CH623352A5 - Process for bonding two substrates together with the aid of a heat-activated thermoplastic adhesive and apparatus for making use of this process - Google Patents

Description

The present invention relates to a method for bonding two substrates together using a thermo-activated thermoplastic adhesive and an apparatus for its implementation.

In industry, thermo-activated or hot-adhering thermoplastic adhesives are widely used for bonding different products. One of the most common applications is packaging and cardboard packaging in which the short curing time of this adhesive is particularly advantageous.

One of the most common problems posed by adhesives of this type is that of the compression of the adhesive after application so that the contact between the adhesive and the substrate to be bonded is sufficient for good bonding. The relatively high viscosity, high surface tension and rapid hardening properties of these adhesives combine to prevent spreading of the adhesive over a large area when the adhesive is applied as a liquid to the substrate. . The liquid hardens in the form of a thick bead instead of spreading. Even during rapid compression, for example between two flaps of a carton, the adhesive spreads with difficulty. In general, when two bonded surfaces are spaced, we see that the bond breaks at the adhesive-substrate interface. Consequently, the larger the surface or interface contact, the higher the resistance of the bond.

The object of the invention is to remedy this drawback. To this end, the method according to the invention is characterized in that it comprises the formation of a mixture of a gas with the thermo-activated adhesive, the placing of the mixture under pressure so that the gas passes into solution in the thermo-activated adhesive, and the distribution of the solution at low pressure, so that the gas is released from the solution and forms a foam of adhesive, this foam of adhesive then being compressed between two substrates so that it forms a bond between these.

The increase in the bonding strength of the glue results at least in part from the fact that the material spreads over an area at least twice that occupied in the absence of foam, for the same compression conditions. . As the resistance of the bond is a function of the surface covered by the bond, this foaming ensures the formation of a bond twice as strong as that of a glue without foam, for the same quantity of glue.

This greater possibility of spreading an adhesive in the form of a foam is to be attributed to several physical characteristics of the foam. First, the conventional thermo-activated molten adhesives are very viscous and similar to molten glass whose flow requires significant energy. On the contrary, the glue in the form of foam is less viscous and requires less energy. In other words, a larger volume of glue foam in the molten state can be flattened and displaced by a given force, per unit of time, relative to the quantity of the same molten glue displaced by the same force, but without it forming a foam. In addition, the melted adhesives are incompressible while the foams are compressible, given the presence of gas bubbles. The foam bubbles tend to reduce the viscosity and density of the adhesive and it is easily compressible.

Glue foam also has other important advantages compared to the same glue applied and used not in the form

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of a foam. More specifically, it can be seen that the opening time during which the material retains its bond strength after distribution on a substrate is extended. It is also found that hardening and adhesion are faster during compression between two substrates, for example two flaps of a carton. These two characteristics are very desirable in cardboard applications, because they eliminate the need for instant closure of the flaps after application and allow the removal of the clamping pressure applied to the bonded faces quickly after application of the pressure. These two characteristics give greater manufacturing tolerances and therefore increase the range of applications of adhesives.

The longer opening time of the adhesive foam compared to the non-foaming material is due to the small cells of the foam containing air or a gas and forming insulating barriers preventing heat loss therefore the solidification of the liquid adhesive. When the adhesive in the form of foam is then spread between the surfaces by application of the clamping pressure, it spreads over a surface twice that which is given by the non-foaming adhesive, so that the greater surface contact allows the glue foam to cool more quickly than glue without foam.

It can be seen, when air, or a gas such as nitrogen, is well mixed with a thermo-activated liquid adhesive and is dissolved with the liquid adhesive by application of a high pressure such as 2.1 • 106 Pa, that the gas passes into solution in the adhesive. When the solution formed is dispensed by a conventional valve device, the gas leaves the solution and is trapped in the glue in the form of a closed cell foam having the desirable adhesive characteristics described above.

In one embodiment, the thermoplastic adhesive is heated and melted in a heated tank. The molten glue is then mixed with air and pressurized by a one or two stage gear pump. In this pump, the gas and the liquid glue mix well and the gas is dissolved in the pump discharge pressure, in the liquid glue. The solution is then transmitted by a valve dispensing gun or valve which dispenses the adhesive at atmospheric pressure. As it leaves the dispenser nozzle, the gas escapes from the solution and forms small bubbles which expand the glue. This, in the uncompressed state, hardens in the form of a solid and homogeneous foam having closed cells containing air or a gas, distributed uniformly in the adhesive.

In another embodiment, the mixture of the thermoplastic adhesive and the blowing agent is heated and melted in a tank heated to a temperature exceeding the melting temperature, but lower than the decomposition temperature of the blowing agent. The mixture of glue and blowing agent is then pressurized by a gear pump and it is transmitted at a pressure of, for example, 2.1 × 10 6 Pa, to a distributor of hot material. The mixture is further heated to a higher temperature between the pump and the outlet of the distributor, so that the blowing agent decomposes and releases a gas such as nitrogen which, at this pressure, passes into solution in the liquid adhesive which can then be distributed as indicated above.

Large bubbles have often been formed in thermoplastic adhesives, but these large bubbles are not in solution in the adhesive and do not cause the formation of a uniform foam of adhesive. On the contrary, these large bubbles simply form cavities randomly placed in the glue and accompanied by small quantities of glue forming a foam of separate droplets and not of small and regularly arranged pockets or cells created by continuous extrusion in the glue. In general, whenever large air bubbles have been formed in the glue, until now, they have come either from the drying of the liquid glue tank, or from the cavitation of the pump, or from incorporating water into the glue which then forms small pockets of water vapor. Whenever these conditions occur, the device projects and spits the adhesive through its outlet nozzle and forms a very irregular deposit of adhesive on the substrate. As soon as this condition appears, an attempt is made to remedy this drawback by eliminating the projection and the bubbles.

The present process is based on the deliberate creation of small regularly spaced pockets of air or gas in the hot melted glue, and not large unwanted bubbles randomly arranged and often created inadvertently or accidentally as a result of poor fusion and poor distribution.

The process is suitable for practically all applications of thermo-activated adhesives, but it is particularly advantageous in packaging and cardboard applications in which, until now, good wetting of large surfaces of substrate has been difficult. glue, given the limited compression forces that can be used. In most of these applications, this process allows a reduction of at least 50% in the total amount of adhesive required for the same or a better bond, and without the material being significantly more expensive, because gas, or l The air used for foaming is available at little or no cost.

When using the adhesive, the surface necessary to obtain a suitable bond with one or more substrates requires a smaller mass of adhesive foam than of the same type of adhesive which does not form foam.

The advantages of the invention appear in the form of a reduction in the weight of adhesive applied, so that consumption is reduced and the cost is reduced for the manufacturer.

"■ The characteristics and advantages of the invention will emerge more clearly from the description of examples which follows, given with reference to the appended drawings in which:

fig. 1 is a perspective view with parts torn off from an adhesive application device;

fig. 1A is a partial diagrammatic perspective, with a sectional section, of the dispensing gun of the apparatus of FIG. 1;

fig. 1B is a section of the gear pump of FIG. 1; fig. 2 is a partial perspective with parts cut away from a part of a variant of the apparatus of FIG. 1;

fig. 3 is a schematic perspective, with parts in section, of a second variant;

fig. 4 is a perspective view of a glue dispenser nozzle, and it indicates the formation of a bead of glue without foam distributed by the nozzle;

fig. 5 is similar to FIG. 4, but it represents the bead of glue foam;

fig. 6 is a section along line 6-6 of FIG. 4;

fig. 7 is a section along line 7-7 of FIG. 5;

fig. 8 is a section along line 8-8 of FIG. 5;

fig. 9 is a section through two substrates between which a bead of foamless adhesive is compressed;

fig. 10 is a view similar to FIG. 9, but it shows the greatest compression obtained with the same force for the same glue as in the case of FIG. 9, but in the form of a foam, and FIG. 11 is a photograph, magnified 20 times, of a cup of glue foam.

Fig. 11 is a photomicrograph of a foam 10 of thermo-activated thermoplastic adhesive produced according to the method. This foam 10 is formed with a conventional thermo-activated adhesive Eastabond A-3 which is a polyethylene-based material manufactured by Eastman Chemical Company, Rochester, New York. Closed cells 11 containing air created by trapping air bubbles released from a solution of air in the molten liquid glue are regularly distributed in the foam 10. They are formed after distribution of the solution of l air in the liquid adhesive by a conventional distributor 12 of such an adhesive under high pressure (fig. 1 A). As shown in fig. 11, the

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cells 11 are distributed relatively uniformly in the foam and are almost the same size everywhere. In the embodiment shown, the cells have a diameter of between 0.1 and 0.7 mm. In other embodiments, satisfactory glue foams can be formed having cells as small as 0.1 mm, evenly distributed in the glue, or as large as 0.7 mm. The size of the cells in the foam is not essential insofar as the foam is homogeneous and has regularly distributed cells. Obviously, the cells cannot be so large that when the foam is subsequently compressed between the substrates, as shown in FIG. 10, it breaks and leaves cavities so large that they pass through the entire thickness of the compressed adhesive.

We refer to fig. 1, which represents an advantageous embodiment of the apparatus which comprises a melting tank 15, a gear pump 16, a gas or air reserve 17, a filter 18 and a distributor 12. In practice, the glue solid thermoplastic in the form of pellets, pieces or blocks is placed in the tank 15, then melted by the devices 19 housed in the bottom wall of the tank. The molten glue then passes by gravity to the inlet 10 (FIG. 1B) of the pump 16. A gas at low pressure, for example air at a pressure slightly higher than atmospheric pressure, is brought from the reserve 17, by means of a pipe 21, to a gas inlet 21a (FIG. 1B) of the pump 16. The air and glue flow passes inside the pump 16 in which the cooperating teeth of two pinions 36a, 37a cause a thorough mixture of the melted glue and the gas (in the same way as the air is mixed with the cream in the form of whipped cream), the gas being introduced under pressure into the liquid glue and forming with it a solution 10. This then leaves the pump via a conduit 22, the filter 18, and reaches the outlet conduits 23 of a distribution block 24 and flexible pipes 25 of the dispensing gun 12. The pump 16 can then increase the pressure of the mixture of the gas and the melted glue to a value of approximately 2.1. 106 Pa, maintained in the conduits 23 of the block 24 and the pipes 25 towards the gun 12. At this pressure. , the air, or the gas, of the glue remains in solution in it, until the distribution by the gun 12.

The gun 12 has a pneumatic piston 12P attached to a control valve 26. When pressurized air is transmitted through the inlet pipe 27 of the gun, it pushes the piston 12P upwards and compresses a spring 28, so that the valve 26 is opened and the molten glue can come out of the gun to a pressure of 2.1 ■ 106 Pa. The solution forms a thin, limpid liquid stream which expands rapidly because tiny bubbles of gas appear. These become visible and the solution takes on the appearance of a foam at a distance of approximately 13 mm from the outlet of the nozzle. The tiny bubbles get bigger and are trapped in the glue which solidifies and thus forms a foam with the cellular structure of fig. 11.

The heated tank 15 and the pump 16 of the device 13 are placed in a box 30 made of sheet metal. This consists of two parts; a control part 31 and a part 32 comprising the reservoir. The two parts are separated by an insulating partition 33 which protects the electrical equipment of the part 31 against the heat released by the tank 15. The control part includes the usual thermostats for adjusting the temperature as well as the measurement gauges and temperature control.

The tank 15 is open at its upper part and has lower walls 34 and 35 which are inclined towards an inlet orifice 20 of the pump 16. The lower walls of the tank contain the heaters 19 which heat the material. solid thermoplastic at a temperature slightly above the melting temperature. This is usually 80 to 175 ° C, in the case of most thermo-activated adhesives.

The pump 16 is a conventional single-stage gear pump having engaged gear teeth which act as many small pistons which suck the liquid entering the pump, pressurize it and distribute it at the outlet. Such pumps create suction at the inlet, so that they suck up the liquid. In the embodiment shown, gas such as air or nitrogen also reaches the inlet 21a of the pump through the tube 21.

The two pinions 36a, 37a in engagement with the pump 16 are mounted on two parallel shafts 36,37. One of the shafts 36 is driven by a motor, for example a pneumatic motor 38, and the other can rotate freely.

Such a gear pump 16 is described in detail in the patent of the United States of America No. 3964645. It comprises two gears in engagement whose teeth play the role of multiple small pistons which suck the liquid entering the pump, the pressurize and distribute it at the outlet.

Fig. 2 shows a variant in which the inlet pipe 21 and the inlet 21a of the pump are removed and the air is simply sucked into the pump from the chamber placed above the molten glue.

In this variant, the nested fins 41 fixed to the hubs 42, 43 are fixed to the motor shaft 36 and the idler shaft 37 without being able to rotate. During operation, the fins 41 rotate and pass over the inlet channel 20 of the pump 16 during the rotation of the two shafts 36, 37. When passing over the channel 20, the fins prevent the formation of a vortex of air and draw in air when there is not enough liquid in the pump. In other words, the fins 41 break any vortex of air and expel the liquid which must enter the inlet channel with the air drawn into the channel by the vacuum created by the gears of the pump.

When the apparatus of fig. 1 or fig. 2, the solid thermoplastic adhesive in the form of granules, pieces or blocks is placed in the reservoir 15 in which it melts and forms a reserve of molten material. The latter descends on the inclined lower walls 34, 35 of the reservoir towards the inlet channel 20 of the pump 16. According to FIG. 1, air, or nitrogen, or any inert gas in the presence of the liquid adhesive, is transmitted at a pressure slightly higher than atmospheric pressure, for example at 3.5-104 Pa or less, through the conduit 21 to the air inlet 21a. In the case of the variant of FIG. 2, the air is drawn into the channel 20 from the chamber which is located above the liquid glue from the reservoir. The liquid and the gas which penetrate simultaneously are then carefully mixed in the pump 16 and expelled under pressure by the outlet of the pump in the fluid conduit 22. In the latter, the combination of the gas and the liquid is at a relatively high pressure of the order of 2.1 • 106 Pa, and it is found that the gas passes into solution in the liquid at this pressure. The solution formed then passes through the filter 18, the conduits 23 and the piping 25 to the dispensing gun 12. After opening the valve 26 of the gun, the solution escapes in transparent and limpid form. Before the solution has moved very far from the nozzle, about 13 mm, the solution forms a foam with tiny bubbles of gas in the liquid which turns into a whitish foam. Fig. 5 represents this condition, the interface 44 of the clear liquid 45 and of the foam 46 being represented above the point of application of the adhesive current on a substrate 47. The bubbles continue to grow and to multiply when the current s away from the nozzle. Even after depositing the bead of glue in the form of a foam on the substrate, the bead continues to grow in width and in height for a significant time, for example 1 min after contact with the substrate 47. FIGS. 7 and S illustrate this growth.

This foam retains its heat and has an opening time (during which it retains adhesion characteristics) significantly greater than that of a pearl 49 (fig. 4) of the same glue distributed under the same conditions, but without bubbles d air or gas in the liquid. This long opening time allows the application and adhesion of the adhesive foam to a substrate during a

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much longer than that allowed by the same adhesive distributed without foam. In addition, as shown in figs. 9 and 10, when the open glue foam is compressed between the substrates 47, 47A, most of the gas is expelled from the foam and the glue spreads over a width W which is approximately equal to twice the width W 'obtained with a bead of the same glue distributed by the same nozzle, but without dissolved gas, then by compression with the same force of the two substrates 49, 49A. This additional compressibility of the adhesive foam compared to the adhesive without foam is particularly advantageous in applications such as packaging and cardboard in which only a limited pressure can be applied to the substrate, for example the flaps of smooth or corrugated cardboard. which must be glued. In many applications, the increased compressibility of the adhesive foam, which can be compressed to a thickness as low as 0.23 mm under a pressure of 2.6-104 Pa, allows the formation of an equal or better bond between two substrates as that obtained with the same amount of glue without foam, for half the total amount of glue.

Fig. 3 shows another variant. In it, the thermo-activated molten glue 50 is transmitted from a reservoir 51 by an inlet channel 52 to a two-stage gear pump 54. As in the embodiment of FIG. 1, air, or a gas at relatively low pressure, for example 3.5-104 Pa, arrives via the channel 52 at the pump 54 simultaneously with the molten glue 50. In the first stage of the pump, the gas and the glue mix and they are transmitted by a conduit 56 to the inlet 57 of the second stage 58 of the pump 54. This second stage 58 has a capacity greater than that of the first stage. It transmits a solution of air or gas in the molten glue to the outlet channel through a conduit which joins a distribution block 55. This has a hole allowing the arrangement of conduits 61 and 62 one inside the other. The internal conduit 61 transmits the solution of the liquid glue containing the air to the block 65 of the recycling distributor gun. The block 65 has a conduit 66 for circulation of molten glue towards the valve 70 for leaving the gun 71. There are also return passages 73 which can return the solution via the external conduit 62, the block 55 and the flexible tubing 75, at the inlet of pump 54. This recycling has two roles: it allows the introduction of a greater quantity of air or gas into the solution, and it gives a better solution than a device without recycling.

As in the case of the apparatus of FIGS. 1 and 2, the apparatus of FIG. 3 forms an adhesive foam from a solid thermoplastic adhesive placed in the tank 51. In the latter, the solid material is heated by electrical resistors 81, placed at the bottom of the tank. The molten glue 50 then leaves the latter via the conduit 52 and reaches the inlet of the pump 54. Simultaneously, a gas such as air, carbon dioxide, or nitrogen, reaches a pressure of approximately 3.5 -104 Pa or less at the same input. The suction of the pump ensures the entry of air or gas and liquid in the first stage which ensures a thorough mixing of the materials. The mixture is transmitted through the conduit 56 to the second stage 58. In the latter, the mixture is at a pressure sufficient for the gas to go into solution. Then, the solution is transmitted through the conduits 61 and 66 to the valve 70 of the dispensing gun 71. After controlling a pneumatic motor 82 of this gun, the valve 70 opens, so that the solution is distributed by the nozzle 80. Shortly after the outlet of the nozzle and when the solution is at atmospheric pressure, the gas of the solution separates and forms tiny closed bubbles in the glue. At this time, the latter has the appearance of a foam which continues to widen and grow, as shown in figs. 7 and 8 during the growth of the bubbles. When the glue has solidified, we see that the bubbles have a diameter between 0.1 and 0.7 mm.

As long as the apparatus of fig. 3 operates, part of the solution passing through the conduit 61 circulates in the passages 37 of the block 65 towards the conduits 62, 74, 75 joining the inlet channel of the pump. The solution which returns is then mixed with the hot liquid glue coming from the reservoir 51. This continuous stream of glue in the block 65 and the duct 74 of the gun ensures the presence in the solution transmitted to the gun of a quantity of gas sufficient for qu 'A foam is formed on leaving the nozzle of the gun, the solution never remaining in the pipe 61 for a time which is sufficient for the separation of the gas from the solution.

The essential advantage of the process described is that it allows the inexpensive formation of a thermo-activated adhesive foam without expensive gas and without the use of expensive equipment. The gas used for the foam is either air which is available free of charge or nitrogen which is relatively inexpensive. Any other inert gas in the presence of liquid glue is also suitable.

When blowing agents are used, a mixture of 100 parts by weight of thermo-activated solid thermoplastic adhesive and one part by weight of powder of blowing agent is placed in the tank 15, and the heaters 10 at the bottom of the tank. melt the glue. This and the blowing agent are chosen so that the latter does not decompose or does not give off gas at the glue melting temperature. The mixture of glue and powdered solid blowing agent descends by gravity to the inlet 20 of the pump 16. The mixture passes through the inlet 20 inside the pump 16 and the teeth in engagement of the two pinions (not shown) put the mixture at a high pressure such as 2.1 -106 Pa, the mixture being pumped at this pressure from the outlet of the pump to line 22, to filter 18, to lines 23 of outlet of block 24 and in the heated conduit 25 to the dispensing gun 12. The conduit 25 is a flexible or unheated conduit in a conventional manner.

In an advantageous embodiment, the adhesive Eastabond A-3 manufactured by Eastman Chemical Company, Rochester, New York, is used as thermo-activated adhesive. 100 parts by weight of this adhesive in the form of granules are mixed with one part by weight of Celogen AZ which is a commercial blowing agent manufactured by Uniroyal Chemical Division, Uniroyal, Inc. The abovementioned adhesive has a melting temperature of between 82 and 93 ° C and an application temperature of 188 ° C approximately. The Celogen AZ material decomposes and gives off nitrogen gas between 180 and 210 ° C. These two materials, the blowing agent powder and the glue granules, are mixed in the indicated ratio, in the solid state. The mixed solids are placed in the tank 10 which heats the mixture to about 121 ° C. At this temperature, the glue melts and forms a mass of molten glue and blowing agent in the reservoir 15. The glue descends along the inclined bottoms 34, 35 towards the inlet 20 of the pump 16. The latter drives out the mixing at the outlet in the duct 22. In the latter, the mixing is at relatively high pressure of the order of 2.1 × 10 6 Pa which is maintained until dispensing by the nozzle of the dispenser. The mixture of line 22 passes through the filter 18 and the lines 23 to the heated flexible pipe 25, then the gun 12. During the passage through the pipe 25, the mixture is further heated to the application temperature of approximately 190 ° C. When the mixture reaches 180 ° C, the blowing agent begins to decompose and releases nitrogen gas. At the pressure of the mixture in line 25, the nitrogen immediately passes into solution in the adhesive. This solution remains in this form until distribution by the piston 12.

The apparatus used is inexpensive and corresponds, for the most part, to the apparatus used for the melting and distribution of thermo-activated adhesives. Consequently, the cost of additional apparatus required is low.

The essential advantage of the process described is the resulting sticky product. The adhesive foam created has a specific weight which is of the order of half that of the non-foaming adhesive. The surface of cooperation with the substrate is increased compared to the same adhesive which does not form foam. The weather

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opening is also increased compared to glue without foam. These characteristics allow a reduction in the cost of the adhesive, in many applications, by at least 50% without reducing the fixing resistance of the bonded surfaces.

Another advantage is the thixotropic property of the glue foam. When ordinary glue is applied to a vertical surface, it tends to run down that surface like a drop of water applied to a vertical substrate. When it descends along. this vertical surface, it constitutes a thin film at the top and a film of increasing thickness towards the bottom of the droplet or of the current. Given the variation in thickness of the material, the opening time is variable, so that the connection has a variable quality. On the other hand, the adhesive foam, given its more accentuated thixotropic nature, presents less risk of deformation or of flow on a vertical surface and therefore forms a more constant or regular quality bond on a surface.

In the present specification, it has been considered that the adhesive is a thermo-activated thermoplastic adhesive. This expression covers all adhesives containing no solvent and which are applied in the molten state and form a bond after cooling in the solid state.

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Claims (21)

623,352 2 1. A method of bonding two substrates together using a thermally activated thermoplastic adhesive, characterized in that it comprises the formation of a mixture of a gas with the thermally activated adhesive, placing the mixture under pressure so that the gas passes into solution in the thermo-activated glue, and the distribution of the solution at low pressure, so that the gas is released from the solution and forms a glue foam, this glue foam then being compressed between two substrates so that it forms a bond between them. 2. Method according to claim 1, characterized in that the mixture is placed at a pressure of at least 6.3 • 105 Pa so that the gas goes into solution. 3. Method according to claim 1, characterized in that the mixture is placed at a pressure of about 2.1 • 106 Pa so that the gas goes into solution. 4. Method according to claim 1, characterized in that the mixture is formed by mechanical agitation of a molten material in the presence of a gas. 5. Method according to claim 1, characterized in that the mixture is formed by heating a solid adhesive and a blowing agent. 6. Method according to claim 1, characterized in that the adhesive foam is allowed to cool after having compressed it between the two substrates. 7. Method according to claim 4, characterized in that it comprises the distribution of the solution at atmospheric pressure. 8. Method according to claim 7, characterized in that the solution is distributed on a first substrate at atmospheric pressure. 9. Method according to claim 8, characterized in that a pressure of 2.8 ■ 104 Pa is applied to the adhesive foam so that it is reduced in the form of a film of approximately 0.23 mm thickness placed between the substrates. 10. Apparatus for implementing the method according to claim 1, characterized in that it comprises a device intended to form a mixture of thermo-activated thermoplastic adhesive and a gas, a device for putting the mixture under pressure so that the gas passes into solution in the adhesive, and a device for distributing the solution at a pressure lower than the pressure for maintaining the gas in solution. 11. Apparatus according to claim 10, characterized in that it comprises a device for heating the mixture of the solid thermoplastic adhesive and the blowing agent. 12. Apparatus according to claim 10, characterized in that it comprises a device for agitating the gas in the molten material. 13. Apparatus according to claim 10, characterized in that the pressurizing device comprises a gear pump. 14. Apparatus according to claim 10, characterized in that a heated tank is intended to receive and to melt the solid thermoplastic adhesive. 15. Apparatus according to claim 10, characterized in that the dispensing device comprises a gun having an outlet nozzle and a valve which can be selectively opened and which regulates the ejection of the adhesive foam by the gun. 16. Apparatus according to claim 15, characterized in that the second heating device comprises a device housed in the dispensing gun. 17. Apparatus according to claim 10, characterized in that the stirring and pressurizing device comprises a two-stage gear pump. 18. Apparatus according to claim 15, characterized in that the dispensing gun is of continuous flow and ensures the constant circulation of the solution of gas and liquid glue in the gun when the valve is closed. 19. Apparatus according to claim 18, characterized in that the constant circulation gun comprises two tubes for circulating the gas solution and the liquid adhesive between the pressurizing device and the gun. 20. Apparatus according to claim 19, characterized in that one of the tubes is placed inside the other. 21. Object obtained by the method according to claim 1.