Nonwovens Basis Weight Uniformity

Introduction

One of the critical issues related to nonwoven fabrics which highly affects their mechanical properties, aesthetic and physical properties, is the uniformity of basis weight of nonwoven fabrics. This factor identifies many factors related to nonwoven fabrics. For example, variation in basis weight indicates inconsistency in web formation. The “uniformity” term is considered in three different aspects: a) variations in fiber orientation and alignment b) fiber size and shapes c) basis weight. Although the variation in fibers size, shape and orientation should be considered, the mass uniformity has been assessed in these papers. As we have been already familiar, the uniformity of nonwoven webs is relevant to its solidity and porosity, so the nonuniformity causes failure in nonwoven products [1]. Furthermore, lightweight nonwovens are nonuniform and it causes inhomogeneity. These effect forces manufacturers produce webs with higher basis weights [2].

Measurement of Uniformity

In this paper, basis weight irregularity is measured with the cut-and-weight method. In this method, samples were randomly chosen from a different part of a nonwoven web then weighed. The coefficient of variance (CV%) of weighed samples counted and its used as an “index” of uniformity. Similar works had been done previously by the cut-and-weight method by using optical methods and they analyzed the basis weight distribution of samples by image analysis and optical density instead of “index” utilized in this present study [1]. In addition, they applied sample images with known uniformity in order to confirm the accuracy of the method. One set of images was simulated images with different uniformity and wet-laid nonwovens with different degree of uniformity. The other sets of nonwovens fabric were illuminated through transmitted light, the intensity of images is related to the optical density of samples and their basis weight.

In this paper, the basis weight of samples was assessed by attenuating a light beam passing through the fabric or web. However, they claimed that there is no applicable theory to this, so they used experimental tests to confirm the results [2]. In this method, a beam passes through the fabric and it was reflected on a bicycle reflector, them it passed back through the fabric and it was measured by a photodetector. They devised a device measuring basis weight uniformity which was used at speeds up to 400m/min.

Limitations and advantages of methods

One of the challenges is that the index is not size invariant and measurement at different size scale will get a different scale [1]. Conversely, the image-based systems are able to monitor basis weight uniformity at higher resolutions and different index scales.

One issue related to evaluating the uniformity is based on detection of defects and their distribution. In this method, they used the concept of outliers in distribution which positive outliers are areas where basis weight or density is high and conversely negative outliers are considered in places where density is low. The issue here was that by which method outliers detected. In this study, they inspired from quadrant analysis previously done by others which they could overcome the size dependence of previous methods. In this work, they extended the previous works to grayscale images and introduced new uniformity index. Moreover, they used grayscale intensity instead of % of black in the image. One advantage here is that they could minimize errors relevant to segmentation to a binary image [1].

One of the issues here is that unfixed position of samples being evaluated causes many errors. But they proved with statistical analysis that the device is accurate enough to use in nonwoven fabrics with three or more times less variable than current commercial nonwovens [2]. In this method, they could investigate the basis weight of nonwovens on-line. In fact, their method is able to analyze the basis weight of a moving web accurately and rapidly which can be applied in nonwovens production.

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