Basic HTML Version
Enthalpy of Melting&Mixing:
Tadmor.et.al (1979) in their book on principles of polymer processing states that melting of
compressed polymer particulate solids such as powders, pellets, beads, flakes, and
granulates is the most important elementary step in polymer processing. The minimum
total heat input to reach the temperature known as the processing temperature that
facilitates the insertion of constituents in the matrix (or seen differently - impregnation of
immiscible material by the wetting medium) is the enthalpy of melting. Additional heat (or
mechanical energy) input is required for mixing. Therefore, enthalpy of mixing includes the
enthalpy of melting. Heat energy required for melting is polymer dependent giving an
indication of bonds that are being loosened to introduce the newer ingredients. Specific
Energy is commonly used term in this context. It is defined as the total input energy per unit
mass of material to accomplish the mixing task. Importantly, in modern co-rotating twin-
screw extrusion, the heat exchange between the vessel and the material is so small and can
be ignored. The specific mechanical energy input based on the motive power provides
enough information on the process.
Kinematics of Melting&Mixing:
Molecular agitation results in increase in temperature. Mixing needs such an agitated state.
Shear facilitates the transfer of gross movement into molecular agitation. Shear is defined
as the time duration the material experiences a given magnitude of shear rate along a
particular direction. Certain requirements such as size reduction are achieved at certain
shear rates. The geometry and configuration of elements affects the transfer of material
and shear experienced by the material during the extrusion process. Melting and mixing is
affected by the intensity as well as uniformity of shear. Uniformity refers to a narrow
distribution in the volume fraction of material experiencing the same magnitude of shear.
Capability enhancement inMelting&Mixing:
Shear rate is directly proportional to speed of rotation of the extruder. Change in diameter
for a constant flight depth / diameter (h/d) ratio does not affect the shear rate. The
clearance between the element's outer-diameter and the barrel’s internal diameter
influences the peak shear rate and a small volume fraction experiences such high rates
leading to most common processing problems. Figure 2 shows the effect of geometry and
the potential effect of clearances on the shear rate.
-4-
Figure 2: Radial shear rate experienced by volume fraction in a fully filled zone
E N H A N C I N G
Capabilities
in a Twin-Screw Extruder
MELTING
AND
MIXING
Melting and mixing is affected by the
intensity as well as uniformity of shear.
A good understanding of the forces
experienced by the material inside the
extruder helps in precise control over
the process to achieve the exact results
that are required from the process.