Recently, a study published in the journal Communications Chemistry proposed a technical solution: introducing lignocellulosic materials extracted from non-recyclable household waste into a biodegradable poly-L-lactide (PLLA) biocomposite system. This technology uses lignocellulosic recycled pulp obtained from contaminated municipal solid waste and secondary hydrolysis residues as reinforcing fillers for commercial PLLA.
The resulting composite has a tensile modulus of approximately 6 GPa and can completely disintegrate within 30 days in a simulated industrial composting environment. This research provides a new technological path for processing household waste that is directly discarded in the recycling process.
Problems posed by recycling waste
In the traditional recycling system, a large amount of contaminated waste cannot be recycled, which hinders the development of a material circular economy. The UK alone generates around 27 million tonnes of household waste each year, but its national recycling rate has long stagnated at around 45%.
When mixed recyclable waste is processed at a Material Recovery Facility (MRF), up to 30% of the material can be rejected due to severe contamination of the material and limitations of sorting technology. Most of this rejected material is sent to landfill or incineration, resulting in high disposal costs for local authorities, with landfill entry fees alone being as high as approximately £150/tonne. The degraded lignocellulose pulp obtained from this type of waste has traditionally had very low economic value, and there is an urgent need to develop new resource utilization methods.
A new process route for high-value waste
Researchers conducted process tests on two types of waste: one is recycled solids (RS) obtained from paper and cardboard components rejected by material recovery facilities; the other is hydrolyzed solid residue (PHS) obtained after enzymatic hydrolysis and saccharification of recycled pulp.
Process flow: First, wash in 80°C hot water for 30 minutes to reduce the microbial load of the material and peel off attached contaminants; then rinse to remove coarse impurities; the solid material is dried and dry-mixed to obtain fiber raw materials with uniform components for subsequent polymer processing.
The washed residue was blended with commercial PLLA particles, and the mass fraction of both fillers (RS or PHS) in the composite was 30 wt%. The mixture is put into a co-rotating twin-screw extruder for processing: the barrel temperature of the feed section is 170°C; zone 1-3 is 180°C; zone 4-6 is 190°C; and the die head temperature is 200°C. After the extruded strips are cut into pellets, they are then subjected to high-pressure injection molding.
Differential scanning calorimetry and tensile testing were used to characterize the composite materials; the washed RS and PHS fillers were observed through scanning electron microscopy. Compost simulation experimental conditions: 58±2℃, lasting 37 days; the sample is made of 0.06mm thick molded film, and the residual dry mass is used to evaluate the degree of material disintegration.
Mechanical properties and compost degradation behavior
Chemical composition analysis shows that the total sugar content of the recycled solid material (RS) is about 68% (mainly cellulose) and lignin is about 21%; the fiber structure of the hydrolysis residue PHS is more fragmented. The original PHS extract content is 47% and the ash content is 24%; after washing treatment, the inorganic components in the material account for about 50%.
Thermal analysis results show that both waste fillers can act as nucleating agents in the PLLA matrix and promote the crystallization of PLLA. The crystallinity of the injection molded composite increased from 20% in the pure PLLA control group to about 40%.
The waste filler also acts as a mechanical reinforcement, so the increase in material stiffness cannot be entirely attributed to increased crystallinity. The biocomposite has a tensile modulus of about 6 GPa; under conditions of comparable crystallinity, unreinforced pure PLLA is only 2.5 GPa. The tensile strength was maintained at approximately 60MPa for all composite formulations. The composites yield and break at lower strains than pure PLLA, reflecting the trade-off between stiffness and ductility.
Compost testing showed that composites with waste filler disintegrated faster. Both RS-reinforced and PHS-reinforced materials can completely disintegrate within 30 days under simulated composting conditions. As a comparison, unmodified PLLA lost only 40% of its mass during the same cycle; it was not until day 37 that the disintegration rate of pure PLLA reached approximately 94%.
The author pointed out that even if the crystallinity of PLLA increases, the degradation is still accelerated. The reason is that the degradation of RS and PHS fillers will form holes inside the matrix, making it easier for moisture and microorganisms to invade the PLLA matrix. Note: The experimental results for complete disintegration in 30 days are based on 0.06mm film specimens, not 3mm injection molded tensile specimens. This conclusion only applies to this test sample form.
Practical applications and scaling prospects
This study prepared biocomposites for structural use, and testing related to packaging applications has not yet been carried out. The study confirmed that washed waste filler without bleaching can be directly blended with thermoplastic polymers, eliminating the need for energy-intensive processes such as bleaching and papermaking. Contaminated waste is recycled to produce polymer composites that have high stiffness and can completely disintegrate under simulated industrial composting conditions.
If applied on a large scale, this process can reduce the amount of urban contaminated waste sent to landfills and incineration, and convert waste originally to be disposed into composite material raw materials. However, the study did not report the cost of industrial processing, nor did it have a complete life cycle assessment of the entire process. The above problems still need to be solved to achieve commercialization.
For composites made entirely from waste
Compared with the team's earlier process, this solution eliminates the bleaching and papermaking processes, bringing significant environmental benefits.
Based on the team's previous "cradle to gate" life cycle assessment model: after eliminating the above process, each kilogram of recycled lignocellulosic pulp can reduce the global warming potential of up to 5.65kg carbon dioxide equivalent. This value refers to the previous model and is not the result of recalculation for the entire new process in this study.
The research team proposed the idea of combining this process with biorefinery technology to convert carbohydrates derived from garbage into lactic acid or lactide, and ultimately realize that the PLLA matrix phase and reinforcing fillers are all prepared from garbage. However, industrial-scale trials are still needed to assess environmental performance, processing conditions and economic feasibility.
