Continuous process for Manufacture of Polyester l i r e Yarn H E l M O HARDUNG-HARDUNG
Process reduces cost and production time of tire cord yarn to yield product of high viscosity
he continuous process for the production of polyester
Ttire cord developed by the research and development
divisions of Vickers-Zimmer starts from pure terephthalic acid (PTA) and ethylene glycol (EG) as feedstock. I t was systematically developed by laboratory research involving the chemistry and kinetics of the process, by model experiments regarding the (hydrodynamic) flow behavior of viscous liquids, and by extensive production studies in no less than three pilot plants. T h e process combines Mobil’s improved continuous esterification process with Vickers-Zimmer’s continuous polycondensation process and their new high-pressure direct-spinning technology. T h e use of pure terephthalic acid as feedstock gives higher yields of polyester related to the input quantity and therefore results in a n obvious saving in material cost. Particular attention, however, must be paid to a number of highly undesirable by-products which are formed during the esterification of terephthalic acid. T h e amount of diethylene glycol (DEG) in the prepolymer is a decisive criterion for the quality of the tire cord. T h e formation of such critical by-products depends decisively on temperatures, residence times, residence time spectra, glycol concentrations, and the catalyst system. However, when optimized in this respect, the continuous process yields a quality much better than hitherto practiced batch processes.
SPINNING MANIFOLD
LI Il
KNEADER
CHIPS
EXCHANGER
TAKE-UP UNIT PREPOLYCONDENSAT1ON POLY CONDENSATION STAGES REACTOR I1 ESTER IF1CAT1ON POLYCONDENSATION DIRECT-SPINNING STAGES REACTOR 1
Figure 7. Continuously operated process for production of polyester tire yarn starting from terephthalic acid VOL. 6 2
NO. 3
M A R C H 1970
13
POLYMER FROM LASTREACTOR
Continuous Process
Esterification and polycondensation stage. A paste of pure terephthalic acid and ethylene glycol passes a special design heat exchanger and reacts in several esterification stages resulting on a precondensed intermediate product which is fed to the polycondensation stage (Figure 1). Compared with D M T as feedstock, the lack of methoxy groups leads stoichiometrically to a 15y0 higher yield per weight unit and accelerates the polycondensation rate, thus reducing the resistance time in the polycondensation reactors. (Thus the valuability of the polymer is decreased and reactors can be smaller for the same throughput.) The esterification reaction is carried out a t temperatures of about 270°C with a relatively short reaction time (less than one hour). The intermediate product of the direct esterification stage is further treated in a twostage polycondensation process producing a polyester with an intrinsic viscosity of 0.30-1 .0 after approximately 6 hr. The intermediate product which enters the polycondensation process is already partially polycondensed. The first polycondensation reactor is equipped with ring disks fixed to a horizontal shaft and operates under a vacuum within the I-mm mercury range and a t a temperature of about 280°C. The polymer melt is extracted from this reactor by a special extraction screw and metered to the second polycondensation stage which shows similar design characteristics but is operated a t a higher vacuum and within about the same temperature range. The high-viscosity product is extracted from this reactor and pumped directly to the spinning manifolds. Provision is made for adding delusterant, pigments, or other additives. The different flow behavior of the viscous polymer melt which a t the beginning of the polycondensation has a viscosity of only a few poises and a t the end several tens of thousands poises (both values related to operating temperatures) can be handled by two reactors designed for the visocsity ranges in question. Spinning and further treatment. Figure 2 shows the spinning procedure and the further treatment of the high viscous polyester melt. As indicated above, the polymer melt passes the spinning manifolds and is directly transferred to the spinning blocks. Having passed the filter and spinnerette, the polyester capillaries are kept in a plastic state by being run through a heated tube to retard cooling and only later are quenched in the blow duct. The spun product is wound up and the resulting bobbins are stored in a certain climate condition where they are subsequently stretched to the final tire cord yarn on a draw twister. Research also has been carried out to combine the winding and stretching steps in one machine (spin-draw), however, the results did not show a marked economic advantage as yet. 14
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
HIGH PRESSURE SPINNING MAN1FOLD
--- SPINNERETTE TEMPERATURE CONTROL AND MEASUREMENT
QUENCH DUCT
I
SPIN DUCT
FILAMENTS
I
1
>
FINISH ROLLS
TAKE- UP UNIT
Figure 2. Direct spinning of polyester tire yarn
The above direct spinning process avoids the necessity for chip production, crystallization, drying and remelting of the chips which means a considerable gain in economy, and, above all, quality. The polyester melt is not brought into contact with water, thus avoiding hydrolytic decomposition and, therefore, viscosity degradation. A further advantage of direct spinning is a smaller residence time of the product which again means a better quality. A high pressure-spinning process (200-500 kg/cm2) for the production of polyester tire cord was developed which keeps the polymer melt relatively cool and which reaches spinning temperatures only a t the last point where these temperatures are necessary. The temperature increase is obtained by utilizing the pressure drop of the polymer melt flow rising in the filter or the spinnerettes. A better temperature uniformity is observed because the heat is created within the polymer and not transferred by heat exchange with a surface (a pressure drop of 100 atm leads to a temperature increase of about 4°C). The
A U T H O R Heimo Hardung-Hardung is employed by VickersZimmer i n Frankfurt, W e s t Germany. T h i s paper w a s presented as part of the Symposium on Novel Processes and Technology of the European and Japanese Chemical Industries, 158th National ACS Meeting, N e w York, N . Y., September 7-12, 1969.
results are low denier variations and good associated specifications. Since the Vickers-Zimmer continuous polycondensation has been described in detail elsewhere, subsequent remarks will be focused on the spinning and drawing problems. There are two methods of obtaining the desired pressure drop within the spinning head, either by dense filters or by spinnerettes with a high l / d ratio of the holes. Vickers-Zimmer decided to place the major part of the pressure drop into the filters, since it is difficult to arrive a t a n even dissipation of heat in the spinnerettes if excessive pressure gradients occur a t this point. Furthermore, the l / d of borings ratio is not freely variable after taking into account maximum permissible shear of the polymer and minimum hole diameter. Sintermetal filters with fine pores or metal screen filters with 10-50,000 meshes/cm2 or combination of both filter types can be used. T h e polyester filament leaving the spinnerettes must be kept in a plastic state until the capillaries achieve their final diameter to avoid a great molecular preorientation (birefringence). They pass a heated tube, the so-called after-heater, before entering in the quench duct. Three main aims sought during the spinning procedure are: (1) A minimal degradation in the intrinsic viscosity ( 2 ) Low preorientation (3) Minimum possibility for capillaries to stick together while passing the plastic zone T h e tendency of capillaries leaving the spinnerette to stick together increases with higher temperatures as does the tendency for viscosity degradation, whereas the preorientation is decreased by higher temperatures. Therefore a compromise must be made in choosing the optimum temperature distribution in the after-heater. T h e capillaries leaving the after-heater are cooled down in the quenching duct. By setting the velocity of the blowing air, a sufficient cooling effect must be reached together with extremely slight turbulence of the capillaries to avoid their collision and minimize denier variations. Having passed the quench duct, the threads are treated with a special preparation which guarantees good adhesiveness of the capillaries. T h e threads on bobbins must be kept for a definite time in a certain climate and are finally stretched on a draw twister. At intrinsic viscosities of 0.9 in the spunthread stretching, ratios in the range of 1:5 to 1 :6 can be applied. To reach a high tenacity and a high elongation a t break, the stretched threads are shrunk approximately 2 to 8%, whereby stretching and shrinking are combined in a two-stage operation on the same machine. Stretching speeds of about 200 m/min seem to be optimal in view of the increases of capillary breaks.
Design Features
T o obtain uniform capillary diameters and a negligible preorientation, temperature control becomes of predominant importance. Spinning pumps and spinnerettes of each manifold must be heated separately. However, this means also that single groups of the spinning system can be operated separately with different temperatures, a n advantage when a denier program must be partly changed. For the spinning procedure, spinnerettes with relatively large diameters--e.g., a t maximum 190-mm diameter with 192 capillaries-are used. These large diameters permit relatively great distances between the capillaries, a fact which guarantees uniform temperature distribution and reduced tendency of the threads to stick together. T h e behavior of the high viscosity liquids was studied in a scale model of the spinning manifold and spinning head applied for the production of polyester tire cord. By means of injected color, the residence behavior of the viscous medium could be observed in pipe lines, spinning pumps, filters and spinnerettes, and especially in “dead spaces.” Many changes led to satisfactory residence time spectra of the high viscous flow in all parts of the system. Short evaluation of physical properties of produced tire cord yarn. With the described continuous process, a n intrinsic viscosity of better than 0.9 can be constantly and safely obtained in the yarn. This high viscosity is a prerequisite for high tensile strength, and results in only slight fatigue and minimal twine loss. The continuous polycondensation process also permits a low carboxyl number (in the range of 20 X 30 X 10-6 equivalents/g a t viscosities close to 1.0) which leads to a high hydrolytic and thermal stability (Goodyear test). As already mentioned, a lower DEG content (below 2%) can be obtained by continuous esterification and polycondensation which means a high melting point and a lower tendency of the polymer to degrade. Values of above 8.5 g per denier can be obtained safely and the combined stretching and relaxing in a single procedure permit values of the elongation a t break of better than 1270. REFERENCES ( 1 ) Ellwood, P., “Continuous polyester condensation is obtained with two reactors,” Chem. Eng., 74, 98-100 (Nov. 1967). (2) Hardung-Hardun H. “Die kontinuierliche Herstellung von Polyester,” Vortrag, Debhema-%aguLg,Frankfurt-am-Main, 1967. (3) Horn F. C Oxley, J . H., and Jacob, “Production of high intrinsic viscosity Dolvethhene ilreohthalate industrial filament varn.” lecture. Williamsbure. “, Va. (4) Hummel U. P and Oxley J. H “The continuous production of polyester fibres from’ dem
