CA2214729C - Energetic copolyurethane thermoplastic elastomer - Google Patents

Abstract

An energetic copolyurethane thermoplastic elastomer is prepared by polymerizing a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less with a diisocyanate at a NCO/OH ratio of about 0.7 to 1.2. The resulting copolymer is easy to incorporate in gun propellant formulations and is recyclable.

Description

ENERGETIC COPOLYURETHANE
THERMOPLASTIC ELASTOMER
FIELD OF THE INVENTION
The present invention relates to a copolyurethane thermoplastic elastomer and is particularly concerned with an energetic copolyurethane thermoplastic elastomer for use in high-energy compositions.
BACKGROUND OF THE INVENTION
High-energy solid compositions such as propellants and composite explosives are usually prepared by combining a variety of materials including oxidizers, binders, plasticizers and a curing agent. Many energetic binders are available for use in the preparation of these high-energy compositions. It has been found that dihydroxyl terminated telechelic energetic polymers having a functionality of two or less are useful as energetic binders. In particular, glycidyl azide polymer (GAP) has been found to be an excellent energetic binder in advanced propellants and explosives due to its energetic, yet insensitive and thermally stable properties.
One application of glycidyl azide polymer is with a low vulnerability ammunition (LOVA) gun propellant to obtain a high energy low vulnerability ammunition (I-IELOVA) gun propellant. 1n this case, the energetic binder is obtained by chemically reacting a hydroxyl terminated azido prepolymer with a diisocyanate; however, the resulting elastomer have irreversible cross-links, and hence is not recyclable. Moreover an irreversible cross-links binder is more difficult to use in the processing of gun propellants than a reversible cross-links binder.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a energetic copolyurethane thermoplastic elastomer, which is easy to process and is recyclable, for use in high-energy compositions.
In accordance with one aspect of the present invention, there is provided an energetic copolyurethane thermoplastic elastomer comprising linear polyurethanes of the formula:
_p_(p_p~_p_ wherein D is a diisocyanate;
P is a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less; and nis1to100 physically cross-linked to one another by hydrogen bonds.
Preferably, the dihydroxyl terminated telechelic energetic polymer has a molecular weight ranging from about 500 to about 10,000 and is selected from the group consisting of glycidyl azide polymer (GAP), poly 3-azidomethyl-3-methyloxetane (AMMO), poly bis 3,3-azidomethyloxetane (BAMO), poly 3-nitratomethyl-3-methyloxetane (NIMMO) and poly glycidyl nitrate (GLYN).

In an embodiment of the present invention, the energetic copolyurethane thermoplastic elastomer includes a chain extender. Suitable chain extenders are low molecular weight diols such as ethylene glycol or a diol of the formula: OH-CHZ-(CHZ)"-CHZ-OIL wherein n is 0 to 8. The chain extenders can also be low molecular weight diamines.
In accordance with another aspect of the present invention, there is provided a process for preparing an energetic copolyurethane thermoplastic elastomer comprising the step of polymerizing a dihydroxyl terminated telecheGc energetic polymer having a functionality of two or less with a diisocyanate at a NCO/OH ratio of about 0.7 to 1.2, and preferably, at a ratio of about one. The resulting copolymer can be solvated and mixed with other components of a high-energy composition such as a gun propellant. Evaporation of the solvent results in a suitable gun propellant.
Preferably, the diisocyanate is aromatic such as 4, 4' methylenebis-phenyl isocyanate (MDI) and toluene diisocyanate (TDI), or aliphatic such as hexamethylene diisocyanate (HMDI) and isophorone diisocyanate (IPDI).
The polymerization process may be carried in a suitable solvent such as dried ethyl acetate to provide a solvated energetic copolyurethane thermoplastic elastomer which can be used, in its solvated form, to prepare high-energy compositions.

The advantages of the present invention are to provide an energetic copolyurethane thermoplastic elastomer, which is easy to process and is recyclable, for use as an energetic binder in high-energy compositions.
DETAILED DESCRIPTION OF THE INVENTION
Thermoplastic elastomers (TPE) are copolymers of the type ABA or AB, where A
and B are a hard segment and a soft segment respectively. The hard segment is capable of crystallization or association and gives the thermoplastic behavior to the copolymer, whereas the soft segment gives the elastomeric behavior to the copolymer. In practice, at room temperature, a thermoplastic elastomer behaves like a rubber because it is cross-linked in the same fashion as a conventional elastomer, but with reversible physical cross-links. Since the physical cross-links are reversible, the thermoplastic elastomer can be melted or dissolved in a solvent, so that the polymer can be mix with other components of, for example, a gun propellant formulation. A gun or rocket propellant or a composite explosive could be isolated upon cooling or evaporating the solvent. Cooling or evaporating the solvent lets the broken physical cross-links reform and the elastomeric properties are recovered. 'therefore, obsolete material can be melted or dissolved before the separation of the components leading to a recyclable material.
Tn the present invention, a recyclable linear copolyurethane having the following chemical structure:

O

/~

ene etic C- C O- ~~~ - dilsocyanate diisocyanate N~~~CH2 ---po~~ -O- ~
~

-\~

H
H

(i) ,.

O O

_ ~~
diiasocyanate - C - O
- CH2 ~ dlisocyanate k O ~
~
-~-D ~~ ~ ~ p ~~
~
~

H
H

is obtained by polymerizing a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less with a diisocyanate. Referring to structure (I), the energetic polymer is the elastomeric B segment and the thermoplastic A segment is provided by the urethane moieties. Suitable hydroxyl terminated energetic polymers are poly-GAP, poly-AMMO, poly-BAMO, poly-NIMMO and poly- GLYN, of molecular weights of about 500 to 10,000.
Preferably, the diisocyanate is aromatic such as 4, 4' methylenebis-phenyl isocyanate and toluene diisocyanate, or aliphatic such as hexamethylene diisocyanate and isophorone diisocyanate.
Dibutyltin dilaurate is used as the catalyst at a concentration of about 0.01 to 0.05% by weight.
In order for the reaction to be successful, the reaction conditions must be controlled carefully.
The NCO/OH ratio must be about 0.7 to 1.2, and preferably about one, to avoid chemical cross-linking between the chains and to obtain the best reproducible copolymer. The exact NCO/OH
ratio will depend, for one thing, on the purity of the samples used.
Generally, the value of the NCO/OH ratio will approach one as the degree of purity of the samples increases. The presence of water should be avoided to ensure reproducibility of the reaction. If present, water will attack the isocyanate group leading to an amine group which will introduce chemical cross-linking.

As shown in structure (I), the urethane groups within the copolymer form hydrogen bonds with the oxygen of another urethane or with the oxygen of an ether, resulting in physical cross-links between the chains. The hydrogen bonds between the urethane groups give the hard segments of the thermoplastic elastomer and therefore the thermoplastic behavior. These hydrogen bonds are reversible, and hence, can be broken by dissolving the copolymer in a solvent.
Generally, it is also possible to brake the hydrogen bonds of most thermoplastic elastomers by melting them.
However, in the case of GAP-based copolyurethane thermoplastic elastomers, the copolyurethane should not be melted as both the decomposition of GAP and the melting point of polyurethanes occur at about 200°C. According to the literature, linear polyurethanes have melting points in the region of 200°C when the thermoplastic content is about 20 to 50% by weight; this is when there are enough hard segments to induce crystallinity.
In accordance with another embodiment of the invention, the polymerization process can be performed in a solvent to avoid the solvation step which can be time consuming. Preferably, the solvent is dried ethyl acetate. By varying the polymer concentration, a wide variety of energetic thermoplastic elastomer (ETPE) solutions suitable for, for example, an HELOVA
formulation process can be obtained.
Chain extenders such as ethylene glycol, 1,3-propanediol, 1,4 butanediol or other low molecular weight diols or diamines may be added to obtain copolyurethanes of dif~'erent molecular weights.
For example, a mixture of a specific proportion of a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less and a chain extender is reacted with a diisocyanate at a NCO/OH ratio of about one to yield a copolymer of desired molecular weight. Hence, by varying the proportion of the energetic polymer and chain extender used, it is possible to vary the molecular weight and the thermoplastic content of the resulting elastomer.
Likewise, the mechanical properties of the copolyurethane thermoplastic elastomer, which are determined by the numbers of hard and soft segments, can also be adjusted according to required needs.
CNEMICALS
GAP M~ 2000 was obtained from 3M company, Minnesota, U.S.A. Dibutyltin dilaurate and 4, 4' methylenebis-phenyl isocyanate were obtained from Aldrich Chemical Co., Milwaukee, Wisconsin, U.S.A. Poly-NIMMO M~ 2000 was obtained from ICI England.

PREPARATION OF GAP Ms 2000 COPOLYURETHANE THERMOPLASTIC ELASTOMER
I. DETERMINATION OF THE CONCENTRATION OF OH IN GAP M, 2000 BY THE EQUIVALENT
WEIGHT (EW) METHOD USING NMR SPECTROSCOPY
0.23 g of GAP M° = 20~ was reacted with 0.4 mL of acetic anhydride in 5 mL of pyridine at 95°C in a corked container for 12 hours. The pyridine was evaporated under vacuum (1 to 5 torts) to give an acetylated polymer and residual pyridine, acetic anhydride and acetic acid. The pyridine, acetic anhydride and acetic acid were removed by dissolution of the acetylated polymer in 30 mL of toluene and co-evaporated under vacuum (1 to 5 torts). The co-evaporation step was repeated twice and completed with a final evaporation under higher vacuum (0.1 to 1 tort) using a mechanical pump.

The resulting acetylated derivative was dissolved in CDC13 and the ~H NMR
spectra was acquired. The equivalent weight determination for GAP was made by integrating the large intensity of the polymer peaks in the region 3.5-4.0 ppm followed by the integration of the acetyl group at 2.1 ppm. The EW was calculated by the following equation:
EW (g/mol): Io,~/5 x MW
IcH3/3 wherein EW is equivalent weight (g/mol of alcohol);
Ion is the area under the GAP peaks in the ~H spectrum (between.3.5 and 4.0 ppm);
kH3 is the area under the CH3 acetyl peak in the'H spectrum (at 2.1 ppm); and MW is the molecular weight of the monomer repetition unity of GAP
(99.1 g/mol).
The EW for GAP M" = 2000 was found to be 1200g/mol.

2. POLYMERIZATION
100 g (0.083 mole of OH) of GAP M" = 2000 was mixed with 0.05 g of dibutyltin dilaurate (0.05%) to ensure its dispersion in the polymer and the mixture was magnetically stirred in a 500 mL round bottom flask and heated to 60°C under vacuum for 16 hours.
10.1042 g (0.404 mole MDI; 0.081 mole NCO) of freshly distilled 4, 4' methylenebis-phenyl isocyanate was added to the hot dried polymer. This gave a NCO/OH ratio of 0.97. The diisocyanate-polyrrier mixture was thoroughly mixed for one minute and put into a preheated desiccator at 60°C, and a vacuum was applied for about five minutes to remove all gases. The desiccator was then placed in an oven at 60°C for about 24 hours to complete the polymerization. A GAP-based copolyurethane thermoplastic elastomer having a molecular weight M" varying from 35,000 to 40,000 was obtained upon cooling, yield 110.1 S g.
The spectroscopic analysis of the above product is as follows:
IR: um~ (FILM) cni ~ : 3400, 3320, 2920, 2860, 2100, 1730, 1590, 1520, 1440, I
410, 1340, 1290, 1210, 1100, 930, 910, 850, 810, 660.
~HNMR: 8 (CDCl3) ppm: 3.1-4.1 (all other protons, m), 5.1 (CIIO-CONH, m), 7.0 (NH m), 7.2 (aromatic protons, AB system, 3J,e,8 = 8.0 Hz ~3CNMR : 8 (CDC13) ppm: 41.2 (phenyl-CHZ), 52.2 (CHZN3), 70.1-72.6 (CHZO), 79.3 (CHO), 119.6 (carbons ortho to NH), 130.0 (carbons meta to NH), 136.2 (carbons para to NH), 137.2 (C-NH aromatic), 153.0 (urethane carbonyl).
IR = infrared, "NNMR = proton nuclear magnetic resonance, "3CNMR = carbon nuclear magnetic resonance, J = coupling constant in hertz (Hz), m = multiplet, s = singles PREPARATION OF GAP M,=2000 COPOLYURETHANE THERMOSPLASTIC ELASTOMER IN
ETHYL ACETATE SOLVENT
100 g of GAP M" = 2000 was mixed with 0.05 g of dibutyltin dilaurate (0.05%) and the mixture was magnetically stirred in a S00 mL round bottom flask and heated to 60°C under vacuum for 16 hours. Dried ethyl acetate was added to the flask in a concentration to obtain 30 to 70% by weight of polymers. 10.1042 g of freshly distilled 4, 4' methylenebis-phenyl isocyanate was to added to the round bottom flask_ The solvated diisocyanate-polymer mixture was stirred at 60°C
for about 24 hours to complete the polymerization. A solvated GAP-based copolyurethane thermoplastic elastomer of molecular weight M" varying from about 35,000 to 40,000 was obtained upon cooling. The solvated elastomer can be used as is in the preparation of high-energy compositions.

PREPARATION OF POLY-NIMMO M~ 2000 COPOLYURETHANE THERMOPLASTIC ELASTOMER
1. DETERMINATION OF THE CONCENTRATION OF OH IN POLY-NIMMO Mo 2000 The concentration of OH in poly-NIMMO M~ 2000 was found to be 1000g/mole using the equivalent weight method as described in example 1.
2. POLYMERIZATION
100 g (0.1 mole of OH) of poly-NIMMO was mixed with 0.05 g of dibutyltin dilaurate to ensure its dispersion in the polymer and the mixture was stirred in a 500 mL bottom flask and heated to 60°C under vacuum for 16 hours. 10 g (0.04 mole of MDI; 0.08 mole of NCO) of freshly distilled 4, 4' methylenebis-phenyl isocyanate was added to the hot dried polymer. This gave a NCO/OH ratio of 0.80. The diisocyanate-NIMMO mixture was thoroughly mixed for one minute and put into a preheated desiccator at 60°C and a vacuum was applied for about five minutes to remove all gases. The desiccator was then placed in an oven at 60°C for 24 hours to complete the polymerization. A NIMMO-based copolyurethane thermoplastic elastomer of molecular weight M" of about 15,000 to 17,000 was obtained upon cooling, yield 110.05 g.
n Alternatively, the NIMMO polymer was purified by precipitation in methanol prior to the polymerization step in order to remove trifunctional oligomer impurities present in the commercial poly-NIMMO sample, and the polymerization was performed as described above to yield quantitatively the copolyurethane thermoplastic elastomer of molecular weight of about 15,000 to 17,000. In this case, the NCO/OH ratio was about 0.95.
IR: um~ (FILMyri': 3400, 3320, 2960, 2930, 2880, 1730, 1630, 1520, 1480, 1450, 1410, 1360, 1280, 1220, 1100, 1060, 980, 860, 750, 700, 630, 610.
~HNMR: 8 (Acetone-D6) ppm: I.0 (CH3, s), 3.3 (CHZ-O, s), 4.1 (phenyl-CH2, s), 4.5 (C'.HZONO2, s), 7.3 (aromatic protons, AB system, 3J,~=8.0 Hz), 8.7 (NH-urethane, s).
~3CNMR: 8 (Acetone-D6) ppm: 17.9 (CH3), 41.5 (phenyl-CHZ), 74.6 (CH20), 76.5 (CH20N02), 119.7 (carbons ortho to NH), 130.3 (carbons meta to NH), 137.3 (carbons para to NH), 138.4 (C-NH
aromatic), 154.8 (urethane carbons).
All the copolyurethanes synthesized according to the process of the present invention are rubber-like material which can easily be dissolved in a solvent such as dried ethyl acetate in a polymer to solvent ratio of about 35:65. The resulting solvated material can be used as an energetic binder is high-energy compositions.

While the foregoing embodiments of the present invention have been described and shown, it is understood that all alternatives and modifications may be made thereto and fall within the scope of the invention.

Claims (12)

1. An energetic copolyurethane thermoplastic elastomer comprising linear polyurethanes of the formula:

-P-(D-P)n-D-wherein D is a diisocyanate;
P is a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less; and n is 1 to 100 physically cross-linked to one another by hydrogen bonds.

2. An energetic copolyurethane thermoplastic elastomer as in claim 1, wherein the dihydroxyl terminated telechelic energetic polymer has a molecular weight ranging from about 500 to about 10,000.

3. An energetic copolyurethane thermoplastic elastomer as in claim 1, wherein the dihydroxyl terminated telechelic energetic polymer is selected from the group consisting of glycidyl azide polymer, poly 3-azidomethyl-3-methyloxetane, poly bis 3,3-azidomethyloxetane, poly 3-nitratomethyl-3-methyloxetane and poly glycidyl nitrate.

4. A energetic copolyurethane thermoplastic elastomer as in claim 1, further comprising a chain extender.

5. An energetic copolyurethane thermoplastic elastomer as in claim 4, wherein the chain extender is selected from the group consisting of a low molecular weight diol and diamine.

6. An energetic copolyurethane thermoplastic elastomer as in claim 5, wherein the low molecular weight diol is selected from the group consisting of ethylene glycol and a diol of the formula: OH-CH2-(CH2)n-CH2-OH wherein n is 0 to 8.

7. A process for preparing an energetic polymer copolyurethane thermoplastic elastomer comprising the step of polymerizing a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less with a diisocyanate at a NCO/OH ratio ranging from about 0.7 to about 1.2.

8. A process as in claim 7, wherein the ratio of NCO/OH is about one.

9. A process as in claim 7, wherein the diisocyanate is selected from the group consisting of 4, 4' meihylenebis-phenyl isocyanate, toluene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate.

10. A process as in claim 7, wherein the polymerization step is performed in a suitable solvent.

11. A process as in claim 10, wherein the solvent is dried ethyl acetate.

12. The use of the energetic copolyurethane thermoplastic elastomer as claimed in claim 1 as an energetic binder for high-energy compositions.