Pele Oy

Effects of Friction in

Papermaking

1

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Preface

Papermaking is a long sequential process, where all process sections

should work to get best possible overall efficiency and product quality.

It is very important to have top knowledge of all parts of the process to get

maximum possible total efficiency and good product quality.

As an example in the following, I will present how paper friction depends

on the total papermaking process and also affects to it and finally to roll

winding and product quality. This really needs more competence than only

winding expertise.

Helsinki, 9 December, 2015

Pekka Komulainen

2

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FRICTION IN PAPERMAKING

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Total efficiency of papermaking process

Papermaking is a long sequential process where all process sections should

work to get saleable production.

If we assume five separate processes (different colors) and very good 99%

material efficiency in each, the total efficiency is 95%. When the efficiency of

each process is as good as 95% then the total efficiency is only 77%.

4

Stock

prep

Paper slitting

and winding

Packaging

Headbox et

drainage

Wet

pressing

Paper

drying Coating /

sizing

Calen-

dering Reeling

Sheeting

Coat

drying

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Paper friction

The development of the tangential force F as a function of time required to

initiate and to maintain relative movement between two solid bodies, where Fs

is the static friction force and Fk is the kinetic friction force.

Coefficient of friction µ is the measured force divided by the normal force.

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The Friction between Paper Surfaces

Garoff, Niklas, Doctoral thesis, 2002

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Possible difficulties of COF variation

High COF

Roll bouncing, vibration and throw-

out in winding

High roll hardness

Dusting, low surface strength

Wear of web contacting machine

parts (wires, felts, rolls, dewatering

elements, knives)

Problems in web converting

High power consumption

Low COF

Web spreading difficulties and crepe

wrinkles in roll winding. Low roll

hardness.

Slipping between driven rolls and

paper.

Sliding of sheets and packages in

treatment, storage and transport.

Sheet grip problems in printing houses,

copier machines and automatic teller

machines.

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Coefficient of friction (COF) should be constant in every process. The level

of COF is also important. In papermaking there are mechanical and

chemical reasons which affect on friction.

More about friction:

www.fpirc.kth.se/Documents/PDF/Friction.pdf

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Measurement speed and COF

Copy paper friction force as a function of time. Measuring speeds were 0.2

and 5 mm/s. A strong stick-slip motion is visible at the lower speed.

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ANNA JOHANSSON et al.

Paper friction—influence of measurement conditions

VOL. 81 NO. 5 TAPPI JOURNAL

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Surface roughness structure and COF

Normally surface roughness has no effect on friction. However, paper is

sometimes a different material.

Fibers rising from paper surface can originally be orientated or will be

orientated after first measurement. It is important to measure paper-to-paper

friction in same direction (also meaning flow from the headbox).

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Higher COF Lower COF

The Friction between Paper Surfaces

Garoff, Niklas, Doctoral thesis, 2002

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Friction sample contamination

Contact-free handling of the test sheets is important. This is demonstrated by

comparing results for contact-free test surfaces with surfaces that had been

touched with the thumb and the hand.

These results show that the friction decreased considerably when the surfaces

were touched before measurement.

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Paper friction—influence of measurement conditions

ANNA JOHANSSON, CHRISTER FELLERS, DENNIS GUNDERSON, AND URBAN HAUGEN

VOL. 81 NO. 5 TAPPI JOURNAL

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PAPER CHEMISTRY EFFECTS ON FRICTION

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Effects of low paper COF in papermaking

Fatty chemicals reducing friction even in small amounts are mixed in the pulp. These can

be fatty acids from wood, deinking chemicals, defoamers, neutral sizes etc. In addition,

lubricants are used in coatings. With coated broke lubricants are returned to the wet end.

These chemicals reduce paper friction and also surface tension thus lowering wet web

strength and having effect on paper winding.

Fatty acids

soaps etc.

More

broke

Bursts

Wrinkles

Winder

breaks

Low winder

capacity

Low winder

capacity

Winder

breaks

Low winder

capacity

More

broke

Winder

breaks

Low winder

capacity

Higher web

tension

Higher

speed More

splices

Wet end

breaks Low fiber

friction

Low paper

friction Low surface

tension

Low wet

web strength

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COF and paper chemicals

Higher COF with the following chemicals (order of effectiveness):

Silica (retention system), calcined clay and titanium dioxide

Ground calcium carbonate and PCC

Clean chemical pulp and starch

More rosin acids than fatty acids from wood (more softwood)

Generally hydrophilic/oleophobic substances with high surface energy

Lower COF with the following chemicals (order of effectiveness):

Silicones/defoamers (several), lubricants and fatty acids from wood

Fatty acid surfactants which are used in the deinking process

Talc and kaolin clay (platy pigments)

Latexes, gums, waxes especially in deinked pulp

Generally hydrophobic/oleophilic (lipophilic) substances with low surface energy i.e. substances having low surface tension such as surfactants

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Fatty acids

Fatty acids are organic acids with a

long carbon chain. Some saturated

fatty acids are:

Butyric: CH3(CH2)2COOH

Lauric (dodecanoic acid):

CH3(CH2)10COOH

Myristic (tetradecanoic acid):

CH3(CH2)12COOH

Palmitic (hexadecanoic acid):

CH3(CH2)14COOH

Stearic (octadecanoic acid):

CH3(CH2)16COOH

Arachidic (eicosanoic acid):

CH3(CH2)18COOH

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Paper COF and type of fatty acids

The coefficient of friction of cellulose surfaces impregnated with different

fatty acids plotted against the number of carbon atoms in the fatty acid.

14 The Friction between Paper Surfaces, Garoff, Niklas, Doctoral thesis, 2002

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Disadvantages of fatty acids

Increasing amount of fatty acids reduces friction and surface tension. This might in some cases be good for winder vibration but not good for wet and dry paper strength.

Fatty acids

Seika Tay: TAPPI Journal, August 2001

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Surface energy and hydrophobicity

Materials with lower surface energy are more hydrophobic. More

hydrophobic surfaces have lower coefficient of friction.

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Picture: Leslie Webb

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Extractive content of American wood species

% of dry wood

Balsam Fir 1.0 - 1.8

Douglas Fir 0.3 - 2.6

Western Hemlock 0.3 - 1.3

Eastern Hemlock 0.2 - 1.2

Western Larch 0.7 - 0.9

White Spruce 0.4 - 2.1

Jack Pine 1.9 - 4.3

Longleaf Pine 2.1 - 9.2

Silver Maple 0.2 - 0.9

White Birch 1.5 - 3.5

Trembling Aspen 1.0 - 2.7

Extractives content has effect on COF. If there is variation in

wood species there is variation in COF.

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Surface-active agents or surfactants

Surfactants are molecules having a dual character.

Part of each molecule is hydrophilic and part is

hydrophobic.

The dual affinity means that surfactant molecules

have a tendency to accumulate at interfaces

between polar and nonpolar phases.

Surfactants can help in dispersing suspension

particles, spread various liquids onto solids, and

stabilize desirable foams.

Surfactants can be a nuisance to papermakers

when they stabilize undesirable foams, hurt dry

strength of paper, or hurt fine-particle retention.

Surfactants are added in deinking and intentionally

to retention aid formulations, biocides, and various

sizing agents to keep such products stable during

storage.

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Initial wet web strength

Friction forces in the wet web form initial

wet web strength. However, there must

be pressure force between the fibers to

get friction force.

This pressure force is formed of surface

tension forces together with fiber stiffness.

There are several chemicals which

reduce surface tension, fiber-to-fiber

friction and paper-to-paper friction. This

would help in winder vibration and

bouncing but at the same time these

chemicals reduce paper strength and

runnability.

Friction force = μN

N

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Influence of wood extractives on COF

S1 and S2 in the picture are static COF:s and K3 is kinetic COF. It can be noted that

clean reference has highest COF.

The same is valid for virgin and recycled pulps: well washed pulp has higher COF.

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THE INFLUENCE OF WOOD EXTRACTIVES ON

PAPER-TO-PAPER FRICTION, 1999 International Paper

Physics Conference Proceedings

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Effects of wood extractives

For winder vibration and roll

bouncing lower friction is good. For

crepe wrinkles higher friction is

better.

Lower sheet strength especially wet

web strength has effect on paper

machine runnability.

Best friction level is always

compromise between paper

machine runnability and winder

runnability as well as final paper

quality.

Some softwood extractives especially in the mechanical pulp

have the following effects:

Components Effects

Triglycerides,

fatty acids

Lower strength

and friction

All hydrophobic

components

Foaming, low

retention

All hydrophobic

components

Lower water

absorbance

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22

COF and fillers

Platy fillers decrease COF (kaolin, talc).

Synthetic silicate and calcined clay increase COF.

Michael C. Withiam, The effect of fillers on paper friction properties

April 1991 Tappi Journal

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Synthetic silicate and COF

Synthetic silicate is very efficient in increasing kinetic COF. Small

amounts up to 2% have the greatest effect.

Michael C. Withiam, The effect of fillers on paper friction properties

April 1991 Tappi Journal

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Retention control and silica

Polyacrylamide and silica

microparticles are often used to keep

tray water consistency constant.

Small additions of silica can increase

paper COF considerably. There can

be high variation in silica content in the

paper - more silica means higher

friction.

Bentonite retention system might be

more slippery – bentonite is a special

type of clay.

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Foam control and paper COF

Foam is controlled by varying defoamer

content according to air content measurement.

The most universal characteristic of any

defoamer is the fact that it is surface active, but

highly insoluble in water. It has to be

formulated so that it will be dispersed as tiny

droplets, i.e. as an emulsion.

Foam control agents in surface size or coating

stay on the paper surface and have very big

effect on paper COF.

Defoamers normally reduce friction. Variation

of defoamer content makes variation in paper

COF.

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Example of water circulation system

Let us suppose that fresh water consumption is high (30 m3/t paper) and

there is no water lock between the pulp mill and the paper machine.

Pulp Mills Paper-

making

Fresh Water 30 m3/t paper

To effluent treatment

From PM 11 m3/t paper

Pulp consistency 10%,

water 9 m3/t pulp,

lot of dissolved &colloidal

materials to paper machine

To effluent

treatment

10 m3/t paper 20 m3/t paper

1 m3/t paper

17 m3/t paper 12 m3/t paper

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Water lock after pulp mill

Let us suppose that there is a double wire press between pulp mill and

paper machine and the solids after double wire press is 33%. Only 2 m3

water/t pulp is then transported to the paper machine.

Pulp

Mills

Paper-

making

Fresh 10 m3/t paper

From PM 11 m3/t paper

If consistency is 33%, water

content is 2 m3/t pulp, less

anionic trash to paper machine

Main flow to effluent from

DIP Plant anionic trash out

0 m3/t paper 10 m3/t paper

1 m3/t paper

0 m3/t paper

9 m3/t paper

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DCS, fresh water and water lock

Practical fresh water usage is 10-20 m3/t. This range has very little effect on

dissolved and colloidal solids concentration. However, moving from integrated

system to interstate washing (water lock) has a big effect on DCS carry-over to

paper machine.

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Base paper porosity and coating coverage

Coating coverage has effect on smoothness after calendering. Coating can have lower friction than base paper.

The final paper friction depends more on the friction difference between coating and base paper than on the final paper roughness.

Variations in fiber components and in their shares in the furnish have effect on paper porosity and coating coverage.

Porous base Dense base

TING HUANG AND PIERRE LEPOUTRE: Effect of basestock surface structure and chemistry on coating holdout and coated paper properties, VOL. 81: NO. 8 TAPPI JOURNAL

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Coating lubricants and COF

The most effective lubricant is wax. Also stearates reduce friction.

Toshiharu Enomae · Naoya Yamaguchi - Fumihiko Onabe

Influence of coating properties on paper-to-paper friction

of coated paper

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FRICTION IN WINDING

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TNTs and two-drum winder

With a two-drum winder the nip force at the end of winding can be too high. If this is

compensated by tapering web tension and torque difference, the combination of heavy

nip load and low tension can create crepe wrinkles.

0

2

4

6

8

10

12

100 300 500 700 900 1100 1300 1500 1700

Roll diameter, mm

Nip

lo

ad

, kN

/m

Nip load

Compensation with rider roll

Nip due to roll’s own weight

Tapering web tension or

torque can

be dangerous

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Winder types for thin printing papers

Winders have been developed to reduce the load between winder drum and paper roll (too heavy load breaks a thin paper). Glossy coated paper has high density and paper is thin. This requires better winder than standard two-drum winder (e.g. Soft drum winder, WinBelt or single drum winder).

Thinner paper or larger roll diameter

Two-drum

Winder

Belt supported

Winder Multi-station

Winder

Multi-station

belt supported

Belt

Belt

Recommended with

soft drum cover(s)

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Winder problems and COF

There is an optimum COF especially for two-drum winders

Papermaking variables have a great influence on the COF level

COF is an important papermaking variable and should be monitored

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Pap

er

co

eff

icie

nt

of

fric

tio

n

0.5

1.0

Vibrating and bouncing range

(uncoated & matt coated grades)

Wrinkling range

(thin and smooth grades)

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Two-drum winder and uneven caliper

The purpose of shaftless winding is to rotate the rolls with same surface speed. In practice this is difficult. Due to axial forces the rolls try to attach to each other and try to have the same rotational speed.

The edge roll can be smaller in diameter. The situation is more difficult, when there is a smaller roll in the middle of the set. Caliper profile should be all the way slightly thinner to the edges to get better winding.

When paper friction is high, touching rolls have big effect resulting in disturbing forces = core misalignment, out-of-roundness, roll bouncing and throw-out.

One remedy is to run some supporting paper layers on the cores slitters still open.

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CD caliper profile

Winder drum

Axial force

Problem

Picture: Voith Paper

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Roll bouncing

General unstable behavior of the roll,

i.e., roll rocking back and forth from

drum to drum caused by roll diameter

differences and high friction paper.

Appears on shaftless two-drum type of

winders or belt supported winders.

Can be seen as blinking in the roll

gaps or shaking of the core locks after

certain roll diameter .

Picture: Metso Paper

36

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Effect of paper COF on coated fine paper winding

When COF is ≥0.5 there is a risk for vibration. This is very common situation when mostly carbonate is used in coating.

When COF is low, roll dishing can be a problem.

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0,35 0,55

Paper Coefficient of Friction

Ris

k f

or

win

din

g p

rob

lem

s

No problems

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Effect of paper COF on newsprint winding

When COF is ≥0.5 there is a risk for vibration and bouncing. This is very common situation when DIP quality is good (no stickies or fatty acids).

For low grammage coated paper wrinkles and burst are a common problem.

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0,3 0,5

Paper Coefficient of Friction

Ris

k f

or

win

din

g p

rob

lem

s

No problems

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Vibration, friction and roll hardness

When newsprint is made of mechanical pulp or lower quality DIP, coefficient of friction (COF) is low and there are no vibration problems in winding.

Nowadays, with better DIP processes and higher amount of carbonate fillers, newsprint tends to have high friction and severe vibration problems (like bond paper in the picture).

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Example of winder vibration and bouncing

Due to vibration, speed of the winder must be increased stepwise. With normal speed there are more web breaks (example in the picture) and to avoid these, the speed must be reduced. Reduced speed, stepwise acceleration and web breaks all reduce winder capacity.

Time, s

Sp

eed

, m

/min

1000

500

1500

2000 Break

40

Roll bouncing

ALEXIS OLSHANSKY

TAPPI Journal, Feb 1997

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Roll deformations and vibration

If paper friction is high and/or paper compressibility is high all impacts to the

roll give a permanent deformation. These tend to have a certain wavelength.

If this wavelength matches roll periphery, vibration will be heavier.

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Roll bouncing

ALEXIS OLSHANSKY

TAPPI Journal, Feb 1997

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Vibration harmonics vs. drum’s natural frequency

Winder drum

natural frequency

Paper roll frequencies vary due

to growing roll diameter

= Heavy

Vibration

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J-line and crepe wrinkles

Internal slipping of paper layers under the roll surface is

measured with J-line measurement.

Shooting a radial line on roll end and then winding layers on

top of that will move the marked layers first to the tightening

direction and then slowly towards the winding direction (=

loosening the original maximum web tension under some top

layers).

If the maximum tension is low and slippage high there will be

a negative tension in the machine direction resulting in a

buckle of the layers (= crepe wrinkle).

Very bad combination is a high nip load, a low web tension

and a low COF. This is most common at the machine reel

edge, where caliper can be high and web length long (=loose

edges). In addition edges can be dry and thus COF is low.

It is best to have curved CD profile = slightly lower caliper at

edges to avoid crepe wrinkles and other problems.

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Sensitivity to crepe wrinkles

There will be slippage and possibility to get crepe wrinkles, when

- F > µ • p • A + S

- F = buckling force from nip action = f (N and nip width)

- A = area of possible sliding

- S = paper stiffness force

To avoid wrinkles paper static COF must be high as well as roll hardness, but nip load should be low. Low paper stiffness and caliper increase possibilities to get wrinkles.

With a soft drum cover the buckling force and J-lines are smaller than with a hard winder drum. Possibility to get crepe wrinkles is very small.

Picture: Metso Paper 44

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Reasons for crepe wrinkles

Low web-to-web coefficient of friction

Slipping of web layers due to nip effect inside the roll surface

When roll weight is high (close to center if center support, close to surface if surface support)

Combination of low web tension and high/increasing nip load

Combination of loose web and high caliper/roll diameter

Low basis weight or thin paper and low MD stiffness

Hard winding on top of soft winding

Variation in CD profiles, loose edges, uneven rider roll contact

Winding direction

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No-problem channel in winding

It is important to find the correct web tension for every paper grade. If tension

is too high web breaks and internal bursts are problems.

If tension is too low crepe wrinkles are problems.

Increasing nip load with roll diameter

No problem area

Too low web tension

wrinkles

Too high web tension

bursts and breaks

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Core burst of single drum winders

Due to core and roll bottom bending

shear and compression forces have

most effect at roll edges against the

core chuck area.

The situation is worst when roll

weight is highest.

COF has effect on the slippage

between layers. Gearing is easier

with low COF.

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Pressure from spool to paper

There is more pressure at reel edge when the spool is thin, reel is wide,

diameter is large and paper density is high.

Thin groundwood papers are more sensitive.

Reel hardness, air between layers and machine speed have influence.

The problem: Wrinkles and bursts on reel bottom.

Bottom broke up to 2% = 2000 m of 100000 m.

Problem areas

48

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Effect of COF on roll traction

Tension change depends on

COF.

Air cushion can reduce the real

COF depending on speed, web

porosity, capstan wrap angle

and web tension.

With very high speeds and

dense webs there can be a

floating situation. Then the

paper-roll COF has no effect

anymore.

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Picture: Getting and Losing Traction

by Jerry Brown

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Traction between web and roller

There are two distinct ways to improve the

traction potential between web and roller:

topography and chemistry.

Topography is using surface roughness to

increase traction. Rough rollers will have a

higher traction with rough webs and with all

webs where air entrainment is significant.

However, smooth rollers might have more

traction with a smooth web at speeds less

than 50 mpm or where grooving is present to

handle the air.

The most common roller roughening

techniques are grooving and spraying with

tungsten carbide.

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Chemistry and roll traction

Chemistry also affects traction. Metal surfaced rollers tend to have a

somewhat similar friction against many webs; a COF of 0.3 is typical.

However, those same webs against common resilient cover materials, such

as polyurethane have considerably higher COF.

The effect of chemistry on COF is very efficient. Basically only molecular

layer on the surface has big effect on COF.

Adding some lubricant or high friction chemicals to the surface size or coating

has more influence than chemicals in base paper. Similarly, contamination of

rollers can have great effect on COF between roller and paper. For example

washing of winder drums is very important to keep good traction.

The only issue here is that higher COF topography or chemistry tend to be

expensive to achieve and expensive to maintain.

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Paper grades and COF

Good deinking removes all fatty acids from the pulp. Fatty acids are lubricants

decreasing paper friction. Good deinked pulp = high paper-to-paper COF =

vibration in winding.

Newsprint

earlier Improved

deinking

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Loosening of fibers

After wire section there are several kind of forces trying to loose fibers from the

paper web.

Fiber detachment depends on the balance between binding forces and

detachment forces. To solve the dust problems it is important to increase binding

forces and reduce detachment forces. With high friction papers the detachment

forces are often higher and paper is more dusty.

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Potential dust fiber

Paper web

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Conclusion

Be aware that COF is important to the runnability of the paper you work with and the performance of your product.

Be aware that COF is sensitive to changes you cannot see or feel.

Include COF as one of the paper properties routinely measured and controlled.

Relate COF to the operating parameters of your equipment. Keep records so that you can compensate for changes in paper properties.

Work out a reasonable specification for COF - and hold to it.

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