FR2811301A1 - Composite sheet for thermoforming into pots for yogurt, etc., has expanded polystyrene layer forming majority of sheet sandwiched between two thin dense layers of polystyrene - Google Patents

Abstract

A composite sheet has an expanded polystyrene layer forming the majority of the sheet sandwiched between two thin dense layers of polystyrene. Independent claims are included for the following: (a) use of the above sheet to form a pot; (b) forming the above sheet by producing a lightened layer of polystyrene using an expansion agent, controlling the morphology of the layer and then co-extruding it between two dense layers of polystyrene. A preprinted polystyrene film is laminated to one or both its external faces and the sheet is then sterilized with hydrogen peroxide.. The sheet is thermoformed to make a container.

Description

1 2811301

  The present invention relates to a polystyrene composite sheet for the manufacture of packaging or containers

  thermoformed, as well as its use in the agro-

  food. Food containers of the yoghurt pot type are conventionally made up of a container of approximately cylindrical shape surmounted by edges projecting from

  which is applied after filling the pot with a sealing cap.

  They are generally based on a compact polymer

  thermoplastic used in one or more layers.

  Current environmental legislation, particularly in Germany, imposes taxes on packaging proportional to the weight of the latter. It is therefore essential for a manufacturer to limit the weight of its packaging to a minimum in order to pay the most taxes.

possible.

  To reduce the weight of packaging and containers, the immediate solution is to reduce the amount of material that composes them. This reduction in material will lead, for example, to a reduction in the thickness of the walls of the containers. However, it should not be done at the expense of reducing resistance, especially to

2 2811301

  compression. In fact, the containers, for their storage or transport, are generally stacked, so they must not collapse or

  crash, especially when full.

  Therefore, one of the aims of the present invention is to provide a lighter material than the materials used for current packaging or containers, without reducing their strength.

  mechanical, especially compression.

  Another object of the invention is to provide a material which allows the printing of information or decorative patterns, this printing having to be stable, and in particular to resist friction and

scratched.

  Another object of the invention is to provide a material which can be thermoformed like conventional containers of the yogurt pot type, on the same machines, for example those commonly used and designated under the name "Form-Fill-Seal">, which allow, according to a continuous operation, from the constituents of the container in the form of sheets, to thermoform the container from said sheets,

fill it in and seal it.

  In the food industry, the sheets of material constituting the container are, before the thermoforming step, optionally passed through a bath of hydrogen peroxide (H 2 O 2) in

3 2811301

  view of their sterilization. To prevent drops from this sterilization bath from "hanging" on the surface of the polymer material, it

  it is essential that this surface be as smooth as possible.

  All these aims are achieved by a three-layer composite sheet of polystyrene, for the manufacture of packaging or thermoformed containers, according to the present invention, in that the sheet comprises as predominant constituent a layer of expanded polystyrene with a blowing agent, covered, on each of its faces, with a layer

  of compact polystyrene of smaller thickness.

  Such a structure, mainly consisting of an expanded layer makes it possible to reduce the weight of packaging or containers from 15 to%, making it possible to reduce all the taxes proportional to the weight

of these.

  The mechanical resistance of such a composite material is equivalent to that of the conventional compact material, for a thickness

almost identical total.

  The three-layer polystyrene composite sheet according to the invention

  allows thermoforming on usual machines.

  According to a preferred embodiment of the composite sheet, the expanded polystyrene layer is formed from a mixture of impact polystyrene and crystal polystyrene, which makes it possible to combine the advantages of a

4 2811301

  rigid and brittle polymer (crystal polystyrene or GPPS: general purpose polystyrene) and those of an impact-resistant polymer, but

  soft (impact polystyrene or HIPS: high impact polystyrene).

  The preferred proportions are approximately 30 to 70% of impact polystyrene and 70 to 30% of crystal polystyrene, for the layer of expanded polystyrene. The presence of impact polystyrene overcomes the weakening of the sheet which may result from the expansion of the central layer. Advantageously, the blowing agent for the central polystyrene layer is used in an amount approximately between 0.5 and 2% by weight per 100% of polystyrene. This blowing agent allows the creation of "air bubbles" within the polystyrene layer, caused by the degradation of an additive during extrusion. Additives or blowing agents can be based on exo or endothermic reactions. The preferred blowing agent here is a combination of sodium hydrogen carbonate and an organic acid (e.g. citric acid) or an acid salt or an ester. The various additives are optionally formulated with a polymer or

  a wax and used in the form of a granular mixture at 0.2-2% approximately.

  The two outer layers of compact polystyrene which "sandwich" the expanded central layer are preferably

2811301

  consisting of approximately 60% by weight of impact polystyrene and approximately 40

% by weight of crystal polystyrene.

  The composite sheet according to the invention can also be covered by lamination with a printed polystyrene film, the printed face of which faces one of the layers of compact polystyrene. The print (or the decor) is thus protected. This laminated film also makes it possible to make the outer surface of the composite sheet even smoother, which is advantageous in the event of sterilization of said sheet in a

hydrogen peroxide solution.

  The composite sheet according to the present invention is used for the manufacture of pots or containers intended for the food industry, for example pots for yogurts, desserts, creams, etc. These thermoformed containers or packaging are produced according to a process comprising the following successive stages: a) manufacture of a layer of light polystyrene in the presence of an expanding agent, b) control of the morphology of the sheet, c) coextrusion of the layer of light polystyrene and, on each of its faces, a layer of compact polystyrene to form a three-layer composite sheet,

6 2811301

  d) application by lamination of a pre-printed polystyrene film on an outer face of the three-layer composite sheet, e) optional sterilization by passing said sheet through a bath of hydrogen peroxide, f) thermoforming of the three-layer composite sheet covered

of the film.

  Preferably the application of the pre-printed film by lamination (step c) is carried out cold, that is to say, without heating the film and the rolls of the calender which are, in operation, at a temperature not exceeding 65 C approx. However, to facilitate the adhesion of the film to the composite sheet, the latter is at a more

  high (for example around 200 to 240 C).

  It appears that the composite sheet according to the invention, comprising an expanded central layer, exhibits a markedly less shrinkage, that is to say a less significant orientation than a sheet of compact polystyrene, at the outlet of the die. 'extrusion, when pulling on the sheet at the calender. Indeed, expansion occurs in three dimensions. This results in a lesser orientation of the macromolecules in the sheet under the effect of the traction exerted by the calender, and a lesser longitudinal orientation of the sheet between the

7 2811301

  die and grille. This less longitudinal orientation (shrinkage of 2 to 3% instead of 10%) therefore leads to a larger useful surface of the sheet. The composite sheet can also be covered by laminating a second film of polystyrene (printed or not) applied to the

  second layer of compact polystyrene.

  The present invention will be described in more detail with the aid of the following illustrative diagrams: FIG. 1 is a section view (enlarged) of a composite sheet according to the present invention; Figure 2 is a diagram illustrating the manufacture of the sheet

  composite according to the present invention.

  An example of a composite sheet (1) is shown in Figure 1.

  It consists here of a central layer (2) of expanded polystyrene.

  This expanded layer was produced by a mixture of impact polystyrene 40% and crystal polystyrene (60%) in the presence of I% (by weight of polystyrene) of expansion agent, consisting of citric acid and hydrogen sodium carbonate, which gives rise by degrading, during extrusion, air bubbles (5) distributed throughout the volume of this layer. In order to control the size of the bubbles (5) and their dispersion, nucleating agents (in the form of charges

8 2811301

  mineral, waxes, polymers ...) can be introduced beforehand

in the mixture.

  This lightened central layer (2) is sandwiched between two compact layers (3 and 4) of polystyrene (PS) formed from 60% impact PS and 40% crystal PS, to form a composite sheet (1)

three-layer according to the invention.

  This composite sheet (1) is covered with a practically transparent film (6), which may have, on its face (6 ′) facing the compact layer (3) a print (information or decoration). The z0 three-layer sheet according to the invention is significantly lighter than a conventional compact sheet (generally consisting of two layers of compact polystyrene, one with 70% impact PS and 30% crystal PS and

  the other having 30% shock PS and 70% crystal PS).

  The comparison of the densities gives: - for a conventional compact sheet of 800 gum thickness = 1.045 - and for a lightened sheet according to the invention of 850 gm thickness

= 0.8 to 0.9

  for the same compressive strength.

  The composite sheet (1) presented above is prepared using

  of an assembly as shown diagrammatically in FIG. 2.

9 2811301

  The three-layer sheet (1) is formed by coextrusion of the three layers (2, 3 and 4) described above. At the exit of the flat die (10), the three-layer sheet, at a temperature of the order of 220 C, is laminated between two cylinders (11 and 11 ') of a calender, unheated, with the film (6) , bearing on one of its faces (6 ') the desired impression. The film (6) and the cylinders (11 and 1 l ') of the calender are then at a temperature between approximately 30 and 60 C. The film (6) comes into contact with the extruded sheet (1) when it passes between the cylinders of the calender. The printed face (6 ') then comes into contact with the sheet, which protects the printing, from the manufacturing stage. The film is intended for

  remain on the outside surface of the pot after thermoforming.

  After filling, the pot is sealed with a sealing sheet which is applied, in a conventional manner, to the layer of compact polystyrene opposite the film with respect to the layer of expanded polystyrene. There is therefore no modification to be expected for the machines

  thermoforming, filling and sealing (Form-Fill-Seal).

2811301

Claims (12)

  1. composite sheet (1) three-layer polystyrene for the manufacture of packaging or thermoformed containers, characterized in that it comprises as majority constituent a layer (2) of expanded polystyrene with an expanding agent, covered on each on its faces, a layer of compact polystyrene (3, 4) thick weaker.   2. Composite sheet (1) according to claim 1 characterized in that the layer of expanded polystyrene (2) is formed of a mixture of   impact polystyrene and crystal polystyrene.   3. Composite sheet (1) according to claim 2 characterized in that the layer of expanded polystyrene (2) consists of impact polystyrene in a proportion approximately between 30 and 70% and of polystyrene crystal in an approximately proportion between 70 and 30% by weight.   4. Composite sheet (1) according to any one of claims   previous characterized in that the blowing agent for the central polystyrene layer (2) is used in an amount approximately between 0.5 and 2% by weight per 100% of polystyrene. il 2811301   5. Composite sheet (1) according to any one of claims   previous characterized in that the blowing agent is a mixture   citric acid and sodium and / or potassium hydrogen carbonate.   6. Composite sheet (1) according to any one of claims   previous characterized in that the two compact layers (3, 4) consist of about 60% by weight of impact polystyrene and   about 40% by weight of crystal polystyrene.   7. Composite sheet (1) according to any one of claims   previous characterized in that it is covered by laminating a film (6) of printed polystyrene, the printed face (6 ') of which is turned   to one of the layers (3) of compact polystyrene.   8. Composite sheet (1) according to claim 7 characterized in that it is covered by rolling a second polymer film   applied to the second layer (3) of compact polystyrene.   1 5 9. Use of the sheet according to any one of   previous claims for the manufacture of pots for food industry.   10. Use according to claim 9 for the manufacture of pots for yogurts, desserts, creams ...   11. Method for manufacturing thermoformed packaging or containers comprising the following successive steps: 12 2811301   a) manufacture of a layer of light polystyrene (2) in the presence of a blowing agent, b) control of the morphology of the sheet, c) coextrusion of the layer of light polystyrene (2) and, on each of its faces, of a layer of compact polystyrene (3, 4) to form a three-layer composite sheet (1) according to claim 1, d) application by rolling of a film (6) of polystyrene preprinted on at least one outer face (3) of the three-layer composite sheet, e) optional sterilization by passing said sheet through a bath of hydrogen peroxide, f) thermoforming of the three-layer composite sheet covered of the film.   12. Method according to claim 11 characterized in that   the film is applied by lamination without heating the film.