The problem
Innovia
Films is the world's leading supplier of speciality Biaxially
Oriented Polypropylene (BOPP) and cellulose films for speciality
packaging, labelling, graphic arts and security products.
With a turnover of more than £400 million Euros, the business
employs some 1,350 people worldwide. Total annual film capacity
currently stands at more than 130,000 tonnes.
They requested evaluation of a coated polypropylene film used in
food packaging, which degraded over a period of time resulting in
increased opacity of the material when stored. The problem
appeared to be related to the processing conditions under which the
films had been manufactured.
The challenge
This particular challenge related to a film used in packaging
food, in this particular case, for a company manufacturing and
packaging potato crisps in transparent bags. Rapid turnaround
time from receipt of samples to results of investigation was
required to allow knowledge gained to be fed back into the
manufacturing process.
The bags are manufactured from two polymer systems - a 'core' of
polypropylene, which is coated on both sides by a 0.6 µm layer of a
random copolymer of propylene and ethylene. Different
parameters during manufacture and processing led to variations,
both in the 'haziness' of the otherwise transparent films, and in
how the transparency changes with time. The challenge facing
Daresbury Laboratory was that
either one of these constituent parts could have been causing the
haziness.
The solution
The multidisciplinary facilities available on the Synchrotron
Radiation Source allowed independent investigation of the bulk
polymer and the surface layer.
Grazing incidence powder diffraction was used to look at the 0.6
µm surface layer - the diffraction patterns from all samples showed
just three broad peaks, which corresponded exactly to the positions
expected for pure polypropylene. Therefore, despite the presence of
up to 10% ethylene in the copolymer, the molecular arrangement of
polypropylene is being adopted in the coating layers' crystallites.
The peaks are sharper in the case of the 'clear' films, implying a
higher level of molecular order within the crystallites.
Small angle X-ray scattering was used to look at the bulk
polymer. Comparison of data recorded for 'clear' and 'hazy'
samples revealed a subtle but significant effect. There was more
scattering in the 'hazy' samples. It may be expected that there
would be more crystallites in the 'hazy' samples - this information
showed that the size of those crystallites is around 200 Å.
Grazing incidence X-ray Diffraction pattern of
polymer surface layer for three samples. The
samples range from "clear" to "hazy" in the order of red,
blue and green respectively.
Benefits
- The crisp packet manufacturer was able to alter its production
conditions to eliminate hazing
- The rapid solution of the problem allowed the manufacturer to
demonstrate a proactive and trouble-shooting approach to its
customers
- Loss of production was kept to a minimum, whilst the
manufacturer's brand reputation was enhanced