Inkjet Printing Ink Formulations and Additives (Worth Keeping)

Ink Formulation Parameters and Additives

01 Surface Tension Adjustment of Ink

The surface tension of ink determines the formation of droplets in the print head and is one of the key factors in the spreading and wetting behavior on substrates. By adjusting solvent components and adding surfactants, the surface tension can be controlled. For example, water-based ink with isopropyl alcohol as a co-solvent significantly lowers its surface tension, from 72.8 dyn/cm (pure water) to 30 dyn/cm, due to the concentration of isopropyl alcohol.

Typically, when the co-solvent concentration is relatively high, the surface tension decreases significantly. Surfactants are usually used in very low concentrations, generally with mass fractions below 1%, and often below 0.1%.

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Surface tension refers to the pull between adjacent parts of a liquid surface in a direction perpendicular to the boundary line between them. Liquids exhibit both cohesive and adhesive properties, both of which are manifestations of molecular attraction. Cohesion allows a liquid to resist stretching, while adhesion allows it to stick to other surfaces.

The phenomenon of surface tension occurs at the boundary between a liquid and a gas, or between two immiscible liquids, due to the molecular attraction between molecules. Imagine a thin film at the surface bearing the stretching force of this surface, and this force is called surface tension.

At room temperature (around 20°C), the surface tension of most liquids is in the range of 20-40 dyn/cm, but some are higher, such as the surface tension of water at 72 dyn/cm and mercury at 470 dyn/cm.

For liquid metals, the surface tension is usually higher; for instance, the surface tension of liquid copper at 1131°C is 1103 dyn/cm. Some elements, which are gaseous at room temperature but liquid at low temperatures, have low surface tension, such as liquid helium at 4.3 K, which has a surface tension of just 0.098 dyn/cm, and liquid hydrogen at 90.2 K with a surface tension of 0.2 dyn/cm.

Theoretical analysis also shows that for the same liquid, surface tension decreases as temperature increases.

The formation of surface tension is closely related to the special forces acting on molecules within the thin layer at the liquid surface.

If surface tension is determined by the composition of the liquid medium, it will not change over time, and its value will be in a balanced state. However, if surface tension is controlled using surfactants, dynamic surface tension should be considered. This parameter is important when a new surface is forme,d and surfactant molecules have not yet covered it (such as when droplets or spreads form). Initially, the new surface has very high surface tension, and the surfactant diffuses to the interface, causing the surface tension to decrease until equilibrium is reached. It should be emphasized that surface tension (both static and dynamic) depends on all components of the ink and their interactions, such as the interaction of surfactants with dissolved polymers, and even the effect of plasticizers due to ink container movement.

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Newton/meter (N/m): This is the standard unit of surface tension in the International System of Units (SI).

Millinewton/meter (mN/m): This is another common unit for surface tension, with 1 millinewton equaling 0.001 newton.

Dyne/centimeter (dyn/cm): Since 1 dyne equals 0.001 newton, 1 dyn/cm is also equal to 1 mN/m.

02 Rheological Control of Ink

Ink rheology plays a crucial role in its performance during inkjet ejection and diffusion on substrates. Ink performance is influenced by many parameters, such as the composition of solvents, the presence of polymer additives, concentration, surfactants, moisturizers, and the quality of dispersion.

Most inkjet inks are Newtonian fluids, with constant viscosity over a wide range of shear rates, although non-Newtonian inks can also be used. Inkjet inks typically have very low viscosity, usually below 20 cP, and the viscosity depends on the print head (typically lower than the viscosity of thermal nozzles, which is 3 cP). Viscosity can generally be controlled by selecting appropriate additives, such as 1%-3% concentration of long-chain glycerol or soluble high-molecular-weight polymers.

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cP: Centipoise (cP) is the unit for dynamic viscosity, which represents the amount of internal friction in a liquid when it flows under a certain shear stress. It is typically expressed in mPa·s in the International System of Units (SI), and habitually referred to as cP.

1 cP = 10^-3 Pa·s (Pascal-second) = 1 mPa·s (millipascal-second).

Viscosity refers to the flowability (or lack of flowability) of a substance, measured in centipoise (cP).

At 20°C, water has a viscosity of 1 cP, which flows easily. In contrast, molasses has a viscosity of 100,000 cP, making it very thick.

03 Foam and Defoamers

Foaming phenomena are often observed in inks, and the presence of surfactants and polymers can cause serious issues for inkjet performance.

The solution to this issue is the addition of defoamers, which are molecules capable of breaking down foam. Defoamers suppress foam stability by two main mechanisms: 1) locally reducing surface tension to a very low value, which causes the foam to rapidly thin, and 2) promoting drainage of the thin liquid film.

The first mechanism is achieved by additives that reach a limited solubility, usually containing an immiscible part (e.g., a silicon derivative). A typical example of a defoamer working through the second mechanism is tri-n-butylphosphate, which reduces surface viscosity.

In cases where non-soluble defoamers are used, the phase separation effect is significant during ejection (changing the wettability of the print head) and after printing (leading to surface defects due to low surface tension). Therefore, defoamers should be avoided or carefully selected to avoid phase separation effects during long-term storage. In any case, the defoamer should be used at the minimum concentration.

04 Wetting Agents

Moisturizers are added to water-based inks to prevent ink evaporation at the print head and to prevent clogging. These moisturizers should be water-soluble, and typical wetting agents for water-based inks include glycerol, diethylene glycol, polyethylene glycol, ethylene glycol, and propylene glycol methyl ether (ethylene glycol monobutyl ether).

In addition to preventing ink evaporation at the print head, wetting agents also increase the drying time of the printed image. Therefore, when selecting the optimal concentration of wetting agents, both the evaporation prevention and drying time factors should be considered. Typically, wetting agents make up a significant portion of the solvent, with mass fractions ranging from 10%-30%.

05 Electrolytes and pH

The presence of electrolytes in pigment-based functional materials can cause stability issues during storage because particles near the compressed double electric layer may cause flocculation. Therefore, the concentration of electrolytes should be kept as low as possible. Multivalent ions, such as Ca2+ ions, are particularly important. Typical chelators used in aqueous formulations, such as ethylenediaminetetraacetic acid (EDTA), are used at concentrations of 0.1%-0.5%.

pH is also crucial for water-based inks because it can significantly affect the solubility of various components and the stability of functional material dispersion particles. When the ink contains polymer binders, the solubility effect is crucial. For instance, acrylic resins with lower solubility at low pH are important for stability.

To control the required pH, some buffers are added to the ink. A typical buffer used in water-based inks is tris(hydroxymethyl)aminomethane.

06 Biocides

Since most dyes are organic molecules, especially in aqueous solutions, they provide an excellent medium for bacterial and fungal growth. Bacterial and fungal reproduction can lead to clogging of the nozzles. To prevent this, biocides should be added to the ink. The choice of biocide depends on the growing biological species.

In theory, biocides should be highly effective and have broad antibacterial activity. Common biocides include 1,2-benzisothiazolin-3-one and 2,6-dimethyl-4-hydroxybenzoate. Biocides are usually added in small quantities, with mass fractions ranging from 0.1% to 0.5%.

07 Binders

Inks usually contain binders, which provide adhesion strength for the printed material to the substrate and may help prevent wear. Binders are typically soluble polymer resins that can be dissolved or dispersed in the ink via heat curing or UV curing. The choice of binder depends on good adhesion and matching with the substrate. Because the ink has low viscosity, the molecular weight of the binder resin is generally below 100,000, and often below 50,000. Typical resins include vinyl chloride/vinyl acetate copolymer, acrylic resins, and polyketone resins.

08 Example Ink Formulations

• Solvent-based Ink: Solvent mixture (10% ethylene glycol ether, 10% hydroxy ketone, 40% alkyl lactate, 22% acetylacetate salt); Blue pigment (Blue 44 or Blue 45, 3.5%); Diluent (methanol, ethanol, or 2-propanol, 10%); Surfactant (polyacrylate or polysiloxane, 0.05%); Resin (vinyl chloride/vinyl acetate copolymer, 4%); UV absorber (benzophenone and benzotriazole, 0.5%)
  
  

• Water-based Ink: Co-solvent mixture (10% 2-pyrrolidone and 5% tetraethylene glycol); Pigment carbon black (C300, 4%); Binder (SMA2000, styrene-maleic anhydride copolymer, vinyl aromatic compound, 0.7%; WAX 85-302-1, polyurethane, 0.7%); Surfactant (non-ionic 465 0.05%; Lithium carboxylate anionic fluorine-containing surfactant, 0.1%; 2-(=ethoxyphenylphosphoryl amine)-1,3-dithiolane 9NP and phosphate ester, 0.3%); Chelating agent (ethylenediaminetetraacetic acid, 0.05%)
  
  

• Hot Melt Ink: Solid medium (hard stearic ketone, 48%; methylene stearic acid, 30%; amino resin, 20%); New neopentyl glycol diester yellow pigment, 2% (melting temperature of the component is 110°C)
  
  

• UV Cured Ink: Photopolymerizable monomers and oligomers (one acrylate of diethylene glycol diacrylate, 23.5%; neopentyl glycol alkoxylated diacrylate, 23.5%; dodecyl acrylate, 17%; dipentaerythritol hexaacrylate, 3%; trimethylolpropane ethoxytriacrylate, 6%); Free radical stabilizer (Genorad 16, 0.5%); Photoinitiator (1-hydroxycyclohexyl phenyl ketone, 4%; 253nm and 368nm Genocure low heat mass, 4% absorption peak; Genocure PMP307nm, 4% absorption peak); Colorant (Magenta dispersions SPF586, 20%).
  
  

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Polymerizable monomers refer to simple compounds capable of linking through polymerization reactions (including addition polymerization and condensation polymerization) to form high molecular compounds.

Here are some common polymerizable monomers and their types:

• Monomers containing unsaturated bonds: Ethylene (C2H4): contains carbon-carbon double bonds, which can undergo polymerization to form polyvinyl chloride (PVC).
  
  

• Methyl acrylate (H3C-CH=CH-COOCH3): contains carbon-carbon double bonds and can undergo polymerization to form poly(methyl acrylate).
  
  

• Maleic anhydride: typically does not directly participate in polymerization reactions but can be converted into a monomer for polymer formation by other chemical reactions.
  
  

• Monomers containing carboxyl groups: Acetic acid (CH3-COOH): can undergo condensation polymerization to form polyacrylic acid.
  
  

Other Types of Monomers:

For example, amino acids are monomers that form proteins, glucose is a monomer for polysaccharides, and nucleotides are monomers for nucleic acids. When selecting polymerizable monomers, the feasibility of their polymerization reactions and the required properties of the polymer should be considered. Different monomers can undergo polymerization through different methods (such as free radical polymerization, anionic polymerization, cationic polymerization, etc.) to form high molecular compounds with specific structures and properties.