Thermal, rheological and morphological properties of biodegradable blends based on poly (lactic acid)
and polycarbonate blends
Nadjat CHELGHOUM. Melia GUESSOUM. Nacerddine HADDAOUI Laboratoire de Physico-Chimie des Hauts Polymères (LPCHP)
Département de Génie des Procédés, Faculté de Technologie, Université Ferhat ABBAS Sétif1 Sétif (19000), Algérie
Abstract: Biopolymers are expected to be an alternative for conventional plastics due to the limited resources and soaring petroleum price which will restrict the use of petroleum based plastics in the near future. In this context, poly(lactic acid) (PLA) has attracted the attention of polymer scientists as a potential biopolymer to substitute the fossil fuel based polymers.
Even the huge advancements in PLA research, there is still many drawbacks that continue to limit its employment in some sectors which require particular mechanical and thermal properties. For this reason, blending with other polymers appears as an attractive strategy to overcome the PLA’s shortcomings and enlarge its application domains. Among possible PLA blends, its combination with polycarbonate (PC) presenting a high inherent thermal stability and an important tensile strength appears the more suitable path to overcome PLA brittleness and poor thermal resistance. Consequently, PLA/PC blends have received considerable attention in research because of their potential applications as friendly to the environment advanced packaging materials and for industrial applications.
The objective of this study was to prepare biodegradable materials based on a bio-based polymer, which is the PLA and an engineering thermoplastic, PC. The properties of the blends were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)and scanning electron microscopy (SEM).The study showed that the blends kneading torque are between those of the homopolymers and are proportional to the rate of PLA. Thermogravimetric analysis showed that PC improved notably the thermal stability of the blends. DSC results pointed out significant changes on the thermal behavior of the PLA phase into the blends.
Keywords: Blends, Biopolymer, Poly(lactic acid), Polycarbonate, Thermal stability.
I. INTRODUCTION
The high brittleness and poor thermal and gas and water vapor barrier properties of PLA are the major obstacles for its diffusion in many usual and technical applications [1, 11].
Thus, a lot of attempts have been devoted to promote PLA performances by blending with other polymers and/or incorporating nanoclays and non-biodegradable and biodegradable fillers [12]. In this context, PLA has been melt blended with many polymers, such as polyethylene terephthalate (PET) [13,14], polyamide (PA) [15], polystyrene
(PS) [16], biopolyethylene [17], poly(butylene adipate-co- terephthalate) (PBAT) [18], poly (vinylphenol) [19], thermoplastic starch (TPS) [20], poly(butylene succinate (PBS) [21], polyhydroxybutyrate-valerate (PHBV) [22], and polycarbonate (PC) [23]. Nanoclays have been added to PLA to reinforce particularly mechanical properties [2, 24-27], fire retardancy [28], thermal properties [11, 29], optical properties [30] and barrier performances [31, 32]. Also, PLA bio- composites have been prepared using wood flour [33], lignin [34, 35], cellulose nanocrystals [6, 8, 9, 36, 37] and olive pit [38]. Furthermore, PLA nanocomposites based on metallic nanoparticles, such as zinc oxide (ZnO) [39] and silver (Ag) [9] as antimicrobial agents and titanium dioxide (TiO2) to promote photodegradability [40] have been investigated.
Even these advancements in PLA research, there is still many drawbacks that continue to limit its employment in some sectors which require particular mechanical and thermal properties. Among PLA above cited blends, its combination with PC presenting a high inherent thermal stability and an important tensile strength and elongation at break appears the more attractive strategy to overcome PLA brittleness and poor thermal resistance.
Recently, several studies on PLA and PC blends have been performed to get an idea on the improvements that can be induced by such a combination and on the factors that could influence its performances [41- 43]. To improve affinity between the two immiscible components, Lee et al. [41]
studied the compatibilizing effect of poly(styrene-g- acrylonitrile)-maleic anhydride (SAN-g-MAH), poly(ethylene-co-octene) rubber-maleic anhydride (EOR- MAH) and poly(ethylene-co-glycidyl methacrylate) (EGMA) on the mechanical, morphological and rheological properties of a (30/70) PLA/PC blend. They concluded that the higher mechanical strength equivalent to the lowest PLA droplet size is obtained at 5 phr of SAN-g-MAH. On the other hand, Phuong et al. [42] tried the reactive compatibilization of PLA/PC blends via interchange reactions using tetrabutylammoniumtetraphenylborate (TBATPB) and triacetin (TA) as a catalytic system. The addition of both TA and TBATPB resulted in improved compatibility through the formation of a PLA-PC copolymer which glass transition temperature (Tg) is situated between the Tg values of PLA
and PC, as obtained bydynamic mechanical thermal analysis.
As the catalyst was added, the Young's modulus of the materials increased as a result of both the improvement of compatibility and chain scission in the PLA phase, enabling the achievement of co-continuous phase morphology for lower PC contents.
In this study, the blending conditions such as temperature and time were selected to permit the occurrence of interchange reactions between polymers. The characterization of the blends focused essentially on the structural, rheological, morphological and thermal properties as a function of the composition.
II. Experimental II.1 Materials
PLA is a PLI 005 resin manufactured by NaturePlast. It is deriving from vegetal resources and has the following characteristics: (melt flow index (MFI) at 190°C and under 2.16 kg: 10-30 g/10min, melting temperature: 155-160°C, degradation temperature: 240-250°C). PC is supplied by Bayer under the trade name of Makrolon 2858 (MFI at 190°C:
10g/10min, density 1.20 g/cm3).
II.2 Processing
Before melt mixing, PC, PLA were dried in an oven at 60°C for 24 h. PLA and PC dried granules were manually mixed in different ratios then melt blended in a 60 cm3Brabender mixer chamber to get (90/10), (70/30) and (50/50) (Vol.%) PLA/PC blends. Mixing was conducted at a temperature of 220°C, a speed of 30 rpm during a mixing time of 15 min. Torque measurements were carried out during the blending.
II.3 Characterization techniques
- Differential scanning calorimetry (DSC).
DSC measurements were carried out to investigate the thermal behavior of the materials with a Perkin-Elmer instrument under inert atmosphere. The analysis was performed on approximately a mass of 10 mg according to a three steps program: first heating from 25 to 250°C at 10°C/min, cooling from 250 to 25°C at 10°C/min, and second heating from 25 to 250°C at 10°C/min. Two minutes isothermal plateau was inserted between the ramps. The thermal properties, in the calorific capacity difference (ΔCp), the glass transition temperature (Tg), the crystallization temperature and enthalpy, respectively Tc and ΔHc and the melting temperature and enthalpy, respectively Tm and ΔHm were determined from the second heating curves.
- Thermogravimetric analysis (TGA).
Thermogravimetric analysis was performed using a Perkin Elmer TGA 4000. Measurements were carried out by heating the samples from 25 to 700 °C at a heating rate of 10°C/min under a flow of nitrogen gas.
- Scanning electron microscopy (SEM).
The morphology of the PLA/PC blends was studied by scanning electron microscopy using a JSM-6301F microscope.
The analysis was performed by the observation of the surfaces
produced on fracturing at liquid nitrogen temperature and after coating with a conductive layer of gold.
III.RESULTS AND DISCUSSION
III.1.Thermal characterization of PLA, PC homopolymers and blends:
Fig. 1 .DSC thermograms of PLA, PC and PLA/PC blends Figure 1 shows the DSC thermograms of PLA/PC blends melt mixed. The Tg of the PLA phase is unchanged while that of the PC phase cannot be observed because it is in the temperature range wherein the PLA melting begins. The no shifting of the PLA phase Tg suggests the total immiscibility of the two polymers for all the compositions.
The PLA weight fractions(ω) inside the samples were calculated using the Cp values at its glass transition (table 1).Also, the blends thermograms reveal that the crystallization temperature of PLA shifts to higher values as the PC rate increases. This indicates that PC, which is still in its glassy state, disrupts the reorganization of PLA chains during the crystallization process. Moreover, at the beginning of PLA melting, PC glass transition occurs avoiding the possibility to calculate the PLA melting enthalpy correctly.
The crystallinity attained by the PLA into the blends after the cold crystallization were calculated, using the PLA weight fraction, by dividing the melting peak area by the PLA mass.
III.2.Measurements of blends torques PLA/PC
To highlight the effects PC on the viscosity of the blends, the variations of the mixing torque as a function of time for the blends have been studied. Usually, torque variations versus time show in the beginning, an increase resulting from the resistance of the polymer that has not yet reached the melt.
Thereafter, the torque apparently decreases when the polymer attains the melt and all the interchain interactions have been broken.
Finally, the torque reaches a constant value as a plateau which expresses the torque at stability for the blends.
TABLE I.DSC thermogram analysis for PLA and PLA/PC blends Glass Transition
(50°C to 67°C)
CrystallizationCold (80°C to 135°C)
Melting (135°C to 170°C)
Samples Tg
(°C)
Cp
(J/g°C) w
(%) Tcc
(°C) AreaPeak
(mJ)
Hcc
(J/g) Tm1
(°C) Tm2
(°C) PeakArea (mJ)
Hm
(J/g)
PLA 59.2 0.517 100 98.8 334.0 33.4 150.5 156 363.1 36.3 PLA/PC
90/10 59.4 0.431 83 100.7 324.4 31.2 150.6 156.5 331.4 31.9 PLA/PC
70/30 60.2 0.349 68 102.2 258.6 24.6 151.3 159 267.3 25.5 PLA/PC
50/50 60.0 0.262 51 102.0 185.9 17.9 151.2 159.3 201.7 19.4
The figure 2 shows that the torque at stability measured after a mixing time of 15 minutes reaches values that vary proportionally with the PC rate. The reduction of the mixing torque as the PLA concentration increases is probably caused by the high sensitivity of this polymer to degradation reactions, especially hydrolysis and thermal decomposition.
0 200 400 600 800 1000
0 5 10 15 20 25 30
PLA PLA/PC (90/10) PLA/PC (70/30) PLA/PC (50/50) PC
Torque (N.m)
Time (s)
Fig.2.Time dependent curves of torque for PLA, PC and PLA/PC blends III.3.Thermal stability of PLA/PC blends
Thermogravimetric analysis was performed to evaluate the effects of PC concentration and the thermal stability of PLA/PC blends. The degradation parameters, including temperatures at which starts and finishes the decomposition, noticed Td0and Tdf,respectively, the temperature at maximum weight loss (Tdmax) and the decomposition rate (Vd) are evaluated from the thermograms representing the variations of the weight loss (TG) and the derivative of the weight loss versus time (DTG).
Figure 3(a) and 3(b) depict, respectively, TG and DTG thermograms for PLA, PC and PLA/PC blends. PLA decomposition is carried out in a single step that extends from
295°C to 388°C. The weight loss at Tdmaxof approximately 358°C is around 56%, corresponding to a degradation rate of 2.83%/min. The thermal stability of PC is higher because degradation, which occurs also in a single stage, begins around 410°C, reaches a maximum at 521°C and ends at 627°C with a solid residue of 23.61%. The maximum rate of weight loss is about 0.93%/min. PLA/PC blends present an intermediate thermal stability. In fact, the thermogram of the (90/10) PLA/PC system shows a decomposition in a single step and reveals a slight increase in the degradation temperatures, whereas the (70/30) and (50/50) blends thermograms exhibit principally two steps corresponding to the decomposition of PLA and PC phases. In these blends, the Tdmaxof the PLA phase is not affected contrary to that of the PC phase which moves to lower temperatures. This fact has already been observed by Phuong et al. [42], who explained that this could be due to the PLA degradation that would have accelerated the PC phase decomposition. The (50/50) PLA/PC blend exhibits a supplementary small peak in the DTG thermogram that could correspond to the blend’s third phase which caused the
slight strengthening effect.
Fig.3.TG (a) and DTG (b) thermograms of PLA, PC and PLA/PC blends.
III.4.Morphological study of PLA/PC blends
The evolution of the PLA/PC blends morphology with the composition. Micrographs (a) and (b) showing the morphology of (90/10) and (70/30) PLA/PC blends respectively, exhibit PC droplets dispersed into the PLA matrix. The interface between the domains of the two polymers reveals the existence of voids attributed to the poor adhesion due to their immiscibility. PC droplets present different sizes varying from 13 to 2μm for the (90/10) PLA/PC blend and from 40 to 2μm for the composition (70/30). The micrographs (c) representing the (50/50) PLA/PC system reveals a change from the droplet/matrix to the co-continuous morphology resulting from the equivalent rates of the two polymers into the blend. PLA domains show a good adhesion to the PC ones even if they are melt mixed. The observed morphology agrees perfectly with the TG results which mentioned the presence of a third phase at the interface.
(a) (b) (c)
Fig.4.Scanning electron micrographs of PLA/PC blends: (a) (90/10), (b) (70/30), (c) (50/50).
IV.CONCLUSION
The torque one is all the more high since the rate of PC in the mixture increases. The reduction in the couple of malaxation with the rate of PLA is undoubtedly caused by the great sensitivity of this polymer to the reactions of degradation, particularly the hydrolysis.
The thermogravimetric analysis showed that the mixtures have thermal stabilities higher than that of the pure PLA.
The thermogram of mixture PLA/PC (90/10) reveals a decomposition in only one stage because of the weak rate of PC But with mixtures PLA/PC (70/30) and (50/50) two stages of decomposition exhibent corresponding to the PLA and to the PC could be allotted to a formed structure copolymer of units PC and PLA and having, consequently, an intermediate thermal stability. In the case of these two systems, T dmax of the PLA was not affected too much by the mixture, contrary to that of the PC which was moved towards lower temperatures which explained than this could be due to the fact that the degradation of the PLA would have accelerated that of the PC.
the mixture in a molten state of the PC with the PLA disturbs the crystallization of this last bus the chains of the PC, being still in a vitreous state, return the process of arrangement of the segments of chains of the more difficult crystallite PLA, which allows crystallization but temperatures higher than that of the PLA alone .
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