Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Patent
1998-01-20
2000-04-04
Koslow, C. Melissa
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
106428, 106441, 106419, 106401, 244158R, C04B 1400
Patent
active
060456094
DESCRIPTION:
BRIEF SUMMARY
The invention relates to white pigments coated with an oxidising agent, a process for their preparation, and paints containing them.
Heat control of spacecraft calls upon a variety of techniques, in particular passive heat control.
The basis of passive heat control is the thermo-optic properties of coatings which are applied to the spacecraft, namely: ##EQU1##
The solar absorption coefficient is integrated over the solar spectrum between 0.2 .mu.m and 2.8 .mu.m. the nature of the coatings.
All bodies emit radiation which is proportional to the fourth power of their absolute temperature, their surface area, time and their emissivity .epsilon..
We thus have the relationship: its environment from which we obtain: ##EQU2##
Passive heat control coatings can be classified into a variety of categories. Particular among these, as a function of the ratio .alpha..sub.s /.epsilon., are: metallised plastic films, quartz mirrors, etc); gilding, polished metals, etc).
White paints are constituted by a binder, at least one pigment and a solvent.
The most frequently used white pigments are: titanium dioxide (TiO.sub.2), zinc oxide (ZnO), tin oxide (SnO.sub.2), zinc orthotitanate (Zn.sub.2 TiO.sub.4), zinc orthostannate (Zn.sub.2 SnO.sub.4), tin orthotitanate (SnTiO.sub.4), etc, also mixtures of these pigments.
When white paints are exposed to a space environment they degrade, particularly in their thermo-optic properties (increase in the solar absorption coefficient: .alpha..sub.s), due to the combined action of different factors in this space environment, essentially due to the effects of solar ultra-violet radiation.
That solar radiation is between 180 nm and 2 800 nm and short wavelength ultra-violet radiation (180 nm to 250 nm) is responsible for almost all of the degradation.
The following hypothesis can be put forward to explain such degradation:
Under the action of UV radiation, white pigments, essentially metal oxides MO.sub.x, lose oxygen to produce a product MO.sub.x-y which has degraded thermo-optic properties (.alpha..sub.s).
The increase in the solar absorption coefficient .alpha..sub.s damages passive heat control and may be a source of overheating which could have grave consequences for the components of the spacecraft and/or the on-board equipment.
There is thus a need for white pigments which suffer reduced or even zero degradation, in particular under solar UV radiation.
The aim of the present invention is to satisfy this need.
More particularly, the invention concerns white mineral pigments in the form of fine particles having increased stability towards ultra-violet solar radiation, characterised in that the pigment particles are coated with an oxidising agent.
We have found that the oxidising agents of the invention can make up the oxygen deficit in the pigments (MO.sub.x-y) as UV radiation causes the degradation. This results in almost complete protection of these pigments.
A pigment of the invention preferably contains 1% to 10% by weight of oxidising agent. Below 1% the improvement in stability is small while above 10% the gain in stability obtained hardly increases.
The invention also concerns a process for improving the stability of white mineral pigments towards ultra-violet solar radiation, characterised in that it consists of coating particles of a white mineral pigment with an aqueous solution of an oxidising agent, then drying the pigment particles to obtain pigment particles which are coated with oxidising agent.
One coating process could, for example, consist of dissolving an oxidising agent in water at a suitable concentration, for example between 2% and 30% by weight, depending on the oxidising agent. If necessary, the mixture can be heated to improve dissolution. The white pigment to be treated is added to the resulting solution and stirring is carried out for several hours to break up any agglomerates of particles which may be present. The pigment is allowed to settle and is then separated from the solution. The pigment is then dried, for example in an oven. Final grinding can be carri
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Guillaumon Jean-Claude
Nabarra Pascale Veronique
Centre National d'Etudes Spatiales
Koslow C. Melissa
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