Stock material or miscellaneous articles – Composite – Of polyamide
Patent
1990-05-17
1992-06-30
Thibodeau, Paul J.
Stock material or miscellaneous articles
Composite
Of polyamide
2642358, 264346, 4284755, 4284761, 428910, B32B 2734, B29C 7102
Patent
active
051262118
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1.Technical field
The present invention relates to biaxially oriented polyamide film, and more particularly, to biaxially oriented polyamide film which has a low degree of anisotropy in heat shrinkage over the entire width of the mill roll. This biaxially oriented polyamide film can be made into bags which are hardly liable to distortion when subjected to heat treatment with hot water or retorting.
2. Background art
Biaxially oriented film of polyamide typified by nylon 6 has outstanding pinhole resistance, heat resistance, low-temperature resistance, and gas barrier property. Owing to these characteristic properties, it is generally used as a packaging material, in the form of composite film with polyethylene sealant, for frozen foods, refrigerated foods, and retort foods.
The production of biaxially oriented polyamide film involves the steps of subjecting a polyamide resin to melt extrusion, cooling and solidifying the extruded film, reheating the unoriented film and subjecting it to drawing in the machine direction (MD) and transverse direction (TD), and heat-setting the drawn film. Drawing is usually accomplished by sequential biaxial drawing in flat form, simultaneous biaxial drawing in flat form, or simultaneous biaxial drawing in tubular form.
After biaxial drawing, biaxially oriented polyamide film undergoes heat setting under tension at a temperature close to the melting point of the film. Heat setting makes the film resistant to shrinkage that takes place during storage or in the subsequent steps such as printing and lamination.
After heat setting, biaxially oriented polyamide film is combined with a sealant such as polyethylene and polypropylene by the aid of an adhesive and fabricated into composite bags, which are used for food packaging. Food packages usually undergo heat treatment at 70.degree.-80.degree. C. or above in hot water, boiling water, or pressurized hot water for the purpose of sterilization of foods.
Unfortunately, biaxially oriented polyamide film has a disadvantage that it greatly shrinks during hot water treatment even after heat setting. This is true particularly of the case in which packaging bags are made from the marginal part of the heat-set film. Such packaging bags are subject to distortion during hot water treatment for sterilization. This distortion deteriorates the appearance of the package and hence lowers the commercial value of the package. In addition, such packaging bags are liable to S-curling (bag distortion) that takes place before hot water treatment, and this often hinders the automatic filling operations.
The distortion of packaging bags occurs because the film as the raw material is anisotropic in heat shrinkage. In other words, the film shrinks in hot water in the machine direction (MD) differently than in the transverse direction (TD). (The maximum shrinkage takes place in the direction of film advance.) When a film with a high degree of anisotropy is made into bags by folding, the resulting bags are subject to distortion because the front side and back side shrink in different directions.
In the present invention, the anisotropy is expressed in terms of the difference between the degrees of shrinkage (caused by hot water) in the directions 45.degree. and 135.degree.deviated to the left from the machine direction (MD).
The above-mentioned distortion is liable to occur in the case where packaging bags are made from marginal parts of film drawn in the flat form by the tenter method. By contrast, such anisotropy does not exist in film drawn in the tubular form. However, this film becomes anisotropic when it undergoes heat setting by the tenter method for the reduction of shrinkage in hot water. This tendency becomes significant in the case of heat setting which is carried out at a temperature close to the melting point of the resin from which the film is made. It is known well that this phenomenon is attributable to bowing. Bowing occurs when film undergoes uneven drawing in the tentering process. Bowing can be visualized when film is tentered
REFERENCES:
patent: 3502766 (1970-03-01), Tsuruta et al.
patent: 4252966 (1981-02-01), Matsuoka et al.
patent: 4352926 (1982-10-01), Matsuoka et al.
Misiro Kouichi
Mizutani Tomoji
Yokota Katuyuki
Kohjin Co., Ltd.
Nakarani D. S.
Thibodeau Paul J.
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