Biaxially-oriented pet foil for use for SMD foil capacitors,...

Details Biaxially-oriented pet foil for use for SMD foil capacitors,... Biaxially-oriented pet foil for use for SMD foil capacitors,...

1999-03-24

2001-07-31

Chen, Vivian (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S339000, C428S480000, C428S910000, C528S308000, C528S308100, C528S308600, C264S288400, C264S290200

Reexamination Certificate

active

06268052

ABSTRACT:

The present invention relates to polyester films, in particular PET films, which, owing to their improved shrinkage properties, are particularly suitable for the production of capacitors for SMD technology. Particularly for capacitors which are used in SMD technology, films having a small thickness and heat resistance are required. This has advantages in terms of the space occupied by the capacitor and in the soldering process.
At present, PEN and PPS films are used for film capacitors in SMD technology. PEN and PPS films have a substantially higher melting point, PEN about 265° C. and PPS about 285° C., than PET films (about 255° C.) and therefore have different shrinkage characteristics. However, the high costs of such PEN and PPS films are decisively disadvantageous. It is known that PET films currently available on the market are unsuitable for the production of capacitors for SMD technology, or are suitable only to a very limited extent under special limiting conditions, such as, for example, using a capsule or with reduction of the maximum permitted soldering temperatures to about 200° C. The PET capacitors are mechanically unstable after the heating required for the production.
To ensure that PET films have the necessary heat stability, the shrinkage must be reduced, particularly in the transverse direction at high temperatures, to such an extent that the capacitors can be produced and that the capacitors are stable to the soldering bath.
According to JP-B-63/004499 (Toray), biaxially oriented polyester films having low shrinkage values are tailored so that a further heat treatment step is additionally carried out.
EP-A-0 402 861 (DHC) and JP-A-63/011326 (Toray) describe films having a very low longitudinal shrinkage. EP-A-0 402 861 describes a first heating step at from 225° C. to 260° C. with a relaxation of from 1 to 15% and a subsequent, second relaxation of from 0.01 to 10% at temperatures below 180° C. These films are disadvantageous because the longitudinal shrinkage described there is so small that the capacitors produced therefrom are not sufficiently strong. This results in poor insulation resistance.
It was the object of the present invention to provide a PET film having SMD capabilities. The film should have a very low transverse shrinkage even at high temperatures, so that the capacitors produced therefrom are mechanically stable after heating and stable to the soldering bath. In addition, the film should have the longitudinal shrinkage required for strengthening the capacitors.
This object is achieved by a biaxially oriented film having a thickness of ≦12 &mgr;m and the following properties:
Ts (TD)
>160° C.
S 200 (TD)
≦5%
S 200 (MD)
2%-5%
Shrinkage force ratio
N
S
220/N
S
160
≧4.5
Film setting peak
≧210° C.
Film density
1.39-1.41 g/cm
3
The invention accordingly relates to a biaxially oriented single-layer or multilayer polyester film which can be coated on one or both sides, having a total thickness of ≦12 &mgr;m, Ts (TD) being the temperature (° C.) at which the shrinkage (measured by means of TMA) in the transverse direction begins, S 200 (TD) being the shrinkage (%) at 200° C. in the transverse direction, S 200 (MD) being the shrinkage (%) at 200° C. in the longitudinal direction and N
s
220 being the shrinkage force (N) at 220° C. and N
s
160 being the shrinkage force (N) at 160° C. in the transverse direction. The film setting peak is the maximum temperature experienced by the film (measured by means of DSC analysis).
The film according to the invention has a thickness of ≦12 &mgr;m, preferably ≦8 &mgr;m and in particular ≦6 &mgr;m.
The temperature (° C.) Ts (TD) at which the shrinkage in the transverse direction begins must be ≧160° C., preferably ≧175° C., in order to prevent mechanical deformation of the resulting capacitors during the soldering process.
The transverse shrinkage of the film at 200° C. is <5%, preferably <3% and particularly preferably ≦1%, since otherwise the capacitor produced from this film is too greatly constricted on soldering.
It has proved to be particularly favorable if the transverse shrinkage of the film at 220° C. is <5%, preferably <3% and particularly preferably ≦2%, since otherwise the capacitor produced from this film is subjected to excessive mechanical deformation at high soldering temperatures.
The shrinkage force ratio N
s
220/N
s
160 in the transverse direction of the film according to the invention is ≧4.5%, particularly preferably ≧5%.
If the longitudinal shrinkage at 200° C. is outside the specification 2% ≦×≦5%, the capacitor produced from the film is no longer mechanically stable, a range of 3% ≦×≦4% having proved to be particularly favorable.
The film setting peak is ≧210° C., preferably ≧215° C. and in particular ≧220° C. This ensures that the film remains heat-stable in the further processing steps, whose temperatures are usually in the range from 220 to 250° C.
The density of the polyester used for film production is in the range from 1.39 to 1.41 g/cm
3
. If this value is above the specification, the film is too crystalline and, in the production of the capacitor, would lead to brittleness of the film and hence to failure of the capacitor. If it is below this range, the thermal stability (hydrolytic stability) of the film is too low.
The film according to the invention is produced from polyester raw materials. Polyester raw materials are understood as meaning compositions which for the predominant part, i.e. to an extent of at least 80% by weight, preferably at least 90% by weight, comprise a polymer selected from the group consisting of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), poly-1,4-dicyclohexanedimethylene terephthalate (PCT), polyethylene naphthalate bibenzoate (PENBB) and blends of these polymers. Polyester raw materials which preferably essentially comprise ethylene terephthalate units and/or preferably up to 30 mol % of comonomer units are preferred, a variation in the glycol and/or the acid component of the comonomer units being possible. The polyesters can be prepared both by the transesterification method using the conventional catalysts, such as, for example, Zn, Ca, Li and Mn salts, or by the direct esterification method.
The polyester raw material optionally contains the additives (particles) conventionally used for improving the slip and sliding properties in the production of capacitor films, for example inorganic pigments, such as kaolin, talc, SiO
2
, MgCO
3
, CaCO
3
, BaCO
3
, CaSO
4
, BaSO
4
, LiPO
4
, Ca
3
(PO
4
)
2
, Mg
3
(PO
4
)
2
, TiO
2
, Al
2
O
3
, MGO, SiC, LiF or the Ca, Ba or Mn salts of terephthalic acid. However, it is also possible to add particles based on crosslinked, infusible, organic polymers, such as, for example, polystyrenes, polyacrylates or polymethacrylates. The particles are preferably used in a concentration of from 0.005 to 5.0% by weight, particularly preferably in a concentration of from 0.01 to 2.0% by weight (based on the weight of the layer). The average particle size is from 0.001 to 10 &mgr;m, preferably from 0.005 to 5 &mgr;m.
The polyester films can be produced by known processes from raw materials described above or from a combination of the above polyester raw materials with further raw materials or conventional additives in a conventional amount of from 0.1 to not more than 10% by weight, both as monofilms and as multilayer, optionally coextruded films having identical or different surfaces, one surface being, for example, pigmented and the other surface containing no pigment. Furthermore, one or both surfaces of the film may be provided with a conventional functional coating by known processes.
In the preferred extrusion process for the production of the polyester film, the molten polyester material is extruded through a slot die and quenched as a substantially amorphous prefilm on a chill roll. This film is then heated again and is stretched in the longitudinal and transverse direction or in the transverse and in the

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