The reinforcement of
The reinforcement of conventional oil-based thermoplastics with short fibres (up to 2 mm length) is a classical method to improve the mechanical properties of the matrix material maintaining its extrusion and injection moulding capabilities. Well known examples are glass fibre reinforced polypropylene (PP) and polyamide (PA) for which a multitude of standard formulations is available on the market. Injection moulded items from these materials are much stronger and stiffer than the unreinforced counterparts, often more than hundred or even several hundred per cent. Moreover, high impact strengths complement the well-balanced property profile of this kind of standard composite materials. An embrittlement caused by the glass component is over compensated by the ductile behaviour of the PP or PA matrix. Fig. 2: Wound bobbins of cellulose rayon tire cord yarn Cordenka ® RT700 PLA meets Rayon Tough PLA compounds reinforced with cellulose rayon for injection moulding For PLA as a sort of biobased commodity polymer, however, such fibre reinforced standard types are not available on the market yet but represent a fruitful topic of applied research. Apart from improving strength and stiffness, special emphasis is put on reducing the brittleness of PLA by appropriate reinforcing fibres. Improved impact strength may, in combination with high strength and stiffness, open up new fields of application for PLA in the technical field in addition to the packaging sector. As seen in Fig. 1, the classical short glass fibre reinforcement does not lead to all the desired results with PLA. Although strength (σ max ), stiffness (E-modulus), and notched Charpy impact strength (a cN ) are improved considerably, Charpy impact strength (a c ), as well as tensile elongation (ε B ) and absorbed energy at break (W B ) fall below the PLA starting level. Obviously, for reinforcing PLA as successful as PP or PA with glass, alternative fibre types are needed capable of improving toughness and ideally with biogenic and biodegradable character. Biobased high performance cellulose vs. glass fibres for reinforcement An alternative is found in high strength, spun cellulose fibres known as cellulose rayon or simply rayon. Highly anisotropic and Value 16 14 12 10 8 6 4 2 0 PLA PLA + 20% Glass fibre PLA + 20% Rayon fibre σ max E-Modulus [MPa*10] [GPa] ε Β [%] Compounding : Kneader Matrix : Ingeo PLA 7000D Fibre : Rayon, RT700, 4mm : Glass, Lanxess CS 7952, 4.5mm W Β [J/10] a cN [kJ/m 2 ] a c [kJ/m 2 *10] Value 12 10 8 6 4 2 0 Tensile strength, σ max [MPa*10] Youngs-modulus, [GPa] Notched Charpy, a cN [kJ/m 2 ] Charpy,a c [kJ/m 2 *10] Compounding : Kneader Matrix : Ingeo PLA 6252D Fibre : Rayon Cordenka RT700, 4mm 0 10 20 30 40 Fibre content, wt.-% Fig. 1: Impact and tensile properties of composites – glass fibre vs. rayon fibre reinforcement Fig. 3: Impact and tensile properties of rayon fibre reinforced composites as a function of fibre content 22 bioplastics MAGAZINE [03/12] Vol. 7
Injection Moulding 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 (MPa) Modulus (GPa) Elongation (%) 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 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. Composites of PLA and rayon 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. 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
