Radiometric Properties of Biodegradable Films for Horticultural Protected Cultivation

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1 Radiometric Properties of Biodegradable Films for Horticultural Protected Cultivation G. Vox, E. Schettini and G. Scarascia-Mugnozza Dipartimento PROGESA, Università di Bari via Amendola 165/a, Bari Italy Keywords: biodegradable films, starch based material, low tunnel, soil mulching, solar transmissivity, PAR transmissivity Abstract The use of low-density polyethylene plastic films for soil mulching and greenhouse covering has been increasing significantly in the world. In order to overcome the environmental pollution due to the plastic wastes production, innovative biodegradable materials, which can be disposed directly in the soil or in composting plant, have been introduced to replace polyethylene films. Biodegradable films are innovative materials, as a consequence their physical properties must be evaluated in order to assess if they meet the agricultural requirements. In the present research the radiometric properties of biodegradable starch based films were tested in laboratory and during experimental field tests. The biodegradable films were used as low tunnel covering and mulching films in a melon cultivation in Southern Italy. Two black biodegradable and one transparent mulching film were used in combination with two biodegradable films for low tunnel covering. Low density polyethylene films were used for comparison too. Laboratory radiometric tests carried out on the biodegradable films for low tunnel covering showed values of the solar transmissivity coefficient between 86 and 87% and values of the long wave infrared coefficient ranging from 10 to 16%. The yield of melon obtained using biodegradable films was comparable with the yield obtained using polyethylene films, especially using the black mulching film rather than transparent mulching films. INTRODUCTION The most commonly plastic films used for soil mulching and for greenhouse covering are made of low density polyethylene (LDPE) and ethylene vinyl acetate (EVA), produced with fossil raw resources. Agricultural plastic films are subjected to degradation due to their exposure to atmospheric agents such as solar radiation, wind, rain, high air temperature and relative humidity, and to pesticides application used during cultivation (Desriac, 1991; Lemaire, 1993). This results in the production of a huge quantity of plastic waste materials which needs a correct collection, disposal and recycling process with increasing costs for the growers and environmental impact. For an environmental friendly agricultural activity an alternative to the plastic films produced with fossil raw resources can be represented by films based on biodegradable and renewable agricultural raw materials (Kaplan et al., 1993; Malinconico et al., 2002). At the end of their life, biodegradable films can be disposed of directly in the soil or in a composting plant. Biodegradable materials which decompose in the soil are subjected to an accelerated degradation due to the action of micro-organisms such as bacteria, fungi and algae, and mineralise into carbon dioxide or methane, water and biomass. On the other hand, biodegradable materials can be incorporated with organic materials, such as food and vegetable residues and manure, in order to generate carbon-rich compost (Narayan, 2001). Research centres and industries have been involved recently in the development of biodegradable materials for agricultural applications (Tocchetto et al., 2002). Experimental tests are actually in progress in Europe in order to study the behaviour and the functionality of biodegradable films used for soil mulching and for low tunnel covering (Scarascia-Mugnozza et al., 2004). The radiometric properties of the films used for greenhouse covering or for soil mulching play an important role in relation with the Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS

2 capacity of the material to modify air and soil temperature under the film. The capacity of greenhouse covering materials, such as plastic films, to transmit solar radiation increasing the greenhouse internal air temperature in comparison with the external one is related to the radiometric properties of the covering material, mainly the transmissivity. Similarly soil temperature is affected strongly by the film transmissivity when films are used for soil mulching. Radiometric properties of greenhouse covering materials and of mulching films are generally defined by means of coefficients calculated as average values of the transmissivity for different wavelength intervals: the solar range, the photosynthetically active radiation (PAR) range and the long wave infrared radiation (LWIR) range (Papadakis et al., 2000). The transmissivity coefficient in the solar wavelength range (300 nm<λ<2500 nm) represents the fraction of solar radiation passing though the material. The higher is the value of the solar transmissivity coefficient the higher is the rise in temperature of the air inside the greenhouse or of the soil temperature under a transparent mulching film. The solar coefficient is calculated as the weighted average value of the transmissivity in the wavelength range nm using the spectral distribution of the solar radiation at the ground level as weighting function (ISO 9050, 1990). The radiation in the PAR range (400 nm<λ<700 nm) is necessary for photosynthesis which is the basic process for crop production (Monteith and Unsworth, 1990). As a consequence the value of the transmissivity coefficient in the PAR range, i.e. the quantity of PAR radiation transmitted by the covering material, has great importance in relation with the crop growth. The PAR coefficient is evaluated, similarly to the solar coefficient, as the weighted average value over the range nm. Black mulching films are opaque to the PAR radiation in order to reduce the weeds growth under the mulching film. Transparent mulching films produce a higher soil temperature in comparison with black mulching films but at the same time they allow the development of the weeds. The appropriate values of the transmissivity coefficients in the PAR and in the solar wavelength range, suitable to maximise crop production, depend mainly on solar radiation level of the site, the cultivation period and the type of the crop. The LWIR range extends for wavelength values higher of 3000 nm. However the LWIR transmissivity coefficient is defined as the average value of the transmissivity in the range nm where the bodies at ambient temperature have the maximum energy emission as expressed by the Planck s spectral distribution of emissive power (Siegel and Howell, 1972; Papadakis et al., 2000). The indoor greenhouse air temperature and the soil temperature under mulching film rise with the decrease of the LWIR transmissivity coefficient of the greenhouse material and of the mulching film, respectively. Biodegradable films for greenhouse covering and for soil mulching are new materials, produced with biodegradeable raw materials, such as starch, different from fossil raw materials. As a consequence radiometric properties of these innovative films must be investigated deeply in order to assess if they meet the agricultural requirements such as traditional LDPE and EVA films actually used in crop protection. The objective of the present paper is to analyze the radiometric characteristics and the crop productive performance of these innovative biodegradable films by means of field tests and laboratory tests. MATERIALS AND METHODS In order to evaluate the radiometric properties of the biodegradable films, field and laboratory tests were carried out. A full scale field test was conducted using the innovative biodegradable transparent and black films as mulching films, and the biodegradable transparent films as covering for low tunnels. The films were tested with a melon crop, at the experimental field of the University of Bari in Policoro (Matera), Southern Italy, on a flat area of 2300 m 2 having latitude 40 13' N, longitude 16 40' E, altitude 31 m. Five different starch based biodegradable films were used during the tests and they were compared with four traditional LDPE films (Table 1). The biodegradable films were manufactured by three industrial companies cooperating in the research project. The starch based raw material was supplied by Novamont Company (Novara, Italy), while the extrusion was made by Pati Company (San Zenone degli Ezzelini, Treviso, Italy) for the 576

3 mulching films named M1, M2 and M9 and by Plastika Kritis Company (Heraklion, Crete, Greece) for the low tunnel films named L1 and L2. Besides, a transparent commercial low tunnel LDPE film named L0, manufactured by Plastika Kritis Company and three commercial LDPE mulching films, named M0, M0T and M0F manufactured by Pati Company, were used for functionality and agronomical comparison. The nine films were combined in six plots replicated three times in a randomized block design: M0-L0, M0T-L0, M0F-L0, M1-L1, M2-L2, M9-L2. On 19 March 2003 mulching films were laid mechanically and a week later covered manually with low tunnel films installed on steel arches, 60 cm high (Fig. 1). The longitudinal axis of the low tunnels was North-South oriented. On 1 April melon plants were transplanted. The harvesting was carried out from the second half of June, During the test both the external and the inside climatic parameters of the low tunnels were recorded by means of an electronic system composed of a data logger provided with sensors. The Tecno El data logger measured the parameters with a 60 s frequency and stored the hourly average values. Air temperature and relative humidity, bare soil temperature, wind velocity and direction, solar radiation and amount of rain precipitation were recorded as external climatic parameters of the site. Inside microclimatic parameters under the low tunnels, as air temperature and relative humidity, mulched soil temperature and humidity at a depth of 10 cm were measured and recorded. Platinum resistance thermometers were used as temperature sensors, a Schenk pyranometer was used to measure the solar radiation in the wavelength range of µm. The radiometric properties of the films were analysed by means of spectrophotometers at the Department PROGESA of the University of Bari. The transmissivity in the wavelength range between 300 nm and 2500 nm was measured by means of a Perkin- Elmer UV-VIS-NIR Lambda 950 spectrophotometer, an integrating sphere was used to evaluate the diffuse fraction of the transmitted radiation. Total transmissivity and direct transmissivity were measured during the laboratory tests, the diffuse transmissivity can be evaluated by subtracting the direct transmissivity from the total transmissivity. The transmissivity coefficients were calculated as weighted average values of the transmissivity over the wavelength interval between 300 and 2500 nm for the solar range, and between 400 and 700 nm for the PAR range, using the spectral distribution of the terrestrial solar radiation as weighting function. The transmissivity in the LWIR range between 2500 nm and nm was measured by means of a Perkin-Elmer FT-IR 1760 X spectrophotometer. The LWIR transmissivity coefficients were calculated as average values over the wavelength interval between 7500 nm and nm. RESULTS AND DISCUSSIONS The values of the transmissivity coefficients of the tested films in the solar, in the PAR, and in the LWIR ranges are shown in Table 2. Concerning the mulching materials the transparent films M0T (LDPE), M0F (LDPE) and M9 (biodegradable) showed a value of the solar transmissivity coefficient equal to 91.7 %, 79.5 % and 87.4% respectively, while black LDPE film M0 and the biodegradable black films M1 and M2 showed a very low transmissivity coefficient (<4%) in the solar radiation range. The biodegradable film M9 showed a higher capacity to diffuse solar radiation in comparison with LDPE films M0T and M0F since M9 had a lower value of the solar direct transmissivity coefficient, it was in fact equal to 13.8% for film M9, equal to 85.4% for M0T and to 72.0% for M0F. Similar results were recorded for the PAR total and diffuse transmissivity coefficients (Table 2). The LWIR transmissivity coefficient of the mulching films ranged from 11.2% for the biodegradable film M1 to 83.1% for the LDPE film M0F. Concerning the low tunnels covering materials the highest value of the solar transmissivity coefficient, equal to 90.4% was reported for the LDPE film L0, while biodegradable films L1 and L2 showed a value of 86.1% and 87.5% respectively. The diffuse fraction of the transmitted radiation was higher for the biodegradable films L1 and L2 which recorded a value of the solar direct transmissivity coefficient equal to 10.5% and 12.4% respectively, in comparison with LDPE film L0 that showed a value of 68.8%. The LWIR transmissivity coefficient of the low tunnel films was lower for the 577

4 biodegradable films L1 (9.9%) and L2 (15.4%) in comparison with the LDPE film L0 (39.7%). The measurements of the air temperature under the low tunnels and of the mulched soil temperature highlighted differences between the plots using different materials. Considering the plots having black mulching films and transparent low tunnel films, i.e. plots M0-L0 (LDPE materials), M1-L1 (biodegradable) and M2-L2 (biodegradable), a different behaviour was recorded between the biodegradable films and the LDPE films. The means of the maximum of the air temperatures recorded during an observation period of two weeks (Fig. 2) showed that the highest value, 32.5 C, was recorded for M0-L0 plot, it was caused by the higher value of the solar transmissivity coefficient of the film L0 that compensates also the higher value of the LWIR transmissivity of the LDPE film L0. Besides L0 film was significantly less diffusive of the films L1 and L2 and it caused an increase of the soil heating due to the higher fraction of direct solar radiation falling on the mulched soil. Among the biodegradable films L1 and L2, the latter showed a higher value of the mean maximum air temperature since it had a higher value of the solar transmissivity coefficient. Minimum average values of the air temperature during the night were similar for the three plots since the biodegradable film L1 and L2 had a lower value of the LWIR transmissivity in comparison with the LDPE film L0 and it compensated the lower quantity of energy accumulated during the day under the biodegradable films. Similarly to the air temperatures the highest value of the average maximum soil temperature was recorded for L0 due to the higher fraction of solar radiation, total and direct, falling on the mulching film (Fig. 3). Considering plots with transparent mulching films and transparent low tunnel films, i.e. plots M0T-L0, M0F-L0 and M9-L2, the plot using the LDPE film L0 as low tunnel covering recorded higher values of the average maximum temperature, due to the higher value of the solar transmissivity coefficient of the film L0 (Table 2). A lower difference was recorded during the night for the mean minimum air temperatures between the two plots covered with L0 film and the plot covered with film L2, due to the lower LWIR transmissivity of the L2 low tunnel film in comparison with the LWIR transmissivity of the film L0. The comparison between the plots using transparent mulching films and the plots using black mulching films showed that both mean maximum and minimum soil temperatures (Fig. 3) were higher under the transparent mulching films as it was to be expected. Concerning the yield, field measurements showed (Fig. 4) that the highest value was recorded for the plot M0F-L0 since it reached sufficient high air and soil temperatures, controlling at the same time the weeds growing. Differently the weak weed control limited the performance of the plots M0T-L0 and of M9-L2 since the films M0T and M9 presented a higher value of the PAR transmissivity in comparison with the film M0F. CONCLUSIONS Biodegradable films represent an economical and environmentally friendly solution for the problem of the disposal and management of the plastic wastes generated by the use of LDPE films. The functionality tests carried out in the research on their radiometric properties showed that biodegradable films can be used to replace low density polyethylene films for low tunnel covering and for soil mulching. Biodegradable films present a good capacity to induce the greenhouse effect because they have a very low value of the transmissivity in the LWIR range. On the other hand the biodegradable films have a lower transmissivity in the solar range in comparison with LDPE films, the research should be addressed in the future in order to improve the solar transmissivity by reducing the thickness of the materials. ACKNOWLEDGMENTS The present work has been carried out under the project Environmentally friendly mulching and low tunnel cultivation - BIOPLASTICS funded by the European Commission (EC RTD QLRT, Contract n QLK5-CT ). 578

5 The contribution to programming and executing of this paper must be equally divided between the authors. Authors thank M. Cosmo, technician of the Department PROGESA - University of Bari, for his cooperation in the spectrophotometric measurements. Literature Cited Desriac, P The effect of pesticides on the life of greenhouse film. Plasticulture, 89: ISO Glass in building determination of light transmittance, solar direct transmittance, total solar energy transmittance and ultraviolet transmittance, and related glazing factors. International Organization for Standardization Kaplan, D.L., Mayer, J.M., Ball, D., McCassie, J. and Stenhouse, S Biodegradable polymers and packaging. Technomic Publishing Co., Lancaster, PA. Lemaire, J Control of the weathering of polymers in plasticulture. Plasticulture, 97: Malinconico, M., Immirzi, B., Massenti, S., La Mantia, F.P., Mormile, P. and Petti, L Blends of polyvinylalcohol and funcionalized polycaprolactone. A study on the melt extrusion and post-cure of films suitable for protected cultivation. J. of Materials Science, 37: Monteith, J.L. and Unsworth, M Principles of environmental physics. Arnold. London. Narayan, R Drivers for biodegradable/compostable plastics and role of composting in waste management and sustainable agriculture. Bioprocessing of Solid Waste and Sludge, 1 (1), Papadakis, G., Briassoulis, D., Scarascia-Mugnozza, G., Vox, G., Feuilloley, P. and Stoffers, J.A Radiometric and thermal properties of, and testing methods for, greenhouse covering materials. J. of Agric. Eng. Research, 77 (1): Scarascia-Mugnozza, G., Schettini, E. and Vox, G Effects of solar radiation on the radiometric properties of biodegradable films for agricultural applications. Biosystems Engineering, 87(4): Siegel, R, and Howell, J.R Thermal radiation heat transfer. McGraw-Hill. NY. Tocchetto, R.S., Benson, R.S. and Dever, M Outdoor weathering evaluation of carbon-black-filled, biodegradable copolyester as substitute for traditionally used, carbon-black-filled, non-biodegradable, high-density polyethylene mulch films. J. of Polymers and the Environment, 9 (2):

6 Tables Table 1. Films used in the research. Film thickness (µm) material colour Mulching films M0 40 LDPE black M1 20 biodegradable black M2 15 biodegradable black M0F 40 LDPE smoke-grey M0T 40 LDPE transparent M9 20 biodegradable transparent Low tunnel films L0 40 LDPE transparent L1 40 biodegradable transparent L2 40 biodegradable transparent Table 2. Transmissivity coefficients of the tested films; PAR, photosynthetically active radiation; LWIR, long wave infrared radiation. Film Transmissivity coefficient (%) solar total solar direct PAR total PAR direct LWIR M M M M0F M0T M L L L

7 Figurese Fig. 1. Low tunnels and mulching films on the experimental field at the end of April Maximum Minimum 27.8 Temperature ( C) M0-L0 M1-L1 M2-L2 M0T-L0 M0F-L0 M9-L2 Plot Fig. 2. Mean of the maximum and minimum air temperature measured under the low tunnels during the period 16-30/4/

8 Maximum Minimum Temperature ( C) M0-L0 M1-L1 M2-L2 M0T-L0 M0F-L0 M9-L2 Bare soil Plot Fig. 3. Mean of the maximum and minimum soil temperature measured under the mulching films during the period 16-30/4/ number weight (kg) Number of melons Weight (kg) M0-L0 M1-L1 M2-L2 M0T-L0 M0F-L0 M9-L2 Plot 0 Fig. 4. Marketable yield: number of melons per plot and weight (kg) per plot. 582