IE43887B1 - Improvements in or relating to the cross-linking of fibres of cellulosic origin - Google Patents

Description

This invention relates to the cross-linking of fibres of cellulosic material, more particularly wood pulp fibres.

The cross-linking of fibres of cellulosic material, for example wood fibres or cotton linters imparts thereto new characteristics, especially a greatly increased water absorption. The links formed between the anhydroglucose units of the cellulose chains hinder the interfibre bond formation, impart stiffness to the fibres and increase the water absorption of the fibres through oapilarity thereby set up. Cross-linked cellulosic fibres find use in the preparation of flannel, pads and babies' napkins, as well as sheet materials of larger size which are given improved bulk and softness as well L5 as reduced tensile strength. If employed with a resinous binder, the modified fibres are particularly useful in the manufacture of non-woven fabrics which are characterized by their improved softness, bulk, handling and absorbency.

According to the present invention, there is provided a process for cross-linking fibres of cellulosic material, comprising the steps of fluffing paper pulp, spraying on the fibres while the fibres are in separated form, a crosslinking agent solution constituted by an aqueous !5 solution of formaldehyde and hydrochloric acid, sufficient of the aqueous solution being sprayed to provide on the fibres from 1 to 6% of formaldehyde based on the weight Q S-z of the fibres, introducing the thus treated fibres into an air stream sufficiently hot to achieve cross-linking of the fibres introduced thereinto and separating the thus cross-linked fibres from gaseous by-products of the cro s s-1inking.

It is preferred that the wetted fibres be maintained in the hot air stream for only a few seconds, more particularly from 1 to 10 seconds, preferably from 2 to 3 seconds. To ensure that the best quality products are obtained and, in particular, to minimise discolouration Ox «.he iMise, ic is preterred that tne entire process be carried out within 1 minute from start to finish.

In order that the fluffed paper pulp remains in separated form when the formaldehyde/hydrochloric acid solution is sprayed thereon, it is preferred that the fibres are aerated. The hot air stream in which the wetted fibres are introduced to a zone in which crosslinking takes place preferably has a velocity of from 1 to 20 metres per second.

The paper pulp supplied to the fluffer in which fluffing by a dry process takes place preferably contains surface active compounds. The fluffing is preferably carried out to obtain individual fibres of length 1 to 3 millimetres and thickness 8 to 10 microns, the fibres being free from knots. The spraying on of cross-linking reagents is preferably carried out in such manner that as uniform a deposit as possible is formed on the fibres either by condensation of the reagent or simply by deposition of the reagent while wet on the fibres. Supply of the reagent is preferably continued until the solids content of the fibres reaches from 70 to 80% by weight.

The reagent employed comprises formaldehyde and hydrochloric acid. The hydrochloric acid may be replaced by an amount of a salt which will yield hydrochloric acid under the reaction conditions in the required amount.

The reagent preferably additionally contains formic acid or acetic acid. When formic or acetic acid is present, the amount thereof used is preferably less than 50% of the total weight of reagent employed. In fact, formic acid or acetic acid may be omitted, but more hydrochloric acid and more formaldehyde must then be used if crosslinking is to be achieved. Charring of the fibres may then occur. Furthermore, in the absence of formic acid, iormaldehyde is more volatile and the reaction is difficux to control. When use of formic acid is dispensed with, the cross-linked fibres obtained are then less water absorbent. Formic acid may be considered to be a weak L5 catalyst, an anchorage agent for the formaldehyde on the fibres and a swelling agent for the fibres.

The formaldehyde is employed in an amount of from 1 to 6% by weight of the fibres. The hydrochloric acid may be used in quantities ranging up to 2% by weight of !0 the fibres. The quantity of hydrochloric acid used is important. An excess of this material causes a yellowing of the fibres. In practice, the hydrochloric acid is preferably used in an amount of from 0.1 to 0.2% by weight of the fibres. Formic acid or acetic acid when employed is preferably not applied to the fibres in an amount of more than 12% by weight thereof. The reagent employed preferably contains water in an amount of from 2 to 19% by weight of the fibres employed.

For the better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example only, to the accompany ing drawing which is a schematic flow-sheet of a preferred method for producing cross-linked cellulosic fibres - 5 4 2 8 8 7 according to this invention.

Referring to the drawing, cellulosic fibres are fed from a paper pulp supply 1 to a fluffer 2 in which the fibres are separated. The fluffed paper pulp fibres may be heated although they may undergo heating sufficiently when undergoing separation in the fluffer. The drying of the fibres caused by the heating is completed when the fibres are fed to a so-called cyclone-type apparatus 3 that is a static vessel into which substances are introduced in such manner as to form a vortex (Cyclone is a Registered Trade Mark), in which appaicuus they arc dried and aerated in order to ensure that the fibres are maintained separate from each other as they are then passed to a spraying unit 4 and a tubular reaction vessel 5 to which a hot air stream 6 is also fed. In the spraying unit 4, a reagent solution as aforesaid is sprayed on to the fibres. Because the fibres are kept wet the use of a hot air stream 6 having a temperature as high as 200°C results in the temperature of the fibres not reaching more than 50° C so that the fibres are not damaged. The cross-linked fibres now obtained are separated in a cyclone 7 and recovered at 8, while air and effluents to be recycled leave the cyclone 7 at 9. If necessary, the fibres can be subjected to further drying in order to remove all contaminants thereon resulting from the crosslinking process.

The process of this invention is particularly convenient in that it does not require any impregnation or ageing step in order to ensure that sufficient formaldehyde enters the fibres to be able to achieve crosslinking of the cellulose thereof. A particular benefit of the process of this invention is that less than 1 minute is required between the fluffing step and the - 6 recovery of the cross-linked fibres.

The following examples illustrate this invention.

Examples 1-5 The procedure described above with respect to the 5 accompanying drawing was carried out five times using different reagents supplied to the spraying unit. In each case, cross-linking was achieved by flash-drying at 180°C over a period of 2.5 seconds in the reaction vessel 5. The reagents used and results obtained are set out in the following table:- μ (0 10 φ >1 Μ Η Η Λ tf r-l ·Η .μ ‘Η Md id μ υ Μ Ο «pQ {3 ω ·η ·η μ μ φ IW tf β β rd μ Φ φ Φ Λ tP ω ι 1 Λ 4-» 0 tf φ μ Ή φ μ •Η Λ Φ oi Λ £ a Λ i a β ω 5 o C •u 0 it) Ή S +J ιη Η Η <η ι Ο π β H — P +J •Η β 10 O o tn a m ο M □ m +i o β ¢) P tn£ tf tn Φ Ή μ φ -ο ? β $Γ Ό a Φ 0 Λί β rd β (0 a rd φ 0 Φ Η Μ μ ί>1 1 Λ Λ S 10 Φ φ μ 0 co μ Μ μ μ •S. rd 0 μ •rd •Η &> r-1 μ tf X! £ •Η Φ υ Ό a ? rd >1 Μ Φ ιη ιη <Ν rd μ • • • tf 00 CN CN ο Ο £ ιο ιη ιη 00 ιη Η 5 Γ* ιη Γ) co ω I CN CN 1 CN ΊΚ ω co ’d* Γ) CN • ♦ * W Η1 ο ο Μ4 Ο CN σ> Η σ» rd co rd rd □ m μ ο φ ο ti rd A β •Η Τ3 Φ β φ rd & Μ φ Φ > Ή μ tf Μ tf I Η.

CN In the examples, the water absorption was measured as follows. A hand sheet produced from 5 g of fibres was formed on conventional equipment and dried for 2 minutes under a pressure of 3.5 kg/cm . The hand sheet was post-dried for 2 hours in an air-forced oven at 105°C. The degree of cross-linking achieved was determinable bybulk determination. The hand sheet was then placed in a cylindrical basket having a conical bottom. The closed basket was dipped into a vessel containing 1 litre of water and allowed to remain there for 3 minutes. The basket was removed and drained -tor 1 minute. The amount of water remaining in the vessel was then measured. By way of comparison, an uncross-linked sheet produced from paper pulp fibres absorbs between 3 and 5 g of water per gram.

Claims (19)

1. A process for cross-linking fibres of cellulosic material, comprising the steps of fluffing paper pulp, spraying on the fibres while the fibres are in separated form a crosslinking agent solution constituted by an aqueous solution of formaldehyde and hydrochloric acid, sufficient of the aqueous solution being sprayed bo provide on the fibres from 1 to 6% of formaldehyde based on the weight of the fibres, introducing the thus treated fibres into an air stream sufficiently hot to achieve cross-lruking of the fxores introduced therein',:o _.na separating the thus cross-linked fibres from gaseous by-products of the cross-linking.

2. A process as claimed in claim 1, wherein the air stream has a temperature of from 60 to 25O°C.

3. A process as claimed in claim 2, wherein the air stream has a temperature of from 170 to 180°C.

4. A process as claimed in any one of claims 1 to 3, wherein the fibres are retained in the air stream for from 1 to 10 seconds.

5. A process as claimed in claim 4, wherein the fibres are retained in the air stream for from 2 to 3 seconds.

6. A process as claimed in any one of the preceding claims, wherein the steps from fluffing to separating off of the cross-linked fibres are carried out within a time of less than 1 minute.

7. A process as claimed in any one of the preceding claims, wherein the air stream has a velocity of from 1 to 20 m/sec.

8. A process as claimed in any one of the preceding claims, wherein the fluffed paper pulp is heated to achieve drying thereof prior to contacting of the fluffed 438S7 - 10 paper pulp with the cross-linking agent solution.

9. A process as claimed in any one of the preceding claims, wherein aeration of the fluffed pulp is carried out to maintain the fibres in an uncollapsed state.

10. A process as claimed in any one of the preceding claims, wherein the cross-linking agent solution is supplied in such amount as to provide hydrochloric acid in a quantity of up to 2% by weight of the fibres.

11. A process as claimed in Claim 10, wherein the cross-linking agent solution is supplied in such amount as to provide hydrochloric acid in an amount ranging from 0.1 to 0.2% by weight of the fibres.

12. » A process as claimed in any one of the preceding claims, wherein the hydrochloric acid is employed in the cross-linking agent solution in the form of a salt thereof which will yield hydrogen chloride under the conditions employed.

13. A process as claimed in any one of the preceding claims, wherein the cross-linking agent solution additionally contains formic acid or acetic acid.

14. A process as claimed in claim 13, wherein the formic acid or acetic acid is employed in an amount of less than 50% by weight of the cross-linking agent solution.

15. A process as claimed in claim 14, wherein the amount of formic acid or acetic acid employed in the cross-linking agent solution is such that the crosslinking agent solution will supply formic acid or acetic acid to the fibres in an amount of not more than 12% by weight of the fibres.

16. A process as claimed in any one of the preceding claims, wherein the cross-linking agent solution supplies water to the fibres in an amount of from 2 to 19% by weight of the fibres.

17. A process for cross-linking fibres of cellulosic origin, substantially as hereinbefore described with reference to the accompanying drawing. 5

18. A process for cross-linking fibres of cellulosic origin, substantially as described in any one of the foregoing Examples 3 to 5.

19. Paper pulp fibres whenever cross-linked by the process claimed in any one of the preceding claims.