Development of ablative materials based on modification of polyphenylene compounds is discussed. Chemical and physical properties are analyzed for application as heat resistant materials. Synthesis of linear polyphenylenes is described. Effects of exposure to oxyacetylene flame and composition of resultant char layer are presented

Hammermesh, C. L.

Heimbuch, A. H.

Parker, J. A.

Vincent, D. N.

English

NASA Technical Reports Server

August 1972	 B72-10451 
NASA TECH BRIEF 
Ames Research Center
	 4V 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Chemical Modification of PoIy(p-Phenylene) for Use in Ablative

Compositions 
The problem: 
Determine whether p-phenylene polymers can be 
structurally altered to limit spalling under high temper-
ature, high heat flux conditions. Polyphenylenes offer 
significant advantages as ablative materials because 
their skeletal structures consist of long chains of 
conjugated aromatic rings which, on thermal or radia-
tive degradation, produce a maximum amount of char, 
in the form of a thermally resistant graphitic structure, 
and yield a minimum amount of gaseous products 
other than hydrogen. However, the integrity of molded 
ablative composites containing poly(p-phenylene) 
is destroyed by spalling (or by exfoliation) when the 
composites are exposed to the high temperatures and 
heat fluxes of an oxyacetylene torch. 
The solution: 
Incorporate chlorine into the polymeric structure. 
This produces potential cross-linking sites for sub- 
sequent pyrolysis, increases the char yield, and in-
creases the physical integrity of the composite's char 
layer. 
The chemical and physical properties of the chlor-
inated polyphenylene composites suggest possible 
applications as brake lining material and thermal 
insulation at high temperatures. 
How it's done: 
In the synthesis of linear polyphenylenes, a small 
amount (less than 1%) of chlorine was incorporated in 
the polymer. The chlorine was derived from the cata-
lysts, cupric and aluminum chlorides. When benzene 
was polymerized in the presence of these two catalysts, 
at temperatures below 35° C and reaction times of less 
than two hours, a usual reaction product contained 
about 0.8% chlorine, a carbon-to-hydrogen ratio of
about 1.50, and no detectable ortho, meta, or nonaro-
matic linkages. Reaction conditions were established 
to produce polymers which contained higher chlorine 
contents and appeared to be polynuclear ring struc-
tures. The polymers containing 4 to 6% chlorine had 
significantly	 improved	 ablation	 characteristics. 
The increased chlorine content of the polymers 
was achieved by two different methods; i.e., direct-
and post-halogenation techniques. Direct halogena-
tion was accomplished by the incremental addition of 
catalyst at the reflux temperature of benzene during 
the polymerization reaction. The post-halogenation 
was accomplished by bubbling chlorine gas into a sus-
pension of the linear polymer in the presence of a 
ferric chloride catalyst. 
Polyphenylenes containing chlorine and poly-
nuclear structures pyrolyze in two distinct stages; 
essentially all of the chlorine content is lost in the 
first stage; and degradation products, which are mark-
edly deficient in aromatic pyrolyates, are formed in 
the second stage (which is apparently the same as that 
of unchlorinated polymers). 
Thermogravimetric and gas chromatographic anal-
yses showed that the "unchlorinated" polymer de-
grades by chain scission,,with multiples of phenyl units 
given off after an initial loss of hydrogen and methane 
at the lower temperatures. Chain scission was de-
creased mainly through nuclear condensation and 
"thermal" cross-linking at the higher temperatures, 
above 550° C, so that a good char yield of 82% was ob-
tained in an inert atmosphere to temperatures of 
900° C. In contrast, the chlorinated polymer under-
went little chain scission, so that the char yield was 
even greater, 88%. This can be explained by dehydro-
halogenation, taking place at temperatures below about 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19720000450 2020-03-17T03:39:37+00:00Z
550° C, effecting chemical cross-linking and nuclear 
condensation at the earlier stages of pyrolysis. Most of 
the weight loss of the chlorinated polymer in the first 
stage of pyrolysis was due to the loss of chlorine (as 
HCI). If the elimination of HCI is made to occur in a 
"curing" step, char yields of up to 94% could be 
obtained from the cured polymer, based on calcula-
tions of carbon retention alone. 
The incorporation of small amounts (4 to 12 parts 
per hundred) of chopped quartz or carbon fiber pre-
vents the cracking of heavy sections during the curing 
step; poor adhesion between the polymer and the fi-
brous filler imparts sufficient porosity to allow the es-
cape of gaseous products generated during the curing 
process. One inch diameter samples, containing chlor-
inated polyphenylene mixed with chopped quartz or 
carbon fiber and cured at 550° C for 30 minutes, were 
exposed for 20 minutes to an oxyacetylene flame under 
conditions of 3.95 watt/meter 2 (350 BTU/ft2 sec) heat 
flux and 2000°C surface temperature. A hard, tightly 
adherent char layer was formed to a depth of about 0.5 
cm, with a surface regression of less than 0.25 cm. In 
contrast, uncured samples or samples containing un-
chlorinated polyphenylerie spailed severly.
Notes: 
1. When the improved composite is exposed to the 
oxyacetylene flame, large sections may fail due to 
thermal stresses; small samples maintain their 
integrity. 
2. Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP72-1045 1 
Patent status: 
No patent action is contemplated by NASA. 
Source: J. A. Parker and A. H. Heimbuch
Ames Research Center, and
D. N. Vincent and C. L. Hammermesh of

North American Rockwell Corp.

under contract to
Ames Research Center 
(ARC-10135)
. 
. 
B72-10451	 Category 04


Chemical modification of poly(p-phenylene) for use in ablative compositions

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