The Chemistry of Butyl Devulcanisation: What Happens Inside the Autoclave

Butyl reclaimed rubber sells at a premium over whole-tyre reclaim because it retains a specific and commercially valuable property — low gas permeability. Understanding why that property survives the reclamation process requires a brief look at the chemistry happening inside the autoclave.

Butyl rubber: the base chemistry

Isobutylene-isoprene rubber (IIR), commercially known as butyl rubber, is a copolymer of isobutylene (97–98%) and isoprene (2–3%). The near-saturated backbone — almost no double bonds along the main polymer chain — is what gives it low gas permeability. Gas molecules have few sites to diffuse through, unlike natural rubber which has a double bond in every repeat unit.

When butyl is vulcanised (in the original tube manufacturing process), sulphur or resin cure systems create cross-links at the isoprene sites. These cross-links give the tube its dimensional stability and elasticity. They are also what must be partially broken during reclamation to make the material reprocessable.

What devulcanisation actually does

Devulcanisation does not fully reverse vulcanisation. That is a common misconception. What the autoclave process does is selectively break the polysulphidic cross-links (C–S–S–C bonds) while leaving the main polymer backbone substantially intact. The goal is to create a material that is soft enough to mill and compound, but has not been degraded so severely that its mechanical properties are gone.

In the autoclave, the shredded butyl feedstock is treated with a combination of steam, elevated temperature (typically 180–220°C), pressure, and a reclaiming agent. For butyl, the reclaiming agent is typically a combination of aromatic process oil and a peptising chemical. The mechanism is thermomechanical: the heat and steam swell the rubber, and the chemical agent weakens the polysulphidic bonds, allowing the pressure and mechanical shear to break them selectively.

Why the isobutylene backbone survives

The key to butyl reclaim’s commercial value is that the isobutylene-rich backbone — the structure responsible for gas impermeability — is chemically resistant to the devulcanisation conditions. The C–C bonds in the backbone are significantly stronger than the polysulphidic cross-links. A correctly controlled autoclave process breaks the cross-links at temperatures that do not yet attack the backbone.

This means the reclaimed material retains a high fraction of the isobutylene repeat units, and therefore a high fraction of the original gas-barrier performance. The cross-link density is lower (which is why reclaim is softer and easier to process than virgin butyl), but the polymer segments between cross-links — the segments that block gas diffusion — are still there.

Where things go wrong: blending and overprocessing

There are two ways a butyl reclaim processor can destroy the gas-barrier value of the product. The first is blending: mixing whole-tyre or natural-rubber feedstock into the autoclave charge. NR and SBR from tyre rubber are not isobutylene-based and do not contribute to gas impermeability. Every percent of non-butyl feedstock dilutes the isobutylene content of the output, linearly reducing its barrier performance.

The second is overprocessing: running the autoclave too hot, too long, or with too much chemical agent. At elevated conditions the main-chain backbone begins to degrade — chain scission produces shorter polymer segments and reduces both mechanical properties and gas barrier performance. Overprocessed butyl reclaim has a characteristic low Mooney viscosity and poor tensile properties.

Both problems are detectable by incoming QC at the buyer’s compounding facility. Specific gravity will be off if there is non-butyl blending (butyl has higher specific gravity than NR/SBR). Mooney viscosity ML(1+4) 100°C below 40 typically indicates overprocessing. Air permeability testing is the definitive check if your compound specification requires it.

Implications for buyers

The chemistry is why single-source feedstock discipline matters commercially. A supplier who guarantees butyl-only input into the autoclave is guaranteeing something about the output chemistry — something you cannot verify visually, only by testing. It is also why sample-and-approve protocols exist: you cannot tell from looking at a bale whether the gas-barrier performance is intact. A one-time test of a sample batch, followed by consistent feedstock control from the supplier, is the practical quality management approach most buyers use.