Method for the production of substantially open-cell polystyrene sheet

Abstract

A method of producing a substantially open-cell, expanded polystyrene sheet, comprising the steps of:

forming a mixture comprising from 30 to 95% of polystyrene incorporating an aliphatic hydrocarbon with 4-6 carbon atoms, from 0 to 65% of polystyrene, and from 0.2 to 10% of a nucleating agent,

mixing the mixture obtained and melting it by heating inside an extruder,

bringing the mixture to a temperature of 130-150°C C. in the final portion of the extruder,

extruding the mixture in a lower-pressure atmosphere to produce an expanded sheet, which has a low density and a high capacity to absorb aqueous liquids, particularly if a surfactant is added to the starting mixture, and which, when coupled with two films of plastics material gives rise to a laminate suitable for the production, by thermoforming, of trays of foods which may release aqueous liquids.

Description

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 2, a tray according to the invention comprises a body 1 with a base 2 having holes 4 and side walls 3.

With reference to FIG. 3, the structure of the tray is constituted by a sheet 5 of substantially open-cell, expanded polystyrene, the upper surface of which is covered by a polystyrene film 6 and the lower surface of which is covered by a polystyrene film 7.

The film 6 has a series of holes 4 with diameters of 0.1-1.5 mm which extend into the thickness of the sheet 5.

When a tray of the type described above is used for packaging foods such as, for example, meat, which may release aqueous liquids, the liquid released by the food finds the holes 4 in the film and penetrates the substantially open-cell expanded polystyrene sheet 4 through the holes.

The penetration of the liquid is preferably facilitated by the presence of a surfactant in the sheet 5; the surfactant in fact considerably reduces the natural water-repellency of the plastics material, increasing the adhesion forces between the solid (plastics material) and the liquid until they exceed the cohesion forces between the liquid molecules and allow the liquid to penetrate the substantially open-cell structure of the sheet 5 through the holes 4.

To facilitate the entry of the liquid into the holes 4, recesses centred around the holes 4 may be formed in the upper surface of the base 2.

The substantially open-cell structure of the expanded polystyrene of which the sheet 5 is made has a dense network of capillary channels which put the individual cells into communication and enable them to receive and retain the liquid which has entered through the holes formed in the upper surface of the base 2 of the tray.

The liquid is thus retained as by a sponge and, because of the strong capillary interactions between the channels and the liquid, it can no longer return to the upper surface of the base 2 of the tray by passing through the holes 4, even when the tray is inclined or even inverted.

A great advantage achieved by the trays according to the invention in comparison with those of the prior art, including the tray produced according to the method of patent application EP-A-0 090 507, is constituted by the absence of unpleasant marks on the surface resulting from the spread of blood released by the foods into the absorbent structure as a whole.

In fact, the surface film of plastics material, which is preferably opacified polystyrene, completely masks the presence of the blood of or any other aqueous liquid released by the foods and retained in the underlying substantially open-cell sheet.

FIG. 4 is a partial transverse section showing a tray 8 having a base 9 and side walls 10 and constituted by a sheet 11 of conventional (closed-cell) expanded plastics material on which an absorbent pad 12 constituted by the substantially open-cell sheet produced by the method according to the invention is superimposed, solely on the base 9 of the tray; a film 13 of thermoplastic material having holes 14 with diameters of 0.1-1.5 mm is applied to the absorbent pad 12. The holes 14 also extend into the absorbent pad 12.

The tray according to the invention and the method of producing it will be described further by means of the following examples, provided by way of non-limiting illustration.

EXAMPLE 1

90 kg of VESTYPOR 10N® expandable polystyrene from HÜLS, containing 6.4% by weight of pentane, was mixed with 4.0 kg of HOSTASTAT SYSTEM E 3904® and 6.0 kg of talc powder and supplied into an extruder with two screws, with a cylinder diameter of 122 mm, a length/diameter ratio (L/D) of 20, and a flow rate of 125 kg/h.

The mixture of the aforementioned components, which had already been melted and amalgamated in the first portion of the extruder, was extruded (head pressure 40 bars) through a circular die having a 1 mm opening.

The temperature profile in the various regions of the extruder, from the supply region towards the extrusion head, was as follows:

T1  194°C C.
T2  199°C C.
T3  213°C C.
T4  146°C C.
T5  149°C C.
T6  149°C C.
T7  151°C C.
T8  149°C C.
T9  144°C C.
T10 139°C C.

The temperature of the molten mixture in the region immediately preceding the output of the extruder was 146°C C.

The cylindrical, tubular body of expanded plastics material output from the extruder was then subjected to the usual steps of cutting, cooling and winding on reels.

The sheet had a weight of 255 g/m, a thickness of 3.7 mm, and a density of 69 g/l.

The sheet thus produced was sent to a hot lamination step in which a bidirectional white polystyrene film 25 μm thick was applied to each of its surfaces.

The double laminated sheet has produced was perforated on one of its surfaces by means of metal needles and was sent for thermoforming in order to form therefrom a tray according to the invention.

After immersion in water for 15 minutes, the tray produced showed a specific absorption of 4.5 g/dm² per mm of thickness.

EXAMPLE 2

76.0 kg of EXTIR A 7000® expandable polystyrene from ENICHEM, containing 6% by weight of pentane, was mixed with 14.0 kg of EDISTIR SR 550® shock-resistant polystyrene, 4.0 kg of HOSTASTAT SYSTEM 3904® and 6.0 kg of talc and supplied into an extruder with two screws with a cylinder diameter of 122 mm, a length/diameter ratio (L/D) of 20, and a flow-rate of 125 kg/h.

The mixture of the aforementioned components, which had already been melted and amalgamated in the first portion of the extruder was extruded (head pressure 50 bars) through a circular die having a 0.8 mm opening.

The temperature profile in the various regions of the extruder, from the supply region towards the extrusion head, were as follows:

T1  195°C C.
T2  193°C C.
T3  212°C C.
T4  140°C C.
T5  145°C C.
T6  144°C C.
T7  143°C C.
T8  145°C C.
T9  145°C C.
T10 139°C C.

The temperature of the molten mixture in the region immediately preceding the output of the extruder was 141.5°C C.

The cylindrical, tubular body of expanded plastics material output from the extruder was then subjected to the usual steps of cutting, cooling and winding on reels.

The sheet had a weight of 250 g/m², a thickness of 3 mm and a density of 83 g/l.

The sheet thus produced was sent to a hot lamination step in which a bidirectional, white polystyrene film 50 μm thick was applied to each of its surfaces.

The double laminated sheet thus produced was perforated on one of its surface by means of metal needles and was sent for thermoforming in order to form therefrom a tray according to the invention.

After immersion in water for 15 minutes, the tray produced had a specific absorption of 4.8 g/dm² per mm of thickness.

EXAMPLE 3

78.0 kg of EXTIR A 7000 expandable polystyrene from ENICHEM, containing 6.5% by weight of pentane, was mixed with 13.0 kg of VESTYRON 314 crystal polystyrene from HÜLS, 4.0 kg of HOSTASTAT SYSTEM E 3904® and 5.0 kg of talc and supplied into an extruder with two screws, with a cylinder diameter of 122 mm, a length/diameter ratio (L/D) of 20, and a flow-rate of 125 kg/h.

The mixture of the aforementioned components, which had already been melted and amalgamated in the first portion of the extruder was extruded (head pressure 40 bars) through a circular die having a 0.8 mm opening.

The temperature profile in the various regions of the extruder, from the supply region towards the extrusion head, was as follows:

T1  200°C C.
T2  206°C C.
T3  212°C C.
T4  149°C C.
T5  151°C C.
T6  150°C C.
T7  152°C C.
T8  145°C C.
T9  144°C C.
T10 140°C C.

The temperature of the molten mixture in the region immediately preceding the output of the extruder was 143°C C.

The cylindrical, tubular body of expanded plastics material output from the extruder was then subjected to the usual cutting and cooling steps.

The sheet had a weight of 245 g/m², a thickness of 3.3 mm and a density of 74 g/l.

The sheet thus produced was sent to a hot lamination step in which a bidirectional, white polystyrene film 100 μm thick was applied to each of its surfaces.

The double laminated sheet thus produced was perforated on one of its surfaces by means of metal needles and was sent for thermoforming in order to form therefrom a tray according to the invention.

After immersion in water for 15 minutes, the tray produced showed a specific absorption of 4.3 g/dm² per mm of thickness.

The tray formed with the sheet produced by the method according to the present invention has a series of advantages in comparison with trays of the prior art.

First of all, in comparison with trays including a pad or layer of absorbent paper material, it offers the advantage of being made solely of plastics material and thus permitting easy recycling. Moreover, the production costs of the tray of the invention are considerably lower than those connected with the manufacture of the above-mentioned trays.

Moreover, when the film of plastics material which covers the absorbent sheet or the absorbent pad included in the tray according to the invention is opacified, any marks from blood or similar liquids released by the foods are masked, which gives the packages produced with the present tray an appearance particularly pleasing to the purchaser.

Claims

1. A method of producing a substantially open-cell polystyrene sheet, comprising the steps of:

(a) forming a mixture comprised of (i) from 30 to 95% of polystyrene beads which contain an aliphatic hydrocarbon with 4-6 carbon atoms, (ii) from 0 to 65% of virgin polystyrene beads, and (iii) from 0.2 to 10% of a nucleating agent,

(b) forming a melt of the mixture obtained in step (a) by heating the mixture inside an extruder

(c) bringing the melted mixture to a temperature of 130-150°C C. in the final portion of the extruder, and

(d) extruding the melted mixture in a lower-pressure atmosphere to produce an expanded sheet thereof.

2. A method according to claim 1, in which the mixture comprises from 5 to 10% of the nucleating agent.

3. A method according to claim 2, wherein the aliphatic hydrocarbon with 4-6 carbon atoms constitutes 1-7% by weight of the total weight of the mixture.

4. A method according to claim 3, wherein the aliphatic hydrocarbon with 4-6 carbon atoms is constituted by pentane.

5. A method according to claim 2, wherein the nucleating agent is selected from the group comprising sodium bicarbonate, citric acid, talc, sodium carbonate, Hydrocerol® and gypsum.

6. A method according to claim 1, wherein the mixture comprises a quantity variable from 0.2 to 10%, of the total weight of the mixture of at least one surfactant.

7. A method according to claim 6, wherein the at least one surfactant is a salt of a sulphonic acid of formula R--SO₃H or of a sulphuric ester of formula R--OSO₃H, in which R is selected from the group comprising alkyl and alkylaryl, with an alkali metal or an alkaline-earth metal.

8. A method according to claim 1, wherein the mixture comprises a quantity of up to 30%, of a shock-resistant polystyrene.

9. A method according to claim 6, wherein the mixture comprises a quantity of up to 30% of shock-resistant polystyrene.

10. A method according to claim 6, wherein the mixture comprises a quantity variable from 1 to 4% of the total weight of the mixture of at least one surfactant.

11. A method according to claim 8, wherein the mixture comprises a quantity of 10 to 20% of the shock-resistant polystyrene.

12. A method as defined in claim 8, wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene.

13. A method as defined in claim 11, wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene.

14. A method as defined in claim 6, wherein the mixture comprises a quantity of 10 to 20% of said shock-resistant polystyrene.

15. A method as defined in claim 6, wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene.

16. A method as defined in claim 14, wherein the shock-resistant polystyrene is a dispersion of polybutadiene in polystyrene.