Solar reflectors

Details Solar reflectors Solar reflectors

1999-06-25

2001-09-04

Nakarani, D. S. (Department: 1773)

Stock material or miscellaneous articles

Composite

Of silicon containing

C428S451000

Reexamination Certificate

active

06284385

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
Various techniques are used for the thermal control of space vehicles, namely active control or passive control.
2. Description of the Prior Art
Active control uses thermal machines or systems for transferring thermal energy from one point to another. Passive control uses coating materials which have well-defined thermooptical properties, such as the solar absorption coefficient &agr;
s
and the infrared emissivity factor &egr;.
Three categories of coating materials may be distinguished:
cold coating materials: low &agr;, high &egr;: &agr;
s
/&egr;<(e.g. white paints, SSM, OSR);
moderate temperature coating materials: &agr;
s
≈&egr;≈0.3, &agr;
s
/&egr;≈1 (e.g. aluminum paints, metals);
hot coating materials: high &agr;
s
, high &egr; with &agr;/&egr;>1 (e.g. black paints, solar absorbers).
With regard to cold coating materials, SSM (Second Surface Mirror) materials consist of a metallized polymeric film and OSR (Optical Surface Reflector) materials consist of a metallized quartz or glass tile.
SSM polymeric films usually consist of Teflon® FEP (a tetrafluoroethylene-hexafluoropropylene copolymer), Mylar® (polyester) or Kapton® (polyimide), all from Du Pont de Nemours, which are metallized using aluminum or silver. The highest performance SSMs are of the Al/Teflon® FEP or Ag/Teflon® FEP type. OSRs usually consist of a metallized tile, 100 micrometers thick, of very pure quartz or cerium-doped glass.
The thermooptical properties of SSMs and OSRs are imparted:
in respect of the solar absorption factor, by the deposited metal (aluminum, &agr;
s
≈0.10; silver, &agr;
s
≈0.08); the more transparent the quartz or polymeric film, the better the solar absorption;
in respect of the infrared emissivity, by the polymeric film or the quartz tile; in the case of the polymeric film, the emissivity depends on the thickness
e
(for example, in the case of Teflon® FEP, ≈=0.48 for e =25 &mgr;m and &egr; 0.75 for e=125 &mgr;m).
The mass per unit area of SSMs and OSRs is high, given the density of Teflon® FEP (d=2.24) in the case of SSMs and of quartz (d=2.20) in the case of OSRs. The cost of these materials is very high, especially in the case of OSRs, since the vacuum deposition of metal can only be carried out on small areas (typically 5×5 cm).
If these coating materials are applied to a spacecraft, there is additionally the cost of placing (by bonding) the SSMs and OSRs (use of very expensive special tools).
There is therefore a need for high-performance thermal control coating materials which are lighter and less expensive than the current coating materials.
The invention aims to satisfy this need.
SUMMARY OF THE INVENTION
More particularly, the invention relates to a thermal control coating material comprising a metal reflecting layer optionally supported by a substrate, wherein the metal reflecting layer is coated with a transparent protective layer made of a polysiloxane which has a structure consisting of two-dimensional or three-dimensional blocks connected together by linear chains or by linking groups, said protective layer being free of chromophore groups capable of absorbing in the ultraviolet band above 170 nm.
This layer must have a very high transparency, so as not to affect the solar absorption coefficient of the metal layer, a high infrared emissivity and a degree of flexibility, in particular when the substrate is itself flexible.
The material of the invention is produced by using any method to deposit a layer of a polysiloxane having a very high transparency (transmission factor≧98% between 200 and 1 000 nm) and a density close to 1 on a thin shiny metal (for example aluminum or silver) layer which may be solid (for example 25 to 100 &mgr;m thick) or which may be deposited beforehand on a substrate. The substrate may be flexible or rigid, but is preferably flexible.
The polysiloxane forming the transparent layer has a structure comprising two-dimensional or three-dimensional blocks connected together by linear chains of greater or lesser length depending on the desired degree of flexibility. An entirely two-dimensional or three-dimensional polysiloxane would be rigid and brittle and would not be suitable for the intended application. As a variant, the two-dimensional or three-dimensional blocks may be connected together by linking groups, such as —O—groups,
groups or other groups, resulting from the reaction or the condensation of two functional groups. The polysiloxanes used here have a structure corresponding to the following models:
Three-dimensional Structure
where R is an alkyl radical, preferably a methyl radical, and n=0, 1, 2, 3 in the case of moderately rigid structures for applications on solid metal substrates and n≧4 for applications on metallized flexible substrates.
The linear chains connecting the two-dimensional or three-dimensional blocks are usually polydimethyl-siloxane chains.
The protective layer must not have any chromophore groups capable of absorbing in the ultraviolet above 170 nm, whether these groups are present in the polysiloxane or come from crosslinking agents or catalysts.
This is because, exposed to ultraviolet radiation in space, such groups would degrade the protective layer, which would impair the transparency and the thermooptical properties, especially the solar absorption factor (&agr;
s
).
The chromophore groups to be avoided include the groups:
There are several ways of obtaining the polymer structures defined above:
1) using polysiloxanes having two-dimensional or three-dimensional structures and comprising radicals which either can coreact mutually, or can be crosslinked with polydimethylsiloxanes to create linear chains between the two-dimensional or three-dimensional blocks;
2) from a polysiloxane of formula
in which R′ is a reactive group which allows co-cross-linking and reaction with reactive groups on other polydimethylsiloxanes.
The polymerization may take place by polyaddition or by polycondensation, depending on the reactive groups used, with or without catalysts and at room temperature or above (up to 250° C.).
Suitable reaction schemes include:
System 1:
self-crosslinking two-dimensional
crosslinking with polydimethylsiloxanes: two-dimensional
where R and R′ are reactive groups and n is an integer equal to or greater than 1.
System 2:
self-crosslinking
where R and R′ are reactive groups and n and m are integers which are the same or different and equal to or greater than 1.
In the case of system 2, the products are more flexible since they comprise more polysiloxane chains than in the case of system 1.
Moreover, the shorter the polysiloxane chains, the more rigid the coatings.
Finally, the use of crosslinking agents makes it possible to increase the number of two-dimensional and three-dimensional structures.
A non-exhaustive list of comonomers which can be used in the context of system 1 is given hereinafter:
polysilesesquioxances of the type
with reactive groups such as —H, —CH═CH
2
, —OH, —Cl, —Br, —O—Me, —O—Et, —O—Ac, —N—Me
2
, —O—N═CR
2
, etc.;
polysiloxanes having a three-dimensional structure of the type:
where R═—H, —CH═CH
2
, —OH, —Cl, —Br, —OMe
2
, —O-Et, —O—Ac, —N—Me
2
, —O—N═CR
2
, etc.;
polydimethylsiloxanes having reactive groups at the ends of the chain, such as : —H, —CH═CH
2
, —OH, —Cl, —Br, —O-Me, —O-Et, —O—Ac, —N—Me
2
, —O—N═CR
2
, etc.
The proportions of each constituent may be varied depending on whether a product of greater or lesser flexibility is desired. These proportions may vary in the range from 10% by weight to 90% by weight for the polysiloxanes having a two-dimensional or three-dimensional structure or polydimethylsilioxany having reactive groups, the other constituents forming the balance up to 100%.
In system 2, non-limiting examples of the comonomers include:
polydiethoxysiloxane, polydimethoxysiloxane
polydimethylsiloxanes having reactive groups at the ends of the chain or in the chain, such as : —

Affiliated with

Also associated with

Rating

Solar reflectors does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Solar reflectors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solar reflectors will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2489270