Print this article | Send to Colleague

The working group chair below is currently working on a draft to propose a new Standard:

WGC: Martti Tuomisto

Please find below the current scope of this new proposal:

New standard for measuring coating color viscosity at high shear rate.
Method: Capillary viscometer, shear rate 750 000 1/s

Coating colors are non-Newtonian materials, and therefore their rheology is complicated. The most important rheological factor is viscosity, and in case of coating colors it is strongly dependent on a shear rate. Coating color undergoes variable shear rates during the coating
process, varying from zero to several million 1/s. Therefore, measurements are needed in the whole shear rate area for the realistic simulation of coating processes. Rotational viscometers are suitable tools for screening the rheology of coating colors below the shear rate of 50 000 1/s, but
capillary viscometers, such as ACA AX, is needed for shear rates of 50 000 to 1.5 million 1/s.

There are two major groups of factors, which determine the rheology of coating colors: surface chemistry and hydrodynamics. Surface chemistry determines the rheology of coating colors in the low shear rate area (rotational viscometers). There are both attractive and repulsive forces
between the particles. If attractive forces are too strong, particles tend to flocculate. Viscosity increases and suspension becomes unstable. When the shear rate is high enough, structures break down, and viscosity depends mostly on hydrodynamic factors.

The most important hydrodynamic factors are:
- Solids content (i.e. volume fraction of particles)
- Particles size
- Particle size distribution
- Viscosity of continuous (water) phase
- Shape of the particles

ACA AX-100 is a capillary viscometer, which is specifically designed for measuring the viscosity of coating colors under hydrodynamic conditions, i.e. in the high shear rate area. Extrusion pressure is measured with a pressure transducer, and the displacement of the sample
cylinder is measured with a magnetic position sensor. Flow rate through the capillary is calculated based on the displacement of the sample cylinder and measurement time.

Well known, and generally accepted, equations of pipe flow are used to determine a viscosity
value as a function of shear rate. It is also described in the literature and generally accepted that
the pure hydrodynamic viscosity (ie. surface chemistry is not influencing anymore) can be measured at shear rates above 500 000 1/s. However, so far there has not been a standard method to describe high shear viscosity at such high shear rates. We are setting now a new standard at
the shear rate of 750 000 1/s. The standard is applicable to coating colors, surface treatments, pigment slurries, latex slurries etc…

Roll, blade, curtain and jet coating are presently the most used technologies for paper coating at high speeds. Coating colors are subjected to high stresses in all coaters, and shear forces are clearly above 500 000 1/s. Therefore, the new standard method can be used for simulations of
different coaters to help papermakers optimize blade pressures, coat weights and raw materials in order to avoid or solve runnability problems caused by coating colors.

If you would like to participate in the Working Group that is working on this draft or if you wish to join the correspondent SSIG to review the proposal once a ballot is opened, please write to standards@tappi.org

TAPPI Standards guidelines require that all TAPPI members receive notification of any proposed new Standard or TIPs. This message is being sent to you because you are a TAPPI member; have purchased TIPs or Standards products from TAPPI; or because you have previously expressed interest in this topic.

TAPPI is the leading technical association for the worldwide pulp, paper and converting industry.
15 Technology Parkway South | Norcross, GA 30092 | www.tappi.org
1-800-332-8686 (US) | 1-800-446-9431 (Canada) | +1-770-446-1400 (Worldwide)