Cyclohexanone Grades Caprolactam Grade Vs Technical Vs Solvent
Cyclohexanone Grades: Caprolactam-Grade vs Technical vs Solvent
Commercial cyclohexanone is not a single-specification material. The same ketone body is distributed under caprolactam-grade, technical-grade, and solvent-grade descriptors that reflect distinct impurity envelopes, distillation cut points, and downstream use tolerances. The substance is identified by CAS 108-94-1, with molecular weight 98.15 g/mol, normal boiling point 155.6 °C at 101.3 kPa, freezing point -47 °C, closed-cup flash point 44 °C, autoignition temperature 420 °C, density 0.9478 g/cm³ at 20 °C, refractive index 1.4507 at 20 °C, and water solubility 8.7 g/100 mL at 20 °C.
| Property | Typical value | Grade-selection relevance |
|---|---|---|
| CAS registry number | 108-94-1 | Regulatory and trade identification |
| Molecular weight | 98.15 g/mol | Stoichiometric calculation in oxime and isocyanate reactions |
| Normal boiling point | 155.6 °C at 101.3 kPa | Distillation cut control for grade assignment |
| Freezing point | -47 °C | Bulk storage in unheated vessels |
| Flash point, Tag closed cup | 44 °C | Flammable liquid classification and storage design |
| Autoignition temperature | 420 °C | High-temperature reactor safety boundary |
| Density at 20 °C | 0.9478 g/cm³ | Mass-to-volume conversion in batching |
| Vapor pressure at 20 °C | 0.5 kPa | Vent emission and drying-rate estimation |
| Water solubility at 20 °C | 8.7 g/100 mL | Aqueous purge and moisture pickup behavior |
| Hansen δD | 17.8 MPa0.5 | Nonpolar resin solvency |
| Hansen δP | 6.3 MPa0.5 | Polar resin and surface-wetting interactions |
| Hansen δH | 5.1 MPa0.5 | Hydrogen bonding with alcohols, water, and urethane systems |
The grade descriptor is therefore an operational tolerance statement, not a difference in molecular identity. Caprolactam-grade cyclohexanone is the highest-purity cut produced for nitrogen-containing polymer intermediates, technical-grade material is a broader multi-purpose intermediate for oxidation and hydrogenation, and solvent-grade material is handled as a formulated component in coatings, inks, and adhesives where solvency and evaporation behavior dominate selection.
What Distinguishes Caprolactam-Grade Purity from Technical and Solvent Bands?
Differences among the three grades are best read through specification limits. Table 2 lists representative published specification bands used in commercial quality agreements and bid documents. The values are not universal; they shift with producer process route, storage age, and analytical method.
| Parameter | Caprolactam-grade | Technical-grade | Solvent-grade |
|---|---|---|---|
| Assay by gas chromatography, area % | ≥ 99.8 | ≥ 99.0 | ≥ 99.0 |
| Water, Karl Fischer ISO 760 | ≤ 0.05 wt% | ≤ 0.10 wt% | ≤ 0.20 wt% |
| Color ASTM D1209 | ≤ 5 Pt-Co | ≤ 10 Pt-Co | ≤ 20 Pt-Co |
| Acidity as acetic acid ASTM D1613 | ≤ 0.005 wt% | ≤ 0.01 wt% | ≤ 0.01 wt% |
| Distillation range ASTM D1078 | ≤ 1.0 °C including 155.6 °C | ≤ 2.0 °C including 155.6 °C | ≤ 2.5 °C including 155.6 °C |
| Cyclohexanol | ≤ 0.1 wt% | ≤ 0.3 wt% | ≤ 0.5 wt% |
| Phenol | ≤ 0.01 wt% | ≤ 0.05 wt% | ≤ 0.10 wt% |
| Non-volatile residue | ≤ 0.005 wt% | ≤ 0.02 wt% | ≤ 0.05 wt% |
The caprolactam-grade limits on water, acidity, and distillation spread are tight because these impurities propagate through oximation and Beckmann rearrangement. Solvent-grade material can tolerate more water and a wider distillation range, but the same limits become binding when the material is used in moisture-sensitive urethane reactions. Technical-grade material is frequently the broadest commercial cut, but it is not automatically dirty; it is defined by fit for oxidation and hydrogenation rather than by coating-solvent performance.
In caprolactam production, cyclohexanone is reacted with an aqueous hydroxylamine salt in a continuous stirred-tank reactor equipped with external circulation cooling. The reaction is exothermic and forms an oxime layer that is separated from the ammonium salt liquor. Temperature and pH profiles are set to avoid oxime hydrolysis; low pH drives hydroxylamine into protonated forms while high pH slows ketone conversion. Aldehydes and α,β-unsaturated ketones present in the ketone feed participate in aldol chemistry and can carry color bodies into the caprolactam product. Water in the ketone is restricted because it changes the phase split and later dilutes the oleum used for rearrangement. Production-scale operating data show that an incoming water content above 0.05 wt% can increase sulfuric acid consumption and raise the energy load of ammonium sulfate recovery. Heat exchangers in the oximation loop are typically sized for a maximum exotherm based on ketone feed rate; a sudden increase in reactive impurities shifts the temperature profile and can initiate off-spec oxime if the cooling system lacks spare duty.
Technical-grade material is used as an intermediate in adipic acid production and cyclohexanol hydrogenation. In adipic acid oxidation, cyclohexanone is introduced into a nitric acid oxidation train; residual cyclohexanol is acceptable because it is also oxidized to adipic acid. Water and non-volatile residues are the main yield penalties. In a nitric acid oxidation system, high water in the ketone feed increases the evaporation load in the product condenser and can reduce nitric acid concentration in the reaction zone. Non-volatile residues accumulate in reboiler and crystallizer legs of the recovery train. Where technical-grade cyclohexanone is hydrogenated to cyclohexanol over copper-based or nickel-based fixed-bed catalyst, sulfur-containing impurities are more restricting than water. Producers requiring low sulfur should request a route-specific analysis. Published data for specific hydrogenation catalyst tolerances across all producers is limited; catalyst supplier qualification protocols remain the binding control.
When Solvent-Grade Material Is Selected for Coating and Resin Systems
Solvent-grade cyclohexanone is specified in vinyl resin coatings, gravure inks, polyurethane adhesives, and epoxy thinning where a medium-to-slow evaporating ketone is acceptable. Its Hansen solubility parameters of δD = 17.8 MPa0.5, δP = 6.3 MPa0.5, and δH = 5.1 MPa0.5 place it near the solubility windows of nitrocellulose, cellulose acetate butyrate, polyvinyl chloride, polyvinyl acetate, and selected acrylic resins. The relative evaporation rate is approximately 0.32 relative to n-butyl acetate at 25 °C; this is lower than methyl ethyl ketone and distinguishes cyclohexanone in spray-applied formulations. In high-solids coatings, the slower evaporation reduces dry spray and improves flow, but it extends open time and can retain solvent in the film after forced drying. Replacement of 20 wt% of an aromatic hydrocarbon diluent with cyclohexanone changes the evaporation profile and may reduce viscosity; the extent is resin-dependent and should be measured by rotational viscometer under ASTM D2196 or ISO 3219. Because cyclohexanone has a low blank viscosity of 2.02 mPa·s at 25 °C, resin interactions dominate the measured curve.
In moisture-cure polyurethane adhesives, water in solvent-grade cyclohexanone is a critical process variable. A water content of 0.20 wt% corresponds to 2 kg of water per 1000 kg of solvent. One mole of water consumes approximately two moles of isocyanate functionality; this represents roughly 222 mol of NCO per 1000 kg batch. Formulators that do not compensate by increasing isocyanate prepolymer may observe viscosity drift, gassing, and reduced adhesion. Solvent-grade material with water above 0.20 wt% is therefore pre-dried over type 3A molecular sieves or diverted to non-isocyanate systems.
In epoxy-amine systems, cyclohexanone is not inert. Primary amine curatives can condense with the ketone to form imines and water, altering hardener stoichiometry and reducing crosslink density. This incompatibility is particularly relevant in high-solids epoxy floor coatings and amine-cured tank linings. Solvent-grade cyclohexanone should be limited to formulations where the amine is pre-reacted into a polyamide or amine adduct, or where the manufacturer has validated the ketone-amine equilibrium. Published kinetic data for all amine hardeners is limited; screening by Fourier transform infrared spectroscopy is used to track imine formation at 25 °C and 50 °C in accelerated storage.
Regulatory classification depends on the end use rather than the grade label alone. Under FDA 21 CFR 175.105, cyclohexanone may be used as a component of adhesives intended for food contact, provided that the adhesive is separated from food by a functional barrier or residual solvent is removed. In the European Union, the substance is registered under REACH and is subject to national workplace exposure limits. Solvent-grade material supplied for coatings, inks, or adhesives must be evaluated for residual solvent in dried films under ASTM D2369 or equivalent volatile organic compound test methods. This is a film-formation and compliance issue rather than a bulk-specification issue.
Storage Stability and Specification-Linked Handling Boundaries
Cyclohexanone is stored in carbon steel or stainless steel tanks fitted with nitrogen blanketing, flame arresters, and grounding. The liquid is flammable with a flash point of 44 °C under ASTM D56, an autoignition temperature of 420 °C, and explosive limits in air of approximately 1.1 vol% to 9.4 vol%. Peroxides can form slowly in air-exposed drums, particularly under ultraviolet light; producers may monitor peroxide content and recommend storage away from heat and direct sunlight. The polar ketone absorbs moisture from humid air. In a drum opened at 60% relative humidity, repeated headspace exchange can raise water content above the original solvent-grade limit. Caprolactam-grade users should sample after each opening and dry or reject the parcel if the water limit is exceeded. Transfer lines should be dedicated or flushed with the same grade; contamination by aromatic hydrocarbons, alcohols, or water can alter downstream performance. Bulk storage does not require agitation because the viscosity at 25 °C is low, approximately 2.02 mPa·s. Heated storage is not required except in regions where ambient temperature approaches the freezing point of -47 °C.
Route-specific impurity profiles persist across all three grades. Cyclohexanone produced by phenol hydrogenation may contain traces of phenol and cyclohexanol. Material produced by cyclohexane oxidation via KA oil carries a different cyclohexanol-to-cyclohexanone ratio and may contain aliphatic acids. These differences influence distillation behavior, color stability, and reactivity with catalysts or isocyanates. Purchasers should require the production route, a chromatographic impurity profile, and the test methods used for water and color. The grade term alone is insufficient for catalyst-sensitive or moisture-sensitive downstream operations.
