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Finishing

Kristian Goldszer

Papermaking Course Montevideo

Part II Paper Processes

March 19 21, 2009

Sources: Metso Paper, KnowPap, Cepatec AB, Skogsindustrierna


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Finishing

Calendering

Coating

Reeling


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Objectives of Calendering

Pre Calendering

Controls caliper profile

Controls bulk

Creates a good smooth surface for coating

Controls porosity dewatering of coating

Finish Calendering

Creates gloss

Creates smoothness

Improves the printing properties


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Effects on Paper Properties

Desirable Changes

Smoothness +++ Gloss +++ Oil absorption - - Air porosity - - - Two-sidedness - -

Undesirable Changes Density +++ Thickness - - -

Stiffness - - - Compressibility - - - Opacity - - Brightness - Tear strength - -

+ = increases- = decreases


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Calendering Process Variables

The main parameters that contributes to the calendering result are:

Pressure Loading Roll hardness and dimension

Temperature Web Roll

Web Moisture

Water Steam

Time Machine speed

Nip length Number of nips


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Specific Pressure ( max ) =

[kN/m]

[mm]

Nip width

Specific pressure [MPa]

Specific Pressure

Linear load (kN/m)

Nip width (mm)


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Specific Pressure and Nip Width

100 kN/m 100 kN/m100 kN/m

Nip width 1-2 mm

Max nip pressure 80 Mpa

Nip width 10-20 mm

Max nip pressure 40 MPa

Nip width 30-250 mm

Max nip pressure 10 MPa


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Effect of Linear Load


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Effect of Temperature

The higher the temperature, the easier it is to plasticize the paper

Lignin and hemi cellulose soften


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Effect of Moisture

The softening temperatures of lignin and hemi cellulose drop as the

moisture content increases


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Effect of Machine Speed


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Effect of Number of Nips


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Calender Types

Hard Calender

Soft Calender

Long Nip Calender

Multi Nip Calender


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Hard Calender

Metal rolls

Narrow nip Short dwell time in nip

High maximum pressure

Used for: pre and final calendering of board

final calendering of news


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Soft Calender

At least one of the rolls forming the nip is a soft cover roll

Can replace a hard calender in all applications More even density profile


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Comparison of Hard and Soft Calender


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Long Nip Calender

Especially for board

Excellent smoothness with reduced loss of bulk


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Soft Calender

Long-Nip Calender

Nip dwell times based on speed 700 m/min

70 mm shoe length

Nip

Pressure(MPa

)

Nip Dwell Time (ms)

Nip Load Soft Calender versus Long Nip Calender

Shoe calender has much higher potential in treating the sheet surface

than a soft roll calender due to lower pressure and longer dwell time


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More Effective Heat Transfer - Melts the Surface

Soft calendered 250 C Shoe calendered 250 C


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Calender Type Bulk versus Printability


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New Technology Metal Belt Calender


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Multi Nip Calender

Mainly for printing papers

Densifies the web, in particularly the surface


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Coating

Paper and Board are coated in order to improve printability

Coating impacts Paper and Board surface in the following way:

Improved smoothness => more uniform print result

Increased porosity in the surface layer => uniform colourabsorption

Increased opacity => less risk for print through

Improved brightness

Increased gloss


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Coated Products


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Main Coating Processes

Two principles:

Application of coating colour in excess, 200 g/m2, metering ofexcess coating colour to final amount, 5 30 g/m2

Pre-metered amount corresponding to desired coat weight isapplied to the substrate to be coated

Drying:

IR dryer

Air dryer

Cylinder dryer


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Coating Colour

Pigments, 80 95 %

Binders, 5 20 % Additives, 1 2 %

Solids content = 40 60 %

Viscosity 1200 mPas

Base Layer:

Coarse pigments

Coverage important Improve smoothness

Even out brightness

Affect colour absorption

Top Layer:

Fine pigments

Increase gloss Increase brightness

Improve smoothness

Desired colour absorption


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Demands on Pigments

Compatible with other substances in coating colour

Easy to disperse in water and good runnability

Low average particle size

Low binder demand

Not wearing

Low price


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Important Pigments

Clay

70 75 % < 2 m High viscosity High Aspect Ratio, 25 - 40 Gloss. smoothness Good coverability => used for low

coat weights such as LWC, ULWC

Calcium Carbonate

90 % < 2 m Irregular spherical particles

Low Aspect Ratio, 3 - 7 Rougher and duller surface

Brightness, opacity Higher pigment brightness than clay,

but not the same coverage

High Aspect Ratio => Gloss


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Important Pigments

Calcined Clay

75 95 % < 2 m Lower density

Higher light scattering

Brightness , Opacity

Titanium Dioxide

Used together with other pigment, 5 15 %

Very high refractive index and small particles, 0.3 0.4 m

Brightness , Opacity

Plastic pigments

Gloss, Opacity


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Clay Calcium Carbonate


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Function of Binder in Coating Colour


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Important Binders

Latex

Styrene Butadiene Styrene Acrylate

PvAc

Starch Low Binder Cost High Binding Power

Good Water Retention

Reduces Solids Content of Total Mix

Increased Risk for Uneven Binder Migration


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Additives

Dispersants

NaPa Increases viscosity

Wet Strength Agent Harden coating, cross link binders Ammonium Zirconium Carbonate Glyoxale

Melamine Formaldehyde Resins

Lubricant Ammonium Stearate

Optical Brightener

Biocide Prevent bacterial growth Glutharaldehyde

Defoamer


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Coating Processes

Excess Processes

Roll application Jet application

Short dwell time application (SDTA)


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Roll Application

Application pressure = 0.5 4 Bar

Dwell time = 15 75 ms Applicator roll:

Rubber covered

Diameter 25 % of backing roll

Speed, 25 % of machine speed 0.5 2 mm gap between applicator roll

and substrate to be coated

Dewatering controlled by pressure in

high speeds and by dwell time in lowspeeds


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Jet Application

Application pressure = 1 Bar

Dwell time = 15 75 ms Dewatering controlled by dwell time


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Short Dwell Time Application

Application pressure = 1 Bar

Dwell time = 1.25 6.25 ms Dewatering controlled by

dwell time


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Blade Coating

Most frequently used coating method

Machine speeds up to 2000 m/min High solids content, 62 67 %

Blades are 0.4 0.5 mm thick and 75 mm wide

Gives a uniform surface

Stiff or bent blade

The coating evens out irregularities

in the base paper filling in method


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Blade Coating

The coat weight is controlled by

applying a blade pressure Important with constant blade

angle for uniformity

For stiff blades the coat weightdecreases with increasedpressure and vice versa

Stiff blades for high speedmachines with low coat weight

Bent blades for low speedmachines with high coat weight


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Roll Blade

Alternative to blade for pre-coating of board

Coat weight controlled with a rubber hose behind the rod blade thatcontrols the pressure

Coat weight not as sensitive to pressure changes as blade coating

Higher roughness compared to blade coating

Solids content around 3 5 % lower than for blade coating

The coating follows the contour ofthe base paper contour method


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Air Knife

Excess coating colour is metered with an air jet

Excess coating colour is collected in a vacuum box Earlier common for top coating of board

Low solids content, 40 45 %

Low speeds, 400 500 m/min

Speed limitation is main reason for replacing air knifes with blade

The coating follows the contour ofthe base paper contour method


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Drying of Coating Colour IR Drying

electrically or gas heated normally 1st stage

evaporation rate 100 140 kg/h,m2

Hot Air Drying steam or gas heated evaporation rate 35 160 kg/h,m2

Cylinder Drying coating surface must be non sticking final drying evaporation rate 10 15 kg/h,m2

Water 30 - 35 % of total coating composition


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Coated White Top Liner


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Liquid Packaging Board


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Light Weight Coated


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Reeling

Production of large diameter parentrolls for further processing in the mill:

Calendering

Coating

Winding to customer roll

Sheeting to customer sheets


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Roll Requirements

Correct roll dimensions

Diameter (also cross CD) Weight Web length

Round rolls Core in centre

Avoid vibrations Straight roll edges

In further processing Storing

Good roll structure Hardness and roll tension

Clean and dust free


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Reeling Processes

The reeling process includes the following processes:

1. Web control before reeling

2. Reeling3. Reel change

4. Reel handling

We control web inspection web tension machine speed vibrations

Reeling roll structure

Reel change deceleration stopping

moving

Reel handling coating calendering

storage converting


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Roll Structure

Radius

Hardness

The first layers are wound as hard as required to prevent slippage

between the layers The hardness decreases continuously from the intermediate zone to

the surface


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Effect of Line Pressure on Roll Structure

Constant Pressure

bad roll structure

Controlled Pressure good roll structure

Line pressure between roll and reel drum

Reel

Reel


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Effect on Paper Properties of Reeling Line pressure, web tension and internal tensions effects paper

properties

The effect is high close to the core and low at the surface

DecreasesPorosity

IncreasesGloss

DecreasesRoughness

DecreasesBulk

DecreasesStiffnessDecreasesThickness

ResultProperty


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Reel Types Pope

Simple model suitable for:

Small rolls

Qualities that are insensitive toline pressure

Compressible paper

Peripheral drive


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Turn Up Pope

High initial nip load due to reel spool weight

Hard to control nip load in transfer from primary- to secondary arms

Sudden increase in nip load at transfer

Nip load


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Center drive torque

Advantages with center drive torque: Bigger rolls

Better roll structure Bigger tolerance to variations in line pressure

Reel Types Center Drive Torque


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Turn Up Methods

Common methods:

Tape Gooseneck

Water Jet

Choice depends on: Grade

Operating speed

Web width

Reel type


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Water Jet Turn-Up

Reel drum

WJ beam

Water pressure max 2000 barHigh speed operation,tail cutting and turn-upwith air blow

Nip

Pick-up air blow


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Tape Turn-Up 1. Control panel2. Tape feeding and cutting

unit

3. Tape transfer rail

4. End of the tape

5. Break6. Empty reel spool in the

primary arms

7. The tape is fed into thenip and the tape sticks

onto the reel spool8. The tape cuts and guides

the tail around the reelspool

9 finishing 4

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