WO2002030657A1

WO2002030657A1 - A method, an apparatus and a matrix for making composite plates

Info

Publication number: WO2002030657A1
Application number: PCT/DK2001/000669
Authority: WO
Inventors: Claus Burchardt
Original assignee: BARSMARK AS
Current assignee: BARSMARK AS
Priority date: 2000-10-13
Filing date: 2001-10-12
Publication date: 2002-04-18
Anticipated expiration: 2003-04-13
Also published as: DK173919B1; DK200001526A; AU2002210381A1

Abstract

A method and an apparatus for making of composite plates wherein a web is provided with electrically conducting fibres for heating the web up with electric current during formation of the web into composite plates.

Description

A method, an apparatus and a matrix for making composite plates

Background of the invention

The present invention concerns a method of production of composite plates as described in the introductory part of claim 1. The invention also concerns an apparatus for production of composite plates, a matrix of fibre, a composite plate, a tubular composite plate, a product containing a composite plate and a method for post-forming of a composite plate.

It is prior art to produce composite plates from matrices consisting of fibres of different materials. A widely used method is making such plates in moulds with heat and pressure applied to the matrix with or without a thermosetting plastic component. In cases where a thermosetting component is not used, the fibres consist of a thermoplastic polymer melted by heating and exposed to pressure, after which the finished plate appears after cooling and hardening. For reinforcement of the material it is a usual practice to mix polymer fibres and glass fibres, where polymer fibres are melted during the formation of composite, whereby the polymer encloses the non-melted glass fibres which subsequently function as reinforcing material.

To produce composite plates with smooth and even surface and more or less homogenous thickness, it is necessary to use long and arrayed fibres, i.e. fibres are to have certain preferred directions, for example, lie at certain angles with respect to the direc- tion of web in the production. Such fibre matrices are called web. This term covers not only woven matrices but also matrices where long fibres in array are assembled by stitching. The term "web" is used for long fibre composites as opposed to "non- woven" materials which are mixtures of short fibres and are used, for example, for insulation mats.

In order to produce composite plates in great lengths, a so-called belt press can be used, which is known e.g. from WO 97/26397. A matrix of fibres is passed through a heating process and a pressing process, where the pressing process takes place between pressure rollers with belts running around them in such a way, that composite plate is produced as lengths between the belts on each side of the plate. However, belt presses are very expensive machines, why other apparatuses have been developed for the production of composite plates in web form without using belts around the rollers. These apparatuses cost only a small fraction of the corresponding belt presses.

Such an apparatus is described in Norwegian patent application NO 984275 based on International patent application WO 98/31857. In this apparatus composite plates are produced in lengths from a web of polymer fibres and reinforcing fibres, preferably glass fibres. The web is led through a first heating zone, where it is heated up by ther- mal radiation to a first temperature. Then the web is led over a number of warm rods that heat the web up to a temperature above melting point. After this heating stage the web is cooled and then pressed to form composite plates. In the process of pressing the melted polymer settles around the glass fibres, thus forming the composite with a more or less smooth surface.

This method has some disadvantages. To achieve a complete melting of web, it is exposed to a strong thermal radiation on a preheating stage. The disadvantage of this preheating stage is that a temperature is reached on the external side of the web, which is close to decomposition temperature for the polymer, for example, 230°C for poly- propylene, abbreviated PP, while the temperature in the center of the web is much lower, for example, 140°C, and often below melting point, which for PP is 165°C. Consequently, the web undergoes a decomposition of external polymer fibres, which is very undesirable but accepted in order to obtain the necessary preheating for the final temperature at which the polymer melts to consolidate the composite plate.

Another disadvantage is lack of homogenization in the material when the web initially is heated to the melting temperature and then pressed at a lower temperature. Homogenization is the process in which the polymer melts completely and then settles, enclosing the reinforcement material.

The prior art methods have to run at a relatively high speed in order to minimize the time span between the heating and the subsequent pressing with cold rollers. But at high speed it is impossible to heat the web up to the correct working temperature, which is somewhat higher than the melting temperature and preferably close to decomposition temperature. This is one of the reasons why the thermal radiation preheating stage is used in the prior art methods and systems. It has, however, not been possible to solve this problem completely as high process speed results in poor preheating and unsatisfactory homogenization of the material, while slow process speed results in better heating but, on the other hand, it takes long time for the web to reach the pressing rollers why the pressing takes place at too low temperature, also resulting in unsatisfactory homogenization.

There are other disadvantages connected with the use of the conventionally manufactured composite plates. It is sometimes desirable to produce other forms of composite plates than great lengths. To a certain extent it is possible to re-melt composite plates and re-form them, but it is a slow and difficult process, which is also energy demanding, and more advanced forms than lengths, such as round containers, are not so easy to shape with webs (lengths) as basic material.

The purpose of this invention is to provide a method and an apparatus for production of composite plates from the web, where the above described disadvantages are avoided, and where preheating of web by thermal radiation is not necessary.

This purpose is achieved by using a method of the type mentioned in the introduction, with the feature indicated in the characterising part of claim 1.

By the above mentioned term, namely matrix of fibres, is meant a composite which can include short, non-woven fibres that are stitched or pressed together, and/or long fibres, woven or sewn together. In the text below the invention will be explained with a composite web as an example, which however shall by no means delimit the invention to not being applicable with so-called non- woven matrices of fibres.

Temperature action for the production of composite plates is achieved by heat transfer from an electrically conducting web or fibre forming part of composite matrix and which is heated by means of electric current. The current conducting fibres may, for example, form part of the matrix during the production of the matrix, but it is also possible to put these fibres or stitch them into the matrix subsequently.

There are different suitable electrically conducting fibres, which are commercially available. These conducting fibres, which are e.g. used for heating clothes, can be heated up to above 200°C, even up to 400° C, which makes them suitable for heating the web. If the matrix is heated to a temperature above the melting point for the polymer in the matrix, for example 200° C for polypropylene, conducting fibres will melt together with the polymer and possible other fibres in the matrix. It is assumed that melting temperature of conducting fibres is higher than melting temperature of the polymer in the matrix.

In the composite production processes known until now as described in the introduction, it has been very difficult to produce composite plates with greater thickness than a few millimeters. The problem is that the web, when heated up by thermal radiation or contact heating, has a considerable temperature gradient from the external side to the core of the web, which implies decomposition of polymer on the external side of the web before the web in the core of the composite is melted. Existing composite plates are therefore incompletely melted, resulting in inferior properties. The composite pla- tes produced according to the invention are free from these disadvantages as even thick webs with embedded current conducting fibres can be heated up from inside. Therefore, this invention is a big step forward in this area. Furthermore, it is possible to evenly heat webs with large thickness variations since the thick areas will have more current conductors than the thin areas, resulting in more power, and therefore more heat, being supplied to the thick areas. Until now, it has not been possible to produce thermoplastic composites, especially in lengths, with distinct variations in thickness.

During production of composite plates, the web is heated up and pressed at a temperature where at least one, however preferably one, of the composite materials are mel- ting. Typically, the composite is made of polymer fibres, which by melting settle around reinforcing fibres, such as glass, and other fibres in the composite, if any. Pressing may occur by placing the web, which is at a temperature higher than the melting temperature of the polymer, between two press plates, which are then moved to- wards each other until a predetermined distance. The composite plate formed thereby will then assume this predetermined dimension at the most. Alternatively, pressing can be performed by leading the web between a pair of rollers adjusted to a certain distance or a certain pressing force whereby the composite plate is formed as a long web which after cooling, when the polymer has set or hardened, can be rolled together, depending on the flexibility, for further transport.

Heating of the web, which occurs by conducting current through the web, can be achieved by conveying the web over a first and a second roller with electric potential bet- ween these rollers for conduction electric current through the conducting fibres in the web between the first and the second roller. Such two rollers, which are acting as electrodes, can with advantage be used respectively before and after the press mold. It is, however, also possible that electric potential is established between a first roller and the press mold itself.

Furthermore, it is possible to subject the web to several press molds, where different electric potentials can be provided between a number of different rollers and press molds by which a gradual heating can be achieved, in case such is appropriate.

The web can be formed in different ways and may comprise fibres of PP, PET or

PTFE, as well as fibres of reinforcing material, such as glass fibre, carbon fibre, ara- mide fibres, kevlar fibres, metal fibres, carbon coated glass fibres, vegetable fibres and other organic fibres, or synthetic fibres.

Besides, the web according to the invention also contains current carrying fibres. Such current carrying fibres in the web can, for example, be carbon fibres or metal fibres woven or stitched into a web of polymer fibres and reinforcing fibres, preferably glass fibres. It is also possible, however, for reinforcing fibres to act as current conducting fibres. Carbon fibres can, for example, constitute the reinforcing material. But also glass fibres as reinforcing material can be made conducting by coating their surface with a conducting material. The web has proved to be particularly usable if the share of reinforcing fibres in the web is between 40% and 80%, preferably between 60% and 70%, and most preferred 60%.

Cooling of the web after pressing can be performed in different ways, but it is preferred that the web is cold pressed by at least one rotating cold roller. The roller acts as an additional mould for the composite plate where, for example, the surface of composite plate can be structured. Such structuring, in which embossing of the surface may form a part in the shape of, for example, patterns or logos, may, however, be done during pressing in hot condition.

It is necessary that contact is established between the electric power source and the conducting fibres in the web. This can be achieved by direct contact with conductive fibres on external side of the web. In a further development of invention, the matrix has been supplied with contact tracks, e.g. alongside the track, for electric contact with skates (like collector shoes) or rollers suited for this purpose.

These contact tracks will also be accessible after heat and pressure treatment of the web during the forming of composite web. By conducting current through the finished composite web, the composite can be heated up when used later. This offers a possibility for an absolutely new application of thermoplastic composites.

Examples of applications, where subsequent heating is desirable, are especially those when the composite has to be post-formed, i.e. formed after actual production of con- solidated composite. Here the composite is heated to a temperature below melting point, yet high enough for plastic deformation of the composite. By conducting more current through some areas in the composite, a differentiated forming may be performed as these areas will be softer than the others. Furthermore, it is possible to deform even relatively thick areas, which has not been possible before as heating from outside caused melting of material at the outer side before the material was heated through and through and plastic. Heating of the composite is also a useful property of a composite according to the invention. Thus, such a composite can very easily be kept frost-free in cold areas, for example, if used for feed or drinking troughs in outdoors farming of domestic animals in the winter time.

If such composite plates are used for walls in cold storage rooms or refrigerators, the defrosting of the walls is achieved in a very easy and quick way by heating the composite. In larger cold stores it would be possible to completely defrost and dry the walls before the room is heated up, thereby saving large amount of energy for heating and subsequent cooling of the cold storage room.

Furthermore, the composite can be very usable for rooms requiring heating and particularly if a very uniform heating is required without significant temperature gradients.

Another application of the composite is on inside walls of caravans or railway cars, which are known for lack of comfort in cold seasons as the walls are cold. By using a composite according to the present invention, the inside walls can be heated up in a simple way, thus avoiding such discomfort.

One more application of the conductive composite material is to use it as a screening wall for electronic equipment, for example in mobile or stationary military installations. Such screening can, for example, be used for protection from tapping.

During production of the composite web it is further possible to add a surface layer on the composite web, for example, a synthetic material or metal layer. Moreover, the surface of the composite web can be structured if this is suitable.

Description of the drawing

Embodiments of the invention will be explained below with reference to the figures, where:

FIG. 1 illustrates the production of a composite plate according to the invention, FIG. 2 illustrates the second production method according to the invention, FIG. 3 illustrates the third production method according to the invention, FIG. 4 illustrates the fourth production method according to the invention, FIG. 5 shows a matrix according to the invention, viewed from the flat side, FIG. 6 shows an alternative production method for composite plates in accordance with the invention, where the composite plate is shaped as a pipe, FIG. 7 shows a second alternative production method for composite plates according to the invention, where the composite plate is shaped as a pipe.

FIG. 1 illustrates production of a composite plate according to the invention. The web

1 is fed into the machine according to the invention by means of two feeding rollers 2, 2'. After that the matrix is carried through a first set of rollers 4, 4' and a second set of rollers 6, 6', between which an electric potential 8 is established, which causes electric current, direct current or alternating current, to pass through web 1 from the first roller set 4, 4' to the second roller set 6, 6 resulting in heating of web 1. The current is chosen in such a way so that it causes melting of polymer fibres in web 1. By means of pressure rollers 3, 3', which can be, but are not necessarily heated up, the partly melted web 1 is pressed together with pressure P to form a composite web 5. This web can be subjected to additional pressing, for example, by rollers 6, 6', before the composite length is cooled for consolidation of the final composite 7.

FIG. 2 illustrates a production method according to the invention, in which the electric potential is established between the first set of rollers 4, 4' and pressure rollers 3, 3'.

FIG. 3 illustrates a production method according to the invention, where the web is subjected to pressure by a set of pressure plates 10, 10'. Electric potential is established between the first set of rollers 4, 4' and pressure plates 10, 10'.

As alternative for pressure plates or pressure rollers, belt presses can be used.

FIG. 4 illustrates a production method according to the invention, where different electric potentials 8, 8', 8" are established between several sets of rollers 4, 4', 9, 9', 3, 3' and 6, 6'. The matrix in this process is subjected to pressure between the rollers 3,3', but it is also possible to subject the matrix to pressure between the other rollers, if this is considered to be necessary.

The distance between different set of rollers, 4, 4', 6, 6', 3, 3', 9, 9' can vary according to need. A long distance between the rollers allows choosing a higher production speed compared to prior art techniques.

FIG. 5 shows a matrix 1 according to the invention seen towards the flat side. A feeding roller 2 is also shown together with one of the pressure rollers, 3. The matrix is furnished with contact tracks 11, 11' which are designed for contacting external contact rollers 12, 12', contact skates, shoes or similar, in order to ensure heating of matrix 1 during forming. Between contact rollers 12, 12' an electric potential 8 is established. In this case it is assumed, that the conductive fibres are running crosswise of the matrix or under a small angle with the transverse direction of the matrix in order to ensure that electric current can run between contact rollers 12, 12'. In a preferred version of the invention the conducting tracks will still be accessible for contacting after consolidation of composite web 7. This way a later post-forming can be performed in a simple way, as discussed above.

FIG. 6 shows an alternative production method for composite plates according to the invention, where the composite plate is shaped as a pipe. Such composite pipe is obtained by pulling a fibre stocking 1, preferably web, over an inner mould 14, for e- xample, a steel cylinder with Teflon coating. After that, the web 1 is heated by conducting current through two contact tracks 11, 11', which run, for example, longitudi- nally or transversely of the stocking, or in diagonal tracks. For exerting pressure on the matrix of fibres, the stocking is encased by an external press mold 13.

Pipe-shaped composite plates can also be produced by diagonal wrapping of a cylindrical steel mold 14 with a long web-shaped matrix according to the invention as illu- strated in FIG. 7. Suitably, the matrix 1 is made with three contact tracks, namely two contact tracks 11, 11' along the sides and a third track 11", for example, in the middle of composite web 1. Diagonal wrapping allows contact tracks 11, 11' to be in contact with each otlier. Passage of current through the matrix is then achieved by the power source suitably contacting the central contact track 11" and at least one of contact tracks 11, 11' at the sides of the matrix.

The making of pipes of the composite according to the present invention implies great advantages. First of all, such a pipe can subsequently be heated by conducting current through conducting fibres. Hereby, a type of pipe is obtained which can be easily kept frost-free in cold surroundings, or which can be used for general heating of liquids in the pipes. Furthermore, it is possible to easily heat up such pipes to a temperature needed to kill bacteria or similar in the pipes. This aspect is of high interest for hospital sector, where the pipes, as per existing practice, cannot be satisfactorily cleaned, or where a thorough cleaning can only be done with considerable cost.

Furthermore, local heating of pipes can be appropriate for bending or other adjustment of pipes during installation.

Heating the composite with conducting fibres can also be supplemented with techniques comprising magnetic induction, where the current is induced in the conductive fibres by means of varying magnetic fields. The advantage of such forms of technique is that there is no need for contact, for example, by means of contact tracks. Besides, such heating does not require the conductive fibres to be oriented in a certain direction or to have a certain pattern.

In the above, the invention has been explained in connection with webs containing a thermoplastic polymer. It is, however, within the ability of the skilled in the art to utilize the invention by using hot fibres in thermosetting composite plates.

Claims

PATENT CLAIMS

1. A method for making composite plates, where a matrix of fibres comprising at least two different materials undergoes a process of heating to a temperature, where at least one of the materials in the matrix is melted and subjected to compression by at least one press mold consisting of at least two fixed pressing members, between which the matrix is pressed in hot condition and then cooled, characterised in that the matrix contains electrically conducting fibres, and that the matrix is heated by electric current conducted through the conducting fibres.

2. A method according to claim 1, characterised in that at least one of two fixed press members belongs to the group consisting of pressure plates, rollers and belt presses.

3. A method according to any of the previous claims, characterised in that the matrix is carried over a first roller and then over a second roller, where there is electric potential between the rollers for establishing an electric current in the conducting fibres of the matrix between the first and the second roller.

4. A method according to any of the previous claims, characterised in that the matrix contains fibres of polymer, preferably PP, PET or PTFE, and fibres of a reinforcing material, preferably glass fibres.

5. A method according to any of the previous claims, characterised in that the conducting fibres contain reinforcing fibres coated with electrically conducting material.

6. A method according to claim 4 or 5, characterised in that reinforcing fibres comprise glass fibres, and that the share of glass fibres in the matrix is between 40% and 80%, preferably between 60% and 70%, and most preferred about 60%.

7. A method according to any of the previous claims, characterised in that the matrix is subjected to surface structuring during pressing in hot condition, and/or by pressing during the cooling process.

8. A method according to any of the previous claims, characterised in that the matrix is at least partly heated by means of magnetic induction.

9. An apparatus for making composite plates, including a heating unit and a press mold, characterised in that the matrix contains electrically conducting fibres and that the heating unit comprises at least two electrodes in electric contact with the conducting fibres in the matrix.

10. An apparatus according to claim 9, characterised in that the press mold comprises at least one from the group consisting of pressing plates, pressing rollers and belt presses.

11. An apparatus according to claim 9 or 10, characterised in that the apparatus is provided with contact rollers or contact skates for contacting contact tracks on the matrix.

12. A matrix of fibres for production of composite plates, where the matrix has at least two types of fibre, where a first fibre type consists of polymer and a second fibre type consists of reinforcing material, characterised in that the matrix contains long fibres, which are capable of conducting an electric current.

13. A matrix according to claim 12, characterised in that the conducting fibres are enclosed by the fibre of the second type.

14. A composite plate produced by means of a method according to any of claims 1 to 10.

15. A composite plate containing a polymer and reinforcing fibres and conducting fibres, where the conducting fibres are enclosed by a third type of fibre or by reinforcing fibres.

16. A tubular composite plate made from a matrix of fibres containing conducting fibres for heating the matrix.

17. A product containing at least a part of a composite plate as described in claims 14, 15 or 16.

18. A method for post-forming of a composite plate according to claims 14, 15 or 16, including establishing electric contact with the conducting fibres in the composite plate and heating the composite plate to a temperature at which the composite becomes plastically deformable by passing electric current through the conducting fibres.