Packaging materials for Foods
than shrink-wrapping (45 - 75 m), or alternatively, linear low-density polyethylene is used at
thicknesses of 17 - 24 m. The cling properties of both films are adjusted to increase adhesion
between layers of the film and to reduce adhesion between adjacent packages.
High-density polyethylene (HDPE) is stronger, thicker, less flexible and more brittle than LDPE
and a better barrier to gases and moisture. Sacks made from HDPE have high tear and puncture
resistance and have good seal strength. They are waterproof and chemically resistant and are
increasingly used instead of paper or sisal sacks.
Polypropylene is a clear glossy film with a high strength and puncture resistance. It has a
moderate barrier to moisture, gases and odours, which is not affected by changes in humidity. It
stretches, although less than polyethylene. It is used in similar applications to LDPE. Oriented
polypropylene is a clear glossy film with good optical properties and a high tensile strength and
puncture resistance. It has moderate permeability to gases and odours and a higher barrier to
water vapour, which is not affected by changes in humidity. It is widely used to pack biscuits,
snackfoods and dried foods.
Polyvinylidene chloride is very strong and is therefore used in thin films. It has a high barrier to
gas and water vapour and is heat shrinkable and heat sealable. However, it has a brown tint
which limits its use in some applications. Polyamides (or Nylons) are clear, strong films over a
wide temperature range (from – 60 to 200°C) that have low permeability to gases and are
greaseproof. However, the films are expensive to produce, require high temperatures to heat seal,
and the permeability changes at different storage humidities. They are used with other polymers
to make them heat sealable at lower temperatures and to improve the barrier properties, and are
used to pack meats and cheeses.
Films are coated with other polymers or aluminium to improve their barrier properties or to
impart heat sealability. For example a nitrocellulose coating on both sides of cellulose film
improves the barrier to oxygen, moisture and odours, and enables the film to be heat sealed
when broad seals are used. Packs made from cellulose that has a coating of vinyl acetate are
tough, stretchable and permeable to air, smoke and moisture. They are used for packaging meats
before smoking and cooking. A thin coating of aluminium (termed ‘metallisation’) produces a
very good barrier to oils, gases, moisture, odours and light. This metallised film is less expensive
and more flexible than plastic/aluminium foil laminates. The properties of selected coated films
are shown in Table 1.
Table 1: Properties of selected packaging materials
Strength Clarity Thickness (m)
* *** * *** 21-40
- ** * ** * 25-200
- *** ** *** * 350-1000
*** *** *** ***
*** *** *** ***
Aluminium *** *** ***
- ** ** *** ** 12-23
*** *** ***
* = low, ** = medium, *** = high. PvDC = polyvinylidene chloride, LPDE = low density
polyethylene, HDPE = High density polyethylene.
Thicker films of each type have better barrier properties than thinner films.