Products
Solvent-Based Flexo Inks
A physically drying flexo system for the work where the substrate is non-polar and the press is fast: surface-printed polyethylene bags with their overprint varnishes, and high-coverage cement sacks on kraft and woven polypropylene.
Flexographic inks
Physical drying, non-polar substrates, and speed
A solvent-based flexo ink dries by evaporation, not by reaction. Pigment is dispersed in a resin — nitrocellulose, polyamide or polyurethane, alone or blended — dissolved in an alcohol or alcohol/ester system. Between stations the hot-air tunnel removes the solvent and the resin lays down as a film. Nothing crosslinks unless the grade is a two-component polyurethane. Because the dried film re-dissolves in its own solvent, viscosity is corrected at the press, wash-up is straightforward, and six to eight colours trap wet-on-dry at high web speed. The single thing the whole system depends on is that the solvent actually leaves the film rather than being sealed into it.
The first named application is surface printing of polyethylene bags. Polyethylene has no polar groups at its surface, so an untreated film sits at roughly 30 to 31 dyn/cm and the ink beads instead of wetting. Corona treatment oxidises the surface and raises its energy; 38 dyn/cm (mN/m) is the working minimum for surface print, and below it adhesion is unreliable no matter which ink is used. Above it, a polyamide-rich resin keys to the oxidised surface and gives a film flexible enough to survive what a bag goes through — creasing, filling, stacking and rubbing against the bag next to it. The print is on the outside with nothing over it, which is why the overprint varnish is not a finishing touch but part of the specification: it is the wear layer, it sets gloss or matt, and it carries the slip additive that fixes the coefficient of friction the bagging line needs.
The second is cement bag printing, on multiwall kraft paper sacks and on woven polypropylene. This is heavy solid coverage — brand, grade and safety text across a large area — run at sack-line speed, so pigment loading has to be high and solvent release fast enough that the deposit is dry at the rewind rather than merely dry on top. Kraft is absorbent and needs holdout, or the colour sinks and strength is lost; woven polypropylene is non-polar like PE and needs the same corona treatment and the same check at the press. The finished sack is then stacked, dragged across a pallet, loaded, transported and often stored in daylight, so rub resistance and light-fastness carry equal weight with colour. Shade ranges for this work are built on high light-fastness pigments and are heavy-metal-free.
The trade-off is worth stating plainly. Solvent systems are chosen for two reasons: adhesion on difficult non-porous substrates, and speed. They are not chosen because they are cleaner or cheaper. They bring a flammable-liquid inventory, area classification and earthing during transfer, lower-explosive-limit control in the dryers, wash-solvent distillation, and either an abatement route or a solvent management plan where consumption passes the regulatory threshold. Where the substrate is absorbent and the job does not carry the line speed, rub and light-fastness demands of a filled sack — general board, corrugated and much paper work — the water-based flexo route is a genuine alternative and frequently the better one, and we will say so. Where the substrate is polyethylene or woven polypropylene, or where a multiwall kraft sack has to be printed at sack-line speed and then survive stacking, dragging and daylight, the solvent route earns its place. Every product carries a Technical Data Sheet, a Safety Data Sheet and a Regulatory Summary.
The range
These are series descriptions rather than grade names. Specification for this range is being finalised, so nothing below should be read as a fixed product claim.
UniFlex-210PA
Polyamide surface print, for corona-treated polyolefin film and bags. Alcohol-led, high gloss, keyed to the oxidised surface, flexible enough to stay intact through creasing, filling and stacking. Supply 50 ± 10 s DIN 4 at 25 °C, press 16–20 s, flash point 12–16 °C.
UniFlex-230NP
Nitrocellulose/polyamide blend, for a wider substrate window: treated polyethylene, BOPP, PET and coated paper. Harder film than straight polyamide, with better block and heat resistance where the job meets a sealing jaw or a warm reel. Flash point 2–10 °C.
UniFlex-250PU
Two-component polyurethane, isocyanate-crosslinked for chemical, heat and abrasion resistance beyond a physically dried film. Ester-only solvent system; mixed at the press with UniFlex-259HD because pot life is 6–8 h at 30 °C.
UniFlex-259HD
Aliphatic polyisocyanate hardener for the 250PU series. Added at 3–5 % w/w on ink. 1 kg and 5 kg packs, 6 months sealed, moisture sensitive.
UniFlex-270KS
Kraft sack series. High pigment loading with controlled penetration so colour stays on the surface of an absorbent multiwall kraft. Solvent balance set for release at sack-line speed; built on Blue Wool 7–8 pigments.
UniFlex-280WP
Woven polypropylene sack series, for corona-treated woven PP and BOPP-laminated woven stock. Adhesion to the tape surface plus the rub resistance a filled, stacked and dragged sack needs. Blue Wool 7–8 pigments.
UniFlex-290OG
Overprint varnish, gloss. The wear layer over a surface print: sets gloss, takes the scuff, and carries the slip additive at the level the bagging line’s coefficient of friction requires. Supply 30 ± 5 s DIN 4 at 25 °C.
UniFlex-291OM
Overprint varnish, matt. Same duty as 290OG at a controlled low gloss; matting agent raises the effective viscosity, so it is run at the top of the press window rather than thinned into it.
UniFlex-295RD
Reducer, matched to each series’ own solvent blend. Ester-only version supplied for the 250PU series.
UniFlex-296SL
Slow blend for hot press halls above 35 °C, n-propanol and n-propyl acetate rich, to stop the film skinning before the solvent is out.
UniFlex-297EX
Extender varnish, for dropping colour strength without thinning the film or changing the solvent balance.
Product data
| Process | Flexography — central-impression, stack and in-line presses; anilox-metered, doctor-bladed. Web speeds 100–400 m/min. |
|---|---|
| Solvent system — house position on ketones | Toluene free, aromatic free and ketone free across every grade on this page. No MEK, MIBK, acetone or cyclohexanone is used in any flexo grade, food contact or not. The carriers are ethanol, n-propanol, ethyl acetate and n-propyl acetate only. This is the house position across the whole solvent range, flexo and gravure alike: ketone free is the standard, not a food-contact option. |
| Solvent system — UniFlex-210PA, polyamide surface print | Ethanol 60–70 %, n-propyl acetate 25–35 %, n-propanol to balance, w/w as supplied. Press reducer UniFlex-295RD is the same blend. UniFlex-296SL substitutes n-propanol for part of the ethanol for press halls above 35 °C, where the film is skinning before the solvent is out. |
| Solvent system — UniFlex-230NP, nitrocellulose/polyamide blend | Ethanol 45–55 %, ethyl acetate 30–40 %, n-propyl acetate 10–15 %, w/w as supplied. The nitrocellulose needs the ester to dissolve; the ethanol carries the polyamide and sets the release rate. |
| Solvent system — UniFlex-250PU, two-component polyurethane | Ester only: ethyl acetate 70–80 %, n-propyl acetate 20–30 %, w/w. Alcohol is excluded from the ink and from the reducer once hardener is in, because hydroxyl reacts with the isocyanate and takes the crosslink out of the film. An alcohol reducer added at the press is the usual reason a two-component job fails its resistance test. |
| Solvent system — UniFlex-270KS and UniFlex-280WP, sack series | Ethanol / n-propyl acetate about 65:35 w/w with 5–10 % n-propanol. The slower alcohol is there for holdout: it keeps the pigment on the surface of an absorbent multiwall kraft instead of letting it travel in with the solvent. |
| Supply viscosity | Process and line shades 50 ± 10 s. Opaque white and sack solids 70 ± 15 s. Two-component base before hardener 45 ± 10 s. Overprint varnishes 30 ± 5 s. All DIN 4 mm to ISO 2431 at 25 °C. |
| Press viscosity | Process and tone work on a 360–500 l/cm anilox: 16–18 s. Line and solid on 180–300 l/cm: 18–20 s. Kraft and woven-sack solids on 100–160 l/cm: 20–22 s. Overprint varnish: 18–22 s. DIN 4 mm to ISO 2431 at 25 °C, set to anilox and web speed. Letdown from supply viscosity is typically 25–35 % w/w of the matching reducer. |
| Viscosity — method note | Quote the cup with the number. A DIN 4 second is not a Ford B4 second and neither is a Zahn 2 second. Sample and cup both at 25 ± 1 °C, cup calibration checked. Efflux readings taken at whatever the press hall happens to be, with no temperature correction, are the commonest reason a shade drifts across a shift. |
| Solids and density, as supplied | Transparent and process shades 32–40 % w/w solids, 0.92–1.02 kg/l. Opaque white 48–56 % w/w, 1.25–1.35 kg/l. Sack shades 40–48 % w/w. Overprint varnishes 30–36 % w/w. Solids by ISO 3251, 1 g for 1 h at 105 °C. |
| Anilox and dry deposit | Indicative 3.5–6 cm³/m² for process work and 6–12 cm³/m² for solids, whites and sack coverage; dry deposit typically 1–3 g/m², rising to 2.5–3.5 g/m² on a kraft solid where holdout is being bought with weight. |
| Two-component addition and pot life | UniFlex-259HD aliphatic polyisocyanate adduct at 3–5 % w/w on ink. Pot life 6–8 h at 30 °C and 4–5 h at 35 °C, taken as the point at which press viscosity has risen 5 s on DIN 4. Hardened ink is not returned to stock. Full property development 5–7 days at 25 °C, so a resistance test run at the delivery is testing the wrong film. |
| Light-fastness — method | Filtered xenon arc to ISO 12040, rated against the ISO 105-B02 blue wool references 1 to 8. The ratings below are on a full-tone print at 1.5–2.5 g/m² dry on the stated substrate, unvarnished. A 1:25 reduction with extender or white loses one to two steps. An overprint varnish carrying a UV absorber gains up to one step, and is not assumed unless it is specified. |
| Blue Wool — process yellow (PY12, PY13, PY83 diarylide) | 4–5 full tone, 2–3 at 1:25. Not specified for outdoor sack work. |
| Blue Wool — outdoor yellow and orange (PY151, PY154, PO64 benzimidazolone) | 7–8 full tone, 6–7 at 1:25. |
| Blue Wool — process magenta (PR57:1 rubine) | 4–5 full tone, 2–3 at 1:25. |
| Blue Wool — lake and warm reds (PR53:1, PR48:2) | 3–4 full tone, 2 at 1:25. Indoor and short-life work only. |
| Blue Wool — outdoor reds (PR122 and PR202 quinacridone, PR254 DPP) | 7–8 full tone, 6–7 at 1:25. |
| Blue Wool — process cyan and green (PB15:3, PG7 phthalocyanine) | 7–8 full tone, 6–7 at 1:25. |
| Blue Wool — reflex blue and violet (PB15:1 with PV23) | 5–6 full tone, 4–5 at 1:25. |
| Blue Wool — black and white (PBk7 carbon black, PW6 rutile titanium dioxide) | 8 full tone; black 7–8 at 1:25. White is rated 8 but yellows under heat rather than under light, which is a separate test. |
| Light-fastness — the sack shade range | Cement and fertiliser sack shades are built only on pigments rating 7–8 in full tone: phthalocyanine, benzimidazolone, quinacridone, DPP and carbon black. The published floor for outdoor sack work is therefore 7–8, not 6–8. A 6 is accepted only where a brand shade cannot be matched inside 7–8, and it is then named on that product’s own data sheet rather than left inside a page range. |
| Surface energy required | 38 dyn/cm (mN/m) minimum on polyethylene and woven polypropylene for surface print, measured at the press with test inks to ISO 8296 on the reel that is about to run, not taken from the film certificate. 40–44 mN/m where the print is later laminated. |
| Adhesion and rub — in-house checks | Adhesion: straight pull with 3M 610 or equivalent tape across a solid, 24 h after printing, reported as the percentage of ink retained; cross-hatch is not used on a 2 µm film on polyethylene because the blade cuts the substrate. Rub: Sutherland rub tester, 4 lb (1.8 kg) block, 100 cycles at 23 °C and 50 % RH — the house load and cycle count across every range, since a 100-cycle pass at 2 lb is not comparable with one at 4 lb. The rubbed faces differ with the application and the reason is stated with the result: print-to-print on sack and bag work, because that is how a filled sack rubs in a stack, and print against unprinted stock on sheet-fed work. Typical results at 1.5–2.5 g/m² dry on treated polyethylene: a varnished surface print shows no significant transfer at 100 cycles and an unvarnished one at 25–50 cycles. Both are typical values on the stated construction and are confirmed on the customer’s own film, varnish and filled sack. |
| Handling — classification | Flammable liquid, GHS Category 2, H225, highly flammable liquid and vapour. Transport UN 1210, Printing ink, flammable, Class 3, Packing Group II. Class A petroleum under the Petroleum Rules 2002 because the flash point is below 23 °C, so bulk storage at the converter is licensed. |
| Flash point by series | UniFlex-210PA and the alcohol-led sack series 12–16 °C. UniFlex-230NP and the ester-rich blends 2–10 °C. UniFlex-250PU, ester only, −4 to +4 °C. Overprint varnishes follow the series they match. Abel closed cup, ISO 13736. The figure on the individual Safety Data Sheet is the one to use for area classification; the range here is the class, not the product. |
| Storage | 5–35 °C, below 30 °C where the store allows it, out of direct sunlight, containers closed and upright, in a flammable-liquids store away from ignition sources and oxidising agents. Earth and bond the container and the receiving vessel before transfer and keep them bonded throughout — a static discharge during decanting is the realistic ignition source, not the press. Hardener is moisture sensitive: reseal at once and never decant it back into a part-used can. |
| Pack sizes | 1 kg and 5 kg lacquered tins for shade and match work. 20 kg lacquered pails for production colours and varnishes. 200 kg mild-steel drums for whites, sack shades and varnishes on continuous runs. Reducers and slow blends in 20 l tins and 200 l drums. UniFlex-259HD hardener in 1 kg and 5 kg only, sized so a pack is finished within a shift. All packaging UN-approved for Class 3, Packing Group II. |
| Shelf life | 12 months from manufacture in the unopened original container at 5–35 °C for one-component inks and varnishes. 6 months for UniFlex-259HD hardener, which takes up atmospheric moisture and gasses the can once it has been opened. |
| Dryer and press-hall safety | Solvent vapour in the drying tunnels held below 25 % of the lower explosive limit, or below 50 % with continuous LEL monitoring and interlocks, to EN 1539. Extraction at the reel and at the ink stations, wash-solvent recovered by distillation, and a solvent management plan or an abatement route where annual consumption crosses the local threshold. |
| Documentation | Technical Data Sheet, Safety Data Sheet and Regulatory Summary with every product. |
Compliance and standards
- Raw materials selected to the EuPIA Charter
- Heavy-metal-free pigments
- REACH-registered raw materials
- Toluene-free
- Aromatic-solvent-free
- Ketone-free grades for food-related work
- TDS, SDS and Regulatory Summary for every grade
White paper
Corona treatment, adhesion and the overprint varnish on polyethylene
Polyethylene is a hydrocarbon with nothing polar at its surface. Untreated, it sits at about 30 to 31 dyn/cm. An ink wets a surface when the substrate’s surface energy exceeds the ink’s surface tension by a useful margin — in practice eight to ten dyn/cm — which is where the 38 dyn/cm floor for surface print comes from. Work reserved for lamination is usually specified higher again, around 40 to 44. Below the floor the symptoms arrive in a familiar order: solids mottle, fine dots fail to transfer cleanly, and the tape test lifts the print.
Corona treatment works by discharging through air at the film surface, breaking C–H bonds and oxidising what is exposed. The result is carbonyl, carboxyl, hydroxyl and peroxide groups a few nanometres deep, plus a small amount of micro-roughening. Adhesion is then polar interaction with the resin, with some mechanical keying. Because it is a surface effect only, it is fragile. Treatment level falls with time in store, falls faster when the reel is stored warm, and falls faster again on film carrying migrating additives.
That last point causes most of the field failures. Slip and antiblock additives — erucamide and oleamide in particular — bloom to the film surface during storage and sit between the ink and the treated polymer. A reel can read 38 on a dyne pen and still fail the tape test, because what the ink has bonded to is a weakly held additive layer rather than the film. The practical answers are to print treated film reasonably fresh, to re-treat inline where the press has a corona station, and to verify at the point of print: dyne pen or dyne solution as an indication, then a cross-hatch or tape pull on dried print, then rub and crinkle to represent what the bag actually experiences. Leave the print to condition before judging adhesion; a film still holding solvent does not read true.
The overprint varnish is then the second half of the specification. On a surface print the ink is the outermost layer of the bag, so the OPV takes the abrasion, sets the gloss or matt appearance, and carries the slip package that determines coefficient of friction. COF is a two-sided problem: too slippery and a stacked pallet walks; too grippy and the bagging machine will not feed, open or turn the bag. The OPV must also survive the sealing jaw without blocking or picking, and must not pick off when a tightly wound reel is stored warm. Both the slip additive level and the OPV film weight move COF, so change one variable at a time and measure on the printed, conditioned bag rather than on a drawdown.
White paper
Solvent release, retained solvent, and the abatement duty
Drying is evaporation, so the solvent blend is a rate specification as much as a solubility one. The blend needs a true solvent for the resin plus diluents that control cost and evaporation rate. Tuned too fast, the ink dries in the anilox cells and on the plate: screening in solids, dirty print, and colour drift across the run. Tuned too slow, the solvent is still in the film at the rewind. On heavy sack coverage this matters most, because that job is simultaneously the thickest deposit and the highest speed.
Retained solvent shows itself as odour, as blocking in the reel, and as adhesion that tested well on press and fails a week later. The dryer is the control. Air turnover and extraction matter more than temperature: a hot tunnel with poor air exchange skins the surface of the film and seals solvent underneath, which is the worst of both outcomes. Where the job is food-related packaging, retained solvent is a limit to be worked to and measured, with ester and odour-active species controlled more tightly than the total, and the blend is supplied ketone-free.
The obligations that come with the system are real and should sit in the decision rather than be discovered afterwards. Solvent vapour is flammable; dryer atmospheres are conventionally monitored and interlocked well below the lower explosive limit. Transfer requires earthing and bonding, the press area requires classification, drums require cool closed storage, and wash solvent belongs in a still rather than a drain. Where annual solvent consumption passes the thresholds set in the Industrial Emissions Directive or the local equivalent, the plant carries either an abatement route — regenerative thermal oxidation, or carbon adsorption with recovery — or a solvent management plan demonstrating equivalent reduction.
Against that, the water-based comparison is honest and short. A water-based flexo system removes the VOC inventory, the flammability classification and the abatement duty, and on absorbent stock — board, corrugated, general paper work — it prints well and is often the correct choice. What it does not match is adhesion on marginally treated non-porous film, and outright speed, because water carries a high latent heat of vaporisation and asks much more of the dryer for the same deposit. On a heavy-coverage sack line, that speed difference is the whole argument, which is why the kraft sack work stays on solvent while lighter absorbent-stock work often does not. So the rule we work to is simple: if the substrate is paper and the line is not running at sack-line speed with sack-line rub and light-fastness demands, ask about the water-based route first; if it is polyethylene or woven polypropylene, or a filled sack at speed, the solvent route is chosen for those specific reasons and not out of habit.
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