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On the rheological characterization of polyethylene melts by using glass capillaries
Polymer Testing 18 (1999) 397–403
Test Apparatus
On the rheological characterization of polyethylene melts by using glass capillaries ´ ´ ´ Jose Perez-Gonzaleza,b,*, Lourdes de Vargasb Metalurgia y Materiales, ESIQIE-ESFM, Instituto Polite´cnico Nacional, C.P. 07300, Apdo. Postal 75-685, Me´xico D.F, Mexico b Departamento de Fı´sica, Escuela Superior de Fı´sica y Matema´ticas, Instituto Polite´cnico Nacional, C.P. 07300, Apdo. Postal 75-685, Me´xico D.F, Mexico
a
Received 23 March 1998; accepted 18 May 1998
Abstract The rheological characterization of polyethylene melts by using glass capillaries was investigated. Different polyethylene melts were characterized in a capillary rheometer by using glass capillaries and the results were compared with those obtained by using standard dies of tungsten-carbide, in addition to some homemade from aluminum. The data from glass capillaries showed an excellent agreement in all the cases with the data obtained with metal capillaries, and differences in the flow curves due to the influence of capillary materials were not detected. These results evidence the good performance and reliability of glass capillaries to perform the rheological characterization of this sort of polymer melt. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The measurement of the rheological properties of polymer melts is very important for those involved in the plastics processing industry, as well as for those working in research and development of polymer based products, since the knowledge of these properties is very useful for quality control and modelling of processes. Apart from compression molding in which polymers are processed at very low shear rates, * Corresponding author. Tel: ⫹ 52-5-729-6000, ext. 55032; Fax: ⫹ 52-5-586-2825; E-mail: [email protected] 0142-9418/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 3 6 - 1
398
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
most polymer processes are carried out at high shear rates, as are the cases for extrusion and injection molding. In such cases, the rheological characterization of polymer melts is usually performed by using rheometers in which a Poiseuille flow is generated. For a complete description of rheometers for molten polymers see Dealy [1]. The most commonly used type of rheometer is the capillary one, in which the flow may be generated at a constant pressure or at a constant shear rate, and the rheological parameters of the fluid are determined by using capillary dies. Such dies can be made up of different materials, those manufactured from metals being the most popular for example tungsten-carbide or stainless steel. In order to have a reliable rheological characterization of the melt by using this sort of rheometer, it is possible that a large number of capillaries will be required, since some corrections may be necessary, as for example Bagley [2] or Mooney [3] corrections. In addition, to study the influence of pressure on polymer melt viscosity, large capillaries are required which are not easily available. Therefore, an important investment of money in capillaries may be necessary depending on the capillary materials; the more resistent the material the more expensive the die. Thus, for example, comercial dies made up of tungsten-carbide have a better dimensional stability and wear resistance than stainless steel dies, but their prices are of the order of four times those of steel. In the present work, it is shown that the determination of rheological properties of polyethylene melts can be carried out by using glass capillaries. In analogy with tungsten-carbide, the borosilicate glass capillaries used in this work show high resistance to wear because of their high hardness and high dimensional stability under several processing conditions due to their low thermal expansion coefficient. Besides, this type of glass (commercially available under the Duran trademark) is highly resistant to water, neutral and acid solutions, as well as acids and organic solvents. Even in long periods of work, at temperatures higher than 100°C, glass is superior in chemical resistance to most metals. Because of their very low price as compared to metal capillaries, glass capillaries can be available in a wide range of length (L) to diameter (D) ratio (L/D), and different diameters. The results obtained in this work by using glass capillaries were compared with those obtained by using standard capillaries of tungsten-carbide, in addition to some homemade from aluminum. The data from glass capillaries showed an excellent agreement in all the cases with metal capillaries, establishing the good performance and reliability of glass capillaries to characterize this sort of polymer melt. 2. Experimental The polymers used in this work were low density polyethylene (LDPE) of industrial grade from Union Carbide and a linear low density polyethylene (LLDPE) from Aldrich. The flow experiments were carried out at T ⫽ 190°C by means of an Instron capillary rheometer in which the glass capillary dies were adapted. Borosilicate glass capillaries were used, all with an entry angle of 180°. In addition, tungsten-carbide (T-C) capillaries, and an aluminum one, were also used to compare with those results obtained by using the glass capillaries. The capillaries’ dimensions are reported in Table 1. The glass capillaries’ internal diameters were easily determined by using mercury, and were cut to the appropriate L/D by means of a diamond wheel. The way in which the glass capillaries were adapted to the rheometer is represented in Fig. 1.
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
399
Table 1 Capillary dimensions and materials L/D D D D D
⫽ ⫽ ⫽ ⫽
0.00067 m 0.0012 m 0.0012 m 0.00127 m
15 15 15 -
Material 20 20
61 60
Glass Glass Aluminum T-C
The adaptation system consists of a stainless steel support with a conical exit (3) to avoid sticking of the polymer due to die swell. This piece has a box for a silicon O-ring (4) to avoid polymer leakage. A second piece of stainless steel (3⬘) allows the seal between the capillary and the adaptation. The set of capillaries (1) and adaptations is put inside the nut for the dies (5), which are screwed on the rheometer barrel. The twisting of the nut in the barrel compresses the top piece (3⬘) which deforms the O-ring and produces the seal. Finally, the seal between the support and the barrel is provided by a teflon O-ring (2). The experiments were performed several times to ensure the reproducibilty of data. The obtained results are presented in the following section. 3. Results and discussion ˙ The data for the apparent shear rate (␥app) and the wall shear stress (w) without corrections are given by the standard equations: 32Q ˙ ␥app ⫽ D3
w ⫽
⌬pD 4L
(1)
(2)
where Q and ⌬p are the volumetric flow rate and the pressure drop respectively. From these quantities the apparent flow curves were obtained for different capillaries. Figs. 2 and 3 show the flow curves for glass and aluminum capillaries with L/D ⫽ 15, and capillaries of glass and tungsten-carbide (T-C) with L/D ⫽ 20 respectively. Observe in both cases the excellent agreement of the flow curves belonging to different capillary materials and same L/D, which shows the good performance of the glass capillaries at low pressures. The highest pressure drop in this case is of the order of 24 MPa. Fig. 4 shows the flow curves corresponding to the glass capillaries of L/D ⫽ 61 and a tungstencarbide capillary of L/D ⫽ 60 with an entry angle of 90°. These flow curves appear almost superimposed, giving evidence of the good performance of the glass capillaries at high pressures, and shows that data obtained with standard capillaries are fully reproducible with glass capillaries. In fact, glass capillaries are able to work at even higher pressures, as it happens when the experi-
400
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
Fig. 1. Schematic representation of the way in which glass capillaries are adapted to capillary rheometer. 1) Glass capillary; 2) polytetrafluoroethylene seal; 3 and 3⬘) stainless steel support for the capillary; 4) silicon O-ring; 5) nut of the rheometer.
ments are performed at lower temperatures or with more viscous polymers as shown in Fig. 5, where the flow curves for a LLDPE obtained with capillaries of L/D ⫽ 15 are plotted. The highest pressure drop reached during these experiments is of the order of 39 MPa. Note that the flow curves have the typical shape found in the literature for polyethylenes in the presence of slip. One point to pay attention to, is that related to the capillary wall’s ability to remove some of
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
WALL SHEAR STRESS (MPa)
2
401
LDPE T = 190° C
10–1 8 7 6
L/D = 15
5
D = 0.0012 m
4 GLASS 3
2 101
ALUMINUM
2
3
4 5
102
2
3
4 5
103
2
3
4
APPARENT SHEAR RATE (1/s)
Fig. 2. Flow curves of LDPE obtained with glass and aluminum capillaries of L/D ⫽ 15 at T ⫽ 190°C.
WALL SHEAR STRESS (MPa)
3 2
LDPE T = 190° C
10–1 7 6 5
L/D = 20
4
D = 0.0012 m; GLASS D = 0.00127 m; T-C
3 2 101
2
3
4 5
102
2
3
4 5
103
2
3
4
APPARENT SHEAR RATE (1/s)
Fig. 3. Flow curves of LDPE obtained with glass and tungsten-carbide capillaries of L/D ⫽ 20 at T ⫽ 190°C.
the heat generated by the viscous heating, since it can be important for polymers with a narrow range of processing temperatures such as polyvinyl chloride. In such a case, the increase in temperature may lead to degradation of the polymer. At low pressures, increase in temperature is negligible and flow curves for different capillary materials are very similar (see Figs. 2 and 3).
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
402 2
WALL SHEAR STRESS (MPa)
LDPE T = 190° C 10–1 9 8 7 6 5 4 D = 0.0012m; L/D = 61; GLASS
3
D = 0.00127m; L/D = 60; T-C
2 101
2
3
4 5 6
2
102
3
4
5 6
103
APPARENT SHEAR RATE (1/s)
Fig. 4. Flow curves for LDPE obtained with glass and tungsten-carbide capillaries of long L/D at T ⫽ 190°C. The tungsten-carbide one is an standard die with an entry angle of 90°.
7 6 LLDPE
WALL SHEAR STRESS (MPa)
5
T = 190° C 4 3
2 L/D = 15 D = 0.0012 m D = 0.00067 m 10–1 9 8 101
2
3
4 5
102
2
3
4 5
103
2
APPARENT SHEAR RATE (1/s)
Fig. 5. Flow curves for LLDPE obtained with glass capillaries of L/D ⫽ 15 at T ⫽ 190°C.
3
4
5
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
403
At high pressures, it can be observed that glass capillaries have similar performance to tungstencarbide capillaries (see Fig. 4). It is worth noting from the flow curves presented above that the capillary material does not have an strong influence on the flow behavior for the combinations of polymer-capillaries used in this work. However, the flow of other type of liquids or polymeric systems should be investigated in order to make wider the scope of the use of this type of dies. A drawback of glass capillaries that should be mentioned is their brittleness, in case of impact they can be broken. However, they are so inexpensive that they can be easily spared. 4. Conclusions The results presented in this work show the reliability of glass capillaries to perform rheological characterization of polymer melts, and open new possibilities to carry out their exhaustive characterization by using capillary rheometers. The major advantages of the use of glass capillaries is their high resistence to wear and thermal dimensional stability, in addition to their low cost and easy availability. Thus, capillary studies requiring a large number of capillaries can be performed at a very low cost, while important differences in flow behavior for these polymers with respect to that found when using metal capillaries are not expected. So, glass capillaries appear to be an appropriate alternative instead of metal capillaries to characterize polyethylene melts. Acknowledgements We want to acknowledge to Alfredo Maciel his help in preparing the experiments with the Instron rheometer. This work was supported by CONACYT and DEPI-IPN. JPG and Lde V are COFFA-fellowships. References [1] Dealy JM. Rheometers for Molten Plastics. Van Nostrand Reinhold, 1982. [2] Bagley EB. End corrections in the capillary flow of polyehtylene. J Appl. Phys 1957;28:624. [3] Mooney M. Explicit formulas for slip and fluidity. J Rheol 1931;2:210.
Test Apparatus
On the rheological characterization of polyethylene melts by using glass capillaries ´ ´ ´ Jose Perez-Gonzaleza,b,*, Lourdes de Vargasb Metalurgia y Materiales, ESIQIE-ESFM, Instituto Polite´cnico Nacional, C.P. 07300, Apdo. Postal 75-685, Me´xico D.F, Mexico b Departamento de Fı´sica, Escuela Superior de Fı´sica y Matema´ticas, Instituto Polite´cnico Nacional, C.P. 07300, Apdo. Postal 75-685, Me´xico D.F, Mexico
a
Received 23 March 1998; accepted 18 May 1998
Abstract The rheological characterization of polyethylene melts by using glass capillaries was investigated. Different polyethylene melts were characterized in a capillary rheometer by using glass capillaries and the results were compared with those obtained by using standard dies of tungsten-carbide, in addition to some homemade from aluminum. The data from glass capillaries showed an excellent agreement in all the cases with the data obtained with metal capillaries, and differences in the flow curves due to the influence of capillary materials were not detected. These results evidence the good performance and reliability of glass capillaries to perform the rheological characterization of this sort of polymer melt. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The measurement of the rheological properties of polymer melts is very important for those involved in the plastics processing industry, as well as for those working in research and development of polymer based products, since the knowledge of these properties is very useful for quality control and modelling of processes. Apart from compression molding in which polymers are processed at very low shear rates, * Corresponding author. Tel: ⫹ 52-5-729-6000, ext. 55032; Fax: ⫹ 52-5-586-2825; E-mail: [email protected] 0142-9418/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 3 6 - 1
398
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
most polymer processes are carried out at high shear rates, as are the cases for extrusion and injection molding. In such cases, the rheological characterization of polymer melts is usually performed by using rheometers in which a Poiseuille flow is generated. For a complete description of rheometers for molten polymers see Dealy [1]. The most commonly used type of rheometer is the capillary one, in which the flow may be generated at a constant pressure or at a constant shear rate, and the rheological parameters of the fluid are determined by using capillary dies. Such dies can be made up of different materials, those manufactured from metals being the most popular for example tungsten-carbide or stainless steel. In order to have a reliable rheological characterization of the melt by using this sort of rheometer, it is possible that a large number of capillaries will be required, since some corrections may be necessary, as for example Bagley [2] or Mooney [3] corrections. In addition, to study the influence of pressure on polymer melt viscosity, large capillaries are required which are not easily available. Therefore, an important investment of money in capillaries may be necessary depending on the capillary materials; the more resistent the material the more expensive the die. Thus, for example, comercial dies made up of tungsten-carbide have a better dimensional stability and wear resistance than stainless steel dies, but their prices are of the order of four times those of steel. In the present work, it is shown that the determination of rheological properties of polyethylene melts can be carried out by using glass capillaries. In analogy with tungsten-carbide, the borosilicate glass capillaries used in this work show high resistance to wear because of their high hardness and high dimensional stability under several processing conditions due to their low thermal expansion coefficient. Besides, this type of glass (commercially available under the Duran trademark) is highly resistant to water, neutral and acid solutions, as well as acids and organic solvents. Even in long periods of work, at temperatures higher than 100°C, glass is superior in chemical resistance to most metals. Because of their very low price as compared to metal capillaries, glass capillaries can be available in a wide range of length (L) to diameter (D) ratio (L/D), and different diameters. The results obtained in this work by using glass capillaries were compared with those obtained by using standard capillaries of tungsten-carbide, in addition to some homemade from aluminum. The data from glass capillaries showed an excellent agreement in all the cases with metal capillaries, establishing the good performance and reliability of glass capillaries to characterize this sort of polymer melt. 2. Experimental The polymers used in this work were low density polyethylene (LDPE) of industrial grade from Union Carbide and a linear low density polyethylene (LLDPE) from Aldrich. The flow experiments were carried out at T ⫽ 190°C by means of an Instron capillary rheometer in which the glass capillary dies were adapted. Borosilicate glass capillaries were used, all with an entry angle of 180°. In addition, tungsten-carbide (T-C) capillaries, and an aluminum one, were also used to compare with those results obtained by using the glass capillaries. The capillaries’ dimensions are reported in Table 1. The glass capillaries’ internal diameters were easily determined by using mercury, and were cut to the appropriate L/D by means of a diamond wheel. The way in which the glass capillaries were adapted to the rheometer is represented in Fig. 1.
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
399
Table 1 Capillary dimensions and materials L/D D D D D
⫽ ⫽ ⫽ ⫽
0.00067 m 0.0012 m 0.0012 m 0.00127 m
15 15 15 -
Material 20 20
61 60
Glass Glass Aluminum T-C
The adaptation system consists of a stainless steel support with a conical exit (3) to avoid sticking of the polymer due to die swell. This piece has a box for a silicon O-ring (4) to avoid polymer leakage. A second piece of stainless steel (3⬘) allows the seal between the capillary and the adaptation. The set of capillaries (1) and adaptations is put inside the nut for the dies (5), which are screwed on the rheometer barrel. The twisting of the nut in the barrel compresses the top piece (3⬘) which deforms the O-ring and produces the seal. Finally, the seal between the support and the barrel is provided by a teflon O-ring (2). The experiments were performed several times to ensure the reproducibilty of data. The obtained results are presented in the following section. 3. Results and discussion ˙ The data for the apparent shear rate (␥app) and the wall shear stress (w) without corrections are given by the standard equations: 32Q ˙ ␥app ⫽ D3
w ⫽
⌬pD 4L
(1)
(2)
where Q and ⌬p are the volumetric flow rate and the pressure drop respectively. From these quantities the apparent flow curves were obtained for different capillaries. Figs. 2 and 3 show the flow curves for glass and aluminum capillaries with L/D ⫽ 15, and capillaries of glass and tungsten-carbide (T-C) with L/D ⫽ 20 respectively. Observe in both cases the excellent agreement of the flow curves belonging to different capillary materials and same L/D, which shows the good performance of the glass capillaries at low pressures. The highest pressure drop in this case is of the order of 24 MPa. Fig. 4 shows the flow curves corresponding to the glass capillaries of L/D ⫽ 61 and a tungstencarbide capillary of L/D ⫽ 60 with an entry angle of 90°. These flow curves appear almost superimposed, giving evidence of the good performance of the glass capillaries at high pressures, and shows that data obtained with standard capillaries are fully reproducible with glass capillaries. In fact, glass capillaries are able to work at even higher pressures, as it happens when the experi-
400
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
Fig. 1. Schematic representation of the way in which glass capillaries are adapted to capillary rheometer. 1) Glass capillary; 2) polytetrafluoroethylene seal; 3 and 3⬘) stainless steel support for the capillary; 4) silicon O-ring; 5) nut of the rheometer.
ments are performed at lower temperatures or with more viscous polymers as shown in Fig. 5, where the flow curves for a LLDPE obtained with capillaries of L/D ⫽ 15 are plotted. The highest pressure drop reached during these experiments is of the order of 39 MPa. Note that the flow curves have the typical shape found in the literature for polyethylenes in the presence of slip. One point to pay attention to, is that related to the capillary wall’s ability to remove some of
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
WALL SHEAR STRESS (MPa)
2
401
LDPE T = 190° C
10–1 8 7 6
L/D = 15
5
D = 0.0012 m
4 GLASS 3
2 101
ALUMINUM
2
3
4 5
102
2
3
4 5
103
2
3
4
APPARENT SHEAR RATE (1/s)
Fig. 2. Flow curves of LDPE obtained with glass and aluminum capillaries of L/D ⫽ 15 at T ⫽ 190°C.
WALL SHEAR STRESS (MPa)
3 2
LDPE T = 190° C
10–1 7 6 5
L/D = 20
4
D = 0.0012 m; GLASS D = 0.00127 m; T-C
3 2 101
2
3
4 5
102
2
3
4 5
103
2
3
4
APPARENT SHEAR RATE (1/s)
Fig. 3. Flow curves of LDPE obtained with glass and tungsten-carbide capillaries of L/D ⫽ 20 at T ⫽ 190°C.
the heat generated by the viscous heating, since it can be important for polymers with a narrow range of processing temperatures such as polyvinyl chloride. In such a case, the increase in temperature may lead to degradation of the polymer. At low pressures, increase in temperature is negligible and flow curves for different capillary materials are very similar (see Figs. 2 and 3).
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
402 2
WALL SHEAR STRESS (MPa)
LDPE T = 190° C 10–1 9 8 7 6 5 4 D = 0.0012m; L/D = 61; GLASS
3
D = 0.00127m; L/D = 60; T-C
2 101
2
3
4 5 6
2
102
3
4
5 6
103
APPARENT SHEAR RATE (1/s)
Fig. 4. Flow curves for LDPE obtained with glass and tungsten-carbide capillaries of long L/D at T ⫽ 190°C. The tungsten-carbide one is an standard die with an entry angle of 90°.
7 6 LLDPE
WALL SHEAR STRESS (MPa)
5
T = 190° C 4 3
2 L/D = 15 D = 0.0012 m D = 0.00067 m 10–1 9 8 101
2
3
4 5
102
2
3
4 5
103
2
APPARENT SHEAR RATE (1/s)
Fig. 5. Flow curves for LLDPE obtained with glass capillaries of L/D ⫽ 15 at T ⫽ 190°C.
3
4
5
J. Pe´rez-Gonza´lez, L. de Vargas / Polymer Testing 18 (1999) 397–403
403
At high pressures, it can be observed that glass capillaries have similar performance to tungstencarbide capillaries (see Fig. 4). It is worth noting from the flow curves presented above that the capillary material does not have an strong influence on the flow behavior for the combinations of polymer-capillaries used in this work. However, the flow of other type of liquids or polymeric systems should be investigated in order to make wider the scope of the use of this type of dies. A drawback of glass capillaries that should be mentioned is their brittleness, in case of impact they can be broken. However, they are so inexpensive that they can be easily spared. 4. Conclusions The results presented in this work show the reliability of glass capillaries to perform rheological characterization of polymer melts, and open new possibilities to carry out their exhaustive characterization by using capillary rheometers. The major advantages of the use of glass capillaries is their high resistence to wear and thermal dimensional stability, in addition to their low cost and easy availability. Thus, capillary studies requiring a large number of capillaries can be performed at a very low cost, while important differences in flow behavior for these polymers with respect to that found when using metal capillaries are not expected. So, glass capillaries appear to be an appropriate alternative instead of metal capillaries to characterize polyethylene melts. Acknowledgements We want to acknowledge to Alfredo Maciel his help in preparing the experiments with the Instron rheometer. This work was supported by CONACYT and DEPI-IPN. JPG and Lde V are COFFA-fellowships. References [1] Dealy JM. Rheometers for Molten Plastics. Van Nostrand Reinhold, 1982. [2] Bagley EB. End corrections in the capillary flow of polyehtylene. J Appl. Phys 1957;28:624. [3] Mooney M. Explicit formulas for slip and fluidity. J Rheol 1931;2:210.