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A study of the flow properties of glass fibre reinforced PBT in extrusion
Journal of
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 707-712
Materials Processing Technology
A study of the flow properties of glass fibre reinforced PBT in extrusion J.Z. Liang, C.Y. Tang
and W.B. Lee**
Department of Chemical Machinery, South China University of Teclmology Guangzhou 510641, People's Republic of China Department of Manufacturing Engineering, Hong Kong Polytechnic, Hong Kong
The extrusion shaping process of polymer materials is a course of polymer deformation, molten flow and solidification shaping. The melt flow character is one of the important parameters of polymer process-ability; it is usually expressed by a viscosity or flow curve. In the present study, the end effect and the flow properties of glass fibre reinforced PBT melt P O C A N B 4 2 3 5 during extrusion have been investigated by using a R h e o v i s 2100 capillary rheometer. It has been found that the suitable extrusion temperature range of the sample is 240 ~ 250 °C. Within the apparent shear rate ranged from 50 to 1000 s 1, the flow properties of the sample melt do not obey the power law. The relationship between the shear stress at the wall and the apparent shear rate is approximately linear. Under the lower extrusion temperature and apparent shear rate, the apparent shear viscosity of the sample melt has been found to be sensitive to the apparent shear rate. The dependence of the apparent shear viscosity on temperature does not agree with the Arrhenins equation. The entry effect of the sample melt is significant and the relationship between the entrance pressure drop and the apparent shear rate basically obeys the power law under the experimental conditions.
1. INTRODUCTION
2. EXPERIMENTAL INVESTIGATION
Polybutylene terephthalate (PBT) is made from butylene glycol and di-methyl terephthaiate. It has desirable properties of low processing costs, good creep, fatigue, and chemical resistance, and good mouldability and dimensional stability. There are many applications of PBT in industries; for example, electrical components, automotive body components, distributor caps, and many metal replacement. In order to improve the physical properties of end products, PBT is filled with a fixed proportion of glass fibres during the manufacturing process. During the processing and shaping of PBT, the filled glass fibres have certain effects on the flow properties of the polymer melt. The significance of these effects would depend on the orientation of glass fibres along the flow and the flow stability of the melt. However, there are only limited publications on the related studies. In this paper, the end effect and the flow properties of glass fibre reinforced PBT melt P O C A N B 4 2 3 5 during extrusion will be discussed.
2.1. Sample The PBT sample, P O C A N B4235, was manufactured by Mobay Corporation of the United States. They were light grey pellets which were filled with 3 0 % glass fibre to improve their mechanical properties. The average density of the sample Co) was 1050 kg m e .
2.2. Apparatus and methods The main experimental apparatus was a capillary rheometer, R h e o v i s 2100 b y Ceast Corporation of Italy. A pressure sensor was located at the top of the piston to measure the total pressure drop in the extrusion process. The capillary tubes had diameter of 1 m m and length to diameter ratios (L/D) are 10, 20, 30, and 40 respectively. The entry angle was set to 90 °. In order to examine the flow properties in the actual processing conditions, the extrusion temperature (7) was varied from 230 to 2 6 0 °C. The piston speeds (v) were in the range of 4 ~ 75
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0924-0136(94)01712-A
SSD!
708
ZZ.
Liang et
aL / Journal o f Materials Processing Technology 48 (1995) 707-712
m/minwhere the corresponding shear rates ( y , )
sensitive to the change in "/a Further investigation
were in the range of 50 ~ lO00 s -1. shows that log APen and log T a have approximately linear relationship (see Figure 2). 3. RESULTS 7 3.1. End pressure losses
During the extrusion of polymer melt, the total pressure drop (AP) includes an entry pressure drop (APen), a pressure drop in the die ( A P e ) , and an exit pressure loss (APex), that is A P = A P e n + APe+
APex
(1)
(2)
APen a is called the end pressure drop. It represents the elastic property of the melt in the flow. For most polymer melts, APex are much smaller than APen a [1], so &Pen may be approximated by APen a. When a polymer melt passes from a larger cross-section reservoir into a small-diameter extrusion die, an entry convergent flow is formed due to the abrupt contraction of the flow section. The flow at the entrance normally comprises of extensional flow and shear flow. Hence, APe, , = AP e + AP s
(3)
where AP e and APs are the pressure drops contributed by extensional flow and shear flow, respectively. In the cases of relatively large crosssection area contractions or large entry angles at the entrance, AP e is the major contribution of APen. Recently, a number of mathematical models [25] have been proposed to predict and estimate the pressure drops of visco-elastic flow at the entrance. In the present study, Bagley drawing method [2] has been used to determine the values of AP~n. Figure 1 shows the dependency of APen on T a-
,..... //
s o
a
4
/ ~ ///[3
~-8 3
or,
ZXP = AP~n a + ZXPa
=
8
2
"'
1
2
° --=--240°C
~.( ........... ~ / /"' ,~ / ~ /.,..~/
"D 245 °C
/
0
-~- 250=C
7
0
200
~ l ~ .---~ 400 600 800 1000 Shear Rate, ~a (s")
Figure I. Entrance pressure drop APen versus Y a 10 %-
<3 £
a
1
I1.
2=u
~
-[]-245 °c
It can be
observed that APen increases as y a increases. When T a is held constant, APen decreases with lowering the processing temperature (7). In the lower temperature range, the change in APe, is more
0.1
10
100
1000
Shear Rate, ~a (s-]) Figure 2. Entrance pressure drop APen versus y a in log-log scale
Z Z . L i a n g e t aL / J o u r n a l o f M a t e r i a l s Processing T e c h n o l o g y 4 8 ( 1 9 9 5 ) 7 0 7 - 7 1 2
709
20 18 16
- = - T = 2 5 0 ° C , r a = 97.3 s - 1 -~- T = 250"C, ra = 501.9 s-1 -~r--T = 240"C, ra = 97.3 s-1
%
14 -'~- T = 240°C, ra = 501.8 s-1
~- 12 O a
o
lO
t,l
tn =
8
ft..
6
............
.D
I--
4 2 I
I
5
10
I 15 L / D ratio
I
t
I
20
25
30
Figure 3. Total pressure drop AP versus L/D ratio
Thus the relationship between APen and y a may be considered obeying the power law, i.e. (4)
APe, , = A y a n
The sample flow curves in logarithmic coordinates are shown in Figure 4. It can be observed that the experimental data deviate from linearity. This implies the sample flow does not well agree with the power law. Nevertheless, ~, is getting larger as y a increases. W h e n y a reaches a certain
where A and B are the constants of the power law. Under the experimental conditions, we get
level, r w will decrease for higher processing temperature. Further investigation shows that the
A = 1.3 ~ l . g M P a ' s
relationship between r w and + a is linear (see Figure
and
B = 1.1 ~ 1.2
5): :3.2. Flow properties In Figure 3, the flow properties of the polymer melt at 240 °C and 250 °C can b e observed. For short die length, L/D = 10, the ratio of APen to AP is as high as 60°/o. Therefore, in the determination of shear stress at the die wall (~w), the following correction for entrance effect should be taken:
xw =
4L
(5)
rw = a + f l ' Y a
(6)
where a and ,8 are constants. Under the experimental conditions, o~has a value of about 2 x 104 Pa and f l = 0.45 ~ 0.84. The magnitude o f f l i s higher for lower extrusion temperature, but changes in the extrusion temperature causes no changes in a. The dependency of apparent shear viscosity r/a on shear rate Y a is shown in Figure 6.
710
ZZ. Liang et aL / Journal of Materials Processing Technology 48 (1995) 707-712
At lower apparent shear rate (Y a
<300 s'~),
12
qa is
10
sensitive to changes in Y a. This characteristic is more significant at lower extrusion temperature. The
I//~~
~
,
8
viscous flow curves tend to flatten when Y a is larger than 300 s -1, this shows the dependency of 9a on Ya is weakened over the respective range. Comparatively, the viscous flow curve changes more gradually at 250 °C. 3.3. Effects of temperature on apparent s h e a r viscosity Figure 7 illustrates the relationship between the extrusion temperature T and the apparent shear viscosity qa of the sample melt. W h e n holding the extrusion speed constant, r/a will decrease with higher the extrusion temperature T. The responsiveness of r/a on T is more significant at higher extrusion speed (v). However, at lower extrusion speed and T < 245 °C, increase in T will cause sigjaificant decrease in qa. On the other hand, 7/~ tends to be stable for T > 245 °C.
~=
2
4 . - 2 5 0 °C
U3
0
0
=
I
500
1000
ShearRate,~'a(s-l) Figure 5. Shear stress at the die wall fwversus Yo
60O .%-
1000 4=- 2 4 0 ° C
-t240
°C
5O0
-[~ 245 °C
400
-*.- 250 °C
II
43- 2 4 5 ° C
> 300
-.- 250°C
200 "o
lOO
IIII
i Ill
IIII IIII
/~
I*1.
IIIII
IIIII
IIIII
O3
<
100 0
I
0
500
I
1000
Shear Rate, "fa (s-l) 09
lO
Figure 6. Apparent shear viscosity 7/, versus Y a 10
100
1000
Shear Rate, "fa (s-l) Figure 4. Shear stress at the die wall *w versus y a in log-log scale
It was also observed that the dependency of the apparent shear viscosity r/a on the extrusion temperature T can be affected by the extrusion speed v. Moreover, the result shows that the relationship
ZZ. Liang et aL / Journal of Materials Processing Technology 48 (1995) 707-712
between log r/a and 1/7" (T' is in absolute temperature scale) is nonlinear (see Figure 8). Therefore, the relationship between the apparent shear viscosity and the extrusion temperature does not agree with the Arrhenius equation. 200 180 160
#
711
It was found that the apparent shear viscosity ~?a is high and the sample melt cannot be effectively extruded at the extrusion temperature lower than 240 °C. When the extrusion temperature is higher than 250 °C, ~?a will decrease rapidly. Gas bubbles will formed or fluctuations in pressure will occur. Hence, the quality of extrudate appearance and the flow stability will be affected. Therefore, the suitable processing temperature range should be between 240
250 °C.
140 41
120 •~
100
o
80
-l-V=40 mm/min
60 40 20
~3
--D-V=50 mm/min
0 240
245
250
Temperature, T (°C)
Figure 7. Apparent shear viscosity r/a versus T 1000
# Itl O
"N 100
t
-m-v=40 mm/min V = 50 mm/min
-D-
D.
<
10
1.91
[
t
I
I
1.92
1.93
1.94
1.95
1/T' (10-s IC1) Figure 8. Apparent shear viscosity qa in log scale versus I/T'
4. DISCUSSION
At the entrance of the convergent flow, the sample melt is accelerated and the polymer molecular chains experienced pulling actions along the flow direction. The sample contained 30% of glass fibres and the glass fibres will be oriented in the convergent flow direction. The molecular chains and the glass fibres are stretched and resulted in alignment. In addition, due to the contribution from shear flow at the entrance, the viscous energy dissipation and the elastic energy storage are significant. Hence the entrance pressure losses are remarkable. From the polymer rheologlcal point of view, flow occurs when slipping of the polymer molecular chains take place. In other words, the flow resistance will depend on the motion of polymer molecular chains and the entanglement force between large molecules. At a constant extrusion temperature, the disentanglement level in the sample molecules and the overall glass fibre alignment are greater for higher extrusion speed (shear rate). Thus the entrance pressure losses will also increase. At a constant extrusion speed, increase in temperature will increase the mobility of molecular chains and weaken the entanglement force between large molecules. Thus, this causes APen to decrease (Figure 1). At lower extrusion temperature, the apparent shear viscosity of the sample melt is relatively higher and the adhesion force between the polymer melt and the glass fibres is stronger. As a result, the fluctuation in pressure is not significant in the extrusion process and the relationship between ~'w and 7 a is closely linear (Figure 5). At higher extrusion temperature, the viscosity of the sample melt is relatively lower and the adhesion force between the polymer melt and the glass fibres is
712
Z Z . Liang et aL / Journal o f Materials Processing Technology 48 (1995) 707-712
weakened. Under the shearing and stretching action, the glass fibres and the polymer melt do not adhere permanently, therefore pressure fluctuations are introduced. When Y a is further increased, stress oscillation will occur. Kutty and his colleagues [6] examined the effects of short glass fibres on the changing flow actions in thermoplastic polyurethane melt. It was found that under certain temperature and shear rate, the existence of glass fibre will increase the nonNewtonian behaviour of the melt. For most polymers, polymer melt basically follows the power law under normal processing conditions. In this study, the glass fibre reinforced PBT has approximately linear relationship between zw and 7a. Thus, it indicates that the existence of glass fibres has significant effects on the flow properties of polymer melt. At lower extrusion rate, the shearing effect is insufficient to disentangle the molecular chains and destroy the adhesion between polymer melt and glass fibre. At lower extrusion temperature, the apparent shear viscosity r/,, of the sample melt is relatively high. Increase in extrusion speed, the disentanglement and aligmnent of molecular chains and glass fibres, as well as the flow properties will be improved. The apparent shear viscosity qa of the sample melt will sharply decrease as illustrated in Figure 6. Furthermore, due to the reasons mentioned above, the apparent shear viscosity q~ of the glass fibre reinforced PBT is more sensitive to the change in temperature (see Figure 7 and 8). The findings of this study is similar to the results obtained in reference [6].
5. CONCLUSIONS Under the experimental conditions, the entrance effect of the sample melt is significant and the relationship APen between 3',~ followed the power law. However, it was found that the flow of the sample melt does not agree with the power law. At lower extrusion temperature, the relationship between r w and ~{a is approximately linear. At higher extrusion temperature, shearing stress
oscillation will occur. Therefore, the suitable extrusion temperature range for the sample is 240 ~ 250 °C. At lower extrusion temperature and speed (T = 240 °C, v = 40 mm/min), rla of the sample melt is sensitive to Ya. It was also found that the extrusion temperature has significant effects on J/a and their relationship does not agree with the Arrhenius equation. Moreover, the presence of glass fibre has significant effects on the flow behaviours of the PBT melt. The effects may be mainly caused by the entrance flow properties, and the glass fibre orientation and spreading level at the die. The exact mechanism will be examined in the future studies. Acknowledgment: We would like to thank Dr. J.N. Ness, the University of Hong Kong, for his support in this study.
REFERENCES
1. 2.
3.
4.
5.
6.
C.D. Han, Rheology in Polymer Processing, Academic Press, New York, 98 (1976). E.B. Bagley, End Corrections in the Capillary Flow of Polyethylene, J. Appl. Phys., 28, 624 (1957). F.N. Cogswell, Converging Flow of Polymer Melts in Extrusion Dies, Polym. Eng. Sci., 12, 64 (1972). A.G. Gibson and G.A. Williamson, Shear and External Flow of Reinforced Plastics in Injection Moulding II., Polym. Eng. Sci., 25, 980 (1985). J.Z. Liang, X.L. Sun and G.J. Tang, Research On Pressure Drop of the Flow of Polymer Melt Through Conical Dies, Proceedings of ChinaJapan Inter. Congr. on Rheology, Beijing University Press, China, 190 (1991). S.K.N. Kutty, P.P. De and G.B. Nando, Rheology of Short Kevlar Fiber-reinforced Thermoplastic Polyurethane, Rubber Compos. Proc. Appl., 15, 23 (1991).
ELSEVIER
Journal of Materials Processing Technology 48 (1995) 707-712
Materials Processing Technology
A study of the flow properties of glass fibre reinforced PBT in extrusion J.Z. Liang, C.Y. Tang
and W.B. Lee**
Department of Chemical Machinery, South China University of Teclmology Guangzhou 510641, People's Republic of China Department of Manufacturing Engineering, Hong Kong Polytechnic, Hong Kong
The extrusion shaping process of polymer materials is a course of polymer deformation, molten flow and solidification shaping. The melt flow character is one of the important parameters of polymer process-ability; it is usually expressed by a viscosity or flow curve. In the present study, the end effect and the flow properties of glass fibre reinforced PBT melt P O C A N B 4 2 3 5 during extrusion have been investigated by using a R h e o v i s 2100 capillary rheometer. It has been found that the suitable extrusion temperature range of the sample is 240 ~ 250 °C. Within the apparent shear rate ranged from 50 to 1000 s 1, the flow properties of the sample melt do not obey the power law. The relationship between the shear stress at the wall and the apparent shear rate is approximately linear. Under the lower extrusion temperature and apparent shear rate, the apparent shear viscosity of the sample melt has been found to be sensitive to the apparent shear rate. The dependence of the apparent shear viscosity on temperature does not agree with the Arrhenins equation. The entry effect of the sample melt is significant and the relationship between the entrance pressure drop and the apparent shear rate basically obeys the power law under the experimental conditions.
1. INTRODUCTION
2. EXPERIMENTAL INVESTIGATION
Polybutylene terephthalate (PBT) is made from butylene glycol and di-methyl terephthaiate. It has desirable properties of low processing costs, good creep, fatigue, and chemical resistance, and good mouldability and dimensional stability. There are many applications of PBT in industries; for example, electrical components, automotive body components, distributor caps, and many metal replacement. In order to improve the physical properties of end products, PBT is filled with a fixed proportion of glass fibres during the manufacturing process. During the processing and shaping of PBT, the filled glass fibres have certain effects on the flow properties of the polymer melt. The significance of these effects would depend on the orientation of glass fibres along the flow and the flow stability of the melt. However, there are only limited publications on the related studies. In this paper, the end effect and the flow properties of glass fibre reinforced PBT melt P O C A N B 4 2 3 5 during extrusion will be discussed.
2.1. Sample The PBT sample, P O C A N B4235, was manufactured by Mobay Corporation of the United States. They were light grey pellets which were filled with 3 0 % glass fibre to improve their mechanical properties. The average density of the sample Co) was 1050 kg m e .
2.2. Apparatus and methods The main experimental apparatus was a capillary rheometer, R h e o v i s 2100 b y Ceast Corporation of Italy. A pressure sensor was located at the top of the piston to measure the total pressure drop in the extrusion process. The capillary tubes had diameter of 1 m m and length to diameter ratios (L/D) are 10, 20, 30, and 40 respectively. The entry angle was set to 90 °. In order to examine the flow properties in the actual processing conditions, the extrusion temperature (7) was varied from 230 to 2 6 0 °C. The piston speeds (v) were in the range of 4 ~ 75
0924-0136/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved 0924-0136(94)01712-A
SSD!
708
ZZ.
Liang et
aL / Journal o f Materials Processing Technology 48 (1995) 707-712
m/minwhere the corresponding shear rates ( y , )
sensitive to the change in "/a Further investigation
were in the range of 50 ~ lO00 s -1. shows that log APen and log T a have approximately linear relationship (see Figure 2). 3. RESULTS 7 3.1. End pressure losses
During the extrusion of polymer melt, the total pressure drop (AP) includes an entry pressure drop (APen), a pressure drop in the die ( A P e ) , and an exit pressure loss (APex), that is A P = A P e n + APe+
APex
(1)
(2)
APen a is called the end pressure drop. It represents the elastic property of the melt in the flow. For most polymer melts, APex are much smaller than APen a [1], so &Pen may be approximated by APen a. When a polymer melt passes from a larger cross-section reservoir into a small-diameter extrusion die, an entry convergent flow is formed due to the abrupt contraction of the flow section. The flow at the entrance normally comprises of extensional flow and shear flow. Hence, APe, , = AP e + AP s
(3)
where AP e and APs are the pressure drops contributed by extensional flow and shear flow, respectively. In the cases of relatively large crosssection area contractions or large entry angles at the entrance, AP e is the major contribution of APen. Recently, a number of mathematical models [25] have been proposed to predict and estimate the pressure drops of visco-elastic flow at the entrance. In the present study, Bagley drawing method [2] has been used to determine the values of AP~n. Figure 1 shows the dependency of APen on T a-
,..... //
s o
a
4
/ ~ ///[3
~-8 3
or,
ZXP = AP~n a + ZXPa
=
8
2
"'
1
2
° --=--240°C
~.( ........... ~ / /"' ,~ / ~ /.,..~/
"D 245 °C
/
0
-~- 250=C
7
0
200
~ l ~ .---~ 400 600 800 1000 Shear Rate, ~a (s")
Figure I. Entrance pressure drop APen versus Y a 10 %-
<3 £
a
1
I1.
2=u
~
-[]-245 °c
It can be
observed that APen increases as y a increases. When T a is held constant, APen decreases with lowering the processing temperature (7). In the lower temperature range, the change in APe, is more
0.1
10
100
1000
Shear Rate, ~a (s-]) Figure 2. Entrance pressure drop APen versus y a in log-log scale
Z Z . L i a n g e t aL / J o u r n a l o f M a t e r i a l s Processing T e c h n o l o g y 4 8 ( 1 9 9 5 ) 7 0 7 - 7 1 2
709
20 18 16
- = - T = 2 5 0 ° C , r a = 97.3 s - 1 -~- T = 250"C, ra = 501.9 s-1 -~r--T = 240"C, ra = 97.3 s-1
%
14 -'~- T = 240°C, ra = 501.8 s-1
~- 12 O a
o
lO
t,l
tn =
8
ft..
6
............
.D
I--
4 2 I
I
5
10
I 15 L / D ratio
I
t
I
20
25
30
Figure 3. Total pressure drop AP versus L/D ratio
Thus the relationship between APen and y a may be considered obeying the power law, i.e. (4)
APe, , = A y a n
The sample flow curves in logarithmic coordinates are shown in Figure 4. It can be observed that the experimental data deviate from linearity. This implies the sample flow does not well agree with the power law. Nevertheless, ~, is getting larger as y a increases. W h e n y a reaches a certain
where A and B are the constants of the power law. Under the experimental conditions, we get
level, r w will decrease for higher processing temperature. Further investigation shows that the
A = 1.3 ~ l . g M P a ' s
relationship between r w and + a is linear (see Figure
and
B = 1.1 ~ 1.2
5): :3.2. Flow properties In Figure 3, the flow properties of the polymer melt at 240 °C and 250 °C can b e observed. For short die length, L/D = 10, the ratio of APen to AP is as high as 60°/o. Therefore, in the determination of shear stress at the die wall (~w), the following correction for entrance effect should be taken:
xw =
4L
(5)
rw = a + f l ' Y a
(6)
where a and ,8 are constants. Under the experimental conditions, o~has a value of about 2 x 104 Pa and f l = 0.45 ~ 0.84. The magnitude o f f l i s higher for lower extrusion temperature, but changes in the extrusion temperature causes no changes in a. The dependency of apparent shear viscosity r/a on shear rate Y a is shown in Figure 6.
710
ZZ. Liang et aL / Journal of Materials Processing Technology 48 (1995) 707-712
At lower apparent shear rate (Y a
<300 s'~),
12
qa is
10
sensitive to changes in Y a. This characteristic is more significant at lower extrusion temperature. The
I//~~
~
,
8
viscous flow curves tend to flatten when Y a is larger than 300 s -1, this shows the dependency of 9a on Ya is weakened over the respective range. Comparatively, the viscous flow curve changes more gradually at 250 °C. 3.3. Effects of temperature on apparent s h e a r viscosity Figure 7 illustrates the relationship between the extrusion temperature T and the apparent shear viscosity qa of the sample melt. W h e n holding the extrusion speed constant, r/a will decrease with higher the extrusion temperature T. The responsiveness of r/a on T is more significant at higher extrusion speed (v). However, at lower extrusion speed and T < 245 °C, increase in T will cause sigjaificant decrease in qa. On the other hand, 7/~ tends to be stable for T > 245 °C.
~=
2
4 . - 2 5 0 °C
U3
0
0
=
I
500
1000
ShearRate,~'a(s-l) Figure 5. Shear stress at the die wall fwversus Yo
60O .%-
1000 4=- 2 4 0 ° C
-t240
°C
5O0
-[~ 245 °C
400
-*.- 250 °C
II
43- 2 4 5 ° C
> 300
-.- 250°C
200 "o
lOO
IIII
i Ill
IIII IIII
/~
I*1.
IIIII
IIIII
IIIII
O3
<
100 0
I
0
500
I
1000
Shear Rate, "fa (s-l) 09
lO
Figure 6. Apparent shear viscosity 7/, versus Y a 10
100
1000
Shear Rate, "fa (s-l) Figure 4. Shear stress at the die wall *w versus y a in log-log scale
It was also observed that the dependency of the apparent shear viscosity r/a on the extrusion temperature T can be affected by the extrusion speed v. Moreover, the result shows that the relationship
ZZ. Liang et aL / Journal of Materials Processing Technology 48 (1995) 707-712
between log r/a and 1/7" (T' is in absolute temperature scale) is nonlinear (see Figure 8). Therefore, the relationship between the apparent shear viscosity and the extrusion temperature does not agree with the Arrhenius equation. 200 180 160
#
711
It was found that the apparent shear viscosity ~?a is high and the sample melt cannot be effectively extruded at the extrusion temperature lower than 240 °C. When the extrusion temperature is higher than 250 °C, ~?a will decrease rapidly. Gas bubbles will formed or fluctuations in pressure will occur. Hence, the quality of extrudate appearance and the flow stability will be affected. Therefore, the suitable processing temperature range should be between 240
250 °C.
140 41
120 •~
100
o
80
-l-V=40 mm/min
60 40 20
~3
--D-V=50 mm/min
0 240
245
250
Temperature, T (°C)
Figure 7. Apparent shear viscosity r/a versus T 1000
# Itl O
"N 100
t
-m-v=40 mm/min V = 50 mm/min
-D-
D.
<
10
1.91
[
t
I
I
1.92
1.93
1.94
1.95
1/T' (10-s IC1) Figure 8. Apparent shear viscosity qa in log scale versus I/T'
4. DISCUSSION
At the entrance of the convergent flow, the sample melt is accelerated and the polymer molecular chains experienced pulling actions along the flow direction. The sample contained 30% of glass fibres and the glass fibres will be oriented in the convergent flow direction. The molecular chains and the glass fibres are stretched and resulted in alignment. In addition, due to the contribution from shear flow at the entrance, the viscous energy dissipation and the elastic energy storage are significant. Hence the entrance pressure losses are remarkable. From the polymer rheologlcal point of view, flow occurs when slipping of the polymer molecular chains take place. In other words, the flow resistance will depend on the motion of polymer molecular chains and the entanglement force between large molecules. At a constant extrusion temperature, the disentanglement level in the sample molecules and the overall glass fibre alignment are greater for higher extrusion speed (shear rate). Thus the entrance pressure losses will also increase. At a constant extrusion speed, increase in temperature will increase the mobility of molecular chains and weaken the entanglement force between large molecules. Thus, this causes APen to decrease (Figure 1). At lower extrusion temperature, the apparent shear viscosity of the sample melt is relatively higher and the adhesion force between the polymer melt and the glass fibres is stronger. As a result, the fluctuation in pressure is not significant in the extrusion process and the relationship between ~'w and 7 a is closely linear (Figure 5). At higher extrusion temperature, the viscosity of the sample melt is relatively lower and the adhesion force between the polymer melt and the glass fibres is
712
Z Z . Liang et aL / Journal o f Materials Processing Technology 48 (1995) 707-712
weakened. Under the shearing and stretching action, the glass fibres and the polymer melt do not adhere permanently, therefore pressure fluctuations are introduced. When Y a is further increased, stress oscillation will occur. Kutty and his colleagues [6] examined the effects of short glass fibres on the changing flow actions in thermoplastic polyurethane melt. It was found that under certain temperature and shear rate, the existence of glass fibre will increase the nonNewtonian behaviour of the melt. For most polymers, polymer melt basically follows the power law under normal processing conditions. In this study, the glass fibre reinforced PBT has approximately linear relationship between zw and 7a. Thus, it indicates that the existence of glass fibres has significant effects on the flow properties of polymer melt. At lower extrusion rate, the shearing effect is insufficient to disentangle the molecular chains and destroy the adhesion between polymer melt and glass fibre. At lower extrusion temperature, the apparent shear viscosity r/,, of the sample melt is relatively high. Increase in extrusion speed, the disentanglement and aligmnent of molecular chains and glass fibres, as well as the flow properties will be improved. The apparent shear viscosity qa of the sample melt will sharply decrease as illustrated in Figure 6. Furthermore, due to the reasons mentioned above, the apparent shear viscosity q~ of the glass fibre reinforced PBT is more sensitive to the change in temperature (see Figure 7 and 8). The findings of this study is similar to the results obtained in reference [6].
5. CONCLUSIONS Under the experimental conditions, the entrance effect of the sample melt is significant and the relationship APen between 3',~ followed the power law. However, it was found that the flow of the sample melt does not agree with the power law. At lower extrusion temperature, the relationship between r w and ~{a is approximately linear. At higher extrusion temperature, shearing stress
oscillation will occur. Therefore, the suitable extrusion temperature range for the sample is 240 ~ 250 °C. At lower extrusion temperature and speed (T = 240 °C, v = 40 mm/min), rla of the sample melt is sensitive to Ya. It was also found that the extrusion temperature has significant effects on J/a and their relationship does not agree with the Arrhenius equation. Moreover, the presence of glass fibre has significant effects on the flow behaviours of the PBT melt. The effects may be mainly caused by the entrance flow properties, and the glass fibre orientation and spreading level at the die. The exact mechanism will be examined in the future studies. Acknowledgment: We would like to thank Dr. J.N. Ness, the University of Hong Kong, for his support in this study.
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