Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-06-15
2002-11-19
Thiobodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
Of metal
C428S480000, C428S910000, C156S308200, C156S309600, C156S309900, C156S321000, C156S322000
Reexamination Certificate
active
06482526
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a polyester resin-covered metal sheet possessing excellent formability. The covered sheet is particularly suited for “heavily formed” use, such as drawing, drawing and ironing, drawing and stretch forming, and ironing after drawing and stretch forming. The present invention also teaches a method for forming said resin-covered metal sheet.
BACKGROUND OF THE INVENTION
Metal containers such as beverage cans or battery containers are typically formed by drawing, drawing and ironing, drawing and stretch forming, or ironing after drawing and stretch forming. These drawing processes expand the interior volume of the metal container by reducing the thickness of the surrounding walls. Subsequent to drawing, the containers are usually laminated with a corrosion resistant coating and printed with desired text and indicia.
This process may be enhanced by first coating the metal sheets with an organic resin, such as polyethylene terephthalate (PET). Initial coating of the subsequently-drawn metal reduces coating costs and mitigates environmental pollution resulting from dispersion of solvents during the application of corrosion resistant coatings. Such resin-coated metal cans have already been utilized in beverage cans.
Suitable organic resins, in the form of a biaxially-oriented film, are heat-bonded to metal sheets prior to the drawing process. These films are manufactured via biaxial elongation of a thermoplastic polyester resin, followed by heat-setting. Their mechanical characteristics, when measured with a tensile tester, are characterized by large yield strength and small elongation (elongation after fracture).
Alternatively, the resin films may be laminated onto a metal sheet with adhesive, so as to avoid the loss of biaxial orientation that results from heat-bonding. However, due to their limited ability to elongate, these resin coatings exhibit numerous fractures and cracks. Furthermore, where there is only limited adhesion between the metal and the resin, the resin tends to peel off during the lamination process. Furthermore, where heat bonding is utilized to laminate the polyester resin onto the metal sheet, the biaxial orientation of the resin film is partially or entirely lost. Consequently, the yield strength of the post-lamination resin film decreases while elongation improves, preventing the film from cracking, peeling-off, or fracturing. Conversely, resin films lacking biaxial orientation have such large permeability that the contents of a container laminated therewith permeate the film and corrode the metal substratum. Such films also tend to generate coarse spherlites during the printing process, and tend to crack readily if containers collide or fall.
In biaxially oriented polyester resin films heat bonded to metal sheets, the elongation after fracture, prior to lamination, is defined in one of the following ways:
1. According to the preferable range described in Laid open Japanese patent Hei 1-249331,
2. As the range of the orientation coefficient showing the degree of biaxial orientation prior to lamination,
3. As the preferable range of elongation after the fracture and tensile strength are defined, as illustrated in Laid open Japanese patent Hei 2-70430.
Processes like those disclosed in Laid open Japanese patents Hei 1-249331 and Hei 2-70430, which utilize heat bonding to laminate a biaxially oriented resin film onto a metal sheet, effectively destroy the resin's biaxial orientation. This alters the values of the post-fracture elongation and the tensile strength. Thus, previously acceptable, biaxially oriented films, subsequent to heat lamination, may no longer exhibit the same favorable biaxial orientation; the film's favorable elongation and tensile strength will also be compromised.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome the deficiencies in the prior art. Accordingly, the present invention produces a polyester resin film-covered metal sheet
The values of the elongation after fracture (disclosed in Laid open Japanese patent Hei 1-249331) and elongation after fracture and disclosure of tensile strength (Laid open Japanese patent Hei 2-70430) are determined prior to lamination of the resin to the metal sheet. characterized by excellent formability and adapted for use in conventional drawing, drawing and ironing, drawing and stretch forming, and ironing after drawing and stretch forming processes.
SUMMARY OF THE INVENTION
According to the present invention, a polyester resin film-covered metal sheet, which retains the biaxial orientation of the resin subsequent to lamination, has a true stress value ranging from 3.0 to 15.0 kg/mm
2
measured at 75° C. and corresponding to a true strain of 1.0. In a preferred embodiment, the polyester resin is a polyethylene terephthalate resin having a low crystallization temperature, (i.e., the temperature of the exothermic peak produced upon heating a quenched sample of the resin in a differential scanning calorimeter) between 130 and 165° C., preferably 140 to 155° C. The polyester resin is preferably a copolyester resin of recurring ethylene terephthalate or butylene terephthalate monomers. Alternatively, it may be a copolyester resin consisting of at least two of the ethylene or butylene terephthalate monomers, or a double layered polyester resin consisting of a laminate of at least two of the afore-mentioned resins.
The method of producing the present invention entails contacting the polyester resin to a metal sheet, heating the composite to a temperature above the melting temperature of the polyester resin, and pinching and pressing the composite into a laminate with a pair of laminating rolls. The laminating rolls form a nip at the exit site of the laminate, said nip being equipped to cool the emerging laminate at a rate of at least 600° C./second. The resulting laminate exhibits a true stress of 3.0 to 15.0 kg/mm
2
measured at 75° C. and corresponding to a true strain of 1.0.
The resin contemplated for use in the present invention consists of a polyester resin, preferably polyethylene terephthalate, having a low crystallization temperature ranging from 130 to 165° C., optimally between 140 to 155° C. Alternatively, the resin may constitute a copolymer of ethylene terepthalate and ethylene isophthalate monomers. Either formulation, when applied to a metal sheet according to the present invention, results in a laminate of decreased yield strength and increased elongation, thereby reducing occurrences of film peeling, cracking, or fracture.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to the coating of metal sheets with a polyester resin film, preferably polyethylene terephthalate (PET), biaxially oriented along its length and width, and having a low crystallization temperature ranging from 130 to 165° C., optimally between 140 and 155° C. This latter value will be explained subsequently.
When an amorphous polyester resin, such as PET, is obtained by heating said resin above its melting temperature, immediately quenching the resin, and then gradually heating with a differential scanning calorimeter (DSC). This process generates an exothermic peak between temperatures of 100 and 200° C., depending upon the resin composition. Resins with exothermic peaks at higher temperatures exhibit lower crystallization velocities compared with those characterized by lower-temperature exothermic peaks. For example, polybutylene terephthalate resins produce an exothermic peak at about 50° C., whereas PET generates a peak at about 128° C. In contrast, an ethylene terephthalate -ethylene isophthalate copolyester resin (typically used in “2-part” cans) exhibits an exothermic peak at about 177° C.
According to the present invention, a biaxially oriented film of PET resin having a low crystallization temperature outside the 130 to 165° C. range can be heat bonded to a metal sheet. However, a PET resin exhibiting a crystallization temperature between 130 and 165° C. is better suited to produce a metal-resin laminate that
Gotoh Fumiko
Iwashita Hiroyuki
Tanaka Atsuo
Browdy and Neimark PLLC
Thiobodeau Paul
Toyo Kohan Co., LTD
Zacharia Ramsey
LandOfFree
Metallic sheet covered with polyester resin film and having... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Metallic sheet covered with polyester resin film and having..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Metallic sheet covered with polyester resin film and having... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2972471