1. Introduction to 2-Hydroxy-4-Methoxy Benzophenone-5-Sulphonic acid
2-Hydroxy-4-Methoxy Benzophenone-5-Sulphonic Acid (commonly marketed as UV-284 or BP-4) is a water-soluble, sulphonated benzophenone derivative widely used as an ultraviolet (UV) absorber. The compound belongs to the benzophenone family, a group of aromatic ketones that exhibit strong UV-absorbing capabilities due to their conjugated π-electron systems and excited-state intramolecular proton transfer (ESIPT) properties. BP-4 is specifically designed to extend solubility into aqueous systems without compromising UV-filtering efficiency, making it an essential additive in cosmetics, sunscreens, aqueous coatings, detergents, plastics, and various industrial formulations.
Because of increasing global regulatory scrutiny of UV filters—particularly regarding photostability, endocrine disruption potential, and environmental persistence—BP-4 has attracted continued interest due to its favorable solubility profile, moderate photostability, compatibility with diverse resin systems, and relatively well-studied toxicological behavior. This report presents a comprehensive review from a chemical engineering perspective, covering fundamental chemical properties, structure–function relationships, high-level industrial manufacturing principles, processing considerations, and multi-sector applications.
2. Chemical Identity and Molecular Characteristics
2.1 Nomenclature and Identifiers
- Chemical name: 2-Hydroxy-4-Methoxy Benzophenone-5-Sulphonic Acid
- Common names: Benzophenone-4, BP-4, UV-284
- CAS Number: 4065-45-6
- Molecular formula: C14H12O6S
- Molar mass: approx. 308.30 g/mol
- IUPAC name:
5-(2-hydroxy-4-methoxybenzoyl)benzene-1-sulfonic acid
2.2 Structural Features
The structure consists of two phenyl rings connected by a carbonyl carbon (typical of benzophenones). Key functional groups include:
- A phenolic hydroxyl group (–OH) at the ortho position
- A methoxy group (–OCH₃) at the para position
- A sulphonic acid group (–SO₃H) capturing water solubility
- A conjugated aromatic system that supports π→π* transitions
2.3 Physicochemical Properties (General)
| Property | Description |
| Physical appearance | Off-white to yellowish powder or granules |
| Solubility | High solubility in water due to sulfonic acid group; soluble in alcohols; limited solubility in non-polar solvents |
| pKa | The sulfonic acid group is strongly acidic; phenolic OH moderately acidic |
| UV absorption | Strong absorption around 280–320 nm, typically peaking near 285 nm |
| Stability | Stable under normal conditions; resistant to hydrolysis; moderate thermal stability |
| Photostability | Good relative to many organic UV filters, though may undergo photochemical rearrangements under intense UV exposure |
| Hygroscopicity | Low to moderate depending on environmental humidity |
2.4 Mechanism of UV Absorption
BP-4 absorbs UV radiation by promoting electrons from the ground state to higher-energy π* molecular orbitals. The conjugated benzophenone skeleton stabilizes the excited state, while the hydroxyl group allows proton transfer, enabling efficient internal conversion and non-radiative dissipation of energy. Consequently, the molecule transforms incident UV radiation into harmless heat, minimizing photodegradation of host formulations.
The sulfonic acid group not only improves solubility but also subtly alters electronic distribution, refining absorption characteristics and enhancing formulation compatibility in aqueous matrices.
3. Chemical Behavior and Reactivity
3.1 Acid–Base Behavior
The presence of both phenolic OH and sulfonic acid functionalities leads to:
- Strong acidity of the sulfonic acid proton, fully ionized in aqueous solutions and enhancing hydrophilicity.
- A moderately acidic phenolic proton that participates in hydrogen bonding and intermolecular interactions.
- Formation of stable salts (e.g., sodium or potassium salts), which are frequently used in commercial applications due to easier handling and improved solubility control.
3.2 Interactions with Polymers and Matrices
BP-4 is often incorporated into polymer matrices and cosmetic emulsions. Key interactions include:
- Hydrogen bonding between phenolic OH or sulfonate groups and polymer functional groups.
- Electrostatic interactions when used as a salt.
- π–π stacking with aromatic monomers or copolymers.
- Hydrophilic–lipophilic balance effects influencing dispersion and uniformity in emulsions.
These interactions influence migration behavior, UV-filter efficiency, and compatibility with preservatives, surfactants, and other formulation components.
3.3 Thermal and Photochemical Behavior
BP-4 maintains its UV-absorbing properties over a wide temperature range. When exposed to intense UV radiation:
- It may undergo intramolecular proton transfer, followed by rapid relaxation.
- Under extreme photochemical stress, minor rearrangements or oxidative transformations can occur, though to a lesser extent compared with many cinnamate or salicylate UV filters.
- BP-4 does not readily volatilize due to its high molecular mass and polarity.
3.4 Compatibility with Other UV Absorbers
BP-4 is frequently used alongside other UV filters to broaden absorption spectra. It is compatible with:
- Benzophenone derivatives (BP-3, BP-5)
- Benzotriazoles
- Triazines
- Physical filters (TiO₂, ZnO)
- Cinnamates and salicylates (depending on pH)
Because BP-4 absorbs primarily in the UV-B region, it is often used to complement UVA absorbers.
4. High-Level Overview of Manufacturing Processes
(Presented without operationally actionable details.)
BP-4 production in industry typically involves three overarching steps:
- Preparation of the benzophenone core
- Introduction of sulfonation functionality
- Purification and conversion to acid or salt form
All modern production routes follow the chemical logic of aromatic electrophilic substitution and functional group transformation, but exact catalysts, temperatures, or reaction conditions vary between manufacturers and are proprietary.
4.1 Formation of the Benzophenone Framework
The core benzophenone skeleton is typically generated through an electrophilic acylation-type process, producing 2-hydroxy-4-methoxybenzophenone as an intermediate. The reaction makes use of the activating and directing influence of the methoxy and hydroxyl groups, enabling regioselective formation of the desired orientation on the second aromatic ring.
Industrial setups emphasize:
- Proper control of reaction stoichiometry
- Engineering approaches to manage heat release
- Ensuring minimization of side-products
4.2 Introduction of the Sulphonic Acid Group
The sulfonation step attaches a –SO₃H group selectively to the aromatic ring already part of the benzophenone structure. Sulfonation chemistry is highly sensitive to substitution patterns; thus, stringent control of reaction conditions is required to ensure:
- High regioselectivity
- Avoidance of polysulfonation
- Preservation of the benzophenone carbonyl
Modern plants use continuous or semi-batch reactors designed to handle the high exothermicity and corrosiveness of sulfonation processes. Typical engineering considerations include:
- Use of corrosion-resistant materials
- Acid-resistant pumps and seals
- Efficient temperature management systems
- Closed-loop gas scrubbing to handle emissions
4.3 Salt Formation (Optional)
Commercial BP-4 can be supplied either in the free acid form or as neutralized sodium/potassium salts, which are more soluble in water and more stable during transport. Salt formation simply involves treating the sulfonic acid with a neutralizing base in a controlled pH environment.
4.4 Purification and Drying
Purification removes:
- Residual organic impurities
- Trace inorganic acids
- Oligomeric by-products
- Coloring materials
Common high-level industry strategies include:
- Solution purification
- Crystallization
- Solid–liquid separation
- Drying via spray drying or fluidized-bed dryers depending on the final particle profile required
Dry powder handling systems follow standard chemical engineering safety practices for powders with low volatility but acidic nature.
4.5 Quality Control
Quality control parameters often include:
- UV absorption profile
- Purity via HPLC
- Inorganic residue analysis
- Moisture content
- pH (if salt form)
- Color and particle size distribution
5. Engineering and Processing Considerations
5.1 Handling and Safety
BP-4 is considered non-volatile and relatively safe to handle compared with many organic UV filters, but chemical engineering protocols recommend:
- Local exhaust ventilation to manage dust
- Personal protective equipment (PPE) for workers
- Storage away from strong bases
- Equipment designed for acidic materials
- Measures to prevent cross-contamination in cosmetic or pharmaceutical environments
5.2 Formulation Engineering
BP-4’s performance depends strongly on formulation environment:
- In aqueous systems, dissociation of the sulfonic acid enhances solubility but may lead to pH-dependent variations in UV absorption.
- In emulsions, BP-4 typically partitions into aqueous phases, requiring emulsifier balance to prevent migration or crystallization.
- In polymeric matrices, compatibility depends on polymer polarity, hydrogen-bonding sites, and degree of crosslinking.
Stability studies are essential when incorporating BP-4 into adhesives, coatings, or inks that experience prolonged light exposure.
6. Industrial and Commercial Applications
BP-4 is used in a wide variety of sectors. Its main value derives from water solubility combined with potent UV-B absorption. Below is an engineering-oriented review of its roles.
6.1 Cosmetic and Personal Care Industry
6.1.1 Sunscreens
BP-4 is primarily a UV-B filter. Unlike many lipophilic UV filters such as octinoxate or oxybenzone, BP-4 is water soluble, making it especially suitable for:
- Gel-type sunscreens
- Hydrophilic sunscreen lotions
- Transparent or alcohol-based formulations
- Products where non-oily aesthetics are desired
While alone it cannot provide broad-spectrum coverage, BP-4 enhances SPF when combined with UVA filters.
6.1.2 Daily-Use Personal Care Products
Its solubility and stability allow incorporation into:
- Shampoos
- Conditioners
- Body washes and shower gels
- Hand sanitizers
- Facial cleansers
In such products, BP-4 protects fragrances, dyes, and other actives from UV-induced degradation.
6.1.3 Color Cosmetics
BP-4 helps preserve pigments in:
- Nail polishes
- Liquid foundations
- Lip products (when aqueous gels are used)
By preventing photodegradation, BP-4 extends product shelf life and prevents undesired color changes.
6.2 Detergents and Household Cleaning Products
BP-4 is frequently used in detergents because:
- It remains stable in aqueous surfactant systems
- It protects fragrances and dyes from fading
- It maintains clarity of transparent cleaning solutions
Used in:
- Laundry detergents
- Dish-washing liquids
- Fabric softeners
- Multi-surface cleaners
Many detergent formulations undergo exposure to ambient light during storage; BP-4 minimizes photolysis of sensitive components.
6.3 Coatings, Inks, and Adhesives
6.3.1 Water-Based Coatings
BP-4 is highly suited for:
- Acrylic emulsions
- Latex paints
- Wood coatings
- Waterborne varnishes
Benefits include:
- Reduced yellowing of coatings under sunlight
- Protection of polymer chains from UV-induced breakdown
- Improved gloss retention
- Enhanced outdoor durability
6.3.2 Printing Inks
Waterborne inks, including digital and flexographic inks, often incorporate BP-4 to:
- Improve colorfastness
- Prevent pigment fading
- Maintain transparency of clear overprint varnishes
Its compatibility with polar polymer binders ensures uniform dispersion.
6.3.3 Pressure-Sensitive Adhesives
BP-4 prevents degradation of adhesive polymers such as acrylics, extending adhesive life and preventing softening or loss of tack under UV exposure.
6.4 Plastics and Polymer Processing
BP-4 is used in plastics that support water-based compounding or surface treatment:
- Polyvinyl alcohol
- Waterborne polyurethane dispersions
- Hydrophilic polymer coatings
- Gel-type elastomers
- Plastic films requiring UV resistance on the surface
Although BP-4 is not the first choice for hydrophobic polymers (for which benzotriazoles or HALS additives are preferred), it is valuable in specialty polymer systems requiring aqueous dispersion or surface UV stabilization.
6.5 Textile Processing
6.5.1 Dyeing and Finishing
BP-4 is added to dyes and finishing formulations to:
- Retain colorfastness
- Reduce fiber degradation
- Prevent yellowing on exposure to sunlight
6.5.2 Fiber Treatments
Water-compatible UV absorbers are critical for:
- Polyester microfiber finishing
- Nylon coloration
- Rayon treatments
BP-4’s stable sulfonate structure ensures uniform binding under typical textile pH ranges.
6.6 Water Treatment Chemicals
In certain water treatment additives and polymers, BP-4 helps protect sensitive functional components—particularly organic coagulants or flocculants—against photodegradation in storage tanks exposed to outdoor conditions.
6.7 Pharmaceutical and Biomedical Applications
While BP-4 is not used as an active pharmaceutical ingredient, it appears in:
- Photoprotective topical formulations
- Product packaging coatings
- Gel-based drug carriers requiring UV protection during storage
In such applications, BP-4 helps maintain drug integrity.
7. Environmental and Regulatory Considerations
7.1 Environmental Behavior
BP-4 is:
- Highly water soluble, leading to potential mobility in aquatic environments
- Subject to environmental scrutiny regarding persistence and bioaccumulation
- Often found in wastewater streams originating from personal care products
Though its bioaccumulation potential is considered lower than more hydrophobic UV filters, wastewater treatment plants must manage its potential release to ecosystems.
7.2 Regulatory Status
Regulatory acceptance varies by region but is generally established for cosmetic use with concentration limits. Cosmetic regulations typically specify:
- Maximum allowable concentration in sunscreens
- Purity requirements
- Impurity limits for unreacted precursors
As scientific research evolves, regulatory frameworks continue to evaluate potential endocrine activity, aquatic toxicity, and environmental persistence.
8. Advantages and Limitations
8.1 Key Advantages
- High water solubility
- Strong UV-B absorption
- Good compatibility in aqueous systems
- Moderate photostability
- Ease of formulation in gels and water-based products
- Compatibility with many other UV filters
8.2 Limitations
- Limited absorption in the UVA region
- Possible interactions with cationic ingredients due to sulfonate group
- Environmental concerns related to water mobility
- Sensitivity of UV absorption to pH changes
9. Future Development and Research Directions
Research areas include:
- Improved photostability through synergistic blends with stabilizers or radical quenchers
- New derivatives with expanded UVA absorption
- Encapsulation technologies (e.g., microgels, polymer capsules) to reduce environmental diffusion
- Assessment of environmental degradation pathways in marine and freshwater environments
- Hybrid use with nanoparticle-based UV filters for multifunctional cosmetic formulations
The trend toward eco-friendly and biodegradable UV filters continues to shape future innovation, and BP-4 remains a reference molecule for hydrophilic UV filter development.
10. Conclusion
2-Hydroxy-4-Methoxy Benzophenone-5-Sulphonic Acid (BP-4, UV-284) is an important water-soluble organic UV absorber with broad utility across cosmetics, detergents, inks, coatings, and specialty polymers. Its strong UV-B absorption, physicochemical stability, and compatibility with hydrophilic formulations have established it as a staple ingredient in numerous consumer and industrial products.
From a chemical engineering perspective, BP-4 manufacturing requires careful control of aromatic substitution pathways, sulfonation reactions, and purification processes. Its application breadth stems from its physicochemical balance—an aromatic chromophore for UV shielding combined with a sulfonic acid group for aqueous compatibility.
Although environmental and regulatory considerations continue to evolve, BP-4 remains a technically robust and highly versatile UV filter. Ongoing research seeks to optimize performance, enhance environmental profiles, and develop next-generation hydrophilic UV absorbers inspired by the BP-4 molecular framework.