Method for determining the concentration of hydroxyl groups in polyols

Abstract

The present invention relates to a method for determining the concentration of hydroxyl groups present in a polyol, comprising the following steps: acetylation of the hydroxyl groups of the polyol to ester groups by reacting the polyol with excess acetic anhydride, hydrolysis of the mixture of acetylated polyol, acetic anhydride, and acetic acid obtained at the end of step (i), and potentiometric determination of the acetic acid obtained at the end of step (ii), step (i) of acetylation being carried out in the presence of at least one 4-dialkylaminopyridine catalyst of formula (1) or one of its salts: (1) in which R and R', identical or different, are C1-C6 alkyl groups, or R and R' together form a C2-C6 ring, and in the presence of at least one solvent selected from: - aromatic solvents selected from toluene, xylene, Anisole, chlorobenzene and bromobenzene, acetone, ethyl acetate, and mixtures thereof. Figure for the summary: Fig. 1.

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 this sample (in g or kg).

[0003] [Form. 1] of =---------------—--- d''= TiiRA d!QH (eg / kg) x

[0004] Precise knowledge of the number of -OH groups present in polyol prepolymers, particularly in polyol prepolymers used as binders in propellants, makes it possible to accurately calculate the amount of isocyanate crosslinking agent to add to obtain the desired degree of crosslinking. This degree of crosslinking determines the mechanical and ballistic properties of the resulting propellant material. In this field, it is essential to have a reliable, repeatable, and precise method for measuring the -OH group content of these polyol prepolymers in order to modulate the desired mechanical and ballistic properties. Precise measurement of the -OH groups also makes it possible to evaluate new grades and batches of polyols, and to monitor batches during aging.

[0005] 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 so-called "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, or even very slow, at room temperature (from a few hours to a few days). The usual practice is to heat the mixture under reflux, for between 30 minutes and 2 hours, at a temperature between 95 and 115°C. However, this method, when it When applied to polybutadiene hydroxytelechelic acid (PBHT), it causes a brown discoloration of the polymer during the acetylation reaction with acetic anhydride, due to side reactions attributable in particular to denaturation of the polymer chain, which consumes the acetic anhydride reagent and consequently distorts the measured OH content. Furthermore, due to its instability, ASTM standards E222 and D4274 recommend preparing the acetic anhydride / pyridine reagent on the same day and not using it if its color is darker than pale yellow.

[0006] 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 gives OH group values ​​for polyols similar to those obtained with the ASTM E122 method, avoids the appearance of brown coloration during acetylation, and allows the use of a reduced amount of solvent. However, heating under reflux for 50 min is still necessary.

[0007] 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 enabled 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 because it is prone to the formation of peroxides.

[0008] 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.

[0009] 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 access to the precise value of the OH group content in a given polyol, by replacing the pyridine solvent-catalyst usually used in the acetylation step with a mixture combining DMAP (catalyst) with a specific solvent. This specific catalyst-solvent system allows the acetylation step to be carried out at room temperature in less than 15 minutes, unlike the 2 hours of reflux heating required by ASTM E222. The absence of heating thus avoids the denaturation of PBHT, which is 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, provided that it uses a low-toxicity solvent. Summary of the invention

[0010] 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 system combining DMAP and a specific solvent during the acetylation step with acetic anhydride. Brief description of the drawings

[0011] Figure 1 shows the acetylation kinetics of a PB HT solution in solvent toluene.

[0012] Figure 2 shows the acetylation kinetics of a PBHT solution in solvent. pyridine, under different operating conditions.

[0013] Figure 3 illustrates the evolution of the color during the acetylation of a solution of PBHT in pyridine solvent, under different operating conditions.

[0014] Figure 4 shows the evolution of the visual appearance of a PBHT solution as a function of of the acetylating reagent (acetic anhydride and pyridine solvent or acetic anhydride and toluene solvent).

[0015] Figure 5 shows the effect of heating on the measured OH content of a solution of PBHT in toluene solvent.

[0016] Figure [6] measures the OH content of a PBHT according to the assay method of the invention, in toluene solvent, in chlorobenzene solvent, and in tetrahydrofuran solvent (commercial THF, unstabilized).

[0017] Figure [7] evaluates the stability of a reactive acetylating agent and catalytic solution mixture, in different solvents.

[0018] Figure 8 evaluates the stability of a mixture consisting of acetylating reagent and solution. catalytic and additional DMF 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. potentiometric titration of the acetic acid obtained at the end of step (ü), step (i) being carried out in the presence of at least one 4-dialkylaminopyridine catalyst of formula (1) or one of its salts:

[0020] (1)

[0021] wherein R and R', identical or different, are alkyl groups in CrC6, and preferably in Ci-C4, or R and R' together form a ring in C2-C6, and preferably in C2-C4,

[0022] and in the presence of at least one solvent, capable of solubilizing and compatible with the polyol, chosen from: - aromatic solvents selected from toluene, xylene, anisole, chlorobenzene and bromobenzene, - acetone, - ethyl acetate, and - their mixtures.

[0023] For the purposes of the present invention, an alkyl group is defined as a saturated, linear or branched, aliphatic hydrocarbon group, in the form of Ci-C6, and preferably CrC4. 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.

[0024] 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.

[0025] In the catalyst of formula (1), R and R' can be identical CrC4 alkyl groups, and preferably identical CrC2 alkyl groups.

[0026] The catalyst of formula (1) of the invention is advantageously 4-dimethylaminopyridine (DMAP).

[0027] In one embodiment, the solvent used in step (i) of acetylation is an aromatic solvent selected from toluene, xylene, anisole, chlorobenzene, and bromobenzene.

[0028] In an advantageous embodiment, the solvent used in step (i) is toluene or chlorobenzene.

[0029] In one embodiment, the solvent can be used in a mixture with at least one co-solvent of a different formula, selected from toluene, xylene, anisole, benzene, chlorobenzene, bromobenzene, cyclohexane, octane, heptane, Dichloromethane, dichloroethane, trichloroethane, carbon tetrachloride, trichloroethylene, chloroform, acetonitrile, dimethylformamide (DMF), dimethylacetamide, ethyl acetate, and acetone. Advantageously, the co-solvent is dimethylformamide (DMF). These co-solvents are advantageously used when the solvent used in step (i) is toluene, particularly when determining the OH content of polyols that are sparingly soluble in toluene.

[0030] The volume ratio between the co-solvent and the solvent advantageously varies from 0 to 1 / 1.

[0031] During the acetylation step (i), the polyol which is reacted with the excess acetic anhydride is preferably hydroxy-telechelic polybutadiene (PBHT) or glycidyl polyazuride (PAG), and more preferably hydroxy-telechelic polybutadiene (PBHT).

[0032] 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.

[0033] The molar ratio of -OH functions / acetic anhydride implemented during the acetylation step (i) advantageously varies from 1 / 4 to 1 / 1.1, and preferably from 1 / 2 to 1 / 1.5.

[0034] In addition to the foregoing provisions, the invention also includes other 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.

[0035] Examples:

[0036] Method of the invention:

[0037] The determination of 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 sample analyzed presents residual or functional acidity).

[0038] 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).

[0039] All tests are carried out at room temperature.

[0040] 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.

[0041] 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.

[0042] 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 concentration TKOho,5n in mol / L of the methanolic potassium solution is given by the relation: [Form. 2] T ___________________ - a20422 x in which: - mHPK: exact mass of potassium hydrogen phthalate weighed (in g), and - Véq calibration0.5n ■ volume of 0.5N methanolic potassium solution poured at the equivalence point (in mL).

[0043] Dosage method: acetylation, hydrolysis and potentiometric titration - First 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. - 2nd step: hydrolysis Once the acetylation step was completed, 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. - 3rd step: potentiometric titration Once the hydrolysis was complete, the mixture was titrated directly in the flask by potentiometric acid-base 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 blank tests were carried out under the same conditions. The equivalent volumes measured for the test portions and blanks are respectively noted as véq0HPE and véq0HBianc-

[0044] Determination of acidity: In a 150 mL wide-necked bottle, 5 g of sample (unacetylated raw material) were weighed, and the exact weighed mass was recorded (re-sample). 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 portions per sample and three blanks were carried out under the same conditions. The equivalent volumes measured for the test portions and blanks are respectively noted as: Veq Acidity PE Ct Veq Acidity Blank •

[0045] Results:

[0046] Crude OH content: The crude OH content (in eq / kg) of each test portion is calculated as follows: [Form. 3] Rate «F OH àrut =---::::

[0047] in which: - average volume of titrant added at the equivalence point (in mL) averaged over all the blanks produced, - Véq ohpe: volume of titrant added at the equivalence point for the test aliquot (in mL), - TKoho,5n: titrant concentration (in mol / L), and - sample ■ mass of the test sample (in g). For each sample, the crude OH level is averaged over all test samples taken.

[0048] Acidity: The acidity (in eq / kg) of each test portion was calculated as follows: [Form. 4] Acm s te =-------------------------------- in which: - average volume of titrant added at the equivalence point (in mL) averaged over all blanks produced - véq ape: volume of titrant added at the equivalence point for the test aliquot (in mL) - TKOho,5n: titrant concentration (in mol / L) - méchantiiion: mass of the test sample (in g)

[0049] For each sample, the acidity is averaged over all the test samples taken.

[0050] OH content: 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] Rate «TCW? (é^ / ^) = iTOAT Rate &fu£

[0051] The OH content of a PB HT of grade R45HT (sample = 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: DM AP 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 [Fig. 1]. These conditions led to very rapid acetylation of the OH groups of PB HT, without any unwanted reactions. The measured OH content is therefore reliable and meets the supplier's specifications.

[0052] The OH content of the R45HT grade PB HT determined according to the method of the invention was compared to the OH content of the same batch of R45HT PB HT measured under different operating conditions: a) Heating a solution of 2.5 g of PB HT and 10 mL of a conventional acetylating reagent (acetic anhydride in pyridine solvent, volume ratio 9 / 191, prepared as previously described for the toluene and acetic anhydride-based acetylating reagent) to 100°C, without DMAP catalyst (the pyridine solvent acts as the catalyst), resulted in 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 a conventional acetylating reagent: acetic anhydride in a pyridine solvent, and without DMAP catalyst (the pyridine solvent acts as the 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 for the toluene-acetic anhydride acetylating reagent), and 2 mL of DM AP catalytic solution (DMAP solution at 43.7 g / L in pyridine). These conditions lead to an unstable reagent. A side reaction consuming acetic anhydride occurs rapidly, distorting the measurement of the OH concentration. This reaction is characterized by the formation of a yellow-orange colored compound in the mixture, as shown in [Fig. 4]. This same reaction also occurs in a toluene solvent, but extremely slowly, making it negligible and therefore not distorting the measurement of the OH concentration. 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 [Fig. 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.

[0053] 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 [Fig. 6]. These conditions led to very rapid acetylation of the OH groups in PBHT. The measured OH content was identical for toluene and chlorobenzene solvents, but significantly higher for tetrahydrofuran solvent, as shown in [Fig. 6]. This difference in the measured OH content in THF solvent is due to a spurious reaction between the peroxides present in THF and the alkene bonds of PBHT, artificially generating OH groups not initially present, leading to an overestimation of the measured OH content.

[0054] The stability of the acetylating reagent (acetic anhydride solution in toluene solvent prepared as described above) was evaluated at room temperature by monitoring the change in the equivalent volume measured after hydrolysis as a function of the storage time of the mixture. No consumption of acetic anhydride was observed. was observed, and no yellowing of the solution, indicating the absence of any parasitic reaction.

[0055] 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 or DMF solvent due to a side reaction consuming the acetic anhydride. The results are shown in [Fig. 7].

[0056] The stability of the mixture consisting of 10 mL of acetylating reagent and 2 mL of catalytic solution (both prepared as described above, 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 main 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 [Fig. 8].

Claims

Demands

1. A method for determining the level of hydroxyl functions present in a polyol, said method comprising the steps of: i. acetylation of the hydroxyl functions of the polyol to ester functions 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: (1) in which R and R', identical or different, are Ci-C6 alkyl groups, or R and R' together form a C2-C6 ring, and in the presence of at least one solvent selected from: - aromatic solvents selected from toluene, xylene, anisole, chlorobenzene and bromobenzene, - acetone, - ethyl acetate, and - mixtures thereof.

2. The method 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 an aromatic solvent selected from toluene, xylene, anisole, chlorobenzene, bromobenzene.

5. A process according to any one of claims 1 to 4, characterized in that the solvent is toluene or chlorobenzene.

6. A process according to any one of claims 1 to 5, characterized in that the solvent is used in a mixture with at least one co-solvent of a different formula, selected from toluene, xylene, anisole,

7.

8.

9.

10. benzene, chlorobenzene, bromobenzene, cyclohexane, octane, heptane, dichloromethane, dichloroethane, trichloroethane, carbon tetrachloride, trichloroethylene, chloroform, acetonitrile, dimethylformamide, dimethylacetamide, ethyl acetate, acetone, and preferably dimethylformamide. A process according to any one of claims 1 to 6, characterized in that the ratio between the co-solvent and the solvent varies from 0 to 1 / 1. A process according to any one of claims 1 to 7, characterized in that the polyol is hydroxytelechelic polybutadiene or glycidyl polyazuride, and preferably hydroxytelechelic polybutadiene. A process according to any one of claims 1 to 8, characterized in that the acetylation step (i) is carried out at a temperature ranging from 15 to 30°C, and preferably from 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. A process according to any one of claims 1 to 9, 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