Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1998-06-02
2001-02-27
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S323000, C428S332000, C428S339000, C428S458000, C428S480000, C428S910000
Reexamination Certificate
active
06194054
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a metallized or ceramically coated, biaxially oriented polyester film with high oxygen barrier and built up from at least one base layer B and, applied to this base layer, at least one outer layer A, where this outer layer A has a defined number of elevations (the terms elevation and protrusion are used interchangeably throughout) of defined height and diameter, and where at least this outer layer A is metallized or ceramically coated. The invention also relates to the use of the film and to a process for its production.
2) Prior Art
In many foodstuff packaging applications, there is demand for a high barrier effect against gases, steam and flavors (this having the same significance as low transmission or low permeability). A well known process for producing packaging of this type consists in high-vacuum aluminum metallizing of the plastic films used. Another well known process consists in coating the films with ceramic materials (e.g. SiO
x
, AlO
x
or MgO
x
). Essentially, the coatings used are transparent.
The barrier effect against the substances mentioned above depends essentially on the type of polymers in the film and the quality of the barrier layers applied. Thus a very high barrier effect against gases, such as oxygen and flavors, is achieved in metallized, biaxially oriented polyester films. A barrier effect against steam is achieved in metallized, biaxially oriented polypropylene films.
The good barrier properties of metallized or ceramically coated films mean that they are used in particular for packaging foodstuffs and luxury foods, for which long storage or transport times create the risk that the packed foodstuffs become spoilt, rancid or lose flavor if there is an inadequate barrier; examples are coffee, snacks containing fats (nuts, chips, etc.) and drinks containing carbon dioxide (in pouches).
If polyester films metallized with an aluminum layer or having an applied ceramic layer are used as packaging material, they are generally a constituent of a multilayer composite film (laminate). Bags produced therefrom can be filled, for example, on a vertical tubular bag forming, filling and sealing machine (vffs). The bags are heat-sealed on their inward side (i.e. on the side facing the contents), the heat-sealable layer consisting generally of polyethylene or polypropylene. The composite film here typically has the following structure: polyester layer/aluminum or ceramic layer/adhesive layer/heat-sealable layer. If the laminate thickness is from about 50 to 150 &mgr;m, the thickness of the metal or ceramic layer is only from 10 to 80 nm. Even this very thin layer is sufficiently effective to achieve adequate protection from light and very good barrier properties.
The oxygen barrier or the oxygen transmission is generally measured not on the laminate or the packaging itself, but on the metallized polyester film. To ensure good quality of the foodstuffs or luxury foods even after relatively long storage times, the oxygen transmission (identical with permeability) of the metallized film may not be greater than 2 cm
3
/m
2
bar d, (i.e. not greater than 2 cubic centimeters of oxygen per square meter of film at a pressure of 1 bar per day) but in particular not greater than 1 cm
3
/m
2
bar d. In future, the demands of the packaging industry will head toward still higher barriers, with attempts to achieve permeability values of significantly less than 1.0 cm
3
/m
2
bar d for metallized or ceramically coated films.
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect of metallized or ceramically coated polyester films nor of how this may be decisively improved. Variables which are clearly important are the surface of the substrate and the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties.
In this connection, Weiss et al., in “Thin Solids Films” 204 (1991), pp. 203-216, studied the influence of the surface roughness of a substrate layer on its permeability. For this, polyester films were coated with lacquer which contained various concentrations of titanium dioxide particles. In his experiments, the concentration of titanium dioxide particles in the lacquer was varied from 2 to 20% by weight. Using this method, the surface roughness of the coated substrate surface could be varied from 43 nm (unlacquered and lacquered film, without titanium dioxide) to 124 nm. In his experiments, increasing roughness (increasing proportion of TiO
2
) of the lacquered surface resulted in markedly higher oxygen transmissions after metallizing with aluminum. However, the largest step increase in oxygen transmission was seen when the lacquered film (0% by weight TiO
2
) was compared with the unlacquered film, although the surface roughness of the substrate was the same in both cases. The lacquering alone of the film gave a deterioration in the barrier from about 0.43 cm
3
/m
2
d bar (plain film) to about 19 cm
3
/m
2
d bar (lacquered film). A further disadvantage of his studies was that the aluminum layer was applied using a laboratory evaporator. When compared with an industrial metallizer, this method achieves essentially low permeability values, and the influence of the substrate surface cannot be seen clearly.
Other detailed results of studies on the influence of the substrate surface of polyester films on their barrier properties can be found in the dissertation by H. Utz (Technische Universität München 1995: “Barriereeigenschaften aluminiumbedampfter Kunststoffolien” [Barrier properties of aluminum-metallized plastic films]).
According to the studies by Utz (p. 66 ff.), there is no direct correlation between the surface roughness (average roughness height R
a
) of the PET film and its oxygen barrier. For example, the film for video applications which has an average roughness height of R
a
=22 nm, is particularly smooth and has an oxygen transmission of 1.3 cm
3
/m
2
bar d compared with a much rougher PET II (R
a
=220 nm) film having an oxygen barrier of 1.2 cm
3
/m
2
bar d.
EP-A-0 124 291 describes a single-layer biaxially oriented polyester film for magnetic recording tape which has the following surface property parameters
a) the average roughness R
a
is from 1 to 16 nm,
b) the coefficient of friction &mgr;k is from 0.01 to 0.20 and
c) the following relationship exists between R
a
and &mgr;k
0.1≦10 *
R
a
+&mgr;k≦
0.31.
These properties are created by using TiO
2
particles (anatase) or TiO
2
and CaCO
3
particles in a proportion by weight of, respectively, from 0.1 to 0.5% and from 0.1 to 0.3%. The diameter of the TiO
2
particles is from 0.1 to 0.5 &mgr;m. The surface of this film is formed by a large number of elevations/protrusions (“the excellent slipperiness of the polyester film of this invention is simultaneously achieved by the presence of the many very minute protrusions”) and these obey a distribution such that the graph described by the following relationship
log y=−
8.0
x+
4.34,
y>
10
is not intersected. In this equation, x (&mgr;m) is a height above a standard level and y is the number of elevations (number/mm
2
) if the elevations are sectioned at a height of x. The distribution of the elevations is determined using standard equipment for measuring roughness.
EP-A-0 490 665 A1 describes a single-layer biaxially oriented polyester film for magnetic recording tape; the film contains
a) from 0.05 to 1.0% by weight of &thgr;-alumina (theta aluminum) having an average particle diameter in the range from 0.02 to 0.3 &mgr;m, and
b) from 0.01 to 1.5% by weight of inert particles of a type other than &thgr;-alumina and having an average particle diameter in the range from 0.1 to 1.5 &mgr;m, these particles being larger than the &thgr;-alumina particles.
The surface of this film is formed by a large number of elevations/protrusions which are described by the relationship
−11.4
x+
4
<log
Hilkert Gottfried
Peiffer Herbert
Mitsubishi Polyester Film GmbH
Thibodeau Paul
Zacharia Ramsey
LandOfFree
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