24 Green plastics for food packaging J.J. de vlieger, TNO Industrial Technology, The Netherlands 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. 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. 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 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 pe 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
24.1 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
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