Composition and spectral reflectance data of barium sulfate polyvinyl alcohol coatin

Schutt, J. B.

Shai, C. M.

English

NASA Technical Reports Server

.CHARLES M. SHAl 
JOHN B. SCHUTT - 
+ - G O D ~ A R R  SPACE F ~ H T  CENTER 
- GREENBELT. MARY [AND 
. / > 
- - 
https://ntrs.nasa.gov/search.jsp?R=19710022844 2020-03-23T15:14:06+00:00Z
2ORMULATION PROCEDIiRE AND SPECTRAL DATA FOR 
A HIGHLY REFLE CTmG COATING FROM 200 ^ ilm TO 2300 
Chmles M. Shai 
,--- 
John B. Schutt 
GODDARD SPACE FLIGHT CENTER 
Greenbelt, Maryland 
FORMULATION PROCEDURE Ah'D SPECTRAL DATA FOR 
A EIIGHLY REFLECTING COATING. FROM 200 nm TO 2300 nm 
Charles M. Shai 
John B. Schutt 
ABSTRACT 
A method for formulating a highly reflecting and mechanically stable 
coating from very pure barium sulphate is carefully detailed. Re- 
flectance data are presented for this coating from 200 nm to 2500 nm 
and compared with the commercially avaihb1e Eastman Kodak pre- 
formulated coating as well as freshly smoked magnesium oxide. Con- 
psred to the Eastman coatbg; the one detailed& &e bodgof_this report 
has the zdvantages of somewhat higher reflectance vslues throughout 
the spectral region of interest as well as greater ease of handling. 
CONTENTS 
...................................................... ABSTRACT 
INTEODUCTXON. ................................................. 
FORMULATATG PRO CEDERE ..................................... 
SURFACE PREPARATION AND APPLICATION PROCEDURE ........ 
SPECTRAL RE FLE CTmCE DATA ................................ 
Applications of MS-125.. ....................................... 
.............. ..Future of Pgnts as pav io l e t  Reflecting Coatings. 
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Page 
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iii 
LIST OF ILLUSTRATIONS 
Figdre 
- 1 Comparisons of Spectra! RefIedance Data for 
Mg 0. MS-125. Eastman White Reflectance Standard 
Page 
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LIST OF TABLES 
Table - - 
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rage 
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1 FormulationMS-125 ...................................... -3.- 
5- 
FORMULATION PROCEDURE. AND SPECTRAL DATA 
FOR GSFC WHITE REFLECTANCE STANDPIRD 
INTRODUCTION 
Continuing interest in highly reflecting diffuse coatings has pompted investiga- 
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tions into the promisi?of such materials as the z&ali metal halides, particularly 
sodium cliloride f the alkaline earth carbonates, particularly magnesium car- 
bonate. &uninum oxide - magnesium oxide - potassium silicate paints, barium 
sulphate powder, barium sulphate powder in polyvinyl alcohol and smoked mag- 
nesium 0~5de to recall a few. Most often .used among these candidates is smoked 
magnesi-ii oxide. When freshly smoked, both its average diffuse reflectance and 
the Irxqpitude of its reflectace from 200 ,am to 2400 a re  unchallenged. This 
property is paticularly true in the ultraviolet. However, the atmospheric 
instability 2.3 and fragility ~f smcked magnesium oxide spurred investigations 
for the purpose of f i n h g  compromise coatin$ got suffering Erom the disad- 
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vantages of smoked magnesi-um oxide. This sear& is r e q & ~ ~ { b l e  for the 'List 
. . 
oE materials and systems given above. 
A!?hnllgh_ bz&i1tm suEphakbe beeo hvestigated ic o-e ~ r a ~ r g p r e v i c t l s l y ,  it 
was not until a high purity barium sulphate powder and a pol~?;in~i zlcohol sus- 
gension of that powder were introduced by the Eastman Kodak Company that 
barium sulphate based diffuse rzfiectance coatings received extensive attention. 
A paper by Grum and ~ u c k e y ~  summarizes the Kodak process for purification 
lStuart, J. U-? Tech. Brief 71-10110. 
2Hammond 111. H. K., J. Opt Soc. .her. 45, 904(1955)- 
3Weber. H., Z Phys. 130. 392i1951). 
4Grum. F. and Luckey, G. Uf., .*p. Opucs, 7. 2W-2234:.1968). 
1 
and precipitation of their high purity barium sulphate and gives the historical 
background serving to generate their interest in this particular powder. Also 
included in their publication is comparative data with smoked magnesium oxide. 
Because these authors a r e  complete in their treatment of the virtues of barium 
suiphate powder and the polyvinyl alcohol coating formulation, we are  conteat 
with meticulously treating the formulation problem. Experiences in our labo- 
ratory with the Eastman paint demonstrated that problems were usually en- 
countered in its application because of phase agglomeration between pigment- 
pigment and pigment binder due ta interactions. Redispersion of theix formula- 
tion has been found difficult. It  is with this aspect of the problem that this 
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write-up is primarily concerned; namely, providing histructions for formulation 
of the coating wherefrom these aforementioned interactions are  minimal. Also, 
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the formulation as given is found zfter standing to be redispersable by hand 
stirring after the addition of water. 
Included also a re  reflectance m v e s  comparing both the Eastman coating and 
our coating with freshly smoked magnesium oxide. For easy reference, data on 
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s o d i . ~ ~  chloride is also incorporated. 
FORMULATING PROCEDURE 
Referring to the formulation presented in Table I, the dominant vehicle, water, 
is sp1.it into two portions. The first portion, weighing 72 grams, is to be com- 
bined with the polyvinyl alcohol while the remaining portion is mixed with the 
absolute alcohol. After weighing out 72 grams of water into a suitable container, 
the water is heated to about 90°C or to a temperature sufficient for melting the 
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polwinyl almi1o"r After the proper melting temperature for the polyvinyl 
Formulation MS-225: 3a1:itm Sulphate Polyvinyl Alcohol Coating 
With Extended White Reflectance 
Ingredient Weight {gms.) Percent Tiemarks 
Barium Eastman Kodak 
Sulphate White Reflectace 
(Ba So, . 2 H,O) 400 55.2 Standard No. 6091 
Water 197 27.2 Distilled 
Polyvinyl 99% Hydrolyzed 
Alcohol 3 0.4 mot water soluble) 
Ethyl Alcohol 125 17.2 Absolute 
alcohol has been reached by the water, the polyvinyl alcohol solids are  sprinkled 
slowly into the water, while the solids are kept under rapid agitation to prevent 
agglomeration and the formation of the PVA into Zarge~globules . Provided 
formulations of this coating are not excessive in sic-, initial operation is 
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most conveniently carried out using a magnetic scirrer. While the polyvinyl 
alcohol is clissolving - a procedure normally r e q u i r e  about fifteen minutes - 
the re-? portion of water (125grams) is mixed with therequirquired!5 grquir+m 
portion of absolute alcohol in a separate container. 
Once the polyvinyl alcohol has achieved complete solvation, the solution is 
dio~ved to cool down to room temperature under continued agitation. Once room 
temperatme has been reached by the solution, i t  is added to a blender jar. Sub- 
sequently the blender is operated at high speed after which the 250 grams of 
alcohol-water solution is added slowly. Care must be taken to prevent dissolu- 
tion of the polyvinyl alcohol. A safe duration time far this step is  two minutes. 
Care must also be 'taken to preirent noticeable heating of the mixture. 
Finally, the required portion of barium sulphate (400 g r a ~ s )  is added to the 
rapidly agitating mixture. The powder may be added rapidly. The required 
dispersion should be obtained in ten to fifteen minutes. Care must be talken 
once more to prevent over heating of the coating composition. 
SURFACE PREPARATION AND APPLICATION PROCEDURE 
Prior to application of the coating compositior to metal surhces, the smfaces 
must be thoroughly degreasec?. Since most applications of this coating are to 
small objects such as integrating s?heres, degreasing may be readily occom- 
plished by wipi- with acetone, methyl ethyl ketone,'etc. Only in unilsul cir- 
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cumstances is alkali cleaning followed by acid etching required. Once the sur- 
faces ire-grease free, a suitable prim2 coat must be zpplied. A satisfactory 
primer for this instance is a flat acrylic white. The intermediate application 
of n_ white pri.mer is required because residual bisulphate or szlfuric acid 
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. \ 
_mieties iire gradually released from tho surfaces of the pigment, which are 
capable of diffusing to'the metal-coating interface and reacting with metal to 
form hydrogen. Sim~ltaaeously, sulfuric-bisulphate migration is gaziilleled by 
the counter~diffusion of metallic particles and ions capable of staining or de- 
stroying the coating. Consequently, surface preparation and suitably precoating 
are important. 
Prior to spray application, the coating composition as in the Formulating 
Procedure must ba thoroughly stirred and diluted with mfficient distilled water 
7 
to allow the paint to regain its freshly formulated consistency. Normally after 
a shelf Iife of two weeks or more agglomeration & occur during spray appli- 
cation. Consequeutly, prior addition of water expedites appliption and provides 
the desired cosmetic appearance. 
The coating composition a s  for~~ulated is best applied using a spray appazatus 
incorporating a pressure pot. A scitable system is BZNKS model number seven. 
As in the procedure recommended by Eastman Kodak, premarking the primed ' 
surface with a pencil. provides a convenient way for monitoring the coverago of 
the mating over the primer. Normally this hide is achieved with about 40 mils 
of the barium sulphate coating. 
After application, the coating is &--cured. Heating is mecessary  and is to be 
avoided. 
SPECTRAL RE F i E  CTANCE DATA 
.. 
Spectral comparisons of the Eastman diffuse ref l~shse  with the one described 
herein (MS-125) are presentee! in Figure 1 dong with smoked Mg 0 and sodium 
chloride-for the sake of completeaess. Data for the Eastman, MS-125, and 
smoke6 Mg 0 coatings were obtained on a Cary model 90 spectrorefledoneter 
m d  mrrnalized t~ AsdlaPsde. F- Ratafar the sdium &aide -coat& w-w-e 
obtained by means of a Beckman PK-2A spectroreflectometer &th a Gier- 
Dunlde attachment. 
The use of barium sulphate dispersed in polyvinyl alcohol as an integrating 
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coating is now nearly universal. Sodium chloride is also currently in use, but 

because application is more time consuming and rewires considerable experi- 
ence, its use les MS-125. When the highest infrared reflectance is  required, 
sodium chloride must be used; however, if high ultrasgolet reflectance below 
295 IUE is required, use of MS-125 is required. / 
Diffuse reflecting white coatings are co_ntjn~&l~ - - ir; de&md for calibrating 
photomultiplier tubes used in flight experiments. Calibration coatings have been 
prepared for OAO-A, B, C and EIEAO. Similarly, the Cerenkov chamber for 
-- HEGE has been coated with this compositi~n because it cv,ently offers the, 
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best reflectance intheneighborhood of 200 nm, as well+,-mechanicaI s t a b i l ~ z ~  - 
. 
Uses for the Nimbus Project a r e  numerous, but the continuing use is for coating 
one inch fused silica integrating spheres flodin on sound rockets to obtain caIi- 
bration points for the BACKSCATTER ULTRAVIOLET EXPERDENT flown - 
For ERTS A, a thirty inch aluminum sphere was coated, again to prnvide a 
suitable surface for calibration of a scanner with sensitivity in the ultraviolet, 
visible as well as infrared regions of the electromagnetic spectrum. 
. 
Future of Paints as  Ultraviolet Refleeti- Coatings - m 
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yithin the context of the term ultraviolet reflecting as used in this section is 
meant Egh ultraviolet reflectance and dX&eness in the neighborhood of 200 nm. 
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h o k e d  magnesium oxicte not only exists a s  the most refiecting coating ininthis 
region but also the most diffuse.5 Optically therefore, b k u m  suIphate and po'g- 
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vinyl alcohol are less sztisfactory. -4 major difficulty in further Sevelopment of a 
SPrivate cornmunicarion from Arlen Kruger. Simbus Pzaject - - .  
10 
nechanical!j. stable coating for this region is purity of the powdcr - not. with- 
:;tanding the fact that a b i ~ d e r  could not be used since all are str~cgl? sbsorbing 
in this regim. The essential wr?cepts req~&red for a choice of pc:vder are the 
ionization ptential of the .yersp_ective catio'l and the electron affirit~i sf the 
perspective anion. To maximize the pbton  energy far reflectance cct-off, the 
dicference beiween these quantities must b e  maximized. From the periodic 
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b h l e  therefore, cesium fluoride fits this condition best, but not opt;-ndiy &;ncc? 
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it is too moisture sensitive.' With elaborate wntainment precs:;ll;ons perhaps it 
could be used. ,Lithium fluoride is a more optimal choice than cesiun fluoride 
because it i s  less moisture sensitive. Carrying on now with the subjest of 
purity, it is clear that whenever a host iike lithium fluoririe, i .L . . one w i t h  a , , 
wide band gap. is impure extraneous absorptions a r e  introduced at the gap 
ener~ees for the 1mpur;ties becacse their. ioniation potential e!e?r;rc~~. sffinity 
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Bifferences a re  less than that of the hoit. It appears therefore that smoked 
magnesim oside and barium sulphate-polyvinyl alcohol aze opt i~ la i  &-;I. tt.c,ir 
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respective mechuiic~i  strengths. 


Formulation procedure and spectral data for a highly reflecting coating from 200 nm to 2300 nm

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