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Preparation and characterization of compatibilized composites of poly(butylene adipate-co-terephthalate) and thermoplastic starch by two-stage extrusion
Journal Pre-proofs Preparation and characterization of compatibilized composites of poly(butylene adipate-co-terephthalate) and thermoplastic starch by two-stage extrusion Wenyong Liu, Shenggong Liu, Zhijie Wang, Jiahao Liu, Bingfeng Dai, Yi Chen, Guangsheng Zeng PII: DOI: Reference:
S0014-3057(19)31617-9 https://doi.org/10.1016/j.eurpolymj.2019.109369 EPJ 109369
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
8 August 2019 15 October 2019 11 November 2019
Please cite this article as: Liu, W., Liu, S., Wang, Z., Liu, J., Dai, B., Chen, Y., Zeng, G., Preparation and characterization of compatibilized composites of poly(butylene adipate-co-terephthalate) and thermoplastic starch by two-stage extrusion, European Polymer Journal (2019), doi: https://doi.org/10.1016/j.eurpolymj.2019.109369
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Preparation and characterization of compatibilized composites of
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poly(butylene adipate-co-terephthalate) and thermoplastic starch by
3
two-stage extrusion
4 5
Wenyong Liua,b,*,[email protected], Shenggong Liub, Zhijie Wangb, Jiahao Liub,
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Bingfeng Daib, Yi Chena, Guangsheng Zenga,**,[email protected]
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aHunan
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of Comprehensive Utilization of Agricultural and Animal Husbandry Waste Resources,
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Hunan International Scientific and Technological Innovation Cooperation Base of
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Biomass Fiber Materials and Application, Hunan University of Technology, Zhuzhou
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412007, China
12
bNational
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Materials Research and Development Technology, Hunan Key Laboratory of Advanced
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Packaging Materials and Technology, College of Packaging and Material Engineering,
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Hunan University of Technology, Zhuzhou 412007, China
Key Laboratory of Biomass Fiber Functional Materials, Hunan Key Laboratory
and Local Joint Engineering Research Center of Advanced Packaging
16 17 18 19 20 21 22
*Corresponding
author: Department of Polymer Materials and Engineering, College of Packaging and Material Engineering, Hunan University of Technology, Zhuzhou 412007, China. **Corresponding author: Department of Polymer Materials and Engineering, College of Packaging and Material Engineering, Hunan University of Technology, Zhuzhou 412007, China.
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Graphical abstract 1
25 26
Highlights
27
PBAT/TPS composites with excellent performance were prepared by simple
28
extrusion
29
Tensile strength increased by 50% after reinforcement and double compatibilization
30
Thermal stability and crystalline structures did not change with additives
31
Composite melt was a typical pseudoplastic fluid, showing easy processibility
32
Improved compatibility was confirmed by SEM, DMA and DSC
33 34 35
Abstract
36
Poly(butylene adipate-co-terephthalate) (PBAT) has good biodegradability and
37
mechanical properties, but its high cost limits its full applications. The incorporation of
38
thermoplastic starch (TPS) into PBAT can significantly lower the cost, but the
39
mechanical properties of the obtained PBAT/TPS composites are dramatically reduced.
40
Herein, the PBAT/TPS composites with excellent mechanical properties were prepared
41
by simple melt-blending extrusion in combination with the reinforcing and
42
compatibilizing strategy. The results showed that the tensile strength of the reinforced
43
and double-compatibilized composite significantly increased by 50%, and its elongation
44
at break increased by 18%. The increased mechanical properties indicated improved
45
compatibility, which was confirmed by SEM. The presence of the additives had no
46
significant effect on the thermal stability and the crystalline structures of the composites.
47
Moreover, the composite melt was a typical pseudoplastic fluid, showing easy
2
48
processibility. The PBAT-based composites with excellent performance and reduced
49
cost by simple melt-extrusion processing will show promising applications.
50 51
Keywords: Poly(butylene adipate-co-terephthalate) (PBAT); Thermoplastic starch
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(TPS); Compatibilized composite; Extrusion
53 54
1. Introduction
55
Nowadays, “white-pollution” from petroleum-based plastics represented by polyolefin
56
is becoming more and more serious. Most of those products cannot be degradable,
57
resulting in adverse effects on the environment [1, 2]. Therefore, biodegradable polymers
58
expected to replace petroleum-based polyolefins have attracted considerable attention in
59
the past few decades [3-8]. As an utterly biodegradable polyester, poly(butylene
60
adipate-co-terephthalate) (PBAT) has similar mechanical properties to polyethylene,
61
which makes it very attractive in practical applications [9-11]. However, the main
62
limitation of PBAT is the high cost, which is not beneficial to its full applications.
63
Therefore, it is strongly expected to reduce the cost by blending with other more
64
cost-efficient materials. Among the cost-efficient materials, as a renewable natural
65
polymer with abundant sources and low cost, starch is exceptionally favored [12-15],
66
which can be plasticized to form thermoplastic starch (TPS) for extensive applications
67
[16-19]. If TPS is blended with PBAT to prepare the PBAT/TPS composite, the cost of
3
68
the used raw materials would be sharply reduced. However, the compatibility between
69
PBAT and starch is weak, leading to the significantly reduced properties of the
70
composites.
71
The major problem of the PBAT/TPS composites is the poor interfacial adhesion
72
between the hydrophobic PBAT and the hydrophilic TPS. Extensive efforts have been
73
made to improve the compatibility between PBAT and TPS by using a variety of
74
compatibilizers, including low molecular compatibilizers and macromolecular
75
compatibilizers [8, 20]. As for the low molecular compatibilizers, MAH [21] was much
76
favored because of its highly reactive anhydride groups and double bond. For the
77
macromolecular compatibilizers, maleated TPS [22], maleated PBAT [21], and
78
styrene-maleic anhydride-glycidyl methacrylate terpolymer [23] were usually used.
79
Among them, poly(ethylene-co-vinyl alcohol) (EVOH) [24-26] with both lipophilic
80
segment and hydrophilic segment might be the right choice for compatibilizing the
81
PBAT/TPS composite [27-29].
82
According to the reported literature about the PBAT/TPS composites, the
83
compatibility between PBAT and TPS had been improved to some extent after
84
compatibilization. However, the mechanical properties were still not satisfactory for the
85
application (such as the weak tensile strength and low elongation at break) because of
86
the limited compatibility. Therefore, a more effective compatibilizer or compatibilizing
87
strategy is very necessary for enhancing the interaction between the lipophilic PBAT and
4
88
the hydrophilic TPS.
89
In addition, to enhance the mechanical properties of a polymer, nano-scale fillers are
90
usually used to prepare polymer-based nanocomposites by the blending method [30-32].
91
Among them, nano-SiO2 has been widely concerned owing to its excellent surface
92
properties. However, the strong interactions between its silanol groups made it easily
93
agglomerate, leading to its inhomogeneous dispersion in composites and poor
94
performance of the obtained composites [33, 34]. Thus, the challenge is to disperse the
95
particles in the polymer matrix homogeneously and to prevent these agglomerates as
96
much as possible. Though the agglomeration of nano-SiO2 can be reduced by its
97
chemical modification, these methods are costly and make the processing more
98
complicated. Therefore, a simple physical blending method [35-37], which can
99
effectively disperse nano-SiO2, would be a better substitute for reducing the cost.
100
Poly(butylene adipate-co-terephthalate) (PBAT) has good biodegradability, and its
101
mechanical properties are close to that of polyethylene. Nevertheless, its high cost limits
102
its full applications. Blending with TPS can significantly lower its cost. In previous
103
reports about PBAT/TPS composites, however, there are still two main problems of poor
104
mechanical properties and compatibility [38-40]. Herein, PBAT-based composites were
105
prepared by a simple melt-blending extrusion. We focused on reducing the cost and
106
improving the compatibility and the mechanical properties of the composites by using
107
nanoparticles and compatibilizers. PBAT was firstly blended with TPS to reduce the cost
5
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by simple extrusion, and the mechanical properties were then improved by reinforcement
109
and compatibilization. It is expected that the compatibility between PBAT and TPS
110
would be improved by the addition of the compatibilizer of MAH or/and plasticized
111
EVOH. The effects of the nanoparticle and the compatibilizers on mechanical properties,
112
thermal properties, morphology, and crystalline structure of the composites were
113
investigated and discussed in detail.
114 115
2. Experimental
116
2.1. Materials
117
PBAT (Ecoflex F Blend C1200, BASF, Germany), cassava starch (Jinguang Starch Co.
118
Ltd., China), and ethylene-vinyl alcohol copolymer (EVOH) with 27 mol % of ethylene
119
(L170B, Kuraray Co. Ltd., Japan) were dried in a drying oven at 80 °C for 48 hours
120
before processing. Glycerol (99.7%, Medical Grade, Xin Jiu Da Chemical Co. Ltd.,
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China), nano-silica (HL-200, GBS High-Tech & Industry Co. Ltd., China), and maleic
122
anhydride (Analytical Grade, Sinopharm Chemical Reagent Co. Ltd., China) were used
123
as received.
124
2.2. Preparation of the composites
125
The preparation of the composites was carried out by two-stage extrusion. Firstly,
126
nano-SiO2 was added into glycerol under stirring and evenly dispersed by ultrasonication.
127
Glycerol with nano-SiO2 was mixed with cassava starch (the mass ratio of starch to
6
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glycerol was 100:40) in a high-speed mixer (SHR-10A, Zhangjiagang Grand Machinery
129
Co. Ltd., China). Then, the first-stage melt-blending extrusion was adopted to prepare
130
the TPS/nano-SiO2 composite by a twin-screw extruder (CTE-35, Coperion Keya
131
Machinery Co. Ltd., China) with the screw speed about 480 rpm in the range of 130-170
132
°C. The extruder has a ratio of length to diameter (L/D) of 42 and a screw diameter of
133
35.5 mm. Finally, the second-stage melt-blending extrusion was used to prepare the
134
multi-component composites of PBAT, TPS/nano-SiO2, MAH or/and plasticized EVOH.
135
The compositions of the composites are shown in Table 1. The samples for testing and
136
characterization were obtained by injection molding (HTF90W1, Haitian Group Co. Ltd.,
137
China) in the range of 160-180 °C. All samples were conditioned at room temperature
138
and 50% relative humidity for 48 hours before the following tests.
139 140
2.3. Characterization
141
2.3.1. Static mechanical tests
142
A universal testing machine (CMT-4104, New Sansi Material Testing Co. Ltd., China)
143
was used to determine the static mechanical properties by tensile and bending tests
144
according to GB/T1040-1992 and GB/T9341-2008 standards (China) at room
145
temperature. A cantilever impact testing machine (550J-2, Wance Testing Machine Co.
146
Ltd., China) was used to obtain the Izod impact strength according to GB/T1843-2008
147
standard (China). At least five specimens for each sample were tested. The strain rate was
7
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20 mm/min for the tensile tests and 2 mm/min for the bending tests, respectively. The
149
dumbbell-type specimens with a length of 150 mm, a width of 10 mm and a thickness of 4
150
mm were used for the tensile tests. The specimens with a length of 80 mm, a width of 10
151
mm and a thickness of 4 mm were used for the bending and impact tests. In the case of
152
impact specimens, there is a notch with 2 mm depth in the middle of one side.
153
2.3.2. Scanning electron microscopy (SEM)
154
The micrographs of the fracture surface of the samples were obtained on a scanning
155
electron microscope (JEOL JSM 6700F, Japan) operated at 5 kV. The specimens were
156
quenched in liquid nitrogen and then fractured. The samples were observed after
157
sputter-coating platinum.
158
2.3.3. Dynamic mechanical analysis (DMA)
159
A dynamic mechanical analyzer (Q8000, PerkinElmer, USA) was used to determine
160
the dynamic mechanical behavior as a function of temperature in a three-point bending
161
mode. The frequency and the strain were 1 Hz and 25 μm, respectively. The range of
162
temperature was from -120 °C to 40 °C at a heating rate of 2 °C/min under nitrogen
163
atmosphere.
164
2.3.4. Thermogravimetric analysis (TGA)
165
The thermal stability of the samples was detected by a thermogravimetric analyzer
166
(Pyris 1 TGA, PerkinElmer, USA). The tests were performed from room temperature to
167
500 °C under nitrogen atmosphere.
8
168
2.3.5. Differential scanning calorimetry (DSC)
169
A differential scanning calorimeter (DSC Q2000, TA Instruments, USA) was used to
170
examine the thermal behavior of the composites. The scanning process consisted of three
171
steps. The samples were first heated from room temperature to 180 °C and remained at
172
this temperature for 5 minutes to eliminate their thermal history. The samples were then
173
cooled from 180 °C to -30 °C and later reheated from -30 °C to 180 °C. All of the tests
174
were performed under nitrogen atmosphere.
175
2.3.6. X-ray diffraction (XRD)
176
A polycrystalline X-ray diffractometer (XRD, Empyrean, PANalytical, Netherlands)
177
with radiation from copper target tube (Cu Kα radiation wavelength of 1.5406 Å) was
178
used to determine the crystalline structure of the composites. The range of the diffraction
179
angle, 2θ, was from 2 to 60°. The angle was increased in steps of 0.03° during the
180
measurement.
181
2.3.7. Rheological behavior tests
182
The steady-state rheological behavior of the samples was tested by a capillary
183
rheometer (Rosand RH7-D, Malvern, UK). The tests were performed at 165 °C and
184
175 °C, respectively. The relationship between shear stress and shear rate of the
185
pseudoplastic fluid can be expressed by the Ostwald-de Wale power-law equation:
186
τ=K∙γn
187
where τ stands for the shear stress, γ corresponds to the shear rate, K is the consistency
9
188
coefficient, and n is the non-Newtonian index. The viscosity was corrected by Bagley,
189
and the shear rate was corrected by Rabinowich. The values of n and K can be obtained
190
from the lg - lg γ curve. For Newtonian fluids, n is equal to 1 while n is lower than 1 for
191
pseudoplastic fluids. The higher the degree of deviation is, the stronger the
192
pseudoplasticity of the material is.
193 194
3. Results and discussion
195
3.1. Static mechanical properties
196
The mechanical properties of the composites are listed in Table 2. The results showed
197
that in comparison with the pure PBAT sample, the P-T sample had a much lower tensile
198
strength (11.6 MPa) and a lower elongation at break (1264%). The significantly reduced
199
tensile strength of the P-T sample indicated the poor compatibility between PBAT and
200
TPS. When nano-SiO2 (0.3 phr) was added into the P-T sample, the P-T-S sample showed
201
the improved tensile strength (15.2 MPa) and a little lower elongation at break (1138%).
202
The result illustrated the noticeable reinforcing effect of nano-SiO2 on the P-T sample.
203
After the addition of MAH, the P-T-S-M sample has almost the same tensile strength as
204
the P-T-S sample, while the former possesses a higher elongation at break and a higher
205
impact strength than the pure PBAT. This result indicated that the toughness of the
206
composite of the P-T-S-M sample became better with the addition of MAH, due to the
207
improved compatibility between PBAT and TPS by MAH. The similar effects of MAH
10
208
were reported by Mohanty [41] for the nanocomposites of PBAT and organically
209
modified layered silicates.
210
When EVOH (4 phr) was used instead of MAH as the compatibilizer, the tensile
211
strength of the P-T-S-E composite increased slightly (from 15.2 MPa to 16.4 MPa), and
212
the elongation at break is a little higher than that of the P-T-S sample. The fact suggested
213
that EVOH played a specific role in compatibilizing the composite of PBAT and TPS.
214
Then, a double-compatibilizer strategy (MAH and EVOH) was used to improve the
215
mechanical properties of the composites. The results showed that both the tensile strength
216
and the elongation at break of the sample P-T-S-M-E were increased significantly. In
217
comparison with the P-T sample, the tensile strength of the P-T-S-M-E sample increased
218
by 50% (up to 17.4 MPa), and its elongation at break increased by 18% (up to 1497%). At
219
the same time, its impact strength increased significantly, showing the improved
220
toughness. These results suggested that the combination of MAH and EVOH had further
221
improved the compatibility and the mechanical properties of the PBAT/TPS composite.
222
In comparison with the previous work under the same ratio of PBAT and TPS[38-40],
223
the PBAT/TPS composites through the reinforcing and the compatibilizing strategy
224
presented better mechanical properties in this work. The results confirmed that the
225
reinforcing and the double-compatibilizing strategy was very useful for improving the
226
mechanical properties of the PBAT/TPS composites. Moreover, the mechanical
227
properties of the obtained composites are superior to those of conventional polyethylene.
11
228
Therefore, the obtained composites with potential biodegradability could be applied
229
instead of the general plastics.
230
The reinforcing and compatibilizing effect could be explained by the interaction
231
between different components. Firstly, nano-SiO2 interacted with TPS to increase the
232
tensile strength of TPS, resulting in the improved tensile strength of the composite
233
P-T-S. Secondly, one anhydride group of MAH might react with the terminal hydroxyl
234
group of PBAT, and the other anhydride group might do with the hydroxyl group of
235
TPS to form ester linkages. The hydrogen atoms on the methylene close to the carbonyl
236
group in PBAT and the double bond on maleic anhydride can form free radicals under
237
high temperature and shearing. Therefore, some of the maleic anhydrides can be grafted
238
onto PBAT to improve its compatibility with TPS [23]. Moreover, the hydrophobic
239
long-chain portion of EVOH can interact with the lipophilic PBAT because of the similar
240
aliphatic segments, while the hydroxyl groups in EVOH can form hydrogen bonds or
241
interact with the hydroxyl groups in TPS [24, 42]. The interaction among EVOH, PBAT
242
and TPS can further increase the compatibility between PBAT and TPS. Thus, the
243
hydrophilic TPS may be encapsulated by PBAT continuous phase, resulting in the
244
improved compatibilization between the two components and the improved mechanical
245
properties of the composites.
246 247
3.2. Morphology
12
248 249 250 251
To further confirm the improved compatibility between PBAT and TPS, the fracture
252
morphologies of the specimens were observed by SEM. The obtained micrographs are
253
shown in Figure 1. As can be seen from Figure 1(a), PBAT forms a continuous phase,
254
while many great TPS phase domains (the particle-like domain) can be found in the
255
PBAT phase, indicating the non-compatibilized status of the P-T-S sample. However, as
256
shown in Figure 1(b), the amount of TPS particles dramatically decreased, and a
257
co-continuous phase was almost formed after the addition of MAH [43], confirming the
258
improved compatibility. As shown in Figure 1(c), when EVOH was used to replace
259
MAH, there was also a co-continuous phase between PBAT and TPS, and fewer TPS
260
particles were observed, which indicated that EVOH also has good compatibilization.
261
When both MAH and EVOH were used together, as shown in Figure 1(d), the fracture
262
structure was more homogeneous, and the continuous phase became more uniform.
263
Therefore, it could be concluded that the double compatibilization of the two
264
compatibilizers is better than each one alone, leading to more improved mechanical
265
properties.
266 267
3.3. Dynamic mechanical analysis
13
268 269 270
The evolution of the storage modulus (E') and loss factor (tanδ) versus temperature of
271
the pure PBAT and the PBAT/TPS composites are shown in Figure 2. As can be seen
272
from Figure 2(a), the storage modulus of all the samples decreased with the increase of
273
temperature, which indicated that the elasticity of the composites weakened with
274
increasing the temperature. In general, if a transition such as the glass transition and
275
secondary transition with temperature occur, a peak will appear in the tanδ curve. The
276
corresponding transition temperature, such as the glass transition temperature (Tg) and
277
secondary relaxation temperature (T), can be determined by the position of the tanδ
278
peaks. As shown in Figure 2(b), there is a single transition at about -30°C, corresponding
279
to the Tg of the pure PBAT. In the case of the P-T sample, the strong peak at about -26°C
280
is attributed to the Tg of the PBAT component, which is a little higher than that of the pure
281
PBAT, probably due to that the addition of TPS and the poor compatibility hindered the
282
movement of PBAT chain. In addition, there is a weak peak at about -46°C,
283
corresponding to the secondary relaxation of the glycerol-rich phase in the P-T sample
284
[44]. However, it is worth noting that the glass transition of the starch-rich phase is not
285
shown in the tanδ curves, probably due to that the glass transition of the starch-rich
286
phase occurred at a higher temperature than the detection temperature [38, 44]. After the
287
addition of nano-SiO2, the two transition temperatures shifted to the higher temperatures
14
288
(-22°C and -41°C, respectively) than those in the P-T sample. This indicated that
289
nano-SiO2 hindered the movement of the PBAT chain and glycerol-rich phase because
290
of the reinforcing function of nano-SiO2. However, after the addition of MAH, the
291
P-T-S-M sample showed the lower transition temperatures (-30°C and -46°C), likely
292
due to the compatibilization and the plasticizing effect of MAH. Compared with those
293
of the P-T-S sample, the P-T-S-E sample also presented the lower transition
294
temperatures (-27°C and -45°C), owing to the promoted movement of the
295
macromolecular chains from the compatibilization of EVOH. When both MAH and
296
EVOH are added simultaneously, the P-T-S-M-E sample also displayed the slightly
297
lower transition temperatures (-28°C and -46°C) than those in the P-T-S-E sample,
298
indicating the more improvement in the compatibility of the components. In a word, the
299
change of the transition temperatures demonstrated the improved interfacial
300
compatibility between PBAT and TPS.
301 302
3.4. Thermo-gravimetric analysis
303 304
The thermogravimetry was used to investigate the thermal stability of the composites.
305
Figure 3(a) shows the TGA curves, and Figure 3(b) presents its derivative curves (DTG)
306
for the samples. The relevant results are summarized in Table 3, listing the temperature
307
values for 10, 25, 50 and 75% mass loss (T10, T25, T50 and T75, respectively), the
15
308
corresponding temperature values of the two main peaks in DTG curves (Tp1 and Tp2) and
309
the residue percent.
310 311
The results showed that PBAT exhibited a single degradation step from 340°C to
312
450°C and the temperature value of the peak in the DTG curve was 413 °C (TpPBAT =
313
413°C). For the P-T sample, a two-step degradation process (270 – 350 °C and 350 –
314
450°C) is observed. The degradation at 270 – 350 °C is attributed to that of TPS, while
315
another degradation at 350 – 450°C is similar to the pure PBAT, corresponding to that of
316
PBAT in the P-T sample. As for the other composites, a distinct two-step degradation
317
process is also visible. The two corresponding temperature values of the two peaks from
318
the different composites (322 – 325°C and 411 – 418°C) were almost the same as the Tp
319
of TPS (TpTPS = 325 °C) in the P-T sample and that of the pure PBAT (TpPBAT = 413°C),
320
respectively. This indicated that the degradation of the composites was a combination of
321
those of TPS and PBAT. Moreover, it could also be confirmed from the similar Tp values
322
that the additives (nano-SiO2 and the compatibilizers) had no apparent effect on the
323
degradation of PBAT and TPS in the composites. These results indicated that the
324
composites had good thermal stability. Besides, the residue existed at high temperature
325
was mainly attributed to nano-SiO2 and the carbonization of the polymer components
326
(TPS, PBAT, and EVOH).
327
16
328
3.5. Differential scanning calorimetry (DSC)
329 330
Figures 4(a) and 4(b) are the DSC curves of the cooling and the reheating process for
331
the samples, corresponding to the crystallization transition and the melting transition,
332
respectively. It could be found that the crystallization transition of the pure PBAT
333
occurred at about 55°C (Tc = 55°C). For the P-T sample, there are two peaks at 84°C
334
and 100°C. The strong peak at 84°C is ascribed to the Tc of PBAT in the P-T sample. The
335
sharp increase (from 55°C to 84°C) is due to the promoted movement and regularity of
336
PBAT molecular chains at a higher temperature by the TPS particles [45]. Another weak
337
peak at 100°C is assigned to the Tc of the TPS phase in the P-T composite. For the P-T-S
338
sample, the Tc-s (85°C and 100°C) are almost the same as those of the P-T sample. After
339
the addition of MAH, however, the Tc of the PBAT component shifted to a high
340
temperature of about 93°C, probably owing to the more favorable crystallization of
341
PBAT at a higher temperature from the compatibilization of MAH. When EVOH instead
342
of MAH was used, there are three peaks of 91°C, 100°C, and 118°C, corresponding to the
343
Tc of PBAT, TPS, and EVOH in P-T-S-E, respectively. The higher Tc of PBAT (91°C)
344
than that for the P-T-S sample (85°C) also indicated the more favorable crystallization of
345
PBAT at a higher temperature promoted by EVOH. When MAH and EVOH were used
346
simultaneously, the higher Tc of the PBAT component (94°C) implied that the more
347
improved compatibility by the double-compatibilizing strategy was more beneficial to
17
348
the crystallization of PBAT at a much higher temperature.
349
As shown in Figure 4(b), the transitions with temperature correspond to the melting
350
transitions, and the corresponding transition temperature is the melting temperature (Tm).
351
For the pure PBAT, a transition is shown at about 125°C, corresponding to the melting of
352
the crystalline PBAT structure (Tm=125°C). After blending TPS with PBAT, two
353
transitions are visible at about 110°C and 125°C, respectively. The transition at 125°C is
354
similar to that for the pure PBAT, which is attributed to the melting of the crystalline
355
PBAT phase in the P-T composite. Another transition at 110°C is ascribed to the melting
356
of the crystalline starch phase.
357
Moreover, for the other composites, there are two melting transitions at 110°C and
358
125°C, respectively, similar to those for the P-T composite. The two transitions had no
359
apparent change with the addition of nano-SiO2 and the compatibilizer (MAH or/and
360
EVOH). This indicated that the additives had no significant influence on the melting
361
transition of the composites. Therefore, the melting processing of the composites would
362
not be affected by the additives, which is beneficial to the melting processing of the
363
composites.
364 365
3.6. X-ray diffraction measurements
366 367
18
368
The wide-angle X-ray diffraction technique was used to investigate the crystalline
369
structures of the composites. The results are shown in Figure 5. The pure PBAT sample
370
shows five peaks at the angles of 16.1º, 17.6º, 20.8º, 23.1º, and 24.9º, which were similar
371
to the characteristic peaks of PBT homopolymer [46]. The result indicated that there
372
were crystalline structures in pure PBAT. For the P-T composite, besides the five peaks,
373
the other two peaks (one strong peak at 21.3º and one weak peak at 18.3º) could be
374
observed, which are corresponding to the crystalline structure of starch [47]. As for the
375
other composites after the addition of nano-SiO2 or/and the compatibilizers, there were
376
also seven peaks without any differences in the positions of the peaks from those of the
377
P-T sample. This result suggested that the crystalline structures in the composites did not
378
change with the addition of nano-SiO2 and the compatibilizers.
379 380
3.7. Rheological analysis
381 382 383 384
Figure 6 shows the relationship between the shear stress and the shear rate for the
385
compatibilized P-T-S-M-E sample at 165°C and 175°C, respectively. The shear stress of
386
the samples increased with the increase of shear rate, indicating the typical pseudoplastic
387
characteristics of the composite melt. The lg - lgγplots maintain a good linear
19
388
correlation after the linear fitting. According to the slope and the intercept of the fitted
389
line, the values of the non-Newtonian index (n) and consistency coefficient (K) could be
390
obtained. With the temperature increase from 165C to 175C, the n value increased, and
391
the K value decreased. This indicated that the pseudo-plasticity of the composite melt
392
became weak, and the Newtonian characteristic increased. Both of the n values were less
393
than 1, confirming a typical pseudoplastic characteristic of the melt. The melt viscosity is
394
relatively low, implying its easy processability [48, 49].
395 396
4. Conclusions
397
The composite of PBAT and TPS with excellent mechanical properties (the tensile
398
strength: 17.4 MPa, the elongation at break: 1496.8%) was obtained when nano-SiO2 was
399
used as a reinforcing agent, and MAH and EVOH were used simultaneously as the
400
compatibilizer by a simple two-stage melt-blending extrusion. The SEM results indicated
401
the improved interfacial compatibility between PBAT and TPS, which was further
402
demonstrated by the decrease in Tg and the increase in Tc of the PBAT component. The
403
TGA results showed that the composites had good thermal stability, which was not
404
affected by the additives. The XRD results confirmed that the crystalline structures of
405
PBAT and starch had no significant changes with the addition of the nanoparticle and the
406
compatibilizers. Moreover, the composite melt was a typical pseudoplastic fluid, showing
407
the weakened pseudo-plasticity and the enhanced Newtonian characteristic with the
20
408
increase of temperature. The PBAT-based composites with good mechanical properties,
409
easy processibility, possible biodegradability, and reduced cost will show broadly
410
potential applications.
411 412
Acknowledgments
413
The work was supported by China Scholarship Council (File No. 201708430086),
414
China Postdoctoral Science Foundation (No. 2016M592444), Natural Science
415
Foundation of Hunan Province of China (No. 2018JJ2088) and Undergraduate
416
Innovation Program of Hunan Province (No. S201911535006).
417 418 419
Data availability Statement The raw/processed data required to reproduce these findings cannot be shared at this
420
time as the data also forms part of an ongoing study.
421
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Polyester/Sepiolite Nanocomposite Properties?, Macromol. Chem. Phys. 208(23) (2010)
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2542-2550.
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26
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528
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529
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531
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534
Preparation and characterization of reinforced starch-based composites with
535
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536
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537
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539
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540
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541
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543
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546
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547
copolyesters, Polym. Degrad. Stab. 83(2) (2004) 289-300.
27
548
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549
starch with maleic acid derivatives by etherification reaction, Carbohydr. Polym. 180
550
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551
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552
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553
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554
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555
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556
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557
film blowing processability, Adv. Polym. Tech. 37(6) (2017) 2061-2074.
558 559 560
Fig. 1. SEM micrographs of the fracture surface of the samples P-T-S (a), P-T-S-M (b),
561
P-T-S-E (c) and P-T-S-M-E (d).
562
Fig. 2. Curves of storage modulus (a) and loss factor (b) versus temperature for the
563
samples.
564
Fig. 3. TGA (a) and DTG (b) curves of the samples.
565
Fig. 4. DSC curves of cooling (a) and second heating (b) process for the samples.
566
Fig. 5. XRD results for the samples.
567
Fig. 6. Relationship between shear stress and shear rate for the P-T-S-M-E sample at two
568
different temperatures. 28
569 570
Table 1. Compositions of the various samples PBAT
TPS
Nano-SiO2
MAH
EVOH
(wt.%)
(wt.%)
(phr)
(phr)
(phr)
PBAT
100
-
-
-
-
P-T
80
20
-
-
-
P-T-S
80
20
0.3
-
-
P-T-S-M
80
20
0.3
1.6
-
P-T-S-E
80
20
0.3
-
4
P-T-S-M-E
80
20
0.3
1.6
4
Samples
571 572
Table 2. Mechanical properties of the samples Tensile
Youngs
Flexural
Flexural
Impact
strength
modulus
strength
(MPa)
(MPa)
(KJ/m2)
Elongation at Samples
strength
modulus break (%)
(MPa)
(MPa)
PBAT
26.70.3
25.10.3
1383.230.2
3.80.1
169.733.9
22.50.5
P-T
11.60.4
30.80.5
1264.314.5
3.70.2
126.411.5
24.30.5
P-T-S
15.20.2
40.90.9
1138.518.9
4.70.2
146.920.1
26.90.5
P-T-S-M
15.50.3
31.31.2
1401.117.2
3.10.2
97.84.6
28.80.8
P-T-S-E
16.40.3
40.20.8
1141.54.8
3.90.2
127.53.8
24.90.7
29
P-T-S-M
27.20.9 17.40.1
112.07.3 1496.850.5
4.00.1
30.61.8
-E
573 574
Table 3. TGA data of the composites Samples
T10 (°C)
T25 (°C)
T50 (°C)
T75 (°C)
Tp1 (°C)
Tp2 (°C)
Residue (%)
PBAT
384
398
410
421
-
413
4.77
P-T
317
381
406
419
325
412
4.15
P-T-S
319
385
407
420
322
416
6.39
P-T-S-M
320
385
405
418
323
411
5.43
P-T-S-E
323
390
410
423
325
418
5.91
P-T-S-M-E
319
386
404
417
324
412
5.75
575 576
30
S0014-3057(19)31617-9 https://doi.org/10.1016/j.eurpolymj.2019.109369 EPJ 109369
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
8 August 2019 15 October 2019 11 November 2019
Please cite this article as: Liu, W., Liu, S., Wang, Z., Liu, J., Dai, B., Chen, Y., Zeng, G., Preparation and characterization of compatibilized composites of poly(butylene adipate-co-terephthalate) and thermoplastic starch by two-stage extrusion, European Polymer Journal (2019), doi: https://doi.org/10.1016/j.eurpolymj.2019.109369
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© 2019 Published by Elsevier Ltd.
1
Preparation and characterization of compatibilized composites of
2
poly(butylene adipate-co-terephthalate) and thermoplastic starch by
3
two-stage extrusion
4 5
Wenyong Liua,b,*,[email protected], Shenggong Liub, Zhijie Wangb, Jiahao Liub,
6
Bingfeng Daib, Yi Chena, Guangsheng Zenga,**,[email protected]
7
aHunan
8
of Comprehensive Utilization of Agricultural and Animal Husbandry Waste Resources,
9
Hunan International Scientific and Technological Innovation Cooperation Base of
10
Biomass Fiber Materials and Application, Hunan University of Technology, Zhuzhou
11
412007, China
12
bNational
13
Materials Research and Development Technology, Hunan Key Laboratory of Advanced
14
Packaging Materials and Technology, College of Packaging and Material Engineering,
15
Hunan University of Technology, Zhuzhou 412007, China
Key Laboratory of Biomass Fiber Functional Materials, Hunan Key Laboratory
and Local Joint Engineering Research Center of Advanced Packaging
16 17 18 19 20 21 22
*Corresponding
author: Department of Polymer Materials and Engineering, College of Packaging and Material Engineering, Hunan University of Technology, Zhuzhou 412007, China. **Corresponding author: Department of Polymer Materials and Engineering, College of Packaging and Material Engineering, Hunan University of Technology, Zhuzhou 412007, China.
23 24
Graphical abstract 1
25 26
Highlights
27
PBAT/TPS composites with excellent performance were prepared by simple
28
extrusion
29
Tensile strength increased by 50% after reinforcement and double compatibilization
30
Thermal stability and crystalline structures did not change with additives
31
Composite melt was a typical pseudoplastic fluid, showing easy processibility
32
Improved compatibility was confirmed by SEM, DMA and DSC
33 34 35
Abstract
36
Poly(butylene adipate-co-terephthalate) (PBAT) has good biodegradability and
37
mechanical properties, but its high cost limits its full applications. The incorporation of
38
thermoplastic starch (TPS) into PBAT can significantly lower the cost, but the
39
mechanical properties of the obtained PBAT/TPS composites are dramatically reduced.
40
Herein, the PBAT/TPS composites with excellent mechanical properties were prepared
41
by simple melt-blending extrusion in combination with the reinforcing and
42
compatibilizing strategy. The results showed that the tensile strength of the reinforced
43
and double-compatibilized composite significantly increased by 50%, and its elongation
44
at break increased by 18%. The increased mechanical properties indicated improved
45
compatibility, which was confirmed by SEM. The presence of the additives had no
46
significant effect on the thermal stability and the crystalline structures of the composites.
47
Moreover, the composite melt was a typical pseudoplastic fluid, showing easy
2
48
processibility. The PBAT-based composites with excellent performance and reduced
49
cost by simple melt-extrusion processing will show promising applications.
50 51
Keywords: Poly(butylene adipate-co-terephthalate) (PBAT); Thermoplastic starch
52
(TPS); Compatibilized composite; Extrusion
53 54
1. Introduction
55
Nowadays, “white-pollution” from petroleum-based plastics represented by polyolefin
56
is becoming more and more serious. Most of those products cannot be degradable,
57
resulting in adverse effects on the environment [1, 2]. Therefore, biodegradable polymers
58
expected to replace petroleum-based polyolefins have attracted considerable attention in
59
the past few decades [3-8]. As an utterly biodegradable polyester, poly(butylene
60
adipate-co-terephthalate) (PBAT) has similar mechanical properties to polyethylene,
61
which makes it very attractive in practical applications [9-11]. However, the main
62
limitation of PBAT is the high cost, which is not beneficial to its full applications.
63
Therefore, it is strongly expected to reduce the cost by blending with other more
64
cost-efficient materials. Among the cost-efficient materials, as a renewable natural
65
polymer with abundant sources and low cost, starch is exceptionally favored [12-15],
66
which can be plasticized to form thermoplastic starch (TPS) for extensive applications
67
[16-19]. If TPS is blended with PBAT to prepare the PBAT/TPS composite, the cost of
3
68
the used raw materials would be sharply reduced. However, the compatibility between
69
PBAT and starch is weak, leading to the significantly reduced properties of the
70
composites.
71
The major problem of the PBAT/TPS composites is the poor interfacial adhesion
72
between the hydrophobic PBAT and the hydrophilic TPS. Extensive efforts have been
73
made to improve the compatibility between PBAT and TPS by using a variety of
74
compatibilizers, including low molecular compatibilizers and macromolecular
75
compatibilizers [8, 20]. As for the low molecular compatibilizers, MAH [21] was much
76
favored because of its highly reactive anhydride groups and double bond. For the
77
macromolecular compatibilizers, maleated TPS [22], maleated PBAT [21], and
78
styrene-maleic anhydride-glycidyl methacrylate terpolymer [23] were usually used.
79
Among them, poly(ethylene-co-vinyl alcohol) (EVOH) [24-26] with both lipophilic
80
segment and hydrophilic segment might be the right choice for compatibilizing the
81
PBAT/TPS composite [27-29].
82
According to the reported literature about the PBAT/TPS composites, the
83
compatibility between PBAT and TPS had been improved to some extent after
84
compatibilization. However, the mechanical properties were still not satisfactory for the
85
application (such as the weak tensile strength and low elongation at break) because of
86
the limited compatibility. Therefore, a more effective compatibilizer or compatibilizing
87
strategy is very necessary for enhancing the interaction between the lipophilic PBAT and
4
88
the hydrophilic TPS.
89
In addition, to enhance the mechanical properties of a polymer, nano-scale fillers are
90
usually used to prepare polymer-based nanocomposites by the blending method [30-32].
91
Among them, nano-SiO2 has been widely concerned owing to its excellent surface
92
properties. However, the strong interactions between its silanol groups made it easily
93
agglomerate, leading to its inhomogeneous dispersion in composites and poor
94
performance of the obtained composites [33, 34]. Thus, the challenge is to disperse the
95
particles in the polymer matrix homogeneously and to prevent these agglomerates as
96
much as possible. Though the agglomeration of nano-SiO2 can be reduced by its
97
chemical modification, these methods are costly and make the processing more
98
complicated. Therefore, a simple physical blending method [35-37], which can
99
effectively disperse nano-SiO2, would be a better substitute for reducing the cost.
100
Poly(butylene adipate-co-terephthalate) (PBAT) has good biodegradability, and its
101
mechanical properties are close to that of polyethylene. Nevertheless, its high cost limits
102
its full applications. Blending with TPS can significantly lower its cost. In previous
103
reports about PBAT/TPS composites, however, there are still two main problems of poor
104
mechanical properties and compatibility [38-40]. Herein, PBAT-based composites were
105
prepared by a simple melt-blending extrusion. We focused on reducing the cost and
106
improving the compatibility and the mechanical properties of the composites by using
107
nanoparticles and compatibilizers. PBAT was firstly blended with TPS to reduce the cost
5
108
by simple extrusion, and the mechanical properties were then improved by reinforcement
109
and compatibilization. It is expected that the compatibility between PBAT and TPS
110
would be improved by the addition of the compatibilizer of MAH or/and plasticized
111
EVOH. The effects of the nanoparticle and the compatibilizers on mechanical properties,
112
thermal properties, morphology, and crystalline structure of the composites were
113
investigated and discussed in detail.
114 115
2. Experimental
116
2.1. Materials
117
PBAT (Ecoflex F Blend C1200, BASF, Germany), cassava starch (Jinguang Starch Co.
118
Ltd., China), and ethylene-vinyl alcohol copolymer (EVOH) with 27 mol % of ethylene
119
(L170B, Kuraray Co. Ltd., Japan) were dried in a drying oven at 80 °C for 48 hours
120
before processing. Glycerol (99.7%, Medical Grade, Xin Jiu Da Chemical Co. Ltd.,
121
China), nano-silica (HL-200, GBS High-Tech & Industry Co. Ltd., China), and maleic
122
anhydride (Analytical Grade, Sinopharm Chemical Reagent Co. Ltd., China) were used
123
as received.
124
2.2. Preparation of the composites
125
The preparation of the composites was carried out by two-stage extrusion. Firstly,
126
nano-SiO2 was added into glycerol under stirring and evenly dispersed by ultrasonication.
127
Glycerol with nano-SiO2 was mixed with cassava starch (the mass ratio of starch to
6
128
glycerol was 100:40) in a high-speed mixer (SHR-10A, Zhangjiagang Grand Machinery
129
Co. Ltd., China). Then, the first-stage melt-blending extrusion was adopted to prepare
130
the TPS/nano-SiO2 composite by a twin-screw extruder (CTE-35, Coperion Keya
131
Machinery Co. Ltd., China) with the screw speed about 480 rpm in the range of 130-170
132
°C. The extruder has a ratio of length to diameter (L/D) of 42 and a screw diameter of
133
35.5 mm. Finally, the second-stage melt-blending extrusion was used to prepare the
134
multi-component composites of PBAT, TPS/nano-SiO2, MAH or/and plasticized EVOH.
135
The compositions of the composites are shown in Table 1. The samples for testing and
136
characterization were obtained by injection molding (HTF90W1, Haitian Group Co. Ltd.,
137
China) in the range of 160-180 °C. All samples were conditioned at room temperature
138
and 50% relative humidity for 48 hours before the following tests.
139 140
2.3. Characterization
141
2.3.1. Static mechanical tests
142
A universal testing machine (CMT-4104, New Sansi Material Testing Co. Ltd., China)
143
was used to determine the static mechanical properties by tensile and bending tests
144
according to GB/T1040-1992 and GB/T9341-2008 standards (China) at room
145
temperature. A cantilever impact testing machine (550J-2, Wance Testing Machine Co.
146
Ltd., China) was used to obtain the Izod impact strength according to GB/T1843-2008
147
standard (China). At least five specimens for each sample were tested. The strain rate was
7
148
20 mm/min for the tensile tests and 2 mm/min for the bending tests, respectively. The
149
dumbbell-type specimens with a length of 150 mm, a width of 10 mm and a thickness of 4
150
mm were used for the tensile tests. The specimens with a length of 80 mm, a width of 10
151
mm and a thickness of 4 mm were used for the bending and impact tests. In the case of
152
impact specimens, there is a notch with 2 mm depth in the middle of one side.
153
2.3.2. Scanning electron microscopy (SEM)
154
The micrographs of the fracture surface of the samples were obtained on a scanning
155
electron microscope (JEOL JSM 6700F, Japan) operated at 5 kV. The specimens were
156
quenched in liquid nitrogen and then fractured. The samples were observed after
157
sputter-coating platinum.
158
2.3.3. Dynamic mechanical analysis (DMA)
159
A dynamic mechanical analyzer (Q8000, PerkinElmer, USA) was used to determine
160
the dynamic mechanical behavior as a function of temperature in a three-point bending
161
mode. The frequency and the strain were 1 Hz and 25 μm, respectively. The range of
162
temperature was from -120 °C to 40 °C at a heating rate of 2 °C/min under nitrogen
163
atmosphere.
164
2.3.4. Thermogravimetric analysis (TGA)
165
The thermal stability of the samples was detected by a thermogravimetric analyzer
166
(Pyris 1 TGA, PerkinElmer, USA). The tests were performed from room temperature to
167
500 °C under nitrogen atmosphere.
8
168
2.3.5. Differential scanning calorimetry (DSC)
169
A differential scanning calorimeter (DSC Q2000, TA Instruments, USA) was used to
170
examine the thermal behavior of the composites. The scanning process consisted of three
171
steps. The samples were first heated from room temperature to 180 °C and remained at
172
this temperature for 5 minutes to eliminate their thermal history. The samples were then
173
cooled from 180 °C to -30 °C and later reheated from -30 °C to 180 °C. All of the tests
174
were performed under nitrogen atmosphere.
175
2.3.6. X-ray diffraction (XRD)
176
A polycrystalline X-ray diffractometer (XRD, Empyrean, PANalytical, Netherlands)
177
with radiation from copper target tube (Cu Kα radiation wavelength of 1.5406 Å) was
178
used to determine the crystalline structure of the composites. The range of the diffraction
179
angle, 2θ, was from 2 to 60°. The angle was increased in steps of 0.03° during the
180
measurement.
181
2.3.7. Rheological behavior tests
182
The steady-state rheological behavior of the samples was tested by a capillary
183
rheometer (Rosand RH7-D, Malvern, UK). The tests were performed at 165 °C and
184
175 °C, respectively. The relationship between shear stress and shear rate of the
185
pseudoplastic fluid can be expressed by the Ostwald-de Wale power-law equation:
186
τ=K∙γn
187
where τ stands for the shear stress, γ corresponds to the shear rate, K is the consistency
9
188
coefficient, and n is the non-Newtonian index. The viscosity was corrected by Bagley,
189
and the shear rate was corrected by Rabinowich. The values of n and K can be obtained
190
from the lg - lg γ curve. For Newtonian fluids, n is equal to 1 while n is lower than 1 for
191
pseudoplastic fluids. The higher the degree of deviation is, the stronger the
192
pseudoplasticity of the material is.
193 194
3. Results and discussion
195
3.1. Static mechanical properties
196
The mechanical properties of the composites are listed in Table 2. The results showed
197
that in comparison with the pure PBAT sample, the P-T sample had a much lower tensile
198
strength (11.6 MPa) and a lower elongation at break (1264%). The significantly reduced
199
tensile strength of the P-T sample indicated the poor compatibility between PBAT and
200
TPS. When nano-SiO2 (0.3 phr) was added into the P-T sample, the P-T-S sample showed
201
the improved tensile strength (15.2 MPa) and a little lower elongation at break (1138%).
202
The result illustrated the noticeable reinforcing effect of nano-SiO2 on the P-T sample.
203
After the addition of MAH, the P-T-S-M sample has almost the same tensile strength as
204
the P-T-S sample, while the former possesses a higher elongation at break and a higher
205
impact strength than the pure PBAT. This result indicated that the toughness of the
206
composite of the P-T-S-M sample became better with the addition of MAH, due to the
207
improved compatibility between PBAT and TPS by MAH. The similar effects of MAH
10
208
were reported by Mohanty [41] for the nanocomposites of PBAT and organically
209
modified layered silicates.
210
When EVOH (4 phr) was used instead of MAH as the compatibilizer, the tensile
211
strength of the P-T-S-E composite increased slightly (from 15.2 MPa to 16.4 MPa), and
212
the elongation at break is a little higher than that of the P-T-S sample. The fact suggested
213
that EVOH played a specific role in compatibilizing the composite of PBAT and TPS.
214
Then, a double-compatibilizer strategy (MAH and EVOH) was used to improve the
215
mechanical properties of the composites. The results showed that both the tensile strength
216
and the elongation at break of the sample P-T-S-M-E were increased significantly. In
217
comparison with the P-T sample, the tensile strength of the P-T-S-M-E sample increased
218
by 50% (up to 17.4 MPa), and its elongation at break increased by 18% (up to 1497%). At
219
the same time, its impact strength increased significantly, showing the improved
220
toughness. These results suggested that the combination of MAH and EVOH had further
221
improved the compatibility and the mechanical properties of the PBAT/TPS composite.
222
In comparison with the previous work under the same ratio of PBAT and TPS[38-40],
223
the PBAT/TPS composites through the reinforcing and the compatibilizing strategy
224
presented better mechanical properties in this work. The results confirmed that the
225
reinforcing and the double-compatibilizing strategy was very useful for improving the
226
mechanical properties of the PBAT/TPS composites. Moreover, the mechanical
227
properties of the obtained composites are superior to those of conventional polyethylene.
11
228
Therefore, the obtained composites with potential biodegradability could be applied
229
instead of the general plastics.
230
The reinforcing and compatibilizing effect could be explained by the interaction
231
between different components. Firstly, nano-SiO2 interacted with TPS to increase the
232
tensile strength of TPS, resulting in the improved tensile strength of the composite
233
P-T-S. Secondly, one anhydride group of MAH might react with the terminal hydroxyl
234
group of PBAT, and the other anhydride group might do with the hydroxyl group of
235
TPS to form ester linkages. The hydrogen atoms on the methylene close to the carbonyl
236
group in PBAT and the double bond on maleic anhydride can form free radicals under
237
high temperature and shearing. Therefore, some of the maleic anhydrides can be grafted
238
onto PBAT to improve its compatibility with TPS [23]. Moreover, the hydrophobic
239
long-chain portion of EVOH can interact with the lipophilic PBAT because of the similar
240
aliphatic segments, while the hydroxyl groups in EVOH can form hydrogen bonds or
241
interact with the hydroxyl groups in TPS [24, 42]. The interaction among EVOH, PBAT
242
and TPS can further increase the compatibility between PBAT and TPS. Thus, the
243
hydrophilic TPS may be encapsulated by PBAT continuous phase, resulting in the
244
improved compatibilization between the two components and the improved mechanical
245
properties of the composites.
246 247
3.2. Morphology
12
248 249 250 251
To further confirm the improved compatibility between PBAT and TPS, the fracture
252
morphologies of the specimens were observed by SEM. The obtained micrographs are
253
shown in Figure 1. As can be seen from Figure 1(a), PBAT forms a continuous phase,
254
while many great TPS phase domains (the particle-like domain) can be found in the
255
PBAT phase, indicating the non-compatibilized status of the P-T-S sample. However, as
256
shown in Figure 1(b), the amount of TPS particles dramatically decreased, and a
257
co-continuous phase was almost formed after the addition of MAH [43], confirming the
258
improved compatibility. As shown in Figure 1(c), when EVOH was used to replace
259
MAH, there was also a co-continuous phase between PBAT and TPS, and fewer TPS
260
particles were observed, which indicated that EVOH also has good compatibilization.
261
When both MAH and EVOH were used together, as shown in Figure 1(d), the fracture
262
structure was more homogeneous, and the continuous phase became more uniform.
263
Therefore, it could be concluded that the double compatibilization of the two
264
compatibilizers is better than each one alone, leading to more improved mechanical
265
properties.
266 267
3.3. Dynamic mechanical analysis
13
268 269 270
The evolution of the storage modulus (E') and loss factor (tanδ) versus temperature of
271
the pure PBAT and the PBAT/TPS composites are shown in Figure 2. As can be seen
272
from Figure 2(a), the storage modulus of all the samples decreased with the increase of
273
temperature, which indicated that the elasticity of the composites weakened with
274
increasing the temperature. In general, if a transition such as the glass transition and
275
secondary transition with temperature occur, a peak will appear in the tanδ curve. The
276
corresponding transition temperature, such as the glass transition temperature (Tg) and
277
secondary relaxation temperature (T), can be determined by the position of the tanδ
278
peaks. As shown in Figure 2(b), there is a single transition at about -30°C, corresponding
279
to the Tg of the pure PBAT. In the case of the P-T sample, the strong peak at about -26°C
280
is attributed to the Tg of the PBAT component, which is a little higher than that of the pure
281
PBAT, probably due to that the addition of TPS and the poor compatibility hindered the
282
movement of PBAT chain. In addition, there is a weak peak at about -46°C,
283
corresponding to the secondary relaxation of the glycerol-rich phase in the P-T sample
284
[44]. However, it is worth noting that the glass transition of the starch-rich phase is not
285
shown in the tanδ curves, probably due to that the glass transition of the starch-rich
286
phase occurred at a higher temperature than the detection temperature [38, 44]. After the
287
addition of nano-SiO2, the two transition temperatures shifted to the higher temperatures
14
288
(-22°C and -41°C, respectively) than those in the P-T sample. This indicated that
289
nano-SiO2 hindered the movement of the PBAT chain and glycerol-rich phase because
290
of the reinforcing function of nano-SiO2. However, after the addition of MAH, the
291
P-T-S-M sample showed the lower transition temperatures (-30°C and -46°C), likely
292
due to the compatibilization and the plasticizing effect of MAH. Compared with those
293
of the P-T-S sample, the P-T-S-E sample also presented the lower transition
294
temperatures (-27°C and -45°C), owing to the promoted movement of the
295
macromolecular chains from the compatibilization of EVOH. When both MAH and
296
EVOH are added simultaneously, the P-T-S-M-E sample also displayed the slightly
297
lower transition temperatures (-28°C and -46°C) than those in the P-T-S-E sample,
298
indicating the more improvement in the compatibility of the components. In a word, the
299
change of the transition temperatures demonstrated the improved interfacial
300
compatibility between PBAT and TPS.
301 302
3.4. Thermo-gravimetric analysis
303 304
The thermogravimetry was used to investigate the thermal stability of the composites.
305
Figure 3(a) shows the TGA curves, and Figure 3(b) presents its derivative curves (DTG)
306
for the samples. The relevant results are summarized in Table 3, listing the temperature
307
values for 10, 25, 50 and 75% mass loss (T10, T25, T50 and T75, respectively), the
15
308
corresponding temperature values of the two main peaks in DTG curves (Tp1 and Tp2) and
309
the residue percent.
310 311
The results showed that PBAT exhibited a single degradation step from 340°C to
312
450°C and the temperature value of the peak in the DTG curve was 413 °C (TpPBAT =
313
413°C). For the P-T sample, a two-step degradation process (270 – 350 °C and 350 –
314
450°C) is observed. The degradation at 270 – 350 °C is attributed to that of TPS, while
315
another degradation at 350 – 450°C is similar to the pure PBAT, corresponding to that of
316
PBAT in the P-T sample. As for the other composites, a distinct two-step degradation
317
process is also visible. The two corresponding temperature values of the two peaks from
318
the different composites (322 – 325°C and 411 – 418°C) were almost the same as the Tp
319
of TPS (TpTPS = 325 °C) in the P-T sample and that of the pure PBAT (TpPBAT = 413°C),
320
respectively. This indicated that the degradation of the composites was a combination of
321
those of TPS and PBAT. Moreover, it could also be confirmed from the similar Tp values
322
that the additives (nano-SiO2 and the compatibilizers) had no apparent effect on the
323
degradation of PBAT and TPS in the composites. These results indicated that the
324
composites had good thermal stability. Besides, the residue existed at high temperature
325
was mainly attributed to nano-SiO2 and the carbonization of the polymer components
326
(TPS, PBAT, and EVOH).
327
16
328
3.5. Differential scanning calorimetry (DSC)
329 330
Figures 4(a) and 4(b) are the DSC curves of the cooling and the reheating process for
331
the samples, corresponding to the crystallization transition and the melting transition,
332
respectively. It could be found that the crystallization transition of the pure PBAT
333
occurred at about 55°C (Tc = 55°C). For the P-T sample, there are two peaks at 84°C
334
and 100°C. The strong peak at 84°C is ascribed to the Tc of PBAT in the P-T sample. The
335
sharp increase (from 55°C to 84°C) is due to the promoted movement and regularity of
336
PBAT molecular chains at a higher temperature by the TPS particles [45]. Another weak
337
peak at 100°C is assigned to the Tc of the TPS phase in the P-T composite. For the P-T-S
338
sample, the Tc-s (85°C and 100°C) are almost the same as those of the P-T sample. After
339
the addition of MAH, however, the Tc of the PBAT component shifted to a high
340
temperature of about 93°C, probably owing to the more favorable crystallization of
341
PBAT at a higher temperature from the compatibilization of MAH. When EVOH instead
342
of MAH was used, there are three peaks of 91°C, 100°C, and 118°C, corresponding to the
343
Tc of PBAT, TPS, and EVOH in P-T-S-E, respectively. The higher Tc of PBAT (91°C)
344
than that for the P-T-S sample (85°C) also indicated the more favorable crystallization of
345
PBAT at a higher temperature promoted by EVOH. When MAH and EVOH were used
346
simultaneously, the higher Tc of the PBAT component (94°C) implied that the more
347
improved compatibility by the double-compatibilizing strategy was more beneficial to
17
348
the crystallization of PBAT at a much higher temperature.
349
As shown in Figure 4(b), the transitions with temperature correspond to the melting
350
transitions, and the corresponding transition temperature is the melting temperature (Tm).
351
For the pure PBAT, a transition is shown at about 125°C, corresponding to the melting of
352
the crystalline PBAT structure (Tm=125°C). After blending TPS with PBAT, two
353
transitions are visible at about 110°C and 125°C, respectively. The transition at 125°C is
354
similar to that for the pure PBAT, which is attributed to the melting of the crystalline
355
PBAT phase in the P-T composite. Another transition at 110°C is ascribed to the melting
356
of the crystalline starch phase.
357
Moreover, for the other composites, there are two melting transitions at 110°C and
358
125°C, respectively, similar to those for the P-T composite. The two transitions had no
359
apparent change with the addition of nano-SiO2 and the compatibilizer (MAH or/and
360
EVOH). This indicated that the additives had no significant influence on the melting
361
transition of the composites. Therefore, the melting processing of the composites would
362
not be affected by the additives, which is beneficial to the melting processing of the
363
composites.
364 365
3.6. X-ray diffraction measurements
366 367
18
368
The wide-angle X-ray diffraction technique was used to investigate the crystalline
369
structures of the composites. The results are shown in Figure 5. The pure PBAT sample
370
shows five peaks at the angles of 16.1º, 17.6º, 20.8º, 23.1º, and 24.9º, which were similar
371
to the characteristic peaks of PBT homopolymer [46]. The result indicated that there
372
were crystalline structures in pure PBAT. For the P-T composite, besides the five peaks,
373
the other two peaks (one strong peak at 21.3º and one weak peak at 18.3º) could be
374
observed, which are corresponding to the crystalline structure of starch [47]. As for the
375
other composites after the addition of nano-SiO2 or/and the compatibilizers, there were
376
also seven peaks without any differences in the positions of the peaks from those of the
377
P-T sample. This result suggested that the crystalline structures in the composites did not
378
change with the addition of nano-SiO2 and the compatibilizers.
379 380
3.7. Rheological analysis
381 382 383 384
Figure 6 shows the relationship between the shear stress and the shear rate for the
385
compatibilized P-T-S-M-E sample at 165°C and 175°C, respectively. The shear stress of
386
the samples increased with the increase of shear rate, indicating the typical pseudoplastic
387
characteristics of the composite melt. The lg - lgγplots maintain a good linear
19
388
correlation after the linear fitting. According to the slope and the intercept of the fitted
389
line, the values of the non-Newtonian index (n) and consistency coefficient (K) could be
390
obtained. With the temperature increase from 165C to 175C, the n value increased, and
391
the K value decreased. This indicated that the pseudo-plasticity of the composite melt
392
became weak, and the Newtonian characteristic increased. Both of the n values were less
393
than 1, confirming a typical pseudoplastic characteristic of the melt. The melt viscosity is
394
relatively low, implying its easy processability [48, 49].
395 396
4. Conclusions
397
The composite of PBAT and TPS with excellent mechanical properties (the tensile
398
strength: 17.4 MPa, the elongation at break: 1496.8%) was obtained when nano-SiO2 was
399
used as a reinforcing agent, and MAH and EVOH were used simultaneously as the
400
compatibilizer by a simple two-stage melt-blending extrusion. The SEM results indicated
401
the improved interfacial compatibility between PBAT and TPS, which was further
402
demonstrated by the decrease in Tg and the increase in Tc of the PBAT component. The
403
TGA results showed that the composites had good thermal stability, which was not
404
affected by the additives. The XRD results confirmed that the crystalline structures of
405
PBAT and starch had no significant changes with the addition of the nanoparticle and the
406
compatibilizers. Moreover, the composite melt was a typical pseudoplastic fluid, showing
407
the weakened pseudo-plasticity and the enhanced Newtonian characteristic with the
20
408
increase of temperature. The PBAT-based composites with good mechanical properties,
409
easy processibility, possible biodegradability, and reduced cost will show broadly
410
potential applications.
411 412
Acknowledgments
413
The work was supported by China Scholarship Council (File No. 201708430086),
414
China Postdoctoral Science Foundation (No. 2016M592444), Natural Science
415
Foundation of Hunan Province of China (No. 2018JJ2088) and Undergraduate
416
Innovation Program of Hunan Province (No. S201911535006).
417 418 419
Data availability Statement The raw/processed data required to reproduce these findings cannot be shared at this
420
time as the data also forms part of an ongoing study.
421
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558 559 560
Fig. 1. SEM micrographs of the fracture surface of the samples P-T-S (a), P-T-S-M (b),
561
P-T-S-E (c) and P-T-S-M-E (d).
562
Fig. 2. Curves of storage modulus (a) and loss factor (b) versus temperature for the
563
samples.
564
Fig. 3. TGA (a) and DTG (b) curves of the samples.
565
Fig. 4. DSC curves of cooling (a) and second heating (b) process for the samples.
566
Fig. 5. XRD results for the samples.
567
Fig. 6. Relationship between shear stress and shear rate for the P-T-S-M-E sample at two
568
different temperatures. 28
569 570
Table 1. Compositions of the various samples PBAT
TPS
Nano-SiO2
MAH
EVOH
(wt.%)
(wt.%)
(phr)
(phr)
(phr)
PBAT
100
-
-
-
-
P-T
80
20
-
-
-
P-T-S
80
20
0.3
-
-
P-T-S-M
80
20
0.3
1.6
-
P-T-S-E
80
20
0.3
-
4
P-T-S-M-E
80
20
0.3
1.6
4
Samples
571 572
Table 2. Mechanical properties of the samples Tensile
Youngs
Flexural
Flexural
Impact
strength
modulus
strength
(MPa)
(MPa)
(KJ/m2)
Elongation at Samples
strength
modulus break (%)
(MPa)
(MPa)
PBAT
26.70.3
25.10.3
1383.230.2
3.80.1
169.733.9
22.50.5
P-T
11.60.4
30.80.5
1264.314.5
3.70.2
126.411.5
24.30.5
P-T-S
15.20.2
40.90.9
1138.518.9
4.70.2
146.920.1
26.90.5
P-T-S-M
15.50.3
31.31.2
1401.117.2
3.10.2
97.84.6
28.80.8
P-T-S-E
16.40.3
40.20.8
1141.54.8
3.90.2
127.53.8
24.90.7
29
P-T-S-M
27.20.9 17.40.1
112.07.3 1496.850.5
4.00.1
30.61.8
-E
573 574
Table 3. TGA data of the composites Samples
T10 (°C)
T25 (°C)
T50 (°C)
T75 (°C)
Tp1 (°C)
Tp2 (°C)
Residue (%)
PBAT
384
398
410
421
-
413
4.77
P-T
317
381
406
419
325
412
4.15
P-T-S
319
385
407
420
322
416
6.39
P-T-S-M
320
385
405
418
323
411
5.43
P-T-S-E
323
390
410
423
325
418
5.91
P-T-S-M-E
319
386
404
417
324
412
5.75
575 576
30