Why Does Rubber Become Hard and Brittle?

A new rubber band stretches easily and snaps back. One that has spent a few summers in a drawer or on a sunny windowsill may feel stiff, crack when you pull it, or even crumble. Wiper blades harden, door seals stop sealing, old hoses develop a web of fine cracks and the rubber feet on a gadget turn rock-hard. Why does a material famous for its stretchiness lose it?

The question matters because rubber does important, often invisible jobs: sealing fuel and water lines, cushioning machinery, keeping weather out of buildings and cars. Understanding what attacks it explains why some rubbers last decades while others fail within a few years, and why the same seal can work on a warm day and fail on a cold one.

The basic principle: rubber is a loosely tied network

Natural and synthetic rubbers are elastomers, polymers whose molecules are very long, flexible chains. In a raw state the chains would simply flow past each other, so manufacturers vulcanise rubber, a curing process that ties neighbouring chains together at occasional points called cross-links. Vulcanisation reduces stickiness and increases strength while leaving long, free sections of chain between the links.

Those free sections are the secret of elasticity. When you stretch rubber the coiled chains straighten out; when you let go, their random thermal jiggling pulls them back into a coiled, disordered state. For this to work, two conditions must hold. The chains must be able to move, and there must be the right number of cross-links: enough to stop the material flowing, but not so many that the chains cannot uncoil. Ageing and cold upset exactly these two conditions.

How it works, step by step

1. An attack begins at the surface. Heat, ultraviolet light or traces of certain metals help oxygen molecules react with the rubber, creating highly reactive fragments called free radicals.
2. A chain reaction spreads. Free radicals react with oxygen and with neighbouring chains, producing more radicals in a self-sustaining cycle.
3. Chains are either cut or joined. Some reactions break chains (chain scission); others form new cross-links between them. Which one dominates depends largely on the type of rubber.
4. Extra cross-links tighten the network. With less free chain between the links, the rubber becomes stiffer, harder and less able to stretch before breaking.
5. Stress turns weakness into cracks. Where the rubber is stretched or bent, ozone and oxidation open up surface cracks, which usually run at right angles to the direction of strain.
6. The part fails. A hardened seal no longer springs back against the surface it is meant to seal, and a cracked hose or band can split under load.

The science behind it

Oxidation: cross-linking versus chain scission

Thermo-oxidative ageing, the combined effect of oxygen and heat, is best understood as two competing processes: old bonds breaking and new bonds forming. When new cross-links win, the aged elastomer ends up stiffer and more brittle; when chain scission wins, it softens. Published guidance from rubber manufacturers notes that rubbers based on butadiene, such as nitrile (NBR) and styrene–butadiene types, tend to harden as new cross-links form, while natural rubber tends to suffer chain scission and soften. In EPDM, a common rubber for car door seals and roofing, both happen but cross-linking usually dominates, so hardening is more likely.

Heat speeds all of this up. Like most chemical reactions, ageing reactions are often described with an Arrhenius-type relationship, in which the reaction rate k rises steeply with absolute temperature T:

k = A · exp(−Ea / RT)

where Ea is an activation energy, R the gas constant and A a constant for the reaction. The practical message is qualitative: a rubber part near a hot engine or in direct summer sun ages much faster than the same part kept cool. Traces of copper, manganese and iron also catalyse oxidation strongly, which is why compounders try to keep them out of rubber.

To slow these reactions, rubber is compounded with antioxidants. Amine types, particularly para-phenylenediamines, are effective at around 1–2 per cent of the mix; hindered phenols are weaker but stain light-coloured products less.

Ozone cracking

Ozone is a reactive form of oxygen present at low levels in outdoor air and produced by electric motors, high-voltage equipment and ultraviolet lamps. It reacts readily with the carbon–carbon double bonds found in the backbone of rubbers such as natural rubber, cutting the chains at the surface. If the rubber is stretched by more than about 10 per cent, the result is a pattern of small, deep cracks. One reference work notes that cracks a millimetre long can appear in unprotected rubber after only a few weeks at a typical outdoor ozone level of about 5 parts per 100 million.

Protection comes from antiozonants, which migrate to the surface and react with ozone before it reaches the rubber, and from microcrystalline waxes, which bloom to the surface to form a thin protective skin. Rubbers whose main chains contain no double bonds, such as EPDM, silicone and fluoroelastomers, are inherently far more resistant to ozone and oxygen.

Sunlight and ultraviolet

Ultraviolet light carries enough energy to break chemical bonds near the surface, starting the same free-radical oxidation described above. The result is a thin, oxidised surface layer that can craze and wear away, a process similar to the one described in our article on why plastic products turn yellow. Carbon black, the filler that makes most tyres and hoses black, has traditionally been credited with absorbing ultraviolet and visible light and turning it into harmless heat; research on reinforcing fillers suggests the way the filler binds to the rubber surface also plays a part.

Cold and the glass transition

Cold hardening is a different phenomenon. Every polymer has a glass transition temperature (T_g): above it the chains can change shape and the material is rubbery; below it they are effectively frozen and the material is glassy and brittle. Laboratory measurements by differential scanning calorimetry illustrate how low this can be for common rubbers:

Stiffening starts well above T_g. In the tests carried out for the inquiry into the 1986 Space Shuttle Challenger accident, the O-ring material’s transition region extended up to about 10 °C (50 °F), and its ability to spring back after being squeezed fell dramatically in the cold. The figures below come from one of those tests, in which O-rings were held compressed for 30 minutes and then released (temperatures converted approximately from Fahrenheit):

The investigators concluded that temperature was the dominant factor controlling how quickly the O-ring could recover. Unlike oxidation, this kind of stiffening is physical rather than chemical, so the rubber normally regains its flexibility when warmed. Low-temperature plasticisers, which act as internal lubricants between the chains, are added to some rubbers to keep them flexible in the cold.

Real-world applications

Choosing the right rubber. Engineers match the polymer to the environment: ozone- and weather-resistant types such as EPDM for outdoor seals, oil-resistant nitrile for fuel and hydraulic systems, and grades with suitably low glass transition temperatures for cold climates.

Accelerated ageing tests. Because real ageing takes years, standards bodies define laboratory tests to speed it up. In the ASTM D573 air-oven test, for example, rubber is held in hot air for a set time and properties such as hardness are measured before and after, so the change can be reported.

Storage of spare parts. Seal suppliers publish storage guidelines based on standards such as ISO 2230 and DIN 7716. One supplier recommends keeping elastomer parts below 25 °C, at relative humidity preferably below 65 per cent, away from sunlight and ozone sources such as electric motors, and free from stretching or folding. With those conditions it gives indicative minimum storage periods by rubber family:

Museum conservation. Conservators keep historic rubber objects cool, dark and dry, and sometimes in oxygen-free packaging, precisely to slow oxidation.

Common misconceptions

Old rubber hardens because it dries out

Loss of oils or plasticisers can contribute in some compounds, but the main cause of permanent hardening is chemical: oxidation adds cross-links and reshapes the network. That is why rubber sealed away from moisture can still harden if oxygen and heat reach it.

All old rubber goes hard

Not necessarily. Depending on its chemistry, degraded rubber may become hard and cracked or soft, spongy and sticky. Natural rubber in particular often softens as its chains are cut.

Rubber that is stiff in the cold has been ruined

Cold stiffening near the glass transition is largely reversible, and the part usually recovers when warmed. The danger is that a cold-stiffened part cannot do its job while cold, and it may crack if forced to flex sharply in that state.

Limitations and exceptions

The picture here is simplified. Commercial rubbers are complex compounds containing fillers, oils, curing agents and protective chemicals, and two products made from the same base polymer can age very differently. Many products also age unevenly, because sunlight and ozone act mainly on the exposed surface. Accelerated tests have their own pitfalls: at high temperatures, oxygen can be used up faster than it diffuses into a thick part, which complicates predictions of real service life.

Chemical exposure is a separate failure route: fuels, oils and solvents can attack rubbers that are not designed for them, plasticisers can migrate out of some compounds, and the right choice of polymer depends on the specific fluid. Nitrile, for instance, resists petroleum oils and fuels well but is damaged by even small amounts of ozone. Where rubber parts perform a safety role, such as brake components, gas and fuel hoses, or seals in pressure systems, inspection and replacement should follow the manufacturer’s instructions, and a qualified professional should be consulted if a part looks cracked, hardened or swollen.

Header photo: Mark Stuckey on Unsplash.