Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2002-09-05
2004-08-31
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S220000, C428S515000, C428S516000, C428S521000, C428S910000
Reexamination Certificate
active
06783848
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains, in general, to matte biaxially oriented polypropylene films having improved matte property and processability. More specifically, The present invention is directed to a matte biaxially oriented polypropylene film having low gloss (45°, <10%) and high haze (>70%), which solves problems related to increase of silver (or bright) spots and roughness of a matte pattern, and a shortened die cleaning cycle due to die build-up.
2. Description of the Prior Art
Generally, films, made from polyolefin resin balanced in terms of price and performance, are excellent in mechanical, thermal and electrical properties, and have superior chemical resistance and formability. Thus, such films have been mainly used for packaging of foods and goods, and for coating of paper and other materials, or as lamination materials. Among the films, a matte film which is able to give the film smooth feelings due to decrease of surface gloss is evaluated as a highly valuable product increasing marketability of packaging, coating or lamination materials, and has been widely used. In particular, a matte biaxially oriented film consisting mainly of polypropylene resin constitutes most polyolefin-based matte films, and has been variously used for food packaging and paper lamination.
The matte function is based on scattered reflection of light by uneven reflecting surface. The methods for providing the matte property to the film surface are classified into: (1) surface treatment such as coating and etching of the surface of a conventional film, (2) use of the difference of flowability, shrinkage and refractive index in an incompatible resin blend, (3) formation of uneven film surface using an inorganic filler and (4) embossing treatment for formation of a cooling roll having an uneven surface. In the matte biaxially oriented polypropylene film, the method described in the above (2) is mainly used. As the incompatible resin, a resin blend of polyethylene (high viscosity/low melt index) and polypropylene (low viscosity/high melt index), having a large difference in viscosity, is typically used.
In the method for providing the matte property by use of the incompatible resin blend having a large difference in flowability, as the degree of incompatibility and difference of flowability between resins in the blend becomes larger, or as a resin composition in a bi-component system approaches 50/50 wt %, the matting effect is increased. However, if the difference of flowability between the incompatible resins is too large, poor mixing and numerous fish eyes are generated upon extrusion due to large difference of viscosities. Poor mixing increases roughness of the matte pattern and fish eyes generate silver (or bright) spots of the matte film during stretching. The term “silver spot” refers to a slightly lighter portion on the film surface, while the term “bright spot” refers to a glittering portion of homo polypropylene core exposed by non-wetting of the matte resin to the skin. The silver (or bright) spots are generated due to the poor stretchability and weak interface of fish eye, tearing of interface during stretching. Such phenomena are undesirable for food packaging and paper lamination film, and decrease marketability of the materials.
In addition, when the incompatible resin blend having large difference in flowability is used to increase the matting effect, problems such as die build-up are caused. As for the die build-up, the incompatible resins flow non-uniformly owing to different flowability thereof and are deposited onto the die like impurities. Very low molecular weight components of resin blend also tend to be deposited onto the die. If the die build-up is excessive, the machine should be shut down to clean the die.
Further, the silver (or bright) spot of the matte film, roughness of the matte pattern and the die build-up are influenced by a molecular weight distribution (MWD) of the used resin. Typically, the matte property improves as the molecular weight distribution increases. However, if the MWD is too broad, increase of the silver (or bright) spots of the matte film and roughness of the matte pattern, due to high molecular weight portion, and increase of the die build-up, due to low molecular weight portion, become extreme. In particular, in the case of having a bimodal molecular weight distribution, the above problems are rendered much worse.
Therefore, as the matte biaxially oriented polypropylene film, a film having haze of 60-70% and gloss (45°) of 10-15% is typically used.
Examples of the conventional techniques disclosed to improve the properties of the matte biaxially oriented film are as follows:
WO 8910839 discloses a biaxially oriented polypropylene film comprising a heat-sealable HDPE (high density polyethylene) as a surface layer, which is advantageous in light of excellent transparency, such as low haze and high gloss, and low friction coefficient.
In U.S. Pat. No. 4,578,316, there is disclosed an oriented film structure comprising a polypropylene film having on at least one surface thereof a layer of a blend of (I) a member selected from the group consisting of a medium density polyethylene, high density polyethylene and mixtures thereof and (II) polypropylene. Such a film has advantages of low friction coefficient, and excellent printability and transparency (low haze and high gloss). As seen from the aforementioned patents, which pertains to the production of films having excellent transparency using polyolefin blend, the matte appearance of the blend not only depends on a kind of the blended polymer resin (polyethylene, polypropylene, etc.) but also a molecular weight and a molecular weight distribution (MWD) of the blended resin.
Furthermore, preparations of a matte surface layer in a biaxially oriented film using a polyolefin resin blend component are disclosed in U.S. Pat. Nos. 5,364,704, 5,366,796, 5,473,016, 5,492,757, 5,494,717, 5,496,600, 5,501,905, 5,516,563, 6,322,894, and EP Publication No. 0 367 613 B1.
In U.S. Pat. Nos. 5,364,704 and 5,366,796, there is disclosed a matte biaxially oriented film having haze value greater than 40%, comprising a resin blend having MI of 1.5-12 g/10 min (230° C.) of polyethylene having MI of 0.5-1.5 g/10 min (190° C.) and polypropylene having MI of 0.5-10 g/10 min (230° C.). But, the above film suffers from the disadvantage of not obtaining sufficient matting effect due to small MI difference between the resins.
In U.S. Pat. No. 5,473,016, there is also disclosed a matte film comprising a resin composition of propylene copolymer (component 1), very linear low density polyethylene (VLLDPE, component 2) and polyethylene resin (component 3) modified with epoxy and hydroxide groups. Optionally, inorganic fillers may be added to the resin composition. However, this film had a drawback, such as high preparation cost due to use of modified polyethylene resin and VLLDPE. Further, the addition of inorganic filler such as CaCO
3
to the biaxially oriented film leads to the better matting effect in the quantitative aspect, while the matte layer is peeled off and silver (or bright) spots may be generated in qualitative aspect.
In EP Publication No. 0 367 613 B1, there is also disclosed a matte film using an incompatible resin blend. As the resin, HDPE and polypropylene, having MI of 1 g/10 min or less, are used, and preferably, inorganic fillers such as SiO
2
may be added thereto. However, the above film is opaque due to addition of the inorganic filler, and easily has silver (or bright) spot generated upon stretching.
In U.S. Pat. No. 5,492,757, there is also disclosed a film comprising a matte layer consisting of a blend of polyethylene and polypropylene, in which polyethylene having MI of 1-50 g/10 min and polypropylene having MI of 1.5-12 g/10 min are used, and CaCO
3
or TiO
2
is added to the matte layer or the other layers, thus increasing whiteness and opacity. As for this film having excellent printability, the reduced matte property because of the resin bl
Cho Soo-Hyun
Lee Tae-Keun
Rhee Jang-Weon
Nakarani D. S.
SK Corporation
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