WO1979000767A1

WO1979000767A1 - Process for coating a substrate with a film of thermoplastic material

Info

Publication number: WO1979000767A1
Application number: PCT/EP1979/000014
Authority: WO
Inventors: C Guignard
Original assignee: Battelle Memorial Institute; C Guignard
Priority date: 1978-03-15
Filing date: 1979-03-14
Publication date: 1979-10-04
Also published as: GB2035844B; JPS55500152A; IT7920988A0; NL7902026A; IT1111547B; BE874830A; ES478661A1; FR2419769A1; SE7909387L; GB2035844A

Abstract

A process for coating a substrate (10) with a film of thermoplastic material, in which fibres or filaments of the material are applied to the substrate and are melted to form a continuous film. The material may be applied by forming, on a donor substrate (5) a layer of said material in the molten state, applying an electrostatic field between the substrates (5, 10) to draw from said layer a plurality of filaments which are deposited on the receiving substrate (10) forming the film on the receiving substrate by causing said filaments to melt. This process can be used in particular to form very thin films. It is applicable to adhesive materials as well as to thermoplastic polymers.

Description

PROCESS FOR COATING A SUBSTRATE WITH A FILM OF THERMOPLASTIC MATERIAL

This invention relates to a process for coating a substrate with a film of thermoplastic material, in particular a thermoplastic polymer or an adhesive having hard and soft states according to its temperature.

British Patent Specification No. 933 250 describes a process which consists in covering a Substrate with a layer of thermoplastic material in powder form by electrostatic means, and melting subsequently the covering powder to obtain a continuoμs film adhering to the Substrate. The thickness of the film depends on the granulation of the powder. However, when it is desired to deposit very thin films, of the order of 10 microns, it becomes difficult to ensure with this method a sufficient uniformity of the film, and the latter may have microporous areas.

US Patent MO. 2723 646 describes an electrostatic atomizer for a liquid, designed to ccat surfaces with this liquid. This apparatus is essentially adapted for the application of substances in solution such as paints. Once the liquid layer has been deposited on the substrates to be coated, the solvent has to be evaporated in order to obtain the desired coating. Apaxt from the fact that such a method is applicable only to substances which can be dissolved, the evaporation of the solvent gives rise to problems and reduces the output to a substantial extent. Furthermore, a process is also-known for producing filaments by electrostatic means from a molten thermoplastic material, being the subject of British Patent No. 1 484 584. This process makes it possible in particular to deposit a non-woven coating on a substrate.

The object of the present invention is to provide a coating process by means of which it is possible to obtain a coating which is continuous and of uniform thickness whatever the shape of the substrates.

According to the invention, a process for coating a Substrate with a film of thermoplastic material is characterized in that fibers or filaments of said material are .distributed uniformly on said Substrate by raising their temperature, at least in the proximity of the Substrate to be coated , to a value at least equal to the melting temperature of the aforesaid material during a period of sufficient length to cause the totality of said fibers or filaments to melt in order to cover said Substrate uniformly with the said material.

This technique, as employed in the case of coating of a Substrate with a film of thermoplastic polymer material, makes it possible to obtain not -only an absolutely uniform but also a particularly thin film, for example of a thickness of 12 μ and less, that is to say about one tenth the thickness of the thinnest uniform films obtained with the above-mentioned powder process. The accompanying drawings illustrate schematically and by way of example several embodiments of installations for performing the process according to the present invention. In the drawings: -

Figure 1 is a perspective view showing a first embodiment and a modification thereof;

-Figure 2 shows a second embodiment in side elevation;

Figure 3 shows a third embodiment in side elevation;

Figure 4 is a partly perspective view of a fourth embodiment; and

Figure 5 is a view in elevation of a fifth embodiment.

The installation illustrated in Figure 1 is identical in concept to that described in Swiss Patent No. (Patent Application No. 15840/77), and therefore only the elements necessary for the linderstanding of the present invention will be described in detail.

The installation shown in Figure 1 comprises a feeder assembly 1 having two parallel endless chains 2 mounted on three pairs of guiding sprocketε 4a, 4b and 4c located at the vertices of a triangle and of which one, 4a, is fast with the drive shaft of a motor M.

Electrical conductor wires 5 are tensionei transversely between the two parallel chains 2 and constitute a plurality of earthed electrodes. These wires are to be heated by the Joule effect with the aid of a direet-current source DC and two busbars. 8 and 9. An electrostatic powder dispenser station is mounted at a location on the path of the wires 5. This station comprises essentially a hopper 6 associated with a vibrator (not shown), and an electrode 7, connected to a terminal of an electrostatic generator GE ₁ supplying a potential of the order of 15 kV for example, located at the outlet of the hopper 6. This electrode 7 is designe to impart an electric charge to the powder contained in the hopper 6 and composed of a dielectric thermoplastic material such as polyethylene, polypropylene, polystyrene a polyamide, a polyester, etc.

In this example, the Substrate to be coated is an aluminum foil 10 passing from a feed roll 11 to a take-off roll 12 by way of two guiding rolls 13 and 14 arranged to convey this foil initally parallel to a portion of the feeder assembly , and then through an oven 15. The foil 10 is connected to a terminal of an electrostatic generator GE ₂ , supplying a voltage of the order of 20 to 30 kV, for example. The shafts of the rolls 11, 12, 13 and 14 are insulated, in order to keep the aluminum foil at the potential of the electrostatic generator GE ₂ . The powdered dielectric thermoplastic material, which is to be used for cαating the foil 10 with a thin film of the order of 5 to 10 μm. thick, is placed in the hopper 6. The powder leaving this hopper is charged electrostatically by comiήg into contact with the electrode 7. The powder thus charged is attracted by the earthed wires.5 and forms a deposit on their surfaces, forming a uniform layer. At the same time, these wires are heated by a current from the direct-current source De. The wires 5 are moved, at right angles to their length round a triangular path by the chains 2 and 3 and the motor M in the direction indicated by the arrow F, whilst the aluminum foil 10 is moved in the direction of the arrow F ₁ .

The heated wires 5 melt the powder deposited on them, and constitute a source or donor substrates for the thermoplastic material. When the thus molten dielectric material arrives at a position facing the foil 10, charged to a high potential by the generator GE ₂ , the forces exerted on this material by the electrostatic field draw out a plurality of filaments which are deposited on the foil. The non-woven product formed by the aecumulation of the filaments on the foil 10 then passes into the oven 15, the temperature of which is at least equal to the melting point of the thermoplastic material, so as to cause the filaments to melt to form . a continuous film on the foil 10. The thickness of the film thus formed obviously depends on the quantity of deposited material and, in consequence, on the mean size of the filaments and on the relative speed between the foil 10 and the feeder assembly 1. Tests have shown that it is possible to form by this process a film having a thickness of the order of 5 to 10 μm, of perfect continuity and devoid of any porosity, ensuring complete coating of the Substrate in spite of the very small thickness of the Movie. If it is taken into aecount that in order to obtain the same result by depositing the powder directly on the foil 10, without passing through the filament stage, it is necessary to deposit a material layer of the order of 50 μm thick, the considerable saving in plastic material achieved by the present process will be appreciated.

The above-described embodiment may undergo certain modifications which do not alter the principle of the process. Thus, the film may be formed, not by melting a non-woven product deposited on the foil 10, but by heating the space between the feeder assembly 1 and the foil 10, for example with the aid of two infrared bars 16 placed one at each end of this space, so as to prevent the solidification of the filaments, thus causing them to be deposited on the foil 10 in the molten state and to form the film progressively without passing through an intermediate non-woven stage as previously described. It is of course understood that in this variant the oven 15 can be dispensed with.

In another embodiment, very schematically illustrated in Figure 2, the filaments are deposited on a Substrate the temperature of which is not less than the melting temperature of the filament material.

In the embodiment shown in Figure 2, the feeder assembly 1 'is mounted around five pairs of pulleys 4'a, 4'b, 4'c, 4'd and 4'e_, insulated from earth. The electrodes 5 'are charged to the potential of the electrostatic generator GE. The thermoplastic powder is distributed by a hopper 6 'at earth potential, and the powder deposited on the electrodes 5' is melted during the movement of these electrodes 5 'through an oven 17. In this example, the Substrate is an aluminum foil 10 ', hot-formed in an oven 18 and leaving the oven at a temperature which is higher than the melting temperature of the thermoplastic material, so that the filaments which are deposited on the Substrate 10 'melt on contact with the latter at the rate at which they are deposited on the hot Substrate, thus forming a film coating on this Substrate as described in connection with Figure 1. It will be appreciated, of course, that this second embodiment is not limited to the coating of a hot-formed substrates, but can also be applied to a flat substrates. It was simply intended as an example in which it is advantageous to deposit the filaments on a hot substrates. Conversely, deposition on a non-flat substrates can be performed in the embodiment illustrated in Figure 1.

The uniformity and the minimum thickness of the deposited film depend on the interfacial tension between the receiving Substrate and the thermoplastic material, which is essentially a function of the wettability of the surface of the Substrate, and on the surface tension of the molten thermoplastic material. In the well-known powder-coating and electrostatic pulverization methods, the surface of the substrates has to be totally covered by the powder, so that the thickness of the film thus formed is a function of the powder grain size. In contrast, in the case of the process in described, the density of the filaments per unit surface area can be selected at will, in particular by controlling the speed of advance of the receiving substrates in front of the donor substrates, the minimum density of filaments required to form homogeneous film on the receiving Substrate being determined by the inter facial tension between the thermoplastic material and the receiving Substrate (it should be noted that the surface tension of the plastic material can be modified by the addition of wetting agents). It is for this reason that the process described above makes it possible to obtain a substantially thinner coating film than the powder or pulverization. processes already known. An important element in determining the thickness of the film is constituted by the possibility of making the fibers much finer than the powder grains. whereas the grain size of the powder is several tens of microns, it is possible to make the filaments or fibers finer than 10 μm, even finer than 1 μm. Further, with a uniform distribution of the fibers, the thickness of the film is directly related to the size of the fibers. Thus it is possible to form very thin films, of less than 10 μm.

Particularly gocd results have been obtained by orienting the fibers parallel to each other in order to form a layer of fibers extending parallel, practically side by side, on the substrates. Such a method ensures a uniform coating without holes, the thickness of which ^" coating is a direct function of the size of the fibers; thus, with fibers between 1 and 10 μ m. Thick it is possible to obtain a layer the mean thickness of which is of the order of 5 μm- Such a deposit of parallel fibers can be obtained by adjusting the relative velocity between the electrode wires 5, 5 'and the receiving Substrate to a value at least equal to the rate of production of the fibers, or substantially higher, for example 100 m / min, which gives a quite regular fiber layer and makes it possible to form a continuous film after heating, of the order of 15 to 20 g / m ² of polypropylene, for example. The orientation of the fibers can also be achieved by means of a current of air.

In a variant of the process, the starting material is already in the form of fibers or filaments instead of a layer of molten material. Figures 3 and 4 illustrate two ways of putting this into effect.

Figure 3 refers to a conventional installation for electrostatic flock formation, which comprises a hopper, 19 filled with fibers, at the outlet of which is an electrode 20 connected to one of the poles of an electrostatic generator GE, and the other pole of which is connected to a rectangular electrode 21 below the path of a foil 10b to be coated. This installation further comprises a blower 22 and a heating unit 23. The fibers leaving the hopper 19 are charged by the electrode 20, which generates an ionized atmosphere at the outlet of the hopper, and are oriented within the field generated between the electrodes 20 and 21 in such a manner that they move, and are oriented, perpen- dicular to the surface of the foil 10b_. The blower 22 drives an air current contrary to the direction of movement of the foil 10b as indicated by the arrow F, which current lays the fibers parallel against each other, in which position they are melted by the heating unit 23.

In the embodiment shown in Figure 4, a wad of filaments 24 is unwound onto the foil 10c to be coated. To this end, and in order to ensure a regular distribution of the fibers, an electrostatic field is generated between a charging electrode 25 which charges the filaments, and an electrode 26 mounted below the foil 10c. A heating unit 27 serves to melt the filament layer spread on the foil 10c.

Figure 5 illustrates a mode of execution in which the filaments are produced by a multiple extruder 28, an electrostatic field being established between this extruder, connected to one of the poles of an electrostatic generator GE, and an electrode 29 connected to the other pole of the same generator. The filaments are deposited on a foil 10d moving in the direction of the arrow F at a speed which at least equals the rate of extrusion of the filaments, in such a manner that the filaments, held at a uniform distance from each other between the extruder 28 and the foil 10d by the effect of the field, are deposited in parallel to each other on the foil.

It is to be understood that the invention is not limited to the coating of electrically conductive substrates. For example, it is possible to coat card-board with a film of plastics material in order to render it impermeable. In such a case, the fibers are deposited on a cold substrate. Good results were achieved in tests, by briefly heating a layer of parallel thermoplastic fibers deposited on cardboard to a temperature substantially higher than the melting temperature of the plastic material. The shorter the heating period, the less the molten plastic will tend to be absorbed by the cardboard. In the tests, a film could be obtained by heating a layer of polypropylene of 15 - 20 g / m ² to about 200 ° C for about 3 seconds. The duration of heating can be further reduced by raising the temperature still higher.

Although in the foregoing a process for coating a Substrate with a film of thermoplastic polymer material has been described, this same process can be used for coating a Substrate with an adhesive film, in order to produce a self-adhesive material or an adhesive tape, for example. Tests were carried out with an adhesive pulverized in a cryogenic process and placed in a feed hopper as in the case of Figures 1 and 2, kept at a temperature below the softening temperature of the adhesive. Following this, the electrode wires 5, 5 'were heated to a temperature of approximately 60 ° C, as was the receiving substrates in order to effect the spreading of the deposited adhesive before cooling to ambient temperature.

Claims

CLAIMS:

1. A process for coating a Substrate with a film of thermoplastic material, characterized in that fibers or filaments formed of said material are uniformly distributed on said Substrate by raising their temperature at least in the proximity of said Substrate to a value at least equal to the melting temperature of the said material for a period of sufficient length to cause the totality of said fibers or filaments to melt and thus to cover the substrates uniformly with said material.

2. A process according to Claim 1, characterized in that the fibers or filaments are disposed on the substrates in the form of a layer of substantially parallel fibers or filaments in a side-by-side arrangement.

3- A process according to Claim 2, characterized in that the size of the fibers is selected as a function of the desired thickness of the coating.

4. A process according to Claim 1, characterized in that said fibers are formed by disposing a filiform donor Substrate to face the first-mentioned Substrate, forming on this donor Substrate a layer of a dielectric thermoplastic material heated to a temperature at which its viscosity enables it to form fibers, and creating between the said substrates an electrostatic field the lines of force of which extend substantially perpendicular to the surface of said donor substrates, in such a mariner that under the action of the forces generated by this field groups of molecules are drawn from discrete points of the said layer along respective lines of force, each thus drawing a fiber of the said material towards the first-mentioned substrates.

5. A process according to Claim 4, characterized in that the speed of relative movement between the said substrates is at least equal to the rate of production of said fibers within the electrostatic field, so as to form on the receiving substrates a layer of parallel fibers.

6. A process according to Claim 1, characterized in that the Substrate is heated to a temperature at least equal to the melting temperature of the said material, in such a manner that the said fibers or filaments melt progressively as they are being deposited on the substrate.

7. A process according to Claim 1, characterized in that the space surrounding the substrate is heated to a temperature at least equal to the melting temperature of said material, in such a manner that the fibers or filaments are heated to the melting temperature of the said material before coming into contact with the substrate.

8. A process according to Claim 1, characterized in that said fibers or filaments are deposited on the substrate at a temperature below the melting point of the material, the substrate is removed from the deposition region of said fibers or filaments, and the temperature of the deposited fibers or filaments is raised to a.value which at least equals the melting temperature of said material.

9. A process according to any one of Claims 4 to 8 in which the material with which the receiving substrate is coated is an adhesive.