A trowelable ablative coating composition is disclosed. The composition comprises an epoxy resin, an amide curing agent, glass microspheres and ground cork. A method for protecting a substrate is also disclosed. The method comprises applying the trowelable ablative coating discussed above to a substrate and curing the coating composition

Headrick, Stephen E.

Hill, Roger L.

NASA Technical Reports Server

United States Patent [191 
Headrick et al. 
[ i i ]  Patent Number: 4,837,250 
[45] Date of Patent: Jun. 6, 1989 
[54] TROWELABLE ABLATIVE COATING 
COMPOSITION AND METHOD OF USE 
[75] Inventors: Stephen E. Headrick, Huntsville; 
Roger L. Hill, Decatur, both of Ala. 
[73] Assignee: USBI Booster Production Company, 
Inc., Huntsville, Ala. 
[21] Appl. No.: 183,268 
[22] Filed: Apr. 11, 1988 
Related U.S. Application Data 
[62] 
[51] Int. Cl.4 ......................... CO8K 7/26; C08K 13/02 
[52] U.S. Cl. .................................... 523/179; 523/223; 
523/446 
[58] Field of Search ................ 523/179, 223, 138, 446 
Division of Ser. No. 76,971, Jul. 23, 1987, Pat. No. 
4,772,495. 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,380,941 4/1968 Dittman et al. ..................... S23/179 
4,077,921 3/1978 Sharpe et al. ........................ 2W2.S 
4,204,899 5/1980 Walker et al. ...................... S23/179 
4,595,714 6/1986 McAllister et al. ................. S23/179 
4,031,059 6/1977 Strauss ................................ S23/179 
Primary Examiner-Lewis T. Jacobs 
Attorney, Agent, or Firm-Kevin E. McVeigh 
[573 ABSTRArn 
A trowelable ablative coating composition is disclosed. 
The composition comprises an epoxy resin, an amide 
curing agent, glass microspheres and ground cork. A 
method for protecting a substrate is also disclosed. The 
method comprises applying the trowelable ablative 
coating discussed above to a substrate and curing the 
coating composition. 
2 Claims, 1 Drawing Sheet 
https://ntrs.nasa.gov/search.jsp?R=20080012333 2019-08-30T03:57:37+00:00Z
U.S. Patent Jun. 6,1989 4,837,250 
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1 1.5 2 2.5 3 4 5 6 7 8 4 10 15 20 25  30 40 SO 
2 Heat Rate - (Btu/ft -sec) 
2 
4,837,250 
1 
The foregoing and other features and advantages of 
the present invention will become more apparent from 
the following description and accompanying drawings. 
TROWELABLE ABLATIVE COATING 
COMPOSITION AND METHOD OF USE 
BRIEF DESCRIPTION OF THE DRAWINGS 
The FIGURE shows the recession rate of the abla- 
The invention described herein was made in the per- 5 
formance of work under NASA Contract No. NAS 
8-36100 and is subject to the provisions of Section 305 
of the National Aeronautics and Space Act of 1958 (72 
Stat. 435; 42 U.S.C. 2457). 
076,971, filed on July 23, 1987, now US. Pat. No. 
4,772,495, issued Sept. 20, 1986. 
DESCRIPTION 
tive coating of the present invention and of an exem- 
plary conventional coating versus the heating rate. 
BEST MODE FOR CARRYING OUT THE 
INVENTION 
The mixture of epoxy resin and amide curing agent of 
the present invention may be of any epoxy resin and any 
curing agent, combined in any proportion that provides 
15 the required pot life, cures at ambient temperature and 
imparts the required strength, adhesion, hardness, and 
thermal stability to the cured trowelable ablative coat- 
ing composition. 
This is a division of copending application Ser. NO. 10 
1. Technical Field 
ne field of art to which this invention pertains is 
2. Background Art 
synthetic coating compositions. 
Ablative materials are used as heat shields to protect Epichlorohydrinhisphenol A type epoxy resins are 
space vehicles as they are subjected to high velocity, 20 preferred. An epichlorohydrinhisphenol A epoxy resin 
re-entry to the earth's atmosphere. Such materials need tice Of the present invention is known as Scotch- 
high temperature conditions during launch and during 
exhibit good adhesion to the substrate, be flexible 
enough to provide thermal strain stability, have good 
thermal stability and have low thermal conductivity. 
Ablative materials in current use may be roughly 
categorized as either solid materials or cured-in-place 
coating compositions. Cured-in-place ablative coating 
compositions may be either low viscosity compositions 3o 
for a spray application or high viscosity compositions 
for hand trowel application. 
that was found to be particularly suitable for the prac- 
Weld @) Epoxy Adhesive 2216B (Translucent) manu- 
factured by 3M Corporation. 
Amide curing agents are preferred. An amide curing 
agent found to be particularly suitable for the practice 
of the present invention is Scotch-Weld @ Epoxy Ad- 
hesive 2216A (Translucent) manufactured by 3M Cor- 
poration. 
It is preferred that the mixture of epoxy resin and 
amide curing agent comprise between about 67 weight 
percent and about 73 weight percent of the ablative 
25 
(e*g* cork sheet Or coating composition, and most preferred that the mix- 
ture comprise between about 69 weight percent and must be conformed and adhered to the substrate to be 
protected. Fabrication of a smooth covering from solid 35 about 71 weight percent of the compos~t~on~ is pre- 
ablative materials and removal of the charred covering ferred that the quantity by weight ofamide curing agent 
from the spacecraft after re-entry are difficult, time in the mixture to be equal to the quantity by weight of 
consuming and expensive processes. epoxy resin in the mixture multiplied by a factor be- 
Spray applied coating compositions such as that dis- tween about 0.97 and about 1.03 and is most preferred 
closed in U.S. Pat. No. 4,077,921 have overcome some 40 that the quantity by weight of amide curing agent be 
of the difficulties associated with solid coverings, but equal to the quantity by weight of epoxy resin multi- 
are not applicable in all Situations. Solid ablative mate- plied by a factor between about 0.99 and about 1-01. 
rial such as cork sheet is still used to protect protuberant The glass microspheres of the present invention may 
StrUCtUreS and high viscosity ablative coating composi- be any hollow discrete spheres of alkali metal silicate 
tions are used for close-out and repair of Primary &la- 45 glass that impart the desired low density and increased 
tive layers. flame resistance to the coating composition. Sodium 
Conventional high viscosity ablative coating compo- borosilicate glass microspheres with a particle size be- 
sitions are each deficient in some significant respect tween about 20 microns and about 200 microns and a 
(e& high cost, short Pot life or hazardous ingredients). bulk density between 0.185 grams/cubic centimeter 
What is needed in this art is an ablative coating com- 50 (g/cm3) and 0.195g/cm3 are preferred. Commercially 
position that overcomes the above limitations. available glass microspheres that were found to be suit- 
able for the practice of the present invention are known 
as IG-101 Glass Eccospheres, manufactured by Emer- 
A trowelable ablative coating composition is dis- son and Cuming. The glass microspheres should corn- 
closed. The composition comprises about 67 weight 55 prise between about 22 weight percent and about 28 
percent to about 73 weight percent of a mixture of an weight percent of the coating composition and most 
epoxy resin and an amide curing agent, about 22 weight preferably comprise between about 24 weight percent 
percent to about 28 weight percent glass microspheres, and about 26 weight percent of the composition. It is 
and about 4.5 weight percent to about 5.5 weight per- preferred that the glass spheres be packaged and stored 
cent ground cork. The quantity of the amide curing 60 in a manner to protect them against moisture absorp- 
. agent is equal to the quantity of the epoxy resin times a tion. 
factor between 0.97 and 1.03. The ground cork of the present invention may be any 
A further aspect of the disclosure involves a method ground cork that imparts the desired low density and 
for protecting a substrate in a high temperature, high low thermal conductivity to the coating composition. It 
velocity, erosive environment. The method comprises 65 is preferred that the particle size of the ground cork be 
applying the trowelable ablative coating composition such that it will pass through a 20 mesh screen yet be 
discussed above to the substrate and curing the compo- retained on a 40 mesh screen. A suitable ground cqrk is 
sition. known as Granulated Cork, 20/40 Screen, manufac- 
DISCLOSURE OF INVENTION 
4,837,250 
3 
tured by Sheller Globe, Incorporated. It is preferred 
that the ground cork comprise between about 4.5 
weight percent to about 5.5 weight percent of the coat- 
ing composition, and most preferred that the ground 
cork comprise between 4.8 weight percent and 5.2 
weight percent to the coating composition. It is pre- 
ferred that the cork be packaged and stored in a manner 
to protect it against moisture absorption. 
The elements of the coating composition of the pres- 
ent invention must be thoroughly mixed to obtain a 
homogeneous mixture. The coating composition of the 
present invention may be mixed using conventional 
mixing equipment of convenient size, such as a Hobart 
mixer, manufactured by Hobart Corporation. To insure 
proper cure, the epoxy and curing agent should be 
mixed together briefly prior to adding the glass micro- 
spheres and ground cork filler materials. It is preferred 
that the epoxy resin and curing agent be mixed for a 
time period between about 30 seconds and about 60 
seconds prior to the addition of the filler materials. It is 
preferred that the composition be mixed for an addi- 
tional time period of between about 3 minutes and about 
5 minutes after addition of the filler materials. 
The coating composition is self-curing at room tem- 
perature, and the rate of cure is sensitive to both tem- 
perature and relative humidity. For example, the com- 
position has a pot life or working time of about 90 min- 
utes in a controlled environment at a temperature be- 
tween 72" F. and 76" F. with the relative humidity be- 
tween 50% and 60%. As a further illustration, in an 
uncontrolled environment at a temperature between 85" 
F. and 95" F. with a relative humidity of between 70% 
and 80% in direct sunlight, the composition has a pot 
life of about 45 minutes. The curing reaction continues 
for several days at room temperature and is complete 
within 8 to 10 days. 
The coating composition may be applied by hand 
troweling or molded during a time period less than or 
about equal to the pot life of the material discussed 
above. 
Example I 
The substrate panels were fabricated from 8 inch 
thick 2219-T87 aluminum cut to 12 inch x 12 inch pan- 
els. The surfaces of the panels were cleaned with per- 
chloroethylene, chromate conversion coated, primed 
(Sikken Primer No. 463-6-3) and painted (Sikken's Top- 
coat No. 443-3-1). The painted surfaces were lightly 
cross-hatch sanded with 100 grit sandpaper and wiped 
clean with perchloroethylene. 
A trowelable ablative coating composition, known as 
Booster Trowelable Ablative (BTA), comprising 35% 
by weight epoxy resin (Scotch-Weld 2216B), 35% by 
weight amide curing agent (Scotch-Weld 2216A), 25% 
by weight glass microspheres (IG-101 Glass Eccos- 
pheres) and 5% by weight Granulated Cork (20/40 
screen) was mixed in a Hobart mixer. The curing agent 
and epoxy resin were added to the Hobart mixer and 
mixed for 45 seconds. The glass spheres and cork were 
added to the mixer and the composition was then mixed 
for an additional 3 minutes. 
The trowelable ablative coating composition was 
applied to the substrate panels by hand troweling. Even 
distribution was obtained by seating the substrate within 
a 12 inch X 12 inch frame and then applying and level- 
ing the coating composition. The panels were allowed 
to cure for 24 hours at ambient temperature before the 
frame was removed and allowed to cure for a total of 10 
- 
e 
4 
days before further testing. Test panels were then cut 
into test specimens of appropriate size as discussed be- 
low. Each specimen was sanded level and smoothed 
with minimal material removal, and then brushed clean 
Tensile specimens were cut from the test panels. The 
specimens were tested with a Satec System tensile tester 
at 75" F. at a crosshead speed of 0.05 inch per minute. 
The total load at failure was used to determine the ten- 
10 sile strength. Average tensile values, in pounds/square 
inch (psi), for 30 2"X2" specimens with i'' thick layers 
of coating are presented in Table 1. The majority of 
failures occurred at the primer/topcoat interface, indi- 
cating that the cohesive strength and adhesion of the 
15 coating composition to the topcoat is stronger than the 
adhesion between the primer and the topcoat. 
Density values were calculated from weight and 
volume measurements. Linear density was calculated 
by measuring the dimensions of a specimen at five sepa- 
20 rate locations for each dimension to obtain average 
dimensions and weighing the specimens on an analytical 
balance. After weighing, the coating composition was 
removed from the substrate leaving primer and paint 
intact. The substrate was weighed and measured for the 
25 final calculation. The average, in pounds/cubic 
foot,(lb/ft3), of two linear density measurements are 
presented in the Table 1. 
The hardness of the cured coating was measured 
using a Shore "C" durometer. These readings were 
30 obtained from each of the tensile samples before tensile 
testing. Average hardness values are presented in Table 
1. 
Parallel testing was performed using an exemplary 
conventional trowelable ablative coating (MTA-2). 
35 Results are also listed in Table 1. 
TABLE 1 
5 of sanding residue. 
BTA MTA-2 
tensile strength (psi) 602 478 
40 density (lb/ft3) 31.0 35.0 
hardness (Shore "C) 75 75 
The trowelable ablative of the present invention (BTA) 
exhibited a higher tensile strength and a lower density 
Example I1 
The ablative coating of the present invention projects 
an underlying substrate by gradual charring and reces- 
50 sion when subjected to high temperature, high velocity 
erosive environments. Testing was performed to char- 
acterize the rate at which the ablative coating of the 
present invention recedes under various conditions and 
to verify the ability of the coating to protect a substrate 
A trowelable ablative coating composition was for- 
mulated mixed as in Example I. A 0.5" thick layer of the 
coating was applied by hand trowel to each of eight 
20" X 20" X 8 "aluminum substrates according to the 
60 method described in Example I. A 0.5" thick layer of 
coating was applied to each of four 20"X 20"X 8 alumi- 
num substrates by injecting the coating between the 
substrate and a covering mold. 
Each of the specimens was mounted on the top sur- 
65 face of a 34" long X12" wide water cooled wedge 
shaped fixture for testing. Each specimen was subjected 
to high velocity (MACH 4), high temperature (1440" 
F.) conditions in a high velocity wind tunnel at each of 
45 than the conventional trowelable ablative (MTA-2). 
55 under simulated re-entry conditions. 
4,837,250 
- 5  6 
four heating rates (10BTU/ft2-sec, 20BTU/ftz-sec, ablative (MTA-2), and indicate a lower recession rate 
30BTU/ft2-sec and 40BTU/ft%c). The apex of the for the BTA at a given heating rate. 
wedge shaped fixture was oriented toward the wind The design results were verified by subjecting trow- 
5 sequence of conditions simulating launch and re-entry. angle of the fixture with respect to the air flow. A sec- All verification points fell below the respective design 
Ond wedge shaped known as a “shock genera- curves and none of the verification specimens exhibited 
tor”, was positioned above the surface of the specimen. 
The angle of the shock generator with regard to the The ablative coating of the present invention has a 
airstream was adjusted to deflect the airstream down- 10 low cost and poses minimal health hazard. The uncured 
ward toward the surface of the specimen and more coating composition has a long pot life, may be applied 
intensely heat a narrow band of the specimen to achieve by hand troweling or molding procedures, and cures at 
the 30 BTU/ftZ-sec and 40 BTU/ft2-sec heating rates. room temperature. The cured coating exhibits low den- 
n e  thickness of the coating layer was periodically sity, high adhesion and high tensile strength. The coat- 
measured with an electronic displacement measuring 15 ing Protects an underlying substrate by providing a 
ring and recession when exposed to high temperature, between the initial thickness of uncharred coating and high velocity environments. 
the final thickness of uncharred coating divided by the Although this invention has been shown and de- 
20 scribed with respect to detailed embodiments thereof, it duration of exposure to a particular heating rate. 
Design curves relating recession rate to heating rate will be understood by those in the art that vari- 
were generated for troweled BTA specimens, for ous changes in form and detail thereof may be made 
molded BTA specimens, and for troweled MTA-2 spec- without departing from the spirit and scope of the 
imens. These curves were of the form: claimed invention. 
Source and the heating rate was varied by varying the eled BTA specimens and BTA specimens to a 
exposure. 
system. Recession rate was calculated as the difference layer of that erodes by gradual char- 
25 I claim: 
;=xp, 1. A trowelable ablative coating composition for 
protecting a substrate under high temperature, high 
velocity, erosive conditions, consisting essentially of: 
about 67 weight percent to about 73 weight percent 
of a mixture of an epoxy resin consisting of an 
epichlorohydrinhisphenol A reaction product and 
an amide curing agent, wherein the quantity by 
troweled BTA 0.08824 1.219 quantity by weight of the epoxy resin multiplied by 
molded BTA 0.02439 1.621 35 a factor between about 0.97 and about 1.03. 
troweled MTA-2 0.03342 1.89 about 22 weight percent to about 28 weight percent 
hollow glass microspheres, and about 4.5 weight 
percent to about 5.5 weight 
where f =recession rate in (mil/sec) and q =heating 
rate in (BTU/ftz-sec). The x and y coefficients for each 
material are listed in Table 2. 
TABLE 2 
30 
X Y weight of the amide curing agent is equal to the 
The design curves are shown in the Figure, where 
curve A is the design curve for troweled BTA, curve B 
is the design curve for molded BTA, and curve C is the 4o 
design curve for troweled MTA-2. It should be noted 
that within the range Of heating rates Of  interest, (i.e. 
lOBTU/ftZ-sec to 40 BTU/ftZ-sec), the design curves 
for the trowelable ablative of the present invention 
(BTA) fall below those for the conventional trowelable 45 
percent ground cork. 
t. ne trowelable ablative of claim wherein the 
glass microspheres comprise sodium borosilicate glass 
microspheres having a diameter between about 20 mi- 
crons and about 200 microns and a bulk density of be- 
tween about 0.185g/cm3 and about 0.195g/cm3. * * * * *  
50 
55 
60 
65 


Trowelable ablative coating composition and method of use

United States Patent [191 [I  11 Patent Number: 4,772,495 
Headrick et al. [45] Date of Patent: Sep. 20, 1988 
[54] TROWELABLE ABLATIVE COATING 
COMPOSITION AND METHOD OF USE 
[75] Inventors: 
[73] Assignee: 
[21] Appl. No.: 
Stephen E. Headrick, Huntsville; 
Roger L. Hill, Decatur, both of Ala. 
United Technologies Corporation, 
Hartford, Conn. 
76,971 
[22] Filed: Jul. 23,1987 
[51] Int. Cl.4 .............................................. B32B 15/00 
[52] U.S. Cl. .................................... 427/386; 523/179; 
523/219; 521/54; 52V84.1; 521/85; 521/102; 
521/178; 521/907; 264/45.3 
[58] Field of Search ................ 427/386; 523/179, 219; 
521/54, 84, 85, 907, 102, 178; 264/45.3 
[561 References Cited 
U.S. PATENT DOCUMENTS 
4,031,059 6/1977 Straws ................................ 523/179 
4,070,020 1/1978 Darro ................................. 264/45.3 
4,077,921 3/1978 Sharpe et al. ....................... 523/179 
4,124,670 1 VI978 Cecka et al. ....................... 264/45.3 
4,129,634 12/1978 Cecka et al. ....................... 264/45.3 
4,595,714 6/1986 McAllister et al. ................. 523/179 
Primary Examiner-Morton Foelak 
Attorney, Agent, or Firm-Harry J. Gwinnell 
P I  ABSTRACT 
A trowelable ablative coating composition is disclosed. 
The composition comprises an epoxy resin, an amide 
curing agent, glass microspheres and ground cork. A 
method for protecting a substrate is also disclosed. The 
method comprises applying the trowelable ablative 
coating discussed above to a substrate and curing the 
coating composition. 
2 Claims, 1 Drawing Sheet 
Heat Rate - (Etu/ft'-sec) 
https://ntrs.nasa.gov/search.jsp?R=20080012391 2019-08-30T04:02:53+00:00Z
US. Patent Sep. 20,1988 4,772,495 
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2 Heat Rate - ( B t u / f t  -sec) 
4,772,495 
1 2 
TROWELABLE ABLATIVE COATING BRIEF DESCRIPTION OF THE DRAWINGS 
COMPOSITION AND METHOD OF USE The FIGURE shows the recession rate of the abla- 
tive coating of the present invention and of an exam- 
The invention described herein was made in the per- 5 plary conventional coating versus the heating rate. 
BEST MODE FOR CARRYING OUT THE formance of work under NASA Contract No. NAS 
INVENT ION 8-36100 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (72 
Stat. 435; 42 U.S.C. 2457). 
DESCRIPTION 
The mixture of epoxy resin and amide curing agent of 
10 the present invention may be of any epoxy resin and any 
curing agent, combined in any proportion that provides 
1. Technical Field the required pot life, cures at ambient temperature and 
The field of art to which this invention pertains is imparts the required strength, adhesion, hardness, and 
thermal stability to the cured trowelable ablative coat- 
2. Background Art 
Ablative materials are used as heat shields to protect EPichlorohYdrin/bisPhenol A type epoxy resins are 
space vehicles as they are subjected to high velocity, Preferred- An ePichlorohYdrin/bisPhenol A epoxy resin 
high temperature conditions during launch and during that was found to be particularly suitable for the prac- 
re-entry to the earth’s atmosphere. Such materials need tice of the present invention is known as Scotch- 
exhibit good adhesion to the substrate, be flexible 2o Weld 8 Epoxy Adhesive 2216B (Translucent) manu- 
synthetic coating compositions. 
15 ing composition. 
enough to provide thermal strain stability, have good factured by 3M Corporation* 
Amide curing agents are preferred. An amide curing thermal stability and have low thermal conductivity. 
Ablative materials in current use may be roughly agent found to be particularly suitable for the practice 
of the present invention is Scotch-Weld 8 Epoxy Ad- categorized as either solid materials or cured-in-place 
25 hesive 2216A (Translucent) manufactured by 3M Cor- coating compositions. Cured-in-place ablative coating poration. compositions may be either low viscosity compositions It is preferred that the mixture of epoxy resin and for a spray application or high viscosity compositions amide curing agent comprise between about 67 weight 
percent and about 73 weight percent of the ablative for hand trowel application. 
must be conformed and adhered to the substrate to be ture comprise between about 69 weight percent and 
protected. Fabrication of a smooth covering from solid about 71 weight percent of the  composition^ It is pre- 
ablative materials and removal of the charred covering ferred that the quantity by weight of amide curing agent 
from the spacecraft after re-entry are difficult, time in the mixture be equal to the quantity by weight of 
35 epoxy resin in the mixture multiplied by a factor be- consuming and expensive processes. 
closed in U.S. Pat. No. 4,077,921 have overcome some that the quantity by weight of amide curing agent be 
of the difficulties associated with solid coverings, but to the quantity by weight of epoxy resin multi- 
are not applicable in all situations. Solid ablative mate- plied by a factor between about 0.99 and about 1.01. 
rial such as cork sheet is still used to protect protuberant 4o The glass microspheres of the present invention may 
Structures and high Viscosity ablative Coating COmpOSi- be any hollow discrete spheres of alkali metal silicate 
tions are used for close-out and repair of Primary abla- glass that impart the desired low density and increased 
tive layers. flame resistance to the coating composition. Sodium 
Conventional high viscosity ablative coating compo- borosilicate glass microspheres with a particle size be- 
Sitions are each deficient in some significant respect 45 tween about 20 microns and about 200 microns and a 
(e& high cost, short Pot life Or hazardous ingredients). bulk density between 0.185 grams/cubic centimeter 
What is needed in this art is an ablative coating com- (g/cm3) and 0.195 g/cm3 are preferred. Commercially 
position that overcomes the above limitations. available glass microspheres that were found to be suit- 
able for the practice of the present invention are known 
50 as IG-101 Glass Eccospheres, manufactured by Emer- DISCLOSURE OF INVENTION 
A trowelable ablative coating composition is dis- son and Cuming. The glass microspheres should corn- 
closed. The composition comprises about 67 weight prise between about 22 weight percent and about 28 
Percent to about 73 weight percent of a mixture of an weight percent of the coating composition and most 
epoxy resin and an amide curing agent, about 22 weight preferably comprise between about 24 weight percent 
percent to about 28 weight percent glass microspheres, 55 and about 26 weight percent of the composition. It is 
and about 4.5 weight percent to about 5.5 weight per- preferred that the glass spheres be packaged and stored 
cent ground cork. The quantity of the amide curing in a manner to protect them against moisture absorp- 
agent is equal to the quantity of the epoxy resin times a tion. 
factor between 0.97 and 1.03. The ground cork of the present invention may be any 
A further aspect of the disclosure involves a method 60 ground cork that imparts the desired low density and 
for protecting a substrate in a high temperature, high low thermal conductivity to the coating composition. It 
velocity, erosive environment. The method comprises is preferred that the particle size of the ground cork be 
applying the trowelable ablative coating composition such that it will pass through a 20 mesh screen yet be 
discussed above to the substrate and curing the compo- retained on a 40 mesh screen. A suitable ground cork is 
sition. 65 known as Granulated Cork, 20/40 Screen, manufac- 
The foregoing and other features and advantages of tured by Sheller Globe, Incorporated. It is preferred 
the present invention will become more apparent from that the ground cork comprise between about 4.5 
the following description and accompanying drawings. weight percent to about 5.5 weight percent of the coat- 
coverings cork sheet Or 30 coating composition, and most preferred that the mix- 
Spray applied coating compositions such as that dis- tween about 0.97 and about 1.03 and is most preferred 
4.772,495 
3 
ing composition, and most preferred that the ground 
cork comprise between 4.8 weight percent and 5.2 
weight percent to the coating composition. It is pre- 
ferred that the cork be packaged and stored in a manner 
to protect it against moisture absorption. 
The elements of the coating composition of the pres- 
ent invention must be thoroughly mixed to obtain a 
homogeneous mixture. The coating composition of the 
present invention may be mixed using conventional 
mixing equipment of convenient size, such as a Hobart 
mixer, manufactured by Hobart Corporation. To insure 
proper cure, the epoxy and curing agent should be 
mixed together briefly prior to adding the glass micro- 
spheres and ground cork filler materials. It is preferred 
that the epoxy resin and curing agent be mixed for a 
time period between about 30 seconds and about 60 
seconds prior to the addition of the filler materials. It is 
-preferred that the composition be mixed for an addi- 
tional time period of between about 3 minutes and about 
5 minutes after addition of the filler materials. 
The coating composition is self-curing at room tem- 
perature, and the rate of cure is sensitive to both tem- 
perature and relative humidity. For example, the com- 
position has a pot life or working time of about 90 min- 
utes in a controlled environment at a temperature be- 
tween 72" F. and 76" F. with the relative humidity be- 
tween 50% and 60%. As a further illustration, in an 
uncontrolled environment at a temperature between 85" 
F. and 95' F. with a relative humidity of between 70% 
and 80% in direct sunlight, the composition has a pot 
life of about 45 minutes. The curing reaction continues 
for several days at room temperature and is complete 
within 8 to 10 days. 
The coating composition may be applied by hand 
troweling or molded during a time period less than or 
about equal to the pot life of the material discussed 
above. 
EXAMPLE I 
The substrate panels were fabricated from 6 inch 
thick 2219-T87 aluminum cut to 12 inchx 12 inch pan- 
els. The surfaces of the panels were cleaned with per- 
chloroethylene, chromate conversion coated, primed 
(Sikken Primer No. 463-6-3) and painted (Sikken's Top- 
coat No. 443-3-1). The painted surfaces were lightly 
cross-hatch sanded with 100 grit sandpaper and wiped 
clean with perchloroethylene. 
A trowelable ablative coating composition, known as 
Booster Trowelable Ablative (BTA), comprising 35% 
by weight epoxy resin (Scotch-Weld 2216B), 35% by 
weight amide curing agent (Scotch-Weld 2216A), 25% 
by weight glass microspheres (IG-IO1 Glass Eccos- 
pheres) and 5% by weight Granulated Cork (20/40 
screen) was mixed in a Hobart mixer. The curing agent 
and epoxy resin were added to the Hobart mixer and 
mixed for 45 seconds. The glass spheres and cork were 
added to the mixer and the composition was then mixed 
for an additional. 3 minutes. 
The trowelable ablative coating composition was 
applied to the substrate panels by hand troweling. Even 
distribution was obtained by seating the substrate within 
a 12 inchx 12 inch frame and then applying and leveling 
the coating composition. The panels were allowed to 
cure for 24 hours at ambient temperature before the 
frame was removed and allowed to cure for a total of 10 
days before further testing. Test panels were then cut 
into test specimens of appropriate size as discussed be- 
low. Each specimen was sanded level and smoothed 
4 
with minimal material removal, and then brushed clean 
of sanding residue. 
Tensile specimens were cut from the test panels. The 
specimens were tested with a Satec System tensile tester 
5 at 75" F. at a crosshead speed of 0.05 inch per minute. 
The total load at failure was used to determine the ten- 
sile strength. Average tensile values, in pounddsquare 
inch (psi), for 30 2 " ~ 2 "  specimens with 4'' thick layers 
of coating are presented in Table 1. The majority of 
10 failures occurred at the primer/topcoat interface, indi- 
cating that the cohesive strength and adhesion of the 
coating composition to the topcoat is stronger than the 
adhesion between the primer and the topcoat. 
Density values were calculated from weight and 
15 volume measurements. Linear density was calculated. 
by measuring the dimensions of a specimen at five sepa- 
rate locations for each dimension to obtain average 
dimensions and weighing the specimens on an analytical 
balance. After weighing, the coating composition was 
20 removed from the substrate leaving primer and paint 
intact. The substrate was weighed and measured for the 
final calculation. The average, in pounds/cubic foot, 
(Ib/ft3), of two linear density measurements are pres- 
ented in the Table 1. 
The hardness of the cured coating was measured 
using a Shore "c" durometer. These readings were 
obtained from each of the tensile samples before tensile 
testing. Average hardness values are presented in Table 
1. 
Parallel testing was performed using an exemplary 
conventional trowelable ablative coating (MTA-2). 
Results are also listed in Table 1. 
TABLE 1 
25 
30 
ETA MTA-2 35 
tensile strength (psi) 602 498 
density (lb/ft3) 31.0 35.0 
hardness (Shore "C") 75 15 
~ 
40 The trowelable ablative of the present invention (BTA) 
exhibited a higher tensile strength and a lower density 
than the conventional trowelable ablative (MTA-2). 
EXAMPLE I1 
The ablative coating of the present invention protects 
an underlying substrate by gradual charring and reces- 
sion when subjected to high temperature, high velocity 
erosive environments. Testing was performed to char- 
acterize the rate at which the ablative coating of the 
50 present invention recedes under various conditions and 
to verify the ability of the coating to protect a substrate 
under simulated re-entry conditions. 
A trowelable ablative coating composition was for- 
mulated mixed as in Example I. A 0.5" thick layer of the 
55 coating was applied by hand trowel to each of eight 
20" x 20" X 6" aluminum substrates according to the 
method described in Example I. A 0.5" thick layer of 
coating was applied to each of four 20" x 20" x 6" alu- 
minum substrates by injecting the coating between the 
Each of the specimens was mounted on the top sur- 
face of a 34" longX12" wide water cooled wedge 
shaped fixture for testing. Each specimen was subjected 
to high velocity (MACH 4), high temperature (1440" 
65 F.) conditions in a'high velocity wind tunnel at each of 
four heating rates (10 BTU/ftz-sec, 20 BTU/ftkec, 30 
BTU/ft*-sec and 40 BTU/ft*-sec). The apex of the 
wedge shaped fixture was oriented toward the wind 
45 
60 substrate and a covering mold. 
5 
4,772,495 
6 
source and the heating rate was varied by varying the sequence of conditions simulating launch and re-entry. 
angle of the fixture with respect to the air flow. A sec- All verification points fell below the respective design 
ond wedge shaped member, known as a “shock genera- curves and none of the verification specimens exhibited 
tor”, was positioned above the surface of the specimen. substrate exposure. 
The angle of the shock generator with regard to the 5 The ablative coating of the present invention has a 
airstream was adjusted to deflect the airstream down- low cost and poses minimal health hazard. The uncured 
ward toward the surface of the specimen and more coating composition has a long pot life, may be applied 
intensely heat a narrow band of the specimen to achieve by hand troweling or molding procedures, and cures at 
the 30 BTU/ft2-sec and 40 BTU/ft2-sec heating rates. room temperature. The cured coating exhibits low den- 
The thickness of the coating layer was periodicah 10 sity, high adhesion and high tensile strength. The coat- 
measured with an electronic displacement IneaSUring ing protects an underlying substrate by providing a 
system. Recession rate was calculated aS the difference layer of insulation that slowly erodes by gradual char- 
between the initial thickness of uncharred coating and ring and recession when exposed to high temperature, 
the final thickness of uncharred coating divided by the high velocity environments. 
duration of exposure to a particular heating rate. Although this invention has been shown and de- 
Design curves relating recession rate to heating rate scribed with respect to detailed embodiments thereof, it 
were generated for BTA specimens, for will be understood by those skilled in the art that vari- 
imens. These curves were of the form: without departing from the spirit and scope of the 
l5 
molded BTA specimens, and for troweled MTA-2 spec- ous changes in form and detail thereof may be made 
2o claimed invention. 
We claim: 
1. A method of protecting a substrate under high where ?=recession rate in (mil/sec) and G= heating rate 
in (BTU/ft2-sec). The and coefficients for each temperature, high velocity, erosive conditions, compris- 
ing applying to the substrate a trowelable ablative coat- material are listed in Table 2. 
about 67 weight percent to about 73 weight percent 
troweled BTA 0.08824 1.219 epichlorohydrin/bisphenol A reaction product and 
molded BTA 0.02439 1.621 an amide curing agent wherein the quantity by 
troweled MTA-2 0.03342 .1.89 30 weight of the amide curing agent is equal to the 
quantity by weight of the epoxy resin multiplied by 
The design curves are shown in the FIGURE, where a factor between about 0.97 and about 1.03, 
curve A is the design curve for troweled BTA, curve B about 22 weight percent to about 28 weight percent 
is the design curve for molded BTA, and curve C is the 35 hollow glass microspheres, and 
design curve for troweled MTA-2. It should be noted about 4.5 weight percent to about 5.5 weight percent 
that within the range of heating rates of interest, (Le. 10 ground cork, and 
BTU/ft%ec to 40 BTU/f@-sec), the design curves for curing the composition. 
the trowelable ablative of the present invention (BTA) 2. The method of claim 1 wherein the glass micro- 
fall below those for the conventional trowelable abla- 4o spheres comprise sodium borosilicate glass micro- 
tive (MTA-2), and indicate a lower recession rate for spheres having a diameter between about 20 microns 
the BTA at a given heating rate. and about 200 microns and a bulk density of about 0.158 
The design results were verified by subjecting trow- g/cm3 and about 0.195 g/cm3. 
eled BTA specimens and molded BTA specimens to a 
LXY, 
25 ing composition consisting essentially of 
TABLE 2 
X Y of a mixture of an epoxy resin consisting of an 
* * * * *  
45 
50 
55 
60 
65 
UNITED STATES PATENT AND TRADEMARK OFFICE 
CERTIFICATE OF CORRECTION 
PATENT NO. : 4 , 7 7 2 , 4 9 5  
DATED : September 20 ,  1988 
lNVENTOR(S) : Stephen E .  Headrick & Roger L .  H i l l  
corrected as shown below: 
It is certified that error appears in the above-identified patent and that said Letters Patent is hereby 
T i t l e  page: 
Huntsville, AL-- 
Assignee:" should be --USBI Booster Production Company, Inc. II 
Column 6 ,  l i n e  41 "0.158" should b e  --0.185-- 
. 
Signed and Sealed this 
Twenty-first Day of March, 1989 
Attest: 
Attesting OBcer 
DONALD I. QUIGG 
Commissioner of Puwrifs und Trudeinurk,~ 


Trowelable ablative coating composition and method of use

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