Method for placing a self-sealing product on the inner surface of a tire

Abstract

The process includes a non-crosslinked self-sealing composition which is introduced into an extrusion means, the geometric and thermodynamic characteristics of which have been specially adapted. The speed and temperature conditions of the extrusion means are adjusted so that, at an application nozzle forming the outlet die of the extrusion means, the self-sealing composition is crosslinked. The application nozzle is brought close to the internal surface of the casing previously set in relative motion with respect to the application nozzle, and an extruded and crosslinked bead that has a given width and profile is deposited directly on the internal surface of the casing.

Claims

1. A method of applying a rubber-based self-sealing composition on an inner surface of a tire casing, comprising the steps of: introducing a non-crosslinked self-sealing composition into an extrusion device adjusting speed and temperature conditions of the extrusion device so that, at an application nozzle forming an outlet die of said extrusion device, the self-sealing composition is crosslinked, and bringing the application nozzle close to the inner surface of said casing previously set in relative motion with respect to the application nozzle, and depositing an extruded and crosslinked bead having a given width and profile directly on said internal surface of the casing.

2. The method according to claim 1, in which the value of the G/G (Tg ) ratio of the self-sealing composition decreases by a value greater than 0.1 when it changes from the non-crosslinked state to the crosslinked state.

3. The method according to claim 2, in which the value of the G/G (Tg ) ratio of the non-crosslinked self-sealing composition is between 1.2 and 3.

4. The method according to claim 2, in which the value of the G/G (Tg ) ratio of the crosslinked self-sealing composition is between 0.66 and 2.

5. The method according to claim 1, in which the properties of shear thinning and of reducing the viscosity as a function of the temperature of the self-sealing composition are combined with the method of supplying heat so as to control the integrity of the elastomeric matrix during crosslinking in the extruder.

6. The method according to claim 1, in which the extrusion device is formed by a screw of length (L) and of diameter (D) rotated inside a barrel.

7. The method according to claim 6, in which the ratio (L/D) between the length and the diameter of the screw is between 15 and 30.

8. The method according to claim 1, in which the temperature and speed conditions of the extrusion means are adjusted so that the composition remains inside the extrusion device at a temperature between 150 C. and 200 C. for a time of between 1 min and 20 min.

9. The method according to claim 1, in which the temperature and speed conditions of the extrusion device are adjusted so that the amount of thermal energy supplied to the self-sealing composition in the form of mechanical energy is less than 75% of the total thermal energy supplied to said self-sealing composition.

10. The method according to claim 1, in which the output speed of the self-sealing composition from the application nozzle is between 1 ms.sup.1 and 4 ms.sup.1.

11. The method according to claim 1, in which the self-sealing composition is deposited by coiling said bead by varying the pitch and the axial position of the application nozzle with respect to the tire casing.

12. The method according to claim 11, in which said bead is applied continuously by moving the application nozzle tangentially to the internal surface of the tire from the equator of the casing to a first shoulder, then by starting again in the opposite direction in the direction of a second opposite shoulder passing through the equator, and by finally coming back to the equator.

13. The method according to claim 1, in which the tire casing is in the vulcanized state.

14. A method of applying a rubber-based self-sealing composition on an inner surface of a tire casing, comprising the steps of: introducing a non-crosslinked self-sealing composition into an inlet of an extrusion device; adjusting speed and temperature conditions of the extrusion device so that, at an application nozzle forming an outlet die of said extrusion device, the self-sealing composition is crosslinked, and bringing the application nozzle close to the inner surface of said casing previously set in relative motion with respect to the application nozzle, and depositing an extruded and crosslinked bead having a given width and profile directly on said internal surface of the casing; wherein the extrusion device includes an assembly comprising a screw rotated in a barrel, the extrusion device further including a duct positioning downstream of the assembly that opens into the application nozzle; wherein the extrusion device includes a plurality of zones, and the method further includes increasing the temperature of the composition in a first zone to a set point temperature between 140 degrees C. and 220 degrees C. during a first period of time, maintaining the temperature at the set point temperature in a second zone for a second period of time, and increasing the temperature in a third zone for a third period of time, wherein the third period of time is shorter relative to the second period of time; wherein the third zone includes the duct and the application nozzle.

15. The method of claim 14, wherein the application nozzle is axially moveable, and the bead is deposited in a coiled manner relative to the tire casing.

16. The method of claim 14, wherein the self-sealing composition is pre-mixed at a ratio of material and the ratio of material is maintained from the inlet to the outlet die.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents an extrusion device capable of implementing the process according to the disclosure,

(2) FIG. 2 represents a graph of distribution of the heat inputs imparted to the self-sealing composition as a function of the throughput and of the rotational speed of the screw,

(3) FIG. 3 represents a diagram illustrating the shear-thinning behaviour of the self-sealing composition, in particular the variation of the viscosity with the shear rate at low temperature and the change thereof with temperature.

DETAILED DESCRIPTION OF THE ENABLING EMBODIMENT

(4) FIG. 1 represents an extrusion device capable of implementing the process according to the disclosure. This extrusion device mounted on a load-bearing frame 13 comprises a screw 20 rotated in a barrel 10 by a motor 12 and a reducer 11.

(5) The barrel comprises, upstream, a feed zone 23 through which the non-crosslinked self-sealing composition is introduced. A feed roller 22 makes it possible to ensure a good regularity of the introduction of the compound.

(6) Positioned downstream of the extrusion assembly is a duct 31 that opens into an application nozzle 30 of known type, and that is described by way of example in publication EP 268 544 or else in publication EP 264 600.

(7) The application nozzle is positioned opposite the internal surface of a tire 40 rotating about its axis, so as to allow the deposition by successive coilings of a bead 41 of the self-sealing composition, of calibrated profile and thickness. A movement of axial displacement in translation is imparted to the nozzle in order to be able to continuously deposit several beads 41 by traverse winding, this nozzle being connected to the outlet of the extruder by a flexible tube.

(8) The barrel 10 comprises heating means 21. These heating means are capable of regulating the temperature of the barrel over a range extending from 140 C. to 220 C. so as to maintain the temperature of the material between 150 C. and 200 C.

(9) Preferentially, means will therefore be chosen such as heater bands, a heat pipe specially suitable for operating at this temperature level, or else means such as pressurized steam, for which it will be pointed out here that they are more difficult to implement in order to regulate the temperature over the range considered. It is also possible to install heating means of the same type as before in the body of the screw in order to supply extra heat to the composition.

(10) The process for crosslinking the self-sealing composition is linked to several parameters that relate just as much to the configuration of the extrusion means as to the rheology and the composition of the self-sealing compound, the general principles of which will be recalled below.

(11) Indeed, it is advisable to selectively adjust the processing parameters of the extrusion device so that the energy supplied to the composition over the transfer time is sufficient to obtain a crosslinked composition at the point of leaving the application nozzle 30.

(12) The thermal change as a function of time of the composition in the course of its movement inside the barrel is represented schematically by the diagram that appears in FIG. 1.

(13) The first zone A, close to the feed zone, has the objective of changing the temperature of the composition from the temperature of introduction, likened to the ambient temperature in the workshop, to the set point temperature between, for most of the self-sealing compositions used, 140 C. and 220 C. and preferentially between 170 C. and 200 C.

(14) In steady state (normal operating state is understood), the heat input during the residence of the composition in the transfer zone B is relatively constant.

(15) Finally, the passage of the composition into the duct and into the application nozzle, corresponding to extrusion zone C, in which the composition experiences greater heating during a relatively brief time (the time axis on the graph from FIG. 1 being solely illustrative and not significant when it comes to the time representation scale from one zone with respect to the other). Thus, during tests carried out in the laboratory, it was established that the residence time or duration of the material in zone C is equal to or less than 10% of the total residence time of the material between the moment when it is heated to the set point temperature in the feed zone A and the outlet at the application nozzle 30, this total residence time constituting the curing time of the material with a view to the crosslinking thereof.

(16) The choice of the set point temperature will depend firstly on the rotational speed of the screw, which will act, on the one hand, on the residence time of the composition inside the barrel, which varies the conductive heat inputs from the walls to the compound and, on the other hand, on the mechanical energy arising from the friction and shearing of the compound between the walls of the screw and of the barrel and that is converted to thermal energy; for this purpose, it is possible to adapt the gap between the flights of the screw and the barrel which defines the shearing undergone by the self-sealing composition.

(17) For practical reasons, and in particular for controlling the cycle times, which may comprise shutdown phases during which the screw is stopped, it may be sought to increase the portion of the mechanical energy converted to heat so as to reduce the energy inputs during the shutdown phases and so as not to impair too significantly the vulcanization reaction due to the extension of the energy input by contact with the walls, the thermal inertia of which is high.

(18) However, it is not possible to let the portion of the mechanical energy increase beyond certain limits. Indeed, it has been demonstrated that the self-sealing compounds are capable of losing their properties when they are subjected to excessively high shear rates. Therefore, it will be ensured that the energy transmitted by the mechanical effects is less than 75% of the total of the thermal input imparted to the compound. This limit, the objective of which is to reduce the effects of conduction, makes it possible to obtain relatively stable crosslinking states, for cycle interruption times that may range up to ten or so minutes.

(19) In order to adjust the conductive heat input, it is possible for example to modify the flight height, the value of the exchange area between the composition circulating in the body of the extrusion means and the heating parts of said extrusion means. It is also possible to act on the set point temperature of the heating means 21.

(20) The graph from FIG. 2, determined for obtainment conditions and for a self-sealing composition as defined below, and in which the speed is given on the x-axis and the outlet throughput on the y-axis, makes it possible to illustrate the distribution of the heat inputs originating from the mechanical energy due to shearing on the one hand and from the energy supplied by conduction by the walls on the other hand. These values were obtained with a screw speed of between 20 rpm and 120 rpm.

(21) In view of what has just been disclosed, a person skilled in the art will observe without difficulty that the more the screw speed increases, the more the heating linked to the mechanical energy increases, and the shorter the residence time in the extrusion means. By carrying out appropriate experimental designs, it is easy to determine the operating parameters that make it possible to regulate this equilibrium.

(22) It is also possible to adapt the length of the screw relative to its diameter, so as to increase the residence time without adversely affecting the outlet throughput. By considering a screw of diameter D and of length L, the ratio L/D may usefully be between 15 and 30.

(23) By way of example of the implementation of the process according to the disclosure, the set point values obtained for a self-sealing composition, the composition of which is described in publication FR 2 955 587 by the applicant, will be indicated below. This composition is based on a blend of at least two solid elastomers, a polybutadiene or butadiene copolymer elastomer and a natural rubber or synthetic polyisoprene elastomer, (the weight ratio between the two elastomers being within a range of from 10:90 to 90:10); on a hydrocarbon resin (between 30 and 90 phr); and on a filler (0 to less than 30 phr).

(24) In an example of a self-sealing composition used for the implementation of the disclosure, this composition comprises a solid unsaturated diene elastomer (blend of 50 phr of solid NR with 50 phr of solid BR; number-average molar mass Mn of the blend of elastomers equal to around 270 000 g/mol in the final composition); a hydrocarbon resin Escorez 2101 from Exxon Mobil (Tg equal to around 44 C.; softening point equal to around 90 C.; Mn equal to around 800 g/mol; PDI equal to around 2.1) at a weight content of around 50 phr; around 15 phr of liquid polybutadiene elastomer (Ricon 154 from Sartomer Cray ValleyTg equal to around 20 C.; Mn equal to around 5000 g/mol and PDI equal to around 1.4); 0.5 phr of sulphur combined with 0.5 phr of DPG; it also comprises a very small amount (around 1 phr) of carbon black (N772) and around 3 phr of antioxidant.

(25) In order to obtain a composition for which the Tg value of the non-crosslinked material at the start of the process is 1.52, and for which the Tg value of the crosslinked material at the outlet of the nozzle is 1.15, the set point values of the parameters of the process in stabilized mode were the following:

EXAMPLE 1

(26) Temperature: 150 C. to 180 C. Screw diameter: 120 mm Screw length: 1800 mm Screw/barrel gap: 0.1 mm Duct length: 500 mm Nozzle outlet cross section: 30 mm.sup.2 Ratio of the screw-barrel exchange areas: 0.5 Throughput: 1.5 kg/min Conduction/self-heating ratio: 1/3-2/3 Average residence time: 8 min

EXAMPLE 2

(27) Temperature: 150 C. to 200 C. Screw diameter: 120 mm Screw length: 1800 mm Screw/barrel gap: 0.1 mm Duct length: 2300 mm Nozzle outlet cross section: 20 mm.sup.2 Ratio of the screw-barrel exchange areas: 0.5 Throughput: 3 kg/min Conduction/self-heating ratio: 1/3-2/3 Average residence time: 4 min

(28) The graph from FIG. 3 illustrates the shear-thinning behaviour of the self-sealing composition used for the implementation of the process of the disclosure where the variation in the viscosity with the shear rate at low temperature and the change thereof with temperature are noted. Thus, it is observed that the viscosity decreases when the velocity gradient increases, which gives an increasingly fluid material. It is understood that other vulcanizable self-sealing compositions may be used with the process of the disclosure as long as they have an equivalent shear-thinning behaviour.

(29) Obviously, it is also possible to implement the process according to the disclosure with the aid of known extrusion devices, different from the one used as a basis for the present description.

(30) By way of example, use may be made of an injection press, and the equilibrium will be sought between the heat supplied by a heating means positioned around the intake pot, and the mechanical energy transmitted to the composition during the transfer in the duct and in the nozzle. The adjustment of the parameters of this device may appear easier, but it nevertheless proves expensive in cycle time in so far as most of the heat input takes place by conduction from the walls of the cylinder.

(31) The deposition of the self-sealing product on the inner surface of the tire is carried out in a known manner by traverse winding of a continuous bead by bringing the outlet of the application nozzle 30 close to said surface, after having previously set the tire rotating and, according to a defined pitch, by displacing the nozzle tangentially to the inner surface corresponding to a direction substantially parallel to the rotational axis of the tire. The bead formed at the outlet of the application nozzle is thus deposited directly on the internal surface of the tire.

(32) The width and the thickness of the bead may be adjusted in order to reduce the application time. Good results appear to be able to be obtained by forming a bead having a width between 10 and 20 mm and a thickness between 1 and 2 mm.

(33) So as to improve the visual perception of this coiling, it is proposed to subject the application nozzle to a continuous and alternating axial movement, starting from the equator of the casing by heading towards the shoulder zone 42, then by starting again in the opposite direction in the direction of the opposite shoulder 43 passing through the equator, and by finally coming back to the equator.

(34) The advantage of the method according to the disclosure is also to be able to deposit a layer of self-sealing compound directly on the inner surface of the tire irrespective of the step of production of the latter, and in particular when the tire is completely vulcanized, as is the case for example in the warehouses intended for the delivery of the tires. This way of proceeding makes it possible to differentiate tires at the last moment, on the basis of the usage options specified by a given client. In which case, the extrusion means are positioned in the delivery store and are placed for the use of the operators responsible for organizing the orders.