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Nucleation and compatibilization of poly (butylene adipate-co-terephthalate) and polylactide biodegradable composite films.

INTRODUCTION

Poly(butylene adipate-co-terephthalate) (PBAT) is one of the most attractive biodegradable polymers because its high elongation, flexibility, and toughness [1, 2]. PBAT could be used in widespread applications; i.e., biomedical material, food container, and film packaging. However, low modulus and low strength were significant limitations of PBAT [3]. Reinforcing PBAT with other polymers is one of well-known techniques to improve its drawbacks. Commercial polymers such as polystylene (PS) and poly(ethylene terephthalate) (PET) may be considered as a good alternative material for strengthening PBAT. On the other hand, the production process, usage, and waste of these polymers lead to addition of PLA could be used to reinforce the strength of PBAT composite films. For example, the tensile strength of PBAT composite films with 50 wt% of PLA was increased about 32 % compared to that of neat PBAT. However, elongation at break of PBAT/PLA (50/50 w/w) composite films was only about 3 % resulting from incompatibility between two phases of PBAT and PLA [7]. Hence, the nucleating agent and the compatibilizer were used to improve tensile properties and compatibility of the PBAT/PLA composite films. As a result, the objectives of this work were focused on the preparation of PBAT/PLA biodegradable composite films at various compositions. Two types of nucleating agent (talc and nano-precipitated calcium carbonate (NPCC)) were investigated. PLA was used as a reinforcing polymer, while toluenediphenyl diisocyanate (TDI) was used as a compatibilizer at different amounts (1 to 9 wt%) based on PLA contents to enhance an interfacial adhesion between PBAT and PLA.

Experimental:

Materials:

PBAT resin (Ecoflex F BX7011) acted as polymer matrix with a density of 1.26 g/cm3 was purchased from BASF Corporation. The glass transition temperature (Tg) and melting tempaerature (Tm) of PBAt were about -30 and 110 [degrees]C (DSC analysis), while its weight average molecular weight (Mw) and polydispersity index (PDI) were 170 kDa and 1.32 (GPC analysis in THF), respectively. Polylactide (PLA 4042D, Mw = 130 kDa, PDI = 1.46, Tg = 58 [degrees]C, Tm = 152 [degrees]C, density = 1.24 g/cm3) was purchased from NatureWork LLC. NPCC and talc used as a nucleating agent were supplied from Behn Meyer Chemical Co., Ltd., Bangkok, Thailand and Siam Cement Group (SCG Chemicals) Co., Ltd., Rayong, Thailand, respectively. TDI obtained from Siam Chemical Industry Co., Ltd., Bangkok, Thailand was used as a compatibilizer for improving interfacial adhesion of PBAT and its composites.

Sample Preparation:

The PBAT/PLA composites were prepared through a counter-rotating twin screw extruder (L/D = 15/1, TSE 16 TC, PRISM). PLA was used as a reinforcing polymer at different concentrations (10 to 50 wt%), whereas NPCC and talc acted as a nucleating agent were fixed at 2 phr, respectively. TDI were used as compatibilizer at various levels from 1 to 9 wt% based on PLA contents for enhancing an interfacial adhesion between two phases of the composite films. Sample formulations and its abbreviations were displayed in Table 1. The temperature profiles and screw speed of twin screw extruder were controlled on three zones ranging from 110 to 180 [degrees]C and 30 rpm, respectively. The extruded pellets were dried in a vented oven at 60[degrees]C overnight and then compression-molded into the film specimens by hydraulic press (Scientific, Labtech Engineering).

Characterization and Testing:

Rectangular film specimens of tensile testing with the size of 15 mm wide, 150 mm long, and about 250 pm thickness were performed using a universal testing machine, according to the ASTM D882-02. Scanning electron microscope (SEM) was employed to characterize the fractured surface of neat PBAT and PBAT/PLA composites.. The fractured surface of the films was coated with a thin layer of gold before being scanned. The SEM was operated at 15 kV to image the films.

Thermal stability of the composite films was evaluated by using a thermogravimetric analyzer (TGA) under nitrogen atmosphere using a heating rate of 20[degrees]C/min from 50 to 600[degrees]C. Thermal behaviors of the films were characterized by a differential scanning calorimeter (DSC). Sample size with an average weight of 10 - 12 mg encapsulated in a hermetically sealed aluminum pan was prepared for each test. Thermal history of all samples was removed by first heat scanning from 25 [degrees]C to 180 [degrees]C and hold for 3 min to eliminate any thermal history of all samples, followed by quenching the sample with liquid nitrogen to -60 [degrees]C, and finally heating again to 180 [degrees]C at cooling and heating rates of 10 [degrees]C/min, respectively. All experiments were carried out under nitrogen atmosphere. The glass transition temperature (Tg), crystallization temperature (Tc), cold crystallization temperature (Tcc), and melting temperature (Tm) were evaluated, respectively.

RESULTS AND DISCUSSION

Tensile strength and elongation at break of neat PBAT and PBAT/PLA composite films with different amounts of PLA (10 to 50 wt%) are displayed in Figure 1a and 1b, respectively. Neat PBAT showed a ductile behavior with very high elongation at break of 111 % but low tensile strength of only about 6 MPa (Table 2). In addition, the presence of PLA in PBAT composite films led to an increment of tensile strength but significant decreasing of elongation at break compared to neat PBAT. These results indicated that the addition of PLA could be used to reinforce the strength of PBAT composite films. For example, the tensile strength of PBAT composite films with 50 wt% of PLA was increased about 32 % compared to that of neat PBAT. However, elongation at break of PBAT/PLA (50/50 w/w) composite films was only about 3 % resulting from incompatibility between two phases of PBAT and PLA [7]. Hence, the nucleating agent and the compatibilizer were used to improve tensile properties and compatibility of the PBAT/PLA composite films.

Physical appearances of neat PBAT and the PBAT/PLA (70/30 w/w) composite films are displayed in Figure 2a and 2b, respectively. The results illustrated that elongation of neat PBAT film was obviously increased by forming a neck within a narrow region of a width, indicating ductile behavior of neat PBAT. In contrast, the PBAT/PLA (70/30 w/w) composite film was brittle material without formation of elongation area which was in agreement with tensile properties. The more PLA content was added the lower elongation at break of the PBAT biodegradable composite films was (as shown in Table 2).

Figure 3 displays the SEM micrographs of fracture surface of neat PBAT, neat PLA, and PBAT composite films with different contents of PLA. In Figure 3, the SEM micrograph of neat PBAT (3a) displayed smooth fracture surface which was a typical characteristic of ductile material; in contrast, that of neat PLA (3d) illustrated rougher fracture surface which was associated with brittle polymer, respectively. For PBAT/PLA composite film, the SEM images showed the obvious phase separation between PBAT and PLA (Figure 3b and 3 c). A large number of PLA particles on the fracture surface of the films were found. Furthermore, many gaps in fracture surface of the composites which were resulting from pulling away and debonding of PLA particles from PBAT matrix were obviously noticed. These results revealed the incompatibility between two phases of PBAT and PLA which was in agreement with tensile properties of the PBAT/PLA biodegradable composite films without compatibilizer. This is a reason why the tensile properties of the films significantly decreased with the increasing of PLA.

The influence of PLA amounts (0 to 40 wt%) on the thermal stability of neat PBAT and its composites is displayed in Figure 4 and Table 3. TGA thermograms indicated that neat PLA had an initial weight loss at about 330 [degrees]C, responding to thermal decomposition of PLA main chains, whereas neat PBAT illustrated a single main stage of thermal degradation at about of 370 [degrees]C, related to the decomposition of PBAT. Furthermore, TGA thermograms of PBAT/PLA films revealed 2 steps of thermal degradation. The first step was approximately 330 to 350 [degrees]C which was attributed to the decomposition of PLA, whereas the second stage was about 370 to 390 [degrees]C which was associated with the loss of PBAT. These results indicated the presence of PBAT and PLA in the biodegradable composite films.

For evaluating the thermal stability of neat PBAT and its biodegradable composites, the temperature at 20 wt% weight loss of sample was calculated. As shown in Table 3, the temperature used for removing 20 wt% of neat PBAT was 401.7[degrees]C. The temperature at 20 wt% weight loss of the blend films apparently decreased (from 395.3 to 367.9 [degrees]C) when the PLA amount increased (from 10 to 40 wt%). These results implied that thermal stability of the PBAT/PLA biodegradable films decreased with the increment of PLA amount. The PBAT/PLA composite films were lower thermal stability than neat PBAT at all compositions. This was resulting from PLA phase with lower thermal stability (Td = 330 [degrees]C). As a result, with increasing of PLA and PBAT ratio, thermal stability of composite was lower than ones of neat PBAT. Furthermore, data from TGA thermograms revealed that percentage of char remaining at 550 [degrees]C of neat PBAT (4.93 %) was higher than that of neat PLA (0.74 %). The results showed that weight of char remaining of the sample was slightly reduced with the increasing of PLA content. It might be explained that thermal stability of PLA significantly affected to the presence of char residue in composite films.

Figure 5 show the cooling (5a) and 2nd heating (5b) curves of DSC thermograms of neat PBAT, neat PLA, and PBAT biodegradable composite films with different concentrations of PLA. Furthermore, the data of thermal transition temperature, i.e., the glass transition temperature ([T.sub.g]), crystallization temperature ([T.sub.c]), cold-crystallization temperature ([T.sub.cc]), and melting temperature ([T.sub.m]), characterized by a differential scanning calorimetry (DSC) are conclusively displayed in Table 4.

The effect of PLA contents on the DSC thermograms was clearly observed. DSC thermogram of neat PLA displayed the [T.sub.g] at about 59.2 [degrees]C and a melting endothermic peak at 152.5 [degrees]C which represented to residual crystallinity. Similar results have been previously reported by Martin and Averous [8]. Besides, [T.sub.cc] peak appeared at 124.9 [degrees]C in the 2nd heating curve of DSC thermogram of neat PLA. This thermal behavior implied that crystallization of neat PLA in cooling step was not completed due to high cooling rate. As a result, [T.sub.cc] peak could be observed in 2nd heating step of DSC thermogram of neat PLA because some parts of PLA chains were re-crystallized. In case of neat PBAT, its thermogram showed a [T.sub.g] at approximately -30 [degrees]C whereas, a broader peak of [T.sub.m] appeared at around 123.2 [degrees]C and no [T.sub.cc] which was similarly reported by Jiang et al and Yeh et al [7, 9]. Neat PLA showed the thermal behavior of a brittle material with higher [T.sub.g] (compared to room temperature); in contrast, neat PBAT exhibited that of a flexible polymer with lower [T.sub.g]. These results indicated the thermal characteristics of neat PLA and neat PBAT.

When higher level of PLA (from 10 to 40 wt%) was added into the PBAT biodegradable films, [T.sub.g] was significantly increased from 45.8 [degrees]C to 53.7 [degrees]C. It might be due to brittle behavior of PLA which prompted chain stiffness of PBAT/PLA biodegradable composite films. In addition, [T.sub.cc] of the PBAT composite films was obviously observed at about 105 to 106 [degrees]C with the increment of PLA levels from 10 to 40 wt%. Both decreasing of [T.sub.c] and increasing of [T.sub.cc] in the DSC thermograms of composite films indicated that the crystallization behaviors of the films became slower with the presence of PLA. In other words, the addition of PLA retarded crystallization process of PBAT/PLA biodegradable composite films. However, from 2nd heating curves of DSC thermograms, the presence of PLA into the composite film led to obvious appearance of [T.sub.m] which related to degree of crystallinity and mechanical properties of composite films. This is one of reasons why the tensile properties of the composite films increased with the increasing of PLA levels.

Influence of different types of nucleating agent and various levels of TDI on the tensile strength and elongation at break of the PBAT/PLA composite films is exhibited in Figure 6. These results indicated that the tensile strength of the PBAT composite films with 2 phr of nucleating agents (both NPCC and talc) increased with the increment of TDI amounts, while the elongation at break sharply increased with 7 wt% TDI based on PLA content. These results were obviously observed in both of the nucleated PBAT/PLA composite films with NPCC and talc. As shown in Table 5, the nucleated PBAT/PLA composite films with 7 wt% TDI gave the highest tensile properties over the whole composition range. The mechanical data indicated that the tensile strength and elongation at break of the PBAT/PLA/talc composite films with 7 wt% TDI increased about 33 % and 15 %, respectively, whereas those of PBAT/PLA/NPCC composite films with 7 wt% TDI increased about 42 % and 20 %, respectively comparing to the films without TDI. Significantly, the addition of compatibilizer composed of diisocyanate functional group led to interfacial adhesion improvement of the binary mixing [10, 11]. These results can be summarized that on the one hand adding too less level of TDI (i.e., less than 7 wt%) was insufficient for improving the interfacial addition between PBAT and PLA phases, but on the other hand, adding too much amount of TDI (i.e., more than 7 wt%) might cause some cracks in the composites, which led to the reduction of the tensile properties. Thus, the addition of 7 wt% TDI in the nucleated PBAT/PLA composite films could be an appropriate concentration to improve and strengthen the interfacial adhesion of PBAT and PLA, leading to the increasing of tensile properties of the films.

SEM images of the fracture surface after tension of nucleated PBAT/PLA composite films without and with 7 wt% TDI based on PLA amount is illustrated in Figure 7. For SEM images of the PBAT/PLA/talc (90/10/2) and PBAT/PLA/NPCC (90/10/2) composite films without compatibilizer (Figure 7a and 7b), the phase separation between PBAT and PLA was noticed at where some PLA particles detached from PBAT matrix prompting to a large number of holes on the composite fracture surface. These results clearly revealed the incompatibility of PBAT and PLA. The cracking mechanism of the nucleated PBAT/PLA composite films without TDI might predominately take place via the matrix of the agglomerated PLA particles than at the interface between PBAT and PLA.

Nevertheless, a smoother and more uniform fracture surface between PBAT and PLA were showed in the SEM images of compatibilized PBAT/PLA biodegradable composite with talc and NPCC (Figure 7c, 7d). The addition of compatibilizer (7 wt% of TDI based on PLA content) significantly influenced to the fracture surface of the films. Both compatibilized PBAT/PLA/talc and PBAT/PLA/NPCC composite films indicated that the PLA particles were finely dispersed into the PBAT composite films, compared to those without compatibilizer. Many cavities on the fracture surface of the nucleated composite films with TDI evidently decreased. These behaviors indicated that some interfacial adhesion between two phases of PBAT and PLA was obviously improved with the addition of TDI. It might be implied that the toughening mechanism of the PBAT/PLA composite films with the presence of TDI occurred through the interface between PBAT and PLA. These results supported the tensile properties of the nucleated PBAT/PLA composite films which previously discussed.

Conclusion:

Tensile strength of the PBAT/PLA biodegradable composite films was improved with the increment of PLA content; in contrast, elongation at break was significantly decreased due to phase separation between PBAT and PLA, supported by SEM results. Furthermore, data from TGA and DSC thermograms revealed that thermal stability of the composite films was slightly decreased and the crystallization was retarded with the addition of PLA. However, incompatibility between two phases of PBAT and PLA could be improved by using compatibilizer. The addition of TDI on the both of PBAT/PLA/talc and PBAT/PLA/NPCC composite films led to the enhancement of interfacial adhesion and the reduction of phase separation into the composites, as evidenced by tensile and morphological studies. From these data, the appropriate compatibilizer concentration for enhancing the properties of the nucleated composite films was 7 wt% of TDI based on PLA amounts.

ARTICLE INFO

Article history:

Received 12 March 2015

Accepted 28 April 2015

Available online 6 June 2015

ACKNOWLEDGMENT

The authors acknowledged the financial support from Faculty of Science Research Fund (2014), Prince of Songkla University. This work was partially supported by Bioplastic Research Unit, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University. W. Phetwarotai gratefully thanks the Development and Promotion of Science and technology Talents project (DPST).

REFERENCES

[1] Racha, A.I., L. Khalid and M. Abderrahim, 2014. Rheological, morphological, and interfacial properties of compatibilized PLA/PBAT blends. Rheologica Acta, 53: 501-517.

[2] Zhang, N.W., Q.F. Wang, J. Ren and L. Wang, 2009. Preparation and properties of biodegradable poly(lactic acid)/poly(butylene adipate-co-terephthalate) blend with glycidyl methacrylate as reactive processing agent. Journal of Materials Science, 44: 250-256.

[3] Ren, J., H.Y. Fu, T.B. Ren and W.Z. Yuan, 2009. Preparation, characterization and properties of binary and ternary blends with thermoplastic starch, poly(lactic acid) and poly(butylene adipate-co- terephthalate). Carbohydrate Polymers, 77: 576-582.

[4] Pillin, I., N. Montrelay and Y. Grohens, 2006. Thermo-mechanical characterization of plasticized PLA: Is the miscibility the only significant factor? Polymer, 47(13): 4676-4682.

[5] Wang, Y., S.M. Chiao, T.F. Hung and S.Y. Yang, 2012. Improvement in toughness and heat resistance of poly(lactic acid)/polycarbonate blend through twin-screw blending: Influence of compatibilizer type. Journal of Applied Polymer Science, 125(SUPPL. 2): E402-E412.

[6] Phetwarotai, W. and D. Aht-Ong, 2011. Reactive compatibilization of polylactide, thermoplastic starch and poly(butylene adipate-co-terephthalate) biodegradable ternary blend films. Materials Science Forum, 696: 178-181.

[7] Jiang, L., M.P. Wolcott and J.W. Zhang, 2006. Study of biodegradable polyactide/poly(butylene adipate-co-terephthalate) blends. Biomacromolecules, 7: 199-207.

[8] Martin, O. and L. Averous, 2001. Poly(lactic acid): plasticization and properties of biodegradable multiphase systems. Polymer, 42: 6209-6219.

[9] Yeh, J.T., C.H. Tsou and C.Y. Huang, 2010. Compatible and Crystallization Properties of Poly(lactic acid)/Poly(butylene adipate-co-terephthalate) Blends. Journal of Applied Polymer Science, 116(2): 680-687.

[10] Phetwarotai, W., P. Potiyaraj and D. Aht-Ong, 2012. Characteristics of biodegradable polylactide/gelatinized starch films: Effects of starch, plasticizer, and compatibilizer. Journal of Applied Polymer Science, 126: E162-E172.

[11] Wang, H., X.Z. Sun and P. Seib, 2001. Strengthening blends of poly(lactic acid) and starch with methylenediphenyl diisocyanate. Journal of Applied Polymer Science, 82(7): 1761-1767.

(1,3) Worasak Phetwarotai, (1) Ekwipoo Kalkornsurapranee, (1) Poonpailin Janhomklai, (1) Tunsuda Suparanon, (2) Neeranuch Phusunti

(1) Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hatyai, Songkhla 90112, Thailand.

(2) Department of Chemistry, Faculty of Science, Prince of Songkla University, Hatyai, Songkhla 90112, Thailand.

(3) Bioplastic Research Unit, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hatyai, Songkhla 90112, Thailand.

Corresponding Author: W. Phetwarotai, Bioplastic Research Unit, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hatyai, Songkhla 90112, Thailand.

E-mail: w.phetwarotai@hotmail.com

Table 1: Sample formulations of neat PBAT and PBAT/PLA
biodegradable composite films with various types of nucleating
agent and different levels of PLA and TDI.

Sample ID       PBAT       PLA       Nucleating   TDI levels
              contents   contents       agent     (wt% based
               (wt%)      (wt%)       contents     on PLA)
                                        fphrl

                                    Talc   NPCC

Neat PBAT       100         0        --     --        --
PBPL10           90         10       --     --        --
PBPL20           80         20       --     --        --
PBPL30           70         30       --     --        --
PBPL40           60         40       --     --        --
PBPL50           50         50       --     --        --
PBPLIOTa         90         10       2      --        0
PBPLIOTaTD1      90         10       2      --        1
PBPL10TaTD3      90         10       2      --        3
PBPL10TaTD5      90         10       2      --        5
PBPL10TaTD7      90         10       2      --        7
PBPL10TaTD9      90         10       2      --        9
PBPL10NP         90         10       --     2         0
PBPL10NPTD1      90         10       --     2         1
PBPL10NPTD3      90         10       --     2         3
PBPL10NPTD5      90         10       --     2         5
PBPL10NPTD7      90         10       --     2         7
PBPL10NPTD9      90         10       --     2         9

Table 2: Tensile properties of neat PBAT and PBAT/PLA
biodegradable composite films with various amounts of PLA from 10
to 50 wt%.

Sample ID    Tensile strength    Elongation at break (%)
                  (MPa)

Neat PBAT   6.38 [+ or -] 0.20    111.02 [+ or -] 11.24
PBPL10      6.95 [+ or -] 0.39     52.40 [+ or -] 7.32
PBPL20      6.79 [+ or -] 0.28     18.19 [+ or -] 3.57
PBPL30      6.84 [+ or -] 0.65     4.29 [+ or -] 0.75
PBPL40      7.27 [+ or -] 0.97     2.13 [+ or -] 0.39
PBPL50      8.43 [+ or -] 0.29     2.77 [+ or -] 0.25

Table 3: Data from TGA thermograms of neat PBAT, neat PLA, and PBAT
biodegradable composite films with different amounts of PLA.

Sample ID    Temperature at     Char remaining at
              20% of Weight     550[degrees]C (%)
            Loss ([degrees]C)

Neat PBAT         401.7               4.93
PBPL10            395.3               4.25
PBPL20            386.3               4.10
PBPL40            367.9               3.43
Neat PLA          356.5               0.74

Table 4: DSC data of neat PBAT, neat PLA, and PBAT composite films
with various amounts of PLA.

Film Samples     Thermal Transitions ([degrees]C)

               [T.sub.g]   [T.sub.c]   [T.sub.cc]

Neat PBAT        -29.6       96.1          --
PBPL10           45.8        75.4        105.9
PBPL20           52.4        74.3        106.1
PBPL40           53.7        69.6        105.4
Neat PLA         59.2        58.4        124.9

Film Samples     Thermal Transitions
                    ([degrees]C)

               [T.sub.m1]   [T.sub.m2]

Neat PBAT        123.2          --
PBPL10           145.4        152.3
PBPL20           145.6        152.4
PBPL40           145.8        154.1
Neat PLA           --         152.5

Table 5: Tensile properties of PBAT/PLA (90/10 w/w) biodegradable
composite films with various types of nucleating agents and
different concentrations of TDI.

Sample ID             Tensile             Elongation at
                   strength (MPa)           break (%)

PB PL10 Ta       6.55 [+ or -] 0.82   85.83 [+ or -] 13.90
PB PL10 Ta TD1   5.67 [+ or -] 0.68    7.40 [+ or -] 1.44
PB PL10 Ta TD3   6.26 [+ or -] 0.21    22.66 [+ or -] 2.76
PB PL10 Ta TD5   8.04 [+ or -] 0.30    39.09 [+ or -] 1.43
PB PL10 Ta TD7   8.72 [+ or -] 0.60   98.67 [+ or -] 10.36
PB PL10 Ta TD9   7.26 [+ or -] 1.09    11.52 [+ or -] 1.40
PB PL10 NP       6.55 [+ or -] 0.82   85.83 [+ or -] 13.90
PB PL10 NP TD1   5.12 [+ or -] 0.59   31.13 [+ or -] 14.86
PB PL10 NP TD3   6.11 [+ or -] 0.68    44.02 [+ or -] 6.30
PB PL10 NP TD5   5.94 [+ or -] 0.79    50.62 [+ or -] 4.95
PB PL10 NP TD7   9.27 [+ or -] 0.54   103.52 [+ or -] 13.65
PB PL10 NP TD9   6.93 [+ or -] 0.47    66.54 [+ or -] 8.44
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Author:Phetwarotai, Worasak; Kalkornsurapranee, Ekwipoo; Janhomklai, Poonpailin; Suparanon, Tunsuda; Phusun
Publication:Advances in Environmental Biology
Article Type:Report
Date:Jun 30, 2015
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