CA1109709A

CA1109709A - Absorbent papers

Info

Publication number: CA1109709A
Application number: CA334,454A
Authority: CA
Inventors: Richard G.C. Henbest
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1978-09-01
Filing date: 1979-08-23
Publication date: 1981-09-29
Also published as: ATE2233T1; EP0009322A1; JPS5536397A; EP0009322B1; DE2964483D1; AU5017379A

Abstract

ABSTRACT OF THE DISCLOSURE

"ABSORBENT PAPERS"

An absorbent paper is formed from 5 - 95% by weight of insoluble amino-formaldehyde resin fibres of 1 - 10 mm length and 1 - 30 µm diameter and correspond-ingly 95 - 5% cellulose pulp. Where the Canadian Standard Freeness of the cellulose pulp is below 310 x + 140 ml.
(where x is the proportion of lignin free pulp in the cellulose pulp) the proportion of amino-formaldehyde fibres is such that the blend has a freeness above 220 x + 400 ml.
The paper may be made by forming an aqueous slurry of the fibrous constituents into sheet form fol-lowed by dewatering.

Description

~)97~9 Abaorbent ~a~ers and a process for their ~roduction This invention relates to absorbent papers: such papers find a variety of uses, such aa facial or other hygienic tissues, towelling and blotting paper. Depending on the desired and use, other factors besides absorbency, for example bulk, softne6s, and strength may also be of importance.
Paper is generally made by a wet-laid process wherein a slur y of fibres in water is formed into sheet form, e.g. by depos-ition of the slurry on to a moving porous surface, e.g. the wire of a Fourdrinier paper making machine, and removing the water, initial-ly by drainage through the porous surface and subsequently by pas-sage of the drained fibrous web through a suitable drier. Generally, to make satisfactory papers, the average length of the fibres should be below lO mm. The fibres used to make the paper are usually cel-lulosic: cellulosic pulps are of two main types viz lignin free,e.g. chemical pulp wherein the raw cellulose i8 converted into a pulp suitable for paper m~king by chemical means such as the well known sulphite or sulphate processes during which the lignin in the wood is extracted, and lignin containing, e.g. mechanical pulp where-in the raw cellulose e.g. wood is ground to the requisite fibre sizewithout lignin removal. Semichemical and thermomechanical pulps are produced by proces~es in which little or none of the lignin is removed and ~o are herein clas~ed with mechanical pulp. Papers made from lignin free, e.g. chemical, pulps have markedly different prop-erties from tho~e made from lignin containingj e.g. mechanical pulps.In some oases the paper may be made from a blend of lignin free and .
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~ 11097~9 lignin containing pulps in order to obtain a desired balance of properties.
One parameter that has a significant effect on the paper properties is the degree of beating or refining of the pulp: in general the greater the degree of beating the stronger and less bulky is the resultant paper. The degree of beating is conveniently assessed by measuring the freeness of the pulp. In this specification freeness refers to the freeness measured by the Canadian Standard Freeness test procedure:
the higher the freeness, the less well beaten is the pulp.
The absorbency of the paper is also affected by the freeness:
generally the higher the freeness, i.e. the less well beaten, the greater the absorbency.
We have found that absorbent papers may be obtained by making the paper from certain mixtures of cellulose pulp and fibres formed from an amino-formaldehyde resin such as a urea-formaldehyde (UF) resin.
Paper formed from mixtures of UF resin fibres and cellulose pulps have been described in our copending Cana-dian patent application No.321608. In that application the cellulose pulp had a Canadian Standard Freeness of less than 400 ml (chemical pulp) or 120 ml (mechanical pulp).
We have now found that absorbent papers can be made using amino-formaldehyde resin fibres mixed with cel-lulose pulps having somewhat higher freenesses. Also, where the proportion of amino-formaldehyde resin fibres is high, absorbent papers can be made with cellulosic pulps having lower freenesses.
By the term absorbent paper we mean that the paper has a water-absorption capacity exceeding 3: this may be 11~)97~9 i - - 2A -determined by saturating a weighed quantity of air dry paper with water, lightly shaking to remove excess water, followed by reweighing. The absorption capacity is calculated at the weight of water absorbed per unit weight of the air dry paper.
In general absorbent papers according to the invention have a greater water-absorption capacity than the most absorbent paper that can be made, under the same paper-making conditions, from the cellulose pulp employed. For example, an -- ~

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~1~97~9 unbeaten or very lightly beaten chemical wood pulp may have a free-ness of about 600 ml and paper made therefrom may have a water absorption capacity of about 3. ~y the addition of 2~/o ~F fibres to the same wood pulp, the water absorption capacity may be increased to about 3.4. Alternatively stronger papers may be made: thus by beating the cellulose pulp to a freeness of about 450 ml prior to blending with the 20% of ~F fibres, a paper can be made that is stronger than the most absorptive pure cellulose pulp paper achiev-able from this cellulose pulp and yet has similar ab~orption characteri~tics.
In order to obtain absorbent papers from cellulose pulp9 that have been fairly well beaten, it i8 necessary to incorporate such an amount of ~mlno-formaldehyde resin fibres that the result-ant blend has a high freeness. The relationship between freeness of a mixture of lignin containing and lignin free, e.g. a mixture of mechanical and chemical, pulps and the freenesses of the individ-ual pulps is, for the purposes of the present invention, sufficiently linear to be quoted as F = fc x + fm (l - x) where F i8 the freeness of the mixture of pulps fc is the freeness of the lignin free pulp fm is the freeness of the lignin containing pulp x is the weight proportion of the lignin free pulp in the mixture.
On the other hand the relationship between the freeness of a blend of amino-formaldehyde resin fibres and a cellulose pulp and the individual freeness of the amino-formaldehyde resin fibres and the cellulose pulp is not linear. However, as a guide, if a lignin free, e.g. chemical, pulp of freeness 400 ml i8 mixed with an equal weight of ~F fibres the resultant blend will have a freeness of about 600 ml. Likewise a lignin containing, e.g. mechanical, pulp of freene~s 120 ml mixed with an equal weight of ~F fibres gives a blend of freene~s about 380 ml.
According to the present invention we provide an absorbent paper product formed from a blend of fibrous constituents comprising 1097~S

5 to 95% by weight of amino-formaldehyde resin fibre~ which are in-soluble in cold water and have an average length between 1 and 10 mm and a mean diameter between 1 and 30 ym, and, correspondingly, 95 to 5% by weight of cellulose pulp, provided that, where the Canadian Standard Freeness of the cellulo3e pulp is below 310 x + 140 ml (where x is the proportion by weight of lignin free pulp in said cellulose pulp~ the proportion of amino-formaldehyde resin fibres in the blend is such that the Canadian Standard Freeness of the blend is above 220 x + 400 ml.
Thus, considering the case where the cullulose pulp is wholly a lignin free, e.g. chemical, pulp, i.e. x = 1, the pulp should have a freeness above 450 ml. Where, however, the pulp free-ness ia below 450 ml., absorbent papers can be made with blends con-taining sufficient Pm;no-formaldehyde resin fibres to give a blend of freeness above 620 ml. ~ikewise where the cellulose pulp is wholly a lignin containing, e.g. mechanical, pulp, i.e. x = o, the pulp freenes~ should be above 140 ml., but where it is below this figure, absorbent papers can be made with blends containing suf-ficient amino-formaldehyde resin fibres to give a blend of freeness above 400 ml.
The amino-formaldehyde resin used to make the fibres is a condensate of an amino compound, preferably a polyamine such as urea or melamine, with formaldehyde. The amino compound is prefer-ably urea, alone or in admixture with up to 5% by weight of melamine.
The molar ratio of formaldehyde to amino groups is preferably between 0.6:1 and 1.5:1, particularly between 0.7:1 and 1.3:1.
The amino-formaldehyde resin fibres may be made by any suitable fibre forming technique such as wet or d~y spinning and are preferably formed by a centrifugal spinning process, for example as described in our German O~S Specification 2810535, which give~, as is preferred, substantially straight and unbranched fibres.
The amino-formaldehyde resin fibres should have an average length, weighted by length, of between 1 and 10 mm, preferably be-twe~n 2 and 6 mm. Preferably substantially all the amino-formpldehyde resin fibres have a length within the range 1 to 10 mm. me ~m;no-.

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~1~97~9 formaldehyde resin fibres should have a mean diameter between 1 and 30 ~m, preferably between 2 and 20 ~m, and particularly between 5 and 15 ~m. Preferably substantially all the amino-formaldehyde resin fibres have a diameter between 1 and 30 ~m.
The ~m;no-formaldehyde re3in fibres preferably have an average strength of at least 50 M~m 2 (which correspond~ approxim-ately to 33 ~mg ), particularly at least 100 M~m ( 67 Nmg ).
~he amino-fo~maldehyde resin fibres should be cured to such an extent that they are insoluble in water: thus their solub-ility in water at 25C should not exceed 1.5% by weight.
Cellulose fibres that may be used include mechanical wood pulp, chemical wood pulp, such as is produced by the sulphate or sulphite pulping processes, thermomechanical and semichemical wood pulps. Alternatively the cellulose pulp may be cotton linters, linen fibres derived from rags, or other cellulose fibres conven-tionally used in paper making. Depending on their source and method of production they may or may not contain substantial quantities of lignin. Thus cotton linters are substantially free of lignin.
The absorbent papers may be made by the conventional wet laid process, e.g. as hereinbefore described, after preparation and blending of the fibrous in OEedients into a paper-making fibrous stock. The absorption capacity and bulk of the paper can be in-creased if the conventional step of pressing the wet paper sheet prior to drying is omitted.
Papers containing a high proportion of amino-formaldehyde resin fibres tend to have relatively poor strengths. The strength of such papers may be increased by incorporating a binder into the paper: the binder can be added to the aqueous slurry of the fibrous constituents or can be incorporated in a sub~equent impregnation or coating stage. Thus the binder may be 3prayed on to the wet web or it may be coated on to the dry or partly dried paper. Different methods are appropriate to different binder systems, as is well known to those ~killed in the paper making art.
Binders may also advantageously be in¢orporated in papers ~ ~L097~9 containing lesser proportions of amino-formaldehyde resin fibres.
Examples of suitable binder~ include starch or modified starch, polymer latices, water soluble polymers such as poly(ethylene im;ne), poly(acrylamide), and poly(vinyl pyrrolidone). The binder is preferably treated to render it cationic in water. Particularly favoured are cationic binders added with the fibres-including, in addition to those binders already mentioned, cationic starch and urea - or melamine~formaldehyde resins, a3 conventionally used to achieve increase~ in paper wet strength. Typically the amount of binder employed is from 0.01 to loYo~ preferably 0.1 to 5%, by weight of the fibrous ingredient3.
The paper products of the present invention contain 5 - 95%
by weight of amino-formaldehyde resin fibres and corre~pondingly 95 - 5% by weight of cellulose fibres. The properties of the paper will vary congiderably as the proportions of the respective fibres are varied: thus as the proportion of amino-formaldehyde resin fibres inoreases, absorbency and bulk inoreases. Generally as the proportion of amino-formaldehyde resin fibres increases 80 the freeness of the cellulose pulp should be reduced in order to obtain adequate strength, although it will be appreciated that for some applications strength is not important, for example in highly absorb-ent products where an absorbent paper layer is attached, e.g. by an adhesive or stitching, to a supporting web.
~he paper products preferably contain at least 15%, and preferably le~s than 80%, by weight of Pm;no_formaldehyde resin fibres.
The paper products may be creped, by conventional means, as a way of increasing their bulk and also, incidentally, to improve their stretch. Creping may not be necessary with papers comprising predominantly amino-formaldehyde resin fibres but in the latter case it may be desirable to incorporate one or more of the aforementioned binders. Where the amount of amino-formaldehyde resin fibres is relatively low, i.e. below about 35% by weight, the paper will be soft and bulky and may require creping on conventional equipment to produce a paper acceptable for absorbent applications. ~owever, ~ ' .

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~ ` 11097~9 because of the higher bulk conferred by the amino-formaldehyde resin fibres, the 3everity of t~e creping process may be reduced as compared with conventional papers.
At intermediate levels (i.e. 35 - 75% by weight) of ~m;no formaldehyde resin fibres, the papers may be acceptable for some absorbent applications, but for others may require to be creped.
At high percentages of amino-formaldehyde resin fibres, i.e. above 75% by weight, a very bulky absorbent paper is produced which will ne~d little, if any, creping. It may be desirable though, and particularly with papers containing more than 90% by weight of amino-formaldehyde resin fibres to incorporate a binder as described above.
The advantages of the papers of the pre~ent invention, which are conferred by the presence of the amino-formaldehyde resin fibres are improved absorbency, softness, porosity and bulk, often with little or no sacrifice in strength, thus enabling a combination of useful characteristics to be obtained that cannot be achieved with wholly oellulose pulps.
Furthermore, the rapid draining characteristics of amino-formaldehyde resin fibre~ also confer advantages in processing. It is possible, for example, to increase the dilution of the stock, while maintaining machine speed, and 80 improve the even formation of the sheet. ~his may be of particular importance with light weight tissue products.
The invention is illustrated by the following examples in which all percentages are by weight. In the examples the amino-formaldehyde resin fibres employed were urea-formaldehyde (~F) fibres made by centrifugal spinning. An aqueous solution of a urea-formalde-hyde resin having a formaldehyde:urea molar ratio of 2:1, a solids content of 65%, and a viscosity of 45 poise, was mixed continuously with lC% of a solution containing 1.66% poly(eth~lene oxide) of weight average molecular weight 600,000 and 6.66% ammonium sulphate.
~he resulting mixture was fed at a rate of 200 g/min. to a 13 cm diameter spinning cup rotating at 10,000 rpm. The resin was spun as fibres from the cup into an atmosphere of air at 150C, and then the 97~9 fibrea were removed therefrom and cured by heating for 3 hour~ at 120C to render them inaoluble in cold water. ~he resultant fibreæ
were 3hredded and further disintegrated in a laboratory valley beater.
The shredded, beaten, fibres had an average fibre length of about 3 mm and an average diameter about 10 ~m. Substantially all the fibres had a length within the range 1 - 10 mm and a diameter with-in the range 3 - 20 ~m~ The strength of the fibres, measured by short span testing of a loose mat of the fibre~, was approximately 120 M~m~2.
In the examples the ~F fibre~ were blended, in specified quantities expressed on a dry fibre basis, with cellulose pulps that had been beaten in a valley beater to specified freenesses. ~he resultant blends were made into paper hand~heets of substance about 60g m 2 using a ~ritish Standard handsheet former. Except where indicated, the papers were wet pressed.
~he papers were tested in the following manner:
Absorption capacit~: a small quantity of the p~per was weighed, with water, lightly shaken to remove excess water, and rewei~hed. ~he absorption capacity was calculated as the weight of water absorbed per gram of air dry paper.
Absorption time: according to the TAPPI Bibulous paper test, T432.
~he time taken to absorb 1 ml of water was measured.
N emm absorbency test: this test was modified to conform with the sizes of the paper samples made and with the fact that highly absorbent materials were being tested. A strip of paper, 150 mm x 15 mm was held vertically with 10 mm of the sample immersed in water. The height of the water rise up this strip after 5 minutes was recorded. (~ormally a time of 10 minutes is recommended for the N emm test).
Oil absorDtion: according to the Patra test using oil S-600.
urst Index: burst pressure is measured by the ~APPI ~403 method and the result in kNm 2 is divided by the substance in gm 2 to give the burst index.

In this example a commercially available birch sulphate, ' )97~9 i.e. a chemical wood, pulp wa~ used. Tn order to assess the maximum absorbence obtainable with this pulp, the pulp was dispersed in water in a laboratory disintegrator without any prior beating or refiningO
~ecause in the absorption capacity test the paper sample tended to 5 fall apart on total immersion in water and ~o rendered it impractical to obtain a meaningful value, paper was also made from the same pulp after it had been lightly beaten.
Papers were also made from blends of the cellulose pulp beaten to variou~ degrees with various amounts of the ~F fibres. The results are shown in the following table.

_ _ IFreene~s (ml) - }~ret Absorb. Klemm Run % ~F Bulk Index Ab~orb. Time ri~e r-~ P"~P ~lend ~ -1 Capacity (~e~) ( = )-1.1* 0 + + _ o.44 _ 146 7o 1.2* 0 604 604 _ 0.652.98 145 56 1.3* 20 284 384 2.06 1.782.70 376 4o 1.4 20 452 510 2.09 1.113.03 177 68 1.5 20 604 612 2.22 o.3o3.38 60 go 1.6 5o 452 621 2.69 o.333.42 54 101 1.7 5o 604 690 2.78 0.113.32 25 126 1.8 75 284 691 3.20 0.114.01 29 106 1.9 75 452 692 3.37 0.014.23 21 118 1.10 75 604 ~ ~.49 _5.20 17 112 * comparative + un~eaten pulp ~uns 1.1 and 1.2 demonstrate that the maximum absorbency obtainable 30 with the birch sulphate pulp alone i9 ab~orption capacity ~u3, absorption tlme n~145 - 146 ~ec., and Klemm riBe n~56 - 70 mm.
This example shows that while a paper with 2~% ~F (run 1.4) has similar ab~orbency properties to the maxi = achievable with the birch sulphate pulp alone, the burst index is much improved. It also 35 demonstrate~ that absorbent paper~ may be made using low freeness 11~)97~9 plllp8, if s~fficient ~F fibres are added - compare runs 1.3 and 1.8.
EXA~Ll~: 2 ~ o demonstrate the effect of a binder, starch wa~ added to the fibrous mixture from which the papers were made. ~he cellulose pulp was a birch sulphate pulp beaten to a freeness of 484 ml.

_--X ~taIoh Freeness of--~=r~t Elemm on fibre ~lend (incl ~ulk Index Absorb. Rise Run % ~F weight starch) ml cm3g-1 K~g-l capacity (mm) .

2.1* o o 484 1.46 2. 30 2.95 41 2.2* o 3 411 1.42 4-30 2.69 38 2.3 5o 3 616 2.90 0.62 3.87 138 2 4 lo 696 ~.46 0 12 5.19 ~140 * comparative ' ~y comparison with starch free systems of similar blend freeness and OF content in Example 1, e.g. compare run 2.3 with r~n 1.6 and run 2.4 with r m 1.9, it is seen that adding starch improves both the 20 absorbency and burst indes.
EU~LE ~
The procedure of Example 1 was repeated using a commercially available unbleached mech~n1cal wood pulp in place of the birch sulphate pulp. In one case, run 3.7, the paper was made omitting the 25 wet pressing atep. The results are shown in the following ~able.
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_ _ Freene~s (ml) - Bur~t Aboor~ Klemm Run % ~F Bulk Index Absorb. Time -Ri~e 5 ~ I ~ 91e~d o=3/g ~ Capaoity (oeo) (~m)

3.1* 0 437 437 _ _ 3.32 205 45 3.2* 20 69 1102.59 0.84 3~01 758 32 3 3 20 437 5032.90 0.27 3.84 195 60 3.4 50 145 4233.24 0.20 3.82 75 69 3.5 5o 437 5483.46 0.11 3.94 138 59 3.6 75 69 5443.54 o.io 4.2 21 108 3.7+ 75 69 5444.8 0.08 5.49 13 122 3.8 75 87 574.51 0.11 4.47 34 100 ~.9 75 ~ 613~46 1 4.61 49 85 * comparative + not wet pressed.
Again it is seen that if sufficient ~F fibres are incorpor-ated, ab~orbent papers can be made with low freeness cellulose pulps.
EXAMPLæ 4 The procedure of F-P~rle 1 was repeated using as the cellulose pulp a mixture of 70% bleached pine sulphate and 30~
bleached birch ~ulphate. In some cases ~, on fibre wei~ht, of starch was added and, in all these runs, the wet pressing step was 25 omitted. The water absorption time quoted is the time taken to absorb 0.1 ml of water rather than 1 ml a8 in the previous examples.
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Freeness (ml) Burst Oil Water Klemm Run % ~F ~hlk Index Absorb. Absorb. Rise l l IPI1P I ~1 ~

4.1* 587 587 3.25 1.83 o.4 5.8 99 4.2* O 638 638 3.50 1.21 0.6 6.5 94 ~.3* 0 650 650 3.62 0.9? 0.8 4.6 108 4-4 20 587 675 3.53 1.03 0.2 5.5 110 4.5 20 638 700 3.48 o.76 <0.1 6.1 103 4.6+ 20 640 705 3.72 1.60 0.2 8.6 90 4.7 20 650 675 2.68 o.55 o.3 4.4 106 4.8 4o 587 712 3.95 o.45 <0.1 3.9 109 4.a 4o 638 725 3.88 0.34 ~0.1 4.6 112 4.10+ 40 640 728 3.76 0.85 0.1 5- 3 94 4.11 4o 650 725 3.97 0.29 <0.1 3-5 114 .
* comparative 20 + etarch added EX,AMPLE ~
~ he procedure of Example 1 wae repeated using as the cellulose ~ulp a birch ~ulphate wood pulp of freenees 425 ml. ~he results are ehown in the followiD6 ~able, together with data for a 25 commercial blottin~ paper and a commercial absorbent paper towellin~.
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1~ )97~9 Blend freeness ~ulk ~urst Index Absorb. Klemm ~un % ~F (ml) (cm3g-1) kNg~lt(isme) r'se _

5.1 10 460 1.72 2.26 44 43 5.2 20 500 1.92 1.93 42 46 5.3 30 535 2.13 1.40 21 64 5.4 40 570 2.28 1.00 18 70 10 5.5 5o 605 2.48 0.80 9 86 5.6 60 642 2.81 0.40 9 102 5.7 70 680 3,06 0.23 6 113 5.8 80 715 3.29 0.11 1* 115 5~9 ~ 90 752 4.42 o.o6 3* 82 15 5 ' 10 blotting paper 1. 78 O .73 44 48 5 11 ab~orbent towellins 4.12 o.56 2 * time to absorb 0.1 ml water ~he paper~ of runs 5.1 to 5.9 all had absorption capacities in excess of 3.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An absorbent paper product formed from a blend of fibrous constituents comprising 5 - 95% by weight of amino-formaldehyde resin fibres which are insoluble in cold water and have an average length between 1 and 10 mm and a mean diameter between 1 and 30 µm, and, correspondingly 95 - 5%
weight of cellulose pulp, provided that, where the Canadian Standard Freeness of said cellulose pulp is below 310 x +
140 ml (where x is the proportion by weight of lignin free pulp in said cellulose pulp), the proportion of amino-formaldehyde resin fibres in the blend is such that the Cana-dian Standard Freeness of said blend is above 220 x + 400 ml.

2. An absorbent paper product as claimed in Claim 1 wherein the fibrous constituents comprise 15 to 80%
by weight of the amino-formaldehyde resin fibres and, cor-respondingly, 85 to 20% by weight of cellulose pulp.

3. An absorbent paper product as claimed in Claim 1 containing 0.01 to 10% by weight, based on the weight of the fibrous constituents, of a binder.

4. An absorbent paper product as claimed in Claim 3 wherein the amino-formaldehyde resin fibres form at least 70%
by weight of the fibrous constituents.

5. An absorbent paper product as claimed in Claim 1 that has been creped.

6. An absorbent paper product as claimed in Claim 5 in which the amino-formaldehyde resin fibres form less than 35% by weight of the fibrous constituents.

7. An absorbent paper product as claimed in Claim 1, 2 or 3 wherein the amino-formaldehyde resin fibres are fibres of a resin formed by condensing formaldehyde with urea and 0 to 5% by weight, based on the weight of urea, of melamine.

8. An absorbent paper product as claimed in Claim 1, 2 or 3 wherein the molar ratio of formaldehyde to amino groups in the amino-formaldehyde resin is between 0.6:1 and 1.5:1.

9. An absorbent paper product as claimed in Claim 1, 2 or 3 wherein the amino-formaldehyde resin have an average fibre length in the range of 2 to 6 mm with substantially all the fibres having lengths within the range 1 to 10 mm.

10. An absorbent paper product as claimed in Claim 1, 2 or 3 wherein the amino-formaldehyde resin fibres have a mean diameter within the range 2 to 20 µm with substantially all the fibres having diameters within the range 1 to 30 µm.

11. A process for the production of an absorbent paper product according to Claim 1 comprising forming an aqueous slurry of the fibrous constituents, forming said slurry into sheet form, and removing the water.

12. A process as claimed in Claim 11 wherein the sheet is not pressed prior to drying.

13. A process as claimed in Claim 11 or 12 wherein the sheet is creped after drying.