WO1998001620A1

WO1998001620A1 - Method for refining cellulose stock

Info

Publication number: WO1998001620A1
Application number: PCT/GB1997/001817
Authority: WO
Inventors: Stuart David Bailey; Pamela Ann Johnson; Calvin Roger Woodings
Original assignee: Courtaulds Fibres (Holdings) Limited
Priority date: 1996-07-08
Filing date: 1997-07-04
Publication date: 1998-01-15
Also published as: AU3451297A; GB9614311D0

Abstract

Aqueous stock which comprises lyocell fibres can be refined more rapidly when the stock comprises from 0.1 to 5.0 percent by weight sodium hydroxide. Paper made from stock refined in this manner may exhibit improved tensile properties, including tensile strength, burst strength and tear strength.

Description

METHOD FOR REFINING CELLULOSE STOCK

Field of the invention

This invention relates to methods for refining aqueous slurries of cellulose fibres, in particular lyocell fibres, notably for use in papermaking. Such slurries are commonly referred to as stocks or papermaking stocks.

Background art

Lyocell fibres are known, and their manufacture is described for example in US-A-4 , 416 , 698. Such fibres are made by extrusion of cellulose dissolved in an aqueous tertiary amine N-oxide solvent through a die into an aqueous coagulating bath. This process may also be referred to as solvent spinning, and the fibres produced thereby may be referred to as solvent-spun cellulose fibres. The lyocell process has a number of advantages over other processes for the manufacture of cellulosic fibres such as the viscose process, for example reduced environmental emissions. Lyocell fibres have found application both in textile and in non-textile applications. O-A-95/ 14398 describes the manufacture of paper from lyocell fibres and the use of such paper in the manufacture of cigarette filters.

According to conventional practice, papermaking pulps are handled as aqueous slurries or stocks in the course of preparation of the furnish used in papermaking. One operation almost always performed on such a slurry is that referred to synonymously as beating or refining, that is to say subjection of the slurry to mechanical action which serves to induce swelling, cutting, maceration and fibrillation of the cellulose fibres. Beating or refining modifies the physical properties of the fibres, allowing the production of paper of greater density, hardness and strength. Disclosure of the invention

According to the present invention there is provided a method for refining an aqueous stock which comprises lyocell fibres, characterised in that the stock comprises from 0.1 to 5.0, preferably from 0.5 to 3.0, percent by weight sodium hydroxide .

The invention further provides a method of making paper comprising lyocell fibres, characterised in that it includes the step of refining an aqueous stock which comprises lyocell fibres and from 0.1 to 5.0, preferably from 0.5 to

3.0, percent by weight sodium hydroxide.

The concentration of lyocell fibres in the stock is conveniently in the conventional range for papermaking using cellulosic fibres, for example in the range from 0.1 to 2.5 percent by weight.

The titre of the lyocell fibres is often in the range from 0.5 to 10 decitex. The lyocell fibres are staple fibres, often having staple length in the range from 0.5 to 10 mm. The lyocell fibres may be unpigmented or may be pigmented, for example with a matting pigment such as titanium dioxide.

The temperature of the stock submitted to refining is commonly at or around ambient temperature. The temperature rises during the refining process because of the mechanical work performed during it.

Suitable refining equipment for use in the method of the invention includes conventional apparatus such as a hollander, hydrapulper, valley beater, disintegrator or disc refiner. The method of the invention may be performed batchwise or continuously.

The stock submitted to the method of the invention may contain one or more additives, for example dispersing or wetting agents, in known manner.

The refining method of the invention permits more rapid refining than when a conventional stock containing no sodium hydroxide is used. Surprisingly, paper made from a stock refined by the method of the invention may be found to exhibit improved tensile properties, including tensile strength, burst strength and tear strength, in comparison with paper made from a stock refined in conventional manner.

The invention is illustrated by the following Example, in which parts and proportions are by weight unless otherwise specified :-

Example

Lyocell fibre (12 g bone dry weight (= 13.2 g conditioned), 1.7 dtex, 5 mm staple, matt; available from Courtaulds Fibres (Holdings) Limited) and demineralised water (2 1), optionally containing dissolved sodium hydroxide, were placed in the bowl of a pulp disintegrator and refined (simulating valley beating or hydrapulping) for either 100,000 or 200,000 disintegrator revolutions. Suitable disintegrators are available from Messmer Instruments Limited, Gravesend, Kent, UK and from Buechel van de Korput BV, Veemendaal, Netherlands. The Canadian Standard Freeness (CSF) of the fibre in the resulting slurry (stock) was measured according to TAPPI Standard T227 om-85 and recorded in ml. The stock was made into paper according to TAPPI Standard T205 om-88 . The results shown in Table 1 were obtained: Table 1

NaOH % CSF Bulk Tensile Burst index Tear index ml ml/g index kPa.nv/g mN . m /g Nm/g 5 100,000 disintegrator revolutions

0 (control) 493 3.23 10.4 1.78 23.3

0.5 453 3.04 14.6 1.92 24.0

1.5 341 3.01 18.0 2.40 40.5

5.0 495 2.85 11.5 1.78 19.6

10 200,000 disintegrator revolutions

0 (control) 101 2.67 19.3 2.60 32.0

0.5 74 2.67 28.9 2.92 39.0

1.5 21 2.51 35.9 3.14 33.3

5.0 77 2.50 25.3 2.60 29.4

15 Tensile, burst and tear indices were measured according to TAPPI Standards T494 om-88, T403 om-85 and T414 om-88, respectively.

It was observed that, in the equipment used, the lyocell fibre in a stock containing 0.3% w/v lyocell fibre 20 tended to float, resulting in poor mixing during the refining step. It is thought that this could have been cured by addition of a suitable wetting or dispersing agent or by use of alternative refining equipment.

Claims

1. A method for refining an aqueous stock which comprises lyocell fibres, characterised in that the stock comprises from 0.1 to 5.0 percent by weight sodium hydroxide.

2. A method according to claim 1, further characterised in that the stock comprises from 0.5 to 3.0 percent by weight sodium hydroxide.

3. A method according to claim 1 or claim 2, further characterised in that the stock comprises from 0.1 to 2.5 percent by weight lyocell fibres.

4. A method according to any one of the preceding claims, further characterised in that the staple length of the lyocell fibres is in the range from 0.5 to 10 mm.

5. A method of making paper, characterised in that it includes the step of refining an aqueous stock which comprises lyocell fibres by the method of any one of the preceding claims.