# Compostable tea bags and coffee capsules

Why heat-sealable filter paper contains polypropylene, what brewing above PLA's glass transition means, the barrier-plus-pressure-plus-degradation conflict in a capsule, and why compostable capsules are screened out of composting.

A paper tea bag is usually about a fifth plastic, because paper cannot be heat-sealed. And an espresso capsule has to hold nineteen bar, exclude oxygen for a year, then fall apart on command.

Source: https://en.bioecon.ru/docs/biochem-industrial/polymers-materials/biodegradable-tea-bags-coffee-capsules/
Updated: 2026-08-25



Both products are small, single-use, and hold hot water — but their engineering problems are different, and each is instructive about a different failure of compostability claims.

## The plastic in a paper tea bag

A conventional tea bag is made of filter paper that must be **heat-sealed** closed at high speed. Cellulose fibres do not soften and fuse on heating; they char. So heat-sealable filter paper contains a thermoplastic — typically **polypropylene fibres at roughly 20–25 per cent** — which melts at the seal and bonds the layers.

That is why a bag which looks and feels like paper is neither compostable nor plastic-free. The polypropylene remains after the cellulose has degraded, as fine fibres in the compost.

**Pyramid and mesh bags** are usually more plastic, not less. Nylon and PET meshes are entirely plastic; PLA meshes are marketed as the compostable answer. The point worth knowing about a PLA mesh is thermal: **PLA's glass transition is roughly 55–60 °C, and brewing water is 90–100 °C.** The mesh is therefore well above its glass transition in the cup — softened and rubbery rather than glassy — which is a different physical state from the one it was tested in at room temperature.

The genuinely plastic-free routes remove the requirement instead of substituting for it: unbleached **abaca or manila hemp** paper closed by **folding, stitching or a stapled string** rather than by a heat seal.

## A capsule has four requirements that fight each other

An espresso capsule must simultaneously:

**Exclude oxygen for months.** Roasted coffee stales by oxidation of its lipids and loss of volatile aromatics; an unprotected roast degrades noticeably in weeks. This demands a barrier close to absolute, which is why **aluminium** is the incumbent — a metal foil is effectively impermeable, and no polymer is.

**Withstand brewing pressure.** Espresso extraction runs at roughly 9 bar at the puck, and machines generate higher upstream. The capsule is a small pressure vessel that is also being pierced.

**Survive a hot, wet, acidic extraction** without contributing flavour.

**Then degrade.** Which is a requirement to be structurally weak, in a body that has just been required to be strong and impermeable.

Compostable capsules resolve this with multilayer construction — a PLA or PBS body with an added barrier layer — and generally accept **shorter shelf life** than aluminium. That trade is the honest description of the product, and it is usually stated in the shelf-life figure rather than in the marketing.

## The operational failure is separate from the material one

A capsule can be genuinely certified to EN 13432 and still not be composted. Industrial composting facilities run **mechanical pre-screening** to remove contamination, and a compostable capsule is not visually or mechanically distinguishable from a conventional one on a sorting line. Many are removed and sent to incineration or landfill regardless of what they are made of.

This matters because it is a different kind of claim from the material one. **The material claim is about chemistry and can be certified; the outcome claim depends on collection systems, facility policy and residence time**, none of which the manufacturer controls. A facility running a 4–6 week cycle may also simply be faster than the 12-week disintegration window the standard permits.