Bronopol—Guardian Preservative in Metalworking Fluids: Applications, Risks and Regulatory Analysis  

Abstract  

Bronopol (2-bromo-2-nitropropane-1,3-diol) is a broad-spectrum antimicrobial that has long been used as an industrial preservative (including in metalworking fluids, MWF), in personal-care formulations and in specialty chemicals. Its advantages—good water solubility, activity against Gram-positive/-negative bacteria and fungi, and ease of use—are offset by potential formation of trace formaldehyde and, under certain conditions, nitrosamines or skin-sensitising species. This review summarises bronopol’s chemistry and antimicrobial mode of action, formulation strategies in MWF, occupational-health concerns relating to formaldehyde release, and current regulatory landscapes in China, the USA and the EU. Risk-management recommendations and future trends are also discussed.

1. Physico-chemical properties and antimicrobial efficacy  

Chemical name: 2-bromo-2-nitropropane-1,3-diol

CAS: 52-51-7

MW: 200.0

MP: 130 °C

Density: ~1.2 g cm⁻³

Vapour pressure: 1.68 × 10⁻³ Pa at 25 °C.  

Bronopol is highly soluble in water and polar solvents, stable at pH 4–7, but undergoes accelerated decomposition at pH > 8 with release of low levels of formaldehyde. It is bactericidal and fungicidal at low mg L⁻¹ concentrations, notably effective against Pseudomonas aeruginosa (MIC 25 mg L⁻¹) and sulfate-reducing bacteria (MIC 12.5 mg L⁻¹).

2. Use in metalworking fluids (MWF)  

Microbial spoilage of water-based MWF causes odour, corrosion, slime and system blockage. Bronopol is added either (i) in-concentrate during MWF manufacture, (ii) on-site as a maintenance (≈10 ppm) or “shock” dose (50–100 ppm), or (iii) via pre-dissolved stock solutions at < 40 °C to avoid thermal decomposition. Compatibility with extreme-pressure additives, emulsifiers and corrosion inhibitors must be validated. Because some traditional biocides (formaldehyde releasers, isothiazolinones) face regulatory or supply pressure, bronopol remains a viable option, especially in closed-loop, low-to-moderate bioburden systems.

3. Stability and formaldehyde-release issue  

Hydrolytic degradation increases with pH and temperature; nevertheless, measured free formaldehyde is usually “low” or below occupational limits. Still, bronopol should not be combined with primary/secondary amines to minimise nitrosamine formation. Workplace assessment must cover (a) bronopol itself (skin/eye irritation, sensitisation), (b) any liberated formaldehyde, and (c) possible co-exposure to other amines or high pH.

4. Toxicology, sensitisation and environmental fate  

Acute toxicity is moderate; bronopol can provoke contact dermatitis in susceptible individuals. Environmental degradation yields brominated by-products with aquatic toxicity (rainbow trout 96-h LC₅₀ 20 mg L⁻¹; Daphnia 48-h LC₅₀ 1.4 mg L⁻¹). Waste streams require neutralisation or biological treatment before discharge.

5. Analytical methods  

HPLC is standard for bronopol quantification; GC-MS/LC-MS identify breakdown products. Formaldehyde may be monitored by DNPH-derivatisation HPLC, enzymatic kits or portable colorimetry. Chinese patent CN108614044A provides an validated HPLC protocol.

6. Global regulatory overview  

6.1 China  

- Occupational exposure: GBZ 2.1-2019 sets limits for formaldehyde (PC-TWA 0.5 mg m⁻³, MAC 0.75 mg m⁻³). Enterprises must survey workplace air and implement controls if bronopol-derived formaldehyde is detectable.  

- Chemical management: bronopol is listed as a hazardous chemical; SDS, labelling and emission-permit rules apply under MEE and Ministry of Emergency Management statutes.

6.2 United States  

- EPA: bronopol is registered as an antimicrobial pesticide for industrial fluids (RED 1988). Labels specify maximum use rates and environmental precautions.  

- OSHA: formaldehyde standard (29 CFR 1910.1048) PEL = 0.75 ppm TWA, 2 ppm STEL; action level 0.5 ppm. ACGIH classifies formaldehyde A1 (confirmed human carcinogen) with a TLV-Ceiling of 0.1 ppm. Facilities must monitor if bronopol can release formaldehyde above these values.

6.3 European Union  

- Cosmetics: bronopol is permitted preservative up to 0.1 % under Reg. (EC) No 1223/2009; SCCS scrutinises formaldehyde release.  

- Biocides: bronopol is approved under the Biocidal Products Regulation (EU) 528/2012 (PT 4, 6, 13) with defined concentration limits and risk-mitigation measures.  

- Worker protection: Directive 2004/37/EC (as amended) sets an IOELV of 0.3 ppm TWA and 0.6 ppm STEL for formaldehyde, with Carc. 1B and Skin-Sens. notations.

7. Risk-reduction strategies  

- Formulation: keep pH ≤ 8, avoid amines, incorporate antioxidants or stabilisers.  

- Rotation/ synergy: combine bronopol with other biocicide classes (isothiazolinones, biguanides, QACs) at reduced individual doses.  

- Engineering controls: enclosed mixing, local exhaust ventilation, automatic dosing.  

- Personal protection: chemical-resistant gloves, goggles, respiratory protection if airborne formaldehyde > 0.1 ppm.  

- Monitoring: routine HPLC for bronopol, DNPH-HPLC or direct-reading instruments for formaldehyde; medical surveillance for dermal symptoms.  

- Waste treatment: neutralise spent MWF, segregate halogenated streams, ensure compliance with local effluent limits.  

- Substitution: evaluate alternative preservatives (e.g., biguanides, certain isothiazolinones) where risks are unacceptable, balancing efficacy, toxicity and cost.

8. Conclusion  

Bronopol delivers high-performance, broad-spectrum microbial control in water-based metalworking fluids, but its use must be tempered by awareness of formaldehyde release, potential nitrosamine formation, and increasingly stringent occupational and environmental standards. A science-based risk-management framework—encompassing prudent formulation, stringent on-site monitoring, engineering and personal protective measures, and responsible waste handling—is essential. Continued development of safer, greener preservation technologies will complement, and may eventually replace, bronopol while maintaining fluid integrity and worker safety.