Pine Bark as a Renewable Feedstock for the Production of Rigid Polyurethane Foam

The use of biomass waste for polymer production is a key component of the global plastic sustainability strategy. Bark contains low molecular weight polyphenolic and carbohydrate compounds rich in hydroxyl groups, making it a potential macromonomer source for plastics. The global annual volume of bark waste is estimated at 300–400 million m³, making it a cheap and highly available raw material.

This study focused on the waste-free processing of pine (Pinus sylvestris) bark into rigid polyurethane (PUR) foam. Extractives, obtained in a yield of 25%, enriched with low molecular weight carbohydrates, were separated via pressurized water extraction at 150 °C. Increasing the extraction temperature above 150 °C significantly reduced both the extractives yield and their hydroxyl content due to carbohydrate dehydration. Extractives were converted into liquid polyols through "green" oxypropylation with propylene carbonate (PC), varying PC/OH ratios. The residual bark was introduced into PUR foam formulation as a filler.

The resulting polyols exhibited viscosities ranging from 8.6 to 210 Pa·s, and hydroxyl values between 580 - 660 mg KOH/g were used to synthesize PUR foams with PMDI at an NCO/OH ratio of 1.1.

In the PUR foam formulations, commercial polyols (Lupranol 3300 and Lupranol 3422) were partially or fully replaced by the bark extractives-based polyols. Key material properties —apparent density, deformation, morphological, and thermal characteristics—were evaluated and compared to foams based on commercial polyols.

Polyol synthesized at a PC/OH ratio of 3 was the most promising for PUR foam production, yielding materials with properties equal to or better than those of foams based on commercial polyols. However by incorporating the residual bark after extraction as a filler led to a slight reduction in mechanical performance.

Acknowledgements:

This research is supported by the project: Innovation in Forest Management and Value Chain for Latvia's Growth: New Forest Services, Products and Technologies (Forest4LV) – Project Nr. VPP-ZM-VRIIILA-2024/2-0002.