Chemistry Reference
In-Depth Information
polyethylenes, i.e., LDPE and LLDPE, respectively (Section 11.5.3). LDPE grades
intended for extrusion into packaging film have relatively low shear viscosities and
high elongational viscosities. As a result, extrusion of tubular film involves reason-
able power requirements and stable inflated film “bubbles” between the die and
film take-off. LLDPEs of comparable MFIs require much more power to extrude.
Their melts can, however, be drawn down to much thinner gauges (an advantage),
but the tubular film bubbles are more prone to wobble and tear (a disadvantage).
The best of both worlds can be realized by blending minor proportions of selected
LDPEs into LLDPEs.
When problems occur during polymer processing it is necessary to perform at
least a preliminary analysis of the particular fabrication process. Experiments on
production equipment are time-consuming, difficult to control and expensive.
Therefore, equivalent laboratory experiments are very desirable. Ideally, one
would be able to analyze the production process in terms of fundamental physical
quantities and measure these with rheological equipment. It is necessary to make
sure that the laboratory measurements correspond to the actual production process
and to select the rheological characteristics that bear on the particular problem.
That is to say, do not measure viscosity to try to get information about a phenom-
enon that is affected mainly by the elastic character of the material. Note in this
connection that most laboratory data are obtained from steady-state measure-
ments, while the polymers in some processes never reach equilibrium condition.
(The ink in a high-speed printing operation is a good example.) If this rheological
analysis is not feasible the production process can sometimes be simulated on a
small, simplified scale, while paying attention to the features that are critical in
the simulation.
There are a number of fine recent rheological references
[28
30]
that should
be consulted for more details than can be considered in an introductory text.
4.14
Effects of Fabrication Processes
An important difference between thermoplastics and other materials of construc-
tion lies in the strong influence of fabrication details on the mechanical properties
of plastic articles. This is exhibited in the pattern of frozen-in orientation and fab-
rication stresses. The manufacturing process can also have marked effects on
crystalline texture and qualities of products made from semicrystalline polymers.
Orientation generally produces enhanced stiffness and strength in the stretch
direction and weakness in the transverse direction. In semi-crystalline polymers,
the final structure is sensitive to the temperature
time sequence of the forming
and subsequent cooling operations and to the presence or absence of orientation
during cooling. Different properties are produced by stretching a crystallized sam-
ple at temperatures between
T
g
and
T
m
, or by orienting the molten polymer before
crystallizing the product.
