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Polypropylene chain scissions and molecular weight changes in multiple extrusion
Polymer Degradation and Stability 60 (1998) 3342 % 1998 Published by Elsevier Saence Limited. All rights reserved Printed in Northern Ireland CC
014l-3910/98/$19.00
Polypropylene chain scissions and molecular weight changes in multiple extrusion* V. A. Gonzilez-Gonzilez, Centro de Investigacidn (Received
G. Neira-Velizquez
& J. L. Angulo-SBnchezl
en Quimica Aplicada, Saltillo, Coahuila, 25 100 Mexico
6 July 1996; revised version
received
10 September
1996; accepted
8 October
1996)
Polypropylene degradation, during multiple extrusion cycles at different temperatures, is studied by following chemical and molecular weight changes. The former was measured using FTIR and the latter was determined by size exclusion chromatography. The molecular weight and polydispersity reduction, and the lack of carbonyl groups and gel formation, show that chain scission and not oxidation is the dominant degradation reaction under the experimental conditions. Molecular weight distribution curves suggest that chain scissions occur close to the middle part of the macromolecule, in agreement with the Bueche theory for mechanical degradation. Three zones with different degradation-temperature behavior were observed: no appreciable degradation occurred between melt temperature and 230°C; from 230 to 250°C there was a rapid increase of chain scissions with rise of temperature; and finally from 250 to 270°C the number of chain scissions did not change with temperature increase. 0 1998 Published by Elsevier Science Limited. All rights reserved
1 INTRODUCTION
recycling in order to determine such things as the number of extrusion cycles that a polymer might undergo before loosing mechanical properties, or the conditions under which the polymer will not degrade. There are always molecular weight averages and molecular weight distribution (MWD) changes.7,8 Depending on the polymer type and even the catalyst used for production, many radical reactions9,i0 can take place during processing. In the case of polyethylene, chain scission and recombination are observed yielding crosslinked material as well as low molecular weight chains.8 However, for polypropylene (PP) the main events reported are chain breaking and polydispersity reduction9 although no information regarding the number or location of chain scissions is reported. The main degradation mechanisms which may occur during processing are thermal, mechanical, oxidative or combinations of these mechanisms. Different polymer and extrusion parameters are important for degradation, such as molecular weight, viscosity, temperature, shear stress, extruder and die geometry, and screw velocity. There are reports on the molecular weight changes taking place during different processes, particularly in the
Extrusion is a common process used to transform polymers, not only for end product manufacturing, but also for the preparation of polymers with specific properties. A good example is polypropylene (PP) which is submitted to extrusion degradation in the controlled rheology process to obtain polymers (CR-PP) with the desired flow properties.ip3 Another common process is polymer mixing, using recycled and unprocessed macromolecules, for waste reduction or recycling. In this case, the low molecular weight species in the partially degraded recycled material can change both rheologic and mechanical properties.4-6 In general during extrusion the macromolecule is submitted to thermal and mechanical stresses which promote chemical reactions, leading to polymer degradation and a lowering of the physical properties of the polymer. This knowledge is of great importance with the increasing need of polymer This work is part of the Masters Degree Thesis of G. NeiraVelazquez at the Coahuila State Autonomal University (U. A. de C.). tTo whom correspondence
should be addressed. 33
V. A. Gonzdez-Gonzdez
34
CR-PP and PP degraded via multiple processing.“p’3 However, most of these papers mainly evaluate mechanical and rheologic properties and there is a dearth of information about the molecular weight change during multiple extrusion at different die temperatures, especially in the absence of peroxides. This information may be important in selecting processing conditions either to promote degradation or to prevent it. The present work aims to study the chain scissions occurring to commercial polypropylene subjected to multiple extrusion cycles at various die temperatures.
et al.
2.3 Degradation experiments
2 EXPERIMENTAL
Die temperatures for processing were defined and set to 240 and 270°C. The lower limit was selected according to exploratory degradation results and the high temperature limit was chosen because of thermogravimetry results showing excessive thermal degradation above 275°C. Virgin polypropylene was passed through the extruder, at the conditions given in Table 2; the extrudate was quenched in a cold water bath (51O’C) and air dried. The extrudate was pelletized and later used for multiple extrusion. This procedure was repeated up to 19 times. Table 3 gives the codes, die temperatures and number of extrusion cycles for the polymer selected for characterization.
2.1 Polymer
2.4 Characterization
Commercial polypropylene (FINA OIL Co.) was used as received. The polymer physical characteristics were determined at our laboratory, and are shown in Table 1.
2.4.1 Infrared spectroscopy (FTIR) Polypropylene films: each sample in Table 3 was hot pressed (T= 200°C and 15 ton ram force) into a polypropylene film which was cooled under pressure. The infrared spectra of these films was found using Nicolet FTIR-710 equipment. The 150& 1800cmp1 region was analyzed for any change in the carbonyl groups.
2.2 Extrusion A single-screw extruder, Killion model 315A, (24:l L/D ratio) with a 2mm diameter circular die, was used for multiple processing. The screw speed was fixed at 50rpm for all experiments. The extruder had four temperature control zones and one melt pressure measuring device. Three control zones correspond to the screw feed, transition and metering sections, and the fourth zone to the die. The temperature profiles for the different experiments are noted in Table 2. Table 1. Physical characteristics of the polypropylene Density Melt flow M, x IO-’ M, x 10m4 Polydispersity index MwIM, (g/cm3) (g/lOmin) 0.902
3.48
3.6
6.6
5.45
Table 2. Extruder temperature profile for multiple extrusion experiments Barrel zone I (“C) 195 195 195 195 195
Barrel zone 2 (“C)
Barrel zone 3 (“C)
Die zone (“C)
215 215 215 215 215
225 225 225 225 225
240 245 250 260 270
methods
Table 3. Polypropylene molecular weight characteristics determined by SEC Cycle number 0 5 8 12 16 19 5 8 12 16 19 5 8 12 16 19 5 8 12 16 19 5 8 12 16 19
Temperature iv 240 240 240 240 240 245 245 245 245 245 250 250 250 250 250 260 260 260 260 260 270 270 270 270 270
M,
Mn
MwIMn
no
5.45 4.89 4.63 3.99 3.87 3.67 4.67 3.93 3.95 3.51 3.12 4.24 3.83 3.77 3.37 3.06 4.04 3.53 3.72 3.25 2.89 3.95 3.43 3.46 3.05 2.76
0.0 0.048 0.049 0.074 0.154 0.146 0.047 0.017 0.213 0.268 0.313 0.011 0.068 0.264 0.283 0.374 0.016 0.072 0.285 0.30 0.382 0.024 0.074 0.240 0.315 0.388
<10-5 x IO-4 3.60 3.08 2.91 2.45 2.21 2.12 2.95 2.55 2.15 1.83 1.57 2.77 2.37 1.97 1.74 1.47 2.62 2.17 1.91 1.64 1.38 2.54 2.11 1.84 1.53 1.31
6.61 6.30 6.30 6.15 5.73 5.71 6.31 6.49 5.45 5.21 5.03 6.53 6.18 5.23 5.15 4.81 6.50 6.16 5.14 5.06 4.78 6.45 6.15 5.33 5.02 4.76
MFI (g/lOmin) 3.48 5.62 6.26 8.36 11.69 14.66 4.80 6.88 13.33 19.95 24.78 6.10 8.90 14.70 24.30 26.30 6.40 10.43 18.63 27.35 30.43 8.54 13.63 22.43 29.37 33.42
Polypropylene chain scissions and molecular weight changes in multiple extrusion
l&O
1766 1733 1700 1667 IS33 WAVENUMBER
1600
l5&
1533
(Cm-‘)
1. Infrared traces of the carbonyl region for polypropylene after indicated extrusion cycles at 270°C.
2.4.2 Crosslinked polymer The presence of crosslinked material was determined by immersing a filter paper cartridge, containing approximately 250mg of sample, in xylene at the boiling temperature (124°C). The dissolution time was 12 h. The crosslinked (insoluble) material remaining on the filter was weighed; these results were confirmed by size exclusion chromatography.
2.4.3 Molecular weight distributions Molecular weight characteristics, of selected samples, were obtained in a Waters 15OC size exclusion chromatography (SEC) instrument equipped with differential refractometer detector and three Ultra Styragel columns having 106, lo4 A and linear mixture nominal porosity. The carrier solvent was 1,2,4trichlorobenze (TCB) at 145°C and 1 ml/min flow rate. All the samples were prepared as 0.1 wt% polypropylene solutions in TCB with 0.2 wt% antioxidant (hindered phenol) added to reduce the occurrence of oxidative degradation during sample preparation and analysis. The samples were heated at 135°C for 4 h to ensure complete dissolution, and filtered before injection into the SEC. The molecular weight averages of six replicates were calculated using a universal calibration curve constructed with narrow MWD polystyrene standards, the reported Mark-Houwink parameters* and a Millennium GPC software. 2.4.4 Meltj?ow index (MFI) The measurements of MFI were done with a Kayeness 7053 melt index equipment according to the ASTM D1238 method test condition ‘L’: 230°C; load 2160 g. The results are shown in Table 3. 2.4.5 Extrusion variables Melt pressure drop and power consumption were measured during the extrusion experiments. The data are analyzed based on molecular weight and cycle number.
80,000
P
42,!jOO
30,000
1 1 0
I 5
Polypropylene
molecular
weight, number
1 15
1 IO CYCLES
Fig. 2.
35
NUMBER
3
(N)
average (M”) as a function
of extrusion
cycles and temperature.
V. A. Gonzdez-Gonzdez
36
et al.
6
V 3.25
CYCLES
Fig. 3.
Molecular
weight, weight average (M,)
P
‘0
T= T= T= T= T=
240 245 250 260 270
C C C C C
NUMBER
of polypropylene
as a function
of extrusion
cycles and temperature.
6.0-
3 : Q 3.0-
220
235
250
265
30
T f“C)
Fig. 4.
3 RESULTS
Polypropylene
molecular
weight increment
AND DISCUSSION
(eqn (1)) with temperature
and extrusion
cycles.
3.2 Molecular weight averages and distribution
3.1 Carbonyl groups Figure 1 contains the FTIR spectrum of the carbony1 region which showed only a small ‘shoulder’ at ca. 1740cm-’ which did not change with multiple extrusion. These results prove that there are almost no oxidative reactions during the extrusion because of the absence of oxygen within the extruder. This behavior is in agreement with other authorsi4,i5 findings on polypropylene thermal and mechanical degradation.
The molecular weights, number (M,) and weight (M,) averages, for the samples are shown in Table 3. The changes in M, and M, are plotted against the cycle number (A’) for the different die temperatures in Figs 2 and 3. There is a small, lineal, reduction for M, on increasing the cycle number at 240°C (Fig. 2) but for higher temperatures the change is linear only up to eight cycles, showing a rapid change between 8 and 16 cycles, and a leveling off from 16 to 19 cycles. There is an
Polypropylene
chain scissions and molecular
apparent lower limit of around 47 000 after 19 cycles with only a slight dependence on the temperature. The change is greater for M, (Fig. 3); the decreasing behavior fits a second order equation. -. The apparent limiting value for it4, IS ca. 130 000. According to the mechanical degradation theory of Bueche16 the probability for chain scissions is higher for the high molecular weight chains to yield smaller molecules of about half the original size.
weight changes
Molecular
A%
=
[(x)Ni+l
-
I
I
I
6.0
5.0
4.0
weight distribution
(~)NilT=constant
(Mw)
curves for polypropylene,
after different
extrusion
cycles at 270°C.
6.OC A 8 V
5 6 12
CYCLES CYCLES CYCLES
.
16
CYCLES
Mw/Mn
3
235I
I
I 265
T I”C)
Fig. 6.
Polydispersity
37
The more evident reduction in M, (60%) is due to the fact that this average is more sensitive to the concentration of high molecular weight species than the M,, (12%). We defined a molecular weight increment (A M,) in eqn (1):
log
Fig. 5.
in multiple extrusion
change with temperature
for polypropylene.
2
Cl)
38
V. A. Gonzdez-Gonzdez
where: (MW)N is the molecular weight average at a given number of cycles and (LV~)~+~ that at the next measuring point at a given temperature. The AM, is plotted for the different temperatures in Fig. 4, two stages are observed for molecular weight change. At low temperatures (240 < T < 250°C) AM, increases with temperature and in the second stage spanning from 250 < T < 270°C -. M, IS almost constant. The curves corresponding to 12 cycles deviate from this behavior, but no explanation is offered at this point. The general behavior suggests that chain breaking depends
1A
5
et al.
upon temperature only for the 24&250°C range and at higher temperatures depends mainly on the number of cycles. Extrapolating in the low temperature region, a limiting value (230°C) was obtained; below this temperature and under these processing conditions there would be no chain scission. This result is in agreement with preliminary measurements where samples processed at 225°C show no measurable molecular weight reduction up to 15 cycles. Finally, the effect of the multiple processing on the molecular weight distribution is a narrowing of the curve due to the removal of the high
CYCLES1
8
8
CYCLES
V
12
CYCLES
0
16
CYCLES
235
250
265
280
T f"CJ
Fig. 7.
Polypropylene
average scission per chain depending
8
T-
*
T = 250
245
C
V
T =
260
C
Melt flow index changes
and extrusion
cycles.
C
CYCLES
Fig. 8.
on die temperature
for polypropylene
NUMBER
after different
IN)
extrusion
cycles and temperatures
39
Polypropylene chain scissions and molecular weight changes in multiple extrusion
molecular weight fraction, without modification of the low molecular weight end. Figure 5 shows the chromatogram traces of samples processed at 270°C after O-19 cycles. The loss of high molecular weight chains is evident while the low molecular weight region remains practically unchanged. This was observed at all processing temperatures and proves that degradation proceeds mainly by long
chain breaking. The absence of very low molecular weight species suggests that the chain breaking takes place far from the chain ends. Polydispersity behaves similar to AM, when plotted against temperature. In Fig. 6 polydispersity decreases on increasing temperature or number of cycles, and the two previously mentioned zones are also observed, though not as evident as in Fig. 4.
a0 ! I L2
Fig. 9.
Relationship
between
melt flow index and molecular weight, geometrical extrusion cycles and temperatures.
b
i
drop during extrusion
l
T = 245
C
*
7- = 2.50
c
l
T=26OC
115
Ib CYCLES
Fig. 10. Melt pressure
mean, for polypropylene
NUMBER
of polypropylene
(NI
at different
temperatures.
submitted
to multiple
V. A. Gonzdez-Gonzcilez et al.
40
3.3 Chain breaking as a function of temperature The number of chain scissions according to eqn (2):i7
(~1~) was calculated
(2)
na =
where IV,, is the molecular weight of the polypropylene after a given number of extrusion cycles and M,, that of the original polymer. The average number of chain scissions is plotted as a function of die temperature in Fig. 7. Here, the number of scissions per chain is zero up to ca. 230°C according to the M, results. At temperatures between 230 and 250°C there is a sharp increase of chain scissions, but at higher temperatures nR does not change. In the high temperature region only the cycle number is important for chain breaking. Mechanical chain breaking, according to Bueche,‘(j occurs due to tensions concentrating close to the middle section of the chain. These tensions are related to entanglements, the viscosity (q) and shear rate (v). The force (F”) exerted on the chain depends on these factors through the product 0y LV,*‘~. Here M, is the molecular weight between entanglements and p is the density. On increasing temperature the viscosity and shear rate decrease, therefore, chain breaking will be reduced and degradation should be lower. The viscosity reduction (on temperature raising) will also favor this.
However, our results show that degradation does not takes place up to ca. 230°C where it sharply increases until reaching 250°C and finally remains almost constant. We believe this behavior is due to a change in Me. At relative low temperatures (below 230°C) the molecules have several entanglement points, M, and P do not reach the energy level required for chain breaking, hence no degradation takes place. In the interval 230-250°C there is an abrupt change in degradation, indicating that chain scissions are taking place rapidly. This could happen if M, is increased (viscosity is lowered) implying a reduction of the entanglements per chain. The maximum molecular weight M, corresponds to half the total chain length, or only one entanglement per chain. We think that at 250°C the maximum M, has been reached, hence, further temperature increments will not affect nR and degradation will only depend on the time the molecule is submitted to the chemical and mechanically excited state (number of cycles). As several recombining steps should take place to form insoluble material, and no crosslinked polymer was detected, the results indicate that chain breaking is the only event taking place in agreement with the thermodynamics of scission versus crosslinking for polypropylene. 3.4 Melt flow index MFI The MFI shows a great dependence on the cycle number and die temperature. Figure 8 shows a
lO.O0
EXPERIMENTAL
l
CAL Cl/LATE/l
1.2'0~
7.5-
0.04 1.0
I
1.62
I
MW
Fig. 11.
Experimental
relationship
between
I
2.25
2.87
3.. 0
m /o-5
melt pressure drop and molecular weight (M,) calculated behaviour according to ref. 20.
for our polypropylene
samples,
and
Polypropylene chain scissions and molecular weight changes in multiple extrusion
second order fit for the data of MFI versus number of cycles (N). The curve is more pronounced at Some authors1*,i9 have higher temperatures. reported that melt flow index and molecular weight can be directly correlated (MFI 0: l/M,) for lineal polymers. When MFI was plotted against molecular weight (M,) third order lines were obtained. However, if we use the geometrical mean (Mwn) defined as M,,
=
(M, M,)“2
a straight line with good correlation in Fig. 9.
(3)
41
in the low molecular weight region (below 264000) may reflect the effect of the peroxide used in that experiment on the degradation kinetics. Polymer output rate decreased with increasing M,, but increases with rising temperature. These effects may be explained by considering that the polymer viscosity is lower for lower molecular weights or higher temperatures. Finally, power consumption was linearly reduced on increasing the cycle number or temperature. This may be related to the system viscosity which decreases on molecular weight reduction.
was obtained 4 CONCLUSION
In MFI = 4.3 - 3.5 In M,,
(4)
That is, according to our results, the polydispersity must be considered. 3.5 Extrusion variables The melt pressure drop (AP) was monitored during extrusion. In Fig. 10 it can be seen that AP decreases on raising the number of cycles or temperature. The lines are almost parallel, but at higher temperatures the slope changes between 4 that the molecular and 7 cycles. Considering weight is decreasing, the linear behavior is normal, but no explanation can be offered for the deviation. The pressure drop for all the samples is plotted against molecular weight (Mw) in Fig. 11, and we found a linear correlation: AP = 1.85 x 10-5K
Pabendiskas
- 1.77
(5)
reported two equations for the AP-M, behavior, a first order equation for M, > 264000 and a third order one for M, < 264000. In Fig. 11 it can be seen that the behavior is similar for both systems, in the high molecular weight region, linear with similar slopes. We consider that the difference between AP’s is due to the equipment characteristics. Pabendiskas used a static mixer unit in the extruder. Extrapolating the line obtained with our results, a limiting molecular weight of around 100 000 is obtained for AP=O. Considering that the pressure change is due to polymer degradation, this value implies that no degradation is taking place and is close to the limit values calculated with the M, versus N data. The behavior observed by Pabendiskas et d2’
No change in the polypropylene chemical structure was detected, and carbonyl groups were observed only in very low concentrations. This strongly suggests that no thermo-oxidative degradation occurs. The molecular weight distribution curves became narrow, and polydispersity was reduced with increasing cycle number or temperature. A similar reduction was observed for the molecular weight averages. There is an apparent M, limit between 100 000 and 130 000 below which no degradation occurs under the experimental conditions. Three zones for the effect of temperature on degradation were observed: first, below 230°C no degradation takes place; second, between 230 and 250°C the temperature is the defining degradation parameter; third, above 250°C but below the initial thermal degradation temperature (275”(Z), the temperature increase had no effect on the degradation. Based on these results, and the Bueche mechanical degradation theory, the polypropylene degradation during multiple extrusion is thought to be thermomechanical, caused by chain scissions (apparently promoted by chain entanglements). The highest molecular weight chains were preferentially broken during the degradation and the scissions may be located close to the macromolecule center. Only minor amounts of thermooxidative degradation, expressed as carbonyl groups, were measured and no recombination reactions occurred during the experiment, proved by the gel absence. The molecular weight and viscosity reduction affect the extrusion variables such as melt pressure drop, power consumption and melt flow index.
42
V. A. Gonzdez-Gonzdez
REFERENCES 11. I. Bellahcene, M. and Bounafa, N., Proc. Ann. Tech. Conf. SPE, 1991, p. 720. 2. Tzoganakis, C., Adv. Polym. Technol., 1989, 9, 321. 3. Ryu, S. H., Gogos, C. G. and Xanthos, M., Adv. Polym. Technol., 1992, 11, 121. 4. Dagh, S. S., Kelleher, P. G., Monroy, M., Patel, A. and Xanthos, M., Adv. Polym. Technol., 1990, 10, 125. 5. Staicu, D., Banica, G. and Stoica, S., Polym. Degrad. Stab., 1994, 46, 259. 6. Nguyen, K. T., Weber, L. and Hebert, L. P., Proc. Ann. Tech. Conf. SPE, 1993, p. 3432. I. Lew, R., Suwanda, D. and Balke, S. T., J. Appl. Polym. Sci., 1988, 35, 1049. 8. Lew, R., Suwanda, D. and Balke, S. T., J. Appl. Polym. Sci., 1988, 35, 1033. 9. Ying, Q., Zhao, Y. and Liu, Y., Makromol. Chem., 1991, 192, 1041.
12. 13. 14. 15. 16. 17. 18. 19. 20.
et al.
Ries, H. and Menges, G., Proc. Ann. Tech. Conf SPE, 1988, p. 339. Shibayama, M., Imamura, K. Y., Katoh, K. and Nomura, S., J. Appl. Polym. Sci., 1991, 42, 1451. Hinsken, H., Moss, S., Pauquet, J. R. and Zweifel, H., Polym. Degrad. Stab., 1991, 34, 279. Billiani, J. and Fleischmann, E., Polym. Degrad. Stab., 1990, 28, 67. Valenza, A. and La Mantia, F. P., Polym. Degrad. Stab., 1988, 20, 63. Al-Malaika. S.. Polvm. Plast. Technol. Eng., 1988,27, 261. Bueche, F., ‘J. Appi Polym. Sci., 1960, 4, 101. David, C., Trojan, M., Daro, A. and Demarteau, W., Polym. Degrad. Stab., 1992, 37, 233. Tzoganakis, C., Vlachopoulos, J. and Hamielec, A. E., Polym. Eng. Sci., 1988, 28, 170. Bremner, T. and Rudin, A., J. Appl. Polym. Sci., 1990, 41, 1617. Pabedinskas, A., Cluett, W. R. and Balke, S. T., Polym. Eng. Sci., 1989, 29, 993.
014l-3910/98/$19.00
Polypropylene chain scissions and molecular weight changes in multiple extrusion* V. A. Gonzilez-Gonzilez, Centro de Investigacidn (Received
G. Neira-Velizquez
& J. L. Angulo-SBnchezl
en Quimica Aplicada, Saltillo, Coahuila, 25 100 Mexico
6 July 1996; revised version
received
10 September
1996; accepted
8 October
1996)
Polypropylene degradation, during multiple extrusion cycles at different temperatures, is studied by following chemical and molecular weight changes. The former was measured using FTIR and the latter was determined by size exclusion chromatography. The molecular weight and polydispersity reduction, and the lack of carbonyl groups and gel formation, show that chain scission and not oxidation is the dominant degradation reaction under the experimental conditions. Molecular weight distribution curves suggest that chain scissions occur close to the middle part of the macromolecule, in agreement with the Bueche theory for mechanical degradation. Three zones with different degradation-temperature behavior were observed: no appreciable degradation occurred between melt temperature and 230°C; from 230 to 250°C there was a rapid increase of chain scissions with rise of temperature; and finally from 250 to 270°C the number of chain scissions did not change with temperature increase. 0 1998 Published by Elsevier Science Limited. All rights reserved
1 INTRODUCTION
recycling in order to determine such things as the number of extrusion cycles that a polymer might undergo before loosing mechanical properties, or the conditions under which the polymer will not degrade. There are always molecular weight averages and molecular weight distribution (MWD) changes.7,8 Depending on the polymer type and even the catalyst used for production, many radical reactions9,i0 can take place during processing. In the case of polyethylene, chain scission and recombination are observed yielding crosslinked material as well as low molecular weight chains.8 However, for polypropylene (PP) the main events reported are chain breaking and polydispersity reduction9 although no information regarding the number or location of chain scissions is reported. The main degradation mechanisms which may occur during processing are thermal, mechanical, oxidative or combinations of these mechanisms. Different polymer and extrusion parameters are important for degradation, such as molecular weight, viscosity, temperature, shear stress, extruder and die geometry, and screw velocity. There are reports on the molecular weight changes taking place during different processes, particularly in the
Extrusion is a common process used to transform polymers, not only for end product manufacturing, but also for the preparation of polymers with specific properties. A good example is polypropylene (PP) which is submitted to extrusion degradation in the controlled rheology process to obtain polymers (CR-PP) with the desired flow properties.ip3 Another common process is polymer mixing, using recycled and unprocessed macromolecules, for waste reduction or recycling. In this case, the low molecular weight species in the partially degraded recycled material can change both rheologic and mechanical properties.4-6 In general during extrusion the macromolecule is submitted to thermal and mechanical stresses which promote chemical reactions, leading to polymer degradation and a lowering of the physical properties of the polymer. This knowledge is of great importance with the increasing need of polymer This work is part of the Masters Degree Thesis of G. NeiraVelazquez at the Coahuila State Autonomal University (U. A. de C.). tTo whom correspondence
should be addressed. 33
V. A. Gonzdez-Gonzdez
34
CR-PP and PP degraded via multiple processing.“p’3 However, most of these papers mainly evaluate mechanical and rheologic properties and there is a dearth of information about the molecular weight change during multiple extrusion at different die temperatures, especially in the absence of peroxides. This information may be important in selecting processing conditions either to promote degradation or to prevent it. The present work aims to study the chain scissions occurring to commercial polypropylene subjected to multiple extrusion cycles at various die temperatures.
et al.
2.3 Degradation experiments
2 EXPERIMENTAL
Die temperatures for processing were defined and set to 240 and 270°C. The lower limit was selected according to exploratory degradation results and the high temperature limit was chosen because of thermogravimetry results showing excessive thermal degradation above 275°C. Virgin polypropylene was passed through the extruder, at the conditions given in Table 2; the extrudate was quenched in a cold water bath (51O’C) and air dried. The extrudate was pelletized and later used for multiple extrusion. This procedure was repeated up to 19 times. Table 3 gives the codes, die temperatures and number of extrusion cycles for the polymer selected for characterization.
2.1 Polymer
2.4 Characterization
Commercial polypropylene (FINA OIL Co.) was used as received. The polymer physical characteristics were determined at our laboratory, and are shown in Table 1.
2.4.1 Infrared spectroscopy (FTIR) Polypropylene films: each sample in Table 3 was hot pressed (T= 200°C and 15 ton ram force) into a polypropylene film which was cooled under pressure. The infrared spectra of these films was found using Nicolet FTIR-710 equipment. The 150& 1800cmp1 region was analyzed for any change in the carbonyl groups.
2.2 Extrusion A single-screw extruder, Killion model 315A, (24:l L/D ratio) with a 2mm diameter circular die, was used for multiple processing. The screw speed was fixed at 50rpm for all experiments. The extruder had four temperature control zones and one melt pressure measuring device. Three control zones correspond to the screw feed, transition and metering sections, and the fourth zone to the die. The temperature profiles for the different experiments are noted in Table 2. Table 1. Physical characteristics of the polypropylene Density Melt flow M, x IO-’ M, x 10m4 Polydispersity index MwIM, (g/cm3) (g/lOmin) 0.902
3.48
3.6
6.6
5.45
Table 2. Extruder temperature profile for multiple extrusion experiments Barrel zone I (“C) 195 195 195 195 195
Barrel zone 2 (“C)
Barrel zone 3 (“C)
Die zone (“C)
215 215 215 215 215
225 225 225 225 225
240 245 250 260 270
methods
Table 3. Polypropylene molecular weight characteristics determined by SEC Cycle number 0 5 8 12 16 19 5 8 12 16 19 5 8 12 16 19 5 8 12 16 19 5 8 12 16 19
Temperature iv 240 240 240 240 240 245 245 245 245 245 250 250 250 250 250 260 260 260 260 260 270 270 270 270 270
M,
Mn
MwIMn
no
5.45 4.89 4.63 3.99 3.87 3.67 4.67 3.93 3.95 3.51 3.12 4.24 3.83 3.77 3.37 3.06 4.04 3.53 3.72 3.25 2.89 3.95 3.43 3.46 3.05 2.76
0.0 0.048 0.049 0.074 0.154 0.146 0.047 0.017 0.213 0.268 0.313 0.011 0.068 0.264 0.283 0.374 0.016 0.072 0.285 0.30 0.382 0.024 0.074 0.240 0.315 0.388
<10-5 x IO-4 3.60 3.08 2.91 2.45 2.21 2.12 2.95 2.55 2.15 1.83 1.57 2.77 2.37 1.97 1.74 1.47 2.62 2.17 1.91 1.64 1.38 2.54 2.11 1.84 1.53 1.31
6.61 6.30 6.30 6.15 5.73 5.71 6.31 6.49 5.45 5.21 5.03 6.53 6.18 5.23 5.15 4.81 6.50 6.16 5.14 5.06 4.78 6.45 6.15 5.33 5.02 4.76
MFI (g/lOmin) 3.48 5.62 6.26 8.36 11.69 14.66 4.80 6.88 13.33 19.95 24.78 6.10 8.90 14.70 24.30 26.30 6.40 10.43 18.63 27.35 30.43 8.54 13.63 22.43 29.37 33.42
Polypropylene chain scissions and molecular weight changes in multiple extrusion
l&O
1766 1733 1700 1667 IS33 WAVENUMBER
1600
l5&
1533
(Cm-‘)
1. Infrared traces of the carbonyl region for polypropylene after indicated extrusion cycles at 270°C.
2.4.2 Crosslinked polymer The presence of crosslinked material was determined by immersing a filter paper cartridge, containing approximately 250mg of sample, in xylene at the boiling temperature (124°C). The dissolution time was 12 h. The crosslinked (insoluble) material remaining on the filter was weighed; these results were confirmed by size exclusion chromatography.
2.4.3 Molecular weight distributions Molecular weight characteristics, of selected samples, were obtained in a Waters 15OC size exclusion chromatography (SEC) instrument equipped with differential refractometer detector and three Ultra Styragel columns having 106, lo4 A and linear mixture nominal porosity. The carrier solvent was 1,2,4trichlorobenze (TCB) at 145°C and 1 ml/min flow rate. All the samples were prepared as 0.1 wt% polypropylene solutions in TCB with 0.2 wt% antioxidant (hindered phenol) added to reduce the occurrence of oxidative degradation during sample preparation and analysis. The samples were heated at 135°C for 4 h to ensure complete dissolution, and filtered before injection into the SEC. The molecular weight averages of six replicates were calculated using a universal calibration curve constructed with narrow MWD polystyrene standards, the reported Mark-Houwink parameters* and a Millennium GPC software. 2.4.4 Meltj?ow index (MFI) The measurements of MFI were done with a Kayeness 7053 melt index equipment according to the ASTM D1238 method test condition ‘L’: 230°C; load 2160 g. The results are shown in Table 3. 2.4.5 Extrusion variables Melt pressure drop and power consumption were measured during the extrusion experiments. The data are analyzed based on molecular weight and cycle number.
80,000
P
42,!jOO
30,000
1 1 0
I 5
Polypropylene
molecular
weight, number
1 15
1 IO CYCLES
Fig. 2.
35
NUMBER
3
(N)
average (M”) as a function
of extrusion
cycles and temperature.
V. A. Gonzdez-Gonzdez
36
et al.
6
V 3.25
CYCLES
Fig. 3.
Molecular
weight, weight average (M,)
P
‘0
T= T= T= T= T=
240 245 250 260 270
C C C C C
NUMBER
of polypropylene
as a function
of extrusion
cycles and temperature.
6.0-
3 : Q 3.0-
220
235
250
265
30
T f“C)
Fig. 4.
3 RESULTS
Polypropylene
molecular
weight increment
AND DISCUSSION
(eqn (1)) with temperature
and extrusion
cycles.
3.2 Molecular weight averages and distribution
3.1 Carbonyl groups Figure 1 contains the FTIR spectrum of the carbony1 region which showed only a small ‘shoulder’ at ca. 1740cm-’ which did not change with multiple extrusion. These results prove that there are almost no oxidative reactions during the extrusion because of the absence of oxygen within the extruder. This behavior is in agreement with other authorsi4,i5 findings on polypropylene thermal and mechanical degradation.
The molecular weights, number (M,) and weight (M,) averages, for the samples are shown in Table 3. The changes in M, and M, are plotted against the cycle number (A’) for the different die temperatures in Figs 2 and 3. There is a small, lineal, reduction for M, on increasing the cycle number at 240°C (Fig. 2) but for higher temperatures the change is linear only up to eight cycles, showing a rapid change between 8 and 16 cycles, and a leveling off from 16 to 19 cycles. There is an
Polypropylene
chain scissions and molecular
apparent lower limit of around 47 000 after 19 cycles with only a slight dependence on the temperature. The change is greater for M, (Fig. 3); the decreasing behavior fits a second order equation. -. The apparent limiting value for it4, IS ca. 130 000. According to the mechanical degradation theory of Bueche16 the probability for chain scissions is higher for the high molecular weight chains to yield smaller molecules of about half the original size.
weight changes
Molecular
A%
=
[(x)Ni+l
-
I
I
I
6.0
5.0
4.0
weight distribution
(~)NilT=constant
(Mw)
curves for polypropylene,
after different
extrusion
cycles at 270°C.
6.OC A 8 V
5 6 12
CYCLES CYCLES CYCLES
.
16
CYCLES
Mw/Mn
3
235I
I
I 265
T I”C)
Fig. 6.
Polydispersity
37
The more evident reduction in M, (60%) is due to the fact that this average is more sensitive to the concentration of high molecular weight species than the M,, (12%). We defined a molecular weight increment (A M,) in eqn (1):
log
Fig. 5.
in multiple extrusion
change with temperature
for polypropylene.
2
Cl)
38
V. A. Gonzdez-Gonzdez
where: (MW)N is the molecular weight average at a given number of cycles and (LV~)~+~ that at the next measuring point at a given temperature. The AM, is plotted for the different temperatures in Fig. 4, two stages are observed for molecular weight change. At low temperatures (240 < T < 250°C) AM, increases with temperature and in the second stage spanning from 250 < T < 270°C -. M, IS almost constant. The curves corresponding to 12 cycles deviate from this behavior, but no explanation is offered at this point. The general behavior suggests that chain breaking depends
1A
5
et al.
upon temperature only for the 24&250°C range and at higher temperatures depends mainly on the number of cycles. Extrapolating in the low temperature region, a limiting value (230°C) was obtained; below this temperature and under these processing conditions there would be no chain scission. This result is in agreement with preliminary measurements where samples processed at 225°C show no measurable molecular weight reduction up to 15 cycles. Finally, the effect of the multiple processing on the molecular weight distribution is a narrowing of the curve due to the removal of the high
CYCLES1
8
8
CYCLES
V
12
CYCLES
0
16
CYCLES
235
250
265
280
T f"CJ
Fig. 7.
Polypropylene
average scission per chain depending
8
T-
*
T = 250
245
C
V
T =
260
C
Melt flow index changes
and extrusion
cycles.
C
CYCLES
Fig. 8.
on die temperature
for polypropylene
NUMBER
after different
IN)
extrusion
cycles and temperatures
39
Polypropylene chain scissions and molecular weight changes in multiple extrusion
molecular weight fraction, without modification of the low molecular weight end. Figure 5 shows the chromatogram traces of samples processed at 270°C after O-19 cycles. The loss of high molecular weight chains is evident while the low molecular weight region remains practically unchanged. This was observed at all processing temperatures and proves that degradation proceeds mainly by long
chain breaking. The absence of very low molecular weight species suggests that the chain breaking takes place far from the chain ends. Polydispersity behaves similar to AM, when plotted against temperature. In Fig. 6 polydispersity decreases on increasing temperature or number of cycles, and the two previously mentioned zones are also observed, though not as evident as in Fig. 4.
a0 ! I L2
Fig. 9.
Relationship
between
melt flow index and molecular weight, geometrical extrusion cycles and temperatures.
b
i
drop during extrusion
l
T = 245
C
*
7- = 2.50
c
l
T=26OC
115
Ib CYCLES
Fig. 10. Melt pressure
mean, for polypropylene
NUMBER
of polypropylene
(NI
at different
temperatures.
submitted
to multiple
V. A. Gonzdez-Gonzcilez et al.
40
3.3 Chain breaking as a function of temperature The number of chain scissions according to eqn (2):i7
(~1~) was calculated
(2)
na =
where IV,, is the molecular weight of the polypropylene after a given number of extrusion cycles and M,, that of the original polymer. The average number of chain scissions is plotted as a function of die temperature in Fig. 7. Here, the number of scissions per chain is zero up to ca. 230°C according to the M, results. At temperatures between 230 and 250°C there is a sharp increase of chain scissions, but at higher temperatures nR does not change. In the high temperature region only the cycle number is important for chain breaking. Mechanical chain breaking, according to Bueche,‘(j occurs due to tensions concentrating close to the middle section of the chain. These tensions are related to entanglements, the viscosity (q) and shear rate (v). The force (F”) exerted on the chain depends on these factors through the product 0y LV,*‘~. Here M, is the molecular weight between entanglements and p is the density. On increasing temperature the viscosity and shear rate decrease, therefore, chain breaking will be reduced and degradation should be lower. The viscosity reduction (on temperature raising) will also favor this.
However, our results show that degradation does not takes place up to ca. 230°C where it sharply increases until reaching 250°C and finally remains almost constant. We believe this behavior is due to a change in Me. At relative low temperatures (below 230°C) the molecules have several entanglement points, M, and P do not reach the energy level required for chain breaking, hence no degradation takes place. In the interval 230-250°C there is an abrupt change in degradation, indicating that chain scissions are taking place rapidly. This could happen if M, is increased (viscosity is lowered) implying a reduction of the entanglements per chain. The maximum molecular weight M, corresponds to half the total chain length, or only one entanglement per chain. We think that at 250°C the maximum M, has been reached, hence, further temperature increments will not affect nR and degradation will only depend on the time the molecule is submitted to the chemical and mechanically excited state (number of cycles). As several recombining steps should take place to form insoluble material, and no crosslinked polymer was detected, the results indicate that chain breaking is the only event taking place in agreement with the thermodynamics of scission versus crosslinking for polypropylene. 3.4 Melt flow index MFI The MFI shows a great dependence on the cycle number and die temperature. Figure 8 shows a
lO.O0
EXPERIMENTAL
l
CAL Cl/LATE/l
1.2'0~
7.5-
0.04 1.0
I
1.62
I
MW
Fig. 11.
Experimental
relationship
between
I
2.25
2.87
3.. 0
m /o-5
melt pressure drop and molecular weight (M,) calculated behaviour according to ref. 20.
for our polypropylene
samples,
and
Polypropylene chain scissions and molecular weight changes in multiple extrusion
second order fit for the data of MFI versus number of cycles (N). The curve is more pronounced at Some authors1*,i9 have higher temperatures. reported that melt flow index and molecular weight can be directly correlated (MFI 0: l/M,) for lineal polymers. When MFI was plotted against molecular weight (M,) third order lines were obtained. However, if we use the geometrical mean (Mwn) defined as M,,
=
(M, M,)“2
a straight line with good correlation in Fig. 9.
(3)
41
in the low molecular weight region (below 264000) may reflect the effect of the peroxide used in that experiment on the degradation kinetics. Polymer output rate decreased with increasing M,, but increases with rising temperature. These effects may be explained by considering that the polymer viscosity is lower for lower molecular weights or higher temperatures. Finally, power consumption was linearly reduced on increasing the cycle number or temperature. This may be related to the system viscosity which decreases on molecular weight reduction.
was obtained 4 CONCLUSION
In MFI = 4.3 - 3.5 In M,,
(4)
That is, according to our results, the polydispersity must be considered. 3.5 Extrusion variables The melt pressure drop (AP) was monitored during extrusion. In Fig. 10 it can be seen that AP decreases on raising the number of cycles or temperature. The lines are almost parallel, but at higher temperatures the slope changes between 4 that the molecular and 7 cycles. Considering weight is decreasing, the linear behavior is normal, but no explanation can be offered for the deviation. The pressure drop for all the samples is plotted against molecular weight (Mw) in Fig. 11, and we found a linear correlation: AP = 1.85 x 10-5K
Pabendiskas
- 1.77
(5)
reported two equations for the AP-M, behavior, a first order equation for M, > 264000 and a third order one for M, < 264000. In Fig. 11 it can be seen that the behavior is similar for both systems, in the high molecular weight region, linear with similar slopes. We consider that the difference between AP’s is due to the equipment characteristics. Pabendiskas used a static mixer unit in the extruder. Extrapolating the line obtained with our results, a limiting molecular weight of around 100 000 is obtained for AP=O. Considering that the pressure change is due to polymer degradation, this value implies that no degradation is taking place and is close to the limit values calculated with the M, versus N data. The behavior observed by Pabendiskas et d2’
No change in the polypropylene chemical structure was detected, and carbonyl groups were observed only in very low concentrations. This strongly suggests that no thermo-oxidative degradation occurs. The molecular weight distribution curves became narrow, and polydispersity was reduced with increasing cycle number or temperature. A similar reduction was observed for the molecular weight averages. There is an apparent M, limit between 100 000 and 130 000 below which no degradation occurs under the experimental conditions. Three zones for the effect of temperature on degradation were observed: first, below 230°C no degradation takes place; second, between 230 and 250°C the temperature is the defining degradation parameter; third, above 250°C but below the initial thermal degradation temperature (275”(Z), the temperature increase had no effect on the degradation. Based on these results, and the Bueche mechanical degradation theory, the polypropylene degradation during multiple extrusion is thought to be thermomechanical, caused by chain scissions (apparently promoted by chain entanglements). The highest molecular weight chains were preferentially broken during the degradation and the scissions may be located close to the macromolecule center. Only minor amounts of thermooxidative degradation, expressed as carbonyl groups, were measured and no recombination reactions occurred during the experiment, proved by the gel absence. The molecular weight and viscosity reduction affect the extrusion variables such as melt pressure drop, power consumption and melt flow index.
42
V. A. Gonzdez-Gonzdez
REFERENCES 11. I. Bellahcene, M. and Bounafa, N., Proc. Ann. Tech. Conf. SPE, 1991, p. 720. 2. Tzoganakis, C., Adv. Polym. Technol., 1989, 9, 321. 3. Ryu, S. H., Gogos, C. G. and Xanthos, M., Adv. Polym. Technol., 1992, 11, 121. 4. Dagh, S. S., Kelleher, P. G., Monroy, M., Patel, A. and Xanthos, M., Adv. Polym. Technol., 1990, 10, 125. 5. Staicu, D., Banica, G. and Stoica, S., Polym. Degrad. Stab., 1994, 46, 259. 6. Nguyen, K. T., Weber, L. and Hebert, L. P., Proc. Ann. Tech. Conf. SPE, 1993, p. 3432. I. Lew, R., Suwanda, D. and Balke, S. T., J. Appl. Polym. Sci., 1988, 35, 1049. 8. Lew, R., Suwanda, D. and Balke, S. T., J. Appl. Polym. Sci., 1988, 35, 1033. 9. Ying, Q., Zhao, Y. and Liu, Y., Makromol. Chem., 1991, 192, 1041.
12. 13. 14. 15. 16. 17. 18. 19. 20.
et al.
Ries, H. and Menges, G., Proc. Ann. Tech. Conf SPE, 1988, p. 339. Shibayama, M., Imamura, K. Y., Katoh, K. and Nomura, S., J. Appl. Polym. Sci., 1991, 42, 1451. Hinsken, H., Moss, S., Pauquet, J. R. and Zweifel, H., Polym. Degrad. Stab., 1991, 34, 279. Billiani, J. and Fleischmann, E., Polym. Degrad. Stab., 1990, 28, 67. Valenza, A. and La Mantia, F. P., Polym. Degrad. Stab., 1988, 20, 63. Al-Malaika. S.. Polvm. Plast. Technol. Eng., 1988,27, 261. Bueche, F., ‘J. Appi Polym. Sci., 1960, 4, 101. David, C., Trojan, M., Daro, A. and Demarteau, W., Polym. Degrad. Stab., 1992, 37, 233. Tzoganakis, C., Vlachopoulos, J. and Hamielec, A. E., Polym. Eng. Sci., 1988, 28, 170. Bremner, T. and Rudin, A., J. Appl. Polym. Sci., 1990, 41, 1617. Pabedinskas, A., Cluett, W. R. and Balke, S. T., Polym. Eng. Sci., 1989, 29, 993.