1. IntroductionPolymers with excellent electrical insulation havebeen extensively used as packing materials in elec-trical devices due to their good process ability, lightweight and low cost. Nevertheless, the thermal con-ductivity of polymers are generally very low (i.e.0.100.25 W/m1K1), and it has been widely rec-ognized that the thermal conductivity of these poly-mers has to be enhanced to lower the energy lossand increase the stability of the devices. With theincreasing demand for high density power andenergy transmission of electronic devices, fabrica-tion of insulating polymer materials with high ther-mal conductivity has become crucial [14].High crystallization and orientation of polymer cangreatly enhance the thermal conductivity along theorientation direction, but they usually have difficul-

ties of processing [5]. Blending of polymer withinorganic fillers is an effective and convenient wayto enhance the polymer thermal conductivity whilemaintaining the electrical insulation, such as poly-mer composites with boron nitride (BN) [6, 7], alu-minum nitride (AlN) [2, 3], silicon nitride (Si3N4)[8, 9], alumina [10], silicon carbride (SiC) [11] andsilica (SiO2) etc. [12] have been investigated. In theinorganic filler/polymer composites, the thermalconductivity increases with the filler content. Veryhigh filler loading is often used to obtain high ther-mal conductivity. However, it seriously alters thepolymer component and can form agglomerates,which induces stress concentration and decrease ofthe tensile strength, modulus and ductility of thematerial [13], so the practical application of thermalconductive polymer composites is limited.

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The effects of particle size and content on the thermalconductivity and mechanical properties of Al2O3/high density polyethylene (HDPE) compositesS. Zhang1,2, X. Y. Cao1, Y. M. Ma1*, Y. C. Ke1, J. K. Zhang1, F. S. Wang1

1Beijing National Laboratory for Molecular Sciences, Laboratory of New Materials, Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100190, P. R. China

2Graduate School of Chinese Academy of Sciences, Beijing 100190, P. R. China

Received 26 October 2010; accepted in revised form 10 January 2011

Abstract. The influences of filler size and content on the properties (thermal conductivity, impact strength and tensilestrength) of Al2O3/high density polyethylene (HDPE) composites are studied. Thermal conductivity and tensile strength ofthe composites increase with the decrease of particle size. The dependence of impact strength on the particle size is morecomplicated. The SEM micrographs of the fracture surface show that Al2O3 with small particle size is generally more effi-cient for the enhancement of the impact strength, while the 100 nm particles prone to aggregation due to their high surfaceenergy deteriorate the impact strength. Composite filled with Al2O3 of 0.5 m at content of 25 vol% show the best syntheticproperties. It is suggested that the addition of nano-Al2O3 to HDPE would lead to good performance once suitably dis-persed.

Keywords: polymer composites, mechanical properties, thermal conductivity

eXPRESS Polymer Letters Vol.5, No.7 (2011) 581590Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2011.57

*Corresponding author, e-mail: maym@iccas.ac.cn BME-PT

Filler size and shape are also important factors forthe thermal conductivity and mechanical properties[14, 15]. Zhou et al. [16] used Al2O3 with differentsize to fill silicone rubber, and reported that nano-sized Al2O3 composite exhibited higher thermalconductivity and mechanical properties than themicro-sized one. The influence of hybrid fillers wasalso investigated [17, 18]. However, reports on howto enhance the thermal conductivity and mechanicalproperties of the polymer composites simultane-ously are still quite limited.The particle size and content in the composite deter-mine the average interparticle distance [19, 20],which is close related to the thermal conductivityand mechanical properties. At the same particlecontent, smaller particle size leads to lower inter-particle distance and more chances for the forma-tion of thermal conductive pathway [2123]. Theparticle size and content affect the interparticle dis-tance and the stress state of the matrix polymer sur-rounding the voids. When the average interparticledistance is in a suitable range, extensive plasticdeformation in the matrix can be easily induced[2426]. So the optimization of the particle size andcontent can be a convenient and feasible way toprepare composites with good synthetic properties.In this paper, Al2O3/HDPE composites with differ-ent alumina particle sizes (diameter of 10, 4.7,0.5 m and 100 nm, denoted as A10, A4.7, A0.5and A0.1) were prepared. The influence of aluminacontent and particle size on thermal conductivity,impact strength and tensile strength are studied. Thesynthetic properties of the composites are optimizedand the mechanism is also discussed.

2. Experimental2.1. MaterialsThe polymer used in this study was high densitypolyethylene (HDPE) 5000S (SinoPec BeijingYansan Petrochemical Co. Ltd, Beijing, China). Ithad a density of 0.95 g/cm3 and a melt flow index

of 0.9 g/10 min (2.16 kg at 190C). Four types ofalumina particles with average diameters of 10, 4.7,0.5 m and 100 nm (denoted as A10, A4.7, A0.5and A0.1 respectively, >!99 wt% purity) were usedas fillers. The suppliers and parameters of the alu-mina particles were listed in Table 1. All of the alu-mina particles were of "-crystalline and the thermalconductivity was about 30 W/(mK). The aluminaparticles were added to the HDPE without surfacepretreatment.

2.2. Sample preparationThe blends of HDPE with various alumina particleswere prepared by a two-roll mill (X(S) K-160, Dou-ble Elephant Group Company, Jiangsu, China) for20 min. The temperature of the front roll and backroll were 140 and 130C respectively. After extru-sion, the samples were cut into small pieces.The samples for thermal conductivity and mechani-cal property tests were prepared by injection mold-ing (SZ-68/400, Liuzhou Rubber and Plastic Machin-ery Factory, Guangxi, China) at 200C. The samplesfor thermal conductivity measurement were squarebars of 50 mm in length and 4 mm in thickness.Tensile and impact bars were prepared according toASTM D638 Type I and ASTM D256.The maximum contents of alumina A4.7 and A0.5in the composites were 50 vol%, but those of A10and A0.1 were 30 vol%. The reasons for the diffi-culties of processing for 10 and 0.1 m particlecomposites are different. The surface area of the0.1 m alumina particles is very large. When fillercontent is higher than 30 vol%, it is very difficultfor HDPE to wet the surface of all nano-particles,and constrained the blending. On another hand, the10 m particles increase the roughness of the com-posite surface. When the 10 m alumina content ishigher than 30 vol%, the surface of the compositebecomes too rough to measure the thermal conduc-tivity for it cant make a good contact with heatsource. The compositions were represented by the

Zhang et al. eXPRESS Polymer Letters Vol.5, No.7 (2011) 581590

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Table 1. The suppliers and data of alumina fillers

*The values were given by the suppliers

Filler code Supplier Surface area*

[m2/g]Mean particle size*

[m]Geometrical standard

deviation*

A10 Shanghai Yuejiang Titanium Chemical Manufacture Co.Ltd. CHN, Shanghai, China 0.6 10.0 1.5

A4.7 Fujian Johnson Mining Co. Ltd. CHN, Xiamen, China 1.5 4.7 2.0A0.5 Wan Jing New Material Co. Ltd. CHN, Hangzhou, China 5.0 0.5 2.8A0.1 Wan Jing New Material Co. Ltd. CHN, Hangzhou, China 12.5 0.1 3.6

alumina volume fraction. The real composition ofthe composites was measured by the calcinationmethod, and the difference between the theoreticalcomposition and the real one is less than 2%.

2.3. Characterization2.3.1. Thermal conductivityThe through-plane thermal conductivity was meas-ured at 30 and 50C with thermal conductivity ana-lyzer (HC-110, EKO Instruments Co. Ltd, SasazukaShibuya-ku, Japan).

2.3.2. Impact and tensile testThe notched Izod impact strength was measured ona ceast pendulum impact strength tester CSI-137Cat 23C (Wuzhongshi Material Tester Co. Ltd,Ningxia, China), according to ASTM D256. Thedrop velocity was 3.5 m/s and the testing resultswere the average of ten parallel measurements. Thetensile measurements were carried out at 23Cusing an Instron 3365 universal materials testingmachine (Instron Corporation, Massachusetts,America), according to ASTM D638. The averagevalues of the mechanical properties in this studywere obtained by five independent measurements.

2.3.3. Scanning electron microscope (SEM)The as prepared composites were cryogenicallyfractured in liquid nitrogen. Both the cryogenicallyfractured and impact fractured surfaces of compos-ites were inspected by SEM ( JSM-6700F, JapanElectron Optics Laboratory Co. Ltd, Tokyo, Japan)at an acceleration voltage of 3 kV. The surfaces ofthe samples were coated with a conductive plat-inum layer before inspection.

3. Results and discussion3.1. Dispersion of Al2O3 in the HDPE matrixThe dispersion of the alumina particles differs withparticle content and particle size, which determinesthe properties of the composites [27, 28]. The com-posites cryogenically fractured surfaces wereinspected by SEM (Figure 1 and Figure 2).For a certain size of alumina particle, the dispersionin the composite changes with content. Their trendsare similar when the particle size #!0.5 m. Thecomposite of A4.7/HDPE is taken as a typical exam-ple to show t