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Process for making a liquid absorbing thermoplastic materialRelated Patent Categories: Synthetic Resins Or Natural Rubbers -- Part Of The Class 520 Series, Involving Inert Gas, Steam, Nitrogen Gas, Or Carbon Dioxide, Processes Of Preparing A Desired Or Intentional Composition Of At Least One Nonreactant Material And At Least One Solid Polymer Or Specified Intermediate Condensation Product, Or Product Thereof, Nonmedicated Composition Specifically Intended For Contact With Living Animal Tissue Or Process Of Preparing; Other Than Apparel, Compositions For Use In Tape Adhesives, Binder Or Impregnate For A Body Fluid Adsorbent Device, E.g., A Surgical Adhesive Tape, Etc.Process for making a liquid absorbing thermoplastic material description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070032565, Process for making a liquid absorbing thermoplastic material. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates to a process for making a liquid absorbing thermoplastic material, comprising a thermoplastic polymeric composition having absorbent particles dispersed therein. Such liquid absorbing thermoplastic materials can be utilized in a number of end uses where liquid absorption is desired, for example in the absorbent core of disposable absorbent articles, such as sanitary napkins, panty liners, interlabial devices, tampons, disposable diapers, incontinence pads, wound dressings, nursing pads and the like for completely or partially substituting said absorbent core. BACKGROUND OF THE INVENTION [0002] In general the absorption and retention of aqueous liquids, particularly body fluids such as urine, menses, etc., are accomplished by use of absorbent articles containing absorbent materials. Such articles include sanitary napkins, panty liners, interlabial devices, tampons, disposable diapers, incontinence pads, wound dressings, nursing pads, and the like. Generally, the most used absorbent materials are cellulose materials (e.g., defiberised wood pulp) and superabsorbent materials. In particular, when referring to disposable diapers or sanitary napkins and the like presently available in the market, the cellulose materials are in the form of bat or sheet, typically further containing particulate absorbent materials, usually referred to in the art as superabsorbents or hydrogelling materials, which allow to manufacture thin but very absorbent core structures. A primary need in incorporating superabsorbent material in particle form within an absorbent structure is its stabilization, in order to counteract the tendency of powdered material to bunch up or agglomerate, hence providing an uneven absorptive capacity in the absorbent structure, or also to dust off the structure itself. Known approaches are for example to adhesively fix the particles into a fibrous structure, or to disperse the powdered superabsorbent material in a fibrous matrix, e.g., cellulose pulp, and fix it in place mechanically, e.g., by calendaring or embossing. An alternative approach is to blend a superabsorbent particulate material into a thermoplastic matrix, e.g., a thermoplastic composition. The superabsorbent containing thermoplastic composition can be typically extruded or coated in any desired position and pattern onto a suitable substrate, to be then incorporated into an absorbent article, thus entirely providing the absorbent material in the article, or alternatively integrating a more traditional fibrous absorbent structure, with no risk of dust off of the particulate material, or displacement within the absorbent structure of the article. For example, EP 1013291 and WO 98/27559 describe a hot melt adhesive containing a superabsorbent polymer. WO 99/57201 illustrates compositions comprising a thermoplastic component and a superabsorbent polymer, said compositions in form of a film layer or applied to a disposable absorbent article with various hot melt adhesive application techniques. Applications WO 03/049777 and WO 04/028427 respectively describe thermoplastic compositions comprising a matrix of a thermoplastic polymeric composition and superabsorbent particles dispersed therein, which have a particularly effective fluid acquisition and handling capacity, and absorbent articles comprising superabsorbent containing thermoplastic compositions arranged in a pattern of unattached spaced apart zones. [0003] The technology of composite thermoplastic materials comprising a matrix of a thermoplastic composition and particles of superabsorbent material dispersed therein has provided a solution to the problem of powdered superabsorbent material "instability" within absorbent structures in absorbent articles, preventing particle displacement as, e.g., dust-off, agglomeration, or bunching up, and safeguarding the end users of absorbent articles virtually from any undesired contact with the superabsorbent particles upon normal use. This technology has also allowed the use in absorbent articles of superabsorbent materials in a much smaller particle size than previously adopted for traditionally manufactured fibrous absorbent structures with superabsorbent materials. Such reduced average particle size, generally in the range below about 150.mu., about 10.mu. to about 40.mu., as disclosed for example in WO 04/028427, is advantageous not only in terms of better liquid handling and absorption capacity of the corresponding thermoplastic absorbent compositions, due to the increased surface/volume ratio provided by smaller particles, but also in an easier processability of the thermoplastic material, which may have lower viscosities at the process conditions, typically a hot melt extrusion onto a substrate. [0004] However, powdered superabsorbent materials also pose health risks to those involved in the manufacturing process, particularly when they are manufactured in very small particle sizes as mentioned above. The finely powdered superabsorbent material can become airborne and can be inhaled by workers. Once inhaled, the superabsorbent material absorbs liquid within the respiratory passages swelling to many time its original size. This can result in blocked air passages and potentially traumatic health complications. [0005] According to the known technique, liquid absorbing thermoplastic materials comprising a matrix of a thermoplastic composition with superabsorbent particles dispersed therein are manufactured by providing a) the superabsorbent material in the desired, and possibly very small, particle size, usually achieved by grinding coarser particles, b) the components of the thermoplastic composition, and c) by uniformly mixing them all upon sufficient heating, in order to melt the components of the thermoplastic composition. Hence the superabsorbent material has to be typically handled in a very small particle size, namely manufactured, shipped and then processed in the production line of the liquid absorbing thermoplastic composition. Shipping of superabsorbent materials in very low particle size is also subjected to severe limitations in many countries, based on health and environmental grounds. On the other hand provision of superabsorbent material in the desired very low particle size starting from coarser and environmentally less demanding particles directly in the same line of production of the final liquid absorbing thermoplastic material is not always convenient, since it necessarily implies addition of a grinding station to the production line, which does not allow much flexibility in the manufacturing process of the liquid absorbing thermoplastic material, and increases complexity. It also implies in any case handling of fine superabsorbent particles at a certain stage of the process. [0006] Hence there is the need for a process for making a liquid absorbing thermoplastic material comprising a matrix of a thermoplastic composition and absorbent particles, typically particles of superabsorbent material, dispersed therein which is more flexible and allows the use of particles in the desired low particle size, without the limitations and drawbacks created by the handling of such very fine particles. [0007] It would, therefore, be desirable to provide an improved process for making a liquid absorbent thermoplastic material, which allows a simpler and safer handling of the absorbent materials in very low particle sizes. It would also be desirable to provide a process for making a typically liquid absorbing thermoplastic material wherein particles of superabsorbent material can be manufactured in any desired particle size, and then possibly further handled, e.g., stocked or shipped, before provision to the liquid absorbing thermoplastic material production line. SUMMARY OF THE INVENTION [0008] The present invention provides a process for making a liquid absorbing thermoplastic material comprising a matrix of a thermoplastic polymeric composition and particles of absorbent material dispersed in the matrix, wherein the thermoplastic polymeric composition comprises at least two components. The process comprises the following steps: [0009] providing the particles of absorbent material, mixing the particles of absorbent material with a first component in the liquid state of the at least two components forming a pre-mix, mixing the pre-mix with a second component of the thermoplastic polymeric composition in the molten state. BRIEF DESCRIPTION OF THE DRAWINGS [0010] FIG. 1 shows a schematic view of a production line for the manufacture of a liquid absorbing thermoplastic material according to the present invention. DETAILED DESCRIPTION OF THE INVENTION [0011] By "liquid" as herein used is meant water based fluids or liquids such as urine, menses, serum, blood, sweat, mucous as well as other aqueous solutions generally defined as body fluids, but it is not intended to exclude other water based fluids. [0012] By "room temperature" as herein used is conventionally meant a temperature of 25.degree. C., as known in the art. [0013] For purposes of the present invention, viscosity has to be meant as melt viscosity at a certain temperature, which is determined with any suitable apparatus as known in the art. Particularly, viscosity can be determined with the procedure and apparatus described on page 22 of the already cited PCT application WO 99/57201, to which reference is made, in the paragraph headed "Melt Viscosity", possibly with the necessary adjustments, e.g., spindle selection, for measuring viscosities below 10 centipoises. [0014] For purposes of the present invention, particle size is defined as the dimension of a particle which is determined by means of any suitable method known in the art for particle sizes comprised in the range according to the present invention. Particularly indicated are laser light scattering analysis or laser diffraction analysis. The average particle size of a given sample is defined as the particle size corresponding to a cumulative distribution of 50% of the particles of the sample. In other words, the average particle size of a given sample of absorbent material particles is defined as the particle size which divides the sample in half on a mass basis, i.e., half of the sample by weight will have a particle size greater than the average particle size and half of the sample by weight will have a particle size less than the particle size. [0015] The process of the present invention will be herein described with reference to the production of a liquid absorbing thermoplastic material wherein the particulate absorbent material is in fact a water-insoluble water-swellable material in particle form, but of course the present invention refers to any liquid absorbing thermoplastic material comprising a different absorbent particulate material. [0016] FIG. 1 shows a production line for the manufacture of a liquid absorbing thermoplastic material comprising a matrix of a thermoplastic polymeric composition and particles of water-insoluble water-swellable absorbent material dispersed therein. The matrix of thermoplastic polymeric composition comprises at least two components. [0017] The particles of water-insoluble water-swellable absorbent material 10 are provided from a supply means, for example a container 15, and the first component 20 in the liquid state of the thermoplastic polymeric composition is provided from a respective suitable supply means, typically a tank or vessel 25. The particles 10 and the first component 20 in the liquid state are conveyed in the desired amounts and proportions by suitable means, e.g., the tubing 27, to a mixing device 30, where they are uniformly mixed to form a pre-mix 35. Preferably, the first component 20 of the thermoplastic polymeric composition is liquid at room temperature (25.degree. C.), such that it can be directly provided in the liquid state from the tank or vessel 25 to the mixing device 30. The pre-mix 35 is such a case is typically a slurry which is more or less viscous or pasty at room temperature. Alternatively, the first component 20 can be solid or semi-solid at room temperature (25.degree. C.), and brought to the liquid state by suitably heating it with known heating means before or upon providing it to the mixing device 35. In one embodiment, the first component 20 is selected such that at room temperature it is liquid, and also has a viscosity below about 30,000 centipoise, below about 10,000 centipoise, below about 5,000 centipoise, between about 1 and about 4,000 centipoise, or between about 10 and about 3,000 centipoise. Low viscosity at room temperature of the liquid first component 20 provides the advantage of an easier and more effective mixing with the particles of water-insoluble water-swellable absorbent material 10 in order to form the pre-mix 35 in the mixing device 30. Less energy is in fact required for the mixing step, as well as for handling and conveying the first component 20, typically from the tank or vessel 25 to the mixing device 35, and of the pre-mix 35 as well further of the mixing device 35. A uniform mixing of the particles of water-insoluble water-swellable absorbent material 10 and of the first component 20 may be more easily and effectively achieved because of the relatively low viscosity of the first component 20 at room temperature. [0018] If desired, a controlled heating can be also provided to a first component which is liquid already at room temperature, in order to further reduce its viscosity and preferably bring it in the desired range for the mixing step where the pre-mix is formed. Such viscosities can be also provided by suitably heating and melting a first component in said mixing step in the less preferred embodiment when the first component is solid at room temperature. [0019] The pre-mix 35 is then further processed in order to form the final liquid absorbing thermoplastic material. This further process can be performed directly in the same production line, right after the formation of the pre-mix 35, or alternatively the pre-mix can be temporarily stored, in order to be subsequently fed to the production line of the liquid absorbing thermoplastic material, in the same facility, or alternatively shipped to another facility for this final production step. In any case, the handling, storing and/or shipping of the pre-mix 35 constituted by the first component 20 preferably liquid at room temperature, comprising the particles of water-insoluble water-swellable absorbent material 10 dispersed therein, is greatly simplified with respect to the handling, storing and/or shipping of the particles of water-insoluble water-swellable absorbent material alone. Continue reading about Process for making a liquid absorbing thermoplastic material... 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