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Issue 03/2022

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Highlights: Injection Moulding Beauty & Healthcare Basics: Biocompatibility of PHA Starch

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BOOK STORE New Edition 2020 New Edition 2020 ORDER NOW www.bioplasticsmagazine.com/en/books email: books@bioplasticsmagazine.com phone: +49 2161 6884463 52 bioplastics MAGAZINE [03/22] Vol. 17

Value 10 Years ago 20 tinyurl.com/2012-PLA-rayon 16 12 8 PLA + 20% Rayon 4 0 Unmodified Coupled with HMDI σ max σ f,max [MPa*10] [MPa*10] Injection Moulding E-Modulus [GPa] Fig. 7: Impact, bending and tensile properties of rayon fibre reinforced PLA: unmodified vs. strong interface Weak interface trough anti-coupling In contrast to covalent coupling, anti-coupling can be generated by making the fibre surface hydrophobic during the compounding process. This is accomplished by adding small amounts (3 wt.-%) of maleic anhydride grafted polypropylene (MAPP) to the PLA pellets. In this way, the energy absorbing debonding and fibre pull-out process is activated and exceptionally tough PLA composites can be obtained (see refs. [6 - 8]), as shown in Fig. 8. Consequently, the absorbed penetration energy (E P ) and damping (ratio of loss to storage work) determined by falling dart impact tests is clearly increased. With a fiber content of 20 % Charpy impact strength values of 95 kJ/m 2 and 26 kJ/m 2 were determined on un-notched and notched specimens, respectively. This represents a five to ten fold increase over the values for unreinforced PLA, and is doubled/tripled compared to unmodified compounds. For a fibre content of 30 % (results not shown) notched Charpy values of 35 kJ/m 2 were measured while un-notched test bars did not show a breaking event. Conclusion Compounding : Kneader Matrix : Ingeo PLA 6252D Fibre : Rayon, Cordenka RT700, 4mm in length W Β [J/10] ε Β [%] a cN [kJ/m 2 ] It was shown that cellulose rayon reinforcement is a viable biobased option to improve the mechanical properties of PLA, in particular the unsatisfactory impact behaviour. In contrast to conventional impact modifiers which reduce stiffness and strength, fibre reinforcement with rayon gives improved toughness, strength, and impact strength at the same time. Apart from stiffness, rayon reinforcement proved to be superior over short glass fibre reinforcement with additional advantages in terms of lower density, reduced abrasiveness, facilitated renewable energy incineration, and biobased character of the whole composite. a c Value [kJ/m 2 *10] 28 24 20 10 8 6 4 2 0 By Jens Erdmann Johannes Ganster Hans-Peter Fink Fraunhofer Institute for Applied Polymer Research IAP Potsdam, Germany Compounding : Extrusion Matrix : Ingeo PLA 4042D Fibre : Rayon, Cordenka RT700 σ max [MPa*10] Fig. 6: SEM cryofracture micrographs of unmodified (top) and with HMDI coupled (bottom) composites of rayon reinforced PLA Injection Moulding E-Modulus [GPa] PLA + 20% Rayon a cN [kJ/m 2 ] Unmodified Anti-coupled with MAPP Melt mixing T > 130°C, t = 5 min Fig. 8: Impact and tensile properties of rayon fibre reinforced PLA: unmodified vs. weak interface - COOH (Carboxyl) - OH (Hydroxyl) ductile (see Tab. 1 for elongation at break), these fibres prove to be capable of simultaneously improving strength, stiffness, and impact strength. The reinforcing cellulose fibre used in these studies and compared with conventional E-glass, is the rayon tyre cord yarn Cordenka ® RT 700 which is produced by Cordenka GmbH, Obernburg, Germany, on a several thousand tonnes scale. The yarn with 1.350 filaments of 1.8 dtex corresponding to a diameter of 12 µm resembling a glass fibre roving is shown in Fig. 2. The mechanical properties of single filaments of rayon and other cellulose spun fibres have been characterized in some detail in this institute (see refs. [1, 2]). Some results are given in Tab. 1 in comparison to glass fibres. With 830 MPa the Cordenka fibre has the highest tensile strength among commercially available man-made cellulose fibres. Compared to glass the properties are considerably lower, however property levels of the composites are in the same range and, moreover, rayon has a series of advantages over glass fibres. Fiber Strength Modulus Elongation (MPa) (GPa) (%) Cordenka RT 700 830 ± 60 a 20 ± 1 13 ± 2 E-glass 3300 ± 500 85 ± 5 4.6 ± 0.6 a Standard deviation Tab. 1: Mechanical properties of single filaments of rayon tire cord and E-glass First, the density of rayon with 1.5 g/cm 3 is lower than the glass density of 2.5 g/cm 3 bearing potential for light weight construction. Then the wear of the processing equipment is much reduced owing to the ‘softer’ character of the fibre (anisotropy) and the thus low abrasiveness. Less fibre breakage is experienced during repeated compounding for the same reason giving advantages at recycling operations. Finally incineration and therefore renewable energy recovery a c [kJ/m 2 *10] Injection Moulding E P 1 wt.-% HMDI - NCO (Cyanate) [J] damping [/10] Polyester backbone Physical bonds (entanglements) - CONH (Amide-linkage) + CO 2 - COONH (Urethane-linkage) is facilitated due to the organic nature of the fibre. On the down side, besides low stiffness, there is the reduced thermal processing window posing difficulties for higher melting thermoplastics, say, above 240°C. Finally, composite preparation might be affected by the hydrophilic nature of rayon. This is obvious from Fig. 1 where with the same fibre weight fraction of 20 % rayon fibres excel in absorbed energy, as well as notched and un-notched Charpy impact strength. Strength is as good as with glass and stiffness reduced. References 1. Ganster, J. and Fink, H.-P., Novel cellulose fibre reinforced thermoplastic materials. Cellulose, 2006. 13(3): p. 271-280. 2. Ganster, J., et al., Cellulose man-made fibre reinforced polypropylene - correlations between fibre and composite properties. Cellulose, 2008. 15(4): p. 561- 569. 3. Ganster, J., Fink, H.-P. and Pinnow M., High-tenacity man-made cellulose fibre reinforced thermoplastics - Injection moulding compounds with polypropylene and alternative matrices. Composites Part A-Applied Science and Manufacturing, 2006. 37(10): p. 1796-1804. 4. Weigel, P., et al., Polypropylene-cellulose compounds - High strength cellulose fibres strengthen injection moulded parts. Kunststoffe-Plast Europe, 2002. 92(5): p. 35-37. 5. Ganster, J. and Fink, H.-P., In: Kalia S., et al., Cellulose Fibers: Bio- and Nano Polymer Composites. Springer- Verlag, Berlin, Heidelberg, 2011. p 479-506. 6. Ganster, J., Erdmann, J. and Fink, H.-P., Tailored PLA Materials with Biobased Fibers. Kunststoffe international, 2011. 101: p. 46-49. 7. Erdmann, J., Ganster, J., Einfluss des Faserdurchmessers auf die Struktur und Mechanik Cellulosefaser-verstärkte PLA-Kompsosite. Lenzinger Berichte, 2011. 89: p. 91-102. 8. Erdmann, J., Ganster, J., Composite composition, method for the production thereof, molded part and use. 2010. WO2011101163 Fig. 5: Possible coupling mechanism at the interphase between PLA matrix molecules, cellulose fibre and hexamethylene diisocyanate during reactive compounding Composites of PLA and rayon Further improvements through tailoring the fibre matrix interface - Strong interface trough coupling Prior to the work with PLA and other biobased matrix materials a wealth of experience was gathered with rayon reinforced petro-based thermoplastics such as polypropylene and polyethylene (see refs. [3, 4]). These materials are on the brink of commercialisation and are offered meanwhile by Cordenka. For PLA as the matrix material positive results for both tensile and impact tests were obtained as well, as shown in Fig. 3 for Ingeo PLA 6252D and 4 mm short cut rayon in the range between 10 und 40 wt.-%. While a linear increase is noticed for the Young’s modulus (700 MPa per 10 % increase in fibre fraction), tensile and notched Charpy impact strengths reach a plateau of 100 MPa and 9 kJ/m 2 , respectively, between 20 % and 30 % fibre fraction. For un-notched Charpy impact strength a maximum of 60 kJ/m 2 is found at 20 % fibres. The non-linear behaviour of the latter properties is caused by the insufficient fibre-matrix adhesion producing flaws in the structure and premature failure. Even without a compatibilisation (coupling agents) homogeneous composites are obtained with completely separated and well dispersed fibres, as demonstrated in Fig. 4. Obviously composite properties will be influenced by the strength of the fibre-matrix interphase: strong, preferably covalent bonds between fibre and matrix will lead to a good stress transfer to the fibres also at high deformations and even close to sample failure and thus improve the composite strength. The method employed for PLA used here is based on diisocyanate coupling agents, namely hexamethylene diisocyanate (HMDI) 1 wt.-% of which was added during compounding. In this way an additional, separate fibre treatment it not necessary which is an economic advantage. The proposed coupling mechanism at the interphase is shown in Fig. 5. Ideally, the plentiful hydroxyl groups at the fibre surface react with the isocyanate moieties such that the fibre surface is functionalized with the isocyanate groups which in turn can react with either hydroxyl or carboxyl groups at the PLA chain ends to give urethane or amide linkages. In that way covalent bonds are established between cellulose fibres and the matrix material and the stress transfer to the matrix is realized through physical bonds also known as entanglements. The improved fibre-matrix adhesion can be verified by the SEM pictures of Fig. 6. For the HMDI system (bottom picture) neither fibre pull-out nor debonding at the interface is visible. The reduction of structural defects through HMDI coupling leads to improved mechanical properties as shown in Fig. 7 for tensile, flexural strength (σ f,max ), and impact values, except for modulus (same) and notched Charpy impact strength (reduced). The latter reduction is caused by the coupling-induced impediment of fibre pull-out and debonding as energy absorbing mechanisms at notched impact failure. However, tensile and flexural strength, elongation and absorbed energy at break, and un-notched Charpy impact strength even surpass the values for composites with 30 % and 40 % fibres and unmodified interface (Fig. 3). In May 2022, Rudolf Einsiedel, Vice President Cordenka Innovations GmbH, Obernburg am Main, Germany says: PLA-Cordenka ® , the cellulose fibrereinforced polylactide material for injection moulding, is an innovation that makes the collaboration of Fraunhofer IAP and Cordenka – as reported by bioplastics MAGAZINE 10 years ago -– commercially viable. Today, the product is available on commercial scale. PLA-Cordenka is a technical biocomposite for injection moulding. Polymer matrix, reinforcement fibre and their composite are biobased and biodegradable. Cordenka cellulose rayon fibres significantly improve the impact resistance of PLA. Further, they can function as nucleating agents, induce crystallinity and raise the heat resistance of PLA to the level of an engineering plastic, if appropriate processing is applied. bioplastics MAGAZINE [03/12] Vol. 7 25 The Cordenka PLAcomposite is the sustainable, fossil carbon free material alternative for the production of technical injection moulded parts such as electronic part housings. Besides its physical properties, as displayed in the preliminary material data sheet available at cordenka-innovations.com, the biocomposite material stands out with its unique touch, feeling unlike plastic or wood, but like sustainable innovation. Product samples are available upon request. For material inquiries, please contact info@cordenkainnovations.com. 24 bioplastics MAGAZINE [03/12] Vol. 7 Fig. 4: Structure of PLA reinforced with 20 wt.-% rayon visualized by scattering electron microscopy (SEM) at low and high magnification bioplastics MAGAZINE [03/12] Vol. 7 23 bioplastics MAGAZINE [03/22] Vol. 17 53

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