Natural fibres as reinforcement in polylactic acid (PLA) composites

Transcription

1 Composites Science and Technology 63 (2003) Natural fibres as reinforcement in polylactic acid (PLA) composites K. Oksman a, *, M. Skrifvars b, J.-F. Selin c a Department of Machine Design and Materials Technology, NTNU, Norwegian University of Science and Technology, Rich. Birkelands vei 2, N-7491 Trondheim, Norway b SICOMP AB, POB 271, SE Pitea, Sweden c Fortum Oyj, POB 310, FIN Porvoo, Finland Accepted 21 February 2003 Abstract The focus in this work has been to study if natural fibres can be used as reinforcement in polymers based on renewable raw materials. The materials have been flax fibres and polylactic acid (PLA). PLA is a thermoplastic polymer made from lactic acid and has mainly been used for biodegradable products, such as plastic bags and planting cups, but in principle PLA can also be used as a matrix material in composites. Because of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax composites in order to improve the impact properties. The studied composite materials were manufactured with a twin-screw extruder having a flax fibre content of 30 and 40 wt.%. The extruded compound was compression moulded to test samples. The processing and material properties have been studied and compared to the more commonly used polypropylene flax fibre composites (PP/flax). Preliminary results show that the mechanical properties of PLA and flax fibre composites are promising. The composite strength is about 50% better compared to similar PP/flax fibre composites, which are used today in many automotive panels. The addition of plasticizer does not show any positive effect on the impact strength of the composites. The study of interfacial adhesion shows that adhesion needs to be improved to optimise the mechanical properties of the PLA/flax composites. The PLA/flax composites did not show any difficulties in the extrusion and compression moulding processes and they can be processed in a similar way as PP based composites. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: PLA; Flax; Renewable raw materials; Composites; Extrusion; B. Mechanical properties; B. Microstructure; DMTA 1. Introduction The growing environmental awareness and new rules and regulations are forcing the industries to seek more ecologically friendly materials for their products. For example automotive applications based on natural fibres with polypropylene as matrix material are very common today. Less work has been done to study composites with matrices, which originate from renewable raw materials. There are many different polymers of renewable materials: for example polylactic acid, cellulose esters, poly hydroxyl butyrates, starch and lignin based plastics. The problems with these polymers have been poor commercial availability, poor processability, low toughness, high price and low moisture stability. * Corresponding author. Tel.: ; fax: address: kristiina.oksman@immtek.ntnu.no (K. Oksman). The long-term properties of renewable materials are also very important especially if the products are not single use applications. Natural fibres have many advantages compared to synthetic fibres, for example low weight, and they are recyclable and biodegradable. They are also renewable and have relatively high strength and stiffness and cause no skin irritations [1 8]. On the other hand there are also some disadvantages: moisture uptake, quality variations and low thermal stability. Many investigations have been made on the potential of the natural fibres as reinforcements for composites and in several cases the results have shown that the natural fibre composites own good stiffness but the composites do not reach the same level of strength as glass fibre composites [1 8]. The manufacturing methods of natural fibre thermoplastic composites have been modified lay-up/press moulding (film stacking method), pultrusion, extrusion and injection moulding [4 8] /03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)

2 1318 K. Oksman et al. / Composites Science and Technology 63 (2003) While many studies have been made on the potential of natural fibres just a few investigations are made on the possibilities to use renewable polymers as matrix for such fibres [9 13]. The studied biopolymers have been soy-oil based epoxy, starch, polycaprolactone (PCL), polyhydroxybutyrate (PHB), modified cellulose, acetic acid, polylactic acid (PLA) and polyester amide [9 13]. Polylactic acid polymers or polylactides are polyesters of lactic acid, and these polymers have recently been introduced commercially for products where biodegradability is wanted. Polylactic acid is a versatile polymer made from renewable agricultural raw materials, which are fermented to lactic acid. The lactic acid is then via a cyclic dilactone, lactide, ring opening polymerised to the wanted polylactic acid. The polymer is modified by certain means, which enhance the temperature stability of the polymer and reduce the residual monomer content. The resulting polylactic acid can be processed similarly as polyolefines and other thermoplastics although the thermal stability could be better. Reinforcing with fibres is one possibility to enhance thermal stability. Polylactide polymers are stiff and brittle materials, and it is therefore necessary to use plasticizers to improve the elongation and impact properties. The polylactide is fully biodegradable. The degradation occurs by hydrolysis to lactic acid, which is metabolised by micro-organisms to water and carbon monoxide. By composting together with other biomass the biodegradation occurs within two weeks, and the material has fully disappeared within 3 4 weeks [14]. There are several promising markets for biodegradable polymers such as polylactide. Plastic bags for household bio waste, barriers for sanitary products and diapers, planting cups, disposable cups and plates are some typical applications. To date no commercial large-scale production of polylactide exists, but this is likely to change in the near future. The starting material, lactic acid, will also need new capacity. Commercial markets for biodegradable polymers are expected to increase substantially in the coming years. The aim of this study was to make an initial investigation how PLA will act as matrix material for natural fibre composites. As PLA can be processed in nearly the same way as polypropylene, it should be possible to prepare flax reinforced composites by extrusion. The mechanical properties of the composites were studied according to the tensile testing. Further, the thermal properties were studied with dynamic mechanical thermal analysis (DMTA), the morphology was studied with scanning electron microscopy (SEM) and the possible degradation of PLA during extrusion was determined by gel permeation chromatography (GPC). 2. Materials and methods 2.1. Materials Matrix: PolyL-lactic acid (PLA), POLLAIT from Fortum was used. The MFI for PLA is between 1 and 2 g/10 min (190 C, 2.16 kg), which is lower than for the used PP. Matrix (ref): Polypropylene (PP), from Adstiff 770 ADXP, Montell polyolefins. This is a PP with specific high MFI, 45 g/10 min (230 C, 2.16 kg). It is a suitable matrix for extrusion of composite materials. Additive: Triacetin, glycerol triacetate ester, from Sigma-Aldrich, was used as plasticizer for PLA. It has good compatibility with the polymer, and increases the elongation of the plastic from less than 10% to more than 250%. Reinforcements: Enzyme retted flax fibres in the form of long heckled fibres were connected together by hand, called hand made roving. The appearance of used fibres can be seen in Fig. 1. Table 1 shows the properties of used materials. PLA has better mechanical properties compared to PP while PP has higher MFI and lower density. The flax fibres have superior mechanical properties compared to PP and PLA, which can be expected to result in good reinforcing effect for both polymers. Various formulations of studied materials are shown in Table 2, totally 12 different formulations were studied Methods Compounding of composite materials The composite materials were manufactured using a twin-screw extruder (Coperion Werner & Pfleiderer Fig. 1. Hand made flax fibre roving.

3 Table 1 Material properties of used raw materials K. Oksman et al. / Composites Science and Technology 63 (2003) Materials Flexural-modulus (GPa) Tensile strength (MPa) Elongation to break (%) MFI (g/10min) (2.16 kg, 190 C) Density (g/cm 3 ) PP PLA a Flax fibres a Data from the manufacturer. Table 2 Compositions of different materials Materials Matrix (wt.%) Flax fibres (wt.%) PP 100 PP/flax PP/flax PLA 100 PLA/flax PLA/flax PLA/triacetin 95 5 PLA/triacetin PLA/triacetin PLA/triacetin/flax PLA/triacetin/flax PLA/triacetin/flax Triacetin (wt.%) ZSK 25 WLE). The flax fibre content was 30 and 40 wt.%. The fibres were fed into the side extruder and the fibre content in the composite was calculated according to feeding speed and the weight of the roving per meter. The processing parameters are shown in Table 3. The liquid triacetin plasticizer was pumped into the extruder Compression moulding Test samples for mechanical testing were compression moulded with a conventional compression moulding press (Fjellman Press Mariestad AB) with a maximum press capacity of 3100 tons. The mould temperature was 50 C and the pressure was about 70 MPa Mechanical testing The tensile testing was performed according to ASTM 3039 standard for tensile testing on an Instron. Impact testing was performed according to ISO 179 unnotched Charpy standard for fibre reinforced composite materials. At least 10 specimens were tested for every material Electron microscopy Fractured surfaces of the materials were studied with a CamScan scanning electron microscope (SEM) with an acceleration voltage of 30 kv. The sample surfaces were sputter coated with gold to avoid charging. Table 3 Processing settings for extrusion Material PP/flax PLA/flax Speed (rpm) Torque (%) Vacuum vent. Yes Yes Temp. profile (C ) Zone (PP, PLA, plastizer) Zone 2 Zone Zone 4 (Flax fibres) Zone Zone Zone 7 Zone Zone Zone 10 Zone Total output (kg/h) GPC Number and weight average molecular weights before and after the extrusion of the PLA were determined by gel permeation chromatography on a Waters GPC system with Styragel columns (10 5,10 4,10 3 and 500 A ) and a refractive index detector. Chloroform was used as effluent, and the analysis was done at room temperature. A universal calibration was used for the calculation of the molecular weights, using Mark Houwink constants for PLA and polystyrene, which was used as standard DMTA Dynamical mechanical thermal analysis (DMTA, Rheometric Scientific Mk III) was performed to initially investigate if the addition of the flax fibres will improve the thermal properties, such as maximum use temperature of PLA. Four materials, PLA, and PLA/flax, PLA/ triacetin and PLA/flax/triacetin, were tested to find the maximum use temperature for the use and also to see possible interaction effects between PLA matrix and flax fibres. DMTA was run in the dual cantilever bending mode and the typical sample dimensions were: thickness 1 2 mm, length 25 mm and width 4 mm. The temperature interval was from room temperature, about 25 C, to 180 C with a heating rate of 1.5 C/min and using a frequency of 1 Hz.

4 1320 K. Oksman et al. / Composites Science and Technology 63 (2003) Results 3.1. Mechanical testing The mechanical properties of PLA/flax composites were compared to PP/flax. Table 4 shows the summary of the results. Generally, the pure PLA has better mechanical properties than pure PP. Fig. 2 shows the tensile stress and Fig. 3 the stiffness of the tested materials. The pure PLA has a tensile strength of 50 MPa and a modulus of 3.4 GPa compared to 28 MPa and 1.3 GPa of pure PP. The addition of flax fibres will not improve the tensile strength, which is an indication of poor adhesion between the flax fibres and the matrix. The stress is not transferred from the matrix to the stronger fibres. The addition of flax will increase the modulus but the higher fibre content will not improve the modulus in the PLA composites as it will for PP composites. A possible explanation of this can be the fibre orientation. The test samples are compression moulded and the fibres can be orientated differently from one sample to another. Cyras et al. [10] have studied starch/pcl/sisal fibre composites and reported a tensile modulus of 0.7 GPa and a maximum strength of 14.4 MPa with a 30 wt.% sisal fibre content. These values are very low compared to our PLA/flax modulus 8.3 GPa and the strength 53 MPa. A research group at DLR in Germany lead by Riedel [11,12] have studied different biocomposites and reported very high mechanical properties of composites they studied. They have used Bioceta, Sconacell and PLA as matrix and unspecified natural fibre mats as reinforcements and reached mechanical properties near by glass fibre mat reinforced plastics. Because of the brittle nature of PLA, triacetin was used to plasticize the pure PLA and for the PLA/flax composites. Triacetin has previously been used for plasticizing of pure PLA with good results. Usually the triacetin content is 12 15%, lower amounts do not give effects. In this work with addition of fibres we also wanted to test lower amounts. The fibre content was held constant during this test, 40 wt.%. Table 5 shows a summary of the mechanical properties of plasticized PLA and PLA/flax composites. Fig. 2. Tensile stress of PLA/flax composite compared to PP/flax. Fig. 3. Tensile modulus of PLA/flax composites compared with PP/ flax. The results show that the tensile stress is decreased with increased triacetin content and this trend was even more visible in PLA/flax composites. The addition of triacetin showed a positive effect on the elongation to break for pure PLA and PLA/flax composites, which was expected because of the softening effect. The highest triacetin addition (15%) clearly shows a negative effect for PLA/flax composites, both the stress and stiffness are strongly decreased (see also Figs. 4 and 5). Fig. 5 shows how the stiffness of PLA and PLA/flax composites is affected by addition of triacetin. The stiffness of PLA/flax composites is strongly decreased Table 4 The mechanical properties of PP/flax and PLA/flax composites Materials Elongation to break (%) S.D. Max. Stress (MPa) S.D. E-modulus (GPa) S.D. PP PP/30% flax PP/40% flax PLA PLA/30% flax PLA/40% flax

5 Table 5 Mechanical testing of plasticized PLA with and without flax fibres K. Oksman et al. / Composites Science and Technology 63 (2003) Materials Elongation to break (%) S.D. Max. stress (MPa) S.D. E-modulus (GPa) S.D. PLA PLA/5% Tri PLA/10% Tri PLA/15% Tri PLA/40% flax PLA/5% Tri/40% flax PLA/10% Tri/40% flax PLA/15% Tri/40% flax Fig. 4. The tensile stress of PLA with 5%, 10% and 15% triacetin content and 40% flax fibres. Fig. 6. The effect of triacetin and 40% flax fibres on the unnotched Charpy impact strength of PLA. content does not show any positive effect on Charpy impact strength of the composites. Notice that the standard deviation was increased with increased triacetin content. The residual moisture content in the flax fibres had most likely a negative effect on the triacetin / PLA /fibre system Electron microscopy Fig. 5. The effect of triacetin and 40% flax fibres on the tensile modulus of PLA. with the triacetin content while the triacetin did not affect the stiffness of pure PLA in the same level. Fig. 6 shows how the impact properties of PLA and PLA/flax composites are affected by the addition of triacetin. It can be seen that the addition of triacetin did not affect the impact properties of the PLA/flax composites at all as expected. The addition of 5% triacetin in PLA shows the best impact strength. Higher triacetin Fig. 7 shows the fracture surface of the PLA/flax composite. It is possible to see that there are many fibre pull-outs and that the fibre surfaces are clean which indicates poor adhesion between the fibres and the PLA matrix. The fibres are also orientated (which can be seen in Fig. 7b). Further, Fig. 7 shows that the flax fibres are in the form of single fibres and that indicates that the fibres have been separated during the extrusion process. The fibres are also very well dispersed in the PLA matrix. Good dispersion of single fibres and fibre orientation should result in very high mechanical properties GPC The GPC analysis showed that the weight average molecular weight was g/mol for the pure PLA.

6 1322 K. Oksman et al. / Composites Science and Technology 63 (2003) occur at temperatures around C. Fig. 8 shows dynamic modulus and tan delta curves of the PLA and PLA/flax composites. It is possible to see in Fig. 8a that thermal properties of PLA are increased with the incorporation of flax fibres. The softening temperature is increased from about 50 C for pure PLA to 60 Cwith flax fibres and it is further increased if the composite is crystallized. The composite will soften after 60 C but the modulus will start to increase again around 80 C which is a typical effect of cold crystallisation [15]. The curve of the crystallized sample (PLA/flax II) shows very good thermal properties. Fig. 8b shows tan delta for PLA and PLA/flax composites and PLA/flax II (cold crystallised). The tan delta peak is not changed due to addition of flax but it is very much affected by the crystallisation. The peak broadens and is also very low compared to uncrystallised sample. Fig. 9 shows PLA and PLA/flax with 10 wt.% triacetin plasticizer. Fig. 9a shows that the plasticizer has decreased the thermal properties of PLA which was even expected because of the plasticizing effect. The addition of fibres and crystallisation will increase the softening temperature. In Fig. 9b tan delta curves are shown, the addition of triacetin will decrease the tan delta peak from 63 to 55 C for PLA but the addition of Fig. 7. Fractured surface of PLA/flax composites: (a) detailed picture, (b) overview. After extrusion with flax fibres, a slightly lower number average molecular weight was recorded, g/mol. The GPC also showed that the molecular weight distribution was unimodal, and that no low molecular weight fraction was present. It could therefore be concluded that the PLA matrix does not degrade chemically during the processing DMTA Dynamic mechanical tests were run to characterize the composites. PLA can be semi-crystallic material but it is usually amorphous because the crystallisation will Fig. 8. DMTA runs for PLA and PLA with flax fibres: (a) dynamic modulus, (b) tan delta.

7 K. Oksman et al. / Composites Science and Technology 63 (2003) The study of interfacial adhesion, which is a wellknown problem for natural fibres and synthetic polymers, also shows that adhesion needs to be improved to optimise the mechanical properties of the PLA/flax composites. The microscopy study of the composite microstructure showed poor adhesion between the fibres and PLA matrix. The flax fibres were well dispersed in the PLA and separated to single fibres due to the compounding process. The thermal properties of the PLA, which are a drawback for PLA, were improved with an addition of flax fibres. Triacetin plasticizer did not improve the composite impact properties, it rather had a negative effect on mechanical and impact properties. The results of mechanical testing indicated that triacetin will change the fibre structure making the fibres more brittle because all mechanical properties were strongly decreased by the use of triacetin. The GPC analysis shows that the PLA was not degraded due to the compounding process and incorporation of flax fibres. The possibility to use conventional manufacturing processes is a very important factor for industrial use of renewable materials. In this case PLA/flax composites did not show any difficulties in the extrusion and compression moulding processes and they can be processed in a similar way as PP based composites. Fig. 9. DMTA run for PLA/flax composites with 10% triacetin: (a) dynamic modulus, (b) tan delta. triacetin resulted in increased tan delta temperature for the composites. Tan delta was increased from 55 Cto70 C which indicates some kind of interaction effect between the fibres and PLA due to the triacetin. It is possible that triacetin will act as a compatibilizer for the PLA/flax system. Further, the peak is moved to the higher temperatures and also broadens for cold crystallised sample. Acknowledgements This work was funded by Vinnova, The Swedish Agency for Innovation Systems. Runar La ngstro m, SICOMP AB, is gratefully acknowledged for performing the composite manufacturing and mechanical testing and Maaria Sela ntaus, Fortum Oil and Gas, is gratefully acknowledged for doing the GPC analysis. 4. Conclusions The objective of this study was to investigate if PLA can be used as matrix in composite systems where natural fibres are used as reinforcements. The preliminary results show that PLA works very well as matrix material for natural fibre composites. The mechanical properties of PLA and flax fibre composites are promising. The composite strength is about 50% better compared to similar PP/flax fibre composites, which are used today in many industrial applications. The stiffness of PLA is increased from 3.4 to 8.4 GPa with an addition of 30 wt.% flax fibres. Generally these results together with earlier reported by DLR [11,12] indicates that PLA natural fibre composites have mechanical properties high enough for use instead of conventional thermoplastic composites. References [1] Hornsby PR, Hinrichsen E, Tarverdi K. Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres, part II analysis of composite microstructure and mechanical properties. J Mater Sci 1997;32: [2] Oksman K, Wallstrom L, Berglund LA, Filho RDT. Morphology and properties of unidirectional sisal-epoxy composites. J Appl Pol Sci 2002;84: [3] Oksman K. High quality flax fibre composites manufactured by the resin transfer moulding process. Journal of Reinforced Plastics and Composites 2001;20(7): [4] Heijenrath R, Peijs T. Natural-fibre-mat-reinforced thermoplastic composites based on flax fibres and polypropylene. Ad Comp Let 1996;5(3):81 5. [5] Oksman K. Mechanical properties of natural fibre mat reinforced thermoplastics. Appl Comp Mat 2000;7: [6] Mieck K-P, Nechwatal A, Knobeldorf C. Potential applications of natural fibres in composite materials. Melliand Textilberichte 1994;11: [in English].

8 1324 K. Oksman et al. / Composites Science and Technology 63 (2003) [7] Sanadi AR, Cauldfield DF, Rowell RM. Reinforcing polypropylene with natural fibers. Plastic Engin 1994;4:27 8. [8] Bledzki AK, Reihmane S, Gassan J. Properties and modification methods for vegetable fibers for natural fiber composites. J Appl Pol Sci 1996;5: [9] Williams GI, Wool RP. Composites from natural fibres and soy oil resin. Appl Comp Mat 2000;7: [10] Cyras VP, Innace S, Kenny JM, Vazques A. Relationship between processing and properties of biodegradable composites based on PCL/starch matrix and sisal fibres. Polymer Composites 2001;22(1): [11] Riedel U, Nickel J. Natural fibre-reinforced biopolymers as construction materials-new discoveries. Die Angewandte Macromoleculare Chemie 1999;272: [12] Herrmann AS, Nickel J, Riedel U. Construction materials based upon biologically renewable resources from components to finished parts. Polymer Degradation and Stability 1998;59: [13] Mishra S, Tripathy SS, Misra M, Mohanty AK, Nayak SK. Novel eco-friendle biocomposites: biofiber reinforced biodegradable polyester amide composites fabrication and properties evaluation. J Reinforced Plastics and Composites 2002;21(1): [14] Meinander K, Niemi M, Hakola JS, Selin J-F. Polylactides degradable polymers for fibres and films. Macromol Symp 1997; 123: [15] Chartoff RP. Thermoplastic polymers. In: Turi EA, editor. Thermal characterization of polymeric materials, vol. 1, 2nd ed. Academic Press; 1997 [chapter 3].