The Distribution of Kapok and Silk Fibers in Cotton Blended Yarns

YING Yufei，ZHOU Menglan，WANG Fumei，2

(1.College of Textiles，Donghua University，Shanghai 201620，China;2.Key Laboratory of Textile Science and Technology，Ministry of Education，Shanghai 201620，China)

0 Introduction

The properties of fibers and the technological factors affect the migration rule of fibers in the yarn.In order to reveal the migration rule of kapok and silk fibers in cotton blended yarns，it needs to study the radial distribution of fibers in the cross-section of the blended yarn.Generally，in the blended yarn，the radial location and quantity of each component is not uniform.Due to the stress-strain behavior and surface friction properties of fibers are different，some fibers may preferentially distribute in outer layers，while others may preferentially locate in inner layers［1］.For kapok blended fabrics，many performances depend on the kapok fiber content，especially the distribution regularity of the kapok fiber in the blended yarn.Because kapok fiber is a natural fine and highly hollow fiber，and the hollow degree is 80%－90%［2］，it’s warm，antibacterial and driving mite.If kapok fibers mainly distribute in the surface layer of yarns，the fabric will have soft and warm handle，prominent anti-microbial and driving mite features，also with the novel visual style.Silk is soft，glossy，elastic，and has other advantages of other fibers.But silk is expensive.So there are more and more imitation products and blended products appearing on the market，especially products blended with the natural cotton fibers.If silk fibers in cotton blended yarns mainly distribute in the surface，the products will present more characteristics of silk.

However，so far，the distribution of kapok and silk fibers in cotton blended yarns has not been studied much.One reason is the lag of overall study in the kapok/cotton and silk/cotton blended yarns，and the other reason is the technical difficulty for kapok fibers:the different hollow degrees of kapok fibers lead to the volume density of the kapok fiber is not a constant，so the traditional calculation method，Hamilton migration index，is not suitable for the calculation of the kapok fiber distribution index.

This paper compares the various researches on graphics software used in the porous network and particle size［3－5］.Based on that，using Photoshop pixel to determine fibers’cross-sectional area in blended yarns is put forward，which is direct，accurate，fast，and avoid using the volume density of fibers.The distribution rule of kapok and silk fibers in the yarn also depends on the properties of other fibers blended with them.This paper mainly studies the migration rule of kapok and silk fibers in the common cotton blended yarns.Practice has shown that blending these fibers can take respective advantages，and these yarns are becoming more and more popular on the market.

1 Experimental

1.1 Materials

Shandong Xia Jin Feng Run Co.，Ltd provided the kapok/cotton，kapok/viscose/cotton blended yarns.The kapok/cotton blended yarns were obtained from Guangdong Esquel Textiles Co.，Ltd.Fibers blended in the opening and cleaning processes to make these kinds of blended yarns.The raw cottons blended with 14.58tex(40S)kapok fibers have higher grade than that blended with 19.67tex(30S)kapok fibers.The blending ratios of four kinds of yarns are shown in Table 1.

Table 1 Blending ratios of yarns

1.2 Making samples

In order to more clearly distinguish the different fibers in the yarn，this study uses a Scanning Electron Microscope(SEM)to take photos of the yarn crosssections to test and analyze of various fibers distribution in the yarn.Firstly，coat the yarn with wool fibers and put it into the fiber microtome to get the cross-sectional slice for taking the yarn cross-sectional images.Actually，it also can use the biological microscope to take photographs of the yarn cross-sectional images，as long as making the slice without causing deformation of the yarn.

Each slice randomly selects 3 or 4 segments of the yarn，and each segment is coated by wool fibers alone before putting into the slicer.For each kind of yarns，ensure to obtain 10 clear slicers from 10 segments of the yarn.

1.3 Traditional Hamilton migration index analysis method

Traditional Hamilton migration index analysis is as follows［6］:

①According to the yarn cross-sectional micrograph，the yarn cross-section is divided into five equally spaced concentric circles(or ellipse)，so that the yarn cross-section is divided into five layers from inside to outside.

②Count the numbers of two kinds of fibers in each layer，NAi，NBi(i=1，2，3，4，5)，then according to the average linear density values of various fibers DA，DB(Tex or dtex)and bulk density values ρAand ρB(g/cm3)，convert the numbers of fibers NAiand NBito area of fibers Aiand Bi(Ai=NAiDA/ρA，Bi=NBiDB/ρB)and total area of all fibers ti(ti=Ai+Bi)in each layer.The notation of each kind of the fibers’total area in each layer and the statistical weight value of each layer are shown in Table 2.

Table 2 A，B fibers area code and statistical weight

③Take the fiber A for an example.Using the formula(1)to calculate the distribution moment FMAof the fiber A in relative to the middle layer(i=3).In the formula，the numbers in front of the area notation are the statistical weight values.

④By the theoretical ratio A/T，using the formula(2)to calculate the distribution moment FMUwhen the fiber A，B are distributed uniformly.

If FMU＜ FMA，then it means the fiber A is prior to outward distribution.

⑤Calculate the maximum outward distribution moment FMOwhen all the fibers A distribute in yarn outer layers.Firstly，fill the total area of the fiber A into the fifth layer.Next，if there is a remainder，fill it into the next layer.Continue above steps until finishing.In a similar way，the maximum inward distribution moment FMIis calculated.Fill the total area of the fiber A into the first layer.If there is a remainder，fill it into the next layer.Continue above steps until finishing.The step is shown by Fig.1.

Fig.1 Schematic diagram of fiber extreme distribution form

⑥Based on Hamilton migration index formula(3)and formula(4)，the migration index Minand Moutof the fiber A are calculated.Minis the inward migration index and Moutis the outward migration index.

The range of Hamilton migration index is－100% ≤ Min≤0，while 0≤ Mout≤100%.If Minis less than zero，it indicates that the fiber A is prior to inward distribution.If Moutis more than zero，which indicates that the fiber A is prior to transfer outward.The calculation method for the fiber B is the same with that used for the fiber A.

1.4 Photoshop pixel method of calculating fibers’cross-sectional area in the yarn

The question of applying traditional Hamilton migration index method to kapok blended yarns is that it’s difficult to calculate fibers’area in the second step of traditional method，because kapok has high hollow degree and the hollow degree or the shape of the cross-section could change in the yarn.In other words，it’s not reasonable to determine kapok fibers’area by using fiber number，radial size and bulk density.Therefore，the second step of traditional method is improved in this paper.The method is using Photoshop software to classify fibers in yarns’cross-sections［7］，and Photoshop pixel method is used to figure out area of different fibers［8－9］.Through that Hamilton migration index M can be calculated.Take a blended yarn for example.The specific steps are as follows:

(1)As Fig.2 shows，Photoshop software is used to process the images of the yarn cross-sectional slice.Firstly，circle the boundary of the yarn and designate the center of the circle.Secondly，divide the radius into five equal partitions and draw five concentric circles.Then the cross-section of the yarn is divided into five rings.

Fig.2 The cross-sectional SEM figures of the yarn before and after Photoshop treatment

(2)Use Photoshop software to color the three kinds of fibers in SEM figure with yellow，blue and red.Yellow is used for the fiber A(kapok fibers)，blue for the fiber B(viscose fibers)，and red for the fiber C(cotton fibers)，as shown in Fig.2.After marking with different colors，the actual distribution in each layer is shown in Fig.3.

(3)The area of each kind fiber(Ai，Bi，Ci)can be obtained in number i ring by the pixel method，namely calculating the pixel points of the three colors in the number i ring by Photoshop histogram function，as shown in Table 3.Specific operation is as follows:open Photoshop software，click on the window option and histogram option，select the target layer and the target color，and get pixel points of the color chosen in the selected circle.The calculation condition of pixel points in Table 3 is that the radius of the outermost circle is 6.95cm，and the resolution is 173 pixel/inch.The resolution refers to the number of pixel points in per square inch that the image has.It is a parameter of the testing system.The number of the pixel points in single fiber’s cross-section is not only related to the fiber fineness and the magnification，also relevant to the resolution of the system.

The smallest cross-sectional area of the single kapok fiber generally occupies 603 pixel points.Therefore，the precision of the method for measuring the fiber’s cross-sectional area is higher than the current diameter measurement method，and its greater advantage is that it can test the cross-sectional area of the non-circular fiber.

Table 3 A，B and C fibers pixel distribution

Fig.3 Schematic diagram of fibers’distribution in each circle layer

(4)During the calculation，take the pixel values in Table 3 as the corresponding area values directly.Then use formulas from the formula(1)to the formula(4)to calculate the migration index.In theory，according the formula(5)，it should convert the pixel values into area values under the testing condition.

Where Sfis the cross-sectional area of fibers;PXis the pixel values;R is the revolution.

However，when use the formula(3)and(4)to calculate the migration index，the numerator and the denominator contain the same multiplication and division factors which can be canceled out.So the magnification of the image and the converting from pixel values to area values has nothing to do with the calculation results of the migration index Minand Mout.

2 Results and Discussion

2.1 Hamilton migration indexes of kapok and silk fibers

It can be observed in Fig.4 that due to the different spun time，there is an obvious difference in the hollow degree of kapok fibers in the yarns.The yarn in(a)is spun in this year，and the yarns in(b)and(c)are spun two years ago.Yarns blended two years ago have higher hollow degree than yarns blended in this year，and kapok fibers in the yarns blended in this year almost entirely get squashed.This shows that kapok fiber has potential to recover hollow structure by itself.How to dig the potential in finishing is worth exploring in the future.

Fig.4 Cross-sections of kapok/cotton and silk/cotton blended yarns

After observing ten cross-sections of each yarn，choose five clear cross-sections for measurement.Hamilton migration indexes of kapok and silk fibers in these kinds of yarns are calculated，as shown in Table 4.

Table 4 Hamilton index analysis of kapok and silk fibers

It can be seen from Table 4 that the Hamilton migration index of kapok fibers is about 20%when blended with the common cotton fibers and cotton type viscose fibers.Kapok fibers have obvious trend of transferring to outer layers of the yarn.And with the increase of the kapok fiber content，the trend of outward transfer grows.When the kapok content is 20%，the average of Hamilton index is 16.2%.When the content is 30%，the average of Hamilton index is 20.7%.When the content increases to 40%，the average index reaches to 22.4%.The Hamilton migration index of silk fibers has a fluctuation.The average of Hamilton index is 10.5%.So it still can reflect that it has strong trend to migrate to the outer layer of the yarn.

These results caused because the inward and outward migration of fibers in the ring-spun yarns.When producing the ring-spun yarns，the parallel arranged fibers are twisted in twisting triangle area，which makes the fibers in the outer layer of the yarn strain and crowd into the yarn core.The inner fibers are extruded out to become the outer fibers.Every fiber will change the location several times from the outside to the inside，and then from the inside to the outside.And the ends of the fiber will be extruded to the surface of the yarn because the ends are unable to be strained.Compared with cotton fibers and viscose fibers，kapok fibers have the smooth surface［10］，the low bending rigidity and the short length，which makes kapok fibers have more opportunities to be pushed out to the surface of the yarn.Silk fiber is also easy to migrate to the yarn surface because of its smooth surface and low initial modulus.

Table 5 Hamilton index analysis of fibers in the same yarn(K/R/C(30/45/25))

Hamilton indexes of different fibers in the same yarns are shown in Table 5.Obviously，the average of Hamilton migration index of kapok fibers is 20.7%，which means kapok fibers have the strong trend to migrate to the outer layer.The reasons are that kapok fibers have the low linear density，the low bending stiffness，the short length and the smooth surface，all these features making fibers transfer outwards.The average index of viscose fibers is－11.0%，and－2.8%is for cotton fibers，showing a trend of inward transfer.This indicates that kapok fibers coat the outer of the yarn，while viscose and cotton fibers hide in the inner layers.The surface of the blended yarn reflects more characteristics of kapok fibers.

2.2 The fiber percentage in the surface of the yarn

The kapok and silk fiber content in the surface of the yarn has a great influence on the appearance and properties of cotton blended yarns.Therefore，the percentage of fibers in the surface layer could well characterize this feature.Use the data in the measurement of migration index to get the ratio of kapok or silk fibers in the outermost layer by dividing the crosssectional area of kapok or silk fibers by the total area of all fibers［11］.

Where SKis the fiber percentage in the surface of the yarn;A5is the cross-sectional area of one kind of fibers in the fifth layer;t5is the total area of all fibers.The results are shown in Table 6.

Table 6 The percentage of kapok and silk fibers in the yarn surface layer

It can be observed in Table 6 that the highest percentage reaches 79.5%and most of kapok fibers distribute on the surface layer.The average percentage of silk fibers in the surface layer is 29.3%，higher than the blending ratio of the yarn.Fig.5 shows the relationship between the kapok fiber content X and the kapok fiber percentage SKin the surface layer.With the increase of the content of kapok fibers，the percentage of kapok fibers in the surface layer also increases gradually.It finds high correlation of two indexes after linear fitting，up to 0.93.

Fig.5 The relationship between the kapok percentage in the surface and the kapok fiber content

At the same time，the experiment also found that the kapok fibers have a cluster or bundle phenomenon.Especially in the surface layer of the yarn，the cluster phenomenon is more obvious.It indicates that the current processing like carding failed to make kapok fibers be single fibrous.In addition，it also found that kapok fibers in the yarn surface keep more complete hollow structure as shown in Fig.6.

As can be seen from the above experiment methods，using Photoshop pixels test method can quantitatively calculate the blending ratios of kapok/cotton and viscose/cotton blended yarns which are in manual testing currently.

Fig.6 Kapok blending yarn global and local enlargement figures

3 Conclusions

Using Photoshop software，it can directly observe and quantitatively test cross-sectional area of all kinds of fibers in the yarn，including non-circular cross-sectional area，and it’s fast and accurate.Thus，it can quantitatively test the radial distribution or the migration rule，the blending ratio and other characteristics of fibers in the yarn.

According to the structure characteristics of kapok/cotton and silk/cotton blended yarns，this paper obtained the following research conclusions:

a)In the current kapok blended yarns，kapok fibers have a tendency to migrate to the surface of the yarn.Hamilton migration index of kapok fibers is about 20%.And with the increase of the kapok content，the trend of outward transfer increases，which makes the percentage of kapok in the yarn surface greater than the average blending ratio.In addition，with the increase of the kapok content，the percentage of kapok in the yarn surface is increasing linearly.It’s the result of the combined effects of the shortness，smooth surface and soft properties of kapok fibers.

b)In the kapok/viscose/cotton blended yarn，kapok fibers migrate to the yarn surface with the strongest trend，and cotton type viscose fibers have the strongest trend of migrating to the yarn core.

c)In the blended yarn，kapok fibers occur bun-dles phenomenon，which means the opening and cleaning process failed to make kapok fibers be single fibrous.The longer time the blended yarn is stored，the higher hollow degree the kapok fibers have.Kapok fibers have hollow reply potential.And kapok fibers in the surface of the ring spun yarn keep the higher hollow degree than fibers inside the yarn.

d)In cotton/silk blended yarn，silk fibers migrate to yarn surface with the strongest trend，and the percentage of silk fibers in the yarn surface is greater than the yarn blending ratio.Silk fiber is easier to transfer to the yarn surface because of its smooth surface and low initial modulus.

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