11. Orientation Development.pdf

Orientation Development in Solid-State Extrusion and

Hot Forming of Polypropylene Tubes

Mohamed Elnagmi, 1 Mukesh Jain, 1 James F. Britten 2

1 Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada L8S4L7

2 Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S4M1

The use of high-strength polymer in automotive struc-

tural components is limited by insufﬁcient understand-

ing of microscopic aspects of deformation for accurate

numerical predictions of the mechanical behavior

during forming processes. One approach to meeting

these critical data needs is a careful examination of

the structure property relationships that directly inﬂu-

ence formability. Different hot forming processes

(solid-state extrusion, axial feed hot oil tube forming,

and biaxial ball stretching test) are utilized in this work

for investigating the effect of process conditions on

the molecular orientation of polypropylene (PP) tubes.

White-Spruiell representation of orientation factors

based on the results form X-ray diffraction (XRD)

patterns is utilized to analyze the development of

orientation under extrusion and various forming condi-

tions. POLYM. ENG. SCI., 51:1446–1454, 2011. ª 2011 Society

of Plastics Engineers

Biaxial orientation factors have been deﬁned in the liter-

ature after White and Spruiell [4] to quantify the orienta-

tion with respect to machine (MD) and transverse (TD)

directions and have been used to characterize the biaxial

orientation features of linear low-density polyethylene

(LLDPE) blown ﬁlms [5–7] as well as low-density polyeth-

ylene (LDPE) tubular blown ﬁlms [8]. In a study dealing

with polypropylene blown ﬁlm, it was shown that the

results of the orientation factors are close to the biaxial line

and progress along this path with continued blowing [9].

Research activities in the area of polymer forming by

the current authors have demonstrated that structural-ori-

ented polypropylene (OPP) tubes can be formed at higher

temperatures by processes such as axial-feed hot oil or

gas forming [10, 11]. This process typically results in

strain paths that lie in the tensile-compressive quadrant of

the forming limit diagram. Recently, OPP tubes have been

also deformed at higher temperature by a new biaxial ball

stretching test (BBST) system [12], to subject the tubes to

equi-biaxial tensile stretching. For the above processes,

studies dealing with microstructural aspects of forming of

structural thermoplastic tubes, and particularly of oriented

polypropylene (OPP) tubes are not available.

In this article, the development of orientation in solid-

state extruded PP tubes, axial feed hot oil tube forming

(AF-HOTF) samples, and BBST samples is studied with

wide angle X-ray diffraction (WAXD). The objective of

this study is to relate the deformation process of solid-state

extrusion and subsequent forming of OPP tubes bulging to

the change in preferred molecular orientation. As the

microstructure development and formability are closely

related, such relationships will be useful in the develop-

ment of suitable extruded thermoplastic tube materials and

for optimization of tube forming processes in the future.

INTRODUCTION

One of the main goals of manufacturing of structural

automotive parts is to create lighter, good quality parts at

lower costs. High modulus structural thermoplastics appear

to be promising candidate materials in this regard. How-

ever, due to their more complex microstructure and defor-

mation behavior compared with metals, a careful control of

the process parameters such as pressure, temperature, and

cooling after forming is required. Achievement of a high

modulus in polymers depends on the draw ratio (DR) (i.e.,

the ratio of initial cross-sectional area to the ﬁnal product).

A high modulus and high strength polymer can be pro-

duced by using solid-state deformation processes, such as

extrusion and drawing that work toward orienting the mol-

ecules of a polymer in the axial direction. Orientation is

used to tailor mechanical properties of ﬁlms, ﬁbers, and

various blow-molded parts (sheets, rods, and tubes) [1–3].

EXPERIMENTAL

Correspondence to: M. Elnagmi; e-mail: elnagmi2@gmail.com

Contract grant sponsors: Decoma International Inc.; PSA Composites Inc.

DOI 10.1002/pen.21909

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

Solid-State Extrusion

The material employed in this study is a polypropylene

homopolymer with a melt ﬂow index, MFI, of 0.75 and a

2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2011

FIG. 1. Axial feed tube bulging system (AF-HOTF).

FIG. 2. Biaxial ball stretching test system (BBST).

density of 0.9071 g/cm 3 . The existing solid cylindrical

bars of polypropylene (70 mm in diameter) were pro-

duced at Polymer Sheet Applications Company (PSAC)

Inc. in Guelph, Ontario. Tubes with different draw ratios

(OPP) used in the hot forming experiments were produced

by ram extrusion in the solid state at PSAC Inc. Also, PP

tubes were obtained by machining solid cylindrical melt

extruded, polypropylene bars. These tubes, referred to as

billet- or melt-extruded polypropylene (EPP) tubes, were

also used in BBST experiments.

actuator resulted in biaxial expansion of the tube sample.

The tube expansion was observed through a die opening

on the opposite side of the punch. Strain measurements

during the BBST tests were obtained through the die

opening using the online ARAMIS optical strain measure-

ment system. The results of strain development are avail-

able in an earlier paper [12].

Wide Angle X-Ray Diffraction

Axial Feed Hot Oil Tube Forming System

A comprehensive picture of the distribution of crystal-

line orientation within a sample was obtained through

pole ﬁgure analyses. Using a rotary steel cutter with cool-

ing water, 15 mm

AF-HOTF system (Fig. 1) was designed and fabricated,

and then used to conduct formability tests on OPP tubes.

This test system was installed on a dual-actuator, servo-

hydraulically controlled MTS test system of 250 kN

capacity. In this system, the tube was placed between the

upper and the lower plugs connected to the hot oil pres-

sure line. The middle of tube was kept free (or unsup-

ported) and open to view to allow for continuous observa-

tion of expansion of the tube using the online ARAMIS

optical strain measurement system [13]. Hot silicon oil

was used as a heating and pressurizing media. A con-

trolled pressurization rate for the expansion and bursting

up to a pressure of 3000 psi was achieved at a range of

temperatures up to 160

0.5 mm specimens were

cut out of billet, extruded and bulged tubes parallel to the

extrusion direction (Fig. 4). A Rigaky RU-200 rotating

anode X-ray generator operating at 50 kV and 90 mA,

with a Cu K a (

0.5 mm

1.54184 ˚ ) parallel focused 0.5 mm

beam was used in transmission mode. Each specimen was

mounted on a Bruker 3-circle D8 single crystal diffrac-

tometer with the sample (extrusion) axis parallel to the

k ¼

-axis and the tube exterior direction oriented along

f ¼

. XRD 2 diffraction images were recorded on a ﬂat

Bruker SMART6000 CCD area detector at a distance of

51.65 mm,

C. The upper actuator was moved

downward to seal the tube as well as to provide axial

feeding of the tube specimen during the bulging process.

The lower actuator was used in tandem with the pressure

intensiﬁer to apply the pressure inside the tube. Further

details were provided in an earlier paper [11]. The axial

feed of the tube results on strain paths in the tension com-

pression side of the forming limit diagram (FLD).

v ¼

54.79

x ¼ 2

172.90

y ¼

0.00

rotation of

) (the sample axis) was made with

20s frames stored at 5

–360

intervals. Data were collected at

three points along the sample axis. Each scan was proc-

essed with the GADDS software package [14] to generate

Biaxial Ball Stretching Test System

A BBST test rig, as shown in Fig. 2, was also designed

and fabricated for stretching OPP tubes with no axial end

feeding to obtain strain paths closer to the equal biaxial

tension strain path. The BBST system was installed on

MTS 250 KN servo-hydraulic test machine ﬁtted with a

TCE-N300 Shimadzu thermostatic chamber for conduct-

ing the elevated temperature tests. In the BBST test, hori-

zontal movement of a punch toward inner surface of tube

(Fig. 3), caused by the vertical movement of the lower

FIG. 3. Tube sample insert between clamps in biaxial stretching ball

test system.

DOI 10.1002/pen

POLYMER ENGINEERING AND SCIENCE—-2011 1447

FIG. 5. A monoclinic crystal system in isotactic polypropylene (a) the

crystal system with carbon chains parallel to c-axis and (b) side view

show the angle between a and a 0 axis.

FIG. 4. Fibers cut out of bulged tubes at middle of bulge area in the

extrusion direction.

pole ﬁgures and determine the ﬁber orientation relative to

the sample axis.

The preferred orientation was recorded using Debye

patterns. Reﬂections associated with all eight Debye rings

were identiﬁed from the inner ring to the outer ring as

(110)

plane of axes b and c, the orientation of the a 0 -axis can

be computed [16]. Wilchinsky [17] has developed an

equation to determine

cos 2

values indirectly by

means of intensity measurements from strongly diffracting

planes. Only two pole ﬁgures, (110)

f c,ED i

and (040)

, are

the billet. The reﬂections were converted to arcing in

extruded samples.

, (040)

, (130)

, and (111)

, (041)

a þ

(

131)

cos 2

required for the evaluation of

in monoclinic

crystal system using the following expression [17]:

f c,ED i

cos 2

f C;ED i¼

099

f 110;ED i

Representation of Orientation of OPP Tubes

cos 2

901

f 040;ED i

(2)

Common morphological measure of orientation, Her-

man’s orientation factor, f j,n [15], for a given plane was

calculated using the following expression:

The Herman’s orientation factor satisfactorily describes

the degree of axial orientation in crystalline ﬁbers. How-

ever, as described in the book by Alexander [15], it is

insufﬁcient to describe the orientation characteristics of

biaxially oriented products such as blown ﬁlms, injection-

molded, and blow-molded container parts. Therefore,

biaxial orientation factors are deﬁned in this work after

White and Spruiell [4] to quantify the orientation with

respect to extrusion (ED) and transverse (TD) directions

as follows:

cos 2

f j;n ¼

ðh

f j;n i

Þ=

(1)

where

f j,n is the angle between the j-crystallographic axis

a, b,orc) and the ﬁber axis as represented by proc-

essing directions; extrusion, transverse, and normal direc-

tions (n

ED, TD, and ND, respectively). The symbol

, .

implies an average over the entire pole ﬁgure.

Table 1 shows the relationship between f j,n and

f j,n for

f ED;j ¼

cos 2

f j;ED iþh

f j;TD i

(3)

the different chain axis orientations.

The Herman’s orientation factors are directly calcu-

lated from (h00) and (00l) poles in Eq. 1 for orthogonal

crystal systems. It is to be noted that there are no strong

(00l) type diffractions in isotactic polypropylene. Polypro-

pylene samples had a monoclinic structure with dimen-

sions a

f TD;j ¼

cos 2

f j;TD iþh

f j;ED i

(4)

cos 2

where

is the average value of the square of the

cosine of the angle between the j-crystallographic axis (j

f j,n i

¼ 6.5 ˚ , b ¼ 99.58 for

the unit cell (Fig. 5a and b). As shown, the (040)

¼ 6.63 ˚ , b

¼ 20.78 ˚ , c

a, b,orc) and the ﬁber axis as represented by the proc-

essing directions (n

planes

are perpendicular to the b-axis. Thus, the b-axis orienta-

tion factors can be directly computed from the 040 inten-

sity distribution. Also, as the angle between a and a 0 axis

is small (9.5

ED, TD).

A schematic of White and Spruiell triangle is illus-

trated in Fig. 6 in terms of the biaxial orientation factors

f ED and f TD . States of uniaxial orientation with respect to

the extrusion and transverse directions lie along the re-

spective coordinate axes. The biaxial orientation factors

take values between

) and the a 0 -axis is perpendicular to the

1 and

1, with 0 representing ran-

TABLE 1. The relationship between f c,ED and f c,ED for the different

chain axis orientations.

dom orientation,

1 representing perfect orientation, and

1 representing the orthogonal orientation. The path

marked by a dashed line through the center of the triangle

represents the case of equal biaxial orientation where the

orientation with respect to the extrusion and transverse

directions is the same. The base of the triangle which is

between the apex (1, 0) and (0, 1) represents the case of

planar deformation (ﬁlm surface) and the middle of the

hcos 2

Orientation

f j,n

f j,n i

f j,n

j-crystallographic axis is parallel to

the ﬁber axis (extrusion direction)

þ1.0

j-crystallographic axis is perpendicular

to the ﬁber axis (extrusion direction)

2 0.5

j-crystallographic axis oriented randomly Random

1/3

0.0

1448 POLYMER ENGINEERING AND SCIENCE—-2011

DOI 10.1002/pen

FIG. 6. Graphical representation of orientation, the orientation triangle

diagram.

FIG. 8. X-ray diffraction intensity proﬁles from PP billet and extruded

samples at different draw ratios.

base represents equal planar orientation. The side of the

triangle between the apex (1, 0) and ( 2 1, 2 1) represents

the case of planar deformation where the machine direc-

tion is perpendicular to the surface.

The data collected from XRD was utilized to obtain an

estimate of % crystallinity from a single frame. This

frame was processed within Gadds software by ﬁrst sub-

tracting the air scatter and then comparing the amorphous

scatter to the crystalline scatter [14]. An estimate of crys-

tallinity, as shown in Fig. 9, was observed to increase

with increasing draw ratio. This is because the alignment

of the polymer chains in OPP tube makes the formation

of a crystalline structure easier [1]. The crystallographic

c-axis (chain axis) of all extruded (OPP) samples was

aligned parallel to the extrusion direction (ﬁber axis) and

perpendicular to the crystallographic b-axis, the orienta-

tion factor f b,ED was

RESULTS

Microstructural Characteristics of Initial Billet, Extruded,

and OPP Tubes

reﬂections, were diffuse and

weak in the billet sample. Also, (060)

The

-phase, i.e., (117)

reﬂec-

tions were weak in all samples (Fig. 7). The Debye rings

in all samples were similar except that the (117)

, and (220)

0.5). In the billet sample, how-

ever, the crystallographic c-axis (chain axis) and b-axis

were distributed randomly and the orientation factor f b,ED

was

(

-phase

was present in the billet (although with a poorly deﬁned

peak) but was nonexistent in the oriented samples. The

results indicate that the crystalline fraction in the billet

contained

0 as shown in Fig. 10.

Figure 11 presents the (040)

pole ﬁgures plotted in

stereographic projection. In the billet sample Fig. 11a, the

pole ﬁgure shows a random orientation around extrusion

direction (ED). All extruded polypropylene tubes at dif-

ferent draw ratios show orientation patterns as uniaxial

and rotationally symmetric around the ED (Fig. 11b–e).

The (040)a pole ﬁgure of BBST samples (billet tube,

(Fig. 11f)) shows a concentration of the b-axis between

forms. Figure 8 shows intensity peaks

(110), (040), (130), (111), (041)

and

(

131), (060), and

(220) for

-phase

reﬂection from the billet. However, the oriented speci-

mens after solid-state extrusion with different draw ratios

contained only

-phase reﬂections and (117) peak for

-phase.

FIG. 7. 2D X-ray diffraction pattern of (a) PP billet (unoriented) and

(b) PP extruded tube at different draw ratios.

FIG. 9. Draw ratio versus % crystallinity of billet and extruded poly-

propylene tubes.

DOI 10.1002/pen

POLYMER ENGINEERING AND SCIENCE—-2011 1449

FIG. 10. Draw ratio versus orientation factor of billet and extruded

polypropylene tubes.

transverse direction and extrusion direction in a broad

band making an angle in the range 20

–60

. For BBST

FIG. 12. White and Spruiell representation of biaxial orientation of

billet, extruded polypropylene, and subsequently formed tube samples at

different draw ratios and bulge samples.

OPP tube (DR

pole

ﬁgure shows orientation patterns similar to the extruded

tube with a small tendency toward transverse direction

(Fig. 11g). The AF-HOTF samples (DR

6.3), on the other hand, the (040)

6.3) are shown

for a range of axial feeds (Fig. 11h–j). With no axial feed

(Fig. 11h), the b-axis shows a concentration in the trans-

verse direction with only a slight difference when com-

pared with the extruded samples. However, with an axial

feed of 8.0 mm (Fig. 11i), the (040)

The development of orientation is affected by two proc-

esses, the uniaxial orientation (i.e., solid-state extrusion)

and axial feed hot forming, resulting in different biaxial

orientation factors from a polypropylene blown ﬁlm.

Biaxial orientation factors for the extruded and bulged

tube samples are summarized in Table 2. The extrusion

direction orientation factors f ED of extruded samples ver-

sus draw ratio are plotted in Fig. 13, where it is shown

that the f ED;c values are positive and increase with the

draw ratio, whereas the f ED;b and f ED;a values are negative

and decrease with the draw ratio. All of the f TD values

(f TD;a , f TD;b , and f TD;c ) are 0, which represents the case of

uniaxial orientation. For the bulge samples, the extrusion

and transverse orientation factors (f ED and f TD ) versus

axial feed are plotted in Fig. 14a and b). As shown, the

f ED;c values are positive and f TD;c values are negative and

both sets of values decrease with the axial feed. On the

other hand, the f ED;b and f ED;a values are negative and the

f TD;b and f TD;a values are positive and both sets of values

increase with the axial feed.

poles are distributed

around the transverse direction in a band making an angle

of about 30

. Furthermore, with an axial feed of 18.0 mm

(Fig. 11j), the (040)

poles are distributed in a broad band

making an angle of about

(35

–60

)

to the extrusion

direction.

In Fig. 12, the White-Spruiell biaxial orientation factor

for the starting billet sample is located at the origin. The

orientation factors of the extruded samples move along

the extrusion direction toward the (0, 1) apex with the

increase in the draw ratio which represents the case of

uniaxial orientation. On the other hand, the orientation

factors of the AF-HOTF samples start at the highest ori-

entation factor on the extrusion direction axis, move along

the side of the triangle with the increases in axial feed,

and approach the apex (

1) with thinning. In bulged

tubes, the results show that the biaxial orientation factors

lie between the planar strain state and equal biaxial strain.

Microstructural Characteristics After BBST

Figure 15 presents X-ray diffraction patterns of

billet

tubes formed in BBST at 150, 160, and 1708C

TABLE 2. Crystalline orientation characteristics of extruded and

bulged polypropylene tubes.

Sample type

f ED;c

f TD;c

f ED;b

f TD;b

f ED;a

f TD;a

Billet 0.000 0.000 0.000 0.000 0.000 0.000

Extrusion DR ¼ 4.5 0.639 2 0.003 2 0.350 0.000 2 0.297 0.000

Extrusion DR ¼ 5.0 0.708 0.002 2 0.370 0.000 2 0.340 0.000

Extrusion DR ¼ 5.7 0.744 0.001 2 0.380 0.000 2 0.367 0.000

Extrusion DR ¼ 6.3 0.756 0.000 2 0.386 0.000 2 0.372 0.000

Axial feed ¼ 0.0 mm 0.693 2 0.029 2 0.330 0.040 2 0.357 0.000

Axial feed ¼ 8.0 mm 0.388 2 0.158 2 0.176 0.120 2 0.201 0.059

Axial feed ¼ 18.0 mm 0.139 2 0.236 2 0.070 0.160 2 0.065 0.094

FIG. 11. The poles ﬁgures of (040) plane of monoclinic a form plotted

in stereographic projection, (a) billet. OPP tube: (b) DR ¼ 4.5, (c) DR

¼ 5.0, (d) DR ¼ 5.7, and (e) DR ¼ 6.3. BBST: (f) billet tube. (g) OPP

tube. AF-HOTF OPP tube: (h) no axial feed, (i) 8.0 mm axial feed, and

(j) 18 mm axial feed.

1450 POLYMER ENGINEERING AND SCIENCE—-2011

DOI 10.1002/pen

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