Chemistry Reference
In-Depth Information
by gradual decay over time, resulting in warpage of the part.
Figure 4.33b
sketches the dependence of the deformation once the steady shearing has stopped.
An inelastic fluid maintains the final strain level, while a viscoelastic substance
will undergo some elastic recoil.
Whether a polymer exhibits elastic as well as viscous behavior depends in part
on the time scale of the imposition of a load or deformation compared to the char-
acteristic response time of the material. This concept is expressed in the dimen-
sionless Deborah number:
response time of material
time scale of process
(This parameter was named after the prophetess Deborah to whom Psalm 114 has
been ascribed. This song states correctly that even mountains flow during the infi-
nite observation time of the Lord, viz., “
The mountains skipped like rams, The
hills like lambs
.”) The process is primarily elastic if
N
Deb
.
N
Deb
5
1 and essentially vis-
cous if
N
Deb
,
1. The response time of the polymer, and its tendency to behave
elastically, will increase with higher molecular weight and skewing of the molec-
ular weight distribution toward larger species.
A number of polymer melt phenomena reveal elastic performance
[27]
.A
common example is extrudate swell (or die swell), in which the cross section of
an extruded profile is observed to be larger than that of the orifice from which it
was produced. Melt elasticity is required during extrusion coating operations
where the molten polymer sheet is pulled out of the sheet die of the extruder by a
moving substrate of paper or metal foil. Since the final laminate must be edge-
trimmed it is highly desirable that the edges of the polymer match those of the
substrate without excessive edge waviness or tearing. This requires a good degree
of melt cohesion provided by intermolecular entanglements which also promote
elasticity. Other elasticity-related phenomena include undesirable extrudate sur-
face defects, called melt fracture and sharkskin, which appear with some poly-
mers as extrusion speeds are increased.
A number of modern devices provide accurate measurements of both viscous
and elastic properties (although some have limited shear rate ranges). An inexact
but very convenient indicator of relative elasticity is extrudate swell which is
inferred from the ratio of the diameter of the leading edge of a circular extrudate
to that of the corresponding orifice. Since MFI is routinely measured, its limited
value can be augmented by concurrent die swell data. As an example, polyethy-
lenes intended for extrusion coating should be monitored for minimum die swell,
at given melt index, while polymers for high-speed wire covering require maxi-
mum die swell values, which can be set by experience.
The emphasis to this point has been on viscous behavior in shearing modes of
deformation. However, any operation that reduces the thickness of a polymeric
liquid must do so through deformations that are partly extensional and partly
shear. In many cases polymers respond very differently to shear and to extension.
A prime industrial example involves
low-density and linear
low-density
