Water based coating: Guidelines to achieve excellence

Micro Inks has recently introduced a whole new range of emulsion coatings based on proven German technology, which includes products for all requirements like gloss coating, matt coating, foil coating, duct coating, gravure coating and primer coating, etc. Prasanta Sarkar, general manager -technical at Micro Inks Limited provides technical information on the processing, application, drying and other physical characteristics of water-based coatings.

All the water-based coatings are supposed to be stored in a cool but frost-free place. Storage at temperature higher than 40º C will increase the viscosity of the coatings. Though, the shelf life of the coating is six-months from the date of manufacturing if the container is not opened, but after opening the container, the coating should be used up as quickly as possible. If the water based coating is stored over a period of several weeks, it may gain in viscosity due to the formation of temporary structure in the coating. However, the coating may be brought into its original condition simply by vigorous stirring and if necessary the coating could be diluted with water for viscosity reduction. A thumb rule as regards to dilution: one percent addition of water dilutes the coating viscosity by five seconds. The characteristics of the coating are not adversely affected by such dilution (max five percent water). The coating need to be stirred well and its viscosity checked before use.

Film formation

The water based coatings are generally formulated for a minimum film forming temperature (MFT) of between +5º and +10º C. For safety, the coating needs to be allowed to adjust to room temperature (during cold season) before it is applied. Working at a temperature below the MFT usually leads to wetting and adhesion problems.

Processing procedure

If applied too quickly, water-based coating tends to squeeze out at the rear end of the sheet. It is therefore absolutely essential to cut out the board backing under the rubber blanket to the size of the surface being coated, in order to eliminate squeeze beads that might tend to stick. The backing should be cut out to a size little smaller than the sheet format. Coating must not be applied beyond the edge. The installation of appropriate IR radiators and hot-air blades with air extraction is essential.

Coating by indirect systems

The basic difference between indirect and direct applications is the fact that the indirect systems involve a longer transport path for the coating, since it travels via the plate cylinder. The drying speed of coatings for such systems therefore often needs to be reduced in order to prevent paper sticking. If a water-based coating is applied from systems that allow the dampening unit to be converted into the coating device, we recommend to replace the fount roller with a coating application roller having hardness of approx 45 Shore and the fount pan with a coating pan. If the sheet does not occupy the entire width of the press, we recommend attaching suitable squeeze rollers or blades to the fount pan rollers. This counteracts any build-up or drying of coating on rollers, printing plate or the rubber blanket.

Coating by direct systems

In this case, the coating film is transferred directly from the forme roller to the forme cylinder and onto to the stock. It is generally advisable to work with compressible rubber blankets that are not too soft. Make sure that the coating is applied uniformly and at correct rate. Too little coating can lead to piling and inadequate surface protection and gloss, whereas too much coating can lead to splashing and drying problems in the stack.

Coating by means of screen roller and chambered doctor blade

This type of coating with flexographic process, where nothing but the pickup volume and the screening determine the amount of coating transferred, acts as the main type of unit in use. Coating film thickness could only be altered by changing the screen roller. The following guideline gives an idea of the amount of coating transferred: about 30-50 percent of the stated pickup volume of the screen roller is transferred to the substrate, e.g. if the pickup volume is 10 cm3, 3 to 5 gsm of wet coating will be transferred to the substrate.

Cleaning the press

When the coating unit is stopped even for a while, the plate and the rubber blanket must be thoroughly cleaned by washing. Special cleaner should be used along with water for better cleaning efficiency. It should be noted that it becomes very difficult to clean the dried acrylic film; therefore it is recommended to immediately clean the coating unit after every stoppage.

Water based coating and glue-ability

Solvent based system are not recommended for subsequent processing of aqua based coated surface (e.g. solvent-based adhesives for film lamination), because the solvent causes the coating film to swell. This results in poor adhesion of the laminated film with the substrate. Water based coated surfaces are suitable for gluing if appropriate quality water-based adhesive is used. Despite the good glue-ability on water based coating, the folding-carton production sector today always leaves blank areas when coating. The reason for this is that the water based adhesives set considerably faster directly on the uncoated, absorbent stock with the result, to achieve shorter cycle times.

Resistance properties of inks

Generally the water-based coatings are slightly alkaline and in some areas contain small quantity of solvents, mainly alcohols. In this case, the offset inks so used need to be solvent-resistant and also alkali-resistant because the inks could otherwise get affected with an unwanted change in colour when coated. With practical experience it has been observed that the risk of colour change increases with intermixed colours having many components and lower the concentration the higher the risk of colour changes. Whenever coating on such inks, always check the stack for any changes in color after starting the print run because it is not possible to stimulate actual practical conditions during laboratory testing. Alternatively it is suggested that to use fast pigments in colour shade matching. However, one exception is the process magenta, as the low alkali fastness, such inks can be coated with water-based coatings with no problems.

Drying principle of water-based coating

Water-based coating systems dry exclusively by physical process as these contain about 55-70 percent water. The drying speed of the coating will be determined by the rate of absorption of the water into the stock. In stack the drying of the coating will also be facilitated by the absorption capability of the back side of the paper/board. These coatings also dry partially by evaporation of water (30-60 percent) and to the tune of 40-70 percent by absorption. Film formation in water-based coatings is largely complete even while 20-30 percent of the water is still present in the coating immobilization point.

Drying aids

A combination of hot air blade and infrared (IR) radiator is the most reliable way to accelerate drying process. Cold air will not be suitable. The moisture carrying hot air must be extracted for efficient drying of the coating. Flow of the hot air and the output of the IR lamps are optimum when the following temperatures are measured in the delivery stack (assuming an optimum room or stack temperature of 23º C):

For paper: approx. 8-10º C above the temperature in the feeder stack.

For Board: approx. 10-12º C above the temperature in the feeder stack.

At high press speed, an extended delivery will be further beneficial. Though, excessive short-wave length IR radiation can cause blocking in the stack, especially if thick ink films are being printed. In particular, dark inks are heated up a lot and cause sticking in combination with the coating film. As a general rule, the stack temperature should not be allowed to exceed 35º C.

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Properties of water-based coatings

Rub resistance

The drying speeds of water-based coatings are set so that the coatings are tack-free in the stack with standard wet film thickness. Nevertheless, light spray powdering – preferably with starch powder is required for very heavy inking jobs with wet-on-wet coating application. The rub resistance of the coated products depends to a great extent on the stock and the coating film thickness. Use of spray powder will reduce the rub resistance.

Heat sealing resistance

Heat sealing resistance depends on many factors

• Sealing temperature (C)
• Sealing time (S)
• Pressure (bar)
• Film type
• Number of film plies

Finishing with hot-stamping film

The adhesion of the hot-stamping film depends on the substrate used (paper or board) as well as the type and degree of dryness on the ink printed on the substrate. As a rule most of the high gloss and standard coatings can be finished with hot-stamping film. The quality of the hot-stamping films also plays a very crucial role in their suitability. It is advisable to carry out a test prior to start of the production run.