Process for Manufacturing a Non-Woven Fabric by Hydrodynamic Needling, and Product of Said Manufacturing Process

In the journal ITB Nonwovens, Industrial Textiles, 4/95, pages 20-25, "Degradables or the Recycling Economy for Disposables," or in the journal Chemical Fibers International, Vol. 46, April 1996, page 102, "A New Water-Soluble PVA Fiber for Nonwovens Application," a special chemical fiber is described made from a hydrophilic synthetic polymer. In addition to other important properties, it has the special ability to dissolve in water at certain temperatures without leaving a residue. The fiber consists of a polyvinyl alcohol and can be processed by weaving or by nonwoven technology to form a sheet material from which any desired article of clothing or the like can be produced. These sheets made of PVA fibers however are used especially in hygienic products because after being used during surgery in a hospital for example, they can be disposed of easily and rapidly if contaminated with blood and the like by dissolving them in hot water.

It is known that these PVA fibers can be processed by means of a card or aerodynamically to form a nonwoven. To lend them sufficient strength, these fleeces are compacted by mechanical needling technology and then processed to produce the desired articles of clothing. It has been found that fleeces with these fibers that have been compacted by mechanical needling technology can be produced at rates that are too low for industry. In addition, when mechanical needling is used, there is a risk of holes being formed, which is disadvantageous for the desired impermeability of the hygienic articles to water.

The goal of the invention is to develop a method by which the nonwoven manufactured as usual from these PVA fibers can be compacted continuously at a higher rate than by the mechanical method, and thus a product can be produced that is uniformly compacted and is essentially impermeable to fluid.

It is known to use hydrodynamic needling alone to produce a compacted endless fleece product. The fleece web coming from a fleece-laying machine such as a card or together with a cross-layer is subjected immediately thereafter to needling water jets to compact the fleece product. Then the wet fleece must be dried. The idea of the invention is to process a nonwoven by this method, said nonwoven consisting at least partially of PVA fibers. Initially it would appear impossible to use this water needling method alone as a compaction method for such a chemical fiber, since the fibers come in contact with water for a prolonged period of time during needling and therefore there is a risk of their dissolving. This danger exists, if not during needling itself, then at least during the drying of the wet fleece product immediately afterward, since drying is not possible without heat.

It has now been found according to the invention that it is nevertheless possible with several special parameters to compact a fleece product made of these fibers using the hydrodynamic method. Thus for example it is advantageous if the previously moistened nonwoven is subjected once on both sides to the water jets and by several water jets in sequence in each case with the energy of the jets of the next nozzle beam always being higher, for example from 50 bar to 120 bar on the first side and from 120 bar to 160 bar on the second side. The last needling on each side should be performed at about 80 bar and performed with a larger number of water jets distributed across the width of the nonwoven in order to produce a uniformly smooth surface. Under these conditions, a fleece weight of 40 to 150 g/m.sup.2 can be compacted. The transport speed of the nonwoven during compaction is 70 m/min or more. The fleece-processing rate depends only on the possible fleece-laying rate. The production rate is adjusted to the respective fleece weight, but it is always lower at higher weights.

It is important how the drying parameters are defined. Initially the needled fleece must be dewatered mechanically before it is dried, by squeezing or by suction for example, in order to achieve a level of moisture that is not more than 100%. Then the fleece must be dried by drying air which is not heated to a temperature greater than 120.degree. C. It is especially advantageous for the drying and the ventilation to be performed at the same time, in this case on a rotating screen drum with internal suction, and to increase the air speed in the fleece by a high fan rpm, up to 4 m/second. Various tests have shown that with this method there is no damage to the PVA fibers. Both during drying and also during the hot final processing that follows, there were no visible disadvantages like the brown spots that usually occur otherwise. Basically, drying is also possible using a belt dryer, with ventilation also being produced, or with an IR dryer, etc.

If the fleece is also to be given impregnation such as foam or liquid impregnation that makes it water-repellent, it is advantageous to perform this step after a first drying down to 30% moisture content unless impregnation takes place wet-in-wet. Then after the first drying, the second drying stage should be performed exactly like the first at a temperature of up to 120.degree. C. and the fleece dried completely. It is also possible to perform impregnation only after drying for example down to 5% moisture content. Following complete drying of the needled fleece, it is no problem to crosslink the fleece at temperatures up to 210.degree. C.

It is known that a fleece made of these PVA fibers can be provided with an additional layer of pulp or paper in order to increase the water-repellent property of the nonwoven. Foam impregnation, liquid impregnation, and also advantageously in the method according to the invention, application of a layer of this kind in pulp form or as tissue paper, can be used, and then bonding the layer to the needled fleece, with said layer being laid down on the fleece prior to the second needling for needling on the back side simultaneously with the fleece, said layer being bonded with the fleece during the needling that then takes place.

The method according to the invention produces a novel product. The subject of the application also extends to a fleece product made of PVA fibers that is compacted by water needling on both sides for example and finally is dried as well.