EP1135555B1 - Paper making method using a novel retention system comprising a precipitated silica and a cationic polymer - Google Patents

Abstract

The invention concerns a novel method for making paper based on cellulose fibre in sheet form, wherein a novel retention system comprising a suspension of precipitated silica and a cationic polymer is used for improving in particular the retention of the incorporated mineral fillers. The invention also concerns a method for making paper using a retention system which substantially improves draining. More precisely, the invention concerns the use of a precipitated silica and a cationic polymer comprising a cationic guar gum and/or a cationic starch.

Description

The present invention relates to a novel process for the production of paper-based cellulose fiber paper in which a novel retention system comprising a suspension of precipitated silica and a cationic polymer is used to improve, in particular, the retention of the incorporated mineral fillers. . The present invention also relates to a method of making paper using a retention system which substantially improves drainage (or dripping), i.e. the speed with which water flows from the suspension of fibers. More specifically, the invention relates to the use of an aqueous suspension of precipitated silica and a cationic polymer comprising a cationic galactomannan and / or a cationic starch.

In addition, the mechanical properties of the paper obtained according to the process of the invention are improved, for example stiffness and tear resistance as well as other properties such as whiteness. In addition, the retention system according to the invention may have advantages as regards the quality and recyclability of the white water from the papermaking process as well as the papers broken during the manufacturing process.

The manufacture of paper poses many problems. One of the ongoing concerns is to reduce the cost of paper by decreasing the amount of cellulosic fiber in the composition of the pulp. Another approach is to reduce the concentration of aqueous discharges due to increasingly severe environmental constraints.

Paper manufacturers have proposed various ways to reduce the cost of paper and try to improve its properties. One of the approaches used is the addition of inexpensive mineral fillers in the papermaking process to replace the fiber. On the other hand, certain mineral fillers are specifically used to improve certain properties of the paper. Thus, for example, titanium oxide is used in its anatase and / or rutile forms to improve the opacity of the papers, particularly in the case of laminated papers.

Unfortunately, the addition of mineral fillers which are micrometric particles encounters the problem of retention: during the formation of the sheet on the web of the paper machine, the mineral particles tend to pass through this fabric; which generates charged white water circuits. This poses problems in the treatment of rejects but also in the quality of the sheet.

To date, the prior art proposes the use of retention agents to reduce the problem of lack of retention. However, many solutions proposed to date are not economically viable to allow their use for the preparation of any type of paper. Indeed, some retention agents or retention systems are complex and expensive products, which therefore does not allow their use for products of ordinary quality.

US-A-4961825 discloses a binder for use in a papermaking process, which binder comprises, on the one hand, a cationic or amphoteric carbohydrate, and, on the other hand, a colloidal silica sol.

At present, the Applicant has developed a novel papermaking process using a novel retention system which significantly increases the retention of mineral fillers, fibers and other materials in the paper sheet.

Another object of the invention is to provide a retention system and a papermaking process in which the properties of the paper obtained, including, for example, the opacity yield of the mineral fillers, the tear strengths, the whiteness and other necessary properties are improved by optimizing the use of mineral fillers. Of course, optimization is done according to the type of load used.

Another object of the invention is to provide a paper having a high concentration of mineral fillers which has tear resistance and other acceptable characteristics.

Other objects and advantages of the invention will follow on reading the description below and in particular in the tests, tables and figures illustrating various features of the invention.

The present invention is based on the development of a retention system and the papermaking process using it which significantly increases the retention of mineral fillers and other characteristics of the paper and which allows the optimization of the action. mineral fillers present in the pulp.

The increased retention of mineral filler and fines in our papermaking process alleviates the problems of white water contamination.

1. (i) une suspension de silice précipitée,
2. (ii) et un polymère cationique choisi parmi un amidon cationique, un galactomannane cationique et/ou leur mélange.

The present invention therefore relates to a papermaking process by forming and drying an aqueous paper pulp containing cellulosic pulp and mineral fillers into which is incorporated into the pulp before the formation of the sheet a system retention comprising:

1. (i) a suspension of precipitated silica,
2. (ii) and a cationic polymer chosen from a cationic starch, a cationic galactomannan and / or their mixture.

The amount of solids in the gelling system is 0.02 to 50% by weight, preferably 1 to 30% by weight, based on the weight of the paper stock or stock.

The ratio of the cationic polymer / precipitated silica should be between 1 and 10 by weight, and preferably, this ratio is between 2 and 6, the latter depending in particular on the degree of substitution of the precipitated silica and the cationic polymer.

As regards the suspension of precipitated silica, this means a suspension of silica consisting of the filter cake resulting from the precipitation reaction. In other words, the precipitation of the silica is carried out, the reaction medium is filtered and a filter cake is obtained which is washed if necessary. This cake is then disintegrated and thus forms a suspension of precipitated silica.

In general and advantageously, in the context of the present invention, use is made of an aqueous suspension of precipitated silica having a dry matter content of between 10 and 40% by weight, a viscosity of less than 4.10 ^-2 Pa.s for a shear of 50 s ^-1 and the amount of silica contained in the supernatant obtained after centrifugation of said suspension at 7,500 rpm for 30 minutes represents more than 50% of the weight of the silica contained in the suspension. More particularly, this aqueous suspension of precipitated silica has a solids content of between 15 and 35% by weight and, in addition, a viscosity which is less than 2.10 ^-2 Pa.s at a shear of 50 s ^-1 . In this regard, reference is made to the patent application WO 96/01787 of the Applicant concerning this type of silica and the contents of which should be considered as part of the present application.

The precipitated silicas are generally precipitated at a pH around the neutral pH or at a basic pH, the gels being obtained at a pH that is usually acidic or very acidic.

In the case where the cationic polymer is a cationic galactomannan, it is preferably selected from galactomannans comprising at least two vicinal hydroxyl groups, in particular cationic guars. As far as guars are concerned, it has been noticed that their reactive centers are particularly accessible, which makes it possible to use small amounts to reach a satisfactory effect.

When using the retention system with cationic guar as one of the components, mineral fillers are retained to a significant degree in the final product and the paper produced has improved strength compared to paper obtained from a process without a retention system.

The base guar in the cationic guar is of a natural type. Natural guar is extracted from the albumen of certain plant seeds, for example Cyamopsis Tetragonalobus. The guar macromolecule is constituted by a linear main chain constructed from monomeric β-D-mannose sugars linked together by (1-4) bonds, and α-D-galactose side units linked to β-D-mannoses. by links (1-6).

The preparation of cationic guars is known per se. By way of example, the cationic guars are formed by reaction between hydroxyl groups of the polygalactomannan and reactive quaternary ammonium compounds.

The degree of substitution of the cationic groups of the guar is generally at least 0.01 and preferably at least 0.05 and up to 1.0. within the scope of the invention, a suitable range is from 0.08 to 0.5. It is assumed that the molecular weight of guar gum ranges from 100,000 to 1,000,000 and generally is around 220,000.

As commercial products, mention will be made by way of non-limiting examples of the products of the MEYPRO-FLOC 130, MEYO-BOND 9806, MEYO-BOND 109, JAGUAR C-13-S, JAGUAR C-14-S and JAGUAR C-15 series. , JAGUAR C-17, JAGUAR C-162 from MEYHALL and RHÔNE-POULENC Inc., GUAR CAT 10 products from CESALPINIA.

In the case where the cationic polymer is a cationic starch, it is generally chosen from those having a degree of substitution of between approximately 0.1 and 0.05 and, preferably, between approximately 0.02 and 0.04 and more particularly, more than about 0.025 and less than about 0.04. As is well known, a starch is made cationic by substitution with an ammonium group by techniques known per se and may have varying degrees of substitution up to 0.1. For this purpose, a wide variety of ammonium compounds can be used. By way of nonlimiting example, the cationic starch can be prepared by treating the basic starch with 3-chloro-2-hydroxypropyltrimethylammonium chloride or 2,3-ethoxypropyltrimethylammonium chloride. which makes it possible to obtain a cationic starch having a degree of substitution of 0.02 to 0.04.

The basic starch used to prepare the cationic starch may be selected from the group consisting of potato starch, wheat starch, corn starch, barley starch, oat starch, rice starch, starch. tapioca, and their mixture.

Depending on the case and / or the nature (s) of the polymers, these will be formulated as aqueous solutions.

The mineral fillers used in the process are varied in nature and are chosen in particular according to the type of paper manufactured and its future use. The mineral filler material that can be used includes any common inorganic filler whose surface is at least partially anionic in character.

Among the mineral fillers, mention may be made, without limitation, of kaolin, clay, chalk, calcium carbonate, titanium dioxide, silica, and mixtures thereof.

The mineral fillers are normally added in the form of an aqueous dispersion at the appropriate concentrations specific to the type of paper manufactured.

Many commercial products can be used as mineral fillers for papermaking. By way of non-limiting examples, mention may be made of kaolin from ECC, Omyafill calcium carbonate from OMYA and Calopake from RHÔNE-POULENC, titanium dioxide from Antntitan from KEMIRA and Rhoditan from RHÔNE -POULENC.

The possibility of adding mineral fillers to the paper pulp is limited by factors such as the retention of the charges on the fabric, the dewatering of the pulp on the fabric, the wet and dry resistance of the paper obtained.

Now, in accordance with our invention, the problems mentioned above due to the addition of these fillers can be substantially overcome or eliminated by using our retention system which also allows to add proportions of these fillers more higher than the normal to get special properties in the paper produced.

Thus, by using the retention system of the invention, it has become possible to produce paper which contains more fillers while retaining its mechanical properties. In this way, therefore, the mechanical properties of the paper (including the modulus of elasticity, tensile index, tensile energy absorption, etc.) have values equal to or even greater than those previously achieved with papers. obtained from conventional paper stocks in which a retention agent of the prior art is optionally used.

The sheet, after drying, has greatly improved strength characteristics when using the method according to the invention. It has also been found that when mineral fillers such as those mentioned above and the like are used in the paste, these mineral fillers are effectively retained in the sheet and, moreover, do not have a negative effect on the strength of the sheet. in contrast with the sheets obtained by a manufacturing method without gelling system according to the invention.

Although the mechanism that occurs in the mother paste during the formation and drying of the paper in the presence of the retention system is not fully controlled, it is believed that the retention system forms an association with the fibers and with the fibers. fillers to form a complex flocculating matrix.

Indeed, the manufacture of the paper sheet necessarily passes through a dewatering step which can profoundly alter the structure of the colloids and their distribution. The changes in the structure of the aggregates of filler loads affect the retention rate of these and the opacity of the paper obtained. Thus, in the presence of the retention system of the invention, during dewatering, a flocculate is formed within the cellulosic network which traps the charges in order to preserve during this critical stage the properties that the particles possess in suspension.

The components of the retention system are added within the paper making device as a mixture or separately. However, according to a preferred embodiment of the invention, the optimum results are obtained when the retention system based on precipitated silica and cationic polymer is formed in situ in the pulp.

Advantageously, this can be done by firstly adding the cationic polymer (guar and / or starch) in the form of an aqueous solution and adding separately to the pulp of the precipitated aqueous silica solution in a mixing tank or in a point of the device where there is appropriate agitation, so that the two components are dispersed with the paper-forming components and thus act simultaneously with each other and with the paper-forming components.

It has been found that in a papermaking process using the gelling system described in the invention, the pH of the mother paste is not excessively critical and is generally less than 11, and preferably 5 to 9 .

Other chemical additives for the paper can be mixed within the mother paste, such as antifoams, bonding agents, etc ... In this regard, it is important to ensure that the content of these other agents does not interfere with the formation of the gel matrix and that the content of the agent (s) in the recycled white water does not increase too much to interfere with the formation of the gel matrix. Therefore, it will be preferred to add the agent (s) to a point in the system after formation of the gel matrix.

The improvements due to the retention system are observed with an effect of the same order as well with chemical pulps as mechanical and thermomechanical pulps.

From the research and work done, it appears that the principles of the present invention are applicable to the manufacture of any type and quality of paper. For example, writing printing papers, packaging papers, laminated papers.

Among one of the paper type preparation possibilities, the write-printing paper is one of the very positive-yielding ways, i.e., increased charge retention phenomenon and improved paper mechanical qualities. In this case, the majority of the fillers used is calcium carbonate.

The amount of retention system to be used varies depending on the desired effect and the characteristics of the particular components that are selected in the preparation of said system. For example, for a given precipitation silica in a retention system, if it contains cationic guar gum with a DS of 0.3 instead of a DS of 0.7, more retention system will be required. .

The examples below illustrate, without limitation, the advantages and properties related to the paper according to the invention and its manufacturing process.

TESTS

• la rétention : quantité de charges retenues sur la feuille de papier, ou pour la rétention totale, quantité totale de fines particules retenues sur la feuille.
• le drainage : qui caractérise la rapidité avec laquelle l'eau s'écoule de la suspension fibreuse.

Retention performance is measured essentially by two parameters:

• the retention: quantity of charges retained on the sheet of paper, or for the total retention, total quantity of fine particles retained on the sheet.
• drainage: which characterizes the speed with which water flows from the fibrous suspension.

• Méthode "Bol BRITT Jar" : elle permet de mesurer la rétention chimique (totale et charges)
• Méthode "Shopper-Riegler" : elle permet de mesurer la rétention chimique et le drainage
• Méthode via "Tireuse de formette" : elle permet de mesurer la rétention en charges.

These two characteristics can be measured using different methods:

• " BRITT Jar Jar " method: it allows to measure the chemical retention (total and charges)
• "Shopper-Riegler" method : it allows to measure chemical retention and drainage
• Method via "Trainer of form": it makes it possible to measure the retention in loads.

Method called "bowl BRITT"

This method consists of measuring the chemical retention of the charges while avoiding the formation of the fibrous mattress responsible for mechanical retention by filtration effect. In fact, from a 500 ml sample of the dispersion of fibers + fillers + retention system + optionally additives to be tested, kept under agitation, only the first 100 ml are withdrawn through a sieve. By determining the respective quantities of fibers and charges passed into the filtrate, the global retention values (fibers + charges) and the retention retention are calculated.

This method of measuring retention is described by K. Britt and J. E. Unbehend in Research Report 75, 1/10 1981, published by ESPRA Empire State Paper Research Institute, Syracuse, N.Y. 13210, USA.

For the measurements, a filter bowl equipped with a 600 μm opening grid was used.

Method called "Shopper-Riegler"

This Shopper-Riegler method is used according to standard NFQ 50-003.

• extrait sec : 27,08%,
• surface = 221 m²/g,
• taille des particules = 40-70 nm.

The precipitated silica solution used in the examples, hereinafter silica A , has the following characteristics:

• dry extract: 27.08%,
• area = 221 m ² / g,
• particle size = 40-70 nm.

• extrait sec : 9,2% ;
• surface = 494 m²/g,
• taille des particules = 10 nm.

The colloidal silica solution used in the examples, hereinafter silica B, has the following characteristics:

• dry extract: 9.2%;
• area = 494 m ² / g,
• particle size = 10 nm.

Example 1

This example shows the chemical retention obtained with the BRITT Jar test.

i) Preparation of the mother suspension and dilution

The suspensions are prepared by defibrating the cellulose in 500 ml of demineralized water in a Dispermat-type bowl for 10 minutes at 2,000 rpm.

Charges are added to the Dispermat and agitation is maintained at 1,000 rpm for 5 minutes.

The mixture obtained is then transferred to a 10-liter mixer with a stirring blade, then diluted to the concentration of 0.5% fiber, and finally kept under stirring all the time manipulations to ensure, during sampling , a perfect homogeneity.

Then, 500 ml of the suspension mixture is taken. These 500 ml are introduced into the BRITT bowl with stirring with a propeller type blade and provided with a 600 μm grid.

The stirring rate is set according to the desired shear and is stirred for 30 seconds.

The starch is added followed by stirring for 30 seconds. The silica is then introduced (in the tests comprising this product). Then stirred for 30 seconds.

Finally, 100 ml of the mixture is withdrawn by gravity.

Prepared formulations

Formulation A Formulation B Formulation C Paper fibers: 70% hardwood-30% softwood (Shopper-Riegler index = 30 -45 ° SR) 100 shares 100 shares 100 shares Charge: Calcium Carbonate Calopake F 20 parts 20 parts 20 parts Hi CAT 168 cationic starch 0.8% dry / fiber 0.8% dry / fiber 0.8% dry / fiber Precipitated silica A 0.2% dry / fiber / / Colloidal silica B / 0.2% dry / fiber /

ii) Filtration

The 100 ml collected are then filtered on BÜCHNER with WHATMANN No. 42 filters (ash-free filters, previously dried for 1 hour at 105 ° C. and then weighed at ± 0.0001 g).

The filter residue is then carefully removed, dried for 1 hour at 105 ° C., then cooled in a desiccator and weighed (± 0.0001). This allows the calculation of the overall retention rate.

The residue thus dried on the filter is then calcined (according to Standard Ash Rates No. NFQ 03-047) to give the rate of retention of the mixture.

iiii) Calculations and Results

• $$\mathrm{Rétention\hspace{1em}globale}=\frac{\left(\mathrm{P}1-\left(5*\mathrm{P}2\right)\right)}{\mathrm{P}1}*100$$
  
  • P1 = Poids du mélange (charges+fibres) dans le prélèvement initial.
  • P2 = Poids du résidu du prélèvement de 100 ml filtré et séché.
• $$\mathrm{Rétention\hspace{1em}en\hspace{1em}charges}=\frac{\left(\mathrm{P}3-\left(5*\mathrm{P}4\right)\right)}{\mathrm{P}3}*100$$
  
  • P3 = Poids de charges dans le prélèvement initial de 500 ml.
  • P4 = Poids du résidu du prélèvement de 100 ml filtré, séché, calciné.

Global retention and retention under load are calculated using the following formulas:

• $$\mathrm{Overall\; retention}=\frac{\left(\mathrm{P}1-\left(5*\mathrm{P}2\right)\right)}{\mathrm{P}1}*100$$
  
  • P1 = Weight of the mixture (fillers + fibers) in the initial sample.
  • P2 = Weight of the residue of the 100 ml sample filtered and dried.
• $$\mathrm{Retention\; in\; expenses}=\frac{\left(\mathrm{P}3-\left(5*\mathrm{P}4\right)\right)}{\mathrm{P}3}*100$$
  
  • P3 = Weight of loads in the initial sample of 500 ml.
  • P4 = Weight of the residue of the 100 ml sample filtered, dried, calcined.

Results

Retention Agent Stirring speed (rpm) Retention in expenses (wt% / rate of charges introduced) Overall retention (wt% / total weight of solids in suspension) Formulation A 500 79 92 900 71 88 Formulation B 500 75.5 91.5 900 67 86.5 Formulation C 500 61.5 88 900 55 83

Example 2

• Mesure de drainage = temps nécessaire au drainage de 600 ml de solution
• Mesure de rétention globale = filtration des 600 ml et pesée du résidu, selon le même mode opératoire que pour la mesure au BRITT Jar.

This example shows the chemical retention and drainage calculated according to the "Shopper-Riegler" method with the following modifications:

• Drainage measurement = time required to drain 600 ml of solution
• Overall retention measurement = 600 ml filtration and weighing the residue, according to the same procedure as for the BRITT Jar measurement.

i) Formulations used

Formulation A ' Formulation B ' Formulation C ' Paper fibers: 60% eucalyptus - 40% long sulphate fiber (Shopper-Riegler index = 24 ° SR) 100 shares 100 shares 100 shares Charge: Calcium carbonate OMYALITE 90 50 shares 50 shares 50 shares Hi CAT 142 cationic starch 0.5% dry / fiber 0.5% dry / fiber 0.5% dry / fiber Jaguar C-17 cationic guar 0.05% dry / fiber 0.05% dry / fiber 0.05% dry / fiber Precipitated silica A 0.2% dry / fiber Colloidal silica B 0.2% dry / fiber

The suspension is prepared as in Example 1, with stirring for 15 minutes after addition of the starch, and then addition of cationic guar. A stirring time of 15 seconds is observed before the optional addition of silica A or B.

ii) Results

Retention Agent Drainage (s) Overall retention (wt% / total weight of solids in suspension) Formulation A ' 17 95 Formulation B ' 19 94 Formulation C ' 20 83

Example 3

• Mesure de drainage = temps nécessaire au drainage de 400 ml de solution.
• Mesure de rétention globale = mesure de turbidité du filtrat dilué 3 fois. Plus la turbidité est élevée, plus la rétention globale est faible.

This example shows the chemical retention and drainage according to the "Shopper-Riegler" method with the following modifications:

• Drainage measurement = time required to drain 400 ml of solution.
• Overall retention measurement = turbidity measurement of the filtrate diluted 3 times. The higher the turbidity, the lower the overall retention.

i) Formulation used

Charge: CaCO3 15 shares Polyaluminum aluminum Aqualenc 18 0.3% dry / fiber Jaguar C-17 cationic guar 0.9% dry / fiber Precipitated silica A (dry extract: 27.08%, surface area = 221 m ² / g, particle size = 40-70 nm) 0.2% dry / fiber

The suspension is prepared in a manner identical to that of Example 1. In a first step, the product Aqualenc 18 is added. Then, the type A silica is added and, finally, the guar is added under a stirring of 1000 rpm. .

ii) Results

Retention Agent Drainage (s) Overall retention (% turbidity) Aqualenc 18 (?) + Jaguar C17 27 50 Cationic Starch + Jaguar C17 + Silica A 16 35

Example 4

This example gives the charge retention obtained on a shredder.

i) Preparation of the paper pulp

The preparation of the dough is carried out as for example 1. Formulation A " Formulation B " Formulation C " Paper fibers: 70% hardwood - 30% softwood (Shopper-Riegler Index = 30-45 ° SR) 100 shares 100 shares 100 shares Charge: Calcium Carbonate Calopake F 10 and 25 parts 10 and 25 parts 10 and 25 parts Hi CAT 168 cationic starch 0.8% dry / fiber 0.8% dry / fiber 0.8% dry / fiber Precipitated silica A 0.2% dry / fiber Colloidal silica B 0.2% dry / fiber

(ii) Manufacturing of the forms

The dough thus defibrated is introduced into a stirred tank of 10 liters. Then diluted with filtered water to a total volume of 4 liters. Then the diluted dough is stirred for 1 minute and about 500 ml of suspension is taken and placed in a test tube.

Starch is added to this suspension. After stirring for 30 seconds, silica A or silica B is added, depending on the case, and the mixture is stirred again for 30 seconds.

The contents of the test tube are poured into the bowl of the tablet machine. It is mixed by bubbling and then the form is made by drawing under vacuum.

The form is then recovered on a cardboard support and placed in a vacuum dryer. We then weigh the form and readjust the sample volume to obtain a grammage of 60 g / m ² .

iii) Results

Retention in charge (% compared to the initial charge rate) Calopake F charging rate Formulation A Formulation B Formulation C 10 parts 82 81 59 25 shares 73 75 57

Claims (14)

1. A process for making paper by forming and drying an aqueous paper pulp containing cellulose pulp and mineral fillers, wherein a retention system comprising :

   (i) a cationic polymer chosen from a cationic starch, a cationic galactomannan and/or a mixture thereof,

   (ii) and silica,

   is incorporated into the stock pulp before.formation of the sheet,
   characterized in that said silica is a suspension of precipitated silica, consisting of the filter cake obtained from the precipitation reaction.
2. The process as claimed in claim 1, characterized in that the aqueous suspension of precipitated silica has a solids content of between 10 and 40% by weight and a viscosity of less than 4 × 10^-2 Pa.s for a shear of 50 s^-1, and an amount of silica contained in the supernatant obtained after centrifuging said suspension at 7500 rpm for 30 minutes represents more than 50% of the weight of the silica contained in the suspension.
3. The papermaking process as claimed in any one of the preceding claims, characterized in that the cationic polymer is a galactomannan containing at least two vicinal groups.
4. The papermaking process as claimed in the preceding claim, characterized in that the galactomannan has a degree of substitution of at least 0.01 and preferably at least 0.05, and which can be up to 1.0.
5. The papermaking process as claimed in the preceding claim, characterized in that the cationic polymer is a cationic starch with a degree of substitution of between about 0.1 and 0.05 and preferably between about 0.02 and 0.04.
6. The papermaking process as claimed in any one of the preceding claims, characterized in that the mineral fillers are chosen from the group consisting of kaolin, clay, chalk, calcium carbonate, titanium oxide and silica, and a mixture thereof.
7. The process as claimed in any one of the preceding claims, characterized in that the pH of the stock pulp is maintained between 5 and 9.
8. The process as claimed in any one of the preceding claims, characterized in that the amount of solids in the retention system is from 0.02 to 50% by weight, relative to the weight of the stock pulp.
9. The process as claimed in any one of the preceding claims, characterized in that the cationic polymer/precipitated silica ratio must be between 1 and 10 by weight, and this ratio is preferably between 2 and 6, depending on the degree of substitution of the precipitated silica and of the cationic polymer.
10. The process as claimed in any one of the preceding claims, characterized in that the retention system, based on precipitated silica and cationic polymer, is formed in situ in the paper pulp.
11. The papermaking process as claimed in the preceding claim, characterized in that the retention system is formed in situ by adding, in a first stage, cationic polymer in the form of an aqueous solution and adding, in a second stage, aqueous precipitated silica solution to the paper pulp in a mixing tank or at a point in the device at which there is suitable stirring.
12. A paper which can be obtained from the process as claimed in any one of the preceding claims.
13. The use of the paper as claimed in claim 12 as laminated paper, writing printing paper or wrapping paper.
14. The use of the paper as claimed in claim 13 as writing printing paper.