Paper Coating Color Preparation
General Aspects of Coating Kitchen Set-up
The objective of a coating color preparation plant is to prepare the desired amount of coating color in the required quality using a combination of mixing, pumping, storing, conveying, metering, and screening processes. Such a plant consists of storage tanks, pipelines, pumps, valves, mixer, screens and dosing equipment. Figure 7.12 shows the key processes starting from unloading chemicals to the coating color application.
7.7 Coating Color Preparation
In the coating kitchen all raw materials needed for coating are combined in the requisite form and sequence. In the past, work in the coating kitchen was largely manual. Today, the same tasks are usually handled by a process controlled system, without much human intervention except for monitoring. The mill operators can track every detail of preparation and coating operations. All the process data are stored for instant recall. This is to support uniform high quality by selection of the appropriate raw materials, their separate or joint preparation with strict adherence to timed dosing sequences, and clean working practices.
Coating kitchens operate either batchwise or continuously. In view of problems such as dusting of powdery products and for operational reasons, many paper mills have switched from dry to liquid storage of their starting materials. This also brings the advantage that individual components can be precisely dosed. In every case a special handling and make-up process for the individual products is necessary.
Dispersing of Pigments
These days, most of the pigments are delivered as slurries or dispersions with 60–78 % solids. These pigments are very well dispersed by the suppliers. Pigments supplied in the dry form have to be carefully dispersed in the paper mill before being temporarily stored and converted in liquid form. Poor dispersing leads to problems such as blade scratches, excess rejects at screens, rheological fluctu¬ations, and runnability disturbances at the coaters.
The first step in dispersing is wetting the particles, eliminating the air layers on the pigment surfaces. Water is absorbed into the agglomerate pores by capillary action. Chemical deflocculation will proceed slowly by diffusion. The resulting dispersion will not be homogenous and will contain agglomerates, which require physical stress to break them down. For good pigment dispersion at high dry solids content, a disperser is used to create the physical shear and mixing forces. Two examples of batch dispersers are shown in Fig. 7.13 and Fig. 7.14. To achieve the required high solids content of the coating colors some chemicals, namely dispers¬ing agent and caustic soda, must be added to the suspension to improve and stabilize the dispersion (see section 18.104.22.168.2).
7.7 Coating Color Preparation
A complete pigment makedown batch process is shown in Fig. 7.15. Railway cars or truckloads are emptied into a hopper. The pigment is conveyed and dosed into a disperser by measuring the weight of pigment with load cells. The same is done with the feed water and dispersing agent plus pH-control chemical.
After disper¬sion the content of the disperser is emptied into an intermediate tank. Screening is a continuous process to ensure maximum capacity and screening efficiency. From the screens, the slurries go to a screening tank. The slurry will then be pumped to a large storage tank where it waits for pumping and dosing into the coating color mixer.
Processing of Binders
Latexes are delivered by the manufacturers as a water-containing dispersion of polymerized synthetic chemicals at dry solids levels of approximately 50 %. Latexes contain certain surface-active chemicals, and the main task in coating color prepa¬ration is not to disturb or break their balance. This means that the whole handling must be clean and without any harmful chemicals. For delivery and storage of the different latexes a truck or railway car is emptied, using either its own pumps or a pump from the mill. The storage capacities of latexes are optimized according to their consumption. Normally these storage tanks are made of stainless steel and not equipped with any agitators. From these storage tanks, latexes are pumped and screened into the coating color mixer.
Starch has been one of the most popular binders for decades, because it is a cheap product, especially when utilizing low-cost unmodified starches. These grades are normally sold at lower costs than pulp. Starch is also a very flexible material and a paper or board coating mill can convert it to their specific coating requirements. The starch producing companies can also modify starch by introducing esters or ethers of starch. A special feature of starch acetates for the paper industry is that these ester groups are efficient in preventing amylose retrogradation (see below). They have extremely good viscosity stability. An ether starch, e. g., is appropriate for its good film-forming property or holdout of organic solvents.
Native or unmodified starch dispersed in cold water, settles out rapidly due to lack of solubility. A dispersion of starch in water has no adhesive power. To become an adhesive, the starch has to be heated in water above the gelatinization tem¬perature of the starch, which differs depending on its plant origin (Table 7.13). When a starch suspension is heated beyond the gelatinization temperature, the individual starch granules begin to swell and, after a time, a colloidal sol or starch paste is obtained with adhesive and binding properties. The hot gelatinized starch paste is a non-Newtonian fluid.
A starch paste derived from unmodified starch has relatively high viscosity at very low solids concentration. In practice, it is nearly impossible to prepare a manageable starch paste exceeding 7 % unmodified starch. With time and temperature decrease, an increase in viscosity or thickening can be observed. This thickening is due to a well-known phenomenon for all unmodified starches called setback. It occurs because during thermal decomposition, e. g., ge¬latinization, the original crystalline arrangement of the starch molecules is lost.
When cooling the starch paste, the molecules cling together again, thus forming insoluble aggregates. As a result of this crystallization process, the paste solution gradually turns turbid, while the viscosity increases. Finally, the viscous paste turns into an opaque mass or gel. In very dilute solutions, there is not enough material
Basic characteristics of starch.
Potato Barley Wheat Corn Waxy maize Tapioca
Source Tuber Grain Grain Grain Grain Tuber
Particle size (m) 10–100 10–35 3–35 5–25 4–30 3–30
Gelatinization temperature (°C) 60–65 80–85 80–85 75–80 65–70 65–70
Moisture at 65 % RH (%) 19 13 13 13 13 13
Protein (%) 0.05–0.1 0.3–0.5 0.3–0.5 0.3–0.5 0.2–0.4 0.05–0.1
Fat (%) 0.05 0.4 0.8 0.7 0.2 0.1
Ash (%) 0.3–0.4 0.1–0.2 0.2–0.4 0.1–0.2 0.1–0.2 0.2–0.3
Phosphorus (%) 0.08 0.02 0.06 0.02 0.01 0.01
7.7 Coating Color Preparation
(b) continuous (source: BVG).
to gel the entire solution so insoluble starch particles sink to the bottom. It is mainly the linear amylose molecules which exert this usually undesirable ten¬dency, called amylose retrogradation or simply retrogradation, which is an irrevers¬ible process. To prevent retrogradation effectively, modification of the starch is necessary. Methods to reduce and stabilize the viscosity of starch by gelatinization and depolymerization are discontinuous or continuous cooking processes.
All starch cooking applications involve an initial dispersion of starch in water to give a slurry. According to the intended application of the starch, the slurry concentration will vary between 5 % and 40 % dry substance.
The batchwise cooking of modified starch takes place unpressurized at tem¬peratures around 95 °C for approximately 30 min under good stirring conditions (Fig. 7.16(a)). Continuous starch cooking is done in the so-called jet cooker at a steam temperature of approximately 120–130 °C. The steam immediately interacts with the starch in a Venturi tube and the gelatinization is completed within a few seconds (Fig. 7.16 (b)).
Another common possibility is the enzyme conversion of native starch, again done either discontinuously or continuously. With the aid of highly active protein molecules from bacteria, so-called alpha-amylase, the starch molecules are broken up at 70–85 °C and the enzyme must be inactivated after approximately 10–20 min, in order to stop further break up. The inactivation can take place through the addition of acids and holding at 95 °C for 10–15 min or by swift heating up to 130 °C. A continuous starch cooking and converting system is shown in Fig. 7.17.
22.214.171.124 Other Binders
Polyvinyl alcohol (PVA) and proteins are delivered to the paper or board mills as dry powder. in sacks or big bags. The handling system for these products has the same features, cooking with steam, as starches. The key equipment here for the batch process is a heavy-duty mixer with steam inlet tubes for even distribution of steam bubbles. The product slurries are heated to 90–100 °C with direct steam heating and then held at this temperature for 20–30 min until there is a clear solution of the product in the batch cooker.
For example, the PVA solution has a viscosity maximum at temperatures of 65–75 °C. The major difference between PVA and starch cooking is that in a PVA cooking system the molecular structure or polymer chain length is not altered. The hot PVA solution is also held warm in its storage vessels in order to avoid viscosity changes due to decrease in temperature.
The key control parameter of a protein solution is the solids content, therefore the metering of water and protein powder must be accurate.
Some pH-controlling chemicals must be added to have the right caustic process conditions. Typical protein processing parameters are 10–25 % solids content, alkali amount (one¬third of sodium hydroxide and two-thirds of ammonium water) around 7 % of the protein, heating to 60 °C and holding at this temperature for between 15 and 30 min. The protein will be cooked until the binder is a lump-free solution.
Additives are, for example, carboxy methyl cellulose (CMC), flow modifiers, pH-controlling chemicals, preservatives (biocides), defoamers, deaeraters, dyes and
Coating Color Preparation
optical brighteners. All products except CMC are delivered as solutions, emul¬sions, or dispersions. Their concentration can be suitable for direct metering into the coating color preparation, or they may be diluted to a lower concentration in order to improve the metering accuracy or to avoid shocks caused by too strong chemicals. Therefore proper investments in handling and metering systems of these additives have to be made.
The mixing, storage and circulation tanks for coating colors are made of stainless steel, since the pH range of raw materials and coating colors tends to corrode standard steels. Most vessels are equipped with an agitator, and some also have a shell/jacket that may be heated or cooled. Agitators are mostly indispensable be¬cause, in low-viscosity colors especially, solid color components are prone to sed¬imentation unless the suspension is stirred continuously.
Screens and Filters
Impurities of any kind in coating colors may lead to serious problems in coating machines so these contaminants have to be removed by means of either open vibrating screens (Fig. 7.18) or closed filters (Fig. 7.19). Open screens are fre¬quently preferred, because the retained dirt is visible and defective screens can be quickly replaced. However, there is a risk of air becoming entrained with the coat¬ing color. Also, color splashes and blocked screens can heavily contaminate the immediate environment. Closed filters are clean by comparison.
In parallel ar¬rangement, they incorporate pressure controls and automatic backwashing func¬tions. In everyday practice, color losses inevitably occur during backwashing. Also, retained dirt particles can only be identified after filter cartridges are removed.
Whichever system is chosen, the mesh of screens or filters has to be small enough to retain all particles > 50 mm.
In modern coating kitchens the various starting materials are usually pre-screened over mesh widths around 50–60 mm before the finished coating color is passed through 100–150 mm screens or filters in the circulation system.
Smaller mesh sizes are inadvisable, because they would largely restrict the passage of the highly viscous color. Coarser screens are frequently installed in the coater recircu¬lation system to retain scraps of paper that may have been introduced into the coating circuit after web breaks.
Degassing of Coating Colors
Coating colors contain air and gas bubbles of various sizes that originate from the raw materials and turbulence during color preparation. Gas content can vary greatly: 10–15 % by volume is typical, although it may exceed 35 % in extreme cases. Gas levels in coatings have become a major quality issue since the introduc¬tion of new jet applicators because every air or gas bubble leaves a crater in the coating surface. This compelled papermakers to install online degassing equip¬ment in coaters with jet applicators. Most of the degassing systems are based on
7.7 Coating Color Preparation
the density differences in a centrifugal field of a cyclone (Fig. 7.20). Modern sys¬tems are capable of reducing gas levels to 4 %. The effiency of such physical de¬gassing apparatuses will be further improved by the additional use of chemical deaerators (see 126.96.36.199.4.5).
Batch Preparation of Coating Colors (Fig. 7.21)
All components of the coating color formula are pumped from their storage tanks into a mixer and hereby screened once again. A recirculation pipe installed be¬tween the component storage tank and the mixer is used for those chemicals that have a settling tendency, like pigment slurries, or to keep certain products homo¬geneous in their temperatures and concentrations, like starch, CMC, or proteins.
The mixer is placed on load cells to measure accurately the batch sizes and weights of the major components. Other precise ways to dose the additives are mass flow¬meters and metering pumps. In batch processes, these metering devices can be optimally calibrated and dimensioned because the flow rates are normally kept constant and the dosing time is varied depending on the amount of each additive used in a coating color formula. Batch processes are also very flexible for a wide dosing range of each component.
The difference between the maximum and mini¬mum rates does not negatively influence their accuracy or the flexibility. One set of metering instruments and valves can easily be used to serve two mixers optimizing investment costs and space in the coating color kitchen. The flexibility of a batch process is obvious in cases where the rheological or other properties of coating colors have to be changed by altering the order of addition of the same chemicals. The mixing time and mixing energy can also be easily changed.
Continuous Coating Color Preparation (Fig. 7.22)
The major advantage of a continuous process is the smaller size and thus lower cost of pumps, valves, flow meters and pipings. So-called “calibration systems” for each line ensure the metering accuracy and are fully automated. These continuous systems are most effective in high-capacity coating with large volumes of coating colors with the same color quality, such as in a one-grade paper mill. Here the number of different coating colors is low and, if there is a grade change, different coatings are usually allowed to be mixed with each other.
This continuous system differs from a batch process in many aspects: The components of coating colors are continuously pumped, screened, and metered into a continuous mixer. Besides the size difference of the piping equipment the mixers are completely different because mixing must be completed during the short time when the components are passing through the mixer. It is equipped with high-shear, high-energy mixing zones, usually two, or two separate mixers in series are operated. Dosing of all additives is crucial for constant coating color quality, they are metered with mass flow meters, magnetic flow meters, or metering pumps.
The mixer feeds its ready-made coating color directly to the coater supply tanks which results in a low storage volume of processed coating color. The advantages are fast control measure¬ments of the coating color properties and the possibility of quick changes in the color composition. On the other hand the range of volumetric capacity of each chemical line is strongly limited for a given installation.
Coating Color Supply Systems for Coaters
The basic function of the coater supply system is to supply coating color to the coating heads to be evenly spread on the paper or board web. In most cases only about 10 % of the pumped coating color will be applied onto the running fiber web. About 90 % of the coating color flows back to the supply tank.
This high internal coating color recirculation is required to ensure homogenous and constant color properties for trouble-free coater operation. The feed of the fresh color into the supply tank is controlled by its level. From the supply tank, the coating color will be pumped first to the screens, which eliminate all impurities larger than 50 mm, depending on the geometry of the screening elements. The second key process is degassing of the coating color.
Depending on the design of the coating heads, the coating color flow is led to the ponds or chambers through one or two inlet pipes (Fig. 7.23) controlled by a throttling valve for accurate flow. The overflow goes back freely to the supply tank. The contents of solids and gas as well as temperature and pH and their variations are monitored continuously.