Acid Number and Solvent Release Criteria for Gravure Ink Resin Selection

In high-speed rotogravure packaging presses operating between 180 m/min and 300 m/min, residual solvent in printed biaxially oriented polypropylene, polyethylene terephthalate, and oriented polyamide is not governed solely by drying-air temperature. The acid number of the binder resin, expressed as milligrams of potassium hydroxide per gram of resin and determined by non-aqueous potentiometric titration according to ISO 2114:2000 or color-indicator titration under ASTM D974-22 for compatible resin solutions, correlates with the density of carboxylic acid sites available for hydrogen bonding with ethyl acetate, ethanol, and propan-2-ol. A higher concentration of free carboxylic acid groups in an alcohol-soluble polyamide or acrylic resin increases both pigment wetting and adhesion to primed polyester film, but simultaneously raises the equilibrium solvent retention under a given drying-hood residence time. On a typical nine-station gravure line equipped with drying hoods delivering 18 m/s to 25 m/s air velocity and web temperatures of 65 °C to 85 °C, the solvent release rate from a printed ink film is controlled by the coupled diffusion and evaporation of solvent molecules through a vitrifying binder matrix. The relationship is not linear: below the resin acid number threshold that maintains an open polymer network, rapid drying can create a surface skin that traps solvent in the lower half of the ink layer; above that threshold, strong carboxyl-solvent interactions dominate and retention again increases. Converter specifications for high-barrier retort laminates frequently require retained ethyl acetate below 5 mg/m² after 24 h of storage at 25 °C, measured by static headspace gas chromatography with flame ionisation detection according to EN 13628-1:2002 or ASTM F1884-18.

Acid number influences solvent release through two distinct mechanisms: free-volume modification and specific solute-solvent interaction. Carboxylic acid groups increase chain-chain hydrogen bonding, which raises the glass transition temperature and reduces the diffusion coefficient of small solvent molecules; they also form transient hydrogen-bonded complexes with ester and alcohol solvents, increasing the enthalpy of desorption. Both effects are measurable by dynamic headspace methods. In practice, formulators titrate acid number and amine value before blending because the neutralisation state of carboxyl groups changes both viscosity and solvent retention. A resin with an acid number of 6 mg KOH/g dissolved at 35 % solids in an ethyl acetate/ethanol blend may show a DIN 4 mm flow time of 18 s at 25 °C; after titration with a low-molecular-weight amine to neutralise 50 % of the carboxyl groups, the same solution can drop to 15 s, but the solvent release rate may slow further because the resulting ionic groups retain polar solvent more strongly. This neutralisation effect is routinely encountered in production batches when amine-based slip agents or antistatic additives are added downstream. The use of ISO 2431:2019 flow cups and ISO 3251:2019 non-volatile content determinations is standard for incoming resin solutions; the solvent release criterion is more often assessed by a laboratory drying test with a wire-wound drawdown bar and an oven set to 70 °C with horizontal airflow of 10 m/s.

How Does Carboxylic Acid Content Influence Dynamic Headspace Retention in Polyamide-Based Gravure Inks?

Alcohol-soluble polyamide binders derived from dimer fatty acid and aliphatic diamines are specified with acid values typically in the range 2–8 mg KOH/g and amine values from 3 mg KOH/g to 6 mg KOH/g. The carboxylic acid end groups in these resins contribute to pigment wetting on TiO₂ and organic pigments, but they also act as high-energy retention sites for ethanol and ethyl acetate. During a dynamic headspace measurement, printed film samples are sealed in headspace vials, equilibrated at 80 °C for 30 min, and sampled with split injection into a gas chromatograph fitted with a polar or intermediate-polarity column and flame ionisation detector. Industrial experience indicates that the residual ethanol for a polyamide-based white ink printed on 12 µm polyester at 200 m/min can shift from below 3 mg/m² to above 7 mg/m² when the binder acid number is increased from 4 mg KOH/g to 9 mg KOH/g within the same drying tunnel. The mechanism is not solely thermodynamic; carboxyl groups at the film surface accelerate solvent evaporation at the interface while simultaneously increasing the viscosity of the underlying wet film through hydrogen-bonded entanglements. This creates a densified surface layer that reduces the effective diffusion coefficient for remaining solvent. The result is a process window that becomes narrow at line speeds above 220 m/min; the drying-hood temperature must be raised no more than 5 °C to avoid film blistering, while lower temperatures leave ethanol above the desired limit. Resin suppliers therefore report acid number and amine value together because an excess of amine groups can partially neutralise acid groups, altering the solvent retention profile and ink water resistance.

In production, the acid number of a polyamide resin is not a single-value safety factor. Batch-to-batch variation of ±1 mg KOH/g is common for commercial dimer acid-based polyamides, and this variation can shift the dynamic headspace residual ethanol by 1–2 mg/m² under fixed drying conditions. When a converter uses a gravure cylinder with 70 lines/cm² and 28 µm cell depth, the wet film thickness at 40 % solids is approximately 11 µm; at this thickness, the diffusion path length for ethyl acetate is short enough that acid number effects are moderate. At 50 lines/cm² with 35 µm cell depth and 18 µm wet film thickness, the same acid number increase can produce a residual ethanol rise of 3–5 mg/m² because the solvent must travel through a longer path within the binder network. This interaction between engraving geometry and acid number is a practical reason why solvent release trials must be repeated whenever cylinder specifications change. The standard laboratory test for retained solvent under EN 13628-1:2002 requires careful sample handling because residual solvents can diffuse into the packaging film substrate; polyethylene terephthalate and oriented polyamide show different partitioning behaviour. For acid-rich polyamide inks, the highest retained solvent is usually observed in the first printed layer next to the substrate, not in the surface layer.

The drying of a polyamide ink film can be described by a two-stage model. During the initial constant-rate period, solvent evaporates from the liquid surface, and the film temperature remains at the wet-bulb temperature even when the supply air is 70 °C. Carboxylic acid groups influence this period mainly through solution viscosity and surface tension; higher acid number increases both, but the effect on evaporation rate is less than 10 %. Once the film solids exceed approximately 70 %, the system enters a falling-rate diffusion-controlled stage. In this stage, the solvent diffusion coefficient through the partially dried polyamide matrix becomes the limiting parameter, and it is here that carboxyl content has the largest measurable effect. The diffusion coefficient of ethanol in a polyamide film with an acid number of 8 mg KOH/g can be several times lower than in a comparable film with an acid number of 3 mg KOH/g at the same temperature. Because the drying tunnel residence time on a modern gravure press is often only 0.4–0.8 s per colour, the diffusion stage is incomplete when the web exits the hood; residual solvent then continues to desorb slowly from the wound reel, which is why converter specifications are based on samples taken after 24 h storage. If the reel is heat-sealed or laminated before this equilibration, the retained solvent can later migrate into the food-contact side, creating organoleptic defects.

On a production-scale Cerutti R960 rotogravure press printing white polyamide-based ink on 12 µm primed polyester at 200 m/min, the engraved cylinder carries a cell density of 70 lines/cm² and a cell depth of 28 µm. Ink viscosity at the trough is adjusted to 18 s by DIN 4 mm cup at 25 °C using a solvent blend of ethyl acetate, ethanol, and propan-2-ol at 60:30:10 by volume. Drying-hood supply air is heated to 70 °C, with an average impingement velocity of 20 m/s; web residence time inside the drying tunnel is approximately 0.5–0.8 s per colour station. Under these conditions, a resin acid-number shift from 4 mg KOH/g to 9 mg KOH/g changes the measured residual ethanol from less than 3 mg/m² to more than 7 mg/m² when sampled at the rewind and analysed by EN 13628-1:2002. This configuration represents a common packaging line; published data for this specific press and cylinder combination is limited, but the trend is consistent with diffusion-controlled drying measurements reported for alcohol-soluble polyamide films. The conflict is most severe at high line speeds because the top of the ink film reaches its glass transition within 0.2 s, while the lower film fraction remains above the solvent boiling point for an additional 0.4 s. The resulting skin layer is enriched in carboxyl groups if the resin acid number is high, because the lower-molecular-weight acid-terminated chains migrate to the surface during solvent evaporation. This surface segregation improves slip and anti-blocking but worsens re-solubility and can leave a measurable ethanol signature in laminating applications.

When the same ink is diluted to 16 s with additional ethyl acetate, the drying tunnel temperature can be reduced from 70 °C to 65 °C, but the residual solvent after 24 h may remain above 5 mg/m² because dilution increases the wet film thickness and the total solvent load. Conversely, raising the drying-hood supply air from 70 °C to 75 °C reduces retained ethanol by 1.5–2.5 mg/m² for the low-acid resin, but the same increase in temperature for the high-acid resin can produce microblisters if the film surface skins over too quickly. This is a process cliff-edge: the permissible temperature window for the high-acid resin is only ±5 °C, whereas the low-acid resin tolerates ±10 °C. Such differences are not captured by the ink’s flow curve alone; they require coupled measurement of acid number, solvent release by headspace gas chromatography, and visual inspection of the dried film after 24 h lamination. A gravure ink with an acid number outside the 2–8 mg KOH/g range may still print acceptably on a low-speed narrow-web line operating at 80 m/min, but it can fail on a high-speed wide-web packaging press because the drying time is too short to offset the increased solvent-binding capacity. Process engineers therefore specify the acid number of incoming resin lots and perform a headspace solvent test after every cylinder change when the engraved cell depth exceeds 30 µm.

Acid Number, Amine Value, and Adhesion to Primed Polyester and Aluminium Foil

Acid number in gravure binders is not selected purely for drying behaviour; it also determines adhesion strength to primed polyethylene terephthalate and aluminium foil. Carboxylic acid groups can form polar interactions with hydroxyl and carboxyl species on corona-treated polyester and with oxide/hydroxide layers on aluminium. A polyamide resin with an acid number of 6 mg KOH/g generally gives higher adhesion in metal foil lamination than a resin with 2 mg KOH/g, but the stronger interaction lengthens solvent retention and can increase water sensitivity. For retortable laminates, the printed ink must withstand 121 °C steam sterilisation for 30 min without delamination. Here, converters often select polyurethane binders with acid numbers in the range 1–5 mg KOH/g and aliphatic isocyanate crosslinkers, because these systems provide hydrolytic stability and low residual solvent. However, if the acid number is below 1 mg KOH/g, adhesion to primed polyester may be insufficient after lamination, and the ink can peel from the film during pouch forming. Acid number therefore has a lower boundary for adhesion and an upper boundary for solvent release and water resistance; the practical window for a solventborne polyurethane laminating ink is often 3–10 mg KOH/g, depending on the choice of polyol and isocyanate.

Amine value interacts with acid number because commercial alcohol-soluble polyamides contain both acid and amine end groups. The ratio of acid number to amine value influences pigment dispersion, viscosity, and solvent release. In TiO₂-based white inks, an excess of amine groups can displace adsorbed dispersant from the pigment surface and cause seeding; an excess of acid groups can reduce viscosity stability and increase retained ethanol. For a stable polyamide-based ink, resin manufacturers commonly maintain an acid number of 2–8 mg KOH/g with an amine value below 6 mg KOH/g. The amine value is determined by non-aqueous titration with perchloric acid in glacial acetic acid; the sample is first neutralised with a known excess of acid and back-titrated. When a nitrocellulose/polyamide blend is formulated, the nitrocellulose itself contributes negligible acid number but substantial hydrogen-bonding sites from hydroxyl and nitrate ester groups. At 20 wt% polyamide addition, the blend retains more ethyl acetate than the polyamide alone because nitrocellulose raises the glass transition of the film; at 50 wt% polyamide addition, the acid number rises and the film becomes softer but also more solvent-retentive due to carboxyl groups. This non-linear behaviour means that solvent release must be measured at each blend ratio, and extrapolation from either component alone is unreliable.

The data in the table are typical commercial ranges rather than universal specifications, and published data for specific configurations is limited. Acid number alone does not predict solvent release; the glass transition temperature, molecular weight distribution, hydroxyl number, and neutralisation state must be considered. For example, an acid-functional acrylic with a high acid number can release solvent rapidly if its molecular weight is low and its glass transition temperature is below 40 °C, whereas a moderate-acid polyamide with a high glass transition domain may retain more solvent. This is why incoming resin specifications should include glass transition temperature by differential scanning calorimetry according to ISO 11357-2:2020 and molecular weight by gel permeation chromatography using DIN 55672-1:2016. The operational boundary is also sensitive to ambient moisture: pre-drying of raw materials and solvent blends is required when relative humidity exceeds 60 %, because absorbed water competes with solvent for hydrogen-bonding sites on carboxyl groups and slows release.

If Isocyanate-Cured Polyurethane Resins Are Selected for Low Retained Solvent in Laminating Structures

When a converter moves from surface printing to retortable lamination, the retained solvent criterion tightens from 20 mg/m² to below 5 mg/m², and the binder selection often shifts from polyamide to a solventborne polyurethane with an acid number between 1 mg KOH/g and 10 mg KOH/g. The carboxylic acid groups in these polyurethanes are introduced by incorporating dimethylolpropionic acid into the polyol backbone; they provide pigment wetting and adhesion to oriented nylon and primed polyester. Because the acid groups can react slowly with aliphatic polyisocyanate crosslinkers, the formulator must maintain the NCO:OH molar ratio between 1.2:1 and 1.5:1 to compensate for carboxyl consumption. At lower NCO:OH ratios, incomplete crosslinking leaves the ink film soft and solvent-retentive; at higher ratios, unreacted isocyanate may migrate and cause laminate bond failure. The solvent release profile of a polyurethane binder is typically faster than that of alcohol-soluble polyamide because the soft-segment glass transition temperature is below 0 °C, and the acid number is lower. However, the presence of even 3 mg KOH/g of acid in the polyurethane can interact with methyl ethyl ketone or ethyl acetate, and residual solvent after 24 h may still exceed 5 mg/m² if the film is over-printed before the previous layer has released enough solvent.

On a tandem extrusion laminator processing 12 µm polyethylene terephthalate at 150 m/min, the printed polyurethane ink is dried in a hot-air tunnel at 70–80 °C with an air velocity of 15–20 m/s. The solvent release of a polyurethane ink with an acid number of 5 mg KOH/g is usually sufficient to meet the 5 mg/m² limit when the wet film thickness is below 12 µm; however, when a white base layer is printed with a 50 lines/cm² cylinder and 35 µm cell depth, the wet film thickness increases to approximately 18 µm and residual methyl ethyl ketone can remain above 8 mg/m² after the same drying time. In this situation, the acid number is not the sole variable; the molecular weight distribution and the soft-segment content of the polyurethane control the diffusion rate. A resin with a glass transition temperature of -20 °C and an acid number of 5 mg KOH/g will release solvent faster than a resin with the same acid number but a glass transition temperature of 30 °C. The operational boundary is therefore resin-specific: pre-drying of raw materials and solvent blends is required when relative humidity exceeds 60 %, because absorbed water competes with solvent for hydrogen-bonding sites on carboxyl groups and slows release. Avoid combining high-acid polyurethane binders with amine-based slip agents or adhesion promoters, because premature neutralisation forms ionic clusters that increase viscosity and retained solvent.

Solvent release criteria for isocyanate-cured polyurethane gravure inks are often validated by measuring retained solvent before and after lamination. A sample is cut from the printed web at the rewind, sealed in a headspace vial within 10 min, equilibrated at 80 °C for 30 min, and injected into a gas chromatograph with a polar column. The values are reported in milligrams per square metre, but the conversion from chromatographic peak area to mass per square metre depends on the extraction time, temperature, and substrate thickness; laboratories therefore follow EN 13628-1:2002 or ASTM F1884-18 with the same sample size and calibration range. If the printed film is laminated before the solvent release test, the measured values may be lower because the polyurethane adhesive layer acts as a diffusion barrier. This does not mean the solvent has disappeared; it has migrated into the adhesive or the substrate. For retortable structures, a simulated retort test at 121 °C for 30 min can release trapped solvent and cause delamination or odour. The acid number of the polyurethane resin should therefore be controlled within the supplier’s specification, and the incoming resin certificate of analysis should include acid number by ISO 2114:2000, non-volatile content by ISO 3251:2019, and residual isocyanate content by titration. Without these data, a production-scale change in resin lot can shift the solvent release curve sufficiently to fail a high-barrier packaging specification.