WO2000039200A1 - Easy-to-peel heat-sealing multilayer film from coextruded, biaxially oriented polypropylene - Google Patents

Abstract

The invention relates to a coextruded BOPP film with easy-to-peel sealing properties which is printed in a non-inked sealing zone with a thermoplastic lacquer having a screen-type pattern. Said lacquer contains at least one polyvinyl butyral (PVB) or at least one ethylene-vinyl acetate copolymer (EVA copo) as a feature-specific solid component. Said lacquer is only weakly sealable against coextruded BOPP. The spacing effect of said lacquer gradually decreases as the softening increases with rising temperatures. The invention further relates to packagings produced from said film.

Description

Heat-sealable multilayer film made of coextruded, biaxially oriented polypropylene with peelable opening behavior

The present invention relates to biaxially oriented, heat sealable, coextruded

Polypropylene films, which are equipped with printing on the inside and outside sealing to achieve a tear-free (peelable) longitudinal seam, and their use for the production of packaging.

Heat-sealable packaging foils and their use for full wrapping with frontal folds and overlapping longitudinal seams (FIG. 2) have long been known. Unfinished raw foils have also been known for a long time, their overlapping sealed longitudinal seams can be peeled open with the appropriate setting of the sealing temperature, the sealing pressure and the sealing time. Examples include cellulose films coated with nitrocellulose and acrylic-coated, biaxially oriented

Polypropylene film (BOPP). In this regard, coextruded heat-sealable BOPP films, which are preferably used because of lower packaging costs, fail because of too tight a seal or a too small temperature range with a sufficiently low seal strength for such applications.

Through cold seal equipment (KS) for the purpose of high packing speeds on horizontally working form-fill-u. Closing machines can be made from peelable packaging that can be peeled open using BOPP films. However, the full-wrap packaging described here with copper fold and longitudinal seam sealed on the outside and on the inside, according to FIG. 2, eludes this KS technology, since KS compounds can only be sealed against themselves and are therefore sealed only for the inside and the inside Longitudinal and transverse seams of tubular bag packs come into consideration.

Furthermore, tear strips offer a possibility to alleviate the deficit of the difficult opening of packages made of coextruded BOPP films and to weaken an already industrially practiced application, as described in EP 0 577 509 B1 the sealing strength of the raw film by grid-like, seal-resistant printing of the sealing zones is so unreliable that only the additional use of a tear strip leads to the desired opening behavior with complete exposure of the packaged goods. This applies in particular when finished packs are subjected to post-heat treatment to improve the pack appearance, and as a result the outstanding tab of the tear strip connects to the wrapping material in an uncontrolled manner and loses its function as an opening aid.

The possibility of using heat-resistant lacquers or printing inks to remove the heat-sealability of sealable raw foils or to reduce them in the case of partial coverage is known, but in practice it is subject to large, unacceptable fluctuations, which depend on the natural tolerances of the raw foil properties, the printing conditions and the application weights are dependent. The effect of such seal-resistant systems based on less softened are determined

Nitrocellulose or two-component polyurethanes due to their permanent spacer function, even under the influence of heat, as a principally desired reduction in sealing contact (area and pressure) of exposed co-extruded BOPP surfaces. In addition, there is a risk here that cannot be avoided with certainty in gravure printing that the desired residual seal strength of the raw film is negated by so-called toning (application of color to undesired areas).

It was therefore the task of providing inexpensive, coextruded BOPP films which, due to the manufacturing process, have unsuitably strong sealing properties, in the course of the draining process without the disadvantages mentioned so reliably to provide the peelable sealing properties so that packaging, preferably full-wrap packaging, produced therefrom as shown in FIG. 2, can be opened with sufficient ease and without cracks and without any tear strips, and that with technically unavoidable fluctuations in raw materials, as well as with variations in the pressure conditions, sealing temperatures and application weights over a wide tolerance range

The seal strength remains largely unchanged. This was achieved according to the invention in that commercially available, coextruded BOPP films were printed in a grid-like manner with thermoplastic lacquer, which itself was only slightly weak against coextruded, by means of repeat controlled gravure printing, preferably on the electrically pretreated side in a color-free sealing area

BOPP is sealable, contains at least one polybutyral (PVB) or at least one ethylene-vinyl acetate copolymer (EVA-Copo) as a feature-determining component. This special selection of the lacquer causes the spacing effect to decrease successively as the temperature increases, and the coextruded BOPP surfaces become increasingly in

Seals contact.

The design of the grid used in printing allows the sealing strength and peeling behavior to be influenced in a targeted manner when sealing inside against outside (a / b sealing).

Table 3 shows particularly suitable grids. It is possible both to carry out the grating uniformly over the length and width of the sealing area and to make the grating variable over the length and / or width of the sealing area within the possibilities of the engraving technique in order to meet special requirements with regard to seam strength and / or to meet peel behavior. According to the grid described, the lacquer application is carried out over part of the area and preferably with a full-tone engraving depth.

The paint used is a commercially available paint / varnish system with

Polyvinyl butyrate (PVB), as is commonly used for counterprinting on coextruded BOPP surfaces (e.g. 15-020613-4, MX 31 series, Siegwerk), or ethylene-vinyl acetate copolymers (EVA copo, e.g. 10- 612764-0 W, Siegwerk) as a feature-determining solid component. Mixtures of several PVB lacquers or several EVA copo lacquers are also possible. The application amount (dry) of varnish for PVB varnishes is preferably 0J to 1.7 g / m ² , in particular 0.8 to 1.2 g / m ² . In the case of EVA copo paints, application amounts (dry) of 0.1 to 2.5 g / m ² , in particular 1.0 to 1.5 g / m ^{2, are} preferred.

The film according to the invention is preferably a coextruded film

BOPP film that was printed outside the color-free sealing areas on the electrically pretreated side using raport-controlled gravure printing.

The printing equipment according to the invention makes it possible to set coextruded BOPP films which, as raw films, have too high seal strengths with tear separation images, so that they can be sealed more weakly and peelably.

It was neither to be expected that an ethanol-thinnable PVB lacquer printed in accordance with the invention would seal sufficiently firmly against untreated co-extruded BOPP, nor that when using a water-thinnable EVA-Copo lacquer, no undesirable tear in the material from the remaining lacquer-free sealing points on the co-extruded surface despite sufficiently firm sealing would go out when the seam was separated, but that, surprisingly, crack-free peeling occurs in both cases and tear strips are unnecessary as an opening aid.

The film according to the invention is preferably produced by multi-use printing of commercially available, largely standardized, coextruded BOPP films in rotogravure or flexographic printing, preferably on conventional multicolor gravure printing machines with register control using engraved cylinders with a cup shape, with frontal or co-extruded suitable for BOPP Counter printing inks can be used in combination with one of the lacquers according to the invention in the color-free range. Finally, a customer-specific final cut is made on commercially available roll cutting machines with side edge control. The arrangement of the color-free areas depends on the shape of the packaging which is to be produced from the coextruded BOPP film. Color-free are preferred

Areas in accordance with FIG. 1 as endless bands on both edges of the roll in whose running direction or as transverse bands are arranged at regular intervals across the running direction of the roll.

The invention further relates to packagings produced from the films according to the invention, in particular full wrapping packagings, which have no additional tear-open aid and can be opened easily and without cracks.

The following examples are intended to explain the invention in more detail.

The relevant properties of the films according to the invention corresponding to the intended use are determined by testing the low-pressure seal strength (NDS) over a film-compatible temperature range (sealing curve) in order to assess their suitability for wrapping packages.

The force in N, based on the test strip width of 15 mm, is determined as the low-pressure seal strength, which is necessary for the splitting separation of a seal seam produced under defined conditions (pressure, temperature, time).

Sealing conditions: pressure 0J5 N / cm ² , time 0.5 sec, temperature 105 to 150 ° C, five

Kelvin graduated wisely.

Measuring devices: Low pressure sealing device from Brugger, with sealing jaws 20 * 50 mm ² , finished, 5 different temperatures. 5 counterweights with 25 * 30 mm ² contact surface, which are covered with 2 to 3 mm thick felt.

Test strip cutter with a cutting width of 15 mm. Tensile testing machine with measuring range ION and pulling speed 100 mm / min.

Qualitative assessment: In addition to a value level> 0.5 N / 15 mm, which is to be regarded as acceptable for wrapping practice, as a necessary condition, the crack-free, peeling separation according to the invention was included in the evaluation as a sufficient condition and the temperature range with the desired sealing behavior was determined (Table 1). It can be seen that seal values> 1.5 N / 15 mm lead to undesirable material tear when the seam is separated.

example 1

Through multicolored counter-gravure printing on the electrically pretreated, later packaging inside of a commercially available 20μ thick Coex-BOPP film (Walothen® C20EHS from Wolff Walsrode AG), with widely used ethanol-thinnable polyvinyl butyral paints (PVB) in the decor area and polyvinyl butyral varnish (15- 020613-4, series MX 31 from Siegwerk Druckfarben GmbH & Co. KG) in the color-free longitudinal seam area were NDS values with various conventional engraving screens 60/1, 70/1, 70/4 and 80/1 (Table 2) , Outside to inside, between 0.5 and 1.5 N / 15 mm from 125 ° C, achieved with crack-free splitting.

Depending on the grid, the applicable temperature range is of different widths and, for the shapes to be favored (60/1 and 70/4), requires precise temperature control of the sealing tools in the packaging operation (Table 1).

Example 2

By replacing the PVB varnish used in Example 1 with a commercially available water-thinnable sealing wax based on an aqueous dispersion of saponified synthetic resins (ethylene-vinyl acetate copolymer) in the form of the sealing wax 10-612764-OW from Siegwerk Druckfarben GmbH & Co KG, with designated

Sealability against untreated Coex-BOPP, the desired NDS values between 0.5 and 1.5 N / 15 mm and peeling separation behavior could be achieved over a wide temperature range (Table 1).

Comparative Example 1

Compared to Examples 1 and 2, the NDS (a / a) of the untreated, unprinted raw film with values = 2.5 N / 15 mm is too high and seam separation is not possible without cracks (Table 1). Comparative Example 2

The NDS (a / b) of the untreated against the pretreated side of the unprinted raw film also shows no usable course over temperature. Although, as expected, the values are lower than for the NDS (a / a) in

Comparative Example 1, the temperature range with sealing strengths of 0.5 to 1.5 N / 15 mm, which require crack-free separation, is too narrow (Table 1).

Comparative Example 3

As an example of the prior art, multicolored counter-rotogravure printing on a film as described in Example 1, but 25 μm thick, with application-dependent different color systems, such as ethanol-thinnable polyvinyl butyral-based ground-color ink, ethanol-ethyl acetate-dilutable NC-based color inks, ethyl acetate-thinnable two-component ink NC ester white color in

Decor area and two-component PU lacquer (seal-resistant) as a grid (Table 3, Y) in the color-free longitudinal seam zone often but not reliably achieved the desired low NDS values. In some cases, the sealing was not sufficient in the particularly practical temperature range of 125 to 135 ° C (Table 1). This result reflects the unsatisfactory state of the art.

Table 1: Low pressure sealing results from examples

a / a = unprinted film side sealed against itself (blank sample) ab = unprinted against film page equipped according to the invention (typical sealing for wrapping packaging) from before (el, pretreated), without the influence of printing ink

Table 2: Marking of the printing cylinder used (engraving with helioclischograph, Hell)

Table 3: Explanation of the squares in Table 1, Example 2 and Comparative Example 3, as well as the differentiation according to areas X, Y and Z, with a uniform engraving grid 70/0.

X Y Z

6: 1 6: 1 10: 1 enlarged

Dimensions in mm, dark areas are printed

Claims

claims 1. Co-extruded BOPP film with peelable sealing properties, characterized in that the film is printed in a grid-like manner in a color-free sealing area with a thermoplastic lacquer which has at least one polyvinyl butyral (PVB) or at least one ethylene-vinyl acetate copolymer (EVA copo ) as a feature-determining solid component and is only weakly sealable against coextruded BOPP. 2. Films according to claim 1, characterized in that the lacquer used is only weakly sealable against untreated co-extruded BOPP surfaces and contains polyvinyl butyral (PVB) as the determining solid component. 3. A film according to claim 1 or 2, characterized in that a mixture of commercially available PVB lacquers is used as the lacquer, as are widely used for counterprinting on coextruded BOPP. 4. Film according to one of claims 1 to 3, characterized in that the lacquer with an application amount of 0.1 to 1.7 g / m ² , preferably from 0.8 to 1, 2 g / m ^{2 is} applied dry. 5. Film according to one of claims 1 to 4, characterized in that the lacquer is defined over part of the surface, with full-tone engraving depth. 6. Film according to claim 1, characterized in that the lacquer used is thermoplastic and sealable against untreated Coex-BOP surfaces and preferably contains saponified synthetic resin (EVA-Copo) as a feature-determining solid component. 7. A film according to claim 1 or 6, characterized in that the lacquer is applied dry with an application amount of 0.1 to 2.5 g / m ² , preferably 1.0 to 1.5 g / m ² . 8. Film according to one of claims 1, 6 or 7, characterized in that the Lacquer defines partial areas, is executed with full-tone engraving depth. 9. Film according to one of claims 1 to 8, characterized in that the grid for the application of the lacquer over the length and width of the sealing area is designed uniformly or variably in order to meet different seam strength requirements. 10. Film according to one of claims 1 to 9, characterized in that the film is printed outside of the sealing area on the electrically pretreated side by means of repeat-controlled gravure printing. 11. A film according to any one of claims 1 to 10, characterized in that the color-free sealing areas provided with the lacquer are designed as endless bands at the edge of the film roll or as transverse bands at uniform intervals transversely to the running direction of the film roll. 12. Packaging, produced from a film according to one of claims 1 to 11.