Indian Journal of Fibre & Textile Research Vol. 27, March 2002, pp. 18-24
Influence of filament core surface structure on tensile properties of DREF-3 yarns
S M Ishtiaque" & K R Salhotra
Department of Text ile Technology, Indian Institute of Technology, New Delhi 11 00 16, India
and
R V M Gowda
Department of Textil e Technology, Bannariamman Institu te of Technology, Sathyamangalam 638 401 , India
Received 16 October 2000; revised received and accepted 8 January 2001
The inOuence of core filament surface structure on tensile properties of friction-sp un yarns has been studied. The yarn spun from Z pre-twist filament core has superior properties while that spun from S pre-twist filament core shows inferior quality as against the yarn spun with Oat fi lament core. Yarns spun with air-jet textured fil ament core exhibit significantly lower tenacity, breaking extension , modulus and energy-lO-break as compared to those made from Oat andlwisted forms. However, in respect of sheath con tribution, the yarns spun with air-je t textured fila ment core perform beller. The sheath cOlllribution and sheath slipping force increase wi th the increase in core filament overfeed during texturing. The sheath cOlllribution is highest fo r the yarn spun from filament with 30% overfeed.
Keywords: DREF-3 yarn, Fi lament- to-fibre frict ion, Flat filament core, Sheath slipping force, Tensile propert ies, Textured filament core, Twisted filament core
1 Introduction Friction spinning has made a substantial progress
during the last two decades and has established a strong positi on in the coarse count range. It has got several distinguishing merits, like high production speed, very high twisting rate and low yarn tension during spi nning. It stands unique in the production of multi-component yarns. However, like other spinning systems, it has some drawbacks, especially in regard to the method of fibre feeding and nature of twisting.
As regards the method of fibre feeding, friction spinning (DREF-2 and DREF-3) employs the standard vertical fibre feeding method where individuali zed fibres carried in the transport duct by aero-dynamic forces are progress ively decelerated as they assemble at the nip of the spinning rollers . This phenomenon, referred as compress ing effed, leads to fibre buckling which results in fibre disorientation, low fibre ex tent and low sheath fibre contribution to the strength of the resultant yarn. Further, the system lacks control over the fibre flow in the transport tube, which results in an uncontrolled fibre assembly in the nip of spinning rollers. The on ly mechan ism of fibre
"To whom all the correspondence should be addressed. Phone: 6591940; Fax: 0091-01 1-6581 103; E- mail : isilt [email protected]
control in both the transport tube and the spinning nip is the fibre-to-fibre, fibre-to-metal and fibre-to-air friction2
.
The twisting in friction spi nning is negative in nature and, therefore, direct twist manipulation is not possible. This causes considerable amount of slippage between the fibre assembly and the fri ction rollers' surface, resulting in the inadequate tightness and frequency of wrapping of sheath fibres over the core component and this leads to differential twist structure.
The above drawbacks have led to low yarn strength and poor surface integrity, which, in turn, limit the end uses of friction-spun yarn as a general purpose yarn. The friction spinning technology has reached the limit in respect of machine design to overcome the aforementioned drawbacks. Nevertheless , it provides the researchers an ample scope to accomplish it through optimization of process parameters and selection of suitable raw mate rial characteri stics. This would enable them to engineer the yarns for specific end uses and hence exploit the system in a better and more useful manner.
It is expected that the fibre characteristics in order of importance for opti mum yarn quality in friction spin ning are fric tion, strength, fineness, length and cleanliness3
,4. Fibre friction plays a vital role during
ISHTIAQUE et al. : TENSILE PROPERTIES OF DREF-3 YARNS 19
the yarn formation and significantly influences the yam properties. There are basically two forms of friction encountered during yarn formation, namely fibre-to-drum surface and fibre-to-fibre friction. The fibre-to-drum surface friction is of both sliding and rolling types5
, while the fibre-to-fibre friction is more of sliding type. These two forms of friction determine the extent of fibre slippage between the drums' surface and yarn surface and the intensity of wrapping of core by the sheath. These two aspects decide the level of radial pressure exerted by the sheath on the core and the cohesive interaction between the two components, which largely govern the mechanical properties of the yarn.
The frictional cohesion between the core and the sheath can be varied in several ways, more commonly by changing the surface structure of either one or both the components. This can be accomplished through mechanical means by twisting and texturing in case of filaments and chemical means like application of fibre finish in case of staple fibres.
In a filament core friction-spun yarn, the core offers good yarn strength and uniformity while the sheath provides spun yarn characteristics, besides being a substrate for the application of special finishes, when the yarn is used for industrial textiles. Nevertheless, these yarns suffer from the disadvantage that the staple fibre sheath slips over the filament core due to the abrasive actions during the subsequent processes which therefore restrict their end uses6
.
The present study was aimed at improving the frictional cohesion between the core and sheath by using different filament cores of varying surface structures and hence the tensile properties of the resultant yarns.
2 Materials and Methods 2.1 Materials
Acrylic fibres of 38 mm and 1.65 dtex were used for sheath. A 110/48 dtex polyester multifilament yarn was used for core in three different forms, namely flat, twisted and textured. The polyester multifilament yarn with zero twist is considered as flat filament. The Z and S twist filaments with 315 tpm (8 tpi) were made from flat filament. This level of twist was more or less optimum so as to bring about a change in its surface structure without causing much deterioration in its mechanical properties. The flat filament was also air-jet textured with three different overfeed rates (10, 20 and 30%) at a temperature of
190°C and air pressure of 9 kg/cm2 to produce three types of filament cores with different loop sizes and surface structures. In total, six types of filament core (FI-F6) were used to spin yarns.
2.2 Fibre!Filament Testing 2.2.1 Tensile Properties
Acrylic fibres were tested on Lenzing Technik 's Vibrodyn for tensile properties at a gauge length of IOmm and a test speed of 20 mm/min. Polyester filament cores were tested for tensile properties on Instron tensile tester at a gauge length of 500 mm and a test speed of 300 mm/min. The tensile properties of various core filaments and acrylic sheath fibres are given in Table 1.
2.2.2 Frictional Properties
Acrylic fibres and core filaments were tested for frictional properties, namely filament-to-fibre and fibre-to-fibre friction using a special device (Fig. I) attached on Instron tensile tester. The fibre fringes of areal density 5 mg/cm2 are prepared by doubling, drawing and combing a sample of fibres taken from sliver. The short and any stray long fibres are removed. The remainder, forming the modal part of the samples with co-terminus ends, is cemented to a piece of card. In case of fibre-to-fibre friction, one fibre fringe is fixed on the metallic platform covered with a sheet of Teflon and the other is hold by the holder slides over the former at the rate of 10 mm/min under an applied normal force of 40 cN. The computer, online with the Instron, plots the friction profile and calculates the frictional force. The test details have already been described by Salhotra et aC. For filament-to-fibre friction, no standard test method is available. However, the following procedure was used to approximately estimate the friction between core-filament surface and the sheath fibres as it happens during the yarn formation. The filament was wrapped onto a thick paper strip with 98 wraps linch. The length and width of the strip were 50 mm and 30 mm respectively. The filaments were firmly attached to the strip at both the ends to avoid any distortion or loss of twist. The paper strip was cut except at the edges, which hold the sheet of filaments fixed to it. This sheet of filaments was fixed at both of its edges on the platform of friction measuring device. The fibre fringe attached to the fibre holder slides over it. The frictional force is computed in the similar way as in case of fibre-to-fibre friction. The frictional characteristics are given in Table 2.
20 INDIAN J. FIBRE TEXT. RES., MARCH 2002
Table I-Tensile properties of core filaments and sheath fibre
Core Filament Linear Breaking load Tenacity Breaking Initial modulus Energy-to-break fil ament code density cN cNltex extension cNltex J
tex %
Fl at F I 11.11 317.7 28 .6 21.1 724.1 25.4x I0·2
(3 .9) ( 10.1) (2.8)
Ztwist F2 11.33 3 18.8 28 .1 20.4 668.6 23 .4x 10-2
(5 .5) (14.2) (2.1 )
S twi st F3 11.33 316.6 27 .9 19.9 672.0 23.0X I0-2
(4.8) (13 .0) (2 .0)
AJT IO F4 11.44 295 .8 25.9 18.4 599.9 18.4xI0·2
(6.6) (11.5) (6.3)
AJT20 F5 11.67 292 .2 25.0 17.7 567.2 14.8x I0·2
(5 .6) ( 15.5) (6.8)
AJT30 F6 12.00 239.7 20.0 16.7 305.2 11.3x 10-2
(6.7) (13.0) (7.1)
Sheath 1.65 dtex 3.6 21.4 51.6 97 .9 1.16x 10-4 (Acrylic fibres) ( 10 .1 ) (14.1) (24.8)
AJT IO - Air-jet textured filament with 10% overfeed; AJT20 - Air-jet textured filament with 20% overfeed; AJT30- Air-jet textured filament with 30% overfeed. Values in parentheses indicate coefficient of variat ion (%)
8
7
3
5
Fig. I - Friction measuring device [I-metallic platform, 2-clamp for fixed fringe, 3-holder for movable fringe, 4-fibre fringes, 5-hook, 6-frictionless pulley, 7-<:otton cord (string), and 8- moving cross-head of instronl
2.3 Preparation of Yarn Samples Using each of six filaments in the core and acrylic
fibres in the sheath, six yarn samples (YI-Y6) were spun on DREF-3 friction spinner using the following process parameters: yarn count, 37 tex; core-sheath ratio, 30:70; spinning drum speed, 4250 rpm; yarn delivery rate, 175 mlmin; friction ratio, 3.5; and suction air pressure, -25 mbar.
All the process parameters were kept constant so that the yarn quality is influenced only by the friction between core and sheath due to the filament-core surface structure.
Table 2 - Frictional properties of core filaments and sheath fibre
Core Fi lament-to- fi bre Fibre-to-fibre filament friction friction
code Frictional Coefficient Frictional Coefficient
force of friction force of friction cN cN
FI 10.5 0.26 17.2 0.43 F2 10.1 0.25
F3 10. 1 0.25
F4 13.4 0.33
F5 15.1 0.38
F6 17.6 0.44
2.4 Yarn Testing The following tests were carried out to assess the
influence of filament core surface structure on tensile properties of friction-spun yarns.
2.4.1 Tensile Properties The yarn samples were tested on Instron tensile
tester for tenacity, breaking extension and other tensile properties at a gauge length of 500mm and test speed of 300 mmlmin. The computer, online with the Instron, directly computes the mean and CVO/O values of tenacity, breaking extension, modulus and energyto-break. The parameters, like ratio of resultant yarn tenacity to core filament tenacity, percentage increase in breaking strength of resultant yarn due to the sheath, and sheath contribution, were determined to assess the influence of core fil ament surface structure on yarn quality.
ISHTIAQUE el at. : TENSILE PROPERTIES OF DREF-3 YARNS 21
2.4.2 Sheath Slippage Resistance The sheath slippage resistance of a filament core
yarn can be expressed in two ways, namely strip resistance and sheath slipping force. The strip resistance is a measure of the number of cycles required to strip the sheath fibres from a certain length of core.
The sheath slipping force, used in the present study, has been taken as the force required to cause sheath fibres to slip over a certain length of core. It can be of practical significance due to the fact that there is possibility for the sheath to slip over the core when the yarn passes at high speeds over sharp, rough and abrading parts of machines during post spinning processes.
A new method has been devised to measure the sheath slipping force. A special type of clamp has been fabricated for this purpose. It consists of two adjustable jaws where the face of one of them is lined with zero gauge emery paper and that of the other carries minute serrations to provide adequate but not a positive grip. There is also a provision to vary the normal force between the two jaws of the clamp through a spring and screw arrangement. Although the normal force cannot be measured accurately, it can be adjusted by trial and error through tensioning or relaxing the spring to an amount just sufficient to cause sheath to slip over the core without damaging the filament. The clamp is connected to the upper moving crosshead of Instron with the help of a hook . The lower end of the yarn is gripped in the fixed jaw of Instron. The upper portion of the yarn is held in between the jaws of the clamp with certain length of yarn projecting out from the clamp. The test was carried out at a gauge length (length of yarn gripped between the fixed jaw of Instron and the jaws of the clamp) of 50 mm and test speed of 500 mm/min. The gauge length of 50 mm was selected so that it is sufficiently longer than the sheath fibre length (38 mm) to avoid any deliberate breakage of sheath fibre. The test was also carried out at a gauge length of 100 mm and test speed of 250 mm/min to observe the effect of test parameters on sheath slipping force. As the test starts, tension develops in the yarn and reaches a maximum (below the level of yarn breaking force). The sheath now may have partially broken and starts slipping over the core filament. The computer, online with the Instron, gives a profile of the sheath slipping force plotted against the sliding distance of 50mm (Fig. 2) . Two important parameters, namely the maximum slipping force (S max) at which the
400r---------------------------~
~ 300 ., u "-
If 0>
.~ 200 a. iii .I: C .. Vi 100
o~~~ __ ~~ __ ~~ __ ~~~~~ o 5 10 15 20 25 30 35 40 45 50
Sliding Oistance.mm
Fig. 2 - A typical profile of sheath slippi ng force
35,--------------------------------, ~F l OIF2 SF3
30
TenaCIty. cN/tex Breaking Extension. %
Fig. 3 - Tenacity and breaking extension of various core filaments (FI-F6)
sheath starts to slip over the core and the average slipping force (Smean) can be determined from the graph.
3 Results and Discussion The flat filament core and the yarn spun from it are
considered as control samples. The results of all other samples are compared with those of the control samples and subjected to tests of significance at 95% level of confidence.
3.1 Properties of Filament Cores 3.1.1 Tensile Properties
Table 1 shows that the filament cores F2 and F3 have significantly lower tenacity and initial modulus than the filament core F 1, which can be attributed to the filament obliquity. The three air-jet textured filament cores F4, F5 and F6 are inferior to FI as they show a significant difference with regard to tenacity, breaking extension, initi al modulus and energy-tobreak (Fig. 3). This can be ascribed to the molecular deformation and disorientation caused by the thermo-
22 INDIAN J. FIBRE TEXT. RES ., MARCH 2002
aerodynamic effects during the process of air-jet texturing.
3.1.2 Frictional Properties Table 2 shows that there is no significant difference
amongst the frictional properties of filament cores FI, F2 and F3. However, filament cores F4, F5 and F6 exhibit considerably higher values of friction as compared to FI, which is due to the surface modification by texturing.
3.2 Yarn Properties 3.2.1 Tensile Properties
Table 3 shows that the tensile properties of the yarn Y2 are superior to those of the yarn Y 1. The yarn Y3 does not differ from yarn Y 1 in respect of tenacity, however, it exhibits lower breaking extension and energy-to-break. These differences are being explained on the basis of cohesion between core and sheath as follows. In friction spinning, the core is false twisted in S direction in the pre-spinning zone and initial part of the spinning zone, and in Z direction during latter part of the spinning zone8
. Due to this false twisting effect, the filament core with Z pre-twist is false twisted in the S direction, causing partial or complete removal of pre-twist (extent depends upon the filament pre-twist level and friction ratio). The removal of filament pre-twist opens up the core and increases the surface area of the filament, resulting in better gripping of sheath fibres at the core-sheath interface. The sheath is Z twisted along with the core in the remaining half of the spinning zone, leading to better cohesion, greater sheath contribution and hence higher strength of the resultant yam. The S pre-twisted filament core, on the other
hand, is further fa lse twisted in the same direction of pre-twist. This makes the core compact and does not allow much anchoring of sheath in the core. This results in less contribution of sheath to the strength of the resultant yarn.
The tenacity values of all the three air-jet textured fil ament core-spun yarns (Y4, Y5 and Y6) are significantly lower than that of the yarn YI (Fig. 4). This observation is in agreement wi th the findings of Ishtiaque et a1. 9
. The tenacity and modulus decrease while the extension and energy-to-break increase with the increase in overfeed. The increase in overfeed increases the filament bulk, which in conj unction with the mechanical deformation reduces the load bearing capacity of the filament core and hence results in lower yam tenacity and modulus. The higher overfeed increases the loop size and frequency , causing greater surface area of the fil ament, which, in turn, increases the filament-to-fibre friction (Table 2). The increased loopy surface captures the sheath fibres, providing a
30 ,----------------------------------, ISI YI m Y2 !:I Y3
20
15
10
o T emci ty, eN Itex Breaking Extension. %
Fig. 4 - Tenacily and breaking eXltension of various resultant yarns (YI-Y6)
Table 3 - Tensile properties of yarns
Core Yarn Linear density Breaking load Tenacity Breaking lnitial Energy-to-filament code tex cN cN/tex extension, % modulus break, J
eN/tex
Flat YI 37.8 5 17.7 13 .7 24.1 214.3 42.9x lO·2
0.3) (5.5) (3.3)
Ztwist Y2 37.0 514.3 13.9 24.6 209.0 43 .0 X 10.2
(3.9) (5 .9) (4.4)
S twist Y3 37.3 510.9 13.7 23 .0 212.6 39. Ix 10.2
(4.9) (9.3) (4.5)
AJT IO Y4 37.0 493.6 13.4 21.6 157.0 32.8 X 10.2
(6.5) (10.3) (9.0)
AJT20 Y5 37.7 499.8 13.3 22.2 155.2 33.9 x lO·2
(6.0) ( 10.2) (8 .3)
AJT30 Y6 37.6 478.8 12.7 23.3 128.8 35.0 X 10.2
(4.4) (7.0) (4.6)
Values in parentheses indicate coefficient of variation (%)
ISHTIAQUE er al . : TENSILE PROPERTIES OF DREF-3 YARNS 23
good cohesion to them and enable the sheath to contribute more to the strength and extension and thus the energy-to-break of the resultant yarn.
3.2.2 Core and Sheath Contribution Both the core and the sheath contribute to the
resultant yarn properties. Their individual contributions cannot be distinctly segregated. However, for the clarity of understanding, certain parameters that signify the individual effect have been calculated.
It is clear from Table 4 that in yarns Y 1, Y2 and Y3, the core and the sheath contributions are not very different. This can be attributed to the fact that the tensile (Table 1) and frictional (Table 2) characteristics of the three filament cores are not very different. Thus, the sheath contributes more or less equally in these yarns.
The yarns Y4, Y5 and Y6 exhibit significantly different sheath contributions as compared to the yarn Y 1, owing to the fact that the tensile (Table 1) and friction (Table 2) characteristics of the respective filament cores are different. The sheath contribution in terms of breaking load and tenacity increases with the increase in filament overfeed in texturing and is highest in case of yarn spun with filament core having
30% overfeed (F6) . It can be observed that the tenacity of filament F6 is around 70% of that of the filament FI, whereas the tenacity of yarn Y6 is about 93% of that of the yarn Yl. This can be attributed to relatively higher filament-to-fibre frictional coefficient in case of F6 than that in case of Fl. The higher frictional cohesion between core and sheath plays a significant role and compensates, to a certain extent, the structural weakness of core component through higher sheath contribution. The ratio of resultant yarn tenacity to core filament tenacity and percentage increase in resultant yarn breaking force due to the sheath contribution (Figs. 5 and 6) are more or less the same in case of Y 1, Y2 and Y3 and some
70 ,--------------------------,
~ Yl ID Y2 cY3 60
~ 50
" ... u 40 .S ;:: ~ 30 ... '" ~ 20
10
OY4 IZI Y5 IDY6
o .L.--'-"-'~~
Fig. 5 - Ratio of resultant yarn tenacity to core filament tenacity
Table 4 - Core and sheath contribution in the resultant yarn
Linear density, tex (Ft) Breaking load, cN (FB) Tenacity, cN/tex (FT)
Linear density, tex (Yt) Breaking load, cN (Y B) Tenacity, cNltex (YT)
Linear density, tex (St) Breaking load, cN (SB) Tenacity, cN/tex (ST)
(Resultant yarn tenacity)/ (core filament tenacity) x 100, %
Core filament FI F2
Il.ll 11.33 317.7 318.8 28.6 28.1
Resultant yarn
Y1 Y2 37.8 37.0
517 .7 514.3 13.7 13.9
Sheath contribution
SI S2 26.7 25.7
200.1 195.5 7.5 7.6
47.9 49.5
Per cent increase in breaking strength of 63.0 61.3 resultant yarn due to sheath contribution"
St=Yt- Ft; SB=YB-FB; ST= SB/ St a increase in breaking strength = (Y B - FB) / FB = SB / FB
F3 11 .33 316.6 27.9
Y3 37.3
510.9 13.7
S3 26.0 194.3 7.5
49.1
61.4
F4 F5 F6 11.44 11.67 12.00 295.8 292.2 239.7 25.9 25.0 20.0
Y4 Y5 Y6 37.0 37.7 37.6
493.6 499.8 478.8 13.3 13.2 12.7
S4 S5 S6 25 .6 26.0 25.6 197.8 207.6 239.1 7.7 8.0 9.3
51.4 52.9 63.7
66.9 71.0 99.7
24 INDIAN J. FlBRE TEXT. RES ., MARCH 2002
120 ,----------------, f'il Yl m Y2 § Y3
100 QY4 r2Y5 DY6 ., ((j
ti 80 .£1
E 60 (,) ... u
Po. 40
20
Fig. 6 - Per cent increase in resultant yarn breaking strength due to the sheath contribution
what higher for Y4 and Y5. This contribution is very high in case of Y6 for the reasons explained earlier.
3.2.3 Sheath Slipping Force Table 5 shows that the sheath slipping force varies
significantly with the specimen gauge length. Both the maximum and average slipping force reduce as the gauge length increases from 50mrn to 100 mm, which can be due to the fact that the tension in both core and sheath reduces with the increase in specimen length, and this may cause the sheath to slide over the core more easily. The test speed in the range of 250-500 mmlmin does not show any significant influence on the sheath slipping force.
Table 6 shows that the sheath slipping force for the yarn Y2 is higher than that for both the yarns Y 1 and Y3. This can be attributed to better anchoring of sheath in core. The yarns Y 4 and Y5 exhibit significantly higher sheath slipping force as compared to yarn Y 1, which can be explained on the similar basis as discussed in section 3.2.1. In case of yarn Y6, this force is so high that it could not be measured. The yarn Y6 either breaks when the normal force between the jaws is increased or the jaws just slide over the sheath without causing it to slip over the core when the normal pressure between them is lowered. This clearly shows that there is a strong frictional cohesion between the core and the sheath of the yarn Y6, resulting in excellent sheath slippage resistance.
4 Conclusions 4.1 The friction between the core and the sheath and the anchoring of sheath fibres in core playa decisive role in the tensile properties of friction-spun yarns.
Table 5 - Sheath slipping force relationship with gauge length and test speed
(Flat fil ament core friction-spun yarn, Y I)
Gauge length Test speed Sheath sl ipping force, cN mm mm/min Maximum Average
force force
50 500 331 .2 (22.5) 108.2 (17.9)
100 500 308.8 (20.5) 96.2 (21 .6)
50 250 329.4 (19.8) 107.2 (19.5)
Values in parentheses indicate coefficient of variation (%)
Table 6 - Sheath slipping force
Yarn code Sheath slipping force a, cN -----
Maximum force A verage force
YI 331.2 (22.5) 108.2 (17 .9)
Y2 350.6 (21.0) 130.5 (16.8)
Y3 317.2 (22.1) 107.2 (14.9)
Y4 392.5 (17.7) 150.4 (17 .6)
Y5 394.7 (17.4) 166.5 (14.5)
a Sheath slipping force for Y6 could not be measured. Values in parentheses indicate coefficient of variation (%)
4.2 The use of a filament core with the pre-twist, opposite in direction to the twist inserted by the DREF-3 machine, results in superior yarn tensile properties as compared to a flat filament core. 4.3 Yarns spun with air-jet textured filament cores exhibit relatively inferior tensile properties, however, they show higher sheath fibre contribution and greater sheath slipping force. 4.4 The use of twisted and textured filaments in core may prove to be costlier due to the additional processes. Nevertheless, the use of filament cores of desired tensile and surface properties ensures requisite y.arn properties and provides scope to engineer yarns, especially for technical textiles.
References I Fehrer E, TeXf Prax Inst, 10 (1986) 2-6. 2 EI Mogahzy Y E & Broughton R M, Text Res J, 63 (1993) 465. 3 Deussen H, Text Month, II (1984) 41. 4 Salhotra K R, Indian J Fibre Text Res, 17 (1992) 255. 5 Shantong Z, J Text Ins!, 81 (1990) 79. 6 Miao M, How Y L & Ho S Y, Text Res J, 66 (1996) 676. 7 Salhotra K R, Ishtiaque S M & Gowda R V M, Friction in
acrylic fibres, Indian J Fibre Text Res (in press). 8 Merati A A, Konda F, Okamura M & Marui E, Text Res J, 68
(1998) 441. 9 Ishtiaque S M & Swaroopa T K, Asian Text J, 10 (1998) 28.
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