WO2014075289A1

WO2014075289A1 - Thermoplastic polymer composition comprising an alkali metal hypophosphite salt

Info

Publication number: WO2014075289A1
Application number: PCT/CN2012/084734
Authority: WO
Inventors: Junli Li; Gary Woodward
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2012-11-16
Filing date: 2012-11-16
Publication date: 2014-05-22
Anticipated expiration: 2015-05-16
Also published as: EP2920234A4; EP2920234A1; CN104781326A

Abstract

A composition comprising at least a thermoplastic polymer matrix, an alkali metal hypophosphite salt, and 0.1-30wt% of an organic dicarboxylic acid or a salt thereof, having a first pkA≤5; based on the weight of the alkali metal hypophosphite salt; and a method for preparing such composition.

Description

THERMOPLASTIC POLYMER COMPOSITION COMPRISING AN ALKALI METAL HYPOPHOSPHITE SALT

The present invention concerns compositions comprising at least a thermoplastic polymer matrix, an alkali metal hypophosphite salt, and an organic dicarboxylic acid or a salt thereof, having a first pkA < 5; based on the weight of the alkali metal hypophosphite salt; and method to prepare such compositions. PRIOR ART

Calcium hypophosphite is known and reported in the industry and the academy as a good flame retardant (FR) for plastics. However, even if calcium hypophosphite has brought a lot of interest no industrial application in thermoplastics are reported today because of its unpredictable thermal stability that can generate phosphine and catch fire.

US20070082995 and US2010160523A claim the use of hypophosphite salts as FR for polyesters and polyamide. In the second patent application they claim the use of additives such as inorganic hydrates and/ organic salt to improve the stability of plastic but looking at the results it looks like that in several instances they burn the plastic during the compounding which could be due to hypophosphite salts make the crossHnking or degradation of polymer in the thermal processing

Moreover, US20070173572 Al & US20050137297 Al claim the use of hypophosphite salts and calcium hypophosphite in particular but in the first patent addition of some additives to react with any phosphine that could be generated is described. These publications state that calcium hypophosphite is not stable and that the decomposition generates some phosphine, their invention is about mopping the phosphine but does not address the stability itself.

Overall, hypophosphite salts have been claimed to be good to excellent flame retardants for a wide range of plastics but so far they have not been successfully commercialized. There is then a need to develop thermoplastic formulations comprising hypophosphite salts as FR additives having an improved melt stability to decrease variations of its viscosity.

INVENTION

It appears now that the use in combination of an alkali metal hypophosphite salt and an organic dicarboxylic acid or a salt thereof, having a first pkA < 5 within a thermoplastic polymer matrix permits to avoid deficiencies of the prior art and notably enables a thermoplastic composition that avoids an increase in viscosity and related crosslinking of the thermoplastic matrix. The combination of the present invention also permits a low variation in the viscosity of the thermoplastic resin, while in comparison the use of other salts of organic acids leads to a drop of matrix integrity, and then also a drop of mechanical properties. Moreover, it appears then that the use of an alkali metal hypophosphite salt and a FR additive may lead to increase the viscosity of the thermoplastic matrix and the related crosslinking of the resin; while this observed behavior can clearly be avoided by the presence of the organic dicaboxylic acid or salt of the present invention.

The invention then provides a convenient method which avoids obtaining a crosslinked polymer as a resul of the use of an alkali metal hypophosphite salt, such as calcium hypophosphite, within the thermoplastic polymer matrix. The invention then improves the processing stability of alkali metal hypophosphite salt formulation at high temperatures, notably around 300°C. This invention can also protect alkali metal hypophosphite salt in thermal processing and reduce the PH₃ generated from alkali metal hypophosphite salt decomposition. The invention provides a way to improve the stability of alkali metal hypophosphite salt in polymer as well as a unique way to measure and track formulation behavior. These two results combined allow a safe use of alkali metal hypophosphite salt as a halogen free flame retardant with a quicker method for formulation development . The present invention first concerns a composition comprising at least :

(a) a thermoplastic polymer matrix,

(b) an alkali metal hypophosphite salt, and

(c) 0.1 to 30 wt % of an organic dicarboxylic acid or a salt thereof, having a first pkA < 5; based on the weight of the alkali metal hypophosphite salt (b).

The present invention also concerns a method to prepare a composition by blending at least :

(a) a thermoplastic polymer matrix,

(b) an alkali metal hypophosphite salt, and

DETAILS OF THE INVENTION

(a) Thermoplastic polymer matrix

Typically, the thermoplastic polymer present in composition of the invention is selected from the group consisting in polyethylene, polypropylene, polyphenylene ethers; polyamides, especially PA66, PA6, PA6.10, high temperature polyamides such as PPA, PA4.6, PA9T, PA66.6T, PAIOT, PA6.6T and blends of polyamides, such as PA/PET, PA/ABS or PA/PP; polyesters as polybutylene terephthalate (PBT); polycarbonates; epoxy resins; phenolic resins; acrylonitrile butadiene styrene (ABS); styrene acrilonitrile (SAN); mixtures of high impact polystyrene (HIPS) and polyphenylene ethers such as PPO/HIPS; styrene butadiene rubber and lattices (SB and SB); expandable polystyrene (EPS); halogenated polymers such as polyvinyl chloride (PVC), and mixtures and blends of these polymers.

Polyamides are preferably chosen among PA66, PA6, PA6.10, PA 10.10, PA11 and PA12, and high temperature polyamides such as PPA, PA4.6, PA9T, PA66.6T, PA10T, PA6.6T and blends of polyamides, such as PA/PET, PA/ABS or PA/PP.

Preferred weight proportion of thermoplastic polymer (a) is comprised between 40% and 90%, based on the total weight of the composition, more preferably between 30% and 60%.

(b) Alkali metal hypophosphite salt

Preferred weight proportion of alkali metal hypophosphite salt (b) is comprised between 1 and 30 wt %, based on the total weight of the composition, more preferably between 8% and 25%.

The hypophosphite salt present in the composition according to the invention is preferably of the formula (1) below : O

Η,Ρ O M

n

(1)

wherein :

n is 1 , 2 or 3 ; and

M is a metal selected from the group consisting alkali metal, alkaline earth metal, aluminium, titanium and zinc. Preferably, M is calcium or aluminium.

The hypophosphite salt may be heat stabilized so as that when it is heated during 3 hours at 298K under a flow of argon flushing at rate 58 mL/mins, it generates less than 0.5 mL of phosphine per gram of hypophosphite salt.

The hypophosphite salt preferably includes and is advantageously a calcium hypophosphite. Whatever its exact nature, the hypophosphite salt present in the compositions of the invention is so heat stabilized that ,when it is heated during 3 hours at 298°C under a flow of argon flushing at rate 58 mL/min, it generates less than 0.5 mL of phosphine per gram of hypophosphite salt. Preferably according to this test it generates less than 0.1, more preferably less than 0.05, particularly less than preferably less than 0.02 mL of phosphine per gram of calcium hypophosphite. The heat stability of the hypophosphite salt at 298 °C may especially be tested by using a Gastec tube to detect P¾, as illustrated in the appended examples.

The hypophosphite salts and, especially, calcium hypophosphite, can be prepared for example from white phosphorus (P₄) reacted under alkaline conditions with calcium hydroxide or calcium oxide and water as taught by US 5,225,052. It is also possible to obtain calcium hypophosphite by reaction of a calcium salt or simply from lime as taught by Chinese patent CN101332982, with hypophosphorous acid. For example the lime suspension is simply neutralized with hypophosphorous acid, the impurities are removed by filtration and the product isolated in a same way as previously described. It is also possible to obtain calcium hypophosphite from other metallic hypophosphites or the acid by ion exchange process. The process for stabilizing the starting hypophosphite salt which is useful for preparing the polymer composition of the invention can be batch, continuous or semi-continuous and be performed in a close or open system under inert atmosphere. That inert atmosphere can be for example carbon dioxide, argon, or nitrogen. The process for stabilizing the starting hypophosphite salt can be performed under atmospheric pressure, under pressure or under vacuum. The quality of the hypophosphite salts may be determined use of thermal analysis tools such as ARC (Adiabatic Reaction Calorimeter) and TGA (Thermal Gravimetric Analysis). The test can be carried out at any stage during the heating process described before.

Another way to check the quality of the heat stabilized hypophosphite salt used in the instant invention, is to perform a stability test at elevated temperature on the product, alone or mixed with plastic and measure the amount of phosphine generated during the test. It is also possible to measure the amount of phosphine generated when the product is compounded with plastics such as polyamide.

(c) Organic carboxylic acid

Preferred weight proportion of the organic dicarboxylic acid or a salt thereof (c) is comprised between 0.1% and 10%, based on the total weight of the composition, more preferably between 1 % and 7%.

The logarithmic constant, pKa, which is equal to -logio a, is also referred to the acid dissociation constant of the organic dicarboxylic acids of the present invention. This constant is classically measured in a water solution at a temperature of 25 C. For an organic dicarboxylic acid of the invention, the constant for dissociation of the first proton may be denoted as Kal and the constant for dissociation of successive protons as Ka2. Then, the first pKa as defined above corresponds to pKal .

Preferably the first pKa of the salt of the organic dicarboxylic acids (c) of the present invention is inferior or equal to 4.3, more preferably inferior or equal to 4.2 and well more preferably comprised between 0 and 4.2.

Preferred organic dicarboxylic acids (c) of the invention are chosen in the group consisting of oxalic acid (pKal=1.25), succinic acid (pkal=4.2), maleic acid (pKal=1.9), adipic acid (pKal=4.3) o-phthalic acid (pKal=2.98), and m-phthalic acid (pKal=3.7).

Salt of an organic dicarboxylic acids (c) may be for example alkali-metal or alkali-earth salts, such as sodium, magnesium, calcium, and/or potassium.

Organic dicarboxylic acid or salt thereof (c) may be used to coat the alkali metal hypophosphite salt (b) previously to the blend with the thermoplastic polymer matrix (a). It is also perfectly possible to blend the organic dicarboxylic acid or salt thereof (c), the alkali metal hypophosphite salt (b) and the thermoplastic polymer matrix (a) without a pre-blend of the organic dicarboxylic acid or salt thereof (c) and the alkali metal hypophosphite salt (b) In a general way, the coating of the organic dicarboxylic acid or salt thereof (c) on the alkali metal hypophosphite salt (b) may be carried out with different methods. As example the alkali metal hypophosphite salt (b) may be surface-coated by intimate contacting of the alkali metal hypophosphite salt (b) and the organic dicarboxylic acid or salt thereof (c), possibly in a solvent, such as water and/or alcohol, and eventually followed by filtering and/or drying of the product thus obtained.

Alternatively, the alkali metal hypophosphite salt (b) may be surface-coated by the organic dicarboxylic acid or salt thereof (c), by mixing them as dry powders. The alkali metal hypophosphite salt (b) may notably be surface-coated by the organic dicarboxylic acid or salt thereof (c) by mechanical grinding in a milling machine and optionally mixing the dry powders, notably in a slow or high speed mechanical mixer.

A binding agent may be used in the surface-coating process improve the adhesion of the coating compound to the surface of the alkali metal hypophosphite salt (b). Illustrative examples of binding agents are organic binders, such as synthetic or natural waxes, modified waxes, liquid hydrocarbons or epoxide resins. (d) Flame retardant additives

The composition of the present invention may also comprise one or several flame retardant additives. Different types of flame retardant additives may be used according to the invention. They can provide several mechanisms of function such as endothermic degradation, thermal shielding, dilution of gas phase, dilution of combustible portion, and radical quenching.

Flame retardant additives for polymer compositions are notably described in Plastics Additives, Gachter/Muller, Hansen, 1996, page 709 and passim. Useful Flame retardant additives are notably cited in the following patents: US6344158, US6365071, US6211402 and US6255371.

Flame retardant additives used in the composition of the instant invention are preferably chosen in the group comprising :

A) Phosphorous containing flame retardant additives, such as:

phosphine oxide such as for example triphenylphosphine oxide, tri-(3-hydroxypropyl) phosphine oxide and tri-(3-hydroxy-2-methylpropyl) phosphine oxide.

phosphonic acids and their salts, and phosphinic acids and their salts, such as for example phosphinic acid of zinc, magnesium, calcium, aluminium or manganese, notably aluminium salt of diethylphosphinic acid, aluminium salt of dimethylphosphinic acid, or zinc salt of dimethylphosphinic acid. cyclic phosphonates, such as diphosphate cyclic esters that is for example Antiblaze 1045.

organic phosphates such as triphenylphosphate.

inorganic phosphates such as ammonium polyphosphates and sodium polyphosphates.

red phosphorous, that can may be found under several shapes such as stabilized, coated, as a powder,

B) Nitrogen containing flame retardant additives, such as : triazines, cyanuric acid and/or isocyanuric acid, melamine or its derivatives such as cyanurate, oxalate, phtalate, borate, sulfate, phosphate, polyphosphate and/or pyrophosphate, condensed products of melamine such as melem, melam, melon, tris(hydroxyethyl) isocyanurate, benzoguanamine, guanidine, allanto^'ine and glycoluril.

C) Halogen containing flame retardant additives, such as:

Bromine containing flame retardant additives, such as polybromodiphenyl oxydes (PBDPO), brominated polystyrene (BrPS), poly(pentabromobenzylacrylate), brominated indane, tetradecabromodiphenoxybenzene (Saytex 120), ethane- l,2-bis(pentabromophenyl) or Saytex 8010 of Albemarle, tetrabromobisphenol A and brominated epoxy oligomers. Notably can be used the following compounds: PDBS-80 from Chemtura, Saytex HP 3010 from Albemarle or FR-803P from Dea Sea Bromine Group, FR-1210 from Dea Sea Bromine Group, octabromodiphenylether (OBPE), FR-245 from Dead Sea Bromine Group, FR- 1025 from Dead Sea Bromine Group and F-2300 or F2400 from Dead Sea Bromine Group.

Chlorine containing flame retardant additives, such as Dechlorane plus® from OxyChem (CAS 13560-89-9).

D) Inorganic flame retardant additives, such as antimony trioxide, aluminium hydroxide, magnesium hydroxide, cerium oxide, boron containing compounds such as calcium borate.

These compounds may be used alone or in combination. Charring agents and charring catalysts may also be used if necessary.

A composition according to the present invention may notably comprise:

1 to 20 % by weight of melamine,

1 to 20 % by weight of melamine cyanurate,

1 to 20 % by weight of melem, and

- 1 to 20 % by weight of red phosphorous, notably a masterbatch made of polymer and comprising red phosphorous. (e) Other additives

More generally, the composition according to the invention may also comprise additives normally used for the manufacture of polymer compositions, especially intended to be moulded. Thus, mention may be include plasticizers, nucleating agents, catalysts, light and/or thermal stabilizers, antioxidants, antistatic agents, colorants, pigments, matting agents, conductive agents, such as carbon black, moulding additives or other conventional additives. For the preparation of a polymer composition, the fillers and additives may be added to by any conventional means suitable, for instance during the polymerization or as a molten mixture. The additives are preferably added to the polymer in a melt process, in particular during a step of extrusion, or in a solid process in a mechanical mixer; the solid mixture may then be melted, for example by means of an extrusion process.

The compositions according to the invention may be used as raw material in the field of plastics processing, for example for the preparation of articles formed by injection-moulding, by injection/blow-moulding, by extrusion or by extrusion/blow-moulding. According to one customary embodiment, the modified polyamide is extruded in the form of rods, for example in a twin-screw extrusion device, said rods then being chopped into granules. The moulded components are then prepared by melting the granules produced above and feeding the molten composition into injection-moulding devices.

As articles obtained from the composition according to the invention mention may, for example, be made of articles in the motor vehicle industry, such as components under the engine hood, bodywork components, tubes and tanks, or articles in the electrical and electronics field, such as connecters.

The invention will now be further illustrated by the following examples.

EXPERIMENTAL PART EXAMPLE 1

CaHypo (102g) from Sinopharm Chemical Reagent Co, Ltd is charged in a reactor and mixed with water (161g). 50% hypophosphorous acid (34g) is then added slowly and the mixture is thoroughly stirred for 30 minutes and the pH is controlled between 4 and 6. Then, the slurry is filtered to afford 75 g of solid. This solid is washed with water (40 g) and then with acetone (75g). 57.8g of wet solid is thus obtained to finally afford 56g of dry CaHypo HT after evaporation of the volatiles under reduced pressure overnight at room temperature. EXAMPLE 2

Oxalic acid

103g CaHypo HT are dispersed into 400mL mixed solvent of EtQH and H₂0 (9:1 w/w) with mechanical stirring. And then 1.59g oxalic acid was added into resulted mixture. After addition, this mixture was heated to refluxing at 100°C for 4h under stirring. When cooled down to room temperature, solid was filtered out with EtOH washing and dried in vacuum oven to give 99g white powder. 1.8g dry product was mixed with 3g glass fiber and 5.2g PA66 26A supplied by Rhodia and then was put into microcompounder at 300°C with screw speed l OOrmp/min for 3mins. In this process, screw force changing was monitored along time. After compounding, matrix was taken out and the viscosity number was analyzed by method of international standared-ISO307.

Succinic acid

103g CaHypo HT are dispersed into 400mL mixed solvent of EtOH and H₂Q (9:1 w/w) with mechanical stirring. And then 2.08g succinic acid was added into resulted mixture. After addition, this mixture was heated to refluxing at 100°C for 4h under stirring. When cooled down to room temperature, solid was filtered out with EtOH washing and dried in vacuum oven to give 97g white powder.

1.8g dry product was mixed with 3g glass fiber and 5.2g PA66 26A and then was put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time. After compounding, matrix was taken out and the viscosity number was analyzed by method of international standared-ISO307. /w-phthalic acid and o-phthalic acid

103g CaHypo HT are dispersed into 400mL mixed solvent of EtOH and H₂O (9: 1 w/w) with mechanical stirring. And then 2.93g isophthalic acid was added into resulted mixture. After addition, this mixture was heated to refluxing at 100°C for 4h under stirring. When cooled down to room temperature, solid was filtered out with EtOH washing and dried in vacuum oven to give 98g white powder. 1.8g dry product was mixed with 3g glass fiber and 5.2g PA66 26A and then was put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time in order to determine the Initial Force (N) at tO and the Final Force (N) after 3 minutes. After compounding, matrix was taken out and the final viscosity number was analyzed by method of international standared-ISO307.

Adipic acid

103g CaHypo HT are dispersed into 400mL mixed solvent of EtOH and H₂0 (9: 1 w/w) with mechanical stirring. And then 2.58g adipic acid was added into resulted mixture. After addition, this mixture was heated to refluxing at 100°C for 4h under stirring. When cooled¹ down to room temperature, solid was filtered out with EtOH washing and dried in vacuum oven to give 98g white powder.

1.8g dry product was mixed with 3g glass fiber and 5.2g PA66 26A and then was put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time. After compounding, matrix was taken out and the viscosity number was analyzed by method of international standared-ISO307.

Results of properties are mentioned in Table 1. Table 1

Weight ratio : organic dicarboxylic acid or a salt thereof (c), based on the weight of the alkali metal hypophosphite salt (b).

It appears then that the use of CaHypo coated by the organic dicarboxylic acid of the present invention permits to obtain a thermoplastic composition that avoid an increase of the viscosity and the related crosslinking of the thermoplastic matrix.

EXAMPLE 3

Dry mixing method that with different ratio of salt

1.8g CaHypo HT was mixed with different ratio of calcium oxalate before formulating. Then the resulting salt mixture with 3g glass fiber and 5.2g PA66 26 A was put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time.

Results are mentioned in Table 2.

Table 2

EXAMPLE 4

Dry mixing method that with different calcium salts

1.8g CaHypo HT was mixed with several amount of calcium salt of different organic acid before formulating. Then the resulting salt mixture with 3g glass fiber and 5.2g PA66 26A was put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time.

Results are mentioned in Table 3.

Table 3

It appears then that the combination of the present invention permits to keep a low variation in the viscosity of the thermoplastic resin, while in comparison the use of other salts of organic acids leads to a drop of matrix integrity, and then also a drop of mechanical properties, even with lower proportions.

EXAMPLE 5

Dry mixing method used in formulation with Exolit OP1230

0.9g CaHypo HT was mixed with 10% calcium oxalate before formulating. Then the resulting salt mixture was formulated with 0.9g Exolit op 1230, 3g glass fiber and 5.2g PA66 26 A and then put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time.

0.9g CaHypo HT was mixed with 10% maleate calcium before formulating. Then the resulting salt mixture was formulated with 0.9g Exolit op 1230, 3g glass fiber and 5.2g PA66 26A and then put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time.

0,9g CaHypo HT was mixed with 10% succinate calcium before formulating. Then the resulting salt mixture was formulated with 0.9g Exolit op 1230, 3g glass fiber and 5.2g PA66 26A and then put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time. 0.9g CaHypo HT was mixed with 10% phthalate calciumbefore formulating. Then the resulting salt mixture was formulated with 0.9g Exolit opl230, 3g glass fiber and 5.2g PA66 26A and then put into microcompounder at 300°C with screw speed lOOrmp/min for 3mins. In this process, screw force changing was monitored along time,

0.9g CaHypo HT was mixed with 0.9g Exolit op 1230, 3g glass fiber and 5,2g PA66 26A and then put into microcompounder at 300°C with screw speed 1 OOrmp/min for 3mins. In this process, screw force changing was monitored along time.

Results are mentioned in Table 4

Table 4

It appears then that the use of CaHypo and Exolit FR additive leads to increase the viscosity of the thermoplastic matrix and the related crosslinking of the resin. This behavior can clearly be avoided by the presence of the organic dicaboxylic acid of the present invention.

EXAMPLE 7

Dry mixing method used in formulation with MCA in PA6

1.3 g CaHypo HT was mixed with 5% wt of calcium oxalate before formulating. Then the resulting salt mixture was formulated with 0.97g melamine cyanurate (MCA), 3g glass fiber and 4.68g PA6 XS 1352 BL from Rhodia and then put into microcompounder at 280-300°C with screw speed lOOrmp/min. In this process, screw force changing was monitored along time.

Results are mentioned in Table 5

Table 5

It appears then that the use of the organic dicarboxylic acid of the invention permits to avoid crosslinking of the PA6 resin.

Claims

WHAT IS CLAIMED IS:

1. Composition comprising at least :

(a) a thermoplastic polymer matrix,

(b) an alkali metal hypophosphite salt, and

2. Composition according to claim 1 wherein the thermoplastic polymer matrix is selected from the group consisting in polyethylene, polypropylene, polyphenylene ethers; polyamides, especially PA66, PA6, PA6.10, high temperature polyamides such as PPA, PA4.6, PA9T, PA66.6T, PA10T, PA6.6T and blends of polyamides, such as PA/PET, PA/ABS or PA/PP; polyesters as polybutylene terephthalate; polycarbonates; epoxy resins; phenolic resins; acrylonitrile butadiene styrene; styrene acrilonitrile; mixtures of high impact polystyrene and polyphenylene ethers such as PPO/HIPS; styrene butadiene rubber and lattices; expandable polystyrene; halogenated polymers such as polyvinylchloride, and mixtures and blends of these polymers.

3. Composition of any of claim 1 to 2, wherein the weight proportion of thermoplastic polymer (a) is comprised between 40% and 90%, based on the total weight of the composition.

4. Composition of any of claim 1 to 3, wherein the weight proportion of alkali metal hypophosphite salt (b) is comprised between 1 and 30 wt %, based on the total weight of the composition.

5. Composition of any of claim 1 to 4, wherein the alkali metal hypophosphite salt (c) is calcium hypophosphite.

6. Composition of any of claim 1 to 5, wherein the hypophosphite salt is heat stabilized so as that when it is heated during 3 hours at 298K under a flow of argon flushing at rate 58 mL/mins, it generates less than 0.5 mL of phosphine per gram of hypophosphite salt.

7. Composition of any of claim 1 to 6, wherein the weight proportion of the organic dicarboxylic acid or a salt thereof (c) is comprised between 0.1 % and 10%, based on the total weight of the composition.

8. Composition of any of claim 1 to 7, wherein the first pKa of the salt of the organic dicarboxylic acids (c) of the present invention is inferior or equal to 4.3.

9. Composition of any of claim 1 to 8, wherein the organic dicarboxylic acids (c) are chosen in the group consisting of oxalic acid, succinic acid, maleic acid, adipic acid, o-phthalic acid, and m-phthalic acid.

10. Composition of any of claim 1 to 9, wherein the salt of an organic dicarboxylic acids (c) is alkali-metal or alkali-earth salt.

11. Composition of any of claim 1 to 10, wherein the composition comprises one or several flame retardant additive.

12. Composition of claim 11 , wherein the flame retardant additive is chosen in the group consisting of :

A) Phosphorous containing flame retardant additives,

B) Nitrogen containing flame retardant additives,

C) Halogen containing flame retardant additives, and

D) Inorganic flame retardant additives.

13. Method to prepare a composition by blending at least :

(a) a thermoplastic polymer matrix,

(b) an alkali metal hypophosphite salt, and

14. Method according to claim 13, wherein the organic dicarboxylic acid or salt thereof (c) is used to coat the alkali metal hypophosphite salt (b) previously to the blend with the thermoplastic polymer matrix (a).

15. Method according to claim 14 wherein the alkali metal hypophosphite salt (b) is surface-coated by intimate contacting of the alkali metal hypophosphite salt (b) and the organic dicarboxylic acid or salt thereof (c), possibly in a solvent.

16. Method according to claim 14 wherein the alkali metal hypophosphite salt (b) is surface-coated by the organic dicarboxylic acid or salt thereof (c) by mixing them as dry powders.