MULTILAYERED FIBROUS PRODUCT

Abstract: In a method of manufacturing a multilayered fibrous product, wherein a layered product is deposited on a continuously moving gas-permeable forming surface by successively supplying streams of fibre-containing gas to said forming surface, at least the first layer formed on said forming surface is consolidated to such a degree that the majority of the fibres of said layer are oriented in a plane parallel to said forming surface.

Description: Multilayered fibrous product This invention relates to a method of manufacturing a fibrous product comprising the steps of forming a multilayered fi¬ brous product consisting of at least two layers on the top surface of a continuously moving gas-permeable forming surface, maintaining a vacuum at the underside of said gas-permeable forming surface and successively supplying streams of a fibre-containing gas to the top surface of said forming surface. In the conventional production of soft fibrous products , such as table napkins, facial towels, rolls of kitchen paper, toilet paper and paper towels, several e. g . three thin fibrous webs which have been manufactured by a wet process are combined on a conver¬ ter. A wet process is a process in. which the starting material for the manufacture of a fibre layer is a slurry of fibres in water and in which said slurry is dewatered on a wire cloth and is subsequently dried. The fibrous product composed of two or more layers may then be crepped or embossed to make the layers adhere to one ano¬ ther. It is essential that the adherence between the layers does not become excessive because the product then becomes too rigid . On the other hand, it is also essential that the adherence is suffi¬ ciently high to prevent the product from delamϊnating during use. When the wet laid fibrous layers have been combined to form the desired multilayered product, the product may be rolled as is the case in the manufacture of toilet paper and rolls of kitchen paper or it may be cut into sheets which may then be suitably folded . ■ The fibrous layers which are used in the production of the above mentioned multilayered fibrous products are ordinarily very thin . Thus, their weight may be 10-15 g/m . Due to low thickness and strength of these layers the manufacture of such layers and their removal from the wire cloth on which they are formed present serious problems at the production rates which ordinarily are used in the production of wet paper, viz. production rates of the order of 1200 m/min . I n particular, the removal of the thin fibrous layers from the wire cloth is difficult to perform and in practice it can only be effected by contacting the wire cloth with the thin moist fibrous layer located thereon with a Yan kee cylinder which is a large intern- ally heated cylinder. As a result of such contact the moist fibrous layer is transferred from the wire cloth to the cylinder surface. However, during this operation the fibrous layer is dried and it may adhere to the cylinder surface. The removal of the fibrous layer from the cylinder surface is effected with a scraper, a socalled doctor's knife, or a crepping knife. Factories for the production of wet-laid fibrous products are highly energy-consuming which is mainly due to the fact that large amounts of water have to be used to form the fibre slurries used as a starting material and that this water subsequently has to be removed during the dewaterϊng and drying steps. Contrary thereto the energy consumption in. a factory in which fibrous pro¬ ducts are manufactured by a dry process is relative small . There- fore, there is a need for utilizing the dry process for the manufac¬ ture of a multilayered fibrous product. There is also a need for providing a method which does not comprise the difficult operation of removing a thin fibrous layer from the wire on which it is formed and the combination of such a layer with one or more additional layers formed on separate wires so as to form the desired multilayered fibrous product. Furthermore, there is a need for multilayered fibrous products which do not unduly delamϊnate during use. These needs are fulfilled by the method of the invention, which method is characterized in effecting at least after the formation of the first fibrous layer and before the formation of the second layer a consolidation of the fibrous layer located on the forming surface such that at least the major proportion of the fibres of said layer are oriented in a plane parallel to the forming surface. As a result of such an orientation of the fibres only a limited number of free fibre ends will extend from the fibrous layer after the consolidation . Thus, the integration between the first layer and a further layer formed on top of the first layer can be reduced to a desired level and Is thus adjustable so that the two layers may be displaced relatively to one another. In other words, the products prepared by the method of the invention have properties which are equal to or better than those of the conventional products consisting of wet formed fibrous layers. A multilayered fibrous product can be prepared by the method of the invention without removing the indi- 3 vidual thin layers from the forming surface, e. g . in the shape of a wire. This presents several advantages in the manufacture of such products and also considerably reduces the formation of products which suffer from defects, e. g . defects which are caused by damages of thin fibrous layers. By causing the fibres to be oriented in a plane parallel to the forming surface, the strength and in particular tear strength of the fibrous product is increased. Such an increase is generally ob¬ tained but is particularly pronounced in cases where moisture and/or binder is added to the fibrous layer prior to the consolidation . By varying the degree of consolidation , e. g . by varying the pressure and/or temperature used in the method of the inven¬ tion, it is possible to prepare layered fibrous products in which the layers adhere to one another to a desired degree and thus to prepare products having properties which are adjusted to the final use of such products. A low degree of adherence between the thin fibrous layers is normally desired in soft light fibrous products, e. g . products 2 having a weight of 30-60 g/m , for use as toilet paper or facial towels, and the method of the invention is particularly suitable for the manufacture of such fibrous products . In the production of such fibrous products a consolidation of each layer is effected after the formation of such layer and the degree of consolidation is such that essentially no free fibre ends extend from the layer. As mentioned above the consolidation of the fibrous layer or layers presents significant advantages in cases where It is desired to increase the strength of the layered fibrous product by the addi¬ tion of binder. When a binder is used in the conventional soft fibrous products, the binder is ordinarily a wet strength binder, e. g . a binder of the latex type. However, it is difficult to cause such binders to migrate into a fibrous layer and it may be necessary to apply binder to both sides of the product. In such case the fibrous product has to be removed from the wire on which it has been formed and then turned around and transferred to another wire. Furthermore, complicated driers are required to effect the drying of the binder-containing fibrous products . By consolidating the fibrous layer or layers and by supply- O FI ϊng binder thereto before an additional fibrous layer is formed on top of the first layer, an improved penetration of the binder is obtained . It Is believed that this is due to the fact that the surface of the layer or layers are made more smooth as a result of the consolidation . Thus, a smaller amount of binder is required to in¬ crease the strength of the fibrous product to a given value by supplying the binder to the Intermediary product than to the final product. If binders of the latex type are used In fibrous products which are used for polishing purposes, e. g. to polish windows and mirrors, the problem may arise that the surfaces of the articles to be polished may be smeared by binder if the binder Is brought into direct contact with such surfaces . This problem can be eliminated by preparing the fibrous product by the method of the invention, viz. by supplying wet strength binder, such as a latex binder, to the fibrous layer only which is located within the product, i. e. which is protected by outer layers . In this case another type of binder may be supplied to these outer layers. Alternatively, the wet strength binder may be eliminated from the central layer and the consolidation of the outer layers may be increased by using a binder and/or by heating and moistening these layers. I n the latter case It is possible to form a particularly voluminous and soft product. The softness of such a product Is primarily due to the fact that each outer layer can be displaced relative to the central layer. A product of the above mentioned type can also be manu¬ factured by preparing the two layers, I . e. a consolidated outer layer and on top of that a fibrous layer which optionally contains a wet strength binder on one forming wire and another consolidated outer layer on a second forming wire and subsequently combining the two layers. The fibrous product thus prepared comprises two consolidated outer layers and a non-consolidated central layer. In cases where the central layer is thin, the outer layers can easily be displaced relative to one another and in that case the central layer actually functions as a separating layer. Conventional latex binders are hydrophobic. Thus, the rate at which liquids are absorbed In latex-containing products may some times be low. This constitutes a serious drawback because the liquid absorption rate of a fibrous product is in many cases a pro¬ perty which is equally- Important as the liquid absorption capacity of M?I said product. The hydrophobic character of the latex can be coun¬ teracted by introducing into the fibrous layer a surfactant. However, such surfactants sometimes have a harmful effect on the remaining properties of the latex. By providing a latex-containing fibre layer as a central layer in a fibrous product prepared by the method of the invention, the undesired effect on the liquid absorption rate originating from the latex may be significantly reduced because the outer layers will exhibit the desired high liquid absorption rate. When a latex binder is to be supplied to an interior layer only, it is possible to use emulsions having significantly higher dry matter contents than the emulsions used in the prior art methods. In such prior art methods the latex .emulsions used ordinarily have a dry matter content of 5-15% by weight and preferably about 10% by weight. In the method of the invention it is possible to use emul¬ sions having a dry matter content of up to about 40% by weight. The use of such highly concentrated emulsions significantly reduces the energy consumption during the drying of such products. The dry matter content of latex In the final product or in the layer containing latex may be from 5 to 15% in a product prepared by the method of the invention. When preparing strong and strongly absorbing fibrous products containing latex as binder, it is ordinarily preferred to make the product as voluminous as possible. In such cases it is possible to eliminate the consolidation after the formation of the central layer(s) and merely to apply latex binder onto said layer(s) by spraying. If it is desired to form a thick central layer, it may be preferable to prepare such layer in two steps and to apply binder twice. This is particularly preferable when the dry matter concentration of the binder is high . The application of the latex binder, e. g . in the form of an emulsion in water, may be followed by a step in which hot air is drawn through the fibrous material on the forming wire in order to evaporate and remove water and to cure the binder. By using suit¬ able amounts of latex binder in the central layer of such a heat- -treated fibrous product, it is possible to impart to such layer such a strength that the final multilayered fibrous product can be washed and wrung . OMPI Instead of heat-treating the fibrous product and conse¬ quently curing the binder shortly after the addition of said binder, the final multilayered product may be heat-treated either while it is still located on the forming wire or after it has been removed from said wire. In both cases a partial transfer of water from the moist latex-containing central layer to the surface layer takes place and the adherence between said layers will be Increased. The stream of air used for curing the binder preferably has a temperature of between 150 and 250°C, e.g. about 200°C. The curing of the binder can also be effected by means of light having a wave length which has been selected dependent of the properties of a surface layer, if any, containing no latex binder. In an embodiment of the .method according to the invention In which a latex binder Is used, it is possible as indicated above to prepare fibrous products which are sufficiently strong and stable to allow them to be wrung and washed. Such products are suitable e.g. as rolls of kitchen paper, table cloths and napkins. A latex binder can be used to provide a voluminous and highly absorbing product, e.g. a fibrous layer having a weight of 2 from 50 to 70 g/m . In such products the latex binder may be used to bond the fibres to one another in an open structure or In other words to keep the fibres spaced from one another. Other binders than latex binders are also suitable. Examp¬ les of other binders which are suitable in the method of the Invention are modified starches and polyvinyl alcohols and polyesters. The latter binders are suitable in the form of fibres, In the form of a powder or In liquid form. Instead of reinforcing the fibrous product by using a binder it is possible to add moisture to the fibrous layer In the form of an aqueous or gaseous medium before the consolidation and to effect the consolidation of the fibrous layer while . heating said layer to a temperature of above 100 C. The Increased strength thus ob¬ tained is believed to be due to the formation of hydrogen bonds between the fibres. A particularly high increase of strength Is obtained by heat-embossing the moistened fibrous layer. During such heat-embossϊng the fibres are brought into Intimate contact in the embossing zones in the presence of moisture and this causes the formation of pronounced strength zones. The moisture content of the fibrous layer prior to the heat embossing is preferably between 6 and 15% by weight and the embos¬ sing temperature used is preferably above 120°C . In such a heat-em¬ bossing the embossing rollers used may have a temperature of up to 200 C provided the embossing rate is sufficiently high . When the fibrous product is embossed in connection with or subsequent to the consolidation, such embossing is preferably effected in a manner so that the strongly embossed zones constitute from 10 to 40% of the total surface area of the product, and so that the remaining zones have pillow shape. Soft fibrous products having any given number of layers and the composition and thickness of each layer can be varied by the method of the invention . It is also possible to prepare products in which the thickness of the layers varies from layer to layer. In very light products, i. e. fibrous products having a weight of from 2 30 to 45 g/m , the fibrous layers ordinarily are about the same 2 weight, e.g . 10-15 g/m , whereas the central layer(s) may be made 2 heavier, e. g. may have a weight of 40-60 g/m in heavier fibrous products comprising three or more layers. The composition of the layers can be varied, both as far as the fibre type as well as the properties of a given fibre type Is concerned. Furthermore, the additives contained in said layers including binder and/or filler may also be varied. It is pointed out that in some cases It is preferable to use powdered binders, e. g . starch, thermoplastic or thermocuring syn- thetic binders and to introduce such binders together with fibres ■which are deposited on the forming surface. Thus, it is possible to incorporate such powdered binders into a central layer of the pro¬ duct. The cellulosic fibrous starting material can be mixed with varying amounts of other types of fibres, e.g. synthetic organic fibres . When only one type of fibres is used, it is possible to vary the relative amounts of short and long fibres. The number of layers is determined by the number of fibre distributors mounted along the forming surface, for example in the shape of an endless forming wire, and the thickness of the layers deposited from each of these fibre distributors is determined by the construction and operation of these fibre distributors . When carrying out the method of the Invention it is prefer¬ able to use fibre distributors of the type which is disclosed in the specification of British patent No. 1 .207.556. This type of fibre distributors consists of a housing having a plane, perforated bottom and comprising two or more stirrers having impellers rotating about axes which are perpendicular to the perforated bottom. A particularly suitable embodiment of such a fibre distribu¬ tor Is disclosed In British patent specification No. 2.008.038. This fibre distributor comprises two or more rows of stirrers provided In the same housing and extending perpendicularly to the direction of movement of a forming wire provided below said housing . When the fibre distributors of the above mentioned type are used for carrying out the method of the Invention , the consolida¬ tion of the fibrous layer Is preferably effected by a set of rollers, of which one roller is provided below and the second roller above the forming wire in such a manner that the fibrous layer deposited thereon is pressed against the forming wire. In that case the pattern of the forming wire is embossed Into the underside of the fibrous layer. In cases where the fibrous layer Is moistened and the consoli¬ dation is effected with a heated roller, the provision of such an embossed pattern will Increase the strength of the fibrous product. As described in the specification of British patent No. 2.015.604 it is possible to prepare a dry-laid fibrous product having a surface layer containing a relatively high concentration of long fib¬ res by using a forming wire having relatively large mesh openings. Such an open mesh forming wire is also suitable for use In the method of the invention . If it Is desired to provide another embossing pattern in the underside of the fibrous product than that of the forming wire, the fibrous layer passing from one fibre distributor to the following one can be slightly raised from said forming wire and thus caused to change position relative to said wire. The fibrous product can be raised from said wire, e. g . by directing a stream of air originating from an air slot, against the underside of the fibrous layer. When the fibrous layer after formation of a further layer on top of it Is passed Into the nip of a pair of rollers mounted after the second fibre distributor, the fibrous layer is in another position relative to the forming wire, and the pattern of said forming wire is consequently embossed into the underside of the fibrous layer in zones which are displaced relative to the corresponding zones of the embossed pattern..already provided in the fibrous layer. o..:?ι If the fibrous layer is moistened again and if a heated consolidation roller is used, the strength of the product can be further increased . The above mentioned raising and displacement of the fibrous layer can be repeated when the fibrous layer has passed the second fibre distributor and further fibre distributors . By providing the underside of the fibrous product with several overlapping and partially displaced embossed patterns, the number of zones in which the product has been strongly consolidated is increased and consequently, the strength of the fibrous product is also increased . Additionally, the opacity of the product is increas¬ ed. Such an increased opacity can also be obtained by introdu¬ cing into an internal layer, e. g . the central layer of a fibrous pro- duct, a powdered filler, such as kaolin, chalk, talc, titanium di¬ oxide and zinc oxide or a pigment. By varying the amount of filler it is also possible to vary the density of the product. Fillers, e. g . kaolin, in amounts of up to 50% of the weight of the fibrous product can be used but when a filler is used , the amount ordinarily is from 5 to 20% by weight. As mentioned above a pigment can be introduced into a central layer. If a three-layered fibrous product comprising a colour¬ ed central layer is embossed , a coloured embossed pattern having a high decorative effect can be provided by embossing . By combining a coloured central layer with surface layers .having different colours, varying colour effects are obtainable. Contrary to what is the case in the prior art methods, the provision of the above mentioned surface decorations can be effected while the fibrous product is still present on the forming wire. This means that the finishing of the product can be effected "on line" which means that it is possible to prepare fibrous products which are fully finished when they leave the forming wire and consequently merely have to be rolled up or cut into sheets . As mentioned above one or more layers of the multilayered product may consist of another type of fibres than the remaining layers. Thus, it is possible to prepare a layered product in which one or more internal layers are composed of relatively cheap fibres, such as waste fibres and re-used fibres . When a three-layered fibrous product is prepared by the c!.:?ι method of the invention, all three fibre layers are formed orr the same forming wire and consequently a production rate of 500 m/min . on such an apparatus corresponds to a production rate of 1500 m/mϊn. on a conventional apparatus on which only one layer Is formed . At production rates of above about 300 m/mϊn. on a dry forming machine, such vibrations in the stream of air containing fibres may arise that the fibres are concentrated in linear zones thus making the fibrous product ununiform. This phenomenon is sometimes referred to as the "beach effect" . By consolidating the fibrous layer shortly after its forma¬ tion as takes place by the method of the invention, this "beach ef¬ fect 11 Is counteracted because the fibres are fixed relative to one another and this fixation Initiates a ' self-adjustment which is due to the fact that larger amounts of fibres are deposited in the thin zones in which the resistance to air flow is minimum rather than in the relative thick zones. Furthermore, the fibrous layer is smoothened out by the method of the invention and consequently the final pro¬ ducts obtained are more uniform than the products obtained by the wet-forming method and products prepared by the conventional dry -forming methods. Fibrous products prepared by the conventional wet process normally have different strengths in the machine and cross-machine directions. Thus the fibres of the fibre slurry deposited on the foraminous wire tend to be oriented predominantly in the machine direction instead of being oriented at random. Therefore, It is diffi¬ cult to manufacture wet-laid fibrous products having the same strength in all directions. In some products, however, it is desired to provide a higher strength in the machine direction than in the cross-machine direction . This applies e.g. to newsprint paper , which should be capable of resisting the high tensions in the machine direction which occur during the printing process. Fibrous products having an increased strength in the machine direction (or in another desired direction) can be manufac¬ tured by a special embodiment of the process of the Invention. Thus, if a thin fibre layer deposited on the forming surface is subjected to the influence of forces which are acting in the machine direction or any other direction, it is possible to impart to the fibres O H of said layer a predominant orientation In said direction. Thus, if water is to be added to the fibre layer before consolidation the water is preferably sprayed onto the fibre layer under an acute angle relative to the surface of the fibre layer, e.g. in the machine direction so as to rearrange the fibres and to orient them predominantly in said direction. The water is preferably spray¬ ed onto the fibre layer when said layer is still located on the forming wire and is under the influence of a vacuum maintained in the suc¬ tion box. The angle under which the water is sprayed onto the fibre layer is preferably between 10° and 30°. The effect described may be improved by directing flows of water and compressed air against the fibre layer. The above mentioned embodiment of the process of the in¬ vention also makes it possible to Increase the strength of the fibrous multilayer sheets in the plane of said sheets and to effect a corres¬ ponding decrease of the strength in a direction perpendicular there¬ to. Experiments have shown that the process of the invention is suitable for the manufacture of both soft products, such as pro- ducts for use as towels and tissue as well as harder paper- and board-like products. In the manufacture of heavy products, e.g . products for the production of multiply folding boxboard, the middle layer(s) is (are) preferably moistened during the consolidation, and it is possib- le to manufacture multilayered products comprising 5-10 layers. In the production of multilayered products comprising 5-10 layers the moistening may also be effected when the last layer or layers have been formed, and the heat consolidation is preferably effected at a pressure exceeding 20-50 kg per cm. In order to obtain a proper balance the bottom layer or layers should be treated In a similar manner with water and should be consolidated and optimally subjected to a conventional finishing treatment. This balancing consolidation may be effected on a transfer wire which is a wire to which the fibrous layer Is transferred after its formation on the forming wire. Thus the transfer wire may have a lower run and a suction box located above said lower run . When the fibrous layer is introduced into the zone below said lower run, it is sucked against the transfer wire and is maintained in contact with said wire until the vacuum is broken . During this period the under- side of the fibrous layer may be moistened and subsequently the product may be consolidated. In some cases the layers located at the surfaces of the multilayered sheets should preferably be strongly consolidated so as to produce a beam effect and to provide a strong and smooth sur¬ face. The intermediate layer or layers should be consolidated to a lower degree thus permitting the product to be folded or bent with¬ out generating the socalled "biscuit effect" . In this embodiment the consolidation and the surface struc- ture may be improved by adding to the surface layer pigments and/or binders either in the form of a powder, a dispersion, or an emulsion . The binders may be added to the water for moistening the fibrous layers or in powder form to the' fibres before or during their deposition on the forming surface. Although the method of the invention mainly has been de¬ scribed in connection with the manufacture of three-layered fibrous products, it Is to be understood that the desired technical effect is also obtained when preparing two-layered products and products containing more than three fibrous layers. O^PI

Claims: C l a i m s 1. A method of manufacturing a fibrous product comprising the steps of forming a multilayered fibrous product consisting of at least two layers on the top surface of a continuously moving gas- -permeable forming surface, maintaining a vacuum at the underside of said gas-permeable forming surface and successively supplying streams of a fibre-containing gas to the top surface of said forming surface, c h a r a c t e r i z e d in effecting at least after the formation of the first fibrous layer and before the formation of the second layer a consolidation of the fibrous layer located on the forming surface such that at least the major proportion of the fibres of said layer are oriented in a plane parallel to the forming surface. 2. A method according to claim 1, c h a r a c t e r i z e d in that the consolidation is effected by rolling said fibrous layer. 3. A method according to claim 1, c h a r a c t e r i z e d in that a binder is supplied to said fibrous layer in connection with the consolidation or after the consolidation but before the formation of a further fibrous layer. 4. A method according to claim 2, c h a r a c t e r i z e d In that the fibrous layer is moistened before the consolidation and that the consolidation is effected while the fibrous layer is heated. 5. A method according to claim 4, c h a r a c t e r i z e d in that the consolidation is effected by heat embossing the fibrous layer. ' 6. A method according to claim 5, c h a r a c t e r i z e d in that the moisture content of the fibrous layer is adjusted at a value of between 6 and 15% by weight before the consolidation. 7. A method according to claim 1 for the manufacture of a soft layered fibrous product, c h a r a c t e r i z e d in effecting after the formation of each fibrous layer such a consolidation that the surface of the consolidated fibrous layer has essentially no fibre ends extending from said surface after said consolidation. 8. A method according to claim 1 for the manufacture of a soft three- layered product, c h a r a c t e r i z e d In effecting the consolidation in the presence of heat and moisture after the formation of the first fibrous layer and after the formation of the third fibrous layer. 9. A method according to claim 1, c h a r a c t e r i z e d in that the second fibrous layer is treated with a wet strength binder before the formation of a further fibrous layer. V

Inventors: KROEYER KARL KRISTIAN KOBS, KROEYER, KARL, KRISTIAN, KOBS
Applicants: KROYER K K K, KROEYER, KARL, KRISTIAN, KOBS
Application year: 1982
Main classification: D21H1
IPC codes: D21F11