1. Field of the Invention
The present invention relates to a resin composition for heat-shrinkable polypropylene-based shrink label (hereinafter sometimes simply referred to as heat-shrinkable propylene shrink label), a film for heat-shrinkable polypropylene-based shrink label, a laminated film for heat-shrinkable polypropylene-based shrink label, and a container having the film or laminated film attached thereto, and more particularly to a resin composition for heat-shrinkable polypropylene-based shrink label having an improved heat-shrinkability, particularly at low temperatures, a film or laminated film using the resin composition, and a container having the film or laminated film attached thereto.
2. Description of the Related Art
In recent years, shrink labels have been widely used for the purpose of sheathing wrapped articles to improve their external appearance, wrapping articles to avoid direct impact of their contents, tightly wrapping articles, label-wrapping glass bottle or plastic bottle to protect the material and display.
Known examples of plastic materials to be used for these purposes include polyvinyl chloride, polystyrene, polyethylene terephthalate, and polypropylene. A polyvinyl chloride label exhibits an excellent heat-shrinkability but is disadvantageous in that when combusted, it produces chlorine gas, which pollutes environment. A polystyrene or polyethylene terephthalate label exhibits a good heat-shrinkability but is disadvantageous in that it differs little in specific gravity from polyethylene terephthalate bottle (hereinafter referred to as “PET bottle”) and thus can hardly be separated from PET bottle by suspension, preventing the recycling of PET bottle. Further, such a plastic label is made of a resin having a poor heat resistance to have a sufficient heat-shrinkability. Thus, when retorted, the plastic label is subject to resin melting that causes the printing ink to flow out.
A polypropylene differs greatly in specific gravity from PET bottle and thus can be easily separated from PET bottle by suspension and exhibits an excellent heat resistance but leaves something to be desired in heat-shrinkability. As approaches for improving low temperature shrinkability there are known a method involving the addition of propylene-butene-1 copolymer to the polypropylene and a method involving the addition of a petroleum resin or terpene resin to the polypropylene (JP-A-62-62846 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”)). However, these methods leave something to be desired in their effect. Thus, it has been desired to further improve the shrinkability of polypropylene-based resin as a base.
Further, the foregoing resin composition having a petroleum resin incorporated therein is disadvantageous in that an unstretched sheet which has been extruded during the formation of a film, particularly during the extrusion of unstretched sheet, can be easily stuck to the cooling roll, deteriorating the formability of the sheet. If a polypropylene-based resin free of petroleum resin is laminated on the unstretched sheet as a surface layer to improve the formability thereof, it is disadvantageous in that the heat-shrinkability is deteriorated because the interlayer has an insufficient heat-shrinkability. In order to solve the foregoing problem, it has been desired to provide an interlayer which exhibits an excellent heat-shrinkability.
No films for polypropylene-based shrink label having a satisfactory balance of the foregoing low temperature shrinkability, transparency, blocking resistance and slipperiness have ever been found. Further, as its heat-shrinkability increases, the conventional polypropylene-based shrink labels exhibit a raised specific gravity that deteriorates the efficiency in suspension separation from PET bottle by water. Thus, it has been keenly desired to provide a film for polypropylene-based shrink label having a low specific gravity and an improved heat-shrinkability.