Absorbent material and production thereof

Abstract

The invention relates to a method for production of paper, wherein cellulosic fibres in an aqueous suspension are treated with a carrier carrying an oil, wherein the carrier has a hydrophobic and porous surface. The invention can impart improved sorptive capacity to the produced paper. The invention also relates to paper obtainable by the method. The invention further relates to a method for producing an absorbent and an absorbent obtainable from the method, and the use of a carrier and an oil carried by the carrier for imparting an increased sorptive capacity to an absorbent comprising cellulosic fibres.

Description

[0001] The present invention relates to paper having improved sorptive capacity and a method for producing such paper. Furthermore, the invention relates to a method for producing an absorbent and an absorbent obtainable by the method. The invention also relates to the use of a carrier and an oil for treating cellulosic fibres to improve the sorptive capacity of an absorbent.

BACKGROUND OF THE INVENTION

[0002] A wide variety of absorbents having sorptive capacity are known today in the art of absorption. In the present context, the term “sorptive capacity” includes both the rate by which an absorbent takes up liquids, such as water or aqueous solutions, including body fluids such as urine, blood and menstrual fluids, and the liquid-retaining capacity of the absorbent. The sorptive mechanism may be adsorption or absorption, or a combination thereof. Absorbents may comprise e.g. fluff, impregnating paper, or tissue paper.

[0003] As stated in Pulp and Paper Manufacture Vol. 2 pages 280-281, Joint Textbook Committee of the Paper Industry, 1987, the term “fluff” applies to fibres which have been separated by mechanical means from dry pulps for application in dry-formed nonwoven webs or pads in the household or sanitary fields. As is evident from this reference, both the rate of liquid uptake and the liquid-holding capacity of a fluff-containing absorbent are important when used in these fields. This is particularly valid with regard to absorbent articles such as a catamenial products (e. g. sanitary napkins, pantiliners, tampons etc.), diapers, bandages, adult incontinence garments, and the like. Good liquid uptake and liquid-holding capacity are important for the function of such articles, constantly subjected to pressure imposed by the weight and the movements of the bearer. Thus, it is important that the liquid-holding capacity is high enough to retain the absorbed liquid also under pressure. Furthermore, in order to give good comfort to the bearer, the article should provide a feeling of dryness, meaning that any rewetting from the article to the skin of the bearer should be avoided, raising the requirements with regard to liquid-holding capacity even higher. The same prerequisites also apply to other types of absorbents, such as tissue paper, or impregnating paper.

[0004] EP 889 993 relates to a method for production of fluff-containing absorbents, wherein cellulosic fibres are treated in presence of water with hydrophobic substances such as zeolites and activated carbon.

[0005] However, problems have been associated with such hydrophobic substances, e.g. the lack of aesthetic appearance of active carbon-containing consumer's goods, such as a catamenial product, being black as a consequence of the active carbon being present. Drawbacks have also been associated in pulp treatment with zeolites, as to the costs of such products, and the relatively low affinity they have to the cellulosic fibres making up the formed sorptive paper. Furthermore, zeolites may give rise to abrasive wear of the paper machine and other equipment used when treating the paper pulp, resulting from the relatively hard surface of zeolites.

[0006] It has been desirable in this art to develop further compositions capable of imparting improved sorptive capacity to paper.

[0007] The main object of the present invention is to provide a paper having improved sorptive capacity as well as the production and use thereof in absorbents. A further object of the present invention is to provide a method for producing paper having improved sorptive capacity without the drawbacks referred to above.

THE INVENTION

[0008] The present invention relates to a method for producing paper comprising adding an oil and a carrier having a porous and hydrophobic surface carrying said oil to an aqueous suspension of cellulosic fibres.

[0009] It has been surprisingly found that the inventive method can dramatically increase the sorptive capacity of the paper.

[0010] The carrier preferably is a macroporous carrier, preferably a macroporous polymer. Any carrier suitable for carrying an oil, especially a carrier having a fairly high affinity to the oil may be used. Such carriers include both organic and inorganic porous material which are wetted more readily by an oil than by the aqueous pulp dispersion surrounding the carrier.

[0011] Preferably, the carrier is a thermoplastic, and more preferably a dimensionally stable macroporous polymer. The average diameter of the pores of the carrier suitably is in the range from about 0.1 to about 50 &mgr;m, preferably from about 0.2 to about 15 &mgr;m.

[0012] For reasons of simplicity, a carrier in the form of a preferred polymer will now be described herein more in detail. However, the preferred properties of the polymer applies to any suitable carrier in accordance with the present invention.

[0013] Generally, favourable results are attained when using a of which at least 15 volume % of the polymer pores are filled with the oil substantially immobilised therein, with optimum results being obtained using a material of which at least 50 volume % and not more than 95 volume % of the pores is filled with the oil. Completely filled up pores may cause problems associated to expansion of the oil, which may cause oil leakage from the pores. If the polymer is totally filled up with oil, such difficulties can be easily overcome by mixing the filled polymer with unfilled (porous) polymer, so that the excess of oil can be taken up by unfilled polymer material. The porous polymer is suitably used in the form of grains having an average particle diameter from about 0.01 to about 5 mm, preferably from about 0.1 to about 1 mm. Alternatively, the polymer, preferably being of organic origin, may be employed in granulated form, as well as in the form of membranes, or fibres which may be hollow or not. The specific surface area of the macroporous polymer may be varied depending on the oil used. However, as long as the oil remains in the pores of the polymer, the polymer may have any specific surface area. A skilled person can arrive at such appropriate surface area by routine experimentation. However, the specific surface area suitably ranges from about 10 to about 200 m2/g, preferably from about 10 to about 40 m2/g, and most preferably from about 20 to about 30 m2/g.

[0014] Examples of suitable polymers include polymethyl(meth)acrylate, styreneacrylonitrile copolymer, and acrylonitrile-butadiene styrene copolymer, all of which may be partially cross-linked or not. Further examples include low pressure polyethylene, high pressure polyethylene, polypropylene, polystyrene, acrylonitril-butadiene-styrene terpolymers, styreneacrylonitrile copolymers, styrene-butadiene copolymers, poly(4-methyl-pentene-1), and polybutene, or mixtures thereof.

[0015] The carrier suitably is selected from thermoplastic polymers such as polyolefins or vinyl polymers, preferably from polyethylene, polypropylene or mixtures thereof.

[0016] Suitably, the procedure used to prepare the oil-containing carriers is as follows: firstly, 5-90 wt %, preferably 10-40 wt %, of a polymer is dissolved by heating the polymer to a temperature above its upper critical phase separation temperature Tc in a 10-95 wt % mixture of one or more miscible oils, e.g. A and B, the mixing ratio of A to B being so selected so as to phase separate the polymer by cooling, resulting in a polymer-rich and a polymer-poor (oily) phase. On further cooling, the formed structure is fixed before the phase separation ends, due to vitrification or crystallisation of the polymer, eventually resulting in a porous polymer material filled with the mixture of compounds A and B which may be used, either as such or after further addition of oil. The size of the formed polymers may thereafter be reduced, e.g. by grinding. Further details concerning the production of the production procedure are disclosed in e.g. EP 653 950 and EP 662 344, the disclosures of which are hereby incorporated by reference.

[0017] By the term “oil” is generally meant a liquid substantially insoluble in water, but substantially soluble in organic solvents, such oil being obtainable from plants, animals, mineral deposits, as well as by synthesis. The oil suitably is a synthetic or a natural oil, or mixtures thereof, in the form of a glycerol ester of one or more, preferably unsaturated, fatty acids. The oil suitably is selected from palmitic oil, olive oil, peanut oil, paraffinic oil, fish oil, triisooctylmellitate oil, herring oil, linseed oil, soybean oil, hydrogenated castor oil, strophanthus seed oils, calendula officinalis seed oils, hydrogenated strophanthus seed oils, hydrogenated calendula officinalis seed oils, cardamine impatiens seed oils, ricinoleic fatty acid oil, kamala oils, mallotus discolor oils, and mallotus claoxyloides oils and/or castor oil or mixtures thereof, preferably from triisooctylmellitate oil, soybean oil, castor oil, or mixtures thereof.

[0018] According to one preferred embodiment of the invention, the carrier is a polypropylene carrying an oil mixture of triisooctylmellitate oil, soybean and castor oils.

[0019] By the term “paper” is meant herein particularly all types of paper being used when absorption is required. Examples of such paper include e.g. fluff, impregnating paper, or tissue paper, or other paper having high absorption speed. It is to be noted in this context that an absorbent may solely comprise paper. Thus, a paper as such may also constitute an absorbent.

[0020] By the term “aqueous suspension” is meant herein any liquid or solid particles, e.g. cellulosic fibres, in contact with water or dispersed therein.

[0021] According to a preferred embodiment of the invention, the cellulosic fibres are treated with the carrier and the oil carried by the carrier in an aqueous suspension prior to being further treated, e.g. before defibration of the fibres when producing fluff. The carrier and the oil may be added before and after the cellulosic fibres in the aqueous suspension have been formed to a sheet.

[0022] In this context, the term “sheet” signifies a sheet or a web. The formation of the sheet below referred to as “pulp sheet” may follow either the wet method analogous to conventional paper making, or the flash drying method. Both methods are described in Pulp and Paper Manufacture, Vol. 1 page 753-757, Joint Executive Committee of Vocational Education Committees of the Paper Industry, 1969.

[0023] The carrier and the oil are suitably added to the aqueous suspension to treat the cellulosic fibres by contacting.

[0024] The amount added of carrier and oil suitably is from about 0.1 to about 10 kg/metric tonne of cellulosic fibres, preferably from about 0.5 to about 5 kg/tonne. The treatment may be carried out at any stage in the paper producing process, which process of course comprises further conventional stages before and after the method described herein. The oil and carrier, suitably being applied in the form of an aqueous dispersion, suitably is added to the stock, i.e. the aqueous suspension comprising the cellulosic fibres, before forming pulp sheets, suitably at a dry cellulose fibre content ranging from about 0.1 to about 7 wt %, more preferably from about 0.2 to about 5 wt %, and most preferably from about 0.3 to about 4 wt %. The oil and carrier may also be added to the formed pulp sheet, suitably by spraying a pulp sheet suitably having a dry fibre content from about 5 to about 50 wt %, preferably from about 25 to about 45 wt %. Preferably, the oil and carrier are contacted with the cellulosic fibres for at least one minute, more preferably at least 30 seconds, and most preferably at least about 5 seconds before drying.

[0025] The cellulosic fibres making up the pulp sheet are usually obtained by disintegrating wood, conventionally in the form of chips, fibres or bundles of fibres. In the present context, the concept of “bundles of fibres” is regarded to be equivalent to the concept of “fibres”. The separated fibres may be obtained by means of any pulp-making method known to a skilled person, e. g. by a method for production of mechanical pulp (MP), stone groundwood pulp (SGW), pressure groundwood pulp (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemi-mechanical pulp (CMP), or chemi-thermomechanical pulp (CTMP), although the preferred pulps are chemical pulps as, for instance, sulphate and sulphite pulps. However, the cellulosic fibres may also advantageously be cotton fibres. Another plausible source of fibres is recycled fibres from wastepaper.

[0026] The present invention also relates to paper obtainable by the method as above described. The paper produced suitably is fluff, impregnating paper or tissue paper or other absorbing paper, preferably fluff or tissue paper.

[0027] The invention also relates to paper, preferably fluff or tissue paper, comprising cellulosic fibres, an oil and a carrier having a porous and hydrophobic surface carrying said oil. The oil and carrier suitably have characteristics as further described herein. Preferably, the oil is selected from triisooctylmellitate oil, soybean oil, castor oil or mixtures thereof. The oil-carrying carrier is suitably homogeneously distributed in the paper.

[0028] The invention also relates to a method for producing an absorbent comprising adding to an aqueous suspension of cellulosic fibres a carrier having a hydrophobic and porous surface and an oil carried by the carrier. Further characteristics are as described herein.

[0029] The invention also relates to an absorbent comprising paper obtainable from the method as described above, said absorbent including e.g. sanitary napkins, pantiliners, tampons, diaper bandages, adult incontinence garments and the like. Preferably, the absorbent comprises fluff or tissue paper.

[0030] The invention further relates to the use of a carrier and an oil carried by the carrier, in which the carrier has a porous and hydrophobic surface, for treatment of cellulosic fibres to improve the sorptive capacity of an absorbent comprising cellulosic fibres.

[0031] The invention further relates to a process for treating cellulosic fibres with a carrier having a porous and hydrophobic surface and an oil carried by the carrier. The treatment is intended to improve the sorptive capacity of an absorbent formed from the treated cellulosic fibres, preferably fluff or tissue paper or an absorbent comprising said fluff or tissue paper. Further characteristics of the carrier and the oil are as described hereabove.

[0032] The present invention is illustrated in more detail below by means of an example. Unless otherwise stated the parts and percentages below are given by weight. In the example below MPP-B available from Akzo Nobel N. V., Arnhem, The Netherlands, was used. MPP-B is a macroporous polymer containing an absorbed oil in the pores thereof, as further described in EP 0 653 950 B1. The particle size was 200-350 &mgr;m and the extractive content 1 g oil/g of polymer.

[0033] In the example below, produced fluff was tested with respect to rewetting and wetting rate. The test method for the uptake rate was SCAN-C 33:80, in which the fluff samples of 3 g, having a diameter of 50 mm, were positioned vertically and loaded with a weight of 500 g on top. Each sample was allowed to absorb water from below, and the time taken until water penetrated through the upper surface of the sample was determined by means of an electronic detector. The shorter the time required to penetrate the upper surface, the higher the uptake rate of the fluff. In the test method for determination of rewetting or liquid holding capacity, a fluff sample of 3 g having a diameter of 50 mm was positioned vertically and loaded with a weight of 1 kg on top for 30 s, and was then unloaded. 10 ml of water was applied to the sample under a time period of 10 s and the liquid was allowed to drain the sample for 30 s, after which the sample was loaded with a weight of 1 kg for 4 minutes. 15 sheets, 8×8 cm, of filter paper were placed on top of the sample and sheets are loaded with a weight of 5 kg for one additional minute, after which the 15 sheets were weighed. The increase of weight of the sheets was due to rewetting. Thus a low increment indicates low rewetting, and consequently, a high liquid retaining capacity, which is favourable.

Example 1

[0034] 10 g of bleached sulphate pulp was defiberised in 500 ml of water together with the MPP-B product in addition rates from 1 to 4 kgltonne of dry pulp for 10 minutes. The composition of MPP-B sample contained 50 wt % polypropylene, and 50 wt % triisooctylmellitate oil (Emkarate 7930 ex ICI). Polymer beads thereof were produced in an extrusion process using soybean oil and castor oil as cell forming agents. The porosity of the polymer was approximately 75% and the polymer had a cellular pore structure with an average pore diameter of 15±5 &mgr;m (as determined from SEM pictures). The polymer material was ground and subsequently sieved in order to obtain a particle size fraction in the range from 200-350 &mgr;m. The thus produced particles were extracted with acetone and subsequently filled with triisooctylmellitate oil. A reference sample without MPP-B was also made. The thus obtained stock was dewatered through the screen cloth of a wire mould, producing pulp sheets having diameters of 210 mm. The sheets were dried at 60° C. for 120 minutes and then dry shredded into fluff in a hammer mill. The fluff was formed into three samples each sample weighing 3 g. The fluff samples thus obtained were tested at ambient conditions of about 23° C. and 50% RH (relative humidity), with regard to rewetting as is set forth in table 1 below. 1 TABLE 1 Addition rate (kg/tonne Rewetting Examples Sample dry pulp (gramme) 1 Reference (without 0 1.98 carrier and oil) 2 MPP-B 1 0.59 3 MPP-B 2 0.52 4 MPP-B 3 0.60 5 MPP-B 4 0.52

[0035] As can be seen, fluff comprising MPP-B imparted a considerably increased liquid retaining capacity compared to the reference, i.e. fluff not treated with MPP-B.

Claims

1. Method for producing paper comprising adding to an aqueous suspension of cellulosic fibers a carrier having a porous and hydrophobic surface, wherein said carrier carries an oil.

2. Method according to claim 1, wherein the carrier is a macroporous polymer.

3. Method according to claim 2, wherein the macroporous polymer has an average pore diameter in the range from about 0.1 to about 50 &mgr;m.

4. Method according to claim 2, wherein the macroporous polymer is a thermoplastic polymer.

5. Method according to claim 1, wherein the oil is triisooctylmellitate oil, soybean oil, or castor oil.

6. Method for producing paper comprising adding to an aqueous suspension of cellulosic fibers a carrier carrying an oil, said carrier having a hydrophobic surface and pores having an average pore diameter in the range from about 0.1 to about 50 &mgr;m.

7. Method according to claim 6, wherein said carrier carrying said oil is added to the aqueous suspension of cellulosic fibers in an amount of from about 0.1 to about 10 kg/metric tonne of cellulosic fibers.

8. Paper obtained by a method comprising adding to an aqueous suspension of cellulosic fibers a carrier having a porous and hydrophobic surface, wherein said carrier carries an oil.

9. Paper according to claim 8, wherein the carrier is a macroporous polymer.

10. Paper according to claim 8, wherein the carrier has pores having an average pore diameter in the range from about 0.1 to about 50 &mgr;m.

11. Paper comprising a carrier having a porous and a hydrophobic surface, wherein said carrier carries an oil.

12. Paper according to claim 11, wherein the carrier is a macroporous polymer.

13. Paper according to claim 11, wherein the carrier has pores having an average pore diameter in the range from about 0.1 to about 50 &mgr;m.

14. Paper according to claim 13, wherein at least 15 volume % of the pores are filled with oil.

15. Paper according to claim 11, wherein the carrier has a specific surface area from about 10 to about 40 m2/g.

16. Paper according to claim 11, wherein the oil is triisooctylmellitate oil, soybean oil, or castor oil.

17. Process for treating cellulosic fibers comprising treating said cellulosic fibers in an aqueous suspension with a carrier having a porous and hydrophobic surface, wherein said carrier carries an oil.