WO2001028596A2

WO2001028596A2 - Superabsorbent plexifibrils, fibrous sheets made out of them and process for their manufacturing

Info

Publication number: WO2001028596A2
Application number: PCT/EP2000/010517
Authority: WO
Inventors: Ivo Edward Ruzek
Original assignee: Ivo Edward Ruzek
Priority date: 1999-10-21
Filing date: 2000-10-19
Publication date: 2001-04-26
Also published as: WO2001028596A3; DE19950728A1

Abstract

The invention relates to plexifibrils made by flash spinning of overheated aqueous gels of superabsorbent polymers based on polyacrylic acid. Due to a flash evaporating of solvent water from pressurised aqueous gels extruded through a spinneret, which occurs instantly after the mass left the pressure area the superabsorbent polymer will be disintegrated in plexifibrils. Plexifibrils have a particular form of a network with typical (macro-) length of 400 to 1000 mm and with a typical (micro-) thickness of less than 4 νm. They can easily be laid in form of nonwoven sheets at a foraminous conveyer belt. Superabsorbent plexifibrils or nonwoven sheets made out of them can be used as constituents in absorption sheets or hygienic items like diapers or sanitary napkins.

Description

Superabsorbent Plexifibrils, Fibrous Sheets Made out of Them and Process for their Manufacturing

The invention relates to a process for manufacturing of superabsorbent plexifibrils and of fibrous sheets made out of them. According to this process, the superabsorbent polymer will be mixed with water in a weight ratio of 1:0.5 to 1:10 to form a gel. This gel will be heated under pressure to a temperature of 120 to 230 °C (spinning temperature) and the mass will be extruded by means of a gear pump under a pressure, which should correspond at least to the equilibrium water pressure at the spinning temperature, through a spinneret into an expansion space which will be kept under a substantially lower pressure, than the spinning gel. By the instant (flash) water evaporating the superabsorbent polymer will be transferred into plexifibrils.

Further, this invention relates to a process where the weight ratio of superabsorbent polymer and water will be kept between 1:0.5 and 1:2. Further, the spinning temperature of the polymer gel may be kept between 130 to 200 °C. In an other embodiment of this invention, the pressure in the expansion space following he spinneret will be atmospheric pressure.

The present invention also includes a process of producing a nonwoven fabric by forming a fibrous sheet at a foraminous conveyer belt evacuated from the bottom.

Finally this invention relates to superabsorbent plexifibrils manufactured according to the described process.

State-of-the-art

Superabsorbent polymers (SAP) are polymers capable to catch and bind large amount of water by forming aqueous gels. As crucial properties of SAP will be evaluating their absorption capacity, absorption speed and transport properties for water in gel form.

Several types of superabsorbent polymers have been developed and patented. Most important among them are SAP based on polyacrylic acid (PAA), which can be described as state-of- the-art SAP. The group name "polyacrylic acid" will be used for polymers containing more than 50 % acrylic acid monomer and which are non soluble in water, but capable to substantially swell in water. Such SAP of the class of polyacrylic acid include crosslinked polyacrylic acid or copolymers, starch grafted polyacrylnitril hydrolysates, starch and acrylic acid grafted crosslinked polymers as well as hydrolysates of copolymers based on vinylacetate and acrylic esters. In such polymers and copolymers, about 60 to 90 % of all carboxyl groups are neutralised by alcalic metals.

For the practical use SAP will be manufactured as powder or fine granulate. Product properties will be defined by absorption capacity, absorption speed, water permeability, gel stability over the time.

It is very difficult to provide all requirements above mentioned, because some of them are contradictory. It is a challenge for the further development to optimise the properties. For instant, SAP with high absorption capacity, which means a gel with low degree of crosslinking, is limited in the absorption speed, in water permeability and gel stability when swollen. In opposite, SAP providing high values of absorption speed, water permeability and gel stability would provide much lower absorption capacity.

There have been numerous attempts to modify the chemical structure of the SAP in whole or at the particle surface, trying to reach more advantageous combinations of properties. Such attempts are known e. g. from the USP 5,883,158. It is possible to reach more advantageous combinations of SAP properties by changing the particle size. By using polymers with high absorption capacity it is possible to increase he absorption speed by using smaller particle size. Nevertheless, there are narrow practical limits for such optimising process. In mixture with wood pulp as basic structure of absorption cores, small size SAP particles are difficult to be anchored and with reduced particle size the tendency of SAP migration outside of such structure increases rapidly.

Further, it has been proposed to transform SAP in thin film, which could help to manage the problem of poor anchoring and fluid transport in the absorption sheet. The disadvantage of this solution is the necessity to use such a film as the bottom ply of combined multiplies absorption sheets.

Absorption cores of hygiene items, as baby and adult incontinence diapers, sanitary napkins etc., where SAP will be used in combination with wood pulp, represent by far the most important market for SAP.

Without respect of the superabsorbent polymers it is known, to spin polymers not water soluble, or not swelling in water, from polymer gels in suitable organic solvents. According to this process, known e. g. from the USP 5,707,580, polymer gels in organic solvents will be heated at a temperature far above the boiling temperature of the solvent used and extruded through a spinneret. When leaving the spinneret holes, the solvent evaporates instantly (flashes) whereby the polymer will be transferred in fine fibrils (plexifilaments) of a typical network structure. It is obvious, that, when using organic solvents, it is necessary to perform the flash spinning process in a closed cabinet. All solvent vapours have to be condensed, recycled and brought back into the process. However and by all means, concessions have to be made to prevent the solvent or its vapour respectively from polluting of the environment.

Objective of the Invention

Objective of this invention is to transfer superabsorbent polymers in a ultrafine fibrillar form allowing to reach a favourable combination of both high absorption capacity of low crosslinked acrylic acid polymers and sufficiently high absorption speed, the latter due to a ultrafine resolution of the SAP in form of plexifibrils.

Scope of the Invention

The objective of the invention will be solved in a way, that aqueous SAP gels, containing 50 to 1000 weight-% water based on SAP will be heated at temperatures of 120 to 230 °C (spinning temperature) and extruded through a heated spinneret (a metal plate with small orifices) by means of a metering gear pump at a pressure at least corresponding to the equilibrium water steam pressure at the spinning temperature into an expansion space where the pressure will be substantially below the water steam pressure at the spinning temperature. The aqueous gel, where the solvent (water) is present under supercritical conditions and instantly evaporates (flashes) just after it left the spinneret holes and so has been released from the high pressure conditions, will be disintegrated in form of ultrafine network of plexifibrils. By controlling of the process conditions it is possible to receive SAP plexifibrils with minimum water content. The water vapour evaporated during the flash spinning process can not pollute the environment and can be released. Subsequently it is possible to perform the flash spinning process in an open space which can be kept under atmospheric pressure.

From the point of view of energy consumption it is advantageous to keep the water portion in the aqueous gel as low as only possible. It has been proven, that the water portion can be reduced up to 50 to 200 weight-%, based on SAP.

To prevent polymer degradation it is further of advantage to keep the spinning temperature in a range of l30 to 200 ^cC.

Ultrafine SAP plexifibrils manufactured according to this flash spinning process have a particular network structure with extremely large specific surface. This improves the water absorption properties. Due to their particular network structure, SAP plexifibrils with a typical (macro-) lengths of 400 to 1000 mm and a typical (micro-) thickness of less than 4 μm, can be easily anchored and fixed in the basic wood pulp structure of typical absorption sheets. They show no trend to migration outside of the absorption core, regardless how much of SAP plexifibrils will be added.

The present invention also includes a process of for producing a nonwoven fabric out of SAP plexifibrils, in which case the flash spun SAP plexifibrils will be collected at a foraminous conveyer belt. Such nonwoven SAP fabric provides excellent properties as an ultimate water absorbent. It can be used as one constituent of multiplies high performance absorption sheets.

A very important advantage of the flash spinning process out of aqueous polymer gels according to this invention is the fact, that no organic solvents, what so ever, will be used, but water, providing with SAP gels, which can be under suitable conditions flash spun into SAP plexifibrils. To use water as solvent represents a substantial economical advantage, because the evaporated solvent - water - need not to be carefully regenerated. On top this process is free of any pollution danger and can not spoil the environment.

Examples

Example 1

In a screw plasticiser heated at 60 °C have been mixed 1 part of a commercial superabsorbent polymer powder based on polyacrylic acid Favor® manufactured by the Stockhausen Company with 1.5 parts water to form a gel. The gel was transferred into an extruder with a screw diameter of 30 mm., heated at 220 °C and by means of a heated metering gear pump extruded through a spinneret equipped with 12 holes. The spinneret package has been heated at 220 °C and has been equipped with a screen filter located between the spinneret entrance and the spinneret plate. In the free space between the filter screen and spinneret plate the overheated gel was allowed to form bubbles due to depressurising of the spinning mass below the equilibrium water pressure at 220 °C. The throughput has been kept at 12 g of the gel per minute.

After the SAP gel extruded through the spinneret holes left the pressurised area and vent in the expansion space below the spinneret kept at atmospheric pressure, the water solvent flashed off (evaporated instantly) from the overheated gel, leaving back a particular three dimensional film-fibril network of SAP. This ultrafine network has been characterised by a (macro-) length of the fibrils in range of about 400 to 1000 mm and by a very low (micro-) thickness of about 4 μm or less. The three dimensionality results apparently from the cross- linking interconnections o the subdenier fibrils. Thus a multitude of individual but interconnected fires have been created from each singular spinneret hole. This particular fibrous structure has been addressed as plexifibrils.

Due to their large specific surface, which has been estimated at about 2 m²/g SAP, plexifibrils as superabsorbent allow an excellent access for water and provide a high absorption speed.

Example 2

In this case a gel of superabsorbent polymer based on polyacrylic acid containing 2.3 weight parts of water on 1 weight part of SAP has been used. This has been an intermediate product resulting directly from the polymerisation of acrylic acid in aqueous solution. This gel was filled in an 30 mm extruder, heated at 200 °C and forwarded by means of a metering gear pump to a primary air spinneret. In this case a "melt blown" spinneret has been used. It consists of a V-shaped spinneret body with orifices flanked by two air slots (one at each side) connected with a air heater. In this case, the spinneret was heated at 200 °C and the pressurised air has been heated at the same temperature. The entrance of the spinneret body formed a "bottle-neck" so, that in the larger space following, the spinning mass has been depressurised and allowed to form bubbles. The spinneret has been equipped with 50 holes. The throughput of the spinning mass has been kept at 0.5 g per hole and minute.

Just after the spinning mass was released from spinneret holes, plexifibrils have been formed in same way, which has been described in Example 1. By means of two hot air streams flanking the V-shaped spinneret body, plexifibrils have been additionally dried and forwarded to a foraminous conveyer belt located in a distance of 500 mm below the spinneret. At the conveyer belt, a uniform sheet of SAP plexifibrils has been formed as a "SAP nonwoven fabric".

Example 3

In an autoclave a SAP gel has been made containing 5 weight parts of water at 1 weight part of SAP. The mass has been heated at 230 °C and forwarded by means of metering gear pump to a melt blown spinneret described in Example 2. The spinneret as well as the air have been heated at 230 °C. The throughput of the spinning mass has been kept at 1 g per hole and minute.

Just after the spinning mass was released from spinneret holes, SAP plexifibrils have been formed in same way, as described in Example 1 and Example 2. Due to higher water content it was necessary to increase the air stream and volume just to receive dry SAP plexifibrils.

Claims

1. Process for manufacturing of superabsorbent plexifibrils and of fibrous nonwoven sheets out of them wherein superabsorbent polymers will be mixed in a weight ratio of 1:0.5 to 1:10 with water to form a gel, heated under pressure to a temperature of 120 to 230 °C and subsequently by means of metering gear pump under pressure corresponding at least to the equilibrate water vapour pressure at the spinning temperature extruded into an expansion space with a pressure substantially below the equilibrate pressure as above, by forming ultrafine plexifibrils due to flash evaporating of water solvent.

2. Process according to claim 1 wherein the weight ratio of superabsorbent polymer and water will be kept from 1:0.5 to 1:2.

3. Process according to claim 1 and 2 wherein the spinning temperature will be kept in a range from 130 to 200 °C.

4. Process according to claims 1 to 3 wherein the expansion space below the spinneret will be kept at atmospheric pressure.

5. Process according to claim 1 to 4 wherein the superabsorbent plexifibrils will be transferred below the spinneret at a foraminous conveyer belt to a nonwoven sheet.

6. Superabsorbent plexifibrils manufactured according to claims 1 to 5.

7. Nonwoven sheets made of superabsorbent plexifibrils manufactured according to claimsl