DE19736398A1 - Bi-axially oriented film of specific thickness with improved mechanical hardening and shrinkage properties - Google Patents

Abstract

A biaxially oriented film of thickness <= 10 mu and having a specific mechanical hardening coefficient. Biaxially oriented film of thickness <= 10 mu , has a mechanical hardening coefficient (MV): where MV ( deg C.mm<2>.N<-1>) = (S200TD(T2%-100)(S200MD-3.35) x 1000)/(ETD-3000) = ?- 2-50; S200TD = widthwise shrinkage after 15 min.(%); S200MD = lengthwise shrinkage after 15 min.(%); T2% = temperature ( deg C) at which a constant tension of 5 N/mm<2> and 2% is achieved; ETD = E-modulus in cross-section (N/mm<2>) An independent claim is also included for production of the film by stretching sequentially in the length and width directions, then thermofixing, whereby the relaxation speed is <= 20% and the maximum temperature \- 210 deg C.

Description

The present invention relates to polyester films, in particular PET films Due to their improved shrink properties, they are particularly suitable for manufacturing of capacitors for SMD technology. Especially for capacitors, that are used in SMD technology, thin films are required and temperature resistance. This brings advantages in the use of space Capacitor and during the soldering process.

PEN and PPS films are currently used for film capacitors in the SMD technology. PEN and PPS films have a significantly higher melting point, PEN approx. 265 ° C and PPS approx. 285 ° C, as PET films (approx. 255 ° C), and therefore have another shrinkage characteristic. However, they are of a decisive disadvantage high cost of such PEN and PPS films. It is known that currently on the market Not available PET films for the production of capacitors for SMD technology or only under certain conditions, such as B. with capsule or suitable by reducing the maximum permitted soldering temperatures to approx. 200 ° C are. The PET capacitors are after the tempering required for manufacture mechanically unstable.

So that PET films have the required thermal stability, a number of Properties (including shrinkage especially in the transverse direction at high Temperatures) do not leave certain areas to the solder bath resistance of the To ensure capacitors.

According to JP-B-63/004499 (Toray), biaxially oriented polyester films are low Shrink values set in that an additional Heat treatment step is carried out. In EP-A-0 402 861 (DHC) and  JP-A-63/011326 (Toray) are films with a very low longitudinal shrinkage described. EP-A-0 402 861 describes a first tempering step at 225 ° C up to 260 ° C with a relaxation of 1 to 15% and subsequent, second Relaxation from 0.01 to 10% at temperatures below 180 ° C. These slides are disadvantageous because the longitudinal shrinkage described there is so low that the capacitors made from it are not sufficiently solidified. This results poor insulation resistance.

The object of the present invention was now to provide an SMD-compatible PET film To make available, which does not have the disadvantages described above. From The capacitors produced in the film are said to be mechanically stable after the tempering as well as solder bath resistance.

This object is achieved by a single-layer or multilayer biaxially oriented film with a thickness of ≦ 10 µm, in which the mechanical strengthening coefficient MV does not leave the ranges given below according to the following equation:

MV is ≧ -2, preferably ≧ 0, in particular ≧ 0.5, particularly preferably ≧ 1 and MV is ≦ 50, preferably ≦ 30 and particularly preferably ≦ 5.

In addition, a maximum of one of the coefficients S200 _TD , (T2% -100), (S200 _MD -3.35) and (E _TD -3000) may be negative.

S200 _TD is the shrinkage in the transverse direction (= TD = Transverse Direction) at 200 ° C after 15 min in%.

S200 _MD is the shrinkage in the longitudinal direction (= MD = Machine Direction) at 200 ° C after 15 min in%.

T2% is the temperature in ° C at which the film expands by 2% under a constant tension of 5 N / mm ² .

E _TD is the elastic modulus (tensile modulus) in the transverse direction at room temperature (21 ° C) in N / mm ² .

The film of the invention has a thickness of ≦ 10 microns, preferably ≦ 8 microns and in particular of ≦ 6 µm.

Two process steps are particularly important for the suitability of a film for the production of SMD capacitors. This is the winding of the wheels with subsequent annealing to solidify the capacitors and on the other hand the heat resistance of the capacitors during the soldering process. It has been shown that the heat resistance, in particular the dimensional stability, of the finished capacitors depends crucially on the shrink properties of the film in MD and TD. Here, the lower the shrinkage at high temperatures (200 ° C) in MD and TD, the better the heat resistance. Low shrinkage values therefore result in the desired small values for MV in the above formula. On the other hand, shrinkage is important for the consolidation of the winding wheels during the tempering step. If the windings are not sufficiently solidified, the resulting capacitors have insufficient insulation resistances and unacceptably high values for the dielectric loss factor tan δ. A balance between particularly low and still sufficient shrinkage is therefore necessary for SMD suitability. Both the transverse shrinkage and the longitudinal shrinkage contribute to the hardening mentioned. The contribution of longitudinal shrinkage is much more important. A low transverse shrinkage can therefore be tolerated rather than a too low longitudinal shrinkage. Values for S200 _MD of less than 3.35% have proven to be very unfavorable. Therefore, the coefficient in the formula becomes negative at lower shrinkage values and leads MV out of the ideal range.

With S200 _TD , values of less than or equal to 1% have proven to be particularly favorable, with values of less than 0.8% being preferred.

The consolidation of the layer package is not only determined by the shrinkage but also by other, in particular mechanical, properties of the film. The pressure on the layers created by the winding tension and the longitudinal shrinkage causes the film to expand and thus to solidify. This extension is e.g. B. depending on the modulus of elasticity at the respective temperature, since a film with high mechanical strength is less easy to expand than a film with a lower modulus of elasticity. The best way to measure the tendency of the film to expand in the transverse direction under pressure and at a certain temperature is not to apply pressure, but to apply a pull to the film with a constant low force. For the properties of the film, it has proven to be advantageous if the film shows an elongation of at least 2% (T2%) at a temperature of 5 N / mm ² at the lowest possible temperature (this temperature is over 150 for standard capacitor films ° C and ideally between 100 and 140 ° C, better even between 100 and 130 ° C for the films according to the invention). This means that the layer pressure that occurs during tempering leads to an early consolidation of the layers. However, it is problematic if the elongation of 2% occurs even at very low temperatures below 100 ° C. Then the tempering temperature must be reduced, which in turn significantly deteriorates the solder bath resistance.

Low values for T2% can e.g. B. can be achieved by low moduli in the transverse direction. Very low values, which are clearly below 4000 N / mm ² , are again unfavorable. Such modules have no direct negative influence on the suitability for SMD, but generally lead to a deterioration in the electrical properties of the film. The fact that the modulus of elasticity is taken into account in the above formula takes this fact into account.

The density of the polyester used for film production is advantageously between 1.385 and 1.410 g / cm ³ . Films with values above 1.410 tend to be too crystalline and could lead to brittleness of the film in the capacitor production and thus possibly to the failure of the capacitor. If the density is below 1.385, the shrinkage of the film may be too high, the modulus of elasticity may be too low, or the sensitivity to hydrolysis may be too high.

The film according to the invention is preferably produced from polyester raw materials. Polyester tubular materials are understood to mean compositions which are used for predominant part, d. H. at least 80% by weight, preferably at least 90% by weight, from 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 mixtures of these polymers exist. Preferred are polyester raw materials, which consist essentially of Ethylene terephthalate units and / or up to 50 mol% from comonomer units are built up, with a variation in the glycol and / or the acid component of the comonomer units is possible. The production of the polyester can both after the transesterification process with the usual catalysts, such as. B. Zn, Ca, Li and Mn salts as well as the direct esterification process.

The polyester raw material optionally contains the usual additives (particles) for the production of capacitor foils to improve the slip and sliding properties, e.g. B. 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, Mn salts of terephthalic acid. However, particles based on crosslinked, infusible, organic polymers such as. B. polystyrenes, polyacrylates, polymethacrylates can be added. The particles are preferably used in a concentration of 0.005 to 5.0% by weight, particularly preferably in a concentration of 0.01 to 2.0% by weight (based on the weight of the layer). The average particle size is 0.001 to 10 μm, preferably 0.005 to 5 μm.

The polyester films can be produced using known methods from those described above Raw materials or in combination of the above polyester raw materials with other raw materials or conventional additives in the usual amount of 0.1 to a maximum of 10 wt .-% both as Monofilms as well as multi-layer, optionally co-extruded films  same or differently formed surfaces are produced, wherein For example, one surface is pigmented and the other surface is not Contains pigment. Likewise, one or both surfaces of the film can follow known methods can be provided with a conventional functional coating.

In the preferred extrusion process for producing the polyester film, the melted polyester material is extruded through a slot die and quenched as a largely amorphous pre-film on a cooling roll. This film is then heated again and stretched in the longitudinal and transverse directions or in the transverse and in the longitudinal direction or in the longitudinal, in the transverse and again in the longitudinal direction and / or transverse direction. The stretching temperatures are generally T _g + 10 ° C. to T _g + 60 ° C., the stretching ratio of the longitudinal stretching is usually 2 to 6, in particular 3 to 4.5, that of the transverse stretching 2 to 5, in particular 3 to 4.5 and that of the optionally carried out second longitudinal stretching at 1.1 to 3. The first longitudinal stretching can optionally be carried out simultaneously with the transverse stretching (simultaneous stretching). Depending on the design of the stretching system used, the known influencing variables stretching ratio, temperature and speed are to be coordinated with one another in such a way that the measured variables modulus of elasticity, shrinkage and T2% are in the desired ranges. Favorable are rather low stretch ratios in the transverse direction (<4) and medium to high (<3.6) stretch ratios in the longitudinal direction. In combination with the fixing temperatures and relaxations given below, the desired values can be achieved, for example, by a transverse stretching ratio of 3.6-4.0, preferably 3.7-3.9, particularly preferably around 3.8 and a longitudinal stretching ratio of 3. Set 6-4.2, preferably 3.7-4.0, particularly preferably by 3.9.

The film is then heat-set at oven temperatures of 200 to 260 ° C, especially at 220 to 250 ° C. Crucial for the production of the film according to the invention is the actual temperature experience of the film and not the ambient temperature in the manufacturing process. For example, at very high speed of the production machine the furnace temperature, d. H. the Ambient temperature, well above the temperature that the film actually has  experienced when passing through this oven. The maximum temperature that the slide actually experienced during their manufacturing process can be done using DSC analysis determine on a finished film (= maximum temperature experience = film fixing peak). This temperature should be between 225 and 245 ° C, preferably between 230 and 240 ° C.

In the fixing zone, the film is relaxed in the transverse direction by a total of 5 to 15%. In addition to this total relaxation of 5-15%, the decisive factor is Relaxation rate, which according to the invention is <20% / s. This is from Significance for the setting of the shrinkage in the transverse direction. The Relaxation rate is the quotient of the relaxation in% and the time in s, in which the relaxation takes place.

The relaxation can be done in several discrete steps, but also in one step take place, the relaxation in several steps, in particular over a longer one Route is preferred. A relaxation rate is particularly favorable of <5% / s, with as much of the relaxation as possible at temperatures <233 ° C, better even at temperatures <210 ° C. At least 0.7% Relaxation should take place at temperatures <210 ° C. Height Relaxation speeds and high relaxation temperatures lead to a less reduction in transverse shrinkage while at the same time significantly reducing the Longitudinal shrinkage, which is unfavorable for the reasons mentioned. The high of Total relaxation must be chosen individually for each production facility be that the specified parameters are in the claimed range. At a total relaxation of 8% in combination with a fixation temperature of 230 ° C, a relaxation rate of 3% / s is suitable around the desired values to achieve for MV.

The film is then cooled and wound up. The one described here Process for the production of polyester films with reduced transverse shrinkage is not only applicable to polyester but also to other thermoplastic polymers.  

The invention is explained in more detail below with the aid of examples.

Examples

The following measurement methods were used to characterize the films obtained:

Shrinkage

Thermal shrinkage was determined on 10 cm squares. The samples are measured precisely (L ₀ ) and annealed for 15 minutes in a forced-air drying cabinet at the temperature specified in each case. The samples are taken and precisely measured at room temperature (L).

Temperature when elongated by 2% (T2%)

To determine the temperature T2%, at which the elongation of the film is at least 2%, the TMA analysis is carried out using a device from Mettler (TMA40). The elongation in% of the 10 mm × 6 mm samples is measured at a heating rate of 4 K / min and a constant force of 5 mN / mm ² . The temperature at the start of the measurement is 30 ° C.

density

Densities are determined in accordance with ASTM D 1505-68 by immersing samples in density gradient columns. Mixtures of CCl ₄ / heptane are used to produce the density gradient column.

E-module

The modulus of elasticity is determined using the Z010 tension-strain gauge from Zwick. The modulus of elasticity is the quotient of the force related to the initial cross section and the change in length in relation to its original length when deformed within the Hook range. The measuring strips are 15 mm wide and the  Measuring length is 100 mm. It is used at a temperature of 21 ° C Take-off speed of 1 mm / min measured.

example 1

PET chips are dried at 160 ° C and extruded at 280 to 310 ° C. The molten polymer is extruded from a slot die and onto a roller electrostatically applied. The film is added by a factor of 3.8 in the machine direction Stretched 115 ° C. In a frame at 120 ° C there is a transverse stretch around the Factor 3.8. The film is then heat set, the maximum The film's temperature experience is 231 ° C. The film is even Steps in the transverse direction 8% with a relaxation rate of 2.5% / s relaxed.

Example 2

A film is produced by the method according to Example 1. The film fixing peak is however at 237 ° C and the transverse stretching ratio at 3.7.

Comparative Example 1

A biaxially oriented film is produced as described in Example 1, with the The difference is that the film is heat set at 210 ° C (= film fixing peak).

Comparative Example 2

A biaxially oriented film is produced as described in Example 1, however without relaxation of the film in the transverse direction.

Comparative Example 3

A biaxially oriented film is produced as described in Example 1, however the film relaxes in the transverse direction with 2.0%.

Comparative Example 4

A biaxially oriented film is produced as described in Example 1, which  However, this time the film fixing peak is at 241 ° C and the film is 10% in the transverse direction relaxed.

Layer capacitors are made from the foils. These are after the Tempering judged in terms of their quality. The quality criteria are the Failure due to insulation resistance, ΔC, dielectric loss factor and Dielectric strength used.

After this assessment, the capacitors become one for reflow soldering exposed to a suitable temperature profile. The peak temperature is 235 ° C. The capacitors are then rated for their solder bath resistance the same criteria as assessed after the tempering.

Table 1 summarizes the properties of the films from the examples.  

Claims (11)

1. Single or multilayer biaxially oriented film with a thickness of ≦ 10 µm, the mechanical strengthening coefficient MV being determined according to the following equation:
and where
MV is ≧ -2 and ≦ 50, and
S200 _TD the shrinkage in the transverse direction at 200 ° C after 15 min in%,
S200 _MD the shrinkage in the longitudinal direction at 200 ° C after 15 min in%,
T2% is the temperature in ° C at which the film expands by 2% under a constant tension of 5 N / mm ² , and
E _{TD is} the tensile modulus in the transverse direction in N / mm ² . 2. Film according to claim 1, characterized in that at most one of the coefficients S200 _TD , (T2% -100 ° C), (S200 _MD -3.35%) or (E _TD -3000 N / mm ² ) is negative. 3. Film according to claim 1 or 2, characterized in that the transverse shrinkage at 200 ° C (S200 _TD ) ≦ 1%. 4. Film according to one or more of claims 1 to 3, characterized characterized in that one or both surfaces of the film with a functional layer are coated. 5. Film according to one or more of claims 1 to 4, characterized characterized in that the film is a polyester film.   6. A method for producing a thermoplastic polymer film, wherein a melted polymer melt extruded onto a chill roll and then sequentially in the longitudinal and then in the transverse direction or in the transverse and then stretched lengthways or simultaneously lengthways and crossways and, if necessary, then again in longitudinal and / or Is stretched transversely and then heat set, the Foil during heat setting with a relaxation rate of < 20% / s is relaxed and the film during the manufacturing process maximum temperature experience of <210 ° C (measured by DSC analysis on the finished film). 7. The method according to claim 6, characterized in that the film during the heat setting is relaxed by 5-15%. 8. The method according to claim 6 or 7, characterized in that the Relaxation rate is <10% / s. 9. The method according to claim 7 or 8, characterized in that the Relaxation rate is <5% / s. 10. The method according to one or more of claims 6 to 9, characterized characterized in that the polymer is a polyester. 11. The method according to claim 10, characterized in that the polyester consists predominantly of polyethylene terephthalate.