Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2002-05-30
2004-03-23
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S341000, C428S409000, C428S523000, C427S535000, C427S536000, C427S322000
Reexamination Certificate
active
06709754
ABSTRACT:
TECHNICAL FIELD
This invention relates to a hydrophilic treatment of films made of styrene resins and application of modifiers to improve, for example, the antistatic properties and slip properties of the films. More particularly, it relates to films to be mechanically processed with, for example, a bag-forming apparatus or a window film applicator, in particular, styrene resin films suitable for window film application.
BACKGROUND ART
Because of being stiff and highly transparent, styrene resin films have been frequently employed as food packaging films for lettuces, raw shiitake and bananas and window films (i.e., films to be applied to envelope windows).
Styrene resin films are employed as food packaging films, since they exhibit excellent food visibility due to the high transparency and high steam-permeability, thereby keeping the freshness of foods with much transpiration (for example, raw shiitake) over a prolonged period of time. Polyethylene films and polypropylene films, which have low steam-permeability, are unsuitable for packaging foods with much transpiration.
In particular, styrene resin films are frequently employed as envelope window films, since less stiff films (for example, polyethylene films, polypropylene films) suffer from troubles such as wrinkling in the step of windowing envelopes and therefore are unsuitable for this purpose.
As an example of the utilization of a styrene resin film in food packaging bags for keeping freshness, JP-A-8-230933 discloses a styrene resin film (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). This film can be processed in sheet form into bags in which foods are filled by hand. However, this film has been subjected to no surface treatment, which brings about a problem of shear in bag-forming because of the insufficient slip properties and opening properties (i.e., easiness in opening the bags for filling foods therein) in the step of automatically filling foods (for example, shiitake) into the bags, in particular, processing the film into bags at a high speed and automatically filling foods in the bags.
In recent years, bananas are packaged in a bag formed by fuse-sealing three sides of a monolayer styrene resin film piece. To wrap a bulky food such as bananas, the bag should be provided with a wide opening. Thus, static electrification due to friction of the upper and lower film sheets makes it difficult to open the bag, thereby causing a problem that bananas can be hardly put into the bag.
To solve this problem, there have been reported styrene resin films coated with modifiers on the film surface. However, these styrene resin films suffer from various problems as will be discussed hereinafter.
As described above, it has been a practice to carry out various surface treatments for improving the slip properties of styrene resin film bases and imparting antistatic properties thereto to thereby give styrene resin films appropriately withstanding mechanical processing, for example, bag-formation and envelope window film application.
For example, JP-A-53-115781 proposes a method whereby a styrene resin film base surface is subjected to a hydrophilic treatment (for example, corona discharge) to give a surface tension of 400 to 550 &mgr;N/cm and then a modifier containing an anticlouding agent and silicone oil is applied on the base surface. According to this technique, one surface of the film base is exclusively subjected to the corona discharge and the modifier is applied onto the thus treated surface. On the other hand, JP-A-10-119978 discloses a method of applying an antistatic agent onto both surfaces of a base material.
Recently, the processing speeds of bag-forming machines, in particular, window film applicators and automatic paper feeders have been elevated to 1,000 sheet/minute or more, over the machines currently in use (i.e., 400 to 600 sheet/minute). With this tendency toward higher processing speeds, it becomes necessary that a window film has two conflicting characteristics, namely, antistatic properties of both surfaces and adhesiveness to paper. It is true that the film disclosed by JP-A-10-119978 suffers from less trouble exclusively from the view point of static electricity. However, it is provided with no means of preventing film scratches. In addition, the problems of wrinkling and positioning error after adhesion are not completely solved in this case. Namely, the problem of static electricity can be solved merely by applying a large amount of an antistatic agent to the film base surfaces to thereby enhance the antistatic properties. However, this treatment brings about another trouble that the antistatic agent applied thickly prevents an adhesive from attaining the film base surfaces and solidifying thereon, thus causing wrinkling and positioning error.
On the other hand, JP-A-2-72050 discloses a styrene resin film containing a waxy antiblocking agent, while JP-A-2-72051 discloses a styrene resin film containing a granular antiblocking agent. Although the films reported in these documents show relieved film damage due to the improved slip properties, they still suffer from the problem of the frequent occurrence of troubles due to static electricity.
By the antistatic treatment on exclusively one surface according to JP-A-53-115781, the obtained film is hardly used for window film application. A styrene resin film roll set in a window film applicator is unwound and then brought into contact with metal rollers or rubber rollers before the adhesion of the film to the envelope paper. Since plural rollers are employed in the contact step, both surfaces of the film come into contact with the rollers and thus electrostatically charged. Unless the film has the antistatic properties on both surfaces, the film sheets, having been cut into a definite size, wind around each other immediately before coming into contact with the envelope paper, thus making continuous processing impossible.
Therefore, double-side application is employed in the thermoplastic resin film for envelope windowing according to JP-A-10-119978. However, this double-side application disclosed in this document suffers from the following problem.
Namely, the film for a window film applicator disclosed in this document is not subjected to any hydrophilic treatment on both surfaces but an antistatic agent is applied on both surfaces of the film in almost the same coating weights, followed by winding into a roll. The film roll thus formed is put into the window film applicator and then unwound before using as a film. In the unwinding step, however, there arises a problem that the antistatic agent, which has been applied onto both surfaces in the same weight, partly (or mostly in an extreme case) migrates from one surface to the opposite surface, depending on the winding tension and other environmental factors (for example, temperature, humidity) in the winding step and the storage conditions (for example, temperature, humidity) of the rolled film. As a result, the coating weight of the antistatic agent widely varies lengthwise. When the film surface having a large amount of the antistatic agent thereon is to be adhered to the envelope paper, the antistatic agent interferes the adhesion and thus causes positioning error between the film and the envelope window, thereby damaging the commercial value of the product.
The invention aims at imparting antistatic properties to both surfaces of a film to thereby provide a film which is suitable for mechanical processing with the use of, for example, a high-speed printer, a bag-forming machine or a high-speed window film applicator, in particular, a styrene resin film having improved adhesion properties to paper as required in an envelope window film applicator, and a process for producing the same.
DISCLOSURE OF THE INVENTION
The present inventors have conducted intensive studies to solve the above-described problems. As a result, they have successfully found out that the object can be achieved by imparting different surface tensions to the front and back surfaces of a
Mizukami Osamu
Tanuma Manabu
Tazuke Yoshiyuki
Yasukata Kouichi
Asahi Kasei Kabushiki Kaisha
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Feely Michael J
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