GB2094323A - A process for the production of a cationic starch derivative - Google Patents

Abstract

In a process for the production of cationic starch derivatives by reacting starch during and immediately after cooking with a quaternary ammonium salt containing epoxide groups, the alkaline starch paste with the derivatisation reagent added thereto is transported in aqueous medium from the cooking zone into a reaction by transportation means such as a pump at a temperature above 100 DEG C, at a high pH-value (e.g. 9 to 13) and at a starch concentration of more than 15% by weight. The reagent used is preferably an epoxy alkyl trialkyl ammonium salt.

Description

SPECIFICATION A process for the production of a cationic starch derivative This invention relates to a process for the production of a cationic starch derivatives by reacting starch during and immediately after the cooking phase with a quaternary ammonium salt containing epoxide groups.

Cationic starch derivatives are widely used in the paper industry because they afford major advantages in paper making by virtue of their favourable self-retention, retention and dewatering accelerating properties.

Even today, standard commercial cationic starches are still primarily produced in batches by an elaborate dispersion process. In this process, native starch is made into a 30 to 50% suspension in water, followed by the addition of an alkaline catalyst and a cationisation reagent. After heating to a suitable reaction temperature, for example, in the range from 40 to 500 C, at which it still does not swell, the starch is left to react for between 10 and 30 hours. In this known process, it is always important to ensure that the geiatinisation point of the starch is not reached because, in that case, further treatment of the batch would be impossible on account of the enormous increase in viscosity involved.The position of the gelatinisation point depends upon several factors such as, for example, the type of starch, the temperature, the quantity of alkali, the addition of foreign salts, the degree of conversion, etc. On completion of the reaction, the product is acidified, separated, washed and dried.

In order to reduce the number of operations involved in the known process referred to above, attempts have also been made to carry out the reaction with a quaternary ammonium salt containing epoxide groups during the - in any case -- unnecessary cooking operation, for example in a jet cooker.

However, it has not hitherto been possible to develop a satisfactory process for reacting starch during and immediately after the boiling phase.

Thus, according to British Patent No.1,333,315 for example, the reaction is carried out in a jet cooker with a starch concentration of less than 1 5%. However, the results obtained by this process are unsatisfactory because, on account of the low starch concentration, most of the reagent is hydrolysed, with the result that few cationic substituents are bound to the starch.

According to European Patent Application No. 4774, temperatures below 1000C are said to be applied, although this necessitates long reaction times of up to 2 hours. Reaction times as long as these are impracticable for continuously carrying out the cationisation process, for example in a paper making factory.

For this reason, it was proposed in British Patent No. 1,504,876 to carry out the cationisation process in a jet cooker at a starch concentration of more than 15%. The viscosity and retrogradation problems which this involves are said to be eliminated by the addition of oxidising agents which, by degrading the starch, simultaneously reduce the viscosity and tendency towards retrogradation of the starch. Unfortunately, this process is attended by the serious disadvantage that the cationic starch derivatives thus degraded are no longer suitable for universal application and, above all, are unsuitable for the in-pulp sizing of paper.

According to British Patent No. 1,505,243, the viscosity and retrogradation problems involved in boiling starch dispersions containing more than 1 5% of starch are solved by working at a higher steam pressure and with a large excess of steam in the cooking zone. However, this process is attended by the disadvantage that in general, sufficiently superheated steam is generally not available in paper making factories, in addition to which the large excess of steam required makes the process unfavourable from the energy point of view.

Accordingly, an object of the present invention is to provide a process for the production of cationic starch derivatives in a jet cooker which, with the assistance of technically simple means, gives technically satisfactory and useful cationic starch ethers for a low energy demand and a high yield of chemicals.

It has now surprisingly been found that the partial retrogradation of the starch which is described in British Patent Nos. 1,504,876 and 1,505,243 does not occur provided that the alkaline starch paste with the cationisation reagent added thereto is delivered by transporting means from the cooking zone into a reaction zone at temperatures above 1000C, at a high pH-value and with a starch concentration of more than 15%. This measure also eliminates the need to use excess steam in the cooking zone.

Accordingly, the present invention provides a process for the production of a cationic starch derivative which comprises reacting starch during and immediately after cooking with a quaternary ammonium salt containing epoxide groups, the alkaline starch paste with the derivatisation reagent added thereto being transported in aqueous medium from the cooking zone into a reaction zone by transporting means at a temperature above 1000C, at a high pH-value and with a starch concentration of more than 1 5%, by weight.

According to the invention, the reaction of the starch with a quaternary ammonium salt containing epoxide groups during and immediately after the cooking phase is carried out in aqueous medium having a solids content of more than 1 5% by boiling the alkalised starch suspension with the ammonium salt added thereto in a jet cooker, delivering the starch paste formed by transporting means into a reaction zone and then optionally diluting it in known manner before it is used for the purpose envisaged.

The process according to the invention has the particular advantage that it may be carried out with ordinary superheated steam which is always available in papermaking factories and that it operates favourably in terms of energy by avoiding an excess of steam. In addition, the use of solids concentrations of more than 1 5% provides for high conversions of 50 to 75% (Yield of ammonium salt reacted, based on the ammonium salt used). The process according to the invention may even be effectively carried out at concentrations of more than 30%. The solutions of cationic starch derivatives thus produced are at least equivalent and, in some cases, even superior in their effectiveness to solutions of standard commercial cationic starch derivatives.

Quaternary ammonium salts containing epoxide groups and corresponding to the following formula are used for the cationisation process according to the invention:

In the above formula, m is an integer of from 1 to 3, Rt, R2 and R3 represent alkyl, cycloalkyl or aryl radicals, two of the radicals R, to R3 may form an alkylidene radical, n is the number 1 or 2 and Xis the anion of a strong to medium-strength inorganic or organic acid.

It is of particular advantage to use an epoxy propyl trialkyl ammonium salt in which R1, R2 and R3 together contain no more than 10 carbon atoms.

It is also possible with advantage to use the stable chlorohydrin analogs of the epoxides which may be converted into the epoxide immediately before the reaction with a strong base.

The temperature prevailing in the cooking compartment of the jet cooker is best in the range from 100 to 1 500C which corresponds to a pressure of from about 1 to 6 bars (gauge). It is of advantage to work at a temperature in the range from 100 to 1 300C.

The ammonium salt is used in quantities of up to 20% by weight and preferably in quantities of up to about 1 0% by weight, so that cationic starch derivatives having a degree of substitution of at most 0.1 and, more advantageously, of up to 0.05 are formed.

The strong bases used for alkalising the starch paste are hydroxides of the alkaline-earth or alkali metals, preferably NaOH, KOH or Ca(OH)2. The pH-value should be in the range from 9 to 13 and, more particularly, in the range from 10 to 1 2.

A pump is best used for transport between the jet boiler and the reaction zone. Gear pumps, eccentric screw pumps and screw pumps are particularly suitable.

The dimensions of the reaction zone are such that residence times of from a few seconds to a few minutes are reached. Preferred residence times are from about 30 seconds to at most 5 minutes.

Any standard commercial native, degraded and other modified starches may be used as the starch. Native starches, for example potato starch, cornstarch, wheat starch, tapioca starch, rice starch or sago starch, are advantageously used.

The invention is illustrated by the following Examples.

General Test Procedure Native starch is continuously introduced by means of a metering screw into an approximately 200 litre capacity vessel equipped with a stirrer. A starch suspension having a dry matter content of from 15 to 35% is prepared by the simultaneous, controlled addition of water with the stirrer switched on.

At the same time, an aqueous solution of the cationisation reagent is introduced into the vessel by means of a metering pump in a quantity of from 2 to 6% of solids, based on starch (expressed as dry matter).

The pH-value is adjusted to between 1 0 and 1 3 by the continuous addition of a 10% aqueous sodium hydroxide solution.

The suspension thus prepared was delivered by means of an eccentric screw pump into a cooking zone where it is cooked by the direct introduction of steam at temperatures in the range from 90 to 1500C.

A second pump takes up the cooked starch paste and transports it into a reaction zone of which the dimensions are such that the residence time amounts to between 30 seconds and 5 minutes.

The starch solution thus obtained is adjusted to the required temperature and concentration in known manner by subsequent dilution with water.

The starch solution then has the required properties and may be directly used as such.

The degree of substitution of the cationic starch or rather the percentaye nitrogen content of the cationic starch was determined by Kjeldahl analysis as the most important criterion of its quality. To this end, the starch is flocculated with alcohol, washed and subjected to nitrogen determination.

EXAMPLE 1 Cationisation reagent: 3-chloro-2-hydroxypropyl trimethyl ammonium chloride Starch: native potato starch Concentration of the starch in the suspension vessel: 30% pH-value in the suspension vessel: 12.5 Temperature in the suspension vessel: 200C Cooking tube temperature: 11 oac Residence time in the reaction zone: 90 seconds Nitrogen content (based on starch) in the suspension: 0.248% in the paste (chemically bound): 0.181n e Yield (based on the cationisation reagent used): approximately 75% EXAMPLE 2 Cationisation reagent: 3-chloro-2-hydroxypropyl trimethyl ammonium chloride Starch: native potato starch Concentration of the starch in the suspension vessel: 20% pH-value in the suspension vessel: 12.0 Cooking tube temperature: 1 1 OOC Residence time in the reaction zone: 60 seconds Nitrogen content (based on starch) in the suspension: 0.268% in the paste (chemically bound): 0.1 63% Yield (based on the cationisation reagent used): approximately 63% EXAMPLE 3 Cationisation reagent: glycidyl trimethyl ammonium chloride Starch: native cornstarch Concentration of the starch in the suspension vessel: 20% pH-value in the suspension vessel: 12.5 Cooking tube temperature: 1 400C Residence time in the reaction zone: 120 seconds Nitrogen content (based on starch) in the suspension: 0.310% in the paste (chemically bound): 0.199% Yield (based on the cationisation reagent used): approximately 66%

Claims (12)

1. A process for the production of a cationic starch derivative which comprises reacting starch during and immediately after cooking with a quaternary ammonium salt containing epoxide groups, the alkaline starch paste with the derivatisation reagent added thereto being transported in aqueous medium from the cooking zone into a reaction zone by transporting means at a temperature above 1000C, at a high pH-value and with a starch concentration of more than 1 5% by weight

2. A process as claimed in Claim 1, wherein a pump is used as the transporting means.

3. A process as claimed in Claim 1 or 2, wherein an epoxy alkyl trialkyl ammonium salt is used as the derivatisation reagent.

4. A process as claimed in any of Claims 1 to 3, carried out at a temperature in the range from 100 to 1500C.

5. A process as claimed in Claim 4, wherein the temperature is from 110 to 1 300C.

6. A process as claimed in any of Claims 1 to 5, carried out at a pH value of from 9 to 1 3.

7. A process as claimed in any of Claims 1 to 5 carried out at a pH-value of from 10 to 12.

8. A process as claimed in any of Claims 1 to 7, wherein the reaction time amounts to from a few seconds to a few minutes.

9. A process as claimed in claim 8 wherein the reaction time is from 30 seconds to 5 minutes.

10. A process as claimed in any of claims 1 to 9, wherein the cationic starch ether produced has a degree of substitution of up to 0.1.

11. A process as claimed in Claim 10 wherein the degree of substitution is up to 0.05.

1 2. A process for the production of a cationic starch derivative substantially as described with particular reference to the Examples.

1 3. A cationic starch derivative when produced by a process as claimed in any of Claims 1 to

1 2.