EFFECT OF RAW MATERIAL AND PROCESSING CONDITIONS ON YARN NEPPINESS

Ma_gorzata Matusiak1, Iwona Frydrych1,2 1) Institute of Textile Architecture e-mail:[email protected] [email protected] 2) Technical University of Lodz e-mail: [email protected]

1. Introduction

One of the most important quality problems of yarns is the nep presence. Neps in a yarn are defined as “point agglomerations of fibers entangled into the yarn causing the increase of the yarn diameter” [1]. On the basis of the statistical data originated from world cotton mills it can be stated that yarn neppiness is still not a diminishing problem. The sources of neps in a yarn are neps and trashes contained in a fiber stream, from which directly the yarn is formed [2]. The number of neps in cotton yarn depends on two main factors: - characteristics of raw material used for the yarn production, - conditions of the technological process in the spinning mill. The development of modern instrumental measurement systems afforded possibilities for the complex evaluation of cotton fibers in raw material and spinning semi-products, among the other assessment of parameters, which influence the neppiness of manufactured cotton yarn in a direct or indirect way. There were created conditions for the development of the methods enabling a prediction of cotton yarn quality in the aspect of its neppiness [3, 4].

2. Changes of cotton fiber stream during processing

Starting from harvesting cotton is exposed to the numerous processes, which in the final effect lead to the yarn creation. Mechanical outer actions during the yarn manufacturing cause significant changes of almost all average parameters of processed cotton. In the preliminary cleaning and ginning process the intensive trash removing already takes a place, but simultaneously it is observed the fiber damage as well as the nep formation. During the technological process in the spinning mill, there are a number of stages, which cause the significant changes of characteristics of the processed fiber stream. The number and kind of operations, to which cotton is exposed, depend on the quality of raw material as well as of the used spinning system. In the result there can be observed the significant changes of fiber stream parameters, like the average length and its irregularity, average linear density and maturity, immature and short fiber content and so on. Contamination of cotton decreases during the particular stages of processing. The most significant reduction of trash and dust content in cotton takes a place in the opening and blending processes, and next during the and processes. In the rotor spinning system trashes are removed also by the opening roller of rotor (Fig. 1).

1 60 g /

t 50 n

C 40

30 h s

a 20 r

T 10 0 Raw Lap after Sliver after Fibers from material carding drawing rotor

Blend A Blend B

Fig. 1. The changes of trash content in cotton during the processing in the rotor spinning system

In the beginning it is observed an increment of the nep number in the preliminary treatment of cotton in the scutching room. It is caused by occurring outer mechanical factors, which are connected with actions of the working machine elements on the fibers as well as by the pneumatic transport of fibers between particular machines in the technological line. The considerable (50 % - 90 %) reduction of the nep number (Fig. 2) occurs in the further stages of cotton processing, mostly during carding and combing process.

300

g 250 / t

n 200 C

150 p

e 100 N 50 0 Raw Lap Sliver after Sliver after Fibers from material carding drawing rotor

Blend A Blend B

Fig. 2. The changes of nep content in cotton during the processing in the rotor spinning system

The mentioned above changes of cotton fiber stream during the technological process in the spinning mill cause that the number of neps and trashes in the spinning semi-products, from which directly the yarn is formed, i.e., in the feeding the frame or in the fiber web in rotor of the rotor spinning frame, differs significantly from the initial nep and trash number in cotton raw material used for production.

3. The model of nep number in the cotton yarn

Neps and trashes contained in cotton, which were not removed during processing in the process of sliver preparation get into the yarn and can be a source of yarn neps. But not all of them are the reason of yarn faults. Neps and trash particles of appropriate small sizes are not visible on the yarn surface and in the same way they are not registered by the measurement device as yarn neps. Only these neps and trashes contained in the fiber stream are registered as yarn neps, whose size is bigger than the given critical value.

2 Taking it into consideration as well as known tendencies of changes of the nep and trash number in cotton during the processing it is possible to formulate the following general equation expressing the number of neps per 1000 m of cotton yarn:

⎡ ⎛ NRE ⎞ ⎛ NRE ⎞ ⎛ CE ⎞ ⎛ CE ⎞⎤ = ⋅ ⋅ ()+ ∆ ⋅ ⎜ − 1 ⎟ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⎜ − k ⎟ + ⋅ ⎜ − 1 ⎟ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⎜ − p ⎟ N yUT Tt y ⎢n N c N t 1 1 u U c 1 ⎜1 ⎟⎥ (1) ⎣⎢ ⎝ 100 ⎠ ⎝ 100 ⎠ ⎝ 100 ⎠ ⎝ 100 ⎠⎦⎥ where: NyUT - the number of neps per 1000 m of yarn according to the Uster® Tester, Tty - linear density of yarn, n - the share of neps of size equal to and higher than the critical nep size for a given linear density of yarn in the total nep number in the fiber stream from which the yarn is created, Nc - the average nep number in cotton used for yarn manufacturing, _Nt - the increment of the nep number in cotton in the preliminary treatment in the scutching room, th NREi - the average nep removing efficiency by machines in the i stage of cotton processing, k - the number of spinning process stages causing the nep removing, u - the share of trash of size equal to and higher than the critical nep size for a given linear density of yarn in the total trash number in the fiber stream, from which the yarn is created, Uc - the average trash particle number in cotton used for yarn manufacturing, th CEj - the average trash removing efficiency by machines in the j stage of cotton processing, p - the number of spinning process stages causing the trash removing.

Parameters characterizing the work effectiveness of machinery in the technological order can be determined basing on the intermill diagnostics carried out in the spinning mill using the AFIS system. The parameter characterizing the carding machine effectiveness in the aspect of nep reduction is the Nep Removing Efficiency (NRE %), which is expressed by equation (2) :

NepCnt / g − NepCnt / g NRE = feed del ⋅100% (2) NepCnt / g feed where: NRE - nep removing efficiency, Nep Cnt/gfeed - the nep number per gram in fiber stream feeding the machine, Nep Cnt/gdel - the nep number per gram in the fiber stream delivered by the machine.

Dependably on the spinning system there occur a few stages in the technological process, in which neps are removed. First of all the nep number reduction takes a place during the carding process. Correctly working carding frames can remove ca. 80 % ÷ 90 % of neps contained in the feeding fiber stream (web or lap). The value of NRE for a given carding machine does not change in a significant way in a longer period of time. It can be maintained on the similar level during 16÷18 weeks after upgrading of the carding frame coverings [5]. Analysis [6] showed that there are the considerable differences between the nep removing

3 efficiency of particular carding machines in the technological line. Therefore, in order to predict the nep number in cotton yarn on the basis of proposed equation (1) the average nep removing efficiency should be calculated for the set of carding machines. In the combed spinning system apart from carding the neps are also removed during the combing process. The nep removing efficiency by set of combing frames is an average value from the nep removing efficiency of all combing machines in the technological line. For particular combing frame the NRE should be calculated as an average from values of these coefficients for all machine heads. Similarly like in the case of carding machines the nep removing efficiency of the combing frames does not change significantly in a long period of time. Moreover, it was stated [6] that at a correct machine adjustment, there are not any significant differences in aspect of nep removing efficiency between the particular machine heads. In the case of the rotor spinning process apart from carding process it should be taken into consideration the action of the opening roller of the rotor spinning frame. Nep removing efficiency NRE of opening roller can be also calculated from equation (2). In this case as a fiber stream delivered by the machine the fiber web accumulating on the rotor circumference should be considered. Cleaning efficiency CE is expressed by the following equation:

TrashCnt / g − TrashCnt / g CE = feed del ⋅100% (3) TrashCnt / g feed where: CE - trash removing efficiency, Trash Cnt/gfeed - the trash number per gram in fiber stream feeding the machine, Trash Cnt/gdel - the trash number per gram in the fiber stream delivered by the machine.

Contaminants contained in cotton are removed during the preliminary treatment in the scutsching room, during carding and combing processes and in the case of the rotor spinning system in the zone of opening roller action. Generally, it can be stated that independently of the applied spinning system the number of stages, in which the reduction of cotton contamination occurs is smaller then the number of stages causing the nep number reduction. The difference results from the preliminary treatment of cotton at the opening and blending machine line. In this stage of cotton processing cotton contamination is removed very intensively; whereas simultaneously the number of neps increases. The increment of nep number in the first stage of spinning process, i.e., during the preliminary treatment does not depend considerably on the initial nep number in raw material, therefore, it should be expressed by the absolute value. The most important factors influencing the nep formation are: fiber properties and process conditions. According to the fiber parameters first of all we should list: fiber fineness, maturity and slenderness, which influence the tendency of fibers to entangle and to form neps [7]. Moreover, opening and cleaning process conditions depends on the quality of processed raw material, especially its contamination, which influences the cleaning ability [8]. Dependably on the cotton cleaning ability, there is chosen an appropriate set of machinery, the number of cleaning points and an adjustment of the working elements of opening-cleaning machinery. Above factors cause that, for example, long roller ginned cotton of a big trash content requires quite different cleaning conditions than saw ginned middle staple cotton. The differences can concern not only the cleaning points and their adjustments, but also machine

4 configuration, what implies the conditions of pneumatic transport, which is one of the most important factors causing the nep increment in cotton during the processing in the scutching room. Therefore, for each group of opening – cleaning machinery for a given type of raw material blend of the mean value of parameter _Nt, characterizing the absolute nep increment during the processing on this kind of machinery should be determined separately.

4. Determination of the critical nep size

Into the derived equation (1) the n and u values were introduced - expressing the content of neps and trashes of sizes equal to and bigger than the value critical for a given linear density of yarn in the total nep number in the fiber stream, from which the yarn is formed. Application of equation (1) for calculation of the predicted nep number in cotton yarn requires first of all knowledge about the value of the critical nep size for a given assortment of yarn. This problem was the object of research work. The critical nep sizes for cotton ring spun yarns of different linear densities were determined experimentally [9 ÷ 11]. Moreover, in the Institute of Textile Architecture the theoretical models enabling calculation of the critical size of neps dependably on the linear density of yarn for ring (4) and rotor (5) yarns were derived [11]:

= ⋅ 0,663 Dncrit 0,107 Tt y (4)

= ⋅ 0,663 DncritOE 0,138 Tt yOE (5)

Next, on the basis of proposed models the values of critical nep size were calculated for different ring and rotor yarns of linear densities typical for Polish spinning mills (Fig. 3).

2000

1500

m 1000 µ

500

0 0 102030405060 linear density of yarn tex ring rotor

Fig. 3. The critical nep size for ring and rotor yarns (11)

Above experiments and theoretical considerations were conducted for ring-spun and rotor-spun yarns with the assumption of sensitivity setting of the Uster® Tester at evaluating the number of neps in yarn for ring spun yarns: + 200% and for rotor yarns +280%. There are the standard settings of Uster® Tester sensitivity used the most often at the assessment of cotton yarn quality. There are also the basic settings for statistical data contained in Uster® Statistics [12, 13]. It is possible to derive the theoretical model on the critical nep size in a function of linear density of cotton yarn and sensitivity on the Uster® Tester at the yarn neppiness measurement. The investigations of this problem are carried out in IAT.

5 Knowing the critical nep size per a given linear density of yarn, it is possible to determine the n and u values. These values can be read directly from histograms illustrating the distribution of sizes of neps and trashes contained in the fiber stream (roving or web). Such histograms can be obtained by means of the AFIS system. Moreover, studies showed [14] that the percentage distribution of nep sizes in the roving is similar for different manufactured in the same spinning system. In Fig. 4, there are presented cumulative diagrams prepared for different roving samples manufactured by a carded systems installed in several Polish cotton spinning mills. They show the growing percentage of neps of different sizes in the direction from the biggest to the smallest ones.

120 %

100

e g

t 80 a n e c

r 60 e p

g 40 n i w o r 20 G

0 475 575 675 775 875 975 1075 1175 1275 1375 1475 1575 1675 1775 1875 1975 2075 2175 Nep size µ m

Fig. 4. Cumulative diagram of the nep content of a given size in the total nep number in the roving samples manufactured from various medium staple cotton blends

On the basis of the presented above results it was calculated the average percentage of neps of sizes equal to and bigger than a given size, corresponding to the value of n introduced into the equation (1) for the number of neps in yarn. However, due to numerous factors connected with a spinning practice it was decided that a range, in which the predicted number of neps per 1000 m of yarn is expected to be found should be determined. Therefore, on the basis of experimental results it was determined minimum (nmin) and maximum (nmax) percentage of neps of sizes equal to and bigger than a given size in the total nep number in the roving. The range prediction seems to be more adequate to the reality of cotton spinning mills, the present state of knowledge and technical level of spinning machines and measuring systems than a point prediction. Moreover, in a practice hand trash particles have insignificant influence on the nep number in yarn. In the sliver after combing, there are usually only individual particles of sizes from 50 µm to 500 µm. After carding the sliver usually contains a few or several particles of hard impurities, most of them of sizes below 800 µm. Thus the number of trash particles, which size exceeds the critical value is in many cases very small and can be neglected. Moreover, a distribution of sizes of hard trash particles is random, and so the content of particles bigger than the critical value is difficult to predict. Taking this fact into account the component standing for trashes in the proposed equations can be omitted, and it does not result in any considerable error. A simplified formula for the nep number per 1000 m of yarn according to the Uster® Tester for ring-spun cotton yarn is:

6 ⎡ ⎛ NRE ⎞ ⎛ NRE ⎞⎤ N ≈ Tt ⋅ ⎢n⋅()N + ∆N ⋅⎜1− 1 ⎟⋅⋅⋅⋅⋅⋅⋅⎜1− k ⎟⎥ (6) yUT y ⎣ c t ⎝ 100 ⎠ ⎝ 100 ⎠⎦

5. Experimental verification of theoretical models

In order to verify the proposed theoretical studies the spinning processes were conducted in one of the Polish cotton spinning mills, two times with a 6-month interval, during processing two different blends of raw material [14]. Having determined by means of AFIS system the effectiveness of machines in the technological line and nep content in the raw material blend used for production, we calculated the predicted minimum and maximum number of neps per 1000 m of produced yarns. Next, the calculated values were compared with corresponding real values obtained in result of yarn measurement by Uster® Tester (Fig. 5).

Sample 1 Sample 2

m 800 1200

m 0 1000 0 600 0 0 Real 0 800 1 0 Real / 1

s 400 Max

/ 600 p

s Max e Min p 400 n

200 e Min

n 200 0 0 0 102030405060 0 1020304050 Linear density of yarn tex Linear density of yarn tex Fig. 5. The comparison of the real number of neps per 1000 m of yarn and the minimum and maximum predicted values In both cases it was found that the real nep number in yarn is within the predicted interval. Thus, the proposed method of predicting the number of neps in cotton yarn is proved to be right.

Suming up

On the basis of carried out considerations it was stated that: 1. There are two main factors influencing the neppiness of cotton yarns: the characteristics of raw material used for production and conditions of the spinning process. 2. The following fiber parameters: fineness, maturity, length and slenderness influence the ability of cotton to entangle and in a some way to nep creation. 3. The average size as well as the distribution of size of neps and trashes contained in cotton semi – products, from which the directly yarn is formed, decide about a visibility of these faults on the yarn surface. 4. A percentage of cotton contaminants as well as their structure influence a cotton cleaning efficiency and decide about a way and conditions of preliminary treatment in the scutching room. 5. The type, adjustment and technical shape of machines in the technological line effect their work effectiveness and the quality of manufactured yarn.

7 Literature

1. Polish Standard PN -76/P-06746, Cotton yarn. Faults, 1976. 2. Kluka A., Matusiak M., Frydrych I., Yarn Nepiness - Influence of Raw Material Quality and Process Technology, Melliand Textilberichte 7-8, E137-138 (1998). 3. Frydrych I., Matusiak M., Cotton Neps in the Technological Process, Fibers and Textiles in Eastern Europe No 1 (24), 22-25 (1999). 4. Färber Ch., Relationships Between Cotton Fiber Properties and Mill Processing Results from Bale to Card Sliver. II World Cotton Conference, Ateny (1998). 5. Furter R., Frey M., Analyse des Spinnprozesses durch Messung von Zahlen und Größe der Nissen, Melliand Textilberichte 7, 504-510 (1991). 6. Kluka A., Matusiak M., Report from Grant no 1986/C/T09-3/97 “Starting the Production of Combed Yarns of Low Linear Density and High Quality Standard by Application of Modern Spinning Techniques on the Basis of Pilot Measurement of Machinery Order”, Lodz (1998). 7. I. Frydrych, M. Matusiak, T. _wi_ch, Cotton Maturity and its Influence of Nep Formation, Textile Research Journal 71 (7), 595 ÷ 604 (2001). 8. Artzt P., Gresser G., Maidel H., Einfluss von Trashgehalt und Reinigungswilligkeit der Baumwollen auf die Garmqulität, ITB Garn- und Flächenherstellung, 2, 14-21 (1995). 9. Färber Ch., Einfluss des AFIS - Störpartikelgehaltes auf die Imperfektionen von Baumwoll-Ring- und Rotorgarnen, Melliand Textilberichte 10, 652-655 (1996). 10. Peters G., Einfluss von Unreinigkeiten und Nissen in der Vorlage auf definierte Eigenschaften von Ringgarnen bei unterschiedlichen Garnfeinheiten, Graduate Thesis Paper, Fachhochschule Coburg – Münchberg (1993). 11. Frydrych I., M. Matusiak, Predicting the Nep Number in Cotton Yarn – Determination of the Critical Nep Size, Textile Research Journal 72 (10), 917-923 (2002), 12. Uster Statistics 1989. Uster News Bulletin No 36 (1989). 13. Uster® News Bulletin No 40, Zellweger Uster, Customer Information Serwis (1997). 14. Matusiak M., Predicting the Quality of Cotton Yarns on the Basis of Metrological Characteristics of Raw Material, Ph. D. thesis at the Technical University of Lodz, (2002).

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