journal of thermoplastic composite effect of both...

Article

Effect of both nano-sizealumina particles andsevere deformation onpolyethylene crystallinityindex

Gh Tadayyon, SM Zebarjad and SA Sajjadi

Abstract

The morphological and microstructural changes during ball milling of PE powder mixed

with different weight percent (5–15 wt%) of Alumina particles (50–100 nm) were stud-

ied. The milling was performed in a planetary ball mill for various times up to 40 h. The

results showed that the addition of hard particles accelerates the milling process, lead-

ing to fracture of polyethylene matrix. The result of X-ray diffraction analysis deter-

mined that the degree of crystallinity of polyethylene decreased by increasing of ball

milling time and weight percent of Alumina.

Keywords

nanocomposite, polyethylene matrix, alumina, ball mill, crystallinity

Introduction

Polymer systems are used because of to their rare properties: ease of production,light weight, and often ductility. One way to enhance their properties is to reinforcepolymers with fibers, whiskers, platelets, or particles.1 The embedding of reinforce-ment in a matrix to produce composites has been a common practice for manyyears. Using this approach, polymer properties can be improved while maintainingtheir suitable characteristics. Improvements in properties can often be found evenat relatively low filler content.1–11 Polymer nanocomposites offer new technological

Department of Materials Science and Engineering, Engineering Faculty, Ferdowsi University of Mashhad, P.O.

Box 91775-1111, Azadi Square, Mashhad, Iran

Corresponding author:

SM Zebarjad, Department of Materials Science and Engineering, Engineering Faculty, Ferdowsi University of

Mashhad, P.O. Box 91775-1111, Azadi Square, Mashhad, Iran

Email: [email protected]

Journal of Thermoplastic Composite

Materials

25(4) 479–490

! The Author(s) 2011

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DOI: 10.1177/0892705711415186

jtc.sagepub.com

and economical benefits. The nanocomposites which include nanoparticles exhibitsuperior properties such as enhanced mechanical and thermal properties.11

Making good samples of polymer matrix nanocomposites is a challenging areathat draws considerable effort. Creating one universal technique for making poly-mer nanocomposites is difficult due to the physical and chemical differencesbetween each system and various types of equipment available to researchers.Each polymer system requires a special set of processing conditions to beformed, based on the processing efficiency and desired product properties. Thedifferent processing techniques in general do not yield equivalent results.1

Researchers have tried a variety of processing techniques to make polymermatrix nanocomposites. These include melt mixing, in situ polymerization, solgel, mechanical milling, and other approaches.1–8 Ball milling, which has beenone of the main methods to prepare traditional polymer microcomposites, canbe used for preparation of polymer nanocomposites.2–4

According to a literature survey done by the authors, there are many articlesthat focus on the polymer nanocomposite, and they can be categorized in severalapproaches.

Vollenberg and Heikens12 were able to produce good nanocomposite samples bythoroughly mixing filler particles with polymer matrix. The polymer matrices usedin these experiments were polystyrene (PS), styrene-acrylonitrile copolymer (SAN),polycarbonate (PC), and polypropylene (PP). The reinforcement was aluminabeads 35 nm and 400 nm in diameter. The results showed that the PS and PPhad a very poor interaction with the particles due to their nonpolar character,while the interfacial interaction was much more noticeable for the SAN and PC.For composites with PS, PC and PP matrix and alumina particles the modulusincreased with decreasing size of particles. For all systems, the elastic modulusincreased with increasing volume fraction of the reinforcement.5–6

Alumina is a ceramic metal oxide of great importance as building material,refractory material, electrical and heat insulator, attributed to its high strength,corrosion resistance, chemical stability, low thermal conductivity, and good elec-trical insulation.13 The incorporation of nano-scaled alumina in PP has improvedthe mechanical properties of the polymer composites14 and the wear resistance ofPET filled by nearly 2� over the unfilled polymer.15

Mohamad et al. reported that the presence of uniformly distributed aluminananoparticles have efficiently hindered polymer chain movement during deforma-tion, and contribute to the high stiffness of the composites.16 Alumina presencethermoplastic and thermosetting polymeric materials are also gaining wide appli-cation as surface coatings. The addition of alumina nanoparticles is meant toenhance the mechanical and thermal properties compared to an absence of suchconstituents. Generally, it has been found that the presence of low loading ofalumina nanoparticles tend to enhance the thermal and mechanical properties ofthe polymer matrix. A challenge has been observed to be overcome to facilitate theenhancement of the properties of the polymer matrix, namely the need to dispersethe nanoparticles uniformly throughout the polymer. At low loading, the

480 Journal of Thermoplastic Composite Materials 25(4)

nanoparticles could be distributed uniformly across the polymer.13 Various prepar-ative methods have been adopted to facilitate good dispersion. This includesmechanical milling Alumina nano fiber17 and mechanical milling followed by hotextrusion.18

The degree of crystallinity of a polypropylene system was found to change verylittle with the addition of CaCO3 nanoparticles. Moreover, the size of the crystal-line domain spherulites was found to change dramatically in different systems.Scanning electron microscopy (SEM) showed that the spherulites in the puresystem had an average size of around 40 mm, but no spherulites could be seenwhen CaCO3 nanoparticles were added to the system. It is possible that the spher-ulites were too small and they were not detected by SEM.1

The addition of nano-size montmorillonite clay platelets to a polyamide-6matrix was found to have no effect on the degree of crystallinity of the polymerwith weight fraction up to 5%. However, the nanoparticles did have an effect onthe size of the crystallites.7 The size of crystallites in the nanocomposites was nearlyan order of magnitude smaller than the size of spherulites in the pure matrixsystem.1,7 Results for a polyamide-6 matrix composite with silica nanoparticlesshowed that neither the size nor the filler content had any effect on the crystallinityof the system.7

Abareshi et al.19 fabricated MDPE/clay nanocomposite using the ball-millingmethod. They showed that both milling time and clay content had not significanteffect on the crystal structure of MDPE matrix. They believed that the microcrys-talline dimension Lhkl of MDPE-clay nanocomposite was less than that of pureMDPE. They also showed that the addition of clay to MDPE caused the reductionof the crystalline size of MDPE, although ball milling could also be an effectiveparameter in reducing the crystallite size of MDPE. Abareshi et al. proved that theball milling had influence on the crystallinity of MDPE, especially during the earlystage of milling, and the crystallinity of MDPE decreased as the clay contentsincreased.

Based on the literature survey, there is no any evidence of study concentrated onpolyethylene-alumina nanocomposites. Thus, the main goal of this study is tofabricate polyethylene-alumina nanocomposites by high energy ball milling as anew method. Also, attempts will be made to elucidate the role of both ball millingand alumina nanoparticles on the crystallinity index of medium-densitypolyethylene.

Experimental

Materials

Medium-density polyethylene was used as matrix. Alumina particles provided fromNanolin company were used as reinforcement. The specification of used MDPEand alumina are summarized in Table 1. Figure 1 shows the TEM micrographtaken from the nano-size alumina.

Tadayyon et al. 481

Composite preparation method

The mixture of pure medium-density polyethylene and different weight percent ofalumina (0–15wt%) were put in to a stainless steel vial with stainless steel balls.The size of the balls varied: 13 balls with 10mm diameter, and 7 balls each with 12and 15mm diameters. The ratio of ball mass to the mixture load was kept constantat 20:1.

Ball milling was carried out using planetary high energy ball-milling machine(PM-200) at 250 rpm, for 5, 10, 20 and 40 hours. Then the produced nanocompo-sites were characterized by X-ray diffraction.

Figure 1. TEM picture of nanoscaled alumina.

Table 1. The specification of starting materials.

Materials Type Properties Particle shape

Resin MDPE Density: 0.937 (g cm�3) Nearly spherical

Melting point: 127�CSemi-solid temperature: 117�C

Reinforcement Alumina Alpha-aluminum oxide Irregular

Purity: 99.5%

Average particle size: 70 nm


Morphological analysis

TEM examinations provided information about PE/Al2O3 nanocomposite. After20 hours’ milling, the initial particles were deformed and a change from irregular tobean shape was noticed (Figure 2a, b).The experimental results indicate thatincreasing ball milling time enhances alumina dispersion and decreases size ofparticles. 40 hours of milling was sufficient to reduce the alumina size to �60 nm(Figure 2c, 2 d). Note that the fragmentation of the bean-like particles and distri-bution of nanoparticles may occur concurrently. It can be expected that for a givenvolume fraction (15wt%), the smaller mean size of particles and the narrower sizedistribution will enhance the homogeneity of the nanocomposite. The shape of theparticles probably also plays a role. It may be guessed that particles with moreregular shapes should be more effective at a given size and volume fraction. It isimportant to highlight here that particles with irregular shapes of alumina can beprepared from this method with a specific size, at least, as an average and with ahomogeneous distribution in matrix.

Figure 2. TEM micrographs of MDPE–5 wt% Al2O3 at various milling times; (a) and (b) 20 h,

(c) and (d) 40 h.

Tadayyon et al. 483

X-ray diffraction (XRD)

XRD patterns were obtained using a Bruker/D8 ADVANCE diffractometer withCu Ka radiation (�=0.15406 nm) in the range of 2y=4–70� at 0.03� increments.

The degree of crystallinity of samples was quantitatively estimated following themethod of Nara and Komiya;20 a smooth curve which connected peak baseline wascomputer-plotted on the diffractogram by Smadchrom software (Figure 3). Thearea above the curve was taken as the crystalline portion, and the lower areabetween the curve and linear baseline which connected the two points of the inten-sity was taken as the amorphous section.21

The ratio of upper area to the total diffraction area was considered as the degreeof crystallinity. The equation of the degree of crystallinity is as follows:

Xc ¼ Ac = ðAc þ AaÞ ð1Þ

where Xc, Ac and Aa refer to the degree of crystallinity; crystallized and amorphousarea on the X-ray diffractogram respectively.

Results and discussions

Figure 4 shows the XRD spectrum of un-milled pure polyethylene and alumina. Asseen in the figure, an amorphous peak of polyethylene at diffraction angle of about20 and two crystalline peaks at diffraction angle of 21.7 and 24 appear. Aluminapeaks at diffraction angle are: 25.7, 35.5, 38, 43.5, 52.7, 57.6, 61.4, 66.7 and 68.3respectively. In fact polyethylene crystallizes in an orthorhombic crystalline struc-ture with lattice dimensions of a=7.40 Å, b=4.93 Å and c=2.534 Å, with thec-direction being the chain direction. Typically only three strong reflections areobserved, the former is the amorphous step and the others are crystalline

Figure 3. Calculation of the relative degree of the crystallinity.21


diffraction peaks. These peaks is due to X-ray diffraction from the (110), (200)planes whereas the (200) being the prominent peak. First peak is superimposed on abroad region of intensity which results from the amorphous component of thepolymer (polymers are usually not 100% crystalline). This amorphous halo reflectsa preferred spacing of PE chains in the amorphous state.

The dependence of XRD profiles of pure MDPE on milling time is shown inFigure 5. The position of the maximum peaks in the patterns of MDPE before andafter ball milling is similar, but the intensity of peaks has remarkably decreased asmilling time increases. In fact, severe plastic deformation of the polyethylenepowder can lead to accumulation of internal stress.2 As the plastic deformation

0 10 20 30 40 50 60 70 80

2 teta

(200)

(110)

Neat PE

Pure Alumina

Figure 4. X-ray diffraction patterns of neat PE and as-received Alumina before ball milling.

Pure PE

5 15 25 35 45 55 65

2 teta

0 hr5 hr

10 hr20 hr

40 hr

Figure 5. X-ray diffractogram of pure MDPE at different milling times.

Tadayyon et al. 485

continues (it means that mechanical milling continues), work hardening of thedeformed particles reached to a critical value, leading to activation of the fractureprocess. As a matter of fact, with increasing milling time, internal strain increasessteadily.2 The peak broadening of the milled powders represents a decrease in thecrystallite size and accumulation of lattice strain. Therefore, high-energy mechan-ical milling of polyethylene results in amorphization, as evidenced by reducedintensities and peak broadening in X-ray diffraction patterns (see Figure 5). Theposition of the XRD peaks doesn’t have any shift that can be related to the lack ofchange in the lattice structure. This result is similar to what Abareshi et al.proposed.12

The dependence of X-ray diffraction patterns of produced MDPE matrix nano-composite powders on nano-size alumina content and milling time are shown inFigures 6 and 7. Indeed the peaks of MDPE do not show any significant shift as thealumina content increases. It implies that all matrix nanocomposites have the samecrystal structure, and that alumina nano-size powders cannot change the crystalstructure. With the alumina loading, the peaks have become wider and their inten-sities have decreased. On the other hand, when hard particles are added to poly-ethylene powder, the chain scission occur earlier, the agglomerations are broken tosmaller pieces, and their morphology changes because of nanodispersed aluminadisrupts the ordered structure of polyethylene. This is because alumina is brittleand acts as a concentration point, and causes failure of crystallites and increasesdegree of amorphous. Figure 8 shows the X-ray diffraction patterns of PE/15wt%Al2O3 powder after selected ball-milling time. The intensity of the peaks decreasesafter the 5 hours of milling time, whereas the peak positions do not change. Furthermilling, to 20 hours, causes the intensity of peaks decreases but it is not dramatic.

Figure 9 shows the estimation method for determining degree of crystallinity.The Area between the new curve and baseline which connected the two points ofthe intensity 2y of 10� and 25.6� in the samples was taken as the amorphous section.

ball milling time: 5 hr

0 10 20 30 40 50 60 702 teta

Pure PE

PE / 5wt% Al2O3

PE / 10wt% Al2O3

PE / 15wt% Al2O3

Figure 6. X-ray diffractogram of MDPE matrix nanocomposite as a function of alumina con-

tent after 5 hours milling.


The ratio of amorphous area to total diffraction area was taken as the degree ofamorphousness. The degree of crystallinity can be obtained using the followingequation:

Xc ¼ 1�Xa ¼ 1� ðAa=ðAcþAaÞÞ ð2Þ

Degree of crystallinity of pure and un-milled polyethylene is 36.5. The additionof nano-size alumina to polymer matrix is found to have an effect on the degree of

ball milling time: 20 hr

0 10 20 30 40 50 60 70

2 teta

Pure PE

PE / 5wt% Al2O3

PE / 10wt% Al2O3

PE / 15wt% Al2O3

Figure 7. X-ray diffractogram of MDPE matrix nanocomposite as a function of alumina con-

tent after 20 hours milling.

PE / 15wt% Al2O3

0 10 20 30 40 50 60 702 teta

0 hr5 hr

10 hr20 hr

Figure 8. X-ray diffractogram of MDPE/15 wt% Al2O3 at different milling time.

Tadayyon et al. 487

crystallinity of the polyethylene with weight fraction up to 10%. After 20 hoursworth of milling, the degree of crystallinity remains nearly constant with a finalvalue of about 24%. Considering these data, results determine that milling time ismore effective than alumina weight percent. The presented results in Figure 10summarize that during the early state of milling, degree of crystallinity decreasesrapidly and slow down afterwards.

The achieved results are similar to those reported by other investigators. Forexample, the addition of nano-size montmorillonite clay platelets to a polyamide-6matrix was found to have no effect on the degree of crystallinity of the polymerwith weight fraction up to 5%. However, the nanoparticles did have an effect on

Pure PE - 40hr ball mill

0

50

100

150

200

250

300

350

400

450

10 12 14 16 18 20 22 24 262 teta

Lin

(co

un

ts) main curve

estimated curve

Figure 9. Estimation of the relative degree of the crystallinity.

22

24

26

28

30

32

34

0 5 10 15 20 25 30 35 40 45ball milling time (hr)

Deg

ree

of c

ryst

allin

ity

Pure PE

PE / 5wt% Al2O3

PE / 10wt%A2O3

PE / 15wt% Al2O3

Figure 10. Degree of crystallinity versus milling time for different samples.


the size of the crystallites. The size of crystallites in the nanocomposites was nearlyan order of magnitude smaller than the size of spherulites in the pure matrixsystem.5 Results for a polyamide-6 matrix nanocomposite with silica nanoparticlesshowed that neither the size nor the filler content had any effect on the crystallinityof the system.8 Park et al.22 found that for (syndiotactic) polystyrene–organoclaynanocomposites, dispersed clay layers act as nucleating agent competing with crys-tal growth in polymers. Thus, exfoliated clay nanocomposites have a lower degreeof crystallinity with faster crystallization rate. However, the crystallinity of crys-talline and semi-crystalline polymers was not affected very much by the addition ofnanoparticles. There may be some changes in particular nanocomposite systems,but overall no major differences in crystallinity of nanocomposites versus neatpolymers were observed in any of the systems examined.1

Conclusions

PE/Al2O3 nanocomposite can be prepared by dry high-energy ball milling frompolyethylene and Alumina nanoparticles. During mechanical milling of PE/Al2O3powders, the morphology and structure of the particles undergo continuouschanges. Plastic deformation, cross-linking, and chain scissions of materials areall dominant mechanisms which influence the characteristics of milled powders.The results show that the addition of nano-scaled alumina particles acceleratedmilling process of polyethylene powder, and small hard particles may act as smallmilling agent. Therefore, the fracture process is started earlier. After ball milling,polyethylene is pulverized to a disorder and amorphous state, while the dispersedAl2O3 particles remain nanocrystalline. With increasing of the volume fraction ofalumina particles, crystallinity percent decreases. During the early stages of milling,the degree of crystallinity decreases rapidly, and slows down afterwards.

References

1. Jordan J, Jacob KI, Tannenbaum R, Sharaf MA and Jasiuk I. Experimental trends in

polymer nanocomposites—a review. Materials Science and Engineering: A 2005; 393:1–11.

2. Nicolais L and Carotenuto G. Metal-polymer nanocomposite. Italy: John Wiley & Sons,INC, 2005, pp.145–170.

3. Pomogalio AD and Kestelman VN. Metallopolymer nanocomposite. Germany: Springer-Verlag Berlin Heidelberg, 2005, pp.120–435.

4. Mai YW and Yu Zh. Polymer nanocomposites, first edn. Cambridge, England: Woodhead

Publishing Limited, 2006.5. Hussain F, Hojjati M, Okamoto M and Gorga RE. Journal of Composite Materials 2006;

40: 1511–1575.

6. Gacitua WE, Ballerini AA and Zhang J. Polymer Nanocomposites: Synthetic andNatural Fillers-A Review Maderas. Ciencia y tecnologı́a 2005; 7(3): 159–178.

7. Ray SS and Okamoto M. Polymer/layered silicate nanocomposites: a review from prep-

aration to processing. Progress in Polymer Science 2003; 28: 1539–1641.

Tadayyon et al. 489

8. Usuki A, Hasegawa M and Kato M. Polymer-clay nanocomposites, first ed., Toyota

Central R&D Labs., Inc., Nagakute, Aichi, Japan: Springer-Verlag, Berlin, Heidelberg,2005, pp.480–1192.

9. Costa Francis R, Abdel-Goad M, Wagenknecht U and Heinrich GN. Nanocomposites

based on polyethylene and Mg-Al layered double hydroxide. I. Synthesis and charac-terization. Polymer 2005; 46: 4447–4453.

10. Zanetti M, Bracco P and Costal L. Thermal degradation behaviour of PE/clay nano-composites. Polymer Degradation and Stability 2004; 85: 657–665.

11. Ray SS and Okamoto M. Polymer/layered silicate nanocomposites: a review from prep-aration to processing. Progress in Polymer Science 2003; 28: 1539–1641.

12. Vollenberg PHT and Heikens D. Particle size dependence of the Young’s modulus of

filled polymers: 1. Preliminary experiments 1989; 30(9): 1656–1662.13. Ridzuan Noordin, M. and Liew, K. (2010). Synthesis of Alumina Nanofibers and

Composites, In: Ashok Kumar (ed.), Nanofibers. INTECH: Croatia, pp.405–418.

Available from: http://sciyo.com/articles/show/title/synthesis-of-alumina-nanofibers-and-composites.

14. Jung CH, Choi JH, Lim YM, Jeun JP, Kang PH and Nho YC. Preparation andCharacterization of Polypropylene Nanocomposites Containing Polystyrene-grafted

Alumina Nanoparticles. Journal of industrial and engineering chemistry 2006; 12:900–904.

15. Bhimaraj P, Burris DL, Action J, Sawyer WG, Toney CG, Siegel RW and Schadler LS.

Effect of matrix morphology on the wear and friction behavior of alumina nanoparticle/poly (ethylene) terephthalate composites. Wear 2005; 258: 1437–1443.

16. Mohamad N, Muchtar A, Ghazali MJ, Hj. Mohd D and Azhari CH. The Effect of

Filler on Epoxidised Natural Rubber-Alumina Nanoparticles Composites. EuropeanJournal of Scientific Research 2008; 24: 538–547.

17. Santos-Beltran A, Gallegos-Orozco V, Estrada-Guel I, Gomez LB, Magana FE,

Yoshida M and Sanchez RM. TEM characterization of Al-C-Cu-Al2O3 compositesproduced by mechanical milling. Journal of Alloys and Compounds 2007; 514: 434–435.

18. Flores-Zamora MI, Estrada-Guel I, Gonzalez-Hernandez J, Miki-Yoshida M andMartinez-Sanchez R. Aluminium-graphite composites produced by mechanical milling

and hot extrusion. Journal of Alloys and Compounds 2007; 434(35): 518–521.19. Abareshi M, Zebarjad SM and Goharshadi EK. Crystallinity bhavior of MDPE-clay

nanocomposites fabricated using ball milling method. Journal of Composite Materials

2009; 43: 2821–2830.20. Nara S and Komiya T. Studies on the Relationship Between Water-satured State and

Crystallinity by the Diffraction Method for Moistened Potato Starch. International

journal for the investigations, processing and use of carbohydrates and their derivatives1983; 35(12): 407–410.

21. Shujun W, Jinglin Y and Wenyuan G. Use of X-ray Diffractometry (XRD) for

Identification of Fritillaria According to Geographical Origin. American Journal ofBiochemistry and Biotechnology 2005; 1(4): 207–211.

22. Park C, Choi WM, Kim MH and Park OOK. Thermal and mechanical properties ofsyndiotactic polystyrene/organoclay nanocomposites with different microstructures.

Journal of Polymer Science Part B: Polymer Physics 2004; 42(9): 1685–1693.


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