Structural parameters and physical properties
Relevant structural parameters, namely loop length, courses/cm, wales/cm etc. were recorded after conditioning the samples. All the parameters were measured and mentioned in Table 5.
Effect of stitch parameters on fabric areal density
Areal density and thickness were measured and compared on the basis of yarn fineness, structural variation and dimensional stability as shown in Fig. 1.
In the samples developed, areal density/GSM is affected by yarn fineness, relaxation stage and stitch type of knitted fabrics. The yarn linear density (yarn count) has direct relationship with fabric areal density. The areal density of knitted fabric for same type of fabric decreases as the yarn becomes finer and increases as the yarn becomes coarser. As yarn becomes finer, the number of fibers per cross-section decrease and thus the weight decreases. Areal density of wet relaxed /bleached fabrics are higher than dry relaxed fabric similar to other researchers (Uyanik et al., 2016). The reason behind this is the fabric shrinkage after washing which increases the loop density. Due to increased loop density, more number of stitches accumulate in a particular area that ultimately increases the areal weight of the fabric. The variation in areal density of different knit structures for the same yarn fineness is also observed. This is due to the variation in number/percentage of tuck stitches (NTS), ratio of tuck to knit stitches (RTKS), and location/positioning of tuck stitches (LTS). Pattern also has some effect, but in present study all structures have same pattern i.e. zigzag (Tyagi et al., 2009). Figure 1 shows that all the fabrics having tuck stitches have higher weight compared to single jersey knitted fabric due to accumulation of yarn at tucking places. DP samples have higher percentage of tuck stitches i.e. 50% as compared to 33% in DL samples. However, DP samples show lower areal density. This can be explained on the basis of LTS. DL has no plain courses as in DP, although both of them have 2 successive tucks in the wales. The courses with tuck stitches squeeze between the plain courses in the lengthwise direction. Thus there is an increase of courses/cm in Lacoste samples which leads to increase in areal density. But for SP and SL this is not followed, as percentage of tuck stitches is double in SP as compared to SL i.e. (50% and 25% respectively). The tuck stitches do not get squeezed in these samples. Both the SP and DP samples have same percentage of knit and tuck stitch i.e. 50%. But DP has higher areal density than SP because the ratio of tuck to knit stitches (RTKS) is different. DP has 2 tuck stitches per 2 knit stitches, while in SP this ratio is 1 tuck stitch per 2 knit stitches. This ultimately results in higher areal density in DP than SP. The combination of knit and tuck loops is the best option to increase the areal density of knitted fabrics.
Under real wearing conditions, as the knitted apparel fits the body, there can be substantial changes in areal density. It might increase or decrease depending upon the multiaxial stretch and resulting compression. The structure of knit is responsible to determine the level of deformation and corresponding change of areal density. Once a knitted structure has been stretched in use, a fabric should contract or recover to its original dimension. It was noticed that, when the structure has tuck stitch, there is a significant increase in width-wise extensibility values, but a significant decrease in length-wise extensibility values. The results show that single jersey structure has higher extensibility and recovery value as compared to its derivatives. This is due to the geometry or shape of the knitted loop and its arrangement, which makes the fabric highly extensible and recoverable, than tuck loop structure. Comparatively, single pique has higher width-wise extensibility and recovery values but lower length-wise extensibility and recovery value. This is due to the fact that, tuck loops reduce fabric length and length-wise elasticity because of the higher yarn tension on the tuck and held loops that cause them to rob yarn from adjacent knitted loops, making them smaller. Therefore, the fabric width is increased because of tuck loops pulling the held loops downward, causing them to spread outwards and make extra yarn available for width-wise extensibility and recovery. Single jersey derivatives with tuck loop are also thicker than plain single jersey due to tucked yarn over the held loop, which increases fabric thickness than a normal knitted loop (Assefa & Govindan, 2020).
Effect of stitch parameters on fabric thickness
Figure 2 shows a comparison of thickness for all the knitted samples with effect of yarn fineness, relaxation stage and stitch type. As yarn becomes finer, thickness decreases due to decrease of number of fibers/cross section.
As the fabric samples undergo wet treatment, thickness increased due to increase of stitch density. Because of the increase in stitch density higher weight and thickness are obtained for all fabric samples. This effect is not very significant for thickness, as the fabric samples are compressed and lose their bulk/porosity during the finishing process.
The thickness of DL fabric sample is higher than that of all other fabrics. It is followed by DP, SP, SL and SJ respectively as is the trend for areal density. Lowest value of thickness is observed for SJ fabric. This shows that thickness of fabric increases with increasing the tuck stitch. The tucked yarn over the held loop increases fabric thickness as compared to a normal knitted loop.
Fabric samples with tuck loop are thicker than fabric having knit loop due to accumulation of yarn at the tucking place in a repeat. The percentage of knit and tuck loops in the structure of knitted fabric has significant effect on thickness. If there are 100% knit loops then yarn accumulation will be minimum and loop lies on two dimensions i.e. in length and width directions only. This means that no yarn will lie in the third direction and as a result the thickness of fabric will be reduced. But as the percentage of tuck stitch increases in combination with knit stitch, the fabric thickness also increases. If we check the thickness value of DL and DP fabrics, it can be seen that DL has higher value of thickness as compared to DP, because of positioning/location (LTS) of tuck stitches (LTS). Similar trends are observed as for the areal density in other structures. In real life condition, as the knitted apparel fits the wearer body, there can be decrease in thickness. Based on the resistance to mechanical deformation, the corresponding changes in thickness will occur. Among the knitted structures, single jersey structure is mechanically less resistant as compared to tuck-knit combinations. Therefore, the decrease of thickness can be higher with respect to undeformed state.
Effect of stitch parameters on knitted fabric comfort properties
Air permeability of knitted single jersey and derivatives
Air permeability is very sensitive indicator for a fabric comfort. It is ability of fabric to allow air/wind to pass through it. It can affect apparel fabric performance in many ways. It defines fabric properties like protection against wind, keeping warm, rate of quickest heat loss etc. (Haghi, 2004; Vigneswaran et al., 2009). Air permeability depends on a number of factors like material properties, dimensional properties and finishing treatment. An in-depth study of the structural factors, that influence air permeability, assumes that air flow occurs through the spaces in between the yarn. Thus, the pores between adjacent yarns contribute to pore volume of fabric. The total airflow is also affected by pores enclosed within the yarn. Air permeability depends upon tightness factors, but in our study tightness factor was kept constant i.e. approximately 14. Comparative analysis of air permeability is shown in Fig. 3.
It can be observed from Fig. 3a, as yarn becomes finer, it results in increase in air permeability, which is due to a smaller number of fibers/cross section. In finer yarns, higher fabric porosity occurs due to low packing factor. Air permeability increases as pores between loops become bigger, which results in higher air transmission. In case of coarser yarns, there are smaller spaces between yarns as well as less air space within yarn which causes lower permeability of air. Air permeability increases for the fabrics made from finer yarns as expected in present study. Finer yarns have lower hairiness values which may be another factor for higher air permeability. In this study, combed yarns were used which show much lower hairiness responsible to block the pores and restrict airflow.
From Fig. 3b it is clear that SJ fabrics show lowest airflow permeability. With all knit stitches, the fabric becomes compact and the spaces between loops reduce, making the fabric air resistant. When tuck stitches are introduced in the fabric structure, it becomes more open and porous. SP structure has maximum NTS, thus it exhibits highest air permeability value followed by DL and SL. The inter-yarn pore is the most important parameter influencing the openness of the fabric structure. The fabric pore chartertics and their distribution in knitted fabric affect the air permeability. In Lacoste structures, there are courses of knit structure which squeeze the tuck stitches. Thus the pore size decreases which leads to lower air permeability as compared to SP knits which have an effect of LTS. Although DP has higher percentage of tuck stitches like SP, but DP has ratio of 2 tuck stitches to 2 knit stitches (RTKS) i.e. double overlapping of yarn in double tuck loops, while in SP it is 1 to 1. Therefore, more accumulation of yarn occurs and ultimately results in higher thickness, which leads to reduction of air permeability. In the current analysis the major factor which affects the air permeability in the loop type. The combination of alternate tuck and knit stitch is the best option for higher air permeability. Under real wearing conditions, as the knitted apparel fits the body, there can be variations in areal density and thickness. These changes will certainly influence the resultant air permeability for the apparel in real use. In most cases, the stretching would cause increase of inter loop spaces and thus an increase of air permeability.
Thermal resistance/insulation of knitted single jersey and derivatives
Thermal insulation is the ability of a material to resist heat flow through it. With a low thermal resistance a gradual loss of heat energy results in cold feeling. The term thermal resistance, refers to value of fabric thermal insulation and it is inversely proportional to thermal conductivity. The thermal resistance is dependent upon thermal resistance of fiber and resistance offered by air trapped within the fabric structure. Effect of yarn linear density and knitted structures on thermal resistance of fabrics was investigated in the present study. Results are shown in Fig. 4.
As shown in Fig. 4a, coarser yarns result in higher thermal resistance due to increase in total pore volume which leads to increase air pockets in the knitted fabric structure. The thermal conductivity of stagnant air is 0.025 Wm−1 K−1 which is much lower than any fiber material. The amount of stagnant air pockets influences the overall resistance of fabrics. When hairiness increases, more air is entrapped in air pores resulting in increased value of thermal insulation.
Thermal properties are influenced by microscopic, mesoscopic and macroscopic porosity. Fabrics from tuck-knit stitch combinations showed higher thermal resistance values than the fabrics from 100% knit stitches. DL fabric exhibits a higher value than the others due to its higher thickness. It is decreased in the order DP, SP, SL and SJ, respectively. The plain courses in DL squeeze tucks between them which changes the pore size due to LTS. So the amount of stagnant air pockets increases which very much influences the overall thermal resistance of the fabric. But in SL structures, pore size increases due to RTKS. In DP structure, due to double tuck loop or in other words due to RTKS, more accumulation of yarn leads to higher thickness and thus a higher thermal resistance. The thermal resistance is highly correlated with thickness in present study as shown in Fig. 6. Similar observations are reported by other researchers in a dry fabric which entirely depends on thickness and, to a lesser extent, on fabric construction and fiber conductivity (Arumugam et al., 2017; Venkataraman et al., 2015a, 2015b).
Under real wearing conditions, as the knitted apparel fits the body, there can be variations in areal density and thickness. These changes will certainly influence the resultant thermal properties. The thickness decrease will result in reduction of thermal insulation. However, the extent of decrease will further depend on corresponding change of areal density. Among the knitted structures, single jersey structure is mechanically less resistant as compared to tuck-knit combinations. Therefore, the thermal insulation will also reduce substantially.
Relative water vapor permeability (RWVP%) of knitted single jersey and derivatives
It is the term used for ability to transfer water vapors through fabric to the external environment which defines transfer of sweat from wearer’s body to environment through fabric. It is a percentage of vapor transfer through the fabric in comparison with equivalent thickness of air.
From the Fig. 5, it is clear that as the yarn linear density increases, RWVP% increases. RWVP% is largely dependent upon the formulation and arrangement of fibers within the yarn. Finer yarn has lower packing fraction, lower diameter, increasing number of pores or spaces between loops and thus contribute to higher RWVP%. A layer having coarser cotton yarn will take more time in transferring water vapor.
Fabric RWVP% is the replacement of fabric–air interface with fabric–water interface. More porous structure offers lower resistance to water vapor. SJ structures exhibit highest RWVP% followed by knit-tuck stitch combinations. In SJ, the structure comprising of knit stitches only, increases the flow of moisture vapor. Another reason may be the lowest thickness in case of SJ fabric structure. Also SJ structure consists of knit loops only as compared to other structures which consist of a combination of knit-and-tuck loops. In SJ, the loop leg orientation is only in the vertical wale direction which helps in better wicking. Faster liquid dispersion in fibrous materials is facilitated by small, uniformly distributed and interconnected pores (Behera & Mishra, 2007).
Due to more porous structures (high total pore volume) the tuck stitches have high liquid retention. Water vapor permeability of SL fabrics is maximum among all the knit–tuck combinations due to minimum percentage of tuck stitches. DL structures exhibit lowest RWVP% due to LTS (tuck stitch squeeze between plain courses). This leads to small pores which are not interconnected. Although it is good for liquid movement due to high capillary pressure, but flow in capillary spaces may stop when geometric irregularities allow the meniscus to reach an edge and flatten. This limits the liquid advancement in the thickness direction (Das et al., 2009; Hsieh, 1995). A combination of high thermal resistance and low relative water–vapor permeability can cause uncomfortable situation to the wearer as the heat stored in the body cannot be dissipated. Increase in RWVP can be attributed to lower fabric density and thickness. Such observations are depicted in Fig. 6.
Water vapor resistance/evaporative resistance
A higher value of water vapor resistance is an indicator of bad moisture transportation and fabric being less breathable to vapor transmission. A fabric having lower water vapor resistance means it takes less time during transportation of moisture thus resulting in drier skin and improved comfort. Due to increase of yarn linear density, fabric thickness increases and thereby increasing evaporative resistance. The knitted fabrics in present research show considerably lower water vapor resistance in the range of 3–5 m2 Pa/W, which indicates these fabrics can provide satisfactory level of comfort to the wearer. As compared to all other structures DL, the thickest fabric shows highest evaporative resistance and lowest relative water vapor permeability. When density of fabrics increases, the resistance to water spreading also increases. So evaporative resistance of DL is highest followed by DP, SP, SL and SJ.
Relation between RWVP% and air permeability
The air permeability trends are compared vis-à-vis relative water vapor permeability (RWVP%) of the single knit derivative fabric as given in Fig. 7. It can be observed that the trends are almost opposite to each other. This is due to replacement of fabric–air interface with fabric–water interface, and thus due to presence of more stagnant air, more resistance for water to evaporate is resulted.
Under real wearing conditions, as the knitted apparel fits the body, there can be variations in density and porosity. These changes will certainly influence the moisture management and RWVP%. It will depend upon the nature of capillarity or distribution of pores under deformed state of the knitted apparel.
Analysis of variance
The ANOVA statistical technique was employed to investigate which input parameter has most significant effect on the thermo-physiological characteristic. The ANOVA analysis given in Table 6 shows the percentage contribution for each parameter on the thermo-physiological comfort.
The analysis of variance shows that the variables like yarn fineness, wet relaxation as well as the type of knitted structure have significant influence on the physical parameters e.g. areal density and thickness of single knit fabric derivatives. Further the yarn tex, wet treatment and change of structure by tuck-knit stitch combinations significantly influences the thermo-physiological properties e.g. air permeability, thermal insulation and relative water vapor permeability.
The yarn fineness has maximum contribution towards changing thickness and areal ensity, followed by knit structure. Further the yarn fineness is most influential factor to change the thermo-physiological properties. The wet relaxation induced structural changes seem to be slightly more influential than the knit structure itself to change the air permeability and thermal resistance. On the other hand RWVP% is rather dominated by the yarn fineness and the knit structure. These findings are in the same line as previous researchers (Arumugam et al., 2017; Celep & Yuksekkaya, 2017; Çoruh, 2015; Vadicherla & Saravanan, 2017).