Agriculture Reference
In-Depth Information
house and for withstanding strong winds.
The resistance to tearing of the plastic is
important to avoid tears due to accidental
cuts of the film, which are not unusual in
the low-cost 'parral'-type greenhouses. The
resistance to impact is needed so that the
film can withstand hail and strong winds.
Direct solar radiation
Diffuse radiation
Fig. 4.6.
Some greenhouse-covering plastic films
have the power to diffuse solar radiation,
increasing the proportion of diffuse radiation inside
the greenhouse. Therefore, on a sunny day, inside
a greenhouse covered with this type of film the
shadows are less defined and sharp than outside.
Durability
The durability of an agricultural film is
defined as the shelf life during which the
film retains, at least, 50% of its initial
mechanical properties (Díaz
et al
., 2001).
The degradation of the film in the green-
house occurs mainly by the action of UV
radiation from the sun, which degrades the
polymer (photodegradation). In addition,
durability is also influenced by other fac-
tors, for example: (i) climate conditions
(temperature and radiation mainly); (ii) the
additives used; (iii) the thickness of the film;
and (iv) the management of the greenhouse
(pesticides used, assembly of the film)
among others (Briassoulis
et al
., 1997a, b).
Both halogenated and sulfurated pesticides
attack the photostabilizers (HALS and nickel
quenchers), shortening the shelf life of the
films (Barahona and Gómez-Vázquez, 1985;
Gugumus, 2000).
The artificial ageing of films in the lab-
oratory using special lamps, which repro-
duce similar conditions (but more intense)
to the natural solar radiation, allow for a
quick estimate of the longevity characteris-
tics of a film.
acquires (droplets or water film). The
condensation on the inner face of the
plastic film can notably reduce the trans-
missivity to solar radiation, depending on
the shape of the droplets (Fig. 4.7). Indeed,
condensation reduces the transmission to
long-wave IR. The dust and dirt accumu-
lated over the film decrease its light trans-
missivity, in the same way as the ageing
of the material which specifically limits
the radiation (Matallana and Montero,
1989;
Montero
and
Antón,
2000b;
Papadakis
et al
., 2000).
When evaluating the transmissivity of
new materials it is important to relate the
results to the prevailing light during the test
(light quality) (Kittas and Baille, 1998; Kittas
et al
., 1999).
Mechanical properties
The mechanical characteristics of the
film depend both on the intrinsic factors
of the material (i.e. type of raw material)
as well as on the conditions during their
transformation into a film (homogeneity
in the distribution of additives, proper
and uniform thickness). The degrading
action of solar radiation also affects these
properties, mainly depending on the
exposure time.
From the point of view of the grower,
the most relevant mechanical properties are
resistance to traction, tearing and impact
(Briassoulis
et al
., 1997b; Marco, 2001). The
resistance to traction, which evaluates the
capacity of the film in the greenhouse to
withstand tensile forces, is important dur-
ing the assembly of the film in the green-
Behaviour with regards to condensation
Anti-dripping additives increase the surface
tension in the film, so that the water vapour
condenses in the shape of a film without
producing droplets, so compared with the
same film without anti-dripping additives
there is an increase of light transmission
due to the reduced reflection of light
(Pearson
et al
., 1995; Von Elsner
et al
.,
2000a). However, the durability of the effect
is normally shorter than the shelf life of the
film (Papadakis
et al
., 2000).
When anti-dripping additives are used
in monolayer films, the external surface of
