A method of applying cork-resin ablative insulation material to complex curved surfaces is disclosed. The material is prepared by mixing finely divided cork with a B-stage curable thermosetting resin, forming the resulting mixture into a block, B-stage curing the resin-containing block, and slicing the block into sheets. The B-stage cured sheet is shaped to conform to the surface being insulated, and further curing is then performed. Curing of the resins only to B-stage before shaping enables application of sheet material to complex curved surfaces and avoids limitations and disadvantages presented in handling of fully cured sheet material

Sharpe, M. H.

Simpson, W. G.

Walker, H. M.

English

NASA Technical Reports Server

RESIN ABLATIVE INSULATION FOR 
COMPLEX SURFACES AND METMOD FQR 
APPLYHNG THE SAME 
Inventors: Hill M. Walker, Guntersville; Max 
M. Sharpe; William G. Simpson, both 
of Huntsville, all of Ala. 
The United States of h e r i a  as 
represented by the Administrator of 
the National Aeronsuties and Space 
AdmhlQratiom, Washington, D.C. 
Assignee: 
Appl. No.: 9QI,7BI 
Filed: Sep. 12,1978 
ht. a . 2  ....................... B29C 17/00; B29C 17/04 
U.S. CL .................................... 156/2lq 156/213; 
156/285; 264/1 18; 264/119; 264/124; 260A7.2 
2W124, 118, 119; 260/17.5,9; 428/326,455, 
456 
Field off Sea~ch ....................... 156/212, 213, 285; 
References cited 
US. PATENT DOCUMENTS 
2,671,733 3/1954 Bstermayer ..................... 264/124 X 
3,055,057 9/1962 Flotron ............................ 264/124 X 
3,284,260 11/1966 Best ...................................... 156/212 
3,591,447 7/1971 Juhl ...................................... 428/326 
3,784,433 1/1974 Garnish et al. .................. 156/330 X 
3,821,135 6/1974 King ......................................... 260/9 
4,053,667 11/1977 Smith ............................... 156/285 X 
Primary Examiner-George F. Lesmes 
Assistant Examiner-Daniel R. Zirker 
Attorney, Agent. or Firm-J. H. Beumer; J. R. Manning; 
L. D. Wofford, Jr. 
r571 AB§ma 
Cork-resin ablative insulation material is prepared in a 
form suitable for application to complex curved sur- 
faces by mixing finely divided cork with a B-stage cur- 
able thermosetting resin, forming the resulting mixture 
into a block, B-stage curing the resin-containing block 
and slicing the block into sheets. The B-stage cured 
sheet is shaped to conform to the surface being insu- 
lated, and further curing is then performed. Curing of 
the resin only to B-stage before shaping enables applica- 
tion of sheet material to complex curved surfaces and 
avoids limitations and disadvantages presented in han- 
dling of fully cured sheet material. 
3 NQ 
https://ntrs.nasa.gov/search.jsp?R=19800017889 2020-03-21T02:52:29+00:00Z
4204,899 
~ ~ ~ ~ ~ ~ ~ E S I N  ABLATIVE INSULATION FOR 
COMPLEX SURFACES AND METHOD FOR 
APPLYING THE SAME 
ORIGIN OF THE INVENTION 
This invention was made by employees of the United 
States Government and may be manufactured and used 
by or for the United States Government for govern- 
mental purposes without payment of any royalties 
thereon or therefor. 
BACKGROUND OF THE INVENTION 
ahis invention relates to cork compositions and more 
particularly to cork-resin ablative insulating material 
and to a method of applying such material to complex 
curved surfaces. 
DESCRIPTION OF THE PRIOR ART 
Natural cork has properties that make it an ideal 
material for providing thermal protection to certain 
exposed surfaces of rocket booster stages. It has a low 
density, low thermal conductivity and a high degree of 
stability in the presence of vibration. Owing to its 
unique cellular structure, cork bums slowly under igni- 
tion, forming successive layers of char. Compositions 
made up of cork particles and a phenolic resin binder 
have been shown to be effective ablative insulating 
materials. 
Cork insulating material has been prepared previ- 
ously by combining cork particles with phenolic resin, 
curing the mixture by application of heat and pressure 
and cutting sheets from the resulting fully cured, con- 
solidated block. Fully cured composition cork sheet 
material can be applied to plane, cylindrical or conical 
surfaces without difficulty simply by cutting, fitting, 
and bonding parts in place. In the case of complex 
curved surfaces, however, application of fully cured 
sheet material has presented much difficulty. Flexibility 
of fully cured compositions is limited, and in most cases 
sheet material breaks or cracks when attempts are made 
to drape the material to conform to contours of complex 
surfaces. Even when cured sheet material is bonded to 
a contoured surface, the composition retains a memory 
of its flat shape and tends to undergo bridging and for- 
mation of residual stresses. As a result, laying up of 
large numbers of small individual pieces has been re- 
quired in applying cork composition to complex sur- 
faces. The use of cork as an ablative insulation material 
5 
10 
15 
20 
25 
30 
35 
40 
45 
2 
into a smooth continuous piece. Problems of cracking 
or bridging and tedious application of small pieces of 
fully cured material are thereby avoided. In addition, 
cork-resin material that is subjected to B-stage curing as 
an intermediate processing step exhibits properties supe- 
rior to conventional cork ablative insulation, in particu- 
lar, lower density and better performance in thermal 
aero-shear environments. 
It is therefore an object of this invention to provide 
cork-resin ablative insulation material in a form suitable 
for application to complex curved surfaces. 
Another object is to provide cork-resin sheet material 
capable of being laid up on complex surfaces and cured 
without undergoing cracking or bridging. 
Another object is to provide a method of applying 
cork-resin ablative insulating material to complex 
curved surfaces. These and other objects and advan- 
tages of the invention will become apparent upon refer- 
ence to the following detailed description. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
Finely divided cork, preferably in the form of gran- 
ules 16 to 50 Mesh (U.S.  Standard) in size, is mixed with 
a resin system selected to provide capability for B-stage 
curing, consistent with desired final properties such as 
strength and temperature stability in the fully cured 
product. In general a resin content of 20 to 60 weight 
percent can be used, and 25 to 45 weight percent is 
preferred. Although other resin systems, such as poly- 
urethanes, silicones, epoxies and melamines can be em- 
ployed, phenolic resins are preferred because of their 
favorable high temperature stability, strength and abla- 
tion characteristics. 
Phenolic resins, which are condensation reaction 
products of an aldehyde with a phenol, are available 
with a wide range of curing characteristics depending 
on molecular structure, catalyst employed and the like. 
In order to provide suitable B-stage curing for the pres- 
ent invention the selected phenolic resin system should 
exhibit a relatively low number of reaction sites so as to 
avoid rapid curing and relatively long molecular chains, 
which stearically hinder cross-linking reactions. Tech- 
niques for tailoring properties of phenolic resin systems 
by means such as modifying the molecular structure of 
reactants are well known in the literature. In addition to 
the aldehyde and phenol reactants, the resin system may 
contain other components such as catalysts requiring would be greatly facilitated by providing cork-resin 50 for curing and a organic solvent. Use of pr; cornposition in a form suitable for laying up as sheet mixed phenolic resin system containing to 2o weight material conforming to complex curved surfaces. percent organic solvent is preferred. B-stage curing of 
SUMMARY OF THE INVENTION such systems can be carried out by heating the resin at 
is provided in the form of B-stage cured, flexi- In a preferred procedure the resin and cork mixture is 
ble sheets capable of being applied to complex curved disposed in a compressible container, the amounts of 
surfaces. The sheet material is prepared by mixing finely resin and cork being selected to provide a given density 
divided cork particles with a thermosetting resin m e -  in the B-stage cured block upon being compressed to a 
nable to B-stage curing, compressing the mixture into a 60 Predetermined VOhme. A density of 25 to 35 Pounds 
block, curing the block to &stage and cutting it into per cubic foot is preferred. The phenolic resin-cork 
sheets. The resulting cork-resin sheets, while in B-stage mixture is then heated to a temperature of 280" to 320" 
condition, are shaped to conform to the surface being F., and preferably about 300" F. until B-stage curing is 
insulated and are then subjected to further curing. B- complete, a period of about 15 minutes being required. 
stage curing of the resin-cork composition provides 65 The heating period can be adjusted to provide optimum 
strength for cutting and handling of sheet material, handling characteristics, that is, maximum strength con- 
consistent with sufficient flexibility to conform to com- sistent with retained flexibility and flowability in sheet 
plex shapes and internal flowability to allow molding obtained from the B-stage cured block. Longer heating 
In the present invention cork-resin ablative insulating 55 an elevated temperature for a predetermined period* 
4,204,899 
3 4 
periods produce more complete curing and higher days. Specimens of the resulting material were tested 
strength. for mechanical and physical properties, and for ablation 
Sheets of B-stage and cork-resin material are pro- characteristics. Flatwise tensile strength values over 
duced by slicing the block at the desired thickness. 100 pounds per square inch were obtained in most cases, 
Sheets up to 5/8 inch thick can be processed according 5 as compared to values of about 75 p.s.i. for conventional 
to the invention. Conventional cutting or slicing de- fully cured cork-resin material. In the case of specimens 
vices can be used for this pu,qose. cured at room temperature, tensile strength values in- 
The cork-resin sheet can be shaped to conform to the creased 25 percent after standing for 2 weeks. Thermal 
surface and applied to the surface in one-step, by "in- conductivities for test specimens were approximately 
situ" molding or shaped over a mold corresponding in 10 the same as for conventional material, and densities 
shape to the surface and subsequently applied to the were 20 percent lower. Ablation rates in hypervelocity 
surface. Although some bonding to the surface will wind tunnel testing were determined to be 20 percent 
occur without use of an adhesive in in-situ molding, it is lower than for conventional material. 
preferred to use an adhesive such as an epoxy, polyure- While a preferred embodiment has been described 
thane or silicone resin to insure complete bonding of the 15 above, such description is for illustrative purposes only, 
sheet to the insulated surface. For in-situ molding the and it is to be understood that changes and variations 
sheet is draped over the adhesive coated surface, and may be made without departing from the spirit or scope 
the assembly is placed in a vacuum bag. Upon evacuat- of the following claims. 
ing of the bag, atmospheric pressure forces the B-stage What is claimed is: 
cured sheet to conform to the surface. In most cases the 20 1. The method of preparing cork-resin ablative insula- 
sheet will conform readily to surface curvatures with- tion material for application to a complex curved sub- 
out cutting or trimming, but such measures may be used strate comprising: 
to remove excess material when required. (a) mixing finely divided cork with a B-stage curable 
The shaped cork-resin material is then subjected to thermosetting resin, said resin comprising 20 to 60 
further curing either at ambient or elevated tempera- 25 weight percent of the mixture; 
ture, depending on the final properties desired. Substan- 
tially complete curing can be obtained by heating at 
280" to 320" F. for 30 to 60 minutes under vacuum. This 
curing cycle provides maximum strength and resistance 
to ablation. However, an alternative room-temperature 30 
curing cycle provides adequate properties for most 
applications and avoids the need for heating. In the 
latter cycle the shaped sheet is merely allowed to stand 
in air at room temperature for at least 3 days. Room- 
temperature curing is particularly advantageous in ap- 35 
plications where provision of curing heat is impractical 
owing to component size or other restrictions. 
The invention is further illustrated by the following 
example. nolic resin. 
(b) forming the resulting mixture into a block; 
(c) B-stage curing the resin-containing block by heat- 
ing at a temperature of 280" F. to 320" F. for a 
period of at least about 15 minutes; 
(d) cutting the B-stage cured block into sheets; 
(e) draping the resultant sheet material over said sub- 
strate or a surface corresponding in shape thereto; 
(9 enclosing the resulting assembly in a vacuum bag; 
(g) evacuating said bag; and 
(h) further curing the resultant shaped sheet material 
by heating said sheet material at a temperature of 
280" F. to 320" F. for at least 30 minutes. 
2. The method of claim 1 wherein said resin is a phe- 
3. The method of preparing cork resin ablative insula- 
tion material for application to a complex curved sub- 
strate comprising: 
(a) mixing finely divided cork with a B-stage curable 
thermosetting resin, said resin comprising 20 to 60 
weight percent of the mixture; 
40 EXAMPLE I 
Test articles comprising a ten-inch metal hemisphere 
having a f inch thick sheet of cork-phenolic resin 
bonded to the surface of the hemisphere were prepared 
by the following procedure: Phenolic resin and cork 45 
granules at approximately equal parts by weight were 
combined to form a mixture, and the mixture was 
formed into a block having a density of 25 pounds per 
cubic foot. The block was then B-stage cured by heat- 
ing at 300" F. Sheets f inch in thickness were then cut 50 
from the block, and, after application of a silicone adhe- 
sive to the hemisphere, sheet material was draped over 
the surface. A vacuum bag was then placed over the 
hemisphere and the bag was evacuated, forcing the 
sheet to conform to the surface. In some cases the 55 
shaped sheet was fully cured by heating at 300" F. for 1 
hour, while in other cases further curing was carried 
(b) forming the resulting mixture into a block; 
(c) B-stage curing the resin-containing block by heat- 
ing at a temperature of 280" F. to 320" F. for a 
period of at least 15 minutes; 
(d) cutting the B-stage cured block into sheets; 
(e) draping the resultant sheet material over said sub- 
strate or a surface corresponding in shape thereto; 
( f )  enclosing the resulting assembly in a vacuum bag; 
(g) evacuating said bag; and 
(h) further curing the resultant shaped sheet material 
by allowing it to stand in air at ambient tempera- 
ture for at least three days. 
shaped sheet to stand in air for 3 * * * * *  
60 


Cork-resin ablative insulation for complex surfaces and method for applying the same

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server