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Procedia Materials Science 2 ( 2013 ) 137 – 143
Materials Science Engineering, Symposium B6 - Hybrid Structures PLA-viscose-composites with continuous fibre reinforcement for structural applications
M. Reinhardt*, J. Kaufmann, M. Kausch, L. Kroll
Chemnitz University of Technology, D-09107 Chemnitz, Germany
Abstract
Aiming a biobased composite, the polymer PLA (polylactide) was selected for matrix material. Because of its likewise natural origin, viscose (rayon) was chosen for unidirectional reinforcement. Filament wet winding was applied for hollow structures from the novel composite. Furthermore unidirectional plates were processed by film stacking. With both methods fibre contents of at least 50 volume percent are feasible for these continuous reinforced composites. Performed examinations illustrate significant increase of mechanical properties in comparison with common natural reinforced PLA (compression moulded non-woven).
© 2013 TheTh eAuthors. Author Publisheds. Published by Elsevier by Elsevier Ltd. Ltd. Selection and and/or peer-review peer-review under responsibilityunder responsibility of Conference of C onferenceorganizers (MSE-Symposiumorganizers (MSE B6).-Symposium B6)
Keywords: biobased; PLA (polylactide); viscose (rayon); unidirectional; composite; filament winding; film stacking; mechanical properties
1. Introduction
Reinforcement of polymers by natural fibres is successfully applied in lightweight engineering. Starting mainly in the 1990s hybrid materials with thermoplastic and natural fibres are commonly processed in compression moulding (Carus 2008). Due to their similar weight-performance-proportion those natural fibre
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2211-8128 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Conference organizers (MSE-Symposium B6). doi: 10.1016/j.mspro.2013.02.016 138 M. Reinhardt et al. / Procedia Materials Science 2 ( 2013 ) 137 – 143
composites (NFC) have substituted partially traditional glass fibre composite (GFC) applications. Technological applications are mainly concentrated on bast fibres (flax, hemp, kenaf, jute and ramie). Their further advantages are given by their low abrasion, reduced recycling effort, plus distinctive acoustic and thermal absorbability. In early 2000s comprehensive studies were performed in order to characterise natural fibres and their composites (e.g. Lampke 2001, Odenwald 2002). The disadvantages of the NFC are mainly caused by their origin. Thus fibre quality and steady supply are affected by growing and harvest conditions. Furthermore, maximum fibre length is naturally limited to size of the plant obtained from. Bio-based matrix materials are used increasingly instead of petroleum based materials. The most important thermoplastic biopolymer produced with chemical synthesis from feedstock is PLA (cf. Endres 2011, p. 82ff). For producing PLA vegetable materials with high carbohydrate content, such as corn starch or sugar cane, is fermented to lactic acid. This monomer is polymerised in steps of oligocondensation to prepolymers, depolymerisation to dilactides and afterwards ring-opening polymerisation to polylactide. This biopolymer is meanwhile affordable on comparatively low prices due to up-scaling effects. Furthermore, the PLA property profile has good advantages compared with standard petroleum based polymer PP (Table 1). Both mentioned resins are commonly reinforced with natural fibres. Although tensile strength and tensile modules are slightly reduced, improvement of stiffness is given by lower density. Furthermore, PLA benefits by the increased elongation at break. For example Jakob Winter GmbH produces cases made from NFC with either PP or PLA (Winter, 2011).
Table 1: Comparison typical matrix materials for NFC
tensile tensile elongation density specific strength specific modulus
strength modulus at break in g/cm³ spec in Espec in in N/mm² E in N/mm² B in % Nm/g Nm/kg PP (homo) 32 1.3 >50 0.9 35.6 1.4 PP with kenaf, hemp, flax 23.8 1.9 3.0 0.8 29.8 2.4 compresion mould 50 % fibre content* PLA without reinforcement 59 3.7 2.5 1.25 47.2 3.0 PLA with kenaf, hemp, flax 45 3.2 19.9 0.9 50.0 3.6 compression mould 35 % fibre content* *customary hybrid non-woven average of types from Quadrant and Isowood
Fibre grades such as viscose (rayon) are manufactured in industrial processes. These synthetic cellulose fibres are characterised by steady fibre quality, but benefits of natural fibres stay with the industrial types. They have a low mass density; they are non-abrasive and are natural originated. Consequently, late 2000s started publication of using viscose for reinforcing PLA and PP resins (e.g. Ganster 2006, Bax 2008, Graupner 2009). Those parts were processed by injection and compression moulding competitively to natural fibres. Though, in this processes fibre lengths are still limited. Chosen viscose and other regenerated fibre grades are manufactured by chemically dissolving wood pulp and reconstituting fibres or films (cf. Endres, p. 114ff). The grades differ in properties, due to the distinctive solvents used for regenerating. Viscose is the most important fibre grade in terms of production volume and furthermore available in form of filament yarns. Therefore, viscose was used to develop a composite with biobased and continuous fibre- reinforcement within the eniPROD research project. Thus, in combination with the biopolymer PLA as matrix material the continuous reinforced composite is completely based on renewable raw materials. M. Reinhardt et al. / Procedia Materials Science 2 ( 2013 ) 137 – 143 139
2. Materials
Two grades of the biobased thermoplastic biopolymer PLA were chosen. Filament winding tests were performed with the Ingeo Natureworks 3251D grade intended for injection moulding applications. This grade is characterised by tensile yield strenght of 48 N/mm², elongation at break 2.5 %, density of 1.24 g/cm³ and melting temperature between 188 °C and 210 °C. For film stacking foils made from an Ingeo Naturworks Grade, provided by Pütz GmbH + Co. Folien KG, were used. Those foils were provided with tensile yield strength of 56 N/mm², elongation at break 6.0 %, density 1.25 g/cm³, and melting temperature between 150 °C and 165 °C. The cellulose based man-made fibre grades, especially the regenerated grades (viscose, tencel, carbamat) were originally developed for textile industry. Therefore, the spun fibres are often cut to suit textile post- processing like twisting yarns or manufacturing non-woven. But for aimed technologies filament fibres are needed. For experiments a filament yarn of viscose with 400 tex, consisting of two low-twisted 200 tex rovings (about 580 μm in diameter each roving, assuming a round cross section) was chosen. Mechanical properties were obtained by testing according DIN EN ISO 2062. To appraise the analysed PLA-viscose-composites a customary glass fibre reinforced material was needed. Chosen Twintex P PP 60 1485 4/1 NATUR from Owens Corning is a woven prepreg of glass fibres and PP fibres. Material data are acquired from the companies’ brochure (Owens Corning 2011).
Nomenclature Tt 1 tex=1 g/km; unit in textile industry to characterise fibres; mass per fibre length
3. Methods and results
3.1. Manufacturing
3.1.1. Filament winding Wet winding method was applied. Therefore, the viscose filament roving was pulled through a resin bath with the PLA. Temperature was stabilised with an oil sump and settled according to appropriate viscosity for processing. The viscose filament was spread within the resin bath for enhanced impregnation. Afterwards, the impregnated filament was placed by the horizontal moving carriage on the turning mandrel. Thus, fibre placement may be controlled according to load conditions. Resin bath temperature was about 200 °C. Although, degradation temperature of viscose is about 175 °C, no significant degradation of viscose reinforcement was observed during the short term exposure in the resin bath. In the first experiments, cracks occurred on the hollow structures. So it was necessary to enhance the polymer by adding some plasticiser. Therefore, a citric acid ester (Citrifol II) was added to the melted PLA, till viscosity of mixture gave best results in filament winding. Moreover, it was critical to separate the hollow structure from the mandrel, due to the huge shrinkage in the structure during cooling. Different elongation of wet (more) and dry (less) viscose fibres effect high tangential forces applied to the mandrel. This issue was diminished by using a demountable mandrel.
3.1.2. Film stacking A novel machine type for continuously processing thermoplastic prepregs was applied, developed in partnership with the Cetex e.V. (Kroll 2009). With this technique several coils of the reinforcement material (viscose) were placed on a creel (Fig. 3). After spreading, the fibres were covered with foil of thermoplastic 140 M. Reinhardt et al. / Procedia Materials Science 2 ( 2013 ) 137 – 143
PLA from both sides. All material was preheated and then pressed by calender roles (heated up to 170 °C). Afterwards, the pre-consolidated prepreg was pulled-out on a reel. Main result in film stacking is that viscose filaments are unsusceptible to the process. The fibres were not degraded by processing temperatures. Moreover the filaments resisted the appearing tensile forces.
Fig. 1. Process scheme film stacking
3.2. Mechanical properties
3.2.1. Fibre material The test results were related to the yarn count (Tt in tex). Modulus J0 was determined with 13.5 N/tex and strenght F was determined with 92.3 N. Tensile strength and modulus are related according equations (1) and (2). In Table 2 the results are compared with flax fibres. Properties of flax fibres vary according fibre length, growing and harvest conditions. Whereas strength and modulus of industrial fibre type viscose is steady, and may be adjusted in certain extend with processing technology.
F (1) Tt
E J 0 (2)
Table 2: Mechanical properties of viscose compared with flax fibres
tensile strength tensile modulus in N/mm² E in kN/mm² Viscose 200 tex filament roving 691.0 20.2 according to DIN EN ISO 2062 test results Flax fibre* 250 - 1100 12 - 64 (Lampke 2001)
3.2.2. Composite Mechanical properties were obtained with specimen from compression moulded plates, consisting of several layers of film stacked prepregs. Mechanical properties were obtained according DIN EN ISO 527. Contrary to natural reinforcement, strength and stiffness of PLA is increased by viscose reinforcement (Fig. 4), whereas density of the novel composite is higher (Fig. 5 left). Tensile strength and modulus are more M. Reinhardt et al. / Procedia Materials Science 2 ( 2013 ) 137 – 143 141 than doubled compared the matrix material. Furthermore, elongation at break of the novel composite is extended in comparison with non-reinforced PLA and common natural reinforced PLA materials (Fig. 5 right). The continuous PLA-viscose-composite has nearly one third of stiffness and strength of a customary near unidirectional PP-glass-composite, but density is less.
600 25000 tensile strength tensile modulus 500 20000 400 15000 300
520 10000 200 22200 5000 3680
100 3150 59 8580 45 134 tensile modulus in N/mm² tensile strength in N/mm² 0 0 PLA moulded continuous twintex PLA hybrid viscose-PLA moulded non-woven PP-GF woven 4-1
Fig. 2: Comparison of tensile strength and modulus
1.6 16 14 1.4 14 1.2 12 1 10 0.8 8 1.49 1.36 0.6 1.25 6
density in g/cm³ 0.4 0.9 4 2.5 2.4 elongation at break in % 0.2 2 0 0 PLA moulded continuous twintex PLA moulded continuous PLA hybrid viscose-PLA moulded hybrid viscose-PLA non-woven PP-GF non-woven woven 4-1
Fig. 3: Comparison of (left) density and (right) elongation
4. Discussion
In this study two manufacturing methods, filament wet winding and film stacking, were successfully applied for processing continuous reinforced biobased composites. Good impregnation was affected with each technology by spreading the rovings. As a result, in this processes untwisted filament yarns should be used for enhanced impregnation. Tensile strength is nearly tripled and tensile modulus is more than doubled compared to the common moulded PLA natural fibre composites, based on non-woven and the polymer itself. But density of the novel PLA-viscose-composite is higher than of the pure polymer, due to higher density of the viscose (1.5 g/cm³). Though, strength and modulus related to density are still doubled (Fig. 4). The continuous PLA-viscose-composite has nearly one third of strength and modulus of a customary near unidirectional PP-glass-composite. Because of similar density also specific strength and specific modulus display this ratio. Therefore, mechanical performance of biobased polymers is exceeded in structural applications.
142 M. Reinhardt et al. / Procedia Materials Science 2 ( 2013 ) 137 – 143
400 16000 specific strength 350 14000 specific modulus 300 12000 250 10000 200 8000 6309 349 150 14899 6000
100 99 4000 2944 47
specific strength Nm/g 50 2000 specific modulus in Nm/kg 0 0 PLA continuous twintex viscose-PLA moulded PP-GF woven 4-1
Fig. 4: Comparison of specific strength and specific modulus
Although, mechanical properties of flax fibres (cf. Table 2) may be higher, the quality of viscose is steady without any influences of growing or harvesting. Furthermore, the reinforcement fibres may be placed in load direction. Due to the steadiness of fibre performance and unidirectional reinforcing the composite may be calculated according to common dimensioning methods with low safety factors. Therefore, mechanical performance of biobased polymers is exceeded in structural applications. Because the cellulose fibre types are optimised for textile industry, they are enhanced in water absorption. Therefore, two problems were observed during testing. First, the material properties (elongation and strength) differ with moisture content but also with temperature. Because of higher elongation of the wet fibres, high tangential forces occurred. But this issue was solved by using a demountable mandrel. Second, the fibre material is swelling due to water absorption. In particular if the layer of matrix material is not closed completely this may cause destruction of the composite. In both processes closed surface of composite is necessary for protecting the fibre. This may be guaranteed by final covering the composite with additional polymer layers. Moreover PLA itself is permeable to water. Therefore fibre swelling may occur with closed composite also. But examinations showed that this issue can be diminished by appropriate choice of viscose grade. It was obtained, that water absorption is in great fraction depended on fibre structure. Fibres with ragged structure absorb more water than those with closed structure (Fig. 6). Therefore, the ragged ones are swelling and already destroy the composite during short term exposure with water. For the other fibres no destroying was observed.
Fig. 5: Microscoping viscose (left) cloud structure (right) heart structure M. Reinhardt et al. / Procedia Materials Science 2 ( 2013 ) 137 – 143 143
Next steps of examination will be characterisation of the material in terms of impact, thermal and dynamic behaviour. Furthermore, process parameter needs to be examined to adjust distinctive structural properties of the composite. One more challenge will be reducing the water susceptibility of the PLA matrix material.
5. Conclusions
To overcome lack of performance of common natural reinforced composites a novel complete biobased and continuous reinforced composite was developed. In this study the thermoplastic biopolymer PLA was reinforced with viscose. This industrial fibre type is made from cellulose. Two manufacturing methods were successfully applied; filament winding for hollow structures and film stacking for pre-consolidated material. Moreover, the materials are harmless and convenient in manufacturing. Mechanical performance of biobased polymers is exceeded in structural applications. Strength and stiffness are considerable better than common NFC based on hybrid nonwoven. Furthermore, mechanical properties reach one third of GFC based on 4-1 woven material. Hence viscose fibres are optimised for textile industry they are enhanced according water absorption. This caused destructing issue in composites. Thus, concepts for diminishing water susceptibility are presented.
Acknowledgements
This work has been supported within the saxon competition “Energy-efficient Product and Process Innovations in Production Engineering” (eniPROD). Acknowledged is Thomas Bauer. He gained some of presented results in his Bachelor Thesis, supervised by Martin Kausch.
References
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