Mechanical behaviour of biodegradable agricultural ﬁlms...

Polymer Degradation and Stability 91 (2006) 1256e1272www.elsevier.com/locate/polydegstab

Mechanical behaviour of biodegradable agricultural filmsunder real field conditions

D. Briassoulis*

Agricultural University of Athens, Department of Agricultural Engineering, Iera Odos 75, 11855 Athens, Greece

Received 7 May 2005; received in revised form 14 August 2005; accepted 6 September 2005

Available online 11 November 2005

Abstract

The mechanical behaviour of various types of biodegradable materials depends on their chemical composition and additives, the processingcharacteristics and the application conditions. The environmental conditions during storage and usage of these materials strongly influence theirmechanical properties and behaviour. Ageing and degradation during the useful lifetime of biodegradable agricultural films causes losses in themechanical performance of the material, as measured by monitoring the evolution of some of the critical mechanical properties. Such losses maybe comparable to the corresponding losses of the conventional polyethylene agricultural films due to ageing, or they may be more drastic. In thepresent paper, the overall mechanical and ageing/degradation behaviour of experimental specially designed and manufactured low-tunnel andmulching biodegradable films, exposed to full-scale field conditions is analysed. Selected critical mechanical properties of these films manufac-tured with different grades of Mater-Bi material and additives, different thickness and processing schemes and exposed to real cultivation con-ditions in four different locations in Europe are investigated in the laboratory and compared against the corresponding behaviour of conventionalagricultural films at various stages of their exposure time.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Biodegradable materials; Agricultural films; Degradation of plastic films; Mechanical properties

1. Introduction

An extensive and steadily expanding use of plastic films(mostly polyethylene) in agriculture and particularly in pro-tected horticulture (mulching, low tunnels, greenhouses) isreported world-wide over the last decades [1]. The market ofplastics used for these purposes in Europe involves hundredsof thousands of hectares and thousands of tons of plastic filmsper year [2]. The increasing interest in the use of mulching andlow tunnels for protected cultivation aims at elimination ofweeds, conservation of water and fertilisation also providinga better micro-environment for the plants and protectionagainst adverse climatic conditions. The conventional agricul-tural plastic films used today are low density polyethylenes

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E-mail address: [email protected]

0141-3910/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2005.09.016

(in some cases HDLE, LLDPE), poly(vinyl chloride), polybu-tylene or copolymers of ethylene with vinyl acetate.

A major negative consequence of this expanding use ofplastics for protected horticulture is related to handling theplastic wastes and the associated environmental impact asonly a small percentage of the constantly rising amount of ag-ricultural plastic waste is currently recycled. A large portion ofthis is left on the field or burnt uncontrollably by the farmersreleasing harmful substances with the associated obvious neg-ative consequences to the environment. The aesthetic pollutionand landscape degradation of regions of natural beauty is an-other well-known negative consequence in the Mediterraneanregions of Europe. Furthermore, burying of these materials inthe agricultural land, a practice unfortunately followed bymany farmers in Europe, represents an imminent threat foran irreversible soil contamination and possibly for the safetyof the food produced in such fields. The reasons for these en-vironmentally dangerous practices are the lack of cost efficient

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1257D. Briassoulis / Polymer Degradation and Stability 91 (2006) 1256e1272

systematic disposal techniques available to the growers andthe high labour cost for the proper collection of the plasticfilms following the end of the cultivation. Thus, the use of bio-degradable materials appears as a challenging attractive alter-native for enhancing sustainable and environmental friendlyagricultural activities related to mulching and low-tunnels cul-tivation applications.

In an effort to cope up with the severe and continuouslygrowing agricultural plastic waste problem specific promisingmaterials have been developed, some of which are already inthe market, even though not widely used yet [3]. Two maincategories of innovative materials, based on two different con-cepts, are at the moment under development with the supportof intensive research efforts world-wide. In particular, amongthe materials developed included are really biodegradablefilms but also partially biodegradable films or even films ofcontrolled photodegradation followed by a questionable fatein the soil [1]. It should be emphasised that in all the casesof agricultural biodegradable films, biodegradability is oneof the major design requirements. Unfortunately, in severalcases, biodegradability also represents a major controversialissue. Thus, it is well known that poly-lactone and poly(vinylalcohol) films are readily degraded by soil micro-organisms.On the other hand, the addition of specialised pro-oxidantsin polyethylene films accelerates the breakdown of polyethylenebut biodegradability of such materials is strongly disputed[4e11].

The efficient and profitable use of commercial biodegrad-able films aiming at achieving high performance throughouttheir useful lifetime and at reducing pollution through practis-ing environmentally friendly, sustainable agriculture, involvesseveral crucial technological questions. In particular, biode-gradable agricultural films should meet a set of minimum de-sign requirements, including: adequate strength and elongationat break for mechanical installation, good mechanical proper-ties with regard to ageing during the useful lifetime of the filmand 100% biodegradation in the soil preferably before the nextcultivation season. Specifically for low-tunnel films, the designrequirements also include an adequate mechanical behaviourof these films to resist various loads and load combinations(wind, hail, snow loads, etc.) [12]. In addition, special addi-tives may be required aimed at adjusting the physical proper-ties of these films depending on the geographical region, theparticular cultivation needs and the season.

A literature review on the mechanical behaviour of varioustypes of biodegradable materials [3] reveals that their mechan-ical properties depend, in general, on their chemical com-position [13,14], the processing parameters, storage andapplication conditions [15,16] and degradation behaviour.However, the mechanical behaviour of biodegradable agricul-tural films has not been the subject of a systematic research yetas these materials are either at the experimental stage of devel-opment or their market share still remains limited. In one case,carbon-black-filled, biodegradable, copolyester mulch film(EA) and commercial carbon-black-filled, high-density poly-ethylene (HDPE) mulch film were exposed for 12 weeks tocommercial vegetable crop growing conditions in the U.S.A.

and the positive results are reported in Ref. [17]. In this casethe biodegradable EA mulch film exhibited higher tensileand elongation at break than the presently used HDPE mulchfilm. It was shown that the mechanical properties, includingthe tensile strength of both the EA and HDPE, were sufficientlypreserved from planting to harvest. The changes in the elonga-tion at break of the EA film proved that it possesses the prop-erties needed for the specific agricultural applications.

The present work concerns with the analysis of the mechan-ical behaviour of the innovative biodegradable mulching andlow-tunnel films developed in the course of a Europeanproject1 aiming at optimising the design of these films. Thebiodegradable material selected and used was Mater-Bi, a materialbased on starch complexed with biodegradable polyesters. Mater-Bi was selected among several alternative biodegradable mate-rials considered, with emphasis placed on the applicability ofthe selected material for agricultural applications (e.g. biode-gradability in soil, no ecotoxicity, acceptable strength oforiginal films, etc.). Time and cost limitations related to thefull-scale experiments did not allow for testing more materialsin the framework of this project. The series of the full-scaleand laboratory experiments described in this work has beencarried out to investigate, among others, the overall mechani-cal performance of the new sets of experimental biodegradablefilms and optimise their design according to the mechanicaldesign requirements established in Ref. [12]. The performanceof several experimental biodegradable films made of differentgrades of Mater-Bi material and additives, different thicknessand processing parameters exposed in the field under realcultivation conditions in several locations in Europe, as wellas in the laboratory, is analysed systematically in the followingsections and compared against the performance of the corre-sponding conventional polyethylene films.

2. Experimental design and materials

2.1. Full-scale experimental design

Three sets of experimental biodegradable low tunnel, directcover and mulching films were developed, tested and their de-sign was optimised over a period of three years. The filmswere used and tested in full-scale experimental cultivationsin four different European locations, in Italy, Greece, Franceand Germany. The biodegradable films were compared againstconventional PE/EVA films used currently in low tunnel,mulching and direct cover applications. During the field testsseveral parameters of the films performance and their effectsin the agronomic productivity, soil characteristics and eco-toxicity were monitored and evaluated continuously. The deg-radation/ageing process of these materials under realcultivation conditions (exposure to solar radiation, wind loads,rain and hail, installation procedures, etc.) was investigated bymeans of mechanical, radiometric and other appropriate

1 Bioplastics: ‘Biodegradable plastics for environmentally friendly mulching

and low-tunnel cultivation’, QLK5-CT-2000-00044.

1258 D. Briassoulis / Polymer Degradation and Stability 91 (2006) 1256e1272

physical tests, soil analyses, etc. Also, the biodegradation be-haviour of the biodegradable films after they were rototilled inthe soil, along with the plant remains, was evaluated systemat-ically. Furthermore, the possible release of harmful substancesin the soil due to the biodegradation process of the materialwas investigated by means of ecotoxicity tests carried out dur-ing the field tests and for a period of three years following thefirst experimental season. The optimisation of the design andmanufacturing of each new set of films was based on the sys-tematic evaluation of the overall performance of the previousset of the films as well as on analytical approaches [12]. Workis in progress on elaborating, evaluating and eventually pub-lishing the results of these extensive investigations [18e20].

In the present work the mechanical behaviour of the biode-gradable films exposed in the four experimental cultivations inAthens, Montpellier, Bari and Hannover, is analysed. Threedifferent experimental biodegradable materials were devel-oped and tested within each set of full-scale experiments(one set a year). Apart from the biodegradable films (denotedby the codes M1, M2 and M3 for the mulching films and L1,L2 and L3 for the low-tunnel films), PE conventional films(denoted by the codes M0 for the mulching film and L0 forthe low-tunnel film) were used as controls.

The low-tunnel field trials in Greece started late in Aprilwith watermelon cultivation. Mulching cultivation withoutcovering started later, in May. The experimental field shownin Fig. 1 is located in the region of Spata northeast of Athens.Mulching films for both covered and uncovered cultivationwere installed by manual work and watermelon plants weretransplanted at one row. The low-tunnel films were installedby manual work as well (over the mulching and the trans-plants). The low-tunnel films were stretched over plasticarches approximately 50 cm high and 60 cm wide (Fig. 1) (in-stallation of low tunnels: April 22, 2002; May 13, 2003; April6, 2004. Installation of mulching films: April 22, 2002; May 9,2003; May 4, 2004; low-tunnel films removal: May 20, 2002;June 27, 2003; May 25, 2004). During the warm days of May,openings were made on the tunnel films, for ventilation rea-sons. The low-tunnel films were dismantled late in May orin June, while the harvesting was performed during the

Fig. 1. The experimental field (watermelon cultivation) of AUA at Spata,

Attiki.

summer period. Analogous full-scale experiments were de-signed in the other locations [18e20].

During the three experimental cultivation periods sampleswere periodically cut out of the exposed biodegradable andconventional films in all four locations following a strictschedule, more frequently during the first period, in order toassess the evolution of their mechanical properties with timeduring their exposure under real field conditions. One of themain targets of this work was to investigate the effects ofthe processing parameters, various stabilisers and the filmthickness on the degradation of the biodegradable agriculturalfilms with the exposure time, aimed at developing optimisedfilms. It appears that it is in fact possible to develop optimisedthin biodegradable agricultural films based on Mater-Bi thatcan perform adequately for the specific applications (on-goingresearch work).

2.2. Critical material characteristics

As shown in Table 1 three different sets of agricultural bio-degradable films were developed, one set each year, over thethree year-period, each set optimised with the developmentof thinner films following the evaluation of the performanceof the corresponding previous set of films. All films were es-pecially designed and optimised for the corresponding agricul-tural applications. Concerning the mechanical performance ofboth the mulching and low-tunnel biodegradable films, themost critical parameters investigated, for the given grades ofMater-Bi material, are (a) material and additives, (b) filmthickness, and (c) processing parameters.

2.2.1. MaterialsThe materials finally selected and used in this work for the

experimental films development are various grades of Mater-Bi, a starch based complexed with biodegradable polyestersbiodegradable material that differ in terms of lifespan underreal cultivation conditions, ranging from 3 to 9 months. Ma-ter-Bi materials are suitable for film cover with a very widerange of properties in terms of processability, mechanicalproperties, transparency, permeability to water, and biodegra-dation rate. They are both biodegradable and compostable ac-cording to the present European standard CEN EN13432 (alsoDIN 54900 and UNI 10785 standards). Mater-Bi was selectedamong several alternative biodegradable materials suitable forthe production of experimental agricultural films based on cri-teria related to the particular agricultural films applications: tobe biodegradable in soil, to exhibit acceptable initial strength,to be free of ecotoxic effects in soil, etc. The number of theparameters under investigation in the effort to optimise thefilm processing and design and the cost and time limitationsassociated with the full-scale experiments did not allow formore biodegradable materials to be studied in this particularproject. The selection made by no means excludes the possi-bility that other innovative biodegradable materials may bedeveloped, that could be better suited for the particular appli-cations (this is an open topic of research work). The


Table 1

Material characteristics of low-tunnel and mulching films exposed in full-scale experiments in Athens

Material code Film material Processing scheme

Mulching film material

Experimental set of 2002

M1-NF 803/PeA-20b-02/A Mater-Bi material: NF 803/P e 20 mm; plus 3.6% carbon blacka A


M3-NF x66eA-18b-02/B Mater-Bi material: NF x66 e 18 mm; plus 5% carbon blackf B

M0-LLDPEeA-25b-02/B LLDPE material e 25 mm; plus 5% carbon blackf B




M3-NF 803/PeA-15b-03/B Mater-Bi material: NF 803/P e 15 mm; plus 7% carbon blackf B





M3-NF 803/PeA-20c-04/B Mater-Bi material: NF 803/P e 20 mm; plus 3.7% green coloure B


Low-tunnel film material


L1-NF 803eA-60-02/B Mater-Bi material: NF 803 e 60 mm; B

L2-NF 803eA-60s-02/B Mater-Bi material: NF 803 e 60 mm; plus 1% UV stabiliserc B

L3-NF 803eA-60s-02/A Mater-Bi material: NF 803 e 60 mm; plus 0.2% Triazineb A

L0-LDPEeA-60-02/B LDPE material e 60 mm; no stabiliser B

Experimental set of 2003L1-NF 803eA-40-03/B Mater-Bi material: NF 803 e 40 mm; no stabiliser B

L2-NF 803eA-40s-03/B Mater-Bi material: NF 803 e 40 mm; plus 1% UV stabiliserc B

L3-NF 803eA-30s-03/A Mater-Bi material: NF 803 e 30 mm; plus 0.2% Triazineb A

L0-LDPEeA-60-03/B LLDPE material e 60 m; no stabiliser B


L1-NF 803/PeA-30-04/B Mater-Bi material: NF 803/P e 30 mm; no stabiliser B

L2-NF 803/PeA-30s-04/B Mater-Bi material: NF 803/P e 30 mm; plus 1% stabiliserd B

L3-NF 803/PeA-30s-04/A Mater-Bi material: NF 803/P e 30 mm; plus 0.2% Triazineb A

L0-LDPEeA-60-04/B Material PE 1010 e 60 mm; no stabiliser B

a VIBA Carbon Black 99545 is a master-batch based on Mater-Bi carrier with 40% of carbon black.b Cyatec UV1164: CYATEC UV1164 is a UV adsorber characterised by a very low migration rate, and suitable for very low thicknesses.c UV master-batch KRITILEN UV 20H (high molecular weight polymeric HALS with a small quantity of processing stabiliser) in LDPE carrier resin produced

using raw materials which comply to the food packaging regulations.d UV master-batch KRITILEN UV 22H (mixture of two high molecular weight polymeric HALS with a small quantity of processing stabiliser) in LDPE carrier

resin produced using raw materials which comply to the food packaging regulations.e Green master-batch KRITILEN GREEN 51670 in LLDPE carrier resin produced using raw materials which comply to the BGA food packaging regulations.f Black master-batch KRITILEN BLACK 438 in LDPE or LLDPE carrier resin produced using raw materials which comply to the food packaging regulations.

characteristics of the experimental biodegradable and conven-tional black mulching and low-tunnel film materials tested inAthens are presented in Table 1. The conventional mulchingfilm material used in these experiments is LLDPE whileEVA films were also used and tested as control mulchingfilm in some other locations. Conventional LDPE films wereused for the low tunnels, without stabilisers. In the case of bio-degradable mulching films, two grades of Mater-Bi materialswere tested: NF 803/P and NF x66 (the second one was testedonly during the first experimental set). In the case of biode-gradable low-tunnel films, two grades of Mater-Bi materialswere tested: NF 803 (first and second experimental sets) andNF 803/P (the same NF 803/P material was used for both,the low-tunnel and the mulching films during the last-third ex-perimental period). Grade NF 803/P was found to be bestsuited for thin agricultural films.

2.2.2. AdditivesAs far as the biodegradable mulching films are concerned,

carbon black was used in most cases (carbon black master-batch based on Mater-Bi for the processing scheme A orcarbon black master-batch based on polyethylene for the pro-cessing scheme B). Any possible stabilising effects of carbonblack on the degradation of Mater-Bi based biodegradablefilms are yet unknown (carbon black is known, however, toact as a strong stabiliser for the conventional film during itsuseful lifetime). In some other cases of biodegradable mulch-ing films, special additives that might be of interest for specificcultivations like colours (especially green) or stabilisers fortransparent mulching films were tested. As far as the biode-gradable low-tunnel films are concerned, the effect of theUV stabilisers belonging to two main groups was investigated,as shown in Table 1: (a) HALS based UV stabilisers and (b)


UV adsorber characterised by a very low migration rate, suit-able for films of very small thickness.

2.2.3. ThicknessThe design of the biodegradable films aimed, among others,

at establishing a minimum possible thickness for the film sothat the price can be reduced and the biodegradability ratecan be enhanced, retaining, however, a satisfactory perfor-mance. As shown in Table 1 it was possible to reduce gradu-ally the design thickness of the biodegradable films from thefirst to the last experimental set, based on the feedback andthe experience gained from the previous experimental test re-sults and following laboratory tests and processing trials and,specifically for low-tunnel films, the engineering design ap-proach introduced in Ref. [12]. It should be noticed, however,that the thickness values of the films shown in Table 1, actuallyrepresent the nominal thickness of the manufactured films.The measured film thickness was in fact found to deviatefrom the nominal one, to a variable degree depending on theprocessing conditions.

2.2.4. Processing parametersTwo different commercial processing schemes were em-

ployed in manufacturing the experimental biodegradable filmswithin each set, as shown in Table 2. The processing parame-ters pertaining to the two schemes differ significantly. A majordifference concerns the size of the commercial extruder used(a rather large machine was used with Scheme B).

3. Critical mechanical properties of biodegradableagricultural films

3.1. Mechanical design requirements of biodegradableagricultural films

Concerning the design requirements for agricultural films,a brief overview is presented in Ref. [12]. As it is explainedin Ref. [12] the recent European Standard on ‘plastics emulching thermoplastic films for use in agriculture and horti-culture’ [21] and the Standard ‘‘covering thermoplastic filmsfor use in agriculture and horticulture’’ [22] require that agri-cultural films should meet specific minimum values of selectedmechanical properties, without, however, relating those prede-termined values with the conditions to which the films will beexposed to and the installation and supporting systems. Somecritical design requirements for biodegradable agriculturallow-tunnel films are derived in Ref. [12] as a result of a system-atic analytical and experimental research work.

3.2. Laboratory assessment of the mechanical behaviourof biodegradable agricultural films

A complete list of standard methods, pertaining to plastics andplastic films in general, which can be used for assessing the me-chanical properties of LDPE films used in protected cultivationwas presented in earlier works [23]. A sub-set of selected critical

standard testing methods is introduced in the European StandardsprEN 13655:2001 [21] and prEN 13206:1998 [22]. Testingmethods for mechanical properties of agricultural films thathave been harmonised at a European level2 were employed inthe course of the present work as follows.

3.2.1. Tensile propertiesThe tensile properties of agricultural films were measured

following the EN ISO 527-3 [24] or its equivalent ASTM D882 [25]. The basic procedure of this method, with appropriateadjustments for agricultural films is described in Ref. [26].

Table 2

Technical characteristics of the two processing schemes used in the corre-

sponding production lines of agricultural biodegradable films

Processing

parameter

Technical characteristics of the processing schemes

A and B

Case A Case B

Diameter of

extruder (mm)

80 100/150/100

Diameter of

head (mm)

300 800

L/D of extruder 25 30

Die gap of

head (mm)

1.2 2.1

Cooling ring Cooling ring: one outlet Two outlets

System used Simple outside cooling

(no IBC)

IBC

Screw RPM 23e44 rpm; most of

production 35e44 rpm

depending upon the

flow rate required to

meet both film width

and thickness specification

21/31/21 rpm

Screw

temperature

profile ( �C)

130, 140, 145, 145, in one

case of Mater-Bi NF

x66 C Mb

Black (12%): 115,

130, 130, 135

110e120 �C

Blow die

temperature

profile ( �C)

145, 145, 145, 145, in

one case of Mater-Bi

NF x66 C Mb Black

(12%): 140, 140,

145, 145

110e120 �C

BUR (blow up ratio) 2.97e4.24

(for film production)

1.6e2.2

Winder speed

(m/min)a6.2 (60 mm film)e29

(15 mm film)

65e80

Bubble radius According to film width According to

film width

Pressure inside

the bubble

Not available Not available

Tensile force

from the wind up

Not available Not available

Wider speed and screw RPM values may vary according the characteristics of

raw material lot, ambient conditions during extrusion, and melt homogeneous-

ness coming out from the die gap.a Winder speed is adjusted in order to obtain the requested thickness.

2 SMT project: New testing methods for plastic films used as greenhouse

covering materials; contract no. SMT4-CT97-2154 (DG 12).


3.2.2. Tear resistanceTear resistance of plastic films is a complex function of its

ultimate resistance to rupture. However, the European Stand-ards prEN 13655:2001 [21] and prEN 13206:1998 [22] donot include this property among the properties to be testedand reported. As shown in Ref. [26] standard testing methodshave been devised to test two different values of tear resis-tance: (a) the force required to initiate the tear and (b) theforce required to propagate the tear. Both properties are impor-tant for agricultural films even though it might occasionally beimpossible to prevent a film from tearing and so resistance totear propagation becomes of special interest. On the otherhand, it is important for a thin film to resist initiation of tearsince resistance to tear propagation is expected to be muchlower. The method used for initial tear resistance is ASTMD 1004-90 [27] (also ISO 34-1 [28]; using Graves angle testpiece with nick). In fact, this testing procedure measures thetotal force required for both, initiation and propagation oftear [27]. Tear propagation resistance can be measured bythe trouser tear method ISO 6383/1 [29]. The standard Elme-dorf method ISO 6383/2 [30] is a method designed to testmainly packaging materials and is more suitable for highly ex-tensible films, because it employs very high speeds of tearing(7.6e46 m/min), when compared to the trouser tear methodwhich employs low speeds of tearing (250 mm/min). Duringa preliminary phase of the present work, the thin biodegrad-able films under investigation were also tested with the trousertear method and were found to exhibit rather low tear propa-gation resistance values, within the error range of the smallerload cell available, and so it was decided to employ only theinitial tear resistance method [27] for simultaneously measur-ing initiation and propagation of tear. Of course, the mecha-nism of pure tear propagation [29,31] is different from thecombined tear initiation and propagation mechanism [28]and the results should be interpreted accordingly.

3.2.3. Impact resistanceAccording to the new prEN 13206:1998 standard [22] the

impact resistance shall be measured with the ISO 7765-1,method A [31] (or ASTM D 1709 [32]). The impact resistanceof the experimental biodegradable films exposed to real fieldconditions was not tested because of the limitations imposedby the large number of samples needed for this test.

4. Mechanical behaviour of biodegradableagricultural films

In the following sections, the results of the mechanical testsperformed on the samples taken at pre-defined time intervalsfrom the low-tunnel and mulching films exposed during thethree full-scale field experiments in Athens and in the otherthree experimental locations are presented. Cleaning of thesamples exposed to the field conditions was proven to bea rather difficult to accomplish, but also a very importanttask, related to the efficiency of the mechanical testing as it af-fects the thickness measurements and also, in some cases, thecondition (degradation) of the samples received.

4.1. Mechanical behaviour of original biodegradableagricultural films

The experimental Mater-Bi based biodegradable films ofthe first full-scale experiment in Italy (University of Bari)were designed and manufactured for a nine-month strawberrycultivation. The mechanical performance of these originalfilms (before exposure) is analysed in this section and com-pared to the corresponding behaviour of the conventionalpolyethylene (or EVA) films (this behaviour was briefly pre-sented in Ref. [3]). The reason for choosing this particular firstset original Mater-Bi based biodegradable films for the com-parative studies is that the thickness of these films was compa-rable to the one of the conventional films as they are designedfor a strawberry cultivation that requires a good mechanicalperformance over a longer cultivation period (note that thefull-scale experiments in the other locations or in the nexttwo experiments in Bari were designed for watermelon, melonand lettuce that require only one to three months covering pe-riod by low-tunnel films and so thinner biodegradable filmswere used in those cases).

The following samples of original films were examined: (a)samples of thin biodegradable films suitable for covering lowtunnels (denoted by ‘L’) and (b) samples of thin biodegradablemulching films (denoted by ‘M’). The nominal thickness ofthe low-tunnel films was 60 mm whereas the measured thick-ness was found to vary in the range of 45e75 mm. Thenominal thickness of the mulching films was 60 mm for theconventional film and 25 mm, 30 mm and 60 mm for the biode-gradable films; the measured thickness was found to vary sig-nificantly though. The conventional films tested were LDPE(denoted by L0-B and M0-A) and three-layer LLDPEeEVAfilms (denoted by L0-A) and LLPPE (M0-B). The mechanicalproperties were determined at the Laboratory of Strength ofMaterials of the Agricultural University of Athens in machinein parallel (P) and in transverse (T) directions according to theprocedure described in Ref. [26].

4.1.1. Tensile strength of low-tunnel filmsThe behaviour of the original (before exposure) thin low-

tunnel biodegradable films shown in Fig. 2 suggests a rathergood mechanical behaviour that is comparable to the corre-sponding behaviour of the conventional low-tunnel films.Elongation at break is higher in the transverse direction whilestrain hardening in the parallel direction is characterised bya higher tangent modulus. Elongation at break does not varyso much as with the mulching films. Variability in the thick-ness was found to be significant.

4.1.2. Tensile strength of mulching filmsThe behaviour of the original very thin mulching biode-

gradable films shown in Fig. 3 is comparable to the one expe-rienced by the corresponding PE mulching films in the paralleldirection while in the transverse direction the elongation atbreak is lower [3]. The elongation at break is again higherin the transverse direction but this property was found to berather sensitive. Another characteristic of the thin


parallel direction

02468

10121416

0 100 200 300 400 500

stress (M

Pa)

M3NFx66B-30M1NFx66A-52M0-LDPE–A-55M0-LLDPE–B-22

transverse direction

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500strain (%)strain (%)

stress (M

Pa)

M3NFx66B-32M1NFx66A-59M0-LDPE–A-52M0-LLDPE–B-24

(a) (b)

Fig. 3. Typical tensile stressestrain curves for very thin biodegradable mulching film in parallel (a) and transverse (b) direction (10 cm specimens) (2-digit

numbers: thickness in mm) [3].

parallel direction

02468

101214161820

0 100 200 300 400 500 600

stress (M

Pa)

L3NF803B-67L0-LDPE–B-55L0-EVA–A-45L1NF803A-67


02468

101214161820

0 200 400 600strain (%)strain (%)

stress (M

Pa)

L1NF803A-65L3NF803B-67L0-LDPE–B-45L0-EVA–A-47

(a) (b)

Fig. 2. Typical tensile stressestrain curves for thin biodegradable agricultural film in parallel (a) and transverse (b) direction (5 cm specimens) (two-digit numbers:

thickness in mm) [3].

parallel direction

0

20

40

60

80

100

120

140

0 20 40 60 80displacement (mm)

in

itial tear resistan

ce

(N

/m

m) L1NF803A-62

L3NF803B-73

L0-LDPE–B-50

L0-EVA–A-49


0

20

40

60

80

100

120

140

0 10 20 30 40 50 60displacement (mm)

in

itial tear resistan

ce

(N

/m

m)

L1NF803A-61

L3NF803B-75

L0-LDPE–B-50

L0-EVA–A-50

(a) (b)

Fig. 4. Typical initial tear resistance curves for thin biodegradable low tunnel films in parallel (a) and transverse (b) direction (two-digit numbers: thickness

in mm) [3].

parallel direction

0

20

40

60

80

100

120

140

0 10 20 30 40displacement (mm)

initi

al te

ar d

irect

ion

(N/m

m) M1NFx66A-54

M3NFx66B-26

M0-LDPE–B-50


0

20

40

60

80

100

120

140

0 10 20 30 40 50 60displacement (mm)

initi

al te

ar re

sist

ance

(N/m

m) M3NFx66B-27

M2NFx66A-35

M0-LDPE–A-49

(a) (b)

Fig. 5. Typical initial tear resistance curves for thin biodegradable mulching films in parallel (a) and transverse (b) direction (two-digit numbers: thickness

in mm) [3].


biodegradable mulching films is the insignificant strain-hard-ening effects. The most important observation for the mulch-ing films is the significant variability of the thickness.Another feature of interest is the significant variation of theelongation at break, especially in the parallel direction. Themain source of this differentiation was attributed to the thick-ness of the film (the thinner the film the higher the variability)and also, to the processing parameters used.

4.1.3. Tear resistanceFinally, the initial tear resistance results, shown in Fig. 4 for four

thin biodegradable low-tunnel films and in Fig. 5 for four thin bio-degradable mulching films indicate a satisfactory behaviour,comparable to that of the corresponding conventional PE films [3].

4.2. Evolution of mechanical properties of mulching filmsexposed under real cultivation conditions

The evolution of the elongation at break, the tensilestrength and the initial tear resistance for three sets, each

consisting of three different mulching films and the conven-tional polyethylene film, exposed in the experimental field ofAUA (Spata, Attiki) during the years 2002e2004, is presentedin this section. Selected typical cases of films exposed in theother three locations and tested in AUA, of special interestfor the present analysis, are also analysed in this section.

4.2.1. Elongation at breakThe evolution of the elongation at break of the biode-

gradable mulching films in the four full-scale experimentallocations (Fig. 6) confirms that in some cases no major de-crease is experienced at least within the first weeks (Fig. 6c:M1, M2; Fig. 6a: M1) whereas, in other cases, the elongationat break value of the original biodegradable film is alreadyvery low (e.g. case of M3-film manufactured by processingscheme B: Fig. 6c,e; M2 thin film (thickness 12 mm; process-ing scheme A) and M3 film (thickness 20 mm; processingscheme B): Fig. 6a). A dramatic drop of elongation at breakto zero is confirmed in the transverse direction for the mulch-ing biodegradable films within the first couple of weeks for

parallel direction

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time (weeks)

elon

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ak (%

)

M0LDPEB-40M1NF803/PB-20M2NF803/PB-12M3NF803/PB-20 (green)

B


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0 1 2 3 4 5 6 7 8time (weeks)

elon

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ak (%

)

M0LLDPEA-25bM1NF803/PA-15bM2NF803/PA-12bM3NF803/PA-20c

B

(a) (b)

parallel direction

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0 1 2 3 4 5 6 7 8 9time (weeks)

elon

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ak (%

)

M0LLDPEA-25

M1NF803/PA-20

M2NF803/PA-15

M3NF803/PA-15

B


0 2 4 6 8 10 12time (weeks)

0

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200

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400

500

600

elon

gatio

n at

bre

ak (%

)

MOLDPEC-25

M4NF803/PC-25(transparent)M5NF803/PC-25(stabilized)M6NF803/PC-25

All B

(c) (d)

parallel direction

0

100

200

300

400

500

600

0 2 4 6 8time (weeks)

elon

gatio

n at

bre

ak (%

)

M0LDPEA-25

M1NF803/PA-20

M2NF803/PA-25

M3NF866A-18

B


0

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600


elon

gatio

n at

bre

ak (%

)

M0LDPEA-25

M1NF803/PA-20

M2NF803/PA-25

M3NF866A-18

B

(e) (f)

Fig. 6. Typical elongation at break in parallel (a,c,e) and transverse (b,d,f) direction curves for thin biodegradable mulching films exposed in Athens (b,c,e,f), Bari

(a), and Montpellier (d) (two-digit numbers: nominal thickness in mm) (e,f and c,d and a,b are the corresponding curves of the first and second and third exper-

imental tests, respectively).


most of the cases (Fig. 6), unless the initial elongation at breakvalues is already very low (mostly processing scheme B orvery thin films; M2 thin film 12 mm: Fig. 6b).

The low initial elongation at break values observed in sev-eral cases of biodegradable mulching films may be partiallyattributed to the processing parameters used for the film extru-sion and film blowing that apparently would need further op-timisation for the particular Mater-Bi material, especially forscheme B. Despite this, it should be noticed that it was possi-ble to produce rather thin films (third experimental set) withperformance analogous to that of thicker films (the film thick-ness was gradually reduced from the first set to the third set;from 25 mm to 12 mm).

In general, the relatively quick drop of the elongation atbreak of the biodegradable mulching films as compared tothe conventional polyethylene films (thicker in most cases) un-der their exposure to field conditions is directly related to thedegradation of this property under a combination of conditions(UV, temperature, stress, humidity, etc.), a behaviour thatappears to be inherent to Mater-Bi made very thin films pro-duced with the specific processing schemes. The additive car-bon black used with the mulching films does not function as

a stabiliser in the case of the Mater-Bi based films in theway it does for polyethylene films.

4.2.2. Tensile strengthThe evolution of the tensile strength in most cases of the

biodegradable mulching films experienced in the four loca-tions (Fig. 7) suggests a slight local drop in the tensile strengthespecially in the transverse direction, during the first weekfollowed by a quick recovery right after it. In some casesthe tensile strength in the transverse direction decreases slightlybelow the initial stress at yield, a behaviour observed at ran-dom time intervals (Fig. 7). In addition, there are also somecases of biodegradable mulching films that exhibit a strengththat is systematically lower than the corresponding initialstress at yield values (e.g. very thin films exposed in Athens,Fig. 7b; it should be noticed that the tensile strength values fol-low closely and asymptotically the evolution of the corre-sponding stress at yield values). This low-tensile strengthbehaviour may be partially attributed to the processing param-eters used, especially for some cases of very thin films, as dis-cussed later on but, most probably, to the higher sensitivity ofthe very thin mulching films to degradation, following their

parallel direction

0

5

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tens

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treng

th (M

Pa)

B


parallel direction transverse direction

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25


tens

ile s

treng

th (M

Pa)

B

(a) (b)

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tens

ile s

treng

th (M

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M0LLDPE-25

M1NF803/PA-20

M2NF803/PA-15

M3NF803/PA-15B

0

5

10

15

20

25

0 2 4 6 8 10time (weeks)time (weeks)

time (weeks)time (weeks)

tens

ile s

treng

th (M

Pa)

B

(c) (d)

parallel direction

0

5

10

15

20

25

0 2 4 6 8

tens

ile s

treng

th (M

Pa)

M0LDPEA-25

M1NF803/PA-20

M2NF803/PA-25

M3NF866A-18

M0LDPEA-25

M1NF803/PA-20

M2NF803/PA-15

M3NF803/PA-15

B


0

5

10

15

20

25

0 2 4 6 8

tens

ile s

treng

th (M

Pa)

M0LDPEA-25

M1NF803/PA-20

M2NF803/PA-25

M3NF866A-18B

(e) (f)

M0LDPEB-40M1NF803/PB-20M2NF803/PB-12M3NF803/PB-20 (green)

M0LLDPEA-25bM1NF803/PA-15bM2NF803/PA-12bM3NF803/PA-20c

Fig. 7. Typical tensile strength in parallel (a,c,e) and transverse (b,d,f) direction curves for thin biodegradable mulching films exposed in Athens (bef) and Bari (a)

(two-digit numbers: nominal thickness in mm) (e,f; c,d; and a,b are the corresponding curves of the first, second and third experimental tests, respectively).


exposure to field conditions. This sensitivity explains the de-crease of the tensile strength below the initial stress at yield,observed in a few cases (note that ageing is in general associ-ated with a slight increase of the stress at yield [33]). Never-theless, with a few exceptions, the tensile strength of thebiodegradable mulching films in the parallel direction isshown to remain remarkably stable and above the stress atyield. Higher sensitivity is observed with the tensile strengthin the transverse direction (as it is also the case with the elon-gation at break), especially for very thin films, than in the par-allel direction suggesting that processing parameters should befurther optimised in order to overcome these problems. In gen-eral, the tensile strength of the thinner mulching biodegradablefilms of the third and second experimental period may be con-sidered to be as satisfactory as the one of the thicker films ofthe first experimental period. This behaviour may be furtherimproved provided that large deviations from the nominal(design) thickness and/or premature degradation of the filmare avoided (e.g. during storage, installation, etc.). The initialstress at yield of well-designed and processed biodegradablemulching films, following a targeted optimisation and refine-ment of the processing parameters, may be safely consideredto represent an asymptotic lower limit for the evolution ofthe tensile strength (a criterion established already for theLDPE films in Ref. [33]).

4.2.3. Initial tear resistance and tear propagationThe evolution of the initial tear strength (tear propagation is

simultaneously measured with the corresponding test [28]) fol-lows in general the corresponding behaviour of the elongation

at break as far as the displacement at maximum load is con-cerned and the tensile strength as far as the tear strength isconcerned, with some variations (Figs. 8 and 9). It should benoted that the tear resistance is not normalised with respectto the film thickness (there is a strong relationship betweentear resistance and film thickness; this relationship is not linearthough). Therefore, care should be taken to account for thefilm thickness in evaluating the film performance in tear resis-tance, as presented in Figs. 8 and 9. There are cases, wheretear resistance of biodegradable mulching films is reducedwithin the first week or start from a rather low value thatremains inferior to the corresponding behaviour of the conven-tional film in both directions (taking into account the filmthickness deviation from the nominal thickness). Thus, for ex-ample, the biodegradable mulching film M3: NF 803/P A-15b-03(i.e. 15 mm thick; processing scheme B) exposed in Athens,exhibits the worst performance in terms of tear resistance inthe transverse direction (Fig. 9b) but also the smallest tear dis-placement at maximum load in both directions from the begin-ning of the cultivation period, as compared to the other twofilms M1 and M2 (Fig. 8c,d). The low values obtained withthe displacement at maximum load imply that tear propagationassociated with the initiation of tear is faster in these cases.A similar behaviour is observed with thicker films manufac-tured with scheme B during the first experimental periodand so this behaviour is considered systematic and it is attrib-uted to the processing of the film by scheme B. The samelower tear resistance of films manufactured by processingscheme B as compared to the corresponding resistance ofthe conventional films is observed in both directions with filmsexposed in all other locations.

(a)

(c) (d)

(b)parallel direction

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25

30


disp

lace

men

t at m

axlo

ad (m

m)

disp

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men

t at m

axlo

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m)

disp

lace

men

t at m

axlo

ad (m

m)

disp

lace

men

t at m

axlo

ad (m

m)

M0LLDPEA-25M1NF803/PA-15M2NF803/PA-12M3NF803/PA-20

B


0

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15

20

25

30

0 2 4 6 8 10 12time (weeks)

M0LLDPEA-25

M1NF803/PA-15M2NF803/PA-12M3NF803/PA-20

B

parallel direction

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20

25

30

0 1 2 3 4 5 6time (weeks)


B


0

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15

20

25

30

0 1 2 3 4 5 6time (weeks)

M0LLDPEA-25M1NF803/PA-20

M2NF803/PA-15M3NF803/PA-15

B

Fig. 8. Typical displacements at maximum load curves in parallel (a,c) and transverse (b,d) direction, due to initial tear for thin biodegradable mulching films

exposed in Athens (two-digit numbers: nominal thickness in mm) (c,d and a,b are the corresponding curves of the second and third experimental tests, respectively).


parallel direction

01234567


M0LLDPEA-25M1NF803/PA-15M2NF803/PA-12

M3NF803/PA-20

B


01234567



B

(a) (b)


01234567

0 1 2 3 4 5 6time (weeks)

tear

resi

stan

ce(1

0-3 k

N)

tear

resi

stan

ce(1

0-3 k

N)

tear

resi

stan

ce(1

0-3 k

N)

tear

resi

stan

ce(1

0-3 k

N) M0LLDPEA-25

M1NF803/PA-20M2NF803/PA-15M3NF803/PA-15

B

parallel direction

01234567

100 5 15time (weeks)

M0LDPEB-40M1NF803/PB-20M2NF803/PB-15M3NF803/PB-20(stabilized)

B

(c) (d)

Fig. 9. Typical initial tear resistance curves in parallel (a,c) and transverse (b,d) direction for thin biodegradable mulching films exposed in Athens (a,b,d) and

Bari (c) (two-digit numbers: nominal thickness in mm) (c,d and a,b are the corresponding curves of the second and third experimental tests, respectively).

4.3. Evolution of mechanical properties of low-tunnelfilms exposed under real cultivation conditions

The evolution of the elongation at break, the tensilestrength and the initial tear resistance for three sets, each con-sisting of three different biodegradable low-tunnel films andthe conventional polyethylene film, exposed in the experimen-tal field of Athens during the years 2002e2004, is presented inthis section. As in the case of mulching films, selected cases ofbiodegradable low-tunnel films exposed in the other three lo-cations, of special interest for the present analysis, are alsopresented in this section.

4.3.1. Elongation at breakThe evolution of the elongation at break of the low-tunnel

films in the full-scale experimental locations (Fig. 10) suggestsa gradual drop of the elongation at break in the parallel direc-tion within the first month of exposure (processing scheme A),or even a more abrupt drop within the first week for processingscheme B and for thinner films (Fig. 10a,c,e). A dramatic dropof elongation at break to zero in the transverse direction is ob-served for the low-tunnel films exposed in the full-scale ex-periments within the first week, especially for processingscheme B (Fig. 10b,f). In the case of very thin direct coverfilms (exposed in Hannover; lettuce cultivation), the elonga-tion at break in the transverse direction was shown to drop sig-nificantly within the first 1e2 weeks. On the contrary, inanother case, some low-tunnel films exhibited a relativelygood behaviour in the transverse direction (films exposed inMontpellier; melon cultivation; Fig. 10d). In general, the elon-gation at break of the low-tunnel films exposed for more than

one month reaches very low values and may go practically tozero in some cases, especially in the transverse direction.

It is apparent that the performance of the Mater-Bi basedlow-tunnel biodegradable films, in terms of the elongation atbreak, is dependent to a large extent on the processing parame-ters (but also on several other factors including thickness, expo-sure conditions, etc.). A further optimisation of the processingparameters should aim at improving the performance of the thinbiodegradable films, especially in the transverse direction thatis the weak point in the performance of these films in compar-ison with the corresponding (thicker) conventional films.

4.3.2. Tensile strengthThe evolution of the tensile strength in the parallel direc-

tion, of the low-tunnel films exposed in Athens as well as inthe other locations, confirms that the tensile strength of thefilms manufactured by processing scheme A remains remark-ably stable and above the stress at yield throughout theirexposure period (Fig. 11a,c,e; the typical cases shown are rep-resentative of the observations made in all four experimentallocations). In a few cases of films produced by processingscheme B, the tensile strength was found to decrease to a lowervalue than the corresponding initial stress at yield value. Ananalogous behaviour to the parallel direction is observed forthe tensile strength in the transverse direction, expect that, insome cases, films manufactured by processing scheme B expe-rience a more significant, randomly occurring, drop in the valueof the tensile strength, below the initial stress at yield value(Fig. 11b,d,f). In most cases of samples of biodegradablelow-tunnel films, especially of those produced by processingscheme A, the tensile strength in the transverse directionremains close to the initial stress at yield, throughout the



0

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0 1 2 3 4 5 6

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elon

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)el

onga

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at b

reak

(%)

elon

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L0LDPEA-60

L1NF803/P-A-30L2NF803/P-A-30(stabilized)L3NF803/P-A-30(stabilized)

parallel direction

parallel direction

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elon

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L0LDPEB-40

L1NF803/P B-30L2NF803/P B-30(stabilized)

L3NF803/P B-30(stabilized)

AA

A

A

A

B

(a)

(c)

(b)


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L0EVAC-50

L1NF803C-40

L2NF803C-40(stabilized)

L3NF803C-30(stabilized)

0

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300

400

500

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0 1 2 3 4 5 6time (weeks)

L0LDPEA-60

L1NF803A-40

L2NF803A-40(stabilized)L3NF803A-30(stabilized)

(d)


0

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300

500

400

600

0 1 2 3 4 5time (weeks)

L0LDPEA-60

L1NF803A-60 (notstabilized)

L2NF803A-60 (notstabilized)L3NF803A-60(stabilized)

parallel direction

0

100

200

300

400

500

600

0 5 10 15 20 25time (weeks)

L1NF803B-60(stabilized)L2NF803B-60 (notstabilized)

L3NF803B-60(stabilized)

L4NF803B-60 (notstabilized)

(e) (f)

Fig. 10. Typical elongation at break in parallel (a) and transverse (b) direction curves for thin biodegradable low-tunnel films exposed in Athens (b,c,f), Bari (a,e)

and Montpellier (d) (two-digit numbers: nominal thickness in mm) (e,f; c,d; and a,b are the corresponding curves of the first, second and third experimental tests,

respectively).

exposure period (e.g. films manufactured by using processingscheme A and exposed in Montpellier; Fig. 11d).

As it was observed with the mulching films, the perfor-mance of the biodegradable low-tunnel films over their usefullifetime may be comparable to, or in some cases, inferior tothat of the corresponding conventional films in terms of tensilestrength in both directions. The performance of the biodegrad-able films in the transverse direction is the weak point in thisparticular comparison with the specific conventional films. Inmost cases though, at the end of the cultivation period, the bio-degradable films experiencing the best performance exhibitbehaviour comparable to the behaviour of the conventionalfilms in terms of tensile strength in both directions tendingto the corresponding initial stress at yield values asymptotically.This could be used as a criterion of successful processing ofbiodegradable films. Therefore, the stress at yield may be con-sidered, in general, as an asymptotic lower limit for the

evolution of the tensile strength for low-tunnel biodegradablefilms, as in the case of mulching films. Higher sensitivity isagain observed with the tensile strength in the transverse direc-tion confirming that this behaviour represents a systematic anddifficult to tackle processing optimisation problem for Mater-Bi based thin films, suggesting also the need for furtheroptimising the relevant processing parameters. The practicalaim should be to optimise the processing parameters in sucha way that the initial stress at yield may be safely consideredto represent an asymptotic lower limit for the evolution of thetensile strength for well-designed and processed biodegradablelow-tunnel films in both directions. As far as stabilisation isconsidered, it appears that the effect of the particular additiva-tion schemes tested on the performance of the biodegradablelow-tunnel films, in terms of tensile properties, is insignificant.Processing problems appear to be dominant and also, morecritical and controllable than stabilisation (comparing the


parallel direction

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0 2 4 6time (weeks)

tens

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L0LDPEA-60L1NF803A-30L2NF803A-30 (stabilized)L3NF803A-30 (stabilized)

A

A

A A

A

A


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tens

ile s

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L0LDPEA-60

L1NF803/P-A-30L2NF803/P-A-30(stabilized)L3NF803/P-A-30(stabilized)

(a) (b)

parallel direction

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L3NF803C-30 (stabilized)


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0 1 2 3 4 5 6time (weeks)

tens

ile s

treng

th (M

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L0EVAC-50

L1NF803C-40

L2NF803C-40 (stabilized)L3NF803C-30 (stabilized)

(c) (d)

parallel direction

0

5

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25

0 1 2 3 4 5time (weeks)

tens

ile s

treng

th (M

Pa)

L0LDPEA-60L1NF803A-60 (notstabilized)L2NF803A-60 (notstabilized)L3NF803A-60(stabilized)


0

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20

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0 1 2 3 4 5time (weeks)

tens

ile s

treng

th (M

Pa)

L0LDPEA-60L1NF803A-60 (notstabilized)L2NF803A-60 (not stabilized) L3NF803A-60(stabilized)

(e) (f)

Fig. 11. Typical tensile strength in parallel (a) and transverse (b) direction curves for thin biodegradable low-tunnel films exposed in Athens (a,b,e,f), Montpellier

(c,d) (two-digit numbers: nominal thickness in mm) (figures e,f; c,d; and a,b are the corresponding curves of the first, second and third experimental tests,

respectively).

stabilisation schemes used against no stabilisation; basic re-search is needed in the direction of developing new additiva-tion schemes appropriately designed for biodegradableMater-Bi films).

4.3.3. Initial tear resistance and tear propagationThe evolution of the initial tear strength of the low-tunnel

biodegradable films follows in general the corresponding be-haviour of the elongation at break as far as the displacementat maximum load is concerned and the tensile strength as faras the tear strength is concerned (resistance to tear propagationis simultaneously measured with the corresponding test). As inthe case of the mulching films, a more gradual reduction intear resistance is observed with time. The low-tunnel biode-gradable films exposed in Athens confirm a systematic differ-ence between processing schemes A and B, in favour ofscheme A (Figs. 12 and 13). It appears that biodegradable

films manufactured with processing scheme A exhibit bettertear resistance and they tear more slowly than the filmsmanufactured with scheme B. The initial tear resistance ofthe low-tunnel biodegradable films (scheme A), if normalisedwith respect to the film thickness, is found to be comparable tothe corresponding resistance of the conventional films andremains relatively stable over most of the exposure period.An analogous behaviour was observed with the films exposedin the other locations (e.g. the tear resistance in the transversedirection of films exposed in Hannover confirms the adequatebehaviour of processing scheme A; Fig. 13f).

The particular stabilisation schemes used with the biode-gradable low-tunnel films do not appear to affect the tear resis-tance of the films and certainly they do not improve the weakbehaviour of the films produced with a processing scheme thathas not been optimised for the specific thin Mater-Bi basedfilms production.


parallel direction

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0 2 4 6time (weeks)

disp

lace

men

t at m

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0 2 4 6time (weeks)

disp

lace

men

t at m

axlo

ad (m

m) L0LDPEA-60

L1NF803A-30L2NF803A-30(stabilized)L3NF803A-30(stabilized)

(a) (b)

parallel direction

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0 1 2 3 4 5 6time (weeks)

disp

lace

men

t at m

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L0LDPEA-60L1NF803A-30L2NF803A-30(stabilized)L3NF803A-30(stabilized)

L0LDPEA-60L1NF803A-40L2NF803A-40(stabilized)

L3NF803A-30(stabilized)


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0 1 2 3 4 5 6time (weeks)

disp

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men

t at m

axlo

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m) L0LDPEA-60

L1NF803A-40L2NF803A-40(stabilized)L3NF803A-30(stabilized)

(c) (d)

A

AA

A

Fig. 12. Typical displacements at maximum load curves due to initial tear for thin biodegradable low-tunnel films exposed in Athens in parallel (a,c) and transverse

(b,d) direction (two-digit numbers: nominal thickness in mm) (c,d and a,b are the corresponding curves of the second and third experimental tests, respectively).

5. Overall evaluation of the mechanical behaviourof the biodegradable films

5.1. The mechanical behaviour of the biodegradablefilms under real field conditions

5.1.1. Low-tunnel filmsThe overall mechanical performance in the field experi-

ments of the low-tunnel biodegradable films that were (a)manufactured with an adapted-optimised processing scheme(e.g. scheme A) and (b) designed in accordance with an appro-priate design procedure [12], was found to be satisfactory ingeneral, comparable to the corresponding tensile strength per-formance of the conventional films of an analogous thickness(a further effort for optimisation of the processing parametersis under way aimed at improving the behaviour with respect toelongation at break). An illustrative example is shown inFig. 14: the mechanical performance of the 30 mm film L3-NF 803/PeA-30s-04/A (Table 1) after 50 days of exposurein the experimental filed of AUA was found to be as good asthe corresponding performance of the 60 mm conventionalLDPE films. Despite the presence of ventilation openings,the film performed very well and no tear propagation or pre-mature failure due to wind pressure was observed. It may beconcluded therefore that the tensile strength criterion, stronglyassociated with the tear resistance of the film, is the major de-sign criterion in the case of the thin low-tunnel biodegradablefilms. Improvement of the film behaviour in terms of retaininga satisfactory percentage of the initial value of the elongationat break with the time of exposure is desirable, but it appearsthat this property is not as critical as the tensile strength for the

overall mechanical performance of the films (it is importantwhen related to tear propagation and possibly to localised fail-ure due to puncture or impact though).

5.1.2. Mulching filmsThe thin biodegradable mulching films with very low elon-

gation at break values were found to degrade quicker than the(thicker) conventional films as a result of tear propagation inthe transverse direction along the holes opened for the trans-plantation (premature failure in cases of using tools withtooth-like edges) or due to gradual degradation (Fig. 15).Such problems were not observed with thicker biodegradableor conventional mulching films (e.g. thickness of 20e25 mm). Therefore, very thin Mater-Bi based biodegradablemulching films need to be further improved in terms of theirmechanical behaviour through optimisation of the processingparameters. Thicker biodegradable films (i.e. at least 15 mm)may be used safely in replacing conventional mulching films.

5.2. Optimising the processing parameters

The performance of the various biodegradable experimentalmulching and low-tunnel films exposed under different fieldconditions (as presented in the previous sections) confirmsthat the processing parameters used for the extrusion andfilm blowing play a crucial role in the overall mechanical be-haviour of these films. It is apparent that some of the process-ing scheme B parameters used were not appropriate for thefilm extrusion and blowing of the particular Mater-Bi materialbased thin films. It is also evident that the processing scheme Aparameters may be further optimised to improve the


parallel direction

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tear

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L0LDPEA-60L1NF803A-30




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L0LDPEA-60L1NF803A-40L2NF803-40(stabilized)L3NF803A-30(stabilized)

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01234567


L0PE1010B-40

L1F803B-40

L2F803B-40(stabilized)

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A

Fig. 13. Typical initial tear resistance curves for thin biodegradable low-tunnel films exposed in Athens in parallel (a,c) and transverse (b,d) direction, in Bari (e)

and Hannover (f) (two-digit numbers: nominal thickness in mm) (c,d; e,f; and a,b are the corresponding curves of the second and third experimental tests,

respectively).

mechanical performance of the biodegradable films, eventhough satisfactory in most cases. Nevertheless, improvementis needed especially in the mechanical performance of the thinand very thin films in the transverse direction. Subsequently,the parameters of the processing schemes A and B havebeen critically evaluated and they are currently under revi-sion/refinement (work in progress).

6. Conclusions

The mechanical performance of several experimental bio-degradable films made of different grades of Mater-Bi material(polymers based on starch complexed with biodegradablepolyesters) and additives, different thickness and processingparameters are investigated and compared against the corre-sponding behaviour of conventional agricultural films underreal cultivation conditions. Investigation of selected criticalmechanical properties describing the mechanical behaviour

of these experimental mulching and low-tunnel biodegradablefilms, exposed to four different locations across Europe, con-firms that a rather good mechanical behaviour is possible forthese films, comparable to the behaviour of conventional agri-cultural films in terms of strength, inferior, however, to theconventional films in terms of elongation at break. The mostimportant points may be summarised as follows:

e The performance of the biodegradable mulching and low-tunnel films over their useful lifetime may be comparableto that of the conventional (thicker) films in terms of ten-sile strength in the parallel direction; the tensile strength ofthin biodegradable films may reach values below the initialstress at yield in the transverse direction.

e The biodegradable mulching and low-tunnel films, espe-cially thin films in the transverse direction, may exhibita very low elongation at break values within the firstweek of their exposure.


e The evolution of the initial tear strength of the Mater-Bibased biodegradable films follows in general the corre-sponding behaviour of the elongation at break as far asthe displacement at maximum load is concerned and thetensile strength as far as the tear strength is concerned;The biodegradable films manufactured with processingscheme A exhibit a better tear resistance and they tearmore slowly than the films manufactured with scheme B(large machine) in both directions.

e The stabilisation schemes used with the biodegradablefilms do not affect the mechanical performance of thesefilms. The particular stabilisation schemes used with thebiodegradable films do not appear to affect significantlythe tensile strength and the tear resistance of the films

Fig. 14. Typical low-tunnel L3-NF 803/PeA-30s-04/A biodegradable film

after 50 days of exposure between low tunnels using conventional

L0-LDPEeA-60-04/B LDPE films.

Fig. 15. Typical very thin mulching M2-NF 803/PeA-12b-04/A biodegradable

film after 21 days of exposure.

and certainly they do not improve the weak behaviour ofthe films produced with processing schemes that havenot been optimised for the specific biodegradable filmsproduction. Processing problems appear to be more criticaland controllable at this moment, than stabilisation. Basicresearch is needed though in the direction of developingnew additivation systems, appropriate for biodegradableMater-Bi based low-tunnel and mulching films.

e In general, the performance of the thin biodegradable filmsin the transverse direction remains the weak point in com-parison with the conventional films (thicker films).

Concerning low-tunnel films, for first time the design oflow-tunnel films was based on a specific design procedureand was not done empirically. It appears that it is in factpossible to develop optimised thin low-tunnel films based onMater-Bi that can perform adequately for these specific appli-cations. This can be achieved despite the fact that Mater-Bibased films degrade faster than the corresponding conventionalfilms.

The Mater-Bi grade NF 803/P was found to be best suitedfor blow extrusion of thin biodegradable agricultural films. Ithas been shown that it is possible to develop very thin bio-degradable films made of this grade (with a thickness of atleast up to 30 mm for low tunnels and 15 mm for mulchingfilms) that perform satisfactory for the specific applicationsand may replace conventional (thicker) polyethylene films.A further optimisation of the processing parameters inblow extrusion of thin biodegradable films is expected toallow for an improvement of their mechanical behaviour inthe transverse direction, and especially in retaining highervalues of the elongation at break during the useful lifetimeof the films. The practical aim is to optimise the processingparameters in such a way that the stress at yield may be safelyconsidered to represent an asymptotic lower limit for theevolution of the tensile strength for well-designed and pro-cessed thin biodegradable low-tunnel films and improve theelongation at break in the transverse direction for both,mulching and low-tunnel films. Work along this direction isin progress.

Acknowledgments

This work has been carried out in the framework of theEuropean research project Bioplastics: ‘Biodegradable plasticsfor environmentally friendly mulching and low-tunnel cultiva-tion’, QLK5-CT-2000-00044, funded by the EU. Specialthanks are due to N. Chronopoulou, O. Thodi and E. Hatzisfor performing the mechanical tests and to Mr. G. Makrisfor collecting the samples and supervising the experimentsin the experimental field in Athens. Also, thanks are due tothe partners of the project who provided the samples exposedto their experimental fields in Hannover (J. Michaelis, B. vonElsner; University of Hannover), in Montpellier (P. Feuilloley,V. Judais; Cemagref) and in Bari (G. Vox, E. Schettini, G.Scarascia; University of Bari).


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