APPLICATION INFORMATION
ADDITIVES FOR CURTAIN COATING

K-A 2

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Additives for curtain coating

Contents
03 Introduction
04 BYK additives for curtain coating
05 Wetting and dispersing
06 Defoamers
07 Dynamic surface tension
09 Rheology modifiers

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Introduction
Curtain coating is an increasingly common pre-metered coating technique
where the liquid coating forms a curtain which falls onto the moving
substrate. Curtain coating allows for:• Fast coating speeds• Thin coating layers• Improved uniformity• Contour matching• Multi-layer coatings
There are however challenges associated with curtain coating including:• The entrapment of foam within the coating color(resulting in pinholes/craters/fish eyes)• Breaking of the coating upon contact with the substrate due tohigh shear forces• Splitting of the curtain as a result of high surface tensions (egging)

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Additives for curtain coating

BYK additives for curtain coating
BYK offers a range of additives to improve the curtain
coating process (G.01) and ensure excellent coating quality
with minimal impact on final coating properties.

Key benefits ofBYK additives• High cost efficiency• Easy to incorporate at any
stage of manufacturing• Long-lasting effects• Compatible with a varietyof formulations

Schematic of a curtain coater

Wetting & dispersing
Curtain forming zone

Color supply
Defoamer
Surfactant

Curtainstabilization
zone

Rheology modifierElongation zone

G. 01

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Additives for curtain coating

Wetting and dispersing
Effective dispersion within the coating color is essential forcurtain coating to provide a uniform coating throughout theproduction run.
The viscosity of the system can be used to evaluate theefficiency of the wetting. The lower the viscosity the better.
The efficiency is system-dependent as illustrated in G. 02.
In addition to ensuring complete dispersion, anti-settlingproperties may also be required, particularly for heavy
particles such as fluorescent pigments. For aqueous systems,
OPTIGEL-WX is highly recommended in low levels.

Wetting and dispersing additive recommendation forcoating pigments
Pigment

Recommended additive

CaCO3

DISPERBYK-190*

KaolinDIPSERBYK-199*

TiO2

BYK-154
T. 01* Biocide-free version available (BF)

Viscosity/mPa s-1BROOKFIELD-RV viscosity measurements for various pigment slurries used by paper coaters
2000

1600

1200

800

400

066 % solids kaolinControl  

BYK-154  DISPERBYK-190  

75 % solids CaCO3DISPERBYK-199

38 % solids acid clay

60 % solids anatase TiO2G. 02

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Additives for curtain coatingDefoamers
The presence of foam is common within the curtain coatingprocess. This can result in surface defects upon coating suchas cratering. For paper curtain coating, BYK’s primary
recommendations are BYK-016 and BYK-025. BYK-016
and BYK-025 significantly reduce the amount foam in a
coating color (G. 03).

Coating color with no additive (left) versus
coating color with 0.2 % BYK-016 (right)

The effectiveness and compatibility of defoamers are
system-dependent and therefore BYK offers a wide range ofdefoamers to cover all eventualities and to provide optimal
defoaming performance.

For microfoam (< 100 µm),a combination of BYK-019
and BYK-024 in a 3:2 ratio
is recommended.

BYK-016• VOC-free• Approved for foodcontact

BYK-025• High-performance• Silicone-containing
defoamerG. 03

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Additives for curtain coating

Dynamic surface tension
BYK-3410 is BYK’s number one recommendation for
reducing the dynamic and static surface tension of thecoating color and preventing curtain splitting (G. 05).
Reducing the dynamic surface tension of the coating
ensures that new surfaces are rapidly stabilized (G. 04),
allowing coating machines to run at faster speeds.

Possible curtain flow diagrams
No curtain = rheology andsurface tension issues“Egging” = surface tension
issuesGood curtain = suitableproperties

G. 04

BYK-3410• Silicone-free• Approved for food contact• Reduces “egging”• Improves substrate wetting• Has minimal impact on
final coating properties

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Additives for curtain coating / Dynamic surface tension

BYK-DYNWET 800 is a high-performing alternative toBYK-3410 which can be used when BYK-3410 is notcompatible in the system (G. 06). BYK-DYNWET 800
has very low foam stabilization and is known to work ina range of high-speed processes.

Another alternative is the silicone-containing BYK-3400
which produces a similar level of surface tension reduction
as BYK-3410 whilst having a low impact on the final
surface slip.

Impact of BYK-DYNWET 800 on a curtain

Surface tension of coating color with the incorporation of BYK-3410

Surface tension (mN m-1)

DST in clay slurry“Egging”
Without additive

80

7060

5040

Full curtain
With 0.5 % BYK-DYNWET 800

3020

10010100

1000
Surface age (ms)Clay slurry w/o additive+ 0.25 % BYK-3410+ 0.50 % BYK-3410+ 0.75 % BYK-3410+ 1.00 % BYK-3410

G. 05

G. 06

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Additives for curtain coating

Rheology modifiers
In the curtain coating process, the coating color is exposed
to high shear forces, especially at the impingement zone
(G. 07).

Filmformingzone

Two things can occur in this zone which can result in poor
coatings:
1. Breaking of the coating upon contact with the paper
2. Turbulent flow resulting in heel formation and defects

Curtain

Schematic ofa falling curtain

HeelImpingement zone

G. 07

10Additives for curtain coating / Rheology modifiers

These are ideal for curtain coating processes as they:• Increase the viscosity in the impingement zone
• Help reduce turbulent flow
• Increase elongational stability• Have minimal impact on pumpability and flow in thelower shear parts of the process

To overcome these issues, a high-shear thickener isrequired which provides elongational stability (G. 08).
RHEOBYK-T 1000 VF, RHEOBYK-T 1010 VF and­RHEOBYK-L 1400 VF are VOC-free associative
thickeners which provide elongational stability (increase
the “stretchiness” of the fluid). They also increase
viscosity at high shear, while having minimal effect onviscosity at low shear (G. 09), which is important for
reducing turbulent flow.

Rheology profiles of 2.0 % addition of RHEOBYK toAQUACER 1061

0.04

η (Pa·s)

Diameter (mm)Elongation of AQUACER 1061 with 2.0 %RHEOBYK addition

100.00

0.03

10.00

0.02

1.00

0.01

0.100.00

0.01-20246810120.01

0.10

1.00

10.00

100.00AQUACER 1061

AQUACER 1061 + 2.0 % RHEOBYK-T 1000 VF

AQUACER 1061 + 2.0 % RHEOBYK-T 1010 VF

AQUACER 1061 + 2.0 % RHEOBYK-L 1400 VF

G. 08

1,000.00

10,000.00
ϔ (s-1)Time (s)AQUACER 1061

2.0 % RHEOBYK-T 1000 VF

2.0 % RHEOBYK-L 1400 VF

2.0 % RHEOBYK-T 1010 VF

G. 09

07/2023
K-A 2

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info@byk.com
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The information herein is based on our present knowledge and experience. The information merely describes the properties of our products but noguarantee of properties in the legal sense shall be implied. We recommend testing our products as to their suitability for your envisaged purpose prior touse. No warranties of any kind, either express or implied, including warranties of merchantability or fitness for a particular purpose, are made regarding
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This issue replaces all previous versions.