Bioplastics: PLA, PHA, bio-PE and starch blends: What 'bioplastic' actually means
Understanding Chemicals / Module 14: Bioplastics: PLA, PHA, bio-PE and starch blends Part 1 of 5What 'bioplastic' actually means
The word bioplastic hides two separate questions. Where did the carbon come from: a crop or an oil well? And what happens at the end: does a microbe eat the material, or does it last like ordinary plastic? Getting these straight is the first step in judging any pitch in this field, because the economics and the environmental case differ completely from one answer to the other.
1.1 Origin and end of life are two separate questions
European standards define bio-based plastics simply as plastics derived from biomass, meaning plant, animal or microbial material that has not fossilised. That is a statement about the beginning of a plastic's life. Biodegradability is a statement about the end: micro-organisms in a named environment, such as an industrial composting plant or an anaerobic digester, break the polymer all the way down to carbon dioxide or methane, water and mineral salts. Standards such as EN 13432, ASTM D6400 and ISO 17088 fix the test conditions and pass mark (Kabasci 2014, section 1.1).
The two properties are independent. Whether a polymer biodegrades depends on its molecular structure, not on where its carbon came from. Polyethylene made from sugarcane is bio-based but resists biodegradation exactly as fossil polyethylene does. Conversely, PBAT and polycaprolactone are made from oil yet pass composting standards. Rosenboom et al. (2022) advise against calling these bioplastics at all.
Compostable is narrower than biodegradable. Composting is biodegradation under managed conditions that yields carbon dioxide, water, heat and humus. An item certified as industrially compostable has passed four tests: at least 90% of its carbon turned into CO2 within six months at 58 C, 90% broken into pieces smaller than 2 mm within three months, no harm to plant growth, and heavy metals below a limit (Rosenboom et al. 2022, Box 2).
Put the two questions on two axes and you get four boxes (figure). The investment logic differs by box. Bio-based durable plastics sell a smaller fossil carbon footprint and plug into existing recycling. Biodegradable ones sell a different end of life, which pays off only if the waste actually reaches a composter or digester.
| Biodegradable (in a named environment) | Durable (not biodegradable) | |
|---|---|---|
| Bio-based carbon | PLA, PHA, thermoplastic starch, bio-PBS, cellulose | Bio-PE, bio-PP, bio-PET, PEF, bio-based polyamides and polyurethanes |
| Fossil carbon | PBAT, PBS, PCL, PVA | PE, PP, PET, PS, PVC |
Starch blends often straddle the left column: the starch is bio-based, but the polyester partner (such as PBAT) is usually fossil-based.
Source: Rosenboom et al. (2022), Box 1 and Table 1; Kabasci (2014), Ch 1, pdf pp. 2, 8 (journal pp. 118, 124); section 1.1- Kabasci (2014), Ch 1 Bio-Based Plastics: Introduction · section 1.1
- Rosenboom et al. (2022), Bioplastics for a circular economy · pdf pp. 1-2, 13 (journal pp. 117-118, 129), Box 1, Box 2
1.2 Drop-in copies and new polymers are different businesses
A second distinction matters even more for cost. A drop-in bioplastic is chemically identical to an existing fossil polymer. Bio-PE is ordinary polyethylene whose ethylene came from sugarcane ethanol instead of a cracker; bio-PET is ordinary PET in which the glycol part came from bioethanol. Because the molecules are the same, they run on the same converting machines, meet the same specifications and go into the same recycling streams (Rosenboom et al. 2022).
A new polymer has no fossil twin. PLA, PHA and thermoplastic starch have their own chemistry, their own processing windows and their own end-of-life routes. That brings properties polyethylene lacks, such as compostability, but converters must learn new settings and sorting plants meet an unfamiliar material.
New bio-based polymers arrive by two routes (Kabasci 2014, section 1.1). Some are natural polymers taken more or less intact from biomass: starch, cellulose, and PHA, which bacteria build inside their own cells. Others are assembled by chemists from a bio-based monomer: PLA is polymerised from lactic acid made by fermentation.
Bio-based content need not be 100%. In PTT, a polyester, the 1,3-propanediol made from corn starch is fully bio-based but the terephthalic acid is fossil; counting carbon atoms gives 3 of 11, or 27% bio-based. In bio-PET, two of ten carbon atoms come from plants: 20% by carbon but 31.25% by mass (Kabasci 2014, sections 1.1 and 14.2). The share is measured, not assumed: plant carbon carries a trace of radioactive carbon-14 from the air while fossil carbon has almost none, and ASTM D6866 measures it (Kabasci 2014, section 14.4).
- Bio-based plastics plastics derived from biomass (CEN definition)
- Natural polymers, extracted and modified physical or chemical processing of a biopolymer
- Starch: thermoplastic starch and blends melted with water and plasticisers
- PHA made and stored inside bacterial cells
- Cellulose and cellulose acetate
- Polymerised from bio-based monomers chemistry applied to fermentation products
- PLA from fermented lactic acid; new polymer
- Bio-PE and bio-PP from bioethanol; drop-in
- Bio-PET and PTT partly bio-based; drop-in or new
- Natural polymers, extracted and modified physical or chemical processing of a biopolymer
- Kabasci (2014), Ch 1 Bio-Based Plastics: Introduction; Ch 14 The Promise of Bioplastics · sections 1.1, 14.2, 14.4
- Rosenboom et al. (2022), Bioplastics for a circular economy · pdf pp. 6, 9 (journal pp. 122, 125)
1.3 A few polymer basics decide what each material can do
Four ideas from polymer science explain most of what follows. First, how chains grow. In chain-growth polymerisation, used for polyethylene, an initiator or catalyst starts a chain and monomer units add one at a time, building a full chain in a fraction of a second. In step-growth polymerisation, used for polyesters such as PET, the same esterification reaction is repeated many times and the by-product water must be driven off to push the equilibrium towards long chains (Wittcoff 2004, section 15.3). PLA can be made either way, and the choice decides its cost.
Second, crystallinity. Chains that are regular and unbranched can pack side by side like pencils in a box. Crystalline regions make a plastic stiffer, stronger and more resistant to heat and solvents; irregular chains stay amorphous (Wittcoff 2004, section 15.5.1). Branching is why high-density polyethylene, 50-90% crystalline, is rigid while low-density polyethylene, 30-40% crystalline, is flexible (Kent 2012, Ch 15).
Third, two temperatures. Below the glass transition temperature (Tg) the amorphous parts are glassy and the plastic tends to be brittle; above it they are rubbery and tough. The crystalline melting point (Tm) is where the ordered regions melt and the material flows (Wittcoff 2004, section 15.5.2). An amorphous cup softens near its Tg.
Fourth, the backbone. Polyethylene's chain is carbon joined to carbon, with no handle for water or enzymes to grip. PLA, PHA and the other aliphatic polyesters have ester links in the chain, which water can cut. That single structural fact separates the compostable materials from the durable ones (Rosenboom et al. 2022).
- Wittcoff (2004), Ch 15 How Polymers Are Made · pdf pp. 495-496, 538-543 (book pp. 469-470, 512-517)
- Kent (2012), Ch 15 Synthetic Resins and Plastics · pdf pp. 596-600
- Rosenboom et al. (2022), Bioplastics for a circular economy · pdf p. 6 (journal p. 122)
See it in the table
H₂O WaterCO2 Carbon dioxideC2H4 EthyleneHDPE High-density polyethyleneEtOH BioethanolCheck your understanding
Polyethylene made from sugarcane ethanol is best described as:
Its carbon comes from plants, so it is bio-based, but it is chemically ordinary polyethylene and resists biodegradation exactly as fossil PE does.Bio-PET in which only the ethylene glycol is plant-derived has what bio-based carbon content?
Two of the ten carbon atoms in each repeat unit come from the glycol, so 20% by carbon. 31.25% is the glycol's share by mass; 27% is the carbon share of PTT.Why do PLA and PHA biodegrade while bio-PE does not?
Biodegradability follows molecular structure, not carbon origin. Ester links can be hydrolysed; polyethylene's carbon-carbon backbone offers no such weak point.