Patent GB-1165865-A (GB)
Title: Preparation of Sheath/Core Filaments
IPC: B29C48/05
Filed: 1968-04-17
Abstract: 1,165,865. Sheath-core filaments. E. I. DU PONT DE NEMOURS & CO. 17 April, 1968 [18 April, 1967], No. 18231/68. Heading B5B. [Also in Division G2] A sheath-core filament is made by coextruding sheath and core polymer streams from a spinneret, the method being characterized by (1) forming a circumferentially pressure-equalized reservoir of molten sheath polymer about the outlet of a core polymer passage, (2) extruding a stream of molten core polymer from said outlet of said passage toward an orifice, and (3) simultaneously with (2) extruding a stream of molten sheath polymer from said reservoir into intimate contact with said core polymer stream, thereby enveloping said core polymer in a sheath and forming a moving annular surface which is the interface between the sheath polymer stream and said core polymer stream, (4) providing a fixed annular surface in the path of said sheath polymer stream which is convergent with respect to said moving annular surface as said moving surface approaches said orifice, thereby forming a circumferential pressure-equalizing region within said sheath polymer stream which centres said core polymer stream in said sheath polymer stream, and (5) drawing said sheathcore filament through said orifice. The fixed surface may be convergent towards said orifice, and in this case the length of said moving annular surface is preferably equal to at least 0À48 times the diameter of said core polymer stream in said orifice, filaments so produced having desirably at least a 4 : 1 core to sheath volume ratio, the maximum thickness of the sheath at said orifice being 0À10 mm., and the eccentricity ratio no greater than 2, and especially no greater than 1À5. Alternatively, the moving surface may be divergent towards said orifice, and in this case the length of the moving annular surface may be equal to at least 0À22 times the diameter of said core polymer stream in said orifice, filaments so produced having desirably at least a 14 : 1 core to sheath volume ratio, the maximum thickness of the sheath at the orifice being 0À10 mm., and the eccentricity ratio no greater than 2, and especially no greater than 1À5. In a further modification, the fixed surface may be convergent towards said orifice and the moving surface divergent towards said orifice. Alternatively, both the fixed surface and the moving surface may be convergent towards said orifice and in this case fixed and moving surfaces are preferably decreasingly monotonically convergent towards said orifice (i.e., the rate of convergence with distance never drops to zero). In the apparatus of Fig. 3, molten polymeric core material is extruded down the passage 22 of injector 12, while molten polymeric sheath material is being extruded through channel 20. Sheath polymer reservoir 14 fills with sheath polymer material, which comes into intimate contact with the core polymer material at the outlet 30 of the ejector 12. The ejector tip 26 is recessed far enough from the fixed annular surface 24 so that the annular space therebetween is less restrictive of the sheath polymer stream than the annular space between the core polymer stream and the block 4 at orifice 16. Thus the pressure of the polymeric sheath material is substantially equal on all sides of the polymeric core material at the outlet 30 of core polymer passage 22 due to the ability of a slowly flowing fluid under pressure to circumferentially equalize pressure within itself. Thus, in operation, a wedge-shaped annular circumferential pressure-equalizing region 28 (see Fig. 4) is formed below the tip 26 of the ejector within the sheath polymer stream. This wedge-shaped region 28 is defined by the fixed annular surface 24 of the reservoir and moving annular surface 32 formed by the interface of the sheath and core polymer streams, and these surfaces are convergent with respect to each other in the direction of orifice 16. Preferably the wedge-shaped region is monotonically decreasingly convergent toward orifice 16. The rate at which the wedge-shaped surfaces converge need not be uniform but the angle between the fixed and moving surface should be in the range of 5 to 75 degrees and preferably from 25 to 60 degrees. As the fixed surface 24 and the moving surface 32 approach orifice 16, the wedge-shaped region 28 can be produced in several different ways: the fixed surface 24 can be convergent towards the orifice while moving surface 32 is neither convergent nor divergent with respect to orifice 16; fixed surface 24 can be convergent toward the orifice while moving surface 32 is divergent toward orifice 16; and both surfaces 24 and 32 can be convergent toward the orifice. Figs. 6, 7 and 8 show enlarged partial views of alternative constructions of a spinneret such as that of Fig. 4 with ejector tip 26 and/or surface 24 being changed so as to produce a reverse wedge-shaped region 40. In Figs. 6 and 7 the sheath polymer passage 20 is highly restricted near the tip of ejector 12. The process is applicable to the production of any sheath-core filament, but is particularly useful in the production of sheath-core fibre optical filaments of small diameter where concentricity of the sheath and core is most important due to the small diameter of the filament and the need to maximize the light transmission at any given fibre diameter. The general extrusion range of apparent viscosities of thermoplastic polymers for which the process is useful is about 10<SP>2</SP> to about 10<SP>5</SP> poises. Specified thermoplastic materials for fibre optic production include polystyrene, polymethyl methacrylate, polyfluoroalkyl methacrylate and fluoroalkyl methacrylate/methyl methacrylate copolymer. In the examples, the sheath polymer is polyfluoroalkylmethacrylate, and the core polymer polymethyl methacrylate moulding resin.