Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Layered – stratified traversely of length – or multiphase...
Reexamination Certificate
2001-06-11
2003-10-28
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Layered, stratified traversely of length, or multiphase...
C264S290200
Reexamination Certificate
active
06638465
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process of manufacturing a EVOH/Polyester (coextruded) bistretched film. The invention also relates to such a film, showing high barrier properties and high mechanical properties.
BACKGROUND OF THE INVENTION
Extrusion of multilayer films is well-known. Stretching of films, being monolayer or multilayer films, is known as well.
The above techniques have been applied to many different types of films, in order to obtain the desired results. One multilayer film that would be of high interest is a film containing a layer of EVOH and a layer of polyester. The layer of EVOH would impart barrier properties such as gas barrier properties against oxygen, carbon dioxide, helium, aroma and flavors, etc. . . . The layer of polyester would impart mechanical properties, such as Young's modulus, tensile strength, heat resistance, clarity, etc. . . . Since these two types of polymers are not compatible, a coextrusion binder is necessary. Thus, one would look for films having one layer of EVOH, one layer of coextrusion binder and one layer of polyester.
The process of manufacturing such films is, however, very delicate. Especially, stretched films are very difficult to manufacture, since the layer of EVOH is very difficult to stretch, especially to bistretch. As a matter of fact, polyester, being nearly amorphous at the exit of the extruder, can be easily stretched, either simultaneously or sequentially, and high quality film is easily obtained. The EVOH layer on the contrary, has a high tendency to crystalise under heat and stress, so that a sequential stretching is not possible: stretching in MD direction forms a crystalline EVOH layer, that breaks whilst the attempt to stretch in TD direction. Simultaneous stretching is thus required. Eventually, the stretched films are heat set, so as to develop the mechanical properties of the film, thanks to the polyester layer.
Thus, there is a need for a method that would allow the production of valuable multilayer EVOH/PET films, but would also be cost effective and easy to carry out.
JP-A-55139263 discloses a process where a multilayer film is bistretched then heat treated at a temperature below the melting point of the EVOH material. This, however, is not satisfactory, since the melting point of all EVOH grades suitable for high barrier applications is below 190° C., preferably below 180° C., most preferably below 170° C. Such low heat-set temperatures are not appropriate to maintain the thermo-mechanical strength of the outer polyester layers, resulting from the biaxial stretching. This would lead to a high thermal shrinkage and the high mechanical strength of polyester film would be lost at higher temperatures.
JP-A-63272548 discloses a process for manufacturing a laminate stretched film comprising coextruding a 5-layer film, especially polyester/binder/EVOH/binder/polyester, simultaneously bistretching it, and heat-treating it under the following conditions:
0.5≦X≦5
EVOH melting point+40° C.≦T≦polyester melting point 8 sec≦t≦25 sec
where:
X=total polyester thickness/EVOH thickness
T=heat treatment temperature
t=heat treatment time
Examples given in this document provide simultaneous bistretching at a temperature of 90° C., at ratios of 3.3×3.3, followed by heat treatment at a temperature of about 230° C., for a duration of about 15 sec. The respective polymers are PET, EVOH and modified PET or modified EVA resin as a binder. The thicknesses (unstretched/stretched) of the EVOH and PET layers are respectively about 54 &mgr;m/5 &mgr;m and about 130 &mgr;m/12 &mgr;m, giving a X value of about 2.5. The EVOH melting point is about 180 (leading to on heatset temperature of 220° C. for EVOH mp+40° C.) while PET melting point is about 260° C. At this high heatset temperatures, the film starts to crystallize and gets brittle very fast. Heatset times higher than 8 seconds will definitely lead to film breakage in the oven and a loss of mechanical properties, especially elongation, due to high crystallization and brittleness thereof. Additionally, running a film line with such high hold-up times in the heatset zones would require either very long heatset zones in the oven or very low line speeds. This is economically not suitable for such a product.
SUMMARY OF THE INVENTION
The object of the present invention is a process where the heat treatment is carried out during a time below 8 sec., and at a temperature preferably below 220° C.
The resulting film shows enhanced properties, especially when the ratio of thicknesses of polyester to EVOH is higher than 5.
DETAILED DESCRIPTION OF THE INVENTION
The polyester used in the invention is any polyester where the major part of it is comprised of any aromatic repeating ester units. The term polyester in this invention refers to a polymer that is obtained by condensation polymerization of an aromatic dicarboxylic acid such as terephthalic acid or 2,6-naphthalene dicarboxylic acid and of an aliphatic glycol such as ethylene glycol, 1,4-butanediol or 1,4-cyclohexane dimethanol. These polymers, in addition to being homopolymers, may also be copolymers having a third component or several components. In this case, the dicarboxylic acid component may be, for example, isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalene dicarboxylic acid, 4,4′-diphenyldicarboxylic acid, adipic acid, sebacic acid, decanedicarboxylic acid and 1,4-cyclohexane dicarboyxlic acid; the oxycarboxylic acid component can be, for example, p-oxybenzoic acid and the glycol component can be, for example, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butanediol, neopentyl glycol, 1,4-cyclohexane dimethanol, polyethylene glycol and polytetramethylene glycol.
Examples of such polyesters are polyethylenenaphthalate (PEN), polybutyleneterephthalate (PBT), polyethyleneterephthalate (PET), the latter PET being the preferred polyester.
Mixtures are also possible, optionally with another polymer different from a polyester. The intrinsic viscosity of the polyester that is used in the invention may vary from e.g. 0.45 to e.g. 0.7, measured in phenoltetrachloreethane at 30° C. The MW may vary within broad limits, e.g. between 10000 to 30000 g/mol.
The binder material is any material that is adhesive and allow the polyester and EVOH layers to show adhesion, with either adhesive rupture or cohesive rupture. The skilled man will choose the binder thanks to its general knowledge or thanks to routine tests.
Examples of such binders include modified polyolefins, polyacrylates, polyurethanes, polyesters, etc.
Examples of binders are the following (co)polymers, grafted with maleic anhydride or glycidyl methacrylate, in which the grafting rate is for example from 0.01 to 5% by weight:
PE, PP, copolymers of ethylene with propylene, butene, hexene, octene, butadiene, EPR, EPDM, containing, for example, 35 to 80% by weight ethylene, as well as any styrene-based block copolymers such as SBS, SIS, SEBS, and the like;
ethylene and vinyl acetate (EVA) copolymers containing up to 40% by weight vinyl acetate;
ethylene and alkyl (meth)acrylate copolymers containing up to 40% by weight alkyl(meth)acrylate;
ethylene and vinyl acetate (EVA) and alkyl (meth)acrylate copolymers, containing up to 40% by weight comonomers.
Further examples of binders are the following (co)polymers, in which ethylene represents preferably at least 60% by weight and where the termonomer represents, for example, 0.1 to 10% by weight of the copolymer:
ethylene/alkyl (meth)acrylate or methacrylic acid/maleic anhydride or glycidyl methacrylate copolymers;
ethylene/vinyl acetate/maleic anhydride or iglycidyl methacrylate copolymers;
ethylene/vinyl acetate/alkyl (meth)acrylate or methacrylic acid/maleic anhydride or glydicyl methacrylate copolymers. The term “alkyl (meth)acrylate” stands for C1 to C6 alkyl, such as methyl, ethyl, butyl and 2-ethylhexyl methacrylates and acrylates. Moreover, these polyolefins can also be cross-linked using any suitable process o
Andreis-Olinger Christine
Feyder Gusty
Dupont Teijin Films US Limited Partnership
Eashoo Mark
LandOfFree
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