Process for the sizing of paper

Description

[0001] The present invention relates to a process for the sizing of paper with rosin and an aluminium compound in the presence of CaCO₃.

[0002] Paper is sized to make it water repellent so that it can be written or printed on with ink without the ink spreading in the paper. Sizing usually consists in applying a layer of rosin on the cellu­lose fibers in the paper pulp and this in the presence of an alumi­nium compound that acts as a source of positively charged ions; the presence of these positively charged ions is required, as both the cellulose fibers and the rosin are negatively charged and therefore repulse one another. By "rosin" is meant hereabove and hereafter the sizing agent that is obtained from softwood trees. Although rosin is the most widely used sizing material, there are applications where it is not comple­tely satisfactory and where it has been replaced by a group of che­mical compounds, which are best described as "reactive sizes" and often called "synthetic sizes", in contrast to rosin sizes consi­dered to be natural products. Synthetic sizes do not require the presence of an aluminium com­pound. However, their reaction with the fibers in the paper pulp can be relatively slow, compared to the essentially instantaneous interaction between rosin, aluminium compound and fiber. More details about the nature of rosin sizes and synthetic sizes and their use in the paper industry can be found in "Pulp and Paper. Chemistry and Chemical Technology. Third Edition. Volume III", 1981 by James P. Casey (John Wiley & Sons, Inc.), which book is incorporated herein by reference.

[0003] CaCO₃ (chalk) can have been included in different ways into a paper pulp to be sized. It can have been added as filler to in­crease the opacity of the paper. It can also have been added as constituent of paper waste to be recycled and containing CaCO₃ as filler or as surface pigment. Moreover, it can have been added partly as filler and partly as constituent of paper waste to be recycled. When CaCO₃ comes into the paper pulp as constituent of paper waste to be recycled, the CaCO₃-content of the paper pulp can sof course strongly vary in function of the time.

[0004] A process for the sizing of paper with rosin and an aluminium compound is known through the article "Zur Problematik des Alumi­niumsulfats in Kalziumkarbonathaltigen Systemen. - Neutralleimung" by R. Thummer and K. Schubert, Wochenblatt für Papierfabrikation, 1979, 107(17), p. 645-649. In this known process aluoiniuo sulfate is used as aluminium compound and the CaCO₃ is present in the paper pulp because CaCO₃-filled paper waste is recycled. Three modes of operation of this process were tested. The first mode of operation consisted in operating in a completely traditional way, namely at a pH of 4.5-5.5. Large quantities of aluminium sulfate had to be used. Moreover, one had to cope with formation of gypsum precipitates and basic aluminium sulfates, which are hard to dissolve, both having repercussions on the quality of the paper. The second mode of operation consisted in sizing at a pH of 6.0-6.7 (pseudoneutral). A satisfying sizing was obtained in this pH-range although the traditional sizing theory prescribes a pH of 4.5-5.5. The above-mentioned problems decreased, but new problems rose. The quantity of aluminium sulfate required to maintain the pH between 6.0 and 6.7 fluctuated strongly, because the CaCO₃-content in the waste was not constant. Sometimes no aluminium sulfate at all had to be added, so that, accordingly, certain parts of the paper were not sized. In the third mode of operation one operated under constant addition of aluminium sulfate to make sure that the paper is always sized. Thereby, however, important fluctuations occurred in the pH (from 4 to 7), giving rise to important fluctuations in the retention of the pulp compounds in the paper and hence in the paper quality. An optimal retention was obtained at pH=6-7. So, this known process gives no satisfying result.

[0005] The aim of the present invention is to provide a process as de­fined before, which avoids the drawbacks of the before-mentioned known process.

[0006] In the process of the invention a poly(aluminium hydroxysul­fate) with an OH/Al-ratio of 1-2.5 is used as aluminium compound.

[0007] Indeed, it was found that, when such an aluminium compound is used in the sizing of paper with rosin in the presence of CaCO₃, on the one hand, a sufficient quantity of aluminium can be used to obtain a good sizing without formation of gypsum and, on the other hand, a pH of 6-7.1 is generated, even when the CaCO₃-content of the pulp fluctuates strongly, whereby an optimal retention is guaran­teed.

[0008] Here, the following should be noted. EP-A-0062015 deals with the sizing of paper with rosin and poly (aluminium hydroxysulfate). However, this document does not disclose the presence of CaCO₃ in the pulp to be sized. EP-A-0104904 discloses compositions comprising mineral particles, e.g. calcium carbonate, in suspension for use in papermaking (incor­poration of filler in the paper pulp) and in sewage treatment (floc­culation/coagulation). It is recommended to add to said composi­tions a known flocculant such as aluminium sulfate, but only when they are to be used for sewage treatment. The above-mentioned James P. Casey reference states on page 1562 that "Papermakers alum is supplied in the form of acid or basic alum. Acid alum contains an excess of sulfuric acid beyond the amount required by the formula Al₂(SO₄)₃, and basic alum contains an excess of Al₂O₃"; the same reference states on page 1564 that "Sizing of stock filled with calcium carbonate presents considerable problems with a rosin size-alum system".

[0009] In the process of the invention the OH/Al-ratio of the poly(aluminium hydroxysulfate) should range between 1 and 2.5. If this ratio is less than 1, then there is risk of formation of gypsum and the pH of the pulp can fall below 6. If the ratio exceeds 2.5, then the poly(aluminium hydroxysulfate) is unstable. Very good results are obtained as from an OH/Al-ratio of 1.4. However, results no longer improve when the OH/Al-ratio exceeds 1.6; the process just becomes more expensive as the cost price of the poly(aluminium hydroxysulfate) then increases. Therefore preference is given to an OH/Al-ratio of 1.4-1.6.

[0010] In the process of the invention the different known types of rosin can be used : unsaponified, saponified and cationic.

[0011] The quantity of poly(aluminium hydroxysulfate) to be used depends of course on the quantity of rosin used, which on its turn depends on a number of factors such as the desired degree of sizing, the type and the grinding grade of the raw materials.

[0012] Poly(aluminium hydroxysulfate) for use in the process of the invention can be made according to the processes described in EP-B-0062015 and EP-A-0110846.

[0013] The process of the invention is illustrated by the following example. In this example the following starting products are used :
- An industrial pulp with a grinding degree of 36°SR and composed of 1/3 pine wood, 1/3 leaf wood, 1/3 CTMP Lijng 230/80 (a semiche­mical and semithermomechanical pulp), 5 % kaolin filler and 2 % Omyalite (trade name for CaCO₃);
- Dynacoll VS 50, which is the trade name for a saponified rosin size with a solids content of 50 % (a product of Akzo);
- An aqueous solution of poly(aluminium hydroxysulfate) with an OH/Al ratio of 1.5 and an Al-content of 1.6 mol/l, which will be called further "PAS".
The pulp is diluted with water to a solids content of 0.5 %, where­after Dynacoll VS 50, diluted itself to a solids content of 4.25 %, is added thereto in such an amount that 4 g of dry rosin are intro­duced per 100 g of solids contained in the pulp. The pulp is then divided into four portions. To the 1^st pulp portion 0.075 ml PAS is added per g of solids contained in said portion, to the 2^nd portion 0.15 ml, to the 3^rd portion 0.225 ml and to the 4th portion 0.30 ml. Then the pH of each pulp is measured, whereafter sheets are made from each pulp. The sheets are kept for a week at 23°C and 50 % relative humidity and then subjected to
- the Cobb test : g of water absorbed per m² of paper in 60 sec.
- the drop test (DT) : time in minutes required for absorbing a drop of water.
- the edge absorption test : height of rise in mm of a coloured liquid, wherein the edge of the sheet is held for 30 minutes.
Finally, the pH of the sheets is measured. The results are as follows

Pulp	PAS addition ml/g	pH pulp	pH paper	Cobb₆₀ g H₂O/m²	DT min	Edge absorption mm
1	0.075	6.9-7.1	7.1	28.2	71	1
2	0.15	7.1	7.1	22.5	89	1
3	0.225	6.7	7.1	24.0	130	1
4	0.30	6.6-7.1	6.8	23.9	87	1

[0014] These results show that the process of the present invention yields an excellent sizing and that the pH of the pulp remains in the range 6.1-7.1 notwithstanding the substantial variation of the ratio between PAS and CaCO₃.

Claims

1. A process for the sizing of paper with rosin and an aluminium compound in the presence of CaCO₃, characterized in that a poly(aluminium hydroxysulfate) with an OH/Al-ratio of 1-2.5 is used as aluminium compound.

2. A process according to claim 1, characterized in that the OH/Al-­ratio of the poly(aluminium hydroxysulfate) ranges from 1.4 to 1.6.