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技術解説The Structure of Rotor-Spun Yarns

Part H:Effects of Fibre Property on Yarn Structure

R.H.Gong and R.A.Jhatial

Department of Textiles,University ofManchester Institute of Science and Technology,Manchester M60 1QD,UK

AbstractThis paper reports the results of an in-depth investigation into why the measuredtwist of

rotor-spun yarns is dependent on the spinning process parameters,especially fibre

properties.The investigation focuses on the study of yarn structure using a visualmeasurement method in conjunction with the use of tracer fibres.The measurement systemis based on computer analysis of digitised video images of the yarn.The system can revealthe twist level within different radial layers of the yam.The results show that rotor-spun

yarn structures are affected by fibre properties and the difference in yarn structure is themain cause for the apparent twist difference reported previously.

Keywords:Rotor spinning,yarn structure,twist measurement.

1. Introduction

Conventional twist testing methods based on theuntwisting-retwisting principle cannot measure the real twistin rotor-spun yams.This is because rotor-spun yams have a

variable twist structure from the yarn surface to the yarn

core and the yarn cannot be truly untwisted.Numerousattempts were made to test the actual twist in rotor-spunyams.Lord tried to measure the twist in rotor-spun yarnswith the Rockbank and torsional equilibrium methods ill,butsimilar to other methods based on the torsional properties ofthe yam this could not overcome the problem of twist

variation in the radial direction of the yam.Louis measuredthe twist on the basis of counting the number of stripes perunit length of the yarn 123,but due to the inherent structure ofrotor yarn,it was difficult to distinguish between wrappingsand stripes.Salhotra measured the twist in the yam core by

removing the wrappings from the yarn surface 131.However,the removal of tight wrappings and wrappings with endsburied in the yarn body distorted the yarn structure.None ofthese methods can give the true twist level reliably and twisttesting is currently still based on the traditional

untwi sting-retwi sting principle.Furthermore,several researchers have reported that in

general the measured twist of rotor yams is lower than themachine twist and when tested under the same conditionsand for the same machine twist,the measured twist isdependent on the process parameters and on the fibre

property in particular [4-7.However, there is no publishedliterature giving conclusive explanations of the causes of this

phenomenon.We report in this paper a visual system that can be used to

examine the yarn twist structure in all radial layers and alsoexperimental results that reveal why the measured twist of

rotor-spun yams is dependent on process parameters.

2. Twist Measurement

In order to examine the twist structure of rotor-spun yams,

a twist measuring system based on computer imaging and

incorporating the tracer fibre technique [8] was developed.Itwas shown recently that using image analysis alone couldonly measure the twist of surface fibres 193.By using blacktracer fibres in a predominantly white yarn,the yam twiststructure can be examined from the tracer fibres after the

white fibres are optically dissolved. As the tracer fibres arerepresented in all redial layers of the yam,it is possible toexamine the twist structure in all the yam layers.Usually,only O.1% of the yam is made up from tracer fibres;in ourmethod,2.3% of the yam is made up from black tracer fibres.The higher amount of black fibres increases the accuracy of

the measurement of twist in all radial layers of the yarn.The

system is schematically shown in Figure 1.The twist testing system comprises of four main

components:a yam carriage unit,a video microscope,aninterfacing card and a personal computer.Transmitted lightis used for the microscope.In order to optically dissolve thewhite fibres,a special liquid of paraffin oil and mono-bromonaphthalene was prepared separately for the yams spun from

polyester,viscose and cotton fibres.The refractive index ofliquid for the yams was calculated by using the followingformula:

(1)

where:

N is the required refractive index;

nl and n2=the refractive indices of the two liquids used;

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(繊 維 工 学)Vol.54,No.10(2001) 31

V1,V2=the volume of the two liquids.The image of the yarn immersed in the liquid is magnified

by the optical microscope.The image from the microscope

is converted into video signal by the video camera.The

video signal is digitised by the TV card which then sends the

digital signal to the CPU for further processing.The images

can be viewed on the PC monitor and/or stored on a disk

with 24-bit resolution.The image quality is very high,

enabling the differentiation of twist and fibre crimp and theeasy measurement of twist.

Wrapper fibres should be ignored during twist

measurement because they do not represent the yarn twist.

To avoid repeat measurement of twist in the same fibre or

the same group of fibres,an interval of at least 75 mm wasset between consecutive measurements by rotating the

winding wheel.For each yarn,100 measurements were

taken and five meters of the yarn was discarded between

every twenty measurements.The number of twist per meter

was calculated by:

(2)

Where:

tf=number of twists per frame;

lif=frame length in mm.

3. Results

In order to validate the visual twist testing system,a set of

ring-spun yarns were made on a SKF laboratory ring frame

from the fibres listed in Table ‡T.The yarn samples were spun

to a linear density of 35 tex and a twist level of 485 turns per

meter.The yarns were tested using the visual system

described above and the Zweigle D 301 automatic twist

tester based on the double untwist-retwist principle.100

measurements were taken using the visual system and 60

using the Zweigle D 301 for each sample.The results are

shown in Table ‡U.

Rotor-spun yarn samples were prepared using a Rieter

RU14 machine with a 40 mm rotor running at 71,000 rpm.

The yarns were all produced with a nominal linear density of

tested using the procedures described above. The results are

shown in Table ‡V.The twist efficiency is the percentage of

measured twist as compared to the machine twist.

Fig. 1 Schematic diagram of

the visual yarn testing system.

Table ‡T Fibre Material

Table ‡U Measured Twist of Ring-spun Yarns

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4.Discussions

From Table ‡T,it is clear that the visual twist testing

method gives similar results to the Zweigle D 301

instrument which is generally accepted to be accurate for

testing ring-spun yarns.Statistical analysis using the"t"test

showed that the differences between the mean values of the

two twist testing methods are not significant within the

confidence interval of 95%.The higher variations in the

visual system results are due to the small field length of the

image during measurement.The typical field length of the

image is only 5 mm as compared to the 500 mm testing

length for the Zweigle D 301.This difference in variation is

consistent with the fact that spun yarns usually have higher

shorter-term irregularity.

Table ‡V Measured twist of rotor-spun yarns

Fig. 2a 1.2 dtex polyester

Fig. 2b 1.6 dtex polyester

Fig. 2c 2.4 dtex polyester

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(繊維 工 学)Vol,54,No.10(2001) 33

Fig. 2d Viscose (2.4/1.7 blend)

Fig. 2e Cotton

Fig. 2 Surface structure of rotor-spun yams.

Fig. 3a 1.2 dtex polyester

Fig. 3b 1.6 dtex polyester

Fig. 3c 2.4 dtex polyester

Fig. 3 Core structure of rotor-spun yams.

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34 繊 維 機 械 学 会 誌

The results in Table ‡V show that the twist level in

rotor-spun yams as measured by the visual system is similar

to the machine twist.However,the twist measured using the

Zweigle D 301 is dependent on the fibre type,which

confirms the results of other researchers.

In order to find out the cause for apparent twist loss as

measured by the traditional double untwist-twist method,the

yarn samples were examined under microscope.Figure 2

shows some examples of the yarn structure.It is clear that

yams spun from fibres that have a lower bending stiffness,

such as the coarser polyester fibres,tend to have more and

tighter wrappings.These structural differences are the main

cause of the widely reported twist loss for polyester

rotor-spun yarns,when twist is measured using methods

based on the untwisting-retwisting method.The difference in

wrapping structure leads to differing resistance to the

untwisting of the yarn;yams with more and tighter

wrappings have a higher untwisting resistance and therefore

giving misleadingly lower twist readings on the Zweigle D

301 or similar testing instruments.Cotton fibres are shorter

and stiffer than most synthetic fibres used for staple yarn

production and hence they have higher resistance to

wrapping.This is the primary reason why yams spun from

cotton have been reported to show the highest twist

efficiency.

To further confirm the above findings,the core twist

structure of the yarn was studied.Figure 3 shows the core

yarn structure after the removal of surface wrappings.

Clearly, the yarns are similar in their core twist structures

regardless of the fibre stiffness.

It should be pointed out that the twisting measurement

system described here is not suitable for routine factory use.

This is because tracer fibres have to be used,which will notbe appropriate for commercial yarn production.

5.Conclusions

The visual method reported in this paper can provideaccurate twist testing for rotor-spun yams.When the twist istested using methods based on the untwisting-retwisting

principle,the twist values are lower than the machine twistfor yarns made from longer fibres with a lower bendingstiffness.This is mainly due to the increased wrapping in the

yarn.However,the core twist structures are similar for yarnsspun from different fibres.

References

[1]Lord,P.R.,and Grady,P.L.,1976.Text.Res.J.,46,123.

[2] Louis,G.L.,1978.Text.Res.J.,48,293.[3] Salhotra,K.R.,1981.Text.Res.J.,51,710.[4] Audivert,R.,1988.J.Text.Inst.,79,333.[5] Barella,A.,Castro,L.Manich,A.M.,and De Castellar,

M.D.,1986.Melliand Textilber.,67,779.

[6] Gong,R.H.and D.Senthil Kumar,1999.Proceedings ofATC'99,Kyoto,Japan,205-208.

[7] Kaushik,R.C.D.,Salhotra,K.R.and Tyagi,G.K.,1994. Indian Journal of Fibre & Textile Research,19,7.

[8] Morton,W.E.and Summers,R.J.,1947.J.Text.Inst, 34,T.19.

[9] Cybulska,M.,1999.Text.Res.J.,69,369.

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