BPA Bisphenol A

Bisphenol A (IUPAC 4,4′-(propane-2,2-diyl)diphenol; CAS 80-05-7; EINECS 201-245-8; molecular formula C₁₅H₁₆O₂; molecular weight 228.29 g/mol) is available in prill, flake, and molten bulk product models. The solid forms are produced by melt crystallization or prilling after vacuum distillation; molten product is supplied in insulated tank cars with nitrogen blanketing at 150–170 °C. A typical technical grade has a p,p′-BPA content of not less than 99.0% by gas chromatography area, free phenol not exceeding 0.10%, iron below 1.0 mg/kg, and moisture below 0.10% by Karl Fischer titration. The material has a melting point of 155–157 °C, a boiling point of 220 °C at 4 mmHg, a density of 1.20 g/cm³ at 25 °C, a water solubility commonly reported between 120 mg/L and 300 mg/L at 25 °C, and a vapour pressure near 5×10⁻⁶ Pa at 25 °C.

BPA is manufactured by the acid-catalysed condensation of phenol and acetone. Industrial reactors use cross-linked sulfonated styrene-divinylbenzene ion-exchange resin with a mercaptan promoter. The crude reaction mixture contains o,p′-BPA, p,p′-BPA, 2,4-BPA isomer, Dianin’s compound, and polyphenol oligomers; these are separated by crystallization or adduct crystallization with phenol. Control of excess phenol and water concentration shifts selectivity toward the desired p,p′-BPA isomer. The largest downstream use is polycarbonate production, followed by liquid and solid epoxy resin synthesis; smaller-volume applications include polysulfone, polyarylate, polycarbonate diol, and tetrabromobisphenol A flame-retardant intermediates.

Regulatory constraints affect grade selection. BPA is included on the REACH Candidate List as a substance of very high concern for endocrine-disrupting properties; EU Regulation 10/2011, as amended by 2018/213, sets a specific migration limit of 0.05 mg/kg food for BPA from plastic food-contact materials, and polycarbonate infant feeding bottles are prohibited in the European Union. REACH Annex XVII entry 66 restricts BPA in thermal paper at or above 0.02% by weight. Formulators using BPA-based epoxy resins for can coatings must verify compliance under FDA 21 CFR 175.300 and confirm that residual BPA migration is below applicable limits.

What limits free phenol and iron in bisphenol A destined for optical polycarbonate?

In optical-grade BPA, free phenol is controlled because residual phenol acts as a chain stopper in phosgene-based interfacial polymerization and contributes to colour formation at melt processing temperatures above 300 °C. Iron is controlled because Fe³⁺ and Fe²⁺ ions promote oxidative degradation of polycarbonate and can form insoluble salts in alkaline aqueous processing streams. Polycarbonate-grade BPA specifications therefore commonly include free phenol at or below 0.05% by gas chromatography area, iron at or below 0.5 mg/kg, and colour at or below 10 APHA when measured as a 50% methanol solution. Batch-to-batch variation in prill size distribution is controlled by prilling tower air flow and melt viscosity; variation in particle size affects dust generation and gravimetric feeding accuracy on continuous compounding lines.

On a production-scale optical polycarbonate line, BPA is mixed with aqueous sodium hydroxide to form the disodium salt. The aqueous phase is contacted with methylene chloride and phosgene in a stirred interfacial reactor. High pH above 11 favours salt formation but increases phosgene hydrolysis; pH below 10 slows condensation. Molecular weight is regulated by monofunctional phenol chain stoppers; residual free phenol in the monomer must therefore be accounted for when calculating the target chain-stopper addition. A shift in interfacial pH of even 0.2 pH units from the setpoint can alter the resulting polycarbonate melt flow rate under otherwise fixed ratio control. After isolation, BPA-polycarbonate powder is pelletized on a vented twin-screw extruder with L/D of 40:1. Barrel temperatures between 270 °C and 300 °C are used, with vacuum devolatilization below 20 mbar to strip residual methylene chloride below 10 ppm and phenol below 5 ppm. Pre-drying to a moisture content below 0.02% at 120 °C for 4 h in a desiccant dryer with a dew point below -40 °C is required before injection molding. Insufficient drying causes hydrolysis at carbonate linkages during melting; this is observed on processing lines as silver streaks, black specks, and a measurable reduction in intrinsic viscosity. For optical parts, contact with cycloaliphatic amines and strong alkaline mould-release agents must be avoided because these accelerate decomposition in the melt and increase haze.

Thermal Degradation Pathways in BPA-Polycarbonate Melt Transesterification

In the phosgene-free melt process, BPA and diphenyl carbonate are polymerised in a cascade of high-viscosity reactors. Temperature is staged from 180 °C to 310 °C as molecular weight increases, while phenol by-product is removed overhead through wiped-film evaporators under vacuum below 1 mbar absolute. The reaction is catalysed by tetrabutylammonium hydroxide or alkali metal salts; catalyst residues are neutralised with acidic additives to avoid uncontrolled branching during later processing. At temperatures above 320 °C, Fries rearrangement generates branched structures and ortho-phenolic groups that reduce thermal stability and increase gel formation. Residual lithium or sodium catalyst concentrations above 1 mg/kg can promote transesterification in the melt and shift the melt flow rate during injection molding. The resulting polycarbonate typically exhibits a glass transition temperature of 145–150 °C by ISO 11357-2:2020, a notched Izod impact strength of 600–800 J/m at 23 °C by ASTM D256-10, and a tensile yield strength of 60–65 MPa by ASTM D638-14. Melt flow rate grades are commonly adjusted from 4 g/10 min to 30 g/10 min at 300 °C/1.2 kg according to ISO 1133-1:2022 for injection molding and extrusion applications.

For epoxy resin synthesis, BPA is condensed with epichlorohydrin in the presence of aqueous sodium hydroxide. Liquid DGEBA resins derived from standard BPA typically have an epoxide equivalent weight of 170–190 g/eq by ASTM D1652-11 and a viscosity at 25 °C of 7,000–10,500 mPa·s by ASTM D2196-20. The product distribution between monomeric DGEBA and higher oligomeric homologues is controlled by the epichlorohydrin-to-BPA molar ratio, caustic addition rate, and water content in the coupling stage. Reducing the epichlorohydrin-to-BPA molar ratio below 3:1 increases the concentration of oligomeric bis-epoxides and raises viscosity, while ratios above 10:1 favour monomeric DGEBA but increase the recovery burden on the epichlorohydrin distillation train. Hydrolyzable chlorine in coatings-grade DGEBA is typically controlled below 0.1% by weight; residual chlorohydrin intermediates are converted by additional caustic dehydrohalogenation and water washing in a continuous liquid-liquid extraction train. BPA-derived epoxy resins are formulated with amine, anhydride, or phenolic hardeners. In coil-coating lines, BPA-based epoxy-phenolic systems are applied by roller coating and cured at peak metal temperatures of 210–230 °C for 45–75 s. The aromatic BPA segments provide rigidity and corrosion resistance but are susceptible to hydrolysis in strongly alkaline media at pH above 12 and temperatures above 80 °C.

When Bisphenol F Replaces Bisphenol A in Low-Viscosity Epoxy Laminating Resin

Bisphenol F (4,4′-methylenediphenol; CAS 620-92-8; molecular weight 200.23 g/mol) is structurally analogous to BPA but lacks the two methyl substituents at the central carbon. The resulting DGEBF liquid resin typically exhibits a viscosity of 3,000–5,000 mPa·s at 25 °C, compared with 7,000–10,500 mPa·s for BPA-based DGEBA. This difference is used in solvent-reduced flooring and resin transfer moulding where lower viscosity improves fibre wetting and permits higher filler loading. The absence of the central dimethyl group reduces steric shielding of the ether linkages and yields cured networks with lower glass transition temperature and greater moisture uptake. When BPA-based DGEBA is cured with a stoichiometric amount of dicyandiamide and 0.5 phr imidazole accelerator at 180 °C, the cured matrix commonly shows a glass transition temperature near 130 °C; direct substitution of DGEBF under the same cure schedule typically reduces the maximum glass transition temperature, although the magnitude depends on hardener type and cure conversion. Published data for this specific comparison is limited; comparative testing with ISO 11357-2:2020 is required for formulation qualification.

Bisphenol S, Bisphenol F, and Hydrogenated Bisphenol A Comparative Performance

Bisphenol S (4,4′-sulfonyldiphenol; CAS 80-09-1; molecular weight 250.27 g/mol) carries a sulfone bridge that imparts higher thermal stability and chemical resistance than the 2,2-propylidene bridge of BPA. Bisphenol S is used in polyethersulfone and polysulfone synthesis; those polymers display glass transition temperatures above 200 °C, while BPA-polycarbonate typically remains near 145–150 °C. Bisphenol F is selected where lower liquid epoxy viscosity is required, but its methylene bridge is less hydrophobic than the central dimethyl bridge of BPA. Hydrogenated bisphenol A is produced by catalytic hydrogenation of the aromatic rings of BPA; it is used in light-stable epoxy formulations and polycarbonate diols for coatings and elastomers. The saturated cyclohexane rings provide improved outdoor weathering and lower ultraviolet absorbance; however, commercial grades require separate handling specifications and may show wider melting ranges than BPA. BPA remains the dominant high-volume monomer because its 2,2-propylidene group balances rigidity, melt processability, and raw material availability from phenol-acetone integration.

In a direct substitution study for a fixed polycarbonate line, replacing BPA with bisphenol S is not feasible without changing the entire polymerisation system because bisphenol S has lower nucleophilicity and requires different solvent and base conditions. Equipment fouling, salt precipitation, and vacuum devolatilization requirements differ significantly between the two monomers. For existing BPA-based polycarbonate plants, the main drop-in modifications are limited to adjusting phenol chain-stopper feed or changing additive packages, not changing the bisphenol backbone.