Benzoyl Peroxide (BPO) CAS 94-36-0: Properties, Uses & Safety Guide

What Is Benzoyl Peroxide (BPO) — CAS 94-36-0

Benzoyl peroxide (BPO), systematically named dibenzoyl peroxide, is an organic peroxide with the molecular formula (C₆H₅CO)₂O₂ and CAS Registry Number 94-36-0. It consists of two benzoyl groups linked by a peroxide (–O–O–) bond — the weakest and most chemically reactive part of the molecule.

In its pure dry form, BPO appears as a white granular or crystalline powder with a faint benzaldehyde-like odor. It has a molecular weight of 242.23 g/mol and a melting point of approximately 103–106°C, at which point decomposition begins rather than clean melting. For this reason, commercial BPO is almost always supplied in wetted, phlegmatized, or paste form — water content of 25–35% is the most common stabilization method used in industrial and pharmaceutical-grade products.

The compound was first synthesized in 1898 and has since become one of the most commercially significant organic peroxides globally. Its utility stems from a single defining chemical behavior: upon moderate heating or UV exposure, the O–O bond cleaves homolytically, generating two highly reactive benzoyloxy radicals. These radicals drive free-radical polymerization, bleaching reactions, and antimicrobial activity — the three pillars of BPO's industrial and pharmaceutical relevance.

Key Physical and Chemical Properties

Accurate specification data is essential when sourcing or formulating with dibenzoyl peroxide (CAS 94-36-0). The following table summarizes the most commonly referenced parameters:

The half-life of BPO in solution is a critical parameter for formulators: at 70°C in benzene, the half-life is approximately 10 hours; at 100°C, it drops to under 30 minutes. This temperature-dependent decomposition rate is what makes BPO selectable across a wide range of curing and polymerization processes by simply adjusting reaction temperature.

Industrial Applications of Dibenzoyl Peroxide

BPO (CAS 94-36-0) is consumed across several major industrial sectors, each exploiting a distinct aspect of its radical-generating chemistry.

Polymer Initiator and Crosslinking Agent

The largest single industrial use of dibenzoyl peroxide is as a free-radical initiator in the production of polymers and resins. It initiates the polymerization of vinyl acetate, styrene, acrylics, and PVC at temperatures of 60–100°C, generating consistent chain-initiation rates that are controllable by concentration and temperature. In unsaturated polyester resin (UPR) systems used in fiberglass composites, BPO is paired with a tertiary amine accelerator (most commonly N,N-dimethylaniline or N,N-diethylaniline) to achieve room-temperature cure — a process central to the hand lay-up, pultrusion, and resin transfer molding methods used in marine, construction, and automotive composite fabrication.

BPO also crosslinks silicone rubber and natural rubber at elevated temperatures, improving mechanical strength, heat resistance, and compression set. Peroxide-cured silicone typically achieves better high-temperature performance than platinum-catalyzed systems, making BPO-crosslinked grades preferred for automotive gaskets, medical tubing, and wire insulation.

Flour and Food Bleaching

Food-grade benzoyl peroxide is approved in many jurisdictions as a flour bleaching and maturing agent. Freshly milled wheat flour has a yellowish tint from carotenoid pigments; BPO oxidizes these pigments to colorless compounds, producing the bright white flour preferred for commercial baking. Permitted usage levels are tightly regulated — the FDA allows up to 50 ppm in flour in the United States, while the EU has banned BPO as a flour additive since 1997, requiring air-aging (oxidation) instead. Food-grade BPO must meet strict purity specifications, with heavy metal limits (particularly lead and arsenic) orders of magnitude tighter than industrial grades.

Dental and Cosmetic Bleaching

At concentrations of 3–10%, BPO is used in professional and over-the-counter tooth whitening strips and gels. It penetrates enamel and dentin, where radical oxidation breaks chromophore molecules responsible for staining. Benzoyl peroxide is generally considered to have a more favorable safety profile than carbamide peroxide for short-contact applications, as it decomposes more rapidly and leaves fewer residual oxidizing species.

Cheese and Dairy Processing

In countries where permitted, food-grade BPO is used to bleach milk for certain white cheese varieties — particularly Blue cheese, where annatto-based coloring agents used in the feedstock milk must be neutralized to achieve a consistent white curd color. This application is tightly controlled and limited to specific regional markets.

Pharmaceutical Grade BPO: Acne and Dermatology

Pharmaceutical-grade benzoyl peroxide is one of the most thoroughly studied topical actives in dermatology. It is a first-line treatment for mild-to-moderate acne vulgaris, available in concentrations of 2.5%, 5%, and 10% in gel, cream, wash, and lotion formulations.

Its mechanism of action is threefold:

• Antibacterial: Generates reactive oxygen species that are bactericidal against Cutibacterium acnes (formerly Propionibacterium acnes). Critically, BPO does not induce antibiotic resistance — a significant advantage as antibiotic-resistant acne strains become more prevalent globally.
• Keratolytic: Mildly loosens and exfoliates the stratum corneum, reducing follicular plugging that leads to comedone formation.
• Sebostatic: At higher concentrations, reduces sebum production modestly, decreasing the substrate available for bacterial proliferation.

Clinical evidence consistently shows that 2.5% BPO is as effective as 5% and 10% concentrations for reducing inflammatory lesion counts, with significantly fewer side effects (dryness, peeling, irritation). Most current dermatology guidelines recommend starting at the lowest effective concentration. Combination products pairing BPO with topical clindamycin or adapalene are among the most prescribed acne regimens worldwide.

Commercial Grades and Formulation Forms

Dibenzoyl peroxide (CAS 94-36-0) is commercially available in several distinct forms, each optimized for a specific handling or application context:

• Dry granular (≥97% active): Used only where absolutely necessary; classified as a Class 5.2 oxidizing substance and subject to strict transport and storage regulations. Rarely specified for new processes due to shock and friction sensitivity.
• Wetted powder (25–35% water content): The most common industrial form. Water content reduces sensitivity to shock and heat while maintaining high active content. Must be kept moist during storage; drying out restores hazard classification.
• Paste in plasticizer (50% active): BPO dispersed in dibutyl phthalate, dioctyl phthalate, or other plasticizers. Preferred for rubber vulcanization and silicone crosslinking — the plasticizer doubles as a processing aid in the compound.
• Dispersion in water (70–78% active): High active-content slurry with rheology modifiers. Used where high initiator loading with minimal solvent introduction is needed, as in suspension polymerization.
• Pharmaceutical micronized powder (USP/EP grade): Particle size controlled to D90 <25 µm for uniform dispersion in topical formulations. Purity ≥98%, with strict limits on benzoic acid and benzaldehyde impurities.

When comparing supplier offerings, always confirm the active oxygen content (AOC) alongside the nominal BPO percentage, as phlegmatizers and water content directly affect the actual initiator loading delivered per kilogram of product.

Storage, Handling, and Safety Requirements

BPO (CAS 94-36-0) is classified as an organic peroxide and must be handled under a dedicated risk management framework. Failures in storage conditions are the leading cause of BPO-related incidents in industrial settings.

• Temperature control: Store at or below 30°C for wetted grades; below 25°C recommended. Never exceed the stated SADT. Refrigerated storage (2–8°C) is mandatory for some high-concentration paste grades and extends shelf life significantly.
• Incompatible materials: BPO reacts violently with reducing agents, strong acids, strong bases, and heavy metal salts (especially iron, copper, and lead compounds, which catalyze decomposition). Keep all such materials out of storage areas.
• Contamination prevention: Even trace contamination with catalytically active materials can trigger runaway decomposition. Dedicated scoops, containers, and transfer equipment must be used — no shared tooling with other chemicals.
• Fire and explosion: Dry BPO is flammable and can deflagrate under confinement. Wetted grades are significantly safer but will release the dry solid if water evaporates. Fire response requires large volumes of water — dry powder extinguishers are ineffective and may spread burning material.
• PPE requirements: Chemical splash goggles, face shield, nitrile or neoprene gloves, and flame-resistant lab coat for handling industrial quantities. Dust mask (minimum FFP2/N95) when handling powdered grades.

Regulatory transport classification: UN 3102 (Organic Peroxide Type B, solid) for dry grades; UN 3106 (Organic Peroxide Type D, solid) for most wetted commercial grades. Specific classification depends on concentration and phlegmatizer content — always verify with the current SDS from your supplier before arranging shipment.

Global Market and Regulatory Landscape

The global benzoyl peroxide market was valued at approximately USD 350–400 million in 2023, with demand driven in roughly equal measure by polymer and rubber processing (industrial sector) and pharmaceutical/personal care applications. Asia-Pacific accounts for the largest share of industrial BPO consumption, led by China's massive polymerization and rubber industries. North America and Europe dominate pharmaceutical-grade consumption.

Key regulatory reference points by region:

• United States: FDA OTC monograph covers topical BPO for acne (2.5–10%). Industrial BPO regulated under EPA and OSHA process safety management rules at sites using large quantities. TSCA listed.
• European Union: Listed in Annex III of the Cosmetics Regulation (EC 1223/2009) for leave-on and rinse-off skin products with concentration limits. Classified as Ox. Sol. 2, H272 under CLP Regulation.
• China: Listed in the Inventory of Existing Chemical Substances in China (IECSC). Pharmaceutical-grade subject to NMPA registration requirements for imported drug substances.

Import and export of dibenzoyl peroxide (CAS 94-36-0) requires compliance with the UN Recommendations on the Transport of Dangerous Goods (Orange Book) in all jurisdictions, with specific packaging certifications required for each UN number and concentration class.