TECHNICAL FIELD

The present invention relates to a method for chemically modifying a specific polymeric part in order to give it flame-retardant properties or to improve the latter, said method involving a covalent reaction with at least one specific flame-retardant compound. The invention also relates to specific flame-retardant polymeric parts.

Generally, the flame-retardant properties of a polymeric part can be modified or improved in different ways such as for example:

- the addition of one or more organic or inorganic flame-retardant charges to the polymer powder before the formation of a composite material (for example, by extrusion, spinning, fusion, thermoforming), however with the possibility that the presence of charges may have a negative effect on the properties of the polymer that are not meant to be modified, all the more so since, in order to be effective, the charges have to be present in a mass content of above 30%, this addition proving to be negative for parts manufactured by 3D printing, fusion phenomena, which are characteristic of this technology, then being inhibited, resulting in major defects in the printed parts; or

-the impregnation of the polymer with one or more chemical agents making it possible to impart or improve the targeted property but with the following disadvantage(s):

*the impregnation only results in surface treatment and does not allow penetration of the part in depth, the targeted property thus only being located on the surface of the part;

*the impregnation does not allow a strong fixing of the chemical agent(s), the targeted property conferred by this/these agent(s) not having satisfactory durability.

In view of the above, the authors of the present invention propose to develop polymeric parts with flame-retardant properties by chemically grafting a specific flame-retardant compound, said flame-retardant properties being maintained after ageing in specific temperature and humidity conditions and also having mechanical properties that are maintained after ageing in specific temperature and humidity conditions.

DISCLOSURE OF THE INVENTION

Thus, the invention relates to a method for chemically modifying a polymeric part in order to give it flame-retardant properties or to improve them, said method comprising a step of covalent reaction of a polymeric part, comprising at least one polymer, comprising, as reactive groups, -NH-CO- amide groups and/or -OH hydroxyl groups, with a flame-retardant compound from the family of alkylphosphonic or arylphosphonic dihalides, said compound reacting with all or a portion of said reactive groups.

A polymeric part is defined, generally, as a part made from a material comprising at least one polymer comprising, as reactive groups, -NH-CO- groups and/or hydroxyl groups, the polymer(s) being shaped into the part, for example by a shaping technique such as 3D printing (for example the specific MJF technique corresponding to the abbreviation Multi Jet Fusion), the extrusion/injection technique, the additive manufacturing technique, the method of the invention thus being able to form part of the manufacturing cycle of a part at the post-processing stage (i.e. the stage of finishing the part after shaping).

A flame-retardant compound is understood to be a compound capable of conferring flame-retardant properties, said compound being from the family of alkylphosphonic or arylphosphonic dihalides, which may correspond to the following general formula (I): wherein:

-X ¹ and X ² represent, independently of one another, a halogen atom;

-R represents an alkyl group or an aryl group.

More specifically, X ¹ and X ² can both represent a chlorine atom, in which case the compound corresponds to an alkylphosphonic or arylphosphonic dichloride compound.

Preferably, R represents an alkyl group, more specifically, an alkyl group comprising 1 to 6 carbon atoms, preferably 1 to 4 carbon atoms, for example an ethyl group.

A specific compound corresponding to these specific details is ethylphosphonic dichloride corresponding to the following formulae (II):

When R is an aryl group, it may represent a phenyl group.

The alkylphosphonic or arylphosphonic dihalide compounds react with -NH-CO groups and/or hydroxyl groups of the polymer with the formation of an HX acid (X representing a halogen atom emanating from said compounds) and are thus chemically grafted covalently to the polymer, thus imparting in a robust manner flame-retardant properties to the polymer. This reaction falls into the category of nucleophilic substitution reactions.

The covalent reaction step can be performed in liquid phase, which means that the reaction step comprises an operation of contacting the polymeric part with a liquid solution comprising the flame-retardant compound (this solution may consist exclusively of said compound, when this exists in liquid form), this contacting may be carried out by any impregnation techniques, such as the dripping or dip-coating technique. The contacting operation can be carried out at room temperature followed by a drying operation at a temperature of 120°C for several hours, when the flame-retardant compound is ethylphosphonic dichloride.

The covalent reaction step can also be carried out advantageously in gaseous phase, which assumes that the flame-retardant compound is capable of existing in this gaseous state in the reaction conditions.

In particular, due to the reaction involving a flame-retardant compound in gaseous form for chemically modifying the polymeric part, the following advantages have been found:

-the improvement of the flame-retardant properties of the polymeric part, regardless of the processing or shaping technique used to obtain the part, without affecting the shape, the geometry and the finishing details of the polymeric part;

-the possibility of carrying out said modification by limiting the quantity of flame-retardant compound used (for example, less than 5% by mass);

-the possibility of minimising the quantity of flame-retardant compound used and functionalising if necessary all the complex reliefs of the part;

- the fixing of the flame-retardant compound, in a robust manner, from the fact that the functionalisation of the polymeric part by this compound consists of a chemical grafting of the part by this compound;

-the versatile nature of the method, which allows the functionalisation of the part(s) regardless of the geometry of the latter and the level of detail.

Furthermore, the method of the invention may have the following advantages:

-an easily industrialisable method including a small number of steps, not generally requiring large quantities of products and allowing the simultaneous treatment of several parts;

-no prior preparation of the surface of the parts to be treated;

-the possibility of treating all the complex reliefs of the parts as necessary.

The polymeric part to be treated according to the method of the invention is a part comprising (or consisting exclusively of) at least one polymer comprising, as reactive groups, -NH-CO- groups and/or hydroxyl groups, -NH-CO- and/or hydroxyl groups reacting in a covalent manner with the flame-retardant compound.

In particular, the polymeric part to be treated according to the method of the invention can be a part comprising (or consisting exclusively of) one or more polymers comprising, as reactive groups, -NH-CO- groups, this or these polymers able to be one or more polyamides. Even more specifically, the polymeric part may be a polyamide-12 part. In particular, the part may a porous or partially porous polyamide-12 and, even more specifically, a polyamide-12 having a densityof less than or equal to 960 kg/m ³ , for example ranging from 650 kg/m ³ to 960 kg/m ³ , preferably less than or equal to 900 kg/m ³ , for example ranging from 700 kg/m ³ to 900 kg/m ³ .

Furthermore, the polymeric part may comprise one or more inorganic charges, for example silica beads with a diameter of between 10 pm and 200 pm.

The reaction step is advantageously carried out exclusively in the presence of the polymeric part and of the flame-retardant compound and in particular in the absence of organic solvent(s).

When the reaction step is performed in gaseous phase, it is in particular performed at a temperature and pressure necessary for maintaining the flame-retardant compound in the gaseous state and for the reaction between the flame-retardant compound and the polymer(s) of the polymeric part.

More specifically, for the implementation of the reaction step in gaseous phase, for a given flame-retardant compound belonging of the compounds from the family of alkylphosphonic or arylphosphonic dihalides, the man skilled in the art can easily choose a pair of temperature and pressure, for which the flame-retardant compound is in the gaseous state, with, of course, a temperature at which the polymeric part remains mechanically stable (for example, a temperature at most equal to 150°C for a PA-12 part), the amount of the flame-retardant compound being advantageously chosen so as to obtain a charge rate (in mass %) of flame-retardant compound allowing the polymeric part thus modified to present, advantageously, a V-0 grade (for example, a charge rate of at least 1.1 % when the flame-retardant compound is ethylphosphonic dichloride and the polymeric part is PA-12).

The temperature and pressure couples for a flame-retardant compound can be determined from a temperature-pressure curve illustrating the evolution of the temperature (in °C) as a function of the pressure (in bar), when flame-retardant compound is in a gaseous state.

For example, when the flame-retardant compound is ethylphosphonic dichloride, the temperature and pressure couples can be chosen from the following couples: (58.7°C ; 0.0079 bar) ; (63.1°C ; 0.01 bar) ; (67.5°C ; 0.0127 bar) ; (76.2°C ; 0.0216 bar) ; (90.5°C ; 0.0391 bar) ; (102.1°C ; 0.0674 bar) ; (113.6°C ; 0.1008 bar) ; (126.6°C ; 0.1709 bar) ; (141.8°C ; 0.2722 bar) ; (149.5°C ; 0.3517 bar).

When the flame-retardant compound is methylphosphonic dichloride, the temperature and pressure couples can be chosen from the following couples: (36. 8°C ; 0.0079 bar) ; (41°C ; 0.01 bar) ; (45°C ; 0.0127 bar) ; (53.3°C ; 0.0216 bar) ; (67°C ; 0.0391 bar) ; (78°C ; 0.0674 bar) ; (89°C ; 0.1008 bar) ; (101.4°C ; 0.1709 bar) ; (116.5°C ; 0.2722 bar) ; (123.3°C ; 0.3517 bar) ; (133°C ; 0.4585 bar) ; (145.3°C ; 0.6311 bar).

When the flame-retardant compound is n-propylphosphonic dichloride, the temperature and pressure couples can be chosen from the following couples: (47.1°C ; 0.0079 bar) ; (51.4°C ; 0.01 bar) ; (55.7°C ; 0.0127 bar) ; (64.2°C ; 0.0216 bar) ; (78.1°C ; 0.0391 bar) ; (89.3°C ; 0.0674 bar) ; (100.6°C ; 0.1008 bar) ; (113.2°C ; 0.1709 bar) ; (128.1°C ; 0.2722 bar) ; (135.7°C ; 0.3517 bar) ; (145.5°C ; 0.4585 bar).

When the flame-retardant compound is phenylphosphonic dichloride, the temperature and pressure couples can be chosen from the following couples: (112.2°C ; 0.0079 bar) ; (117.2°C ; 0.01 bar) ; (122.3°C ; 0.0127 bar) ; (132°C ; 0.0216 bar) ; (148.2°C ; 0.0391 bar).

From the point of view of its implementation, the step of covalent reaction, when performed in gaseous phase, may include the following operations:

-an operation of placing the polymeric part in a first reactor, the reactor being heated to the temperature at which the method is carried out;

-an operation of pressurising the reactor to the pressure at which the method is carried out; -an operation of vaporising the flame-retardant compound(s) in a second reactor connected to the first reactor, this vaporisation operation being, for example, carried out at a temperature of 110°C and a pressure of 100 mbar, when the flameretardant compound is ethylphosphonic dichloride;

-an operation of injecting the flame-retardant compound(s) in the gaseous state into the first reactor;

-an operation of maintaining the temperature and pressure of implementation of the method until the reaction is completed.

At the end of the reaction step, the polymeric parts are thus chemically modified and are covalently bonded to (or covalently grafted to) residues of the flameretardant compound (the residues being what remains after they have reacted with the -NH-CO- groups and/or -OH hydroxyl groups of the polymer(s) of the polymeric part).

After the reaction step, the polymeric part modified in this way can be subjected to drying, for example, by heating or under vacuum.

The invention also relates to a polymeric part which can be obtained by the method of the invention as defined above, with at least one polymer comprising:

*groups corresponding to at least one of the following formulae (III) and (IV):

(III) (IV) and optionally -NH-CO- groups; and/or

*groups corresponding to one of the following formulae (V) and (VI):

(V) (VI) and optionally -OH groups;

R and X forformulae (III), (IV), (V) and (VI) being as defined above, namely R representing an alkyl or aryl group and X representing a halogen atom.

It is understood that the groups of formulae (III) and (IV) are groups resulting from the reaction of the -NHCO- groups of the initial polymer(s) with an alkylphosphonic or arylphosphonic dihalide compound, the possible presence of -NHCO- groups occurring when the alkylphosphonic or arylphosphonic dihalide compound does not react with all of the -NHCO- groups of the initial polymer(s). The bracket at the bond of the phosphorus atom indicates that it is bonded to another part of the polymer (for example by reaction of another -NHCO- group with the -P-X group) or another polymer chain.

It is understood that the groups of formulae (V) and (VI) are groups resulting from the reaction of -OH groups of the initial polymer(s) with an alkylphosphonic or arylphosphonic dihalide compound, the possible presence of -OH groups occurring when the alkylphosphonic or arylphosphonic dihalide compound does not react with all of the -OH groups of the initial polymer(s). The bracket at the bond of the phosphorus atom indicates that it is bonded to another part of the polymer (for example by reaction of another -OH group with the -P-X group) or to another polymer chain.

Polymers which can be used to form the polymeric parts of the invention are in particular polymers comprising groups corresponding to at least one of the following formulae (III) and (IV):

(III) (IV) and optionally -NH-CO- groups, R and X being as defined above, it being possible for these polymers to be derived from the reaction of an initial polymer from the polyamide family with an alkylphosphonic or arylphosphonic dihalide compound.

Even more specifically, polymers according to the invention are in particular polymers comprising groups corresponding to at least one of the following formulae (III) and (IV):

(III) (IV) and comprising -NH-CO- groups,

R and X being as defined above.

In particular, R, when it is an alkyl group, may represent an ethyl group and X a chlorine atom.

Other advantages and features of the invention will become apparent from the non-limiting description below.

BRIEF DESCRIPTION OF THE DRAWINGS

[Fig. 1] is a diagram illustrating a device for performing the method of the invention. DETAILLED DESCRIPTION OF PREFERRED EMBODIMENTS

EXAMPLE 1

This example illustrates the implementation of a specific mode of the chemical modification method of the invention consisting of a chemical modification of a polyamide-12 part, so as to improve its flame-retardant properties with a flame-retardant compound: ethylphosphonic dichloride.

The reaction of polyamide-12 with the flame-retardant compound flameretardant mentioned above can be represented by the following reaction scheme: the other chlorine atoms can also be engaged in a nucleophilic substitution reaction with other -NH groups of polyamide-12.

The method was carried out in a device illustrated schematically in the accompanying Figure 1.

The polyamide-12 sample to be treated (reference 1) is suspended in the deposition reactor 3, hermetically sealed, and magnetically agitated, heated previously to the treatment temperature, then the latter is pressurised by drawing a vacuum of up to 40 mbar by a vacuum pump 5 by opening the valve 7. Once the desired pressure is reached, the valve 7 is closed in order to keep the deposition reactor 3 under vacuum and isolated. In an adjacent reactor 9 connected to the deposition reactor 3, a known quantity of flame-retardant compound 11 is injected, at a temperature such that the latter is preheated or even in a gaseous state in order to facilitate its vaporisation.

When the temperature and pressure conditions allow the flameretardant to be maintained in its vapour form, then the valve 13 between the deposition reactor3 and the adjacent reactor9 is opened. The compound isthen vaporised in the pipe 15 introduced into the deposition reactor 3 and as a result of the pressure difference between the two reactors 3 and 9.

The flame-retardant compound is then introduced into the deposition reactor 3 via an injection nozzle 17 connected to the pipe 15. A plate 19 forming a physical barrier is located above the injection nozzle 17 to avoid any liquid projection of the flameretardant compound onto the sample to be treated (possibly its condensation) between the flame-retardant compound and the sample to be treated thus takes place.

At the end of the processing time (1 to 30 minutes), purge cycles are performed in the reactor to recover excess unreacted flame-retardant compound. The pressure is broken then the treated sample is removed and placed in an oven (5 minutes to 1 hour), in order to completely remove the unreacted flame-retardant.

More specifically, three tests were carried out with the operating conditions specified in the table below.

For clarification, the charge rate (in mg) corresponds to the quantity of flame-retardant compound deposited on the sample, while the charge rate (% by mass) corresponds to the mass ratio of the quantity of flame-retardant compound deposited on the total mass of the sample after treatment.

For these different tests, a characterisation by spectroscopic analysis (XPS and ToF SIMS) of the treated samples made it possible to demonstrate and confirm the formation of covalent bonds between the polymer and the flame-retardant compound. Indeed, there is a modification of the XPS spectrum of the N element obtained in the treated polyamide-12. The additional ToF-SIMS analyses confirmed the formation of -N P(O)- groupings.

Consequently, these characterisations confirm that this post treatment is not a simple surface deposit. It is really a covalent chemical grafting of the flame-retardant compound onto the PA-12. This feature makes the deposit much more robust. Its adhesion to the PA-12 is then very strong.

For these various tests, flame tests were also carried out to determine whether the samples resulting from these tests (with a length of 125 mm, a width of 13 mm and a thickness of 5 mm) belonged to the fire classes V-0, V-l, V-2 according to a test representative of the standard UL94V. By multiple ignition both the residual burning and flaming time and the flow of ignited drips from the sample are evaluated.

The results obtained correspond to V-0, i.e. the results:

-a residual burning time after ignition of less than 10 seconds;

-a residual burning time after the second ignition of less than 10 seconds;

-no flow of ignited drips;

-no complete burning of the samples.

In addition, a comparative fire test was carried out with:

-a polyamide-12 part treated with diphosphoryl chloride according to a protocol defined above (with a charge rate of 140 mg, i.e. 1.7 %);

-an untreated polyamide-12 part. On observation, it appears that, unlike the untreated part, the treated part has very low (even zero) ignition times during the 2 ignitions. In addition, no flaming drip likely to ignite the cotton was observed with the treated part.

The parts treated with ethylphosphonic dichloride were subjected to temperature and hydrolysis conditions (typically 7 days at 70°C under 95% relative humidity). The parts have an identical appearance to that of the parts before "ageing". Indeed, no "sticky" aspect was identified.

In comparison, polyamide-12 parts treated in a similar way with trimethylsilylchlorosulfonate (with a charge rate of in the order of 1% by mass) appear sticky after being subjected to temperature and hydrolysis conditions (more specifically, 7 days at 70°C at 95% relative humidity), which attests to a decrease in mechanical properties. Without being bound by theory, this decrease could be due to a hydrolysis of the flame-retardant compound grafted onto the polyamide-12, this hydrolysis being materialised by the breaking of -N-S- covalent bonds due to the acidic environment generated. It also results in a decrease in flame-retardant performance as the parts only have a V-2 grade instead of a V-0 grade.

EXEMPLE 2

This example illustrates the implementation of a specific mode of the chemical modification method of the invention consisting of a chemical modification of a polyamide-12 part, so as to improve its flame-retardant properties with a flame-retardant compound: ethylphosphonic dichloride.

This example is carried out under conditions similar to those of Example 1 with, in particular, the following specific conditions:

-Introduction of 2 samples into the deposition reactor with respective initial masses: 8.4422 g and 7.9032 g ;

-Temperature in the deposition reactor : 120°C ;

-Volume of the flame-retardant compound injected into the reactor : 3 mL ;

-Initial pressure in the deposition reactor : 11 mbar ; -Pressure in the deposition reactor at the start of the injection : 60 mbar ;

-Final pressure in the deposition reactor at the end of the injection : 103 mbar ;

-Duration of contact : 2 minutes.

At the end of the duration of contact, the samples are dried at 120°C for 10 hours.

At the end of the process, the two samples respectively have a mass of 8.5549 g and a mass of 8.0126 g, that is to say a charge rate (in mass %) of 1.3% and 1,4% respectively.

For these various tests, flame tests were also carried out to determine whether the samples resulting from these tests (with a length of 125 mm, a width of 13 mm and a thickness of 5 mm) belonged to the fire classes V-0, V-l, V-2 according to a test representative of the standard UL94V. It appears from these tests that the two samples belong to fire class V-0.

EXAMPLE 3

This example illustrates the implementation of a specific mode of the chemical modification method of the invention consisting of a chemical modification of a polyamide-12 part, so as to improve its flame-retardant properties with a flame-retardant compound: ethylphosphonic dichloride.

This example is carried out under conditions similar to those of Example 1 with, in particular, the following specific conditions:

-Introduction of 4 samples into the deposition reactor with respective initial masses: 8.3995 g, 8.5205 g ; 8.4622 g and 8.6278 g ;

-Temperature in the deposition reactor : 120°C ;

-Volume of the flame-retardant compound injected into the reactor : 2 mL ;

-Initial pressure in the deposition reactor : 13 mbar ;

-Pressure in the deposition reactor at the start of the injection : 52 mbar ; -Final pressure in the deposition reactor at the end of the injection : 88 mbar ;

-Duration of contact : 2 minutes.

At the end of the duration of contact, the samples are dried at 120°C for 10 hours.

At the end of the process, the four samples respectively have a mass of 8.4762 g, a mass of 8.5998 g, a mass of 8.538 g and a mass of 8.7046 g, that is to say a charge rate (in mass %) of 0.9% for each of these samples.

For these various tests, flame tests were also carried out to determine whether the samples resulting from these tests (with a length of 125 mm, a width of 13 mm and a thickness of 5 mm) belonged to the fire classes V-0, V-l, V-2 according to a test representative of the standard UL94V. It appears from these tests that the four samples belong to fire class V-0.

EXAMPLE 4

This example illustrates the implementation of a specific mode of the chemical modification method of the invention consisting of a chemical modification of a polyamide-12 part, so as to improve its flame-retardant properties with a flame-retardant compound: ethylphosphonic dichloride, this specific mode being carried out in liquid phase.

To do this, a sample of PA-12 with a mass of 4.5612 g is immersed in a test tube filled with the flame-retardant compound at room temperature and atmospheric pressure for a period of 10 to 30 seconds. After removing the sample from the tube, the sample is dried at 120°C for 10 hours. The sample has a mass of 4.7477 g, i.e a charge rate (in mass %) of 3.9%.

Flame tests were also carried out to determine whether the sample resulting from these tests (with a length of 125 mm, a width of 13 mm and a thickness of 5 mm) belonged to the fire classes V-0, V-l, V-2 according to a test representative of the standard UL94V. It appears from these tests that the sample belongs to fire class V-0.