ACAV Ultra High Shear Presentation
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Transcript of ACAV Ultra High Shear Presentation
ACAV Viscometers by1
ACAV
Coating ColorRunnabilityAnalyzers
Picture: Voith
ACAV Viscometers by
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
2
ACAV Viscometers by
Viscosity Measurement
Viscosity Ranges of Viscometers
3
ACAV Viscometers by
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!
4
Coat weight g/m2 10 10 8
Color solids % 62 62 62
Machine speed m/min 1000 1500 2000
Color density kg/m3 1500 1500 1500
Coating 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 coater
Film thickness
ACAV Viscometers by
Viscosity Measurement
Shearing History of Coating Color
5
Applicator NipPumping
Mixing
Blade
Screens
Rod
0 102 103 104 106105 107
Curtain hitting point
ACAV Viscometers
Coating Color Circulation
Conventional
viscometers measure
ONLY low shear
rates!
Share Rate (1/s)
= Conventional viscometers
ACAV Viscometers by
Viscosity Measurement
Why Full Shear Rate Range Measurement?
Measure the full range of shear rates to check the Impact of different factors on the viscosity
6
Chemical factors
- Van der Waals
- Electrostatic Repulsion/Attraction
- Steric Factors
Hydrodynamic factors
- Particle size
- Distribution of Particle size
- Shape of Particles
- Viscosity of Water Phase
- Solid Fraction
PCC-Slurry
0.0 2.0x105 4.0x105 6.0x105 8.0x1050
500
1000
1500
2000
Vis
cosity
(mP
as)
Shear rate (1/s)
= Conventional viscometers
ACAV Viscometers by
Viscosity Measurement
Dominating Factors at Different Shear Rates
7
SURFACE 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,
rotational
and capillary
viscometers
Capillary and
SLIT viscometers
ACAV Viscometers by
Viscosity Measurement
Shear Rate Ranges of Viscometers
8
ACAV Viscometers by
Viscosity Measurement
Additional Notes on Shear Rate Because viscosity is often strongly dependent on shear rate, it is
necessary to know what is the actual shear rate in the process. If shear rate is not known, it doesn’t make sense to compare viscosity of samples
to their performance during the process.
Usually it doesn’t matter is shear rate 50 000 1/s or 60 000 1/s.
it matters is shear rate 10 000 or 100 000 1/s.
Often it is enough to know order of magnitude of shear rate.
9
ACAV Viscometers by
Viscosity Measurement
Viscometers & Shear Rate
10
Rheological 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
ACAV Viscometers by
Coating optimization
Effect of Particle Size Distribution on Ultra High Shear Viscosity
11
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
ACAV Viscometers by
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.
12
= Measurement range of Conventional viscometers
Coating cost saving: 153 000 EUR/year!
ACAV Viscometers by
Coating optimization
More fibers in coating circulation caused streaking problems
13
= Measurement range of Conventional viscometers
Conventional
viscometers
measure ONLY
low share rates!
ACAV Viscometers by
Coating optimization
Runnability and Particle Size Distribution
14
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
ACAV Viscometers by
Coating optimization
Temperature Effects
15
60ºC
50ºC
30ºCCoating shear rates
Logical behaviour
below 100 000 1/s
= Measurement range of Conventional viscometers
Increase of coating color
temperature destroys the
surface active ingredient
of the latex.
ACAV Viscometers by
Coating optimization
New coating recipe provided higher solid content at lower ultra high shear viscosity level
16
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 by
ACAV Viscometers
ACAV Viscometers are the most powerful tools for runnabilityanalysis of coating colors and pigment slurries.
17
A2 A4
ACAV Viscometers by
ACAV Viscometers
Blade versus Slit Geometry
18
BLADE
0.5 mm
SLIT
Coating Color~ 0.5 mm
~ 2
0 µ
m
Base paper8
0 µ
m
Similar Velocity Profiles
with Blade and Slit
ACAV Viscometers by
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
19
ACAV Viscometers by
ACAV Viscometers
Slit Assembly
20
ACAV Viscometers by
ACAV Viscometers
SLIT Viscosity versus Blade Load
21
Pilot coater speed 1200 mpm
Blade pressure needed to
achieve 12 g/m2 coat weight
Aalto University
ACAV Viscometers by
ACAV Viscometers
ACAV SLIT Viscosity vs Scratch Counts
22
Amount of dispersant added
is 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 -86
Dr. Rajan R. Iyer, Ray Hollingsworth, Dr. David R. Skuse, Imerys, Sandersville,USA
ACAV Viscometers by
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.
23
ACAV Viscometers by
ACAV Viscometers
Stability Test with SLIT/Agglomerates
24
Coating colors with strong
structures are more likely to form
agglomerates. Therefore dry
bleeding and scratching are more
probable.
Formation of agglomerates, Extrusion Pressure is Constant
ACAV Viscometers by
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.
25
Results: TAPPI Coating Conf. 2002, Dr. Koji Okomori, Masato Yamaguchi,
Masahito Suzuki and Hirokazu Morii, R&D Division, Nippon Paper, Tokyo
ACAV Viscometers by
ACAV Viscometers
Guidelines for Interpreting Graphs
26
Viscosity
(mPas)
100
1000
Optimum
Rheology
Behaviour High viscosity at ultra high shear rates:
1. Blade bleeding and streaking
2. Problems in maintaining target coat weight
3. Web breaks, scratches, agglomerates
Low viscosity at ultra high shear
rates: uneven coat weight
High viscosity at low shear rates:
pumping and start-up problems
High viscosity at high shear rates:
problems in screens, rod, jet and
applicators
110
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 by
ACAV Viscometers
Operational Limits of Blade Load
27
Desired coat weight (c.w.) =
Blade load
Co
at w
eig
ht Operational area
Suitable viscosity:
even coating and no breaks.
Too low viscosity: c.w. is
not achieved due to uneven
coat weight.
Too high viscosity: c.w. is
not achieved due to web
breaks.
Web
breaks
Uneven
coat weight
Color viscosity must
be suitable for each
coat weight and
base paper
ACAV Viscometers by
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).”
28
By ECCI, TAPPI Coating ´98
By Imerys, TAPPI Coating´01
ACAV Viscometers by
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 %
29
ACAV Viscometers by
Pigment Slurry Analysis
Kaolin with Wide Particle Size Distribution
30
ACAV Viscometers by
Pigment Slurry Analysis
Kaolin with Narrow Particle Size Distribution
31
ACAV Viscometers by
Pigment Slurry Analysis
Dispersion Tests of Slurries of New Engineered Kaolin Pigment
32
High viscosity and shear thickening
region cause screening problems.
Ultra high shear viscosity showed
good correlation to screening
The new kaolin pigment has a special
particle size distribution (the smallest
particles had been cut off)
0,22 %0,24 %
0,28 %
0,26 %
= Measurement range of Conventional viscometers
ACAV Viscometers by
Pigment Slurry Analysis
Dispersion Tests of Slurries of Engineered Kaolin Pigment
33
ACAV Viscometers by
Pigment Slurry Analysis
Dosage of Dispersing Agent was deficient
34
ACAV Viscometers by
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
35
ACAV Viscometers by
Analysis of Coating Color Formulations
Viscosity Curves of “Typical” Coating Colors and Coating Colors
with Narrow Particle Size Distribution.
36
Wide particle size
distribution
& traditional latexNarrow particle
size distribution
& traditional latex
Reproduced from R. Knappich et al.
ACAV Viscometers by
Analysis of Coating Color Formulations
Viscosity Curves of Coating Color with Narrow Particle Size Distribution and Latex A/B
37
Narrow particle
size distribution
Ultra high shear
viscosity correlates
directly on blade
loads.
Reproduced from R. Knappich et al.
ACAV Viscometers by
Analysis of Coating Color Formulations
Effect of Medium Co-binder Addition
38
KAOLIN BASED COATING
PVOH
Starch
Latex A
Latex B
CMC
ACAV Viscometers by
Analysis of Coating Color Formulations
Effect of Particle Size of Latex
39
ACAV Viscometers by
Analysis of Coating Color Formulations
Thickeners – Example of Shear Rate Effect
40
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.
ACAV Viscometers by
Analysis of Coating Color Formulations
Comparison and Effect of Thickeners
41
Property No thickener CMC StarchDewatering rate 118 g/m2 49 g/m2 43 g/m2
Storage modulus 1.55 Pa 101 Pa 36 Pa
Visc. of water phase 1 mPas 3.5 mPas 8.3 mPas
Brookfield viscosity 100 mPas 1200 mPas 750 mPas
High shear viscosity 12 mPas 40 mPas 55 mPas
High de-watering rate = poor water retention
High storage modulus = high elasticity
Starch has relatively low formational structure -> low
elasticity and low viscosity at low shear rate. Thickens
water phase -> high viscosity at high shear rates.
ACAV Viscometers by
Comparison of shear rates
42
ViscometerShear Rate
1/sComparison to process
Brookfield 10 Shear rate 200,000 times lower
Rotational 40,000 Shear rate 50 times lower
Capillary
ACAV A4500,000
Shear rate comparable to most real
processes
Capillary
ACAV A22,000,000
Shear rate same as in high speed
coating process
ACAV SLIT 10,000,000Possibility to simulate even highest
shear rates
ACAV Viscometers by
Comparison of Shearing Times
43
Viscometer Shearing Time Compared 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
ACAV Viscometers by
Service of ACAV A2
44
Hydraulic oil
& oil filter,
every 2nd
year
Piston seal,
300-500
analyses
Door safety switches,
once a year
ACAV Viscometers by
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.
45
ACAV Viscometers by
Extensional Viscosity
Advantages of EXTV Option Of ACAV A2
Excellent repeatability
Easy to use and calibrate
Method based on the well-known Euler number
The Euler number can be measured at different shear rates to examine
possible changes in extensional viscosity versus shear rate.
The higher the Euler number, the higher the extensional viscosity of the
sample.
46
ACAV Viscometers by
Extensional Viscosity
Extensional viscosity measurement (EXTV) option.
47
EXTV
ACAV Viscometers by
Extensional Viscosity
Extensional Viscosity vs. Shear Rate
48
Coating colors under
development for
curtain coater
LWC coating color
Water
ACAV Viscometers by
Extensional Viscosity
Influence of Thickener on EXTV
49
Source: IMERYS TECHNICAL GUIDE
ACAV Viscometers by
Curtain Coating
Shear rate
50
ACAV Viscometers by
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.
51
Alleborn, N., Südderhauf, H., Raszillier, H.
High-Speed curtain coating of paper
PTS 20. Streicherei-Symposium 2001
ACAV Viscometers by
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.
52
Conclusions of PTS 2007 article of Curtain Coaters:
M.Ojanen / Kemira; T.Sinkko and L.Kunnas / UPM
ACAV Viscometers by
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.”
53
Conclusions of PTS 2007 article of Curtain Coaters:
M.Ojanen / Kemira; T.Sinkko and L.Kunnas / UPM
ACAV Viscometers by
Film Coating
54
Premetering
rod
Paper
web
Backing
roll
Transfer
roll
Application
Nip
Shear rate
about 1 000 000 1/s
Coated
paper
Film
split
Transfer
nip
Normally a film coating
is dual-sided,
simultaneously
Shear Rate
~ 1 000 000 1/s
ACAV Viscometers by
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.
55
ACAV Viscometers by
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.
56
Picture: Voith
ACAV Viscometers by
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.
57
ACAV Viscometers by
Air Content Measurement
Air Content Measurement Option of ACAV
58
ACAV Viscometers by
Air Content Measurement
59
Coating MethodCritical Air content
[%]
Blade & Roll
Application6 - 8
Film 8 - 12
Jet & Blade 2
Curtain 0,2 - 0,5
ACAV Viscometers by
Air Content Measurement
Repeatability of ACM Option of ACAV
60
Two samples were taken
from the same coating
color. Air content of both
samples was measured
10 times.
ACAV Viscometers by
Spray Nozzle Option
‘Viscosity’ and flow resistance of spray nozzle
ACAV equipped with Spray Nozzle Option can be used as coating head for laboratory coaters (Helicoater, CLC, …)
61
ACAV Viscometers by
Spray Nozzle Option
62
Spray NozzleSame geometry and
flow as in process
version
Quick Connector
ACAV Viscometers by
Comparison of ACAV A2 and A4
ACAV Viscometers
Shear rates 100 - 10 000 000 1/s
Viscosity range - 30 000 mPas
Over 120 units sold since 1997
Best correlation to blade loads
Best correlation to runnability
problems
Capillary, SLIT, Spray, DWR, Air
Content and EXTV technologies
63
ACAV Viscometers by
Comparison of ACAV A2 and A4
Unique Measurement Method
ACAV is the only unit which can forecast the probable occurrence of the
following runnability problems:
too high blade loads
formation of agglomerates
number of scratches
blade bleeding
blade streaking
problems in maintaining target coat weight
blocks in screens
other problems in coaters and applicators
64
ACAV Viscometers by
Comparison of ACAV A2 and A4
ACAV A4
Shear Rates up to 4,000,000 1/s
Capillary, SLIT and Spray Nozzle options
Dynamic Water Retention (DWR)
Best correlation to blade load
65
ACAV Viscometers by
Comparison of ACAV A2 and A4
66
ACAV Viscometers by
Comparison of ACAV A2 and A4
67
ACAV Viscometers by
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.”
68
Erkki Laiti, PhD, Manager of R&D Services at Ahlstrom Research Corporate Center
ACAV Viscometers by
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.”
69
Matti Lindeman, Group Leader, UPM, R&D laboratory, Lappeenranta, Finland
ACAV Viscometers by
Slow Speed – No Runnability Problems
70In past the runnability wasn’t a problem.
ACAV Viscometers by
High Speed – Need for Latest Technology
71
Nowadays we do need the latest technology to
prevent and to solve runnability problems.