Study on Physical-mechanical Parameters of Ring-, Rotor-and Air-jet-spun Modal and Micro Modal Yarns

The main physical-mechanical parameters of modal yarns (unevenness, faults, hairiness and spectrograms) were compared with the parameters of micro modal yarns of the same fi neness and end-use. The diff erence in tenacity and elongation at break of diff erent types of modal and micro modal-spun yarns is determined by yarn structure. The highest tenacity was achieved in the oriented structure of ring-spun yarn, followed by air-jet-spun and rotor-spun yarn, in the case of both modal and micro modal fi bres. All types of modal yarns diff er in overall unevenness and in terms of micro modal fi bres. The values of the overall unevenness of ring-, rotorand air-jet-spun modal yarns are greater than or equal to the same values of micro modal yarns. The spinning technique, and thus the yarn structure, determine the level of overall yarn evenness. The number of faults at diff erent levels of sensitivity measurement to detect the highest number of thin and thick places and neps (–30%, +35% and +140%) is greater in rotorand air-jet-spun yarn than in ring-spun yarn for both levels of fi bre fi neness. Periodic faults of short wavelengths with signifi cant amplitude increase the number of yarn faults to a certain extent. Rotor-spun micro modal yarn shows the highest deviation from ideal unevenness, while ring-spun modal yarn shows the lowest deviation. Yarn hairiness depends on the spinning technique. Finer fi bres cause lower hairiness in all yarn types.


Izvleček
Glavne fi zikalno-mehanske lastnosti modalnih prej (neenakomernost, napake, dlakavost in spektrogrami) so bile primerjane z lastnostmi mikromodalnih prej enake fi noče in končnega namena uporabe.Razlike v trdnosti in pretržnem raztezku različnih vrst modalnih in mikromodalnih prej določa njihova struktura.Najvišjo trdnost je dosegla prstanska preja z orientirano strukturo, tej sta sledili obe preji iz modalnih in mikromodalnih vlaken, izdelani po postopku z zračnim curkom (t.i. postopku air-jet) in rotorskem postopku.Modalne preje se med seboj razlikujejo v enakomernosti preje, prav tako tudi mikromodalne preje.Vrednosti enakomernosti prstanske, rotorske in z zračnim curkom (t.i. air-jet) spredene modalne preje so večje ali enake vrednostim mikromodalnih prej.Tehnika predenja in s tem struktura preje vplivata na njeno enakomernost.Število napak, merjeno pri različnih stopnjah občutljivosti, da bi zaznali najvišjo raven tankih in debelih mest ter nopkov (-30 %, +35 % in +140 %), je večje pri rotorski preji in preji z zračnim curkom (t.i. preji air-jet) kot pri prstanski preji pri obeh fi nočah vlaken.Na kratkih dolžinah ugotovljene periodične napake z veliko amplitudo povečajo število napak preje v določenem obsegu.Rotorska mikromodalna preja ima največje, prstanska modalna preja pa najmanjše odstopanje od idealne enakomernosti.Kosmatost preje je odvisna od postopka predenja.Finejša vlakna povzročajo manjšo kosmatost prej kot bolj groba vlakna.Ključne besede: kemična celulozna vlakna, predivna preja, mikrovlakna, periodične napake 1 Introduction Today, there is a broad range of applications for modal fi bres in the clothing industry for making lighter-weight knitted garments and articles worn next to the skin.Knitted fabric characteristics are largely dependent on the raw material composition of yarn and the yarn type from which a knitted fabric is made.Parameters of mass irregularity, which include overall unevenness, yarn faults defi ned as thin places, thick places, neps and hairiness, aff ect the appearance and other characteristics of a fabric.In general, specifi c samples must be made to reduce the number of input parameters, in order to fi nd a more precise relationship between the eff ect of fi bre type, fi bre properties and the spinning process (including the type of spinning machine) on the basic physical-mechanical properties of yarn (and thus on the appearance and properties of a knitted fabric).A number of researchers have dealt with the properties of ring-spun yarn [1], the dynamic properties of air-jet-spun yarn and rotor-spun yarn [2], the geometric analysis of spun yarns with the aim of assessing the tensile properties of air-jet-spun yarn [3], the assessment of the tensile properties of air-jet-spun yarn [4], and the tensile properties of rotor-spun yarn [5].Yarn unevenness and hairiness has also been studied by a number of researches [6−9].Th e eff ect of periodic faults on overall unevenness and/or on the number of faults in yarn has only been studied to a small extent [10,11].However, it is not evident from literature that a more signifi cant study of the eff ect of fi bre fi neness on the basic physical and mechanical properties of different yarn types has been undertaken.In order to reduce the number of diff erent input parameters and to fi nd a clearer connection between parameters, yarn with one type of diff erent fi bre parameters should be spun using the same technological process, the same twist coeffi cient and the same fi neness.Th us, the eff ect of the spinning technique (rotor and ring) used for micro modal fi bres on unevenness [12] and the unevenness of air-jet-spun yarn relative to rotor-and ring-spun micro modal yarn has already been studied [13].Th e latter studies still continue, so that modal yarns are also included.Th us, the aim of this paper is to present a comparison of the basic physical-mechanical properties of ring-, rotor-and air-jet spun yarn made from modal fi bres, and to extend that comparison to the same types of micro modal yarns.

Experimental part
For the purposes of this study, ring-, rotor-and airjet-spun yarns were spun with a nominal count of 20 tex (Nm 50) from 1.3 dtex fi bres with a length of 38 mm.Th e basic properties of these yarns were compared with the properties of yarns having the same end-use (knitting) from 1 dtex micro modal fi bres with a length of 39 mm.Modal yarns were spun under the same technological conditions as micro modal yarns [12,13]: a) Ring-spun modal yarn was produced using the carding manufacturing process, comprising fi bre preparation phases (opening, blending and carding), spinning preparation (drawing, pre-spinning and ring spinning), winding and cleaning.A Zinser 351 ring spinning machine connected to an Autoconer X5 winding machine was used for the ring spinning process.b) Rotor-spun modal yarn was produced using the carding manufacturing process, comprising fi bre preparation phases (opening, blending and carding), spinning preparation (drawing) and rotor spinning.A Schlafh orst A8 rotor spinning machine was used for spinning.c) Air-jet modal yarn was produced using the carding manufacturing process, comprising fi bre preparation phases (opening, blending and carding), spinning preparation (three drawing passages) and air-jet spinning.A Rieter J 20 machine was used for spinning.d) Yarn unevenness was determined in accordance with standard ASTM D1425/D1425M-14 Standard Test Method for Evenness of Textile Strands Using Capacitance Testing Equipment [16].Unevenness, the number of yarn faults and hairiness were determined on an Uster Tester 4-S with a yarn throughput speed of 400 m/minute through the measuring fi eld with a test time of 2.5 minutes.One measurement of each cross-wound package of each yarn type was performed.e) Th e tensile properties of yarn were determined in accordance with standard ISO 2062:2009 Textiles -Yarns from packages -Determination of single-end breaking force and elongation at break using constant rate of extension (CRE) tester [17].Measurements were performed on an Uster Tensorapid 4 instrument.A total of 100 measurements per package of each yarn type were performed.
Th e number of fi bres in the cross-section (n f ) was determined from the ratio between yarn fi neness (T ty ) and fi bre fi neness (T tf ) using the following equation: Th e limiting irregularity CV lim was calculated for man-made fi bres and cotton using the following equation [18]: Th e index of irregularity I was determined from the ratio between the coeffi cient of variation of unevenness (obtained by measuring CV m ) and limiting irregularity (CV lim ) using the following equation: Th e index of irregularity is a measure of the unevenness deviation of a certain fi bre type that will be spun in an ideal situation where I = 1.It also shows how well the machines used in certain technological phases run and whether any deviations in their operation occur.Fibre distribution in yarn is not completely controlled.In practice, the orientation of fibres in yarn is regarded as a random occurrence.Th e goal to be achieved in production is to maintain a constant number of fi bres in any yarn cross section.Non-periodic faults can be thin places, thick places and neps.Th e level of measurement sensitivity is typically as follows: -30%, -40%, -50% and -60% for thin places; +35%, +50%, +70%, +100% for thick places; and +140%, +200%, +280% and +400% for neps.Periodic faults or system faults occur due to the periodic irregular motions of individual machine elements and/or due to damage to those elements (e.g.rollers, gears, belts, vibrations, etc.).Periodic faults are shown using a spectrogram.If unevenness reaches the ideal value (I = 1), the spectrogram has an ideal curve.Th e spectrogram function takes the following mathematical form [18]: where: S = spectrogram, n f = number of fi bres in the yarn cross-section, l o = fi bre length and λ = wave-length.The real spectrogram curve deviates from the ideal curve.

Results and discussion
Th e fi neness of all modal yarn types is very consistent and ranges from 20.13 to 20.22 tex, and from 20.04 to 20.15 tex in the case of micro modal yarns.Th e coeffi cient of variation of fi neness among crosswound packages is very low, generally below 1% (Table 1).Th is indicates the very high consistency of high-quality production.Th e number of fi bres in the yarn cross section is calculated using equation 1, from which it is apparent that fi bre fi neness determines the number of fi bres in the yarn cross-section of the same yarn fi neness.Th e number of modal fi bres in the yarn cross-section ranges from 155 to 156, while the number of micro modal fi bres in the yarn cross-section ranges from 200 to 202.Th e number of twists in all yarn types is uniform and is determined according to the end-use of yarn (knitting).Th e twist coeffi cient ranged from 3,280 to 3,350 m -1 tex 0.5 .

Tensile properties
Th e values of basic physical-mechanical parameters and the breaking elongation properties of yarns with a nominal count of 20 tex used to make knit garments are given in Table 1 and shown in Figure 1.Ring-spun modal yarn has the highest tenacity (23.81 cN/tex), followed by aerodynamic (20.77 cN/tex) and rotors with the lowest tenacity (15.39 cN/tex).Yarn elongation at break follows yarn tenacity.Th e highest elongation at break among modal yarns was found in ring-spun yarn (10.83%), followed by air-jet-spun yarn (9.25%), while the lowest elongation at break was found in rotor-spun yarn (8.17%).Th e diff erence in tenacity and elongation at break of diff erent types of spun modal yarns (as well as micro modal yarns) is caused by yarn structure, as the result of the spinning technique.In principle, a greater number of fi bres increases the contact surface among fi bres, and thus causes a greater overall friction force for a particular yarn type.With the same number of twists, ring and or spun modal yarn has a lower tenacity (23.81 and 15.38 cN/tex respectively) than ring-and rotor-spun micro modal yarn (24.09 cN/ tex and 15.86 cN/tex respectively).However, yarns from fi ner micro modal fi bres did not impart signifi cantly greater tenacity than yarns from coarser modal fi bres in all yarn types.Th e highest performed work of rupture of modal yarn is observed in ring-spun yarn due to its oriented structure (14.84 N cm), followed by air-jet-spun yarn (11.22 N cm), while the lowest value of the performed work of yarn rupture is observed in rotor-spun yarn (7.66 N cm).Th e same sequence is also observed in micro modal yarns.By comparing modal and micro modal yarns, ring-and air-jet-spun yarn from fi ner micro modal fi bres perform less work of yarn rupture (14.26 N cm, 10.78 N cm) than the same types of modal yarns (14.84 N cm, 11.22 N cm).In rotor-spun yarn, the higher fi neness of micro modal fi bres had practically no eff ect on the work of yarn rupture (7.70 N cm, 7.66 N cm).

Unevenness
Th e values of the unevenness parameters of all types of modal and micro modal yarns are given in Table 2 and shown in Figures 2−8.All types of modal yarns diff er in overall unevenness and in micro modal fi bres.Rotor-spun yarn has the greatest overall unevenness in the case of modal fi bres (13.95 %), while ring-spun yarn has the lowest unevenness

Table 1: Main yarn parameters and tensile properties of ring-, rotor-and air-jet-spun modal and micro modal yarn
Type of material Linear density N a) Twist Tenacity Elongation Work x - x -

Thin places
Th e number of thin places at diff erent levels of sensitivity for all types of modal and micro modal yarns are given in Table 3 and shown in Figure 10.Rotor-(2316.8)and air-jet-spun yarn (1187.2) have a significantly higher number of thin places in modal yarn at a level of measurement sensitivity of -30% than ringspun yarn (198.8).Comparing the same types of yarns spun from diff erent fi bres, it is evident that ring-and rotor-spun modal yarn have a greater number of thin places (198.8,2316.8)than micro modal yarn (130.2, 1250.4).However, air-jet-spun modal yarn has a slightly smaller number of thin places (1187.29)than micro modal yarn (1197.4).Th e difference is 0.8% and can be ignored.At fi rst glance, these results indicate that the greater fi neness of micro modal fi bres has no eff ect on the appearance of thin places in air-jet-spun yarn.However, because two periodic faults occurred with short wavelengths and signifi cant amplitude in two out of 10 packages (Figure 8), the same faults increased the number of thin places.Accordingly, coarse modal fi bres in ring and rotor spinning, particularly the spinning techniques (ring and rotor), create a greater number of thin places at a level of measurement sensitivity of -30% (greater weight reduction than the reference/ average weight of 100%) than fi ner micro modal fibres.To determine the eff ect of periodic faults on the number of thin places, more in-depth studies are necessary, with the specifi c preparation of yarn samples.At the usual sensitivity level of -50%, which is typically used in spinning mills, rotor-spun yarn has a markedly greater number of thin places (8.1) than airjet-spun (2) and ring-spun yarn (0.9) in the case of modal yarns.By comparing all types of modal yarns with micro modal yarns, yarns made from coarser modal fi bres have a greater or a nearly equal number of thin places.Periodic faults of short wavelengths affect the number of thin places, as well as the coefficient of variation of unevenness of 10 packages CV b .
Figure 8 shows the systemic periodic fault of short wavelengths in air-jet-spun yarn, which increases the number of thin places and the value of CV b .

Thick places
Th e number of thick places and the coeffi cient of variation of thick places of 10 packages are given in Table 4 and shown in Figure 11.Th e number of thick places in all types of modal yarns at a level of measurement sensitivity of +35% is greatest in rotor-spun yarn (446), followed by air-jetspun yarn (101.1), while the lowest number is seen in ring-spun yarn (29.7).In other words, the number of thick places in rotor-spun modal yarn is 15 times

Figure 9: Number of thin places of modal and micro modal ring-, rotor-and air-jet-spun yarns for diff erent levels of sensitivity
greater than in the ring-spun modal yarn, while the number of thick places in air-jet-spun modal yarn is 3.4 times greater than in ring-spun modal yarn.At levels of measurement sensitivity of +50%, +70% and +100%, the sequence of thick places in modal yarns was the same as it was at a sensitivity level of +35%.Th e latter is analogous to micro modal yarns.By comparing the number of thick places at a level of measurement sensitivity of +35%, ring-spun yarns made from coarser modal and fi ner micro modal fi bres have nearly the same number of thick places (29.7 and 30.4).Th us, the eff ect of fi bre fi neness on the number of thick places in ring-spun yarn at a sensitivity level of +35% is minimal.How-ever, at a level of measurement sensitivity of +35%, a higher scattering of the number of thick places CV b in ring-spun micro modal yarn (26.38%) is visible in relation to modal yarn (17.54%).Th e eff ect of the fi neness of modal fi bres on the number of thick places at a level of measurement sensitivity of +35% is signifi cant in rotor-spun yarn.Th us, rotor-spun yarn from coarser modal fi bres has a greater number of thick places at all levels of measurement sensitivity (446, 32.4 and 0.70) than rotor-spun yarn from fi ner micro modal fi bres (245.8,12.9, 0.1 and 0).In air-jet-spun micro modal yarn, the number of thick places at a level of measurement sensitivity of + 35% does not decrease, but increases relative to or   is equal to (132, 5.4, 0.4 and 0) air-jet-spun modal yarn (101.1, 3.4, 0.4 and 0), which is analogous to the number of thin places in the same yarn.Here, too, the eff ect of the structure of air-jet-spun yarn and periodic faults in two out of 10 packages resulted in an increase in the number of thick places.
In spinning mills, the number of thick places is usually considered at a level of measurement sensitivity of +50%.Th e eff ect of fi ner micro modal fi bres on the number of thick places is diff erent and depends on the spinning technique.Th e use of micro modal fi bres resulted in a 222.2% increase in the number of thick places at a level of measurement sensitivity of +50% in ring-spun yarn and a 28.8% increase in air-jet-spun yarn (including the eff ect of periodic faults), while a 60.2% reduction in the number of thick places was seen in rotor-spun yarn.

Neps
Since neps are actually thick places shorter than 4 mm, the number of neps in modal fi bres follows the number of thick places (Table 5, Figure 12).Th us, the greatest number of neps in modal yarns at a sensitivity level of +140% is the smallest in the ring-spun yarn (72.7) followed by the air-jet-spun yarn (80.5) and the greatest in the rotor-spun yarn (1273.5).By comparing modal and micro modal yarns, it is evident that the ring-and air-jet-spun micro modal yarn has a greater number of neps (98.9, 280.6) than the modal yarn of the same type (72.7, 80.5).Only in   In other words, at a level of measurement sensitivity of +140% fi ner micro modal fi bres increase the number of neps in the ring-and air-jet spinning, while their number decreases in the rotor spinning.
In the air-jet-spun yarn the infl uence of periodic faults of short wavelengths with signifi cant amplitude is, as with thin and thick places, probably signifi cant.At a level of measurement sensitivity of +200% the sequence of the number of neps is diff erent.In modal yarns the smallest number of neps is found in the air-jet-spun yarn (5.3), followed by the ring-spun yarn (11) and fi nally the rotor-spun yarn (68.5).By comparing modal and micro modal yarns, it is apparent that the sequence of the number of neps is retained as well as at a level of measurement sensitivity of + 280% and +400%.Th us, fi ner micro modal fibres increase the number of neps in the ring-and airjet-spun yarn at a level of measurement sensitivity of +200% but also at a level of measurement sensitivity of +280.Th us, by using micro modal fi bres at a level of measurement sensitivity of +200% the number of neps increased in the ring-and air-jet-spun yarn by 159.1% and 166.0%respectively, and the number of neps in the rotor-spun yarn was reduced by 54.5%.

Hairiness
Th e values of hairiness of ring-, rotor-and air-jet-spun modal and micro modal yarns are given in Table 6 and shown in Figure 12.Yarn hairiness does not follow the number of faults, but is most dependent on the spinning machine type, i.e. the yarn formation technique and fi bre fi neness for the same end-use.In the case of modal yarns, the yarn with the lowest hairiness is the yarn produced using an air-jet spinning technique (3.71), followed by the yarn spun using a rotor spinning machine (4.34), while the yarn spun using traditional ring spinning (6.09) shows the highest hairiness.Th e eff ect of fi bre fi neness on the hairiness of yarns made using diff erent spinning techniques is equally expressed.Namely, all yarns from fi ner micro modal fi bres show lower hairiness than yarns from coarser modal fi bres.Th e reduction of hairiness through the use of micro modal fi bres is greatest in ring-spun yarn (13.3%), followed by rotor-spun yarn (5.99%), and lowest in relative terms in air-jet-spun yarn (4.04%).Air-jet-spun yarn (6.88%) has the highest scattering of hairiness among yarn packages. .Th e cause of the latter is the appearance of periodic faults with signifi cant amplitudes in three packages of air-jet-spun modal yarn and in two packages of air-jet-spun micro modal yarn at diff erent wavelengths.
Rotor-spun modal and micro modal yarn shows • the highest deviation from ideal unevenness, followed by air-jet-spun yarn, while ring-spun modal yarn shows the lowest deviation.Th e number of faults at diff erent levels of measu- • rement sensitivity to detect the highest number of thin and thick places and neps (-30%, +35% and +140%) is greater in rotor-and air-jet-spun yarn than in ring-spun yarn for both fi ber fi neness.Th e eff ect of fi ber fi neness on the hairiness of • yarns made using diff erent spinning techniques is signifi cant: in the case of modal yarns, the yarn with the lowest hairiness is the yarn produced using an air-jet spinning technique (3.71), followed by the yarn spun using a rotor spinning machine (4.34), while the yarn spun using traditional ring spinning (6.09) shows the highest hairiness.
All yarns from fi ner micro modal fi bres show lo-• wer hairiness than yarns from coarser modal fibres.

Figure 1 :
Figure 1: Tenacity and elongation of modal and micro modal ring-, rotor-and air-jet spun yarn

Figure 2 :
Figure 2: Unevenness of ring-, rotor-and air-jet-spun modal and micro modal yarns at diff erent cut lengths

Figure 10 :
Figure 10: Number of thick places of modal and micro modal ring-, rotor-and air-jet yarns for diff erent levels of sensitivity

Figure 11 :
Figure 11: Number of neps of modal and micro modal ring-, rotor-and air-jet-spun yarns for diff erent levels of sensitivity

Table 2 :
).Furthermore, the values of the overall unevenness of ring-, rotor-and air-jet-spun modal yarn (10.21%, 10.95% and 12.33%) are higher than the same values of the micro modal yarns (9.67%, 12.69% and 12.12%).Th us, the spinning technique and consequently the yarn structure determine the level of overall yarn evenness.Th e coeffi cient of variation of the overall unevenness obtained as the mean value of 10 packages CV b is highest in air-jetspun modal yarn (2.44%) and air-jet-spun micro modal yarn (4.69%).Th e cause of the latter is the appearance of periodic faults with signifi cant amplitudes in three packages of air-jet-spun modal yarn (1, 2 and 3) and in two packages of air-jet-spun micro modal yarn (3 and 4) at diff erent wavelengths (Fig-ures 7 and 8).Most yarns do not contain periodic faults with signifi cant amplitudes.Th is is the reason CV b values are low (Figures 3−6).Overall yarn unevenness CV m represents the eff ective (actual) value of yarn mass irregularity obtained through measurement.Th e limit value irregularity of yarn CV lim is determined using equation 2, while the unevenness index of irregularity (I) is determined using equation 3. Th e value I of all modal yarn types ranges from 1.27 to 1.74.Because I is the value of the deviation of the achieved unevenness in production of one yarn type and one number of yarn twists in relation to ideal unevenness, the nearest value to the ideal value is obtained in ring-spun yarn (1.27), while the lowest value is obtained in the rotor-spun yarn Unevenness of ring-, rotor-and air-jet yarn spun from modal and micro modal fi bres at diff erent cut lengths

Table 4 :
Th ick places of ring-, rotor-and air-jet spun yarn from modal and micro modal fi bres at diff erent levels of sensitivity

Table 5 :
Neps of ring-, rotor-and air-jet spun yarn from modal and micro modal fi bres at diff erent level of sensitivity

Table 6 :
Hairiness (H) of the ring-, rotor-and air-jetspun modal and micro modal fi bres yarn from modal and micro modal fi bres Th e diff erence in tenacity and elongation at break • of diff erent types of modal yarns (as well as micro modal yarns) is caused by the yarn structure, as the result of the spinning technique.Th e highest performed work of rupture of modal • yarn is observed in ring-spun yarn due to its oriented structure, followed by air-jet-spun yarn, while the lowest value of the performed work of rupture is observed in rotor-spun yarn.Th e higher fi neness of micro modal fi bres in • rotor-spun yarn had practically no eff ect on the performed work of rupture (7.70 N cm and 7.66 N cm).Th e spinning technique and consequently the • yarn structure determine the level of overall yarn evenness.All types of modal yarns diff er in overall uneven-• ness and in micro modal fi bres.Th e values of the overall unevenness of ring-, ro-• tor-and air-jet-spun modal yarn (10.21%, 10.95% and 12.33%) are higher than the same values of micro modal yarns (9.67%, 12.69% and 12.12%).Most yarns do not contain periodic faults with si-• gnifi cant amplitudes.Th is is the reason CV b values are low.Th e coeffi cient of variation of the overall uneven-• ness obtained as the mean value of 10 packages CV b is the highest in air-jet-spun modal yarn (2.44%) and air-jet-spun micro modal yarn (4.69%) Figure 12: Hairiness H and CV of 10 cones of modal and micro modal ring-, rotor-and air-jet-spun yarns