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J. of Supercritical Fluids 82 (2013) 50– 55

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

jou rn al hom epage: www.elsev ier .com/ locate /supf lu

reparation of microcellular polystyrene/polyethylene alloy foams byupercritical CO2 foaming and analysis by X-ray microtomography

he Xinga,b, Mouhua Wanga, Guohao Dua, Tiqiao Xiaoa, Weihua Liua,iang Doua, Guozhong Wua,∗

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, ChinaUniversity of Chinese Academy of Sciences, Beijing 100049, China

r t i c l e i n f o

rticle history:eceived 9 December 2012eceived in revised form 31 May 2013ccepted 6 June 2013

a b s t r a c t

Non-destructive X-ray microtomography at a third generation synchrotron facility was applied to ana-lyze 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 cellstructure 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

eywords:olystyreneolyethyleneolymer alloyupercritical carbon dioxideicrocellular foam

improve the cell morphology. The effects of PS/PE composition and foaming conditions (temperatureand pressure) on the cell structure of foams were investigated in detail. The optimal SEBS content for thefoaming of PS/PE (70:30) alloys was found to be 5 wt%. The cell size and cell density were also dependenton the foaming temperature and the saturation pressure.

© 2013 Elsevier B.V. All rights reserved.

-ray microtomography

. Introduction

Polystyrene (PS) and polyethylene (PE) are two of the mostidely consumed commodity polymers [1]. PS has been widelysed for the production of foam material due to its high modulus,xcellent dimensional stability and thermal insulation. PE hasxcellent properties such as good toughness, elasticity and pro-essability. It is often used to blend with the PS matrix to improvets ductility and impact performance [2,3]. Many studies haveeen performed on PS/PE blends since the 1970s. Unfortunately,S/PE blends are mutually immiscible or incompatible due to veryow entropy of mixing and mostly positive enthalpy of mixing4,5]. They typically exhibit coarsening and instability of phase

orphology due to weak interfacial adhesion between the twohases [6]. The weak phase interfacial adhesion of PS and PElends leads to poor mechanical properties [7,8], an unattractiveppearance and poor foaming behavior. The miscibility betweenS and PE can be improved by the addition of a compatibilizer.lock copolymers with sequences chemically identical or similar tohe blend components are typically used as compatibilizers, such

s styrene–butadiene (SB) [9–13], styrene–butadiene–styreneSBS) [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: wuguozhong@sinap.ac.cn (G. Wu).

896-8446/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.supflu.2013.06.003

compatibilizer because it contains chain blocks with chemicalstructural 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 mechan-ical properties, chemical resistance, low thermal and electricalconductivity and good sound insulation. For the preparation ofmicrocellular PS/PE alloy foams, supercritical carbon dioxide(scCO2) is an ideal and clean blowing agent because CO2 ischemically stable and non-toxic. Moreover, it has a mild criticaltemperature (31 ◦C), moderate critical pressure (7.38 MPa) and arelatively high solubility in polymers. In previous studies, scCO2has been used to produce various polymer alloy foams, includingpoly(methyl methacrylate)/poly(l-lactic acid) (PMMA/PLA) [19],poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends [20,21], poly(methyl methacrylate)/poly(methyl-methacrylate-co-butylacrylate-co-methylmethacrylate) (PMMA/MAM) blends [22], polystyrene/poly(styrene-co-butadiene-co-methylmethacrylate) (PS/SBM) blends [22], PS/PMMA blends[23], polyethylene/polypropylene (PE/PP) blends [24,25] andpolypropylene/polystyrene (PP/PS) blends [26]. However, thestudy of foaming of PS/PE alloys with scCO2 has seldom beenreported, 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 char-acterized by high-resolution X-ray computed tomography at theX-ray imaging and biomedical application station (BL13W1) of theShanghai Synchrotron Radiation Facility. X-ray microtomography

Z. Xing et al. / J. of Supercritical Fluids 82 (2013) 50– 55 51

S/PE (7

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Fig. 1. SEM microphotography of the P

XMT) is a non-destructive and three-dimensional imaging tech-ique in which a series of radiographic images is used to reconstruct

3D intensity distribution map of a specimen’s X-ray absorption27,28]. It can be used to reconstruct interior structural details withigh resolution [29,30]. The cell structures of PS/PE foams wereompared in detail, and the effects of PS/PE ratio and SEBS addi-ion on cell size, cell density and the dimensional uniformity of theells were also investigated. The cell morphology of PS/PE foamsith the SEBS compatibilizer is significantly improved comparedith the foam without SEBS. The PS/PE alloy foams with SEBS have

more uniform cell size distribution, smaller mean cell diameternd higher cell density. The optimal SEBS content for the foamingf PS/PE alloys was found to be 5 wt% after a comparison of the cellize distribution of the samples with various SEBS content.

. Experimental

.1. Materials

Polystyrene (GPPS 525) pellets, supplied by Sinopec Guangzhouompany, have a density of 1.04 g/cm3, melt flowing index of.0 g/10 min and Tg of 97 ◦C. LDPE (LD100-AC) pellets, supplied byinopec Beijing Yanshan Company, have a density of 0.9225 g/cm3,elt flowing index of 2.0 g/10 min and Vicat softening tempera-

ure of 90 ◦C. The SEBS copolymer (Kraton G1652), purchased fromhell Chemical Co., Ltd., has a melt flowing index of 1.0 g/10 min,nd Mw values of the EB (ethylene/butylene) and the PS blocks are5,000 and 7500. Carbon dioxide (>99.5% purity) was obtained fromoutang Special Gases of Shanghai.

.2. Mixing and foaming process

Binary and ternary blends of various compositions werelended at 200 ◦C in a two-screw mixer (Thermo Haake PolyDrive,ermany). The PS/PE blending weight ratios were 90/10, 80/20,0/30, 60/40 and 50/50. The proportion of SEBS in the PS/PE (70:30)lends was 0, 2, 5 and 10 wt%. After mixing all the materials, thelends were hot pressed for 5 min at 160 ◦C and a pressure of2 MPa into thin sheets (1 mm in thickness) using a plate vulca-izing machine. The PS/PE alloy sheets were foamed, as previouslyescribed [31]. The soaking time and depressurization rate were

h and 0.7 MPa/s, respectively. Five different soaking tempera-ures (80, 90, 100, 105 and 110 ◦C) and three different saturationressures (15, 20 and 25 MPa) were used in the foaming experi-ents.

0:30) foams with SEBS of 0 and 5 wt%.

2.3. X-ray microtomography

The XMT tests were performed at the X-ray imaging andbiomedical application station (BL13W1) of the Shanghai Syn-chrotron Radiation Facility. The foam sample was cut into a prismwith a length of 5 mm and a diameter of 1 mm and fixed onto a thinplastic pipe. The plastic pipe was mounted on a multi-dimensionalplatform with a rotator to position the sample and to rotate aroundthe beam. The PS/PE foam was irradiated by a synchrotron radia-tion X-ray beam with photon energy of 18 keV that passed througha slit with a length of 45 mm and a width of 5 mm. For eachsample, approximately 15 cm of sample-to-detector distance, 2 sof exposure time and 1200 equiangular radiographic images of1000 × 1000 pixels were acquired over a total angular range of 180◦

(angular step: 0.15◦). The X-ray beams through the samples wererecorded using a CCD camera with an effective pixel size of 0.74 �m.At the same time, dark current and reference images were alsorecorded 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 LEO1530 VP scanning electron microscope at an acceleration voltageof 10.0 kV. The samples were immersed in liquid nitrogen and frac-tured at −196 ◦C, then mounted on objective table. Before SEMobservation, the fractured surfaces were sputter-coated with goldfor observation.

2.5. Image analysis

Microstructures of the PS/PE polymeric alloy foams wereobtained by statistical analysis of the cell parameters. The sliceimages of the PS/PE foams were measured using the Image Pro soft-ware. Cell density (Nf) is determined by the number of cells per unitvolume of foam, which was calculated using Eq. (1):

Nf =(

nM2

A

)3/2

(1)

where n, M and A are the number of cells in the micrograph, themagnification of the micrograph and the area of the micrograph(cm2), respectively.

The average cell diameter was calculated using Eq. (2):

D =∑

dini∑ni

(2)

52 Z. Xing et al. / J. of Supercritica

Fig. 2. The cell size distribution of the PS/PE (70:30) foams with SEBS of 5 wt%obtaining from SEM and XMT.

Fig. 3. Reconstructed slices of PS/PE (70:30) foams with different

l Fluids 82 (2013) 50– 55

where ni is the number of cells with an area-equivalent diameterof di.

3. Results and discussion

3.1. Comparison of SEM and XMT for cell observation

It is known that PS/PE blend is immiscible and has a weakinteraction between the dispersed phase and matrix. Hence, bothPS/PE blend and its foam have poor mechanical performance. Fig. 1shows the microphotographs of the section of the PS/PE (70:30)foams. As shown in Fig. 1(a), perfect cells cannot be observed forthe PS/PE foam without SEBS. The cell morphology of the foamis much improved with the addition of SEBS, but broken cellsstill exist (as shown in Fig. 1(b)). The poor cell morphology of thePS/PE foam can be attributed to the destruction of cell structure byfracture in liquid nitrogen. Therefore, we employed in this worknon-destructive XMT for the observation of cell morphology. Fig. 2shows 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 bySEM, the XMT analysis gives a smaller peak value of cell size anda wider size distribution. This is because the large cells are easyto crack in the sample preparation for SEM observation, due to

SEBS content: 0 wt% (A), 2 wt% (B), 5 wt% (C) and 10 wt% (D).

Z. Xing et al. / J. of Supercritical Fluids 82 (2013) 50– 55 53

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ig. 4. Cell size distribution of PS/PE (70:30) foams with different SEBS content.

heir larger stress concentration and thinner cell wall. This can bexactly avoided in the XMT observation.

.2. Effect of SEBS on cellular structure

SEBS was added to improve the compatibility of PS and PE. Annitial investigation was performed to confirm its effect on the mor-hology of PS/PE alloy foams. Several slice images of the PS/PE alloyoams captured through high-resolution X-ray computed tomogra-hy are shown in Fig. 3. The content of the compatibilizer in theS/PE (70:30) foams was 0, 2, 5 and 10 wt% SEBS. The foams wererepared by saturating with scCO2 at 25 MPa and 105 ◦C for 8 hnd then quickly depressurizing to atmospheric pressure. It can belearly observed that the cell morphology of the PS/PE foams ismproved after addition of SEBS. The cells of the PS/PE foams areiscrete, nearly spherical and surrounded by thick walls. There-ore, the foams have well-defined closed cells under the foamingonditions. For the foam without any SEBS (Fig. 3(A)), some largerregular bubbles or cracks can be observed, and the homogeneityf the cells is inferior. In contrast, the cell morphology of the PS/PEoams is evidently meliorated after addition of SEBS into the alloy

atrix. As shown in Fig. 3(B)–(D), the size of the large bubbles andracks decreased gradually with increasing SEBS content, and theell size also became more uniform.

For an explicit comparison of cell size, a statistical calculationf cell size and cells number was performed using the slice images.he cell size distributions of the PS/PE (70:30) foams presented inig. 3 were shown in Fig. 4. The width of the cell size distributioneak, which indicates the dispersity of the cell size, decreased with

ncreasing SEBS content. The foam morphology indicates that theniformity of the cell size is improved with the addition of SEBS, andhe cell size increased with an increase in the SEBS content. It is clearhat the presence of SEBS can drastically affect the nucleation andnflation of the cells, resulting in an improved cell size distribution.

In heterogeneous nucleation theory, homogeneous dispersiveicron-size particles are employed as nucleating agents to induce

eterogeneous nucleation for the formation of a large number ofucleation sites in the early nucleation period [32]. Due to the

nherent incompatibility of PE with PS, their blend (PS/PE = 70/30)s in fact a two-phase material, i.e., a soft polymer dispersed into aard polymer matrix phase [17]. Without the SEBS triblock copoly-

er, the PS/PE blend has a poor interfacial adhesion between the

wo phases, and micro-voids are formed at the interface of thewo phases [4,17]. During rapid depressurization in the foamingrocess, CO2 gas molecules are supersaturated in the matrix of

Fig. 5. Cell size distribution of PS/PE foams (5 wt% SEBS) with different PS/PE con-tent.

the polymer and nucleate. Subsequently, the nuclei grow to formbubbles by diffusion of CO2 molecular into the nuclei, however, dif-fusion of gas molecules into the micro-voids at the interface of thetwo phases is easier than nucleation. Expansion of the micro-voidsprior to the nucleation therefore results in non-uniformity of thecell size and a lower cell density of foam. When SEBS is incorporatedinto the PS/PE blend, there is better interfacial adhesion betweenthe two phases and a much finer and more uniformly dispersedphase [4,5]. The nucleation of CO2 gas in the matrix of the polymeris rapid and uniform due to the improvement of the compatibilityof PS with PE. As a result, PS/PE foams with finer cell morphologyare 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% SEBShas the smallest cell size and the largest cracks; therefore its celluniformity is poor. The average cell size of the PS/PE foam with5 wt% SEBS is smaller than that with 10 wt% SEBS, and the cell sizedistribution of the PS/PE foam with 5 wt% SEBS is narrower thanthat with 10 wt% SEBS. It is indicated that the cell morphology of thePS/PE foams with more than 5 wt% SEBS is not improved. The opti-mized proportion of SEBS in PS/EP alloys should be 5 wt% in termsof cell morphology and the size distribution of the PS/PE foam. Thus,specimens with 5 wt% SEBS were selected to investigate the effectsof 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 soak-ing in scCO2 for 8 h at 25 MPa and 105 ◦C. The effect of PS/PE blendratio on the cell size distribution and the foam structure parame-ters can be observed in Figs. 5 and 6. The PS/PE foam with lower PScontent has a larger cell size and wider cell size distribution. Theaverage cell diameter decreased from 19 �m to 13.5 �m, and thecell density increased from 4.7 × 107 cm−3 to 1.1 × 108 cm−3 withincreasing PS content. These results are probably due to the highermelt elasticity and viscosity of PS compared with PE at the foamingtemperature [5,17], which prevents the growth and fusion of bub-bles during the foaming process. The PS/PE foams with higher PScontent 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) foamsprepared under different foaming temperatures were statistically

54 Z. Xing et al. / J. of Supercritical Fluids 82 (2013) 50– 55

Fig. 6. Effect of PS/PE ratio on the average cell diameter and cell density of PS/PEfoams with 5 wt% SEBS.

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ig. 7. Cell size distribution of PS/PE (70:30) foams with 5 wt% SEBS produced atemperatures from 80 ◦C to 110 ◦C.

ounted, and the statistical results are illustrated in Figs. 7 and 8.

ith an increase in foaming temperature, the cell size and theidth of the cell size distribution peak increased significantly.

he average cell diameter increased from 5.2 �m to 21.9 �mith an increase in the temperature from 80 ◦C to 110 ◦C, and

ig. 8. Effect of foaming temperature on the average cell diameter and cell densityf PS/PE (70:30) foams with 5 wt% SEBS.

Fig. 9. Cell size distribution of the PS/PE (70:30) foams with 5 wt% SEBS producedat saturation pressures of 15, 20 and 25 MPa.

conversely, the cell density decreased from 2 × 109 cm−3 to3.3 × 107 cm−3. The weak surface tension and melt elasticity of thePS/PE blends at higher temperature lead to a weaker resistanceto 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 weakmelt strength at higher temperature, leading to a decreased celldensity.

Figs. 9 and 10 show the pressure dependence of cell size distri-bution, average cell diameter and cell density of the PS/PE/SEBS(70:30:5) foam. The cell size and the width of cell size dis-tribution peak obviously decrease with raising the saturationpressure. The average cell diameter decreases from 18.6 �m to9.2 �m with the saturation pressure, and reversely the cell densityincreases from 2.5 × 107 cm−3 to 1.6 × 108 cm−3. In homogeneousor heterogeneous nucleation theory [33,34], a higher saturationpressure leads to a lower nucleation energy barrier and highercell nucleus density, and the reverse is true during the foamingprocess. As CO2 concentration dissolved in polymer is constant,the smaller cell size results from the higher cell nucleus den-sity. Therefore, the cell size of the PS/PE foam is reduced and

the cell density of the PS/PE foam increased at higher saturationpressures.

Fig. 10. Effect of saturation pressure on the average cell diameter and cell densityof the PS/PE (70:30) foams with 5 wt% SEBS.

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. Conclusions

Microcellular PS/PE alloy foams were prepared by a supercriti-al CO2 foaming process. The triblock copolymer SEBS was addedo the PS/PE alloys as a compatibilizer to improve the cell mor-hology of the PS/PE foams. The PS/PE alloy foams with addedEBS have a smaller mean cell diameter and higher cell densityhan those without SEBS. However, if the SEBS content is too high,

larger cell size is induced. The optimal SEBS content for theoaming of PS/PE alloys is 5 wt%. PS/PE foams with a higher PSomponent have a slightly decreased cell size and increased cellensity. A higher foaming temperature results in an increase in theell diameter from 5.2 �m to 21.9 �m and a decrease in the cellensity from 2 × 109 cm−3 to 3.3 × 107 cm−3. The saturation pres-ure dependence of the cell size and density exhibited an oppositerend. With increasing the saturation pressure, the cell diameterecreased from 18.6 �m to 9.2 �m, and the cell density increasedrom 2.5 × 107 cm−3 to 1.6 × 108 cm−3.

cknowledgements

We acknowledge our colleagues at the X-ray imaging andiomedical application station (BL13W1) at the SSRF for their

nstructive help during experimental data collection and imageeconstruction. This work was supported by the National Naturalcience Foundation of China under Grant No. 11079048.

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