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PBAT blends
Polymer Testing 43 (2015) 27e37
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Material properties
Influence of chain extender on mechanical, thermal and morphological properties of blown films of PLA/PBAT blends rio Elida Suman Bretas b, Liliane Cardoso Arruda a, *, Marina Magaton b, Rosa a Marcelo Massayoshi Ueki a ~o Cristva ~o, rio de Processamento de Polímeros, P2CEM, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Sa Laborato Cep:49100-000, Sergipe, Brasil b ~o Carlos, Rod. Washington Luiz, Km 235, Sa ~o Carlos, Departamento de Engenharia de Materiais, Universidade Federal de Sa ~o Paulo, Brasil Cep:13565-905, Sa
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2015 Accepted 13 February 2015 Available online 24 February 2015
Blown films of poly (lactic acid)/poly (butylene adipate-co-terephthalate) (PLA/BAT) blends were prepared in compositions of 40/60 and 60/40. A multifunctional epoxy chain extender, Joncryl ADR 4368, was used as compatibilizer. Both the morphology and the mechanical behavior of the blown films were investigated. The results showed that their morphology is dramatically affected by the chain extender and the blend composition. The mechanical properties were significantly changed due to the generated morphology. All blends showed increase in the modulus of elasticity, but the elongation and the stress at break showed significant increase in the 40% PLA composition without the chain extender, but in compositions with 60% PLA only with the chain extender. These results were substantiated by the change in morphology, which was caused by changes in the interfacial tension due to the chain extender, and in the viscosity ratio due to the composition. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biodegradable polymers Chain extender Blown film Polymer blends Mechanical properties
1. Introduction The large use of plastics in the packaging industry is increasing the interest in using biodegradable polymers to replace conventional polymers in order to reduce the environmental impact caused by the inappropriate disposal of this type of material [1]. The poly(lactic acid) e PLA is a thermoplastic, biodegradable, biocompatible polymer that can be,manufactured from renewable resources [2]. It is known that PLA has low toughness at room temperature and low melt strength when compared with conventional polymers, factors that limit its application in large-scale processes, such as blown film extrusion, blow molding and foaming, for which melt stability is an important feature [3,4]. PLA melt strength needs to be * Corresponding author. Tel./fax: þ55 79 21056972. E-mail address: [email protected] (L.C. Arruda). http://dx.doi.org/10.1016/j.polymertesting.2015.02.005 0142-9418/© 2015 Elsevier Ltd. All rights reserved.
improved in order to enlarge its processing window, and thus its application scope. Copolymerization, the addition of plasticizers and the blending with other polymers are some of the main ways to achieve improvements in PLA properties [5]. PLA blends with flexible polymers, such as poly(butylene-succinate-co-adipate) (PBSA) [6], poly(butylene succinate) (PBS) [7], poly(butyl acrylate) (PBA) [8] and poly(butylene-adipate-co-terephthalate) (PBAT) [9,10] have been studied to improve the mechanical properties of the PLA, mainly to increase its toughness. Poly (butylene-adipate-co-terephthalate) e PBAT is an aliphatic aromatic copolyester manufactured from petroleum-based resources consisting of two types of comonomer, a rigid BT segment (butylene terephthalate) consisting of 1.4 butanediol and terephthalic acid monomers, whereas the flexible BA section (butylene adipate) consists of 1.4 butanediol and adipic acid monomers [11]. It has high elongation at break and low modulus of elasticity,
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with mechanical behavior similar to a thermoplastic elastomer, and thus PBAT has been considered a good alternative to toughening PLA. PLA and PBAT, when working together, are considered complementary [4,12,13]. However, the PLA/PBAT blend is seen as immiscible. Thus, having good interface compatibility between PLA and PBAT is essential to obtain a blend with satisfactory properties [14]. Recently, additives called chain extenders have been developed to improve melt strength, thermal stability as well as to work as compatibilizers in polymer blends. Chain extenders enable re-linking polymer chains that have broken due to any kind of degradation reaction, and thereby increase the molecular weight of the polymer [2,15]. Branched chains can be obtained according to the functionality of the chain extender used and its concentration in the polymer [16]. Multifunctional chain extenders containing epoxy groups have been extensively used in polyesters [15]. The epoxy groups in the chain extender may react with the hydroxyl and carboxyl groups in the polyester chain-ends. This occurs due to reactions involving the epoxy group ring opening and the creation of covalent bonds with hydroxyl formation [16]. Their use in blends promotes the formation of copolymers during compounding in the molten state. They work as physical compatibilizers by reducing the size of the dispersed phase [11,17]. The use of chain extenders in PLA/PBAT blends has been reported in some studies. Zhan et al. [4] studied a random terpolymer of ethylene, acrylic and glycidyl methacrylate (T-GMA) as the reactive processing agent in PLA/PBAT blends. They found that blends with T-GMA showed better miscibility and higher hardness without severe loss in tensile strength. Al-Itry et al. [15] studied the compatibilization of PLA/PBAT blends using the multifunctional epoxide Joncryl as chain extender. The blend rheological and mechanical properties were investigated. The presence of chain extender led to increase in the tensile modulus, elongation at break and in the complex viscosity of the blends. Blown film extrusion is the most important industrial process in the production of polymeric films, mainly for packaging in general. However, the literature is very scarce with respect to PLA/PBAT blend blown films. In this process, the molten polymer is pumped through an annular die and then the extruded polymer in tube shape is drawn by nip rolls longitudinally to the die exit. Simultaneously, the extruded melt is air-cooled near the die via air-ring and blown by injecting air through the center of the die mandrel, thus forming a bubble. The film thickness and diameter is determined by controlling the extruder output, the bubble blow-up ratio and the film haul-off rate (or takeup speed). As the extruded molten polymer is drawn in both the transverse (TD) and longitudinal machine (MD) directions, the film blowing process may be rheologically treated as biaxial elongational flow. The polymer molecular orientation and the final morphology of the blown film result from a complex interaction between the polymer rheology and the process parameters [18]. The current paper aimed to study the chain extender action on the mechanical behavior of high-dispersed phase concentration PLA/PBAT blends obtained by blown film
extrusion. Morphological, thermal and rheological experiments were conducted to explain chain extender effect on mechanical behavior. 2. Experimental section 2.1. Materials and methods 2.1.1. Materials PLA e Ingeo™ Biopolymer 2003D (Nature Works) was obtained from Cargill-Dow in the form of pellets with melt flow index of 6 g/10 min (210 C, 2.16 kg). PBAT e Ecoflex® F BLEND C1200 was obtained from BASF in the form of pellets with melt flow index of 2.5e4.5 g/10 min (190 C, 2.16 Kg). The commercially available multifunctional epoxide, Joncryl ADR®4368, was kindly supplied by BASF, with epoxy equivalent weight of 285 g/mol. In the current paper, the term “Joncryl” is used to represent Joncryl ADR®4368. 2.1.2. Processing The neat polymers were dried under vacuum at 80 C for 4 hours, before each processing step (melt-compounding and blown film extrusion) and rheological characterizations, in order to avoid hydrolytic degradation. Joncryl was used as received. Compositions containing 0, 40, 60 and 100% PLA in weight were studied, respectively designated PBAT, 40PLA, 60PLA and PLA, in formulations without chain extender. For the formulations containing chain extender, the designation was X/0.3JC and X/0.6JC, indicating formulations with 0.3 and 0.6% in weight, respectively. All the formulations were melt-compounded in a single screw extruder (Wortex) with screw diameter of 30 mm and L/D ratio of 34. The screw used in the current study has a MADDOCK mixing element located at the end of the metering zone. The melt compounding temperature at different zones was independently controlled in order to achieve a temperature profile in the range of 155e170 C. The screw speed was set at 60RPM. The blown films were prepared in a single screw extruder IMACON with 25 mm diameter screw, L/D 22. An annular die of 50 mm diameter, with die gap of 0.85 mm was used to shape the initial tube dimensions. The processing temperature at different zones was set from 160 to 190 C for PLA, 160e190 C for blends and from 140 to 150 C for PBAT. The screw speed in the film extruder was of 20RPM in formulations with neat PLA and 35RPM in the blends and neat PBAT. The take-up speed was set to 2.8 m/ min. 2.1.3. Rheological characterizations Aiming at evaluating the reaction between the chain extender and the neat polymers and their blends, the current study conducted a measurement of torque versus time using an internal mixer HAAKE, model Rheomix 600p, with Roller type rotor, at 170 C and rotor speed of 60RPM. The rheological properties in oscillatory mode of the PLA and PBAT, with and without chain extender, were assessed using an oscillatory shear rheometer ANTON PAAR, model MCR302. The samples were tested using
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parallel-plate geometry (d ¼ 25 mm) with a gap of 1 mm at 170 C. The following tests were performed: a) dynamic time sweep test using angular frequency of 10 rad/s and 1% strain amplitude to assess thermal stability and chain extender reaction; and b) small amplitude oscillatory frequency sweep at angular frequency from 600 to 0.1 rad/s and strain amplitude of 1% to achieve complex viscosity of neat and chain extender modified PLA and PBAT to evaluate viscosity ratio. A strain sweep test was initially conducted to determine and guarantee the linear viscoelastic regime for each formulation. 2.1.4. Crystallinity evaluation by X-ray diffractometry (XRD) X-ray diffraction patterns were recorded using a Shimadzu XRD-6000 diffractometer to qualitatively evaluate the crystallinity of the films. Scans were carried out from 5 to 30 at a scan rate of 2 /min using Ni-filtered CuKa radiation. 2.1.5. Thermal characterization by differential scanning calorimetry (DSC) A NETZSCH differential scanning calorimeter (DSC), model 200F3 Maia, equipped with a liquid nitrogen cooling system was used. The DSC cell was constantly purged with nitrogen at a flow rate of 50 mL/min. Approximately 7e9 mg of sample were sealed in aluminum pans. The temperature programming consisted of three steps; the samples were first cooled to 50 C, heated to 200 C at 10 C/min and held at 200 C for 2 min. They were then cooled to 50 C at 10 C/min and reheated to 200 C at 10 C/min. All the calculated data are the average of two measurements. 2.1.6. Morphological characterization by scanning electron microscopy (SEM) The cross-section of the blown films parallel to the drawn direction (MD) and transversal to the drawn direction (TD) was the object of the morphological analysis characterized by scanning electron microscope (SEM) (JEOL JSM-5700) at an accelerating voltage of 5 kV. The crosssection for the analysis was achieved by cryofracturing the films in liquid nitrogen and then covering with a thin layer of gold. 2.1.7. Mechanical properties The mechanical properties of blown films were obtained by tensile tests, according to ASTM D-882. The tests were conducted using an INSTRON Universal Testing Machine (model 3367) with load cell of 100N and crosshead speed of 50 mm/min. Rectangular specimens were cut from the blown film with the length parallel to the take-up direction or machine direction (MD). 3. Results and discussion 3.1. Rheological characterization of neat and chain-extender modified PBAT and PLA Rheological tests carried out by torque rheometry on PLA and PBAT with and without chain extender are shown in Fig.1. The experiment was conducted to evaluate the
Fig. 1. Torque versus time for PBAT (a) and PLA (b) with and without chain extender.
evolution of the chain extender reaction with the polymer over the mixing time in the molten state. It is known that the torque increase is related to molten polymer viscosity increase, which is caused by molecular weight increase. After mixing for 15 minutes it was observed that the PBAT with 0.3 and 0.6% chain extender showed torque increase of 19% and 23%, respectively, when compared with the neat PBAT torque. The PLA formulations with 0.3% and 0.6% Joncryl showed, respectively, a torque 36% and 59% greater than that of neat PLA, thus indicating greater chain extender reactivity with the PLA than with PBAT. The molten polymer thermal stability over time is an indicator of the chain extender effectiveness. Since the thermal degradation process of the polymers leads to decrease in melt viscosity and elasticity, it is possible to monitor such change by measuring the storage modulus (G0 ) based on time. Fig. 2 shows the evolution of the PLA and PBAT (with and without chain extender) reduced storage modulus (G 0 (t)/G' (0)) against time. After 20 minutes, there was reduction of approximately 4% in the standard storage modulus of the PBAT without chain extender and reduction of 12% in the PLA without chain extender. This decrease was three times greater than that of the PBAT, and showed the lower thermal stability of PLA
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Fig. 2. Reduced storage modulus (G0 (t)/G0 (0)) versus time of neat and extended polymers.
when compared to PBAT. For the PBAT/0.6JC and PLA/0.6JC formulations, there was respective increase of 29% and 47% in the reduced storage modulus, showing more prominent chain extending reaction in the PLA. Al-Itry et al. [15] studied the thermal stability of PLA, PBAT and their blends with 0.25, 0.5 and 1.0 Joncryl, and found similar results. Thus, the use of chain extender not only prevents polymer degradation but also increases melt stability. The complex viscosity plots of the neat polymers and of polymers modified by chain extenders were obtained from rheological tests in oscillatory mode and are shown in Fig.3. It can be seen that complex viscosity decreases with angular frequency in both polymers. This is typical shearthinning behavior. The PBAT/0.6JC complex viscosity increased by 62%, taking the frequency of 0.1rad/s as reference, when compared with that of the neat PBAT. On the other hand, the PLA/0.6JC complex viscosity increased by 209% when compared with the PLA without chain extender. As expected, according to the torque rheometry data, the chain extender showed higher reactivity with the PLA when compared with the PBAT. The increase in complex viscosity is related to the increase in the molecular weight of the polymers due to the reaction of the chain extender epoxy group with both chain-ends of the polymer [19]. This is evidence of the chain extension of the macromolecules, thus confirming the chain extender reactivity.
3.2. Characterization of the films 3.2.1. X-ray diffraction The results from the XRD analysis of PLA and PBAT films and their blends without chain extender are shown in Fig.4. The X-ray diffractogram of the PLA film showed an amorphous halo only. However, four diffraction peaks close to 17.6 , 20.5 , 22.9 and 24.6 [20] were observed in the X-ray diffractogram of the PBAT film, thus indicating the presence of a crystalline structure. The PLA/PBAT blends showed
Fig. 3. Complex viscosity versus angular frequency of neat and modified PBAT (a) and PLA (b).
PBAT-crystalline-phase characteristic peaks and an amorphous halo relating to the PLA. These results indicate that the PLA is amorphous within the blends and that the crystallinity found refers to the PBAT crystalline phase. Similar results were found in films with addition of Joncryl (data not shown). 3.2.2. Thermal properties Table 1 shows the cold crystallization temperature (Tcc), melting temperature (Tf), melting enthalpy (DHf), cold crystallization enthalpy (DHcc) and the degree of crystallinity (Xc) of the films related to the first heating scan. The glass transition temperature (Tg) values were determined in the second heating scan in order to evaluate the miscibility of the systems. Fig. 5 shows the DSC curves of the films of the polymers and of their blends. PLA showed a double endothermic peak (melting temperature) at 155.4 C and 151 C. This double peak is common in polyesters, and the literature justifies its presence by a melt/recrystallization/re-melt mechanism [21]. The PBAT film showed two endothermic peaks at 60.4 C and 126.4 C. The peak at 60.4 C is attributed to the melting of crystalline phase of BA fraction in the sample [22]. For the PLA/PBAT blends, there was an overlap between the PBAT melting region and the PLA cold-crystallization region. Therefore, it
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Fig. 4. X-ray diffraction patterns obtained from blown films of PLA, PBAT and their blends.
Table 1 Melting and crystallization behaviour of the films of PLA, PBAT and PLA/PBAT blends. Formulation
Tag ( C)
Tbg ( C)
Tcc ( C)
DHcc (Jg1)
Tm ( C)
DHm (Jg1)
Xcc (%)
PLA PBAT 40PLA 40PLA/0.3JC 40PLA/0.6JC 60PLA 60PLA/0.3JC 60PLA/0.6JC
60.9 e 58.9 57.9 58.9 59.1 59.2 59.3
e 33.2 34.5 34.5 33.0 35.1 35.3 34.8
115.5 e 95.7 98.6 101.3 98.8 97.4 96.8
25.6 e 7.2 6.0 6.6 12.9 12.8 11.5
151/155.4 60.4/126.5 153.7 154.1 153.7 154.1 153.6 153.0
25.6 21.8 15.7 13.8 15.2 19.4 19.6 19.5
0.0 19.1 12.4 11.4 12.6 14.2 14.9 17.5
Tag ¼ glass transition temperature of PLA, Tbg ¼ glass transition temperature of PBAT, Xcc ¼ degree of crystallinity of PBAT.
was not possible to determine the PBAT melting temperature in the blends. Only the PLA melting temperature was determined, and is presented in Table 1. All blends showed two distinct glass transition temperatures relative to the PLA and PBAT phases. This indicated that the formed blends are thermodynamically immiscible. Similar results were reported by Signori et al. [23] regarding PLA/PBAT blends containing 75wt% of PLA. The PBAT incorporation to the PLA decreased the coldcrystallization temperature by 20 C, thus indicating that the PBAT enabled improvement of the crystallization ability of PLA in the blend, as previously reported [24]. As for the 40PLA blend, the presence of the chain extender led to slight increase in the cold-crystallization temperatures of the blends, which ranged from 95.7 C in the 40PLA to 101.3 C in the 40PLA with 0.6% chain extender.
The XRD data showed that PLA is amorphous in the films whereas PBAT is crystalline, hence just the PBAT fraction was considered as being the crystalline phase of the films. The PLA cold-crystallization enthalpy was subtracted from the sample melting enthalpy to determine the PBAT degree of crystallinity (Xc), which is given by Equation (1):
Xc ¼
DHm DHcc WPBAT H0 m
100%
(1)
where DHf is the sample melting enthalpy, DHcc is the coldcrystallization enthalpy, in this case from PLA. WPBAT is the mass fraction of the PBAT in the blend, DH f is the melting enthalpy of 100% crystalline PBAT. The DH f value used for the PBAT was of 114 J.g1 [25]. The PBAT phase in the blends reduced its degree of crystallinity from 19% in the neat
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Fig. 5. First heating DSC thermograms obtained from blown films of PLA, PBAT and their blends.
PBAT to 12% and 14% in the blends with 40% and 60% PLA, respectively. Since PLA is a lower chain flexibility and higher viscosity component, it restricts the mobility of PBAT chains in the blend, thus hampering the molecular reorganization and thereby reducing the degree of crystallinity. 3.2.3. Morphological characterization The formation of the morphology of immiscible polymer blends is the result of interaction between process variables (temperature, deformation types and rate) and blend components properties (composition, viscosity ratio, interfacial tension, continuous phase viscosity and elasticity of the components). The final morphology is the combination of these factors. The initial particle size, the polymer elasticity, the dispersed phase percentage and the draw ratio [26] are the main factors affecting the morphology formed during drawing. When the dispersed phase concentration gets close to 1:1, complex structures, such as ribbon- or sheet-like, platelet, stratified and cocontinuous structures, are formed. However, the prevalence of one or other structure is basically controlled by factors such as the flow type and the intensity during its processing in the molten state, the viscosity ratio and the interfacial tension. Fig. 6 shows micrographs of the 40PLA film, with and without chain extender, obtained in cross-section longitudinal and transverse to the take-up direction of the film. It is possible to see a characteristic morphology of immiscible blends with low adhesion, in which the PLA is the dispersed phase. Regarding the 40PLA film without chain extender, the PLA dispersed phase presents itself as an elongated and fibrillar structure, preferably arranged towards the drawn direction of the film (take-up direction). This fibrillar morphology is caused by the elongational strain derived from the film drawing process. In 40PLA films containing 0.3 and 0.6% Joncryl, the dispersed phase appears as ellipsoids oriented towards the film drawing. The literature has shown that the chain extenders in the blends can produce copolymers from the link of a polymer chain with the chain of another polymer, in this case the PLA-co-PBAT, thus
improving the compatibility between the PLA and PBAT phases [27]. The formation of this copolymer may acts as compatibilizer and lead to the reduction of the interfacial tension that stabilizes the dispersed phase into smaller sizes, thus leading to refinement of the dispersed phase. Finer morphology as well as viscosity ratio (hd>hm) greater than 1, as in the case of 40PLA/0.3JC and 40PLA/0.6JC, favor the formation of ellipsoids rather than fibrils. Despite the improvement in the interaction between matrix and dispersed phase, probably through the formation of a copolymer (PBAT-co-PLA), it was not enough to promote better adhesion between the matrix and dispersed phase, as can be seen in the micrographs. Fig. 7 shows the micrographs of 60PLA films, with and without chain extender, in the longitudinal and transversal to the take-up direction of the film. These compositions present PBAT as dispersed phase. The characteristic morphology in these blends is ribbon- or sheet-like aligned to drawing direction. A structure similar to the skin-core type can be noticed, in which the dispersed phase is coarse in the middle of the thickness (core region) of the film but, as the dispersed phase gets closer to the film surface (skin region), the morphology presents a finer, fibrillar structure. During the flow through the annular die, the molten polymer is subjected to a pressure-driven flow. This flow type shows maximum shear rate in the region near the die wall, i.e., in the film surface. The shear rate decreases as it gets closer to the middle of the film thickness, where the shear rate is zero. This shear rate gradient produces a coarse morphology in the center, where the shear rate is low to zero. As the dispersed phase gets closer to the surface of the film, it reduces its size due to the high shear rate. As the molten polymer exits the die, the flow becomes elongational in two directions: the drawn direction (MD) and the transversal direction (TD), which is commonly known as biaxial drawing. At this time, the spherical particles are drawn and orientated, thus forming the elongated structures. The skin-core structure type also becomes evident in the blend with 40% PLA without chain extender. The 60PLA film without chain extender showed a coarser dispersed phase than 60PLA blends with chain extender. In the case of 60PLA/0.3JC and 60PLA/0.6JC, the combination of lower interfacial tension and viscosity ratio < 1 led to a more refined fibrillar structure. 3.2.4. Mechanical properties Figs. 8e10 show results for modulus of elasticity, stress at break and elongation at break, respectively, for all studied formulations. Regarding the PLA, with or without chain extender, the tensile-strain behavior showed a marked yield stress (sy) of approximately 55e59MPa with failure by neck instability, and tensile modulus ranging from 2200 to 2584MPa, due to increase in the chain extender content. The increase in these properties may be related to PLA molecular weight increase due to PLA chain end reaction with Joncryl. As for the PBAT, the chain extender increased the tensile strength and the elongation at break. The increase in molecular weight, measured by torque rheometry, as well as in the degree of crystallinity, measured by DSC, may be responsible for the improvement in these properties (Fig.10).
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Fig. 6. Micrographs of the 40PLA film: (a) 40PLA (MD and TD), (b) 40PLA/0.3JC (MD and TD), (c) 40PLA/0.6JC (MD and TD).
From the analysis of the properties of the 40% PLA blends, it is worth highlighting that the dispersed phase consists of PLA with brittle polymer behavior, high modulus of elasticity and high tensile strength, compared with the PBAT. With respect to blends with 60% PLA, the dispersed phase is the PBAT with elastomeric behavior, high ductility and low modulus of elasticity. The graph in Fig.8 shows that the modulus of elasticity meets the additivity rule, since the theoretical elastic moduli were calculated in all studied blends. Table 2 shows the value of the modulus experimentally obtained and calculated by the mixing rule in which good agreement is observed in blends with 60% PLA. Equation (2) was used to calculate the modulus by the mixing rule:
Eteorico ¼ EPBAT VPBAT þ EPLA VPLA
(2)
where EPBAT and EPLA are the moduli experimentally obtained from neat PBAT and PLA, respectively, in each formulation. Such values are shown in Fig.8. VPBAT and VPLA
represent the PBAT and PLA volume fractions, respectively. The density values of 1.25 kg/m3 for the PLA, and of 1.26 kg/ m3 for the PBAT provided by the manufacturers were adopted to calculate the volume fraction. Since in the case of 40PLA, the matrix is PBAT, comparison with respect to neat PBAT modulus, shows an increase of at least 800%. However, there was small decrease in the modulus of elasticity in the case of 40PLA formulations with chain extender when compared with those without chain extender. As discussed in the previous section, the morphology change from elongated and fibrillar structure (40PLA) to deformed droplets in the 40PLA/0.3JC and 40PLA/0.6JC blends (ellipsoids) explains the decrease in the modulus of elasticity in the presence of chain extender, as well as the values lower than those of the mixing rule. Regarding the stress at break, the PLA in the blend led to increase from 11MPa (neat PBAT) to 20.8MPa (40PLA), which corresponds to a 90% increase. As for the 40PLA/0.3JC and 40PLA/0.6JC blends, the chain extender addition did
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Fig. 7. Micrographs of the 60PLA film: (a) 60PLA (MD and TD), (b) 60PLA/0.3JC (MD and TD), (c) 60PLA/0.6JC (MD and TD).
Fig. 8. Elastic modulus for PLA, PBAT and PLA/PBAT blends with different Joncryl concentrations.
Fig. 9. Tensile Stress at Break for PLA, PBAT and PLA/PBAT blends with different Joncryl concentrations.
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Fig. 10. Elongation at break for PLA, PBAT and PLA/PBAT blends with different Joncryl concentrations.
not improve this property and the stress-at-break values were even lower than those of the 40PLA without chain extender, thus indicating that the presence of chain extender did not provide effective reinforcement. It is noteworthy that in blends with 40% PLA, the PLA is the dispersed phase with stress at break of 41MPa for the PLA without extender, and up to 54MPa for the PLA with 0.3% chain extender. These results are explained by the morphology in these blends. In the 40PLA blend, the PLA phase appeared as elongated fibrils in the loading direction during the tensile test. Even with low interaction between the die matrix and the dispersed phase, the elongated fibrillar structure of the dispersed phase was enough to act as mechanical reinforcement, which characterizes the morphology of microfibrilar composites (MFC) [27]. However, in 40PLA blends with chain extender, the PLA phase appeared as ellipsoids, i.e., deformed droplets with low aspect ratio and weak interaction between the matrix and the dispersed phase, and which acted as stress concentrators in the PBAT matrix. These statements become more evident when observing the elongation at break in these blends (Fig.10). For the 40PLA blends, there was a reduction of more than 900% in elongation at break in the presence of chain extender compared with the 40PLA blend without chain extender. This shows that, in this system, the increased interfacial interaction provided by the chain extender (verified by the morphology stabilization in the form of droplets) was not enough to compensate the property losses due to the change in morphology from elongated fibrillar to deformed droplets. As for the 40PLA
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Fig. 11. Tensile stressestrain curves of the PLA and PBAT with different Joncryl concentrations.
formulation, the elongation at break was 325%, and in blends with chain extender it was 58% for 40PLA/0.3JC and 29% for 40PLA/0.6JC. On the other hand, regarding the 60PLA blend, the presence of chain extender allowed 900% increase in elongation at break when compared with the 60PLA blend without chain extender. It also showed 1200% increase when compared with neat PLA (Fig.10), thus enabling a significant improvement in ductility. As for 60PLA blends with chain extender, there was the refinement of the dispersed phase that changed from coarse sheets or platelet to elongated fibrils of PBAT phase. This change in morphology is probably responsible for the increase in elongation at break. A significant improvement in stress at break was also found, which changed from 22.3MPa (without chain extender) to 34e35MPa with chain extender. The mechanical behavior of the studied formulations can be best seen in Figs. 11 and 12, which show
Table 2 Values of experimental and calculated moduli. Formulation
Experimental modulus (MPa)
Theoretical modulus (MPa)
40PLA 40PLA/0.3JC 40PLA/0.6JC 60PLA 60PLA/0.3JC 60PLA/0.6JC
1101 995.3 858.2 1382.3 1598.8 1562.1
959.4 1062 1103.2 1379.6 1537 1596.8
Fig. 12. Tensile stressestrain curves of the 40PLA and 60PLA blends with different Joncryl concentrations.
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stressestrain curves of neat polymers and blends as well as the chain extender effect on the mechanical behavior. The 40PLA, 60PLA/0.3JC and 60PLA/0.6JC blends showed welldefined yielding, followed by stress softening and final strong strain hardening, i.e., stable neck growth during drawing. In the case of formulations with PBAT dispersed phase (60PLA), i.e., when the dispersed phase is of rubbery material, cavitation and interfacial de-bonding may occur [28,29]. In the literature, de-bonding is the proposed mechanism for toughening PLA/PBAT blends [9,30]. There is dispersed phase refinement in the presence of chain extender, thereby reducing the inter-particle distance and increasing the interfacial adhesion between the PLA and PBAT phases. Thus, stress and strain can be more efficiently transferred from the PLA matrix to the PBAT dispersed phase, thus increasing the ductility. Therefore, deformation probably occurs by shear yielding.
4. Conclusions The current study investigated the chain extender effect on the morphology, as well as on the mechanical and rheological properties, of PLA/PBAT blends in the form of blown films. Both PLA and PBAT polymers showed improvement in thermal stability and increase in complex viscosity with the addition of chain extender, however, the chain extender showed greater reactivity with PLA than PBAT. By analyzing the morphology and thermal properties, it was concluded that the chain extender, at the contents used in this work, was not enough to provide increased adhesion between the phases, but it significantly changed the morphology of the blends. Two factors account for the change in morphology: a) the reduction in the interfacial tension, which is probably caused by the formation of PLA/PBAT copolymers in the presence of chain extender and b) the change in the viscosity ratio due to the composition of the blends. Films with 40% PLA, without chain extender, showed PLA dispersed phase in fibrillar and elongated forms oriented towards the film drawing direction. This was responsible for the reinforced and increased ductility of this blend. There was morphology refinement in the presence of chain extender, but the viscosity ratio greater than 1 made the PLA dispersed phase appear as ellipsoids oriented towards the film drawing direction. The low interfacial adhesion between the matrix and the dispersed phase made the PLA rigid particles act as stress concentrators and dramatically reduce the elongation at break of 40PLA films with chain extender. The films with 60% PLA showed skin-core type morphology. Without the presence of the chain extender, the PBAT dispersed phase showed a coarse ribbon- or sheet-like structure in the middle of the film thickness and fine fibrils near the film surface. However, the low adhesion with the PLA matrix led to the reduction of its ductility. Furthermore, in the 60PLA/0.3JC and 60PLA/0.6JC blends, the combination of lower interfacial tension and viscosity ratio < 1 led to finer and homogeneous morphology with elongated fibrillar structure, which was responsible for the significant improvement in the ductility of the films in these formulations.
Acknowledgments The authors thank CAPES, CNPq (CNPq Processo 558320/2010-4) and FAPITEC/SE for the financial support, BASF for providing JONCRYL ADR® 4368 and Polymer Processing Laboratory from SENAI CIMATEC/BA for preparation of the tubular films. References [1] R. Zullo, S. Iannace, The effects of different starch sources and plasticizers on film blowing of thermoplastic starch: correlation among process, elongational properties and macromolecular structure, Carbohydrate Polymers 77 (2) (2009) 376e383. [2] H. Li, M.A. Huneault, Effect of chain extension on the properties of PLA/TPS blends, Journal of Applied Polymer Science 122 (2011) 134e141. [3] R. Al-Itry, K. Lamnawar, A. Maazouz, Reactive extrusion of PLA, PBAT with a multi-functional epoxide: physico-chemical and rheological properties, European Polymer Journal 58 (2014) 90e102. [4] N. Zhang, Q. Wang, J. Ren, L. Wang, Preparation and properties of biodegradable poly(lactic acid)/poly(butylene adipate-coterephthalate) blend with glycidyl methacrylate as reactive processing agent, Journal of Materials Science 44 (2009) 250e256. [5] P. Zhao, W. Liu, Q. Wu, J. Ren, Preparation, mechanical, and Thermal properties of biodegradable Polyesters/Poly(Lactic acid) blends, Journal of Nanomaterials 2010 (2010) 1e8. [6] S. Lee, J.W. Lee, Characterization and processing of biodegradable polymer blends of poly(lactic acid) with poly(butylene succinate adipate), Korea-Australia Rheology Journal 17 (2) (2005) 71e77. [7] A. Bhatia, R.K. Gupta, S.N. Bhattacharya, H.J. Choi, Compatibility of biodegradable poly (lactic acid) (PLA) and poly (butylene succinate) (PBS) blends for packaging application, Korea-Australia Rheology Journal 19 (3) (2007) 125e131. [8] B. Meng, J. Deng, Q. Liu, Z. Wu, W. Yang, Transparent and ductile poly(lactic acid)/poly(butyl acrylate) (PBA) blends: structure and properties, European Polymer Journal 48 (1) (2012) 127e135. [9] M. Kumar, S. Mohanty, S.K. Nayak, M.R. Parvaiz, Effect of glycidyl methacrylate (GMA) on the thermal, mechanical and morphological property of biodegradable PLA/PBAT blend and its nanocomposites, Bioresource Technology 101 (2010) 8406e8415. [10] R. Muthuraj, M. Misra, A.K. Mohanty, Biodegradable Poly(butylene succinate) and Poly(butylene adipate-co-terephthalate) Blends: reactive Extrusion and Performance Evaluation, Journal of Polymer and the Environment 22 (2014) 336e349. [11] T. Kijchavengkul, R. Auras, M. Rubino, S. Selke, M. Ngouajio, R.T. Fernandez, Biodegradation and hydrolysis rate of aliphatic aromatic polyester, Polymer Degradation and Stability 95 (2010) 2641e2647. [12] S.-Y. Gu, K. Zhang, J. Ren, H. Zhan, Melt rheology of polylactide/ poly(butylene adipate-co-terephthalate) blends, Carbohydrate Polymers 74 (2008) 79e85. [13] H.-T. Chiu, S.-Y. Huang, Y.-F. Chen, M.-T. Kuo, T.-Y. Chiang, C.Y. Chang, Y.-H. Wang, Heat Treatment effects on the mechanical properties and Morphologies of poly (Lactic Acid)/Poly (Butylene adipate-co-terephthalate) blends, International Journal of Polymer Science 2013 (2013) 1e11. [14] S. Lin, W. Guo, C. Chen, J. Ma, B. Wang, Mechanical properties and morphology of biodegradable poly(lactic acid)/poly(butylene adipate-co-terephthalate) blends compatibilized by transesterification, Materials and Design 36 (2012) 604e608. [15] R. AL-Itry, K. Lamnawar, A. Maazouz, Improvement of thermal stability, rheological and mechanical properties of PLA, PBAT and their blends by reactive extrusion with functionalized epoxy, Polymer Degradation and Stability 97 (2012) 1898e1914. [16] Y. Corre, J. Duchet, J. Reignier, A. Maazouz, Melt strengthening of poly (lactic acid) through reactive extrusion with epoxyfunctionalized chains, Rheologica Acta 50 (2011) 613e629. [17] S. Sun, M. Zhang, H. Zhang, X. Zhang, Polylactide toughening with epoxy-functionalized grafted acrylonitrileebutadienee styrene particles, Journal of Applied Polymer Science 122 (5) (2011) 2992e2999. [18] C.D. Han, Rheology and Processing of Polymeric Materials Volume 1: Polymer Technology, Oxford University Press, New York, 2007. [19] Q. Meng, M.-C. Heuzey, P.J. Carreau, Control of thermal degradation of polylactide/clay nanocomposites during melt processing by chain extension reaction, Polymer Degradation and Stability 97 (10) (2012) 2010e2020.
L.C. Arruda et al. / Polymer Testing 43 (2015) 27e37 [20] C.-S. Wu, Utilization of peanut husks as a filler in aliphatic-aromatic polyesters: preparation, characterization, and biodegradability, Polymer Degradation and Stability 97 (2012) 2388e2395. [21] P.M. Ma, R.Y. Wang, S.F. Wang, Y. Zhang, Y.X. Zhang, D. Hristova, Effect of fumed silica on the crystallization behavior and thermal properties of poly(hydroxybutyrate-co-hydroxyvalerate), Journal of Applied Polymer Science 108 (2008) 1770e1777. [22] Z. Gan, K. Kuwabara, M. Yamamoto, H. Abe, Y. Doi, Solid-state structures and thermal properties of aliphaticearomatic poly(butylene adipate-co-butylene terephthalate) copolyesters, Polymer Degradation and Stability 83 (2004) 289e300. [23] F. Signori, M. Coltelli, S. Bronco, Thermal degradation of poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) and their blends upon melt processin, Polymer Degradation and Stability 94 (2009) 74e82. [24] L. Jiang, M.P. Wolcott, J. Zhang, Study of biodegradable Polylactide/ Poly(butylene adipate-co-terephthalate) Blend, Biomacromolecules 7 (1) (2006) 199e207.
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[25] H. Xiao, W. Lu, J.-T. Yeh, Crystallization behavior of fully biodegradable Poly(Lactic Acid)/Poly (Butylene adipate-co-terephthalate) blends, Journals of Applied Polymer Science 112 (2009) 3754e3763. [26] S. Ebnesajjad, Plastic Films in Food Packaging: Materials, Technology and Applications, Elsevier, Amsterdam, 2012. [27] M. Evstatiev, S. Simeonova, K. Friedrich, X.-Q. Pei, P. Formanek, MFCstructured biodegradable poly(L-lactide)/poly(butylene adipate-coterephatalate) blends with improved mechanical and barrier properties, Journal of Materials Science 48 (2013) 6312e6330. [28] S. Wu, Phase structure and adhesion in polymer blends: a criterion for rubber toughening, Polymer 26 (1985) 1855e1863. [29] C.B. Bucknall, D. Clayton, W.E. Keast, Rubber-toughening of plastics, Journal of Materials Science 8 (1973) 514e524. [30] W. Dong, B. Zou, Y. Yan, P. Ma, M. Chen, Effect of chain-extenders on the properties and hydrolytic degradation behavior of the poly(lactide)/poly(butylene adipate-co-terephthalate) blends, International Journal of Molecular Sciences 14 (10) (2013) 20189e20203.
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
Influence of chain extender on mechanical, thermal and morphological properties of blown films of PLA/PBAT blends rio Elida Suman Bretas b, Liliane Cardoso Arruda a, *, Marina Magaton b, Rosa a Marcelo Massayoshi Ueki a ~o Cristva ~o, rio de Processamento de Polímeros, P2CEM, Universidade Federal de Sergipe, Av. Marechal Rondon s/n, Sa Laborato Cep:49100-000, Sergipe, Brasil b ~o Carlos, Rod. Washington Luiz, Km 235, Sa ~o Carlos, Departamento de Engenharia de Materiais, Universidade Federal de Sa ~o Paulo, Brasil Cep:13565-905, Sa
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2015 Accepted 13 February 2015 Available online 24 February 2015
Blown films of poly (lactic acid)/poly (butylene adipate-co-terephthalate) (PLA/BAT) blends were prepared in compositions of 40/60 and 60/40. A multifunctional epoxy chain extender, Joncryl ADR 4368, was used as compatibilizer. Both the morphology and the mechanical behavior of the blown films were investigated. The results showed that their morphology is dramatically affected by the chain extender and the blend composition. The mechanical properties were significantly changed due to the generated morphology. All blends showed increase in the modulus of elasticity, but the elongation and the stress at break showed significant increase in the 40% PLA composition without the chain extender, but in compositions with 60% PLA only with the chain extender. These results were substantiated by the change in morphology, which was caused by changes in the interfacial tension due to the chain extender, and in the viscosity ratio due to the composition. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Biodegradable polymers Chain extender Blown film Polymer blends Mechanical properties
1. Introduction The large use of plastics in the packaging industry is increasing the interest in using biodegradable polymers to replace conventional polymers in order to reduce the environmental impact caused by the inappropriate disposal of this type of material [1]. The poly(lactic acid) e PLA is a thermoplastic, biodegradable, biocompatible polymer that can be,manufactured from renewable resources [2]. It is known that PLA has low toughness at room temperature and low melt strength when compared with conventional polymers, factors that limit its application in large-scale processes, such as blown film extrusion, blow molding and foaming, for which melt stability is an important feature [3,4]. PLA melt strength needs to be * Corresponding author. Tel./fax: þ55 79 21056972. E-mail address: [email protected] (L.C. Arruda). http://dx.doi.org/10.1016/j.polymertesting.2015.02.005 0142-9418/© 2015 Elsevier Ltd. All rights reserved.
improved in order to enlarge its processing window, and thus its application scope. Copolymerization, the addition of plasticizers and the blending with other polymers are some of the main ways to achieve improvements in PLA properties [5]. PLA blends with flexible polymers, such as poly(butylene-succinate-co-adipate) (PBSA) [6], poly(butylene succinate) (PBS) [7], poly(butyl acrylate) (PBA) [8] and poly(butylene-adipate-co-terephthalate) (PBAT) [9,10] have been studied to improve the mechanical properties of the PLA, mainly to increase its toughness. Poly (butylene-adipate-co-terephthalate) e PBAT is an aliphatic aromatic copolyester manufactured from petroleum-based resources consisting of two types of comonomer, a rigid BT segment (butylene terephthalate) consisting of 1.4 butanediol and terephthalic acid monomers, whereas the flexible BA section (butylene adipate) consists of 1.4 butanediol and adipic acid monomers [11]. It has high elongation at break and low modulus of elasticity,
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with mechanical behavior similar to a thermoplastic elastomer, and thus PBAT has been considered a good alternative to toughening PLA. PLA and PBAT, when working together, are considered complementary [4,12,13]. However, the PLA/PBAT blend is seen as immiscible. Thus, having good interface compatibility between PLA and PBAT is essential to obtain a blend with satisfactory properties [14]. Recently, additives called chain extenders have been developed to improve melt strength, thermal stability as well as to work as compatibilizers in polymer blends. Chain extenders enable re-linking polymer chains that have broken due to any kind of degradation reaction, and thereby increase the molecular weight of the polymer [2,15]. Branched chains can be obtained according to the functionality of the chain extender used and its concentration in the polymer [16]. Multifunctional chain extenders containing epoxy groups have been extensively used in polyesters [15]. The epoxy groups in the chain extender may react with the hydroxyl and carboxyl groups in the polyester chain-ends. This occurs due to reactions involving the epoxy group ring opening and the creation of covalent bonds with hydroxyl formation [16]. Their use in blends promotes the formation of copolymers during compounding in the molten state. They work as physical compatibilizers by reducing the size of the dispersed phase [11,17]. The use of chain extenders in PLA/PBAT blends has been reported in some studies. Zhan et al. [4] studied a random terpolymer of ethylene, acrylic and glycidyl methacrylate (T-GMA) as the reactive processing agent in PLA/PBAT blends. They found that blends with T-GMA showed better miscibility and higher hardness without severe loss in tensile strength. Al-Itry et al. [15] studied the compatibilization of PLA/PBAT blends using the multifunctional epoxide Joncryl as chain extender. The blend rheological and mechanical properties were investigated. The presence of chain extender led to increase in the tensile modulus, elongation at break and in the complex viscosity of the blends. Blown film extrusion is the most important industrial process in the production of polymeric films, mainly for packaging in general. However, the literature is very scarce with respect to PLA/PBAT blend blown films. In this process, the molten polymer is pumped through an annular die and then the extruded polymer in tube shape is drawn by nip rolls longitudinally to the die exit. Simultaneously, the extruded melt is air-cooled near the die via air-ring and blown by injecting air through the center of the die mandrel, thus forming a bubble. The film thickness and diameter is determined by controlling the extruder output, the bubble blow-up ratio and the film haul-off rate (or takeup speed). As the extruded molten polymer is drawn in both the transverse (TD) and longitudinal machine (MD) directions, the film blowing process may be rheologically treated as biaxial elongational flow. The polymer molecular orientation and the final morphology of the blown film result from a complex interaction between the polymer rheology and the process parameters [18]. The current paper aimed to study the chain extender action on the mechanical behavior of high-dispersed phase concentration PLA/PBAT blends obtained by blown film
extrusion. Morphological, thermal and rheological experiments were conducted to explain chain extender effect on mechanical behavior. 2. Experimental section 2.1. Materials and methods 2.1.1. Materials PLA e Ingeo™ Biopolymer 2003D (Nature Works) was obtained from Cargill-Dow in the form of pellets with melt flow index of 6 g/10 min (210 C, 2.16 kg). PBAT e Ecoflex® F BLEND C1200 was obtained from BASF in the form of pellets with melt flow index of 2.5e4.5 g/10 min (190 C, 2.16 Kg). The commercially available multifunctional epoxide, Joncryl ADR®4368, was kindly supplied by BASF, with epoxy equivalent weight of 285 g/mol. In the current paper, the term “Joncryl” is used to represent Joncryl ADR®4368. 2.1.2. Processing The neat polymers were dried under vacuum at 80 C for 4 hours, before each processing step (melt-compounding and blown film extrusion) and rheological characterizations, in order to avoid hydrolytic degradation. Joncryl was used as received. Compositions containing 0, 40, 60 and 100% PLA in weight were studied, respectively designated PBAT, 40PLA, 60PLA and PLA, in formulations without chain extender. For the formulations containing chain extender, the designation was X/0.3JC and X/0.6JC, indicating formulations with 0.3 and 0.6% in weight, respectively. All the formulations were melt-compounded in a single screw extruder (Wortex) with screw diameter of 30 mm and L/D ratio of 34. The screw used in the current study has a MADDOCK mixing element located at the end of the metering zone. The melt compounding temperature at different zones was independently controlled in order to achieve a temperature profile in the range of 155e170 C. The screw speed was set at 60RPM. The blown films were prepared in a single screw extruder IMACON with 25 mm diameter screw, L/D 22. An annular die of 50 mm diameter, with die gap of 0.85 mm was used to shape the initial tube dimensions. The processing temperature at different zones was set from 160 to 190 C for PLA, 160e190 C for blends and from 140 to 150 C for PBAT. The screw speed in the film extruder was of 20RPM in formulations with neat PLA and 35RPM in the blends and neat PBAT. The take-up speed was set to 2.8 m/ min. 2.1.3. Rheological characterizations Aiming at evaluating the reaction between the chain extender and the neat polymers and their blends, the current study conducted a measurement of torque versus time using an internal mixer HAAKE, model Rheomix 600p, with Roller type rotor, at 170 C and rotor speed of 60RPM. The rheological properties in oscillatory mode of the PLA and PBAT, with and without chain extender, were assessed using an oscillatory shear rheometer ANTON PAAR, model MCR302. The samples were tested using
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parallel-plate geometry (d ¼ 25 mm) with a gap of 1 mm at 170 C. The following tests were performed: a) dynamic time sweep test using angular frequency of 10 rad/s and 1% strain amplitude to assess thermal stability and chain extender reaction; and b) small amplitude oscillatory frequency sweep at angular frequency from 600 to 0.1 rad/s and strain amplitude of 1% to achieve complex viscosity of neat and chain extender modified PLA and PBAT to evaluate viscosity ratio. A strain sweep test was initially conducted to determine and guarantee the linear viscoelastic regime for each formulation. 2.1.4. Crystallinity evaluation by X-ray diffractometry (XRD) X-ray diffraction patterns were recorded using a Shimadzu XRD-6000 diffractometer to qualitatively evaluate the crystallinity of the films. Scans were carried out from 5 to 30 at a scan rate of 2 /min using Ni-filtered CuKa radiation. 2.1.5. Thermal characterization by differential scanning calorimetry (DSC) A NETZSCH differential scanning calorimeter (DSC), model 200F3 Maia, equipped with a liquid nitrogen cooling system was used. The DSC cell was constantly purged with nitrogen at a flow rate of 50 mL/min. Approximately 7e9 mg of sample were sealed in aluminum pans. The temperature programming consisted of three steps; the samples were first cooled to 50 C, heated to 200 C at 10 C/min and held at 200 C for 2 min. They were then cooled to 50 C at 10 C/min and reheated to 200 C at 10 C/min. All the calculated data are the average of two measurements. 2.1.6. Morphological characterization by scanning electron microscopy (SEM) The cross-section of the blown films parallel to the drawn direction (MD) and transversal to the drawn direction (TD) was the object of the morphological analysis characterized by scanning electron microscope (SEM) (JEOL JSM-5700) at an accelerating voltage of 5 kV. The crosssection for the analysis was achieved by cryofracturing the films in liquid nitrogen and then covering with a thin layer of gold. 2.1.7. Mechanical properties The mechanical properties of blown films were obtained by tensile tests, according to ASTM D-882. The tests were conducted using an INSTRON Universal Testing Machine (model 3367) with load cell of 100N and crosshead speed of 50 mm/min. Rectangular specimens were cut from the blown film with the length parallel to the take-up direction or machine direction (MD). 3. Results and discussion 3.1. Rheological characterization of neat and chain-extender modified PBAT and PLA Rheological tests carried out by torque rheometry on PLA and PBAT with and without chain extender are shown in Fig.1. The experiment was conducted to evaluate the
Fig. 1. Torque versus time for PBAT (a) and PLA (b) with and without chain extender.
evolution of the chain extender reaction with the polymer over the mixing time in the molten state. It is known that the torque increase is related to molten polymer viscosity increase, which is caused by molecular weight increase. After mixing for 15 minutes it was observed that the PBAT with 0.3 and 0.6% chain extender showed torque increase of 19% and 23%, respectively, when compared with the neat PBAT torque. The PLA formulations with 0.3% and 0.6% Joncryl showed, respectively, a torque 36% and 59% greater than that of neat PLA, thus indicating greater chain extender reactivity with the PLA than with PBAT. The molten polymer thermal stability over time is an indicator of the chain extender effectiveness. Since the thermal degradation process of the polymers leads to decrease in melt viscosity and elasticity, it is possible to monitor such change by measuring the storage modulus (G0 ) based on time. Fig. 2 shows the evolution of the PLA and PBAT (with and without chain extender) reduced storage modulus (G 0 (t)/G' (0)) against time. After 20 minutes, there was reduction of approximately 4% in the standard storage modulus of the PBAT without chain extender and reduction of 12% in the PLA without chain extender. This decrease was three times greater than that of the PBAT, and showed the lower thermal stability of PLA
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Fig. 2. Reduced storage modulus (G0 (t)/G0 (0)) versus time of neat and extended polymers.
when compared to PBAT. For the PBAT/0.6JC and PLA/0.6JC formulations, there was respective increase of 29% and 47% in the reduced storage modulus, showing more prominent chain extending reaction in the PLA. Al-Itry et al. [15] studied the thermal stability of PLA, PBAT and their blends with 0.25, 0.5 and 1.0 Joncryl, and found similar results. Thus, the use of chain extender not only prevents polymer degradation but also increases melt stability. The complex viscosity plots of the neat polymers and of polymers modified by chain extenders were obtained from rheological tests in oscillatory mode and are shown in Fig.3. It can be seen that complex viscosity decreases with angular frequency in both polymers. This is typical shearthinning behavior. The PBAT/0.6JC complex viscosity increased by 62%, taking the frequency of 0.1rad/s as reference, when compared with that of the neat PBAT. On the other hand, the PLA/0.6JC complex viscosity increased by 209% when compared with the PLA without chain extender. As expected, according to the torque rheometry data, the chain extender showed higher reactivity with the PLA when compared with the PBAT. The increase in complex viscosity is related to the increase in the molecular weight of the polymers due to the reaction of the chain extender epoxy group with both chain-ends of the polymer [19]. This is evidence of the chain extension of the macromolecules, thus confirming the chain extender reactivity.
3.2. Characterization of the films 3.2.1. X-ray diffraction The results from the XRD analysis of PLA and PBAT films and their blends without chain extender are shown in Fig.4. The X-ray diffractogram of the PLA film showed an amorphous halo only. However, four diffraction peaks close to 17.6 , 20.5 , 22.9 and 24.6 [20] were observed in the X-ray diffractogram of the PBAT film, thus indicating the presence of a crystalline structure. The PLA/PBAT blends showed
Fig. 3. Complex viscosity versus angular frequency of neat and modified PBAT (a) and PLA (b).
PBAT-crystalline-phase characteristic peaks and an amorphous halo relating to the PLA. These results indicate that the PLA is amorphous within the blends and that the crystallinity found refers to the PBAT crystalline phase. Similar results were found in films with addition of Joncryl (data not shown). 3.2.2. Thermal properties Table 1 shows the cold crystallization temperature (Tcc), melting temperature (Tf), melting enthalpy (DHf), cold crystallization enthalpy (DHcc) and the degree of crystallinity (Xc) of the films related to the first heating scan. The glass transition temperature (Tg) values were determined in the second heating scan in order to evaluate the miscibility of the systems. Fig. 5 shows the DSC curves of the films of the polymers and of their blends. PLA showed a double endothermic peak (melting temperature) at 155.4 C and 151 C. This double peak is common in polyesters, and the literature justifies its presence by a melt/recrystallization/re-melt mechanism [21]. The PBAT film showed two endothermic peaks at 60.4 C and 126.4 C. The peak at 60.4 C is attributed to the melting of crystalline phase of BA fraction in the sample [22]. For the PLA/PBAT blends, there was an overlap between the PBAT melting region and the PLA cold-crystallization region. Therefore, it
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Fig. 4. X-ray diffraction patterns obtained from blown films of PLA, PBAT and their blends.
Table 1 Melting and crystallization behaviour of the films of PLA, PBAT and PLA/PBAT blends. Formulation
Tag ( C)
Tbg ( C)
Tcc ( C)
DHcc (Jg1)
Tm ( C)
DHm (Jg1)
Xcc (%)
PLA PBAT 40PLA 40PLA/0.3JC 40PLA/0.6JC 60PLA 60PLA/0.3JC 60PLA/0.6JC
60.9 e 58.9 57.9 58.9 59.1 59.2 59.3
e 33.2 34.5 34.5 33.0 35.1 35.3 34.8
115.5 e 95.7 98.6 101.3 98.8 97.4 96.8
25.6 e 7.2 6.0 6.6 12.9 12.8 11.5
151/155.4 60.4/126.5 153.7 154.1 153.7 154.1 153.6 153.0
25.6 21.8 15.7 13.8 15.2 19.4 19.6 19.5
0.0 19.1 12.4 11.4 12.6 14.2 14.9 17.5
Tag ¼ glass transition temperature of PLA, Tbg ¼ glass transition temperature of PBAT, Xcc ¼ degree of crystallinity of PBAT.
was not possible to determine the PBAT melting temperature in the blends. Only the PLA melting temperature was determined, and is presented in Table 1. All blends showed two distinct glass transition temperatures relative to the PLA and PBAT phases. This indicated that the formed blends are thermodynamically immiscible. Similar results were reported by Signori et al. [23] regarding PLA/PBAT blends containing 75wt% of PLA. The PBAT incorporation to the PLA decreased the coldcrystallization temperature by 20 C, thus indicating that the PBAT enabled improvement of the crystallization ability of PLA in the blend, as previously reported [24]. As for the 40PLA blend, the presence of the chain extender led to slight increase in the cold-crystallization temperatures of the blends, which ranged from 95.7 C in the 40PLA to 101.3 C in the 40PLA with 0.6% chain extender.
The XRD data showed that PLA is amorphous in the films whereas PBAT is crystalline, hence just the PBAT fraction was considered as being the crystalline phase of the films. The PLA cold-crystallization enthalpy was subtracted from the sample melting enthalpy to determine the PBAT degree of crystallinity (Xc), which is given by Equation (1):
Xc ¼
DHm DHcc WPBAT H0 m
100%
(1)
where DHf is the sample melting enthalpy, DHcc is the coldcrystallization enthalpy, in this case from PLA. WPBAT is the mass fraction of the PBAT in the blend, DH f is the melting enthalpy of 100% crystalline PBAT. The DH f value used for the PBAT was of 114 J.g1 [25]. The PBAT phase in the blends reduced its degree of crystallinity from 19% in the neat
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Fig. 5. First heating DSC thermograms obtained from blown films of PLA, PBAT and their blends.
PBAT to 12% and 14% in the blends with 40% and 60% PLA, respectively. Since PLA is a lower chain flexibility and higher viscosity component, it restricts the mobility of PBAT chains in the blend, thus hampering the molecular reorganization and thereby reducing the degree of crystallinity. 3.2.3. Morphological characterization The formation of the morphology of immiscible polymer blends is the result of interaction between process variables (temperature, deformation types and rate) and blend components properties (composition, viscosity ratio, interfacial tension, continuous phase viscosity and elasticity of the components). The final morphology is the combination of these factors. The initial particle size, the polymer elasticity, the dispersed phase percentage and the draw ratio [26] are the main factors affecting the morphology formed during drawing. When the dispersed phase concentration gets close to 1:1, complex structures, such as ribbon- or sheet-like, platelet, stratified and cocontinuous structures, are formed. However, the prevalence of one or other structure is basically controlled by factors such as the flow type and the intensity during its processing in the molten state, the viscosity ratio and the interfacial tension. Fig. 6 shows micrographs of the 40PLA film, with and without chain extender, obtained in cross-section longitudinal and transverse to the take-up direction of the film. It is possible to see a characteristic morphology of immiscible blends with low adhesion, in which the PLA is the dispersed phase. Regarding the 40PLA film without chain extender, the PLA dispersed phase presents itself as an elongated and fibrillar structure, preferably arranged towards the drawn direction of the film (take-up direction). This fibrillar morphology is caused by the elongational strain derived from the film drawing process. In 40PLA films containing 0.3 and 0.6% Joncryl, the dispersed phase appears as ellipsoids oriented towards the film drawing. The literature has shown that the chain extenders in the blends can produce copolymers from the link of a polymer chain with the chain of another polymer, in this case the PLA-co-PBAT, thus
improving the compatibility between the PLA and PBAT phases [27]. The formation of this copolymer may acts as compatibilizer and lead to the reduction of the interfacial tension that stabilizes the dispersed phase into smaller sizes, thus leading to refinement of the dispersed phase. Finer morphology as well as viscosity ratio (hd>hm) greater than 1, as in the case of 40PLA/0.3JC and 40PLA/0.6JC, favor the formation of ellipsoids rather than fibrils. Despite the improvement in the interaction between matrix and dispersed phase, probably through the formation of a copolymer (PBAT-co-PLA), it was not enough to promote better adhesion between the matrix and dispersed phase, as can be seen in the micrographs. Fig. 7 shows the micrographs of 60PLA films, with and without chain extender, in the longitudinal and transversal to the take-up direction of the film. These compositions present PBAT as dispersed phase. The characteristic morphology in these blends is ribbon- or sheet-like aligned to drawing direction. A structure similar to the skin-core type can be noticed, in which the dispersed phase is coarse in the middle of the thickness (core region) of the film but, as the dispersed phase gets closer to the film surface (skin region), the morphology presents a finer, fibrillar structure. During the flow through the annular die, the molten polymer is subjected to a pressure-driven flow. This flow type shows maximum shear rate in the region near the die wall, i.e., in the film surface. The shear rate decreases as it gets closer to the middle of the film thickness, where the shear rate is zero. This shear rate gradient produces a coarse morphology in the center, where the shear rate is low to zero. As the dispersed phase gets closer to the surface of the film, it reduces its size due to the high shear rate. As the molten polymer exits the die, the flow becomes elongational in two directions: the drawn direction (MD) and the transversal direction (TD), which is commonly known as biaxial drawing. At this time, the spherical particles are drawn and orientated, thus forming the elongated structures. The skin-core structure type also becomes evident in the blend with 40% PLA without chain extender. The 60PLA film without chain extender showed a coarser dispersed phase than 60PLA blends with chain extender. In the case of 60PLA/0.3JC and 60PLA/0.6JC, the combination of lower interfacial tension and viscosity ratio < 1 led to a more refined fibrillar structure. 3.2.4. Mechanical properties Figs. 8e10 show results for modulus of elasticity, stress at break and elongation at break, respectively, for all studied formulations. Regarding the PLA, with or without chain extender, the tensile-strain behavior showed a marked yield stress (sy) of approximately 55e59MPa with failure by neck instability, and tensile modulus ranging from 2200 to 2584MPa, due to increase in the chain extender content. The increase in these properties may be related to PLA molecular weight increase due to PLA chain end reaction with Joncryl. As for the PBAT, the chain extender increased the tensile strength and the elongation at break. The increase in molecular weight, measured by torque rheometry, as well as in the degree of crystallinity, measured by DSC, may be responsible for the improvement in these properties (Fig.10).
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Fig. 6. Micrographs of the 40PLA film: (a) 40PLA (MD and TD), (b) 40PLA/0.3JC (MD and TD), (c) 40PLA/0.6JC (MD and TD).
From the analysis of the properties of the 40% PLA blends, it is worth highlighting that the dispersed phase consists of PLA with brittle polymer behavior, high modulus of elasticity and high tensile strength, compared with the PBAT. With respect to blends with 60% PLA, the dispersed phase is the PBAT with elastomeric behavior, high ductility and low modulus of elasticity. The graph in Fig.8 shows that the modulus of elasticity meets the additivity rule, since the theoretical elastic moduli were calculated in all studied blends. Table 2 shows the value of the modulus experimentally obtained and calculated by the mixing rule in which good agreement is observed in blends with 60% PLA. Equation (2) was used to calculate the modulus by the mixing rule:
Eteorico ¼ EPBAT VPBAT þ EPLA VPLA
(2)
where EPBAT and EPLA are the moduli experimentally obtained from neat PBAT and PLA, respectively, in each formulation. Such values are shown in Fig.8. VPBAT and VPLA
represent the PBAT and PLA volume fractions, respectively. The density values of 1.25 kg/m3 for the PLA, and of 1.26 kg/ m3 for the PBAT provided by the manufacturers were adopted to calculate the volume fraction. Since in the case of 40PLA, the matrix is PBAT, comparison with respect to neat PBAT modulus, shows an increase of at least 800%. However, there was small decrease in the modulus of elasticity in the case of 40PLA formulations with chain extender when compared with those without chain extender. As discussed in the previous section, the morphology change from elongated and fibrillar structure (40PLA) to deformed droplets in the 40PLA/0.3JC and 40PLA/0.6JC blends (ellipsoids) explains the decrease in the modulus of elasticity in the presence of chain extender, as well as the values lower than those of the mixing rule. Regarding the stress at break, the PLA in the blend led to increase from 11MPa (neat PBAT) to 20.8MPa (40PLA), which corresponds to a 90% increase. As for the 40PLA/0.3JC and 40PLA/0.6JC blends, the chain extender addition did
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Fig. 7. Micrographs of the 60PLA film: (a) 60PLA (MD and TD), (b) 60PLA/0.3JC (MD and TD), (c) 60PLA/0.6JC (MD and TD).
Fig. 8. Elastic modulus for PLA, PBAT and PLA/PBAT blends with different Joncryl concentrations.
Fig. 9. Tensile Stress at Break for PLA, PBAT and PLA/PBAT blends with different Joncryl concentrations.
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Fig. 10. Elongation at break for PLA, PBAT and PLA/PBAT blends with different Joncryl concentrations.
not improve this property and the stress-at-break values were even lower than those of the 40PLA without chain extender, thus indicating that the presence of chain extender did not provide effective reinforcement. It is noteworthy that in blends with 40% PLA, the PLA is the dispersed phase with stress at break of 41MPa for the PLA without extender, and up to 54MPa for the PLA with 0.3% chain extender. These results are explained by the morphology in these blends. In the 40PLA blend, the PLA phase appeared as elongated fibrils in the loading direction during the tensile test. Even with low interaction between the die matrix and the dispersed phase, the elongated fibrillar structure of the dispersed phase was enough to act as mechanical reinforcement, which characterizes the morphology of microfibrilar composites (MFC) [27]. However, in 40PLA blends with chain extender, the PLA phase appeared as ellipsoids, i.e., deformed droplets with low aspect ratio and weak interaction between the matrix and the dispersed phase, and which acted as stress concentrators in the PBAT matrix. These statements become more evident when observing the elongation at break in these blends (Fig.10). For the 40PLA blends, there was a reduction of more than 900% in elongation at break in the presence of chain extender compared with the 40PLA blend without chain extender. This shows that, in this system, the increased interfacial interaction provided by the chain extender (verified by the morphology stabilization in the form of droplets) was not enough to compensate the property losses due to the change in morphology from elongated fibrillar to deformed droplets. As for the 40PLA
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Fig. 11. Tensile stressestrain curves of the PLA and PBAT with different Joncryl concentrations.
formulation, the elongation at break was 325%, and in blends with chain extender it was 58% for 40PLA/0.3JC and 29% for 40PLA/0.6JC. On the other hand, regarding the 60PLA blend, the presence of chain extender allowed 900% increase in elongation at break when compared with the 60PLA blend without chain extender. It also showed 1200% increase when compared with neat PLA (Fig.10), thus enabling a significant improvement in ductility. As for 60PLA blends with chain extender, there was the refinement of the dispersed phase that changed from coarse sheets or platelet to elongated fibrils of PBAT phase. This change in morphology is probably responsible for the increase in elongation at break. A significant improvement in stress at break was also found, which changed from 22.3MPa (without chain extender) to 34e35MPa with chain extender. The mechanical behavior of the studied formulations can be best seen in Figs. 11 and 12, which show
Table 2 Values of experimental and calculated moduli. Formulation
Experimental modulus (MPa)
Theoretical modulus (MPa)
40PLA 40PLA/0.3JC 40PLA/0.6JC 60PLA 60PLA/0.3JC 60PLA/0.6JC
1101 995.3 858.2 1382.3 1598.8 1562.1
959.4 1062 1103.2 1379.6 1537 1596.8
Fig. 12. Tensile stressestrain curves of the 40PLA and 60PLA blends with different Joncryl concentrations.
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stressestrain curves of neat polymers and blends as well as the chain extender effect on the mechanical behavior. The 40PLA, 60PLA/0.3JC and 60PLA/0.6JC blends showed welldefined yielding, followed by stress softening and final strong strain hardening, i.e., stable neck growth during drawing. In the case of formulations with PBAT dispersed phase (60PLA), i.e., when the dispersed phase is of rubbery material, cavitation and interfacial de-bonding may occur [28,29]. In the literature, de-bonding is the proposed mechanism for toughening PLA/PBAT blends [9,30]. There is dispersed phase refinement in the presence of chain extender, thereby reducing the inter-particle distance and increasing the interfacial adhesion between the PLA and PBAT phases. Thus, stress and strain can be more efficiently transferred from the PLA matrix to the PBAT dispersed phase, thus increasing the ductility. Therefore, deformation probably occurs by shear yielding.
4. Conclusions The current study investigated the chain extender effect on the morphology, as well as on the mechanical and rheological properties, of PLA/PBAT blends in the form of blown films. Both PLA and PBAT polymers showed improvement in thermal stability and increase in complex viscosity with the addition of chain extender, however, the chain extender showed greater reactivity with PLA than PBAT. By analyzing the morphology and thermal properties, it was concluded that the chain extender, at the contents used in this work, was not enough to provide increased adhesion between the phases, but it significantly changed the morphology of the blends. Two factors account for the change in morphology: a) the reduction in the interfacial tension, which is probably caused by the formation of PLA/PBAT copolymers in the presence of chain extender and b) the change in the viscosity ratio due to the composition of the blends. Films with 40% PLA, without chain extender, showed PLA dispersed phase in fibrillar and elongated forms oriented towards the film drawing direction. This was responsible for the reinforced and increased ductility of this blend. There was morphology refinement in the presence of chain extender, but the viscosity ratio greater than 1 made the PLA dispersed phase appear as ellipsoids oriented towards the film drawing direction. The low interfacial adhesion between the matrix and the dispersed phase made the PLA rigid particles act as stress concentrators and dramatically reduce the elongation at break of 40PLA films with chain extender. The films with 60% PLA showed skin-core type morphology. Without the presence of the chain extender, the PBAT dispersed phase showed a coarse ribbon- or sheet-like structure in the middle of the film thickness and fine fibrils near the film surface. However, the low adhesion with the PLA matrix led to the reduction of its ductility. Furthermore, in the 60PLA/0.3JC and 60PLA/0.6JC blends, the combination of lower interfacial tension and viscosity ratio < 1 led to finer and homogeneous morphology with elongated fibrillar structure, which was responsible for the significant improvement in the ductility of the films in these formulations.
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