Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment

Polymer Degradation and Stability 91 (2006) 2801e2808 www.elsevier.com/locate/polydegstab

Mechanical properties decay and morphological behaviour of biodegradable films for agricultural mulching in real scale experiment Giacomo Scarascia-Mugnozza a, Evelia Schettini a, Giuliano Vox a, Mario Malinconico b,*, Barbara Immirzi b, Stefania Pagliara b a

Department of Engineering and Management of the Agricultural, Livestock and Forest Systems (PROGESA), University of Bari, Via Amendola 165/a, 70126 Bari, Italy b Institute of Chemistry and Technology of Polymers, CNR, Via Campi Flegrei, 34 e Comprensorio Olivetti, 80078 Pozzuoli (Napoli), Italy Received 17 October 2005; received in revised form 20 January 2006; accepted 10 February 2006 Available online 21 June 2006

Abstract The use of plastic materials in agriculture causes the serious drawback of huge quantities of waste. The introduction of biodegradable materials, which can be disposed directly into the soil, can be one possible solution to this problem. Biodegradable materials are actually innovative materials; therefore, their physical properties must be evaluated in relation to their functionality during the use in field. In the present research results of experimental tests carried out on biodegradable films used in strawberries protected cultivation are presented. The decay of some relevant physical parameters of biodegradable films during the cultivation period was monitored by laboratory tests (SEM analysis, mechanical tensile tests and infrared reflectance spectroscopy). Infrared spectroscopy clearly indicated that the mechanical degradation starts from the starch component of the material. Tensile tests showed that the value of elongation at break of biodegradable materials decreased in some cases by 300% after 10 days of field application. Ó 2006 Published by Elsevier Ltd. Keywords: Biodegradable material; Agriculture; Mulching film; Physical properties; Degradation; Solar radiation

1. Introduction The world consumption of plastic materials in agriculture amounts yearly to 6.5 million tons; more than 10% of the total consumption refers to plastic films for soil mulching [1]. The use of plastic films for soil mulching mainly for horticultural cultivation reduces the growth of the weeds, irrigation water consumption, the washout of nutrients into the ground water, the development of the plant diseases coming from the soil and the use of pesticides. Therefore, soil mulching contributes to a more sustainable agricultural production. Unfortunately the lifetime of plastic mulching films is reduced owing to their prolonged exposure to climatic agents

* Corresponding author. Tel.: þ39 081 8675212; fax: þ39 081 8675230. E-mail address: [email protected] (M. Malinconico). 0141-3910/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2006.02.017

such as solar radiation, rain, hail, wind, high air temperature, high air relative humidity and by chemical products used during the crop cycle. As a consequence, due also to the necessity of soil tilling for agricultural needs, plastic mulching films can be used only for one or two cultivation periods, then they must be disposed off. Therefore, there is a huge consumption of oilbased material; moreover the recycling of mulching films is expensive and time-consuming due to the high labour cost for the collection, caused also by the direct contact of mulching film with the soil. Sometimes used plastic films are left on the field or burnt uncontrollably by the farmers producing the release of harmful substances with the associated obviously negative consequences to the environment [2]. A solution to this problem can be the introduction in agriculture of films produced with biodegradable raw materials such as starch [3e6]; biodegradable films can be disposed directly into the soil or into a composting system at the end of their lifetime [7,8].

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Biodegradation in the soil is a natural process due to the action of micro-organisms such as bacteria, fungi and algae. Residual breakdown products of biodegradable films after the cultivation period should not be toxic or persist in the environment, and should be completely mineralised into carbon dioxide or methane, water and biomass by means of soil micro-organisms in a reasonable time frame [9e11]. On the other hand such materials must be functional during use. Particularly they must maintain suitable mechanical and physical properties when exposed to climatic agents and pesticides during the crop cultivation [12e14]. The degradation of low density polyethylene (LDPE) agricultural films has been thoroughly investigated [9,15,16] while few studies have been made to study the variation of the physical and mechanical properties of starch-based biodegradable films for agricultural application [13]. The aim of the present research is to give an innovative contribution to the study of the behaviour of the starch-based biodegradable films in field, investigating the variation of some relevant physical parameters and the mechanism of the degradation. The starch-based biodegradable films were developed and used within the project ‘Environmentally friendly mulching and low tunnel cultivation’ funded by the European Commission.

2. Biodegradable materials A number of naturally occurring polysaccharides, proteins and polyesters, as well as synthetic biodegradable polymers are being considered for biodegradable products [17]. In addition, various polysaccharide and protein materials being considered for edible coatings and films for pharmaceutical applications also have potential as biodegradable packaging films. Starch-based polymers, as well as polyhydroxybutyrate/valerate (PHBV) copolymers and polylactic acid (PLA) polymers, are believed in particular to have considerable promise as biodegradable materials [18,19]. The five largest potential product areas for biodegradable resins (in no specific order) are shown in Table 1. For biodegradable materials to replace non-biodegradable synthetics, obtained from non-renewable sources, the basic mechanical, radiometric and/or barrier properties for the intended application will have to be matched. Use of conventional synthetic plastic processing technology will probably be necessary in order to ensure economic viability. In addition, the biodegradable characteristics must not limit product storage conditions, result in insect infestation, or produce safety problems in food applications. Thus, the challenge for biodegradable Table 1 Potential markets for biodegradable resins Product

Current resin consumption (Mtons)

Films Fibres and non-wovens Rigid and thermal forming Blow moulding Coated paperboard

4.6 1.6 3.6e4.5 2.0 10.6

polymer products is controlled lifetime, performing intended functions, remaining stable during storage and use, and then biodegrading at the intended time and conditions. In this research, the biodegradable films are made by a starch-based polymer produced by Novamont company (Novara, Italy), under the trade name Mater-Bi, especially formulated to meet the requirement for mechanical installation, duration according to type and time of cultivation, safe disposal directly into the cultivation soil upon rototilling. Several field tests carried out in Europe have shown that the material can be successfully applied for several crops. 3. Experimental tests 3.1. Materials and methods The functionality of starch-based biodegradable mulching films was investigated by means of cultivation field tests and by laboratory tests. The variation of mechanical and physical properties of the innovative biodegradable films during their use in field was studied in relation to the climatic parameters at the field site. 3.1.1. The field test The full scale cultivation test was conducted at the experimental station of the University of Bari in Policoro (Matera), on a flat area of 5100 m2 having latitude 40
130 N, longitude 16
400 E, altitude 31 m. The trial was carried out using biodegradable black mulching films during a protected cultivation of strawberry in Southern Italy from September 2001 to July 2002. The black mulching biodegradable films were manufactured in co-operation with three industrial companies participating in the research project. The starch-based raw material (Mater-Bi), made of destructurised starch complexed with biodegradable polyesters [3,20], was supplied by Novamont company while the extrusion was made by Pati company (San Zenone degli Ezzelini, Treviso, Italy) for the mulching films named M1 and M2, and by Plastika Kritis company (Heraklion, Crete, Greece) for M3 film. Mater-Bi grades can contain renewable raw material in a range which is between 30 and 90% depending on the application and the required performances. In addition, a commercial LDPE black mulching film produced by Pati company was used for comparison. This film was chosen because it is a commercial mulching film widely used for strawberry cultivation in Southern Italy. Some characteristics of the mulching films are reported in Table 2. Table 2 Characteristics of the mulching films

Average thickness (mm) Max thickness (mm) Min thickness (mm) Film width (mm) Density (kg/dm3) Carbon black (ppm)

M2 black

M1 black

M3 black

35 39 30 1400 1.29

45 53 39 1400 1.29

25 26 22 1400 1.29

32,000

32,000

90,000

LDPE black 50 54 46 1400 0.94 32,000

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Several cultivation blocks were realised with different biodegradable mulching films and with traditional LDPE materials for comparison; the cropping methods were the same for all the blocks. Mulching films were installed mechanically in September 2001 and strawberry plants were transplanted a few days after. Mulching installation consists of laying the film on top of the planting site burying the mulching film edges (Fig. 1). Thus a part of the mulching film remains buried in the soil (coded ‘‘in-soil’’) and the other part of film (coded ‘‘out-soil’’) has a face towards the soil (coded ‘‘out-soil D’’) and the opposite face towards the air (coded ‘‘out-soil U’’). At the end of January 2002, after about 130 days of exposure in field, transparent biodegradable films supported by steel arches were installed on the mulched soil for the strawberry protection with low tunnel. Until the middle of May 2002 the low tunnel films remained on the mulched soil. At the end of July 2002, after strawberries harvesting, the soil was tilled in order to shatter the biodegradable mulching films and to bury them with plant residues inducing degradation in the soil. During the test the weather conditions and microclimatic parameters of the soil and of the air inside the low tunnels were automatically recorded by means of sensors and a data logger. Data were acquired with a 60 s frequency and recorded as average hourly values. Ambient air temperature and relative humidity, wind velocity and direction, solar radiation falling on the mulching films and amount of rain precipitation were gathered as weather conditions of the site. Besides the values of the air temperature and of the relative humidity under the low tunnels, of the soil temperature and of the soil dampness under the mulching films were measured and collected. Platinum resistance thermometers were located under the mulching films in order to measure the temperature of the soil in contact with the mulching film; soil dampness was measured by tensiometers. A Schenk pyranometer was used to measure the solar radiation in the wavelength range 0.3e3 mm. 3.1.2. The laboratory tests Morphological and mechanical analyses were performed on film samples gathered at different times of exposure during the experimental crop cycle. Mulching film samples were collected periodically, with higher frequency at the beginning of the test when the degradation of the materials is expected to be faster. Scanning electron microscopy (SEM) analysis was performed by using a SEM Philips model XL 20 on film surfaces coated with Au/Pd alloy. Different types of surface were analysed: the surface of the film buried inside the soil (in-soil); the face towards the soil (out-soil D) and the opposite face (outsoil U) of the film outside the ground. Before the analysis, the samples were washed in ethanol in an ultrasonic bath for 10 min and then dried in air. Additionally, tensile tests were performed on in-soil and out-soil samples, taken always in the film extrusion direction (MD direction), having checked that the properties in MD directions are more regular than those in transverse direction. Tensile tests were executed on samples gathered in different

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periods of the cultivation trial, on specimens of 4 mm width at a crosshead speed of 100 mm/min. Young’s modulus of elasticity (E ), stress at break (sB) and deformation at break (3B) were evaluated to obtain information on the ageing of the materials. Moreover, a qualitative surface analysis by attenuated total reflectance (ATR) infrared spectroscopy at two different times of exposure has been carried out to evaluate chemical modifications occurring on film surface. The ATR attachment uses a multiple reflection system (Benchmark from SPECAC, UK) with horizontal geometry equipped with a KRS-5 crystal (the angle of incidence was 45
, and the number of reflections was equal to 6). 3.2. Results and discussion SEM analysis and infrared reflectance spectroscopy of film surfaces, decay of mechanical tensile parameters executed on the film samples taken in real conditions are considered as very effective tests to evaluate variations of the films’ behaviour during their use in field. Micrometric evaluations did not show relevant variations in overall thickness of different films in the course of degradation process, even in the part of the films left under soil. This observation may shed some light on the degradation mechanism, which must be confirmed by the following microscopic observation. The SEM analysis of new M1, M2 and M3 films reveals a rather homogeneous dispersion of gelatinised starch particles embedded in a continuous matrix (constituted by the synthetic polymeric component of the formulation) (Fig. 2). The surfaces of the in-soil samples taken from M1, M2 and M3 films at the end of the cultivation period are shown in Fig. 3. The samples’ surface clearly shows the presence of holes whose diameters closely resemble those of the original starch particles. This finding can explain why there is no reduction in thickness: the soil micro-organisms evidently ‘‘eat’’ the starch first, leaving a sort of porous (cheese like) surface with minimal variation in lateral dimension. In M1 in-soil film sample (Fig. 3a) it is still possible to see the network of micro-organisms (probably fungi) lying on the film surface and protruding their hyphae. Different behaviour is recorded for the part of the film outside the soil. In this case we have analysed the surfaces facing the sunshine (out-soil U) and facing the soil (out-soil D) at the end of cultivation. The three films behave similarly, so we report only one example, related to M2. The out-soil U portion is reported in Fig. 4a and it is evident that no holes are found on the surface, and only some swelling of the starch particles occurs, compared to the new film (Fig. 2b), probably due to the effect of humidity from the soil. The portion out-soil D of the film, being in contact with soil, seldom shows some evidence of micro-organisms but any eventual degradation is strongly delayed, compared to the in-soil portion of the films (Fig. 3a), as evidenced by the absence of holes. ATR Spectroscopy performed on biodegradable film M1 at the beginning of soil deposition and after 59 days for in-soil and out-soil portions of the film is reported in Fig. 5. It is

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Fig. 1. Mulching films in the experimental field in October 2001.

evident how the most relevant difference among the spectra is the diminution of the peak centred at 3200 cm1, which corresponds to the eOH stretching vibration of the starch component. This finding is consistent with the morphological observation of the disappearance from the surface of the starch granules. From the spectra it is also evident that the inside soil film has a less amount of starch left than the outside soil film, as expected due to the microbial nature of the attack. Concerning the mechanical properties of the biodegradable films it is worth to highlight that while the initial modulus (not reported) seems to be less influenced by ageing, the parameters at break, as elongation at break and stress at break, are more significant for the evaluation of the effect of degradation in such highly deformable materials as the mulching films under investigation. Table 3 reports the values of the elongation at break (3B) and stress at break (sB) of the out-soil and in-soil portions of biodegradable films together with the cumulated solar radiation falling on the films and the cumulated soil temperature under the mulching films. Cumulated solar radiation is the quantity of the solar energy falling on the mulching film during the in field exposure period of the biodegradable materials. In the same way the cumulated temperatures measured in the soil under the different biodegradable mulching films were calculated. Data reported in Table 3 were restricted to the first 124 days of exposure in field, because after this period it was difficult to carry out tests in laboratory on biodegradable film samples, particularly those of M3, due to the state of degradation. Since the film installation consists of burying its edges, the portions of the biodegradable films inside the soil biodegrade due to the action of soil-borne micro-organisms present in the soil. Even though laboratory mechanical tests were difficult to be carried out after 124 days of exposure, the biodegradable films satisfied their mulching function for the required cultivation period of 9 months. In fact the biodegradable films hugged the soil bed by stretching film and covered the cultivation soil beds from planting to harvesting. The variation of the elongation at break (3B) as a function, respectively, of cumulated soil temperature and solar radiation during the testing period is calculated in order to correlate

Fig. 2. SEM micrographs of the new biodegradable films: M1 film (a); M2 film (b); M3 film (c).

mechanical properties of the biodegradable materials and the weather conditions of the experimental field site (Figs. 6 and 7). The cumulated soil temperature and the cumulated solar radiation are calculated from the day of the mulching installation as sum of the hourly values. The sudden decreases of the elongation at break of the in-soil portions of the biodegradable films M1 and M2 are considerable in the early period of ageing while there is very little variation beyond the cumulated soil

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Fig. 4. SEM micrographs of the out-soil portions of M2 biodegradable film after 269 days of exposure in field: out-soil U (a); out-soil D (b).

Fig. 3. SEM micrographs of the in-soil portions of the biodegradable films after 269 days of exposure in field: M1 film (a); M2 film (b); M3 film (c).

temperature limit point of about 66  103 K (Fig. 6a); the in-soil portion of M3 shows a lower variation of the elongation at break for all the period analysed, starting anyway from a lower value of the elongation at break for the new material (Fig. 6a). The out-soil portions of biodegradable films exhibit a slighter decrease for all the stage of the study in comparison with the in-soil portions (Fig. 6b). In fact, the elongation at break of M1 and M2 films decreases strongly before a cumulated soil

temperature limit of 215  103 K and slowly after this limit. The behaviour of M3 film is not well defined in all the observation period: almost constant up to the cumulated soil temperature limit point of 215  103 K and unstable after this point. The correlation between the elongation at break of the outsoil portion of the films and the cumulated solar radiation shows a considerable decrease of the elongation at break especially for the films M1 and M2 in the first period of ageing, up to the cumulated solar radiation limit point of 440 MJ/m2. M3 exhibits an almost constant value of the elongation over the cumulated solar radiation up to a limit point of 293 MJ/m2; after this point the variation is unstable (Fig. 7). Possible explanation of the behaviour of M3 film may be related to some defects of fabrication of the film which is thinner, and hence more difficult to process. The degradation of the biodegradable films is also affected by the soil moisture [7,14]. The suitable parameter generally applicable in different soil conditions useful to define the soil moisture is the soil water potential or tension, measured with tensiometers. During the cultivation period water and fertilizers were supplied to the mulched soil by a drip irrigation system controlled by tensiometers, placed under the mulching films. The soil water potential was maintained at a constant

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Fig. 5. ATR Spectroscopy performed on biodegradable M1 film new and after 59 days for in-soil and out-soil portions of the film.

pressure of 30 kPa measured by tensiometers characterised by pressure ranging from 0 kPa for the wet soil to 100 kPa for the dry soil. The humidity soil level was maintained quite high by the irrigation system action for crop requirements. At the same time a rain precipitation of 470 l/m2 was recorded during the film exposure time in field. Water irrigation and rain did not significantly reduce the mulching function of the biodegradable films tested.

Biodegradable starch-based mulching films are materials at an experimental stage, thus their requirements concerning the mechanical properties are not defined by international standards. Traditional plastic films for mulching, transparent, thermic or black, used in agriculture and horticulture must be in accordance with European Standard [21]. The Standard states that black plastic mulching films must have a tensile stress at break higher than 16 MPa and a tensile elongation at break

Table 3 Variation of the mechanical parameters of biodegradable mulching films as a function of the exposure time in field Exposure time (days)

Cumulated solar radiation (MJ/m2)

Cumulated soil temperature (K)

Out-soil sB (MPa) (20%)

3B (%) (20%)

sB (MPa) (20%)

3B (%) (20%)

M1 0 10 20 31 45 59 98 124

0 154 293 440 597 707 963 1155

0 66,067 136,694 214,436 311,963 408,863 671,306 844,828

11 8 7 9 6 11 9 6

380 247 166 145 96 108 98 44

11 4 3 4 4 4 1 1

380 37 37 14 8 8 21 34

M2 0 10 20 31 45 59 98 124

0 154 293 440 597 707 963 1155

0 66,138 137,356 215,063 312,649 409,631 672,423 846,088

11 10 11 9 10 13 6 7

262 180 159 111 173 207 164 153

11 3 8 7 6 6 2 4

262 11 72 28 56 11 30 90

M3 0 10 20 31 45 59 98 124

0 154 293 440 597 707 963 1155

0 67,167 138,116 215,706 313,260 410,254 673,170 846,946

14 17 19 14 8 19 2 14

94 95 94 80 48 89 32 126

14 19 4 5 6 15 5 2

94 88 20 12 10 57 57 23

sB, Stress at break; 3B, elongation at break.

In-soil

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a

400 M1 M2 M3

350 300

Elongation at break εB (%)

Elongation at break εB (%)

400

250 200 150 100 50

250 200 150 100 50

M1 M2 M3

350 300 250 200 150 100 50

6 1.

2E

+0

6 1.

0E

+0

5 8.

0E

+0

5 6.

0E

+0

5 +0 0E 4.

2.

0E

+0

0

5

0 +0

200

400

600

800

1,000

1,200

1,400

Cumulated solar radiation (MJ/m2)

5

5

0E +0 9.

7.

8.

0E

+0

+0

5

5 0E

5

0E +0 6.

+0 0E

+0 4.

5.

0E

0E

+0

5

5

05 0E +

+0

2.

0E 1.

3.

5

0 +0 0E 0.

Elongation at break εB (%)

400

0E

300

0

Cumulated soil temperature (K)

0.

M1 M2 M3

350

0

0

b

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Fig. 7. Elongation at break of the out-soil portions of biodegradable films as a function of cumulated solar radiation during the test.

for M1, 19.2 t/ha for M2, 18.2 t/ha for M3, while the lowest yield, 17.8 t/ha, was reported for the LDPE film. Also the earliness at the first day of the harvest was significantly different among the theses: at 23 April 2002, M3 recorded 35.7% of its total yield, M2 recorded 31.2%, M1 recorded 25.1% while LDPE recorded 20.8%. Additionally, observations on the experimental field area were carried out during and after the cultivation period until July 2005. The degradation rate of the biodegradable residues buried in the soil showed that 12 months after the tillage, the weight of the film residues present in the soil was less than 4% of the initial weight of the installed film. Moreover, ecotoxicity tests showed no evidence of ecotoxicity in the soil after the burying of the biodegradable film [22].

Cumulated soil temperature (K) Fig. 6. Elongation at break of the in-soil (a) and the out-soil portions (b) of biodegradable films as a function of cumulated soil temperature during the test.

higher than 180e250% ranging as a function of the thickness. The tested biodegradable materials showed lower values of tensile stress and elongation at break, especially the M3 film, in comparison with the values of the European Standard (Table 3). However, their mechanical properties were sufficiently in the range necessary to be used in the period from planting to harvesting applying the same cultivation techniques currently used for plastic mulching films. The biodegradable mulching films were installed without any problem by means of a tractor moving at the same speed and following the same procedure generally used for traditional films. As mentioned above, during the 9 months of experimental crop cycle the edges of the biodegradable films buried continued to satisfy their function to hug the soil bed by stretching the film. Besides all the biodegradable mulching films used during the field tests continued to have their mulching function to avoid the direct contact of the berries with the soil. The parts of mulching films in contact with the air underwent the decreasing of their mechanical properties even though no relevant holes were detected and films continued to have their functionality. The agronomic response of the innovative materials evaluated by means of the marketable total yield was higher in comparison with LDPE: 21.5 t/ha was reported

4. Conclusions The use of biodegradable materials in agriculture can promote sustainable and environmentally friendly cultivation reducing the contamination of the soil, enhancing the protection of the landscape in rural areas against pollution, and increasing the use of renewable non-oil raw materials such as starch. At the end of their lifetime, in fact, these biodegradable materials can be shattered and buried in the soil or disposed in composting system, instead of being left or burnt without any control in the country, or collected for an expensive and energy consuming recycling. It has been shown that the initial performances and duration of starch-based films are consistent with their ‘‘safe’’ use for soil mulching in protected strawberry cultivation. Biodegradable films in a similar way to LDPE plastic films used for crop protection undergo variation of their physical and mechanical properties due to the prolonged exposure of the material to the climatic agents. Laboratory tests carried out in the present research indicated that the mechanical degradation starts from the starch component of the material. The type of degradation is essentially microbiological in nature, as evidenced by the different mechanical behaviours and morphological appearances of the same film whether sampled inside the soil or outside. The starch component is degraded

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first, leaving a sort of sponge-like material that can be safely buried in the soil for the final complete composting. The results obtained highlight how the biodegradable material continues to have the functionality needed for agricultural applications, during the crop cycle. The experimental tests, the data processing and the editorial work must be shared, within the competencies of the two research groups, equivalently among the authors.

Acknowledgements The present work was carried out under the project ‘‘Environmentally friendly mulching and Low tunnel cultivation e BIOPLASTICS’’ financed by the European Commission (EC RTD QLRT, Contract no. QLK5-CT-2000-00044). The comments of Dr. Francesco Degli Innocenti e Dr Sara Guerrini of Novamont Company are gratefully acknowledged. We thank Mr. Giuseppe Orsello of ICTP-CNR for technical support in morphological analysis. References [1] Joue¨t JP. Plastics in the world. Plasticulture 2001;120(2):108e26. [2] Picuno P, Scarascia-Mugnozza G. The management of agricultural plastic film wastes in Italy. In: Proceedings of the international agricultural engineering conference, Bangkok (Thailand); 6e9 December 1994. p. 797e808. [3] Bastioli C. Properties and applications of Mater-Bi starch-based materials. Polymer Degradation and Stability 1998;59:263e72. [4] Gasper M, Benko Z, Dogossy G, Reczey K, Czigany T. Reducing water absorption in compostable starch-based plastics. Polymer Degradation and Stability 2005;90:563e9. [5] Halley P, Rutgers R, Coombs S, Kettels J, Gralton J, Christie G, et al. Developing biodegradable mulch films from starch-based polymers. Starch 2001;53:362e7. [6] Lo¨rcks J. Properties and applications of compostable starch-based plastic material. Polymer Degradation and Stability 1998;59:245e9. [7] Chandra R, Rustgi R. Biodegradable polymers. Progress in Polymer Science 1998;23:1273e335.

[8] Narayan R. Drivers for biodegradable/compostable plastics and role of composting in waste management and sustainable agriculture. Bioprocessing of Solid Waste and Sludge 2001;11(1). [9] Feuilloley P, Cesar G, Benguigui L, Grohens Y, Pillin I, Bewa H, et al. Degradation of polyethylene for agricultural purposes. Journal of Polymers and Environment 2005;13(4):349e55. [10] Kaplan DL, Mayer JM, Greenberger M, Gross RA, McCarthy SP. Degradation methods and degradation kinetics of polymer films. Polymer Degradation and Stability 1994;45:165e72. [11] Swift G. Requirements for biodegradable water-soluble polymers. Polymer Degradation and Stability 1998;59:19e24. [12] Briassoulis D. An overview on the mechanical behaviour of biodegradable agricultural films. Journal of Polymers and the Environment 2004;12(2):65e81. [13] Scarascia-Mugnozza G, Schettini E, Vox G. Effects of the solar radiation on the radiometric properties of biodegradable films for agricultural applications. Biosystems Engineering 2004;87(4):479e87. [14] Tocchetto RS, Benson RS, 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. Journal of Polymers and the Environment 2002;9(2):57e62. [15] Nijskens J, Deltour J, Albrecht E, Grataud J, Feuilloley P. Comparative studies on the ageing of polyethylene film in the laboratory and in practical use. Plasticulture 1990;87(3):11e20. [16] Ramirez E, Martinez JG, Sanchez S, Balderas CL. Prediction of useful life of greenhouse films with artificial ageing equipment. Plasticulture 1995;105:5e12. [17] Okada M. Chemical syntheses of biodegradable polymers. Progress in Polymer Science 2002;27:87e133. [18] Krochta JM, De Mulder-Johnston CLC. Biodegradable polymers for agricultural products. In: Fuller G, McKeon TA, Billis DD, editors. ACS symposia ‘‘Agricultural materials as renewable resources non food and industrial applications’’; 1996. p. 121. [19] Narayan R. Polymeric materials for agricultural feedstock. In: Fishman ML, Friedman RB, Huang SJ, editors. ACS symposia ‘‘Polymers from agricultural coproducts’’; 1994. p. 2. [20] Bastioli C, Bellotti V, Gilli G. The use of agricultural commodities as a source of new plastic materials. In: Biodegradable packagings and agricultural films, Paris; May 1990. [21] EN-13655 Plastics: mulching thermoplastic films for use in agriculture and horticulture. Brussels: Comite´ Europe´en de Normalisation (C.E.N.); 2002. [22] Kapanen A. VTT Biotechnology Finland. Personal communication; 2005.