Stock material or miscellaneous articles – Hollow or container type article – Nonself-supporting tubular film or bag
Reexamination Certificate
2001-01-26
2002-01-29
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Hollow or container type article
Nonself-supporting tubular film or bag
C428S034900, C428S035500, C428S327000, C428S332000, C528S295300
Reexamination Certificate
active
06342281
ABSTRACT:
FIELD OF INVENTION
The present invention relates to heat-shrinkable polyester films, and more particularly, to heat-shrinkable polyester films suitable for label use, which may cause only rare occurrence of shrinkage spots, wrinkles, strains, longitudinal sinking, and other defects during heat shrinkage, and which may further have excellent break resistance.
BACKGROUND OF THE INVENTION
In the past, heat-shrinkable films have been widely used for various applications, such as shrink-wrap films, shrinkable labels, and cap seals, by utilization of their property of causing shrinkage by heating. In particular, heat-shrinkable stretched films made of vinyl chloride resins, polystyrene resins, polyester resins, or other resins have been used as labels on various vessels, such as polyethylene terephthalate (PET) vessels, polyethylene (PE) vessels, and glass vessels.
However, vinyl chloride resins have serious problems including low heat resistance and evolution of hydrogen chloride gas in their incineration. In addition, when heat-shrinkable films of vinyl chloride resins are used as shrinkable labels on PET and other vessels, the labels should be separated from the vessels in the process of recycling the vessels.
In contrast, films of polystyrene resins or polyester resins cause no evolution of harmful substances such as hydrogen chloride gas in their incineration, and therefore, these films have been expected to take the place of vinyl chloride resin films as shrinkable labels on vessels.
However, polystyrene resin films, although they exhibit good shrinkage finish in appearance after shrunk, have poor solvent resistance, so that they require the use of special ink in their printing. They also have serious problems in their disposal, e.g., they require incineration at high temperatures, in which case they may cause evolution of black smoke and bad smell in large quantities.
As materials that can solve the above problems, polyester resin films have been extremely expected to serve, and there has been a steady increase in their amounts for use. The conventional heat-shrinkable polyester films as described above, however, cannot have satisfactory heat-shrinkage characteristics. More particularly, they easily cause the occurrence of shrinkage spots or wrinkles during heat shrinkage, and they further have some serious problems, when used for covering the bodies of vessels such as PET bottles, PE bottles, and glass bottles, and then shrunk, including distortion of letters or patterns after the shrinkage, which have been previously printed on the films before the shrinkage, and further including insufficient adhesion of the films to the vessels. In addition, they have poor shrinkability as compared with heat-shrinkable polystyrene films, so that they should be shrunk at higher temperatures to attain the desired degree of shrinkage, which further causes serious problems including the deformation of bottles and the occurrence of whitening.
In general, when heat-shrinkable films are used for covering vessels and then shrunk on a large scale for industrial production, there has been a method in which the films formed into labels, tubes, bags, or other shapes are fitted on the vessels, and then allowed to pass, while being carried on a belt conveyor, through a shrinkage tunnel of such a type that the films are heat shrunk by steam blowing (i.e., steam tunnel) or a shrinkage tunnel of such a type that the films are heat shrunk by hot air blowing (i.e., hot air tunnel). The efficiency of heat transmission in the steam tunnel is higher than that in the hot air tunnel, and therefore, the use of a steam tunnel can result in heat shrinkage with higher uniformity to give good shrinkage finish as compared with the use of a hot air tunnel. However, the conventional heat-shrinkable polyester films as described above are inferior in shrinkage finish after shrunk through a steam tunnel to heat-shrinkable vinyl chloride resin films and heat-shrinkable polystyrene resin films. On the other hand, the irregularity of internal film temperature easily occurs during heat shrinkage through a hot air tunnel. As a result, in particular, there easily occur shrinkage spots, wrinkles, strains, and other defects. For this reason, the conventional heat-shrinkable polyester films as described above are also inferior in shrinkage finish after shrunk through a hot air tunnel to heat-shrinkable vinyl chloride resin films and heat-shrinkable polystyrene resin films.
SUMMARY OF THE INVENTION
Under these circumstances, the present inventors have extensively studied to provide heat-shrinkable polyester films suitable for label use, which have excellent shrinkage characteristics over a wide range of temperature extending from low temperatures to high temperatures, particularly in a low temperature range, which may cause only rare occurrence of shrinkage spots, wrinkles, strains, longitudinal sinking, and other defects during heat shrinkage, and which may further have excellent break resistance. As a result, they have found that such heat-shrinkable polyester films can be obtained by the control of dynamic viscoelasticity and heat shrinkability after treatment in hot water.
Thus the present invention provides heat-shrinkable polyester films wherein the value of tan &dgr; for dynamic viscoelasticity in a main shrinkage direction of the film is 0.15 or higher at 65° C. and takes a maximum of 0.40 or higher at a temperature of 65° C. to 100° C. both inclusive (unless otherwise indicated, the range of numerical values referred to herein includes those at both upper and lower limits), and the heat shrinkability in the main shrinkage direction of the film after treatment in hot water at 80° C. for 10 seconds is 30% or higher.
DETAILED DESCRIPTION OF THE INVENTION
For the heat-shrinkable polyester film of the present invention, the value of tan &dgr; for dynamic viscoelasticity in the main shrinkage direction of the film should be 0.15 or higher, preferably 0.20 or higher, at 65° C.
As used herein, the value of tan &dgr; refers to a value defined by tan &dgr;=G′/G″ where G′ and G″ are storage modulus and loss modulus, respectively, which can be determined by applying sine stress to a sample and measuring the delay of sine strain as the response of the sample.
In the process of industrial production where heat-shrinkable films formed into labels, tubes, or other shapes are fitted on vessels and then heat shrunk through a shrinkage tunnel, the temperature on the surface of the vessels in contact with the heat-shrinkable films, although it may vary with the type of process or vessel used, is generally kept at a temperature of 85° C. or lower. The value of tan &dgr; for dynamic viscoelasticity in the main shrinkage direction of the film at such low temperatures is a factor determining the occurrence of shrinkage spots, wrinkles, strains, and other defects during heat shrinkage. In particular, if the value of tan &dgr; for dynamic viscoelasticity in the main shrinkage direction of the film is 0.15 or higher at 65° C., the process of industrial production where heat-shrinkable films are heat shrunk through a shrinkage tunnel involves only rare occurrence of shrinkage spots, wrinkles, strains, and other defects.
The value of tan &dgr; for dynamic viscoelasticity in the main shrinkage direction of the film may preferably be 0.05 or higher, more preferably 0.10 or higher, at 60° C., in which case the film has particularly excellent shrinkage characteristics at low temperatures and exhibits particularly good shrinkage finish.
In addition, the value of tan &dgr; for dynamic viscoelasticity in the main shrinkage direction of the film should take a maximum at a temperature of 65° C. to 100° C. both inclusive. If the value of tan &dgr; for dynamic viscoelasticity in the main shrinkage direction of the film takes a maximum at a temperature lower than 65° C., the film has deteriorated break resistance at room temperature and comes to easily cause a change in physical properties with the lapse of time. For example, during
Hayakawa Satoshi
Morishige Chikao
Ohashi Hideto
Sato Maki
Tabota Norimi
Acquah Samuel A.
Morrison & Foerster / LLP
Toyo Boseki Kabushiki Kaisha
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