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Nitrocellulose
Nitrocellulose, also known as cellulose nitrate, is a nitrated cellulose ester polymer that is used as the main compound in many types of ammunition, propellants and explosives as well as a wide range of other materials.
Braconnot discovered in 1833 that mixing nitric acid with carbohydrates yielded inflammable materials which he called “xyloidines” (Miles 1955). This material was of much lower purity, probably only containing 5-6% nitrogen compared to nitrocellulose that Schönbein later produced and called guncotton (Urbański 1965). Schönbein was the first one to see the potential in using nitrocellulose in explosive material (Miles 1955). In the beginning the application of nitrocellulose was limited and it took several years before its use as a reliable explosive.
Nitrocellulose has been industrially produced since the 19th century. Even if the process has changed to become more automatic the manufacturing principle has not changed much in the last hundred years. Mixing cellulose with a sulfonitric mixture of sulphuric acid, nitric acid and water is still the common way to produce nitrocellulose with high nitrogen content. Nitrocellulose is similar to the cellulose in structure. It is produced through nitrification of one, two or three of the hydroxyl groups that are connected to carbons C2, C3, and C6 of the cellulose (see Figure 1). Each cellulose monomer have three hydroxyl groups that can be substituted. This gives nitrocellulose the chemical formula [C6H7O2(OH)3-x(ONO2)x]n, where x is the number of hydroxyl groups substituted by nitro groups and n is the number om monomers. The nitrogen level of nitrocellulose is often described/measured in degree of substitution (D.S.) which gives a number that represents the average number of hydroxyl groups that has been substituted. The following equation can be used to calculate D.S.
Equation 1. . . = 3.6 × nitrogencontent(%) 31.13 − nitrogencontent(%)
The theoretical maximum substitution would yield a D.S. of 3 which equates to a nitrogen content of 14.1 %. The highest reported nitrification is a D.S. of 2.9 (≈13.9% of nitrogen content) (Miles 1955, Selwitz 1988).
The amount of nitrogen in the nitrocellulose affects the properties, such as solubility, viscosity and flammability. At low amounts of nitrogen the solubility in ether-alcohol increases when the nitrogen levels increases, peaking at 11 – 12% (see table 1). Higher amounts of nitrogen decreases in solubility and amounts nearing the theoretical maximum (14.1%) of nitrogen groups have a very low level of solubility in ether-alcohol. At these high amounts the nitrocellulose is commonly dissolved in acetone, etylacetate or ether-alcohol.
Different methods are used to achieve different degrees of substitution since different levels of nitrogen contents are used in the industries. Lower amount of nitrate nitrocellulose are used in a wide variants of products such as lacquer, plastic film and ink while higher percentage of nitrogen are used in propellants and explosive materials.
Especially the military applications requires a reliable product that behave as expected, and as such thorough quality controls are done. There are however two major difficulties in achieving this. The first is that cellulose, which nitrocellulose is produced from, is a natural product. Its characteristics are therefore affected by numerous variables, such as its geographical origin and the season of the year it is grown. The type of plant the cellulose is refined from also affects the characteristics of the end product. In table 2 is some examples of differences between different celluloses (table 2 is borrowed from Chemistry and technology of explosives Vol. II by Urbański, 1965). The characteristics of the nitrocellulose have large effect on the ballistic properties of ammunitions and propellants (Fernández de la Ossa et al. 2012, Johansson 2009).
The second problem is the way nitrification of the cellulose is controlled. Nitrogen content of the nitrocellulose is monitored by taking out samples from the reaction chamber during production for measurements. Other characteristics, e.g. density and viscosity, are measured on the end product. Different batches often have to be mixed to achieve consistency in characteristics. The problem, that off-line measurements results in, has been approached by trying to develop a mathematical model to calculate the ideal batch time for the cellulose before nitrification (Barbosa, et al. 2005). The current standard method used to measure the nitrogen content of the nitrocellulose only measures the amount of nitrogen per mass of nitrocellulose, it does not take in consideration how the nitrogen is distributed (MIL-DTL-244B 1996, MIL-STD-286C 1991).
Analytical techniques
The study of nitrocellulose is a complicated task given its high chemical and structural complexity. A wide range of techniques are being used for the analysis of this ester polymer. Here follows different techniques commonly used for characteristic studies of nitrocellulose.
SEC
Size-exclusion chromatography (SEC) is an effective technique for analysing the polymeric characterisations of nitrocellulose. An effective set-up to use when analysing nitrocellulose is a SEC with triple detection (refractometry, viscometry and lightscattering) (Fernández de la Ossa, et al. 2011). SEC with simple detection system (refractometry) have been used to analyse the polymeric properties of nitrocellulose using polystyrene standards as a substitute (Fernández de la Ossa et al. 2011).
It has been demonstrated that the molecular weight reproducibility of data acquired by analysing nitrocellulose with SEC has a low reproducibility when comparing between different research teams (Fernández de la Ossa et al. 2012). In a study nine laboratories from eight different countries used the same SEC method to analyse nitrocellulose with nitrogen content of 11.6 – 13.5%. The result was that the main cause of the low reproducibility was differences in the drying process of the nitrocellulose and the lack of a definition of similar and good baseline in the obtained chromatograms (Fernández de la Ossa et al. 2012). This shows the complexity of generating reliable data when analysing nitrocellulose by SEC.
FTIR
Fourier transform infrared spectroscopy (FTIR) is a common method for analysing nitrocellulose. It has been used to analyse the morphologic and thermal properties of nitrocellulose as well as the degradation of it (e.g., Kovalenko et al. 1994, Phillips et al. 1955, Schroeder et al 2001). By observing a decrease in the NO2 signal and an increase in the OH- signal using FTIR and 13C-nuclear magnetic resonance (NMR) spectroscopy showed that highly nitrated nitrocellulose is not resistant to biodegradation (Tarasova et al. 2005). Using FTIR has also been proposed as a method to do quantitative analyses of nitrogen content in nitrocellulose (Gensh et al. 2011). A study of triple-bas gunpowder that used scanning electron microscope (SEM) and micro-reflectance FTIR was able to show among others that only 10µm of the top layer was affected in the ignition process (Schroeder et al. 2001).
GC/MS
Gas chromatography (GC), alone or coupled to a mass spectrometer (MS), has commonly been used to analyse degradation of nitrocellulose that has been thermally treated (Fernández de la Ossa et al. 2011, Katoh et al. 2005). A study of fractions from pyrolysis of gunpowder by GC/MS showed that nitrocellulose was the main source of by-products. (Cropek et al. 2001b). The same study also showed that when thermally treated, nitrocellulose produced almost no heavy weight fractions. GC/MS has also been applied in characterising emissions of energetic material and energetic waste, there among analysing the incineration of nitrocellulose fines (Cropek et al. 2001a).
The common system used when studying the characteristics using GC/MS is to have a pyrolysis chamber installed to the injector (Fernández de la Ossa et al. 2011). This setup transports the gases produced from pyrolysis directly into the injector. This method is very effective to look at the characteristic of by-products and samples does not have to be dissolved but it does not give any data of the complete nitrocellulose molecule.
Objective
The objective is to establish a method that allows the characterisation of nitrocellulose and to be able to differentiate between nitrocellulose with different properties. That information could then be used to find correlations between the characteristics of nitrocellulose and properties of ammunition and explosives.
Method
Samples
The nitrocellulose samples were received from Eurenco Bofors AB. The nitrocellulose samples were from two different manufacturers, one from Finland and one from France.
GC/MS
Gas chromatograph was chosen for this project due to its high sensibility and reproducibility which are key qualities for this project. Samples were analysed with an Agilent HP 6890 Gas Chromatography System coupled to an Agilent HP 5973 Mass Spectrometer and an Agilent HP 7683 Injector tower with Agilent HP 7683 Autosampler. The separation was carried out using an Agilent DB-5MS (30m × 0.250mm, 0.25µm film thickness). The initial temperature was set to start at 90 ˚C because of the boiling point of the solvents and then raise to a final temperature of 350 ˚C which was the maximum of this GC system. The scan range of the mass detector was set to scan for 30.0-500.0 amu. For a more specific description of the temperature program and mass spectrometry parameters see appendix table A1. The tune profile (Figure A1) and tune scan (Figure A2) have been included in the appendix.
A series of solvents where tested to see if acetone could be avoided as solvent (see table 3). Unfortunately no other solvent then acetone was able to dissolve a satisfactory amount of nitrocellulose. Acetone was therefore used as the solvent for all analysis carried out with GC/MS. Chromatogram and mass spectrometry data were analysed using MassLynx V4.1 software. Peak areas were analysed statistically using SIMCA V13.0.3.
Table of contents :
1. Introduction
1.1. Nitrocellulose
1.2. Analytical techniques
1.4. Objective
2. Method
2.1. Samples
2.2. GC/MS
2.3. FTIR
3. Results and analysis
3.1 Gas chromatography
3.2. Mass spectrometry
3.3 Multivariable statistical evaluation
3.4. FTIR
4. Discussion
4.1. Problems during the project
4.2. Further experiments
4. Conclusion
5. Consideration
6. References
7. Apendix
