GB2174386A - Iron tannate production - Google Patents

Abstract

Iron tannate is produced by reaction of an iron (II) salt with a naturally occurring tanning extract in the presence of a remover of oxonium ions, such as an alkali metal acetate. The tanning extract used is preferably the aqueous phase obtained by chilling and centrifuging a natural extract, which aqueous phase has a defined degree of purity. This aqueous phase may be spray dried and then reconstituted for use. Alternatively the natural tanning extract may be used if an elevated temperature, from 60-90 DEG C is used for the reaction. Preferred iron (II) salt and tanning extract are, respectively, ferrous sulphate and mimosa extract.

Description

SPECIFICATION Iron complexes of tanning extracts This invention relates to the production of iron complexes of extracts of natural tannins, in particular mimosa extracts (Acacia mearnsii).

The invention also relates to the use of such iron complexes in anti-corrosive non-aqueous film-forming compositions.

The production of compounds of polyvalent metals with naturally occurring tanning extracts and their use in anti-corrosive non-aqueous film-forming compositions has previously been described, for example in U.K. Specification 860,277.

Particular emphasis is placed in that specification on the production and use of lead tannate. In a specific example mimosa extract solids dissolved in water are reacted with lead acetate to give a lead tannate pigment which is separated and dried. This is the pigment which is then incorporated into a non-aqueous system to form priming paint.

The use of lead compounds has come under scrutiny in recent years because of the effect of lead on the environment. Therefore the use of lead tannate in primer paints is really no longer acceptable.

We have been seeking alternatives to lead tannate and have in particular been working in the field of iron tannate. We have found that there are considerable difficulties in making an iron tannate which has uniform properties and which is suitable as an anti-corrosive agent in a primer paint. We have now found processes by which iron tannate pigments having anticorrosive properties may be produced in a reproducible manner.

The invention therefore in its broad aspect relates to an improved process for the production of iron tannate and its use as an anticorrosive agent in primer paint.

In the production of iron tannate, a naturally occurring tannin extract, in particular an extract of mimosa may be reacted with an iron (II) salt under oxidative conditions in the presence of an oxonium ion remover, for example an alkali metal acetate. The reaction involves both oxidation and chelation, the iron being oxidised from the iron (II) to the iron (III) state and the iron (III) being chelated by the extract.

We have found that the prior treatment of the extract which is used and the conditions under which it is used are most important in producing an iron tannate which is of uniform and reproducible quality and which can be used as an anti-corrosive agent in a primer paint.

Extracts of naturally occurring tannins, such as mimosa are products of commerce and are well known. These extracts are of indefinite composition and contain a number of high molecular weight colloidal-type materials. If the mimosa extract of commerce is used in the production of iron tannate then the iron tannate which is obtained is not of a quality or uniformity required for its use as an anti-corrosive agent in primer paints. Thus if one merely uses the mimosa extract then the process is not really reproducible and leads to non-uniform results.

We have found according to the invention that if the aqueous extract of natural mimosa is chilled to 0"-5"C, and is then centrifuged a purified aqueous phase is produced and unwanted solid materials are removed; then this purified extract can be used in the production of iron tannate to give a material which is both of uniform properties and consistently usable as an anti-corrosive agent in anti-corrosive primer paints.

The centrifuging of the mimosa extract is carried out on the chilled natural mimosa extract and is carried to the point where the aqueous phase has a certain clarity which can be judged by a simple test described later.

According to one aspect of the invention therefore there is provided a process for the production of iron tannate wherein an iron (II) salt is reacted with a naturally-occurring tanning extract in the presence of a remover of oxonium ions in which the tanning extract used is the aqueous phase obtained by chilling and centrifuging of a natural extract, optionally with spray drying and reconstitution of the said aqueous phase having a defined degree of purity.

The iron (II) salt used in the reaction is preferably ferrous sulphate. The oxonium ion remover used in the reaction is preferably an alkali metal acetate, such as sodium acetate.

The tanning extract is preferably mimosa extract.

Thus in the preferred case a solution of extract of natural mimosa is prepared. This solution is centrifuged after chilling to produce a purified aqueous phase and an unwanted solid fraction. The purity of the aqueous phase is judged by reference to its clarity. The aqueous phase meeting this test of purity is separated and this is then reacted under oxidative conditions with the iron (II) sulphate in the presence of the sodium acetate to precipitate an iron tannate of reproducible composition, which is then dried, and which is then suitable for incorporation in a primer paint having anti-corrosive properties.

It may be convenient not to use the aqueous phase prepared by centrifuging immediately it is prepared because the preparation of the iron tannate may be carried out at a different location. Under these circumstances the Iqiuid extract can be spray dried and can be redissolved when required for use. This saves the difficulties associated with the transport of liquids.

The reaction of the purified mimosa extract with the iron (II) sulphate and in the presence of sodium acetate under oxidative conditions may be carried out at approximately room temperature or at slightly elevated temperature.

However we have found that it may well be advantageous to carry out the whole reaction at a temperature which is markedly in excess of room temperature. Thus, the precipitation and filtration of the iron tannate may be effected at a temperature within the range of from 60 -90 C, a particularly preferred temperature being within the range of 70"-75"C.

Under these conditions it is indeed not absolutely necessary to use as the tanning extract the purified aqueous phase of a centrifuging process as described above. Thus, when one is using elevated temperatures for precipitation, then the natural mimosa extract can be used directly without prior chilling and centrifuging.

This is another aspect of the present invention and according to this aspect of the invention there is provided a process for the preparation of iron tannate in which a naturally occurring tanning extract and an iron (II) salt are reacted in the presence of an oxonium ion remover, in which the reaction temperature is maintained throughout the reaction at a temperature within the range of 60 to 90"C, preferably within the range of 70"-75"C.

In this hot precipitation process one may also use an extract which has been subjected to centrifuging to recover an aqueous phase of a certain purity as described above and thus in this case two features of the present invention are combined.

We have found when carrying out the present invention that not all the tannins present in the tanning extract are indeed precipitated in a filterable condition in any one precipitation step.

Thus, if after filtering off the iron tannate produced in the first precipitation, the filtrate and washings are reacted with a further quantity of iron (II) sulphate and under oxidative conditions in the presence of an oxonium ion remover a further substantial quantity of iron tannate is precipitated. This precipitation can be repeated a number of times should it be of interest to do this. The precipitate obtained each time is usable as an anti-corrosive agent in a primer paint.

The iron tannates which are prepared according to the present invention can be used as pigments having anti-corrosive properties.

The iron tannate is recovered in a suitable form which is then incorporated as a pigment with or without extenders in suitable resin binders, such as alkyd resins.

In order that the invention may be further understood the following examples are given by way of illustration.

Example 1 1400 grams of the spray dried powder from the cold centrifuging of natural mimosa extract, which aqueous phase meets the test of clarity set out below, was dissolved in 4.1 litres of the tap water at 60-70"C. The spray dried mimosa was sprinkled into a well stirred solution over a period of about 10 minutes.

,The solution was made up to 5.0 litres at room temperature.

A sample of the mimosa solution was rechecked for clarity. For this purpose 25 cm3 are run into an 8 cm diameter crystallising dish (having a clear plane undistorted glass base). This was then placed on a sheet of clear glass over a lit 40 watt bulb with a white background. In order to meet the necessary standard of clarity this solution should be clear and red wine coloured. The lettering on the bulb should be perfectly clear to read. The clarity should also persist if the mimosa solution is chilled down to 5"C.

The mimosa extract was then reacted with iron (II) sulphate and in the presence of sodium acetate. 1.144 1 of the mimosa extract was run into a 30 cm diameter pneumatic trough with a depth of 15 cm and then 0.5 1 of an iron (II) sulphate solution containing 56 g of FeS04.7H2O and 0.5 1 of a sodium acetate solution containing 53 g of CH3COONa.3 H20 was added.

Alternatively the pH of the mimosa solution may be raised by the addition of the sodium acetate solution thereto before the addition of the iron (II) sulphate solution. The iron (II) sulphate solution may then be added to the strongly stirred mimosa and sodium acetate solution over a period of 20 minutes.

The iron tannate produced was then filtered off, and was washed with distilled water to the point where there was no residual sulphate or acetate ion.

The washed iron tannate was then dried, ground to a fine particle size suitable for a paint formulation by grinding with ceramic high density balls in a one litre ball mill.

The pigment was then incorporated in a primer paint formulation such as A or B below: Formulation A Iron tannate 100 Alkyd resin (55% N-V) 180 White Spirit 50 330 Formulation B Iron tannate 100 Extender 100 Alkyd resin 180 White spirit 50 430 Example 2 Example 1 was repeated with the exception that a working temperature of 70"C was main tained throughout the whole of the precipitation filtration and washing process. Also the mimosa extract which was used was a natural mimosa extract and not the aqueous phase from cold centrifuging.

Example 3 In a further experiment the aqueous phase from cold centrifuging was used as described in Example 2.

In the production of the iron tannate the ratio of the iron (Il) sulphate to the sodium acetate and the mimosa extract may vary quite widely. In the above Examples however a ratio of these components is used such that the iron and acetate ions are present in a mole ratio of approximately 1 to 2 and the mimosa extract provides a mole equivalent assuming the average tannin molecule contains a flavanoid units and is approximately 70% flavanoid.

Claims (11)

1. A process for the production of iron tannate wherein an iron (II) salt is reacted with a naturally occurring tanning extract in the presence of a remover of oxonium ions, in which the tanning extract used is the aqueous phase obtained by chilling and centrifuging of a natural extract, optionally with spray drying and reconstitution of the said aqueous phase, which aqueous phase has a defined degree of purity.

2. A process as claimed in any of claims 1 to 4 in which the temperature at which the iron (II) salt is reacted with the naturally occurring tanning extract at a temperature of from 60-90"C.

3. A process as claimed in claim 1 or claim 2 in which the iron (II) salt is ferrous sulphate.

4. A process as claimed in any of claims 1 to 3 in which the oxonium ion remover is an alkali metal acetate.

5. A process as claimed in any of claims 1 to 4 in which the tanning extract is mimosa extract.

6. A modification of the process as claimed in any of claims 2 to 5 in which in place of the aqueous phase obtained by the chilling and centrifuging of a natural extract, the natural extract itself is used.

7. A process as claimed in any of claims 1 to 6 in which the filtrate and washings from the reaction of the iron (II) salt and the natural tanning extract, is reacted at least once with a further quantity of the iron (II) salt in the pres ence of an oxonium ion remover to provide a further quantity of iron tannate.

8. A process for the preparation of iron tannate substantially as herein described with reference to the Examples.

9. Iron tannate when prepared by a pro cess as claimed in any of claims 1 to 8.

10. An anti-corrosive primer composition comprising an anti-corrosive agent an iron tannate as claimed in claim 8.

11. A primer composition as claimed in claim 10, substantially as herein described with reference to the Examples.