EP1994222A1

EP1994222A1 - Improved process for the manufacture of paper and board

Info

Publication number: EP1994222A1
Application number: EP07726292A
Authority: EP
Inventors: John Stuart Cowman
Original assignee: Clariant International Ltd
Current assignee: Clariant Finance BVI Ltd
Priority date: 2006-02-20
Filing date: 2007-02-02
Publication date: 2008-11-26
Also published as: WO2007096242A1; US20090025895A1; TW200801061A; AR059584A1

Abstract

The present invent ion provides a process for the manufacture of paper or board in which the anionic charge at the fibre surface is artificially increased by adding a substantive water soluble anionic compound, providing more sites and enhancing the adsorption of cationic papermaking additives. The preferred anionic compound is a phenolic polymer.

Description

IMPROVED PROCESS FOR THE MANUFACTURE OF PAPERAND BOARD

The instant invention relates to a process for the manufacture of paper or board in which the anionic charge at the fibre surface is artificially increased, providing more sites and enhancing the adsorption of cationic papermaking additives.

The use of water soluble cationic papermaking additives in the manufacture of paper and board is a well established technique for the provision of fibre and filler retention, water removal from the wet web, wet and dry strength improvement and anionic trash control. In papermaking systems where the use of fresh water, as a raw material, is unrestricted, the performance of such cationic additives is generally adequate. It is however becoming more common to limit the use of fresh water and, for environmental reasons, to recycle the process water. Recycling in this manner leads to an increase in dissolved and colloidal substances, and hence conductivity, within the circuit water, especially when the cellulosic fibre source is derived from waste paper. It is generally accepted that higher conductivities, in the water phase, suppress the anionic charge on the cellulosic fibre surface. Anionic (negatively charged) groups at the fibre surface act as anchor points for cationic (positively charged) additives and control the adsorption and hence the performance of such additives.

The reduced performance of cationic papermaking additives in higher conductivity conditions has attracted much attention in the last 5 to 10 years. Most of the emphasis in research and development has been focussed on the chemical and molecular structure of the cationic additive and how these properties could be modified to improve performance.

Cellulose, a naturally occurring polymer derived from trees and processed to yield a raw material for papermaking, contains both hydroxyl -OH and carboxyl -COOH groups. The latter is the result of oxidation and subsequent reduction in molecular weight during delignification and bleaching. As a result, the zeta potential of virgin cellulosic fibres is always negative or anionic. Case 2006CH003 2

Zeta potential is a representation of surface charge and is normally evaluated by taking an aqueous suspension of cellulosic fibres, forming a fibre plug on a metallic screen electrode and allowing a flow of water to pass through the plug. A potential difference (known as the streaming potential) between the screen and a second ring electrode, a short distance away, is measured and, from this value, the zeta potential is automatically calculated. Instruments, which record the zeta potential of fibre surfaces, are available from several manufacturers.

Cellulosic fibre, for the purposes of papermakmg, is available in a range of qualities ranging from fully bleached, with almost all the lignin and non-cellulosic components removed, to varieties of post-consumer brown wastepaper. The cellulosic content, especially at the fibre surfaces, depends heavily on the quality of this raw material. The virgin fibre in unbleached pulps contains more than 50% of lignins, wood resins and other non-cellulosic contaminants, leading to a reduction in carboxyl groups and associated anionic charge at the fibre surface. During the recycling process, contaminants are partially removed and re-deposited on the fibre surface, reducing even further the number of available anionic carboxyl groups After 5 or 6 recycling sequences, contaminants can cover as much as 90% of the fibre surface.

It is well documented that the zeta potential of an aqueous fibre slurry is influenced by the level of conductivity in the water phase. The surface of a negatively charged cellulosic fibre has a fixed layer of oppositely charged cations (often referred to as the Stern layer) and a diffuse layer of counter ions on top of the fixed layer. This concept has become known as the electrical double layer theory. As the fibre moves in water, the layer of cations (Stern layer) is carried with it. The ions m the diffuse layer, on the other hand, do not move with the fibre. The boundary between the Stern layer and the diffuse layer is known as the slip plane In low conductivity systems, the slip plane will be relatively far from the surface of the fibre but as conductivity is increased, more ions are introduced into the diffuse layer. As a result, the double layer is compressed and more cations are forced into the Stern layer, reducing the anionic charge on the fibre.

Zeta potential measurements carried out on different pulps, at 1% consistency, and with varying levels of conductivity are listed m Table A Case 2006CH003

Table A

OCC = Old Corrugated Containers (brown wastepaper) ONP = Old Newspapers

The performance of cationic additives, which rely on ionic interaction with the cellulosic fibre surface for their affinity to the substrate, is proportional to the level of conductivity in the water phase of the papermaking system. There is, therefore, a requirement for compounds, that can increase the anionicity of the surface charge (zeta potential), especially under conditions of high conductivity, and provide more ionic bonding sites for cationic papermaking additives.

It is known that many anionic direct dyestuffs have affinity for cellulose, mainly due to close alignment of the dyestuff molecule on the cellulosic surface, coupled with the formation of van der Waals and hydrogen bonds between dyestuff and fibre. The flat linear stilbene structure is particularly suited for efficient alignment and there are many dyestuffs, most of which contain sulphonic acid salt groups for increased water solubility, based on this chemistry.

In a similar manner, stilbene-based optical brightening agents (OBA) also have strong affinity for cellulosic fibres, even though their solubilising groups are usually sulphonic Case 2006CH003 4

or carboxylic acid salts, bestowing an anionic charge on the molecule. If adsorption were due to ionic attraction only, one would expect repulsion between OBA, dyestuff and fibre

The cellulosic fibres, OBAs and dyestuffs each have a charge density, normally recorded as milhequivalents per gram of substance (mequiv/g). A typical range of values for cellulosic fibres would be within the range 0.005 to 0 5 mequiv/g. Dyestuffs and OBAs have a higher charge density than the average value for cellulose (usually in the range 0.5 to 1.5 mequiv/g) and, when adsorbed on the fibre surface, these additives increase the zeta potential of the cellulose. In other words, the cellulosic fibre surface charge becomes more negative

It is of course accepted that the compounds described above are functional additives, providing colour and fluorescence to the finished paper sheet, properties which are not universally desired

It has now been found that certain anionic phenolic polymers also have affinity for papermakmg fibres, especially those that are brown in colour due to residual lignm Here too, the charge density of the phenolic polymers is higher than that of the cellulosic substrate When adsorbed on the fibre surface, the phenolic polymers increase the zeta potential The effect of adsorbed additive on the zeta potential of various pulps is shown in Table B

Case 2006CH003

Table B

OCC = Old Corrugated Containers (brown wastepaper) ONP = Old Newspapers Dyestuff = Direct Yellow 11 (30% active liquid)

OBA = tetrasulphonated stilbene compound (25% active liquid) CAS No. 16470-24-9 APP = Anionic phenolic polymer (30% active liquid) CAS No. 94094-87-8

There are a multitude of papermaking additives possessing a predominantly anionic charge. The majority of these additives have little or no affinity for cellulosic surfaces (the repulsion rule applies). As a consequence, additives such as polyacrylic acid, fatty Case 2006CH003 6

acid soaps, carboxymethyl cellulose and anionic starch have no value as pre-treatments in the present invention.

It has now been found that by first adding a substantive water soluble anionic compound to the aqueous fibres slurry the negative surface charge on the cellulosic fibres is increased, thus providing more sites and enhancing the adsorption of cationic papermaking additives. This technique allows the use of any cationic additive, modified or otherwise, and provides an increase in performance, especially under higher conductivity conditions. The instant invention demonstrates that cationic papermaking chemicals, added for purposes such as improved drainage, fibre and filler retention, dry and wet strength, deposit control and sizing, benefit from a more anionic zeta potential.

Therefore an object of the instant invention is a process for making paper or paper board comprising • in a first step the addition to an aqueous suspension of cellulosic fibres of a substantive water soluble anionic compound (1), which adsorbs on the fibre surface and increases the negative surface charge of the fibre, recorded by means of zeta potential measurements, and

• in a second step the addition of a cationic papermaking additive (2) selected from a retention or drainage aid, a wet or dry strength polymer, a cationic fixative, a softener or debonder, a cationic sizing chemical

This artificial increase provides more sites and enhances the adsorption of cationic papermaking additives. In the present invention, substantive anionic compounds (1) are employed, which adsorb on the fibre surface and increase the anionic charge. Additives, which are anionic but have no affinity for cellulosic fibres, do not demonstrate this effect.

The process for making paper or paper board according to the invention comprises, continuously forming an aqueous cellulosic fibre suspension, to which is added a substantive water soluble anionic compound (1), followed by one or more water soluble cationic additive (2) and optionally an inorganic coagulant, draining the suspension on a screen to form a wet sheet and drying the sheet. Case 2006CH003

The substantive water soluble anionic compound (1) is characterized in that it increases the negative surface charge on the cellulosic fibres within the suspension, providing additional anchor points for water soluble cationic additives (2). The adsorption potential and hence the performance of water soluble cationic additives (2) is improved.

The water soluble cationic additives (2), with improved performance, provide higher retention values and/or faster drainage speeds for the cellulosic fibrous suspension, and/or higher wet and/or dry strength values of the dried paper sheet.

Cationic additives are widely used in the paper industry and may be applied to control the papermaking process and/or to add functionality to the paper sheet. Cellulosic fibre retention and water removal are two important process variables, controlled by retention and drainage aids, respectively, polymeric additives, which are mostly cationic in nature and derived from acrylamide-dialkylammoalkyl methacrylic or acrylic ester copolymers. Diallyldimethylammonium chloride (DADMAC) is also a popular monomer and is available, both as a homopolymer and in polymer combinations with other monomers.

Cationic dry strength additives are based on either natural or synthetic polymers. Starch and guar may be cationised in a reaction involving epoxypropyl-trimethylammonium chloride. Synthetic additives for dry strength are numerous but include products based on polyvmylamine, polyamine, polyamide and glyoxylated polyacrylamide chemistry. Wet strength additives are predominantly polyamideamme or polyallylamme chemistry, further reacted with epichlorohydrin.

Cationic fixatives are generally polymers with a high charge density and include polyamine (reaction products of aliphatic amines with epichlorohydrin), poly- DADMAC, polyvinylamme and acrylamide-dialkylammoalkyl methacrylic or acrylic ester chemistries. Softener and debonder chemistry is generally non-polymeric and based on cationic quaternary ammonium derivatives of fatty amines (often alkoxylated), fatty acid esters or imidazole compounds Case 2006CH003 8

In a preferred embodiment the substantive water soluble anionic compound (1) is a phenolic polymer, which has strong affinity for cellulosic fibres, especially cellulosic fibres that have not been fully bleached and are brown in colour due to residual ligmn. By preference, the substantive water soluble anionic compound (1) is a phenolic polymer, consisting of recurring units of the formula

- [P - CH₂J_n - [Q - CH₂]_m -

wherein P is an hydroxyl-substituted phenyl ring, wherein the phenyl ring is not further substituted or is substituted with sulphomc acid, sulphonic acid salt, carboxylic acid or carboxylic acid salt groups and Q is P or an aromatic sulphonic acid or sulphonic acid salt and m = 1 to 5 and n = 1 to 20.

Preferred P, as a constituent of the phenolic polymer, is phenol, phenol sulphonic or carboxylic acid, cresol, cresol sulphonic or carboxylic acid, dihydroxy diphenyl sulphone, dihydroxy diphenyl sulphone sulphonic or carboxylic acid, naphthol sulphonic or carboxylic acid and Q is P or naphthalene sulphonic acid, xylene sulphonic acid, cumene sulphonic acid, cresol sulphonic acid or benzene sulphonic acid.

The sulphonic or carboxylic acid groups are present in the form of sodium, potassium, lithium, ammonium, ammo or hydroxyalkylammo salts.

The molecular weight of the phenolic polymer generally is between 2'00O and 30O00 Daltons, preferably between 10'0OO and 30O00 Daltons.

In a further embodiment the substantive water soluble anionic compound (1) is a dyestuff, which has strong affinity for cellulosic fibres. Preferably it is Direct Yellow 11.

In another embodiment the substantive water soluble anionic compound (1) is a dyestuff based on stilbene sulphonic acid chemistry. The substantive water soluble anionic compound (1) is by preference an optical brightening agent. More preferred, the substantive water soluble anionic compound (1) is a an optical brightening agent based Case 2006CH003 9

on stilbene sulphonic acids. Even more preferred, the substantive water soluble anionic compound (1) is an optical brightening agent based on stilbene sulphonic acids, containing 2, 4, 6 or more sulphonic acid groups, optionally neutralised with any alkaline compounds, but preferably with sodium, potassium or lithium hydroxides.

The water soluble cationic additives (2) display improved performance in papermaking systems where the level of conductivity in the water circuits is greater than 1000 micro Siemens, and especially where the conductivity is greater than 2500 micro Siemens.

The cellulosic fibres are derived from bleached, semi-bleached or unbleached wood pulp, deinked pulp or waste paper.

The amount of substantive water soluble anionic compound (1), added to the cellulosic fibre suspension prior to any cationic additive (2), is 0.001 to 10%, more preferably 0.01 to 2% of dry compound, based on the dry weight of cellulosic fibres.

The amount of cationic papermaking additive (2) is 0.01 to 2% of dry compound, based on the dry weight of cellulosic fibres.

According to the present invention, in papermaking systems where the conductivity levels are higher than 1000 micro Siemens, preferably higher than 2500 micro Siemens, the addition of a substantive water soluble anionic compound (1), followed by a typical cationic papermaking additive (2), enhances the performance of such additives. Retention and drainage aids, wet and dry strength additives, cationic fixatives for trash control, all display improved performance. In turn, increased productivity and paper machine cleanliness are useful secondary effects of the present invention. Case 2006CH003 10

Examples

The following examples will serve to illustrate the invention. The dosage rates of the additives mentioned in the examples are based on product, as supplied, as a percentage of the dry weight of cellulosic fibre. All measurements mentioned in the present invention were carried out using a SZP-06 System Zeta Potential from BTG Mϋtek GmbH.

Example 1

A 1% slurry of cellulosic fibres was sampled in a paper mill, during the manufacture of test liner from old corrugated container wastepaper. The sample was removed at a point, before the addition of the retention aid, which in this case was a high molecular weight cationic polyacrylamide powder (cationic monomer content amounts to 10% molar) The water circuits in this mill were classed as relatively closed, with a fresh water usage of 3 m per tonne of paper. With such a low fresh water consumption, dissolved and colloidal substances had increased the conductivity to around 4000 μS/cm. Zeta potential measurements indicated that the surface charge of the cellulosic fibres lay between 0 and -5 mV, inferring that the number of available -COOH groups for ionic bonding was very small. The cationic polyacrylamide was performing rather inefficiently, clearly evident from the large distance between flow box and wet line on the paper machine wire. Drainage of water from the cellulosic fibres through the machine wire was rather slow, reflected in a below-target machine speed There was clearly a requirement to reduce the drainage times for a fixed volume of backwater Retention aids, in the form of cationic polymers, are generally employed to coagulate and flocculate cellulosic fibres, thereby accelerating the water removal process

The sampled fibre slurry, with a pH of 6.2 and stirring at 1000 rpm in a Dynamic Drainage Jar, was pre-treated with a dyestuff (Direct yellow 11, 30% active liquid) and then the usual amount of retention aid The valve on the jar was opened and the time taken to collect 400 ml of water (drained through machine wire) was recorded. A summary of the results is tabled below. Case 2006CH003 11

Table 1

Drainage times are clearly quicker with a pre-addition of Direct Yellow 11

Example 2

Drainage measurements were carried out, using the same fibre slurry and in the same manner as in Example 1 , but with a pre-treatment comprising a tetrasulphonated stilbene optical brightening agent (OBA, 25% active liquid) CAS No 16470-24-9 A summary of the results is tabled below,

Table 2

Drainage times are clearly quicker with a pre-addition of OBA

Example 3

Drainage measurements were carried out, using the same fibre slurry and m the same manner as in Example 1, but with a pre-treatment comprising an anionic phenolic polymer (APP, 30% active liquid) CAS No 94094-87-8 A summary of the results is tabled below, Case 2006CH003 12

Table 3

Drainage times are clearly quicker with a pre-addition of APP.

Example 4

This example demonstrates the effect, when a fibre slurry is pre-treated with a compound, which is anionic but not substantive. Drainage measurements were carried out, using the same fibre slurry and in the same manner as in Example 1 , but with a pre- treatment comprising an anionic ammonium polyacrylate (40% active liquid CAS No. 9003-03-6). A summary of the results is tabled below;

Table 4

Drainage times (and hence water removal) are slower than with the cationic polyacrylamide alone, indicating that the ammonium polyacrylate is not adsorbed on the fibre surface, but is instead contributing to the level of dissolved and colloidal anionic Case 2006CH003 13

substances in the water phase and consuming some of the cationic polymer, which would otherwise be available for fibre flocculation.

Example 5 Cationic polymeric wet strength agents are in common use in the paper industry Large volumes of resm are often required to achieve the desired level of wet strength m the paper sheet and when the zeta potential of the cellulosic fibre is too low to accept such high addition levels, excess polymer remains in the water phase and the wet strength of the sheet is disappointingly low. This example demonstrates how a pre-treatment of a substantive anionic compound increases the zeta potential of the cellulosic fibre and the amount of adsorbed cationic wet strength polymer, leading to higher values of wet strength m the paper sheet.

Old corrugated containers (OCC) were re-pulped in tap water at 4% consistency and refined to a value of 40⁰SR (Schopper Riegler). This pulp was then diluted with tap water to 1 % consistency and the conductivity adjusted to 1000 μS/cm with sodium sulphate. The pH was measured at 6.8 The pulp was used to make 2 g (equivalent to 100 gsm) hand sheets with the British Standard Sheet Forming Apparatus.

1 litre of stock was placed in a suitable container and stirred at 500 rpm. A pre-addition of anionic phenolic polymer (APP, 30% active liquid, CAS No. 94094-87-8 ) was dosed (see table for details) and then stirred for 60 sees. A cationic polyamideamine- epichlorohydrm resin (PAE, 12% active liquid, CAS No. 70914-39-5) was then added and stirred for a further 30 seconds 200-ml samples of the treated pulp were then taken and formed into a hand sheets For each test, 4 hand sheets were made, to obtain a meaningful average.

The sheets were then pressed onto stainless steel plates at 4.0 bar for 4 minutes, then placed into drying rings and dried at 100⁰C for 30 minutes. After conditioning at 50⁰RH and 23 °C for a minimum period of 12 hours the sheets were evaluated using the following tests: Case 2006CH003 14

Burst strength - Sheets immersed in water for 1 minute. The excess water was then blotted off and the sheets subjected to strength testing (TAPPI Standard T403 OM-91, Bursting Strength of Paper), using a laboratory burst tester.

Tensile - Evaluated using a Lloyd WRK5 Tensile Tester. 15mm wide strips were cut from each sample and 5 drops of water were placed at the centre of the strip and allowed to stand for 30seconds. The strip was then clamped in the jaws of the Lloyd WRK5 and left for 60seconds. The tensile test was then carried out.

Table 5

The Burst Index = Burst result The Tensile Index = Tensile result

Sheet weight in gsm Sheet weight in gsm

Similar improvements in performance were noted when the wet strength resin was replaced with proprietary cationic dry strength products.

Claims

Case 2006CH003 15Claims

1. A process for making paper or paper board comprising

• in a first step the addition to an aqueous suspension of cellulosic fibres of a substantive water soluble anionic compound (1), which adsorbs on the fibre surface and increases the negative surface charge of the fibre, recorded by means of zeta potential measurements, and

• m a second step the addition of a cationic papermakmg additive (2) selected from a retention or drainage aid, a wet or dry strength polymer, a cationic fixative, a softener or debonder, or a cationic sizing chemical.

2. A process according to claim 1, wherein the substantive water soluble anionic compound (1) is a phenolic polymer consisting of recurring units of the formula

- [P - CH₂J_n - [Q - CH₂]_m -

wherein

P is an hydroxyl-substituted phenyl ring, wherein the phenyl ring is not further substituted or is substituted with sulphonic acid, sulphonic acid salt, carboxylic acid or carboxylic acid salt groups, and

Q is P or an aromatic sulphonic acid or sulphonic acid salt, and m is 1 to 5, and n is 1 to 20.

3. A process according to claim 2, wherein

P is phenol, phenol sulphonic or carboxylic acid, cresol, cresol sulphonic or carboxylic acid, dihydroxy diphenyl sulphone, dihydroxy diphenyl sulphone sulphonic or carboxylic acid, naphthol sulphonic or carboxylic acid, and

Q is P or naphthalene sulphonic acid, xylene sulphonic acid, cumene sulphonic acid, cresol sulphonic acid or benzene sulphonic acid. Case 2006CH003 16

4. A process according to claim 2 or 3, wherein the sulphomc or carboxylic acid groups are present in the form of sodium, potassium, lithium, ammonium, ammo or hydroxyalkylamino salts

5. A process according to any of claims 2 to 4, wherein the molecular weight of the phenolic polymer is between 2'00O and 30O00 Daltons.

6. A process according to claim 5, wherein the molecular weight of the phenolic polymer is between 10'0OO and 30O00 Daltons.

7. A process according to claim 1, wherein the substantive water soluble anionic compound (1) is a an anionic direct dyestuff with strong affinity for cellulosic fibres or an optical brightening agent or mixtures thereof.

8. A process according to claim 7, wherein the anionic direct dyestuff is Direct Yellow 11.

9. A process according to claim 7, wherein the optical brightening agent is based on stilbene sulphomc acids containing 2, 4, 6 or more sulphonic acid groups which may be neutralized with sodium, potassium or lithium hydroxides.

10. A process according to any of the preceding claims wherein the amount of substantive water soluble anionic compound (1), added to the cellulosic fibre suspension prior to any cationic additive (2), is 0.001 to 10% of dry compound, based on the dry weight of cellulosic fibres.

11. A process according to claim 10 wherein the amount of substantive water soluble anionic compound (1) is 0 01 to 2% of dry compound.

12. A process according to claim 1, wherein the amount of cationic papermaking additive (2) is 0.01 to 2% of dry compound, based on the dry weight of cellulosic fibres. Case 2006CH003 17

13. A process according to any of the preceding claims, wherein the cellulosic fibres are derived from bleached, semi-bleached or unbleached wood pulp, deinked pulp or waste paper.

14. A process according to any of the preceding claims, wherein the level of conductivity in the water circuits is greater than 1000 micro Siemens.

15. A process according to any of the preceding claims, wherein the level of conductivity in the water circuits is greater than 2500 micro Siemens.