Patent GB-815788-A (GB)
Title: Improvements in or relating to conversion of cellulose derivatives into granular form
Inventor: ANDREW EUGENE A
IPC: C08B5/06
Filed: 1956-02-20
Abstract: <PICT:0815788/IV (a)/1> <PICT:0815788/IV (a)/2> Minute hard globules of plasticized or unplasticized cellulose derivatives are obtained by extruding a viscous lacquer consisting of a solution of the plasticized or unplasticized cellulose derivative in a solvent, of which no more than 24 per cent by weight is soluble in water at 20 DEG  C. and which boils below 100 DEG  C., through one or more orifices submerged in an aqueous medium, severing the resultant streams of lacquer into short lengths or slugs while the streams are submerged in the aqueous medium, immediately suspending the slugs of lacquer in the aqueous medium, subjecting the slugs to liquid shear while suspended in the aqueous medium to disintegrate the slugs and disperse the lacquer globules thus formed in the aqueous medium, and removing the solvent from the dispersed globules by heating the dispersion to a temperature above the boiling point of the solvent. The cellulose derivative may be nitrocellulose or cellulose acetate. The nitrocellulose may be dry, water-wet, alcohol-wet, fibrous or pre-gelatinized before being formed into the lacquer.  The nitrocellulose solvent may be ethyl or isopropyl acetate, methyl ethyl ketone, or a mixture of four parts of methyl ethyl ketone with one part of toluene.  The diameter of the extrusion orifices is preferably 0.015-0.030 inch.  The aqueous suspending medium may be a mixture of 94 parts of water, two parts of gum arabic, two parts of ethyl acetate and two parts of sodium sulphate.  As shown in Fig. 1, an apparatus which may be used for the process consists of a cylindrical vessel having a side wall 1 and end plates 2, 3 which are drawn up into sealing engagement with the ends of side wall 1 by a series of bolts 4 furnished with wing nuts.  In the bottom of the vessel there is provided an extrusion head 5 having an orifice plate 6 with extrusion orifices 7.  The apparatus is provided with a rotor having a shaft 8, on the lower end of which is fixed a member having a number of radially extending cutter blades 9 which ride upon the upper surface of the orifice plate 6 and which, as they pass over the orifices 7, sever the extruded streams of lacquer to form slugs of lacquer.  The lacquer is fed to the extrusion head under pressure through the inlet 14.  Within the side walls 1 a stator 10 has a series of axially spaced baffles 11 extending towards the axis of the vessel and dividing the free space in the latter into a series of shear chambers 12.  Turbine blades 13 fixedly mounted on the shaft 8 are accommodated in the shear chambers with sufficient clearance to effect the desired lacquer particle size reduction.  The shaft 8 is driven from an external source at a relatively high speed, e.g. 1500 r.p.m.  The turbine blades are preferably pitched to oppose the upward pressure exerted by the incoming liquids, i.e. the lacquer entering the apparatus through inlet 14 and the aqueous suspension medium fed into the apparatus through the inlet 15.  The lacquer dispersion is continuously discharged from the vessel through a tangential outlet 16 into a still in which the dispersion is heated to a sufficiently high temperature to drive off the solvent from the lacquer globules.  The nitrocellulose is thereby converted into the form of hard globules of nearly spherical shape, 95 per cent or more of which are below 75 microns in diameter.  The suspension of lacquer slugs may also be subjected to liquid shear in a conventional homogenizing apparatus such as that shown in Fig. 2, the suspension having previously been prepared in an apparatus such as that shown in Fig. 1 and preferably subjected to mild shearing in that apparatus. In an example, a lacquer consisting of one part of nitrocellulose dissolved in four parts of ethyl acetate is extruded and cut into slugs and suspended in double its weight of an aqueous liquor in the apparatus shown in Fig. 1.  After being subjected to mild shearing in the chamber 12 the suspension is passed twice at the rate of 1 or 2 gallons per minute through the homogenizer shown in Fig. 2.  This has a cylindrical wall 21, end closures 22, 23, a rotor 26 mounted on shaft 27 driven at 5000 r.p.m., inlet 24 and outlet 28.  The clearance between the rotor 26 and the stator 25 is 0.0005 inch.  The homogenized lacquer emulsion is then passed into a vessel in which the solvent is distilled off from the lacquer droplets.