ultrasonic characterization of phase morphology of high density polyethylene/polyamide 6 blend melts

Ultrasonic Characterization of Phase Morphology ofHigh Density Polyethylene/Polyamide 6 Blend Melts

Shan Wang,1 Congmei Lin,2 Huimin Sun,1 Fan Chen,1 Jiang Li,1 Shaoyun Guo1

1 The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of SichuanUniversity, Chengdu 610065, China

2 Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China

The high density polyethylene (HDPE) and polyamide 6(PA6) blend melts with a droplet-matrix microstructurewere investigated using ultrasonic diagnosis system.The blend composition, as well as the particle size ofthe dispersed PA6 phase controlled by adding variousamounts of the reactive compatibilizer HDPE graftedwith maleic anhydride (HDPE-g-MAH), was, respec-tively, correlated with the ultrasonic velocity andattenuation. The results showed that ultrasonic velocitywas insensitive to the particle size but varied linearlywith the blend composition. However, the decrease ofultrasonic attenuation with the increasing content ofHDPE-g-MAH suggested that the attenuationdepended greatly on the particle size. Further investi-gations revealed that there was a good linear relation-ship between the excess attenuation and the size ofthe dispersed phase. Our results present that ultra-sonic technique may be served as a promising tech-nique for exploring phase morphology of polymerblends during processing. POLYM. ENG. SCI., 52:338–345,2012. ª 2011 Society of Plastics Engineers

INTRODUCTION

The phase morphology of multiphase polymer blends,

as well as its evolution, is a basic issue in polymer proc-

essing area [1]. The size, shape, spatial distribution, and

time evolution of the dispersed phase, which are the focus

of the morphology control, decide to a large extent the

macro-properties of the materials. Therefore, characteriz-

ing the morphology of multiphase polymer blends has

been considered as the key to investigate the relationship

of structure and properties.

Presently, the morphology characterization is mostly

based on microscopy (e.g., scanning electron microscopy

(SEM), transmission electron microscopy (TEM), and

atom force microscopy (AFM)), from which the micro-

structure can be observed directly. However, all the meth-

ods mentioned above are mostly confined to off-line char-

acterizing the samples in the solid phase. Apart from

time-consuming, the analytic results are not always pre-

cise as the samples just come from a small portion of the

whole specimen [2]. Moreover, measurements usually

require separate sample preparation (e.g., quenching the

melts rapidly), which may significantly alter the original

state due to the different thermo-physical property of each

phase. Thus, research and development of the rapid in situ

characterization techniques aiming at the phase morphol-

ogy of blend melts have been deemed to be a necessary

and meaningful work.

In recent years, optical-related technologies have been

successfully applied to polymer processing for real-time

monitoring the morphological evolution during the melt

blending. Leukel et al. [3] placed an optical microscope

in an extruder die for real-time image observation and

proved that this method is effective for characterizing the

morphology of polymer blends resulting from the extru-

sion process. Light scattering techniques, which are based

on the interaction between the light and the polymer

melts, have been used to predict the concentration and the

particle size of dispersed phase. A succession of represen-

tative studies were performed by the following research

groups, i.e., Sheng et al. [4, 5], Schlatter et al. [6], and

Hobbie et al [7, 8]. However, the need of sapphire optical

windows and the desire of good light transmission proper-

ties for materials greatly limited the application of these

techniques.

Ultrasonic diagnosis, as a novel method with the favor-

able fast, nonintrusive, real-time and convenient charac-

teristics, appears to be appealing in this aspect for the

measurements can be conducted in highly concentrated or

optically opaque dispersions [9]. Parameters such as ultra-

sonic velocity (or transit time) and attenuation can pro-

vide useful information on the morphology of polymer

blends. Experimental studies of ultrasound signal depend-

Correspondence to: Jiang Li; e-mail: [email protected] or Shaoyun

Guo; e-mail: [email protected]

Contract grant sponsor: National Natural Science Foundation of China;

contract grant number: 50973075.

DOI 10.1002/pen.22087

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2012

ence on the compatibility of polymer blends have been

widely performed in polymeric solution and solid phase.

Hourston et al. [10–12] and Sing et al. [13–15] suggested

that ultrasonic velocity varied linearly with the blend

composition for miscible blends, while it deviated from

linearity if the miscibility of polymer blends decreased.

He et al. [16] used ultrasonic attenuation to investigate

blend morphology, and the results showed that attenuation

depended on the size of dispersed phase and a discontinu-

ity of scattering attenuation was always observed as phase

inversion occurred.

Few authors have tried to study blend melts using

ultrasound due to the heavy absorption and strong inter-

ference of detection signal, as well as the high tempera-

ture which limits the application of the transducer. Thanks

to the work of Jen’s group [17], a novel ultrasound trans-

ducer equipped with a clad metallic buffer rod can be

successfully applied to the polymer melts and recently a

series of work have been carried out. Based on the fact

that the alteration in molecular chain orientation could

result in a change in the sonic velocity of polymer melt,

Li et al. [18–20] in-line investigated the relaxation of ori-

entation and disorientation of various polyolefins. Villa-

nueva et al. [21] fixed the ultrasound sensors (US) onto

the extruder exit, and real-time studied the effects of

screw configuration and clay nature on the dispersion of

nanoclays in a LDPE matrix. Other works such as in-line

monitoring the composition ratio [22], the residence time

distribution [23], and the polymer degradation [24] were

also conducted. However, few attentions have been paid

to the blend morphology and the attempt to associate the

acoustic parameters with the phase morphology is rarely

reported [25].

High density polyethylene (HDPE) and polyamide 6

(PA6) are semicrystalline plastics, which have versatile

industrial applications. HDPE has excellent low tempera-

ture flexibility, low cost, and good resistance to moisture

permeation [26], while PA6 shows high strength, favor-

able thermo-mechanical characteristics, and good resist-

ance to oxygen permeation and hydrocarbons [27]. Thus,

it is conceivable that blends of HDPE and PA6 (HDPE/

PA6) can synergically combine the merits of both compo-

nents, provided that the blends are appropriately compati-

bilized and the corresponding phase morphology gets

optimized.

In this work, the immiscible HDPE/PA6 blends with

10, 20, and 30 wt% of a dispersed PA6 phase were pre-

pared. Different amounts of HDPE grafted with maleic

anhydride (HDPE-g-MAH), as a reactive compatibilizer,

were added to obtain the different PA6 phase size. The

morphology of the samples was firstly characterized by

the conventional SEM. Then ultrasonic diagnosis as a

novel technique was performed to investigate the influen-

ces of the PA6 concentration and the HDPE-g-MAH con-

tent on the ultrasonic velocity and attenuation. This work

aims to explore the relationship between the ultrasonic pa-

rameters and the phase morphology for the droplet-matrix

blend melts, which lays the foundation for the potential

application of in-line ultrasonic diagnosis in the polymer

processing.

EXPERIMENTAL

Materials

A commercial HDPE (5000S) with a density of 0.946

g/cm3 and a melt flow index of 1.0 g/10 min (1908C, 2.16kg) was purchased from Lanzhou Petrochemical Co.,

China. PA6 (M3400) with a density of 1.14 g/cm3 and a

flow melt index of 6 g/10 min (2308C, 2.16 kg), was

obtained from Guangdong Xinhui Meida-DMS Nylon

Chips Co., China. HDPE-g-MAH with maleic anhydride

content of 0.8 wt% and a flow melt index of 1.2 g/10 min

(1908C, 2.16 kg) was supplied by Ningbo Nengzhiguang

New Materials Technology Co., China.

Blending Preparation

Compatibilized and uncompatibilized HDPE/PA6

blends were prepared by melt mixing with an intermesh-

ing corotating twin-screw extruder (Haake Rheomex,

PTW16/25). The temperatures of different zones were set

to 2408C, except that those of feeder and die were 190

and 2358C, respectively. The feeding and screw speed

were separately 40 and 60 rpm. Prior to melt blending,

the pellets of PA6 and HDPE-g-MAH were dried in a

vacuum oven at 858C for 24 h to avoid the effects of

moisture. The extruded strands were cooled by air and

then pelletized.

Three composition ratios (wt/wt) of HDPE/PA6 blends

were 90/10, 80/20, and 70/30. For each blend composi-

tion, the concentrations of HDPE-g-MAH ranging from 0

to 10 wt% with respect to the HDPE/PA6 resin were

used.

Morphological Analysis

Samples for the morphological analysis were prepared

through compression molding at 2308C. The phase mor-

phology was examined by Philips XL30FEG SEM oper-

ated at an accelerating voltage of 10 kV. To create better

contrast, samples were fractured in liquid nitrogen, etched

by formic acid to remove the PA6 particles, then sputter-

coated with Au before SEM observation.

The SEM micrographs were analyzed by the JEOL

SmileView software. At least 300 diameters were meas-

ured per sample. The number average diameter (Dn) was

then calculated by:

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 339

Dn ¼P

i NiDiPi Ni

; (1)

where Di and Ni were the diameter and number of par-

ticles, respectively.

Ultrasonic Instrumentation and Measurement Principles

Ultrasonic measurements were performed on a home-

made slit die which was fitted to the barrel exit of the

capillary rheometer (RH7, Malvern). Details of the setup

were illustrated in Fig. 1. Two flush-mounted US were

diametrically opposed across the die to emit and receive

longitudinal waves with a central frequency of 5.0 MHz.

Two pressure transducers (P2, P3) with J-type thermocou-

ples were used to measure the melt pressure and tempera-

ture (Tm). A wrapped-on heating jacket with a piecewise

proportion integral differential program was employed to

control the die temperature (Tdie), which ensured that the

Tm in the slit die was consistent with that in the capillary

rheometer. All the sensors were connected to the GIMI

system (PACE Simulations) composed of an ultrasonic

pulser-receiver, a sampling digital oscilloscope, and an

automated data acquisition system. And by this device,

we could simultaneously track the evolution of tempera-

ture, pressure, and ultrasonic velocity and attenuation of

the polymer melt in the slit die. In this work, pellets were

first held in the barrel of capillary rheometer for melting,

and then the melts were forced into the slit die at a con-

stant speed of 10 mm/min and finally stayed in the slit

die. All the measurements were performed at 2308C after

enough relaxation until the melt pressure was zero and

the ultrasonic velocity was unchanged. Each sample was

taken for five measurements, and the mean values of the

sound velocity and attenuation were then obtained,

respectively.

Figure 2 showed the schematic of ultrasound propaga-

tion in the polymer melt which was confined between two

aligned steel buffer rods. The longitudinal waves with

amplitude of A0 traveled to the steel/polymer interface

where part of its energy went through the molten polymer

with a thickness d ¼ 2 mm. The acoustic wave was then

reflected back and forth between the two interfaces. This

produces a series of echo signals, A1, A2. . ., exiting from

the second interface and directed toward the receiving

transducer (Fig. 3). Usually, the first and second echoes

were used for calculation of the ultrasonic velocity and

attenuation. By measuring the flying time (t1 and t2) and

the amplitude (A1 and A2) of the two successive echoes,

the ultrasonic velocity (C) and attenuation (a) in the poly-

mer melt can be separately expressed as follows:

C ¼ 2d

t2 � t1; (2)

a ¼20 log10 A1

.A2

8: 9;2d

: (3)

FIG. 1. Schematic drawing of the ultrasonic monitoring setup.

FIG. 2. Schematic drawing of ultrasound propagation in the polymer

melts.

FIG. 3. Successive ultrasonic echoes obtained by the receiving trans-

ducer, Ai refers to the ith echo transmitted from the polymer melt.

340 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

RESULTS AND DISCUSSION

Morphological Observations

The SEM images of the 90/10, 80/20, and 70/30

HDPE/PA6 blends with different amounts of HDPE-g-

MAH are presented in Figs. 4, 5, and 6, respectively. It

can be seen that PA6 phase is dispersed in HDPE matrix

in the form of spherical particles, which is known as the

typical droplet-matrix morphology. As is expected, the

phase morphology takes on increasingly refined structure

with the increasing amount of HDPE-g-MAH due to the

effect of reactive compatibilization. The statistic average

sizes of PA6 particles as a function of HDPE-g-MAH

content for the three compositions are depicted in Fig. 7.

It can be seen that for all the compositions, the particle

FIG. 4. SEM images of the 90/10 HDPE/PA6 blends with various amounts of HDPE-g-MAH: (a) 0 wt%,

(b) 1 wt%, (c) 2 wt%, (d) 5 wt%, and (e) 10 wt%.




size first decrease sharply with the increase of the HDPE-

g-MAH content, and then gradually levels off, following

a typical behavior of an emulsion curve [28]. The follow-

ing exponential equation [29] provides a good estimate of

the dependency of the particle size on the HDPE-g-MAH

concentration.

Dn;c � Dn;1� �

Dn;0 � Dn;1� � ¼ exp �ncð Þ; (4)

where Dn,c is the number average diameter for a concen-

tration c of HDPE-g-MAH, Dn,0 is the number average di-

ameter for a blend without HDPE-g-MAH, Dn,1 is a con-

stant representative of the limiting particle size, and n is a

constant that determines the efficiency of the HDPE-g-

MAH as an emulsifier.

Table 1 presents the constants Dn,1 and n for Eq. 4. Itcan be seen that, with the increase of PA6 concentration,

Dn,1 gets increased while n shows the opposite. It is sug-

gested that it should add more amounts of compatibilizer

for the blends with higher concentration of PA6, to ensure

the same emulsifying effect. This can be confirmed by the

fact that the particle size increases with the increasing

PA6 concentration when the content of HDPE-g-MAH is

fixed, which is also shown in Fig. 7.

Ultrasonic Measurements

On the basis of the above SEM results, the relationship

between the ultrasonic parameters, i.e., ultrasonic velocity

and attenuation, and the phase morphology will be inves-

tigated in the following section. Before we conducted ul-

trasonic measurements for the HDPE/PA6 blends, the

acoustic properties of neat HDPE and PA6 were first

measured at 2308C, and the results were listed in Table 2.

For such homogeneous polymer melts, the differences

in both the velocity and the attenuation for HDPE and

PA6 are mainly originated from the different molecular



FIG. 7. Particle size of the dispersed PA6 phase as a function of

HDPE-g-MAH content for the 90/10, 80/20, and 70/30 HDPE/PA6

blends.

TABLE 1. Fitting parameters of Eq. 4.

Blend compositions (HDPE/PA6) Dn,1 n

90/10 0.72 1.0

80/20 0.96 0.63

70/30 1.2 0.51


structure. However, as for the HDPE/PA6 blends, the

blend composition and the phase size may have additional

effects on the propagation of ultrasound wave considering

the presence of the interface.

Figure 8 presents the ultrasonic velocity as a function

of the HDPE-g-MAH content for the 90/10, 80/20, and

70/30 HDPE/PA6 blends. The velocities seem unchanged

irrespective of the HDPE-g-MAH content for the three

systems. According to the results of the SEM observation,

it is known that the particle size decreases with the

increasing HDPE-g-MAH content. Therefore, the constant

velocity means that ultrasonic velocity is insensitive to

the variation of the particle size. On the other hand, Fig.

8 also shows that there are velocity differences among the

three blend compositions, and the velocity is larger for

the blends with more concentration of PA6. Further rela-

tionship between ultrasonic velocity and PA6 concentra-

tion are shown in Fig. 9. It can be observed that the ultra-

sonic velocity of the blends increases linearly with the

PA6 concentration up to 30 wt% in our experiment. In

terms of the above discussion, it is suggested that the ul-

trasonic velocity could be used to investigate the concen-

tration of the dispersed phase, but it is unavailable for

predicting the phase size.

As it has shown that ultrasonic attenuation is more sen-

sitive than velocity to characterize the compatibility and

morphology in solid blend systems [16], it is expected

that this sensitivity could remain in the blend melts as

well. Figure 10 shows the relationship between the ultra-

sonic attenuation and the HDPE-g-MAH content for the

90/10, 80/20, and 70/30 HDPE/PA6 blends. It should be

noted that the attenuation used here is also called the total

attenuation. Contrary to the behavior of ultrasonic veloc-

ity, it is evident that the total attenuation shows great de-

pendence on the HDPE-g-MAH content for the three

blend compositions. In general, the attenuation shows an

initial sharp decline with the increasing HDPE-g-MAH

content, and then gradually levels off, showing the similar

behavior as that of the particle size. Further observations

can be found that the changes of the total attenuation with

the HDPE-g-MAH content are well fitted by the exponen-

tial equation as follows.

atotal;c � atotal;1� �

atotal;0 � atotal;1� � ¼ exp �n1cð Þ; (5)

where atotal,c is the total attenuation for a concentration cof HDPE-g-MAH, atotal,0 is the total attenuation for a

blend without HDPE-g-MAH, atotal,1 is a constant repre-

sentative of the limiting total attenuation, and n1 is a con-

stant that determines the efficiency of HDPE-g-MAH to

decrease the total attenuation.

TABLE 2. Acoustic properties of HDPE and PA6 measured at 2308C.

Material Velocity (m/s) Attenuation (dB/cm)

HDPE 1016 4.95

PA6 1352 3.26

FIG. 8. Ultrasonic velocity as a function of HDPE-g-MAH content for

the 90/10, 80/20, and 70/30 HDPE/PA6 blends.

FIG. 9. Ultrasonic velocity as a function of the concentration of PA6.

FIG. 10. Total attenuation (atotal) as a function of HDPE-g-MAH con-

tent for the 90/10, 80/20, and 70/30 HDPE/PA6 blends.


Table 3 presents the constants atotal,1 and n1 for Eq. 5.It can be found that, with the increase of PA6 concentra-

tion, atotal,1 gets increased while n1 decreases, suggesting

that the higher concentration of PA6 in the blend composi-

tions, the more amounts of HDPE-g-MAH should be added

to ensure the same reducing effect. This can also be con-

firmed in Fig. 10, which displays the fact that the total

attenuation shows higher value in the blend with more PA6

concentration when the content of HDPE-g-MAH is fixed.

In terms of the similarity between the changes of the

total attenuation and that of the particle size with the

HDPE-g-MAH content, it is reasonable to speculate that

the attenuation could indirectly reflect the change of the

particle size of the dispersed phase. However, as men-

tioned above, the ultrasonic attenuation in Fig. 10 refers

to the total attenuation (atotal), which is mainly originated

from the intrinsic wave absorption (aintrinsic), as well as

the scattering effect due to the presence of the inclusion

(aexcess). aintrinsic is simply the sum of the absorption of

each individual component and can be expressed by

aintrinsic ¼ (1 2 f)aPE þ faPA6, where aPE and aPA6 are,

respectively, the attenuation of the pure HDPE and PA6,

and f is the weight fraction of PA6. Herein, aexcess is

mainly concerned, because it depends largely on the

inclusion size. The value of aexcess can be obtained by

subtracting the intrinsic absorption from the total attenua-

tion, and the equation is expressed as follows [25].

aexcess ¼ atotal � aintrinsic ¼ atotal � ð1� fÞaPE þ faPA6½ �:(6)

Figure 11 displays aexcess as a function of the HDPE-g-

MAH content for the three blend compositions. Similar to

the behavior of the total attenuation, the excess attenua-

tion decrease sharply with the increase of HDPE-g-MAH

content, then gradually level off. Also, the experimental

data can be reasonably well described by the following

exponential equation.

aexcess;c � aexcess;1� �

aexcess;0 � aexcess;1� � ¼ exp �n2cð Þ; (7)

where aexcess,c is the excess attenuation for a concentra-

tion c of HDPE-g-MAH, aexcess,0 is the excess attenuation

for a blend without HDPE-g-MAH, aexcess,1 is a constant

representative of the limiting excess attenuation, and n2 is

a constant that determines the efficiency of HDPE-g-

MAH to decrease the excess attenuation.

The constants aexcess,1 and n2 for Eq. 7 are also dis-

played in Table 3. Consistent with the tendency of atotal,1and n1, aexcess,1 and n2 show the increase and decrease

with the increasing concentration of PA6, respectively.

Apart from the difference for the 90/10 HDPE/PA6

blends, n1 and n2 keep the same for the 80/20 and 70/30

HDPE/PA6 blends. This might be ascribed that the contri-

bution from the scattering effect is not large enough for

90/10 HDPE/PA6 blends, while it plays the main role for

the other blend compositions. Moreover, if comparing the

n2 with the n in Table 1, it is found that, for each blend

composition, the constant n2 is not entirely consistent with

but more or less smaller than n. As we mentioned above,

the constant n2 or n determine the efficiency of the

HDPE-g-MAH to decrease the excess attenuation or parti-

cle size, so it indicates that ultrasonic results show some

delays in reflecting the efficiency of the HDPE-g-MAH

compared with the SEM results. Possible reasons may be

due to the difference in thermal history, causing a slight

TABLE 3. Fitting parameters of Eqs. 5 and 7.

Blend compositions

(HDPE/PA6) atotal,1 n1 aexcess,1 n2

90/10 5.39 1.2 0.60 0.94

80/20 6.27 0.53 1.66 0.53

70/30 6.59 0.46 2.15 0.46

FIG. 11. Excess attenuation (aexcess) as a function of HDPE-g-MAH

content for the 90/10, 80/20, and 70/30 HDPE/PA6 blends.

FIG. 12. Excess attenuation (aexcess) as a function of particle size for

the 90/10, 80/20, and 70/30 HDPE/PA6 blends.


difference in the particle size for the samples used for

SEM observation and ultrasonic measurement.

Although some differences in the particle size exist for

the two characterization method, it is still encouraging to

seek the relationship between the present particle size and

the excess attenuation. By combination of Figs. 7 and 11,

the quantitative relationship between aexcess and the parti-

cle size of dispersed phase can be obtained in Fig. 12.

Figure 12 shows that aexcess increases linearly with the

increase of the dispersed particle size for all the three

blend compositions. By the linear fitting of experimental

data, three equations can be obtained as follows, respec-

tively.

For 90=10 HDPE=PA6 blends; aexcess ¼ 0:18þ 0:6� Dn

(8)


(9)


(10)

Thus, after in-line measuring aexcess of HDPE/PA6 blend

melts, the particle size of the dispersed phase can be eas-

ily calculated using these linear equations. However, it

needs to note that, the Eqs. 8–10 have not applied to

other immiscible polymer blends up to present, and more

experiments need to be conducted in the future to verify

the generality of these equations.

CONCLUSIONS

In this work, a reliable ultrasonic method was first

used to measure the concentration and particle size of the

dispersed PA6 phase for PA6/HDPE blend melts. It

showed that ultrasonic velocity was insensitive to the par-

ticle size but varied linearly with the blend composition

in our experimental region. However, the decrease of the

ultrasonic attenuation with the addition of HDPE-g-MAH

suggested that the attenuation depended greatly on the

particle size of the dispersed phase. Similar to the evolu-

tion of the particle size, the relationship between the

excess attenuation and HDPE-g-MAH content could be

reasonably well described by the exponential decay

model. Further investigations revealed that there was a

good linear relationship between the excess attenuation

and the particle size of the dispersed phase, which made

it promising for in-line monitoring the morphological evo-

lution by measuring the attenuation of the blend melt dur-

ing polymer processing.

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ultrasonic characterization of phase morphology of high density polyethylene/polyamide 6 blend melts

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