Note: Descriptions are shown in the official language in which they were submitted.
419P27CA
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WO 9S/3225 1 PCT/DE95/00644
Coating substance with low emissivity in the heat radiation range
s
The invention relates to a coating substance which has a low emissivity in the heat
radiation range.
Known coating substances are substantially composed of binders, pigments and
10 different additives. With standard coating substances, the majority of the binders
and the dispersed pigm~nt~ have a high degree of absorption in the heat radiation
range, and thereby also have a high degree of heat radiation emission.
A silicon-based wall coating for an external house wall is described here by way of
lS an example. The dispersed pigments which are mainly composed of lime, have, as
does the silicon-based binder, high absorption bands in the heat radiation range of
the thermal infra-red range of 3 to 100 ,um. The degree of emission of the housewall in the heat radiation range is thus over 90%. This means that in addition to
the heat losses by convection, that is to say the heat loss to air, the house wall
20 radiates heat energy at Ms = ~ . a . T4. With a wall tel~lpelature of 0 celsius, that
is to say 273 Kelvin, this means that with an ~ of 0.9, heat is radiated at 2.3 W-m-2.
It is important to know that these heat losses from a house by heat radiation are
additional, that is to say completely independent of the heat losses by convection.
25 This can be explained in that air is transparent to heat radiation over a wide range,
and for heat radiation a drop in temperature is not dependent on the air temperature,
but instead dependent on the radiation temperatures of the environment and the sky.
When the sky is clear, these temperatures are significantly lower tnan that of t'ne air.
In addition to avoiding heat losses on the external wall of a house, it is also
wo~ vhile to reduce the transfer of heat by radiation to the inside of the external
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wall of a house. All items such as furniture, floors, and also in particular theinternal walls of the house emit heat in the form of heat radiation according to the
regularities described. People themselves also give off heat in the form of radiation
towards the inside of the external wall. In particular, heaters naturally also give off
S radiated heat and although this is in the direction of the internal space it is,
however, equally towards the inside of the external wall.
In this case, in accordance with the degree of emission ~ > 0.9 (degree of emission
= degree of absorption) heat radiation is absorbed by over 90%, and transported
10 by heat conduction to the external wall.
To avoid direct transfer of heat by radiation from heaters into the wall, so-called
"reflective foils for heaters" are commercially available. The metallic surface of the
reflective foils only absorbs approximately 10 to 20% of the heat radiation. The15 difference from 100% is reflected back into the room, in this case to the heater.
Unfortunately these reflective foils are not popular, probably because of their
m~t~llic appearance, and are therefore seldom used. In any case, complete liningof a home with such reflective foils would not be sensible as they have no, or very
little, moisture diffusion capability and on the other hand would turn the room into
20 a Faraday cage in which nobody would wish to live. It also would not correspond
to our aesthetic concepts of interior design.
With respect to energy-saving, it would however be worthwhile to, as it were,
inwardly metal-coat a room so that the heat radiation is reflected back into the25 room. Nevertheless this must relate to a moisture permeable layer which does not
make the room a Faraday cage and which also complies with aesthetic requirements.
The increasing air pollution which is to a great extent caused by the burning offossil fuels for heating houses, and also the knowledge that the reserves of fossil
30 fuels will at some time be e~rh~l)ste~, make it n~cess~ry to use all possibilities for
minimi~ing the energy requirement.
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The object of the invention is to provide an improved coating substance, with the
aid of which energy can be saved. Further, a process for m~nnfactllring coating
pigmrnt~ should be found which can be used with these coating substances.
5 This object is solved according to the features described in claim 1 and claim 8.
Advantageous further developments of the subject-matter of the invention are
described in the dependent claims. Further, the object is solved with respect to the
process in claim 11.
10 In an unexpected manner it has been shown that in accordance with the invention,
a coating substance with properties of low emissivity in the heat radiation range can
be m~mlf~ctllred by dispersion of particles which are highly transparent in the heat
radiation range, and the refractive index of which is greater or smaller in the heat
radiation range and is in any case different to the refractive index of the binder,
15 wherein the binder has a high degree of permeability in the heat radiation range.
Such a coating substance has no disadvantageous effects in the visible range.
Particularly good results are obtained when the product of the refractive index of the
individual particles in the thermal infra-red range and the particle diameter is20 substantially equal to half the wave length of the wavelength range in which the
coating subst~nre should have a low emissivity effect. Slight shifts result from the
refractive index of the binder in which the particles are dispersed. The greater the
refractive index of the binder, the greater the shift in the average wavelength
towards the longer wavelength range. Preferably, the pelcelltage of the filling ratio
25 of the particles in the binder should be at 20 to 70%, in particular 30 to 50%, with
lefelellce to the volumes of the total coating .
The degree of the reflection or of the emission is determined by the difference
between the refractive power of the binder and the refractive power of the dispersed
30 particles. The greater the difference, the higher the desired reflectivity present.
The refractive indices of binders with high transparency in the heat radiation range
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are normally in the range of 1.3 to 1.7. A large difference in the refractive index
can above all be produced when the refractive index of the particles is greater than
that of the binder. Preferably it should be in the range of 2 to 4, but higher
refractive indices of the particles are also conceivable. If the refractive index of the
5 particles is less than that of the binder, it should if possible be in the range of that
of air, that is to say 1.
The band width of the range in which the low emission and high reflectivity has to
be produced is also dependent on the size of the difference between the refractive
10 index of the binder and that of the particles. The greater the difference in the
refractive index of the two materials, the greater the band width by the averagewavelength selected. With a difference in refractive index of 2(nbjnder = 1.5; np~njcles
= 3.5) a band width results for the first resonance of approximately 6 ~m. With
this, the range of the atmospheric window, which is of military relevance, can be
15 design~cl with low emissivity or reflectivity of 8 - 14 ~m. The range relevant for
- 300 kelvin radiators can thus be clesign~.d with low emissivity and reflectivity at 8 -
14 ~m, where the atmosphere has a high transparency and thus allows energy
through into space. Subsequent resonances are also produced in the relevant
atmospheric window at 3 - 5 ~m up to the range of visible light.
As material for the dispersed particles, all materials with high transparency in the
heat radiation range can be considered, which have a greater or smaller refractive
index than the binder in the heat radiation range.
25 Particularly advantageous materials within the framework of the invention for the
particles which are dispersed in the binder can be selected in particular from the
group of the following: ger~n~ni~lm, silicon, metal sulphides such as, for example,
lead sulphide, metal selenides such as, for example zinc selenide, metal tellurides
or tellurium itself, chlorides such as, for example, sodium and potassium chloride,
30 fluorides such as, for example, calcium fluoride, lithium fluoride, barium fluoride
and sodium fluoride, and antimonides such as, for example, indium antimonide.
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. s
The choice of materials which are transparent in the wavelength range of heat
radiation and also have a different refractive index to the binder is limited.
According to the invention, particles with an artificially increased or reduced
refractive index can also be used for this application.
To provide particles with an artificially increased refractive index, organic orinorganic binders with high transparency in the heat radiation range are loaded with
10 to 50 percent by volume of colloidal metal powder with a particle size in therange of 0.05 to 1 ,um, such that the colloidal particles are uniformly distributed in
10 the binder. The binder loaded in this way is dried and after drying is comminllted
to the desired grain size conforming to the refractive index of the material obtained.
Because of the extremely small size of the colloid metal particles there is no
disadvantageous increase in reflectivity in other wavelength ranges.
15 Depending on the degree of loading with colloidal metal powder and the refractive
index of the binder, particles can be m~nnfactl-red with a refractive index
significantly above that of the initial material. With a filling ratio of 30 percent by
volume of colloidal copper, the average particle size of which was below 0.5 ,um,
the refractive index of the molten polyethylene mass used as the binder could be20 increased from 1.5 to 2.2. The polyethylene loaded in this manner was
subsequently cooled with liquid nitrogen and comminllted to the desired particle size
of 2.5 ,~m.
As the low emissivity of the coating substance according to the invention is achieved
25 above all because the refractive indices of the dispersed particles and the binder are
different, according to the invention low emissivity can also be obtained by
dispersion of air, that is to say a ffller with a low refractive index, in a binder. In
principle the same conditions apply here as in the case already described. An
- op~ ulll effect is obtained when the di~mPter of the air-filled hollow spaces is
30 subst~nti~lly the same size as half the average wavelength of the range in which a
low emissivity and high reflectivity is desired. The hollow spaces can also be
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placed in the binder by mech~ni~pl means using spray techniques, or by means of
known ch~lnir~l reactions.
With the methods described until now for producing a low emissivity coating
5 substance, it was possible to determine the wavelength ranges in which the colours
should be of low emissivity or reflectivity by means in particular of the size but also
to a limited extent by the filling volume or the filling ratio of the particles dispersed
in a binder. If, however, a low emissivity colour with as wide a band width as
possible is desired, pre-formed hollow micro-spheres known per se, the wall
10 material of which must nonetheless be transparent in the heat radiation range and
can be composed from the materials described above, are suitable for this purpose.
It is also possible in this case to artificially increase the refractive index of the wall
material by dispersion of colloidal metal particles. The loading ratio of a binder
with the hollow micro-spheres transparent in the heat radiation range is not crucial,
15 but the higher the loading ratio, the lower the heat emissivity of a coating so
produced. The ~ m~ter of the hollow micro-spheres should be in the range of 5 -
500 ,~4m, but in particular 10 to 200 ,um.
A further way to produce a low emissivity coating substance is to disperse plate-
20 like, flaky pigm~n~ which are made from materials which are transparent in theheat radiation range and can originate from the range of the materials already
described or materials transparent in the heat radiation range per se, the refractive
index of which is artificially adjusted by the dispersion of colloidal metal particles.
Such plate-like hltelrelel1ce pigments are known from the area of special effectpaints and varnishes for the cosmetic industry or also for the automotive industry.
In DE OS 32 21 045 pearl gloss pigments based on coated mica chips are described.
Their effectiveness is limited to the visible range, however, as their interference
30 producing shapes are specially ~im~rt.cioned for the visible light range and because
the materials used are not Llanspal~l1t in the heat radiation range and act in a
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absorbent maMer. Various methods for m~nll~ct~ring such plate-like pigments are
known. In most cases, substances are chemically deposited on mica plates.
However, m~nl-f~cturing methods are also known in which paint layers are appliedto a moving drying belt, for example with a squeegee, for subsequent commin~ltion
S to produce pi~ments.
With the latterly described method particularly inexpensive interference pigments
with good effectiveness in the heat radiation range can be produced in the following
manner. Preferably three layers of, in particular, organic materials transparent in
10 the heat radiation range are applied, by means of which a different refractive index
is created by the different loading ratio of colloidal metal particles. Firstly a layer
with as high a refractive index as possible is applied, then follows a layer with the
lowest possible refractive index, and the last layer again has a high refractive index,
wherein before application of the next layer, each layer is dried so that the layers
15 do not run into one another. The designation of the highest possible or lowerrefractive index of the material for the respective layer is in accordance with the
relationship to the refractive index of the binder used.
After drying and pulverisation, interference pigments are obtained with high
20 reflectivity and low emissivity in the heat radiation range which are dispersed in a
binder permeable in the heat radiation range and these together produce a coating
substance effective in the heat radiation range.
Binders are pfer~ ,d in the framework of the invention which are highly transparent
25 in the heat radiation range such as, for example, cyclised or chlorine rubber and
bitumen binders. If good resistance to oil, benzine and chemic~ls is required,
binders are preferred within the framework of the invention which are selected from
the group including the polyurethane, acrylate, PVC polymer mixtures, polyethylene
/ vinyl acetate polymer mixtures, butyl rubber and silicon alkyd resins groups.
30 Depending on requirements, modified aqueous polyethylene-based binders, such as
Poligen PE and Poligen WE1 from BASF Ludwigshafen can be used. Mixtures of
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polyethylene binders with aqueous acrylate binders.
Some examples of m~mlf~etllring of the coating substance according to the invention
are given hereinafter.
s
Example 1
40 percent by volume, based on the solids content of the binder, of a silicon powder
with an average grain size of 1.7 ,um and a refractive index of approximately 3.5
10 in the heat radiation range is applied to a conventional chlorine rubber-based paint
binder with a refractive index of approximately 1.6 in the heat radiation range. In
order to largely avoid settling of the particles in the binder, the paint film is
subjected to rapid drying at 80 degrees celsius in a furnace. When subsequent
measurement of the reflectivity and emissivity of the dark grey paint is done, an
15 average emissivity of 20% (80% reflectivity) is determined in the 4.5 to 6 ~m and
8 to 13 llm wavelength range.
Example 2
20 Multiple sheets of polyethylene are sprayed onto a primed metal plate up to a total
thickness of 0.5 mm with an air-pressure driven spray pistol for hot glue with ametered air supply. Due to the metered air supply, hollow micro-spheres occur inthe polyethylene, the diameter of which is in the range of S to 10 ,um. A ratio of
50 percent by volume of air to binder. was determined by weighing. During
25 subsequent measurement of the reflectivity and emissivity properties of the layer an
average degree of emissivity of 65~ (35% reflectivity) was determined in the 4.5to S~m and 8 to 12 ~m wavelength range.
Example 3
30 percent by volume of copper particles with an average grain size of 0.5 ~m are
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dispersed in a molten polyethylene mass and distributed in the molten mass usinga standard working method. The polyethylene loaded in this way was subsequently
cooled with liquid nitrogen and ground to an average particle size of 3.5 ~m. The
particles obtained in this way were dispersed at up to 35 percent by volume in a5 conventional cyclised rubber-based binder. The mixture was coloured green withconventional transparent colorants and painted onto a primed metal plate. Duringthe subsequent measurement of the reflectivity and emissivity properties of the
coating substance a wide banded degree of emissivity of 75 % (25 % reflectivity) in
the whole heat radiation wavelength range was determined, with deviations in the10 4.5 to 5 ~m and 8 to 12 ~Lm ranges. In these ranges, the degree of emissivity was
35% (65% reflectivity).
Example 4
15 Up to 50% by volume of hollow micro-spheres made from a silicon-based material
transparent in the heat radiation range and calcium fluoride and various oxides were
dispersed in an aqueous dispersion of Poligen WE1, a polyethylene oxidate from
BASF, for reducing the melting point. The rli~meter of the hollow micro-spheres
was in the range of 30 to 80 ~m with wall thirl~n~sses in the range of 1 to 3 ~m.
20 The mixture was coloured white with ultrafine (less than 1 ~m diameter) whitepigmentS made from zinc sulphide and subsequently measured with respect to its
emissivity properties in the heat radiation range. A degree of emissivity of 30%(70% reflectivity) was deterrnined over the whole heat radiation range. Only in the
4 to 6 ~m range was the degree of emissivity 65% (35% reflectivity).
Example 5
30 percent by volume of copper particles with an average diameter of less than 0.5
,um was dispersed in a conventional cyclised rubber-based binder highly transparent
30 in the heat radiation range. The mixture was diluted with a solvent such that after
drying of t-h-e paint sprayed onto a Teflon plate there was a film thickn~s5 of 1 to 1.5
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~m. A further film of a cyclised rubber paint without the copper particles, the layer
thickness of which was 2 to 3 ~m after drying and hardening, was sprayed onto the
hardened ~llm. Afterwards the layer with the copper particles was then applied to
this second layer. The layer obtained in this way was scraped off the Teflon plate
5 and crushed in the mortar. After sieving out excessively finely ground dust
particles, the plate-like layer pigments, transparent in the heat radiation range, were
viewed under the microscope. The dimensions of their area were between 10 to 20
,um and the thickness of the layer 4 to 8 ,~4m. Due to layer building using different
refractive indices, the layer pigm~ntc had a high reflectivity in the heat radiation
10 range. 25 percent by volume of the layer pigments were dispersed in a modified
Poligen WE1 dispersion from BASF and after colour tinting were coloured white
with ultra-fine (less than 1 ,~4m ~i~meter) white pigments measured in the wavelength
range of heat radiation. The emission in the 6 to 14 ~m wavelength range was 35 %
(reflectivity 65%) and in the 2 to 5 ,um wavelength range was 70% (30%
15 reflectivity).
