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In contrast to other cellulose-based products, in the case of nitrocellulose the degree of substitution is indicated indirectly via the nitrogen content (in relation to dry substance). Theoretically, a nitrogen content of 14.14% is possible (all three positions in an anhydroglucose unit are converted, i.e. substituted).
In actual practice, the nitrogen content cannot exceed around 13.6 to 13.8%. The nitrogen content of Walsroder Nitrocellulose and Walsroder NC-Chips for coatings and printing inks is between 10.7 and 12.3%. Nitrocellulose with a nitrogen content above 12.6% is classed as an explosive.
The degree of substitution determines the solubility of Walsroder Nitrocellulose and Walsroder NC-Chips in organic solvents. Walsroder Nitrocellulose and Walsroder NC-Chips are divided up according to their solubility as follows:
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A Grade
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10.7-11.3
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Soluble in alcohol, esters, ketones and glycol ethers.
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AM Grade
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11.3-11.8
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Soluble in esters, ketones and glycol ethers, highly compatible and dilutable with alcohols.
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E Grade
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11.8-12.3
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Soluble in esters, ketones and glycol ethers; dilutable with alcohols.
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The maximum permissible nitrogen content is governed by legislation and varies from country to country between 12.2% and 12.6%.
The degree of substitution is coded differently in various regions thus:
Viscosity
The viscosity of Walsroder Nitrocellulose or Walsroder NC-Chips depends on the molar mass (molecular weight) of the nitrocellulose. As determining the molar mass is far more complex than determining the viscosity, and as the viscosity of the solution is significant in practical terms, a viscosity index is stated and the viscosity of the nitrocellulose in a specific solvent mixture is specified.
The viscosity of the individual nitrocellulose grades available from Wolff Cellulosics is determined by the Cochius method. To compare the viscosity of nitrocellulose from various suppliers, ISO 14446 is a useful standard to follow.
Phlegmatisers/Stabilisers
A further grading feature of Walsroder Nitrocellulose and Walsroder NC-Chips is its damping agent content and the type of damping agent. Industrial nitrocellulose is required by law to contain at least 25% damping agent (e.g. alcohol, water) or 18% plasticiser.
The purpose of damping agents or plasticisers is to phlegmatise or stabilise the nitrocellulose in order to deactivate the hazardous properties of dry nitrocellulose (high flammability, high burning rates). The nitrocellulose grades offered by Wolff Cellulosics are damped with at least 30% alcohol or water, or in the case of NC-Chips phlegmatised with 20% plasticiser.
The difference between Walsroder Nitrocellulose and Walsroder NC-Chips is the type of phlegmatizer used (alcohol or plasticizer). The noticeable difference is only their physical form and the method of handling them. Within the mean viscosity indicated, the nitrocellulose in both Walsroder Nitrocellulose and Walsroder NC-Chips is the same.
Production
Nitrocellulose is produced by causing cellulose to react with nitrating acid (a mixture of nitric acid and sulfuric acid). Following complex washing and stabilizing stages, damping agents (alcohols or water) or plasticizers are added to the nitrocellulose which is then marketed as Walsroder Nitrocellulose or Walsroder NC-Chips.
The schematic flow chart gives an outline of the individual stages of the process.
The raw material used for producing Walsroder Nitrocellulose and Walsroder NC-Chips is carefully selected and well-characterized types of cellulose, the exact specifications having been agreed with the cellulose suppliers. Compliance with these specifications is regularly checked during incoming goods inspections. This also applies to all other raw materials, thus assuring the consistent quality of Walsroder Nitrocellulose and Walsroder NC-Chips.
The cellulose is caused to react with the nitrating acid (a mixture of nitric acid and sulfuric acid) in the nitrating process. A new nitrating plant came on stream at Wolff Cellulosics in 1996. It is a state-of-the-art production unit incorporating 125 years of nitrocellulose manufacturing experience.
After nitration, centrifuges separate the nitrocellulose from the spent nitrating acid used in excess quantities. This spent nitrating acid is separated into nitric acid, sulfuric acid and water, and completely recycled back into the process.
The next stage is the pre-stabilization stage where the nitrocellulose is washed with water, thus separating off any acid left on the fibres. Next, the viscosity of the nitrocellulose is adjusted by pressure boiling (heating under pressure to temperatures above 100°C).
This is followed by the post-stabilization stage where the nitrocellulose is washed with water and heated to temperatures below 100°C. The water is subsequently separated off to leave a water content of 35%. This water-wet nitrocellulose can then be packaged. To produce alcohol-damped Walsroder Nitrocellulose, the water is replaced with the appropriate damping alcohol. To produce Walsroder NC-Chips, the appropriate plasticizer is added to water-wet nitrocellulose which is then dried.
In the course of the entire production process all relevant properties of the nitrocellulose and of the process are constantly monitored to ensure the consistent high quality of Walsroder Nitrocellulose and Walsroder NC-Chips.
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