Effect of Fibre Properties on the Structure, Strength, and Thermal Conductivity of Foam-Formed and Air-Laid Cellulosic Lightweight Fibrous Materials

The growing demand for sustainable materials is driving interest in cellulose fibres as eco-friendly alternatives to traditional insulation and cushioning materials, including expanded polystyrene (EPS), Extruded Polystyrene (XPS), and mineral wool, which pose environmental challenges. Foam forming has been extensively studied as a method for producing lightweight structures from cellulose fibres, but air-laying—a common nonwoven method—has been less explored. This study examines how wood fibre type and fibrillation level affect the structure, insulation, and strength of foam-formed and air-laid materials. A novel binding method is introduced for air-laying, involving post-laying water spraying to enhance bonding. Foam-formed materials had an average pore size of 300 to 600 μm with a wide distribution, including millimetre-scale pores; while air-laid materials had a smaller, more uniform pore size of 100 to 150 μm. Mechanical refining increased the pore size in foam-forming. Thermal conductivity decreased with decreasing fibre length, pore size, and increasing tortuosity of the fibre phase. The highest compression stress was achieved with refined chemi-thermomechanical pulp (CTMP), and the best recovery with unrefined bleached softwood kraft pulp (BSKP) and mixtures of acacia and BSKP. The findings suggest that mixing short hardwood fibres with longer softwood fibres in foam-forming could enhance performance in thermal insulation applications.