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Effects of polytrimethylene terephthalate on crystallization and melting behavior of beta-polypropylene in the blends
Journal of Industrial and Engineering Chemistry 19 (2013) 926–931
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of polytrimethylene terephthalate on crystallization and melting behavior of beta-polypropylene in the blends Zhidan Lin *, Baofeng Xu, Zixian Guan, Chao Chen College of Science and Engineering, Jinan University, Guangzhou 510632, PR China
A R T I C L E I N F O
Article history: Received 8 October 2012 Accepted 13 November 2012 Available online 22 November 2012 Keywords: Polypropylene (PP) Polytrimethylene terephthalate (PTT) b-Nucleation Morphology Crystallization Polymer blends
A B S T R A C T
It has been observed that the formation of b-crystal is suppressed when b-polypropylene (b-PP) is blended with crystalline polymers, but the reason is still uncertain. In this study, we investigated the influence of the crystallization conditions of polytrimethylene terephthalate (PTT), such as melt crystallization, cold crystallization, isothermal crystallization temperature, and crystallization time on the b-nucleation behavior of PP phase in the b-PP/PTT blends. The results showed that the b-crystal content of PP phase in the blends decreases with increasing of PTT content. Cold crystallization of PTT would mainly induce the formation of a-crystal in PP component, whereas melt crystallization would induce the formation of b-crystal PP in their blends. The degree of crystallinity of PTT increases by improving the crystallization temperature or extending the time of crystallization, which leads to the production of more a-crystal in PP component. This means that the second component could inhibit the formation of b-crystal in PP. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Isotactic polypropylene (PP) is a polycrystalline material that exists in a, b, or g crystal forms [1,2]. b-Nucleated PP is generally attractive because of good thermal and mechanical properties [3– 6]. Toughness and heat distortion temperature of b-PP is much larger than those of a-PP. These two characteristics are very important from an applied point of view, however, b-PP has less crystal stability than a-PP and large b-crystal contents PP can only be obtained in special crystallization conditions. For example, the introduction of b-nucleating agents, temperature gradient, and shear melting lead to production of more b-crystal [7–11]. The yield strength and elastic modulus of b-PP are smaller than those of a-PP. One important way to improve effectively the b-PP performance is blending PP with other polymers [12]. It was noted that, the b-PP blends can easily be obtained when PP and some atactic polymers (such as elastomer) are blended [13– 16]. The most important factor to prepare blends containing b-PP is the ability of the second component to induce a-nucleating effect [13]. When the crystallization temperature of the second component with a-nucleating effect is below the crystallization temperature of PP, it will not affect the formation of b-PP,
* Corresponding author. Tel.: +86 20 85223271; fax: +86 20 85223271. E-mail address: [email protected] (Z. Lin).
otherwise it will inhibit the formation of b-PP. For example, in blends of b-PP/poly(vinylidene difluoride) (PVDF) and b-PP/PA6, it is hard to form b-PP, though the high efficient b-nucleating agents to be added; because PVDF and PA6 have very strong a nucleating effect and higher melting points than PP. Menyhar and Varga found that in blends of b-PP/PA6 with no compatibilizer modification, PP matrix containing a-PP is formed [17]. On the contrary, in the presence of maleic anhydride grafted PP (PP-g-MA) compatibilizer, mainly b-PP matrix formed [18]. The formation of a-PP with no compatibilizer is associated with the phenomenon that bnucleating agent is wrapped by polar PA6 component in the blend. Yang et al. studied the crystallization and melting behavior of PP/PA6 blends before and after etching with sulfuric acid [19]. Their studies revealed that b-nucleating agent is mainly distributed at the PA6 phase or the interface of PP and PA6. The authors also studied the effects of preparation conditions and compatibilizer types on the b-PP content in the b-PP/PA6 blends [20,21]. According to our literature survey, there are only few in-depth studies on how a polymer, as the second component, affects the bPP component contents in blends of b-PP with polymers having the solidification temperature above that of PP. The effects of the second component interface on the microstructure have not been reported. PTT is a crystalline polymer with a medium crystallization rate with solidification temperature above that of PP [22]. Its degree of crystallinity can easily be controlled through the thermal
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.11.009
Z. Lin et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 926–931
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Before blending, all the materials were adequately dried in a vacuum oven at appropriate temperatures for 12 h. 0.1 wt% calcium pimelate was added to PP to prepare b-nucleated PP using SHJ-20 twin-screw extruder (L/D = 40, D = 21.7 mm) at 220 8C with screw rotation of 150 rpm and residue time of 120 s. Extrudates were cooled in a water bath and cut into pellets using a pelletizer machine. The blends with various ratios of bnucleated PP and PTT (see Table 1) were also prepared on the SHJ20 twin-screw extruder at 255 8C with the screw rotation of 150 rpm and residue time of 120 s.
100 8C at a cooling rate of 10 8C/min, and were heated to 260 8C at a heating rate of 10 8C/min. The second heating curves were recorded. (c) Effects of isothermal crystallization time of PTT component on the crystallization and melting of PP component in b-PP/PTT blends: bPP60PTT40 blend samples were rapidly heated to 260 8C and held for 3 min. Then, they were rapidly cooled to 184 8C for various isothermal times (0, 5, 10, 15, and 20 min). The samples were then rapidly cooled to 150 8C and were cooled to 100 8C at a cooling rate of 10 8C/min. Finally, they were heated to 260 8C at the heating rate of 10 8C/min and the second heating curves were recorded. (d) Effects of cold crystallization temperature (between Tg and Tm) of PTT component on crystallization and melting of PP component in the b-PP/PTT blends: bPP60PTT40 blend sample was rapidly heated to 260 8C, held for 3 min, and then quenched in the liquid nitrogen to make sure the sample was cooled very rapidly. Next, the sample was rapidly heated to various cold crystallization temperatures (198, 200, 202, and 204 8C) in DSC in nitrogen atmosphere and held for 15 min. After that, it was rapidly cooled to 150 8C and subsequently, it was cooled to 100 8C at a cooling rate of 10 8C/min, followed by heating to 260 8C at a heating rate of 10 8C/min. The second heating curves were recorded. (e) Effects of cold crystallization time of PTT component on the crystallization and melting behavior of PP component in the bPP/PTT blends: a bPP60PTT40 blend sample was rapidly heated to 260 8C, held for 3 min and then quenched in the liquid nitrogen. Next, it was rapidly cooled to 202 8C for different cold crystallization times (0, 5, 10, 15, and 20 min) and rapidly cooled to 150 8C. Subsequently, it was cooled to 100 8C at a cooling rate of 10 8C/min. Then, the sample was heated to 260 8C at a heating rate of 10 8C/min. The second heating curves were recorded.
2.3. DSC characterization
2.4. WAXD characterization
TA Instruments Q200 differential scanning calorimeter (DSC) was used to study the thermal behavior of blend samples. Approximately 5–10 mg of samples were weighed for DSC examinations. All DSC experiments were performed in nitrogen atmosphere. To study various effects, we conducted the following methods:
Wide-angle X-ray diffraction (WAXD) experiment was conducted with a Rigaku Geigerflex Model D/Max-III A rotating anode X-ray diffractometer. Graphite monochromatic Cu Ka radiation was used as radiation source. The scanning range was between 5 and 408 with a rate of 48/min and a step length of 0.02. The content of the PP crystal (Kb) was determined according to standard procedures described in the literature [24], using Eq. (1)
history. In this paper, the effects of PTT degree of crystallinity on the crystalline and melting behavior of PP/PTT blends were discussed. The b-PP/PTT blend samples, as templates were prepared using various crystallization conditions, isothermal crystallization temperatures and times, and then quenching in liquid nitrogen after various annealing conditions of temperatures and times to control the crystallinity of PTT. In this way, we investigated the effects of the degree of crystallinity and thermal history of the second component on the b-crystal content of PP component in the blend. 2. Experimental 2.1. Materials Isotactic polypropylene (F401, MFR 3) was supplied by Sinopec Yangzi Petrochemical Company Ltd., China. PTT (Corterra Polymer 9200) was supplied in pellet form by Shell Chemicals, Canada, with Tm of 228 8C and intrinsic viscosity of 0.92 dL/g. Calcium pimelate, as b-nucleating agent, was prepared in our laboratory according to the method described in the literature [23]. 2.2. Samples preparation
(a) Effects of PTT content on the crystallization and melting of PP phase in the b-PP/PTT blends: a blend sample was rapidly heated to 260 8C, held for 3 min at this temperature, and then cooled to 100 8C at a cooling rate of 10 8C/min. Then, the cooling and second heating curves of the sample were recorded. (b) Effects of isothermal crystallization temperature of PTT phase on the crystallization and melting of PP component in the b-PP/ PTT blends: bPP60PTT40 blend sample was rapidly heated to 260 8C and held for 3 min at this temperature. Then, after rapidly cooling to and keeping at various isothermal temperatures (180, 182, 184, and 186 8C) for 15 min, they were rapidly cooled to 150 8C. Subsequently, the samples were cooled to Table 1 Compositions of the b-PP/PTT blends. Sample
b-PP (%)
PTT (%)
bPP bPP90PTT10 bPP80PTT20 bPP70PTT30 bPP60PTT40 bPP50PTT50
100 90 80 70 60 50 –
– 10 20 30 40 50 100
PTT
Kb ¼
Hb ð300Þ Hb ð300Þ þ Ha ð110Þ þ Ha ð040Þ þ Ha ð130Þ
(1)
where HV (hkl) denotes the intensity of respective (hkl) peak belonging to phase V. 2.5. SEM examinations The fracture surfaces of the broken specimens in the impact property test conditions were sputter-coated with gold before conducting scanning electron microscope (SEM) examinations. The fracture surfaces morphologies of the blends samples were examined using a Philips XL-30 environmental scanning electron microscope (ESEM) with an acceleration voltage of 20 kV. 3. Results and discussion 3.1. Morphology of b-PP/PTT blends Fig. 1 shows the SEM micrographs of fracture surface of pure bPP, b-PP/PTT blends with various PTT contents. The morphology
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Fig. 1. SEM micrographs of pure b-PP and b-PP/PTT blends with various PTT contents.
of the fracture surface of pure b-PP (Fig. 1a) was smooth with some small white particles. These particles should be calcium pimelate that did not melt during the PP melting process. The fracture surfaces of b-PP/PTT blends (Fig. 1b–f) were rugged. The spherical particles of bare PPT with size of 1–15 mm and the voids of PPT shedding were observed. The obvious interface between the PPT phase and PP phase indicates strong interfacial tension between the two polymers. The PPT particle size depended on its content. The large contents of PTT resulted in larger PTT particles; when the PTT contents was more than 40 wt%, the size of PTT particles were larger than 10 mm, which gradually leveled off. In these conditions, the phases are easier to touch each other to form a continuous phase. 3.2. Effects of PPT content on crystallization and melting behavior of PP Authors found that the strong a-nucleating effect of PA would inhibit the formation of b-PP in b-PP/PA blends [17]. Under this effect, PP matrix containing a-PP is formed in blends. Both of PA and PTT are crystalline polymer. When the molten b-PP/PTT blends are cooled, initially PTT crystallizes that will affect the crystallization of PP phase.
Fig. 2a shows the crystallization curves of b-PP/PTT blends compatibilized with 0.1 wt% calcium pimelate. With the addition of PTT, the crystallization temperature of PP phase decreased. It shows that PTT crystallites inhibit the crystallization of PP. The crystallization temperature of PP decreased as a result of PTT presence. The crystallization temperature peak of PTT could be observed only when the PTT content increased above 40 wt%. The crystallization temperatures of PTT component in the blends were observed much above that of pure PTT. Combined with the SEM observations, the obvious crystallization temperature curve can be observed when the PTT particles are big (bigger than 10 mm). The melting of PP phase induces the formation of crystal PTT that can significantly increase the crystallization temperature curve of PTT. Fig. 2b shows the melting curves of b-PP/PTT blends nucleated with 0.1 wt% calcium pimelate. It reveals the b-crystal promotional effect of calcium pimelate in PP. The melting peaks of PP bcrystals and a-crystals were observed at 152.9 and 163.5 8C, respectively, in the melting curves. The addition of PTT strengthened the a-crystal melting peak and weakened the b-crystal melting peak of b-PP component of the blends. This observation indicated again that PTT would inhibit the effect of b-crystal agent, thus, the b-crystal content in the b-PP/PTT blends decrease. Because intensity of a-crystalline melting peak changes a lot when
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Fig. 3. Melting curves of b-PP/PTT blends with different crystallization methods.
shows that this method can induce the formation of a-crystal. Maybe the cold crystallization is more conducive to the improvement of the PTT degree of crystallinity, inducing the formation of more a-crystal. This phenomenon was also confirmed by WAXD examination results (see Fig. 4). 3.4. Effects of crystallization temperature on melting behavior of PP in b-PP/PTT blends
Fig. 2. Crystallization and melting curves of pure b-PP and b-PP/PTT blends with various PTT contents.
Polymer isothermal crystallization conditions also affect the degree of crystallinity of the polymer [25]. We designed melt isothermal and cold isothermal crystallizations at temperatures above melting point of PP to conduct the isothermal crystallization of PTT. Effects of different degrees of crystallinity of PTT phase at various isothermal crystallization temperatures on the melting behavior of PP phase in b-PP/PTT blends were studied. Fig. 5 shows the melt curve of PP phase in b-PP/PTT blends crystallized isothermally at various temperatures for 20 min and then cooled at a rate of 10 8C/min. No significant change between a-crystal and bcrystal melting peaks strengths in the bPP60PTT40 blends were observed (Fig. 6). We assume that PTT was fully crystallized after the isothermal crystallization at the experiment temperature after
the amount of PTT is above 40 wt%, we selected bPP60PTT40 blends as research subjects. Effects of crystallization conditions of PTT on b-crystal of PP in the b-PP/PTT blends are discussed in the next section. 3.3. Effects of crystallization methods on melting behavior of PP phase in b-PP/PTT blends There are two crystallization ways of PTT: melt crystallization (crystallization of the blend during the cooling process from high temperature melt) and cold crystallization (crystallization of the blend during the heating process from room temperature). These methods form different crystal morphologies and content that influence the proportion of b-crystal and a-crystal of PP in the bPP/PTT blends. Fig. 3 shows the DSC melting thermograms of b-PP/ PTT blends (bPP60PTT40) crystallization in different ways. The intensity of b-crystal peak is stronger than a-crystal in the melting curves of the cold crystallization and melt isothermal crystallization. It shows that these crystallization methods can induce the formation of b-crystal. The intensity of a-crystal peak is stronger than b-crystal in the melting curves of cold crystallization, which
Fig. 4. WAXD curves of b-PP/PTT blends with different crystallization methods.
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Fig. 5. Melting curves of b-PP/PTT blends after PTT phase iso-crystallization at various temperatures. Fig. 7. Melting curves of b-PP/PTT blends after PTT phase iso-crystallization at 180 8C for various times.
Fig. 6. Melting curves of b-PP/PTT blends after PTT phase cold crystallization at various temperatures.
melting curves of the quenched blends. The b-crystals of PP are mainly formed in blends, because quenched PTT forms amorphous morphology. Amorphous PTT does not inhibit the b-nucleating effect of PP. With the extension of annealing time at 180 8C, the crystallization degree of PTT increased. Consequently, b-nucleating effect was significantly inhibited in b-PP/PTT blends. Then, the melting peak of a-crystal was strengthened and the melting peak of b-crystal was on the contrary in the blends. Fig. 8 shows the melting curves of bPP60PTT40 quenched samples annealed at 198 8C for various times. With the extension of annealing time at 198 8C, the degree of crystallinity of PTT increased. Consequently, b-nucleating effect was significantly inhibited in b-PP/PTT blends. These results indicate that the bnucleating effect of PTT on PP depends on the degree of crystallinity of PTT—larger crystallinity of PTT leads to greater a-nucleating effect on PP. In summary, reducing the degree of crystallinity of the second component of the b-PP blend is one way to weaken the anucleating effect of the second component on PP. This is also an effective way to obtain large b-crystal content in the b-PP blends.
20 min. The same degree of crystallinity of PTT phase indicated the same b-nucleating effect. Fig. 6 shows the melt curve of PP phase in b-PP/PTT blends that after 20 min crystallization at different cold crystallization temperature were cooled at a rate of 10 8C/min. When the cold crystallization temperature was in the range of 198–204 8C, the melting peak strength gradually reduced when the cold crystallization temperature increased. In such conditions, the b-crystal content of iPP reduced. This observation indicates that the degree of crystallinity of PTT increases when we raise the cold crystallization temperature, which leads to stronger a-nucleating effect of PTT on the PP component of the blends. This observation also reveals that b-nucleating effect of PP in b-PP/PTT blend is suppressed when degree of crystallinity of PTT is large. 3.5. Effects of crystallization time on melting behavior of PP in b-PP/ PTT blends To obtain amorphous or low crystalline PTT, blend melt samples were quenched in liquid nitrogen. The quenched samples annealing was performed under various conditions to control the PTT crystallinity. Fig. 7 shows the melting curves of bPP60PTT40 quenched samples that were annealed at 180 8C for different times. The melt peak of b-crystal appeared in the
Fig. 8. Melting curves of b-PP/PTT blends after PTT phase cold crystallization at 198 8C for various times.
Z. Lin et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 926–931
4. Conclusions In this study, we observed that the formation of b-crystal is suppressed when b-PP is blended with crystalline polymer, but the reason for this observation is still uncertain. The influences of crystallization conditions of PTT (melt and cold crystallizations) and isothermal crystallization temperature and crystallization time on b-nucleation behavior of PP phase in b form of nucleated PP/PTT blends were investigated and the results were discussed. The b-crystal content of PP in the PP/PTT blends decreased when the amount of PTT increased. Cold crystallization of PTT would mainly induce the formation of a-crystal, but melt crystallization would induce the formation of b-crystal in PP component of the blend. The b-crystal content in the b-PP/PTT blend changed when the degree of crystallinity of PTT was changed. The b-crystal content decreased with the increase of the PTT crystallinity. These observations confirm that the PTT component degree of crystallinity in the blends control the a-nucleating effect and inhibits the b-nucleating effect of PP in b-PP/PTT blends. It can be concluded that reducing the degree of crystallinity of the second component in the b-PP blends is one way to weaken the a-nucleating effect of the second component, which is also an effective way to obtain high b-crystal content of b-PP blends. Acknowledgement This research project was supported by Natural Science Foundation of China and Project of Science (Grant No. 21101076).
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Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Effects of polytrimethylene terephthalate on crystallization and melting behavior of beta-polypropylene in the blends Zhidan Lin *, Baofeng Xu, Zixian Guan, Chao Chen College of Science and Engineering, Jinan University, Guangzhou 510632, PR China
A R T I C L E I N F O
Article history: Received 8 October 2012 Accepted 13 November 2012 Available online 22 November 2012 Keywords: Polypropylene (PP) Polytrimethylene terephthalate (PTT) b-Nucleation Morphology Crystallization Polymer blends
A B S T R A C T
It has been observed that the formation of b-crystal is suppressed when b-polypropylene (b-PP) is blended with crystalline polymers, but the reason is still uncertain. In this study, we investigated the influence of the crystallization conditions of polytrimethylene terephthalate (PTT), such as melt crystallization, cold crystallization, isothermal crystallization temperature, and crystallization time on the b-nucleation behavior of PP phase in the b-PP/PTT blends. The results showed that the b-crystal content of PP phase in the blends decreases with increasing of PTT content. Cold crystallization of PTT would mainly induce the formation of a-crystal in PP component, whereas melt crystallization would induce the formation of b-crystal PP in their blends. The degree of crystallinity of PTT increases by improving the crystallization temperature or extending the time of crystallization, which leads to the production of more a-crystal in PP component. This means that the second component could inhibit the formation of b-crystal in PP. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Isotactic polypropylene (PP) is a polycrystalline material that exists in a, b, or g crystal forms [1,2]. b-Nucleated PP is generally attractive because of good thermal and mechanical properties [3– 6]. Toughness and heat distortion temperature of b-PP is much larger than those of a-PP. These two characteristics are very important from an applied point of view, however, b-PP has less crystal stability than a-PP and large b-crystal contents PP can only be obtained in special crystallization conditions. For example, the introduction of b-nucleating agents, temperature gradient, and shear melting lead to production of more b-crystal [7–11]. The yield strength and elastic modulus of b-PP are smaller than those of a-PP. One important way to improve effectively the b-PP performance is blending PP with other polymers [12]. It was noted that, the b-PP blends can easily be obtained when PP and some atactic polymers (such as elastomer) are blended [13– 16]. The most important factor to prepare blends containing b-PP is the ability of the second component to induce a-nucleating effect [13]. When the crystallization temperature of the second component with a-nucleating effect is below the crystallization temperature of PP, it will not affect the formation of b-PP,
* Corresponding author. Tel.: +86 20 85223271; fax: +86 20 85223271. E-mail address: [email protected] (Z. Lin).
otherwise it will inhibit the formation of b-PP. For example, in blends of b-PP/poly(vinylidene difluoride) (PVDF) and b-PP/PA6, it is hard to form b-PP, though the high efficient b-nucleating agents to be added; because PVDF and PA6 have very strong a nucleating effect and higher melting points than PP. Menyhar and Varga found that in blends of b-PP/PA6 with no compatibilizer modification, PP matrix containing a-PP is formed [17]. On the contrary, in the presence of maleic anhydride grafted PP (PP-g-MA) compatibilizer, mainly b-PP matrix formed [18]. The formation of a-PP with no compatibilizer is associated with the phenomenon that bnucleating agent is wrapped by polar PA6 component in the blend. Yang et al. studied the crystallization and melting behavior of PP/PA6 blends before and after etching with sulfuric acid [19]. Their studies revealed that b-nucleating agent is mainly distributed at the PA6 phase or the interface of PP and PA6. The authors also studied the effects of preparation conditions and compatibilizer types on the b-PP content in the b-PP/PA6 blends [20,21]. According to our literature survey, there are only few in-depth studies on how a polymer, as the second component, affects the bPP component contents in blends of b-PP with polymers having the solidification temperature above that of PP. The effects of the second component interface on the microstructure have not been reported. PTT is a crystalline polymer with a medium crystallization rate with solidification temperature above that of PP [22]. Its degree of crystallinity can easily be controlled through the thermal
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.11.009
Z. Lin et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 926–931
927
Before blending, all the materials were adequately dried in a vacuum oven at appropriate temperatures for 12 h. 0.1 wt% calcium pimelate was added to PP to prepare b-nucleated PP using SHJ-20 twin-screw extruder (L/D = 40, D = 21.7 mm) at 220 8C with screw rotation of 150 rpm and residue time of 120 s. Extrudates were cooled in a water bath and cut into pellets using a pelletizer machine. The blends with various ratios of bnucleated PP and PTT (see Table 1) were also prepared on the SHJ20 twin-screw extruder at 255 8C with the screw rotation of 150 rpm and residue time of 120 s.
100 8C at a cooling rate of 10 8C/min, and were heated to 260 8C at a heating rate of 10 8C/min. The second heating curves were recorded. (c) Effects of isothermal crystallization time of PTT component on the crystallization and melting of PP component in b-PP/PTT blends: bPP60PTT40 blend samples were rapidly heated to 260 8C and held for 3 min. Then, they were rapidly cooled to 184 8C for various isothermal times (0, 5, 10, 15, and 20 min). The samples were then rapidly cooled to 150 8C and were cooled to 100 8C at a cooling rate of 10 8C/min. Finally, they were heated to 260 8C at the heating rate of 10 8C/min and the second heating curves were recorded. (d) Effects of cold crystallization temperature (between Tg and Tm) of PTT component on crystallization and melting of PP component in the b-PP/PTT blends: bPP60PTT40 blend sample was rapidly heated to 260 8C, held for 3 min, and then quenched in the liquid nitrogen to make sure the sample was cooled very rapidly. Next, the sample was rapidly heated to various cold crystallization temperatures (198, 200, 202, and 204 8C) in DSC in nitrogen atmosphere and held for 15 min. After that, it was rapidly cooled to 150 8C and subsequently, it was cooled to 100 8C at a cooling rate of 10 8C/min, followed by heating to 260 8C at a heating rate of 10 8C/min. The second heating curves were recorded. (e) Effects of cold crystallization time of PTT component on the crystallization and melting behavior of PP component in the bPP/PTT blends: a bPP60PTT40 blend sample was rapidly heated to 260 8C, held for 3 min and then quenched in the liquid nitrogen. Next, it was rapidly cooled to 202 8C for different cold crystallization times (0, 5, 10, 15, and 20 min) and rapidly cooled to 150 8C. Subsequently, it was cooled to 100 8C at a cooling rate of 10 8C/min. Then, the sample was heated to 260 8C at a heating rate of 10 8C/min. The second heating curves were recorded.
2.3. DSC characterization
2.4. WAXD characterization
TA Instruments Q200 differential scanning calorimeter (DSC) was used to study the thermal behavior of blend samples. Approximately 5–10 mg of samples were weighed for DSC examinations. All DSC experiments were performed in nitrogen atmosphere. To study various effects, we conducted the following methods:
Wide-angle X-ray diffraction (WAXD) experiment was conducted with a Rigaku Geigerflex Model D/Max-III A rotating anode X-ray diffractometer. Graphite monochromatic Cu Ka radiation was used as radiation source. The scanning range was between 5 and 408 with a rate of 48/min and a step length of 0.02. The content of the PP crystal (Kb) was determined according to standard procedures described in the literature [24], using Eq. (1)
history. In this paper, the effects of PTT degree of crystallinity on the crystalline and melting behavior of PP/PTT blends were discussed. The b-PP/PTT blend samples, as templates were prepared using various crystallization conditions, isothermal crystallization temperatures and times, and then quenching in liquid nitrogen after various annealing conditions of temperatures and times to control the crystallinity of PTT. In this way, we investigated the effects of the degree of crystallinity and thermal history of the second component on the b-crystal content of PP component in the blend. 2. Experimental 2.1. Materials Isotactic polypropylene (F401, MFR 3) was supplied by Sinopec Yangzi Petrochemical Company Ltd., China. PTT (Corterra Polymer 9200) was supplied in pellet form by Shell Chemicals, Canada, with Tm of 228 8C and intrinsic viscosity of 0.92 dL/g. Calcium pimelate, as b-nucleating agent, was prepared in our laboratory according to the method described in the literature [23]. 2.2. Samples preparation
(a) Effects of PTT content on the crystallization and melting of PP phase in the b-PP/PTT blends: a blend sample was rapidly heated to 260 8C, held for 3 min at this temperature, and then cooled to 100 8C at a cooling rate of 10 8C/min. Then, the cooling and second heating curves of the sample were recorded. (b) Effects of isothermal crystallization temperature of PTT phase on the crystallization and melting of PP component in the b-PP/ PTT blends: bPP60PTT40 blend sample was rapidly heated to 260 8C and held for 3 min at this temperature. Then, after rapidly cooling to and keeping at various isothermal temperatures (180, 182, 184, and 186 8C) for 15 min, they were rapidly cooled to 150 8C. Subsequently, the samples were cooled to Table 1 Compositions of the b-PP/PTT blends. Sample
b-PP (%)
PTT (%)
bPP bPP90PTT10 bPP80PTT20 bPP70PTT30 bPP60PTT40 bPP50PTT50
100 90 80 70 60 50 –
– 10 20 30 40 50 100
PTT
Kb ¼
Hb ð300Þ Hb ð300Þ þ Ha ð110Þ þ Ha ð040Þ þ Ha ð130Þ
(1)
where HV (hkl) denotes the intensity of respective (hkl) peak belonging to phase V. 2.5. SEM examinations The fracture surfaces of the broken specimens in the impact property test conditions were sputter-coated with gold before conducting scanning electron microscope (SEM) examinations. The fracture surfaces morphologies of the blends samples were examined using a Philips XL-30 environmental scanning electron microscope (ESEM) with an acceleration voltage of 20 kV. 3. Results and discussion 3.1. Morphology of b-PP/PTT blends Fig. 1 shows the SEM micrographs of fracture surface of pure bPP, b-PP/PTT blends with various PTT contents. The morphology
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Fig. 1. SEM micrographs of pure b-PP and b-PP/PTT blends with various PTT contents.
of the fracture surface of pure b-PP (Fig. 1a) was smooth with some small white particles. These particles should be calcium pimelate that did not melt during the PP melting process. The fracture surfaces of b-PP/PTT blends (Fig. 1b–f) were rugged. The spherical particles of bare PPT with size of 1–15 mm and the voids of PPT shedding were observed. The obvious interface between the PPT phase and PP phase indicates strong interfacial tension between the two polymers. The PPT particle size depended on its content. The large contents of PTT resulted in larger PTT particles; when the PTT contents was more than 40 wt%, the size of PTT particles were larger than 10 mm, which gradually leveled off. In these conditions, the phases are easier to touch each other to form a continuous phase. 3.2. Effects of PPT content on crystallization and melting behavior of PP Authors found that the strong a-nucleating effect of PA would inhibit the formation of b-PP in b-PP/PA blends [17]. Under this effect, PP matrix containing a-PP is formed in blends. Both of PA and PTT are crystalline polymer. When the molten b-PP/PTT blends are cooled, initially PTT crystallizes that will affect the crystallization of PP phase.
Fig. 2a shows the crystallization curves of b-PP/PTT blends compatibilized with 0.1 wt% calcium pimelate. With the addition of PTT, the crystallization temperature of PP phase decreased. It shows that PTT crystallites inhibit the crystallization of PP. The crystallization temperature of PP decreased as a result of PTT presence. The crystallization temperature peak of PTT could be observed only when the PTT content increased above 40 wt%. The crystallization temperatures of PTT component in the blends were observed much above that of pure PTT. Combined with the SEM observations, the obvious crystallization temperature curve can be observed when the PTT particles are big (bigger than 10 mm). The melting of PP phase induces the formation of crystal PTT that can significantly increase the crystallization temperature curve of PTT. Fig. 2b shows the melting curves of b-PP/PTT blends nucleated with 0.1 wt% calcium pimelate. It reveals the b-crystal promotional effect of calcium pimelate in PP. The melting peaks of PP bcrystals and a-crystals were observed at 152.9 and 163.5 8C, respectively, in the melting curves. The addition of PTT strengthened the a-crystal melting peak and weakened the b-crystal melting peak of b-PP component of the blends. This observation indicated again that PTT would inhibit the effect of b-crystal agent, thus, the b-crystal content in the b-PP/PTT blends decrease. Because intensity of a-crystalline melting peak changes a lot when
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Fig. 3. Melting curves of b-PP/PTT blends with different crystallization methods.
shows that this method can induce the formation of a-crystal. Maybe the cold crystallization is more conducive to the improvement of the PTT degree of crystallinity, inducing the formation of more a-crystal. This phenomenon was also confirmed by WAXD examination results (see Fig. 4). 3.4. Effects of crystallization temperature on melting behavior of PP in b-PP/PTT blends
Fig. 2. Crystallization and melting curves of pure b-PP and b-PP/PTT blends with various PTT contents.
Polymer isothermal crystallization conditions also affect the degree of crystallinity of the polymer [25]. We designed melt isothermal and cold isothermal crystallizations at temperatures above melting point of PP to conduct the isothermal crystallization of PTT. Effects of different degrees of crystallinity of PTT phase at various isothermal crystallization temperatures on the melting behavior of PP phase in b-PP/PTT blends were studied. Fig. 5 shows the melt curve of PP phase in b-PP/PTT blends crystallized isothermally at various temperatures for 20 min and then cooled at a rate of 10 8C/min. No significant change between a-crystal and bcrystal melting peaks strengths in the bPP60PTT40 blends were observed (Fig. 6). We assume that PTT was fully crystallized after the isothermal crystallization at the experiment temperature after
the amount of PTT is above 40 wt%, we selected bPP60PTT40 blends as research subjects. Effects of crystallization conditions of PTT on b-crystal of PP in the b-PP/PTT blends are discussed in the next section. 3.3. Effects of crystallization methods on melting behavior of PP phase in b-PP/PTT blends There are two crystallization ways of PTT: melt crystallization (crystallization of the blend during the cooling process from high temperature melt) and cold crystallization (crystallization of the blend during the heating process from room temperature). These methods form different crystal morphologies and content that influence the proportion of b-crystal and a-crystal of PP in the bPP/PTT blends. Fig. 3 shows the DSC melting thermograms of b-PP/ PTT blends (bPP60PTT40) crystallization in different ways. The intensity of b-crystal peak is stronger than a-crystal in the melting curves of the cold crystallization and melt isothermal crystallization. It shows that these crystallization methods can induce the formation of b-crystal. The intensity of a-crystal peak is stronger than b-crystal in the melting curves of cold crystallization, which
Fig. 4. WAXD curves of b-PP/PTT blends with different crystallization methods.
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Fig. 5. Melting curves of b-PP/PTT blends after PTT phase iso-crystallization at various temperatures. Fig. 7. Melting curves of b-PP/PTT blends after PTT phase iso-crystallization at 180 8C for various times.
Fig. 6. Melting curves of b-PP/PTT blends after PTT phase cold crystallization at various temperatures.
melting curves of the quenched blends. The b-crystals of PP are mainly formed in blends, because quenched PTT forms amorphous morphology. Amorphous PTT does not inhibit the b-nucleating effect of PP. With the extension of annealing time at 180 8C, the crystallization degree of PTT increased. Consequently, b-nucleating effect was significantly inhibited in b-PP/PTT blends. Then, the melting peak of a-crystal was strengthened and the melting peak of b-crystal was on the contrary in the blends. Fig. 8 shows the melting curves of bPP60PTT40 quenched samples annealed at 198 8C for various times. With the extension of annealing time at 198 8C, the degree of crystallinity of PTT increased. Consequently, b-nucleating effect was significantly inhibited in b-PP/PTT blends. These results indicate that the bnucleating effect of PTT on PP depends on the degree of crystallinity of PTT—larger crystallinity of PTT leads to greater a-nucleating effect on PP. In summary, reducing the degree of crystallinity of the second component of the b-PP blend is one way to weaken the anucleating effect of the second component on PP. This is also an effective way to obtain large b-crystal content in the b-PP blends.
20 min. The same degree of crystallinity of PTT phase indicated the same b-nucleating effect. Fig. 6 shows the melt curve of PP phase in b-PP/PTT blends that after 20 min crystallization at different cold crystallization temperature were cooled at a rate of 10 8C/min. When the cold crystallization temperature was in the range of 198–204 8C, the melting peak strength gradually reduced when the cold crystallization temperature increased. In such conditions, the b-crystal content of iPP reduced. This observation indicates that the degree of crystallinity of PTT increases when we raise the cold crystallization temperature, which leads to stronger a-nucleating effect of PTT on the PP component of the blends. This observation also reveals that b-nucleating effect of PP in b-PP/PTT blend is suppressed when degree of crystallinity of PTT is large. 3.5. Effects of crystallization time on melting behavior of PP in b-PP/ PTT blends To obtain amorphous or low crystalline PTT, blend melt samples were quenched in liquid nitrogen. The quenched samples annealing was performed under various conditions to control the PTT crystallinity. Fig. 7 shows the melting curves of bPP60PTT40 quenched samples that were annealed at 180 8C for different times. The melt peak of b-crystal appeared in the
Fig. 8. Melting curves of b-PP/PTT blends after PTT phase cold crystallization at 198 8C for various times.
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4. Conclusions In this study, we observed that the formation of b-crystal is suppressed when b-PP is blended with crystalline polymer, but the reason for this observation is still uncertain. The influences of crystallization conditions of PTT (melt and cold crystallizations) and isothermal crystallization temperature and crystallization time on b-nucleation behavior of PP phase in b form of nucleated PP/PTT blends were investigated and the results were discussed. The b-crystal content of PP in the PP/PTT blends decreased when the amount of PTT increased. Cold crystallization of PTT would mainly induce the formation of a-crystal, but melt crystallization would induce the formation of b-crystal in PP component of the blend. The b-crystal content in the b-PP/PTT blend changed when the degree of crystallinity of PTT was changed. The b-crystal content decreased with the increase of the PTT crystallinity. These observations confirm that the PTT component degree of crystallinity in the blends control the a-nucleating effect and inhibits the b-nucleating effect of PP in b-PP/PTT blends. It can be concluded that reducing the degree of crystallinity of the second component in the b-PP blends is one way to weaken the a-nucleating effect of the second component, which is also an effective way to obtain high b-crystal content of b-PP blends. Acknowledgement This research project was supported by Natural Science Foundation of China and Project of Science (Grant No. 21101076).
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