Bio-based barrier films

Flexible packaging exists to keep oxygen, water vapour and grease away from food. The reason it is almost always a laminate — and therefore almost always a recycling and composting problem — comes from a single conflict in polymer physics.

Permeability is two properties multiplied

A gas crosses a dense polymer film by solution–diffusion: it dissolves into the polymer at the high-concentration face, diffuses through as individual molecules hopping between transient gaps in the chain packing, and desorbs at the other face.

Permeability is therefore the product of solubility — how much gas the polymer will take up — and diffusivity — how fast it moves once in. A good barrier needs both to be low, and they are governed by different things. Diffusivity is reduced by dense chain packing, high crystallinity, chain stiffness and a glass transition above use temperature. Solubility is governed by chemical likeness between the gas and the polymer.

The polarity trade-off is the whole problem

Oxygen is small and non-polar. It dissolves poorly in polar polymers, so the best oxygen barriers are strongly polar: EVOH, PVOH, polyamide, and the biological polar polymers — starch, cellulose, chitosan and proteins. These have high cohesive energy density and extensive hydrogen bonding, which also packs the chains tightly and keeps diffusivity low.

Water is small and polar. It dissolves readily in exactly those polymers. Worse, absorbed water breaks the interchain hydrogen bonds that were providing the tight packing and acts as a plasticiser, lowering the glass transition and increasing chain mobility. An EVOH or starch film’s oxygen barrier can fall by an order of magnitude as relative humidity rises. The barrier is destroyed by the very thing it must be protected from.

Conversely, non-polar polyolefins — PE, PP — have excellent water-vapour barrier because water does not dissolve in a hydrocarbon, and poor oxygen barrier for the same reason applied to oxygen.

No single polymer is good at both, and this is a structural fact rather than a gap in the materials catalogue. Hence the laminate: a polyolefin or wax layer to keep water out, a polar layer to keep oxygen out, tie layers to bond them, and often a metallised or inorganic-coated layer for the highest demands.

Why laminates are the end-of-life problem

A multilayer structure is chemically several materials bonded inseparably. Mechanical recycling needs a single polymer stream; a laminate cannot be sorted into one. Composting is only slightly easier, because every layer must independently meet the standard — a compostable polyester with a thin non-degradable barrier coating fails as a whole.

The routes out are all constrained versions of the same trade-off. All-compostable laminates pair PLA or PBAT with a bio-polar layer, keeping the architecture but making each layer certifiable. Cellulose-based films — regenerated cellulose and its derivatives — offer good oxygen barrier and inherent degradability, and need a coating for moisture. Protein and polysaccharide coatings applied thinly to a compostable substrate add oxygen barrier without a separate structural layer. Wax and lipid layers supply the water barrier from renewable material.

Each is a real product. None escapes the underlying physics: something polar must stop the oxygen, and it must be kept dry by something non-polar.

Two things are worth checking on any claim in this category. Barrier data quoted without a relative humidity is not usable for a polar film, because that is precisely the variable it depends on. And compostability certifications name an environment — a film certified for industrial composting at 58 °C is making no claim about a garden heap.