Multilayer plastic film

Field of invention

This invention relates to a multilayer plastic film intended particularly for high-speed welding or packaging of petty articles, such as toys, small parts, sweets, spare parts, handkerchiefs, hotel soaps and other means of personal hygiene.

State of the art

As regards construction and related welding or design of packages and packaging technology, the subject of known published solutions, particularly of patent specifications, are means of personal hygiene, such as napkins and sanitary menstrual napkins.

For instance, the European patent No. 0532649 describes an overall arrangement ("design") of a hygienic article (particularly of a sanitary menstrual napkin). As a packaging a siliconised release paper (paper coated with a layer of silicone) is used here. Neither the considerably complicated design of the hygienic mean nor the material composition of the packaging are suitable for high-speed welding or packaging.

Siliconisation of films or paper over their entire surface for hygienic means is the subject of the European patent No. 0639147 while a partial siliconisation of selected areas of the packaging film by printing is covered by another European patent, namely patent No. 1234564. However, both above inventions concern solely surface finish of the films for packaging of hygienic means. The aim is to achieve primarily a favourable peel effect during opening of the package before use of the hygienic mean (a film for an easy opening) but optimisation of properties of the packaging film with respect to requirements for high-speed packaging is not dealt with.

The technology according to European Patent application No. 1707338 could be considered to be a certain attempt to solve the above problem. The patent application covers blowing of coextruded polyolefin film and subsequent orientation of the film in two independent stages, the second orientation following immediately after the first one. According to the patent application at least the first orientation is advantageously biaxial and the film is heated to a temperature of 90 to 180 °C prior to the second orientation. Generally, also the orientation ratio is specified, namely by the range from 3:1 to 25:1 (preferably 7:1). However, composition of the coextruded film layers is characterised very generally only, without an attempt to achieve some more substantial optimisation in this direction. Consequently, not even the films produced by the above process have optimum properties

with respect to the requirements for the high-speed welding or packaging of small products. Practice shows that such optimisation must consists in selecting a suitable material composition of the film.

Nature of the invention

The multilayer plastic film according to this invention contributes to a considerable extent to elimination of the above shortcomings. The nature of the invention can be described as follows. The multilayer plastic film comprises an inner layer made of PE-LD/PE-HD blend with outer layers of identical composition based on a special bimodal PE-LLD and forming matt surfaces of non-plastic character being attached symmetrically onto both sides of the inner layer. Total thickness of the multilayer film is 0.015 to 0.045 mm and thickness ratio of the separate layers in the order outer - inner - outer layer ranges between 1:1:1 and 1:4:1.

The polymer compound of the inner layer contains advantageously 10 to 90 per cent by weight of PE-LD and 90 to 10 per cent by weight of PE-HD. Further, it can contain PE- LLD and/or granulate and chiefly colourants, fillers, UV stabilizers and/or flow modifiers as additives.

The bimodal PE-LLD of the outer layers possesses a very broad molecular weight distribution (MWD) ranging from 15 to 20 (compared to normal monomodal PE-LLD, MWD of which ranges from 4 to 6). The material of the outer layers can also contain up to 10 per cent by weight of polymer components of a different composition.

The bimodal PE-LLD of the outer layers forms a migration barrier for the additives contained in the inner layer.

At least one of the outer layers of the multilayer plastic film can be provided with a coating of an antiadhesive agent - notably by a layer of polyamide or silicone - applied onto entire surface of the film or locally onto selected areas.

The main advantage of the multilayer plastic film according to the invention consists in the fact that thanks to its general construction - a combination of layers of a defined material composition - the film is optimised for high-speed welding or packaging of petty articles such as toys, small parts, sweets, spare parts, handkerchiefs, hotel soaps and other means of personal hygiene (napkins, sanitary menstrual napkins /menstrual pads/, etc.).

Invention embodiments

Example 1

The multilayer plastic film in the exemplary embodiment of the invention comprised 3 layers having a total thickness of 0.030 mm and thickness ratio of separate layers in the order outer - inner - outer of 1 :2: 1.

The inner layer was made of a blend comprising 75 per cent by weight of PE-LD and 25 per cent by weight of PE-HD. The melt flow index (melt flow rate /MFR/) according to standard ISO 1133 (at the temperature of 190 °C and weight of 2.16 kg) of the PE-LD used was 2 grams/ 10 minutes and density (according to ISO 1183-2) 918 kg/m ³ . The melt flow index (melt flow rate) of the PE-HD was 0.2 g/10 min. and density 936 kg/m ³ .

The outer layers were made of bimodal PE-LLD having molecular weight distribution (MWD) ranging from 15 to 20 (Note: molecular weight distribution of a standard - monomodal - PE-LLD ranges from 4 to 6).

Melt flow rate (MFR) of the bimodal PE-LLD used in this specific case was 0.2 and density 927 kg/m ³ (measured by the same methods as in case of the materials of the inner layer).

The film of the above construction comprising separate layers of the above material composition of was suitable for welding process running at the speed of up to 400 cycles/min.

Example 2

Construction, total thickness of the film as well as thickness ratio of the film separate layers was identical with those given in Example 1. Also the material of the outer layers was the same.

The inner layer was made of a blend comprising 50 per cent by weight of PE-LD and 50 per cent by weight of PE-HD with the characteristics (MFR and density) of these polymer components being again identical with those given in the Example 1.

The film of the above construction and material composition was suitable for welding process running at the speed of up to 1000 cycles/min.

Example 3

The construction and material composition of the outer layers of the multilayer plastic film were identical with those given in the Example 1.

10 per cent by weight (related to the total weight of the 75:25 PE-LD/PE-HD blend) of a granulate prepared by regranulation of film trimmings, scrap resulting from changes of the technological process and from cutting out of products during manufacture and conversion of the film was added to the inner layer.

The film of the above construction and material composition was suitable for welding process running at the speed of up to 400 cycles/min.

Example 4

The construction and material composition of the outer layers of the multilayer plastic film were identical with those given in the Example 2.

15 per cent by weight (related to the total weight of the 50:50 PE-LD/PE-HD blend) of a colour concentrate containing 50 per cent of a colorant was added to the inner layer.

The film of the above construction and material composition was suitable for welding process running at the speed of up to 1000 cycles/min.

Example 5

The construction and material composition of the outer layers of the multilayer plastic film were identical with those given in the Example 2.

10 per cent by weight (related to the total weight of the 50:50 PE-LD/PE-HD blend) of a granulate identical with that used in the Example 3 and 14.2 per cent by weight (related to the total weight of the blend including the regranulate) of the colour concentrate containing 50 per cent of a colourant was added to the inner layer.

The film of the above construction and material composition was suitable for the welding process running at the speed of up to 1000 cycles/min.

Example 6

The construction and material composition of the separate layers of the multilayer plastic film were identical with those of some of the Examples 1 to 5.

A polyamide layer was applied by lamination process onto the film surface that forms the inner side of the packaging during welding. The polyamide surface has antiadhesive properties, which means that for instance products of ladies' hygiene provided with an adhesive layer on one side, packed in this film during high-speed welding can be taken out of the packaging (container) without causing the adhesive layer to adhere onto the inner side of the packaging (container).

The film of the above construction and material composition was suitable for welding process running at the speed of up to 400 or 1000 cycles/min., depending on the composition of the inner layer (Examples 1 to 5), regardless the surface layer.

Example 7

The construction and material composition of the separate layers of the multilayer plastic film were identical with those of some of the Examples 1 to 5.

A polyamide layer was applied by co-extrusion process onto the film surface that forms the inner side of the packaging during welding. The polyamide surface has antiadhesive properties, which means that for instance products for ladies' hygiene provided with an adhesive layer on one side, packed in this film during high-speed welding can be taken out of the packaging (container) without causing the adhesive layer to adhere onto the inner side of the packaging (container).

The film of the above construction and material composition was suitable for welding process running at the speed of up to 400 or 1000 cycles/min., depending on the composition of the inner layer (Examples 1 to 5), regardless the surface layer.

Example 8

The construction and material composition of the separate layers of the multilayer plastic film were identical with those of some of the Examples 1 to 5.

A silicone layer was applied locally by flexo printing process according to a specified pattern onto surface of the film that forms the inner side of the hygienic means packaging (container) during welding. The silicone coating was applied on places in which the packaging (container) will be in contact with the adhesive layer of the hygienic means. This will make possible for the products provided with the adhesive layer and packed in this film during the high-speed welding to be taken out of the packaging (container) without causing the hygienic means to adhere onto the packaging (container).

The film of the above construction and material composition was suitable for welding process running at the speed of up to 400 or 1000 cycles/min., depending on the composition of the inner layer (Examples 1 to 5), regardless the surface layer.

Example 9 (comparing)

The multilayer plastic film in the exemplary embodiment of the invention comprised 3 layers having a total thickness of 0.030 mm and thickness ratio of separate layers in the order outer - inner - outer of 1 :2: 1.

The outer layers were made (similarly as in the preceding Examples) of bimodal PE- LLD having molecular weight distribution (MWD) ranging from 15 to 20.

But the inner layer was made of PE-LD alone having melt flow index (melt flow rate /MFR/) according to ISO 1133 (at the temperature of 190 ⁰ C and weight of 2.16 kg) 2 grams/10 minutes and density (according to ISO 1183-2) 918 kg/m ³ .

The film was suitable for welding processing up to only 200 cycles/min.