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An Experimental Study of the Needled Nonwoven Process: Part I: Fiber Geometry Before Needle Punching

Posted on: Friday, 30 April 2004, 06:00 CDT

This series of papers reports three experimental methods that have been used in a recent study of the wool needle-punched nonwoven process. Although the measurement results are specific to wool fibers, the experimental techniques can be applied generally to study the needle-punching process. The first paper presents a method of quantifying the two-dimensional geometric characteristics of wool fibers in the card web for feeding to the needle-punching machine. The overall characteristics of the fiber curve are important to the fiber geometry change during the transformation of a card web to a consolidated nonwoven fabric. We use a minimum-width rectangle technique to characterize the overall fiber shape in the card web. Fiber characteristics, such as extent, shape factor, orientation, and area density of the fibers, are measured from the minimum-width rectangle. The fiber geometry is further classified into hooks (trailing and leading), straight fibers, U-bends, and loops and entanglements. The fiber geometric features are analyzed statistically based on over 400 fibers. Hook fibers account for 56% of the card web, straight fibers for 21%, U-bends for 15%, and loops and entanglements for the remaining 8%. Longer fibers are more likely to form hooks, while shorter fibers are more likely to form U- bends and straight fibers.

Previous studies of fiber geometry formed during carding have all considered yarn making, whereas the fiber web formed on the doffer is "folded" into a sliver form, converting a fiber curve substantially in plane into a primarily one-dimensional configuration. Fibers in card sliver have been classified according to the position of hooks along their length, being referred to as leading hooks, trailing hooks, and hooks on both ends. Most fibers in card sliver are trailing hooks, which are formed when the fibers are transferred from the swift to the doffer. The controlling carding parameters for the formation of hooks are swift/doffer speed ratio and weight of the resultant sliver [I]. A higher swift/doffer ratio or a heavier resultant sliver is usually associated with an increased number of fiber trailing hooks in card sliver. The presence of fiber hooks in sliver reduces the effective fiber length in the resultant yarn. In combed yarn production, for example, worsted spinning and combed cotton spinning, these fiber hooks are corrected in subsequent processes, such as drawing (gilling in the worsted process), combing, roving, and spinning. The presence of fiber hooks influences combing noilage, drafting irregularity, and yarn properties. Hooks at the trailing ends of fibers when entering drafting systems or at the leading ends when entering the comb are more effectively straightened. While this primarily one-dimensional description of fiber geometry in card sliver has provided sufficient information for yarn spinning studies, the planar or two- dimensional features of fibers in the card web are vitally important to nonwoven manufacturing.

In studying nonwoven structural mechanics, detailed knowledge of fiber geometric characteristics in non-woven fabrics is required. Existing mechanics models rely on "microscopic" or localized fiber features, such as orientation and the curl factor. Both of these parameters have been defined in relation to a very short fiber length [2, 3, 4]. The curl factor, for instance, is calculated from fiber segments in the order of a length equal to the mean bond-to- bond distance. In needle-punched nonwoven fabrics, these bonds can slip when the fabric is extended, and thus the macroscopic fiber geometry (i.e., over the whole fiber length) is required to derive the local fiber shape after one bond slips.

In this study, we will focus on fiber batts formed by cross- lapping carded webs for feeding to the needle punching machine used in nonwoven fabric production. Except for the fibers at the two edges of the batt, the primarily planar fiber configuration formed in the card web is largely unchanged up to the needle-punching stage. The needle-punching process modifies this planar fiber configuration to a 3D curve by transferring segments of the fiber in the direction perpendicular to the plane of the original 2D fiber curve. Due to needle punching, parts of the fiber (or the whole fiber) slip in their original plane to allow for the fiber length required for vertical transfer. The shape of a fiber in the final nonwoven fabric is determined by its initial shape and the needle action under the constraints of neighboring fibers, contacting needles, and fiber-stripper plates.

Preparing Card Web

A three-worker mini-card was used. The fiber web stripped from the doffer was rolled up on a winding drum. There was no nominal draft between the doffer and the winding drum. For this study, the wool top had a 27 /xm diameter. A small proportion of the wool was dyed black and carded together with the undyed wool. This tracer fiber mixture was then further "diluted" by mixing with additional undyed wool in a second carding process. This allowed a thorough mixing of the tracer fibers and compensated for the smaller number of workers in the mini-card.

Before starting the card, A4-sized paper sheets were attached to the winding drum. The card was then switched on for a short time to build up a layer of fibers on the drum surface. An A4-sized transparent film was used to cover the web from the top, so that it was held between the paper and the film. The web was then carefully cut out around the edge of the film. The paper and the transparency sheets were then stapled to hold the piece of card web securely. The direction in which the web emerged from the card was marked on the sheet for later use in measuring fiber orientation. all samples were prepared under the same carding conditions, which were considered to be suitable for carding the kind of wool in this experiment.

Minimum-Width Rectangle Technique

The dimensions and orientations of the minimum-width rectangles enclosing 412 randomly chosen fibers were measured with a sliding rectangular gauge, which was moved around the fiber until the minimum distance between two opposing sides of the rectangle was achieved. This formed the narrowest rectangle that enclosed the curved fiber, as shown in Figure 1. The length and width of the rectangle were then measured, along with the angle between the long side of the rectangle and the carding direction ?. The fiber shape was classified (described later), and then the fiber was carefully taken out from the sheets and its straightened length measured. Aspect ratios were obtained by dividing the width of each rectangle by its length (i.e., a small aspect ratio represents a narrow rectangle).

FIGURE 1. Framing fiber in a minimum-width rectangle.

The distribution of the rectangle length, shown in Figure 2, was skewed with the median on the left side of the mean. Figure 3 is the distribution of the straightened length of the fibers (based on 104 fibers). The mean length of the rectangles was about one-half and the mean width about one-quarter of the mean straightened length of the fibers. Note that the fiber length histogram skews to the same side as that in Figure 2.

FIGURE 2. Length distribution of the minimum-width rectangle.

The distribution of the rectangle aspect ratios (width divided by length) also showed a slight bias, with the median on the left-hand side of the mean (see Figure 4). As the fibers are transferred across the web thickness by needle punching, they will be straightened in their original plane, and the dimensions of these minimum-width rectangles will somewhat decrease in the needled fabric.

FIGURE 3. Distribution of fiber length after straightening.

FIGURE 4. Distribution of rectangle aspect ratio.

FIGURE 5. Fiber orientation distribution.

The distribution of the orientation angles of the rectangles (Figure 5) was approximately symmetrical around the direction of card web production, as would be expected from a normal carding machine.

Classification

We used a classification based on the overall features of the fiber geometry, instead of localized details such as fiber curvature or crimp. The fibers were classified into four major categories: hooks, U-bends, straight fibers, and loops and entanglements:

Hooks: Reversal at either or both ends of the fiber. While it is common practice to distinguish between leading and trailing hooks in yarn-spinning studies, this distinction probably does not bear the same significance to the study of the nonwoven process.

U-bends: Gradual change of direction. The main differences between U-bends and hooks are that the curvature at the bending area of a U-bend is much smaller and the bend is approximately at the middle of the fiber length. Such a definition inevitably leaves room for ambiguity in the subjective classification. An alternative would be to group the U-bends into the hooks category and differentiate them by the aspect ratio of the minimum-width rectangle.

Straight fibers: Fibers could be approximated by a straight line through the neutral centers of crimps.

Loops: Fibers making a gradual change of direction greater than 360.

Entanglements: A long length of fiber confined in a relatively small radius area.

The three kinds of hooked fibers (leading, trailing, and hooks at both ends) constituted about 56% o\f the total number of fibers in the card web. Trailing hooks predominated, while very few fibers had hooks at both ends. Straight fibers accounted for about 21% of all fibers, and U-bends, which sit between straight and hooked fibers, about 15%. Loops and entanglements together formed about 8% of the web. Many of the entanglement fibers (about 3% in total) would very likely have been the neps that we normally observe in card webs.

Table I shows the aspect ratios of the minimum-width rectangles around the fibers for each class. The hooked fibers had an average aspect ratio of 0.43 (i.e., they were more than twice as long as they were wide). The straight fibers had rectangles with the smallest aspect ratios and the U-bends the largest.

Table II demonstrates how the straightened length of the fibers relates to the fiber geometric characteristics in the card web. Longer fibers are more likely to form hooks, and shorter fibers are more likely to form U-bends and straight fibers. This is due to the drag force that the wire on one carding surface exerts on a fiber to pull it through the wires of the opposing carding surface. The longer fibers require greater drag forces, which are achieved by forming sharp-bend hooks around the holding wires. The ratio between the length of the minimum-width rectangle and the straightened fiber length reflects the level of straightening in the card web. The straight fiber class has a much higher ratio than the other three major classes. Another measure of the extent, or crookedness, of a fiber in the card web is the ratio between the straightened fiber length and the area of the minimum-width rectangle drawn around the fiber. This parameter defines the area fiber density. Note that when measured in this way, all fiber classes appear to have approximately the same mean value.

TABLE I. Distribution of rectangle aspect ratio according to fiber class.

TABLE II. Relationship between fiber length and fiber class.

Conclusions

We have experimentally studied the two-dimensional characteristics of fibers in the card web by using a minimum-width rectangle method. This method enables the analysis of various characteristics of the fiber's geometry, such as the extent, area of spread, orientation, and crookedness. The fibers are also classified according to their overall geometric features into hooked, U-bend, straight, loop, and entanglement. The characteristics of each class are analyzed using the results from the minimum-width rectangle study. Longer fibers are more likely to form hooks, and short fibers are more likely to form straight fibers and U-bends. This can be related to the different drag forces the fibers experience in carding.

ACKNOWLEDGMENTS

We thankfully acknowledge the support for this work by the New Zealand Foundation for Research, Science and Technology.

Literature Cited

1. Ghosh, G. C., and Bhaduri, S. N., Studies on Hook Formation and Cylinder Loading on the Cotton Card, Textile Res. J. 38, 535- 543 (1968).

2. Hearle, J. W. S., and Stevenson, P. J., Nonwoven Fabric Studies, Part III: The Anisotropy of Nonwoven Fabrics, Textile Res. J. 33, 877-888 (1963).

3. Pourdeyhimi, B., Ramanathan, R., and Dent, R., Measuring Fiber Orientation in Nonwovens, Part II: Direct Tracking, Textile Res. J. 66, 747-753 (1996).

4. Pourdeyhimi, B., Ramanathan, R., Dent, R., and Davis, H., Measuring Fiber Orientation in Nonwovens, Part III: Fourier Transform, Textile Res. J. 67, 143-151 (1997).

Manuscript received March 18, 2003; accepted June 23, 2003.

MENGHE MIAO AND HEATHER E. GLASSEY

Canesis Network Ltd.,1 Christchurch, New Zealand

1 Formerly WRONZ.

Copyright Textile Research Institute Apr 2004

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