Injection Moulding By
Injection Moulding By Jens Erdmann Johannes Ganster Hans-Peter Fink Fraunhofer Institute for Applied Polymer Research IAP Potsdam, Germany Melt mixing T > 130°C, t = 5 min 1 wt.-% HMDI - COOH (Carboxyl) - OH (Hydroxyl) - NCO (Cyanate) Polyester backbone Physical bonds (entanglements) - CONH (Amide-linkage) + CO 2 - COONH (Urethane-linkage) Fig. 5: Possible coupling mechanism at the interphase between PLA matrix molecules, cellulose fibre and hexamethylene diisocyanate during reactive compounding Further improvements through tailoring the fibre matrix interface - Strong interface trough coupling 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. Fig. 6: SEM cryofracture micrographs of unmodified (top) and with HMDI coupled (bottom) composites of rayon reinforced PLA 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). 24 bioplastics MAGAZINE [03/12] Vol. 7
Injection Moulding 20 16 PLA + 20% Rayon Unmodified Coupled with HMDI Compounding : Kneader Matrix : Ingeo PLA 6252D Fibre : Rayon, Cordenka RT700, 4mm in length 28 24 20 Compounding : Extrusion Matrix : Ingeo PLA 4042D Fibre : Rayon, Cordenka RT700 PLA + 20% Rayon Unmodified Anti-coupled with MAPP Value 12 8 Value 10 8 6 4 4 2 0 σ max [MPa*10] σ f,max [MPa*10] E-Modulus [GPa] W Β [J/10] ε Β [%] a cN [kJ/m 2 ] a c [kJ/m 2 *10] 0 σ max E-Modulus [MPa*10] [GPa] a cN [kJ/m 2 ] a c [kJ/m 2 *10] E P [J] damping [/10] Fig. 7: Impact, bending and tensile properties of rayon fibre reinforced PLA: unmodified vs. strong interface Fig. 8: Impact and tensile properties of rayon fibre reinforced PLA: unmodified vs. weak 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 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. 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 bioplastics MAGAZINE [03/12] Vol. 7 25
