Issue 02/2017
bioplasticsMAGAZINE_1702
bioplasticsMAGAZINE_1702
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Agriculture/Horticulture<br />
Petroleum based plastics are largely used in agriculture<br />
as plastic films for crop protection and soil mulching,<br />
pipes, containers for seedlings transplanting and pots.<br />
After the cultivation period is complete, however, agricultural<br />
plastic waste is coated with soil, organic matter, and agrochemicals<br />
and must therefore undergo the correct collection,<br />
disposal, and recycling processes. One sustainable solution<br />
to the serious problem of the environmental pollution<br />
is the employment of biodegradable polymeric materials in<br />
agriculture; such materials are able to be integrated directly<br />
in the soil, at the end of their lifetime, where the bacterial<br />
flora transforms them in water, biomass, carbon dioxide or<br />
methane. Many of the possibly suitable biodegradable polymers,<br />
however, show unsuitable mechanical performance or<br />
processability and may be not cost effective if compared to<br />
petroleum based plastics. Due to increasing environmental<br />
awareness, researchers continue to seek new materials that<br />
can be used as ecologically friendly alternatives to agricultural<br />
materials based on synthetic petrochemical polymers.<br />
The research teams around the authors have developed<br />
biodegradable polymeric materials—for spray mulching<br />
coatings and plant pots—by using protein hydrolysates that<br />
are derived from waste products of the leather industry and<br />
functional poly(ethylene glycol) as a crosslinking agent [1, 2]<br />
Protein-based waste materials are especially suited for this<br />
purpose because they have an intrinsic agronomic values for<br />
soil fertilization due to their high nitrogen content.<br />
Several experimental tests were carried out to prove the<br />
feasibility to generate in situ mulching films and coatings<br />
showing good mechanical performances and environmental<br />
durability (Figure 1a and Figure 1b). Their functionality as well<br />
as their mechanical and physical behaviors was investigated in<br />
standard and controlled experimental conditions [1, 2].<br />
To assess whether or not water suspensions would be<br />
capable of achieving a consistent coating when sprayed directly<br />
onto soil, poly(ethylene glycol) diglycidyl ether (PEGDGE) and<br />
protein hydrolysates were chosen as starting materials. The<br />
scientists prepared the novel derivatives in water solutions<br />
following a synthetic procedure based on the reaction between<br />
protein hydrolysate amino groups and functional end groups<br />
of PEGDGE. They also added wood-cellulose microfibers (up<br />
to a final 18wt %) to enhance the mechanical properties of the<br />
composite, and carbon black to obtain black films (and thus<br />
prevent photo-oxidation and weed photosynthesis).<br />
The bioplastic solutions were then distributed with an<br />
airbrush using a spray machine. In this way, it was possible<br />
to completely cover the growing substrate around the plants<br />
with a thick mulching coating that dried to a hard consistency<br />
(Figure 2a). The biodegradable coatings maintained their<br />
mulching effect for a period ranging from one to nine months<br />
achieving weed suppression (Figure 2b and Figure 2c).<br />
The lifespan of the coating depends on its thickness as well<br />
as the temperature and moisture content of the soil, but is<br />
mainly dependant on the structure and morphology of the<br />
material. Morphological analysis performed on a sample<br />
that was directly sprayed onto the soil and exposed for six<br />
months—see Figure 3—shows a different pattern depending<br />
on its exposure. The side exposed to solar radiation does<br />
not differ significantly from the original film, and there is<br />
no indication of degradation. The surface facing the soil,<br />
however—see Figure 3—consists almost exclusively of fibers,<br />
thus indicating that degradation begins in the polymeric<br />
component of the material. These results show that the<br />
biodegradation process occurs more rapidly where there is<br />
direct contact between the film and micro-organism and the<br />
remaining fibers act as a barrier, modulating the environmental<br />
duration of the blend thus promoting slow release of fertilizers.<br />
More recently tests were carried out using novel biodegradable<br />
containers for seedlings. The objectives of this research are to<br />
develop new biodegradable materials starting from renewable<br />
biobased raw products, and to engineer the properties of these<br />
materials so that they can be used to produce biodegradable<br />
plant pots that guarantee no damage to roots, no transplant<br />
shock, an enhancement to plant growth, and the slow release<br />
of fertilizing protein-based compounds during their degradation.<br />
The preparation of these novel biodegradable polymeric<br />
materials began from an aqueous solution of protein hydrolysate<br />
(derived from waste products of the leather industry), PEGDGE,<br />
and natural fillers (i.e., sawdust or wood flour). Compounding<br />
was then performed in a Brabender mixer at 60°C and<br />
subsequently the pots were prepared by compression molding<br />
the biocomposites (which were the consistency of a paste)<br />
and drying them at 70°C: see Figure 4. It was found that the<br />
biodegradable containers for seedlings showed good resistance<br />
during the first stage of use (i.e., when the seedlings were grown<br />
from seed, before transplanting): see Figure 5(a). After the<br />
transplant, the containers (which were buried in soil) degraded<br />
in roughly two weeks, allowing the roots to pass through<br />
the container walls and thus enabling the overall growth of<br />
the plants: see Figure 5(b). As a result of the slow release of<br />
proteinaceous material, the containers showed a soil-positive<br />
fertilizing effect.<br />
To test the efficacy of this approach, the researchers<br />
implemented them in the cultivation of pepper plants. At<br />
harvest, the mean height of the pepper plants grown inside the<br />
biodegradable pots was 0.94m. In contrast, the control plants<br />
(grown in non-biodegradable containers) were characterized<br />
by a mean height of 0.67m [3].<br />
In summary, the newly developed biodegradable sprayable<br />
mulches and plant pots (for transplanting seedlings) could<br />
promote valid ecologically sustainable cultivations, enhance the<br />
protection of the landscape against pollution in rural areas, and<br />
increase the use of renewable non-oil raw materials.<br />
The teams are currently experimenting with these<br />
approaches by applying them to different cultivations. They<br />
hope to prove the feasibility of their novel biodegradable<br />
materials by investigating their functionality as well as their<br />
physicochemical and mechanical behavior in standard and<br />
controlled experimental field conditions, and by following their<br />
biodegradation process during plant cultivation.<br />
References<br />
[1] L. Sartore, G. Vox, E. Schettini, 2013. Preparation and Performance of<br />
Novel Biodegradable Polymeric Materials Based on Hydrolyzed Proteins for<br />
Agricultural Application J. Polym. Environ. 21 (3), pp718-725. doi: 10.1007/<br />
s10924-013-0574-2<br />
[2] E. Schettini, L. Sartore, M. Barbaglio and G. Vox, Hydrolyzed protein based<br />
materials for biodegradable spray mulching coatings. Acta Horticulturae<br />
(ISHS) 952, pp.359-366, 2012.<br />
[3] L. Sartore, E. Schettini, F. Bignotti, S. Pandini, and G. Vox, Biodegradable<br />
plant nursery containers from leather industry wastes, Polym. Composite.<br />
2016. doi:10.10<strong>02</strong>/pc.24265.<br />
www.uniba.it | www.brescia.edu<br />
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