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