Straight out of the tree, natural rubber is not much use. The Polymer Science Learning Center puts it plainly: the latex is polyisoprene, it gets runny and sticky when warm, and it gets hard and brittle when cold. A tire made from it would only work somewhere that stays at seventy degrees all year.
In 1839 Charles Goodyear, by then a failed inventor, laid a piece of fabric coated with latex, sulfur and a little white lead on a hot stove. It sizzled. When he looked, the rubber no longer melted in heat or cracked in cold. Sulfur had built bridges between the polymer chains, and a vulcanized rubber object is, chemically, one giant molecule.
That is the first thing to know about the word “rubber.” It names a behavior, stretch and snap-back, and more than one kind of chemistry can deliver it.
Two origins
Natural rubber is latex tapped from Hevea trees. A Mongabay report from a Thai plantation describes the work: a couple with 500 trees on 1.6 hectares, cutting a line in the bark of each tree from midnight to 3:30 a.m. so the latex will flow. They are typical. Roughly 1.7 million smallholders produce 90% of Thailand’s natural rubber, and Thailand is the world’s leading producer.
Synthetic rubber is the other branch: SBR, EPDM, neoprene, and the thermoplastic elastomers sold as TPE and TPU. These are made in a plant from petrochemical building blocks, and our standard counts every one of them as plastic.
The two are not minor rivals. The International Rubber Study Group forecasts 15.73 million tonnes of natural rubber demand in 2025 against 16.77 million tonnes of synthetic. Nearly half of all rubber is still tapped from a tree. The same report calls natural rubber supply “structurally tight,” with a projected shortfall of 488,000 tonnes this year.
Where we draw the line
We don’t count natural rubber as plastic, and we do count synthetic rubber. The reason is origin, not molecular shape. Polyisoprene is a polymer either way, so shape isn’t the test. The test is whether a reactor built the chain or a tree did. How we draw this and the other lines is on our standard page.
It is a judgment call, and I’d rather say so than pretend the chemistry settles it. The tree has costs. The Mongabay piece notes that the EU’s deforestation regulation, due to take effect in January 2027, will require suppliers to prove their land wasn’t cleared after December 31, 2020. Rubber is one of the commodities it covers, and the people carrying the paperwork are mostly farmers with a few hectares.
There is a health cost too. The sap contains a protein that can trigger allergic reactions. When the FDA cleared the first guayule latex glove in 2008, it noted that anywhere from 3 percent to 22 percent of health care workers are sensitized to traditional latex. That is a good reason a glove may not be natural rubber. A bottle seal that touches only a lid and a bottle neck is a different exposure, though I’d still want a maker to say what the seal is.
Reading a label
Here is the practical problem. “Rubber” on a product page is almost never specific enough. A grip, a seal, a sole, a band: any of them might be natural rubber or a synthetic, and the one word covers both. If a maker writes only “rubber,” we treat that as an unanswered question.
The good answer is specific. The soulbottles help page for its swing-top seal says the rubber is latex from Sri Lanka, processed in Germany without petrochemical-based chemicals or plasticizers. In the maker’s German: “Naturgummi und Naturkreide, das ist alles,” natural rubber and natural chalk, that’s all. Since June 2018 the seals have used Fair Rubber certified material.
That is a seal I can reason about. It is why the soulbottles glass bottle is on our list as Bronze, and why its closure is steel wire, porcelain and natural rubber, with no silicone in it. We still don’t know the glaze on the little logo or what the shipping box is made of, and we’ve asked.
Ask any maker two questions. Which rubber, and who says so? Natural or synthetic, the answer is a name and a source. “Rubber” alone is a shrug.