Chemistry Reference
In-Depth Information
weld lines are relatively minor with ductile amorphous plastics like ABS and
polycarbonate and with semicrystalline polymers such as polyoxymethylene. This
is because these materials can reduce stress concentrations by yielding
[23]
.
Semicrystalline polymers are impact resistant if their glass transition tempera-
tures are much lower than the test temperature. The impact strength of such mate-
rials decreases with increasing degree of crystallinity and particularly with
increased size of supercrystalline structures like spherulites. This is because these
changes are tantamount to the progressive decrease in the numbers of tie mole-
cules between such structures.
The impact strength of highly cross-linked thermoset polymers is little
affected by temperature since their behavior is generally glassy in any case.
4.13
Rheology
Rheology is the study of the deformation and flow of matter. The processing of
polymers involves rheological phenomena. They cannot be evaded. It is important,
therefore, that practitioners have at least some basic knowledge of this esoteric sub-
ject. This section is a very brief review, with the aim of guiding the perplexed to at
least ask the right questions when confronted with a rheological problem.
Following is a summary of the major points, which are elaborated below.
1.
The rheological behavior of materials is generally very complex, and
polymers are usually more complex than alternative materials of construction.
2.
A complete rheological characterization of a material is very time consuming
and expensive and much of the data will be irrelevant to any particular
process or problem.
3.
Rheological measurements must be tailored to the particular process and
problem of interest. This is the key to successful solution of rheological and
processing problems. Relevant rheological experiments are best made at the
same temperatures, flow rates, and deformation modes that prevail in the
process of interest.
4.
Following are some important questions that should be asked in the initial
stages of enquiry:
a.
Is the process isothermal? Most standard rheological measurements are
isothermal; many processes are not.
b.
Is the material behavior entirely viscous or does it also comprise elastic
components?
c.
Does processing itself change the rheological properties of the material?
d.
Do the material purchase specifications ensure rheological behavior?
e.
Do steady-state rheological measurements characterize the material in a
particular process?
f.
Is it best to look to rheological measurements or to process simulations for
answers to a particular problem?
