Agriculture Reference
In-Depth Information
EVA copolymer
is a copolymer obtained
by the same polymerization system as the
LDPE. Its optical properties are slightly dif-
ferent from those of PE having in general a
higher PAR transmission and a lower tur-
bidity but thermal properties are better
(Table 4.3). These thermal performances
(transmissivity to long-wave IR) depend on
the content of vinyl acetate, being 12-14%
in most cases. Its main disadvantage is its
higher tendency to fluency when cold
('creep'), which is quite inadequate for
windy areas (the film stretches but does not
recover well to the initial shape). As it gets
dirty easily, it is usually incorporated in
multilayer films. The EVA films can be very
transparent to light (crystal variant) or if it is
of the opaline type (translucent) it has great
diffusing power.
PVC film
has similar optical properties
to EVA, but better thermal properties
(Table 4.3). It attracts dust, as does EVA. Its
widths are limited (6.5 m with extrusion
and 2 m with calendering) and it has very
little resistance to tears which is solved by
incorporating a fabric, preventing it from
tearing and breaking. After its use it is dif-
ficult to dispose of and for this reason, and
its cost (which is higher than PE), its use is
limited in the Mediterranean area.
PVF film
has excellent optical and
mechanical properties and is very durable,
lasting for a long time, but its price is really
high, so it is seldom used.
Polyethylene
terephthalate (PET)
also has excellent opti-
cal properties but is very expensive. Both
are difficult to fix to the structure.
t h e r m a l
f i l m s
.
To improve the transmissivity
to long-wave IR the standard PE films are
enriched with additives that block the long-
wave IR, resulting in thermal PE or infrared
polyethylene (IR-PE) that prevents thermal
inversion in the greenhouse, in relation to
standard PE (Fig. 4.8). These additives
slightly decrease the PAR transmissivity
(Table 4.3).
Nowadays, additives are being used
(new thermal loads) which can vary the dif-
fusing power of light (Salmerón
et al
.,
2001).
Another way of increasing the opacity
to long-wave IR in the EVA films is to
increase the proportion of vinyl acetate.
A film can be considered as thermal
when its transmissivity to long-wave IR
(7-14 mm) is lower than 25% for 200 mm
thickness
films
(European
regulation
EN-13206).
a n t i
-
d r i p p i n g
f i l m s
.
The anti-dripping films
are hardly used as monolayer films, but as a
part of multilayer films, being usually
placed on the inner surface. There are also
products which, when applied to the plastic
film as a spray, provide an anti-dripping
effect.
m u l t i l a y e r
f i l m s
.
With the aim of incorporat-
ing in a single film the best characteristics
of several types, the multilayer films were
created, consisting of two or more plastic
films (layers) welded by coextrusion (a
manufacturing process).
Nowadays the use of the three-layer
type is increasing. One of the most com-
monly used has an EVA film in the middle
of two PE films, the external film containing
UV additives (for longer durability) and the
internal film containing anti-dripping addi-
tives. In addition, it may contain other spe-
cific additives.
The multilayer films have displaced
the use of anti-dust monolayer films.
Special films
l o n g
d u r a t i o n
f i l m s
.
The incorporation of
photostabilizing additives (see section 4.5.3)
is a common practice in the manufacture of
films to extend their lifespan. LD-PE (long
duration PE), also called UV-PE, may last
three growing seasons, in high annual radia-
tion areas. In areas of lower radiation it will
last longer. The guarantee usually given for
the durability of these films is on the condi-
tion that there will be no abusive use of pes-
ticides, quantified as a maximum permissible
content of chloride and sulfur on the film.
p h o t o s e l e c t i v e
f i l m s
.
Initially, photoselective
films were based on the 'waterfall' effect (flu-
orescence) by means of luminescence addi-
tives which converted UV radiation in blue
light and green into red (see section 4.5.3),
