WO2006130029A2

WO2006130029A2 - Silica gels with moisture indicator, a method of obtaining them, and use of phthalein dyes

Info

Publication number: WO2006130029A2
Application number: PCT/PL2006/000039
Authority: WO
Inventors: Waclaw Lanowy; Krystian Lukaszczyk; Przemyslaw Kasprzycki
Original assignee: Poch S.A.
Priority date: 2005-06-03
Filing date: 2006-06-01
Publication date: 2006-12-07
Also published as: WO2006130029A3; PL375514A1

Abstract

The invention concerns the application of phthalein dyes as indicators for colouring silica gels. The achieved silica gels with moisture indicators are excellent drying agents which are not harmful to humans and do not burden the natural environment. They possess very clear and distinct colour transitions of the hydrated and unhydrated forms and have greater sensitivity of colour change in the course of the gel's hydration.

Description

Silica gels with moisture indicator, a method of obtaining them, and use of phthalein dyes

FIELD OF THE INVENTION

Silica gels with moisture indicators are widely used as drying agents or humidity indicators in many commercial fields and everyday life. They are produced on the basis of silica gels which, chemically, are mixtures of silicic acids. Silicic acids are weak acids and produce sols from which gels form containing large amounts of water. The fresh gel may contain from a few to several dozen water molecules for each Siθ₂ molecule. Such a gel can be gradually dehydrated, by which it has the property of a reversible gel, i.e. it can bind water again.

BACKGROUND OF THE INVENTION

1 Silica gel is a chemical substance of low chemical activity; according to the structure and composition of the gel we differentiate, for example, fine-pored and macroporous gels. A silica gel saturated in the proper manner with an indicator solution becomes, after roasting, a dehumidifying gel with moisture indicator which changes colour as it absorbs moisture from its immediate surroundings, indicating the loss of its absorbent capability, which substantially increases the utility of the gel itself.

Until now, cobalt salts have been customarily used for dying silica gels (the indicator) in large-scale applications. Cobalt salts are harmful and carcinogenic substances, and thus dehumidifying gels with a moisture indicator produced with cobalt salts should also qualify as harmful and carcinogenic substances, which significantly limits their application. Moreover, the observed colour change of the dye (from blue to pink-violet) which takes place during the course of hydration may be difficult to observe and it occurs relatively close to the complete saturation of the gel. In some technical applications where moisture is a critical parameter, this may present a substantial inconvenience. THE GOAL OF THE INVENTION

An aim of the invention is to provide silica gels with moisture indicator by which the gels would not be harmful to humans and animals and would not be burdensome to the natural environment.

A further aim of the invention is to provide a gel in which the colour change occurs at a lower degree of hydration and would be clear and distinct, in particular that it assures a greater contrast of the change in colour of the hydrated and unhydrated forms than is the case of known silica gels with moisture indicator, e.g. a cobalt indicator.

The above aims were unexpectedly realized in this invention.

SUMMARY OF THE INVENTION

The first aspect of the invention is a silica gel with moisture indicator, whereby the moisture indicator contains at least one phthalein dye. Preferably, the phthalein dye is a compound with the formula:

where:

R indicates H or CH(CH₃)₂

Ri indicates H or CH3.

Particularly preferably, phthalein dye is phenolphthalein, thymolphthalein, or a mixture of them. The next aspect of the invention is use of phthalein dye as defined above for the manufacture of a silica gel with moisture indicator. Preferably, the silica gel with moisture indicator is a drying agent. Particularly preferably, the silica gel with moisture indicator is a humidity indicator.

The next aspect of the invention is a method of obtaining a silica gel with moisture indicator characterised in that the silica gel is saturated with a solution containing a phthalein dye and then the saturated gel is dried, whereby preferably the phthalein dye solution contains a solvent from among the group comprising: water, an alcohol, an ether, a ketone, an alkaline solvent, an acidic solvent, or any mixture of these. Preferably in the method according to the invention the phthalein dye is a compound as defined above. Preferably, in the method according to the invention, the drying of the saturated silica gel with moisture indicator is conducted at a temperature of 150-250⁰C until a water content less than 2% is achieved. Particularly preferably, the drying is conducted to regenerate the damp gel.

The next aspect of the invention is a method of obtaining silica gel with moisture indicator characterised in that a phthalein dye is added to the solution of silicates or acid at the production stage of a silica gel. Preferably, the phthalein dye is a compound as indicated above.

The particular subject of the invention is the use of phthalein dyes, in particular 1) phenolphthalein, 2) thymolphthalein, or a mixture of these, as an indicator to colour silica gels which is not harmful to healthy persons and which does not burden the environment.

In the first case one obtains, after saturation of the silica gel and its roasting, orange-red silica gel which changes its colour to white-cream when absorbing water.

In the second case, applying thymophthalein solutions, has been obtained, after saturation of the silica gel and its roasting, dark-burgundy silica gel which changes its colour to white-cream when absorbing water.

The range of the colour change of the indicator in relation to the absorbent capacity of the gel, which depends on the type of gel, its granulation, and other factors, is approx. 45-60%, which guarantees that the colour change of the gel occurs before it loses its ability to absorb moisture. This is very important in a variety of applications. Moreover, the colour change is very clear and distinct.

After the above colour change of the silica gel, in the case of fine-pored gels they can be submitted to reactivation (restoration of their ability to absorb moisture) by heat treatment. This process can be repeated several times and the silica gel with the indicators according. to the invention does not lose its functional properties. A silica gel with the indicators according to the invention or mixtures of them may be won through saturation of the silica gel with aqueous, alcohol, ether, or acetone solutions, alkaline solutions, acidic solutions, or their mixtures, containing the dissolved dye. During saturation, the gel should be mixed to ensure even saturation. After saturation of the silica gel, it should be drained and then dried at a temperature of 150-250⁰C until a uniform colour and the defined water content (less than 2%) is obtained. An alternative method of obtaining a gel according to the invention may consist of adding the phthalein dyes at the production stage of the silica gel.

After desiccation, the product should always be packed in watertight containers.

THE VIRTUES OF THE INVENTION

Silica gels with the moisture indicators according to the invention have numerous practical virtues which substantially differentiate them from the silica gels with cobalt indicators now in use:

• They are excellent drying agents, and the moisture indicators are not harmful to humans or animals and are not burdensome to the natural environment.

• The colour change is very clear and distinct (greater contrast of the colour change of the hydrated/unhyd rated forms than in silica gels with cobalt indicators).

• Silica gels in which the phthalein dyes phenophthalein, thymolphthalein, or mixtures of them are used as indicators are characterized by greater sensitivity of the indicator, that is the colour change occurs at a lower degree of hydration of the gel.

Some embodiments of this invention are described in the following examples.

Example 1

A dye solution to saturate silica gel is prepared by dissolving 2.1 g of solid sodium hydroxide in 1 dm³ of water, and then 1.1 g phenolphthalein. The resultant solution is poured over 2.5 kg of silica gel. The gel is mixed, and approx. 30-40 minutes later the excess liquid is drained off and the gel is dried at a temperature of 150-250⁰C. The silica gel with indicator is mixed during desiccation until a uniform colour and a water content of less than 2% are achieved. The finished product is paced in a special container. The resultant product is orange-red in colour.

Example 2

A dye solution to saturate silica gel is prepared by dissolving 1.44 g of thymolphthalein or phenolphthalein in 1.0 kg of methanol. The resultant solution is poured over 2.8 kg of silica gel. The gel is mixed, and approx. 30-40 minutes later the excess liquid is drained off and the gel is dried initially at a temperature of 80-

100⁰C, then at a temperature of 150-250⁰C. The silica gel with indicator is mixed during desiccation until a uniform colour and a water content of less than 2% are achieved. The finished product is paced in a special container. The resultant product is dark-brown in colour in the case of thymolphthalein and orange-red in the case of phenolphthalein.

The above-described merits of silica gels with the moisture indicators according to the invention were illustrated in the following experiments.

Experiment 1 : Confirmation of the functional properties of silica gel containing a phthalein dye (phenolphthalein or thymolphthalein) as the moisture indicator.

A 27% sulphuric acid solution was poured into a desiccator of ca. 180 mm in diameter so that the height of the layer of sulphuric acid in the lower compartment of the desiccator was ca. 5 cm. Approximately 2.0000 g of silica gel with moisture indicator was placed on a scales pan which had been weighed to an accuracy of 0.0002 g (previously placed in the desiccator with the sulphuric acid, the diameter of the scales pan being ca. 30 mm). The scales pan with the silica gel with moisture indicator was placed in the desiccator and the colour change of the gel was observed. When the indicator was phenolphthalein, the colour change was from orange-red to white-cream, and when it was thymolphthalein from dark-burgundy to white-cream. After the colour change, the scales pan with gel was weighed and the mass increase (moisture) determined; then it was left in the desiccator for 24 hours and the increase in mass (moisture) was determined again.

Results:

• The fine-pored silica gel with moisture indicator according to the invention (phenolphthalein, thymolphthalein) absorbs, depending on its granulation (2-7 mm), temperature, and composition of the gel, ca. 15-18% moisture in relation to its mass (complete saturation of the gel with moisture).

• The colour change occurred about 8-9% earlier than the complete saturation of the gel with moisture (i.e. ca. 7-9% in relation to the mass of the gel).

The range of the colour change of the moisture indicator in relation to the silica gel's ability to absorb moisture guarantees relatively optimal exploitation of the absorbent capacity of the gel (in several layers) and no possibility of a situation in which loss of this capacity goes unnoticed.

Experiment 2: A comparison of the properties of the gel according to the invention with a known silica gel containing a cobalt salt as moisture indicator.

Three desiccators with diameters of ca. 180 mm were filled with:

1. a 27% sulphuric acid solution, corresponding to 80% relative humidity of the air in equilibrium with the sulphuric acid solution

2. a 43% sulphuric acid solution, corresponding to 50% relative humidity

3. a 58% sulphuric acid solution, corresponding to 20% relative humidity in such a manner that the height of the layer of sulphuric acid solution in the lower compartment of the desiccator was ca. 5 cm.

In each desiccator, three identical scales pans were placed, each containing ca. 2.000 g of gel:

1. the first scales pan contained silica gel saturated with phenolphthalein solution (orange-red)

2. the second contained silica gel saturated with thymolphthalein solution (dark-brown)

3. the third contained silica gel with the classic moisture indicator, a cobalt salt (blue)

After placing the scales pans in the desiccators, the colour changes of all the silica gels where continuously monitored and the dependence of total colour fading of the particular gels with the defined indicators and the percent increase of absorbed water was determined. Total colour fading was defined by comparing previously prepared standards (samples of hydrated gels). Also observed was at what minimum percent increase in absorbed water the fading of the gel is clearly visible. The quantity of absorbed water was determined by weighing to an accuracy of 0.001 g. The test was conducted at a temperature of 20-22⁰C.

The fading rates of the gels depended on the relative humidity and were greatest in desiccator no. 1 (conditions corresponding to 80% humidity of the air). After total, permanent fading of the gel, the sample was weighed and the percent of absorbed water (X_water) was determined according to:

( M₁ - M₂) X 100%

■K water ⁼ M₂

where: Mi - the mass of the hydrated gel

M₂ - the mass before hydration

The results are presented in the following table.

* in accordance with the norm TWT/Och/13/99

The total absorbency (of water) of silica gels with indicators depends on the properties of the silica gel itself, and not on the kind of indicator used. Conclusions:

Silica gels with the moisture indicators according to the invention have much greater sensitivity of their colour changes during water absorption. The colour change is more distinct and occurs far before the point of complete absorption of water (achieving total saturation of the gel). This was especially evident in the moisture absorption in desiccator no. 3 (humidity of 20%), where prolonged hydration of the samples could in principle be monitored only in the silica gels with the indicators according to the invention.

Claims

1. Silica gel with moisture indicator, whereby the moisture indicator contains at least one phthalein dye.

2. Silica gel as claimed in claim 1 , whereby the phthalein dye is a compound with the formula:

where:

R indicates H or CH(CH₃)₂

Ri indicates H or CH₃.

3. Gel as claimed in claim 1 , whereby the phthalein dye is phenolphthalein, thymolphthalein, or a mixture of them.

4. Use of phthalein dye for the manufacture of a silica gel with moisture indicator.

5. Use according to claim 4, wherein the phthalein dye is a compound with the formula:

where:

R indicates H or CH(CH₃)₂

Ri indicates H or CH₃.

6. Use according to claim 4, wherein the phthalein dye is phenolphthalein, thymolphthalein, or a mixture of them.

7. Use according to claim 4, wherein the silica gel with moisture indicator is a drying agent.

8. Use according to claim 4, wherein the silica gel with moisture indicator is a humidity indicator.

9. A method of obtaining a silica gel with moisture indicator characterised in that the silica gel is saturated with a solution containing a phthalein dye and then the saturated gel is dried.

10. The method according to claim 9, whereby the phthalein dye solution contains a solvent from among the group comprising: water, an alcohol, an ether, a ketone, an alkaline solvent, an acidic solvent, or any mixture of these.

11. The method according to claim 9, whereby the phthalein dye is a compound with the formula:

where:

R indicates H or CH(CH₃)₂

Ri indicates H or CH₃.

12. The method according to claim 9 or 11 , whereby the phthalein dye is phenolphthalein, thymolphthalein, or a mixture of them.

13. The method according to claim 9, whereby the drying of the saturated silica gel with moisture indicator is conducted at a temperature of 150-250⁰C until a water content less than 2% is achieved.

14. The method according to claim 9, whereby the drying is conducted to regenerate the damp gel.

15. A method of obtaining silica gel with moisture indicator characterised in that a phthalein dye is added to the solution of silicates or acid at the production stage of a silica gel.

16. The method as claimed in claim 15, whereby the phthalein dye is a compound as indicated in claims 2-3.