Solvent based printing inks

Flexo and gravure applications

D. MANTISDFH Druckfarben

Liquid inks - definition

• Printing inks are coatings which contain colorants (inorganic and organic pigments, soluble toners, dyes)

• Printing inks act also like adhesives, they must set on the printed substrates for the full service life of the packaged structure

Inks ≠ ~ adhesives

• Inks must be stable during storage, dilution and flowing into the closed circuit of the printing machine (not thickening – gelling, not flocculating – settling)

• BUT must adhere instantly and permanently after application

• On absorbent substrates the binder should not leave pigment unprotected on the surface (easy rub of, powder-off).

• On non-absorbing substrates the resin system must film form and cross-link with the functional groups of the substrate (often using catalysts / adhesion promoters).

• In some cases solvents used to thin / dilute inks penetrate / swell polymeric films to activate / enhance physical + chemical bonding

Ink absorption

Inks and ingredients

• INKS: Dispersions of colorants in a carrier.

• CARRIER: resin solution/emulsion/dispersion

• SOLVENTS: dilute resins, regulate viscosity and drying time

• ADDITIVES: solve printing problems, maximize service life of the printing ink film

Liquid inks - scope

• Printing inks - transfer a message from

the printed area- protect the printed surface- emerge a packaging style- protect the consumer

from a defective product (active packaging)

- control the packaging (intelligent packaging)

Liquid inks - substrates

• Printing inks applied on a wide range of substrates:

- Paper (coated, uncoated, calendered …)

- Films mono or bi-oriented, laminated, co-extruded: PP, PE, PA, PET ….

- Alum foil (washed, primed, coated)

- Metallized paper / films- Laminated structures: - Coated films: Al-Ox, SiOx

coated OPA, PET, OPP- Woven films (PP sacks)

Check ink – substrate compatibility

STUDY + EXPERIMENT + LEARN • Test in small scale before using in large scale• Learn before printing• First experiment in the lab and then in production

(adhesion. blocking, solvent retention, heat resistance etc………….)

• Study the special characteristics for the substrate (treatment, coating, absorption ..)

• focus on the final requirements of the packaging structure (sealing, lamination, steam sterilization, microwavable, outdoor exposure …)

Inks: visual effect

• Color: - Hue defines the nature of the colour

(shade like red, blue, green etc)- Strength = intensity = saturation =

chroma- Purity = value: brightness, darkness• Transparency vs. opacity: percentage

of light scattered from or transmitted through the printed image (colorant type, good dispersion, substrate)

• Gloss: percentage of reflected light (evenness - smoothness of ink film, high resin content, higher ink thickness, better pigment dispersion increase gloss)

Inks and drying models

• Radiation:IR, UV, EB inks• Chemical drying: (e.g. 2K or

PUR inks) followed by thermal polymerization (heat activates and catalyzes inks)

• Oxidation:offset oil based inks + metal catalysts)

• Absorption:into porous substrates like carton-boxes)

• Evaporation:controlled removal of solvent(s) followed by film-forming of resins + colorants on the substrate

Resistances of printed area

• Heat resistance (heat sealing, pasteurization, sterilization etc)

• Light-fastness (resistance to fading under UV exposure)

• Weathering (exposing to weathering and chemical elements e.g. rainfall)

• Abrasion (rub, scuff, crinkle): proper formulation in terms of resins, pigment/resin ratio, additives, dispersion

• Chemical resistance (acid, alkali, oil, butter, lactic acid,

Must focus TO:

• Conditioning of the ink: control rheology and viscosity

• Keep the ink clean from any kind of contamination

• Keep the ink in motion through proper pumping / circulation

• Keep the ink under control (viscosity, pH for the w/b systems, temperature, solvent level, foam level etc…..)

Gravure s/b inks

Special characteristics:• Low viscosity (15-25 Zahn cup 2, watch out for screening,

halo), near Newtonian flow• Easily re-solubilized (re-wetted) in the engraved cells• Fast evaporation of solvents (fast drying) and instant setting

on the substrate• Rapid flow out of the engraved cells and transfer onto the

substrate forming a smooth film• Heavy wet ink film: 8-12 μm maximum thickness• Low odor, minimum solvent retention• One nip point – compression of the ink• The ink acts as a lubricating fluid during doctor blade wipping• Low residual tack of the dried ink (to avoid pick-off, set-off).• Low abrasiveness (choose the right pigments, fine tune

formulation)

Gravure s/b inks

• The metering blade provide a clean wipe. The pressure and the application angle of the blade must be stable to ensure consistent colour reproduction and minimization of hazing and streaking. The contact area must be low enough to minimize blade and cylinder wear

• The ink must not dry to fast, to avoid solvent evaporation from the ink left over in the engraved cell – drying in.

• The ink must not dry too slow, to avoid blocking, dirty printing

• Control air streams to minimize ink skinning causing hazing, streaks

• Control agitation in ink trough to minimize foam, ink skipping

Flexo s/b inks - I

Special characteristics• Medium to low fluidity of diluted inks: 22 – 30 sec Zahn cup 2 (higher

viscosity for process printing)• High delivery viscosity: 50 – 90 Zahn cup 2, to prevent severe pigment

settling and to allow using different solvent mixtures for diluting the ink (according to the printing conditions: speed, drying temperature etc…)

• High viscosity: dirty printing – more thick ink layer on plates → fill-ins• 2 or 3 nip points, compression of ink• Shear thinning of liquid inks (excess shearing of ink during transport in

the closed circuit)• Wet ink film: 2 – 10 μm, depending on the tools used and the printing

conditions (speed, metering unit, working practice). Higher pigment loading than gravure inks.

• Solvent used for thinning must not swell / destroy photopolymer plates – stereo

• Instant solvent evaporation after application using BCD (between color driers), final setting in the long drying tunnel.

Flexo s/b inks - II

• Severe loss of solvents in open systems, check periodically viscosity lever and correct if needed.

• Low viscosity: low printing density, low resistance of the printed area, distortion of high-lights + tonal shift, quality drift in shadow and midtones (dot structure, coverage, print density)

Flexo s/b inks

• Rubber plates: high surface energy, more accepting of ink from the anilox but transfer less ink on the substrate

• Photopolymer plates: have lower but more consistent surface energies than rubber plates (less affinity with the ink) but transfer more easily to the substrate

• The solvent(s) in the ink must not solubilize the resin of the plate, otherwise we observe dimensional changes in the image area of the plates (swelling) and will create some tackiness (which will decrease the transferability of the ink)

Ink transferability

• Pigments dispersion characteristics and resin selection define runnability of the ink through the cylinders (engraved cylinder, anilox to plate and plate to substrate)

• Ink must have a flexible but in the same time tough body to avoid misting (spraying out) and excessive shear thinning (especially in high speeds).

• Inks must be kept soluble when circulating (in the closed dosing system of the press) but must release easily the solvents immediately after application onto the substrate. The higher the solubilization of the resins (extend, expand linearly, fluid and flexible) in the solvent mixture the better transferability but the poorer solvent release exhibit.

• Inks must not be oxidized or thicken during printing (respect guidelines when using the inks, check out the self life of each product, look after the phanal based and metallic inks). Some coatings of the substrates re-solubilized during printing – combine efficiently inks vs. substrates.

Anilox the metering station

Compatibility solvents vs. flexographic plates

s/b inks – handling

• Store inks in a conditioned warehouse (avoid freezing and / or overheating).

• Condition inks, varnishes, solvents, substrates in the production before using.

• Dilute the inks before entering the printing deck.• Re-circulate the inks using the appropriate pumps for at least 10

minutes. • Prepare a solvent mixture to regulate drying conditions• Use ex-proof equipment, train the personnel• Minimize solvent escape during use, don’t lave containers open, filter

remaining quantity before re-using.• Use the proper thinning varnish to reduce ink density• Avoid over-dilution to prevent kick-out (large aggregates and

agglomerates will form).

Filtering the inks

• Abrasive pigments (usually TiO2, metallic), contamination (metal fines, debris), not adequately dispersed inks (large clusters of agglomerates, aggregates) wear tools of printing presses → quality issues, increase of rejected printed material Install a magnet trap in the filtering unit (max 50 μm)

• Common printing defects: (streaking, comets, scoring. drag-out)

Viscosity and solvents

• Apply balanced solvent mixture• Use the right solvents• Do not over-dilute s/b inks (if the viscosity

approaches that of solvent mixture then there is not enough binder in our system resulting to poor layout, decreased properties of printing area, settling of pigment, wear of machine elements etc.. Lack of printing quality…)

• Study the dilution curves.• Do not dilute cold inks with warm solvents or

vice versa (preferable conditions 21 – 25 °C), to avoid shocking phenomenon. Add the varnish (low solid system) into the base (high solid system) under adequate stirring. Then add solvent into inks under stirring.

• A cold ink is not performing as well as a hot ink (especially in chambered doctor blades)

Solvents for thinning and regulating drying speed

Establish solvent balance

• Solvent added into the ink to regulate printing viscosity

• Solvent loss from the ink sump, printing station

• Composition of escaping solvents (solvent blends) determined by: solvent molecule, resin interaction, ambient conditions, press settings etc..

• A solvent blend must be added to replace loss and prevent imbalance (resin kick-out, ink souring, gloss reduction, viscosity increase, loss of adhesion)

• Major issues of solvent imbalance in jobs where ink consumption is limited (spot colours, process printing)

• Prepare a blend of balanced solvents and add it into the diluted ink establishing a more balanced drying behavior.

Viscosity cups

• Periodically check of flow cups. Replace if worn.

• Use a standard cup as a reference tool.

• Handle cups with care (do not drop, squeeze). Inspect the exit aperture.

• Keep viscosity caps wetted with solvents (do not let ink dry), store in vertical position.

• Use metallic and not plastic cups for s/b inks.

Viscosity cups

• Use standard calibrated viscosity cups

• Compare viscosity readings between different cups

• Test the accuracy between reading and operators

• Define the temperature of inks and related varnishes

Automatic viscometers

• Viscosity drift during long runs create problems in: color matching, coverage, transferability

• 1 second increase in viscosity can increase consumption up to 25 %

• Automatic instruments measure and if needed control viscosity minimizing wasted and rejected (off-specs) materials, press setup time, returned ink after the production.

• Automatic reliable measurements, adjustments can be carried out in real time

• Less time needed to check the ink (manual reading must take place every 15-20 minutes)

Automatic viscometers

Falling ball viscometers operate in a bypass line from the ink pumped tothe printing press. Stopping the ink flow allows the ball to fall providing a timed measurement proportional to the terminal velocity and inversely proportional to the viscosity. The measurement is taken periodically and is not a continuous measurement. The separate fluid bypass is quasi- independent of the main loop to the printing press. It requires additional space to accommodate the separate flow path and associated instrumentation.

The falling piston viscometer is composed of a cylinder and piston

assembly. The piston is raised drawing the ink to be measured into

the cylinder through an inlet path. The piston is allowed to fall by

gravity, expelling the ink sample out through the same inlet path. The

time of fall in seconds is a measure of viscosity that can be

correlated to other units of viscosity, such as cup seconds or

centipoise.

Automatic viscometers

The third approach consists of a straight metal rod maintainedin resonant vibration by a continuously applied power source. Installed in-line to the fluid flow (vibrating rod viscometers), the sensor is between the ink pump and printing deck. The operating frequency is in the audible range (typically 300-500 Hz). High-pitched sounds and vibration in the press that is close to the resonant or harmonic frequency of the rod can affect the reading. Based on the Mechanical operation of printing presses, it may not be Uncommon to encounter these frequencies, but they can be suppressed with mechanical isolation to allow acceptable operation.While falling piston, falling ball and vibrating rod viscometers have been successfully utilized in the industry, a new solid- Tate solution further simplifies the integration of the viscometer due to its small size, ease of use, and output interface options.Equally important, the sensor allows users to configure the viscosity control solution that is optimal for their application.

Automatic viscometers

The solid-state sensor is a fraction of the size of Previous viscometers, has no moving parts, is insensitive to vibration, and provides an alternate design approach for users. Offered strictly as a sensor, Vectron’s ViSmart sensor solution allows system integrators and OEMs to implement viscosity control system designs without having to

compromise their requirements.The sensing solution readily measures aqueous or solvent based printing inks. The ViSmart viscosity sensor system consists of two to three components depending on the application requirements. For solvent-based printing inks and other hazardous applications, the viscosity measurement solution consists of a hazardous Location certified viscosity sensor, a hazardous location certified shunt-diode barrier and a DIN rail mount converter

Automatic viscometers

viscosity solid state sensor system components can be incorporated within a flexographic printing press. The sensor easily integrates in-line with the fluid to be measured, while the shunt diode barrier ensures excessive energy does not get supplied to the sensor in the hazardous classified area. The DIN rail mount converter allows continuous and easy access to the sensor temperature and viscosity data on a CAN or RS-485 physical layer utilizing a variety of protocols.

Solvent based inks

THE END

Thank you very much

Questions ???