Method for producing a flame-retardant polyester

Abstract

Method for producing a flame-retardant polyester, comprising the following steps: - Reaction of a chain-forming monomer (A) with a phosphorus-containing polyester (B) and at least a difunctional alcohol (C) to form a reaction mixture, wherein the reaction is carried out at a temperature at which alcoholysis of the phosphorus-containing polyester (B) takes place, wherein the phosphorus-containing polyester (B) is obtainable by polycondensation of phosphorus-containing monomers (B1) with ester-forming monomers (B2), wherein the phosphorus-containing monomer (B1) is selected from the addition products of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) and core-substituted DOPO derivatives to unsaturated compounds from the group of mono- and polyvalent carboxylic acids and their anhydrides, and the ester-forming monomer (B2) is selected from the group consisting of mono- and polyhydric alcohols, mono- and polyhydric carboxylic acids and mixtures thereof, and - Producing a flame-retardant polyester from the reaction mixture, wherein the flame-retardant polyester comprises units of the chain-forming monomer (A), the phosphorus-containing polyester (B) and the at least difunctional alcohol (C).

Description

[0001] The invention relates to a method for producing a flame-retardant polyester.

[0002] Flame-retardant thermoplastic polymers can be obtained by adding non-reactive flame retardants, which can be incorporated into a base polymer through simple physical mixing or dissolution. In contrast, the production of flame-retardant thermoplastic polymers with reactive flame retardants—that is, thermoplastic polymers in which the flame retardant is incorporated into the base polymer—always requires at least one or more chemical process steps, which are generally carried out during the production of the base polymer.

[0003] Non-reactive flame retardants allow for a so-called "late addition" process, in which the flame retardant is added to the base polymer shortly before processing, usually during extrusion, for example as a masterbatch. In contrast, such a process is normally not possible when using reactive flame retardants, as these undergo a chemical reaction with the base polymer and, under the temperature and pressure conditions prevailing during extrusion, in most cases cause polymer degradation.

[0004] For the production of flame-retardant polyamides, a number of non-reactive, halogen-free flame retardants have long been used in technical applications. These offer advantages over halogenated flame retardants in terms of both process engineering and human and ecotoxicological safety.

[0005] DE 26 46 218 A1 discloses phosphorus-containing flame retardants obtained by adding 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) to an unsaturated compound having at least one ester-forming functional group and further reacting it with an ester-forming compound selected from dicarboxylic acids or ester-forming derivatives thereof, diols or ester-forming derivatives thereof, and oxycarboxylic acids or ester-forming derivatives thereof. These reactive, phosphorus-containing flame retardants are then reacted with dicarboxylic acids such as terephthalic acid and a glycol to form flame-resistant polyesters.

[0006] CN 104419003 A describes the use of a DOPO-based flame retardant for polyamides, wherein the flame retardant is bonded to the polymer chain by graft copolymerization.

[0007] From DE 28 16 100 C2, flame retardants are known which were obtained by polycondensation of the addition product of DOPO or DOPO derivatives to itaconic acid with polyhydric alcohols and have a molecular weight of 1,000 to 20,000 g / mol and a phosphorus content of 5.3 to 8.5 wt.%.

[0008] DE 10 2011 116 178 A1 describes a flame retardant that is preferably used for the flame-resistant treatment of thermoplastics and thermosets. It is obtained by reacting a phosphinic acid derivative from the group of DOPO and core-substituted DOPO derivatives with an optionally substituted polydiene containing isolated C-C double bonds.

[0009] From EP 2 284 208 A1, a DOPO-based flame retardant is known that is suitable for the flame-retardant treatment of curable synthetic resins. It is produced from DOPO or core-substituted DOPO derivatives by several reaction steps involving the introduction of reactive functional groups.

[0010] Also suitable as a flame retardant for curable synthetic resins is an addition product obtained by reacting DOPO and core-substituted DOPO derivatives with unsaturated compounds, as described in DE 10 2008 009 298 A1.

[0011] CN 111777641 A discloses the synthesis of a bifunctional DOPO derivative and its use as a flame retardant for epoxy resins.

[0012] WO 2016 / 008074 A1 describes the use of a similarly bifunctional DOPO derivative, which is used as a melt additive to impart flame-retardant properties to semi-aromatic polyamide or a mixture of semi-aromatic and aliphatic polyamide.

[0013] Suitable DOPO derivatives can also be incorporated into the chains of polyurethane copolymers as flame retardants, as shown in WO 2016 / 008074 A1 for hydrous polyurethanes.

[0014] In DE 10 2009 020 211 A1, a polyester is used for the production of molded parts. This polyester is produced by polycondensation and, in a second step, compounded with a di- or polyfunctional compound to obtain a polyester with a higher molecular weight. The functional compounds serve to extend the polymer chain and control the material properties, but have no other functionalities (e.g., flame-retardant effects).

[0015] EP 2 144 950 B1 provides halogen-free, non-reactive flame retardants based on phosphorus-containing polyesters, for example according to formula (I), which are non-toxic and can be processed well as a masterbatch together with thermoplastic molding compounds in melt spinning processes or other extrusion processes at elevated temperatures.

[0016] These phosphorus-containing polyesters are obtained by polycondensation of phosphorus-containing monomers with ester-forming monomers. The phosphorus-containing monomers are selected from the addition products of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) and core-substituted DOPO derivatives to unsaturated compounds from the group of mono- and polyvalent carboxylic acids and their anhydrides.

[0017] However, in contrast to polyamide, for example, it is particularly advantageous in the production of flame-retardant polyester not to add the phosphorus-containing ester-forming monomer as a non-reactive flame retardant in polymerized form according to formula (I) in the extrusion process of a spinning mass, but to incorporate it directly into the polyester chains as a reactive comonomer in the polycondensation process during the production of the polyester, since chain degradation can occur if it is added during extrusion.

[0018] In US patent 2004 / 0097621 A1, the use of an ester-forming monomer, available under the name "Ukanol™ ES", as a comonomer in the direct esterification to flame-retardant modified polyethylene terephthalate (PET) is described.

[0019] German patent DE 20 2013 103 502 U1 describes the use of a corresponding oligomeric flame retardant (available under the name “Ukanol™ GK-8” from Schill + Seilacher GmbH, Böblingen) according to formula (II) as a comonomer combined with 3-hydroxyphenylphosphinyl propionic acid (3-HPP) as a second comonomer in the polycondensation process. However, it notes an undesirable tendency for the flame-retardant modified polyesters to gray, which may necessitate the use of graying inhibitors.

[0020] In addition to the tendency to turn grey, the prolonged reaction time when adding comonomers according to formula (II) is problematic.

[0021] For the synthesis of flame-retardant polyesters, preferably polyethylene terephthalate, the monoethylene glycol diester of the DOPO-itaconic acid adduct is used as a monomer. This monomer is available, for example, under the name "Ukanol™ ES" from Schill + Seilacher, Böblingen, Germany. The active content of this commercial product is approximately 65%. The remainder is monoethylene glycol (MEG) as a solvent, which serves to obtain a liquid product.

[0022] The use of the liquid dilute comonomer has several disadvantages due to the solvent. The active content is relatively low at approximately 65%, meaning that free solvent is reintroduced into the polymerization process relatively late in the manufacturing process when the dilute comonomer is added. This leads to an extension of the polymerization time. Furthermore, the solvent, in this case MEG, can generate oxidation products during storage, which in turn can have a reducing and / or deactivating effect on the catalysts and additives used in the polymerization. This also leads to an extension of the polymerization time and potentially to an undesirable change in the color of the polyester.

[0023] The object of the invention is to provide a method for producing a flame-resistant polyester which in particular has shorter polymerization times.

[0024] The problem is solved by a process for producing a flame-retardant polyester, comprising the following steps: A chain-forming monomer (A) is reacted with a phosphorus-containing polyester (B) and at least a difunctional alcohol (C), the reaction being carried out at a temperature at which alcoholysis of the phosphorus-containing polyester (B) occurs. The phosphorus-containing polyester (B) is obtained by polycondensation of phosphorus-containing monomers (B1) with ester-forming monomers (B2), wherein the phosphorus-containing monomer (B1) is selected from the addition products of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and core-substituted DOPO derivatives to unsaturated compounds from the group of mono- and polyhydric carboxylic acids and their anhydrides. The ester-forming monomer (B2) is selected from the group consisting of mono- and polyhydric alcohols, mono- and polyhydric carboxylic acids and mixtures thereof.The flame-retardant polyester is subsequently produced from the reaction mixture, wherein the flame-retardant polyester comprises units of the chain-forming monomer (A), the phosphorus-containing polyester (B) and the at least difunctional alcohol (C).

[0025] It was discovered that the phosphorus-containing polyester (B) can be split in situ by means of alcoholysis into the monomer units of the phosphorus-containing polyester (B), which are subsequently reacted with the chain-forming monomer (A) to form the flame-resistant polyester according to the invention.

[0026] As is known in the prior art, the production of polyesters is usually carried out in several steps. In the so-called first esterification reactor, a first condensation stage takes place at temperatures above 100 °C and a pressure of 0.5 bar or more, allowing the water or methanol produced in the condensation reaction to be distilled off. Subsequently, in the following steps, condensation is completed at temperatures typically above 225 °C and under vacuum in order to achieve a sufficiently high degree of conversion.

[0027] The process according to the invention allows the phosphorus-containing polyester (B) intended for flame retardancy to be added directly in the first esterification reactor, so that no additional solvent needs to be introduced into the reaction and distilled off in the subsequent manufacturing process. This allows the reaction time to be shortened accordingly, resulting in a particularly simple process flow. Chain-forming monomer (A)

[0028] The chain-forming monomer (A) is in particular selected from the group consisting of at least difunctional carboxylic acids, their diesters of monoalcohols and combinations thereof.

[0029] Preferably the chain-forming monomer (A) is terephthalic acid, isophthalic acid, phthalic acid, 2,6-napthalendicarboxylic acid, adipic acid, 1,4-butanedioic acid (also known as succinic acid) and / or a dimethyl ester of the aforementioned acids.

[0030] The chain-forming monomer (A) is used in particular in a proportion of 20 to 70 percent by weight, based on the total mass of the reaction mixture, preferably in a proportion of 30 to 60 percent by weight, and particularly preferably in a proportion of 35 to 60 percent by weight. Polyester containing phosphorus (B)

[0031] The average molecular weight M nThe phosphorus content of the polyester (B) is in particular 15,000 g / mol or greater, preferably between 15,000 and 100,000 g / mol, particularly preferably between 20,000 and 35,000 g / mol.

[0032] The molecular weight of the phosphorus-containing polyester can be determined by end-group determination.

[0033] The average degree of polymerization P n The phosphorus content of the polyester (B) is in particular at least 40 and is preferably between 50 and 270, particularly preferably between 50 and 90.

[0034] The phosphorus-containing polyester (B) has in particular a glass transition temperature of 60 °C or more, preferably a glass transition temperature in the range of 65 to 85 °C, for example of 75 °C.

[0035] In other words, the phosphorus-containing polyester is primarily a solid, which further enhances its handling and storage stability. In particular, the phosphorus-containing polyester (B) can be stored without the addition of solvents, thus preventing solvent oxidation products from interfering with the polymerization process.

[0036] A preferred phosphorus-containing polyester is the polyester according to formula (I):

[0037] In formula (I) R1 represents hydrogen, methyl or ethyl, R2 is a residue -(CH2) m -O-R1, n is an integer between 40 and 270, A represents a branched or unbranched alkylene group with 2 to 10 carbon atoms or an optionally substituted aromatic bridging group. The substituents on the aromatic bridging group are alkyl, alkoxy, aryl, aryloxy, aralkyl and / or alkylaryl.

[0038] The phosphorus-containing polyester is used in particular in a proportion of 1 to 20 percent by weight, based on the total mass of the reaction mixture, preferably in a proportion of 3 to 15 percent by weight, and particularly preferably in a proportion of 5 to 10 percent by weight.

[0039] The total phosphorus content of the flame-retardant polyester is determined by the proportion of phosphorus-containing polyester.

[0040] The flame-retardant polyester produced by the method according to the invention has in particular a total phosphorus content in the range of 0.1 to 2 percent by weight, preferably 0.6 to 1.5 percent by weight.

[0041] The phosphorus-containing polyester (B) is obtainable by polycondensation of phosphorus-containing monomers (B1) with ester-forming monomers (B2).

[0042] An example of a phosphorus-containing phosphorus ester (B) that can be used in the process according to the invention is the commercial product Ukanol™ FR80 from Schill + Seilacher GmbH, Böblingen. Phosphorus-containing monomer (B1)

[0043] The phosphorus-containing monomer (B1) is selected from the addition products of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) and core-substituted DOPO derivatives to unsaturated compounds from the group of mono- and polyvalent carboxylic acids and their anhydrides.

[0044] A preferred phosphorus-containing monomer (B1) is represented in formula (III):

[0045] In formula (III), A has the same meaning as in formula (I).

[0046] Formula (IV) is a generalized representation of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) and core-substituted DOPO derivatives that can be used in the production of the phosphorus-containing polyester:

[0047] In formula (IV), R1 and R2 are the same or different and each independently alkyl, alkoxy, aryl, aryloxy or aralkyl, n and m are integers from 0 to 4, and R3 represents a residue derived from an unsaturated dicarboxylic acid or its anhydride. Preferably, R1 and R2 are each C 1-8 -Alkyl or C 1-8 -Alkoxy, n and m are preferably 0 or 1. Ester-forming monomer (B2)

[0048] The ester-forming monomer (B2) is selected from the group consisting of mono- and polyhydric alcohols, mono- and polyhydric carboxylic acids and mixtures thereof.

[0049] Preferably, the ester-forming monomer (B2) used is selected from the group of saturated mono- and polyhydric alcohols.

[0050] Particularly preferred ester-forming monomers (B2) are aliphatic diols such as monoethylene glycol (MEG), diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, copolymers of ethylene glycol and propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, hexanediol and 1,10-decanediol.

[0051] Preferred polyhydric alcohols are tris-2-hydroxyethyl isocyanurate (THEIC), glycerol, trimethyl olethane, trimethylolpropane and pentaerythritol, as well as sugar alcohols such as mannitol. At least difunctional alcohol (C)

[0052] The at least difunctional alcohol serves as a solvent, to carry out the alcoholysis of the phosphorus-containing polyester (B) and as a reaction partner of the chain-forming monomer (A).

[0053] Preferably, the at least difunctional alcohol (C) is selected from the group of saturated mono- and polyhydric alcohols.

[0054] Particularly preferred at least difunctional alcohols (C) are aliphatic diols such as monoethylene glycol (MEG), diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, copolymers of ethylene glycol and propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, hexanediol and 1,10-decanediol.

[0055] The at least difunctional alcohol (C) monoethylene glycol (MEG) is particularly preferred.

[0056] In one variant, the at least difunctional alcohol (C) corresponds to the ester-forming monomer (B2). In other words, the same alcohol used in the alcoholysis of the phosphorus-containing polyester (B) is used as the solvent in the reaction mixture. This method yields particularly uniformly structured flame-retardant polyesters.

[0057] The at least difunctional alcohol (C) is used in particular in a proportion of 20 to 70 percent by weight, based on the total mass of the reaction mixture, preferably in a proportion of 30 to 65 percent by weight, and particularly preferably in a proportion of 35 to 60 percent by weight.

[0058] To ensure complete alcoholysis of the phosphorus-containing polyester (B), the molar ratio of at least difunctional alcohol (C) to phosphorus-containing polyester (B) is in particular 1:1 or higher.

[0059] In addition, a molar ratio of acid functionalities to difunctional alcohol (C) of 1:2 or higher is established in the reaction mixture. Reaction accelerator (D)

[0060] Optionally, a reaction accelerator (D) is added to the mixture of chain-forming monomer (A), phosphorus-containing polyester (B) and at least difunctional alcohol (C).

[0061] The reaction accelerator (D) is at least a difunctional ester, preferably a bisglycol ester of terephthalic acid, isophthalic acid and / or phthalic acid, particularly preferably a terephthalic acid bisglycol ester.

[0062] The reaction accelerator (D) is used in particular in a proportion of 10 to 50 percent by weight, based on the total mass of the reaction mixture, preferably in a proportion of 20 to 40 percent by weight, and particularly preferably in a proportion of 25 to 35 percent by weight.

[0063] When a reaction accelerator (D) is used, the proportion of the chain-forming monomer (A) in the reaction mixture decreases accordingly. For example, when 10% by weight of reaction accelerator (D) is used, the proportion of the chain-forming monomer (A) is reduced by 10% by weight, in each case based on the total mass of the reaction mixture. Procedure

[0064] The production of the reaction mixture preferably takes place in a single stage in the first esterification reactor, i.e. the components used are mixed and converted to the reaction mixture by heating to a temperature at which alcoholysis of the phosphorus-containing polyester (B) takes place.

[0065] The reaction takes place at a temperature of 150 °C or higher, preferably 250 °C or higher. The maximum temperature can be determined depending on various process parameters, especially the desired viscosity of the reaction mixture and the selected pressure.

[0066] The pressure during the conversion to the reaction mixture is in particular 0.5 bar or more, preferably 0.5 to 2 bar.

[0067] Parallel to the alcoholysis, polycondensation of the components in the reaction mixture already takes place. Therefore, the temperature at which the alcoholysis is carried out corresponds, in particular, to the temperature provided for the polycondensation reaction in the first esterification reactor. Although a lower temperature is sufficient for alcoholysis than for the polycondensation reaction, this approach allows for a particularly simple adaptation of existing production processes to the process according to the invention.

[0068] The flame-resistant polymer is then produced from the reaction mixture in further steps, using methods known in the prior art.

[0069] The subsequent steps are preferably carried out at a temperature above 225 °C and under vacuum.

[0070] The units originating from the phosphorus-containing polymer (B) are incorporated, in particular statistically, into the flame-retardant polyester.

[0071] Further advantages and features will result from the following description of preferred embodiments, which, however, should not be understood in a restrictive sense. Definitions and measurement methods

[0072] To determine the intrinsic viscosity, the polymer samples were dissolved in phenol / tetrachloroethane (1:1 v / v) and the flow times were determined in an Ubbelohde viscometer at 20 °C. The intrinsic viscosity was calculated according to [η]=−1+1+1.4ηsp0.7⋅c with c = 0.1 g / dl and ηsp=η−ηLηL, where n L denotes the viscosity of the pure solvent and η the measured viscosity.

[0073] The melting range, glass transition temperature Tg, and recrystallization temperature were determined using differential scanning calometry (DSC). A Netzsch DSC 214 instrument was used for this purpose. The heating and cooling rates were 10 K / min.

[0074] The phosphorus content was determined after digestion by a mixture of sulfuric acid and nitric acid and oxidation with peroxodisulfate using the LCK 350 / LCK 349 cuvette test from Dr. Lange.

[0075] The target torque was determined using a torque sensor built into the stirrer. Preliminary experiments on alcoholysis

[0076] The following are preliminary experiments for carrying out alcoholysis. All experiments were conducted at atmospheric pressure (1 bar). Comparative example 1

[0077] 10 g of UKANOL™ ES are dissolved in 90 g of monoethylene glycol (MEG) at room temperature.

[0078] A clear, colorless and homogeneous solution with a viscosity of 22.5 mPas at 25°C is obtained. Comparative example 2

[0079] 10 g of UKANOL™ FR 80 are stirred into 90 g of MEG for 30 minutes at 100 °C. A milky white, inhomogeneous emulsion is formed.

[0080] Determining the solution viscosity at 25 °C was not possible because solid components formed again when the emulsion cooled. Example 3

[0081] 10 g of UKANOL™ FR 80 are stirred in 90 g of MEG for 30 minutes at 190 °C.

[0082] The result is a clear, colorless and homogeneous solution.

[0083] The solution viscosity at 25 °C is 24 mPas. This corresponds to the comparative experiment 1, in which a phosphorus-containing monomer was used.

[0084] Thus, alcoholysis of the phosphorus-containing polyester UKANOL™ FR 80 is possible at elevated temperature and atmospheric pressure. Production of a flame-retardant polyester: Comparative example 4 - Synthesis of terephthalic acid bisaglycol ester (TPDMEG)

[0085] 32.4 wt% MEG, 37.6 wt% terephthalic acid, and 30 wt% terephthalic bisglycol ester (TPDMEG) are placed in a reaction vessel and heated to 200 °C. During the reaction, water is released and collected in a feed flask. After completion of the reaction, the product is removed, cooled, and crushed. Example 5 - Synthesis of the bisglycol ester of UKANOL™ FR 80 (DOPODMEG 1)

[0086] 25.2 wt% MEG, 33.4 wt% terephthalic acid, 6.7 wt% UKANOL™ FR 80, and 31.9 wt% terephthalic bisglycol ester (TPDMEG) are placed in a reaction vessel and heated to 200 °C. During the reaction, water is released and collected in a feed flask. After completion of the reaction, the product is discharged, cooled, and crushed. Example 6 - Synthesis of the bisglycol ester of UKANOL™ ES (DOPODMEG 2)

[0087] 15.6 wt% MEG, 37.3 wt% terephthalic acid, 16.3 wt% UKANOL™ ES, and 30.8 wt% terephthalic bisglycol ester (TPDMEG) are placed in a reaction vessel and heated to 200 °C. During the reaction, water is released and collected in a feed flask. After completion of the reaction, the product is discharged, cooled, and crushed. Polymerization

[0088] The bisglycol ester mixtures shown in examples 4 to 6 are polymerized to polyethylene terephthalate (PET) or polyethylene terephthalate copolymers (CoPET) using Sb2O3 as a catalyst in a stainless steel reactor until a target torque of approximately 3 Nm is reached.

[0089] The reaction conditions are listed in Table 1, where the condensation time corresponds to the reaction time until the target torque is reached. Table 1: Reaction conditions for polymerization. PET (Comparison example 4) CoPET 1 (Example 5) CoPET2 (Example 6) Monomer TPDMEG DOPODMEG 1 DOPODMEG 2 Reactor temperature [°C] 275 274-279 273-278 Pressure [mbar] 0,5 - 1,1 1,6 - 2,2 3,4 - 3,5 Condensation time [min] 40 50 55 Torque [Nm] 2,98 - 3,31 2,98 - 3,25 3,09 - 3,33 Stirrer speed [rpm] 140 143 142 Distillate [g] 117,44 98,24 81,83 Characterization of polyester

[0090] Table 2 lists the properties of the polymer samples obtained from Examples 4 to 6, as well as two further industrially produced samples.

[0091] Comparison example 7 is a PET that was industrially produced with the addition of the phosphorus-containing polyester UKANOL™ ES.

[0092] Comparison example 8 is a PET that was industrially produced without the addition of a phosphorus-containing flame retardant.

[0093] Comparison examples 7 and 8 additionally contain TiO2 as a filler. Table 2: Properties of the polyesters according to examples 4 to 8. PET (Comparison example 4) CoPET 1 (Example 5) CoPET2 (Example 6) PET FR (Comparison example 7) PET (Comparison example 8) Phosphorus content [wt.%] 0 0,7 0,7 nb nb Intrinsic viscosity 0,5875 0,5554 0,5711 nb nb Melting range [°C] 73,4 67,4 67,6 76,3 76,3 Tg [°C] 73,4 67,4 67,6 76,3 76,7 Recrystallization temperature [°C] 126,5 133,8 133,0 137,6 128,1 nb = not determined

[0094] Table 2 shows that the comparative example 4 has similar calorimetric properties and a similar intrinsic viscosity to the flame-retardant polyesters which are obtainable via a method according to the invention.

[0095] In comparison to the industrially manufactured examples 7 and 8, similar calorimetric properties can also be observed.

[0096] Therefore, the flame-retardant polyesters obtainable according to the inventive method are expected to exhibit similar behavior and possible application profiles as known polyesters, but with the additional benefit of integrated flame protection.

Claims

A process for producing a flame-retardant polyester, comprising the following steps: - Reacting a chain-forming monomer (A) with a phosphorus-containing polyester (B) and at least a difunctional alcohol (C) to form a reaction mixture, wherein the reaction is carried out at a temperature at which alcoholysis of the phosphorus-containing polyester (B) takes place, wherein the phosphorus-containing polyester (B) is obtainable by polycondensation of phosphorus-containing monomers (B1) with ester-forming monomers (B2), wherein the phosphorus-containing monomer (B1) is selected from the addition products of 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) and core-substituted DOPO derivatives to unsaturated compounds from the group of mono- and polyhydric carboxylic acids and their anhydrides, and the ester-forming monomer (B2) from the mono- and polyhydric alcohols, The group consisting of mono- and polyvalent carboxylic acids and mixtures thereof has been selected.and- producing a flame-retardant polyester from the reaction mixture, wherein the flame-retardant polyester comprises units of the chain-forming monomer (A), the phosphorus-containing polyester (B) and the at least difunctional alcohol (C). The method according to claim 1, characterized in that the chain-forming monomer (A) is selected from the group consisting of at least difunctional carboxylic acids, their diesters of monoalcohols and combinations thereof. The method according to claim 2, characterized in that the chain-forming monomer (A) is terephthalic acid, isophthalic acid, phthalic acid, 2,6-napthalendicarboxylic acid, adipic acid, 1,4-butanedioic acid and / or their dimethyl esters. Method according to one of the preceding claims, characterized in that the phosphorus-containing polyester (B) is used in a proportion of 1 to 20 percent by weight, based on the total mass of the reaction mixture. Method according to one of the preceding claims, characterized in that the at least difunctional alcohol (C) is an aliphatic diol. Method according to one of the preceding claims, characterized in that the at least difunctional alcohol (C) corresponds to the ester-forming monomer (B2). Method according to one of the preceding claims, characterized in that the molar ratio of at least difunctional alcohol (C) to phosphorus-containing polyesters (B) is 1:1 or higher. Method according to one of the preceding claims, characterized in that a reaction accelerator (D) is additionally used in the reaction mixture, wherein the reaction accelerator (D) is at least a difunctional ester. Method according to one of the preceding claims, characterized in that the conversion to the reaction mixture is carried out at a temperature of 150 °C or higher.