Nitrocellulose dissolution

The first plastics were actually developed during the last half of the 19th century. Paper is composed to a significant extent of the natural polymer cellulose and closely related substances. Treatment of paper with nitric acid produced the first (semi-) artificial polymer, nitrocellulose. Dissolution of nitrocellulose in alcohol/ether gave a viscous solution (collodion) which forms a hard film upon solvent evaporation. The polymer thus formed was quite flammable. An improved product based on nitrocellulose, termed celluloid, was molded into... 

These opposing tendencies may defeat the purpose of the fractional precipitation process. The fractional precipitation of crystalline polymers such as nitrocellulose, cellulose acetate, high-melting polyamides, and polyvinylidene chloride consequently is notoriously inefficient, unless conditions are so chosen as to avoid the separation of the polymer in semicrystalline form. Intermediate fractions removed in the course of fractional precipitation may even exceed in molecular weight those removed earlier. Separation by fractional extraction should be more appropriate for crystalline polymers inasmuch as both equilibrium solubility and rate of solution favor dissolution of the components of lowest molecular weight remaining in the sample. 

The initial dissolution is now usually carried out in copper tubs with a water jacket which maintains a temperature of 45-50°C in the tub. The tubs contain 100 kg of a mixture of nitroglycerine and nitrocellulose. Their contents are stirred with wooden paddles and then allowed to stand for 20-30 min. This is sufficient time for complete dissolution of nitrocellulose at this temperature. 

Soon after the manufacture of nitrocellulose smokeless powder began it was established that the powder obtained by the partial dissolution of nitrocellulose in a mixture of alcohol and ether (partly colloidal powder) has a chemical stability inferior to that of the nitrocellulose from which it derived. Thus Vieille [11] reports that on heating to a temperature of 110°C CPj guncotton undergoes denitration with the evolution of 0.04 cm3 NO/hr/gramme whereas the powder obtained from these substances without a stabilizer undergoes denitration at more than twice the rate, namely 0.10-0.15 cm3 NO/hr/gramme of substance. 

For safety purposes nitrocellulose is delivered to the factory in a wet state and before it is partially dissolved in a mixture of alcohol and ether the water must be removed since this prevents the process of swelling and dissolution. 

The total content of residual solvent is determined by partially dissolving a weighed sample of the powder in a solvent (a mixture of alcohol and ether) and adding water so as to precipitate the nitrocellulose from the solution in llocculant form. The weight of this nitrocellulose is determined by evaporation to dryness, repeated dissolution and precipitation with water, and by final drying. The difference between the weight of the powder sample and that of the nitrocellulose is the weight of the residual solvent. 

Microscopic investigations of the gelatinization and dissolution of nitrocellulose seem to indicate that a protective cover exists on the surface of nitrocellulose fibres which dissolves with difficulty (Mangenot and Raison [134], Fenson and Fordham [133], p. 286). 

According to investigations reported by Jenkins and G. Davies [28] and by T. Urbafiski [29] the dissolution and gelatinization of nitrocellulose begins at the ends of fibres cut in the beating process. 

C at room temperature they undergo dissociation. Summing up it seems improbable that these compounds are of any importance in the dissolution process of nitrocellulose. The Highfield theory [36] (p. 248 and below) is more feasible. 

The influence of the polarity of solvents on their ability to dissolve nitrocellulose has been also demonstrated by Wo. Ostwald [50], who has introduced the value fi2/e as an expression characterizing the strength of solvent ( —dipole moment, e-dielectric constant). Good solvents of acetyl celluloses are characterized by a high iM2/s value. The physical significance of /i2/e is not clear, however, and its introduction has not helped to clarify the process of dissolution. 

Ageing of nitrocellulose solutions. The tendency of nitrocellulose solutions to undergo changes in the course of time (to age ) has been known for many years. It consists mainly in a decrease of viscosity during the first ten to twenty hours after dissolution. At room temperature the decrease of viscosity stops after 1-2 days. At an elevated temperature it continues, and the viscosity approaches a certain asymptotic limiting value, that may be estimated by means of extrapolation. 

Identical results of fractionating nitrocellulose by successive dissolution and precipitation have also been reported by other authors, e.g. Brunswig [102] Kumi-chel [103], Lacape [55], Glikman [104]. 

In other experiments, the opposite method of fractionation by dissolution was applied. In this way G. Meissner [107] prepared a soluble fraction with a nitrogen content of 10.28%, in quantity about 4% by extracting a specimen of nitrocellulose with 12.17% N using 50 50 ether-alcohol. The insoluble part was composed of nitrocellulose of 12.32% N. 

Unpublished microscopic experiments of T. Urbanski [29] on the dissolution of nitrocellulose fibres provide additional evidence that there exists a layer hindering the action of solvents, since the fibre starts to dissolve at the ends which have been tom and frayed by passage through the beaters. 

Attempts to obtain nitrocellulose containing more than 12.7% of nitrogen failed. When acid diluted below 77% was used partial dissolution of the product occurred. As an explanation for this, Vieille suggested that the soluble substances represented products which were hydrolysed or oxidized under the influence of the relatively dilute nitric acid. The reactions of hydrolysis and oxidation occur less readily the more concentrated the nitric acid, and the more hydroxyl groups are esterified. It also seems that nitrate groups counteract the side processes (such as oxidation). 

The nitrogen content should be within the range 11.0-12.3% N, i.e. within such limits that ensure complete dissolution in organic solvents. For cheaper varnishes, in which ethyl alcohol is used as a solvent, nitrocellulose of lower nitrogen value, e.g. 10-10.5%, is used. Such a nitrogen content endows the product with complete solubility in ethyl alcohol. 

Nitrostarch dissolves in alcohol more easily than nitrocellulose, and at a nitrogen content of 10.0-11.5% is wholly soluble in ethyl alcohol. Incomplete dissolution indicates nitrostarch of higher or lower nitrogen percentage. A product containing 12.1% N has a solubility of 32.5%. 

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