Kinds of Stretch:
Stretch is one of the characteristics people interpret as “comfort”.
That is, an inelastic product is seldom viewed to be as desirable as one
that has an appropriate level of stretch. Stretch in itself is useful
e.g. medical gown belt, conformable packaging, under cast padding, and
other low tension end uses where regain isn’t desirable.
Different applications require
different kinds of stretch
Stretch is usually measured as extension or a
percent of the compacted form. Limited stretch has advantages that allow
the designer to customize and control maximum extension. In some cases
too much extension is undesirable. There may be a need to have
controlled (limited) tension. An elastic (ex. rubber, spandex)
continually increases tension and can’t be tensionless. A creped
nonwoven can have extension controlled and thereby meeting design
requirements.
Regain (recovery) from stretch varies depending on the substrates weight
and especially the polymer. Nylon and polyester have good resilience.
Polypropylene is softer and has lower resilience (regain) but may be
adequate. In some cases polypropylene regain can be increased by adding
weight. Regain is usually expressed as a % of the original length after
stretching at a specific energy level (ex. 80% recovery after 50%
extension).
Resilience is often acceptable at a level of extension less than maximum
(ex. 25% in use where 50% is the maximum).
Regain can be greatly influenced by the angle of fibers relative to the
direction of stretch. For example a substrate that stretches in the MD
whose fibers are mostly oriented in that direction (i.e., a carded web)
will, all other things being equal, have much better regain than a
spunbond with fibers that lay in random direction and on average have
less than 50% MD fiber orientation. The closer creped fibers are
oriented to the machine direction, the more they can help your nonwoven
regain or recover from stretch.
Two-Way (sometimes referred to as Four-Way, Bi-Axial, Harmonized) Stretch:
In some applications stretch is desired both MD and CD. By combining The
Micrex Process with a substrate with CD stretch (such as spunlace
without cross-lapping) two-way stretch can easily be achieved.
Other Properties:
Secondary properties are of equal importance. In interlining,
requirements include washing, machine drying, solvent dry cleaning, and
steam pressing / ironing. In diapers the key secondary characteristics
are softness and attachment to other components. Industrial applications
have a broad variety of additional requirements such as controlled
internal volume (for wicking), abrasion resistance, etc. An example of
how stretch can influence secondary characteristic performance, a
nonwoven that stretches when in an abrasive environment may allow fibers
to absorb the abrasive energy by extending rather than breaking. |
