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Orientation development in solid-state extrusion and hot forming of polypropylene tubes.

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. However, due to their more complex microstructure and deformation 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 produced by using solid-state deformation processes, such as extrusion and drawing that work toward orienting the molecules 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) [l-3].

Biaxial orientation factors have been defined in the literature after White and Spruiell [4] to quantify the orientation 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 polyethylene (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-oriented 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 development of suitable extruded thermoplastic tube materials and for optimization of tube forming processes in the future.

EXPERIMENTAL

Solid-State Extrusion

The material employed in this study is a polypropylene homopolymer with a melt flow index, MFI, of 0.75 and a density of 0.9071 g/[cm.sup.3]. The existing solid cylindrical bars of polypropylene (70 mm in diameter) were produced 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.

[FIGURE 1 OMITTED]

Axial Feed Hot Oil Tube Forming System

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, servohydraulically 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 pressure line. The middle of tube was kept free (or unsupported) and open to view to allow for continuous observation 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 controlled pressurization rate for the expansion and bursting up to a pressure of 3000 psi was achieved at a range of temperatures up to 160[degrees]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 compression side of the forming limit diagram (FLD).

[FIGURE 2 OMITTED]

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 servohydraulic test machine fitted with a TCE-N300 Shimadzu thermostatic chamber for conducting the elevated temperature tests. In the BBST test, horizontal movement of a punch toward inner surface of tube (Fig. 3), caused by the vertical movement of the lower 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 measurement system. The results of strain development are available in an earlier paper [12].

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Wide Angle X-Ray Diffraction

A comprehensive picture of the distribution of crystalline orientation within a sample was obtained through pole figure analyses. Using a rotary steel cutter with cooling water, 15 mm * 0.5 mm * 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.sub.[alpha]] ([lambda] = 1.54184 [Angstrom]) parallel focused 0.5 mm beam was used in transmission mode. Each specimen was mounted on a Bruker 3-circle D8 single crystal diffractometer with the sample (extrusion) axis parallel to the [phi]-axis and the tube exterior direction oriented along [phi] = 0[degrees]. [XRD.sup.2] diffraction images were recorded on a flat Bruker SMART6000 CCD area detector at a distance of 51.65 mm, [chi] = 54.79[degrees], [omega] = -172.90[degrees], 2[theta] = 0.00[degrees]. A rotation of [phi] (0[degrees]-360[degrees]) (the sample axis) was made with 20s frames stored at 5[degrees] intervals. Data were collected at three points along the sample axis. Each scan was processed with the GADDS software package [14] to generate 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)[alpha], (040)[alpha], (130)[alpha], and (111)[alpha], (041)[alpha] + (-131)[alpha] in the billet. The reflections were converted to arcing in extruded samples.

Representation of Orientation of OPP Tubes

Common morphological measure of orientation, Herman's orientation factor, [f.sub.j,n] [15], for a given plane was calculated using the following expression:

[f.sub.j.n] = (3([cos.sup.2][[empty set].sub.j.n])-1)/2 (1)

where [[phi].sub.j,n] is the angle between the j-crystallographic axis (j=a, b, or c) and the fiber axis as represented by processing directions; extrusion, transverse, and normal directions (n = ED, TD, and ND, respectively). The symbol <...> implies an average over the entire pole figure. Table 1 shows the relationship between [f.sub.j,n] and [[phi].sub.j,n] for the different chain axis orientations.
TABLE 1. The relationship between [f.sub.c,ED] and
[[phi].sub.c,ED] for the different chain axis
orientations.

Orientation                 [[phi].     ([cos.sup.2]    [f.sub.j,n]
                            sub.j,n]  [[phi].sub.j,n])

j-crystallographic axis            0                 1         +1.0
is parallel to the fiber
axis (extrusion direction)

j-crysiallographie axis is        90                 0         -0.5
perpendicular to the fiber
axis (extrusion direction)

j-crystallographic            Random               1/3          0.0
axis oriented randomly


The Herman's orientation factors are directly calculated from (h00) and (001) poles in Eq, 1 for orthogonal crystal systems. It is to be noted that there are no strong (001) type diffractions in isotactic polypropylene. Polypropylene samples had a monoclinic structure with dimensions [alpha] = 6.63 [Angstrom], b = 20.78 [Angstrom], c = 6.5 [Angstrom], [beta] = 99.5[degrees] for the unit cell (Fig. 5a and b). As shown, the (040)[alpha] planes are perpendicular to the b-axis. Thus, the b-axis orientation factors can be directly computed from the 040 intensity distribution. Also, as the angle between a and a' axis is small (9.5[degrees]) and the a' axis is perpendicular to the plane of axes b and c, the orientation of the a'-axis can be computed [16]. Wilchinsky [17] has developed an equation to determine ([cos.sup.2] [[phi].sub.c,ED]) values indirectly by means of intensity measurements from strongly diffracting planes. Only two pole figures, (110)[alpha] and (040)[alpha], are required for the evaluation of ([cos.sup.2][[phi].sub.c,ED]) in monoclinic crystal system using the following expression [17]:

([cos.sup.2][[empty set].sub.C,ED]) = 1-1.099([cos.sup.2][[empty set].sub.110,ED])-0.901([cos.sup.2][[empty set].sub.040,ED]) (2)

The Herman's orientation factor satisfactorily describes the degree of axial orientation in crystalline fibers. However, as described in the book by Alexander [15], it is insufficient to describe the orientation characteristics of biaxially oriented products such as blown films, injectionmolded, 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:

where ([cos.sup.2][[phi].sub.j,n]) is the average value of the square of the cosine of the angle between the j-crystallographic axis (j = a, b, or c) and the fiber axis as represented by the proc essing directions (n = ED, TD).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[FIGURE 5 OMITTED]

A schematic of White and Spruiell triangle is illustrated in Fig. 6 in terms of the biaxial orientation factors [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] States of uniaxial orientation with respect to the extrusion and transverse directions lie along the respective coordinate axes. The biaxial orientation factors take values between +1 and -1, with 0 representing random 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 base represents equal planar orientation. The side of the triangle between the apex (1, 0) and (-1, -1) represents the case of planar deformation where the machine direction is perpendicular to the surface.

RESULTS

Microstructural Characteristics of Initial Billet, Extruded, and OPP Tubes

The [gamma]-phase, i.e., (117)y reflections, were diffuse and weak in the billet sample. Also, (060)[alpha], and (220)[alpha] reflections were weak in all samples (Fig. 7). The Debye rings in all samples were similar except that the (117) [gamma]-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 [alpha] and [gamma] forms. Figure 8 shows intensity peaks (110), (040), (130), (111), (041) + (-131), (060), and (220) for [alpha]-phase reflections and (117) peak for [gamma]-phase reflection from the billet. However, the oriented specimens after solid-state extrusion with different draw ratios contained only [alpha]-phase.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

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 subtracting the air scatter and then comparing the amorphous scatter to the crystalline scatter [14]. An estimate of crystallinity, 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 r-axis (chain axis) of all extruded (OPP) samples was aligned parallel to the extrusion direction (fiber axis) and perpendicular to the crystallographic 6-axis, the orientation factor [f.sub.b,ED] was --(-0.5). In the billet sample, however, the crystallographic c-axis (chain axis) and b-axis were distributed randomly and the orientation factor [f.sub.b,ED] was ~0 as shown in Fig. 10.

Figure 11 presents the (040)[alpha] pole figures plotted in stereographic projection. In the billet sample Fig. 12a, the pole figure shows a random orientation around extrusion direction (ED). All extruded polypropylene tubes at different draw ratios show orientation patterns as uniaxial and rotationally symmetric around the ED (Fig. 1 lb--e). The (040)[alpha] pole figure of BBST samples (billet tube, (Fig. 11f)) shows a concentration of the b-axis between transverse direction and extrusion direction in a broad band making an angle in the range 20[degrees]-60[degrees]. For BBST OPP tube (DR = 6.3), on the other hand, the (040)[alpha] pole figure shows orientation patterns similar to the extruded tube with a small tendency toward transverse direction (Fig. 1lg). The AF-HOTF samples (DR = 6.3) are shown for a range of axial feeds (Fig. llh--j). With no axial feed (Fig. llh), the b-axis shows a concentration in the transverse direction with only a slight difference when compared with the extruded samples. However, with an axial feed of 8.0 mm (Fig. 1li), the (040)[alpha] poles are distributed around the transverse direction in a band making an angle of about 30[degrees]. Furthermore, with an axial feed of 18.0 mm (Fig. 1lj), the (040)[alpha] poles are distributed in a broad band making an angle of about (35[degrees]-60[degrees]) to the extrusion direction.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

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 orientation factor on the extrusion direction axis, move along the side of the triangle with the increases in axial feed, and approach the apex (-1, -1) with thinning. In bulged tubes, the results show that the biaxial orientation factors lie between the planar strain state and equal biaxial strain.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The development of orientation is affected by two processes, 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 [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] of extruded samples versus draw ratio are plotted in Fig. 13, where it is shown that the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] c values are positive and increase with the draw ratio, whereas the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values are negative and decrease with the draw ratio. All of the fBTD values ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) are 0, which represents the case of uniaxial orientation. For the bulge samples, the extrusion and transverse orientation factors ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) versus axial feed are plotted in Fig. 14a and b). As shown, the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values are positive and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values are negative and both sets of values decrease with the axial feed. On the other hand, the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] values are negative and the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII],h and fBTDc values are positive and both sets of values increase with the axial feed.
TABLE 2. Crystalline orientation characteristics of extruded and bulged
polypropylene tubes.

Sample type           [MATHEMATICAL  [MATHEMATICAL  [MATHEMATICAL
                        EXPRESSION       EXPRESSION       EXPRESSION
                           NOT            NOT            NOT
                      REPRODUCIBLE   REPRODUCIBLE   REPRODUCIBLE
                        IN ASCII]      IN ASCII]      IN ASCII]

Billet                        0.000          0.000          0.000
Extrusion DR = 4.5            0.639         -0.003         -0.350
Extrusion DR = 5.0            0.708          0.002         -0.370
Extrusion DR = 5.7            0.744          0.001         -0.380
Extrusion DR = 6.3            0.756          0.000         -0.386
Axial feed = 0.0 mm           0.693         -0.029         -0.330
Axial feed = 8.0 mm           0.388         -0.158         -0.176
Axial feed = 18.0 mm          0.139         -0.236         -0.070

Sample type           [MATHEMATICAL  [MATHEMATICAL  [MATHEMATICAL
                        EXPRESSION       EXPRESSION       EXPRESSION
                           NOT            NOT            NOT
                      REPRODUCIBLE   REPRODUCIBLE   REPRODUCIBLE
                        IN ASCII]      IN ASCII]      IN ASCII]

Billet                        0.000          0.000          0.000
Extrusion DR = 4.5            0.000         -0.297          0.000
Extrusion DR = 5.0            0.000         -0.340          0.000
Extrusion DR = 5.7            0.000         -0.367          0.000
Extrusion DR = 6.3            0.000         -0.372          0.000
Axial feed = 0.0 mm           0.040         -0.357          0.000
Axial feed = 8.0 mm           0.120         -0.201          0.059
Axial feed = 18.0 mm          0.160         -0.065          0.094


Microstructural Characteristics After BBST

Figure 15 presents X-ray diffraction patterns of billet tubes formed in BBST at 150, 160, and 170[degrees]C corresponding to effective strains of 0.78, 0.78, and 0.8, respectively, for different orientation positions ( [phi] = 0[degrees], 45[degrees], and 90[degrees]). The Debye rings in all billet tube samples were similar to the starting billet sample.

[FIGURE 12 OMITTED]

Figure 16 shows the X-ray diffraction patterns of OPP tube samples after BBST tests at 150, 160, and 170[degrees]C and effective strains of 0.31, 0.52, and 0.48, respectively, for the three [phi] angles. The patterns at 150[degrees]C and 160[degrees]C are quite similar and show the same arcing as in OPP tube sample at DR = 6.3 (refer to earlier Fig. 7). For the OPP tube samples at 170[degrees]C deformed to an effective strain of 0.48, however, the arcing appears to be getting broader with increasing [phi] angle, i.e., by fiber rotation from 0[degrees] at TD-ED plane to 90[degrees] at ND-ED plane.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

In Fig. 17, the White-Spruiell representation of biaxial orientation factor for the various deformed states of the tube materials are shown. The undeformed billet tube sample is located at the origin. The orientation factor of the BBST billet tube sample at 150[degrees]C move slightly toward positive side of the transverse direction from the origin. The BBST billet tube sample at 160[degrees]C shows that the orientation factor moves further toward the positive side between the extrusion and transverse directions and closer to the equibiaxial strain state. At 170[degrees]C, the orientation factor of BBST billet tube samples moved slightly away from the origin along the extrusion direction in the positive direction. At this temperature, the material becomes more temperature sensitive and the tube samples started to sink in the transverse direction during the BBST tests. On the other hand, the orientation factor of the BBST OPP tube samples at 150, 160, and 170[degrees]C, started at the highest orientation factor (OPP tube DR = 6.3) on the extrusion direction axis (uniaxial direction) and moved toward the right side of the triangle (i.e., planar film surface, refer to Fig. 6) slightly away from the apex (0, 1). The results show that the biaxial orientation factors lie on the planar strain state toward the equal biaxial strain state and close to uniaxial direction.

[FIGURE 16 OMITTED]

DISCUSSION OF DEVELOPMENT OF MORPHOLOGY AND ORIENTATION

Initial Billet, Extruded, and Formed OPP Tubes

The results of pole figure analysis (Fig. 11) indicate that spherulite structure of the melt extruded billet was transformed into the fibrillar structure in the solid-state extruded OPP tubes. The monoclinic [alpha]-form was found in all solid-state extrusion specimens extruded at different draw ratios (refer to Fig. 8). The (040) pole figures of the extruded polypropylene tube (DR = 6.3) (refer to Fig. 11) are dominated by the uniaxial c-axis orientation, the c-axis of the monoclinic unit cell is parallel to ED and a-and b-axis are randomly distributed in the TD-ND plane. The pole figures of the bulge samples with no axial-feed (again refer to Fig. 11) show that the width of the b-axis orientation distribution increased significantly. This is because the chain axis that corresponds to the c-axis orientation in the extrusion direction moved in the ED-ND plane as a result of bulge tube being under internal pressure only. However, with an increase in axial-feed, the b-axis orientation distributed in an even broader band in ED-TD plane and is more intensely concentrated in the transverse direction. The combination of axial feed and internal pressure rotates the b-axis of crystallites from being randomly distributed in the ND-TD plane to the transverse direction.

Significant differences in orientation factor for bulge tube compared with the extruded tube were observed (refer to Fig. 12). Biaxial orientation factor represented by White and Spruiell triangle shows that with an increase in axial feed, the chain axis reorients from the positive uniaxial direction toward the negative biaxial direction and lies between the planar strain state and the equal biaxial strain state. The negative biaxial orientation factors result from the compression axial feed that is applied continuously at the tube ends during AF-HOTF test. These microstructural changes are schematically presented in Fig. 18.

Referring to Fig. 12, at the origin, the orientation factor for e-axis is 0 (for a random billet sample), and an increase in orientation factor with increase in draw ratio is observed. This is because more molecular chains are oriented in the extrusion direction during solid-state extrusion. Both a-axis and b-axis show the same trend and exhibit a perpendicular orientation to the r-axis as indicated for the monoclinic crystal system (refer to Fig. 5). For bulge samples, however, the c-axis started with a high orientation factor, parallel to the extrusion direction (extruded tube at DR = 6.3), and decreased with an increase in axial feed (also refer to Fig. 14a). This trend shows that reorientation of the c-axis occurs away from the extrusion direction and toward the transverse direction.

Biaxial Ball Stretching Test

White-Spruiell biaxial orientation factor for the starting melt extruded billet sample is located at the origin (Fig 17). From this origin, orientation factor of BBST billet tube samples tested at 160[degrees]C moves toward equal biaxial strain state, whereas the orientation factor for the BBST billet tube sample at 170[degrees]C moves in the extrusion direction only. Again, this is because the material softens considerably due to its proximity to its melting point and the material starts to sink in transverse direction resulting in uniaxial orientation (ED). This phenomenon of material softening or sagging has been observed in sheet thermo-forming as well [l]. Sagging is often countered by tensioning the sheet in commercial practice. Some systems are equipped with independent edge heating zones to compensate for heat sink effect.

The development of biaxial orientation factors in BBST billet tube (EPP) was different from OPP tube (Fig. 17). This is because the starting material structure was different as shown in Fig. 18. In the case of EPP, the strain history of these samples consists of melt extrusion followed by BBST process. The samples are randomly oriented after melt extrusion and therefore start at the origin in the White-Spruiell diagram. The Spherulite structure is slightly oriented toward equal biaxial direction as shown schematically in Fig. 19 based on the results from the X-ray diffraction patterns (refer to Fig. 15), pole figures (refer to Fig. 11), and biaxial orientation factor (refer to Fig. 17). For the OPP, the strain history of these samples consists of solid-state extrusion followed by BBST process. The OPP samples start at a high orientation factor, parallel to the extrusion direction and are reoriented by BBST away from extrusion direction and toward the transverse direction (parallel to the equal biaxial direction). This is shown schematically in Fig. 20 where a noticeable fibril alignment exists in the equal biaxial direction, as the starting tube material was highly oriented in extrusion direction (DR = 6.3). This is in agreement with the results from the X-ray diffraction patterns (refer to Fig. 16) and pole figures (refer to Fig. 11).

In conventional metal tube hydroforming process, no reorientation of grains occurs with axial feed and therefore no significant change in anisotropic properties has been reported. For thermoplastic polymers such as PP studied in this work, however, the formed tubes become more isotropic with increased axial feed due to the reorientation of molecular chains as shown in Figs. 21 and 22 [11]. The observed microstructure development is consistent with anisotropic mechanical properties of extruded tubes and more isotropic properties of the bulged tubes. It is therefore critical that more biaxial deformation modes are applied during solid-state extrusion and subsequent tube forming process OPP tubes to attain more isotropic and ductile response in the postformed properties.

[FIGURE 17 OMITTED]

[FIGURE 18 OMITTED]

CONCLUSIONS

Different hot forming processes (solid-state extrusion, AF-HOTF, and BBST) were utilized for investigating the effect of process conditions on the orientation development in PP tubes. White-Spruiell representation of orientation factors offers a useful method to compare the above forming processes. In bulged tubes, the results show that the biaxial orientation factors lie between the planar strain state and the equal biaxial strain state. These orientations arise from two different processes of solid-state extrusion and axial feed tube forming (AF-HOTF) that deform material under different strain paths. The resulting biaxial orientation factors are also different from polypropylene blown film as reported in the literature. The development of biaxial orientation factors after BBST tests on OPP tube and the billet tube samples is also different from the extruded samples and AF-HOTF samples and again dependent on the strain path associated with these processes. This suggests that the microstructure of the formed PP tubes is strongly dependent on the starting material structure and the forming process conditions. Uniaxial orientation arising from extrusion can lead to considerable anisotropy of mechanical properties. This anisotropy is reduced during subsequent biaxial bulging of the tubes.

ACKNOWLEDGMENTS

The authors wish to thank Dr. M. Bruhis of McMaster University for help in the development of forming test systems.

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

1 Department of Mechanica (l) Engineering, McMaster University, Hamilton, Ontario, Canada L8S4L7 (2) Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S4M1

Correspondence to: M.Elnagmin; 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).

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Date:Jul 1, 2011
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