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high-density polyethylene blends and their glass fiber reinforced composites
Polymer Testing 54 (2016) 90e97
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
Morphology and properties of bio-based poly (lactic acid)/highdensity polyethylene blends and their glass fiber reinforced composites Xiang Lu a, b, Lei Tang b, Lulin Wang b, Jianqing Zhao a, *, Dongdong Li b, Zhaomian Wu b, Peng Xiao b, ** a
Key Laboratory of Polymer Processing Engineering of the Ministry of Education, College of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China Kingfa Scientific and Technological Co. Ltd., Guangzhou, 510663, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2016 Accepted 29 June 2016 Available online 1 July 2016
Poly (lactic acid) (PLA)/high-density polyethylene (HDPE) blends of various proportions and their glass fiber reinforced composites were prepared by melt-compounding. The miscibility, phase morphology, thermal behavior and mechanical properties of the blends were investigated. The blends were immiscible systems with two typical morphologies, spherical droplet and co-continuous, and could be obtained at various compositions. Water contact angle and SEM images indicated that PLA and HDPE are immiscible but can be successfully compatibilized by ethylene-butyl acrylate-glycidyl methacrylate (PTW). Thermal degradation of all blends led to two weight losses, for PLA and HDPE. The incorporation of HDPE improved the thermal stability of the blend. With the addition of 5 wt% PTW, the impact strength of PLA/HDPE/PTW blends (60/40/5 w/w/w) is increased to 18.0 kJ/m2. The effect of glass fiber (GF) on the morphology and mechanical properties of PLA/HDPE/PTW blends (60/40/5 w/w/w) was also investigated. The addition of GF improved the thermal stability of the PLA/HDPE/PTW blends to some extent. The tensile strength of these blends increased and impact strength decreased with increasing GF content. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Poly (lactic acid) High-density polyethylene Blend Properties Morphology
1. Introduction Plastic waste has become a menace to the environment due to the non-biodegradable nature of the most used plastics [1,2]. Poly (lactic acid) (PLA) is one of the environmentally benign polymers due to its biodegradability [3e5]. Because of its high strength and stiffness, excellent transparency and biodegradability, PLA is a promising alternative to some petroleum-based polymers and has been applied in various medical applications, trash bags, table utensils, films, paper coating, fiber and cloth [6,7]. Unfortunately, the inherent brittleness of PLA is the major drawback that has restricted its broader applications that need plastic deformation at high stress levels. Blending PLA with other polymers is the most
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhao), [email protected] (P. Xiao).
(X.
http://dx.doi.org/10.1016/j.polymertesting.2016.06.025 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
Lu),
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attractive and practical route towards modifying its physical properties. General methodologies for toughening PLA materials include the introduction of a rubbery domain in block copolymers, plasticization with a miscible component, or blending with an immiscible rubbery material [8e10]. Various biodegradable or nonbiodegradable polymers, such as poly (butylene succinate) (PBS) [10], poly-[(butylene succinate)-co-adipate] (PBSA) [11], natural rubber (NR) [12], poly (butyleneadipate-co-terephthalate) (PBAT) [13], polyurethane elastomer prepolymer (PUEP) [14], poly(ether) urethane (PEU) elastomer [15] and polypropylene (PP) [16] have been blended with PLA, acting as toughening components. However, PLA might not be miscible with some of these polymers, phase separation usually occurs and leads to the deterioration of properties. Therefore, various studies have aimed at modifying the morphology to increase the compatibility between the two phases [3,11,17e19]. High-density polyethylene (HDPE) is one of the cheaper synthetic polymers with a combination of outstanding physical, chemical, mechanical, thermal and electrical properties. Because of
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its many advantages, such as higher flexibility, higher impact strength and elongation at break, and excellent processability, it is widely used in flexible packaging and containers [20]. As mentioned before, the main problems of PLA are its low impact strength and elongation at break. Blending PLA with HDPE may be one of the simple approaches to overcome these limitations and PLA/HDPE blends are clearly of great technical interest, but rarely has work been done on these two important classes of thermoplastics. In this study, we prepared immiscible and ethylene-butyl acrylate-glycidyl methacrylate (PTW) terpolymer compatibilized PLA/HDPE blends with different compositions. According to the literature [21e23], the epoxide group in the backbone of PTW can react efficiently with the end eOH of polyester under melt processing to form graft copolymers that are interfacially active. Therefore, the effect of weight ratio of PLA/HDPE blends, compatibilizer (PTW) and glass fiber (GF) content on the morphology and properties of blends was investigated. Surprisingly, a co-continuous phase morphology and high impact toughness were observed in the PLA/HDPE blend, which has never been reported before, and could lead to a number of specific end-use applications.
2. Experimental section 2.1. Raw materials Commercial grade PLA (PLA 4032D), with glass transition temperature Tg z 60 C and melting temperature Tm z 170 C, was obtained from Natureworks, LLC (USA). The high-density polyethylene (model DMDA 8007), with a melt flow rate of 8.3 g/10 min (190 C, 2.16 kg), was purchased from the Dow Chemical Company. Ethylene-butyl acrylate-glycidyl methacrylate terpolymer pellets (Elvaloy PTW, designated ‘PTW’) with a melt index of 12 g/10 min (190 C, 2.16 kg), were provided by DuPont Co.; its E/BA/GMA monomer ratio is 66.75/28/5.25 (w/w/w). E-glass fibers (GF), designed as RO99 P319, were supplied by Saint-GobainVetrotex (Vado Ligure, Italy).
2.2. Preparation of PLA/HDPE blends and its GF reinforced composites Before melt blending, the PLA and HDPE pellets were dried in vacuum at 80 C for 4 h. PLA/HDPE blends with different compositions (100/0, 80/20, 60/40, 50/50, 40/60, 20/80, 0/100, w/w) were prepared in a Brabender counter-rotating twin-screw extruder (Germany) with a screw diameter of 25 mm and a length/diameter ratio of 20:1. The profile temperatures were 100, 190, 200, 200, 200, 180, 180, and 210 C, and the screw speed was 60 rpm. To investigate the effect of compatibilizer (PTW) and GF on the morphology and properties of the blends, PLA/HDPE/PTW (100/0/0, 80/20/5, 60/ 40/5, 50/50/5, 40/60/5, 20/80/5, 0/100/0, w/w/w) blends and PLA/ HDPE/PTW/GF (60/40/5/x, x ¼ 0, 5, 10, 20, 30, w/w/w/w) composites were melt-compounded using the same process parameters. All extruded blends were immediately cooled in a water bath, and pelletized. These pellets were dried at 80 C for more than 4 h. After being oven-dried, the pellets were injection-molded into standard specimens for ISO 527 and ISO 179 (EC-75N-Ⅱ, TOSHIBA Machine Co. Ltd., Japan). The temperature profile of the injection barrels was 200, 200, 200 and 200 C from the first heating zone to the nozzle. The injection pressure was set at 55 MPa. All test specimens were conditioned for 48 h at 25 C and 50% relative humidity, prior to testing and characterization.
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2.3. Characterization The fracture surfaces of the blends were studied using a scanning electronic microscope (SEM, HITACHI Se3400N), specimens (4 mm thick) were submerged in liquid nitrogen for about 15 min and fractured to expose the internal structure. Before recording the morphological observations, the sample surfaces were sputtercoated with Au to prevent build-up of electrostatic charge during observations. Contact angle tests were performed with an OCA40 apparatus (Dataphysics Co., Ltd., Germany), static contact angles of distilled water were measured by depositing a drop of 3e5 mL on the sample surface, and the values were estimated as the tangent normal to the drop at the intersection between the sessile drop and the surface. Thermogravimetric analysis (TGA) was performed on samples of about 10 mg using a Netzsch TG209 instrument over 30e600 C in a N2 atmosphere (250 ml/min) with a 10 C/min heating ramp, all thermal parameters were determined as an average of three experiments. Differential scanning calorimetry (DSC) was performed using a Netzsch DSC instrument (model 204c, Germany) equipped with a liquid nitrogen-cooling accessory. Specimens were first heated from room temperature to 200 C at a rate of 10 C/min, kept there for 3 min to eliminate any thermal history, and then cooled to 30 C at a rate of 10 C/min under a nitrogen atmosphere. A second scan was performed by reheating from 30 to 200 C at a rate of 10 C/min. Tensile tests to determine the tensile strength and elongation at break were carried out using an Instron universal machine (model 5566, USA), in accordance with ISO 527 at a single-strain rate of 20 mm/min at room temperature. An Instron POE2000 pendulum impact tester was used for impact testing according ISO 179-1 using type C notch. All the tensile strength, elongation at break and impact strength values were determined in an average of five repeats. 3. Results and discussion 3.1. Miscibility and phase morphology of the PLA/HDPE blends Contact angle measurement is a traditional procedure used for the assessment of the surface energy of solids. Fig. 1 shows digital photos of water contact angle for the PLA/HDPE blends and PLA/ HDPE/PTW blends. The water contact angle of PLA/HDPE blends were 89.7 (80/20), 94.2 (60/40), 96.8 (50/50), 99.1 (40/60) and 102.9 (20/80), respectively, which lie between those of the pure polymers (PLA 79.9 and HDPE 107.2 ). With the addition of 5 wt% PTW to PLA/HDPE blends, the water contact angle of blends 80/20/ 5, 60/40/5, 50/50/5, 40/60/5 and 20/80/5 (PLA/HDPE/PTW) decreased to 82.1, 85.2 , 91.8 , 94.1 and 97.5 , respectively. There were approximately 7.6 , 9.0 , 5.0 , 5.0 and 5.4 lower than that of PLA/HDEP blends (80/20, 60/40. 50/50, 40/60 and 20/80), respectively. This indicated that the compatibility of PLA/HDPE blends was successfully improved with the addition of 5 wt% PTW. Fig. 2 gives the SEM images of fracture surface for the PLA/HDPE blends and PTW compatibilized PLA/HDPE/PTW blends with various blending ratios. A typical island-sea type morphology, where discrete droplets of the minor phase (HDPE) are dispersed in the matrix (PLA), and the average domain size is about 3e5 mm. Surprisingly, on increasing the HDPE content, a co-continuous phase morphology was observed in the PLA/HDPE blends (60/40 and 50/50). During processing, the minor phase is generally broken up to form the dispersed phase. As shown in equ. (1), the viscosity ratio (K) of the dispersed phase and matrix phase, the matrix viscosity (hm), shear rate (g) and interfacial tension (a) are related to
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Fig. 1. Digital photos of water contact angle for the PLA/HDPE blends and PLA/HDPE/PTW blends with various blending ratios, (a) 80/20, (b)60/40, (c) 50/50, (d) 40/60, (e) 20/80, (a’) 80/20/5, (b’) 60/40/5, (c’) 50/50/5, (d’) 40/60/5, (e’) 20/80/5.
the average diameter of the dispersed phase (d) [24]. In this study, the formation of this co-continuous phase is under investigation and will be reported in the future. For a further increase in the HDPE content, the PLA/HDPE (40/60 and 20/80) blend again exhibited distinct island-sea type morphology (with PLA being the island phase and HDPE being the sea phase). Moreover, all the SEM micrographs of PLA/HDPE blends (Fig. 2aee) show that the cryofractured surfaces of the blends were smooth, indicating that PLA and PP are immiscible thermo-dynamically.
d¼
4aK ±0:84
g hm
(1)
In addition, the influence of 5 wt% PTW on the morphology of the PLA/HDPE blends was also studied. As shown in Fig. 2(a’-e’), with the introduction of PTW, the size of discrete droplets decreased for PLA/HDPE/PTW (80/20/5, 40/60/5, 20/80/5) blends and the co-continuous phase morphology was replaced by uniform dispersion (with tiny and uniform droplets of the dispersion phases) for PLA/HDPE/PTW (60/40/5 and 50/50/5) blends. The transformation of morphology indicated that obvious interfacial compatibility reactions occurred between the end eOH of PLA and epoxy groups of PTW, which is in agreement with the results of water contact angle measurements. Hence, it indicated that PTW successfully reduces the surface tension between PLA and HDPE and improves their compatibility. 3.2. Mechanical properties of PLA/HDPE blends The mechanical properties of pure PLA, pure HDPE, PLA/HDPE blends and PLA/HDPE/PTW blends with different compositions are
presented in Fig. 3(aed). Compared to the pure PLA, the tensile strength of the PLA/HDPE blends decreased monotonically with increasing HDPE content. For example, the tensile strengths of PLA/ HDPE blends are 53.4 MPa (80/20), 36.9 MPa (60/40), 34.2 MPa (50/ 50), 30.4 MPa (40/60), 24.5 MPa (20/80), respectively, which lie between those of the pure polymers (PLA 68.2 MPa, HDPE 22.8 MPa). Furthermore, all the PLA/HDPE blends show lower elongation at break (10%) except the 20/80 blend (60.3%), and all the PLA/HDPE blends show lower impact strength (5 kJ/m2). However, with the addition of 5 wt% PTW, the change of tensile strength is not obvious, but the elongation at break of PLA/HDPE/ PTW blends increased to 23.4% (80/20/5), 28.8% (60/40/5), 40.8% (50/50/5), 40.1% (40/60/5), 114.2% (20/80/5), respectively. The impact strength of PLA/HDPE/PTW blends increased to 6.9 kJ/m2 (80/20/5), 18.0 kJ/m2 (60/40/5), 16.1 kJ/m2 (50/50/5), 8.2 kJ/m2 (40/ 60/5) and 12 kJ/m2 (20/80/5), which are approximately 1.6, 4.1, 4.6, 3.9, 4.1 times than that of PLA/HDPE blends (80/20, 60/40, 50/50, 40/60, 20/80), respectively. The improvement of elongation at break and impact strength indicated that PTW successfully acts as a compatibilizer for PLA/HDPE blends.
3.3. Thermal properties of PLA/HDPE blends PLA and HDPE are semicrystalline polymers, so the crystallinity and melting behavior of the blends may strongly depend on the proportion of the blends [3,25]. To confirm the influence of proportion on the thermal properties of PLA/HDPE blends, DSC was employed to measure the crystallinity and melting difference between the pure PLA, pure HDPE and PLA/HDPE blends. Fig. 4(a) shows the cooling curves of the pure PLA, pure HDPE and PLA/HDPE
Fig. 2. SEM images for the PLA/HDPE blends and PLA/HDPE/PTW blends with various blending ratios, (a) 80/20, (b) 60/40, (c) 50/50, (d) 40/60, (e) 20/80, (a’) 80/20/5, (b’) 60/40/5, (c’) 50/50/5, (d’) 40/60/5, (e’) 20/80/5.
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Fig. 3. Mechanical properties of PLA/HDPE and PLA/HDPE/PTW blends, (a) stress-strain curves, (b) tensile strength, (c) elongation at break, (d) impact strength.
blends. For neat PLA, no apparent peaks were noted because it is hardly crystallized during cooling [26]. The HDPE showed a sharp crystallization peak at about 116 C. For the PLA/HDPE blends, there was no apparent crystallization peak appeared for PLA component, and no significant change for the crystallization peak of HDPE. The corresponding heating curves and summary of the DSC results are shown in Fig. 4 (b) and Table 1. The relative crystallinity (Xc) of each component in all samples was calculated as follows:
XcðHDPEÞ ¼
XcðPLAÞ ¼
DHmðHDPEÞ o DHmðHDPEÞ wf ðHDPEÞ
DHmðPLAÞ DHccðPLAÞ o DHmðPLAÞ wf ðPLAÞ
(2)
(3)
where DHm is the measured melting enthalpy (from DSC), DHom is the enthalpy of the original polymer crystal (292 J/g [27] for HDPE and 93 J/g [14] for PLA), DHcc is the cold crystallization enthalpies measured form DSC, and wf is the weight fraction of each component in the blends. It shows that the relative crystallinity of PLA (Xc(PLA)) increased with the HDPE loading, but only slight change in the relative crystallinity of HDPE (Xc(HDPE)) was observed with the addition of PLA. This is probably because the presence of HDPE caused the PLA macromolecule chains to form perfect crystals, and it dominated the increase of Xc(PLA) [26,28].
3.4. Thermal stability of PLA/HDPE blends TGA analysis was also performed on the PLA/HDPE blends. The TGA and corresponding first derivative TGA (DTG) curves of PLA/ HDPE blends are shown in Fig. 5. Finally, Table 2 summarizes the TGA and DTG data for the onset degradation temperature (T5) (the temperature at which 5 wt% degradation occurred), and for the maximum degradation temperature (Tmax) (the peak temperature of the DTG curve). All results are reported as averages of three independent tests. The results indicated that the thermal degradation of all the PLA/HDPE blends consisted of two weight losses between 300 C and 550 C, corresponding to PLA and HDPE, respectively. Table 2 shows that T5 values of the pure PLA, PLA/HDPE blends (80/ 20, 60/40, 50/50, 40/60, 20/80) and pure HDPE are 334.2 C, 337.6 C, 343.2 C, 348.7 C, 358.4 C, 362.5 C and 447.3 C, respectively, which increased with the HDPE content. The Tmax(PLA) of the 80/20, 60/40, 50/50, 40/60, 20/80 blends (PLA/HDPE) appear at approximately 366.1 C, 367.2 C, 369.4 C, 370.5 C and 373.3 C, which are approximately 2.7 C, 3.8 C, 6.0 C, 7.1 C and 9.9 C higher than that of pure PLA (363.4 C), respectively. Simultaneously, the Tmax(HDPE) of the 80/20, 60/40, 50/50, 40/60, 20/80 blends (PLA/HDPE) appear at approximately 482.7 C, 482.3 C, 482.2 C, 482.5 C and 482.4 C, respectively. This is almost the same as that for pure HDPE (482.3 C). All the results indicate that the incorporation of HDPE improved the thermal stability of the PLA component, which is consistent with the results of reference [16].
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Fig. 4. DSC cooling (a) and heating (b) curves of pure PLA, pure HDPE and PLA/HDPE blends.
Table 1 Crystallization temperature (Tc), cold crystallization temperatures (Tcc), melting temperature (Tm), melting enthalpy (DHm), cold crystallization enthalpies (DHcc) and relative crystallinity (Xc) for PLA/HDPE blends. PLA/HDPE (w/w)
Tc(HDPE) ( C)
Tcc(PLA) ( C)
Tm(PLA) ( C)
Tm(HDPE) ( C)
DHm(PLA) (J/g)
DHm(HDPE) (J/g)
DHcc(PLA) (J/g)
Xc(PLA) (%)
Xc(HDPE) (%)
100/0 80/20 60/40 50/50 40/60 20/80 0/100
e 117.2 116.9 116.7 116.3 116.3 116.5
100.7 94.3 94.8 93.2 95.1 e e
169.1 167.9 168.5 167.7 168.5 168.2 e
e 132.4 132.6 131.9 132.7 132.9 133.7
39.8 29.3 24.5 21.3 16.9 8.5 e
e 40.3 83.2 102.3 123.4 163.1 201.0
22.6 12.1 9.1 7.1 4.9 e e
18.5 23.1 27.6 30.5 32.3 45.7 e
e 69.0 71.2 70.1 70.4 69.8 68.8
Fig. 5. TGA (a) and DTG (b) curves of pure PLA, pure HDPE and PLA/HDPE blends.
Table 2 Onset degradation temperature at 5% weight loss (T5) and temperature at maximum degradation rate (Tmax), for pure PLA, pure HDPE and PLA/HDPE Blends. Samples
T5 ( C)
Tmax(PLA) ( C)
Tmax(HDPE) ( C)
Pure PLA 80/20 60/40 50/50 40/60 20/80 Pure HDPE
334.2 337.6 343.2 348.7 358.4 362.5 447.3
363.4 366.1 367.2 369.4 370.5 373.3 e
e 482.7 482.3 482.2 482.5 482.4 482.3
3.5. Dispersion of GF in PLA/HDPE/PTW blends The dispersion state and morphology of GF in the PLA/HDPE/ PTW (60/40/5) matrix with different GF loadings characterized by SEM and the SEM images of the impact fracture surface are shown in Fig. 6. It can be observed that the GF was obviously oriented with the melt flow direction, and no obvious reunion phenomenon was observed, thus illustrating that GF could be distributed homogeneously in the PLA/HDPE/PTW polymer matrix.
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Fig. 6. SEM images for the GF reinforced PLA/HDPE/PTW blends (60/40/5) with various GF loading, (a)5 wt%, (b) 10 wt%, (c) 20 wt%, (d) 30 wt%.
3.6. Effect of GF on thermal stability of PLA/HDPE/PTW blends Fig. 7 and Table 3 show the TGA and the corresponding DTG results for the PLA/HDPE/PTW (60/40/5) blend and PLA/HDPE/ PTW/GF (60/40/5/x) composites with various GF loading. All the samples decomposed in a two-step process (which corresponding to PLA and HDPE, respectively), and the TGA curve of the GF reinforced composites shifted to a higher temperature compared with that of the PLA/HDPE/PTW blend. The T5 of the PLA/HDPE/PTW/GF composites is about 17 C higher at 30 wt% GF loading than that of the pure PLA/HDPE/PTW blend (314.1 C). Furthermore, the
Tmax(PLA) and Tmax(HDPE) of the PLA/HDPE/PTW/GF (60/40/5/30) composites appear at approximately 365.2 C and 478.7 C, which are approximately 10.7 C, and 18.6 C higher than that of the PLA/ HDPE/PTW (60/40/5) blend (Tmax(PLA) ¼ 354.5 C, and Tmax(HDPE) ¼ 460.1 C), respectively. These results indicate that the addition of GF can improve the thermal stability of the PLA/HDPE/ PTW/GF composites to some extent. The improvement in thermal stability can be attributed to the “tortuous path” effect of GF, and this effect delays the escape of the volatile degradation products and the char formation [29,30].
Fig. 7. TGA (a) and DTG (b) curves of GF reinforced PLA/HDPE/PTW blends (60/40/5) with various GF loading.
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Table 3 Onset degradation temperature at 5% weight loss (T5) and temperature at maximum degradation rate (Tmax), for PLA/HDPE/PTW/GF blends. GF content (wt%)
T5 ( C)
Tmax(PLA) ( C)
Tmax(HDPE) ( C)
0 5 10 20 30
314.1 318.9 323.2 324.9 331.3
354.5 359.9 361.7 362.6 365.2
460.1 467.9 472.7 476.9 478.7
at break. Fig. 8(d) shows the impact strength of the PLA/HDPE/ PTW/GF composites with varying concentrations of GF. For the unreinforced PLA/HDPE/PTW (60/40/5) blend, the impact strength is 18.0 kJ/m2. The impact strength of PLA/HDPE/PTW/GF (60/40/5/ x) blends decreases to 8.0 kJ/m2, 8.8 kJ/m2, 7.4 kJ/m2 and 7.6 kJ/m2, respectively, when 5.0, 10.0, 20.0 and 30.0 wt % GF are added. This is also probably due to the introduction of the rigid GF and the poor interfacial interaction between GF and resin matrix [29,33e35].
4. Conclusions 3.7. Effect of GF on mechanical properties of PLA/HDPE/PTW blends Fig. 8 shows the variation of tensile strength, elongation at break and impact strength of the PLA/HDPE/PTW (60/40/5) blends with the GF content of the blends. As presented in Fig. 8, with the increase of GF content from 0 to 30 wt%, the tensile strength of PLA/ HDPE/PTW/GF composites (60/40/5/x, x ¼ 0, 5, 10, 20, 30) increased greatly from 38.9 MPa to 68.2 MPa while the elongation at break decreased from 28.8% to 4.9%. A similar tendency between the filler content and the tensile properties was also observed for other GF reinforced composites [31]. It can be thought that the increase in the tensile strength is due to the introduction of the higher modulus GF [32]. The reason of decrease in the elongation at break is probably because GF in the matrix destroyed the phase morphology of PLA/HDPE/PTW (60/40/5). Additionally, the interfacial adhesion between rigid GF and PLA/HDPE/PTW blends is not being good enough may also contribute to the decline of elongation
In this study, PLA/HDPE and PLA/HDPE/PTW blends of different compositions were prepared and studied. In addition, the effect of GF on the properties of the PLA/HDPE/PTW blends was investigated. Water contact angle and SEM images indicated that PLA and HDPE are immiscible and the compatibility between them can be successfully improved by PTW. Surprisingly, a co-continuous phase morphology was observed for the 60/40 and 50/50 PLA/HDPE compositions. The tensile strength and elongation at break of blends decreased monotonically with increasing HDPE content, but the impact strength of PLA/HDPE/PTW (60/40/5) blends increased to 18.0 kJ/m2. The TGA results indicated that the incorporation of HDPE improved the thermal stability of the PLA component. Furthermore, tensile strength of PLA/HDPE/PTW/GF composites increased much from 38.9 MPa to 68.2 MPa with the addition of 30 wt% GF, and the elongation at break and impact strength decreased to some extent. The addition of GF can also improve the
Fig. 8. Mechanical properties of glass fibers reinforced PLA/HDPE/PTW (60/40/5) composites.
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thermal stability of the PLA/HDPE/PTW/GF composites. Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51435005) the National Natural Science Foundation of China-Guangdong Joint Foundation Project (U1201242), 973 Program (2012CB025902), the Special-funded Program on National Key Scientific Instruments and Equipment Development of China (Grant No. 2012YQ230043). References [1] Y.-X. Weng, Y.-J. Jin, Q.-Y. Meng, L. Wang, M. Zhang, Y.-Z. Wang, Biodegradation behavior of poly(butylene adipate-co-terephthalate) (PBAT), poly(lactic acid) (PLA), and their blend under soil conditions, Polym. Test. 32 (2013) 918e926. [2] R. Muthuraj, M. Misra, A.K. Mohanty, Biodegradable poly(butylene succinate) and poly(butylene adipate-co-terephthalate) blends: reactive extrusion and performance evaluation, J. Polym. Environ. 22 (2014) 336e349. [3] R. Al-Itry, K. Lamnawar, A. Maazouz, Rheological, morphological, and interfacial properties of compatibilized PLA/PBAT blends, Rheol. Acta 53 (2014) 501e517. [4] M.-B. Coltelli, S. Bronco, C. Chinea, The effect of free radical reactions on structure and properties of poly(lactic acid) (PLA) based blends, Polym. Degrad. Stab. 95 (2010) 332e341. [5] C.M. Thurber, Y. Xu, J.C. Myers, T.P. Lodge, C.W. Macosko, Accelerating reactive compatibilization of PE/PLA blends by an interfacially localized catalyst, ACS Macro Lett. 4 (2015) 30e33. [6] H. Zhang, J. Huang, L. Yang, R. Chen, W. Zou, X. Lin, J. Qu, Preparation, characterization and properties of PLA/TiO2nanocomposites based on a novel vane extruder, RSC Adv. 5 (2015) 4639e4647. [7] L. Xie, H. Xu, B. Niu, X. Ji, J. Chen, Z.M. Li, B.S. Hsiao, G.J. Zhong, Unprecedented access to strong and ductile poly(lactic acid) by introducing in situ nanofibrillar poly(butylene succinate) for green packaging, Biomacromolecules 15 (2014) 4054e4064. [8] 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, Polym. Degrad. Stab. 97 (2012) 1898e1914. [9] N. Fu, G. Li, Y. Li, H. Ding, X. Yu, N. Wang, Q. Zhang, X. Qu, Poly(butylene terephthalate)/polyacrylic blends with enhanced toughness, Polym. Plast. Technol. Eng. 53 (2014) 1654e1663. [10] T. Yokohara, M. Yamaguchi, Structure and properties for biomass-based polyester blends of PLA and PBS, Eur. Polym. J. 44 (2008) 677e685. [11] M. Nofar, A. Maani, H. Sojoudi, M.C. Heuzey, P.J. Carreau, Interfacial and rheological properties of PLA/PBAT and PLA/PBSA blends and their morphological stability under shear flow, J. Rheol. 59 (2015) 317e333. [12] Y. Chen, D. Yuan, C. Xu, Dynamically vulcanized biobased polylactide/natural rubber blend material with continuous cross-linked rubber phase, ACS Appl. Mater Interfaces 6 (2014) 3811e3816. [13] L. Goebel, C. Bonten, Influence of the phase morphology on the weldability of PLA/PBAT-blends by using butt-welding, AIP Conf. Proc. 1593 (2014) 312e315. [14] X. Lu, X. Wei, J. Huang, L. Yang, G. Zhang, G. He, M. Wang, J. Qu, Supertoughened poly(lactic acid)/polyurethane blend material by in situ reactive interfacial compatibilization via dynamic vulcanization, Ind. Eng. Chem. Res. 53 (2014) 17386e17393. [15] Y. Li, H. Shimizu, Toughening of polylactide by melt blending with a biodegradable poly(ether)urethane elastomer, Macromol. Biosci. 7 (2007) 921e928.
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[16] H. Kang, X. Lu, Y. Xu, Properties of immiscible and ethylene-butyl acrylateglycidyl methacrylate terpolymer compatibilized poly (lactic acid) and polypropylene blends, Polym. Test. 43 (2015) 173e181. [17] 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, Int. J. Mol. Sci. 14 (2013) 20189e20203. [18] G.-X. Chen, H.-S. Kim, E.-S. Kim, J.-S. Yoon, Compatibilization-like effect of reactive organoclay on the poly(l-lactide)/poly(butylene succinate) blends, Polymer 46 (2005) 11829e11836. [19] H. C., Flavio Vazquez, Katharina Landfester, Hu Guo-Hua, Tha Pith, Morand Larnbla, Reactive blends of thermoplastics and latex particles, Polym. Adv. Technol. 6 (1995) 309e315. [20] J. Zhang, G.M. Rizvi, C.B. Park, M.M. Hasan, Study on cell nucleation behavior of HDPEewood composites/supercritical CO2 solution based on rheological properties, J. Mater. Sci. 46 (2011) 3777e3784. [21] M. Kumar, S. Mohanty, S.K. Nayak, M. Rahail Parvaiz, Effect of glycidyl methacrylate (GMA) on the thermal, mechanical and morphological property of biodegradable PLA/PBAT blend and its nanocomposites, Bioresour. Technol. 101 (2010) 8406e8415. [22] Li Yang, Hong Chen, Shikui Jia, Xiang Lu, Jintao Huang, Xingxing Yu, Kunhao Ye, Guangjian He, J. Qu, Influences of PTW on morphology and mechanical properties of PBTPOE blends, J. Appl. Polym. Sci. (2014), http:// dx.doi.org/10.1002/APP.40660. [23] C. Zhang, G. Dai, Mechanical properties and reactions of PBT/PTW blends, J. Mater. Sci. 42 (2007) 9947e9953. [24] S. Wu, Formation of dispersed phase in incompatible polymer blends_ interfacial and rheological effects, Polym. Eng. Sci. 27 (1987) 335e343. [25] S. Ran, Z. Guo, L. Han, Z. Fang, Effect of a lewis acid catalyst on the performance of HDPE/BFR/GNPs composites, Ind. Eng. Chem. Res. 53 (2014) 4711e4717. €ussler, U. Wagenknecht, [26] L. As’habi, S.H. Jafari, H.A. Khonakdar, L. Ha G. Heinrich, Non-isothermal crystallization behavior of PLA/LLDPE/nanoclay hybrid: synergistic role of LLDPE and clay, Thermochim. Acta 565 (2013) 102e113. [27] S.K. Swain, A.I. Isayev, Effect of ultrasound on HDPE/clay nanocomposites: rheology, structure and properties, Polymer 48 (2007) 281e289. [28] H. Ebadi-Dehaghani, M. Barikani, H.A. Khonakdar, S.H. Jafari, Microstructure and non-isothermal crystallization behavior of PP/PLA/clay hybrid nanocomposites, J. Therm. Anal. Calorim. 121 (2015) 1321e1332. [29] Z. Shuidong, T. Lingcao, L. Jizhao, H. Hanxiong, J. Guo, Relationship between structure and properties of reprocessed glass fiber reinforced flame retardant poly(butylene terephthalate), Polym. Degrad. Stab. 105 (2014) 140e149. [30] X. Wang, L. Song, W. Pornwannchai, Y. Hu, B. Kandola, The effect of graphene presence in flame retarded epoxy resin matrix on the mechanical and flammability properties of glass fiber-reinforced composites, Compos. Part A Appl. Sci. Manuf. 53 (2013) 88e96. [31] X.-L. Xie, Z.-Z. Yu, Q.-X. Zhang, G.-W. Zheng, Y.-W. Mai, Synergistic effect of SEBS-g-MA and epoxy on toughening of polyamide 6/glass fiber composites, J. Polym. Sci. Part B Polym. Phys. 45 (2007) 1448e1458. [32] J.M. Lee, Z.A. Mohd Ishak, R. Mat Taib, T.T. Law, M.Z. Ahmad Thirmizir, Mechanical, thermal and water absorption properties of kenaf-fiber-based polypropylene and poly(butylene succinate) composites, J. Polym. Environ. 21 (2012) 293e302. [33] Y.-H. Feng, Y.-J. Li, B.-P. Xu, D.-W. Zhang, J.-P. Qu, H.-Z. He, Effect of fiber morphology on rheological properties of plant fiber reinforced poly(butylene succinate) composites, Compos. Part B Eng. 44 (2013) 193e199. [34] Z. Liang, P. Pan, B. Zhu, T. Dong, Y. Inoue, Mechanical and thermal properties of poly(butylene succinate)/plant fiber biodegradable composite, J. Appl. Polym. Sci. 115 (2010) 3559e3567. [35] Y. Zhang, C. Yu, P.K. Chu, F. Lv, C. Zhang, J. Ji, R. Zhang, H. Wang, Mechanical and thermal properties of basalt fiber reinforced poly(butylene succinate) composites, Mater. Chem. Phys. 133 (2012) 845e849.
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Material properties
Morphology and properties of bio-based poly (lactic acid)/highdensity polyethylene blends and their glass fiber reinforced composites Xiang Lu a, b, Lei Tang b, Lulin Wang b, Jianqing Zhao a, *, Dongdong Li b, Zhaomian Wu b, Peng Xiao b, ** a
Key Laboratory of Polymer Processing Engineering of the Ministry of Education, College of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China Kingfa Scientific and Technological Co. Ltd., Guangzhou, 510663, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2016 Accepted 29 June 2016 Available online 1 July 2016
Poly (lactic acid) (PLA)/high-density polyethylene (HDPE) blends of various proportions and their glass fiber reinforced composites were prepared by melt-compounding. The miscibility, phase morphology, thermal behavior and mechanical properties of the blends were investigated. The blends were immiscible systems with two typical morphologies, spherical droplet and co-continuous, and could be obtained at various compositions. Water contact angle and SEM images indicated that PLA and HDPE are immiscible but can be successfully compatibilized by ethylene-butyl acrylate-glycidyl methacrylate (PTW). Thermal degradation of all blends led to two weight losses, for PLA and HDPE. The incorporation of HDPE improved the thermal stability of the blend. With the addition of 5 wt% PTW, the impact strength of PLA/HDPE/PTW blends (60/40/5 w/w/w) is increased to 18.0 kJ/m2. The effect of glass fiber (GF) on the morphology and mechanical properties of PLA/HDPE/PTW blends (60/40/5 w/w/w) was also investigated. The addition of GF improved the thermal stability of the PLA/HDPE/PTW blends to some extent. The tensile strength of these blends increased and impact strength decreased with increasing GF content. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Poly (lactic acid) High-density polyethylene Blend Properties Morphology
1. Introduction Plastic waste has become a menace to the environment due to the non-biodegradable nature of the most used plastics [1,2]. Poly (lactic acid) (PLA) is one of the environmentally benign polymers due to its biodegradability [3e5]. Because of its high strength and stiffness, excellent transparency and biodegradability, PLA is a promising alternative to some petroleum-based polymers and has been applied in various medical applications, trash bags, table utensils, films, paper coating, fiber and cloth [6,7]. Unfortunately, the inherent brittleness of PLA is the major drawback that has restricted its broader applications that need plastic deformation at high stress levels. Blending PLA with other polymers is the most
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhao), [email protected] (P. Xiao).
(X.
http://dx.doi.org/10.1016/j.polymertesting.2016.06.025 0142-9418/© 2016 Elsevier Ltd. All rights reserved.
Lu),
[email protected]
attractive and practical route towards modifying its physical properties. General methodologies for toughening PLA materials include the introduction of a rubbery domain in block copolymers, plasticization with a miscible component, or blending with an immiscible rubbery material [8e10]. Various biodegradable or nonbiodegradable polymers, such as poly (butylene succinate) (PBS) [10], poly-[(butylene succinate)-co-adipate] (PBSA) [11], natural rubber (NR) [12], poly (butyleneadipate-co-terephthalate) (PBAT) [13], polyurethane elastomer prepolymer (PUEP) [14], poly(ether) urethane (PEU) elastomer [15] and polypropylene (PP) [16] have been blended with PLA, acting as toughening components. However, PLA might not be miscible with some of these polymers, phase separation usually occurs and leads to the deterioration of properties. Therefore, various studies have aimed at modifying the morphology to increase the compatibility between the two phases [3,11,17e19]. High-density polyethylene (HDPE) is one of the cheaper synthetic polymers with a combination of outstanding physical, chemical, mechanical, thermal and electrical properties. Because of
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its many advantages, such as higher flexibility, higher impact strength and elongation at break, and excellent processability, it is widely used in flexible packaging and containers [20]. As mentioned before, the main problems of PLA are its low impact strength and elongation at break. Blending PLA with HDPE may be one of the simple approaches to overcome these limitations and PLA/HDPE blends are clearly of great technical interest, but rarely has work been done on these two important classes of thermoplastics. In this study, we prepared immiscible and ethylene-butyl acrylate-glycidyl methacrylate (PTW) terpolymer compatibilized PLA/HDPE blends with different compositions. According to the literature [21e23], the epoxide group in the backbone of PTW can react efficiently with the end eOH of polyester under melt processing to form graft copolymers that are interfacially active. Therefore, the effect of weight ratio of PLA/HDPE blends, compatibilizer (PTW) and glass fiber (GF) content on the morphology and properties of blends was investigated. Surprisingly, a co-continuous phase morphology and high impact toughness were observed in the PLA/HDPE blend, which has never been reported before, and could lead to a number of specific end-use applications.
2. Experimental section 2.1. Raw materials Commercial grade PLA (PLA 4032D), with glass transition temperature Tg z 60 C and melting temperature Tm z 170 C, was obtained from Natureworks, LLC (USA). The high-density polyethylene (model DMDA 8007), with a melt flow rate of 8.3 g/10 min (190 C, 2.16 kg), was purchased from the Dow Chemical Company. Ethylene-butyl acrylate-glycidyl methacrylate terpolymer pellets (Elvaloy PTW, designated ‘PTW’) with a melt index of 12 g/10 min (190 C, 2.16 kg), were provided by DuPont Co.; its E/BA/GMA monomer ratio is 66.75/28/5.25 (w/w/w). E-glass fibers (GF), designed as RO99 P319, were supplied by Saint-GobainVetrotex (Vado Ligure, Italy).
2.2. Preparation of PLA/HDPE blends and its GF reinforced composites Before melt blending, the PLA and HDPE pellets were dried in vacuum at 80 C for 4 h. PLA/HDPE blends with different compositions (100/0, 80/20, 60/40, 50/50, 40/60, 20/80, 0/100, w/w) were prepared in a Brabender counter-rotating twin-screw extruder (Germany) with a screw diameter of 25 mm and a length/diameter ratio of 20:1. The profile temperatures were 100, 190, 200, 200, 200, 180, 180, and 210 C, and the screw speed was 60 rpm. To investigate the effect of compatibilizer (PTW) and GF on the morphology and properties of the blends, PLA/HDPE/PTW (100/0/0, 80/20/5, 60/ 40/5, 50/50/5, 40/60/5, 20/80/5, 0/100/0, w/w/w) blends and PLA/ HDPE/PTW/GF (60/40/5/x, x ¼ 0, 5, 10, 20, 30, w/w/w/w) composites were melt-compounded using the same process parameters. All extruded blends were immediately cooled in a water bath, and pelletized. These pellets were dried at 80 C for more than 4 h. After being oven-dried, the pellets were injection-molded into standard specimens for ISO 527 and ISO 179 (EC-75N-Ⅱ, TOSHIBA Machine Co. Ltd., Japan). The temperature profile of the injection barrels was 200, 200, 200 and 200 C from the first heating zone to the nozzle. The injection pressure was set at 55 MPa. All test specimens were conditioned for 48 h at 25 C and 50% relative humidity, prior to testing and characterization.
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2.3. Characterization The fracture surfaces of the blends were studied using a scanning electronic microscope (SEM, HITACHI Se3400N), specimens (4 mm thick) were submerged in liquid nitrogen for about 15 min and fractured to expose the internal structure. Before recording the morphological observations, the sample surfaces were sputtercoated with Au to prevent build-up of electrostatic charge during observations. Contact angle tests were performed with an OCA40 apparatus (Dataphysics Co., Ltd., Germany), static contact angles of distilled water were measured by depositing a drop of 3e5 mL on the sample surface, and the values were estimated as the tangent normal to the drop at the intersection between the sessile drop and the surface. Thermogravimetric analysis (TGA) was performed on samples of about 10 mg using a Netzsch TG209 instrument over 30e600 C in a N2 atmosphere (250 ml/min) with a 10 C/min heating ramp, all thermal parameters were determined as an average of three experiments. Differential scanning calorimetry (DSC) was performed using a Netzsch DSC instrument (model 204c, Germany) equipped with a liquid nitrogen-cooling accessory. Specimens were first heated from room temperature to 200 C at a rate of 10 C/min, kept there for 3 min to eliminate any thermal history, and then cooled to 30 C at a rate of 10 C/min under a nitrogen atmosphere. A second scan was performed by reheating from 30 to 200 C at a rate of 10 C/min. Tensile tests to determine the tensile strength and elongation at break were carried out using an Instron universal machine (model 5566, USA), in accordance with ISO 527 at a single-strain rate of 20 mm/min at room temperature. An Instron POE2000 pendulum impact tester was used for impact testing according ISO 179-1 using type C notch. All the tensile strength, elongation at break and impact strength values were determined in an average of five repeats. 3. Results and discussion 3.1. Miscibility and phase morphology of the PLA/HDPE blends Contact angle measurement is a traditional procedure used for the assessment of the surface energy of solids. Fig. 1 shows digital photos of water contact angle for the PLA/HDPE blends and PLA/ HDPE/PTW blends. The water contact angle of PLA/HDPE blends were 89.7 (80/20), 94.2 (60/40), 96.8 (50/50), 99.1 (40/60) and 102.9 (20/80), respectively, which lie between those of the pure polymers (PLA 79.9 and HDPE 107.2 ). With the addition of 5 wt% PTW to PLA/HDPE blends, the water contact angle of blends 80/20/ 5, 60/40/5, 50/50/5, 40/60/5 and 20/80/5 (PLA/HDPE/PTW) decreased to 82.1, 85.2 , 91.8 , 94.1 and 97.5 , respectively. There were approximately 7.6 , 9.0 , 5.0 , 5.0 and 5.4 lower than that of PLA/HDEP blends (80/20, 60/40. 50/50, 40/60 and 20/80), respectively. This indicated that the compatibility of PLA/HDPE blends was successfully improved with the addition of 5 wt% PTW. Fig. 2 gives the SEM images of fracture surface for the PLA/HDPE blends and PTW compatibilized PLA/HDPE/PTW blends with various blending ratios. A typical island-sea type morphology, where discrete droplets of the minor phase (HDPE) are dispersed in the matrix (PLA), and the average domain size is about 3e5 mm. Surprisingly, on increasing the HDPE content, a co-continuous phase morphology was observed in the PLA/HDPE blends (60/40 and 50/50). During processing, the minor phase is generally broken up to form the dispersed phase. As shown in equ. (1), the viscosity ratio (K) of the dispersed phase and matrix phase, the matrix viscosity (hm), shear rate (g) and interfacial tension (a) are related to
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Fig. 1. Digital photos of water contact angle for the PLA/HDPE blends and PLA/HDPE/PTW blends with various blending ratios, (a) 80/20, (b)60/40, (c) 50/50, (d) 40/60, (e) 20/80, (a’) 80/20/5, (b’) 60/40/5, (c’) 50/50/5, (d’) 40/60/5, (e’) 20/80/5.
the average diameter of the dispersed phase (d) [24]. In this study, the formation of this co-continuous phase is under investigation and will be reported in the future. For a further increase in the HDPE content, the PLA/HDPE (40/60 and 20/80) blend again exhibited distinct island-sea type morphology (with PLA being the island phase and HDPE being the sea phase). Moreover, all the SEM micrographs of PLA/HDPE blends (Fig. 2aee) show that the cryofractured surfaces of the blends were smooth, indicating that PLA and PP are immiscible thermo-dynamically.
d¼
4aK ±0:84
g hm
(1)
In addition, the influence of 5 wt% PTW on the morphology of the PLA/HDPE blends was also studied. As shown in Fig. 2(a’-e’), with the introduction of PTW, the size of discrete droplets decreased for PLA/HDPE/PTW (80/20/5, 40/60/5, 20/80/5) blends and the co-continuous phase morphology was replaced by uniform dispersion (with tiny and uniform droplets of the dispersion phases) for PLA/HDPE/PTW (60/40/5 and 50/50/5) blends. The transformation of morphology indicated that obvious interfacial compatibility reactions occurred between the end eOH of PLA and epoxy groups of PTW, which is in agreement with the results of water contact angle measurements. Hence, it indicated that PTW successfully reduces the surface tension between PLA and HDPE and improves their compatibility. 3.2. Mechanical properties of PLA/HDPE blends The mechanical properties of pure PLA, pure HDPE, PLA/HDPE blends and PLA/HDPE/PTW blends with different compositions are
presented in Fig. 3(aed). Compared to the pure PLA, the tensile strength of the PLA/HDPE blends decreased monotonically with increasing HDPE content. For example, the tensile strengths of PLA/ HDPE blends are 53.4 MPa (80/20), 36.9 MPa (60/40), 34.2 MPa (50/ 50), 30.4 MPa (40/60), 24.5 MPa (20/80), respectively, which lie between those of the pure polymers (PLA 68.2 MPa, HDPE 22.8 MPa). Furthermore, all the PLA/HDPE blends show lower elongation at break (10%) except the 20/80 blend (60.3%), and all the PLA/HDPE blends show lower impact strength (5 kJ/m2). However, with the addition of 5 wt% PTW, the change of tensile strength is not obvious, but the elongation at break of PLA/HDPE/ PTW blends increased to 23.4% (80/20/5), 28.8% (60/40/5), 40.8% (50/50/5), 40.1% (40/60/5), 114.2% (20/80/5), respectively. The impact strength of PLA/HDPE/PTW blends increased to 6.9 kJ/m2 (80/20/5), 18.0 kJ/m2 (60/40/5), 16.1 kJ/m2 (50/50/5), 8.2 kJ/m2 (40/ 60/5) and 12 kJ/m2 (20/80/5), which are approximately 1.6, 4.1, 4.6, 3.9, 4.1 times than that of PLA/HDPE blends (80/20, 60/40, 50/50, 40/60, 20/80), respectively. The improvement of elongation at break and impact strength indicated that PTW successfully acts as a compatibilizer for PLA/HDPE blends.
3.3. Thermal properties of PLA/HDPE blends PLA and HDPE are semicrystalline polymers, so the crystallinity and melting behavior of the blends may strongly depend on the proportion of the blends [3,25]. To confirm the influence of proportion on the thermal properties of PLA/HDPE blends, DSC was employed to measure the crystallinity and melting difference between the pure PLA, pure HDPE and PLA/HDPE blends. Fig. 4(a) shows the cooling curves of the pure PLA, pure HDPE and PLA/HDPE
Fig. 2. SEM images for the PLA/HDPE blends and PLA/HDPE/PTW blends with various blending ratios, (a) 80/20, (b) 60/40, (c) 50/50, (d) 40/60, (e) 20/80, (a’) 80/20/5, (b’) 60/40/5, (c’) 50/50/5, (d’) 40/60/5, (e’) 20/80/5.
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Fig. 3. Mechanical properties of PLA/HDPE and PLA/HDPE/PTW blends, (a) stress-strain curves, (b) tensile strength, (c) elongation at break, (d) impact strength.
blends. For neat PLA, no apparent peaks were noted because it is hardly crystallized during cooling [26]. The HDPE showed a sharp crystallization peak at about 116 C. For the PLA/HDPE blends, there was no apparent crystallization peak appeared for PLA component, and no significant change for the crystallization peak of HDPE. The corresponding heating curves and summary of the DSC results are shown in Fig. 4 (b) and Table 1. The relative crystallinity (Xc) of each component in all samples was calculated as follows:
XcðHDPEÞ ¼
XcðPLAÞ ¼
DHmðHDPEÞ o DHmðHDPEÞ wf ðHDPEÞ
DHmðPLAÞ DHccðPLAÞ o DHmðPLAÞ wf ðPLAÞ
(2)
(3)
where DHm is the measured melting enthalpy (from DSC), DHom is the enthalpy of the original polymer crystal (292 J/g [27] for HDPE and 93 J/g [14] for PLA), DHcc is the cold crystallization enthalpies measured form DSC, and wf is the weight fraction of each component in the blends. It shows that the relative crystallinity of PLA (Xc(PLA)) increased with the HDPE loading, but only slight change in the relative crystallinity of HDPE (Xc(HDPE)) was observed with the addition of PLA. This is probably because the presence of HDPE caused the PLA macromolecule chains to form perfect crystals, and it dominated the increase of Xc(PLA) [26,28].
3.4. Thermal stability of PLA/HDPE blends TGA analysis was also performed on the PLA/HDPE blends. The TGA and corresponding first derivative TGA (DTG) curves of PLA/ HDPE blends are shown in Fig. 5. Finally, Table 2 summarizes the TGA and DTG data for the onset degradation temperature (T5) (the temperature at which 5 wt% degradation occurred), and for the maximum degradation temperature (Tmax) (the peak temperature of the DTG curve). All results are reported as averages of three independent tests. The results indicated that the thermal degradation of all the PLA/HDPE blends consisted of two weight losses between 300 C and 550 C, corresponding to PLA and HDPE, respectively. Table 2 shows that T5 values of the pure PLA, PLA/HDPE blends (80/ 20, 60/40, 50/50, 40/60, 20/80) and pure HDPE are 334.2 C, 337.6 C, 343.2 C, 348.7 C, 358.4 C, 362.5 C and 447.3 C, respectively, which increased with the HDPE content. The Tmax(PLA) of the 80/20, 60/40, 50/50, 40/60, 20/80 blends (PLA/HDPE) appear at approximately 366.1 C, 367.2 C, 369.4 C, 370.5 C and 373.3 C, which are approximately 2.7 C, 3.8 C, 6.0 C, 7.1 C and 9.9 C higher than that of pure PLA (363.4 C), respectively. Simultaneously, the Tmax(HDPE) of the 80/20, 60/40, 50/50, 40/60, 20/80 blends (PLA/HDPE) appear at approximately 482.7 C, 482.3 C, 482.2 C, 482.5 C and 482.4 C, respectively. This is almost the same as that for pure HDPE (482.3 C). All the results indicate that the incorporation of HDPE improved the thermal stability of the PLA component, which is consistent with the results of reference [16].
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Fig. 4. DSC cooling (a) and heating (b) curves of pure PLA, pure HDPE and PLA/HDPE blends.
Table 1 Crystallization temperature (Tc), cold crystallization temperatures (Tcc), melting temperature (Tm), melting enthalpy (DHm), cold crystallization enthalpies (DHcc) and relative crystallinity (Xc) for PLA/HDPE blends. PLA/HDPE (w/w)
Tc(HDPE) ( C)
Tcc(PLA) ( C)
Tm(PLA) ( C)
Tm(HDPE) ( C)
DHm(PLA) (J/g)
DHm(HDPE) (J/g)
DHcc(PLA) (J/g)
Xc(PLA) (%)
Xc(HDPE) (%)
100/0 80/20 60/40 50/50 40/60 20/80 0/100
e 117.2 116.9 116.7 116.3 116.3 116.5
100.7 94.3 94.8 93.2 95.1 e e
169.1 167.9 168.5 167.7 168.5 168.2 e
e 132.4 132.6 131.9 132.7 132.9 133.7
39.8 29.3 24.5 21.3 16.9 8.5 e
e 40.3 83.2 102.3 123.4 163.1 201.0
22.6 12.1 9.1 7.1 4.9 e e
18.5 23.1 27.6 30.5 32.3 45.7 e
e 69.0 71.2 70.1 70.4 69.8 68.8
Fig. 5. TGA (a) and DTG (b) curves of pure PLA, pure HDPE and PLA/HDPE blends.
Table 2 Onset degradation temperature at 5% weight loss (T5) and temperature at maximum degradation rate (Tmax), for pure PLA, pure HDPE and PLA/HDPE Blends. Samples
T5 ( C)
Tmax(PLA) ( C)
Tmax(HDPE) ( C)
Pure PLA 80/20 60/40 50/50 40/60 20/80 Pure HDPE
334.2 337.6 343.2 348.7 358.4 362.5 447.3
363.4 366.1 367.2 369.4 370.5 373.3 e
e 482.7 482.3 482.2 482.5 482.4 482.3
3.5. Dispersion of GF in PLA/HDPE/PTW blends The dispersion state and morphology of GF in the PLA/HDPE/ PTW (60/40/5) matrix with different GF loadings characterized by SEM and the SEM images of the impact fracture surface are shown in Fig. 6. It can be observed that the GF was obviously oriented with the melt flow direction, and no obvious reunion phenomenon was observed, thus illustrating that GF could be distributed homogeneously in the PLA/HDPE/PTW polymer matrix.
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Fig. 6. SEM images for the GF reinforced PLA/HDPE/PTW blends (60/40/5) with various GF loading, (a)5 wt%, (b) 10 wt%, (c) 20 wt%, (d) 30 wt%.
3.6. Effect of GF on thermal stability of PLA/HDPE/PTW blends Fig. 7 and Table 3 show the TGA and the corresponding DTG results for the PLA/HDPE/PTW (60/40/5) blend and PLA/HDPE/ PTW/GF (60/40/5/x) composites with various GF loading. All the samples decomposed in a two-step process (which corresponding to PLA and HDPE, respectively), and the TGA curve of the GF reinforced composites shifted to a higher temperature compared with that of the PLA/HDPE/PTW blend. The T5 of the PLA/HDPE/PTW/GF composites is about 17 C higher at 30 wt% GF loading than that of the pure PLA/HDPE/PTW blend (314.1 C). Furthermore, the
Tmax(PLA) and Tmax(HDPE) of the PLA/HDPE/PTW/GF (60/40/5/30) composites appear at approximately 365.2 C and 478.7 C, which are approximately 10.7 C, and 18.6 C higher than that of the PLA/ HDPE/PTW (60/40/5) blend (Tmax(PLA) ¼ 354.5 C, and Tmax(HDPE) ¼ 460.1 C), respectively. These results indicate that the addition of GF can improve the thermal stability of the PLA/HDPE/ PTW/GF composites to some extent. The improvement in thermal stability can be attributed to the “tortuous path” effect of GF, and this effect delays the escape of the volatile degradation products and the char formation [29,30].
Fig. 7. TGA (a) and DTG (b) curves of GF reinforced PLA/HDPE/PTW blends (60/40/5) with various GF loading.
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Table 3 Onset degradation temperature at 5% weight loss (T5) and temperature at maximum degradation rate (Tmax), for PLA/HDPE/PTW/GF blends. GF content (wt%)
T5 ( C)
Tmax(PLA) ( C)
Tmax(HDPE) ( C)
0 5 10 20 30
314.1 318.9 323.2 324.9 331.3
354.5 359.9 361.7 362.6 365.2
460.1 467.9 472.7 476.9 478.7
at break. Fig. 8(d) shows the impact strength of the PLA/HDPE/ PTW/GF composites with varying concentrations of GF. For the unreinforced PLA/HDPE/PTW (60/40/5) blend, the impact strength is 18.0 kJ/m2. The impact strength of PLA/HDPE/PTW/GF (60/40/5/ x) blends decreases to 8.0 kJ/m2, 8.8 kJ/m2, 7.4 kJ/m2 and 7.6 kJ/m2, respectively, when 5.0, 10.0, 20.0 and 30.0 wt % GF are added. This is also probably due to the introduction of the rigid GF and the poor interfacial interaction between GF and resin matrix [29,33e35].
4. Conclusions 3.7. Effect of GF on mechanical properties of PLA/HDPE/PTW blends Fig. 8 shows the variation of tensile strength, elongation at break and impact strength of the PLA/HDPE/PTW (60/40/5) blends with the GF content of the blends. As presented in Fig. 8, with the increase of GF content from 0 to 30 wt%, the tensile strength of PLA/ HDPE/PTW/GF composites (60/40/5/x, x ¼ 0, 5, 10, 20, 30) increased greatly from 38.9 MPa to 68.2 MPa while the elongation at break decreased from 28.8% to 4.9%. A similar tendency between the filler content and the tensile properties was also observed for other GF reinforced composites [31]. It can be thought that the increase in the tensile strength is due to the introduction of the higher modulus GF [32]. The reason of decrease in the elongation at break is probably because GF in the matrix destroyed the phase morphology of PLA/HDPE/PTW (60/40/5). Additionally, the interfacial adhesion between rigid GF and PLA/HDPE/PTW blends is not being good enough may also contribute to the decline of elongation
In this study, PLA/HDPE and PLA/HDPE/PTW blends of different compositions were prepared and studied. In addition, the effect of GF on the properties of the PLA/HDPE/PTW blends was investigated. Water contact angle and SEM images indicated that PLA and HDPE are immiscible and the compatibility between them can be successfully improved by PTW. Surprisingly, a co-continuous phase morphology was observed for the 60/40 and 50/50 PLA/HDPE compositions. The tensile strength and elongation at break of blends decreased monotonically with increasing HDPE content, but the impact strength of PLA/HDPE/PTW (60/40/5) blends increased to 18.0 kJ/m2. The TGA results indicated that the incorporation of HDPE improved the thermal stability of the PLA component. Furthermore, tensile strength of PLA/HDPE/PTW/GF composites increased much from 38.9 MPa to 68.2 MPa with the addition of 30 wt% GF, and the elongation at break and impact strength decreased to some extent. The addition of GF can also improve the
Fig. 8. Mechanical properties of glass fibers reinforced PLA/HDPE/PTW (60/40/5) composites.
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thermal stability of the PLA/HDPE/PTW/GF composites. Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51435005) the National Natural Science Foundation of China-Guangdong Joint Foundation Project (U1201242), 973 Program (2012CB025902), the Special-funded Program on National Key Scientific Instruments and Equipment Development of China (Grant No. 2012YQ230043). References [1] Y.-X. Weng, Y.-J. Jin, Q.-Y. Meng, L. Wang, M. Zhang, Y.-Z. Wang, Biodegradation behavior of poly(butylene adipate-co-terephthalate) (PBAT), poly(lactic acid) (PLA), and their blend under soil conditions, Polym. Test. 32 (2013) 918e926. [2] R. Muthuraj, M. Misra, A.K. Mohanty, Biodegradable poly(butylene succinate) and poly(butylene adipate-co-terephthalate) blends: reactive extrusion and performance evaluation, J. Polym. Environ. 22 (2014) 336e349. [3] R. Al-Itry, K. Lamnawar, A. Maazouz, Rheological, morphological, and interfacial properties of compatibilized PLA/PBAT blends, Rheol. Acta 53 (2014) 501e517. [4] M.-B. Coltelli, S. Bronco, C. Chinea, The effect of free radical reactions on structure and properties of poly(lactic acid) (PLA) based blends, Polym. Degrad. Stab. 95 (2010) 332e341. [5] C.M. Thurber, Y. Xu, J.C. Myers, T.P. Lodge, C.W. Macosko, Accelerating reactive compatibilization of PE/PLA blends by an interfacially localized catalyst, ACS Macro Lett. 4 (2015) 30e33. [6] H. Zhang, J. Huang, L. Yang, R. Chen, W. Zou, X. Lin, J. Qu, Preparation, characterization and properties of PLA/TiO2nanocomposites based on a novel vane extruder, RSC Adv. 5 (2015) 4639e4647. [7] L. Xie, H. Xu, B. Niu, X. Ji, J. Chen, Z.M. Li, B.S. Hsiao, G.J. Zhong, Unprecedented access to strong and ductile poly(lactic acid) by introducing in situ nanofibrillar poly(butylene succinate) for green packaging, Biomacromolecules 15 (2014) 4054e4064. [8] 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, Polym. Degrad. Stab. 97 (2012) 1898e1914. [9] N. Fu, G. Li, Y. Li, H. Ding, X. Yu, N. Wang, Q. Zhang, X. Qu, Poly(butylene terephthalate)/polyacrylic blends with enhanced toughness, Polym. Plast. Technol. Eng. 53 (2014) 1654e1663. [10] T. Yokohara, M. Yamaguchi, Structure and properties for biomass-based polyester blends of PLA and PBS, Eur. Polym. J. 44 (2008) 677e685. [11] M. Nofar, A. Maani, H. Sojoudi, M.C. Heuzey, P.J. Carreau, Interfacial and rheological properties of PLA/PBAT and PLA/PBSA blends and their morphological stability under shear flow, J. Rheol. 59 (2015) 317e333. [12] Y. Chen, D. Yuan, C. Xu, Dynamically vulcanized biobased polylactide/natural rubber blend material with continuous cross-linked rubber phase, ACS Appl. Mater Interfaces 6 (2014) 3811e3816. [13] L. Goebel, C. Bonten, Influence of the phase morphology on the weldability of PLA/PBAT-blends by using butt-welding, AIP Conf. Proc. 1593 (2014) 312e315. [14] X. Lu, X. Wei, J. Huang, L. Yang, G. Zhang, G. He, M. Wang, J. Qu, Supertoughened poly(lactic acid)/polyurethane blend material by in situ reactive interfacial compatibilization via dynamic vulcanization, Ind. Eng. Chem. Res. 53 (2014) 17386e17393. [15] Y. Li, H. Shimizu, Toughening of polylactide by melt blending with a biodegradable poly(ether)urethane elastomer, Macromol. Biosci. 7 (2007) 921e928.
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