What Parafilm M is for, and what it will not do
Almost everyone who has worked in a laboratory has used it, and almost nobody has read its limits. They are short, and they decide whether it is the right closure for what you are about to do.
What it is
A blend of polyolefins and paraffin waxes, with no plasticisers in it. Odourless, colourless, semi-transparent and chemically inert. It carries no adhesive and needs no heat: pulled to over twice its length and pressed down, it grips itself as it recovers. The seal is the stretch.
That is also why it seals to itself rather than to glass. Wrap it past the rim and down the neck, so that the film has its own surface to close against.
The five limits
Heat accounts for three of them. The other two spoil an experiment rather than a seal.
- Working range
- −45 °C to +50 °C
- Cold is not the problem. Warmth is.
- Melting point
- 60 °C
- A warm room is fine; a drying oven is not.
- Autoclave
- No
- A cycle runs at 121 °C, twice the melting point. It will not survive it.
- Gases
- Permeable
- Oxygen, carbon dioxide and water vapour pass through it.
- Polar solvents
- About 48 hours
- It resists them for a working day or two, not indefinitely.
The one that catches people out
Moisture proof and airtight are not the same claim, and the film is only the first. It holds a liquid in and slows evaporation to almost nothing, which is what it is for. It does not hold an atmosphere: oxygen, carbon dioxide and water vapour cross it. Anything that depends on the headspace — anaerobic culture, a controlled gas, a long incubation where slow exchange matters — wants a stopper, not a film.
Which width
Width is the only real decision. Four inches closes a flask neck or a beaker in a single wrap and is what most benches keep. Two inches is for tubes, plates and small joints, and a roll of it goes about twice as far because you are not throwing half of each piece away.
Length follows from that rather than the other way round. See what we stock →
Questions we are asked
- Can you autoclave Parafilm M?
- No. Parafilm M melts at 60 °C and an autoclave cycle runs at 121 °C. Seal vessels after sterilisation, not before.
- What temperature can Parafilm M take?
- The manufacturer's stated working range is −45 °C to +50 °C. It stays usable in a freezer; it softens and then melts with heat, at 60 °C.
- Is Parafilm M airtight?
- No. It is a barrier against evaporation and volume loss, but it is permeable to oxygen, carbon dioxide and water vapour. For a controlled headspace or anaerobic work it is the wrong closure.
- How far does Parafilm M stretch?
- Over 200% of its original length. The stretch is what makes the seal: it grips itself as it recovers, with no adhesive and no heat.
The figures here are the manufacturer's and its distributors', restated rather than improved — Amcor, who make it. We are an authorized reseller: what we can tell you about the film is what they publish about it, and we would rather say so than round it off.
Buying by the case for a school or a laboratory? Ask for a quote — priced per roll and per case, with a W-9 the same day.