US 20090025895 A1
The present invention 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.
1. A process for making paper or paper board comprising the steps of:
adding a water soluble anionic compound (I), which adsorbs on the fibre surface and increases the negative surface charge of the fibre, recorded by means of zeta potential measurements, to an aqueous suspension of cellulosic fibres,
wherein the water soluble anionic compound (1) is a phenolic polymer consisting of recurring units of the formula
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,
Q is the same as P or an aromatic sulphonic acid or sulphonic acid salt,
m is 1 to 5,
n is 1 to 20;
subsequently adding a cationic papermaking additive (2) selected from the group consisting of: a retention aid a drainage aid, a wet strength polymer, a dry strength polymer, a cationic fixative, a softener, a debonder, and a cationic sizing chemical.
2. A process according to
P is selected from the group consisting of: phenol, phenol sulphonic, phenol carboxylic acid, cresol, cresol sulphonic, cresol carboxylic acid, dihydroxy diphenyl sulphone, dihydroxy diphenyl sulphone sulphonic, dihydroxy diphenyl sulphone carboxylic acid, naphthol sulphonic, and naphthol carboxylic acid, and
Q is the same as P or is selected from the group consisting of: naphthalene sulphonic acid, xylene sulphonic acid, cumene sulphonic acid, cresol sulphonic acid and benzene sulphonic acid.
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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.
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 papermaking, 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 Stem 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 (Stem layer) is carried with it. The ions in the diffuse layer, on the other hand, do not move with the fibre. The boundary between the Stem 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 Stem 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 in Table A.
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 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 milliequivalents 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 papermaking fibres, especially those that are brown in colour due to residual lignin. 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.
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 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
This artificial increase provides more sites and enhances the adsorption of cationic papermaking additives. In the present invention, substantive anionic compounds (I) 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.
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-dialkylaminoalkyl 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 polyvinylamine, polyamine, polyamide and glyoxylated polyacrylamide chemistry. Wet strength additives are predominantly polyamideamine or polyallylamine 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, polyvinylamine and acrylamide-dialkylaminoalkyl 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.
In a preferred embodiment the substantive water soluble anionic compound (I) 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 lignin. By preference, the substantive water soluble anionic compound (I) is a phenolic polymer, consisting of recurring units of the formula
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=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, amino or hydroxyalkylamino salts.
The molecular weight of the phenolic polymer generally is between 2'000 and 30'000 Daltons, preferably between 10'000 and 30'000 Daltons.
In a further embodiment the substantive water soluble anionic compound (I) 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 (I) is a dyestuff based on stilbene sulphonic acid chemistry. The substantive water soluble anionic compound (I) is by preference an optical brightening agent. More preferred, the substantive water soluble anionic compound (I) is a an optical brightening agent based 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.
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 Mütek GmbH.
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 m3 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.
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;
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 phenolic polymer (APP, 30% active liquid) CAS No. 94094-87-8. A summary of the results is tabled below;
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;
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 substances in the water phase and consuming some of the cationic polymer, which would otherwise be available for fibre flocculation.
Cationic polymeric wet strength agents are in common use in the paper industry. Large volumes of resin are often required to achieve the desired level of wet strength in 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 in the paper sheet.
Old corrugated containers (OCC) were re-pulped in tap water at 4% consistency and refined to a value of 40° SR (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 secs. A cationic polyamideamine-epichlorohydrin 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 100° C. for 30 minutes. After conditioning at 50° RH and 23° C. for a minimum period of 12 hours the sheets were evaluated using the following tests:
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 T4030M-91, Bursting Strength of Paper), using a laboratory burst tester.
Tensile—Evaluated using a Lloyd WRK5 Tensile Tester. 15 mm 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 30 seconds. The strip was then clamped in the jaws of the Lloyd WRK5 and left for 60 seconds. The tensile test was then carried out.
Similar improvements in performance were noted when the wet strength resin was replaced with proprietary cationic dry strength products.