Foaming-free sustainable and scalable insulating foams derived from sunflower stalk pith for thermal insulation and sound absorption

Developing biomass-based materials with no or less chemical modifications represents a more sustainable strategy. Sunflower ( Helianthus annuus L. ) stalks are known as typical agricultural waste, most of which are burned or buried, causing environmental pollution. Relying on the porous and low-density feature, sunflower pith (SP) inside the stalk is a potential foam material. However, the problems like small size, irregular shape and anisotropy hinder the practical applications of SP. Here, we reported a simple breaking-stacking-molding (BSM) strategy, which involves breaking SP into particles, followed by spread of phenol-formaldehyde (PF) resin on the particles, and then stacking and molding the particles into panels with desired shape and size. The resulted foams demonstrate improved mechanical strength, isotropy, excellent thermal-insulation and noise-reduction capabilities. Specifically, these foams with different apparent densities exhibited thermal conductivity of 0.038–0.048 W · m –1 ·K –1 . By processing these foams into a vacuum insulation panel (0.023 W ·m –1 ·K –1 ), their applications for refrigerators, cold-chain transportation and building, etc., become practical. Further, the 10mm-thick SPF panel has high noise-reduction coefficient of 0.34 at a frequency range of 250–3000 Hz, suggesting its great potential as high-performance sound absorption material. Additionally, the SPF material shows excellent fire safety and retains good biodegradability. Technically speaking, the BSM strategy does not involve any chemical modifications on SP, and is basically physical treatment, requiring simple processing equipment, and making production of SPF scalable.