- Email: [email protected]
Breakup promotion of deformed EPDM droplets by the migration of nanoparticles during extrusion
Polymer Testing 86 (2020) 106445
Contents lists available at ScienceDirect
Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest
Breakup promotion of deformed EPDM droplets by the migration of nanoparticles during extrusion Lei Gong a, *, Shu-hua Chen a, Yang Yu a, Bo Yin b, Ming-bo Yang b a
Department of Environment and Chemical Engineering, Dalian University, Dalian, 116622, Liaoning, People’s Republic of China College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Morphology Evolution Migration NanoPP
The effect of migration of calcium carbonate (CaCO3) nanoparticles on the breakup dynamics of EthylenePropylene-Diene Monomer (EPDM) droplets in Polypropylene (PP) matrix during melt extrusion was investi gated in situ. The breakup process of EPDM droplets was sped up dramatically when the migration of CaCO3 nano-particles from dispersed phase to matrix was introduced to PP/EPDM melts. It was found that both the total breakup time and the shape stability of slender EPDM droplets decreased with the increase of CaCO3 concen tration. Both the maximum value in equivalent diameter d and aspect ratio AR of EPDM droplets were also reduced by increasing the composition of CaCO3 nanoparticles. Results were discussed in consideration of interfacial tension and migration of CaCO3 nanoparticles. Reduction in interfacial tension is mainly responsible for the improved breakup process in the two-step composites with CaCO3 nanoparticles (<2 wt%). Higher composition of CaCO3 (�2 wt%) induced the CaCO3 aggregates in the EPDM phase. These aggregates acted as stress concentration when the EPDM droplets break up.
1. Introduction Blending two or more polymers physically is attracting a lot of in terests and growing rapidly [1–4]. Without introducing new chemistry, blending two or more polymers may exert synergistic effects on each other in terms of physical properties. After mixing two immiscible polymer melts, a micro-scale phase region with specific arrangement called micro-structure is acquired. In general, micro-structure is gov erned by melt rheology and shear history, and dictates the ultimate properties of the blends, such as optical, rheological and transport properties [5]. The evolution of micro-structure for polymer blends, especially the formation of sea-island structure, has been widely explored when it is suddenly subjected to flow. The micro-structural evolution of dispersed droplets undergoes deformation, coalescence, retraction, and different types of breakup [6,7], which is dependent on the capillary number, viscosity ratio, and field character (elongation or shear) [8–10]. Based on the quantitative description of the relationship between the critical capillary number and viscosity ratio of droplets under both uniaxial tensile and simple shear measured by Grace [6], abundant theories of explanation for the breakup of dispersed droplets in polymer blends have been deduced [10–12].
In consideration to the research history of polymer blends with nanofillers, lots of research has been done on to micro-structural evolution in polymer blends during melt processing since the sixties [13]. The results reported since then discovered that the influence of nanoparticles on the final micro-structure of blends (sea-island or co-continuous morphology) was strongly relied on the selective distribution of nano particles in the blends [2,14–16]. Generally, the relaxation and breakup of deformed dispersed droplets are disrupted when the filler localized in dispersed phase [15,17], and no effect on the critical capillary number when the filler is situated in the liquid-liquid interface [18]. Enhanced morphology stability and domain size can be achieved when the nano particles were selectively dispersed in the matrix or at the interface [16, 19]. Different mechanisms are deduced to explain the morphological changes in polymer blends with nanoparticles. Some authors suggested that the active surface of nanoparticles is important to decrease the interfacial tension between the two polymer phases. Some believed that the increase of coalescence barriers between dispersed droplets was owing to the rigid layer formed by nanoparticles [20,21]. When intro ducing nanoparticles into polymer blends, the final equilibrium morphology, the preferable distribution of the nanoparticles within the blends and the dispersion state of nanoparticles cannot be achieved
* Corresponding author. E-mail address: [email protected] (L. Gong). https://doi.org/10.1016/j.polymertesting.2020.106445 Received 5 November 2019; Received in revised form 31 January 2020; Accepted 15 February 2020 Available online 23 February 2020 0142-9418/© 2020 Published by Elsevier Ltd.
L. Gong et al.
Polymer Testing 86 (2020) 106445
immediately due to the high viscosity of polymers [19]. Accompanied by the micro-structural evolution of the dispersed droplets, the migration of nanoparticles from initial phase to the selective phase is proved by the experimental observations in polymer blends, if the nanoparticles not in the selective phase initially [22–25]. In other words, during nano particles transferring from one phase to another, the dispersed droplets deform, elongate, orientate, breakup and finally relax into a stable sphere simultaneously. The mechanism on the transfer of nanoparticles from one phase to another [26–28], and the influence of the geometry of nanoparticles on the nanoparticle migration [22] have been widely explored. However, it is unclear that how the micro-structural evolution of dispersed droplets is with the consideration of nanoparticles’ migration, because the two processes are hard to separate. As far as we know, the migration of nanoparticles has not been investigated as a considerable factor for the micro-structural evolution of dispersed droplets during the melt blending. It is crucial to understand the dependence of the evolution of the dispersed droplets on the migration of rigid nanoparticles from the dispersed phase to the matrix. In our study, a simple model blend including CaCO3 nanoparticle-filled Ethylene-Propylene-Diene Mono mer (EPDM) droplets blending with polypropylene (PP) melt, is pro posed for the investigation in the influence of the nanoparticles migration on the micro-structural evolution of EPDM droplets during extruding. Such system is introduced to evaluate the final morphology by interfacial tension of ternary blend in melt blending situation [29,30] and to study the dynamics of nanoparticles movement from the dispersed phase into the matrix. On the one hand, such route could ensure a pre distribution of CaCO3 nanoparticles in the dispersed phase; on the other hand, it could provide the possibility of migration of nanofillers from dispersed phase to matrix. Furthermore, the depen dence of the breakup mechanism of a single droplet on the composition of nanoparticles can be investigated. The effect of nanoparticle incor poration on the viscoelastic and interfacial properties, and the breakup and relaxation process of dispersed droplets during melt blending were studied in this paper.
representation of the screw configuration is shown in Fig. 1. In this study, the outer diameter D2 of the screw is 22.5 mm, and the inner diameter D1 is 21 mm. Besides, the channel depth h and the screw width b are equal to 1.5 mm and 0.5 mm respectively. The distance from the root to the center of each channel (along the screw helix) can be expressed as (n-1/2)S, where S is the screw pitch (in this case, S ¼ 12 mm), and n represents the number of the screw channel. Specimens were collected from the center of each channel after the melting of solid (from n ¼ 10). 2.3. Microscopy Cross sections of extruded samples perpendicular to the flow direc tion were obtained in liquid nitrogen. The samples were then etched in dimethylbenzene for 2 h, and cleaned with acetone for several times. The etched samples were air-dried for 24 h at room temperature. The dried samples were coated by a thin layer of gold and the surface to pographies of the samples were characterized by JSM-5900LV scanning electron microscopy (SEM) with an accelerating voltage of 20 kV. The particle sizes of the droplets were enumerated from each SEM image using Image Pro Plus (IPP). The length (L) and width (B) of the elliptical droplet were determined using image analysis software for SEM images. An equivalent diameter, d, is defined for each particle as the diameter of a circle with the same cross-sectional area [31]: n pffiffiffiffiffiffiffiffi P Bi Li
d ¼ i¼1
n
where n is the number of droplets. 3. Results and discussion 3.1. Results The aspect ratio, AR ¼ L/B is introduced to describe the deformation degree of a moderated or highly stretched droplet, where L and B are the length and width of the droplet, respectively. When the AR is above a critical aspect ratio (ARcr), the thread-like droplet would break up into smaller droplets due to Rayleigh instability. In contrast, when the AR is below the critical value, the droplet would retract back to a spherical shape. For intermediate AR, the droplet would break up from the ends into smaller droplets (ending-pinching) [16,32,33]. Fig. 2 describes the breakup and relaxation process of pristine PP/ EPDM and PP/EPDM/CaCO3 (2 wt%) during extrusion process. Fig. 3 describes d and AR of pristine PP/EPDM and PP/EPDM/CaCO3 with varying CaCO3 composition as a function of extrusion distance (D), respectively. Apparently, PP is served as the continuous phase, while the Etched holes and the bright smaller particles belong to EPDM dispersed droplets and CaCO3 nanoparticles, respectively. For the pristine PP/
2. Experimental section 2.1. Materials Polypropylene (PP, T30S) with a melt flow rate (MFR) of 2.6 g/10 min and density of 0.91 g/cm3 (based on ASTM D1505-68) was supplied by Lanzhou Petrochemical Co. Ltd, China. EPDM (Nordel 4725p) with Mw of 135,000 g/mol was purchased from Dow Elastomers L.L.C., USA, which consists of 70% ethylene, 25% propylene and 4.9% ethylidene norbornene (ENB). CaCO3 nanoparticles was purchased from Huaxin Nanomaterial Co. Ltd, China, modified with stearic acid and particle diameter of 10–40 nm particle diameter. 2.2. Mixing procedures Two-step composite system was introduced to investigate the influ ence of CaCO3 nanoparticles migration on the microstructural evolution in polymer blends. CaCO3 nanoparticles was dispersed in EPDM to obtain EPDM/CaCO3. Three EPDM/CaCO3 with different composition CaCO3 were studied: 0.5 wt%, 2 wt%, and 4 wt%, respectively. PP was then mixed with each EPDM/CaCO3 using twin-screw extruder on a constant mass ratio of 80:20 (CTE35, KEYA Company, Ltd, China), yielding EPDM/PP/CaCO3. EPDM/PP blend without CaCO3 was extruded as the control. Three-stage setting temperature was adopted from the stage of feeding to melting: 175 � C, 200 � C and 200 � C. The die temperature was 5 � C lower than that of melting zone. The screw was quenched immediately and pulled out rapidly when the melts were blended homogeneously. The effects of CaCO3 transfer on the morphological evolution of dispersed droplet for EPDM/PP blend was studied with identical screw configuration. The schematic
Fig. 1. The screw configuration used in this study. 2
L. Gong et al.
Polymer Testing 86 (2020) 106445
Fig. 2. SEM micrographs of PP/EPDM blend (a) and two-step PP/EPDM/CaCO3 composites (b) respectively at different position along the extruder axis.
Fig. 3. The equivalent diameter (a) and aspect ratio (b) at different positions along the extruder axis for PP/EPDM/CaCO3 composites with varying CaCO3 contents.
EPDM, the thread-like fibril droplets dispersed in PP melt at the begin ning of the extrusion (Fig. 2a, extrusion distance, D ¼ 126 mm), and subsequently broke up owing to Rayleigh instability (capillary-wave instability) at 174 mm spot, where the droplets reached the ultimate strength (Fig. 3, d ¼ 7.53 μm, AR ¼ 5.91). The droplets then began to retract back to ellipse-like or spherical droplets with an average diam eter of 0.98 μm at 248 mm spot and eventually stabilized to spherical droplets with an average diameter of 0.41 μm at 280 mm. It is interesting to find out that there was a dramatic change in the breakup process of dispersed droplets for the blends with CaCO3 nanoparticles. Multifarious morphology of EPDM droplets, e.g. slender fibril, short fibril, and sphere, was observed in the PP/EPDM/CaCO3 (2 wt%) nanocomposites at the beginning of extrusion (Fig. 2b). The EPDM droplets present both a smaller d (1.66 μm) and a shorter AR (5.21) at 114 cm compared to these of pristine PP/EPDM. The equilibrium between breaking and retracting of dispersed droplets was reached at 150 mm for the PP/ EPDM/CaCO3 (2 wt%) nanocomposites, much quicker than that of pristine PP/EPDM. The EPDM dispersed droplets were also observed to stabilize at 162 mm with an average diameter of 0.39 μm in PP/EPDM/ CaCO3 (2 wt%) nanocomposites. In comparison with pristine PP/EPDM blend, a slight change in AR of EPDM droplets was observed in the
composites with 0.5 wt% CaCO3 nanoparticles (Fig. 3). Both d and AR reduced upon increasing composition of CaCO3 nanoparticles. These results imply that the addition of CaCO3 nanoparticles affected the shape stability and d of EPDM dispersed droplets remarkably. An improvement in the breakup dynamics of EPDM droplets was observed with the addition of CaCO3 nanoparticles. The total breakup distance of EPDM droplets to reach the equilib rium state varying with the composition of CaCO3 nanoparticles is shown in Fig. 4. The total breakup distance for EPDM droplets from the very beginning to spherical particles (AR ¼ 1) reduced after introducing CaCO3 nanoparticles into the PP/EPDM blend. In general, the higher the content of CaCO3 nanoparticles, the shorter the total breakup distance (AR ¼ 1). It was observed that higher concentration of CaCO3 nano particles has much little effect on the total breakup distance of EPDM droplets. The total breakup distance of EPDM dispersed droplets upon the addition of 0.5 wt%, 2 wt% and 4 wt% of CaCO3 nanoparticles reduced to 174 mm, 150 mm, and 138 mm, respectively, compared to 248 mm of pristine PP/EPDM blends. This result of total breakup dis tance implies that the deformation and breakup time for droplets of EPDM/PP filled with CaCO3 nanoparticles was remarkably shorter than that of pristine PP/EPDM blend. Interestingly, the breaking dynamics of 3
L. Gong et al.
Polymer Testing 86 (2020) 106445
Fig. 4. Distance of EPDM droplets broken to spherical particles (AR ¼ 1) varying with the CaCO3 contents in the PP/EPDM/CaCO3 composites.
Fig. 5. Viscosities of pure PP, pure EPDM and their binary composites at 200 � C plotted as a function of frequency.
dispersed droplets was drastically sped up in the PP/EPDM/CaCO3 (0.5 wt%) composites. In brief, the results demonstrated that the total breakup distance and the shape stability of EPDM droplets were signif icantly reduced by increasing the composition of CaCO3 nanoparticles.
where p is defined as p ¼ ηd/ηm; and, ηd is the viscosity of dispersed phase. Rheological measurements (Fig. 5) show that there was no obvious variation in the complex viscosities for PP/CaCO3 and EPDM/CaCO3 compared with that for PP and EPDM respectively. Results also indicated that little interactions between CaCO3 nanoparticles and the polymer chains when a low composition of CaCO3 nanoparticles was added [36]. Guan Gong observed that high composition of CaCO3 (at least 15 wt%) can reduce polymer molecular mobility [37]. Fig. 5 demonstrates that the viscosity ratio,p is equal to 0.624 in PP/EPDM blend under our shear rate (150 s 1). A slight variation of p to 0.517 was observed in two-step composites. According to Equation (5), the Cacrit of all samples in this study is almost equivalent to 0.45. Based on Grace theory [6], for the range of p of all samples in this study, the droplet breaks from the middle part gradually until two sub-droplets were formed when Cacrit < Ca in single flow field (uniaxial stretch or simple shear). The speculation of breakup mechanism of the dispersed droplet was verified by SEM micrographs (Fig. 2). According to Equation (1), the critical equivalent diameter, Rcrit, of a dispersed droplet is expressed as follows:
3.2. Discussion The competitive relationship of interfacial tension and viscous stresses can be expressed by the capillary number, Ca (Equation (1)): (1)
Ca ¼ ηm RG=σ
where, ηm is the viscosity of matrix, G is the shear rate, and σ is the interfacial tension. Ca represents the ratio of the relaxation time of interfacial tension (ηmR/σ) to the deformation time induced by shear (G 1). It is a dimensionless measurement for the size of droplet under a fixed shear rate [5]. The shear rate of twin-screw extruder can be evaluated by Equation (2), which has been established by Burkhardt et al. [34]: � G¼ðν0 =hÞ½ðh2 2πDnηÞðdP=dzÞf2y=h 1gþcosθ�þð2ν0 =hÞf2 ð3y=hÞgsinθ (2)
Rcrit ¼
where the first and second brackets on the right are affected by the down-channel flow and cross flow, respectively; D is the screw diameter; v0 ¼ πDn is the linear circumferential screw velocity; n is the screw revolution per minute; h is the channel depth; θ is the helix pitch angle; dP/dz is the pressure gradient in the z direction (along the screw helix); y is the channel depth when y ¼ 0 on the barrel surface; and, η is the melt viscosity. Equation (2) can be deduced to Equation (3) due to (h2/ 2πDnη)(dP/dz)((2 y/h)1] ≪cosθ under the present shear rate of the screw:
γ AB ¼ γ A þ γB
A rotational speed of 150 rpm was employed for this study; thus, the shear rate induced by the twin screw was 150 s 1. When the Ca of a droplet is below the critical capillary number, Cacrit, the droplet main tains steady orientation and shape. In contrast, the droplet breaks up eventually when it has a Ca above the Cacrit. The relationship between Cacrit and viscosity ratio, p was expressed by de Bruijn [35] (Equation (4)): log Cacrit ¼
0:506
0:0995 log p þ 0:124ðlog pÞ2
log p
0:115 log 4:08
(5)
When the equivalent diameter, R is less than Rcrit, the deformation and orientation of the droplet remain similar. On the other hand, the droplet breaks up eventually when R is larger than Rcrit. The Rcrit is proportional to Cacrit and interfacial tension, respectively, but inversely proportional to the viscosity of the matrix or shear rate. The values of surface tension, dispersion and polar components are listed in Table 1. The interfacial tension of each pair can be calculated from surface tension by using the geometric mean equation deduced by Wu [38]:
(3)
G�n
Cacrit σ ηm G
1=2
2ðγdA γdB Þ
1=2
ðγ pA γ pB Þ
(6)
where γ ¼ γdþγp, γ is surface tension, d is dispersion component and p is polar component; γAB is the interfacial tension; and, γA and γB are the surface tensions of the two materials in contact. The calculated inter facial tensions of all possible polymer/elastomer are also given in Table 1. An obvious reduction of interfacial tension was demonstrated from 1.7 mN/m to less than 0.92 mN/m after the introduction of CaCO3 nano-particles to PP/EPDM blend. This result is in good agreement with the result of PA6/PS/SiO2 system reported by Kong et al. [16]. The interfacial tension of PP and EPDM were slightly varied by increasing
(4)
4
L. Gong et al.
Polymer Testing 86 (2020) 106445
Table 1 Summary of interfacial tension of each pair. Polymer
γ(mN/ m)
γp(mN/ m)
γd(mN/ m)
Interfacial tension σ with PP(mN/m)
PP EPDM EPDM/CaCO3 20/0.5 20/2 20/4
40.04 32.19 35.00
0.26 1.22 0.94
39.79 30.98 34.06
1.7 0.82
34.25 35.17
0.91 1.01
33.34 34.16
0.92 0.86
the composition of CaCO3 nanoparticles. According to the report of Kong et al. [16], the change of interfacial tension relies on the concentration of nanoparticles at the interface between the matrix and the dispersed drops [26]. Results demonstrated that the composition of CaCO3 nano particles at the interface may be close to saturation when 0.5 wt% of CaCO3 nanoparticles was loaded and increase in composition of CaCO3 nanoparticles would probably exert no effect on the interfacial tension. Based on Equation (5), the theoretical values of Rcrit for PP/EPDM blends filled with 0 wt%, 0.5 wt%, 2 wt% and 4 wt% of CaCO3 nano particles were calculated to be 13.75 μm, 6.63 μm, 7.44 μm and 6.96 μm, respectively while the experimental Rcrit values for the samples were calculated to be 7.53 μm, 2.31 μm, 1.66 μm and 0.89 μm, respectively (Fig. 3a). The reason of the difference between the theoretical Rcrit and the experimental Rcrit is unclear and probably due to the influence of nanoparticles migration from dispersed phase to matrix. However, both the theoretical and experimental Rcrit imply that the breakup dynamic of deformed droplets was sped up by introducing CaCO3 nanoparticles into PP/EPDM blend (Fig. 2). The transfer of CaCO3 from EPDM dispersed droplets to PP matrix was appeared voluntarily accompanied by the evolution of the dispersed droplets, when EPDM/CaCO3 master batch compounded with PP melt in the twin screw extruder. According to Elias’s theory, the velocity field inside the dispersed droplet caused by shear was dominant one among the three migration mechanisms, when nanoparticles tend to transfer from the dispersed droplets to the matrix. In the control of such mech anism, the migration was divided into three stages. In the first stage, CaCO3 nanoparticles migrated from interior of dispersed droplets to two-phase interface induced by the shear. The moving of the nano particles inside the droplet might bring an unstable capillary wave, which probably led to the subsequent break up of the deformed droplet. And then the rigid nanoparticles spent some time at the interface until the chains of dispersed phase were desorbed gradually from the CaCO3 surface and were totally substituted by the PP chains [18], which could be belonged to the second stage. After CaCO3 particles move to the interface induced by shear field, the chains of the preferential distri bution phase (PP) near the interface will be close to the CaCO3 surface and subsequently replace those of EPDM. A defect formed within the elongated dispersed droplet is resulted from a thermodynamically un stable curvature as shown in Fig. 6. The defect is probably evolved to a initial craze under the shear, which lead to the final breakup of the deformed dispersed droplet due to the poor interface between nano particles and the dispersed phase. It is primly payed the responsibility to smaller aspect ratio and equivalent diameter of the dispersed droplet in two-step composites than that in binary system during the breakup process of the dispersed droplet, especially for the one with lower con centration of CaCO3. however, it is still unclear why the aspect ratio and equivalent diameter of the two-step composites decreased with the in crease in CaCO3 nanoparticles. In the third stage, the CaCO3 nanoparticles crossed the interface into matrix. Andreas et al. deduced a low migration velocity and a high interfacial stability between the two polymers for nanoparticles with relative low aspect ratio, such as carbon black (CB) [22]. It means that CaCO3 nanoparticles would spend quite a lot of time at the interface during the second stage, much longer than the time of the first and third stages.
Fig. 6. Nano-CaCO3 at the PP/EPDM blend interface during extruding.
The second migration mechanism is described for PP/EPDM/CaCO3 nanocomposites: the exposure of CaCO3 nanoparticles to cross-sections during the breakup of EPDM droplets. The aggregates of CaCO3 nano particles play a role as stress concentration points during the breakup process of EPDM droplet. A great quantity of deformed dispersed droplets which broke up at the CaCO3 aggregates were observed in the two-step composites with 2 wt% and 4 wt% CaCO3 nanoparticles, but absent in the one with 0.5 wt% nanoparticles. A schematic drawing representing the second migration mechanism for CaCO3 nanoparticles during the breakup process is shown in Fig. 7. Orientation and defor mation of EPDM dispersed droplets were probably happened under shear. However, the CaCO3 aggregates in EPDM dispersed droplets could hardly follow the deformation of the droplets, which would acted as stress concentration points under load owing to the poor interfacial adhesion between CaCO3 and EPDM, and then lead to the breakup of the droplets. Liu et al. [39] discovered that the presence of large SiO2 ag glomerates within the PP matrix depressed the elongation of droplets at break. Lee et al. [40] indicated a poor interaction between the non-polar polyolefin matrix and the filler. Such viewpoints are in accordance with the cold-drawing theory of inorganic particles filled with brittle poly mers. This implication may be the major reason for the decreasing trend in aspect ratio and equivalent diameter of the two-step composites with increasing composition of CaCO3 (�2 wt%).
Fig. 7. SEM micrographs of PP/EPDM/CaCO3 (80/20/2) composites prepared by two-step extrusion at 126 mm along the extruder axis (a) (b) and schematic diagram for the breakup of dispersed droplets. 5
L. Gong et al.
Polymer Testing 86 (2020) 106445
4. Conclusions
[10] S. Comas-Cardona, C.L. Tucker III, Measurements of droplet deformation in simple shear flow with zero interfacial tension, J. Rheol. 45 (1) (2001) 259–273, 1978present. [11] S. Guido, M. Minale, P.L. Maffettone, Drop shape dynamics under shear-flow reversal, J. Rheol. 44 (6) (2000) 1385–1399, 1978-present. [12] H. Yamane, et al., Observation of deformation and recovery of poly (isobutylene) droplet in a poly (isobutylene)/poly (dimethyl siloxane) blend after application of step shear strain, J. Rheol. 42 (3) (1998) 567–580, 1978-present. [13] M. Walters, D. Keyte, Heterogeneous structure in blends of rubber polymers, Rubber Chem. Technol. 38 (1) (1965) 62–75. [14] F.Y. Liu, L.M. Feng, et al., Segregated polypropylene/cross-linked poly(ethyleneco-1-octene)/multi-walled carbon nanotube nanocomposites with low percolation threshold and dominated negative temperature coefficient effect: towards electromagnetic interference shielding and thermistors, Compos. Sci. Technol. 159 (2018) 152–161. [15] C. Zhang, et al., Multi-walled carbon nanotube in a miscible PEO/PMMA blend: thermal and rheological behavior, Polym. Test. 75 (2019) 367–372. [16] M. Kong, et al., Retarded relaxation and breakup of deformed PA6 droplets filled with nanosilica in PS matrix during annealing, Polymer 52 (22) (2011) 5231–5236. [17] T. Merkel, et al., Emulsification of particle loaded drops in simple shear flow, Colloid. Surface. Physicochem. Eng. Aspect. 470 (2015) 179–187. [18] Y. Pan, X. Liu, X. Hao, D.W. Schubert, Conductivity and phase morphology of carbon black-filled immiscible polymer blends under creep: an experimental and theoretical study, Phys. Chem. Chem. Phys. 18 (2016) 32125–32131. [19] L. Elias, et al., Morphology and rheology of immiscible polymer blends filled with silica nanoparticles, Polymer 48 (20) (2007) 6029–6040. [20] P. Clegg, Fluid-bicontinuous gels stabilized by interfacial colloids: low and high molecular weight fluids, J. Phys. Condens. Matter 20 (11) (2008) 113101. [21] G. Filippone, et al., The role of organoclay in promoting co-continuous morphology in high-density poly (ethylene)/poly (amide) 6 blends, Polymer 49 (5) (2008) 1312–1322. [22] A. G€ oldel, et al., Shape-dependent localization of carbon nanotubes and carbon black in an immiscible polymer blend during melt mixing, Macromolecules 44 (15) (2011) 6094–6102. [23] Y. Li, H. Shimizu, Conductive PVDF/PA6/CNTs nanocomposites fabricated by dual formation of cocontinuous and nanodispersion structures, Macromolecules 41 (14) (2008) 5339–5344. [24] O. Meincke, et al., Mechanical properties and electrical conductivity of carbonnanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene, Polymer 45 (3) (2004) 739–748. [25] D. Wu, et al., Selective localization of multiwalled carbon nanotubes in poly (ε-caprolactone)/polylactide blend, Biomacromolecules 10 (2) (2009) 417–424. [26] L. Elias, et al., Immiscible polymer blends stabilized with nano-silica particles: rheology and effective interfacial tension, Polymer 49 (20) (2008) 4378–4385. [27] R. Stark, M. Kappl, H.-J. Butt, Interaction of solid surfaces across binary mixtures of polymer melts, Macromolecules 40 (11) (2007) 4088–4091. [28] K. David, N. Dan, R. Tannenbaum, Competitive adsorption of polymers on metal nanoparticles, Surf. Sci. 601 (8) (2007) 1781–1788. [29] C. Ma, M. Zhang, M. Rong, Morphology prediction of ternary polypropylene composites containing elastomer and calcium carbonate nanoparticles filler, J. Appl. Polym. Sci. 103 (3) (2007) 1578–1584. [30] H. Yang, et al., Largely improved toughness of PP/EPDM blends by adding nanoSiO 2 particles, Polymer 48 (3) (2007) 860–869. [31] X.-h. Chen, et al., Study on morphology evolution and fractal character of the miscible blend between isotactic polypropylene and copolymer of ethylene and propylene, Polymer 50 (14) (2009) 3347–3360. [32] H.A. Stone, Dynamics of drop deformation and breakup in viscous fluids, Annu. Rev. Fluid Mech. 26 (1) (1994) 65–102. [33] G.I. Taylor, The viscosity of a fluid containing small drops of another fluid, in: Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 1932, pp. 41–48. [34] K. Burkhardt, H. Hermann, S. Jakopin, Plasticizing and mixing principles of intermeshing co-rotating and counter-rotating twin-screw extruders, SPE ANTEC Tech. Papers 24 (1978) 498–506. [35] R.A. Debruijn, Deformation and Breakup of Drops in Simple Shear Flows, 1991. [36] G. Lei, et al., Morphology and properties of PP/EPDM binary blends and PP/EPDM/ nano-CaCO3 ternary blends, J. Appl. Polym. Sci. 123 (1) (2012) 510–519. [37] G. Guan, et al., Dynamic rheological behavior of isotactic polypropylene filled with nano-calcium carbonate modified by stearic acid coating, J. Macromol. Sci. 48 (2) (2009) 329–343. [38] S. Wu, J. Macromol. Sci. Rev. Macromol. Chem. 10 (1) (1974) 1–73. [39] L. Yiqun, et al., The structure and physical properties of polypropylene and thermoplastic olefin nanocomposites containing nanosilica, Polymer 47 (22) (2006) 7731–7739. [40] L. Hyuk-soo, et al., TPO based nanocomposites. Part 1. Morphology and mechanical properties, Polymer 46 (25) (2005) 11673–11689. [41] A. G€ oldel, G. Kasaliwal, P. P€ otschke, Selective localization and migration of multiwalled carbon nanotubes in blends of polycarbonate and poly (styreneacrylonitrile), Macromol. Rapid Commun. 30 (6) (2009) 423–429.
The transfer of rigid fillers (MWCNTs, SiO2, CaCO3) from the dispersed phase to matrix during melt blending and its influence on the final morphologies of the ternary composites, and the possible mecha nism deduced to the phenomenon had been studied previously [41]. However, the effect of the migration of CaCO3 nanoparticles from the dispersed phase to the matrix on the micro-structural evolution of dispersed droplets has not been explored during extrusion blending of polymer blend with nanoparticles [18]. The breakup process of EPDM dispersed droplets was improved dramatically and the breakup mecha nism was changed under the transfer of CaCO3 nanoparticles from EPDM phase to PP melt, as compared to that of pristine PP/EPDM blend. The total breakup time and the shape stability of EPDM droplets were reduced by increasing the composition of CaCO3 nanoparticles. The key reason is the reduction in interfacial tension caused by the introduction of CaCO3 nanoparticles for the two-step composites with low content of CaCO3 (<2 wt%). While the CaCO3 aggregates in deformed EPDM droplets acted as the stress concentration points are mainly responsible for the reduced maximum equivalent diameter d and aspect ratio AR of EPDM droplets when the composition of CaCO3 was high (�2 wt%). The reduction in interfacial tension becomes an unimportant cause in this situation. Deeper understanding on the micro-structural evolution in ternary composites during melt blending in conventional extruder is important. In brief, the migration of nanoparticles promotes the development of functional polymer materials with high price-performance ratio. CRediT authorship contribution statement Lei Gong: Conceptualization, Methodology, Software, Writing original draft, Supervision. Shu-hua Chen: Data curation, Software. Yang Yu: Visualization, Investigation. Bo Yin: Writing - review & editing. Ming-bo Yang: Writing - review & editing, Supervision. Acknowledgements The authors gratefully acknowledge the financial support from the Science and Technology Star Program of Dalian (Contract No. 2017RQ039). References [1] Y.S. Lipatov, Polymer blends and interpenetrating polymer networks at the interface with solids, Prog. Polym. Sci. 27 (9) (2002) 1721–1801. [2] Y.M. Pan, X.H. Liu, et al., Reversal phenomena of molten immiscible polymer blends during creep-recovery in shear, J. Rheol. 61 (2017) 759–767. [3] G. Jiang, H. Wu, S. Guo, Reinforcement of adhesion and development of morphology at polymer–polymer interface via reactive compatibilization: a review, Polym. Eng. Sci. 50 (12) (2010) 2273–2286. [4] M. Sumita, et al., Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black, Polym. Bull. 25 (2) (1991) 265–271. [5] C.L. Tucker III, P. Moldenaers, Microstructural evolution in polymer blends, Annu. Rev. Fluid Mech. 34 (1) (2002) 177–210. [6] H.P. Grace, Dispersion phenomena in high viscosity immiscible fluid systems and application of static mixers as dispersion devices in such systems, Chem. Eng. Commun. 14 (3–6) (1982) 225–277. [7] J. De Bruijn, et al., Determination of octanol/water partition coefficients for hydrophobic organic chemicals with the “slow-stirring” method, Environ. Toxicol. Chem. 8 (6) (1989) 499–512. [8] B. Bentley, L. Leal, An experimental investigation of drop deformation and breakup in steady, two-dimensional linear flows, J. Fluid Mech. 167 (1986) 241–283. [9] A.S. Almusallam, R.G. Larson, M.J. Solomon, A constitutive model for the prediction of ellipsoidal droplet shapes and stresses in immiscible blends, J. Rheol. 44 (5) (2000) 1055–1083, 1978-present.
6
Contents lists available at ScienceDirect
Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest
Breakup promotion of deformed EPDM droplets by the migration of nanoparticles during extrusion Lei Gong a, *, Shu-hua Chen a, Yang Yu a, Bo Yin b, Ming-bo Yang b a
Department of Environment and Chemical Engineering, Dalian University, Dalian, 116622, Liaoning, People’s Republic of China College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Morphology Evolution Migration NanoPP
The effect of migration of calcium carbonate (CaCO3) nanoparticles on the breakup dynamics of EthylenePropylene-Diene Monomer (EPDM) droplets in Polypropylene (PP) matrix during melt extrusion was investi gated in situ. The breakup process of EPDM droplets was sped up dramatically when the migration of CaCO3 nano-particles from dispersed phase to matrix was introduced to PP/EPDM melts. It was found that both the total breakup time and the shape stability of slender EPDM droplets decreased with the increase of CaCO3 concen tration. Both the maximum value in equivalent diameter d and aspect ratio AR of EPDM droplets were also reduced by increasing the composition of CaCO3 nanoparticles. Results were discussed in consideration of interfacial tension and migration of CaCO3 nanoparticles. Reduction in interfacial tension is mainly responsible for the improved breakup process in the two-step composites with CaCO3 nanoparticles (<2 wt%). Higher composition of CaCO3 (�2 wt%) induced the CaCO3 aggregates in the EPDM phase. These aggregates acted as stress concentration when the EPDM droplets break up.
1. Introduction Blending two or more polymers physically is attracting a lot of in terests and growing rapidly [1–4]. Without introducing new chemistry, blending two or more polymers may exert synergistic effects on each other in terms of physical properties. After mixing two immiscible polymer melts, a micro-scale phase region with specific arrangement called micro-structure is acquired. In general, micro-structure is gov erned by melt rheology and shear history, and dictates the ultimate properties of the blends, such as optical, rheological and transport properties [5]. The evolution of micro-structure for polymer blends, especially the formation of sea-island structure, has been widely explored when it is suddenly subjected to flow. The micro-structural evolution of dispersed droplets undergoes deformation, coalescence, retraction, and different types of breakup [6,7], which is dependent on the capillary number, viscosity ratio, and field character (elongation or shear) [8–10]. Based on the quantitative description of the relationship between the critical capillary number and viscosity ratio of droplets under both uniaxial tensile and simple shear measured by Grace [6], abundant theories of explanation for the breakup of dispersed droplets in polymer blends have been deduced [10–12].
In consideration to the research history of polymer blends with nanofillers, lots of research has been done on to micro-structural evolution in polymer blends during melt processing since the sixties [13]. The results reported since then discovered that the influence of nanoparticles on the final micro-structure of blends (sea-island or co-continuous morphology) was strongly relied on the selective distribution of nano particles in the blends [2,14–16]. Generally, the relaxation and breakup of deformed dispersed droplets are disrupted when the filler localized in dispersed phase [15,17], and no effect on the critical capillary number when the filler is situated in the liquid-liquid interface [18]. Enhanced morphology stability and domain size can be achieved when the nano particles were selectively dispersed in the matrix or at the interface [16, 19]. Different mechanisms are deduced to explain the morphological changes in polymer blends with nanoparticles. Some authors suggested that the active surface of nanoparticles is important to decrease the interfacial tension between the two polymer phases. Some believed that the increase of coalescence barriers between dispersed droplets was owing to the rigid layer formed by nanoparticles [20,21]. When intro ducing nanoparticles into polymer blends, the final equilibrium morphology, the preferable distribution of the nanoparticles within the blends and the dispersion state of nanoparticles cannot be achieved
* Corresponding author. E-mail address: [email protected] (L. Gong). https://doi.org/10.1016/j.polymertesting.2020.106445 Received 5 November 2019; Received in revised form 31 January 2020; Accepted 15 February 2020 Available online 23 February 2020 0142-9418/© 2020 Published by Elsevier Ltd.
L. Gong et al.
Polymer Testing 86 (2020) 106445
immediately due to the high viscosity of polymers [19]. Accompanied by the micro-structural evolution of the dispersed droplets, the migration of nanoparticles from initial phase to the selective phase is proved by the experimental observations in polymer blends, if the nanoparticles not in the selective phase initially [22–25]. In other words, during nano particles transferring from one phase to another, the dispersed droplets deform, elongate, orientate, breakup and finally relax into a stable sphere simultaneously. The mechanism on the transfer of nanoparticles from one phase to another [26–28], and the influence of the geometry of nanoparticles on the nanoparticle migration [22] have been widely explored. However, it is unclear that how the micro-structural evolution of dispersed droplets is with the consideration of nanoparticles’ migration, because the two processes are hard to separate. As far as we know, the migration of nanoparticles has not been investigated as a considerable factor for the micro-structural evolution of dispersed droplets during the melt blending. It is crucial to understand the dependence of the evolution of the dispersed droplets on the migration of rigid nanoparticles from the dispersed phase to the matrix. In our study, a simple model blend including CaCO3 nanoparticle-filled Ethylene-Propylene-Diene Mono mer (EPDM) droplets blending with polypropylene (PP) melt, is pro posed for the investigation in the influence of the nanoparticles migration on the micro-structural evolution of EPDM droplets during extruding. Such system is introduced to evaluate the final morphology by interfacial tension of ternary blend in melt blending situation [29,30] and to study the dynamics of nanoparticles movement from the dispersed phase into the matrix. On the one hand, such route could ensure a pre distribution of CaCO3 nanoparticles in the dispersed phase; on the other hand, it could provide the possibility of migration of nanofillers from dispersed phase to matrix. Furthermore, the depen dence of the breakup mechanism of a single droplet on the composition of nanoparticles can be investigated. The effect of nanoparticle incor poration on the viscoelastic and interfacial properties, and the breakup and relaxation process of dispersed droplets during melt blending were studied in this paper.
representation of the screw configuration is shown in Fig. 1. In this study, the outer diameter D2 of the screw is 22.5 mm, and the inner diameter D1 is 21 mm. Besides, the channel depth h and the screw width b are equal to 1.5 mm and 0.5 mm respectively. The distance from the root to the center of each channel (along the screw helix) can be expressed as (n-1/2)S, where S is the screw pitch (in this case, S ¼ 12 mm), and n represents the number of the screw channel. Specimens were collected from the center of each channel after the melting of solid (from n ¼ 10). 2.3. Microscopy Cross sections of extruded samples perpendicular to the flow direc tion were obtained in liquid nitrogen. The samples were then etched in dimethylbenzene for 2 h, and cleaned with acetone for several times. The etched samples were air-dried for 24 h at room temperature. The dried samples were coated by a thin layer of gold and the surface to pographies of the samples were characterized by JSM-5900LV scanning electron microscopy (SEM) with an accelerating voltage of 20 kV. The particle sizes of the droplets were enumerated from each SEM image using Image Pro Plus (IPP). The length (L) and width (B) of the elliptical droplet were determined using image analysis software for SEM images. An equivalent diameter, d, is defined for each particle as the diameter of a circle with the same cross-sectional area [31]: n pffiffiffiffiffiffiffiffi P Bi Li
d ¼ i¼1
n
where n is the number of droplets. 3. Results and discussion 3.1. Results The aspect ratio, AR ¼ L/B is introduced to describe the deformation degree of a moderated or highly stretched droplet, where L and B are the length and width of the droplet, respectively. When the AR is above a critical aspect ratio (ARcr), the thread-like droplet would break up into smaller droplets due to Rayleigh instability. In contrast, when the AR is below the critical value, the droplet would retract back to a spherical shape. For intermediate AR, the droplet would break up from the ends into smaller droplets (ending-pinching) [16,32,33]. Fig. 2 describes the breakup and relaxation process of pristine PP/ EPDM and PP/EPDM/CaCO3 (2 wt%) during extrusion process. Fig. 3 describes d and AR of pristine PP/EPDM and PP/EPDM/CaCO3 with varying CaCO3 composition as a function of extrusion distance (D), respectively. Apparently, PP is served as the continuous phase, while the Etched holes and the bright smaller particles belong to EPDM dispersed droplets and CaCO3 nanoparticles, respectively. For the pristine PP/
2. Experimental section 2.1. Materials Polypropylene (PP, T30S) with a melt flow rate (MFR) of 2.6 g/10 min and density of 0.91 g/cm3 (based on ASTM D1505-68) was supplied by Lanzhou Petrochemical Co. Ltd, China. EPDM (Nordel 4725p) with Mw of 135,000 g/mol was purchased from Dow Elastomers L.L.C., USA, which consists of 70% ethylene, 25% propylene and 4.9% ethylidene norbornene (ENB). CaCO3 nanoparticles was purchased from Huaxin Nanomaterial Co. Ltd, China, modified with stearic acid and particle diameter of 10–40 nm particle diameter. 2.2. Mixing procedures Two-step composite system was introduced to investigate the influ ence of CaCO3 nanoparticles migration on the microstructural evolution in polymer blends. CaCO3 nanoparticles was dispersed in EPDM to obtain EPDM/CaCO3. Three EPDM/CaCO3 with different composition CaCO3 were studied: 0.5 wt%, 2 wt%, and 4 wt%, respectively. PP was then mixed with each EPDM/CaCO3 using twin-screw extruder on a constant mass ratio of 80:20 (CTE35, KEYA Company, Ltd, China), yielding EPDM/PP/CaCO3. EPDM/PP blend without CaCO3 was extruded as the control. Three-stage setting temperature was adopted from the stage of feeding to melting: 175 � C, 200 � C and 200 � C. The die temperature was 5 � C lower than that of melting zone. The screw was quenched immediately and pulled out rapidly when the melts were blended homogeneously. The effects of CaCO3 transfer on the morphological evolution of dispersed droplet for EPDM/PP blend was studied with identical screw configuration. The schematic
Fig. 1. The screw configuration used in this study. 2
L. Gong et al.
Polymer Testing 86 (2020) 106445
Fig. 2. SEM micrographs of PP/EPDM blend (a) and two-step PP/EPDM/CaCO3 composites (b) respectively at different position along the extruder axis.
Fig. 3. The equivalent diameter (a) and aspect ratio (b) at different positions along the extruder axis for PP/EPDM/CaCO3 composites with varying CaCO3 contents.
EPDM, the thread-like fibril droplets dispersed in PP melt at the begin ning of the extrusion (Fig. 2a, extrusion distance, D ¼ 126 mm), and subsequently broke up owing to Rayleigh instability (capillary-wave instability) at 174 mm spot, where the droplets reached the ultimate strength (Fig. 3, d ¼ 7.53 μm, AR ¼ 5.91). The droplets then began to retract back to ellipse-like or spherical droplets with an average diam eter of 0.98 μm at 248 mm spot and eventually stabilized to spherical droplets with an average diameter of 0.41 μm at 280 mm. It is interesting to find out that there was a dramatic change in the breakup process of dispersed droplets for the blends with CaCO3 nanoparticles. Multifarious morphology of EPDM droplets, e.g. slender fibril, short fibril, and sphere, was observed in the PP/EPDM/CaCO3 (2 wt%) nanocomposites at the beginning of extrusion (Fig. 2b). The EPDM droplets present both a smaller d (1.66 μm) and a shorter AR (5.21) at 114 cm compared to these of pristine PP/EPDM. The equilibrium between breaking and retracting of dispersed droplets was reached at 150 mm for the PP/ EPDM/CaCO3 (2 wt%) nanocomposites, much quicker than that of pristine PP/EPDM. The EPDM dispersed droplets were also observed to stabilize at 162 mm with an average diameter of 0.39 μm in PP/EPDM/ CaCO3 (2 wt%) nanocomposites. In comparison with pristine PP/EPDM blend, a slight change in AR of EPDM droplets was observed in the
composites with 0.5 wt% CaCO3 nanoparticles (Fig. 3). Both d and AR reduced upon increasing composition of CaCO3 nanoparticles. These results imply that the addition of CaCO3 nanoparticles affected the shape stability and d of EPDM dispersed droplets remarkably. An improvement in the breakup dynamics of EPDM droplets was observed with the addition of CaCO3 nanoparticles. The total breakup distance of EPDM droplets to reach the equilib rium state varying with the composition of CaCO3 nanoparticles is shown in Fig. 4. The total breakup distance for EPDM droplets from the very beginning to spherical particles (AR ¼ 1) reduced after introducing CaCO3 nanoparticles into the PP/EPDM blend. In general, the higher the content of CaCO3 nanoparticles, the shorter the total breakup distance (AR ¼ 1). It was observed that higher concentration of CaCO3 nano particles has much little effect on the total breakup distance of EPDM droplets. The total breakup distance of EPDM dispersed droplets upon the addition of 0.5 wt%, 2 wt% and 4 wt% of CaCO3 nanoparticles reduced to 174 mm, 150 mm, and 138 mm, respectively, compared to 248 mm of pristine PP/EPDM blends. This result of total breakup dis tance implies that the deformation and breakup time for droplets of EPDM/PP filled with CaCO3 nanoparticles was remarkably shorter than that of pristine PP/EPDM blend. Interestingly, the breaking dynamics of 3
L. Gong et al.
Polymer Testing 86 (2020) 106445
Fig. 4. Distance of EPDM droplets broken to spherical particles (AR ¼ 1) varying with the CaCO3 contents in the PP/EPDM/CaCO3 composites.
Fig. 5. Viscosities of pure PP, pure EPDM and their binary composites at 200 � C plotted as a function of frequency.
dispersed droplets was drastically sped up in the PP/EPDM/CaCO3 (0.5 wt%) composites. In brief, the results demonstrated that the total breakup distance and the shape stability of EPDM droplets were signif icantly reduced by increasing the composition of CaCO3 nanoparticles.
where p is defined as p ¼ ηd/ηm; and, ηd is the viscosity of dispersed phase. Rheological measurements (Fig. 5) show that there was no obvious variation in the complex viscosities for PP/CaCO3 and EPDM/CaCO3 compared with that for PP and EPDM respectively. Results also indicated that little interactions between CaCO3 nanoparticles and the polymer chains when a low composition of CaCO3 nanoparticles was added [36]. Guan Gong observed that high composition of CaCO3 (at least 15 wt%) can reduce polymer molecular mobility [37]. Fig. 5 demonstrates that the viscosity ratio,p is equal to 0.624 in PP/EPDM blend under our shear rate (150 s 1). A slight variation of p to 0.517 was observed in two-step composites. According to Equation (5), the Cacrit of all samples in this study is almost equivalent to 0.45. Based on Grace theory [6], for the range of p of all samples in this study, the droplet breaks from the middle part gradually until two sub-droplets were formed when Cacrit < Ca in single flow field (uniaxial stretch or simple shear). The speculation of breakup mechanism of the dispersed droplet was verified by SEM micrographs (Fig. 2). According to Equation (1), the critical equivalent diameter, Rcrit, of a dispersed droplet is expressed as follows:
3.2. Discussion The competitive relationship of interfacial tension and viscous stresses can be expressed by the capillary number, Ca (Equation (1)): (1)
Ca ¼ ηm RG=σ
where, ηm is the viscosity of matrix, G is the shear rate, and σ is the interfacial tension. Ca represents the ratio of the relaxation time of interfacial tension (ηmR/σ) to the deformation time induced by shear (G 1). It is a dimensionless measurement for the size of droplet under a fixed shear rate [5]. The shear rate of twin-screw extruder can be evaluated by Equation (2), which has been established by Burkhardt et al. [34]: � G¼ðν0 =hÞ½ðh2 2πDnηÞðdP=dzÞf2y=h 1gþcosθ�þð2ν0 =hÞf2 ð3y=hÞgsinθ (2)
Rcrit ¼
where the first and second brackets on the right are affected by the down-channel flow and cross flow, respectively; D is the screw diameter; v0 ¼ πDn is the linear circumferential screw velocity; n is the screw revolution per minute; h is the channel depth; θ is the helix pitch angle; dP/dz is the pressure gradient in the z direction (along the screw helix); y is the channel depth when y ¼ 0 on the barrel surface; and, η is the melt viscosity. Equation (2) can be deduced to Equation (3) due to (h2/ 2πDnη)(dP/dz)((2 y/h)1] ≪cosθ under the present shear rate of the screw:
γ AB ¼ γ A þ γB
A rotational speed of 150 rpm was employed for this study; thus, the shear rate induced by the twin screw was 150 s 1. When the Ca of a droplet is below the critical capillary number, Cacrit, the droplet main tains steady orientation and shape. In contrast, the droplet breaks up eventually when it has a Ca above the Cacrit. The relationship between Cacrit and viscosity ratio, p was expressed by de Bruijn [35] (Equation (4)): log Cacrit ¼
0:506
0:0995 log p þ 0:124ðlog pÞ2
log p
0:115 log 4:08
(5)
When the equivalent diameter, R is less than Rcrit, the deformation and orientation of the droplet remain similar. On the other hand, the droplet breaks up eventually when R is larger than Rcrit. The Rcrit is proportional to Cacrit and interfacial tension, respectively, but inversely proportional to the viscosity of the matrix or shear rate. The values of surface tension, dispersion and polar components are listed in Table 1. The interfacial tension of each pair can be calculated from surface tension by using the geometric mean equation deduced by Wu [38]:
(3)
G�n
Cacrit σ ηm G
1=2
2ðγdA γdB Þ
1=2
ðγ pA γ pB Þ
(6)
where γ ¼ γdþγp, γ is surface tension, d is dispersion component and p is polar component; γAB is the interfacial tension; and, γA and γB are the surface tensions of the two materials in contact. The calculated inter facial tensions of all possible polymer/elastomer are also given in Table 1. An obvious reduction of interfacial tension was demonstrated from 1.7 mN/m to less than 0.92 mN/m after the introduction of CaCO3 nano-particles to PP/EPDM blend. This result is in good agreement with the result of PA6/PS/SiO2 system reported by Kong et al. [16]. The interfacial tension of PP and EPDM were slightly varied by increasing
(4)
4
L. Gong et al.
Polymer Testing 86 (2020) 106445
Table 1 Summary of interfacial tension of each pair. Polymer
γ(mN/ m)
γp(mN/ m)
γd(mN/ m)
Interfacial tension σ with PP(mN/m)
PP EPDM EPDM/CaCO3 20/0.5 20/2 20/4
40.04 32.19 35.00
0.26 1.22 0.94
39.79 30.98 34.06
1.7 0.82
34.25 35.17
0.91 1.01
33.34 34.16
0.92 0.86
the composition of CaCO3 nanoparticles. According to the report of Kong et al. [16], the change of interfacial tension relies on the concentration of nanoparticles at the interface between the matrix and the dispersed drops [26]. Results demonstrated that the composition of CaCO3 nano particles at the interface may be close to saturation when 0.5 wt% of CaCO3 nanoparticles was loaded and increase in composition of CaCO3 nanoparticles would probably exert no effect on the interfacial tension. Based on Equation (5), the theoretical values of Rcrit for PP/EPDM blends filled with 0 wt%, 0.5 wt%, 2 wt% and 4 wt% of CaCO3 nano particles were calculated to be 13.75 μm, 6.63 μm, 7.44 μm and 6.96 μm, respectively while the experimental Rcrit values for the samples were calculated to be 7.53 μm, 2.31 μm, 1.66 μm and 0.89 μm, respectively (Fig. 3a). The reason of the difference between the theoretical Rcrit and the experimental Rcrit is unclear and probably due to the influence of nanoparticles migration from dispersed phase to matrix. However, both the theoretical and experimental Rcrit imply that the breakup dynamic of deformed droplets was sped up by introducing CaCO3 nanoparticles into PP/EPDM blend (Fig. 2). The transfer of CaCO3 from EPDM dispersed droplets to PP matrix was appeared voluntarily accompanied by the evolution of the dispersed droplets, when EPDM/CaCO3 master batch compounded with PP melt in the twin screw extruder. According to Elias’s theory, the velocity field inside the dispersed droplet caused by shear was dominant one among the three migration mechanisms, when nanoparticles tend to transfer from the dispersed droplets to the matrix. In the control of such mech anism, the migration was divided into three stages. In the first stage, CaCO3 nanoparticles migrated from interior of dispersed droplets to two-phase interface induced by the shear. The moving of the nano particles inside the droplet might bring an unstable capillary wave, which probably led to the subsequent break up of the deformed droplet. And then the rigid nanoparticles spent some time at the interface until the chains of dispersed phase were desorbed gradually from the CaCO3 surface and were totally substituted by the PP chains [18], which could be belonged to the second stage. After CaCO3 particles move to the interface induced by shear field, the chains of the preferential distri bution phase (PP) near the interface will be close to the CaCO3 surface and subsequently replace those of EPDM. A defect formed within the elongated dispersed droplet is resulted from a thermodynamically un stable curvature as shown in Fig. 6. The defect is probably evolved to a initial craze under the shear, which lead to the final breakup of the deformed dispersed droplet due to the poor interface between nano particles and the dispersed phase. It is primly payed the responsibility to smaller aspect ratio and equivalent diameter of the dispersed droplet in two-step composites than that in binary system during the breakup process of the dispersed droplet, especially for the one with lower con centration of CaCO3. however, it is still unclear why the aspect ratio and equivalent diameter of the two-step composites decreased with the in crease in CaCO3 nanoparticles. In the third stage, the CaCO3 nanoparticles crossed the interface into matrix. Andreas et al. deduced a low migration velocity and a high interfacial stability between the two polymers for nanoparticles with relative low aspect ratio, such as carbon black (CB) [22]. It means that CaCO3 nanoparticles would spend quite a lot of time at the interface during the second stage, much longer than the time of the first and third stages.
Fig. 6. Nano-CaCO3 at the PP/EPDM blend interface during extruding.
The second migration mechanism is described for PP/EPDM/CaCO3 nanocomposites: the exposure of CaCO3 nanoparticles to cross-sections during the breakup of EPDM droplets. The aggregates of CaCO3 nano particles play a role as stress concentration points during the breakup process of EPDM droplet. A great quantity of deformed dispersed droplets which broke up at the CaCO3 aggregates were observed in the two-step composites with 2 wt% and 4 wt% CaCO3 nanoparticles, but absent in the one with 0.5 wt% nanoparticles. A schematic drawing representing the second migration mechanism for CaCO3 nanoparticles during the breakup process is shown in Fig. 7. Orientation and defor mation of EPDM dispersed droplets were probably happened under shear. However, the CaCO3 aggregates in EPDM dispersed droplets could hardly follow the deformation of the droplets, which would acted as stress concentration points under load owing to the poor interfacial adhesion between CaCO3 and EPDM, and then lead to the breakup of the droplets. Liu et al. [39] discovered that the presence of large SiO2 ag glomerates within the PP matrix depressed the elongation of droplets at break. Lee et al. [40] indicated a poor interaction between the non-polar polyolefin matrix and the filler. Such viewpoints are in accordance with the cold-drawing theory of inorganic particles filled with brittle poly mers. This implication may be the major reason for the decreasing trend in aspect ratio and equivalent diameter of the two-step composites with increasing composition of CaCO3 (�2 wt%).
Fig. 7. SEM micrographs of PP/EPDM/CaCO3 (80/20/2) composites prepared by two-step extrusion at 126 mm along the extruder axis (a) (b) and schematic diagram for the breakup of dispersed droplets. 5
L. Gong et al.
Polymer Testing 86 (2020) 106445
4. Conclusions
[10] S. Comas-Cardona, C.L. Tucker III, Measurements of droplet deformation in simple shear flow with zero interfacial tension, J. Rheol. 45 (1) (2001) 259–273, 1978present. [11] S. Guido, M. Minale, P.L. Maffettone, Drop shape dynamics under shear-flow reversal, J. Rheol. 44 (6) (2000) 1385–1399, 1978-present. [12] H. Yamane, et al., Observation of deformation and recovery of poly (isobutylene) droplet in a poly (isobutylene)/poly (dimethyl siloxane) blend after application of step shear strain, J. Rheol. 42 (3) (1998) 567–580, 1978-present. [13] M. Walters, D. Keyte, Heterogeneous structure in blends of rubber polymers, Rubber Chem. Technol. 38 (1) (1965) 62–75. [14] F.Y. Liu, L.M. Feng, et al., Segregated polypropylene/cross-linked poly(ethyleneco-1-octene)/multi-walled carbon nanotube nanocomposites with low percolation threshold and dominated negative temperature coefficient effect: towards electromagnetic interference shielding and thermistors, Compos. Sci. Technol. 159 (2018) 152–161. [15] C. Zhang, et al., Multi-walled carbon nanotube in a miscible PEO/PMMA blend: thermal and rheological behavior, Polym. Test. 75 (2019) 367–372. [16] M. Kong, et al., Retarded relaxation and breakup of deformed PA6 droplets filled with nanosilica in PS matrix during annealing, Polymer 52 (22) (2011) 5231–5236. [17] T. Merkel, et al., Emulsification of particle loaded drops in simple shear flow, Colloid. Surface. Physicochem. Eng. Aspect. 470 (2015) 179–187. [18] Y. Pan, X. Liu, X. Hao, D.W. Schubert, Conductivity and phase morphology of carbon black-filled immiscible polymer blends under creep: an experimental and theoretical study, Phys. Chem. Chem. Phys. 18 (2016) 32125–32131. [19] L. Elias, et al., Morphology and rheology of immiscible polymer blends filled with silica nanoparticles, Polymer 48 (20) (2007) 6029–6040. [20] P. Clegg, Fluid-bicontinuous gels stabilized by interfacial colloids: low and high molecular weight fluids, J. Phys. Condens. Matter 20 (11) (2008) 113101. [21] G. Filippone, et al., The role of organoclay in promoting co-continuous morphology in high-density poly (ethylene)/poly (amide) 6 blends, Polymer 49 (5) (2008) 1312–1322. [22] A. G€ oldel, et al., Shape-dependent localization of carbon nanotubes and carbon black in an immiscible polymer blend during melt mixing, Macromolecules 44 (15) (2011) 6094–6102. [23] Y. Li, H. Shimizu, Conductive PVDF/PA6/CNTs nanocomposites fabricated by dual formation of cocontinuous and nanodispersion structures, Macromolecules 41 (14) (2008) 5339–5344. [24] O. Meincke, et al., Mechanical properties and electrical conductivity of carbonnanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene, Polymer 45 (3) (2004) 739–748. [25] D. Wu, et al., Selective localization of multiwalled carbon nanotubes in poly (ε-caprolactone)/polylactide blend, Biomacromolecules 10 (2) (2009) 417–424. [26] L. Elias, et al., Immiscible polymer blends stabilized with nano-silica particles: rheology and effective interfacial tension, Polymer 49 (20) (2008) 4378–4385. [27] R. Stark, M. Kappl, H.-J. Butt, Interaction of solid surfaces across binary mixtures of polymer melts, Macromolecules 40 (11) (2007) 4088–4091. [28] K. David, N. Dan, R. Tannenbaum, Competitive adsorption of polymers on metal nanoparticles, Surf. Sci. 601 (8) (2007) 1781–1788. [29] C. Ma, M. Zhang, M. Rong, Morphology prediction of ternary polypropylene composites containing elastomer and calcium carbonate nanoparticles filler, J. Appl. Polym. Sci. 103 (3) (2007) 1578–1584. [30] H. Yang, et al., Largely improved toughness of PP/EPDM blends by adding nanoSiO 2 particles, Polymer 48 (3) (2007) 860–869. [31] X.-h. Chen, et al., Study on morphology evolution and fractal character of the miscible blend between isotactic polypropylene and copolymer of ethylene and propylene, Polymer 50 (14) (2009) 3347–3360. [32] H.A. Stone, Dynamics of drop deformation and breakup in viscous fluids, Annu. Rev. Fluid Mech. 26 (1) (1994) 65–102. [33] G.I. Taylor, The viscosity of a fluid containing small drops of another fluid, in: Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 1932, pp. 41–48. [34] K. Burkhardt, H. Hermann, S. Jakopin, Plasticizing and mixing principles of intermeshing co-rotating and counter-rotating twin-screw extruders, SPE ANTEC Tech. Papers 24 (1978) 498–506. [35] R.A. Debruijn, Deformation and Breakup of Drops in Simple Shear Flows, 1991. [36] G. Lei, et al., Morphology and properties of PP/EPDM binary blends and PP/EPDM/ nano-CaCO3 ternary blends, J. Appl. Polym. Sci. 123 (1) (2012) 510–519. [37] G. Guan, et al., Dynamic rheological behavior of isotactic polypropylene filled with nano-calcium carbonate modified by stearic acid coating, J. Macromol. Sci. 48 (2) (2009) 329–343. [38] S. Wu, J. Macromol. Sci. Rev. Macromol. Chem. 10 (1) (1974) 1–73. [39] L. Yiqun, et al., The structure and physical properties of polypropylene and thermoplastic olefin nanocomposites containing nanosilica, Polymer 47 (22) (2006) 7731–7739. [40] L. Hyuk-soo, et al., TPO based nanocomposites. Part 1. Morphology and mechanical properties, Polymer 46 (25) (2005) 11673–11689. [41] A. G€ oldel, G. Kasaliwal, P. P€ otschke, Selective localization and migration of multiwalled carbon nanotubes in blends of polycarbonate and poly (styreneacrylonitrile), Macromol. Rapid Commun. 30 (6) (2009) 423–429.
The transfer of rigid fillers (MWCNTs, SiO2, CaCO3) from the dispersed phase to matrix during melt blending and its influence on the final morphologies of the ternary composites, and the possible mecha nism deduced to the phenomenon had been studied previously [41]. However, the effect of the migration of CaCO3 nanoparticles from the dispersed phase to the matrix on the micro-structural evolution of dispersed droplets has not been explored during extrusion blending of polymer blend with nanoparticles [18]. The breakup process of EPDM dispersed droplets was improved dramatically and the breakup mecha nism was changed under the transfer of CaCO3 nanoparticles from EPDM phase to PP melt, as compared to that of pristine PP/EPDM blend. The total breakup time and the shape stability of EPDM droplets were reduced by increasing the composition of CaCO3 nanoparticles. The key reason is the reduction in interfacial tension caused by the introduction of CaCO3 nanoparticles for the two-step composites with low content of CaCO3 (<2 wt%). While the CaCO3 aggregates in deformed EPDM droplets acted as the stress concentration points are mainly responsible for the reduced maximum equivalent diameter d and aspect ratio AR of EPDM droplets when the composition of CaCO3 was high (�2 wt%). The reduction in interfacial tension becomes an unimportant cause in this situation. Deeper understanding on the micro-structural evolution in ternary composites during melt blending in conventional extruder is important. In brief, the migration of nanoparticles promotes the development of functional polymer materials with high price-performance ratio. CRediT authorship contribution statement Lei Gong: Conceptualization, Methodology, Software, Writing original draft, Supervision. Shu-hua Chen: Data curation, Software. Yang Yu: Visualization, Investigation. Bo Yin: Writing - review & editing. Ming-bo Yang: Writing - review & editing, Supervision. Acknowledgements The authors gratefully acknowledge the financial support from the Science and Technology Star Program of Dalian (Contract No. 2017RQ039). References [1] Y.S. Lipatov, Polymer blends and interpenetrating polymer networks at the interface with solids, Prog. Polym. Sci. 27 (9) (2002) 1721–1801. [2] Y.M. Pan, X.H. Liu, et al., Reversal phenomena of molten immiscible polymer blends during creep-recovery in shear, J. Rheol. 61 (2017) 759–767. [3] G. Jiang, H. Wu, S. Guo, Reinforcement of adhesion and development of morphology at polymer–polymer interface via reactive compatibilization: a review, Polym. Eng. Sci. 50 (12) (2010) 2273–2286. [4] M. Sumita, et al., Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black, Polym. Bull. 25 (2) (1991) 265–271. [5] C.L. Tucker III, P. Moldenaers, Microstructural evolution in polymer blends, Annu. Rev. Fluid Mech. 34 (1) (2002) 177–210. [6] H.P. Grace, Dispersion phenomena in high viscosity immiscible fluid systems and application of static mixers as dispersion devices in such systems, Chem. Eng. Commun. 14 (3–6) (1982) 225–277. [7] J. De Bruijn, et al., Determination of octanol/water partition coefficients for hydrophobic organic chemicals with the “slow-stirring” method, Environ. Toxicol. Chem. 8 (6) (1989) 499–512. [8] B. Bentley, L. Leal, An experimental investigation of drop deformation and breakup in steady, two-dimensional linear flows, J. Fluid Mech. 167 (1986) 241–283. [9] A.S. Almusallam, R.G. Larson, M.J. Solomon, A constitutive model for the prediction of ellipsoidal droplet shapes and stresses in immiscible blends, J. Rheol. 44 (5) (2000) 1055–1083, 1978-present.
6