Background of the Invention: Thermoplastic polymers are widely used to manufacture packaging for foods, beverages, and other products. The packaging may be in the form of blow-molded containers, thermoformed sheet, film, injection-molded containers, and other forms well known to those skilled in the art. After the primary fabrication process, secondary operations such as cutting, welding, sealing, and decorating may be required to complete the package. For example, cutting is often a necessary secondary operation in the thermoforming process. A package is thermoformed from a flat plastic sheet and the molded article is removed from the surrounding sheet by using steel rule, forged, or matched-metal cutting dies. It is desirable that during the cutting process the thermoplastic perform creating minimal wear on the cutting die and producing little or no defects on the cut edges.

Thermoplastics such as poly (vinyl chloride) (PVC) and oriented polystyrene (OPS) generally satisfy the criteria for successful cutting. In contrast, amorphous poly (ethylene terephthalate) (APET) is know to be extraordinarily difficult to cut, particularly using steel rule dies. This has prevented, or been an impediment to, many manufacturers using APET in thermoforming applications.

Minimal understanding of the cutting behavior of APET exists in the art. In C. Arcona and T. A. Dow, Journal of Materials Science 31 (1996) 1327, the role of knife sharpness in the slitting of PET films was studied. This study determined that the force to cut PET sheet increases as the blade becomes dull. In D. Bollen, J. Denier, E. Aernoudt, and W. Muylle,

Journal of Materials Science 24 (1989) 2957, biaxially oriented PET was shear cut by using two rotating circular knives. The effect of cutting speed, film thickness, and knife angle was studied to determine these interrelationships on the formation of fibers during the cutting of the PET. Neither reference addresses the predominately compressive forces found when cutting by steel rule dies.

Accordingly, there exists a significant need in the art for a process for improving the cutting behavior of APET without adding significant cost to the overall package or upsetting conventional recycle streams.

There is a further need for improving the cutting behavior of amorphous PET using conventional steel rule dies.

There is yet a further need for improving the quality of the cut of amorphous PET.

Yet another need is to improve the die sharpness cut time for amorphous PET.

Accordingly, it is to the provisions of such that the present invention is primarily directed.

SUMMARY OF THE INVENTION It has unexpectedly been discovered that the cutting characteristics of a thermoplastic film or sheet and particularly a polyester film or sheet and more particularly an amorphous polyester film or sheet can be significantly improved by coating at least a portion of at least one surface of the base film or sheet with a second material which has an improved cutting characteristic relative to the base film or sheet. Broadly, the method of the present invention includes the steps of forming a multi-layer structure having a first or base layer of thermoplastic, wherein the first layer has a first surface; coating at least a portion of the first surface with a second material to form a second layer, wherein a monolayer of the second material has an improved cutting characteristic relative to a monolayer of the base layer; and cutting the multi-layer structure in a manner so that second layer is penetrated by a cutting device before the first layer is penetrated by the cutting device.

It is an object of the present invention to provide a method for improving the cutting characteristics of a thermoplastic.

It is another object of the present invention to provide a method for improving the cutting characteristics of a polyester and more particularly an amorphous polyester.

These and other objects and advantages of the invention will become more apparent to those skilled in the art in view of the following description and the accompanying drawings.

It is to be understood that the inventive concept is not to be considered limited to the constructions disclosed herein but instead by the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical representation of the cutting properties of a 20-mil (0.508 mm) thick films of : (A) a monolayer of amorphous poly (ethylene terephthalate) homopolymer (PET); (B) monolayer of a first copolyester (COP 1) ; (C) multi-layer COP 1/PET structure having an 18-mil (0.457 mm) thick layer of PET and a 2-mil (0.051 mm) thick layer of COP 1 with the cutting die penetrating the COP 1 layer first; and (D) multi-layer COP 1/PET structure having a 19-mil (0.483 mm) thick layer of PET and a 1-mil (0.0254 mm) thick layer of COP 1 with the cutting die penetrating the COP 1 layer first.

FIG. 2 is a graphical representation of the cutting properties of four-20-mil (0.508 mm) thick films of : (E) a monolayer of PET; (F) monolayer of a second copolyester (COP2); (G) multi-layer PET/COP2 structure having a 15-mil (0.381 mm) thick layer of PET and a 5-mil (0.127 mm) thick layer of COP2 with the cutting die penetrating the PET layer first; and (H) multi-layer PET/COP2 structure having a 15-mil (0.381 mm) thick layer of PET and a 5-mil (0.127 mm) thick layer of COP2 with the cutting die penetrating the COP2 layer first.

FIG. 3 is a graphical representation of the cutting properties of four-20-mil (0.508 mm) thick films of : (I) a monolayer of PET; (J) monolayer of the first copolyester (COP 1) ; (K) multi-layer COP 1/PET structure having a 15-mil (0.381 mm) thick layer of COP 1 and a 5-mil (0.127 mm) thick layer of PET with the cutting die penetrating the PET layer first; and (L) the multi-layer structure (K) with the cutting die penetrating the COP 1 layer first.

FIG. 4 is a graphical representation of the cutting properties of a (M) a monolayer of PET; (N) monolayer of the first copolyester, (COP 1) ; and (O) and (P) a three-layer, sandwich type structure having PET positioned between layers of COP 1 where the COP 1 layer is penetrated on top first (O), then on the bottom first (P).

DETAILED DESCRIPTION OF THE INVENTION The present invention is a method for improving the cutting characteristics of thermoplastic materials and particularly polyesters, and more particularly certain poly (ethylene terephthalate) (PET) based polyesters. As used herein, the term"cutting characteristics"means to the capability of a material being cut to not dull a cutting edge of a cutting apparatus or device, such as a blade or steel rule die, sufficiently to increase the pressure exerted on the cutting device by greater than 50 percent after 5000 cutting operations. Thus, improvement in cutting characteristics of a material means that the cutting device can have more quality cuts for a longer period of time before having to be sharpened or replaced. The cutting characteristics of thermoplastics and particularly certain types of polyesters can be improved by preparing a multi-layer structure wherein the base thermoplastic layer is at least partially coated on one surface thereof with a second material having improved cutting characteristics. It is important to the invention that the second material have superior cutting characteristics relative to the base thermoplastic wherein such cutting characteristics are determined in their respective monolayer form. It is also important to the invention that during the cutting operation the layer or coating of the second material be penetrated prior to the cutting device penetrating the base thermoplastic.

With regard to PET, the cutting characteristic is improved by having a base layer of PET and at least a portion of one surface thereof at least partially coated with a layer of a copolyester and cutting the multi-layer structure in a manner wherein the copolyester surface layer in contact with the cutting device so that copolyester layer is penetrated prior to the cutting device penetrating the base PET. It is important to the present invention that the copolyester, in a monolayer form, possess cutting characteristics superior to the PET based polyester. If the PET base layer is penetrated first, no improvement in cutting is observed.

Structures containing more that two layers will also show an improvement in cutting if the copolyester surface layer is on the exterior of the structure and if the steel rule die penetrates this layer first. For purposes of describing the present invention, PET, and particularly amorphous PET, is used as a base material coated with a second layer of a copolyester, which is described in greater detail below. However, it will be apparent to one skilled in the art, that the process of the present invention is suitable for other thermoplastics and polyesters that are difficult to cut. For example, polymers that are particularly useful in

this process besides PET include PEN, and copolyesters containing up to about 50 mole % of modifying dibasic acids and/or glycols and blends thereof.

In accordance with the present invention, the second layer should have a thickness that is from about 5 to 25 % of the total thickness of the multi-layered structure. Desirably, the second layer thickness is from 10 to about 20 %, and preferably from about 10 to about 15 % of the total thickness of the multi-layered structure. Thinner second material layers will produce only marginal improvements in cutting over a monolayer base thermoplastic.

Describing the invention in greater detail, and particularly with regard to PET, the PET base layer comprises a diacid component and a diol component. In the polyester, the mole percentages of the acids equals a total of 100 mole % and the mole percentages of the diols equal a total of 100 mole %. It is preferred that the polyester include from 85 to 100 mole %, preferably 90 to 100 mole %, and more preferably from 95 to 100 mole % of terephthalic acid wherein the diacid is based on the mole percentages of the dicarboxylic acid component of the polyester equaling a total of 100 mole %. By terephthalic acid, suitable synthetic equivalents, such as dimethyl terephthalate, are included. It should be understood that"dicarboxylic acids"includes the corresponding acid anhydrides, esters, and acid chlorides of these acids.

The polyester can further include from 0 to 15 mole %, preferably 0 to about 10 mole %, and more preferably, from 0 to about 5 mole % of dicarboxylic acids other than terephthalic acid, wherein the mole percentages of the dicarboxylic acid component of the polyester equal a total of 100 mole %. The other dicarboxylic acids include, but are not limited to, aromatic dicarboxylic acids preferably having 4 to 40 carbon atoms, more preferably, 8 to 14 carbon atoms; aliphatic dicarboxylic acids having, preferably 4 to 40 carbon atoms, more preferably, 4 to 12 carbon atoms; or cycloaliphatic dicarboxylic acids having 4 to 40 carbon atoms, more preferably, 8 to 12 carbon atoms. Particularly preferred examples of other dicarboxylic acids useful in forming the polyester of the invention include, but are not limited to, isophthalic acid, dimethyl isophthalate, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, 1,4-cyclohexanediacetic acid, diphenyl-4,4'-dicarboxylic acid, naphthalenedicarboxylate, dimethyl-2,6-naphthalene-dicarboxylate, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, and mixtures thereof. Of these, isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid and naphthalenedicarboxyate are preferred, either singly or in combination. Highly useful naphthalene dicarboxylic acids

include the 2,6-, 1,4-, 1,5-, or 2,7- isomers but the 1,2-, 1,3-, 1,6-, 1,7-, 1,8-, 2,3-, 2,4-, 2,5-, and/or 2,8- isomers may also be used. When cyclohexanedicarboxylic acid is used as a comonomer in the context of the invention, trans-, cis-, or cis/trans mixtures may be used.

The diol component of this invention, the mole percentages referred to herein equal a total of 100 mole %.

In the invention, it is preferred that the glycol component of the polyester contain from about 85 to 100 mole %, preferably 90 to 100 mole %, and more preferably from 95 to 100 mole % of ethylene glycol. The remaining mole %, if any, of the diol component comprising the PET can include a modifying diol selected from cycloaliphatic diols preferably having 6 to 20 carbon atoms or aliphatic diols preferably having 2 to 20 carbon atoms. Examples of such diols are: diethylene glycol, triethylene glycol, propane-1,3-diol, butane-1,4-diol, pentane-1, 5-diol, hexane-1, 6-diol, 3-methylpentanediol- (2, 4), 2- methylpentanediol- (1, 4), 2,2,4-trimethylpentane-diol- (1,3), 2-ethylhexane-diol- (1, 3), 2,2- diethylpropane-diol- (1, 3), hexanediol- (1, 3), 1, 4-di- (hydroxyethoxy)-benzene, 2,2-bis- (4- hydroxycyclohexyl)-propane, 2,4-dihydroxy-1,1,3,3-tetramethyl-cyclobutane, 2,2-bis- (3- hydroxy-ethoxy-phenyl)-propane, decalin diol, and 2,2-bis- (4-hydroxypropoxyphenyl)- propane and 1,4-cyclohexanedimethanol. Particularly preferred modifying diols include diethylene glycol, 1,4-cyclohexanedimethanol, including trans-, or cis/trans mixtures of 1,4- cyclohexanedimethanol, 1,4-butanediol, neopentyl glycol, and mixtures thereof. Of the aforementioned, preferably, the PET base layer is poly (ethylene terephthalate) homopolymer comprising 100 mole % terephthalic acid and 100 mole % ethylene glycol.

In accordance with the present invention, a layer of a second material covers at least a portion of at least one surface of the base PET. Desirably, the second material is a copolyester (COP) that is compatible with the base PET. As used herein,"compatible" means that the surface energies of the two polyesters are sufficiently similar as to not cause delamination of the multi-layered structure. The second layer has superior cutting characteristics, as determined in their respective monolayer forms, relative to the base polyester layer. Desirably, the COP covers from about 5 % to 100 % of the surface area of at least one surface of the base polyester. Preferably the COP covers from about 50 % to 100 % and more preferably from about 85 % to 100 % of the surface area of at least one surface of the base polyester.

The COP surface layer comprises a diacid component comprising repeat units from 80 to 100 mole % terephthalic acid and from 20 to 0 mole % of a modifying acid and a diol component comprising repeat units from 25 to 100 mole % 1,4-cyclohexanedimethanol, from 75 to 0 mole % ethylene glycol and from 0 to 20 mole % of a modifying diol. The mole percentages of the acids in the COP equals a total of 100 mole % and the mole percentages of the diols in the COP equals a total of 100 mole %. The modifying acids and diols are the same as discussed above and more preferably include, but are not limited to, isophthalic acid, dimethyl isophthalate, dimethyl-2,6-naphthalenedicarboxylate, 2,6- naphthalenedicarboxylic acid, diethylene glycol, 1,4-butanediol, neopentyl glycol, and mixtures thereof.

Methods of preparing the polyester and copolyester resins are well known in the art and are commercially available. For example, methods for their preparation are described in United States Patents 2,465,319 and 3,047,539 the entire disclosures of which are incorporated herein by reference.

Methods for forming multi-layer structures are known. Suitable methods include, either singly or in combination, coextrusion; extrusion coating; solution coating; emulsion or latex or dispersion coating; liquid coating methods whereby the coating is cured by exposure to ultraviolet light, or a flux of electrons, or by reaction with atmospheric oxygen; vapor deposition; vacuum deposition; co-injection ; injection over molding; and lamination. The above listed methods may be followed by or be preceded by one or more other processing operations such as stretching or drawing, thermoforming, blow molding, heat sealing, or other processing or fabrication techniques.

In the following examples, cutting properties were determined using a laboratory-scale testing procedure. A freshly extruded plastic sheet having the specified thickness was repeatedly cut using a MTS servohydraulic testing machine (available from Baughman Manufacturing Company) with a medium hard (41-44 Rockwell Hardness), 3-point, 0.042 inch, (1.067 mm), steel rule die with a single bevel. The cuts were made against a base plate of hardened 4140 steel.

In order to obtain complete separation of the sheet during a cut, the steel rule die had to penetrate the sheet completely and impact the base plate. In tests on 20-mil (0.508 mm) thick sheet, the penetration depth of the die was adjusted so that the load with which the steel rule die impacted the base plate after the first complete cut was 600 pounds per lineal inch of

steel rule die, which is approximately twice the load to cut through the sheet. The penetration depth of the die was then held constant for the remainder of the test. The steel rule die was moving at a displacement rate of approximately 15 inches (0.381 m) per minute for each cut. The load to cut the sheet per lineal inch of steel rule die was measured as a function of the number of cuts performed. A fresh steel rule die was used for each cutting test.

EXAMPLE 1 Referring to FIG. 1, the cutting properties of four-20-mil (0.508 mm) thick films of polyester and copolyester film structures were evaluated. The first film, (A), is a monolayer of amorphous poly (ethylene terephthalate) homopolymer (PET). The second film, (B), is a monolayer of a copolyester, (COP 1), containing repeat units of 100 mole % of terephthalic acid, 69 mole % ethylene glycol and 31 mole % 1,4-cyclohexanedimethanol, based on 100 mole % diacids and 100 mole % diols. Significant die wear occurs after the first few cuts in both monolayer materials, although the wear is much more acute in the PET. After 1000 cuts, the load to cut PET is nearly twice the load to cut the COP 1.

The third and fourth films, each having a 20 mil (0.508 mm) thickness, were two, multi-layered, coextruded structures comprising PET and COP 1. The third film, (C), consists of 18 mils (0.457 mm) of PET and 2 mils (0.051 mm) of COP1. The fourth film, (D), consists of 19 mils (0.483 mm) of PET and 1 mil (0.0254 mm) of COP 1. For this test, the steel rule die penetrated the COP 1 layer first. Within the reproducibility of the test, this example shows that the sheet containing a layer of COP 1 having a thickness of 10% of the total multi-layer structure thickness cut like a monolayer of COP 1. This example further illustrates that a multi-layer PET/COP structure can be cut using significantly lower cutting pressures after the same number cuts than a monolayer PET structure. The sheet having 1 mil of COP 1 did not cut as well as the sheet containing 2 mils (0.051 mm) of COP 1, although the sheet containing 1 mil (0.0254 mm) of COP 1 did cut better than monolayer PET.

EXAMPLE 2 Referring to FIG. 2, the cutting properties of four-20-mil (0.508 mm) thick films of polyester and copolyester film structures were evaluated. The first film, (E), is a monolayer

of PET as described above in Example 1. The second film, (F), is a monolayer of a second copolyester, COP2, having repeat units of 17 mole % isophthalic acid and 83 mole % terephthalic acid, and 100 mole % 1,4-cyclohexanedimethanol, based on 100 mole % diacids and 100 mole % diols.

The third and fourth films, each having a 20 mil (0.508 mm) thickness, were two, multi-layered, coextruded structures comprising PET and COP2. The third film, (G), consists of 15 mils (0.381 mm) of PET and 5 mils (0.127 mm) of COP2 where during the cutting operation the PET layer was penetrated first. The fourth film, (H), was the same as (G), except the COP2 layer was penetrated first.

A similar improvement in cutting behavior was observed in the coextruded, multi- layered film of PET/COP2. The coextruded PET/COP2 film cut similar to the COP2 monolayer when the multi-layered film was cut so that the COP2 side was penetrated first, and like PET when the mulit-layered film was cut so that the PET side was penetrated first.

Thus, the presence of a more facile-cutting COP improves cutting only when the steel rule die penetrates the COP surface layer first. This observation is illustrated more dramatically in the following example.

EXAMPLE 3 Referring to FIG. 3, the cutting properties of four-20-mil (0.508 mm) thick films of polyester and copolyester film structures were evaluated. The first film, (I), was a monolayer of PET as described above in Example 1. The second film, (J), was a monolayer of COP 1 as described above in Example 1. The third and fourth films, each having a 20 mil (0.508 mm) thickness, were two, multi-layered, coextruded structures comprising COP 1 and PET. The third film, (K), consists of 15 mils (0.381 mm) of COP 1 and 5 mils (0.127 mm) of PET where during the cutting operation the PET layer was penetrated first. The fourth film, (L), was the same as (K), except the COP 1 layer was penetrated first.

Although the sheet contained 75% of the easier-to-cut COP 1, the sheet cut like PET when cut from the PET side. When cut from the COP1 side, the sheet cut like COP 1.

EXAMPLE 4 As illustrated in Examples 2 and 3, the direction of the cutting, i. e., which layer is penetrated first is important when cutting a two layer, coextruded film structure. This

problem can be avoided if sandwich structure of the form COP/PET/COP is used.

Referring to FIG. 4, a three layer structure was evaluated. The structure consisted of two- 4-mil (0.102 mm) thick outer layers of COP I and a 14-mil (0.356 mm) thick middle layer of PET. The same cutting performance was obtained when the sheet was cut from either side.

The approach of using a sandwich layer type structure with a core of amorphous PET and exterior layers of a better-cutting copolyester has the advantage of eliminating the need to identify which side of the sheet is coated with the copolyester Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting to the invention described herein. No doubt that after reading the disclosure, various alterations and modifications will become apparent to those skilled in the art to which the invention pertains. It is intended that the appended claims be interpreted as covering all such alterations and modifications as fall within the spirit and scope of the invention.