Nonisothermal Thermophysical Evaluation ofPolypropylene/Natural Rubber Based TPEs: Effect ofBlend Ratio and Dynamic Vulcanization

Aju Joseph,1 Sigrid Luftl,2 Sabine Seidler,2 Sabu Thomas,3 Kuruvilla Joseph11 Department of Chemistry, St. Berchmans’ College, Changanacherry, Kerala, India

2 Institute of Materials Science and Technology, Vienna University of Technology, Austria

3 Department of Polymer Science and Technology, School of Chemical Sciences, Mahatma Gandhi University,Kerala, India

Ozawa macrokinetic model is applied to describe thenonisothermal melt crystallization process of polypropyl-ene (PP), in natural rubber/polypropylene (NR/PP) ther-moplastic elastomers (TPEs) as a function of blend ratioand dynamic vulcanization of the rubber phase. It wasfound that as cooling rate increases, the crystallizationtemperature Tc and half time for crystallization t1/2 getdiminished. Dynamic vulcanizates show a similar trendin crystallization as that of neat PP. Crystallization rateconstant and Ozawa exponent were found out for differ-ent temperatures by linear regression method usingOzawa analysis. Ozawa exponent, nO, showed variationin values when the conversion proceeded. For neat PP,the value of nO changes from 1.4 (1028C) to 3 (1128C).The nO values for NR/PP 50/50 blend were higher (e.g.4.1 at 1168C). Crystallization rate constant KO shows amaximum at 0.5 relative crystallinity. The highest crystal-lization rate constants were found for NR/PP 50/50TPEs. The activation energy for the melt crystallizationwas found to vary with the degree of conversion, as wellas with the concentration of NR in the TPEs. Finally,attempts have been made to correlate the crystallizationprocess with morphology of the blend. POLYM. ENG. SCI.,49:1332–1339, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

The development of commercial thermoplastic olefins

(TPOs) in the automotive industry has resulted in

improved bumper systems. This class of materials consists

of blends of polypropylene (PP) with rubber. These rub-

ber inclusions could modify the impact property of the

PP. Also during the processing, these rubber domains

could influence the recrystallization kinetics. Additionally,

the influence of the constant temperature at which post-

processing operations such as coating, painting and so on

are performed requires an understanding of both isother-

mal and nonisothermal process involved. Manufacturing

blends with semicrystalline components often relies on

established methods of evaluating temperature based on

melting profiles with regard to the recrystallization tem-

peratures of these materials [1].

Bulk crystallization of a polymer would lead to various

degrees of crystallinity [2]. It is well understood that the

physical, chemical, and mechanical properties of the crystal-

line polymers depend on the morphology, the crystalline

structure, and degree of crystallization [3]. To control the

rate of the crystallization and the degree of crystallinity and

to obtain the desired morphology and properties, a great

deal of effort has been devoted into studying the crystalliza-

tion kinetics and change in the material properties [4, 5]. In

practical processing, crystallization usually proceeds under

isothermal and nonisothermal conditions [6]. Therefore, it is

important to investigate the dynamic crystallization kinetics

of the polymer. Natural rubber/polypropylene (NR/PP) based

thermoplastic elastomers (TPEs) are very important thermo-

plastic elastomers which could be used for a variety of

applications in automotive field. To the best of our knowl-

edge no studies have been reported on the crystallization

behavior of NR/PP TPE’s. Blends of PP and NR possess

excellent impact properties due to the presence of elastomer

phase, the blends also possess excellent procesability of PP

[7, 8]. In this article, we intended to investigate the effect of

blend ratio and dynamic vulcanization on the nonisothermal

crystallization behavior of PP/NR blends at various cooling

rates. Attempts have also made to predict the crystallization

mechanism using Ozawa theoretical approach. We have also

examined the morphology of the system as a function of

rubber content using SEM. Attempts have been made to

correlate the morphology with crystallization.

Correspondence to: Kuruvilla Joseph; e-mail: kuruvillajoseph@sancharnet.in

and kurupolym@sify.com

DOI 10.1002/pen.21219

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

Theoretical Background

The most common approach used to describe the over-

all isothermal crystallization kinetics is the Avrami model

[9–11] in which the relative crystallinity as a function of

time y(t) can be expressed as

yðtÞ ¼ 1� exp �ðKatÞna½ � (1)

where Ka and na are the Avrami crystallization rate con-

stant and the Avrami exponent, respectively. The Avrami

rate constant Ka is written in the form of the composite

Avrami rate constant Ka (i.e. Ka ¼ Kan). Since the units of

Ka should be preferable. It should be noted that both Ka

and na are constants specific to a given crystalline mor-

phology and type of nucleation for a particular crystalliza-

tion condition [2] and that based on the original assump-

tions of the theory, the value of the Avrami exponent nashould be an integer ranging from 1 to 4.

In the study of nonisothernal crystallization using

DSC, the energy released during the crystallization pro-

cess appears to be a function of temperature rather than

time as in the case of isothermal crystallization. As a

result, the relative crystallinity as a function of tempera-

ture y(T) can be formulated as

yðTÞ ¼R TT0ðdHc=dTÞdT

DHc

(2)

where T0 and T represent the onset and an arbitrary tem-

perature, respectively, dHc is the enthalpy of crystalliza-

tion released during an infinitesimal temperature range dTand DHc is the overall enthalpy of crystallization for a

specific cooling condition.

To use Eq. 1 for the analysis of nonisothermal crystal-

lization data obtained by DSC, it must be assumed that

the sample experience the same thermal history as desig-

nated by the DSC furnace. This may be realized only

when the thermal lag between the sample and the furnace

is kept minimal. If this assumption is valid, the relation

between the crystallization time t and the sample tempera-

ture T can be formulated as

t ¼ T0 � T

f(3)

where f is the cooling rate. According to the Eq. 3, thehorizontal temperature axis observed in a DSC thermo-

gram for a nonisothermal crystallization data can readily

be transformed into the time scale. On the basis of the

mathematical derivation of Evans [12], Ozawa [13]

extended the Avrami theory [9–11] to be able to describe

the nonisothermal crystallization data. Mathematically, the

relative crystallinity function of temperature y(T) can be

represented as a function of cooling rate as

yðtÞ ¼ 1� exp�KO

fnO

� �(4)

where KO is the Ozawa crystallization rate function, and

nO is the Ozawa exponent. It should be noted that the

Ozawa kinetic parameters (i.e., KO and nO) holds similar

physical meaning those of the Avrami ones.

EXPERIMENTAL

Materials

Isotactic PP (Koylene M3060) having melt flow index

(MFI) of 8 g/10 min, density 0.905 g/cm3 at 238C was

kindly supplied by Indian Petro Chemical Ltd, Vadodara,

India. Natural rubber (NR, ISNR-5) was supplied by Rub-

ber Research Institute India, Kottayam, India.

Blend Preparation

PP/NR blends were prepared by melt mixing PP with

NR in a Rheocord at 1808C. Before feeding into the mix-

ing chamber, natural rubber was masticated for 5 min at

room temperature in two roll mill to reduce the molar

mass which leads to the efficient mixing of the compo-

nent polymers. The rotor speed was optimized as 6282

rad/s (60 pm). PP was first melted for 3 min followed by

the addition of NR and the mixing continued for another

6 min. For the dynamic vulcanization, sulfur based cura-

tives were added to the blend in the mixing chamber fol-

lowing the time–torque curve, where the vulcanization is

completed in 10 min.

Morphological Characterization

The morphology of the blend was analyzed by electron

microscopy, using a SEM Philips microscope. The SEM

samples were prepared as follows: the strips cut from com-

pression-molded samples were cryo-cut using glass knife.

The NR phase was extracted using benzene (unvulcanized

blends). After etching the NR phase, the samples were

dried in a vacuum oven at 708C for 12 h and were coated

with gold using sputter coater and micrographs were taken.

DSC Analysis

A TA Waters DSC 2920 apparatus was used to follow

the crystallization kinetics of PP and PP/NR blends. Base-

line calibration, calibration for the temperature scale, and

cell constant were carried out using a pure indium stand-

ard (TM ¼ 156.68C) according to the procedure suggested

by TA instrument in the instrument manual. To minimize

the thermal lag between polymer sample and DSC fur-

nace, each sample pan was loaded with a cut piece from

compression-molded sheets of the blend having the same

thickness. Sample mass was kept around 10 mg. Also for

each measurement new specimens were used and the

specimen compartment was flushed with dry N2 50 ml/

min to avoid any kind of oxidative degradation. The fol-

lowing conditions were applied for the study of the melt-

ing and crystallization behavior of PP/NR blends.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1333

Stage 1. Heating of the sample in the calorimeter at a

rate of 20 K/min from 220 to 2208C.Stage 2. Isothermal annealing at 2208C for 5 min; this

was done to remove any crystalline nuclei if present.

Stage 3. Cooling of the sample in the calorimeter to

258C at various cooling rate.

Stage 4. Heating the sample in the calorimeter again to

2208C at a rate of 20 K/min.

Each sample was measured at least two times.

From the exothermic curves, the crystallization kinetics

parameters were estimated.

RESULTS AND DISCUSSION

Influence of cooling rate on the crystallization behavior

of the neat PP is shown in Fig. 1. As expected, with

increasing cooling rate the onset of crystallization changes

very slightly, but the Tc and end set of crystallization

shifted to lower temperatures. At lower cooling rate the

macromolecules get sufficient time to crystallize and crys-

tallization has taken place at narrow temperature region,

but at faster cooling rates the reorganization of the macro-

molecules took place in a wider temperature region due

to the insufficient time for the molecular orientation. Rel-

ative crystallinity developed for neat PP during various

cooling rate is shown in Fig. 2. It can be seen that 100%

relative crystallinity is achieved by slower cooling in a

narrow temperature region. As the cooling rate increases,

the relative crystallinity development has taken place in a

wider temperature regime.

Effect of Blend Ratio on the Morphology

Influence of blend ratio on the development of relative

crystallinity at a cooling rate of 12 K/min is shown in

Fig. 3. This figure shows how the dispersed NR domains

affect the development of relative crystallinity as a func-

tion of NR content in the blend. It could be interesting to

note that blend with higher amount of NR (50 wt%) initi-

ate the crystallinity development at lower temperature

than the blend with lower content of NR (10, 20 wt%). In

other words 50/50 NR/PP blend crystallization starts at

117.58C, whereas for the 10/90 and 20/80 blend the crys-

tallization start at 1158C. This behavior could be related

to the morphology of the blend. The morphology of 10/

90, 20/80, and 50/50 NR/PP blends are given in Fig. 4a–c.

In the case of 10/90 and 20/80 NR/PP system, the NR

phase is dispersed as domains in the continuous PP phase.

The interphase area per unit volume of the blend in both

cases has been estimated by using the equation 3FA/Rwhere FA is the volume fraction of the dispersed phase

and R is the radius of the dispersed phase [7]. The interfa-

cial areas in both cases are 0.79 and 0.92/lm. Addition-

ally, the interfacial thickness in NR/PP system is

extremely small (\3 nm), considering these factors, the

FIG. 1. Influence of cooling rate on the crystallization behavior of neat PP.

FIG. 2. Development of relative crystallinity changes with cooling rate

for neat PP.

FIG. 3. Influence of blend ratio on the development of relative crystal-

linity of NR/PP blends with various NR content at 128C/min.

1334 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

impurity migration from the rubber phase to PP phase is

less than that of NR/PP 50/50 blend system. On the other

hand in the case of 50/50 NR/PP system, the blend has a

co-continuous interpenetrating network structure (Fig. 4c).

Because of the interpenetrating network type structure the

more impurity can early migrate from rubber phase to the

PP phase. It can induce further crystallization process as

seen in Fig. 3. It is clear from this micrograph that

amount of NR showing a significant influence of the num-

ber of NR domains, size, and the interfacial surface area

provided by these blends. In the case of lower amount of

NR, the surface area provided by the elastomer phase was

smaller and ultimately impurity migration from NR phase

to the PP phase become smaller [14–16].

We would like to further add that crystallization pro-

cess is completed faster in the case of 30/70 and 50/50

NR/PP blend. During the crystallization process, either

the elastomer domains could be engulfed or rejected from

the crystallization growth front. In the case of lower

amount of elastomer, NR domain will be rejected and at

higher concentration it will be engulfed in the crystalline

growth front. This will ultimately affect the width of the

relative crystallinity development.

The influence of cooling rate on the crystallization

behavior of NR/PP 30/70 blend is given in Fig. 5. Com-

paring Fig. 5 with Fig. 2 it is obvious that under the same

conditions, the crystallization process, crystallization starts

and ends at higher temperature for all chosen cooling

rates for NR/PP 30/70 blend than for neat PP. This is

associated with the nucleating action of the impurity in

the rubber phase.

In Table 1 crystallization kinetic parameters following

the Ozawa model [13] are summarized. During the cool-

ing process crystallization takes place along with the melt

conversion and this significantly influences the crystalliza-

tion rate constant KO. In the case of NR/PP 30/70 blend,

in the initial temperature KO having a value of 2.64 at

1188C, when the crystallization proceeds it reaches a

value 1.3 3 104 at 1148C then it decreases and reaches a

value of 2.1 3 101 at 1028C.

Influence of Dynamic Vulcanization

The vulcanization of the rubber phase during mixing

has been investigated as a way to improve the physical

properties of several thermoplastic elastomers based on

rubber/thermoplastic blends [17]. During dynamic vulcan-

ization the crosslinked rubber phase becomes finely and

uniformly distributed in the thermoplastic matrix and

FIG. 4. Scanning electron micrographs of fracture surfaces of different

NR/PP blends. (a) NR/PP 10/90, (b) NR/PP 20/80, (c) NR/PP 50/50.

FIG. 5. Influence of cooling rate on the relative crystallinity develop-

ment of NR/PP 30/70 blend at various cooling rates.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1335

attains a stable morphology. This is schematically shown

in Fig. 6. Dynamic vulcanization has been applied to the

several rubber/thermoplastic blends. Influence of dynamic

vulcanization on the relative crystallinity development of

iPP in dynamically vulcanized NR/PP 50/50, at 12 K/min

as a function of temperature are given in Fig. 7. It can be

seen that the process dynamic vulcanization decreases the

crystallization rate. This could be related to the morphol-

ogy of the blend. During the dynamic vulcanization, the

co-continuous morphology is transformed into dispersed/

matrix morphology. As a result the chance of impurity

migration is reduced. This will reduce the onset of crys-

tallization process. Simple blending influences the relative

crystallinity development in the TPEs, here initiation of

crystallization takes place at higher temperature and it

proceeds much faster as for neat PP. But during the

dynamic vulcanization the NR phase morphology changes

from co-continuous to crosslinked NR domains dispersed

in continuous PP matrix and here the crystallinity devel-

opment takes place similar to neat PP. Also the interac-

tion with the PP phase is changed due to the crosslinking

because of the formation of covalent bond and it restrict

the migration of lower-molar-mass molecule to the PP

phase [18]. Thus, the vulcanization vanished the acceler-

ated crystallization effect observed for the NR/PP 50/50

blend. Also it can be argued that NR proteins in the NR

phase may react with the vulcanization system, so they

can no longer act as additional nucleating agents.

Ozawa Analysis

Linear regression analysis is used to calculate the crys-

tallization kinetic parameters nO and KO using Ozawa

method. Results in Table 1 show that the Ozawa exponent

values obtained are nonintegral. In practice the frequent

occurrence of such values can be explained in many

ways. As a rule explanations are based on models of the

nucleation mechanism [19, 20]. In the case of the given

work, the interpretation of results must take into account

the specificity of the nonisothermal crystallization process.

This specificity arises from the fact that the only parame-

ter which changes considerably in the successive measure-

ments is the cooling rate of the polymer. Thus, it can be

claimed that the cooling rate is one of the factors respon-

sible for the diversity of nO-values. Typical Ozawa plots

of ln(/) against ln[2ln(1 2 y(T))] for neat PP and NR/

PP 50/50 blends are given in Figs. 8 and 9. The results

from these plots are summarized in Table 1. In the case

of neat PP Ozawa exponent, nO shows a maximum value

2.99 at 1108C, as the crystallization precedes the nO value

and crystallization rate constant increases and they reach

a maximum at a particular temperature and then decrease.

While looking at the crystallization parameters developed

for NR/PP 05/95 blend system maximum crystallization

rate is achieved earlier (1148C, nO ¼ 4.41) than for neat

PP, this means that maximum relative crystallinity is

achieved at higher temperature. While comparing to neat

PP, KO value increased 10 times. Further increase in NR

content results in the similar result as that of NR/PP 05/

95 blend system but the KO value increases. While look-

ing at the nO and KO value of NR/PP 50/50 blend and

NR/PP 50/50 (vulcanized blend), the system showed simi-

lar result as those of neat PP while uncrosslinked blend

showed an increased rate of crystallization.

Influence of cooling rate on the Tc of neat PP, TPEs

having different NR content, and dynamically vulcanized

TABLE 1. Nonisothermal crystallization kinetics of PP, NR/PP TPEs, and TPV, Ozawa analysis.

T/8C

PP NR/PP 10/90 NR/PP 20/80 NR/PP 30/70 NR/PP 50/50 NR/PP 50/50D

no Ko 3 102 no Ko 3 102 no Ko 3 102 no Ko 3 102 no Ko 3 102 no Ko 3 102

102 1.4 0.96 1.2 0.59 1.7 2.2 0.7 0.21 1.1 0.55 1.4 0.98

104 1.6 1.3 1.4 0.78 1.7 1.8 0.9 0.30 1.3 0.81 1.9 2.2

106 2 2.2 1.6 1 2.2 3.6 1.2 0.47 1.6 1.2 2.4 4.6

108 2.3 2.5 2.1 1.9 2.4 4 1.6 0.98 2.2 4.1 3 8.9

110 3 5.1 2.7 4.4 3.3 17 2.3 3.4 2.8 13 3.1 5.7

112 2.5 0.56 3.2 5 4 32 3.1 14 3.8 53 3.1 2.3

114 2.9 0.35 3.1 0.89 3.6 3.4 4.5 130 4.1 45 3.1 0.86

116 2 0.013 1.8 0.012 2.2 0.027 4.6 37 4 9.5 3 0.23

118 2.9 0.0035 0.3 0.013 1 0.026 3.6 0.69 3 0.27 2.8 0.032

FIG. 6. Schematic representation of morphology development during dynamic vulcanization.

1336 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

blend with a 50/50 blend ratio is shown in Fig. 10. Tcvalue decreases with increasing cooling rate. This obser-

vation is generally true for all blend compositions, addi-

tionally it can be seen from the Fig. 10 that as the NR

content increases, Tc also rises. It is already shown that

elastomer can influence the Tc of the semicrystalline poly-

mers [21, 22]. In contrast, dynamic vulcanizates of NR/

PP 50/50 shows the lowest crystallization temperature.

Half time for crystallization t1/2 value as a function of

cooling rate is given in Fig. 11. As the cooling rate

increases the t1/2 value get diminished. Neat PP showed the

highest t1/2 value. NR/PP 30/70 and 50/50 blends show the

lowest ones. This clearly indicate that the impurity in the

NR phase act as nuclei for the crystallization of PP.

Effective Energy Barrier of Nonisothermal Crystallization

Apart from the Ozawa macroscopic kinetic models it is

also interesting to evaluate the effective energy barrier,

DE, for a nonisothermal crystallization process. Consider-

ing the variation of the peak temperature (Tp) with the

cooling rate a, several mathematical procedures have been

proposed in literature for the calculation of DE [5].

Among them the Kissinger’s method has been widely

applied in evaluating the overall effective energy barrier:

lnaT2p

!¼ constant� DE

RTp(5)

Recently [23–25], a major concern has been raised for

the use of these procedures in obtaining DE, since they

FIG. 7. Influence of dynamic vulcanization on the relative crystallinity

development behavior of NR/PP 50/50 blend system at 128C/min as a

function of temperature.

FIG. 8. Typical Ozawa analysis for neat PP.

FIG. 9. Typical Ozawa analysis for PP/NR 50/50 sample at different

cooling rates.

FIG. 10. Influence of cooling rate on the Tc of NR/PP blend having

varying blend ratio.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1337

have been formulated for heating experiments (i.e., posi-

tive values of a). Vyazovkin [23] has demonstrated that

dropping the negative sign for ‘‘a’’ is a mathematically in-

valid procedure that generally makes the Kissinger equa-

tion inapplicable to the processes that occur on cooling.

Moreover, the use of this invalid procedure may result in

erroneous values of the effective energy barrier, DE. Fur-thermore, the publication [26] summarizing the results of

the International Confederation for Thermal Analysis and

Calorimetry (ICTAC) Kinetic Project has recommended

the use of multiple heating rate methods for the evaluation

of energy barrier, such as isoconversional methods. An iso-

conversional method can in principle be applied to noniso-

thermal crystallization for evaluating the dependence of

the effective activation energy on conversion and tempera-

ture. Such dependencies have been quite helpful in detect-

ing and elucidating complex kinetic processes in polymeric

systems. The more popular representatives of these meth-

ods are the isoconversional methods of Flynn and Wall

[27] and Ozawa [28]. According to these methods different

effective energy barriers are calculated for every degree of

crystallinity, according to the following equation:

lnðUiÞ ¼ Constant� DEX

RTX;i(6)

where DEX is the effective energy barrier at a given degree

of crystallinity X, TX,i is the set of temperatures related to a

given X at different cooling rates, Fi, and the subscript irefers to every individual cooling rate used.

According to this method, appropriate degrees of crys-

tallinity are selected and the temperature corresponding to

each degree for every cooling rate, TX, is recorded. Then

by plotting the logarithmic values of the cooling rate as a

function of 1/T a straight line must appear, with a slope

equal to the effective energy barrier. However, according

to Vyazovkin and Sbirrazzuoli [29] these methods are

likely inapplicable to melt crystallization because they

require calculation of the logarithm of the heating rate,

which is negative for a cooling process and the use of the

absolute value may invalidate the calculations. Therefore,

the differential isoconversional method of Friedman [30]

and the advanced integral isoconversional method of Vya-

zovkin [31] are the most appropriate. In this work, the

method of Friedman was used.

The Friedman equation is expressed as

lndX

dt

� �X;i

¼ Constant� DEX

RTX;i(7)

where dX/dt is the instantaneous crystallization rate as a

function of time at a given conversion X. According to

this method, the X(t) function obtained from the integra-

tion of the experimentally measured crystallization rates

is initially differentiated with respect to time to obtain the

instantaneous crystallization rate, dX/dt. Furthermore, by

selecting appropriate degrees of crystallinity (i.e. from 2

to 98%) the values of dX/dt at a specific X are correlated

to the corresponding crystallization temperature at this X,i.e. TX. Then by plotting the left hand side of Eq. 7 with

respect to 1/TX a straight line must be obtained with a

slope equal to EX/R. Supaphol et al. [24, 32] also used the

isoconversional method of Friedman in obtaining results

on the effective energy barrier of PP.

The dependence of the effective energy barrier on the

extent of relative crystallization for the neat PP, PP/NR

blends, and dynamically vulcanized blend (TPV) is pre-

sented in Fig. 12. As it can be seen, DE increases with

the increase in the relative degree of crystallinity. Blends

and TPV show a similar shaped graph as that of neat PP

[25]. However, for neat PP a plateau region was observed

at relative degrees of crystallinity less than 25%. In all

cases DE takes higher negative values at low extent of

FIG. 11. Influence of cooling rate on the t1/2 of NR/PP blends and

dynamic vulcanizates having diverse blend ratio.

FIG. 12. Plots of the effective energy barrier for nonisothermal melt

crystallization of PP, NR/PP TPEs and NR/PP 50/50 TPV as a function

of the relative melt conversion.

1338 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

conversion that correspond to temperatures closer to the

melting point. Increase in activation energy as conversion

proceeds suggest that as the crystallization proceeded it

was more difficult for each polymer system to crystallize.

Also the diffusion of the crystallizing molecular segments

from the equilibrium melt to the growth front will be re-

tarded by the rejected molecular species. For all the

blends, the effective energy barrier takes values close to

zero only at very high degrees of relative crystallinity

(greater than 90%). It can also be seen from the Fig. 12

that at lower conversion activation energy is having a

higher negative value for neat PP than for the blends. As

conversion proceeds activation energy is shifted to less

negative value and blends show a high negative value

which is less than that of neat PP. In the initial stages of

melt conversion activation energy is higher for the PP

than blend. However at higher conversion the trend is

reversed.

SUMMARY

Nonisothermal crystallization kinetics of NR/PP blends

have been analyzed as a function blend ratio and dynamic

vulcanization of the rubber phase. Addition of rubber to

PP altered the rate of crystallization. The effect is more

pronounced when the rubber is in dispersed phase. When

the rubber becomes the major phase, the impurity migrates

to the PP phase and then act as the nuclei fro crystalliza-

tion. However rubber is the dispersed phase the rate of

crystallization was unaffected. Dynamic vulcanization of

the rubber phase in the 50/50 blend reduced the crystalli-

zation rate on account of the morphology transformation

from co-continuous to dispersed/matrix phase. Ozawa

exponent and crystallization kinetic rate constant have

been calculated using typical Ozawa plot. The value of nOand KO of the blends showed a similar trend as that of neat

PP. The influence of cooling rate on crystallization process

was studied. As expected as cooling rate increases, the Tcvalues get decreased for all the blends. The t1/2 values alsodecreased with the increase of cooling rate.

The effective energy barrier for nonisothermal crystalli-

zation was estimated as a function of the relative degree of

crystallinity using the isoconversional analysis of calori-

metric data. This was found to vary with the degree of con-

version, as well as with the presence of NR. In the lower

melt conversion the activation energy is higher for neat PP

and lower for TPEs. As the crystallization proceeds activa-

tion energy increases and the values reach close to zero

when the melt conversion reaches more than 90%.

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DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1339