CS265385B1 - Method for determining hydroxyl groups in epoxy resins - Google Patents

Abstract

The solution simplifies, refines and accelerates the procedure for determining free hydroxyl groups by the acetylation method when using imidazole derivatives as acetylation catalysts. The catalysts in this case are 1-alkylimidazoles with the number of carbon atoms in the alkyl group from 1 to 6, which allow for a rapid acetylation at a temperature of 15 to 30 0 C, therefore without undesirable competitive reactions. The invention is applied in the evaluation of technical parameters of epoxy resins during their production and further processing.

Description

The subject of the invention is the determination of hydroxyl groups in epoxy resins by a new method based on acetylation with acetic anhydride in the environment of a suitable catalyst.

Knowledge of the content of hydroxyl groups in epoxy resins is important from the point of view of the kinetics of the curing process and is conditioned by a reliable and rapid analytical method. Strictly selective determination of free hydroxyl groups by the commonly used acetylation method in the environment of pyridine as a catalyst is not possible, because during the reaction, the acetic acid formed at an elevated temperature given by the boiling point of the reaction mixture opens the oxirane ring. The resulting hydroxyl group reacts in a known manner with acetic anhydride to form free acetic acid with subsequent reaction of the epoxy group. Even though this subsequent reaction is slow and cannot be used analytically, it still has an effect on positive errors in the determination of hydroxyl groups. For these reasons, the content of hydroxyl groups in epoxy resins is often calculated as the difference between the ester mass equivalent and the content of epoxy groups (Lidařik M. et al.: Epoxidové hříscie, SNTL Praha (1983), p. 192). In this procedure, by appropriate adjustment of the experimental conditions, the aforementioned acetylation by acetic anhydride in pyridine in the presence of pyridinium chloride can be used to determine the reaction itself, which first opens the oxirane ring to form chlorohydrin groups, and in the subsequent process, the original and chlorohydrin hydroxyl groups are esterified by acetic anhydride. At the same time, a separate determination of the epoxide groups is carried out and the content of hydroxyl groups in the epoxy resin is calculated from the results of both methods. In this analytical procedure, errors arise depending on two different titration conditions.

265 385 determinations using various reagents that degrade the accuracy of the results. For the direct determination of hydroxyl groups in epoxy resins, the determination of active hydrogen based on the reaction of the analyzed sample with lithium aluminum hydride in tetrahydrofuran with the formation of hydrogen gas is used (Ulbrich V., Makeš 0.: Chem. prům. 8/33 (1958), p. 163). Most methods based on the determination of active hydrogen are modifications of the classical Zerewitino method and, in addition to requiring appropriate gas-measuring equipment, bring experimental difficulties consisting in the slow reaction of the analyzed epoxy resins, in the difficult preparation of the reagent and in the necessity to perform the analysis without access to air moisture. For the reasons stated, this method is unsuitable for operational analytical control. Another method is the simultaneous determination of epoxy and free hydroxyl groups in epoxy resins after reaction with acetyl bromide, which is based on several reactions taking place simultaneously (Kerker R., Stori M.: Chimia 33 (1979), p. 84). The reaction of acetyl bromide with a free hydroxyl group produces hydrobromic acid, which opens the epoxy ring to form the corresponding bromohydrin, whose OH group reacts with acetyl bromide and the reaction produces an equivalent amount of hydrobromic acid. Subsequent hydrolysis decomposes the unused acetyl bromide into acetic and hydrobromic acids, which are determined in two stages by potentiometric titration with a volumetric alcoholic solution of potassium hydroxide. The content of epoxy and hydroxyl groups in the analyzed sample is calculated from the difference in the consumption for the titration of both acids determined in the blank experiment and the actual determination. The disadvantage of this method is the relatively long reaction time, the strong affinity of acetyl bromide for air humidity, and the risk of certain errors resulting from the fact that the results involve two interdependent consumptions of volumetric solution determined from one two-step titration.

The shortcomings of the above methods are eliminated by the present invention, the subject of which is a method for determining hydroxyl groups in epoxy resins by acetylation with acetic anhydride in the presence of imidazole derivatives as catalysts with the exclusion of cones.

- 3,265,385 ence reaction of epoxide groups. The essence of the invention lies in the fact that the acetylation is carried out in the presence of a catalyst from the group of 1-alkylimidazoles with the number of carbon atoms in the alkyl substituent from 1 to 6 at a temperature of 15 to 30 °C.

The method of determination according to the invention has a number of advantages over existing methods. It shortens the time required for analysis several times, enables direct determination of hydroxyl groups without the need for determination of epoxy groups, is applicable in a wide concentration range from determination of low concentrations of hydroxyl groups next to high concentrations of epoxy groups to determination of the groups in the opposite concentration ratio. In addition, it provides very reproducible and accurate results.

The weighed amount of the analyzed sample is dissolved in a suitable solvent, preferably in dimethylformamide or dimethylacetamide, a catalyst is added to the sample solution, preferably 1-methylimidazole, which, in addition to its progressive catalytic properties, is also an excellent solvent, and an acetylation mixture prepared from acetic anhydride dissolved in dimethylformamide or dimethylacetamide. The reaction mixture is allowed to react at a temperature of 15 to 30 °C for 5 to 7 minutes and after this time the excess of unconsumed acetic anhydride is hydrolyzed by adding distilled water. The precipitated reacted resin is dissolved by adding a suitable solvent mixture (e.g. acetone + toluene, acetone + dimethylformamide, etc.) and the solution is titrated with a volumetric solution of aqueous or alcoholic sodium hydroxide NaOH visually using thymolphthalein as an indicator or potentiometrically. A blank experiment without a sample is performed in the same way. The content of hydroxyl groups in the analyzed sample is calculated from the difference in consumption.

The accuracy and reproducibility of the method according to the invention was verified on mixtures of model samples (Table I) and on samples of a number of technical epoxy resins (Table II). The tables show the achieved results, from which it is evident that the method is applicable in a very wide concentration range of the analyzed component with excellent results. In 4 parallel determinations, a maximum standard deviation of the average of 1.5% rel. was achieved.

- 4,265,385

Table I

Calculated epoxy- groups (mol.kg” of hydroxyl groups 1 ) Determined epoxy groups determined β idabs. (mol.kg”) hydroxy groups determined by β 4abs, 0.49 8.56 0.49 0.49 to.00 8.59 8.49 -0.07 0.49 8.42 0.49 8.46 1.83 6.18 1.80 1.80 -0.03 6.16 6.14 -0.04 1.80 6.14 1.81 6.11 2.48 5.00 2.47 2.48 to.00 4.99 5.00 this, 00 2.50 5.00 2.47 5.00 3.65 2.92 3.66 3.65 to.00 2.93 2.93 + 0.01 3.64 2.92 3.65 2.93 4.74 0.97 4.76 4.75 +0.01 0.99 · 1.00 + 0.03 4.75 1.00 4.75 1.01 5.03 0.45 5.03 5.03 IO.00 0.48 0.48 + 0.03 5.03 0.48 5.02 0.48

Octylglycidyl ether and glycerin octyl ether were used to prepare model mixtures, in which epoxy and hydroxyl groups were determined. Epoxy groups were determined in model mixtures and in basic substances by reaction with cetyltrimethylammonium bromide, and hydroxyl groups by the method according to the invention.

Table II

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Sample labeling weighing n titrant consumption V(ml) OH content (mol„kg found groups 0 abs. average standard <, deviation (s*) rÍUt.(%) Epicotee 1.4794 1.74 0.59 0.59 0.003 0.52 828 1.5966 1.87 0.59 1.6770 1.96 0.59 1.7391 2.02 0.58 Epicotee 1.1952 4.24 1.78 1.78 0.006 0.37 834 1.2889 4.58 1.78 1.0539 3.72 1.77 1.1738 4.14 1.77 Epicotee 0.4582 2.57 2.81 2.87 0.042 1.47 1001 0.4958 2.88 2.91 0.5440 3.13 2.89 0.5067 2.90 2.87 Epicotee 0.7002 4.38 3.14 3.15 0.031 0.99 1004 0.6652 4.13 3.11 0.6460 4.05 3.14 0.6072 3.86 oh, 19 Epicotee 0.5005 3.56 3.57 3.57 0.018 0.51 1007 0.6143 4.35 3.55 0.5192 3.69 3.56 0.4925 3.53 3.59 Epicotee 0.5582 4.25 3.82 3.80 0.021 0.55 1009 0.5903 4.46 3.79 0.5728 4.36 3.82 0.5750 4.33 3.78

□an aqueous volumetric solution of NaOH was used as the titrant, c (NaOH) = 0.5 mol . I“ ¹ , f = 1.003.

Example 1

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Weigh exactly 1.1952 g of epoxy resin on an analytical balance and dissolve the sample in 4 ml of dimethylformamide while stirring with an electromagnetic stirrer. Add 4 ml of 1-methylimidazole as a catalyst to the solution and pipette exactly 5 ml of the acetylation mixture, which is prepared by diluting 6 ml of acetic anhydride with dimethylformamide to a total volume of 50 ml. The reaction mixture is mixed and left to stand for 7 minutes at 16 °C. After this time, the unreacted acetic anhydride is hydrolyzed by adding 5 ml of distilled water, or the precipitate formed is dissolved by adding 20 ml of a mixture of acetone and toluene in a ratio of 2:1 and after 5 minutes, 15 drops of 0.2% ethanolic thymolphthalein solution are added as an indicator and, while stirring, it is titrated to a blue color with an aqueous standard solution of sodium hydroxide NaOH, c (NaOH) = 0.5 mol . l ^- ^. Consumption of titration solution = 20.40 ml. A blank experiment is carried out in the same way without weighing the sample with a shortened reaction time of 3 minutes. Consumption of titration solution - 24.68 ml, standard solution factor = 1.003.

The result is (24.68 - 20.40) . 1.003 . 0.5

-- = 1.78 mol. kg ^-

1.1952

Example 2

A weight of 0.5971 g of epoxy resin is dissolved under stirring in a mixture of 4 ml of dimethylacetamide and 4 ml of 1-pentylimidazole and after dissolution, exactly 5 ml of the acetylation mixture prepared by diluting 6 ml of acetic anhydride with dimethylacetamide to a total volume of 50 ml is added. The reaction mixture is stirred and left to stand for 5 minutes at a temperature of 28 °C. After this time, the unreacted acetic anhydride is hydrolyzed by adding 5 ml of distilled water, the precipitate of the reacted resin is dissolved by adding 20 ml of a mixture of acetone and toluene in a ratio of 2:1 and after 5 minutes, 15 drops of 0.2% ethanolic thymolphthalein solution are added as an indicator and, while stirring, it is titrated to a blue color with an aqueous volumetric solution of sodium hydroxide.

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NaOH, c (NaOH) = 0.5 mol . I” ¹ . The consumption of titration solution was 20.03 ml. A blank experiment was carried out in the same way without weighing the sample with a shortened reaction time of 3 min. The consumption of titration reagent was 24.50 ml, factor 1.003.

The result is (24.50 - 20.03) . 1.003 . 0.5

..............-...........-Η.-..,,..,................—.,...... = 3.75 mol * kg

0.5971

Claims (1)

SUBJECT OF THE INVENTION Method for the determination of hydroxyl groups in epoxy resins by acetylation with acetic anhydride in the presence of imidazole derivatives as catalysts excluding competitive reaction of epoxy groups, characterized in that the acetylation is carried out in the presence of a catalyst of the 1-alkylimidazole group having 1 to 6 carbon atoms at 15 to 30 ° C. Printed by Moravian Printing Works, Center 100, Studentská tř.5, OLOMOUC Price: 2,40 Kčs