Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-02-24
2002-03-05
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S331000, C428S340000, C428S3550EN, C428S423500, C428S424200
Reexamination Certificate
active
06352762
ABSTRACT:
TECHNICAL FIELD
This invention relates to an easily adhesive film and in particular, to an easily adhesive polyamide film that possesses improved adhesiveness with print ink, for use in lamination with other films as well as other coating mixtures, especially under wet conditions. The film is suitable for boiling sterilization, retort sterilization, and packaging of liquid products.
PRIOR ART
Polyamide film offers good transparency, physical strength, chemical stability, machine workability, and gas protection ability, and is mainly used for packaging. However, it is normally used with surface treatment. Such treatment includes compounded processes such as printing, metal vapor deposition, lamination with other types of film, or coating. To improve adhesiveness, the film surface is conventionally treated by means of corona discharge, low-temperature plasma processes, flaming, or chrome nitrating.
Despite the use of such adhesiveness-improving processes, polyamide film suffers from insufficient adhesiveness for applications involving print ink, metal vapor deposition, lamination with other films, or use of other coating mixtures, especially under moistened conditions.
For example, in Japanese Patent Application Laid-open No. Hei 1-244847, a method is proposed in which adhesiveness between the print ink layer and the sealant layer is improved by forming a layer of improved adhesiveness of the surface of Biaxial Oriented PolyAmide film (hereafter BOPA) with a specified surface tension; however, the higher the BOPA's surface tension before creating this improved-adhesiveness layer, the weaker the film's adhesive strength under wetted conditions. In general, because BOPA's crystallinity increases under heat treatment after the film is stretched, polar groups such as NH, CO, OH group, are hydrically bonded among molecules in the resin. As a result there are fewer free polar groups at the film surface, which significantly affects adhesiveness with the improved-adhesiveness layer; also, surface tension declines. Consequently, with film having high crystallinity after heat treatment, it is not possible to obtain sufficient adhesiveness with the improved-adhesiveness layer. Particularly under moistened conditions, the finished film win have notably low adhesiveness. In addition, because the proposed method is an “off-line” one, in which the coating process is performed after biaxial orientation, production costs are high.
To solve these problems, the present inventors have proposed a measure (Japanese patent No. 2576886) that improves adhesiveness, especially under moistened conditions, by coating one side of the film with a water-soluble coating mixture whose main constituents are a water-soluble (metha) acrylic acid ester and water-soluble epoxy compound. The inventors achieved a polyamide film with improved adhesiveness by coating the film with the said water-soluble coating mixture, and then stretching, heating, and treating the surface Japanese Patent Application Laid-open No. Hei 8-258232.
Although these proposals significantly improved adhesiveness under moistened conditions, adhesiveness was not always sufficient when preparing packages from the compound film, or performing boiling sterilization or retort sterilization when food is packed. The latter two applications have posed particular difficulty.
Japanese Patent Application No. sho 57-26236 proposes a measure to create an easily adhesive polyester film by coating a polyester film that has not experienced crystal orientation with a composite made of polyurethane water dispersion solution and a nonion or anion surface active agent, then stretching the film. However, this measure creates spotted or whitened surface during vapor deposition due to the presence of a defoamant and surface active agent in the coating mixture. To solve this problem, Japanese Patent Application Laid-Open No. Hei 3-55302 proposes a plastic film with improved adhesiveness which is created without using defoamant or surface active agent.
However, producing water polyurethane resin without using defoamant and surface active agent is problematic. Foam, which is generated when ultimately eliminating organic solution after polymerizing water polyurethane resin in solvent, tends to create a thin paint film at the surface which comes in contact with air. This foam does not dissolve in water, and forms bubbles on the film surface. Also when no defoamant or surface active agent are used, i.e. during transportation, mixing coating, and the coating process, the water polyurethane absorbs air, generates foam, and forms bubbles, thus causing defects during printing and in the film's overall appearance.
DISCLOSURE OF THE INVENTION
As a result of examination to solve the above problems, the inventors found that it is possible to eliminate known difficulties from the mixing and coating processes, improve adhesiveness and ink printability, and significantly improve adhesiveness, especially under, wet conditions, for applications involving the use of print ink, laminate film, and other coating mixtures without lowering the film's transparency, slippage, or blocking resistance. These improvements are achieved by coating unstretched or uniaxially stretched non-heated polyamide film with a certain amount of coating mixture which includes a specified surface active agent, then stretching the film in at least one direction and heating it.
In other words, the present invention provides an easily adhesive polyamide film from unstretched or uniaxially stretched non-heated polyamide film through the following processes: coating the film with a water-base coating mixture, whose main constituents are A, B, and C as defined below, of which the solid-content weight ratio is (98-30)/(2-70)/(0.1-10) respectively; stretching the coated film in at least one direction; ensuring that the film's coating level after stretching is between 0.005 and 0.030 g/m
2
, in terms of solid content; and heating the film.
A: water polyurethane resin containing acetylene glycol in which each carbon atom immediately adjacent to the triple-bonded carbon atom is substituted with a hydroxyl group and a methyl group, and/or an ethylene oxide addition product of the acetylene glycol.
B: water-soluble polyepoxy compound
C: particles with an average diameter of between 0.001 and 1.0 &mgr;m
The unstretched or uniaxially stretched non-heated polyamide film used in the present invention has a large number of free polar groups at the film surface which, in general, significantly affect adhesiveness. In addition, through the stretching and heating processes that follow the coating process, a significant amount of heat is added to the film, improving the adhesive strength between the polyamide film and the coated layer. Compared with off-line coating processes, which are performed after the stretching and heating processes, the newly invented method produces a film having stronger adhesiveness.
To improve adhesiveness, especially under moistened conditions, it is effective to maintain a water contact angle between 45 and 60° via, for example, corona discharge; for the strength of the corona discharge, 30 W ·min/m
2
or less is appropriate.
It is preferable to use a self-emulsion type of water polyurethane resin for the present invention because such resins have small-diameter particles and good stability. Particle diameter should be between 10 and 100 nm.
It is recommended that the glass transition point (Tg) of the water polyurethane resin used in the present invention be between 40 and 150° C. A water polyurethane resin with a Tg of less than 40° C. causes blocking when the film is rolled after coating; the remains of the blocking then cause transparent spots to form. In the worst case, it is not possible to rewind the roll, and the film may tear if the roll is forcibly rewound. Because this invention produces an in-line coating in which the polyamide film is stretched after being coated, if Tg is much higher than the temperature applied for drying and stretching after coating, it is di
Ichiki Makoto
Shimizu Shinji
Tominaga Makio
Yanagida Masahiro
Kohjin Co., Ltd.
Nakarani D. S.
Oliff & Berridg,e PLC
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