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polyethylene alloy foams by supercritical CO2 foaming and analysis by X-ray microtomography
J. of Supercritical Fluids 82 (2013) 50–55
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Preparation of microcellular polystyrene/polyethylene alloy foams by supercritical CO2 foaming and analysis by X-ray microtomography Zhe Xing a,b , Mouhua Wang a , Guohao Du a , Tiqiao Xiao a , Weihua Liu a , Qiang Dou a , Guozhong Wu a,∗ a b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 9 December 2012 Received in revised form 31 May 2013 Accepted 6 June 2013 Keywords: Polystyrene Polyethylene Polymer alloy Supercritical carbon dioxide Microcellular foam X-ray microtomography
a b s t r a c t Non-destructive X-ray microtomography at a third generation synchrotron facility was applied to analyze the cell morphology of microcellular polystyrene (PS)/polyethylene (PE) alloy foams. This method, differing from the observation of cross section of cell by SEM, enables one to observe a complete cell structure in the polymer foam. PS/PE foams were prepared using a supercritical CO2 foaming process. A styrene–ethylene–butylene–styrene (SEBS) copolymer was used as the compatibilizer of PS and PE to improve the cell morphology. The effects of PS/PE composition and foaming conditions (temperature and pressure) on the cell structure of foams were investigated in detail. The optimal SEBS content for the foaming of PS/PE (70:30) alloys was found to be 5 wt%. The cell size and cell density were also dependent on the foaming temperature and the saturation pressure. © 2013 Published by Elsevier B.V.
1. Introduction Polystyrene (PS) and polyethylene (PE) are two of the most widely consumed commodity polymers [1]. PS has been widely used for the production of foam material due to its high modulus, excellent dimensional stability and thermal insulation. PE has excellent properties such as good toughness, elasticity and processability. It is often used to blend with the PS matrix to improve its ductility and impact performance [2,3]. Many studies have been performed on PS/PE blends since the 1970s. Unfortunately, PS/PE blends are mutually immiscible or incompatible due to very low entropy of mixing and mostly positive enthalpy of mixing [4,5]. They typically exhibit coarsening and instability of phase morphology due to weak interfacial adhesion between the two phases [6]. The weak phase interfacial adhesion of PS and PE blends leads to poor mechanical properties [7,8], an unattractive appearance and poor foaming behavior. The miscibility between PS and PE can be improved by the addition of a compatibilizer. Block copolymers with sequences chemically identical or similar to the blend components are typically used as compatibilizers, such as styrene–butadiene (SB) [9–13], styrene–butadiene–styrene (SBS) [14–16] and styrene–ethylene–butylene–styrene (SEBS) [4,5,17,18]. In the present work, SEBS was chosen as the
∗ Corresponding author. Tel.: +86 21 39194531. E-mail address: [email protected] (G. Wu). 0896-8446/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.supflu.2013.06.003
compatibilizer because it contains chain blocks with chemical structural units similar to LDPE and PS. Microcellular foams of PS/PE alloys can be applied in building, packaging, sports and leisure articles due to their good mechanical properties, chemical resistance, low thermal and electrical conductivity and good sound insulation. For the preparation of microcellular PS/PE alloy foams, supercritical carbon dioxide (scCO2 ) is an ideal and clean blowing agent because CO2 is chemically stable and non-toxic. Moreover, it has a mild critical temperature (31 ◦ C), moderate critical pressure (7.38 MPa) and a relatively high solubility in polymers. In previous studies, scCO2 has been used to produce various polymer alloy foams, including poly(methyl methacrylate)/poly(l-lactic acid) (PMMA/PLA) [19], poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/ PMMA) blends [20,21], poly(methyl methacrylate)/poly(methylmethacrylate-co-butylacrylate-co-methylmethacrylate) (PMMA/ MAM) blends [22], polystyrene/poly(styrene-co-butadiene-comethylmethacrylate) (PS/SBM) blends [22], PS/PMMA blends [23], polyethylene/polypropylene (PE/PP) blends [24,25] and polypropylene/polystyrene (PP/PS) blends [26]. However, the study of foaming of PS/PE alloys with scCO2 has seldom been reported, probably due to the poor compatibility of PS and PE. In this paper, PS/PE alloys were foamed using a scCO2 foaming process, and the section morphology of the PS/PE foams was characterized by high-resolution X-ray computed tomography at the X-ray imaging and biomedical application station (BL13W1) of the Shanghai Synchrotron Radiation Facility. X-ray microtomography
Z. Xing et al. / J. of Supercritical Fluids 82 (2013) 50–55
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Fig. 1. SEM microphotography of the PS/PE (70:30) foams with SEBS of 0 and 5 wt%.
(XMT) is a non-destructive and three-dimensional imaging technique in which a series of radiographic images is used to reconstruct a 3D intensity distribution map of a specimen’s X-ray absorption [27,28]. It can be used to reconstruct interior structural details with high resolution [29,30]. The cell structures of PS/PE foams were compared in detail, and the effects of PS/PE ratio and SEBS addition on cell size, cell density and the dimensional uniformity of the cells were also investigated. The cell morphology of PS/PE foams with the SEBS compatibilizer is significantly improved compared with the foam without SEBS. The PS/PE alloy foams with SEBS have a more uniform cell size distribution, smaller mean cell diameter and higher cell density. The optimal SEBS content for the foaming of PS/PE alloys was found to be 5 wt% after a comparison of the cell size distribution of the samples with various SEBS content. 2. Experimental 2.1. Materials Polystyrene (GPPS 525) pellets, supplied by Sinopec Guangzhou Company, have a density of 1.04 g/cm3 , melt flowing index of 8.0 g/10 min and Tg of 97 ◦ C. LDPE (LD100-AC) pellets, supplied by Sinopec Beijing Yanshan Company, have a density of 0.9225 g/cm3 , melt flowing index of 2.0 g/10 min and Vicat softening temperature of 90 ◦ C. The SEBS copolymer (Kraton G1652), purchased from Shell Chemical Co., Ltd., has a melt flowing index of 1.0 g/10 min, and Mw values of the EB (ethylene/butylene) and the PS blocks are 35,000 and 7500. Carbon dioxide (>99.5% purity) was obtained from Loutang Special Gases of Shanghai. 2.2. Mixing and foaming process Binary and ternary blends of various compositions were blended at 200 ◦ C in a two-screw mixer (Thermo Haake PolyDrive, Germany). The PS/PE blending weight ratios were 90/10, 80/20, 70/30, 60/40 and 50/50. The proportion of SEBS in the PS/PE (70:30) blends was 0, 2, 5 and 10 wt%. After mixing all the materials, the blends were hot pressed for 5 min at 160 ◦ C and a pressure of 12 MPa into thin sheets (1 mm in thickness) using a plate vulcanizing machine. The PS/PE alloy sheets were foamed, as previously described [31]. The soaking time and depressurization rate were 8 h and 0.7 MPa/s, respectively. Five different soaking temperatures (80, 90, 100, 105 and 110 ◦ C) and three different saturation pressures (15, 20 and 25 MPa) were used in the foaming experiments.
2.3. X-ray microtomography The XMT tests were performed at the X-ray imaging and biomedical application station (BL13W1) of the Shanghai Synchrotron Radiation Facility. The foam sample was cut into a prism with a length of 5 mm and a diameter of 1 mm and fixed onto a thin plastic pipe. The plastic pipe was mounted on a multi-dimensional platform with a rotator to position the sample and to rotate around the beam. The PS/PE foam was irradiated by a synchrotron radiation X-ray beam with photon energy of 18 keV that passed through a slit with a length of 45 mm and a width of 5 mm. For each sample, approximately 15 cm of sample-to-detector distance, 2 s of exposure time and 1200 equiangular radiographic images of 1000 × 1000 pixels were acquired over a total angular range of 180◦ (angular step: 0.15◦ ). The X-ray beams through the samples were recorded using a CCD camera with an effective pixel size of 0.74 m. At the same time, dark current and reference images were also recorded with the same exposure time for flat-field corrections. 2.4. Scanning electron microscope (SEM) analysis The SEM micrographs of PS/PE foams were captured by a LEO 1530 VP scanning electron microscope at an acceleration voltage of 10.0 kV. The samples were immersed in liquid nitrogen and fractured at −196 ◦ C, then mounted on objective table. Before SEM observation, the fractured surfaces were sputter-coated with gold for observation. 2.5. Image analysis Microstructures of the PS/PE polymeric alloy foams were obtained by statistical analysis of the cell parameters. The slice images of the PS/PE foams were measured using the Image Pro software. Cell density (Nf ) is determined by the number of cells per unit volume of foam, which was calculated using Eq. (1):
Nf =
nM 2 A
3/2 (1)
where n, M and A are the number of cells in the micrograph, the magnification of the micrograph and the area of the micrograph (cm2 ), respectively. The average cell diameter was calculated using Eq. (2): D=
dn i i ni
(2)
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where ni is the number of cells with an area-equivalent diameter of di . 3. Results and discussion 3.1. Comparison of SEM and XMT for cell observation
Fig. 2. The cell size distribution of the PS/PE (70:30) foams with SEBS of 5 wt% obtaining from SEM and XMT.
It is known that PS/PE blend is immiscible and has a weak interaction between the dispersed phase and matrix. Hence, both PS/PE blend and its foam have poor mechanical performance. Fig. 1 shows the microphotographs of the section of the PS/PE (70:30) foams. As shown in Fig. 1(a), perfect cells cannot be observed for the PS/PE foam without SEBS. The cell morphology of the foam is much improved with the addition of SEBS, but broken cells still exist (as shown in Fig. 1(b)). The poor cell morphology of the PS/PE foam can be attributed to the destruction of cell structure by fracture in liquid nitrogen. Therefore, we employed in this work non-destructive XMT for the observation of cell morphology. Fig. 2 shows a comparison of cell size distributions of the PS/PE (70:30) foams with 5 wt% SEBS by using SEM and XMT. The corresponding XMT is illustrated in Fig. 3(C). Compared to the observation by SEM, the XMT analysis gives a smaller peak value of cell size and a wider size distribution. This is because the large cells are easy to crack in the sample preparation for SEM observation, due to
Fig. 3. Reconstructed slices of PS/PE (70:30) foams with different SEBS content: 0 wt% (A), 2 wt% (B), 5 wt% (C) and 10 wt% (D).
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Fig. 4. Cell size distribution of PS/PE (70:30) foams with different SEBS content.
their larger stress concentration and thinner cell wall. This can be exactly avoided in the XMT observation. 3.2. Effect of SEBS on cellular structure SEBS was added to improve the compatibility of PS and PE. An initial investigation was performed to confirm its effect on the morphology of PS/PE alloy foams. Several slice images of the PS/PE alloy foams captured through high-resolution X-ray computed tomography are shown in Fig. 3. The content of the compatibilizer in the PS/PE (70:30) foams was 0, 2, 5 and 10 wt% SEBS. The foams were prepared by saturating with scCO2 at 25 MPa and 105 ◦ C for 8 h and then quickly depressurizing to atmospheric pressure. It can be clearly observed that the cell morphology of the PS/PE foams is improved after addition of SEBS. The cells of the PS/PE foams are discrete, nearly spherical and surrounded by thick walls. Therefore, the foams have well-defined closed cells under the foaming conditions. For the foam without any SEBS (Fig. 3(A)), some large irregular bubbles or cracks can be observed, and the homogeneity of the cells is inferior. In contrast, the cell morphology of the PS/PE foams is evidently meliorated after addition of SEBS into the alloy matrix. As shown in Fig. 3(B)–(D), the size of the large bubbles and cracks decreased gradually with increasing SEBS content, and the cell size also became more uniform. For an explicit comparison of cell size, a statistical calculation of cell size and cells number was performed using the slice images. The cell size distributions of the PS/PE (70:30) foams presented in Fig. 3 were shown in Fig. 4. The width of the cell size distribution peak, which indicates the dispersity of the cell size, decreased with increasing SEBS content. The foam morphology indicates that the uniformity of the cell size is improved with the addition of SEBS, and the cell size increased with an increase in the SEBS content. It is clear that the presence of SEBS can drastically affect the nucleation and inflation of the cells, resulting in an improved cell size distribution. In heterogeneous nucleation theory, homogeneous dispersive micron-size particles are employed as nucleating agents to induce heterogeneous nucleation for the formation of a large number of nucleation sites in the early nucleation period [32]. Due to the inherent incompatibility of PE with PS, their blend (PS/PE = 70/30) is in fact a two-phase material, i.e., a soft polymer dispersed into a hard polymer matrix phase [17]. Without the SEBS triblock copolymer, the PS/PE blend has a poor interfacial adhesion between the two phases, and micro-voids are formed at the interface of the two phases [4,17]. During rapid depressurization in the foaming process, CO2 gas molecules are supersaturated in the matrix of
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Fig. 5. Cell size distribution of PS/PE foams (5 wt% SEBS) with different PS/PE content.
the polymer and nucleate. Subsequently, the nuclei grow to form bubbles by diffusion of CO2 molecular into the nuclei, however, diffusion of gas molecules into the micro-voids at the interface of the two phases is easier than nucleation. Expansion of the micro-voids prior to the nucleation therefore results in non-uniformity of the cell size and a lower cell density of foam. When SEBS is incorporated into the PS/PE blend, there is better interfacial adhesion between the two phases and a much finer and more uniformly dispersed phase [4,5]. The nucleation of CO2 gas in the matrix of the polymer is rapid and uniform due to the improvement of the compatibility of PS with PE. As a result, PS/PE foams with finer cell morphology are obtained by employing SEBS as a compatibilizer. The cell size distributions of the PS/PE foams with 2 wt%, 5 wt% and 10 wt% SEBS were compared. The PS/PE foam with 2 wt% SEBS has the smallest cell size and the largest cracks; therefore its cell uniformity is poor. The average cell size of the PS/PE foam with 5 wt% SEBS is smaller than that with 10 wt% SEBS, and the cell size distribution of the PS/PE foam with 5 wt% SEBS is narrower than that with 10 wt% SEBS. It is indicated that the cell morphology of the PS/PE foams with more than 5 wt% SEBS is not improved. The optimized proportion of SEBS in PS/EP alloys should be 5 wt% in terms of cell morphology and the size distribution of the PS/PE foam. Thus, specimens with 5 wt% SEBS were selected to investigate the effects of other factors on the cell morphology of the foams. 3.3. Effect of PS/PE weight ratio on cellular structure The PS/PE sheets with various ratios were expanded after soaking in scCO2 for 8 h at 25 MPa and 105 ◦ C. The effect of PS/PE blend ratio on the cell size distribution and the foam structure parameters can be observed in Figs. 5 and 6. The PS/PE foam with lower PS content has a larger cell size and wider cell size distribution. The average cell diameter decreased from 19 m to 13.5 m, and the cell density increased from 4.7 × 107 cm−3 to 1.1 × 108 cm−3 with increasing PS content. These results are probably due to the higher melt elasticity and viscosity of PS compared with PE at the foaming temperature [5,17], which prevents the growth and fusion of bubbles during the foaming process. The PS/PE foams with higher PS content therefore have a smaller cell size and higher cell density. 3.4. Effect of foaming condition on cellular structure The cells in the slice images of PS/PE/SEBS (70:30:5) foams prepared under different foaming temperatures were statistically
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Fig. 6. Effect of PS/PE ratio on the average cell diameter and cell density of PS/PE foams with 5 wt% SEBS. Fig. 9. Cell size distribution of the PS/PE (70:30) foams with 5 wt% SEBS produced at saturation pressures of 15, 20 and 25 MPa.
Fig. 7. Cell size distribution of PS/PE (70:30) foams with 5 wt% SEBS produced at temperatures from 80 ◦ C to 110 ◦ C.
counted, and the statistical results are illustrated in Figs. 7 and 8. With an increase in foaming temperature, the cell size and the width of the cell size distribution peak increased significantly. The average cell diameter increased from 5.2 m to 21.9 m with an increase in the temperature from 80 ◦ C to 110 ◦ C, and
Fig. 8. Effect of foaming temperature on the average cell diameter and cell density of PS/PE (70:30) foams with 5 wt% SEBS.
conversely, the cell density decreased from 2 × 109 cm−3 to 3.3 × 107 cm−3 . The weak surface tension and melt elasticity of the PS/PE blends at higher temperature lead to a weaker resistance to the expansion of the bubbles than at lower temperature. Therefore, the bubbles can expand to a larger size. Moreover, the bubbles can more readily break up and fuse due to the weak melt strength at higher temperature, leading to a decreased cell density. Figs. 9 and 10 show the pressure dependence of cell size distribution, average cell diameter and cell density of the PS/PE/SEBS (70:30:5) foam. The cell size and the width of cell size distribution peak obviously decrease with raising the saturation pressure. The average cell diameter decreases from 18.6 m to 9.2 m with the saturation pressure, and reversely the cell density increases from 2.5 × 107 cm−3 to 1.6 × 108 cm−3 . In homogeneous or heterogeneous nucleation theory [33,34], a higher saturation pressure leads to a lower nucleation energy barrier and higher cell nucleus density, and the reverse is true during the foaming process. As CO2 concentration dissolved in polymer is constant, the smaller cell size results from the higher cell nucleus density. Therefore, the cell size of the PS/PE foam is reduced and the cell density of the PS/PE foam increased at higher saturation pressures.
Fig. 10. Effect of saturation pressure on the average cell diameter and cell density of the PS/PE (70:30) foams with 5 wt% SEBS.
Z. Xing et al. / J. of Supercritical Fluids 82 (2013) 50–55
4. Conclusions Microcellular PS/PE alloy foams were prepared by a supercritical CO2 foaming process. The triblock copolymer SEBS was added to the PS/PE alloys as a compatibilizer to improve the cell morphology of the PS/PE foams. The PS/PE alloy foams with added SEBS have a smaller mean cell diameter and higher cell density than those without SEBS. However, if the SEBS content is too high, a larger cell size is induced. The optimal SEBS content for the foaming of PS/PE alloys is 5 wt%. PS/PE foams with a higher PS component have a slightly decreased cell size and increased cell density. A higher foaming temperature results in an increase in the cell diameter from 5.2 m to 21.9 m and a decrease in the cell density from 2 × 109 cm−3 to 3.3 × 107 cm−3 . The saturation pressure dependence of the cell size and density exhibited an opposite trend. With increasing the saturation pressure, the cell diameter decreased from 18.6 m to 9.2 m, and the cell density increased from 2.5 × 107 cm−3 to 1.6 × 108 cm−3 . Acknowledgements We acknowledge our colleagues at the X-ray imaging and biomedical application station (BL13W1) at the SSRF for their instructive help during experimental data collection and image reconstruction. This work was supported by the National Natural Science Foundation of China under Grant No. 11079048.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21] [22]
References [1] B. Xu, J. Simonsen, W.E. Rochefort, Mechanical properties and creep resis-
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
tance in polystyrene/polyethylene blends, Journal of Applied Polymer Science 76 (2000) 1100–1108. S.A. Xu, S.C. Tjong, Deformation mechanisms and fracture toughness of polystyrene/high-density polyethylene blends compatibilized by triblock copolymer, Journal of Applied Polymer Science 77 (2000) 2024–2033. S.A. Xu, S.C. Tjong, Effect of compatibilizer content on the tensile properties and fracture toughness of high density polyethylene/polystyrene blends, Polymer Journal 32 (2000) 208–214. T.Y. Kim, D.M. Kim, W.J. Kim, T.H. Lee, K.S. Suh, Effects of poly[styrene-b(ethylene-co-butylene)-b-styrene] on the charge distributions of low-density polyethylene/polystyrene blends, Journal of Polymer Science Part B: Polymer Physics 42 (2004) 2813–2820. M.K.R. Aghjeh, M. Khodabandelou, M. Khezrefaridi, Rheology and morphology of high impact polystyrene polyethylene blends and the effect of compatibilization on their properties, Journal of Applied Polymer Science 114 (2009) 2235–2245. M.N. Bureau, H. ElKadi, J. Denault, J.I. Dickson, Injection and compression molding of polystyrene/high-density polyethylene blends – phase morphology and tensile behavior, Polymer Engineering and Science 37 (1997) 377–390. Z. Stary, I. Fortelny, Z. Krulis, M. Slouf, Effect of the molecular structure of ethene–propene and styrene–butadiene copolymers on their compatibilization efficiency in low-density polyethylene/polystyrene blends, Journal of Applied Polymer Science 107 (2008) 174–186. T.S. George, A. Krishnan, N. Joseph, R. Anjana, K.E. George, Effect of maleic anhydride grafting on nanokaolinclay reinforced polystyrene/high density polyethylene blends, Polymer Composites 33 (2012) 1465–1472. I. Fortelny, D. Michalkova, J. Hromadkova, F. Lednicky, Effect of a styrene–butadiene copolymer on the phase structure and impact strength of polyethylene/high-impact polystyrene blends, Journal of Applied Polymer Science 81 (2001) 570–580. I. Fortelny, J. Mikesova, J. Hromadkova, V. Hasova, Z. Horak, Effect of molecular structure of styrene–butadiene block copolymers on morphology, rheological properties, and impact strength of polystyrene/polyethylene blends, Journal of Applied Polymer Science 90 (2003) 2303–2309. I. Fortelny, M. Slouf, D. Hlavata, A. Sikora, Interfacial activity of styrene–butadiene block copolymers in low-density polyethylene/polystyrene blends, Composite Interfaces 13 (2006) 783–799. I. Fortelny, M. Slouf, A. Sikora, D. Hlavata, V. Hasova, J. Mikesova, C. Jacob, The effect of the architecture and concentration of styrene–butadiene
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
55
compatibilizers on the morphology of polystyrene/low-density polyethylene blends, Journal of Applied Polymer Science 100 (2006) 2803–2816. D. Michalkova, J. Pospisil, I. Fortelny, M. Slouf, Z. Krulis, Effect of a N,N disubstituted phenylene iamine stabilizer and SB/EPDM compatibilizer on toughness and morphology of blends of pre-aged polyethylene with highimpact polystyrene, Journal of Vinyl and Additive Technology 12 (2006) 58–65. Z. Stary, Z. Krulis, J. Hromadkova, M. Slouf, J. Kotek, I. Fortelny, New multicomponent compatibilization system for polyolerin/polystyrene blends, International Polymer Processing 21 (2006) 222–229. H.Q. Xie, D.G. Liu, D. Xie, J.G. Guan, Behavior of ionomers of sulfonated styrene–butadiene–styrene triblock copolymer in polymer blends with crystalline polyolefins and as compatibilizer, Journal of Applied Polymer Science 99 (2006) 1887–1894. D. Michalkova, J. Pospisil, I. Fortelny, J. Hromadkova, F. Lednicky, P. Schmidt, Z. Krulis, Restabilization of the aging resistance of compatibilized blends of pre-aged low density polyethylene and high-impact polystyrene (LDPE/HIPS), Polymer Degradation and Stability 94 (2009) 1486–1493. A. Chirawithayaboon, S. Kiatkamjornwong, Compatibilization of polystyrene/high-density polyethylene blends by high-impact styrene/ethylene–butylene/styrene block copolymer, Journal of Applied Polymer Science 91 (2004) 742–755. Z. Bartczak, A. Galeski, M. Pluta, Ternary blends of high-density copolymers: polyethylene–polystyrene-poly(ethylene/butylene-b-styrene) properties and orientation behavior in plane-strain compression, Journal of Applied Polymer Science 76 (2000) 1746–1761. D. Velasco, L. Benito, M. Fernández-Gutiérrez, J. San Román, C. Elvira, Preparation in supercritical CO2 of porous poly(methyl methacrylate)–poly(l-lactic acid) (PMMA–PLA) scaffolds incorporating ibuprofen, The Journal of Supercritical Fluids 54 (2010) 335–341. S. Siripurapu, Y.J. Gay, J.R. Royer, J.M. DeSimone, R.J. Spontak, S.A. Khan, Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process, Polymer 43 (2002) 5511–5520. Y.-T. Shieh, T.-T. Hsiao, Morphological changes of PVDF/PMMA blends via CO2 treatments, The Journal of Supercritical Fluids 48 (2009) 64–71. J.A.R. Ruiz, M. Pedros, J.-M. Tallon, M. Dumon, Micro and nano cellular amorphous polymers (PMMA, PS) in supercritical CO2 assisted by nanostructured CO2 -philic block copolymers – one step foaming process, The Journal of Supercritical Fluids 58 (2011) 168–176. I.H. Lin, P.F. Liang, C.S. Tan, Preparation of polystyrene/poly(methyl methacrylate) blends by compressed fluid antisolvent technique, The Journal of Supercritical Fluids 51 (2010) 384–398. H.X. Huang, J.K. Wang, X.H. Sun, Improving of cell structure of microcellular foams based on polypropylene/high-density polyethylene blends, Journal of Cellular Plastics 44 (2008) 69–85. C. Liu, X.L. Jiang, T. Liu, L. Zhao, W.X. Zhou, W.K. Yuan, Multifractal analysis of the fracture surfaces of foamed polypropylene/polyethylene blends, Applied Surface Science 255 (2009) 4239–4245. H.X. Huang, H.F. Xu, Preparation of microcellular polypropylene/polystyrene blend foams with tunable cell structure, Polymers for Advanced Technologies 22 (2011) 822–829. L. Salvo, P. Cloetens, E. Maire, S. Zabler, J.J. Blandin, J.Y. Buffière, W. Ludwig, E. Boller, D. Bellet, C. Josserond, X-ray micro-tomography an attractive characterisation technique in materials science, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 200 (2003) 273–286. A.M. Trater, S. Alavi, S.S.H. Rizvi, Use of non-invasive X-ray microtomography for characterizing microstructure of extruded biopolymer foams, Food Research International 38 (2005) 709–719. P.J. Schilling, B.R. Karedla, A.K. Tatiparthi, M.A. Verges, P.D. Herrington, X-ray computed microtomography of internal damage in fiber reinforced polymer matrix composites, Composites Science and Technology 65 (2005) 2071–2078. M. Karimi, M. Heuchel, T. Weigel, M. Schossig, D. Hofmann, A. Lendlein, Formation and size distribution of pores in poly(-caprolactone) foams prepared by pressure quenching using supercritical CO2 , The Journal of Supercritical Fluids 61 (2012) 175–190. Z. Xing, G. Wu, S. Huang, S. Chen, H. Zeng, Preparation of microcellular crosslinked polyethylene foams by a radiation and supercritical carbon dioxide approach, The Journal of Supercritical Fluids 47 (2008) 281–289. W. Zhai, J. Yu, L. Wu, W. Ma, J. He, Heterogeneous nucleation uniformizing cell size distribution in microcellular nanocomposites foams, Polymer 47 (2006) 7580–7589. N.S. Ramesh, D.H. Rasmussen, G.A. Campbell, The heterogeneous nucleation of microcellular foams assisted by the survival of microvoids in polymers containing low glass transition particles. Part I. Mathematical modeling and numerical simulation, Polymer Engineering and Science 34 (1994) 1685–1697. S. Siripurapu, J.A. Coughlan, R.J. Spontak, S.A. Khan, Surface-constrained foaming of polymer thin films with supercritical carbon dioxide, Macromolecules 37 (2004) 9872–9879.
Contents lists available at SciVerse ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Preparation of microcellular polystyrene/polyethylene alloy foams by supercritical CO2 foaming and analysis by X-ray microtomography Zhe Xing a,b , Mouhua Wang a , Guohao Du a , Tiqiao Xiao a , Weihua Liu a , Qiang Dou a , Guozhong Wu a,∗ a b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 9 December 2012 Received in revised form 31 May 2013 Accepted 6 June 2013 Keywords: Polystyrene Polyethylene Polymer alloy Supercritical carbon dioxide Microcellular foam X-ray microtomography
a b s t r a c t Non-destructive X-ray microtomography at a third generation synchrotron facility was applied to analyze the cell morphology of microcellular polystyrene (PS)/polyethylene (PE) alloy foams. This method, differing from the observation of cross section of cell by SEM, enables one to observe a complete cell structure in the polymer foam. PS/PE foams were prepared using a supercritical CO2 foaming process. A styrene–ethylene–butylene–styrene (SEBS) copolymer was used as the compatibilizer of PS and PE to improve the cell morphology. The effects of PS/PE composition and foaming conditions (temperature and pressure) on the cell structure of foams were investigated in detail. The optimal SEBS content for the foaming of PS/PE (70:30) alloys was found to be 5 wt%. The cell size and cell density were also dependent on the foaming temperature and the saturation pressure. © 2013 Published by Elsevier B.V.
1. Introduction Polystyrene (PS) and polyethylene (PE) are two of the most widely consumed commodity polymers [1]. PS has been widely used for the production of foam material due to its high modulus, excellent dimensional stability and thermal insulation. PE has excellent properties such as good toughness, elasticity and processability. It is often used to blend with the PS matrix to improve its ductility and impact performance [2,3]. Many studies have been performed on PS/PE blends since the 1970s. Unfortunately, PS/PE blends are mutually immiscible or incompatible due to very low entropy of mixing and mostly positive enthalpy of mixing [4,5]. They typically exhibit coarsening and instability of phase morphology due to weak interfacial adhesion between the two phases [6]. The weak phase interfacial adhesion of PS and PE blends leads to poor mechanical properties [7,8], an unattractive appearance and poor foaming behavior. The miscibility between PS and PE can be improved by the addition of a compatibilizer. Block copolymers with sequences chemically identical or similar to the blend components are typically used as compatibilizers, such as styrene–butadiene (SB) [9–13], styrene–butadiene–styrene (SBS) [14–16] and styrene–ethylene–butylene–styrene (SEBS) [4,5,17,18]. In the present work, SEBS was chosen as the
∗ Corresponding author. Tel.: +86 21 39194531. E-mail address: [email protected] (G. Wu). 0896-8446/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.supflu.2013.06.003
compatibilizer because it contains chain blocks with chemical structural units similar to LDPE and PS. Microcellular foams of PS/PE alloys can be applied in building, packaging, sports and leisure articles due to their good mechanical properties, chemical resistance, low thermal and electrical conductivity and good sound insulation. For the preparation of microcellular PS/PE alloy foams, supercritical carbon dioxide (scCO2 ) is an ideal and clean blowing agent because CO2 is chemically stable and non-toxic. Moreover, it has a mild critical temperature (31 ◦ C), moderate critical pressure (7.38 MPa) and a relatively high solubility in polymers. In previous studies, scCO2 has been used to produce various polymer alloy foams, including poly(methyl methacrylate)/poly(l-lactic acid) (PMMA/PLA) [19], poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/ PMMA) blends [20,21], poly(methyl methacrylate)/poly(methylmethacrylate-co-butylacrylate-co-methylmethacrylate) (PMMA/ MAM) blends [22], polystyrene/poly(styrene-co-butadiene-comethylmethacrylate) (PS/SBM) blends [22], PS/PMMA blends [23], polyethylene/polypropylene (PE/PP) blends [24,25] and polypropylene/polystyrene (PP/PS) blends [26]. However, the study of foaming of PS/PE alloys with scCO2 has seldom been reported, probably due to the poor compatibility of PS and PE. In this paper, PS/PE alloys were foamed using a scCO2 foaming process, and the section morphology of the PS/PE foams was characterized by high-resolution X-ray computed tomography at the X-ray imaging and biomedical application station (BL13W1) of the Shanghai Synchrotron Radiation Facility. X-ray microtomography
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Fig. 1. SEM microphotography of the PS/PE (70:30) foams with SEBS of 0 and 5 wt%.
(XMT) is a non-destructive and three-dimensional imaging technique in which a series of radiographic images is used to reconstruct a 3D intensity distribution map of a specimen’s X-ray absorption [27,28]. It can be used to reconstruct interior structural details with high resolution [29,30]. The cell structures of PS/PE foams were compared in detail, and the effects of PS/PE ratio and SEBS addition on cell size, cell density and the dimensional uniformity of the cells were also investigated. The cell morphology of PS/PE foams with the SEBS compatibilizer is significantly improved compared with the foam without SEBS. The PS/PE alloy foams with SEBS have a more uniform cell size distribution, smaller mean cell diameter and higher cell density. The optimal SEBS content for the foaming of PS/PE alloys was found to be 5 wt% after a comparison of the cell size distribution of the samples with various SEBS content. 2. Experimental 2.1. Materials Polystyrene (GPPS 525) pellets, supplied by Sinopec Guangzhou Company, have a density of 1.04 g/cm3 , melt flowing index of 8.0 g/10 min and Tg of 97 ◦ C. LDPE (LD100-AC) pellets, supplied by Sinopec Beijing Yanshan Company, have a density of 0.9225 g/cm3 , melt flowing index of 2.0 g/10 min and Vicat softening temperature of 90 ◦ C. The SEBS copolymer (Kraton G1652), purchased from Shell Chemical Co., Ltd., has a melt flowing index of 1.0 g/10 min, and Mw values of the EB (ethylene/butylene) and the PS blocks are 35,000 and 7500. Carbon dioxide (>99.5% purity) was obtained from Loutang Special Gases of Shanghai. 2.2. Mixing and foaming process Binary and ternary blends of various compositions were blended at 200 ◦ C in a two-screw mixer (Thermo Haake PolyDrive, Germany). The PS/PE blending weight ratios were 90/10, 80/20, 70/30, 60/40 and 50/50. The proportion of SEBS in the PS/PE (70:30) blends was 0, 2, 5 and 10 wt%. After mixing all the materials, the blends were hot pressed for 5 min at 160 ◦ C and a pressure of 12 MPa into thin sheets (1 mm in thickness) using a plate vulcanizing machine. The PS/PE alloy sheets were foamed, as previously described [31]. The soaking time and depressurization rate were 8 h and 0.7 MPa/s, respectively. Five different soaking temperatures (80, 90, 100, 105 and 110 ◦ C) and three different saturation pressures (15, 20 and 25 MPa) were used in the foaming experiments.
2.3. X-ray microtomography The XMT tests were performed at the X-ray imaging and biomedical application station (BL13W1) of the Shanghai Synchrotron Radiation Facility. The foam sample was cut into a prism with a length of 5 mm and a diameter of 1 mm and fixed onto a thin plastic pipe. The plastic pipe was mounted on a multi-dimensional platform with a rotator to position the sample and to rotate around the beam. The PS/PE foam was irradiated by a synchrotron radiation X-ray beam with photon energy of 18 keV that passed through a slit with a length of 45 mm and a width of 5 mm. For each sample, approximately 15 cm of sample-to-detector distance, 2 s of exposure time and 1200 equiangular radiographic images of 1000 × 1000 pixels were acquired over a total angular range of 180◦ (angular step: 0.15◦ ). The X-ray beams through the samples were recorded using a CCD camera with an effective pixel size of 0.74 m. At the same time, dark current and reference images were also recorded with the same exposure time for flat-field corrections. 2.4. Scanning electron microscope (SEM) analysis The SEM micrographs of PS/PE foams were captured by a LEO 1530 VP scanning electron microscope at an acceleration voltage of 10.0 kV. The samples were immersed in liquid nitrogen and fractured at −196 ◦ C, then mounted on objective table. Before SEM observation, the fractured surfaces were sputter-coated with gold for observation. 2.5. Image analysis Microstructures of the PS/PE polymeric alloy foams were obtained by statistical analysis of the cell parameters. The slice images of the PS/PE foams were measured using the Image Pro software. Cell density (Nf ) is determined by the number of cells per unit volume of foam, which was calculated using Eq. (1):
Nf =
nM 2 A
3/2 (1)
where n, M and A are the number of cells in the micrograph, the magnification of the micrograph and the area of the micrograph (cm2 ), respectively. The average cell diameter was calculated using Eq. (2): D=
dn i i ni
(2)
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where ni is the number of cells with an area-equivalent diameter of di . 3. Results and discussion 3.1. Comparison of SEM and XMT for cell observation
Fig. 2. The cell size distribution of the PS/PE (70:30) foams with SEBS of 5 wt% obtaining from SEM and XMT.
It is known that PS/PE blend is immiscible and has a weak interaction between the dispersed phase and matrix. Hence, both PS/PE blend and its foam have poor mechanical performance. Fig. 1 shows the microphotographs of the section of the PS/PE (70:30) foams. As shown in Fig. 1(a), perfect cells cannot be observed for the PS/PE foam without SEBS. The cell morphology of the foam is much improved with the addition of SEBS, but broken cells still exist (as shown in Fig. 1(b)). The poor cell morphology of the PS/PE foam can be attributed to the destruction of cell structure by fracture in liquid nitrogen. Therefore, we employed in this work non-destructive XMT for the observation of cell morphology. Fig. 2 shows a comparison of cell size distributions of the PS/PE (70:30) foams with 5 wt% SEBS by using SEM and XMT. The corresponding XMT is illustrated in Fig. 3(C). Compared to the observation by SEM, the XMT analysis gives a smaller peak value of cell size and a wider size distribution. This is because the large cells are easy to crack in the sample preparation for SEM observation, due to
Fig. 3. Reconstructed slices of PS/PE (70:30) foams with different SEBS content: 0 wt% (A), 2 wt% (B), 5 wt% (C) and 10 wt% (D).
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Fig. 4. Cell size distribution of PS/PE (70:30) foams with different SEBS content.
their larger stress concentration and thinner cell wall. This can be exactly avoided in the XMT observation. 3.2. Effect of SEBS on cellular structure SEBS was added to improve the compatibility of PS and PE. An initial investigation was performed to confirm its effect on the morphology of PS/PE alloy foams. Several slice images of the PS/PE alloy foams captured through high-resolution X-ray computed tomography are shown in Fig. 3. The content of the compatibilizer in the PS/PE (70:30) foams was 0, 2, 5 and 10 wt% SEBS. The foams were prepared by saturating with scCO2 at 25 MPa and 105 ◦ C for 8 h and then quickly depressurizing to atmospheric pressure. It can be clearly observed that the cell morphology of the PS/PE foams is improved after addition of SEBS. The cells of the PS/PE foams are discrete, nearly spherical and surrounded by thick walls. Therefore, the foams have well-defined closed cells under the foaming conditions. For the foam without any SEBS (Fig. 3(A)), some large irregular bubbles or cracks can be observed, and the homogeneity of the cells is inferior. In contrast, the cell morphology of the PS/PE foams is evidently meliorated after addition of SEBS into the alloy matrix. As shown in Fig. 3(B)–(D), the size of the large bubbles and cracks decreased gradually with increasing SEBS content, and the cell size also became more uniform. For an explicit comparison of cell size, a statistical calculation of cell size and cells number was performed using the slice images. The cell size distributions of the PS/PE (70:30) foams presented in Fig. 3 were shown in Fig. 4. The width of the cell size distribution peak, which indicates the dispersity of the cell size, decreased with increasing SEBS content. The foam morphology indicates that the uniformity of the cell size is improved with the addition of SEBS, and the cell size increased with an increase in the SEBS content. It is clear that the presence of SEBS can drastically affect the nucleation and inflation of the cells, resulting in an improved cell size distribution. In heterogeneous nucleation theory, homogeneous dispersive micron-size particles are employed as nucleating agents to induce heterogeneous nucleation for the formation of a large number of nucleation sites in the early nucleation period [32]. Due to the inherent incompatibility of PE with PS, their blend (PS/PE = 70/30) is in fact a two-phase material, i.e., a soft polymer dispersed into a hard polymer matrix phase [17]. Without the SEBS triblock copolymer, the PS/PE blend has a poor interfacial adhesion between the two phases, and micro-voids are formed at the interface of the two phases [4,17]. During rapid depressurization in the foaming process, CO2 gas molecules are supersaturated in the matrix of
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Fig. 5. Cell size distribution of PS/PE foams (5 wt% SEBS) with different PS/PE content.
the polymer and nucleate. Subsequently, the nuclei grow to form bubbles by diffusion of CO2 molecular into the nuclei, however, diffusion of gas molecules into the micro-voids at the interface of the two phases is easier than nucleation. Expansion of the micro-voids prior to the nucleation therefore results in non-uniformity of the cell size and a lower cell density of foam. When SEBS is incorporated into the PS/PE blend, there is better interfacial adhesion between the two phases and a much finer and more uniformly dispersed phase [4,5]. The nucleation of CO2 gas in the matrix of the polymer is rapid and uniform due to the improvement of the compatibility of PS with PE. As a result, PS/PE foams with finer cell morphology are obtained by employing SEBS as a compatibilizer. The cell size distributions of the PS/PE foams with 2 wt%, 5 wt% and 10 wt% SEBS were compared. The PS/PE foam with 2 wt% SEBS has the smallest cell size and the largest cracks; therefore its cell uniformity is poor. The average cell size of the PS/PE foam with 5 wt% SEBS is smaller than that with 10 wt% SEBS, and the cell size distribution of the PS/PE foam with 5 wt% SEBS is narrower than that with 10 wt% SEBS. It is indicated that the cell morphology of the PS/PE foams with more than 5 wt% SEBS is not improved. The optimized proportion of SEBS in PS/EP alloys should be 5 wt% in terms of cell morphology and the size distribution of the PS/PE foam. Thus, specimens with 5 wt% SEBS were selected to investigate the effects of other factors on the cell morphology of the foams. 3.3. Effect of PS/PE weight ratio on cellular structure The PS/PE sheets with various ratios were expanded after soaking in scCO2 for 8 h at 25 MPa and 105 ◦ C. The effect of PS/PE blend ratio on the cell size distribution and the foam structure parameters can be observed in Figs. 5 and 6. The PS/PE foam with lower PS content has a larger cell size and wider cell size distribution. The average cell diameter decreased from 19 m to 13.5 m, and the cell density increased from 4.7 × 107 cm−3 to 1.1 × 108 cm−3 with increasing PS content. These results are probably due to the higher melt elasticity and viscosity of PS compared with PE at the foaming temperature [5,17], which prevents the growth and fusion of bubbles during the foaming process. The PS/PE foams with higher PS content therefore have a smaller cell size and higher cell density. 3.4. Effect of foaming condition on cellular structure The cells in the slice images of PS/PE/SEBS (70:30:5) foams prepared under different foaming temperatures were statistically
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Fig. 6. Effect of PS/PE ratio on the average cell diameter and cell density of PS/PE foams with 5 wt% SEBS. Fig. 9. Cell size distribution of the PS/PE (70:30) foams with 5 wt% SEBS produced at saturation pressures of 15, 20 and 25 MPa.
Fig. 7. Cell size distribution of PS/PE (70:30) foams with 5 wt% SEBS produced at temperatures from 80 ◦ C to 110 ◦ C.
counted, and the statistical results are illustrated in Figs. 7 and 8. With an increase in foaming temperature, the cell size and the width of the cell size distribution peak increased significantly. The average cell diameter increased from 5.2 m to 21.9 m with an increase in the temperature from 80 ◦ C to 110 ◦ C, and
Fig. 8. Effect of foaming temperature on the average cell diameter and cell density of PS/PE (70:30) foams with 5 wt% SEBS.
conversely, the cell density decreased from 2 × 109 cm−3 to 3.3 × 107 cm−3 . The weak surface tension and melt elasticity of the PS/PE blends at higher temperature lead to a weaker resistance to the expansion of the bubbles than at lower temperature. Therefore, the bubbles can expand to a larger size. Moreover, the bubbles can more readily break up and fuse due to the weak melt strength at higher temperature, leading to a decreased cell density. Figs. 9 and 10 show the pressure dependence of cell size distribution, average cell diameter and cell density of the PS/PE/SEBS (70:30:5) foam. The cell size and the width of cell size distribution peak obviously decrease with raising the saturation pressure. The average cell diameter decreases from 18.6 m to 9.2 m with the saturation pressure, and reversely the cell density increases from 2.5 × 107 cm−3 to 1.6 × 108 cm−3 . In homogeneous or heterogeneous nucleation theory [33,34], a higher saturation pressure leads to a lower nucleation energy barrier and higher cell nucleus density, and the reverse is true during the foaming process. As CO2 concentration dissolved in polymer is constant, the smaller cell size results from the higher cell nucleus density. Therefore, the cell size of the PS/PE foam is reduced and the cell density of the PS/PE foam increased at higher saturation pressures.
Fig. 10. Effect of saturation pressure on the average cell diameter and cell density of the PS/PE (70:30) foams with 5 wt% SEBS.
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4. Conclusions Microcellular PS/PE alloy foams were prepared by a supercritical CO2 foaming process. The triblock copolymer SEBS was added to the PS/PE alloys as a compatibilizer to improve the cell morphology of the PS/PE foams. The PS/PE alloy foams with added SEBS have a smaller mean cell diameter and higher cell density than those without SEBS. However, if the SEBS content is too high, a larger cell size is induced. The optimal SEBS content for the foaming of PS/PE alloys is 5 wt%. PS/PE foams with a higher PS component have a slightly decreased cell size and increased cell density. A higher foaming temperature results in an increase in the cell diameter from 5.2 m to 21.9 m and a decrease in the cell density from 2 × 109 cm−3 to 3.3 × 107 cm−3 . The saturation pressure dependence of the cell size and density exhibited an opposite trend. With increasing the saturation pressure, the cell diameter decreased from 18.6 m to 9.2 m, and the cell density increased from 2.5 × 107 cm−3 to 1.6 × 108 cm−3 . Acknowledgements We acknowledge our colleagues at the X-ray imaging and biomedical application station (BL13W1) at the SSRF for their instructive help during experimental data collection and image reconstruction. This work was supported by the National Natural Science Foundation of China under Grant No. 11079048.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21] [22]
References [1] B. Xu, J. Simonsen, W.E. Rochefort, Mechanical properties and creep resis-
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
tance in polystyrene/polyethylene blends, Journal of Applied Polymer Science 76 (2000) 1100–1108. S.A. Xu, S.C. Tjong, Deformation mechanisms and fracture toughness of polystyrene/high-density polyethylene blends compatibilized by triblock copolymer, Journal of Applied Polymer Science 77 (2000) 2024–2033. S.A. Xu, S.C. Tjong, Effect of compatibilizer content on the tensile properties and fracture toughness of high density polyethylene/polystyrene blends, Polymer Journal 32 (2000) 208–214. T.Y. Kim, D.M. Kim, W.J. Kim, T.H. Lee, K.S. Suh, Effects of poly[styrene-b(ethylene-co-butylene)-b-styrene] on the charge distributions of low-density polyethylene/polystyrene blends, Journal of Polymer Science Part B: Polymer Physics 42 (2004) 2813–2820. M.K.R. Aghjeh, M. Khodabandelou, M. Khezrefaridi, Rheology and morphology of high impact polystyrene polyethylene blends and the effect of compatibilization on their properties, Journal of Applied Polymer Science 114 (2009) 2235–2245. M.N. Bureau, H. ElKadi, J. Denault, J.I. Dickson, Injection and compression molding of polystyrene/high-density polyethylene blends – phase morphology and tensile behavior, Polymer Engineering and Science 37 (1997) 377–390. Z. Stary, I. Fortelny, Z. Krulis, M. Slouf, Effect of the molecular structure of ethene–propene and styrene–butadiene copolymers on their compatibilization efficiency in low-density polyethylene/polystyrene blends, Journal of Applied Polymer Science 107 (2008) 174–186. T.S. George, A. Krishnan, N. Joseph, R. Anjana, K.E. George, Effect of maleic anhydride grafting on nanokaolinclay reinforced polystyrene/high density polyethylene blends, Polymer Composites 33 (2012) 1465–1472. I. Fortelny, D. Michalkova, J. Hromadkova, F. Lednicky, Effect of a styrene–butadiene copolymer on the phase structure and impact strength of polyethylene/high-impact polystyrene blends, Journal of Applied Polymer Science 81 (2001) 570–580. I. Fortelny, J. Mikesova, J. Hromadkova, V. Hasova, Z. Horak, Effect of molecular structure of styrene–butadiene block copolymers on morphology, rheological properties, and impact strength of polystyrene/polyethylene blends, Journal of Applied Polymer Science 90 (2003) 2303–2309. I. Fortelny, M. Slouf, D. Hlavata, A. Sikora, Interfacial activity of styrene–butadiene block copolymers in low-density polyethylene/polystyrene blends, Composite Interfaces 13 (2006) 783–799. I. Fortelny, M. Slouf, A. Sikora, D. Hlavata, V. Hasova, J. Mikesova, C. Jacob, The effect of the architecture and concentration of styrene–butadiene
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
55
compatibilizers on the morphology of polystyrene/low-density polyethylene blends, Journal of Applied Polymer Science 100 (2006) 2803–2816. D. Michalkova, J. Pospisil, I. Fortelny, M. Slouf, Z. Krulis, Effect of a N,N disubstituted phenylene iamine stabilizer and SB/EPDM compatibilizer on toughness and morphology of blends of pre-aged polyethylene with highimpact polystyrene, Journal of Vinyl and Additive Technology 12 (2006) 58–65. Z. Stary, Z. Krulis, J. Hromadkova, M. Slouf, J. Kotek, I. Fortelny, New multicomponent compatibilization system for polyolerin/polystyrene blends, International Polymer Processing 21 (2006) 222–229. H.Q. Xie, D.G. Liu, D. Xie, J.G. Guan, Behavior of ionomers of sulfonated styrene–butadiene–styrene triblock copolymer in polymer blends with crystalline polyolefins and as compatibilizer, Journal of Applied Polymer Science 99 (2006) 1887–1894. D. Michalkova, J. Pospisil, I. Fortelny, J. Hromadkova, F. Lednicky, P. Schmidt, Z. Krulis, Restabilization of the aging resistance of compatibilized blends of pre-aged low density polyethylene and high-impact polystyrene (LDPE/HIPS), Polymer Degradation and Stability 94 (2009) 1486–1493. A. Chirawithayaboon, S. Kiatkamjornwong, Compatibilization of polystyrene/high-density polyethylene blends by high-impact styrene/ethylene–butylene/styrene block copolymer, Journal of Applied Polymer Science 91 (2004) 742–755. Z. Bartczak, A. Galeski, M. Pluta, Ternary blends of high-density copolymers: polyethylene–polystyrene-poly(ethylene/butylene-b-styrene) properties and orientation behavior in plane-strain compression, Journal of Applied Polymer Science 76 (2000) 1746–1761. D. Velasco, L. Benito, M. Fernández-Gutiérrez, J. San Román, C. Elvira, Preparation in supercritical CO2 of porous poly(methyl methacrylate)–poly(l-lactic acid) (PMMA–PLA) scaffolds incorporating ibuprofen, The Journal of Supercritical Fluids 54 (2010) 335–341. S. Siripurapu, Y.J. Gay, J.R. Royer, J.M. DeSimone, R.J. Spontak, S.A. Khan, Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process, Polymer 43 (2002) 5511–5520. Y.-T. Shieh, T.-T. Hsiao, Morphological changes of PVDF/PMMA blends via CO2 treatments, The Journal of Supercritical Fluids 48 (2009) 64–71. J.A.R. Ruiz, M. Pedros, J.-M. Tallon, M. Dumon, Micro and nano cellular amorphous polymers (PMMA, PS) in supercritical CO2 assisted by nanostructured CO2 -philic block copolymers – one step foaming process, The Journal of Supercritical Fluids 58 (2011) 168–176. I.H. Lin, P.F. Liang, C.S. Tan, Preparation of polystyrene/poly(methyl methacrylate) blends by compressed fluid antisolvent technique, The Journal of Supercritical Fluids 51 (2010) 384–398. H.X. Huang, J.K. Wang, X.H. Sun, Improving of cell structure of microcellular foams based on polypropylene/high-density polyethylene blends, Journal of Cellular Plastics 44 (2008) 69–85. C. Liu, X.L. Jiang, T. Liu, L. Zhao, W.X. Zhou, W.K. Yuan, Multifractal analysis of the fracture surfaces of foamed polypropylene/polyethylene blends, Applied Surface Science 255 (2009) 4239–4245. H.X. Huang, H.F. Xu, Preparation of microcellular polypropylene/polystyrene blend foams with tunable cell structure, Polymers for Advanced Technologies 22 (2011) 822–829. L. Salvo, P. Cloetens, E. Maire, S. Zabler, J.J. Blandin, J.Y. Buffière, W. Ludwig, E. Boller, D. Bellet, C. Josserond, X-ray micro-tomography an attractive characterisation technique in materials science, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 200 (2003) 273–286. A.M. Trater, S. Alavi, S.S.H. Rizvi, Use of non-invasive X-ray microtomography for characterizing microstructure of extruded biopolymer foams, Food Research International 38 (2005) 709–719. P.J. Schilling, B.R. Karedla, A.K. Tatiparthi, M.A. Verges, P.D. Herrington, X-ray computed microtomography of internal damage in fiber reinforced polymer matrix composites, Composites Science and Technology 65 (2005) 2071–2078. M. Karimi, M. Heuchel, T. Weigel, M. Schossig, D. Hofmann, A. Lendlein, Formation and size distribution of pores in poly(-caprolactone) foams prepared by pressure quenching using supercritical CO2 , The Journal of Supercritical Fluids 61 (2012) 175–190. Z. Xing, G. Wu, S. Huang, S. Chen, H. Zeng, Preparation of microcellular crosslinked polyethylene foams by a radiation and supercritical carbon dioxide approach, The Journal of Supercritical Fluids 47 (2008) 281–289. W. Zhai, J. Yu, L. Wu, W. Ma, J. He, Heterogeneous nucleation uniformizing cell size distribution in microcellular nanocomposites foams, Polymer 47 (2006) 7580–7589. N.S. Ramesh, D.H. Rasmussen, G.A. Campbell, The heterogeneous nucleation of microcellular foams assisted by the survival of microvoids in polymers containing low glass transition particles. Part I. Mathematical modeling and numerical simulation, Polymer Engineering and Science 34 (1994) 1685–1697. S. Siripurapu, J.A. Coughlan, R.J. Spontak, S.A. Khan, Surface-constrained foaming of polymer thin films with supercritical carbon dioxide, Macromolecules 37 (2004) 9872–9879.