Halogenated Butyl Rubber Production Process: Chlorination and Bromination

What Halogenation Changes in Butyl Rubber

Halogenated butyl rubber (BIIR and CIIR, designated according to the nomenclature of ISO 1629) is produced by substituting a small fraction of the isoprene units in the isobutylene-isoprene backbone with bromine or chlorine. The base polymer is a butyl rubber with a typical unsaturation of about 1.0 - 2.2 mol %, and the halogenation reaction introduces allylic halogen at a level of roughly 2 % bromine for BIIR and about 1.2 % chlorine for CIIR. The modification does not change the saturated backbone that gives butyl rubber its very low gas permeability; it creates reactive cure sites. The result is faster vulcanisation, the ability to cure with zinc oxide alone, better adhesion to other rubbers and to metals, and improved compatibility in blends.

Because the halogen content controls cure rate, scorch safety and ageing, the whole manufacturing sequence is built around keeping the reaction uniform and the halogen distribution reproducible.

Solvent Preparation and Feed Conditioning

The process starts with a hydrocarbon solution. High-purity butyl rubber bales are cut and dissolved in an alkane solvent, normally hexane or cyclohexane, to a controlled solids concentration that keeps the viscosity low enough for pumping and heat transfer. Moisture and metal ions are the main poison risks for the downstream reaction: water deactivates the acidic species involved in halogenation and promotes unwanted hydrolysis, while traces of iron and other transition metals catalyse degradation. Solvent is therefore dried and distilled, and the solution is filtered before it enters the reactor. Dissolution temperature and holding time are controlled to avoid thermal degradation of the high molecular weight polymer.

Chlorination and Bromination Routes

In both routes the halogen feed rate and reaction time are the primary levers on substitution degree. Over-chlorination causes chain scission and cross-linking that raise gel content and destroy processability, so halogen content, solution colour and viscosity are monitored during the reaction rather than after it.

Neutralisation, Washing and Removal of Residual Halogen

The reactor discharge is acidic and still contains unreacted halogenating agent. It is neutralised with a dilute alkaline solution, for example sodium carbonate or sodium hydroxide, and then washed in several stages with water or dilute acid to strip soluble salts and residues. Washing efficiency is governed by the number of stages, the water-to-cement ratio, temperature and settling time; too few stages leave ash and corrosivity in the finished rubber, while excessive washing costs solvent and energy. Residual halogen and acidity are the quality gates at this point.

Devolatilisation, Drying and Finishing

Solvent and light by-products are removed by steam stripping or vacuum devolatilisation, often in a twin-screw or kneader configuration that handles the high viscosity of the cement. The rubber crumb is then dried with hot air or under vacuum to a low volatile content, and granulated or baled. Drying temperature is critical: too high a temperature promotes secondary cross-linking and a rise in Mooney viscosity, which shows up later as poor milling behaviour. Mooney viscosity is measured to ASTM D1646 and cure characteristics to ASTM D2084 as the release tests for the finished product.

Quality Control and Typical Specification Ranges

Process development has moved towards continuous halogenation reactors, in-line halogen analysers and closed-loop control of feed rate and temperature, which reduce batch-to-batch variation and cut energy and waste. The common thread in every step is the same: keep the reaction uniform and the residual chemistry clean, and the cured compound will deliver consistent permeability, heat ageing and adhesion.

Frequently Asked Questions

Q: How does halogenated butyl rubber differ from regular butyl rubber?
A: The saturated isobutylene backbone is unchanged, but a small number of allylic halogen sites are added. This raises cure activity so the rubber can cross-link with zinc oxide, sticks better to other rubbers and metals, and blends more easily.

Q: What halogen content is typical in BIIR and CIIR?
A: Brominated grades normally carry about 2 % bromine and chlorinated grades about 1.2 % chlorine. The halogen level is the main control on cure rate and scorch time.

Q: Why must moisture and metal ions be controlled before halogenation?
A: Water deactivates the reactive species, lowers substitution efficiency and promotes hydrolysis, while iron and other transition metals catalyse polymer degradation. Both are kept low by solvent distillation and filtration.

Q: What is the biggest processing risk in the reactor stage?
A: Over-halogenation. Excess substitution causes chain scission and cross-linking, raising gel content and viscosity, so halogen feed rate, temperature and residence time are monitored in-line.

Q: How is the finished rubber released for shipment?
A: Halogen content, Mooney viscosity to ASTM D1646, cure characteristics to ASTM D2084, volatile matter and ash content are reported. Mooney viscosity is the most sensitive indicator of thermal damage during drying.

Q: Which downstream industries use halogenated butyl rubber?
A: Tyre inner liners and curing bladders, pharmaceutical stoppers and closures, chemical protective clothing, seals and adhesives, all markets where low gas permeability plus reliable cure and adhesion are required.