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Previously,
the film used for wrapping was PVC, or polyvinylchloride with
plasticiser, which has an amorphous crystalline structure. Some years
ago, PVC was largely replaced by polyolefin shrink film.
PVC film continues to be used
in some applications, however, as it the easiest of any film to seal and
shrink in spite of its strength and storage issues. Its sealing
temperature is close to its degradation temperature. Degradation
by-products include very small quantities of hydrogen chloride (HCL) gas
and carbon deposits on the sealer. The HCL is handled through
ventilation. Polyolefin films also require ventilation. In
manufacturing, good practices regarding the by-products of combustion
should always be considered for the safety of machine operators. Carbon
deposits should be regularly cleaned and the presence of HCL requires
regular maintenance of the sealer. As polyolefins have no chlorine, they
do not produce HCL.
The plasticiser in PVC hardens under cold conditions and softens under
hot conditions. This can decrease strength in cold weather and
machineability in hot weather. As polyolefin has no plasticiser,
temperature is not an issue with this film. PVC film must be stored at
normal room temperature, otherwise it will start to shrink. Polyolefin
tolerates higher storage temperatures.
The overall machineability of PVC gives it some market share, but
advances have made it increasingly easier to deal with polyolefin’s
lower machineability. Given the benefits of polyolefin noted above, it
is now the main material used for display applications.
Display wrapping literally bags the product. This is accomplished using
an L-sealer with center-folded film. The sealer accommodates the
continuous production of wrapped packs, which are next processed through
a shrink tunnel.
L-sealers may be manual, semi-automatic, and fully automatic. Many
manual and semi-automatic models have been replaced by combined seal and
shrink machinery commonly referred to as chamber machines. Fully
automatic L-sealers are versatile, cost effective, and widely used.
Flow wrappers with side seal or overlap mechanisms achieve the highest
output speeds. Regardless, all the different types of sealers produce a
two-dimensional bag wrap.
With its variety of attributes, polyolefin films can be used for a wide
range of applications. Although not generally used in food applications,
exceptions include pizza, produce, and egg wrapping. More precisely,
polyolefin is not used to extend the shelf life of a food product except
when shrink barrier film, a specialized product, is used.
The display applications of polyolefin have virtually no limits, as it
has been engineered to deal with any given requirements. Soft shrink,
perforation, and printing are standard features available. More
specialized are slip and anti-fog treatments.
Some types of polyolefin shrink film offer better machineability and
overall performance than most films on the market. Examples
distinguished by technical differentiation in manufacturing include
irradiated grades with enhanced strength and multiple-layered complexes
tailored to a particular application. Polyolefin manufacture typically
uses three- or five-layer extrusion to enhance performance benefits.
The key to market movement is addressing specific requirements through
the development of materials for specific applications. There is no
doubt that polyolefin shrink film will continue to be developed to
address even more application requirements.
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