Agriculture Reference
In-Depth Information
decreasing the global heat transfer coeffi-
cient (Table 5.2). However, condensation in
the form of droplets is harmful, resulting in
burn on the leaves as each droplet acts as a
lens (magnifier) to solar radiation and as the
droplets fall over the crop this favours the
development of cryptogamic diseases
(Papadakis
et al
., 2000). The size of the
droplet depends on the contact angle
(Fig. 4.7), decreasing the transmissivity if
the angle is less than 40° and reaching a
higher reduction with contact angles of 90°,
which is the case for PE-composed films
(Papadakis
et al
., 2000; Díaz
et al
., 2001).
The condensation of water vapour on
the inner surface of the plastic film needs a
minimum angle of the roof of 20-25° for the
droplets to slide and not fall over the crop
(Von Elsner
et al
., 2000a). The snow slides
over the roof if the angle is equal to or greater
than 26.5°; equivalent to a 1:2 slope (Von
Elsner
et al
., 2000b).
weights (less than 200 g m
−2
) minimizing
the loads on the greenhouse structure. Their
mechanical properties are good, withstand-
ing hail better than glass, although they sof-
ten at high temperatures and are fragile at
very low temperatures. Under normal cli-
mate conditions they are stable. Due to
static electricity they accumulate dust on
their surface, especially EVA and PVC,
which is usually countered by adding anti-
dust additives. Their chemical resistance is
generally good, but the use of some pesti-
cides (containing sulfur or halogens) may
decrease their lifespan (affecting the HALS
additives).
The thermal dilatation coefficient of
plastic materials is higher than in metals,
which must be considered during the assem-
bly of the greenhouse.
The ageing of plastic films involves a
decrease of their mechanical and radiomet-
ric properties, as they degrade with time.
The
polyethylene
used in greenhouses
is of the low density type (LDPE), obtained
by radical polymerization in high pressure
processes. Its transmission of solar radia-
tion is good, although decreases with time
as the film gets dirty and old, but its thermal
behaviour is mediocre, due to its transpar-
ency to long-wave IR when there is no con-
densation on the film (Table 4.3). The
incorporation of thermal additives solves
this problem.
To improve their shelf life, additives
are incorporated which protect the
degrading action of the UV rays, calling
them long duration or long-life polyethyl-
ene (LD-PE) or ultraviolet polyethylene
(UV-PE).
4.5.5
Plastic films most commonly
used in greenhouses
Normal films
The plastic films most commonly used in
greenhouses are LDPE, EVA and their deriv-
atives, frequently incorporating several lay-
ers of different materials in multilayer films
(Robledo and Martin, 1981). The plasticized
PVC films are seldom used, except in east-
ern Asian. Equally, polyester and polyvinyl
fluoride (PVF) films are seldom used.
All these films can be manufactured in
small thicknesses and this allows for low
Table 4.3.
Characteristics of several flexible materials for greenhouse covers (adapted from CPA, 1992
and Tesi, 2001).
Pe
UV-Pe
IR-Pe
eVA
PVC
Thickness (mm)
0.10
0.18
0.18
0.18
0.18
Weight (g m
−2
)
92
165
173
179
230
Direct PAR transmissivity (%)
91
88-90
85-86
90
90
Diffuse PAR transmissivity (%)
90
86
86
76
89
Long-wave IR transmissivity (%)
68
63-65
≤
25
18-27
10-15
Durability under non-aggressive climate
(cropping seasons) (years)
1
3 or more
3
3
2
