Methods for modifying nitrocellulose having lyophobic properties

Abstract

Described herein are methods for chemical modification of nitrocellulose to generate lyophobic properties which are useful in propellant compositions. Such methods include the steps of: a) dissolving neat nitrocellulose in an organic solvent; b) adding a silyl based isocyanate and a catalyst to the solution; (d) stirring the solution in a moisture free environment; e) hydrolyzing the solution by exposing said solution to moisture; and (e) adding fluorinated oxysilane. The modified nitrocellulose retains its energetic properties while exhibiting high water and organic solvent phobicity, effectively functioning as a barrier to migration or diffusion of liquid components in propellant compositions.

Claims

1. A process for preparing modified nitrocellulose having lyophobic properties comprising: (a) dissolving neat nitrocellulose in an organic solvent to form a solution; (b) adding silyl based isocyanate to the solution; (c) adding a catalyst to the solution; (d) stirring the solution in a moisture free environment; (e) hydrolyzing the solution by exposing said solution to moisture; (f) adding fluorinated oxysilane to form modified nitrocellulose.

2. The process of claim 1, wherein the fluorinated oxysilane is (tridecafluoro-1,1,2,2-tetrahydrooctyl) trimethoxysilane.

3. The process of claim 2 wherein the solution is stirred for at least 24 hours after adding the (tridecafluoro-1,1,2,2-tetrahydrooctyl) trimethoxysilane.

4. The process of claim 1 wherein the organic solvent is an ester, ketone or alcohol.

5. The process of claim 1 wherein the organic solvent is tetrahydrofuran.

6. The process of claim 5 wherein the nitrocellulose to tetrahydrofuran is at a ratio of 2:25.

7. The process of claim 1 wherein the silyl based isocynate is 3-triethoxysilyl propyl isocyanate.

8. The process of claim 1, wherein the catalyst is dibutyl tin dilurate.

9. The process of claim 1 wherein stirring the solution in a moisture free environment is for at least 24 hours.

10. The process of claim 1, wherein the modified nitrocellulose produced from step (f) is separated using water.

11. The process of claim 1, further comprising filtering, washing and drying the modified nitrocellulose.

12. The process of claim 11, wherein the modified nitrocellulose is filtered and washed with water.

13. A process for preparing modified nitrocellulose having lyophobic properties comprising: (a) dissolving neat nitrocellulose in an organic solvent to form a solution; (b) adding 3-triethoxysilyl propyl isocyanate to the solution; (c) adding dibutyl tin dilurate to the solution; (d) stirring the solution in a moisture free environment; (e) hydrolyzing the solution by exposing said solution to moisture; (f) adding (tridecafluoro-1,1,2,2-tetrahydrooctyl) trimethoxysilane to form modified nitrocellulose.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a Differential Scanning calorimetry (DSC) thermal scan of the modified nitrocellulose.

(2) FIG. 2 is a Differential Scanning calorimetry (DSC) scan of a baseline (unmodified) nitrocellulose.

(3) FIG. 3 is a FTIR scan of functionalized nitrocellulose vs. baseline nitrocellulose showing formation of new peaks that represented functional groups.

(4) FIG. 4 is an image of a water droplet dispensed on a modified nitrocellulose surface.

(5) FIG. 5 is an image of a nitroglycerin droplet on nitrocellulose.

DETAILED DESCRIPTION

(6) The present invention discloses methods for modifying neat (i.e. raw) nitrocellulose to create a lyophobic surface interface that prevents the migration or diffusion of energetic plasticizers such as nitroglycerin in propellants. This is achieved by chemically modifying the surface of the nitrocellulose with fluorinated silanes, oligomers and polymers. The modified lyophobic nitrocellulose produced herein has equivalent energy output, ignition temperature, and decomposition rate to neat nitrocellulose.

(7) The process for preparing modified nitrocellulose is initiated by dissolving neat nitrocellulose in an organic solvent and reacting the surface hydroxyl groups on the nitrocellulose with a silyl based isocyanate in the presence of a catalyst. The silyl isocyanate is covalently attached to the nitrocellulose which results in additional hydroxyl groups for further modification by crosslinking effects of siloxanes. Crosslinking of siloxanes occurs in the presence of water which in turn promotes hydrolysis. The crosslinking moiety acquires more hydroxyl groups for further reaction to promote increased phobicity of the modified surface by the addition of tridecafluorotrimethyoxysilane.

(8) The chemical process for modifying raw nitrocellulose is illustrated by Scheme I show below:

(9) ##STR00001##

(10) Tridecafluoro-1,1,2,2-tetrahydrooctyl-trimethoxysilane is preferred, however, other fluorinated silanes, oligomers, and polymers including fluoro-alkyl containing polymers can be used in the present invention. Lyophobic modification of nitrocellulose are not only achieved by covalent bonding with fluorine but other low energy molecules can be added as well. These include alkyl functional groups and acyl/alkyl silanes, as well as linear/cyclic oligomers. Representative examples of these groups include propionyl chlorides, dimethyldichlorosilanes, n-octadecyldimethylchlorosilane, and epoxy acrylate oligomers.

(11) A non-limiting example of the present invention is illustrated in the following example.

Example 1

(12) Dissolve 2.0 grams of either 11.05% or 13.14% nitrocellulose in 50 ml of tetrahydrofuran (THF) and stir for forty eight hours in a moisture free environment. This allows sufficient time to completely expose the nitrocellulose polymer chain for hydroxyl group interaction with the isocyanate. Add 1 ml of 3-(triethoxysilyl) propyl isocyanate dropwise. Add 3 milliliters of dibutyl tin dilurate as the reaction catalyst and stir for a minimum of 4 hours up to 24 hours to ensure complete reaction of the isocyanate. Open reaction vessel for slow exposure to atmospheric moisture for slow hydrolysis. Excess THF may be added to compensate for solvent evaporation. Add 1 ml of (tridecafluoro-1,1,2,2-tetrahydrooctyl) trimethoxysilane and stir for 24 hours to allow for complete reaction to occur. Separate the reacted material with 50 ml of water or other suitable solvent. The solid material is then filtered and washed several times with excess water. The functionalized nitrocellulose is then dried at 60 C. for one hour.

(13) Testing and Characterization of the Modified Nitrocellulose

(14) Modified nitrocellulose was characterized to determine the degree of surface functionality and to determine if the performance of nitrocellulose was changed. Water sink tests were performed and showed that the functionalized nitrocellulose remained buoyant compared to neat nitrocellulose which sinks immediately when exposed to a water bath.

(15) Chemical characterization. Differential scanning calorimetry (DSC) was utilized to compare baseline (neat) nitrocellulose and modified nitrocellulose. The results as shown in FIG. 1 (modified) and FIG. 2 (baseline) confirm that ignition temperature and enthalpy is comparable to baseline. FIG. 3 is Fourier Transform Infrared (FTIR) analysis showing new functional groups in the fingerprint region of the modified nitrocellulose compared to baseline nitrocellulose. The results also show that the nitro groups are still intact after synthesis.

(16) Size exclusion chromatography was performed on the functionalized nitrocellulose. Table 1 illustrates an increase of molecular weight distribution for the functionalized nitrocellulose versus the neat nitrocellulose.

(17) TABLE-US-00001 TABLE 1 Molecular Weight Distribution of Modified vs. Unmodified Nitrocellulose. Mn (kDa) Mw (kDa) 11.04% nitrated reference NC in THF 78588 878899 11.04% nitrated reference NC in THF 76447 994608 11.04% nitrated reference NC in THF 75397 1.99E+06 11.04% nitrated reference NC in THF 75740 1.87E+06 11.04% functionalized NC in THF 91301 1.18E+06 11.04% functionalized NC in THF 86306 9.35E+05 11.04% functionalized NC in THF 96289 2.27E+06 11.04% functionalized NC in THF 113222 2.79E+06 13.15% nitrated reference NC in THF 76434 780969 1315% nitrated reference NC in THF 80773 808801 13.15% nitrated functionalized NC in THF 94417 616836 13.15% nitrated functionalized NC in THF 76606 979713

(18) Table 2 provides the contact angle using hexadecane, water, and nitroglycerin as probe fluids. Compared to the neat nitrocellulose, the modified nitrocellulose revealed high contact angles which is evidence of functionalization on the material's surface. FIG. 4 illustrates a water droplet 200 beading on the surface of the modified nitrocellulose 300. FIG. 5 illustrates a nitroglycerin droplet 400 on modified nitrocellulose 300. In another experiment, a comparison of the surface functionalized nitrocellulose with that of neat nitrocellulose was performed with a droplet of nitroglycerin dropped on each surface. The results indicated that functionalized surface is phobic to nitroglycerin whereas complete wetting occurs on the neat nitrocellulose.

(19) TABLE-US-00002 TABLE 2 Contact Angles of Treated vs. Untreated Nitrocellulose Surface mod w/ Surface mod w/addition Neat NC triethoxysilyl NCO of tridecafluorotri- 13.1% Contact angle methoxysilane Contact angle Probe fluid avg.() Contact angle avg.() avg.() H20 (static) 105 115 complete wetting Hexadecane 33/0 80/55 complete Advancing/ wetting Receding

(20) Elemental analysis showed the presence of fluorine and silicon concentrations for the functionalized nitrocellulose.

(21) While the invention has been disclosed with reference to certain preferred embodiments and examples; numerous changes, alternations and modifications are possible without departing from the spirit and scope of this invention as defined in the appended claims and equivalents thereof.