PROCESS FOR THE PRODUCTION OF HIGH-FILLED MINERAL FIBRE BASED SHEETS

Abstract: Process for the production of sheet products replacing asbestos. A basic sheet is produced consisting mainly of mineral fibres and a thermosetting binder, whereupon the sheet is impregnated with a binder containing large amounts of filler. By varying the type of binder and the filler a large number of products can be produced in accordance with this process: backing materials, brake linings, seals and packings.

Description: - Process for the production of high-filled mineral fibre based sheets Asbestos based sheet products are used in many technical applications. Examples in point are packings, seals and backing materials. With all these applications the good thermal and mechanical characteristics of asbestos come into play. Similarly, the good chemical resistance of asbestos is utilised in e.g. cylinder head gaskets for petrol or diesel engines. A number of sheets containing asbestos can be laminated and pressed to con¬ siderable density. Such a product can be used as a friction material in fabric clutches or mechanical brakes. In backing . materials consisting of asbestos with an appropriate binder what counts is the fact that asbestos does not decay and is stable in respect of moisture variations. Examples of such products are backing materials for PYC coated flooring and wall products. During recent years medical studies on human beings previously or currently in contact with asbestos have shown that they are more subject to certain lung diseases than might have been expected. This in its turn has led to prohibitions of the use of asbestos or has at least surrounded the application of asbestos with so many stringent legal requirements as to amount to a prohibition, inasmuch as the prescribed dust removal arrangements and other equipment become altogether too costly in economic terms. Unlike asbestos, so-called "man-made" fibres such as mineral, glass or slag fibres do not exhibit these medical disadvantages. On the other hand the properties of these fibres are far from being equally good as regards their thermal, chemical or mechanical effects. This has made it difficult to replace asbestos with this type of artificial fibres. In one case a substitute for a backing material a*- v i"^ some success. This product c- --" - s in equal parts of mineral fibres (rock fibres), cellulose and a latex-type binder. The high proportion of cellulose jeopardises the resistance to decay, and it is therefore necessary to add a decay inhibitor. The high proportion of cellulose also causes insufficient moisture stability (dimensional stability), which is compensated, for "BU REA OMPI instance, by additions of polyethylene glycol (PEG). A similar product is described, in the Swedish patent application 76085 8-8• It may however be pointed out that an alternative material has appeared in the market, in regard of which an attempt has been made to-get away from the relatively expensive raw materials used in the product described above. This is achieved by adding to the stock large amounts of filler such as kaolin. With this method of production it has proved that problems arise as regards retention, and the yield of material in this process is poor. This product is described in the European patent application 5481. A relatively moderate success has been achieved with this product. As regards the above mentioned application spheres packings and friction materials no new products have so far had any great success. As for mechanical brakes monolithic materials containing a large proportion of metals have enjoyed some success. In summary we may state that it continues -to be very difficult to repla.ce materials containing asbestos. A flexible process suitable for use with more than one of the above application spheres does not exist. The current sheet forming methods applied in the production of asbestos substitutes are generally intended only for one of the product spheres here discussed. The present invention relates to a process for manufacturing sheet-type products for the above applications by basically one and the same machine. The process aims at producing in a first stage a loosely bound sheet which in a later stage is impregnated with a mixture of binder and filler. The loosely bound sheet initially produced contains mostly mineral, fibres (type rock fibres) but can also contain certain smaller amounts of cellulose fibres (including mechanical or tb T -.chanical wood pulp) or glass _ I '".- of the continuously drawn type with a fibre length which clearly exceeds the average fibre length of rock fibres. The reason why rock fibres are chosen as the principal primary sheet formers consists in the fact that a sheet is produced con¬ sisting of a fibre inert to water with relatively good theimal and mechanical characteristics without being too costly. So as to impart a certain tear resistance to the sheet, but in particular by way of a process aid in the form -of a wet strength imparting agent a certain amount of cellulose is acceptable as long as the dimensional stability in the presence of moisture variations is not lost. The chopped long-fibred glass fibres which are costly are replaced to some extent inasmuch as rock fibres are not long. In order to obtain at a later stage after the impregnation treatment a sheet with isotropic or at least sufficiently isotropic characteristics as possible,the sheetmust be formed in such a way as to ensure that the interstices between the fibres become, as far as possible, equally large. This can be achieved by making use of a sheetforming process applied in connection with the non-woven technology. This process is characterised in that use is made of an inclined wire, the stock concentrations being in addition lower than with ordinary flat wire machines. It should be noted at this point that it is also possible to achieve the same effect with flat wire machines if the stock is sufficiently diluted. In order to achieve a sheet sufficiently strong for later impregnation the fibre mix deposited must be bonded. The bond shall be sufficiently stable to moisture to enable subsequent attainment of adequate wet strength for the impregnation stage. This is brought about by making use of plastic hardening agents with good moisture stability, in the first place phenolic resins. Other types of plastic hardening agents such as carbamide or melamine resins are also feasible. In certain cases a more elastic component may be added to this binder in order to avoid excessively pronounced brittleness. This can be achieved with the aid of e.g. SBR latex or other types of latex. Commercial products are Urecoli παxcα with Acronal. The addition of this binder takes place through a secondary breast box located above the wire. For reasons of hygiene it is ' advisable to provide in addition a special wire for this secondary breast box. - ' -he transfer from the first to the second wire can be of the type "poor man's pickup". The secondary breast box has a O. P slot from which the above binder compound flows evenly over the sheet resulting from the previous fibre formation. A suction box must be provided which extends from below the slot for a certain distance beyond the binder feed arrangement. This box which is connected to a vacuum pump serves to suck the binder through the sheet in order to achieve a homogeneous distribution. The amount of binder added can be regulated either by the concentration or by varying the negative pressure in the box. As regards variations of concentration it should perhaps be added that it is also possible to control the binder take-up by varying the viscosity with the aid of various additives. Once the binder has been added it must be hardened/dried. This can advantageously be done with the aid of conventional drying cylinders but also by means of hot air which is blown on to or through the layer of fibres_ The final stage in the formation of the product consists in impregnation. This can be carried out direct after the production of the basic sheets or only after storage during a relatively long or short period of time. The impregnation can be carried out in a number of different types of equipment. If a filler contained in the surface is preferred, it is usual to make use of knife or blade coating. If a more homogeneously impregnated product is required, use is made either of a saturation method or a pressing process such as a size press. The choice of equipment depends on the type of filler used. This depends primarily on the characteristics of the impregnating liquid. It is usual for considerably higher concentrations of both binder and filler to be used. The dry substances can amount to as much ,J. This is of eour?- -s to the rheological character¬ istics of the impregnating liquid. In connection with the manufacture of e.g. dimensionally stable backing materials it has proved to be advantageous to make use of cheap materials such as chalk, which are not affected by moisture. Certain clay minerals have proved to yield interesting friction OMPI materials. Furthermore, these substances can advantageously be combined with a certain proportion of metal powder in order to increase their heat conduction and to achieve different friction coefficients. Barytes may also be used in combination with the above materials. In certain cases the friction may become too high and it may then be necessary to make use of a lubricating material such as graphite or molybdenum sulphide. Use may of course also be made of asbestos but this is not advisable in the light of what has been said in the preamble concerning the danger of health inherent in asbestos. Silicon carbide, silicon nitride or aluminium oxide may be used to produce ground wood pulp manufactured in accordance with the method described in the invention. By way of an additional description ' of the process forming the subject of the invention a plant for the carrying out of the process is described below. This description is offered by way of a speeimen embodiment and does not in any way limit the process. Figure 1 contains a schematic sketch of a machine capable of producing the basic sheet. The figure does not show the equipment for preparing the stock nor the circulation network for the white water. Instead "it shows how the primarly breast box (1) receives the pulp from below. The endless wire (11) filters out the fibre supplied to the stock in a manner typical for inclined wire machines, whereby an especially good fibre distribution is achieved, in particular if use is made by lower concentrations than with con- ventional paper machines. Dewatering may be effected with the aid of a suction box (2), whereupon as shown in the figure the sheet is transferred to another wire (12) with the other breast box (3) located above this wire. Here too a suction box is provided which in this application controls the distribution of the binder in the vertical direction of t -;-• et (4)- The binder replied to the secondary breast box through the line (8) connected after pump (7), which in turn ' is linked with the tank (5). The tank is provided with an agitator (6) to ensure an even concentration. The slot (13) is relatively small (2-5 mm) and must be kept clean in order, to achieve even distribution of the binder. The basic sheet having O ΛPl iPO been impregnated with a binder mix suitable for the basic sheet drying/hardening is required. To this end use may be made of a cylinder with contact heat transfer or by means σf blowing on to the surface or through the layer of fibres. Once this has been done the product can be rolled up on a reeling machine. The figure shows the drying cylinder (9) as a yankee dryer but there is no reason why " several consecutive cylinders should not be used as with a conventional paper machine. It is also feasible directly to replace the reeling machine by the impregnation stage. The impregnation stage is described by figure 2. Figure 2 shows a system suitable for impregnating the basic sheet with an impregnating solution consisting of a binder and a large amount of filler. In the present case a size press (14) is shown. The latter receives material from a roll (10). The impregnating material is passed from a container to the press nip through a line (20) and pumped by pump (21) whic'i is connected to the container (22). The press nip is so adjusted as to partly control the amount of material retained in the sheet. Following the pressing stage an IR stage is provided, the purpose of which is to dry the surface of the material in order to reduce any tendency to stick during subsequent cylinder drying. The IR stages are marked (24) and the cylinders (15) to (18) these being followed by a reeling machine (19) of the same type as that described above (lθ) A specimen embodiment is given below. In this example a test formulation is described. This is the principal method by which a large range of products for technical purposes can be produced. On an inclined wire machine type "Hydroformer" a sheet is produced with a basis weight of 100 g/m . The sheet consists of t o o i. rock fibres of a type t^ which tensides are added for e?'! ' ^ dispersion in water. In addition Θ/o glass fibres are used of the type "Vetrotex" having a length of 20 mm and a diameter of 13 μ. Also β'o tall sulphate cellulose is added, which has been ground to 35 degrees Schopper-Riegler. By way of binder up to Θfo use is made of Urecoll 147 C and Acronal 240 D at a ratio of 5: 5 (absolutely dry) . OiV.PI This sheet is then passed through a size press, the pressing nip of which has been adjusted to 2.6 kgf/cm length of press nip. The impregnating material is composed as follows: CIAGO 1541 E type binder and Millicarb type filler. The weight ratio (absolutely dry) was 1:3- In addition 0.3 dispersion agent Calgon PTΞ was added calculated in relation to the amount of Millicarb. A further additive consisted in 0.1$ defoamer Nopco NXZ calculated in relation to the amount of absolutely dry binder. Also 0.1$ antioxidant was " added (in relation to the amount of binder). The content of dry substance amounted to 70$ and was adjusted by the addition of water. The material was passed at a rate of 30 m/min through a size press, whereby the basis weight of the sheet increased from 100 g/m to 300 g/m (more precisely 298 g/m ). The product thus produced was characterised by very good dimensional stability (0.1$ compared with 0.2$ in the case of commercially traded backing products). The breaking load in the direction of the machine amounted to 10.5 kgf/15 mm as against 11.5 according to the above comparison and in the transverse direction to 9«8 (by comparison with 6.9). The stretch at break amounted on average to 2.3$ (as against 3«5)- The product thus produced has been successfully tried as a backing material for PVC mats. OiWPI fa. _WlPO

Claims: Patent claims 1. A process for the production of impregnated mineral fibre based sheets for use as backing materials, friction materials and packings or similar products characterised in that a basic sheet with the following fibre composition 60-100$ mineral fibres 0-40$ cellulose or mechanical wood pulp 0-40$ chopped glass fibres (type drawn glass fibres) is formed on a wire which is inclined or in that the pulp con- centration, if use is made of a flat wire, is below 1 g/l. To this sheet is added at a later stage a hardening binder to between 5-20$ of the sheet weight, from a second breast box arranged above the sheet. This sheet is hardened either above drying cylinders or by hot air flowing through. The bonded sheet thus produced can either first be stored for subsequent treatment or directly treated with an impregnating material in an impregnating st:_.t_.on, the said impregnating material consisting of a latex-type organiG binder mixed with a filler. The binder/filler ratio amounts to between 1:1 and 1:10 by weight. 2. A process in accordance with claim 1 characterised in that the binder of hardening type, which is added at a second breast box, is based on phenolic resin, urea resin or melamine resin or a mixture of these, which can also be modified by an elastic component of type SBR latex to between 1-20$. 3« process in accordance with claim 1 characterised in that the binder mixed with the filler consists of latex-type products or mixtures thereof, whereby use is made primarily of SBR and NBR-type latex products. ' A. process in accordf.'*'" 1 ™ith claim 1 characterised in tr-+ the filler in the impregnating material is one or a mixture of the following: , Chalk, talcum, mica, kaolin or another clay mineral, baryte, metal powder (mainly iron and copper), molybdenum sulphide, graphite, asbestos, zeolites, aluminium oxide, silicon carbide, silicon nitride. OKPI

Inventors: AOBERG ULF LENNART, AOBERG, ULF, LENNART
Applicants: AOBERG ULF LENNART, AOBERG, ULF, LENNART
Application year: 1983
Main classification: C04B43
IPC codes: B28B1, C04B14, C04B26