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J. of Supercritical Fluids 47 (2008) 281–289

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com/ locate /supf lu

reparation of microcellular cross-linked polyethylene foams byradiation and supercritical carbon dioxide approach

he Xinga,b, Guozhong Wub,∗, Shirong Huanga, Shimou Chenb, Hongyan Zenga

College of Chemical Engineering, Xiangtan University, Xiangtan, Hunan 411105, ChinaShanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading, Shanghai 201800, China

r t i c l e i n f o

rticle history:eceived 29 February 2008eceived in revised form 31 July 2008

a b s t r a c t

A detailed investigation was carried out on the foaming of cross-linked low-density polyethylene (x-LDPE) sheets with CO2 as a blowing agent on a batch scale. The x-LDPE sheets were produced through twodifferent sequences, i.e., part formation after irradiation (Sequence A), and irradiation after part formation

ccepted 20 August 2008

eywords:DPEupercritical carbon dioxideadiation

(Sequence B). The foaming behavior of x-LDPE samples prepared by both Sequence A and B was comparedin terms of cell size distribution, cell size, cell density and volume expansion ratio. The optimum radiationdose for the foaming of x-LDPE was found to be 50 kGy. At a foaming temperature of 100 ◦C, the cell sizeof x-LDPE foam was less than 10 �m, and the maximal value of cell density reached 1011 cells/cm3. The SAand SB series of x-LDPE foams both had a microcellular structure. It may be possible to realize a continuous

-LDP

mopccareco

wOlbsfFfm

ross-linkingicrocellular foam

manufacture process for x

. Introduction

Microcellular polymeric foam is a new class of material thatypically has cell sizes on the order of 10 �m and cell densitiesf more than 109 cells/cm3. Microcellular polymeric foams offerultiple advantages over solid polymers, including a reduction inaterial cost and weight, good mechanical properties and chem-

cal resistance, low thermal and electrical conductivity, and goodound insulation [1–3]. They have been widely applied in insu-ation and packaging, as well as in structures and filters. Martini4] first produced microcellular polymeric foams by using CO2s a physical-blowing agent in the early 1980s. Over the past 25ears, various polymers have been used in the development oficrocellular polymeric foams, including poly(methyl methacry-

ate) (PMMA) [3,5], polystyrene (PS) [6], polycarbonate (PC) [7],oly(ethylene terephthalate) (PET) [7], poly(ether sulfone) [8],olysulfone [8], poly(ether imide) [9], poly(l-lactide) (PLA) [10],olyethylene/polypropylene (PE/PP) blends [11], poly(vinylideneuoride)/poly(methyl methacrylate) (PVDF/PMMA) blends [12],E/PP-wood fiber composites [13], and polyethylene-octene elas-

omer (POE)/organoclay nanocomposite [14].

Low-density polyethylene is widely used because of its excellentroperties such as softness, elasticity, processibility and insula-ion. Cross-linking is often applied to improve the thermal and

∗ Corresponding author. Tel.: +86 21 59554531; fax: +86 21 59558905.E-mail address: wuguozhong@sinap.ac.cn (G. Wu).

(tsiiit[

896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2008.08.009

E foam with the Sequence A.© 2008 Elsevier B.V. All rights reserved.

echanical properties of polyethylene, owing to the formationf a three-dimensional network. The cross-linking can be accom-lished either by high-energy irradiation, or through the use ofhemical cross-linking additives [15–17]. Compared to chemicalross-linking, radiation cross-linking has some advantages, suchs the formation of more uniform links in the polymer matrix, noesidual chemical cross-linking additives, and no byproducts gen-rated during the decomposition of cross-linking additives, whichan bring to bear a deleterious effect on the electrical performancef polyethylene [18].

Polyethylene foam is one of the most important plastic foams,hose commercial production started in the early 1940s [19].wing to its high heat and sound insulation capacity and excel-

ent shock absorption capability, PE foam is widely used in theuilding, packaging, and automotive industries, and by producers ofports and leisure articles. Typically, the cell size of commercial PEoams is 0.1–1 mm and their cell density is less than 106 cm−3 [20].or the production of commercial PE foams, chemical or physicaloaming agents are universally harmful to human and environ-

ent. For example, foaming agents such as chlorofluorocarbonsCFCs) and hydrochlorofluorocarbons (HCFCs) cause the destruc-ion of the ozone layer. It is well known that CO2 is chemicallytable, non-toxic, and easy to obtain from air. It also has a mild crit-

cal temperature (31 ◦C) and moderate critical pressure (7.38 MPa)n comparison with other supercritical fluids [21–23]. In addition,t can plasticize most polymers and decrease their glass transi-ion temperature, and has a relatively high solubility in polymers24,25]. Accordingly, supercritical carbon dioxide (scCO2) is an ideal

2 itical F

ria

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2

2

fs(

2

Sws

vHfbstasSsc(S

2

sr1cpuncdhathtttdtstbitdit

2

82 Z. Xing et al. / J. of Supercr

eplacement of traditional foaming agents and has been increas-ngly employed in the polymeric microcellular forming process asclean blowing agent.

The effect of saturation pressure, foaming temperature andepressurization rate on the morphology of polyolefin foams haseen extensively studied [26–32]. The results show that higher sat-ration pressure leads to higher cell density and smaller cells, whileigher foaming temperatures result in lower cell density and largerells. It has also been found that a higher depressurization ratenduces a higher number of cell nuclei. However, the study of foam-ng of irradiated PE with scCO2 has seldom been reported. In ourrevious work, foaming of PMMA microspheres [33] and microp-rous PVDF membranes [34] was carried out by using scCO2. In thisaper, we prepared cross-linked PE microcellular foam by a com-ination of radiation cross-linking and the scCO2 foaming process.wo processing sequences were used to prepare radiation-cross-inked low-density polyethylene (x-LDPE) sheets: (1) irradiationollowed by screw extrusion and hot pressing (Sequence A), and2) hot pressing followed by irradiation (Sequence B). The specimen

ade by Sequence A is named SA and the one made by Sequence B isamed SB. The Sequence A adopts the idea of radiation modificationf PE pellets prior to part formation [35]; therefore, it is an easy-to-ccomplish extrusion foaming technology. The cell structures of SAnd SB foams were compared in detail, and the effects of radiationose on cell size (D), cell density (Nf) and volume expansion ratioRv) were also investigated. The samples produced by Sequence Aan be expanded into microcellular foam and have higher volumexpansion ratio than the ones made by Sequence B. The Sequenceprovides a new way to prepare microcellular cross-linked LDPE

oams. The mean diameter of cells of x-LDPE foams is less than0 �m, and the value of cell densities of x-LDPE foams is more than09 cells/cm3 when all x-LDPE samples were foamed at the foamingemperature of 100 ◦C. The optimal radiation dose for the foamingf x-LDPE is found to be 50 kGy. Irradiation led to a wider foamingemperature range of LDPE.

. Experimental

.1. Materials

Low-density polyethylene (LD100-AC, pellet) was purchasedrom Sinopec Beijing Yanshan Chemical Corporation, with a den-ity of 0.9225 g/cm3 and melt index of 2.0 g/10 min. Carbon dioxide>99.5% pure) was from Loutang Special Gases of Shanghai.

.2. Irradiation of LDPE

The �-irradiation was carried out in the 60Co source of thehanghai Institute of Applied Physics. Two processing sequencesere used to prepare �-ray irradiated LDPE (Fig. 1). In the first

equence, PE pellets were first irradiated at different doses in sealed

2

bvf

Fig. 1. Process flow chart of two kinds

luids 47 (2008) 281–289

essels at room temperature, and then mixed in the mixer (Thermoaake PolyDrive, Germany) and hot pressed at 160 ◦C and 18 MPa

or 8 min into thin sheets in 1 mm thickness. The specimen madey this procedure is labeled sample of Sequence A (SA). In theecond sequence, PE pellets were initially hot pressed into 1 mmhin sheets under the same conditions, and subsequently irradi-ted at different doses in sealed vessels at room temperature. Thepecimen processed by this procedure is denoted as sample ofequence B (SB). The non-irradiated sheet is labeled sample S0. Allpecimens were cut into 15 mm × 8 mm pieces for the foaming pro-ess. The dose rate was 1.09 kGy h−1 and irradiation doses were 25SA25, SB25), 50 (SA50, SB50), 75 (SA75, SB75) and 100 kGy (SA100,B100), respectively.

.3. Foaming process

In this study, the saturation pressure, soaking time and depres-urization rate were kept constant at 22 MPa, 8 h and 0.7 MPa/s,espectively. Five different temperatures (100, 105, 110, 120, and30 ◦C) were used as foaming temperatures (Tf). The supercriti-al experimental setup was schematically depicted in the previousaper [33]. The foaming process consists of three stages: (1) sat-ration by CO2 at the desired temperature and pressure; (2) cellucleation and growth during the release of CO2 pressure; and (3)ell stabilization in the cooling process associated with the CO2epressurization. In the first stage, the specimens sealed into theigh-pressure vessel were preheated to the desired temperaturefter air was expelled from the vessel by CO2 gas. Subsequently,he CO2 pressure was increased up to the desired pressure using aigh-pressure liquid pump; simultaneously the temperature insidehe autoclave was increased and maintained at a predeterminedemperature using a band heater for temperature control. Whenhe temperature was stable, the specimens were kept at the pre-etermined pressure and temperature for 8 h. In this length ofime, the sample was completely saturated by scCO2 [36]. In theecond stage, in order to supersaturate the specimen with CO2,he autoclave was quickly depressurized to atmospheric pressurey opening a depressurization valve. The thermodynamic instabil-ty resulted in the formation of a large number of cell nuclei inhe LDPE samples. The temperature inside the autoclave quicklyecreased during the rapid depressurization. Eventually, the spec-

mens were removed from the vessel and cooled to ambientemperature.

.4. Foam characterization

.4.1. Morphology of foamMicrostructures of the foamed LDPE samples were characterized

y a LEO 1530 VP scanning electron microscope at an accelerationoltage of 5.0 kV. The samples were immersed in liquid nitrogen andractured at liquid nitrogen temperatures, then mounted on stubs.

of irradiated LDPE specimens.

Z. Xing et al. / J. of Supercritical Fluids 47 (2008) 281–289 283

a) and

TtPnu

N

wm(

biP

�

wtw

d

R

D

wo

2

etaad

X

wmv

2

cdmwf

3

3

iLLopfttc

teifSTLatment of more macromolecular segments. Survival radicals in theirradiated polymer can react with oxygen to induce oxidativedegradation during the mixing. In addition, scission and cross-linking of polyethylene chains can be simultaneously induced by�-irradiation. Oxidative scission prevails at low radiation doses

Table 1Melting point and enthalpy for SA and SB series of samples

Radiation dose (kGy) SA series SB series

Fig. 2. DSC curves of the melting transition of SA (

he fractured surfaces were sputter-coated with gold for observa-ion. The SEM photographs were analyzed by the software Imagero. With this method, the cell density (Nf) is determined by theumber of cells per unit volume of foam, which was calculatedsing Eq. (1):

f =(

nM2

A

)3/2

(1)

here n, M and A are the number of cells in the micrograph, theagnification of the micrograph, and the area of the micrograph

cm2), respectively.The densities of solid and foamed specimens were determined

y deducing the Archimedes Law involving weighing polymer foamn water with a sinker using an electronic analytical balance (HANG-ING FA2104), as described by Eq. (2):

=(

a

a + b − c

)�w (2)

here a, b and c are the weight of the specimen in air without sinker,he totally immersed sinker and the specimen immersed in waterith sinker, respectively, and �w is the density of water.

The volume expansion ratio (Rv) is the ratio of the solid polymerensity (�s) and the foam density (�f), as given by Eq. (3):

v = �s

�f(3)

The average cell diameter (D) was calculated using Eq. (4):

=∑

dini∑ni

(4)

here ni is the number of cells with an area-equivalent diameterf di.

.5. Melting point and crystallization

A Mettler Toledo DSC822e differential scanning calorimeterquipped with a data station was used for scanning the meltingransitions of virgin and �-irradiated samples of about 5–10 mg inluminum pans. The samples were heated from 0 ◦C up to 150 ◦Ct a heating rate of 10 ◦C/min under 50 mL/min nitrogen flow. Theegree of crystallization was calculated using Eq. (5):

c(%) = �Hf

�Hf0× 100 (5)

here �Hf is the melting enthalpy of LDPE and �Hf0 is the specificelting enthalpy for 100% crystalline polyethylene, which has a

alue of 287.3 J/g [37]. 1

SB (b) series of LDPE samples at different doses.

.6. Gel content

The gel content was determined using a 72 h Soxhlet extractionycle using xylene as the solvent. Approximately 1 g of the irra-iated LDPE was placed in a pre-weighed stainless steel fine wireesh. After extraction, the solid remainder was dried to a constanteight in a vacuum oven at 60 ◦C and then re-weighed. The gel

raction was sequentially obtained.

. Results and discussion

.1. Effect of irradiation on LDPE

Fig. 2 shows DSC curves of melting transitions of virgin andrradiated LDPE specimens. It is shown that melting point ofDPE decreases slightly after irradiation. The melting point ofDPE is listed in Table 1. Since the melting point is an indicationf the crystallite size and perfection, a decrease in the meltingoint implies smaller and/or less-perfect crystallites are beingormed in polymer matrix after irradiation [38]. The imperfec-ion in the polymeric chains caused by the irradiation restrainshe crystallization process, leading to smaller and less-perfectrystallites.

Fig. 3(a) shows the effect of the radiation dose on the crys-alline content of the irradiated samples (SA and SB). The meltingnthalpy of LDPE samples is listed in Table 1. The crystallinity ofrradiated LDPE decreased slightly with an increase in the doserom 25 to 100 kGy. However, the crystallinity of LDPE samplesA25 and SA50 was higher than that of non-irradiated LDPE.his can be attributed to the mixing process of the irradiatedDPE pellet in the mixer at 160 ◦C. The frequency of motionnd rearrangement of the macromolecular chain increases duringhe mixing process, which in turn may lead to regular arrange-

Tm (◦C) �Hf (J/g) Tm (◦C) �Hf (J/g)

0 113.1 148.9 – –25 107.9 161.2 108.1 144.350 108.0 153.9 108.1 141.375 108.1 145.9 108.0 141.000 108.2 140.5 107.9 139.2

284 Z. Xing et al. / J. of Supercritical Fluids 47 (2008) 281–289

les. (b

acpo

Fig. 3. (a) Crystallinity of irradiated LDPE samp

nd cross-linking dominates at higher doses [39–41]. The shorthains derived from degradation crystallize at a higher degree, butroduce smaller and less-perfect crystallites. The rearrangementf the macromolecular chains, and the degradation occurring at

lL

f

Fig. 4. SEM micrographs for the SA series of LDPE foams produced at 100

) Gel content of irradiated LDPE samples of SA.

ower radiation doses, promoted the crystallization of the irradiatedDPE.

Generally, the degree of polymer cross-linking can be estimatedrom gel fraction determination. Fig. 3(b) shows the percent-

◦C and at different doses (kGy): (a) 0; (b) 25; (c) 50; (d) 75; (e) 100.

Z. Xing et al. / J. of Supercritical Fluids 47 (2008) 281–289 285

F ced a

adis

ig. 5. Effect of radiation dose on the cell morphology parameters of SA foams produ

ge of the gel formed in the LDPE matrix at various radiationoses. It can be clearly seen that the degree of cross-linking

ncreased with increasing radiation dose over the range of valuestudied.

3

w

Fig. 6. SEM micrographs for the SA series of LDPE foams produced at 105

t 100 ◦C: (a) cell size distribution, (b) average cell diameter (D) and cell density (Nf).

.2. Morphology of cross-linked LDPE foams

The micrographs of specimens foamed at various temperaturesere also investigated. A statistical calculation on cell size and sum

◦C and at different doses (kGy): (a) 0; (b) 25; (c) 50; (d) 75; (e) 100.

286 Z. Xing et al. / J. of Supercritical Fluids 47 (2008) 281–289

d at 1

oiuL

ocFacctccwftsathft

Stcscs

ost

fb

dttituaLsi

tsftmtdttittcam

Fig. 7. SEM micrographs for the SB series of LDPE foams produce

f cells was performed using the SEM images. For a better compar-son, the characterization of the non-irradiated specimen foamednder the same conditions is presented together with the irradiatedDPE.

Fig. 4 shows the effect of radiation dose on the cell morphol-gy of SA samples foamed at 100 ◦C. It can be observed that theells were discrete, nearly spherical and surrounded by thick walls.ig. 5 shows the cell morphology parameters of SA samples foamedt 100 ◦C, such as cell size distribution, average cell diameter andell density. The width of the cell size distribution peak, which indi-ates the dispersity of the cell size, first decreased at 25 kGy andhen increased with increasing dose, and the widest width of theell size distribution peak was found at 100 kGy in Fig. 5(a). It is indi-ated that the uniformity of cell size became poor at higher dose,hich can be also clearly observed in the micrograph of the SA100

oam (Fig. 4(e)). The mean cell diameter was less than 10 �m andhe minimum cell density was more than 109 cm−3 for all foamingamples in Fig. 5(b). However, the dose-dependences of the aver-ge cell diameter and cell density were not regular. This is becausehe cells grew up slowly at a lower foaming temperature due to theigher elastic modulus and stiffness of the LDPE. Consequently, the

oaming behavior of irradiated LDPE cannot be clearly identified forhe foaming at relatively low temperature, e.g., 100 ◦C.

Fig. 6 shows typical SEM images of the fracture surfaces of theA series of LDPE foams produced at 105 ◦C. It should be notedhat homogeneous cells were formed and all foams had closed-ell structures. For the LDPE foams at 0 and 25 kGy, the walls ofome cells contained wrinkles and small holes. Polygonal closed-ell structures were observed at 50 kGy, while continuous spheres

urrounded by thick walls were observed at 75 and 100 kGy.

Fig. 7 shows SEM images of the fracture surface of the SB seriesf LDPE foams produced at 105 ◦C, in a similar manner as the SAeries of foams shown in Fig. 6. The uniformly closed-cell struc-ure of foams is clearly shown. It was found that cells of all the

otasp

05 ◦C and at different doses (kGy): (a) 25; (b) 50; (c) 75; (d) 100.

oams retained continuous spheres surrounded by a thick wall andecame smaller in contrast to those of the SA series of foams.

Fig. 8(a) and (c) shows the dose-dependence of cell size and sizeistribution. With an increase in radiation dose, the cell size andhe width of cell size distribution peak changed significantly forhe SA foaming samples, but only slightly for the SB series of foam-ng samples. The statistical results indicated that the morphology ofhe SA foams was different from that of the SB samples. For foamingnder the same conditions, the SA series of foams had larger cell sizend a wider dispersity of size distribution than the non-irradiatedDPE samples, whereas the cell size of the SB foams decreasedlightly and the change in the dispersity of size distribution wasnsignificant.

Fig. 8(b) and (d) shows the dose-dependence of the foam struc-ural parameters. It is clearly seen that the dose-dependence of cellize and cell density were completely opposite for the SA and SBoams. The average cell diameter of the SA samples increased ini-ially and then decreased with increasing radiation dose, having a

aximum value at 50 kGy. To the contrary, the average cell size ofhe SB foams first decreased and then increased with increasingose, having a minimum size at 50 kGy. The dose-dependence ofhe cell density also has a similar pattern. The difference betweenhe SA and SB methods in the dose-dependence of cell structures ascribed to the difference in the processing of LDPE (Fig. 1). Forhe SA samples, mixing after irradiation caused the transforma-ion of microstructure, e.g., motion and rearrangement of molecularhains. Therefore, cross-linking networks were partly destroyed,nd a separation of gel and sol phases probably took place in theatrix of the SA samples [42–46]. The influence of hot mixing

n the phase variation in LDPE was more significant at 50 kGyhan at lower or higher doses, because gel and sol fractions arelmost equal at this dose. For the SB samples, the cross-linkingites generated by irradiation were uniformly distributed in theolymer.

Z. Xing et al. / J. of Supercritical Fluids 47 (2008) 281–289 287

F oss-lina ion of

SpstSaioosubmcmv

adwfdictltfs

ig. 8. Effect of radiation dose on the cell morphology parameters of irradiation-crverage cell diameter (D) and cell density (Nf) of the SA foams, (c) cell size distribut

Fig. 9 shows the variation of volume expansion ratio (Rv) of theA and SB series of foams as a function of dose for foaming tem-eratures of 100, 105, 110, 120 and 130 ◦C, respectively. Our resultshow that the Rv of these series of foams generally increased withemperature, and that the Rv of SA foams was higher than that ofB foams. The Rv increased initially, and then decreased as radi-tion dose increased. The maximum Rv was observed at 50 kGy,mplying an optimum radiation dose of 50 kGy for the foamingf LDPE. The change in Rv is attributed to the effect of radiationn the chain structure of LDPE. It is known that radiation causestructural changes in LDPE, including cross-linking, degradation,nsaturation, branching and oxidation [39,41]. Cross-linking and

ranching enhance the chain entanglement and interaction amongacromolecules, resulting in higher melt strength and viscosity. In

ontrast, degradation and oxidative scission lead to a decrease ofacromolecular chain length, resulting in lower melt strength and

iscosity [30,35]. The oxidative scission mechanism occurs mainly

w

iat

Fig. 9. Dose-dependence of volume expansion ratio (Rv) for the

ked LDPE foams produced at 105 ◦C: (a) cell size distribution of the SA foams, (b)the SB foams, (d) average cell diameter (D) and cell density (Nf) of the SB foams.

t low radiation dose, while the cross-linking prevails at higherose [39,41,47]. The melt strength of LDPE irradiated at 25 kGy waseak in comparison with LDPE irradiated at a higher dose. There-

ore, the cell wall was breached easily, and cells collapsed at theose of 25 kGy. However, the melt strength of cross-linking LDPE

rradiated at 100 kGy became so strong that the development ofell nuclei was restrained, leading to a decrease in cell density andhe Rv of LDPE foam. As a result, lower or higher doses led to aower Rv of the foam, and cells grew well only at moderate radia-ion dose, e.g., 50 kGy. In addition, the volume of the SA series ofoams was larger than that of the SB samples after expansion at theame foaming conditions, indicating that foaming of the SA sample

as easier than that of the SB sample under the same conditions.

Fig. 10 shows the micrographs of fracture surfaces of three spec-mens (S0, SA50 and SB50) foamed at 130 ◦C. As for S0, cells fusionnd the breaking up of cell walls are shown clearly. This is due tohe foaming temperature of 130 ◦C being too high for the foaming of

SA (a) and SB (b) series of foams produced at 100–130 ◦C.

288 Z. Xing et al. / J. of Supercritical Fluids 47 (2008) 281–289

cimen

tssaogsom

4

�ssteSIreua

A

t

R

[

[

[

[

[

[

[

Fig. 10. SEM micrographs for three foaming spe

he original LDPE. Under this condition, the elasticity modulus andtiffness of LDPE are so weak that cells collapse during the depres-urization stage. In the case of SA and SB, the cells remained intactnd displayed an impinging polygonal closed-cell structure. More-ver, cells of the SA50 foam inflated more than those of SB50, inood agreement with the measurement of Rv. It has been demon-trated that irradiation treatment results in significant expansionf the foaming temperature range for LDPE due to the increase inelt strength and viscosity.

. Conclusions

We have investigated the foaming behavior of two series of-irradiated LDPE by using scCO2 as a blowing agent. The twoeries of samples revealed a reverse pattern of cell structure (cellize distribution, cell size and cell density) with increasing radia-ion dose. The SA series of specimens possessed a higher volumexpansion ratio, greater cell size and lower cell density than theB series of specimens at the same dose and foaming conditions.rradiation led to a wider foaming temperature range of LDPE. Theadiation dose of 50 kGy is optimum for the foaming of LDPE. Thexperimental sequence of SA samples may be suitable for contin-al industrial production of cross-linking PE foams by connectingscrew extruder to a scCO2 supply system.

cknowledgement

This work was supported by the “Hundred talents” project ofhe Chinese Academy of Sciences.

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