Production of polyester filaments of high strength possessing an unusually stable internal structure employing improved processing conditions

In a process for the production of improved polyester filaments of high strength having an unusually stable internal structure which particularly are suited for use at elevated temperatures, comprising:

• (a) extruding a molten melt-spinnable polyester which contains 85 to 100 mol percent polyethylene terephthalate and 0 to 15 mol percent of copolymerized ester units other than polyethylene terephthalate having an intrinsic viscosity of 0.5 to 2.0 deciliters per gram through a shaped extrusion orifice having a plurality of openings to form a molten filamentary material,
• (b) passing the resulting molten filamentary material in the direction of its length through a solidification zone having an entrance end and an exit end wherein said molten filamentary material is uniformly quenched and transformed into a solid filamentary material,
• (c) withdrawing said solid filamentary material from said solidification zone while under a substantial stress of 0.015 to 0.150 gram per denier measured immediately below the exit end of said solidification zone,
• (d) continuously conveying said resulting as-spun filamentary material from the exit end of said solidification zone to a stress isolation device at a rate in excess of 500 meters per minute up to 3000 meters per minute with said filamentary material as it enters said stress isolation device exhibiting a relatively high birefringence of +9.times.10.sup.-3 to +70.times.10.sup.-3,
• (e) continuously conveying said resulting filamentary material from said stress isolation device to a first draw zone,
• (f) continuously drawing said resulting filamentary material at a draw ratio of 1.01:1 to 3.0:1 while present in said first draw zone, and
• (g) subsequently thermally treating said previously drawn filamentary material while under a longitudinal tension and present at a temperature above that of said first draw zone to achieve at least 85 percent of the maximum draw ratio of said as-spun filamentary material and impart a tenacity of at least 7.5 grams per denier to the same, with at least the final portion of said thermal treatment being conducted at a temperature within the range from about 90.degree. C. below the differential scanning calorimeter peak melting temperature of the same up to below the temperature of which filament coalescence occurs;
• the improvement comprising providing said first draw zone in which step (f) is carried out, throughout at a temperature below the glass transition temperature of said as-spun filamentary material thereby facilitating a savings of energy when compared with polyethylene terephthalate drawing procedures of the prior art, and concomitantly enabling said drawing step (f) to be carried out in combination with the other process steps on a stable basis in the substantial absence of filament neck drawing.