Polyimide film

A shaped polyimide article is disclosed having finely divided barium sulfate as an additive which lowers the limiting oxygen index in combustion and substantially eliminates ash as a combustion product.

BACKGROUND OF THE INVENTION 
Polyimide film and sheet materials are well known and have long been used 
in applications requiring high temperature stability. Polyimide has proven 
itself to be chemically stable at high temperatures and in a variety of 
hostile atmospheres and environments. Films and sheets of polyimide have 
found wide acceptance in electrical and electronic applications, 
especially in applications wherein the polyimide will not be burned or, if 
burned, the presence of ash will not be detrimental. 
The present invention provides an improved polyimide exhibiting most 
qualities of the polyimide of the prior art substantially unchanged and, 
also, exhibiting a uniform combustion front in an oxygen-rich atmosphere 
resulting in substantially ash-free combustion products. 
DESCRIPTION OF THE PRIOR ART 
Films and sheets of polyimide material have long been known. A variety of 
additives has, also, been disclosed as useful in polyimide films and 
sheets. For instance, U.S. Pat. No. 3,287,311 discloses the use of 
particulate dielectric additives, specifically, titania, barium and 
potassium titanate, magnesium sulfate, asbestos, ferrous and ferric oxide, 
potassium and sodium tartrate, ammonium hydrogen phosphate, alumina, 
silica and the like. U.S. Pat. No. 3,295,940 discloses abrasive additives, 
specifically, green rouge, silica, alumina, boron carbide, silicon 
carbide, steel, diamond and the like. U.S. Pat. No. 3,356,759 discloses 
the use of particulate inert lubricant additives, specifically, 
polyhalocarbon resins such as polytetrafluoroethylene, 
polychlorotrifluoroethylene, and the like. Canadian Pat. No. 708,896 
discloses electrically conductive additives such as carbon black. 
DESCRIPTION OF THE INVENTION 
According to the present invention there is provided a shaped article 
comprising a continuous matrix of polyimide and a finelydivided, 
particulate, barium sulfate additive distributed substantially 
homogeneously throughout. The shaped article of the present invention can 
be in any form but is most usually in the form of sheets or thin film, 
either as a web of indefinite length or cut off. The shaped articles of 
this invention will, therefore, most often be termed a film or sheet 
herein. 
The barium sulfate additive is used in the article of this invention 
because it has been discovered that barium sulfate alters the combustion 
characteristics of polyimide material in a way heretofore unknown. The 
barium sulfate can be used in concentrations of from 0.1 to 10 weight 
percent based on total weight of the shaped article; and is preferably 
used in concentrations of about 0.2 to 2.0 weight percent on that basis. 
As the concentration of barium sulfate is decreased, the degree of benefit 
is, of course, decreased. As the concentration of barium sulfate is 
increased, the presence of the barium sulfate has an increased influence 
on the physical properties of the polyimide shape article. At 
concentrations of greater than about 2 weight percent, the optics are 
adversely affected and at concentrations of greater than about 4 weight 
percent, the tensile and electrical properties are altered. Generally, 
more than about 4 weight percent barium sulfate should not be used. 
The barium sulfate additive is generally used in a finelydivided 
particulate form. As previously stated, barium sulfate is used primarily 
as an additive herein for the alterations which it causes in the 
combustion characteristics of a polyimide matrix. It has been learned that 
an added benefit can be derived from the use of barium sulfate if the 
particle size is properly selected and, especially, if the shaped article 
is a film or sheet. Barium sulfate having a properly selected particle 
size is useful to improve the handling qualities of films or sheets of 
polyimide. Finely particulate barium sulfate not only alters the 
combustion characteristics of the film or sheet; but, also, serves as a 
slip additive without seriously influencing the optical character or the 
physical and electrical qualities of the film or sheet. 
The average size of barium sulfate particles useful in the practice of this 
invention is from 0.2 to 20 micrometers but, to obtain the additional 
benefits in film or sheet of lower coefficient of friction (increased 
slip) without undue harm to other film qualities, the average barium 
sulfate particle size should preferably be 1 to 4 micrometers. A barium 
sulfate concentration of 0.1 to 1.0 weight percent is, also, preferred to 
obtain the above-mentioned additional benefits. 
Polyimide is useful in the articles of this invention independent of the 
method for making it. Such polyimide is generally the reaction product of 
a dianhydride and a diamine. 
A few of the dianhydrides suitable for use in polyimide manufacture of the 
present invention include: pyromellitic dianhydride; 
2,3,6,7-naphthalene tetracarboxylic dianhydride; 
3,3',4,4'-diphenyl tetracarboxylic dianhydride; 
1,2,5,6-naphthalene tetracarboxylic dianhydride; 
2,2',3,3'-diphenyl tetracarboxylic dianhydride; 
3,3',4,4'-benzophenone tetracarboxy dianhydride; 
2,2-bis(3,4-dicarboxyphenyl) propane dianhydride; ; 
bis(3,4-dicarboxyphenyl) sulfone dianhydride; 
3,4,9,10-perylene tetracarboxylic dianhydride; 
bis(3,4-dicarboxyphenyl) propane dianhydride; 
1,1-bis(2,3-dicarboxyphenyl) ethane dianhydride; 
1,1-bis(3,4-dicarboxyphenyl) ethane dianhydride; 
bis(2,3-dicarboxyphenyl) methane dianhydride; 
bis(3,4-dicarboxyphenyl) methane dianhydride; 
bis(3,4-dicarboxyphenyl) sulfone dianhydride; and the like. 
A few of the diamines suitable for use in polyimide manufacture of the 
present invention include: 4,4'-diaminodiphenyl propane; 
4,4'-diaminodiphenyl methane; benzidine, 3,3'-dichlorobenzidine; 
4,4'-diamino diphenyl sulfide; ; 3,3'-diamino diphenyl sulfone; 
4,4'-diamino diphenyl sulfone; 4,4'-diamino diphenyl ether; 1,5-diamino 
naphthalene; 4,4'-diamino diphenyl diethylsilane; 4,4'-diamino diphenyl 
diphenylsilane; 4,4'-diamino diphenyl ethyl phosphine oxide; 4,4'-diamino 
diphenyl N-methyl amine; 4,4'-diamino diphenyl N-phenyl amine; and the 
like. 
The polyimide is generally made by preparation of a polyamide acid from the 
dianhydride and the diamine followed by conversion of the polyamide acid 
to polyimide by chemical or thermal means. U.S. Pat. No. 3,179,630 
discloses conversion by chemical means and U.S. Pat. No. 3,179,634 
discloses conversion by thermal means. 
The barium sulfate additive is incorporated into the polyimide by being 
initially dispersed into the polyamide acid or into the dianhydride or the 
diamine. Once dispersed in a component, the barium sulfate will remain 
dispersed throughout reaction of the component to completion of the 
polyimide. 
The present invention is an article comprising barium sulfate dispersed in 
a matrix of polyimide. The invention, and what is believed to provide 
patentability, is that the barium sulfate additive alters the combustion 
characteristics of the polyimide. Polyimide with no additive or with other 
inert additives is capable of combustion in an atmosphere of 35-45 volume 
percent oxygen in an oxygen and nitrogen atmosphere while polyimide 
articles of the present invention are capable of combustion in an 
atmosphere of 26-32 volume percent oxygen in oxygen and nitrogen. More 
importantly, polyimide with no additive or with other inert additives 
leaves a charred residue after burning; while polyimide articles of the 
present invention burn substantially completely leaving little or no 
residue. This characteristic can be demonstrated by conducting the 
Limiting Oxygen Index test (LOI) as described hereinbelow. 
The polyimide of this invention retains the well known high temperature 
performance qualities of polyimide without barium sulfate and also 
exhibits the above-mentioned qualities of LOI and ash-free combustion. 
Because the polyimide of this invention burns ash-free, there may be 
reduced tendency for arc tracking through insulative layers of the 
material. The polyimide of this invention finds particular use in fuses 
wherein there is a desire to have a combustible material which burns with 
substantially no ash residue. 
Limiting Oxygen Index (LOI) is the designation of a test used to quantify 
the oxygen content of atmospheres which will support combustion of 
materials such as the polyimide material of this invention. The LOI is 
described in the American Society for Testing and Materials Standards and 
is designated therein as ASTM D-2863-74. The test determines the minimum 
concentration of oxygen in a mixture of oxygen and nitrogen flowing upward 
in a test column that will just support combustion under equilibrium 
conditions of candle-like burning. The LOI may vary slightly depending 
upon the thickness of the sample--a thicker sample resulting in a slightly 
hiher LOI. To prepare for the test, a sample of material to be tested is 
attached to a frame and held vertically in a glass column which is fitted 
with a gas flame ignition source and means for introduction of an 
oxygen-nitrogen gas mixture. The sample should be about 140 millimeters 
long and 52 millimeters wide. An oxygen-nitrogen mixture of predetermined 
proportions is introduced into the column and is continuously supplied at 
a flow rate through the column of 4.+-.1 centimeters per second (gas 
volume flow rate divided by column cross sectional area). To commence the 
test, the ignition source is applied to the entire top edge of the sample 
and then the source is removed and the burning of the sample is observed. 
If more than about 10 centimeters of the sample length burns, the test 
should be repeated with reduced oxygen in the atmosphere. The test is 
repeated until the lowest oxygen concentration is found which will support 
combustion. 
Slide angle slip is a measure of the angle at which a specified weight will 
slide across a surface of the film as prescribed in accordance with the 
method described in TAPPI, Volume 50, number 1, January 1967. It is 
generally accepted that the coefficient of friction of the surface being 
tested can be calculated as the tangent of the slide angle. Slide angle 
slip is often used to indicate the effectiveness of a slip additive. Lower 
slide angles indicate more effective slip additives.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Example A-Precursor 
Preparation of polyamide acid. A solution was made of 20 weight parts of 
4,4'-oxydianiline (ODA) and 330 weight parts of N,N'-dimethylacetamide 
(DMAC); and, over a 20-25 minute period, with agitation and under a 
nitrogen blanket, 21.8 weight parts of dry pyromellitic dianhydride (PMDA) 
were added to the solution. During the addition of PMDA, the temperature 
of the solution was maintained at less than about 60.degree. C. The 
solution was agitated for about 30 minutes after addition of the PMDA to 
complete polymerization to the polyamide acid during which time the 
temperature was permitted to decrease to about 45.degree. C. The solution 
was stored at about -20.degree. C. until used. 
Example B-Comparative 
Preparation of polyimide by thermal conversion. Polyamide acid solution, 
prepared as described in Example A, above, was warmed to about 
20.degree.-25.degree. C. and spread onto a glass plate as a wet film about 
0.5 millimeter thick. The glass plate and wet film were heated on a hot 
plate for about 25 minutes at 112.+-.2.degree. C. After the heating, the 
plate and film were cooled to about 40.degree. C. and the film was 
stripped from the plate, clamped into a frame to prevent shrinkage, and 
heated in an oven for about 30 minutes at 310.+-.5.degree. C. 
Example C-Comparative 
Preparation of polyimide by chemical conversion. Polyamide acid solution, 
prepared as described in Example A, above, was warmed to about 
20.degree.-25.degree. C. and spread onto a glass plate as a wet film about 
0.5 millimeter thick. The glass plate and wet film were immersed in a 1:1, 
by volume, solution of acetic anhydride and beta-picoline for 3-5 minutes 
at about 20.degree.-25.degree. C.; and the resulting, self-supporting, gel 
film was removed from the plate, clamped into a frame to prevent 
shrinkage, and heated in an oven for about 30 minutes at 310.+-.5.degree. 
C. 
EXAMPLE 1 
Preparation of polyimide by thermal conversion and having 1 weight percent 
BaSO.sub.4. Polyamide acid was prepared as described in Example A, above, 
with the exception that 0.8 weight parts of a 52 weight percent slurry of 
BaSO.sub.4 in DMAC was added to the ODA-DMAC solution before addition of 
the PMDA. The BaSO.sub.4 had an average particle size of about 3.+-.1.7 
micrometers. 
Polyamide acid, as prepared in this Example 1, was used to make polyimide 
as described in Example B, above. 
EXAMPLE 2 
Preparation of polyimide by chemical conversion and having 1 weight percent 
BaSO.sub.4. Polyamide acid, as prepared in Example 1, above, was used to 
make polyimide as described in Example C, above. 
EXAMPLE 3 
Preparation of polyimide by thermal conversion and having 0.5 weight 
percent BaSO.sub.4. Equal weight parts of polyamide acid as prepared in 
Example A, above, and in Example 1, above, were combined and used to make 
polyimide as described in Example B, above. 
EXAMPLE 4 
Preparation of polyimide by chemical conversion and having 0.5 weight 
percent BaSO.sub.4. The polyamide acid combination of Example 3, above, 
was used to make polyimide as described in Example C, above. 
EXAMPLE 5 
Preparation of polyimide by thermal conversion and having 0.25 weight 
percent BaSO.sub.4. Equal weight parts of polyamide acid as prepared in 
Example A, above, and in Example 3, above, were combined and used to make 
polyimide as described in Example B, above. 
EXAMPLE 6 
Preparation of polyimide by chemical conversion and having 0.25 weight 
percent BaSO.sub.4. The polyamide acid combination of Example 5, above, 
was used to make polyimide as described in Example C, above. 
Results of tests conducted on films of the above examples are presented in 
Table I. 
TABLE I 
______________________________________ 
BaSO.sub.4 
Slide 
Example (%) Angle Slip 
LOI Ash 
______________________________________ 
B (Control) 
0 60+ 36 Present 
C (Control) 
0 60+ 36 Present 
1 1 26 26 None 
2 1 23 26 None 
3 1/2 27 27 None 
4 1/2 23 27 None 
5 1/4 29 28 None 
6 1/4 25 28 None 
______________________________________ 
The films of Examples B, C and 1-6 were all about 0.041 to 0.046 millimeter 
thick. The results of the tests demonstrate that the BaSO.sub.4 additive 
significantly reduces the slide angle slip, reduces the LOI of polyimide 
film materials, and more importantly, substantially eliminates ash as a 
combustion product. 
EXAMPLE 7 
Comparison of the polyimide of this invention with polyimide containing 
other additive materials. Polyamide acid was prepared as described in 
Example A, above, with the exception that an amount of finely divided 
additive material was added to the ODA-DMAC solution before addition of 
the PMDA. Various amounts of several additives were used to make polyimide 
films and those films were tested and compared with films of this 
invention made and tested under the same conditions. The films were all 
about 0.025 micrometer thick and were made using the procedure described 
in Example B, above. Results of tests conducted on the films of this 
Example 7 are presented in Table II. 
TABLE II 
______________________________________ 
Additive LOI 
Material Wt. % (x) Ash 
______________________________________ 
alumina 0.7 35 &lt; x &lt; 38 Present 
calcium 1.0 32 &lt; x &lt; 35 Present 
pyrophosphate 
silica 0.7 35 &lt; x &lt; 38 Present 
BaSO.sub.4 0.3 28.2 = x None 
0.7 27.1 = x None 
1.2 26.9 = x None 
None (Control) 
0 36 Present 
______________________________________ 
The results of the tests demonstrate that the BaSO.sub.4 additive is the 
only additive in the tests which reduces the LOI of the polyimide. 
Moreover, it was noted that all film samples of these tests except those 
having BaSO.sub.4 additive burn with a flame and leave an ash of 
significant volume and substantial electrical conductivity. The polyimide 
of this invention burns with a uniform front without leaving an ash 
residue. Similar tests with boron nitride and carbon black as the 
additives resulted in polyimide with LOI greater than the LOI for 
additive-free polyimide.