Plastic film

[Detailed Description of the Invention]

[Industrial Field of Utilization]

The present invention relates to plastic film. The objective of the invention lies in providing a film which exhibits little fogging due to adhering water droplets even when kept at low temperature and high humidity.

[Prior Art]

Film with a variety of functions is already widely known. In particular, in the field of packaging for foodstuffs and plants such as flowers, etc, a suitable degree of moisture permeability and of transparency are demanded from the point of view of preservation and appearance. Thus, if the moisture permeability is too great then the contents will dry-out whereas, conversely, if the moisture permeability is too low then moisture collects inside and, when the contents comes into contact therewith, rotting or spoiling occurs. Furthermore, when kept at low temperatures in a refrigerator, or the like, the moisture condenses and forms water droplets, which adhere to the film, making the contents hard to see and becoming a cause of spoilage. For the purposes, in particular, of preventing the adhesion of water droplets, hydrophilic materials such as fatty acid esters or ethylene oxide adducts have been incorporated in or applied to the film surface or, alternatively, a treatment has been carried out to raise the water repellency so that even when water droplets are formed they fall away.

[Problem to be Resolved by the Invention]

However, these techniques suffer from the disadvantage that the surface is made sticky or the incorporated material oozes out, which is unsatisfactory as a commercial product, and so a drying stage is required after the application, and this entails costs. The present invention offers an antifogging film which does not require such application or drying, and which can be obtained merely by coextrusion or lamination.

[Means for Resolving the Problem]

The present invention is a plastic film which is characterized in that, in plastic film of moisture permeability, as prescribed in JIS Z0208, of less than 1000g/m²/24h, there is lamination or coextrusion with a film of moisture permeability at least 1000g/m²/24h; furthermore, the invention also comprises the aforesaid plastic film, characterized in that the film materials are at least one type selected from the group consisting of polyether-polyamide block copolymers, thermoplastic polyester elastomers and thermoplastic polyurethanes.

In the present invention, the "moisture permeability" is the value measured under conditions B (40°C, 90% relative humidity) by the method prescribed in JIS Z0208.

In the present invention, "plastic film" applies to film of moisture permeability less than 1000g/m²/24h. In the case of film with a value of 1000g/m²/24h or more, there is little water droplet adhesion, and so the present invention is not required. Preferably, the value lies in the range 10 to 900, and more preferably in the range 50 to 800g/m²/24h.

The plastic film of the present invention is a laminate with film of moisture permeability at least 1000g/m²/24h. In regard to the lamination method, conventionally-known methods can be employed such as the coextrusion method where both components are simultaneously extruded, the so-called extrusion lamination method where one component is first extruded in the form of film and then the other resin extruded on top, or the method whereby two films are hot pressure-bonded.

The resins from which the film of the present invention is constructed may be of any kind providing they have the aforesaid properties but, from the point of view of moisture permeability, it is preferred that they comprise at least one type selected from the group consisting of polyether-polyamide block copolymers, thermoplastic polyester elastomers and thermoplastic polyurethanes.

In the present invention, "polyether-polyamide block copolymers (abbreviated below to PEBA)" are polymers in which poly(oxyalkylene) chains (a), and polyamide chains (b) comprising polymers of aminocarboxylic acids or lactams with 6 or more carbons or polymers of salts of dicarboxylic acids and diamines with 6 or more carbons, are linked together. Those in which (a) and (b) are linked together via a dicarboxylic acid (c) with from 4 to 20 carbons are generally referred to as polyetheresteramides. These polymers are, for example, produced by the method of Japanese Examined Patent Publication No. 56-45419. Specifically, they are exemplified by Pebax (Elf Atochem Co.), ELY (Ems Co.) and <i>VESTAMID</i> (Hüls Co.). The type and amount of the polyamide component and of the polyether component in the PEBA used in the present invention will be selected according to the objective and the application.

In the present invention, "thermoplastic polyester elastomer (abbreviated below to TPEE)" are polymers in which poly(oxyalkylene) chains (a), or aliphatic polyester chains (a'), and polyester chains (c), which are polymers of hydroxycarboxylic acids with 6 or more carbons, or of aromatic dicarboxylic acids and dihydroxy compounds with 2 or more carbons, are linked together. Further, there may also be jointly used two or more types of the (c) component. This polymer is described in detail in, for example, US Patent 4,739,012. Specifically, they are exemplified by Hytrel (DuPont), <i>Pelprene</i> (Toyobo) and <i>Rekuse</i> (Teijin), etc. The weight ratio of the aforesaid (a) and/or (a') component to the (b) <i>{sic}</i> component in the TPEE used in the present invention will be determined by the objective and application.

In the present invention, as examples of the "thermoplastic polyurethane (abbreviated below to TPU)", there are polyether urethanes and polyester urethanes (adipate type, caprolactone type). Specifically, they are polyurethanes which are normally obtained by the reaction of an organic diisocyanate and a polyether or polyester diol of molecular weight in the range 500 to 6000, with chain extension sometimes being conducted in the present of a catalyst. As the isocyanate, there is preferably used toluylene diisocyanate, diphenylmethane diisocyanate or the like; as the polyether, there is preferably used polyoxytetramethylene, polyoxypropylene or the like; and as the polyester, there is preferably used (poly) butylene adipate or {poly} caprolactone, etc.

In the present invention, providing the moisture permeability lies within the aforesaid range, the PEBA, TPEE and TPU may used either on their own or as mixtures or, in the case of block copolymers, there may be used a mixture of two or more such polymers in which the soft segment/hard segment ratio of the respective resins differs, or again there may be employed blends with other resins.

In the plastic film of the present invention, there can be freely included, within a range that does not impair the properties of the film of the present invention, conventional antioxidants, thermal decomposition preventives, UV absorbers, hydrolysis-resisting modifiers, colouring agents (dyes and pigments), antistatic agents, electrical conducting agents, crystallization nucleating agents, crystallization promoters, plasticizers, ready-sliding agents, lubricants, mould release agents, flame retardants and flame retarding auxiliaries, etc.

Preferably the plastic film of the invention is used such as the face of the higher moisture permeability is on the side on which water droplet could occur. When used as a packaging the side of the higher moisture permeability is advantageously in the inside.

[Effect]

The plastic film of the present invention is excellent in its moisture permeability and there is little water droplet adhesion, and so it is valuable in particular as film for the preservation of foodstuffs and plants such as flowers, etc.

[Examples]

Below, the present invention is explained in more specific terms by means of examples, but it goes without saying that the invention is not to be restricted merely to these examples.

The various properties were measured or evaluated using the following methods.

(1) Moisture permeability

This was measured under conditions B (40°C, 90% relative humidity) based on JIS Z0208.

(2) Water droplet adhesion test

80ml of water of temperature 40°C was introduced into a 100ml beaker, the mouth of the beaker then covered with the film, and the beaker placed in a refrigerator at 5°C. After 4 hours it was removed, and the state of the film observed for adhering water droplets.

[Example 1, Comparative Example 1]

After drying Pebax A and B, which are polyether-polyamide block copolymers (made by Elf Atochem), a two-layer film was extruded at 200°C using a multilayering single screw extruder, such that the thickness was 20µ on the Pebax A side and 10µ on the Pebax B side. Pebax A is made of PA-12 blocks and polytetramethylene glycol (PTMG) blocks and has a shore hardness D of 63. Pebax B is made of PA-12 blocks of Mn 2600 and polyethylene glycol (PEG) blocks of Mn 650. The moisture permeability of the film obtained was 700g/m²/24h, and when a beaker containing water at 40°C was covered with this film such that the Pebax B side was on the inside, there was no adhesion of water droplets. In comparison, the moisture permeability of 30µ film comprising only the Pebax A was 660, and in the water droplet adhesion test numerous water droplets were seen on the film. Now, the moisture permeability of 25µ film comprising the Pebax B alone was 2700g/m²/24h, and there was absolutely no adhesion of water droplets.

[Example 2, Comparative Example 2]

Lamination was performed by extruding a 5µ thickness of Pebax C onto a 25µ film of Pebax D made by Elf Atochem. Pebax C is made of PA-12 blocks and PEG blocks and has a shore hardness D of 40. Pebax D is made of PA-12 blocks and PTMG blocks and has a shore hardness D of 70. The moisture permeability of this two-layer laminated film was 300, and in a water droplet adhesion test with the Pebax C side on the inside, no adhesion of water droplets was noted. In comparison, the moisture permeability of 30µ film comprising only the Pebax D was 260, and in the water droplet adhesion test the adhesion of numerous water droplets was noted. Further, the moisture permeability of 25µ film comprising Pebax C alone was 4300g/m²/24h.

[Comparative Example 3]

10 parts of Pebax C was blended with 90 parts of Pebax D, and then 25µ thickness film formed using a single screw extruder. The moisture permeability of this film was 400, and in the water droplet adhesion test the adhesion of numerous water droplets was noted.

[Examples 3 and 4]

30µ thickness films were obtained by performing lamination onto Pebax D film as in Example 2 but employing Hytrel 4767 (thermoplastic polyester elastomer made by DuPont) or Toyobo Urethane E3080A (made by Toyobo) instead of the Pebax C (Examples 3 and 4 respectively). The Hytrel and Toyolo films alone have moisture permeability above 1000 g/m²/24 h. The moisture permeability was 270 and 280 g/m²/24 h respectively, and in neither case were water droplets seen in the water droplet adhesion test.

[Example 5]

Co-extrusion of a 28µ thickness urethane/Pebax two-layer film was carried out in the same way as in Example 1 except that Toyobo Urethane A3064D was used instead of the Pebax A in Example 1 but has the same moisture permeability. The moisture permeability of this film was 600 g/m²/24 h, and no adhering water droplets were noted.

[Effects of the Invention]

As explained above, the plastic film of the present invention has a suitable degree of moisture permeability and low water droplet adhesion characteristics, and so when fruit, vegetables or flowers, etc, are wrapped therein and kept at low temperatures, very little fogging of the film by water droplets may be expected.