Bio-Wrappers
Biodegradable
packaging, although a complex phenomenon is regarded by scientists as
the technology of the future. Dr NG Mokashi deciphers its success quotient.
Though
there has been genuine concern for environment in recent times, the
continued global population growth has increased pollution at all levels.
And polymers, the foremost in this chemical chain are the first to be
targetted by environmentalists since they are unable to degrade at a
reasonable rate. This led to develop degrading polymers, to sufficiently
low molecular mass (typically about 10,000), which are then accessible
to biodegradation. Edible films produced from biological materials,
offers numerous advantages over conventional synthetic packaging materials.
Potential applications of edible films are numerous (internal moisture
or solute barriers of heterogeneous foods, individual protection of
food pieces, encapsulation of food additives).
Biopolymers
Could be used to formulate biodegradable packaging
i.e. to replace short shelf life plastics. These can be classified in
four categories: polysaccharide, proteins, lipids and polyesters. Obtained
by controlled vegetal or bacterial biosynthesis films primarily composed
of polysaccharide and proteins (cellulose and its derivatives, starch,
gum, gelatin, zein and gluten) have overall suitable mechanical and
optical properties and are highly sensitive to moisture and show poor
water vapour barrier properties. In contrast, films obtained from lipids
(waxes, lipids or derivatives or polyesters (poly D-B-hydroxybutyrate,
polylactic acid) show good water vapour barrier properties but are usually
opaque and relatively stiff. Lipid films could be fragile and unstable
to rancidity.
Important methods of degrading polymers
Photodegradation
It involves use of inherently photo-unstable polymer or use of photo-degrading
additives. Example, ethylene, carbon monoxide polymer where absorption
of light by ketone group leads to chain scission. The polymer becomes
brittle and forms a powder. Alternatively addition of photo-degradant,
which is an UV wave absorber, can be done, instead of dissipating the
absorbed energy as heat (normal) here highly reactive intermediates
are formed which degrade the polymer-iron dithiocarbamate.
Hydrolysis
A co-polymer based on polyester technology, the
film has a lower melting point (200 degree celsius) and all other properties
similar to polyester films. Three different commoners are used, depending
upon end use, and these provide weak points in polymer chain making
them susceptible to degradation, via hydrolysis. In practice, larger
molecules are split by moisture present during composting into smaller
molecules, which degrade into carbon dioxide and water, by microbes.
During composting, hydrolysis takes place reducing molecule weight followed
by biodegradation. BAK 1095 Resin by Bayer (a polyesteramide resin)
shows properties like polyethylene. Polyvinyl alcohol with appropriate
hydroxyl content dissolves it in water though is washed away when used
for water-soluble packaging. Polyethylene film containing about 5-15
per cent cornstarch is an excellent example...
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