Patent GB-793861-A (GB)
Title: De-watering solutions of organic acid esters of cellulose
IPC: C08B3/22
Filed: 1956-06-21
Abstract: <PICT:0793861/IV (a)/1> Water-containing solutions of carboxylic acid esters of cellulose in a water-immiscible solvent (which may be soluble in water to an appreciable but small extent only) are de-watered by feeding into a body of the solution a substantially dry band or like structure comprising film, filaments or the like of hydrophilic polymeric material that is insoluble in the solution, allowing the band to remain in the body of solution long enough to take up a substantial proportion of water, and removing the band from the body of solution.  Examples of suitable hydrophilic materials are cellulose, sodium carboxymethyl cellulose, wool, regenerated casein fibres and silk.  For each part by weight of water in the solution there may be used 2-5 parts by weight of the polymeric material, and de-watering can usually be completed in about 5-20 minutes.  The concentration of cellulose ester in the solution may be 2-10 per cent and the solution may contain about 1-3 per cent by weight of water, part of which may be present in the form of an emulsion.  The solution may result from the removal of water-soluble impurities from a cellulose esterification solution by extraction with an aqueous extractant, for example, as described in Specification 779,802.  The cellulose ester may be an acetate, propionate, butyrate, acetate propionate, acetate-butyrate or benzoate.  The solvent may be nitromethane, m-cresol, cyclopentanone, pyrrole, furfural, trichlorethanol and especially chlorinated hydrocarbons such as methylene chloride, trichloroethane or chloroform or a mixture of two or more of these.  In addition, there may be present a small proportion of a lower aliphatic alcohol.  The process may be carried out in the apparatus shown in the drawing, in which the solution to be de-watered enters the tank 11 through the inlet 12, and travels in the tortuous path defined by the baffles 14, 15 to the other end of the tank, which it then leaves by the outlet 13.  At the same time an endless band 16 of a hydrophilic polymeric material, in the form of a film or fabric, enters the tank between a pair of squeeze rolls 17 at its outlet end, passes through the tank along a sinuous path in counter-current to the solution over a series of guide rolls 18, taking up water from the solution during its passage through the tank.  The band then leaves the inlet end of the tank between a pair of squeeze rolls 19 and passes between a pair of squeeze rolls 21 into a rinsing tank 22, through which it travels in a sinuous path over guide rolls in countercurrent to a flow of a solvent for the cellulose ester.  This solvent, which is preferably identical with that in the solution to be de-watered, enters the tank 22 through the inlet 26, travels in a sinuous path defined by the baffles 23, 24 to the other end of the tank, which it then leaves by the outlet 16.  By this means the band is rendered substantially free from the cellulose ester.  The used solvent leaving through the outlet 27 may advantageously be added to the solution to be de-watered before it enters the inlet 12. After leaving the tank 22 through the squeeze rolls 29 the band passes over the suction box 31, whereby a stream of air is caused to flow over or through it to remove residual solvent. It then passes between the squeeze rolls 33 into a drying chamber 34, through which it is guided in a series of festoons by means of rolls 35, leaves the chamber between squeeze rolls 39, and returns to the tank 11 through the squeeze rolls 17.  In order to move the band through the apparatus, any or all of the rolls 18, 25 and 35 may be driven.  Hot air enters the drying chamber 34 through inlet 36, flows through the chamber in countercurrent to the band along a sinuous path defined by the baffles 38 nearly touching the rolls 35, and leaves the chamber through the outlet 37.  In the example, a dry fabric of viscose rayon is immersed for 15 minutes in a cloudy methylene chloride solution of cellulose acetate of 6 per cent concentration containing 2.2 per cent of water, the solution/fabric weight ratio being 1 : 0.059.  The solution clears and its water content is reduced to below 0.5 per cent.  The fabric is squeezed to remove solution from it, rinsed with methylene chloride and dried.