Melt Flow Instabilities in Capillary Flow of Two-Phase Polymer Systems

C. D. HAN and R. R. LAMONTE

Department of Chemical Engineering Polytechnic Institute of Brooklyn

Brooklyn, New York

An experimental study was made of melt flow instabilities in extrusion of two-phase polymer systems. For the study, blends were prepared from two polymers: polystyrene (Dow Chemical Sl'YRON 686) and high density polyethylene (Union Carbide DMDJ 4309). The experimental technique used in the present study was the same as that described in a previous paper by the authors. The study shows that there are abrupt increases both in exit pressure and in the recoverable shear strain (defined as the ratio of the exit pressure to shear stress) at the critical flow conditions. It has also been found that an addition of a small amount of high density polyethyl- ene (2.5 wt-% and 5.0 wt-76) increases the critical shear rate of polystyrene and hence results in a higher throughput rate before extrudate distortion is actually observed. This result is explained in terms of the independently determined melt elasticity of the two-phase systems investigated.

INTRODUCTION or many years the occurrence of melt fracture

F h a s been one of the primary concerns of polymer processing industries, mainly because i t limits the throughput rate. Consequently, many studies ( 1-4) have been made to investigate the causes of melt fracture. However, almost all of the previous studies dealt with melt fracture phenomena of single-phase homopolymer systems, but very few with two-phase, incompatible polymer systems.

It is interesting to note, however, that many in- dustrially important polymer systems contain addi- tives, such as plasticizers, lubricants, fillers, and ther- mal stabilizers, etc., in order to make polymer proc- essing easier. Often, these additives play an im- portant role in controlling the flow properties of molten polymers in various processing devices. Na- tov and Djagarowa ( 5 , 6 ) made extensive studies of the effect of low molecular weight organic ma- terials on the flow properties of various polymer melts, reporting that the additives reduced the melt flow viscosity considerably.

On the other hand, it is a well-established fact that the elastic properties, rather than viscous proper- ties, of a polymer melt are closely related to the occurrence of melt fracture. Bagley and Sehreiber ( 3 ) attempted to relate the recoverable shear strain of polymer melts to the critical flow conditions in

POLYMER ENGlNEERlNG AND SCIENCE, MARCH, 1972, Vol. 72,

terms of shear rate and shear stress at which melt fracture starts to occur. In a recent paper (7 ) , the authors have described an attempt to correlate the critical flow conditions with exit pressure and also with recoverable shear strain, which is defined as the ratio of exit pressure to shear stress.

This paper presents some new experimental results, which show how the addition of a small amount of high density polyethylene affects the critical flow conditions of polystyrene melts, which then form a two-phase, incompatible system.

EXPERIMENTAL Two blends were prepared from general purpose

polystyrene (Dow Chemical STYRONB 686) and high density polyethylene (Union Carbide DMDJ 4309). These were: 2.5 wt-% polyethylene/97.5 wt-% polystyrene, and 5 wt-% polyethylene/95 wt-% poly- styrene. Polymer blends were prepared by mixing the two polymers, in the form of pellets, in a tumbling operation. Fortunately, the pellet sizes of both polymers were almost identical (about 1/8 in.) and hence no particle segregation was observed when the blends were fed to an extruder.

The apparatus and experimental procedure used have been described in previous papers (7-9). The present study used a capillary having an L/D ratio of 20 and a flat die entry ( 180" included angle).

No. 2 77

C. D. Han and R . R . Lamonte

I"--

moo

6000

5 0 0 0

4000 - W E - g 3000-

5 >

0 - ' 2000

1500

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

AXIAL DISTANCE (in.)

Fig. 1 . Representative pressure profiles for a 97.5 wt-% poly- styrene/2.5 wt-% polyethylene blend at 197°C.

-

-

-

-

-

-

RESULTS AND DISCUSSION In the present study the viscous and elastic prop-

erties of mixtures of polystyrene (STYROND 686)- high density polyethylene (DMDJ 4309) were in- vestigated near and at the critical shear rate at which melt fracture occurred. In order to evaluate these properties the pressure profiles were measured as a function of volumetric flow rate. Typical pres- sure profiles are shown in Fig. 1 for a 97.570 poly- styrene-2.S% polyethylene mixture. This information was then used to obtain the true shear stress, true shear rate, viscosity, and exit pressure (8 ,9 ) .

the effect of the addition of small amounts of polyethylene on the flow curve of poly- styrene is given. These curves cover the range of shear rates from below to above the critical point, with the critical shear rate noted on each curve. It is seen that the flow curves presented are single-valued whereas several authors (2, 10, 11) have presented double-valued flow curves. This question was dis- cussed in detail in an earlier paper by the authors (7) and will not be presented here.

Figure 2 shows that the three mixtures are in the power law region and that the addition of small amounts of polyethylene significantly lowers the flow curve for polystyrene. This is also shown in Fig. 3, which gives the viscosity-shear rate relationship for these mixtures. The latter figure shows that a small amount of polyethylene significantly reduces the melt viscosity of polystyrene. In other words, the poly- ethylene appears to act as a lubricant for the poly- styrene even though, at the temperature used, the

78

In Fig. 2

polyethylene has a higher viscosity than the poly- styrene. As mentioned previously, Natov and Djaga- rowa (5, 6) reported similar behavior for additions of small amounts of low molecular weight organic substances. In the present study, however, the molec- ular weight of the polyethylene is of the same order of magnitude as that of the polystyrene. This effect has also been reported by Han and Yu (12) for the polystyrene-polyethylene system, but for much larger amounts of polyethylene added. This deinon- strates that the lubricating effect reported by Natov and Djagarowa is not limited to low molecular weight materials but may also include high niolecu- lar weight additives as well.

Since the addition of small amounts of polyethyl- ene drastically alters the flow behavior of poly- styrene, it is logical to ask how does its elastic behavior change. The investigation of the elastic behavior of these materials was carried out by means of the exit pressure. The exit pressure is determined by extrapolating the pressure profile (see Fig. 1) to the end of the capillary. The non-zero gauge pressure remaining is called the exit pressure. It has been shown (8, 9 ) that the exit pressure is a measure of the elasticity of the material. Figure 4

30 1 l

0 PS

A P S l P E = 9 7 . 5 / 2 . 5

P S l P E = 9 5 . 0 / 5 . 0 ; 20 v)

n U

SHEAR RATE (SEC-1)

Fig . 2. F low curues for P S / P E blends at 197°C.

0 PS

A PSlPE = 97.5l2.5

P S I PE = 95,015.0

1000 I I , I I ,

100 150 2M) x)O 400 5 0 0 600 700E

SHEAR RATE (SEC- I )

Fig. 3 . Viscosity os shear rate for P S / P E blends at 197°C.

POLYMER ENGINEERING AND SCIENCE, MARCH, 1972, Yo\. 12, No. 2

Melt Flow lnstabilities in Capillary Flow of Two-Phase Polymer Sys tems

60

5 0 - 0 PS

A PS/PE = 97.512.5

40 - P S ~ P E = 9 5 0 / 5 0 ./@' w

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I

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a 20- t w ,AAA t

15 - /. I.

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100 150 200 300 400 500 600 700800

temperature independent for homopolymers. It also provides a way for determining which polymers are more elastic than others. A plot of this type is given in Fig . 6. This figure also shows a break in the curves, but now at the critical shear stress. It is seen that as polyethylene is added, the curves shift progressively lower (i.e., the elasticity of the

the break point (critical shear stress) is roughly constant. Several authors (14, 15) have reported that the product of molecular weight and critical shear stress is constant, while others (10, 16-18) have reported that the critical shear stress itself is roughly constant. The results of the present study tend to agree with the latter group.

mixtures is decreasing). However, the location of

At this point it is convenient to reconsider Fig. 5

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that a change in the elasticity of a ziven polimer system will alter the location of the critical shear rate. Figure 6 shows that, for the range of con- centrations investigated, the melt elasticity decreases with increasing polyethylene concentration. There- fore, the result given in Fig. 5 demonstrates that the critical shear rate increases as the melt elasticity decreases.

Another measure of the elasticity of polymers is the recoverable shear strain (S,) . The value of S , for polymer melts may be defined by

Wt- 'lo POLYETHYLENE Fig. 5. Critical shear rate us wt % of polyethylene.

shows how the exit pressure varies with shear rate for the materials tested. It should be noted that for each material, the curve of exit pressure versus shear rate breaks sharply upwards at the critical shear rate. This break point (critical point) in- creases with increasing amounts of polyethylene added. This is seen more dramatically in Fig. 5 which gives the critical shear rate as a function of the percent of polyethylene added. This result is of some significance since the presence of a small amount of additive material (even high molecular weight material, which increases the critical shear rate) permits much higher throughput rates before visible melt fracture occurs.

A more useful way of viewing the exit pressure data is by plotting exit pressure versus shear stress. This plot was first introduced by Han (13) and is

U) a Y

W 12: 3 v) cn W a a I- - X w

4 PS/PE =950/5.0

A PS/PE =97.5/2 1 SHEAR STRESS (PSI)

Fig. 6. Exit pressure us shear stress for P S / P E blends at 197°C.

79 POLYMER ENGINEERING AND SCIENCE, MARCH, 1972, VoI. 12, No. 2

C . D. Han and R . R . Lamonte

[L v)

U PS/PE =95.0/5.0

2.2 -0-0

I I I I I - 1.0 ' 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0

SHEAR STRESS (PSI)

Fig. 7. Recoverable shear strain os shear stress for PS/PE blends at 197°C.

where Pexit is the pressure and rW is the true shear stress at the wall ( 9 ) . Values of S R were determincd for the materials studied and are presented in Fig. 7. This graph also shows that the elasticity decreases as the percentage of polyethylene increases. The break points shown (critical shear stress) are at approxi- mately the same value of stress, even though the cor- responding values of S, are greatly different. Figures 6 and 7 therefore seem to indicate that even though the elasticity of the mixtures is decreasing, the maxi- mum shear stress which the fluid can undergo before melt fracture remains essentially unchanged within the experimental error. Also, above the critical shear stress, the curves indicate that S R increases mow rapidly with shear stress than below the critical point. This indicates that the ability of the material to store elastic energy is abruptly changing at the critical point.

Regarding the slope of these curves, Bagley (19) has pointed out that he has observed the S R vs rzL curves increased sharply (as in Fig. 7) , leveled off, or decreased. Therefore, one can infer from Fig. 7 that at the critical shear stress a sudden change in S R is a characteristic of the elastic material, but the direction of this change is not evident beforehand.

In conclusion, it was found that the addition of small amounts of high density polyethylene to poly- styrene reduced both the viscosity and the elasticity of polystyrene melts, Interestingly enough, the criti- cal shear rate is seen to increase with the percent of polyethylene added (a t least for the range of concen- trations investigated). This is attributed to the de- crease in melt elasticity of the two-phase systems in- vestigated. However, the critical shear stress remains approximately constant in this range of concentra- tions. It should be noted that a more complete un- derstanding of the flow properties of incompatible polymer blends can be gained only from the analysis of the local properties of the mixture. This is beyond the scope of the present work and an investigation in this area is being pursued by the authors.

ACKNOWLEDGEMENT The work was supported in part by the Textile

Fibers Department, E. I. duPont de Nemours and Company, American Can Company, and Mobil Chemical Company, for which the authors are gratc- ful. The authors are also grateful to Dow Chemical Company and Union Carbide Corporation, which supplied the authors with a large quantity of polyrncr samples. This work is partly taken from the disserta- tion of R. R. Lamonte, submitted to the Faculty of the Polytechnic Institute of Brooklyn in partial ful- fillment of requirements for a PhD degree, 1972.

REFERENCES 1. J. P. Tordella, J. Appl. Phys., 27, 454 (1963). 2. J. P. Tordella, J. Appl. Polym. Sci., 7, 215 (1963). 3. E. B. Bagley, and H. P. Schreiber, Trans. SOC. Rheol., 5,

4. E. B. Bagley, Trans. SOC. Rheol., 5, 355 ( 1961). 5. M. A. Natov and Y. K. Djagarowa, Polym. Sci. U.S.S.R.,

G. M. A. Natov and Y. K. Djagarowa, Makromol. Chem.,

7. C. D. Han and R. R. Lamonte, Polym. Eng. Sci., 11, 385

8. C. D. Han, M. Charles, and W. Philippofi, Trans. SOC.

9. C. D. Han and M. Charles, Polym. Eng. Sci., 10, 148

10. M. Shida and L. V. Cancio, ACS Preprints, Div. of Polym.

11. J. M. Luptom, Chem. Eng. Progr. Symp. Ser., 60, 49, 17

12. C. D. Han and T. C . Yu, J. Appl. Polyin Sci., 15, 1163

13. C. D. Han, AIChE J., 14, 1087 (1970). 14. R. S. Spencer and R. E. Dillon, J. Colloid Sci., 4, 241

15. E. R. Howells and J. J. Benbow, Plast. Znst. London

16. S. M. Barnett, Polym. Eng. Sci., 7, 168 (1967). 17. L. L. Blyler, Jr., and A. C. Hart, Jr., Polym. Eng. Sci., 10,

18. R. W. Myerholtz, J. Appl. Polym. Sci., 11, 687 (1967). 19. E. B. Bagley, Private Communications (1971).

341 (1961).

8,2032 ( 1966).

100,126 (1967).

(Sept. 1971).

Rheol., 13,455 (1969); ibid., 14, 393 (1970).

(1970).

Chem., 10,144 (April 1968).

(1964).

(1971).

( 1949).

Trans., 30,240 ( 1962).

193 ( 1970).

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