Lignin Inter-Diffusion Underlying Improved Mechanical Performance of Hot-Pressed Paper Webs

Area of Science:

• Materials Science
• Forest Products Chemistry
• Pulp and Paper Engineering

Background:

• Bio-based fibers offer sustainable alternatives to conventional packaging materials.
• Improving the mechanical properties of fiber-based materials is crucial for broader application.
• Hot-pressing is an effective technique for enhancing the strength of lignin-containing paper webs.

Purpose of the Study:

• To investigate the impact of hot-pressing conditions on the mechanical properties of thermomechanical pulp (TMP) webs.
• To compare the performance of TMP with other fiber types under varied pressing conditions.
• To elucidate the microstructural changes and bonding mechanisms induced by hot-pressing.

Main Methods:

• Hot-pressing of TMP webs at varied temperatures and solids content.
• Standard mechanical strength testing (wet and dry strength).
• Structural characterization using X-ray microtomography and scanning electron microscopy (SEM).

Main Results:

• Wet strength increased with pressing temperature up to 270°C, attributed to lignin macromolecular inter-diffusion.
• Lignin diffusion exhibited an activation energy of approximately 26 kJ mol⁻¹, dominating wet strength development.
• Optimum dry strength was observed around 200°C, with higher temperatures leading to hemicellulose degradation.
• Highest wet strength was achieved with dry TMP webs; dry strength showed less correlation with initial solids content.

Conclusions:

• Hot-pressing is a viable method to significantly enhance the wet and dry strength of TMP-based materials for packaging.
• Lignin softening and subsequent diffusion are key mechanisms for improving wet strength at elevated temperatures.
• Careful control of pressing temperature and solids content is necessary to optimize mechanical properties and avoid degradation.