Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
1999-09-02
2001-05-29
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S212000, C428S213000, C428S215000, C428S323000, C428S480000, C428S690000, C428S690000, C428S910000, C264S171110, C264S173160, C264S289300, C264S290200
Reexamination Certificate
active
06238782
ABSTRACT:
FIELD OF THE INVENTION
Biaxially oriented polyester film having more than one layer, process for its production, and its use as a magnetic tape film with optimized electromagnetic properties and winding performance.
The invention relates to an at least three-layer, biaxially oriented polyester film which with improved electromagnetic properties compared with prior art films has improved winding performance together with good abrasion properties. The film according to the invention is composed of at least one base layer B and, applied to both sides of this base layer, outer layers A and C, where these film surfaces are defined by the roughness profile gas flow. The film has, furthermore, a haze per unit thickness of <0.4%/&mgr;m. The invention further relates to a process for producing the film, and to its use, in particular as a magnetic tape film.
DESCRIPTION OF RELATED ARTS
Because of their excellent mechanical properties, polyester films have long been used as a substrate material for magnetic recording media. An ideal magnetic recording medium with good electromagnetic properties has a very “smooth” surface. However, for good processing performance in the production of the substrate film and in the coating to give the magnetic tape (winding performance), and also subsequently during use of the final product (abrasion resistance), the surface should have some degree of roughness. If monofilms (single-layer films) are used, some compromises have to be accepted in order to meet these intrinsically contradictory requirements. This is because optimizing one of the properties in these films always adversely effects the other property.
Currently, however, there are known coextruded films (AB, ABA and AB′) which can be used to create what is known as “dual-surface” characteristics. Using this method, the two film surfaces can be given properties, including roughness and topography, which differ to a limited extent.
EP-A-0 135 451, for example, describes a “dual-surface” film of A/B type, where the two film surfaces have different R
a
values. However, a disadvantage of these films is that they cannot be produced cost-effectively, since the two layers (corresponding to two mutually superimposed monofilms) have to be provided with particle systems, and therefore do not permit any cost savings in comparison with monofilms. The quality of the magnetic tape film will be impaired, furthermore, by the regenerated film material (regrind) which arises in all commercial production processes. The regrind must be reused, and it must be incorporated into at least one surface layer of the A/B film. EP-A-0 609 060 and EP-A-0 663 286 achieve an improvement with respect to this quality problem by the principle of A/B/A coextrusion, enabling the effect of the regenerated material on the film surface to be reduced by the screening effect of the two outer layers A. However, ANB/A films outwardly exhibit the character of (thin) monofilms, i.e. the advantage of creating differing surface properties has been lost. However, “dual-surface” characteristics can be regained to a limited extent in ANB/A films through differing thicknesses of the outer layers A. Films of this type are then termed A/B/A′ films (which are distinct from A/B/C films). A disadvantage of these films, however, is the limited flexibility which remains in the design of the surface topographies of the two film surfaces. There is, however, no difference in the roughness profile gas flow of the A sides of an A/B/A film. On converting to a A/B/A′ film, i.e., on increasing the thickness of one A side, the roughness of the side in question changes, but there is hardly any change in its roughness profile gas flow. The increase in roughness brought about by increasing the layer thickness does not lead to any significant improvement in running properties, since the rise in layer thickness introduced by virtue of the pigments essentially creates only flat-topped elevations. Although they increase the R
a
value, they make no contribution toward the object of improving running properties.
A/B/C and A/B/C/B films have been produced and described in a few cases (e.g., EP-A-0 502 745), but here again at least one surface layer comprises regenerated material—with the disadvantages described above—or else the information on A/B/C films is nonspecific (EP-A-0 347 646). The multilayer films disclosed in EP-A-0 502 745 (A/B/C/B and A/B/C films) vary the roughnesses of the film surfaces by changing the particle concentration at constant particle size. This can have only a limited effect on the roughness profile gas flow.
It has already been proposed (German Patent Application file reference 198 14 710.5) that the surface topographies of an at least three-layer film can be controlled by giving it two surfaces composed of outer layers A and C, with a base layer B between these outer layers. The outer layer A has an R
a
value of ≦15 nm and an R
z
value of ≦150 nm and a number N
a
of elevations/protrusions per 0.36 mm
2
related to their respective heights h
a
as follows:
A
l
·e
−B
1
·h
≦N
n
≦A
2
·e
−B2·hn
(1)
where
A
1
=
300
,
A
2
=
7000
B
1
=
7.0
B
2
=
8.0
0.01
μm
≤
h
a
≤
1.0
μm
and the outer layer C has an R
a
value which is greater than that of the outer layer A and a number N
c
of elevation/protrusions per 0.36 mm
2
which is related to their respective heights h
c
as follows:
N
c
≦F·e
−G·h
c
(2)
where F=20,000 and G=9.0
and the haze per unit thickness is <0.4%.
The text in question does not describe films with gas flow values in the roughness profile of >1000 sec and <2000 on surface A and of <600 sec on surface C.
DESCRIPTION OF THE INVENTION
The object of the present invention is to provide a coextruded, biaxially oriented polyester film having more than one layer, and which is suitable as a substrate material for magnetic recording media, and at the same time having a defined (i.e. “smooth”) surface A (for good electromagnetic properties in the magnetic tape) and a defined (i.e. “rough”) surface C. The defined surface C provides for good winding performance during production of the substrate film (and also during coating on high-speed coating systems), and for good running performance during subsequent operations with the tape (together with low abrasion).
The film should also be cost-effective to produce. Another decisive factor is that the suitability of the films as substrate films should extend to thicknesses of <2.5 &mgr;m of the magnetic layer applied to the substrate film. Magnetic tapes with low magnetic layer thicknesses of this type place extremely high requirements upon the topography of the substrate film, since clearly large elevations on the film surface have a more marked effect on the “thin” magnetic layer applied, and this results in poorer electromagnetic properties.
This object is achieved by means of a biaxially oriented, coextruded, at least three-layer polyester film whose two surfaces are formed by outer layers A and C, where between these outer layers there is a base layer B, wherein the outer layer A has a roughness profile gas flow of >1000 sec and <2000 sec and the outer layer C has a roughness profile gas flow of <600 sec.
To achieve good electromagnetic properties in outer layer A, a “smooth” surface is required. For the purposes of the present invention, “smooth” refers not so much to small R
a
values, but rather that the outer layer A has high roughness profile gas flow, i.e. low gas flow. The roughness profile gas flow of the outer layer A must be lower than that of the outer layer C.
To achieve good winding performance (running performance in general) and also good abrasion performance, a “rough” reverse side (outer layer C) is required with small gas flow values in the roughness profile, i.e. high gas flow. The roughness profile gas flow of the outer layer C must be greater than that of the outer layer A.
It has proven particularly usefu
Hellmann Joerg
Hilkert Gottfried
Chen Vivian
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Mitsubishi Polyester Film GmbH
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