DE69308035T2 - METHOD FOR PRODUCING A HYDROPHOBIC SIZE - Google Patents

Abstract

The present invention relates to a process for the preparation of hydrophobic sizing agent dispersions used in the paper and board industry. The invention is both a simple and an economical way of preparing a hydrophobic sizing agent dispersion either in the premises of a sizing agent supplier or a paper maker. The invention is based on a usage of a so-called high-pressure digester in the preparation of a hydrophobic sizing agent dispersion.

Description

Hydrophobic sizing involves making paper or cardboard more or less water-repellent. This property can be measured, for example, by the so-called Cobb test, in which the penetration of water into paper or cardboard is measured as a function of time. The result is given as absorption g/m². In a suitably hydrophobically sized product, the value is generally < 25 g/m², whereas in a product that has not been treated with sizing agents, there is an increase in hydrophobicity, whereby the value can be > 100 g/m².

Conventional compounds used as hydrophobic sizing agents include wood-based resin acids and their derivatives.

Nowadays, the paper and cardboard industry has increasingly shifted to using so-called neutral sizing agents as hydrophobic sizing agents, which are generally alkyl ketene dimer (AKD) or alkenyl succinic anhydride (ASA)-based compounds.

Hydrophobic sizing agents are generally supplied to paper and board mills as a water dispersion in which the active agent (AKD, ASA, resin) content of the product is within the range of 5 to 50%. If an ASA type sizing agent is involved, the sizing agent dispersion is first prepared at a paper or board mill.

The hydrophobic sizing agents are added to a pulp at the paper or board machine at the initial stage of the manufacturing process. In certain cases, these sizing agents can also be added at a later stage of the manufacturing process, e.g. at the size press. The amounts of hydrophobic sizing agents used are generally 0.05 to 2.0% of the dry weight of the paper or board.

In addition to the active agent (AKD, ASA or wood resin), a hydrophobic sizing agent dispersion contains stabilizers, surfactants and other additional chemicals. As a stabilizer, e.g. cationic starch, cationic polyethyleneimines, cationic polyacrylamides, gelatin or casein can be used.

Sodium lignosulfonate or alkylsulfonates are used as surfactants.

Typical additional chemicals include biocides, alum and pH adjusting chemicals.

A conventional way of preparing a hydrophobic sizing agent dispersion from these components is to prepare a homogeneous premix containing, in an appropriate ratio, water, an active agent, a stabilizer, surfactants and other potential additional chemicals by mixing and heating in a reactor.

After obtaining the homogeneous premix, it is generally circulated several times using a high-pressure homogenizer, a colloid mill or a high-speed centrifugal pump until a sufficiently small particle size is obtained (generally 0.1 to 10 µm). The pressure varies within the range of 50 to 500 bar and the temperature within 50 to 100ºC. The resulting dispersion is then fed into a product container via a heat exchanger. In the heat exchanger, the temperature of the dispersion is reduced to < 30ºC.

Surprisingly, it has now been found that a high-pressure digester or pulp cooker, a steam jet digester or pulp cooker, as designed primarily for the cooking of starch, acts as an extremely good dispersing device in the production of a hydrophobic sizing agent dispersion.

The invention is explained in more detail with reference to the accompanying drawing. A steam jet pulp digester process works as follows:

A premix is prepared in a reactor 1 according to the same principle as in the above-mentioned conventional process. Subsequently, the mixture is pumped into a digester section 2 itself, in which the mixture contacts a high pressure steam flow 3 and increases the temperature and pressure (the pressure generally to 2 to 15 bar and the temperature to 100 to 200ºC). The dispersion formed is then transferred to a tubular reactor 4, after which the pressure and temperature are reduced in an expander device 5 and the excess steam can be separated by means of a steam condenser 6. The homogeneous product formed is then transferred via a heat exchanger to a product vessel.

This procedure differs from a conventional procedure as follows:

Conventional method:

Pressure 50 to 500 bar

Temperature 50 to 100ºC

Steam jet digester process:

Pressure 2 to 5 bar

Temperature 100 to 200ºC

The preparation of a hydrophobic sizing agent dispersion in a steam jet digester type process allows high solids contents to be used during processing and since the residence time in the digester and reactor tubes is extremely short, usually < 2 minutes, it enables very small units to be used with a large production capacity.

example 1

A stearin/palmitin alkyl ketene dimer (AKD) premix is prepared on a laboratory scale with the following composition:

Water 800g

AKD 200 g

Orotan SN* 3.0 g

Raisamyl 150E** 22 g

* Naphthalene formaldehyde sulphonate condensate, sodium salt

** Cationic potato starch (DS = 0.05).

The starch was boiled before addition.

The premix was separated into two portions, one of which was passed three times through the high pressure laboratory homogenizer (Rannie). The pressure was 200 bar and the temperature was 70ºC. The other portion was passed once through a laboratory scale steam jet digester, the temperature was 120ºC and the pressure was 3 bar. Both emulsions were diluted to a solids content of 12.5% and cooled to room temperature.

The following properties were measured of the dispersions: Viscosity (Brookfield) and average particle size.

High pressure homogenizer/steam jet pulp cooker

Viscosity 10 cP 8 cP

average particle size 0.56 µm 0.60 µm

The results show that the dispersions are almost identical in their properties.

The dispersions were kept at room temperature for one month and the same measurements were repeated, obtaining the following results:

High pressure homogenizer/steam jet pulp cooker

Viscosity 15 cP 16 cP

average particle size 0.86 µm 0.95 µm

The results show that the storage stabilities of the two dispersions are of the same order of magnitude.

Example 2

An alkenylsuccinic anhydride* (ASA) premix with the following composition was prepared on a laboratory scale:

Water 900g

ASA 50 g

Orotan SN 0.5 g

Raisamyl 150E 100 g

* Anhydrides derived from straight-chain C₁₃-C₂₀-olefins were used as alkylenylsuccinic anhydride.

The premix was also divided into two portions, one of which passed through a high pressure homogenizer (pressure 200 bar, temperature 50ºC) and the other portion through a steam jet digester, the temperature being 110ºC and the pressure 1.5 bar.

The prepared dispersions were diluted to a solids content of 7.5% and their viscosity and average particle size were measured.

High pressure homogenizer/steam jet pulp cooker

Viscosity 600 cP 520 cP

average particle size 0.50 µm 0.65 µm

The results show that the dispersions are almost identical.

A value of < 1 µm is considered to be a generally acceptable particle size for products of this type.

Claims (1)

Process for producing a hydrophobic sizing agent, in particular for a papermaking process, in which sizing agent base components consisting of alkenyl succinic acid (ASA), alkyl ketene dimer (AKD) or resin and additives preferably added to this component are prepared by heating and mixing a sizing agent premix and this premix is subjected to a thermal and mechanical treatment under pressure to obtain a sizing agent dispersion, characterized in that the pressure treatment is carried out at a pressure of 2 to 15 bar and a temperature of 100 to 200°C for a period of less than 2 minutes, the sizing agent premix being subjected to high-pressure steam guided at a high speed.