Zinc orthotitanate suitable for use as a pigment for spacecraft thermal control coatings is prepared by heating a slightly zinc deficient reaction mixture of precipitated oxalates of zinc and titanium. The reaction mixture can be formed by coprecipitation of zinc and titanium oxalates from chloride solution or by mixing separately precipitated oxalates. The mixture is first heated to 400 to 600 C to remove volatiles and is then rapidly heated at 900 to 1200 C. Zinc orthotitanate produced by this method exhibits the very fine particle size needed for thermal control coatings as well as stability in a space environment

Gates, D. W.

Gilligan, J. E.

Harada, Y.

Logan, W. R.

English

NASA Technical Reports Server

United States Patent [191 [111 
Fletcher et ai. 1451 Aug. 2, I977 
[54] METHOD OF PREPARING ZINC 
ORTHOTITANATE PIGMENT 
[76] Inventors: 
[21] Appl. No.: 
[22] Filed: 
James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of; Daniel W. Gates, 
Huntsville, Ala.; Yoshiro Harada, 
Skokie, Ill.; William R. Logan, 
Wheaton, Ill.; John E. Gilligan, 
Prospect Heights, 111. 
696,989 
June 17,1976 
[51] Int. (3.2 ........................... CO9C 1/36; C09C 1/04 
[52] U.S. Cl. .................................... 106/292; 106/296; 
106/299 
[58] Field of Search ........................ 106/292, 296, 299 
[561 References Cited 
U.S. PATENT DOCUMENTS 
2,140,235 12/1938 Lederle et al. ....................... 106/296 
3,337,358 8/1967 Vossen ................................. 106/292 
3,607,338 9/1971 Webb ................................... 106/292 
FOREIGN PATENT DOCUMENTS 
493,871 10/1938 United Kingdom ................. 106/292 
Primary Examiner-Patrick P. Garvin 
Assistant Examiner-J. V. Howard 
Attorney, Agent, or Firm-L. D. Wofford, Jr.; Joseph H. 
Beumer; John R. Manning 
P I  ABSTRACT 
Zinc orthotitanate (Zn2Ti04) suitable for use as a pig- 
ment for spacecraft thermal control coatings is prepared 
by heating a slightly zinc-deficient reaction mixture of 
precipitated oxalates of zinc and titanium. The reaction 
mixture can be formed by co-precipitation of zinc and 
titanium oxalates from chloride solution or by mixing 
separately precipitated oxalates. The mixture is first 
heated to 400" to 600" C to remove volatiles and is then 
rapidly heated at 900" to 1200" C. Zinc orthotitanate 
produced by this method exhibits the very fine particle 
size needed for thermal control coatings as well as sta- 
bility in a space environment. 
5 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19770023293 2020-03-20T07:55:29+00:00Z
4,039,347 
1 2 
lengths below 380 nm, indicating absorption by small 
amounts of uncombined zinc oxide. 
SUMMARY OF THE INVENTION 
In the present invention zinc orthotitanate is prepared 
by heating a slightly zinc-deficient mixture of precipi- 
tated zinc oxalate and titanium oxlate. The oxalate mix- 
ture, which can be prepared by combining separately 
precipitated zinc oxalate and titanium oxalate or by 
10 co-precipitation of mixed oxalates from chloride solu- 
METHOD OF PREPARING ZINC 
ORTHOTITANATE PIGMENT 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 Of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC2487). 
5 
tion, is heated to 400" to 600" C to remove volatile 
decomposition products and is then heated at 900" to 
1200" c. This method produces zinc orthotitanate p i -  
mdy made up of in the very fine she range 
Grinding or milling requirements are minimized so 
that a purer product, with better reflectance and 
ity in the space environment, is obtained. ~ ~ f l ~ ~ ~ ~ ~  
properties at critical wavelengths are also enhanced by 
ric, reaction mixture. 
improved method of preparing zinc orthotitanate. 
Another object is to provide if preparing zinc or- 
thotitanate pigment wherein the pigment is initially 
formed in submicron size particles. 
Another object is to provide a method of preparing 
zinc orthotitanate wherein requirements for milling or 
grinding are minimized. 
Yet another object is to provide zinc orthotitanate 
BACKGROUND OF THE INVENTION 
This invention relates to Zinc titallate pigments and to 
spacecraft thermal control coatings utilizing such pig- 
ments. 
Long term operation of vehicles and equipment in the 
space environment requires the availability of effective 
thermal control coatings. In the absence of a stable 
white coating Or other Protective measures, surfaces 
become overheated, largely because of absorption of 
manfunction when its temperature limitations were 
exceeded. Coatings for such applications should exhibit 
a very low and stable ratio of solar absorptance (as) to 25 
infrared emittance (e) along with good mechanical in- 
tegrity and ease of application. 
Various problems have been presented in the devel- 
opment of pigments and coatings for space applications. 
Many pigments which have favorable reflectance prop- 30 
erties for terrestrial uses undergo a severe degradation 
bined conditions of vacuum and ultraviolet radiation 
l5 needed for thermal control coating pigments. 
which are exposed to radiation would 2o use of a slightly zinc-deficient, rather than stoichiomet- 
energy. 'pacecraft equipment It is therefoye an object ofthis invention to provide an 
of reflectance upon prolonged exposure to the corn- pigment having improved and resistance to degradation in the 'pace environment* 
encountered in outer space. Most of the binders and DESCRIPTION OF THE PREFERRED 
EMBODIMENT vehicles used for conventional coatings cannot be used 35 
in space because they are rapidly damaged by solar 
ultraviolet radiation. In some cases pigments and bind- The Oxalate from which 
ers which are stable to the space environment may be the zinc orthotitanate is prepared can be readily ob- 
incompatible with one another. tained by precipitation from zinc and titanium chloride 
progress has been made in recent years in determining 40 solution with an aqueous oxalic acid solution. Although 
degradation mechanisms and providing stable binders not critical, it is preferred to use chloride solutions at a 
and pigments for space applications. For example, U.S. concentration of lo to 30 weight per cent and 
Pat. Nos. 3,576,656 and 3,607,338 disclose methods of oxalic acid at a concentration of 10 to 20 weight Per 
treating zinc oxide and zinc orthotitanate pigments, cent. The titanium chloride solution can be prepared by 
respectively, to improve their initial reflectance and 45 dropwise addition of TiC141iquid to water in an ice bath 
resistance to loss in reflectance upon exposure to ultra- to avoid hydrolysis of the TiC14. Zinc and titanium 
violet radiation in vacuum. A need exists, however, for oxalate can be Precipitated separately and mixed subse- 
further improvement in pigment properties, particularly quently Or mixed initially by Co-PreciPitation. Although 
for zinc orthotitanate, which exhibits higher intrinsic co-precipitation iS advantageous in that an intimate 
reflectance than the zinc oxide and titanium oxide from 50 reaction mixture is f m m d  without requiring powder 
which it has been conventially prepared by solid-state mixing or blending, separate precipitation with subse- 
reactions. One Source of pigment damage and loss of quent mixing is preferred because better control of reac- 
stability has been the grinding or milling steps required tion stoichiometry is realized and problems associated 
to obtain the very small particle she (submicron to 5 with formation of coarse zinc oxlate particles in co- 
microns, and primarily about .6 micron) needed for 55 precipitation are avoided. 
optimum reflectance of solar radiation. Zinc orthotita- Precipitation from either separate or mixed zinc and 
nate prepared by previous zinc oxidetitanium oxide titanium chloride solutions can be readily carried out by 
reactions has a much larger particle size, and the pro- mixing with oxalic acid solution and maintaining the 
longed grinding or milling required to obtain finer parti- reaction mixture under agitation and mild heating. For 
cles results in decreased reflectance owing to particle 60 co-precipitation, a preferred procedure includes the 
damage (commonly known as "mill yellowing"). In steps of mixing the zinc-titanium chloride and oxalic 
addition, contaminating impurities are inherently intro- acid solutions at a temperature of about 40" C and heat- 
duced, and degradation resistance is decreased owing to ing at about 90" C under agitation for about one to four 
the creation of active sites for degradation mechanisms. hours, depending on batch size. In the case of separately 
Initial formation of pigment particles in the desired size 65 precipitating zinc and titanium oxalate, a temperature of 
range would alleviate these problems. Another problem 10" C to 30" C is preferred for zinc and 80" C to 95" C 
encountered with zinc orthotitanate pigments is that for titanium oxalate, for a reaction time of at least about 
they frequently show decreased reflectance at wave- one hour in either case. 
3 
4,039,347 
The precipitated zinc and titanium oxalates, or mix- 
ture thereof, can be recovered by filtration, washing 
and drying. Filtration is readily performed on a labora- 
tory scale by use of a Buchner funnel system under 
slight vacuum. Conventional equipment and techniques 
can be used for larger amounts of material. The filtered 
precipitate is then washed thoroughly with hot water 
until there is no evidence of acid in the filtrate, thus 
showing removal of any hydrochloric acid and oxalic 
acid. The washed precipitate is thoroughly dried by 
light heating in air, with a temperature of 60" to 80" C 
for a period of at least 10 to 36 hours being suitable for 
this purpose. 
For co-precipitated oxalates the resulting dried pow- 
der is ready for calcining. In the case of separately 
precipitated oxalates, the separate powders are com- 
bined and mixed thoroughly, for example, by dry blend- 
ing for 2-6 hours. (This blending step, since it is per- 
formed prior to conversion to zinc orthotitanate, does 
not degrade pigment properties appreciably). 
The zinc and titanium oxalates, or in the case of co- 
precipitated oxalates the zinc and titanium chlorides, 
are combined at a molar proportion slightly deficient in 
zinc, as compared to the stoichiometric 2:l zinc-to- 
titanium molar ratio represented by the formula (Zn2Ti- 
04) for zinc orthotitanate. Zinc-to-titanium molar ratios 
of 1.92:l to 1.98:l can be used, and about 1.951 is pre- 
ferred. Larger amounts of zinc will result in decreased 
reflectance at wavelengths shorter than 380 nm, the 
characteristic absorption edge of zinc oxide. Although 
it would be expected that use of a zinc-deficient mixture 
would result in the presence of unconverted titanium 
oxide, thereby producing decreased reflectance, such is 
not the case. Free titanium oxide is not present when the 
above-stated molar ratios are used; apparently non-stoi- 
chiometric complex molecules are formed along with 
zinc orthotitanate, the reflectance of such molecules 
being equivalent to that shown by stoichiometric zinc 
orthotitanate. At ratios below the range given above, 
reflectance properties are initially favorable, but long- 
term stability under ultraviolet radiation in vacuum is 
decreased. 
Conversion of the oxalate mixture to zinc orthotita- 
nate is carried out in two steps: A pre-calcine step of 
heating to 400" to 600" C in order to remove volatiles 
released in decomposition of precursor compounds and 
a final, rapid calcine at 900" to 1200" C. The initial step 
is required to avoid violet reactions which would other- 
wise occur if the mixture was immediately subjected to 
higher temperature conditions. In the final calcine the 
powder is heated at 900" to 1200" C, and preferably at 
about 1ooo" C to 1050" C until conversion to zinc or- 
thotitanate is completed. Higher temperatures are to be 
avoided since they result in loss of fine particle size by 
sintering. Rapid final firing also serves to maintain fine 
particle size. In a preferred procedure the powder mix- 
ture, in a container such as a fused quartz vessel, is 
inserted in a furnace that has been pre-heated to the 
desired temperature, thus avoiding a prolonged heat-up 
time. At the preferred temperature a firing period of 1 
to 18 hours can be used, depending on batch size. The 
fired powder is then removed from the oven to avoid a 
long cool-down time. 
The product obtained in the two-step firing proce- 
dure is zinc orthotitanate primarily made up of particles 
in the submicron range, with near-ideal size distribution 
for solar-reflector pigment use; in particular, most of the 
particles are submicron in size, with a sufficient amount 
4 
of particles at or near .6 microns in size so that optinum 
reflectance is provided. Any agglomeration can be bro- 
ken up by light ball-milling and larger particles can be 
removed by screening. 
The resulting pigment is suitable for use in thermal 
control coatings for spacecraft as well as for other ap- 
plications. For thermal-control coating applications the 
pigment is combined with a suitable degradation-resist- 
ant binder. Most organic resins are not stable in the 
10 space environment but certain silicones can be used. 
Examples of suitable binders include the polymethyl- 
siloxane resin available commercially from the Owens- 
Illinois Glass Company under the designation "01-650" 
and poly-dimethylsiloxane elastomer available commer- 
15 cially from the General Electric Company under the 
designation "RTV-602". Further details regarding for- 
mulation of thermal control coating are available in 
U.S. Pat. No. 3,576,656 and pigments prepared by the 
present invention can be used in place of zinc oxide in 
The invention is further illustrated by the following 
5 
20 the coatings described therein. 
examples: 
EXAMPLE 
Zinc orthotitanate samples (approximately 250 grams 
product per sample) were prepared from mixtures of 
titanium oxalate and zinc oxalate at varied zinc-to- 
titanium molar ratios by the following procedure: Tita- 
nium oxalate was precipitated by reaction of a titanium 
30 chloride solution with an oxalic acid solution under 
agitation for one hour at 90" C. Zinc oxalate was precip- 
itated by reaction of a zinc chloride solution with an 
oxalic acid solution under agitation for one hour at 40" 
C. The oxalate precipitates were then filtered, washed 
35 and thoroughly dried. The dried oxalate powders were 
combined at zinc-to-titanium molar ratios of 1.9O:l; 
1.951; 2.00:l; and 2.051 by dry mixing for 16 hours. All 
sample mixtures were then fied at 600" C for 2 hours in 
air. One-half of each sample was then fired at 900' C in 
40 air for 2 hours and the other half was fved at 1200" C in 
air for two hours. In all cases "flash firing" was used, 
that is, the samples in a quartz container were inserted 
into an oven pre-heated to firing temperature and with- 
drawn directly to room temperature for cooling after 
Particle size of the fired samples was determined by 
SEM (scanning electron microscope) analysis. All sam- 
ples fired at 900" C showed a particle size range of 
submicron to 2 microns, and mostly submicron. The 
50 1200" C fired samples showed a particle size in the range 
of 1 to 5 microns, with only minor amounts of submi- 
cron particles, thus indicating that some sintering or 
"necking" of particles had occurred. 
Spectral reflectance of the samples was determined 
55 by forming a water suspension of the powder particles, 
spraying the suspension on aluminum blocks to build up 
a layer 4 to 10 mils thick, inserting the blocks in a reflec- 
tometer and measuring reflectance at wavelengths from 
0.32 to 0.5 microns (325 to 500 nm). Samples fired at 
60 900" C and prepared from mixtures having zinc-to- 
titanium ratios of 2.00:l and 2.051 showed a sharp 
break, or "knee" in their reflectance spectra curves 
below the 380 nm. characteristic absorption edge for 
zinc oxide, reflectance decreasing to 35 to 50 per cent at 
65 slightly lower wavelengths. For 900" C samples having 
zinc-to-titanium ratios of 1.90:l and 1.951, the break in 
the spectral curve was minor or non-existent, indicating 
absence of any free zinc oxide. 
25 
45 firing. 
4,039,347 
5 6 
The samples were then subjected to irradiation testing 
under simulated space conditions (ultraviolet radiation 
in Vacuum) to determine their long term stability. After 
loo0 ESH (equivalent sun hours) a sample fired at 900" 
C and having a zinc-to-titanium molar ratio of 1.951 5 
showed an increase in absorptance of only 0.02 over its 
original valve of 0.11, thus indicating excellent stability. 
The above example is merely illustrative, and one 
skilled in the art can make various changes and modifi- 
cations in the procedures given therein without depart- 10 
ing from the spirit and scope of the invention. 
form of very fine particles which comprises forming a 
mixture of precipitated oxalates of zinc and titanium 15 oxalate and titanium oxalate. 
having a zinc-to-titanium molar ratio of 1.92:l to 1.98:1, 
heating said mixture first at a temperature of 400" to 
600" c and then at a temperature of 900" to 1200" C. 
2. The method of claim 1 wherein the final heating is 
at a temperature of about 1~ to 1250" c. 
3, ne method of 1 wherein said heating at a 
temperature of includes the step of 
inserting a container of said mixture into an Oven 
heated to 900" to 1200" C. 
wherein said mixture is 
prepared by coprecipitation of said oxalates from a 
chloride solution. 
prepared by combining separately precipitated zinc 
to 1200" 
4* The method Of 'Iaim 
What is claimed is: 
1. m e  method of preparing zinc orthotitanate in the 5. The method of claim 1 wherein said mixture is 
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Method of preparing zinc orthotitanate pigment

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