Water-based or water-borne inks first came into use for flexography in the 1940s and are used most commonly with absorbent substrates such as newsprint, pressure sensitive label stock and corrugated board. Water-based inks dry on these substrates by a combination of evaporation and absorption into the paper.
Water-based inks are composed of pigmented suspensions with water soluble resins and/or resin emulsions. Typically, water-based inks have a higher percentage of solids, which means that there is less liquid to be removed. Due to the nature of the resins available in water systems, a thinner layer of ink is often used. When water is combined in an azeotropic mixture with certain solvents, the evaporation can be accelerated. These factors can reduce the demands put on the ink dryers, if they are present on the press. Ink dryers are hot air high velocity ovens that use air to remove the water and co-solvents from the printed ink film.
Gas forced-air and radiant heat dryers, which may consume significant amounts of energy. Even more energy is required to dry the water-based ink when printing on less porous substrates (i.e., film). Water has a much greater latent heat of vaporization (low vapor pressure or evaporation rate) than typical flexographic organic solvents. In comparison to ethyl alcohol, water requires three times the amount of energy to vaporize.
Water-based inks became much more common in flexo printing in the 1980s following passage of the Clean Air Act (CAA). Printing with water-based inks on nonabsorbent materials such as polyethylene film became possible when the inks were formulated with small amounts of fast drying organic solvents. Use of aqueous inks to this day is not common with printer applying ink to plastic films. The drying speed of water-based inks is slower than that of solvent-based inks and print quality can suffer under the extreme conditions used to dry the ink.
With the increased use of water-based inks, it is more common to see in-line corona discharge surface treatment on flexo presses. The printing of various films is greatly improved when the film is pretreated in-line with a corona treater, because it improves ink lay and ink adhesion by increasing the compatibility of the water-based ink and the substrate.
The corona treatment of the substrate results from the bombardment and penetration of ions into the molecular structure at the surface of the substrate. It is the oxidation and polar-group formation in the surface molecules, in conjunction with etching of the surface from the ion bombardment that induces the increase in surface energy of the treated substrate.
The term water-based ink does not mean that it is solvent free. However, the industry continues to work on improving the use of water-based inks to address environmental concerns. Organic solvent in the inks is the source of volatile organic compound (VOC) and Hazardous Air Pollutants (HAPs) emissions from inks. Fewer VOC emissions are becoming the rule today as ink companies respond to their customers need for less solvent (organic solvent) usage.
Inks containing five percent or less VOC are becoming common. Further reductions in VOC are dependent on new resins that facilitate drying without the volume of organic solvent. Some HAPs are being eliminated by the use of propylene glycol derivatives. HAPs from water-based inks should not be a problem in the future.
Typical water-based ink formulations (from Flexography: Principles and Practices).
|35% Pigment Dispersion
|Acrylic Solution Polymer
|Polyethylene Wax Compound
Water will not dissolve water based ink once it is cured or dried. A solvent-based or caustic (high pH) material must be used to break down the ink. Water-based inks will begin to dry on plates, rollers, anilox rollers, and other parts of the press during short periods of press down time. In order to keep the equipment properly operating, more frequent equipment cleaning is required. If the equipment is not adequately cleaned the ink will require highly volatile solvents (VOC and or HAP emissions) and or costly reconditioning.
Conversion to water-based inks has been a success where this change has been accompanied by an increase in press drying capacity (the addition of natural gas or infrared dryers), improved ink metering systems, and the replacement of press components based on electrically different metals (e.g., copper, iron) that come into contact with the ink.
Retraining press operators to become familiar with use of different ink systems also may be necessary.
Best Management Practices & Pollution Prevention
Waste Water Discharge
The assumption that water-based inks and their wash-up wastes can simply go down the drain without prior treatment is misleading. Different federal, state and local regulations apply to the wastewater generated by industrial facilities. Depending on where or from what system a facility discharges wastewater determines which regulations and limitations apply.
Wastewater associated with the use of water-based inks is contaminated with colorants (pigments and dyes), vehicles (alkali-soluble, emulsions or colloidal-dispersion chemistries), auxiliary solvents (alcohols, glycols and glycol ethers) and additives (waxes, plasticizers and defoamers). Colored wastewater may also be an aesthetic concern.
The ink also may contain various metals such as copper, barium, nickel, magnesium compounds, and cobalt. Depending on the state environmental regulations the wastewater may be considered hazardous due to the concentration of specific regulated metals. Trace amounts of chromium and lead compounds may also be found.
Septic systems are designed to treat sanitary wastewater only. Industrial wastewater is not permitted to be discharged to septic tanks or septic systems. Discharging industrial wastewater into a septic system can result in groundwater contamination requiring costly cleanup. For those operations in regions that rely on septic systems as a means to process their wastes, wastewater treatment is not as much an option as a requirement. In many areas, not even treated industrial waste waster may be discharged to a septic system. This can be verified with state or local authorities, commonly the Health Department.
Conversion to a Water-based Ink System
Converting to a water-based ink system from a solvent system requires new perspectives and modification of the flexographic printing press and its functional components.
- The ink transfer system requires attention to ensure the metering capabilities of the components and the structural nature of the rollers. It is imperative that metering rollers are in continuous motion when the water inks are in the print station. Fountain (ink) and anilox rollers in the transfer systems have to replaced with new rollers that have the properties that will facilitate the use of water formulated inks. A harder durometer rubber roller is needed to transfer ink to the anilox roll. Anilox cells need to be adjusted, as less water based ink volume is required on the printing plate.
- Drying systems for solvent inks can be inadequate for water-based inks. They have to be rebuilt or equipped with better air movement and higher exhaust rates. The volume of vapors from a water system is essentially 10 times the volume of solvent given the same liquid volume. Amines (ammonia compounds) are required in the formulations to hold the resins and pigments in suspension and must be completely removed through the drying process.
- Film surfaces must be treated to raise the surface tension during printing with a corona treater.
- Enclosed doctor blade systems can greatly improve quality of printing and reduces the amount of solvent or water (diluent) that will vaporize in the pressroom.
- Mechanical changes are required in auxiliary systems including:
- Ink pumps used for solvent-based inks are not compatible with water-based inks and must be replaced.
- Printing plates used to transfer ink to the substrate must be replaced with plates comprised of materials compatible with water-based ink.
- Solvent recovery equipment that will need to be replaced with water treatment systems.
- Substrate may not be compatible with water-based inks.
- Personnel will need training
For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.
The U.S. EPA has developed and published guidelines for state/local air pollution control agencies that enable them to ascertain which technologies are acceptable to reduce the impact of pollutants in the process. These are called Control Technology Guidelines (CTGs). CTGs set the minimum air pollution control requirement for a given industry or process. The basic CTG for flexographic printers was promulgated in 1978 and provides for three control technologies: add-on controls, high solids ink, and water-based ink. The carrier, water, must represent at least 75% of the liquid volume of the ink as applied. These requirements are applied to only certain printing operations, those designated as "major sources". The definition of a major source depends upon the geographic location of the facility and its actual and potential emissions of VOCs. All states have set the emission levels for major sources.
Waste Water - Wastewater based inks, water containing ink and cleaning compounds are generated during the process printing and press cleaning. The wastewater may be restricted from discharging into the sanitary sewer depending on the type of inks and cleaning products used. Depending on the pigments in the waste, which may contain regulated metals, the waste may be classified as hazardous. The regulated metals differ among each state.
Discharge into a Publicly Owned Treatment Works (POTW - sewage treatment plant) is the most common means of wastewater discharge. Some POTWs require all businesses that discharge a process waste to obtain a local discharge permit while others have an agreement based on local ordinances that the POTW will accept the waste water as long as no disturbances or problems as a result of the waste water the printer is discharging. In those cases the printer may be required to perform monitoring and reporting to the agency and may be required to obtain a permit with specified discharge limits. The waste discharged to the POTW should always be consistent with what is specified in the local ordinances or facility specific permit limits.
To discharge wastewater directly into a waterway a facility must obtain a permit specifying allowable pollutants and flow of discharge. This permit is known as a National Pollution Discharge Elimination System (NPDES) or in some cases a State Pollution Discharge Elimination System (SPDES) permit. These permits have very stringent limits on the allowable level of pollutants that can be discharged and require monitoring of effluent, periodic testing of the water being discharged and extensive monitoring, record keeping and reporting.
The most efficient method of determining if a wastewater discharge permit is required or specific discharge limits apply due to your industrial activities is to first identify your waste streams that may affect the wastewater quality and then contact the local governing body and inquire about industrial waste water discharge regulations. Be prepared to be required to pay for analytical costs associated with evaluating your wastewater.
Health & Safety
Use of proper protective equipment, such as safety glasses, gloves and an apron should be used when handling the ink and ink waste.