WO1997013922A1

WO1997013922A1 - Method of fixing cellulose fibres

Info

Publication number: WO1997013922A1
Application number: PCT/SE1996/001095
Authority: WO
Inventors: Ingegerd Hjorth
Original assignee: Ab Klippans Finpappersbruk
Priority date: 1995-10-09
Filing date: 1996-09-04
Publication date: 1997-04-17
Also published as: SE9503483D0; SE9503483L; SE503887C2; AU7232796A

Abstract

A method of fixing cellulose fibres and rendering them dimensionally stable is described. The cellulose fibres are comprised in a material containing cellulose fibres, preferably a paper material. The method is characterised by first treating the cellulose fibres with a carboxylic acid, preferably in combination with an inorganic acid, such as sulphuric acid, and thereafter with a polyethylene glycol which is then cross-linked by means of a cross-linking agent. The contents of carboxylic acid preferably are about 3-15 % by weight, calculated on dry cellulose fibres, and the contents of polyethylene glycol preferably are about 3-15 % by weight, calculated on dry cellulose fibres. The carboxylic acid preferably is acetic acid and the polyethylene glycol preferably has a molecular weight of about 200-400. In the treatment of paper material the carboxylic acid and the polyethylene glycol may be added to the stock, the carboxylic acid being added first and the polyethylene glycol about 1-30 min thereafter.

Description

METHOD OF FIXING CELLULOSE FIBRES

The present invention relates to a method of fixing cellulose fibres, primarily cellulose fibres in paper and board.

By "fix" is to be understood herein treating the cellulose fibres to make them dimensionally stable, i.e. to be capable of retaining their dimensions and configu¬ ration upon variations in moisture content.

By the expression "cellulose fibres" is intended fibres in materials that contain cellulose fibres, such as solid wood, veneer, paper, paperboard, board, corru¬ gated fibreboard, and the like. Preferably are intended cellulose fibres in paper materials, the expression paper materials being intended to comprise all types of mate¬ rials produced by depositing a starting material contain- ing cellulose fibres onto a support, such as deposition of stock on a wire or air-deposition of cellulose fibres on a wire. The expression "cellulose material" thus com¬ prises paper as well as paperboard, board, corrugated fibreboard and similar two-dimensional products contain- ing cellulose fibres.

It is a well-known problem that materials contain¬ ing cellulose fibres, such as wood, paper material and the like respectively absorb and give off water, depend¬ ing on the environmental conditions, as a result of which the material swells or shrinks, i.e. it changes its dimensions. Such dimensional changes cause the cellulose fibres to move, which sometimes leads to upending of the fibres in the wood material and curling and waviness in the paper material. It is a long-felt desideratum to be able to counteract or to eliminate such undesired mois¬ ture-dependent dimensional changes in materials contain¬ ing cellulose fibres, and a number of different treat¬ ments has been suggested as indicated below.

NO 149,415 describes treatment with heat and pres- sure to render wood dimensionally stable. The treatment 4 HO ( CH₂CH₂0 ) _nH ( I ) wherein n < 20 , and c) an agent crosslinking the polyethylene glycol.

Preferably, the carboxylic acid has 1-8 carbon atoms and more preferably it is chosen from the group consist¬ ing of formic acid, acetic acid, propionic acid, oxalic acid, maleic acid, citric acid, thioglycol acid, acetic acid being the most preferable carboxylic acid.

It is preferable to treat the cellulose fibres with about 3-15% by weight of carboxylic acid, calculated on dry cellulose fibres.

As indicated above, the polyethylene glycol used in accordance with the method of the invention has the for¬ mula ( I ) HO(CH₂CH₂0)_nH (I) wherein n ≤ 20. More preferably, n = 3-20 and most pre¬ ferably n = 4-9, i.e. the degree of polymerisation (DP) of the polyethylene glycol is < 20, more preferably 3-20 and most preferably 4-9. This corresponds to a molecular weight of the polyethylene glycol of approximately < 900, more preferably about 150-900 and most preferably about 200-400.

Preferably, the cellulose fibres are treated with approximately 3-15% by weight, more preferably about 3-7% by weight polyethylene glycol, calculated on dry cellu¬ lose fibres.

The relative proportions of the carboxylic acid to the polyethylene glycol are not critical in accordance with the method of the invention, provided that the con- tents are within the ranges defined above, but preferably the weight ratio of carboxylic acid to polyethylene gly¬ col is from approximately 2:1 to approximately 1:2 and most preferably approximately 1:1.

The method in accordance with the invention is gene- rally carried out by first bringing the material contain¬ ing the cellulose fibre material into contact with the amine/silane complex rendering the cellulose material water-repellent.

As a further example of prior-art technology may be mentioned CH 614,882 which relates to treatment of wood flour with an acid solution, such as oxalic acid, acetic acid, or salicylic acid, whereupon the wood flour is dried to dryness. The treated wood flour is used as an insert between glass sheets for protection against oxidation. US 4,847,088, finally, relates to an antimicrobal composition consisting of a quaternary ammonium silane compound and an acid, such as acetic acid. The composi¬ tion may be used e.g. for treatment of paper.

Although some effects have been obtained as a result of the treatments in accordance with the prior-art tech¬ nology these prior-art methods have proved unsatisfac¬ tory, since they are not able to produce complete fixa¬ tion, i.e. dimensional stability, of the treated cellu¬ lose fibre. Among other things this is thought to be a consequence of the prior-art technology primarily having been concerned with treatments that affect only the sur¬ face of the cellulose material and do not penetrate deep into the material.

The present invention has for its object to eli i- nate the disadvantages inherent in the prior-art techno¬ logy and to provide a treatment producing improved fixa¬ tion, i.e. dimensional stability, of materials containing cellulose fibres, primarily paper materials.

The object of the invention is achieved by treating the cellulose fibres with a carboxylic acid and a poly¬ ethylene glycol that is crosslinked.

The invention thus provides a method of fixing cel¬ lulose fibres, which is characterised by treating the cellulose fibres, in sequence, with a) a carboxylic acid, optionally in combination with an inorganic acid, b) a polyethylene glycol having the general formula ( I ) 6 ethylene glycol to be immobilised and fixed in the mate¬ rial containing the cellulose fibres. Various crosslink¬ ing agents that may be used with polyethylene glycols are known to the expert in the field and there is no need for an extensive enumeration of the crosslinking agents of this kind. A preferred group of crosslinking agents to be used in connection with the present invention is a group consisting of ammonium-zirconium carbonate, glyoxal, and epichlorohydrine-modified polyamides. Especially good results have been achieved with ammonium-zirconium car¬ bonate. The amount of crosslinking agent to be used is the one that suffices to crosslink the polyethylene gly¬ col. Usually an amount of at most approximately 0.5% by weight, calculated on the polyethylene glycol, is suffi- cient and preferably the amount of crosslinking agent is in the range of approximately 0.1-0.5% by weight.

To establish the contact between the cellulose fibres and the carboxylic acid respectively the polyethy¬ lene glycol any suitable method may be used that will bring the cellulose fibres into intimate contact with the reagent. In the case of paper material containing cellu¬ lose fibres the contact is preferably established in con¬ nection with the very production of the paper material by adding the carboxylic acid and the polyethylene glycol to the stock, or at an earlier stage of the process. How¬ ever, it is likewise possible, although less preferable, to treat the finished paper material by coating or spray¬ ing it with the carboxylic acid and the polyethylene gly¬ col. The treatment may be carried out batchwise but a continuous treatment is preferred.

In the case of other types of material containing cellulose fibres, such as solid wood or veneer, the con¬ tact preferably is effected by way of impregnation, the material being immersed in the carboxylic acid and the polyethylene glycol, respectively. Alternatively, the carboxylic acid and the polyethylene glycol are applied carboxylic acid, preferably in the form of an aqueous solution, and the polyethylene glycol is added when the acid has been allowed to work for a sufficient length of time, preferably about 1-30 min, more preferably about 10-15 min. It has then been found according to the inven¬ tion that the order in which the material is treated with carboxylic acid and polyethylene glycol is critical. Thus, it is essential that the material containing the cellulose fibre material is first treated with carboxylic acid and only thereafter with polyethylene glycol. If the order is changed, for instance such that the material containing the cellulose fibre is first treated with polyethylene glycol and then with carboxylic acid or simultaneously with carboxylic acid and polyethylene gly- col, the desired fixation and dimension stability of the cellulose fibres are not achieved.

As indicated earlier, the treatment with carboxylic acid preferably is carried out in combination with an inorganic acid. It seems that the inorganic acid acts as a catalyst, accelerating the effects of the carboxylic acid. If an inorganic acid is used, it is preferably add¬ ed at the same time as the carboxylic acid. However, the inorganic acid could also be added prior to or after the carboxylic acid, although in the latter case the inorga- nic acid must be added prior to the treatment with poly¬ ethylene glycol. The inorganic acid may be chosen from a variety of different inorganic acids, such as sulphuric acid, hydrochloric acid, nitric acid. Sulphuric acid is particularly preferred. The amounts of inorganic acid to be added preferably are approximately 0.3% by weight at the most, preferably about 0.1-0.3% by weight and more preferably about 0.2% by weight, calculated on the amount of carboxylic acid.

After treatment of the material containing cellulose fibres with polyethylene glycol the latter is crosslinked in accordance with the invention with a polyethylene gly¬ col crosslinking agent. The crosslinking causes the poly- 8 means of the carboxylic groups of the carboxylic acid. If, as is preferred in accordance with the invention, an inorganic acid, such as sulphuric acid, is added in addi¬ tion to the carboxylic acid this inorganic acid acts as a catalyst accelerating the opening-up of the cellular structure of the cellulose-containing material. As a con¬ sequence of the cellulose fibre having been treated with carboxylic acid, the treatment of the fibre with poly¬ ethylene glycol becomes an efficient one and is not restricted to being a treatment affecting the surface of the cellulose fibre but one that also penetrates into the interior of the cellulose fibre. It is assumed that the polyethylene glycol is attached by being bonded to the free hydroxyl groups of the cellulose fibres. Because the polyethylene glycol penetrates into the fibres and is bonded to them via hydroxyl groups, the polyethylene gly¬ col is fixed in a stable manner to the cellulose fibres. In addition, the polyethylene glycol is fixed through crosslinking with the aid of a crosslinking agent, as described in the foregoing. In this manner, a cellulose fibre is created which is essentially unaffected by changes in the ambient moisture conditions, i.e. the cel¬ lulose fibre becomes fixed or dimensionally stable. The strong fixation of the polyethylene glycol in accordance with the invention differs from prior-art treatments of material containing cellulose fibres with polyethylene glycol, in accordance with which the polyethylene glycol was applied to the surface of the fibre from whence it could easily be removed. It should be noted that in accordance with the invention it has been found that it is essential that the second component is a polyethylene glycol. The method in accordance with the invention thus cannot be carried out by using other polyalkylene glycols, such as polypropy- lene glycol, as the second component. The reason therefor is not clear. In addition, it is not suitable in accor¬ dance with the invention to perform the method by using on the surface of the material by spraying, roller-appli¬ cation or by some other coating method.

In accordance with the method of the invention the treatment reagent is present in liquid or gaseous form, preferably in liquid form.

The temperature and the pressure of the treatment of the cellulose fibres with carboxylic acid and polyethy¬ lene glycol are not critical but for economical reasons the treatment preferably is carried out at ambient tempe- rature and ambient pressure. Some increase of the tempe¬ rature, for instance to approximately 40-80°C, may in¬ crease the reactivity and make the treatment more effi¬ cient, but in order to avoid evaporation of the reagent the temperature should not exceed approximately 100°C. The pressure may be increased to above the atmospheric pressure, for instance up to approximately 20 bar, e.g. to approximately 5-10 bar, in order to render the treat¬ ment more efficient, for instance in the impregnation of wood. When the cellulose fibres are initially treated with carboxylic acid an acid pH value results, this value usually amounting to approximately 3-4. Surprisingly, it has been found in accordance with the invention that the effects of the treatment with polyethylene glycol is im- proved when the pH value is increased to about 6-8 before the polyethylene glycol is added.

In the method in accordance with the present inven¬ tion both the carboxylic acid and the polyethylene gly¬ col are essential components. Without restricting the invention to any particular theory it is assumed that the carboxylic acid component acts as a reagent that attacks the cellulose fibre, "opening" it up and thus making it accessible to treatment with the polyethylene glycol. The carboxylic acid is then assumed to produce holes in the cell wall of the cellulose fibre and to be bonded to hydroxylic groups of the cellulose fibre. The bonds to the hydroxylic groups are in the form of ester bonds by 10 all specifications related to percentage and proportions are by weight unless otherwise specified.

Example 1

To a slurry of cellulose fibres consisting of approximately 1800 kg sulphate pulp of softwood and water having a cellulose fibre concentration of approximately 10% by weight were added, with stirring, in a pulper of brand Grubbens, 5% by weight of acetic acid and, as a catalyst, 0.2% by weight, calculated on the acetic acid, of sulphuric acid. The mixture was stirred thereafter at 225 rpm for about 15 min, whereupon the pH value was increased to 7.0 by means of NaOH, whereafter 3% by weight of polyethylene glycol having a molecular weight of approximately 200, (i.e. n is approximately 4 in For- mula I) was added. To the mixture was also added 0.2% by weight, calculated on the amount of polyethylene glycol, of ammonium zirconium carbonate as a crosslinking agent.

From the thus treated cellulose fibre slurry was manufactured paper, having a grammage of 100 g/m^, in the conventional manner on a Foudrinier machine. In order to test the dimensional stability of the paper, paper sheets of A4-size were cut. Three samples of the sheets of stan¬ dard A4-size of paper having been treated in accordance with the invention were selected as also three samples of sheets of standard A4-size of paper not having been treated in accordance with the invention. On all sheets the lengthwise direction (i.e. the machine direction) was indicated as also the cross direction. In addition, on each sheet was marked a distance of exactly 200 mm in both directions by means of a sliding rule having

"prongs" thereon (caliper type). The sheets were placed in a water bath having a temperature of 20±2°C and were left in the bath for 10 min. The sheets were thereafter removed from the water bath and a rubber roller was manually rolled backwards and forwards across the A4 sheets. The swelling in percentage was measured in the cross direction and considered as a measurement of the any type of polyethylene glycol but, as already mention¬ ed, the polyethylene glycol should have a degree of poly¬ merisation below approximately 20. When a polyethylene glycol having a degree of polymerisation above 20 is used, the fixation of the cellulose fibre will be unsa¬ tisfactory. The reason therefor is not either clear but in the light of the assumed function during treatment, referred to above, one explanation could be that poly¬ ethylene glycol having a degree of polymerisation above 20 has a molecular size that is too large to allow the molecules to efficiently penetrate into the cellulose fibre. Optimum function is, as already mentioned, obtained with polyethylene glycol having a degree of polymerisation of approximately 3-20, most preferably about 4-9.

Although the present invention is useful generally for fixing cellulose fibres of all types of materials containing cellulose fibres, the invention is particu¬ larly useful for fixing the cellulose fibres of paper material. Materials of this kind conventionally are pro¬ duced from paper pulp that is prepared into stock which is dewatered on a wire on which the desired paper product is formed. Depending on the starting material, the paper pulp could for instance be of sulphite type, sulphate type, recycled pulp or mixtures thereof. The method in accordance with the invention is applicable to all types of paper pulp. In the manufacture of paper the method in accordance with the invention typically is carried out by adding the carboxylic acid in the pulper to which the polyethylene glycol is likewise added. However, it is quite possible to add the components at an earlier stage of the process. In addition, the method in accordance with the invention is applicable to acid-sized as well as neutral-sized paper. In order to further facilitate the understanding of the invention it will be described by way of some eluci¬ dating although not limiting Examples. In the Examples 12

Example 5

The procedure of Example 1 was repeated, with the exception that the acetic acid was replaced by 5% by weight of formic acid. The following results were obtain- ed when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.23 0.68

Example 6

The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted of sulphate pulp of hardwood and that the acetic acid was replaced by 5% by weight of formic acid. The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.21 0.58

Example 7 The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted of recycled pulp and that the acetic acid was replaced by 5% by weight of formic acid. The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.25 0.74

Example 8

The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted to 20% by weight of sulphate pulp of softwood and to 80% of sul- phate pulp of hardwood and that the acetic acid was replaced by 5% by weight of formic acid. The following results were obtained when determining the dimensional stability of the sheet. dimensional stability. It is easily understood that the lower the percentage of swelling of the sheet, the higher the dimensional stability of the sheet. The results which are the mean value from measurements on three sheets, are indicated below.

Treated sheet Untreated sheet Change, in % 0.17 0.68

Example 2

The procedure of Example 1 was repeated, with the exception that the slurry of cellulose fibre consisted of sulphate pulp of hardwood instead of sulphate pulp of softwood. The following results were obtained when measuring the dimensional stability.

Treated sheet Untreated sheet Change, in % 0.14 0.58

Example 3

The procedure of Example 1 was repeated, with the exception that the slurry of cellulose fibre consisted of recycled pulp. The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.19 0.74

Example 4 The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted to 20% of sul¬ phate pulp of softwood and to 80% of sulphate pulp of hardwood. The following results were obtained when deter¬ mining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.18 0.62 14

Treated sheet Untreated sheet

Change, in % 0.16 0.62

Example 12

The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted to 20% of sul¬ phate pulp of softwood and to 80% of sulphate pulp of hardwood, that the polyethylene glycol had a molecular weight of approximately 400, and that no sulphuric acid catalyst was added. The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet

Change, in % 0.16 0.62

Example 13 (comparative)

The procedure of Example 1 was repeated, with the exception that the polyethylene glycol having a molecular weight of approximately 200 was replaced by a polyethy¬ lene glycol having a molecular weight of approximately 1000 (i.e. n is approximately 22 in Formula I). The fol¬ lowing results were obtained when determining the dimen¬ sional stability of the sheet.

Treated sheet Untreated sheet

Change, in % 0.58 0.68

As appears from these values only a very insignificant and unsatisfactory improvement of the dimensional stabi¬ lity was obtained compared with that in the untreated sheet. A reason therefore may be that the polyethylene glycol does not have a sufficiently low molecular weight and consequently cannot penetrate into the fibre cell cavities.

Example 14 (comparative)

The procedure of Example 1 was repeated, with the exception that no polyethylene glycol was used. The foi- Treated sheet Untreated sheet

Change, in % 0.24 0.62

Example 9

The procedure of Example 1 was repeated, with the exception that the polyethylene glycol having a molecular weight of approximately 200 was replaced by a polyethy¬ lene glycol having a molecular weight of approximately 400 (i.e. n equals approximately 9 in Formula I). The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet

Change, in % 0.14 0.68

Example 10

The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted to 20% of sul¬ phate pulp of softwood and to 80% of sulphate mass of hardwood, and that the polyethylene glycol having a mole- cular weight of approximately 200 was replaced by a poly¬ ethylene glycol having a molecular weight of approximate¬ ly 400. The following results were obtained when deter¬ mining the dimensional stability of the sheet.

Treated sheet Untreated sheet

Change, in % 0.15 0.62

Example 11

The procedure of Example 1 was repeated, with the exception that the fibre slurry consisted to 20% of sul¬ phate pulp of softwood and to 80% of sulphate pulp of hardwood, that the polyethylene glycol had a molecular weight of approximately 400 and that as a crosslinking agent was used 0.2% by weight of glyoxal, calculated on the amount of polyethylene glycol, instead of ammonium zirconium carbonate. The following results were obtained when determining the dimensional stability of the sheet. 16

The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.68 0.68 As appears from these values no improvement of the dimensional stability was obtained when the polyethylene glycol was replaced by spent liquor.

lowing results were obtained when determining the dimen¬ sional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.68 0.68 As appears from the results there was no improvement of the dimensional stability in the absence of the polyethy¬ lene glycol.

Example 15 (comparative)

The procedure of Example 1 was repeated, with the exception that no acetic acid or sulphuric acid were used. The following results were obtained when determin¬ ing the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.68 0.68 As appears from these values the dimensional stability was not improved in the absence of carboxylic acid.

Example 16 (comparative)

The procedure of Example 1 was repeated, with the exception that the polyethylene glycol was replaced by a polypropylene glycol having a molecular weight of approximately 400. The following results were obtained when determining the dimensional stability of the sheet.

Treated sheet Untreated sheet Change, in % 0.68 0.68 As appears from these results, no improvement of the dimensional stability was obtained when the polyethylene glycol was replaced by polypropylene glycol.

Example 17 (comparative)

The procedure of Example 1 was repeated, with the exception that the polyethylene glycol having a molecular weight of approximately 200 was replaced by spent liquor. 18 fibres are treated with about 3-15% by weight of poly¬ ethylene glycol, calculated on dry cellulose fibres.

9. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the cellulose fibres are first treated with carboxylic acid in combina¬ tion with sulphuric acid.

10. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the treatment with polyethylene glycol is carried out at a pH value of 6-8.

Claims

1. A method of fixing cellulose fibres, c h a r - a c t e r i s e d by treating the cellulose fibres, in sequence, with a) a carboxylic acid, optionally in combi¬ nation with an inorganic acid, b) a polyethylene glycol having the general formula ( I )

HO(CH₂CH₂0)_nH (I)

wherein n < 20, and c) an agent crosslinking the polyethylene glycol.

2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d by treating the cellulose fibres with a carboxylic acid having 1-8 carbon atoms.

3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d by treating the cellulose fibres with a carboxylic acid chosen from the group consisting of formic acid, acetic acid, propionic acid, oxalic acid, maleic acid, citric acid, thioglycol acid.

4. A method as claimed in claim 3, c h a r a c ¬ t e r i s e d in that the cellulose fibres are treated with acetic acid.

5. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the cellulose fibres are treated with about 3-15% by weight of carboxy¬ lic acid, calculated on dry cellulose fibres.

6. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the cellulose fibres are treated with a polyethylene glycol according to formula (I), wherein n = 3-20.

7. A method as claimed claim 6, c h a r a c t e r ¬ i s e d in that the cellulose fibres are treated with a polyethylene glycol according to formula ( I ) , wherein n = 4-9.

8. A method as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the cellulose