FR3035382B1 - PERFECTLY VALVED DOSING VALVE FOR DISTRIBUTING AN AEROSOL COMPRISING A VALVE ROD - Google Patents

Abstract

The metering dispensing valve (1) has a metering chamber (2) and a valve stem (3). The valve stem (3) is provided with an end disposed on the side of a reservoir (9) and an opposite dispensing end. The valve stem (3) is slidably mounted in the dosing chamber (2), sealingly through a first seal called a valve seal (7) and a second seal called a chamber seal (8). The valve stem (3) is molded from a material comprising a mixture of polybutylene terephthalate and silicone.

Description

Advanced metering valve for dispensing an aerosol comprising a valve stem

The present invention relates to a metering valve for dispensing an aerosol comprising a valve stem.

Valves are well known in the art. They find their main application with aerosol containers for the distribution of products in powder form using a propellant (gas under pressure). When metered, the valves generally comprise a valve body enclosing a metering chamber delimited axially by two annular seals, a top seal called the valve seal and a bottom seal called the chamber seal, and a valve stem movable in particular between a rest position and an actuation position. According to an example described in publication EP 0 858 420, the valve stem is urged by an elastic member towards its rest position, in which a stem shoulder formed on said valve stem rests on the lower surface of said seal. valve. To actuate the metering valve, the valve stem which slides in the valve body inside the annular seals is pressed up to its actuation position, in which a dose of the product is expelled. The elastic member then returns the valve stem to its rest position.

A problem which arises with the valve stems of the valves, in particular of the metering valves, relates in particular to the seal at the level of the valve seal, and the friction appearing when the valve stem slides through the seals.

A solution to these drawbacks has been provided in European patent EP 0 858 420 by proposing a valve stem molded from a material comprising a mixture of an acetal resin, polytetrafluoroethylene (PTFE) and silicone, in particular in the form of silicone oil. .

However, such a solution is not economical since it requires the use of PTFE in a proportion of at least 10%. Furthermore, such prior art valve stems are not the most suitable for a medical device comprising a sterile medicament to be dispensed. In fact, the acetal resins used, such as polyoxymethylene (POM), are poorly resistant to sterilization processes by irradiation, and are liable to deteriorate in use by releasing formaldehyde in the product to be distributed, product toxic for user.

Patent application JP 2006 131287 proposes to solve a problem linked to the adhesion of the products to be dispensed on the valve stems made of polyoxomethyl (POM) or of polybutylene terephthalate (PBT). To do this, the valve stem is molded in POM or PBT comprising 0.1 to 1% of silicone in order to prevent the product to be dispensed from sticking to the valve stem. This document specifies that if the silicone concentration is too low the silicone will end up being too easily removed from the molded part, and on the contrary if the silicone concentration is too high, the dispersion of the silicone will not be good, and will interfere with the molding.

Also, there is a need to provide new, more suitable valves. The object of the invention is in particular to propose a metering valve provided with a valve stem making it possible to remedy the problems of sealing and friction.

Also, the invention relates to a metering valve for dispensing an aerosol comprising - a metering chamber, and - a valve stem provided with an end intended to be disposed on the side of a reservoir and with a dispensing end opposite, slidingly mounted in the metering chamber under the effect of a spring between an upper rest position and a final lower position, the valve stem sliding through a first seal called valve seal or high seal and d '' a second seal called chamber seal or bottom seal, the valve stem being molded from a material comprising a mixture of polybutylene terephthalate (or PBT) and Polydimethylsiloxane said silicone, said silicone representing by weight more than 1.5% by ratio of the total weight of the valve stem.

Thus, the inventors have made the surprising observation that the molding of the valve stem in a material comprising a mixture of PBT and a particular proportion of silicone improves the sliding of the valve stem through the valve seals, by limiting thus friction when the valve stem slides between the valve and chamber seals. As a result, a metering valve is provided which overcomes the problems of sealing and friction, while being more economical to manufacture. In addition, the metering valve includes a valve stem which is adapted for radiation sterilization.

Another advantage of the valve stem having good sliding properties is that it is not necessary to add an external lubricant on the sliding parts (the valve stem and / or the valve seals) at the areas sealing, which limits on the one hand the contact of the lubricant with the product to be dispensed, and limits or even eliminates the amount of lubricant in the dose administered to the user.

The metering valve is therefore devoid of added lubricant, with the exception of a residual quantity of lubricant or sliding aid elements required for the manufacture and handling of certain components of the metering valve and in particular of the valve seals and chamber when made of an elastomeric material.

It is understood that one includes in “a mixture of polybutylene terephthalate (PBT) and of silicone” a mixture, on the one hand of PBT or of a derivative of PBT, and on the other hand, of silicone or of a silicone derivative.

It is also understood that PBT belongs to the family of thermoplastic polyesters and is a semi-crystalline polymer used mainly in injection molding for technical applications. PBT is produced by polycondensation of terephthalic acid or dimethyl terephthalate with 1,4-butanediol, according to methods well known to a person skilled in the art.

In the metering valve, the PBT used to mold the valve stem is used alone which is the most advantageous, but can also be associated with fillers, in particular glass fibers which can represent more than 10% of the composition of PBT . An example of PBT loaded with fiberglass is marketed by the company DuPont under the name of Crastin® SF.

In the metering valve, the silicone used is preferably polydimethylsiloxane (PDMS) used in the form of an oil. So we can talk about silicone oil.

Unlike a coating, for example by spraying, of silicone oil, the proposed valve stem is molded from a material which includes PBT and silicone in its mass. Thus, the silicone is found partly distributed on the surface of the polymer and can exercise its function of limiting friction, but most of it remains trapped in the mass of PBT and is therefore less easily eliminated, in particular by tearing off during friction or by dissolution. in propellants, during use of the metering valve.

In an advantageous embodiment, the silicone has a kinematic viscosity of more than 1000 cSt or mm2.s'1 at 25 ° C. The viscosity data are given for normal temperature and pressure conditions, i.e. an ambient temperature of 25 ° C on average (in a range from 19 ° C to 27 ° C) and at atmospheric pressure at sea of 1 atmosphere.

Silicones with a viscosity of more than 1000 cSt have a molar mass of more than 40,000 g.mol'1.

These viscosities are measured with viscometers known to a person skilled in the art, and in particular an Engler viscometer based on the measurement of the flow time of certain liquids through a calibrated orifice of determined dimensions. This viscometer is a purely empirical device giving a conventional value for viscosity, expressed in Engler degrees. Using concordance tables, it is possible to determine the kinematic viscosity in cSt.

The inventors have found that molding the valve stem of the metering valve with PBT and a silicone oil having a kinematic viscosity as defined above has numerous additional advantages, in addition to its lubricating properties. In fact, due to the high molecular weight, the siloxane polymers are entangled in the PBT and are unlikely to be eliminated during the operation of the metering valve. As a result, the valve stem has good sliding properties over the valve seals over time, and, unlike silicone oils with a viscosity of less than 1000 cSt, they are not likely to be "torn off" »Of the valve stem in the presence of propellant gas and dispensed with the drug.

In another advantageous embodiment, the silicone has a kinematic viscosity at 25 ° C, at a pressure of 1 atmosphere (at; 1 at = 98,066.5 Pa) at sea level, from 1000 cSt to 100,000 cSt, in particular a kinematic viscosity at 25 ° C of more than 10,000 cSt to about 100,000 cSt.

By viscosity from 1000 cSt to 100,000 cSt, at 25 ° C at a pressure of 1 at sea level, here is meant a viscosity of 1000 cSt, 2000 cSt, 3000 cSt, 4000 cSt, 5000 cSt, 6000 cSt, 7000 cSt, 8000 cSt, 9000 cSt, 10000 cSt, 11000 cSt, 12000 cSt, 13000 cSt, 14000 cSt, 15000 cSt, 16000 cSt, 17000 cSt, 18000 cSt, 19000 cSt, 20000 cSt, 21000 cSt, 22000 cSt, 23000 cSt, 24,000 cSt, 25,000 cSt, 26,000 cSt, 27,000 cSt, 28,000 cSt, 29,000 cSt, 30,000 cSt, 31,000 cSt, 32,000 cSt, 33,000 cSt, 34,000 cSt, 35,000 cSt, 36,000 cSt, 37,000 cSt, 38,000 cSt, 39,000 cSt, 40,000 cSt , 41,000 cSt, 42,000 cSt, 43,000 cSt, 44,000 cSt, 45,000 cSt, 46,000 cSt, 47,000 cSt, 48,000 cSt, 49,000 cSt, 50,000 cSt, 51,000 cSt, 52,000 cSt, 53,000 cSt, 54,000 cSt, 55,000 cSt, 56,000 cSt, 57,000 cSt, 58,000 cSt, 59,000 cSt, 60,000 cSt, 61,000 cSt, 62,000 cSt, 63,000 cSt, 64,000 cSt, 65,000 cSt, 66,000 cSt, 67,000 cSt, 68,000 cSt, 69,000 cSt, 70,000 cSt, 71,000 cSt, 72,000 cSt, 73,000 cSt, 74,000 cSt, 75,000 cSt, 76,000 cSt, 77,000 cSt, 78,000 cSt, 79,000 cSt, 80,000 cSt, 8 1000 cSt, 82000 cSt, 83000 cSt, 84000 cSt, 85000 cSt, 86000 cSt, 87000 cSt, 88000 cSt, 89000 cSt, 90000 cSt, 91000 cSt, 92000 cSt, 93000 cSt, 94000 cSt, 95000 cSt, 96000 cSt, 97000 cSt , 98,000 cSt, 99,000 cSt or 100,000 cSt.

Such silicone oils are commercially available in particular from Bluestar Silicones, Dow Corning, Wacker Chemie or also from Bluestar Silicones in particular under the brand Rhodorsil® Oils 47.

In yet another advantageous embodiment, the mixture of polybutylene terephthalate and silicone comprises more than 1.5% by weight of silicone, and in particular from 1.5 to 10% by weight of silicone relative to the total weight of material valve stem.

By "more than 1.5% by weight of silicone, and in particular from 1.5 to 10% by weight of silicone" is meant in the invention amounts of 1.5%, 2%, 2.5%, 3 %, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, or 10% by weight of silicone based on the total weight of the valve stem material.

More advantageously, the mixture of PBT and silicone oil comprises by weight of 1.5 to 5% of silicone, and in particular of 2 to 5% by weight, and in particular 3% by weight of silicone relative to the total weight valve stem material.

Thus, in view of the proportions of silicone used, the PBT represents from 90% to 98.5% by weight relative to the total weight of material of the valve stem. This means that PBT represents 90%, 90.5%, 91%, 91.5%, 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95 , 5%, 96%, 96.5%, 97%, 97.5%, 98% or 98.5 by weight based on the total weight of the valve stem material.

Of course, the PBT mixed with the silicone can also comprise other compounds such as additives, antioxidants, dyes, flame retardants, stabilizers (for example against ultraviolet (UV), or any other additive known to a man. The list is given by way of example and cannot be limiting and limit the scope of the invention Advantageously, the additives represent up to 2%, preferably 1% of the total composition of the material in which the valve stem is molded.

In yet another advantageous embodiment, the valve and chamber seals are arranged respectively at the outlet and at the inlet of the metering chamber.

In the present description, the expressions “top” and “bottom” refer to the position of the valve stem relative to the reservoir on which the metering valve is attached. Thus, a low position of the valve stem corresponds to a position in which the valve stem is closer to the reservoir than in the case of a high position of the valve stem.

The proposed metering valve is also generally intended to be mounted on a neck of a reservoir containing a fluid product to be dispensed in the form of an aerosol, most particularly a pharmaceutical product. It is intended to be used in the inverted position, also called "upside down".

Even more advantageously, the valve and chamber seals are made of an elastomeric material.

Advantageously, the elastomers used in the seals of the metering valve can be made of ethylene-propylene-diene monomer (EPDM), of butadiene-acrylonitrile (NBR) copolymers, of fluoroelastomers such as Viton® marketed by DuPont, or of copolymer of 'isobutylene and isoprene, halogenated or not. This list is not exhaustive, and a person skilled in the art can use various known elastomeric materials having elastic properties and compatible with medical use. One of the advantageous elastomeric materials used for the seals of the metering valve is chlorobutyl rubber (ClIR)

In yet another embodiment, at least one of the valve and chamber seals is molded and / or covered with a material having properties that reduce friction when the valve stem slides.

In order to increase the sliding of the valve stem through the seals of the metering valve, and thus limit friction, it may be advantageous to cover the elastomeric material of the seals of the metering valve with a material making it possible to limit friction, such as silicone oils. In such a case, it may be advantageous to use an uncovered chamber seal and a covered valve seal, which makes it possible to limit the risks of interactions between the silicone oil and the product to be dispensed. It may also be advantageous, in combination or not with the above-mentioned coating, to provide elastomeric seals further comprising a filler of a material comprising fluorine, in particular polytetrafluoroethylene (PTFE).

With such combinations, the friction between the valve stem and the valve seals is very greatly reduced, thereby increasing the service life of the metering valve.

According to another advantageous embodiment, the metering chamber has a cylindrical top compartment and a bottom compartment, the diameter of the top compartment being greater than the diameter of the bottom compartment, a shoulder of the chamber being formed at the interface between the two top and bottom compartments, the final bottom position of the valve stem being defined by abutment of a stem shoulder formed on the valve stem with the chamber shoulder at the interface between the two top and bottom compartments .

The metering valve may further include one or more of the following characteristics: - the stem shoulder is arranged so that the abutment surface extends in a plane perpendicular to the axis of the valve stem. It preferably comprises a protrusion formed on the valve stem, the protrusion being able to be annular or semi-annular, for example in the form of an annular gadroon. The stem shoulder may also include an annular or semi-annular projection, - the stem shoulder is arranged so that the abutment surface extends in a plane not perpendicular to the axis of the valve stem. For example, it comprises an annular or semi-annular projection cooperating with a conical or spherical support with the metering chamber, or else a conical or spherical surface cooperating with an annular or semi-annular projection formed in the metering chamber, - l the stem shoulder is carried by a projection also providing a stop function for the valve stem with a high wall of the metering chamber when the valve stem is in the rest position. Thus, the same projection is used to ensure both the stop function defining the final bottom position and that defining the rest position. It appears that more precise doses are obtained, since the metering valve only presents the manufacturing tolerances of this projection, whereas in the case where the stops would be provided by two distinct parts and distant from each other , the metering valve would present the manufacturing tolerances for each of these parts. The invention also relates to a method of manufacturing a valve stem for a metering valve for dispensing an aerosol, said method comprising: - a step of mixing polybutylene terephthalate with from 1.5 to 10% by weight of silicone relative to the total weight of the mixture, in particular with 3% by weight of silicone relative to the total weight of the mixture, - an injection / molding step of the above mixture to obtain a valve stem.

PBT offers high strength and rigidity for a wide range of applications.

This material is sensitive to hydrolysis at high temperatures. This is why it is particularly advantageous to dry it before molding. Recommended drying conditions (in air) are as follows: 120 ° C for 6 to 8 hours (or 150 ° C for 2 to 4 hours). The humidity level should advantageously be less than 0.03%. If a desiccator is used, it is recommended to dry at 120 C for 2.5 hours. Generally, PBT is melted at a temperature in the range of 220 ° C to 280 ° C, 250 ° C being the optimum melting temperature. The viscosity of PBT in the molten state is quite low and, due to the rapid crystallization rates, the cycle times are generally short. It is therefore easy to mix other materials, in particular silicone oils as described above.

During the injection molding step, it is advantageous to use a mold heated to a temperature of 15 ° C to 80 ° C depending on the type of PBT used (loaded or not). Heat dissipation must be rapid and uniform.

In the proposed method, the molten PBT and the silicone are mixed, in a preferential PBT: silicone ratio of 98.5: 1.5 to 90: 10, more advantageously in a ratio of 98: 2 to 95: 5, and in particular in a ratio of 97: 3. In these ratios, only the proportion of PBT and silicone is considered, other fillers or additives that can be added are not taken into account. The injection of the mixture should advantageously be carried out as quickly as possible due to the rapid solidification of the PBT. It is generally recommended to inject in a heated mold as defined above at a pressure of 150 MPa.

Advantageously, the silicone, or polysiloxane, mixed with polybutylene terephthalate has a kinematic viscosity at 25 ° C of at least 1000 cSt.

It is also more advantageous to use polysiloxanes having a kinematic viscosity at 25 ° C, at a pressure of 1 atmosphere at sea level, from 1000 to 100,000 cSt, and in particular from 10,000 to 100,000 cSt. The invention will be better understood on reading the appended figures, which are provided by way of examples and have no limiting character, in which: - Figure 1 is a view in longitudinal section of a metering valve according to one mode of embodiment, in the high rest position, the metering chamber being isolated from the outside of the dispensing device and from the inside of the reservoir; - Figure 2 is a view similar to Figure 1, in which the metering valve is in an intermediate position, in which the metering chamber communicates with the interior of the reservoir and is isolated from the exterior of the dispensing device; - Figure 3 is a view similar to Figure 1, in which the metering valve is in the final low position, in which the metering chamber communicates with the outside of the dispensing device and is isolated from inside the tank;

The metering valve 1 shown in FIGS. 1 to 3 is a metering valve type valve for the distribution in aerosol form of a fluid product, in particular a medicinal product, by means of a propellant gas, in particular of the HFA type. Of course, the present invention can also be applied to valves of another type or used in different fields, such as perfumery or cosmetics, and with other propellants, for example CFC or compressed gas .

The metering valve 1 is suitable or adapted to operate in the inverted position, that is to say in the position as shown in the drawings. In other words, the metering valve 1 is intended to be used in a position where the metering valve 1 is located under the reservoir 9 containing the product to be dispensed, taking the direction of gravity as a reference. The metering valve 1 is also adapted to be mounted on any container or bottle forming a reservoir 9.

Referring to Figures 1 to 3, the metering valve 1 comprises a valve body 11 in which a ring 10 defining a metering chamber 2 is attached. The metering valve 1 is intended to be secured to the reservoir 9 by means of a ferrule 12 or by any other equivalent means. The valve body 11 is separated from the reservoir 9 by a ring 13 housed in the neck of the reservoir 9. The metering valve 1 further comprises a valve stem 3 which is slidably mounted in the metering chamber 2, between a first high position , called the rest position, represented in FIG. 1, a second position called intermediate filling of the metering chamber 2, represented in FIG. 2, and a third dispensing position, or final low position, represented in FIG. 3, wherein the valve stem 3 is pressed axially inwardly, or downward, of the metering valve 1, being abutted. The valve stem 3 is biased towards its rest position by a spring 6, or elastic return means, which compresses when a user actuates the metering valve 1 and pushes the valve stem 3 axially inside the valve when the user releases its actuating force, the compressed spring 6 recalls the valve stem 3 from its dispensing position to its rest position.

Referring to Figure 1, the valve stem 3 is in the high rest position and cooperates axially with the valve seal 7 carried by the valve body 11 and separating the ring 10 from the ferrule 12. A seal is then created between the upper part 5 of the valve stem 3 and the valve seal 7. Advantageously, the radial interference of the outside diameters of the valve stem 3 and inside of the valve seal 7 is less than 1 mm, and in particular approximately 0, 6 mm, in the sealing zone, which is likely to generate friction when the valve stem 3 is actuated.

In order to overcome these drawbacks related to the need to create a seal at the top 5 of the valve stem 3 in the zone of cooperation with the valve seal 7, the valve stem 3 is advantageously molded from a material comprising PBT and silicone having a kinematic viscosity of more than 1000 cSt.

Advantageously, the valve stem 3 can be provided with a stem shoulder 31 or annular or partially annular projection. This rod shoulder 31, in the rest position, abuts against the valve seal 7 thus increasing the tightness of the upper part of the metering chamber 2.

Still in the rest position, the lower part of the metering chamber 2 is also isolated from the reservoir 9 by the abutment of a cap 41 with the chamber seal 8 housed in the valve body 11. By crushing the seal chamber 8, the metering chamber 2 is therefore hermetically isolated from the reservoir 9, so that, if the metering valve 1 is positioned in the "head up" position (the metering valve 1 being in the opposite position to that shown in the figures ), the product contained in the metering chamber 2 cannot escape to the reservoir 9.

Referring to Figure 2, the metering valve 1 is shown in an operating position called the intermediate position. It is understood from this figure that the valve stem 3 can slide through the valve seals 7 and of chamber 8. The friction of the upper part of the valve stem 3 with the valve seal 7 is reduced, in particular due to the presence of silicones with a viscosity greater than 1000 cSt in the material forming the valve stem 3.

In this intermediate position, the valve stem 3 is mobilized downwards from the metering valve 1, exerting a push on the spring 6, the metering valve 1 in its intermediate position allows the aerosol to enter from the reservoir 9 towards the metering chamber 2, through the lower part of the metering chamber 2. In fact, the axial mobilization of the valve stem 3 will peel off the cap 41 of the chamber seal 8, thus leaving a space between the lower part of the valve stem 3 and the valve seal 7. To allow such a space, the lower part 4 of the valve stem 3 has a diameter less than both the diameter of the upper part 5 of the valve stem 3, and less the diameter of the chamber seal orifice 8.

Referring to Figure 3, the metering valve 1 is shown in its final lower position called the dispensing position. It can be understood from this figure that the valve stem 3 has slid through the valve seals 7 and of chamber 8. In this position, the metering chamber 2 is in communication with the outside via a radial channel 30 leading to an axial distribution channel 32 formed in the upper part 5 of the valve stem 3.

In the lower part of the metering chamber 2, a seal has been created at the chamber seal 8 by axial cooperation with a part of the valve stem 3 intermediate between the upper part 5 and the lower part 4. Thus, the metering chamber 2 is isolated from the reservoir 9 thus preventing the contents of the reservoir 9 from escaping entirely to the outside.

Advantageously, as shown in Figures 1 to 3, the metering chamber 2 consists of a top compartment 21 and a bottom cylindrical compartment 22, the diameter of the top compartment 21 being larger than the diameter of the bottom compartment 22. A chamber shoulder 23 is formed at the interface between the two upper 21 and lower 22 compartments. In other words, the metering chamber 2 of the metering valve 1 is made up as follows: - the upper compartment 21 of the metering chamber 2 is essentially cylindrical of circular section, of a first determined diameter, and - the bottom compartment 22 of the metering chamber 2 is essentially cylindrical of circular section, of a second determined diameter; the second diameter of the lower part of the metering chamber 2 being smaller than that of the upper part of the metering chamber 2, the two cylinders of the upper 21 and lower 22 compartments being coaxial and juxtaposed in the axial direction, one being an extension of the other, the sum of the volumes of the two cylinders, to which the volume of the valve stem 3 is subtracted in this zone, defining the volume of the dose dispensed by the metering valve 1, when it is actuated .

In the final low position, the rod shoulder 31 comes into contact with the chamber shoulder 23 thus limiting the axial actuation of the valve rod 3, and in particular allowing the spring 6 to not be stressed to its maximum. use of the valve stem 3 molded in the material described above therefore allows better sliding through the valve seals 7 and chamber 8, while ensuring the necessary seals, even in the context of use propellants creating pressure at the joints. This better sliding also has the effect of increasing the speed of movement of the valve stem 3 and of reducing the stiffness of the spring 6. The invention is not limited to the embodiments presented and other embodiments will appear clearly to a person skilled in the art. It is in particular possible to propose improvements in the structure of the valve body, of the metering chamber or also of the valve stem in order to improve the precision of the distribution of the pharmaceutical product intended to be delivered, or to improve the longevity of the metering valve.

Claims (10)

1. Metering valve (1) for dispensing an aerosol, characterized in that it comprises: - a metering chamber (2), and - a valve stem (3) provided with an end intended to be placed on the side a reservoir (9) and an opposite dispensing end, slidably mounted in the metering chamber (2) under the effect of a spring (6), between a high rest position and a final low position, the valve stem (3) sliding through a first seal called a valve seal (7) and a second seal called a chamber seal (8), the valve stem (3) being molded from a material consisting of a mixture of polybutylene terephthalate (PBT) and of Polydimethylsiloxane said silicone, said silicone representing by weight more than 1.5% relative to the total weight of the rod. 2. Metering valve (1) according to claim 1, in which the silicone has a kinematic viscosity of more than 1000 cSt at 25 ° C. 3. Metering valve (1) according to claim 1 or 2, wherein the silicone has a kinematic viscosity at 25 ° C from 1000 cSt to 100,000 cSt, in particular a kinematic viscosity at 25 ° C from more than 10,000 cSt to about 100,000 cSt. 4. Metering valve (1) according to any one of the preceding claims, in which the mixture of polybutylene terephthalate and silicone comprises from 1.5% to 10% by weight of silicone relative to the total weight of material of the valve stem (3), and in particular 3% by weight of silicone relative to the total weight of material of the valve stem (3). 5. Metering valve (1) according to any one of the preceding claims, in which the valve (7) and chamber (8) seals are disposed respectively at the outlet and at the inlet of the metering chamber (2) . 6. Metering valve (1) according to any one of the preceding claims, wherein the valve seals (7) and chamber (8) are made of an elastomeric material comprising chlorobutyl. 7. Metering valve (1) according to any one of the preceding claims, in which at least one of the valve (7) and chamber (8) seals is molded and / or covered with a material having properties which reduce friction. when the valve stem (3) slides. 8. Metering valve according to any one of the preceding claims, in which the metering chamber (2) comprises a top compartment (21) and a bottom compartment (22) cylindrical, the diameter of the top compartment (21) being greater than the diameter of the bottom compartment (22), and in which a chamber shoulder (23) is formed at the interface between the two top and bottom compartments (21,22), the final bottom position of the valve stem (3) being defined by an abutment of a rod shoulder (31) formed on the valve rod (3) with the chamber shoulder (23). 9. A method of manufacturing a valve stem (3) for a metering valve (1) for dispensing an aerosol, said method comprising: - a step of mixing polybutylene terephthalate with from 1.5 to 10% by weight of silicone relative to the total weight of the mixture, - a step of injecting / molding the above mixture to obtain a valve stem (3) molded from a material consisting of a mixture of polybutylene terephthalate (PBT) and Polydimethylsiloxane says silicone. 10. Method according to the preceding claim, wherein the silicone mixed with polybutylene terephthalate has a kinematic viscosity at 25 ° C of at least 1000 cSt.