Patent Application
20080032141
This invention is directed to a cold seal release film, which preferably is a biaxially oriented multilayer polymeric film. In accordance with the broadest aspects of this invention both the film and method of making the film can be employed in connection with cold seal release films having either a matte surface or a non-matte surface. However, in the most preferred embodiments of this invention the release properties are enhanced in connection with a matte surface cold seal release film. The matte surface is provided in an outer skin layer by blending two or more incompatible copolymers. Such skin layers are well known in the art and are specifically disclosed in a number of the Wilkie patents identified earlier herein. For example, note Wilkie U.S. Pat. No. 5,981,047 and Wilkie U.S. Pat. No. 6,022,612. These patents already have been fully incorporated herein.
In accordance with this invention the reference to a matte finish refers to a surface having a 45 degree gloss less than 30; more preferably 15 or less and most preferably 10 or less.
In the most preferred embodiment of this invention the cold seal release film is a multilayer structure having an internal core and opposed, relatively thin skin layers. Although the preferred embodiment of cold seal release films within this invention includes three layers, it is within the scope of this invention to include additional layers, including tie layers between the core layer and skin layers, as is necessary.
The core preferably is formed of polypropylene (as defined hereinafter) with one of the outer skin layers employing conventional matte-surface polymers to be described hereinafter and modified in accordance with this invention. The opposed skin layer preferably is a printing/laminating layer that is oxidatively treated to receive reverse printing thereon and also to receive a laminating adhesive for securing the release film to a separate, inner web or film to form a composite, packaging film with the release surface being the outer surface thereof. The inner surface of the innermost layer of the inner web or film, in the preferred embodiments, is a cold seal receptive surface. In accordance with the broadest aspect of this invention the innermost layer of the inner web or film can be any of the well known skin layers employed or known in the art as providing a receptive layer to receive and retain a cold seal adhesive thereon.
The cold seal release films of this invention also can be employed by themselves as packaging films, without being laminated to a separate, inner web or film. In these latter release films the skin layer opposed to the cold seal release layer preferably is oxidatively treated, such as by corona or flame treatment, to receive and retain a cold seal adhesive thereon. Most preferably the oxidative treatment is corona treatment.
The multilayer, cold seal release films in accordance with this invention typically have a thickness in the range of about 40-200 gauge (0.4-2 mils), with the thickness of the cold seal release skin layer being in the range of 4-20 gauge (0.04-0.2 mils), the thickness of the opposed cold seal receptive layer being in the range of 2-10 gauge (0.02-0.1 mils), and the core layer constituting the remaining thickness of the film.
In accordance with a unique aspect of the present invention, as will be described in detail hereinafter, the cold seal release layer is formulated to include a silicone oil slip agent therein, and is then oxidatively treated, preferably by corona treatment, to provide enhanced cold seal release properties. Most preferably the silicone oil slip agent is present in an amount of at least 0.05% (500 PPM); more preferably at least approximately 0.1% (1000 PPM) and preferably in the range of 0.1% (1000 PPM) and 0.2% (2000 PPM) based upon the weight of the skin layer. It should be understood that the balance of the skin layer can include polymeric materials that are disclosed as being conventionally employed in release layers.
In the following examples, the values of cold seal release force were determined in accordance with the following procedure:
Referring to Table I, four products including different commercial skin layer compositions demonstrate the effect of oxidatively treating (preferably corona treating) commercial skin layers to which a silicone oil slip agent had been added. The specific formulation of each of the skin layers in Table I is set forth in Table II.
The Ampacet 400700D and SKMF502H employed in samples 391-1 and 391-2, respectively, are commercial matte skin polymers including a formulation that is treated as proprietary by the suppliers. The Ampacet 400700D polymer is provided by Ampacet Corporation located in Terre Haute, Ind., and the SKMF502H polymer is provided by SK in Seoul, Korea.
Sample 391-3 is a 50/50 blend, by weight, of Fina 6540, sold by Total Petrochemicals, and Basell PB8340, sold by Basell USA, in Elkton, Md. The detailed formulation of 391-3 is set forth in Table II.
Sample 391-4 is a clear skin layer made of Fina 6450. The specific formulation of the skin layer is shown in Table II.
As will be noted in Table I, the highest cold seal adhesive release force for each sample was experienced with the sample that had no corona treatment.
It should be noted that the two numbers reported for each sample under the title CSR (cold seal release) force are the force required to initiate the peeling action and the force to continue the peeling action once it has been started, respectively. For example, sample 391-1, when exposed to high corona treatment, had an initiation seal release force of 31 g/in. and a seal release force of 21 g/in. to continue the release after it had been started. It is the second value; namely, the seal release force to continue release that is the significant value in this invention. Unless otherwise indicated, all further references to the cold seal release force in this application refers to the second value; namely, the force required to continue the releasing function after release has been initiated.
Except for sample 391-3, a reduction in CSR force in excess of 50% was experienced when comparing the skin layer with no corona treatment with the skin layer having a high corona treatment. For example, the peel strength for sample 391-1 went from 82 g/in. with no corona treatment to 21 g/in. with a high corona treatment. Sample 391-2 went from 75 g/in. with no corona treatment to 23 g/in. with high corona treatment.
The lack of satisfactory results for sample 391-3 was believed to result from the incomplete mixing of the two polymer formulations. However, even in this sample a reduction in CSR force was experienced by corona treating the layer. Moreover, as is reported later in this application, a very similar formulation that was properly blended gave results that were expected in this invention.
Sample 391-4 experienced a CSR of 125 g/in. without corona treatment and a CSR of 26 g/in. with a high corona treatment. This latter sample had a clear finish, as opposed to a matte finish, as evidenced by the 45° gloss value of 85.
The column farthest to the right shows both the static and kinetic coefficient of friction for the various samples. In the preferred embodiments of this invention the static coefficient of friction should be less than 0.5 and the kinetic coefficient of friction should be less than 0.4. More preferably the static coefficient of friction should be 0.35 or less and the kinetic coefficient of friction should be 0.25 or less. As is illustrated in Table I, the static coefficient of friction for all of the samples was less than 0.5 and the kinetic coefficient of friction was less than 0.4.
Table III below illustrates four additional samples; each including the same commercial matte skin polymer composition, but differing from each other in the particular amount and/or type of polydimethylsiloxane employed in the mix. The specific formulation of samples 397-1-397-4 are shown in following Table IV. It should be noted that the matte skin polymer composition is treated as proprietary by the manufacturer.
Referring to Table III it should be noted that each of the three samples 397-1-397-3 had a reduction in cold seal release force well in excess of 50% by corona treating that layer. For example, 397-1 with no corona treatment had a cold seal release force of 92 g/in., which was reduced to 15 g/in. by high corona treatment. In sample 397-2 the cold seal release force without corona treatment was 85 g/in., which was reduced to 31 g/in. by high corona treatment. A similar result is shown for sample 397-3.
Sample 397-4 is a prior art structure that clearly confirms the unique and unexpected result achieved in this invention by including a silicone oil slip agent in a cold seal release formulation that then is oxidatively treated. Specifically, 397-4 included a commercial matte polymer formulation but with no silicone oil slip agent therein. In this product exposing the layer to high corona treatment, as opposed to no corona treatment, resulted in an increase in the cold seal release force from 62 g/in. to 90 g/in. This is exactly what was expected in the prior art when corona treating a cold seal release surface.
However, in accordance with the present invention, exposing samples having 0.1% of different molecular weight polydimethylsiloxane, as illustrated in samples 397-1 and 397-2, resulted in a substantial reduction in the cold seal release force. In a like manner, employing as much as 0.2% of polydimethylsiloxane in the blend, as illustrated in sample 397-3, resulted in a substantial reduction in the cold seal release force by the application of corona treatment. These results were completely unexpected.
An additional set of test product is identified in Table V. All of the samples identified in Table V include 0.1% of the same polydimethylsiloxane but different polymer blends of conventional matte formulations disclosed in the prior art. In other words, the specific blends in samples 399-1-399-6 are all reported in the literature as providing a matte surface.
Table VI illustrates the specific formulation of the matte skin layers for each of samples 399-1-399-6.
The gloss and C.O.F. values were not recorded for samples 399-1-399-6. As will be noted in Table V, exposing the various skin layers to high corona treatment resulted in a substantial reduction in the cold seal release force. In particular, the reduction in all samples was well in excess of 50%, as compared to the same skin layer without corona treatment.
An additional point which should be noted is that sample 399-5 essentially is the same formulation as sample 391-3 reported in Table I, but with the specific polymers having been adequately blended. It should be noted that the adequate blending of the polymers resulted in a cold seal release force of 14 g/in. when the layer was exposed to high corona treatment. This compares to a cold seal release force of 57 g/in. in sample 391-3, wherein the blend was not considered to be adequately mixed or blended.
Although the various examples disclosed herein included oxidative treatment by corona treatment, applicant has determined that a similar reduction in cold seal release force can be obtained by flame treatment. Applicant believes that other oxidative treatment processes may also be usable to obtain the desired benefits of this invention. Although oxidative treatment by flame treatment is considered to be within the scope of this invention, such treatment resulted in other film properties being less desirable than the samples treated with corona treatment. Therefore, corona treatment of the release layer is preferred in the present invention.
In accordance with the broadest aspect of applicant's invention, the oxidative treatment of the skin layer should reduce the cold seal release force by at least 25%, as compared to a non-oxidatively treated layer. For example, if the peel force of a non-oxidatively treated layer were 80 g/in., then the oxidatively treated layer in accordance with this invention should have a cold seal release force of no greater than 60 g/in. [80−(25% of 80)=60].
In the most preferred embodiments of this invention, the cold seal release force should be reduced by an amount in excess of 50% from that of a non-oxidatively treated layer. The high corona treatment of each of the samples reported in Tables I, III and IV, except for sample 391-3 which was not properly manufactured and sample 397-4 which did not employ the present invention, achieved a reduction in cold seal release force well in excess of 50%, as compared to the same layer with no corona treatment.
In accordance with the preferred embodiments of this invention the percentage of silicone oil slip agent employed, in the release layer should be at least 0.05%; more preferably at least 0.1% and most preferably in the range of 0.1%-0.2%.
It should be understood that it is not possible to accurately quantify a value representative of medium or high corona treatment due to the insensitivity of the dyne solution and contact angle surface energy measurement techniques that commonly are employed. However, individuals skilled in the art understand the levels of treatment that generally qualify as medium and high. More significantly, based upon the teachings in this application individuals skilled in the art can clearly adjust the level of corona treatment to obtain the desired and/or necessary reduction in cold seal release force of the release layer in a cold seal release film.
It should be understood that the particular cold seal adhesive utilized in this invention does not constitute a limitation on the broadest aspects of this invention. It is believed that most commercially available, well known cold seal adhesives can be employed in the present invention. For example, the cold seal adhesives specifically described in the various Wilkie patents that already have been incorporated herein by references are believed to be suitable for use in the instant invention. Stating this another way, applicant is not aware of any special properties that are required in cold seal adhesives to achieve the benefits of this invention, as already have been described in detail.
While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
