Process for manufacturing sintered polybenzimidazole article

Provided is a process for manufacturing sintered polybenzimidazole (PBI) articles at high yields, said articles having neither voids nor porous, low-strength parts resulting from oxidative deterioration. To produce said sintered PBI articles from a particulate PBI resin, said PBI resin is compacted in a closed mold, the mold is heated up to a predetermined temperature of from 350.degree. to 600.degree. C. without applying any external pressure thereto, then, after having heated up to said predetermined temperature, the mold is pressed to have a predetermined external pressure of from 140 to 1400 kg/cm2 at said predetermined temperature whereby the resin is sintered for from 60 to 350 minutes under that conditions, and finally the sintered PBI article is cooled and taken out of the mold.

FIELD OF THE INVENTION 
This invention relates to a process for manufacturing sintered 
polybenzimidazole (hereinafter referred to as PBI) articles. 
BACKGROUND OF THE INVENTION 
PBI is known as a polymer which can be used in a broad temperature range 
while having excellent characteristics such as high mechanical strength, 
high chemical resistance, high solvent resistance, high radiation 
resistance and high flame resistance. As one example of manufacturing 
sintered PBI articles by sintering PBI, known is a method of sintering a 
mixture comprising a PBI polymer and a PBI prepolymer as a sintering aid 
under sufficient heat and pressure (see U.S. Pat. No. 3,340,325). However, 
as producing a mixed gas comprising phenol vapor and water vapor, this 
method is problematic in that care must be taken in the sintering 
operation and that many voids remain in the products resulting in the 
failure in manufacture of thick products. 
As opposed to this, the sintered PBI articles developed by Hoechst Celanese 
Corporation are superior to conventional ones as having good physical 
properties and being able to be practicably thick. Said excellent articles 
are manufactured according to the Hoechst Celanese's sintering process for 
manufacturing them, which is as follows (see U.S. Pat. No. 4,814,530): 
A particulate PBI resin having a particle size not larger than 100 
mesh-screen, having a water and volatiles content of not larger than 0.1% 
by weight and having an inherent viscosity of at least 0.4 is compacted in 
a mold at room temperature for at least 1 minute under a pressure of from 
2000 to 20000 psi, then the thus-compacted PBI resin is heated at 
temperatures falling between 825.degree. F. and 950.degree. F. with 
maintaining said pressure range (heating step), and, after the PBI resin 
has been heated up to the predetermined temperature falling within said 
range, the pressure is removed, and thereafter said PBI resin is further 
heated at least at said temperature for 4 hours (sintering step). After 
this, the resulting PBI article is again compressed under a pressure 
falling within said range, and then, after once cooled to 800.degree. F. 
or lower, this is again heated and kept at a temperature falling between 
825.degree. F. and 950.degree. F. for at least one hour under the same 
pressure (post-curing step). Through this process, manufactured are the 
sintered PBI articles. 
One typical profile of the relationship between the time-dependent 
temperature and pressure in said Hoechst Celanese's PBI sintering process 
is shown in FIG. 1, in which the horizontal axis indicates the varying 
time and the vertical axis in the upper graph indicates the varying 
temperature while that in the lower graph indicates the varying external 
pressure to be applied to the mold. In the Hoechst Celanese's process, a 
PBI resin from which water and volatiles have been removed in order to 
prevent the formation of voids is first loaded into a mold, and then 
compacted therein under pressure (time c). The time of from c to d is for 
the heating step of heating the PBI resin up to the sintering temperature 
prior to being sintered. In this heating step, the PBI resin is heated 
while pressure is applied to the mold. The heating temperature is a 
predetermined one to fall between 440.degree. C. and 510.degree. C. The 
time for the heating is from 1 hour and 30 minutes to 2 hours and 30 
minutes. The time of from d to e is for the sintering step of sintering 
the PBI resin, before which the pressure to the mold is removed and in 
which the predetermined temperature is maintained. The time of from e to f 
is for the post-curing step, which follows said sintering step and in 
which the pressure that is the same as that employed in the heating step 
is again applied to the mold in order to prevent the sintered PBI from 
being expanded, and the sintered PBI is once cooled to below Tg of PBI 
resin and then again heated to the temperature that is the same as that 
employed in the sintering step while said elevated pressure is maintained 
to thereby post-cure the sintered PBI. At the time f, the pressure is 
removed, and then the sintered PBI is cooled and thereafter taken out of 
the mold. 
This process could produce sintered PBI with good characteristics, but is 
still problematic in various points such as those mentioned below. In this 
process, since pressure is applied to the PBI resin all the time during 
the heating step, the decomposed gas resulting from the heating of said 
PBI resin could not escape from within the sintered PBI article but 
remains therein as voids. These voids cause the cracking of the article, 
resulting in the increase in defective products. The decomposed gas 
comprises the vapor resulting from the decomposition of the 
non-polymerized PBI resin itself and the vapor resulting from the reaction 
of lithium chloride (LiCl) (this is added to the PBI resin as a 
stabilizer) with the PBI resin at high temperatures. Concretely, the 
decomposed gas may include, for example, CO, CO.sub.2, CH.sub.4, 
chloroform and phenol. This vapor composition was confirmed through gas 
chromatographic mass analysis of the gas that results from the heating of 
the PBI resin at about 500.degree. C. On the other hand, in the sintering 
step, since the pressure is not applied to the mold, the sintered PBI 
article expands, resulting in the failure in obtaining products having 
desired shapes. For this, two essential factors are referred to; one being 
the expansion (spring back) of the sintered article itself due to the 
none-pressure applying to the mold, and the other being the generation of 
the decomposed PBI gas to cause the expansion of the sintered article. 
Since the part of the PBI resin that has expanded due to said spring back 
of itself is contacted with oxygen in air that may penetrate into the 
mold, it could not be sintered but is oxidized to be porous. The strength 
of said porous part is lowered to be about 80% as compared with the other 
sintered part. Thus, the sintered PBI articles having such porous parts 
are unsuitable to practical use. 
The subject matter of the present invention is to provide a process for 
manufacturing sintered PBI articles with no porous part to be caused by 
the spring back of PBI, while minimizing the voids that shall result from 
the gas to be generated by the heating of PBI. 
SUMMARY OF THE INVENTION 
We, the present inventors have assiduously studied so as to attain the 
above-mentioned subject matter, and, as a result, have found that, when 
the pressing device of a mold, into which a PBI resin to be sintered is 
put, is fixed and restrained in such a manner that the PBI resin being in 
the mold is well compacted therein in the absence of any external pressure 
being applied to said PBI resin, and when said PBI in said condition is 
heated up to a sintering temperature and thereafter sintered under a 
predetermined pressure, then the formation of voids in the sintered PBI 
article can be prevented and the expansion of the sintered PBI article due 
to its spring back can also be prevented. On the basis of these findings, 
the inventors have completed the present invention. 
Accordingly, the present invention provides a process for manufacturing 
sintered PBI articles by sintering a PBI resin, which comprises the 
following steps in that order: 
(1) a step of putting a PBI resin into a mold having a predetermined shape; 
(2) a step of closing the mold to thereby compact the PBI resin therein, 
followed by heating the mold up to a predetermined temperature of from 
350.degree. C. to 600.degree. C. in the absence of any external pressure 
to the mold, 
(3) a step of sintering the resin, after having reached the predetermined 
temperature, in such a manner that said elevated temperature is kept as it 
is for from 0 to 100 minutes, then the pressure to the mold is increased 
up to a predetermined pressure of from 140 to 1400 kg/cm.sup.2, and 
thereafter said elevated temperature and said increased pressure are kept 
as they are for from 60 to 250 minutes; 
(4) a step of cooling the mold down to a temperature of from 50.degree. to 
400.degree. C.; and 
(5) a step of taking out the sintered PBI article from the mold.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
In general, PBI resins of the following formula may be used for 
manufacturing the sintered PBI articles of the present invention. 
##STR1## 
wherein R that constitutes the recurring units is a tetravalent aromatic 
nucleus with the nitrogen atoms forming the benzimidazole rings being 
paired upon adjacent carbon atoms, i.e., ortho carbon atoms, of the 
aromatic nucleus; R' that also constitutes the recurring units is a 
divalent group of the class consisting of an aromatic ring, an alkylene 
group (preferably having 4 to 8 carbon atoms) and a heterocyclic ring such 
as pyridine, pyrazine, furan, quinoline, thiophene and pyran; and R and R' 
both may be the same or different in the recurring polymer chains. 
Examples of PBIs of the above-mentioned formula may include the following 
polymers and copolymers: 
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole; 
poly-2,2'-(biphenylene-2",2"')-5,5'-bibenzimidazole; 
poly-2,2'-(biphenylene-4",4"')-5,5'-bibenzimidazole; 
poly-2,2'-(1",1",3"-trimethylindanylene)-3",5"-p-phenylene-5,5'-bibenzimida 
zole; 
2,2'-(m-phenylene)-5,5'-bibenzimidazole/2,2'-(1",1",3"-trimethylindanylene) 
-5",3"-(p-phenylene)-5,5'-bibenzimidazole copolymer; 
2,2'-(m-phenylene)-5,5'-bibenzimidazole/2,2'-biphenylene-2",2"'-5,5'-bibenz 
imidazole copolymer; 
poly-2,2'-(furylene-2",5")-5,5'-bibenzimidazole; 
poly-2,2'-(naphthalene-1",6") -5,5'-bibenzimidazole; 
poly-2,2'-(naphthalene-2",6") -5,5'-bibenzimidazole; 
poly-2,2'-amylene-5,5'-bibenzimidazole; 
poly-2,2'-octamethylene-5,5'-bibenzimidazole; 
poly-2,2'-(m-phenylene)-diimidazobenzene; 
poly-2,2'-cyclohexenyl-5,5'-bibenzimidazole; 
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)ether; 
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfide; 
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfone; 
poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)methane; 
poly-2,2'-(m-phenylene)-5,5"-di(benzimidazole)propane-2,2; and 
poly-ethylene-1,2,2,2"-(m-phenylene)-5,5"-di(benzimidazole)ethylene-1,2 
where the double bonds of the ethylene groups are intact in the final 
polymer. 
One preferred polymer is poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole. 
As the starting PBI resins, in general, usable herein are those having the 
physical properties mentioned below. 
PBI resins having an inherent viscosity (IV) of not smaller than 0.4 dl/g 
at 25.degree. C. as a solution prepared by dissolving 0.4 g of PBI in 100 
ml of 97 wt. % sulfuric acid; 
PBI resins having a particle size of not larger than 500 .mu.m, preferably 
not larger than 150 .mu.m; and 
PBI resins having a water and volatiles content of not larger than 0.1% by 
weight. 
The restriction of the particle size of the starting PBI resins is based on 
the reason because, if the particle size is larger than 500 .mu.m, the 
complete removal of voids initially existing in the PBI particles is 
difficult. Since PBI resins which are solid at room temperature are 
hygroscopic, they generally contain from about 2-3% by weight of water. If 
these PBI resins having such a water content are sintered, the sintered 
PBI articles will inevitably have voids. Therefore, it is desirable to dry 
the PBI resins, for example, at 150.degree. C. for 12 hours or longer, or 
at 177.degree. C. for 4 hours or longer, thereby making the PBI resins 
have a water and volatiles content of not larger than 0.1% by weight. The 
volatiles as referred to herein indicate phenol and others that result 
from the partial decomposition of PBI during the synthesis of PBI. 
The sintering process of this invention comprises the heating step, the 
sintering step and the cooling step. 
If the mold is pre-heated, before or after the step of putting a PBI resin 
thereinto, at a temperature falling between 100.degree. C. and 400.degree. 
C., preferably between 250.degree. C. and 300.degree. C., rapid heat 
transfer to the PBI resin is possible and the total processing time for 
the production of sintered PBI articles can be shortened. Therefore, the 
process of this invention may optionally comprise said pre-heating step. 
This pre-heating can be effected by putting the mold in an oven having 
forced air convection therein, or by heating the mold with a heater as 
built in said mold. 
To carry out the process of this invention, a PBI resin, which has been 
preferably pre-dried, is first charged into a mold (charging step). Next, 
prior to the next heating step, it is desirable to compact the PBI resin 
by pressing it, thereby degassing the mold. The pressure for this 
compacting may be suitably from 50 to 350 kg/cm.sup.2. Though depending on 
the size and the shape of the intended sintered article, the compacting 
time may be generally within 30 minutes or shorter. A filler having a 
minimized volatiles content, such as graphite, glass, glass fiber or 
carbon fiber, may be added to the starting PBI resin, depending on the 
intended characteristics of the sintered PBI to be obtained. After the PBI 
resin has been well compacted in the mold, the pressing device is fixed 
and restrained so that it is not moved from the predetermined position. 
Where pressure is applied for the compacting mentioned above, this is 
removed to be 0 kg/cm.sup.2. 
Next, before the sintering step, the mold is heated up to the sintering 
temperature (heating step). In this heating step, it is important that any 
external pressure should not be applied to the mold. In other words, the 
mold is fixed and restrained at the pressing position at which the PBI 
resin was compacted in the mold. In this condition, no external pressure 
shall be applied to the pressing device of the mold, or that is, the 
pressure to the mold shall be 0 kg/cm.sup.2. The highest temperature to 
which the PBI resin in the mold is heated is selected from the range 
between 350.degree. C. and 600.degree. C. The temperature at which the 
heating is started may be from 100.degree. C. to 400.degree. C., if the 
mold is pre-heated; or may be room temperature when the mold is not 
pre-heated. In this heating step, no pressure is applied to the PBI resin 
in the mold. Therefore, even if PBI is decomposed to give gas, said gas 
may be easily removed out of the mold via its slits, resulting in great 
reduction in the formation of voids (these voids are inevitable in 
conventional sintered PBI articles) in the sintered PBI articles to be 
finally obtained herein. This heating may be effected by the heater as 
built in the mold or by any other heating means capable of attaining the 
intended heating. Considering the next heating step, the mold to be used 
herein is advantageously one as combined with a pressing device. The time 
for the heating step may be preferably from 1 hour and 30 minutes to 2 
hours and 30 minutes. 
After this heating step, pressure is applied to the mold thus heated up to 
the sintering temperature, with maintaining said elevated temperature, and 
the PBI resin is sintered in this mold (sintering step). In this sintering 
step, the PBI resin is still kept at the elevated temperature of from 
350.degree. C. to 600.degree. C., to which is applied a constant pressure 
which is necessary for the sintering and which falls between 140 
kg/cm.sup.2 and 1400 kg/cm.sup.2. In this step, it is desirable that the 
predetermined temperature and pressure be kept constant as much as 
possible within the defined ranges. For this, for example, suitable 
apparatus is a thermostat combined with a device, by which the temperature 
and/or the pressure are/is restored to the predetermined one(s), if having 
varied within a predetermined range of deviation. The time for the 
sintering may be suitably defined, depending on the size, thickness and 
shape of the intended article to be obtained after the sintering. In 
general, the time may fall between 60 minutes and 350 minutes. 
Since almost all the decomposed gas of PBI is discharged out of the mold in 
the previous heating step, sufficient pressure can be applied to the PBI 
not containing the decomposed gas in this sintering step. Therefore, the 
contact area of the particulate PBI resin can be enlarged in this 
sintering step, in which, in addition, said PBI resin is shielded from air 
since pressure is applied to the mold, being different from the PBI resin 
to be sintered according to the above-mentioned Hoechst Celanese's 
process. Accordingly, since the PBI resin being sintered according to the 
process of this invention is not contacted with oxygen in air, it is 
hardly oxidized and decomposed. Therefore, the process of this invention 
can produce sintered PBI articles with high strength. 
Finally, the sintered PBI article is cooled and then taken out of the mold 
(cooling step). In the process of this invention, it is desirable that the 
pressure be not removed immediately after the sintering step but be 
gradually reduced after the temperature of the sintered PBI article has 
become lower than the glass transition point (Tg) of the starting PBI 
resin when the sintering temperature is higher than Tg of the starting PBI 
resin. This is because, if the pressure is removed while the temperature 
of the sintered PBI article is still higher than said Tg, the removal of 
the pressure will cause spring back of the sintered PBI article, resulting 
in the expansion of the volume of the article. If so, sintered PBI 
articles with desired shapes are difficult to obtain. 
Therefore, if the sintering temperature is higher than the glass transition 
point (Tg) of the starting PBI resin, it is desirable that the constant 
pressure kept during the sintering step be still kept even in the cooling 
step until the temperature of the sintered PBI articles reaches the glass 
transition point of the starting PBI resin. For example, where 
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole is used as the starting PBI 
resin, the pressure can be lowered to 350 kg/cm.sup.2 when the temperature 
of the sintered PBI article has become lower than the glass transition 
point of said PBI resin, 427.degree. C. The pressure to be applied to said 
PBI article during cooling step may be obtained with reference to the 
relationship between the compressive strength (kg/cm.sup.2) of the 
sintered article and the temperature thereof. From said relationship, the 
compressive strength of the sintered article corresponding to temperature 
is obtained, and the pressure to be applied to said article at the lowered 
temperature must be controlled to be not higher than said compressive 
strength of the article at said lowered temperature. In particular, when 
the temperature of the sintered PBI article is slightly lower than Tg of 
the starting PBI resin, the strength of the sintered article is not so 
high. In this condition, if a pressure higher than its compressive 
strength is applied to the sintered article, the article is broken. 
Therefore, it is important that the pressure to be applied to the sintered 
article be well controlled to evade such breakage of the article. 
On the other hand, where the sintering temperature is set to be not higher 
than Tg of the starting PBI resin, the sintered PBI article is slightly 
apt to show spring back as compared with a case where the sintering 
temperature is set to be higher than Tg of the starting PBI resin. Also in 
this case where the sintering temperature is not higher than Tg of the 
starting PBI, therefore, it is still desirable that the sintered article 
be cooled under suitable pressure. 
After the sintered article has been cooled to 300.degree. C. or lower, it 
is taken out of the mold. 
The time for the cooling step varies, depending on the size, thickness and 
shape of the sintered article, but may be generally from 2 hours to 6 
hours. 
In order to explain the present invention in more detail, one preferred 
embodiment of the sintering process of this invention is referred to 
hereinunder with reference to FIG. 2 which shows the time-dependent 
relationship between the temperature and the pressure employable in the 
process of this invention. In FIG. 2, the horizontal axis indicates the 
varying time; and the vertical axis in the upper graph indicates the 
varying temperature while that in the lower graph indicates the varying 
pressure. 
The time from a to b is for the pre-heating step of pre-heating the mold to 
a temperature falling between 100.degree. C. and 400.degree. C., 
preferably between 250.degree. C. and 300.degree. C. This pre-heating step 
is an optional one to be effected, if desired, for shortening the time for 
the entire process of this invention. 
The time from b to c is for the pre-treatment step of charging a PBI resin, 
which may be optionally dried, into a mold and compacting it therein. The 
pressure for this step may be generally from about 50 kg/cm.sup.2 to about 
350 kg/cm.sup.2. If this step follows the previous pre-heating step, it is 
desirable that the pre-heated condition be still kept in this step. 
The time from c to d is for the heating step of heating the compacted PBI 
resin to a temperature at which said PBI resin is sintered in the next 
sintering step. In this heating step, it is necessary that the pressing 
device of the mold be fixed at the position at which the PBI resin has 
been compacted in the previous step of from b to c, and that no external 
pressure is applied to the mold. Thus, the external pressure to be applied 
to the pressing device in this condition shall be 0 kg/cm.sup.2. Where no 
pre-heating is operated in the previous step, the PBI resin in the mold is 
heated from room temperature up to a predetermined temperature falling 
between 350.degree. C. and 600.degree. C.; but where mold has been 
pre-heated in the previous step, the PBI resin in the mold is heated from 
said pre-heated temperature up to said predetermined temperature falling 
said range. The time for the heating step may be generally from 1 hour and 
30 minutes to 2 hours and 30 minutes. 
The time from d to e is for the sintering step where the PBI resin as 
heated in the mold up to said predetermined temperature is sintered while 
a predetermined pressure is applied to the mold. The application of the 
pressure to the mold is started from 0 to 100 minutes after the finish of 
the previous heating step, preferably after almost all the decomposed gas 
has been removed from the heated PBI resin, for example, from 5 to 60 
minutes after the finish of the previous heating step. The time for the 
sintering step that includes the step of removing the decomposed gas and 
the step of pressing the mold may be from 60 to 350 minutes. 
The time from e to f is for the cooling step of cooling the sintered PBI 
article after the sintering step. Where the sintering temperature employed 
in the previous sintering step is not lower than Tg of the starting PBI 
resin, sudden full removal of the pressure at a temperature higher than 
said Tg will cause the spring back of the sintered PBI article, often 
resulting in failure in the production of the desired article. In this 
case, therefore, it is desirable that the sintered PBI article be first 
cooled to the stage e1 (at which the temperature of the sintered PBI 
article is lower than Tg of the starting PBI resin) with no removal of the 
pressure, and thereafter the pressure is removed. If the sintered article 
has a complicated structure, the pressure may be removed in 2 or more 
stages. In addition, depending on the conditions for this step, the 
pressure may also be removed in 2 or more stages. As one example where 
poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole is used as the starting PBI 
resin, the pressure is lowered to and set at 350 kg/cm.sup.2 after the 
sintered PBI article has been cooled to lower than the glass transition 
point of the starting PBI resin, 427.degree. C. (e1). Next, when the 
sintered PBI article has been further cooled to 350.degree. C. (e2), the 
pressure can be lowered to and set at 190 kg/cm.sup.2. 
The sintered PBI articles obtainable according to the process of this 
invention have high chemical resistance to, for example, ketones, organic 
acids, oil well brines, oil well sour gas, and aromatic, aliphatic and 
halogenated hydrocarbons. Therefore, the sintered PBI articles obtainable 
according to the process of this invention are particularly effective in 
applications where requirements cannot be met by other resins--in extreme 
high temperatures, in harsh chemical environments, or in applications 
where durability and wear resistance are important. The sintered BPI 
articles obtainable according to the process of this invention are 
especially preferably used in manufacture of gaskets, seals, o-rings, 
bearings, semiconductors-tooling devices, gears, bearings and valves in 
the field of petroleum, geothermal, petrochemical industry and in other 
industrial applications. 
The present invention will be described in more detail by means of the 
following examples, which, however, are not intended to restrict the scope 
of the invention. 
EXAMPLES 
A particulate PBI resin, poly-2,2'-(m-phenylene)-5,5'-bibenzimidazole 
having an inherent viscosity of 0.55 dl/g was dried for 12 hours in a 
forced air convection oven at 150.degree. C., to thereby remove the water 
and volatile content from the resin. 
A mold of 320 millimeters square.times.35 millimeters thick in size was 
pre-heated at 200.degree. C., and 6 kg of the dried powdery PBI resin was 
loaded into the mold, and pressed on a 650-ton oil press at 100 
kg/cm.sup.2 to thereby compact the resin in the mold. Next, the upper ram 
of this press was fixed at the compacted position, and the pressure was 
removed to be 0 kg/cm.sup.2 in the mold. The resin in the mold was heated 
up to 470.degree. C. with the heater built in the mold, without imparting 
any external pressure thereto. This heating step took 2 hours. After 30 
minutes, 600 kg/cm.sup.2 pressure was applied to the mold still at 
470.degree. C., and kept for 3 hours also still at 470.degree. C. 
After this, the mold was cooled in the following manner. First, after the 
mold was cooled to lower than 427.degree. C., the pressure to the mold was 
lowered to 350 kg/cm.sup.2. Next, after the mold was further cooled to 
about200.degree. C., the sintered article was taken out of the mold. This 
sintered article had a tensile strength of 1800 kg/cm.sup.2 on an average 
at 23.degree. C. 
TABLE 1 
__________________________________________________________________________ 
Physical Example Compara. 
Properties 
Condition 
standard 
Unit 
1 2 3 4 5 Average 
Example 
__________________________________________________________________________ 
Tensile 
23.degree. C. 
JIS kg/cm.sup.2 
1640 1790 
1880 
1840 
1850 
1800 1630 
Strength at K7113 
Breakage 
Tensile 
23.degree. C. 
% 4.0 4.0 4.0 4.0 4.0 4.0 3.0 
Elongation 
at 
Breakage 
Modulus of 
23.degree. C. 
kg/cm.sup.2 
65600 
67000 
66600 
-- -- 66400 
66000 
Elasticity 
Mean 24 to 150.degree. C. 
ASTM 
1/.degree.C. 
2.4 .times. 10.sup.-5 
-- -- -- -- 2.4 .times. 10.sup.-5 
2.3 .times. 10.sup.-5 
Thermal 
200 to TMA 1/.degree.C. 
3.5 .times. 10.sup.-5 
-- -- -- -- 3.5 .times. 10.sup.-5 
-- 
Expansion 
300.degree. C. 
Coefficient 
Poisson 0.35 0.35 
-- -- -- 0.35 0.34 
Ratio 
Dielectric 
27.degree. C. in 
JIS kV/mm 
22 25 26 24 22 24 21 
Breakdown 
ordinary oil 
C2110 
Strength 
__________________________________________________________________________ 
COMATIVE EXAMPLE 
The same particulate PBI resin as in Example 1 was dried in the same manner 
as in Example 1. In order to obtain a sintered article of 270 
millimeters--outside-diameter, 180 millimeters--inside-diameter, 30 
millimeters--thick in size, the resin was loaded into a mold, which was 
then immediately pressed at 350 kg/cm.sup.2. After this was heated up to 
460.degree. C., the pressure was removed, and the resin was kept in the 
mold at 460.degree. C. for 4 hours. After this, the sintered PBI article 
was again pressed at 350 kg/cm.sup.2, and once cooled to 420.degree. C. 
and thereafter again heated up to 460.degree. C. and kept at said elevated 
temperature for 90 minutes at said pressure. Finally, this was cooled to 
200.degree. C., and taken out of the mold. However, the article was 
cracked. This indicates that the gas generated during the heating remained 
as voids in the sintered article, and the sintered article was deformed 
and broken due to the stress as generated by the expansion force of the 
gas. 
Under the same conditions as above except that the time for the sintering 
at 460.degree. C. was varied to 6 hours, the sintered article was 
vitrified as having been oxidized and decomposed. This indicates the 
oxidation and decomposition of the PBI during the sintering step. 
In Table 1, shown are the physical properties of the sintered articles as 
obtained in the above-mentioned Examples of this invention and those of 
the sintered article as obtained in the above-mentioned Comparative 
Example which demonstrates the Hoechst Celanese's process. For the 
physical properties, measured were the tensile strength at breakage, the 
tensile elongation at breakage and the modulus of tensile elasticity which 
represent the mechanical properties, the mean thermal expansion 
coefficient which represents the thermal property, and the dielectric 
breakdown strength which represents the electric property. 
As is known from the data shown in Table 1, the characteristics of the 
sintered PBI articles as obtained according to the process of this 
invention are better than those of the sintered PBI article as obtained 
according to the known process. This is because the sintered articles as 
obtained according to the process of this invention have few voids 
therein, and since the sintered articles being manufactured according to 
the process of this invention are not contacted with oxygen in air during 
the sintering step, they are hardly oxidized and deteriorated, and 
therefore their strength is prevented from being lowered. 
In addition, since the sintered PBI articles as obtained according to the 
process of this invention have few voids therein and are oxidized and 
deteriorated little, there is little dispersion in the quality of the 
products as obtained according to the process of this invention. 
Therefore, the process of this invention can produce uniform sintered 
articles at high yields. Moreover, since the decomposed gas of the PBI can 
be removed from the sintered PBI articles in their manufacture according 
to the process of this invention, it is easy to produce thick sintered PBI 
articles according to the process of this invention. 
As has been described in detail hereinabove, in the process of this 
invention, formation of voids in the sintered PBI articles can be 
prevented, and spring back of the sintered PBI articles, which makes the 
articles have oxidized and deteriorated porous parts, can also be 
prevented. Therefore, according to the process of this invention, it is 
possible to produce sintered PBI articles having high strength but having 
neither voids nor porous parts at high yields, and it is easy to produce 
even thick sintered PBI articles also at high yields. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.