Agro-food industry waste Polysaccharides Renewable by-products Eco-sustainability Mechanical properties Transplanting

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Author's personal copy Resources, Conservation and Recycling 70 (2013) 9–19

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Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec

Recycled wastes of tomato and hemp fibres for biodegradable pots: Physico-chemical characterization and field performance Evelia Schettini a , Gabriella Santagata b,∗ , Mario Malinconico b , Barbara Immirzi b , Giacomo Scarascia Mugnozza a , Giuliano Vox a a b

Department of Agricultural and Environmental Science (DiSAAT) – University of Bari, Via Amendola 165/a, 70126 Bari, Italy Institute of Chemistry and Technology of Polymers, CNR, Via Campi Flegrei 34, Comprensorio Olivetti, 80078 Pozzuoli, Napoli, Italy

a r t i c l e

i n f o

Article history: Received 24 January 2012 Received in revised form 30 October 2012 Accepted 2 November 2012 Keywords: Agro-food industry waste Polysaccharides Renewable by-products Eco-sustainability Mechanical properties Transplanting

a b s t r a c t Wastes and by-products of agro-food industries and paper–textile manufacturing companies, such as tomato peels and seeds, and hemp, were glued with sodium alginate in order to produce biodegradable pots for plant transplanting in agriculture, thus aiming both to reduce such wastes and also to fight the accumulation of plastic pot wastes produced in plant nurseries. Laboratory tests performed on polysaccharide films and biocomposites based sheets prepared with the same materials developed for preparing the pots, were carried out in order to understand the chemico-physical correlations between resin, ionic crosslinking agent, reinforcing fibres and water. To this aim, mechanical tests, water vapour permeability tests, water up-take evaluations and morphological analysis were carried out. It was found a strong physical interaction between sodium alginate and calcium ions in the development of a threedimensional network. The crosslinked structure was able to physically entrap the reinforcement fibres by means of hydrogen bonding, as evidenced by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. SEM analysis performed on fracture surfaces of the biocomposites evidenced that the fibres were well embedded inside the three-dimensional network, even if their dispersion needed some improvements; EDS analysis revealed the presence of calcium in rather all the selected internal micro-zones, thus suggesting a well structured network. In order to assess the agronomic performance of the novel biodegradable pots in seedling transplanting activity, the pots were tested in real field condition during 2009 at the experimental farm of the University of Bari, Italy. From the analysis of the young plants transplanted in the field, it was inferred that the biodegradable containers had enhanced the roots plants development and the plant growing, avoiding transplant shock and root deformation. After the using time, they completely degraded into the soil within 2 weeks. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The agro-food industries and the paper–textile manufacturing companies produce huge amount of wastes and by-products handled as material of negative impact both in terms of the environmental concern, due to the increasing of pollutant disposal, and in terms of the industrial sustainability due to the high costs related to their management. In Europe, for example, the agro-food processing industries produce about 250 million tonnes per year of by-products and wastes (AWARENET, 2004), whereas only in Italy,

∗ Corresponding author at: Institute of Chemistry and Technology of Polymer (ICTP), National Council of Research (CNR), Via Campi Flegrei, 34-80078 Pozzuoli, NA, Italy. Tel.: +39 081 8675214; fax: +39 081 8675230. E-mail address: [email protected] (G. Santagata). 0921-3449/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.resconrec.2012.11.002

the paper and textile industries generate each year about 37 million tonnes of wastes (OECD/Eurostat, 2005). In the past most of the edible scraps were often recovered as raw material for the production of animal feed, compost and fertilizer; otherwise they were discarded in common dumps. Recently, in order to reduce the trash collection and disposal fees, new methods and policies for waste handling and treatment have been introduced (Riggi and Avola, 2010) to recover, recycle and convert the by-products and wastes into upgraded products (Federici et al., 2009; Laufenberg et al., 2003; Rousu et al., 2002). A valid example of this policy is represented by the European tomato industries that produce and process each year about 10 million tonnes of tomatoes; the deriving wastes, quantified in approximately 0.1 million tonnes per year, are made up of processing residues, fibrous parts, seeds and peels, containing high amounts of polysaccharides, such as pectin and other important chemicals like carotenoids, especially lycopene, whose antioxidant

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activity is well set up (Zhao et al., 2002). Their recovery should be highly cost-effective (Naviglio et al., 2008), particularly considering that lycopene cost is about 50,000 D per kg. Among wastes coming from agro-food–textile manufacturing, the fibres represent sizeable and functional component. In particular natural fibres, such as jute, kenaf, flax, hemp, and agriculture residues including stalks of most cereal crops, coconut fibres, peanut shells and tomato seeds and peels, are becoming very attractive as reinforcing fibres of biocomposites (Sreekumar et al., 2008). Natural fibres coming from wastes of agro-food industries and paper–textile manufacturing companies provide environmental and technological profits when used to reinforce composites both in terms of high strength and stiffness performance in low density materials, and in terms of positive environmental impact (Uma Devi et al., 2004; Lopez et al., 2012). In this paper, these fibres were employed as mechanical reinforcement of biodegradable polymers in the setting up of biodegradable pots that can be used for transplanting. The process of removing a plant from the place where it has been growing to another growing location, known as transplanting, is a worldwide cultural practice. Growers use pots and cell trays of different materials, sizes, shapes, and colours, to suit crop species, growing methods, and marketing strategies (Evans and Hensley, 2004; Evans and Karcher, 2004). At nurseries and greenhouses, seeds, bulbs, and young plants are allocated in cell trays or pots containing growing substrate both to start the growth under uniform cultivation conditions, with a better control of crop density and uniformity, and to preserve seeds, bulbs, and plants from soil pests and diseases, if compared to direct seeding in soil, thus improving the growth of vigorous seedlings and plants. One of the limitations of un-permeable and rigid containers is that roots tend to circle the outer perimeter of the root ball, which can result in reduced plant growth, health, and survival once transplanted (Evans and Karcher, 2004; Struve, 1993). So transplanting is necessary to allow a more natural development of plant’s root structure; anyway, during the transplanting, the roots can be damaged. Most of the containers used for transplanting are made of non-renewable oil-based raw materials, such as polystyrene, polyethylene and polypropylene, with suitable mechanical properties, chemical and microbial degradation resistance, durability, as well as low costs. After use, plastic pots result contaminated with soil, organic matter and agro-chemicals. Nevertheless the high costs related to a correct collection, disposal of and recycling process of post-use plastic pots, determine their wild neglecting in landfill or their uncontrolled combustion with the subsequent emission of toxic substances both into the atmosphere and into the soil. A valid alternative to the employment of petroleum based thermoplastic pots may be represented by the use of biodegradable pots (Evans and Hensley, 2004; Evans and Karcher, 2004; Horinouchi et al., 2008; Kowalska et al., 2002; Yamauchi et al., 2006; Wu et al., 2003). Once buried, the biodegradable pots are subjected to biodegradation process, being transformed in biomass and inorganic products (e.g., carbon dioxide and water). Biodegradable pots, already produced by worldwide companies, such as William Sinclair Horticulture Ltd. (Lincoln, England, http://www.william-sinclair.co.uk/), Enviroarc (Scoresby, Australia, http://www.enviroarc.net/), Fertil SA (Boulogne Billancourt, France; www.fertilpot.com), and Jiffy Products International AS (Stange, Norway, http://www.jiffygroup.com/), are made of plant fibre, rice, starch, grasses and vegetable oils, with a lifetime ranging from few months to 1–2 years. Although being biodegradable, they generally need a composting site to completely decompose (Trojanowsky and Huttermann, 2002). Moreover, some of these pots show unsuitable mechanical performances hindering the roots to pass throughout; in addition they may forth

odd smell and may be not cost effective if compared to plastic pots. Recently researches have been focusing the attention on the development of novel biodegradable and cost-competitive pots or multiple nurseries. These pots are biocomposites whose continuous phase is characterized by biopolymers coming from renewable and available origin, such as polysaccharides (Malinconico et al., 2002; Avella et al., 2007), and whose solid phase, dispersed within the polymeric matrix, is represented by natural fillers and fibres coming from wastes of agro-food and texˇ 2008). The arrangement tile processing industries (Simkovic, obtained from the phases combination produces a system with improved structural, mechanical and chemico-physical performance (Mohanty et al., 2000a; Hatami-Marbini and Pietruszczak, 2007). Among the wide range of biopolymers, polysaccharides can be used as binders for pots applications because they are biodegradable, biocompatible and non-toxic polymers, widely available and renewable (Malinconico et al., 2002; Avella et al., 2007; Schettini et al., 2007; Kapanen et al., 2008). Moreover, as a consequence of the presence of polar groups (OH− , NH3 + , COO− ) on macromolecular chains and following to the intrinsic physico-chemical properties, polysaccharides show high affinity with water molecules. As a matter of fact, they induce the development of hydrogels, i.e., threedimensional water stable networks, structured by means of ionic, covalent, thermoreversible or pH-reversible crosslinked processes (Coviello et al., 2007; Russo et al., 2007). In biodegradable pots carrying out, the natural fibres act as a reinforcement able to enhance the strength and stiffness of the resulting composite structures. Nevertheless, the mechanical performances of natural fibres are poorer if compared to those of the most widely used competing reinforcing man-made fibres. The conventional fibres, such as glass, carbon, and aramid, can be produced with a definite range of properties, whereas the characteristic properties of natural fibres depend on the plant source, plant age, separating technique, moisture content, etc. (Lewin and Pearce, 1985). Anyway, because of low density of natural fibres, the specific properties (property-to-density ratio), such as strength and stiffness, of plant fibres are comparable with the ones of glass fibres (Wambua et al., 2003). In literature, the influence of plant fibres on the mechanical properties of biodegradable polymers has been widely explored (Cyras et al., 2001; Hepworth and Bruce, 2000). Moreover the chemical similarity between polysaccharides and natural fibres, consisting mainly of cellulose, resulted in an increased tensile strength of the reinforced polymers. This finding has been ascribed to the good adhesion between fibre and matrix, promoted by the similar highly hydrophilic character of both components (Mohanty et al., 2000a). On the base of the above remarks, in this paper the development, the implementation, and the field test of innovative biodegradable pots realized by means of sodium alginate, as polymeric matrix, and tomato and hemp fibres, as natural reinforcing dispersed phase, are investigated. The mechanical performances of the new biocomposites strictly depend on the physico-chemical interactions among polymeric matrix and fibres (Mohanty et al., 2000b). To this aim, the hydrogen bonding between the active functional groups of the biocomposites components (Mohanty and Misra, 1995) could entail suitable effects (Shinde and Nagarsenker, 2009). Aim of the paper is both to investigate the functionality, the physico-chemical and mechanical behaviour of these novel biodegradable pots in standard and controlled experimental field conditions, and to follow the assessment of the biodegradation process of these pots during plant cultivation in soil.

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Fig. 1. Structure of alginate: ␣-l-guluronic acid (G) ␤-d-mannuronic acid (M) and MG block copolymer (a); schematic drawing of the egg-box model and calcium coordination to homopolymeric blocks of ␣-l-guluronate residues (b).

2. Materials and methods 2.1. Experimental part 2.1.1. Materials Sodium alginate, purchased from Lianyungang Zhongda Seaweed Industrial Co. Ltd. (XuGou, Lianyungang, Jiangsu, China), was characterized by 37 ± 1% of guluronic fraction. The weight average molecular weight (Mw) was equal to 1.2 × 106 Da and the number average molecular weight (Mn) was equal to 2.3 × 105 Da. In aqueous media the guluronate residues of sodium alginate are able to provide ionic crosslinking process in presence of multivalent cations, such as calcium ions (Draget et al., 2006) (Fig. 1). Due to the natural presence of calcium into the soil, it is predictable that biocomposites based on sodium alginate, once buried, undergo to spontaneous ionic crosslinking, thus providing water resistant biodegradable pots (Immirzi et al., 2009; Malinconico et al., 2008). The natural fibres used to reinforce the polymeric matrix were flexible and short fibres from tomato peels and seeds combined to more rigid, stiff and long fibres from hemp strands (Ashori and Nourbakhsh, 2010). Hemp fibres, consisting of about 70% of cellulose, 15% of hemicelluloses, 5% of lignin and wax and up to 10% of moisture (Amar et al., 2005), were provided from ASSOCANAPA (Pistoia, Italy), and were on average 200 ␮m long, 10 ␮m wide and 5 ␮m thick. Tomato fibres were wastes of tomato-processing industry of Lycopersicon esculentum “S. Marzano”, tomato cultivar purchased from Bulsem (Salerno, Italy) (Tommonaro et al., 2008). The dry matter of tomato peels and seeds was mostly composed of about 50% of fibres and 20% of crude protein, while the remaining part was characterized by fats and carotenoids, such as lycopene and ␤-carotenoid. The cell walls of fibres contained cellulose, hemicelluloses and pectin, while starch was present as energetic and preserving plant source (Knoblich et al., 2005). After the extraction of high added value bioactive molecules, such as polysaccharides, carotenoids and polyphenols, from peels and seeds, the residual dried fibres were used as received. Tomato fibres were on average 80 ␮m long, 10 ␮m wide and 10 ␮m thick (Tommonaro et al., 2008). Polyglycerol, as plasticizer, was supplied from Solvay (Milan, Italy) and used as received. Calcium chloride used to crosslink sodium alginate was provided from Sigma Aldrich (Milan, Italy) and used as received.

2.1.2. Laboratory-scale films, sheets and pots setting up In order to identify and characterize the blends to be used for the pots, two different laboratory-scale films, based on sodium alginate and calcium alginate, were prepared to perform physico-chemical analysis helpful to predict the polymers performance as bonding agent inside the pots. The sodium alginate films (coded A) were prepared by casting from an aqueous solution obtained by dissolving 2.0 g of sodium alginate in 100 ml of distilled water (2%, w/v) under stirring at the temperature of 70–75 ◦ C. Successively 1.0 g of polyglycerol (1%, w/v) was added as plasticizer and dissolved under stirring. The solution was filtered and kept few minutes under vacuum for degassing. After this treatment, the solution was firstly poured, avoiding bubble formation, into a polyester mould kept in plane, in this way assuring the homogeneity of film thicknesses, and afterwards placed under ventilated hood for 3 days at room temperature, thus allowing the water evaporation. The crosslinked calcium alginate films (coded ACr) were prepared by soaking for 15 min the A films in a 5% (w/v) aqueous solution of calcium chloride. The drying process of the ACr crosslinked films followed the procedure previously described for the A un-crosslinked films. Prior to testing, all the films were equilibrated at 45% relative humidity by storing them in a desiccator over a saturated solution of calcium nitrate at room temperature. The pots and the sheets were obtained from the same biocomposites, soaking 50.0 g of aggregate of fibres in 100 ml of a 2% (w/v) sodium alginate water solution. The components were thoroughly mixed by means of a Brabender Plastograph blender (Rheomix, Haake Rheometer, Germany) for 30 min at room temperature (cold process) and at a rate of 16 revolutions per minute (rpm). Successively the paste was used to shape the two different forms (pot and sheet). The pots were obtained by using a stainless steel device, appositely produced, made with pots shaped closed moulds (Fig. 2). After distributing the paste inside the moulds, the device was fixed between the cold plates of a press where the moulds were allowed to close, left for few minutes for completely fill the cavity with the wet paste and then opened, in this way providing the wet shaped pots. A following drying process, carried out in an oven at 40 ◦ C under air flow for 24 h, allowed to remove the water content up to obtain a final constant weight. The pots were characterized by a height of 40 mm, an end base diameter of 40 mm, a top base

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permeability tests, density and porosity tests and morphological analyses were carried out on the A and ACr laboratory-scale films.

2.1.3.1. Mechanical test. Young’s modulus, stress and strain at break were evaluated by means of a dynamometer (model 4301, Instron, Canton, MA, USA) following the ISO 5893 standard (ISO 5893, 2002). The dumbbell specimens tested were characterized by a thickness of 75 ␮m for the A films and of 108 ␮m for the ACr films, a width of 4 mm, and a length of about 28 mm. For each composition six specimens were tested. All the measurements were carried out at room temperature and at crosshead rate of 2 mm min−1 . The values of the Young’s modulus are within ±10%, while the stress values are within ±15%, and the elongation at break fluctuates in the range of ± 20%. Fig. 2. Stainless steel pots mould.

diameter of 55 mm, a thickness of 4 mm, and a weight of 9.0 g (Fig. 3). As concerning the preparation of sheets, about 2.5 g of the wet paste was distributed into stainless steel moulds of 70 mm2 with a rubber frame of 3 mm and successively shaped in parallelepiped sheets by cold pressing. After the drying process, carried out as in the case of the pots, sheets of about 3 mm thick were obtained. Three different compositions of biocomposites (both for sheets and for pots) were prepared varying the percentage of tomato and hemp fibres added to sodium alginate water solution; the first one included 100% of tomato fibre (coded ATH100), the second one consisted of 90% of tomato fibre and 10% of hemp fibre (ATH90) and the last one contained 70% of tomato fibre and 30% of hemp fibre (ATH70). The crosslinked sheets (ATH100Cr, ATH90Cr, ATH70Cr) were achieved by soaking the sheets for 15 min in a 5% (w/v) deionized water solution of calcium chloride; the drying process of the crosslinked sheets was the same as described for the pots. The films and sheets thickness was determined with an electronic digital micrometre with accuracy of 0.001 mm. The thickness was measured after the drying and the conditioning at room temperature and 45% relative humidity, averaging the values measured over 10 different points. 2.1.3. Laboratory film tests With the aim to predict and highlight the physico-chemical behaviour of the pots based on the sodium alginate and calcium alginate polymeric resins, mechanical tests, water vapour

Fig. 3. Experimental biodegradable pot.

2.1.3.2. Water vapour permeability, density and porosity tests. Water vapour permeability (WVP) was calculated on the base of the ASTM standard method E 96–80 (ASTM E 96–80, 2002) following the equation: WVP = WVTR

 L  p

(1)

where WVTR (g m−2 s−1 ) is the water vapour transmission rate of films, L (m) is the film thickness, and p (Pa) is the water pressure difference between both sides of the film. WVTR assessment consists of evaluating the rate of water vapour diffusion through the specimen by means of gravimetric measurements, making periodic weightings. For the tests it was used the desiccant method by which three test specimens for each sample were sealed to the open mouth of a CEAST metal cup, with a 3000 mm2 exposed area for the test and filled with silica gel; the metal cups were placed in an environmental chamber set at air temperature of 23 ◦ C and air relative humidity of 45%. In order to better explain some peculiar results of chemicophysical properties of the films and composites, such as permeability and mechanical performance, the density and porosity of the films and sheets were evaluated. The tests were carried out via a liquid displacement method (Bundela and Bajpai, 2008), using n-hexane as the displacement liquid, due to its ability to easily penetrate the pores of the sheets without inducing shrinkage, swelling or solubilization of the polymeric matrix (Zhang and Ma, 1999). A sample of weight W was immersed in a graduated cylinder containing a known volume (V1 ) of n-hexane. The sample was kept in n-hexane for 10 min, and then a series of brief evacuation–repressurization cycles were conducted to force the solvent into the pores of the composite. The cycling was continued until no air bubbles emerged from the sheets. The total volume of the n-hexane plus the n-hexane-impregnated sheets was recorded as V2 and the residual n-hexane volume was recorded as V3 . The n-hexane impregnated sheet was removed from the cylinder. The density of the sheet was expressed as d = W/(V2 − V3 ) and the porosity of the open pores in the composite was obtained with ε = (V1 − V3 )/(V2 − V3 ) × 100. 2.1.3.3. Scanning electron microscopy (SEM) analysis. Morphological analysis on the A and ACr sample surfaces was performed by means of scanning electron microscopy (SEM) (Quanta 200 FEG, FEI, Eindhoven, The Netherlands). Air dried samples were fixed onto aluminium stubs through carbon adhesive disks and observed with a low-vacuum secondary electron detector using the accelerating voltage of 5.0 kV. The samples were observed at room temperature and at an internal water vapour pressure of 66.66 Pa.

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2.1.4. Laboratory biocomposites tests The sheets of un-crosslinked and crosslinked biocomposites were used to perform mechanical and morphological analyses, as well as water up-take and biodegradation tests. 2.1.4.1. Mechanical tests: flexural test and puncture test. In order to simulate the natural pressure exerted by roots against the pot wall and bottom, two different mechanical tests were performed: the flexural test, according to ASTM D790 normative (ASTM D790-10, 2010), and the puncture test, an empiric test previously used by Immirzi et al. (2009). All the mechanical analyses were performed on six specimens, previously conditioned at room temperature and at air relative humidity equal to 45%. Flexural loads combined tensile, compression and shear loads. The upper surface of each specimen was put into compression, the central portion experienced shear, and the lower face underwent tension. The flexural samples were cut into specimens of about 3 mm thick, 5 mm wide and 7 mm long; the flexural modulus was determined using an universal testing machine (4504, Instron, Canton, MA, USA) in accordance with ASTM D790 standard (ASTM D790-10, 2010). Three-point bend testing was performed at a crosshead speed of 1 mm min−1 with load cell of 1 kN, to evaluate both flexural strength and flexural modulus; each test was performed until failure occurred. The distance between the span was 48 mm, so the span to depth ratio, i.e., the length of the outer span divided by the thickness of the specimen, was 16 which is an acceptable value to minimize the area of shear stress created along the midline of the specimen that could invalidate the test (Lee et al., 2009). The puncture test consisted in penetrating the specimens until the laceration of the same (Malinconico et al., 2008; Immirzi et al., 2009). More specifically, the samples to test, opportunely cut with a punch cutter of 42 mm circular section, were trapped in cups fixed on the inferior traverse of the INSTRON instrument (Fig. 4). They underwent the action of a force exerted by a spherical dart linked to a steel rod fixed on the upper traverse of the apparatus; the dart, moving down at a fixed rate of 2 mm min−1 , penetrated the sample until the rupture of the same. The applied load as a function of the displacement was recorded; the parameters obtained were normalized with respect to the ½ of the area of the sphere and with respect to an estimated value of the displacement of 10 mm. 2.1.4.2. Scanning electron microscopy analysis. Morphological analysis was carried out on cryogenically fractured surfaces of films and sheets by means of a SEM (Quanta 200 FEG, FEI, Eindhoven, The Netherlands) in high vacuum mode, using a secondary electron detector and an accelerating voltage of 20.0 kV. Before the electron microscopy observation, the fractured surfaces were coated with Au–Pd alloy with a SEM coating device (MED 020, Bal-Tec AG, Tucson, AZ, USA). The coating provided the entire sample surfaces with a homogeneous layer of the metal of 18 ± 0.2 nm. By means of energy dispersive X-ray spectroscopy (EDS) it was possible to perform the chemical analysis of selected microscopic regions. EDS was performed in the SEM by means of an Oxford Inca Energy 250 System equipped with an INCAx-act LN2-free detector, using an accelerating voltage of 20.0 kV. 2.1.4.3. Water up-take test. In order to reproduce the mechanical resistance of biodegradable pots in wet soil, pre-weighted dry sheets were immersed in deionized water for 60 min at room temperature and weighed every 15 min. The samples tested were characterized by a length of 20 mm and a width of 15 mm, a weight of 0.2 g and a thickness of 3 mm for the un-crosslinked samples while a weight of 0.5 g and a thickness of 5 mm for the crosslinked samples. Since the ATH100, ATH90 and ATH70 samples were not crosslinked, the polymer chains were more flexible and able to interact, with water molecules; for this reason it was predictable

Fig. 4. Apparatus used for the “puncture test”: a test cup for “puncture test” analysis (a) and the penetrating dart fixed on Instron device (b).

that some of the fibres could disentangle from the polymeric network flowing out from the mat. To take into account this drawback, all the samples were placed in Petri dish covered with filter papers, previously dipped in water and weighed. The sheets were withdrawn from the solutions and their wet weights were determined after first blotting with a filter paper to remove the surface water. The water up-take percentage was calculated using the equation: Wst (%) =

W − W  t d Wd

× 100

(2)

where Wst (wt%) is the water sorption of the sheets, Wd and Wt respectively are the weights of the samples in the dry state and in the swollen states at different times. For each kind of sample, the data were averaged on three specimens. 2.1.4.4. Laboratory biodegradation tests. To simulate the biodegradation process undergone by biodegradable pots when buried in soil for transplanting, biodegradation tests were performed on biocomposites thick sheet samples. Among the biocomposites, the ATH100Cr and ATH70Cr crosslinked samples were analysed since it was expected that the un-crosslinked biodegradable pots, when buried and regularly watered, endured the crosslinking process promoted by the calcium ions present in the soil and naturally available to the polymeric resin. The biodegradation tests, carried out on a laboratory-scale, followed the standard test method described in ASTM Standard D5338-98 (2003) regarding the biodegradation in soil. Following this test, the mineralization of the sample, i.e., the amount of organic carbon of the material which is converted into CO2 , was measured. The CO2 production

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Fig. 5. Experimental biodegradable pots setting up.

measured for a material, expressed as a fraction of the calculated carbon content, was reported with respect to time, from which the degree of biodegradability was assessed. The soil used, characterized by a pH equal to 7.76, total solid content equal to 86.83% and volatile solid content equal to 5.55%, was loaded with salts and compost, according to the rate of 1 g of compost in 25 g of soil. The soil was mixed with the test material and the CO2 evolved was measured. Testing and observation were followed for 60 days. Crystalline cellulose was used as control. 2.2. The field test The biodegradable pots for seedling transplanting were tested in real field condition during 2009. The test was carried out inside a steel-constructed greenhouse (30.00 m × 10.00 m), North-South oriented, at the experimental farm of the University of Bari in Valenzano (Bari), Italy, having latitude 41◦ 05 N, longitude 16◦ 53 E, altitude 85 m asl. The greenhouse was covered with an ethylene vinyl acetate (EVA) film (PATILUX, Pati Co., Treviso, Italy), having a thickness of 200 ␮m and the following radiometric coefficients: solar radiation range (200–2500 nm) total transmissivity equal to 90.9%, solar direct transmissivity equal to 56.7%, and long wave infrared range (LWIR, 7500–12500 nm) transmissivity equal to 22.5%. Greenhouse temperature was controlled by means of a natural and a forced ventilation system. Ventilation was provided automatically when inside air temperature exceeded 27 ◦ C. The experiment was conducted in the greenhouse from July 21 to October 30, 2009. Besides the ATH100, ATH90 and ATH70 biodegradable pots, commercial pots made of 100% polystyrene (coded PS) with drainage holes were tested and used as control. All the pots were filled with potting soil. During the period from seeding to seedlings transplanting, from July 21 to August 25, 2009, the pots were allocated in a steel bench, characterized by an area of 2.0 m × 1.0 m and a depth of 0.3 m. The bench, covered with an anti root film (Tenax FL, Tenax SPA, Viganò, Lecco, Italy), was filled with perlite (Agrilit2, Perlite Italiana srl, Perlite Italiana, Corsico, Milan, Italy) for almost half of its depth. The bench area was divided in equal parts. The experimental design was a completely randomized design; the four treatments (ATH100, ATH90, ATH70 and PS) had four replicates with four pots per replicate. The pots were allocated inside the perlite for 2/3 of their height (Fig. 5). On July 21, 2009 three pepper seeds were put into the growing substrate in each pot. After emergence only one plant per pot was kept. The irrigation was provided by means of an intensive fogging system, realized with a PVC irrigation pipe (diameter of 1.6 mm) placed all around the bench with plastic nebulizer nozzles every 0.5 m. The nozzles watered over 180◦ . During the

Sample

Density (g/cm3 )±10%

Porosity (%)±20%

A ACr ATH100 ATH90 ATH70 ATH100Cr ATH90Cr ATH70Cr

0.43 0.38 0.29 0.27 0.22 0.28 0.21 0.19

21.7 29.2 23.8 25.7 27.4 27.1 32.7 36.1

period from seeding to seedlings transplanting, the irrigation was provided 3 times per day for 4 min at a time. On August 25, 2009 the seedlings together with the biodegradable pots were transplanted in the soil, without transplant shock. The control plants, i.e., the plants grown inside the PS containers, were transplanted following the procedure of removing the plastic pots. Peppers were harvested on October 30, 2009. Air temperature and relative humidity were continuously measured inside the greenhouse during the cultivation period by means of a Hygroclip-S3 sensor (Rotronic, Zurich, Switzerland); the data, measured with a frequency of 60 s, were averaged every 15 min and stored in a data logger (CR10X, Campbell, Logan, USA). 3. Results and discussion 3.1. Laboratory films and biocomposites evaluation A significant increase in thickness was recorded upon crosslinking films and sheets in calcium chloride solution. The thickness of the A and ACr samples was respectively about 75 ␮m and 108 ␮m, with an increase of 44% upon cross-linking; the thickness of the ATH100, ATH90 and ATH70 sheets was about 3 mm, while the ATH100Cr, ATH90Cr and ATH70Cr sheets showed a thickness of about 5 mm, with an increase of about 67%. This thickness increasing may be explained considering that the crosslinking process of alginate molecules, responsible of tie points development and stabilizing, often referred in literature as “egg-box” type process (Grant et al., 1973), occurred during the samples soaking in water solutions of calcium chloride; sodium alginate film swelled while crosslinked, i.e., it up-took some water during the formation of three-dimensional stable network of calcium alginate. For this reason the conformation of crosslinked polymeric backbone attained in wet condition did not change after drying and reconditioning process (Russo et al., 2007, 2010). This outcome induced the increasing of free volume and consequently of the macromolecular chain mobility, influencing in unusual way some physico-chemical properties of both crosslinked films and biocomposites (Lee et al., 2009). The hypothesis of free volume increasing was supported by the evaluation of biocomposites porosity. The results, reported in Table 1, could highlight the analysis of some atypical behaviours of films and sheets. In particular, the increasing of both free volume and porosity of crosslinked samples could better explain the permeability results of the ACr film and the mechanical performances of crosslinked sheets. The mechanical properties, analysed by means of the tensile test method, and the permeability of the A and ACr films are reported in Table 2. As a result of the stiffness rising due to the crosslinked network formation, the increasing of Young modulus and the dropping off of elongation at break were evidenced in the ACr samples. As concerning permeability data, it was possible to observe that the ACr films experienced higher permeability with respect to the A films. In literature it is reported that generally crosslinked films afforded lower water vapour permeability if compared to un-crosslinked ones, since the tie points produced

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Table 2 Mechanical properties and water vapour permeability of the sodium alginate (coded A) and calcium alginate (coded ACr) laboratory films. Film

Young modulus (MPa) ±10%

Tensile stress at break (MPa) ±15%

Tensile strain at break (%) ±20%

Permeability (ng s−1 m−1 Pa−1 )

A ACr

2170 4270

74 98

18 4

0.072 0.128

during the development of the three-dimensional network represented a hindrance to the permeation of molecules (Crank and Parker, 1968). Nevertheless in this research it was found the opposite result. A plausible explanation to this unusual outcome could be found both in the increasing of free volume of the crosslinked ACr films and in their enhanced porosity with respect to the uncrosslinked samples (Table 1), that allowed water molecules to easily cover a straight pathway throughout the films surface (Russo et al., 2007, 2010; Lee et al., 2009). The SEM analyses of the A and ACr film surfaces were reported in Fig. 6: the micrograph of the A film (Fig. 6a) showed a tracked but homogeneous and smoothed surface, typical of a quite regular chain distribution, while the micrograph of the ACr film (Fig. 6b) showed an indented and rough surface, with zones of higher material agglomeration, characteristic of a well structured crosslinked network.

In Table 3 the data related to flexural properties of the tested sheets are detailed; both set of the un-crosslinked and crosslinked samples showed a general increase of the mechanical parameters when hemp fibres were added to tomato peels. The mechanical enhancement was expected, as it is known that fibres, due to their increased aspect ratio (length/diameter) compared to non fibrous filler, can improve mechanical properties of composites (Gironès et al., 2011). In the case of the un-crosslinked samples, when the nominal hemp fibre weight fraction increased beyond a critical point, as in the case of the ATH70 sample, a slight lowering of both Young modulus and stress at break were observed. This outcome could be due to the irregular dispersion of fibres inside the polymeric matrix, responsible of the voids concentration rising in the biocomposite, as confirmed by the porosity values reported in Table 1. Generally, the stiffness of fibre reinforced plastic composites depends on the properties of the constituent fibres, the matrix and the load transfer capability between the matrix and fibres through the shear stress (Luo and Netravali, 1999). If voids are produced in the composites, in the matrix or between the fibre and matrix, they significantly weaken the load transfer capability, due to the stress concentration around them (Luo and Netravali, 1999). Moreover, by increasing the concentration of hemp fibre, their wettability due to the physical interaction among hydroxyl groups of both resin and fibre, decreased, in this way reducing the strength of adhesion holding polymeric resin and fibres together (Lee et al., 2009) and, consequently, allowing their mechanical performance fading. Concerning the crosslinked biocomposites, it was possible to evidence a general reduction of rigidity and strength with respect to the corresponding un-crosslinked samples (Table 3). It is known, from previous studies (Gleghorn et al., 2008) that when alginate is cross-linked, it loses its adhesive properties since most of its active carboxylated and hydroxyl groups are strongly engaged in physical interaction with calcium ions (Russo et al., 2007); therefore the bonding strength between the matrix and the fibres is substantially reduced and, consequently, the mechanical parameters are negatively affected. Moreover, as previously described for the ACr samples, the three-dimensional network of crosslinked biocomposites, obtained in wet condition, did not undergo conformational changing or drastic shrinking after drying process; this experimental finding implies both the enhancing of the sheets porosity, as shown in Table 1, and the increasing of free volume of crosslinked biocomposites; as a consequence, the macromolecular chain mobility was increased and the flexural mechanical properties underwent a fading (Russo et al., 2007, 2010). Anyway, in crosslinked samples, a slight rising of stiffness was observed passing by ATH100Cr to ATH70Cr samples; the higher

Table 3 Flexural properties of the laboratory biocomposites sheets.

Fig. 6. Scanning electron micrograph of the A surface sample (a) and of the ACr surface sample (b).

Sheets

Young modulus (MPa) ±10 MPa

Tensile stress at break (MPa) ±0.2 MPa

ATH100 ATH90 ATH70 ATH100Cr ATH90Cr ATH70Cr

63.62 97.08 81.70 48.05 62.51 78.21

0.71 1.20 0.92 0.46 0.68 0.85

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a

Table 4 Maximum load and displacement of the biocomposites sheets evaluated by the puncture test.

1.6

1.4

Sheets

Maximum load (N) ±10%

Displacement (mm) ±5%

ATH100 ATH90 ATH70 ATH100Cr ATH90Cr ATH70Cr

8.7 10.5 14.8 10.4 11.2 15.0

3.59 4.07 4.75 3.22 3.98 4.02

Water up-take (%)

1.2

0.8

0.6

0.4 ATH100

ATH90

ATH70

ATH100Cr

ATH90Cr

ATH70Cr

0.2

0.0 0

b

10

20

30 40 Dipping time (min)

50

60

70

200.0 180.0 160.0 140.0

Water absorpition (%)

concentration of longer and stiffer fibres, such as hemp strands, together with the strong three-dimensional network, probably induce the attainment of improved matrix/fibre interfacial region, responsible of the increasing of flexural strength (Luo and Netravali, 1999). Table 4 shows the results of the puncture tests reporting a significant increase of the resistance to break when hemp fibres were added, both in the un-crosslinked samples and in the crosslinked ones. The larger aspect ratio of fibres, compared to the tomatoes powder, was the main reason of such increase. The influence of polymer cross-linking became evident at high fibres content (ATH70Cr), where the break resistance of the sample was much higher, as occurred in the flexural tests. The enhanced displacement measured for the un-crosslinked samples was probably due to the presence of voids and points of discontinuity provided by irregular dispersion of higher un-wettable hemp fibre concentration. To explain the reduced sensitivity of the puncture test to cross-linking, in comparison to the flexural test, it is important to consider the different way to apply loads in both the mechanical tests: for the puncture test a local mechanical solicitation is applied on the sample, whereas in the flexural test tensile, compressive and shear stresses are exerted on the lower face, upper surface and central portion of the specimen, respectively. In Table 5 the weighting increases of the biocomposites as a consequence of their swelling in water at different times are reported; the final water uptake of the sheets, Wst = 60 min (%), calculated by Eq. (2), are reported too. The swelling kinetics of the samples, Wst = 60 min (%), and the water up-take percentage, Wst (%), are shown in Fig. 7 as a function of sampling time. The un-crosslinked samples and the crosslinked ones behaved in different way with two different kinetics of water absorption (Fig. 7a): the un-crosslinked samples swelled following a slow and regular profile during all the experimental time, reaching a swelling plateau after 45 min; the crosslinked samples, on the other hand, experienced a faster kinetic of water uptaking mainly during the first minutes of water dipping, absorbing most of the water and reaching the swelling equilibrium after the first 30 min. This probably suggests that all the available binding sites of crosslinked polymeric networks were easily and swiftly engaged in hydrogen bonding with water. In addition, it is worthwhile to underline that the water absorption percentage was about 130% for the un-crosslinked samples, and in the range of 150–190% for the crosslinked samples, as shown in Fig. 7b. These experimental results were in agreement with the previous

1.0

120.0 100.0 80.0 ATH100

ATH90

ATH70

ATH100Cr

ATH90Cr

ATH70Cr

60.0 40.0 20.0

0.0 0

10

20

30 40 Dipping time (min)

50

60

70

Fig. 7. Water up-take percentage (a) and water absorption percentage (b) as a function of the dipping time of the sheets of un-crosslinked and crosslinked biocomposites.

hypothesis related to the increasing of free volume of crosslinked polymer. The thickness rising measured on dried and reconditioned biocomposites suggests that the three-dimensional network was formed and stabilized during the swelling phenomena, inducing the increasing of free volume and hence the availability of binding sites to water molecules. Moreover, the analysis of Table 1 showed that a particular enhancing of porosity was recorded in the case of crosslinked samples when hemp fibres were added to tomato peels. For these reasons the water uptaking process of crosslinked biocomposites resulted enhanced if compared to the water absorption of untreated samples. Moreover, the conformational stability of crosslinked structures allowed water molecules to easily cover

Table 5 Water up-take values of the laboratory biocomposites sheets; Wd : weight of the sample in the dry state (t = 0 min); Ws : weight of the sample in the swollen states evaluated every 15 min. Sample

Wdt = 0 min (g)

Wst1 = 15 min (g)

Wst2 = 30 min (g)

Wst3 = 45 min (g)

Wst4 = 60 min (g)

Wst (%) evaluated by Eq. (2)

ATH100 ATH90 ATH70 ATH100Cr ATH90Cr ATH70Cr

0.220 0.190 0.210 0.550 0.580 0.490

0.358 0.331 0.335 0.750 0.930 0.953

0.482 0.420 0.455 1.330 1.450 1.380

0.500 0.425 0.488 1.350 1.450 1.400

0.502 0.435 0.490 1.350 1.470 1.400

128 129 133 145 153 186

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25

biodegradability (%)

20 ATH100Cr

ATH70Cr

15

10

5

0 0

10

20

30

40

50

60

biodegradation time (days) Fig. 9. Biodegradation (%) in soil of the ATH100Cr and ATH70Cr sheets as a function of time.

Fig. 8. Scanning electron micrograph of ATH100 fracture surface (a) and of ATH100Cr fracture surface (b).

the straight pathway of biocomposites network, hastily engaging most of the free binding sites and swiftly reaching the swelling plateau. The SEM micrograph of fractured surface of the ATH100 sample highlighted a smooth and regular coating of polymeric matrix on tomato fibre surface and, at same time, an irregular distribution of fibres (Fig. 8a). The SEM micrograph of fractured surface of the ATH100Cr sample showed a rough fracture surface with the presence of voids dispersed throughout the biocomposite surface, probably due to the irregular distribution of fibres among macromolecular chains, exchanged with zones of regular polymer matrix diffusion (Fig. 8b). Table 6 reports the chemical elemental analysis, by EDS, of four selected microscopic regions. EDS revealed the presence of calcium in rather all the selected internal micro-zones; this result suggests the formation throughout the whole sample of stable tie points responsible of free volume development, as previously described. This effect is also detectable in SEM micrographs

Table 6 Chemical elemental analysis by energy dispersive X-ray spectroscopy (EDS). Spectrum

C

O

Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4

71.13 52.16 69.09 37.43

28.87 45.75 29.62 52.96

Na

Ca

0.60

1.49 1.28 6.64

2.96

of ATH90Cr and ATH70Cr samples (data not reported) (Russo et al., 2007, 2010). In Fig. 9 the percentages of biodegradation of the ATH100Cr and ATH70Cr samples as a function of the time are reported; it was opportunely selected and followed the biodegradation pattern of the ATH70Cr samples due to the higher concentration of hemp fibres with respect both the ATH100Cr and ATH90Cr samples. During the first 60 days of permanence in testing soil, it was observed that the ATH100Cr samples degraded faster than the ATH70Cr samples; in particular the ATH100Cr samples achieved about 20% of biodegradation whereas the ATH70Cr sample reached 15%. This finding could be ascribed to the different kind of fibres used. Tomato peels and seeds are short fibres based on pectin, starch and cellulose, whose hydrophilic hydroxyl groups along polymeric chains are easily available to microbial population (Knoblich et al., 2005). As a matter of fact, being short sized, tomato fibres supply a higher contact surface to soil microorganisms, thus providing a relative faster biodegradation. On the other hand, hemp fibres are longer and mostly based on microcrystalline cellulose (Amar et al., 2005) and for these reasons the hydrophilic macromolecular chains are not easily susceptible to microorganism attack. Moreover it is worthwhile to highlight the sizeable difference between the biodegradation rates of films and crosslinked biocomposites. The films are based on amorphous polymeric structure easily available to soil bacteria flora, whereas the biocomposites, due to the presence of cellulose based fibres, show a more structured chemical arrangement able to hinder and to slow the microbial attack. Moreover, if equivalent exposed surfaces of films and biocomposites are considered, it is possible to detect a faster biodegradation of the film surface, since in biocomposites microorganisms firstly mineralize the polymeric matrix and following transform the fibres, as shown by Immirzi et al. (2007). 3.2. Field test result During the first stage of use, when the seedlings were grown from the seeds before the transplant, all the biodegradable pots showed sufficient mechanical resistance to guarantee material functionality. All the biodegradable pots remained intact throughout the entire period of 35 days from seeding to seedlings transplanting: the parts of the pots in contact both with the perlite and the air did not show visual biodegradation. In this period the pots were subjected to a mean greenhouse air temperature of 31.6 ◦ C and to a mean greenhouse relative humidity of 53.8%. The biodegradable pots allowed the development of the root structure with good branching structure and the development of

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Fig. 10. Roots development in the ATH100 and ATH90 biodegradable pots and in the polystyrene pot (PS) at seedlings transplanting.

secondary branching, with which the plants up took water and nutrients. At the transplant, a significantly dense network of root hairs developed in ATH100 pots, a dense one in ATH70 pots and a less dense network in ATH90 pots (Fig. 10). In PS pots, used as control, long roots dominated the root system, reducing overall root development (Fig. 10). The different pots significantly influenced the seedlings height. The highest seedlings were those grown inside the ATH100 pots (0.119 m) and inside PS pots (0.115 m); shorter seedlings were grown inside the ATH90 (0.099 m) and ATH70 pots (0.100 m) suggesting that the presence of the filler probably influenced the seedlings growth. After the transplant, the biodegradable containers degraded completely in 16 days allowing the passage of the roots through the containers walls and the growth of the plants. The roots spread in a radial fashion; no root rot or similar symptoms were observed. At the end of crop cycle, the different pots influenced pepper height. The mean plant height was 0.76 m for the plants grown inside the ATH90 pots, 0.73 m inside the ATH100 and ATH70 pots. The control plants were characterized by a mean plant height of 0.67 m.

4. Conclusion The research showed that wastes and by-products of agro-food industries and paper–textile manufacturing companies, such as natural fibres, may be converted in upgraded products used as reinforcing solid phase of biocomposites designed for the production of innovative biodegradable pots to be employed for transplanting process in agriculture. The wastes and by-products, recycled by means of cost-friendly processes, are environmentally friendly, fully biodegradable and easily available. In addition biodegradable pots overstay and degrade into the soil with a consequent positive environmental impact, in this way representing a valid eco-sustainable alternative to the traditional oil-derived containers, very often cause of environmental pollution, since the improper post-using discarding. The use of the biodegradable pots implies the drastic reduction of man labour related to the pots recovering, cleaning and plants transplanting. The eco-friendly methodology of preparation, involving a cold process, entails low productive costs. Moreover the reduction of the costs related to the correct disposal of the wastes generated by agro-food industries and paper–textile manufacturing companies must be considered. The research showed that the mechanical properties of the biocomposites made of tomatoes seeds and peels, combined to hemp fibres and bound together into the cross-linked alginate network, are very interesting for the selected application. Laboratory testing was performed on films and sheets, to well understand the influence of water and calcium uptake on the structure of the materials. It was found a correlation between cross-linking, water diffusion and mechanical performance. SEM analysis on fracture

surface showed that the fibres dispersion needed some improvement, while EDS analysis following the calcium profile, evidenced a quite regular diffusion of the cross-linker inside the biocomposite. In terms of horticultural performance, the biodegradable pots did not cause damage to the plants during the test period but allowed to develop very dense and active root hair; moreover, during the transplanting operations, no transplant shock and root deformation were detected. Future research should be addressed towards the use of further wastes of agro-food industries such as processing residues of hazelnuts, citrus fruits, olive oil and wine. Acknowledgements The authors shared programming and editorial work equivalently within the areas of their expertise. M. Malinconico, B. Immirzi and G. Santagata kindly acknowledge the financial support of CNR-MISM Program “Sviluppo di imballaggi da fonti rinnovabili per una logistica sostenibile” (2007–2011). The authors thank Mr. Giuseppe Narciso (CNR) for supporting in Scanning Electron Microscopy, Mr. Vincenzo di Liello (CNR) for supporting in pots preparation and Dr. Okay Rukaesih, Head of Centre of Textile in Jawa Barat (Indonesia), for sustaining in biodegradation tests. References Amar KM, Manjusri M, Lawrence TD. Natural fibers, biopolymers, and biocomposites. Tailor & Francis: CRC Press; 2005. Ashori A, Nourbakhsh A. Bio-based composites from waste agricultural residues. Waste Management 2010;30:680–4. ASTM Standard E 96–80. Standard test methods for water vapor transmission of materials. West Conshohocken, PA: ASTM International; 2002, doi:10.1520/E0096 E0096M-10 www.astm.org ASTM Standard D5338-98. Standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions. West Conshohocken, PA: ASTM International; 2003, doi:10.1520/D5338-98R03 www.astm.org ASTM D790-10. Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials. West Conshohocken, PA: ASTM International; 2010, doi:10.1520/D0790-10 www.astm.org Avella M, Di Pace E, Immirzi B, Impallomeni G, Malinconico M, Santagata G. Addition of glycerol plasticizer to seaweeds derived alginates: influence of microstructure on chemical–physical properties. Carbohydrate Polymers 2007;69:503–11. AWARENET. Handbook for the prevention and minimization of waste and valorization of by-products in European agro-food industries. Agro-food waste minimization and reduction network (AWARENET). Grow Programme, European Commission co-ordinated by the Spanish Technology Centre GAIKER. Transactions of the Wessex Institute; 2004. pp. 1–7. http://dx.doi.org/10.2495/WM020321 Bundela H, Bajpai AK. Designing of hydroxyapatite–gelatin based porous matrix as bone substitute: correlation with biocompatibility aspects. eXPRESS Polymer Letters 2008;2(3):201–13. Coviello T, Matricardi P, Marianecci C, Alhaique F. Polysaccharide hydrogels for modified release formulations. Journal of Controlled Release 2007;119:5–24. Crank J, Parker GS. Diffusion in polymers. New York: Academic Press; 1968. Cyras VP, Martucci J, Iannace S, Vaˇızquez A. Influence of the fiber content and the processing conditions on the flexural creep behaviour of sisal–PCL–starch composites. Journal of Thermoplastic Composite Materials 2001;14:1–13.

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