Paper manufacture

Abstract

 A method of making paper by depositing cellulose fibres as a layer from an aqueous suspension, and de-watering said layer of cellulose fibres, in which method, to improve the dry strength of the paper product, the cellulose fibres are treated successively with a cationic non-formaldehyde poiymer and an anionic polymer which is an aminoplast resin whose repeating unit is anionic.
The method can be employed to give paper of usefully Improved dry strength, without generating an unacceptable level of free formaldehyde in the working environment.
Treatment of the cellulose fibres is preferably carried out by successively adding the cationic polymer and the anionic polymer to the aqueous suspension from which the cellulose fibres are deposited.

Description

[0001] This invention relates to the manufacture of paper, and is particularly concerned with making paper of improved dry strength.

[0002] In the manufacture of paper, cellulose fibres are deposited as a layer from an aqueous suspension, and the layer is subsequently de-watered, consolidated and dried. To improve the wet strength of the paper product it is known to treat the cellulose fibres with a melamine acid colloid, which is an acidic solution of an aminoplast resin of relatively high formaldehyde content, and this treatment also gives a good improvement in dry strength. However, use of such a resin as ordinarily practised leads to a concentration of free formaldehyde in the working atmosphere which is coming to be regarded as unacceptably high.

[0003] The present invention provides a method which can be employed to produce paper of usefully improved dry strength, without generating an unacceptable level of free formaldehyde in the working atmosphere.

[0004] According to the invention there is provided a method of making paper by depositing cellulose fibres as a layer from an aqueous suspension, and de-watering said layer of cellulose fibres, in which method, to improve the dry strength of the paper product, the cellulose fibres are treated successively with a cationic polymer not derived from .formaldehyde and an anionic polymer which is an aminoplast resin whose repeating unit is anionic.

[0005] This property in the repeating unit of being 'anionic' arises from the presence in the unit of an acidic group in salt form e.g. -O.SO₂- or -S0₃-. (The
cation of the salt is of no particular importance, provided that it does not interfere with the ready solution or dispersion of the salt in water. It is therefore conveniently an alkali metal ion such as that of sodium or potassium, or an organic cation, e.g. N(CH₃)₄⁺, which is small in comparison with the repeating unit.)

[0006] Preferably, treatment of the cellulose fibres is carried out by successively adding the cationic non-formaldehyde polymer and the anionic polymer to the aqueous suspension from which the cellulose fibres are deposited. The polymers dissolve or disperse in the aqueous suspension, and become successively deposited on the fibres.

[0007] Whatever process of treatment is employed, enough time (usually at least 0.5 minute is desirable) has to be allowed to elapse after addition of the cationic polymer in order to allow it to become substantially completely adsorbed by the cellulose fibres with which it is brought into contact. Premature addition of the anionic aminoplast resin would enable it to react with the appreciable content of cationic polymer still present in the ambient aqueous medium, and thus cause formation of a neutral water-insoluble polymeric complex, whose precipitation onto the cellulose fibres under treatment is not effective in developing dry strength in the paper to be formed from those fibres.

[0008] The anionic aminoplast resin is preferably one derived by condensation of (A) melamine (B) formaldehyde (C) an alkali metal salt of sulphurous acid. Such resins are disclosed in our British Patent Specification 751501 as useful for incorporation in drilling fluids for the drilling of deep wells. Preferably, the formaldehyde employed in the condensation is used in an amount of 1.75-4 moles per mole of melamine.

[0009] The anionic aminoplast resin is suitably used in an amount forming 0.1-5% by weight of the cellulose fibres.

[0010] The cationic non-formaldehyde polymer used in the method of the invention is preferably one whose repeating unit is cationic as a result of the presence in the unit of an amino group in salt form bound to aliphatic carbon atoms. The polymer is preferably a polyamide, suitably one of the kind described in British Patent Specification 865727, that is to say one derived by the condensation of a C₃ - C₁₀ saturated aliphatic dibasic acid (e.g. malonic, succinic, adipic, azelaic acid) and a polyalkylene polyamine containing 2-8 alkylene groups (e.g. diethylene triamine, triethylene tetramine, tetraethylene pentamine, dipropylene triamine), followed by reaction of the condensate with epichlorhydrin. Such cationic polymers are conventionally employed to impart wet strength to paper.

[0011] In carrying out the present invention, the cationic polymer is suitably used in an amount forming 0.1-5% by weight of the cellulose fibres.

[0012] The invention is illustrated by the following Example,

Example

[0013] Bleached sulphite cellulose pulp (Modocrown) was beaten to a freeness of 32° Schopper-Riegler and then diluted to a cellulose fibre concentration of 1.5% by weight.

[0014] This aqueous suspension of cellulose fibres was then brought to a pH of 6 with dilute sulphuric acid, and cationic polymer and anionic polymer were successively added to it with stirring, each polymer being in solution in water and being added in an amount forming 2% by weight (resin solids) of the cellulose fibre content of the suspension.

[0015] The cationic polymer was a polyamide condensate obtained from diethylene triamine (2.1 moles), adipic acid (2 moles) and epichlorhydrin (2.5 moles), following generally the procedure of Example 1 in British Patent Specification 865727, and containing a repeating unit which was predominantly

[0016] The anionic aminoplast resin was a condensate obtained from melamine (1 mole), formaldehyde (3 moles) and sodium metabisulphite (enough to provide 1 mole of sulphite radical, S032-), following generally the procedure
of Example 1 in British Patent Specification 751501, and containing a repeating unit which was predominantly

[0017] Two minutes elapsed between addition of the cationic polymer and addition of the anionic aminoplast resin.

[0018] After addition of the anionic polymer, the aqueous suspension was left for 1/2 hour and then diluted to a cellulose fibre concentration of 0.3% by weight. This diluted suspension was then used to make handsheets of 60g./m², using an entirely conventional apparatus (Papermakers Association Standard Apparatus for Pulp Evaluation), and the handsheets were air-dried and then heated at 127°C for 10 ninutes to develop dry strength fully.

[0019] For purposes of comparison, the procedure was repeated:-

(i) Without the use of polymers

(ii) Using, instead of cationic and anionic polymers used successively, a melamine-formaldehyde acid colloid of the kind conventionally used to impart wet and dry strength, whose use is associated with a high level of formaldehyde (5-10 parts per 10⁶ by volume) in the working atmosphere.

[0020] The breaking strength of the various handsheets was then measured.

[0021] The results were:-

[0022] In following the cationic/anionic procedure in a paper mill, we have found the atmospheric concentration of formaldehyde in the vicinity of the drying section of the mill to be no higher than about 0.5 parts per 10⁶ by volume.

Claims

1. A method of making paper by depositing cellulose fibres as a layer from an aqueous suspension, and de-watering said layer of cellulose fibres, in which method, to improve the dry strength of the paper product, the cellulose fibres are treated successively with a cationic non-formaldehyde polymer and an anionic polymer which is an aminoplast resin whose repeating unit is anionic.

2. A method according to claim 1, in which treatment of the fibres is carried out by successively adding cationic polymer and anionic polymer to the aqueous suspension from which the cellulose fibres are deposited.

3. A method according to claim 1 or 2, in which the aminoplast resin is one derived by condensation of (A) melamine (B) formaldehyde (C) an alkali metal salt of sulphurous acid.

4. A method according to claim 3, in which the formaldehyde is employed in an amount of 1.75-4 moles per mole of melamine.

5. A method according to any one of claims 1 to 4, in which the anionic polymer is employed in an amount forming 0.1-5% by weight of the cellulose fibres.

6. A method according to any one of claims 1 to 5, in which the cationic polymer is a polyamide.

7. A method according to claim 6, in which the cationic polymer is one derived by the condensation of a C₃-C_lO saturated aliphatic dibasic carboxylic acid and a polyalkylene polyamine containing 2-8 alkylene groups, followed by reaction of the condensate with epichlorhydrin.

8. A method according to any preceding claim in which the cationic polymer is employed in an amount forming 0.1-5% by weight of the cellulose fibres.

9. A method of making paper substantially as herein described.