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01 | 2010

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Basics Basics of

Basics Basics of Cellulosics Article contributed by S. Zepnik, A. Kesselring, R. Kopitzky, C. Michels, all Fraunhofer UMSICHT, Oberhausen, Germany CH 2 OH H OH O H H O OH H OH H H H H O O H OH CH 2 OH n Fig. 1: Molecular structure of cellulose [3]. S OH O HO S OH O O NaOH + CS 2 O O O OR O C S - Na + O O O O OH HO OH n RO OR RO OR R = C S - Na + Fig. 2: Treatment of cellulose with alkali and carbon disulfide [5]. Cellulose, as a major component of plants, is the most abundant raw material and therefore one of the oldest and most widely used chemical in the world. Cellulose (Fig. 1) is a polysaccharide consisting of anhydroglucose units (D-glucose units) linked together by ß-(14) glycosidic bonds to form linear chain structures [1]. The degree of crystallinity and the crystal structure depends on the origin and pretreatment of the cellulose. In general the polymer is not processable as a thermoplastic, it is very stiff and is insoluble in water and most common organic solvents as a result of the very strong hydrogen bond network formed by the hydroxyl groups and the ring and bridge oxygen atoms [1]. The cohesion between the chains is favoured by the high spatial regularity of the hydrogen-bond forming parts [2]. Cellulose is derived either from wood pulp or cotton linters by delignification in a multi-step process and because of its unprocessable behaviour the raw cellulose is modified. The modification of cellulose is often combined with depolymerisation by oxidation, acid or alkaline reactions and laundering [3]. Viscose solutions, cellulose esters and ethers are the major groups of chemically modified cellulose derivatives. They have been used in a wide range of applications such as fibres, films or plastics. Viscose Solutions Pure cellulose is treated with a strong base e.g. sodium hydroxide (‘alkalization’) and then mixed together with carbon disulfide to obtain cellulose xanthate (Fig. 2) [4]. This viscose is extruded into an acid solution either through a slit die to produce cellophane or through a spinneret to receive rayon fibres. Rayon was the first man-made manufactured fibre based on renewable raw cellulose. Today there are two basic processes to produce rayon – the viscose method and the cupramonium method (cuprammonium silk). Other methods such as the nitrocellulose process are negligible due to their inefficiency. Different types of rayon – regular rayon, high wet modulus rayon, high tenacity rayon, crupamonium rayon – can be produced. The properties of rayon fibres are more similar to those of other natural fibres such as cotton rather than those of thermoplastic fibres such as nylon. Rayon exhibits a silk-like appearance coupled with a good maintenance of its brilliant colours [6]. As a natural fibre, rayon is a highly moisture absorbent and breathable material which is easy to dye. The fibre shows antistatic behaviour and does not pill during fabrication [6]. In general, rayon as a cellulose-based fibre shows high flammability but the use of a flame retardant can 44 bioplastics MAGAZINE [01/10] Vol. 5

Basics improve the flame protection. A major advantage is its ability and versatility to blend with other fibres. Rayon is used in a wide range of applications, e.g. yarns, textiles or reinforcements (Fig. 3). Cellophane Cellophane is a cellulose-based thin and highly transparent film made from viscose solutions under special process condition to obtain a nonbrittle plasticized film. Finally the film is dried and rolled up through heated mills. In the 90’s the Fraunhofer Institute IAP developed a new process based on an amine oxide method to produce blown films from cellophane [7]. Thanks to its biodegradability and low permeability to air, oils, greases, and bacteria but with a coincidental high permeability to water vapour, cellophane is widely used for food packaging. The films are printable and weldable. Further applications of cellophane are self-adhesive tapes, semipermeable membranes or even displays. Cellophane is a brand of Innovia Films Ltd (Cumbria, UK) [8]. Fig. 3: Example of Rayon yarn (photo: Wikipedia) Cellulose Esters (organic and inorganic) Due to its structure with three reactive OH-groups on each anhydroglucose units, cellulose can be transformed into various numbers of organic and inorganic acid esters [9]. However industrial esterification is limited to derivatives with reproducible properties. Therefore esterified organic esters are obtained only from a small range of saturated aliphatic organic acids with up to four carbon atoms [9]. The most important organic cellulose esters which are in large-scale production are cellulose (di)acetate (CA), cellulose (tri)acetate (CTA), cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP). CA, CAB and CAP are white amorphous materials whereas CTA is semi-crystalline. They are commercially available as powders or flakes [9]. Major suppliers of the raw esters are Acetati Spa, Celanese, Daicel, Eastman or Rhodia. Property CA CAB CAP Density [g/cm³] 1,23 - 1,32 1,16 - 1,21 1,19 - 1,21 Flexural Modulus [MPa] 758 - 4210 827 - 1790 1160 - 1860 Tensile Strength [MPa] 39,5 - 125 15,9 - 51,5 22,1 - 41,5 Tensile Elongation [%] 2,2 - 70 30 - 51 3 - 45 Rockwell Hardness 38 - 112 40 - 83 55 - 96 Notched Izod Impact [J/m] 51 - 195 80 - 534 80,6 - 533 Table 1: Some properties of CA, CAB and CAP [according to 10]. Fluctuation range is due to plastizer and additive content They are non-toxic, odorless and less flammable than nitrocellulose. Furthermore these esters show good resistance to weak acids, mineral and fatty oils as well as petroleum [9]. Typical properties of CA, CAB and CAP are compared in table 1. Because of the narrow window between the melting and decomposition temperature as well as the strong interactions between the non-esterified OH-groups these cellulose esters must be additivated to produce thermoplastic materials. The easiest way is plasticization, whereas blending is another possibility [11], but due to high hydrogen Hansen solubility parameters blending is limited. On the other hand the incorporation of a second substituent into CA (e.g. CAB) weakens the strong hydrogen network and enhances the miscibility with plasticizers or polymers. Therefore the mouldability and modification of CAB and CAP is generally better than for CA. bioplastics MAGAZINE [01/10] Vol. 5 45

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