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Cleaner technology processes
for washing off water-based flexographic inks
The objective of this project was to describe cleaner technologies in the process of
washing off water-based flexographic inks in the packaging printing industry.
The project concentrates on environmental assessment of processes and chemicals in
relation to waste water. This includes cleaning in connection with the printing on
absorbing materials (e.g. corrugated cardboard) and non-absorbing materials (e.g. plastic
film). As regards types of industries, the project covers those using water-based
flexographic inks in the production of packaging such as cardboard boxes, paper bags,
envelopes, cartons and flexible packaging (e.g. plastic bags).
The tools used in this review of the current state of the process of washing off
water-based flexographic inks comprise investigations at six packaging printing companies,
a questionnaire survey (twenty packaging printing companies), literature search,
participation in trade fairs etc. and information from more than thirty individuals with a
thorough understanding of the packaging printing industry. Chemicals used, i.e. inks and
cleaning agents, were hazard screened by use of ranking systems for the aquatic
environment and health.
In the cleaner technology work, i.a. laboratory tests, computer simulations,
investigations at pilot plants and at a system in operation at one of the participating
printing firms were used. Furthermore, immediate possibilities of substitution of
chemicals are given.
In Denmark today, the process of washing off water-based flexographic inks is mainly
carried out within the corrugated cardboard printing industry (approx. 15 companies) but
also envelope printing firms (approx. 3), "paper bag" printing firms (approx.
2), a single cardboard printing company and 1-2 flexible packaging printing firms are
washing off water-based flexographic inks. The latter, however, only on a very limited
scale (only lac/coating and white ink). For technical reasons, proper printing with a
single or more inks on non-absorbing material (flexible packaging, plastic) is still not
particularly widespread and was only recorded at a single printing firm printing on
plastic rubbish bags.
30% (corresponding to approx. 75 production units) of the total production of packaging
printing in Denmark are considered to be produced by use of flexographic printing
techniques. Of these, 20-30 production units print with water-based flexographic inks.
In order to transport the ink from the ink container to the plate, which transfers the
ink to the printing object (e.g. corrugated cardboard), some ink units (ink feeding
systems) are placed on the flexographic printing machines. These ink units consist of i.a.
pumps, tubes, pipes and an open ink fountain or a closed doctor blade system placed in
close contact with the anilox-roller. The anilox-roller secures the transfer of the ink to
the plate. The ink unit (including anilox-roller) and the plate are the parts of the
flexographic printing machine that get in contact with the ink and thus require cleaning.
Apart from the waste of ink during the actual washing of the ink unit, spillage of ink
also occurs during the printing operation due to leakage.
In today’s Denmark, it is estimated that there exist approx. 300 ink units with
open ink fountains (earlier systems) and 150 with doctor blade systems (recent systems).
Estimates indicate that there have only been installed high pressure nozzle washing
units at 40-50 ink units and automatic flow at 5-10 ink units in the printing industry
today. The bulk of the ink units are thus manually washed.
Furthermore, it is estimated that a total of around 150,000 wash-offs of water-based
flexographic inks are made a year and that more than 90% of these wash-offs are made
within the corrugated cardboard packaging printing industry.
Within today’s printing industry, direct ink spillage during operation of ink
units with open ink fountains is estimated typically at 5-10 kg ink calculated per
wash-off. Correspondingly, direct spillage using doctor blade systems is estimated at
0.2-2 kg per wash-off. Direct ink spillage at minor ink units (e.g. for bag and envelope
printing) is in all probability less.
The ink waste from the actual wash-off is estimated at around 3-6 kg per wash-off for
open ink fountain and at 2-4 kg per wash-off for doctor blade systems.
Within the flexographic printing industry today, the total ink spillage (direct
spillage + waste during wash-off) is estimated at around 8 kg ink per wash-off, which
amounts to a total annual ink waste in this industry of approx. 1,200 tons corresponding
to around DKK 38 mill.
The annual consumption of water-based flexographic inks is estimated to amount to
around 3,000 tons, of which water-based flexographic inks for printing on non-absorbing
materials account for less than 1%. The total ink waste of 1,200 tons a year constitutes
40% of the consumption.
Within the flexographic printing industry, the annual consumption of cleaning agents
and water for washing of ink units is estimated to constitute 20 tons and 50.000 m3,
respectively.
Estimates indicate that around half the printing firms are treating their waste water
prior to discharge to sewer. The most frequently applied technique is flocculation and a
few companies use membrane filtering and evaporation. On this basis, it is estimated that
around 600 tons of ink is annually ending up in the sewerage systems.
The water-based flexographic inks contain components that, based on hazard screening,
are assessed to be undesirable in the sewerage system or unacceptable for health reasons.
These components might be certain binders (rosin compounds), a single solvent (ethylene
glycol monomethyl ether), organic pigments, certain dispersing agents (alkyl phenol
ethoxylates, certain quaternary ammonium compounds), certain cross-linking agents
(aziridine), moisteners and almost all applied preservatives.
Wash water from washing off water-based flexographic inks may be characterised by a
weakly basic pH value, an AOX content of up to around 30 mg/L dependent on type(s) and
actual amount of pigment, COD/BOD5 ratio of around 20-30 and weak to moderate
inhibition of nitrification.
Practically all washing of plates is reckoned to be performed by separate (off-press)
manual washing and in approx. half the cases using water only. An overall estimate for the
packaging printing industry indicates that around 500,000 plate washes are made annually
with a consumption of approx. 15 tons of cleaning agents. The volume of the ink residues
that are washed off is insignificant compared to the volume of ink residues from washing
of ink units. The consumption of fresh water is estimated at 2,500 m3 and the
wash water is typically treated together with the wash water from the washing of ink
units.
The typical content in cleaning agents used in this industry is predominated by
substances considered relatively unproblematic if they are discharged to public wastewater
treatment plants in not too large volumes. However, substances may occur that are
undesired in the sewage system or unacceptable for health reasons. I.a. they are some
cationic detergents and some complexing agents (trisodium nitrilo acetate and nitrilo
acetate).
Wash water from plate washing, in which an alkaline detergent-based cleaning agent has
been used, may be characterised by a weakly basic pH value, an AOX content of up to around
1.5 mg/L dependent on type(s) and actual amount of pigment, COD/BOD5 ratio of
around 2 and moderate to unacceptable inhibition of nitrification (dependent on type of
cleaning agent).
Today, the predominant technique used in separate (off-press) wash of anilox-rollers is
blasting with baking powder. This will typically take place twice a year at the most.
An overall estimate indicates that the volume of chemical waste and air emission
generated during wash-off of water-based flexographic inks is insignificant.
As an overall assessment of the wash-off of water-based flexographic inks, it can thus
be concluded that the process, which is predominantly the most potential hazard to the
environment, is the washing of ink units and that the impact primarily originates from the
ink residues washed off.
Candidates for substitution among components in water-based flexographic inks are
especially found among binders and emulsifiers. I.a. they are rosin-based binders, which,
for environmental reasons, should be replaced by environmentally less harmful altenatives
as e.g. polyacrylates with a very low monomer content («200 ppm). Among
emulsifiers/dispersing agents are i.a. alkyl phenol ethoxylates and traditional quaternary
ammonium compounds, which should be replaced by linear alcohol ethoxylates and more recent
quaternary ammonium compounds containing ester compounds (DEEDMAC, DEEDMAMS),
respectively. Other functional groups containing ink components, which, for health or
environmental reasons, should be substituted, comprise i.a. antifoaming agents,
moisteners, preservatives and cross-linking agents. To these may be added pigments, of
which the majority is lacking studies to clarify especially their environmental
properties. There are few candidates for substitution among the components in the cleaning
agents. They are only the traditional quaternary ammonium compounds and certain complexing
agents (NTA).
Measurements made during this project on working printing machines at a corrugated
cardboard packaging printing firm show that the first litre of rinsing water from two not
optimised washing processes, i.e. manual washing of open ink fountain and high pressure
nozzle washing of doctor blade system, may contain approx. 25% and 40%, respectively, of
the ink residues (the ink dead volume) in the ink unit. In both cases, the ink content of
the rinsing water was 70-80%. If this first litre of rinsing water ends up in the ink
container with a content of 15 litres of ink, it will only result in a dilution of approx.
1%. At optimised high pressure nozzle units with very low ink dead volumes (0.6 litres),
the use of short water boosts may result in recovery percentages above 80 by adding the
first litre of rinsing water to the ink container with a resultant dilution of only around
1%. Investigations of the impact of minor dilutions with water on the printing properties
of typical flexographic inks strongly indicate that dilution in the order of 1-8% does not
decisively influence the ink. Furthermore, it is assessed that rinsing water with a
content of ink residues (in any case black) will be recyclable as ink with an addition of
concentrated ink. Presumably, this should also apply to other colours.
With the existing technology within this printing industry, an overall estimate is that
simply by catching the first litre of rinsing water and recycling it as ink, 25-50% of the
ink waste from wash-offs may be avoided. By paying attention to the avoidance of ink
spillage during operation (leak-tight gaskets, optimal doctor blade systems, avoiding
overflow and long-term idle running), it is considered possible to achieve a reduction of
the direct ink spillage of at least an equivalent percentage. It is thus assumed that with
the existing technology by relatively simple measures, savings of at least 25-50% of the
present total ink waste may be achieved, corresponding to 300-600 tons of ink annually
(DKK 10-20 mill.). By optimising these initiatives, even higher recovery percentages may
be achieved. By introducing more recent technology, i.e. optimised nozzle washing systems
with doctor blade systems and ink drip trays, it is estimated that ink recovery
percentages of more than 80% might be achieved and notably from a very small ink residual
volume (approx. 0.6 litres). On this basis, it is estimated that the ink waste may be
reduced to around 1-2% of the estimated current level.
By relatively simple measures (attention to avoidance of unnecessary consumption of
fresh water, i.e. "good housekeeping"), it is estimated that, with the existing
technology, the water consumption of the packaging printing industry could be reduced by
70-80%. If the newest optimised technology is implemented, a reduction in the order of 95%
compared to the estimated current level may be achieved. If these measures are combined
with reverse flow flushing, a reduction of more than 98% may be achievable.
If the above measures regarding the minimising of ink waste are combined with reverse
flow flushing in the rinsing processes, and the concept regarding addition of concentrated
ink to ink-containing rinsing water is combined with the introduction of automatic
continuous control of viscosity and pH, an additional considerable reduction of the ink
waste may be achieved. Printing companies introducing optimised water treatment such as
ultrafiltration (possibly combined with after-polishing) or evaporation may achieve
wastewater-free operation. If potential investigations of the possibilities of recycling
of concentrate from ultrafiltration turn out well, almost 100% recycling of the ink
residues should be achievable.
Some development is required, e.g.: Development of ink units with very small ink dead
volumes (quantity of ink residues), testing/development of concepts regarding recycling of
rinsing water with addition of concentrated ink and testing/development of the concept
regarding recycling of concentrate from water treatment. As for applied chemicals and
discharge of waste water to sewer, especially a clarification of the environmental
properties of the pigments is essential.
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