Nonisothermal Crystallization Behavior of Biocomposites from Poly

An ASABE Meeting Presentation Paper Number: 131621882

Nonisothermal Crystallization Behavior of Biocomposites from Poly(lactic acid) and Bamboo Fiber Shaoping Qian1, hailiang Mao1, Kuichuan Sheng*1, yifan Luo1 & Jun Lu1 College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, China

Written for presentation at the 2013 ASABE Annual International Meeting Sponsored by ASABE Kansas City, Missouri July 21 – 24, 2013 Abstract. Poly(lactic acid)/bamboo fiber (BF) bio-degradable composites, incorporating various amounts of alkali solution pretreated BF (BF:1, 5, 10, 20 wt %), were prepared by mixing. The nonisothermal crystallization kinetics of pure PLA and PLA/BF composites were comparatively investigated by using combined AvramiOzawa models with differential scanning calorimetry at constant cooling rate of 1.2, 2.5, 5, 10 °C/min, respectively. The DSC plots revealed that BF increased the overall crystallization rate of PLA and the crystallization enthalpy (∆Hc) as well. It was found out that BF acted as nucleating agents in PLA/BF composites, which was also in good agreement with activation energy values for nonisothermal crystallization obtained from Kissinger equations. The change in thermal properties such as crystallization temperature, and melting temperature was monitored. Keywords. biodegradable material; PLA; bamboo fiber; DSC; nonisothermal crystallization

1. Introduction Poly(lactic acid) or PLA is an important environmentally friendly polymer that boasts a high modulus, high strength and good biocompatibility. therefore, it has been used in pharmacy and medical device in the past few years (Madhavan Nampoothiri, Nair and John, 2010). Recently, PLA has been finding an increasing number of applications in the packaging industry due to its good mechanical properties, transparency and compostability. One general drawback of the PLA of material is that it exhibits a lower glass transition temperature (Tg), up to about 60 °C (Saeidlou et al., 2012), compared to competing polyesters, which limits its application in packaging industry. As we know, plasticization and heterogeneous nucleation are two effective ways to enhance crystallization behavior of PLA and the improvement of crystallinity can promote PLA thermal properties. For example, the heat deflection temperature (HDT) and Vicat penetration temperature were increased more than 30 and 100 °C, respectively, after amorphous samples were fully crystallized (Song et al., 2012; Madhavan Nampoothiri et al., 2010). Bamboo is widely used in people's production and life in China, because of its well-adapted, rapid growth, high productivity, mutli-purpose and good benefits on economy, society and environment (Qisheng and Fengwen, 1999). Bamboo is rich in fiber, which has high strength and toughness, and it can make up for the inadequacy of PLA materials. Furthermore, bamboo particles can be applied as the nucleating agent for The authors are solely responsible for the content of this meeting presentation. The presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Meeting presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author’s Last Name, Initials. 2013. Title of Presentation. ASABE Paper No. ---. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a meeting presentation, please contact ASABE at [email protected] or 269-932-7004 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).

crystallization on PLA/BF composite process if the particles are fine enough (Pan et al., 2012; Gui et al., 2012). The crystallization behavior of neat PLA (Zhang et al., 2004; Saeidlou et al., 2012), PLA stereocomplexes (Schmidt and Hillmyer, 2001; Tsuji and Tezuka, 2004), and PLA/other polymer or fiber blends (Nam et al., 2006; Hwang et al., 2012) has been investigated extensively by both isothermal and nonisothermal methods. Natural fibre filler have been the focus of academic and industrial attention in recent years, because the final composites often exhibit a desired enhancement of physical and thermal properties compared to the neat polymer matrix (Zhou et al., 2009). Ryota Kose et al. (2013) investigated a desirable size of cellulose fibers for enhancing the crystallization of poly(lactic acid) (PLA) in composites by comparison among cellulose nanofibers with different widths on nanoscales. It is possible that an optimum width range of 60 nm in the cellulose nanofibers may exist in balance between the favorable self-assembly as an enthalpy effect and the tendency toward dispersion as an entropy effect. Nguyen et al. (2010) studied the nonisothermal crystallization kinetics of a recycled polypropylene (RPP) and recycled polypropylene/short bamboo fiber (RPPSBF) composites. Research of the PLA crystallization behavior is available to find way to improve the crystallization property. Actually, PLA compound processing is often under nonisothermal conditions. As a result, it is important to study the nonisothermal crystallization behavior of biocomposites from poly(lactic acid) and bamboo fiber, while information and explanation for it is scanty. In this paper, an attempt was made to prepare different weight fraction of bamboo fiber (1, 5, 10, 20 wt %) compounded PLA composite using alkali treatment as the fiber surface modifier. Differential scanning calorimetry (DSC) was employed to measure the crystalline parameters of the composite under different cooling rate, such as crystallization temperature (Tc), crystallization enthalpy (∆Hc) and the degree of crystallinity. Moreover, nonisothermal crystallization behavior and kinetics and activation energy were examined by using combined Avrami-Ozawa models and Kissinger models, respectively.

2. Materials and Methods 2.1 Materials PLA pellets (ES701) obtained from Tongjieliang Biomaterials Co., Ltd. Shanghai, China were used as matrix material. PLA had a number average molecular weight of 52,000 g mol-1, Tg of 57.5 °C and melting point (Tm) of 153.9 °C. BF was obtained from Bamboo Processing Factory, Zhejiang Province, China. 2.2 Preparation of PLA/BF composites Bamboo particles were ground to ～75 μm with a hammer mill and screened through mesh size of 200, then immersed in NaOH solution with the concentration of 1wt % in a 35.0 °C water batch with a solid-to-liquid ratio of 1: 15 for 3 h. After that, BF was rinsed with distilled water and washed with acetic acid solution for neutralization. Meanwhile a mesh of 300 size was used to screen the BF again. Then they were further rinsed with distilled water and filtered under vacuum for 5 min and finally dried at 105 °C for 24 hours. PLA was dried for 12 h at 65 °C under vacuum before processing, PLA to BF ratio was kept at 99:1, 95:5, 90:10, 80:20, respectively. The blends about 50 g for each were prepared in a co-rotating twin screw lab-scale compounder under the conditions of 150 °C and 50 rpm for 5 min. Then samples were kept in desiccator for further characterization. 2.3 Thermal property Differential scanning calorimetry (200F3, Netzsch) was adopted to study the thermal properties of the different blending ratio composite. To study the nonisothermal melting and crystallization behavior of PLA and its blends, the samples (ca. 10 mg) were firstly heated to 200 °C and held for 5 min to eliminate any thermal history. Then they were cooled to 50 °C at cooling rates of 1.2, 2.5, 5, 10 °C /min, respectively. Nitrogen was used as purging gas at a rate of 50 mL min-1. Crystallinity (Xc) was estimated according to the following equation, X c (%) 

H c  100 %  H 0  X PLA

(1)

where, ∆Hc refers to the crystallization enthalpy of PLA/BP composite; ∆H0 refers to the enthalpy value during 100% crystallization of PLA, which is 93.6 J/g (Saeidlou et al., 2012); XPLA refers to the weight fraction of PLA in PLA/BP composite.

3. Results and discussion DSC analysis was taken to compare the effects of pure PLA and its blends on the crystallization behavior. 2013 ASABE Annual International Meeting Paper

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Fig. 1(a-e) shows the nonisothermal crystallization endothermic curves of the composites under different cooling rates. The parameters of interest, that is, crystallization temperature (Tc), crystallization enthalpy (∆Hc) are summarized in table 1 and table 2. Only one crystalline exothermic peak can be observed from crystallization curve of different blending ratio composite. It is seen that crystalline exothermic peak of pure PLA and PLA/1%BF blends are wide and almost no crystallization under certain cooling rate while the other blends’ are narrow and sharp. That is mainly because of the improvement of surface compatibility due to the elimination of hemicelluloses and part of lignin after alkali treatment of the BF. Moreover, BF could be good act as the role of nucleating agent when the BF content and granularity were proper in BF/PLA composite (Fortunati et al., 2012). In addition, the single crystalline exothermic peak indicate that it was one crystallization process from melting to crystallization, which means there was a single crystal forms in the PLA/BF composite (Hwang et al., 2012).

（ a）

（ b）

（ c）

（d）

（ e） Figure 1. Crystallization curve of different blending ratio composite a： 100:0； b： 99:1； c： 95:5； d： 90:10； e： 80:20

2013 ASABE Annual International Meeting Paper

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Table 1. Crystallization temperature of different blending ratio composite with different cooling rate Crystallization temperature of different blending ratio composite (°C) Cooling rate (°C /min) Pure PLA

99:1

95:5

90:10

80:20

1.2

119.8

—

—

100.3

101.6

2.5

124.5

115.1

105.3

94.3

98.4

5

125.7

125.3

108.1

96.5

100.2

10

127.1

126.6

115.7

99.9

104.1

Table 2. Crystalline enthalpy of different blending ratio composite with different cooling rate Crystalline enthalpy (ΔHc) of different blending ratio composite (J/g)

Cooling rate (°C /min)

Pure PLA

99:1

95:5

90:10

80:20

1.2

14.44

—

—

3.172

2.111

2.5

17.54

27.92

20.86

11.14

17.46

5

3.298

23.76

32.92

31.50

30.72

10

2.308

14.34

33.16

36.08

34.55

It can be seen from Table 1 that the crystallization temperature of the composite increased with the increase of cooling rate under three cooling rate of 2.5, 5 and 10 °C/min. Crystallization temperature of pure PLA and PLA/BF: 99:1 blend are similar, but with the increasing of the BF content, it firstly decreased and then slightly increased. This observation indicates that a certain amount of BF after alkali treatment can act as nucleating agent in the process of crystallization of PLA/BF composite, and have a great influence on crystallization temperature. Crystalline region of polymer matrix decreased due to the uneven dispersion when more BF was used. And this will lead to an inadequate crystallization. The values of crystallization enthalpy (∆Hc) calculated from DSC thermogram in different cooling rate are given in Table 2. Crystallization enthalpy of pure PLA decreased with the increase of cooling rate, and then increased significantly after adding BF. It is quite similar to the blends (PLA/BF： 99:1). However, to the blends of proportions of 95:5, 90:10, 80:20, the crystallization enthalpy increased with the increase of cooling rate, the trend was contrary to the pure PLA. It means that the crystallization behavior changed when the content of bamboo fiber in composite exceed 5%, indicating that the 300 mesh BF after alkali treatment can act as a nucleating agent during the process of crystallization (Liu et al., 2013). Meanwhile, The higher cooling rate and larger crystallization enthalpy were, the shorter time required for crystallization process. Addition of BF to PLA significantly affected the rate of crystallization due to more crystal nucleus formation by heterogeneous nucleation which was conducive to the growth of the crystal nucleus and thickening of crystal plate and region. To determine the crystallization rate, the relative crystallinity can be plotted as a function of time. Relative crystallinity to time of different blending ratio composite by calculating the crystallization curve integral are given in Figure 2 (a-e). The crystalline time of pure PLA reduced with the increase of cooling rate, because there was not enough time to a conformational arrangement for PLA molecular segment to go into crystalline state if the cooling rate is too fast. In other words, the movement of PLA molecular segment could not keep up with the change in temperature resulted in irregular lattices and a decrease of crystalline time and lower crystallinity with the increase of cooling rate. The crystallization time was not significantly changed with the increasing of cooling rate after the addition of BF, indicating that the crystallization behavior of PLA changed due to BF and the molecular segment movement rate was increased during crystallization, thus reducing the influence of cooling rate on the crystalline time and the crystallinity of PLA.


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（a）

（b）

（c）

（d）

（e） Figure 2. Relative crystallinity to time of different blending ratio composite a 100:0；b 99:1；c 95:5；d 90:10；e 80:20

Generally, Avrami model is used to explain the isothermal crystallization behavior of semicrystalline polymers. The Avrami equation is given as follows (Gumus et al., 2012):

1  X t  exp(  Z t t n )

(2)

where Xt is the relative degree of crystallinity at time t, Zt is the rate constant giving the information about the nucleation and the growth rate. The Avrami exponent n gives information about the nucleation type and the morphology of the crystallite formed. As a theory, it is not suitable for analyzing nonisothermal crystallization, because the kinetic parameters of physical meaning is not clear with analysis of nonisothermal crystallization behavior using Avrami equation, and the Avrami index n is not equivalence to isothermal Avrami index value. Jeziorny (1978) argued that Avrami crystallization rate constant could be corrected by cooling rate (Φ) when the cooling rate was constant, The Jeziorny equation is given as follows:


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lg Z c 

lg Z Φ

(3)

where Z is the rate constant, Zc is Jeziorny crystallization rate constant corrected by cooling rate. The Avrami theory was extended to nonisothermal crystallization by Ozawa. The equation was obtained by assessing the relationship between crystallization temperature and crystallinity (Ozawa, 1965).

1  X (T )  exp(

P(T ) ) Φm

(4)

where X(T) is the relative crystallinity at temperature T, Φ is cooling rate, P(T) represents the cooling function and m is the Ozawa exponent. Ozawa equation was used to determine the Ozawa exponent of some semicrystalline polymers, while it had an assumption: m had nothing to do with temperature, but in fact, many studies suggested that the value of m was related to temperature. Liu et al.(1997) proposed a different kinetic approach to describe a nonisothermal process by combining Avrami and Ozawa equation at a given Xt as follows (eqs.(5) and (6) ):

lg Z  n lg t  lg P (T )  m lgΦ

(5)

lgΦ= lg F (T )   lg t

(6)

The kinetic parameter F (T )  [ P (T ) ]1/ m is the required cooling rate to reach a certain degree of crystallinity at Z

a unit time. Therefore, high F(T) values mean higher cooling rates that are needed to reach the certain value of relative crystallinity in the unit time, α is equal to the ratio of the Avrami exponent n to the Ozawa exponent m. The slope gives the value of α and the intercept gives the kinetic parameter F(T) by plotting lgΦ against lgt. Figure 3 shows the plots of pure PLA and its blends at different relative crystallinity values. The values of the kinetic parameters obtained from the plots are given in Table 3. From the relationship between lgΦ and lgt, it follows the linear rule though a few data points are not. This observation indicates that the combining Avrami and Ozawa method is suitable for the analysis of nonisothermal crystallization kinetics of PLA/BF composite. From the data in Table 3, it can be seen that with the increase of BF, the value of F (T) decreased significantly, that means the crystallization rate increased, thus addition of the BF increased the crystallization rate of PLA matrix. Furthermore, when the BF content reached the biggest, the fastest crystallization rate was. Moreover, as α value changed from positive to negative, this shows that BF could change the crystallization properties of PLA matrix and the crystal morphology.

(a)


(b)

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(c)

(d)

(e) Figure 3. LogΦ to Logt of different blending ratio composite a：100:0；b：99:1；c：95:5；d：90:10；e：80:20

There are many mathematical approaches to evaluate the activation energy (∆E) of the crystallization process. The approach proposed by Kissinger were used in this study (Gumus et al., 2012). The equation of Kissinger are given in eqs. (8):

d ln( x / Tp ) E  d（1/ Tp） R 2

(8)

where Tp is the peak temperature, x is the cooling rate, and R is the universal gas constant. Crystallization activation energy can be achieved by plotting ln(x/Tp2) against 1/Tp. The calculated values of activation energy (Fig. 4) are given in Table 4. It can be concluded that the different proportions of PLA/BF composite data points are close to form 5 lines, the Kissinger equation is valid (Gumus et al., 2012). As the table listed， the absolute value of the crystallization activation energy decreased significantly with the addition of BF, indicating a great influence of reinforcing fillers adding to PLA. Furthermore, the movement of PLA molecular segments and crystallization getting easier, thus BF can greatly promote the crystallization of PLA. This can be attributed to the effect of BF after alkali treatment, played a good role of nucleating agent during crystallization process in PLA matrix. However, when the BF content exceeded 10%, maybe due to the distribution of fiber with PLA molecules, although a part of PLA molecules went into crystal lattice by heterogeneous nucleation, formed dense crystal, the others were hampered by excessive BF, therefore made a slightly higher of activation energy.


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Table 3. Nonisothermal Crystalline kinetic parameter of different blending ratio composite Relative crystallinity Different blending ratio

Parameter 20%

40%

60%

80%

F(T)

7.47

17.2

35.4

76.3

α

2.1

2.4

2.7

3.0

F(T)

1.46

1.97

2.35

2.75

α

-0.110

-0.093

-0.094

-0.096

F(T)

1.54

0.27

0.06

0.02

α

-2.89

-4.54

-5.62

-5.81

F(T)

3.65

0.96

0.16

0.02

α

-1.98

-4.11

-6.27

-8.12

F(T)

0.532

0.034

0.00012

1.14E-12

α

-4.3

-7.3

-13.2

-31.5

100:0

99:1

95:5

90:10

80:20

2)

Figure 4. ln(x/Tp to 1/Tp of different blending ratio composite

Table 4. Crystallization active energy of different blending ratio of PLA/BF composite Different blending ratio

100:0

99:1

95:5

90:10

80:20

∆E(KJ/mol)

320.9

100.7

109.0

175.5

182.4

CONCLUSIONS A comparison of thermal properties of pure PLA and its blends with BF under different cooling rate were performed. In addition, the nonisothermal crystallization behavior was investigated by using combined Avrami and Ozawa kinetic models. PLA/BF composite was only one crystalline peak, and the more BF added to the 2013 ASABE Annual International Meeting Paper

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blends, the narrower crystalline peak was. Crystallization temperature decreased with the increasing of BF content, but no significant change could be seen with the different BF content composite. Crystallization enthalpy of the composite increased with the increasing of cooling rate when the BF content exceed 5 wt %. Nonisothermal crystallization kinetic of PLA/BF composite well fit combined Avrami and Ozawa models. The crystallization activation energy of PLA matrix reduced after BF’s addition, indicating a promotion of BF to PLA matrix on nonisothermal crystallization. Acknowledge The authors are grateful to the Special Fund for Agro-scientific Research in the Public Interest of China (No. 201003063) for financial support.

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