Biodegradable plastics like PLA (polylactic acid) break down through hydrolysis: water molecules attack the ester bonds linking monomer units, cutting the long polymer chain into shorter fragments that microbes then metabolize into CO₂, water and biomass. This only happens fast enough to matter above the polymer's glass transition temperature (~55–60 °C for PLA) — exactly what industrial composting provides and home heaps or landfills usually don't.
Bond scission is modelled as pseudo-first-order kinetics driven by a simplified Arrhenius temperature term, scaled by moisture and microbial activity:
dX/dt = k(T) · (1 − X)
k(T) = k0 · exp(−Ea/R / T) · (M/100) · (B/100)
X = fraction of ester bonds hydrolyzed (0→1)
T = temperature, Kelvin
M = moisture level (%), B = microbial activity (%)
Ea/R = 8000 K (reduced activation temperature)
- Temperature slider — sets T in the Arrhenius term; small changes near 55–60 °C swing k by orders of magnitude.
- Moisture slider — water is a hydrolysis reactant; dry conditions stall degradation.
- Microbial activity slider — sets the population consuming exposed chain fragments into CO₂.
- Industrial / Home / Landfill — realistic condition presets (58 °C/60 %/80 % vs 25 °C/40 %/30 % vs 15 °C/10 %/5 %).
- Reset chain — regrows an intact polymer chain to run the experiment again.
This is why "compostable" plastic left in a landfill or backyard bin can persist for years — it needs the heat, moisture and microbial density of a certified industrial facility to actually mineralize.