Food-Contact & Pharma Use of Cyclohexanone: What Is Allowed?

Cyclohexanone (CAS 108-94-1) is a cyclic ketone with a normal boiling point of 155.6 °C, a closed-cup flash point of 44 °C, and a log P of approximately 0.81. These physical properties govern its performance as a solvent in food-contact coating and adhesive systems and its analytical behavior in pharmaceutical residual solvent testing. The substance is permitted in specific food-contact material categories under U.S. FDA regulations when used as a solvent or component and removed under good manufacturing practice. In pharmaceutical manufacturing, cyclohexanone is not harmonized in the ICH Q3C(R8) Class 1 or Class 2 residual solvent tables, so its use triggers a qualification burden that differs from common compendial solvents such as methanol or dichloromethane.

The matrix below consolidates the principal regulatory and control statuses for cyclohexanone in food-contact and pharmaceutical contexts.

Within resinous and polymeric coatings for metal substrates, 21 CFR 175.300(b)(3) lists cyclohexanone among permitted solvents. No fixed migration limit is assigned to cyclohexanone in that section; instead, the cured coating must comply with general food-contact good manufacturing practice under 21 CFR 174.5. In a vinyl chloride-vinyl acetate copolymer can coating formulated with cyclohexanone as the primary let-down solvent, the solvent is present at roughly 30–35 wt% of the wet lacquer and is removed in multi-zone forced-air ovens. The high boiling point of cyclohexanone relative to methyl ethyl ketone or ethyl acetate means that the final oven zones must maintain substrate surface temperatures above 120 °C for sufficient residence time to prevent solvent retention in the film. Production-scale experience with reverse roller coating lines indicates that retained cyclohexanone is more likely in thick edge-bead zones and in films applied above 12 g/m² dry coat weight; such retained solvent can partition into 10% ethanol aqueous food simulant and fail sensory testing. Compliance testing for residual cyclohexanone in coatings is not based on a specific FDA numerical migration limit but on the requirement that the residual solvent be reduced to the lowest level consistent with 21 CFR 174.5 and that no organoleptic or functional adulteration of food occur.

Cyclohexanone is not a direct food additive and has no 21 CFR 172 listing. Direct addition to food or use as a food-contact substance outside the specified sections requires separate authorization. Where cyclohexanone is used in a food-contact article outside the listed adhesive, coating, and paper sections, the manufacturer must establish an effective Food Contact Notification under 21 U.S.C. 348(h) or a Threshold of Regulation exemption. In such cases, migration testing under 21 CFR 170.39 is required. Cyclohexanone’s low log P means the aqueous food simulants are the most stringent; partitioning into n-heptane is typically lower by a factor of two to three, though published data for this specific comparison is limited. The safety assessment for an FCN is based on estimated dietary intake rather than a solvent-specific SML, and cyclohexanone generally must be removed or reduced to trace levels in the finished article. This is different from the listed sections, where the solvent may be present in the wet formulation and then removed during processing.

Does 21 CFR 175.105 Cover Cyclohexanone in Food-Contact Adhesive Formulations?

Cyclohexanone is included in the substance list in 21 CFR 175.105(c) for adhesives used in food-contact laminates and labels. The regulation permits cyclohexanone as a solvent or component provided the adhesive is either separated from food by a functional barrier or the quantity that migrates to food is no more than negligible under 21 CFR 174.5. In flexible packaging, a two-component polyurethane laminating adhesive diluted with cyclohexanone to 20–25 wt% solids is applied by gravure cylinder and dried at 70–90 °C before lamination. Unlike ester solvents, cyclohexanone has a lower evaporation rate and a moderate aqueous partition; its log P of approximately 0.81 means it is more readily extracted by aqueous food simulants than by fatty food simulants. Residual cyclohexanone in the dry adhesive is therefore a particular concern for water-based food contact. On production laminators, inline non-dispersive infrared analyzers are used to monitor solvent retention before lamination; retained solvent is driven down by passing the coated web through a second drying tunnel or by reducing line speed. Published data for specific numerical limits in commercial adhesive lines is limited, but industrial practice generally targets residual solvent below 1 mg/m² before lamination for cyclohexanone-containing adhesive systems. The relevant regulatory requirement remains migration-based rather than formulation-based: if the food-contact side is a functional barrier film such as oriented polyester or aluminum foil, the residual solvent in the adhesive does not violate 21 CFR 175.105 provided the overall laminate does not transfer solvent to food above negligible levels.

For paper and paperboard, cyclohexanone is permitted as a component under 21 CFR 176.170 for aqueous and fatty foods and under 21 CFR 176.180 for dry foods. The substance functions as a slow-evaporating ketone solvent in nitrocellulose or vinyl-based paper coatings and printing inks. In a coated paperboard line, the dry loft must be operated at temperatures of 100–110 °C with adequate air turnover because cyclohexanone retention in the fiber web is influenced by the low headspace volatility and the affinity of the ketone for cellulosic fibers. Extraction testing under 21 CFR 176.170(d) uses food simulants including 10% ethanol, 50% ethanol, and n-heptane depending on the intended food type. Aqueous simulant extracts are analyzed by headspace gas chromatography with flame ionization detection. Paperboard samples that are dried at low web tension may show higher retained cyclohexanone in the center of the reel; this spatial variability is controlled by process limits on drying temperature and by release of finished material only after replicate extraction values fall below the analytical method quantitation limit. The absence of a specific FDA migration limit for cyclohexanone in paper and paperboard means that measured values are evaluated against 21 CFR 174.5 general safety and organoleptic criteria rather than a fixed numerical cutoff.

Residual Solvent Assessment in Oral Solid Dose Manufacturing Under ICH Q3C(R8)

In pharmaceutical manufacturing, cyclohexanone may be used as a reaction solvent or crystallization medium for intermediates, but its residual solvent status is not harmonized under the ICH Q3C(R8) Class 1 or Class 2 table. Consequently, no default PDE or concentration limit of 0.1% applies to cyclohexanone in drug substances or excipients under the ICH framework. A sponsor or contract manufacturing organization that uses cyclohexanone after the final synthetic step must qualify the solvent as an unlisted residual solvent using pharmacopoeial principles. The USP <467> residual solvents chapter does not include a specific target limit for cyclohexanone; any specification is therefore risk-based and supported by toxicological justification. In practice, API batches crystallized from cyclohexanone are dried under vacuum at 60–70 °C for 8–12 hours to minimize residual solvent. Gas chromatographic quantification with flame ionization detection or mass spectrometric confirmation is performed after dissolution in dimethyl sulfoxide or N,N-dimethylformamide. Because cyclohexanone is a high-boiling, moderately polar ketone, headspace sampling may exhibit poor recovery from polar matrices; method validation per ICH Q2(R1) should include recovery studies across a spiked concentration range and should account for matrix-induced signal enhancement. Published regulatory limits for cyclohexanone in pharmaceutical dosage forms are limited; the operational specification is typically derived from toxicological review and is not a compendial acceptance criterion.

When cyclohexanone is used as a cleaning solvent between batches, the cleaning validation protocol must treat the solvent as a residual active cleaning agent rather than a residual solvent from synthesis. Swab sampling of stainless steel equipment followed by extraction in dimethyl sulfoxide and GC-FID analysis is used. Acceptable residue limits are calculated from toxicological data and expressed as mass per surface area; published regulatory limits for cyclohexanone as a cleaning agent are limited, so the limit must be established by risk assessment. This situation arises most often in non-aqueous granulation trains where the solvent is selected for dissolution of binders or water-sensitive actives. Equipment surfaces made of polytetrafluoroethylene or silicones can retain cyclohexanone longer than electropolished stainless steel; cleaning validation therefore includes surface-specific recovery studies.

In modified-release tablet coating, cyclohexanone can serve as a solvent for ethylcellulose or methacrylic acid copolymer systems at solids content of 6–10 wt%. The organic solvent is sprayed from a pan coater with inlet air temperature of 50–60 °C, and the outlet air is maintained below 35 °C to avoid polymer melt and sticking. The solvent must be removed during the coating process to residual levels below the qualified specification. Process failures such as insufficient exhaust air or overspray can trap cyclohexanone under the coating layer; this trapped solvent is detectable only after dissolution of the dosage form in dimethylformamide. Published data for this specific configuration is limited.

When Cyclohexanone Migrates from Nylon-6 Food-Contact Materials and Drug Packaging

When nylon-6 films, molded components, or multilayer packaging are used, cyclohexanone can appear as a residual process solvent or as an impurity related to caprolactam production. In food-contact applications, nylon-6 is permitted under 21 CFR 177.1500, but residual monomers and impurities are controlled under general GMP and any applicable specifications. During extractables screening for pharmaceutical packaging, cyclohexanone is routinely included in volatile and semivolatile compound libraries. A nylon-6 film extracted in 50% ethanol at 70 °C for 2 hours may release cyclohexanone in the low nanogram per square centimeter range, although published data for this specific configuration is limited. The low log P of cyclohexanone favors partitioning into aqueous simulants, making it a relevant extractable for injectable drug products stored in nylon-based overwraps or blister films. Under USP <1663> and USP <1664>, cyclohexanone detected in an extractables study must be evaluated for potential leachability under long-term storage; if the drug product solution contains substantial water, the partitioned concentration may be higher than in oils. Limits are set as analytical thresholds and toxicological concern levels derived from the route of administration, and cyclohexanone does not have a harmonized extractables-specific limit. This is a critical boundary: detection of cyclohexanone in packaging extractables does not automatically render the system noncompliant, but it triggers an evaluation that cannot rely on a compendial residual solvent limit.

Analytical control of cyclohexanone in both regulatory domains depends on headspace gas chromatography with either flame ionization detection or selected ion monitoring mass spectrometry. Method detection limits in polymer matrices typically range from 0.1 μg/g to 1 μg/g, depending on sample preparation, but published round-robin data for cyclohexanone specifically is limited. The detector response is linear over 0.5–100 μg/mL in solvent-based standards. In food-contact compliance testing, the extraction solvent must match the food type: aqueous simulants for high-moisture foods, n-heptane for fatty foods, and 50% ethanol for emulsion or alcoholic foods. The headspace GC method must be revalidated when switching from laminated film to paperboard because the fiber matrix shifts recovery and matrix-induced signal enhancement. Cyclohexanone quantitation in pharmaceutical cleaning swabs requires low-volume extraction with dimethyl sulfoxide and a polar stationary phase to separate the ketone from dimethylformamide or N-methylpyrrolidone residues.