Rotor Spinning – influence of fibre properties, yarn quality compared to ring spinning, preparatory and post spinning

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N.Balasubramanian, 9869716298

Retd Jt Director (BTRA) & Consultant

. Abstract
Influence of fibre properties and preparatory and post spinning processes on yarn quality and spinning performance of rotor spinning is critically reviewed based on Research and development work over the years. Accumulation of microdust and wedging of trash in rotor groove is one of main factors for end breakage rate and periodic irregularity in yarn. Though rotor yarn has lower strength, strength of weak places is comparable to that of ring yarn because of lower variability. Improvement in quality is more in coarse mixings. Addition of waste without affecting quality is possible. Lower card production rate improves rotor yarn quality. Size pick up is higher in rotor yarns and concentration and formulations have to be adjusted. Yarn and fabric appearance are better but tear strength is lower with rotor yarn.
Influence of fibre properties
Fibre Fineness
Fibre fineness has the maximum influence on quality of rotor yarns. A greater number of fibres per cross section is required in rotor yarns than in ring yarns. Minimum number of fibres to enable spinning increases at high rotor speeds. Minimum number of fibres required for various materials are as under
1. Cotton 100 – 140
2. Polyester/cotton – 80 – 120
3. Acrylics – 80 – 100
Fibre maturity is less critical in rotor spinning than in ring spinning. One study however shows that yarn appearance is adversely affected by low micronaire cotton though end breakage rate is reduced1.
Stronger the fibre stronger will be the rotor yarn. Since lower strength is one of the drawbacks of rotor yarn, stronger cottons should be used.Louis2 reported that fibre strength has more influence on yarn strength than fibre length.
Though fibre length is important its effect is not that pronounced as in ring yarns. With increase in fibre length, reduction in yarn strength with respect to ring spinning will be higher and reduction in irregularity will be less. This is partly because of higher disorientation and wrapper fibre incidence with longer fibres. Louis2 found more wrapper fibres with longer cotton (31.8mm) than a shorter cotton(25.1mm) Heap and Price3 found higher shrinkage of bleached knitted fabrics with shorter, coarser and weaker cottons compared to long, fine and strong cottons. Long staple cottons have poorer utilization of fibre strength because of higher incidence of wrapper fibres1.
Variability in length is not that critical in rotor spinning as in ring spinning as there is no drafting system. So comber noil and flat strips are added to some extent to reduce raw material cost. Manohar,Rakshit and Balasubramanian4 found that improvements in yarn irregularity and imperfections are more marked in waste mixing and the fall in strength is also lower. Taher5 et al found that yarn count, rotor parameters and speed have as much influence on yarn quality as waste % added in mixing. Up to 25 % waste could be added to mixing without affecting yarn quality if rotor diameter and speed are properly chosen. 92% of yarn strength is accounted by fibre properties in rotor yarns as against 94 % in ring yarns6.
Micro dust and Trash
Rotor spinning is more sensitive to trash and micro dust in cotton than ring spinning. Micro dust and trash increase end breakages and contribute to periodic irregularities in yarns that result in rejections. Irregularity, hairiness, abrasion resistance and resistance to abrasion deteriorate with yarns produced without periodic cleaning of rotor7.Trash content in input sliver should be below 0.2 % to get satisfactory performance. Trash particles get wedged in the rotor and result in periodicity with wavelength equal to rotor circumference. Micro dust in cotton should be within 5 – 35 mg/kg. Small deposits first form in rotor which acts as nuclei for further deposits until a complete ring of deposits is formed. End breaks occur when the ring size increases beyond a level8. Naarding9 found that micro dust in cotton consisted of particles of 5 to 40 ΅m diameter. Langley and Haasma10determined particle size distribution on rotor wall and groove using a microscopic method for measuring micro dust, This is found to be of the type given in Fig 1
Fig 1 : Particle size distribution in rotor wall and groove
Mean size of the particle on rotor wall is 2 ΅m and that in groove is 3.8 ΅m. However, this study has been made on laboratory model spintester without trash cleaner and so the results have to be used with caution. Using coulter counter Simpson11 found that particle size in the rotor ranged between 4 – 11 ΅m. His results also showed a larger particle size in groove than that in rotor wall. Mid infra red spectroscopy can be used for qualitative identification of cotton contaminants. This technique showed that type of trash and dust accumulating in rotor is of hull and shale rather than seed coat fragments12.Barella and Virgo13 found a gradual deterioration in cotton yarn strength, regularity, imperfections, faults, hairiness and abrasion resistance from start to end of package build on rotor positions where no end break has occurred. Progressive appearance of periodicities in the yarn in wavelength range 20 – 100mm is found as package builds up. This is because of accumulation of dust, trash and dust as the rotor has not been cleaned. With viscose, the yarn shows deterioration in central layers which disappears afterwards. This is because of accumulation of impurities in rotor which result in a break followed by rotor cleaning. These studies emphasise the need for regular cleaning of rotor groove. Nield and Abadeer14 found that front take off of yarn gives inward rotation which pushes dust particles into the groove. Back take off gives outward rotation which keeps rotor groove clean. Rotor groove shape affects rotor deposits15.Rotation of yarn around its axis during peeling off has considerable influence on movement of trash in rotor groove16.
Man made Fibre
Rotor yarns from man made fibres and blends are 20-30 % weaker than ring yarns with lower elongation. Improvements in evenness and imperfections are negligible. Wrapper fibre incidence is higher, because of longer fibre length, leading to inferior fabric appearance. Problems with polyester are
1. Static charge development
2. Rapid wear and tear of opening roller, rotor and transport tube due to titanium dioxide. Special fibres with lower TiO2 content of .03 % are preferred for rotor spinning.
3. Powdery deposit in rotor due to broken fibre tips and oligomeric material.
4. Crimp in the fibre should be lower as higher crimp hampers opening and affect smooth flow in transport tube
5. Pilling tendency with knitted fabric is encountered especially with high tenacity fibre.
Fibre stiffness
Stiffness in fibre resists bending and twisting and integration into yarn. Fibre type, length and diameter influence stiffness. Ratio of diameter to fibre length is a measure of stiffness. Shorter fibre of same type and diameter will be stiffer. Larger diameter fibre of the same type and length will be stiffer. Twist efficiency will be lower with stiffer fibres.
Yarn quality
Yarn strength is generally lower in rotor yarns than ring yarns by 15 – 25% and the extent of difference depends upon quality of mixing, count and upon type of strength test (Lea or single thread). Manohar, Rakshit and Balasubramanian4 found that reduction in lea strength ranges from16 – 22% with normal mixing and from 5 – 14% in waste mixing which means that the drop in strength is of much lower order with inferior mixing. Balasubramanian17 showed that the difference in strength between rotor and ring yarns depends upon the type of strength test, being more in single thread than in lea. This is because of higher lea ratio in rotor yarns which again is due to lower strength variability. Sultan and Elhawary18 confirmed that the extent of reduction in strength of rotor yarns compared to ring yarns can be brought down by spinning coarser counts from shorter cottons. Jameel19 et al reported that loss in strength of open end yarns in relation to ring yarns increases from 6 % in 10s to 12.5 % in 16s, 23.3 % in 20s and 19.4% in 25s. 20 tex yarn from P/C blend has higher tenacity than 30 tex from the same blend20. Tenacity and elongation increase with increase in polyester content. Lower strength of rotor yarns is because of presence of hooked fibres, poor load distribution, low migration and lower spinning tensions21. Drop in strength of rotor yarn with respect to ring yarn varies with fibre length of material as shown in Fig 2. With short fibres and waste with fibre length between 10 – 15 mm, drop in strength is minimum and with length beyond 30 mm drop in strength is maximum. By optimising rotor spinning parameters, 15-25 % waste can be added in rotor spinning22.
Fig 2 : Effect of fibre length on reduction in strength of rotor yarn, ROs = Rotor yarn strength, RIs = Ring yarn strength
Variability in Count and strength
Lower strength in rotor yarn is partly compensated by lower variability in count and strength. Manohar, Rakshit and Balasubraamanian4 showed that the spread in frequency distribution of single thread strength is considerably reduced in rotor yarns compared to ring yarns. This will be clear from Fig 3. As a result though mean strength of rotor yarn is lower than ring yarn, strength of weakest place is comparable or even lower than that of ring yarns. This explains the better performance of rotor yarns than ring yarns in weaving and knitting.
Fig 3 : Comparison of frequency distribution of single thread strength of rotor and ring yarns, 12.5s
Irregularity and Imperfections
25 – 30 % reduction in irregularity of yarn as estimated by Uster is found with rotor yarns compared to ring yarns. The improvement is more prominent in coarser counts and short fibres and waste mixing4. Reduction in thin and thick places is even more prominent to the extent of 80 – 90%. Neps however show lower order of reduction and are comparable to ring yarns at high rotor speeds. The reduction in irregularity and thin and thick places is because of absence of drafting waves, better control over short fibres and back doubling during deposition on rotor groove. Neps do not show reduction because closely wound wrapper fibres get counted as neps.
Fig 4 : Effect of fibre length on ratio of U% of Rotor and Ring yarns, Ro = Rotor, Ri = Ring Fig 4 shows how reduction in U% of rotor yarns with respect to ring yarns varies with fibre length.
With fibres of length 10mm reduction in U% in rotor yarn is around 25% and with fibres beyond 30 mm reduction is negligible. Yarn regularity is better in 20tex than 30 tex yarns20. Unevenness and thick places increase with polyester content in P/C blend in 20 tex. But in 30 tex neps as well as thick places are more in P/C blends than 100 % polyester or cotton yarns20.Technique of factor analysis by SPSS showed lower utilization of fibre properties in rotor spinning than in ring spinning23. Higher impact of fibre length in rotor spinning is indicated by this study.
Pillay24 et al found that the total length as well as mean length of protruding hairs and loops is lower in rotor yarns than ring yarns. Weight loss in singeing is also lower by 1-2% in rotor yarns than ring yarns. This is because of better control of fibres in rotor spinning. In ring spinning there is no control over trailing fibres after they leave front roller nip and contribute to bulk of hairiness. Mohammed25 et al confirmed that rotor yarns are less hairy than ring yarns but have higher variability in hairiness, with Polyester/cotton blends. Loops are more prominent in rotor yarns. Hairiness of rotor yarns increases with increase in polyester content. Sirang26 et al observed that though rotor yarns have higher hairiness, % of extremely short (<0.25mm) and extremely long (>3mm) protruding hairs of rotor yarns are greater than that of ring yarns. Further between bobbin CV of hairiness of rotor yarn is higher than that of ring yarns. This means that there are variations in surface characteristics of rotor and navel from one spinning position to another. Rotor yarns have lower CV of diameter which must be because of the lower weight/unit length variability. Another finding is higher within bobbin diameter CV than between bobbin CV, which is contrary to that in ring yarns. Prediction of yarn hairiness of ring and rotor yarns using back propagation neural network algorithm showed model based on HVI data is superior to those using AFIS and FMAT. Longer fibres with higher elongation and lower maturity have less hairiness27.
Abrasion Resistance
There is a general opinion that wear of needles is higher in knitting with rotor yarns because of higher abrasion resistance. However, based on the attrition caused to the wire in Lawson Hemphill CTT tester, Jones28 concluded that the abrasion caused by rotor yarn is only slightly higher than that by ring yarn. Chand29 however opined that that rotor yarn has a lower abrasion resistance than ring yarn because of the soft nature of sheath. Barella and Manich30 report that abrasion resistance increases with twist. Cotton yarns have higher abrasion resistance than viscose and acrylics.
Mean fibre position and rootmean square deviation, proposed by Hearle to quantify migration, are lower in rotor yarns than in ring yarns24.This indicates core is denser in rotor than ring yarns and migration is lower in former. Rate of change of helix envelope is lower in rotor than ring yarns when actual number of turns is kept the same. Tendency for long fibres to occupy preferentially core position found in ring yarns is not that prominent in rotor yarns. Kumar31 et al confirmed that rotor yarns have lower mean fibre position, lower root square deviation but higher migration intensity. Ishtiaque32 et al found that rotor yarns have lower fibre extent, fibre overlap index and fibre-pair-overlap length than ring yarns. Contrary to the work of other authors, Salhotra33 et al report that all migration parameters reduce with twist reach a minimum and increase with further increase of twist. Initial reduction may be because of increase in yarn compactness at the peeling point. At higher twist, this is offset by reduced number of fibres in yarn cross-section. Migration parameters do not follow any particular trend with blend proportion in polyester viscose blends. 100% polyester has lower mean fibre position and higher mean migration intensity than 100% viscose. 33/67 PV blend has maximum root mean square deviation34.
End breakage rate
End breakage rate in rotor spinning depends upon yarn strength at the peeling point and spinning tension. It also depends upon disturbing factors like trash and microdust, lay of fibres in groove. End breakages will increase with rotor speed if suitable action is not taken on optimizing mixing and parameters of machine, especially rotor diameter. Rotor diameter has to be reduced and optimum navel has to be used, with increase in rotor speed to offset the increase in tension. Das and Ishtiaque35, who classified end breaks as per the shape of the broken end, reported that end breaks with tapered end and embedded trash increase with rotor speed. With increase in opening roller speed, end breakages increase because of increased fibre breakages and fibre disorientation in transport tube. At the same time, end breaks due to seed coat are reduced. Grey rotational analysis and Taguchi method have been integrated to find optimum process parameters for combed rotor yarns. While rotor speed and feed rate influence strength, unevenness is affected by rotor surface speed and feed rate36. Use of finer drawing sliver and lower draft in rotor reduces end breakage rate1. Relationship between probability of end breakage and breaking strength, yarn tension and tension variability has been studied by Uriniy37.
Fabric Quality
Though tensile strength of fabric from rotor yarn is lower than that of ring yarns, fall in strength comes down from yarn to fabric stage38.This is because of better fabric assistance derived from rotor yarns than ring yarns because of higher bulkiness and greater area of contact at interlacements with former. Fabric assistance also varies with make of rotor machine. Higher warp way strength than weft way strength is found in rotor yarns. This is because of better fabric assistance provided by weft than warp because of higher crimp. Appearance Fabric appearance is distinctly superior with rotor yarn than ring yarn38. The superior appearance comes from higher uniformity, lower level of thick and thin places and faults and kitties. The cover of the fabric is also much better with rotor yarn fabrics. Tear Strength Tear strength reduces by 25 – 30 % in fabrics from rotor yarns compared to that from ring yarns. This is one of the major drawbacks of rotor yarn.
Bursting Strength
Bursting strength of knitted fabric from rotor yarn is found to be much lower than that from ring yarn39.
Abrasion Resistance
Abrasion resistance as estimated by weight loss, after a preset number of cycles, reduces38 by 20 to 55%. The nature of twisting is different in ring and rotor yarns as shown in Fig 5. While surface of ring yarn is relatively smooth, there are helical corrugations in rotor yarns corresponding to twisting. The corrugations initially come into contact with abrading medium and prevent disintegration of surface fibres leading to higher abrasion resistance.
Fig 5 : Comparison of twisting in rotor and ring yarns
Chand29 however attributed the higher abrasion of rotor yarn fabrics, in normal square constructions, to the greater bearing surface arising from soft outer layer of the yarn. However with fabrics having long floats of warp or weft like satin or terry towel, rotor yarn fabrics will have lower abrasion resistance. Iqbal and Kolhatkar40 confirmed higher abrasion with rotor yarns than ring yarns in denim fabrics. They attributed this to the presence of wrapper fibres in rotor yarns which can slide over core material and avoid abrasion with abrading material. Rameshkumar39 et al also found abrasion resistance to be higher in knitted fabrics from rotor yarns compared to that from ring yarns. Rotor yarns however gave more breaks in knitting.
Rubbing and Wash Fastness
Wash and wear rating of rotor yarn fabrics is better than ring yarn fabrics38. However Iqbal and Kolhatkar40 found inferior rubbing and washing fastness of denim fabrics made from rotor yarns is comparable to that from ring yarns. Shrinkage in Knitting
Heap and Price3 found that shrinkage of bleached knitted fabric from rotor yarn increased by 1.5 % in length and 1 % in width with increase of twist factor from 3.6 to 4. Influence of Preparatory
Preparatory is similar to ring spinning except for the absence of speed frame and use of smaller size cans in finisher drawing. Since trash and micro dust have a critical influence, Blowroom cleaning efficiency should be given great importance. The line should also be equipped with micro dust extractors or de dusters. Passage of material over perforated drum with suction inside helps to remove micro dust.
Rakshit and Balasubramanian41 showed that lower production rate in carding reduced trash level in sliver and brought down the opening roller waste in rotor spinning.55 % reduction in end breakages in rotor spinning and markedly improved yarn are other benefits from lower card production rate. Use of crush rolls in carding pulverizes trash and is recommended in rotor spinning. Simpson and Louis42 found that double carding reduces trash accumulation in rotor, brings down end breakages and improves yarn quality. Drawing
Normally two drawing passages are recommended. The second passage not only improves fibre parallelization and sliver uniformity but also acts like a de dusting equipment to remove micro dust and small particles. But in waste mixing having very short fibres single drawing passage with autolevelling is adequate particularly in applications where demand on quality is not stringent. In extreme cases, direct feed of card sliver with draw frame module is done for counts 100Ne and below. Idzite43 concludes that one drawframe would be adequate for spinning polyester yarn if card is equipped with draft regulator. Yarns can be produced conforming to Uster 25 % standards with this set up. Barella and Vigo44 concluded that omission of one drawing passage affects tenacity significantly but has only marginal effects on evenness and imperfections with acrylic fibre. Technique for order preference by similarity to ideal solutions (TOPSIS) was used for determining optimum back zone setting, break draft and delivery speed for rotor yarn in terms of knitting performance. Parameters obtained by this method gave best knitting performance45.
Relative Humidity
Mean fibre extent is affected by relative humidity in the spinning room and as a result yarn quality is affected46. Post Spinning Processes
Assembly winding prior to doubling will avoid wrap twisting as both ends will be at the same tension. Further it minimizes singles at the time of breakage or run out of one of the packages.
Tenacity improvement with doubling is higher than ring yarns by 25 % for S/Z and 20 % for Z/Z yarns. Soft twisted singles give maximum improvement in strength
Fig 6 : Relationship between Doubling twist to singles twist ratio (D/S) for maximum tenacity s twist in rotor and ring yarns, 16s Ne

Palaniswamy46 et al found that twist liveliness of S/Z rotor ply yarn increases as D/S increases. D/S ratio of .45 shows minimum twist liveliness and knitted fabrics made from the same show least spirality in both wet and dry state. Unal48 found that the handle of fabrics woven with doubled rotor yarn as weft was stiffer than that of doubled ring yarns as weft. Fabrics made from doubled S/S rotor weft yarns were stiffer than those from doubled Z/Z rotor weft yarns.
Winding operation is normally not be necessary as
• Fault incidence is low
• Conical packages with different angles are possible in modern machines
• Waxing attachment is available
• Fault clearing system is integral part of modern machines
Winding is however done in some cases to ensure higher level of uniformity in length of yarns and to reduce hard waste caused by remnants. Use of accurate yarn length measuring systems and efficient clearing may obviate winding. Winding efficiency is higher with rotor yarns because of lower fault incidence, longer length of yarn and lower order of tension variations from start to end. Tension in winding should be kept lower to preserve elongation. Economics of rotor spinning will however come down if winding is used.
Special wave shaped wire rings are used as balloon breakers in the creel with rotor packages to reduce peak tensions. Yarn tension must be kept lower to preserve elongation. Yarn leaving the package should rotate in the same direction in all positions to minimize streakiness in fabric. As rotor yarns are lively they should be conditioned for 24 hrs at 90 % RH and 400 C.
• Since rotor yarns are bulkier and more absorbent, they take up to 3 – 10 % higher size than ring yarns. So size concentration and formulations have to be adjusted.
• Size pickup should be 10 % less than ring yarns to minimize loss in elongation. Elongation should not reduce by more than 25 – 30 %.
• Size concentration should be reduced by 20 – 30 %
• Stretch should be kept within 1.5 %
• Rotor yarns can be overdried particularly when machine runs at slow speed and this could increase end breaks.
• Moisture content of cotton sized yarns should be kept at 8.3% - 8.8%
• Squeeze pressure should be kept lower to minimize surface roughness. With lower size concentration, pressure can be kept higher.
Size penetration will be better with rotor yarns because of their bulkiness. This should result in greater strength improvement. However, contradictory findings have been reported in regard to the effect of sizing on yarn tensile properties. Schwarz49 et al report a lower order of strength improvement in rotor yarns compared to ring yarns upon sizing. As a result difference in strength between ring and rotor yarns increase by 75% upon sizing. At the same time, difference in elongation between the two yarns reduces upon sizing. Difference in hairiness between the two yarns is also reduced substantially upon sizing. However Sengupta50 et al and Moreau51 found greater improvement in strength upon sizing with rotor yarns compared to ring yarns. Moreau51 found that the strength improvement from sizing in rotor yarns is higher in coarser count. Strength retention upon abrasion of sized rotor yarns depends upon the type of size. Strength retention of sized rotor yarns increases with twist factor and becomes higher than ring yarns at high twist. Sengupta50 et al also found better strength retention with rotor yarns compared to ring yarns upon subjection to fixed number of abrasion cycles in Zweigle weavability tester.
Loom shed working
A joint study by ATIRA, BTRA, and SITRA38 showed that warp breakage rates in loom shed are brought down by 20 – 70 % with rotor yarns because of higher elongation, fewer weak places and faults, lower strength variability and ability to withstand cyclic loads. At any strain imposed on yarn during shedding, stress in rotor yarn is lower than that of ring yarns. Weft breaks however do not show much reduction. Githaiga52 et al found that fibre quality and rotor yarn production parameters have a significant influence on weft insertion rate in Air-jet loom. Frequent cleaning of rotor of microdust helps to improve weaving performance.
One drawback of rotor yarns is the harsh feel of the fabric. To overcome this, finishing treatments with a softener like silicone elastomers or condensation products of fats is required. But finishing treatment will be associated with a fall in strength. So fabric construction has to be altered to get strength requirements in ultimate fabric. However Mukhopadyay53 et al found that reduction in tear strength of ring and rotor yarns diminishes from grey to finished stage in military khaki. Presence of wrapper fibres will impede raising/napping. So a second passage may have to be given in the raising machine. Fabrics from rotor yarn dye to deeper shade but brilliancy is lower. Lord21 opined that increase in bulkiness upon finishing is of lower order than that of ring yarns.
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