New process makes' biodegradable 'plastics truly compostable! (2)

Main Points

1. Enzyme processing with surface exposed active sites can be realized through engineering enzyme - protectant - polymer complex. Depolymerization of polycaprolactone and polylactic acid can be achieved in a few days in systems containing less than 2% of the enzyme.

2. The study found that up to 98% of the polymers in standard soil compost and home tap water can be converted to small molecules, completely eliminating the current need to separate and landfill their products in composting facilities.

3. Oxidase embedded in polyolefin maintains its high catalytic activity during the whole catalytic degradation process. Hydrocarbon polymers do not bind to enzymes as tightly as polyester polymers, and the resulting active radicals do not chemically modify the macromolecular host.

4. This study provides molecular guidance for the selection of enzyme-polymer and enzyme protectant to regulate substrate selectivity and optimize biocatalysis pathways.

Fig. 1 Degradation of polymers by biocatalysis of embedded enzymes.

At the enzyme load of 0.02wt %, RHP lipase nanoclusters were evenly distributed throughout the whole region (Fig. 2a). RHP-BC lipase clusters, approximately 50 nm to 500 nm in size, are located between bundles of PCL tablets (Fig. 2C). Nanoscale dispersion and trace additives are the key to maintain the properties of the main body. The formation and change of nanopore structure during the degradation process were characterized by SAXS, DSC and scanning electron microscopy, and the conversion rate reached 98% within 24 h.

The new process, designed by the authors, involves embedding enzymes in edible polyester during the plastic manufacturing process. These enzymes are protected by a simple polymer coating that prevents the enzyme from unravelling and becoming useless. When exposed to heat and water, the enzyme breaks away from the polymer's protective coating and begins to break down the plastic polymer into PLA components, reducing it to lactic acid, which feeds the soil microbes in the compost. Polymer packaging also degrades. Microplastics are a by-product of many chemical degradation processes and are themselves a pollutant. Plastics made using the techniques studied by the authors were 98% degradable.

Fig. 2 Characterization and degradation of PCL-RHP-BC lipase

The authors designed molecules called "random heteropolymers" (RHPs), which wrap around enzymes and gently anchor them together without restricting their natural flexibility. RHP consists of four types of monomer subunits, each of which has the chemical properties of interacting with chemical groups on the surface of a particular enzyme. They degrade under ultraviolet light, and their concentration is less than 1% of the weight of plastic. The authors coated the enzyme in RHP and embedded billions of these nanoparticles in plastic resin beads. The study found that the enzymes coated with RHP didn't change the properties of the plastic, which can be melted and extruded into fibres at temperatures around 170 degrees Celsius, just like ordinary polyester plastic. All you need to do to trigger degradation is add water and a little heat. At room temperature, 80% of the modified PLA fibers were completely degraded in about a week. 

The higher the temperature, the faster the degradation rate. Under industrial composting conditions, the modified PLA was degraded within 6 days at 50 ° C. Another polyester plastic, PCL(polycaprolactone), degrades in 2 days under industrial composting conditions of 40 ° C. Protease K used in the PLA and lipase used in PCL are both inexpensive and readily available. This rapid degradation is well suited to urban composting, which usually takes 60 to 90 days to turn food and plant waste into usable compost. Industrial composting takes less time at high temperatures, but the modified polyesters also break down more quickly at these temperatures.