Technologies
Solvent-Based Technology
Fast release, wide substrate latitude and the bond strengths flexible packaging needs — with solvent balance and retention as the things that have to be controlled.
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The technology
Solvent-Based Technology
A solvent-based ink is a resin solution carrying pigment, and the film forms purely by evaporation. Nothing reacts. That simplicity is the reason the technology still holds the lamination and surface-print work it holds: it releases fast, it wets low-energy film readily, it takes a wide range of resins and so a wide range of adhesion and resistance behaviour, and it re-dissolves cleanly for wash-up and for re-use of returned ink.
The resin chooses the job. Nitrocellulose gives fast release, gloss and crisp print, and is the backbone of surface-printed work on treated polyolefin. Polyurethane gives the flexibility, thermal resistance and bond strength that a laminated pouch needs, and is what carries a retort or a hot-fill construction. Vinyl and acrylic resins carry specific adhesion cases. Polyamide is used for surface print on polyolefin where slip and gloss are wanted. Most production inks are a designed blend, and the blend is what a Technical Data Sheet is actually describing.
The solvent blend is a second formulation in its own right. A fast blend releases quickly and can trap solvent by skinning the surface before the depth of the film has emptied. A slow blend empties the film properly and costs speed, and on a surface-printed job can leave the print soft at the rewind. What goes in the press-side thinner has as much effect on the finished laminate as what came in the drum.
How it works
Solvent retention is the property that matters
Retained solvent is solvent that never left the film before the job was wound, laminated or sealed. It causes the three faults that bring a flexible-packaging job back: odour in the pack, taint transferred to the product, and bond failure where the retained solvent plasticises the adhesive layer. It is invisible at the rewind and expensive at the customer.
It is controlled, not hoped for. Dryer temperature profile rising through the tunnel rather than flat, genuine air turnover and extraction rather than recirculated saturated air, a solvent blend matched to the speed and the film weight, and a check on the finished reel. Residual solvent is measured by static headspace gas chromatography on a cut sample — our house acceptance for laminated food-contact constructions is typically not more than 5 mg/m² total residual solvent, with individual species and any brand-owner limit applied on top. The number belongs to the construction and the line, so it is confirmed on your own job.
On the press
Lamination bonds and what breaks them
A lamination ink sits between the printed film and the adhesive, so every bond in the pack passes through the ink film. Bond strength is a system property: the ink, the adhesive, the treatment level on both webs, the nip and the cure of the adhesive all contribute. A PU-based ink is used because it carries the load and tolerates the thermal cycle; a nitrocellulose surface ink put into a laminate will usually deliver a bond that looks acceptable on day one and fails after ageing.
Bonds are read by T-peel on 15 mm strips after the adhesive has cured — 24 h for a solventless system, up to 7 days for a two-component solvent-based adhesive — and again after the pack’s own condition: retort, hot fill, freezer, or accelerated ageing at 50 °C. A bond quoted without the ageing condition is not a specification. Typical values for an adhesive-laminated PET/PE construction with our PU inks are in the region of 1.5–3.0 N/15 mm before ageing, qualified on the actual structure.
Process window
| Resin systems | Nitrocellulose, polyurethane, polyamide, vinyl and acrylic, singly or blended. NC for release, gloss and surface print; PU for lamination bond, flexibility and thermal resistance; PA for slip and gloss on surface-printed polyolefin; vinyl and acrylic for specific adhesion and resistance cases. |
|---|---|
| Solvents | Ethyl acetate, n-propyl acetate, ethanol and isopropanol blends. Toluene-free and aromatic-solvent-free across the range; ketone-free grades for food-related work. Press-side thinners are supplied as matched blends, because an arbitrary single solvent changes the release curve and the retention behaviour. |
| Viscosity | 15–20 s DIN 4 (ISO 2431, 4 mm) at 25 °C for gravure; 18–25 s for flexo. Measured at press temperature. Automatic viscosity control with the correct thinner blend, not with whichever solvent is on the gantry. |
| Substrates and treatment | Corona-treated PET, BOPP, PE, CPP, nylon, metallised film, aluminium foil, paper and coated board. Film verified at ≥ 38–42 mN/m with test inks to ASTM D2578 within the shift of printing; treatment decays in the reel and slip additive blooms, so a certificate from the film supplier is not a substitute for a test at the press. |
| Drying | Hot-air tunnel with a rising temperature profile, typically 40–50 °C at entry to 60–80 °C at exit, with genuine air turnover and extraction at the lower explosive limit set by the line’s safety system. A flat, hot profile skins the surface and traps solvent underneath. |
| Residual solvent | House acceptance for laminated food-contact constructions: typically not more than 5 mg/m² total, with individual species limits and any brand-owner limit applied on top. Static headspace GC on a cut sample of the finished reel. The figure belongs to the construction, the line speed and the dryer, so it is confirmed on your own job before it becomes a specification. |
| Lamination bond strength | Indicative 1.5–3.0 N/15 mm for an adhesive-laminated PET/PE construction with our PU-based inks, T-peel at 300 mm/min after full adhesive cure. Retort, hot-fill and frozen constructions are qualified against their own condition, and the value after ageing is the one that matters. |
| Film weight | Typically 1.2–2.5 g/m² dry for gravure process colours and higher for solid white. Heavier films empty more slowly and raise the retention risk, so coverage, speed and dryer capacity are set together. |
| Cylinder and anilox | Gravure: electromechanically engraved chrome, typically 70–120 lines/cm with stylus angle and cell depth chosen for the tone and the ink. Flexo: 300–500 lpi for process, 200–300 lpi for solids. Doctor blade pressure and angle affect laydown as much as viscosity does. |
| Resistance testing | Cure and resistance are read on the converted structure: 100 dry rub cycles at 1.8 kg with no transfer (Sutherland block, 23 °C / 50 % RH), scuff and block resistance on the reel, and where the pack requires it, seal strength, retort survival and product-contact resistance on filled packs. |
| Emissions and recovery | Solvent-laden air is handled by the converter’s abatement or recovery system — regenerative thermal oxidation or activated-carbon recovery. Ink formulation affects the load: lower film weight, higher pigment strength and a matched thinner blend reduce solvent throughput for the same print. |
| Storage and shelf life | 12 months in closed original containers at 15–25 °C in a designated flammable store, away from ignition sources and direct sun, earthed and bonded when decanting. Handling detail is on the Safety Data Sheet for each grade. |
Where it is used
Products and papers that build on this technology.
Solvent-Based Flexo Inks
PE bag surface print, cement bags and OPVs.
Solvent-Based Gravure Inks
PU lamination inks for flexible pouches.
Functional Coatings
Heat-seal lacquers, barrier coatings and primers.
Flexible Packaging
The market these technologies serve.
Fault finding
Where a solvent-based job usually goes wrong, and what to change.
Odour or taint in the finished pack
Retained solvent until proved otherwise. Cut a sample from the reel and run static headspace GC rather than judging by nose. If retention is high, correct the dryer profile first — rising, not flat, with real extraction — then slow the blend down, then reduce film weight. A recirculating dryer running on saturated air will not empty a film at any temperature.
Bond strength fails after ageing but passes at the rewind
Either retained solvent plasticising the adhesive, or the wrong resin in the ink. Measure residual solvent first. If it is low, check that the ink is a PU-based lamination grade rather than a surface-print NC ink, and confirm the adhesive coat weight and cure time. Test T-peel after the pack’s real ageing condition, not only at 24 h.
Print is soft or blocks in the reel
Film not empty at the rewind. Check dryer exit temperature and extraction, reduce winding tension so a soft film is not forced into contact, and let the reel stand before slitting. If the job must run at speed, move to a faster solvent blend and accept the higher retention risk only with a headspace check to confirm it.
Ink dries on the cylinder or the plate
Blend too fast for the conditions, or the ink is running warm. Move a fraction of the blend to the slower component, check that the duct and pan are not being heated by the dryer, and confirm viscosity at running temperature rather than at the start of the shift.
Poor adhesion on treated film
Treatment, in almost every case. Test with dyne pens or ISO 8296 inks on the reel you are about to run: corona decays and slip additive blooms over weeks in store. Below 38 mN/m, re-treat in line. If treatment is good and adhesion still fails, the resin system is wrong for the film, not under-dosed.
Colour drifts through a long run
Solvent balance. As the fast fraction evaporates preferentially from an open pan, the remaining ink changes both viscosity and pigment dispersion, and the colour moves with it. Keep the pan covered, recirculate, dose with the matched thinner and control viscosity automatically. Correcting colour with pigment while the blend is drifting chases the fault.
Key points
Fast release
Evaporative film formation: high line speeds on gravure and CI flexo.
Lamination bonds
PU systems built for the thermal and mechanical load of a flexible pack.
Toluene-free
Toluene-free and aromatic-free throughout; ketone-free grades for food-related work.
Retention controlled
Residual solvent measured by headspace GC, not assumed from the dryer setting.
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