Water-Based Gravure Inks for Flexible Packaging

Executive summary

Gravure is the highest-quality, highest-volume process for flexible packaging, and it has been almost entirely solvent-based since its inception: toluene, ethyl acetate and alcohol blends dry fast, wet polyolefin films and give the resolubility that gravure’s open cells need. Water changes every one of those properties. This paper explains the four technical barriers to water-based gravure on film — drying, wetting, resolubility and lamination bond — and how Uniform Industries’ water-based gravure inks for flexible packaging address them, delivering surface-print and lamination inks with zero retained solvent and VOC below regulatory thresholds.

1. Why water is hard

Water has a latent heat of evaporation of 2260 kJ/kg, roughly five times that of ethyl acetate, and a surface tension of 72 mN/m against 24 mN/m for the solvent. The first fact means the dryer must deliver five times the energy per kilogram of carrier; the second means the ink will not wet a polyolefin film at 38 – 42 mN/m without help. Add that water-based binders must be alkali-soluble to stay dispersed and become water-resistant only after the amine leaves the film, and that dried water-based ink does not redissolve in water the way solvent ink redissolves in solvent, and the four barriers are defined.

2. Drying — engineering the film, not just the dryer

The energy problem is real but overstated. Water-based gravure inks are formulated at 40 – 50 % solids, against 25 – 35 % for solvent inks, so less carrier is applied per unit area; with cells engraved 15 – 25 % shallower than for solvent, the water load per square metre falls by half. The remaining requirement is met by dryer airflow rather than temperature: high-velocity impingement at 70 – 90 °C with adequate exhaust removes water faster than higher temperature with poor airflow, and keeps heat-sensitive films flat. Co-solvents — a few per cent of ethanol or isopropanol — lower the surface tension and the initial evaporation energy without moving the ink out of low-VOC classification.

Figure 1 – Evaporation energy per m² falls steeply with ink solids (illustrative).

3. Wetting and adhesion on film

Surface tension is reduced from 72 to about 30 mN/m with surfactant packages and co-solvent, but wetting is only the beginning. Adhesion on corona-treated PE and BOPP comes from binder selection: acrylic-polyurethane hybrids and specific styrene-acrylic emulsions with functional groups that interact with the oxidised film surface, supported where necessary by a water-based primer. In-line corona to 40 – 42 mN/m is mandatory, and films with migrating slip additives must be qualified individually.

4. Resolubility and press stability

Gravure cells that dry in during a stoppage print a starved, mottled image. Solvent inks resolve instantly; water-based inks must be formulated to do so, with a balance of alkali-soluble resin (which redissolves in fresh ink) and emulsion polymer (which gives resistance once dry). Uniform Industries’ films are engineered with a resoluble resin fraction and a pH-stable amine system, so that cells that skin during a short stop are cleaned by the ink on restart; a pH stabiliser and a defoamer are supplied for long runs.

5. Lamination inks

For reverse-printed laminates the ink is buried between film and adhesive, and the measure of success is bond strength. Water-based lamination inks are formulated with binders that are compatible with both solvent-free and solvent-based polyurethane laminating adhesives, with pigments that do not bleed into the adhesive, and with the low retained-water content that prevents delamination during the adhesive’s cure. Because there is no retained solvent, the classic gravure defect of laminate odour disappears entirely.

Figure 2 – Carrier applied and VOC released per m² at equal wet film (illustrative).

6. The sustainability arithmetic

A solvent gravure press printing 12 g/m² of ink at 30 % solids releases about 8 g/m² of solvent; on a 1.2 m web at 200 m/min that is 115 kg of VOC per hour to be incinerated or recovered. The water-based equivalent releases water and a few grams of ethanol. For the converter this means no solvent-recovery plant or thermal oxidiser, a pressroom below occupational exposure limits without solvent-vapour monitoring, no retained-solvent testing on food packaging, and a printed reel that can be laminated or slit without an odour hold.

7. Uniform Industries water-based gravure inks for film

The range comprises surface-print inks for treated PE, BOPP and PET, lamination inks for BOPP and PET reverse printing, high-opacity whites, and a matched set of extenders, pH stabiliser, defoamer, retarder and primer. Inks are supplied press-ready at 18 – 22 s (Zahn 2) and are matched to customer shades in our laboratory; each series has a technical data sheet with cylinder engraving, drying, adhesion and lamination-bond data.

Key takeaways

Water’s high heat of evaporation is offset by higher solids and shallower cells; airflow, not temperature, dries the ink.

Wetting and adhesion on film are binder and surface-treatment problems, both solvable.

Resolubility must be formulated in; it does not come for free as in solvent inks.

Zero retained solvent removes VOC, odour and the recovery plant in one step.

References: ISO 8296 (film surface tension); ASTM F88 / F904 (seal and bond strength); ISO 11890 (VOC content of coatings).