Creep
As mentioned, when a polymer such as nylon is deformed and
held at constant stress (load), the chain segments are extended
and bonds are stretched. The polymer is uncomfortable in
this position, and the various chain segments will rearrange
themselves in a direction of lower stress if enough time is
allowed. If the load continues to be applied, chain movement
continues—in the direction of the applied load. This
phenomenon, called creep, is largely irreversible, causing
permanent deformation.
Stress Relaxation
If the polymer is deformed and held at constant strain
(deformation) instead of constant stress, the same chain segment
rearrangement occurs. If allowed enough time, they will finally
find positions of zero stress or nearly so. Having done so, they
will continue in their new positions even after the material is
released. This phenomenon is called stress relaxation, or stress
decay, and is really another form of creep. To reach zero stress
requires a very long period of time, perhaps years. For shorter
periods of time, rearrangement is less than complete, and many
chain segments can return to, or in the direction of, their original
position. This affords partial recovery.
These processes are both time- and temperature-
dependent.
Therefore, a polymer will have more complete recovery from
deformation at room temperature than if the material is
deformed and held at an elevated temperature for the same
length of time. A polymer will also deform easier and recover
faster at elevated temperatures than at room temperature.
Factors Affecting Bend Recovery
The following is a detailed discussion of the factors affecting
bend recovery of brush filaments.
Inherent Bend Recovery of Brush Filaments
Bend recovery of a material is partly an inherent property of that
material and partly influenced by the manufacturing process. In
the case of man-made filaments, the raw materials from which
they are manufactured prescribe a range for bend recovery.
Bend recovery can be changed within this range by filament
manufacturing techniques.
A relative measure of the bend recovery of one material to
another may best be obtained by taking measurements of the
materials in question under identical standard test conditions.
Comparative data for several materials are shown in Table 4.
Temperature
Bend recovery of Tynex® is maximum at room temperature and
slightly above. Bend recovery of 66 nylon is more temperaturesensitive
than that of Tynex®.
Table 5. Bend Recovery versus Temperature
Humidity
The bend recovery of Tynex® is unaffected by changes in
relative humidity.
Table 6. Bend Recovery versus Relative Humidity
Strain
Small and rapid deflection of nylon is perfectly elastic or
springlike and results in complete recovery. The larger the strain,
the lower the bend recovery. When larger filaments are deflected,
they are strained more, which results in higher stress.
Table 7. Bend Recovery versus Filament Caliper
Time Under Load
The longer a filament is held in a deflected position, the less
complete the recovery.
Table 8. Bend Recovery versus Time Under Load
Relaxation Time
The longer a filament is allowed to relax, the more complete
the recovery.
Table 9. Bend Recovery versus Relaxation Time