Article pubs.acs.org/Langmuir
Novel Dynamic Elongational Flow Procedure for Reinforcing Strong, Tough, Thermally Stable Polypropylene/Thermoplastic Polyurethane Blends Shikui Jia, Jinping Qu,* Chengran Wu, Weifeng Liu, Rongyuan Chen, Shufeng Zhai, Zan Huang, and Fuquan Chen National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of the Ministry of Education, South China University of Technology, Guangzhou 510640, China ABSTRACT: Thermoplastic polyurethane (TPU)/polypropylene (PP) blends of different weight ratios were prepared with a self-made vane extruder (VE), which generates global dynamic elongational flow, and a traditional twin-screw extruder (TSE), which generates shear flow. High-resolution scanning electron microscopy and polarizing microscopy showed a structure feature of fiber morphology and a clear interlocking structure of spherulites of PP/TPU blends prepared with a VE. The wide-angle X-ray diffraction results showed that the TPU/PP blend based on dynamic elongational flow had evident crystalline structure of the β form as a function of PP (90 wt %), compared to that of the conventional shear flow processing techniques. A significant improvement of the mechanical properties was obtained; the samples prepared with a VE had superior mechanical properties compared to those of the samples prepared with a TSE. Interestingly, differential scanning calorimetry curves showed that dynamic elongational flow could successfully improve the crystallinity of the PP/TPU blends. Furthermore, dynamic thermomechanical and thermogravimetric analysis curves revealed the apparent partial miscibility and strong interaction of the PP/TPU blends influenced by dynamic elongational flow, compared to that of TSE-extruded. Further research will provide significant understanding of the spherulite interface and high-performance manipulation of PP/TPU blends under dynamic elongational flow, achieving superior PP/TPU blends.
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INTRODUCTION Among thermoplastic polymer alloys, the combination of polypropylene (PP) with thermoplastic polyurethane (TPU) exhibits more significant advantages compared to the pure components. TPU is a linear copolymer composed of microphase-separated hard and soft segments. The soft segments usually form an elastomer matrix responsible for the elastic and low-temperature properties of TPU, whereas the hard segments act as multifunctional tie points and function as physical cross-links and reinforcing fillers.1 TPU can mainly enhance the toughness of PP at lower temperature whereas PP can slightly improve the crystallization of TPU, which can further increase the blend stiffness. The correlation of the mutual modification between the PP and the TPU matrices is very significant in polymer processing. Utilizing thermoplastics available through recycling technology can be good for the sustainable development of the environment and resource conservation. Generally, TPU and PP blends are highly immiscible because of the large differences in their polarities and their high interfacial tensions,2 limiting the usefulness of the blends and calling for methods to improve their compatibility. Previous studies on the binary blend included work by Kalfoglou et al.,3 who compared the compatibilizing efficiency for PET/PP © XXXX American Chemical Society
blends of various compatibilizers and indicated that the effect of aging on the strong deformation behavior of the blends depended on different compatibilizers. Potschke et al.4,5 reported that, at similar viscosity ratios, blends with polyether-based TPU had finer morphology than those with polyester-based TPU because the free energy of the soft segment surfaces of polyether is lower than that of polyester. Moreover, reactive compatibilization can generate graft or block copolymers in situ during the process of melt blending and appears to produce the best blend compatibilization.6−9 In addition, to improve the two-phase interaction some researchers adopted high-shear-field, high-energy electrons and core−shell copolymerization.10−14 The results revealed that the high force field could successfully enhance the mechanical and thermal properties. Schneider et al.15 studied a new class of microfluidic systems and graft photopolymerization for surface modification. Whangbo et al.16 studied the characterization of the morphology and nanostructures by tapping-mode atomic force microscopy, and the results showed Received: June 18, 2013 Revised: September 19, 2013
A
dx.doi.org/10.1021/la4023079 | Langmuir XXXX, XXX, XXX−XXX
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Table 1. Processing Conditions of Blends processing temp (°C)
die (L/D)
output
blends
VE
TSE
VE
TSE
VE
TSE
PP/TPU
175-180-190-200
170-175-180-185-190-195-200-200
19.2
18.7
37 kg/h
34 kg/h
dynamic elongational flow and conventional shear flow were examined, respectively. These results could provide a theoretical background for the simulation and prediction models of an immiscible polymer system in a VE. Emphasis is placed on the effect of the dynamic elongational field on the molecular chain conformation and interlocking capacity such as spherulite interactions, crystal perfection, and fine dispersion particles compared to the shear field.
that different thermal treatments could also influence the microcompatibility of blends. Moreover, Macosko et al.17 studied the compatibility of PP-gNH2 and PP-g-NHR with TPU and compared it to that of a maleated PP (PP-g-MA). The results revealed that compatibility of the three functionalized PPs with TPU was ranked in increasing order as follows: PP-g-NHR ≥ PP-g-NH2 ≥ PP-gMA. To enhance the mechanical properties of the PP/TPU blends further, some researchers produced the PP/TPU nanocomposites with clay by melt mixing using a twin-screw extruder.18 The results indicated that the clay could reduce the surface energy of the TPU hard segments and make them more compatible with the nonpolar PP. Furthermore, the dispersity in melt-mixed immiscible blends was influenced by material parameters such as the viscosity and polarity ratios, blend composition, and processing conditions.19−22 Potschke et al.23 obtained the relationship between the processing parameters and the maximum stable drop. Thus, the processing conditions play a very important role in compatibility during melt compounding, especially at higher concentrations of the dispersed phase or at high viscosity ratios. The aforementioned work was managed in a twin-screw extruder (TSE) or batch mixer governed by conventional shear flow. Qu et al.24 improved the shear flow by the extra vibrational force and studied the effect of the vibrational shear flow field in a capillary dynamic rheometer on the crystallization behavior of PP. The result showed that the αform crystal of the PP became more perfect under the influence of the vibrational shear field. Moreover, several studies have reported that melted drops in polymer processing were more efficiently broken under elongational flow than under shear flow.25−29 Various attempts have been made to generate elongational flow based on converging channels, but most of these elongational flows were local and fixed.30−32 Qu33 invented novel nonscrew plasticizing processing equipment known as the vane extruder (VE). This equipment consists of certain groups of vane plasticizing units that could generate higher stress and dynamical elongational flow. The materials in the plasticating and conveying process can generate elongational deformation, which could remarkably shorten the duration of thermomechanical processing, reduce energy consumption, and improve the blending performance.34,35 The effect of dynamically converging channels on fiber organization and damage during vane extrusion on sisal-fiberreinforced PP composites revealed that VEs generate elongational flow in polymer processing.36 The droplet size in the dispersed phase of VE-prepared PP/polyamides and PP/ polystyrene was much smaller than that of TSE-prepared blends. The results indicated that the dynamic elongational deformation field was more effective at mixing for immiscible polymer blends.37 In the present work, we compared the reinforcement mechanical properties and enhancement compatibilizing efficiency for PP/TPU blends of various weight ratios on the basis of novel VE and conventional TSE. Mechanical properties, morphological properties, spherulite interfaces, and thermal behaviors of the blends based on
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EXPERIMENTAL SECTION
Materials and Preparation. PP (grade T30S, isotaxy type) was obtained from China Petroleum and Chemical, and the melt flow index and density provided by the supplier were 3.0 g/10 min (230 °C/2.16 kg) and 0.91 g/cm3, respectively. TPU (grade WHT1195, polyester type) was supplied by Yantai Wanhua Polyurethanes Co., Ltd., and the shore A hardness and density provided by the supplier were 70 A and 1.2 g/cm3, respectively. All of the materials were dried in an oven at 80 °C for 4 h before processing. With a constant output and draw ratio of the die, three compositions with PP/TPU weight ratios of 10/90 to 90/10 were prepared in the VE and TSE. The blends (10/90, 50/50, and 90/10) were used to study the interface morphology and thermal properties. Table 1 presents the main parameters of the VE and TSE applied in this investigation. The specimens used for the measurement of mechanical properties, wideangle X-ray diffraction and dynamic thermal mechanical properties were compression-molded in a hydraulic press at 200 °C. The specimens used to measure the phase morphology, thermal behavior, and spherulites were new extruder blends, and the specimen were dried in an oven at 80 °C for 4 h before measuring. Mechanical Testing. A type Instron 5566 universal testing machine with a tensile speed of 20 mm/min was used, according to the GB/T 1447-2005 standard. The dimensions of testing specimens were carefully machined to be 150 mm × 10 mm × 4 mm. All tests were performed at ambient temperature (25 °C), and five specimens were used in each test to obtain the average value. Scanning Electron Microscopy (SEM). A Quanta 200 scanning electron microscope (FEI Co.) was used to investigate the morphology that can be observed for the dispersion-phase interfaces of TPU/PP blends. The samples were fractured in liquid nitrogen for 30 min and covered with gold before being examined with the microscope. Polarizing Optical Microscope (POM). The crystallization morphology of PP/TPU was observed through a polarizing microscope (Axioskop40) with 200× or 500× magnification. Small fragments of all samples were inserted between two microscope cover glasses and placed on a hot stage. The fragments were heated to 210 °C, cooled to 120 °C, and kept at this temperature for 15 min. The morphology of the spherulites was collected by taking their microphotographs after the fragments were naturally cooled to room temperature. Wide-Angle X-ray Diffraction (WAXD). A D8 ADVANCE (Bruker, Germany; Cu Kα, λ = 0.154 nm, 40 kV, 40 mA) was used for WAXD analysis of TPU/PP blends. The measurement were performed at a 2θ angle of 5−55°, a scanning rate of 2°/min, and a scanning step of 0.02°. To obtain a clear X-ray scattering intensity, a cubic patch with a width of 4 mm, a length of 10 mm, and a thickness of 6 mm was carefully processed from the compression-mold specimens. The WAXD results could be employed to evaluate the orientation, crystalline morphology, and crystallinity. Differential Scanning Calorimetry (DSC). DSC measurements were performed using a 204C differential scanning calorimeter (Netzsch, Germany) under a nitrogen atmosphere. The samples B
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were heated from 25 to 230 °C at a heating rate of 10 °C/min, melted at 230 °C, and allowed to stand for 3 min to eliminate the thermal history. Afterward, the samples were cooled to −80 °C at a cooling rate of 10 °C/min, held at −80 °C for 3 min, and reheated to 230 °C at a heating rate of 10 °C/min for the second heating run. Sample weights varied from 4 to 5 mg. Dynamic Mechanical Thermal Analysis (DMA). To study the thermomechanical properties and distinct insights into the interactions of the PP/TPU blends on the molecular level, dynamic mechanical thermal properties were investigated using a DMA 242 (Netzsch, Germany). The three-point bending method was used at a frequency of 1 Hz and a heating rate of 3 °C/min. All of the specimens were rectangular trips with dimensions of 60 mm × 10 mm × 4 mm. The storage modulus (E′), loss modulus (E″), and loss factor (tan δ) were measured as a function of temperature (−70 to 120 °C). Thermogravimetric Analysis (TG). TG measurements were performed using a 209 thermogravimetry analyzer (Netzsch, Germany) under a nitrogen and air atmosphere in order to study the thermal stability and obtain the partial compatibility of PP/TPU blends. Samples of approximately 6 mg were heated from 25 to 700 °C at a heating rate of 10 °C/min. From the TG curves, thermal degradation features such as the onset degradation (Tonset), temperature at maximum rate of degradation (Tmax), and temperature at different mass losses were presented.
increases under the influence of the novel dynamic elongational flow field, compared to that of the conventional shear flow field. To analyze the stiffness and toughness of the PP/TPU blends further, the tensile modulus and impact strength are illustrated in Table 2. Apparently, the strength modulus and impact strength of the PP/TPU blends based on VE are also greater than those extruded by conventional TSE. As the TPU content (10, 50, and 90 wt %) increases, the tensile modulus is successfully improved from 926.78, 196.79, and 11.65 MPa for the PP/TPU blends based on the conventional TSE of up to 1030.98, 212.33, and 14.23 MPa for VE-extruded ones, respectively. More substantial promotion exists in the TPU/ PP blends extruded by novel VE with obtained impact strengths of 4.12, 9.91, and 51.64 kJ/m2, whereas they are 3.64, 4.69, and 38.07 kJ/m2 for those extruded by conventional TSE, respectively. The above-mentioned results (i.e., tensile strength, tensile modulus, elongation at break, and impact strength) suggest that the toughness and strength of the PP/TPU blends under the influence of the novel dynamic elongational flow are significantly better than in conventional shear flow. Moreover, high-resolution SEM observations are performed to examine the differences in dispersion particle morphology in the PP/TPU blends extruded by novel VE and conventional TSE. Figure 1 shows the dispersion-phase morphology of the PP/TPU blends from fresh extruder dies of VE and TSE. In the conventional TSE-extruded specimens (Figure 1A−C), a small number of fine spherulites are observed in the PP matrix when the TPU content is 10 wt % (Figure 1a). As the TPU content increases to 50 wt %, the bicontinuous phase is observed in Figure 1B, and a clear interlocking phenomenon between PP and TPU matrices is also observed. When the TPU content is 90 wt % (Figure 1C), the number of dispersion particles is shown in the TPU matrix. The result reveals that the PP viscosity is smaller than the TPU viscosity. Thus, PP is easily broken in the TPU matrix for shear flow field processing. More importantly, dispersed TPU forms long fibers with a large aspect ratio because of the elongational flow from the Figure 1a observation. The interface between the TPU and PP matrices is sharp because of high interfacial tension. The formation of TPU droplets is clearly observed. The phenomenon is attributed to the fact that some fibers are broken up and droplets are changed under the influence of the elongational deformation field. A bicontinuous phase is also observed with a TPU content of up to 50 wt % in Figure 1b. Interestingly, smaller TPU particles can be found in the cross section of the TPU component, which has not been previously reported.18,38 The existence of such particles indicates the effective dispersive mixing of materials by the elongational flow generated by VE. The processing conditions significantly affected the morphology development during melt mixing, especially at a higher content of the dispersed phase and at a high viscosity ratio.28 In particular, Figure 1c shows the dispersed phase of the PP
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RESULTS AND DISCUSSION Mechanical Properties and Interface Morphology Analysis. In the present study, our first concern is to investigate the influence of dispersion particle morphology on the mechanical properties of PP/TPU blends on the basis of novel dynamic elongational flow and conventional shear flow, respectively. Figure 1 shows the tensile strength, elongation at
Figure 1. Mechanical properties and interface morphology of the PP/ TPU blends: (A−C) extruded by TSE; (a−c) extruded by VE.
break, and dispersion particles morphology of the PP/TPU blends. The tensile strength of the PP/TPU blends extruded by VE is evidently superior to that of TSE-extruded specimens, and the elongation at break of the PP/TPU blends significantly
Table 2. Mechanical Properties of PP/TPU Blends Extruded by VE and TSE blend content (PP/TPU) tensile strength (MPa) tensile modulus (MPa) impact strength (kJ/m2)
90/10 VE TSE VE TSE VE TSE
30.98 26.78 1030.98 926.78 4.12 3.64 C
± ± ± ± ± ±
50/50 1.7 1.6 23.2 17.8 0.3 0.3
12.33 6.79 212.33 196.79 9.91 4.69
± ± ± ± ± ±
1.2 0.9 13.5 9.7 0.6 0.4
10/90 17.23 ± 1.4 11.65 ± 1.2 14.23 ± 1.3 11.65 ± 0.8 51.64 ± 2.3 38.07 ± 1.6
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transferred from a fibrillar to a droplet structure, and its particle size can evidently be smaller than that of a TSE-extruded specimen when the TPU content is 90 wt %. Given the finer morphology and stronger interfacial adhesion responsible for efficient stress transfer across interfaces, the mechanical properties of the blends is considerably improved under the effect of the novel dynamic elongational flow. Spherulites and Interface Analysis. The POM photographs of the VE- and TSE-extruded PP/TPU blends are shown in Figure 2. Figure 2A−C shows the spherulite structure
10 and 50 wt %. At a TPU content of 90 wt %, more complete PP spherulites are shown in Figure 2F compared to VEextruded specimens. The result suggests that the interaction between PP and TPU matrices is weaker compared to that of VE-extruded samples. Thus, the partial interlocking effect of the TPU and PP chains may lead to the novel result as a function of the dynamic elongational deformation field that generates the high-pressure and convergent elongation deformation. Many of the PP chains can remain in the TPU melt islands, and the TPU chains (as a transport medium) partially included in the amorphous intraand interspherulitic regions of the PP matrix lead to the apparent partial miscibility in amorphous regions.39 The mixing of the PP and TPU chains in the soft amorphous phases reduces the mobility of the macromolecular chains, creating stiffer PP/TPU blends when PP is the matrix phase.40 However, the conventional shear flow mainly generates flat drag motion, resulting in a weaker mass transfer and interaction between PP and TPU matrices compared to the situation for VE-extruded. Crystalline Structure Analysis. To reveal specific structural information on interlocking spherulites in the novel VE-extruded PP/TPU blends, we utilized the microbeam for wide-angle X-ray diffraction (WAXD). Figure 3 shows WAXD spectrograms of the neat PP and PP/TPU blends extruded by novel VE and conventional TSE with the addition of the TPU. Evidently there are no differences between the patterns of PP/ TPU specimens extruded by VE and TSE and those observed in Figure 3B. Typical reflections of the α-form crystal of PP can be observed (i.e., the typical reflections of the α-form crystal at 2θ = 14.14, 16.93, and 18.54° corresponding to lattice plans (110), (40), and (130), respectively). And three obvious diffraction peaks with a stronger diffraction intensity of α-form crystals are exhibited in the spectrograms of both VE-extruded and TSE-extruded specimens. The result proves that TPU acts as a good nucleating agent when its content is lower (
