CS233594B1 - Processing of laminated packing material - Google Patents

Abstract

The invention relates to a method for producing laminated packaging materials by extrusion coating degraded pigmented polyolefins onto papers and cardboards. It applies a web of degraded polypropylene and low-density polyethylene with a flow index of 20 to 40 g/min. containing 3 to 20% by weight of heat-treated inorganic pigments, stabilizers and processing additives onto unbleached kraft papers and pigments. This achieves high whiteness and strength of the laminated packaging material.

Description

The invention relates to the production of laminated packaging materials by extrusion coating of polyolefin mixtures onto paper and cardboard.

The most common packaging means are paper and cardboard. Their advantages are mainly good mechanical properties and low cost, disadvantage on the contrary barrier properties, resistance to molds and the like. Also plastics have their advantages, which means mainly barrier properties, elasticity, heat weldability, but also shortcomings, such as poor printability, difficult bonding by gluing and higher cost. Thus, it is apparent that a combination of paper or cardboard and plastics can produce a packaging material of high utility properties. The most common combinations are plastic - paper and plastic - paper - metal foil combinations.

Frequently technology for manufacturing a laminated material is extrusion blow-molding, whose characteristic features are: a relatively high working temperature of the melt _and 320 ° C, high working speed (substrate moving speed of 600 m / min) and that the polymer melt is longitudinally stretched flow, a situation similar to that of synthetic fibers.

Nowadays, low density polyethylene (LDPE) is the most widespread for extrusion coating technology. In addition, polypropylene is used, which has the following advantages over polyethylene: higher rigidity, hardness, abrasion resistance of the film, better resistance to elevated temperatures, better resistance against fats and oils, especially at higher temperatures, better water vapor barrier, better gas and aroma barrier, lower bulk density

233 594 (compared to LDPE by about 2.5 '□) and hence the possibility to cover a proportionately larger area, better resistance to chemicals.

Disadvantages are: Improved printability, more demanding thermal welding conditions, which means higher temperatures and a narrower temperature range, more demanding extrusion technology and higher cost. »,

The production of laminated packaging materials is based on bleached papers and cartons on which unpigmented polyolefins with high flow index are applied by extrusion. mechanical strength than bleached paper and board, so thinner pads can be used. In addition, they are cheaper because bleaching technology is not required in their manufacture, a production operation that involves a large amount of waste water pollution. For environmental reasons, the production of bleached paper and cardboard is limited in the vast majority of production plants.

The object of the invention is to solve the production of laminated packaging materials from unbleached kraft paper and cardboard and polypropylene.

In order to use an unbleached backing, the polyolefin film must contain white pigments with sufficient opacity. However, inorganic pigments and fillers deteriorate the processing stability of the polyolefin melt, by which means the decomposition of polyolefins resulting in the formation of cracks in the coating film. Pigmented filled polyethylene for extrusion coating is known from the literature, however, this problem has not been solved with polypropylene yet due to failures occurring during extrusion coating of polypropylene, pulsation during stretching, which causes longitudinal and transverse unevenness of polymer deposition, melt tapering and mainly film tearing .

% with a flow index of 20 to 40 g / 10 minutes (ČSN 64 08 61, condition 12), containing 3 to 20% by weight of heat-treated inorganic pigments such as titanium dioxide, zinc oxide or calcium carbonate, phcaolin or

233 594 mixtures thereof, stabilizers and processing additives for unpacked kraft paper and cardboard.

In the process of the present invention, unbleached kraft papers and high strength cartons are used, so that even the laminated packaging material has higher strength, or the substrate weight can be reduced compared to bleached papers and cartons.

The use of filled and pigmented polypropylene improves the overall appearance of the refined papers and cartons, since the surface of the polyolefins is dull and gives a more natural impression. Increasing the surface polarity facilitates the use of adhesives for bonding laminated packaging materials and improving the weldability of polyolefins to other materials. A higher degree of corona discharge surface treatment is achieved.

Depending on the type of pigment used, the barrier properties of laminated packaging materials such as water vapor, oxygen and other gas permeability can be varied by increasing or decreasing the surface friction coefficient of the pigmented and filled polyolefins.

Degraded modified types of polypropylene are used to produce the laminated packaging materials of the present invention. The preparation of natural, i.e., unpigmented or unfilled types, is described in Czechoslovak and foreign patents, such as in AO 210 738, US 1 348 836, Brit. No. 992,388 or Japanese Patent 5,221,539.

Pigments and fillers are generally used in the form of a concentrate of these materials in low density polyethylene, which are in the form of a granulate or agromerate and usually contain 30 to 70% by weight of parts of the inorganic component.

Pigments or fillers may also be included in one or both of the ingredients used in the preparation of the polypropylene deposition type according to said patents.

The basic problem in extrusion application of pigmented or filled types of polypropylene is the possibility of working in such a range of temperatures and residence times of the material in the machine that does not yet deeply degrade the polymer (manifested by the formation of gaseous.

233 594 (degradation products and resulting cracks in the film) and at the same time the temperature of the deposited material is sufficient to perfectly anchor the deposited material to the substrate. This can be achieved by suitable composition of the stabilization system, heat treatment of pigments or fillers and, of course, a combination of both.

Useful means to increase the adhesion of the deposited polymer to the backing are also the use of a well adhering polymer to modify it, as described, for example, in Japanese Patent No. 5,238,539, or by treating the surface of a laminated paper or cardboard with a corona discharge or adhesive adhesive.

The components of the stabilizing system necessary to achieve the above high processing stability can be contained in the polymer, inorganic substance concentrate, or both. Stabilization systems for the extrusion coating of a mixture of polypropylene modified with an inorganic pigment or filler need to be stabilized with a synergistic mixture of phenolic stabilizers, phosphite stabilizers, epoxy stabilizers and secondary sulfur antioxidants.

The thermal treatment of pigments and fillers carried out just prior to mixing with the polypropylene avoids the unfavorable effects of the filler on the polypropylene film, in particular the formation of bubbles, which leads to tearing of the melt curtain. Particularly suitable pigments are titanium dioxide, zinc oxide, calcium carbonate and optionally magnesium, kaolin or mixtures of these fillers and pigments. These fillers should have an average particle size of less than 10 µm, preferably about 1 µm, and should be hydrophobized by known methods to achieve a good bond between the pigment particles and the polypropylene.

Suitable stabilizer system composition, heat treatment of pigments or fillers or combinations thereof can be obtained by materials which can then be extensively applied to paper or board at a substrate speed of up to 300 m / min and coatings from 20 g / m with good adhesion to the substrate It is free of cracks and defects and is uniformly longitudinally and transversely, achieving uniform dispersion of the pigment and / or filler of the polyolefin deposit, which results in uniform surface pigmentation.

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Example 1

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The polyolefin mixture was prepared according to Brit. U.S. Pat. 992 388.

80 wt. parts of polypropylene with IT 38 g / 10 minutes and 20 wt. parts of low-density polyethylene with IT 7 g / 10 minutes (ČSN 64 0861, condition 4). The resulting IT mixture was 33 g / 10 minutes (ČSN 64 0861, condition 12). The mixture was stabilized with 0.15 wt. parts of phenolic stabilizer (2,6-di-tert-butyl p-cresol) and additive with 0.1 wt. parts of calcium stearate.

To the granulate of the above mixture was added a granulated concentrate of TiO ₂ in low density polyethylene containing 60 wt. parts TiO ₂ 95 wt., parts granulate of polyolefin mixture and 5 wt. parts of the TiO ₂ concentrate were mixed in a cone mixer and placed in the extruder hopper.

A mixture of polyolefins and a pigment concentrate was extruded onto a strip of unbleached kraft paper having a basis weight of 60 g / m < 2 >

The melt application of this mixture was carried out on an extruder with a screw diameter of 114.4 mm and a screw length / diameter ratio = 28/1 at a melt temperature of 290 ° C, a slot width of 0.5 mm and an air gap of 12 cm.

The paper surface was pretreated with a 0.8 mA Quran discharge. The melt film is bonded to a web of paper in front of the laminating unit interface in which the paper is pressed at 400 kPa, then cooled by passing through a chill roll with a chrome shiny surface.

Under these conditions, cracks occurred in the melt film * and the material produced was virtually unusable,

Example _d 2

The mixture of polyolefins was prepared according to the art. AO 210 738. It contained 80 wt. parts of polypropylene and 20 wt. parts of low density polyethylene and its IT. was 35 g / 10 minutes (ČSN 64 0861, condition 12). The mixture was stabilized with 0.15 wt. parts by weight of phenolic stabilizer (2,6-di-tert-butyl p-cresol), 0.12 wt. parts of sulfur secondary antioxidants (distearylthiodipropionate) and 0.15 wt. % phosphite stabilizer (tetrakis- (2,4-di-tert-butylphenyl) -4,4'-diphenylylenediphosphonite) and additive 0.04 wt. parts of stearate

233 594 calcium. To the granulate of the above mixture was added a granulated concentrate of TiO ₂ in low density polyethylene containing 60 wt. parts of TiO which had previously been heat-treated with 95 wt. parts of the granulate of a mixture of polyolefins and 5 wt. parts of the TiO ₂ concentrate were mixed in a cone mixer and placed in the extruder hopper.

A mixture of polyolefins and a pigment concentrate was extruded onto a strip of unbleached kraft paper having a basis weight of 60 g / m 2, moving at a speed of 80 m / min.

The melt application of this mixture was carried out under the conditions of Example 1.

Under the given deposition conditions, an even deposit was achieved in the longitudinal and transverse directions. The coating had sufficient adhesion to the substrate and was free of cracks. The basis weight of the coating was 30 g / m. The pigment dispersion in the film was uniform without clumps.

Example 3

A mixture of polyolefins identical to Example 2 having the same composition of the stabilizing system was used. To the granulate of this mixture was added a granular ZnO concentrate in low density polyethylene containing 60 wt. parts of ZnO and granular concentrate CaC0 ₃ in a low density polyethylene containing 60 parts by weight CaCOg. Both inorganic concentrates were heat treated prior to mixing with the granulate with the polyolefin blend.

parts by weight of the polyolefin mixture granulate, 5 parts by weight of ZnO concentrate and 5 parts by weight of CaCO3 concentrate were mixed in a cone mixer and placed in the extruder hopper.

The above blend of granulates was extruded onto a strip of unbleached kraft paper having a basis weight of g / m < 2 >

The melt application of this mixture was carried out under the conditions of Example 1.

Under the given deposition conditions, an even deposit was achieved in the longitudinal and transverse directions. The coating had sufficient adhesion to the substrate and was free of cracks. The basis weight of the coating was 36 g / m @ 2.

233 594

Example 4

The procedure was analogous to Example 3, except that the TiO ₂ concentrate used in Example 2 was used. Prior to mixing with the polyolefin granulate, the concentrates of the inorganic compounds were heat treated.

The melt application of the granulate mixture was carried out under the conditions given in Example 1,

Under the given deposition conditions, an even deposit was achieved in the longitudinal and transverse directions. The coating had sufficient adhesion to the substrate and was free of cracks. The basis weight of the coating was 32 g / m ² .

Claims (1)

SUBJECT OF THE INVENTION 233 594 A process for the production of laminated packaging materials by extrusion deposition of degraded pigmented polyolefins on paper and cardboard, characterized in that a blend of degraded polypropylene and low density polyethylene having a flow index of 20 to 20 is applied. 40 g / 10 min containing 3 to 30 wt% of thermally treated inorganic pigments such as titanium dioxide, zinc oxide, calcium carbonate, kaolin or mixtures thereof, stabilizers and processing additives for unbleached kraft papers and cartons.