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Solid-state extrusion of isotactic polypropylene through a tapered die: 2. Structure and some properties of extrudates
Solid-state extrusion of isotactic polypropylene through a tapered die: 2. Structure and some properties of extrudates K. Nakamura*, K. Imada and M. Takayanagi Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka, Japan (Received 19 July 1973; revised 8 February 1974) The structure and some properties of the solid-state extrudates of isotactic polypropylene (PP) were examined. The crystal modifications of the PP extrudates differed as the extrusion temperature changed. The formation of smectic crystals was observed in the samples extruded at temperatures below 70°C, while the monoclinic modification was predominant above 70°C. The crystal orientation factor, f~, increased with increasing extrusion ratio (EFt) and reached 0.988 when ER was 6.3, which was considered to be an upper limit of EFt of PP extrusion. The mechanical properties and the thermal shrinkage of the extrudates were also examined. From these measurements the PP extrudates were considered to have structures similar to the drawn PP.
INTRODUCTION In recent years we have investigated the plastic deformation and the solid-state extrusion of crystalline polymers under high pressures, and reported the work in a number of papers 1-6. Before being applied to polymeric materials, solid-state extrusion had been developed as one of the cold working processes for metals. The theoretical background of metal extrusion has been well established 7, 8 and many metals are now extruded successfully on a large scale. The solid-state extrusion of polymeric materials has been investigated recently9 and difficulties are apparent. Polymeric materials are quite different from metals in that the molecules are long and orient easily during deformation. At this stage then, it is important to clarify how the molecular orientation and strain hardening affect practical processing. In previous papers we have reported the process of solid-state extrusion of high density polyethylene (HDPE) I-z, blend systems of HDPE and n-paraffin 4, isotactic polypropylene (pp)5 and nylon-61, 2, from the viewpoint of the theory of deformation of plastic materials, and we proposed an equation to predict the extrusion pressure from tensile data z, 5. We have also examined the crystal orientation, superstructure and some properties of HDPE extrudates extruded at high temperatures (80-II0°C) 6. The properties and the structure of HDPE extrudates, which have excellent transparency in appearance and excellent thermal shrinkage characteristics, are shown to resemble those of HDPE drawn samples annealed with free ends after having been stretched. * On leave from the Central Research Laboratory of Sumitomo Chemical Industries Co. Ltd. 446
POLYMER, 1974, Vol 15, July
In the present paper, the crystal modification, crystal orientation, mechanical properties and thermal shrinkage of the PP extrudates are considered. EXPERIMENTAL
Material PP resin used in the solid-state extrusion was a commercial grade of isotactic polypropylene, Noblen D501 (Sumitomo Chemical Co.). The viscosity-average molecular weight, Mv---~,of the resin was 4.7 x 105 and the melt index (MI) was 0.46. Billets of 10mm diameter were prepared by melting the PP pellets at 200°C in a mould placed in a conventional hydraulic press, and cooled slowly to room temperature in the mould under pressure. The billets thus formed were then turned into cylindrical rods on a lathe. The density of the billets was about 0.904 g/cm 3 at 25°C.
Extrusion method Figure 1 shows the extrusion device comprising a piston-cylinder system similar to that reported previously 5. Tapered dies had half angle o~ of 25 ° and an inlet radius R of 5 mm. By using dies of various outlet radii as listed in Table 1, the extrusion ratio (ER) was altered. Extrusion ratio, ER, is defined as the ratio of the cross-sectional areas of the entrance, (zrR2), and the exit, (zrr2), of the die, i.e. ER=(R/r)L Deformation ratio, DR, is also defined, for the samples which show the 'spring back' or die swell phenomenon at the die exit, as the ratio of the cross-sectional areas of the original billet, (~rRZ), and that of the extrudate, (,rr'Z), i.e. DR=(R/r') 2. The value of DR is usually smaller than ER by the factor of (r/r')L Die swell ratio is expressed by (r'-r)/r. Extrusion temperatures, Te, covered
Structure and properties of solid-state extrudates of polypropylene : K. Nakamura et al.
~
hydrostatically extruded isotactic polypropylene was isotropic with reference to the cross-section directions. This was not applied to our extrudates, but instead, testpieces cut out from the extrudates including their central axes were always used. The orientation factor, f , of the c axis of the PP monoclinic crystal in the extrusion direction was evaluated from the intensity distribution of (110) and (040) planes along azimuthal angle and by using Wilchinskii's equationlk The orientation factor f~ of the b axis was determined directly from the intensity distribution of (040) plane and f " of the a* axis was determined by using the following equation based on the orthogonality between the a*-, b- and c-axes:
-2
A
r 2r
b
f'+fB+f,=O C
G
D
....
a
Figure I Extrusion device. (a) Piston cylinder; (b) vertical section of the die. A, Piston; B, cylinder; C, die; D, support; E, plate of press; F, eye hole; G, thermocouple; H, heater; I, material
Table 1 Outlet diameter and extrusion ratio of the dies used
Outlet diameter (mm)
Extrusion ratio
7"0 6-0 5'0 4'8 4'5 4"2 4"0 3"5
2"0 2-8 4"0 4'3 4"9 5'7 6"3 8'2
a range of 40 to 150°C and was controlled manually within an accuracy of 1°C. Extrusion was conducted after preheating for 1 h, putting the billet in the extrusion device at a prescribed temperature. Extrusion rate was also controlled manually between 5 to 10mm/min. Structure and properties of the extrudates were ascertained not to be affected by extrusion rate in this range.
Structure analysis For the analyses of the crystal modifications and crystal orientation of the PP extrudates, the wide angle X-ray scattering (WAXS) intensity was measured at room temperature by using a fibre specimen holder and a Geiger-Mfller counter. Ni-filtered CuK~ radiation with a point collimation system was used. Samples for X-ray measurements were sheets of 0"5 to 1.0 mm thickness cut from the extrudates including their central axes. In the cases of HDPE 6, axis symmetric orientation of the a axis of the orthorhombic unit cell along the radial directions of the extrudate were observed. Williams I° reported that from measurements of birefringence the structure of
The fracture surface of the extrudate was observed with a scanning electron microscope (SCEM). Small angle X-ray scattering (SAXS) photographs were taken at room temperature by using a small angle camera.
Measurement of the properties of extrudates The stress-strain curves of the extrudates at room temperature ( ~ 23°C) were obtained by using a Tensilon UTM-III (Toyo-Baldwin Co.). Samples for testing were fiat sheets 0-5mm thick and 2mm wide, cut out from the extrudate including its central axis. The initial distance between the crossheads of the testing machine was 20mm and the extension rate was 5 mm/min. The measurements of the dynamic modulus of the extrudates were made by using a Rheovibron DDV-II at room temperature at l l0Hz. These mechanical properties were measured along the extrusion direction. The thermal shrinkage of the extrudates was measured by using a Rheovibron. The samples used were sheets of about 0-5 mm thick and 0.8 mm wide. By separating a specimen holder so as to maintain a small constant tension of about 105N/m z and heating the specimen at the rate of 1.5°C/min, the thermal shrinkage of the extrudate was measured as the change of the distance between specimen holders. RESULTS AND DISCUSSION
Extrusion behaviour Table 2 lists the extrusion pressure under typical extrusion conditions. The upper limit of ER was 6.3 in the range of extrusion temperatures of 70 to 150°C. At ER of 8.2 cracks appeared in the extrudate and stable extrusion could not be attained. At Te of 40°C extrusion under higher ER values, such as 6.3, was not conducted because of the limit of the strength of the apparatus. According to Williams 1° the upper limit of DR of hydrostatically conducted PP extrusion at 100°C was 5. He obtained extrudates of higher DR by further drawing of the extruded samples. We examined the extrusion of lower molecular weight PP resin Noblen Wl01 ( - ~ = 2 . 7 x 1 0 5 , MI=7.90, Sumitomo Chemical Co.), of which limiting ER was also 6.3. Table 2 shows that the extrusion pressure increases with decreasing extrusion temperature and with increasing ER. As we reported previously, the extrusion pressure P can be estimated by the following equation from the tensile data of the polymer1-5: P =(1 +/zcot~) f201n(S/r) Y(,).exp(~/~cot~)dc
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Structure and properties of solid-state extrudates of polypropylene : K. Nakamura et al. Table 2 Extrusion pressure, die swell and deformation ratio under typical extrusion conditions Te
Pressure
Die swell
(°C)
ER
x 10; (N/ma)
(%)
DR
40 40 40 70 70 70 70 100 100 100 100 130 130 130 130 130
2"0 2.8 4'0 2'0 2"8 4'0 6'3 2'0 2-8 4-0 6.3 2.0 2"8 4'0 4.9 6'3
7"7 12"3 20.7 2.6 5.1 8-8 37"0 2-0 2.4 5"1 23"0 2"0 2.0 2-6 4"1 11-5
11 "1 11 "7 12.0 10'1 11 '2 11 '0 8"5 10"7 10"0 8'0 7'5 9"3 8-3 9.0 10'0 7-5
1.6 2-2 3-2 1 "7 2"2 3"2 5"3 1.7 2.3 3"4 5'4 1 '7 2'4 3-4 4"1 5.4
Die
became larger than the outlet diameter of the die. Thus DR is to be expressed as the ratio, (R/r') 2, of the cross-sectional areas of the material before and after the deformation by using r', a radius of the extrudate, instead of the extrusion ratio ER=(R/r) 2. Table 2 lists the die swell and the DR of the extrudates. From preliminary experiments die swell was found to be affected by the molecular weights of the resins and the moulding conditions of the billets. Generally speaking, die swell increases with increasing molecular weight and with increasing annealing temperature of the billet.
Structural analysis of the extrudates Figure 2 shows the equatorial WAXS profiles of the samples extruded at Te of 40 to 130°C under ER of 4.0.
swell=(r'-r)/r
-A
tn
c-
C
D
E
I
12
I
I
l
I
16 20 2g(dcgrees)
I
I
F
24
Figure 2 Equatorial diffraction curves of samples extruded at various temperatures under ER of 4.0. Te: A, 40°; B, 55°; C, 70°; D, 85°; E, 100°; F, 130°C
where /~ is a frictional coefficient between the material and the die wall and Y ( 0 is the yield stress or deformation resistance of the material as a function of the strain E. In the case of PP a reasonable value of extrusion pressure could be obtained by assuming the value of/~ to be 0-30 5 In contrast to the HDPE extrusion, die swell occurred at the exit of the die and the diameters of the extrudates
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POLYMER, 1974, Vol 15, duly
WAXS profiles of the samples extruded above 85°C showed characteristic features of the monoclinic crystals of PP, while the samples obtained at lower temperatures gave very broad peak profile. The peak maximum of the sample extruded at 40°C locates at 20= 14.4 °, which differs from the correct peak position of (110) reflection of monoclinic crystal (20--14.1°). WAXS observations on original billets are characteristic of monoclinic crystals, so the transition of the crystal structure is considered to occur during low temperature extrusion process. It is well known that the monoclinic crystals of PP a r e converted into smectic ones which have broad (hkO) peak at 20=15.3 ° by uniaxial drawing at low temperatures12, la. Natta14 has shown that the oriented smectic structure can be formed by drawing the quenched PP sheet below 70°C. From these results of drawn PP it is reasonable to consider that the broadening and the peak shift of the WAXS profiles of the samples obtained at extrusion temperatures below 70°C are due to the partial transition of the monoclinic crystals to smectic ones. The WAXS profile of the sample extruded at 40°C, as shown in Figure 2, resembles that of the overlap of diffraction intensity curves 15 for films composed of the monoclinic crystals and quenched films composed of the smectic crystals. Figure 3 shows the orientation factors of a*-, b- and c-axes of monoclinic crystal of PP extrudates of various deformation ratios extruded at 130°C. The orientation factor fe increases with increasing DR and reached 0.988 at DR of 5.4, which indicates that the molecular chains in the crystal orient along the extrusion direction almost completely. Figure 4 shows the SCEM photograph of the fracture surface of this highly deformed extrudate. In this SCEM image many fibrils are observed to run along the extrusion direction, which corresponds to the excellent c-axis orientation of the sample. In those samples a*- and b-axes orient perpendicular to the extrusion direction as a foregone consequence of the c-axis orientation. However, the orientation of the a*-axis in the plane perpendicular to the extrusion direction is not equal to that of the b axis but slower than that of the b-axis. Such tendency of the orientation had been also found in drawn pp15. The lag of the a* axis orientation may probably be caused by the fact that the a* axis orients along extrusion direction in the early stages of the deformations. The a* axis orientation was positively observed in the sample drawn at high temperature in. The same a* axis orientation was also observed in the extrudate from the billet annealed at
Structure and properties of solid-state extrudates of polypropylene : K. Nakamura et al. from the meridional scattering gives 196A. According to Balt{t-Calleja and Peterlin 17 the long period of PP drawn at 130°C is about 200A, along the stretched direction. This value corresponds to that of the extrudate. The long periods of the extrudates obtained by extrusions at I00 and 150°C also correspond to those of the samples drawn at 100 and 150°C, respectively. The results described above concerning the crystal modification, crystal orientation and the SAXS of the extrudates of PP agree with those of drawn PP. The structure of the PP extrudate is quite similar to that of the drawn PP. Such similarity of the structure observed between the extrudate and the drawn sample make it possible to estimate the extrusion pressure on the basis of the tensile data, as reported previously3, 5
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u
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04
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Mechanical properties Figure 6 shows nominal stress vs. nominal strain curves of the original sample and the extrudates extruded
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2
3
4
5
6
I
(R/rl} 2 Figure 3 Orientation factor of samples extruded at 130°C, ©, &;
e,f~; x,f~
Figure 5 SAXS photograph of the sample extruded at 130°C at deformation ratio of 5.4
25 /
!
/
/
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!/
C
e z
~5
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;
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Figure 4 Scanning electron micrograph of the fracture surface of the sample extruded at 130°C at deformation ratio of 5.4
160°C for 2h. The orientation factor f " of the samples extruded at 130°C at DR of 1.4 and 1-9 from annealed billets were positive at 0.030 and 0-049, respectively. This shows that a* axis orientation along the extrusion direction really occurs in the early stages of the extrusion. Figure 5 gives the SAXS photograph of the extrudate of D R = 5.4 obtained at 130°C. This photograph exhibits a streak along the equator and weak ones where the centres lie on the meridian. These features resemble those of drawn samples. The long period calculated
c,o
I
0
I
20
I
I
40
Strain (%) Figure 6 Stress-strain curves of undeformed sample ( ), three samples extruded at 130°C (. . . . ) under deformation ratios of 3-4 (A), 4.1 (B), and 5'4 (C), and a sample drawn to a draw ratio of 6.2 (. . . . )
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Structure and properties of so~M-state extrudates of polypropylene : K. Nakamura et al. stages of heating owing to thermal expansion, shrinkage occurs at high temperatures. The temperatures at which shrinkage begins are about 100, 130 and 150°C for the samples extruded at 100, 130 and 150°C, respectively. We also examined the thermal shrinkage of the samples extruded at ! 30°C at different E R . The thermal behaviour was almost the same as shown in Figure 8 for other samples of different ER. Thus PP extrudates are found to shrink at about the temperatures at which the extrusions were conducted. As reported previously6, H D P E extrudate has high dimensional stability up to 120°C when it has been extruded at the temperatures above 100°C, while PP extrudates shrink considerably above the extrusion temperatures. To obtain a PP extrudate having high thermal stability, it is necessary to conduct the extrusion at the temperatures as high as would be encountered after the processing.
4
'0 -~ 2
C
i
I
3
I
I
I
I
5 Deformotion rQtio
7
Figure 7 Dynamic modulus of the extrudates and drawn sample ( 0 ) measured at 110Hz
(1) The upper limit of the extrusion ratio in the solidstate extrusion of PP was (R/r)~ = 6"3 in the temperature range of 70 ° to 150°C. At higher extrusion ratio cracks were produced in the extrudate. (2) The crystal structure of the PP extrudates was affected by the extrusion temperature. Smectic crystals were formed in the samples extruded below 70°C, while on the extrudates prepared at higher temperatures the monoclinic crystals were observed predominantly. (3) The crystal orientation increased with increasing deformation ratio and the orientation factor reached 0.988 at the limiting extrusion ratio, (R/r)2=6.3. (4) Mechanical properties of PP were improved by solid-state extrusion in comparison with the original unoriented samples. The extent of improvement was about the same as observed in drawn sample. (5) Thermal shrinkage of PP extrudates was observed to begin at about extrusion temperatures and seemed to be independent of the degree of deformation.
I
o t-
"'X'x-'~ X X X''XX -2[" I
I
40
,
I
I
80
I
12O
i
I
16O
Temperoture PC)
Figure 8 Thermal shrinkage of the sample extruded at 100° (O), 130° (©) and 150°C ( x ) under extrusion ratio of 6.3
at 130°C under various deformation ratios. For comparison, a curve of the sample drawn to a draw ratio of 6.2 at 130°C is also presented as a broken line in Figure 6. T h e mechanical properties of PP are observed to be improved by solid-state extrusion. Figure 7 presents dynamic modulus, E ' , at l l 0 H z of the same samples as those used in the stress-strain measurements. The modulus, E ' , increases linearly with increasing extrusion ratio. The curve can be extrapolated to the point of the drawn sample (solid circle). Thus the mechanical properties of the extrudate and the drawn sample are the same when they are compared at the same values of the deformation ratio and the draw ratio. Thermal shrinkage Figure 8 shows the thermal shrinkage of the samples extruded at 100, 130 and 150°C at E R of 6.3. Although
the length of each extrudate increases in the early
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P O L Y M E R , 1974, Vol 15, July
CONCLUSIONS
REFERENCES I Imada, K., Kondo, Y., Kanekiyo, K. and Takayanagi, M. Rep. Progr. Polym. Phys. Japan 1971, 14, 393 2 Imada, K., Kondo, Y., Kanekiyo, K. and Takayanagi, M. Proc. 1971 Int. Conf. Mech. Behavior Mater. 1972, 3, 476 3 Imada, K. and Takayanagi, M. Int. J. Polym. Mater. 1973, 2, 89 4 Maruyama, S., Imada, K. and Takayanagi, M. Int. 3". Polym. Mater. 1973, 2, 105 5 Nakamura, K., Imada, K. and Takayanagi, M. Int. J. Polym. Mater. 1972, 2, 71 6 Imada, K., Yamamoto, T., Shigematsu, K. and Takayanagi, M. J. Mater. Sci. 1971, 6, 537 7 Hill, R. 'The Mathematical Theory of Plasticity', Clarendon Press, Oxford, 1950 8 Avitzur, B. 'Metal Forming Process and Analysis', McGrawHill, New York, 1968 9 Buckley, A. and Long, H. A. Polym. Eng. ScL 1969, 9, 115 10 Williams, T. J. Mater. Sci. 1973, 8, 59 11 Wilchinskii, Z. W. J. Appl. Polym. Sci. 1963, 7, 923 12 Sobue, H. and Tabata, I. J. Appl. Polym. Sci. 1959, 2, 66 13 Takahara, H. and Kawai, I~. Rep. Progr. Polym. Phys. Japan 1967, 10, 277 14 Natta, G. Makromol. Chem. 1960, 35, 94 15 Takahar, H., Kawai, H. and Yamada, T. Sen-i Gakkaishi 1967, 23, 102; 1968, 24, 219 16 Awaya, H. Nippon Kagaku Kaishi 1961, 82, 1575 17 Balt~i-Calleja, F. J. and Peterlin, A. J. Macromol. Sci. (B) 1970, 4, 519
INTRODUCTION In recent years we have investigated the plastic deformation and the solid-state extrusion of crystalline polymers under high pressures, and reported the work in a number of papers 1-6. Before being applied to polymeric materials, solid-state extrusion had been developed as one of the cold working processes for metals. The theoretical background of metal extrusion has been well established 7, 8 and many metals are now extruded successfully on a large scale. The solid-state extrusion of polymeric materials has been investigated recently9 and difficulties are apparent. Polymeric materials are quite different from metals in that the molecules are long and orient easily during deformation. At this stage then, it is important to clarify how the molecular orientation and strain hardening affect practical processing. In previous papers we have reported the process of solid-state extrusion of high density polyethylene (HDPE) I-z, blend systems of HDPE and n-paraffin 4, isotactic polypropylene (pp)5 and nylon-61, 2, from the viewpoint of the theory of deformation of plastic materials, and we proposed an equation to predict the extrusion pressure from tensile data z, 5. We have also examined the crystal orientation, superstructure and some properties of HDPE extrudates extruded at high temperatures (80-II0°C) 6. The properties and the structure of HDPE extrudates, which have excellent transparency in appearance and excellent thermal shrinkage characteristics, are shown to resemble those of HDPE drawn samples annealed with free ends after having been stretched. * On leave from the Central Research Laboratory of Sumitomo Chemical Industries Co. Ltd. 446
POLYMER, 1974, Vol 15, July
In the present paper, the crystal modification, crystal orientation, mechanical properties and thermal shrinkage of the PP extrudates are considered. EXPERIMENTAL
Material PP resin used in the solid-state extrusion was a commercial grade of isotactic polypropylene, Noblen D501 (Sumitomo Chemical Co.). The viscosity-average molecular weight, Mv---~,of the resin was 4.7 x 105 and the melt index (MI) was 0.46. Billets of 10mm diameter were prepared by melting the PP pellets at 200°C in a mould placed in a conventional hydraulic press, and cooled slowly to room temperature in the mould under pressure. The billets thus formed were then turned into cylindrical rods on a lathe. The density of the billets was about 0.904 g/cm 3 at 25°C.
Extrusion method Figure 1 shows the extrusion device comprising a piston-cylinder system similar to that reported previously 5. Tapered dies had half angle o~ of 25 ° and an inlet radius R of 5 mm. By using dies of various outlet radii as listed in Table 1, the extrusion ratio (ER) was altered. Extrusion ratio, ER, is defined as the ratio of the cross-sectional areas of the entrance, (zrR2), and the exit, (zrr2), of the die, i.e. ER=(R/r)L Deformation ratio, DR, is also defined, for the samples which show the 'spring back' or die swell phenomenon at the die exit, as the ratio of the cross-sectional areas of the original billet, (~rRZ), and that of the extrudate, (,rr'Z), i.e. DR=(R/r') 2. The value of DR is usually smaller than ER by the factor of (r/r')L Die swell ratio is expressed by (r'-r)/r. Extrusion temperatures, Te, covered
Structure and properties of solid-state extrudates of polypropylene : K. Nakamura et al.
~
hydrostatically extruded isotactic polypropylene was isotropic with reference to the cross-section directions. This was not applied to our extrudates, but instead, testpieces cut out from the extrudates including their central axes were always used. The orientation factor, f , of the c axis of the PP monoclinic crystal in the extrusion direction was evaluated from the intensity distribution of (110) and (040) planes along azimuthal angle and by using Wilchinskii's equationlk The orientation factor f~ of the b axis was determined directly from the intensity distribution of (040) plane and f " of the a* axis was determined by using the following equation based on the orthogonality between the a*-, b- and c-axes:
-2
A
r 2r
b
f'+fB+f,=O C
G
D
....
a
Figure I Extrusion device. (a) Piston cylinder; (b) vertical section of the die. A, Piston; B, cylinder; C, die; D, support; E, plate of press; F, eye hole; G, thermocouple; H, heater; I, material
Table 1 Outlet diameter and extrusion ratio of the dies used
Outlet diameter (mm)
Extrusion ratio
7"0 6-0 5'0 4'8 4'5 4"2 4"0 3"5
2"0 2-8 4"0 4'3 4"9 5'7 6"3 8'2
a range of 40 to 150°C and was controlled manually within an accuracy of 1°C. Extrusion was conducted after preheating for 1 h, putting the billet in the extrusion device at a prescribed temperature. Extrusion rate was also controlled manually between 5 to 10mm/min. Structure and properties of the extrudates were ascertained not to be affected by extrusion rate in this range.
Structure analysis For the analyses of the crystal modifications and crystal orientation of the PP extrudates, the wide angle X-ray scattering (WAXS) intensity was measured at room temperature by using a fibre specimen holder and a Geiger-Mfller counter. Ni-filtered CuK~ radiation with a point collimation system was used. Samples for X-ray measurements were sheets of 0"5 to 1.0 mm thickness cut from the extrudates including their central axes. In the cases of HDPE 6, axis symmetric orientation of the a axis of the orthorhombic unit cell along the radial directions of the extrudate were observed. Williams I° reported that from measurements of birefringence the structure of
The fracture surface of the extrudate was observed with a scanning electron microscope (SCEM). Small angle X-ray scattering (SAXS) photographs were taken at room temperature by using a small angle camera.
Measurement of the properties of extrudates The stress-strain curves of the extrudates at room temperature ( ~ 23°C) were obtained by using a Tensilon UTM-III (Toyo-Baldwin Co.). Samples for testing were fiat sheets 0-5mm thick and 2mm wide, cut out from the extrudate including its central axis. The initial distance between the crossheads of the testing machine was 20mm and the extension rate was 5 mm/min. The measurements of the dynamic modulus of the extrudates were made by using a Rheovibron DDV-II at room temperature at l l0Hz. These mechanical properties were measured along the extrusion direction. The thermal shrinkage of the extrudates was measured by using a Rheovibron. The samples used were sheets of about 0-5 mm thick and 0.8 mm wide. By separating a specimen holder so as to maintain a small constant tension of about 105N/m z and heating the specimen at the rate of 1.5°C/min, the thermal shrinkage of the extrudate was measured as the change of the distance between specimen holders. RESULTS AND DISCUSSION
Extrusion behaviour Table 2 lists the extrusion pressure under typical extrusion conditions. The upper limit of ER was 6.3 in the range of extrusion temperatures of 70 to 150°C. At ER of 8.2 cracks appeared in the extrudate and stable extrusion could not be attained. At Te of 40°C extrusion under higher ER values, such as 6.3, was not conducted because of the limit of the strength of the apparatus. According to Williams 1° the upper limit of DR of hydrostatically conducted PP extrusion at 100°C was 5. He obtained extrudates of higher DR by further drawing of the extruded samples. We examined the extrusion of lower molecular weight PP resin Noblen Wl01 ( - ~ = 2 . 7 x 1 0 5 , MI=7.90, Sumitomo Chemical Co.), of which limiting ER was also 6.3. Table 2 shows that the extrusion pressure increases with decreasing extrusion temperature and with increasing ER. As we reported previously, the extrusion pressure P can be estimated by the following equation from the tensile data of the polymer1-5: P =(1 +/zcot~) f201n(S/r) Y(,).exp(~/~cot~)dc
POLYMER, 1974, Vol 15, duly
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Structure and properties of solid-state extrudates of polypropylene : K. Nakamura et al. Table 2 Extrusion pressure, die swell and deformation ratio under typical extrusion conditions Te
Pressure
Die swell
(°C)
ER
x 10; (N/ma)
(%)
DR
40 40 40 70 70 70 70 100 100 100 100 130 130 130 130 130
2"0 2.8 4'0 2'0 2"8 4'0 6'3 2'0 2-8 4-0 6.3 2.0 2"8 4'0 4.9 6'3
7"7 12"3 20.7 2.6 5.1 8-8 37"0 2-0 2.4 5"1 23"0 2"0 2.0 2-6 4"1 11-5
11 "1 11 "7 12.0 10'1 11 '2 11 '0 8"5 10"7 10"0 8'0 7'5 9"3 8-3 9.0 10'0 7-5
1.6 2-2 3-2 1 "7 2"2 3"2 5"3 1.7 2.3 3"4 5'4 1 '7 2'4 3-4 4"1 5.4
Die
became larger than the outlet diameter of the die. Thus DR is to be expressed as the ratio, (R/r') 2, of the cross-sectional areas of the material before and after the deformation by using r', a radius of the extrudate, instead of the extrusion ratio ER=(R/r) 2. Table 2 lists the die swell and the DR of the extrudates. From preliminary experiments die swell was found to be affected by the molecular weights of the resins and the moulding conditions of the billets. Generally speaking, die swell increases with increasing molecular weight and with increasing annealing temperature of the billet.
Structural analysis of the extrudates Figure 2 shows the equatorial WAXS profiles of the samples extruded at Te of 40 to 130°C under ER of 4.0.
swell=(r'-r)/r
-A
tn
c-
C
D
E
I
12
I
I
l
I
16 20 2g(dcgrees)
I
I
F
24
Figure 2 Equatorial diffraction curves of samples extruded at various temperatures under ER of 4.0. Te: A, 40°; B, 55°; C, 70°; D, 85°; E, 100°; F, 130°C
where /~ is a frictional coefficient between the material and the die wall and Y ( 0 is the yield stress or deformation resistance of the material as a function of the strain E. In the case of PP a reasonable value of extrusion pressure could be obtained by assuming the value of/~ to be 0-30 5 In contrast to the HDPE extrusion, die swell occurred at the exit of the die and the diameters of the extrudates
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WAXS profiles of the samples extruded above 85°C showed characteristic features of the monoclinic crystals of PP, while the samples obtained at lower temperatures gave very broad peak profile. The peak maximum of the sample extruded at 40°C locates at 20= 14.4 °, which differs from the correct peak position of (110) reflection of monoclinic crystal (20--14.1°). WAXS observations on original billets are characteristic of monoclinic crystals, so the transition of the crystal structure is considered to occur during low temperature extrusion process. It is well known that the monoclinic crystals of PP a r e converted into smectic ones which have broad (hkO) peak at 20=15.3 ° by uniaxial drawing at low temperatures12, la. Natta14 has shown that the oriented smectic structure can be formed by drawing the quenched PP sheet below 70°C. From these results of drawn PP it is reasonable to consider that the broadening and the peak shift of the WAXS profiles of the samples obtained at extrusion temperatures below 70°C are due to the partial transition of the monoclinic crystals to smectic ones. The WAXS profile of the sample extruded at 40°C, as shown in Figure 2, resembles that of the overlap of diffraction intensity curves 15 for films composed of the monoclinic crystals and quenched films composed of the smectic crystals. Figure 3 shows the orientation factors of a*-, b- and c-axes of monoclinic crystal of PP extrudates of various deformation ratios extruded at 130°C. The orientation factor fe increases with increasing DR and reached 0.988 at DR of 5.4, which indicates that the molecular chains in the crystal orient along the extrusion direction almost completely. Figure 4 shows the SCEM photograph of the fracture surface of this highly deformed extrudate. In this SCEM image many fibrils are observed to run along the extrusion direction, which corresponds to the excellent c-axis orientation of the sample. In those samples a*- and b-axes orient perpendicular to the extrusion direction as a foregone consequence of the c-axis orientation. However, the orientation of the a*-axis in the plane perpendicular to the extrusion direction is not equal to that of the b axis but slower than that of the b-axis. Such tendency of the orientation had been also found in drawn pp15. The lag of the a* axis orientation may probably be caused by the fact that the a* axis orients along extrusion direction in the early stages of the deformations. The a* axis orientation was positively observed in the sample drawn at high temperature in. The same a* axis orientation was also observed in the extrudate from the billet annealed at
Structure and properties of solid-state extrudates of polypropylene : K. Nakamura et al. from the meridional scattering gives 196A. According to Balt{t-Calleja and Peterlin 17 the long period of PP drawn at 130°C is about 200A, along the stretched direction. This value corresponds to that of the extrudate. The long periods of the extrudates obtained by extrusions at I00 and 150°C also correspond to those of the samples drawn at 100 and 150°C, respectively. The results described above concerning the crystal modification, crystal orientation and the SAXS of the extrudates of PP agree with those of drawn PP. The structure of the PP extrudate is quite similar to that of the drawn PP. Such similarity of the structure observed between the extrudate and the drawn sample make it possible to estimate the extrusion pressure on the basis of the tensile data, as reported previously3, 5
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Figure 4 Scanning electron micrograph of the fracture surface of the sample extruded at 130°C at deformation ratio of 5.4
160°C for 2h. The orientation factor f " of the samples extruded at 130°C at DR of 1.4 and 1-9 from annealed billets were positive at 0.030 and 0-049, respectively. This shows that a* axis orientation along the extrusion direction really occurs in the early stages of the extrusion. Figure 5 gives the SAXS photograph of the extrudate of D R = 5.4 obtained at 130°C. This photograph exhibits a streak along the equator and weak ones where the centres lie on the meridian. These features resemble those of drawn samples. The long period calculated
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Strain (%) Figure 6 Stress-strain curves of undeformed sample ( ), three samples extruded at 130°C (. . . . ) under deformation ratios of 3-4 (A), 4.1 (B), and 5'4 (C), and a sample drawn to a draw ratio of 6.2 (. . . . )
POLYMER, 1974, Vol 15, duly
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Structure and properties of so~M-state extrudates of polypropylene : K. Nakamura et al. stages of heating owing to thermal expansion, shrinkage occurs at high temperatures. The temperatures at which shrinkage begins are about 100, 130 and 150°C for the samples extruded at 100, 130 and 150°C, respectively. We also examined the thermal shrinkage of the samples extruded at ! 30°C at different E R . The thermal behaviour was almost the same as shown in Figure 8 for other samples of different ER. Thus PP extrudates are found to shrink at about the temperatures at which the extrusions were conducted. As reported previously6, H D P E extrudate has high dimensional stability up to 120°C when it has been extruded at the temperatures above 100°C, while PP extrudates shrink considerably above the extrusion temperatures. To obtain a PP extrudate having high thermal stability, it is necessary to conduct the extrusion at the temperatures as high as would be encountered after the processing.
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Figure 7 Dynamic modulus of the extrudates and drawn sample ( 0 ) measured at 110Hz
(1) The upper limit of the extrusion ratio in the solidstate extrusion of PP was (R/r)~ = 6"3 in the temperature range of 70 ° to 150°C. At higher extrusion ratio cracks were produced in the extrudate. (2) The crystal structure of the PP extrudates was affected by the extrusion temperature. Smectic crystals were formed in the samples extruded below 70°C, while on the extrudates prepared at higher temperatures the monoclinic crystals were observed predominantly. (3) The crystal orientation increased with increasing deformation ratio and the orientation factor reached 0.988 at the limiting extrusion ratio, (R/r)2=6.3. (4) Mechanical properties of PP were improved by solid-state extrusion in comparison with the original unoriented samples. The extent of improvement was about the same as observed in drawn sample. (5) Thermal shrinkage of PP extrudates was observed to begin at about extrusion temperatures and seemed to be independent of the degree of deformation.
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Figure 8 Thermal shrinkage of the sample extruded at 100° (O), 130° (©) and 150°C ( x ) under extrusion ratio of 6.3
at 130°C under various deformation ratios. For comparison, a curve of the sample drawn to a draw ratio of 6.2 at 130°C is also presented as a broken line in Figure 6. T h e mechanical properties of PP are observed to be improved by solid-state extrusion. Figure 7 presents dynamic modulus, E ' , at l l 0 H z of the same samples as those used in the stress-strain measurements. The modulus, E ' , increases linearly with increasing extrusion ratio. The curve can be extrapolated to the point of the drawn sample (solid circle). Thus the mechanical properties of the extrudate and the drawn sample are the same when they are compared at the same values of the deformation ratio and the draw ratio. Thermal shrinkage Figure 8 shows the thermal shrinkage of the samples extruded at 100, 130 and 150°C at E R of 6.3. Although
the length of each extrudate increases in the early
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CONCLUSIONS
REFERENCES I Imada, K., Kondo, Y., Kanekiyo, K. and Takayanagi, M. Rep. Progr. Polym. Phys. Japan 1971, 14, 393 2 Imada, K., Kondo, Y., Kanekiyo, K. and Takayanagi, M. Proc. 1971 Int. Conf. Mech. Behavior Mater. 1972, 3, 476 3 Imada, K. and Takayanagi, M. Int. J. Polym. Mater. 1973, 2, 89 4 Maruyama, S., Imada, K. and Takayanagi, M. Int. 3". Polym. Mater. 1973, 2, 105 5 Nakamura, K., Imada, K. and Takayanagi, M. Int. J. Polym. Mater. 1972, 2, 71 6 Imada, K., Yamamoto, T., Shigematsu, K. and Takayanagi, M. J. Mater. Sci. 1971, 6, 537 7 Hill, R. 'The Mathematical Theory of Plasticity', Clarendon Press, Oxford, 1950 8 Avitzur, B. 'Metal Forming Process and Analysis', McGrawHill, New York, 1968 9 Buckley, A. and Long, H. A. Polym. Eng. ScL 1969, 9, 115 10 Williams, T. J. Mater. Sci. 1973, 8, 59 11 Wilchinskii, Z. W. J. Appl. Polym. Sci. 1963, 7, 923 12 Sobue, H. and Tabata, I. J. Appl. Polym. Sci. 1959, 2, 66 13 Takahara, H. and Kawai, I~. Rep. Progr. Polym. Phys. Japan 1967, 10, 277 14 Natta, G. Makromol. Chem. 1960, 35, 94 15 Takahar, H., Kawai, H. and Yamada, T. Sen-i Gakkaishi 1967, 23, 102; 1968, 24, 219 16 Awaya, H. Nippon Kagaku Kaishi 1961, 82, 1575 17 Balt~i-Calleja, F. J. and Peterlin, A. J. Macromol. Sci. (B) 1970, 4, 519