AQUEOUS COATINGS
SHEETFED OFFSET PRESSROOM
ARE AQUEOUS COATINGS DIFFERENT FROM PASTE LITHO INKS?
Aqueous coatings differ from paste litho inks as liquid flexo and gravure inks do. Aqueous coatings look a bit like milk or cream and pour similarly. Unlike paste inks that are formulated to stay open and not dry up on the ink roller train, aqueous coatings are formulated to dry very fast and consequently must not be allowed to enter the ink roller train. They are made from thermoplastic resin solutions and emulsions, most will be about 40% solids and 60% water and other volatiles. Importantly, hard drying takes place as the water /volatiles are evaporated or absorbed.
HOW ARE AQUEOUS COATINGS PACKAGED?
Aqueous coatings are normally supplied in 55 gal. disposable fiber drums, or they may be supplied in 330 gal. (6 drum) corrugated disposable or returnable totes, or if volume is high, by 5000 gal. tanker, Sample quantities are supplied in 5 gal. plastic kits.
WHAT SHELF LIFE LIMITS ARE THERE?
Cork aqueous coatings typically have a 6-month shelf life under ambient storage conditions. It is recommended that coating inventory be used in a fist in first out basis.
HOW SHOULD AQUEOUS COATINGS BE STORED
Cork aqueous coatings must not be frozen. Precautions will be taken to assure that the product is not frozen in shipment. The customer must also take steps when handling and inventorying product so that it is not frozen. It is also important to note that aqueous coatings are similar to single viscosity motor oils in that lower temperatures thicken, and higher temperatures thin them. Any changes from its room temperature, (70 degree F) design viscosity will change the coating running characteristics. It is recommended that the aqueous coatings be stored and used at as close to 70 degrees F as possible so that running results can be optimized.
SHOULD A NEW DRUM OF COATING STIRRED
There are components of some coatings that may tend to settle out over a period of time but there are other volatile components that will tend to be freed into the headspace. It is therefore advised as a good practice to rock, roll or stir the kit, drum or tote to assure a good blend of fresh coating. Also, since the normal smell of most Cork aqueous coatings is sweet, ammonia, the chance of opening a drum and finding a sharp release of volitalized ammonia is reduced if the package is thoroughly agitated before opening .
ARE REDUCTIONS OR ADDITIONS EVER RECOMMENDED?
It is recommended that Cork aqueous coatings be used as received at the viscosity supplied. Formulations will be supplied at viscosities tailored to the requirements of specific coater or pressroom specifications. If a specific formulation is designed to accept a press side reduction or other addition, it will be noted on the products technical data sheet.
WHAT PUMPING DEVICES ARE RECOMMENDED?
Diaphragm pumps are recommended to both pump and circulate aqueous coating from the supply container to the coater and back. The pump should be run at a rate that satisfies coater demand and does not recirculate coating excessively, creating foam. The return line must not suck and pump air into the coating supply, creating excessive foam. Proper circulation assures that a thoroughly blended coating is supplied from the top to the bottom of a supply container, as it is emptied. Aqueous coatings are a complex chemical composition where the balance of each component is key to performance. Thorough coating blending throughout a run assures that the chemical balances that are formulated into a coating are consistent, allowing optimal performance to be achieved.
HOW CAN EXCESSIVE FOAMING BE AVOIDED?
The amount of amine or ammonia used to solubilize acrylic resin is a primary determinant of a coatings' pH or acidity/alkalinity balance. Improper pH can contribute to foaming. The amine or ammonia component of a coating will evaporate from an open drum, which is why it is recommended that a coating container always be covered. Coating flowing back from the fountain via the pumping system will carry some entrained air with it. Simply putting a loop in the return line allows some of the air to escape. For example, direct the return line flow through a perforated pipe leading from a modified drum cover to the bottom of the supply drum. Don't just pump a stream of return line coating splashing into the supply container. Also watch the use of detergent wash-up solutions that will aggravate foaming conditions if all traces of the wash-up are not thoroughly rinsed away. Beware of adding defoamers! Sometimes it may be advised by a Cork representative that a particular defoamer solution be added in order to overcome a bad situation. Follow the advice carefully and with caution. While a little defoamer addition may help, overuse can be harmful, disturbing the chemical balance of a coating, causing transfer, and other performance properties to suffer.
HOW IMPORTANT IS VISCOSITY?
Viscosity of an aqueous coating is very important to its running characteristics and the final dry film properties that are achieved. A viscosity measurement tells us how thick or thin a liquid is by measuring the fluids' resistance to flow. Temperature affects the viscosity of aqueous coatings, hotter = thinner, colder = thicker, Cork manufactures to exact specifications, measuring viscosity at 70 degrees F., using a sophisticated Brookfleld viscometer. Cork retains batch samples for a minimum of one year. Field measurement of viscosity is not an exacting science when Zahn cups are used. Many factors can affect a viscosity measurement, including the exacting procedure required at an exact temperature, cleanliness and a dent free cup, a thoroughly blended coating sample, a foam free sample, etc., etc. Field viscosity checks are best when they are used to make a side by side comparison to another sample, especially a known NIST standard traceable oil. See Cork 'TECHTALK NEWS Sep 92", for more information. Cork recommends that its' coatings be used as received. Field viscosity adjustments are not advised because it is too easy to upset the chemical balance of a formulation adversely affecting both the running and final dry film design properties.
HOW SHOULD THE COATER BE SETUP TO RUN?
Cork's advice, is that the coater should be set up following the manufacturers recommendations. Typically though it can be said that a kiss impression, where there is minimum printing pressure between the plate and blanket and/or the blanket and impression cylinder, will produce the best coating results. Also repetitive stops and starts should be avoided. All make-ready operations should be completed before coating is supplied to the fountain.
WHAT OTHER ADVICE DO YOU HAVE ABOUT BLANKETS?
The latest advice seems to be that the harder the blanket durometer the better, which minimizes ink dryback to the blanket, especially on high speed presses. Sticky-back blankets have been used to pattern coat from plate cylinders by precise cutting to relieve glue flaps. Engravers type blankets, which permit easy cutting and don't delaminate at high press speed have been used with blanket coaters. The latest in spot coating technology are photo polymer plates (Cyrel), offering superior registration, transfer and lay.
WHAT HAS TO BE DONE WHENEVER THE PRESS IS STOPPED?
Again, follow the press/coater manufacturers' recommendations. But EVERY TIME that the press is stopped, any surfaces that are not rotating and being rewet and replenished with coating, MUST be sprayed or rewet with water to avoid the drying of the coating on these surfaces. Before restarting these surfaces must be washed so that they are tack free. Remember, aqueous coatings are designed to dry very fast to produce a tack free sheet that will not setoff.
WHAT WASH-UP SOLUTIONS ARE RECOMMENDED?
Straight water will wash-up most wet aqueous coatings, however a few formulations will cleanup better with a 90% water/10% alcohol mix. Whenever a coating begins to dry, it will begin to resist wash-up, and it will be necessary to resort to a 10%-25% ammonia/water solution or commercial blends such as "Fantastic" or "409". Once a coating is completely dry it becomes very difficult to remove except by physical means such as mechanically scraping.
HOW DO PRESS VARNISHES AND AQUEOUS COATINGS DIFFER?
Press varnishes are composed basically like offset inks, but without pigment. Most varnishes are about 75% solids, but new high solids types may range as high as 95% solids. They set like offset inks by absorption and dry chemically by an oxidation/polymerization process. Varnishes are used like inks, and can be pattern or spot printed easily. While they are applied in a thin film they are still slow drying (hours or days), require the use of copious amounts of spray powder to prevent setoff and yellow with age. Some jobs will require short pile, or rack delivery. Aqueous coatings by contrast are unlike paste offset inks in that they typically range to 45% solids with the remainder water and minor amounts of other volatiles. They dry when the volatiles are evaporated and absorbed leaving the solid materials to form a tack free film. Aqueous coatings will be dry to the touch as the sheet reaches delivery in a property equipped and properly running line. Most printers still rely on using a light volume of a small particle size starch spray powder. However, some printers with adequate drying capability, run spray powder free, drying primers and very fast drying coatings effectively. Aqueous coatings will dry water clear and formulations that contain optical brighteners will even dry whiter looking than the sheet. Pattern coating is done routinely using the techniques described earlier, above.
ARE SPECIAL INKS REQUIRED WHEN USING AQUEOUS COATING?
It must be remembered that offset inks are oil based and aqueous coatings are water based. Since oil and water do not mix aqueous coatings must be formulated with special surfactants that allow the water coating to wet out, and lay smoothly and completely over the printed surface. No special inks are required when aqueous coating, but inks that contain PTFE Teflon waxes, or silicone additions are to be avoided. In the early days of aqueous coating wax free inks were required but today high rub inks containing some non-PTFE waxes are being coated. Caution is still the watchword and the best advice is always to test to be sure and remember that all inks are not alike, and there are different coating formulations too. It can also be mentioned that radiation curable LV and EB offset inks are being successfully overcoated with Cork aqueous coatings, as are Waterless inks - the latest ink technological advance. More-over, print quality is paramount since a poor print will not be enhanced by coating but will show reduced gloss and print fidelity.
HOW IS IT POSSIBLE TO DRY THESE COATINGS IN-LINE BEFORE DELIVERY?
The most important thing to remember is that these coatings are approximately 60% water, which must be evaporated or absorbed before the coating solids, can form a tack free, micro-porous (dry) film. Technically, the coating initially dries when the resin solids molecules link, or join together, to form a thin film by a process called coalescence. This drying phase accounts for about 90% of a coating's drying, with the remaining 10% occurring over perhaps 24 hours when full properties are completely developed. In order for these fast drying coatings to be dry enough to be handled in delivery without blocking and setoff, it is necessary to equip a press with an adequate combination of drying air and dwell time. It is especially important to strip laminar airflow, a physical phenomena that holds evaporated water vapor close to the sheets surface, away with an air knife. Removal of this boundary layer moisture allows efficient evaporation. IR emitters (short and/or medium wave) are used to accelerate ink setting via absorption and oxidation, and by elevating the temperature of the coating, evaporation is increased. It is also essential that the high volume of moisture-laden air be removed or extracted from within the drying zone or drying will be impeded. Regarding maintenance, IR reflectors should be routinely cleaned to guarantee consistency of rated energy output, or drying will be slowed.
WHAT CAN BE SAID ABOUT PILE OR LOAD TEMPERATURE?
Press manufacturers customarily make recommendations about pile temperatures, especially related to ink drying. Cork recommends a temperature of 95 degrees F. in the pile when coating, measuring the temperature in areas of ink coverage. When running Work & Turn, coating both sides of a sheet, tower 85~8 degree F. pile temperatures are recommended for the second pass to prevent re-softening of the still drying first pass coating. Remember, too high a load temperature may cause setoff, especially in higher loads where greater pressures are a factor. A pile temperature of 110 degrees F. might cause blocking and setoff under the right circumstances. Inks also must be considered since just because they are covered with a coating, doesn't mean they have dried any faster. Sure, IR heat accelerates the setting and drying of inks, but they are still soft and not fully oxidized/polymerized because this process is time dependent.
HOW BLOCK RESISTANT ARE AQUEOUS COATINGS?
Aqueous coatings are thermoplastic, and even when completely dry they will become soft and moldable under conditions of elevated heat, pressure and humidity. Most fully dried/cured Cork coatings will be block resistant under conditions of 140 degrees F., for 24 hours at 95% RH, under a 2 PSI load, face to face. A few formulations will be resistant up to 130 degrees F. A fully dried cured state is not reached until approximately 24 hours after printing/coating. However, it must be noted that many factors can compromise these anticipated results, such as extremely slow inks, very heavy ink coverage, high moisture content in printed inks or paper-paper/board, non-absorbent stock (PE, Vinyl, Polyester, Foil, laminated), synthetic stocks, very humid drying conditions, etc.
CAN WE WORK & TURN SUCCESSFULLY COATING TWO SIDES?
W & T production is being done successfully, but every job can be different and success is dependent on the variables discussed above being right. The time to W & T varies and is dependent on all of the things that go into a job. Essentially, the first side inks must be set enough, and the coating dry enough. The second pass should not be attempted if the load temperature remains over 90 degrees F. The time to W & T successfully is an individual press room determination. Your experience based on your equipment capability, inks, coating, substrate, and job, will dictate what can be done in terms of time. As a rule of thumb, the fastest inks and the fastest coatings should be used with the minimal second pass IR and heat, to effectively dry the coating for delivery. Shorter loads and slower press speeds may be required or simply more time may be needed until the second pass is attempted.
HOW ARE THE HIGHEST GLOSS COATINGS ANY DIFFERENT?
The highest gloss coatings will have higher solids and consequently there will be more dry film on the sheet after the volatiles are evaporated. Considering the resins involved, these coatings tend to be more difficult to dry. Also, the highest gloss coatings will offer less rub resistance because waxes that are added to improve rub reduce gloss, therefore the smallest amounts are used. Mother thing to watch is heavy ink coverage (over 350%), because gloss may be lost if the coating is absorbed into the heavy ink film and crawling or shrink cracks may appear in the dry coating film.
HOW WOULD YOU SUMMARIZE?
TEST to learn and thoroughly understand the limitations of your printing/coating production operation! Each specific job can be different, and unlike previous work. Never assume that all conditions and variables that can affect the final acceptable result will remain the same. TEST AND KNOW! STOCK INK FOUNTAIN SOLUTION. ENVIRONMENTAL CONDITIONS are all interrelated and can, and do, affect the quality and acceptability of the work produced. TEST to learn and thoroughly understand the limitations of your printing/coating production operation! Each specific job can be different, and unlike previous work. Never assume that all conditions and variables that can affect the final acceptable result will remain the same. TEST AND KNOW! STOCK INK FOUNTAIN SOLUTION. ENVIRONMENTAL CONDITIONS are all interrelated and can, and do, affect the quality and acceptability of the work produced. TEST to learn and thoroughly understand the limitations of your printing/coating production operation! Each specific job can be different, and unlike previous work. Never assume that all conditions and variables that can affect the final acceptable result will remain the same. TEST AND KNOW! STOCK INK FOUNTAIN SOLUTION. ENVIRONMENTAL CONDITIONS are all interrelated and can, and do, affect the quality and acceptability of the work produced.
AQUEOUS COATINGS are not all the same. Coating manufacturers have the ability, with the raw material selection available, to formulate for a wide variety of requirements. Always discuss both your and your customers requirements with your coating supplier, so that you can expect to have the best formulation available for the particular work that you are attempting. AQUEOUS COATINGS are not all the same. Coating manufacturers have the ability, with the raw material selection available, to formulate for a wide variety of requirements. Always discuss both your and your customers requirements with your coating supplier, so that you can expect to have the best formulation available for the particular work that you are attempting. AQUEOUS COATINGS are not all the same. Coating manufacturers have the ability, with the raw material selection available, to formulate for a wide variety of requirements. Always discuss both your and your customers requirements with your coating supplier, so that you can expect to have the best formulation available for the particular work that you are attempting.
Elmer W. Griese - revised 1/96
Cork Coating and Adhesives
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Last modified: October 03, 2002
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