Unscrew the cap of a steel water bottle and run a fingernail around the inside. There’s usually a soft ring in there. The Klean Kanteen TKPro has two. Of the steel bottles we’ve looked at that name the ring, one, the soulbottles steel bottle, seals with natural rubber. The rest use silicone. Whether silicone counts as plastic decides whether many bottles, jars and lunch boxes sold as plastic-free are what they say.
The disagreement starts at the top. Pura, which sells a steel bottle with a silicone sport top, says on its care page that silicone “is a natural material; it does not contain any plastic or latex” (the quote and its source are on our page for that bottle). Wacker, a German chemical company that makes silicone, writes in its own brochure: “Silicones are a special group of plastics.” It qualifies that at once: “plastic” normally means organic materials that don’t occur in nature, and Wacker calls silicones semi-organic, since silicon is more metallic than carbon.
Sand, a furnace and a gas well
The sand story is true for the backbone. Silicone starts as quartz, smelted to silicon metal in submerged electric arc furnaces, with coal, petroleum coke, charcoal and wood chips supplying the carbon that strips the oxygen away. The EU’s reference document for the industry lists 10,800 to 12,000 kWh of furnace electricity per metric ton of silicon. Elkem, which makes silicon as well as silicone, puts the reaction at about 1,500 to 2,000 °C.
Then comes the step that makes it silicone. Silicon powder reacts with methyl chloride over a copper catalyst, a route Müller and Rochow found independently in 1940 and 1941, and the main product is dimethyldichlorosilane. The methyl chloride is made from methanol and hydrogen chloride, and 89% of all chloromethane goes into silicones. Methanol, in turn, is about 65% natural gas and 35% coal, with about 0.2% from renewable sources.
Water strips off the chlorine, and the pieces join into chains and small rings, one of them a four-silicon ring called D4 that comes back later in this story. The chains get longer, fumed silica goes in for strength, and the mix is cured into a rubbery network with either a platinum catalyst or a peroxide. A well-made food part then goes into an oven. Wacker’s processing guide calls for at least 4 hours at 200 °C to drive off volatiles, and for food contact it calls that step “recommended or even essential.”
Do the arithmetic on the repeating unit, (CH3)2SiO, and it comes out about 38% silicon and 22% oxygen by weight, which leaves roughly 40% as methyl groups. The silicon came from a quarry. The methyl groups almost certainly came from a gas well or a coal seam. The industry’s own life-cycle study puts silicone at about 6.3 kg of CO2 equivalent per kg, two thirds of it from making the silicon metal.
Plastic depends on who is sorting
The reason the authorities disagree is more useful than any one answer, because each of them sorts materials for a different job.
For a chemist the question is what the chain is made of. IUPAC, the body that names chemicals, files polysiloxanes under inorganic polymers, those “with a skeletal structure that does not include carbon atoms.”
Engineering standards care about behavior instead. ISO 472, the vocabulary standard for plastics, defines plastic as a material that can be “shaped by flow” and then adds a note: “Elastomeric materials, which are also shaped by flow, are not considered to be plastics.” The American standard, ASTM D883, leaves out rubber as well. Silicone rubber is an elastomer. So are natural rubber and EPDM, and that gap is why the EU’s chemicals agency refused to build its microplastics rule on the ISO wording: it “specifically excludes elastomeric materials,” which include tire wear.
Food-contact law asks what ends up in the food, and the EU’s main plastics rule for it says plainly that it “shall not apply to” ion exchange resins, rubber or silicones, which are left to measures of their own.
Pollution rules cast a wider net, because they care about what persists. The EU’s single-use plastics directive builds its definition on the chemicals law’s idea of a polymer, and says the definition should cover “polymer-based rubber items”, sparing only natural polymers that haven’t been chemically modified. The EU restriction on intentionally added microplastics exempts polymers “that do not contain carbon atoms in their chemical structure”. Every methyl group on a silicone chain carries a carbon, so silicone stays in.
Then there’s the shopper, who means whatever they meant when they typed “plastic-free.” In 2025 a maker told the UK’s advertising regulator that “plastic” and “polymer” are different things. The regulator ruled that consumers “would understand the claims ‘plastic-free’ and ‘zero microplastics’ as meaning the products about which the claims were made, and their packaging, did not contain plastic.” That case was about a dissolvable PVOH film. I found no ruling in the UK, EU, US, Australia, the Netherlands, France or Germany that applies it to silicone sold as plastic-free.
Even the plastic-free movement splits. Beth Terry, who writes the blog My Plastic-free Life, answered a reader’s question with “Yes, it’s technically plastic,” and then admitted she didn’t know whether it was safer than the carbon-based kind. Plastic Free July’s own site suggests silicone bowl toppers as an alternative to cling wrap.
What it gives off
Silicone has a reputation for being inert, and mostly it earns it. The exceptions are specific.
It leaves the factory carrying small siloxane molecules, including the rings D4, D5 and D6. The EU named all three substances of very high concern in 2018, for how long they last and build up in the environment, and found D4 toxic as well (Regulation 2024/1328). Since 6 June 2026 the same regulation bars them at 0.1% or more in substances and mixtures, but it exempts their use “in the production of articles,” which, as I read it, leaves a finished gasket outside the rule.
Those small molecules move into food, and fat pulls them out. In a German study of silicone baking molds, a meat loaf took up 177 mg of siloxanes per kg while milk-based foods stayed under 2.4. Molds that hadn’t been post-cured migrated considerably more. A 2025 study measured that difference directly: small siloxanes moving into shortcrust pastry fell from 153 mg/kg to 9.5 when the mold was post-cured. The larger rings migrated either way. Use helps. Molds in a pizza bakery went from 11 to 18 mg/kg down to below 1 mg/kg over about 1,700 bakes. Germany’s risk agency recommends a limit of 0.5% on volatile release, and when Bavaria’s food authority tested 60 silicone baking items in 2025, three were faulted for releasing too much.
Particles are the newer question, and the evidence is thin. A 2022 study put four brands of silicone baby bottle teats through 60 cycles of ten-minute steaming and found flakes from 0.6 to 332 micrometers in the rinse water. The authors estimated a baby could swallow more than 660,000 of them in a year. Their supplementary data put the total mass at under 0.1 mg per teat over those 60 cycles, and teats that were never steamed released none. A Swedish study of a silicone pacifier got a more mixed result: boiling it didn’t by itself release a significant number of nanoparticles, which appeared when the pacifier was blended, and in larger numbers when it had been boiled first. It measured particles of roughly 100 to 180 nanometers, far smaller than the teat study’s flakes. I found no independent replication of the teat result.
The study that matters most here is the one I couldn’t find: what a silicone gasket in a water bottle gives off into cold water over a year of daily use.
What it leaves behind
Silicone oils do break down. In soil, clay catalyzes them into dimethylsilanediol, which then biodegrades or evaporates and oxidizes in sunlight, with silica, carbon dioxide and water as the expected end products. Most of that research was done in silicone makers’ own labs, Dow Corning’s above all. A 2025 review by two McMaster University chemists, who both hold posts at a start-up that makes fillers for silicone rubber, puts complete conversion of simple silicone oils, the small rings included, at one to five years, depending on climate.
A cured gasket is a different animal, and I found no measured breakdown rate for silicone rubber anywhere. The same review says commercial silicone elastomers usually end up in the environment, and that the more crosslinked a polymer is, the slower it degrades. Silicone rubber already turns up in microplastic surveys: a Spanish study of 200 catsharks found it among the four most common polymers in their digestive tracts.
Burned, it becomes amorphous silica, carbon dioxide and water, according to a 2024 review of silicone recycling. Recycled, barely. That review’s estimate for 2024 is 35,000 to 45,000 metric tons chemically recycled worldwide in its abstract and 35,000 to 40,000 in its text, against roughly 3 million metric tons made. One of its two authors is employed by a silicone chemicals company, and I found no other count. The City of Calgary tells residents to put worn-out silicone in the garbage.
Where it earns its place
None of this cancels what silicone is good at. Parker’s O-ring handbook rates standard silicone from about -54 °C to 204 °C, with good resistance to ozone and weather. Natural rubber is rated to 158 °F, about 70 °C, in dry heat. It cracks where ozone finds a stretched surface, so compounders add waxes and antiozonants. Weck, which still seals its canning jars with natural rubber, tells owners to hand wash the rings, because dishwasher detergent causes dry rot. Latex allergy counts against it too: in 1997 NIOSH put sensitization at 1% to 6% of the general population, though the FDA notes that molded “dry” rubber carries lower levels of the allergenic proteins than dipped latex.
Silicone has its weak spots too. Parker describes “poor tear strength and little wear resistance,” lists silicone as incompatible with superheated steam above 121 °C, and recommends EPDM for hot water and steam. EPDM is a synthetic as well.
Plastic-free seals do best where the contents are cool and the washing is by hand. Weck’s rings are natural rubber that “doesn’t deform even at -40 degrees”. The soulbottles glass bottle on our list seals porcelain to glass with natural rubber and chalk. Heat narrows the field without closing it: Le Parfait’s canning washers are natural rubber, as is the gasket in Bialetti’s aluminum moka pot. Past that, silicone mostly takes over. That covers pressure cooker rings, which Instant Pot says to replace every 12 to 18 months, and the caps of insulated bottles, where I found no maker sealing with anything else. Baking is the exception, where the plastic-free answer is to need no seal at all: glass, steel or cast iron.
Reading a plastic-free label with silicone on it
Find the silicone and ask what job it does. A ring inside a steel cap is a small part doing a job natural rubber does badly in a dishwasher. A sport top you drink through, or a bag that holds warm leftovers, puts silicone against a mouth or hot food. The release studies above covered baking molds, baby teats and a pacifier. I found none on a sport top or a food bag.
If the contents stay cool and you wash by hand, natural rubber or no seal at all will do the job.
Ask the maker whether the silicone is platinum-cured and whether it meets the 0.5% limit on volatile release in Germany’s BfR Recommendation XV. A maker who knows will answer in a line.
Clean new silicone before it touches food, and keep it within the maker’s temperature limit. Bavarian researchers drew that advice from their measurements of silicone baking molds.