non-isothermal crystallization behavior of polypropylene with nucleating agents and nano-calcium...

EUROPEAN

European Polymer Journal 41 (2005) 2753–2760

www.elsevier.com/locate/europolj

POLYMERJOURNAL

Non-isothermal crystallization behavior of polypropylenewith nucleating agents and nano-calcium carbonate

Huang Yiping *, Chen Guangmei, Yao Zhen, Li Hongwu, Wu Yong

School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China

Received 5 October 2004; received in revised form 6 February 2005; accepted 9 February 2005Available online 19 August 2005

Abstract

The non-isothermal crystallization kinetics of isotactic polypropylene (iPP) and nucleated iPP was investigated byDSC. The crystalline morphology of iPP was observed by polarized light microscopy. It was found that the crystalli-zation rate increased with the addition of nanometer-scale calcium carbonate (nm-CaCO3) particles. The addition ofdibenzylidene sorbitol (DBS) could greatly reduce the spherulite size of iPP. The crystallization temperature for theiPP with DBS was higher than for non-nucleated iPP. DBS was an effective nucleating agent for iPP. The results ofmeasurements suggested that there was a coordinated action to the crystallization of iPP when the organic nucleatingagents (DBS) and nm-CaCO3 were added to iPP together. Comparison to the modified Avrami equation and Ozawaequation, another method—Mo�s method can describe the non-isothermal crystallization behavior of iPP and nucleatediPP more satisfactorily.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Nucleating agent; Non-isothermal crystallization, Isotactic polypropylene; Dibenzylidene sorbitol

1. Introduction

In recent years, the effects of nucleating agents on thecrystallization process of polypropylene (PP) werewidely studied [1–4]. It is well known that isotactic poly-propylene (iPP) can form large spherulites when it crys-tallizes from the melt. iPP containing large spherulitesusually shows unsatisfactory impact strength at low tem-peratures and its products are opaque. Several meanshave been used in industrial practice to overcome theshortcoming of iPP. One of the important methods isadding nucleating agents to change the crystalline mor-

0014-3057/$ - see front matter � 2005 Elsevier Ltd. All rights reservdoi:10.1016/j.eurpolymj.2005.02.034

* Corresponding author. Tel./fax: +86 055 151 07 304.E-mail address: [email protected] (Y. Huang).

phology of iPP. Usually, as the nucleating agents areadded to iPP, the crystallization temperatures of iPPincrease, the crystallization rate is accelerated and thecrystallinity increases slightly. It was reported that 0.8wt% of an organic phosphorus nucleating agent addedin iPP increased the nucleation density by six orders ofmagnitude, and the crystallization temperature wasenhanced by 12 K [5]. Alkali dehydroabietate added toPP can greatly increase the rate of crystal nucleation innucleated PP although it hinders PP chains folding onthe front of crystals and decreases the rate of crystalgrowth, since it increases the fold surface energy re [4].In fact, the effectiveness of various nucleating agentson iPP is inconsistent. In most cases, the sorbitol deriv-atives, which are effective nucleating agents for PP, atconcentrations between 0.4% and 1.5% in PP, increase

ed.

mailto:[email protected]

Table 1The contents of DBS and nm-CaCO3 in 100 g isotacticpolypropylene for different samples

Sample PP-1 PP-2 PP-3 PP-4 PP-5

Content of DBS (g) 0 0 0.2 0.4 0.4Content of nm-CaCO3 (g) 0 3.0 3.0 3.0 0

2754 Y. Huang et al. / European Polymer Journal 41 (2005) 2753–2760

the rate of crystallization and the temperature at whichthe maximum rate of crystallization occurs upon coolingfrom the molten state [6]. At the same time, it was re-ported the fold surface energy re of PP decreased [7].

On the other hand, the mechanical and thermal prop-erties of polymers can be greatly improved by the addi-tion of nanometer-scale inorganic particles. Whennanometer-scale calcium carbonate (nm-CaCO3) parti-cles were added to PP, the higher tensile and flexuralstrength of PP were obtained [8]. It is natural to askwhether there is coordinated action to the crystallizationof iPP when the organic nucleating agents (DBS) andnm-CaCO3 are added to iPP together. Moreover, thestudy of non-isothermal crystallization of polymers isof great technological significance, since most practicalprocessing techniques proceed under non-isothermalconditions. In this report, the non-isothermal crystalliza-tion of the iPP samples in the presence of dibenzylidenesorbitol (DBS) and nm-CaCO3 was investigated bydifferential scanning calorimetry (DSC). Wide-angleX-ray diffraction (WAXD) and polarizing light micros-copy (PLM) were also used to research the crystalliza-tion morphology of iPP. The validity of the modifiedAvrami equation, Ozawa equation and Mo�s methodfor these systems was discussed.

360 400 440 360 400 440

(A)

PP-1

2.5K/min

5K/min

10K/min

20K/min

40K/min

EN

DO

>

Temperature/K

PP-3

(B)

2.5K/min

5K/min

10K/min

20K/min

40K/min

Fig. 1. Non-isothermal melt crystallization curves for PP-1 (A)and PP-3 (B) at the indicated cooling rates.

2. Experiments

2.1. Materials

Commercially available iPP (supplied by Qilu Petro-leum & Chemical Corp., melting index 2.86 g/10 min at503 K and 2.16 kg) was used in this study. iPP was char-acterized by a gel permeation chromatography (GPCV2000, produced by Waters Co.) in 1,2,4-trichloroben-zene at 433 K. The weight average molecular weight(Mw) and the polydispersity of iPP were 3.46 · 105 and5.9, respectively. The tacticity of iPP was characterizedby 13C NMR method recorded at 403 K on anAMX400 Bruker spectrometer in pulse Fourier trans-form mode. About 300 mg samples were dissolved in2.5 ml 1,2,4-trichlorobenzene and 0.5 ml hexadeutero-benzene (C6D6). Experimental conditions were: pulseangle 90�, sweep width 0–220, relaxation delay 11 s, timedomain 64 K. The tacticity of iPP was 94.7%.

The nm-CaCO3 was commercial product and theparticle size was about 50 nm in average. DBS was syn-thesized in our laboratory. The melting point for DBSwas 496 K,and the crystallization temperature was455 K.

2.2. Specimen preparation

Before mechanical mixing, iPP was dried in an ovenat 373 K for 4 h and then cooled down to room temper-

ature. DBS and nm-CaCO3 particles were dried at 395 Kfor 5 h. Homogeneous solid samples of iPP with appro-priate nucleating agent or nm-CaCO3 were prepared in atwin screw Brabender machine under a nitrogen blanketat 473 K. The screw speed was 45 rpm and the mixingtime in the extruder was 10 min. The contents of DBSand nm-CaCO3 in iPP are shown in Table 1.

2.3. Apparatus and experimental procedures

A Perkin–Elmer Pyris-1 DSC was used for calorimet-ric investigations of non-isothermal crystallization. Cal-ibration was performed using pure indium at a heatingrate of 10 K/min. About 5 mg of each sample wereplaced in a DSC pan and heated at a rate of 40 K/minfrom ambient temperature to 485 K for 5 min to removeall memory of previous thermal and mechanical history(1st run). After that, the sample was cooled to 323 K.Heating at a rate of 40 K/min followed each coolingtreatment to 485 K (2nd run). The constant cooling ratesR used were 2.5, 5, 10, 20 and 40 K/min.

All wide-angle X-ray diffraction measurements wereobtained at ambient temperature on a MXP18AHFX-ray diffractometer using Cu Ka radiation. Samplesfor wide-angle X-ray diffraction were injection moldedunder nitrogen at 485 K and transferred at ambient tem-perature to a circular mold with a diameter of 20 mmand a thickness of 1 mm.

Y. Huang et al. / European Polymer Journal 41 (2005) 2753–2760 2755

Samples containing DBS or/and nm-CaCO3 for mor-phology observation were prepared by fusion a scrap ofsamples placed between two cover glasses. After meltingat 485 K for 3 min, they were kept in a quasi-isothermalstate at 393 K for 48 h.

Table 2The crystallization temperature (TP), the half-width of the peak(DT1), t1/2 and crystallinity for all samples at different coolingrates

Specimens PP-1 PP-2 PP-3 PP-4 PP-5

40 K/min TP/K 374.6 381.2 388.6 387.8 392.5DT1/K 11.2 6.1 7.1 10.5 13.0t1/2/min 0.42 0.29 0.23 0.35 0.36

3. Results and discussion

3.1. Non-isothermal crystallization

Fig. 1 shows the crystallization curves for PP-1 (A)and PP-3 (B) obtained at different cooling rates. It is

320 360 400 440320 360 400 440

(B)

PP-5

PP-4

PP-3

PP-2

PP-1

40 K/min

(A)

PP-5

PP-4

PP-3

PP-2

PP-1

10 K/min

EN

DO

>

Temperature (K)

Fig. 2. DSC cooling traces of all samples recorded at thecooling rates 10 K/min (A) and 40 K/min (B).

-1.8 -1.2 -0.6 0.0 0.6 1.2

-4

-3

-2

-1

0

1

2(A)

PP-3

ln(-

ln(1

-Xt))

lnt

40201052.5

Fig. 3. Plots of ln(�ln(1 � Xt)) versus ln t for non-isothermal melt cryrepresent for the cooling rates.

found for iPP and nucleated iPP that the crystallizationtemperature TP decreased with the increase of coolingrate R. In fact, the similar results can be observed for

-1.8 -1.2 -0.6 0.0 0.6 1.2 1.8

-4

-3

-2

-1

0

1

2(B)

PP-5

lnt

40201052.5

stallization of PP-3 (A) and PP-5 (B). Series of numbers in figure

Cr/% 36.2 35.2 35.9 36.3 37.5

20 K/min TP/K 379.2 385.2 392.6 394.9 396.5DT1/K 7.7 4.3 4.7 6.0 9.6t1/2/min 0.68 0.42 0.37 0.55 0.58Cr/% 36.7 35.6 37.2 36.9 38.3



2.5 K/min TP/K 389.2 395.1 399.4 404.4 405.5DT1/K 4.2 3.0 3.2 4.9 7.4t1/2/min 5.20 2.92 2.36 3.92 3.44Cr/% 38.8 36.4 38.7 38.4 39.5

Table 3The values of Avrami exponent n, the rate parameter ln Zc forall samples at different cooling rates

Samples Cooling rates R, K/min

2.5 5.0 10.0 20.0 40.0

PP-1 n 3.6 3.1 3.7 4.2 4.2ln Zt �6.601 �3.242 �1.524 1.084 3.296ln Zc �2.640 �0.648 �0.152 0.054 0.082

PP-2 n 3.8 3.9 4.5 4.5 5.4ln Zt �4.667 �2.235 0.183 3.343 6.478ln Zc �1.867 �0.447 0.018 0.167 0.162

PP-3 n 4.9 5.0 5.0 5.2 5.2ln Zt �4.642 �1.473 1.556 5.095 7.592ln Zc �1.857 �0.295 0.156 0.255 0.190

PP-4 n 4.6 4.9 4.3 4.5 4.4ln Zt �6.827 �3.814 �0.231 2.22 4.560ln Zc �2.731 �0.763 �0.023 0.111 0.114

PP-5 n 4.3 4.9 5.2 4.2 4.6ln Zt �5.612 �3.683 �0.760 2.092 4.801ln Zc �2.245 �0.737 �0.076 0.105 0.120


three other specimens in this research. In Fig. 2 the DSCtraces obtained at cooling rates of 10 K/min (A) and40 K/min (B) for neat iPP and iPP with the nucleatingagent DBS and/or nm-CaCO3 are shown. From thecurves, it can be observed that TP increases stronglyupon addition of DBS to iPP.

The values of the crystallization temperatures TP, thehalf-life t1/2, the half-widths of the crystallization peaksDT1 and degree of crystallinity Cr for all specimens arecollected in Table 2. Here, the crystallization tempera-tures TP are those corresponding to the exothermic peakmaxima, the half-life t1/2 is the time taken for 50% of thetotal crystallization to occur. The degree of crystallinityCr has been calculated from the enthalpy of crystalliza-tion, using the value of 237 J/g for the enthalpy of fusionof 100% crystalline iPP [9]. The results in Table 2 indi-cated that the crystallization temperatures of iPP changewith the cooling rates and the addition of nucleatingagents and nm-CaCO3. The DBS as nucleating agentof iPP can increase the crystallization temperatures TPabout 15–18 K at different cooling rates, while the addi-tion of nm-CaCO3 increases the TP of iPP about 4 Konly. When DBS and nm-CaCO3 are added to iPP to-gether, the difference of TP between nucleated iPP andpure iPP increases from 10 K to 14 K with increase ofDBS in specimens. The sharpness of the crystallizationpeaks, as measured by DT1, is considerably higher forthe addition of nm-CaCO3 than for pure iPP. The exo-therms for PP-2, which contain nm-CaCO3 only, arethe sharpest in those of all specimens. But the addition

1 3

-4

-2

0

2

PP-3

(A)

ln(-

ln(1

-X(T

)))

lnR

387K

389K

393K

395K

397K

2 4

Fig. 4. Ozawa plots of non-isothermal melt crystallization of PP-3 (crystallization temperatures.

of DBS decreases the sharpness of the crystallizationpeaks for iPP obviously. When iPP contains 0.4% DBSonly, the DT1 of the specimen is the largest in all sam-ples. The values of t1/2 also indicate that the additionof nm-CaCO3 can increase the crystallization rate ofiPP obviously but the addition of DBS cannot. Fromthe degree of crystallinity Cr, it can be found that the

1 3

-4

-2

0

2

PP-5

(B)

lnR

391K

393K

397K

399K

401K

403K

2

A) and PP-5 (B). Series of numbers in figure represent for the

Table 4Kinetics parameters of non-isothermal crystallization for iPPand nucleated iPP at different relative crystallinity by thecombination of the Avrami–Ozawa equation

Samples Relative crystallinity, %

10 30 50 70 90

PP-1 a 1.2 1.2 1.1 1.2 1.2F(T) 7.90 11.24 13.31 15.10 17.74

PP-2 a 1.2 1.1 1.1 1.2 1.2F(T) 5.24 7.01 8.13 9.19 10.66

PP-3 a 1.1 1.2 1.2 1.2 1.2F(T) 4.26 5.60 6.64 7.74 9.87

PP-4 a 1.1 1.1 1.1 1.1 1.1F(T) 6.93 8.98 10.32 11.82 14.04

PP-5 a 1.2 1.2 1.2 1.2 1.2F(T) 6.63 9.05 11.15 13.67 16.93


addition of DBS increases the crystallinity of iPPslightly, while the addition of nm-CaCO3 decreases theCr of iPP inconsiderably.

Generally, isothermal crystallization of polymers canbe described by Avrami equation [10], which assumedthat the relative degree of crystallinity developed withcrystallization time t,

1� X t ¼ expð�ZttnÞ

where the exponent n is a mechanism constant, whichdepends on the type of nucleation and growth process;Xt is the relative degree of crystallinity at time t; Zt isa composite rate constant involving both nucleationand growth rate parameters in Avrami equation. Con-sidering the non-isothermal character of the processinvestigated, Jeziorny pointed out that the value of rateparameter Zt should be adequately corrected [11].Assuming constant or approximately constant R, the fi-nal form of the parameter characterizing the kinetics ofnon-isothermal crystallization was given as follows:

lnZc ¼ ðlnZtÞ=R

By using Avrami equation, the plots of ln(�ln(1 �Xt)) versus ln t are shown in Fig. 3. Each curve showsan initial linear portion, subsequently tending to leveloff. Usually, the linear portion is considered to be dueto the primary crystallization and the deviation to bedue to the secondary crystallization. From Fig. 2, itcan be seen that the exotherm peaks for PP-5 are dual.We can deduce there is the secondary crystallizationoccurring. So PP-5 has obvious deviation in Fig. 3(B).The values of n and ln Zc, determined from the slope

-0.8 -0.4 0.0 0.4

0.4

0.8

1.2

1.6

PP-3

(A)

logR

logt

1030

5070

90

Fig. 5. Plots of log R versus log t for non-isothermal melt crystallizat

and intercept of the initial linear portion in Fig. 3, arelisted in Table 3. The average values of the Avrami expo-nent for PP-1 to PP-5 were 3.8, 4.4, 5.1, 4.5 and 4.6,respectively. The values of the rate parameter Zc in-crease with increasing the cooling rate R. At the samecooling rate, PP-2 and PP-3 have the higher values ofthe Zc.Another method—Ozawa�s theory is also adopted to

describe the non-isothermal crystallization [12]. In thispresent work, the overall non-isothermal crystallizationkinetics of iPP and nucleated iPP is studied using theOzawa formalism. This is based on the equation

-0.8 -0.4 0.0 0.4 0.8

0.4

0.8

1.2

1.6

PP- 5

(B)

logt

10

305070

90

ion of PP-3 (A) and PP-5 (B) at indicated relative crystallinity.


lnð� lnð1� X ðT ÞÞÞ ¼ KðT Þ � m lnR

where X(T) is the volume fraction of material crystal-lized at temperature T, R is the constant cooling rate,m is Owaza exponent that depends on the crystal growthand nucleation mechanism, and K(T) is the cooling crys-tallization function. Fig. 4(A) shows the results of Oza-wa analysis for specimen PP-3. A series of lines areobtained for different temperatures, and it indicates theOzawa equation describes satisfactorily the non-isother-mal crystallization behavior of iPP containing 0.2% DBSand 3% nm-CaCO3. However, Ozawa analysis for thenon-isothermal crystallization of PP-5, which contains0.4% DBS only, is not appropriate, as shown inFig. 4(B). For PP-5, some non-linear plots are obtained.

Fig. 6. Polarized light micrographs for iPP with DBS

According to the curves in Fig. 4, the average value of m

for iPP is about 2.4.When nm-CaCO3 is added to iPP, theaverage value of m reaches 3.3. The addition of DBSdecreases the value of m. The values of m for PP-3 andPP-4 are 2.8 and 2.3.

Obviously, the Ozawa equation does not describe sat-isfactorily the non-isothermal crystallization behavior ofall specimens in this research. In fact, the DSC curves inFig. 2 have shown that the exotherm peaks for PP-5 aredual, that is to say, the further perfection of crystal oc-curs. The Ozawa equation was proven to fail to ade-quately describe the non-isothermal crystallizationkinetics for some polymers, in which the further perfec-tion of crystal occurs [13,14]. In order to analyse thenon-isothermal crystallization for all specimens better,

and/or nm-CaCO3 obtained at 393 K for 48 h.

10 15 20 25 30 35

21.8

21.1

18.6

16.8

14.1

PP-4

PP-2

PP-1

2 θ

Fig. 7. Ambient temperature X-ray diffractograms for PP-1,PP-2 and PP-4.


another method is adopted to deal with non-isothermaldata by combining the Avrami equation with the Ozawaequation, as follows [15]:

logR ¼ log F ðT Þ � a log t

where the parameter F(T) = [K(T)/Zt]1/m refers to the

value of cooling rate, which has to be chosen at unitcrystallization time when the measured system amountsto a certain degree of crystallinity; a is the ratio of theAvrami exponent n to the Ozawa exponent m. Fig. 5shows a series of straight lines of log R versus log t forPP-3 and PP-5. The kinetic parameter F(T) and a forspecimens are listed in Table 4. For each specimen, thevalues of a change slightly with the relative degree ofthe crystallinity, and contents of DBS and nm-CaCO3in iPP. F(T) for each specimen increases as increasingthe relative crystallinity. It is interesting that the valuesof F(T) for PP-2 and PP-3 are lower than those of theother samples at same relative crystallinity. It indicatesthat it has shorter time for PP-2 and PP-3 than for theothers when the measured system arrives to a certain de-gree of crystallinity.

3.2. Crystallization morphology

Fig. 6 shows the optical micrographs viewed withcross polars for some specimens obtained at 393 K for48 h after cooling from the molten state. It is obviousto see that the larger spherulitic dimensions on the orderof about 20 lm are observed for PP-2, which containsnm-CaCO3 only. In fact, the largest spherulites about100 lm in diameters can be observed for pure iPP (notshown in Fig. 6). Much smaller spherulitic dimensionsappear in iPP samples that contain DBS with and with-out nm-CaCO3. In the micrographs of these samples, weonly can see large numbers of spots no more than 5 lmin diameters. It indicates that DBS is an effective nucle-ating agent for iPP.

Wide-angle X-ray diffraction measurements in Fig. 7reveal that pure iPP and nucleated iPP crystallize in themonoclinic a-form via reflections at 2h = 14.1�, 16.8�,18.6� and 21.8� [16,17]. The presence of the b-form ofiPP via reflection at 2h = 21.1� is also can be seen. Acomparison of the crystallographic reflections at scatter-ing angles of 21.1� and 21.8� in Fig. 7 suggests thatnm-CaCO3 does not affect the fraction of iPP to crystal-lize in the b-form, while the addition of DBS can de-crease the fraction of iPP to crystallize in the b-formslightly.

In summary, DBS increases the crystallization tem-perature of iPP by 15–18 K, increases the nucleationdensity but does not increase the crystallization rate.The addition of nm-CaCO3 increases the crystallizationrate of iPP. Either DBS or nm-CaCO3 does not affect thefraction of polymer that crystallizes in the b-form obvi-ously. When nm-CaCO3 and a little DBS are added to

iPP (such as PP-3), the significant coordinated actionoccurs.

4. Conclusion

Three methods are used to deal with the non-isother-mal crystallization process for iPP and nucleated iPP.Avrami equation and Ozawa equation cannot describesatisfactorily the non-isothermal crystallization behaviorof PP-5 in this research, while Mo�s method can. For allother samples, three methods are appropriate. Accord-ing to the results obtained by Avrami equation, the pri-mary crystallization stage for non-isothermal meltcrystallization might correspond to a three-dimensionalspherical growth with thermal nucleation. The valuesof the rate parameter Zc, the kinetics parameter F(T)and the half-widths of the crystallization peaks DT1 re-veal that the addition of nm-CaCO3 increases the crys-tallization rate of iPP from the molten state obviously.The results of DSC and the polarized light microscopydisplay DBS can increase the crystallization tempera-tures and the nucleation density of iPP. DBS is aneffective nucleating agent for iPP. When DBS and nm-CaCO3 are added to iPP together, the crystallizationrate decreases and crystallization temperature TP


increases with the increasing amount of DBS. Obvi-ously, there is coordinated action for the crystallizationof iPP when the organic nucleating agents (DBS) andnm-CaCO3 are added to iPP together. X-ray diffracto-grams indicate that the fraction of the b-form of iPPalmost do not change with the addition of DBS andnm-CaCO3.

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