Free DEP Content in Stored Nitrocellulose Gravure Ink Films

During extended warehouse storage of printed reelstock, nitrocellulose-based gravure ink films containing diethyl phthalate as a plasticizer undergo slow physical re-equilibration that redistributes the plasticizer between the nitrocellulose matrix, pigment-binder interfaces, and the film surface. Gravure inks based on nitrocellulose with a nitrogen content of 10.7–12.2 wt% are dried at web speeds of 150–450 m/min on multi-colour presses, leaving an ink film thickness commonly between 1 µm and 5 µm on substrates such as corona-treated oriented polypropylene, polyethylene terephthalate, and paper. Diethyl phthalate has CAS number 84-66-2, a molar mass of 222.24 g/mol, and a boiling point of approximately 298–299 °C at 101.3 kPa; it is not covalently bonded to the nitrocellulose macromolecule. The analytical term “free DEP” is therefore an operationally defined fraction: the plasticizer that can be removed from comminuted film under mild extraction without complete dissolution of the nitrocellulose or thermal stress sufficient to degrade the polymer. This fraction is the migration-relevant pool because the molecule must diffuse through the binder to reach a food-contact surface or packaging interface. Quantification of free DEP in stored films is driven by the need to demonstrate conformity with Commission Regulation (EU) 10/2011 Annex I Table 1 and Annex V, and with FDA 21 CFR §175.300 where nitrocellulose is permitted as a resinous and polymeric coating component. Stored film samples often display surface blooming, odour development, or changes in block resistance when the free plasticizer fraction exceeds the compatibility limit of the nitrocellulose matrix; such changes are more pronounced in high-gloss, low-pigment films printed at high solvent dilution.

How Does Residual Solvent Influence the Extractability of Diethyl Phthalate?

Organic solvents retained in dried gravure ink films act as temporary plasticizers and can reduce the activation energy for DEP diffusion, thereby increasing the amount of plasticizer obtained during short room-temperature extraction. The principal retained solvents in nitrocellulose gravure systems are ethyl acetate, ethanol, isopropanol, and n-propyl acetate; their boiling points span 77.1–101.6 °C, and their polarities overlap with the unmodified hydroxyl groups of nitrocellulose. When residual solvent content, determined in accordance with ASTM D2369-20 or ISO 11890-2:2020, exceeds 2.5 wt% of the dried ink film, the film is notably softer and the free DEP fraction determined by 24 h methyl tert-butyl ether extraction can be elevated relative to a solvent-free reference. This effect is especially problematic when printed reels are tested within days of press output, because solvent concentration gradients persist through the film thickness and plasticizer diffusion follows a non-Fickian case-II front in the swollen surface layers. Conversely, residual solvent below approximately 0.5 wt% permits the nitrocellulose network to densify and reduces the extractability of DEP under mild conditions; the same total DEP content may then report a lower free concentration unless the extraction time or temperature is increased. The threshold is not a single fixed value because it depends on nitrocellulose nitrogen content, pigment volume concentration, and the ratio of fast to slow solvents retained in the film. Laboratories therefore record residual solvent content alongside free DEP data to avoid misclassifying a formulation as underplasticized or overplasticized. A robust testing sequence for stored gravure films includes ASTM D2369-20 volatiles determination, EPA 3540C Soxhlet extraction for total DEP, and a parallel 24 h mild extraction at 22 ± 2 °C using methyl tert-butyl ether for the free fraction.

When stored films are exposed to cyclic relative humidity in uncoated paperboard or in polyethylene wrappers with high water vapour transmission rates, moisture uptake by nitrocellulose modifies the hydrogen-bond network and can force DEP toward the film surface. Cycling between 30% RH and 80% RH at 23 °C produces dimensional changes that exceed those of the underlying polyester or polypropylene substrate, generating microcracks at the ink-substrate interface and creating pathways for plasticizer migration. Nitrocellulose is hygroscopic due to residual hydroxyl groups; absorbed water acts as an antiplasticizer at low concentrations but as a plasticizer above approximately 2 wt% moisture on dry film, altering the activation volume for DEP diffusion. Surface bloom can be detected by attenuated total reflectance infrared spectroscopy when the carbonyl stretch of phthalate esters at 1715–1725 cm⁻¹ intensifies relative to the nitrocellulose nitrate band near 1650 cm⁻¹. The free DEP content under these conditions may be operationally separated into a surface fraction, collected by a dry wipe or adhesive tape lift, and a bulk fraction, extracted after film dissolution in a solvent blend such as acetone:ethyl acetate 1:1 v/v. If the surface fraction exceeds 0.05 µg/cm², visual haze and blocking of stored reels are commonly observed, though published data for this specific configuration is limited. Films wound under high radial pressure from differential rewinding, typically 0.2–0.5 MPa at core layers, show accelerated plasticizer exudation because the compressive stress increases chemical potential at the film-film interface. Conditioning samples for 48 h at 50 ± 5% RH and 23 ± 2 °C before extraction reduces variability caused by transient humidity history but does not reverse permanent film densification.

Chromatographic Resolution of Diethyl Phthalate from Co-Extracted Phthalate Contaminants

Gas chromatography with flame ionisation detection remains the default screening tool for free DEP in stored nitrocellulose films because of its tolerance for low injection volumes and its ability to separate common ortho-phthalates in under 20 min. A typical program uses a 30 m × 0.25 mm × 0.25 µm film column with a 5% phenyl/95% dimethylpolysiloxane stationary phase, splitless injection at 250 °C, and a temperature ramp from 80 °C to 300 °C at 15 °C/min. Retention time locking may be performed against an internal standard of deuterated diethyl phthalate or against benzyl benzoate, and quantification is carried out in selected ion monitoring mode when mass spectrometry is available; for electron ionisation, the common phthalate fragment ion at m/z 149 is used as a screening ion, while m/z 177 and m/z 222 support confirmation of DEP. The major analytical difficulty is not detector response but co-extraction of other phthalates and nitrocellulose degradation products that co-elute near DEP. Diisobutyl phthalate, dibutyl phthalate, and phthalate impurities from packaging adhesives can be present in recycled substrates and can produce elevated total phthalate readings unless chromatographic resolution is verified with a mid-column polarity confirmatory column such as a 50% phenyl/50% dimethylpolysiloxane stationary phase. Table 1 summarizes extraction and instrumental approaches used for free DEP in gravure films. The choice between total and free DEP measurements is not interchangeable: exhaustive Soxhlet extraction with a solvent capable of dissolving nitrocellulose reports total DEP, while a cold solvent rinse reports only weakly surface-associated DEP.

Table 1. Analytical extraction and instrumental conditions for free DEP in stored nitrocellulose gravure ink films.

In a typical production-control laboratory, a 100 cm² printed film specimen is cut into 5 mm × 5 mm pieces and extracted without grinding to avoid thermal shear heating that can cause local nitrocellulose decomposition. Grinding is deliberately avoided because the energetic impact of a laboratory mill can generate local hot spots above 160 °C, at which point nitrocellulose begins to autocatalyze through nitrate ester cleavage. Sample comminution is therefore performed with stainless steel scissors or a cryogenic knife mill under liquid nitrogen, keeping the polymer below 0 °C to embrittle the film without depolymerization. The cut film is weighed to 0.1 mg, combined with a known mass of internal standard, and extracted in an amber vial with a polytetrafluoroethylene-lined cap. For free DEP, the extraction time and temperature are kept deliberately low to avoid swelling the nitrocellulose matrix; a common protocol uses methyl tert-butyl ether at 22 ± 2 °C for 24 h, with orbital agitation at 150 rpm. The extract is filtered through a 0.45 µm polytetrafluoroethylene syringe filter and injected without concentration if the expected DEP level lies between 0.1 µg/mL and 50 µg/mL. Matrix-matched calibration is required because nitrocellulose extracts contain polar degradation products that alter injection port activity and detector response. A blank nitrocellulose film, prepared without DEP, is used to confirm that no phthalate contamination arises from solvents, septa, vials, or the substrate itself. When the measured free DEP content is near a regulatory migration limit, the sample is re-extracted by pressurized fluid extraction according to EPA 3545A to determine total DEP and to verify that the mild extraction has not left a hidden reservoir of migration-capable plasticizer.

When High-Performance Liquid Chromatography Is Preferable to GC-FID for Aged Nitrocellulose Samples

Liquid chromatography with ultraviolet or diode-array detection is preferred when stored gravure films contain nitrocellulose degradation products that can condense in a gas chromatographic inlet or produce reactive nitrogen oxides during thermal desorption. In reversed-phase mode, a C18 column of 150 mm × 4.6 mm × 5 µm, a mobile phase of acetonitrile:water 60:40 v/v ramped to 90:10 v/v over 12 min, and a flow rate of 1.0 mL/min provide baseline separation of diethyl phthalate from common packaging contaminants such as diisobutyl phthalate and dibutyl phthalate. Ultraviolet detection at 275 nm exploits the aromatic phthalate ester chromophore, while the nitrate ester of nitrocellulose absorbs only weakly at this wavelength, reducing matrix interference. HPLC is also the technique of choice when the sample extract contains high-boiling slip agents, optical brighteners, or UV absorbers that would require injector maintenance after repeated GC runs. The principal limitation of HPLC is its lower chromatographic resolution relative to capillary GC for structural isomers; therefore, a positive finding near a regulatory threshold should be confirmed by GC-MS in selected ion monitoring mode with retention time and ion ratio criteria. The choice of extraction solvent remains critical: acetonitrile is compatible with direct injection into reversed-phase HPLC but may not fully dissolve or swell nitrocellulose, so free DEP recoveries can be lower than with methyl tert-butyl ether unless the film is first cut to fine fragments and extracted for a longer period of 48 h.

Stored Nitrocellulose Films Release Free DEP Only When the Polymer Matrix Is Swollen

Because the operational definition of free diethyl phthalate depends on matrix swelling, nitrocellulose films form dense hydrogen-bonded networks that restrict plasticizer diffusion in the dry state. Diffusion coefficients for low-molecular-weight plasticizers in plasticized nitrocellulose are generally reported in the range of 10⁻¹³ cm²/s to 10⁻¹⁵ cm²/s at 25 °C, depending on nitrogen content, plasticizer loading, and residual solvent; published data for this specific configuration is limited. A dry film with a glass transition well above room temperature behaves as a glassy barrier, and only the DEP molecules near the surface or in microvoids are available for extraction. When the film is exposed to a solvent that swells the matrix, the free volume increases and the apparent free DEP fraction rises. This phenomenon creates a methodological conflict: extraction conditions that are too mild may underreport the mobile plasticizer pool, while conditions that are too aggressive dissolve nitrocellulose and report total DEP, including plasticizer that would not migrate under food-contact conditions. The threshold between these regimes is not abrupt but occurs over a solvent-activity band in which the nitrocellulose passes through a glass-to-gel transition. For methyl tert-butyl ether, this transition is observed near 22–25 °C for films containing more than 10 wt% DEP, whereas for ethanol the swelling threshold is lower because ethanol interacts with nitrocellulose hydroxyl groups. The practical consequence is that free DEP measurements should always be accompanied by a statement of extraction solvent, time, temperature, and the final moisture content of the film. Without these parameters, comparison between laboratories is not technically valid.

European Commission Regulation (EU) 10/2011 Annex I Table 1 and Annex V provide the compliance framework for plasticizer migration from food-contact materials, but the regulatory position of diethyl phthalate is not identical to that of the four ortho-phthalates restricted under REACH Annex XVII entry 51 and RoHS Delegated Directive (EU) 2015/863. Diethyl phthalate is not generally subject to the REACH Annex XVII entry 51 restrictions that apply to dibutyl phthalate, benzyl butyl phthalate, di(2-ethylhexyl) phthalate, and diisobutyl phthalate in toys and childcare articles. In food-contact applications, nitrocellulose-based coatings may be evaluated under FDA 21 CFR §175.300, which sets extractive limitations for resinous and polymeric coatings, and under Commission Regulation (EU) 10/2011 where the substance is listed in Annex I Table 1 with a specific migration limit if applicable. Compliance testing for gravure ink films is not performed on the dry film alone; the printed substrate is tested as a final food-contact article under the intended worst-case time and temperature conditions defined in Annex V, using food simulants such as 10% ethanol for aqueous foods, 3% acetic acid for acidic foods, and vegetable oil or 95% ethanol for fatty foods. The measured migration of DEP is expressed in mg/kg food simulant, and the analytical method must demonstrate a limit of quantification below the applicable limit. Table 2 summarizes the regulatory instruments most often referenced in free DEP testing of stored nitrocellulose gravure ink films. Because national legislation may impose additional restrictions, compliance cannot be inferred solely from total or free DEP content in the dry film; migration testing under end-use conditions remains the reference method.

Table 2. Regulatory instruments relevant to diethyl phthalate in stored nitrocellulose gravure ink films.