SHAPED PRODUCT COMPRISING A THERMOSET, FOAMED, PLASTICS SHEET MATERIAL REINFORCED WITH COHERENT NATURAL-FIBRE MATERIAL, AND A PROCESS FOR MAKING THE SHAPED PRODUCT

Abstract: The invention provides a novel shaped product comprising a sheet material of polyurethane resin material reinforced with binder-free natural-fibre material, such as jute, flax, hemp, coir, ampas, ramie or cotton, possibly combined with polypropylene, polyethylene and/or glass fibres. A preferred fibre material is jute, preferably in the form of jute needled felt. The invention also provides a process for making the product. In the process, a binder-free mat of natural-fibre material having a water content of at least 4 % which is impregnated with a known per se mixture thermosettable to form a polyurethane material, comprising a polyether-polyol component and an isocyanate component, followed by maintaining the temperature of the mixture at 20-50°C, and setting the impregnated fibrous mat in a closed mould at a pressure of 3.5-20 kg/cm² and a temperature of 20-75°C.

Description: The present invention is directed to a sandwich panel, as well as to a process for preparing a sandwich panel. The production of sandwich panels and more in particular light weight sandwich panels has been known in the art. Known sandwich panels are generally so made that ready outer sheets are unilaterally provided with a layer of adhesive whereafter the core layer is glued between and with these outer sheets. Although this prior process can be effected both continuously and discontinuously, it is clear that it is always necessary first to make the outer sheets before proceeding to glue these sheets to the core layer. It is an object of the present invention to provide a shaped product of the above kind, which can be obtained through a simple procedure, is environmentally friendly and can be processed, and which, by using the properties of the natural-fibre material and of the plastics material, can lead to material combinations having particular properties. The invention is directed to an improved sandwich panel, that is easy to make and has a strong bonding between core material and top layers. The sandwich panel of the invention comprises a core layer firmly bonded on opposite sides to a thermoset polyurethane resin sheet material reinforced with cohesive binder-free natural fibre material, said sandwich panel having been obtained by combining two sheets of cohesive binder-free natural fibre material containing at least 4% water, impregnated with polyurethane resin with said core layer between said sheets to form a laminate composite and thereafter setting the composite in a mould. Preferably the core layer has a honeycomb structure. According to the invention, a binder-free natural-fibre material is used to exclude a possible interfering effect of a binder. As compared with the prior panels, the present invention provides a sandwich panel made in an integrated manner, that is to say, the outer sheets are formed as the sandwich panel is formed. One advantage of the invention as compared with the prior panels is therefore that one process stage is saved, while a further advantage is that no adhesive material needs to be used, because use is made of the excellent bonding characteristics of polyurethane with a multiplicity of materials. Moreover, in the prior manufacture of a sandwich panel using a honey-comb structure as a core layer, the adhesive is commonly applied to the outer sheets. The effective adhesive area or bonding area is therefore determined by the cross-sectional area of the material of the honey-comb. By combining the impregnated fibrous mat with the honey-comb and subsequently allowing this packet to set at the pressure and temperature to be used in the method according to the invention, a sandwich panel with an excellent bond between the outer sheets and the core layer is obtained. This good bond is caused by the foam effect of the polyurethane, as a result of which the polyurethane bonds not only to the material surface of the cross-section, but also to the inner and outer sides of the raised edges of the honey-comb structure. The sandwich panel may be a flat product or one that is profiled to a greater or lesser extent, for example, corrugated or curved. The core layer may include a lattice or honeycomb structure, a full-layer structure or a highly profiled structure. If the core layeris a lattice or honey-comb structure, it may be made of a material such as, for example, paper, cardboard, aramide,polystyrene, polycarbonate. In the case of a core layer with a full-layer structure, the layer may be formed of, for example, a foam layer of polyurethane, polystyrene or polyvinyl chloride. In the case of a polyurethane foam layer as the core layer, it may be filled, for example, with glass fibres or expanded clay granules, or the core layer may consist of balsa wood or a mineral filling material as, for example rock wool or ceramic fibre material. If the core layer is a profiled material, it may be formed, for example, of corrugated cardboard, ABS, GVK, polypropylene, polystyrene, aramide. It is also possible to use a profiled core layer obtained from a parallel arrangement of substantially contacting tubes of polyvinyl chloride, polystyrene or polypropylene. A product according to this invention may have the form of a flat sheet, as such, but also, for example, of a specifically shaped product substantially made of the sandwich panel according to the invention, such as a self-supporting interior part, for example, a door panel, of an automobile. Within the world of building too, there are many uses for such embodiment. More in particular, the following examples of uses of the sandwich panels may be mentioned: Stand construction (exhibitions). At present, heavy panels are used for this purpose (15-30 kg/m²). The panel according to this invention has a mass of 3-5 kg/m². Building industry. Examples are doors, roof panels, partitions, floor plates, insulation panels. Project building. Use as store shelves, counters and the like. Furniture industry. Couches, beds. The invention also relates to a method of making a sandwich panel, which comprises impregnating two mats out of cohesive binder-free fibre material having a water content of at least 4%, with a reaction mixture thermosettable to polyurethane, comprising a polyether-polyol component and an isocyanate component, the temperature of which mixture is maintained at 20-50°C; combining the impregnated fibrous mats with a core layer between them to form a laminate composite; transferring the composite to a mould and maintaining it therein at a temperature of 20-75°C and a pressure of 3.5-20 kg/cm² to obtain setting of the polyurethane material; and thereafter removing the resulting product from the mould. Preferably the water content of the natural-fibre material is 4-20%. In accordance with a further elaboration of the method according to this invention, the percentage of resin of the impregnated fibrous mat is 35-70% by weight. The polyether-polyol component and the isocyanate component from the mixture settable to form the polyurethane resin material, with which the fibrous mat is impregnated,react directly with each other after being mixed in accordance with the polyaddition principle, the basic reaction of which reads as follows: This reaction may, if desired, be accelerated with specific catalysts,which are added to the polyol, if desired with other required adjuvants, such as surfactants and flame retarders. From the polyurethane chemistry, it is known that the isocyanate component is capable of reacting with water according to the following reaction equation: The present invention is based upon the idea, based on the reaction of isocyanate with water, to make good use of the water naturally available in natural-fibre materials, the content of which is at least 4%, to produce a polyurethane resin material which exhibits a foaming effect as a result of the carbon dioxide gas released during the reaction of the isocyanate component with the water. It has now been found that the product obtained by the method according to the invention has a certain porosity. This porosity can be controlled to a certain extent by varying the proportion of isocyanate in the mixture. The porosity caused during the process according to the invention also results in a product having properties which are attractive for the application thereof, i.e., a good shock absorption and a low specific gravity of 700-850 kg/m³. By virtue of the above foaming effect, the invention relates to the following fibre materials, which all have a certain moisture content: Jute, flax, hemp, coir, ampas, ramie and cotton, as well as the combinations of these with polypropylene, polyethylene and glass fibres, provided the minimum moisture content is ensured. The preferred form of the natural-fibre material is jute needled felt. This material is cheap and available as a standard material, while owing to the nature of the felting process (web formation followed by needling), there is a certain bond between the fibres without the presence of interfering binders. Depending on the application of the fibre-reinforced material, fibres or combinations thereof suitable for such purpose can be selected. One advantage of a polyurethane is that, owing to the nature of the chemical reaction, the components react fully. Moreover, no solvents are present, and no harmful substances or smells are released. It is true that the isocyanate component by itself is aggressive to eyes, skin and respiratory organs, but owing to the low vapour pressure, the MAC value of 0.02 ppm will not soon be exceeded. The polyol component consists of non-volatile compounds, which are not dangerous during normal treatment. The polyurethane material reinforced with natural-fibre material according to the invention has a good impact resistance, which is caused, among other reasons, by the tough character of the polyurethane and the good fibre-polyurethane bond. Owing to the many possibilities of the polyurethane chemistry, the properties of the material are adjustable within reasonable limits when the same raw materials are used. The procedure in the process according to the invention is simple and comprises the following stages a. Preparatory work, b. Wetting or impregnating the fibres, c. Moulding and pressing, and possibly, d. The after-treatment. Among the preparatory work is the punching or cutting of the coherent natural-fibre material, for example, jute needled felt, to crude peripheral shapes. In this way, practically the only waste material is jute waste, because no resin is applied to excess jute, and resin wastes remain limited. For certain uses (for example, flat sheet materials), the above treatment can be done without. The resin is made up in accordance with the desired formulation. The addition of 0.1 to 3% of catalyst and 0.1 to 3% of of surfactant to the polyol component takes place with quiet stirring to prevent the introduction of air, and hence moisture, as much as possible. For the impregnation of the jute fibre mat provided, the mixture comprising the polyether-polyol component and the isocyanate component is applied to the jute felt mat with a high-pressure two-components machine by the so-called "airless-spray" method. Depending on product requirements, the percentage of resin can range from 35 to 70%. By means of a robot or manipulator system, impregnation can be accurately carried out. By adjusting the process parameters, the mechanical properties can be influenced within certain limits. For example, the temperature of the resin is adjusted by means of a heat exchanger. In accordance with the present invention, this temperature is 20-50°C. This operating temperature, as well as the amount of catalyst determine the interval between impregnation and pressing. After the impregnation of the felt mat, it is transferred to a mould placed in a press. The mould has been pre-treated with a release agent to prevent "sticking" of the product, and thereby to shorten the demoulding time. As soon as the mat is within the mould, the latter is closed, and the setting reaction can be fully continued. According to the invention, a temperature of 20-75°C is maintained during moulding. Unlike, for example, moulding compounds of polyester and phenol resins, the pressure can be kept at a low value. Depending on the desired material, the pressure according to the present invention is 3.5-20 kg/cm³. The period of pressing depends on the initial temperature of the components, catalyst concentration, mould temperature and thickness of the product. After completion of the setting process, the product can be released and is ready for any after treatment. The after-treatment may include painting the product or coating it with a plastics film. Owing to its porosity, the product has a certain air permeability. This makes it possible for the product to be coated with a plastics film or other air-tight coating material using vacuum equipment. According to the process according to this invention, the procedure is such that the impregnated fibrous mats with a core layer between them are joined to form a laminate composite, whereafter the composite is set in the mould. In the process according to the invention it may be desirable to cut both the fibrous mats and the core layer, which may be made of one of the types of structure and/or materials specified hereinbefore, to the desired size. The process according to the invention can be carried out both continuously and discontinuously. In the case of a continuous embodiment, the mould plates are technically adapted, for example, formed as endless belts of steel. The principle of the impregnation technique remains substantially the same, albeit that in the continuous embodiment the coherent natural-fibre material, for example jute, is processed from a roll thereof. In the process according to the invention, a suitable mixture thermosettable to polyurethane may in addition to the polyol component and the isocyanate component comprise 0.1-3% by weight of catalyst of the type of DBTL (dibutyl tin laureate), Fox-o-cat (tradename, given by Resina Chemie, of Foxhol, the Netherlands) or Dabco, and 0.1-3% by weight of a surfactant. In the preparation of polyurethane resins, the art mainly uses raw materials having bi-functional or poly-functional hydroxyl groups, for example, or polyethers having terminal hydroxyl groups capable of reacting with bi-functional or poly-functional isocyanates. In the framework of the process according to the present invention, use is made of polyether-polyols. The functionality, that is to say, the number of reactive hydroxyl groups, is in this case 3 or more to obtain ultimately a three-dimensionally branched, thermoset, polyurethane material having good mechanical properties such as impact resistance and bending strength. Examples of such polyols are of the type commercially available under the trademarks of Resicast, Desmophen, Fox-o-pol, Elastophen, Rütapur. The isocyanate component preferably used in the process according to the invention is an aromatic isocyanate, for example, TDI and MDI (toluene-diisocyanate (2, 6) and methylene-diphenyldiisocyanate (4, 4)). Isocyanates of such a type are commercially available under the trademarks of Urestyl, Desmodur, Elastan, Baymidur.

Claims: 1. Sandwich panel comprising a core layer firmly bonded on opposite sides to a thermoset polyurethane resin sheet material reinforced with cohesive binder-free natural fibre material, said sandwich panel having been obtained by combining two sheets of cohesive binder-free natural fibre material containing at leat 4% water, impregnated with polyurethane resin with said core layer between said sheets to form a laminate composite and thereafter setting the composite in a mould. 2. Sandwich panel according to claim 1, wherein the core layer is a honeycomb structure. 3. Sandwich panel according to claim 1 or 2, wherein the binder-free natural-fibre material is jute, flax, hemp, coir, ampas, ramie or cotton. 4. Sandwich panel as claimed in claim 3, wherein the natural-fibre material is combined with polypropylene, polyethylene and/or glass fibres. 5. Sandwich panel as claimed in claim 3, wherein the natural-fibre material is jute. 6. Sandwich panel as claimed in claim 5, wherein the binderfree jute fibre material is jute needled felt. 7. A process for making a sandwich panel according to claims 1-6, which comprises impregnating two mats out of cohesive binder-free fibre material having a water content of at least 4%, with a reaction mixture thermosettable to polyurethane, comprising a polyether-polyol component and an isocyanate component, the temperature of which mixture is maintained at 20-50°C; combining the impregnated fibrous mats with a core layer between them to form a laminate composite; transferring the composite to a mould and maintaining it therein at a temperature of 20-75°C and a pressure of 3.5-20 kg/cm² to obtain setting of the polyurethane material; and thereafter removing the resulting product from the mould. 8. A process as claimed in claim 7, wherein the water content of the natural-fibre material is 4-20%. 9. A process as claimed in claim 7, wherein as the natural-iibre material jute, flax, hemp, coir, ampas, ramie, or cotton is used. 10. A process as claimed in claim 9, wherein the natural-fibre material is combined with polypropylene, polyethylene and/or glass fibres. 11. A process as claimed in claim 9, wherein jute is used. 12. A process as claimed in claim 11, wherein the jute is used in the form of a jute needled felt. 13. A process as claimed in claim 11, wherein the resin percentage of the impregnated fibrous mat is 35-70% by weight.

Inventors: VAN DER KOOY, IJEDE
Applicants: DE GROOT AUTOMOTIVES B.V.
Application year: 1988
Main classification: C08J5
IPC codes: C08J5, B32B5