《食品包装技术》（英文版）Chapter 24 Green plastics for food packaging

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,

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520 Novel food packaging techniques nowadays 30% by volume 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 The durability of plastics is beyond dispute. Some plastics need to be durabl 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-calle 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 24.2 The range of biopolymers 24.21 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. 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

nowadays 30% by volume3 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

Green plastics for food packaging 521 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, NatureWorksPLA. 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 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 60C and its melting temperature is 155C. Tensile strength of oriented polymers is reported to be 80-11O0Mpa with elongation at break of up to 30%. Poly lactide 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, whicl occurs naturally in food and in the body. 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 60C 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. However, PLA is not yet found in large applications of food packaging today

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 microorganisms and growing plastic in corn and other crops. Cargill Dow has scaledup the process of turning sugar into lactic acid and subsequently polymerises it into the polymer polylactic acid, NatureWorksTMPLA. 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% Llactide 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

522 Novel food packaging techniques 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 levelopment of totally degradable products for specific market needs. The two main components of starch are polymers of glucose: amylose(MW 10-10),an essentially linear molecule and amy lopectin(MW 10'-10%), 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 ource.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 hermoplastic 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 re 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 /m2/min/bar compared to 1880cm/m2/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/m- compared to 0.7cm/m" 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

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 105 –106 ), an essentially linear molecule and amylopectin (MW 107 –109 ), 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 (43cm3 /m2 /min/bar compared to 1880cm3 /m2 /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 (4708cm3 /m2 compared to 0.7cm3 /m2 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

Green plastics for food packaging 523 New concepts are required to solve the intrinsic problem of the hydrophilicity and mechanical instability of starch-based bioplastics without too much added 24.2.7 Polyhydroxyalkanoates An industrial fermentation process in which microorganisms converted plant sugars into polyhydroxyalkanoates was developed by ICl, 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 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. Although its water-resistant properties give it a cutting edge performance advantages other than biodegradability. I erpary turned out to cost in food packaging compared to other bioplastics, the plastic substantially more than its fossil fuel-based count 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 fficient stiffness due to the intrinsic softness of the polyester.12,13,provide content but for thermoforming applications such blends cannot 24 2.9 Polycaprolactone and polyvinylalcohol Polycaprolactone is made from synthetic(petroleum)sources, and has seen limited use, apart from being used in starch-blends because of its low glass transition temperature of -60C and melting temperature of 60C Another polymer being used in packaging applications is polyvinylalcohol (PVOH), although its biodegradability is disputed. Some polymers like PVOH

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 ÿ60ºC 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

524 Novel food packaging techniques 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. 24.3 Developing novel biodegradable materials 243.1 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 ssociated 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. 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 mechanica properties but also an increased long-time stability of these properties and a related reduction of ageing effects

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 nonpermeable 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 welldispersed 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

Green plastics for food packaging 525 hydrophillic and cationic part hydrophilic and H-bond active part (clay compatible) (starch compatible) Fig. 24.1 Example of possible modifiers for starch-clay nano-composites and quirements for clay modification. 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 odified 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 ha been proven by XRD analysis that most of the layers are indeedswollen'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. 243. 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 toughne

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

526 Novel food packaging techniques 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. 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. 5 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 perio 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

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

528 Novel food packaging techniques defined as a material that must be capable of physical, chemical, thermal and/or decomposes completely into carbon dioxide and water. as compost ultimately biological degradation such that this material used According to European directive No. 94/62 the producer or importer of ackaging 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 hat 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 omposting activities in which it will be treated. A draft standard, prEN 13432 as been made with requirements for compostable products. According to this standard, the following criteria are relevant for a compostable product 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 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 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 tha 90% of the theoretical value 3. The period of application of the test methods shall be a maximum of six Unless technically impossible, the packaging, packaging materials or packaging component shall be tested for disintegration in the form in which it will

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

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