There are many parameters to consider when
designing plastic mulch films for agricultural applications. These
include the type and level of ultraviolet stabilizer, the antioxidant
package, the guage of the mulch film, the polymer to be used for production,
the opacity of the film and the color of the mulch film. Some the
factors which will influence these various choices are the mulch film
service lifetime, the geographic location for use, the soil type and
condition, the mulch laying equipment and the crops to be grown on
the mulch film.
Ultraviolet Stabilizers
The choice of the ultraviolet inhibitor stabilization package for
mulch film is important in designing a mulch film for several reasons.
The type and level of the ultraviolet inhibitor (UVI) will determine
the outdoor lifetime of the film, i.e., whether the film will last
for six months, or one year, etc. In order to choose not only the
correct UVI, but also the proper level, one must be familiar with
the geographic location for which the mulch will be used as well as
the types and levels of agrochemical contact that will be involved.
The level of UVI needed to properly stabilize a plastic mulch film
will vary depending upon guage, agricultural location, service lifetime
and mulch film color.
Antioxidants
Another important factor to be considered when designing a plastic
mulch film is the choice of antioxidant additives. Antioxidants (AOs)
are additives that interrupt the autooxidation (thermal decomposition)
process of the polymer. This autoxidation of the polymer can lead
to a loss of the film’s physical and optical properties similar
to that experienced with UV degradation. Antioxidants are important
to protect the polymer from degradation both during mulch film manufacture
(Primary AO) as well as during field exposure (Secondary AO). Secondary
AOs also help protect the polymer and additives during the masterbatch
manufacture. Masterbatch is the form that the additives and the colorants
come in that is used by the film manufacturer to produce the mulch
film.
Polymer Choice
The selection of the polymer resins used to produce the mulch films
is significant for determining the physical characteristics of the
finished mulch film. Some of the most commonly selected types of polymers
to produce mulch films include low density polyethylene (LDPE), linear
low density polyethylene (LLDPE), high density polyethylene (HDPE),
and metallocene LLDPE (mLLDPE). The resin blends used to produce a
mulch film can impact such properties as the tear and puncture resistance
of the film, the mechanical stretch of the film, the strength of the
film and the moisture and vapor permeability of the film. All of these
variables are important to designing a mulch film. However, generally
all resins cannot be used together to impart all the best properties.
Therefore, it is necessary to design the polymer matrix to yield the
properties necessary to suit the growers farming needs and mechanical
mulch film laying capabilities. Ideally, the mulch film should have
enough retained strength and elasticity to be easily removed from
the field.
Processing and Fabrication
The processing of the resin blends is also a crucial factor affecting
the physical properties. The film manufacturer must control the rate
of film production, the cooling rate of extruded film, the tension
of the film winding equipment, as well as the level of additives for
each film. These factors can contribute to the crystallinity of the
film, which can affect the strength and permeability of the film in
addition to the tear and puncture properties. The thickness of the
film (gauge) can also impact the pyhsycal properties as well as the
service lifetime of the mulch film.
Colored Mulch Film
Black, white and clear mulch films are standards in plasticulture
with many other colors now being evaluated as well. Plastic mulch
films now include such colors as silver, red, blue, yellow, green,
olive and brown. These mulches are being evaluated not only for their
effect upon plant growth and fruit, but also for their ability to
suppress weed growth, control insect infestation and warm/cool the
soil as needed.
White Mulch Film
The choice of various pigments in order to make the colored mulch
film is also important and can have an impact upon the type and level
of UVI needed to sufficiently stabilize the plastic mulch film for
its service lifetime. The pigment titanium dioxide (TiO2) is used
to color the film white. There are many different types and grades
of TiO2 available for a number of applications. It is important however,
to choose a grade which is suitable for outdoor exposure when designing
a plastic mulch film. Weatherable grades of TiO2 are surface coated
to make them more stable outdoors. Untreated grades of TiO2 can actually
accelerate the degradation of film outdoors even inthe presence of
UVI.
Black Mulch Film
Carbon black is another pigment that is available in a wide variety
of grades and particle sizes for a multitide of applications. Smaller
particle sized carbon blacks tend to perfom better in outdoor applications
than in the larger particle sized carbon blacks. All pigments used
to make colored mulch films need to be outdoor stable to ensure that
the color does not fade and that they do not contribute to premature
film degradation.
Conclusion
In essence, there is a wide array of factors that must be taken into
account when desiging a plastic mulch film. Everything from thickness,
geographic location and film gauge to ultraviolet stabilizers, antioxidants,
and resin blends must be considered to ensure a plastic mulch film
that will perfom well throughout the growing season. These variables
make it almost impossible to produce one mulch film for all climates,
geographic regions, crops and service lifetimes.
