24.1 Introduction: the problem of plastic packaging waste
Polymers and plastics are typical materials of the last century and have made a
tremendous growth of some hundreds of tons/year at the beginning of the 1930s
to more than 150 million tons/year at the end of the 20th century with 220
million tons forecast by 2005. Western Europe will account for 19% of that
amount. Today, the use of plastic in European countries is 60kg/person/year, in
the US 80kg/person/year and in countries like India 2kg/person/year.
1
The basic
materials used in packaging include paper, paperboard, cellophane, steel, glass,
wood, textiles and plastics. Total consumption of flexible packaging grew by
2.9% per year during 1992–1997, with the strongest growth in processed food
and above average growth in chilled foods, fresh foods, detergents and pet foods.
Plastics allow packaging to perform many necessary tasks and provide thereby
important properties such as strength and stiffness, barrier to gases, moisture,
and grease, resistance to food component attack and flexibility.
2
Plastics used in
food packaging must have good processability and be related to the melt flow
behaviour and the thermal properties. Furthermore, these plastics should have
excellent optical properties in being highly transparent (very important for the
consumer) and possess good sealabilty and printing properties. In addition,
legislation and consumers demand essential information about the content of the
product.
Compared to the total amount of waste generated in for example the EU,
packaging accounts for only a small part, about 3%. Nevertheless, the actual
total amount of packaging waste in Europe is still at least 61 million tons per
year and this amount has a big impact regarding the waste streams produced by
households. In the Netherlands the fraction of plastics in municipal waste is
24
Green plastics for food packaging
J.J. de Vlieger, TNO Industrial Technology, The Netherlands
nowadays 30% by volume
3
and in the US 21%. Disposal costs are high, in
Europe 125 Euro per ton, in the USA 12–80 Euro per ton but in countries like
Japan even 250 Euro per ton.
4
The durability of plastics is beyond dispute. Some plastics need to be durable
but many plastics have only a limited life or are used only once and therefore
durability is not essential. A recent governmental action against litter in the
streets in The Netherlands shows a billboard with a plastic cup lying on the
highway with the message that if nobody picks it up, this cup will still be there
after 90 years. The persistence of these petrochemical-based materials in the
environment beyond their functional life is a problem. To bring this waste
disposal under control, integrated waste management practices including
recycling, source reduction of packaging materials, composting of degradable
wastes and incineration have to be introduced. However, these measures will not
help to decrease dependency on petroleum-based products and part of the
solution can perhaps be found in the development and introduction of so-called
biodegradable packaging materials that will degrade naturally into harmless
degradation products at the end of their life cycle. This had led in the past to
some misconceptions about how these materials could help solve the problem
because policy has always been strong on supporting recycling of present plastic
materials. On the other hand politicians have also reacted by introducing
legislation for degradability requirements and thus providing a platform for
natural polymer producers to obtain a larger market share in the non-food area.
Specific applications where biodegradability is required are sacks and bags
that can be used for composting waste, foamed trays, cups and cutlery in the fast
food sector, soluble foams for industrial packaging, film wrapping, laminated
paper, foamed trays in food packaging, mulch films, nursery pots, plant labels in
agricultural products and diapers and tissues in hygiene products.
5
24.2 The range of biopolymers
24.2.1 Introduction
The development of biodegradable packaging alternatives has been the subject
of much research and development in recent times, particularly with regard to
renewable alternatives to traditional oil-derived plastics. Biopolymers, polymers
synthesised by nature such as starch and polysaccharides, are an obvious
alternative. However, these natural polymers on their own do not demonstrate
the same material properties as traditional plastics, limiting potential
applications of the technology. There are two major groups of biodegradable
plastics currently entering the marketplace or positioned to enter it in the near
future: polylactic acid (PLA) and starch based polymers.
6
These new polymers
are truly degradable but full degradability will occur only when products made
from these polymers are disposed of properly in a composting site.
520 Novel food packaging techniques
24.2.2 Lactic acid
The efforts of biotechnology and agricultural industries to replace conventional
plastics with plant derived alternatives have seen recently the following three
approaches: converting plant sugars into plastic, producing plastic inside micro-
organisms and growing plastic in corn and other crops. Cargill Dow has scaled-
up the process of turning sugar into lactic acid and subsequently polymerises it
into the polymer polylactic acid, NatureWorks
TM
PLA. Lactic acid can be
produced synthetically from hydrogen cyanide and acetaldehyde or naturally
from fermentation of sugars, by Lactobacillus. Fermentation offers the best
route to the optically pure isomers desired for polymerisation. Condensation
polymerisation of lactic acid itself generally results in low molecular weight
polymers. Higher molecular weights are obtained by condensation
polymerisation of lactide, the intermediate monomer. When racemic lactides
are used, the result is an amorphous polymer, with a glass transition temperature
of about 60oC, which is not suitable for packaging.
7
24.2.3 Polylactic acid
Polylactic acid (PLA) is a polymer that behaves quite similarly to polyolefines
and can be converted into plastic products by standard processing methods such
as injection moulding and extrusion. It has potential for use in the packaging
industry as well as hygiene applications. Currently a main obstacle is the high
price of the raw material and the lack of a composting infrastructure in the
European, Japanese and US markets. The current global market for lactic acid
demand is 100,000 tons per annum, of which more than 75% is used in the food
industry. Perhaps the biggest opportunities for PLA lie in fibres and films. For
instance, worldwide demand for non-woven fabrics for hygiene application is
400,000 tons per annum. Other important market niches can be found in the
agricultural industry such as crop covers and compostable bags.
The polymer of choice for most packaging applications may be 90% L-
lactide and 10% racemic D,L-lactide. This material is reported to be readily
polymerised, easily meltprocessable and easily oriented. Its Tg is 60oC and its
melting temperature is 155oC. Tensile strength of oriented polymers is reported
to be 80–110Mpa with elongation at break of up to 30%. Polylactide films are
reported to be very similar in appearance and properties to oriented polystyrene
films. Residual lactide is not a concern since it hydrolyses to lactic acid, which
occurs naturally in food and in the body.
7
Therefore, PLA polymers are designed
for food contact. Cargill Dow, the largest producer of PLA polymers, has
confirmed that one of their grades is GRAS (Generally Recognised As Safe),
permitting its use in direct food contact with aqueous, acidic and fatty foods
under 60oC and aqueous and acidic drinks served under 90oC. In Europe, lactic
acid is listed as an approved monomer for food contact applications in
Amendment 4 of the Monomers Directive, 96/11/EC. All PLA polymer
additives have appropriate EU national regulatory status.
8
However, PLA is not
yet found in large applications of food packaging today.
Green plastics for food packaging 521
24.2.4 Native starch
Starch is nature’s primary means of storing energy and is found in granule form
in seeds, roots and tubers as well as in stems, leaves and fruits of plants. Starch is
totally biodegradable in a wide variety of environments and allows the
development of totally degradable products for specific market needs. The two
main components of starch are polymers of glucose: amylose (MW 10
5
–10
6
), an
essentially linear molecule and amylopectin (MW 10
7
–10
9
), a highly branched
molecule. Amylopectin is the major component of starch and may be considered
as one of the largest naturally occurring macromolecules. Starch granules are
semi-crystalline, with crystallinity varying from 15 to 45% depending on the
source. The term ‘native starch’ is mostly used for industrially extracted starch.
It is an inexpensive (< 0.5 Euro/kg) and abundant product, available from potato,
maize, wheat and tapioca.
9
24.2.5 Thermoplastic starch
Thermoplastic starch (TPS) or destructurised starch (DS) is a homogeneous
thermoplastic substance made from native starch by swelling in a solvent
(plasticiser) and a consecutive ‘extrusion’ treatment consisting of a combined
kneading and heating process. Due to the destructurisation treatment, the starch
undergoes a thermo-mechanical transformation from the semi-crystalline starch
granules into a homogeneous amorphous polymeric material. Water and glycerol
are mainly used as plasticisers, with glycerol having a less plasticising effect in
TPS compared to water, which plays a dominant role with respect to the
properties of thermoplastic starch.
24.2.6 Water resistance of starch-based products
Thermoplastic starch behaves as a common thermoplastic polymer and can be
processed as a traditional plastic. TPS shows a very low permeability for oxygen
(43cm
3
/m
2
/min/bar compared to 1880cm
3
/m
2
/min/bar of LDPE) which makes
this material very suitable for many packaging applications. In contrast, the
permeability of TPS for water vapour is very high (4708cm
3
/m
2
compared to
0.7cm
3
/m
2
of LDPE). This sensitivity to humidity (highly hydrophilic) and the
quick ageing due to water evaporation from the matrix makes thermoplastic
starch as such unsuitable for most applications. Due to this drawback there are
no products available at the moment made from pure thermoplastic starch,
which are form-stable (or even hydrophobic) in a wet atmosphere and
mechanically stable over a sufficiently long period of time.
Producers of starch-based products overcome this problem by blending the
thermoplastic starch with hydrophobic synthetic polymers (biodegradable
polyesters) or by the production of more hydrophobic TPS derivatives (starch
ester). Unfortunately, all theses production processes make the starch-based
products rather expensive in comparison to the common plastic alternatives.
522 Novel food packaging techniques
New concepts are required to solve the intrinsic problem of the hydrophilicity
and mechanical instability of starch-based bioplastics without too much added
cost.
9
24.2.7 Polyhydroxyalkanoates
An industrial fermentation process in which microorganisms converted plant
sugars into polyhydroxyalkanoates was developed by ICI, later Zeneca. Almost
all living organisms may accumulate energy storage materials (e.g. glycogen in
muscles and in livers, starch in plants and fatty compounds in all higher
organisms) whereby polyhydroxyalkanoates (PHAs), as polyesters, represent the
group of energy storage materials (e.g. carbon source that is exclusively found
among bacteria). Generally PHAs are thermoplastic, water-insoluble
biopolyesters of alkanoic acids, containing a hydroxyl group and at least one
functional group to the carboxyl group. The FDA approved Biopol, the PHA
produced by Monsanto who acquired the technology from Zeneca, as a food
contact material. Important aspects were the biopolymer itself and the presence
of breakdown products as crotonic acid. Also the incorporation of fermentation
by-products – the microorganism Ralstonia eutrophus is not food grade – was of
major concern. Other types of PHAs have not been approved for food contact
applications yet.
10
Although its water-resistant properties give it a cutting edge
in food packaging compared to other bioplastics, the plastic turned out to cost
substantially more than its fossil fuel-based counterparts and offered no
performance advantages other than biodegradability.
11
29.2.8 Synthetic polyesters
These (aliphatic) polyesters are formed by polycondensation of glycols and
dicarboxylic acids. They have tensile and tear strengths comparable to low
density polyethylene and can be coextruded and readily heat-sealed. They can be
processed into blown or extruded films, foams and injection moulded products
and used in refuse and compost bags and cosmetic and beverage bottles. Due to
their high price, aliphatic polyesters are used only in combination with starch.
When tested, starch-polyester blends show in all cases an important decrease in
water sensitivity whatever the thermoplastic starch and polyester type and
content but for thermoforming applications such blends cannot provide
sufficient stiffness due to the intrinsic softness of the polyester.
12, 13, 14
24.2.9 Polycaprolactone and polyvinylalcohol
Polycaprolactone is made from synthetic (petroleum) sources, and has seen only
limited use, apart from being used in starch-blends because of its low glass
transition temperature of 60oC and melting temperature of 60oC.
Another polymer being used in packaging applications is polyvinylalcohol
(PVOH), although its biodegradability is disputed. Some polymers like PVOH
Green plastics for food packaging 523
and starch are so water sensitive that they can in fact be water soluble. The most
widely used water soluble polymer PVOH is prepared by hydrolysis of
polyvinylacetate. Its water solubility can be adjusted to render it soluble in both
hot and cold water or in hot water only. Control of the degree of hydrolysis can
give control over the water solubility of the resulting resin. PVOH is not used as
food packaging but in unit doses for agricultural chemicals, dyes and pigments,
as well as water-soluble laundry bags for hospitals and detergent pouches.
7
24.3 Developing novel biodegradable materials
24.3.1 Introduction
One of the major problems connected with the use of most of the natural
polymers, especially of carbohydrates, is their high water permeability and
associated swelling behaviour in contact with water. All this contributes to a
considerable loss of mechanical properties, which prohibits straightforward use
in most applications. Because of the hydrophilic and low mechanical properties
of starch the property profile of these materials is insufficient for advanced
applications like food packaging. The few applications for just thermoplastic
starch, which do not involve the use of polymeric substances to form blends, are
packaging chips, packaging for capsules and as packaging for food products
(e.g. separate layers in boxes of chocolates) but never in direct contact with
food. Their hydrophilic character, their reduced processability (with respect to
polyolefines), and their insufficient mechanical properties represent particular
drawbacks in this respect. Special processing or after-treatment procedures are
necessary to sustain an acceptable product quality. As indicated before,
presently applied methods for decreasing the hydrophility and increasing and
stabilising the mechanical properties are blending with different, hydrophobic,
biodegradable synthetic polymers (polyesters) and the application of
hydrophobic coating(s). One recent new technology involves the application
of the nano-composite concept that has proven to be a promising option.
9
24.3.2 Barrier effect of nano clay particles in a biopolymer matrix
The incorporation of nano-clay sheets into biopolymers has a large positive
effect on the water sensitivity and related stability problems of bioplastic
products. The nature of this positive effect lies in the fact that clay particles act
as barrier elements since the highly crystalline silicate sheets are essentially non-
permeable even for small gas molecules like oxygen or water. This has a large
effect on the migration speed of both incoming molecules (water or gases) as
well as for molecules that tend to migrate out of the biopolymer, like the water
used as a plasticiser in TPS. In other words, nano-composite materials with well-
dispersed nano-scaled barrier elements will not only show increased mechanical
properties but also an increased long-time stability of these properties and a
related reduction of ageing effects.
524 Novel food packaging techniques
In order to achieve the final clay-starch nano-composite material, a ‘clay
modification’ and an ‘extrusion’ processing step can be distinguished, which are
described below. For the preparation of nano-composite materials consisting of
starch and clay, the use of special compatibilising agents (modifier) between the
two basic materials is necessary as depicted in Fig. 24.1.
Layered silicates are characterised by a periodic stacking of mineral sheets
with a weak interaction between the layers and a strong interaction within the
layer. The space between the layers is occupied by cations. By cation exchange
reactions between the clay and organic cations (such as alkyl ammonium salts)
the layered silicate can be transformed into organically modified clay. The inter
layer distance will increase by using voluminous modifiers. If this modifier is
compatible with starch as well, a homogeneously and nanoscaled distribution
(exfoliation) of the clay sheets can be effected in the polymer matrix. The
modified clay can be analysed by X-ray investigation (XRD) to determine the
inter-layer distance. The pure clay shows an interlayer distance of 1.26nm. It has
been proven by XRD analysis that most of the layers are indeed ‘swollen’ after
the modification reaction. The interlayer distance changes to 2.34nm – an
increase of nearly 100% compared to the pure clay.
24.3.3 Extrusion
The starch and the modified clay are mixed at temperatures above the softening
point of the polymer by polymer melt processing (extrusion). At these
temperatures the polymer melt intercalates. The success of the polymer
intercalation depends on the modification of the clay, on the degree of increased
interlayer distance and on the interaction between the modifier and the matrix
material. A full destructurisation is needed for a successful polymer melt process
of starch. Therefore, it is very important to find the optimal starch/clay/
plasticiser content, the most effective geometry of the screws and the right
temperature profile within the extruder.
24.3.4 Properties of the starch-clay nanocomposites
A homogeneous incorporation of clay particles into a starch matrix on a true
nano scale has proved to be possible. The addition of clay during processing
supports and intensifies the destructuring process of starch, providing a means of
easier processing. The obtained starch/clay nanocomposite films show a very
strong decrease in hydrophilicity. The stiffness, the strength and the toughness
Fig. 24.1 Example of possible modifiers for starch-clay nano-composites and
requirements for clay modification.
Green plastics for food packaging 525
of the nanocomposite material are improved and can be adjusted by varying the
water content. Clay will decrease the water permeability to some extent
(maximal with a factor 2). Clay will reinforce the starch blends only when it is
fully exfoliated.
Hot pressed films made out from material indeed showed a great advantage
compared to films made from pure thermoplastic starch. Ordinary TPS
evaporates water very quickly upon ageing. Figure 24.2a shows a photograph
of a hot pressed film of pure thermoplastic starch (after ageing the granulates for
three hours at room temperature following the extrusion step). The apparent
morphology indicates that it is not possible to form a true film any more. In
contrast Fig. 24.2b shows a hot pressed film of a starch/clay nano-composite.
Transparent and homogeneous films can be formed which show an increased
mechanical stability and toughness as well.
24.4 Legislative issues
It is important to remark that biodegradability and compostability are different
concepts.
2
While biodegradation may take place as a result of the disposal of a
material in landfills, composting usually requires a pre-treatment of municipal
solid waste; it is necessary in fact to remove all bulky non-compostable items
before beginning the composting process, separating organic from inorganic
waste. Moreover, before composting other steps are necessary: particle size
reduction, magnetic removal of metals, moisture addition and mixing. Under
ideal conditions the decomposition of organic material can take 30 to 60 days.
International Standards Research (ISR) at the request of ASTM studied the
performance of biodegradable plastics in full-sized composting facilities and
under laboratory conditions. The ISR work determined that plastics needed to
meet three criteria to be compostable. According to this standard ASTM D6400
they must be able to:
1. demonstrate inherent biodegradability at a rate and degree similar to natural
biodegradable polymers
2. disintegrate during active composting, so that there are no visible,
distinguishable pieces found on the screens
3. have no ecotoxicity – nor impact the ability of the resultant compost to
support microbial and plant growth.
15
A standard world-wide definition for biodegradable plastics has not been
established, nevertheless all the definitions already in place (ASTM, CEN, ISO)
correlate the degradability of a material to a specific disposal environment and to a
specific standard test method which simulates this environment in a time period
which determines its classification. The European Parliament on 20 December
1994 adopted a directive (94/62 EC) in order to harmonise national measures
concerning the management of packaging and packaging waste, to provide a high
level of environmental protection and to ensure the functioning of the internal
526 Novel food packaging techniques
market. In the 94/62 EC Directive a very brief part is dedicated to compostable and
biodegradable materials. In item three, ‘compostability’ is defined as organic
recycling and it is pointed out that compostability can take place only under
controlled conditions and not in landfills. Moreover, ‘biodegradable packaging’ is
Fig. 24.2 Compression moulded films of a) pure TPS granulate and b) starch/clay nano-
composite granulate
Green plastics for food packaging 527
defined as a material that must be capable of physical, chemical, thermal and/or
biological degradation such that this material used as compost ultimately
decomposes completely into carbon dioxide and water.
2
According to European directive No. 94/62 the producer or importer of
packaging is responsible for the recovery of a substantial fraction of the annual
amount of packaging it produces in the market. It states that at least 65% must
be recovered, at least 45% must be recovered by material recycling and at least
15% of each packaging material must be recycled. The term recovery denotes
the sum of recycling (material recovery), incineration (energy recovery) and
composting (organic recovery). Furthermore, the directive prohibits packaging
that does not fulfil the essential requirements. For products to be designed to be
compostable the requirement is that ‘they should be of such a biodegradable
nature that it does not hinder the source-separated collection of biowaste, nor the
composting activities in which it will be treated’. A draft standard, prEN 13432,
has been made with requirements for compostable products. According to this
standard, the following criteria are relevant for a compostable product.
16
1. The individual packaging components shall be completely biodegradable.
2. The total product shall disintegrate completely during a composting
process.
3. The addition of the product to the biowaste shall not have negative effects
on the composting process.
4. The addition of the product to the biowaste shall not have negative effects
on the quality of the final compost.
To demonstrate biodegradability, it is possible to use several internationally
accepted standard methods for determining the biodegradability of organic
compounds. Both aquatic tests and tests with high solids environments are
allowed, although tests under controlled composting conditions are preferred.
Evaluation criteria follow.
1. For a packaging material or the constituents of a packaging material which
consists of only one polymer (homo-polymer or random copolymer)
without any additives, the degree of biodegradation based on carbon dioxide
release or oxygen consumption shall be more than 60% of the theoretical
value.
2. For a packaging material or the constituents of a packaging material
comprised of different components (polymer blends), or block copolymers
and after addition of low molecular additives, the degree of biodegradation
based on carbon dioxide release or oxygen consumption shall be more than
90% of the theoretical value.
3. The period of application of the test methods shall be a maximum of six
months
Unless technically impossible, the packaging, packaging materials or packaging
component shall be tested for disintegration in the form in which it will
528 Novel food packaging techniques
ultimately be used. In practice, packaging materials are tested and from this it is
concluded that a complete packaging will be disintegrated if all its materials are
capable of disintegration. A complete packaging should, however, be tested in
cases where a direct conclusion is not possible, e.g., if two or more packaging
materials are firmly joined together forming a fixed multi-layer structure.
Due to the nature and analytical condition of the disintegration test, the test
results cannot differentiate between biodegradation and biotic disintegration but
they are required to demonstrate that a sufficient disintegration of the test
materials is achieved within the specified treatment time of biowaste. By
combining these observations with the information obtained from the laboratory
tests it can be concluded whether a test material is sufficiently biodegradable
under the known conditions of biological waste treatment.
Food Contact Materials are all materials and articles intended to come into
contact with foodstuffs, including not only packaging materials but also cutlery,
dishes, processing machines, containers, etc.
17
The term also includes materials
and articles that are in contact with water intended for human consumption, but
it does not cover fixed public or private water supply equipment.
The harmonisation at EU level of the legislation on Food Contact Materials
fulfils two essential goals: the protection of the health of the consumer and the
removal of technical barriers to trade. Food contact materials shall be safe and shall
not transfer their components into the foodstuff in unacceptable quantities. The
transfer of constituents of the food contact materials into the food is called
migration. To ensure the protection of the health of the consumer and to avoid
adulteration of the foodstuff two types of migration limits have been established in
the area of plastic materials:
18
an overall migration limit of 60mg (of substances)/
kg (of foodstuff or food simulants) that applies to all substances that can migrate
from the food contact material to the foodstuff and a specific migration limit
(SML) which applies to individual authorised substances and is fixed on the basis
of the toxicological evaluation of the substance. The SML is generally established
according to the acceptable daily intake or the tolerable daily intake set by the
Scientific Committee on Food. To set the limit, it is assumed that, every day
throughout his/her lifetime, a person of 60kg eats 1kg of food packed in plastics
containing the relevant substance at the maximum permitted quantity.
24.5 Current applications
24.5.1 Introduction
Despite the problems still encountered in properties and production of
biopolymers, biodegradable food packaging products enter the market because
of supermarkets being increasingly tricked by the marketing effect of a green
image. Novamont has begun to supply Tesco with nets for fruits and bags to
Swiss and German supermarkets. Albert Heijn in The Netherlands uses
biodegradable packages from Natura Verpackung while Sainsbury in the UK
is doing composting trials for food waste at 75 of its stores, using Mater-Bi bags.
Green plastics for food packaging 529
Table 24.1 gives some properties of polymers used in packaging materials.
19
Among the biodegradable polymers PLA seems to be the polymer that can
compete in terms of mechanical properties with conventional polymers. The
drawback here is the low Tg of PLA.
20
The main players in the biodegradable
polymer market are shown in Table 4.2. Despite the slow entry into of use of
biodegradables in food packaging, some trends are visibly shifting towards
sustainable chemistry and green plastics being applied in niche markets. Most of
the efforts these days seem to be focused on foamed products for food
packaging.
24.5.2 Starch based foams in food packaging
The use of foamed polymer packaging, for example, polystyrene (PS)
clamshells, by prominent users such as McDonald’s Restaurants has recently
decreased significantly because of perceived environmental disadvantages.
Table 24.1 Properties of polymers used as packaging materials
Polymer Tensile strength Tensile modulus Max. use
MPa GPa temperature oC
LDPE 6.2–17.2 0.14–0.19 65
HDPE 20–37.2 121
PET 68.9 2.8–4.1 204
PS 41.3–51.7 3.1 78
PA 6 62–82.7 1.2–2.8 –
PP 33–37.9 1.1–1.5 121
PLA 40–60 3–4 50–60
Table 24.2 Main players in biodegradable polymers and their trade names
Material Supplier Trade name
Starch based Novamont MaterBi
Starch based Biotec Bioplast
Thermoplastic starch Avebe Paragon
Thermoplastic starch National Starch Ecofoam
(Novamont licensee) Envirofil
Polylactide/PLA Cargill Dow Nature Works PLA
Polylactide/PLA Mitsui Lacea
Polylactide/PLA Hycail
Polylactide/PLA Galactic Galactic
(Co)polyester BASF Ecoflex
(Co)polyester Eastman Chemical Eastar Bio
(Co)polyester Du Pont Biomax
(Co)polyester Showa Highpolymer Bionolle
Polycaprolactone Union Carbide Tone polymer
Polycaprolactone Solvay CAPA
530 Novel food packaging techniques
Polystyrene is derived from non-renewable resources, is non-degradable and for
its processing blowing agents were used in the past that contributed to the
depletion of the ozone layer. Paper-based products have a more favourable
environmental perception but do not share the mechanical properties of
polystyrene foams. It is well known that starch, containing sufficient moisture,
can provide stable foams.
A step forward has been the introduction recently by Novamont of a new
foamed tray based on starch particularly for the ‘McDonalds’ type of
applications.
21
Apack has introduced another tray made from a baked starch
formulation that has a coating of EastarBio aliphatic-aromatic copolyester.
Sainsbury, a leading retailer in the UK, has been packaging its organic fruit and
vegetables in these starch-based materials by Apack.
22
Paperfoam has patented Paperfoam that is produced from a viscous
suspension containing starch, cellulose fibres and water. The suspension is
injected into the mould. Due to the mould’s temperature (c. 200oC) the starch
granulate gelatinises and the water evaporates. The manufacturing cycle bakes
from five seconds to two minutes, depending on wall thickness, from starch,
natural fibres and water using an energy-efficient, one-step production
technology. It can be recycled with paper and is biodegradable. Paperfoam
combines a foamed inner structure with a smooth outer face and is applicable for
a wide variety of uses. At present Paperfoam is used in the packaging of hand-
held electronic consumer goods, such as telephones, but not yet in food
packaging.
23
Other types of foamed products at different stages of development are blends
of starch with poly(vinyl alcohol-co-40%-ethylene), PVOH-40, a degradable,
water resistant polymer that can be processed into viable alternatives to PS foam
packages via wafer baking technology, extrusion, or expanded-bead moulding
24
and starch-based dough made by a baking process for various food containers.
25
Although one of the most versatile technologies for the production of starch-
based foam is via this type of baking process where a starch dough is heated
under pressure to form a moulded foam product, these starch products are
moisture sensitive and have poor mechanical properties. Both of these attributes
can be improved by the inclusion of fibres and/or fillers in the dough
formulation. The resulting products are starch-based foam composites with
mechanical and thermal properties rivalling those of polystyrene.
26
24.5.3 PHA in food packaging
PHA properties show that it might be a very good alternative for conventional
polymers in food contact packaging. However, when Monsanto bought the
Biopol process in 1995 profitability still remained elusive.
11
The approach with
the most potential was to grow PHA in plants, modifying the genetic make-up of
the crop so that it could synthesise plastic as it grew and eliminate the
fermentation process. However, it was found that producing one kilogram of
PHA from genetically modified corn plants would require about 300% more
Green plastics for food packaging 531
energy than the 29 megajoules needed to manufacture 1kg of fossil-fuel-based
polyethylene. This finding prompted Monsanto to terminate this method of
producing PHA. The Biopol assets were obtained by Metabolix but food
packaging is not a product line they intend to focus on in future. Proctor and
Gamble together with Kaneka Corporation are working on a new development
in PHA production but this product might not become available in large
quantities until 2005.
24.5.4 PLA in food packaging
With PLA it is a similar story in terms of energy balance to PHA when one looks
at the production site of Cargill Dow with its Nature Works
TM
PLA in Blair,
Nebraska. The Blair plant with a capacity of 140 000 metric tons per year
produces 1kg PLA with 56 megajoules of energy. However, in principle PLA
processes can require between 20–50% fewer fossil resources than making
plastics from oil but it is still significantly more energy intensive than most
petrochemical processes.
Packaging solutions from Nature Works can be extruded, thermoformed and
blow moulded, unlike other traditional products, such as paper. For packaging
they have two film grades and one grade for thermoforming. The film grades are
designed for applications like candy twistwrap and for laminations for
packaging such as flavoured crewels, coffee packs and pet foods because of
the additional advantageous properties such as the barrier to flavour and grease
and superior oil resistance. The potential applications of thermoformed products
within packaging is multifold, dairy containers, food service ware, transparent
food containers, blister packs and cold drink cups. PLA polymer has been shown
to biodegrade similarly to paper under simulated composting conditions (ASTM
D5338, 58oC). Degradation of PLA packaging depends both on exposure
conditions and on amount and type of plasticiser. Sainsbury would like to use
PLA but will not do this because of the lactic acid coming from GM crops.
21
The consumer might not accept this at the moment although the GM label is
destroyed at the fermenting stage. Much of the PLA of Cargill Dow is for fibre
applications but the company is already working with many leading European
packaging converters, including Trespaphan on oriented PLA films, Klo¨cknes
pentaplast on thermoformed trays and lids, and Autobar on thermoformed dairy
pots. Food retailers are also increasingly involved.
24.5.5 Proteins in food packaging
Proteins have long and empirically been used to make biodegradable, renewable,
and/or edible packaging materials. Numerous vegetable proteins (corn zein,
wheat gluten, soy proteins) and animal proteins (milk proteins, collagen,
gelatine, keratin, myofibrillar proteins) are commonly used. Although protein
materials have been studied extensively
27
a breakthrough is not yet imminent
although some of the properties have increased extensively recently. Protein
532 Novel food packaging techniques
materials can be processed into transparent and water-resistant films by casting
or thermo-forming. In packaging, collagen sausage casings are the best known
of the commercial applications.
28, 29
24.6 Future trends
When one looks at the present market for biodegradable food packaging
materials then that market is still virtually non-existent compared to any
conventional plastics used in food packaging. The reason for this is the high
price, the sometimes inferior cost/performance relation and the fact that still
only a few materials have received FDA approval.
Since 2001 the market for biodegradable/compostable products has definitely
been growing after remaining at the same level of 20,000 tons worldwide for the
last five years and although few in number, new products for food packaging
have been introduced since.
24.7 References
1. JOGDAND S N, Welcome to the world of eco-friendly plastics,
www.members.rediff.com, 1999 .
2. AVELLA M, BONADIES E, MARTUSSCELLI E, RIMEDIO R, ‘European current
standardization for plastic packaging recoverable through composting and
biodegradation’, Polymer Testing, 2001, 20, 517–521.
3. www.rivm.nl/mieucompendium: C6.6 Hoeveelheid kunststoffen en PVC
in huishoudelijk restafval.
4. GRUBER P ‘Sustainable design of polymers and materials’, Abstracts, 6th
International Scientific Workshop on Biodegradable Polymers and
Plastics, Honolulu, Hawaii, 2000.
5. BASTIOLI C, ‘Global status of the production of biobased packaging
materials’, Conference proceedings The Food Biopack Conference,
Copenhagen, August 27–29, edited by C J Weber, 2–7, 2000.
6. BILBY G D, ‘Degradable polymers’, www.angelfire.com, 2000.
7. SELKE S, ‘Biodegradation and packaging’ (2nd edn), Pira International
Reviews, 2000.
8. CARGILL DOW LLC, Product information, published June, 2000.
9. FISCHER S, VLIEGER J J, DE KOCK T, BATENBURG L-, FISCHER H, ‘Green’ nano-
composite materials – new possibilities for bioplastics’ Materialen, 2000,
16, 3–12.
10. WEUSTHUIS R A, WALLE G A M, VAN DER EGGINK G, ‘Potential of PHA based
packaging materials for the food industry’, Conference proceedings The
Food Biopack Conference, Copenhagen, August 27–29, edited by C J
Weber, 24–7, 2000.
11. GERNGROSS T U, SLATER S C, ‘How green are green plastics’, Scientific
Green plastics for food packaging 533
American, feature article of August isssue, 2000.
12. AVE
′
ROUS L, FAUCONNIER N, MORO L, FRINGANT C, Blends of thermoplastic
starch and polyesteramide: Processing and properties, J. Appl. Polym. Sci.,
1999, 76, 1117.
13. AVE
′
ROUS L, MORO L, DOLE P, FRINGANT C, ‘Properties of thermoplastic
blends: Starch-Polycaprolactone’, Polymer, 1999, 41, 4157.
14. AVE
′
ROUS L, FRINGANT C, to be published.
15. NARAYAN R, ‘The scientific rationale behind biodegradable/compostable
standards’, Abstracts 6th International Scientific Workshop on
Biodegradable Polymers and Plastics, Honolulu, Hawaii, 2000.
16. ZEE M VAN DER, ‘Compostable products-Legislation standards and future
policy’, Green-Tech Newsletter April 1999, 2, 2, I.
17. LEBARON P C, WANG Z, PINNAVIA T J, ‘Polymer-layered silicate
nanocomposites: an overview’, Applied Clay Science, 1999, 15, 11–29.
18. Final report Bionanopack project 1999-2001, FAIR CT98-4416, Co-
ordinator J J de Vlieger, 2002.
19. BRODY A L, ‘Packaging materials’, Encyclopedia of Polymer Science and
Engineering, 1987, 10, 684–710.
20. SO
¨
DERGA
?
RD A, ‘Lactic acid polymers for packaging materials for the food
industry’, Conference proceedings The Food Biopack Conference,
Copenhagen, August 27–29, edited by C J Weber, 19–23, 2000.
21. Modern Plastics International, December 2001, 44.
22. European Plastic News, January 2002, 21.
23. Green-Tech Newsletter 2000, vol. 3, no 1, 4 Paperfoam, www.paper
foam.nl.
24. ORTS W, NOBES G A R, GLENN G M, GRAY G M, HANSEN L U, HARPER M V,
‘Blends of starch with poly(vinyl alcohol)/ethylene copolymers for use in
foam containers’, Abstracts 6th International Scientific Workshop on
Biodegradable Polymers and Plastics, Honolulu, Hawaii, 2000.
25. GLENN G M, NOBES G A R, GRAY G M, ‘Insitu laminating process for baked
starch based foams’, Abstracts 6th International Scientific Workshop on
Biodegradable Polymers and Plastics, Honolulu, Hawaii, 2000.
26. NOBES G A R, ORTS W J, GLENN G M, ‘Use and effeects of agricultural fibers
and fillers in baked starch-based foam composites’, Abstracts, 6th
International Scientific Workshop on Biodegradable Polymers and
Plastics, Honolulu, Hawaii, 2000.
27. GUILBERT S, ‘Potential of the protein based biomaterials for the food
industry’, Conference proceedings The Food Biopack Conference,
Copenhagen, August 27–29, edited by C.J. Weber, 13–18, 2000.
28. GUILBERT S, CUQ B, GONTARD N, ‘Recent innovations in edible and/or
biodegradable packaging’, Food Additives and Contaminants 14, 2000, 6,
741–751.
29. KROCHTA J M, MUKDER-JOHNSTON C DE, ‘Edible and biodegradable
polymer films: challenges and opportunities’, Food Technol., 1997, 51,
61–73
534 Novel food packaging techniques