the single float stitch, measurements were taken in feeders 2 and 4, while for the 2 × 1 rib, measurements were taken in feeders 1 and 3.The loop length was calculated as the average of all measurements within each feeder, divided by the number of loops formed by the feeder in the designated area, in accordance with the standards GOST 8846-87 14970. 69In the case of the double knit structure, the designated area consisted of 50 wales (50 × 2 needles) as per GOST 8846-87. 69The presented results represent an average of 20 measurements for both the loop length of the single float stitch l 1 and the 2 × 1 rib l 2 .3. The weight W s in the g/m 2 determined as an average from the mass of five samples, each having an area 200 cm 2 according to the EN 12127:1997. 70.The fabric thickness t in mm determined as an average from the thickness at 10 different places on every sample according to the ISO 5084:1996. 71he fabric thickness tests were carried out on the Thickness Gage SM-124 device at a pressure of 2.5 N/cm 2 (25 kPa) with test surface area 1 cm 2 .
After the measurements of the dimensional values had been submitted, the fabric stitch density S, the average length of yarn l a in the mm, run-in ratios l 1 /l 2 and fabric tightness K have been calculated.The fabric stitch density S in loops/cm 2 was determined using the following equation ( 1): The average length of yarn l a in mm was calculated as average length of yarn in unit repeat, according to the equation ( 2) in accordance with the GOST 8846-87 standards. 69The calculation procedure is outlined as follows: where l 1 -the loop length for single float stitch in mm, l 2 -the loop length for 2 × 1 rib in mm, n 1 and n 2 represent the number of loops in unit repeat in % for single float stitch and 2 × 1 rib, respectively (in this case, n 1 = 25%, n 2 = 75%).
The fabric tightness K was calculated according to the equation (3) as follows: where T -the linear density of the yarn in tex, l a -the average length of yarn in mm.

Determination of the dimensional change after washing the knit fabrics
The dimensional change of the knitted fabrics during washing DC was analyzed through the percentage of change in length DC l and width DC w .After being knitted, the fabrics were for several days laid out on a flat surface under a standard atmosphere to facilitate recovery from the stress imposed by knitting and were washed in a household fully automatic washing machine with a cotton program at 30 o C containing 3 g/l of an efficient wetting agent.After the washing cycle, the fabrics were laid out, with minimum stress, on a flat surface under a standard atmosphere for at least 24 h.The samples were exposed to four washing treatments (washing relaxations WR) according to the ISO 6330:2021 72 and ISO 5077:2007. 73he mean percentage of dimensional change (shrinkage "-" or expansion "+") in length DC l and width DC w is calculated according to the equation (4): where "original measurement" is measurement before first washing.
The presented results represent an average of five measurements per variant of fabrics in both directions (in the length and width).

Determination of non-creasing properties of the knit fabrics
Non-creasing properties of materials are characterized by the non-creasing coefficient, which quantitatively characterizes the degree of influence of the linear dimensions of the knitted fabric after removing the deforming crushing load and an appropriate rest period.The non-creasing properties of knitted fabrics were measured on technical face and determined according DSTU 2995-95. 74he device for the determination of the non-creasing properties is presented on Figure 2, a. Sample preparation consists of the following: two strips for each direction (length and width) measuring 160x120 mm ±1 mm are sewn with a stitch with a frequency of 2-3 stitches per 10 mm according to GOST 12807-2003. 75The total size of the sample should be (320 mm ±1 mm) x (120 mm ±1 mm) (Figure 2(b)).Measurements are carried out in three places of the sample (1-1, 2-2, 3-3) according to the Figure 2(b).
After being conditioned to a standard atmosphere according to ISO 139:2005, 62 the sample is placed on cylinder 2 so that the edge of the sample is in contact with the base 3. Several weights are then placed on the flange of the hollow rigid cylinder 1 so that the total mass of the flange 3 and the weights is 10.0 kg ± 0.1 kg, in accordance with GOST 7328-2001. 76 The sample is then removed from the cylinder by wrapping it, without deforming it, and clamps are attached at points C and D. The mass of the clamps is 10 g.Using one pair of clamps, the sample is placed on a flat surface, after which the clamps are removed.The sample is then left to rest for a period of 10 min ±1 min.After that, the distance between the marks 1-1, 2-2 and 3-3 is measured.The measured change in width of the sample h i due to crushing in mm is calculated as the arithmetic value of these three distances according to Figure 2(b).
The non-creasing properties coefficient CR in both directions (in the length CR l and width CR w ) is calculated according to the equation ( 5): where h o -the distance between the marks along the vertical axis of the sample, mm (h o = 100 mm); h i -the distance between the marks along the horizontal axis of the sample, mm (h i = 150 mm); n -number of elementary samples.For each fabric, five measurements were recorded for both directions (of the crease resistance values in length CR l and width CR w ).

Determination of the air permeability of the knit fabrics
The comfort properties of the knitted fabrics were assessed by analyzing the air permeability AP of the knitted fabrics on the technical face, in accordance with the ISO 9237:1995 standard. 77Air permeability tests were carried out on the Tester III device in the laboratory of the Institute of Natural Fibers (Poland) at a pressure drop of 100 Pa with the test surface area of 20 cm2 .The presented results represent an average of five measurements per each variant of fabrics.

The dimensional and mechanical properties of the yarn
The knitting process of the Swiss double piqué knit fabrics depends significantly on the physical and mechanical properties of the yarn, above all: smoothness (evenness), breaking elongation and strength (breaking force and tenacity).During processing on knitting machines and the use of the products, the yarn undergoes the influence of various forces, which can lead to its breakage.Therefore, to characterize the ability of yarn to withstand tensile loads without destruction, indicators of breaking force and breaking elongation are introduced. 64he experimental value of the properties of the yarn are presented in Table 1.
The cotton/flax yarn (type I) was produced using the rotor spinning system, while the PAN/flax yarn (type II) was produced using the ring spinning system in accordance with GOST 32086-2013 for blended yarn intended for knitwear production. 78The pure flax yarn (type III) was produced using the wet spinning process from middle length flax fiber, in compliance with GOST 10078-85. 79t is apparent from Table 1 that all types of yarn are characterized by high irregularity (CV) of the experimental yarn diameter d e .Pure flax yarn is characterized by high strength unevenness, which leads to higher breakage during the process of knitting, higher resource consumption and a decrease in equipment productivity.The pure flax yarn, as opposed to cotton/flax and PAN/flax yarn, is characterized by less breaking elongation E and higher tenacity P u .However, high irregularity (CV) of the twist T, tenacity P u and breaking elongations E of the pure flax yarn cause difficulties during knitting operations.
Despite the approximately equal tenacity Р u of cotton/ flax and PAN/flax yarn, the breaking elongation E of investigated yarns differs significantly.The greatest breaking elongation E is found in PAN/flax yarn, which is 3,7 times higher than the breaking elongation E of cotton/flax yarn.This is because the breaking elongation E is affected by the composition of the yarn.It is known that the PAN fibers have a high breaking elongation E (12.0%-35.0%)and flax fibers have a low breaking elongation (1.5%-2.5%). 80Thus, the introduction of 30% flax fibers into the yarn composition reduces the breaking elongation E of PAN/flax yarn but, despite this, PAN/flax yarn has the greatest breaking elongation E (11.24%) compared to cotton/flax (5.43%) and pure flax (2.05%) yarns.Additionally, it is worth mentioning that pure flax yarn has the lowest breaking elongation compared to blended flax yarns, as mentioned in. 4 Overall, the breaking elongation of yarns is influenced by their composition, with PAN/flax yarns demonstrating the highest breaking elongation due to the characteristics of PAN fibers.The addition of flax fibers reduces the breaking elongation, but the PAN/flax yarn still outperforms the other yarn types mentioned.
The holding loops, under the influence of force, increase the loop height by robbing the yarn from the floats and legs of adjacent loops of the corresponding courses, subsequently increasing the length of the yarn itself . 61Based on the findings of a study 61 on circular knitting machines it is recommended to use twisted yarn to prevent brake up (failure) of fabrics causing all the knit courses to be discarded, which is caused by a disruption of the yarn during the knitting process of the fabrics.However, as mentioned in the work, 61 using a combined knitted structure can help reduce the rate of yarn breakage when using a single yarn.Furthermore, the utilization of twice-piled yarn can mitigate this issue.As it could be seen from the Table 1, the blended flax yarn is characterized by unequal tensile strength, which could be the cause of the disruption of the yarn during knitting process on a multi feed circular machine.Preliminary studies on the knitting process of the French piqué structure revealed challenges compared to the Swiss piqué structure.This was an additional reason for choosing the Swiss double piqué structure for investigation, as it is widely used in various upper garments.As previously mentioned in the introduction, the Swiss piqué structure demonstrates a less pronounced diagonal effect compared to French piqué, as discussed in the research conducted by. 23

The structural characteristics of the knit fabrics
In this section, we analyze the influence of three different yarn types (cotton/flax, PAN/flax, and pure flax) used to produce Swiss double piqué knit fabrics on the fabrics' structural parameters.By utilizing yarns for production of knitted fabrics of varying loop lengths, we explore how this affects the overall structure of the knitted fabric.
The number of wales W and courses C, stitch density S, average loop length of yarn l a , weight W s , and fabric thickness t are all considered to be the most important fabric structural parameters, as they dictate the behavior of the fabric during wearing and washing.The structure of the technical face and the technical back sides of the Swiss double piqué knit fabrics from cotton/flax, PAN/flax and pure flax yarns at 20× magnification are presented in the Figure 3.
The results of measurements of structural characteristics of Swiss double piqué knit fabrics after four washing treatments are presented in the Table 2 and Figure 4.
The results in Table 2, Figures 4 and 5 demonstrate a clear dependence of the fabrics' structural parameters on the type of yarn used.This tendency is corroborated in paper. 21he fabrics made from cotton/flax and PAN/flax yarns have a similar range of the number of wales W and courses C (Figure 4).Fabrics made from pure flax yarn typically have fewer wales W and courses C compared to fabrics knitted from cotton/flax and PAN/flax yarns.This can be attributed to the larger average length of yarn and lower elasticity of    pure flax yarn, as discussed in reference. 2Since the elastic deformation is taking place during the process of bending the yarn into loops, the yarn bended into the loop tends to straighten into the maximum possible loop yarn length and occupy the maximum loop surface.That is, as a rule, a characteristic of the cotton/flax and PAN/flax yarn.The elastic elongation of the yarn has a big influence on the loop form. 18y processing yarn with higher elastic elongation, it is possible to obtain knitted fabrics with smaller loop sizes compared to the loop size of the yarn with low elastic elongation.As such, the number of wales W and courses C and other dimensional characteristics may differ depending on the types of the yarn, as outlined in articles. 18,19The number of courses per 1 cm C of fabrics is 1.2-1.5 times greater than the number of wales per 1 cm W of knitted fabrics made from all types of yarns (Table 2), as illustrated in Figure 5 which can be explained by the structure of the Swiss double piqué knit fabrics.The relation between the wales W and courses C, weight W s and average loop length l a of the cotton/flax, PAN/flax and pure flax yarns, for Swiss double piqué knitted fabrics, are presented in Figure 5.
After the analysis of the investigated results (Table 2, Figures 4 and 5), it can be concluded that the structural characteristics of Swiss double piqué knitted fabrics made of cotton/flax, PAN/flax and pure flax yarns, in the scope of this research, depend on the type of yarn and the length of loop yarn.The works 11,12,43 mention the influence of both the type of yarn and the length of loop yarn on the structural characteristics.In this case (Table 2, Figures 4  and 5) the wales W, courses C, fabric density S and weight W s , of knitted fabrics are linearly related to the average loop length l a , except for the number of wales W for pure flax yarn (Figures 4(a) and 5(c)).The same trend is discussed in the works. 14,51s can be seen from the data in Table 2 , the average loop length l a of knitted fabric from all types of yarn increases, resulting in decreases in the respective structure parameters.Notably, the change in the structure parameters of knitted fabrics from cotton/flax yarn is the most significant, due to its greatest range of change in average loop length l a .Conversely, the lowest range of change in average loop length l a of knitted fabrics from pure linen yarn is accompanied by the least considerable decrease in the structure parameters of this knitted fabric.
The results given in Table 2 and Figure 4(c), indicated that the weight W s and thickness t of knitted fabrics depends on the type of yarn and yarn diameter (Table 1) as discussed in work. 42The knitted fabrics made from pure flax yarn possess the thinnest thickness t (1.14-1.39mm), which can be attributed to its smallest experimental yarn diameter d e of 0.30 mm, resulting in the lowest weight W s (310-328 g/m 2 ).The knitted fabrics from PAN/flax yarn have the highest thickness t (1.96-2.08 mm) and weight W s (442-517 g/m 2 ), which can be attributed to their highest experimental yarn diameter d e of 0.56 mm.

The dimensional change in washing of the knit fabrics
The results of investigations of the dimensional change DC of the Swiss double piqué knitted fabrics from cotton/ flax, PAN/flax and pure flax yarns after four washing treatments (shrinkage "-" or expansion '+') are presented in Table 3 The relation between dimensional change in length DC l, , width DC w and the number of washing treatments for Swiss double piqué knit fabrics from cotton/flax (a), PAN/ flax (b) and pure flax (c) yarn is presented in Figures 6 and  7 .
The analysis of the investigated results showed that the dimensional change DC after washing treatment of Swiss double piqué knitted fabrics made from cotton/flax, PAN/ flax and pure flax yarns, within the confines of this study, depends on the type of yarn, the length of loop of yarn and the number of washing treatments as shown in research. 53y comparing the results obtained for the knitted fabrics from cotton/flax, PAN/flax and pure flax yarns (Table 3    and Figure 6 and 7 ), it could be concluded that the type of yarn influences both the level and trend of dimensional changes in both directions.The dimensional change in both directions of knitted fabrics from the PAN/flax and pure flax yarn follow the same trend -they behave isotropically, meaning it experiences length and width shrinkages concurrently, as mentioned in the paper. 53he dimensional change of the cotton/flax yarn knitted fabrics variants 1-5 show isotropic behavior, meaning they experience concurrent length and width shrinkages.Conversely, fabric variants 6-7 exhibit anisotropic behavior, where a length shrinkage is usually accompanied by a width expansion.This difference in dimensional behavior can be explained by the influence of type of yarn and loop length on the relaxation process.In this case, the results (Table 3, Figure 6  The non-creasing properties of the knit fabrics The non-creasing properties are one of the indicators of physical and mechanical properties of knit fabrics, which depend on the type of yarn, geometric characteristics, processing and the manner of exploitation.It is also a significant characteristic of the appearance of the knit fabric or product.The study of non-creasing properties are of great practical importance, since low non-creasing properties of textile materials diminish the appearance of the final product, distort the shape given to it and accelerate the wear of the material along the folds.The non-creasing properties are a characteristic of the elasticity of a knitted fabric, which is the property of the latter to resist the formation of unwanted creases during exploitation under the influence of crushing loads.
A study was conducted to investigate the effect of yarn type and knitted fabric structure parameters on the noncreasing properties of knitted fabrics.The results of measurements of non-creasing properties in the length CR l and width CR w of Swiss double piqué knit fabrics after washing are presented in the Figure 8 .
The results of the research revealed that the non-creasing properties coefficient of the studied knitted fabrics is mainly dependent on the type of yarn, rather than the loop length of the yarn.Knitted fabrics produced with pure flax yarn had the lowest non-creasing properties coefficient in both length and width, with values of 80%-89% and 88%-91%, respectively.The non-creasing properties coefficient of knitted fabrics along the length of cotton/flax and PAN/ flax yarns were in the same range, at 91%-95%.For knitted fabrics in width from cotton/flax yarn, the non-creasing properties coefficient ranged from 88%-95%, while the non-creasing properties coefficient of knitted fabrics in width from PAN/flax yarn had a narrower range of 93%-94%.

The air permeability of the knit fabrics
Air permeability is an important property of knit fabrics which is frequently requested by retail buyers.The air permeability of a knit fabric is a measure of how well it allows the passage of air through it.The air permeability of knitted fabrics depends on the type of yarn, type of knit structure, stitch density and other factors.
The results of investigations of the air permeability AP of the knitted fabrics from blended flax and pure flax yarns are presented in Figure 9 .
The air permeability AP of Swiss double piqué knitted fabrics made of cotton/flax, PAN/flax and pure flax yarns, in the scope of this research, depends on the type of yarn, the average loop length l a , fabric tightness K, and stitch density S as previously reported in. 54,55In this case (Figure 9 ) the air permeability AP of knitted fabrics from all types of yarn is linearly related to the average loop length l a , fabric tightness K, stitch density S and weight of fabrics W s , which was also concluded in work. 59With the increase of the average loop length values l a of cotton/flax yarn by 25%, the fabric tightness K decreases by 20.2%, the fabric stitch density S and weight of fabrics W s decrease (by the same level of 25%), which results in an increace of air permeabilily AP by 2.5 times.Reducing the variations in the average yarn length l a , and as result the fabric tightness K, fabric stitch density S and weight W s , of PAN/flax and pure flax knitted fabrics leads to less fluctuation in their air permeability AP.With the increase of the average loop length l a of PAN/flax yarn by 14%, the fabric tightness K decreases by 12.4%, the fabric stitch density S and weight of fabrics W s decrease (by practically the same level of 15%) and as a result the air permeability AP increases 1.6 times.With the increase of the average loop length l a of pure flax yarn by 11%, the fabric tightness K decreases by 11.2%, the fabric stitch density S and weight of fabrics W s decrease by the 7% and air permeability AP increases 1.4 times.As such, the fabrics with the largest average loop length, lowest fabric tightness and fabric density value have the highest air permeability, as mentioned in the work. 10he knit fabric from pure flax yarn has the highest air permeability AP in comparison to fabrics from blended (cotton/flax and PAN/flax) yarns.This can be attributed to the thinner structure of the knitted fabric (thickness) and the experimental diameter of the yarn d e , as discussed in the experimental section regarding the parameters of the knitwear structure (Table 2).In contrast, the influence of the thickness of the knit fabric and the experimental diameter d e of cotton/flax and PAN/flax yarns demonstrate a different trend.

Conclusion
This study provides valuable results concerning the structural parameters, dimensional changes during washing, non-creasing properties, and air permeability of Swiss double piqué knitted fabrics made from cotton/flax (70% cotton, 30% flax), PAN/flax (70% PAN, 30% flax), and pure flax (100% flax) yarns.The analysis of the obtained research results allowed us to draw the following conclusions: The dimensional change in both directions of knitted fabrics from the PAN/flax and pure flax yarns is isotropic, meaning that they experience concurrent length and width shrinkages, as mentioned in the paper. 53The dimensional change of the cotton/flax yarn knitted fabrics has a different trend, which can be explained by the influence of the type of yarn and loop length on the relaxation process.• The findings of the study indicated that the noncreasing characteristics coefficient of the examined knitted fabrics is mainly influenced by the type of yarn, rather than the average loop length l a .• • The air permeability AP of knitted fabrics is affected by the type of yarn, the average loop length l a , the fabric tightness K and stitch density S as was previously indicated. 54,55The air permeability AP of knitted fabrics from different types of yarns is directly proportional to the average loop length l a , the fabric tightness K, stitch density S and weight of fabrics W s .The knit fabric from pure flax yarn has the highest air permeability AP when compared to fabrics from blended (cotton/flax and PAN/flax) yarns.This can be explained by the thinner fabric and the experimental diameter of the yarn.On the other hand, the influence of the thickness of the knit fabric and the experimental diameter of cotton/flax and PAN/flax yarns shows a different trend.
The results of this investigation proved possibilities of using pure flax and blended flax yarn in the manufacturing process of knitted fabrics on circular knitting machines of a specified gage.As far as pure flax yarn is concerned, attention should be devoted to the properties such as fineness, smoothness, and uniformity.

Declaration of conflicting interests
The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figure 1 .
Figure 1.The graphical representation (a) and knitting notation (b) of the Swiss double piqué knit structures.
After 40 min ±1.0 min, the weights and flange are removed.Without removing the sample from the cylinder, clamps are attached at points A and B (Figure 2(b)) to secure the sample.

Figure 2 .
Figure 2. Device for determination of non-creasing properties (a) and elemental sample of the material (b).

Figure 3 .
Figure 3.The structure of technical face and technical back sides of the Swiss double piqué fabrics from cotton/flax (a and b), PAN/flax (c and d) and pure flax (e and f) yarns at 20× magnification.

Figure 4 .
Figure 4.The wales W, courses C, the loop length for the single float stitch l 1 and 2 × 1 rib l 2, average loop length l a and weight W s for Swiss double piqué knit fabrics from cotton/flax I (a), PAN/flax II (b) and pure flax III (c) yarns.

Figure 5 .
Figure 5. Relation between wales W, courses C, weight W s and average loop length l a for Swiss double piqué knit fabrics from cotton/flax (a), PAN/flax (b) and pure flax (c) yarns.

Figure 6 .
Figure 6.Relation between dimensional change in the length DC l and the number of washing treatments for Swiss double piqué knit fabrics from cotton/flax (a), PAN/flax (b) and pure flax (c) yarn.

Figure 7 .
Figure 7. Relation between dimensional change in the width DC w and the number of washing treatments for Swiss double piqué knit fabrics from cotton/flax (a), PAN/flax (b) and pure flax (c) yarn.
and 7 ) demonstrate that: • • The main dimensional change of knitted fabrics of all yarn types occurs during the first and the second washing treatments.The findings of the study were corroborated by research. 40• • The knitted fabrics from PAN/flax yarn experienced the least dimensional change in length DC l (shrinkage range is 1.83%-4.67%)and width DC w (shrinkage range is 2.83%-8.29%).This can be explained by the effect of the composition of the yarn on the dimensional change because the PAN fibers practically do not swell.The more significant dimensional change occurred in knit fabrics made from cotton/flax and pure flax yarns in both directions because of the great swelling of these fibers in comparison to PAN fibers.• • With the increase of average loop length values l a , the dimensional change in length DC l increased, but the dimensional change in the width DC w decreased for all type of yarns.The length of the loop is one of the main factors influencing the dimensional change of garments during washing, as discussed in the pap ers. 11,12,43,44

Figure 8 .
Figure 8.The non-creasing properties in the length CR l and width CR w of the of the Swiss double piqué knit fabrics from cotton/ flax, PAN/flax and pure flax yarn.

••
The dimensional and mechanical properties of the investigated types of yarn are influenced by the composition of the yarn.The pure flax yarn, in contrast to cotton/flax and PAN/flax yarn, is characterized by a lower breaking elongation E and higher tenacity P u .Despite the approximately similar tenacity P u of cotton/flax and PAN/flax yarn, the breaking elongation E of the investigated yarns differs considerably.• • The structural characteristics of Swiss double piqué knitted fabrics, in the scope of this research, depend on the type of yarn and the average loop length l a .With the increase of the average loop length l a of knitted fabric from all types of yarns, the investigated structural characteristics such as the number of wales W and courses C, fabric density S, weight W s , and fabric tightness K decrease.Notably, the change in the structure parameters of the knitted fabric from cotton/flax yarn is the most significant due to its greatest range of change in average loop length l a .Conversely, the lowest range of change in average loop length l a of knitted fabric from pure linen yarn is accompanied by the least considerable decrease in the structure parameters of this knitted fabric.• • The weight W s and thickness t of knitted fabrics depend on the type and experimental diameter of the yarn used.Knitted fabrics made from pure flax yarn possess the thinnest thickness t, which is due to its smallest experimental yarn diameter d e , resulting in the lowest weight W s .Conversely, knitted fabrics made from PAN/flax yarn have the highest thickness t and weight W s , which can be attributed to their largest experimental yarn diameter d e .• • The level and trend of the dimensional change in washing of knitted fabrics from cotton/flax, PAN/ flax, and pure flax yarns depend on the type of yarn.

Figure 9 .
Figure 9. Relation between the air permeability AP and average loop length l a , the air permeability AP and fabric tightness K for Swiss double piqué knit fabrics from cotton/flax (a), PAN/flax (b) and pure flax (c) yarns.

Table 1 .
The experimental value of the dimensional and mechanical properties of the yarn.

Table 2 .
The experimental value of structural characteristics of Swiss double piqué knit fabrics.
Type of yarns Variants of fabric W, loops/cm C, loops/cm S, loops/ cm 2 l 1 , mm l 2 , mm l 1 /l 2 l a , mm W s , g/m 2 t, mm K

Table 3 .
The values of dimensional change in the length DC l and in the width DC w of Swiss double piqué knit fabrics after four washing treatments.
• • The primary dimensional alteration of knitted fabrics of all yarn types usually occurs during the first and second washing treatments.The results of the study were confirmed by another research. 40