Accordingly,a higher value of the coefficient indicates a larger surface area of the pores.Before the wear trial test,the surface area of open pores was the lowest for PAN knitted fabric,which can be easily observed visually in Fig.1.After the wear trial test,the stitch surface coefficient reduced for all the knitted fabrics,even for the PAN knit whose stitch density did not change after the wear and care cycles.This can be explained by the changes in the PAN yarn geometry.Besides,the difference between the knits in the size of the interstitial pores became smaller after undergoing the wear trial test which also can be seen in Fig.1d,e and f.The stitch volume coefficient indicates the ratio between the volume occupied by a stitch and the volume filled with yarn within the stitch.Thus,a higher value of the stitch volume coefficient indicates higher open space in a stitch.The interaction of the yarn and knit parameters resulted in the highest stitch volume coefficient for the wet relaxed hemp knitted fabric indicating the less volume within a stitch filled with yarn.After the wear trial test,the hemp knit still had the highest volume of open pores in the stitches.The changes,in percentage,in the geometrical parameters of the knitted fabrics after the wear trial test are summarized in Table 6.As can be noted,the hemp/PAN knitted fabric has undergone the biggest change in the porosity and stitch surface coefficient.This may be due to the fact that this knit was actually produced from two yarns with different intrinsic properties which in turn were induced by different fibre type.As expected,the smallest change was observed in the geometry of the PAN knit.Although conduction,convection and radiation mechanisms synchronize in the heat transfer through textile materials,cannabis grow euipment the conduction of heat by fibres is considered to be the most dominant.
However,the convective heat transfer through single jersey knitted fabrics can be an important factor considering their distinctive open structure.Therefore,besides the thermal conductivity of the knitted fabrics,their heat transfer coefficient was determined.According to the obtained results,the hemp knitted fabric exhibited the highest heat transfer coefficient meaning that its ability to transfer heat was higher,whereas the hemp/PAN and PAN knits were characterized by the same heat transfer coefficient.On the other hand,the hemp and hemp/PAN knitted fabrics exhibited comparable thermal conductivity.This led to the conclusion that the heat convection was favoured to a greater extent in the hemp knit which can be attributed to its higher open space.The thermal conductivity of the PAN knit was higher as compared to other knits,although a higher amount of air in the PAN yarn was supposed to slow down the conduction of heat especially in the pure PAN knit.In addition,the thermal conductivity of acrylic fibres is known to be lower than that of cellulose fibres.We believe that this can be explained by the findings of Kawabata according to which the thermal conductivity of a fibre is about ten times higher along its axis than across its width,and Stankovic et al.who confirmed that yarn hairiness is responsible for an increase in thermal conductivity of the fabric since the protruding fibres are arranged parallel to the heat flow.The acrylic yarn was much hairy comparing to the hemp yarn which must have contributed to an increase in thermal conductivity of the PAN knit.Despite more intimate contact among fibres in the hemp yarn in comparison to the PAN yarn,which is a presumption of higher thermal conductivity,the introduction of the hemp yarn into the hemp/PAN knit diminished the positive effect of fibres orientation,and reduced the thermal conductivity of the knit.On the other hand,the lower diameter of the hemp yarn increased the size of interstices in the hemp/ PAN knitted fabric contributing to the convection of heat,and therefore,the heat transfer coefficient was the same for the hemp/ PAN and PAN knits.In other words,the PAN and hemp/PAN knitscan transfer the same amount of heat but the conduction mechanisms prevailed in the PAN knitted fabric.The thermal properties of the knitted fabrics changed after undergoing wear and care cycles in the way that the differences in the thermal parameters among the knits became more distinctive,as can be noted in Fig.3.With an increase in stitch density of the hemp knit,the size of open pores reduced after the wear trial test,which must have resulted in a reduction in the convective heat transfer.This is an indication that the conduction of heat became the dominant mechanism of heat transfer through the hemp knit.
The explanation for this is in the fact that with an increase in stitches density,the number of interlacing points between yarn segments per unit area of the knit increases.In such a way,the contact area between the fibres was increased which improved the conduction of heat.Unlike the same values of the heat transfer coefficient for the hemp/PAN and PAN knitted fabrics before the wear trial test,the heat transfer coefficient of the hemp/PAN knit became higher than that of the PAN knit after the period of wear and care of the garments.This can be attributed to an increase in the stitch density of the hemp/PAN knitted fabric after the wear trial test by which the higher contact area between the fibres was enabled; hence the higher thermal conductivity and heat transfer coefficient.The thermal conductivity of the PAN knit reduced after the wear trial test which was attributed to the reorientation of the fibre ends as already explained.The fibre ends became perpendicularly oriented to the heat flow which reduced the conduction of heat.To determine the transient thermal parameters of the knitted fabrics,their specific heat was measured and given in Table 7 along with their volume heat capacity which was calculated by multiplying specific heat by bulk density of the material.Heat capacity of polymer materials is determined by the molecular structure and state of the polymer,and therefore the wet relaxed hemp and PAN knitted fabrics differed between each other in the specific heat capacity.Although the specific heat capacity of the hemp/PAN knitted fabric was quite close to that of the hemp knit,they differed in the volume heat capacity due to the difference in their bulk density.After the wear trial test,the specific heat capacities of the knitted fabrics changed as a result of the changes in the fibres morphology,which influenced the thermal vibrational motions at microscopic level.The pure hemp knit exhibited the biggest change in the specific heat capacity which can be explained by the fact that it was entirely produced of hydrophilic fibres.Although the hemp/PAN knit experienced the biggest change in the geometry at the macroscopic level,it seems that the changes at the microscopic level were closer to the PAN knit.Since the thermal diffusivity is a measure of rapidity of the heat transmission through the fabric,the higher value of this parameter indicates that the thermal balance will be reached more quickly,which is desirable for both summer and winter clothing textiles.The thermal diffusivity is directly proportional to the thermal conductivity and inversely proportional to the volume heat capacity causing the PAN knit to have the highest value of thermal diffusivity which is about 76% and 42% higher than that of the hemp and hemp/PAN knits,respectively.
The thermal absorptivity quantifies the transient heat conduction when human skin touches a fabric,which normally has the lower temperature than that of the skin due to which heat flows away from the skin.The higher the fabric thermal absorptivity,the higher the temperature drop in the skin,and the more intense the feeling of coolness at the first moment of contact with the fabric.It has been known that hygroscopic fibres provide cooler feeling to the fabric,whereas acrylic fibres are considered to be among the warmest.Indeed,the hemp knitted fabric was characterised by the highest thermal absorptivity,followed by the hemp/PAN and PAN knits.The changes in the transient thermal properties of the knitted fabrics after the period of wear and care of the garments are graphically shown in Fig.5.It can be observed that the order of the knits in terms of thermal diffusivity changed.Having the lowest thermal diffusivity before the wear trial test,the hemp knit exhibited the highest thermal diffusivity after the test.Since the specific heat capacity of the worn and washed hemp knit reduced,it seemed that increased thermal conductivity had a more important effect on the thermal diffusivity of this knit.The thermal diffusivity of the hemp/ PAN knitted fabric changed from being between those of the other two knits to being the lowest one.This is a consequence of the highest specific heat capacity of the hemp/PAN knit due to which it was characterized by even the highest volume heat capacity.The thermal absorptivity of the knits altered after the wear trial test,mobile grow system but the extent and direction of these changes seemed dependent on the changes in the geometry and thermal characteristics of the knits.Considering that the square of thermal absorptivity of a fabric is directly proportional to its volume heat capacity and thermal conductivity,it seems that the highest value of thermal absorptivity of the hemp knit resulted from its highest thermal conductivity which compensated for a reduction in the specific heat capacity.Observed increase in the thermal conductivity of the hemp/PAN knit after the wear trial test led to an increase in the thermal absorptivity regardless of the reduced specific heat capacity which was compensated by the increased bulk density of the knit.Although the specific heat capacity of the PAN knit was reduced to the lowest extent,and its bulk density was almost unchanged,the decrease in thermal conductivity after the wear trial test caused the thermal absorptivity of the PAN knit to become lower.
Discussed effects of the repeated wear and care cycles of the knitted fabrics are summarized in Table 8.Considering all parameters describing the thermal behaviour of the knitted fabrics in both steady-state and transient conditions,the hemp knitted fabric underwent the biggest changes.The most drastic change was the increase in the thermal diffusivity,followed by the thermal conductivity increase.The thermal absorptivity increased to the lowest extent as a consequence of the decrease in the volume heat capacity.It seems that the changes in the geometry of the PAN knit and fibre morphology,which occurred during the repeated wear and care cycles,exhibited a synergetic effect and caused both steady-state and thermal parameters to decrease.Hemp/ PAN knitted fabric exhibited the similar increase in the heat transfer coefficient,thermal conductivity and thermal diffusivity,while the increase in the thermal absorptivity was a bit lower due to unchanged volume heat capacity.Even though the hemp/PAN knitted fabrics underwent the biggest changes in the geometry after the wear trial test,the increase in thermal characteristics was moderate in comparison to that of the hemp knit.This can be explained by the fact that the hemp knit after undergoing the wear trial test was characterized by the densest arrangement of fibres and yarns within.Generally,the most important is the positive trend of the investigated thermal comfort parameters of the hemp and hemp/PAN knitted fabrics during their usage.After the wear trial test,the hemp/ PAN knitted fabric exhibited the thermal parameters which were approximately the average of those of the hemp and PAN knits,which could be attributed to the same linear density of the hemp and acrylic yarns.Therefore,it seems reasonable to expect that the thermal properties of the hemp/PAN blends can be designed by choosing an adequate combination of yarns with different linear density.By comparing the values of the thermal parameters of the hemp/PAN knitted fabric after the wear trial test with those of the hemp knit before the test,it can be concluded that the hemp/PAN knit has kept the “hemp character” after its usage.Within the broader context,the applied design of the experiment which included wear trial testing of textile products in real life conditions made it possible to observe changes in designed fabric’s properties at the exploitation stage.In such a way,durability of product’s performances can be evaluated.In spinning sector,blended yarns can be produced as a mixture of different fibres in the blow room following the steps of carding,drawing,roving and spinning or by the combination of different fibre containing slivers in the drawing stage of the yarn spinning.The blending method proposed in this study offers the knitwear companies a new possibility for designing their own fibre blends exclusively by choosing yarns available on the market,and consequently for savings and for increase their market competitiveness.In consideration of the fact that the production of knitted goods has an advantage over woven textiles in terms of tactile comfort,production cost and environmental point of view,this study aims to offer a more sustainable design approach to the development of comfortable cellulose based clothing textiles.