An Overview on the Mechanical Behaviour of Biodegradable Agricultural Films

Journal of Polymers and the Environment, Vol. 12, No. 2, April 2004 ( 2004)

An Overview on the Mechanical Behaviour of BiodegradableAgricultural Films

D. Briassoulis1

The mechanical behavior of various types of biodegradable materials depends, mainly, on theirchemical composition and the application conditions. Various additives are added into thebioblends to improve their properties, which sometimes even reach the levels of the conventionalplastics. It is well known that the environmental conditions during production, storage, and usageof these materials inuence their mechanical properties. Ageing during the useful lifetime alsocauses great losses in the elongation. In the present paper, the overall mechanical behaviorof biodegradable lms, which may be considered suitable for agricultural applications, but also ofpartially biodegradable lms, is reviewed and analyzed. Selected critical mechanical propertiesof lms before their exposure to biodegradation are investigated and compared against those ofconventional agricultural lms.

KEY WORDS: Biodegradable materials; agricultural lms; plastic lms; mechanical properties.

1 Agricultural University of Athens, Department of AgriculturalEngineering, Iera Odos 75, 11855, Athens, Greece. E-mail:[email protected]

INTRODUCTION

Recently, there is an increasing interest in the useof mulching and low tunnels for protected cultivation[1]. The market of plastics used for these purposes inEurope involves hundreds of thousands of hectares andthousands of tons of plastic lms per year [2]. A largeportion of these is left on the eld or burnt uncontrol-lably by farmers, releasing harmful substances withthe associated obvious negative consequences to theenvironment. The reasons for these environmentallydangerous practices are the lack of cost-efcient sys-tematic disposal techniques available to the growersand the high labor cost for the proper collection of theplastic lms following the end of the cultivation. Thususe of biodegradable materials appears as a challenging,attractive alternative for enhancing sustainable and en-vironmental friendly agricultural activities in mulchingand low-tunnels cultivation.

Several types of biodegradable agricultural lmssuch as starch with poly(vinyl alcohol) but also partially

biodegradable lms [e.g., starch with poly(ethylene-co-acrylic acid)] have been produced in the USDA labo-ratories [1] and elsewhere. Polylactone and poly(vinylalcohol) lms are readily degraded by soil microorgan-isms, whereas the addition of iron or calcium acceleratesthe breakdown of polyethylene. Commercially availablebiodegradable soil mulching lms have already beenproduced. Innovative biodegradable starch-PCL (poly(-caprolactone) based mulching lms and lms for lowtunnels are under development in the course of a currentEuropean project.2 The performance of these lms com-pared to the performance of the conventional polyethyl-ene lms in the eld under real cultivation conditions (inseveral locations in Europe), as well as in the laboratory,is currently under investigation. In another case, carbonblacklled, biodegradable copolyester mulch lm andcommercial carbon blacklled, high-density polyethyl-ene (HDPE) mulch lm were exposed for 12 weeks tocommercial vegetable crop growing conditions in theUnited States, the positive results are reported in [3]. Theefcient and protable use of biodegradable lms aimingat reducing pollution through practicing environmentally

651566-2543/04/0400-0065/0 2004 Plenum Publishing Corporation

2 Bioplastics: Biodegradable plastics for environmentally friendlymulching and low tunnel cultivation, QLK5-CT-200000044.

friendly, sustainable agriculture, involves several crucialtechnological questions. Among them, a dominant ques-tion concerns their mechanical behavior.

The adequacy of various formulations of biodegrad-able agricultural lms depends primarily on their me-chanical behavior with regard to the specic applicationsunder consideration and, of course, on their biodegrad-ability behavior. Developing biodegradable lms with anoptimum combination of desirable mechanical propertiesand biodegradation performance represents a ratherchallenging multidisciplinary problem. A literaturereview on the mechanical behavior of various types ofbiodegradable materials reveals that their mechanicalproperties depend, in general, on their chemical compo-sition [4,5]; the production, storage, and applicationconditions [6,7], and ageing.

In the present paper, a rather focused literature re-view on the mechanical behavior of biodegradable andpartially biodegradable materials, with special emphasisplaced on lms, is presented. It should be made clearthat this review does not concern biodegradable materi-als in general but only the mechanical behavior ofbiodegradable lms. Even though the relevant publishedresearch work in this specic eld is limited, the infor-mation collected is considered to represent a valuablebasis for understanding the mechanical behavior experi-enced by the biodegradable agricultural lms made bymaterials already available on under development andthe mechanical behavior experienced by more genericbiodegradable lms, which may be used for agriculturalapplications under certain conditions and possible modi-fications. The mechanical behavior of agriculturalbiodegradable lms currently in use, or new formula-tions under development, can be better evaluated ifthe critical issues pertaining to the overall mechanicalbehavior of biodegradable materials are understood.In addition, to enrich the limited information available inthe literature, some selected critical mechanical proper-ties of the experimental (starch-PCL) fully biodegradableagricultural lms are presented and compared againstthose of conventional agricultural lms. Only the me-chanical behavior of original materials is consideredherein. Ageing, biodegradation, and storage effects arenot considered in this work.

BIODEGRADABLE MATERIALS

Categories of Biodegradable Materials

Between the nonbiodegradable petroleum-basedplastics and the renewable sourcebased biodegradablebioplastics, the chemical industry is also thinking in

terms of aliphatic/aromatic ratio by using chemicalprocess engineering to achieve petroleum-basedbiodegradable plastics [8]. According to Narayan [9],biodegradable plastic technologies can be classifiedunder three broad categories and several subcategories:

1. Aliphatic polyester based (petrochemical feedstock;agricultural feedstock; microbial synthesis)

2. Natural polymer based (starch and starch derivatives[starch esters]; cellulose and cellulose esters; proteins,other polysaccharides, and amino acids)

3. Blends, alloys, and graft copolymers of natural poly-mers and polyesters.

Following the comprehensive review of [1],biodegradable materials may be grouped as follows:

1. Natural biodegradable polymers or biopoly-mers: Polymers formed in nature during the growthcycles of all organisms. Included are:

a. Polysaccharides: The most important polysac-charides of concern to material applications are celluloseand starch. Increasing attention is being given recently tomore complex carbohydrate polymers produced by bac-teria and fungi, such as xanthan, curdlan, pullulan, andhyaluronic acid [1]. Starch: Starch is a well-known polymer, naturally pro-

duced by plants in the form of granules (mainly frompotatoes, corn, and rice). Starch granules vary fromplant to plant but are in general composed of a linearpolymer, amylose (in most cases up about 20 wt%of the granule), and a branched polymer, amylopectin[1,7,9].

Cellulose: Cellulose is another widely known poly-saccharide produced by plants. The molecular chain ofcellulose is very long, consisting of one repeating unit(cellobiose), and occurs naturally in a crystalline state[1,1012].

Chitin and chitosan: Chitin is a macromolecule foundin the shells of crabs, lobsters, shrimp, and insects. Itconsists of 2-acetamide-2-deoxy-b-D-glucose throughthe b-(14)-glycoside linkage [1,1315].

Alginic acid: Many polysaccharides in solution uponthe introduction of counterions, form gels. Alginatesare able to form gels in the presence of divalentcations [1,1618].

b. Polypeptides of natural origin: The proteins thathave found applications as materials are, for the most part,neither soluble nor fusible without degradation, so theyare used in the form in which they are found in nature [1].

c. Bacterial polyesters: This category includes nat-ural polyesters, which are produced from renewableresources by a wide variety of bacteria as intracellular

66 Briassoulis

reserve materials. Bacterial polyesters are thermoplasticbiopolymers. The basic polymer with R CH3, poly-b-hydroxybutyrate (PHB) is highly crystalline [1,1926].

2. Polymers with hydrolysable backbones:Polymers with hydrolysable backbones have been foundto be susceptible to biodegradation. Among them aliphaticpolyesters, polycaprolactone, polyamides, polyurethanesand polyureas, polyanhydrides and poly(amide-enamine)s[1]:

a. Polyesters: Polyesters derived from diacids ofmedium sized monomers (C6C12) [e.g., poly(glycolicacid)] are more readily degraded by fungi (Aspergillusniger and Aspergillus avus), than those derived fromlonger or shorter monomers [27]. The reason for that, aswell as for the fact that exible aliphatic polyesters aredegradable, whereas rigid aromatic polyesters are not, isthat biodegradability by enzyme catalysts requires thatthe synthetic polymer chain t into the enzymes activesite [1,28,29]. Poly(glycolic acid) (PGA) is the simplestlinear, aliphatic polyester.

b. Polycaprolactone: Poly(e-caprolactone) (PCL)[3033] is generally prepared from the ring-openingpolymerization of e-caprolactone. PCL is biodegradatedby fungi and can be degraded enzymatically [1,34].

c. Polyamides: The strong interchain interactions ofpolyamides and so their higher crystallinity results in lowrate of biodegradation. Low-molecular-weight oligomersmay be degraded by enzymes and microorganisms[1,3537].

d. Polyurethanes and polyureas: Polyurethanes havethe structural characteristics of polyesters and polyamides,whereas polyureas might be viewed as poly(diamide)s[1]. If the prepolymer is a polyester, then polyurethanesare readily biodegradable, whereas polyether-basedpolyurethanes are resistant to biodegradation [38].

e. Polyanhydrides: Polyanhydrides are a group ofpolymers with two sites in the repeating unit susceptibleto hydrolysis [1]. These are ber-forming polymers thatare very susceptible to hydrolysis [39].

f. Poly(amide-enamine)s: Poly(amide-enamine)shave been found to be susceptible to hydrolysis andbiodegradation, both by fungi and enzymes [40].

3. Polymers with carbon backbones: These poly-mers (e.g., vinyl polymers) require an oxidation processfor biodegradation because they are not susceptible tohydrolysis in general. Most of the biodegradable vinylpolymers contain an easily oxidizable functional groupand catalysts are added to promote their oxidation orphotooxidation, or both [1].

a. Poly(vinyl alcohol) and poly(vinyl acetate):Poly(vinyl alcohol) (PVA) is the most readily biodegrad-able of vinyl polymers [1] (microbial degradation as well

as enzymatic degradation by secondary alcohol peroxi-dases isolated from soil bacteria of the Pseudomonasstrain) [41,42].

b. Polyacrylates: Poly(alkyl acrylate)s and poly-cyanoacrylates generally resist biodegradation [1].Weight loss in soil-burial tests has been reported forcopolymers of ethylene and propylene with acrylic acid,acrylonitrile, and acrylamide [43].

Biodegradable Films

Not all of the biodegradable materials are suitablefor production of biodegradable lms. Even though thereare rapid developments in the manufacturing and mar-keting areas of biodegradable materials, at least threemajor commercially available biobased degradable poly-mers groups from which biodegradable lms may beproduced can be identied within the above main cate-gories (apart from cellulose-based lms): starch-basedpolymers, polyhydroxybutyrate (PHB) polymers, andpolylactides (PLA). In the category of nonbiobasedbiodegradable polyesters from petrochemical feedstock,commercial lms have been developed from biocopoly-esters (e.g., Eastar) or from starch-PCL blends [9]. Ofcourse, other biodegradable materials have also beenused for lm production (e.g., blends of soy protein andbiodegradable polyesters, etc.) [9]. Intensive ongoingresearch efforts around the world aim at the developmentof laboratory samples of innovative degradable polymersor the improvement of available materials, some ofwhich could be used as biodegradable agricultural lms.The basic characteristics of the most important cate-gories of biodegradable materials from which biodegrad-able lms are produced are briey presented below.

Polyhydroxybutyrate (PHB)A major type of commercially available biobased

polymer consists of polyhydroxybutyrate (PHB) andpolyhydroxyvalerate (PHV) polymers, which were ini-tially produced by fermenting a sugar feedstock (glucoseis currently being used) with a naturally occurring mi-croorganism [1926]. One of the commercially availableproducts, for example, is technically a family of linearpolyesters of three hydroxybutyric and three hydroxyva-leric acids produced in nature from the fermentation ofsugars by the bacterium Alcaligenes eutrophus. Thisbiodegradable product is stable when stored in air and isquite stable when stored even in humid conditions.Degradation to carbon dioxide and water will occur onlywhen they expose the polymer to microorganisms found

An Overview on the Mechanical Behaviour of Biodegradable Agricultural Films 67

naturally in soil, sewage, river bottoms, and other simi-lar environments. The rate of degradation is dependenton the material thickness and the amount of bacteriapresent. The fact that this material decomposes morerapidly without oxygen present is signicant becauseoxygen is not present in modern landlls. This kind ofPHB based resins can be converted into various typesof plastic products, depending on the physical propertiesof the resin used. Major products are likely to be plasticlms and coatings.

Starch-Based Polymers

There are several degradable polymers groups madefrom starch [1,7,9]. For example, a fully biodegradablestarch-based polymer is manufactured primarily fromcorn or potato starch, along with smaller amounts offood-grade additives (not intended for human consump-tion). This resin is suitable for manufacturing injection-molded pieces, lms, and a starched-based loose-llpackaging material. This kind of resin degrades in anactive biological environment like polyactides. Anotherstarch-based commercial resin contains performance-enhancing additives, such as synthetic linear polymers,plasticizers, and compounds that trigger or acceleratedegradability. These kinds of materials are intended to bemixed with synthetic polymers to create plastic products,making the products more degradable than traditionalsynthetic plastics (i.e., these materials may be onlypartially biodegradable). A typical product would con-tain about 43% starch, 50% synthetic polymer, and 7%proprietary ingredients. Many biodegradable starch-based thermoplastic blends have been developed andstudied extensively, such as starch/polycarpolactone,starch/cellulose acetate, and starch/ethylene-vinyl alco-hol copolymer [4447].

Polylactides (PLA)These biodegradable polymers are made from lactic

acid produced via starch fermentation. PLA polymers aregenerally derived by fermenting carbohydrate crops suchas corn, wheat, barley, cassava, and sugar cane [1,9]. Theprocess involves the fermentation of sugars to producelactic acid, which is converted to PLA through low-cost,high-yield catalytic polymerization. PLA-based polymersare completely degradable under compost conditions.Although PLA is not water-soluble, microbes in marineenvironments can degrade it into water and carbondioxide. PLA-based resins can be modied to adapt tomany applications, from disposable food-service items,

sheet extrusion, or coatings for paper. PLA isa hard material, similar in hardness to acrylic plastic witha hardness on the Rockwell H Scale of more than 60.Because of the hardness, the PLA fractures along theedges, creating a product that cannot be used. To over-come these limitations, PLA must be compounded withmaterials to adjust the hardness (e.g., such a commercialproduct has a hardness of 20.6 on the Rockwell H Scale).

THE MECHANICAL BEHAVIOR OFBIODEGRADABLE MATERIALS: A BRIEFLITERATURE REVIEW

The mechanical behavior of various biodegradablematerials reported in the literature concerns a variety ofcritical properties and parameters. The most importantfactors inuencing the mechanical behavior of biodegrad-able or partially biodegradable materials, are presentedherein, with the aim of underlining the critical issues withrespect to the evaluation of the mechanical properties ofthese materials.

Polyhydroxybutyrate (PHB)One of the main groups of biodegradable materials

studied with regard to their mechanical behavior isthe group of the bacterial thermoplastic polyesters poly(3-hydroxyalkanoate), known as PHAs, produced by thefermentation of renewable materials (e.g., sugars and mo-lasses) [48]. The pure homopolymer PHB, produced inlarge quantities, is a brittle material (elongation at breakbr 10%, impact strength 3 kJ/mm2) with a large elas-tic modulus (E 1.7 GPa) and high fracture stress (u35 MPa). The pure copolymer PHB/V is also brittle (br 15%, E 1.2 GPa, u 25 MPa). It should be notedthat a polymer is designated as brittle if br 20% [48].Being brittle, PHB and PHB/V are not suitable for tech-nological applications such as lms, etc. The brittlenessof PHB is attributed to [48]: (1) the secondary crystal-lization of the amorphous phase during storage time atroom temperature leading to rapid decrease of elongationat break. As a result of secondary crystallization in theamorphous region, density, crystallinity, stress and E-modulus increase while the material becomes brittle andhard with much lower elongation at break; (2) the glasstemperature (Tg) being close to room temperature; (3) thelow nucleation density, which results in the developmentof large spherulites exhibiting interspherulitic cracks.

Efforts to improve the poor mechanical behavior ofPHB and PHB/V by mixing them with other polymershad limited success because of problems of chemical

68 Briassoulis

incompatibility (i.e., poor mixture). In an effort to pro-duce biodegradable blends with improved mechanicalproperties, aiming at desirable values such as fracturestress of 1827 MPa, elongation at break 400%660%,improved impact strength, and long-time stability (com-parable to those of PE), PHB and PHB/V (possibly alsoincluding some other polymers; e.g., 4%20% wt PVAc)are mixed with various additives, all of which arebiodegradable materials. The mechanical properties ofthe PHB/V copolymer can also be tailored by varying theHV content to produce different grades with exibilitiesand tensile strengths comparable to those of polyethyleneand polypropylene [49]. Possible modications of PHBand PHB/V to improve their mechanical propertiesinclude the addition of the following [48]:

Nucleating agents: Nucleation agents and crystalliza-tion conditions inuence the average size of thespherulites [48]. In the case of PHB, large spherulitesare developed with cracks and splitting around thecenter, which are responsible for the poor mechanicalbehavior of the material [48]. In the case of blendsproduced by the addition of nucleating agents (smallcrystalline particles; e.g., 0.5% wt saccharin), numer-ous small, ne imperfect, spherulites are developedwhile the crystallization rate is increased. Note thatduring fast cooling from the melt, the degree ofcrystallinity is lower and small crystallines are formed[48]. Concerning the impact strength, impact dependson the number and size of the crystalline structuresand the degree of crystallinity (samples with highcrystallinity 40% are brittle, and samples with 30%crystallinity are ductile; note: crystallinity of PHBis 60%, PHB/V is 45%). For the same crystallinity,material with big spherulites is more brittle thanmaterial with ne spherulites.

Plasticisers: High elongation at break and high degreeof exibility require that glass temperature must belower than testing temperature. In general, elongationand impact strength depend on glass temperatureand morphology [48]. By adding plasticisers (e.g.,9.5% wt glycerol, 7.5% wt triacetin, 5.5% wt tribu-tyrin [48]), the rigidity of PHB and the fracture stressare reduced (E 743250 MPa; u 1826 MPa)as a result of enhanced molecular motion and lowerglass temperature, while the elongation and the impactresistance are increased (br 150660%, impactstrength above 20 kJ/mm2) [48]. The plasticisers mayalso affect the adhesion at the ller/matrix interface[49].

Lubricant in processing: Addition of lubricant inprocessing (e.g., 2% wt glycerolmonostearate, 2% wt

glyceroltristearate) results in reduced degradation ofchains and so decrease in molecular weight and meltviscosity [48].

Apart from using additives, it has also been reportedthat feeding the bacteria with a variety of carbon sourcesled to the production of different copolymers, and a ma-terial was obtained with better mechanical propertiesthan PHB [1,2426].

Another class of PHB-based biodegradable materi-als can be designed by blending the basic biodegradablematerial with other biodegradable (or environmentalbenign) polymers, llers and plasticizers, for specic ap-plications (e.g., blends with cellulose esters, starch, andpolylactic acid). Thus the inclusion of granular starch(25%) and CaCo3 (10%) in the PHBV matrix (8% HV,5% plasticiser) results in a drop of the tensile strength to16 MPa, an increase of the exural modulus to 2.0 GPa,and a signicant reduction in the cost [49]. In such acase, PHBV composites with cornstarch [50] had poormechanical behavior as a result of poor adhesionbetween the starch granules and the PHBV matrix (e.g.,u 10 MPa). The large-size spherical starch granules(15 m) are responsible for development of stressconcentrations and crack growth [50]. Using PEO (poly-ethylene oxide; molecular weight 20000; partially com-patible with PHBV; note that pure PEO is softer andmore exible than pure PHBV) as the interfacial layerresulted in improved adhesion of the starch to the PEO(probably by hydrogen bonding) and/or resistance tocrack growth, with a signicant improvement in the me-chanical properties of the composite. In fact, a signicantimprovement was obtained in the mechanical propertiesof the composite (50/50 blend of PEO-coated starch andPHBV:tensile strength 19 MPa, elongation at break23%) [50]. No effect on the crystallinity of PHBV wasdetected by the addition of PEO [50]. However, ques-tions arise on the lower degree of biodegradability of theso-produced material.

The effect of blending starch on the physicalproperties of PHB was studied in the work of [51], byadopting a lm casting from a common solvent ap-proach. The blend lms had a single glass transitiontemperature for all proportions of PHB-starch, and theyall were found to be crystalline. The tensile strength ofthe blend lm with the ratio of 0.7:0.3 PHB/starchwas maximum (31.45 MPa) compared to virgin PHB(18.29 MPa). In another case of blending with starch,poly(butylene succinate adipate) (PBSA), an aliphaticthermoplastic biodegradable polyester available com-mercially, however, of high cost, was blended with gran-ular corn starch [52]. Pure PBSA lm has been studied


for various applications, including agricultural filmapplications. It was shown that the PBSA/starchbiodegradable system (up to a starch content of 30%) hasmechanical properties useful for blown lm applications.Both elongation at break and tensile strength were shownto decrease with increasing starch content (from 780% to500% and from 32 MPa to 20 MPa, respectively).

Very-high-strength lms may be developed by uni-axial orientation. Thus uniaxially oriented lms with highmechanical properties were processed from ultra-high-molecular-weight poly(R)-3-hydroxybutyrate (P(3HB))by two-step drawing procedure [53]. Maximum growthrate of spherulites was observed in one-step hot-drawnlms along with orientation of molecular chains and anger-joint structure of lamellar crystals with tensilestrength up to 277 MPa and elongation at break 84%.The tensile strength of two-step drawing process reached400 MPa and remained unchanged for 2 months as a re-sult of planar zigzag conformation and a highly orderedstructure with nger-joint structure of lamellar crystals,avoiding in this way secondary crystallization [53].

Starch-Based Polymers

The main motivations for developing starch-basedpolymers include several advantages [54]: exibility inadjusting the material properties to the needs of theparticular application by modifying appropriately the com-position, low-cost blending process as opposed to innova-tive synthetic material development, and biodegradability.

Starch

As far as starch is concerned, starch granules arehydrophilic and so the water content of starch changeswith relative humidity changes [55] (e.g., corn starchcontains 6% moisture at 0% RH and 20% moisture at80% RH). Starch contains crystalline areas within thebranched amylopectin component (and so retains itsstructure when immersed in water), whereas the linearamylose is mostly amorphous. Granules are ruptured(gelatinized) in water at 130C and at lower temperaturesin alkaline solutions. In general, granular starch may beused in two ways in biodegradable plastics. (1) It may becompounded into plastics in the form of biodegradableller [55]. Gelatinized starch may also be used as smallsize ller in plastics (if dried during processing). (2) Itmay be plasticized with water (5%20%) [55] and com-patibilized with other polymers to become part ofthe plastic matrix. The extruded starch remains in theamorphous state when stored at room temperatures and

humidities up to 80% (Tg is at or above room tempera-ture room at 15%25% water content and is lowered bymoisture absorption). At 15% water content, extrudedstarch has poor mechanical properties, not adequate forlm applications (initial tensile strength 2030 MPa andelongation at break 10%15%). These properties arevery much affected by changes in the relative humidityand so by the corresponding changes in Tg [55]. It hasbeen shown that the fracture mechanism of starch ex-trudes turns from brittle to plastic for water contentabove 15% [56]. Ageing of extruded starch at constanttemperature and moisture (only in a few hours at 20%moisture levels) leads to further reduction of elongationat break. Mechanical properties, moisture resistance, andtemperature stability of starch granules can be improvedin several ways [55]: by increasing the crystallinity toimprove mechanical properties, reduce ageing effectsand water sensitivity; by adding nonvolatile plasticizersto get a more exible product and lower Tg below roomtemperature, so that ageing is eliminated; by blendingwith other polymers (e.g., blending gelatinized starchwith poly(ethylene-co-acrylic acid) copolymer (EAA) toimprove all properties. However, it should be taken intoaccount that EAA is non biodegradable

Especially concerning lms made of starch only,they have high tensile strength, but they are brittle andexhibit almost no elongation [50]. It has been suggestedthat lms containing starch as a biodegradable compo-nent should meet the following minimum target values:tensile strength 27.6 MPa, elongation at break 100%, tearstrength 78N/mm, impact resistance 4.2 kJ/m2[57].Usually only the tensile strength value is met.

Among other techniques, modied starch has beenused to improve the mechanical properties of starch.Acetylation of starch, for example, is a well-knownreaction leading to starch with a high content (70%) oflinear amylose [58] and is a relatively easy derivative tosynthesize [59]. The resulting high-amylose starch ac-etate is considerably more hydrophobic than starch, hasbeen shown to have better retention of tensile propertiesin aqueous environments, has improved solubility com-pared to starch, and is easily cast into lms from simplesolvents. High-amylose starch also exists naturally.Thus, for example, in some varieties of peas, amylose upto 50% was measured, out of which a part was found tobe complexed by native lipids [60]. In another work, ineld peas, starch content was shown to have reached themaximum (44% in DM) 26 days after owering, with45% amylose in total starch [61]. In garden peas, 25 daysafter owering, starch content had only reached half ofits maximum (32% in DM), with an extremely high amy-lose proportion (88%). Differences have also been found

70 Briassoulis

in the morphology of the spherulites [62]; spherulites areeasily formed in mung bean and potato starch, whereasordinary maize starch yields less numerous, not well de-veloped coarse spherulites. Acid-modied maize starchyields a large number of very round spherulites, whereaswheat and tapioca starches form very poorly developedspherulites. No spherulitic morphology was observed inoat, rice, sorghum, amaranth, and yellow pea starches.

Starch crystallinity has been investigated exten-sively. Crystallinity of amylose and amylopectin thinlms with 0, 10%, and 30% of glycerol and stored at RH0, 54%, and 91%, prepared of water cast dilute solutions,was studied in [63]. Each fresh amylose lm showed B-type crystalline structures, and depending on the glyceroland water contents, the amount of crystallinity variedfrom 6% to 32%, whereas the fresh amylopectin lmswere completely amorphous. The crystal formation inthe highly plasticized amylopectin lm was suggested tobe due to its rubbery state under the storage conditions.Amylose lms were stable in water, unlike amylopectinlms, which dispersed quickly in water. About 35% ofthe amylose lms were resistant to -amylase, whereasamylopectin films were hydrolyzed wholly. It wasconcluded that even if part of the amylose lms wereamorphous, these amorphous regions were more resist-ant to hydrolysis than the amorphous amylopectinstructures.

Cross-linking has been related to crystallinity andthe mechanical properties of starch lms. Thus, for ex-ample, mechanical tensile properties, water vapor trans-mission rate (WVTR), and oxygen permeability coef-cients of cast high amylose starch lms were determinedas a function of cross-linking degree and percentage offree humidity [64]. The effect of using increased amountsof cross-linking agents, was an increased cross-linkingdegree that tends to reduce the degree of crystallinity,thus modulating mechanical properties, water vapor per-meability, and oxygen permeability coefcients. Maximalvalues of yield strength, tensile strength at break, andWVTR versus cross-linking degree were reached formoderate cross-linking degree. Optimal crystalline/amorphous ratio in the lms may induce interactionsand balanced effects, which would be responsible forthe nonlinear behavior of some of the investigated prop-erties [64]. By cross-linking with epichlorohydrin inthe range of 110 g cross-linker/100 g polymer, themechanical properties of the lms were found to bestill related to water content while water vapor perme-ability remains high compared to that of some syntheticpolymeric materials.

Natural ber reinforcement also has been used toimprove the mechanical behavior of starch lms. In such

a case, native corn starch and hydroxypropylated starch(HPS) based plastic lms were prepared using the shortpulp ber as the reinforcement and the glycerol as theelasticizer [65]. The crystallinity of lms increased withpulp content, but decreased with glycerol content and de-gree of substitution by hydroxypropylation. The resultsof tensile test showed that the strain and stress at breakand elastic modulus increased with pulp content [66].With glycerol content, the strain at break increased con-siderably, but the breaking stress and elastic modulusdecreased. The brittleness problem of the lms was over-come by the pulp, glycerol, and water content. The hy-droxypropyl starch lms showed results similar to thoseof the native starch lms. The exibility of the lms wasfound to be improved by the hydroxypropylation.

Aimed at meeting the needs of specic applications,multilayer starch-polyesters lms have also been devel-oped. Multilayer lms based on plasticized wheat starch(PWS) and various biodegradable aliphatic polyestershave been prepared through at lm coextrusion andcompression molding [67]. Poly(lactic acid) (PLA), poly-esteramide (PEA), poly(epsilon-caprolactone) (PCL),poly(butylene succinate adipate) (PBSA), and poly(hy-droxybutyrate-co-valerate) (PHBV) were chosen as theouter layers of the stratied polyester/PWS/polyesterlm structure. The multilayer lms may be suitable forapplications in food packaging or disposable articles.

To improve the overall mechanical performance ofstarch-based lms, intensive research work has also beendevoted to developing starch blends with synthetic read-ily biodegradable polymers such as PVA (poly(vinylalcohol) [57]. Starch blends with nonbiodegradable poly-mers such as EAA (poly (ethylene-co-acrylic acid) orLDPE have also been developed and studied extensively.However, polyethylene/starch blends or any other blendsof starch with nonbiodegradable polymers are notconsidered to be biodegradable materials but possiblyonly partially biodegradable (synergism of biologicaldegradation of starch component (when above 30%) andchemistry of ageing; the question on possible successivebiodegradation of PE, over a long time, remains a contro-versial issue [1,54,68]). Therefore these blends may notbe considered (or labeled) as biodegradable or used assuch in agricultural lm applications but may be consid-ered only as biodisintegradable. The analysis of their me-chanical behavior is, however, of more general interest,applicable as a reference in several aspects to other reallybiodegradable starch-based polymers, and it is thereforeincluded in the present literature review. This is combinedwith the fact that the relevant information in the technicalliterature on the mechanical behavior of starched-based,fully biodegradable polymers is rather limited.


EAA, PVA/Starch Blends (EAA/Starch Blends: PossiblyOnly Partially Biodegradable)

Hydrophobic synthetic polymers are incompatiblewith starch [55]. Increasing starch component in starch-EAA lms results in decrease of elongation at break(from 260% at 10% starch to 92% at 40%) without anymajor change in tensile strength. In particular, additionof EAA was shown to increase viscosity of starch solu-tions by complexing with more than one starch molecule.As a result, starch-EAA mixtures containing 40% starchcould be processed to produce good-quality (not fullybiodegradable though) lms by extrusion blowing [57].Replacement of EAA with LDPE (also nonbiodegrad-able) for reducing cost, results in decrease of bothelongation at break and tensile strength. Thus, at 40%starch, there is a decrease in the elongation at break from92% (starch blends with no PE) to 34% (starch blendswith ratios per hundred: 20 EAA:40 PE) and in tensilestrength from 26.7 MPa to 20.1 MPa, respectively.Replacing part of EAA by PVA results in increase oftensile strength and decrease of elongation at break [55].EAA forms a helical inclusion complex with starch, butalso it can form complexes with PVA; thus a singleEAA molecule could be used to bind together starch andPVA in a starch-EAA-PVA complex.

In the work of Lawton and Fanta [57], glycerol wasused as plastcizer. An increase in the glycerol content,used as plastcizer, was shown to result in a decrease of thetensile strength [57], as happens with plasticizers usedwith synthetic polymers in general, whereas no signicanteffect from PVA or EAA was detected on tensile strengthin this case. On the contrary, elongation at break wasshown to increase with glycerol, EAA, and PVA content(i.e., with less starch content). A three-way interaction be-tween the three variables was established. In particular, itwas shown that the addition of small amounts of EAAimproves elongation of cast lms containing starch, glyc-erol, and PVA (too much EAA leads to brittle lmsbecause of complexes with starch, too little EAA leads tophase separation between PVA and starch) [57].Concerning glycerol plasticized lms prepared fromstarch-PVA mixtures with starch over 50%, it was foundthat a mixture of 55.6% starch, 2.8% EAA, 28.3% PVA,and 13.3% glycerol produced lms with elongation atbreak at least 100% and tensile strength of 25 MPa [57].

In another case, starch-PVA lms (with a smallamount of EAA added to the lm formulation; 41%starch, 41% PVA, 3% EAA, and 15% glycerol on a drybasis) were investigated after storage in different tem-peratures for 7 days [7]. It was observed that elongationincreases as %RH increases. This effect is attributed to

the fact that starch is hygroscopic so the lm will gain orlose water to be in equilibrium with the air. In addition,because water acts as plasticizer, changing the watercontent in the lm will change the properties of thelms. In general, increasing plasticizer levels increasesthe value of elongation. Of course, lms with goodproperties should not change with humidity.

On the other hand, tensile stress of the samestarch/PVA cast lms appears to decrease with the in-crease of the relative humidity, which is consistent forpolymers containing increasing amounts of plasticizers.For tear resistance, the behavior of this particular mate-rial was different [7]. Tear resistance was decreased atlow and high relative humidity. High RH means lowerstrength in general as a result of the effect of the wateracting as plasticizer. Films stored at 51% and 76% did nottear at all, because they were above their Tg. For low RH,materials in the lm are below their Tg and so the lmsdo not have much motion in their chains to absorb thestress of being torn [7]. As far as the starch type is con-cerned, it was shown that all lms except for those con-taining high-amylose cornstarch, experienced a decreasein elongation at break with ageing in 168 days; however,the elongation at break did not drop below 100%. Filmsmade from waxy corn starch had lower impact strengthand experienced the greatest loss in elongation at breakwith ageing (from 144% in 7 days to 34% in 168 days)and a decrease in tensile strength with ageing. Filmsmade of high-amylose corn starch had the most consis-tent properties over the entire range of test condition [7].

The effect of the starch ratio on PVA/starch blendswas studied [69]. In particular, Poly(vinyl alcohol)(PVA) starch (ST) blends (1/1, 1/3, and 1/5) wereprepared by gelation/crystallization from semidilutesolutions in dimethyl sulfoxide (Me2SO) and water mix-tures and elongated up to eight times. The elongation upto eight times could be done for the 1/1 blend, but anyelongation was impossible for blends whose ST contentwas beyond 50%. To avoid phenomena of solubility forblends whose the ST content was beyond 50%, cross-linking of PVA chains was carried out by formalizationunder formaldehyde vapor. For the 1/1 blend, the amountof ST dissolved in water at 23C was less than 3% for theundrawn state and 25% for the drawn lm. The Youngsmodulus of the drawn lms with a draw ratio of eighttimes was 2 GPa at 20C.

LDPE/Starch Blends (Possibly Only PartiallyBiodegradable)

The mechanical properties of LDPE/granular starchcomposites [4] have been extensively investigated as

72 Briassoulis

functions of the starch volume fraction , granular size,and presence of compatibilizers. A strong negativedependence on of elongation at break ( 1/3) andtensile strength ( ~ 2/3) has been conrmed. In fact,several models have been presented for determiningthe composite mechanical properties, taking also intoaccount the particle size distribution etc [56]. The elasticmodulus, on the other hand, increases with because ofthe stiffening effect of the granule (note: several modelshave been proposed for describing the composite modu-lus of a material with two crystalline and two amorphousmicrophases [56]. It has also been reported [4] that thereduction in tensile strength and elongation at break in-creases with the granule size. Starch granule size variesfrom source to source (e.g., average diameters [4]: rice:3 m, potato: 35 m, corn 10 m). As a result of thestarch volume fraction effect, most of the LDPE/starchblends have shown lower tensile strength and percentageelongation compared to LDPE [70]. It appears that inbinary LDPE/starch blends, the binding of starch gran-ules to the LDPE matrix is not so strong because of therelatively smooth surface of starch particles resulting inpremature fracture [70].

It has been established that during straining theamorphous phase is rst deformed, followed by a highorientation of both phases and the activation of crystal-lographic mechanisms [56]. Yielding of LDPE startsfrom the activation of skeletal bonds motions, occurringat temperature much lower that the one of the testingconditions [56]. The fracture energy of LDPE/starchblends is due to crack development, debonding of theparticles-matrix interface, yielding of LDPE bridgesand so the formation of microlaments during the elasticdeformation of LDPE. For LDPE/starch blends withstarch content less than 20%, it is the LDPE matrix thatdominates the plastic deformation because of smallnumber of starch particles resulting in fewer crazes [56].However, even in such cases, in the zones near starchparticles, stress concentrations appear as a result of mis-t of the two phases. Concerning debonding phenomena,it has been conrmed that when a particulate-lled poly-mer is subjected to stress, debonding between the llerparticles and the matrix occurs when the stress exceeds acritical value [49], and so adhesion between the twophases is lost. Then, as the matrix remains the sole load-bearing phase, the effective cross section is reduced andso the tensile strength is reduced. The ller decreases theelongation at break because the polymer matrix elonga-tion is much greater than the macroscopic elongation ofthe composite [49]. In some cases, rigid llers increasethe elongation at break by acting as inhibitors for crackgrowth [4].

For a given composition of LDPE/starch blend, themechanical behavior was shown to depend also on theconditioning of the sample over certain relative humidi-ties. As the water content increases, under specicrelative humidity conditioning, the tensile strength andthe modulus of elasticity of a given composition ofLDPE/starch blend decrease signicantly, while theelongation at break increases dramatically. This plasti-cization of LDPE/starch blends may be due to penetra-tion of water and lling of the voids [56], with the wateracting as elasticizer. Tensile stress appears to decreasewith the increase of the relative humidity, which isconsistent for polymers containing increasing amounts ofplasticizers [7]. However, a case has been reported inwhich no signicant effect from RH on the mechanicalproperties of LDPE/granular starch composites wasobserved [4].

It has been observed that starch/polyolen blends,being incompatible, result in larger phase domainswhich, in turn give rise to larger nondegradable residuesand subsequently to secondary pollution and to dimin-ished mechanical properties of the blends [70]. To copewith this problem, various techniques have been intro-duced to ensure homogeneity in blends at microscopiclevel and reduce the interfacial tension and so improveor retain the characteristics of the blend components.

One approach consists of the development ofstarch/PE blends by using EAA as a compatibilizer (e.g.,composite lms have been developed using starch andcopolymer of ethylene and EAA). The addition of acompatibilizer (e.g., EAA) in LDPE/granular starchcomposites does not signicantly affect the elongation ortensile strength, but signicantly increases the compositetensile modulus because of the improved adhesionbetween the starch and the matrix [4]. In another case,however, it has been suggested that adhesion has greatereffect on the tensile strength than granules size [4]. Infact, dry starch/PE lms are brittle and tear easily [6].A weak tear resistance of PE/starch composites was at-tributed to bad dispersion of two phases (starch/PE) withdifferent viscosity. It has been shown [6] that the EAAlevel improves mainly tear resistance (the improved tearresistance still remains low though). Susceptibility ofstarch-EAA-PE lms to tear propagation, because ofline-wise propagation without any branching, rendersthese lms inadequate for many applications (tear resist-ance 4 N/mm at 40% starch content).

Another proposed approach consists of the modi-cation of starch by grafting of various monomers ontostarch (e.g., acrylamide, acrylonitrile, etc.), thus obtain-ing small domain size in blends [70]. In that way, avariety of starch esters such as starch acetate and some


fatty acid esters have been used for biodegradableblends. In a specic case, partially biodegradable blendsof LDPE and esteried starch were developed (starchphthalate, stath, a hydrophobic derivative obtained byphthalation of starch). It was observed that most of theblends had lower tensile strength and elongation at breakthan LDPE (9.92 MPa and 92.5%, respectively) and thatthe tensile strength and elongation at break increased,while the modulus decreased as the starch was substi-tuted by stath [70]. Morphology of binary LDPE/stathblends showed improved adhesion, leading to enhancedmechanical properties compared to LDPE/starch blends[70]. The roughness of the stath particles brings aboutincreased adhesion as a result of the improved state of in-terface, so that fracture occurs only after much higherelongation has taken place. LDPE/starch blends haveshown higher modulus than pure LDPE and binaryblends of LDPE/stath [70], attributed to the decreasedhydrogen bonding in stath as a result of substitution,despite the fact that stath is more crystalline thanstarch. The increasing modulus with increasing incorpo-ration of starch in LDPE can be explained by thecrystallinity, hydrogen bonding, and stiffening effect ofthe starch granules [1] (the estimation of the modulusof granular starch of 15 GPa in [70], is considerablygreater than most unlled synthetic polymers of com-mercial importance, but signicantly lower than themodulus of cellulose; it is also greater than anotherreported value of 2.7 GPa for starch). Hydrogen bondingdominates over crystallinity in these LDPE/starchblends [70]. Concerning impact resistance, all blendscontaining stath had higher impact resistance thanbinary LDPE/starch blends, suggesting smaller particlesize in stath blends, but lower impact resistance thanLDPE.

In some cases, blends of LDPE/starch/PCL havebeen developed. Thus, low-density polyethylene/plasti-cized starch/polycaprolactone blends were processed byconventional extrusion, injection molding, and filmblowing techniques in the work of [71]. The ne disper-sion of polycaprolactone phase in the polyethylene/starchmatrix of lms resulted in mechanical property increase,whereas in injection specimens there was propertydecrease as a result of phase coalescence. In anotherwork [72], a ner starch phase dispersion was achievedin injection-molded products than in lms, probably be-cause of the development of higher shear rates at injec-tion molding. Starch incorporation in polycaprolactoneresulted in a material with decreased strength and elon-gation at, both yield and break, whereas the modulus in-creased. At high starch content, particle coalescence wasassociated with a further mechanical property decrease.

Modied PE/Starch Blends (Possibly PartiallyBiodegradable)

Graft copolymerization of thermoplastic polymersonto starch provides another method for preparingstarchpolymer composites [1]. Graft copolymerizationallows starch and synthetic polymers to be held togetherby chemical bonding rather than as simple physicalmixtures. As a result, separation of the two polymerphases is less likely to occur. Among several starchgraft copolymers of special interest proved to be thestarch- g-poly(methyl acrylate) (S-g-PMA). In particular,S-g-PMA copolymers having grafted side chains withmolecular weights of less than 500,000 can be easily ex-truded into a lm that shows excellent initial tensilestrengths and elongations. This material may have anapplication as a fully biodegradable plastic mulch [1].

Reactive blending represents another economicaland commercially viable approach in which graft orblock copolymers are formed in situ during the blendpreparation by using polymers containing reactive func-tional groups [1]. Reactive blending is known to improvethe compatibility and interfacial adhesion of two immis-cible polymers. According to this method, synthetic poly-mers having functional groups such as carboxylic acid,anhydride, epoxy urethane, or oxazoline, can react withhydroxyl or carboxyl groups (in modied starch) to forma blend with stable morphology. In such an application[1] starch was used with oxidized polyethylene, andLDPE to produce lms. The addition of high-molecular-weight oxidized PE (OPE) in ratios OPE/starch 0.30.5improved the tensile strength and elongation as a resultof hydrogen bonding between carboxyl groups in OPEand hydroxyl groups on the starch surface. Once again, itwas reported that as the percentage of starch in the blendis increased, the tensile strength and the percentage elon-gation decrease. In an alternative approach starch isblended with polymers containing polar functionalgroups that can interact with starch. In that way, blendsof starch and copolymers of an olen and, optionally, apoly(mono)olen or poly(mixed)olen were developedthat were then injection molded or lm blown intocommercial articles [1]. Nevertheless, it was again con-rmed that an increase in the starch percentage adverselyaffected the physical properties of the blends.

In some cases, it was shown that the anhydridecompounds improve the properties of composites madefrom cellulosic llers. However, analogous results forreactive blending of starch with anhydride functionalpolymers are limited [1]. Thus, for example, it has beenshown that maleated high-density polyethylene (HDPE)improved the tensile strength of composites containing

74 Briassoulis

wood our with increasing concentration of ller. Blendsof corn starch (50%80% wt) and synthetic polymerscontaining anhydride groups (styrene maleic anhydridecopolymer [EPMA], and the corresponding nonfunc-tional polystyrene and ethylene propylene copolymers)have been investigated. Blends containing EPMA ab-sorbed less water than SMA blends containing the sameweight fraction of starch. The tensile strengths of blendscontaining functional groups were superior compared tothe blends made from nonfunctional polymers. When thestarch contents increased from 60% to 70%, the tensilestrength remained unchanged for the SMA blend butincreased for the EPMA blend. All samples supportedthe growth of microorganisms, which increased withincreasing starch content.

Starch-Based Blends for Films (Possibly PartiallyBiodegradable)

In cases of partially biodegradable lms for foodpackaging applications [56], made from LDPE, wheatstarch, and soluble starch, it was shown that the presenceof starch at contents higher than 30% had an adverseeffect on the mechanical properties of the blends (lowertensile strength and modulus; note that the modulus ofwheat starch is rather low, 40.3 MPa). In another case,mechanical properties such as tensile strength andelongation at break of composites made from sago starch(SS) and LLDPE were shown to decrease with increas-ing starch content, whereas the modulus increased [73].The incorporation of sago starch into LLDPE led to anincrease in the modulus of the composites because thestarch granules are stiffer than the LLDPE matrix inwhich they are dispersed. On the other hand, the yieldstress can be increased by the addition of llers andreduced by the addition of plasticizers. The increase inyield strength with increase in ller content is assumedto be due to the ller carrying higher loads than the ma-trix. Because the polymer matrix is subjected to a largerstrain than the macroscopic strain (the ller particles donot elongate), the material yields at a lower macroscopicstrain than the unlled material. The optimum ller con-tent was found to be 15%, above which a sharp dropin the mechanical properties occurred. It has been sug-gested [73] that this is a useful range for trash bags andagricultural mulch, in which mechanical properties suchas tensile strength and elongation at break are not verycritical. Urea and polyols were also added to starch-EAAformulations to facilitate the preparation and to improvethe economics and quality of the starch-based lms [1]by improving the gelatinization of starch at low levels ofwater, thus allowing direct extrusion of a uniform lm

from a semidry blend (16% water content). Initial tensilestrengths of urea-containing starch-EAA based lm weregenerally lower than those made by the premix method,but after water soaking to remove the urea, the tensilestrengths were nearly equal to those made without urea.On the contrary, glycerol and starch-derived polyols canbe added to starch-EAA systems to increase the percent-age of the biodegradable component without adverselyaffecting the physical properties of the lms.

Starch/PE copolymer EAA composites for blownlms were examined with regard to various compositionsof starch and EAA content, and water, ammonia, andurea contents as percentage of starch [6]. The best lms,containing 20% starch, were almost equivalent to PElms (tensile strength 20% lower than that of PE). Thesecond inuential variable affecting the lm perform-ance was the water content, because it is considerednecessary for gelatinizing and plasticizing starch to makeit exible enough for the process of lm blowing(best content 0.15 g water per gram of starch in the for-mulations examined in ref [6]; urea allows for completegelatinization).

Starch PCL-Based Blends for Films (FullyBiodegradable)

Poly(e-caprolactone) (PCL), produced by the ringopening polymerization of e-caprolactone, belongs tothe category of polyesters derived from petrochemicalfeedstocks. This material is fully biodegradable. Filmextrusion of PCL has been reported, however, with apoor impact and tear strength behavior [9]. To overcomethis problem, blends, alloys, and graft copolymers ofPCL with starch have been reported, including commer-cial lms based on starch-PCL [9]. In one approach,plasticized starch instead of granular starch is used,resulting in reduced mactrascopic dimensions (i.e., lmthickness), smaller domain size, and improved strengthand processing characteristics. The properties of theso-produced lms are comparable to LDPE lms andbetter than PCL lms. The mechanical behavior of someexperimental PCL-starch lms is presented in a latersection.

In another case, investigation of the mechanicalbehavior of two types of PCL/sago starch composites(dried granulated sago starch and undried thermoplasticsago starch) [74] has shown that in both types all majormechanical properties, tensile strength, modulus of elas-ticity, and yield stress decrease as sago starch concentra-tion increases. This is also attributed to the hydrophobicnature of PCL leading to poor interfacial interactionand so adhesion on sago starch granules on the fracture


surfaces. The dried granulated sago starch acts better asller in terms of mechanical properties and the easy ofbiodegradation, whereas the second type was shown toimpart better yield stress to the composites [74]. Inanother study, starch blends with PCL as well as withethylene, vinyl alcohol, cellulose acetate were chemi-cally modied by cross-linking [75]. The blends ob-tained had reduced water-uptake and stiffness (secantmodulus), especially for the case starch-PCL.

The behavior of some PCL-modied starch blendshas been investigated [76]. Seventy-micron lms wereprepared from modied starch and poly-CL (100%polycaprolactone; 50% modified starch and 50%polycaprolactone blend; 50% nonmodied starch and50% polycaprolactone blends). It was observed that withthe addition of starch the Youngs modulus of poly-caprolactone was increased and became less ductile,whereas tensile strength and elongation at break valuesdecreased. The modulus of the nonmodied starch-PCLblend was 273.0 MPa, which was signicantly higherthan that of the pure PCL, which was 167.6 MPa. Notethat using modied starch instead of native starch didnot change the elastic modulus signicantly, probablybecause of the more compatible interface between mod-ied starch particles. The decrease in tensile strength wasabout 50%, which is similar to the values reported byother investigators in the literature [77,78]. Decrease inthe elongation at break was about 40%.

Polylactides (PLA)Poly-L-lactic acid (PLLA) is formed by a chemical

condensation reaction of the lactic acid monomer and hasa tensile strength at break of 4570 MPa and an elonga-tion of 85%105%. Concerning the mechanical proper-ties of representative commercial products, some typicalvalues are glass transition temperature 63.8C, tensilestrength 32.22 MPa, tensile stress at break 21.87 MPa,elongation at break 30.72%). In a particular case [79], thebehavior of PLLA, PCL (polycaprolactone) and threedifferent copolymers based on PLLA and polyglycolicacid (PLLA-co-PGA) and their composites with hydrox-yapatite obtained from bovine bone (ossein: a biologicalhydroxyapatite) were investigated. The addition of 5%ossein did not affect signicantly the mechanical proper-ties. It was conrmed, as in many other cases, that therigid llers increase the modulus and decreases the ten-sile strength and elongation at break for both polymers.The yield stress also decreased, except for PCL, in whichit increased. The size of ossein particles was found to becritical for the improvement of the mechanical properties.

The mechanical properties of blends of PLA andstarch using conventional processes are rather poor be-cause of incompatibility (below one half of the values ofthe original PLA materials; elongation below 5%) [80].Plastizers (with hydrophilichydrophobic structure) areused to increase elongation (br above 75%) and obtainbiodegradable material that can be used for lm blowing.Also, PLA and starch were blended with a reactive agentduring the extrusion process by adding a catalyticamount of some reactants (peroxides, anhydrides). Themechanical properties of the so-obtained reactive blendsimproved signicantly because of the good compatibilityand the cross-linking or coupling reactions betweenmulticomponents [80].

Other kinds of PLA blends have been investigatedas well. Poly(L-lactic acid) (PLLA) was melt-blendedwith a small amount of poly(aspartic acid-co-lactide)(PAL) or poly(sodium aspartate-co-lactide) (PALNa)and processed into homogeneous press lms in the workof [81]. The mechanical properties and transparency ofsuch blends were found to be comparable to that of thenonblended PLA lm. It was found that PAL andPALNa are effective additives for accelerating thehydrolysis of PLA. The nonenzymatic hydrolysis rates ofpoly(butylene succinate) (PBS) and polycaprolactone(PCL) were also enhanced by the presence of PAL in theblend.

Graft polymerization has also been applied to PLA.Cellulose diacetate-graft-poly(lactic acid)s (CDA-g-PLAs) were synthesized successfully over a wide rangeof composition in a combination of different ways ofgraft polymerization: a copolycondensation of lacticacid; a ring-opening copolymerization of L-lactide indimethyl sulfoxide; and a copolymerization similar to thesecond, but in bulk, each initiated at residual hydroxylpositions on CDA [82]. All the copolymer productsgave a single Tg. During tensile measurements con-ducted at 80100C for lm sheets of melt-quenchedCDA-g-PLAs, it was observed that their drawability in-creased drastically with increasing PLA content and, at acertain w(PLA) of MS greater than or equal to 14, theelongation at rupture reached a maximum of 2000%.

PolysaccharidesProteins Composite Films

Preparation of composite lms through combineduse of compatible polysaccharides and proteins to im-prove properties of biopolymers has been reported in theliterature [83]. Such complexes in many cases can dis-play better functionality than proteins or polysaccharidesalone. For example, the use of xylan together with a

76 Briassoulis

protein like wheat gluten may result in lms having quiteunexpected and useful properties, depending on thenumerous possible interactions and bonding with gluten[83]. The possibility of using xylan as an agriculturalby-product for production of composite lms in combi-nations with wheat gluten was investigated [83]. Filmswere obtained with added xylan without decreasing lm-forming quality. Wheat gluten/xylan composite lmshaving different characteristics can be produced depend-ing on xylan type, composition, and process conditions.

Films made only of proteins exhibit severalproblems. Soy protein lms, for example, without othersecondary components besides proteins do not have goodmechanical and barrier properties. However, addingcystein and gluten allows the production of lms withincreased tensile strength and better barrier properties(8.24 MPa without and 8.68 MPa with cysteine addition[84]). Still, such values are rather low for agricultural ap-plications. In another case [85], homogeneous lms wereobtained by dissolution of isolate of sunower proteinsin alkaline water (pH 12), addition of a plasticizer, cast-ing, and drying. The use of ionic bases (LiOH, NaOH)capable of interfering with the interproteic noncovalentbonds resulted in the greatest tensile strength (3.9 MPa)and elongation at break (215%251%). Plasticizersconferred diverse tensile properties to the lms; theuse of 1,3-propanediol resulted in the highest (tensilestrength (27.1 MPa), and glycerol resulted in the greatestelongation at break (251%). Different mechanicalproperties were obtained by using mixtures of theseplasticizers.

In a study on the effect of chemical, physical, andageing treatments on the mechanical properties of wheatgluten lms [86] it was shown that ageing of 360 h ledto a 75% and 314% increase in tensile strength (initialvalue 1.7 MPa) and Youngs modulus, respectively, anda 36% decrease in elongation (initial value 500%).Severe thermal (above 110C, 15 min) and formaldehydetreatments highly improved the mechanical resistance ofthe lms (up to 376% and 654% increase in tensilestrength and Youngs modulus and up to 66% decreasein elongation). However, in all these cases the maximumtensile strength of the lms still remains rather low foragricultural applications.

Edible lms were formulated [87] using variousproteins (casein, gelatin, albumin) in combination withstarch and nonthermal as well as intense thermalblending. Nonthermal blended lm showed a double Tg,indicating poor miscibility of the components and,hence, a poor lm-forming property. However, all thelms based on intense thermal blending showed a singleTg, indicating complete molecular miscibility of the

components. Casein-based lm gave better lm proper-ties with a lower water-vapor transmission rate, watergain at different relative humidity conditions, and highertensile strength compared to counterparts containinggelatin and albumin.

MECHANICAL BEHAVIOR OF THIN FULLYBIODEGRADABLE AGRICULTURAL FILMS

Biodegradable Agricultural Films

The signicant interest in the use of mulching andlow tunnels for protected cultivation, aimed at elimina-tion of weeds, conservation of water and fertilization,and providing a better microenvironment for the plantsand protection against adverse climatic conditions, hasled to a rapid increase of the market of agriculturalplastic lms used for these purposes in Europe andworldwide involving hundreds of thousands of hectaresand thousands of tons of plastic lms per year [2]. Theconventional agricultural plastic lms used today arelow-density polyethylenes (in some cases HDLE),poly(vinyl chloride), polybutylene, or copolymers of eth-ylene with vinyl acetate. A side effect arising from theuse of these materials, however, concerns the problem ofhandling tons of accumulated agricultural plastic wastes.In an effort to cope with this severe and continuouslygrowing problem, polymer types (mainly polyethylenewith special light-sensitizing prooxidation additives)have been designed for controlled degradation [88, 89].Among the special additivation systems proposed area mixture of ferric and nickel dibutyldithiocarbamates,the ratio of which is adjusted to provide protectionfor specic growing periods before a rapid photodegra-dation starts and a combination of substituted benzophe-nones and titanium or zirconium chelates [1]. Also, ithas been reported that the addition of iron or calciumaccelerates the breakdown of polyethylene [1]. However,there is a signicant controversy over the possible rateof real biodegradation and/or accumulation of thefragments of the rapidly photodegraded remains ofthese agricultural polyethylene lms, along with ques-tions about possible irreversible contamination of theagricultural soils.

In parallel, really biodegradable agricultural lmshave been already produced specically designed formulching [90]. Among them [1], biodegradable lmsbased on starch with poly(vinyl alcohol) [91] andpoly(vinyl chloride) [92] have been developed in theUSDA laboratories. Also acylated starch-plastic mulchlm was developed and evaluated without any adverseeffects on soil microora [93]. In another case a variety


of starch-based polymers were blended with high-performance biodegradable polyester polymers to deter-mine the applicability of lms to be processed on a lmblowing line and to perform well in mulch lm eldtrials (thickness 3040 m, tensile strength 27 Mpa,elongation at break 300%) [94]. Poly-lactone andpoly(vinyl alcohol) lms are readily degraded by soilmicroorganisms.

Biodegradation agricultural lms should meet a setof minimum design requirements, including adequatestrength and elongation at break for mechanical installa-tion, good mechanical properties with regard to ageingduring the useful lifetime of the lm and 100% biodegra-dation in the soil, preferably before the next cultivationseason. In addition, depending on the geographicalregion, the cultivation, and the season, special additivesmay be required to adjust the physical properties of thesefilms. Specifically for low-tunnel films, the designrequirements also include an adequate mechanicalbehavior of these lms to resist various loads and loadcombinations (wind, hail, snow loads, etc).

A question is raised subsequently: have these designrequirements been established? The recent EuropeanStandard on plastics: mulching thermoplastic lms foruse in agriculture and horticulture[95] and the StandardCovering thermoplastic lms for use in agriculture andhorticulture[96] require that agricultural lms shouldmeet specic minimum values of selected mechanicalproperties, without, however, relating those predeter-mined values with the conditions to which the lms willbe exposed and the installation and support systems.Aiming at dening design requirements for biodegrad-able agricultural low-tunnel lms a systematic analyticaland experimental research work is in progress and therst results will be published soon [97].3

Mechanical Behavior of Agricultural Films

In the case of carbon blacklled, biodegradable,copolyester agricultural mulch lm and commercial car-bon blackfilled, high-density polyethylene (HDPE)mulch lm, reported in the work of [3], the biodegradableEA mulch lm exhibited higher tensile and elongation atbreak than the presently used HDPE mulch lm. It wasshown that the mechanical properties, including the ten-sile strength of both the EA and HDPE, were sufcientlypreserved from planting to harvest. The changes in theelongation at break of the EA lm proved that it possesses

the properties needed for the agricultural application andcould be installed using the same techniques currentlyused for traditional HDPE plastic mulch lm.

In another case, PCL-starch biodegradable lmshave been developed for mulching and low-tunnel appli-cations in the framework of a project in progress.2 Somepreliminary results obtained from the investigation ofthese fully biodegradable experimental thin agriculturallms are presented in this section to offer an overview ofthe mechanical performance of these lms (before use)compared to the corresponding behavior of the conven-tional synthetic lms. Two sets of biodegradable andconventional agricultural plastic thin lms were exam-ined: a set of thin biodegradable lms suitable for cov-ering low tunnels (denoted by L; thickness 6080 )and another set of very thin biodegradable lms (denotedby M; thickness 2560 ), which may be used asmulching lms. The conventional lms tested wereLDPE (denoted by L0b) and three-layer LLDPE-EVAlms (denoted by L0a). The tensile properties, constitut-ing a major category of mechanical properties of thinlms, were determined in the Laboratory of Strength ofMaterials of the Agricultural University of Athens in themachine (P; parallel) and in the transverse (T) directionaccording to the procedure described in [97].

The behavior of the thin L biodegradable lmsshown in Fig. 1 suggests a rather good mechanical be-havior comparable to the one experienced by the corre-sponding conventional low-tunnel lms. Elongation atbreak is higher in the transverse direction while strainhardening in the parallel direction is characterized by ahigher tangent modulus. These curves tend to follow thecorresponding curves of greenhouse LDPE lms exceptthat the elongation at break values are about 80% of thegreenhouse thick LDPE lm values [98].

The behavior of the very thin M biodegradablelms shown in Fig. 2 is probably different from the be-havior of the low-tunnel lms. It is comparable to the oneexperienced by the corresponding PE mulching lms inthe parallel direction, whereas in the transverse directionthe elongation at break is lower. The elongation at breakis again higher in the transverse direction, but this propertywas found to be rather sensitive. Of course, the specicapplication of mulching is not very demanding in terms ofthe mechanical behavior of the thin lms. Another char-acteristic here is the higher stress at yield values obtainedwith the biodegradable lms, especially in the transversedirection along with insignicant strain-hardening effects.

Finally, the initial tear resistance results, shown inFig. 3 for four thin biodegradable low tunnel lms and inFig. 4 for four thin biodegradable mulching lms indi-cate a satisfactory behavior, comparable to that of the

78 Briassoulis


corresponding conventional PE lms. It should be notedthat values of the tear resistance normalized with respectto thickness (N/mm) are comparable only for lms of thesame thickness (i.e., the tear resistance is not linearlydependent on the lm thickness).

CONCLUSIONS

The mechanical behavior of various types ofbiodegradable and partially (starch-based) biodegradablematerials used for lms is shown to depend, mainly, on


Fig. 1. Typical tensile stress-strain curves for thin biodegradable agricultural lm in parallel (5-cm specimens) (a) and transverse (b) (5-cmspecimens) direction (two-digit numbers: thickness in ).

Fig. 2. Typical tensile stress-strain curves for very thin biodegradable mulching lm in parallel (a) and transverse (b) direction (10-cm specimens)(two-digit numbers: thickness in ).

Fig. 3. Typical initial tear resistance curves for thin biodegradable low-tunnel lms in parallel (a) and transverse (b) direction (two-digit numbers:thickness in ).

their chemical composition and the environmentalconditions during production, but also on storage andusage conditions not considered in this work. Reportedinformation in the technical literature concerning on theoverall mechanical behavior of biodegradable lms,which may be used for agricultural applications, but alsoof partially biodegradable lms, is rather limited.

Investigation of selected critical mechanical proper-ties describing the mechanical behavior of thin readilybiodegradable starchPCL experimental lms, which maybe used for low tunnels and soil mulching, conrm a rathergood mechanical behavior comparable to those of conven-tional agricultural lms. Additional aspects, such as othermechanical properties, for example, impact and creep,degradation effects, and application conditions, describingthe mechanical behavior of thin agricultural biodegradablelms from other points of view and also their evolutionwith time, are currently under investigation.

Both, biodegradable and conventional agriculturallms are used in a rather empirical way, whereas the rel-evant standards do not cover adequately the complexity ofthe various conditions to which these lms are exposed.The open questions concerning the design requirementsof biodegradable agricultural lms are currently underinvestigation in the framework of a European project.4

ACKNOWLEDGMENT

This work has been carried out in the framework ofthe European research project Bioplastics: Biodegradableplastics for environmentally friendly mulching and lowtunnel cultivation, QLK5-CT-200000044, funded bythe EU.

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An Overview on the Mechanical Behaviour of Biodegradable Agricultural Films