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MMT nanocomposites
Composites: Part B 37 (2006) 399–407 www.elsevier.com/locate/compositesb
Study on mechanical properties, thermal stability and crystallization behavior of PET/MMT nanocomposites Yimin Wang a,*, Junpeng Gao a, Yunqian Ma b, Uday S. Agarwal b a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 200051, People’s Republic of China b Department of Chemistry and Chemical Engineering, Technical University of Eindhoven, Eindhoven 5600MB, The Netherlands Received 17 February 2005; received in revised form 13 May 2005; accepted 28 May 2005 Available online 4 April 2006
Abstract PET/montmorillonite (MMT) nanocomposites were prepared via melt-blending and its nano-dispersion morphology was confirmed by X-ray diffraction and transmission electron microscopy. Its non-isothermal crystallization behavior was studied by DSC. It is found that the crystallization rate of PET nanocomposite was increased significantly. The Avrami equation parameters related to crystallization, such as n, Zc and t1/2, were calculated and analyzed. The thermal property and mechanical property of the composite were studied. When the MMT content was 1%, the composite has a desired comprehensive property. At this composition, the thermal degradation onset temperature and the thermal deformation temperature of PET were increased by 12 and 35 8C, respectively, and the tensile strength of the PET was increased by 25% with slightly increase of the notched impact strength. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Particle reinforcement; A. Polymer-matrix composites (PMCs); B. Mechanical properties; E. Thermal analysis
1. Introduction Poly(ethylene terephthalate) (PET) is a thermoplastic with many excellent physical properties. However, the disadvantages such as low rate of crystallization and low thermal distortion temperature and low modulus have limited its application as an engineering plastic [1]. One way to enhance its property is to utilize the nanocomposite of PET/layered silicate [2–11]. Polymer/layered silicate nanocomposite can be produced via three routes, i.e. solution, in situ polymerization and melt blending. Solution and in situ polymerization routes are commonly used in research to make nanocomposites, but they are expensive processes. Melt blending is an economically favourable process and a few of experiments have demonstrated some success of this process [12–14]. Because there are various blending parameters and raw materials that can be selected, more study is needed to understand the mechanism and achieve better particle dispersion and mechanical properties [15,16]. In this paper, the effect of compounding MMT with PET on the crystallization behavior, thermal stability, and * Corresponding author. Tel.: C86 21 62379785; fax: C86 21 62379309. E-mail address: [email protected] (Y. Wang).
1359-8368/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2006.02.014
mechanical property of the composite are examined. Organically modified montmorillonite (organo-MMT) is used as the layered clay to prepare polymer/layered silicate nanocomposite in this research because of its wide availability and its easiness to be nano-dispersed in the matrix. 2. Experimental 2.1. Raw materials PET, Intrisic viscosity 0.82 dL/g, Shanghai Jinshan Petrochemical Corporation; Organic Montmorillonite (DK2), Zhejiang Fenghong Clay Corp. Ltd. 2.2. Equipments Twin screw extruder, TSSJ-25/40, D25 mm, L/D 40, Chenguang Plastic Machinery Research Institute. Injection molder, HTB110XB, Zhejiang Ningbo Haitian Plastic Machinery Corp. Ltd. X-ray diffractometer, D/max-rA, Rigaku. Microtome (LKB-V), Sweden. Transmission electron microscopy (TEM), JEM-1200 EX 2, JEOL. Differential scanning calorimeter, 822e, Mettler Toledo, Switzerland.
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Thermal gravity analysis (TGA), Perkin-Elmer TGA-7, USA Perkin-Elmer. Mechanical Experimental Instrument, CMT5204, Tianshui Sansi Hightech Corp. Ltd. Impact testing machine, Resil P/N 6957.000, Italian CEAST Scientific Instrument Corp. Heat distortion temperature/Vicat softening point instrument, XWB-300D, Chengde Laboratory Instrument Corp. Ltd. Polarized light microscopy (PLM), OLYMPUS-BX51.
were kept at 200 8C for 3 min before PLM pictures were taken. Enlargement is 1000 times. Tensile strength, bending strength, impact strength and thermal deformation strength are measured following the procedure in ASTM D638-2000, ASTM D790-2000, ASTM D256-2000 and ASTM D648-2000.
3. Results and discussion 3.1. X-ray diffraction analysis
2.3. PET/MMT blending PET is dried at 120 8C under vacuum for 48 h. OrganoMMT is dried at 80 8C under vacuum for 6 h. PET and OrganoMMT with various ratios are mixed first in high speed mixer, then extruded into pellets via a twin screw extruder. Zone temperature of the barrel is 265 8C. The ratios of PET and Organo-MMT are listed in Table 1. 2.4. Preparation of sample for thermal and mechanical property testing The composite pellets are dried at 120 8C under vacuum for 2 h, and then the samples for testing are injected via an injecting molder. The barrel temperature is 260 8C. Temperature of the molder is kept between 90 and 110 8C. Duration of injection is 2 s. Pressing time is 25 s for preparing tensile and thermal deformation temperature samples, and 15 s for preparing impact strength and bending samples. All samples made are kept at room temperature for 24 h before measuring. 2.5. Testing instruments X-ray diffraction (XRD): Cu Ka, Ni filler, 40 kV, 200 mA, scanning rate 18/min, 2q(18–108). Transmission electron microscopy (TEM): accelerating voltage 100 kv. PET/MMT composite are microtomed into layers with thickness of about 100 nm. Differential scanning calorimeter (DSC) is conducted in the following procedure: the sample is heated from 30 to 290 8C at certain heating rate, kept for 5 min, and cooled to 30 8C at certain hearting rate. Thermal gravity analysis (TGA) is conducted from 25 to 600 8C at 20 8C/min in nitrogen atmosphere with N2 flow rate of 40 mL/min. Morphology observation by polarized light microscopy (PLM): samples are first heated to 290 8C and kept for 5 min, then cooled to 200 8C at a rate of about 130 8C/min. Sample
Fig. 1 is XRD of organo-MMT. At 2qZ3.378, there is a strong diffraction peak. The distance between layers can be calculated according to the Bragg equation 2d sin q Z nl
d q l n
(1)
average distance between MMT layers diffraction angle inlet X-ray wavelength (0.154 nm) diffraction number.
The calculated distance between MMT layers is 2.62 nm. XRD and TEM can be used to decide whether nanocomposite is intercalated type or exfoliated type [1]. Fig. 2 shows the XRD graphs of PET/organo-MMT composites with various amount of organo-MMT. As can be seen from the figure, the 2q value of corresponding XRD peaks decreases with the addition of MMT into PET. The layer distance calculated from Eq. (1) increases from 2.62 nm for pristine PET to 3.46, 3.28 and 3.20 nm for the composites with organo-MMT content of 1, 3 and 5%, respectively. It suggests that polymer molecules have entered the silicate layers of MMT. Because the intensity and sharpness of the diffraction peaks are still evident, especially in PET3M and PET5M, intercalation rather than exfoliation has probably happened.
Table 1 PET/MMT mass ratio of composite samples
OrganoMMT (DK2) PET
PET
PET1M
PET3M
PET5M
0
1
3
5
100
99
97
95
Fig. 1. X-ray diffraction curve of organic montmorillonite DK2.
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Fig. 4. TEM image of PET3M. Fig. 2. X-ray diffraction curve of PET1M, 3M and 5M.
3.2. Transmission electron microscopy (TEM) analysis
conglomerating, which is beneficial to forming the nano particles when there is more polymer with less MMT. Another reason is, when the MMT content is high, it is difficult for macromolecules to enter between the MMT layers due to the congregation of MMT. The TEM results are in agreement with XRD observation and suggest that melt blending can be used in preparing intercalating type nanocomposite only when MMT content is small.
The dispersion state of MMT inside PET matrix is further examined by TEM. Figs. 3–5 are TEM of PET1M, PET3M and PET5M, respectively. Fig. 3 shows that sufficient PET molecules have entered into MMT layers when the content of MMT is 1%. The MMT layers are well distributed in the matrix and the layer distance is about 3 nm. When the content of MMT is increased to 3% (Fig. 4), PET molecules have entered between the MMT layers, but the MMT layers are not well dispersed in the matrix and the layer distance is smaller than PET1M. Fig. 7 shows that the MMT crystallites in PET5M are pressed and squeezed to a distorted state during melt blending process and the middle of the MMT clay layers are not evident to have been intercalated. Fig. 5 also shows that the MMT particles have certain degree of conglomeration (the color of MMT in PET5M in TEM picture is darker than other samples) although outer part has been intercalated. In general, intercalation is less obvious in PET5M than PET1M and PET3M. One possible reason is that the MMT layers’ probability of meeting each other by collision is small when the content of MMT in the matrix is low. What is more, the matrix polymer molecules can prevent MMT layers’ from
Pristine PET is a well-known semi-crystallization polymer with a character of low rate of crystallization. It is necessary to improve its crystallization rate in order to use it as an engineering polymer. The effect of MMT on the crystallization behavior of PET during heating and cooling is examined by DSC (Figs. 6 and 7, and Table 2). In DSC, crystallization temperature from melt (Tmc) and its half peak width (width of the peak at half height) are two parameters that can characterize the crystallization rate. When Tmc is higher and half peak width is less, the rate of crystallization is higher. Fig. 6(a) and Table 2 shows that adding MMT will increase Tmc, increase the sharpness of crystallization peak, and decrease the half peak width. Hence, the nanocomposite of PET/MMT gives higher crystallization
Fig. 3. TEM image of PET1M.
Fig. 5. TEM image of PET5M.
3.3. PET/MMT crystallization behavior
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Fig. 6. DSC of PET/MMT (a) curves of cooling, (b) curves of heating ((1) pristine PET, (2) PET1M, (3) PET3M, (4) PET5M). Rate of heating and cooling are both 10 8C/min.
rate then pristine PET. One reason of crystallization rate enhancement is that MMT can act as an effective heterogeneous nucleating agent. Another possible reason is that MMT nano-structure itself can help the PET molecules stack on each other to grow into crystallites, thus leads to the higher crystallization rate. While the composites give much high rate of crystallization than pristine PET, the difference between composites with different MMT composition are much less. Differential scanning calorimetry (DSC) curves obtained from PET and PET/MMT nanocomposites with different amount of MMT and different heating and cooling rate are showed in Fig. 7. PET/MMT nanocomposites have higher crystallization temperature with the increase of MMT content when the cooling rates are same. For cooling rate of 40 8C/min,
pristine PET shows quite small exotherm peak, while PET/MMT composites still show large crystallization peak. It indicates the both the rate and crystallinity of the composites are increased comparing with pristine PET. DSC result also shows that the crystallization temperature (Tmc) during cooling decrease and the crystallization temperature peak broadens with the increase of cooling rate for the same sample. Ozawa method and Jeziorny method are commonly used to study the non-isothermal crystallization of polymer. Our previous work [17] showed that Ozawa method [16] does not give reasonable result. In this work, Jeziorny method [18] is adopted to study the non-isothermal crystallization of PET. The relative crystallization (Xt) [19], as a function of temperature is defined as
Fig. 7. The DSC curves at different cooling rate (a) PET, (b) PET1M, (c) PET3M, (d) PET5M.
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Table 2 DSC results of PET/MMT composites Sample
Tcc (8C)
Hcc (J/g)
Tm (8C)
Hm (J/g)
Tmc (8C)
Hmc (J/g)
PET PET1M PET3M PET5M
122.25 120.00 117.55 120.58
15.24 22.12 21.28 22.64
257.13 252.28 253.42 255.12
K42.15 K52.35 K50.34 K45.65
188.73 195.57 198.72 202.24
53.59 57.61 53.19 52.30
Note: Tcc, Tm, Tmc are cold crystallization temperature, melting temperature, and crystallization temperature from melt, respectively. Hcc, Hm, Hmc are cold crystallization enthalpy, melting enthalpy and crystallization enthalpy from melt, respectively. Rate of heating and cooling are 10 8C/min.
ðT
Xt Z ðdHc =dTÞ= T0
TðN
ðdHc =dTÞdT
(2)
For non-isothermal crystallization at certain cooling rate, Jeziorny’s [19] method is to modify Zt by the equation Log Zc Z ðlog Zt Þ=F
T0
where T0 and TN are the onset and end of crystallization temperature, respectively. Hc is the heat flow at temperature T. Polymer isothermal crystallization can be described by Avrami equation [20] and expressed as 1KXt Z expðKZt tn Þ
(3)
where n is Avrami index, t is crystallization time, Xt is crystallinity at time t and Zt is kinetic crystallization rate. This can be re-written as: ln½Klnð1KXt Þ Z ln Zt C n ln t
(4)
(5)
where F is the cooling rate, Zc is the kinetics crystallization rate. The curves of (1KXt) vs. T (Fig. 8) indicate that the higher the cooling rate, the lower the temperature at which the composites begin to crystallize. Higher crystallization temperature and higher crystallinity can be observed with the increase of MMT content. Plotting ln[Kln(1KXt)] vs. ln t, we have straight lines as shown in Fig. 9. It is clear that all samples present a linear relation in a relative broad range of crystallinity. Parameters n and ln Zt can be obtained from the slop and intercept of each line. Zc can be calculated from Eq. (5). The result of Avrami
Fig. 8. The curves of (1KXt)–T at different cooling rates (a) PET, (b) PET1M, (c) PET3M, (d) PET5M.
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Fig. 9. The curves of ln[Kln (1KXt)] vs. ln t of the samples at different temperature PET (b) PET1M (c) PET3M (d) PET5M.
index n, kinetics crystallization rate Zc and half-time of crystalliztion t1/2 are summarized in Table 3. The value of Zc and n of PET/MMT composite are much larger than pristine PET. However, the difference between the composites with different MMT contents is less obvious. These parameters values indicate that crystallization ability of PET/MMT Table 3 Parameters calculated from non-isothermal crystallization process Sample
F/8C minK1
n
Zc
t1/2/s
PET
5 10 20 40 5 10 20 40 5 10 20 40 5 10 20 40
4.0 3.5 2.9 2.7 4.7 4.0 4.6 3.6 4.4 4.2 4.1 4.4 4.8 3.9 4.8 4.2
0.21 0.65 0.89 0.95 0.40 0.83 0.94 0.97 0.42 0.80 0.99 0.97 0.38 0.87 0.98 0.96
80.80 61.12 55.01 53.39 67.44 57.42 56.15 54.66 67.23 57.90 55.00 55.20 66.00 56.40 55.80 55.20
PET1M
PET3M
PET5M
nanocomposites is remarkably improved in comparison with the pristine PET. 3.4. Morphology observation by polarized light microscopy (PLM) The morphology during the crystallization of PET/MMT nanocomposite is observed by PLM. The picture in Fig. 10 shows that pristine PET gives small spherulites morphology, while PET/MMT nanocomposites consists of much higher amount of crystallites with much smaller size. The clay layers can act as nucleating agent, thus greatly increase the nuclei number, resulting the observed morphology. 3.5. PET/MMT composites thermal property 3.5.1. TGA analysis Fig. 11 shows that pristine PET onset temperature of degradation (Tonset, weight loss 2.5%) is 310 8C, while Tonset of PET1M, PET3M and PET5M are 322, 332, and 319 8C, respectively. Fig. 12 shows the temperatures at maximum mass loss rate (Tpeak) for the pristine PET, PET1M, PET3M and PET5M are 362, 374, 385 and 375 8C, respectively. Both the Tonset and Tpeak are increased significantly by adding MMT. It
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Fig. 10. The photos of PET/MMT under PLM (a) PET, (b) PET1M, (c) PET3M. (Samples are first heated to 290 8C and kept for 5 min, then cooled to 200 8C and stayed for 3 min before the picture were taken. Enlargement 1000).
is interesting to notice that Tonset and Tpeak of PET5M are lower than that of PET3M, and are similar to PET1M. This is perhaps related to the poor dispersion of MMT in PET matrix in PET5M sample. Therefore, when MMT content is in the low range (!5%), the observed thermal stability behavior of PET/MMT is decided by both MMT content and its dispersion quality.
3.5.2. Heat distortion temperature analysis Table 4 shows that MMT can enhance the HDT significantly. When MMT is just 1%, the HDT is 35 8C higher than that of pristine PET. There are several possible reasons. Firstly, MMT increases the crystallinity of the composite. Secondly, the interaction force between MMT and the matrix is strong. Thirdly, PET chain movements are restricted between the crystallites, which have huge number because of the
Fig. 11. TGA of PET/MMT ((1) pristine PET, (2) PET1M, (3) PET3M, (4) PET5M).
Fig. 12. DTGA of PET/MMT ((1) pristine PET, (2) PET1M, (3) PET3M, (4) PET5M).
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Table 4 Heat distortion temperatures of PET/MMT composites Sample
PET
PET1M
PET3M
PET5M
HDT (8C) (1. 80 MPa)
71
106
112
120
nano-dispersion, and the chain mobility is hindered by crystallites, therefore, the resistance to heat deformation under loading is increased. 3.6. Mechanical property The tensile strength of PET1M with 1% MMT content is enhanced significantly (Fig. 13(a)). It is increased by 25% from 54 to 67.7 MPa. The flexural strength of this sample is also
enhanced (Fig. 13(b)). The notch impact strength is 26.9 J/m when the MMT content is 1% (Fig. 13(c)), which is also higher compared with that of the pristine PET (26.0 J/m). The bending modulus is increased significantly by 43.8%. When the MMT content is 3% or more, the tensile and flexural strength, elongation at break and impact strength are all less than that of pristine PET. The flexural modulus is only a little higher than 1% MMT composite. The enhancement of strength and modulus when adding 1% MMT in the composite is due to the good dispersion of MMT within the polymer matrix, the strong interaction of MMT with PET matrix, and the two-dimension strength enhancement by the MMT nano particles that have large l/d ratio. Nanoparticles have large specific area and large surface-active centers and therefore have more physical and chemical interactions with the polymer matrix. In consequence, the nano particles are difficult to disconnect from the matrix when the composite is under impact, and can resist and transfer the imposed force, leading to the dispersion of the impact energy into surrounding matrix. The strong interaction of MMT with PET matrix is because of the large specific area of MMT nano particles. However, when adding higher amount of MMT, the dispersion of MMT is not ideal and the intercalation is not complete, therefore the tensile, flexural and impact strength and elongation at break decreases.
4. Conclusion
Fig. 13. Mechanical property of PET/MMT composites vs. MMT contents: (a) tensile strength and elongation at break, (b) flexural strength and modulus, (c) Izod Impact strength.
(1) XRD and TEM indicate that melt blending can produce intercalating type of PET/organo-MMT nanocomposite. The dispersion is best when MMT content is 1%. (2) Addition of Organo-MMT (DK2) to PET can enhance the crystallinity and the rate of crystallization, which is desirable for improving PET’s processing ability, for instance, reducing the mold temperature during injection molding. Adding MMT can increase the Tmc, decrease Tcc and reduce half peak width of crystalliztion peak during cooling in DSC. Higher crystallization temperature and nucleation rate of PET during cooling can be achieved by the formation of organic MMT/PET nanocomposite. The composites have higher nucleating rate because MMT can act as a nucleating agent [4,9,11,19,23] and MMT particles’ nano dispersion can help the crystallization as well. When the content of MMT in the composite increases further than 1%, the increase of crystallization rate is not as remarkable as that of 1% content. (3) The composite has the optimum comprehensive mechanical property when the organo-MMT content is 1%. When the amount of MMT is 3% or more than, properties such as tensile strength, impact strength, flexural strength, and elongation at break are less than that of the pristine PET. (4) Adding organo-MMT (DK2) will improve the thermal stability of PET. When MMT content is 1%, onset temperature of degradation of PET/MMT nanocomposite increases by 12 8C, and peak degradation temperature rises by 35 8C.
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(5) HDT can be greatly enhanced by PET/organo-MMT nanocomposite.
Acknowledgements The authors are grateful to Science and Technology Commission of Shanghai Municipality for its financial support (02JC41010). References [1] Scheirs J, Long T, editors. Modern polyesters. Chichester: Wiley; 2003. [2] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28:1539–641. [3] Sa´nchez-Solı´s A, Romero-Ibarra I, Estrada MR, Calderas F, Manero O. Mechanical and rheological studies on polyethylene terephthalate– montmorillonite nanocomposites. Polym Eng Sci 2004;44(6):1094–102. [4] Wan T, Chen L, Chua YC, Lu X. Crystalline morphology and isothermal crystallization kinetics of poly(ethylene terephthalate)/clay nanocomposites. J Appl Polym Sci 2004;94:1381–8. [5] Ou CF, Ho MT, Lin JR. Synthesis and characterization of poly(ethylene terephthalate) nanocomposites with organoclay. J Appl Polym Sci 2004; 91:140–5. [6] Chang J-H, Park D-K. Nanocomposites of poly(ethylene terephthalate-coethylene naphthalate) with organoclay. J Polym Sci Part B 2001;39:2581–8. [7] Chang J-H, Park D-K. Various organo-clays based nanocomposites of poly(ethylene terephthalate-co-ethylene naphthalate). Polym Bull 2001; 47(2):191–7. [8] Chang J-H, Kim SJ, Joo YL, Im S. Poly(ethylene terephthalate) nanocomposites by in situ interlayer polymerization: the thermomechanical properties and morphology of the hybrid fibers. Polymer 2004;45(3):919. [9] Ke YC, Long C, Qi Z. Crystallization, properties, and crystal and nanoscale morphology of PET-clay nanocomposites. J Appl Polym Sci 1999;71:1139–46.
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[10] Takekoshi T, Khouri FF, Campbell JR, Jordan TC, Dai KH. PET nanocomposites prepared by in situ incorporation of varying amounts of four different organoclays. US Patent 5,530,052 (General Electric Co.); June 25, 1996. [11] Tsai TY. Polyethylene terephthalate-clay nanocomposites. In: Pinnavaia TJ, Beall GW, editors. Polymer-clay nanocomposites. England; Wiley; 2000. p. 173–89. [12] Pegoretti A, Kolarik J, Peroni C, Migliaresi C. Recycled poly(ethylene terephthalate)/layered silicate nanocomposites: morphology and tensile mechanical properties. Polymer 2004;45(8):2751–9. [13] Kalgaonkar RA, Jog JP. Copolyester/layered silicate nanocomposites: the effect of the molecular size and molecular structure of the intercalant on the structure and viscoelastic properties of the nanocomposites. J Polym Sci Part B Polym Phys 2003;41:3102–13. [14] Davis CH, Mathias LJ, Gilman JW, Schiraldi DA, Shields JR, Trulove P, et al. Effects of melt-processing conditions on the quality of poly(ethylene terephthalate) montmorillonite clay nanocomposites. J Polym Sci Part B Polym Phys 2002;40:2661–6. [15] Lam CK, Cheung HY, Ling HY, Lau KT. Effect of ultrasonic sonication in nanoclay clusters of nanoclay/epoxy composites. Mater Lett 2005; 59(11):1369–72. [16] Lam CK, Cheung HY, Lau KT, Zhou LM, Ho MW, Hui D. Cluster size effect in hardness of nanoclay/epoxy composites. Compos Part B Eng 2005;36(3):263–9. [17] Gao J. Study on preparation and properties of PET/MMT nanocomposites. Master Degree Thesis. Donghua University (China); 2005. [18] Ozawa T. Kinetics of non-isothermal crystallization. Polymer 1971;12: 150. [19] Jeziorny A. Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by DSC. Polymer 1978;19:1142. [20] Kong X, Yang X, Li G, Zhao X, Zhou E, Ma D. Eur Polym J 2001; 29(3):2. [23] Xu W, Ce M, He P. Non-isothermal crystallization kinetics of polyoxymethylene/montmorillonite nanocomposite. J Appl Polym Sci 2001;82:2281.
Study on mechanical properties, thermal stability and crystallization behavior of PET/MMT nanocomposites Yimin Wang a,*, Junpeng Gao a, Yunqian Ma b, Uday S. Agarwal b a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 200051, People’s Republic of China b Department of Chemistry and Chemical Engineering, Technical University of Eindhoven, Eindhoven 5600MB, The Netherlands Received 17 February 2005; received in revised form 13 May 2005; accepted 28 May 2005 Available online 4 April 2006
Abstract PET/montmorillonite (MMT) nanocomposites were prepared via melt-blending and its nano-dispersion morphology was confirmed by X-ray diffraction and transmission electron microscopy. Its non-isothermal crystallization behavior was studied by DSC. It is found that the crystallization rate of PET nanocomposite was increased significantly. The Avrami equation parameters related to crystallization, such as n, Zc and t1/2, were calculated and analyzed. The thermal property and mechanical property of the composite were studied. When the MMT content was 1%, the composite has a desired comprehensive property. At this composition, the thermal degradation onset temperature and the thermal deformation temperature of PET were increased by 12 and 35 8C, respectively, and the tensile strength of the PET was increased by 25% with slightly increase of the notched impact strength. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Particle reinforcement; A. Polymer-matrix composites (PMCs); B. Mechanical properties; E. Thermal analysis
1. Introduction Poly(ethylene terephthalate) (PET) is a thermoplastic with many excellent physical properties. However, the disadvantages such as low rate of crystallization and low thermal distortion temperature and low modulus have limited its application as an engineering plastic [1]. One way to enhance its property is to utilize the nanocomposite of PET/layered silicate [2–11]. Polymer/layered silicate nanocomposite can be produced via three routes, i.e. solution, in situ polymerization and melt blending. Solution and in situ polymerization routes are commonly used in research to make nanocomposites, but they are expensive processes. Melt blending is an economically favourable process and a few of experiments have demonstrated some success of this process [12–14]. Because there are various blending parameters and raw materials that can be selected, more study is needed to understand the mechanism and achieve better particle dispersion and mechanical properties [15,16]. In this paper, the effect of compounding MMT with PET on the crystallization behavior, thermal stability, and * Corresponding author. Tel.: C86 21 62379785; fax: C86 21 62379309. E-mail address: [email protected] (Y. Wang).
1359-8368/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2006.02.014
mechanical property of the composite are examined. Organically modified montmorillonite (organo-MMT) is used as the layered clay to prepare polymer/layered silicate nanocomposite in this research because of its wide availability and its easiness to be nano-dispersed in the matrix. 2. Experimental 2.1. Raw materials PET, Intrisic viscosity 0.82 dL/g, Shanghai Jinshan Petrochemical Corporation; Organic Montmorillonite (DK2), Zhejiang Fenghong Clay Corp. Ltd. 2.2. Equipments Twin screw extruder, TSSJ-25/40, D25 mm, L/D 40, Chenguang Plastic Machinery Research Institute. Injection molder, HTB110XB, Zhejiang Ningbo Haitian Plastic Machinery Corp. Ltd. X-ray diffractometer, D/max-rA, Rigaku. Microtome (LKB-V), Sweden. Transmission electron microscopy (TEM), JEM-1200 EX 2, JEOL. Differential scanning calorimeter, 822e, Mettler Toledo, Switzerland.
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Thermal gravity analysis (TGA), Perkin-Elmer TGA-7, USA Perkin-Elmer. Mechanical Experimental Instrument, CMT5204, Tianshui Sansi Hightech Corp. Ltd. Impact testing machine, Resil P/N 6957.000, Italian CEAST Scientific Instrument Corp. Heat distortion temperature/Vicat softening point instrument, XWB-300D, Chengde Laboratory Instrument Corp. Ltd. Polarized light microscopy (PLM), OLYMPUS-BX51.
were kept at 200 8C for 3 min before PLM pictures were taken. Enlargement is 1000 times. Tensile strength, bending strength, impact strength and thermal deformation strength are measured following the procedure in ASTM D638-2000, ASTM D790-2000, ASTM D256-2000 and ASTM D648-2000.
3. Results and discussion 3.1. X-ray diffraction analysis
2.3. PET/MMT blending PET is dried at 120 8C under vacuum for 48 h. OrganoMMT is dried at 80 8C under vacuum for 6 h. PET and OrganoMMT with various ratios are mixed first in high speed mixer, then extruded into pellets via a twin screw extruder. Zone temperature of the barrel is 265 8C. The ratios of PET and Organo-MMT are listed in Table 1. 2.4. Preparation of sample for thermal and mechanical property testing The composite pellets are dried at 120 8C under vacuum for 2 h, and then the samples for testing are injected via an injecting molder. The barrel temperature is 260 8C. Temperature of the molder is kept between 90 and 110 8C. Duration of injection is 2 s. Pressing time is 25 s for preparing tensile and thermal deformation temperature samples, and 15 s for preparing impact strength and bending samples. All samples made are kept at room temperature for 24 h before measuring. 2.5. Testing instruments X-ray diffraction (XRD): Cu Ka, Ni filler, 40 kV, 200 mA, scanning rate 18/min, 2q(18–108). Transmission electron microscopy (TEM): accelerating voltage 100 kv. PET/MMT composite are microtomed into layers with thickness of about 100 nm. Differential scanning calorimeter (DSC) is conducted in the following procedure: the sample is heated from 30 to 290 8C at certain heating rate, kept for 5 min, and cooled to 30 8C at certain hearting rate. Thermal gravity analysis (TGA) is conducted from 25 to 600 8C at 20 8C/min in nitrogen atmosphere with N2 flow rate of 40 mL/min. Morphology observation by polarized light microscopy (PLM): samples are first heated to 290 8C and kept for 5 min, then cooled to 200 8C at a rate of about 130 8C/min. Sample
Fig. 1 is XRD of organo-MMT. At 2qZ3.378, there is a strong diffraction peak. The distance between layers can be calculated according to the Bragg equation 2d sin q Z nl
d q l n
(1)
average distance between MMT layers diffraction angle inlet X-ray wavelength (0.154 nm) diffraction number.
The calculated distance between MMT layers is 2.62 nm. XRD and TEM can be used to decide whether nanocomposite is intercalated type or exfoliated type [1]. Fig. 2 shows the XRD graphs of PET/organo-MMT composites with various amount of organo-MMT. As can be seen from the figure, the 2q value of corresponding XRD peaks decreases with the addition of MMT into PET. The layer distance calculated from Eq. (1) increases from 2.62 nm for pristine PET to 3.46, 3.28 and 3.20 nm for the composites with organo-MMT content of 1, 3 and 5%, respectively. It suggests that polymer molecules have entered the silicate layers of MMT. Because the intensity and sharpness of the diffraction peaks are still evident, especially in PET3M and PET5M, intercalation rather than exfoliation has probably happened.
Table 1 PET/MMT mass ratio of composite samples
OrganoMMT (DK2) PET
PET
PET1M
PET3M
PET5M
0
1
3
5
100
99
97
95
Fig. 1. X-ray diffraction curve of organic montmorillonite DK2.
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Fig. 4. TEM image of PET3M. Fig. 2. X-ray diffraction curve of PET1M, 3M and 5M.
3.2. Transmission electron microscopy (TEM) analysis
conglomerating, which is beneficial to forming the nano particles when there is more polymer with less MMT. Another reason is, when the MMT content is high, it is difficult for macromolecules to enter between the MMT layers due to the congregation of MMT. The TEM results are in agreement with XRD observation and suggest that melt blending can be used in preparing intercalating type nanocomposite only when MMT content is small.
The dispersion state of MMT inside PET matrix is further examined by TEM. Figs. 3–5 are TEM of PET1M, PET3M and PET5M, respectively. Fig. 3 shows that sufficient PET molecules have entered into MMT layers when the content of MMT is 1%. The MMT layers are well distributed in the matrix and the layer distance is about 3 nm. When the content of MMT is increased to 3% (Fig. 4), PET molecules have entered between the MMT layers, but the MMT layers are not well dispersed in the matrix and the layer distance is smaller than PET1M. Fig. 7 shows that the MMT crystallites in PET5M are pressed and squeezed to a distorted state during melt blending process and the middle of the MMT clay layers are not evident to have been intercalated. Fig. 5 also shows that the MMT particles have certain degree of conglomeration (the color of MMT in PET5M in TEM picture is darker than other samples) although outer part has been intercalated. In general, intercalation is less obvious in PET5M than PET1M and PET3M. One possible reason is that the MMT layers’ probability of meeting each other by collision is small when the content of MMT in the matrix is low. What is more, the matrix polymer molecules can prevent MMT layers’ from
Pristine PET is a well-known semi-crystallization polymer with a character of low rate of crystallization. It is necessary to improve its crystallization rate in order to use it as an engineering polymer. The effect of MMT on the crystallization behavior of PET during heating and cooling is examined by DSC (Figs. 6 and 7, and Table 2). In DSC, crystallization temperature from melt (Tmc) and its half peak width (width of the peak at half height) are two parameters that can characterize the crystallization rate. When Tmc is higher and half peak width is less, the rate of crystallization is higher. Fig. 6(a) and Table 2 shows that adding MMT will increase Tmc, increase the sharpness of crystallization peak, and decrease the half peak width. Hence, the nanocomposite of PET/MMT gives higher crystallization
Fig. 3. TEM image of PET1M.
Fig. 5. TEM image of PET5M.
3.3. PET/MMT crystallization behavior
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Fig. 6. DSC of PET/MMT (a) curves of cooling, (b) curves of heating ((1) pristine PET, (2) PET1M, (3) PET3M, (4) PET5M). Rate of heating and cooling are both 10 8C/min.
rate then pristine PET. One reason of crystallization rate enhancement is that MMT can act as an effective heterogeneous nucleating agent. Another possible reason is that MMT nano-structure itself can help the PET molecules stack on each other to grow into crystallites, thus leads to the higher crystallization rate. While the composites give much high rate of crystallization than pristine PET, the difference between composites with different MMT composition are much less. Differential scanning calorimetry (DSC) curves obtained from PET and PET/MMT nanocomposites with different amount of MMT and different heating and cooling rate are showed in Fig. 7. PET/MMT nanocomposites have higher crystallization temperature with the increase of MMT content when the cooling rates are same. For cooling rate of 40 8C/min,
pristine PET shows quite small exotherm peak, while PET/MMT composites still show large crystallization peak. It indicates the both the rate and crystallinity of the composites are increased comparing with pristine PET. DSC result also shows that the crystallization temperature (Tmc) during cooling decrease and the crystallization temperature peak broadens with the increase of cooling rate for the same sample. Ozawa method and Jeziorny method are commonly used to study the non-isothermal crystallization of polymer. Our previous work [17] showed that Ozawa method [16] does not give reasonable result. In this work, Jeziorny method [18] is adopted to study the non-isothermal crystallization of PET. The relative crystallization (Xt) [19], as a function of temperature is defined as
Fig. 7. The DSC curves at different cooling rate (a) PET, (b) PET1M, (c) PET3M, (d) PET5M.
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Table 2 DSC results of PET/MMT composites Sample
Tcc (8C)
Hcc (J/g)
Tm (8C)
Hm (J/g)
Tmc (8C)
Hmc (J/g)
PET PET1M PET3M PET5M
122.25 120.00 117.55 120.58
15.24 22.12 21.28 22.64
257.13 252.28 253.42 255.12
K42.15 K52.35 K50.34 K45.65
188.73 195.57 198.72 202.24
53.59 57.61 53.19 52.30
Note: Tcc, Tm, Tmc are cold crystallization temperature, melting temperature, and crystallization temperature from melt, respectively. Hcc, Hm, Hmc are cold crystallization enthalpy, melting enthalpy and crystallization enthalpy from melt, respectively. Rate of heating and cooling are 10 8C/min.
ðT
Xt Z ðdHc =dTÞ= T0
TðN
ðdHc =dTÞdT
(2)
For non-isothermal crystallization at certain cooling rate, Jeziorny’s [19] method is to modify Zt by the equation Log Zc Z ðlog Zt Þ=F
T0
where T0 and TN are the onset and end of crystallization temperature, respectively. Hc is the heat flow at temperature T. Polymer isothermal crystallization can be described by Avrami equation [20] and expressed as 1KXt Z expðKZt tn Þ
(3)
where n is Avrami index, t is crystallization time, Xt is crystallinity at time t and Zt is kinetic crystallization rate. This can be re-written as: ln½Klnð1KXt Þ Z ln Zt C n ln t
(4)
(5)
where F is the cooling rate, Zc is the kinetics crystallization rate. The curves of (1KXt) vs. T (Fig. 8) indicate that the higher the cooling rate, the lower the temperature at which the composites begin to crystallize. Higher crystallization temperature and higher crystallinity can be observed with the increase of MMT content. Plotting ln[Kln(1KXt)] vs. ln t, we have straight lines as shown in Fig. 9. It is clear that all samples present a linear relation in a relative broad range of crystallinity. Parameters n and ln Zt can be obtained from the slop and intercept of each line. Zc can be calculated from Eq. (5). The result of Avrami
Fig. 8. The curves of (1KXt)–T at different cooling rates (a) PET, (b) PET1M, (c) PET3M, (d) PET5M.
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Fig. 9. The curves of ln[Kln (1KXt)] vs. ln t of the samples at different temperature PET (b) PET1M (c) PET3M (d) PET5M.
index n, kinetics crystallization rate Zc and half-time of crystalliztion t1/2 are summarized in Table 3. The value of Zc and n of PET/MMT composite are much larger than pristine PET. However, the difference between the composites with different MMT contents is less obvious. These parameters values indicate that crystallization ability of PET/MMT Table 3 Parameters calculated from non-isothermal crystallization process Sample
F/8C minK1
n
Zc
t1/2/s
PET
5 10 20 40 5 10 20 40 5 10 20 40 5 10 20 40
4.0 3.5 2.9 2.7 4.7 4.0 4.6 3.6 4.4 4.2 4.1 4.4 4.8 3.9 4.8 4.2
0.21 0.65 0.89 0.95 0.40 0.83 0.94 0.97 0.42 0.80 0.99 0.97 0.38 0.87 0.98 0.96
80.80 61.12 55.01 53.39 67.44 57.42 56.15 54.66 67.23 57.90 55.00 55.20 66.00 56.40 55.80 55.20
PET1M
PET3M
PET5M
nanocomposites is remarkably improved in comparison with the pristine PET. 3.4. Morphology observation by polarized light microscopy (PLM) The morphology during the crystallization of PET/MMT nanocomposite is observed by PLM. The picture in Fig. 10 shows that pristine PET gives small spherulites morphology, while PET/MMT nanocomposites consists of much higher amount of crystallites with much smaller size. The clay layers can act as nucleating agent, thus greatly increase the nuclei number, resulting the observed morphology. 3.5. PET/MMT composites thermal property 3.5.1. TGA analysis Fig. 11 shows that pristine PET onset temperature of degradation (Tonset, weight loss 2.5%) is 310 8C, while Tonset of PET1M, PET3M and PET5M are 322, 332, and 319 8C, respectively. Fig. 12 shows the temperatures at maximum mass loss rate (Tpeak) for the pristine PET, PET1M, PET3M and PET5M are 362, 374, 385 and 375 8C, respectively. Both the Tonset and Tpeak are increased significantly by adding MMT. It
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Fig. 10. The photos of PET/MMT under PLM (a) PET, (b) PET1M, (c) PET3M. (Samples are first heated to 290 8C and kept for 5 min, then cooled to 200 8C and stayed for 3 min before the picture were taken. Enlargement 1000).
is interesting to notice that Tonset and Tpeak of PET5M are lower than that of PET3M, and are similar to PET1M. This is perhaps related to the poor dispersion of MMT in PET matrix in PET5M sample. Therefore, when MMT content is in the low range (!5%), the observed thermal stability behavior of PET/MMT is decided by both MMT content and its dispersion quality.
3.5.2. Heat distortion temperature analysis Table 4 shows that MMT can enhance the HDT significantly. When MMT is just 1%, the HDT is 35 8C higher than that of pristine PET. There are several possible reasons. Firstly, MMT increases the crystallinity of the composite. Secondly, the interaction force between MMT and the matrix is strong. Thirdly, PET chain movements are restricted between the crystallites, which have huge number because of the
Fig. 11. TGA of PET/MMT ((1) pristine PET, (2) PET1M, (3) PET3M, (4) PET5M).
Fig. 12. DTGA of PET/MMT ((1) pristine PET, (2) PET1M, (3) PET3M, (4) PET5M).
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Table 4 Heat distortion temperatures of PET/MMT composites Sample
PET
PET1M
PET3M
PET5M
HDT (8C) (1. 80 MPa)
71
106
112
120
nano-dispersion, and the chain mobility is hindered by crystallites, therefore, the resistance to heat deformation under loading is increased. 3.6. Mechanical property The tensile strength of PET1M with 1% MMT content is enhanced significantly (Fig. 13(a)). It is increased by 25% from 54 to 67.7 MPa. The flexural strength of this sample is also
enhanced (Fig. 13(b)). The notch impact strength is 26.9 J/m when the MMT content is 1% (Fig. 13(c)), which is also higher compared with that of the pristine PET (26.0 J/m). The bending modulus is increased significantly by 43.8%. When the MMT content is 3% or more, the tensile and flexural strength, elongation at break and impact strength are all less than that of pristine PET. The flexural modulus is only a little higher than 1% MMT composite. The enhancement of strength and modulus when adding 1% MMT in the composite is due to the good dispersion of MMT within the polymer matrix, the strong interaction of MMT with PET matrix, and the two-dimension strength enhancement by the MMT nano particles that have large l/d ratio. Nanoparticles have large specific area and large surface-active centers and therefore have more physical and chemical interactions with the polymer matrix. In consequence, the nano particles are difficult to disconnect from the matrix when the composite is under impact, and can resist and transfer the imposed force, leading to the dispersion of the impact energy into surrounding matrix. The strong interaction of MMT with PET matrix is because of the large specific area of MMT nano particles. However, when adding higher amount of MMT, the dispersion of MMT is not ideal and the intercalation is not complete, therefore the tensile, flexural and impact strength and elongation at break decreases.
4. Conclusion
Fig. 13. Mechanical property of PET/MMT composites vs. MMT contents: (a) tensile strength and elongation at break, (b) flexural strength and modulus, (c) Izod Impact strength.
(1) XRD and TEM indicate that melt blending can produce intercalating type of PET/organo-MMT nanocomposite. The dispersion is best when MMT content is 1%. (2) Addition of Organo-MMT (DK2) to PET can enhance the crystallinity and the rate of crystallization, which is desirable for improving PET’s processing ability, for instance, reducing the mold temperature during injection molding. Adding MMT can increase the Tmc, decrease Tcc and reduce half peak width of crystalliztion peak during cooling in DSC. Higher crystallization temperature and nucleation rate of PET during cooling can be achieved by the formation of organic MMT/PET nanocomposite. The composites have higher nucleating rate because MMT can act as a nucleating agent [4,9,11,19,23] and MMT particles’ nano dispersion can help the crystallization as well. When the content of MMT in the composite increases further than 1%, the increase of crystallization rate is not as remarkable as that of 1% content. (3) The composite has the optimum comprehensive mechanical property when the organo-MMT content is 1%. When the amount of MMT is 3% or more than, properties such as tensile strength, impact strength, flexural strength, and elongation at break are less than that of the pristine PET. (4) Adding organo-MMT (DK2) will improve the thermal stability of PET. When MMT content is 1%, onset temperature of degradation of PET/MMT nanocomposite increases by 12 8C, and peak degradation temperature rises by 35 8C.
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(5) HDT can be greatly enhanced by PET/organo-MMT nanocomposite.
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