Improvements in or relating to polymeric materials

Abstract

A binder material suitable for energetic compositions comprises a cross-linked material formed from a hydroxyl containing compound (preferably hydroxy terminated polybutadiene), a cross-linker having

Description

The present invention relates to novel polymeric materials which inter alia may be used as binder materials for energetic materials and to improved energetic materials containing such a binder. The invention also provides a process for the manufacture of these novel polymeric materials. Energetic materials are materials that contain a high amount of stored chemical energy that can be realised. Typical classes of energetic materials are propellants such as rocket propellants, oxidizers, fuels, pyrotechnics and explosives and they are materials that can undergo, contribute to or cause rapid exothermic decomposition, deflagration or detonation. These materials include chemical compounds or mixtures thereof that when subject to heat, impact, friction, detonation or other forms of initiation undergo a rapid chemical change with the evolution of large volumes of gasses, usually heated gasses that exert pressures in the surrounding medium. Energetic materials can take various forms and the present invention relates to novel polymeric materials which inter alia are applicable as binders that may be used in many different forms of energetic materials. For example, the invention is applicable to propellants that may be hybrid propellants or solid propellants, pyrotechnic materials and explosives. A hybrid Propellant is at least two components one of which is stored in the liquid phase (usually the oxidizer, which can be cryogenic, e.g. liquid oxygen or non-cryogenic, e.g. hydrogen peroxide) and the other component is in the solid phase (e.g. cross-linked hydroxylterminated polybutadiene (HTPB)). Pyrotechnic Material includes explosive or chemical ingredients, including powdered metals, used in the manufacture of pyrotechnic devices which includes all devices and assemblies containing or actuated by propellants or explosives, with the exception of large rocket motors. Pyrotechnic devices include items such as initiators, ignitors, detonators, safe-and-arm devices, booster cartridges, pressure cartridges, separation bolts and nuts, pin pullers, linear separation systems, shaped charges, explosive guillotines, pyrovalves, detonation transfer assemblies (mild detonating fuse, confined detonating cord, confined detonating fuse, shielded mild detonating cord, etc.), thru-bulkhead initiators, mortars, thrusters, explosive circuit interrupters, and other similar items. An example of a complete device that derives its thrust from ejection of hot gases generated from propellants carried in the vehicle is a rocket, the rocket motor being the portion of the complete rocket or booster that is loaded with solid propellant. A Solid Propellant is a solid composition used for propelling projectiles and rockets and to generate gases for powering auxiliary devices. It can be a rubbery or plastic-like mixture of oxidizer, fuel and a binder optionally including other ingredients that has been processed into a finished propellant grain. The term solid propellant is sometimes used to refer to the processed but uncured product or the individual ingredients, such as the fuel or the oxidizer. There are two types of solid propellants that are commonly in use, viz. Double-base and Composite propellants. Double-base propellants are usually made from a homogeneous propellant grain such as nitrocellulose, into which liquid nitroglycerine is absorbed (usually plus additives). This material is a combined fuel and oxidizer. Composite propellants are a heterogeneous propellant grain with the oxidizer crystals (such as ammonium perchlorate (AP)) and a powdered fuel (usually Aluminium) held together in a matrix of synthetic rubber (or plastic) binder (such as hydroxy terminated polybutadiene (HTPB)). This mixture may be hardened by a curing or cross-linking agent which cross-links the binder. Polymer bonded energetic materials comprising an energetic filler material, usually in the form of a solid crystalline powder, formed into a consolidated mass having suitable mechanical properties and insensitivity by a polymeric binder. Such materials are well known and are used in a variety of military and civilian applications such as high explosives for use in demolition, welding, detonating, for example in mining applications, cutting charges and munition fillings, as propellants for guns and rockets, as gas generators and as pyrotechnics. Binders used in polymer bonded energetic materials need to be (amongst other things) compatible with the other ingredients of the material and suitably processed together with the other ingredients into the appropriate shapes required in the various applications. The binders should also be stable, remain compatible with the active material over a wide temperature range to enable munitions to be stored over time in locations of widely varying ambient temperature. It is preferred that the binder materials have a Tg at least as low as -54°C and are stable at temperatures up to +71 °C. A temperature range of -60°C to +90°C is a particularly desired temperature range. Polymeric binders may be classified generally into chemically cured materials and thermoplastic materials. Chemically cured materials, e.g. thermosetting resins, rely on the chemical reaction between different components to provide the desired cross-linked polymeric structure. Thermoplastic binders allow energetic materials containing them to be processed at elevated temperatures, usually outside the in-service envelope of the end product, which cool to give dimensionally stable sheet, bars, cylinders and other shapes. Reject materials may be recycled by re-heating. This may not normally be achieved with materials based on chemically cured binders. Where thermoplastic materials are used we prefer that they have a number average molecular weight (Mn) of 20,000 or greater in order to provided sufficient strength to the energetic material. In PCT publication WO2018 / 064102 we describe binders for energetic materials comprising a polymer which is cross-linked and containing a tackifying resin (called such because of their common use in adhesives and not to be confused with tackifying polymers such as polyisobtutylene) which can be a rosin ester derived from plant sources. Such tackifying resins decrease the entanglement density of the polymer. We have now found that if the tackifying resin is bonded to the binder polymer by means of a cross-linking agent the elongation at break and the tensile strength of the binder material can be significantly increased and the low temperature properties can be improved. United States patent 6740180 also relates to the use of tackifier resins such as the rosin ester Foral 105 in ethylene vinyl acetate copolymer based binder systems for energetic materials. The ethylene vinyl acetate copolymer may be cross-linked. In these references the tackifier resins are blended with the binder polymer and are used to modify the physical properties of the binder polymer. The present invention is concerned with reacting rosin esters with the binder polymers. The rosins can be derived from Tall oil or Gum rosins from pine trees and are generally obtained by the distillation of crude such oil and is typically a blend of various acids and the acids can be converted to esters by reaction with alcohols. However, to enable the ester to be reacted with a cross-linking agent it is important that the ester contain free hydroxyl groups. The ester may contain one or more free hydroxyl groups. The use of such hydroxyl containing rosin esters in an energetic material has been found to reduce or eliminate the need for plasticisers such as phthalate or adipate esters in the energetic formulations as is also described in PCT publication WO2018 / 189158. The use of plasticisers being undesirable as they can migrate within the formulation reducing adhesion to a casing, liners and particles in the formulation. A particular problem is migration of the plasticiser into the rubber liner material. The polymer or polymers that are used in the energetic material formulations of this invention are cross-linked. Accordingly, like the rosin ester, the base polymer from which the inventive polymers are derived must have functional terminations or functional pendant groups to enable reaction with a cross-linking agent. For example, the polymers may be carboxyl terminated, hydroxy terminated, amino terminated or vinyl terminated. However, when a diisocyanate cross-linking agent is used, in order to be able to react with such a cross-linking agent that also reacts with the hydroxyl groups of the rosin ester resin at least some of the termination comprise hydroxyl groups. Note that “terminated, termination, etc.” here means that it is accessible for further cross-linking reactions and can be at the ends of the polymer chains or at other parts of the polymer chain off pendant chain or branch points. A preferred polymer is hydroxy terminated polybutadiene which has an average of 2.5 hydroxyl groups per polymer chain, i.e. There are chain-terminal hydroxyls and a number of chain-pendant hydroxyls. Accordingly, we have now developed polymeric materials by reacting both a hydroxyl group containing polymer and a hydroxyl group containing rosin ester resin with a cross-linking agent that bonds with the hydroxyl groups of both the polymer and the rosin ester resin. Chemically cross-linkable polymer systems, e.g. for use as binders in energetic materials, need functional points of attachment at the ends and / or along the polymer chain which react to form an immobile but flexible network-like structure in which to embed and bind particles such as energetic material particles. The cross-linking is created by adding a separate crosslinking agent (e.g. a multi-isocyanate, e.g. isophorone diisocyanate, to a blend of a hydroxyl containing polymer, e.g. hydroxy-modified polybutadiene) and a hydroxy containing rosin ester so that the hydroxyl containing polymer is also reacted with the hydroxyl groups of a hydroxyl containing rosin ester. It is preferable to add the hydroxyl containing rosin ester to the polymer prior to cross-linking the system. The resulting polymer-resin blend may be stored and transported as a complete system of any desirable concentration used as a total component and possibly diluted with the neat polymer as the application demands. Polymers comprising acrylonitrile / carboxyl terminated butadienes may include as copolymerized monomer units optionally substituted alkyl chains, e.g. dimethylene optionally substituted with a carboxyl group. Carboxyl terminated acrylonitrile / butadiene copolymers also containing hydroxyl groups and hydroxy terminated polybutadiene have been found to be particularly useful in this invention and are readily cross-linked. The polymers of this invention can be grafted with various functional groups before or after the addition of the tackifying resins / rosins and further cross-linked, such an example is butacene (ferrocene-grafted hydroxy-terminated polybutadiene). Other hydroxy containing polymers may be used and can be modified by cross-linking with the tackifying resins / rosins. The materials of this invention are particularly useful as binders for energetic materials and the process of the invention is particularly useful for the production of binder materials for energetic materials which may be obtained by reacting a rosin ester resin containing free hydroxyl groups with the crosslinking agent (e.g. a diisocyanate) through the free hydroxyl groups and the cross-linking agent additionally reacts with one or more of the free hydroxyl groups of a polymeric binder material. The reaction may not be complete to the extent that unreacted polymer and unreacted rosin ester may remain and some self-cross-linking of the polymer and or the rosin ester may occur however in order to obtain the benefits of this invention at least some of the desired cross-linking must occur and that this has occurred may be determined by Soxhlet extraction of the material removing any unbound rosin ester and leaving cross-linked material for identification. Additionally, we have found that the binder may be further improved if the polymer and the functionalised rosin ester are cross-linked under certain conditions. It is also preferred that the amount of hydroxyl group containing rosin ester that is reacted with the polymer is from 10 to 75 wt% based on the weight of the polymer with which it is reacted. It is also preferred that at least 10% of the rosin ester molecules contain hydroxyl groups. The hydroxyl containing rosin ester resin may be prepared by reacting rosin with a polyol such as pentaerythitol, glycerol, sorbitol, ethylene glycol, polyethylene glycol or polyvinyl alcohol. Any of the polyols used in the polyurethane manufacturing industry can be used in this invention e.g. polyalkylene oxide polyether polyols. The amount of polyol that is mixed with the hydroxyl polymer being such that free hydroxyl groups will remain after esterification and be available for reaction with the cross-linking agent. In a preferred embodiment the polymer comprises hydroxyl terminated polybutadiene which is cross-linked in the presence of a hydroxyl containing rosin ester by an isocyanate group containing cross-linking agent preferably containing at least two isocyanate groups to produce the final polymeric material of the invention. We prefer that the cross-linking reaction is performed at a temperature of 60°C or less. We have found that the incorporation of the hydroxyl containing rosin ester in the system enables the cross-linking reaction to provide a material in which the hydroxyl group containing polymer is linked to the rosin ester (molecules which have a free hydroxyl group). This has been found to produce a material with improved desirable mechanical properties such as increased adhesion, increased tensile elongation and strength and improved low temperature properties and a low Tg as are desired for binders for energetic materials. Additionally, we have found that use of the polymers of this invention avoids (or reduces) the need for plasticisers in energetic material formulations. The rosin ester resins used in this invention should contain free hydroxyl groups to enable reaction with the cross-linking agent. Rosins are naturally occurring products which are mixtures of carboxylic acids. The rosin esters used in this invention are obtained by esterification of the acids with poly-hydroxyl containing materials such as pentaerythritol and glycerol. The amount of the hydroxyl containing material that is used for esterification being such that after esterification the esterified material contains free hydroxyl groups. The binders of this invention are preferably made by first mixing the hydroxyl group containing polymer binder and the hydroxyl containing rosin ester and subsequently adding the cross-linking agent and activating the system to perform the cross-linking reaction. If a full energetic composite material is needed, the solid energetic particles, fuel particles and any other ingredients, are added prior to cross-linking. A cure catalyst may also be employed if necessary, an example of such a catalyst is dibutyltin dilaurate. Energetic materials typically comprise one or more active components which can be activated by energy input, e.g. heat, impact, agitation as is required according to the particular use envisaged for the energetic material. In the final composition the active components are bound together within a matrix of the cross-linked polymer binder of this invention. The performance of these energetic materials including their processing and the energy generated per unit of the active components can depend upon the distribution of the active components throughout the matrix of the polymer binder. We have found that this is significantly improved when the polymer of this invention is included. In this invention the mixture of the non-hydroxyl containing rosin ester and the hydroxyl containing rosin ester should be miscible with the hydroxyl containing polymer used in the formulation. The rosin ester should be miscible with the polymeric binder such that the integrity of the blend of the polymer and the rosin ester should be maintained over a temperature range of -54°C to +71°C. preferably from -60°C to +90°C and it is therefore preferred that the hydroxy containing polymer has a number average molecular weight in the range of 1,000 to 20,000. In a further embodiment the invention provides an energetic material formulation comprising i) one or more active components ii) a binder matrix comprising a hydroxyl group containing polymer cross-linked to a hydroxyl group containing rosin ester. It may also contain a non-hydroxyl containing rosin ester. Examples of suitable rosin esters for use in this invention are rosin esters derived from plant rosin which may be converted to rosin ester. Three types of rosin are used for resin manufacture, gum rosin, wood rosin and tall oil rosin, and they are all generated from trees, e.g. pine trees. Tall oil rosin is obtained by distillation of crude tall oil, a by-product of the kraft sulphate pulping process used in paper making. Crude tall oil typically contains 70-90% acidic material, which is composed essentially of fatty acid and tall oil rosin. Tall oil rosin (TOR) has a tendency to crystallize and usually contains 200-600 ppm sulfur. Highly distilled TOR can produce esters which have been found to be useful in this invention. Gum rosin esters from pine trees are effective and can equally be used and are exemplified herein. Rosin acids are typically a blend of acids such as the following different molecules. Abietic type Abietic Neoabietic Palustric Levopimaric Dihydroabietic Pimaric Isopimaric Sandaracopimaric Rosin molecules can have poor stability caused by unsaturation and stability can be improved by various methods such as disproportionation and hydrogenation. Rearrangement of the double bonds by disproportionation leads to improved stability as shown below. ;.:<S Alette ©eld Oihydmabistic ©©id Another method to improve stability is to hydrogenate the rosin molecules as follows. The rosin containing carboxylic acids can be converted to rosin ester by esterification with various alcohols. The number of alcohol groups and molecular weight of the alcohol determines the softening point of the subsequent ester. In this invention the alcohol should be polyhydric enabling one hydroxyl group to react with the rosin carboxylic acid group and leaving free hydroxyl groups to react with the cross-linking agent. Glycerol and pentaerythritol are the most commonly used and preferred alcohols as are sorbitol, plolyethylene glycol and polyvinyl alcohol. Any of the polyols used in the polyurethane manufacturing industry can be used in this invention e.g. polyalkylene oxide polyether polyols. The esterification should not be 100% in order to leave unreacted free hydroxyl groups in the rosin ester as required for reaction with the cross-linking agent. Rosin esters have a wide span of compatibilities and they have been found to be particularly useful with the hydroxyl containing polymers as in the present invention. When used as a binder for energetic materials the relative proportions of the components of the energetic material will depend upon the type of application for which the material is to be used. The present invention may be used in for example a plastic bonded explosive in which the binder forms between 0.5 and 30% by weight and the energetic filler material forms between 99.5 and 70% by weight. The binder contains at least some of the novel polymers of this invention, preferably at least 10 wt% of the polymer of this invention. Extenders may be used as part of the binder formulation to improve the processibility and flexibility of the product. For example, heavy grade liquid paraffin (up to 3% by weight of the binder formulation) may be employed in the binder. Lecithin and other additives are also used in some formulations. The formulation may also include bonding agents, burn rate modifiers and other ingredients found to be useful in energetic materials. Antioxidants are useful additives and are usually added to the binder polymer and / or rosin ester prior to mixing. Plasticisers may be used but the tackifying resin is capable of replacing some or all of it. The cross-linked binder matrix of this invention is used at a ratio of 1:99 to 90:10 in relation to the total of the other components in the formulation. Preferably from 5:95 to 40:60 more preferably from 10:90 to 30:70. Examples of active components (sometimes known as energetic fillers) include organic secondary explosives. Alicyclic nitranes such as RDX (1,3,5-cyclotrimethylene-2,4,6,-trinitramine) and HMX (1,3,5,7-cyclotetramethylene-2,4,6,8-tetrar,itramine) and TATND (tetranitro-tetraminodecalin) and mixtures thereof. The following active components may also be used as the main or as a subsidiary energetic component in plastic bonded explosives-nitroguanidine, aromatic nitramines such as tetryl, ethylene dinitramine, nitrate esters such as nitroglycerine, butanetriol trinitrate and PETN (pentaerythritol tetranitrate). Other nitroaromatic compounds such as trinitrotoluene (TNT) triaminobenzene (TATB) triaminotrinitro benzene (TATNB) and hexanitrostilbene may also be used. Alternatively active components such as inorganic fillers such as ammonium nitrate and alkaline earth metal salts provide suitable high explosive materials. Metallic fuels such as powdered aluminium, magnesium or zirconium may be used to fuel the exothermic reaction of the oxidation of the energetic material. The metallic fuel may comprise up to 50% by weight of the energetic filler. The energetic materials of this invention may alternatively comprise a gun propellant. In such a material the content of the active component is generally in the range 70 to 90% by weight of the binder / fiIler mixture and may be selected for example from nitroglycerine, RDX and HMX or a combination thereof, optionally with other highly active components such as those listed above. The binder of such a material may comprise in addition to the polymer of this invention a cellulosic material e.g. nitrocellulose e.g. forming from 5 to 95%, e.g. 30 to 70% by weight of the binder. The energetic material may alternatively comprise a gas generation material as the active component for example, for power cartridges for actuators: for base burning, reduced base drag, extended range projectiles: and for control gas jets for missile and projectile guidance systems and the like. Such material is similar in nature to a propellant, but in general contains a lower content of active component, e.g. 45% to 65% by weight optionally together with a surface burning rate inhibitor, e.g. ethyl cellulose. As an example of a suitable rocket propellant embodying the invention the propellant composition may include as active component ammonium perchlorate (20 to 95% by weight) together with aluminium as fuel (5 to 50% by weight of its mixture with the active component), the binder forming for example 5 to 30% by weight of the composition together with the tackifier resin. The energetic material may also comprise a polymer bonded pyrotechnic material, e.g. containing an inorganic nitrate or perchlorate of ammonium, barium or strontium (forming 20 to 80% by weight of the energetic filler), a metallic fuel such as magnesium or zirconium (forming 5 to 60% by weight of the filler), the binder comprising 5 to 30% by weight of the overall composition. Although the use of non-viscous plasticisers may be avoided by use of the polymer bonded energetic materials because the cross-linked polymers of this invention can have a plasticising effect upon the polymer, non-viscous plasticisers may optionally be incorporated in low concentrations in the compositions according to the present invention. We have however found that the use of the cross-linked binder system of the present invention may avoid the need for plasticisers in the formulation, which is beneficial as the plasticiser when used can migrate within the system during storage. Where plasticisers are used, common plasticisers which are dialkyl esters of phthalic, adipic and sebacic acids may be used as the optional plasticiser, e.g. the plasticiser may comprise dibutyl phthalate, disobutyl phthalate, dimethyl glycol phthalate, dioctyl adipate or dioctyl sebacate preferably less than 10% by weight of the binder. In addition, or alternatively, energetic plasticisers such as BDNPAIF (bis-2-dinitropropylacetral / formal), bis-(2-fluoro-2,2-dinitroethyl) formal, diethylene glycol dinitrate, glycerol trinitrate, glycol trinitrate, triethylene glycerol dinitrate, trimethylolethane trinitrate butanetriol trinitrate, or 1,2,4-butanetriol trinitrate, may be employed in concentration less than 10% by weight of binder in the materials according to the present invention. Examples of suitable additional inert or non-energetic binder materials are cellulosic materials such as the esters, e.g. cellulose acetate, cellulose acetate butyrate, and synthetic polymers such as polyurethanes, polyesters, polybutadienes, polyethylenes, polyvinyl acetate and blends and / or copolymers thereof. Various other minor additives may be added to the energetic material formulations of the present invention. Examples of material that may be used include surfactants and antifoam. Preferably, the additives content comprises no more than 10% by weight, desirably less than 5% by weight, of the overall energetic material composition. For example in propellant and gas generator compositions the additive may for example comprise one or more stabilisers, e.g. carbamite or PNTYIA (para-nitromethylaniline); and / or one or more ballistic modifiers, e.g. carbon black or lead salts; and / or one or more flash suppressants, e.g. one or more sodium or potassium salts, e.g. sodium or potassium sulphate or bicarbonate. Other modifiers particularly for ballistics include iron oxide or catacene. Antioxidant in an extent of up to 2% by weight of the overall composition of the energetic materials may usefully be incorporate in the materials. Phenolic antioxidants such as 2,2' -methylene-bis (4 -methyl-6 -butyl) phenol has been found to be suitable. Preferably, where the energetic material according to the present invention is a plastic bonded explosive it contains the following components (in percentage parts by weight): RDX: SO-99.5%, preferably about 88%; binder material of the invention: 20-0.5%, preferably about 12%; 0 to 2% antioxidant, the overall percentages (excluding further optional additives) adding to 100 in each case. In an embodiment of the invention the energetic material formulations of the present invention may be processed into manufactured products by processes which are generally known per se. For example, for the manufacture of plastic bonded explosives the binder material may be produced by mixing the polymeric material and the rosin ester in a blender (such as a simple stirred vessel) at temperatures in excess of the tackifying resin’s Softening Point (SP), e.g. at least 110°C if the SP is 100°C. To study the properties of the binder mixture, the cross-linking agent is added at a temperature in the range 20°C to 60°C. After mixing and effecting the cross-linking reaction the binder may then be added, before it becomes too viscous to add efficiently, to the active component by a solventless process or a solvent lacquer process. Alternatively, all the components may be added at the same time, however, if the rosin ester is solid at the mixing temperature, it must be added to the binder polymer prior to adding the other ingredients. Such a mixing method used, making a pre-blend of the polymer and rosin ester is the preferred method as polymer-rosin ester resin compatibility / miscibility is important. The polymer-rosin ester mixture should ideally be completely compatible / miscible and produce a clear mixture / solution. Although some incompatibility / immiscibility is acceptable providing the mixture is homogeneous throughout the volume. The cross-linking agent for the polymer and the rosin ester is preferably added after the polymer has been blended with the rosin ester. In a solvent lacquer process, the binder of this invention may be dissolved in an organic solvent at a moderately elevated temperature, e.g. 40°C to 80°C and the active component is subsequently stirred into the solvent lacquer after cooling to about 20°C to give a slurry. The slurry is then mixed under vacuum at an elevated temperature, e.g. 50°C to 90°C, preferably 75°C to 90°C. In a solventless process for example, for the production of plastic bonded nitramines the required quantity of pre-dried active component is wetted with water or an aqueous solution and heated to an elevated temperature, e.g. 80°C-100°C. The binder of this invention is then added to the active component and the components are mixed together at that temperature. Any water remaining in the composition is removed under vacuum. Materials produced in the ways described above or in other known ways may, depending on the material composition and its intended use, be shaped into products in known ways. For example, the material may be pressed, moulded, extruded or cast into a desired shape e.g. for use as blocks, sheet explosive or for filling of shells, warheads, rocket motor casings and the like. The latter technique is especially suitable for the manufacture of gun propellant materials, e.g. stick or tubular propellants of known cross-sectional shape. In summary, the energetic materials of the present invention may, depending upon their specific composition and properties, be used in any one or more of the following well known applications: (i) Explosives including general demolition; (ii) Explosive welding; (iii) Active armour; (iv) Detonating cord; (v) Linear cutting charges; (vi) Shell fillings; (vii) Mine fillings; (viii) Grenade fillings; (ix) Shaped-charge warhead fillings; (x) rocket propellants and gas generator propellants, (xi) pyrotechnics. An energetic material needs to be a stable system which can be handled, stored and transported. The conditions under which it should be stable will vary from one energetic material to another and according to the use to which the energetic material is to be put. However generally energetic materials need to be prepared, handled, stored and transported at temperatures in the range from -50°C to 71 °C or higher (preferable range of -60°C to +90°C). We have found that the binder material of this invention has increased the adhesion to particles and liners / casings, increased strength and elasticity as shown by stress / strain testing and also improve the low temperature performance. Long-term ageing studies have found that the formulations are therefore more robust than existing energetic material formulations indicating their usefulness inter alia as binders for energetic materials. The invention is illustrated by reference to the following Examples Examples The reaction sequence in one embodiment of this invention and the production and structure of a new polymeric material of the invention are illustrated in Figure 1. Figure 1 shows a reaction sequence according to the present invention in which dihydroxy terminated polybutadiene (1) (HTBP from SAMYANG FINE CHEMICAL Co., Ltd. #125, Heohyeon 1-gil, Yangsan-si, Gyeongnam, KOREA 626-210) is blended with 10 wt% of a hydroxy containing rosin ester (2) (Dercol PE100 from Diamantino Malho &C.a Lda., Rua da Cerca, 18, 3100-081 Albergaria dos Doze, PORTUGAL). It should be noted that the polymeric structures in Figures 1 and 2 are simplifications and Structures (1), (4) and (6) can have various molecular weights and microstructures. In Structure (4), R represents a rosin acid moiety and the remaining isocyanate group is free to further react with another hydroxylcontaining polymer or hydroxyl containing rosin ester as in Structure (6). The blending is performed in a suitable mixer at 110°C until a clear liquid results. Prior to cross-linking the blends were dried in a vacuum oven at 60°C for 24 hours. The amount of isocyanate cross-linking agent used was calculated to achieve an equivalent ratio (the ratio of HTPB hydroxyl groups to isocyanate groups). The blend is then reacted with toluene diisocyanate cross-linking agent (3) at 60°C for about 30 minutes to produce a product of the final compound (4) and its structure was determined by proton NMR. Respective amounts of pentaerythritol (PER) at 5, 10 and 15 wt% and ethylene glycol (GER) at 5, 10 13.57 and 15 wt% loadings were dissolved in HTPB at 110°C, followed by constant stirring for 15 minutes at the same temperature. Once prepared, the blends were dried under vacuum in an oven for 24 hours at 60°C. For the cross-linking reaction toluene diisocyanate (TDI) in an amount to provide a 1:1 molar ratio of hydroxyl group between TDI and HTPB was added to the HTPB / PER and HTPB / GER blends under magnetic stirring for 1 hour at 60°C. The blends were then moulded (rectangular; 295 mm x 85 mm x 5 mm) and heated in an over at 60°C for 5 days to ensure maximum cross-linking. The polymers were characterised by FTIR spectra in transmission mode using a Bruker Tensor 27 FTIR spectrometer with a resolution of 2 cm -1 in the range of 4000 to 500 cm-1; 64 scans were accumulated. 1H NMR measurements were carried out using a Bruker DPX-400 spectrometer and CDCh with a tetramethyl silane standard. GPC was performed using an Agilent 390-LC system under Tetra Hydro Furan with 2% trimethylamine (TEA) and 0.01% butylated hydroxytoluene (BHT) with a flow rate of 1 mL / min and calibrated using polymethyl methacrylate standards. DSC was carried out using a Mettler Toledo DSC 1 under a nitrogen environment; pure PER and GER were heated from 0 to 130°C at 5°C / min to allow for complete softening, while the HTPB / PER and HTPB / GER blends were measured from -120 to 100°C at 5°C / min. DMTA was performed on a Triton Tritec 2000 DMA in tensile mode at a single frequency of 1 Hz at 0.05 mm (out of 10 mm) from -105 to 100°C at 3°C / min. Tensile mechanical testing was carried out on a Shimadzu Autograph AGS-X tensile tester using a strain rate of 50 mm / min. Figure 2 shows a comparative reaction sequence in which the hydroxy terminated polybutadiene (1) is blended with a fully esterified hydrogenated rosin ester free of hydroxy groups (6). Comparison of Figures 1 and 2 shows that in Figure 1 the hydroxyl containing rosin ester forms part of the final molecule (4) via the cross-linking agent whereas Figure 2 shows that the cross-linking agent (3) simply joins up two of the hydroxy terminated polybutadiene molecules to give the compound (5). The two rosin esters were compared by proton NMR showing the presence of hydroxyl groups in the resin (2) and the absence from the resin (6). The cross-link density, stress and strain values for the hydroxy terminated polybutadiene (HTPB) on its own and T2 and T3 and blends with a hydrocarbon resin tackifier (as in the prior art) versions are shown in Figures 3 to 8. In Figures 3 to 6 the different products are labeled RHTPB - the hydroxy terminated butadiene on its own RTi - 10% and RTi - 20% The product of the RHTPB and 10% and 20% respectively of the hydrocarbon tackifier Escorez 5960 RT2 - 10% and RT2 - 20% The product of the RHTPB and 10% and 20% respectively of comparative material T2 RT3- 10% and RT3-20% The reaction product containing 10% and 20% of the hydroxy containing rosin ester T3 according to the invention The curves in Figure 3 are small angle x-ray scattering plots. Those in Figure 4 are wide angle x-ray scattering plots. Figure 5 is a zoomed view of Figure 3 showing a plot of inter domain spacing. Figure 6 is a plot of inter-domain spacings. Figure 7 is a measure of the cross-link density showing that T3 has a significantly lower crosslink density. Figure 8 compares the maximum elongation to breath of the materials. Every Figure showing the difference of the T3 based materials. The T3 materials have a lower cross-linking density and greater extensional behaviour which enhances the materials adhesion characteristics. The relative results from the tensile testing of these samples are given in the chart (with the reference HTPB sample as 100%) which is Figure 9. The analyses show that the hydroxyl containing rosin ester participates in the cross-linking reaction whereas the hydroxyl-free material does not. The incorporation of the hydroxyl containing rosin ester has been found to result in a 600% increase in elongation of break of a formulation containing 20 wt% of the polymer and a 275% increase in a formulation containing 10 wt%. The cross-linked polymer of the invention was used in the production of an inert version of an energetic material formulation comprising 80 wt% ammonium sulphate (substituting for the energetic counterpart, ammonium perchlorate) and 0.5 wt% Iron Oxide (a burning rate modifier). The remainder of the composition was the cross-linked polymer-tackifying resin combination. The cross-linked polymer of this invention was used (T3) and compared with similar formulations where the polymer was based on the hydroxy-free rosin ester SYLVALITE 2100 (T2) from the Kraton Corporation and the hydroxy-free hydrocarbon resin Escorez 5690 (T1) such as that used in United States Patent 6740180. These samples were compared against the tackifying resin-free sample (HTPB). The mixing was performed in a Resonant Acoustic Mixer (RAM) for mixing an inert composite propellant with a solid loading of 80wt%. A bi-modal distribution of ammonium sulphate was used at a dso of -210pm (60wt%) and ~25pm (20wt%) with 0.5wt% of iron oxide. The remainder of the formulation was the binder that was either unmodified or modified HTPB. An acceleration of 30G was applied for 3 mins followed by 60G for 15 minutes with vacuum at 0.55±0.05bar. X-ray CT was used to study the distribution of the mix components, based on density differences. Analysis of the 2D slices in the XY plane provide vol% of the matrix and coarse ammonium sulphate throughout the vessel. The standard deviation (SD) of the vol% can provide a measure of distributive mixing, with a low standard deviation indicating good distributive mixing. Therefore, the lower the SD, the better the mixing and the more consistent of the distribution of the solid components. Figure 10 shows the advantage of the cross-linked polymer combination of this invention (T3) using the total solids distribution and the inert ammonium sulphate distribution.

Claims

1. A cross-linked polymeric material comprising a hydroxyl group containing compound reacted with a cross-linking agent having at least two reactive groups per molecule at least one of which is reacted with one or more hydroxyl groups of the hydroxyl containing compound and at least another of the reactive groups is reacted with a hydroxyl group of a hydroxyl containing rosin ester resin.

2. A cross-linked polymeric material according to Claim 1 in which the hydroxy group containing polymer is hydroxy terminated polybutadiene.

3. A cross-linked polymeric material according to Claim 1 or Claim 2 in which the hydroxyl group containing rosin ester resin is a partially esterified rosin material.

4. A cross-linked polymeric material according to any of the preceding claims in which the cross-linking agent is a polyisocyanate.

5. A cross-linked polymeric material according to Claim 4 in which the cross-linking agent is a diisocyanate.

6. A cross-linked polymeric material according to any of Claims 1 to 3 in which the rosin ester comprises rosin acid esterified with a polyol.

7. A cross-linked polymeric material according to Claim 6 in which the polyol is polyethylene glycol.

8. A cross-linked polymeric material according to Claim 6 in which the polyol is pentaerythritol.

9. A cross-linked polymeric material according to Claim 6 in which the polyol is glycerol.

10. A cross-linked polymer according to any of the proceeding claims in which the hydroxyl group containing polymer has a number average molecular weight in the range of from 1,000 to 20,000.

11. A cross-linked polymeric material according to any of the preceding claims having a Tg below -54°C.

12. A cross-linked polymeric material of the formulaWhere R represents a rosin acid moiety and the structure within the brackets is base polymer of varying molecular weights and microstructure. The remaining isocyanate group is free to further react with another hydroxyl-containing polymer or hydroxyl containing rosin ester.

13. A process comprising blending a hydroxyl group containing polymer containing at least two reactive hydroxyl groups per molecule and a hydroxyl group containing rosin ester and providing a cross-linking agent containing at least two reactive cross-linking groups to the blend and causing the cross-linking agent to effect a cross-linking reaction between the hydroxyl group containing polymer and the hydroxyl group containing rosin ester.

14. A process according to Claim 13 in which the cross-linking agent is a polyisocyanate.

15. A process according to Claim 13 or Claim 14 in which the hydroxyl group containing rosin ester comprises rosin reacted with a polyol16. A process according to Claim 15 in which the polyol is polyethylene glycol.

17. A process according to Claim 15 in which the polyol is pentaerythritol.

18. A process according to Claim 15 in which the polyol is glycerol.

19. The use of a cross-linked polymeric material according to any of Claims 1 to 12 as a binder in energetic formulations.

20. The use according to Claim 19 as a bonding layer.

21. An energetic material comprising an active material and a binder comprising a crosslinked polymeric material according to any one of Claims 1 to 12.

22. An energetic material according to Claim 21 wherein the active material comprises ammonium perchlorate.

23. An energetic material according to Claim 21 or Claim 22 containing from 10 to 25 wt% of the cross-linked polymer composition.

24. A process for the production of an energetic material comprising blending a cross-linked polymeric material according to any of Claims 1 to 12 with an active material.

25. A process according to Claim 24 in which the active material comprises ammonium perchlorate.

26. A process for the production of an energetic material comprising blending (i) a hydroxyl containing polymer containing at least two hydroxyl groups (ii) a hydroxyl containing rosin ester (iii) an active material and subsequently adding a cross-linking agent containing at least two reactive groups and causing the cross-linking agent to cross-link the hydroxyl group containing polymer with the hydroxyl containing rosin ester.w

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