The synthesis of halogenated butyl rubber uses butyl rubber as a raw material. Chlorine or bromine atoms are introduced into the molecular chain through a halogenation reaction, thereby endowing the material with higher vulcanization activity, adhesion, and compatibility with other rubbers. The synthesis method involves not only the selection of reaction type but also precise control of reaction conditions, catalyst systems, and post-processing to achieve controllable substitution degree, structural homogeneity, and performance optimization.
Industrially, two main synthetic routes are used-chlorination and bromination-each with its own emphasis on halogenating agents, reaction mechanisms, and product characteristics. The chlorination method uses chlorine gas as the halogenating agent and can be carried out under free radical initiation systems or Lewis acid catalytic systems. Free radical chlorination is typically initiated at lower temperatures (e.g., 0–50°C) using light or organic peroxides. Chlorine reacts with the saturated carbon in butyl rubber via free radical substitution, producing chlorobutyl rubber. Lewis acid-catalyzed chlorination utilizes aluminum trichloride, boron trifluoride, etc., to form active complexes with proton donors, promoting ionic substitution reactions under mild conditions. This method exhibits higher position selectivity and reduces side reactions. The advantages of chlorination are its mature technology, readily available raw materials, and the resulting chlorobutyl rubber with good processing flowability, making it suitable for general applications such as large-scale tire airtight layers.
Bromination uses bromine or bromides (e.g., N-bromosuccinimide, hydrogen bromide-peroxide systems) as halogenating agents. The reactions are mostly based on free radical or electrophilic substitution mechanisms. Bromine has a larger atomic radius and slightly lower electronegativity than chlorine, resulting in higher reactivity. It can achieve uniform substitution at lower temperatures (e.g., -10–30°C) and has less impact on the isoprene double bonds in butyl rubber, which helps retain the original aging resistance and low permeability properties. Brominated butyl rubber exhibits faster vulcanization rates and superior adhesion, making it particularly suitable for high-end applications requiring strong interfacial bonding, such as automotive sealing and pharmaceutical packaging.
Regardless of the halogenating agent used, the synthesis process must be based on a butyl rubber solution system. Solid butyl rubber is typically dissolved in an alkane solvent (such as hexane or cyclohexane) to form a homogeneous solution, with strict control over moisture and metal ion content to prevent catalyst poisoning or increased side reactions. The halogenation reaction temperature, halogen introduction rate, reaction time, and catalyst concentration must be optimized based on the target degree of substitution: excessively high temperatures can lead to chain breakage or excessive cross-linking, while excessively low temperatures result in incomplete reactions; rapid halogen introduction can lead to excessively high local substitution, creating uneven performance zones. During the reaction, real-time monitoring of halogen content (e.g., through titration or an online halogen analyzer) and solution color changes is necessary, with timely adjustments to process parameters.
After halogenation, the product undergoes neutralization and washing. Residual halogenating agents and acidic byproducts are neutralized using alkaline solutions (such as dilute sodium carbonate or sodium hydroxide solutions). Multi-stage water washing or dilute acid washing removes soluble salts and unreacted halogens, ensuring product purity. Subsequent devolatilization and drying processes employ vacuum distillation or stripping to remove solvents and low-molecular-weight byproducts, followed by hot air circulation or vacuum drying to reduce moisture content to extremely low levels. For products requiring granulation, extrusion and pelletizing can be performed in the molten state, followed by cooling to obtain uniform granules for easier subsequent processing and metering.
In recent years, the optimization of synthesis methods has focused on greening and precision: developing recyclable solid acid catalysts to reduce corrosion and wastewater discharge; employing continuous flow reaction technology to improve heat transfer efficiency and safety; and combining online monitoring and closed-loop control to achieve real-time regulation of substitution degree and molecular weight distribution, thereby reducing batch variation and improving production efficiency.
Overall, the synthesis method of halogenated butyl rubber revolves around the halogenation of butyl rubber solution, introducing halogen atoms through chlorination or bromination reactions. Combined with meticulous neutralization washing, devolatilization drying, and granulation molding, a scalable and customizable preparation system is formed. The synergy and precise control of each step not only ensures the uniformity of the product's structure and properties but also provides high-quality basic materials for downstream applications in tires, seals, pharmaceuticals, and chemical protection.