Identifying Critical Process Variables in Poly(ethylene Terephthalate) Melt Spinning 1

Abstract

A quantitative procedure for identifying critical operating conditions and material properties for the melt spinning of poly(ethylene terephthalate) (PET) is considered. The method is based on the experimental observation that for vitrified polymers like polystyrene and PET spun at speeds below 3000 m/minute, the molecular orientation of the as-spun fiber is uniquely determined by σ L , the spinline stress at the glass transition temperature. Using a constant tension model that predicts σ L , a sensitivity analysis of the fiber orientation to changes in process variables is investigated. Extrusion temperature, melt intrinsic viscosity, feed rate, and the takeup velocity are the key variables for PET melt spinning, as they strongly affect the freeze line location and the as-spun fiber orientation.The production of poly(ethylene terephthalate) (PET) fibers or filaments involves melt spinning of the molten polymer followed by solid state drawing and annealing. The final fiber properties such as tensile strength, elongation, shrinkage, and dyeability are determined by structural parameters such as orientation and crystallinity. The fiber morphology is the result of the combined influence of the spinning, drawing, and annealing steps. Although the desired orientation and crystallinity are achieved primarily through the control of process parameters in the drawing and annealing steps, the fiber-line processibility in terms of drawability and level of broken and fused filaments is determined by the orientation and uniformity in asspun fibers. Also the final product quality parameters such as the coefficient of variation of fiber denier and tenacity, level of overlength fibers, etc., are influenced mainly by controlling the spinning process variables. This is because the basic spatial arrangement of polymer molecules with respect to the fiber axis is formed in the melt spinning step.The orientation developed in the spinning threadline is the result of tensile deformation under stress in the flowing melt. The net orientation in the flowing melt is determined by the relative rates of extensional flow and the disorienting thermal relaxation phenomena. These rates are governed by the velocity and temperature variation along the threadline. Figure 1 shows a schematic representation of the melt spinning process. Molten polymer is extruded through a spinneret into FIGURE 1. A schematic representation of the melt spinning process. a quench air stream blowing across the spinline. The spinline so formed cools and finally solidifies at a distance from the spinneret known as the &dquo;freeze line.&dquo; The solidified filament is then wound on takeup rolls at a speed significantly greater than the extrusion velocity. This difference in speed causes the filament to stretch, and the final cross-sectional area is considerably smaller (about 100 to 200 times) than the initial extruded area. In the absence of crystallization in the spinline, the orientation of the solidified filament rep-' NCL Communication No. 3185.

A physical–mathematical model for the prediction of fiber diameter of the polyethylene terephthalate (PET) spunbonding nonwoven fabrics

“…where is the polymer mass flow rate, is the filament fiber diameter, is the filament fiber mean velocity in the spinning direction, and is the specific density of the polymer. As the polymer density varies with polymer temperature, the following correlation was also introduced: where is the polymer temperature.…”

“…The correlation of the specific heat capacity of the PET melt with the polymer temperature is as follows: …”

“…The values of c and m are 0.42 and 0.334 [24], respectively. The correlation of the specific heat capacity of the PET melt with the polymer temperature [13,20,21] is as follows: C pf 5ð0:316310 24 TÞ34:182310 7 (9)…”

Recognition of fault process parameters for diameter uniformity variation in melt spinning

“…In the working process of melt spinning machine, the diameter of spinning can be used to evaluate the fiber structure of crystallinity and orientation degree, so as to change the internal quality of fiber [1]. When the diameter variation is small, the spinning quality is more uniform.…”

Parameter optimization in melt spinning by neural networks and genetic algorithms

“…Gupta et al [2] described influence of the speed of a metering pump on throughput rates. Dutta et al [3] used various materials, throughput rates, spinning temperatures, quench air speeds, quench air temperatures, and take-up speeds to yield different properties of as-spun fibers. Ziabicki [4] suggested that the process parameters acting on properties of as-spun fibers include the chemical compositions, molecular structures and physical behaviors of materials; spinning temperatures, dimensions and number of spinneret orifices; and throughput rates, take-up speeds, quench air temperatures, and lengths of spinning paths.…”