Band 4, Heft 2

The demands of the fiber and film producers, the result of scientific polymer research studies and the ideas and work of machine manufacturers determine the trends and developments in fiber and film extrusion. The main goal is the improvement of the fiber- and filmproduct-quality and of the process efficiency. Judged by their results, we believe the most significant new developments in the fiber and filmextrusion are: Polymer cleaning by melt filtration, polymer blends by dynamic polymer mixing, coextrusion of different polymers, product specific orientation, precision in melt metering, high speed spinning resp. high speed film extrusion, inline processes with automatic process control.

Extrusion rates of polyethylene are limited by the onset of surface defects in the extrudates, which are variously termed sharkskin, melt fracture and so on. Several expedients are effective in relieving this problem. We report here that extrudate surface distortion can be eliminated under some conditions by cooling the die, at normal melt extrusion temperatures. This technique appears to be generally effective for LLDPE and HDPE. The process has been patented [10] for polymers with narrow molecular weight distributions.

We have studied the development of the sharkskin defect of a linear low density polyethylene, by using complementary experimental techniques. Roughness measurements have allowed to accurately characterize the surface of the extrudates. Flow birefringence observations have permitted to visualize the flow conditions inside the die. The development of the defect is strongly dependent on the temperature and quasi-independent on the geometry (die length, entry angle). Theoretical computations of the flow through the slit die have allowed the correlation of the amplitude of the defect with the stress distribution. Sharkskin defect appears as a continuous and regular phenomenon of surface degradation without intervention of wall slip.

Polyethylene terephthalate (PET) was spun into fibers in the speed range of 3000 to 5000 m/min. Threadline dynamics was modified by altering the threadline temperature profile. This was achieved using a combination of on-line zone cooling and heating(OLZCH). Experimental results showed that the structure development in the threadline was significantly influenced by the OLZCH conditions. The orientation, crystallinity and crystal dimensions of the resulting fibers were greatly increased by the introduction of OLZCH in the take-up speed range of 3000 to 5000 m/min. Various characterization techniques confirmed that the fiber structure was obviously changed with OLZCH. The fiber tensile properties were noticeably improved via this approach. It is concluded that the fiber structure development may be dominantly enhanced by modifying the threadline dynamics in the high speed melt spinning process.

Air jet melt spinning for multifilament system was studied both theoretically and experimentally. Basic equations describing the multifilament system of air jet melt spinning were derived by considering air temperature and velocity of cooling flow surrounding the individual spinlines. Numerical computations of the fundamental equations for air jet melt spinning were carried out for various spinning conditions. Steady-state solutions obtained give the spinline cross sectional area, spinline temperature, spinning tension and spinline velocity. These are described as a function of the distance measured downward from the spinneret. Air jet melt spinning experiments were conducted for poly-(ethyleneterephthalate) (PET) having an intrinsic viscosity of 0.61. The spinning variables were changed mainly in terms of throughput, distance from spinneret point to ejector and air pressure in the ejector. Good correlations between theory and experiment were obtained for the final spinline speed or as-spun fiber denier. Further, the molecular orientation of asspun fiber can be estimated using the computed stretching speed at the ejector point. It was found that the simulation program of multifilament air jet spinning is very useful in finding the appropriate spinning conditions for the industrial spinning process and in estimating fiber properties.

The development of crystallinity and orientation in melt spun and subsequently drawn poly p phenylene sulfide (PPS) fibers is investigated using differential scanning calorimetry (DSC), birefringence and wide angle x-ray diffraction (WAXS) techniques. The melt spun PPS fibers were found to possess small amounts of crystallinity at moderate draw-down ratios. The fibers may be crystallized to a significant extent by subsequent drawing over a hot plate. The levels of crystallinity and orientation in the largely glassy melt spun PPS fibers has been determined as well as subsequently drawn fibers. Uniaxial mechanical properties were determined as a function of spinning and drawing conditions. In general, elongation to break decreases while modulus and tensile strength increase with an increase of melt spinning draw down ratio and solid draw ratio. The Young's modulus and tensile strengths are increasing functions of fiber birefringence and the elongation to break a decreasing function.

In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was devoted to the development and application of a thermomechanical model of stretching in air and of a purely thermal model of cooling on the roll, taking polymer crystallization into account. The structures and morphologies in a series of extruded polypropylene films were investigated using density and sonic modulus measurements, wide-angle X-ray diffraction, small angle light scattering and optical microscopy. These experimental results combined with theoretical predictions obtained from the model allow to discuss the respective effects of stretching in air and cooling on the roll. Stretching in air essentially affects spherulite nucleation and growth during subsequent crystallization on the roll. Dissymmetrical cooling on the roll induces an inhomogeneous microstructure within the film. The presence of a zone with a finer microstructure near the roll has been correlated with the cooling rates and thermal gradients calculated by the model.

The air ring has an effect on blown film dynamics. By using a momentum macrobalance, an analytical solution for the pressure on the outer surface of the bubble is obtained which is in fairly good agreement with our aerodynamic experimental data. Since the pressure is related to the hoop stress, a more complete prediction of the bubble shape is attained by taking the air ring effect into account.

Composition dependence of the polyblends' properties is usually associated with a synergism (the positive deviation from additivity). Using the bicubic spline interpolation technique and the melt viscoelasticity data the effects of shear stress on the location and intensity of the synergism is demonstrated for a set of binary mixtures of commercial polyolefins. Principal patterns of the “transient” and the “set-in” synergisms are identified.