Thermoformable foamed plastic sheet material for making open containers

The invention relates to the field of the packaging industry and relates to a thermoformed foam sheet material according to the generic part of the first independent claim, the sheet material comprising or comprising at least one layer of a thermoformed and expandable foam material.

Open containers, such as trays or bowls often used for packaging and / or for consuming foodstuffs, are typically made by drawing or deep drawing a piece of flat sheet material made of thermoformable plastic. Such open containers have a bottom wall and a side wall or walls. The sheet material is typically thermoformed

185 847 between a pair of forming tools, i.e., between a nesting tool with a cavity against which the heated sheet material is placed, and a male tool moving into the cavity of the nesting tool to pull the sheet material therethrough.

The use of a sheet material composed of a foamed material or having at least one layer of foam material is advantageous in the manufacture of such containers, since a foam sheet or a container made of such a sheet is usually stiffer than a corresponding solid material with the same surface weight. The sheet material and containers composed at least in part of the foam material are opaque. These containers must have a certain minimum mechanical strength appropriate to their use, and the better they are accepted, the more uniform their opacity and the smoother and shinier their surface.

A further advantage of producing open containers from sheet material that is composed of or includes at least one layer of foam material is that certain portions of the walls of the container can be expanded by applying reduced pressure to the foam while it is in a thermoformable state. whereby the gas trapped in the foam is expanded, and by cooling the foam while it is still under reduced pressure. Such processes are described e.g. in US-3846526 or JP-60192615. In addition, a method and apparatus for drawing and expanding foam sheet material to form open containers is described in a co-pending patent application (with the same filing date as the present application).

According to this co-pending application, the containers are shaped by a plug tool engaging a cavity of the nesting tool to thereby pull the heated sheet material disposed over the cavity of the nesting tool. During this movement of the plug-in tool, the pressure on at least one side of this sheet material is reduced so that it reaches a value (expansion pressure) below the ambient pressure that is low enough to expand the foam material when the plug-in tool reaches its end position or immediately thereafter. . The tools are then held in the end position and the expansion pressure is maintained until the sheet material is cooled to a temperature below its plasticizing temperature. The shaped container is then removed from the mold. Before the moving plug-in tool comes into contact with the heated sheet material, the material may be pre-stretched by applying a pressure difference (low pressure on the socket tool side) therein. Such pre-stretching is especially advantageous when producing containers by deep drawing (draw ratio> 1) and / or difficult-shaped containers (non-round bottom wall, steep side walls).

By applying the method described in the last paragraph and using tools which are designed to remove air between the sheet material and the tool surface, a pattern of holes on at least these parts of its surface, which are to form the side walls of the container (as described in a subsequent co-pending patent application with the same filing date as the present application), it is possible to manufacture in a minimum cycle time from sheet material at least partially composed of foam material open containers with wall thickness and wall density, which are mainly determined by the mechanical properties the container is intended to possess. This means that the wall of the container may have a varying thickness, in particular a varying density, depending on the degree of drawing and the expansion.

Using known sheet materials composed of or containing e.g. polystyrene or polypropylene in expanded form for the production of open containers according to the above-described process, problems may be encountered in that the produced containers have areas of variable draw and / or expansion that are visible or even cause mechanical defects, which happens especially when the cycle time is short and / or the draw ratio is large or the shape of the container is difficult, such as e.g. for containers with a non-circular bottom wall and / or very steep side walls.

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It is therefore an object of the invention to provide a sheet material composed of or containing at least one layer of foam material, the sheet material having good draw properties and an opacity that varies little with density. so that this sheet material can be processed using the method of the simultaneously pending patent application, in particular with very short cycle times and with high draw ratios and with difficult container shapes, obtaining containers which are of good mechanical and aesthetic quality. Moreover, both matter! sheet material and the containers made of this sheet material are easily recyclable and are compatible with foodstuffs as widely as possible (solid and liquid, hot and cold, etc.).

This object is achieved by a sheet material according to the claims.

In order to fulfill the condition of recyclability and food compatibility, the sheet material according to the invention is mainly made of polypropylene. This means that it is made entirely of polypropylene foam or comprises at least one layer of polypropylene foam and an additional layer or further layers, for example solid polypropylene or other thermoformed materials, e.g. barrier materials.

The opacity of the foamed plastic is caused by the light scattering on the pore walls. The smaller the pores in the walls, the more times the light falling on the material is scattered and the more opaque the material has. It has been found that the smaller the pores of the walls, the less the appearance of the material changes when it is subjected to drawing and / or expansion to a fairly large extent. It has also been found that the smaller the pores in the walls, the better the draw properties of the foam. This is because the presence of pores that are large enough to locally alter the ductility of the foam significantly or to be ripped open by drawing becomes more likely as the pore size increases, that is, as the pore scattering rate decreases. in the wall structure.

A measure of the degree of pore dispersion in a foam plastic wall structure is the number of pores per cubic millimeter. Sheet materials with polypropylene foams with a pore number greater than 300 pores per cubic millimeter and processed by the process of the co-pending patent application have been found to provide good quality containers with a relatively low draw ratio (container depth divided by opening diameter: in the range of 0.5- 1, depending on the shape of the container and the expansion factor). Sheet material with polypropylene foam with a pore number of up to 3000 pores per cubic millimeter produces good quality containers with draw ratios of 0.8-1.5 (depending on container shape and expansion ratio). In the manufacture of containers such as cups and trays for packaging and / or drawing foodstuffs as described in co-pending patent application, a polypropylene foam sheet material with a pore number of 800-2500 pores per cubic millimeter, preferably 1000-2000 pores per cubic millimeter. cubic millimeter is very suitable.

The polypropylene foam of the sheet material according to the invention has a density of 0.1-0.7 g / cm³preferably 0.25-0.5 g / cm³, even more preferably 0.36-0.50 g / cm³. The sheet material has a thickness of 0.5-3 mm, preferably 1-2 mm, and even more preferably 1.2-1.6 mm.

If, depending on the density of the polypropylene foam and the thickness of the sheet material produced, a polypropylene blend containing 50-100% polypropylene with a high melt resistance of about 24 cN is used, then it is possible to extrude a sheet material with the above-mentioned properties without any problems. Highly melt resistant polypropylene (long chain branched polypropylene) is commercially available as a homopolymer or as a copolymer.

The greater the melt resistance of a polypropylene blend (i.e., the greater the content of high melt resistance polypropylene), the easier it is to produce a good quality sheet material with the desired number of pores. In addition, since the high percentage of polypropylene with high melt resistance is not the only thing

185 847 has an economic disadvantage, but also causes the containers to have rather brittle walls, it is preferable to reduce the percentage of high melt resistance polypropylene to 50% if possible by adding other types of polypropylene (homo or copolymers) to thereby reduce the melt resistance of this mixture even up to 15 cN, preferably up to 18-20 cN.

A sheet material suitable for use in the thermoforming method of co-pending patent application for the manufacture of cups and trays for packaging and / or consumption of food, which cups and trays have a mechanical stability suitable for the purpose, preferably consisting of or including foam polypropylene containing 60-90%, preferably 60-80% polypropylene with high resistance to melting.

For the production by extrusion of a sheet material composed of or containing polypropylene foam, a solid or gaseous blowing agent is added to the polymer blend in a manner known per se. In addition, other known extrusion-improving additives may be added in known concentrations, as well as e.g. pigments for coloring the foam material.

The number of pores of the extruded foam material is dependent not only on the composition of the extruded polymer and the additives used, but also on the extrusion parameters. For this reason, appropriate extrusion parameters must be experimentally determined to produce a polypropylene foam sheet material having the composition given above and having a pore number within a certain range.

Example:

Sheet material:

polypropylene mixture: 80% polypropylene with high resistance to melting,

20% polypropylene copolymer additives: 1.1% blowing agent foam thickness: 1.4 mm density: 0.43 g / cm³ number of pores: 1450 pores per mm3 pre-stretching thermoforming process: cavity width: 1.2 mm sheet temperature:> 160 ° C expansion pressure:> 0.2 bar (absolute) i.e. <0.2 • 10⁵Pa shaping time (pre-stretching, drawing and expansion): 1.8 -s container made of this sheet material:

round cup: hole diameter: 75mm depth: 80mm wall thickness: 1-1.2mm

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