Flow-Induced Morphology of iPP Solidified in a Shear Device

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Flow-Induced Morphology of iPP Solidified in aShear Device

Roberto Pantani,* Luigi Balzano, Gerrit W. M. Peters

The morphology of molded parts is the result of a competition between deformation rate,crystallization kinetics, relaxation times, and cooling. In this work, samples are solidifiedisothermally, so that the differences in morphology can be ascribed only to flow, and after astep shear, so that the effect of crystallinity on flow can be neglected. The resultingmorphology is characterized, so thatthe data can be adopted for any furtheranalysis. A comparison is conductedamong the resulting structures and anattempt is made to identify a keyparameter able to justify the differences.It is found that the main morphologicalfeatures can be correlated to a singlerheological parameter: the maximumattained value of molecular strainduring the pulse of flow.

Introduction

Themorphologyofan injectionmoldedpart is the resultof a

competition among deformation rate, crystallization

kinetics, relaxation times, and cooling rate.[1] All these

parameters play a role in producing amultilayer skin/core-

type of structure that, eventually, controls the properties of

the final object.[2] Microscopy together with X-rays reveal

that different morphological building blocks exist in the

skin/core structure of inject molded parts. The skin is filled

with densely packed shish-kebab crystallites oriented in

R. PantaniIndustrial Engineering, University of Salerno, via Ponte donMelillo, I-84084 Fisciano (SA), ItalyE-mail: [email protected]. Balzano, G. W. M. PetersMaterials Technology, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands

Macromol. Mater. Eng. 2012, 297, 60–67

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the flow direction whereas the core is filled with randomly

oriented lamellar crystals assembled into spherulites

whose size varies along the thickness.

These differences in the morphology are the result of

different structure formation processes.[3] Close to the wall

of the mold, where deformation rates are the highest,

molecules are severely oriented and stretched by the flow.

The stretched molecular conformation promotes the

formation of long fibrillar bundles ofmolecules (precursors

of crystallization) that rapidly crystallize,[4] forming

shishes that are subsequently overgrown by lamellar

crystals (kebabs).[5] The skin layer is thus formed.[6]

Increasing the distance from the wall, the deformation

rate decreases and molecules experience lower degrees of

orientation and stretch. As a consequence, in the core,

fibrillar flow-induced nuclei are rather short and the

subsequent lamellar overgrowth leads to spherulitical

crystallites.[7,8] Therefore, low degrees of molecular orien-

tation lead to isotropic morphology, similar to quiescent

library.com DOI: 10.1002/mame.201100158

Table 1. Rheological parameters for iPP1 and iPP2.

Sample h0[Pa � s]

K0

[s]

n T0[-C]

C1 C2[-C]

iPP1 4849 0.2674 0.3878 190 2.149 225.8

iPP2 8569 0.8428 0.3286 220 1.114 165

Flow-Induced Morphology of iPP Solidified in a Shear Device

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crystallization. Nevertheless, it is well known that also

a low molecular orientation can have large influence

on nucleation rate.[9,10] Hence, the number of nuclei

formed under the influence of flow is considerably

larger than in quiescent conditions and leads to smaller

spherulites.

In the recent years, many attempts have been made to

predict the final morphology of injection molded parts.

Often, the goal is to model the structure formation

processes that take place over the thickness of the sample.

One of the problems of this type of approach is the lack of

experimental data. In fact, basic relations like the effect of

the deformation history on nucleation density or the effect

of crystallinity on relaxation times are only qualitatively

established.

In this work, we carry out experiments in well-defined

conditions to study the effect of shear flow on structure

formation. To isolate the effect of shear, experiments are

carried out at a constant temperature and using a short

term shear protocol. In this way, little or no material

changes take place during flow. Therefore, the flow

conditions can be characterized assuming a melt-like

rheological behavior.[11] We find that shear rate, time,

and temperature affect the position of the boundary

between skin and core and the distributions of spherulite

dimensions inside the core layer. Interestingly, all results

can be rationalized in terms of a single rheological

parameter: the maximum attained value of molecular

strain during the pulse of flow.

Experimental Section

Materials

Two materials were adopted in this work: a linear isotactic

poly(propylene) (iPP, grade HD120MO, Borealis) with a weight-

average molecular weight (Mw) of 365kg �mol�1 and a polydis-

persity (PD) of 5.2; an iPP also provided by Borealis (coded 34225-2)

with aMw of 576 kg �mol�1 and a PD of 7.5. The two grades will be

named here iPP1 and iPP2, respectively.

Bothmaterialswerepreviouslycarefullycharacterized forall the

properties of interest for this work.[9,12,13]

The shear viscosities of the materials were well described by a

Cross model,

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h T; _gð Þ ¼ aT Tð Þh01þ aT Tð ÞK0 _gð Þ1�n (1)

in which the shift factor aT could be described by a WLF equation

aT Tð Þ ¼ 10� C1 T�T0ð Þ

C2þT�T0 (2)

Figure 1. Schematic view of the geometry adopted in this workand of the location where the micrographs were taken.

The parameters to be used in Equation (1) and (2) are reported in

Table 1.

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The linear viscoelastic behavior of the materials was

described[12,13] by a discrete Maxwell relaxation time spectrum.

The Rouse time of the viscosity-averaged relaxation mode tR,avg at

the temperature of 165 8C (which is one of the temperatures at

which the tests were conducted in this work) was estimated[13] as

0.15 s for iPP1and1.3 s for iPP2.Byadoptingtheshift factor reported

in Equation 2, the values of tR,avg at the temperature of 145 8C(namely the other temperature at which the tests were conducted

in this work) are 0.22 s for iPP1 and 3.5 s for iPP2. It can be noticed

that the relaxation times for iPP2 are one order ofmagnitude larger

than those measured for iPP1.

Procedure

A shear device, namely aMultipass Rheometer[13] was adopted for

the experiments. The device was equipped with a slit geometry

120mm long, 6mmwide, and 1.5mm thick (Figure 1). The thermal

history experienced by the material is reported in Figure 2. The

material was held above its equilibrium melting temperature at

220 8C for 10min, in order to erase the effects of previous

thermomechanical history. Subsequently, it was cooled down to

the test temperature, Ts, at which it was kept for 1 h. The material

was then cooled to room temperature, which was reached about

5min after the cooling start. The flow was applied according to a

step-shear protocol, also represented in Figure 2: after about 2min

from the start of the isothermal step, during which the sample

reached a homogeneous temperature, a constant velocity was

imposed to the pistons and thus the material inside the slit was

forced to reach a constant average velocity, Vav. This velocity was

held for a chosen shearing time, ts.

The experiments performed by changing the test temperature,

the average velocity and the shearing time are summarized in

Table 2.

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Table 2. Set of experimental conditions applied. _gW is the wallshear rate.

Sample Ts[-C]

Vav

[mm � s�1]

ts[s]

_gW

[s�1]

iPP1 145 12.5 3 71

iPP1 145 25 3 146

iPP1 145 50 0.75 297

iPP1 145 100 0.375 600

iPP1 165 100 0.375 590

iPP1 165 200 0.188 1 195

iPP2 145 12.5 3 83

iPP2 145 12.5 6 83

iPP2 145 50 0.75 336

iPP2 165 100 0.375 667

iPP2 165 200 0.188 1 339

Figure 2. Thermal and mechanical protocol applied. Vav¼ averagevelocity during the shearing time, ts¼ shearing time. The timeelapsed between the beginning of the isothermal condition andthe start of the shear flow was always 2 s.

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R. Pantani, L. Balzano, G. W. M. Peters

Results

Themorphology of the obtained sampleswas characterized

ex situ via polarized optical light microscopy. Micrographs

were taken from thin (smaller than 10mm) cross sections

cut from the center of the slit as depicted in Figure 1. The

morphologies resulting after the tests carried out on iPP1 at

the temperature of 145 8C are reported in Figure 3.

For all the samples it is clear that the spherulite

dimensions increase on increasing the distance from

the skin. The sample sheared with an imposed

Vav¼ 100mm � s�1 (Figure 3d) presents an oriented layer

about 150–200mm thick at the sample skin, followed by a

fine-grained layer of very small spherulites which is about

100mmthick. A very thin oriented layer, about 50mmthick,

canbeobservedalso in thesample shearedwithan imposed

Macromol. Mater. Eng.


Vav¼ 50mm � s�1 (Figure 3c). All the other samples do not

present an oriented layer, but just a fine-grained layer

whose thickness increases on increasing the imposed Vav,

and thus their imposed shear rate.

Similar comments hold also for the morphology of the

samples of iPP1 resulting after the tests carried out at the

temperature of 165 8C, whose micrographs are reported in

Figure 4. The sample sheared with an imposed

Vav¼ 200mm � s�1 (Figure 4b) presents an oriented layer

about 50mm thick at the sample skin, followed by a fine-

grained layer of very small spherulites which is about

150mmthick. The other sample (Figure 4a) doesnot present

an oriented layer, but just a fine-grained region, and an

evident increase of the spherulite dimension on increasing

the distance from the skin.

The morphology resulting after the tests carried out on

iPP2at the temperatureof145 8Care reported inFigure5.All

the samples present an oriented layer at the skin, which is

about 250–300mm thick for the sample sheared with

Vav¼ 12.5mm � s�1 for 3 s (Figure 5a) and about 350mm for

the samples sheared with Vav¼ 12.5mm � s�1 for 6 s

(Figure 4b) and with Vav¼ 50mm � s�1 for 0.75 s

(Figure 5c). These latter two samples, in particular, present

a very similar morphology, despite of the much different

shear rate experienced (Table 2). For all the samples

reported in Figure 5, the core layer presents small

spherulites, so that it was not possible to measure the

dimensions from the micrographs.

Similar observations can be made also for the morphol-

ogy of the samples of iPP2 resulting after the tests carried

out at the temperature of 165 8C, whose micrographs are

reported in Figure 6. Both samples clearly present an

oriented layer about 250–300mm thick at the sample skin.

The spherulitic core presents smaller spherulites for the

samples sheared with Vav¼ 100mm � s�1 for ts¼ 0.375 s

with respect to the sample shearedwithahigherVav but for

a shorter time. In both cases, however, the sperulite

diameters are very small (smaller than about 10mm).

An analysis on the micrographs obtained on samples of

iPP1, reported in Figure 3 and 4, allowed to determine the

distribution of the spherulite dimensions along the

thickness direction. The results are reported in Figure 7,

and clearly show that the diameters increase on increasing

the distance from the skin. It was not possible to clearly

resolve spherulites having a diameter smaller than about

40mm, and thus the fine-grained layer could not be

characterized. It is interesting to notice that, at the same

distance from the skin, smaller spherulites are found the

larger is the imposed shear rate.

Simulation

Thetestcarriedoutweremodeledbyconsidering theflowof

a Non-Newtonian fluid in a cavity. The viscosity was

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Figure 4. Polarized optical micrographs for samples of iPP1 sheared at the test tempera-ture of 165 8C. The flow direction is horizontal. The samples are 1 500mm thick.(a) Vav¼ 100mm � s�1, ts¼0.375 s; (b) Vav¼ 200mm � s�1, ts¼0.188 s. The vertical scalesrefer to the parameter L, defined in the discussion section. Only the rightmost verticalscale represents the absolute distance from the skin in mm.

Figure 3. Polarized optical micrographs for samples of iPP1 sheared at the test temperature of 145 8C. The flow direction is horizontal. Thesamples are 1 500mmthick. (a)Vav¼ 12.5mm � s�1, ts¼ 3 s; (b) Vav¼ 25mm � s�1, ts¼ 3 s; (c) Vav¼ 50mm � s�1, ts¼0.75 s; (d) Vav¼ 100mm � s�1,ts¼0.375 s. The vertical scales refer to the parameter L, defined in the discussion section. Only the rightmost vertical scale represents theabsolute distance from the skin in mm.


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described by Equation (1) and (2). The terms related to axial

convection, transverse conduction, and viscous dissipation

were considered in the energy equation. A fixed value at

sample skin was imposed as boundary condition for

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Macromol. Mater. Eng. 2012, 297, 60–67

� 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe

temperature. Thermal conductivity and

specific heat were considered as a func-

tion of temperature and the correspond-

ing description was taken from Hie-

ber.[14] This allowed to calculate the

shear rate distribution along thickness

direction at the location where the

micrographs were taken. The values of

wall shear rate reached during the shear

for each test are reported in Table 2. Also

the temperature evolution due to viscous

dissipation could be calculated by the

simulations. It was noticed that for most

of the tests the maximum temperature

rise due to viscous dissipation is within

about two Celsius degrees and it was

limited to a period of a few tenths of a

second and to the thin high shear rate

zone. In the following of this report, only

data for which the maximum viscous

temperature increase was within about

two Celsius degrees will be compared.

Discussion

A parameter which is often adopted to

describe the effect of flow on crystallization is the

Weissenberg number, Wi, given by the product between

the shear rate and the characteristic relaxation time. For

Wi�1, the flow is effective in orienting themolecules, and

im63

Figure 5. Polarized optical micrographs for samples of iPP2 sheared at the test temperature of 145 8C. The flow direction is horizontal. Thesamples are 1 500mm thick. (a) Vav¼ 12.5mm � s�1, ts¼ 3 s; (b) Vav¼ 12.5mm � s�1, ts¼6 s; (c) Vav¼ 50mm � s�1, ts¼0.75 s. The vertical scalesrefer to the parameterL, defined in the discussion section. Only the rightmost vertical scale represents the absolute distance from the skinin mm.

Figure 6. Polarized optical micrographs for samples of iPP2 sheared at the test tempera-ture of 165 8C. The flow direction is horizontal. The samples are 1 500mm thick.(a) Vav¼ 100mm � s�1, ts¼0.375 s; (b) Vav¼ 200mm � s�1, ts¼0.188 s. The vertical scalesrefer to the parameter L, defined in the discussion section. Only the rightmost verticalscale represents the absolute distance from the skin in mm.

64 Macromol. Mater. Eng. 2012, 297, 60–67

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thus in enhancing the crystallization

kinetics, whereas for Wi� 1 the effect

of flow can be neglected. In thiswork, the

characteristic relaxation time was cho-

sen as the Rouse time of the viscosity-

averaged relaxationmode tR,avg, which is

associated with the fast chain retraction

mechanism. One limitation in the Weis-

senberg number defined above is that it

does not take into account the shearing

time. Recently, a non linear formulation

of the elastic dumbbell model was

successfully applied to describe the

orientation in injection molded sam-

ples.[15,16] In particular, the model

describes the deformation of the popula-

tionofmacromoleculewith respect to the

disordered equilibrium state through a

tensor (named ‘‘A’’) according to the

following equation

im

D

DtA� ry

� �T � A�A� ry� �

¼ � 1

tAþ ry

� �þ ry� �T

(3)


Figure 7.Distribution along the thickness of the spherulite diameter for all the samples of iPP1 analyzed. a) tests carried out at 145 8C, b) testscarried out at 165 8C. The lines are just guides for the eye.


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In steady, simple shear, the xx component (x¼flow

direction) of the tensor A, which will be named Axx, also

coincides with the trace of A and, for Wi of the order 1 or

larger, with themaximum eigenvalue ofA. The square root

ofAxx, whichwill be namedL in the following, can be thus

adopted as a measure of the molecular strain. It ts is the

shearing time, and t is the characteristic relaxation time

(taken here as tR,avg), the time evolution of L can be

expressed as

Figure 8. Dependence of the ratio of the parameter L to theWeissenberg number, according to Equation 3 for the materials

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L ¼ Wi

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 1�e �ts

tð Þ 1þ tst

� �� s(4)

and the temperatures adopted in this work. The dotted verticallines identify the shearing times adopted in this work.

Whendefinedaccording toEquation4, theparameterL is

strictly connected to the Weissenberg number, but

describes the fact that, for short shearing times, the

molecular deformation cannot have the time to reach its

plateauvalue. The dependence of the ratioL/(WiH2) on the

shearing time, according to Equation 4 is reported in Figure

8: being the relaxation time for iPP1 of the order of 0.1 s, for

most of the testsL is nearly equal to theplateauvalue,with

the exceptions of the tests carried out with the highest Vav

for which the shearing time is of the same order of the

relaxation time; on the contrary, for iPP2, whose relaxation

times are of the order of 1 s, for all the tests L keeps

considerably lower than WiH2.

For each shear rate and shearing time, Equation 4 allows

to calculate the value of L at the end of the shearing step

(namely the maximum attained value during the test).

Since, for a given Vav, a different value of shear rate is

present at each distance from the skin, Equation 4 can be

used to describe a distribution of the maximum attained

value for L.

The knowledge of the average velocity during shear and

of the viscosity dependence upon both temperature and

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shear rate allows to calculate the profile of shear rates

which develops during the shear for all the samples

analyzed. The value of the wall shear rate is reported in

Table 2. On its turn, the profile of shear rates allows to

calculate a profile of Weissenberg number and, through

Equation 4, a profile of L along the sample thickness for

each sample. The results are reported in Figure 9 for all tests

of both materials and reveal that, at the end of the shear

step, theparameterL reachesvaluesmuchhigher than1for

most of sample thickness, thus indicating that the flow is

effective in orienting the macromolecules and enhancing

the crystallization kinetics. Furthermore, literature indica-

tions[10,12] suggest that for the values found for the

Weissenberg number the crystallization half times are

reduced of orders of magnitude, thus assuring that most of

the crystallization process is complete during the isother-

mal step. A further confirmation of this is given by in-line

012, 297, 60–67


Figure 9. Profile over the thickness of the parameter L for the samples of both materials sheared at (a) Ts¼ 145 8C and (b) Ts¼ 165 8C.

Figure 10. Spherulite diameter versus the parameterL for iPP1. (a) Tests carried out at 145 8C, (b) tests carried out at 145 8C. The filled symbolsrepresents the spherulites found in the region where the parameter L is smaller than 1.

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birefringence measurements carried out on the tests,[13]

which are not reported in this work.

Assuming that the number of flow-induced nuclei is

directly connected to the maximum attained local value of

the parameterL (namely the data reported in Figure 9), at a

given temperature, the same number of nuclei per unit

volume should be present in different samples at distances

from the skin where the same values of L are reached. For

isothermal crystallization, the same number of nuclei

means the same spherulite diameter, and thus the same

morphology. The parameterL is reported as vertical scales

in the figures from 2 to 5 with the aim of allowing a direct

comparison between the morphologies: it can be noticed

that the thickness and the position of the oriented layers

and of the fine-grained layers are quite well captured.

In Figure 10 the spherulite diameters are reported versus

the parameter L for all the tests. It can be seen that, apart

from a scatter due to uncertainties in the measurements,

there is a clear correlation between the spherulite diameter

distributions and the parameterL. In the regionwhere the

parameter is smaller than 1, and thuswhere the flow is less

effective in orienting the macromolecules, crystallization

Macromol. Mater. Eng.


takesplaceessentially inquiescent conditions, andpossibly

during the cooling following the isothermal step.

TheparameterL should also correlatewith the thickness

of the oriented layer close to the skin of the samples, if

values of the parameter larger than 1 do enhance the

nucleation rate, the transition from spherulitic to fibrillar

structures should reasonably take place (for the same

temperature) at a certain (much larger than 1) value of L.

Figure 11 reports the value, Lc, of the parameter L,

calculated at the distance from the skin marking the

boundary between the oriented layer and the fine grained

layer. Namely oriented fibrillar layers were observed for

values ofL larger than the value ofLc reported in Figure 11,

and a spherulitic morphology was observed for smaller

values of L. Consistently, a fibrillar layer is present when

theskinvalueof theparameterLwascalculated tobe larger

than the critical valueLc reported in Figure 11, for the same

temperature.

The values of Lc, as expected, are always very high.

Furthermore, for the same material, Lc increases on

increasing temperature, probably because of the slower

crystallization kinetics; at the same temperature, Lc is

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Figure 11.Lc (value of the parameterL at the end of the orientedlayer) for all the samples analyzed in this work.


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larger for IPP1 than for IPP2 (which, having a longer

relaxation time, may undergo a slower relaxation after the

step shear).

Conclusion

In thiswork, some testswere conducted in a shear device in

order to study the effect of flow on the resulting

morphology. The tests were conducted isothermally and

according to a step shear protocol. The resulting morphol-

ogywaswell characterized, so that the data can be adopted

for any further analysis. A dumbbell model was applied at

each test adopting as relaxation time the Rouse time of the

viscosity-averaged relaxation mode, which is associated

with the fast chain retraction mechanism. The dumbbell

model allowed to define a parameter describing the

molecular strain, which is strictly connected to the

Weissenberg numberWi. It was found that the distribution

of the spherulite dimensions could be directly correlated to

this parameter. Furthermore, also the boundary between

the oriented skin layer and the spherulitic core, at each

temperature and for each material, was characterized by a

single value of the chosen parameter. The results obtained




can be adopted as a basis for a future modeling of flow-

induced morphology.

Received: May 7, 2011; Published online: July 25, 2011; DOI:10.1002/mame.201100158

Keywords: flow-induced crystallization; isotactic poly(propy-lene); morphology; orientation; skin/core

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Flow-Induced Morphology of iPP Solidified in a Shear Device