Propylene Glycol Tyre Sealant

This once-radiant and now greying green gloop is a liquid puncture prevention system for bike tyres. This slime can be squirted into an inner tube (and sometimes even directly into a tubeless tyre) where it coats the inside of the tyre and oozes around in there until needed, instantly sealing any punctures as they occur. 

Liquid tyre sealants are usually either latex-based or glycol-based. Latex-based sealants have a much longer history and have been used in pneumatic tyres since at least the 1920s. They rely on the influx of air caused by a puncture to evaporate the water and/or ammonia matrix surrounding the dispersed latex molecules, causing them to come together, coagulate and cross-link (chemically change from a liquid to a solid rubber) at the site of the puncture. The downside of these older latex sealants is that as air is pumped in and out of the tyre during everyday use they begin to solidify, quickly losing their efficacy. These sealants are also rendered ineffective if they freeze, which is not uncommon in tyres that are exposed to the elements.

Glycol-based tyre sealants like this one, on the other hand, last a lot longer as they don’t rely on a chemical change to solidify them so they can stay liquid for longer, and they can function down to -37°C. Sealants like this one are made up of hundreds of tiny fibres and inorganic fillers that are suspended in a carrier of either propylene glycol or ethylene glycol and water. The glycol and water element acts as both a carrier and an anti-freeze, allowing the fibres and fillers to move around inside the tyre until a puncture occurs. When a hole forms in the rubber tyre, the pressure of the air escaping the tyre pushes the sealant into the cavity. The hundreds of tiny fibres in the sealant snag on the rubber and each other, intertwining and bonding to the rubber cavity. As the tyre rotates, the rubber flexes and more fibres are packed into the hole until no more will fit. The tiny particles of filler then pack in between the fibres and form an airtight seal.

The fibres can be synthetic (e.g. Kevlar or another aramid), ceramic, woollen or even cellulose, and the fillers are often something like finely ground marble, rubber, mica, bentonite clay or quartz. The more dense the fibres and more viscous the fluid, the better the sealant, but the harder it is to insert into the tyre in the first place. 

The photos of this sample were taken a while ago, but having spent the last 15 years inside a glass jar, this once-gelatinous and glowing gloop has since dehydrated into a jellied clot, with its fibres now very visible where the fluid has dried into a skin on the sides of the jar.