Assessing cloudiness in nonwovens

TL;DR

This paper introduces a robust method to quantify cloudiness in nonwoven filter media using the power spectrum of local areal weight, aiding quality control and standardization.

Contribution

It proposes a theoretically sound and practical approach to measure cloudiness via the power spectrum, addressing spectral band selection for better assessment.

Findings

The power spectrum effectively captures cloudiness in nonwovens.

The method is robust and suitable for standardization.

Spectral band choice correlates with perceived cloudiness.

Abstract

The homogeneity of filter media is important for material selection and quality control, along with the specific weight (nominal grammage) and the distribution of the local weight. Cloudiness or formation is a concept used to describe deviations from homogeneity in filter media. We suggest to derive the cloudiness index from the power spectrum of the relative local areal weight, integrated over a selected frequency range. The power spectrum captures the energy density in a broad spectral range. Moreover, under certain conditions, the structure of a nonwoven is fully characterized by the areal weight, the variance of the local areal weight, and the power spectrum. Consequently, the power spectrum is the parameter that exclusively reflects the cloudiness. Here, we address questions arising from practical application. The most prominent is the choice of the spectral band. It certainly depends on the characteristic "size of the clouds", but is limited by the size and lateral resolution of the images. We show that the cloudiness index based on the power spectrum of the relative local areal weight is theoretically well founded and can be robustly measured from image data. Choosing the spectral band allows to capture the cloudiness either visually perceived or found to be decisive for product properties. It is thus well suited to build a technical standard on it.