Note: Descriptions are shown in the official language in which they were submitted.
13~4~17
POLYIMIDE FILM I~AVING IMPROVED AD~IESIVE PROPERTIES
BACKGROUND OF THE I~VENTION
1. Field of the Invention:
The present invention relates to a heat-resistant polyimide film,
and more particularly, it relates to a polyimide film having improved
adhesive properties which i5 produced by forming a film from a raw
material having incorporated therein a finely divided inorganic powder
that forms minute projections on the film surface and subsequently
sub~ecting the film to corona discharge treatment.
2. DescriPtion of the Prior Art:
It is known that polyimide film has good heat resistance, low
temperature resistance, chemical resistance, electrical insulation
properties, and mechanical strength, and other good properties. It finds
general use as electrical insulation film, heat insulation film, and base
film for flexible printed circuit boards. A polyimide film as the base
film for a flexible printed circuit board is bonded to a copper foil with
an adhesive. Often the bonding is poor on account of the chemical
structure and high chemical (solvent) resistance of polyimde film. At
present, this drawback is mitigated by subjecting polyimide film to
surface treatment such as alkali treatment, corona discharge treatment
and sandblasting.
The conventional alkali treatment or sandblasting encounters some
problems because it has to be performed separately after the film-making
step and requires the additional steps of washing and drying. All this
reduces productivity, increases production cost, and impairs stable
production. In contrast, corona discharge treatment has an advantage
over the above-mentioned two methods of treatment. It has long been used
to improve the printability of paper and plastics moldings, and it can be
easily performed in-line on the film-making machine because it requires
only a simple apparatus.
The present inventors investigated corona discharge treatment as a
means of improving the adhesive properties of polyimide film. As a
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13145~7
r~sult, it w<l~ f`o~lnd tilat corona discharee treatment has a slight effect
in the c.lse of polyimide film with a s-nooth surface produced by casting
from a raw material contaillillg no inorganic fine particles. However, the
adhes;ve strength which is slightly improved by corona discharge
treatment is however generally low and uneven on the film surface. It
was also found that the corona-treated film causes blocking when it is
wound into a roll and that the blocking is accompanied by organic
decomposition products which presumably result from the corona discharge
treatment. All this indicates that the corona discharge treatment is not
completely practical.
SUMM RY OF THE INVENTION
The present invention provides a heat-resistant polyimide film
having improved adhesive properties.
The present invention also provides a heat-resistant polyimide film
having improved adhesive properties, said film having uniform adhesive
strength on the film surface and not being subject to blocking when it is
wound into a roll.
Other and further features and advantages of the invention will
appear more fully from the following description.
In order to achieve the above-mentioned object, the present
inventors carried out research which led to the finding that a polyimide
film having practically improved adhesive properties can be obtained by
producing a polyimi~e film from a raw material having incorporated
therein a finely divided inorganic powder which forms minute projections
on the film surface and subseqùently subjecting the film to corona
discharge ereatment. The present invention was completed on the basis of
this finding.
DETAILED DESCRIPTION OF THE INVFNTION
According to this invention, the polyimide film is produced from a
raw material having incorporated therein a finely divided inorganic
powder which forms minute projections on the film surface and the film is
subjected to corona discharge treatment.
~ 3 ~
The polyimide film of this invention is one which is obtained from a
raw material composed solely or mainly of aromatic tetracarboxylic acid
dianhydride and aromatic diamine.
It is one feature of this invention to use a polyimide film having
minute projections on the film surface. Such projections can be formed
by incorporating in the polyimide film a finely divided inorganic
powder. The inorganic powder must be stable chemically and physically
throughout the entire process of producing the polyimide film. In
addition, it should not adversely affect the physical properties of the
polyamide film and the characteristic properties required for the
application in which the polylmide film is used. With this in mind, a
proper selection should be made with respect to the kind, particle size,
and amount to be added. Examples of chemical compounds as the inorganic
powder include SiO2, TiO2, CAHP04, and Ca2P207. The inorganic powher
should be selected from electrically insulating materials meeting the
above-mentioned requirements.
The particle size of the inorganic powder should not be larger than
100 ~m in particle diameter, preferably not smaller than 1 ~m and not
larger than 5 ~m. An inorganic powder having large particle size (in
excess of 5 ~m) is not desirable for polyimide film which usually has a
thickness of 7.5 to 125 ~m, because it forms such large projections on
the film surface that it decreases the mechanical strength of the
resulting film. On the other hand, an excessively fine inorganic powder
having a particle diameter smaller than 1 ~m does not form projections of
adequate size required for the improvement of adhesive properties by
corona discharge treatment (mentioned later), although it has little
effect on the mechanical properties of the polyimide film. The amount of
the inorganic powder to be added varies depending on the type of chemical _--compound. In the case of CAHP04, for example, it is about 0.05 to 0.5
wt% based on the weight of the film. Too small an amount of the
inorganic powder does not form projections sufficient for the improvement
of adhesive properties. An excessive amount of the inorganic powder does
not uniformly disperse in the polyimide resin and consequently causes the
properties of the polyimide film to fluctuate. The inorganic powder
should be added in the process of polyimide film production while the
13145~7
polyimide resin or a precursor thereof is still fluid, in such a rnanner
that a uniform dispersion is ensured.
The polyimide film of this invention has improved runnability
because the minute projections formed on the film surface reduce the
film-to-film or film-to-object contact area and consequently reduce the
coefficient of friction of the film surface. According to the
conventional technology, the improvement in runnability was achieved by
using an inert inorganic or organic compound or catalyst residues that
form projections on the film surface.
The polyimide film having projections on the surface thereof formed
by the addition of inorganic powder is subjected to corona discharge
treatment for improvement of adhesive properties. The electric power for
corona discharge treatment may be properly selected. It is preferably
not lower than 20 W min/m2 and more preferably not lower than 50
W min/m2. Corona discharge treatment with an electric power lower than
20 W min/m2 is not enough to impart sufficient adhesive strength to
polyimide film as the base film for flexible printed circuit boards.
Rlectric power in excess of 500 W min/m2 decomposes the resin on the film
surface and adversely affects the film properties. The treatment with
such high electric power can only be carried out at a sacrifice of
production speed. Therefore, it is not practical from the view point of
productivity and production cost.
The polyimide film of this invention having surface projections is
more uniform and stable in adhesive properties than the smooth polyimide
film without surface projections. This is because it has good
runnability as mentioned above. In other words, the polyimide film
having good runnability can receive uniform corona discharge treatment
because it runs smoothly without wrinkles. Conventional polyimide film,
which is poor in runnability, does not undergo uniform corona discharge
treatment because it runs unsmoothly and wrinkles upon contact with the
film guide rolls and corona discharge roll.
According to this invention, the polyimide film with surface
projections is superior in adhesive properties to a polyimide film with a
smooth surface. The reason for this is presumed as follows: It is
generally believed that the outermost resin layer taking part in adhesion
131~17
has a weak boundary layer (abbreviated as WBI. hereinafter) which inhibits
adhesion. This is true of polyimide film. The W~L is closely related to
the lower resin layer, and it is thin on the layer formed on the
projections. On the other hand, not only does the corona discharge
treatment modify the surface through the introduction of
oxygen-containing groups, but it also substantially removes the WBL.
Therefore, the WBL formed on the surface projections is effectively
removed by corona discharge treatment and the interface effective for
adhesion is exposed. Thus, the film as a whole has good adhesive
properties.
Polyimide film having a smooth surface causes blocking, and organic
decomposition products are deposited on the blocked surface, when wound
into a roll after corona discharge treatment; whereas the polyimide film
of this invention with surface proJections does not cause blocking.
Presumably, this is because the projections greatly reduce the
film-to-film contact area, permitting the diffusion of organic
decomposition products resulting from the corona discharge treatment.
According to the present invention, it is possible to improve the
adhesion of polyimide film to copper foil, which has been a problem when
polyimide film is applied to its major application, flexible printed
circuit boards. The polyimide film of this invention is also improved in
runnability. Thus, the present invention is of great industrial value
for the production and use of polyimide film.
The invention is now described in more detail with reference to the
Z5 following examples, which are not intended to limit the scope of this
invention.
The physical properties in the examples were measured according to
the following methods. ~--
(a) Kinetic coefficient of friction (Uk) of film: Measured in
the machine direction according to ASTM D-1894-63, as an
index of film runnability.
(b) Adhesion strength: Measured according to JIS C-6481 (1976),
Section 5.7 (method of testing the copper-clad laminate for
printed circuit board). A specimen of polyimide film is
bonded to a copper foil with an epoxy~nylon adhesive.
S
~31~
After curing at 1600C for 12 hollrs, the peel test was
conducted, with the film fixed onto a hard board.
COMPARATIVE EXAMPLE 1
A polyimide film having a nominal thickness of 25 ym was prepared
from a polyimide of the following formula produced from pyromellitic
dianhydride and 4,4'-diaminodiphenyl ether.
0, 0
Il 11
-O ~ N ~ N ~
O O (n = 5 to 500)
COMPARATIVE EX~MPLE 2
0.15 wt% (based on resin) of anhydrous calcium secondary phosphate
(CaHP04) having a particle size distribution of 1 to 5 ~m, with an
average particle diameter of 2.0 ~m was incorporated in a polyiMide of
the same formula as in Comparative Example 1. The resu].ting polyimide
was made lnto a polyimide film with surface projections and having a
nominal thickness of 25 ~
15 COMPARATIVE EXAMPLE 3
The same polyimide film as in Comparative Example 1 was subjected to
corona discharge treatment at 100 W :nintm2 to give a treated polyimide
film.
EXA~PLES 1 to 6, COMPARATIVE EXA~PLE 4
The same polyimide film as in Comparative Example 2 was subjected to
corona discharge treatment at 5 to 400 W min/m2 to give treated polyi.~.ide
films.
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131~517
The polyimicle films obtained in Comparative l~xampl.es 1 to 4 and
Examples 1 to 6 were exami.ned for bond strength and kinetic coefficient
of fri.ction They were also examined for the presence or absence of
organic decomposition products deposited on the film surface. The
results are sho~n in Table 1.
It is apparently noted from Table 1 that the polyimide films having
incorporated therein an inorganic powder and subjected to corona
discharge treatment are remarkably improved in adhesive properties as
compared with the polyimide film which does not include an inorganic
powder and is not subjected to corona discharge treatment. The polyimide
films in the examples are suitable for flexible printed circuit boards
and many other applications.
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