MXPA00005066A - Stretched polyester foamed sheets and containers obtained therefrom - Google Patents

Abstract

Polyester resin foamed sheet, mono e biaxially stretched usable for the production of containers for beverages and foods, having a density lower than 700 kg/m<3>and a crystallization rate such that, when the sheet is heated at 120 DEG C. for 5 minutes, the crystallinity does not exceeds 15%. The aromatic polyester resin used for preparing the foamed sheet can be mixed with an aliphatic polyester resin to impart biodegradability and/or compostability to the sheet or with 1-10%by weight of a polyamide.

Description

STRETCHED POLYESTER CELLULAR LAMINATES AND CONTAINERS OBTAINED FROM THEMSELVES The present invention relates to stretched polyester resin cell sheets, which are heat-settable, and suitable for the production of containers for beverages and food, in which a design can be stamped from which the shape of the can be developed by folding. container. The invention also relates to the containers obtained from said sheet. It relates in particular to stretched sheets which have gas impermeability properties and which are suitable for the production of beverage containers, such as fruit juices, medium shelf-storage milk, tea and the like. The materials used to date for the production of containers for beverages and food, such as containers for fruit juices, milk or others, include substantially a functional layer of cardboard that provides mechanical properties to the container, above all, rigidity, and adhered to the cardboard in some cases, a layer of a thin sheet of coated aluminum, on the side that comes into contact with the beverage or food, of a polyethylene film or similar polymeric material. It is difficult to recycle the containers produced with said multi-layer materials due to the different chemical nature of the different layers.

The possibility of recycling a material that constitutes a container for food or drink is a very important requirement, both from the point of view of the savings that recycling allows to obtain, and from the ecological point of view. In the sector, it is very important to have a recyclable material. There are containers for beverages and liquids, manufactured from polymeric materials, but they do not offer characteristics of rigidity comparable with those of cardboard and for this reason they are not suitable for the production of rigid containers. The containers obtained from these materials fall into the category of small bags (bags). To date, any attempt to manufacture containers that exhibit the necessary rigidity using polymeric materials has failed. The rigidity in the containers depends on the thickness of the wall and, more precisely, varies according to the cube of the thickness of the wall. The use of polymeric material such as polyolefins for the production of containers that have sufficient rigidity would give rise to thicknesses that are not economical and, in addition, that could not be treated due to the difficulty of folding during the closing phase of the container. Other material such as cellular polystyrene can not be used due to its brittleness when layered. They are known, from the patent literature (document No. USP 5,000,991) rigid sheets that are used for the preparation of thermoformed containers for food, formed from a sheet of cellular polyester material and a film of the same nature than the sheet, or other polymeric material. EP-A-836937 discloses semirigid sheets having a thickness of 0.5 to 1.5 mm comprising a layer of polyester resin foam with a density of 0.7 to 1 g / cm3 in which a layer adheres which has properties of gas impermeability, formed by a polymeric material that is not that of the layer constituted by polyester foam. The sheets are used for the preparation by thermoforming of articles for packaging. Patent materials are not known in the literature that comprise a layer of polyester foam capable of being folded to form, by folding, the shape of the container, according to the pattern stamped on the material. The ability of a single layer or multilayer polymeric cellular material to form folds according to a fixed design to develop, by folding, the shape of a container, and the ability of the material to be folded according to said design, are essential requirements for the production of containers for beverages and meals when the technique of folding and folding is applied. A material is suitable for folding if the design that has been stamped on it remains stable over time, and if during the stamping process of said design, there is no breakage that could prevent the folding of the material. In addition, the material must be thermosettable so that it is possible to close the container. Currently, cell sheets of aromatic polyester resin, which are suitable for the production of beverage and food containers, have unexpectedly been discovered, satisfy the aforementioned folding requirements, and have sufficient stiffness to replace the cardboard and a high impact resistance. . The sheets of the present invention are mono or biaxially stretched cells, have a crystallization ratio of the resin so that by heating at 120 ° C for 5 minutes the crystallinity does not exceed 15% and they are stamped on them from the which can be developed by folding a container. The polyester resin that is preferably used for the preparation of the stretched cell sheets is a copolyethylene terephthalate in which between 2% and 20% by mole of terephthalic acid units are substituted by units derived from isophthalic acids and / or naphthalene dicarboxylic acids. Preferably, the copolyethylene terephthalate comprises between 4% and 10% of units derived from isophthalic acid.

The intrinsic viscosity of the resin is greater than 0.8 dl / g (measured in a solution of 60/40 by weight of phenol and tetrachloroethane at 25 ° C, according to ASTM D 4603-86). The melting force is greater than 2 centinewtons at 280 ° C, preferably between 10 and 100 centinewtons and the melt viscosity is at least 1.200 Pa.s at 280 ° C and the shear strength tends to zero, preferably between 2,000 and 20,000 Pa.s. Preferably, the resin is obtained by reclassifying the resin having the intrinsic viscosity of less than about 0.7 to 0.75 dl / g at a temperature between 160 ° C and 210 ° C, carried out in the presence of a dianhydride of an aromatic tetracarboxylic acid, preferably pyromellitic dianhydride used in an amount of between 0.04% and 2% by weight. The preparation of the cell sheets is carried out according to known methods. A preferred method is disclosed in the document USP 5,362,763, the disclosure of which is incorporated herein by reference. The melting strength and the melt viscosity are measured according to the procedures specified below. The melting strength of the resin constituting the sheet presents lower values than those of the resin used for the preparation of the sheet.

The stretched cell sheets according to the present invention are obtained by stretching according to known methods of cell sheets having a bulk density comprised between 50 and 700 kg / m3, preferably 200 to 400 kg / m3, and a thickness between 0.5 and 5 The thickness of the stretched sheets is reduced depending on the stretching ratio applied and, generally, is between 0.2 and 3 mm, preferably between 0.6 and 1.2 mm. The density of the biaxially oriented sheets is generally higher than that of the sheets prior to drawing. When a liquid hydrocarbon is used as the foaming agent, the density decreases. The monoaxially stretched sheets have a density lower than the density before stretching (the lower density is due to the apparatus used for the monoaxial stretching that differs from that used in biaxial stretching). The stretching ratio that can be applied in mono axial or biaxial stretching is from 1.1: 1 to 5: 1. The draw ratio of 1.2: 1 is sufficient in the case of the sheets stretched monoaxially to confer the desired mechanical properties, in particular, improved impact resistance. The high impact strength properties combined with the low density make the monoaxially stretched sheets particularly suitable for the manufacture of beverage and food containers. The biaxial stretching (in the machine direction and in the transverse direction) can be carried out simultaneously or sequentially. The stretching temperatures are generally between 80 ° C and 120 ° C. The residence time varies between a few seconds and 60 seconds or more. Monoaxial stretching can be carried out on a series of rollers heated to a temperature between 95 ° C and 110 ° C and rotating at a different peripheral speed. The monoaxial stretching can be carried out in the machine direction and in the transverse direction. In many cases, it is convenient to subject the stretched sheets to heat setting at temperatures generally comprised between 160 ° C and 220 ° C. As a result of the thermofixing, the dimensions of the sheets are stabilized (shrinkage is considerably reduced) and the mechanical properties are increased. In order to improve the gas impermeability properties of the cellular sheet, said sheet may be coated with a polyester resin film or other material having gas impermeability properties.

In the event that the sheet is coated with a polyester film, the impermeability properties of said film are conferred by subjecting the film to a surface treatment that provides it with waterproofing properties or by applying materials having impermeability properties., such as aluminum and aluminum and silica oxides (AI2O3 and SiOx). A representative surface treatment is the coating of the film with a layer of lithium or potassium polysilicates. The treatment allows very slow regimes of oxygen permeability, which can be 0.3 ml / m2 / 24h / atm or less. The application of aluminum layers and Al and / or Si oxides is carried out according to known procedures. The surface treatments and the arrangement of impermeable materials are selected and carried out in such a way that oxygen pollen film properties are generally conferred to the polyester film generally lower than 70 ml / m2 / 24h / atm (ASTM 1434). In the case of a film metallized with Al or coated with Al and / or Si oxides, the oxygen permeability ratio can be reduced to values lower than 0.3 ml / m2 / 24h / atm. Preferred values lower than 10 ml / m2 / 24h / atm. Aluminum metallized films are commercially available under the name Nu Roll from Nurol S.p.A. (Nu Roll is a registered trademark of Nurol S.p.A.).

In the event that a film having waterproof properties is adhered to the sheet, the untreated side of said film is that which comes in contact with the beverage or food inside the container. It is also possible, depending on the different requirements such as for printing with inks or others, to use as an outer layer a film that meets the above requirements, the inner layer being the cellular sheet to which the film adheres. Adhesion of the film having impermeability or other film properties to the cellular sheet can be carried out according to known methods, by gluing, hot rolling or co-extrusion. It is also possible to cover the polyester cell sheet with a layer of a thermosetting polymer other than polyester resin. The stiffness of the stretched cell sheets is a function of the thickness thereof, that is to say it increases (not proportionally) according to the thickness. The use of inorganic fillers (which can be used up to approximately 20% by weight) makes it possible to increase the rigidity of the sheet. Examples of fillers are silica, alumina, titanium dioxide, calcium carbonate and the like. The production of the containers is carried out according to known methods, folding according to a design stamped on the sheet to develop the shape of the container.

The containers may have different shapes and volumes according to their final use. The cubic, rectangular or pyramid shape can be used. In general, the volume of the containers for drinks and juices is between 0.2 and 2 liters. The surface characteristics of the stretched cell sheets make said sheets suitable for easy inking. The high impact resistance of the drawn sheets makes the containers suitable for use in conditions that require the property of impact resistance. The polyester resin that is used for the preparation of cell sheets stretched according to the invention, and having the aforementioned characteristics, can be obtained by means of the polycondensation of an aromatic bicarboxylic acid, preferably terephthalic acid, with a diol of 2. to 12 carbon atoms, such as ethylene glycol, 1,4-butanediol, or by transesterification of a dimethyl ester of the aromatic bicarboxylic acid with a diol and the polycondensation of the hydroxy ester. Preferably, the resin is selected from polyethylene terephthalate copolymers in which up to 20 mol% of units derived from terephthalic acid are substituted by units deriving from isophthalic acid and / or naphthalene dicarboxylic acid.

In order to confer properties of biodegradability and / or compostability to the cellular sheets, the polyester resin used for the preparation of the sheets is mixed with between 10% and 35% by weight of a biodegradable aliphatic polyester resin. , and the mixture is subjected to a polyaddition reaction in the solid state in the presence of a dianhydride of a tetracarboxylic acid, preferably aromatic. The preferred dianhydride is pyromellitic anhydride and is used in an amount of between 0.05% and 2% by weight of the total amount of resin. The polyaddition reaction in solid state is carried out at a temperature comprised between 150 and 220 ° C, during periods of time and with a sufficient concentration of dianhydride to obtain intrinsic viscosity values of the resin greater than 0.7 dl / g (viscosity measure to ° C in a mixture of 60/40 by weight of phenol and tetrachloroethane) and fusion strength values high enough to achieve foaming of the resin. The intrinsic viscosity is generally between 0.8 and 1.2 dl / g. The appropriate values of the fusion strength are greater than 8 centinewtones, and preferably greater than 20 centinewtones. The aliphatic resin is obtained by polyconcentration of a hydroxy acid with 2 to 22 carbon atoms or its lactone or lactide, or by polycondensation of an aliphatic bicarboxylic acid having between 2 and 22 carbon atoms with an aliphatic or aromatic diol with between 2 and 22 carbon atoms. The preferred polyester is polycaprolactone. The aforementioned polyesters exhibit biodegradable characteristics.

Measurement methods Rheological measurements are taken according to the standard ASTM D 3835 using a Goeffert rheometer at 280 ° C. The fusion force is determined by measuring the force of cN needed to stretch the extruded material from the capillary of the 2002 rheometer.

Geoffert For the determination, a Rheotens device is applied to the outlet of a capillary from Goeffert's 2002 rheometer. The extrusion conditions are as follows: piston speed 0.2 mm / s matrix diameter 2 mm capillary length 30 mm test temperature 280 ° C acceleration used 2.4 cm / s' Repeat each test twice and the average of the two results is the value taken into account. The crystallinity is measured according to the ATR (Alternate Total Reflectance) procedure of the relationship between the trans and left configuration. The ATR instrument was applied to a 2000 FT-IR model by Perkin. As a reference band, absorption was used at 1410 cm "1.

The following examples are provided to illustrate the invention, and do not limit the scope thereof.

EXAMPLE 1 A monoaxially stretched sheet (1: 1: 1 draw ratio) of cellular COPET (copolyethylene terephthalate containing 4% by weight of isophthalic acid) having a crystallinity ratio such that when heated to 120 ° C for 5 hours. minutes, the crystallinity does not exceed 15%, a thickness of 0.7 mm and a density of 375 kg / m3, is folded according to a suitable design to develop, after folding, a container having a parallelepiped shape. The stretched sheet was obtained from a cell sheet having a density of 450 kg / m3, a thickness of 1.1 mm and an average diameter of the cells of 300 microns and a degree of crystallinity of 8%. The total impact energy that was determined using an impact device from Franctovis Ceast was 0.41 J; the energy after drawing (draw ratio 1, 2: 1) was 0.59 J. The cell sheets were obtained from COPET having a melting force of 150 cN at 280 ° C, a melt viscosity of 1, 800 Pa.s at 300 ° C and 10 rad / second and an intrinsic viscosity of 1.24 dl / g, prepared by reclassification of the COPET with intrinsic viscosity of 0.7 dl / g at 210 ° C in the presence of 0.4% by weight of pyromellitic dianhydride.

The container is closed by heat setting. The closure is made hermetically and is easily opened by tearing, cutting or other methods.

EXAMPLE 2 A monoaxially stretched cell sheet having the characteristics mentioned in Example 1, is glued with adhesive compatible with the recycling properties of the container, to a Nu Roll metal film of the company Nurol S.p.A., which has a thickness of 15 microns. The sheet is used for the production of containers for liquids.

EXAMPLE 3 A biaxially stretched cell sheet (drawing ratio of 3: 1 in both directions) obtained from a cell sheet prepared from copolyethylene terephthalate containing 4% isophthalic acid units, mixed with 10% by weight of polycarprolactone UC PCL 787 from Union Carbide, and then subjected to polyaddition in solid state at 180 ° C, until the intrinsic viscosity of 0.85 dl / g is obtained, which has a thickness of 1.2 mm, a density of 180 kg / m3 and such a crystallization ratio of the resin that, when heated to 120 ° C for 5 minutes, the crystallinity does not reach values above 15% », is pressed according to a design intended to be developed by folding a container in a parallelepiped shape. The folded sheet is used for the preparation of containers for medium shelf-life milk and for fruit juices. The containers are closed by heat setting. The seal is maintained and easily opened by tearing, cutting or other methods. The containers subjected to composting under normal conditions of composting, are compostable.

EXAMPLE 4 A cellular PET film having the characteristics of Example 3, is adhered with glue compatible with the recycling properties of the container, to a Nu Roll metallic film with a thickness of 15 microns of the company Nurol S.p.A. The sheet is used for the production of containers for liquids, said containers having impermeability properties.

Claims (19)

NOVELTY OF THE INVENTION CLAIMS

1. - Mono or biaxially stretched cellular sheet, of an aromatic polyester resin, suitable for the production of containers for beverages and food, which has a density lower than 700 kg / m3 and is stamped thereon a design according to which it can be developed by folding a container, in which the crystallization rate is such that, by heating at a temperature of 120 ° C for 5 minutes, the crystallization rate does not exceed 15%.

2. Sheet according to claim 1, wherein the sheet is obtained from copolymer terephthalate containing between 2% and 20% by mole of units deriving from isophthalic acid and / or naphthalene dicarboxylic acids.

3. Sheet according to claims 1 and 2, wherein the sheet is stretched with a ratio of between 1: 1 and 5: 1.

4. Sheet according to claims 1 to 3, in which the polyester resin has a melting strength greater than 1 cN and a melt viscosity greater than 1, 500 Pa.s with a shear strength that tends to zero (measured at 280 ° C).

5. Sheet according to claims 1 to 4, wherein the polyester resin is mixed with between 1% and 10% by weight of a polyamide.

6. - Sheet according to claims 1 to 5, in which a polyester film is adhered, said polyester film having an oxygen permeability ratio of less than 70 ml / m2 / 24h / atm (measured according to ASTM 1434) .

7. Sheet according to claim 6, wherein the polyester film is metallized with aluminum or coated with a layer of alumina or silicon oxide, or with potassium or lithium polysilicates.

8. Sheet according to claims 1 to 5, wherein a film having gas impermeability properties is adhered to the stretched cell sheet by gluing or hot rolling.

9. Sheet according to claims 1 to 8, wherein the cellular sheet has a density comprised between 60 and 500 kg / m3.

10. Sheet according to claim 9, wherein the cellular sheet has a density comprised between 100 and 400 kg / m3. 1.

Sheet according to claims 1 to 10, which has a thickness comprised between 0.5 and 3 mm.

12. Sheet according to claim 1, having a thickness comprised between 0.7 and 1.5 mm.

13. Sheet according to claims 1 and 11, including an inorganic filler.

14. Sheet according to claims 1 and 13, which is obtained from aromatic polyester resin mixed with between 10% and 30% by weight of an aliphatic polyester resin.

15. - Sheet according to claims 1 to 14, which is stretched monoaxially.

16. Container for beverages or food, manufactured from the sheets according to claims 1 to 15.

17. Container according to claim 16, which has a closure formed by heat setting.

18.- Container for fruit juices or for sterilized milk, manufactured from a sheet according to claims 1 to 17, to which a film having oxygen impermeability properties corresponding to values of permeability ratio lower than 70 ml / m2 / 24h / atm.

19. The container according to claim 18, wherein the film adhered to the sheet has an oxygen permeability ratio lower than 10 ml / m2 / 24h / atm.