The present invention relates to intermediate molecular weight shaped polyethylene articles such as polyethylene fibers exhibiting relatively high tenacity, modulus and toughness, and to products made therefrom. The polyethylene article is made by a process which includes the step of stretching a solution of polyethylene dissolved in a solvent at a stretch ratio of at least about 3:1.
Polyethylene fibers, films and tapes are old in the art. An early patent on this subject appeared in 1937 (G.B. No. 472,051). However, until recently, the tensile properties of such products have been generally unremarkable as compared to competitive materials, such as the polyamides and polyethylene terephthalate. Recently, several methods have been discovered for preparing continuous low and intermediate molecular weight polyethylene fibers of moderate tensile properties. Processes for the production of relatively low molecular weight fibers (a maximum weight average molecular weight, Mw, of about 200,000 or less) have been described in U.S. Pat. Nos. 4,276,348 and 4,228,118 to Wu and Black, U.S. Pat. Nos. 3,962,205, 4,254,072, 4,287,149 and 4,415,522 to Ward and Cappaccio, and U.S. Pat. No. 3,048,465 to Jurgeleit. U.S. Pat. No. 4,268,470 to Cappaccio and Ward describes a process for producing intermediate molecular weight polyolefin fibers (minimum molecular weight of about 300,000).
The preparation of high strength, high modulus polyolefin fibers by solution spinning has been described in numerous recent publications and patents. German Off. No. 3,004,699 to Smith et al. (Aug. 21, 1980) describes a process in which polyethylene is first dissolved in a volatile solvent, the solution is spun and cooled to form a gel filament, and, finally, the gel filament is simultaneously stretched and dried to form the desired fiber. U.K. Patent Application No. 2,051,667 to P. Smith and P. J. Lemstra (Jan. 21, 1981) discloses a process in which a solution of a polymer is spun and the filaments are drawn at a stretch ratio which is related to the polymer molecular weight, at a drawing temperature such that at the draw ratio used, the modulus of the filaments is at least 20 GPa (the application notes that to obtain the high modulus values required, drawing must be performed below the melting point of the polyethylene; in general, at most 135.degree. C.). Kalb and Pennings in Polymer Bulletin, Volume 1, pp. 879-80 (1979), J. Mat. Sci., Vol. 15, pp. 2584-90 (1980) and Smook et al. in Polymer Mol., Vol 2, pp. 775-83 (1980) describe a process in which the polyethylene is dissolved in a non-volatile solvent (paraffin oil), the solution is cooled to room temperature to form a gel which is cut into pieces, fed to an extruder and spun into a gel filament, the gel filament being extracted with hexane to remove the parafin oil, vacuum dried and stretched to form the desired fiber.
Most recently, ultra high molecular weight fibers have been disclosed. U.S. Pat. No. 4,413,110 to Kavesh and Prevorsek describes a solution spun fiber of from 500,000 molecular weight to about 8,000,000 molecular weight which exhibits exceptional modulus and tenacity. U.S. Pat. Nos. 4,430,383 and 4,422,993 to Smith and Lemstra also describe a solution spun and drawn fibers having a minimum molecular weight of about 800,000. U.S. Pat. No. 4,436,689 to Smith, Lemstra, Kirschbaum and Pijers describes solution spun filaments of molecular weight greater than 400,000 (and an Mw/Mn&lt;5). In addition, U.S. Pat. No. 4,268,470 to Ward and Cappacio also discloses high molecular weight polyolefin fibers.
In general, the known processes for forming polyethylene and other polyolefin fibers may be observed as belonging in one of two groups: those which describe fibers of low average molecular weight (200,000 or less) and those which describe fibers of high average molecular weight (800,000 or more). Between the two groups, there is a molecular weight range which has not been accessible to the prior art methods for preparing fibers of high tensile properties.
There are advantages to the molecular weight ranges thus far mastered. Lower molecular weight polymers are generally synthesized and processed into fibers more easily and economically than high molecular weight fibers. On the other hand, fibers spun from high molecular weight polymers may possess high tensile properties, low creep, and high melting point. A need exists for fibers and methods which bridge this gap, combining good economy with moderate to high tensile properties. Surprisingly, our process makes it possible to accomplish these results.