ACAV Ultra High Shear Viscometer

ACAV Viscometers by1

ACAV

Coating Color Runnability Analyzers

Picture: Voith


Viscosity Measurement

Coating colors are rheologically complex materials: Their viscosity is strongly dependent on shear rate Viscosity is dependent on shearing time They are viscoelastic They may have significant extensional viscosity and/or normal

stresses


Viscosity Measurement Shear Rates of Modern Coaters

Real shear rates are higher because part of the coating is already in the paper pores and the paper surface is not even!

Coat weight g/m2 10 10 8Color solids % 62 62 62Machine speed m/min 1000 1500 2000Color density kg/m3 1500 1500 1500Coating thickness under blade

µm 21,5 21,5 17,2

Average shear rate Million 1/s 1,4 2,3 3,7

Shear Rate =

Speed of coaterFilm thickness

ACAV Viscometers by

Viscosity Measurement Shearing History of Coating Color

4

Applicator NipPumping

Mixing

BladeScreensRod

0 102 103 104 106105 107

Curtain hitting point

ACAV Viscometers Coating Color Circulation

Conventional viscometers measure ONLY low share rates!

Share Rate (1/s)

= Conventional viscometers


Viscosity Measurement Dominating Factors at Different Shear RatesSURFACE CHEMISTRY AT LOW SHEAR• Solids content• Electric attraction/repulsion• Van der Waals attraction• Steric repulsion• Viscosity of water phase

HYDRODYNAMICS AT ULTRA HIGH SHEAR• Solids content (i.e. volume fraction of particles)• Particles size• Shape of the particles• Particle size distribution• Viscosity of water phase

Brookfield, rotationaland capillary viscometers

Capillary andSLIT viscometers


Viscosity Measurement Viscometers & Shear RateRheological Properties of Starch Latex Dispersions and Starch Latex-Containing Coating Colors

1Jae Y. Shin, 2Nathan Jones, 1,3Do Ik Lee, 1Paul D. Fleming, 1Margaret K. Joyce, 3Ralph DeJong, and 3Steven Bloembergen*.

The unique characteristics and properties of internally crosslinked starch latex binders for paper coating were presented. While low shear Brookfield and Hercules rheograms are commonly used in the industry to assess the runnability of coatings, these results demonstrate that such low shear techniques can be extremely misleading when it comes to the prediction of coating performance on high speed metered size press, rod and blade coaters. The use of more specialized “ultra-high” shear equipment such as the ACAV might be needed to better understand the rheological performance under commercial coating conditions. PaperCon 2012 http://www.tappi.org/Downloads/Conference-Papers/2012/12PAPERCON/12PAP11.aspx

http://www.tappi.org/Downloads/Conference-Papers/2012/12PAPERCON/12PAP11.aspx




Coating optimization Effect of Particle Size Distribution on Ultra High Shear Viscosity

Narrow distribution, 67%

Wide distribution, 67%

Narrow distribution, 65% solids

Wide distribution, 65% solids

= Measurement range of Conventional viscometers

Curves are identical below 200 000 1/s


Coating optimization Runnability of new cheaper co-binder system is same as

earlier. Production of the grade was 60 000 t/a Paper mill wanted to save money,

because of lowered paper prices. They used a large amount (1,2) of

expensive co-binder and a small amount of cheap co-binder (0,35).

After testing several co-binder combinations Color A was best with only 0,4 parts of expensive binder and 0,55 parts of cheap binder.

Runnability and water retention of Color A is exactly same as earlier.

Runnability of Color B is not as good as in Reference.


Coating cost saving: 153 000 EUR/year!


Coating optimization

More fibers in coating circulation caused streaking problems


Conventional viscometers measure ONLY low share rates!


Coating optimization Runnability and Particle Size Distribution

POOR RUNNABILITY, wrong particle size distribution

GOOD RUNNABILITY, right particle size distribution

Curves are identical below 0.25 million 1/s

Less Breaks with high speed and reduced blade pressure


Coating optimization Temperature Effects

60ºC

50ºC

30ºCCoating shear rates

Logical behaviour below 100 000 1/s


Increase of coating color temperature destroys the surface active ingredient of the latex.


Coating optimization New coating recipe provided higher solid

content at lower ultra high shear viscosity level

Precoat old, 62 % Solids

Precoat new, 68 % Solids

BENEFITS:• Coating costs reduced 10 %• Saved drying energy 50 kWh/t• Speed increase 33 % from 900

to 1200 m/min!

Results provided by: Rohm & Haas and Burgo Ardennes


ACAV Viscometers Blade versus Slit Geometry

BLADE

0.5 mmSLIT

Coating Color~ 0.5 mm

~ 2

0 µm

Base paper80

µm

Similar Velocity Profiles with Blade and Slit


ACAV Viscometers

ACAV Slit Best correlation to blade load Best correlation to formation of agglomerates Best correlation to scratch count Enables analysis of most viscous pigment slurries Enables analysis of highest shear rates and simulation of

fastest blade coaters


ACAV Viscometers Slit Assembly


ACAV Viscometers

ACAV SLIT Viscosity vs Scratch Counts

Amount of dispersant addedis given as a percentage

1,60 %

1,60 %

0,80 % 0,80 %

0,40 %0,40 %

1,60 %1,60 %

Results: TAPPI, 2001 Coating and Graphics Arts Conference and Trade Fair, pages 77 -86Dr. Rajan R. Iyer, Ray Hollingsworth, Dr. David R. Skuse, Imerys, Sandersville,USA


ACAV Viscometers Possible formation of Agglomerates / SLIT

Some pigment and/or chemical combinations cause agglomeration behavior, when the color is under a high shear. The pigment and the chemical combinations can form big blocks (agglomerates), which cause problems in coating machines and for example in screens.

The possible formation of the agglomerates can be measured by the SLIT. With the agglomerate measurement, we use same pressure during the whole measurement and we measure the flow. If there is the formation of the agglomerates, they block the SLIT and the flow will decrease or even stop, if there is a strong formation.

If the SLIT is blocked, a customer can open the SLIT and take a sample of the block and use chemical analyzers (IR, AAS, etc.) to find out the reason (pigment or/and chemical) , which caused the problem.


ACAV Viscometers Runnability on High-Speed Blade Coating

Evaluation of the temperature change of coating (TCH) under high-shear rate viscosity with a SLIT die in laboratory experiments is one of the best ways to simulate coating fluidity under the blade.

Water retention, high shear rate viscosity with a SLIT die, and properties of the base paper are needed to predict blade coating runnability.

Results: TAPPI Coating Conf. 2002, Dr. Koji Okomori, Masato Yamaguchi, Masahito Suzuki and Hirokazu Morii, R&D Division, Nippon Paper, Tokyo


ACAV Viscometers Guidelines for Interpreting Graphs

Viscosity(mPas)

100

1000

Optimum RheologyBehaviou

rHigh viscosity at ultra high shear rates: 1. Blade bleeding and streaking2. Problems in maintaining target coat

weight3. Web breaks, scratches, agglomerates

Low viscosity at ultra high shear rates: uneven coat weight

High viscosity at low shear rates: pumping and

start-up problemsHigh viscosity at high

shear rates: problems in screens, rod, jet and

applicators

1 10 2 3 4 5 6 710 10 10 10 10 10 Shear Rate 1/s

Low viscosity at all shear rates: water retention problem


ACAV Viscometers Blade Pressure Correlation

“The blade run-in’s were compared to coating viscosity measurements at various shear rates to confirm the validity of using ultra high shear viscometer as a predictive tool in runnability”

“Ultra high shear viscosity, obtained by the capillary viscometer (ACAV A2), showed GOOD CORRELATION to the blade pressures required to obtain a target coat weight, even though the colors studied had significantly different pigments (e.g. delaminated or chemically engineered).” By ECCI, TAPPI Coating ´98

By Imerys, TAPPI Coating´01


Pigment Slurry Analysis

Maximum viscosity range up to 12 000 mPas (Capillary)

Viscosity range up to 30 000 mPas (SLIT)

Shear rates from 100 1/s up to millions 1/s

Solids up to 76 %


Pigment Slurry Analysis Kaolin with Wide Particle Size Distribution


Pigment Slurry Analysis Kaolin with Narrow Particle Size Distribution


Analysis of Coating Color Formulations

Different thickeners cause different viscosities at different shear rate ranges

different behaviour in screens and applicators different blade loads

Particle size of thickeners has an important impact on particle size distribution of coating colors


Analysis of Coating Color Formulations Viscosity Curves of “Typical” Coating Colors and Coating

Colors with Narrow Particle Size Distribution.

Wide particle size distribution

& traditional latexNarrow particle

size distribution& traditional

latex

Reproduced from R. Knappich et al.


Analysis of Coating Color Formulations Viscosity Curves of Coating Color with Narrow

Particle Size Distribution and Latex A/B

Narrow particle size distribution

Ultra high shear viscosity correlatesdirectly on blade loads.

Reproduced from R. Knappich et al.


Analysis of Coating Color Formulations Effect of Medium Co-binder Addition

KAOLIN BASED COATING

PVOH

StarchLatex A

Latex B

CMC


Analysis of Coating Color Formulations Thickeners – Example of Shear Rate Effect

Same coating color, five different thickeners. Pigment is mostly clay. (Data: Sandås, Salminen)• Note logarithmic scale.• No correlation between

low and high shear rates.• E.g. PVOH: low viscosity at

low shear rates, high viscosity at high shear rates.


Analysis of Coating Color Formulations Comparison and Effect of ThickenersProperty No thickener CMC StarchDewatering rate 118 g/m2 49 g/m243 g/m2Storage modulus 1.55 Pa 101 Pa 36 PaVisc. of water phase 1 mPas 3.5 mPas 8.3 mPasBrookfield viscosity 100 mPas 1200 mPas 750 mPasHigh shear viscosity 12 mPas 40 mPas55 mPasHigh de-watering rate = poor water retentionHigh storage modulus = high elasticityStarch has relatively low formational structure -> low elasticity and low viscosity at low shear rate. Thickens water phase -> high viscosity at high shear rates.


Comparison of Shearing TimesViscometer Shearing

TimeCompared to

process

Brookfield 1 s Tens of thousand times

Rotational 1 s Tens of thousand times

ACAV Capillar 1 ms About 30 times longer

ACAV SLIT 30 µs Same as under blade


Extensional Viscosity Accelerating Flow and Extensional

Viscosity Extensional viscosity (EXTV) is a color’s

resistance to accelerating flow. Coating colors contain polymeric

thickeners, which have considerable effect on extensional viscosity.

Without velocity gradient, polymer chains are random coils. When flow speed increases (arrows are representing flow velocity), the accelerating flow field stretches and orientates these coils. This causes resistance to accelerating flow, i.e. extensional viscosity.


Extensional Viscosity Extensional viscosity measurement (EXTV) option.

EXTV


Extensional Viscosity

Extensional Viscosity vs. Shear Rate

Coating colors under development forcurtain coater

LWC coating colorWater


Extensional Viscosity

Influence of Thickener on EXTV

Source: IMERYS TECHNICAL GUIDE


Curtain Coating Shear rate


Curtain Coating

Nozzle is 0.3 mm, solids content 10 g/min, target coating thickness 10 µm and if coater speed is 1,200 m/min => it is necessary to feed coating color 0.2 l/s every meter of the web.

The flow rate of the coating color is about 0.7 m/s, so the shear rate inside the nozzle is about 13,000 /s.

At the moment when the coating color hits the web the shear rates can be >1,000,000 /s.

If the ultra high shear viscosity is too high or there is a lack of extensional viscosity in the coating color, it could cause uncoated areas on the base paper.

Alleborn, N., Südderhauf, H., Raszillier, H.High-Speed curtain coating of paper PTS 20. Streicherei-Symposium 2001


Curtain Coating Curtain Coaters and ACAV EXTV

ACAV’s EXTV option was found to be capable of measuring extensional viscosity. Based on the measurement data, special thickeners, dosage of special thickeners and solids content of the coating colour all effect the extensional viscosity of the coating colour which was the expected result.

Surfactants or other normally used thickeners did not increase extensional viscosity which was expected, based on the runnability of the curtain coater. Pigment system had no measureable influence on extensional viscosity.

Lower coating colour viscosity enables lower flow rates in extensional viscosity measurement, so extensional viscosity can be measured at lower shear rates. Lower flow rates can also be obtained by using an orifice with a larger hole.Conclusions of PTS 2007 article of Curtain Coaters:

M.Ojanen / Kemira; T.Sinkko and L.Kunnas / UPM


Curtain Coating Curtain Coaters and ACAV EXTV (continues)

“Based on the measurement data and experience with a pilot curtain coater, the Euler number of the coating colour should be >5 at shear rate of 20,000 1/s. If the Euler number is < 5, there is a strong likelihood that problems caused by lack of extensional viscosity might occur.”

“To ensure a reliable measurement, ACA EXTV device should be cleaned and washed carefully after each measurement. Even tiny impurities can cause errors. This can be seen as instability in the graph, especially at low flow rates. Otherwise taking a measurement is quick and easy: the measurement accuracy is only dependant on the solids content of coating colour.”

Conclusions of PTS 2007 article of Curtain Coaters: M.Ojanen / Kemira; T.Sinkko and L.Kunnas / UPM


Film Coating

Premeteringrod

Paper web

Backing

roll

Transfer

roll

ApplicationNip

Shear rate about 1 000 000 1/s

Coatedpaper

Filmsplit

Transfer nip

Normally a film coating is dual-

sided, simultaneously

Shear Rate ~ 1 000 000 1/s


Film Coating Requirements

The shear rate of a coating color in the application nip is about same as the shear rate under the blade in a blade coating process: both the coating film thickness and speed gradients are about same, if both coaters run the same speed.

The relevant area to measure viscosity is around 1 million 1/s in both cases.

Rod pressure has operational limits, and therefore ultra high shear rate viscosity has to be suitable for achieving desired coat weight.

Too high viscosity or solids content, on the other hand, leads to misting and spitting of coating color at the nip outlet.


Film Coating

Rod misting phenomenon is comparable to blade bleeding in blade coating process.

Rod misting may be reduced by: Reducing high shear rate viscosity Reducing solids content Reducing amount of plate-like particles (clay)

Extensional viscosity may also be important, as there is a converging flow field at the entrance of the application nip.

Picture: Voith


Film Coating It is believed that high extensional viscosity is bad for

runnability: filaments elongate more before they break up, and therefore misting is more problematic.

According to experience, misting may be reduced by: Lowering high shear rate viscosity Reducing amount of blocky pigment particles (GCC) Increasing solids content Splitting of coating film seems to be more regular and

smooth, when the solids content of the coating color is high.

In addition, relatively high viscosity at high shear rate range has a positive effect on orange peel formation.


Air Content MeasurementCoating Method

Critical Air content [%]

Blade & Roll Application 6 - 8

Film 8 - 12

Jet & Blade 2

Curtain 0,2 - 0,5


Air Content Measurement Repeatability of ACM Option of ACAV

Two samples were taken from the same coating color. Air content of both samples was measured 10 times.


Spray Nozzle OptionSpray Nozzle

Same geometry and flow as in

process version

Quick Connector


Customer Opinions “Coating formulations frequently used for specialty paper

application are of exceptional character and often very demanding from the rheological point of view.”

“Comprehension of the flow characteristics at realistic shear rates is of paramount importance for formulating of new advanced specialty coatings.”

“The ACAV 2 viscometer was our natural choice as we found it by far the most suitable for our purposes. In addition we have also had a very good earlier experience from ACAV 2 at another Ahlstrom site.”

Erkki Laiti, PhD, Manager of R&D Services at Ahlstrom Research Corporate Center


Customer Opinions Experience of Slurry Measurements

“According to our experiences in screening problems caused by special slurries, we can see the difference between slurries, which flow easily through the screens compared to slurries, which block them. The difference can be seen from shear thickening behaviour at high shear rates by using the ACAV2.”

Matti Lindeman, Group Leader, UPM, R&D laboratory, Lappeenranta, Finland


Slow Speed – No Runnability Problems

In past the runnability wasn’t a problem.


High Speed – Need for Latest Technology

Nowadays we do need the latest technology to prevent and to solve

runnability problems.