Rheological Additives for Crop Protection - Elementis Specialties

Crop Protection Agents and Rheology
4
Fig 1
The rheological properties required of
crop protection formulations vary
according to the intended use of the
product.
Rheology is the study of flow. Flow
behaviour is measured in terms of its
viscosity. Viscosity is the resistance to
flow and defined as:
Viscosity =
Shear Stress
Shear Rate
Shear stress is the force per unit area
applied and shear rate is the resulting
velocity gradient. Crop protection agents
experience shear rates varying from
0.001s -1 , from gravitational forces, to
over 20,000s -1 , when sprayed. The way
the viscosity changes at varying shear
rates greatly influences the overall
product stability and performance. A
material which maintains a constant
viscosity, regardless of shear rate, has
Newtonian flow (Fig 1). Dilatant flow
(Fig 1) is a form where viscosity
increases with shear, sometimes
encountered in highly pigmented/filled
systems. Pseudoplastic flow (Fig 1),
also known as shear-thinning behaviour,
and typical of many commercial
systems, is when viscosity decreases
with increasing shear rate.
Flow may require a specific stress to be
initiated. This particular stress value is
known as the yield point. A system
having both shear-thinning and a yield
point can be described as having plastic
flow.