Process for preparing free flowing spheroidal nitrocellulose particles

Abstract: Fibrous water-wet nitrocellulose containing 4.5 to 5.5% moisture sprayed with a solvent/cosolvent mixture converts to spheroid, densified nitrocellulose via bilateral agitation. A free-flowing dust-free product having a lower burn rate is produced which dissolves more rapidly for lacquer production. A preferred apparatus is an orbital shaker and a preferred solvent/cosolvent is isopropyl acetate/isopropyl alcohol.

Description: The invention relates to a process for the preparation of an improved nitrocellulose product. In particular, the invention relates to a process for preparing a spheroid nitrocellulose with improved performance and safety features. Industrial nitrocellulose is available in various grades of nitration, i.e., high 11.8 - 12.2% and medium 11.2 - 11.8% and low 10.8 - 11.2%. For over 30 years the production of nitrocellulose has not experienced many significant changes. GB-A-336,235 to Imperial Chemical Industries Limited (ICI) describes a method of cutting-cellulose into pieces prior to the nitration process such that the nitrocellulose produced thereby has good bulk flow properties. US-A-2,028,080 to Hercules Powder Co. describes the preparation of a fibrous form of nitrocellulose. GB-A-1,175,607 to Nitrochemie Gesellschaft discloses that compacted and pourable nitrocellulose can be produced by moistening alcohol wet nitrocellulose with a solvent/plasticizer mixture and kneading to increase bulk density from 120g/l up to 365g/l. US-A-3,284,253 to Hercules Incorporated discloses the steps of heating nitrocellulose, treating with a solvent, distillation of solvent, separating excess water, displacing absorbed water and recovering nitrocellulose granules. US-A-3,325,315 to Hercules Incorporated discloses the production of a spheroid form of nitrocellulose by the steps: (a) forming a slurry with agitation of nitrocellulose fibers which are substantially free of water and also substantially free of fiber aggregates in a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent in the absence of a water-soluble protective colloid; (b) the organic liquid mixture being only a softening and swelling agent for the nitrocellulose fibers; (c) subjecting the resulting slurry of nitrocellulose fibers to high shear agitation, whereby the fibers become softened and swollen and break down into tiny, smooth-surfaced, rounded particle; (d) initiating hardening of the resulting shaped particles of softened and swollen nitrocellulose by diluting the slurry with hydrocarbon diluent while continuing high shear agitation; and (e) removing substantially all the nitrocellulose solvent by distillation in the presence of excess hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles. US-A-3,325,572 to Hercules Incorporated discloses a densification process for nitrocellulose in which a water-soluble polymer is used such as a cellulose ether. GB-A-1,000,876 and GB-A-1,051,561 to ICI describe plasticized nitrocellulose. US-A-3,985,691 to Societe Nationale Des Poudres et Explosives discloses a particulate nitrocellulose composition comprising 10 to 30% by weight of a hydroxylic plasticizer containing at least one free hydroxyl group capable of reacting with an isocyanate group. US-A-4,590,019 to Wolff Walsrode describes producing free-flowing nitrocellulose using a screw-extruder. From US-A-3,325,315 a process for the preparation of densified spheroidal nitrocellulose of high viscosity and casting termperatures is known wherein a methyl ethyl ketone/heptane solvent system should be used. From said document the moisture content of the nitrocellulose fibres and a bilateral agitation in preparing nitrocellulose particles as claimed in the present invention can not be derived. However, despite the considerable effort as evidenced by the patent literature, ease of handling and safety consideration for solid nitrocellulose had remained a long sought after property prior to the present invention. The invention provides a process for converting fibrous nitrocellulose to a free flowing form, characterized in that the process comprises the steps: (1) removing water from a water and fibrous nitrocellulose mixture of water-wet nitrocellulose until a moisture content of from 4.5 to 5.5 percent by weight is obtained based on the weight of fibrous nitrocellulose; (2) adding a liquid solvent/cosolvent mixture as a means for conversion of the resultant mixture from fibrous to spheroid form; and (3) using a means for bilateral agitation wherein conversion from fibrous to spheroid form occurs to produce a nonfibrous, densified nitrocellulose with improved flow properties and extended burn time in comparison with an alcohol-wet nitrocellulose. A preferred solvent/cosolvent mixture is isopropyl acetate/isopropyl alcohol and a preferred means for bilateral agitation is an orbital shaker. Fig. 1 Fibrous nitrocellulose of prior art. Fig. 2 Spheroid nitrocellulose prepared according to the invention. Fig. 3 Two grade nitrocellulose phase diagram. Suitable solvents are common esters and ketones. Cosolvents for nitrocellulose are usually alcohols. Aliphatic and aromatic solvents are diluents for nitrocellulose. It has been discovered that an improved spheroid form of densified nitrocellulose can be produced from nitrocellulose having a moisture content between 4.5 to 5.5% by weight using bilateral agitation of a solvent/cosolvent treated fibrous form. Unexpectedly, this novel nitrocellulose exhibits a higher dissolution rate and slower burning rate than alcohol wet nitrocellulose. Further, the higher bulk density allows more product to be packed for shipping while still enabling the product to flow freely due to the spheroid shape. It was further discovered that: (a) 10% moisture in nitrocellulose is the limit beyond which the method of the invention becomes impractical because agglomeration cannot take place; and (b) agglomeration occurs through a two step process. First the nitrocellulose becomes soft and tacky upon contact with a partial solvent, then it is compacted and shaped by an external force through shaking. The partial solvent can be a pure component (i.e., 2B alcohol) or a solvent/co-solvent mixture (i.e., isopropyl acetate/isopropyl alcohol) having the corresponding solubility parameter and hydrogen bonding index lying between the soluble and insoluble regions on the respective nitrocellulose phase diagram. Therefore, any solvent/co-solvent system will work as long as the mixture meets the criteria in terms of solubility parameter and hydrogen bonding index. While it would not have been possible to predict the mechanism and improvements provided by the present invention from thermodynamic first principles, it is now possible to apply scientific prediction to what has been empirically observed. The comparisons with the prior art and the parameters of operability are better understood by reference to the Figures. Fig. 1 Is a photograph of the prior art fibrous nitrocellulose which is manufactured and sold by Aqualon a Hercules Incorporated Co; Fig. 2 is a photograph of the spheroid nitrocellulose prepared according to the invention produced from the Fig. 1 starting material; and Fig. 3 is a phase diagram illustrating how solvent/cosolvent ratios would differ with two different grades of nitrocellulose, indicated by the solid and dotted lines. In the upper portion of the diagram it can be seen that pure ethanol can be either a solvent or a cosolvent depending on the grade of nitrocellulose used, while ethanol 2B is a solvent for both. While being illustrated in more detail by the examples which follow, the process of the invention can be summarized by the following steps. The first step of a preferred process is to dry water wet nitrocellulose until the moisture content falls within 4.5 to 5.5% by weight based on the weight of the nitrocellulose. Continuous monitoring of the material with automatic sensoring can be used to avoid over drying. The second step of the process is to apply a solvent/cosolvent mixture to the 4.5 to 5.5% moisture nitrocellulose. Spraying is a preferred application technique. Reference can be made to a phase diagram such a Fig. 3 to determine the correct solvent/cosolvent ratio to be used for the spray application. Isopropanol/isopropyl acetate is a preferred solvent/cosolvent mixture for softening and binding the nitrocellulose. The third step of the process is to place this material in an apparatus which can apply bilateral force to the softened nitrocellulose comprising specified moisture and absorbed solvent/cosolvent mixture. Such an apparatus is an orbital shaker. With a 150 to 200g sample it can take from 10 to 30 minutes for conversion to be accomplished. In detail the advantages provided by the spheroid nitrocellulose prepared according to the invention shown in Fig. 2 over the previously produced material shown in Fig. 1 are as follows: A. A bulk density increase of up to 50% allows more material to be packed in a drum. B. The free-flowing characteristics allow the drums to be unloaded by tilting. C. The product is dust-free so that product yield and housekeeping are improved. D. Faster dissolution in lacquer formulation increases productivity. E. Slower burning indicates the product is safer to store and handle. The apparatus used for fibrous to spheroid conversion was a laboratory bottle shaker in which the moisture and solvent/cosolvent nitrocellulose softened material was placed in glass bottles with paper-lined lids to prevent static electricity generation. The shaker is a rack with eight (8) multiple arms radially mounted on top of an eccentric drive unit. Each arm is equipped with a bottle holder which can securely hold a bottle up to 10 cm in diameter. The drive provides orbital motion to each bottle at approximately 300 cycles per minute. The two-dimensional motion has vertical oscillation of 6.3 cm and horizontal oscillation of 1.3 cm amplitudes. The invention has Industrial Applicability for the production and use of nitrocellulose for coatings and finishes. The following examples illustrate the practice of the invention without being limiting. EXAMPLE 1 Water-wet nitrocellulose with a moisture content of about 30% was placed in a brass container having screens on the top and bottom and dried with a hot air dryer. After 90 minutes a sample of 50g was obtained which was determined to have a moisture content of approximately 5% by gravametric analysis. This procedure was repeated with five different grades of nitrocellulose characterized as follows: Viscosity Type Degree Polymerization Intrinsic Viscosity RS 1/8 (18-25 mPa.s) 156 0.48 RS 1/8 (30-35 mPa.s) 187 0.56 RS 1/4 215 0.67 RS 1/2 292 0.90 RS 5-6 600 1.84 Wetting agents of different isopropyl acetate concentrations in isopropyl alcohol were made up for different viscosity grades of nitrocellulose by mixing the appropriate amounts of acetate and alcohol as follows: Viscosity Type Acetate in Mix (parts) Alcohol in Mix (parts) Resultant Mix in Alcohol (Wt. %) RS 1/8 (18-25 mPa.s) 5 95 5 RS 1/8 (30-35 mPa.s) 8 92 8 RS 1/4 10 90 10 RS 1/2 14 86 14 RS 5-6 18 82 18 The dried nitrocellulose was placed in a 405 cm 3 [16 oz. (5.7 cm D x 15.9 cm L)] bottle and sprayed with the wetting agent to 70% total solids. The acetate-alcohol-wetted nitrocellulose was then placed on the shaker for ten to thirty minutes of mechanical conditioning to achieve the following bulk densities: Viscosity Type Shaking Time Minutes Bulk Density (dry basis) lb/ft 3 g/cm 3 RS 1/8 (18-25) 0 29.48 0.47 10 36.74 0.59 (with 5% acetate in alcohol) RS 1/8 (30-35) 0 27.12 0.43 10 32.69 0.52 (with 8% acetate in alcohol) RS 1/4 0 26.01 0.42 20 31.61 0.51 (with 10% acetate in alcohol) RS 1/2 0 19.55 0.31 25 29.73 0.48 (with 14% acetate in alcohol) RS 5-6 0 16.25 0.26 30 22.58 0.36 (with 18% acetate in alcohol) A sample of spheroid nitrocellulose produced from grade RS 1/8 (18-25) was subjected to a burn test and observed to take 90 seconds to burn compared to 60 seconds for untreated control material. Example 2 Nitrocellulose with 4.5 to 5.5% moisture was treated with solvent/cosolvent mixtures of methanol/butanol; ethyl acetate/isopropanol; methyl ethyl ketone/ethanol, etc. as selected from the phase diagram of Fig. 3 following the procedure illustrated in Example 1. In all case, spheroid nitrocellulose was produced with up to thirty minutes bilateral shaking. Example 3 A test was repeated as in Example 1 except that all eight bottles contained the same identical compositions. Testing of the eight resulting samples showed reproducable results for the production of spheroid nitrocellulose with improved flow and extended burn time. Example 4 Test lacquer formulations were prepared using samples of the materials illustrated in Fig. 1 and Fig. 2. Whereas it took more than an hour for the prior art nitrocellulose to dissolve to form a clear lacquer, it required only thirty minutes to prepare a clear lacquer using the spheroid nitrocellulose of the invention. Example 5 (comparative) RS 1/8 (18-25) and RS 1/4 nitrocellulose was tested for water content toleration. Starting with about 30% moisture, the samples were dried to various moisture levels using a hot-air blower. The samples were sprayed with isopropyl acetate and ispropyl alcohol as in Example 1. The wetted samples were then mechanically conditioned by agitating on a shaker. All samples were wetted to 70% total solids prior to shaking. The finished products were spheres in various sizes. Results of the experiment are tabulated in Table 1. Results on Agglomeration as a Function of Nitrocellulose Content NC Type Wetting Agent Moisture Content Product Appearance RS 1/8 (18-25) 5% IPAC, 95% IPA 0 2 mm size balls 5% IPAC, 95% IPA 3 2 mm size balls 5% IPAC, 95% IPA 7 3 mm size balls 5% IPAC, 95% IPA 10 2 mm size balls 5% IPAC, 95% IPA 12 big, gummy lumps 8% IPAC, 92% IPA 12 big, gummy lumps RS 1/4 10% IPAC, 90% IPA 0 1 mm size balls 10% IPAC, 90% IPA 8 2 mm size balls 10% IPAC, 90% IPA 10 2 mm size balls 10% IPAC, 90% IPA 12 big, gummy lumps 12% IPAC, 88% IPA 12 big, gummy lumps IPAC - isopropyl acetate IPA - isopropyl alcohol Mixtures comprised of solvents and co-solvents/diluents were used to wet the nitrocellulose prior to shaking. Again the nitrocelluloses used in this experiment were RS 1/8 (18-25) and RS 1/4. The solvents selected were common esters and ketones. The balance of the formulation was either alcohols (generally regarded as co-solvent for nitrocellulose) or common organic solvents (aliphatic or aromatic, generally regarded as diluents for nitrocellulose). After wetting with the appropriate solvent system, the nitrocellulose was subjected to the 2-dimensional shaking motion and the results are summarized on Tables 2. Results on Agglomeration as a Function of Wetting Mixture Composition for RS 1/8 (18-25) Nitrocellulose NC Type Wetting Agent Moisture Content Product Appearance RS 1/8 (18-25) 100% IPA 7 2 mm size balls 100% 2B 7 3 mm size balls 5% EAC, 95% IPA 7 2 mm size balls 5% IPAC, 95% IPA 10 3 mm size balls 5% IPAC, 90% amyl alc. 12 2 mm size balls 8% IPAC, 92% IPA 12 Powderous, no agglomeration RS 1/4 10% acetone, 90% IPA 7 2 mm size balls 10% acetone, 98% 2B 3 2 mm size balls 10% acetone, 90% amyl alc. 3 2 mm size balls 10% acetone 90% toluene 7 Powderous, no agglomeration 10% acetone 90% hexane 7 Powderous, no agglomeration 10% acetone 90% hexane 0 Powderous, no agglomeration 10% MEK 90% IPA 7 2 mm size balls 10% cellosolve 90% IPA 7 1 mm size balls 10% cellosolve 90% toluene 7 Powderous, no agglomeration IPAC - isopropyl acetate IPA - isopropyl alcohol EAC - ethyl acetate MEK - methyl ethyl ketone 2B - 92.5% ethyl alcohol The results from Table 2 indicated that agglomeration can be achieved by wetting nitrocellulose with a mixture comprised of an active solvent and a co-solvent. However, agglomeration is not possible using a solvent diluent mixture.

Claims: A process for converting fibrous nitrocellulose to a free flowing form, characterized in that the process comprises the steps: (1) removing water from a water and fibrous nitrocellulose mixture of water-wet nitrocellulose until a moisture content of 4.5 to 5.5 percent by weight is obtained based on the weight of fibrous nitrocellulose; (2) adding a liquid solvent/cosolvent mixture as a means for conversion of the resultant mixture from fibrous to spheroid form; and (3) using a means for bilateral agitation wherein conversion from fibrous to spheroid form occurs to produce a nonfibrous, densified nitrocellulose with improved flow properties and extended burn time in comparison with an alcohol-wet nitrocellulose. The process of claim 1 wherein the solvent/cosolvent mixture is isopropyl acetate/isopropyl alcohol. The process of claim 2 wherein an explosion proof equipment is used throughout the process. The process of claim 1 wherein an orbital shaker is used in step 3.

Inventors: FUNG KWAN F, FUNG, KWAN F.
Applicants: AQUALON CO, AQUALON COMPANY
Application year: 1992
Main classification: C08B5
IPC codes: B01J2, B29B13, C08B5, C08J3, B29K1