JPS6212278B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to polyurethane compositions, and more particularly to fully cured rocket motor chambers or casing liners for solid propellant motor manufacturing. Liner layers are required in solid propellant motors for several reasons. To make the motor casing gas impermeable,
It is used to provide an insulating film between the casing and the propellant, and also as a material inserted between the propellant and the chamber walls to which the propellant may stick. It has been found that solid propellants based on polyurethane binders, especially those made from hydroxy-terminated polybutadiene (HTPB) as one of the carbamate bond-forming compounds, exhibit very poor adhesion to the liner. This severely affected motor system reliability. Although the initial bond of the propellant to the liner may appear satisfactory, over time the bond weakens and its reliability becomes questionable. The most likely cause for changes in bonding properties upon aging is the migration of plasticizers and other reactive components from the propellant to the liner during curing of the propellant. This impairs propellant-binder curing and causes the bonded polymer phase to be weaker than the propellant matrix. HTPB propellants, which typically contain as much as 35% plasticizer, are generally combined into unplasticized liner formulations. When plasticizer migration begins from the propellant to the liner, several effects occur. Transfer of pure plasticizer from the propellant to the liner is generally not considered detrimental to adhesion as long as the plasticizer is soluble in both the propellant and support liner. However, migration of impure plasticizers can cause problems. The propellant is an antioxidant, a non-functional prepolymer,
and various materials such as soluble burn rate additives. These materials normally have little tendency to migrate from the propellant. However, the presence of plasticizer in the propellant binder loosens the structure of the polymer slightly, which sometimes allows the material to migrate with the plasticizer. Migration of unreacted diisocyanate from the propellant binder into the plasticizer is the most significant cause of poor propellant-liner bond formation. This compromises the stoichiometry of the binder polymer layer at the interface, resulting in a weak boundary layer.
Sometimes this phenomenon becomes so severe that the interfacial material becomes extremely soft and sticky, resulting in very low bond strength. Various methods have been tried to improve interfacial bonding. In one approach, a Washcoat solution containing a curing agent such as toluene diisocyanate (TDI) in chloroethane was sprayed onto the surface of the cured liner layer prior to application of the propellant. However, the liner did not exhibit consistently good adhesion to the propellant, again causing poor reliability. Experienced poor processability and short shelf life. Excess TDI
Formulations of liners and/or propellants with curing agents such as 1,2,2, and 2, have also been attempted, but have proven unsatisfactory due to the adverse effect on the physical properties of the liner or propellant. Partially cured liners were also considered, but proved unsatisfactory due to the limited time available to flow the propellant onto the partially cured liner, and again the flow of propellant to the liner was Adhesion of the liner to the chamber is not always as good as desired, due to the severity of the time required, which is often only a few hours, before propellant casting occurs. It is. It is an object of the present invention to provide a polyurethane propellant substrate, or subbing layer, which is fully cured when the polyurethane propellant layer is cast and does not require the use of washcoats containing active ingredients. The present invention provides a polyurethane-forming component having dispersed therein a stable solid polyisocyanate additive in excess of the amount needed to form the polyurethane and which is reactive with the active hydrogen-containing compound after curing of the composition. A composition comprising: The compositions of the invention are used to improve the adhesion of a first composition layer containing an active hydrogen compound to a surface. applying the composition of the invention to the surface; curing the film; applying said layer to the film; and reacting said compound with said excess additive to form said film and said film. A method is provided that includes the steps of forming a bonded boundary between layers. The compositions of the present invention further include: dispersed therein a stable solid polyisocyanate additive which is in excess of the amount necessary to form the polyurethane and which is reactive with the active hydrogen-containing compound after curing of the composition. a first layer comprising a polyurethane-forming component; and a second layer comprising an active hydrogen compound that reacts with said additive to form an interfacial bond between the layers. used for giving. The preferred composition disclosed herein is +80〓 and
It has been found to have a useful bond life of at least 30 days at 30% relative humidity and is suitable for application using existing spray equipment. This desirable composition is also a thermal insulating liner that provides a reliable adhesive bond with an adhesive strength at least equal to the adhesive strength of the propellant. Superior bonding is provided by formulating the chamber liner composition with a thermally stable solid isocyanate precursor material that does not migrate.
After complete curing of the liner composition, the above materials bind to the hydroxyl groups contained within the layer of polyurethane propellant subsequently applied thereon to strengthen the liner/propellant interfacial polymer bond without the use of washcoat techniques. It is thus possible to optionally react with active hydrogen-containing compounds, such as substituted compounds. The liner composition will be fully cured, so its physical properties will be optimized before casting the propellant so that the time interval before casting the propellant can be completely varied. The interfacial bond is completely reliable and the method of the present invention simplifies rocket motor production techniques with a liner system that provides superior bond strength. In the accompanying drawings: FIG. 1 is a sectional view of a solid propellant rocket motor in series; and FIG. 2 is a sectional view of a rocket motor according to the invention. If you refer to Figure 1 here, the diagram shows a solid propellant -
Depicting the various layers present during the formation of the containing motor chamber. Motor casing or chamber 10 is typically formed of a reinforced fabric such as woven glass or metal assembly. A heat insulating layer 11 such as an elastomer filled with asbestos or carbon is applied to the casing 1.
May be applied to 0. then typically 1/64 to 1/4
A liner 12 having a thickness of inches is inserted to provide a bonding support for adhering the propellant 14 to the walls of the chamber 10. If liner 12 contains an insulating filler, insulating layer 11 may not be necessary.
Liner 12 forms a liner/chamber interface 16 with chamber 10 or a liner/insulator interface 16 if an insulating liner is present, and a propellant/liner interface 18 with the propellant. Since the surface of the cured liner is polymer-rich, the interfaces 16, 18
A polymer-rich layer 20 is formed in each of the layers. This also applies to the propellant, which is the interface 1
At 8, a binder-rich layer 22 is formed. Binder-rich layer 22 and polymer-rich layer 20 are less strong than the solid reinforcing binder and propellant. During propellant curing, the binder's reactive curing material may migrate into the liner. This significantly weakens the strength of the propellant's binder-rich bonding layer 22. This phenomenon causes the formation of a weak binder layer which results in a poor liner/propellant bond. The polymerization reaction for hydroxyl-terminated polybutadiene (HTPB) propellants is generally expressed as follows: In the formula, n, m and y are integers greater than 1. HTPB may be of free radical-initiated (FR-HTPB) or lithium-initiated (Ii-HTPB) types. In the case of FR-HTPB, cross-linking is achieved through adjustment of isocyanate levels since the prepolymer has functionality greater than 2. Li-
Since HTPB has a functionality of less than 2, it therefore requires a trifunctional compound, which is usually a trifunctional hydroxyl-containing compound such as the propylene oxide adduct of hexanetriol.
Using the above reaction, approximately 5000 units of HTPB is 84
Reacts with unit isocyanate. Migration of isocyanate from the propellant binder interface into the liner has a substantial effect on cure stoichiometry resulting in a reduction in the cohesive strength of the polymer layer. As discussed above, prior techniques used to counteract isocyanate migration problems include (1) the addition of additional isocyanate at the interface by the method of isocyanate-containing washcoats placed on the surface of the cured liner or (2) moieties. A hardened liner was used. In the present invention, an excess of isocyanate material is provided at the interface by using a thermally stable isocyanate precursor in excess of the amount of isocyanate normally used for stoichiometric curing of the liner. The precursor homogeneously distributed throughout the liner is thermally stable and the precursor in the adjacent propellant layer
It contains latent isocyanate-containing groups that can optionally react with hydroxyl-containing species such as HTPB to form urethane groups that provide a strong and reliable interfacial bond. Referring now to FIG. 2, a liner polymer-rich layer 20 in accordance with the present invention includes a highly stable isocyanate precursor material 24 that is reactive with the ROH compound 26 in the binder-rich layer 22 of the propellant layer 18. include. After applying the propellant layer 18 to the liner layer 20, the material 24 is converted or rearranged into a functionally reactive compound, which reacts with the hydroxyl-substituted material 26 to form crosslinks 28, which form the liner layer 20. Layer 20 is firmly adhered to propellant layer 18. The liner compositions of the present invention generally include any polyurethane composition that is capable of wetting the surface of an adjacent layer, such as a propellant layer, that layer has a high solids loading, and isocyanate precursors in the liner. Contains available hydroxyl groups reactive with. Liner compositions generally range from about 30 to 65% by weight.
1-10% of various additives such as solid fillers and accelerators, stabilizers and thixotropic modifiers, the remainder being polyurethane-forming ingredients. Desirable liner compositions contain little or no volatile solvents or plasticizers. The composition consists essentially of 20-65 parts by weight of liquid hydroxyl-terminated prepolymer, 15-35 parts of inert solid filler, 4-10 parts of stabilizer and flow control additives, 3-20 parts of stabilizer and flow control additives. a reactive polymeric modifier such as one or more diols or polyols;
It contains an equivalent amount of polyisocyanate sufficient to react with the prepolymer and hydrogen-substituted modifier and a superstoichiometric excess of 1.5 to 5 parts of a stable isocyanate precursor. The equivalent weight of the liquid prepolymer is at least 1000 and usually not more than 5000. In order to form the final elastomeric polymer of molecular weight of at least 20,000 by cross-linking and chain extension, the functionality of the prepolymer is conveniently about
1.7 to about 3.0, preferably about 1.9 to about 3.0.
It is 2.3. Since high molecular weight prepolymers will require heat to reduce the fineness, the molecular weight should be 1000.
to 3000 is desirable. Liquid hydroxyl-terminated prepolymers for liner compositions are desirably of the type that form elastomeric polymers, and are preferably of the type that forms elastomeric polymers, such as polyethylene glycol, polypropylene glycol, and higher alkylene oxide adducts of aliphatic glycols and triols. Hydroxyl terminated polymeric diols or polyether glycols. The liquid prepolymer is preferably of the diene elastomer type, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-
Homopolymers or copolymers of conjugated dienes containing 4-12 carbon atoms such as butadiene, 1,3-pentadiene (piperylene), 3-methyl-1,3-pentadiene and the like. In the case of diene copolymers, the comonomer is added to the polymer to retain its elastomeric properties.
Should not exceed 35%. Suitable comonomers are vinyl substituted aromatic and aliphatic compounds such as acrylonitrile, butene, styrene and the like. Polyisocyanates for curing prepolymers have the general formula R(NCC) _n , where R is a divalent or polyvalent organic group containing 2-30 carbon atoms and m is 2, 3 or 4. ) you can choose from. R can be alkylene, arylene, aralkylene or cycloalkylene. such as the presence of reactive groups that are unable to react with functional groups capable of forming urea or carbamate bonds, which would interfere with the desired reaction.
It is desirable that the organic group be hydrocarbon in nature, although the presence of non-reactive groups containing elements other than carbon and hydrogen is acceptable. Examples of suitable compounds of this type include benzene-1,
3-diisocyanate, hexane-1,6-diisocyanate, toluene-2,4-diisocyanate (TDI), toluene-2,3-diisocyanate, diphenylmethane-4,4' diisocyanate, naphthalene-1,5-diisocyanate, diphenyl-3,3 'dimethyl-4,4' diisocyanate, diphenyl-3,3'-dimethoxy-4,
4' diisocyanate diethyl ether, 3-(diethylamino)pentane-1,5-diisocyanate, butane-1,4-diisocyanate, cyclohexene-1,2-diisocyanate, benzene-1,3,4-triisocyanate, naphthalene-1, Includes 3,5,7-tetraisocyanate, naphthalene-1,3,7-triisocyanate, toluidine-diisocyanate, isocyanate-terminated prepolymers, polyaryl polyisocyanates, and the like. The polyol is preferably, but not limited to,
Diols or triols, and either saturated or unsaturated aliphatic, aromatic or certain polyester or polyether products are possible. Typical compounds include glycerol,
Ethylene glycol, propylene glycol, neopentyl glycol, pentaerythritol,
including alkylene oxide adducts of aniline such as trimethylolethane, glycerol triricinolate, or isonol, which is N,N-bis-(2-hydroxypropyl)aniline, and many other polyols well known in the art; These can be incorporated into the liner composition to adjust the degree of cross-linking. The particular compound and amount used will depend on the functionality and nature of the hydroxyl terminated prepolymer and polyisocyanate used in the liner composition. Since the functionality of Li-HTPB is generally slightly less than 2, the polyol is preferably a triol in order to provide cross-linking between the polymer chains upon reaction with the polyisocyanate. Typical polyols include glycerol triricinolate (GTRO) and isonol (propylene oxide adduct of aniline), i.e., N,N-bis-(2-
Hydroxypropyl) - Aniline can be mentioned. The polyisocyanate is present in the amount necessary to satisfy stoichiometry, ie, the amount of HTPB and any other polyols present in the composition. The polyisocyanate may be di-, tri- or higher functionality and may be aliphatic in nature such as hexane-diisocyanate, but aromatic polyisocyanates such as TDI are preferred. Catalytic curing accelerators can also be used. These chemicals are metal salts such as metal acetylacetonates, preferably thorium acetylacetonate (ThAA), since iron acetylacetonate and other iron salts are HTPB.
This is because it will act as an oxidation catalyst for the unsaturated bonds inside. The stable solid isocyanate precursor additive is present in the liner composition in excess of the amount of polyisocyanate needed to satisfy stoichiometry. Preferably, the precursor is a material that is solid up to a temperature of at least about 150°C. Typically, the precursor is present in an amount of 2-10% of the weight of the composition. The precursor can be the only polyisocyanate present, but is preferably utilized in the presence of a low molecular weight liquid polyisocyanate as described above.
The polyisocyanate should provide substantially all of the curing agent necessary for stoichiometric curing of the liner, and then the precursor should be present in controlled excess. Typical additives are dimers or trimers of organic isocyanates selected from compounds of the formula: In the formula, R is the same or different aliphatic or aromatic group, preferably containing at least one --NCO group, preferably lower alkyl or aryl, and n is an integer from 0 to 1. Aromatic isocyanates can form dimers or trimers, but aliphatic isocyanates usually form trimers. The dimeric ring (uretidinedione) is more unstable and more reactive than the trimeric ring (isocyanurate) and the dimer dissociates more easily and forms the monomeric isocyanate at a faster rate. give. It is known that the NCO component contained in the dimer ring directly reacts with active hydrogen compounds. The dimer is preferably synthesized from aromatic diisocyanates such as toluene diisocyanate, 4,4'-diphenylmethane diisocyanate and biphenyl diisocyanate to produce a dimer of the following formula: (wherein R ² is phenylene, alkylphenylene or phenylalkylene). Ortho-substitutions have been found to significantly reduce the rate of dimer formation and should be avoided. Dimerization is strongly catalyzed in the presence of trialkylphosphine, such as tributylphosphine, and is more gently catalyzed by tertiary amines, such as pyridine. The reaction is a reversible equilibrium reaction. The conversion to dimer at equilibrium is
increases with decreasing temperature and with removal of dimer from the reaction mixture. On the contrary, dimer dissociation is promoted by higher temperatures and removal of monomeric isocyanate. A preferred dimeric material is a TDI dimer, which is readily available or can be synthesized from commercially available monomers. TDI dimer is a solid crystalline material that can be isolated and added to the liner composition before applying the liner to the chamber. The solid filler may be selected from antimony oxide, carbon black, fibrous silica or the like. These provide both structural strength and thermal insulation properties to the product. Thermal bonding agents generally contain both Sb ₂ O ₃ and silica fibers. Liners with low thermal insulation performance may include only carbon black filler. The stabilizer is G-
It is possible to choose from aromatic amines such as β-naphthyl-p-phenylene diamine, thioesters such as ditridecyl-thiodipropionate, sulfur and chromic chloride. A preferred flow control additive is Thixcin E;
This is a thixotropic agent (Baker Castor Oil Co.). Common and specialty liner compositions are shown in the table below:

Table: The liner is made by forming a premix of all ingredients except the isocyanate and the precursor. The premix may be heated to a temperature typically between 100° and 160° to melt the ingredients if necessary. The isocyanate and precursor are then added into the premix and mixed. The liner composition is then applied onto the EPR rubber insulation layer by spraying, brushing or casting and allowed to fully cure in-situ. Typical liner thickness is
Ranges between 1/64 and 1/4 inch. During curing of the liner composition, there is a competitive reaction between the liquid polyisocyanate, the isocyanate groups of the solid dimer, and the hydroxyl groups of the polybutadiene and triol. However, because the dimer is present in superstoichiometric excess, even if some of the NCO groups on it or on the dione ring react with the hydroxyl groups, they are not available for reaction with the propellant. There is a surplus of water. Moreover, the reaction with the liner contributes to localize the dimeric molecules from migration and serves to strengthen the interfacial bond with the propellant. The propellant is cast onto the surface of the fully cured liner. Propellant formulations typically contain between 70〓 and
Made at a temperature of 150㎓. Propellant compositions usually include a high percentage of combustible solids, typically 65% or more by weight, a small percentage of binders, typically less than 15% by weight, and a small amount of burn rate enhancers, typically less than about 3% by weight. include. Combustible solids typically include oxidizing agents such as ammonium perchlorate and finely divided metals such as aluminum powder. Example 3 As the sole isocyanate curing agent, TDI dimer was mixed with the insulation premix as made in Example 1 at a ratio of 1.4 g dimer to 9.3 g premix. After approximately 1 hour at 135°, the sample appeared almost cured. Example 4 Insulating premix of Example 1 with TDI dimer in less than stoichiometric amounts and 0.75 g of dimer vs. 9.3 g
The premixes were mixed in the following proportions. After 16 hours at 135°, the Example 3 sample was fairly hard and tough, while the Example 4 sample was soft and had poor strength. Example 5 Sample (A) was made containing 1.0 g of dimer to 9.37 g of the insulating premix of Example 1. another sample
(B) 9.37g vs. 0.34g dimer and 0.75g TDI
It was prepared by including the above premixes. Both samples were placed in a 135° oven for 10 hours. Sample 5A was a fairly strong material, but cracked when bent at a 90° angle. The shiny surface became cloudy after standing at room temperature and approximately 30% relative humidity (RH). Sample 5B was a strong material and did not crack even when bent. The glossy surface became cloudy after standing for approximately 30 minutes at room temperature and 30% RH. Example 6 A A liner composition was made containing 4.36 g of dimer (150% equivalent as TDI) and 37.5 g of the premix of Example 1. B 1.36g dimer (47% equivalent), 3.0g TDI
A liner composition was made containing (103% equivalent) and 37.5 g of the premix. A liner composition was made containing 3.64 g of C TDI (125% equivalent) and 37.5 g of the premix. Each composition was cured for 10 hours at 135°C. The formulation of Example 6A was grainy indicating that the dimer was insoluble and did not disperse well due to hand stirring. Example 6A was repeated using a more finely divided dimer to yield a usable liner composition. The Example 6B formulation first dispersed the dimer in TDI with stirring to form a paste.
The paste and premix can then be easily combined. Example 6C was made by mixing TDI with premixes. Test results for the liner formulations of Examples 6A, 6B and 6C are shown in the table. Example 7 73 parts by weight of ammonium perchlorate,
Li containing 88% solids was prepared by mixing together 15 parts aluminum powder, 1 part burn rate accelerator and 11 parts Li-HTPB binder agent system containing a stoichiometric amount of TDI.
- Created HTPB-based propellant. Example 8 70 parts ammonium perchlorate in parts by weight,
12 parts FR with 16 parts aluminum powder, 1 1/4 parts burn rate accelerator and stoichiometric amount of TDI.
FR mixed with HTPB and containing 86% solids—
Created HTPB propellant. Adhesion of various liners to these propellants was determined by casting the propellant onto the precursor liner and testing the initial bond strength and assemblage at a temperature of 160°C for 30°C.
% RH by performing a time course bond strength test after 7 weeks of exposure. The data are shown in the table below.

[Table] A bond tensile strength of 90-95psi or higher is considered to be very good. This data is based on excess TDI (actual,
The use of TDI dimer in combination with TDI monomer (actual, 6B) does not give a satisfactory product and the use of TDI dimer alone (actual, 6B) does not give a satisfactory product.
6A) provides a much superior increase in bond strength. When a mixture of monomer and dimeric TDI is used, the dimer is present in excess of that required to cure the liner. Similar comparative results would be obtained by using other dimers or trimers in place of the TDI dimer. The adhesion of the liner of Example 6B was compared to the adhesion of a prior art liner composition that did not contain dimer.
The data given in the table are all directly comparable since all combined samples were made from the same batch of each type of propellant.

【table】

[Table] Na.
Although all of the above provide satisfactory bond strength, the most desirable bond is obtained with the liner formulation according to the present invention. When using TDI-Chlorocene thin coating,
The bond life, ie, the time interval between washcoat application and propellant casting, is only a few hours, regardless of the humidity to which the liner is exposed. The non-hazardous time of casting propellant onto the liner in accordance with the present invention is illustrated by the following test. The liner of Example 6B was tested for extended bond life as shown in the table below.

【table】

[Table] For two propellants (Table V, Table 8 and Table 7, Table 7) Bond life (time that the hardened liner remains intact before the propellant is cast)
is at least 10 days with humidity as high as 50%RH. The extended bond life is a significant advantage in rocket motor manufacturing, since the liner time schedule does not have to be related to the propellant time schedule, and the propellant can be stored for a significant period of time before being cast. This is because it is possible. It is understood that only the preferred embodiments of the invention have been described and that numerous substitutions, modifications and improvements may be made without departing from the spirit and scope of the invention as defined in the appended claims. It should be.

[Brief explanation of the drawing]

Of the accompanying drawings, FIG. 1 is a cross-sectional view of a conventional solid propellant rocket motor; and FIG. 2 is a cross-sectional view of a rocket motor in accordance with the present invention.

Claims (1)

Claims: 1. In a composition for use in a rocket motor chamber liner using a polyurethane propellant, expressed in parts by weight, from 20 to 65 parts liquid hydroxyl-terminated prepolymer, from 15 to 35 parts unsaturated. active solid filler, 4 to 10 parts stabilizer and flow control additives, 3
~20 parts of a reactive polymeric modifier such as one or more diols or polyols, an equivalent amount of polyisocyanate sufficient to react with the prepolymer and hydrogen-substituted modifier, and a greater than stoichiometric excess. 1.5 to 5 parts of a stable isocyanate precursor, wherein the isocyanate precursor has the formula: (wherein R is the same or different and is an aliphatic or aromatic group containing at least one -NCO group, and n represents an integer of 0 or 1). A liner composition characterized by being an isocyanate dimer or trimer. 2. The composition of claim 1, wherein said precursor is solid up to a temperature of 150°C. 3. The composition according to item 1 above, wherein R is lower alkyl or aryl. 4 The precursor has the formula, The composition according to the above item 1, which is an isocyanate dimer selected from the compounds of the following formula (wherein R ² is arylene, alkylarylene, or arylalkylene). 5 the precursor is toluene diisocyanate,
5. The polyurethane composition according to item 4, which is a dimer of a member selected from the group consisting of 4,4'-diphenylmethane diisocyanate and bis-phenyl diisocyanate. 6. The polyurethane composition according to item 4 above, wherein the precursor is a dimer of toluene diisocyanate. 7. The method of claim 5, comprising at least one polyol and a sufficient amount of polyisocyanate to react with the polyol, and wherein the precursor is present in a superstoichiometric amount for curing. Composition of. 8. The composition according to item 7, wherein the polyisocyanate is toluene diisocyanate. 9. The composition of paragraph 1, wherein the prepolymer is a conjugated diene polymer having an equivalent weight of 1000 to 5000 and a functionality of 1.7 to 3.0.