Presentation on theme: "بسم الله الرحمن الرحيم بسم الله الرحمن الرحيم ( وَقُلِ اعْمَلُواْ فَسَيَرَى اللّهُ عَمَلَكُمْ وَرَسُولُهُ وَالْمُؤْمِنُونَ ) التوبة ( 105) التوبة ( 105)"— Presentation transcript:
بسم الله الرحمن الرحيم بسم الله الرحمن الرحيم ( وَقُلِ اعْمَلُواْ فَسَيَرَى اللّهُ عَمَلَكُمْ وَرَسُولُهُ وَالْمُؤْمِنُونَ ) التوبة ( 105) التوبة ( 105) صدق الله العظيم صدق الله العظيم
Coating of Paper and Paperboard By Dr. Essam Saber Abd. El -Sayed Cellulose and Paper Department Organic Chemical Industrial Research Division National Research Center Oct, 2006
Coating of paper and paperboard »» »» Definition of Coating (Coating Process) »» Coated Paper and Board Grades »» Requirements of Coated Paper »» Components of Coating Colors »» Pigments »» Dispersing Agents »» Binders or Adhesives »» Other Additives »» Coating Color Formulations »» Coating Color Preparation »» Types of Coating Machines
Paper defined by a felted sheet formed on a fine screen from a water suspension of fibers and non-fibers materials.
Paper is made by: Pulping and bleaching, to separate and clean the fibers Beating and refining the fibers Forming a web of fibers on a thin screen Drying to remove the remaining moisture Finishing, to provide a suitable surface Converting processes, to produce different paper products.
Wet Converting »Coating » Laminating » Corrugating » Impregnating » Embossing » Wet creping Dry Converting » Bag making » Box making » Envelope making » Cutting & Folding » Hot pressing » roll winding Paper Converting
Coating Processes Coating of Paper and Board » » Coating is a process by which a mixture of water, white pigments, binder, and various additives are applied to one or both sides of paper sheet. » The main objectives of coating process for paper and paperboard are to improve their appearance, and printability. » Coating impart smoothness, gloss, brightness, and opacity to the base sheets for improved appearance, and provide them with enhanced printability. On-MachineOff-Machine
Factors Affecting on Coated Sheet Properties - 1- The base sheets (fiber types, sheet formation, internal sizing, and basis weight) 2- Coating materials (pigment types, binder types, water retention aids, lubricant, and defoamers) 3- Coating formulations (ratios of coating components, solids, and pH ’ s) 4- Coating process (coating application types and speed) 5- Coating weights 6- Drying conditions (dryer types, drying temperature, drying time, and final moisture level.) etc.
Coated Paper and Board grades Classification of coated grads has not yet been standardized. However, certain terms are recognized throughout Europe as: Art paper Chromo-papers Machine-coated papers Light-weight coated (LWC) papers Cast-coated papers Folding boxboard and chromo-board
Chromo-paper Art paper This term refers to: wood-free or slightly wood containing paper That is coated on both sides The weight of the coating exceeds 20g/m 2 per side Art paper is used for high quality printed product Chromo paper is coated only on one side the weight and quality of the coating correspond to that of art paper. Much of this paper is employed for packaging purposes
Requirements of Coated papers Coating must have: 1- adequate stiffness 2- good brightness 3- low yellowing tendency 4- good aging resistance 5- good compressibility 6- high degree of smoothness 7- adequate receptivity for the ink solvent 8- low odor, flexibility, glueability, and crease resistance 9- alkali resistance (labels) 10- washability (wallpaper) 11- solvent resistance
Properties and components of coating colors Coating colors are fairly complicated multi-components systems, which all water-based components. The main components of a coating color are 1. Pigment coating, 2. binders, in addition to, a variety of auxiliary agents such as: 3. Dispersing agents for the pigments 4. Additives for reducing wet abrasion 5. Products for control of viscosity and water retention 6. Foam control agents 7. Shading dyes 8. Optical brighteners 9. Lubricants to improve paper calendaring
Coating colors are classified according to their solids content: Low concentrated coating colors Moderate concentrated coating colors High concentrated coating colors Solid content 35-45%45-55%55-70% Viscosity mPa/s (at rpm 100,20 0 c) mPa/s (at rpm 100,20 0 c) mPa/s (at rpm 100,20 0 c) Coating process Air knife Bar coater Roller blade Roller coater Blade coater Roller coater Blade coater Roller blade Flow behavior NewtonianNewtonian to slightly pesudoplastic Newtonian to pesudoplastic
Factors affected in the coating process Solids content greatly influences the runability of the coating color in the machine Viscosity of the coating color and its flow behavior at varying shear stress in coating machine pH of the coating color which processed at range The water retention capacity of the coating color i.e., the ability of the coating color to retain water in spite of the sucking action of the coating base paper.
Pigments The pigment is a highly important component of coated paper. Pigments form the actual "body" of paper coatings. They usually account for at least 80% of the total dry coating weight. Thus, pigments largely determine the quality and cost of the coating. The principle function of the pigment is to fill in the irregularities of the paper surface, to produce an even and uniformly absorbent surface for printing and to improve the appearance of the coated sheet.
: A good pigment should posses all or most of the following properties: Good dispersibility in water Small particle size distribution High opacifying power High brightness Low water absorption Chemical inertness Compatibility with other coating component low adhesive demand Be continued
Types of Pigments The most important types of pigments are Clay, Ground and precipitated calcium carbonate, Calcium sulfoaluminate (satin white), Titanium dioxide, Talc, Barium sulfate, Gypsum (calcium sulfate ), Calcined and delaminated clays and synthetic pigments based on aluminum silicate are employed for special applications Zinc Pigments (Zinc oxide, zinc sulfide and zinc composite), Calcium sulfide, Calcium sulfate Be continued
Specialty types of pigment coating * Luminescent Pigments Fluorescent pigment [ Sulfides of Zn, Cd, Strantium or calcium] Phosphorescent pigment [ Combination of Zn & Cd sulfides, Ca & Strontium sulfides] * Colored Pigments Zn, Cd, iron oxides for yellow color Iron oxides for brown color Cobalt greens for green color Sienna, red lead for red color Carbon for black color
Dispersing Agents The first stage in the production of coating colors is the conversion of the coating pigments, into a flowable and pumpable aqueous slurry. This dispersion process consists of two steps: The mechanical dispersion of the coarse pigment agglomerates in an aqueous medium, and The chemical stabilization of the slurry towards reagglomeration. Suitable dispersing agents are ordinary alkali as NaOH, Na 2 CO 3, Sodium silicate, trisodium phosphate, and polyanions. e.g.. salts of polyphosphoric acids (pyrophosphates, hexametaphosphates) or poly(acrylic acids).
Binders * The function of the binder or adhesive is to bind the pigment particles to each other and to fix the coat to the base paper. * In spite of their low proportion by weight in the coating color (generally 5 -25% based on the pigment), binders have a large influence on the coating and processing properties of the coating color.
Properties necessary in coating adhesives An adhesive, to be satisfactory, should have: › A high pigment bonding adhesive Good color The correct viscosity for the solid content Strong filming properties to prevent excessive penetration of coating Compatibility with the pigment Enough plasticity Be continued
Binders Natural binders Synthetic binders The most widely employed binders of this Types are: Animal glue, gum arabic, Starch & Its Derivatives (oxidized, etherified, esterfied and hydrolyzed starches) Cellulose ethers (CMC, hydroxyethyl- Cellulose) Soybean protein, casein, and alginate. Polymer dispersion give the coat a higher Water resistance, better flexibility, higher Gloss and better printability. For example: copolymers of several Monomers, e.g., Styrene-butadiene Acrylic & ester Vinyl acetate and acrylonitrile Acrylamide, acrylic acid, maleic acid, and Methacrylamine used as modifier agents.
Other Additives Waterproofing agents, such as water-insoluble polymer dispersions to make the coating be resistance to water. Hardening agents to harden the coating. Three main classes of products are employed as hardening agents: melamine- formaldehyde resins, urea-formaldehyde resins and bifunctional aldehydes such as glyoxal, which is added only to coatings containing starch. products based on ammonium zirconium carbonate or epoxides are being used as cross-linking agents in some cases. Anti-Foam agent to prevent the formation of foam Defomer agent to destroy or kill the foam bubbles. Defoamers include mixtures of higher alcohols, salts of fatty acids, and water-emulsible phosphate esters.
Foam-control agents Foam is a dispersion of air in water. There are different agents that prevent foam formation (foam inhibitors) and those that destroy foams (defoaming agents). Defoamer: An additive mixture, usually containing a water-insoluble surfactant and often containing hydrophobic particles, that destabilizes foam bubbles.surfactantfoam Antifoam: A defoamer product that has been formulated with the aim of preventing the formation of visible foam, not killing existing visible foamdefoamer Defoamers include mixtures of higher alcohols, salts of fatty acids, and water- emulsible phosphate esters. Defoamers are usually diluted with water and added to the coating mixture dropwise. Amounts ranging from 0.01 to 0.05 vol% are usually sufficient.
Titanium dioxide, and blue-violet dyes (shading dyes) are often used to increase the brightness, Optical brighteners such as diaminostilbenedisulfonic acid are added to increase the brightness and the luminosity of coating. polyvinyl alcohol, carboxymethyl cellulose, casein, starch, and some synthetic cobinders used as stabilizing agent for attached of optical brighteners. calcium or ammonium stearate are used in many illustration paper as lubricants to reduce dusting on the calender and produce a higher paper gloss. Other additives
ComponentExamplesFunction PigmentClays PCC, TiO 2 Plastic pigments (polystyrene) Builds a fine porous structure Provide a surface properties Adhesives (Binders) Glues, starches, gums, casein, Soya protein, latexes, acrylics, polyvinyl acetate, etc. Binds pigment particles together. Binds coating to paper. Reinforces the base sheet. Fills the pores of the pigment structure. Additives -Waterproofing agents Formaldehyde donors Glyoxal Lattices Make the coating less sensitive to water.
PlasticizersStearates Wax emulsions Azite Improves the flexibility of coating films Rheology Control Agents (Thickeners) Natural polymers Cellulose derivatives Synthetic polymers Controls coating viscosity and water retention properties. DispersantsPolyphosphates Lignosulphonates Silicates Optimizes pigment dispersion. PreservativesFormaldehyde Beta-naphthol Prevents spoilage of formulation between runs DefoamersProprietary agentsControl foam problems. Elimination air bubbles DyesDirect dyes Acid dyes Used for tinted formulations.
Coating machines Brushes coater Distributor coater Air knife Bar Coater Size Press Film Press Bill Blade Inverter Blade Roller Blade Rod Blade
Coating color Formulations Board (in parts)LWC-Web Offset Paper LWC- Rotogravure paper Components 100 Pigments (clay, CaCo 3 ) Dispersing agent (as required) Binder Harding agent Calcium (ammonium) stearate Solid content %