Study of phthalic anhydride and glycerol polycondensation

. In the article, the basic physical and chemical properties of polymers based on glyphthalic resin are considered. The main advantages and disadvantages are revealed. The areas of application and the main methods for obtaining glyphthalic resins are considered. The research on studying the course of the polycondensation process on the change of acid number during the reaction of obtaining glyphthalic resin of polyatomic alcohol of glycerol and phthalic acid anhydride is carried out. It is revealed that the results of acid number determination during polycondensation can be used for directed regulation of synthesis of alkyd oligomers and their subsequent curing to obtain a polymer with specified properties. The data was obtained that the polymer under study can be further modified to give special properties that extend the area of its application, as well as to ensure the environmental safety of the waste generated during its use. On the basis of the available data, it is concluded that further research of glyphthalic resin is necessary.


Introduction
Currently, the main direction of fundamental research in the polymer field, especially physical and physicochemical research, is the study of various structures and regularities of their formation in multicomponent polymer systems, the study of the properties of such systems in relationship to their morphology, as well as the development of chemical and physical approaches to optimise these properties.
Polymer synthesis is one of the stages of further technological processes that a polymer undergoes to transform into the desired product [1].Understanding of the regularities of these processes and the ability to control them by changing the characteristics of the resulting highmolecular polymer materials allows us to move to certain products, the creation of which is the ultimate goal of production .
Glyptal resins -are the most common types of polyester (alkyd) resins and are polycondensation products of glycerol and phthalic anhydride.For various needs resins are produced in pure and modified form [24].
The process of resin formation proceeds according to the scheme (Fig. 1, 2): The industry produces unmodified (in the form of powder and solution) and modified glyphthalic resins.Modification of resin is carried out in the process of its synthesis by adding modifying reagents that form cross-links between macromolecules.Modification of glyphthalic resins allows to use them for production of hot and cold drying varnishes and enamels, as well as adhesives.Lacquers based on modified glyphthalic resins are used for insulation of electrical machines operating at high temperatures (150 °С and above).
The technological process of production of resin № GF-1350 consists of the following: glycerine is loaded into the boiler, which is heated to 120 °С.Then phthalic anhydride is added and melted under stirring.After the phthalic anhydride is melted the mass in the boiler is heated to 190-200 °С and kept at this temperature until the finished resin is obtained, which is drained into trays through the bottom drain cock.After cooling, the resin is crushed into a fine powder on a crusher.[25] Unmodified resin has a hard and brittle structure, transparent in a thin layer.The colour ranges from yellow to dark brown.The physical properties of the powder depend on the degree of polymerisation.The softening point of this substance should lie within the range of 80-95 °С.There are three stages of polymerisation: stage A-melting resin, soluble in acetone and unstable to water; stage B-melting resin, insoluble in acetone and stable to water; stage C-unmelting resin, insoluble in water and stable to its action.Under strong heating glyphthalic resin decomposes, forming a white deposit of phthalic anhydride [26].
The resin must be hard, mirror smooth in the fracture, homogeneous in colour and must not contain any extraneous impurities.Resin of the third group may have a spongy structure.If the mesh frequency index is high, the products are characterised by hardness, nonsmoothness and resistance to swelling.
Unmodified glyphthalic resins have good electrical insulating ability and therefore are used for production of electrical insulating varnishes at that they are dissolved in alcohol or in water depending on the degree of polycondensation.After applying the varnish, the resin film is cured.Its disadvantage is considerable brittleness.Unmodified glyphthalic resins are rarely used due to poor solubility, incompatibility with vegetable oils, as well as insufficient water resistance and brittleness of the resulting coatings.Sometimes glyphthalic resins are introduced as additives in varnishes based on cellulose nitrates, and also used for production of electrical insulating varnishes.Glyphthalic resin is used as a bonding agent in the manufacture of micanite [27].
To obtain varnishes, the resin is dissolved in alcohol-toluene mixture (1:1).Glyphthalic varnishes are prepared with different resin content.Varnishes have high adhesive ability, they dry rather quickly in the air with formation of adhesive varnish film, which softens when heated.When heated, the lacquer film slowly transforms into a non-fusible and insoluble state, it has insufficient elasticity when drying.Glyphthalic resins and varnishes are used to form hard mica materials and products.They are used mainly for primers and fillers, and enamels for indoor coatings Pure glyphthalic resins in the form of benzene-alcohol varnishes are used to produce coatings on metals.According to American data, a coating of such varnish applied to a metal object and secured by heating to 150 °С for 3-6 h, well resists the action of aqueous solutions of acids, alkalis, sea water and mineral oil.However, high brittleness of pure unmodified glyphthalic resins limits their application.[28] In addition to pure (unmodified) glyphthalic resins, resins modified with vegetable oils or fatty and resin acids are also widely used [29].
AMK glyphthalic glue is a solution of glyphthalic resins in organic solvents with the addition of a siccative.It is used for gluing woollen, glass and cotton thermal insulation to metal.Suitable for bonding at 18-23 °С.adhesive joint readiness 30 h [30].

Materials and methods
According to the synthesis procedure phthalic anhydride and glycerin react in the ratio 3:2.
To observe this ratio it is necessary to take 0.15 mol of phthalic anhydride and 0.1 mol of glycerin.Calculation of the volume of glycerol and mass of phthalic anhydride: m(pht.anh.)=0.15mol × 148 g/mol=22.2g m(glyc.)=0.1 mol × 92 g/mol=9.2g V(glyc.)=9.2 g ÷1.261 g/cm 3 =7.3cm 3 For the titration of samples taken from the reaction medium, prepare 1 l 0.5 Н alcohol solution KON, by making the following calculations: Мэ(KON)=56÷1=56 g/mol m(KON)=56×0.5=28g V(KON)=28÷2.044=13.7 cm 3 =0.014l V(alcohol)=1 l -0.014 l=0.986 l 22.2 g phthalic anhydride and 7.3 mL glycerin are placed in a porcelain beaker and the whole is covered with a tilted glass funnel with a thermometer.The work is carried out in a fume cupboard.The mixture should be heated rapidly on a sand bath to 160°C and maintain this temperature for 1.5 h.Then the reaction mixture is heated to 200°С and kept at this temperature until the polymer, which is difficult to dissolve in acetone, is formed.The reaction mixture should be stirred continuously.

Results and discussion
During the synthesis samples of the reaction mixture are taken to determine the acid number.The first sample is taken after melting of phthalic anhydride, the following ones after 15, 30, 45 min and after 1, 2, 3 hours.Phthalic anhydride burns up on heating and crystallizes on the cold walls of the funnel.[31] E3S Web of Conferences 458, 02034 (2023) EMMFT-2023 https://doi.org/10.1051/e3sconf/202345802034Acid number determined by titration.Acid number -number of milligrams of potassium hydroxide (KOH), required to neutralise all the acidic components contained in 1 g of the substance under test.The acid number is a measure of the sum of carboxylic acids in an organic compound, such as fatty acids, or in a mixture of compounds.Usually, a known amount of sample dissolved in an organic solvent (most often a mixture of polar and nonpolar solvents) is titrated with a potassium hydroxide solution of known concentration and phenolphthalein as an indicator [32,33].In general, the titration proceeds by the following reaction:

R−COOH+KOH→R−COOK+H2O
For this purpose, a sample of the substance is dissolved in acetone, phenolphtalein is added and titrated with alcoholic KOH solution until a light pink colour appears.
All the obtained data are placed in Table 1.

Conclusions
After synthesis of one of the common polyesters -glyphthalic resin by polycondensation of polyatomic alcohol glycerol and phthalic acid anhydride, new possibilities of its application were revealed.While investigating the course of the polycondensation process, a change in the acid number during the reaction was observed.The results of acid number determination during polycondensation can be used for directed control of alkyd oligomer synthesis and their subsequent curing to obtain a polymer with desired properties.The obtained polymer can be further modified to give special properties that extend the scope of its application, as well as to ensure the environmental safety of waste generated during its use.

Table 1 .
Dependence of acid number on synthesis timeWith a gradual decrease in the acid number, the idea that the emerging carboxyl groups gradually enter into the esterification reaction with the hydroxyl groups of glycerol is The acid number values lie almost in a straight line.Small deviations of the delayed points are explained by the error of the experiment, which does not exceed 5%.
Fig. 3. Dependence of acid number on synthesis time