Biochar-reinforced LDPE foams with dynamic covalent networks: Structure–property control and exploring pathways toward reprocessable polyolefin foams

Polyolefin foams are widely used in packaging, insulation, and structural applications, but are typically rendered non-recyclable by the permanent crosslinking required to achieve sufficient melt strength during foaming. Here, low-density polyethylene (LDPE) foams, biochar (BC) composite foams, and vitrimer-modified foams were developed to establish quantitative structure–processing–property relationships while enabling repair and recycling in crosslinked foam systems. Increasing azodicarbonamide (ADC) foaming agent content from 2 to 7 wt% increased foam expansion to ∼260% and reduced density from 0.91 to 0.02 g cm −3 (97% reduction), accompanied by a six-fold increase in average cell area and a modest decrease in crystallinity. Incorporation of BC strongly modified rheology and cellular morphology: 3 wt% BC reduced the average cell area by ∼85% without suppressing expansion, whereas 9 wt% BC induced cell collapse, limiting expansion and increasing hardness and compressive modulus due to elevated melt viscosity. Dynamic covalent crosslinking was introduced through vitrimerization of the optimized LDPE/BC formulation, as confirmed by FTIR-ATR analysis. Vitrimer foams exhibited slightly reduced foaming efficiency but retained comparable mechanical and thermal insulation performance while enabling effective repair and one complete recycling and re-foaming cycle. Although replenishment of the chemical blowing agent was required during re-foaming, this work demonstrates a viable strategy for combining renewable fillers and dynamic networks to advance the circularity of LDPE-based foam materials.