METHOD AND APPARATUS FOR PREPARATION OF MINERAL FIBRE BASED GROWTH MEDIUM, A PLANT GROWTH MEDIUM AND USE OF IT

Abstract: A process for the preparation of a plant growth medium having a density gradient, comprising the steps of providing a primary web comprising mineral fibre material and a curable binding agent, compressing the primary web in at least one longitudinally extending zone in such a manner that the applied pressure increases substantially continuously in the latitudinal direction of the zone, forming a secondary web by bringing the primary web overlap itself, optionally compressing said secondary web and curing the binding agent.

Description: METHOD AND APPARATUS FOR PREPARATION OF MINERAL FIBRE BASED GROWTH MEDIUM, A PLANT GROWTH MEDIUM AND USE OF IT The present invention relates to a process for the preparation of a mineral fibre based plant growth medium having a density gradient. It is known in the art to use products composed by mineral fibre material as plant growth media. These plant growth media have the advantage that they are very porous and thus enable the roots of the plants to obtain both the required water, nutrition and air. Typically, the fibres take up less than or about 5 volume percent of the medium ideally leaving about 95 volume percent to the roots, air, water and/or plant nutrition. A mineral fibre based plant growth medium is generally light, thus easy to handle and economical to transport, and such a medium is furthermore both sufficiently firm and chemically inert to resist degradation when used for hosting plant roots for a prolonged period of time. To absorb and retain water is one of the primary tasks for a plant growth medium. If the medium has too poor a water retention ability, the water will run right through the medium at watering and the plant will dry out during the periods between watering or watering will have to be performed very often causing a significant waste of water. On the other hand, if the plant growth medium has a too effective water retention ability, the plant will not be able to extract the water from the medium and furthermore the plant roots might suffocate due to lack of air in the medium. From DK 170 034 Bl it is known that mineral fibre material based plant growth products exist which have an advantageous water retention ability. These products have a density gradient or a coarseness gradient in the direction of height of the product, in terms of having the highest density or the least coarse fibres at the top and the lowest density or the most coarse at the bottom. In this manner, it has proven possible to diminish the effect of gravity on the water by means of the capillary effect. By increasing the medium density or decreasing the medium coarseness in the height direction, the pore size is generally decreasing in size, which then again increases the capillary effect and thus draws the water upwards in the medium. According to DK 170 034 Bl, such an advantageous plant growth medium can be obtained by stacking and joining a number of discretely produced layers of glass fibres each having an increased surface weight or decreased fibre coarseness relative to the prior layer. However this is a troublesome way of producing such plant growth media. Furthermore, in order to obtain such a product composed by a number of discretely produced layers having sufficiently cohesiveness between said layers, it is necessary to use an amount of binder between the layers, thus impairing the water diffusion abilities between said layers and increasing the cost of production. US 5 009 030 discloses a similar soil-free growth substrate for plants exhibiting a controlled water gradient over the thickness of the product. The hydroretentivity gradient is obtained by means of a density or fineness gradient in the product. The varied density or fineness is obtained according to the document by superimposing a number of layers of glass wool webs having each a different density or fineness. Even though an acceptable hydroretentivity gradient can be obtained according to US 5 009 030, the process for their manufacture is rather troublesome, as several layers of mineral fibre webs must be combined into one. These layers will furthermore, have to be adhered to each other in order to provide a sufficient strength against delamination, and such placement of a binder between the different layers might impair the migration of water and other substances over the layer boundaries. Even further, the product itself merely provides a stepwise hydroretentivity gradient, not a continuous gradient . Accordingly, it is desirable to obtain an improved process for the preparation of an improved mineral fibre based plant growth medium. This object is achieved by the process according to the invention which is characterized in providing a primary web comprising mineral fibre material and a binding agent, compressing the primary web in at least one longitudinally extending zone in such a manner that the applied pressure increases substantially continuously in the transverse direction of the zone so as to induce a compression gradient in said direction, forming a secondary web by bringing the primary web to overlap itself by laying it in a number of layers in the transverse direction of the secondary web and dividing the secondary web to form a plant growth medium. The invention is based on the recognition that by inducing a compression to the primary web, it is possible to essentially maintain the compression until the binding agent in the product is cured or hardened. It is thus possible to compress some parts of the primary web more than other parts and obtain products having varying density. Furthermore, by knowing which parts of the primary web go where, when the primary web is made to overlap itself, it is possible to predict where the more dense parts end up in the final product. The typical and preferred way of bringing the primary web to overlap itself is by means of a pendulum distributer as known in the art. By this method, a secondary web is formed by a number of mutually slightly displaced layers of the primary web which has been laid out in a direction transverse to the longitudinal direction of said secondary web. The secondary web is optionally compressed and the binding agent in the product is cured or hardened. The thus cured secondary web can be cut out in a number of products having virtually any desired shape. A preferred method of laying out a mineral fibre web by pendulum distribution is i.a. disclosed in WO 88/03509 or WO 97/03509. By this method, the one side-edge of the primary web will eventually constitute the top layer of the final product and the opposite side-edge will constitute the bottom layer of the product. Accordingly, by compressing the primary web with an continuously increasing compressive force in the direction from the one edge to the opposite, it is possible to obtain a density gradient in the direction of height (from the bottom to the top) of the final product. The process according to the invention is more simple relative to the prior art process in terms of the number of steps of operation. Accordingly, it is now possible to perform the process as a continuous process and no stacking and/or joining step is necessary. Furthermore, it is an advantage of the process according to the invention, that the absolute density and the density gradient profile can be controlled in detail by simply controlling the pressure applied by the pressure applying means. The compression can be performed by any means capable of inducing a substantially continuous gradient of compression. The compression can e.g. be performed by means of conveyor belts, one or more cylindrical rollers having more or less angular axial suspension relative to the surface plane of the primary web and/or rollers having a conical shape. It is preferred to compress the web by means of a number of rollers. Preferably a number of rollers are used each having separate suspension. By applying a number of individually suspended compression means, it is possible to control the compression force applied by each of the means separately and thus also possible to swiftly switch between compression profiles and/or configurations of compressed zones. It is particularly preferred to use rollers having a diameter of 5-70 cm, more preferable 15-50 cm and even more preferable 20-40 cm. If the rollers are too small, the primary web will build up in front of the rollers and eventually the process must be stopped for cleaning. Too large rollers are generally more difficult to handle. Any number of rollers can be used for compressing the primary web, however it has proven particularly advantageous to use rollers having a width of around SO- 600 mm, more preferably around 50 to 300 mm, and even more preferably around 80 to 150 mm. The rollers can be disposed in a side by side manner or in a displaced manner along both the longitudinal and transversal direction of the web. It has proven to be particularly expedient to arrange the rollers in two rows arranged one after the other in the longitudinal direction of the primary web in which rows the rollers are placed side by side in the transversal direction of the web with a gap between the rollers and, and in such a manner that the gaps between the rollers of the first row are covered by the rollers of the second row. According to a preferred embodiment of the process according to the invention, it has proven advantageous to use rollers or wheels being somewhat resilient. Preferred rollers comprise a flexible suspension and/or a flexible coating, e.g. as a sort of tyres. In this way, the rollers are able to compensate for unevenness of the primary web. Furthermore, , by using rollers having such a resilient surface or suspension, it is possible to reduce the damage induced on the primary web by the compression in terms of fewer damaged or broken fibres. However, it has proven particularly advantageous to use rollers having a smooth non-sticky pressure applying surface of e.g. metal or polytetrafluoroethylene . Hereby it is obtained that the primary web does not stick to the rollers, which might otherwise become a severe problem during the pre-compression step. The compression can be performed in one or more operations using any number of compression means. The density gradient profile can be linear or non-linear depending on the shape or operation of the compression means. The surface of the compression means may be smooth or have a pattern. Naturally, it is possible to compress only a part or several parts of the primary web resulting e.g. in products having more density gradients in the direction height. Furthermore, the secondary web can be split into more products having one or more density gradients. However, it is preferable to induce the density gradient in the entire width of the primary web, i.e. compressing the entire web, thus creating a density gradient in the entire height of the secondary web. It should be noted that the benefits of the present invention may be obtained by any process of the above kind where at least a part of the primary web is compressed before the primary web, e.g. by pendulum distribution or by preceding cutting off in sections as described in EP 0 297 111 Bl, is made to overlap itself under formation of a secondary web which is optionally compressed and eventually cured, and where it is possible to obtain a substantially continuous density gradient in the direction of height of the final product. When compressing the web using rollers, it is preferred to compress the primary web in an amount of about 0 to 400 kg/m (roller width) , preferably 0 to 250 kg/m and more preferably 0 to 100 kg/m in the one side of the zone and in an amount of about 500 to 3000 kg/m, preferably 750 to 2500 kg/m and more preferably 1000 to 2000 kg/m in the opposite side. By compressing the primary web using a continuous compression gradient not only a product having the desired density properties is obtained. By applying the pressure within the above disclosed ranges, it can be obtained that an amount of fibres decompose or break where the highest pressure is applied and substantially no fibres decompose or break where the lightest pressure is applied. Hereby also a continuous gradient of fibre coarseness is provided which even enhances the products water retention ability in terms of controlled capillary effect. It is preferred that the resulting density of the product is 40-180 kg/m 3 more preferable 50-120 kg/m " at the top surface and 10-120 kg/m', more preferable 30-120 kg/nr at the bottom surface, and that the products has a mean density of 30-180 kg/m 3 , more preferably 30-120 kg/m 3 a d even more preferably 40-80 kg/m 1 . According to a preferred embodiment of the process according to the invention 0.5-10 weight-", more preferable 0.5-5 weight-?-, and even more preferable 1-3 weight-% binding agent is added to the primary web. All binding agent weight-percentages is relative to the end- product. It is furthermore a preferable embodiment of the process according to the invention to compress the secondary web in the longitudinal direction, i.e. as disclosed in e.g. US 4 632 685, CH 620 861 or US 2 500 690. Even though the product obtainable by the process according to the invention is particularly suitable for use as a plant growth medium, it can also be employed as heat or sound insulation. The invention also relates to a mineral fibre based plant growth medium obtainable by the process according to the invention. The product obtainable by the process according to the invention is superior over the prior art products in that it does not comprise any significant assembly planes and that the density gradient is substantially continuous rather than stepwise. The products according to the invention can be tailored to meet almost any requirements regarding water retention, air penetration and firmness. The products according to the invention can accordingly be made to suit the needs of a comprehensive variety of plants including tomatoes, cucumbers, roses and kalanchoe. The invention furthermore relates to the use of the product obtainable by the process according to the invention for culturing plants and in particular for the above mentioned plants as well as to an apparatus for carrying out the process according to the invention. Accordingly, the apparatus according to the invention is characterized in comprising means for providing a primary web comprising mineral fibre material and a binding agent, means for compressing the primary web in at least one longitudinally extending zone said means being capable of compressing the primary web in such a manner that the applied pressure increases substantially continuously in the transverse direction of the zone so as to induce a compression gradient in said direction, means for forming a secondary web said means being capable of bringing the primary web to overlap itself by laying it in a number of layers in the transverse direction of the secondary web and means for dividing the secondary web to form plant growth media. The invention will now be further explained by means of illustrations : Fig. 1 Shows a density gradient in the width direction of the primary web. Fig. 2 Shows a product according to the invention. Fig. 3 Illustrates a preferred way of producing the plant growth medium according to the invention. Figure 1 shows how the density varies in the width direction of the primary web after compression using a number of rollers compressing at an axis load of around 0- 1000 kg/m each, and each having a width of around 100 mm. After formation of the secondary web by pendulation the substantially continuous density gradient extends in the height direction of the secondary web. Accordingly, products having such substantially continuous density gradient can be cut from the secondary web. Figure 2 shows a product according to the invention which has a relatively low density at the bottom, a relatively high density at the top and a substantially continuous density gradient from the bottom to the top. In Fig. 3, the steps of producing the secondary web from which the products eventually are cut is illustrated. The first step involves the formation of mineral fibres from a mineral fibre forming melt which is produced in a furnace 1 and which is supplied from a spout 2 of the furnace 1 to a total of four rapidly rotating spinning-wheels 3 to which the mineral fibre forming melt is supplied as a mineral fibre forming melt stream 4. As the mineral fibre forming melt stream 4 is supplied to the spinning-wheels 3 in a radial direction relative thereto, a gas stream is simultaneously supplied to the rapidly rotating spinning-wheels 3 in the axial direction thereof causing the formation of individual mineral fibres or bunches or tufts of mineral fibres which are expelled or sprayed from the rapidly rotating spinning-wheels 3 as indicated by the reference numeral 5. The gas stream may constitute a so-called temperature treatment gas stream, normally a cooling gas stream. The mineral fibre spray 5 is collected on a continuously operated first conveyer belt 6 forming the primary mineral fibre web 7. A heat-curable bonding agent is also optionally added to the primary mineral fibre web 7 either directly to the primary mineral fibre web 7 or at the stage of expelling the mineral fibres from the spinning-wheels 3, i.e. at the stage of forming the individual mineral fibres . The binder can of course be any known binder for use in combination with mineral fibres, i.e. also a thermoplastic binder. The first conveyer belt 6 is, as is evident from Fig. 3, composed of two conveyer belt sections. A first conveyer belt section which is sloping relative to the horizontal direction and relative to a second substantially horizontal conveyer belt section. The first section constitutes a collector section, whereas the second section constitutes a transport section. This can of course be made in any other way known in the art. The conveyor belt(s) used for collecting the fibres are preferably foraminous and provided with means (not shown) for the suction of air through the belts to facilitate the layering of the fibres. This increases the homogeneity of the primary web 7 even further by ensuring a better distribution of the fibres, i.e. in terms of spots with low fibre density having the highest airflow through the belt which then leads to layering of more fibres there, etc. The compression of the primary web 7 is performed by roller 8 having an inclination from one side of the web to the other side and thus inducing a continuous compression gradient in the entire width-direction of the primary web and in such a way that the primary web 9 after longitudinal compression maintains a substantial amount of the compression induced, i.e. more compression in one side than in the other. The inclination of the roller can of course be adjusted to provide any desired compression profile. A similar effect can be obtained by using a number of rollers or bands and/or conically shaped rollers. In most cases it is preferable to compress the primary web in a zone along one side as much as possible while essentially not compressing at all along the other side. Various known means, e.g. in terms of compression aids or the like can advantageously be employed to ensure the correct density gradient is maintained/obtained in the final product. The first section of the first conveyer belt 6 constitutes as stated above a collector section, whereas the second section of the conveyor belt 6 constitutes a transport section by means of which the primary mineral fibre web having the continuous compression gradient in the width direction is transferred to a second and a third continuously operated conveyer belt designated the reference numerals 10 and 11, respectively, which are operated in synchronism with the first conveyer belt 6 sandwiching the compressed primary mineral fibre web 9 between two adjacent surfaces of the second and third conveyer belts 10 and 11, respectively. The second and third conveyer belts 10 and 11, respectively, communicate with a fourth conveyer belt 12 which constitutes a collector conveyer belt on which a secondary mineral fibre web 13 is collected as the second and third conveyer belts 10 and 11, respectively, are swung across the upper surface of the fourth conveyer belt 12 in the transversal direction relative to the fourth conveyer belt 12. The secondary mineral fibre web 13 is consequently produced by arranging the primary mineral fibre web 9 in overlapping relation generally in the transversal direction of the fourth conveyer belt 12. By producing the secondary mineral fibre web 13 from the compressed primary mineral fibre web 9 as disclosed in Fig. 3, a secondary web is produced in which the density varies substantially continuous in the height-direction (thickness), in terms of being lowest at the bottom and highest at the top. Bricks cut out of the preferably cured/hardened secondary web are exceptional for use as plant growth media, due to the controlled hydroretentivity gradient, i.e. improved water retention ability. The term primary mineral fibre web as used herein designates a newly formed mineral fibre web of a typical height (thickness) of 3-7 cm. which is meant for being sandwiched with a number of corresponding primary web layers, preferably constituted by the same primary web in order to obtain a secondary web. A particularly preferred way of obtaining such primary and secondary mineral fibre webs is disclosed in WO 97/01006. The term water retention ability as used herein defines a products ability to retain water as measured in volume percent. The term mineral fibre based plant growth medium as used herein comprises any products suitable as host for the roots of plants and which products comprise a substantial amount of mineral fibres, preferably in an amount of around 1-10 volume- o, more preferable around 2-5 volume-%. The plant growth medium according to the invention may comprise a number of components besides mineral fibres, e.g. lignite, clay and organic compounds such as coco peat. The term mineral fibre as used herein comprises all types of man-made mineral fibres, such as rock, glass or slag fibres, in particular fibres used in materials for thermal or sound insulation purposes, and as filler in cement, plastics or other substances, or which are used as culture medium for plants . The term binding agent as used herein comprises any material which is suited as binding agent in mineral fibre materials for the above products, e.g. phenol formaldehyde urea, acrylic-copolymer, resorsinole, furan or melamine resin. Such binding agents are preferably supplied to the mineral fibre material in the form of aqueous suspensions. In the following, the invention will be described in more detail by way of an example. Example 1 : Mineral fibres are obtained by spinning in a spinning chamber, and are made to deposit on a conveyor belt under formation of a primary web having a width of about 1.8 m, and a surface weight of about 500 g/m 2 . A binding agent comprising phenol formaldehyde urea in aqueous suspension is continuously distributed to the fibres in the air in the spinning chamber prior to the fibres settling on the conveyor belt, the binding agent being added in an amount corresponding to a final concentration of phenol formaldehyde urea of about 3 ° in the final product. The primary web is compressed in its full width by means of a cylindrical roller suspended in such a manner that it is pressing against the web at one edge of the primary web at a pressure of 0 kg/m and at a pressure of 1000 kg/m at the opposite edge of the primary web and pressing at a substantially linear interpolated compression rate between these two values between the primary web side-edges. The primary web is laid out by pendulum distribution under formation of a secondary web having a width of about 2 m, the secondary web as seen in cross-section comprising about 12 layers of primary web. The secondary web obtained is compressed to a height of 100 mm, and cured in a curing oven. Finally, the plant growth media are provided by cutting off in the desired sizes from the thus cured secondary web. Plant growth media prepared by the process according to the example have a continuous density gradient ranging from 35 kg/m 3 at the bottom to 130 kg/m 3 at the top. The plant growth media according to the invention show no signs of delamination, and have an excellent water retention/diffusion ability.

Claims: C l a i m s : 1. A process for the preparation of a mineral fibre based plant growth medium having a density gradient, characterized in providing a primary web comprising mineral fibre material and a binding agent, compressing the primary web in at least one longitudinally extending zone in such a manner that the applied pressure increases substantially continuously in the transverse direction of the zone so as to induce a compression gradient in said direction, forming a secondary web by bringing the primary web to overlap itself by laying it in a number of layers in the transverse direction of the secondary web and dividing the secondary web to form a plant growth medium. 2. A process according to claim 1, characterized in that the compression is performed using one or more substantially cylindrical rollers. 3. A process according to claim 1, characterized in that the compression is performed using one or more substantially conically shaped rollers. 4. A process according to any of the preceding claims, characterized in that the compression is performed at a pressure of 0-400 kg/m at the one edge of the compressed zone and at a pressure of 100-2000 kg/m at the opposite edge of the compressed zone. 5. A process according to any of the preceding claims, characterized in that the compression gradient induced in the compressed zone has a linear profile. 6. A process according to any of claim 1-4, characterized in that the compression gradient induced in the compressed zone has a non-linear profile. 7. A process according to any of the preceding claims, characterized in that the compression zone extends over only part of the transverse direction of the primary web. 8. A process according to any of the claims 2 to 6, characterized in that two or more discrete zones of the primary web are compressed, and that the secondary web is divided into layers in planes parallel to the main-surface planes in web. 9. A process according to any of claim 1-4, characterized in that the entire primary web is compressed, using a single roller or set of rollers. 10. A plant growth medium obtainable by the process according to any of the preceding claims. 11. A plant growth medium according to claim 10, characterized in having a mean density of 30 to 120 kg/m 1 . 12. Use of the plant growth medium according to claim 10 or 11 for culturing plants. 12. An apparatus for the preparation of a mineral fibre based plant growth medium having a density gradient, characterized in comprising means for providing a primary web comprising mineral fibre material and a binding agent, means for compressing the primary web in at least one longitudinally extending zone said means being capable of compressing the primary web in such a manner that the applied pressure increases substantially continuously in the transverse direction of the zone so as to induce a compression gradient in said direction, means for forming a secondary web said means being capable of bringing the primary web to overlap itself by laying it in a number of layers in the transverse direction of the secondary web and means for dividing the secondary web to form plant growth media.

Inventors: CRIDLAND IAN, KIRKEBAEK JOERGEN, NOERGAARD LUIS, CRIDLAND, IAN, KIRKEBAEK, JOERGEN, NOERGAARD, LUIS
Applicants: CRIDLAND IAN, KIRKEBAEK JOERGEN, NOERGAARD LUIS, ROCKWOOL INT, CRIDLAND, IAN, KIRKEBAEK, JOERGEN, NOERGAARD, LUIS, ROCKWOOL INTERNATIONAL A/S
Application year: 1999
Main classification: E04B1
IPC codes: A01G1, A01G31, D04H1, D04H13