Methylchloroisothiazolinone

Methylchloroisothiazolinone (5-Chloro-2-methyl-4-isothiazolin-3-one, CAS No. 26172-55-4) is an organic compound with broad-spectrum antibacterial activity. Its molecular formula is C₄H₄ClNOS, and its molecular weight is 149.6. As a white liquid with a pungent, aromatic odor and slightly soluble in solvents such as chloroform and DMSO, it has become an indispensable preservative and antimicrobial agent in both industrial and daily chemical applications.

Chemical Properties and Synthesis Process

The melting point of methylchloroisothiazolinone is 42–45°C, its boiling point is 109.7°C, and its density in a 14% aqueous solution is 1.25. The core structure—the isothiazolinone ring—exerts a bactericidal effect by disrupting microbial protein bonds. It is incompatible with strong oxidizing agents and should be stored under low-temperature, dry, and well-ventilated conditions. Industrial synthesis typically employs a two-step process: starting from methyl acrylate, a Michael addition reaction is carried out to produce methyl 3,3'-dithiopropionate; this intermediate then reacts with methylamine to form N,N'-dimethyl-3,3'-dithiopropionamide; finally, chlorine gas is introduced for chlorination and cyclization. Chinese patent CN117843585A improves product purity to 99.9% by controlling temperature in staged steps (10–18°C) and gradually adding chlorine gas in three stages (25–30%), while also reducing the formation of the byproduct 2-methyl-4-isothiazolin-3-one.

Industrial Applications and Market Value

As an efficient water treatment agent, methylchloroisothiazolinone exhibits excellent miscibility and can be compounded with chlorine, corrosion inhibitors, surfactants, and other additives. Its broad-spectrum antimicrobial activity covers more than 200 types of microorganisms, including sulfate-reducing bacteria, iron-oxidizing bacteria, and molds. In circulating water systems used in industries such as steel smelting, oilfield water injection, and papermaking, a concentration of just 1–5 ppm is sufficient to achieve a bactericidal efficiency exceeding 99%. In the personal care industry, its combination formulation with MIT (2-methyl-4-isothiazolin-3-one), such as Kathon CG, is widely employed in products like shampoos, facial cleansers, and wet wipes, effectively extending product shelf life to 2–3 years. On the global market, China dominates with 405 manufacturing enterprises; the price for reagent-grade products reaches 558 yuan per 5 grams, while industrial-grade products are supplied in 25 kg batches at a price of 237.5 yuan per batch.

Health Risk and Safety Management

Although methylchloroisothiazolinone is biodegradable, its potential hazards cannot be overlooked. Its acute toxicity manifests as skin irritation (Category 2), dermal irritation (Category 2), and sensitization (Category 1). Prolonged exposure may lead to contact dermatitis, chronic eczema, and impairment of the skin barrier function. Under the EU REACH regulation, its maximum allowable concentration in leave-on cosmetic products is limited to 0.0015%, while rinse-off products are still permitted to contain up to 0.0015%. In occupational exposure studies, the incidence of hand dermatitis among workers on cosmetic production lines reached as high as 12%, primarily associated with direct contact with high-concentration raw materials. Animal studies indicate that the oral LD50 in rats is 140 mg/kg, and the inhalation LC50 is 0.12 mg/L. Although the neurotoxicity risk has not been definitively established, inhalation exposure still warrants precautionary measures.

### Alternatives and Industry Trends

To reduce allergy risks, the industry is accelerating the development of mild preservative systems. For example, the combination of phenoxyethanol and ethylhexylglycerin has already achieved a penetration rate of 65% in infant and toddler products, while new organic acids such as capryloyl hydroxamic acid—thanks to their superior pH stability—are now accounting for as much as 28% of leave-on products like facial masks and serums. In addition, research on plant-derived preservatives like cinnamaldehyde and rosmarinic acid has made breakthroughs; their minimum inhibitory concentration (MIC) values can be as low as 0.02%, though cost remains a key barrier to large-scale adoption. At the policy level, China’s “Cosmetic Safety Technical Specifications” (2025 edition) has introduced a new prohibition on methylchloroisothiazolinone in spray products, compelling companies to reform their formulations.

Usage Recommendations and Consumer Guide

For ordinary consumers, when selecting products, prioritize checking the ingredient list and avoid using products that contain a combination of methylchloroisothiazolinone (MCI), MIT, and formaldehyde-releasing agents. Individuals with sensitive skin are advised to perform a patch test behind the ear; only after no redness or swelling is observed within 24 to 48 hours should the product be used. Individuals exposed in occupational settings must strictly wear nitrile gloves, safety goggles, and N95 masks, and the workplace ventilation rate should reach at least 15 m³/h per person. If symptoms such as erythema or blisters appear, immediately rinse the affected area with running water for 15 minutes and seek medical attention for specific IgE testing.

The dual nature of methylchloroisothiazolinone—its high efficacy in killing bacteria and its potential hazards—calls for the industry to strike a balance between technological innovation and safety management. As the concept of green chemistry continues to deepen, future antimicrobial systems will increasingly favor multi-mechanism synergistic approaches and low-concentration, highly effective solutions, ultimately achieving a seamless integration of product safety and efficacy.