HAp-PLA

Composites: Part A 40 (2009) 404–412 Contents lists available at ScienceDirect Composites: Part A journal homepage: www.elsevier.com/locate/compositesa Mechanical properties of PLA composites with man-made cellulose and abaca ﬁbres Andrzej K. Bledzki a,*, Adam Jaszkiewicz a, Dietrich Scherzer b a b Institut für Werkstofftechnik, Kunststoff- und Recyclingtechnik, University of Kassel, Mönchebergstr. 3, 34109 Kassel, Germany BASF SE, Global Polymer Research – Biopolymers, 37056 Ludwigshafen, Germany a r t i c l e i n f o a b s t r a c t PLA biocomposites with abaca and man-made cellulose ﬁbres were processed by using combined moulding technology: two-step extrusion coating process and consecutively injection moulding. By adding 30 wt% of man-made cellulose, the Charpy impact strength at ambient temperature increased by factor 3.60, compared to unreinforced PLA. Tensile strength rose by factor 1.45 and stiffness by approx. 1.75. Reinforcing with abaca ﬁbres (30 wt%) enhanced both E-Modulus and tensile strength by factor 2.40 and 1.20, respectively. The Charpy A-notch impact resistance of PLA/abaca could be improved by factor 2.4. SEM photographs show ﬁbre pull-outs from the polymer matrix. The ﬁbre orientation was analysed via optical microscopy. The after-process ﬁbre length was signiﬁcantly affected already during compounding process. Ó 2009 Published by Elsevier Ltd. Article history: Received 10 July 2008 Received in revised form 19 December 2008 Accepted 2 January 2009 Keywords: A. Polymer-matrix composites (PMCs) B. Mechanical properties E. Extrusion E. Injection moulding 1. Introduction 1.1. Motivation By using biopolymers as a composite matrix some common composites, the matrix of which is made of crude oil, can be substituted. However, to achieve high mechanical parameters, the biogenous matrix needs to be reinforced. By using natural ﬁbres only, a full ‘‘biocomposite” can be produced. One of the main advantages of natural ﬁbres (NF) is their ‘‘light weight potential”. Natural ﬁbres have a signiﬁcant lower density than glass ﬁbres (GF), which allows the construction of lighter parts, compared to parts reinforced with GF or ﬁlled with minerals. Polylactide has still not found any meaningful market acceptance as an engineering resin, because of its non-satisfying impact resistance and low heat distortion temperature. Therefore, manufacturing of PLA light-weight parts with a high impact resistance would lead to new application ﬁelds, e.g. automotive or electrical industry. In both sectors the major engineering resin is still the PP based composite, as its cost is low and properties are good enough. Adding new biocomposites to these existing and well working material systems requires speciﬁc fundamental basics, for example competitive prices and comparable or improved mechanical properties. Therefore, material data of biocomposites, which are in competition to PP equivalents, should reference to adequate values of polypropylene reinforced with natural ﬁbres (PP NF). For this purpose a choice of tailor-made reinforcing ﬁbres is of dominant importance. Although natural ﬁbres showing potential as reinforcement for composites, their properties vary depending on several factors, such as species, grade, harvest quality and yield, etc. Only natural ﬁbres of technical quality guarantee sufﬁcient reproducibility of mechanical characteristics. Hence, composites described in this paper were reinforced with abaca ﬁbre grade, which is used in PP compounds for under-ﬂoor covering of the Mercedes A Class. Man-made cellulose ﬁbres from Cordenka are well-known highquality technical textiles in the automobile industry (tyre cord in high-speed tyres). The objective of this investigation was to show the potential of PLA short-ﬁbres biocomposites reinforced with cellulosic ﬁbres of such technical quality in reference to equivalent PP NF composites. In order to present comparable and reproducible results only injection moulded specimens were investigated. 1.2. Biopolymers Permanent crude oil price stagﬂation as well as the ecological aspect of both production and disposal of standard oil-based plastics are presently two of the main concerns worldwide. Substitute materials can be novel, renewable biopolymers. In comparison to other common polymer groups the progress of biopolymers R&D, as well as their production have been the fastest for several years [1–7]. The production capacity and the amount of manufacturers and processors have also increased signiﬁcantly [8–13]. State of art within the biogene thermoplastic group is the starch-based lactic family or other bio-polyesters, like * Corresponding author. Tel.: +49 561 804 3690; fax: +49 561 804 3692. E-mail address: [email protected] (A.K. Bledzki). 1359-835X/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.compositesa.2009.01.002 A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 405 poly(hydroxyalkanoates) (PHA) synthesised by microorganisms. Other examples are thermoplastic starch and cellulose derivatives. At present only few groups of the mentioned biopolymers are of market importance [14–18]. The principal reason for the neglected importance is their price level, which is not yet competitive – current turnover in this ﬁeld is not really proﬁtable due to high synthesis costs, unsatisfactory production capacity, poor and not transparent regulations. Until now biopolymers found market acceptance in the packaging and medicine sector, but they are still underestimated as engineering resins due to the poor thermo-mechanical properties. Engineering resins are characterised by a very high stiffness and strength as well as long-term behaviour under circular loading. To achieve these valuable parameters a reinforced matrix is required. Indeed, most of the common technical non-reinforced resins are unable to meet all requirements. Additional disadvantages of these materials are their price level and ﬁnally recycling, particularly when being reinforced with glass ﬁbres. Nearly all substantial requirements can be satisﬁed by renewable resource based materials with natural ﬁbres [19]. 1.3. Abaca and man-made cellulose ﬁbres The abaca ﬁbre is a vegetable ﬁbre that grows rapidly. It belongs to the family of the abaca-banana tree ‘‘musa textilis”. The Philippines are the worldwide leader in the production of abaca ﬁbres with 85% of the market share. The abaca ﬁbres are used in many technical applications [20,21], mostly because of their high tensile properties [22]. Cellulose is a basic component of plants, thus it is a nearly inexhaustible source of raw material. Man-made or ‘‘regenerated” cellulose ﬁbres are produced in chemical–technical viscose processes, in which raw cellulose is modiﬁed into cellulose ﬁbres and derivatives [23,24]. In the last years options for reinforcement with natural and cellulose spun ﬁbres have also been intensively researched, especially regarding common polymers like PP, PVC or PE [25,26]. The idea of the development deals with the combination of two inexpensive commodity materials, polypropylene and natural ﬁbres or Rayontyre cord [27]. By combining them into a polymer composite, production of stronger components can be achieved. In addition, it would enter the application ﬁeld of engineering materials, such as PP-GF or PC/ABS [19,28]. 1.4. Biocomposites on PLA basis PLA biocomposites have been studied for numerous research groups [29–34]. Pluta et al. investigated PLA/montmorillonite micro- and nanocomposites, and showed that the microcomposites have formed a phase-separation between the matrix and reinforcement. Furthermore, nanocomposites can be very easily processed by using nanoﬁllers; however, the biodegradability of the composite was affected. Moreover, the thermal stability in oxidative atmosphere of PLA/montmorillonite could be improved [29]. The inﬂuence of chopped glass and recycled newspaper in PLA composites was assayed by Huda et al. With addition of 30 wt% of both ﬁbres the stiffness rose from 3.3 GPa (origin PLA) to 5.4 GPa by recycled newspaper ﬁbres and to 6.7 GPa by glass ﬁbres. The tensile strength was improved from 62.9 MPa (origin polymer) to 67.9 and 80.2, respectively [30]. Also wood polymer composites (WPC) on PLA basis were studied [31]. With addition of different contents of wood ﬂour (WF) the ﬂexural strength improvement from 98.8 to 114.3 MPa could be observed. The modulus of elasticity increases proportionately as the amount of WF increases. However, the thermal stability decreases with the increase of WF. A study on PLA composites with microcrystalline cellulose (MCC) [32] briefed that both PLA/WF and PLA/wood pulp (WP) have better mechanical properties than PLA/MCC. This is due to the poor interfacial adhesion of PLA and MCC, which was observed via scanning electron microscopy. Also a higher level of matrix crystallinity of PLA/WF and PLA/WP as PLA/MMC could be seen. It was considered as a reason for the improved stiffness, if compared to PLA/MCC. The inﬂuence of cellulose reinforcement on crystallinity of PLA composites was studied in later paper by Mathew et al. [33]. They showed shown that MCC has better potential as nucleating agent and causes higher degree of crystallinity like WF or cellulose ﬁbres which were investigated. Biocomposites of PLA/abaca were investigated by Shibata et al. [34]. In this research three different kinds of chemical treatment were used: esteriﬁcation, mercerization, cyanoethylation. It has been shown that for reinforced PLA the ﬂexural strength could not be improved signiﬁcantly, even if the ﬁbres were treated. In contrast the stiffness increases continuous from approx. 3.6 to 6.0 GPa, depending on the ﬁbre content and treatment method. The biodegradation rate of PLA/untreated abaca ﬁbre (90/10) was much higher, as the neat PLA and PLA/esteriﬁcated abaca ﬁbre (90/10). Other natural ﬁbres used as reinforcement for PLA composites were jute [35], ﬂax [36] and kenaf [37]. The reinforcing effect of most of them brought an increase in stiffness and sometimes in strength. The ductility of the composites was overall worst, which resulted in lower impact strength. To improve this drawback, some other ﬁbrous reinforcement must be used. However, to keep the concept of ‘‘biocomposites”, the reinforcement should be obtained from renewable raw materials, for example a viscose ﬁbre made of native cellulose (man-made cellulose) [26]. Ganster and Fink [28] published results of PLA/man-made cellulose in comparison to PP composites. They reported that with addition of 30 wt% of regenerated cellulose an improvement of most mechanical characteristics can be achieved. The unnotched and notched Charpy impact strength can be improved by factor 3.8 and 2.0, respectively. The modulus and strength increase to 150% of origin PLA. Generally in research papers about bioplastics and their composites, the most described processing methods are compression moulding, mixing or other, non-common techniques for thermoplastic polymers. However, the injection moulding is in common use for thermoplastics; this technology was underestimated for short-ﬁbre biocomposites and was not systematically investigated [25,38]. 2. Experimental part 2.1. Materials The tested composites consist of polylactide matrix with natural and man-made ﬁbres. The components were prepared at a weight ratio of 70/30 (polymer/ﬁbre). The matrix used is PLA 4042D, Mw = 166,000, MVR = 5.7 (190 °C/2.16 kg) manufactured by NatureWorks LLC/USA. Abaca ﬁbres from Manila Cordage (with the friendly assistance of Rieter Automotive) were processed as endless ﬁbres (ﬁlament yarn). The density of the ﬁbre is 1.5 g/cm3, single ﬁbre diameter is 10–30 lm, tensile strength over 900 MPa [39]. The ﬁbre quality owing to FIDA (Fiber Industry Development Authority) is S3. Manmade cellulose ﬁbre (CordenkaÒ 700 Super3) from the company Cordenka/Germany, was delivered as industrial yarn; linear density (nominal) dtex = 2440, number of ﬁlaments 1350, breaking force 128.6 N, single ﬁbre diameter 12 lm. All ﬁbres were used as delivered, without any chemical treatment; for PLA composites no coupling agent was used. 406 A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 2.2. Fabrication of composites The compounds were processed by using a special coating technique. In a ﬁrst step the PLA matrix was compounded together with endless ﬁbres via a coating die and cooled to ambient temperature (twin-screw extruder from Haake, Rheomex PTW 25/32, L/ D = 32, D = 25 mm). The ﬁbre cord was pre-dried online (umpteen seconds at 170 °C). Further, the pellets were dried and compounded on single-screw extruder (Schwabenthan, Polytest 30P, L/D = 25, D = 30 mm) and again processed into pellets (cut length 15 mm) (Fig. 1). The melt temperature in the coating was of approx. 200 °C, screw speed was set at 100 rpm. Process temperature during compounding was approx. 180 °C, at 20 rpm screw speed. To prevent hydrolysis and pores’ formation, the matrix polymer as well as pellets were dried in a convection oven with fan before further processing (moisture content after drying at 80 °C/16 h: PLA 6 0.02%; composite pellets 6 0.2%). To compare the properties with common composites, all values are referenced to PP homopolymer (Sabic/Germany, PP 575P) and its composites (with the addition of 5 wt% of MAH-PP coupling agent relate to the ﬁbre weight) that were processed separately by using same processing methods. The standard test specimens according to DIN EN ISO 527 (‘‘dog-bone” tensile test specimen with shoulders), were produced using an injection moulding machine Klöckner Ferromatik FM 85 at a temperature of approximately 180 °C in the melt, injection pressure was about 500 bar and injection speed was 200 mm/s. Additionally, because of the strongly hydrophilic nature of cellulosic ﬁbres, the injection moulding machine’s hopper was heated to 80 °C and rinsed with nitrogen gas with approx. 4 l/min (N2 P 99.999 vol %, Air Liquide Deutschland, Düsseldorf/ Germany). The test samples according to DIN EN ISO 178 and 179-II (ﬂexural and Charpy tests) were cut out according to DIN EN ISO 2818 from the injection moulded standard test specimens. 2.3. Methods 2.3.1. Mechanical testing The mechanical characteristics were established on a universal mechanical testing machine (Zwick/Roell 1446), using a quasi-statically ﬂexural and tensile test in line with DIN EN ISO 178 and 527. Charpy A-notch impact test was performed on a Zwick/Roell impact-machine due to DIN EN ISO 179-II. All presented results are the average values of 10 measurements. 2.3.2. Dynamic mechanical analysis (DMA) The storage modulus, loss modulus and loss factor were measured as a function of temperature (from 0 to 90 °C). In this paper only storage and loss modulus are presented. A TA Q800 Dynamical Analyser with a dual-cantilever bending ﬁxture in Multi-Frequency-Strain modus was used. The frequency of 1, 3 and 10 Hz at the amplitude of 10 lm and the heating rate of 3 K/min were investigated. The geometry of the sample was 35 Â 10 Â 4 mm. 2.3.3. Optical and scanning electron microscopy To test the ﬁbre/matrix adhesion and composites morphology, scanning electron microscopy (SEM) and optical microscopy (OM) investigations were undertaken. The SEM fracture surfaces were prepared in a cryo-break process (the ‘‘dog-bone” test specimens were stored in liquid nitrogen for at least 10 min, after that removed and immediately broken). A CamScan MV 2300 scanning electron microscope with a wolfram cathode emission gun and acceleration voltage of 20 kV was used. Optical microscopy specimens were cut out from ‘‘dog-bone” samples and incorporated into transparent epoxy resin. After cross-linking, the specimens were ﬁne polished and the surfaces were observed under Zeiss Axioplan optical-microscope. 2.3.4. Evaluation of ﬁbre length To analyse the ﬁbre length of pellets after compounding as well as injected dog-bone specimens, composites were dissolved in dichloromethane (DCM) at ambient temperature. Single ﬁbres Fig. 1. Depicting of two-step extrusion principle (above) with successive injection moulding (below). A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 407 were extracted from whole investigated volume and their length was measured under the Zeiss Axioplan optical microscope. Approximately 500 ﬁbres were measured. 3. Results and discussion 3.1. Mechanical properties Fig. 2 shows tensile and ﬂexural strength results of tested composites. It can be observed that composites reinforced with manmade cellulose ﬁbres achieve the highest strengths of around 92 MPa in tensile and 152 MPa in ﬂexural test. The origin PLA shows tensile strength of 63 MPa and modulus of 3400 MPa. With the addition of 30 wt% of man-made cellulose ﬁbres, an improvement of tensile strength by factor 1.45 and tensile modulus by 1.70 can be achieved (Fig. 3). In the ﬂexural test an upgrade in strength is even higher and it can be improved by factor 1.50. The ﬂexural modulus’s change is the same as in tensile test (factor 1.7). Oksman et al. [36] investigated the inﬂuence of natural ﬁbre reinforcement on mechanical properties of compression moulded PLA composites. They showed that by the addition of 30 wt% of ﬂax ﬁbre tensile strength increases slightly. The ﬁbre content of 40 wt% decreases the tensile stress. The modulus increase was almost exact as in our research, from approx. 3.7 to 8 GPa at matrix/ﬁbre ratio 70/30, and it increases with increasing ﬁbre load. Abaca ﬁbres cause in general better stiffness (improvement in tensile modulus by factor 2.4, ﬂexural modulus by factor 2.1) (Fig. 3), than man-made cellulose. In comparison to unreinforced polymer, abaca ﬁbre improves mechanical characteristics. However, this increase is not as signiﬁcant as by using man-made cellulose ﬁbre, where the tensile strength rise up to 74 MPa (improvement by factor 1.2) and ﬂexural strength up to 124 MPa (factor 1.15). The ductility of each composite decreases, if compared to native PLA. Shibata et al. [34] reported that with the increasing abaca ﬁbre content the modulus enhancement is linearly. This result is similar to our observations and the values of stiffness are also comparable. However, the strength was not improved, even though several ﬁbre treatments were tested [34]. The unchanged composites’ ﬂexural strength can be explained with other materials preparation procedure, where the compound was agglomerated on twin-rotary mixer and not like in this research in two-step extrusion process. Also moisture content of the ﬁbres and composites was not described, what in case of aliphatic polyesters and hydrophilic ﬁbres can be arbitrative, e.g. in view of hydrolysis of polymer chain. Tensile [MPa] PLA composites 8000 Flexural PP composites 6000 4000 2000 3373 3690 8032 7890 5846 6510 1497 1318 4931 4192 4005 3661 0 PLA PLA/abaca PLA/cellulose PP PP/abaca PP/cellulose Fig. 3. Tensile and ﬂexural E-Modulus of PLA and PP composites (DIN EN ISO 524 and 178). +23°C [kJ/m²] PLA composites 12 -30°C PP composites 9 6 3 2,2 2,7 5,3 5,2 7,9 7,4 3,5 1,4 5,3 4,1 11,1 9,8 0 PLA PLA/abaca PLA/cellulose PP PP/abaca PP/cellulose Fig. 4. A-notch Charpy impact strengths of PLA composites (DIN EN ISO 179-2/1eA). Tensile [MPa] PLA composites 160 Flexural PP composites 120 80 40 63 109 74 124 92 152 29 42 44 72 72 104 0 PLA PLA/abaca PLA/cellulose PP PP/abaca PP/cellulose Fig. 2. Tensile and ﬂexural strengths of PLA and PP composites (DIN EN ISO 524/ 178). Fig. 4 represents results of Charpy A-notch impact tests, both at +23 °C and at À30 °C. It can be seen that with the addition of 30 wt% of man-made cellulose ﬁbres, A-notch impact strength at +23 °C increases by factor 3.60. The virgin PLA has impact strength of 2.2 kJ/m2, in contrast to 7.9 kJ/m2 of PLA/cellulose. Furthermore, at À30 °C this value remains almost without any change. Ganster et al. [28] tested composites of PLA/man-made cellulose and they showed a similar tendency. In our test the Charpy impact resistance increased by factor 3.60, compared to factor 2.0 of Ganster at al. [36]. They described a comparable improvement of unnotched samples. In general, the improvement of ﬁbrous reinforcement by Charpy is more signiﬁcant with unnotched specimens than with notched ones. So the enhanced value by factor 3.6 of A-notched samples that we achieved is especially noticeable. The signiﬁcant increase in impact strength can be referenced to much lower diameter (higher aspect ratio) and smooth surface of man-made cellulose ﬁbre, in comparison to the irregular abaca ﬁbre. It affects the ﬁbre/matrix interaction, whereby pull-outs appear more often with regenerated cellulose than with abaca ﬁbre. We suppose that for this reason the path length of initiated crack is being enlarged which increases the energy amount needed to break the sample (Fig. 5). The addition of 30 wt% of abaca ﬁbres leads to an improvement by factor 2.4 at +23 °C and 2.0 at À30 °C. Oksman et al. [36] presented a decreased Charpy impact strength of the PLA/ﬂax composites (60/40). This result is in opposite to our observation that the addition of 30 wt% of abaca ﬁbre enhanced the impact resistance signiﬁcantly. 408 A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 Reinforcing PLA with natural ﬁbres leads to better chemical bonding on the interphase as for PP composites without coupling agent. In addition PLA is from the nature very stiff, and contrasting to PP the impact resistance of PLA is very poor. 3.2. Dynamic mechanical properties The DMA investigation was performed to study the effect of ﬁbrous reinforcement incorporation on the dynamic-mechanical performance of native PLA and its composites. In this case the temperature range within 0 and 90 °C was taken under examination. The secondary crystallisation for PLA begins above 90 °C [41]. Fig. 6 shows the dynamic storage modulus curves of the origin PLA and PLA-based composites as a function of temperature. The storage modulus of PLA composites is much higher then of PLA. This is due to ﬁbre reinforcement, which allows better stress transfer from the matrix to the ﬁbre. The highest value of the dynamic modulus can be achieved for PLA/abaca, than for PLA/cellulose and PLA. The tendency corresponds to the statically tensile and bending test as well. The abaca ﬁbre from the nature is very stiff, where the man-made cellulose is more ﬂexible. This performance allows achieving a higher modulus of abaca reinforced composites and better impact resistance of cellulose ones. It can also be seen that below 70 °C native PLA characterises with the lowest storage modulus. In this temperature range PLA based composites have slightly higher values of the storage modulus. The ﬁbrous reinforcement affects the chain mobility. The composite remains stiffer and the polymer ﬂow is restricted. This effect can also be observed in changing of the glass transition temperature (Tg) derived from the loss modulus curves (Fig. 7). It is obvious that incorporating of ﬁbres enhances the Tg. The slightly shifting (few degrees) of Tg to higher temperature indicates an affected mobility of polymer chains. As the volume fraction of man-made cellulose is higher than abaca (because of the ﬁbre diameter), the Tg of PLA/cellulose is higher and amounts to approx. 68 °C, if compared to 66 °C of PLA/abaca and 64 °C for native PLA. It corresponds to the loss of storage modulus below 70 °C. Mathew and Averous et al. [32,42] discuss, that the shift to higher temperature usually indicates restricted molecule movement because of better interaction between Fig. 5. Scheme of initiated crack path: left, before break; right, after break. Composites of PP/abaca are deﬁned with increased values of notched impact strength as compared to origin PP. This is due to ﬁbre reinforcement and strong interfaces, which results from MAH-PP addition. Because of very good adhesion, ﬁbres are able to transfer stress better as by PLA composites. Moreover, PP is more ductile as PLA. In consequence PP composites characterise with higher impact values. All mechanical properties of PLA composites in contrast to polypropylene made with comparable processing conditions show much better results (Figs. 2 and 3). Exceptions are Charpy impact strength (Fig. 4) and tensile/bending elongation, where PP composites show slightly better values than PLA composites caused by their much higher ductility. The differences in mechanical properties are due to molecular structure of both, polypropylene and polylactide. Regarding to methyl group, polypropylene is a highly non-polar polyoleﬁn. For that reason the bonding between well polar cellulosic ﬁbres (–OH groups) and PP is very poor; however, it can be signiﬁcantly improved by adding coupling agents, e.g. maleic acid anhydride grafted PP copolymers. Basically, PP is very ductile, but only mediocre stiff. As a result, this polymer characterises with lower stiffness expressed via E-Modulus but improved toughness shown in impact strength. PLA is in opposite to polypropylene, aliphatic polyester that contains functional ester groups in the main chain. 6000 ––––––– –––– ––––– · PLA PLA-Abaca PLA-Cellulose Storage Modulus (MPa) 4000 3000 2000 2000 1000 0 60 0 0 20 40 60 80 Universal V4.1D TA Instruments 65 70 75 80 85 Temperature (°C) Fig. 6. Temperature dependence of storage modulus of PLA and PLA-based composites (f = 1 Hz). A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 1200 409 ––––––– PLA PLA-Abaca –––– ––––– · PLA-Cellulose 1000 800 Loss Modulus (MPa) 1000 800 600 600 400 400 200 0 60 65 70 75 80 85 200 0 0 20 40 60 80 Universal V4.1D TA Instruments Temperature (°C) Fig. 7. Temperature dependence of loss modulus of PLA and PLA-based composites (f = 1 Hz). the ﬁbre and the polymer matrix. Furthermore, the a-relaxation involves the movements of polymer chains. The presence of crystalline structures or reinforcement could also act as a physical crosslinking, decreasing the mobility of amorphous regions and consequently increasing the composite’s stiffness. 3.3. Morphology injection moulding. This can explain the higher values of ﬂexural strength, where the ﬁbre orientation is much more favourable, if the load direction is taking into consideration. Moreover, a ﬁne dispersion of ﬁbres in the polymer can be seen, due to two-step processing; it can be achieved by using an additional compounding step. 3.4. After process ﬁbre length The morphology of fracture surfaces was studied by SEM as shown in Fig. 8. The photographs indicate ﬁbre pull-outs at the scanned surface. Furthermore, abaca ﬁbres seem to be better coated with PLA matrix as man-made cellulose. Probably, this is due to the surface roughness, which is very smooth with regenerated cellulose and quite irregular with abaca ﬁbres. The adhesion is not strong in any of the composites. Because of different ﬁbre diameters (Abaca ﬁbres approx. 150 lm, cellulose ﬁbres of around 12 lm) the chosen magniﬁcation had to be different accordingly. The optical microscopy photographs (Fig. 9) show typical after-process ﬁbre orientation, which is elliptical and axially almost symmetric in reference to the melt ﬂow direction during Regarding mechanical properties of short-ﬁbres composites, the main attention should be paid to the ﬁbre length. This property is one of the key parameters in the composites fracture mechanic. While the ﬁbres’ length in compounds is not of major importance, their length in the composite can be the main factor determining failure mechanisms of the cross-section. Fibres’ length distribution in pellets after compounding and in samples after injection moulding is depicted in Fig. 10. The measurement was evaluated on approx. 500 ﬁbres. A signiﬁcant decrease in the ﬁbre length after compounding on the single-screw extruder can be observed, from the cut length of 15 mm to about 1.0–2.0 mm. Moreover, the injection moulded samples show only Fig. 8. SEM photographs of PLA/cellulose (500Â) and PLA/abaca (100Â). 410 A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 Fig. 9. Optical microscopy photographs of PLA/cellulose and PLA/abaca composites (magniﬁcation 17Â). an insigniﬁcant drop of length, if put side by side to pellets, which indicates to massive ﬁbre length reduction during extrusion process. Processing on the single-screw extruder already affected the length signiﬁcantly. Compounding on twin-screw extruders, which is often carried out in composites processing, can surpassingly inﬂuence the ﬁbre length even more. The minimal ﬁbre length is already being achieved during compounding and is not being affected during injection moulding process; however, the occurrence of the ﬁbre length seems to be more uniform. As a result, most of the ﬁbres, which remain longer than 2 mm after compounding, are being shortened below 2 mm during injection moulding. Critical ﬁbre length for PLA/abaca composites was determined according to Eq. (3.1) and amounts Lc = 1.843 mm. Lc ¼ rmax D f 2s ð3:1Þ Fig. 10. After process ﬁbre length of PLA/abaca 70/30 composites. A.K. Bledzki et al. / Composites: Part A 40 (2009) 404–412 411 where Lc, critical ﬁbre length; 1.843 mm, D, ﬁbre diameter; 0.271 mm, rmax , ﬁbre tensile stress at maximum; 489 MPa, s, interf facial or matrix shear strength; 35.96 MPa. s was determined according to Eq. (3.2); rmax m s ¼ pﬃﬃﬃ ¼ 0:58rmax m 3 ð3:2Þ rmax , matrix tensile stress at maximum; 62 MPa. m Fibre aspect ratio according to Eq. (3.3) amounts kc = 6.8 kc ¼ L c rf ¼ D 2s max ð3:3Þ From literature the range of abaca ﬁbre tensile stress is quite wide and includes values from 400 to 1100 MPa [16,34,40]. For that reason we determined this property by ourselves and calculated it to 489 MPa. The value was estimated in a tensile test of a single ﬁbre bundle from 30 measurements. To determine the ﬁbre diameter, an optical microscopy was undertaken. The average value was calculated from 140 measurements of the middle section of the ﬁbre and estimated at 0.271 mm. Hence, for calculation of ﬁbre crosssection in the tensile test, a value of 0.271 mm was used. Most of the tested ﬁbres have a measured ﬁbre length of 1.0– 1.5 mm, whereas the critical ﬁbre length is 1.8 mm. In such case the load cannot be transferred from the polymer matrix to the ﬁbre completely. As a result, tested specimens break too early. Moreover, the pull out mechanism is taking place. This scenario can be partly seen in SEM pictures. However, the theory assumes a perfect adhesion on the ﬁbre/matrix interphase (absence of any structure defects). A perfect adhesion cannot be achieved, especially if no coupling agent was used. This requirement could partially be fulﬁlled only for PP with natural ﬁbre reinforcement, in composites of which, the maleic acid anhydride (MAH) was applied. Specht [43] discussed the inﬂuence of MAH in PP NF composites. He showed that by adding small quantities of coupling agent a signiﬁcant enhancement of shear strength could be achieved. It indicates an improvement of ﬁbre/matrix interaction, which results in better adhesion on the interphase. Moreover, he showed that addition of maleic acid anhydride to PP matrix induces gentler processing. Consequently, decreased ﬁbres damage was observed; probably, because of lubricant effect of MAH in PP. cess ﬁbre orientation, which is almost perfectly symmetric due to the ﬂow direction of polymer melt. The ﬁbre dispersion is very ﬁne, because of the two-step processing. The investigated ﬁbre length was signiﬁcantly reduced during the compounding step on single-screw extruder already. The minimal ﬁbre length is already being achieved during compounding and is not being signiﬁcantly affected during injection moulding. Further research will focuses on improvement of ﬁbre/matrix interphase, e.g. by optimising the process parameters, using chemical ﬁbre treatment, or coupling agents. The after-process ﬁbre length and structure are very important, as both strongly inﬂuence the properties of the end-product. The process optimisation as regards more gentle process parameters or other screw geometry will be undertaken in the near future. Acknowledgements The authors express their appreciation to BASF, Ludwigshafen/ Germany, Rieter Automotive, Sevelen/Switzerland and IAP Potsdam-Golm/Germany for supplying the PLA polymer, abaca and man-made cellulose ﬁbres. References [1] Bioplastics Online. 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