Making a sheet of paper compostable without giving up its barrier is a coating problem, and there are three routes in commercial use: water-based coating, PHA coating and PLA film. They are often presented as a progression — old, newer, newest — and that framing leads buyers into the wrong specification. They are three different trades on the same four axes, and which one is right depends on which property the pack cannot give up.
What the coating is actually doing
An uncoated sheet has no barrier worth the name: oil passes through in minutes and moisture follows. The coating exists to close that gap while keeping the sheet compostable. Everything that follows is about how each route closes the gap and what it gives up in return.
One distinction worth fixing early: this article is about what the coating is. What the coating must not contain — the fluorine question — is a separate subject, covered in our work on PFAS-free packaging. Both matter, and they are not the same decision.
The three routes
Water-based coatings are dispersions applied like a conventional coating and dried. They are the most widely converted of the three, the most mature on food contact, and the established fluorine-free route — the technology behind our water-based coated paper. Their limit is barrier ceiling: they handle ordinary grease contact and moderate moisture, and they fall away on heavy, long-contact fat loads.
PHA coatings are a microbially produced polyester applied as a dispersion. They give a better barrier than water-based systems — grease, water and a useful moisture-vapour barrier — and some grades are claimed to degrade in soil and marine environments as well as industrial composting. The cost is supply and cost position: fewer producers, fewer grades, and converting parameters that cannot be carried over from a polyethylene line.
PLA film is a lamination rather than a coating, applied as a film and heat-sealed or glued to the sheet. It is mature as a film, gives a strong grease and moisture barrier, and converts on familiar equipment. Its limit is the disposal side: PLA needs industrial composting conditions, and the laminate only composts if the film separates and degrades at the same rate as the fibre — which it does not do in a home heap.
The three routes compared
| Route | Barrier (grease / water / moisture vapour) | Composting condition | Food-contact maturity | Converting compatibility | Cost position |
|---|---|---|---|---|---|
| Water-based coating | Good grease; moderate water; limited MVTR | Industrial; some grades also certified home | Most mature of the three | Conventional coating and drying lines | Lowest of the three |
| PHA coating | Good grease; good water; useful MVTR | Industrial; soil and marine degradation also claimed for some grades | Narrower — check the specific grade | Coating lines, but settings are not transferable from PE | Highest; supply is the constraint |
| PLA film lamination | Strong grease; strong moisture; heat-sealable | Industrial only — not home compostable | Mature as a film | Familiar film and lamination equipment | Mid; adds a separate process step |
Choosing between them
The choice follows from what the pack cannot give up:
- If the barrier requirement is ordinary — dry goods, light grease, short contact — water-based coating is the default. It is the cheapest route, the most mature on food contact, and the easiest to convert.
- If the barrier requirement is real — a genuine moisture-vapour barrier, or a fat load the water-based route cannot hold — PHA is the route to price, with the supply question answered first.
- If the pack must be heat-sealed or needs film-level moisture resistance, PLA is the practical answer, on the condition that the disposal route is industrial composting and the customer accepts that.
Where a paper also has to work as a greaseproof sheet in its own right rather than as a coated base — for bakery liners and similar uses — the base sheet matters as much as the coating, and the sheet is specified on its own terms. Our food-grade greaseproof paper is the reference point for that family.
Common misuse
- Assuming a PLA laminate is home compostable. It is not, and the laminate does not separate in a garden heap.
- Carrying PE converting settings across. Water-based and PHA coatings need the line set for them, particularly on drying.
- Specifying "compostable coating" without a grade. Within one family, grades differ in barrier and in the condition they were tested for.
FAQ
Is PHA better than a water-based coating?
It is stronger on barrier and weaker on cost and availability, so it is a different trade rather than a better one. Choose it when the water-based route cannot hold the barrier the product needs.
Can PLA be used where home composting is the route?
No. PLA needs industrial composting conditions. Where the disposal route is a garden heap, PLA lamination is the wrong choice regardless of the paper underneath.
Do these coatings change the base paper?
They change what the sheet can do, not what it is. The base sheet still has to be chosen for strength, grammage and food-contact compliance, and a coated sheet is a new specification rather than a treated version of the old one.
Browse the range
These coatings are applied to a base sheet, and the paper family they most often sit on is covered under vegetable parchment paper. For the packs most coated paper ends up in, start at eco paper bags.