New Low Impact Paper Coating Technology

Vilho Nissinen takes a look at OptiSpray coating technology

1 Introduction

Over time we have seen a decrease in the trend of paper product prices. In recessionary times, low value products lose their profit marginal quite easily. To achieve good profitability, the value of the product must increase while at the same time production costs decrease. The dream of every papermaker is to produce a high-value product with economic raw materials while maintaining good runnability at high speeds.

The demand for lower impact in the paper coating process is well known, as are the difficulties in blade coating. Film transfer technology has taken a big step in the right direction. The weaker base papers can be coated. Still there is heavy web contact with many dependencies and interactions between coater and paper in the film transfer nip. The coat weight variations caused by unevenness of the base paper and changes in the cross directional tensile profile over the coater are ready examples.

Thanks to film transfer technology, the coated and improved newspaper grades are already available. With the help of an improved print image, four-color newspaper printing will gradually attract the commercial advertising business. Even traditional newspapers will be four-color printed on light-weight coated news grade, making them as attractive for product advertising as the newspaper inserts are today. Every cold-set newspaper printing house is seeking business use for available day time hours.

2 Development History Of OptiSpray

In order for new products to be created, a bit of history is essential. The first steps in the creation of the fascinating OptiSpray were taken in the painting laboratory with a couple of liters of traditional blade coating color.

Spray painting itself has been a popular technique for decades. The paper samples we sprayed were closely inspected for quality, and of course droplets were detected here and there on the paper.

A gradual diluting of the clay-containing coating color solids brought some success, atomization and droplet spreading on the paper surface were in acceptable condition. But still the pressure level had to be rather high.

Encouraging results in hand sheet testing led to the first pilot trials in early 1994. Spray guns were installed at the pilot coater to test the sprayed quality in the real coating process.

Instead of long absorption time and slow free sheet drying, initially we used normal infrared and air flotation drying. After the overspray suction chamber was buildt around the spray nozzles, the full-scale pilot trial was possible to carry out.

One meter wide paper rolls were coated by 5, 7.5 and 10 g/m coat weight. Film-transfer vs. Spray comparison was made to see the real potential. Coating, calendering, and even printing went well. The spray samples didn't reach as high a gloss as the film transfer coated ones, but the print image and evenness of the samples was good. The printers evaluated this new paper and found it to be high level. Thus, the first essential breakthrough! It was possible to use the spray technology on paper suited to calendering and printing.

During 1995 and 1996 a couple of studies were undertaken to investigate nozzle wear characteristics, the paper boundary layer influence on spraying, the spray pattern impulse force on paper and the various types of spray guns. Both airsprays and airless sprays were carefully tested.

By the end of 1996 the decision was made to create pilot research equipment that would allow high-speed, full scale spray coating. The coater was started up in September of 1997.

A first attempt showed that the coating color was not properly transferred onto the paper, so the applicator was redesigned to include multiple nozzle rows equipped with three nozzles in each row. The first idea was to have many rows with few nozzles in each row at quite a long distance from the paper web in order to get a good profile. The distance from the web needed to be small enough to get good color transfer to the web and especially to get good spreading onto the web. During the first trial run, a speed of 2500 m/min was reached without any problems. The biggest surprise was that the transfer efficiency didn't decrease when the speed was increased. The coating was shown to be possible via airless high-pressure spray technology.

Board was coated first. It seemed almost possible to replace the air knife with the new spray technology. On the other hand, the trend has been to replace the air knives with blade coaters because the runnability of the blade coater is not an issue in board coating. Yet, in triple coating it would be beneficial to coat the high coverage intermediate layer with spray.

The greatest amount of attention was paid to coating the weak, newspaper-like base papers. From a space-saving point of view it was interesting to see whether or not both sides of the paper could be coated at the same time. Clearly, the spray nozzles could not be positioned above the web because of the dropping problem and thus the vertical position had to be tested.

Using this technology the improved, spray-coated newspaper was developed. The coating structure has proven to be very beneficial for cold-set printing.

3 Challenges To Overcome

The painting industry often has only a few nozzles spraying at the same time. The nozzle change due to wear is not a big problem. The misting has been controlled by electric field or ventilation system and the occasional cleaning of the nozzles is easy.

The continuous process of paper mill production is challenging. There are hundreds of nozzles and no time to stop production for changes or washing. Both sides of the paper must be coated at the same time and some overspray is inevitable. The color has to spread onto the porous, absorbing paper. The droplets have to be small enough. The high-speed paper web takes the mist without transferring it further to the machine hall and even to the printer blankets. Everywhere that the wet mist contacts the equipment, a liquid film forms and the film flows down. Every sharp edge forms drops. Air flow dries the color, creating build-up. And finally, all the spray patterns must fit well together for an adequate CD profile.

4 Production Scale Pilot Machine

The pilot-scale trials were targeted to overcome spray related challenges. This continued until the end of 1999 when the green light was given to build the production scale pilot machine in the technology center.

After the intensive research program the high-pressure airless technology was selected as the basis for the OptiSpray coating process. The liquid film hits the atmosphere after the nozzle, loses speed, and forms small droplets.

With high-pressure technology, rather high viscosity can be used, high color transfer efficiency can be achieved, a high enough impulse and good drop spreading onto the paper surface can be reached, and build-ups at the tip of the nozzles can be eliminated. At the same time, a huge amount of atomizing air, and the related costs to create and handle it, are not necessary.

Top to bottom web run was fulfilled first, as both the overspray mist and the down -flowing color can be collected at the same suction channel at the bottom of the OptiSpray coater. The down-flow helps in keeping the spraying zone and suction channel clean as well. In the suction channel the heavy turbulence throws color film all over the walls, avoiding the possibility of having the color dry in that area.

The spraying zone has cooled walls to avoid color build-ups by water condensation onto the walls. Further, there is an introduction of steam along with the paper web which keeps the internal humidity high enough to get good condensation onto the cooled surfaces.

The OptiSpray nozzle pipes extend only slightly to eliminate drop forming in the spray fans. Color mist forms color film on the pipes, which can easily be transferred to the paper web by the ejector phenomenon near the spray tip.

In the coater are two separate spray nozzle beams to ensure many days, even many weeks, of continuous OptiSpray operation. If there is a need for nozzle changes or washing, a reserve beam can be quickly started up. The idea is that OptiSpray can work continuously between paper machine normal washing stops when there are no paper breaks.

To further ensure reliable operation, internal and external washing sequences are available. With every stop or beam change, the sequences will run automatically. After longer production periods the internal filter wash procedure can be run. The dirt and foreign particles from the screen exit from a separate channel. All the tips can be easily washed simultaneously by specific tip washer . The spray beam can be pulled out from the spraying zone, turned 180 and moved to the washing chamber. To check the condition of the nozzles, there is a window in the washing chamber. Furthermore, the transparent door at the back of the coater allows the operator to observe all sequences during OptiSpray operation. The high-pressure color spraying system is best located in the closed service chamber.

In the production machine the color-containing wash water is collected and reused in the color preparation process. In a tail threading situation and after a paper break, the coater units are moved 400 mm away from the web line. Between the units is an 800 mm wide corridor for the operator to do service work if needed. All the movements are naturally locked and ensured mechanically during service. The coater is cleaned automatically by water spray. Used wash water is collected and used in the color preparation.

The overspray phenomenon with the paper web boundary layer is eliminated by using an air knife in the web running direction. The mist-containing suction air is handled by an electric air cleaner. The cleaner reject is used in the color preparation. Coating color is pressurized by a hermetically sealed multi-piston pump.

5 Surface Tension

The spray process can be defined in terms of the increase in liquid surface area resulting from atomization. The surface area before breakup is that of the liquid cylinder as it emerges from the nozzle. After atomization the final area is the sum of all the individual droplets. Surface tension (work which is needed to increase the area) is an important factor because it represents the energy needed for formation of new surface area. The minimum energy for atomization is the surface tension multiplied by the increase in liquid surface area.

In the atomizing process, surface tension tends to pull the liquid into the form of a sphere because this has the minimum surface energy. For most pure liquids the surface tension decreases with an increase in temperature and it is independent of the age of the surface. It is possible to reduce surface tension by adding surfactants which adsorbs on the liquid/air interface having positive effect for atomization.

Viscosity is the most important liquid property with regard to spray characteristics. Its importance arises especially from the fact that it affects not only the droplet size distributions in the spray, but also the nozzle flow rate and spray pattern. An increase in viscosity lowers the Reynolds number and makes turbulence generation more difficult.

High viscosity prohibits the development of any natural instabilities in the forming liquid jet or sheet. The total effect is to delay disintegration and increase the size of the droplets in the spray. The droplet size increases because for high viscosity liquids, viscous losses are large and less energy is available for atomization. This leads to a coarser spray result.

The easiest way to carry energy to the atomization process is liquid pressure. The pressure energy is changed to the kinetic energy in the nozzle tip orifice. The theoretical maximum speed (no pressure losses) in the orifice is calculated from the equation

Theoretical mass flow (no pressure losses) through the nozzle is calculated from the equation

6 Atomization

The tip geometry is such that the liquid comes out as a thin film, which spreads continuously. When this spreading high speed film hits the air, it atomizes to the small droplets according to Rayleigh's and wave theory.

Typically 90% of the droplets are in the size range of 20 - 60 m.

7 Color spreading

The diameter of the color drop is bigger than the thickness of the coating layer. Therefore the spreading of the drop onto the surface plays a major role in spray coating.

When liquid is at equilibrium with the other phases in contact, the contact angle that the liquid makes is related to the interfacial free energies per unit area of those phases. When the liquid has reached equilibrium with the gas (usually air) and the solid substrate, it is possible to diagram the contact angle as shown in Figure 14.

Wetting is, in general, displacement of one element from a surface by another; usually displacement of air from a solid surface by water or aqueous solution. Three types of wetting phenomena have been accepted. These are adhesional, spreading and immersional wetting. Only adhesional and spreading wetting have importance in coating processes. Spreading is dependent on a so-called spreading coefficient

The droplet adhesion is mainly influenced by the ability of the liquid to penetrate into the porous solid.

A liquid not originally in contact with the substrate adheres to it, as seen in Figure 15.

The driving force of adhesional wetting is known as a work of adhesion.

The reduction of surface energy of either liquid or substrate decreases the tendency of adhesion occurrence. The reduction of contact angle indicates increasing adhesion tendency.

The work of self-adhesion of a liquid is known as the work of cohesion. Cohesion work means the work required to produce two unit areas of interface from an original unbroken column of the liquid

The difference between Wa and Wc is the spreading energy. If Wa > Wc liquid spreads spontaneously over the substrate to form a thin film. In the situation where Wa < Wc liquid forms droplets over the substrate with a finite contact angle. To spread in the last case, the drop has to get energy from the outside of the system.

In spreading wetting liquid is already in contact with the substrate and the liquid spreads, displacing the other fluid (Figure 16).

During the process the interfacial area between the solid and the fluid being displaced decreases by an amount Ai. At the same time the area between solid and liquid increases by an equal amount and the interfacial area between both fluids increases during the process. The total decrease in the energy of the system is Wa - Wc.If Wa - Wc is positive, spreading can occur spontaneously.

If the atomization and the color spreading onto the surface are not in the correct condition, the coating result will include drops here and there.

The system must have enough energy to atomize the liquid, to transfer the drop through the paper boundary layer and finally to spread the drop properly onto the paper surface. Therefore the possibility to reach good coverage requires low enough color viscosity, low enough water phase surface tension, high drop speed, low contact angle and high surface energy of the paper at the surface.

8 Coating Coverage

The spray-coated surface is much more contoured than the surface coated by traditional blade or film transfer coating. This can be clearly seen in small-scale coat weight distribution measured by the laser induced plasma spectrography.

In spray coating there are far fewer low and high coat weight spots. With traditional coating methods, the "hills" have gotten less coating and the "valleys" have gotten much more coating. In the figure 17, with spray coating, the coat weight distribution is sharper. This is beneficial for fiber roughening and for ink mottling as well as for brightness and opacity -- not to forget the print image.

9 Profiles

The optimal fan profile has smooth edges for ease of fitting together to reach a uniform CD profile.

The distance to the web, the fan width and the angle must be optimized.

The typical spray coating peak-to-peak profile has been  5% of the coat weight level. In the trials the nozzles on both sides have been at the same position. In simultaneous double sided coating the nozzles on the second side will be cross directionally located at the middle of the nozzle to nozzle distance on the first side. The total coat weight profile will be even better in the future.

10 Paper Quality

In visual evaluation of the printed samples, the spray-coated printed sheet has very good graduation. Less fiber roughening, better appearance, good touch and not so much mottle have been noted as better than many existing commercial magazine papers.

Some trends compared with traditional coating methods can be clearly seen. With OptiSpray higher brightness and opacity, as well as stiffness, can be reached. A more open coating structure allows higher moisture without any risk of blistering. The porous structure leads to slightly higher ink consumption, but at the same time gives good set off which is very advantageous especially in the cold-set process. Structural issues, along with the contour nature of the surface, gloss and smoothness are not necessarily at the same level as when coated by traditional methods. By optimizing the base paper, the coating color and the calendering process - the gap will surely disappear.

The optimal base paper for the spray process has a hydrophobic inner structure and a hydrophilic surface. Too much absorbency leads to binder migration to the base paper and decreases surface strength. The biggest issue in base paper development will be the administration of low viscosity liquid penetration to the inner structure.

The coating color window is smaller than in traditional coating application methods. Low viscosity and surface tension , as well as the abrasiveness of the color lead to a round pigment particle shape. The solids content has to be lower also. However, with calcium carbonate and suitable latex a solids content of over 60 % is realistic.

11 OptiSpray Possibilities

Newspaper grades
Numerous studies have been performed in order to test the possibilities of OptiSpray. As described earlier, the porous structure of the coating layer gives a clear benefit for cold-set printing. The very important set of properties can be adjusted so that the paper product will be robust for many types of processes and printing inks. This technology provides a good opportunity to expand cold-set printing to the commercial field. The newspaper printing houses can thus get more daytime work. Typical newspapers may become completely four-color printed, with high-quality advertisements throughout.

By sizing, newspaper linting can be eliminated and consequently the paper can be printed using the heat-set process.

Magazine papers
The porous coating structure gives good blistering resistance during the heat-set process. Furthermore, a higher final product moisture can be used and lower air temperatures will be needed in the ink drying process.

Good coverage gives an excellent brightness-opacity ratio and fiber roughening is at a lower lever than traditional coating methods when used with wood-containing grades. The paper touch is good and the visual impact is fascinating.

Many SC-paper grades need surface treatment to bind the filler completely onto the paper surface for better off-set printability. OptiSpray is a compact solution for this.

Board
The spray process doesn't smooth the rough surface so much as cover it well. Thus the spray coated layer is extremely well-suited in the intermediate layer of high quality board coating as a replacement for the air knife. As seen in the case of car painting, a smooth base surface leads to a smooth finished surface. In the future the whole process must be thought of in new ways. Perhaps the product should be pre-calendered and then coated afterwards.

Because of the porous nature of the coating, the barrier products will not be very easily handled via the spray process, but further development of chemicals may make this possible in the future.

Woodfree
The woodfree field has thus far not been adequately investigated so little can be said in this regard. However spray sizing, pre-coating and top coating just after calendering of the precoating are areas to be studied in the near future.

12 The SprayWay To Success

There is no access to markets without sufficient product quality, and no success in the markets without economy. These rules have guided development of OptiSpray in order to allow it to reach a commanding position when compared with other coating methods. We have waited a long time for low impact coating. What does it mean in practice?

Traditionally, paper has been in contact with either the blade or the film transfer nip. In both methods the coat weight formation is strongly related to base paper properties such as porosity, roughness and moisture. As these properties are not even throughout the paper surface, coatweight variations result. Even the best base papers show variations. The paper can freely "drink" coating color during interaction between the coater and paper. The base paper properties have to be even and all the profiles have to be at a high quality level for optimal success in coating. In the OptiSpray process, this requirement is not as great. The nozzles consistently spray the same amount of coating color onto the paper, regardless of base paper variations.

Due to low impact, the base paper strength can be lower than traditionally required. More economical raw materials can be used and base paper faults are better tolerated. Higher base paper moisture can be used, providing greater flexibility when it comes to locating the coater in the paper machine.

There are other savings due to the nature of the OptiSpray design:

• No rotating rolls in the spray coater itself
• No noise caused by vibrations
• No roll changes (and related roll change time)
• No roll covering
• No roll grinding facility and staff
• No blade or rod change time

The double spray-beam solution makes OptiSpray service, such as nozzle change, possible during operation. Ultimately, the coater can operate continuously during paper machine regular washing interval, which normally is one to two weeks. Compared with the blade coater, 5 to 6 % higher production efficiency can be reached.

When compared with the film transfer coater, 2 % higher production efficiency is possible.

A non-contact process is beneficial in high-speed solutions as well. At high speeds the color window of existing coaters narrows, which will partly decrease the coating color formulation differencies in the future.

Fully automated wash sequences make operation easy. Eventually a fully-automated OptiSpray process will be seen, as the need for man-power is already less than with conventional coaters.

Blade or rod streaking is no longer an issue. Coarser, cheaper pigments can be used. The functional criteria of the spray itself is the color coming from the couple of hundred microns diameter nozzle hole.

The spray coater doesn't limit the coat weight range, with the exception of the minimum coat weight at low speeds. The nozzle hole cannot be too small. Paper wetting and the down-flowing coating color on the vertical paper surface are the main limiting factors for the maximum coat weight.

The new OptiSpray coater can be located in the existing dryer section. The base paper is not permitted to wet more than 25% to avoid excessive elongation and decrease of tensile strength.

The maximum moisture before coating will be in the range of 15 to 20% related to coat weight and solids content. The powerful TurnDry drying method does an excellent job of eliminating color absorption into the base sheet: the coating color will be completely immobilized within half a second after coating. Thus, the base paper between the wet coating layers, has no time to wet too much.

There are two possible layout solutions. Either the TurnDry dryer is located in the basement or both the TurnDry and OptiSpray are in the basement.

Figure 27. OptiSpray on the machine floor level

The most important aspect of OptiSpray will be the dramatic improvement in production economy. In the following example some paper machine concepts are compared to show the obvious advantages of OptiSpray.

It's clear that the product value, raw material cost and efficiency cost are the dominant factors in profitability. Drying, service and personnel costs are about the same in each concept.

The higher product value when compared with newsprint and SC grades, cheaper furnish, cheaper pigment and higher efficiency compared with existing technology are the keys to the OptiSpray way to more profitable paper making.

Successful papermakers are looking into the OptiSpray coating method - the SprayWay to Success!