Non-isothermal crystallization kinetics of kaolin modified polyester by scRKqmi


									   Non-isothermal Crystallization Kinetics of Kaolin Modified Polyester

                        ZHANG Ruixin1, GU Mingbo2, CHEN Guoqiang1,*

(1. College of Textile and clothing Engineering, Soochow University, Suzhou, Jiangsu, 215021,


2. College of Chemistry, Chemical Engineering and Materials Science, Soochow University,

Suzhou, Jiangsu, 215123, China)

  Abstract: Fiber-class modified kaolin and PET have been blended in the twin-screw and

granulated to chips containing 4 wt% of kaolin. Non-isothermal crystallization process of kaolin

modified polyester has been investigated using a differential scanning calorimetry (DSC), and

the addition of kaolin enhances either the melting temperature (Tm) or the crystallization

temperature (Tc). The morphology of kaolin modified polyester, the melt of which is cooled at

different cooling rate, has been observed by scanning electron microscope (SEM). The

relationship between Tc and cooling rate Φ has been studied. Semi-crystalline phase t1/2

makes an exponential decline with increasing Φ, and the higher the cooling rate, the shorter

the time of crystallization completion. Non-isothermal crystallization kinetics parameters and

the activation energy have been calculated, which indicates that the higher rate of cooling

needs the higher relative crystallinity in the unit crystallization time, the crystallization rate has

increased while speeding up the temperature reduction, and the activation energy E has

calculated to be -204.1566 kJ/mol for the non-isothermal crystallization processes by the

Kissinger’s methods.

  Keywords: kaolin; polyester; crystallization kinetics; Non-isothermal

1 Introduction

  The inorganic-polymeric hybrid materials have attracted great attention because the hybrid

materials have a superior performance than the single material[1-3]. The researchers have paid

special attention to modify the function of polyester fiber because its highest yield in synthetic

ZHANG Ruixin (张瑞欣): Lecturer; Ph D Candidate; E-mail:
*Corresponding author: CHEN Guoqiang (陈国强): Prof.; Ph D; E-mail:

fiber and the widest application in textile area. Preparing functional fiber through adding

inorganic particles into polyester is convenient and attracts fiber manufactures. For example,

carbon black can be added into polyester to improve the conductivity, antistatic nature,

titanium dioxide to control fiber luster, and ceramic powder to improve thermal properties.

  Poly(ethylene terephthalate), PET, is a slow crystallizing polymer, which besides its wide

industrial success has also become a subject of many research activities. This is because of its

high melting point and good thermal resistance as well as its excellent mechanical properties. It

can be crystallized from the melt over a wide range of supercooling conditions or it may be

quenched to the "amorphous" phase and crystallized by simple heating or drawing thus giving a

relatively higher possibility of controlling product final structure by acting on processing

conditions. The slow crystallization behavior of PET, on the other hand, offers a broader

experimental access to the study of its crystallization kinetics. In fact, owing to these attractive

and suitable properties, almost every aspect of its crystallization behavior has been dealt with

by many scholars over the years[4-9]. Jun et al[10] prepared poly(ethylene terephthalate)

nanocomposites reinforced with multiwall carbon nanotubes (MWCNTs) through melt

compounding in a twin-screw extruder. And they found that the presence of MWCNTs

enhanced the crystallization of PET through heterogeneous nucleation.

  Owing to the relatively easier experimental access to different crystallinity levels in PET, it

has been possible to investigate the spherulite growth rate in the whole range of the

crystallization temperatures even as far back as the works of Baranov et al[11]. As regards the

development of crystallization in PET, Misra and Stein[12], among others, focused on its

crystallization condition at its early stages and concluded that the spherulites evolved from

rod-like nuclei.

  A common approach that has often been used in the attempt of understanding the underlying

mechanisms of crystallization in PET is the one based on the study of its multiple melting

behaviors. A number of studies have been conducted in this regard. In most cases, the data used

in the kinetic equations, however, are rather limited as they do not embrace the range of cooling

rates that are normally employed in polymer processing. Ge et al[13] studied non-isothermal

melt crystallization kinetics of poly(ethylene terephthalate)/barite nanocomposites. And they

found that the Ozawa method failed to describe the non-isothermal crystallization behavior of

PET, while a combined Avrami and Ozawa equation was used to more accurately model the

non-isothermal crystallization kinetics of PET.

  Kaolin or china clay consists chiefly of the mineral kaolinite and has been selected based on

its abundant availability. Kaolinite [Al4Si4O10(OH)8] is a dioctahedral aluminosilicate, which is

built with a tetrahedral SiO4 sheet and an octahedral AlO4(OH)2 sheet linked together by

oxygen atoms. The crystal consists of several of these layers extending in a two-dimensional


  Kaolin is widely used in the fields of ceramics, paper, rubber, plastic, ceramic and other

industries[15]. And the research in using it for polyester fiber area has just begun. The fiber

industries have made some studies about kaolin modified polyester, and have achieved some

success[16]. In the paper, researches of the non-isothermal crystallization kinetics of polyester

added kaolin are to reveal the effect of kaolin on the polyester crystallization process.

2 Experimental

2.1 Materials and Preparation

  Fiber-class modified kaolin, with an average diameter of about 0.8 microns, was offered by

the China Kaolin Company. And PET, with intrinsic viscosity 0.68, was offered by the

Shanghai Petrochemical Company Limited, China. These two materials were blended in the

twin-screw and granulated to chips containing 4 wt% of kaolin, and the chip sample was

marked k-3. The samples were kept in a vacuum oven at 120 ℃ for one hour before use.

2.2 Experimental methods and conditions

  Non-isothermal crystallization kinetics was carried out using a Pyris-1 DSC differential

scanning calorimeter (Perkin-Elmer company of the USA), with empty crucible for reference,

calibrated the temperature with high-purity indium. All DSC curves were performed under

nitrogen atmosphere, and sample weights were between 3 and 5 mg.

  The samples were heated from room temperature to 260 ℃ at the heating rate of 10 ℃/min.

The crystallization temperature (Tc) and melting temperature (Tm) were determined by the

DSC curve.

  Similarly, the samples were heated from room temperature to 300 ℃ at the heating rate of

20 ℃/min, so that the sample was molten, kept for 5 minutes in order to eliminate thermal

history of sample. Then the melt were cooled down at the rates of 10 ℃/min, 12 ℃/min,

15 ℃/min, 18 ℃/min, 20 ℃/min, respectively. So there were 5 thermal analysis atlases by

different crystallization processes, that is, 5 non-isothermal crystallization curves. Parameters

of non-isothermal crystallization kinetics could be studied by analyzing and calculating these

experiments data.

3 Results and Discussion

  At the present time, the studies on isothermal crystallization kinetics of various materials

have been improving day by day and tended to be perfect. Avrami equation has already been

determined, but the actual productions, particularly the polymer processing usually carry on in

the condition of non-isothermal. So it is much more practically important to research on

non-isothermal crystallization of material. However, the non-isothermal crystallization

process is complex because the relationship between temperature and time has to be


3.1 Impact of kaolin on the melting temperature Tm and crystallization temperature Tc

  Fig. 1 shows the DSC curves of k-3 and pure PET at the heating rate of 20 ℃/min and the

cooling rate of 80 ℃/min. It can be seen that the melting temperature Tm of k-3 is 263.43 ℃

and the crystallization temperature Tc is 209.76 ℃, while the melting temperature Tm of pure

PET is 255.71 ℃ and the crystallization temperature Tc is 169.84 ℃. It means that the addition

of kaolin enhance either the melting temperature Tm or the crystallization temperature Tc.

                         Fig. 1 The DSC curves of k-3 and pure PET.

3.2 Relationship between crystallization temperature and the cooling rate

  The non-isothermal crystallization exothermic peaks of k-3 on DSC curves at various

cooling rates, Φ, are shown in Fig. 2. It is apparent that the crystallization processes are all

single peaks at the cooling rate Φ from 10 to 20 ℃/min. With the increasing of Φ, the position

of peak moves from high to low temperature region and the shape of peak widens gradually,

which indicates that range of crystallization temperature increases.

  It is indicated that with the increasing the cooling rate, the molecular chain is difficult to

rearrange regularly during the crystallization process. So the nucleation lags, which postpones

the initial crystallization. So the initial crystallization temperature moves to the low area on

the DSC, that is, the crystallization peak moves to the low temperature region.

           Fig. 2 Heat flow versus temperature during non-isothermal crystallization

                            of k-3 at different cooling rates by DSC.

  When the cooling rate Φ is low, the molecular chain has enough time to rearrange and enter

the crystal lattice during the process of cooling. Thus it can crystallize under high temperature.

But when Φ is high, the system has been reduced to lower temperature before molecular chain

entered the crystal lattice. Hence, it can crystallize only at a lower temperature region. Fig. 3

shows morphology of k-3 after its melt was cooled at different cooling rate. It seems that

crystallization at different cooling rate effects morphology of the polyester: smooth and dense

at low cooling rate, while loose and cracked at high cooling rate.

          Fig. 3 Morphology of k-3 after its melt was cooled at different cooling rate.

                                    (a) 10 ℃/min, (b) 20 ℃/min.

3. 3 Relationship between semi-crystalline phase and the rate of cooling

  The relationship between crystallization temperature Tc and crystallization time t during the

non-isothermal crystallization process can be given by Equation (1) as follows[17]:

                                    To  Tc
                               t                                                             (1)

where To is the initial temperature when crystallization begins (t = 0).

  Therefore, the relationship between relative crystallinity Xt and crystallization time t in the

process of non-isothermal crystallization can be shown in Fig. 4.

  As shown in fig. 4, these series of curves become s-shaped. And the curves tend to be a

relative platform in the later period of crystallization. It is possibly caused by mutually

colliding and extrusion of spherulite boundary in the later stage, which slows down the rate of


                   Fig. 4 Relative crystallinity at different time in the process

                            of non-isothermal crystallization for k-3.

  A straight line was made along Xt = 50 % which is parallel with t-axis, and then the

perpendicular lines was made at the point of intersection the straight line with each Xt-t curve.

The points of intersection of the perpendicular line and the t-axis are considered the

semi-crystalline phases at each rate of cooling.

  Fig. 5 shows the relationship between lnΦ and the logarithm value of semi-crystalline

phase lnt1/2. They have a good linear relationship, indicating that semi-crystalline phase t1/2

makes an exponential decline with increasing the cooling rate Φ, and that the higher the

cooling rate, the shorter the time of crystallization completion.

      Fig. 5 Plot of lnt1/2 versus lnΦ in the process of non-isothermal crystallization of k-3.

3. 4 Non-isothermal crystallization kinetics parameters

  The non-isothermal crystallization process is very complex due to considering the

relationship between temperature and time. So far, there are dozens of data processing

methods about non-isothermal crystallization kinetics, but no one of the methods is accepted


  In order to find a method to describe exactly the non-isothermal crystallization process, Mo

et al[18] adopted a novel kinetic approach by combining the Avrami equation with the Ozawa

equation[19, 20] and successfully dealt with the non-isothermal crystallization behavior:

                             lgΦ = lgF(T) - algt                                                   (2)

where a = n/m, n is the Avrami index, and m is the Ozawa index. The equation is used to deal

with the system of kaolin modified polyester.

  The physical meaning of F(T) is a value of cooling rate that one system of polymer

crystallization must choose for reaching certain crystallinity in the unit time. F(T) expresses

the rate of crystallization[21]. The larger the F(T), the lower the crystallization rate of system.

  Table 1 shows that F(T) increases gradually along with the improvement of the relative

crystallinity Xt. It indicates that the higher cooling rate needs the higher relative crystallinity

in the unit crystallization time.

                             Table 1 lgF(T) and a of k-3 at different Xt

Xt (%)       10        20           30      40         50        60        70         80          90

  a       0.8818    0.9114    0.9537        0.9674       0.9688   0.9771   0.9697     0.9485   0.9190

lgF(T)    0.8487    0.9026    0.9380        0.9738       1.0093   1.0459   1.0910     1.1472   1.2212

3.5 Non-isothermal crystallization activation energy

  Due to the influence of different cooling rate during the non-isothermal crystallization

process, the non-isothermal crystallization activation energy can be worked out using


                         d [ln( / T p )]            E

                                                                                                   (3)
                             d ( 1 / Tp )            R

where Φ is cooling rate, Tp is the temperature peak in the process of cooling at a same rate,

E is the crystallization activation energy, R is the gas characteristic constant. The data of
lg(Φ/Tp2) to 1/Tp is shown in Table 2.

                   Table 2 Non-isothermal crystallization kinetics datum of k-3
                                             1/Tp 10 /(K )            Φ/Tp 10
                                                2        6   -2             2     6
      Φ /(℃/min)         Tp /K                                                           ln(Φ/Tp )

         10             488.513                4.190325                41.90325          -10.0801

         12             486.923                4.217736                50.61284          -9.89131

         15             484.175                4.265749                63.98623          -9.65684

         18             483.179                4.283353                77.10036          -9.47040

         20             481.532                4.312704                86.25409          -9.35821

  Fig. 6 shows a straight line which derives from the data in the table 1. It is easy to carry out

the non-isothermal crystallization activation energy of k-3 by the slope of the straight line,

that is, E = -204.1566 kJ/mol.

                          Fig. 6 A straight line of ln(Φ/Tp ) to 1/Tp10 .
                                                          2             3

4 Conclusions

  Investigation of property of kaolin modified polyester using a differential scanning

calorimetry (DSC) shows the melting temperature Tm is 263.43 ℃ and the crystallization

temperature Tc is 209.76 ℃. The addition of kaolin enhance either the melting temperature Tm

or the crystallization temperature Tc.

  The non-isothermal crystallization process is successfully described. And it is elicited that

the higher the cooling rate, the shorter the time of crystallization completion.

  The non-isothermal crystallization activation energy of kaolin modified polyester can be

worked out using Kissinger, that is, E = -204.1566 kJ/mol.


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