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high fluid polypropylene copolymer blends
Accepted Manuscript Title: Thermal stability and non-isothermal crystallization kinetics of metallocene poly (ethylene-butene-hexene) /high fluid polypropylene copolymer blends Author: Jingyi Wang Hongbing Jia Yingying Tang Xiaogang Xiong Lifeng Ding PII: DOI: Reference:
S0040-6031(16)30338-0 http://dx.doi.org/doi:10.1016/j.tca.2016.11.016 TCA 77644
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
29-9-2013 23-11-2016 24-11-2016
Please cite this article as: Jingyi Wang, Hongbing Jia, Yingying Tang, Xiaogang Xiong, Lifeng Ding, Thermal stability and non-isothermal crystallization kinetics of metallocene poly (ethylene-butene-hexene) /high fluid polypropylene copolymer blends, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2016.11.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermal
stability
and
non-isothermal
crystallization
kinetics
of
metallocene
poly
(ethylene-butene-hexene) /high fluid polypropylene copolymer blends Jingyi Wang1,2, Hongbing Jia*3, Yingying Tang3, Xiaogang Xiong3, Lifeng Ding4
1
College of Material Engineering, Nanjing Institute of Technology, Nanjing 211167, China
2
Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing
Institute of Technology, Nanjing 211167, China 3
Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing
University of Science and Technology, Nanjing 210094, China 4
Department of Chemistry, Xi'an Jiaotong-Liverpool University, Suzhou Dushu Lake Higher Education
Town, Jiangsu Province 215123, China
*Corresponding author, Hongbing Jia, E-mail:[email protected] Telephone: +86-25-84303329, Fax: +86-25-84431939
Highlight Thermal degradation and crystallization behavior of mEBHC/HF-PP were studied. The thermal degradation activation energies increased with the adding mEBHC. mEBHC acted as heterogeneous nucleation agent, and improve crystallization rate.
Abstract: High-fluidity polypropylene (HF-PP) was melt blended with different amount of metallocene poly (ethylene-butene-hexene) copolymer (mEBHC) using a twin-screw extruder. The thermal degradation and non-isothermal crystallization behavior of the blends were investigated using thermo-gravimetric analysis (TGA) and differential scanning calorimeter (DSC). Our investigation revealed that the thermal degradation activation energies of mEBHC/HF-PP blends, calculated from thermo-gravimetric data by Kissinger, Flynn-Wall-Ozawa and Friedman method, increased with the amount of mEBHC added. Therefore, the thermal stability of mEBHC/HF-PP blends was enhanced in comparison to pure HF-PP. Modified Avrami and Mo models and Hoffman-Lauritzen theory were used to evaluate the influence of mEBHC on the non-isothermal crystallization kinetics of HF-PP. The results showed that mEBHC acted as heterogeneous nucleation agent, the non-isothermal crystallization rate of HF-PP matrix was improved by adding mEBHC.
Key words: HF-PP blends, thermal stability, non-isothermal crystallization kinetics, high-fluidity polypropylene
1 Introduction In recent years, high-fluidity polypropylene (HF-PP) has attracted much interests for its application in injection molding of large parts and automobile bumpers due to its outstanding processing and mechanical properties [1-4]. However, high notch sensitivity and poor impact resistance of HF-PP [5] have limited its applications as an engineering plastic, especially under severe conditions. Currently, metallocene catalytic polymerization of polyolefin copolymers, such as metallocene ethylene-octene copolymer (mEOc), metallocene propylene-1-octene copolymer (mPPOc) and metallocene liner low density polyethylene (mLLDPE) have been widely used as toughening agents for PP and PE[6-14], which are mainly owning to their narrow molecular weight distribution, co-monomer distribution, and excellent physical and mechanical properties. For example, McNally et al.[6] found that mEOc could significantly improve the impact strength of PP at both room temperature and cold temperature. Kukaleva et al.[7] also found that the mLLDPE has great influence on the impact strength of “high-crystallinity” polypropylene (hcr-PP). Metallocene poly (ethylene-butene-hexene) copolymer (mEBHC) is a novel metallocene catalytic polymerized polyolefin copolymers with high melt flow rate. Compared to the conventional toughening agents, such as, ethylene propylene rubber (EPR) or ethylene propylene diene monomer (EPDM), mEBHC exhibits the advantage of high melt flow properties. And it also has similar viscosity with HF-PP. In our previous study[15], we focused on the preparation and characterization of metallocene mEBHC toughened high-fluidity PP (HF-PP) blends. The incorporation of mEBHC in HF-PP was found to enhance both the crystallinity degree and impact strength of HF-PP. It is known the lifetime of polymer products depends not only on their physical properties but also on thermal stability [13, 16]. Physical properties of semi-crystalline polymers, like PP, depend greatly on their crystalline [17-19]. Crystallization occurs during the manufacturing of the polymer product. Therefore, understanding polymers’ crystallization mechanism is necessary for macroscopic structure design and final product properties control. In addition, faster crystallization of blends can cause shorter fabrication time in the injection molding process, which can also lead to reduced cost of the finished part.
To the best of our knowledge, there hasn’t been any investigation on the degradation behavior and crystallization kinetic of mEBHC/HF-PP, which is of great importance for developing durable industrial products of HF-PP. This study investigated the thermal degradation kinetic parameters and non-isothermal crystallization kinetics of mEBHC/HF-PP using thermo-gravimetric (TG) analysis and differential scanning calorimeter (DSC). The effects of mEBHC on the thermal stability and non-isothermal crystallization kinetic of HF-PP were also analyzed using different mathematical models. 2 Experimental 2.1 Materials HF-PP (YJP422, M w 285035 , M n 56166 , M w / M n 5.07 , Sinopec-Yangzi Petro Co., Ltd, Nanjing, China) with the melt flow rate (MFR) of 22 g/10 min (ASTM D1238, 230°C and 2.16kg) was used as the matrix polymer. mEBHC ( M w 54844 , M n 19583 , M w / M n 2.8 , ρ = 0.919 g/cm3, MFR = 22 g/10 min (190 °C and 2.16 kg)) was synthesized with the degree of branching (CH3/10000C) = 17.1 (Sinopec-Qilu Petro Co., Ltd, Shandong, China). 2.2 Sample Preparation HF-PP and mEBHC were both dried at 80 ºC for 24h and then were mixed together using a high-speed mixing machine (model CH-10DY) for 10 min. Melt blending was performed using a twin-screw extruder (TE-35, Gelan Machinery Co., Ltd, Zhangjiagang, China) with a rotation speed of 200 rpm. The temperature along the barrel was increased gradually from 175 ºC to 210 ºC: 175 ºC, 190 ºC, 190 ºC, 195 ºC, 200 ºC and 210 ºC, respectively. The blends were cooled down before they were pelletized. The obtained blend pellets were dried again at 80 ºC in a vacuum oven before the injection molding and then were molded into standard sample by injection-machine (JN55-E, Chen Hsong Machinery Co., Ltd, Ningbo, China). The mass fractions of mEBHC in the blends were 0, 5, 10, 20, 30 and 100 wt%, and the weight ratios of mEBHC/HF-PP were designated as 0/100, 5/95, 10/90, 20/80, 30/70, and 100/0, respectively. 2.3 TG analyses Thermo-gravimetric analyses were performed using a DTG-60/60H Simultaneous DTA-TG
Apparatus (Shimadzu Corporation, Japan). The measurements were conducted at heating rates of 5, 10, 20 and 40 °C/min in a dynamic nitrogen atmosphere at a flow rate of 20 ml/min. Sample with a mass of about 10 mg was heated from 30 °C to 600 °C. The data obtained from TGA were further analysed through several kinetic methods summarized in Table 1. Table 1 Kinetic methods used in evaluating degradation activation energy in this study Method
Expression
Plots
Ref.
Kissinger
AR Ea ln 2 ln Tmax g( ) Ea RTmax
1 ln 2 against Tmax Tmax
[20]
Flynn-Wall-Ozawa
Friedman
log
0.457 Ea AEa log 2.315 RT Rg ( )
log against
1 T
1 d ln against T dt
Ea d ln ln Af ( ) RT dt
[21]
[22]
In Table 1, β is the heating rate (K/min), Tmax is the temperature of the peak of derivative TGA curves, α is the degree of conversion (α = (wo−wt)/(wo−wf), wo, wt and wf are the initial, actual and final mass of the sample in the TGA curves, respectively), t is the reaction time (s), T is the absolute temperature (K), the g(α) and f(α) are related to the reaction mechanism and the rate constant k, Ea is the activation energy (kJ/mol), A is the pre-exponential factor, and R is the gas constant (8.314 J/(mol·K)). 2.4 DSC analyses The thermal behaviors of HF-PP and mEBHC/HF-PP blends were analyzed using a Netzsch 200 F3 differential scanning calorimeter (DSC) under nitrogen atmosphere. Approximately 5 mg samples were placed in aluminum pans and tested at a temperature range of 20-200 °C. Samples were heated to 200 ºC with a heating rate of 10 ºC/min and kept for 3 min to erase thermo-mechanical histories, then cooled down to 20 ºC by four different cooling rates of 5, 10, 15 and 20 ºC/min, respectively. As a result, the melting enthalpies (H) as a function of temperature (T) of all blends were obtained. From the DSC vurves, the values of the relative crystallinity (XT) at various cooling rates can be calculated. The XT as a function of temperature (T) can be described as [23, 24]: Tc
XT ( T0
dH ) dT
Te
T0
(
dH ) dT
(1)
where Tc, T0 and Te are the instantaneous, initial and terminal crystallization temperature, respectively. dH represents the change of enthalpy for the crystallization at dT temperature range. In addition, the crystallization temperature can be converted to crystallization time t using the following equation[25]: t (T0 Tc )
(2)
where T0 and Tc are the initial crystallization temperature and the instantaneous temperature at crystallization time (t), respectively, and Φ is the cooling rate. Thus the relative crystallinity as function of time, XT, is defined as: tc
Xt ( t0
dH )dt dt
te
t0
(
dH ) dt
(3)
Results and discussion 3.1 Thermal stability of mEBHC/HF-PP blends Figure 1 shows the TGA and derivative curves (DTG) of pure HF-PP, mEBHC and their blends at a heating rate of 10°C/min under nitrogen atmosphere. The thermal degradation of HF-PP shows one stage of weight loss, which exhibit the onset degradation temperature (Tonset, at 5% weight loss, Fig. 1(a) inset) at 373 °C and maximum loss temperature (Tmax,，the peaks of DTG curves in Fig. 1(b)) at 456 °C, while the Tonset and Tmax of mEBHC are 368 °C and 484 °C, respectively. Incorporating mEBHC was found to be able to increase the thermal stability of HF-PP with the increased Tonset and Tmax (Fig. 1(a) inset, and Fig. 1(b)). Overall, the more mEBHC is incorporated, the higher Tonset and Tmax of the blend will be. With the addition of mEBHC up to 30wt%, the Tmax of the blends could rise 19°C compared to that of pure HF-PP.
Fig. 1 (a) TGA and (b) DTG curves of mEBHC/HF-PP blends at the heating rate of 10 °C/min under nitrogen atmosphere
3.2 Activation energy of degradation
2 Figure 2 shows the linear plots of ln / Tmax
against 1/ Tmax for the various blends from the
Kissinger method, and activation energy (Ea) are calculated and summarized in Table 2. The Ea of pure HF-PP is 199 kJ/mol, which is consistent with the wide range from 120 to 280 kJ/mol reported in the literature [26-28]. The Ea of the blends with 5 wt%, 10 wt%, 20 wt%, 30 wt% of mEBHC increases to 229, 235, 247, and 266 kJ/mol, respectively. It suggests that the thermal stability of HF-PP is proportional to the amount of mEBHC incorporated, which accords with the result of 3.1 Section.
2 Fig. 2 Plots of ln / Tmax against 1/ Tmax for various blends in Kissinger methods
Table 2 Activation energy (Ea) calculated by three methods Kissinger mEBHC/HF-PP
Flynn-Wall-Ozawa
Ea
Ea Rc2
(kJ·mol-1)
Friedman Ea
Rc2 (kJ·mol-1)
Rc2 (kJ·mol-1)
0/100
199
0.997
178
0.989
171
0.985
5/95
229
0.994
192
0.991
192
0.994
10/90
235
0.998
209
0.979
206
0.996
20/80
247
0.996
216
0.984
216
0.987
30/70
266
0.979
238
0.987
238
0.990
Even though Kissinger’s method is a special case in determining Ea, it may not display overall trend of Ea since only data from certain conversion rate is used. For Flynn-Wall-Ozawa and Friedman methods, they are integration methods, which lead to Ea/R for a given value of α by plotting logβ versus 1/T and ln(dα/dt) against 1/T, respectively. The plots of Flynn-Wall-Ozawa and Friedman methods show a general trend of activation energy. As an example, the Figure 3 gives the plots of pure mEBHC/HF-PP blends (10/90) using the Friedman and Flynn-Wall-Ozawa methods, respectively. As can be seen in Fig. 3a and b, the fitted lines are nearly parallel, which indicate approximate activation energies at different conversions and consequently implies the possibility of single reaction mechanism [22]. The trends of other blends which are not shown in Fig. 3 are similar to those shown in this figure, indicating the possible single reaction mechanism. Table 2 shows the average apparent activation energies calculated from the conversion range of 10%-90% through Flynn-Wall-Ozawa and Friedman methods for all the blends. As shown in Table 2, the Ea at different mEBHC/HF-PP blends obtained from the Flynn-Wall-Ozawa and Friedman methods are similar to the Kissinger method although the values are slightly higher than the former. These methods demonstrate that the thermal stability of HF-PP could be enhanced via incorporation of mEBHC.
Fig. 3 Plots of mEBHC/HF-PP blends (10/90) in (a) Friedman and (b) Flynn-Wall-Ozawa method at different conversions 3.3 Non-isothermal crystallization kinetics Figure 4 shows the development of Xt of pure HF-PP and mEBHC/HF-PP (20/80) blends with time t at different cooling rates. All the curves have similar sigmoidal shapes and shift to the left with an increase in coling rate. The time to reach 50% of relative crystallinity (t1/2) is an indicative of the
crystallization rate of the polymer and it can be obtained from the curves in Fig. 4. Table 3 shows that t1/2 strongly decrease with the increase of the cooling rate, indicating that the crystallization is faster at high cooling rates. However, t1/2 changes slightly with incorporating mEBHC. For crystallization peak temperature (Tp), it can be seen that Tp of all samples clearly decrease with increase of cooling rate. This is because at lower cooling rate, HF-PP molecules have enough time to form the necessary nuclei for crystallization, resulting in a higher Tp[29]. Compared with pure HF-PP, Tp of the blends is lower, implying that mEBHC prolongs induction period at the initial crystallization stage of HF-PP.
Fig. 4 Plots of relative crystallization (Xt) versus crystallizaton time (t) for mEBHC/HF-PP blends at various crystallization rates: (a) 0/100; (b) 20/80 Table 3 Non-isothermal crystallization parameters of mEBHC/HF-PP blends Cooling rate (oC/min)
Samples
Non-isothermal
(mEBHC/HF-PP)
behavior
5
10
15
20
0/100
t1/2(min)
1.90
1.10
0.67
0.56
Tp(oC)
122.28
118.97
118.17
115.37
n
4.54
4.42
4.06
3.98
Zc(min-1)
0.46
0.88
1.07
1.08
t1/2(min)
1.94
1.02
0.63
0.57
Tp(oC)
118.50
114.41
111.57
109.97
n
4.43
3.96
3.43
3.92
5/95
10/90
20/80
30/70
Zc(min-1)
0.49
0.94
1.08
1.09
t1/2(min)
1.98
1.12
0.63
0.52
Tp(oC)
118.90
115.00
112.05
110.91
n
4.53
4.41
4.01
3.84
Zc(min-1)
0.47
0.90
1.09
1.11
t1/2(min)
1.77
0.99
0.61
0.51
Tp(oC)
118.46
114.21
112.34
109.41
n
4.00
3.85
4.00
3.58
Zc(min-1)
0.58
0.96
1.08
1.10
t1/2(min)
1.90
0.91
0.51
0.47
Tp(oC)
118.12
113.87
111.28
109.48
n
4.58
3.82
3.16
3.70
Zc(min-1)
0.52
0.99
1.12
1.12
In order to further understand the non-isothermal crystallization, the modified Avrami equation has been adopted, and the detail is described as following:
ln ln 1-X t n ln t ln Zc
(4)
where n is the Avrami exponent, Φ is the constant cooling rate, and Zc is growth rate constant of the non-isothermal crystallization. Figure 5 shows the Avrami plot of ln ln 1-X t versus ln t pure HF-PP and mEBHC/HF-PP blends (20/80) as examples. Each curve shows a slight deviation at the beginning of crystallization, which is due to an induction period at the initial crystallization stage[30]. All the curves show good linear relationship in addition they are almost parallel to each other in Fig. 6. It implies that the nucleation mechanism and crystal growth geometries are similar at different cooling rate[31]. Towards the end of crystallization, the lines have a trend to level off, which may be due to the secondary crystallization. The secondary crystallization is usually considered to be caused by the slower
crystallization and further perfection of crystals in the later stage [32, 33]. The Avrami parameters n and Zc calculated from the linear segment of Fig. 6 are listed in Table 3.
Fig. 5 Avrami plots of mEBHC/HF-PP blends at various crystallization rates: (a) 0/100; (b) 20/80 As seen in Table 3, values of n of mEBHC/HF-PP blends are not integers, indicating a complicated crystallization nucleation mode [34]. At the same cooling rate, the n value of pure HF-PP is larger than that of mEBHC/HF-PP blends, suggesting that mEBHC plays a role of heterogeneous nucleation in the crystallization of HF-PP, which changes the crystallization nucleation and growth of HF-PP[34]. The growth rate constants, Zc, for all samples reduce with the cooling rate decrease. And this is reasonable since Zc is a measure of overall crystallization rate, which gets faster with super cooling rate. Furthermore, the value of Zc of mEBHC/HF-PP blends is bigger than that of pure HF-PP at the same cooling rate, due to its higher fluidity and diffusivity of molecular chains of mEBHC/HF-PP blends[12]. Although Avrami equation has been routinely applied to crystallization of polymers, the intrinsic value of the resulting does not reflect such key phenomena as incomplete crystallization or chain folding [35]. Mo et al [36] developed a new form of kinetic equation by combing Avrami and Ozawa equation for the nonisothermal crystallization. Equation can be expressed as Eq. (5)
log ln 1-X t log k n log t log K T m log
(5)
then Eq. (5) can be formulated as follows:
log log F (T ) a log t
(6)
where F (T ) K (T ) / k 1/ m , and a is the ratio of the Avrami exponent (n) divided by Ozawa exponent
(m). F(T) refers to the required cooling rate at which the measured system reaches a certain degree of crystalline at the unit crystallization time[36]. The values of F(T) and a for all the samples are listed in Table 4. The F(T) values increase with the increasing of relative crystallinity for the same sample, indicating that it requires higher cooling rate to achieve higher degree of crystallinity within unit time[34, 37]. For different mEBHC/HF-PP at the same crystallinity, the F(T) values of the blends show a tendency of decrease with the amount of mEBHC added. This suggests that the incorporation of mEBHC can increase the crystallization rate of HF-PP, which agrees well with the result of modified Avrami analysis. In addition, the a value of the blends decreases with the increase of mEBHC content. In the case of pure HF-PP and mEBHC/HF-PP (5/95) blend, a values are bigger than 1, indicating that the Avrami exponent is bigger than the Ozawa exponent[38]. Table 4 Non-isothermal crystallization parameters of mEBHC/HF-PP blends at different relative crystallinity based on Mo method mEBHC/HF-PP
Xt (%)
10%
30%
50%
70%
90%
0/100
F(T)
6.70
8.56
9.97
11.84
14.97
a
1.09
1.13
1.18
1.23
1.37
F(T)
6.61
8.61
10.02
11.82
14.62
a
1.01
1.05
1.06
1.13
1.20
F(T)
6.57
8.67
10.13
11.71
13.78
a
0.91
0.95
1.01
1.07
1.14
F(T)
5.97
7.95
9.40
11.11
13.82
a
0.99
1.04
1.07
1.13
1.18
F(T)
6.29
7.94
9.20
9.88
12.35
a
0.86
0.91
0.97
0.99
1.04
5/95
10/90
20/80
30/70
The evolution of the crystallization activation energy as a function of relative crystallinity for HF-PP and its blends was investigated using the iso-conversional method of Friedman [39], which can be
described as: E dX ln T ln Af ( X T ) RT dt X T
(7)
where (dXT/dt) is the instantaneous crystallization rate for a given relative crystallinity (XT), A is the pre-exponential factor, ΔE is the effective activation energy for the crystallization process, f(XT) is the conversion function that depends on the reaction mechanism. Thus, at a given XT, the ΔE can be calculated from the slope of the plot of ln(dXT/dt) against 1/T for a set of cooling rates. The dependence of effective crystallization activation energy of HF-PP on the extent of relative crystallization degree calculated using the Friedman’s method is presented in Fig. 6a. For all the samples, ΔE increased with increasing relative melt conversion. At the same conversion degree, the activation energy decreased with the mEBHC content, suggesting the accelerating of crystallization rate with the addition of mEBHC, which agrees well with the result of Avrami and Mo analysis.
Fig. 6 (a) Dependence of effective activation energy and average temperature for mEBHC/HF-PP blends on extent of relative crystallization degree and (b) dependence of effective activation energy on average temperature As described above, the mEBHC acts as heterogeneous nucleation agent in the crystallization of HF-PP, and accelerates the rate of crystallization. In order to understand the crystallization stage, Modified Hoffman-Lauritzen equation[40] was further used to calculate the crystallization parameters, including nucleation constant Kg and activation energy of molecular diffusion across the interfacial boundary between melt and crystals U*. The equation was defined as:
E U *
Tm2 T 2 TmT T2 K R g (T T )2 (Tm T )2 T
(8)
where T is the average temperature associated with the relative crystallization degree used to calculate ΔE, T∞ and Tm are the temperature where diffusion stops and the equilibrium melting point, respectively. Fig. 6b shows the dependence of the effective activation energy on average temperature for HF-PP and mEBHC/HF-PP. In this figure, the experimentally calculated data are presented with discrete points, while the solid lines represent fits of eq.(8) with T∞ =231.2K and Tm =481K [39]. The values of Kg and U*are presented in Table 5. The values were increased with the addition of mEBHC, indicating that the mEBHC prolongs induction period at the initial nucleation stage of HF-PP. In the case of U*, the value was reduced. It implied that the molecular mobility of blends was increased with addition of mEBHC, which improves the rate of crystallization. These results agree well with the results of Avrami and Mo analysis. Table 5 Parameters of Hoffman-Lauritzen from non-isothermal crystallization of mEBHC/HF-PP blends mEBHC/HF-PP
Kg (×103 K2)
U* (kJ/mol)
0/100
2.86
118.7
10/90
3.04
113.2
20/80
3.10
101.0
30/70
3.15
86.7
4 Conclusions Thermal
degradation
kinetic
parameters
and
non-isothermal
crystallization
kinetics
of
mEBHC/HF-PP were investigated. The results showed that the degradation behavious of mEBHC and HF-PP were similar. And overall, the addition of mEBHC improved the onset degradation temperature, maximum loss degradation temperature, and thermal degradation activation energy of HF-PP. Non-isothermal crystallization kinetics analysis performed using Avrami and Mo models and Hoffman-Lauritzen theory. The results indicated that the crystallization peak temperature (Tp) shifted to low temperatures due to the addition of mEBHC. The lower Avrami exponent (n) of the blends implied that mEBHC acts as heterogeneous nucleation site in the crystallization of HF-PP. The larger parameter
Kg and lower U* of the blends indicated the slow crystallization nucleation and fast crystallization growth of HF-PP. Overall, the incorporation of mEBHC improves the crystallization rate of HF-PP, and decreases the effective crystallization activation energies (ΔE). Acknowledge This work was financially supported by Scientific Foundation of Nanjing Institute of Technology with granted number YKJ201508 and CKJB201601, Key Research and Development Plan of Jiangsu Province with granted number BE2015158 and Priority Academic Program Development of Jiangsu Higher Education Institutions.
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S0040-6031(16)30338-0 http://dx.doi.org/doi:10.1016/j.tca.2016.11.016 TCA 77644
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
29-9-2013 23-11-2016 24-11-2016
Please cite this article as: Jingyi Wang, Hongbing Jia, Yingying Tang, Xiaogang Xiong, Lifeng Ding, Thermal stability and non-isothermal crystallization kinetics of metallocene poly (ethylene-butene-hexene) /high fluid polypropylene copolymer blends, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2016.11.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermal
stability
and
non-isothermal
crystallization
kinetics
of
metallocene
poly
(ethylene-butene-hexene) /high fluid polypropylene copolymer blends Jingyi Wang1,2, Hongbing Jia*3, Yingying Tang3, Xiaogang Xiong3, Lifeng Ding4
1
College of Material Engineering, Nanjing Institute of Technology, Nanjing 211167, China
2
Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing
Institute of Technology, Nanjing 211167, China 3
Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing
University of Science and Technology, Nanjing 210094, China 4
Department of Chemistry, Xi'an Jiaotong-Liverpool University, Suzhou Dushu Lake Higher Education
Town, Jiangsu Province 215123, China
*Corresponding author, Hongbing Jia, E-mail:[email protected] Telephone: +86-25-84303329, Fax: +86-25-84431939
Highlight Thermal degradation and crystallization behavior of mEBHC/HF-PP were studied. The thermal degradation activation energies increased with the adding mEBHC. mEBHC acted as heterogeneous nucleation agent, and improve crystallization rate.
Abstract: High-fluidity polypropylene (HF-PP) was melt blended with different amount of metallocene poly (ethylene-butene-hexene) copolymer (mEBHC) using a twin-screw extruder. The thermal degradation and non-isothermal crystallization behavior of the blends were investigated using thermo-gravimetric analysis (TGA) and differential scanning calorimeter (DSC). Our investigation revealed that the thermal degradation activation energies of mEBHC/HF-PP blends, calculated from thermo-gravimetric data by Kissinger, Flynn-Wall-Ozawa and Friedman method, increased with the amount of mEBHC added. Therefore, the thermal stability of mEBHC/HF-PP blends was enhanced in comparison to pure HF-PP. Modified Avrami and Mo models and Hoffman-Lauritzen theory were used to evaluate the influence of mEBHC on the non-isothermal crystallization kinetics of HF-PP. The results showed that mEBHC acted as heterogeneous nucleation agent, the non-isothermal crystallization rate of HF-PP matrix was improved by adding mEBHC.
Key words: HF-PP blends, thermal stability, non-isothermal crystallization kinetics, high-fluidity polypropylene
1 Introduction In recent years, high-fluidity polypropylene (HF-PP) has attracted much interests for its application in injection molding of large parts and automobile bumpers due to its outstanding processing and mechanical properties [1-4]. However, high notch sensitivity and poor impact resistance of HF-PP [5] have limited its applications as an engineering plastic, especially under severe conditions. Currently, metallocene catalytic polymerization of polyolefin copolymers, such as metallocene ethylene-octene copolymer (mEOc), metallocene propylene-1-octene copolymer (mPPOc) and metallocene liner low density polyethylene (mLLDPE) have been widely used as toughening agents for PP and PE[6-14], which are mainly owning to their narrow molecular weight distribution, co-monomer distribution, and excellent physical and mechanical properties. For example, McNally et al.[6] found that mEOc could significantly improve the impact strength of PP at both room temperature and cold temperature. Kukaleva et al.[7] also found that the mLLDPE has great influence on the impact strength of “high-crystallinity” polypropylene (hcr-PP). Metallocene poly (ethylene-butene-hexene) copolymer (mEBHC) is a novel metallocene catalytic polymerized polyolefin copolymers with high melt flow rate. Compared to the conventional toughening agents, such as, ethylene propylene rubber (EPR) or ethylene propylene diene monomer (EPDM), mEBHC exhibits the advantage of high melt flow properties. And it also has similar viscosity with HF-PP. In our previous study[15], we focused on the preparation and characterization of metallocene mEBHC toughened high-fluidity PP (HF-PP) blends. The incorporation of mEBHC in HF-PP was found to enhance both the crystallinity degree and impact strength of HF-PP. It is known the lifetime of polymer products depends not only on their physical properties but also on thermal stability [13, 16]. Physical properties of semi-crystalline polymers, like PP, depend greatly on their crystalline [17-19]. Crystallization occurs during the manufacturing of the polymer product. Therefore, understanding polymers’ crystallization mechanism is necessary for macroscopic structure design and final product properties control. In addition, faster crystallization of blends can cause shorter fabrication time in the injection molding process, which can also lead to reduced cost of the finished part.
To the best of our knowledge, there hasn’t been any investigation on the degradation behavior and crystallization kinetic of mEBHC/HF-PP, which is of great importance for developing durable industrial products of HF-PP. This study investigated the thermal degradation kinetic parameters and non-isothermal crystallization kinetics of mEBHC/HF-PP using thermo-gravimetric (TG) analysis and differential scanning calorimeter (DSC). The effects of mEBHC on the thermal stability and non-isothermal crystallization kinetic of HF-PP were also analyzed using different mathematical models. 2 Experimental 2.1 Materials HF-PP (YJP422, M w 285035 , M n 56166 , M w / M n 5.07 , Sinopec-Yangzi Petro Co., Ltd, Nanjing, China) with the melt flow rate (MFR) of 22 g/10 min (ASTM D1238, 230°C and 2.16kg) was used as the matrix polymer. mEBHC ( M w 54844 , M n 19583 , M w / M n 2.8 , ρ = 0.919 g/cm3, MFR = 22 g/10 min (190 °C and 2.16 kg)) was synthesized with the degree of branching (CH3/10000C) = 17.1 (Sinopec-Qilu Petro Co., Ltd, Shandong, China). 2.2 Sample Preparation HF-PP and mEBHC were both dried at 80 ºC for 24h and then were mixed together using a high-speed mixing machine (model CH-10DY) for 10 min. Melt blending was performed using a twin-screw extruder (TE-35, Gelan Machinery Co., Ltd, Zhangjiagang, China) with a rotation speed of 200 rpm. The temperature along the barrel was increased gradually from 175 ºC to 210 ºC: 175 ºC, 190 ºC, 190 ºC, 195 ºC, 200 ºC and 210 ºC, respectively. The blends were cooled down before they were pelletized. The obtained blend pellets were dried again at 80 ºC in a vacuum oven before the injection molding and then were molded into standard sample by injection-machine (JN55-E, Chen Hsong Machinery Co., Ltd, Ningbo, China). The mass fractions of mEBHC in the blends were 0, 5, 10, 20, 30 and 100 wt%, and the weight ratios of mEBHC/HF-PP were designated as 0/100, 5/95, 10/90, 20/80, 30/70, and 100/0, respectively. 2.3 TG analyses Thermo-gravimetric analyses were performed using a DTG-60/60H Simultaneous DTA-TG
Apparatus (Shimadzu Corporation, Japan). The measurements were conducted at heating rates of 5, 10, 20 and 40 °C/min in a dynamic nitrogen atmosphere at a flow rate of 20 ml/min. Sample with a mass of about 10 mg was heated from 30 °C to 600 °C. The data obtained from TGA were further analysed through several kinetic methods summarized in Table 1. Table 1 Kinetic methods used in evaluating degradation activation energy in this study Method
Expression
Plots
Ref.
Kissinger
AR Ea ln 2 ln Tmax g( ) Ea RTmax
1 ln 2 against Tmax Tmax
[20]
Flynn-Wall-Ozawa
Friedman
log
0.457 Ea AEa log 2.315 RT Rg ( )
log against
1 T
1 d ln against T dt
Ea d ln ln Af ( ) RT dt
[21]
[22]
In Table 1, β is the heating rate (K/min), Tmax is the temperature of the peak of derivative TGA curves, α is the degree of conversion (α = (wo−wt)/(wo−wf), wo, wt and wf are the initial, actual and final mass of the sample in the TGA curves, respectively), t is the reaction time (s), T is the absolute temperature (K), the g(α) and f(α) are related to the reaction mechanism and the rate constant k, Ea is the activation energy (kJ/mol), A is the pre-exponential factor, and R is the gas constant (8.314 J/(mol·K)). 2.4 DSC analyses The thermal behaviors of HF-PP and mEBHC/HF-PP blends were analyzed using a Netzsch 200 F3 differential scanning calorimeter (DSC) under nitrogen atmosphere. Approximately 5 mg samples were placed in aluminum pans and tested at a temperature range of 20-200 °C. Samples were heated to 200 ºC with a heating rate of 10 ºC/min and kept for 3 min to erase thermo-mechanical histories, then cooled down to 20 ºC by four different cooling rates of 5, 10, 15 and 20 ºC/min, respectively. As a result, the melting enthalpies (H) as a function of temperature (T) of all blends were obtained. From the DSC vurves, the values of the relative crystallinity (XT) at various cooling rates can be calculated. The XT as a function of temperature (T) can be described as [23, 24]: Tc
XT ( T0
dH ) dT
Te
T0
(
dH ) dT
(1)
where Tc, T0 and Te are the instantaneous, initial and terminal crystallization temperature, respectively. dH represents the change of enthalpy for the crystallization at dT temperature range. In addition, the crystallization temperature can be converted to crystallization time t using the following equation[25]: t (T0 Tc )
(2)
where T0 and Tc are the initial crystallization temperature and the instantaneous temperature at crystallization time (t), respectively, and Φ is the cooling rate. Thus the relative crystallinity as function of time, XT, is defined as: tc
Xt ( t0
dH )dt dt
te
t0
(
dH ) dt
(3)
Results and discussion 3.1 Thermal stability of mEBHC/HF-PP blends Figure 1 shows the TGA and derivative curves (DTG) of pure HF-PP, mEBHC and their blends at a heating rate of 10°C/min under nitrogen atmosphere. The thermal degradation of HF-PP shows one stage of weight loss, which exhibit the onset degradation temperature (Tonset, at 5% weight loss, Fig. 1(a) inset) at 373 °C and maximum loss temperature (Tmax,，the peaks of DTG curves in Fig. 1(b)) at 456 °C, while the Tonset and Tmax of mEBHC are 368 °C and 484 °C, respectively. Incorporating mEBHC was found to be able to increase the thermal stability of HF-PP with the increased Tonset and Tmax (Fig. 1(a) inset, and Fig. 1(b)). Overall, the more mEBHC is incorporated, the higher Tonset and Tmax of the blend will be. With the addition of mEBHC up to 30wt%, the Tmax of the blends could rise 19°C compared to that of pure HF-PP.
Fig. 1 (a) TGA and (b) DTG curves of mEBHC/HF-PP blends at the heating rate of 10 °C/min under nitrogen atmosphere
3.2 Activation energy of degradation
2 Figure 2 shows the linear plots of ln / Tmax
against 1/ Tmax for the various blends from the
Kissinger method, and activation energy (Ea) are calculated and summarized in Table 2. The Ea of pure HF-PP is 199 kJ/mol, which is consistent with the wide range from 120 to 280 kJ/mol reported in the literature [26-28]. The Ea of the blends with 5 wt%, 10 wt%, 20 wt%, 30 wt% of mEBHC increases to 229, 235, 247, and 266 kJ/mol, respectively. It suggests that the thermal stability of HF-PP is proportional to the amount of mEBHC incorporated, which accords with the result of 3.1 Section.
2 Fig. 2 Plots of ln / Tmax against 1/ Tmax for various blends in Kissinger methods
Table 2 Activation energy (Ea) calculated by three methods Kissinger mEBHC/HF-PP
Flynn-Wall-Ozawa
Ea
Ea Rc2
(kJ·mol-1)
Friedman Ea
Rc2 (kJ·mol-1)
Rc2 (kJ·mol-1)
0/100
199
0.997
178
0.989
171
0.985
5/95
229
0.994
192
0.991
192
0.994
10/90
235
0.998
209
0.979
206
0.996
20/80
247
0.996
216
0.984
216
0.987
30/70
266
0.979
238
0.987
238
0.990
Even though Kissinger’s method is a special case in determining Ea, it may not display overall trend of Ea since only data from certain conversion rate is used. For Flynn-Wall-Ozawa and Friedman methods, they are integration methods, which lead to Ea/R for a given value of α by plotting logβ versus 1/T and ln(dα/dt) against 1/T, respectively. The plots of Flynn-Wall-Ozawa and Friedman methods show a general trend of activation energy. As an example, the Figure 3 gives the plots of pure mEBHC/HF-PP blends (10/90) using the Friedman and Flynn-Wall-Ozawa methods, respectively. As can be seen in Fig. 3a and b, the fitted lines are nearly parallel, which indicate approximate activation energies at different conversions and consequently implies the possibility of single reaction mechanism [22]. The trends of other blends which are not shown in Fig. 3 are similar to those shown in this figure, indicating the possible single reaction mechanism. Table 2 shows the average apparent activation energies calculated from the conversion range of 10%-90% through Flynn-Wall-Ozawa and Friedman methods for all the blends. As shown in Table 2, the Ea at different mEBHC/HF-PP blends obtained from the Flynn-Wall-Ozawa and Friedman methods are similar to the Kissinger method although the values are slightly higher than the former. These methods demonstrate that the thermal stability of HF-PP could be enhanced via incorporation of mEBHC.
Fig. 3 Plots of mEBHC/HF-PP blends (10/90) in (a) Friedman and (b) Flynn-Wall-Ozawa method at different conversions 3.3 Non-isothermal crystallization kinetics Figure 4 shows the development of Xt of pure HF-PP and mEBHC/HF-PP (20/80) blends with time t at different cooling rates. All the curves have similar sigmoidal shapes and shift to the left with an increase in coling rate. The time to reach 50% of relative crystallinity (t1/2) is an indicative of the
crystallization rate of the polymer and it can be obtained from the curves in Fig. 4. Table 3 shows that t1/2 strongly decrease with the increase of the cooling rate, indicating that the crystallization is faster at high cooling rates. However, t1/2 changes slightly with incorporating mEBHC. For crystallization peak temperature (Tp), it can be seen that Tp of all samples clearly decrease with increase of cooling rate. This is because at lower cooling rate, HF-PP molecules have enough time to form the necessary nuclei for crystallization, resulting in a higher Tp[29]. Compared with pure HF-PP, Tp of the blends is lower, implying that mEBHC prolongs induction period at the initial crystallization stage of HF-PP.
Fig. 4 Plots of relative crystallization (Xt) versus crystallizaton time (t) for mEBHC/HF-PP blends at various crystallization rates: (a) 0/100; (b) 20/80 Table 3 Non-isothermal crystallization parameters of mEBHC/HF-PP blends Cooling rate (oC/min)
Samples
Non-isothermal
(mEBHC/HF-PP)
behavior
5
10
15
20
0/100
t1/2(min)
1.90
1.10
0.67
0.56
Tp(oC)
122.28
118.97
118.17
115.37
n
4.54
4.42
4.06
3.98
Zc(min-1)
0.46
0.88
1.07
1.08
t1/2(min)
1.94
1.02
0.63
0.57
Tp(oC)
118.50
114.41
111.57
109.97
n
4.43
3.96
3.43
3.92
5/95
10/90
20/80
30/70
Zc(min-1)
0.49
0.94
1.08
1.09
t1/2(min)
1.98
1.12
0.63
0.52
Tp(oC)
118.90
115.00
112.05
110.91
n
4.53
4.41
4.01
3.84
Zc(min-1)
0.47
0.90
1.09
1.11
t1/2(min)
1.77
0.99
0.61
0.51
Tp(oC)
118.46
114.21
112.34
109.41
n
4.00
3.85
4.00
3.58
Zc(min-1)
0.58
0.96
1.08
1.10
t1/2(min)
1.90
0.91
0.51
0.47
Tp(oC)
118.12
113.87
111.28
109.48
n
4.58
3.82
3.16
3.70
Zc(min-1)
0.52
0.99
1.12
1.12
In order to further understand the non-isothermal crystallization, the modified Avrami equation has been adopted, and the detail is described as following:
ln ln 1-X t n ln t ln Zc
(4)
where n is the Avrami exponent, Φ is the constant cooling rate, and Zc is growth rate constant of the non-isothermal crystallization. Figure 5 shows the Avrami plot of ln ln 1-X t versus ln t pure HF-PP and mEBHC/HF-PP blends (20/80) as examples. Each curve shows a slight deviation at the beginning of crystallization, which is due to an induction period at the initial crystallization stage[30]. All the curves show good linear relationship in addition they are almost parallel to each other in Fig. 6. It implies that the nucleation mechanism and crystal growth geometries are similar at different cooling rate[31]. Towards the end of crystallization, the lines have a trend to level off, which may be due to the secondary crystallization. The secondary crystallization is usually considered to be caused by the slower
crystallization and further perfection of crystals in the later stage [32, 33]. The Avrami parameters n and Zc calculated from the linear segment of Fig. 6 are listed in Table 3.
Fig. 5 Avrami plots of mEBHC/HF-PP blends at various crystallization rates: (a) 0/100; (b) 20/80 As seen in Table 3, values of n of mEBHC/HF-PP blends are not integers, indicating a complicated crystallization nucleation mode [34]. At the same cooling rate, the n value of pure HF-PP is larger than that of mEBHC/HF-PP blends, suggesting that mEBHC plays a role of heterogeneous nucleation in the crystallization of HF-PP, which changes the crystallization nucleation and growth of HF-PP[34]. The growth rate constants, Zc, for all samples reduce with the cooling rate decrease. And this is reasonable since Zc is a measure of overall crystallization rate, which gets faster with super cooling rate. Furthermore, the value of Zc of mEBHC/HF-PP blends is bigger than that of pure HF-PP at the same cooling rate, due to its higher fluidity and diffusivity of molecular chains of mEBHC/HF-PP blends[12]. Although Avrami equation has been routinely applied to crystallization of polymers, the intrinsic value of the resulting does not reflect such key phenomena as incomplete crystallization or chain folding [35]. Mo et al [36] developed a new form of kinetic equation by combing Avrami and Ozawa equation for the nonisothermal crystallization. Equation can be expressed as Eq. (5)
log ln 1-X t log k n log t log K T m log
(5)
then Eq. (5) can be formulated as follows:
log log F (T ) a log t
(6)
where F (T ) K (T ) / k 1/ m , and a is the ratio of the Avrami exponent (n) divided by Ozawa exponent
(m). F(T) refers to the required cooling rate at which the measured system reaches a certain degree of crystalline at the unit crystallization time[36]. The values of F(T) and a for all the samples are listed in Table 4. The F(T) values increase with the increasing of relative crystallinity for the same sample, indicating that it requires higher cooling rate to achieve higher degree of crystallinity within unit time[34, 37]. For different mEBHC/HF-PP at the same crystallinity, the F(T) values of the blends show a tendency of decrease with the amount of mEBHC added. This suggests that the incorporation of mEBHC can increase the crystallization rate of HF-PP, which agrees well with the result of modified Avrami analysis. In addition, the a value of the blends decreases with the increase of mEBHC content. In the case of pure HF-PP and mEBHC/HF-PP (5/95) blend, a values are bigger than 1, indicating that the Avrami exponent is bigger than the Ozawa exponent[38]. Table 4 Non-isothermal crystallization parameters of mEBHC/HF-PP blends at different relative crystallinity based on Mo method mEBHC/HF-PP
Xt (%)
10%
30%
50%
70%
90%
0/100
F(T)
6.70
8.56
9.97
11.84
14.97
a
1.09
1.13
1.18
1.23
1.37
F(T)
6.61
8.61
10.02
11.82
14.62
a
1.01
1.05
1.06
1.13
1.20
F(T)
6.57
8.67
10.13
11.71
13.78
a
0.91
0.95
1.01
1.07
1.14
F(T)
5.97
7.95
9.40
11.11
13.82
a
0.99
1.04
1.07
1.13
1.18
F(T)
6.29
7.94
9.20
9.88
12.35
a
0.86
0.91
0.97
0.99
1.04
5/95
10/90
20/80
30/70
The evolution of the crystallization activation energy as a function of relative crystallinity for HF-PP and its blends was investigated using the iso-conversional method of Friedman [39], which can be
described as: E dX ln T ln Af ( X T ) RT dt X T
(7)
where (dXT/dt) is the instantaneous crystallization rate for a given relative crystallinity (XT), A is the pre-exponential factor, ΔE is the effective activation energy for the crystallization process, f(XT) is the conversion function that depends on the reaction mechanism. Thus, at a given XT, the ΔE can be calculated from the slope of the plot of ln(dXT/dt) against 1/T for a set of cooling rates. The dependence of effective crystallization activation energy of HF-PP on the extent of relative crystallization degree calculated using the Friedman’s method is presented in Fig. 6a. For all the samples, ΔE increased with increasing relative melt conversion. At the same conversion degree, the activation energy decreased with the mEBHC content, suggesting the accelerating of crystallization rate with the addition of mEBHC, which agrees well with the result of Avrami and Mo analysis.
Fig. 6 (a) Dependence of effective activation energy and average temperature for mEBHC/HF-PP blends on extent of relative crystallization degree and (b) dependence of effective activation energy on average temperature As described above, the mEBHC acts as heterogeneous nucleation agent in the crystallization of HF-PP, and accelerates the rate of crystallization. In order to understand the crystallization stage, Modified Hoffman-Lauritzen equation[40] was further used to calculate the crystallization parameters, including nucleation constant Kg and activation energy of molecular diffusion across the interfacial boundary between melt and crystals U*. The equation was defined as:
E U *
Tm2 T 2 TmT T2 K R g (T T )2 (Tm T )2 T
(8)
where T is the average temperature associated with the relative crystallization degree used to calculate ΔE, T∞ and Tm are the temperature where diffusion stops and the equilibrium melting point, respectively. Fig. 6b shows the dependence of the effective activation energy on average temperature for HF-PP and mEBHC/HF-PP. In this figure, the experimentally calculated data are presented with discrete points, while the solid lines represent fits of eq.(8) with T∞ =231.2K and Tm =481K [39]. The values of Kg and U*are presented in Table 5. The values were increased with the addition of mEBHC, indicating that the mEBHC prolongs induction period at the initial nucleation stage of HF-PP. In the case of U*, the value was reduced. It implied that the molecular mobility of blends was increased with addition of mEBHC, which improves the rate of crystallization. These results agree well with the results of Avrami and Mo analysis. Table 5 Parameters of Hoffman-Lauritzen from non-isothermal crystallization of mEBHC/HF-PP blends mEBHC/HF-PP
Kg (×103 K2)
U* (kJ/mol)
0/100
2.86
118.7
10/90
3.04
113.2
20/80
3.10
101.0
30/70
3.15
86.7
4 Conclusions Thermal
degradation
kinetic
parameters
and
non-isothermal
crystallization
kinetics
of
mEBHC/HF-PP were investigated. The results showed that the degradation behavious of mEBHC and HF-PP were similar. And overall, the addition of mEBHC improved the onset degradation temperature, maximum loss degradation temperature, and thermal degradation activation energy of HF-PP. Non-isothermal crystallization kinetics analysis performed using Avrami and Mo models and Hoffman-Lauritzen theory. The results indicated that the crystallization peak temperature (Tp) shifted to low temperatures due to the addition of mEBHC. The lower Avrami exponent (n) of the blends implied that mEBHC acts as heterogeneous nucleation site in the crystallization of HF-PP. The larger parameter
Kg and lower U* of the blends indicated the slow crystallization nucleation and fast crystallization growth of HF-PP. Overall, the incorporation of mEBHC improves the crystallization rate of HF-PP, and decreases the effective crystallization activation energies (ΔE). Acknowledge This work was financially supported by Scientific Foundation of Nanjing Institute of Technology with granted number YKJ201508 and CKJB201601, Key Research and Development Plan of Jiangsu Province with granted number BE2015158 and Priority Academic Program Development of Jiangsu Higher Education Institutions.
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