Issue 02/2017

Agriculture/Horticulture
Petroleum based plastics are largely used in agriculture
as plastic films for crop protection and soil mulching,
pipes, containers for seedlings transplanting and pots.
After the cultivation period is complete, however, agricultural
plastic waste is coated with soil, organic matter, and agrochemicals
and must therefore undergo the correct collection,
disposal, and recycling processes. One sustainable solution
to the serious problem of the environmental pollution
is the employment of biodegradable polymeric materials in
agriculture; such materials are able to be integrated directly
in the soil, at the end of their lifetime, where the bacterial
flora transforms them in water, biomass, carbon dioxide or
methane. Many of the possibly suitable biodegradable polymers,
however, show unsuitable mechanical performance or
processability and may be not cost effective if compared to
petroleum based plastics. Due to increasing environmental
awareness, researchers continue to seek new materials that
can be used as ecologically friendly alternatives to agricultural
materials based on synthetic petrochemical polymers.
The research teams around the authors have developed
biodegradable polymeric materials—for spray mulching
coatings and plant pots—by using protein hydrolysates that
are derived from waste products of the leather industry and
functional poly(ethylene glycol) as a crosslinking agent [1, 2]
Protein-based waste materials are especially suited for this
purpose because they have an intrinsic agronomic values for
soil fertilization due to their high nitrogen content.
Several experimental tests were carried out to prove the
feasibility to generate in situ mulching films and coatings
showing good mechanical performances and environmental
durability (Figure 1a and Figure 1b). Their functionality as well
as their mechanical and physical behaviors was investigated in
standard and controlled experimental conditions [1, 2].
To assess whether or not water suspensions would be
capable of achieving a consistent coating when sprayed directly
onto soil, poly(ethylene glycol) diglycidyl ether (PEGDGE) and
protein hydrolysates were chosen as starting materials. The
scientists prepared the novel derivatives in water solutions
following a synthetic procedure based on the reaction between
protein hydrolysate amino groups and functional end groups
of PEGDGE. They also added wood-cellulose microfibers (up
to a final 18wt %) to enhance the mechanical properties of the
composite, and carbon black to obtain black films (and thus
prevent photo-oxidation and weed photosynthesis).
The bioplastic solutions were then distributed with an
airbrush using a spray machine. In this way, it was possible
to completely cover the growing substrate around the plants
with a thick mulching coating that dried to a hard consistency
(Figure 2a). The biodegradable coatings maintained their
mulching effect for a period ranging from one to nine months
achieving weed suppression (Figure 2b and Figure 2c).
The lifespan of the coating depends on its thickness as well
as the temperature and moisture content of the soil, but is
mainly dependant on the structure and morphology of the
material. Morphological analysis performed on a sample
that was directly sprayed onto the soil and exposed for six
months—see Figure 3—shows a different pattern depending
on its exposure. The side exposed to solar radiation does
not differ significantly from the original film, and there is
no indication of degradation. The surface facing the soil,
however—see Figure 3—consists almost exclusively of fibers,
thus indicating that degradation begins in the polymeric
component of the material. These results show that the
biodegradation process occurs more rapidly where there is
direct contact between the film and micro-organism and the
remaining fibers act as a barrier, modulating the environmental
duration of the blend thus promoting slow release of fertilizers.
More recently tests were carried out using novel biodegradable
containers for seedlings. The objectives of this research are to
develop new biodegradable materials starting from renewable
biobased raw products, and to engineer the properties of these
materials so that they can be used to produce biodegradable
plant pots that guarantee no damage to roots, no transplant
shock, an enhancement to plant growth, and the slow release
of fertilizing protein-based compounds during their degradation.
The preparation of these novel biodegradable polymeric
materials began from an aqueous solution of protein hydrolysate
(derived from waste products of the leather industry), PEGDGE,
and natural fillers (i.e., sawdust or wood flour). Compounding
was then performed in a Brabender mixer at 60°C and
subsequently the pots were prepared by compression molding
the biocomposites (which were the consistency of a paste)
and drying them at 70°C: see Figure 4. It was found that the
biodegradable containers for seedlings showed good resistance
during the first stage of use (i.e., when the seedlings were grown
from seed, before transplanting): see Figure 5(a). After the
transplant, the containers (which were buried in soil) degraded
in roughly two weeks, allowing the roots to pass through
the container walls and thus enabling the overall growth of
the plants: see Figure 5(b). As a result of the slow release of
proteinaceous material, the containers showed a soil-positive
fertilizing effect.
To test the efficacy of this approach, the researchers
implemented them in the cultivation of pepper plants. At
harvest, the mean height of the pepper plants grown inside the
biodegradable pots was 0.94m. In contrast, the control plants
(grown in non-biodegradable containers) were characterized
by a mean height of 0.67m [3].
In summary, the newly developed biodegradable sprayable
mulches and plant pots (for transplanting seedlings) could
promote valid ecologically sustainable cultivations, enhance the
protection of the landscape against pollution in rural areas, and
increase the use of renewable non-oil raw materials.
The teams are currently experimenting with these
approaches by applying them to different cultivations. They
hope to prove the feasibility of their novel biodegradable
materials by investigating their functionality as well as their
physicochemical and mechanical behavior in standard and
controlled experimental field conditions, and by following their
biodegradation process during plant cultivation.
References
[1] L. Sartore, G. Vox, E. Schettini, 2013. Preparation and Performance of
Novel Biodegradable Polymeric Materials Based on Hydrolyzed Proteins for
Agricultural Application J. Polym. Environ. 21 (3), pp718-725. doi: 10.1007/
s10924-013-0574-2
[2] E. Schettini, L. Sartore, M. Barbaglio and G. Vox, Hydrolyzed protein based
materials for biodegradable spray mulching coatings. Acta Horticulturae
(ISHS) 952, pp.359-366, 2012.
[3] L. Sartore, E. Schettini, F. Bignotti, S. Pandini, and G. Vox, Biodegradable
plant nursery containers from leather industry wastes, Polym. Composite.
2016. doi:10.1002/pc.24265.
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