The present invention relates to moisture indicators capable of indicating the absorbent capacity of a desiccant material or more generally the moisture conditions present in the medium in which the indicator is placed.
The moisture indicators according to the invention comprise an active principle which is used in particular for colouring amorphous silica or silica gel, for colouring solid supports in the form of labels or for colouring bentonite or calcium sulphate pellets, or for impregnating molecular sieves, alumina, fossil flour, diatomaceous earth or the like.
The main types of solid static desiccants that are commercially available are silica gel, bentonites, molecular sieves and alumina.
The main applications are:
Dehydration of industrial liquids and gases.
Protection of materials, chemical products and pharmaceuticals, metal parts and electronic circuits against atmospheric moisture, using desiccant sachets, packets or capsules placed inside the protective packaging.
Drying of air in electrical apparatus and in transformers.
It is always necessary to demonstrate the absorbent capacity of the desiccant material, not only at the time of the first sale but also at the time of use.
For these reasons, in any application in which it is necessary to rigorously test the desiccant power of silica, blue silica gel is used (amorphous silica containing 0.5% by weight of cobalt chloride).
Blue silica gel indicates when it is spent by changing colour from blue (maximum desiccant activity) to pink (inactive as a desiccant).
However, when products based on desiccant clay (material which shows no obvious differences between its maximum activity and depletion of the desiccant power) are packaged, moisture-indicating labels are introduced into the packaging. In this way, the purchaser of the product personally checks the desiccant activity of the material purchased.
In addition, these indicator labels are commonly used for indicating the moisture conditions present inside industrial packagings, thus pointing out any penetration of moisture into these packagings and revealing the ambient conditions present during transportation.
A portion of the area of the indicator labels is impregnated with a cobalt chloride solution. This zone changes colour from blue to pink depending on the moisture present in the medium in which the label is placed; the colour change is reversible and thus allows the label to be regenerated for further use.
The concentration of the cobalt chloride solution used for colouring the indicator labels determines the sensitivity to the various moisture contents and thus the possibility of changing from blue to red in the way complying most with the ambient conditions in which the moisture indicator is required to work.
In Amendment XXV of European Directive 67/548, the cobalt chloride used hitherto in the field of desiccant materials inside silica or on labels as a moisture indicator has been relabelled and reclassified.
The consequences of this regulation include a series of warnings as regards the handling and use of products containing inorganic cobalt salts.
Since in blue silica gel, in indicator labels and in bentonite pellets and similar support materials, the coloration is given by cobalt chloride at a certain concentration, the problem arises of identifying one or more components capable of replacing cobalt chloride in its function as a substance capable of changing colour according to atmospheric moisture.
It is this problem which it is the aim of the invention to solve, and thus to provide moisture indicators whose characteristics are at least equal to those currently existing, which make use of cobalt chloride.
Essentially, according to the invention, copper chloride (CuCl2) is used as active principle in combination with a mixture of hygroscopic salts containing the chloride ion for the purpose of colouring solid supports which, besides optionally being desiccants, change colour according to the moisture present in the medium with which they are placed in contact.
In particular, with this active principle, the Applicant has developed a novel coloured silica gel whose moisture-absorbing capacities are no different to those of the previous silica gel, which can be regenerated without problems, and which undergoes a colour change from brown-yellow (when dry) to aquamarine (when wet).
In addition, with this active principle it has been possible to identify many different water-alcohol and aqueous mixtures which allow the production of moisture-indicator cards and dry mixtures of desiccant salts and materials so as to produce moisture-indicating desiccant pellets or manufactured products of any type having these characteristics.