The physics of high-bulk wool batting

Abstract

Knoppy web is a product developed by FibreTech New Zealand Ltd, consisting of small spherical clusters of wool and PLA fibres suspended in a non-woven wool/PLA fibrous batt. It offers improved resilience against compression over regular wool batts, as well as better drape and washability. Knoppy web is targeted to compete with down clusters in bedding and apparel products.

As part of an ongoing commercialisation process, FibreTech New Zealand Ltd desired insight into the production process, optimised knoppy web specifications, and a physical understanding of knoppy web’s resilience against compression. In this thesis we met these objectives, and tested several hypotheses about the benefits of both knops and PLA in non-woven fibrous batting.

We examined the process for producing knoppy web, extracted the relevant physics from each step, and linked theoretical insights to the available control strategies for various production parameters. We then examined the available literature on compression of spheres and random fibrous assemblies. Building upon these, we developed the first mathematical model describing the compression mechanics of a knoppy web.

In the model, a knoppy web is represented as a uniform array of knops embedded in a fibrous web. Each knop is treated as a hollow spherical membrane, to which a series of assumptions are applied such that the resulting sphere captures the core physical intuition of knop compression. The web is treated as a random fibrous assembly using van Wyk’s equations. The model was developed using the energy method, and implemented using a variety of numerical techniques and computational packages.

A series of experiments were conducted to examine key parts of the knoppy web production process. It was observed that knops can provide significantly better compressional properties than both down clusters and goose feathers, and that they can be packaged at high strain for extended periods while retaining their desirable properties (after recovery via steaming). Additionally, an investigation of the parameter space for knoppy web specifications showed that there is significant tunability available in the properties of knoppy web.

The model was compared to compression curve data taken during the investigation of the specification parameter space. Although its predictive powers are limited, the model was able to provide satisfactory first-order fits to the experimental data.

Finally, the various theoretical and experimental results were used to develop optimum knoppy web specifications for use in overbody, underbody and apparel products. These specifications have been subsequently used by FibreTech New Zealand Ltd in the ongoing development of end-user products.