DAMPENING SOLUTION AND DAMPENING SOLUTION ADDITIVES
Ideally, the dampening solution should possess a water hardness of 8 ° dH to 12 ° dH and a pH-balance of 4.8 to 5.5. The typical dampening solution temperature ranges between 10 °C to 15 °C. At the same time, a printer must know that at low temperatures condensation water collects on tubes and in the dampening solution vats, and this may lead to the formation of water droplets. Dampening solution additives are complex material systems with various components included to promote adequate emulgation and wetting (surface tension). They are important for pH-Balance adjustment and for stabilisation (buffer systems), protection against corrosion, for a cooling effect, and in avoiding slime formation (biocide).
WATER and Water Treatments
Water (H2O) consists of hydrogen and oxygen, but as tap water it reaches the printer in different qualities and different degrees of purity, depending on its origin (groundwater or other). Even in clean rainwater, soluble gasses and dirt particles are present. To assess the quality of water the water’s hardness is measured, which largely depends on the quantities of calcium and magnesium present. In any case, the hardness of the water must be calculated before any additives are introduced, since hardness is no longer easily determined in a prepared dampening solution. Depending on the concentration of calcium and magnesium salts, water is classified from hard to soft. Hardness is expressed in degrees. One degree German hardness (1°dH) is equal to 10 mg of calcium oxide per liter water.
Just as we cannot drink water from every tap in the world,we cannot expect perfectly balanced water to be available for printing purposes around the globe. There are several types of water treatment methods and one of them mostly used is reverse osmosis.In the process,the water is pressed against a membrane.Water treated like this, emerges with a very low residual salt content. Subsequently, this osmosis water is reconditioned with salts, until it reaches a degree of hardness ranging from 8° dH to 12° dH.
pH is the unit of measurement for acidity or alkalinity.It is derived from the Latin (Potentia Hydrogenii) and represents a logarithmic description of the concentrations of hydrogen ions. In other words, the pH-Balance is a measure used to determine the acid or alkaline content of aqueous solutions. Solutions with a lower pH are called acidic, and solutions with a higher pH are called alkaline. The pH scales ranges from 0 to 14 (7 is neutral solution). Low pH (acidic) conditions cause water to be corrosive.
Acids will cause pitting of concrete, dissolve metals, wrinkle vinyl, and irritate skin and eyes. High pH (alkaline) conditions cause scaling: minerals (calcium, copper, iron etc) precipitate out of the water and those minerals will block filters and pipes. Depending on the pH value, calcium carbonate contained in the paper will react or not react with the fountain solution. At high pH values, calcium carbonate will be stable, but at low values, there can be an interaction between paper and water.
To keep the pH on a stable level, the fountain solution must be buffered. The pH can be influenced by an interaction between fountain solution, paper and ink. For this reason, fount systems are always buffered to avoid pH fluctuations. To design pH levels and to make a stable fountain solution, a salt combination is necessary. Correct combinations of acid in the buffer are needed for thorough plate wetting without any deposition. In order to achieve the necessary thorough plate wetting without, on the other hand,
causing deposition, the buffer must contain the correct combinations of acid. In modern dampening solution admixtures, the correct pH-Balance is automatically predetermined, if dosages are mixed in according to instructions. Buffering prevents paper and ink from altering the pH-Balance.
Naturally, in order to determine the quality of the dampening solution, its conductivity should also be determined.
Conductivity = μS/cm
Conductivity describes how electricity is conducted through a liquid; impurities in the dampening solution allow conductivity to increase. Conductivity varies depending on the water and additives. The temperature, and the concentration of alcohol also influence conductivity.
By increasing Isopropanol (IPA), conductivity declines. Modern conductivity gauges also measure for temperature. It is important that the conductivity gauge in the central dampening solution be regularly cleaned and recalibrated. Conductivity should be determined using a “freshly prepared dampening solution”, so that this measure can then serve as a “benchmark” when the dampening solution is later exchanged. When the conductivity in the dampening solution has climbed by approx. 1000 μs/cm, this should be taken as a signal that it is time to change the dampening solution. In order to guard against printing problems, it is recommended that the dampening solution be renewed every 14 days. By introducing optional dampening solution filters (e. g. softflow), the useful life of the dampening solution can be substantially prolonged.
The pH-Balance, the temperature, as well as conductivity can be measured by means of a universal test control device. All electronic measuring instruments must be regularly re-calibrated.
The use of IPA
IPA (isopropyl alcohol) in varying doses has been used in sheetfed offset printing for almost 25 years.
Arguments for and effects of the use of IPA:
- Reduction of the surface tension in order to achieve thorough wetting of the printing plate (thin and homogeneous dampening film)
- Increase of fountain solution viscosity in order to achieve uniform fountain solution transport from the water pan to the printing plate
- Rapid evaporation of IPA generates a cooling effect
- Improvement of emulsification and stable ink / water emulsion
- Antibacterial effect
- Reduction of foaming
Arguments against the use of IPA
- Environmental damage due to the presence of VOC (Volatile Organic Compound)
- International legislation aimed at a reduction or total elimination of VOC emission
- Additional taxation in many countries IPA in the air is limited to max. 150 mg/m3 MAC (= the Maximum Allowed Concentration) in many countries
- IPA in the air may cause increased physical strain (e.g. respiration difficulties)
- The ignition temperature of fountain solution with IPA is lower than 50 °C. This implies a danger of fire and explosion, especially in case of incorrect handling and technical errors
- IPA is expensive
The measurement of alcohol in the central dampening solution is conventionally performed by using the density of the dampening solution (float gauge). However, the density of the dampening solution is not only influenced by the IPA-content, but also by the temperature, the kind of additive being employed, and the degree of pollution. Consequently, regular cleaning is vital. Modern measuring procedures, such as infrared or ultra-sound, are largely unaffected by foreign substances.
DAMPENING SOLUTION PRINTING PROBLEMS
build-up on the rubber blanket: Attack on the paper coating from acidic dampening solution.
blank runs: Deposits on rollers, the rubber blanket, and the plate.
plate deterioration: The printing layer is destroyed, the additives are too aggressive. Incorrect machine calibration.
plate corrosion: Plate oxidizes, protection of the plates by means of additives is not sufficient.
over emulsifying: pH-Balance is too high, too much water, the water is too soft, the additives are too high, the rollers are incorrectly adjusted, too much IPA, very little ink reduction
lathering: Circulating detergent, runback performance set too high, additives are too foamy.
poor drying: pH-Balance too low, incorrect print substrate ink combination, pH-Balance of the substrate to be printed is too low
poor freewheeling: pH-Balance is too high, the IPA is too low, plate protection is insufficient, roller calibration is incorrect, ink/dampening solution mixture is not correct. slime, odor: An underdose of the additive, germ infested water, the formation of resistant bacteria.
smearing: Too little dampening solution, dampening solution no longer fit for use, contaminated, incorrect machine adjustment.
spattering: Overemulsification, incorrect balance of ink/dampening solution.
scumming: pH-Balance too high, plate protection insufficient, plate poorly developed, ink-guide set too high, deposits on the plate or rubber blanket, IPA too low, the ink/dampening solution balance is incorrect, tempering is incorrect.
tapered mullers: Too little hydrophylic substance in the dampening solution, chromium is taking the ink.
accretions: Wrong mix, over-emulsification, pH-Balance too high, IPA too low.