CATALYTIC COMPOSITION FOR LINKING OF POLYURETAN BINDERS AT ROOM TEMPERATURE

Description

[0001] The present invention relates to the crosslinking of polyurethane binders. It can be applied to the field of propellant charges for tactical weapons (missiles, rockets), more particularly to solid composite propellants based on polyurethane binders, obtained by a manufacturing process involving mixing, casting, and curing. The invention concerns a catalytic composition for the room-temperature crosslinking of polyurethane binders, notably used in propellant charges.

[0002] Polyurethanes are polymers that contain the urethane group. The urethane function is less commonly called the carbamate function: R'-NH-CO-OR.

[0003] This function results from the condensation reaction between a mobile hydrogen compound of the alcohol type R-OH and an isocyanate R'-N=C=O. To form a polyurethane, it is necessary to react polyols (at least two alcohol functions per molecule) with polyisocyanates (at least two isocyanate functions per molecule).

[0004] The reaction of a diol with a diisocyanate leads to a linear polyurethane. HO-R-OH + O=C=N-R'-N=C=O → -[-ORO-CO-NH-R'-NH-CO-]-

[0005] The urethane functional group, obtained by the addition of an isocyanate to an alcohol, has been known since the late 19th century, and the reactions of isocyanates with various compounds containing mobile hydrogen were studied in the early 20th century. These reactions are generally exothermic.

[0006] The catalysts for the isocyanate-hydroxyl reaction belong to two main families: organometallic complexes (Lewis acid catalysis) or nucleophilic organic compounds (basic catalysis). Among these, it is common to use organometallic salts (such as tin dibutyl dilaurate, or DBTDL) for the isocyanate-hydroxyl reaction, or mixtures of organometallic salts combined with aromatic tertiary polyamines (1,4-diazabicyclo[2.2.2]octane, abbreviated DABCO, is the most significant example of such an amine) when modulating the competition between the isocyanate-hydroxyl and isocyanate-water reactions in polyurethane foams. Conversely, acids and acid chlorides are inhibitors of the isocyanate-hydroxyl reaction.

[0007] The addition of any additive or filler that can significantly alter the acid-base character of a polyurethane formulation is likely to alter its kinetics.

[0008] The state of the art in this field describes the use of catalytic systems to accelerate the kinetics of the polyurethane crosslinking reaction (i.e., the reaction between an isocyanate and a polyol). The most commonly described catalyst in this field is tin dibutyl dilaurate (DBTDL). It is a tin-based organometallic salt. This is referred to as organometallic salt catalysis. Many other organometallic salt catalysts are known in this field; the most common are based on tin, iron, lead, copper, and, more recently, bismuth or zinc.

[0009] In the prior art, the use of a tertiary amine type organic catalyst (basic type catalysis) is also commonly described.

[0010] The nature of the isocyanate (whether aliphatic or aromatic) also allows for modulation of the reaction kinetics. Documents US2018 / 086956A1, CN110305289A, EP2455412A1, US596214 A, WO2005 / 058996A1, and WO2018 / 122495A1 describe prior art compositions.

[0011] In the specific field of composite propellants, the state of the art describes the use of crosslinking catalysts to reduce the curing time of polyurethane-bound composite propellants. The book "Solid Propellant Technology - Davenas" explains that the use of crosslinking catalysts is necessary to reduce propellant curing time. Beyond their kinetic effects, they can play a significant role in mechanical properties by promoting certain reactions and thus guiding the formation of the polymer network. Crosslinking catalysts are generally organic salts of transition metals (tin, iron, chromium).

[0012] The kinetics of the crosslinking reaction must be slow enough to allow implementation operations (this latency period of several hours is called "pot life") but fast enough not to require excessively long crosslinking times (cooking time).

[0013] Cooking temperatures generally range from 40 to 65 °C and the cooking times required to stabilize mechanical properties generally range from approximately 7 to 20 days.

[0014] The purpose of baking is to accelerate cross-linking reactions, thus rapidly hardening the propellant. This is achieved by heating the propellant in an oven (or a pouring well for large blocks) and leaving it there for a certain period of time. This time is shorter the higher the baking temperature.

[0015] In the field of composite propellants based on polyurethane binders, the use of two types of catalytic compositions is commonly described: a. DBTDL (tin dibutyl dilaurate), alone or often associated with a tertiary amine (typically tris(dimethylaminomethyl)phenol or DMP30); b. Tri-phenyl Bismuth (TPB).

[0016] The cooking temperatures applied are generally in the range of 40 to 65°C.

[0017] The catalytic compositions usually used in the field of composite propellants (DBTDL / amine DMP30 or TPB pair) do not allow access to sufficiently rapid crosslinking to be compatible with baking at room temperature and for a sufficiently short time (by short we mean durations compatible with industrial logistics constraints and production rates), including by increasing the catalyst content in the propellant formulation.

[0018] One possible solution would be to increase the catalyst content of conventional catalytic compositions (DBTDL + tertiary amine). However, this approach is not satisfactory because it would require a very significant increase in catalyst content, with expected adverse impacts: a significant increase in the tertiary amine content risks having a detrimental impact on the mechanical properties of the propellant material (it is known in the field that DMP-30 has a favorable effect on the mechanical properties of the propellant when introduced in small quantities, but a detrimental effect when introduced in high concentrations), and exacerbating the risk of gas release upon contact with ammonium perchlorate (the two chemical species being known to be chemically incompatible in intimate mixtures);a significant increase in the level of DBTDL will have a detrimental impact with regard to the toxicity criterion (DBTDL is a CMR chemical constituent, classified according to the current CLP regulations as "Toxic to reproduction" category 1B and "Mutagenic" Category 2).

[0019] Another solution would be to use a single catalytic agent such as amidine (DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene, or DBN, 1,5-diazabicyclo[4.3.0]non-5-ene), which is described in the technical literature as having a catalytic power superior to that of simple tertiary amines (such as DMP-30). However, this approach is not satisfactory, firstly due to a very strong chemical incompatibility between this type of amidine constituent (due to its very high basicity) and the constituent Ammonium Perchlorate (the main ingredient of the composite propellants considered within the scope of the present invention), which precludes their use in the field covered by the present invention.On the other hand, it is preferable to remain with a mixed system based on the combination of an organometallic catalyst coupled with a basic catalyst in order to benefit from a synergistic effect allowing the limitation of the amine compound content (for reasons of chemical compatibility mentioned below) but also in order to have the possibility of modulating the reactivity of the catalytic system, therefore the kinetics of polyurethane formation and consequently adjusting the cooking time (in connection with the desired industrial flexibility) - via the adjustment of the rate of only one of the 2 constituents of the catalytic system without having to resort to a significant increase in the overall content of the catalytic system within the propellant.

[0020] The invention aims to overcome all or part of the problems mentioned above by proposing a catalytic composition allowing crosslinking at room temperature of polyurethane binders, particularly used in propellant charges, over a short period.

[0021] To this end, the invention relates to a catalytic composition for the room-temperature crosslinking of polyurethane binders, characterized in that it comprises: a. A bismuth organic salt catalyst; b. A basic tertiary amine catalyst; c. An unblocked difunctional aliphatic isocyanate.

[0022] The organic bismuth salt type catalyst is bismuth neodecanoate.

[0023] The basic tertiary amine catalyst is tris(dimethylaminomethyl)phenol (DMP30).

[0024] The unblocked difunctional aliphatic isocyanate is isophorone diisocyanate (IPDI).

[0025] The invention will be better understood and other advantages will become apparent upon reading the detailed description of an embodiment given by way of example, a description illustrated by the accompanying drawing in which: [ Fig.1 ] there figure 1 represents the conversion rate of isocyanate to urethane as a function of time at a temperature of 55°C and at room temperature for the reference catalytic system of the DBTDL / DMP-30 domain associated with an MDCI (4,4'-methylenedicyclohexyl diisocyanate) isocyanate; [ Fig.2 ] there figure 2 represents the conversion rate of isocyanate to urethane over time at room temperature using a single prior art basic catalyst (amine or amidine) associated with the same MDCI isocyanate; Fig.3 ] there figure 3 represents the conversion rate of isocyanate to urethane as a function of time at room temperature using a prior art organometallic catalyst combined with a prior art amine catalyst and the same MDCI isocyanate; Fig.4 ] there figure 4 represents the conversion kinetics of isocyanate to urethane (through monitoring of the viscosity value) as a function of time at a temperature of 55°C (reference) and at room temperature using a single prior art catalyst; Fig.5 ] there figure 5 represents the conversion kinetics of isocyanate to urethane (through monitoring of viscosity value) as a function of time at a temperature of 55°C (reference) and at room temperature with the use of a catalytic composition according to the invention.

[0026] The invention relates to the room-temperature crosslinking of polyurethane binders, particularly those used in composite propellant loadings. However, the catalytic compositions described in the invention can also find profitable use in the formulation of inert materials for internal fittings associated with propellant loadings, such as polyurethane-based binders.

[0027] The object of the invention lies in the use of a so-called mixed catalysis system, based on the combination of a non-CMR organic bismuth salt catalyst (replacing tin salts due to their high CMR toxicity; the term CMR originates from regulations on the prevention of chemical risks and is also used to designate certain industrial processes that have carcinogenic, mutagenic, or reprotoxic effects on exposed professionals) and a basic tertiary amine catalyst. This allows, on the one hand, for the synergistic effect induced by the combination of the two catalysts, and on the other hand, for easier modulation of the crosslinking kinetics by adjusting the concentration of either one or both catalysts.Finally, this approach advantageously allows for limiting the total amount of amine catalyst in the propellant, thereby reducing its impact on functional properties, particularly energy performance. Organometallic catalysts have the advantage of being introduced in very small quantities, less than basic amine catalysts. Furthermore, due to their high basicity, amine catalysts are likely to interact chemically with ammonium perchlorate, which constitutes the primary energy charge in composite propellants. For this reason, it seems advisable to use a minimal amount of basic catalyst and to rely on the addition of a second catalyst, such as bismuth salt (for which no risk of chemical incompatibility with ammonium perchlorate has been identified), to achieve the desired crosslinking kinetics.

[0028] Furthermore, it is known in the field that the amine catalyst content can adversely impact the final mechanical properties of the propellant when introduced in high concentrations. Therefore, it seems advisable to keep the amount of tertiary amine introduced to the minimum necessary level, and to modulate only the bismuth salt catalyst content to adjust the crosslinking kinetics.

[0029] Counter-example #1: impact of temperature on reaction kinetics for the reference catalytic composition (DBTDL / DMP-30) associated with a reference isocyanate crosslinker MDCI (MDCI, also known as H 12 MDI, is a di-functional isocyanate crosslinker very commonly used in the field of composite propellants).

[0030] There figure 1 represents the conversion rate of isocyanate to urethane as a function of time at a temperature of 55°C (curve 10) and at room temperature (curve 11). Curve 10, measured at 55°C (100% conversion rate obtained after 20 hours), is considered the reference kinetics to be achieved at room temperature using optimized catalytic compositions.

[0031] There figure 1 This shows that lowering the temperature from 55°C (reference temperature representing the cooking temperature of composite propellants, curve 10) to room temperature (30°C, curve 11) induces a strong lowering of the reaction kinetics with a prior art reference catalytic composition (isocyanate crosslinker MDCI, catalyst no. 1 DBTDL, catalyst no. 2 tertiary amine DMP30).

[0032] The progress of the reaction is represented here by the evolution of the conversion rate of isocyanate to urethane over time, measured by infrared spectrometry using equipment with an ATR (Attenuated Total Reflectance) device on model mixtures (MDCl / DBTDL / DMP30). Kinetic monitoring is obtained by quantifying the intensity of the isocyanate characteristic band at approximately 2260 cm⁻¹ and the urethane characteristic band at approximately 1700 cm⁻¹ over different time periods.

[0033] There figure 1 This shows that the prior art MDCI / DBTDL / DMP30 catalytic composition is not suitable for the objective of crosslinking at room temperature for the same durations as those used under the reference conditions at 55°C. MDCI is a non-blocked difunctional aliphatic isocyanate.

[0034] Counter-example 2: impact of catalyst(s) on reaction kinetics at room temperature.

[0035] There figure 2 represents the conversion rate of isocyanate to urethane as a function of time at room temperature with the use of a single prior art basic catalyst (associated with an MDCI isocyanate crosslinker).

[0036] There figure 2 This shows that a 100% conversion rate after 20 hours cannot be achieved at room temperature using only a basic catalyst such as amidine (DBU: curve 20, DBN: curve 21), guanidine (MTBD, abbreviated as 7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene: curve 24), or amine (DMP30: curve 22, DABCO: curve 23). The DBU and DBN compounds exhibit the fastest conversion kinetics. Advantageously, no saturation or inhibition effect, as described in the prior art technical literature for polyurethanes, is observed with DBN.

[0037] There figure 3 represents the conversion rate of isocyanate to urethane as a function of time at room temperature with the use of a mixed system consisting of a prior art organometallic catalyst associated with a prior art amine catalyst (DABCO) and an isocyanate crosslinker MDCI.

[0038] There figure 3 This shows that a 100% conversion rate after 20 hours cannot be achieved at room temperature using only an organometallic catalyst such as DBTDL (curve 30), Triphenyl Bismuth (TPB: curve 31), or a molybdenum-based catalyst (Bis(acetylacetonato)dioxomolybdene: curve 32). TPB advantageously exhibits conversion kinetics close to those of the reference DBTDL.

[0039] Example 1: Kinetics of conversion of isocyanate to urethane as a function of time at a temperature of 55°C (reference) and at room temperature with the use of a prior art catalyst and the use of the catalytic composition according to the invention.

[0040] There figure 4 This represents the conversion kinetics of isocyanate to urethane as a function of time at a temperature of 55°C (reference curve 40 with the MDCI, DBTL, DMP30 composition) and at room temperature using a prior art catalyst. The conversion kinetics are represented here by monitoring the dynamic viscosity value (in Pa·s), the measurements having been carried out on inert propellant formulations. i.e. incorporating all the propellant components except for the energetic charge (ammonium perchlorate), which is replaced by an inert charge (potassium chloride) of similar density and particle size. The catalytic compositions tested are IPDI isocyanate (aliphatic diisocyanated isophorone crosslinking agent) with DMP-30 (amine catalyst) and zinc ethylhexanoate (organometallic catalyst) at different concentrations: x300 curve 44, x600 curve 43, x1000 curve 41) and IPDI isocyanate (diisocyanate isophorone) with DMP-30 (amine catalyst) and zinc neodecanoate (organometallic catalyst) at a high concentration (x1000 curve 42). Zinc neodecanoate has the advantage of not being highly toxic (it is not a CMR-classified substance), which allows for its incorporation in significantly higher concentrations compared to DBTDL.Zinc ethylhexanoate, on the other hand, has high toxicity (classified as "toxic to reproduction" category 2) but nevertheless less restrictive than that of DBTDL ("Toxic to reproduction" category 1B and "Mutagenic" Category 2).

[0041] On the figure 4 As can be seen, the use of a zinc neodecanoate catalyst (curve 42), despite the use of a very high concentration and an IPDI isocyanate, does not allow for sufficiently high kinetics. This demonstrates that achieving the target kinetics cannot be reliably guaranteed by using just any neodecanoate-based organometallic salt, but rather requires the appropriate choice of catalyst, in the correct concentration, and in combination with a highly reactive isocyanate, exceeding that of the MDCI reference.

[0042] There figure 5 This represents the conversion kinetics of isocyanate to urethane over time at a temperature of 55°C (reference with the MDCI, DBTL, and DMP30 compositions) and at room temperature using a catalytic composition according to the invention. Here again, the conversion kinetics are represented by monitoring the dynamic viscosity value (in Pa·s), the measurements having been carried out on inert propellant formulations. The catalytic compositions tested are IPDI isocyanate (isophorone isocyanate) with DMP-30 and with bismuth neodecanoate at different concentrations (curve 52: x5, curve 51: x100). Bismuth neodecanoate has the advantage of not being highly toxic (it is not classified as a CMR substance, unlike DBTDL), which allows for its incorporation at a significantly higher concentration in the propellant.As an illustration, Bismuth Neodecanoate used in the formulations of curves 51 and 52 is a product marketed by the Borchers company under the name BorchiKat 315.

[0043] There figure 5This illustrates the possibility of achieving a high level of kinetics at room temperature (30°C), sufficiently close to the reference kinetics at 55°C, through the use of a mixed catalytic composition according to the invention. In this example, the catalytic composition according to the invention consists of bismuth neodecanoate (an organometallic catalyst) combined with DMP30 (a basic tertiary amine catalyst), provided that a high level of bismuth neodecanoate is used (100 times the reference level of DBTDL catalyst, corresponding to curve 51) and an IPDI isocyanate exhibiting significantly higher catalytic activity than MDCI isocyanate is used. The level of bismuth neodecanoate could, if necessary, be advantageously increased beyond 100 times (without affecting the overall toxicity of the catalytic system) to more closely approach the reference kinetics.

[0044] The catalytic composition according to the invention exhibits high reactivity, eliminating the need for a tin-based organometallic salt (highly toxic), an aromatic isocyanate (highly reactive but more toxic than aliphatic isocyanates), or a basic amidine-type catalyst (highly reactive but chemically incompatible with the ammonium perchlorate energy charge). The catalytic composition according to the invention also advantageously maintains the DMP-30 amine catalyst content at a typical reference level, thus avoiding the risk of detrimental impact on the mechanical properties of the propellant and the risk of exacerbating gas evolution upon contact with the ammonium perchlorate energy charge.Furthermore, the catalytic composition according to the invention makes it possible to reproduce a crosslinking kinetics at room temperature (20-30°C) equivalent (or at least very close) to that obtained at higher temperature (50-60°C) with conventional catalytic compositions in the field formulated on the combination of DBTDL / DMP30 or TPB alone.

Claims

1. Catalytic composition for crosslinking polyurethane binders at room temperature, characterised in that it comprises: a. An organic bismuth-salt catalyst, the organic bismuth-salt catalyst being bismuth neodecanoate; b. A tertiary amine basic catalyst, the tertiary amine basic catalyst being tris(dimethylaminomethyl)phenol; c. A non-blocked difunctional aliphatic isocyanate, the non-blocked difunctional aliphatic isocyanate being isophorone diisocyanate.