Polysaccharides
Starch polymers
Starch is a polymer of D-glucose organised in two major constituents of huge molecular weights: amylose and amylopectin. Amylose contains amorphous and crystalline regions. It forms a linear structure constituted by repeating units of 1-4a-glucose (Fig. 1.1). Amylopectin is branched on amylose in starch (Fig. 1.2) (Moore and Saunders, 1997; Flieger et al., 2003). The natural crystalline structure of starch must be dismantled in order to produce a thermoplastic material. It is achieved by the application of heat, pressure, mechanical work or by addition of plasticisers such as glycerine, polyols or water.
HO OH HO OH
1.1 Structure of amylose.
HO OH HO OH
1.1 Structure of amylose.
- CH,OH
Oil HO OH
1.2 Structure of amylopectin.
Oil HO OH
1.2 Structure of amylopectin.
First generation of starch polymers
Historically, this category was one of the first generation of biopolymers. To improve their resistance to shock and moisture, polyolefins were added in small quantities (about 10-15%) or in large proportions up to 85-95% to starch (Flieger et al., 2003). Those polymer mixtures disappeared during the biodegradation process leaving small fragments whose degradation time was a function of their carbon chain length.
This type of product gave a very poor image to the first 'biodegradable' polymers. Today most of them are not produced any longer but they gave birth to a new generation of blended plastics used for soil environment applications. They are composed of starch and polyolefin polymers including a catalyst. The catalyst improves the photo and thermo-oxidative degradation of the polyolefin phase (Bastioli, 2002; Arnaud et al., 1994; Scott, 1971). The first step of starch microbial degradation initiates further polyolefin degradation by increasing the porosity, the void formation and the loss of the plastic skeleton integrity.
Currently, plastic films used in agricultural mulch are made with low-density polyethylene (LDPE) containing transition metal compounds soluble in the thermoplastics matrix (the catalyst) and about 6-15% of starch. However, the degradation duration is still high and can reach a few years for some of these products that do not respond to certain norms of biodegradability. Sometimes, a pre-treatment of the starch with a silane coupling agent is required in order to improve compatibility with the hydrophobic phase of the thermoplastic. This technology can also be applied to other matrixes such as PVC or polyester derivatives.
Second generation of starch based polymers
This second generation of polymers includes two kinds of products. The first one is produced from flour (flour biopolymer) and the second one is produced by plasticisation of starch with another biopolymer (Lourdin et al., 1999). The starch appears here more as filler. The flour biopolymers are made from rye, wheat or corn. They are generally cheaper than the second category and are suitable for use in catering (cutlery, forks, dishes, etc.). In this category, we can also classify the biopolymers produced from the whole plant (including starch, cellulose fibres, hemicellulose, lipids). Some commercial biopolymers in this category are listed below.
Supol (Supol, Germany)
Potato flour is submitted to a thermal treatment under pressure. Pellets can be injected to produce single-use dishes which are microwavable and which are compostable or can be added to animal food.
Evercorn (Cornstarch, Japan)
Plasticised maize starch can be injected in order to make small parts for catering or for horticultural applications. This product is compatible with other biopolymers such as PHBV, PLA, PCL polyesters.
Vegemat (Vegemat, France)
This material, made with the complete corn plant, is relatively cheap (1 €/kg) (Forest, 2000). Grades have been developed for injection moulding and the maximum wall thickness is one millimetre. Without treatment this material is sensitive to humidity and is completely biodegraded after eight weeks.
Paragon (Paragon Products BV, The Netherlands)
This is a thermoplastic starch made from potatoes, wheat, maize or tapioca. The applications are found in food packaging, dog toys and veterinary accessories, and for injection moulding of complex parts.
Clean Green Packing (Starchtech Inc., USA) This is soluble in water and is compostable.
To compensate for the inconvenience of plastics made with pure starch, it can be chemically treated to improve its resistance to humidity. One treatment consists of the acetylation or of the esterification of the free hydroxide groups present in the chain by an anhydrous propionic acid. Another possibility is to add to the formulation of the polymer hydrophobic substances such as natural wax or biodegradable plastics which are not sensitive to humidity, however, all these treatments increase the production costs. Novon (Novon International, USA) was originally developed by Warner Lambert for the fabrication of pharmaceutical capsules, includes up to 80-90% of starch. Some grades are edible; others have been developed for thermoforming, sheet and film extrusion or for injection moulding applications.
The second way to improve the mechanical properties of this cheap material (i.e., starch) is to blend it with a more expensive one that has better properties (e.g., polyesters, PVA, cellulose acetate). Some commercial blends are described below.
Starch + PVA
This blending category is commercialised mainly by Mater-bi (Novamont), Envirofil (Enpac), Greenfill (Green Light Products Ltd).
Mater-Bi (Novamont, Italy)
Mater-Bi is one of the main biopolymers commercialised in Europe (Bastioli, 2001). It is a copolymer of thermoplastic starch with natural plasticisers. Following grading, it can also contain cellulose derivatives or polyesters such as e-caprolactone or ethylene vinyl alcohol. This family of materials is compostable. The main applications are for the production of mulch films, shopping bags, food packaging (yogurts), nappies and personal hygiene products (Facco and Bastioli, 2000). The film production capacity of Mater-Bi is about 20,000 tons/year (2003). In Europe, hundreds of cities use Mater-Bi bags for the collection of organic waste (Bastioli, 2002).
Starch + aliphatic polyester
Blends of biodegradable synthetic aliphatic polyesters and starch are used to produce sheets and films for packaging by film extrusion or blown film methods. Up to 50% of the synthetic polyester can be replaced with starch. A polyester synthesised from the poly-condensation of 1,4-butanediol and a mixture of adipic and succinic acids has been blended with wheat starch by Lim (1999) (Nolan-ITU Pty Ltd, 2002). The blends were found to have melting points near that of the polyester alone. Plasticisers were also added to the starch to improve flexibility and processability of the blend. The modified blends were found to retain a high tensile strength and elongation, even at high starch concentrations.
Starch + PCL
Blending starch with degradable synthetic aliphatic polyesters such as PCL has been studied. Biodegradable plastics can be prepared by blending up to 45% starch with degradable PCL. Due to a low melting point of 60 0C and poor mechanical properties, the applications for starch-PCL blends are limited (Nolan-ITU Pty Ltd, 2002).
Bioplast (Biotec, Germany)
Bioplast grades are formulated for injection, blowing injection and flat extrusion. These grades are blends of starch and polycaprolactone. They are moisture sensitive. These blends have been developed for biodegradable film applications like lawn and leaf collection compost bags, agricultural mulch film, etc. The technology involves the following steps:
• plasticisation of the starch using glycerol as plasticiser
• polymerisation with e-caprolactone directly in an extruder
• compounding of the new, branched polymer by reactive blending with thermoplastic starch during the extrusion polymerisation operation
• preparation of compatibilised poly(-caprolactone)-thermoplastic starch blends.
This new starch-PCL resin is being marketed under the name ENVAR for film applications like compost, trash and retail carrier bags. Properties are comparable to LDPE films and better than pure polycaprolactone film.
Two other companies, Novamont (Italy), and Milleta (Biotech Division, Germany), manufacture and sell starch-PCL blends for film applications (e.g. compost bags, trash bags) (SINAS, undated).
Starch + Poly(lactic acid) PLA (Ecostar (Novon))
Blends of PLA and 10-20% starch have been commercialised by Novon as additives for traditional thermoplastics in order to make them biodegradables (Flieger et al., 2003).
To improve the interfacial adhesion between starch/biodegradable phases, the performance of two compatibilisers has been studied by Gormal (2002) to create a mechanically improved blend for food packaging film applications.
Starch + cellulose acetate (Bioflex and some Bioplast (Biotec, Germany))
Bioflex is a blend of starch and cellulose acetate which is rapidly degraded by composting. It is mainly intended for the production of trash bags and films. The material is resistant to oils and greases and can be printed by flexography or by offset (without corona surface pre-treatment). BIOPLAST® GF 105/30 is a plasticiser-free thermoplastic material suitable for injection moulding as well as sheet film extrusion. Applications are short-life products, film coating for foamed starch and fibre trays and as a substitute for food wrapping paper, packaging, etc. (Biotec, undated).
Cellulose and cellulose derivative
For industrial applications, cellulose comes mainly from wood and in small proportions from stalks of sugar cane bagasse (dry pulp after juice extraction in sugar cane). Raw cellulose is a cheap material costing 0.5-1 €/kg. The main uses of cellulose are for paper, membranes, dietary fibres, explosives and textiles.
Figure 1.3 represents schematically the structure of cellulose. The strong glucosidic bonds ensure the stability of the cellulose in various media. Cellulose is generally insoluble and highly crystalline. Chemical reactions such as etherification and esterification are conducted on the free hydroxyl groups to
CH2OH HO OH
1.3 Structure of cellulose.
CH2OH HO OH
1.3 Structure of cellulose.
improve its thermoplastic behaviour. Numerous derivatives are commercialised such as cellulose acetate, ethyl cellulose, hydroxyl ethyl cellulose, hydroxyl propyl cellulose, hydroxyl alkyl cellulose, carboxy methyl cellulose, fatty acid esters of cellulose (Chiellini et al., 2002).
Bio-Compo (Mitsufuku, Japan)
This material is made from cellulose powder and is suitable for thermoforming. The main applications are found in horticulture.
Cellophane
Cellophane films are obtained by dissolution of cellulose in a sodium hydroxide and carbon disulphide solution (Xanthation) and than by recasting in a sulphuric acid bath. The aspect is brilliant and transparent. Degradation takes place after six weeks of composting. Cellophane films are mainly used in food packaging where they are appreciated for their barrier properties against micro-organisms, gases and smells. The other main properties are resistance to infra-red light, oil, heat and transparency to the microwave. Labels are easy to stick on cellophane which is also printable.
The cellulose (di/tri) acetates
Cellulose acetate contains COCH3 radicals in place of free OH groups on sugar (Fig. 1.4) (Flieger et al., 2003). Cellulose acetate is mainly used in the synthesis of membranes for reverse osmosis.
Bioceta (Mazzucchelli, Italy)
Bioceta (developed by Rhone Poulenc) is a cellulose diacetate. It is produced from cotton linter or from wood pulp. The modified cellulose is mixed with a colourant, a stabiliser, a natural plasticiser catalysing the biodegradation. This product is transparent and can be injected, extruded or blown depending on the grade type. It can also be recycled or incinerated. The applications are
Cet In lose acetate
1.4 Structure of cellulose acetate.
Cellulose triacetate
Cellulose diacetate
Rj + COCHJ, R2andR3=H
1.4 Structure of cellulose acetate.
packaging, flower pots, small objects (tooth brushes, etc.). Biocellat comes from the same family of material.
EnviroPlastic Z (Planet Polymer Technologies, USA)
This is made from modified cellulose acetate by using a high temperature process which improves the biodegradability of the material. The composting duration is low (about one or two years). This product can be injected or film extruded for packaging applications.
Celgreen (Daicel Chemical Industries, Japan)
Daicel commercialises various biopolymers using this label. The grade P-CA is produced with cellulose acetate.
Lignin and wood powder blends
Lignin is one of the main constituents of wood. It is a very stable and complex product, insoluble in water and resistant to a number of physical and chemical treatments. The composition of lignin slightly changes from one plant species to another and is a function of the growing conditions but it is always a three-dimensional biopolymer composed of three different units of the phenyl propane family: p-hydroxy phenyl, Guaiacyl and syringic aldehydes (Fig. 1.5). These units are linked by aliphatic and aromatic carbon bonds and ether bonds. In wood, the lignin is closely associated with cellulose and bound to plant polysaccharides in order to form hemicellulose. This complex chemistry and polymer architecture is the reason why it is really difficult to isolate and to plasticise lignin by a cheap process (Chiellini et al., 2002).
The usual source of commercial lignin is waste liquor from the wood pulp industry. It contains sodium ligninates or lignin sulfonates. Previously, liquefaction of lignocellulosic products was achieved using several hard treatments. One consisted of treatment at 320-400 0C in aqueous or organic solvents (Widsten et al., 2002). A second treatment used an acidic catalyst
- 1.5 Structure of main subunits of lignin.
solution at a temperature between 80-150 0C. Today phenols can be used for the liquefaction of wood and lead to the production of thermosetting materials. Sulphuric, oxalic or phosphoric acids also enhance the liquefaction of wood. The derivative product is then a kind of novolac based resin which can be used in adhesives, mouldings or fibres.
Sugar cane waste is another raw material that can be treated in a hot solution of concentrated acetic acid in hydrochloric acid solution. After re-concentration, the lignin is then precipitated in warm water and finally recovered by dissolution in acetone. Due to all this complex chemistry, the major commercialised 'wood polymers' are blends. These plastics contain wood powder, starch or lignin. The presence of lignin as a filler in other polymers improves the quality of the biodegradation. Some of those products are reinforced with flax or hemp.
Arboform (Tecnaro, Germany)
Arboform is a thermally treated mixture of lignin, flax and hemp. This product can be injected and presents a good dimensional stability. Applications are found in car dashboard panels, computer or television frames, GSM housings.
Fasal (IFA, Austria)
Fasal products are made with wood waste, corn floor, natural resins and small quantities of a plasticiser, lubricants and a colourant. It can be processed by injection or extrusion without previous drying. The products look slightly like wood and can be milled, painted, or varnished in the same way as wood.
Treeplast
Treeplast is a product of the same kind developed thanks to a European CRAFT project and is still not commercialised (Eilbracht, 2001).
Lignopol (Borregaard LignoTech, Germany)
Lignopol is a natural biodegradable composite blended with natural proteins, wood, lignin, and natural resins. It is in the form of pellets, which can be processed by extrusion or injection. Products look like wood and can be milled.
Ecoplast (Groen Granulaat, Holland)
This product is composed of wood powder, starch and a binder. It can be injected or thermoformed. Objects made from this material are composted in six weeks.
Napac (Napac, Switzerland)
Napac results from the transformation of Chinese reeds with a natural binder (starch and pine tree resin). These raw materials can be mixed with a colourant and extruded in pellets. The fibre concentration is around 70-75%. Pellets are then moulded by hot compression. This material is perfectly stable outdoors and is formulated to resist exposure to UV light. The applications are flower pots, CD boxes, interior car parts and non-food packaging.
Finally, to complete the picture of natural biopolymers, one can mention the existence of materials produced by lignin/styrene copolymerisation and by lignin/methyl methacrylate copolymerisation. In both cases, the increase of lignin improves the biodegradation of the product by fungi.
New research is being conducted into the idea of modifying lignin polymer using enzymes like peroxydase or laccase. The latter enzyme has now been commercialised by a Danish company, Novo Nordisk, and will certainly promote the commercial appearance of new lignin products.
Chitin and chitosan
Chitin is one of the most widespread polysaccharides in nature and is particularly abundant in the cell walls of insect cuticles, of many fungal species and of shellfish or mollusc exoskeletons. The chemical composition of chitin is based on the repetition of the unit (1-4) 2 acetamide-2-desoxy-D-glucose (or N-acetylglucosamine) (Flieger et al., 2003). Chitin is composed of a linear chain of acetylglucosamine groups (Fig. 1.6) (Lim and Hudson, 2003).
Most chitins and derivatives are extracted from crab shells, lobsters and shrimps or from the waste of fungi fermentation (e.g., Aspergillus sp.) in concentrated NaOH solution. The swelling involves a modification of its natural crystal structure (a or 3). After washing in water, the recovered a structure is chemically resistant due to the hydrogen bonds between the chains. The 3-chitin
- CHjOH H NHCOCH
1.6 Structure of chitin.
is a better candidate to promote the production of derivative products like benzyl chitin or carboxymethyl chitin. It is also better adapted to be transformed by reactions such as acetylation, tosylation, tritylation and acetolysis. Chitosan is produced by the complete or partial elimination of acetyl groups (CH3-CO -deacetylation) which are replaced by an amino-group (Fig. 1.7) (Rathke and Hudson, 1994).
The properties of chitosan depend strongly on the molecular characteristics (molecular weight and degree of acetylation). Chitosan is soluble in water and in some organic solvents. The difference between chitin and chitosan is defined by their solubility in a dilute solution of weak acids. Chitosan dissolves in dilute acetic acid. It presents a unique combination of properties, brought about by its polysaccharide structure, large molecular weight, and a cationic character. Chitin and chitosan are biocompatible and present antithrombogenic and hemostatic properties. These polymers can be extruded to make films for packaging applications. They are edible and can be used in the agricultural (crop protection) and food sectors, and also in wastewater treatment, textiles or cosmetics and toiletries.
They are also used for biomedical applications (biomedical devices, and drug delivery systems). Chitosan and its derivatives form air permeable films. This property facilitates cell regeneration when the films are used to protect tissues against microbiological attack. For this reason chitin and chitosan are also good candidates for artificial skin, and biodegradable sutures. Producers of chitine and chitosan will not be presented here because there are 63 main companies; 30 are located in Asia, 14 in the USA, 12 in Europe, 6 in Canada, and one in Russia.
- CH;OH H NH2
1.7 Structure of chitosan.
14 Biodegradable polymers for industrial applications 1.2.2 Proteins
For thousands of years people have been using natural proteins such as wool, silk and hair (a-keratin) for clothes, adornment or to display their wealth. Proteins are natural chains of a-amino acids joined by amide linkages. They are degraded by enzymes (proteases). The first industrial applications of protein as polymer were in the early 1930s and 1940s with casein and with soy protein. Even though protein biopolymers did not develop as quickly as starch derivates, they remained present in some niche markets such as encapsulates (pharmaceutical), coatings (food industry), adhesives or surfactants (Guilbert, 2002). They can be classified with animal proteins (casein, whey, keratin, collagen and gelatine) and in plant proteins (wheat, corn, soy, pea and potato proteins) (Chiellini et al., 2002).
Collagen and gelatine
Collagen and gelatine represent the most well-known animal polymers. Collagen is a relatively non-extensible protein presenting good stiffness. Gelatine derives from the physical and chemical denaturising of collagen. The good quality of gelatine depends on its high solubility in hot water, its polyampholite character and its intrinsic ability to form thermally reversible gels. Gelatine grades are also available in a wide range of viscosities. The classical applications are for the manufacturing of pharmaceutical products (drug caps), for X-rays, photographic film development and food processing. As a biocompatible material, gelatine displays several advantages. It does not show antigenity and is resorbable in vivo. Its physico-chemical properties can be suitably modulated. Gelatine can be plasticised thanks to the addition of water or of glycerol. There is, however, a limit to the use of this interesting material because there is a risk of viral animal contamination. Finally blends of polyvinyl alcohol and gelatine are the object of studies and researches.
Casein
Casein is a natural polymer extracted from skim milk proteins. It represents a small but important percentage of all the natural polymers used for the manufacturing of water-based adhesives. The casein formulations are highly soluble in alkaline solutions and in water. Casein polymers (modified or not) are mainly used in the manufacture of adhesives and the packaging industry for breweries, wineries and refrigerated products. Casein is also a binder for paints and an additive for adhesives formulations. It can also be used as a plasticiser for concrete. Beyer Richard (2002) demonstrated the feasibility of preparing casein polymer to make edible films and for food products containing this polymer.
Wheat and corn gluten
Polymers made from gluten are flexible, resistant, transparent, and completely biodegradable. They are thermoplastic and present a yellow or slightly brown look. They are relatively impermeable to oxygen and to C02 but are sensitive to humidity and do not give protection against desiccation. Potential applications are the production of soluble pockets for the controlled release of a chemical product (e.g., toilet detergent). The world-wide production of wheat gluten is about 400,000 tons per year. Moreover, as an edible material, gluten is a good candidate for food packaging or single units of coffee or other food.
Soy proteins
Soy beans contain about 18% oil, 38% protein, 30% saccharides (15% soluble saccharides and 15% starch) and 14% moisture and ash. In 1940, Henry Ford presented a car body made from soybean-based materials. Soy proteins allow the development of various biodegradable materials. They are mainly formaldehyde-based thermoset composites. Water resistance can be improved by adding polyphosphate fillers (Otaigbe and Adams, 1997). Many applications have been developed thanks to its very high Young's modulus. A grade has also been formulated for medical applications. The plasticiser is the glycerol and 7-aminopropyltriethoxi silane is used as coupling agent.
In India, many studies have been undertaken into the production of co-extruded films of soy proteins with an aliphatic polyester. The research goal is to decrease the brittle character of the material. The main commercialisation of soy protein plastics is done in the USA (Heartland, Resource Technologies, Iowa, Urethane Soy System Company, Illinois; and Dow Chemical with SoyOil® and BioBalance®) (Flieger et al., 2003).
Polypeptides of aspartic acid and lysine
The wetting level of these polypeptide polymers in water is very high. They are now commercialised by Mitsui Chemical for horticultural applications.
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