GB1388881A - Polymeric pulp for paper and process of preparing polymeric pulp - Google Patents

Abstract

1388881 Preparation of pulp CROWN ZELLERBACH INTERNATIONAL INC 2 June 1972 [3 June 1971 28 Dec 1971] 25928/72 Heading D2A A synthetic polymer pulp for paper-making comprising discrete elongate rolls formed from films of thermoplastic polymer and extending in random directions from one another, is produced by a process comprising discharging a solution of thermoplastic polymer from a zone of higher pressure into a zone of lower pressure at a temperature high enough to effect flash avoporation of solvent, under conditions such that only part of the solvent is vaporised while polymer is precipitated in residual liquid solvent to form a fibrous gel and then refining the gel to disentangle the fibres. The solution of the polymer is heated in a jacketed pressure vessel 10 provided with a stirrer 11. Heating fluid passes to the jacket via conduits 12, 13, and the solvent and a polyolefin enter the vessel 10 via conduits 14, 15. An inert gas may be introduced via a conduit 16. Alternatively, the vessel 10 may be a polymerisation reactor vessel in which an olefin or mixture of olefins is polymerized, or the polymerization is carried out in another vessel and the polyolefin solution or slurry is fed to the vessel 10. From the vessel 10 the solution is fed via a heated conduit 18 and through a nozzle 22 into a precipitation conduit 23, the pressure and temperature of the solution being adiabatically reduced and a portion of the solvent vaporised during passage through the nozzle 22 to produce the fibrous gel which is passed to a vessel 26, e.g. a cyclone, in which the solvent vapour is removed for recycling. From the vessel 26 the gel passes to a refiner 28, e.g. a disc refiner or beater. The polymer concentration in the gel before refining is less than 5% by weight and the gel may be diluted with solvent to attain this concentration. Some solvent vaporization may be effected before the solution passes through the nozzle 22 to cause vapour cutting of the fibres in the gel. The nozzle 22 which is an adjustable throttling ball valve creates sufficient pressure drop to cause violent vaporization of the solvent so that the resulting turbulence also causes the fibres in the gel to break up. After the refiner 28 free solvent may be removed from the gel by filtering, decanting or centrifugation to leave a more concentrated suspension of pulp in the solvent. Paper may be made directly from this pulp but it is preferred to convert the pulp to a suspension of the fibres in water by solvent exchange, i.e. displacement of the solvent by another which can be displaced by water. Alternatively, steam stripping or freeze drying of the solvent may be effected. The fibres are treated with a water dispersing agent prior to or during steam stripping to prevent fibre agglomeration, the agent being preferably polyvinyl alcohol. The fibres formed by the elongate rolls of film have a rough skin texture. The thermoplastic polymer is preferably a crystalline olefin polymer, e.g. polyethylene or polypropylene. The solvent may be liquid or gaseous at room temperature and may be benzene, toluene, butane, pentane, hexane, heptane, octane, cyclohexane, methylene chloride, carbon tetrachloride, chloroform, ethyl chloride, methyl chloride, alcohols, esters, ethers, ketones, nitriles, amides, fluorohydrocarbons, sulphur dioxide, nitromethane or mixtures of these. In an example a handsheet was formed on a stainless steel screen, couched, cold-pressed against a polished caul on both sides and dried on a rotary drier. In a second example synthetic polymer fibres were blended with bleached alder kraft fibres and a paper formed on a small paper-making machine.