Monitoring of the degradation dynamics of agricultural films by IR thermography

Monitoring of the degradation dynamics of agricultural films by IR thermography

Polymer Degradation and Stability 92 (2007) 777e784 www.elsevier.com/locate/polydegstab Monitoring of the degradation dynamics of agricultural films ...

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Polymer Degradation and Stability 92 (2007) 777e784 www.elsevier.com/locate/polydegstab

Monitoring of the degradation dynamics of agricultural films by IR thermography P. Mormile a, L. Petti a,*, M. Rippa a,c, B. Immirzi b, M. Malinconico b, G. Santagata b a

Istituto di Cibernetica-CNR, Dipartimento Materiali e Dispositivi, via Campi Flegrei 34, 80078 Pozzuoli, Italy b Istituto di Chimica e Tecnologia dei polimeri-CNR, via Campi Flegrei 34, 80078 Pozzuoli, Italy c Universita` degli Studi di Napoli Federico II e Corso Umberto I, 80138 Napoli, Italy Received 27 September 2006; received in revised form 24 January 2007; accepted 8 February 2007 Available online 25 February 2007

Abstract The high consumption of plastic materials for use in agriculture is generating serious problems regarding environmental impact. Biodegradable materials are at present the only solution to this type of problems. Research in material science is daily proposing different materials with this requirement for application in various sectors. One of the problems is to analyse the degradation states of the films before their application in order to know the time and the modality of degradation. Here we propose a novel method based on thermography vs time which allows the degradation dynamics of agricultural films to be evaluated. In our experiment we put some biodegradable materials of our interest on soil in an open field. The main characteristic of these materials is that they are originally liquid and they are sprayed on soil and immediately cross-link into a film state. Through an IR thermal camera we monitored the state of the films day by day and analysed the thermal images in order to detect the continuous changing of their physical properties. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Biodegradability; Polymeric materials; Degradation dynamics; Thermography; Agricultural films

1. Introduction The consumption of plastic materials for application in agriculture has dramatically increased in the last decades posing serious problems of environmental concerns. The confirmation of such an increase is shown in Fig. 1 where the positive trend for each type of plastic product used in agriculture is evident. On the other hand, the enormous consumption of plastic films, gives rise to plastic waste which is increasing more and more. In Table 1 we report the incidence (%) on the total plastic waste for each item. Even if the percentage of plastic waste in agriculture seems quite low compared to the other uses, it represents one of the main dangers to the environment due to the difficulties in entering into the recycling chain. In other words, the landfill of million and million tons of plastic films represents a serious * Corresponding author. Tel.: þ39 081 8675037. E-mail address: [email protected] (L. Petti). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.02.015

problem for the environment also because, while consumption continuously increases, the recycling system in any country doesn’t work as it should. Practically most of these films are collected by the farmers and left on the ground borders to be incinerated. The same collection by the farmer is difficult; these films being thinner and damaged, their removal through automatic means is almost impossible. Biodegradable materials are the ideal solution to this kind of problem which are involving the whole food chain year by year. In this area the effort of the research world, including materials engineers, chemists, physicists, and agronomists, focused on the design and synthesis of new materials with biodegradable properties to be used in agriculture [1e5]. This research requires new investigation tools in order to evaluate the real biodegradability of new materials, the degradation time together with its dynamics, and the performances when they are applied on soil. In this view, our recent activity aims to look for innovative methods [6]. Here we report the first experimental evidence of a novel method which uses IR

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Fig. 1. Quantities distribution (tons) of plastic products used in agriculture in the last two decades.

thermography to monitor the degradation states of the proposed materials. 2. Materials and methods In the last three years, within a European ‘‘LIFE-Environment’’ Project funded by EC and entitled ‘‘Biodegradable coverages for sustainable agriculture (BIOCOAGRI)’’, we devoted our research activity, together with other partners, to prepare, characterize, study and analyse new type of materials with biodegradable properties useful for agriculture uses (mulch and solarization). The goal of our Life project was to obtain biodegradable materials in a liquid state in order to spray them on the soil as mulch or solarization film. The films should be biodegradable, non-toxic and originate from renewable resources and therefore polysaccharides were chosen to be the basic ingredients in the waterborne formulations developed. In the recent years starch-based materials have been proposed to produce agriculture film for mulching. The limits of this type of products are the degradation times and the very high cost. Recently, the research has focused on an innovative approach where a sprayable water-based varnish is applied on soil. Such

Table 1 Reported values in percentages of plastic waste in different application fields Sources Household Packaging Automotive Building Electric/electronic Agriculture

63.0% 15.0% 5.5% 5.0% 5.0% 2.0%

varnish, made of biodegradable polysaccharides mixture, contains biodegradable plasticizers to allow the film to remain elastic for the time needed. Moreover, a set of fibrous natural fillers have been tested, which can be mixed in the varnish or can be preventively applied on soil. We considered several materials with different origin and performed many measurements in order to test and verify their chemical and physical properties. Among these materials we focused our interest on three types of polysaccharides materials with the best appealing properties. 2.1. PSS This formulation consists of a mixture of the galactomannans Locust bean gum and Guar gum, in water, to which has been added a non-gelling concentration of agarose. This formulation dries into a clear film and stable toward rain. Locust bean gum, as it is shown in Fig. 2(A), is a galactomannan similar to guar gum consisting of a (1,4)-linked b-D-mannopyranose backbone with branch points from their 6-positions linked to a-D-galactose (i.e. 1,6-linked a-D-galactopyranose). There are about 3.5 (2.8e4.9) mannose residues for every galactose residue. It is less soluble and low viscose than guar gum as it has fewer galactose branch points. Locust bean gum is extracted from the seed (kernels) of the carob tree (Ceratonia siliqua). It forms a food reserve for the seeds and helps to retain water under arid conditions. Guar gum (called guaran), shown in Fig. 2(B), is extracted from the seed of the leguminous shrub Cyamopsis tetragonoloba, where it acts as a food and water store. It is made up of non-ionic polydisperse rod-shaped polymers consisting of molecules (longer than found in locust bean gum) made up of about 10,000 residues [7,8].

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Fig. 2. Materials formulations; (A) locust bean gum and (B) guar gum.

2.2. Chitosan Chitosan (from crustaceans) acts as a preservative coating and biofungicide when sprayed on plants and vegetables. Chitosan is a natural product derived, by deacetylation, from chitin, a polysaccharide found in the exoskeleton of shellfish like shrimp or crabs (see Fig. 3). It is insoluble in water but soluble in acid solution, like acetic acid or citric acid. Plant seeds are soaked in a very low concentrations of chitosan aqueous solutions to both prevent microbial infections and increase the plant production. Chitosan, in fact, can trigger defensive mechanism in plants against infections and parasite attacks [9,10]. 2.3. Alginates The alginate films are based on a polysaccharide, the sodium alginate obtained from brown seaweed. The sodium

Fig. 3. Formulation of chitosan.

alginate is a complex mixture of oligo-polymers, polymannuronic acid (MM), polyguluronic acid (GG) and a mixed polymer (MG) where sequences like GGM and MMG co-exist too (see Fig. 4). In water solution and in presence of divalent cations such as calcium ion, the peculiar buckled backbone of G segments gives rise to water insoluble gels due to the strong interaction between the divalent cations and the COOe groups of the base residual of guluronic acid; they can trap the cations in a stable, continuous and thermo-irreversible three-dimensional network, whose conformation is typical of an egg box. In particular the sodium alginate used comes from China and by NMR analysis, it results rich in guluronic acid, responsible of the rapid cross-linking in presence of calcium; on the base of this important property, it has been prepared water solutions based on Chinese sodium alginate and sprayed them on the soil; the calcium naturally present in the soil should assure the cross-linking of the polymer, allowing its permanence for the lifetime compatible with the agricultural cultivation [11,12]. In Table 2, we report the measurements of the optical characteristics of some materials considered for mulch or solarization applications. According to their properties we selected a lot of them for trials in open fields. The radiometric tests show that one of the sprayable formulations (alginate/polyglycerol) exhibits a good capacity to reduce weed growth in the soil and in the substrate, due to its low PAR (Photo Active Region) transmissivity. The film has a brown colour. For this reason we selected this material for mulching. At the same time some of them have a high capacity of storing energy in the soil due to their low transmissivity in the LWIR (long wave

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Fig. 4. Chemical formula of sodium alginate.

infrared) range. Due to their low transmission in LWIR range these materials are good candidate for solarization application. In the table we report a summary of the functional properties of the films according to the radiometric tests planned for the project, with the value of thermicity that indicates the capability of the coverage film to confine the thermal energy in the soil. The agronomic performances have been measured, together with mechanical properties and degradation behaviour. As for the mechanical properties of the studied films, it is important to underline that when we spray a polymer solution on soil we obtain a sort of coating, so, we cannot apply the classical way to characterize this type of materials as the sample is dirty and not homogeneous. A coating is not a self-standing material, it takes its support from the soil beneath, it is less sensitive to wind and it does not need to be folded into the soil for stability. Moreover, sticking on the soil, in case of damage, the fracture propagation follows a totally different path. Due to this behaviour, we are forced to recognize a new method to evaluate the mechanical response during time. We set up a ‘‘puncture test’’, a test in which the specimens, taken at different times, are penetrated by a dart until the laceration of the same occurs. We can apply this new test to PSS and alginate-based samples, but not to chitosan-based materials. In this last case it was impossible to take significant samples to

be tested, probably because chitosan interacts very strongly with fillers and soil. In the case of alginate-based materials, it is possible to individuate a quite regular trend toward lower values both of the load and of the displacements as reported in Fig. 5(A) and (B). This outline is consistent with the loosing of plasticizer during the time of permanence of the coating on the soil; so the film, resulting more rigid shows both a lower resistance to rupture and a lower displacement. In the case of PSS-based film we noted the same behaviour, but the values are higher, because the material is stronger than alginate-based (see Fig. 6(A) and (B)). The results seem to confirm the technical feasibility of such approach in developing a sustainable plasticulture. In order to give a better understanding of the materials proposed we believe that it is useful to report also some mechanical and optical properties of a traditional film and another biodegradable film (see Table 3). One of the activities planned for the project was the acquisition of information about the degradation of the films. Normally in order to study this process several pictures are taken at different times and then analysed. The comparison between the photos of the first week, of the second one and so on, indicate that the film is modifying itself. The novelty that we

Table 2 Averaged values of light transmission in the ultravioletevisibleenear infraredeinfrared regions and an estimation of thermicity value Film

Light transmission in PAR (%) (400e700 nm)

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PSS Spray Alginate/Starch C./Glycerol Alginate/PolyGlycerol Chitosan/PolyGlycerol

91 96 3,3 94

90 95 69 95

0,98 2,43 4,92 1,05

98 97 94 98

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introduced is the use of thermography to monitor the degradation. This approach allows us to evaluate the local modifications of the material investigated as a function of time and according to the changing of the thermal properties induced by the degradation dynamics. This means that the degradation induces structural modifications (including the decrease of the film thickness) thus resulting in a variation of the thermal capacity of the film. In our experiment we put some of the materials of interest on soil in open fields. The main characteristic of these materials is that they are originally liquid and they are sprayed on soil and immediately after they cross-link into a film state. This is the first time that sprayable films are proposed in agriculture for mulching soil instead of the traditional plastic ones. The IR thermal camera which we employed is an FTI6 supplied by Land Co. which provides accurate and highly stable industrial radiometric imaging and temperature measurement from 20 to 2000  C with a spectral response ranging from 3.2 to 4.2 mm and a display resolution of 0.1  C. 3. Results and discussion Through an IR thermal camera we monitored the state of the films (shown in Fig. 7) for two weeks and we analysed the thermal images to detect the continuous changing of them [13,14], studying also the temperature profiles in a selected target area of each material.

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In Figs. 8 and 9 we report as an example the images after an elaboration describing the dynamic process of the degradation of a PSS spray and of the alginate-based spray, respectively. In Fig. 10, it is reported the ratio of the average temperature (Tm) per unit area (TA) of the films on the soil temperature as a function of time. Tm is the average temperature of the whole film and TA is the temperature of a single area where the degradation is in progress. The value of this ratio decreases according to the degradation rate. In the case of the curves (a) and (b) of Fig. 10, a degradation process doesn’t appear (the ratio Tm/TA is quite constant); while the curves (c) and (d) exhibit clear degradation arising in the area A of the considered Table 3 Comparison of mechanical and optical properties of a traditional material (Goblin LDPE) and a commercial biodegradable film (Mater B) Mechanical properties Creep MD (Mpa) Creep TD (Mpa) Elongation MD (%) Elongation TD (%) Trouser Tear test (N/mm) Optical properties IR Thermal Retention (%) Total PAR 400-700 Diffused PAR (%)

Goblin LDPE 29 28 598 711 131 Goblin LDPE 21.30 93.20 5.70

Mater-Bi NF 803 25 29 653 738 187 Mater-Bi NF 803 94.50 84.10 67.50

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Fig. 7. Materials under study; (A) PSS spray, (B) biodegradable commercialised film and (C) alginate-based spray.

samples (the ratio Tm/TA is decreasing). The dynamic of the process is quite easy: the thinning of the film induces an increasing heat flux and, as a consequence, an increases the temperature per unit area that results in a decreasing of the ratio Tm/TA. It is worthwhile to notice that the values of two different sprayable biodegradable materials (c and d) after two weeks come to converge with the value of bare soil (b) and of a commercialised biodegradable film (a).

The new approach introduced here, represents a valid alternative to the traditional ones, based on the visual control at different weeks. While the analysis in the visible region gives information on the effect of the ‘‘phenomenon degradation’’, the monitoring day by day through IR thermography could extend the study to the causes of the degradation. As well known, the degradation of a film is affected by many factors ranging from different species of bacteria in soil to the

Fig. 8. Thermal images of a PSS spray analysed vs time.

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Fig. 9. Thermal images of an alginate-based spray analysed vs time.

chemical nature of the soil, from the local climate conditions to the chemical agents used by the farmer. In principle, thanks to our method, it is possible to correlate these factors and to evaluate the role of each component and its contribution to the degradation state. Furthermore, it’s possible to monitor the dynamic of degradation, acquiring information before the tearing of the film. As explained before, in correspondence of a modification of the physical and chemical structures of the film, its

emittivity changes, giving rise to a higher heat transmission which is detected in the tearing area. This behaviour is confirmed by the data reported in Fig. 11. The bigger the tearing (degradation state) the higher is the temperature detected. In conclusion, our experimental results show that thermography can be a very good tool to monitor the degradation of a film in agriculture but also to give information on the degradation mechanism related to the soil state. Furthermore MB ABLACK PSS5 TEST

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we believe that the ratio values of temperatures reported in Fig. 10 could be considered as a reference parameter in order to indicate the degradation state of a material employed in agriculture. Acknowledgments This work has been partially supported by the European Life Project Bio. Co. AGRI contract 03/377. References [1] Bastioli C. Properties and applications of Mater-Bi starch-based materials. Polym Degrad Stab 1998;59:263e72. [2] 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. [3] Lo¨rcks J. Properties and applications of compostable starch-based plastic material. Polym Degrad Stab 1998;59:245e9. [4] Chandra R, Rustgi R. Biodegradable polymers. Prog Polym Sci 1998; 23:1273e335.

[5] Feuilloley P, Cesar G, Benguigui L, Grohens Y, Pillin I, Bewa H, et al. Degradation of polyethylene for agricultural purposes. J Polym Environ 2005;13(4):349e55. [6] Berardi PG, Cuccurullo G, Di Maio L, Thermography for polymers film. Proceedings of QIRT-98 Eurotherm Seminar, vol. 60; 1998: p. 265e71. [7] Petkowicz CLO, Reicher F, Mazeau K. Conformational analysis of galactomannans: from oligomeric segments to polymeric chains. Carbohydr Polym 1998;37:25e39. [8] Daas PJH, Schols HA, de Jongh HHJ. On the galactosyl distribution of commercial galactomannans. Carbohydr Res 2000;329:609e19. [9] Gomez-Rivas L, Escudero-Abarca BI, Guadalupe Aguilar-Uscanga M, Hayward-Jones PM, Mendoza P, Ramırez M. Selective antimicrobial action of chitosan against spoilage yeasts in mixed culture fermentations. J Ind Microbiol Biotechnol 2004;31:16e22. [10] Durango AM, Soares NFF, Andrade NJ. Microbiological evaluation of an edible antimicrobial coating on minimally processed carrots. Food Control 2006;17:336e41. [11] Haug A, Larsen B, Smidrod O. Acta Chem Scand 1966;20:183e90. [12] Haug A, Smidrod O. Acta Chem Scand 1965;19:1221e6. [13] Bruening RJ, Mordfin L. Infrared thermography standards for nondestructive testing. In: Snell JR, editor. Thermosense XVI. SPIE Proc, 2245; 1994. p. 220e3. [14] Maldague X. Applications of infrared thermography in nondestructive avaluation. In: Rastogi P, editor. Trends in optical nondestructive testing. New York: Elsevier; 2000. p. 591e609.