Characterisation of Fibre Lengths and Breakage Behaviour of Cotton Fly in Knitting Process Opredelitev dolžine in oblike pretrgov bombažnih letečih

In this research work, the fi bre fl y from diff erent zones in the knitting process, such as the cone unwinding, guide and the knitting zones, were collected and characterised. The results show that in the cone unwinding zone were formed 18% of shortest (i.e. ≤1 mm), 40% of shorter (i.e. 2–6 mm), 32% of short (i.e. 7–11 mm) and 10% of long (i.e. ≥12 mm) fi bres, followed by shortest (14%), shorter (38%), short (30%) and long (18%) in the guide zone, and shortest (30%), shorter (36%), short (28%) and long (6%) in the knitting zone. The surface characteristics and breakage behaviour of fi bre fl y collected from three diff erent zones were studied using a scanning electronic microscope. The majority of fi bre fl y are highly twisted, with kink bands with longitudinal and transverse cracks, and twist break in the form of ductile, granular and individual fi bril break due to torsional fatigue under high tensile stress in the cone unwinding zone. The majority of fi bre fl y are found in twisted form with a fl at surface, are bent with kink bands, surface peeling, a long axial crack and twist break, and either fi bril failure or independent fi bril failure due to the combination of fl ex and torsional fatigue under tensile stress in the guide zone. Surface peeling, bent along with the kink bands, and twist break, either with two and multiple splits or granular and ductile form, were observed in the fl y of the knitting zone due to the combined eff ect of torsional, fl ex and abrasion fatigue under tensile stress.


Introduction
Fibre fl y liberation from cotton spun yarn is a common phenomenon in weft knitting. Cotton spun yarn has inherent characteristics of yarn hairiness that comprises fi bre ends and loops protruding from the yarn surface. Many of these fi bres are either fractured or pulled out from the surface of the yarn and deposited in and around the diff erent zones of a knitting machine, i.e. the cone unwinding, guide and knitting zones, during the knitting process. Th ese deposited fi bres are either picked up by the incoming yarn, break yarn/needles or deteriorate the fabric appearance. Th e generation of fi bre fl y reduces machine effi ciency by 5-15%, while fabric faults increase by 15-25% and fabric weight decreases by 0.5-1.0 % [1][2][3]. Apart from these specifi c problems, at high concentrations, suspended particulate matter of cotton dust and fi bre fl y pollute the indoor atmosphere, which poses health hazards to staff , particularly those susceptible to respiratory illness [4,5]. In this respect, many researchers have studied the length distribution of fl y in a particular zone of a knitting machine with the aim of predicting the possible mechanism of fl y generation during the knitting process. Researchers [2,[6][7][8][9][10][11] have studied in detail the length distribution of fi bre fl y collected in diff erent sectors, i.e. the cone unwinding zone, guide zone and knitting zone of a circular weft knitting machine. Th ey reported that most fi bre fl y of less than 10 mm in length is found in the cone unwinding zone and knitting zone, while fi bre fl y of more than 10 mm in length is found at the top stop-motion and the positive feed device. However, one researcher [6,7,12] observed that edge fi bres are highly abraded where the fi neness of the fi bre fl y is much less than that of virgin fi bres. Th ey also reported that fi bre fl y may be released in diff erent zones of a knitting machine due to the fracturing of previously damaged protruded fi bre or the pulling out of protruded fi bre from the yarn surface. Th e mechanism of pulling and fracturing the protruded fi bre of yarn may be diff erent for each zone of a knitting machine. To date, however, no one has studied in detail the breakage behaviour of fi bre fl y in specifi c zones of a knitting machine. Th is study thus aimed to classify the fi bre length of the fl y with percentages in the specifi c zones of a knitting machine and to characterise the nature of fi bre break in the three diff erent zones of a knitting machine.

Materials
To observe the length distribution, surface characteristics and breakage behaviour of fi bre fl y, 100% J-34 cotton was used to prepare combed ring spun yarn (Table 1). Th e unwaxed yarn samples (cones) were processed in a laboratory knitting machine (Table 1). Th e diff erent sectors of the knitting machine were separated by plastic sheets to segregate the fi bre fl y liberated in these zones. Th is fi bre fl y was collected separately via air suction, using a vacuum cleaner and by changing the fi lter media.

Length distribution of fi bre fl y
To determine the length distribution of cotton fi bre fl y, a sample of fi bre fl y was pasted on a slide with the help of seed oil. Ten slides were prepared from each individual zone in the same way. A random selection method was used to select collected fi bre fl y for spreading on a slide. Th e prepared slide was placed on millimetre graph paper. Th e length of fi bre was then observed manually using a pick glass as shown in Figure 1. Th e lengths of fi bre fl y were classifi ed to four groups of lengths: shortest (≤1 mm), shorter (2-6 mm), short (7-11 mm) and long (12 mm and above) as shown in Table 2. Th e shortest lengths of fi bres were used according to the reference [3,4,10] because the fi bres may be inhaled by staff in the knitting process, while other fi bres were classifi ed as researchers reported in their work [10].
To measure the length distribution of fi bre fl y randomly, 100 fi bres were selected from each of the three zones of the knitting machine.

Surface characteristics and nature of breaks of fibre fl y
To study fi bre fl y morphology and the nature of breaks, a small amount of fi bre fl y was selected randomly from the collected fi bre fl y for all three zones. A JSM 6510 LV scanning electron microscope (SEM) was used to view the microscopic details of fi bre fl y. Th e test samples coated with gold were fi xed on a fl at sample holder with the help of an adhesive.

Results and discussions
Cone unwinding zone Th e fi bre fl y length distribution of the cone unwinding zone is given in Figure 2. It shows the classes of fi bre length with respect to fi bre fl y percentages in the cone unwinding zone of the knitting machine. It is evident from the fi gure that the shortest classes of fi bre lengths are 18%, shorter (40%), short (32%) and long (10%) in the cone unwinding zone. It is clear from the fi gure that 90% of fl y had a fi bre length of less than 11 mm. Fibres of these lengths may be released as fl y due to the fracturing of entangled fi bre [7] because the length of the fl y is much less than the average length of the fi bre. A total of 10% of fl y had a fi bre length in the range of 12-16 mm. Long, protruded fi bre may be pulled out from the yarn structure due to loose gripping in the yarn structure during cone unwinding. Th is is in line with reported work [7,10] in which it was observed that 90% of fl y with a length of less than 10 mm is generated due to the fracturing of the surface fi bre of cotton yarn during the unwinding of the cone package. Long, but loosely held fi bre in the yarn structure is pulled out during cone unwinding. Furthermore, the surface of the broken fi bre and the breakage pattern of fl y collected from the cone unwinding zone were observed using a scanning electron microscope (SEM), and are shown in  in the guide zone are 14%, 38%, 30% and 18% respectively. A total of 82 % fl y have a fi bre length of less than 11 mm. It can be said that these three classes of length may be released as fl y due to the fracturing of protruded fi bre from the yarn surface. Th e protruded fi bre may be subject to strong frictional resistance by the yarn surface and guide surface against the rotational speed of the yarn. In this zone, fl y with a length of 12-16 mm is generated due to the detachment of a few, loosely held long protruded fi bres, either over the twisting of protruded fi bre or the twist loss of yarn at the rotational speed of the yarn [10,11]. Th e surface of the broken fi bres and the breakage pattern of the fl y were observed under a scanning electron microscope (SEM), as shown in Knitting zone Figure 6 shows the distribution of fi bre lengths released as fl y in the knitting zone of the knitting machine. It was observed that the fl y length of the shortest, shorter, short and long fl y was 30%, 36%, 28% and 6% respectively in the knitting zone. A total of 94 % of fl y have a fi bre length of less than 11 mm. It was noted here, as well, that fi bre lengths of less than 11 mm may be released as fl y due to the fracture of protruded fi bre from the yarn surface.
Protruded fi bres may be subject to strong frictional resistance to the rotational speed of the yarn surface itself, the surface of needles, sinkers and the surface of old loop yarn during the formation of new loops [10]. It was also observed that only 6% of fl y are more than 12 mm in length, which is the shortest length of all three zones. A length of fl y may be released due to the momentary twist loss of yarn during the bending and unbending of yarn in the knitting zone [10,11]. Fibre fl y in the knitting zone was examined under a scanning electron microscope, as shown in Figures 7-8. Figure 7a shows a twisted form with surface peeling; Figure 7b shows a twist break with two splits; Figure 7c shows twisted and bent kink bands with an independent fi bril break; Figure 7d shows a twist break with a fi bril break, Figure 7e shows a twist break with multiple splits; Figure 7f shows a twist break with two splits; Figure  8a shows a twist break in granular form with surface peeling; Figure 8b shows a twist break with two splits and surface peeling; Figure 8c shows a twisted form with a fi bril break, Figure 8d shows a twist break with independent fi bril tearing; Figure 8e

Conclusion
Between 82 and 94% of liberated fi bre fl y are less than 11 mm in length. Th is behaviour was observed for all the three zones where the fl y was collected. Th e longest fi bre length was observed in the guide zone (where 18% of fl y was longer than 11 mm in length) due to the momentary twist loss of yarn. Th e surface of the broken fi bre and the breakage pattern of fi bre fl y collected in the unwinding zone were highly twisted, with kink bands with longitudinal and transverse cracks, a twist break in the form of a ductile, granular or individual fi bril break due to torsional fatigue under tensile stress in the cone unwinding zone. Th e fl y in the guide zone was in twisted form, with a fl at surface, bent with kink bands, surface peeling, a long axial crack and a twist break from either fi bril failure or independent fi bril failure due to the combination of fl ex and torsional fatigue under tensile stress. Surface peeling, bent along with kink bands, a twist break with either two and multiple splits of granular or ductile form were observed in the fl y in the knitting zone due to the combined eff ect of torsional, fl ex and abrasion fatigue under tensile stress.