paper & board coating

Paper & Board Coating

Atrak Pulp & Paper Industries Ltd.

Why is Paper Coated ??

Cellulose Fiber Size :

• Soft Wood : Length 3 ~ 4 mm Diameter 25 ~ 50 Micron• Hard Wood : Length 0.8 ~ 1.5 mm Diameter 10 ~ 35 Micron

Pores between Fibers : 10 ~ 100 Micron.

Printing Dot Diameter : 25 ~ 50 Micron.

Coating the paper surface with Fine Pigments (Diameter 1~2 Micron) to cover the fibers and fill the pores.

Functionality

Reduce Ink Consumption

Glueability

Why is Paper Coated ??

Printability

Smoothness Uniformity Ink Receptivity

Visual Properties

Brightness Opacity Gloss

The whole purpose of a coating is to attach uniform layer of pigment to the base sheet.

Coatings Purposes

A Binder is used to adhere the pigment to the fibers.

There are additional additives in the coating formula to help in processing or enhance final sheet properties.

Photo of Un-coated Paper

Magnification 200x

Photo of Coated Paper

Magnification 2000x

Cross Section of Coated Paper

Magnification 2000x

Coating Ingredients

Natural Pigment: Clay, Calcium Carbonate, Talc.

Synthetic Pigment: PCC, Plastic Pigment, TiO2, Silica, Satin White.

Synthetic Binder: Latex.

Hydrosolubles Binder: Starch, CMC, PVA, Casein.

Additives: Dispersant, Thickener, Cross Linker, OBA, Lubricant, De-Foamer, etc.

Water.

Coating Ingredients, Dry Weight %

Function of Coating Ingredients

Pigment : Govern optical & printing properties of the paper.

Binders : Give the cohesion of the coating & adhesion of pigments to the base paper without affecting the properties brought by pigments.

Dispersant : To disperse the pigments.

Cross Linker : Provide wet pick resistant.

OBA : To increase brightness.

Lubricant : Improves flow properties, prevent coating adhesion on to calendar.

Thickener : Modifies the rheology and/or the water retention of the colour.

De-Foamer : Reduces foam.

Pigments

ClayInterest : paper & ink gloss, rotogravure printability, stiffness.

Draw back : lack of brightness, closed coating.

GCCInterest : brightness, low viscosity, high solid, ink porosity.

Draw back : gloss, low water retention, fiber coverage.

TiO2

Interest : opacity, brightness, whiteness

Draw back : cost, latex demanding.

Pigments

Plastic Pigment

Interest : high gloss, brightness, low specific gravity.

Draw back : price, more latex demand.

PCCInterest : brightness, viscosity, low abrasivity.

Draw back : cost, more latex demand.

SilicaInterest : brightness, high specific area, opacity.

Draw back : cost, need higher cohesion binder.

Pigments

TalcInterest : smoothness, non abrasive, best for gravure.

Draw back : gloss.

Satin White

Interest : high gloss.

Draw back : work ability.

Pigment Structure

Magnification 5000x

Pigments Properties

Brightness : 84 ~ 96 % ISO

Particle Size Distribution : 0.22 ~ 2.00 Micron

Surface Area :

Fine Clay 20 ~ 25 m2/gramStandard Clay 18 ~ 20 m2/gramCalcined Clay 13 ~ 17 m2/gramGCC 8 ~ 13 m2/gram

Pigments Properties

Refractive Index :

Interface

Substrate

Angle I

Angle R

R.I. is amount of lights refracted by the pigments.

High R.I. = High Opacity.

Light Comes

Light Refracted

Pigments Properties

Pigments Composition Particle Size Shape R.I. Brightness

Clay Al2O3.2SiO2.2H2O 0.3 ~ 5.0 Hexagonal Platy 1.56 80 ~ 90

GCC CaCO3.MgCO3 0.7 ~ 2.0 Cubic Prismatic Platy 1.65 87 ~ 97

Talc MgO.4SiO2.H2O 0.3 ~ 5.0 Platy 1.57 85 ~ 90

Gypsum CaSO4.2H2O 0.2 ~ 2.0 Roundish 1.52 92 ~ 94

Titanium TiO2 0.2 ~ 0.5 Rod Like 2.55 98 ~ 99

Titanium TiO2 0.2 ~ 0.5 Roundish 2.70 97 ~ 98

PCC CaCO3 0.1 ~ 1.0 Variable Rod Like 1.59 96 ~ 99

Calcined C. Al2O3.2SiO2 0.7 Aggregated Platy 1.56 93

Binders

Latex

Hard Latex : Higher gloss, low binding power, mainly for top coat.Web Offset Latex : Blistering resistance, low binding power.Soft Latex : Lower cost, high binding power, best for pre coat.

Starch

Interest : Stiffness, price.Draw Back : Mottling, low binding power, rheology at high speed.

Binders

CMCInterest : Water retention, rheology, binding power.Draw Back : Handling.

PVAInterest : Excellent binding power, gloss, brightness.Draw Back : Price, rheology at high level.

ProteinInterest : Water retention, binding power, bulkier coating layer.Draw Back : Handling, rheology at high level, microbiology control.

What Does Binder Do ?

1. To hold the pigments on the base paper.2. To help give good printability.3. To give special characteristic.

Pigment

Pigment

Binder

Latex Structure

Latex Particles

Pigments Particles

Main Binder Types

1. Styrene Butadiene.2. Styrene Acrylic.3. Styrene Butadiene Acrylic.4. Vinyl Acetate.

The Influence of Binder

1. Dry and Wet Pick Strength.2. Sheet and Ink Gloss.3. Printability.4. Blister Resistance.5. Ink Drying.6. Yellowing.

The Influence of Binder

Sheet Gloss Ink Gloss

Pigment

Binder

Coat Weight

Solid Content

Increase low shear viscosity to allow proper application.

Adjust high shear viscosity to provide coat weight control and run-ability.

Increase water retention.

Can enhance optical and printing properties of the finished sheet.

Rheology Modifier (Thickener)

Rheology Modifiers

Starch Protein Poly - Acrylates Alco Gum Sodium Alginate Urethane Modified Acrylates Poly Phobe Cellulosic Carboxyl Methyl Cellulose (CMC),

Hydroxyl Ethyl Cellulose (HEC)

Atrak Pulp & Paper Industries Ltd.

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