- Email: [email protected]
Studies of melt flow properties during capillary extrusion of polycarbonate
Jourualof
Materials Processing Technology ELSEVIER
Journal ofMaterials Processing Technology 63 (1997) 501-504
Studies of Melt Flow Properties During Capillary Extrusion of Polycarbonate lZ. Liang", C.Y. Tangb "Department of Chemical Machinery, South China University of Technology Guangzhou 510641, China bDepartment of Manufacturing Engineering, The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong
Industrial Summary Under temperatures of 100°C - 310°C and apparent shear rates of 60s' 1 - 1000s'!, the melt flow properties and their effects on the extrusion of two different types of polycarbonate (PC) resins are studied using a capillary rheometer. The results indicate that the shear flow property of the melts follows the Newtonian flow law. Under the same extrusion conditions, the end effect caused by the higher molecular weight (Mw) melt is relatively more significant than that of the lower M w melt. On the other hand, the viscosity of the lower Mw melt is comparatively more sensitive to temperature change. On the basis of the experimental results, a mathematical model that describes the relationship between the shear viscosity and the temperature of PC is proposed. Thus, this model can be used as a practical tool for the injection moulding of PC resin.
1. Introduction
PC is an engineering plastic, which has desirable properties of high impact strength, a wide working-temperature range, good electrical insulating properties, and good dimensional stability. There are many applications of PC in industry; for example, automotive body components, electrical components, and many metal replacements. However, stress cracking, high notch sensitivity, low hardness, and difficult in processing are the major limitations of PC: especially, its poor melt flow properties cause significant drawbacks in moulding. The flow properties of the melt play an important role in the moulding process of polymeric materials, being important properties that govern the mould design and optimum process conditions. Mount and Chung [1] used heating slide metal plates to study the melting properties of PC, and Lord [2] investigated the correlation of viscosity and pressure of PC melt in injection moulding. However, there are only limited publications on the melt flow properties of PC. In this study, the melt flow properties of two different types of PC and their effects on extrusion under practical conditions have been investigated using a capillary rheometer.
Table 1 MFR and Mw of the PC Samples Sample Melt Flow Rate, MFR A 109/ 10 min B 15g /10 min
Molecular Weight, Mw 27,000 25,000
2.2 Apparatus and Methods The main experimental apparatus was a capillary rheometer, Rheovis 2100 by Ceast Corporation of Italy. The diameter and the working length of the barrel were 9.5mm and 280mm, respectively. A pressure sensor was located at the top of the piston to measure the total pressure drop (M) in the extrusion process. The capillary tubes were 1 mm in diameter and their length-to-diameter ratios (UD) were 10, 20, 30, and 40, respectively. The entry angle was set to 180°. In order to examine the flow properties in the actual processing conditions, the extrusion temperature (1) was varied from 180 to 320°C. The piston speeds (li) were in the range of 5 -75mm/rnin, with the corresponding shear rates (r) in the range of 60 - 1000s· 1 Bagley's graphical method [3] was used to determine the values of the end pressure loss of the melt.
2. Experimental Investigation 3. Results 2.1 Samples 3.1 End pressure losses In the experimental study, two PC samples named as Sample A and Sample B with product codes Calibre 700-10 and Calibre 300-15, respectively, produced by Dow Chemical Ltd. of the United States were used. The melt flow rate (MFR) and the molecular weight of the samples are listed in Table 1. 0924-0136/97/$15.00 @ 1997 Elsevier Science SA All rights reserved
I'll 80924-0136(96)02672-6
When the polymer melt entered the capillary tube from the barrel, there was an entry pressure loss due to the abrupt contraction of the flow section. At the exit, re-distribution of the flow velocity and residual stress cause an exit pressure loss.
J.z. Liang, C. Y. Tang / Journal of Materials Processing Technology 63 (1997) 501-504
502
Therefore, the end pressure loss (Mend) includes the entry pressure loss (Men) and an exit pressure loss (Mex) , i.e.: (1)
where Mend represents the viscoelastic property of the melt in the flow.
10 o Sample A
9
(1I=30mmhnin)
8
o Sample A (1I=50mmhnin)
7
lil 6 0..
6
~
Under different temperatures, the dependence of Mend on the shear stress at the die wall rw is shown in Fig. 2. For Sample A, Mend increases with increasing Tw at 300 0 e and the relationship between them is approximately an exponential function. When the temperature rises by lOoe (i.e. at 310°C), the value of Mend decreases significantly. Moreover, Mend decreases with increasing Tw when the value of Tw is greater than 401kPa. For Sample B, the situation is similar to that for Sample A, except that the extent of the increase of Mend with Tw is comparatively smaller. Under the same extrusion conditions, the values of Mend for Sample A are greater than those for Sample B and they are strongly sensitive to temperature change, as illustrated in Figure 2.
o$ample B (1I=30mmhnin)
5
3.2
4
(ll=70mmhnin)
Fig. 2 shows that the end effect of the sample melts is evident under the present test conditions. Mend has an significant contribution to the total pressure loss M, especially in the case of Sample A. Therefore, the end correction should be made when the value of Tw is to be determined, i.e.:
3
2
o +----+----+---t----+---+------j o
5
15
10
20
25
30
LiD
(!'P -
Fig. 1. M versus LID.
Tw =
Fig, 1 illustrates the relationship between the total pressure drop M and LID during capillary flow of the sample melts at 31o o e. Within the measurement range, the variation of M versus LID are linear under constant extrusion rate V. The values of Mend can be determined from the y-intercepts of the lines in Fig. 1. Therefore, it can be observed that Mend increases with increasing V when the temperature is kept constant. The relationship of M and LID also depends on V, the effect of the piston speed V being remarkable for Sample B.
"Sample A (300 deg C)
" 4
o Sample A (310 deg C)
° SampleB 3
~
1 Q.,
(2)
o Sample A (300 deg C)
120
to
"Sample A (310 deg C)
100
o Sample B (295de9 C)
80
<> Sample B (310deg C)
0..
~
60 40
5
0..
!'P.nd )D 4L
140
~
.
Flow Curves
",Sample B
20 0
"
+--~-+-----+-·---t-I ---+-----1
0
200
400
(295 deg C)
r.
<>SampleB (310 deg C)
Fig. 3.
600
800
1000
(5") Tw
versus
Ya
2
The flow curves plotting <>
0 0
100
50
r w (kPa)
Fig. 2. Mend versus Tw
150
Tw
versus
Ya
of the two sample
melts at various temperatures are shown in Fig. 3 where it can be seen that the curves of Tw versus Ya are linear. The linear relationship indicates that the shear flow of the melts obeys the Newtonian fluid flow law under the extrusion conditions, i.e., the apparent shear viscosity 1]a is almost constant (the slope of the curve for Tw versus Ya is fixed). Thus: (3)
J.Z. Liang,
c.Y. Tang / Journal of Materials Processing Technology 63 (1997) 501-504
Eqn(3) is a constitutive equation for Newtonian fluids, where Ya is given by: . Ya
2VD/
=
(4)
15D3
where DR and D are the diameters of the reservoir and the capillary in nnn, V is in mm/min, and Ya is in S·l Difference between the slopes of the flow curves for the two sample melts are not evident, as shown in Fig. 3, which implies the melts have similar shear sensitivity. On the other hand, the effects of temperature on the shear sensitivity of the melts are relatively noticeable, especially in the case of Sample B. 3.3 Dependence ofthe melt viscosity on temperature
The shear viscosity of polymer melts is temperature dependent. The relationship between the shear viscosity and the temperature of the two sample melts can be obtained by plotting In1]a against liT (T is in the absolute temperature scale), as illustrated in Fig. 4. Under a given extrusion rate, the curves of In1]a versus liT are closely linear, the linearity suggesting that the dependence of the sample melt shear viscosity on temperature can be described by the Arrhenius equation, i.e.: (5)
where A is the constant related to the melt viscosity, E is the activation energy for viscous flow, and R is the universal gas constant.
5.6
5.4
..
.
5.2
~
I::-
It can be seen in Fig. 4 that the slope of the In1]a versus liT curve for Sample B is clearly greater than that of Sample A at the same extrusion rate. As is suggested by Eqn.(5), the stiffer is the slope of the curve the greater is the value of E and hence the more evident it is that the melt shear viscosity is temperature sensitive. In addition, under the same extrusion conditions, the value of 1]a for Sample A melt is greater than that of Sample B.
3.4 Discussion
The flow of polymer is caused by the transition of the chain segment. For the melt flow of PC, it is necessary for the thermal motion energy of the molecular chain to be sufficiently high to overcome the internal revolve potential energy since the molecular chain of PC is rigid: as a result, the viscous flow temperature of PC is high. The more rigid is the molecular chain, or the greater is the force between molecules, the greater is the viscous flow activation energy and the more remarkable the resulting temperature sensitivity of the melt viscosity. As illustrated by the slopes of the lines in Fig. 4, the values of E for the two samples are high, especially in the case of Sample B. In general, the chain segment of a rigid molecular chain is long and the conformation change is difficult. Thus the change in resistance is not significant, in other words, the melt viscosity remains constant (see Fig. 3). With an increase in molecule weight M w , the acting force between molecules will increase as will the melt viscosity. It can be determined from the slope of the lines in Fig. 4, that the value of 1]a for Sample A melt with greater molecular weight is greater than that of Sample B under the same extrusion conditions. Since the melt viscosity is more sensitive to temperature than it is to shear, it would be effective to improve the flow properties of PC during shaping and processing by increasing the operation temperature properly.
The slope of the melt flow curve for Sample A is slightly greater than that of Sample B at the same temperature as shown in Fig. 3, which may imply that the molecular weight distribution of the former is wider than that of the latter. In polymer melt flow, the viscoelastic behaviour of the melts is related closely to their molecular weight and molecular weight distribution. Therefore, the phenomenon, that the end effect of Sample A melt is greater than that of Sample B (see Fig 2) under the same operation conditions may be attributed to the differences in their molecular weight and molecular weight distribution.
5.8
Ul
503
5
.E 4.8
4. Conclusions 4.6
4.4
4.2 1.6
1.7
1.8
1.9
liT (10· 3K)
Fig. 4. In1]a versus liT
2.1
2.2
2.3
Under the present experimental conditions, the die extrusion flow of the two PC sample melts obeys the flow law of Newtonian fluids. The melt shear viscosity depends heavily on the temperature and the relationship between them can be described by the Arrhenius equation. Compared with the temperature sensitivity, the shear sensitivity of the melt viscosity is lower. For this reason, it would be more effective to improve the flow properties of PC by increasing the operation
504
J.z. Liang. C.y. Tang I Journal of Materials Processing Technology 63 (1997) 501-504
temperature in shaping and processing. In the die flow, the end effect of the two sample melts is evident, especially in the case of Sample A at lower temperature. Under the same extrusion conditions, there are significant differences in the flow properties between the two sample melts, which can be attributed to the difference in the molecular structure parameters: molecular weight and molecular weight distribution.
References [1] E.M. Mount and C.I. Chung, Polym. Eng. Sci., 18, (1978) 711. [2] H.A. Lord, Po/ym. Eng. Sci., 19, (1979) 469. [3] E. B. 1. Bagley,Appl. Phys., 5, (1957) 634.
Materials Processing Technology ELSEVIER
Journal ofMaterials Processing Technology 63 (1997) 501-504
Studies of Melt Flow Properties During Capillary Extrusion of Polycarbonate lZ. Liang", C.Y. Tangb "Department of Chemical Machinery, South China University of Technology Guangzhou 510641, China bDepartment of Manufacturing Engineering, The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong
Industrial Summary Under temperatures of 100°C - 310°C and apparent shear rates of 60s' 1 - 1000s'!, the melt flow properties and their effects on the extrusion of two different types of polycarbonate (PC) resins are studied using a capillary rheometer. The results indicate that the shear flow property of the melts follows the Newtonian flow law. Under the same extrusion conditions, the end effect caused by the higher molecular weight (Mw) melt is relatively more significant than that of the lower M w melt. On the other hand, the viscosity of the lower Mw melt is comparatively more sensitive to temperature change. On the basis of the experimental results, a mathematical model that describes the relationship between the shear viscosity and the temperature of PC is proposed. Thus, this model can be used as a practical tool for the injection moulding of PC resin.
1. Introduction
PC is an engineering plastic, which has desirable properties of high impact strength, a wide working-temperature range, good electrical insulating properties, and good dimensional stability. There are many applications of PC in industry; for example, automotive body components, electrical components, and many metal replacements. However, stress cracking, high notch sensitivity, low hardness, and difficult in processing are the major limitations of PC: especially, its poor melt flow properties cause significant drawbacks in moulding. The flow properties of the melt play an important role in the moulding process of polymeric materials, being important properties that govern the mould design and optimum process conditions. Mount and Chung [1] used heating slide metal plates to study the melting properties of PC, and Lord [2] investigated the correlation of viscosity and pressure of PC melt in injection moulding. However, there are only limited publications on the melt flow properties of PC. In this study, the melt flow properties of two different types of PC and their effects on extrusion under practical conditions have been investigated using a capillary rheometer.
Table 1 MFR and Mw of the PC Samples Sample Melt Flow Rate, MFR A 109/ 10 min B 15g /10 min
Molecular Weight, Mw 27,000 25,000
2.2 Apparatus and Methods The main experimental apparatus was a capillary rheometer, Rheovis 2100 by Ceast Corporation of Italy. The diameter and the working length of the barrel were 9.5mm and 280mm, respectively. A pressure sensor was located at the top of the piston to measure the total pressure drop (M) in the extrusion process. The capillary tubes were 1 mm in diameter and their length-to-diameter ratios (UD) were 10, 20, 30, and 40, respectively. The entry angle was set to 180°. In order to examine the flow properties in the actual processing conditions, the extrusion temperature (1) was varied from 180 to 320°C. The piston speeds (li) were in the range of 5 -75mm/rnin, with the corresponding shear rates (r) in the range of 60 - 1000s· 1 Bagley's graphical method [3] was used to determine the values of the end pressure loss of the melt.
2. Experimental Investigation 3. Results 2.1 Samples 3.1 End pressure losses In the experimental study, two PC samples named as Sample A and Sample B with product codes Calibre 700-10 and Calibre 300-15, respectively, produced by Dow Chemical Ltd. of the United States were used. The melt flow rate (MFR) and the molecular weight of the samples are listed in Table 1. 0924-0136/97/$15.00 @ 1997 Elsevier Science SA All rights reserved
I'll 80924-0136(96)02672-6
When the polymer melt entered the capillary tube from the barrel, there was an entry pressure loss due to the abrupt contraction of the flow section. At the exit, re-distribution of the flow velocity and residual stress cause an exit pressure loss.
J.z. Liang, C. Y. Tang / Journal of Materials Processing Technology 63 (1997) 501-504
502
Therefore, the end pressure loss (Mend) includes the entry pressure loss (Men) and an exit pressure loss (Mex) , i.e.: (1)
where Mend represents the viscoelastic property of the melt in the flow.
10 o Sample A
9
(1I=30mmhnin)
8
o Sample A (1I=50mmhnin)
7
lil 6 0..
6
~
Under different temperatures, the dependence of Mend on the shear stress at the die wall rw is shown in Fig. 2. For Sample A, Mend increases with increasing Tw at 300 0 e and the relationship between them is approximately an exponential function. When the temperature rises by lOoe (i.e. at 310°C), the value of Mend decreases significantly. Moreover, Mend decreases with increasing Tw when the value of Tw is greater than 401kPa. For Sample B, the situation is similar to that for Sample A, except that the extent of the increase of Mend with Tw is comparatively smaller. Under the same extrusion conditions, the values of Mend for Sample A are greater than those for Sample B and they are strongly sensitive to temperature change, as illustrated in Figure 2.
o$ample B (1I=30mmhnin)
5
3.2
4
(ll=70mmhnin)
Fig. 2 shows that the end effect of the sample melts is evident under the present test conditions. Mend has an significant contribution to the total pressure loss M, especially in the case of Sample A. Therefore, the end correction should be made when the value of Tw is to be determined, i.e.:
3
2
o +----+----+---t----+---+------j o
5
15
10
20
25
30
LiD
(!'P -
Fig. 1. M versus LID.
Tw =
Fig, 1 illustrates the relationship between the total pressure drop M and LID during capillary flow of the sample melts at 31o o e. Within the measurement range, the variation of M versus LID are linear under constant extrusion rate V. The values of Mend can be determined from the y-intercepts of the lines in Fig. 1. Therefore, it can be observed that Mend increases with increasing V when the temperature is kept constant. The relationship of M and LID also depends on V, the effect of the piston speed V being remarkable for Sample B.
"Sample A (300 deg C)
" 4
o Sample A (310 deg C)
° SampleB 3
~
1 Q.,
(2)
o Sample A (300 deg C)
120
to
"Sample A (310 deg C)
100
o Sample B (295de9 C)
80
<> Sample B (310deg C)
0..
~
60 40
5
0..
!'P.nd )D 4L
140
~
.
Flow Curves
",Sample B
20 0
"
+--~-+-----+-·---t-I ---+-----1
0
200
400
(295 deg C)
r.
<>SampleB (310 deg C)
Fig. 3.
600
800
1000
(5") Tw
versus
Ya
2
The flow curves plotting <>
0 0
100
50
r w (kPa)
Fig. 2. Mend versus Tw
150
Tw
versus
Ya
of the two sample
melts at various temperatures are shown in Fig. 3 where it can be seen that the curves of Tw versus Ya are linear. The linear relationship indicates that the shear flow of the melts obeys the Newtonian fluid flow law under the extrusion conditions, i.e., the apparent shear viscosity 1]a is almost constant (the slope of the curve for Tw versus Ya is fixed). Thus: (3)
J.Z. Liang,
c.Y. Tang / Journal of Materials Processing Technology 63 (1997) 501-504
Eqn(3) is a constitutive equation for Newtonian fluids, where Ya is given by: . Ya
2VD/
=
(4)
15D3
where DR and D are the diameters of the reservoir and the capillary in nnn, V is in mm/min, and Ya is in S·l Difference between the slopes of the flow curves for the two sample melts are not evident, as shown in Fig. 3, which implies the melts have similar shear sensitivity. On the other hand, the effects of temperature on the shear sensitivity of the melts are relatively noticeable, especially in the case of Sample B. 3.3 Dependence ofthe melt viscosity on temperature
The shear viscosity of polymer melts is temperature dependent. The relationship between the shear viscosity and the temperature of the two sample melts can be obtained by plotting In1]a against liT (T is in the absolute temperature scale), as illustrated in Fig. 4. Under a given extrusion rate, the curves of In1]a versus liT are closely linear, the linearity suggesting that the dependence of the sample melt shear viscosity on temperature can be described by the Arrhenius equation, i.e.: (5)
where A is the constant related to the melt viscosity, E is the activation energy for viscous flow, and R is the universal gas constant.
5.6
5.4
..
.
5.2
~
I::-
It can be seen in Fig. 4 that the slope of the In1]a versus liT curve for Sample B is clearly greater than that of Sample A at the same extrusion rate. As is suggested by Eqn.(5), the stiffer is the slope of the curve the greater is the value of E and hence the more evident it is that the melt shear viscosity is temperature sensitive. In addition, under the same extrusion conditions, the value of 1]a for Sample A melt is greater than that of Sample B.
3.4 Discussion
The flow of polymer is caused by the transition of the chain segment. For the melt flow of PC, it is necessary for the thermal motion energy of the molecular chain to be sufficiently high to overcome the internal revolve potential energy since the molecular chain of PC is rigid: as a result, the viscous flow temperature of PC is high. The more rigid is the molecular chain, or the greater is the force between molecules, the greater is the viscous flow activation energy and the more remarkable the resulting temperature sensitivity of the melt viscosity. As illustrated by the slopes of the lines in Fig. 4, the values of E for the two samples are high, especially in the case of Sample B. In general, the chain segment of a rigid molecular chain is long and the conformation change is difficult. Thus the change in resistance is not significant, in other words, the melt viscosity remains constant (see Fig. 3). With an increase in molecule weight M w , the acting force between molecules will increase as will the melt viscosity. It can be determined from the slope of the lines in Fig. 4, that the value of 1]a for Sample A melt with greater molecular weight is greater than that of Sample B under the same extrusion conditions. Since the melt viscosity is more sensitive to temperature than it is to shear, it would be effective to improve the flow properties of PC during shaping and processing by increasing the operation temperature properly.
The slope of the melt flow curve for Sample A is slightly greater than that of Sample B at the same temperature as shown in Fig. 3, which may imply that the molecular weight distribution of the former is wider than that of the latter. In polymer melt flow, the viscoelastic behaviour of the melts is related closely to their molecular weight and molecular weight distribution. Therefore, the phenomenon, that the end effect of Sample A melt is greater than that of Sample B (see Fig 2) under the same operation conditions may be attributed to the differences in their molecular weight and molecular weight distribution.
5.8
Ul
503
5
.E 4.8
4. Conclusions 4.6
4.4
4.2 1.6
1.7
1.8
1.9
liT (10· 3K)
Fig. 4. In1]a versus liT
2.1
2.2
2.3
Under the present experimental conditions, the die extrusion flow of the two PC sample melts obeys the flow law of Newtonian fluids. The melt shear viscosity depends heavily on the temperature and the relationship between them can be described by the Arrhenius equation. Compared with the temperature sensitivity, the shear sensitivity of the melt viscosity is lower. For this reason, it would be more effective to improve the flow properties of PC by increasing the operation
504
J.z. Liang. C.y. Tang I Journal of Materials Processing Technology 63 (1997) 501-504
temperature in shaping and processing. In the die flow, the end effect of the two sample melts is evident, especially in the case of Sample A at lower temperature. Under the same extrusion conditions, there are significant differences in the flow properties between the two sample melts, which can be attributed to the difference in the molecular structure parameters: molecular weight and molecular weight distribution.
References [1] E.M. Mount and C.I. Chung, Polym. Eng. Sci., 18, (1978) 711. [2] H.A. Lord, Po/ym. Eng. Sci., 19, (1979) 469. [3] E. B. 1. Bagley,Appl. Phys., 5, (1957) 634.