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On the Dependence of Avrami Indexes of MDPE on Milling Time

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On the Dependence of Avrami Indexes of MDPE on Milling Time
   PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [University of Tehran]  On: 5 December 2010  Access details: Access Details: [subscription number 926807965]  Publisher Taylor & Francis  Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Polymer Plastics Technology and Engineering Publication details, including instructions for authors and subscription information: On the Dependence of Avrami Indexes of MDPE on Milling Time T. Ebrahimi Sadrabadi a ; S. M. Zebarjad a ; J. Vahdati Khaki a ; S. Sahebian aa  Engineering Faculty, Department of Materials Science and Engineering, Ferdowsi University of Mashhad, Azadi Square, Mashhad, IranOnline publication date: 15 September 2010 To cite this Article  Sadrabadi, T. Ebrahimi , Zebarjad, S. M. , Khaki, J. Vahdati and Sahebian, S.(2010) 'On the Dependenceof Avrami Indexes of MDPE on Milling Time', Polymer-Plastics Technology and Engineering, 49: 12, 1284 — 1288 To link to this Article: DOI: 10.1080/03602559.2010.496395 URL: Full terms and conditions of use: article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.  On the Dependence of Avrami Indexes of MDPE onMilling Time T. Ebrahimi Sadrabadi, S. M. Zebarjad, J. Vahdati Khaki, and S. Sahebian Engineering Faculty, Department of Materials Science and Engineering, Ferdowsi University of Mashhad, Azadi Square, Mashhad, Iran In the current study, the role of milling time on thermal proper-ties and crystallization behavior of medium density polyethylene(MDPE) was investigated. For this purpose, high energy ball milland differential scanning calorimetry (DSC) techniques were used.The results of DSC tests indicated that the milling process affectedon crystallization behavior and thermal properties of MDPE. Risingcrystallization temperature, specific heat capacity and crystallinityindex and decreasing Avrami index were caused by ball milling;yet melting temperature was not changed, even at long milling times.In summary, thermal properties of the polymer were not changedsignificantly by increasing milling time.Keywords  Ball milling; Crystallization behavior; DSC; MDPE;Thermal properties INTRODUCTION Ball milling is being used widely to improve the physicalproperties of materials. This technique can be applied toelemental crystalline powders with different objectives; par-ticle size reduction, amorphization, procurement of nano-crystaline structures, etc. It can be also used for alloyingpowders in the solid state, the so called mechanical alloying.Although most of the studies employing this techniquehave been limited to metallic systems [1–3] , but recently, ballmilling has been applied to polymeric materials.The use of ball milling is one of the best methods todispersion of secondary phase to polymer matrix. Gooddispersion and lack of filler agglomeration in matrix andthe possibility of blending incompatible polymer mixturesare among the main advantages of this process. Firstresults of applying ball milling to polymers showed thatthis procedure is adequate for making polymer blends. Thistechnique overcomes some problems associated with con-ventional methods, such as thermal degradation due toexcessive heating in the melting process or the difficultyin removing the polymer from the solvent if the solutionmethod is used [4–6] .Based on a literature survey done by the authors, thereare many studies concentrated on milling of polymer sys-tems [7–13] . For example Font et al. showed that the binarymixtures of semicrystalline = noncrystalline polymers couldbe formed by ball milling. Also some researchers mademixtures of PEEK = PEI, PET = PEI, PBT = PEI, and PBT = PET using the milling method. The results showed thattheir thermal behavior were comparable with that of blendsobtained by melting [9–13] .Although using mechanical milling in composite pro-ducing has some advantages, but the main question iswhether any change takes place in polymer structurethrough milling or mechanical mixing or not. Some modi-fications of polymer mechanical properties such as strengthand hardness have been obtained as a consequence of themilling treatment [14,15] . Intensive structural changesinduced by milling are described in the literature [16,17] . Fontet al. investigated the effect of milling on thermal behaviorof poly ethylene terephthalate [17] .Research results showed that the energy supplied bygrinding caused an increase in the amount of amorphousmaterial. The crystallization, on heating the glassy stateresulted by milling, took place in two steps. One of themwas near the glass transition and the second occurred justat the beginning of melting. When PET was pre-annealed,some differences in the amorphization behavior, caused bymechanical milling, were encountered [17] . Font et al. inves-tigated the effect of ball milling on semicrystalline bisphe-nol A polycarbonate (PC) [18] .They showed that the semicrystalline PC could be amor-phized by ball milling. The amorphous state reached bythis method was different from what obtained by quench-ing the melt. For polymer amorphized by grinding, a smal-ler heat capacity jump in the glass transition temperaturewas observed. In this case, an additional annealing treat-ment gave rise to a partial recrystallization which was notobserved for quenched PC.The degree of crystallinity after this recrystallizationdepended on the temperature of thermal treatment priorto the amorphization by ball milling [18] . The Avramitheory was applied for crystallization kinetics analysis. Address correspondence to S. M. Zebarjad, EngineeringFaculty, Department of Materials Science and Engineering,Ferdowsi University of Mashhad, Azadi Square, Mashhad, Iran,P.O. Box 91775-1111. E-mail: Polymer-Plastics Technology and Engineering , 49: 1284–1288, 2010Copyright # Taylor & Francis Group, LLCISSN: 0360-2559 print = 1525-6111 onlineDOI: 10.1080/03602559.2010.496395 1284  D o w nl o ad ed  B y : [ U ni v e r si t y  of  T eh r a n]  A t : 10 :26 5  D e c e mb e r 2010  Values of the Avrami index that provide a qualitativeindication of the nucleation mechanism and on the formof crystal growth were determined for different polymericsamples [19,24–30] .Xu et al. investigated the Non-isothermal crystallizationkinetics of exfoliated and intercalated polyethylene = montmorillonite nanocomposites. In this research theAvrami plots showed that the crystal growth of PE in theintercalated sample is two-dimensional, while it is three-dimensional in the exfoliated sample. The crystallizationactivation energy of the intercalated sample is slightlysmaller than that of the exfoliated sample [25] .Apiwanthanakorn et al. [26] investigated the non-isothermal melt-crystallization kinetics and subsequentmelting behavior of poly(trimethylene terephthalate) (PTT)by differential scanning calorimetry (DSC). The Avrami,Tobin and Ozawa equations were applied to describe thekinetics of the crystallization process. The Avrami andthe Tobin models were all found to describe the non-isothermal melt-crystallization data of PTT fairly well,with the Avrami model being the better of the two.Both of the Avrami and Tobin crystallization rate para-meters (i.e., KA and KT, respectively) were found toincrease with increasing cooling rate [26] . Ziaee et al. investi-gated the non-isothermal melt- and cold-crystallization andsubsequent melting behavior of poly(3-hydroxybutyrate)(PHB). The crystallization kinetics was studied by a directfitting of the experimental data to various macrokineticmodels, namely Avrami, Tobin, and Ozawa models.All of the macrokinetic models were found to describethe experimental data for both the melt- and the cold-crystallization processes fairly well. They showed that thetemperature at 1% relative crystallinity, the temperatureat the maximum crystallization rate, and the temperatureat 99% relative crystallinity were all shifted towards lowertemperatures with increasing cooling rate for melt-crystallization or shifted towards higher temperatures forcold-crystallization with increasing heating rate. For boththe melt- and the cold-crystallization processes, the appa-rent incubation period, the crystallization time at differentrelative crystallinity values and the apparent total crystalli-zation period showed a linear relationship with cooling orheating rate in their log–log plots [27] .Thanomkiat et al. investigated the overall isothermalmelt-crystallization and subsequent melting behavior of metallocene-catalyzed syndiotactic polypropylene resinsof various molecular weights. The kinetics of the overallisothermal melt-crystallization process was analyzed basedon various macrokinetic models, i.e., the Avrami, Malkin,and Urbanovici–Segal models. The experimental data werefound to be best described by the Urbanovici–Segal model,followed by the Avrami and the Malkin ones, respectively.For a given resin, all of the overall crystallization rateparameters were found to decrease in their values withincreasing the crystallization temperature, a characteristicof the crystallization in the nucleationcontrolled region [28] .In spite of the importance of MDPE the effect of ballmilling on its structural changes and its thermal propertieshas not been under attention. Therefore, the main goal of current research is to find out the effect of ball millingon thermal properties and crystallization behavior of MDPE. Also it will be tried to clarify the role of millingtime on its Avrami indexes. EXPERIMENTAL PROCEDUREMaterial Medium density polyethylene (MDPE) with density(0.937g = cm 3 ), vicat softening point (117  C) and meltingflow index (MFI) (4.2g = 10min), supplied by Tabriz Petro-chemical Complex, Iran, was used as beginning material. Specimen Preparation Milling of polymer was performed at room temperatureusing a high energy ball mill with constant rotation speedof 250rpm. A cylindrical stainless steel vessel and steel ballswere used. The weight ratio of balls to MDPE powders waskept constant at 30. The 3.5 gram samples were milledfor different times (i.e., 0, 0.5, 1.5, 3, 7, and 30 hours). Justfor simplicity, the polymers at different milling timewere coded as MDPE-0h, MDPE-0.5h, MDPE-1.5h,MDPE-3h, MDPE-7h, and MDPE-30h. Differential Scanning Calorimetery The thermal properties of pure MDPE were examinedusing heat flow differential scanning calorimetry device(Shimadzu DSC-60). Samples were heated from room tem-perature to 150  C at a heating rate of 10  C = min, followedby cooling at the same rate to room temperature. After thatsamples were heated in the second heating cycle. Thesecond heating cycle and the first heating cycle were exactlythe same. Each sample was subjected to DSC test and aver-age values of DSC results for at least two fresh specimensof each sample were reported for thermal properties. RESULTS AND DISCUSSIONCrystallization Behavior Figure 1 shows the variations of heat flow versus timefor MDPE-0h and MDPE-0.5h at heating, cooling andsecond heating cycles. Based on Figure 1, the amount of heat flow of milled MDPE powders is more than thatunmilled MDPE. It is worth noting that the rising trenddue to an increase in milling time was observed for allstudied samples.The heat of fusion (H f  ) and solidification (H s ), melting(T pf  ) and solidification (T cp ) temperatures and initial (T ci ,T mi ) and final (T cf  , T mf  ) temperatures of solidification AVRAMI INDEXES OF MDPE AND MILLING TIME  1285  D o w nl o ad ed  B y : [ U ni v e r si t y  of  T eh r a n]  A t : 10 :26 5  D e c e mb e r 2010  and fusion peaks were obtained from DSC curves andsummarized in Table 1.Based on Table 1 and what appears in Figure 2, at firstheating cycle, melting temperature including initial,maximum (peak of the curve) and final temperatures of melting have not changed significantly as milling timeincrease up to 7 hours, while melting temperatures slightlyincrease for MDPE-7h and MDPE-30h respect to othersamples. The reason of this effect can be related to severedeformation applied on MDPE powders during long timeof milling process, which causes to increase interactionand rigidity of polyethylene chains significantly [24] .At initial cooling process, maximum and final tempera-tures promote to higher amounts by progressing of milling.The experimental data indicate that the nucleation sites inMDPE, realized from T ci , increase with increase in millingtime. At the second heating cycle, fusion temperatures of all samples is almost the same. But heat of fusion in ballmilled MDPE samples is higher than that of MDPE-0h.On the other hand, melting temperature of ball milledMDPE almost do not change by variation of milling time.The reason of this effect can be attributed to the variationof structure of polymer chains during milling process.Considering solidification cycle, the relative crystallinityfraction can be calculated by the following Eq. (1) [20] : X  ð t Þ ¼ R  t 0 dH dt      dt R  1 0 dH dt      dt ð 1 Þ the first integral is the heat generated at time t and thesecond one is the total heat when the crystallization iscompleted.Relative crystallinity versus temperature for MDPEwith different milling time is shown in Figure 3. As seen,solidification in milled MDPE occurs in a wide temperaturerange, and crystallization starts at a temperature higherthan MDPE-0h.Figure 4 shows the variation of relative crystallinity withlogarithm of solidification time for milled and unmilledMDPE samples. As it can be realized, both solidificationtime and solidification rate depend strongly on mechanicalmilling. Relative crystallinity changes over time can bereadily illustrated in the following equation: X  ð t Þ ¼  1    exp ð kt n Þ ð 2 Þ where X(t) is a relative crystallinity at arbitrary time t, n isthe Avrami index that provides a qualitative indication of the nucleation mechanism and on the form of crystalgrowth, and k is the constant concluding nucleation andgrowth parameters [19] . Equation (2) can be transformedinto logarithmic form:ln ½ ln ð 1    X  t Þ ¼  ln ð k  Þ þ  n ln ð t Þ ð 3 Þ By applying Avrami theory and plotting ln[  ln(1  X  t )]against ln( t ), a straight line is obtained with the slopeand the intercept as n and ln(k), respectively. The plot of ln[  ln(1  X  t )] versus ln( t ) is shown in Figure 5, TheAvrami indexes and ln(k) achieved from Figure 5 aresummarized in Table 2.As reported by other investigators, Avrami index 3suggests an instantaneous nucleation with spherulitegrowth geometry [21] . It may be concluded that ball millinghas no effect on the nucleation mechanism and spherulite FIG. 1. The variation of heat flow versus time for MDPE-0h andMDPE-0.5h. TABLE 1Thermal properties of ball milled MDPE achieved from DSC testFirst heating cycle Second heating cycle Third heating cycleMaterials T mi  T mp  T mf   H f   T ci  T cp  T cf   H s  T mi  T mp  T mf   H f  MDPE-0h 121.39 127.47 130.20 122.53 115.88 113.81 110.19 125.43 122.88 129.07 131.69 133.20MDPE-0.5h 121.24 126.81 129.55 141.67 116.76 114.17 110.49 144.08 122.19 128.53 131.25 156.78MDPE-1.5h 121.07 126.42 129.07 141.06 116.48 114.22 110.70 143.78 122.01 128.09 130.75 151.30MDPE-3h 122.60 127.35 130.92 139.68 117.32 114.93 110.63 136.93 121.67 128.73 131.36 146.83MDPE-7h 123.60 128.36 132.00 152.27 117.75 115.39 111.39 141.25 122.43 128.80 131.39 146.34MDPE-30h 125.27 129.42 132.08 159.26 117.79 115.84 111.22 141.89 122.20 128.67 131.10 145.361286  T. E. SADRABADI ET AL.  D o w nl o ad ed  B y : [ U ni v e r si t y  of  T eh r a n]  A t : 10 :26 5  D e c e mb e r 2010  growth of MDPE, But unmilled MDPE has a differentnucleation and crystallized morphology respect to ballmilled MDPE and all milled samples have the same crys-tallized morphology and nucleation mechanism. FIG. 2. Melting and crystallization temperatures of milled MDPEplotted against milling time.FIG. 4. Relative crystallinity of MDPE versus logarithm of solidificationtime at different milling times.FIG. 3. Relative crystallinity versus temperature of MDPE as functionof different milling time.FIG. 5. Plots of the variation of ln[  ln(1  X  t )] versus ln(t) for MDPEball milled at different milling time.AVRAMI INDEXES OF MDPE AND MILLING TIME  1287  D o w nl o ad ed  B y : [ U ni v e r si t y  of  T eh r a n]  A t : 10 :26 5  D e c e mb e r 2010
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