Storage-stable compounds of antioxidants containing ascorbic acid

Abstract

Composition comprising at least ascorbic acid, an antioxidant and a glycerol acetal or ketal of general formula 1 where R1 and R2 independently mean: hydrogen or a C 1-10 - Alkyl group, or R1 and R2 together form a carbon chain with 2 to 5 carbon atoms, where the antioxidant a phenolic antioxidant or an amine antioxidant or a mixture of a phenolic and an amine antioxidant; and wherein the antioxidant is present in the composition at a concentration of 5 to 55 wt% and the ascorbic acid at a concentration of 0.1 to 5 wt%, based on the total weight of the composition (= 100 wt%).

Description

AREA OF INVENTION

[0001] The present invention relates to storage-stable compositions comprising ascorbic acid and an antioxidant and preferably a suitable solvent, the use of the composition to improve the oxidation resistance of fatty acid esters such as biodiesel, and compositions comprising a fatty acid ester and the storage-stable composition. BACKGROUND OF THE INVENTION

[0002] Numerous publications in scientific literature deal with phenolic antioxidants and their influence on improving the oxidative stability of biodiesel and vegetable oils (e.g., The Biodiesel Handbook, 2nd Edition, ISBN-13: 978-1893997622, ​​ISBN-10: 1893997626).

[0003] Liquid formulations of phenolic antioxidants, which are used as additives to improve the oxidation stability of, for example, biodiesel, are disclosed, for example, in US 8 075 804 and EP 2 896 682.

[0004] WO 2007 / 062304 discloses a composition comprising biodiesel, which may contain one or more antioxidants, wherein the antioxidants may be phenolic antioxidants as well as non-phenolic antioxidants. For example, fatty acids may be used as non-phenolic antioxidants.

[0005] WO 2007 / 102948 discloses a composition containing phenolic antioxidant solutions. This application further discloses compositions containing both biodiesel and at least one antioxidant concentrate solution, and mixed fuel compositions containing biodiesel and biodiesel, which may be mixed with other fuels.

[0006] An overview of commercially available phenolic antioxidants is provided by the "no harm list of AGQM biodiesel" (http: / / www.aqqm-biodiesel.de / de / services / no-harmliste / ). Well-known phenolic antioxidants include, for example, BHT (CAS No. 128-37-0), TBHQ (CAS No. 1948-33-0), BKF (CAS No. 119-41-1), TBC (CAS No. 98-29-3), propyl gallate (CAS No. 121-79-9), gallic acid (CAS No. 149-91-7), pyrogallol (CAS No. 87-66-1), hydroquinone (CAS No. 123-31-9), and catechol (CAS No. 120-80-9).

[0007] From O+P, 11-12 / 2004, pages 2 to 5, it is known to stabilize lubricants based on ester oils against oxidation by adding ascorbic acid or a salt thereof in the presence of phenolic antioxidants such as 2,6-di-tert-butyl-4-methylphenol (BHT).

[0008] Liquid formulations of phenolic antioxidants, which are used as additives to improve the oxidative stability of oils and fats used in human and animal foods, are disclosed, for example, in WO 2004 / 090062.

[0009] WO 2015 / 106997 A1 discloses an oxidation-stabilized biodiesel and a process for its production.

[0010] US 3752832 A discloses a method for stabilizing lipids against oxidation. The method utilizes the effects caused by tocopherol and unsaponifiables of rice oil or tocopherol and synergists.

[0011] WO 2007 / 062304 discloses biodiesel fuel compositions exhibiting improved oxidation stability. These biodiesel fuel compositions comprise at least one antioxidant that enhances the fuel's oxidation stability. The biodiesel fuel compositions may also contain an antioxidant mixture or an antioxidant mixture in combination with a polar and / or nonpolar solvent, further enhancing the fuel's oxidation stability.

[0012] During the development of new additives for the oxidative stabilization of biodiesel, we discovered that highly effective phenolic antioxidants such as TBHQ and hydroquinone, when in concentrated solution, are oxidized by atmospheric oxygen during storage. This process forms colored quinones and their derivatives, such as p-benzoquinones from hydroquinone. The oxidation of the phenolic antioxidant reduces its stabilizing effectiveness, which is detrimental to the stabilization of a fatty acid ester like biodiesel.

[0013] Amino antioxidants are also sensitive to oxidation by atmospheric oxygen. TASKS OF INVENTION

[0014] The object of the invention was to develop antioxidant compositions in which this loss of effectiveness due to oxidation by atmospheric oxygen during storage does not occur or occurs comparatively slowly. Such formulations could then be used as additives to improve the oxidative stability of fatty acid esters, preferably biodiesel. SUMMARY OF THE INVENTION

[0015] The inventors of the present invention have now surprisingly discovered that ascorbic acid, particularly in suitable solvents, is capable, even in small quantities, of slowing down or preventing the oxidation of phenolic antioxidants. Therefore, such compositions can be used to improve the oxidation resistance of fatty acid esters, preferably biodiesel.

[0016] The invention relates to the following items: 1. Composition comprising at least ascorbic acid, an antioxidant and a glycerol acetal or ketal of general formula 1 where R1 and R2 independently mean: hydrogen or a C 1-10 - Alkyl group, or where R1 and R2 together form a carbon chain with 2 to 5 carbon atoms, where the antioxidant a phenolic antioxidant or an amine antioxidant or a mixture of a phenolic and an amine antioxidant; wherein the antioxidant is present in the composition at a concentration of 5 to 55 wt% and the ascorbic acid at a concentration of 0.1 to 5 wt%, based on the total weight of the composition (= 100 wt%). 2. Composition according to Item 1, wherein the compound of formula 1 is isopropylideneglycerol (CAS No. 100-79-8). 3. Composition according to item 1 or 2, further comprising one or more of the following compounds: water; an alcohol of the general formula R3-OH, wherein R3 is a C 1-10 -alkyl group; or a glycol or a glycol ether of general formula 2 R4-O-CHR5 - CHR6-(O-CHR5 - CHR6) n -OR7 Formula 2 where R4, R5, R6 and R7 independently H, a C 1-10 - alkyl group, phenyl or benzyl, and n means an integer from 0 to 100. 4. Composition according to any of items 1 to 3, wherein the composition comprises ascorbic acid and the compound of formula 1 in a weight ratio of 1 : 50 to 1 : 1,000. 5. Composition according to any of items 1 to 4, wherein the composition comprises ascorbic acid and the compound of formula 1 in a weight ratio of 1 : 0.25 to 1 : 10. 6. Composition according to any of articles 3 to 5, comprising a mixture of a glycerol acetal or ketal of formula 1 and a glycol ether of formula 2 in any mixing ratio. 7. Composition according to any one of items 1 to 6, wherein the phenolic antioxidant is selected from the following group: BHT (CAS No. 128-37-0), TBHQ (CAS No. 1948-33-0), BKF (CAS No. 119-41-1), TBC (CAS No. 98-29-3), propyl gallate (CAS No. 121-79-9), gallic acid (CAS No. 149-91-7), pyrogallol (CAS No. 87-66-1), hydroquinone (CAS No. 123-31-9), catechol (CAS No. 120-80-9), bisphenol A (CAS No. 80-05-7), or two or more thereof. 8. Composition according to item 7, wherein the composition comprises TBHQ. 9. Composition according to item 8, wherein the composition comprises hydroquinone. 10. Composition according to any one of items 1 to 9, wherein the amine antioxidant is selected from the group consisting of: N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (CAS No. 793-24-8), N-Isopropyl-N'-phenyl-p-phenylenediamine (CAS No. 101-72-4), N,N-Di-sec-butyl-p-phenylenediamine (CAS No. 101-96-2), N'-Bis(1,4-dimethylpentyl)-p-phenylenediamine (CAS No. 003081-14-9), N-Isopropyl-N'-phenyl-p-phenylenediamine (CAS No. 101-72-4) and N,N'-Di-sec-butyl-p-phenylenediamine (CAS No. 101-96-2), or two or more thereof. 11. Composition according to any one of items 1 to 10, further comprising one or more compounds from the group consisting of: dyes; inorganic or organic acids, in particular citric acid; heavy metal inhibitors, in particular an aryltriazole or a salicylidenediamine; and biocides. 12. Method for improving the oxidation resistance of a fatty acid ester, comprising step (C): (C) Contacting a fatty acid ester with a composition as defined in any of items 1 to 11; or using a composition according to any of items 1 to 11 to improve the oxidation resistance of fatty acid esters. 13. The process or use according to item 12, wherein the fatty acid ester is a fatty acid methyl ester. 14. The process or use according to item 12 or 13, wherein the fatty acid ester is a biodiesel. 15. Composition comprising at least one fatty acid ester and a composition as defined in any one of items 1 to 11. 16. Composition according to item 15, wherein the fatty acid ester is a fatty acid methyl ester. 17. Composition according to item 15 or 16, wherein the fatty acid ester is a biodiesel.

[0017] With the composition according to the invention, it is possible not only to reduce or prevent the oxidation sensitivity of phenolic antioxidants, but also to dissolve and keep in solution both the strongly hydrophilic ascorbic acid and the very hydrophobic (lipophilic) phenolic antioxidant. This is due to the choice of solvent, i.e., the glycerol acetal or ketal of formula 1 in the composition. This also applies when the composition according to the invention is used for the oxidative stabilization of fatty acid esters, preferably biodiesel.

[0018] In the compositions according to the invention, the active ingredient content of the phenolic antioxidant remains stable even over extended periods. Little or no oxidation occurs in the air during storage. This is also evident from the fact that a composition containing, for example, hydroquinone turns yellow or red without the addition of ascorbic acid. This discoloration can be reduced by the addition of ascorbic acid.

[0019] The composition of the present invention is generally liquid even at low storage temperatures in the range of 0 °C to 4 °C and does not tend to crystallize immediately. This property is particularly advantageous for use in fatty acid esters such as biodiesel, since the products can be transported and stored at low temperatures without significant heating or insulation requirements.

[0020] In the following, all terms enclosed in quotation marks are defined in the sense of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The invention relates to a composition comprising at least ascorbic acid, an antioxidant and a glycerol acetal or ketal of general formula 1, wherein R1 and R2 independently denote: hydrogen or a C 1-10 - Alkyl group, or wherein R1 and R2 together form a carbon chain with 2 to 5 carbon atoms, wherein the antioxidant is a phenolic antioxidant or an amine antioxidant, or a mixture of a phenolic and an amine antioxidant, as defined in claim 1.

[0022] The term "ascorbic acid" is used synonymously with the term "vitamin C". Therefore, in one embodiment, ascorbic acid (CAS No. 50-81-7) is preferably used in the form of L-ascorbic acid.

[0023] In one embodiment, the composition contains neither salts of ascorbic acid nor reaction products of ascorbic acid with fatty acids.

[0024] The term "phenolic antioxidant" means any phenol that can slow down or prevent the oxidation of a chemical compound, preferably a fatty acid ester.

[0025] Preferably, the phenolic antioxidant is selected from the group consisting of: BHT (2,6-di-tert-butyl-4-methylphenol; 2,6-bis(1,1-dimethylethyl)-4-methylphenol; CAS No. 128-37-0), TBHQ (tert-butylhydroquinone; CAS No. 1948-33-0), BKF (6,6'-di-tert-butyl-2,2'-methylenedicresol; CAS No. 119-47-1), TBC (p-tert-butylcatecholine; CAS No. 98-29-3), propyl gallate (CAS No. 121-79-9), gallic acid (CAS No. 149-91-7), pyrogallol (CAS No. 87-66-1), hydroquinone (CAS No. 123-31-9), catecholine (CAS No. 120-80-9), Bisphenol A (CAS No. 80-05-7), or two or more of them.

[0026] In another embodiment, the phenolic antioxidant is a doubly or triply substituted hydroxyl-substituted benzene.

[0027] In one embodiment, the composition comprises TBHQ.

[0028] In one embodiment, the composition comprises TBHQ and hydroquinone.

[0029] In one embodiment, the weight ratio of TBHQ and hydroquinone is in the range of 1:5 to 5:1.

[0030] The term "aminic antioxidant" means any aromatic, aliphatic or cyclic amine that can slow down or prevent the oxidation of a chemical compound, preferably a fatty acid ester.

[0031] Preferably, the amine antioxidant is selected from the group consisting of: N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (CAS No. 793-24-8), N-Isopropyl-N'-phenyl-p-phenylenediamine (CAS No. 101-72-4), N,N-Di-sec-butyl-p-phenylenediamine (CAS No. 101-96-2), N'-Bis(1,4-dimethylpentyl)-p-phenylenediamine (CAS No. 003081-14-9), N-Isopropyl-N'-phenyl-p-phenylenediamine (CAS No. 101-72-4), and N,N'-Di-sec-butyl-p-phenylenediamine (CAS No. 101-96-2), or two or three thereof.

[0032] Preferably, the antioxidant is present in the composition at a concentration of 5 to 55 wt%, and the ascorbic acid at a concentration of 0.01 to 5 wt%, based on the total weight of the composition (= 100 wt%). An amount of ascorbic acid in the range of 0.1 to 5 wt% is also preferred.

[0033] Preferably the composition comprises ascorbic acid and the compound of formula 1 in a weight ratio of 1:50 to 1:1,000, or 1:100 to 1:800, provided that the composition contains ascorbic acid and the compound of formula 1.

[0034] Preferably, the composition comprises the antioxidant and the compound of formula 1 in a weight ratio of 1:0.25 to 1:10, or 1:0.5 to 1:5, provided that the composition contains ascorbic acid and the compound of formula 1.

[0035] Preferably the composition comprises ascorbic acid and the compound of formula 2 in a weight ratio of 1:50 to 1:1,000, or 1:100 to 1:800, provided that the composition contains ascorbic acid and the compound of formula 2.

[0036] Preferably, the composition comprises the antioxidant and the compound of formula 2 in a weight ratio of 1:0.25 to 1:10, or 1:0.5 to 1:5, provided that the composition contains ascorbic acid and the compound of formula 2.

[0037] Preferably the composition comprises a mixture of a glycerol acetal or glycerol ketal of formula 1 and a glycol ether of formula 2 in any mixing ratio, provided that the composition comprises a compound of formula 1 and formula 2.

[0038] Preferably, the composition comprises ascorbic acid and water in a weight ratio of 1:1 to 1:5, provided that the composition includes water.

[0039] Preferably the composition comprises ascorbic acid and the alcohol R3-OH in a weight ratio of 1:1 to 1:5, provided that the composition includes an alcohol.

[0040] If the composition includes alcohol, then the alcohol is preferably selected from methanol, ethanol or isopropanol.

[0041] The glycerol acetals or ketals of formula 1 are known or can be prepared by known processes by reacting glycerol with the corresponding aldehydes or ketones. Preferably, isopropylidene glycerol (formula 1, R1 = R2 = CH3; CAS No. 100-79-8) and the acetal of glycerol with formaldehyde (R1 = R2 = H; CAS Nos. 99569-11-6, 5464-28-8, and 4740-78-7) can be used.

[0042] The glycerol acetals or ketals of formula 1 can also exist as a structural isomeric 6-membered ring, in which the alcohol function at positions 1 and 3 of the glycerol is acetalized or ketalized. Technical products are mostly found as mixtures of both structural isomers in varying proportions.

[0043] In a particularly preferred embodiment, the composition comprises isopropylidene glycerin, provided that the composition comprises a compound of formula 1.

[0044] The term “C 1-10 - Alkyl group" as used herein, for example in connection with a compound of formula 1 or an alcohol of the general formula R3-OH, means a straight-chain, a branched, a cyclic or a cycloaliphatic alkyl group.

[0045] The glycols and glycol ethers of formula 2 are also known and can be produced by known methods.

[0046] Suitable products of formula 2 are preferably ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monophenyl ether, ethylene glycol monohexyl ether, ethylene glycol monobenzyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-butyl ether, diethylene glycol mono-n-hexyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, propylene glycol monohexyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, dipropylene glycol monohexyl ether, polyethylene glycol ether and polypropylene glycol ether.

[0047] The composition according to the invention may include further compounds. Preferably, such compounds are selected from the group consisting of: dyes, inorganic or organic acids, in particular citric acid, heavy metal inhibitors, in particular aryltriazoles and salicylidenediamines, and biocides.

[0048] Particularly suitable as colorants are fat-soluble dyes, of which the Macrolex® types from Lanxess are especially appropriate.

[0049] The composition according to the invention can be produced by mixing the components, if necessary with the application of heat.

[0050] The invention further relates to the use of the composition to improve the oxidation stability of fatty acid esters.

[0051] The invention therefore further relates to a method for improving the oxidation resistance of a fatty acid ester, comprising step (C): (C) Contacting a fatty acid ester with the composition according to the invention.

[0052] The term "bringing into contact" is used synonymously with the term "mixing".

[0053] The term "fatty acid ester" means esters of fatty acids with preferably 10 to 40 carbon atoms with any monohydric or polyhydric alcohol.

[0054] Preferably, the fatty acid ester is a biodiesel.

[0055] The term "biodiesel" refers to a fatty acid methyl ester obtained by alkaline or acid transesterification of vegetable oils such as rapeseed oil, soybean oil, or palm oil with methanol. The term biodiesel also includes fatty acid methyl esters derived from used vegetable oils and fats, as well as animal oils and fats.

[0056] In one embodiment, the term ‘biodiesel’ includes C 8-36 -Fatty acid methyl esters.

[0057] In one embodiment, the fatty acid glycerides can be animal fatty acid glycerides.

[0058] An example is tallow, such as beef tallow or pork tallow.

[0059] In another embodiment, the fatty acid glycerides can be vegetable fatty acid glycerides.

[0060] Examples include rapeseed oil, soybean oil, sunflower oil, palm oil, coconut oil or algae oil.

[0061] In another embodiment, the fatty acid glycerides are waste oils and fats, i.e., oils and fats that are not suitable for human consumption.

[0062] The invention further relates to a composition comprising at least one fatty acid ester and the composition according to the invention.

[0063] Preferably, the fatty acid ester is a methyl ester.

[0064] Preferably, the fatty acid ester is a biodiesel.

[0065] The composition according to the invention, which contains a fatty acid ester, can be prepared by mixing the fatty acid ester, which is preferably liquid at room temperature, with the composition, if necessary with the application of heat.

[0066] The production of the new storage-stable compositions for the manufacture of additives for biodiesel is described using selected examples. EXAMPLES Example 1

[0067] 80 g of isopropylidene glycerin were heated to 60 °C. Then, 0.2 g of L-ascorbic acid was added and heated to 80 °C until dissolved. This was followed by the addition of 20 g of hydroquinone. Stirring was carried out for approximately 1 to 2 hours until the hydroquinone was completely dissolved. The solution was then cooled to room temperature. A clear, almost colorless solution was obtained.

[0068] A composition produced in this way could be colored by adding, for example, 500 ppm Macrolex® Green (Lanxess). Example 2

[0069] 2.0 g of L-ascorbic acid were dissolved in 10 g of water at 40 °C. 70 g of isopropylidene glycerol were added, followed by the addition of 20 g of hydroquinone. The mixture was heated to 60–80 °C and stirred for approximately 1–2 hours until the hydroquinone was completely dissolved. The mixture was then cooled to room temperature. A clear, almost colorless solution was obtained.

[0070] A composition produced in this way could be colored by adding, for example, 500 ppm Macrolex® Green (Lanxess). Example 3

[0071] The procedure was as described in Example 2. However, the same amount of methanol was used instead of water. Example 4

[0072] The procedure was the same as described in Example 1, except that only 50 g of isopropylidene glycerin was used instead of 70 g. The solution was prepared by heating it to 60–100 °C. Then, 20 g of biodiesel was added. The mixture was stirred until homogeneous and then cooled.

[0073] A composition produced in this way could be colored by adding, for example, 500 ppm Macrolex® Green (Lanxess). Example 5

[0074] The procedure was the same as in Example 1, however, instead of isopropylidene glycerin, the same amount of n-butyl glycol was used as the solvent.

[0075] Further examples of compounds are listed in Table 1. Table 1 Nr. L-Ascorbic acid [amount g / g] Solvent [amount g / g] Phenolic antioxidant [amount g / g] Solvent [amount g / g] Fatty acid esters [amount / g] method 6 0,2 Water0 Hydroquinone 20 Isopropylidene glycerin 80 - 1 7 1,0 Water 10 Hydroquinone 40 Butyl glycol 50 - 2 8 0,2 Water 1.0 Hydroquinone 20 Butyl glycol 80 - 2 9 0,2 Water 1.0 Brenz Catechesis 20 Butyl glycol 80 - 2 10 0,2 Water 1.0 BHT20 Butyl glycol 80 - 2 11 0,2 Water 1.0 BKF20 Butyl glycol 80 - 2 12 0,2 Water 1.0 TBHQ20 Butyl glycol 80 - 2 13 0,2 Water 1.0 Gallic acid 15 Butyl glycol 85 - 2 14 0,2 Water 1.0 Propyl gallate 20 Butyl glycol 80 - 2 15 0,2 Water 1.0 Hydroquinone 20 Butylglycol 60 Biodiesel20 2 16 0,2 Water 1.0 Hydroquinone10TBHQ20 Butyl glycol 70 - 2 17 0,2 Water0 Hydroquinone 10TBHQ30 Isopropylidene glycerin 60 1 18 0,2 Water0 Hydroquinone 10TBHQ 30Citric acid 1.5 Isopropylidene glycerin 60 1 19 0,2 Water0 Hydroquinone 10TBHQ30 Isopropylidene glycerin 40 Biodiesel20 1 Nr. L-Ascorbic acid [amount g / g] Solvent [amount g / g] Phenolic antioxidant [amount g / g] Solvent [amount g / g] Fatty acid esters [amount / g] method 20 0,2 Water0 Hydroquinone 10TBHQ 30Citric acid 1.5 Isopropylidene glycerin 40 Biodiesel20

[0076] It was found that the formulations containing L-ascorbic acid had a significantly longer shelf life than the same formulation without L-ascorbic acid. For example, in the formulation according to Example 1, when stored in an air-permeable polypropylene or polyethylene bottle at temperatures of 20 to 25 °C, a hydroquinone content of 20% since its manufacture could still be detected in the composition after 6 months.

[0077] In contrast, in the same composition from Example 1, but without the addition of L-ascorbic acid, a hydroquinone content of only 12% g / g was found after 6 months of storage. The remaining hydroquinone was converted to p-benzoquinone and other oxidized derivatives by air oxidation. This was clearly evident from the color of the formulation, which was a deep dark red after 6 months.

[0078] The advantageous properties of the compositions according to the invention were further proven by the following tests:

[0079] The efficacy of the formulations according to the invention was demonstrated using a Rancimat apparatus. The method is explained in detail in DIN EN 14214 (EN 14112 / EN 15751). Oxidation stability is expressed as the Oxidation Stability Index (OSI). DIN EN 14214 requires a minimum value of 6 hours.

[0080] The results are listed in Table 2: Table 2 Biodiesel produced from Composition of example number Dosage in ppm(g / g) OSI (hours) Waste fat 1 0 4,8 100 5,6 150 5,9 200 6,8 300 7,8 500 9,6 1.000 11,8 rapeseed oil 17 0 6,8 Soybean oil 500 12,7 [Rapeseed oil methyl ester 70%; Soybean oil methyl ester 30%] 750 15,7 1.000 17,7 2.000 25,3

[0081] Surprisingly, ascorbic acid has a synergistic effect. This is shown in Table 3. Table 3 Biodiesel produced from Composition of example number Dosage in ppm(g / g) OSI (hours) rapeseed oil 17 (but without ascorbic acid) 0 7,5 300 11,8 500 14,4 1.000 19,6 2.000 29,8 rapeseed oil 17 (with 0300 7,512,2 Ascorbic acid 500 16,0 1.000 23,3 2.000 32,7

[0082] Both formulations from Table 3 were stored in polyethylene bottles. Unlike glass, polyethylene is permeable to air. The formulations were stored for one month. Subsequently, the OSI values ​​were determined. The results are listed in Table 4. Table 4 Biodiesel produced from Composition of example number Dosage in ppm(g / g) OSI (hours) rapeseed oil 17 (but without ascorbic acid) 0 7,5 300 9,4 1.000 14,4 rapeseed oil 17 (with ascorbic acid) 0 7,5 300 9,7 1.000 19,6

[0083] According to the results in Table 4, the stabilization of biodiesel with a formulation containing ascorbic acid was better than the stabilization with a respective formulation without ascorbic acid, since in the formulation without ascorbic acid some of the phenolic antioxidant had already been oxidized by atmospheric oxygen.

[0084] This was also observed when determining the Hazen color index. The results are listed in Table 5: Table 5 Composition from example number 17 (but without ascorbic acid); Hazen color index 17 (with ascorbic acid); Hazen color index One day after production 206 210 One month after production 325 131 Example 6

[0085] Table 6 lists examples containing an amine antioxidant. The amine antioxidants were distilled prior to formulation. The compositions were prepared according to the procedure of Example 1. Table 6 Nr. L-Ascorbic acid [amount g / g] Amine antioxidant [amount g / g] Isopropylidene glycerin [amount g / g] Biodiesel [amount g / g] 21 0,4 CAS No. 793-24-8 80 20 22 0,4 CAS No. 101-72-4 70 30 23 0,8 CAS No. 101-96-2 60 40 24 0,4 CAS No. 101-96-2 60 10 TBHQ 30 25 0,4 CAS No. 101-96-2 30 40 30

Claims

[1] Composing at least ascorbic acid, an antioxidant and a glycerol acetal or ketal of general formula 1 where R1 and R2 independently mean: hydrogen or a C 1-10 - Alkyl group, or R1 and R2 together form a carbon chain with 2 to 5 carbon atoms, where the antioxidant a phenolic antioxidant or an amine antioxidant or a mixture of a phenolic and an amine antioxidant; and wherein the antioxidant is present in the composition at a concentration of 5 to 55 wt% and the ascorbic acid at a concentration of 0.1 to 5 wt%, based on the total weight of the composition (= 100 wt%). [2] Composition according to claim 1, wherein the compound of formula 1 is isopropylideneglycerol (CAS No. 100-79-8). [3] Composition according to claim 1 or 2, further comprising one or more of the following compounds: water; an alcohol of the general formula R3-OH, wherein R3 is a C 1-10 -alkyl group; or a glycol or a glycol ether of general formula 2 R4-O-CHR5 - CHR6-(O-CHR5 - CHR6) n -OR7 Formula 2 where R4, R5, R6 and R7 independently H, a C 1-10 - alkyl group, phenyl or benzyl, and n means an integer from 0 to 100. [4] Composition according to any one of claims 1 to 3, wherein the composition comprises ascorbic acid and the compound of formula 1 in a weight ratio of 1 : 50 to 1 : 1,000. [5] Composition according to any one of claims 1 to 4, wherein the composition comprises ascorbic acid and the compound of formula 1 in a weight ratio of 1 : 0.25 to 1 :

10. [6] Composition according to any one of claims 3 to 5, comprising a mixture of a glycerol acetal or glycerol ketal of formula 1 and a glycol ether of formula 2 in any mixing ratio. [7] Composition according to any one of claims 1 to 6, wherein the phenolic antioxidant is selected from the following group: BHT (CAS No. 128-37-0), TBHQ (CAS No. 1948-33-0), BKF (CAS No. 119-41-1), TBC (CAS No. 98-29-3), propyl gallate (CAS No. 121-79-9), gallic acid (CAS No. 149-91-7), pyrogallol (CAS No. 87-66-1), hydroquinone (CAS No. 123-31-9), catechol (CAS No. 120-80-9), bisphenol A (CAS No. 80-05-7), or two or more thereof. [8] Composition according to claim 7, wherein the composition comprises TBHQ. [9] Composition according to claim 8, wherein the composition comprises hydroquinone. [10] Composition according to any one of claims 1 to 9, wherein the amine antioxidant is selected from the group consisting of: N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (CAS No. 793-24-8), N-Isopropyl-N'-phenyl-p-phenylenediamine (CAS No. 101-72-4), N,N-Di-sec-butyl-p-phenylenediamine (CAS No. 101-96-2), N'-Bis(1,4-dimethylpentyl)-p-phenylenediamine (CAS No. 003081-14-9), N-Isopropyl-N'-phenyl-p-phenylenediamine (CAS No. 101-72-4) and N,N'-Di-sec-butyl-p-phenylenediamine (CAS No. 101-96-2), or two or more thereof. [11] Composition according to any one of claims 1 to 10, further comprising one or more compounds from the group consisting of: dyes; inorganic or organic acids, in particular citric acid; heavy metal inhibitors, in particular an aryltriazole or a salicylidenediamine; and biocides. [12] Method for improving the oxidation resistance of a fatty acid ester, comprising step (C): (C) Contacting a fatty acid ester with a composition as defined in any one of claims 1 to 11. [13] Method according to claim 12, wherein the fatty acid ester is a fatty acid methyl ester. [14] Method according to claim 12 or 13, wherein the fatty acid ester is a biodiesel. [15] Composition comprising at least one fatty acid ester and a composition as defined in any one of claims 1 to 11. [16] Composition according to claim 15, wherein the fatty acid ester is a fatty acid methyl ester. [17] Composition according to claim 15 or 16, wherein the fatty acid ester is a biodiesel.