Importance of Rheology and Water Retention in Paper and ...

Importance of Rheology and Water

Retention in Paper and Board Coatings

© 2020 ACA Systems Oy

2 © 2020 ACA Systems Oy

Content

• Introduction ACA Systems Oy

• Paper and board coating in general

• Viscosity

• Water retention

• ACA devices

ACA Systems Oy

We are a leading supplier of process and quality control sensors, analyzers and value

added services for the roll industry all over the world

We are well known in the paper & packaging, nonwovens, plastic films & engineered

materials, fabrics and related market segments

We have worked 30 years closely together with our customers and developed solutions

based on their concerns about roll quality.



Lab Analyzers

ACA AX-100 – High shear rate viscometer

ACA Flow – Coating / substrate flow tester

ACA Permi Lab – Air permeability tester

ACA RoQ – Roll quality analyzer

Simulate real process conditions

in laboratory

Identify good and bad roll fast and easy

Global references



Global ACA Services


Paper & Board coating - General

• Paper and board grades are

sometimes coated to improve

• Printability

• Visual properties

• Functionality of the sheet

• Coating formulas typically

contain three categories of

ingredients:

• Pigments

• Binders

• Additives


Paper & Board coating – Coating materials

• Typical coating color for graphic applications contains (in dry weight)

• 80…95 % Mineral pigments like Calcium carbonate or clay. The amount of pigment is always

represented as 100 pts. Pigments are used to improve printing and optical properties of the sheet.

• 5…20 % Binders like SB/SA-latex (synthetic) or Starch (natural). Binders are used to adhere the

pigment particles to each other and to the base sheet

• 1…5 % Additives like CMC, PVOH, HASE, ASE… Additives are used either for assisting the coating

process ie. optimizing the coating color rheology, enhancing the final quality or creating functionalities

Here is an example of a real coating color formulation (Top coat / fine paper) and its key properties:

Recipe Key properties Rheology

GCC 80 pts Solids content: 68 % Low shear visc. BF 1200 mPas

Clay 20 pts pH 9 High shear visc. ACA 55 mPas

SB Latex 11 pts Temp 25 c Water retention 70 g/m2

CMC 1.0 pts Coat weitght 12 gsm

Coater speed 1600 m / min


Paper & Board coating – Coating process

• The coating process can be roughly divided into different phases where the coating turns from liquid to dry

• Application of coating color onto the base paper.(Liquid)

• Metering of the coating to the final coat weight (Liquid)

• Drying of the coating (Liquid / Dry)

• Finishing of the dried coating layer (Dry)

Film coating (Valmet) Blade coating (Valmet) Curtain coating

Coating consolidation

Liquid Dry


Paper & Board coating – Coater runnability

• The word “runnability” is often used for describing how well coating process works.

• Coating consolidation is a process during which the liquid coating color on the base paper transforms into an

immobilized (dry) coating layer

• The runnability of a coating color is obviously determined by the properties of the liquid color, whereas the

the final quality of the coated paper is dependent on the dry coating structure

• Transformation from wet to dry is a combined process of penetration into the base paper and evaporation

during the drying process

• To simulate these conditions and manage the rheology of the coating color it is necessary to measure the

rheology of the coating color

Liquid DryCoating consolidation


Paper & Board coating – Rheology measurements

• Key rheology measurements to control coating consolidation

• Low shear viscosity / Brookfield viscosity. Describes the chemical interaction and structural viscosity under low shear

rate. It is relevant for compatibility of coating components, pumping and mixing of the coating color.

• High shear viscosity / ACA Viscosity / TAPPI method XYZ. Describes the hydrodynamic effects of the coating color

(non-structural viscosity). ACA method @ 500 000 1/s shear rate. The viscosity curve shape and mPas level are very good

indicators of coater runnability.

• Water retention / TAPPI method T-701. Measures water holding properties of the coating color. Water retention is a very

important property in terms of coater runnability.

• Managing rheology = huge cost saving potential via optimized runnability

High shear capillary viscometer –

ACA AX-100

Water retention analyzer –

ACA Flow WR


Paper & Board coating – Viscosity


Paper & Board coating – Viscosity

• Quality control for standard recipe using ACA AX-100 viscometer at 500 000 1/s shear rate

• Out of control area hydrodynamic viscosity too high and runnability poor

• It is important to control the quality continuously. Even standard recipes fluctuates


Paper & Board coating – Water retention

• When coating color comes into contact with base

paper, certain amount of water phase of coating color

penetrates into the paper web.

• The loss of water starts immediately when coating color

comes into contact with the surface paper loss of

water is mainly due to the capillary actions (porous

structure) of paper surface

• Also external pressure pulses such as pressure in the

nip or under the blade promotes dewatering

• The ability of coating color to hold water is called water

retention.

• Water retention can be controlled to some extend via

thickeners used in the coating color

• Water retention can be measured according to TAPPI

T-701.

• ACA Flow WR is a modern water retention analyzer

that measures according to standard T-701


Paper & Board coating – Water retention

• During the consolidation process, the solids content of the wet coating layer increases, which modifies the

rheology of the coating before it comes into the contact with the blade. In the worst case, the coating can

form a thick, immobile “filter cake”, which can cause streaking and may even cause web break.

• The water taken up by the paper causes a reduction in its internal bonding strength Base paper becomes

weak and may cause web breaks

• The solids content of the coating color recirculated from the coater head can increase significantly during a

short period of time if water retention is poor.

• It is very difficult to apply an even coating layer if filter cake is too thick or solids content too high.

• Note! High shear viscosity curve shape together with water retention measurement gives a good

estimation of coating color behavior during consolidation process

IT IS VERY IMPORTANT TO MEASURE AND CONTROL WATER

RETENTION OF THE COATING COLOR!!


• Poor profiles, uneven coat weight

• Scratching, streaking

• Blade bleeding (wet, dry), film splitting, misting, uneven curtain…

• Optical variations, print mottle etc…

Cost impact

• Coating problems may costs ~ 10 M€ a (300 000 t/a coated fine paper)

• 10 % reduction = 1 M€ / a or 3…5 €/ton

• 25 % reduction = 2.5 M€ / a or 7…15 €/ton

Paper & Board coating – Economics, poor rheology


• Solids content increase

• Binder reduction

• Cheaper formulations

• Better surface and print quality

• Etc…

Cost impact

• Solids content increase from 66 % to 70 % ~600 k€

• 1 pts reduction in latex binder ~250 k€

• Pigment system changes ~200…500 k€

Paper & Board coating – Economics good rheology


Paper & Board coating – Coating development

• Water based barrier coatings to replace plastics

• Coated packaging grades for superior impression

• Coated liner grades

• Digital printed packaging grades

• Functional coatings

• Special applications

• High value low volume market

• And others…

Coating development is nowadays more versatile than ever. Functional recipes require improved

monitoring and new instrumentation. ACA Systems Oy is no 1 partner for coated board and paper

manufacturers as well as coating raw material suppliers and institutes when it comes to connecting the

rheology and runnability.


ACA FLOW WR

Water retention analyzer


ACA Flow WR

ACA FLOW WR®

• A semi-automated water retention tester for wide

operating range.

• Basic operation according to standard T 701 pm-01

Gravimetric method for measuring dewatering of

coating colors.

• The unit measures water retention properties of coating

colors under static conditions.

• Machine is self-calibrating capable – there is no need to

manually change calibration.

• Even calibration check is done automatically and without

interrupting normal usage.

• Start up is “Plug and Play”


ACA Flow WR - Connectors

Connectors

1. 24 VDC connector for voltage input.

2. Pneumatic inlet

Installation environment

1. Voltage to included transformer 200 -230 V, European

style connector.

2. Compressed air minimum 4-bar, dry instrument air. 6 mm

pipe connection at the backside of machine.

3. Temperature about 20C (Normal room temperature)


ACA Flow WR - Features

Main screen• Time left, shows how many seconds there are

left for measurement.

• Stop, Start, measurement start/stop.

• READY, information field which shows

measurement phase and possible errors.

• Settings opens the settings screen.

Setting screen• Measuring time (s), for adjusting the measurement

time (normally 90 seconds)

• Filling pause (s), for setting the pre-fill time. (normally

15 seconds)

• Measuring pressure (p), for setting the pressure.

(normally 500 pascals)

• Back, save values and return to main screen


ACA Flow WR – Measurement procedure

Measurement procedure

1. Weigh the base or blotter paper on analytical scale and insert it to the

base plate.

2. Add the filter paper glossy side upwards on the top of the paper.

3. Insert sample holder upper body on the base plate.

4. Start the measurement from touch screen button Start

5. When test is over the cylinder will open automatically

6. Remove the sample assembly, weigh the base paper and calculate the

WR value using Excel app.

7. Wash the sample assembly and store in dry place.

Sample assembly


ACA Flow WR – Excel app.


ACA Flow WR – Delivery

Delivery package

ACA Flow WR main unit

Sample cylinder

Testing mat with magnetics

AC Power supply

Filter papers (200 pieces)

Blotter papers (200 pieces)

Traveler case


ACA AX-100 viscometer

ACA AX-100 – New capillary viscometer

• Capillary viscometer designed for fast evaluation of coating rheology at high shear rate range

• Very good correlation to runnability of a coater

• Can predict rheology based coating defects

• TAPPI standard method XYZ (in process)

• Very user-friendly equipment

Stand-alone

Small sample size

Fast and reliable measurement

Flexible data handling

Robust design for production conditions (60 kg, 120 cm x 30 cm x 45 cm)

ACA AX-100 Measurement principle

• Principle is to measure flow rate

through the capillary, and pressure

difference required for maintaining

the flow.

• Dimensions of capillary (L,D) are

known: shear rate, shear stress and

viscosity may be calculated.

Flow

Pressure

differenceL

D

ACA AX-100 – Application range

Choosing the right diameter capillary will maximize the application range

ACA AX-100 – Scope of delivery

• AX unit

• 24VDC CE power source

• installed software

• measurement cylinder assembly

• capillaries with holder and seals

AX is meant to measure only non explosive water based liquids. It is using high pressure as 100bar maximum.

The pressure is reached with pneumatically actuated cylinder.

This cylinder is pushing the piston inside the measurement cylinder and it is causing the flow through the capillary.

Pressure and the flow values are taken up and AX software is giving viscosity, shear rate etc. results on Touch Screen.

The results can be extracted via Ethernet connection or can be saved into memory stick.

All movements are pneumatic.

ACA AX-100 – Technical data

• Accuracy of position potentiometer +/- 0,05 mm

• max. peak pressure at 0,7 Mpa 100bar

• Accuracy of pressure transducer +/- 0,3%

• Air consumption 7 bar minimum and 100 l/min

• Electrical connection for the unit supply 230 VAC/50Hz / 6A

• Weight, the main unit <60 kg

• Dimensions, the main unit (depth×width×height) 458 mm × 304 mm × 1176 mm

• Mounting on the table with load capacity >200 kg

• Recommended table size (depth×width×height) 600 mm × 800 mm × 750-800 mm

• Room temperature nominal +20 Celsius (+17…+25 Celsius)

• Humidity nominal 40% RH

• Pneumatic inlet fitting as std. type CEJN 320-5152

External booster as an option


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Importance of Rheology and Water Retention in Paper and ...

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