Note: Descriptions are shown in the official language in which they were submitted.
7~
COPOLYIMIDES WITH A COMBINATION OF
FLEXIBILIZING GROUPS
This invention relates to novel copolyimides in
general and more particularly to novel copolyimides
derived from the reaction of one or more aromatic
dianhydrides with a meta-substituted phenylene diamine
and a bridged aromatic diamine. The incorporation of
the meta-substituted phenylene diamine derived units and
bridged aromatic diamine derived units into the linear
aromatic polymer backbone results in a copolyimide of
improved flexibility, processability and melt-flow
characteristics.
Aromatic polyimides are generally difficult to
process because they exhibit only a limited degree of
flow even at high temperatures and when subjected to
high pressure. These materials are, however,
exceptionally thermally stable and resist attack by most
solvents. Additionally, they generally have very high
glass transition temperatures because of their aromatic
character. Because of desirable properties such as
resistance to solvents and the high glass transition
temperature, many attempts have been made to prepare
aromatic polyimides which can be readily processed.
Prior art methods have included a solution by
incorporating sulfur linkages into a polyimide backbone,
the use of particulated oligomeric polyetherimide acids
which may be converted to a high molecular weight
, ~1
polymer system by melt polymerization, the use of varied
proportions of different polyetherimide segments in an
attempt to reach the optimum balance between
processability and solvent resistance, and preparing
certain polyetherimides which can be reinforced with
various fillers to form composites. The prior art
further teaches that the incorporation of flexible
moieties into the backbone of a polyimide can increase
thermoplastic character, and that the incorporation of
phenylene ether units into polyimides can improve
melt-flow properties. Even though all of the above
prior art systems have contributed in various ways to
improving the processibility of linear aromatic
polyimides, there is still a definite need in the art
for enhanced melt-flow properties in polyimides in order
that they may be used for applications such as holt-melt
adhesives or as matrix resins for fiber-reinforced
composites.
Accordingly, the present invention provides
polyimides with improved flow properties, low melt
viscosities, high glass transition temperatures and
which soften to a high degree above their glass
transition temperatures. The polyimides are resistant
to solvents, can be solvent or hot-melt coated onto
fibers for preparation of compositions, and can be used
as hot-melt adhesives. In addition, the polyimides
become more planar when exposed to temperatures above
their glass transition temperature. The invention
further provides a process for making polyimides of
enhanced melt-flow characteristics and provides for the
use of the polyimides as hot-melt adhesives for bonding
materials together by surface attachment.
According to the present invention, the desired
improvements are achieved by incorporating the proper
mix of flexibilizing units into the backbone of a linear
aromatic polyimide. The flexib]e units independently
~'~.7~
have been shown by others to enhance thermoplastic
character in polyimides, however, in the present
invention the proper incorporation of the prepreg
combinations of these units leads to unexpected
synergistic improvements in softening, thermoplastic and
flow behavior of the resulting polymers.
Generally, the poly(amide-acid) prepared in solution may be
converted to a polyimide by heating the solution at a temperature
of from 150 to 400C for a time sufficient to form the
polyimide.
The polymer with improved softening, thermoplastic
and flow behavior consists essentially of chemically
combined recurring units of the formulas:
O O O O
_ - N R N - Z I - - a n d--~ ~ _
ll 11 ll ll
O O O O
wherein R may differ between the recurring units and is
an aromatic tetravalent radical; and wherein Zl is a
bridged radical of the formula:
~ ~Jm
~B
3a ~ ~ f~ Sj
where Y may differ within the radical and is selected
from the group consisting of -O-, -S-, -CO-, -S02-,
I ' ( 3)2 ~ C(CH3)2 where R is
alkyl or aryl and m is 0, 1, 2 or 3; and wherein Z2 is
of the formula:
~E
)'7 ~1~
~ Z
where Z is selected from the group consisting of -Br-,
C.s~ -F-, -CF3-, -aryl-, or -alkyl.
Suitable R's include:
~ X ~ ~ X ~ X ~ ~ X ~ X ~ X
~, ~X~
wherein X may differ within the R radical and is
selected from the group consisting of -O-, -S-, -CO-,
SO NH-, -SO-, -C(CF3)2-, -C(CH3)2 R'
where R' is alkyl or aryl, as well as others that would
be obvious to those skilled in the art.
Suitable bridged radicals include:
O CF3
~, O ~ C ~ , o ~ C ~ o ~
O O
~0~ S ~0~0~ ~ ~ S~
s
as well as others that would be obvious to those skilled
in the art.
Examples of the Z2 radicals are:
C~ R Ar CF3 F Br
wherein R = alkyl and Ar = aryl.
A special case is also made for polyimides of the
following structure:
O O . O O
_ 11 1
_ ~N~CN ~N~N~ ~ ~--
O O O O
wherein y and m are as defined above and x and y are at
least equal to one and are positive whole numbers.
Representative polymers are prepared according to
the present invention by dissolving the appropriate
dianhydrides and diamines in solvents such as
N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP),
N,N-dimethylformamide (DMF), and bis(2-methoxyethyl)-
ether. Other solvents such as tetrahydrofuran, dioxane,
dimethylsulfoxide as well as others obvious to those
skilled in the art can be used.
6 1'~7~i7~sj
Ater the polymerization has occurred in the
particular solvent, the resultîng poly(amide-acid) is
precipitated in a non-solvent such as water, an alcohol,
or a hydrocarbon, The precipitated poly(amide-acid) is
then filtered from the solution, dried at a temperature
of 20C-30C for ten to fifteen hours, and heated in
an oven for approximately one hour at about 100C, then for
approximately one hour at about 200C and then for approximatel~
one hour at about 300C.
Chemical imidization can also be appropriate utilizing
dehydrating agents such as acetic anhydride in pyridine,
propionic anhydride in pyridine, butyric anhydride in
pyridine, trifluoroacetic acid in pyridine,
dicyclohexylcarbodiimide, or others obvious to those
skilled in the art. The a~ddition of such dehydrating
agents to the poly(amide-acid) solution serves to lower the
temperature at which imidization will take place.
The conversation of the
poly(amide-acid) to the corresponding isoimide should
appropriately be between 5 and 100%.
Copolyimides are prepared using three dianhydrides.
The structures of these copolyimides are as follows:
O O O O
~N~CN ~N~CN ~ O ~
11 11 x 11 11 Y n
O O O O
:'~ O O O O O O
11 11 11 ~ 11 11 11 \
_ [~ ~C ~N ~N ~C ~CN ~~ I n
O O O
O ' O
_
_ ~ ~ ~S~ ~11 ~
O O
O 0
\ ~ ~s ~ ~ ~ n
7~j'7 ~1i
where x:y = 1:1 and n is between 5 and 100.
In these three cases, films are prepared from the
polymers by applying a 0.15 inch coating of the
poly(amide-acid) in DMAc onto a piece of plate glass,
evaporating tne solvent and heating the film for
approximately one hour at about 100C, then for
approximately one hour at about 200C, and then for
approximately one hour at about 300C. After removal
of the films from the glass, they all exhibit a
smoother, more uniform surface than the corresponding
polymer films where the ratio x:y is 1:0 or 0:1. In
order to more quantitatively assess the effect of using
these two flexibilizing groups in conjunction with each
other a series of copolyimides of the following
compositions are prepared in DMAc and thermally
converted to the imide.
4 N ~ ~ ~ ~ N
t ~ ~ s ~ ~ç ~
where y:x is 1:3, 2:2 and 3:1, and where n is 5-100.
The polymer designation (Tables I-V) for the 3:1
structural formula is 431, that for the 2:2 structural
formula is 422, and the designation for the 1:3
structure is 413.
In the case where x = 0, this polymer is designated
BDSDA/4,4'-ODA, and where y is 0, this polymer is
designated BDSDA/_-PDA. Another polymer of the
following structure with the designation BDSDA/3,3'-ODA
~'~7~
was prepared for comparison:
O O
t- ~ ~ S
O O
The glass transition temperatures of these polymers are
shown in Table I.
TABLE I
Copolymer Molar Glass Transition
Polymer Ratio Temperature
y:x Tg, C
BDSDA/_-PDA 0:4 224
413 1:3 220
422 2:2 2~6
431 3:1 217
BDSDA/4,4'-ODA 4:0 217
BDSDA/3,3'-ODA -- 196
The rheological properties of the polymers were
evaluated using a capillary rheometer. This technique
allows one to quantify melt viscosity as a function of
strain rate using a mechanical screw-driven extruder.
The data presented for viscosity are shown as apparent
viscosity because no correction was made for wall
friction in the capillary. Therefore all data are
relative and not absolute.
j7,~1i
The two polymers, BDSDA/_-PDA and BDSDA/4,4'-ODA,
were run as extremes in comparison for the three
copolymers, 413, 422, and 431. The BDSDA/3,3'-ODA was
prepared as an example of a similar polymer with two
types of flexibility in the diamine unit. Attempts to
extrude the BDSDA/4,4'-ODA through the capillary were
unsuccessful because it would not flow within the
capability limits of the rheometer (5 x 106 Pa-sec).
Comparative data for all of the systems are shown in
Table II (all runs were made at 350C).
TABLE II
Strain Rate, Apparent Viscosity
Polymer sec~l Pa-sec
BDSDA/4,4'-ODA 0.404No flow
BDS~A/_-PDA 0.4042.10 x 105
BDSDA~3,3'-ODA 0.4047.95 x 105
413 0.4043.04 x 105
422 0.4041.50 x 105
431 0.4042.62 x 105
The smaller the apparent viscosity number the
greater is the ease of flow. From this Table, it is
clear that the 422 system has the lowest viscosity.
This test was also performed at a higher strain rate
of 13.456 sec 1 and this data is shown in Table III
(350).
~ ~7~j7,~1j
TABLE III
Strain Rate, Apparent Viscosity
Polymer sec~l Pa-sec
BDSDA/4,4'-ODA 13.456No flow
BDSDA/_-PDA 13.4560.323 x 105
BDSDA/3,3'-ODA 13.4560.669 x 105
413 13.4560.346 x 105
422 13.4560.227 x 105
431 13.4560.338 x 105
Again, the 422 system has the lowest viscosity of
all the systems.
The highest strain rate of which the rheometer was
capable was investigated for these same polymers to see
if this trend held. This data is in Table IV (350C).
TABLE IV
StrainlRate, Apparent Viscosity
Polymer sec~ Pa-sec
BDSDA/4,4'-ODA 134.560No flow
BDSDA/_-PDA 134.5600.109 x 105
BDSDA/3,3'-ODA 134.5600.134 x 105
413 134.5600.107 x 105
422 134.5600.079 x 105
431 134.5600.085 x 105
-
~ ~7~.7~
11.
Again even at this highest strain rate (134.56
sec ) the 422 polymer exhibited the lowest
viscosity. It was quite surprising that in each case
the BDSDA/3,3'-ODA exhibited the highest viscosity since
this system has flexibility due to the oxygen bridging
group as well as the added flexibility introduced
through the meta-linkages. However, in each case this
polymer was extrudable where the corresponding
BDSDA/4,4'-ODA with only one flexibilizer was not.
The key feature illustrated in these Tables is that
the 422 copolymer always exhibits the lowest viscosity
and all of the copolymers (413, 422 and 431) flow
through the capillary whereas the BDSDA/4,4'-ODA does
not. In addition the 422 copolymer exhibits a lower
viscosity in each case than does the BDSDA/_-PDA. This
data proves convincingly that the 1:1 copolymer (422)
exhibits higher flow or lower viscosity than the pure
polymers BDSDA/m-PDA and BDSDA/4,4'-ODA. This attribute
is an unexpected result which makes this copolymer very
attractive for fabrication procedures.
The mechanical properties of these polymers are
shown in Table V. The tensile tests were performed on
the extrudate from various capillary rheometer runs.
All mechanical properties were determined at room
temperature using an Instron Testing Machine Model TT-C.
~.~7~
~ i 3 a) o ~
14 ~ ~ Z S
~ ~ _ _ _
~ ~ , 0
~ Ct~ ~ ~ oo _ o -- `J
I `~
~O CO O r- 0
W ~ ____o
Z o~ o ~
W ~ ,_ ~`I _ o
E~ ~ o~
Z ~ ~ ~ CO o ~_
cr~
_ _ _ _ _~
.
¢ ,v V
~ _ o oo ~ o
:~:
C~ o ~ o
V~ ¢ ¢
_l ~ l
w,
O ~ ~ C~
P~ t~
~ ~ C~
~ ~,7~j~7 ~1j
Each value reported in Table V is the average of one
polymer extruded at six different strain rates in the
range 0.404 - 134.560 sec 1, and the melt fraction was
measured at the highest strain rate (134.560 sec 1~.
Of particular note is that the 422 copolymer and the
B~SDA/_-PDA polymer exhibit nearly identical tensile
strengths and moduli and the variability in strength is
slightly lower for the 422 copolymer. This shows that
no sacrifice in mechanical properties results due to
copolymerization. Also of importance is that the degree
of melt fracture (undesirable) is only moderate for the
422 copolymer and the BDSDA/m-PDA. In all other cases
the degree of melt fracture was higher.
Thus, copolymers of polyimides with both flezible
bridging groups and meta-linkages in the diamine-derived
portion of the polymer have afforded flow properties
superior to either of the corresponding homopolymers.
This technique and these compositions clearly lead to
polyimides with unusual and unezpected flow properties
and thus offer an improvement over the state-of-the-art
systems.
The 422 copolymer was further evaluated as a
hot-melt adhesive. The evaluation, based on lap shear
strengths, involved the determination of a bonding cycle
and thermal ezposure.
An adhesive tape comprising a glass cloth coated
with several layers of the 422 copolyimide was placed
between heated and primed surfaces of two titanium
adherends using a surface overlap of about 1.25 cm. The
assembly was placed in a bonding press and subjected to
a bonding cycle comprising the steps of heating the
polyimide tape under 50-500 psi pressure at a rate of
about 7-10C/min from ambient temperature to
340-350C, holding the polyimide at 340-350C
for approximately one hour, and cooling the assembly
B
~'~7~7'~j
14
under pressure to -120 to -160 C. The assembly was
then removed from the bonding press.
Tests were then performed on bonded adherends formed
in this manner to determine the effects on lap shear
strengths for thermal exposure for 1000 hours at
204C. Thermal exposure was performed in a forced air
oven controlled within +1% of exposure temperature. Lap
shear tests were conducted at room temperature, 177C,
and 204C before (controls) and after exposure giving
lap shear strengths of 35.5, 22.0 and 15.0 MPa,
respectively (before exposure) and 34.0, 22.5, and 19.5
MPa, respectively (after exposure). No significant
difference in lap shear strength is noted for those
tested at room temperature and 177C for the controls
and the thermally exposed specimens. A substantial
increase in average lap shear strength at 204C was
obtained for those thermally exposed at 204C compared
to the controls. Tested specimens failed 100%
cohesively except for the 204C control specimens
which failed primarily cohesively with some adhesive
type failure.
A significant increase in Tg (18 C) was determined
for the thermally exposed specimens which is a common
occurrence due to further polymer cure, polymer
oxidation, and/or elimination of trapped volatiles.
This possibly could account for the increase in lap
shear strength seen for the thermally exposed specimens
tested at 204C.
It is apparent from these tests that the polymers of
the invention are useful as adhesives and that the 422
polymer has exceptional characteristics for this
application.
~Pecific Examples
Example 1
10.2096 9 (0.02 moles) of 4,4'-bis(3,4-dicarboxy-
phenoxy) diphenylsulfide dianhydride (BDSDA), 1.0012 9
~ .7~j'7 ~;
(0.005 moles) of 4,4'-diaminodiphenyl ether (4,4'-ODA),
and 1.6222 g (0.015 moles) of 1,3-diaminobenzene (m-PDA)
were allowed to react in 51.33 g of N,N-dimethyl-
acetamide (DMAc) at room temperature for about two5 hours. Molecular weight build up occurred during this
time as evidenced by an increase in solution viscosity.
Inherent viscosity of the solution as determined at 0.5%
concentration in DMAc at 25 C was 0.378 (Copolyimide
413).
Example II
10.2096 9 (0.02 moles) of BDSDA, 2.0024 9 (0.01
moles) of 4,4'-ODA, and 1.08144 9 (0.01 moles) of m-PDA
were allowed to react in 53.172 9 of DMAc at room
temperature for two hours. The inherent viscosity was
0.441 (Copolyimide 422).
Example III
10.2096 (0.02 moles) of BDSDA, 3.0036 g (0.015
moles) of 4,4'-ODA, and 0.54072 9 (0.005 moles) of m-PDA
were allowed to react in 55.01 9 of DMAc at room
temperature for two hours. The inherent viscosity was
0.459.
ExamPle IV
10.2096 g (0.02 moles) of BDSDA and 4.0048 g (0.02
moles) of 3,3'-diaminophenyl ether (3,3'-ODA) were
allowed to react in 56.85 9 of DMAc at room temperature
for two hours. The inherent viscosity was 0.424.
Example V
20.4192 9 (0.04 moles) of BDSDA and 4.325B 9 (0.04
moles) of m-PDA were allowed to react in 98.98 9 of DMAc
at room temperature for two hours. The inherent
viscosity was 0.606.
Example VI
5.1048 g (0.01 moles) of BDSDA and 3.2212 g (0.01
moles) of benzophenonetetracarboxylic dianhydride (BTDA)
were mixed with 2.0024 9 (0.01 moles) of 4,4'-ODA and
1.08144 9 (0.01 moles) of m-PDA in 51 9 of DMAc. This
7~i7 ~i
16
mixture was allowed to react for two hours at room
temperature. The inherent viscosity was 0.890.
Example VII
4.3624 g (0.02 moles) of pyromellitic dianhydride
(PMDA), 2.0024 g (0.01 moles) of 4,4'-ODA, and 1.0814 g
(0.01 moles) of m-PDA were allowed to react for two
hours at room temperature in 42.16 g of DMAc. The
resultant inherent viscosity was 1.058.
Example VIII
3.2224 g (0.01 moles) of BTDA, 1.0012 g (0.005
moles) of 4,4'-ODA, and 0.54072 g (0.005 moles) of m-PDA
in 26.97 g of DMAc were allowed to react at room
temperature for two hours. The inherent viscosity was
1.154.
Example IX
The poly(amide-acid) solution from Example VII was
cast onto a glass plate at a thickness of 0.02 inch and
the solvent was allowed to evaporate. The resulting
polymer film was heated for approximately one hour at
about 100C, then for approximately one hour at about
200C and finally for approximately one hour at
300C. Removal of the film from the glass plate
yielded a very flexible, smooth yellow film.
Example X
The polymer solution from Example VIII was treated
as in Example IX to yield a very flexible, smooth yellow
film.
Example XI
The six polymer solutions from Examples I - VI were
each separately poured into water in a blender to
precipitate the polymer. In each case the solid polymer
was collected via suction filtration. Each polymer was
air dried at 20-30C for ten to fifteen hours and
then subjected to a thermal profile in an air oven for
approximately one hour at about 100C, for
approximately one hour at about 200C, and finally for
1.~7{i 7~f )
approximately one hour at about 300C. The individual
polymers were chopped to a granular consistency.
Example XII
Each polymer solid from Example XI was evaluated for
flow properties by placing each one in a capillary
rheometer and heating them to about 350C. At this
temperature, they were subjected to strain rates from
134.560 to 0.404 sec 1 in order to extrude them and to
measure their viscosities. Each of the six extrudates,
approximately 0.17 cm in diameter and 2.54 cm in gage
length, was measured for its tensile properties in the
direction of extrusion at a crosshead speed of 0.51
cm/min. ASTM Standard D638-82a was used as a guide, but
due to lack of material, sample size was decreased from
recommended ASTM size.
Example Xl I I
Adhesive tape was prepared by brush-coating a 422
polyamic-acid, 20% solids solution in diglyme, ~~7inh
(inherent viscosity) of 0.789, onto 112 E-glass cloth
with A-llO0 finish ( r -aminopropysilane). Prior to
coating, the glass cloth (tightly mounted in a metal
frame) was initially oven-dried for ten minutes at
100C. The 0.01 cm thick glass cloth served as a
carrier for the adhesive as well as for bondline
thickness control and an escape channel for solvent.
Coatings of the polymer solution were applied to the
glass cloth until a thickness of 0.020-0.025 cm was
obtained. After a primer coat (approximately 4% solids
solution) was applied, each coat application thereafter
was air-dried for one-half hour, placed in a forced air
oven, heated from room temperature to about 100C,
held approximately one hour at about 100C, then
heated to about 150C, held approximately two hours at
about 150C, and then heated to about 175C, and
held approximately three hours at about 175C. Some
1~7~j7,~',
18
blistering of the polymer occurred due to the above
treatment.
Example XIV
The prepared adhesive tape (Example XIII) was used
to bond titanium adherends (Ti 6Al-4V, per Mil-T-9046E,
Type III Comp. C) with a nominal thickness of 0.13 cm.
The four-fingered Ti(6Al-4V) panels were surface treated
with a Pasa-Jell 107 (tradename for a titanium surface
treatment available from Semco, Glendale, California)
treatment to form a stable oxide on the surface. The
treated adherends were primed within one hour of the
surface treatment by applying a thin coat, approximately
2.5 x 10 2mm of the 20% solids solution on the surface
to be bonded. They were then air dried in a forced-air
oven for approximately fifteen minutes at about 100C
and approximately fifteen minutes at about 150C. The
primed adherends were stored in a polyethylene bag and
placed in a desiccator until needed. Lap shear
specimens were prepared by inserting the adhesive tape
between the primed adherends using a 1.27 cm overlap
(ASTM D-1002).
The specimens were assembled in a bonding jig in
such a manner as to hold the specimens securely while
being bonded. The assembly was placed in a hydraulic
press and 50-500 psi pressure was applied. The
temperature, which was monitored by a thermocouple spot
welded next to the bondline of one of the specimens, was
increased at a rate of 7-10C/min up to
340-345C. The specimens were held at
340-345C and 50-500 psi for one hour. The press
was then cooled to -120 to -160C, still under
pressure. The bonded specimens were thereafter removed
from the press and the bonding jig and listed for lap
shear strengths.
Specimens were soaked at temperature in a
clam-shell, quartz-lamp oven and were held at
~ ~ao/~ /''lark
19
temperature for ten minutes prior to testing.
Temperatures were controlled to within +3C for all
tests. Bonded thickness was determined as the
difference between the total bonded thickness and the
titanium adherend thickness. The average bondline
thickness was 0.20 mm with a range of 0.12 mm and 0.24
mm.
A rather severe 72 hour water-boil test was
conducted in laboratory glassware containing boiling
distilled water. Lap shear specimens were immersed
above the bonded area at all times during the 72 hour
period. Lap shear strengths were determined at room
temperature, 177C, and 204C. The test produced
decreased strengths at all test temperatures indicating
a lack of resistance to the effects of water on the
adhesive system, i.e., the adhesive and treated titanium
surface. The lap shear strength values decreased by 20%
at room temperature, 41% at 177C, and 70% at
204C. A more realistic test would be to expose the
lap shear specimens to a controlled cyclic humidity
condition more representative of what an adhesive system
would experience during the intended application.
Example XV
16.273 g (0.050 moles) of benzophenonetetracarboxy-
lic dianhydride (BTDA) was slurried (at room temperaturein a 1000 ml cylindrical reaction flask with a removable
four-necked top) with a mixture of 35 g 2-methoxyethyl
ether (diglyme) and 100 g N,N-dimethylacetamide (DMAc).
5.006 g (0.025 moles) of 4-4'-diaminodiphenyl ether
(ODA) was added and stirred for fifteen minutes when the
reaction mixture became transparent due to all the
materials going into solution as this initial reaction
occurred. 2.704 g (0.025 moles) of
meta-phenylenediamine (MPD) was added and the solution
stirred for an additional thirty-five minutes. The
resulting polyamic-acid polymer solution was decanted
i7 ~i
from the vessel and an inherent viscosity of 0.517 dl/g
was obtained at 0.5% solids in DMAc. In the above
reaction a small quantity of a chain stopper, e.g.,
phthalic anhydride or aniline (0.002 moles) could be
employed to control molecular weight.
Example XVI
Adhesive tape was prepared by brush coating a primer
solution of the polyamic-acid solution of Example XV
(diluted to approximately 7.5 wt/% solution in DMAc)
onto 112 E-glass cloth with A-llO0 finish
( r -aminopyropysilane). Prior to coating, the glass
cloth ~tightly mounted on a metal frame) was dried in a
forced-air oven for thirty minutes. The 0.01 cm thick
glass cloth served as a carrier for the adhesive as well
as for bondline control and an escape channel for
solvent. Coatings of the polymer solution were applied
to the glass cloth until a thickness of 0.20-0.25 cm was
obtained following the coating procedure defined in
Example XIII. The adhesive tape as prepared was used to
bond titanium adherends for determination of reasonable
bonding conditions to use in further investigations.
The rather involved procedure to prepare the tape was
necessary to drive-off solvent and reaction product
volatiles when converting the polyamic-acid to the
polyimide which generally occurs above 160C with the
degree of conversion being a function of time and
temperature.
Example XVII
The prepared adhesive tapes (Example XVI) were used
to bond titanium adherends (Ti 6Al-4V, per Mil-T-9046E,
Type III Comp. C) with a nominal thickness of 0.13 cm.
The four-fingered Ti(6AL-4V) panels were grit blasted
with 120 grit aluminum oxide, washed with methanol, and
treated with a Pasa-Jell 107 (Tradename for a titanium
surface treatment available from Semco, Glendale,
California) treatment to form a stable oxide on the
~ ~7~.7^~i
21
surface. The adherends were washed with water, dried in
a forced-air oven at 100C for five minutes and primed
within two hours of the surface treatment by applying a
thin coat of the polyamic-acid solution of the
respective adhesives on the surfaces to be bonded. They
were then air dried for thirty minutes in a forced-air
oven, for fifteen minutes at 100C, and fifteen
minutes at 150C. The primed adherends were stored in
a polyethylene bag and placed in a desiccator until
needed. Lap shear specimens were prepared by inserting
the adhesive tape between the primed adherends using a
1.27 cm overlap (ASTM D-1002) and applying 2.07 MPa
pressure in a hydraulic press during the heating
schedule. Bonding temperature was monitored using a
type K thermocouple spot-welded to the titanium adherend
at the edge of the bondline.
Several bonding cycles for the adhesive (STPI/LARC)
were investigated during this study to determine a
bonding process which produced good strengths. The
following processing cycles were used:
Cycle 1: (1) 2.1 MPa pressure, heating rate
approximately 8.2C/min, RT -~ 329C; (2) hold
fifteen minutes at 329C; and (3) cool under pressure
to approximately 150C and remove from bonding press.
Cvcle 2: Same as Cycle 1 except RT -~ 343 C.
Cvcle 3: Same as Cycle 1 except RT --~ 343C, hold for
one hour.
A bonding cycle was selected from the above cycles
~Cycle 3) and used to determine the effects of an
additional heat treatment of the adhesive tape prior to
bonding based on the lap shear strengths obtained.
Based on those results, lap shear specimens were
prepared for thermal exposure for 500 and 1000 hours at
~'~'7~7~
204C. Thermal exposure was performed in a forced-air
oven controlled within +1% of exposure temperature. Lap
shear tests were conducted at room temperature, 177C,
and 204C before (controls) and after exposure giving
average lap shear strengths of 22.2, 23.9 and 24.3 MPa,
respectively.
Summary
It can thus be seen that the present invention
yields copolymers with a combination of flexible
linkages that exhibit flow properties which make them
particularly well suited for a wide range of
applications including adhesives, molding resins,
laminating resins, dielectric coating and protective
coatings.
