STUDY OF CRYSTALLIZATION OF POLYLACTIC ACID COMPOSITES AND NANOCOMPOSITES WITH NATURAL FIBRES BY DSC METHOD

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1 STUDY OF CRYSTALLIZATION OF POLYLACTIC ACID COMPOSITES AND NANOCOMPOSITES WITH NATURAL FIBRES BY DSC METHOD Luboš BĚHÁLEK, Miroslava MARŠÁLKOVÁ, Petr LENFELD, Jiří HABR, Jiří BOBEK, Martin SEIDL Technical University of Liberec, Studentská 2, Liberec, Abstract This paper deals with the evaluation of crystalline structure and thermal properties of injection molded parts based on polylactic acid (PLA) matrix reinforced by natural fibres (NF). Using differential scanning calorimetry (DSC) were evaluated thermal properties and crystalline structure of PLA/NF composites, depending on the amount, size and type of natural fibres. We observed that some natural fibres (e.g. coir and banana fibres) work like natural nucleating agents and thus subsequently improve the material properties of PLA. We also observed process of melt (primary) and cold (secondary) crystallization of PLA/NF composites depending on cooling rate. Keywords: Biocomposites, Nanocomposites, Crystallization, Natural fibres, Natural nucleating agents, Polylactic acid 1. INTRODUCTION Polylactic acid (PLA) is a linear aliphatic thermoplastic polyester, produced from renewable resources. PLAs and also other biopolymers offer environmental benefits such as biodegradability, greenhouse gas emissions, and renewability of the base material. PLA can be commonly synthesized by ring-opening polymerization of lactide or by direct polycondensation of lactic acid. Used lactic acid monomers are obtained from the fermentation of sugar feed stocks. Based on the stereoregularity (L and D) of lactic acid, L, D and D-L type lactides can be used in PLA production. An equimolar mixture of D and L isomer results in D-L lactide (so called meso - lactide). It is well established that the properties of polylactides vary to a large extent depending on the ratio and the distribution of the two isomers [1]. Generally, commercial PLA grades are copolymers of poly(l-lactic acid) (semi-crystalline PLLA) and poly(d,l-lactic acid) (amorphous PDLLA), which are produced from L-lactides and D,L-lactides, respectively. For a commercial PLA a blend of a higher amount of L-lactide and a lower amount of D-lactide is used [2], [3]. The ratio of L- to D,L-enantiomers is known to affect the properties of PLA, such as the melting temperature, degree of crystallinity and even the degradation characteristics [3]. Semi-crystalline nature of PLA has been observed during cooling of melt. The crystallization behavior of polylactides depends on the thermal history (i.e. annealing) [4], [5], molecular mass [4], amount and type of additives [4, 6, 7] and stereo sequence distribution [1], [4]. When semi-crystalline PLA is processed, its crystallinity changes depended on the cooling rate [5]. Miyata and Masuko studied the non-isothermal crystallization of PLLA at various cooling rates ranging from 0.5 to 20 C min -1. The amount of crystallinity developed in PLLA depended closely on cooling rate. Samples cooled at rates higher than 10 C min -1 could not crystallize and remained amorphous [8]. Typically during injection molding, due to the high cooling rates, totally amorphous PLA products could be produced. In the differential scanning calorimetry (DSC) an exotherm curve of this amorphous PLA can be found, related to the cold crystallization above Tg, and an endotherm curve, related to the melting. When PLA is recrystallized prior to measurement, the exotherm in the DSC curve is not present. Recrystallized PLA products also carry a huge advantage compared to amorphous products. This advantage is manifested

2 in the much higher heat deflection temperature (HDT); while amorphous PLA products have an HDT of 50 C, semi-crystalline PLA products have an HDT of 130 C. Although PLA can be recrystallized after processing into a product, the products shrink and significant warpage occurs during recrystallization due to the increasing crystallinity [4]. 2. EXPERIMENTAL 2.1 Preparation of PLA/NF composites and nanocomposites We prepared the (nano)composites by adding the coir or banana fibres into polylactic acid matrix NatureWorks biopolymer 3251D within ratio 10, 20 and 30 (wt. %) into the extrusion line with twin screw (Fig. 1), temperature profile of 180 C up to 200 C and cold granulation on the end of the line. For the composite material preparation it was necessary to remove fine organic impurities and greases from fibres and to modify their length by milling at the shear mill (n = 3000 min -1 ) with trapezoidal holes of 0.75 mm (the length of milled fibres was within 0.5 mm up to 2 mm). The same method was used for preparing of nanocomposites with coir reinforcement. Ratio of coir s nanoparticles were 2, 4 and 6 (wt. %). Size of nanoparticles: 790 nm 15 %. We prepared nanoparticles by instrument FRITSCH PULVERISETTE 7 premium line. Milled fibres were than dried. Fig. 1 Scheme of technological process for the preparation of NF composites From these granulates of NF composites were for the subsequent evaluation of physical and thermal composites properties injected on injection molding machine the multipurpose test specimen according to ISO DSC characterization of PLA/NF composites and nanocomposites Study of crystallization and thermal analysis was carried out with a DSC 6 Perkin Elmer which was calibrated against an indium standard. An empty aluminium pan was the reference. For injection moulding thermal history evaluation in relation to the amount of natural fibres, the samples amount was (10 0,2) mg and analysis run at 10 C min -1 heating/cooling ramp in one heating-cooling cycle in a nitrogen atmosphere (flow rate 50 ml min -1 ). From the DSC thermograms (Fig. 3, 4) we studied effect of natural fibres on crystallization of PLA and thermal properties such as exothermic cold crystallization temperature (TCC), endothermic

3 melting temperature (Tm), melt crystallization temperature (TC), cold crystallization enthalpy ( HCC), melting enthalpy ( Hm), melt crystallization enthalpy ( HC) and degree of crystallinity ( C). The degree of crystallinity ( C) of PLA injection moulding composites/nanocomposites samples can be calculated based on the enthalpy value of a 100 % crystalline PLA [1] from the following equation: χ C = H m H CC [%] (1) where Hm is the melting enthalpy, HCC is the cold crystallization and value 93 is the melting enthalpy in J.g -1 of totally crystallized PLA sample [1]. 2.3 Crystallization of PLA composites and nanocomposites - influence of natural fibres DSC thermograms of PLA and PLA composites/nanocomposites with 10 wt. % up to 30 wt. % natural fibres or 2 wt. % up to 6 wt. % nanoparticles of natural fibres are summarized in the Table 1. Example of DSC thermograms of PLA and PLA composites with 10 wt. % up to 30 wt. % banana fibres are shown in Fig 3. Table 1 Effect of natural fibres on crystallization of PLA Polymers TCC1 HCC1 TCC2 cold (secondary) crystallization HCC2 Tm melting Hm TC HC melt (primary) crystallization PLA natur none none 8.7 PLA + 10CF none none 8.6 PLA + 20CF PLA + 30CF PLA + 10BF none none 13.2 PLA + 20BF PLA + 30BF PLA + 2nCF none none 10.9 PLA + 4nCF none none 11.1 PLA + 6nCF Note: CF = coir fibres; BF = banana fibres; ncf = nanoparticles of coir fibres C [%] Coir and banana fibres which were added to PLA matrix increased crystallization abilities of the injected parts. At the pure PLA polymer during cooling rate 10 C.min -1 was not observed sharply defined phase transformation associated to melt crystallization of polymer, in contrast to PLA composites reinforced by coir and banana fibres. During heating of the injected part from PLA (heating ramp 10 C min -1 ) there was observed the glass transition temperature Tg 59 o C and cold crystallization within the temperature interval 80 C to 120 C (TCC1) and further within the temperature interval 150 C to 159 C (TCC2). Such cold crystallization of material is a reason for rapid cooling of the melt in the mould cavity during the injection process. It influencing negative not only the structure change and material properties but also in shape and dimensional injected part instabilities (under temperature over 80 C). It was proved that adding the banana and the coir fibres/nanoparticles to the PLA matrix increases the melt crystallization and decreases the cold crystallization enthalpy which also moved to the lower temperatures (with increasing amount of fibres in the PLA matrix is temperature shift higher - see Table 1 and Fig. 3). By adding 30 wt. % of coir or banana fibres and by adding 6 wt. % of coir nanoparticles there is decrease of real injected parts cold crystallization ( HCC1) almost by 25 % or 30 % or 21 %, respectively.

4 Fig. 3 DSC results for PLA and PLA composites reinforced with banana fibres Fig. 4 DSC results for PLA nanocomposites reinforced with nanoparticles of coir fibres 2.4 Crystallization of PLA composites and nanocomposites - influence of fillers and cooling rate At the second experimental phase in which we have studied influence of the cooling rate on the composites crystallinity, the samples were analyzed at a 10 C min -1 heating ramp and 10, 5 or 3 C min -1 cooling ramp in two heating cycles in a nitrogen atmosphere. In the first cycle, PLA composites samples were heated from 25 C to 190 C with 10 C min -1 heating ramp and isothermal stabilized for 1 min; cooled back to 25 C with 10, 5 or 3 C min -1 cooling ramp. In the second cycle, PLA samples were again heated from 25 C to 190 C

5 with 10 C min -1 cooling ramp. Example of DSC thermograms of PLA nanocomposites reinforced by 6 wt. % nanoparticles of coir fibres subjected to different cooling rates (10, 5 and 3 C min -1 ) are shown in Fig 4. Thermal properties of PLA composites and effect of natural fibres and cooling rate on crystallization of PLA are summarized in Table 2 and Table 3. Table 2 Effect of natural fibres and cooling rate on temperature characteristics of PLA composites heating/cooling/ heating rate [ o C min -1 ] TCC1/1 TCC2/1 PLA composites with 30 wt. % banana fibres (BF) Tm/1 TC TCC1/2 TCC2/2 first heating cycle cooling second heating cycle 10/10/ /5/ none none /3/ none none PLA nanocomposites with 6 wt. % nanoparticles of coir fibres (ncf) 10/10/ /5/ /3/ none Tm/2 Table 3 Effect of natural fibres and cooling rate on crystallization of PLA composites heating/cooling/ heating rate [ o C min -1 ] HCC1/1 H CC2/1 H m/1 PLA composites with 30 wt. % banana fibres (BF) HC HCC1/2 HCC2/2 Hm/2 first heating cycle cooling second heating cycle 10/10/ /5/ none none /3/ none none PLA nanocomposites with 6 wt. % nanoparticles of coir fibres (ncf) 10/10/ /5/ /3/ none C [%] From DSC thermograms, Table 2 and Table 3 is obvious that at lower cooling rate there is increasing portion of the melt crystallization of composite material, which starts at higher temperature (area C in the Fig. 4), the degree of crystallinity is also increasing and there is decrease of the cold crystallization (area CC1/2 and CC2/2) on the contrary. Final degree of crystallinity for PLA composite is within the interval 9 % till 48 % according to the cooling rate, type and amount of fibres in matrix. Areas of cold crystallization disappeared not until the cooling rate 3 C min -1 for nanocomposites with the nanoparticles of coir fibres, while for composites with the banana fibres they disappeared already under cooling rate 5 C min -1. Cold crystallization for PLA injected parts can be eliminated by lower cooling rate of melt and also by adding plant fibres. 3. CONCLUSION Application of natural fibres represents the very important material replacement. With regard to the environmental point of view, natural fibres can replace synthetic fibres in the composite materials branch. The similar reality concerns the polymers from the renewable resources (bioplastics) which have became a very attractive material during last years. There is mainly reinforced effect [9] - [11] by adding natural fibres to the synthetic or biodegradable polymer matrix but their another benefits, in comparison to the other fibrous

6 materials, rest in their low weight, low abrasion, combustibility, biodegradability, non-toxicity, and mainly their low price which does not depend on the oil price [12], [13]. Some natural fibres (e.g. banana or coir fibres) in the biopolymer matrix can also influence routine and kinetics of the crystallization. In both cases were proved that adding banana and coir fibres to the PLA matrix increases the degree of crystallization but mainly also lowers the cold crystallization which slips to the lower temperatures or more precisely it lowers its temperature interval. The cold crystallization occurs at material as a result of its intense cooling and it is a reason for shape and dimensional injected part instabilities. The cold crystallization and degree of crystallinity developed in PLA is truly depended closely on the cooling rate. All samples with the cooling rate higher than 5 C min -1 reveal the cold crystallization at subsequent heating of material in the case of PLA composites with 30 wt. % banana fibres, in the case of coir nanoparticles then always under the cooling rate higher than 3 C min -1. PLA composite injected part (multipurpose test specimen according to ISO 3167 prepared according to ISO 294) with 30 wt.% of banana fibres or 6 wt.% nanoparticles of coir fibres reveals the degree of crystallinity ca. 14 % or 12 %, at cooling by 5 C min -1 there is the degree of crystallinity 43% for the banana fibres, 24% for the hemp fibres respectively. We observed that natural fibres work like natural nucleating agents. Owing to this behaviour is possible not only to improve material properties by using natural fibres like reinforcement, but also to improve the composite properties with usage of natural fibres like nucleating agents. We observed different nucleation activity of the coir and banana fibres. It can be dedicated to the different surface structure of the fibres. Thus implementation of different natural fibres could lead to different nucleation activity according to the fibre surface structure. ACKNOWLEDGMENT This paper was written with research project TA support. REFERENCES [1] MOHANTY, A.K. et al. Natural Fibers, Biopolymers and Biocomposites. 1CRC Press, [2] MARTIN, O., AVÉROUS, L. POLY(lactic acid): plasticization and properties of biodegradable multiphase systems. Polymer, 2001, vol. 42, pp [3] TÁBI, T. et al. Crystalline structure of annealed polylactic acid and its relation to processing. express Polymer Letters, 2010, vol. 4, no. 10, pp [4] AHMED, J. et al. Thermal properties of polylactides, Effect of molecular mass and nature of lactide isomer. Journal of Thermal Analysis and Calorimetry, 2009, vol. 95, pp [5] DI LORENZO, M.L., SILVESTRE, C. Non-isothermal crystallization of polymers. Progress in Polymer Science, 1999, vol. 24, pp [6] KE, T., SUN, X. Melting Behavior and Crystallization Kinetics of Starch and Poly(lactic acid) Composites. Journal of Applied Polymer Science, 2003, vol. 89, pp [7] LI, H., HUNEAULT, M.A. Effect of nucleation and plasticizion on the crystallization of poly(lactic acid). Polymer, 2007, vol. 48, pp [8] MIYATA, T., MASUKO, T. Crystallization behaviour of poly(l-lactide). Polymer, 1998, vol. 39, pp [9] OKSMAN, K. et al. Natural fibres as reinforcement in polylactic acid (PLA) composites. Composites Science and Technology, 2003, vol. 63 pp [10] YU, T. et al. Preparation and properties of short natural fibre reinforced poly(lactic acid) composites. Transactions of Nonferrous Metals Society of China, 2009, vol. 19, pp [11] SINGHA, A.S., THAKUR, V.K. Mechanical properties of natural fibre reinforced polymer composites. Bulletin Material Science, 2008, vol. 31, pp [12] KOZLOWSKI, R. Handbook of natural fibres. Processing and applications (Vol. 2). Woodhead Publishing, [13] MUKHOPADHYAY, S. et al. Banana Fibers - Variability and Fracture Behaviour. Journal of Engineered Fibersand Fabrics, 2008, vol. 3, pp