Rigid polyphosphazene foam and process for making same

Low density rigid foamed polyphosphazene articles having excellent physical and flammability properties are made by masticating a mixture of high molecular weight linear polyphosphazene, a curing agent (e.g., sulfur), an accelerator, a blowing agent and optionally fillers, processing aids and the like to form a substantially homogeneous blend and then stopping the masticating, heating to pre-cure and then heating to activate the blowing agent and complete the cure thereby forming a flexible foamed polyphosphazene composite, forming the flexible foamed polyphosphazene composition into a shaped composition, and thereafter heating the flexible foamed shaped polyphosphazene composition at a temperature and for a length of time which causes the flexible foamed polyphosphazene composition to become a rigid foamed polyphosphazene composition of the same shape as the flexible foamed polyphosphazene composition.

BACKGROUND 
Cellular plastics have been available for many years. One of the first of 
such materials was cellular rubber dating to the 1910-1920 period. 
Subsequently, cellular compositions were made from latex, 
phenol-formaldehyde resins, urea-formaldehyde resins, PVC, polyurethane, 
cellulose acetate, polystyrene, polyethylene, epoxides, ABS resins, 
silicones and very recently polyphosphazenes. Polyphosphazene foams have 
very desirable properties in that they are highly fire resistant and when 
subjected to direct flame do not produce toxic smoke which is encountered 
with many other common foamed materials, notably, polyurethanes. 
Polyphosphazenes are polymers containing a plurality of 
##STR1## 
groups wherein substituents are bonded to phosphorus. The polyphosphazenes 
which are the concern of this invention are high molecular weight linear 
polyphosphazenes containing 50 or more of the above units and having 
molecular weights from about 10,000 up to about 5,000,000 or higher. They 
are substantially linear and have little, if any, cross-linking. In 
general, they are soluble in benzene, toluene, cyclohexane and 
tetrahydrofuran and are relatively insoluble in linear aliphatic 
hydrocarbons such as hexane or heptane. Groups substituted on phosphorus 
include phenoxy, alkylphenoxy, alkoxyphenoxy, aminoalkylphenoxy, 
alkylaminoalkylphenoxy, dialkylaminoalkylphenoxy, halophenoxy (e.g., 
para-chlorophenoxy, meta-bromophenoxy, trifluorophenoxy and the like), 
haloalkylphenoxy (e.g., trifluoromethylphenoxy), alkoxy, haloalkoxy (e.g., 
trifluoroethoxy), alkenylphenoxy (e.g., ortho-allylphenoxy and the like). 
Methods of making cellular polyphosphazenes are known. Various procedures 
are described in U.S. Pat. Nos. 4,026,838; 4,055,520; 4,055,523; 
4,107,108; 4,189,413; 4,536,520 and others. In general, the foams are made 
by mixing the polyphosphazene gum, a blowing agent and a peroxide or 
sulfur-type curing agent and heating the blended components to activate 
the blowing agent and cure the resultant foam. 
Because these foams are widely recognized to possess the excellent 
flammability properties required for demanding applications such as pipe 
insulation and cushions, they have become items of considerable commercial 
significance. However, due to their flexible elastomeric nature they are 
potentially unsuitable for many foam-specific applications which require 
good thermal insulating materials having excellent flammability 
properties, but depend on a rigid foam to achieve the desired end-use 
performance as in the case of rigid pipe insulation, for example, where a 
flexible insulation would not take the compressive loads imposed by the 
application design or in the case of composite core material for use in 
aircraft, marine and aerospace applications. Also, it would be highly 
desirable if such a rigid polyphosphazene foam could be produced in a wide 
variety of shapes including rigid slabstock foam sheets and pipe 
insulation having varying degrees of curvature or complex contours to 
accommodate specific end uses. In response to this need, there is now 
provided a low density rigid polyphosphazene foam having excellent 
flammability and compression resistant properties which can be produced in 
a wide variety of shapes and designs. 
SUMMARY OF THE INVENTION 
The process by which the rigid cellular polyphosphazene foams of the 
present invention are made utilizes flexible polyphosphazene foam as a 
precursor allowing for an extremely wide variety of shaped rigid foamed 
articles to be produced therefrom. In accordance with the process, a 
flexible elastomeric polyphosphazene foam is first made by forming a 
composition comprising a substantially linear high molecular weight 
polyphosphazene gum, a curing agent, a blowing agent and optionally a 
plasticizer, an inorganic filler, an accelerator and processing aids. The 
composition is masticated or mixed until it forms a substantially 
homogeneous blend whereupon mixing is stopped. The resultant composition 
is shaped into conventional slabs or sheets or is extruded into a hollow 
cylindrical form prior to curing and then heated in an unconfined 
environment to a temperature which activates the blowing agent causing the 
composition to expand into a foamed composition and completing the cure of 
the foamed composition to produce a flexible elastomeric foamed 
polyphosphazene composition. Optionally, the shaped composition can be 
pre-cured prior to curing. That is, the shaped composition or homogenous 
blend can be aged at a temperature above the predetermined maximum mixing 
temperature but below the activation temperature of the blowing agent. 
This operation causes a limited amount of cross-linking to occur raising 
the viscosity of the composition such that the blowing gas does not escape 
during the blowing operation. This step is usually done in a pre-cure 
oven. After curing, while the foamed material is flexible, it can easily 
be re-shaped into a variety of configurations and designs and processed 
into a rigid foam having a specific shape by heating the shaped flexible 
polyphosphazene foamed material to a temperature and for a length of time 
sufficient to cause the shaped flexible composition to become rigid. By 
this process, rigid foamed sheets, slabs, pipes and the like having 
varying degrees of curvature or complex contours possessing excellent 
compression resistant and flammability properties can be produced.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Thus, a preferred embodiment of the invention is a process for making a low 
density rigid polyphosphazene foam having excellent flammability and 
compression resistant properties, said process comprising: 
(i) forming a composition comprising a substantially linear high molecular 
weight polyphosphazene gum, a curing agent, a blowing agent and optionally 
a plasticizer, an inorganic filler, an accelerator and processing aids, 
(ii) masticating said composition to form a substantially homogeneous 
blend, and then stopping said masticating, 
(iii) heating said composition in an unconfined environment to a 
temperature which activates said blowing agent causing said composition to 
expand into a foamed composition and completing cure of said foamed 
composition thereby forming a flexible foamed polyphosphazene composition, 
(iv) forming said flexible foamed polyphosphazene composition into a shaped 
composition, and thereafter 
(v) heating said flexible foamed shaped polyphosphazene composition at a 
temperature and for a length of time sufficient to cause said flexible 
shaped foamed polyphosphazene composition to become a rigid foamed 
polyphosphazene composition having the same shape as said flexible foamed 
polyphosphazene composition. 
Since the invention also contemplates low density foamed shaped rigid 
polyphosphazene compositions having excellent flammability and compression 
resistant properties made by the process of the invention as 
aforedescribed, another embodiment of the invention is a low density 
foamed shaped rigid polyphosphazene composition made by a process 
comprising: 
(i) forming a composition comprising a substantially linear high molecular 
weight polyphosphazene gum, a curing agent, a blowing agent and optionally 
a plasticizer, an inorganic filler, an accelerator and processing aids, 
(ii) masticating said composition to form a substantially homogeneous 
blend, and then stopping said masticating, 
(iii) heating said composition in an unconfined environment to a 
temperature which activates said blowing agent causing said composition to 
expand into a foamed composition and completing cure of said foamed 
composition thereby forming a flexible foamed polyphosphazene composition, 
(iv) forming said flexible foamed polyphosphazene composition into a shaped 
composition, and thereafter 
(v) heating said flexible foamed shaped polyphosphazene composition at a 
temperature and for a length of time sufficient to cause said flexible 
shaped foamed polyphosphazene composition to become a low density foamed 
shaped rigid polyphosphazene composition having the same shape as said 
flexible foamed polyphosphazene composition. 
High molecular weight linear polyphosphazenes are known polymers. Their 
preparation is described in the literature and in patents such as U.S. 
Pat. Nos. 3,515,688; 3,700,629; 3,702,833; 3,838,073; 3,843,596, 
3,844,983, 3,853,794; 3,883,451; 3,888,799; 3,888,800; 3,896,058; 
3,943,088; 3,948,820; 3,970,533; 3,972,841; 3,994,838; 4,006,125; 
4,116,785; 4,123,503; 4,128,710 and 4,129,529. 
In general, linear polyphosphazenes consist essentially of 
##STR2## 
in which n can range from about 50 to 50,000 or more and wherein any of a 
large number of groups can be substituted on phosphorus. Substituent 
groups can include alkoxy, substituted alkoxy such as haloalkoxy or 
alkoxyalkoxy, aryloxy, substituted aryloxy wherein the substituents can be 
alkyl, alkoxy, halo, alkenyl, haloalkyl, amino, alkylamino, dialkylamino 
and the like. Other phosphorus substituents can be halogen (e.g., 
chlorine), alkenoxy and the like. 
In developing the present invention, excellent results have been achieved 
using polyphosphazene in which the substituents were a random mixture of 
phenoxy, para-ethylphenoxy and ortho-allylphenoxy groups. The 
ortho-allylphenoxy groups impart curing properties to the polyphosphazene 
gum. A preferred ratio is about 30-60 mole percent phenoxy, 30-60 mole 
percent paraethylphenoxy and 1-20 mole percent ortho-allylphenoxy. 
In making a foam, the polyphosphazene gum is first blended with other 
ingredients to give a formulation. An essential component of the 
formulation is a blowing agent. The amount of blowing agent should be that 
which will evolve sufficient gas to give a foam of the desired density but 
not an excessive amount which results in splitting of the foam. Blowing 
agents decompose to evolve gas upon heating. This decomposition 
temperature varies over a wide range with different foaming agents. Many 
foaming agents are azo compounds which evolve nitrogen when undergoing 
thermal decomposition. Examples of blowing agents include 
dinitrosopentamethylenetetramine, 4,4'-oxybis(benzenesulfonyl hydrazide), 
axodicarbonamide, ammonium carbonate, ammonium bicarbonate, sodium 
bicarbonate, ammonium nitrite, tertbutylamine nitrite, guanidine nitrite, 
guanylurea nitrite, sodium borohydride, potassium borohydride, urea, 
biuret, N-nitro urea, diazomaniobenzene, 
2,2'-azobis(2-methylpropionitrile), 2,2'-azobisisobutyronitrile, 
1,1'-azobiscyclohexanecarbonitrile, azobisisobutyramidoxime, 
azobisformamide, N,N'-di-tert-butylazobisformamide, 
N,N'-diphenylazobisformamide, phenylhydrazine, benzylmonohydrozone, 
benzenesulfonyl hydrazide, methyl carbanilate, 4,4'-oxybis(benzenesulfonyl 
hydrazide), 3,3'-sulfonylbis(benzenesulfonyl hydrazide), cyanuric 
trihydrazide, 4,4'-oxybis(benzenesulfonyl semi-carbizide), benzoylazide, 
p-tertbutylbenzoylazide, diphenyl-4,4'-disulfonyldiazide, 
N,N'-dimethyl-N,N'-dinitroso terephthalamide and the like. 
Curing agents encompass a broad range of compounds which serve to promote 
cross-linking of the polyphosphazene. One class of curing agents is made 
up of peroxides. The most important curing agent used to make the present 
foamed compositions are the sulfur-type curing agents generally referred 
to as vulcanizing agents. A typical sulfur vulcanizing system comprises 
sulfur, an accelerator and promoters. Zinc oxide is usually included with 
the sulfur. Other accelerators include zinc dialkyldithiocarbamates (e.g., 
zinc dimethyldithiocarbamate, zinc dibutyldithiocarbamate and the like). 
Other useful accelerators are zinc benzothiazylsulfide, 
N-cyclohexyl-2-benzothiazylsulfenamide, 4,4'-dithiomorpholine, fatty acids 
in combination with zinc oxide such as stearic acid, zinc fatty acid salts 
such as zinc stearate, tetraalkylthiuram monosulfide, tetraalkylthiuram 
disulfide, 2-benzothiazoyl disulfide, zinc benzothiazolyl mercapto, 
mercaptobenzothiazole, 2-benzothiazolysulfenamide, amines, diphenyl 
guanidine, thiobisamines and the like. 
Another component that is usually included in polymer foam compositions is 
a filler. These are usually inorganic materials although some organic 
materials are used. Examples of fillers are clay, talc, mica asbestos, 
feldspar, bentonite, wollastonite, fullers earth, pumice, pyrophillite, 
rottenstone, slate flour, vermicullite, calcium silicate, magnesium 
silicate, alumina, hydrated alumina, antimony oxide, magnesia, titania, 
zinc oxide, silica, calcium carbonate, barium carbonate, magnesium 
carbonate, barium sulfate, calcium sulfate, lime, magnesium hydroxide, 
carbon black, graphite, metal powders, fibers and whiskers, barium 
ferrite, magnetite, molybdenum disulfide, glass fibers or flakes, ground 
glass and the like. 
The polyphosphazene formulations which are foamed according to the present 
invention generally include a plasticizer. These can be liquids which when 
blended with the polyphosphazene gum and the other components tend to 
reduce the viscosity of the mass and assist in making a homogenous blend. 
Useful plasticizers include tricresylphosphate, triphenylphosphate, 
cresyldiphenylphosphate, butyl octyl phthalte, dibutyl phthalate, 
dicyclohexyl phthalate, diisodecyl phthalate, di-2-ethylhexyl phthalate, 
ditridecyl phthalate, isooctylisodecyl phthalate, diisodecyl adipate, 
di-2-ethylhexyl adipate, octyldecyl adipate, diisobutyl adipate, 
diisooctyl adipate, di-2-ethylhexyl azelate, diisodecyl azelate, dibutyl 
maleate, glycerol ricinoleate, isopropyl myristate, isopropyl palmitate, 
butyl oleate, glycerol trioleate, methyl oleate, 2-ethylhexyl oleate, 
dibutyl sebacate, di-2-ethylhexyl sebacate, butyl stearate, 2-ethylhexyl 
stearate, triethyleneglycol dicaprate, ethylene glycol terephthalate 
polyesters, diethylene glycol dipelargonate, polyethylene glycol 200 
dibenzoate, polyethylene glycol 600 dibenzoate, glycerol 
triacetylricinoleate, adipic acid glycol polyester 6,000 and the like. 
The amount of the different components in the formulation can vary widely 
based upon parts by weight per 100 parts by weight of polyphosphazene gum. 
A useful range is given in the following table: 
______________________________________ 
Parts by Weight 
______________________________________ 
Polyphosphazene gum 
100 
Sulfur 0.5-20 
Accelerator 0.1-5 
Blowing agent 10-50 
Filler 50-300 
Plasticizer 5-50 
______________________________________ 
The components in the formulated compositions are then subjected to 
mechanical mixing or mastication to form a substantially uniform blend. 
This mixing is conducted in the same type equipment used in compounding 
rubber prior to vulcanization. Suitable mixing equipment on the laboratory 
scale is marketed under the trademark "Brabender". Larger mixing equipment 
is marketed under the "Banbury" trademark. These are heavy duty mixers 
that crush and masticate the formulation until it forms a homogenous 
blend. 
After the mixing operation, the composition can be shaped into useful forms 
such as sheets and slabs for use in insulation or cushions, or it can also 
be extruded into hollow cylindrical forms for use as pipe insulation. 
Prior to curing, the shaped composition optionally can be aged at a 
temperature above the pre-determined maximum mixing temperature but below 
the activation temperature of the blowing agent. This operation causes a 
limited amount of cross-linking to occur raising the viscosity of the 
composition such that the blowing gas does not escape during the blowing 
operation. This step is usually done in a pre-cure oven. Good results have 
been achieved when the shaped composition is maintained at a pre-cure 
temperature of about 100.degree.-20.degree. C. for a period of about 5-20 
minutes. 
In the next operation, the shaped pre-cured composition is heated in an 
unconfined environment high enough to activate the blowing agent. The 
composition then expands forming a flexible cellular polyphosphazene 
article. The term "unconfined environment" means that there is space 
available into which the shaped polyphosphazen composition can expand 
during cell development. As it reaches its final volume it may again be 
confined in some form or mold. 
Because of the flexible nature of the cellular polyphosphazene article thus 
formed, it can be re-shaped into a variety of geometrical forms and 
configurations prior to its conversion to a rigid polyphosphazene 
material. For example, flexible hollow cylindrical tubes can be re-shaped 
into tubes having varying degrees of curvature or bend or complex contours 
such as "S" or "L" configurations and the like and processed into rigid 
tubes of the same configuration or design and used as insulation for pipes 
of the same shape. In the final step of the process, the shaped flexible 
elastomeric foamed polyphosphazene is heated to a temperature and for a 
length of time which is sufficient to transform the flexible, shaped 
cellular polyphosphazene into a rigid foamed composition of the same shape 
as the flexible composition. Heating can be effected, for example, by 
radiation heating (e.g., infra-red or microwave) or by convection heating. 
Suitable temperatures range from about 75.degree. to 600.degree. C., more 
preferably 100.degree. to 300.degree. C. with useful heating times varying 
from about 10 minutes to 25 days, preferably 8 to 24 hours. The time and 
temperature can be adjusted depending on the geometry and thickness of the 
flexible article, the degree of rigidity desired and the heating mode. 
Excellent results have been achieved when the shaped flexible elastomeric 
polyphosphazene composition is heated to a temperature of about 
200.degree. C. for a period of time of about 16 hours. 
Thus, another embodiment of the present invention is a shaped article made 
from a rigid foamed polyphosphazene composition made by the process of the 
present invention. 
A foaming operation was carried out. The following formulation was used: 
______________________________________ 
Parts by Weight 
______________________________________ 
Polyphosphazene gum 
100 
Hydral 710 W.sup.1 
180 
Zinc stearate 10 
Silastic HA-2.sup.2 
10 
Carbowax 3350.sup.3 
2 
Celogen AZ 130.sup.4 
25 
Carbon black 10 
Sulfur 1.25 
Plasticizer.sup.5 
12 
______________________________________ 
.sup.1 Alcoa brand hydrated alumina 
.sup.2 DowCorning brand of silicon filled methyl vinyl silicone 
.sup.3 Union Carbide brand polyethyleneoxide 
.sup.4 Naugatuck brand azodicarbonamide 
.sup.5 An oil made by substituting trimer with phenoxy, pethylphenoxy and 
O--allylphenoxy groups 
The polyphosphazene gum was a high molecular weight linear polymer 
substituted with about 52 mole percent phenoxy, 42 mole percent 
p-ethylphenoxy and 6 mole percent o-allylphenoxy groups. 
The components were blended in a Banbury mixer until a substantially 
homogeneous blend was obtained. 
A curing concentrate was separately formulated as follows: 
______________________________________ 
Parts by Weight 
______________________________________ 
Polyphosphazene gum.sup.1 
100 
Hydral 710W.sup.2 
150 
Silastic HA-2.sup.3 
25 
Altax.sup.4 23.75 
Vanax 552.sup.5 50 
Butyl Zimate 12.5 
______________________________________ 
.sup.1 High molecular weight linear polymer substituted with about 52 mol 
percent phenoxy, 42 mole percent pethylphenoxy and 6 mole percent 
oallylphenoxy groups 
.sup.2 Alcoa brand hydrated alumina 
.sup.3 DowCorning brand of silicon filled methyl vinyl silicone 
.sup.4 Vanderbilt brand benzothiazyl desulfide oil modified and treated 
with 1% zinc stearate 
.sup.5 Vanderbilt brand piperidinium pentamethylene dithiocarbamate 
The above concentrate was mixed in a Banbury mixer at 60 rpm and then in a 
2-roll mill. Finally, the first formulation above was placed on one roll 
of a 2-roll mill and 14.9 parts by weight of the concentrate were randomly 
dropped into the nip of the 2-roll mill as the blend rotated on one roll. 
The blend was then cut from the roll and was homogenized by 20 passes 
through the mill with folding after each pass to form a sheet. The sheet 
was cut to form a rectangular slab which was placed in a mold. The mold 
was placed in a pre-cure oven maintained at 103.3.degree. C. which is 
below the activation temperature of the blowing agent for approximately 
seventeen minutes and placed in a foaming oven for a twenty-five minute 
period. The foaming oven was maintained at 160.degree. C. which is above 
the activation temperature of the foaming agent. The resultant foam was 
allowed to cool to ambient temperature and then placed into an oven and 
heated to 200.degree. C. for about 16 hours to produce a rigid low density 
cellular polyphosphazene foam. Test specimens were prepared from the foam 
and foam quality, water absorption, compression resistance, density, water 
vapor permeability, tensile strength, and flexural modulus measurements 
were obtained on the test specimens. The results are shown in the 
following Table 1. 
TABLE 1 
______________________________________ 
Foam quality good 
Water absorbance (%).sup.1 
1379 
Compression resistance (lb/in.sup.2).sup.2 
9.26.sup.3 
Density (lb/ft.sup.3).sup.4 
3.64 
Water vapor permeability (perms-in).sup.5 
.026 
Tensile strength (lb/in.sup.2).sup.6 
17.066.sup.7 
Flexural modulus (lb/in.sup.2).sup.8 
3334.8.sup.9 
______________________________________ 
.sup.1 ASTM D 1056 (1 test specimen used instead of 3) 
.sup.2 ASTM D 1056 (except deflection was maintained at 25% and the load 
observed and recorded 60 seconds after 25% deflection was reached) 
.sup.3 Average of 5 specimens each having a surface area of 1 inch.sup.2 
and an average thickness of 0.581 inch 
.sup.4 ASTM D 1667 
.sup.5 MILI-24703 Section 4.6.15 (1 test specimen used instead of 3) 
.sup.6 ASTM D 412 
.sup.7 Average of 3 specimens each having a thickness of 0.25 inch 
.sup.8 ASTM D 790 (3 test specimens used instead of 5) 
.sup.9 Average of 3 specimens each having a thickness of 1 inch 
Flammability properties were also measured for the rigid polyphosphazene 
foam prepared as described above by measuring test specimens of the foam 
for acid gas generation, flame spread index, thermal conductivity, 
specific optical density, rate of heat release and limiting oxygen index. 
The results are shown in the following Table 2. 
TABLE 2 
______________________________________ 
Acid gas generation (mg-HCl/gm).sup.1 
0 
Flame spread index.sup.2 9.51.sup.3 
Thermal conductivity (BTU-in./hr. ft.sup.2 F.).sup.4 
0.275 
Specific optical density.sup.5 
Non-flaming mode 19.5.sup.6 
Flaming mode 34.5.sup.7 
Rate of heat release.sup.8 
Max RHR (kw/m.sup.2) 35.46.sup.9 
2 Min HR (kw-min/m.sup.2) 
33.81.sup.10 
Max SRR (smk/min,m.sup.2) 
22.39.sup.11 
Limiting Oxygen Index (%).sup.12 
&gt;56 
______________________________________ 
.sup.1 MILI-24703 Section 4.6.18 
.sup.2 ASTM E 162 
.sup.3 Average of 4 specimens each 6 inches .times. 18 inches .times. 1 
inch 
.sup.4 ASTM C 518 
.sup.5 ASTM E 662 (2 test specimens used instead of 3) 
.sup.6 Average of 2 test specimens each 3 inches .times. 3 inches .times. 
0.5 inch 
.sup.7 Average of 2 test specimens each 3 inches .times. 3 inches .times. 
0.5 inch 
.sup.8 ASTM E 906 
.sup.9 Average of 3 test specimens each 6 inches .times. 6 inches .times. 
1 inch 
.sup.10 Average of 3 test specimens each 6 inches .times. 6 inches .times 
1 inch 
.sup.11 Average of 3 test specimens each 6 inches .times. 6 inches .times 
1 inch 
.sup.12 ASTM D 2863 (5 test specimens used instead of 10) 
The results demonstrate the excellent flammability properties of this 
material which include high resistance to ignition, little contribution to 
flame spread and extremely low levels of smoke generation. Also, the heat 
release as measured by ASTM E 906 is very low. 
In addition to providing for the production of rigid polyphosphazene foamed 
material of various geometrical shapes and designs, another feature of the 
present process is that it permits the formation of joint bonds between 
two or more individual pieces of flexible foam so that when the individual 
pieces of flexible foam are converted to a rigid foam by the process of 
the invention, they bond with one another to form a single unitary 
composite piece of rigid material. For example, the edges of two or more 
separate pieces of flexible foam can be abutted against each other and 
heated in accordance with the process of the invention and converted to a 
unitary composite piece of rigid foam. During the process, a bond forms 
between the individual pieces of the flexible foam where the edges of the 
pieces contact one another and a single, unitary composite piece of rigid 
foam is produced thereby. This has significant practical application in 
that it allows for several individual pieces or sections of hollow 
cylindrical flexible foam to be bonded together end to end to form one 
unitary section of rigid polyphosphazene material which can be used as 
pipe insulation for exceptionally long sections of pipe. In addition, the 
present process also can be used to bond two pieces of flexible foam 
together each having a cut edge with a 45 degree angle to produce a 90 
degree piece of rigid foam. This feature of the invention virtually 
eliminates or reduces the need for the use of conventional adhesives such 
as solvent-dispersed synthetic rubber resin adhesives to bond separate 
pieces of foamed materials together which often contribute to smoke 
generation in a fire situation. 
To determine the integrity of the bond formed by the present process, the 
tensile strength of a bonded piece of rigid foam was measured and compared 
to that of a single non-bonded piece of rigid foam. An identical 
formulation as previously described was prepared, placed in a mixing 
chamber and mixed until a substantially homogeneous blend was obtained. 
The blended formulation was removed from the mixer and passed between the 
rolls of a two-roll mill about 20 passes with folding between passes to 
form a 0.5 cm sheet. The sheet was cut to form a rectangular slab which 
was placed in a mold. The mold was placed in a pre-cure oven maintained at 
103.3.degree. C. which is below the activation temperature of the blowing 
agent. The slab was then removed from the mold and placed in a foaming 
oven for a twenty-five minute period. The foaming oven was maintained at 
177.degree. C. which is above the activation temperature of the foaming 
agent. Two strips were cut from the foamed flexible composition each 2.5 
inches in length, 0.2 inch in thickness and 1 inch wide and placed end to 
end with the two ends of the strip touching each other. The strips were 
then placed in an air circulation oven, heated to 200.degree. C. and 
maintained at that temperature for 16 hours to convert the individual 
pieces of flexible foam to a solitary composite rigid foam. The sample was 
taken out of the oven, allowed to cool to ambient temperature where 
adhesion between the two foamed pieces was observed to be very good. The 
tensile strength of the bonded piece of rigid foam was measured by the 
ASTM D 412 method and found to be approximately 17 psi which is about the 
same as the tensile strength of the non-bonded rigid foam test specimen 
reported in Table 1 above. 
Thus, another embodiment of the present invention is a method of bonding 
one piece of cured flexible foamed polyphosphazene elastomer material to 
at least one other piece of cured flexible foamed polyphosphazene 
elastomer material to form a single unitary piece of rigid foamed 
polyphosphazene composite material said process comprising placing said 
pieces of cured flexible foamed polyphosphazene elastomer material in 
juxtaposition with and abutting one another and heating said pieces at a 
temperature and for a length of time sufficient to cause said pieces to 
bond together and form a single unitary piece of rigid foamed 
polyphosphazene composite.