Homogeneity of annular extrusion of polystyrene

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-Note to the Editor Homogeneity of annular extrusion of polystyrene Toshikazu Fujimura and

Takashi

Kawamura

Department of Polymer Chemistry, Faculty of Engineering, Yamagata University, Yonezawa-shi992, Japan (Received 14 May 1973; revised 2 January 1974)

INTRODUCTION The blown film process with a circular die has the advantages of a simple apparatus, low operating costs, easy biaxial orientation, and less trimming loss compared to the fiat film process with a slot die, but is said to show poorer gauge tolerance and troublesome control by means of decentring of the die lip-ring. The rotation of die or the take-up is not only a superficial solution of these problems, but also requires complex and expensive machinery. In order to mitigate these disadvantages, many designs of circular die have been suggested 1, 2. Concerning the design variables, the slot die s, 4 and the spiral mandrel die were recently analysed5, but operation variables were studied only partly in an article on the behaviour of sheet extrusion6. The purpose of this investigation was to show experimentally the effects of operating conditions on the homogeneity of the annular extrusion of especially polystyrene, which has a melt viscosity highly sensitive to shear and temperature. For this purpose, the variance of melt flow rate between quadrants of the circular die was determined, changing the output rate with the rotational screw speed, and the specified temperature at various positions of the apparatus, i.e. its longitudinal and circumferential distribution. To facilitate the discussion of various factors, we have devised a special spider-type die, which can control the pressure distribution between quadrants of the die lip, and a differential thermometer, which can measure accurately the melt temperature distribution in the die channels.

Die constants of each of the parts were calculated on the assumption of infinite parallel plate or circular pipe, and the observed pressure drops through the die are given in Table 1.

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EXPERIMENTAL The material used was high-impact polystyrene made by Dainippon Ink and Chemicals, Inc.: density, 1045 kg/m3; melt index, 0.10g/10min; and, as power law fluid, a flow behaviour index, 2-3. The melt was fed from a 30ram diameter extruder (L/D 20) through the adapter to a vertical inlet at the bottom of the circular die. As shown in Figure 1, the die had a construction, which conveyed the melt into four crosswise located manifolds with valves at the upward turnings, then through horizontal spreading channels followed by a vertical annular channel, up to the die lip. The total channel length was designed to be the same around the annular channel by adjusting the inclination of the spreading channel. The profile of the land was corrected by means of trial-and-error, to ensure a uniform flow rate around the die lip at the constant gap.

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P O L Y M E R , 1974, Vol 15, A u g u s t

Figure 1 Design configuration of a spider-type die. I, inlet;

M, manifold; P, pressure control valves; H, horizontal spreading channels; V, vertical annular channel; R, R', die lip rings; S, spider mandrel; B, body; T, thermometers; A, blowing air inlet (R' is omitted in this experiment)

Table I

Calculated die constant and pressure drop distribution

Parts Manifolds Spreading and annular channel Die lip

Die Pressure constant drop x 10-zo (m3) (% to total) 0.244 4-655

26.1 11-3

0.053

62.6

flow behaviour index of 2.00

Note to the Editor The ratio of back pressure flow to drag flow was estimated from the difference between the extrusion rates with and without die. With pressure control by means of valves at each of the manifolds, the variance of flow rate between the quadrants could be reduced to within 1%. The extrusion pressure was measured at the inlet of the adapter with a Bourdon-type gauge. The temperature at the barrel was controlled automatically. The circumferential temperature in the die was controlled manually by band heaters sectioned in quadrants. The temperature of melt in the die was estimated with a differential thermometer made by Toa Electronic Co. It has two pairs of thermistors, which can detect the differences in the protecting sheath due to the conduction to the die wall, following the suggestion by Van Leeuwen 7. The temperature of the tip was read up to + 0.1 °C, controlling the root temperature within +0.02°C. The uniformity of flow rate around the die lip was estimated from the weights of the film portions, which were extruded without blowing and divided in quadrants. It is expressed as the coefficient of variance, VC(Q), or the deviation from mean value.

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fixed temperatures on the variance of extrusion rate (©), the variance of temperature (C]), the pressure drop (V), and the back pressure flow ratio (A), for various rotational speeds of screw. , 20rev/min; . . . . , 50rev/min. Temperatures at other controlled points are kept at 190°C

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on the variance of extrusion rate ( ), the variance of temperature at die ( . . . . ), the pressure drop ( . . . . . . ), and the back pressure flow ratio ( . . . . -). ~7,170°C; x , 190°C; A , £10°C; O, 230°C

peratures on the variance of extrusion rate, the variance of temperature at die, the pressure drop, and the back pressure flow ratio, for various speeds of screw. Temperatures at other controlled points are fixed at 190°C. (Same notations as Figure 3)

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POLYMER,

1974, V o l 15, A u g u s t

537

Note to the Editor RESULTS As shown in Figure 2, increase in the rotational speed of the screw, i.e. output rate, increased the pressure drop through the die and the fraction of back pressure flow. The VC(Q) increased with output rate at lower temperature, but less significant at higher temperature. The coefficient of variance in temperature, VC(T), was held within experimental error. Higher specified temperature reduced the heterogeneity of flow rate and pressure drop. As shown in Figures 3 and 4, the heterogeneous temperature profile in the longitudinal direction increased VC(Q). The temperature change at the adapter seemed to have a more marked effect on VC(Q) and VC(T). In these cases, more back pressure flow showed a tendency of less VC(T), which suggests more homogeneous mixing, but it did not always have a correlation with VC(Q). The distribution of extrusion rate around the die lip, shown in Figure 5, became more heterogeneous by varying the temperature from other specified temperatures. The effects of circumferential temperature distribution are given in Figure 6. The flow rate increased at the locally more heated quadrants and decreased at the

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extrusion flow deviation, and the distance between weld lines, for various rotationalspeeds of screw. • ..... ,20rev/min ; . . . . , 50rev/min; A, at the varied quadrant; O, at the opposite quadrant. Reference temperature at die is 210°C. - represents the calculated flow rate deviation

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opposite quadrant. The distance between weld lines decreased or increased, respectively, at the more heated quadrant or at the opposite quadrant. Therefore, the linear velocity at the more heated quadrant was higher.

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DISCUSSION

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The homogeneity of extrusion rate from the die is dependent on many variables. As estimated in Table 1, the pressure dropped mostly across the die lip in our experiment. Therefore, the problem was simplified and the discussion of flow rate was confined at the die lip, which was approximated as infinite parallel plates• The flow rate deviation in non-dimensional terms as a fraction of mean flow can be expressed as follows:

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Figure 5 Effect of temperature at die on the extrusion flow rate distribution and the temperature distribution of melt in die, for various rotational speeds of s c r e w . - - , 20rev/min; . . . . , 50rev/min. O, 210°C; /% 170°C. Temperatures at other controlled

points are fixed at 210°C 538

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POLYMER, 1974, Vol 15, August

(1)

where Q is the flow rate per unit of circumference, L is the die land length, H is the gap width of lip, P is the pressure applied to die lip, */0 is the viscosity at reference shear, and n is the flow behaviour index expressed in the equation, log,/= log*/°- (n - 1)log(P[Po). This equation indicates that the temperature of melt, at the die lip, affects the flow rate significantly through the considerable change of viscosity. Although the longitudinal temperature profile is specific to the extruder and die combination, the non-uniform controlling

Note to the Editor/Letters condition would result in the higher variance of extrusion rate, probably due to insufficient heat transfer. However, the more mixing with reduced output or more back pressure flow could compensate the difference. In the experiment with more uniform temperature profile, shown in Figure 2, VC (Q) increased with pressure drop, despite higher mixing degree by back pressure flow. Hence, the pressure distribution at the die lip could have an important effect. The calculated value plotted in Figure 6, which corresponds to a higher flow rate due to reduced viscosity at higher temperature was steeper than the observed lines and opposite to the width between weld lines. Therefore, the flow pattern in the annular channel of the die might change and compensate for the heterogeneous pressure drop. Warmuth 6 observed the similar effect of the rotational screw speed on polystyrene and polypropylene, but different results were obtained on polybutene and polyethylene. Since the rotational screw speed changes the flow pattern, depending on design, material parameters, and operational conditions, this phenomenon would be no definite effect. From the above discussion, it is important to devise a die which can compensate the heterogeneity of resistance around the lip, in addition to precise manufacturing, as suggested by Proctor 4. In our design, the gross balancing can be obtained with valves at manifolds and the small difference can be corrected by the profile of land length at constant gap width because, based on equation (1), the land length is less critical and more easily adjustable than the gap width. Since the flow behaviour index affects the effect of die design,

our lip-ring can be replaced corresponding to various types of material. Because the flow behaviour index n exhibits serious effects on the homogeneity of annular extrusion, according to equation (1), uniform gauge might be difficult in the case of polystyrene, which has higher flow behaviour index and higher activation energy of flow. CONCLUSIONS (1) Temperature distribution, particularly near the die, and pressure drop distribution, which are dependent on operation variables, i.e. rotational screw speed, back pressure or stock temperature, appear to have serious effects on the homogeneity of annular extrusion. But some gauge heterogeneity could be scattered in transit through the annular channel of die, probably owing to the change of flow pattern, and compensations for temperature and pressure drop. (2) The die design, which can adjust the resistance around the lip, would be useful particularly in the case of material with high shear-sensitivity. REFERENCES 1 Kock, K. Ind. Anz. 1968, 90, (80), 26 2 Caton, J. A. Br. Plast. 1971, 44, (4), 140 3 Mckelvey, J. M. and Ito, K. Polym. Eng. Sci. 1971, 11, 258 4 Knappe, W. and Sch6newald, H. Kunststoffe 1970, 60, 657; 1971, 61, 497 5 Proctor, B. SPE J. 1972, 28, (2) 34 6 Warmuth, J. Plastverabeiter 1972, 23, (2), 95 7 Van Leeuwen, J. Kunststoffe 1965, 55, 491; Polym. Eng. Sci. 1967, 7, 98

Letters '3C n.m.r, spectra of styrene-butadiene copolymers on solid samples Many papers have been published on the determination of polymer microstructure in solution by n.m.r. In the last few years laC n.m.r, has proved to be a powerful tool in this field. However, very few experiments have been reported on solid samples of polymers. The attainment of a good resolution in solid samples depends on the structural state of macromolecular chains; but more strictly on the relaxation times relative to the different nuclei, which are dependent on the mobility of the chain. As a consequence for a random polymer with sufficient segmental motion, we might expect a taC n.m.r, spectrum which is sufficiently satisfactory and not very different from the analogous one in solution. On the other hand, a very poor spectrum should be obtained in the case of crystalline polymers or in the case of restricted segmental motions. In fact during a previous study I characterizing ethylene-vinyl acetate copolymers we were able to obtain spectra on solid samples, showing a sufficient degree of resolution. This result prompted us to try a similar approach on different copolymers. The present communication concerns some preliminary results on the 13C n.m.r, spectra of solid samples of styrene-butadiene copolymers.

13C n.m.r, spectra of these copolymers in solution will be discussed in a later paper 2. The spectrum of a random copolymer is shown in Figure 1, in solution (a) as well as in the solid state (b). The high field portion of the spectrum (25-47ppm from TMS) is due to the CH and CH2 backbone resonances and the low field portion (l10-147 ppm) portion is due to the aromatic, vinyl and vinylenic 13C atoms.

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POLYMER, 1974, Vol 15, August 539