Crossflow Technology for Ink Jet Fluids

Enabling a Greener Future

Improved pigmented dispersions that are being developed have more complex chemistries, which cause the formation of micro-sized contaminant byproducts.
Global competition has forced many ink jet ink and colorant formulators to reevaluate their processes and investigate new filtration and separation technologies to drive down processing costs. In addition, the move toward greener practices in industry has placed emphasis on water recovery and closed-loop regenerable processes.

Pall crossflow membranes can be cleaned and reused, providing a regenerable filtration solution that can last for years. This translates into reduced waste and associated disposal costs. Crossflow membranes allow formulators to conserve water through the recovery of wastewater produced during formulation processes.

Advantages of Crossflow Filtration

Crossflow filtration has a number of distinctive features that are associated with significant benefits. These benefits make crossflow the preferred filtration method for certain applications involving ink jet fluids.

 
Features Benefits
Generates two outlet streams (retentate and permeate) from one inlet feed stream Allows separation of contaminants sized below or above the membrane rating at a relatively sharp particle-size cutoff
Only a small percentage of the feed stream passes through the membrane on each pass Prevents immediate plugging of the membrane by fluids with a relatively high percentage of suspended solids
Membranes are available in ultrafiltration (UF) molecular weight ratings Allows removal or separation of low-molecular-weight impurities
Membranes can be regenerated via backwash or reverse flow in process Increases permeate volume, improving filtration economics
Membrane flux can be restored with chemical cleaning Extends membrane life significantly, reducing waste and associated disposal costs
 


Crossflow Technology for Ink Jet Ink and Colorant Applications

Ink jet ink fluids are comprised of components and contaminants of various sizes. They range from 100 Da to more than 100 μm.

Relative sizes of components and contaminants commonly found in ink jet fluids
Figure 3: Relative sizes of components and contaminants commonly found in ink jet fluids
* Based on the molecular structure of Dextran


Crossflow technology can be used to separate these components and contaminants in the following applications.
  • Removal of impurities from pigmented aqueous colorants
  • Classification of pigmented dispersions for inks and colorants
  • Dye purification
  • Waste stream management

Removal of Impurities from Pigmented Aqueous Colorants

The process of formulating pigmented ink jet colorants includes removing low-molecular-weight impurities such as salts, excess polymer, and surfactant. Crossflow in diafiltration mode can be used to accomplish this. A membrane with the correct rating allows the impurities and carrier fluid to pass through, while retaining the pigment. The ink colorant recirculates through the crossflow module. A portion of the impurities and fluid pass through the membrane, exiting the module as the permeate stream.

Separation of Low-Molecularweight Impurities from a Pigmented Dispersion
Figure 4: Separation of low-molecularweight weight dispersion
Note: As fluid flows across the membrane, only a small percentage of upstream components pass through.


In diafiltration mode, deionized (DI) water is added to replace the volume of the exiting permeate stream, and the impurities are effectively washed out of the pigmented colorant. The amount of water added is typically expressed as a multiple of the initial starting batch volume, also known as diafiltration volume. As the number of diafiltration volumes increases, the salt concentration in the retentate decreases.

Relationship Between Diafiltration and Salt Concentration

 
Diafiltration Volume Retentate Salt Concentration (%)
0 100.0
1 36.8
2 13.5
3 5.0
4 1.8
5 0.7
6 0.2
7 0.1
 

Classification of Pigmented Dispersions for Inks and Colorants

Crossflow microfiltration (MF) membranes can successfully remove larger contaminants and agglomerates from pigmented dispersions. The particles that are specifically targeted are those larger than the desirable particle size distribution of the pigmented dispersion.

The pigmented dispersion passes through the membrane, and any larger contaminants and agglomerates remain in the retentate fluid. Water is continuously added to the system to maintain the concentration of the retentate fluid stream and minimize flux decay. The ink or colorant, which is the desirable product, gets diluted by the water. An additional step is then necessary to return the permeate to the initial concentration.

Separation of Larger Contaminants and Agglomerates from a Pigmented Dispersion
Figure 5: Separation of larger contaminants and agglomerates from a pigmented dispersion
Note: As fluid flows across the membrane, only a small percentage of upstream components pass through.


Dye Purification

The filtration of dyes is a necessary step in the formulation of dye-based ink jet inks and colorants. Dyes that have not been filtered may contain insoluble hard or soft particles that result from dye synthesis and mixing processes. The dye molecules, which are a low molecular weight and size, easily pass through the membrane into the permeate stream, while the larger insoluble particles become part of the retentate stream. As the dye-based ink or colorant is recirculated across the membrane, the permeate stream is collected in the form of the purified dye product. The retentate stream is concentrated into a sludge-like waste, which is disposed of.

Separation of Oversized Contaminants from Dye
Figure 6: Separation of oversized contaminants from dye
Note: As fluid flows across the membrane, only a small percentage of upstream components pass through.


Waste Stream Management

Wash water used in processing pigmented ink and colorant can be reclaimed by filtering it through a crossflow ultrafiltration membrane. The water passes through the membrane, and the undesirable particulate, including pigment and environmental debris, is retained and concentrated over multiple passes through the crossflow module. The filtered water can be reused as wash water or purified for use in ink or colorant formulation.

Separation of Pigment and Oversized Contaminant from Process Water
Figure 7: Separation of pigment and oversized contaminant from process water
Note: As fluid flows across the membrane, only a small percentage of upstream components pass through.


Membralox Production System
Membralox production system

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Crossflow Filtration
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