Abstract: The various types of rheology modifiers used in inks are reviewed from the point of their chemical composition, mechanism and application properties.
The rheological additives discussed range from organically modified clays to organic products like castor oil derivatives, diamides, hydrophobically modified polyether polyols (PEPO), hydrophobically modified polyurethanes (HEUR) and hydrophobically modified acrylic acid co-polymers (HASE). The key properties of each type are discussed in view of their use in solvent, oil, water and UV ink systems.
The control of rheology in printing inks has always been a challenge for ink formulators. Typically, when ink chemists formulate a new product, the rheological additives are not “the first consideration” in the formulation. Generally, polymer (or resin), solvent and pigment types determine the intrinsic rheology of the ink formulation.
Ideally, the formulators hope to achieve all the rheological properties from the resin system they choose. However, to fine-tune the rheology and improve overall performance properties of their inks, formulators most often seek“rheological additives.”
The rheological modifiers are one of the most important additives utilized in printing ink formulations. The additives are utilized to modify the rheological properties of printing inks to meet the requirements of various types of printing presses. The ink application method determines the type of rheology needed for the specific ink formulation.
In oil and solvent- based offset ink applications, typical rheological additives such as organoclays provide the required consistency for the lithographic printing process. As for low viscosity inks such as flexo, gravure and ink jet ink applications, generally there is no “need” for increasing viscosity or yield value of the inks.
However, the rheological additives are utilized to control properties like flow, temperature stability, ink penetration/hold out or to control the settling of the pigments or fillers of low viscosity ink systems.
A basic understanding of rheology, the science dealing with deformation of flow of matter, is essential to any ink formulator designing an ink system. A typical rheological model is a rectangular body of liquid made up of very thin layers piled on top of each other. (Figure 1).
If a force F is applied on top of an area A and this in turn pulls sideways on top, the pulling action is defined as Shear Stress to F/ A.
As the top layer starts to move under shear stress, it pulls the layer directly underneath along with it. In turn, the second layer pulls the third, the forth, and so on. If the velocity of the top layer is V and the thickness of the liquid pool is X, then the viscosity gradient is defined as Shear Rate. (Figure 2).
Viscosity is defined as the “resistance to flow” of a liquid which is the ratio of force applied (Shear Stress) to the rate of flow resulting (Shear Rate) (Figure 3).
There are various types of flow profiles. The most important flow types for ink systems are Newtonian, Pseudoplastic and Thixotropic. Newtonian flow is a type of rheology profile that shows constant viscosity regardless of changes in shear rate. (Figure 4).
Pseudoplastic flow (known as shear thinning) is a typical profile for many ink systems in which viscosity decreases as shear rate increases. Thixotropic flow also provides a shear thinning profile; however, once the shear is removed, the recovery of the viscosity takes some time and a hysteresis loop is formed by the upper and lower curves. The area between two curves is a measure of the system’s thixotropy. (Figure 5).
Dilatant flow is where viscosity increases with increasing shear. This type of flow is not generally observed in ink applications. Some high solids systems and quicksand are typical examples.
Clays and Organoclays
Naturally occurring clay minerals, such as hectorite and bentonite, are hydrophilic in nature. When dispersed in water, the clay particles swell and separate into individual clay platelets. Due to platelet interaction, a three-dimensional “house of cards” structure is developed. This collodial structure provides thickening properties (Figure 6).
The natural clays may be utilized in water-based flexo and gravure ink systems to improve rheological and antisettling properties of the inks.
In oil or solvent systems, however, the natural clays do not disperse and do not provide rheological properties. In order to thicken oil and solvent systems, the hydrophilic clays are modified with various types of hydrophobic quaternary ammonium compounds.
To make an organoclay, the smectite clay is reacted with a quaternary ammonium compound as shown. (Figure 7).
The quaternary amine ion (CL-) exchanges with the sodium cations on the surface of the clay. The resultant product is an organoclay, where the organic component is firmly bonded to the clay surface. During this reaction, the salt produced is washed out. (Figure 8).
Supplied as powders, the organoclays are in the form of agglomerated platelet stacks. A combination of wetting and mechanical energy is needed to deagglomerate these platelet stacks. Addition of a polar group containing materials like low molecular alcohols, water or propylene carbonate help force the clay platelets further apart, resulting in a completely dispersed rheological structure. (Figure 9).
Today, rheology of lithographic inks is controlled either by reactive (chemical) or non-reactive (physical) gellants or a combination of both types of rheology modifiers. The reactive gellants include aluminum soaps and compounds, organic titanates, oxides/hydroxide of Ca, Mg, Zn and polyamino-acids.
The reactive chemical gellants can not be directly added to ink due to heat activation and process requirements. These gellants are rather incorporated into the ink varnishes during manufacturing. They thicken by cross-linking the resin. (Figure 10).
Most require heat activation to as high as 300°F. Manufacturing problems may be encountered with improperly processed chemical gellants, such as reacting too quickly with resin to produce seeding (small particles of gelled particles) and batch to batch consistency/handling problems. The gelled varnishes are typically used in sheetfed, heatset and UV offset ink applications. The gelled varnishes provide excellent rheology and gloss properties.
Today, there is a lot of effort toward research on new resin technologies, such as “self-structured” resins with adequate rheological properties to eliminate the aluminum gellants. However, the gelled varnishes and the “self-structured” resins may not provide all the required ink performance characteristics, which may show as poor misting, emulsification and penetration.
The most commonly used physical gellants include organoclays and fumed silicas. Both additives can be directly added to the ink formulation without the need for temperature activation.
The organoclays have been successfully used for more than 50 years in a variety of ink applications. The organoclays contribute not only excellent viscosity and yield but they also help control misting and ink penetration onto porous substrates. The organoclays can be used as the sole rheological additive or can be used in combination with gelled varnishes to fine-tune the rheology. (Figure 11).
In recent years, there have been great improvements in organoclay type products to improve the ease of dispersability of these products. The new types of organoclays offer a more open structure compared to the conventional organoclays.
The open structure helps dispersability of the product under low shear and short processing times and eliminates the milling stage of the ink manufacture. In most cases, the overall throughput is improved by reducing the production cost.(Figures 12 and 13).
The organic thixotropic type of rheological additives can be either directly added to ink or can be incorporated into an ink varnish. These organic thixotropes are based on castor oil derivatives, polyester-amides and polyamide-type additives. The organic thixotropes must be incorporated at temperatures in the range of 140°F to 220°F, depending on the ink system and type of additive utilized. Following the application of shear and temperature over a period of time (usually 15-20 minutes), the particles show deagglomeration, softening and swelling.
The functional mechanism is hydrogen bonding through the available groups in the ink system. They do not cross-link with the system. (Figure 14).
The organic thixotropes may be utilized in a variety of solvent and UV ink applications due to their excellent thixotropic rheological properties. (Table 1).
The polyester-amides and polyamides may also be utilized to gel an ink varnish. A recent laboratory study showed that when the polyester-amide or polyamide was incorporated into a heatset free flow varnish, it produced excellent rheological properties. (Figure 15).
(ASE and HASE)
These thickeners are based on acidic acrylic copolymers with carboxylic acid groups and can be further modified with hydrophobic groups. When incorporated in water, while the system is acidic, no rheological activity occurs. However, when the carboxyl groups are neutralized with alkaline additives, they swell and produce excellent viscosity increase.
The ASE and HASE thickeners are commonly used in a variety of water based ink applications including screen, flexo, inkjet ink and gravure inks. (Figure 16).
HEUR and PEPO
HEUR (hydrophobically modified polyurethane) is obtained by reacting isocyanates with diols and hydrophobic components. HEUR thickeners consist of hydrophilic polyethylene oxide polyethers and hydrophobic aliphatic or aromatic residues, which are linked with isocyanates via urethane groups. The hydrophilic segments used are based on polyethers such as polyethylene glycols or derivatives. (Figure 17).
PEPO (polyether polyol) thickeners consist of a polyether polyol linked with hydrophobic aliphatic groups. Due to their hydrophilic component, polyol, the PEPO thickeners provide more Newtonian flow characteristics. (Figure 18).
HEUR and PEPO thickening mechanisms are based on interaction and association with hydrophobic and hydrophilic components in ink formulation. The thickeners interact with pigment and emulsion particles and form a continuous network throughout the system and increase viscosity. (Figure 19).
Both thickeners are excellent rheology modifiers for water-based ink systems. These additives can be modified to produce various types of flow characteristics. (Figure 20).
As the ink manufacturing process changes toward faster throughput and full automation, the ink manufacturers demand easy to disperse, pumpable raw materials in their formulations so that lengthy processing and milling are eliminated.
Ink formulators now have a wide range of rheological additives available to them. It is up to the ink manufacturer to select an ink rheology modifier that will provide the required rheology for their inks while improving their processing costs.
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8) Testa, C., Rhone Poulenc Chemicals, “Developments in Rheology Modifiers for Lithographic Ink Varnishes”, OCCA Conference (November 1991).