Polymers & materials
Compostable disposable tableware
Why oleophobicity is a surface-energy problem that only fluorochemistry solved, what molded pulp actually is, the blocking strategies that replaced PFAS, and why grease-proofing and compostability constrain each other.
Molded fibre tableware — bagasse, bamboo, moulded wood pulp — is compostable as a substrate and always has been. The difficulty has never been the fibre; it is making it hold a hot, greasy meal.
Why grease is harder than water
A liquid spreads on a solid when the solid’s surface energy exceeds the liquid’s surface tension, and beads up when it does not. This gives a clean way to see why the two problems differ.
Water has a surface tension around 72 mN/m — very high, because of hydrogen bonding. Almost any hydrophobic surface sits below it, so making something water-repellent is easy: a wax, a silicone, a hydrocarbon coating all work.
Oils and fats have surface tensions around 25–35 mN/m — far lower. To repel oil, a surface must be below that, and the available chemistry is narrow. Hydrocarbon surfaces sit near 30 mN/m and fail. Silicones reach roughly 20–24. Only densely packed fluorocarbon surfaces — a CF₃-terminated layer is near 15 mN/m — go convincingly lower.
That is the whole reason PFAS became standard in greaseproof food packaging. It was not inertia or cost; fluorochemistry was doing something no other chemistry could do, and doing it in a very thin layer that did not change the paper.
What replaced it, and by what mechanism
Because the repelling strategy is essentially unavailable without fluorine, PFAS-free products block instead. The distinction is worth holding onto: a repellent surface makes oil bead up; a barrier lets oil sit there and stops it getting through.
Film lamination. A thin PLA, PBS or PHA layer on the fibre. Effective and simple, but it adds a polymer layer that must itself meet the compostability standard — and a PLA lining brings PLA’s constraints with it, including the industrial-composting condition and a service temperature limited by its glass transition.
Densification and refining. Beating the pulp more, and pressing harder, closes the pore structure so there is no easy path through. This works with no additive at all, at the cost of stiffness, weight and drainage rate in production.
Mineral and biopolymer coatings. Clay, nanocellulose, chitosan and starch coatings form a tortuous or continuous layer that oil cannot cross quickly. Nanocellulose is particularly effective because its fibrils pack into a dense film, and it is chemically the same material as the substrate.
Internal sizing with waxes or bio-based sizing agents raises water resistance and slows oil penetration without producing true oleophobicity.
Grease resistance is measured, not asserted — the Kit test (TAPPI T 559) rates resistance against a graded series of castor-oil/toluene/heptane mixtures of decreasing surface tension, and higher Kit numbers correspond to lower-surface-tension liquids being held out. A grease claim without a Kit rating or an equivalent hot-oil test is not comparable between products.
The regulatory picture is what forced the change
PFAS are persistent by design: the carbon–fluorine bond is the strongest single bond in organic chemistry, which is exactly why the surface works and exactly why the molecules do not degrade. Denmark banned PFAS in paper and board food contact materials in 2020, the EU has restricted them in food contact, and several US states have followed.
The property that made them useful and the property that got them banned are the same property. A weaker C–F bond would degrade and would also not deliver the surface energy.