Bioplastics: PLA, PHA, bio-PE and starch blends: End of life, climate and land

4.1 Composting and digestion work only in the right plant

Industrial composting runs hot. The standard tests hold the material at 58 C, the temperature of a compost pile's hottest phase, and require 90% conversion of its carbon to CO₂ within six months and 90% disintegration within three (Rosenboom et al. 2022, Box 2; Kabasci 2014, section 14.5). Real composting plants, though, turn their material round in 6-8 weeks. That is why composters often reject PLA bags and utensils (Rosenboom et al. 2022).

The table gives measured times. PLA degrades in 6-9 weeks under industrial conditions, right at the edge of a typical plant's cycle; PBAT takes 2-3 months, PBS 2-5 months and P₃HB 1-4 months. Water first cuts PLA's ester links without microbial help, and only then do the rare PLA-degrading microbes consume the fragments (Kabasci 2014, section 8.10). The rate of that hydrolysis rises sharply once the temperature exceeds PLA's Tg of about 58 C (Kabasci 2014, section 8.8.2), which a hot compost pile provides and a garden heap does not.

Home composting is cooler. The 'home compost' label is not based on a legislative standard: TÜV Austria adapted EN 13432 to 20-30 C and doubled the test times (Rosenboom et al. 2022, Box 2). Novamont designs some starch-polyester grades for home composting and soil biodegradation (Kabasci 2014, section 2.4.5).

Anaerobic digestion breaks biodegradable polymers down without oxygen to methane, which is captured and burned for energy. It works for thermoplastic starch, PCL and PHA, and for PLA at elevated temperatures (Rosenboom et al. 2022). The same chemistry is a liability in an ordinary landfill: buried biodegradable plastic can rot without air to methane, a greenhouse gas, which escapes unless it is captured as it is in a digester.

• Rosenboom et al. (2022), Bioplastics for a circular economy · pdf pp. 8, 11-13 (journal pp. 124, 127-129), Box 2
• Kabasci (2014), Ch 2 Starch; Ch 8 Poly(Lactic Acid); Ch 14 · sections 2.4.5, 8.8.2, 8.10, 14.5

4.2 Recycling favours the drop-ins; PLA and PHA lack a stream

Rosenboom et al. (2022) rank recycling as the best end of life for bioplastics, ahead of composting, but for most of them the recycling streams do not yet exist. Here the drop-ins win outright. Bio-PE and bio-PET are chemically identical to their fossil twins, so they go straight into established PE and PET recycling (Kabasci 2014, section 14.5). And recycling pays: the greenhouse gas burden of mechanically recycled PE and PP is about a third of virgin resin's, and of recycled PET about half.

PLA is the opposite case. It carries resin code 7, 'other', and is typically not recycled. Its chains shorten during use and it tolerates heat poorly, so the melt-and-remould methods used for PET suit it badly (Kabasci 2014, section 8.14). Near-infrared scanners can pick PLA out with 98% accuracy, and laboratory remelting of PLA and PHA works, with loss of strength (Rosenboom et al. 2022).

Chemical recycling suits these polyesters well, because their ester links cut cleanly. PLA hydrolyses to 95% lactic acid in two hours at 160-180 C without a catalyst, using a quarter of the energy needed to make virgin lactic acid; with zinc catalysts it gives back about 90% lactide in six hours. The monomer can be made into virgin-quality PLA (Rosenboom et al. 2022). The obstacles are dedicated plants, separate collection and volume; chemical recycling handles under 1% of recycled plastic. Polyolefins, bio-PE included, can be chemically recycled only by thermolysis, cracking them at about 200-800 C back to oils and gases.

Incineration recovers energy and, for plant carbon, adds no net fossil CO₂. Europe incinerated about 40% of its end-of-life plastic in 2017, the US about 20% in 2014 (Rosenboom et al. 2022).

• Rosenboom et al. (2022), Bioplastics for a circular economy · pdf pp. 10-13 (journal pp. 126-129), Box 2
• Kabasci (2014), Ch 8 Poly(Lactic Acid); Ch 14 · sections 8.14, 14.5

4.3 Outside a managed plant, most of them persist

Some 1-5% of all plastic ends up in land or ocean environments through poor waste management, and about 80% of ocean plastic comes from land (Rosenboom et al. 2022).

Bio-PE behaves like any polyethylene. Polyolefins have no cleavable groups, repel water and have molecular weights in the millions, while only fragments below about 5,000 daltons are thought to be metabolised. Partial degradation of PE film by bacteria, 5-20% over one to two months, is a laboratory curiosity (Rosenboom et al. 2022).

PLA is surprisingly persistent. In sea water at 25 C it lost little weight over ten weeks compared with PHB and PCL. In soil, a calculation from measured rates of chain shortening gave 508 months, about 42 years, for high-molecular-weight PLA to break down to lactic acid (Kabasci 2014, section 8.10).

PHA is the exception. Microbes in soil and water readily attack it, and P₃HB degrades in ocean water in one to six months. That is the strongest argument for PHA wherever loss to the environment is part of the use.

Starch itself biodegrades readily in soil, compost and water (Kabasci 2014, section 2.1), but a blend is only as degradable as its partner. Starch-filled polyethylene, sold in the past with 6-15% starch and metal additives that speed oxidation, merely disintegrates; a buried carrier bag was claimed to fall apart in three to six years instead of hundreds. Novamont's authors argue that such materials fail biodegradation standards, add to marine debris by fragmenting, and can impair recycling of ordinary plastics (Kabasci 2014, section 2.3).

• Rosenboom et al. (2022), Bioplastics for a circular economy · pdf pp. 3, 12, 16 (journal pp. 119, 128, 132)
• Kabasci (2014), Ch 2 Starch; Ch 8 Poly(Lactic Acid); Ch 14 · sections 2.1, 2.3, 8.10, 14.5

4.4 Life-cycle studies: lower carbon, heavier farm footprint

Narayan separates two footprints (Kabasci 2014, sections 14.2-14.3). The material footprint is the carbon in the polymer itself. Polyethylene is 85.7% carbon, so burning or rotting 100 kg releases 314 kg of CO₂; if that carbon came from this season's crop, the next crop reabsorbs it and the net is zero. The process footprint is the energy and emissions of making the plastic. On his figures, PE's process footprint is about 200 kg of CO₂ per 100 kg and PLA's about 385 kg, so PLA is worse on process but better in total once PE's 314 kg of fossil material carbon is added (he gives 519 kg for PE; 200 plus 314 makes 514).

Cradle-to-gate studies, from raw material to factory gate, show wide ranges (table). Bio-PE scores 0.68 t CO₂ equivalent per tonne against 1.8-2.6 for HDPE. PLA spans 0.5-2.9 against 2.4-5 for PET, and P₃HB anything from -2.3 to 4, a spread showing how much assumptions matter (Rosenboom et al. 2022). The Nebraska PLA plant (corn, natural gas, coal power) emitted about 1.2 kg CO₂ per kg of PLA; the same process burning lignin in a lignocellulosic biorefinery would be a net sink of 0.3 kg, or 1.7 kg with green electricity (Kamm 2006, Vol 2 Ch 14).

The burden shifts to the farm. Acidification is 30 kg SO₂ equivalent per tonne for bio-PE against 6-22 for HDPE, and PLA can score worse than oil-based polymers on eutrophication, ecotoxicity and ozone depletion, largely from fertiliser use (Rosenboom et al. 2022; Kabasci 2014, section 8.14). Bioplastics often score badly in landfill scenarios and better once recycling is counted.

Land is not yet a constraint. Bio-based plastics used about 0.02% of farmland, and their land use was put at about 250 times smaller than that of biofuels, with an effect on food prices below 0.1% (Kabasci 2014, sections 13.3 and 13.7).

• Kabasci (2014), Ch 8; Ch 13; Ch 14 · sections 8.14, 13.3, 13.7, 14.2, 14.3
• Rosenboom et al. (2022), Bioplastics for a circular economy · pdf pp. 4, 8, 10 (journal pp. 120, 124, 126)
• Kamm (2006), Vol 2 Ch 14 Polylactic Acid from Renewable Resources · pdf p. 855 (book p. 402)

See it in the table

Check your understanding

A composting plant turns its material round in 6-8 weeks. Which certified-compostable item is most likely to come out undigested?

Bio-PE has a lower cradle-to-gate global warming potential than HDPE but a higher acidification potential. Why?

Which route returns PLA to monomer?