# Property Variations of Binder-Free Lignin-Rich Fiber Networks Driven by Forming Processes and Hot Pressing

## TL;DR

This study examines how different forming methods and hot pressing affect the properties of lignin-rich fiber sheets, finding that extreme pressing conditions enhance strength regardless of initial structure.

## Contribution

The paper introduces how extreme hot-pressing conditions can override initial structural differences in lignin-rich fiber networks, enhancing material strength.

## Key findings

- Compressed air-laid sheets showed highest air permeance and smallest pore size, beneficial for particle filtering.
- Foam-laid sheets had weaker performance at moderate pressing but matched water-laid sheets under extreme conditions.
- Extreme pressing conditions led to high tensile and compression strength due to polymer interdiffusion overriding hydrogen bonding.

## Abstract

Sheets made from
lignin-rich fiber raw materials can be bonded
by hot pressing without external binders. This paper explores how
air-laid, foam-laid, and water-laid web formation methods, initial
sheet moisture content, as well as hot-pressing conditions (5 MPa,
100–260 °C, 1–60 s), impact the physical properties
of board-like materials made of chemi-thermomechanical softwood fibers.
In addition to the structural characterization of the hot-pressed
materials by X-ray microtomography, air permeance, water contact angle,
dry and wet tensile strength, and in-plane compression properties
were measured. Despite the significant structural densification, characteristics
of the forming method were retained after hot pressing in the final
sheet properties. The compressed air-laid sheets had the highest air
permeance and the smallest mean pore size, which could be beneficial
for particle filtering. At moderate pressing temperatures and times,
the significant proportion of large pores in the foam-laid sheets
made them weaker than the corresponding water-laid sheets. However,
under extreme pressing conditions, the foam- and water-laid sheets
reached similar values of high tensile and in-plane compression strength.
This suggests that polymer interdiffusion becomes the dominant factor
for material strength under these conditions, superimposing the hydrogen
bonding created during aqueous forming.