EP0574335A1 - Process for making paper or paperboard having an increased retention - Google Patents

Abstract

Process for the manufacture of a paper or cardboard having improved retention, in which a polyacrylamide and bentonite are added to the fibrous suspension, characterised in that the polyacrylamide is a branched polyacrylamide which is easily water-soluble and is introduced in the dissolved powder form at a concentration of 0.03 to 1.0 per thousand (0.03 to 1.0 per 1000 ) by weight of the dry weight of the fibrous suspension.

Description

The invention relates to a method for manufacturing paper or cardboard with improved retention.

• l'augmentation de la production et la diminution des coûts de fabrication : économie énergétique, marche plus régulière de la machine, rendement plus élevé en fibres, fines, charges et de produits d'ennoblissement anioniques, plus faible acidité dans le circuit liée à une diminution de l'utilisation de sulfate d'alumine et donc amoindrissement des problèmes de corrosion ;
• l'amélioration de la qualité : meilleure formation et épair ; amélioration du taux d'humidité de la feuille, de l'opacité, du lissé, du pouvoir absorbant et diminution de la porosité du papier.

During the manufacture of paper, cardboard or the like, it is well known to introduce retention agents into the pulp, the function of which is to retain a maximum of fines and fillers in the sheet. The beneficial effects that result from the use of a retention agent are essentially:

• the increase in production and the decrease in manufacturing costs: energy saving, smoother running of the machine, higher yield of fibers, fines, fillers and anionic finishing products, lower acidity in the circuit linked to a reduction in the use of alumina sulphate and therefore lessening of corrosion problems;
• improving quality: better training and awareness; improvement in the humidity of the sheet, the opacity, the smoothness, the absorbency and reduction of the porosity of the paper.

It has been proposed for a long time to add bentonite to the pulp, which can optionally be added to other mineral products, such as aluminum sulphates, or even synthetic polymers, in particular polyethyleneimine (see for example documents DE-A-2 262 906 and US-A-2 368 635).

In document US Pat. No. 3,052,595, it has been proposed to combine bentonite with a linear polyacrylamide. This process has hardly developed, since it has found itself in competition with systems that are easier to implement while also being efficient. Furthermore, even with current linear polyacrylamides, the retention power is still insufficient.

In document EP-A-0 017 353, it has been proposed, for the retention of lightly loaded pastes (at most 5% of fillers) to combine with bentonite a weakly anionic linear nonionic copolyacrylamide. This process has hardly been developed, since these polymers are relatively ineffective in terms of retention, in particular of loaded pastes, no doubt due to an insufficient synergy between these copolymers and bentonite which has little tendency to recoagulate.

In document EP-A-0 235 893, it has been proposed to use cationic polyacrylamides of molecular weight greater than one million, preferably thirty million and more, essentially linear, even partially crosslinked. In this way we obtain a retention effect that is certainly satisfactory, but still considered insufficient in the paper application, because the use of bentonite causing water treatment difficulties, users only select this system in the event of advantages significant.

In the article by TAPPI, published in Abstracts Bulletin of the Institute of Paper Science and Technology (vol. 62, n ° 10, April 1992 page 1165), the mechanism of the supercoagulation of activated bentonite in the presence of a cationic copolyacrylamide, without specifying its exact nature. This process has the same drawbacks as above.

The invention overcomes these drawbacks.

It relates to an improved process of the type in question, which consists in adding to the fibrous suspension, a polyacrylamide and bentonite and which makes it possible to obtain a markedly improved retention of fines and fillers and this without reverse effect.

This improvement is characterized in that the polyacrylamide is branched and is easily soluble in water, and is introduced into the suspension in the form of powder dissolved, at a rate of 0.03 to one per thousand (0.03 to 1% o) by weight, of the dry weight of the fibrous suspension.

In other words, the invention consists, among all the polyacrylamides, in using the branched polyacrylamides in the form of powder put in solution. Unexpectedly, this selection allows, in the paper application for the retention of fillers and fines, to reach a level of performance unmatched before. The use of branched polymers also makes it possible to better retain the bentonite on the sheet, and therefore to limit its negative effects on the subsequent water treatment. In addition, the choice of this branched polyacrylamide increases the fixing power of the bentonite on the sheet, thereby bringing about a synergy, therefore a re-coagulation which reduces the content of the bentonite in the white waters.

Advantageously, in practice the branched polyacrylamide is a cationic copolymer of acrylamide and of an unsaturated cationic ethylenic monomer, chosen from the group comprising dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl methacrylate (MADAME), quaternized or salified , dimethyldiallylammonium chloride (DADMAC), chloride acrylamide propyltrimethylammonium (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC). In known manner, this copolymer is branched by a branching agent consisting of a polyfunctional compound having at least two reactive groups chosen from the group comprising double bonds, aldehyde bonds or epoxy bonds. These compounds are well known and are described for example in document EP-A-0 374 458 (see also document FR-A-2 589 145 of the Applicant).

As we know, a branched polymer, designated in the American language by the expression "branched", is a polymer which has all over the chain of branches, groups or branches arranged globally in one plane and no longer in the three directions , as is a crosslinked polymer; such branched, high molecular weight polymers, easily soluble in water, are well known as flocculating agents. These branched polyacrylamides are distinguished from crosslinked polyacrylamides (frequently designated by the American expression "crosslinked"), by the fact that in the latter, the groups are arranged three-dimensionally to lead to products which are practically insoluble and of infinite molecular weight;

In practice, the branching agent is methylene bis acrylamide (MBA), introduced in an amount of five to two hundred (5 to 200) moles per million moles of monomers.

Advantageously, the amount of branched polyacrylamide introduced is between thirty and one thousand grams / ton (30 and 1000 g / t) of dry paste; it has been observed that if the amount is less than 0.03% o, no significant retention is obtained; similarly, if this quantity exceeds 0.1% o, no proportional improvement is observed; however, unlike linear cationic polyacrylamides, as described in the documents EP-A-0 017 353 and 235 893 referred to in the preamble, no reverse effect of dispersion by recirculation in the closed circuits of the excess of polymer not retained on the sheet is observed. Preferably, the amount of branched polyacrylamide introduced is between 0.05 and 0.5 per thousand (% o) of the amount of the dry paste.

As already said, it is important that the branched polymer is used in the form of a diluted powder; in fact, if a branched emulsion polymer is used, the essential presence in these emulsions of surfactants promotes the formation of foams during the manufacture of paper and the appearance of disparities in the physical properties of the finished paper (modification of the absorbance at the places where part of the oil phase of the emulsion is retained on the sheet).

Bentonite, also known as "swecting smectic clay", from the montmorillonite family, is well known and does not need to be described here in detail; these compounds, formed of microcrystallites, have sites on the surface with a high cation exchange capacity capable of retaining water (see for example document US-A-4,305,781 and FR-A-2,283,102). Preferably, a semi-sodium bentonite is used, which is introduced just upstream from the headbox, at a rate of 0.1 to 0.5 percent (0.1 to 0.5%) of the dry weight of the fibrous suspension.

In an advantageous embodiment, the powdered branched polyacrylamide is first dissolved in water, then this solution is introduced into the paste castle of the fibrous suspension circuit, at a rate of 0.05 to 0, 5 per thousand (0.05 to 0.5% o) by dry weight of the dry weight of this fibrous suspension, then the mixture is stirred and sheared, and finally we always add with stirring, upstream of the headbox, bentonite at 0.1 to 0.5 percent (0.1 to 0.5%) by weight dry fiber suspension. The branched polymer dissolved beforehand is introduced at a concentration of between 0.1 and 3 g / liter in front of the pulp feed pump in the pulp circuit, preferably in the pulp castle, and the bentonite is introduced just upstream of the headbox.

The manner in which the invention can be implemented and the advantages which ensue therefrom will emerge more clearly from the embodiments which follow.

Preparation of a weakly cationic branched polyacrylamide

• 13 240 kilogrammes d'acrylamide à 30 % dans l'eau;
• 1600 kilogrammes de chlorure de triméthylaminoéthylacrylate (ADAME) à 75 % en solution dans l'eau;
• 50 kilogrammes d'eau et 100 kilos d'acide adipique;
• et 0,129 kilogrammes de méthylènebisacrylamide (MBA) (soit 25 ppm par rapport à la matière active) comme agent ramifiant.

In a reactor, the following are mixed at ambient temperature:

• 13,240 kilograms of 30% acrylamide in water;
• 1600 kg of trimethylaminoethylacrylate chloride (ADAME) 75% in solution in water;
• 50 kilograms of water and 100 kilos of adipic acid;
• and 0.129 kilograms of methylenebisacrylamide (MBA) (ie 25 ppm relative to the active ingredient) as a branching agent.

A solution is obtained whose pH is 3.6 to which is added, still with stirring, one thousand (1000) ppm of catalyst: isobutyronitrile (AZDN) (ie 15 kg).

The solution is cooled to 0 ° C. and degassed by bubbling with nitrogen. A transfer agent (mercaptoethanol) is then added at a rate of ten (10) ppm relative to the feed, (ie 0.15 kg) as a reaction limiter.

Then 4.2 ppm of ammonium persulfate (63 grams) and 0.86 ppm of iron in the form of Mohr salt (6 ppm of Mohr salt, or 90 grams) are added. The exothermic reaction is allowed to continue for approximately one hour, until the temperature of 92 ° C. is reached.

A gel is then obtained which is left to age for two hours, then which is ground, dried with hot air and rewired again, until a particle size of less than one millimeter is obtained.

A perfectly soluble white powder is then obtained up to forty grams per liter (40 g / l) at room temperature, having an insoluble content of less than 0.02 percent (0.02%). This branched polyacrylamide powder has a Brookfield viscosity close to 2.6 cps (UL, 0.1% in a 1 M Nacl solution at 25 ° C at sixty revolutions per minute (60 rpm)).

This polymer has a cationicity of ten (10) mole percent and a cationicity, measured by the colloidal method, lower than the theoretical cationicity. However, after shearing, this cationicity increases, which attests to the branched and non-linear nature of this polymer. Finally, this polymer has an increase in cationicity after shearing of the order of 20%.

As this polymer is highly soluble in water (insoluble <0.01%), it is therefore not crosslinked.

Example 1

In known manner, a paper pulp is prepared comprising 80% of pulp proper at the rate of thirty five percent (35%) of bleached hardwood, ten percent (10%) of broken-coated and thirty five percent (35%) of bleached kraft, and twenty percent (20%) calcium carbonate.

In neutral medium, bonding is done with 2.0% dimeric ketene alkyl.

The fibrous suspension is dissolved in water at the rate of 2.5 grams / liter. The pH of this suspension is 7.5.

650 cm3 of this fibrous suspension are introduced into the bowl of an automated CTP (registered trademark of Center Technique du Papier, Cartons et Celluloses) form. 200 grams / tonne (or 0.2 per thousand) of the weakly cationic branched polyacrylamide previously prepared is then added. We stir for thirty seconds.

Then added 1400 grams / ton (0.14%) of bentonite, of the type sold by the Applicant under the name CP-B1, having a density of 900 kilos / cubic meter, a bulking power of 40 ml / 2g, a cation exchange capacity of 85 meq / 100 g dry, and an average size less than 75 microns. Stirred again for thirty seconds, then drain by vacuum.

The turbidity in white waters is then measured by weighing the dry matter as well as the weight of the dried formed sheet. The mass balance makes it possible to establish the retention figure according to the formula: $$\frac{\text{Sheet weight}}{\text{Leaf weight + dry weight in white waters}}\text{x 100}$$

This gives a retention of 89.5%.

In the same way as above, retention tests are repeated at variable doses of the weakly cationic branched polymer previously prepared according to the method described above. The results are summarized in Table 1 below. Table 1 % o polymer % retention 0.03 75 0.05 80 0.2 89.5 0.3 92 0.4 92 0.5 92.1 0.75 96.1

An improvement in retention is observed directly linked to the amounts of polymer. Excess polymers do not have reverse effects.

Example 2

Example 1 is repeated, replacing the branched polyacrylamide with a linear cationic polyacrylamide of the type described in document FR-B-2 390 983, marketed by the Applicant under the name FO 4190 PG, of viscosity UL 2.9 and in common use for stationery retention. The following results are obtained. % o FO 4190 PG % retention 0.2 70 0.5 78 0.75 75

It is observed that the retention obtained by means of the branched product (example 1) is 18% higher than that obtained with a linear copolyacrylamide with the same cationic charge (example 2).

In addition, with an excess of cationic linear polymer, a reverse effect is quickly observed.

Example 3

By repeating the teachings of document EP-A-0 202 780, a crosslinked emulsion is prepared containing ten mole percent of chloromethylated ADAME with 10 ppm of MBA but without limiter, having a UL viscosity of 2.75.

This polyacrylamide is completely crosslinked, therefore sparingly soluble.

For an equal active ingredient content, this emulsion is used as in Example 2 in the amount of 0.2 per thousand.

We then obtain a retention rate of 40%.

Example 4

The same emulsion is used as in Example 3. It is put in water, then it is sheared by an ULTRATURRAX mixer, sold by the company IKA (Germany), rotating at ten thousand revolutions per minute. After thirty minutes, a maximum increase in cationicity of 35% is then obtained.

At the same dosage, a retention close to 75% is obtained.

In document EP-A-0 202 780, it is explained that crosslinked products such as in Example 3 need to be sheared in order to obtain optimum efficiency. This is confirmed by the tests of Examples 3 and 4. However, the crosslinked product then sheared has a much lower efficiency than a copolymer of the same composition and of the same branched cationicity.

Preparation of a moderately cationic branched polyacrylamide

• 7848 kilogrammes d'acrylamide à 30 % dans l'eau;
• 7000 kilogrammes de chlorure de triméthylaminoéthylacrylate (ADAME) à 75 % en solution dans l'eau ;
• 152 kilogrammes d'acide adipique;
• et 0,380 kilogramme de méthylénebisacrylamide (MBA) (soit 50 ppm par rapport à la matière active) comme agent ramifiant.

In a reactor, the following are mixed at ambient temperature:

• 7848 kilograms of 30% acrylamide in water;
• 7000 kilograms of trimethylaminoethylacrylate chloride (ADAME) at 75% in solution in water;
• 152 kilograms of adipic acid;
• and 0.380 kg of methylenebisacrylamide (MBA) (ie 50 ppm relative to the active ingredient) as a branching agent.

A solution is obtained whose pH is 3.6 to which is added, still with stirring, one thousand (1000) ppm of catalyst: isobutyronitrile (AZDN) (ie 15 kg).

The solution is cooled to 0 ° C. and degassed by bubbling with nitrogen. A transfer agent (mercaptoethanol) is then added at a rate of fifty (50) ppm relative to the feed (ie 0.75 kg) as a reaction limiter.

Then 4.2 ppm of ammonium persulfate (63 grams) and 0.86 ppm of iron in the form of Mohr salt (6 ppm of Mohr salt, or 90 grams) are added. The exothermic reaction is allowed to continue for approximately one hour until the temperature of 92 ° C. is reached.

A gel is then obtained which is left to age for two hours, then which is ground, dried with hot air and rewired again, until a particle size of less than one millimeter is obtained.

A perfectly soluble white powder is then obtained up to forty five grams per liter (45 g / l) at room temperature, having an insolubility rate of less than 0.02 percent (0.02%). This branched polyacrylamide powder has a Brookfield viscosity of 1.8 (UL, 0.1% in a 1M NaCl solution at 25 ° C at sixty revolutions per minute (60 rpm)).

This polymer has a cationicity of forty five (45) mole percent and a cationicity, measured by the colloidal method, lower than the theoretical cationicity. However, after shearing, this cationicity increases, which attests to the branched and non-linear nature of this polymer. Finally, this polymer has an increase in cationicity after shearing of the order of 23%.

As this branched polymer is highly soluble in water, (% insoluble <0.02%), it is therefore not crosslinked.

Example 5

In a known manner, a pulp is prepared comprising thirty percent (30%) of recovered paper, thirty percent (30%) of bleached kraft, twenty percent (20%) of calcium carbonate, ten percent (10%) of broken -bed and ten percent (10%) of bleached hardwood.

This fibrous suspension is dissolved in water at the rate of 2.5 grams / liter. The pH of this suspension is 7.6.

The retention tests are carried out in the same manner as in Example 1 with the branched moderately cationic polymer prepared above, then compared to a linear polyacrylamide of the same cationicity with a viscosity UL of 2.2 sold by the Applicant under the name FO 4550 BPM.

The results are reported in table no. 2 Table 2 FO 4550 BPM% retention BPMR% retention Quantity of polymers in% o 64 78 0.03 72 89 0.05 80 93 0.2 82 95.5 0.3 80 95.3 0.5 79 96.4 0.7

There is a clear advantage of the moderately cationic branched polymer compared to a linear copolyacrylamide with the same cationic charge. The branched product sees its effect start much faster and provides access to very high retention figures.

Example 6

Kaolin retention was studied in a slightly acidic medium. The fibrous composition is 40% refined bleached kraft and 60% bleached hardwood. 20% load is introduced relative to the fibers. Bonding is carried out with reinforced rosin at a rate of 1.3 percent dry; the pH is adjusted to 5 with alumina sulfate.

On this suspension, retention tests were carried out comparatively with the branched powder polyacrylamide used in Example 1 according to the invention, the linear powder polymer of the prior art used in Example 2 (FO 4190 PG); the sheared emulsion polymer also from the prior art used in Example 4.

We obtain the following results: Polymers used % o dry polymer compared to dry paper total retention% Linear powder FO 4190 PG 0.2 78 0.5 87 Branched powder Example 1 0.2 92 0.5 96.5 Sheared crosslinked emulsion Example 4 0.2 78 0.5 82

Example 7

In the same way as described previously, a range of moderately cationic polymers (45 mol%) is prepared with different branching rates as described in Table 3 below. Table 3 Polymer MBA mole / million mole UL cps viscosity AT 25 1.7 B 50 1.8 VS 75 2.1 D 100 2.2

With these polymers, retention tests are carried out on the fibrous suspension as described in Example 5.

The results obtained are as follows: Product insoluble rate% % o of dry paper polymer total retention% AT <0.01 0.2 81 B <0.01 0.2 93 VS <0.01 0.2 95 D <0.01 0.2 97

These results show that the retention effect is greater the greater the branching.

The process according to the invention, which consists, among the polyacrylamides, of having chosen the branched polyacrylamides in powder form in association with bentonite, not only makes it possible to improve the retention rate unexpectedly compared to the other polyacrylamides, therefore the effectiveness of the treatment, but also makes it possible to improve the clarity of the water under canvas and this, without opposite effect. In addition, it makes it possible to successfully process loaded pasta.

Compared to the combination of bentonite and linear polyacrylamide powder, an improvement in the retention rate of around ten to twenty percent (10 to 20%) is thus observed, which results in a consequent reduction in pollution and allows better recirculation of fines and charges in the machine circuit, and better control of these machines. In addition, there are fewer bacterial deposits in the circuit, therefore fewer faults, fewer breaks, fewer holes in the paper.

Compared to mixtures of bentonite and polyacrylamide emulsions, there is less rejection of oil or surfactants, which as already mentioned, affect the properties of the finished paper.

Finally, compared to other powdered polyacrylamides, the use of branched polyacrylamides allows high dissolution rates, avoids over-flocculation, therefore the absence of flocking on the paper, and as already said, the absence of reverse effect in overdose.

Claims (5)

1 / Process for the production of paper or cardboard with improved retention, in which a polyacrylamide and bentonite are added to the fibrous suspension, characterized in that the polyacrylamide is a branched polyacrylamide, easily soluble in water, and is introduced in the form of powder dissolved in the proportion of 0.03 to 1.0 per thousand (0.03 to 1.0% o) by weight of the dry weight of the fibrous suspension. 2 / A method according to claim 1, characterized in that the branched polyacrylamide is a cationic copolymer of acrylamide and of an unsaturated cationic ethylenic monomer, chosen from the group comprising dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl methacrylate (MADAME), quaternized or salified, dimethyldiallylammonium chloride (DADMAC), acrylamide propyltrimethylammonium chloride (APTAC), and methacrylamidopropyltrimethylammonium chloride (MAPTAC), and in that this polyacrylamide is branched by a branching agent consisting of a polyfunctional compound having at least two reactive groups chosen from the group of double bonds, aldehyde bonds or epoxy bonds. 3 / A method according to claim 2, characterized in that the branching agent is methylene bis acrylamide (MBA), introduced in an amount of five to two hundred (5 to 200) moles per million moles of monomers. 4 / Method according to one of claims 1 to 3, characterized in that the bentonite is a semi-sodium bentonite. 5 / Method according to one of claims 1 to 4, characterized in that the branched polyacrylamide powder is first dissolved in water, then in that the solution obtained is then introduced into the dough castle '' a dough circuit at a rate of 0.05 to 0.5% o by dry weight of the dry weight of the fibrous suspension, then in that the mixture is stirred and sheared, and finally, in that one adds , still with stirring, upstream from the headbox, bentonite, at a rate of 0.1 to 0.5 percent (0.1 to 0.5 %) of the dry weight of the fibrous suspension.