Agriculture Reference
In-Depth Information
monomer it is called a homopolymer, and
when it is made from two or more polymers
it is called a copolymer (Díaz
et al
., 2001).
Almost all the monomers used in the plas-
tics industry come from petroleum.
tain properties or improve their character-
istics, without altering the molecular
structure of the polymer. They can be
processing additives (that ease the fabrica-
tion process) or functional additives (that
give certain qualities to the plastic), reach-
ing up to 10% of the final weight of the
product (Díaz
et al.
, 2001).
The most important functional addi-
tives are: (i) the photostabilizers; (ii) the
anti-acids; (iii) the long and short-wave IR
radiation blockers; (iv) additives that mod-
ify the surface tension; and (v) the lumines-
cence additives (Díaz
et al.
, 2001).
Photostabilizers prolong the life of the
material by delaying ageing of the material;
ageing being precipitated by polymer degra-
dation due to UV light of the Sun's rays.
Common photostabilizers are the nickel
quenchers and diverse organic compounds,
such as the 'HALS' (hindered amine light
stabilizers) (Díaz
et al.
, 2001). The latest
generation of 'HALS' are known as the 'nor-
HALS'. The latter are more stable than the
conventional HALS, in the presence of pes-
ticides and other aggressive acid conditions,
and are being used for long-life films and in
those greenhouses where sulfur is com-
monly applied (e.g. rose culture).
The nickel quenchers give a yellowish-
green colour to the films and their use is
diminishing, due to their residues. The
HALS additives are non-coloured.
The anti-acid additives improve the
resistance of the HALS, increasing their
useful life. Zinc oxide, used as anti-acid,
gives the films a light-diffusing effect.
4.5.2
Plastic materials commonly
used in agriculture
The plastics have a great variety of uses in
agriculture, not just as films but in many
other forms (tubes, boxes, threads).
The plastic films have a low weight, as
compared with glass. For instance, 1 m
2
of
polyethylene (PE) film, 0.2 mm thick,
weighs a bit less than 200 g, while 1 m
2
of
glass, 2.7 mm thick, weighs 6.5 kg. This
characteristic has favoured the use of PE as
greenhouse cladding material.
The most common plastic materials
used as agricultural films (Table 4.2) are the
low density polyethylene (LDPE, with a
density less than 0.93 kg m
−3
), the copoly-
mer of ethylene and vinyl-acetate (EVA) and
the plasticized polyvinyl chloride (PVC).
The more common materials, used as rigid
panels, are polycarbonate (PC), polymethyl
methacrylate (PMMA), rigid PVC and poly-
ester reinforced with fibreglass.
4.5.3
Plastic additives
Additives are incorporated in the process
of plastic making in order to provide cer-
Table 4.2.
Materials commonly used for greenhouse cladding.
Plastics
Glass
Rigid panels
Flexible films
a
Screens
Fibreglass reinforced polyester
Low density polyethylene (LDPe)
Polycarbonate (PC)
Conventional
Polyvinyl chloride (PVC)
Long life or UV
Polymethyl methacrylate (PMMA)
Thermal or IR
ethylene vinyl-acetate copolymer (eVA)
PVC
Others: Polyvinyl fluoride (PVF) (Tedlar), Mylar,
Polyester
a
May be used as multilayer films.
