Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Patent
1999-06-30
2000-10-10
Boykin, Terressa M.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
528193, 528271, C08G 6302
Patent
active
061303119
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to polyester films, in particular PET films whose improved shrinkage properties make them particularly suitable for producing capacitors for SMD technology. Films having a small thickness and heat resistance are required in particular for capacitors which are used in SMD technology. 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 markedly higher melting point, PEN about 265.degree. C. and PPS about 285.degree. C., than PET films (about 255.degree. C.) and therefore have different shrinkage characteristics. However, a decisive disadvantage is the high price of such PEN and PPS films. It is known that PET films currently available on the market are unsuitable for producing capacitors for SMD technology, or have only very restricted suitability within particular limiting conditions, such as with encapsulation or through reducing the maximum permitted soldering temperatures to about 200.degree. C. The PET capacitors are mechanically unstable after the heat treatment required for production.
In order that PET films have the required heat stability, a large number of properties (inter alia the shrinkage, particularly transverse shrinkage at high temperatures) must remain within particular ranges, to ensure the capacitors have soldering-bath stability.
According to JP-B-63/004499 (Toray) low shrinkage values for biaxially oriented polyester films are obtained by carrying out an additional heat-treatment step.
EP-A-0 402 861 (DHC) and JP-A-63/011326 (Toray) describe films with very low longitudinal shrinkage. EP-A-0 402 861 describes a first heat-treatment step at from 225 to 260.degree. C. with a relaxation of from 1 to 15% and a subsequent, second relaxation of from 0.01 to 10% at temperatures below 180.degree. C. These films are therefore disadvantageous, since the longitudinal shrinkage described is so low that the capacitors produced therefrom are not sufficiently consolidated. This results in poor insulation resistance.
It was an object of the present invention to provide a PET film having SMD capabilities, and not having the disadvantages described above. The capacitors produced from the film should, after heat-treatment, be mechanically stable, and also soldering-bath-stable.
This object is achieved by means of a biaxially oriented film having one or more layers and a thickness of .ltoreq.10 .mu.m, where the mechanical consolidation coefficient MC, given by the following equation, lies within the ranges given below: particularly preferably .gtoreq.1, and is .ltoreq.50, preferably .ltoreq.30 and particularly preferably .ltoreq.5. ##EQU2## In addition, not more than one of the coefficients S200.sub.TD, (T2%-100), S200.sub.MD -3.35) and (E.sub.TD -3000) may be negative. after 15 min at 200.degree. C. 15 min at 200.degree. C. under a constant tension of 5 N/mm.sup.2. temperature (21.degree. C.) in N/mm.sup.2. .mu.m and in particular .ltoreq.6 .mu.m.
Two steps are particularly important for the suitability of a film for producing SMD capacitors. These are firstly the winding of the wound elements followed by heat-treatment to consolidate the capacitors, and secondly the heat resistance of the capacitors during the soldering process. It has been found that the MD and TD shrinkage properties of the film are decisive factors in the heat resistance, in particular the dimensional stability, of the finished capacitors. The relationship here is that the lower the MD and TD shrinkage at high temperatures (200.degree. C.) the better the heat resistance. Low shrinkage values therefore give the desired small MC values in the formula above. However, on the other hand the shrinkage is important for consolidation of the wound elements during the heat-treatment step. If the windings are not sufficiently consolidated the resultant capacitors have inadequate insulation resistances and unacceptably high values for the dielectric dissipation factor t
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Bennett Cynthia
Bursch Annegrete
Hora Franz
Kliesch Holger
Kuhmann Bodo
Boykin Terressa M.
Mitsubishi Polyester Film GmbH
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