112 Biodegradable Polymers for Food Packaging a Review

8/17/2019 112 Biodegradable Polymers for Food Packaging a Review

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Review

Biodegradable

polymers for food

packaging: a review

Valentina Siracusaa,*, Pietro

Rocculib, Santina Romanib andMarco Dalla Rosab

aDepartment of Physical and Chemical Methodologyfor Engineering, Engineering Faculty, University of

Catania, Viale A. Doria 6, 95125 Catania, Italy(Tel.: D39 095 7382755; fax: D39 095 333231;

e-mail: [email protected])bDepartment of Food Science, Alma Mater Studiorum,University of Bologna, Cesena (FC), Piazza Goidanich

60, c.a.p. 47023, Italy

For a long time polymers have supplied most of common pack-

aging materials because they present several desired features

like softness, lightness and transparency. However, increased

use of synthetic packaging films has led to a serious ecological

problems due to their total non-biodegradability. Although their

complete replacement with eco-friendly packaging films is just

impossible to achieve, at least for specific applications like food

packaging the use of bioplastics should be the future. The aim of

thisreview was to offer a complete viewof the state ofthe art on

biodegradable polymer packages for food application.

IntroductionThe current global consumption of plastics is more than

200 million tonnes, with an annual grow of approximately

5%, which represents the largest field of application for crude

oil. It emphasises how dependent the plasticindustry is on oil

and consequently how the increasing of crude oil and natural

gas price can have an economical influence on the plastic

market (www.european-bioplastics.org). It is becoming in-

creasingly important to utilize alternative raw materials. Un-

til now petrochemical-based plastics such as polyethylene

terephthalate (PET), polyvinylchloride (PVC), polyethylene

(PE), polypropylene (PP), polystyrene (PS) and polyamide

(PA) have been increasingly used as packaging materials be-

cause their large availability at relatively low cost and be-

cause their good mechanical performance such as tensile

and tear strength, good barrier to oxygen, carbon dioxide, an-

hydride and aroma compound, heat sealability, and so on.But

nowadays their use has to be restricted because they are not

non-totally recyclable and/or biodegradable so they pose se-rious ecological problems (www.european-bioplastics.org;

Sorrentino, Gorrasi, & Vittoria, 2007). Plastic packaging ma-

terials are also often contaminated by foodstuff and biologi-

cal substance, so recycling these material is impracticable

and most of the times economically not convenient. As a con-

sequence several thousands of tons of goods, made on plastic

materials, arelandfilled, increasingeveryyear the problem of

municipal waste disposal (Kirwan & Strawbridge, 2003).

The growing environmental awareness imposes to packaging

films and process both user-friendly and eco-friendly attri-

butes. As a consequence biodegradability is not only a func-

tional requirement but also an important environmental

attribute.The compostability attribute is very important for bio-

polymer materials because while recycling is energy expen-

sive, composting allows disposal of the packages in the soil.

By biological degradation it produced only water, carbon

dioxide and inorganic compounds without toxic residues.

According to the European Bioplastics, biopolymers

made with manufactures renewable resources have to be

biodegradable and especially compostable, so they can

act as fertilizers and soil conditioners. Whereas plastics

based on renewable resources do not necessary have to be

biodegradable or compostable, the second ones, the bio-

plastic materials, do not necessary have to be based on re-newable materials because the biodegradability is directly

correlated to the chemical structure of the materials rather

than the origin. In particular, the type of chemical bond de-

fines whether and in which time the microbes can biode-

grade the material. Several synthetic polymers are

biodegradable and compostable such as starch, cellulose,

lignin, which are naturally carbon-based polymers. Vice

versa, same bioplastics based on natural monomer, can

loose the biodegradability property through chemical mod-

ification like polymerization, such as for example Nylon 9

types polymers obtained from polymerization of oleic acid

monomer or Polyamid 11 obtained from the polymerization

of castor oil monomer.* Corresponding author.

0924-2244/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.tifs.2008.07.003

Trends in Food Science & Technology 19 (2008) 634e643

mailto:[email protected]:[email protected]://www.european-bioplastics.org/http://www.european-bioplastics.org/http://www.european-bioplastics.org/http://www.european-bioplastics.org/mailto:[email protected]://-/?-


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Plastics are compounds based on polymers and several

other chemicals like additives, stabilizers, colourants, pro-

cessing aids, etc., which quantity and type change from

a polymer to another, because each final products have to

be optimized with regard its processing and future applica-

tion (Guilbert, Cuq, & Gontard, 1997; Petersen et al.,1999). For these reasons, manufacture a product using

a 100% renewable resources is neither impossible in the

early future and the tendency is to utilize the highest pro-

portion of renewable resources possible. Until now bioplas-

tics contain more than 50% weight of renewable resources

(www.european-bioplastics.org). Many bioplastics are

mixes or blends containing synthetic components, such as

polymers and additives, to improve the functional proper-

ties of the finished product and to expand the range of ap-

plication. If also additives and pigments can be based on

renewable resources, we can obtain a polymer with approx-

imately 100% weight of biodegradation compounds.Bioplastics, like plastics, present a large spectrum of ap-

plication such as collection bags for compost, agricultural

foils, horticultures, nursery products, toys, fibres, textiles,

etc. Other fields such as packaging and technical application

are gaining importance (www.european-bioplastics.org).

The performance expected from bioplastic materials

used in food packaging application is containing the food

and protecting it from the environment and maintaining

food quality (Arvanitoyannis, 1999). It is obvious that to

perform these functions is important to control and modify

their mechanical and barrier properties, that consequently

depend on the structure of the polymeric packaging mate-

rial. In addition, it is important to study the change thatcan occur on the characteristics of the bioplastics during

the time of interaction with the food (Scott, 2000). As de-

scribed in Biodegradable polymers applications in food

packaging field section, the study of the literature shows

up that only a limited amount of biopolymers are used

for food packaging application. Unlike the usual wrap,

films, labels and laminates came from fossil fuel resources,

the use of biodegradable polymers represents a real step in

the right direction to preserve us from environmental pollu-

tion. Several University Research Center in the world (Italy,

Ireland, France, Greece, Brazil, USA and so on) and several

Industry like NatureWorks LLC, are focalizing their atten-tion to the study of these bio-based materials.

From our point of view, it is important to understand not

only the physical and mechanical properties of such mate-

rials for the task but also the compatibility with the food,

which has been recognized as a potential source of loss

in food quality properties (Halek, 1988).

Chemistry of degradationThe bioplastic aim is to imitate the life cycle of biomass,

which includes conservation of fossil resources, water and

CO2 production, as described in Scheme 1 (www.

european-bioplastic.org).

The speed of biodegradation depends on temperature

(50e70 C), humidity, number and type of microbes. The

degradation is fast only if all three requirements are present.

Generally at home or in a supermarket biodegradation occurs

very low in comparison to composting. In industrial com-

posting bioplastics are converted into biomass, water andCO2 in about 6e12 weeks (www.european-bioplastic.org).

Polymer-based products are required to biodegrade on

a controlled way: natural polymer (like rubber, lignin, hu-

mus) and synthetic polymer like polyolefins biodegrade fol-

lowing an oxo-biodegradation mechanism (Arvanitoyannis,

1999) and consequently cannot satisfy the rapid mineraliza-

tion criteria requested for standard biodegradation. Also, at

ambient temperature, oxo-biodegradation is a slower pro-

cess than hydro-biodegradation as well described by Scott

and Wiles (2001). These authors explained that during the

oxo-degradation of carboxylic acid, molecules of alcohols,

aldehydes and ketones biodegradable with low molar massare produced by peroxidation, initiated by heat or light,

which are the primary cause of the loss of mechanical prop-

erties of hydrocarbon polymers. Than bacteria, fungi, en-

zymes start the bioassimilation giving rise to biomass and

CO2 that finally form the humus. Generally synthetic poly-

mers contain antioxidants and stabilizers added to protect

the polymer against mechano-oxidation during the process-

ing operation and to provide the required shelf-life. So,

from one hand antioxidant is necessary to improve the per-

formance of these materials but, on the other hand, for the

biodegradation process it is better to not add these mole-

cules during polymer processing.

Hydro-biodegradation is the well-known process thatgives bioassimilable products from cellulose, starch, poly-

esters, etc. Aliphatic polyester is hydrolyzed and bioassimi-

lated rapidly in an aqueous environment in much the same

way as starch and cellulose (Scott & Wiles, 2001).

Polyolefin were selected as a basis for the study of biode-

gradable polymer becausetheyhad already achieveda central

position for packaging application, thanks to their combina-

tion of flexibility, toughness, excellent barrier properties, all

at low cost because coming from low value oil fraction.

Synthetic and natural polymers stand at the opposite

ends of a spectrum of properties: polyolefins are hydrocar-

bon hydrophobic polymers, resistant to peroxidation, bio-degradation, highly resistant to hydrolysis, which is their

main attribute in packaging, and not biodegradable. To

make it biodegradable it is necessary to introduce pro-

oxidant additives which promote the oxo-biodegradation

by producing low molar mass oxidation compounds bioas-

similate from the microorganisms. Natural compounds, like

cellulose, starch and so on, are hydrophilic polymers, water

wettable or swellable and consequently biodegradable.

They are not technologically useful for food packaging

where water resistant is required. Between these two ex-

tremes are the hydro-biodegradable aliphatic polyesters

such as polylactic acid (PLA) and the poly(hydroxyacid)

(PHA) (Scott & Wiles, 2001).

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Although hydrocarbons polymers make a positive contri-

bution to environment because they can be mechanically re-

cycled if clean, incinerated with energy recovery, with

a calorific value almost identical to the oil from which they

coming on, they are not compostable. According to the Euro-

pean standard norm UNI EN 13432 (2002), a product to be

defined compostable must be biodegradable and disintegr-

able in brief time, or rather it must be turned from the micro-

organisms into water, carbonic and fertile anhydride

compost. Finally, to be defined compostable, the manufac-

tured article must result compatible with a process of com-posting, that means it must not release dangerous

substances and must not alter the quality of the produced

compost. The last Financial Lawout as objectivethe dismiss-

ing of the mono-use pouches not biodegradable, for food staff

transportation, within the 2010 (Scott & Wiles, 2001).

The use of long-lasting polymers as packaging materials

for short application is not justified, also because physical

recycling of these materials is often impractical because

food contamination. So there is an increasing demand on

the use of biodegradable polymer which could be easily re-

newable (Kale, Auras, & Singh, 2006). While most of the

commercialized biopolymer materials are biodegradable,

these are not fully compostable in real composting condi-

tions, which vary with temperature and relative humidity.

Barrier propertiesThe determination of the barrier properties of a polymer

is crucial to estimate and predict the product-package shelf-

life. The specific barrier requirement of the package system

is related to the product characteristics and the intended

end-use application. Generally plastics are relatively per-

meable to small molecules such as gases, water vapour, or-

ganic vapours and liquids and they provide a broad range of

mass transfer characteristics, ranging from excellent to low

barrier value, which is important in the case of food

products. Water vapour and oxygen are two of the main

permeants studied in packaging applications, because they

may transfer from the internal or external environment

through the polymer package wall, resulting in a continuous

change in product quality and shelf-life (Germain, 1997).

Carbon dioxide is now important for the packaging in mod-

ified atmosphere (MAP technology) because it can poten-

tially reduce the problems associated with processed fresh

product, leading a significantly longer shelf-life. For exam-

ple, for fresh product respiration rate is of a great impor-

tance in MAP design so identify the best packaging isa crucial factor. The most important barrier properties of

polymer films used in packaging application are described.

Oxygen transmission rate (OTR)The oxygen barrier property of a food packaging container

for fresh product (e.g. fruits, salad, ready-to-eat meals) plays

an important role on its preservation. The oxygen barrier is

quantified by theoxygen permeabilitycoefficients(OPC) which

indicate the amount of oxygen that permeates per unit of area

and time in a packaging materials [kg m m2 s1 Pa1]. So,

when a polymer film packaging has a low oxygen permeability

coefficients, the oxygen pressure inside the container drops tothe point where the oxidation is retarded, extending the shelf-

life of the product. Generally the biodegradable polymers pres-

ent a value one or more order of magnitude below the synthetic

polymer used in the same field like PET and OPS. Several

authors reported in literature the oxygen permeability coeffi-

cients of one of the most commercialized biodegradable poly-

mer like the PLA (Auras, Harte, & Selke, 2004; Auras, Singh,

& Singh, 2006; Auras, Singh, & Singh, 2005; Lehermeier,

Dorgan, & Way, 2001; Oliveira et al., 2004).

Together with the permeability coefficient the oxygen

transmission rate (OTR), expressed in cc m2 s1 is given.

The OPC is correlated to the OTR by the following

equation:

Scheme 1. Life cycle www.european-bioplastics.org.

636 V. Siracusa et al. / Trends in Food Science & Technology 19 (2008) 634e643

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OPC ¼ OTR l =D P ð1Þ

where l is the thickness of the film (m), D P is the difference

between oxygen partial pressure across the film [Pa].

D P ¼ p1 p2, where p1 is the oxygen partial pressure at

the temperature test on the test side and p2 is equal tozero on the detector side.

Water vapour transmission rate (WVTR)The water vapour barrier properties for the packaged

product whose physical or chemical deterioration is related

to its equilibrium moisture content, are of great importance

for maintaining or extending its shelf-life. The water va-

pour barrier is quantified by the water vapour permeability

coefficients (WVPC) which indicate the amount of water

vapour that permeates per unit of area and time in a packag-

ing materials [kg m m2 s1 Pa1]. For fresh food products

it is important to avoid dehydration while for bakery or del-icatessen is important to avoid water permeation. The

WVPC of the PLA biodegradable polymer is reported in

the literature (Auras, Harte, Selke, & Hernandez, 2003;

Auras et al., 2005, 2006).

Together with the permeability coefficient is given the

water vapour transmission rate (WVTR), expressed in

cc m2 s1 (or g m2 day1). The WVPC is correlated to

the WVTR as described up in Eq. (1) for the oxygen

parameter.

Carbon dioxide transmission rate (CO2TR)Like the oxygen and water vapour barrier properties,

also the carbon dioxide barrier property is of particular im-portance on food packaging application. The carbon diox-

ide barrier is quantified by the carbon dioxide

permeability coefficients (CO2PC) which indicates the

amount of carbon dioxide that permeates per unit of area

and time in a packaging materials [kg m m2 s1 Pa1]. To-

gether with the permeability coefficient is given the carbon

dioxide transmission rate (CO2TR), expressed in cc m2 s

1 ( o r g m2 day1). The CO2PC is correlated to the

CO2TR as described up in Eq. (1).

Mechanical properties

It is well-known that the polymer architecture plays animportant role on the mechanical properties, and conse-

quently on the process utilized to modelling the final prod-

uct (injection moulding, sheet extrusion, blow moulding,

thermoforming, film forming). In addition, many packaging

containers are commercially used below room temperature,

so it is important to assess the mechanical performance un-

der these conditions (Auras et al., 2005).

Tensile test analyses are made to determine the tensile

strength (MPa), the percent elongation at yield (%), the per-

cent elongation at break (%) and the elastic modulus (GPa)

of the food polymer packaging material. These values are

important to get mechanical information of the biopolymer

materials to be compared with the commercial

nonbiodegradable ones (ASTM D882-02, Standard Test

Method for Tensile Properties of Thin Plastic Sheeting).

Impact properties test is a method utilized to determine

the energy that causes the plastic to fail under specific im-

pact conditions, conducted following the ASTM D1709-03,

Standard Test Methods for Impact Resistance of PlasticFilm by the Free-Falling Dart Method.

The compression test is normally conducted on thermo-

formed sample, according to the ASTM D642, Standard

Test Method for Determining Compressive Resistance of

Shipping Containers, Components, and Unit Loads. Natu-

rally the compression strength is function of the material

and of design (shape and size).

Chemical resistance propertiesProducts that could be packaged in this kind of con-

tainers may have weak or strong acid characteristics; so it

is necessary to assess the performance and the suitabilityof biopolymers stored with common food packaging solu-

tion as a function of time. The interaction and absorption

between chemical compounds and polymer may affect the

final mechanical properties of a polymer (Auras et al.,

2005). Normally the chemical resistance is tested measur-

ing the tensile stress, elongation at break and modulus of

elasticity of sample submerged in weak and strong acid so-

lutions as a function of time, simulating real conditions, at

ambient temperature (23 C) and at 18 C, 23 C and29 C. The weak acid solution is prepared with aceticacid while the strong acid solution is prepared with hydro-

chloric acid (Auras et al., 2005).

Some important production considerationCurrently, there are several types of bio-based polymers

on the market: same coming from petrochemical monomer,

like certain types of polyester, polyester amides and polyvi-

nyl alcohol, produced by different manufacturer, used prin-

cipally as films or moulding. Four other bio-based polymers

are starch materials, cellulose materials, polylactic acid

(Polyester, PLA), polyhydroxy acid (polyester, PHA). Until

now, the PHA polymer is a very expensive polymer because

it is commercially available in very limited quantities. PLA

is becoming a growing alternative as a green food packag-

ing materials because it was found that in many situations itperforms better than synthetic ones, like oriented polysty-

rene (OPS) and PET materials (Auras et al., 2005).

Different types of materials can be combined to form

blend or compounds or semifinished products such as films.

The degradation of the materials is normally studied under

real compost conditions and under ambient exposure by dif-

ferent techniques. The polymer degradation rate is deter-

mined by the nature of the functional groups and the

polymer reactivity with water and catalysts.Any factor which

affects the reactivity such as particle size and shape, temper-

ature, moisture, crystallinity, isomer percentage, residual

monomer concentration, molecular weight, molecular weight

distribution, water diffusion, metal impurities from the



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catalyst, will affect the polymer degradation rate (Kale et al.,

2006). In general high temperature and humidity will degrade

more rapidly the polymer. By visual inspection the packages

are observed for colour, texture, shape and change in dimen-

sion. Generally a digital camera is used to take the pictures.

The thickness of the packages is determined by a thick-ness gauge according to the ASTM norm D4166-

99(2004)e1, Standard Test Method for Measurement of

Thickness of Nonmagnetic Materials by Means of a Digital

Magnetic Intensity Instrument, or by micrometer according

to ASTM D 374-99, Standard Test Methods for Thickness

of Solid Electrical Insulation.

By gel permeation chromatography it is possible to de-

termine the molecular weight of samples dissolved in the

appropriate solvent. Molecular weight variations are an

indication of the degradation rate of the polymers and

give information about when the main fragmentation oc-

curs in a polymer.By differential scanning calorimetry (DSC) it is possible

to determine the glass transition temperature (T g), melting

temperature (T m) and crystallinity of the polymer sample

(ASTM D3418, Standard Test Method for Transition Tem-

peratures and Enthalpies of Fusion and Crystallization of

Polymers by Differential Scanning Calorimetry). The crys-

tallinity is determined according to ASTM D3417-97 and

using the following equation, well-known in the literature,

x cð%Þ ¼ 100 ðD H c þD H mÞ=D H cm ð2Þ

where D H c is the exothermic enthalpy of cold crystalliza-

tion, D H m is the endothermic enthalpy of fusion, D H cm isthe endothermic heat of melting of purely crystalline poly-

mer under study (for example: for PLA is 135 J g1, Kale

et al., 2006; for PET is 125.6 J g1, Auras et al., 2003).

By thermo-gravimetric analysis (TGA) it is possible to

obtain the decomposition temperature, according to the

ASTM E1131-03, Standard Test Method for Compositional

Analysis by Thermogravimetry.

The determination of the pH of the sample surrounding

is one of the most important factors of hydrolytic polymer

degradation since pH variations can change hydrolysis

rates by few order of magnitude. The chemical resistance

is normally determined exposing the materials to weak acid (pH ¼ 6, acetic acid solution) and strong acid(pH¼ 2, hydrochloric acid solution) for a period of 0, 1,3, 5 and 7 days.

The most important analysis for film used in food pack-

aging application is the determination of the oxygen, car-

bon dioxide and water vapour transmission rate (OTR,

CO2TR and WVTR, respectively). These tests are per-

formed according to the ASTM norm described before.

Concerning the mechanical properties, the samples

could be analysed by Impact tests, Tensile properties and

Compression Test of Thermoformed Containers. Generally

these parameters are studies at ambient temperature (22 C)

and at frozen food storage temperatures of 18 C,

23 C and 29 C, since fresh produce packaging anddeli containers are generally used commercially at this

conditions.

Biodegradable polymers applications in foodpackaging fieldThe field of application of biodegradable polymer in

food-contact articles includes disposable cutlery, drinking

cups, salad cups, plates, overwrap and lamination film,

straws, stirrers, lids and cups, plates and containers for

food dispensed at delicatessen and fast-food establish-

ments. These articles will be in contact with aqueous,

acidic and fatty foods that are dispensed or maintained at

or below room temperature, or dispensed at temperatures

as high as 60 C and then allowed to cool to room temper-

ature or below (Conn et al., 1995).

In the last few years, polymers that can be obtained from

renewable resources and that can be recycled and com-

posted, have garnered increasing attention. Also their opti-

cal, physical and mechanical properties can be tailored

through polymer architecture so as a consequence, biode-

gradable polymers can be compared to the other synthetic

polymers used in fresh food packaging field, like the

most common oriented polystyrene (OPS) and polyethylene

terephthalate (PET).

Depending on the production process and on the source,

biopolymers can have properties similar to traditional ones.

They are generally divided into three groups: polyesters;

starch-based polymer; and others.

PolyestersThese materials can be:

i. Polymers directly extracted from biomass like pro-

teins, lipids, polysaccharides, etc.

ii. Polymeric materials synthesized by a classical poly-

merization procedure such as aliphaticearomatic

copolymers, aliphatic polyesters, polylactide aliphatic

copolymer (CPLA), using renewable bio-based mono-

mers such as poly(lactic acid) and oil-based mono-

mers like polycaprolactones.

iii. Polymeric materials produced by microorganisms andbacteria like polyhydroxyalkanoates.

Aliphaticearomatic copolymersThese materials are a combination of polyetilene tere-

phthalate (PET), resistant to microbial attack, with three or

more biodegradable aliphatic polyesters. It is soft, pliable

with a good touch but with a melting point of around 200 C,

too highfor a degradable material. The aliphatic monomer cre-

ates a weak spots in the aromatic polymeric chain which makes

them susceptible to degradation through hydrolysis.

Also if it is totally biodegradable, coming from fossil

fuels such as oil, coal and natural gas, PET production



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causes a consume of non-renewable and finite resources

and with an heavy impact on global waste disposal. If prop-

erly disposed it degrades in 8 weeks but if it is trash without

any control in the environment, the degradation process can

take 50 years to breakdown the structure. The long polymer

molecules are cleaved by moisture into smaller ones, whichare naturally consumed by microbes and converted to car-

bon dioxide and water. This material is commonly used

for eating utensils (like fork, knife, spoon, dishes and so

on) and bottles but it costs twice as much than commercial

ones (Salt, 2002). DuPont (Tennessee) produce the PET/hy-

dro-biodegradable polyester under the trade name of Bio-

max, exported in all over the world.

Aliphatic polyestersThese materials have properties similar to PE and PP

polymers, they are biodegradable but with lack in thermal

and mechanical properties. These materials come from

polycondensation reaction of glycol and aliphatic dicarbox-

ylic acid, both obtained from renewable resources. They are

odourless and can be used for beverage bottles and they

biodegrade in soil and in water giving carbon dioxide and

water, in a period of 2 months (e.g. for a 0.04 mm thick

film) (www.designinsite.dk ).

A commercially available aliphatic copolyester is pro-

duced by Procter and Gamble Co. (P&G, Cincinnati, OH)

with the trade name of Nodax and it can degrade in aerobic

and anaerobic environmental conditions. The other one

is the Eastar bIo, produced from the Eastam Chemical

Company (Hartlepool, UK).

Polylactide aliphatic copolymer (CPLA)This material is a mixture between renewable resources

as lactide and aliphatic polyesters like dicarboxylic acid or

glycol, with hard (like PS) and soft flexible (like PP) prop-

erties, depending on the amount of aliphatic polyester pres-

ent in the mixture. It is easy to process and thermally stable

up to 200 C. The heating value and the quantity of carbon

dioxide generated during combustion are about the half of

that generated from commercial polymer like PE and PP,

and incineration does not produce toxic substances. In nat-

ural environment it starts to degrade in 5e6 months, witha complete decomposition after 12 months. If composted

with food garbage, it begins to decompose after 2 weeks.

Polycaprolactone (PCL)It is a fully biodegradable polymer coming from the po-

lymerization of not renewable raw material, like crude oil.

It is a thermoplastic polymer with good chemical resistance

to water, oil, solvent and chlorine, with a melting point of

58e60 C, low viscosity, easy to process and with a very

short degradation time. It is not used for food application

but if mixed with starch it is possible to obtain a good bio-

degradable material at a low price, used for trash bags.

Poly(lactic acid) (PLA)One of the most promising biopolymer is the poly(lactic

acid) (PLA) obtained from the controlled depolymerization

of the lactic acid monomer obtained from the fermentation

of sugar feedstock, corn, etc., which are renewable resources

readily biodegradable (Cabedo, Feijoo, Villanueva, Lagarón,& Giménez, 2006). It is a versatile polymer, recyclable and

compostable, with high transparency, high molecular weight,

good processability and water solubility resistance. In gen-

eral commercial PLA is a copolymer between poly(L-lactic

acid) and poly(D-lactic acid). Depending on the L-lactide/ D-

lactide enantiomers ratio, the PLA properties can vary con-

siderably from semicrystalline to amorphous ones. Re-

searches carried out to improve the performance quality of

this material are made on PLA with D-lactide content less

than 6%, which is the semicrystalline polymer. However

the amorphous one, containing 12% of D-lactide enantiomer,

is easy to process by thermoforming, which is the actual tech-nology in the food packaging sector, and it shows properties

like polystyrene. This material is commercialized by differ-

ent companies with different commercial names, like for ex-

ample the Natureworks PLA produced by Natureworks

LLC (Blair, NB). Currently it is used in food packaging appli-

cation only for short shelf-life products.

In Table 1 physical characteristics of PLA obtained from

Auras et al. (2006) are reported.

Kale et al. (2006) studied the compostability of three

commercially available biodegradable packages made of

PLA, in particular water bottles, trays and deli containers,

to composting and to ambient exposure. They investigate

the properties breakdown of these packages exposed tocompost conditions by several experimental procedure in-

volving gel permeation chromatography (GPC), differential

scanning calorimetry (DSC), thermal gravimetric analysis

(TGA) and visual inspection. The compost pile was pre-

pared with cow manure, wood shaving and waste feed, at

Table 1. Physical experimental data for PLA (Auras et al., 2006)

Experimental data PLA

T g (C) 62.1 0.7

T m (C) 150.2 0.5

DH cm (J g

1) 93Percent crystallinity (x c) 29.0 0.5Oxygen transmission rate (OTR)(ccm2 day)a

56.33 0.12

Oxygen permeabilityrate (OPC) (kgm m2 s1 Pa1)b

4.33E-18 1.00E-19

Water vapour transmissionrate (WVTR) (g m2 day1)a

15.30 0.04

Water vapour permeabilityrate (WVPC) (kg m m2 s1 Pa1)c

1.34E-14 3.61E-17

a Thickness of 20.0 0.2.b OPC ¼OTR l / DP , with l is the thickness in m and DP is the

difference in oxygen partial pressure across the film.c WVPC ¼WVTR l / DP , with l is the thickness in m and DP is

the difference in water vapour partial pressure across the film.


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a temperature above 60 C. After 3 weeks the pile was put

on asphalt pad. The initial pile temperature, relative humid-

ity and pH was 65 5 C, 63 5% and 8.5 0.5, respec-tively. The packages were subjected to composting for 1,

2, 4, 6, 9, 15 and 30 days and packages exposed to ambient

conditions were also studied. This is because it is well-known that PLA absorb water, resulting in the hydrolysis

and cleavage of the ester linkages, autocatalyzed by the car-

boxylic acid end groups (Scheme 2).

They found that the degradation rate changes with the ini-

tial crystallinity and L-lactide content of the packages, with

lower degradation for the polymer with the highest content

of L-lactide (96 versus 94%) due to the higher crystallinity,

which makes more difficult the degradation of the whole

structure. During the hydrolysis process the sample decreased

in size (reduction of the thickness) and became tough (in-

crease in fragility) with a trend that depend to the shape.

The dimension of the containers before and after compostingwas calculated by measuring the variation on width, length,

heightand thickness. During thefirst periodof theambient ex-

posure (within the first 15 days) the molecular weight of the

sample M w showed a small increase probably due to UV or

gamma radiation which produces chain cleavage and subse-

quent recombination, which can result in crosslinking and

hence an increase in the M w. Theincreasein molecular weight

produced an increase in the glass transition temperature T gand leading to slower degradation since glassy polymers de-

grade more slowly than rubbery ones. Thesame trend was ob-

served for the sample exposed to the compost pile; the M wincrease could be attributed to crosslinking or recombination

reactions. Subsequently, during exposure, the degradationproduced a molecular weight decrease that followed a first or-

der kinetic associated to a first order hydrolysis process af-

fected by the initial crystallinity, thickness and shape of the

sample (Tsuji & Ikada, 1998). Since the hydrolysis occurs

randomly, longer PLA chain is more susceptible to cleavage

than the shorter ones. The fragmentation process, which pro-

duces decomposition of the macromolecules into shorter olig-

omer chainsand monomers,took place giving an initial rise of

the polydispersity index (PDI). Afterwards, polymer frag-

mentation took place with a decrease in PDI until 1.00 value,when only oligomers of the PLA chains are present.

After the first 4 days where an increase of T g was ob-

served correlated to a short increments of M w, the value

showed a reduction which was obviously associated to

the molecular weight reduction, starting from about 62 C

to 30 C. The T g reduction of the PLA packages exposed

to compost conditions followed a linear trend, with a reduc-

tion in C for day which depend on the shape of the con-

tainers (Kale et al., 2006). The T m variation as a function

of time did not follow a linear relationship, with a slight in-

crease of T m for the samples submitted to compost condi-

tions at the beginning of the composting process.The decomposition temperature T D, determined by

thermo-gravimetric analysis (TGA), associated to depoly-

merization (rapid reduction of polymer mass with a slow

reduction in molecular weight) and random degradation

(slow loss of polymer mass with an exponential decrease

in molecular weight) was determined for PLA samples ex-

posed to ambient and composting conditions. No variation

of T D as a function of time was observed for the sample ex-

posed to ambient condition while for the other ones a reduc-

tion of T D was observed, with a linear variation.

PLA packages will compost in municipal or industrial

facilities but the PLA degradation is driven by hydrolysis

which needs higher temperatures to take place, so a com-pletely compost will be difficult. Further studies will be

necessary to find method and techniques that can assess

the degradability of the biodegradable food packaging un-

der real composting conditions.

nHO

O

O

O

O poly

O

OO

OCH3

CH3

CH3

CH3

H2O

HO

O

O

O

O poly

poly

O

O

OCH3

CH3

CH3

CH3OH OHn

HO

O

O

O

H

OO

OCH3

CH3

CH3

n

+

O

O

CH3

HO

Scheme 2. PLA hydrolysis and molecular cleavage.



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As reported from Kale etal. (2006), Pomettoetal. inapre-

vious research studied the banana field exposition of PLA

film in Costa Rica. They found that this film lost its mechan-

ical properties faster than during exposure in simulated con-

ditions in the laboratory, with a degradation enhanced by an

increase in temperature and relative humidity. This data notconcern complete packages degradation.

Concerning the PLA toxicology for human safety, Conn

et al. (1995) studied the migration of small molecules com-

ing from the hydrolytic degradation phenomena of PLA

polymer films of food-contact articles, which are lactic

acid (a safe food substance), the monomer lactide and the

linear dimer of lactic acid which is lactoyllactic acid. In

any case dimers and oligomers hydrolyse in aqueous sys-

tem to lactic acid, which is a common food ingredient

that has been shown to be safe in food at levels far in excess

of any small amount that might result from the intended

uses of PLA. They studied the PLA components migrationby extraction tests in which samples of the polymer were

exposed to food-simulating solvents under conditions that

reproduce the most severe temperature/time conditions to

which food would be exposed while in contact with PLA.

The examined contact was with aqueous, acidic and fatty

foods. It was found that in any case migrants from PLA

other that lactic acid (dimers, trimers, etc.) represented

very small and safe amounts. Migrating quantities of these

species hydrolyse to lactic acid in the aqueous and acidic

media commonly found in food and in the stomach. Lactide

has demonstrated low intrinsic toxicity in testing while the

lactoyllactic acid is normally present in commercially

available lactic acid that is an evidence of its safety.Concerning the optical, physical and mechanical perfor-

mance of the oriented PLA polymer (OPLA) in food appli-

cation, Auras, Singh & Singh (2006) made a comparison

with two of the commonly used materials used for fresh

food packaging application, which are polyethylene tere-

phthalate (PET) and oriented polystyrene (OPS). The phys-

ical experimental data obtained on PLA samples are

reported in Table 1.

Concerning the physical data, OPLA presents the lowest

T g and T m data respect the PET and OPS polymers, while

the crystallinity is very similar to that of PET (OPS is atactic

and does not crystallize so it is highly transparent). About theoxygen transmission rate it was found that OPLA is a good

film for food liketomatoand other breathableproducts where

the oxygen and carbon dioxide barrier requirements are spe-

cifically matched to the respiration rate of the fresh produce.

In order to maintain the freshness property and shelf-life of

fruit and vegetable, it is necessary to control their storage

conditions, like humidity and quantity of gases (oxygen, car-

bon dioxide and ethylene). Usually the specific gas require-

ments are achieved by controlling the type of films used as

packaging materials for different atmospheric conditions.

As far as mechanical properties are concerned, it was found

that at room temperature the three polymers showed similar

tensile strength while at temperature besides that, were

higher than those for PETand OPS. The modulus of elasticity

showed a similar trend with the best value for the OPLApoly-

mer, while the elongation at break was similar at room tem-

perature for the three polymers but was much higher for PET

at value below theroom temperature. Thecompression test of

thermoformed containers had point out that OPLA and OPShave similar compression strength while the PET containers

showed the best value but in this case it was not possible to

give an conclusive information about the overall perfor-

mance because the containers had different shapes.

Theresults forchemical resistance tests showedthat expo-

sure to acid and vegetable oil resulted in a minimal strength

degradation PLA (and also for the other two polymer PET

and OPS). PLA studied in these conditions has showed that

when it is submerged to weak acid solution there is an in-

crease of tensile strength, it becomes more ductile and there

is a reduction in the modulus of elasticity as a function of

time. For sample submerged in strong acid solution therewas an increase of tensile stress, no variation in the elonga-

tion at break, it becomes more brittle with an increase in

the modulus of elasticity which is an indication of the brittle-

ness of the sample as a function of time.

The same mechanical properties were measured when

PLA sample containers were exposed to vegetable oil and

it was found that there was a decrease of the tensile stress,

a reduction of the elongation at break and an increase of the

modulus of elasticity.

Based on the experimental researches made until now it

has been found that PLA is safe and generally recognized

as safe for its use in food-contact articles.It has the advantage

of easily tailoring their physical properties by changing thechemical composition (amount of L- and D-isomer) and the

processing conditions. PLA packages perform, as well as

other containers made on synthetic polymer like PET, PS,

etc., at room and low temperature, suggesting that PLA

would also be suitable for the same food application. How-

ever, same properties such as flexural properties, gas perme-

ability, impact strength, processability, etc., are often not

good enough for this application. This material shows good

barrier to aroma but the most important limitation on the

use of PLA for food application packaging is the medium

barrier to gases and vapours and the brittleness properties.

A possible strategy to decrease the brittleness is to makea blend between PLA and others polymer. Cabedo et al.

(2006) studied the blend of PLA with polycaprolactone

(PCL), which is also a biodegradable semicrystalline poly-

mer obtained from the polymerization of 3-caprolactone. It

showed low tensile strength, high elongation at break, and

processing temperature similar to the PLA and it can be uti-

lized like plasticizer to increase the gas permeability of the

PLA as a consequence of the poor gas barrier properties of

PCL. In this research to the PLA/PCL blend they added

also kaolinite nanocomposites by melt mixing using a con-

ventional polymer extrusion process. By SEM analysis

they found that this blend is immiscible across the composi-

tion range studied, but is was observed a plasticization effect



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of the blend compared to the PLA matrix (by Dynamic-

mechanical analysis, DMA) and a slight increase in its ther-

mal stability (by Thermo-gravimetric analysis, TGA) with an

increment of this effect with the PCL amount increment. The

gas barrier properties showed a significant decrease propor-

tional to the amount of PCL added to the blend, which wascompensated in the sample containing kaolinite which shows

an increase in the gas barrier properties. Anyway, these

changes were clearly discernible but small. The effect of

the nanocomposites is currently under study but it is clear

that these compounds could be a valid route to decrease the

inherent rigidity of some biopolymers and to enhance their

applications.

Further studies on PLA products must be performed to de-

termine the range of compatibility of this polymer and to de-

termine the performance in real shelf-life studies. A study of

the Life-Cycle Assessment (LCA) for the PLA polymer was

just made by Bohlmann (2004), who made a comparisonwith the most used polypropylene (PP) in food packaging ap-

plication. He found that PLA is more energy efficient than PP

polymer because PLA consumes no feedstock energy. How-

ever, when it is taken in consideration the uncertainty of the

estimation, the difference between the two polymers be-

comes marginalized. He found also that the PLA and PP

greenhouse gas emission are equivalent.

Fang et al. (2005) studied the possibility to make a multi-

layer filmwith PLA and a natural materiallike modifiedstarch

to have equal or better performance characteristics to those of

existing product not biodegradable like polyethylene/polyvi-

nylidenchloride/polystyrene (PE/PVDC/PS) multilayer films.

Starch is a totally biodegradable polymer coming from agri-culture; it is abundant, renewable, safe and economics but as

a component of biodegradable laminate film, it shows no plas-

tic behaviour, no adequate mechanical properties and it ther-

mally degrades at around 260 C. When it is extruded in

combination with plasticizers it becomes thermoplastic,

mouldable and an amorphous material with an excellent oxy-

gen barrier characteristic, but it is extremely sensitivity to the

environmental humidity giving rapid biodegradation.

Polyhydroxyalkanoates (PHA)These polymers are produced in nature by bacterial fer-

mentation of sugar and lipids. They can be thermoplastic orelastomeric materials, with a melting point between 40C

and 180 C, depending on the monomer used in the synthesis.

These polymers, alone or in combination with synthetic plas-

tic or starch give excellent packaging films (Tharanathan,

2003). The most common type is the polyhydroxybutyrate

(PHB), coming from the polymerization of 3-hydroxybuty-

rate monomer, with properties similar to PP but more stiffer

and brittle. The copolymer polyhydroxybutyrate-valerate

(PHBV), used as packaging material, is less stiff and tougher.

Thepriceisveryhighbutitdegradesin5e6weeksinamicro-

biology active environments, giving carbon dioxide and wa-

ter in aerobic condition. In anaerobic environment the

degradation is faster, with production of methane.

Yu, Chua, Huang, Lo, and Chen (1998) used different types

of food wastes as carbon source to produce several PHA poly-

mers with different physical and mechanical properties, like

flexibility, tensile strength, melting viscosity. The use of

food waste is a good way to reduce the cost of bioplastics pro-

duction, but until now it is only an experimental procedurewithout any possibility to have a commercial application.

Starch-based polymerCommercial polymer coming from the synthesis of oil-

based monomer can be mixed with different percentage

(10, 50 and 90%) of starch used as additive. Depending

on starch percentage and other materials like additives (col-

ouring additives, flame retardant additives) the properties of

these materials can be varied a lot, becoming stable to un-

stable for example in hot/cold water.

Starch, consumed by microbial action, accelerates the

disintegration or fragmentation of polymer chain by pro-ducing pores in the materials which weaken them. This pro-

cess is quite slow, it can be accelerated only if the starch

added to the mixture exceed 60%. Depending on the type

of the thermoplastic starch materials, they can degrade in

5 days in aqueous aerobic environment, in 45 days in con-

trolled compost and in water.

In 1993 LDPE-starch blend were commercialized under

the trade name Ecostar. Other commercial trade names

are Bioplast (from Biotec GmbH) and NOVON (from

NOVON International) (www.designinsite.dk ).

Starch can be transformed also into a foamed material us-

ing water steam, replacingthe polystyrene foam as packagingmaterial. It can be pressed into trays or disposable dishes and

dissolves in water leaving a non-toxic solution, consumed by

microbic environment in about 10 days giving only water and

carbon dioxide as by-products. The commercial trade names

are Biopur (from Biotec GmbH), Eco-Foam (from Na-

tional Starch & Chemical) and Envirofill (from Norel).

Others biomaterials not used in food applicationAnother natural plastic material, the casein formalde-

hyde, can be generated from a natural protein obtained

from milk, horn, soy bean, wheat, etc. It looks like cellu-

loid, ivory or artificial horn and it is insoluble in water, in-flammable and odourless. This material is used to make

buttons, pins, cigarette-cases, umbrella handles and so on

but not in food application.

The cellulose acetate (CA) is an amorphous tough ther-

moplastic obtained by introducing the acetyl radical of ace-

tic acid into cellulose (cotton or wood). To decrease its

inflammability it is used with additives, with self-extin-

guishing properties. Cellulose acetate is an insulator mate-

rial with a little tendency to electrostatic chargin, brittle

under freezing point. Horn is an organic thermoplastic ma-

terial containing 80% of keratin; it can be pressed in vari-

ous objects and laminas, like buttons, combs, pens, etc.

(www.designinsite.dk ).


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ConclusionsBioplastics development is just beginning; until now it

cover approximately 5e10% of the current plastic market,

about 50,000 t in Europe. The European countries with the

highest utilization of bioplastics are France, Germany, En-

gland, Netherland and Italy but other countries like Bel-gium, Austria, Spain and Switzerland are going to utilize

it in individual applications. The principal field regards

the use of films packaging for food products, loose film

used for transport packaging, service packaging like carry

bags, cups, plates and cutlery, biowaste bags, in agri- and

horticultural fields like bags and compostable articles.

Their development costs are high and yet they do not have

the benefit of economic scale. The increased utilization of

biomass as energy source and raw materials is necessary in

the long term due to the fact that the crude oil and natural

gas resources are limited, but it is tobe keepin mindthatthese

materials have to be found place in a very strong internationalmarket of synthetic ones, with an annual plastics consump-

tion of approximately 200 million tons, with approximately

a 5% average growth per annum. However, plastics and bio-

plastics cover an abundance of types, each with its own indi-

vidual profile, so they present an enormous diversity which

makes them so successful in numerous applications.

It was shown that polyolefins present the same oxo-bio-

degradability of biopolymers, but they are more economical

and effecting during use, so certain they will remain the

materials of choice for packaging application.

Bio-based polymers have already found important appli-

cations in medicine field, where cost is much less important

than function. It seems very unlikely that biodegradable oil-based polymers will be displaced from their current role in

packaging application, where cost is more important for the

consumer market than environmental acceptability.

Biopolymers fulfill the environmental concerns but they

show some limitations in terms of performance like thermal

resistance, barrier and mechanical properties, associated

with the costs. Then, this kind of packaging materials needs

more research, more added value like the introduction of

smart and intelligent molecules (which is the nanotechnology

field) able to give information about the properties of the food

inside the package (quality, shelf-life, microbiologicalsafety)

and nutritional values. It is necessary to make researches onthis kind of material to enhance barrier properties, to ensure

food properties integrity, to incorporate intelligent labelling,

to give to the consumer the possibility to have more detailed

product information than the current system.

References

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bioplastics.org

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112 Biodegradable Polymers for Food Packaging a Review

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