Wrapped yarns have been developing since 70s. Due to its special structure, the yarns have many advantages such as high tenacity and regularity ^[1], these kinds of spinning methods cater the needs of fancy and high performance yarns ^[2].

Wrap spinning works on the principle of wrapping. The feed stock, generally a drawn sliver drafted in a five-roller drafting system. The drafted strand runs through a hollow spindle without receiving true twist. Simultaneously, a continuous filament yarn, unwound from a bobbin on the hollow spindle is wrapped helically over the drafted strand of fibres forms wrap spun yarn. The wrap spun yarn therefore consists of a twistless core (parallel fibres) wrapped helically by a continuous filament as shown in Figure 1. These yarns are also known as cover-spun yarns and the method of spinning is also known as hollow-spindle spinning.

Hollow-spindle Spinning System

Parafil® spinning

Many manufacturers offered different process of wrap spinning and popularly known wrap yarns (PL yarns) from Spindelfabrik Suessen, Germany. It uses four or five roller drafting arrangements, depending on the raw material to be processed ^[3].

The Parafil® hollow spindle rotates up to 35,000 rpm. The hollow spindle is designed as a false twisting assembly. The fibre strand does not pass directly after entering the vertical spindle instead, shortly after entering the spindle; this strand is led out again and back around the spindle with a wrap of about one-quarter of the spindle periphery. In this way, as the spindle rotates, the strand is provided with twist between the drafting arrangement and head of the hollow spindle. These turns of twist are cancelled out again in the spindle head in accordance with the false twist principle. This twist prevents the strand from falling apart in the length prior to filament staple fibres wrapping with filament.

A wrap spun yarn consists of non-twisted parallel bundle of staple fibres held together by helically wrapped filament as shown in Figure 2. The proportion of the filament in the yarn is about 2 - 5%. Due to helical wrapping of the filament and by radial pressure, necessary cohesion between the individual staple fibres is improved. This imparts desired strength to the yarn. The number of wraps per unit length in a standard PL yarn is approximately the same as the amount of twist in a comparable ring spun yarn.

S-Allma® Wrap Spinning 

Saurer-Allma SM series of hollow-spindle wrap-spinning machines designed to produce count in the range of 0.6 Nm to 50 Nm. S-Allma® wrap spinning is available in three levels of traverse length (SM-6, SM-8 and SM-10 with traverse length of 15 cm, 20 cm and 25 cm respectively). It has 48 spindles with speeds up to 30,000 rpm and production rate 200 m/min. The delivery roller and spindle speeds can be varied infinitely and draft is adjustable according to the linear density ^[4].

The 3 or 4 roller drafting assembly allows selection of either the clamping point or draft methods with a draft range of 5 - 59. Fibre length between 50 - 220 mm can be processed effectively. Electronic monitoring used to identify the faults and improves yarn quality. Special features of the drafting assembly include self-alignment of the spinning triangle to the centre of the spindle and continuous suction at the draft rollers, which eliminates roller lapping and assist for self cleaning in spinning zone.

Pneumatic threading is an optional attachment to hollow spindle and speeds up the operating time. A photoelectric cell monitor equipped to detect end-breaks to stop the spindle when a break occur ^[4].

Gemmill & Dunsmore® Wrap Spinning

Another one wrap spinning apparatus for the wrap yarn production is Gemmill & Dunsmore® wrap spinning system. It has five-roller drafting system and provides draft up to 300. Heavy slivers and ropes are also processed without much difficulty. The inclined (45°) drafting system is also adjustable according to process parameters^[5].

'X' Direction Filament-Wrapping System

Similar to Hollow-spindle system 'X' direction filament wrap spinning system^[6] is used for fancy and high performance yarns production. This yarns produced by wrapping around a twisted or twistless fibrous core by filament or yarn. Twist can be applied in either the right hand (Z) or left hand (S) direction or both as shown in Figure 3.

Filament-wrapped yarns can be produced at a much higher production rate than conventional ring spun yarn^[7]. It can eliminate slashing operation^[8]. Twistless cover spun yarns also produced by using wrapper as water soluble adhesive filaments and core as no twist fibres.

In conjunction with the no-twist yarn production^[9], X direction filament-wrapped yarns developed as shown in Figure 1. The theory behind the X-wrapping technique is the adaptation of the Chinese puzzle principle, in which a braided, finger-size tube becomes tighter around a finger when a pulling force is being applied along the longitudinal axis in an attempt to pull the tube off from finger.

A simplified explanation for the tightness is that a compression force is being generated along the wall when being pulled. The Chinese puzzle phenomenon is analogous to the X-wrap yarn when used in dynamic processing. The stress exerted on the X wrap yarn during weaving/knitting will actually cause the criss-cross filament wraps to tighten around the fibrous core and thus cause the yarn to become stronger rather than weaker. This approach is similar in principle to that of the Machie Spinmach system, which was designed primarily for worsted and semi worsted yarns ^[9]

Investigation shows that X-wrap yarn has higher yarn strength and elongation than a conventional wrap yarn and it can be produced as one-step process. X-wrap yarn costs more to produce in terms of filament, but from preliminary yarn data it may be feasible to make X-wrap yarns with fewer wraps/metre than Z or S wrap yarns and thus hold down filament cost.

Comparing 120 wraps per metre (wpm) over 512 wraps per metre, yarns with higher wpm possess higher strength and elongation than the lower wraps per metre yarns. A comparative study on twistless cotton fibres and polyester wrapper filaments, the following results were realised at 99% confidence levels.

Double Z-wrap yarn was about 8% stronger than single Z-wrap yarn, its elongation was only about 73% that of the Z-wrap yarn. The double Z-wrap yarn might have been over-wrapped and thus caused the low elongation. On the other hand, the X-wrap yarn was 54% stronger than Z-wrap yarn, and its elongation was about 17% higher.

Fancy Yarns Production in Wrap Spinning System

Fancy yarns made by the wrap-spinning process with suitable machine parameters, filaments and yarns.

A particular boucle-derivative yarn (terry-pile profile) can be produced from wrap spinning.

The designing and development of fabrics involve skillful use of materials, structures, colours, patterns, finishes, and textures. This last quality can be in great demand at certain periods in the fashion cycle and can best be exploited by the application of fancy wrap yarns.^[10]

Terry-Pile Fancy Wrap Yarns

The terry pile was produced by a profusion of short fibres in the profile, overfed to give a raised effect in fabric similar to that of a chenille yarn. Profile evenness was required to give good structure, colour, and surface effect on the fabric, and the amount of overfeed imparted to the components would determine the pile height and thus the aesthetics, both visual and tactile.

Suitable end-uses for the yarn would be in sportswear/leisurewear, in pile fabrics for appeal such as terry, stretch terry, and toweling. Also provides raised effects in furnishing and household fabrics for toweling, bedspreads. Absorbency, easy-care wash and wear, durability, softness, abrasion resistance, good colour and wash-fastness, etc, are achieved with terry pile wrap yarns.

To suit these performance requirements, cotton or blends are often used. Cotton provides soft, absorbent properties, and, when it is blended with other types of fibre, such as polyester or nylon cater the needs of durability, abrasion resistance and strength can be improved and the costs reduced through the use of the cheap synthetic fibres. It is important to consider basic 'Pile' height and evenness.

The 'Terry-pile' cotton yarn retains softness and absorbency. It had a simple design appeal through its use of natural fibres. Differential dyeing process can be used to achieve deep colouring. The produced yarns used for decorative and functional purpose with necessary texture and colour. The actual properties and its aesthetic appeal were determined by the type of blend, the fibre length and fineness, and the inherent qualities of the fibres.

The design potential realised in using the 'Terry-pile' profile in fabrics was immense. Very interesting three-dimensional effects were produced from matt, shiny, and reverse dyeing. This method of colouring the yarns after spinning allowed for hank-dyeing of the yarns rather than dyeing on cone, which would give a softer and bulkier yarn. The piece-dyeing of fabrics would allow for a rapid response time to meet the demands of fashion markets and enable the spinner to avoid stocking uneconomic quantities of dyed-fibre stocks. Furthermore, the 'Terry-pile' yarn are a cheaper type of pile yarn, capable of being produced at very high speeds on the most up-to-date machinery^[10].

High performance Yarns Production in Wrap Spinning System

The wrap spinning system also used to produce high performance wrap yarns by utilising the high performance fibres in the core or wrap especially in the cut resistance gloves. Some production methods of high performance yarns are listed here ^[11].

Three-ply Wrap Yarn (One core & two wrap components) Three-ply yarn has a core strand and two covering strands. A first covering strand may be wire or fibreglass, is wrapped around the core strand, and second covering strand, which is preferably a yarn or filament wrap around both the core strand and first covering strand in opposite directions, relative to each other is shown in Figure 5.

Four-ply Wrap Yarn (Two core & two wrap components)

Four-ply yarn has two core strands placed parallel or twisted to each other and wrapped by two covering strands in opposite direction relative to each other around the core. The first core is of multifilament and second core is of wire (metallic) component and two covering strand are of suitable multifilament, which is shown in Figure 6.

Five-ply Wrap Yarn (Two core & three wrap components)

 Core component is formed by two strands preferably metallic wire and fibreglass. The cover component has three strands, a first or innermost strand wrapped around the core, a second strand wrapped around the innermost strand and a third or outermost strand wrapped around second strand. Strands are wrapped in opposite directions relative to each other around the core. A typical figure of two core & three-wrap component is shown in Figure 7.

Five-ply Wrap Yarn (Three core & two wrap components)

Five-ply wrap yarn consists of three core strands and two covering strands which is shown in Figure 8. The three core strands may be twisted or braided together and placed parallel to each other (metallic wire, fibreglass and high tenacity synthetic filaments). The covering strands are wrapped in opposite directions relative to each other about the core strands.

Six-ply Wrap Yarn (Two core & four wrap components)

Six-ply wrap yarn is shown in Figure 9, a pair of covering strands form first cover component and two additional covering strands forming second cover component. Wrapping was performed in opposite directions relative to each other^[11]. The core may include two strands one of high-density polyethylene and second of metallic wire, either twisted together or placed parallel to each other.

Elastomeric Core & Staple fibre Wrap Yarn

The Figure 10 shown is a schematic illustration of the elastomeric core & staple fibre wrap yarn. These yarns are distinguishable from ring core spun yarns. The wrapper fibres are twisted around the exterior and encase the core in elastomeric core & staple fibre wrap yarns. Whereas in ring core spun yarns, it do not include such twisted outer wrapper fibres. Additionally, there is no residual twist in the present in the wrap yarns as is present in ring core spun yarns^[12].

Differential twist wrapped yarns

Xin Li et al discussed on the differential twist wrapped yarns production with wrap and plied wrap yarns. The wrap yarn and differential twist wrapped yarns (wrap yarns are combined and plied by conventional twister) are shown in Figure 11 a and b. False twist is inserted by hollow spindle to the core and that is removed when wrapper yarn is wrapped over it. Three natures of twists influence on the differential twist wrap yarns. Those are self-twist, wrapping twist and twisting twist.

The self twists of core yarns and wrap yarns changes the wrap qualities. This difference leads to differential twist in wrapping strand. Wrapper yarn twist leads to wrapping yarn and core yarn deformation due to torsion and bending. The curvature of two kinds of deformations are given by^[13]

 t = ß Sin ß/R

k = Sin2 ß/R.

Where

t = torsion

k = bending

R = radius of helical line in the yarn.

ß = Helical angle when the helical radius is R.

Torsion deformation results from yarn relative turning in to its section bringing about the twist. The bending deformation makes the wrapping yarn wind on to the core yarn. Torsion deformation is in the twisting course while plying the wrap yarns by conventional twisting. This provides the core yarn with self-twist while the wrapping yarn has both torsional and bending deformations. Torsional deformation of core induces self-twist, while the wrapping yarn has both torsional and bending deformations. Its torsional deformation provides self-twist and bending deformation that allows it to wind on core yarns.

The torsion/unit length (t') in radians along the axis is given by

t'= t = Sin ß Cos ß/R.-----1

If the twist in the twisting course are t radians and yarn length change (which alter twisting) is not considered than

Sinß = tR Cos ß -----2

from eq 1 and 2

t'= t Cos2 ß------3

t' = Twist change in twisting course

The helical angle ß of the core yarn when nearing to zero differ from ßw of the wrap yarn. The twist changes are different. For the core yarn, its twist changes is given as

t'c = tc ± t Cos2ßc

While the wrapping yarn's twist changes t'w = tw ± t Cos2ßw tc = Core yarn twist before twisting course

t'c = Core yarn twists after twisting course

tw = Wrapper yarn twist before twisting course

t'w = Wrapper yarn twist after twisting course +ve sign shows the twist in same direction of core and wrapper yarn former twist. - sign shows the twist in opposite direction of core and wrapper yarn former twist.

Bend deformation makes the wrap yarn to wind on core yarns. So wrapping twist after twisting course is (plying of wrapped yarn by conventional twisting)

Tw'= Tw ± t

T = Wrapping twist before plying of wrapped yarn by conventional twisting.

Tw' = Wrapping twist after plying of wrap yarn by conventional twist. + ve sign shows the twist in same direction as wrap former - ve sign shows the twist in opposite direction .

When twist and twist direction of tc', tw' and Tw' are different than it form differential twist wrapped yarn. Two single yarns with same structure and twists after twisting course do not show differential twist.

The wrap spun yarns have four kinds of structures:

1) Same directions of wrapping and twisting twist.

2) Opposite direction of wrapping and twisting twist (with wrapper twist is greater than twisting twist.).

3) Opposite direction of wrapping and twisting twists (with wrapper twist is equal to the twisting twist).

4) Opposite direction of wrapping twist with twisting twist (with wrapping twist is lesser than twisting twist).

The first and second kind of twist assists for improvement in regularity and tenacity. If the angle of wrap is lesser than the wrapping yarn makes surplus wrap yarn over the core component and structure becomes unstable^[13].

Influencing Parameters for Wrap Yarns ^[14]

Wrap Density and Core-wrapper ratio

Unevenness, imperfections and mechanical properties are greatly influenced by both blend compositions (core-wrapper ratio) and wrap density.

With increase in wrap density the breaking extension, abrasion resistance and hairiness index are decrease, whereas initial modulus, toughness and flexural rigidity increase. Initially tenacity increases and then decreases with increase in wrap density, whereas percentage of coefficient of variation does not show any trend.

Core-wrapper ratio will be selected according to the nature of utility. Normally the wrap yarns core% will be in the range of 80 - 98%. It may be increased depending on the yarn engineering.

Materials

By using different types of fibres, yarns and filaments (such as cotton, polyester, nylon, Kevlar, glass, jute, acrylic, aramid, HDPE etc,) in the core and staple fibres and or filaments as wrappers, required fancy and high performance yarns can be produced. Properties of Wrap Spun yarns

The hollow-spindle wrap spinning process allows for innovative and unique design effects. Its versatility can be seen when compared and contrasted with other systems and types of yarn available.

Jute/Acrylic Wrap Spun Yarn

Jute/acrylic blended wrap yarn properties at different blend proportions were studied^[18]. Wrap spinning yarns (140 tex) were made with 100% jute, 100% acrylic and different proportions of jute/acrylic with polyester multifilament (50 denier/27 filaments). Wrapper yarns were produced in Suessen Parafil 2000 wrap spinning machine and analysed for 280,315 and 350 wraps per metre^[14].

Tenacity

The higher extensibility of acrylic fibre helps wrapper filament to orient along the yarn axis with increased radial packing, resulting into greater strength. Apart from this, increase in tenacity may also be attributed to the increase in number of fibres in the yarn cross-section because of the use of finer acrylic fibres. Hence, in this case, decrease in stronger jute component and packing co-efficient with the increase in acrylic fibres in blend does not have much bearing on the tenacity of the hollow spindle spun yarns(Parafil yarn).

However, with the increase in wrap density at a fixed blend ratio in jute/acrylic blended wrap spun yarn, the tenacity increases up to 315 wraps/metre and then decreases with the further increase in wrap density^[14].

This result may be viewed under the effect of two opposing influences.

1) With increase in wraps/metre, the yarns becomes more compact, ie, the fibres in the yarn pack more closely resulting in higher inter-fibre friction due to higher radial compressive force by higher number of wraps/metre, ultimately causing increase in tenacity.

2) With the very large number of wraps/metre the contribution in tensile load sharing by the wrapper filaments in the parallel direction of the yarn axis reduces, which results in decrease in tenacity.

Extension at Break

With the increase in proportion of acrylic fibre in jute/acrylic blended wrap yarn, the extension at break increases continuously showing highest breaking extension for 100% acrylic wrap yarn. This may be due to the increase in proportion of higher extensible acrylic fibre in the core material. However this must be carefully examined that the variation of wrap density for any wrap yarn does not have much bearing on breaking extension.

Initial Modulus

With the variation of blend ratio the initial modulus does not show any definite trend, but 100% jute wrapped yarn exhibit higher initial modulus than that of 100% acrylic yarn. This phenomenon may be due to the higher initial modulus of jute fibre as compared to acrylic fibre. Again, a gradual decrease in initial modulus is noticed with increase in wrap density for all types of yarn due to load bearing mechanism of wrap spun yarns.

Toughness and Flexural Rigidity

With the increase in acrylic content in the yarn with definite wrap density, the toughness increases which is expected due to the greater extensibility of acrylic fibre. It is also noticed that with the increase in wraps/metre for a yarn of same blend ratio, the toughness index increases.

The specific flexural rigidity decreases with the increase in acrylic component at all levels of wrap density. This may be due to the less rigidity of acrylic fibre as compared to jute fibre. Interestingly, with the increase in wraps/metre, the specific flexural rigidity first increases and then decreases. It is also observed that the conventional jute yarn shows lower flexural rigidity as compared to wrapped jute yarn, which may be due to the migratory fibre structure of conventional jute yarn.

 Abrasion Resistance

The flex abrasion resistance increases with the increase in acrylic content in the yarn at comparable wrap density. The above phenomenon may be due to the inherent property of the acrylic fibre. Again with the increase in wrap density the abrasion resistance shows decreasing trend for all type of yarn. The above trend may be due to greater cutting effect because of increased yarn packing^[7].

 Jute Viscose Wrap Spun Yarn

Jute viscose yarns of HDPE and PP wrapper show the same trend in tenacity values ie, with increase in the wrap density. At very low wrap density the force exerted by the wrapper filament is not strong enough to pack closely the fibrous core, which results in low inter-fibre friction between the individual fibres. With increase of wrap density, the fibrous core becomes more compact and more coherent. This may due to the action of higher radial compressive forces exerted by continuous wrapper filament. With increase of wrap density up to 240 wraps/metre in case of jute yarns, the hairiness index values decreased and their after with further increase in wrap density up to 240 wraps/metre, the filament to fibre contact area is increased which in turn might have reduce hairiness index values^[15].

Limitations of Wrap

Yarns Some of the limitations of filament-wrapped yarn are:

1)Tenacity of yarn depends on filament strength.

2) Filament yarn is expensive, although the extra cost may partially be offset by the elimination of slashing costs.

3)The wrapped yarn with a twistless core lacks abrasion resistance and therefore tends to pill or shed during weaving when used as warp yarns.

4) More specifically, when tension is applied on the one-directional wrapped yarn (either Z or S), such as in the weaving operation, stress starts to build up on the filament wrapper and elongation begins. Consequently, wraps per unit length of the core decrease, resulting in less control of the core fibres by the wrapping filament^[6].

5) When a heald or reed rubs against the fibres in the yarn during weaving, the fibres will shed from the yarn and may form small pills. Fibre shedding and pilling weakens the yarn and causes end breakage and loom stoppage during weaving. The remedy is to develop a wrapping technique that will keep the fibres in the twistless core in the wrapped yarn under control during weaving and knitting ^[6].

Advantages of Wrap Yarns

Wrap spinning system is used to fulfill increasing demands on style changes in knitting and carpet industries. Strength and elasticity are quite comparable with those of conventional yarns. These two properties depend on the selection of the correct degree of filament twist for particular yarn and its end use and the angle of wrap is also important. The relaxation characteristics of wrap yarns are similar to those of conventional two-fold yarns. Pliability of the wrapped yarns is superior to that produced by other systems. With conventional yarns, neps and slubs tend to come to the surface, but with wrapped yarns they tend to stay in the centre, because of the absence of twist. There are fewer yarn breaks, and because of the larger packages from the machine there are fewer knots or splices compared with conventional systems.

References

1. Audivert R (1974): Advantages of Staple-fibre Yarns Covered with a Filament, Textile Inst Ind 12, pp 271-272.

2. Plummer H S (1985): Wrap Spinning New Life, Am Textiles 5, pp 45-49

3. Mahendra Gowda R V (2006): "Wrap spinning", New Spinning Systems, Second Edition (Published by NCUTE), pp 159-163.

4. Machinery Catalogue, Gemmill & Dunsmore Ltd, Venture Works, Lund Street, Preston PR1 1YH, UK, (October 1983) "G&D's Latest in Wrap Spinning and Fancy Yarn Production", Textile Horizons, pp 39

5. Machinery Catelog, Saurer-Allma GmbH, Postfach 2580, D-8960 Kempten, BRD (July 1983), "S-Allma enter wrap-spun market", Textile Horizons.

6. Gain L Louis and Harold L Salaun (March 1986), "X-Direction-Wrapped Yarn", Textile Research Journal, Vol 56, No. 161

7. Seidel, Leon E (1979): An Answer to Spun-Like Yarns, Textile Ind. 143 (3), pp 32- 36.

8.Graham C O, Shepard C L and Kullman R M H (1980): Cotton Wrapped Yarns -- A Process to Eliminate Sizing and Desizing, Textile Research Journal, Vol 50, pp 108-114.

9. Oxenham W (Feb-1984): Showcase for New Systems of Yarn Making, Textile Month, pp 34-28.

10. Mole K and Knox J S (1989): The Properties and Uses of Specific Hollow-spindle Yarns, Journal of Textile Institute, No. 3, pp 44-453.

11. Nathaniel H Kolmes et al: Harold F Plemmons (June 13, 1995), "Surgical Glove and Yarn", United States Patent, US005423168A, pp 1-8.

12. John Joseph M Rees, Signal Mountain, Tenn; Leonard L Hixon Jr, Dalton, Ga.,(Dec-1997): System for Forming Elastomeric Core/Staple Fibre Wrap Yarn Using a Spinning Machine, United States Patent Application Publication Rees et al, No: (US005701729A).

13. Xin Li, Jianchun Zhng, Jiexin Li (Feb 2002): Differential Twist Wrapped Yarns Made on Hollow Spindle Spinning Machine, Textile Research Journal,Vol 72, pp 181-185.

14. Sharma I C, Mukhopadhyay A & Ray N C (June 1997): Some studies on Properties of Wrapped Jute/Acrylic (Parafil) Yarns, Indian Textile Journal for Fibre & Textile Research, Vol 22, pp 89-93.

15. Atin Chaudhuri (1996): Jute & Jute-Viscose Wrap Spun Yarn, The Indian Textile Journal, September, pp 78-83.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal December 2008 issue.

Chidambaram Rameshkumar

Bannari Amman Institute of Technology, Sathyamangalam, Tamil Nadu.

R Manimaran

Bannari Amman Institute of Technology, Sathyamangalam, Tamil Nadu.

S Praveen Prakash Bannari

Amman Institute of Technology, Sathyamangalam, Tamil Nadu.

S Josuva K Kumar

Bannari Amman Institute of Technology, Sathyamangalam, Tamil Nadu.

Dr N Anbumani

PSG College of Technology, Coimbatore