MULTILAYER POLYPROPYLENE THERMOPLASTIC SHEET AND METHOD

TO PRODUCE THE SAME

Technical field

This application relates to multi-layered thermoplastic sheet and a method to produce said sheet.

Background art

Nowadays, the Form, Fill and Seal (FFS) lines installed in the market that produce multipacks run only sheets produced in Polystyrene (PS), since this material has demonstrated that it is the most suitable polymer for its correct operation. The wide temperature range that this polymer offers in its transformation, combined with the ease with which it is possible to detach the packages, by breaking the packs by simple flexion, have been the main reasons why this preferred polymer to be used in the production of these sheets .

Polystyrene has a large range of temperatures during its transformation and that characteristic, combined with the fracture mechanism provided by this material, makes PS an ideal candidate to produce multipacks in FFS lines. Moreover, PS has a low diffusion barrier for oxygen and water vapor, which in some cases affects the food products that can be packed in this type of material. Thus, there is a great need to use thermoplastic materials to produce sheets that can be used in FFS lines, without the need to adapt the current production lines, as well as maintaining the properties, such as density and breaking capacity, required for the production of multipacks. Document US20150266643 describes a packaging material for producing deep-drawn plastics material packagings, particularly of multipack packagings, wherein the material is constructed to be breakable, and packaging produced therefrom. The present technology differs from this document in the sense that it uses polypropylene to produce the multi layered sheet by extrusion, wherein the minerals are added to the desired layers during the extrusion process.

Document W02017005360A1 discloses a multilayer polymer film for multipacks comprises at least two layers consisting of polyester and additives, wherein a first layer is porous and a second layer contains an inorganic filler. Multipacks thermoformed from the multilayer polymer film are equipped with snap incisions. The present technology differs from this document in the sense that it uses polypropylene to produce the multi-layered sheet by extrusion, wherein the minerals are added to the desired layers during the extrusion process .

Nowadays, whenever thermoplastics other than PS are selected to produce multipacks in FFS lines, minerals are added to the material in order to improve its properties. These minerals are mixed with the thermoplastic material previously to the extrusion to produce the final sheets or films. This extra step increases the time and cost of the process, often leads to deficiencies in homogenization between the minerals and the material, and there is low control over the quantities of minerals added. Additionally, when opting for this strategy, additives are required to improve the homogenization. However, the addition of such additives alter the organoleptic characteristics of the thermoplastic, particularly the smell.

Thus, the present application presents a solution to avoid the dependency on polystyrene in the production of multilayer sheets for the production of multipacks in FFS lines, as well as discloses a process to overcome the current drawbacks associated with the incorporation of minerals in order to maintain the breaking characteristics required for multipacks .

Summary

The present application relates to a multilayer polypropylene sheet, comprising:

- at least three polypropylene layers, wherein at least one layer is foamed and at least two are outer layers;

-- a global density between 0.95 and 1.20 g/cm ³ and a thickness between 600 and 1500 pm;

wherein the foamed layer is positioned between at least two outer layers and both the foamed layers and the outer layers comprise a mineral.

In one embodiment the outer layers comprise 40 to 70% w/w of polypropylene and 30 to 60% w/w of a mineral.

In another embodiment the foamed layer comprises 70 to 95% w/w of polypropylene and 5 to 30% w/w of a mineral.

In yet another embodiment the mineral is a powder selected from talc, calcium carbonate, wollastonite, silica or mixtures thereof. In one embodiment the multilayer polypropylene sheet further comprises at least one extra layer comprising 90 to 100% w/w of polypropylene and 0% w/w of minerals.

In another embodiment least one layer comprises additives in a percentage between 1 and 10% w/w.

In one embodiment the additive is a pigment masterbatch.

The present application also relates to a method of producing the multilayer sheets comprising the following steps:

preparation of the foamed layer in one twin screw extruder comprising the steps of:

introducing the polypropylene to the twin screw extruder;

introducing a mineral to the twin screw extruder; introducing a physical foaming agent to the twin screw extruder;

mixing all components added in the twin screw extruder;

- preparation of the outer layers in two twin screw extruders comprising the steps of:

introducing polypropylene to the twin screw extruder;

introducing a mineral to the twin screw extruder; mixing all components added in the twin screw extruder;

- the mixture of the foamed layer prepared in one twin screw extruder is conveyed to the central part of a three channel die and the mixture prepared in the two other twin screw extruders is routed to the outer channels of the die. In one embodiment an extra layer without minerals is further prepared in a twin screw extruder that prepares an outer layer .

In one embodiment the physical foaming agent is a gas selected from carbon dioxide, nitrogen or water vapor.

In another embodiment the additives are added in a percentage between 1 to 10% w/w.

In yet another embodiment the additive is a pigment masterbatch .

In one embodiment the operating temperature is between 210 and 250 °C.

In another embodiment a percentage between 0 and 15% w/w of minerals and additives are fed through a main inlet of the twin screw extruder.

In yet another embodiment additional mineral between 0 and 25% w/w is added to each sidefeeder of the twin screw extruder .

General description

The present application relates to a multi-layered thermoplastic sheet to be used in Form, Fill and Seal (FFS) lines to produce breakable multipacks. Moreover, the present application also relates to a method to produce said multilayer sheets by an extrusion process. The multipacks produced in the FFS lines with the presently disclosed multilayer sheets are easily separable, when submitted to flexion, by breaking the areas previously marked for this purpose. The multilayer sheet of the present application comprises at least three layers, wherein at least one layer is foamed.

Currently, the preferred material in FFS lines for multipacks is polystyrene (PS) . However, due to the various drawbacks associated with using such material, the present application proposes an alternative material, and method, do produce a multilayered sheet to be used in existent FFS lines, breaking the dependency on polystyrene. The industry of FFS lines is currently highly dependent on this polymeric material and if there is a lack of its availability or its price rises, the industry might stop.

The sheets described herein are made from polypropylene (PP) , allowing to reduce the consumption of fossil materials, lowering the carbon footprint, lowering the costs of production and breaking the dependency on PS as well. PP is the second most chosen polymer worldwide and can be produced from either naphtha or natural gas. The use of PP in the present technology allows to incorporate large quantities of minerals, increase the oxygen and water vapor barrier of the final product as well as increasing the shelf life packed product. Another advantage in choosing polypropylene instead of polystyrene, is that the fat, such as the one present in certain food products that might be packed in the multipacks, compromise the chemical stability of polystyrene, which does not occur with polypropylene.

Nonetheless, polypropylene does not melt at the same temperature as polystyrene, thus the current equipment used in FFS lines are not adapted to use polypropylene. The solution to overcome this problem is the mineralization of the polymeric material.

Additionally, the sheets produced with this polymer do not have the breaking characteristic by themselves, which is necessary for the multipacks produced in FFS lines. PP comprises a ductile fracture mechanism at room temperature, or temperatures close to room temperature, which increases the difficulty of breaking each pack from the multipack. Therefore, to produce easily breakable multipacks, the material requires the ability to break after an incision is made in the sheet and flexion is exercised upon the material. In order to obtain the breaking characteristic, a mineral has to be incorporated into the layers of the multilayer sheet in well-defined and precise percentages. By doing so, a breaking mechanism, similar to the one currently observed in polystyrene sheets, can be achieved in the sheets described herein. However, the incorporation of minerals into the layers increase the density of the final multilayer sheet by lowering its yield per kilogram. Hence, the existence of a foamed layer in the sheets allows to decrease the overall density of the final product, the multi-layer sheet .

The multilayer sheet is produced by a co-extrusion process wherein at least one of the layers is foamed, and the mineral is added to the layers of the sheet directly in the extruder, providing a greater control of the added amounts.

Brief description of drawings

The present technology is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

Figure 1 illustrates a schematic representation of the multilayer sheet wherein the references are: A - foamed layer, B - outer layers.

Figure 2 illustrates a schematic representation of the multilayer sheet wherein the references are: A - foamed layer, B - outer layers, C - additional outer layers.

Figure 3 illustrates a schematic representation of the multilayer sheet wherein the references are: A - foamed layer, B - outer layers, D - extra layer.

Figure 4 illustrates a schematic representation of the extrusion process to produce the multilayer sheets.

Description of embodiments

Now, preferred embodiments of the present application will be described in detail with reference to the annexed drawings. However, they are not intended to limit the scope of this application.

The present application relates to a multilayer thermoplastic sheet to be used in Form, Fill and Seal lines to produce breakable multipacks.

The multilayer sheet of the present application is produced from a thermoplastic, in a preferred embodiment the thermoplastic is polypropylene, which is co-extruded and foamed in order to obtain a final density of the sheet between 0.95 and 1.20 g/cm ³ , and a thickness between 600 and 1500 mpi. The presently described multilayer sheet comprises at least three layers, wherein at least one layer is foamed.

In order to obtain the breaking characteristic, a mineral has to be incorporated into the layers of the multilayer sheet. Thus, to ensure that the sheet has the breaking characteristics required to produce multipacks, and can be used in current FFS lines, it is necessary to incorporate the maximum possible amount of minerals in order to increase the dimensional stability of the sheet during the thermoforming process. The higher the mineral content incorporated, the greater dimensional stability is achieved. Additionally, the addition of minerals also provides higher stiffness to the final sheet. A higher mineral incorporation increases the compressing strength, allowing the production of packages with less thickness and equal quality, in comparison to the packages produced nowadays with PS.

The mineral is incorporated in the foamed layer and the outer layers also comprise minerals.

The outer layers represent any of the layers that surround the foamed layer. The outer layers require a significant mineral content since otherwise polypropylene alone does not confer the breaking characteristic required to produce multipacks in FFS lines.

In one embodiment the mineral is a powder selected from a list comprising talc (Mg3Si40io (OH) ₂ ) , calcium carbonate (CaC0 ₃ ) , wollastonite (CaSi03), silica or mixtures thereof. The chosen minerals require to be as inert as possible, and with a particle size similar to the ones listed herein. Talc is used with a particle size between 4 and 16 pm. CaCo3 is used with a particle size between 1 and 3 pm.

It is well known that the incorporation of a mineral into the layers of a thermoplastic multilayer sheet increases its overall density, therefore in order to obtain an overall density between 0.95 and 1.20 g/cm ³ , at least one layer of the sheet is foamed. The foamed layer allows to lower the overall density of the multi-layer sheet, since the foamed layer produced through the method herein described presents a density that is between 0.4 and 0.8 g/cm ³ . The density of an outer layer comprising a mineral is between 1.10 and 1.60 g/cm ³ .

In a preferred embodiment, the foamed layer is positioned between at least two outer layers comprising a mineral. The expansion of this particular layer is obtained by the addition of a physical foaming agent into the melted mass of this layer, thus obtaining a foamed layer. In a preferred embodiment the physical foaming agent is a gas. The gaseous physical foaming agent is selected from a list comprising carbon dioxide, nitrogen or water vapor.

The presence of a mineral in the foamed layer is particularly important, not only because the mineral acts as a nucleating agent for the physical foaming agent improving the bubble structure of this layer, but also because minerals improve the fracture mechanism necessary for the presently described sheet .

The composition of the presently disclosed multilayer sheet is as follows:

Outer layer with minerals: Polypropylene - 40% to 70%;

- Minerals - 30% to 60%.

Foamed layer:

Polypropylene - 70% to 95%;

Minerals - 5% to 30%.

In one embodiment the multilayer sheet further comprise at least one extra layer without minerals:

- Polypropylene - 90 - 100%

- Minerals - 0%

The density of the extra layer without minerals is between 0.9 and 1.1 g/cm ³ . When required, this optional extra layer acts as a functional barrier between the outer layers comprising minerals and the packed goods within the multipacks made of the multilayer sheets. Also, this extra layer provides a gloss surface to the finished multilayer sheet .

In one embodiment, additives are added to at least one of the layers. These additives are pigments in a percentage between 1-10%. In one embodiments, the pigments used is a masterbatch colorant.

All percentages are represented in w/w in relation to all the components comprising the sheet.

The thickness of each of the layers varies according to the thickness and density of the sheet. Additionally, the density of the final sheet varies according to the thickness and density of each layer.

The presently disclosed multilayer sheet is co-extruded in a line with co-rotating twin-screw extruders, with direct introduction of the minerals and direct injection of the foaming physical agent into the extruder that processes the foamed layer.

As opposed to the methods used nowadays, wherein the mineral is incorporated into the polymer with additives to produce a formulation in an extrusion step previous to the extrusion process to obtain the final sheet, the present technology proposes the addition of a mineral directly into the extruder with the polymeric material in order to produce the multilayer sheet. The direct introduction of a mineral, without additives, during the extrusion process, allows a greater control and precision on the amount of minerals incorporated into the layers, allows to obtain a sheet with excellent organoleptic characteristics, since the use of additives is one of the main causes of alteration on the organoleptic characteristics of the final product. The incorporation of a mineral as described herein allows to mix the polymer more homogeneously with the mineral and therefore ensure that the properties of the sheet remain constant and homogeneous. The addition of minerals in this manner also eliminates a transformation step, which is the production of a formulation comprising the polymer material and the mineral to be used in the extrusion process afterwards. The elimination of this step enables the overall production process to be energetically and environmentally efficient and consequently economically and environmentally friendly.

The direct introduction of the minerals during the extrusion process also leads to a greater control and precision of the amount of minerals constituting the layer, which is of fundamental importance, since small deviations in these quantities can change the properties of the polymer and completely change final properties of the multilayer sheet.

The method to produce the multilayer sheet of the present application comprises the following steps applied in three twin screw extruders, as seen in Figure 4:

- One twin screw extruder to prepare the foamed layer comprising the steps of:

introducing PP to the twin screw extruder;

- introducing a mineral to the twin screw extruder;

- introducing the physical foaming agent to the twin screw extruder;

mixing all components added in the twin screw extruder.

In one embodiment, additives, such as pigments, are added to the twin screw extruder in a percentage of 1 to 10%.

Mixing the PP, mineral, physical foaming agent and any additives in the co-rotating twin screw extruder will form a composite mixture, which will be able to be used as the foamed layer of the multilayer sheet.

- Two twin-screw extruders to prepare the outer layers comprising the steps of:

introducing PP to the twin screw extruder;

- introducing a mineral to the twin screw extruder;

- mixing all components added in the co-rotating twin screw extruder.

In one embodiment, additives, such as pigments, are added to the twin screw extruder in a percentage between 1 and 10%. In one embodiment, the two twin-screw extruders produce all outer layers with minerals. Mixing the PP, mineral, and any additives in the co-rotating twin screw extruder will form a composite mixture, which will be able to form the outer layers of the multilayer sheet .

In one embodiment the two twin screw extruders that produce the outer layers, can also produce the extra layers without minerals .

The polymer, a percentage of 0-15% by weight of mineral and additives if used, are fed through the main inlet (A) of the twin screw extruder.

Additional mineral, up to the maximum percentage, is introduced to the screws at two side position by means of sidefeeders (B) . The percentage of weight of the mineral of each sidefeeder is 0-25%.

The extruder barrel, consequently the operating temperature, is set at a temperature between 210 and 250 °C. Once the extruder/feeding system assembly starts, the polymer is melted by the action of two screws inside the extruder. The twin screws have a 52 L/D design with four kneading zones in order to achieve a perfect and homogeneous mix between polymer and mineral without the need of additives.

The introduction of the physical foaming agent, the gas, occurs in the last 10D of the screws from the extruder (C) producing the foamed layer.

The mixture of the foamed layer is conveyed to the central part of a three channel die, especially designed for this multilayer sheet. The mixture prepared in the two other extruders is routed to the outer channels of the die. The outer channels of the die can accommodate up to two different layers. This allows to have up to five layers at the exit of the die. The desired layer configuration is obtained to the change of a selector plug.

Figure 1 is a schematic representation of a multilayer sheet comprising three layers. In the present application, the foamed layer (A) has a density between 0.4 and 0.8 g/cm ³ , and the outer layers (B) , surrounding layer (A) , each have a density between 1.10 and 1.60 g/cm ³ .

In one embodiment, such as Figure 2, the multilayer sheet comprises five layers in an C/B/A/B/C structure, wherein the composition is:

Layer B - Layer with mineral:

Layer density - between 1.10 and 1.60 g/cm ³ ;

Thickness - 20 pm and 500 pm;

Polypropylene - 40% to 70%;

Minerals - 30% to 60%;

Additives- 0% to 10%;

Layer C - Layer with mineral:

Layer density- between 1.10 and 1.60 g/cm ³ ;

Thickness - 20 pm and 500 pm;

Polypropylene - 40% to 70%;

Minerals - 30% to 60%;

Additives- 0% to 10%;

Layer A - Foamed layer:

Layer density - between 0.4 and 0.8 g/cm ³ ;

Thickness - 200 pm and 1000 pm; Polypropylene - 70% to 95%;

Minerals - 5% to 30%;

Additives - 0% to 10%.

In one embodiment, such as Figure 3, the multilayer sheet comprises four layers in an D/B/A/B structure, wherein the composition is:

Layer D - Layer without minerals

Layer density - between 0.9 and 1.1 g/cm ³ ;

Thickness - 20 pm and 100 pm;

Polypropylene - 90% to 100%;

Minerals - 0%;

Additives- 0% a 10%;

Layer B - Layer with mineral:

Layer density - between 1.10 and 1.60 g/cm ³ ;

Thickness - 100 pm and 500 pm;

Polypropylene - 40% to 70%;

Minerals - 30% to 60%;

Additives- 0% a 10%;

Layer A - Foamed layer:

Layer density - between 0.4 and 0.8 g/cm ³ ;

Thickness - 200 pm and 1000 pm;

Polypropylene - 70% to 95%;

Minerals - 5% to 30%;

Pigments and Additives - 0% a 10%.

Examples :

Example 1 :

Multilayer sheet comprising five layers in an C/B/A/B/C structure with 0.80 mm of thickness and density 1.01 g/cm ³ , in which the composition is: Layer C - Layer with mineral:

Thickness - 50 pm;

Density- 1.20 g/cm ³ ;

Polypropylene - 62%;

Mineral: Talc in 35%;

Additives: Masterbatch (pigments) - 3%;

Layer B - Layer with mineral:

Thickness - 180 pm;

Density- 1.36 g/cm ³ ;

Polypropylene - 48.5%;

Mineral: Talc in 48.5%;

Additives: Masterbatch (pigments) - 3%;

Layer A - Foamed layer:

Thickness - 340 pm;

Density- 0.57 g/cm ³ ;

Polypropylene - 80%;

Mineral: Talc in 20%;

Physical foaming agent - Nitrogen

Example 2 :

Multilayer sheet comprising three layers in an B/A/B structure with 0.82 mm of thickness and density 0.98 g/cm ³ , in which the composition is:

Layer B - Layer with mineral:

Thickness - 210 pm;

Density- 1.36 g/cm ³

Polypropylene - 48.5%;

Mineral: Talc in 48.5%;

Additives: Pigments - 3%; Layer A - Foamed layer:

Thickness - 400 pm;

Density- 0.57 g/cm ³

Polypropylene - 80%;

Mineral: Talc in 20%;

Physical foaming agent - Nitrogen

Example 3 :

Multilayer sheet comprising four layers in an D/B/A/B structure with 0.80 mm of thickness and density 1.01 g/cm ³ , in which the composition is:

Layer D - Layer without mineral:

Thickness -50 pm;

Density - 0.92 g/cm ³

Polypropylene -97%;

Additives: Pigments - 3%;

Layer B - Layer with mineral:

Thickness - 210 pm;

Density- 1.36 g/cm ³ ;

Polypropylene - 48.5%;

Mineral: Talc in 48.5%;

Additives: Pigments - 3%;

Layer A - Foamed layer:

Thickness - 330 pm;

Density- 0.57 g/cm ³

Polypropylene - 80%;

Mineral: Talc in 20%;

Physical foaming agent - Nitrogen This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.