Isothermal Crystallization Kinetics of Acrylonitrile ...shodhganga.inflibnet.ac.in/bitstream/10603/22422/21/21_chapter10.pdf.pdfpolypropylene and maleic anhydride modified polypropylene

Chapter 10

Isothermal Crystallization Kinetics of Acrylonitrile Butadiene Rubber/Polypropylene Blends

Abstract

The phase morphology of polypropylene/acrylonitrile butadiene rubber (PP/NBR) blends were analysed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The uncompatibilized 60/40 NBR/PP blend showed a co-continuous morphology. Dynamic vulcanisation transformed the co-continuous phase morphology to particle/matrix morphology. Rubber content significantly influences the interparticle distances in thermoplastic vulcanizates (TPVs). The crystallization kinetics of iPP and its blend with nitrile rubber (NBR) has been analysed isothermally using differential scanning calorimetry (DSC). During the isothermal crystallization, relative crystallinity developed with the time dependence has been described by the Avrami equation. The crystallization temperature (Tc) was found to have a significant influence on the Avrami exponent. The crystallization half time (T1/2) for the blend was found to decrease with increase in the nitrile rubber content. This confirms the increase in nucleation site for PP upon the addition of NBR. In the case of TPVs, a lower molar mass foreign substance (additives) could act as nucleation centres for crystal growth. The T1/2 as well as the Avrami exponent (n) showed a slight increase with the increase in nitrile rubber content concentration in the blend. However, the overall kinetic rate constant seems to be influenced by the presence of the NBR phase. Tobin model for kinetics of crystallization was applied and it was found that Tobin Index is affected by rubber content. Kinetic rate constant is dependent on Tc and elastomer content. The influence of physical compatibilization by the addition of graft copolymer on the crystallization process was analysed. Wide angle X-ray diffraction (WAXD) analysis proved that two-crystal forms α and β were present in the isothermally crystallised specimens. Dynamic vulcanization promotes α-crystals. Crystallinity values calculated from WAXD measurements showed that NBR addition does not influence the overall crystallinity of the blend.

The result of this chapter has been submitted to Rubber chemistry and technology for publication.

278 Chapter 10

10.1. Introduction

The development of commercial TPE is of tremendous industrial

importance. Its use in the automotive industry has resulted in improved

bumper systems. This class of materials consists of blends of polypropylene

(PP) with rubber. The inclusion of rubber could modify the properties of the

polypropylene. [1]. It is well understood that the physical, chemical and

mechanical properties of the crystalline polymers depend on the morphology,

the crystalline structure and degree of crystallization. In order 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 taken to

study the crystallization kinetics and change in the material properties [2,3].

Compared with the conventional rubbers, TPEs have commercial

advantages because they can be processed on thermoplastic machinery and

require no separate vulcanization stage. The most significant advantage is that

TPEs can be recycled [4]. TPEs also give higher output rates in injection molding

and extrusion than the conventional elastomers. With these advantages

thermoplastic elastomers are currently finding markets in many applications

where vulcanized rubbers have been traditionally used [5].

The physical properties of semi-crystalline based polymeric materials

strongly depend on their microstructure and crystallinity, because failure of

the materials takes place at the microscopic level. The crystalline form can be

obtained by slowly cooling the melt or by isothermal crystallization at a

temperature between the crystalline melting point and the glass transition

temperature. From this point of view, the crystallization kinetics of isotactic

polypropylene (iPP) has been widely studied by different methods. Isothermal

crystallization kinetics, in general, has been well described by the Avrami

Isothermal Crystallization Kinetics of NBR/PP TPEs 279

equation [6–17] though it is limited when used to describe the crystallization

of TPE that having a commercial potential. The topics of TPEs from

rubber/plastic blends have been extensively reported in the literature for the

last 25 years [18-26].

Blends of PP and NBR possess the oil resistant properties of NBR and

the excellent procesability and mechanical properties of polypropylene.

However these blends are incompatible with poor physical and chemical

interactions across the phase boundaries. Hence the system requires

compatibilization to improve the properties. The effect of phenolic-modified

polypropylene and maleic anhydride modified polypropylene as

compatibilizers on the properties of PP/NBR blend was investigated [27,28].

This chapter deals with the effect of blend composition, compatibilization and

dynamic vulcanisation on the isothermal crystallization behaviour of these

blends and TPVs at various Tcs. Attempts have also made to predict the

crystallisation mechanism using Avrami and Tobin approach. The morphology

of the system was examined as a function of rubber content using AFM and

SEM.

10.2. Isothermal crystallization- Theoretical approach

Overall crystallization of semi-crystalline polymers involves two main

processes: primary and secondary crystallization. Primary crystallization

relates to macroscopic development of crystallinity as a result of two

consecutive microscopic mechanisms: primary and secondary nucleation (i.e.

subsequent crystal growth). Formation of chain folded lamellae leads to

further growth of the lamellae through the processes of branching and splaying

[13]. The primary crystallization is assumed to cease when no additional

molecular stems can transport onto a growth face. This may be due to the

280 Chapter 10

impingement of the crystalline aggregates onto one another. Secondary

crystallization refers to any process that leads to further increase in

crystallinity (after the closure of the primary crystallization process). Two

important processes are envisaged: (a) crystal perfection and/or thickening

of the primary lamellae; and (b) formation of secondary lamellae from

crystallizable melt trapped between two different lamellae in the same

stack (i.e. inter-lamellar crystallizable melt) or between two different stacks

of lamellae (i.e. interfibrillar crystallizable melt). The thickening

mechanism is thermodynamically driven by the reduction of the specific

surfaces of the crystals (hence less free energy spent for the formation of

surfaces), but is hampered by the kinetics factors (e.g. molecular mobility).

Even though it is clear that secondary lamellae have to somehow originate

from either inter-lamellar or inter-fibrillar crystallizable melt (or both)

trapped within the crystalline aggregates (e.g. axialites, spherulites, etc.)

[15-17] after their impingement, the mechanisms by which the formation of

the secondary lamellae formed are uncertain and are still matters of

ongoing research [29-33].

Several models have been proposed to study crystallization kinetics of

semicrystalline polymers (34-43). Avrami and Tobin models were found to be

most satisfactory in describing the experimental data therefore only these two

macro-kinetic models were applied to describe the experimental data in the

present chapter.


10.3. Results and Discussion

10.3.1. Morphology

10.3.1.1. Effect of blend ratio

SEM micrographs shows the morphology of the uncrosslinked blends.

Figures 10.1a-b, where figure 10.1a showing NBR/PP 30/70 and 10.1.b shows

the 60/40 blend. NBR/PP 30/70 blend system, exhibits a dispersed NBR phase

in continuous PP matrix.

a) b)

10.1 a) SEM images of NBR/PP 30/70 1 b) 50/50 blend, NBR phase was etched using chloroform from the faces of cryogenically cut specimens.

Here the NBR phase was selectively extracted using aniline. From these SEM

micrographs we could observe that, when the NBR content increases dispersed

phase morphology changed to a co-continuous morphology. And it is shown in

figure 10.1b.

10.3.1.2. Influence of dynamic vulcanisation

AFM phase images of cross-linked blends (TPVs) having various

blend ratios are shown in figures 10.2a-c, where 10.2a belongs to NBR/PP

50/50, 2b belongs to 60/40 and 10.2c to 70/30. In these systems the cross-

linked NBR phase is dispersed in the PP matrix. The dark phase belongs to the

282 Chapter 10

cross-linked NBR phase and the brighter part of the micrograph belongs to the

PP phase.

a) b) c)

10.2 a-c) AFM images of TPVs from NBR and PP. a) NBR/PP 50/50 b) NBR/PP 60/40 c) NBR/PP70/30.

As the amount of NBR in the TPVs increases, the continuous PP phase

becomes narrow. This is clear from the atomic force micrographs 2a-c. In the

case of dynamic vulcanisation the number of the cross-linked rubber particles

increases with increase in rubber content of the blend. Also it could be seen

from the AFM micrograph that submicron PP domains were entrapped in the

cross-linked NBR phase. It can also be noted that elongated continuous ribbon

of NBR phase could be seen.

10.3.2. Crystallization behaviour

10.3.2.1. Effect of blend ratio: Uncompatibilized TPEs

Young and Lovell [44] has defined crystallization as the process by

which an ordered structure is produced from a disordered phase such as a melt.

The crystallization proceeds in two steps: nucleation and crystal growth.

Theoretical values of ‘n’ and the isothermal rate constant ‘k’ for different

morphologies and nucleation mechanism [45,46] is given in section 5.2.

(Table 5.1.). Nucleation is either homogeneous or heterogeneous. During


homogeneous nucleation, nuclei are formed randomly throughout the melt,

whereas during heterogeneous nucleation the crystals grow from inclusions

such as impurities, fillers and fibers. Tc and cooling rate dominantly influence

the nucleation mechanism [47]. Heterogeneous mechanism is dominant at

lower cooling rate, where as homogeneous nucleation takes place at higher

cooing rate.

Fig. 10.3. Effect of blend ratio on the isothermal crystallization of PP/NBR blends crystallized at 120ºC.

Effect of blend ratio on the isothermal crystallization behavior of pure

iPP and its blend at 120°C is shown in figure 10.3. It could be seen that as the

amount of rubber content increases, the crystallization peak becomes narrow

and the process completes much faster. PP crystallized at 120ºC shows a

broader peak among them.

10.3.2.2. Influence of compatibilization and dynamic vulcanization

Influence of compatibilization and dynamic vulcanization on the

isothermal crystallization of PP/NR 40/60 TPEs and TPVs crystallized at

0 2 4 6 8 10

Hea

t flo

w (W

/g)

Time (min)

PP120 NBR/PP 10/90 NBR/PP 20/80 NBR/PP 30/70 NBR/PP 60/40

284 Chapter 10

120ºC are shown in figure 10.4. It is clear from figure 10.4.a. The narrowing

of the width of the crystallization peak confirms that compatibilization simply

enhances the crystallization rate.

a) b)

Fig.10.4. a) Effect of compatibilization and dynamic vulcanisation on the isothermal crystallization of PP/NBR blends crystallized at 120ºC. b) Isothermal crystallization behaviour of PP and TPVs having various rubber contents. All the samples were crystallized at 120ºC.

Compatibilizer could act as nucleation centres for the crystallization or

compatibilizer could act as plasticizer and it could accelerate the mobility of the

chains. Effects of blend ratio and dynamic vulcanization on the isothermal

crystallization of NBR/PP TPVs crystallized at 120ºC were shown in figure 10.4.b.

Dynamic cross-linking further enhances the crystallization. Goossens et.al. and

Huang et. al. recently reported that these heterogeneities are residues of catalyst,

impurities, incomplete melted crystallites or solvent. (48, 49)

10.3.2.3. Isothermal crystallization-Avrami approach

Experimental data and values predicted using Avrami’s model is

shown in figures 10.5a-c. The symbols and the lines represent experimental

and thermal predictions respectively.

0 2 4 6 8 10

Hea

t flo

w (W

/g)

Time (min)

PP NBR/PP 60/40 NBR/PP 60/40 Com NBR/PP 60/40 D

0 2 4 6 8 10

Hea

t flo

w (W

/g)

Time (min)

PP NBR/PP 60/40 NBR/PP 60/40D NBR/PP 50/50D NBR/PP 70/30D


a) b)

c)

Fig. 10.5.a-c) Evolution of the relative crystallinity and predicted data using Avrami model of PP and its blend with NBR isothermally crystallized at 120ºC. The symbols and the lines represent experimental data and predicted data. a) Effect of blend ratio. b) NBR/PP blend having a blend ratio 60/40, influence of compatibilizer and dynamic vulcanisation c) PP and TPVs of NBR and PP.

Relative crystallinity data calculated from the heat flows of

uncompatibilized blend specimen isothermally crystallized at 120ºC is shown in

figure 10.5.a Influence of compatibilization and dynamic vulcanization on the

0 2 4 6 80.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e cr

ysta

llini

ty, X

t(%)

Time (min)

PP NBR/PP 60/40D NBR/PP 50/50D NBR/PP 70/30D

0 2 4 6 80.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e cr

ysta

linity

, Xt (%

)

Time (min)

PP NBR/PP 10/90 NBR/PP 20/80 NBR/PP 30/70 NBR/PP 60/40

0 2 4 6 80.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Rel

ativ

e cr

ysta

linity

,Xt (%

)Time (min)

PP NBR/PP 60/40 NBR/PP 60/40Com NBR/PP 60/40D

286 Chapter 10

relative crystallinity data of NBR/PP 40/60 blend isothermally crystallized at 120ºC

was shown in the figure 10.5.b. The influences of blend ratio and dynamic

vulcanization on the relative crystallinity data developed and predicted using

Avrami were shown in figure 10.5.c.

Crystallization rate constant ‘k’ and Avrami exponent ‘n’ were determined

using curve fitting method and values were shown in the table 10.1. In almost all

cases, the values of ‘n’ are non-integer, in contrast with the theoretical prediction

(50). Non integral values are generally accounted by the mixed growth and/or

surface nucleation and two-stage crystallization. Grenier and Prud’homme (14)

have shown that experimental factors such as an erroneous determination of ‘zero’

time or the melting enthalpy of the polymer at a given time can cause ‘n’ to be non-

integer.

Table 10.1. Avrami parameters (k and n) for the pure PP, its TPEs with different NBR content and TPVs having various NBR content at distinct Tc’S.

1160C 1200C 1240C 1280C Blend ratio PP/NBR k n k n k n k n

100/00 0.4 2.1 7×10-2 2.2 4.6×10-3 2.4 2.5×10-3 2.6

90/10 0.7 2.2 1.5×10-1 2.6 3.8×10-2 2.2 1×10-2 2.0

80/20 0.5 2.4 1.2×10-1 2.7 2.9×10-1 1.7 2.3×10-3 2.6

70/30 0.2 3.2 4.2×10-1 2.0 2.9×10-1 1.7 2.9×10-2 1.9

40/60 0.7 2.4 1.6×10-1 2.7 3×10-2 2.3 1×10-2 2.0

60/40Com 1.4 3.0 1.0 2.3 2.8×10-1 2.2 4.6×10-2 2.1

50/50D 0.8 4.4 2.9 3.0 9.5×10-1 2.3 2.6×10-1 2.5

40/60D 3.2 2.5 2.6 2.7 8.1×10-1 2.2 1.4×10-1 3.1

30/70D 0.5 4.2 3.6 2.3 1.4 2.1 1.3×10-1 2.6


Crystallization gets accelerated as the Tc decreases. This could be

explained on the basis that at lower crystallization temperature more nuclei got

activated and this impart more centres for macromolecular attachment leading

to faster crystallization. An increase of 12ºC (116 to 128ºC) in Tc will decrease

the rate constant by 10-3 times the value shown at 116ºC for neat PP.

Comparing the Avrami exponent shown by neat PP at different Tcs, there is

only a slight increase from 2.1 to 2.4 as the Tc increased from 116 to 128ºC.

The kinetic rate constant shown by TPEs having different elastomer content,

increases slightly even in the presence of a mass fraction of 10wt% NBR. ‘k’

value increases from 0.4 to 0.7 while the Avrami exponent ‘n’ remains same

when we compare the values for neat PP and NBR/PP 10/90 which crystallized at

116ºC. Compatibilization accelerates the rate constant ‘k’ for the crystallization

and it get doubled in the case of NBR/PP 60/40 TPEs at 116ºC and reaches a ten

times value at 120 and 124ºC. Avrami exponent shows a slight variation in its

values at lower Tcs and remains unaltered at elevated Tcs.

Dynamic vulcanisation process (sample 60/40D) increased its kinetic

rate constant value from 1.4 to 3.2 at 116ºC while the exponent shows a

substantial increase in its ‘k’ value which is associated with compatibilized

sample. In the case of TPVs, as the Tc increases from 116 to 128ºC, the ‘n’

values remain same. For the NBR/PP system kinetic rate constant calculated

using Avrami approach revealed slight increase in the presence of elastomer

and dynamic vulcanization accelerates the crystallization rate. External

additives which include the cure system could impart nucleation in the PP

phase. These could be the reason for the increased ‘k’ value for the TPEs and

TPVs. As the amount of NBR in the TPV increases, the crystallization rate

decreases for all Tcs. Avrami exponent reaches the value 3 after

288 Chapter 10

compatibilization, which means that the spherulite growth is spherical. From

the figures 10.5.a-c it could be clear that Avrami’s model suitably fits for the

PP/NR system.

10.3.2.4. Tobin’s model

Modified version of Avrami model proposed by Tobin, (eq. 2.9 in

chapter 2) is also applied for the crystallization of TPEs prepared from PP and

NBR. Tobin considered the spherulite impingement too for his updated theory

and he claimed that his method is better than Avramis’ model. Tobin index- ‘nt’

and crystallization rate constant ‘kt’ calculated using eq. 2.9 (in chapter 2) are

given in table 10.2.

Table 10.2. Tobin Parameters (kt, isothermal crystallization rate constant and nt,, Tobin exponent) for the pure PP, its TPEs with different NBR content and TPVs having various NBR content at distinct Tc’S.

1160C 1200C 1240C 1280C Blend ratio

PP/NBR Kt nt Kt nt Kt nt Kt nt

100/00 5.2×10-1 3.3 3×10-2 3.5 4.6×10-4 3.9 6.0×10-6 4.0

90/10 1.15 3.4 1.1×10-1 4.1 1×10-2 3.4 1.6×10-3 3.1

80/20 7.6×10-1 3.8 7×10-2 4.2 2×10-2 3.3 1.8×10-4 3.9

70/30 2.2×10-1 4.7 5.2×10-1 3.2 2.6×10-2 2.7 7.9×10-3 3.0

40/60 1.12 3.6 1.1×10-1 4.1 1×10-2 3.6 1.7×10-3 3.0

60/40Com 3.30 4.3 2.1 3.6 2.8×10-1 3.5 1.5×-2 3.3

50/50D 1.5 6.6 10.1 4.6 1.8 3.6 2.5×10-1 3.9

40/60D 18.2 4.0 8.37 4.2 3.4 3.3 1×10-1 4.8

30/70D 8×10-1 7.7 15.2 3.6 7×10-1 4.1 8.4×10-2 4.0

Experimental data and values predicted using Tobin’s model was

shown in figures 10.6a-c. The symbols and the lines represent experimental


data for the relative crystallinity and prediction based on Tobin’s approach,

respectively. Tobin crystallization rate constant decreases from 5.2×10-1 to

6×10-6 as the Tc increased from 116 to 128°C, it could be due to the

inconsistent nucleation density at different Tcs (table 10.2). Also the Tobin

index increases from 3.3 to 4 as the Tc increases from 116 to 128°C. Tobin

exponent could be increased up to 6. The influence of elastomer content on the

crystallization behaviour of PP at lower Tc (116°C) makes clear that Tobin

exponent increases slightly with a lower content of NBR (10wt% NBR) and

further increase in elastomer content up to 40wt% increases the Tobin index

up to 3.6. The maximum value of Tobin exponent is shown for NBR/PP 30/70

at 116°C (4.7). But it is difficult to explain the changes in the Tobin index

shown by PP/NBR TPEs crystallized at higher Tcs. From the experimental

data and predicted values Tobin’s proposed model is also a useful tool to

predict the crystallization kinetics.

290 Chapter 10

a) b)

c)

Fig. 10.6.a-c) Evolution of the relative crystallinity and predicted data using Tobin model for PP and its blend with NBR isothermally crystallized at 120ºC. The symbols and the lines represent experimental data and predicted data. a) Effect of blend ratio. b) Influence of compatibilization and dynamic vulcanization. c) Impact of blend ratio on TPVs.

Verifying the influence of compatibilization and dynamic

vulcanization using Tobin crystallization method, it could be seen that

compatibilization step up the crystallization process. In the case of NBR/PP

60/40 TPEs compatibilization accelerate the crystallization rate ten times at

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e cr

ysta

llini

ty,X

t (%)

Time (Min)

PP NBR/PP 60/40D NBR/PP 50/50D NBR/PP 70/30D

0 2 4 6 80.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e cr

ysta

llini

ty, X

t(%)

Time (Min)

PP NR/PP 10/90 NR/PP 20/80 NR/PP 30/70NR/PP 60/40

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0

PP NBR/PP 60/40 NBR/PP 60/40Com NBR/PP 60/40D

Rel

ativ

e cr

ysta

llini

ty, X

t (%)

Time (Min)


128°C. Dynamic vulcanization of the same blend again increases the ‘Kt’

value ten fold when compared to the value shown by the compatibilized

NBR/PP 60/40 TPEs.

10.3.2.5. Crystallization half-time

Table 10.3. shows the crystallization half times of PP and its blends. It

could be observed that as the crystallization temperature increases, the time

taken for the completion of the half of the crystallization is also increased.

This observation is easy to explain because a higher Tc means a lower super

cooling and at this higher temperature the number of nuclei formed will be

less and hence a delayed crystallization leads to higher crystallization half

time. In the case of pure PP fifteen times increase of T1/2 is shown when the Tc

is increased from 116 to 128ºC. While considering the case of TPEs having

different elastomer content, it could be seen that the T1/2 increases with Tc.

Interestingly as the nitrile rubber content increases, T1/2 decreases. This

confirms the increase in nucleation site for PP in the presence of rubber.

Fig. 10.7. Influence of blend ratio on the half time for the crystallization (T1/2) of NBR/PP TPEs.

0 10 20 30 400

2

4

6

8

10

12

14

16

18

20

T 1/2 (m

in)

Weight percentage of NBR

116oC 120oC 124oC 128oC

292 Chapter 10

Table 10.3. T1/2 values of crystallization for the pure PP, its TPEs and TPVs having different weight percentage of NBR and its dynamic vulcanizates at different Tc’S.

Blend ratio NBR/PP

116ºC

(T1/2) min

120ºC

(T1/2) min

124ºC

(T1/2) min

128ºC

(T1/2) min

PP 1.24 2.78 7.20 18.73

10/90 1.2 1.87 3.73 8.40

20/80 1.1 1.97 3.39 9.23

30/70 1.1 1.47 1.70 5.51

60/40 0.7 1.90 3.96 8.77

60/40 Com 0.5 0.99 1.50 3.61

50/50D 0.62 0.68 0.85 1.45

60/40D 0.60 0.59 0.96 1.64

70/30D 0.6 0.53 0.72 1.89

Nitrile rubber may contain non-reacted catalytic residue or impurities.

It may act as heterogeneous nucleus for the crystallization. Graphical

representation of the influence of elastomer content on T1/2 of TPEs is shown

in figure 10.3. Influence of elastomer content on the T1/2 value is maximum at

higher Tc. At higher Tc the T1/2 value showed a maximum for PP and as NBR

content increases the T1/2 value decreases and levels off at higher NBR

content. At lower Tc the influence of rubber content is minimum and the

weight percentage of NBR does not show much influence and the curve

becomes a flat one. But compatibilization appreciably influences the T1/2


values which is shown in table 10.3. It could be noticed that dynamic

vulcanization and compatibilization decrease the T1/2 and the effect becomes

prominent at higher temperatures. Compatibilization introduces nucleation

centres or it could accelerate the mobility of the macromolecule chains and

hence the T1/2 value diminishes. T1/2 shown by dynamically vulcanised

PP/NBR blend having a 50wt% elastomer content does not exhibit much

reduction at lower Tc. But at higher Tc, all the TPVs showed a reduction in T1/2

values. When we compare the TPEs (vulcanised) and TPVs (unvulcanised)

having the same rubber/plastic ratio we could observe that dynamic

vulcanization accelerates the crystallization and T1/2 value gets reduced. This

may be due to the fact that some of the cure system could influence the

crystallization, because during the dynamic cross-linking there is chance for

the curative to enter into the PP phase thereby causing faster crystallization

during cooling.

10.3.3. Melting behaviour of the isothermally crystallized TPEs

Melting behaviour of isothermally crystallized TPEs of NBR/PP at

different Tcs is given in figure 10.8.a Melting point of the PP remained

unchanged even after the blending and compatibilization process. Melting

behaviour of the compatibilized 60/40 NBR/PP blend is shown in figure

10.8.b. But crystallization temperature has a slight impact on the Tm of neat PP

and its TPEs. When the Tc is increased from 116 to 128ºC, Tm shows a shift of

3ºC to higher melting region because highly ordered crystals are formed due to

the isothermal crystallization at higher Tc [44,51]. Exotherms of the

isothermally crystallized TPEs having a NBR content ≤60-wt% are shown in

figure 10.8.b.

294 Chapter 10

a) b)

c)

Fig.10.8.a-c) Melting behavior of NBR/PP blends. a) Influence of blend ratio on the melting behaviour of PP/NBR blends. b) Influence of compatibilization and dynamic vulcanization on the melting behaviour of NBR/PP 60/40 blend systems. c) Impact of blend ratio on the dynamically vulcanized systems.

From these exotherms it is clear that the crystals developed were in α-

monoclinic form. Influence of compatibilization and dynamic vulcanization on

the NBR/PP 60/40 TPEs and TPVs were given in figure 10.8.b.

Compatibilization does not make any predominant effect on the melting

behaviour of these TPEs. But Dynamic vulcanization increases the Tm (neat

120 140 160 180 200 220

NBR/PP 60/40

NBR/PP 30/70

NBR/PP 20/80

PP

NBR/PP 10/90Hea

t flo

w (W

/g)

Temperature (oC)

120 140 160 180 200 220

NBR/PP 60/40D

NBR/PP 60/40 Com

NBR/PP 60/40

PP

Hea

t flo

w (W

/g)

Temperature (oC)

120 140 160 180 200 220

NBR/PP 70/30D

NBR/PP 60/40D

NBR/PP 50/50D

PP

Hea

t flo

w (W

/g)

Temperature (oC)


PP=165°C) to 170°C. Melting behaviours of TPVs with various NBR content

are shown in figure10.8c. It could be seen that only the TPV with 70 mass% of

NBR shows a difference from other TPVs.

10.3.3.1. WAXD Study

In order to determine the degree of crystallinity and the possible

formation of different possible polymorphic forms, static WAXD

measurements were performed on isothermally crystallized thin specimens.

Figures 10.9a-b represent WAXD intensities for the homopolymer PP and

TPEs having different NBR content up to 60 mass %.

a) b)

Fig.10.9.a-b) WAXD spectrum of a) neat PP and TPEs having varying NBR content, crystallized at 124ºC. b) neat PP and TPVs having varying rubber content crystallized at 124ºC.

PP having a broad amorphous background superimposed on six

distinct peaks at the same values of 2θ angles. These peaks values of 2θ are

14.05º, 16.85o, 18.45o, 21.6o and 28.65o. These correspond to the significant

peak values of α-monoclinic crystalline form of isotactic polypropylene [52].

There is a small additional peak at 15.6o on the PP curve that corresponds to

the β-hexagonal crystalline form of PP. Contribution from β-crystal forms is

0 10 20 30 40 50

NBR/PP 60/40NBR/PP 30/70NBR/PP 20/80NBR/PP 10/90iPP

Inte

nsity

(au)

2θ0 10 20 30 40

NBR/PPD70/30

NBR/PPD 60/40

NBR/PPD 50/50

PP

Inte

nsity

(au)

2θ

296 Chapter 10

clear from figure 10.9a-b. It also reveals that the β crystalline phase is almost

absent and dynamic vulcanization does not promote the growth of β crystalline

phase.

Table 10.4. Normalized percentage crystallinity of NBR/PP TPEs and TPVs calculated from WAXS analysis.

Blend ratio

NBR/PP

Xc (%)

124ºC

100/00 54

10/90 54.6

20/80 53.8

30/70 53.4

40/60D 54.2

50/50D 54

70/30D 54.1

Crystallinity values calculated from the WAXS examination is given in

table 10.4. Normalized percentage crystallinity of PP is almost unaffected by

the presence of NBR. Crosslinked elastomer phase could not influence the

ultimate crystallinity of the PP phase. Hence dynamic vulcanisation has no

influence on the final crystallinity of the TPVs. However, it could affect the

crystallization rate.


10.4. Conclusions

Morphology of the unvulcanised (TPEs) and the vulcanised (TPVs)

blends were analysed using SEM and AFM. Dispersed morphology is shown

at lower concentration of the NBR where NBR is in the dispersed in PP

matrix. A co-continuous morphology is shown by NBR/PP 60/40 TPEs. Phase

inversion is observed due to the dynamic vulcanization. Melting and

crystallization behaviour of blends are analysed using DSC. TPEs and TPVs

were analysed using Avrami’s and Tobin’s theoretical approaches.

Crystallization rate constant calculated using Avrami method has shown that

blend ratio has only slight influence on the ‘k’ value. Compatibilization

predominantly influences crystallization rate of the TPEs. Compatibilization

increases the ‘k’ value in the case of NBR/PP 60/40 TPEs. Dynamic

vulcanization increases the rate constant for the crystallization. Only a slight

influence is shown by the NBR content on the ‘k’ value shown by TPVs. The

‘n’ value shown by the TPEs and TPVs prove that crystallization were taken

place due to the heterogeneous nucleation. Most of the values shown by

Avrami exponent are in between 2-3 and hence the crystal growths were in

spherical in dimension.

Tobin’s theoretical approach is also used for calculating the

crystallization rate constant and Tobin exponent. The Kt value for the

uncompatibilized TPEs shows that blend ratio has only a mild influence on the

crystallization rate. Compatibilization increases the ‘Kt’ value at higher Tcs.

There is significant increase in the ‘Kt’ value calculated for the TPVs

compared to the TPEs having same NBR content. Blend ratio do not have

influence on the Kt value shown by the TPVs. Tobin exponent ‘nt’ also shows

similar trend like the values shown by ‘Kt’.

298 Chapter 10

For the compatibilized TPEs, T1/2 value reduced to half of its value

observed for uncompatibilized blends at higher Tc s. Dynamic vulcanization

further reduces the T1/2 values compared to the uncompatibilized and

compatibilized blends. Blend ratio has no further influence on the T1/2 values

shown by the TPVs.

WAXD measurements prove that dynamic vulcanization promotes the

growth of α-crystals which are present in TPEs and absent in TPVs. Xc values

calculated from WAXD measurements prove that overall crystallinity is

unaffected by the compatibilization and dynamic cross-linking even though

there is significant influence on the rate of crystallization. Melting behaviour

of the TPEs and TPVs were almost unaffected by changing the NBR content

and dynamic vulcanization.


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