Method for assaying the quantity of hydroxyl functions in polyols

Abstract

The present invention relates to a method for assaying the quantity of hydroxyl functions present in a polyol, the method comprising the following steps: (i) acetylating the hydroxyl functions of the polyol into ester functions by reacting the polyol with acetic anhydride in excess; (ii) hydrolyzing the mixture of acetylated polyol, acetic anhydride and acetic acid obtained at the end of step (i); and (iii) assaying by potentiometry the acetic acid obtained at the end of step (ii), the acetylation step (i) being carried out in the presence of at least one 4-dialkylaminopyridine catalyst of formula (1) or one of the salts thereof: wherein R and R', which are identical or different, are C1-C6 alkyl groups, or R and R' together form a C2-C6 ring, in the presence of at least one solvent selected from toluene, and in the presence of at least one cosolvent selected from xylene, anisole, benzene, chlorobenzene, bromobenzene, cyclohexane, octane, heptane, dichloromethane, dichloroethane, trichloroethane, carbon tetrachloride, trichloroethylene, chloroform, acetonitrile, dimethylformamide, dimethylacetamide, ethyl acetate, acetone, pyridine.

Description

Description Title of the invention: Method for determining the concentration of hydroxyl groups in polyols Technical Field

[0001] The invention relates to a method capable of precisely measuring the level of hydroxyl functions present in a polyol. State of the art

[0002] The hydroxyl function rate, commonly referred to as the "OH rate" or "hydroxyl index" of a material, corresponds to the number of moles of hydroxyl functions -OH that it contains (expressed in molar equivalent or in mg KOH equivalent) relative to the mass of that sample (in g or kg). [Form. 1]

[0003] Accurate knowledge of the number of -OH groups present in polyol prepolymers, particularly those used as binders in propellants, allows for the precise calculation of the amount of isocyanate crosslinking agent to add to achieve the desired degree of crosslinking. This degree of crosslinking determines the mechanical and ballistic properties of the resulting propellant material. In this field, a reliable, repeatable, and precise method for measuring the -OH group content of these polyol prepolymers is essential to modulate the desired mechanical and ballistic properties. Accurate measurement of the -OH groups also allows for the evaluation of new grades and batches of polyols, and for monitoring batches during aging.

[0004] Numerous standard procedures for measuring the OH group content already exist, such as ASTM procedures E222, D4274, E326, E1899, D6342, and D1957. The most widely used method for measuring the OH group content in hydroxylated polymers is the acetylation method (e.g., ASTM). E222), which consists of acetylating the hydroxyl groups to ester groups using acetic anhydride, then hydrolyzing and titrating the excess unreacted acetic anhydride by potentiometric back titration. This method is usually carried out in a pyridine solvent, which also acts as a reaction catalyst. However, this reaction is described as slow, even very slow, at room temperature (from a few hours to a few days). Therefore, it is customary to heat the mixture under reflux for 30 minutes to 2 hours at a temperature between 95 and 115°C. However, when applied to hydroxyl-telechelic polybutadiene (PBHT), this method results in a brown coloration of the polymer during the acetylation reaction with acetic anhydride, due to side reactions attributable in particular to a denaturation of the polymer chain, which consumes the acetic anhydride reagent and consequently distorts the measured OH content.In addition, due to its instability, ASTM E222 and D4274 standards recommend preparing the acetic anhydride / pyridine reagent on the same day and not using it if its color is darker than pale yellow.

[0005] To accelerate the analysis of OH group content and / or to work at room temperature, the use of catalysts more efficient than pyridine has been proposed in the literature, including N-methylimidazole (NMI). Alex et al. (Journal of Energetic Materials, 2017, Vol. 35, No. 3, 292-299), for example, proposed substituting NMI for the pyridine catalyst. According to the authors, this method yields OH group values ​​for polyols similar to those obtained with the ASTM E122 method, avoids the appearance of brown coloration during acetylation, and allows for the use of a reduced amount of solvent. However, heating under reflux for 50 minutes remains necessary.

[0006] Pant et al. (Analytical Chemistry: An Indian Journal, Vol. 16, Issue 12, 2016, 532-534) also proposed substituting pyridine with 4-dimethylaminopyridine (DMAP), combined with the solvent tetrahydrofuran (THF), which allowed the acetylation reaction to be carried out at room temperature in 10 minutes. However, THF is toxic and classified as CMR C2; it is also hygroscopic, highly volatile, and unstable due to its susceptibility to peroxide formation.

[0007] It therefore appears necessary to develop a new catalyst-solvent system that is more efficient, less toxic, more stable and does not generate parasitic reactions during the acetylation of polyol with acetic anhydride.

[0008] The inventors have thus discovered a new catalyst-solvent system that is more stable than the system used in the standard method and can be stored for several days in a sealed container. The invention therefore relates to a new acetylation assay method that allows for the precise determination of the concentration of OH groups present in a given polyol. This is achieved by replacing the pyridine solvent-catalyst typically used in the acetylation step with a mixture combining DMAP (catalyst) with a specific solvent / co-solvent mixture. This specific catalyst-solvent / co-solvent system allows the acetylation step to be performed at room temperature in less than 15 minutes, compared to the 2 hours of reflux heating required by ASTM E222. The absence of heating thus avoids the denaturation of PBHT, a side reaction that alters its color and turns it brown.Furthermore, it meets the principles of green analytical chemistry in terms of increased operator safety, elimination or reduction of the use of harmful chemicals, insofar as it uses a low-toxicity solvent. Summary of the invention

[0009] The invention therefore consists of a particularly reliable acetylation assay method allowing access to the true value of the rate of OH functions present in a hydroxylated polymer, said method implementing a particular catalyst-solvent / co-solvent system combining DMAP and a specific solvent / co-solvent mixture during the acetylation step with acetic anhydride. Brief description of the drawings

[0010] Figure 1 represents the acetylation kinetics of a PBHT solution in toluene solvent.

[0011] Figure 2 represents the acetylation kinetics of a PBHT solution in pyridine solvent, under different operating conditions.

[0012] Figure 3 illustrates the evolution of the color during the acetylation of a PBHT solution in pyridine solvent, under different operating conditions.

[0013] Figure 4 shows the evolution of the visual appearance of a PBHT solution as a function of the acetylating reagent (acetic anhydride and pyridine solvent or acetic anhydride and toluene solvent).

[0014] Figure 5 shows the effect of heating on the measured OH content of a PBHT solution in toluene solvent.

[0015] Figure 6 measures the OH level of a PBHT according to the assay method of the invention, in toluene solvent, in chlorobenzene solvent, and in tetrahydrofuran solvent (commercial, unstabilized THF).

[0016] Figure 7 evaluates the stability of a reactive acetylating agent and catalytic solution mixture in different solvents.

[0017] Figure 8 evaluates the stability of a mixture consisting of acetylating reagent, catalytic solution and additional DMF co-solvent.

[0018] Figure 9 evaluates the stability of a mixture consisting of acetylating reagent, catalytic solution and additional acetonitrile co-solvent. Description of the invention

[0019] The present invention relates to a method for determining the concentration of OH groups present in a polyol, said method comprising the following steps: (i) acetylation of the hydroxyl groups of the polyol to ester groups by reaction of the polyol with excess acetic anhydride, (ii) hydrolysis of the mixture of acetylated polyol, acetic anhydride and acetic acid obtained at the end of step (i), and (iii) titration by potentiometry of the acetic acid obtained at the end of step (ii), step (i) being carried out in the presence of at least one 4-dialkylaminopyridine catalyst of formula (1) or one of its salts: in which R and R', identical or different, are alkyl groups in C1-C2, and preferably in C1-C4, or R and R' together form a ring in C2-C6, and preferably in C2-C4, in the presence of at least one solvent, compatible and capable of solubilizing the polyol, said solvent being selected from toluene, and in the presence of at least one co-solvent selected from xylene, anisole, benzene, chlorobenzene, bromobenzene, cyclohexane, octane, heptane, dichloromethane, dichloroethane, trichloroethane, carbon tetrachloride, trichloroethylene, chloroform, acetonitrile, dimethylformamide, dimethylacetamide, ethyl acetate, acetone, pyridine.

[0020] For the purposes of this invention, an alkyl group is defined as a saturated aliphatic hydrocarbon group, linear or branched, in the C1-C2 configuration, and preferably in the C1-C4 configuration. The term "branched" means that at least one lower alkyl group, such as a methyl or ethyl group, is attached to a linear alkyl chain. Examples of alkyl groups include methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, and n-pentyl.

[0021] The salts of the 4-dialkylaminopyridine catalyst of formula (1) include the following salts: 4-dialkylaminopyridinium acetate, 4-dialkylaminopyridinium chloroacetate, 4-dialkylaminopyridinium dichloroacetate, 4-dialkylaminopyridinium trichloroacetate, 4-dialkylaminopyridinium propanoate, 4-dialkylaminopyridinium butanoate, 4-dialkylaminopyridinium pentanoate, 4-dialkylaminopyridinium benzoate.

[0022] In the catalyst of formula (1), R and R' can be identical C1-C4 alkyl groups, and preferably identical C1-C2 alkyl groups.

[0023] The catalyst of formula (1) of the invention is advantageously 4-dimethylaminopyridine (DMAP).

[0024] The solvent used in step (i) of acetylation is toluene.

[0025] The solvent is used in a mixture with at least one co-solvent of a different formula, selected from xylene, anisole, benzene, chlorobenzene, bromobenzene, cyclohexane, octane, heptane, dichloromethane, dichloroethane, trichloroethane, carbon tetrachloride, trichloroethylene, chloroform, acetonitrile, dimethylformamide (DMF), dimethylacetamide, ethyl acetate, acetone, and pyridine. Advantageously, the co-solvent is chosen from dimethylformamide (DMF), acetonitrile, and pyridine. Even more advantageously, the co-solvent is dimethylformamide (DMF). These co-solvents allow for the determination of the OH content of polyols that are sparingly soluble in toluene.

[0026] The volume ratio between the co-solvent and the solvent varies advantageously from 0 to 1 / 1.

[0027] During the acetylation step (i), the polyol that is reacted with excess acetic anhydride is preferably hydroxytelechelic polybutadiene (PBHT) or glycidyl polyazuride (PAG), and more preferably hydroxytelechelic polybutadiene (PBHT).

[0028] In an advantageous embodiment, the acetylation step (i) is carried out at a temperature ranging from 15 to 30°C, and preferably at room temperature, i.e., at a temperature ranging from 18 to 25°C. Advantageously, the duration of this acetylation step (i) is less than one hour, preferably between 10 and 30 minutes, and more preferably between 10 and 20 minutes.

[0029] The molar ratio of -OH functions to acetic anhydride implemented during step (i) of acetylation advantageously varies from 1 / 4 to 1 / 1.1, and preferably from 1 / 2 to 1 / 1.5.

[0030] In addition to the foregoing provisions, the invention includes further provisions which will become apparent from the following supplementary description, which relates to the determination of the rate of hydroxyl functions present in polyols according to the process of the invention. Examples: Method of the invention:

[0031] Determining the OH content of a sample takes place in two stages: - measurement of the crude OH level, and - measurement of acidity (to correct the crude OH level, in the case where the analyzed sample presents residual or functional acidity).

[0032] These two measurements can be carried out in any order. The OH content (in eq / kg) corresponds to the sum of the crude OH content (in eq / kg) and the acidity (in eq / kg).

[0033] All tests are carried out at room temperature.

[0034] Preparation of the acetylating reagent: In a 200 mL glass volumetric flask, 150 mL of toluene was added, followed by 9 mL of acetic anhydride. The mixture was stirred to homogenize it, then the volume was brought up to 200 mL with more toluene.

[0035] Preparation of the catalytic solution: In a 50 mL glass volumetric flask, 2.185 g of DMAP were weighed out, and then 40 mL of toluene was added. The solid was stirred until completely dissolved, and then the volume was brought up to 50 mL with toluene.

[0036] Calibration: Approximately 1 g of potassium hydrogen phthalate was weighed into a beaker, and the exact mass recorded. The mixture was dissolved in a volume of distilled water sufficient to allow immersion of the pH electrode. The mixture was potentiometrically titrated with 0.5 N potassium methanol solution using a 20 mL automatic burette and a combined pH electrode suitable for acid-base titration in aqueous media. The title T KThe concentration of OH O,SN in mol / L of the methanolic potassium solution is given by the relation: [Form. 2] in which: - m H pK: exact mass of potassium hydrogen phthalate weighed (in g), and - Véq calibration 0.5N: volume of 0.5N methanolic potassium solution poured at the equivalence point (in mL).

[0037] Dosage method: acetylation, hydrolysis and potentiometric titration - L ère step: acetylation In a 150 mL wide-necked bottle, a sample mass (2.5 g for R45HT grade PBHT) corresponding to approximately 2 molar meq of -OH groups was weighed, and the exact mass was recorded (sample). 10 mL of acetyling reagent was added to the bottle using a 10 mL automatic burette, along with 2 mL of catalytic solution. The bottle was resealed and placed on a magnetic stirrer at approximately 500 rpm for 15 min to ensure acetylation of all -OH groups in the sample. - 2 ème step: hydrolysis Once the acetylation step was complete, 75 mL of pyridine was added to the flask. 2 mL of distilled water was then added, and the flask was resealed and placed on a magnetic stirrer at approximately 500 rpm for 1 hour. - 3 ème step: potentiometric titration Once hydrolysis was complete, the mixture was titrated directly in the flask by acid-base potentiometric titration with the previously prepared 0.5N methanolic potassium hydroxide solution. The volume of titrant added at the equivalence point was recorded. Three test portions per sample and three blanks were performed under the same conditions. The equivalent volumes measured for the test portions and blanks are respectively denoted Ve. q OH PE and Vé q OH White.

[0038] Determination of acidity: In a 150 mL wide-necked bottle, 5 g of sample (unacetylated raw material) were weighed, and the exact weight was recorded. 85 mL of pyridine solvent was added to the bottle and then placed on a magnetic stirrer to homogenize the mixture. Once the mixture was homogeneous, it was titrated directly in the bottle by potentiometric acid-base titration with a 0.1 N methanolic potassium hydroxide solution prepared in the same manner as before. The volume of titrant added at the equivalence point was recorded. Three test aliquots per sample and three blank aliquots were performed. same conditions. The equivalent volumes measured for the test samples and blanks are respectively noted Veq Acidity PE Ot Veq Acidity Blank- Results :

[0039] Crude OH content: The crude OH content (in eq / kg) of each test sample is calculated as follows: [Form. 3] I to in which: Average white balance (Veq OH): volume of titrant added at the equivalence point (in mL) averaged over all blanks performed. - Véq OH PE: volume of titrant added at the equivalence point for the test aliquot (in mL), - T K OH 0.5N: titrant concentration (in mol / L), and - méchantiiion: mass of the test sample (in g). For each sample, the crude OH level is averaged over all test samples taken.

[0040] Acidity: The acidity (in eq / kg) of each test portion was calculated as follows: [Form. 4] in which: Average white point (Veq A): volume of titrant added at the equivalence point (in mL) averaged over all blanks performed - Véq A PE: volume of titrant added at the equivalence point for the test portion (in mL) - T K OH 0.5N: titrant concentration (in mol / L) - méchantiiion: mass of the test sample (in g)

[0041] For each sample, the acidity is averaged over all the test samples taken.

[0042] OH level: The OH content (in eq / kg) is obtained by summing the previously determined crude OH content (in eq / kg) and acidity (in eq / kg): [Form. 5]

[0043] The OH content of a PBHT of grade R45HT (mechanuiion = 2.5 g) was measured according to the method of the invention, at room temperature in the presence of acetic anhydride, toluene, and DMAP, at different concentrations of DMAP: DMAP No. 1: molar ratio DMAP / acetic anhydride = 0.15, DMAP No. 2: molar ratio DMAP / acetic anhydride = 0.10, DMAP No. 3: molar ratio DMAP / acetic anhydride = 0.05, and DMAP No. 4: molar ratio DMAP / acetic anhydride = 0.025. The results are shown in Figure 1. These conditions led to very rapid acetylation of the PBHT OH groups, without any unwanted reactions. The measured OH content is therefore reliable and meets the supplier's specifications.

[0044] The OH content of R45HT grade PBHT determined according to the method of the invention was compared to the OH content of the same batch of R45HT PBHT measured under different operating conditions: a) By heating to 100°C a solution consisting of 2.5 g of PBHT and 10 mL of a conventional acetylating reagent: acetic anhydride in a pyridine solvent (in volume proportions acetic anhydride / pyridine = 9 / 191, prepared as previously described for the toluene-based acetylating reagent), and without DMAP catalyst (the pyridine solvent acts as the catalyst). These conditions correspond to those of ASTM E222, and led to denaturation of the PBHT and an overestimation of the measured OH content. b) By stirring at room temperature a solution consisting of 2.5 g of PBHT and 10 mL of classic acetylating reagent: acetic anhydride in pyridine solvent, and without DMAP catalyst (the pyridine solvent acts as catalyst).Acetylation is very slow and is still not complete after 7 hours of reaction, but converges to a more reliable value than under conditions a), as shown in Figures 2 and 3. c) By stirring at room temperature a solution consisting of 2.5 g of PBHT and. 10 mL of a conventional acetylating reagent: acetic anhydride in a pyridine solvent (acetic anhydride / pyridine volume ratio = 9 / 191, prepared as previously described for the toluene-acetic anhydride acetylating reagent), and 2 mL of DMAP catalytic solution (DMAP solution at 43.7 g / L in pyridine). These conditions result in an unstable reagent. A side reaction consuming acetic anhydride occurs rapidly, distorting the OH concentration measurement. This reaction is characterized by the formation of a yellow-orange colored compound in the mixture, as shown in Figure 4. This same reaction also occurs in a toluene solvent, but extremely slowly, making it negligible and therefore not distorting the OH concentration measurement.d) Heating a solution of 2.5 g of PBHT and 10 mL of acetylating reagent based on acetic anhydride and toluene, prepared as before (in volume proportions of acetic anhydride / toluene = 9 / 191 and without DMAP catalyst), to 100°C accelerates the reaction but triggers side reactions that lead to yellowing of the PBHT (attributed to its denaturation) and an overestimation of the measured OH content, as shown in Figure 5. e) Shaking a solution of 2.5 g of PBHT and 10 mL of acetylating reagent based on acetic anhydride and toluene, prepared as before (in volume proportions of acetic anhydride / toluene = 9 / 191), and without DMAP catalyst, at room temperature: toluene has no catalytic activity, so the reaction Acetylation does not occur. This configuration does not allow for the measurement of OH levels.

[0045] The OH content of a PBHT of grade R45HT (sample=2.5 g) was measured according to the assay method of the invention, at room temperature, in toluene solvent, then by substituting the toluene solvent with chlorobenzene solvent and with tetrahydrofuran solvent (commercial, unstabilized THF). The results are shown in Figure 6. These conditions led to very rapid acetylation of the OH groups of PBHT. The measured OH content was identical for toluene and chlorobenzene solvents, but significantly higher for tetrahydrofuran solvent, as shown in Figure 6. This difference in the measured OH content in THF solvent is due to a side reaction between the peroxides present in THF and the alkene bonds of PBHT, artificially generating OH functions not initially present, leading to an overestimation of the measured OH level.

[0046] The stability of the acetylating reagent (acetic anhydride solution in toluene solvent prepared as previously described) was evaluated at room temperature by monitoring the change in the equivalence point volume measured after hydrolysis as a function of the mixture's storage time. No consumption of acetic anhydride was observed, and no yellowing of the solution, indicating the absence of any unwanted reactions.

[0047] The stability of the mixture consisting of 10 mL of acetylating reagent and 2 mL of catalytic solution (both prepared as previously described) was evaluated in various solvents. These tests demonstrate that this mixture is stable in toluene solvent for at least 4 h at room temperature, but is unstable in pyridine solvent and highly unstable in DMF solvent due to a side reaction consuming the acetic anhydride. The results are shown in Figure 7.

[0048] The stability of a mixture consisting of 10 mL of acetylating reagent and 2 mL of catalytic solution (both prepared as described previously, in toluene solvent) and x mL (x ranging from 0 to 10) of additional DMF co-solvent was evaluated at room temperature. These tests demonstrate that when DMF is used as a co-solvent and not as the primary solvent, the side reaction of acetic anhydride consumption is negligible for at least 4 h, thus making it possible to use this solvent as a co-solvent. The results are shown in Figure 8. Similar results were observed when pyridine was added as an additional co-solvent.

[0049] The stability of the mixture consisting of 10 mL of acetylating reagent and 10 mL of catalytic solution (both prepared as described previously, in toluene solvent) and 10 mL of additional acetonitrile co-solvent was evaluated at room temperature. These tests demonstrate that when acetonitrile is used When used as a co-solvent rather than the primary solvent, the side reaction of acetic anhydride consumption is negligible for at least 4 hours, thus making it possible to use this solvent as a co-solvent. The results are shown in Figure 9.

Claims

DEMANDS 1. A method for determining the concentration of hydroxyl groups present in a polyol, said method comprising the steps of: (i) acetylation of the hydroxyl groups of the polyol to ester groups by reaction of the polyol with excess acetic anhydride, (ii) hydrolysis of the mixture of acetylated polyol, acetic anhydride and acetic acid obtained at the end of step (i), and (iii) potentiometric titration of the acetic acid obtained at the end of step (ii), characterized in that step (i) is carried out in the presence of at least one 4-dialkylaminopyridine catalyst of formula (1) or one of its salts: in which R and R', identical or different, are Ci-Ce alkyl groups, or R and R' together form a C2-C6 ring, in the presence of at least one solvent selected from toluene, and in the presence of at least one co-solvent selected from xylene, anisole, benzene, chlorobenzene, bromobenzene, cyclohexane, octane, heptane, dichloromethane, dichloroethane, trichloroethane, carbon tetrachloride, trichloroethylene, chloroform, acetonitrile, dimethylformamide, dimethylacetamide, ethyl acetate, acetone, pyridine.

2. A process according to claim 1, characterized in that in the catalyst of formula (1), R and R' are identical C1-C4 alkyl groups.

3. A process according to claim 1 or 2, characterized in that the catalyst is 4-dimethylaminopyridine.

4. A process according to any one of claims 1 to 3, characterized in that the solvent is used in a mixture with at least one co-solvent of a different formula, selected from dimethylformamide, acetonitrile, pyridine, and preferably dimethylformamide.

5. A process according to any one of claims 1 to 4, characterized in that the ratio of the co-solvent to the solvent varies from 0 to 1 / 1.

6. A process according to any one of claims 1 to 5, characterized in that the polyol is hydroxytelechelic polybutadiene or glycidyl polyazuride, and preferably hydroxytelechelic polybutadiene.

7. A process according to any one of claims 1 to 6, characterized in that the acetylation step (i) is carried out at a temperature of 15 to 30°C, and preferably 18 to 25°C, for a period of less than one hour, preferably between 10 and 30 minutes, and more preferably between 10 and 20 minutes.

8. A process according to any one of claims 1 to 7, characterized in that, in step (i), the polyol / acetic anhydride molar ratio varies from 1 / 4 to 1 / 1.1, and preferably from 1 / 2 to 1 / 1.5.

Images