EP1692218A1

EP1692218A1 - Polylactic acid-based degradable foams and process for their production

Info

Publication number: EP1692218A1
Application number: EP04798769A
Authority: EP
Inventors: Cesare Vannini; Fabio Fiordelisi; Walter Movilli; Federico Lanzani
Original assignee: Coopbox Europe SpA
Current assignee: COOPBOX Eastern sro
Priority date: 2003-11-03
Filing date: 2004-11-03
Publication date: 2006-08-23
Also published as: WO2005042627A1; EP1528079A1

Abstract

Degradable foamed lactic acid polymer products, such as trays and containers in general, are produced by the extrusion of the polymers by additioning a foaming agent selected from nitrogen, carbon dioxide and mixtures thereof, in the absence of additional additives as foaming adjuvants selected for example from foaming aids, blowing agent stabilizers, thickeners, and at a temperature within the range of Tm+ 10°C and Tm-30°C, particularly Tm and Tm -10°C wherein Tm is the melting temperature of the polymer.

Description

"PLA-BASED DEGRADABLE FOAMS AND PROCESS FOR THEIR PRODUCTION" BACKGROUND OF THE INVENTION The present invention relates to degradable polylactic acid (PLA) based foams and to a process for their production.

More particularly, the invention relates to products obtained by foaming polymers of lactic acid (PLA), such as sheets, trays, containers in general, cups and the like, used for packaging in general and in particular for food packaging, as well as to their production by means of an innovative process. STATE OF THE ART

The above mentioned containers, particularly cups and trays, are largely used in everyday life; they are usually made of foamed plastics, whereby light products are obtained, having very good mechanical characteristics, which are equivalent to or better than those of the non-foamed containers, despite having lower weight and thus using a smaller amount of raw material.

These containers have some drawbacks. Due to the very large use thereof, and in view of the high volume to weight ratio, a solution for the disposal of used containers, which are not as easily recyclable as other types of plastics products such as bottles and containers, must be urgently found. Furthermore, containers made of foamed plastic materials are not suited as such for some applications: e.g. the trays used for meat should absorb the exuded liquids to prevent meat from sitting in the exuded liquids. Accordingly, trays made of foamed polystyrene, which are laminated to have an inner layer of absorbent cellulose, not visible from the outside, sandwiched between two layers of polystyrene have been proposed; in addition, the inner layer of polystyrene is perforated in order to allow the passage of liquids into the cellulose layer.

EP1329299 describes the use of specific fillers, including glass microspheres, to have an "open" cell structure and to allow the exuded liquid to penetrate into the interstitial spaces of the tray shaped foamed product; this technique however requires the use of surfactant products in order to make hydrophilic the surface of the tray, which is in itself normally hydrophobic, and allow the exuded liquid to reach the inside of the laminate or penetrate into the cavities of the foamed product. These solutions have therefore shown themselves to be expensive. In order to solve the disposal problem of the containers, it was suggested to replace the plastics materials currently used for producing foamed containers, with lactic acid polymers; indeed, depending on the conditions, products based on polylactic acid (PLA) degrade in a substantially complete manner, into CO₂ and water, within a time of few weeks to about a year. Rigid and non-foamed PLA based containers, mainly used as containers for fruit and vegetables, are known. These containers are however more expensive than those traditionally used.

EP-B-0507554 describes a manufacturing process for polymers of lactic acid or copolymers of lactic acid and hydroxycarboxylic acids. The polymerisation of the lactic acid occurs, for example, through the opening of the lactide ring in the presence of catalysts. The polymers are extrusion foamed, using chemical foaming agents such as azodicarbonamide, azobisisobutyronitrile and the like. Evaporation foaming agents, selected from hydrocarbons and halogenated hydrocarbons such as butane, chloroform and chloro- fluorohydrocarbons are also disclosed. This technique has the drawbacks of requiring the use of foaming gases deriving from chemical substances which decompose during the extrusion process or of a gas foaming by evaporation such as hydrocarbons or chlorofluorohydrocarbons, i.e. obtained from non- renewable sources. EP-B-0510999 describes a degradable high polymer network comprising a PLA based polymer which, according to the patent, can be foamed into an open cell sheet if it is mixed with up to 20% by weight of plasticisers such as, for example, phthalic acid derivatives, adipic acid derivatives, maleic acid derivatives, citric acid derivatives, lactic acid, straight chain lactic acid oligomers, cyclic lactid acid oligomers. The extruded sheet is stretched after its extrusion. The lactic acid based polymer is effectively plasticized by the addition of the plasticizers and the resulting resin composition becomes flexible. When the amount of the plasticizer is 5% by weight or more, flexibility can be clearly observed. The resulting product is however poorly suitable to be used as a food container because it does not have the required physical and mechanical characteristics. In fact, it is suggested for use as an absorbing medium for oil and for exuded liquids, such as a filter in ventilation systems, and as a wrapping around the roots of plants. Accordingly, said invention relates to a "polymer network" and hence not to a "foam" having a cell structure capable of maintaining the mechanical performances sufficient to make a container such as a food tray.

JP 2000007816 describes a composition for producing foam containing a polylactic acid based resin, an inorganic nucleating agent such as, for example, talc, a foaming aid such as, for example, a stearic acid based compound or montanoic acid and a volatile blowing agent such as, for example, a hydrocarbon, a chlorinated hydrocarbon, carbonic acid gas, water. Further agents and additives may be added. The foamed product obtained according to JP 2000007816 may be used for producing lunch boxes, tableware, shock absorbing material. JP 2000007815 describes a composition for producing foam by adding a specific thickener and a volatile blowing agent to a biodegradable polylactic acid based resin. The preferred thickener is added up to 20% by weight and is selected among a multivalent isocyanate, a polybasic acid anhydride or the like. JP 2000044716 discloses a foamable composition comprising a degradable copolymer, a blowing nucleator such as, for example, talc, a blowing aid or foaming assistant such as, for example, stearic acid system compounds (calcium stearate) and/or montanoic acid system compounds, and a volatile blowing agent. The copolymer according to JP 2000044716 is obtained by condensing a polysaccharide with lactic acid or polylactic acid. US 5,210,108 describes degradable foam materials for food packaging applications, comprising closed cell foam degradable thermoplastic polymeric resins containing at least 50% by moles of one or more star-shaped polymers. The star-shaped polymer comprises a central residue derived from a polyfunctional compound having from 5 to 100 carbon atoms and originally having at least 5 amino or hydroxyl groups, and a plurality of polymeric branches or arms formed, for example, of polylactide. The polymer is converted to foam by a blowing agent such as nitrogen, carbon dioxide or HFC (HydroFluoroCarbon) compounds.

WO 02/34823 discloses the preparation of a multimodal thermoplastic polymer foam by using a blowing agent stabilizer (up to 50% by weight) which is predominantly located proximate to large cells. The resulting foams are used as thermal insulating materials. The blowing agent stabilizer is a compound that forms a second phase of dispersed discrete domains in a thermoplastic polymer resin and has an affinity for a blowing agent such that the blowing agent preferentially concentrates into or around those domains. WO 99/65977 describes a biodegradable product made from a foamed, cellular material, which is obtained by introducing expanding agents such as carbon dioxide and nitrogen into the molten material, which comprises a base polymer and a cell forming agent. The exemplified base polymer is selected among polyester and copolyester and is in a mixture with cellulose, chitin and thermoplastic starch or starch derivatives; the cell forming agent is for example selected from citric acid, sodium bicarbonate and the like. JP 10128826 describes a manufacturing method for a biodegradable resin (aliphatic polyester) foamed sheet of large expansion ratio. The foaming agent is injected into the melted resin. Detailed process conditions are given. All the above mentioned documents teach that in order to obtain a polylactic (co)polymer foamed article at least some kind of additive (i.e. an additive for the foaming step) is required. The additives are requested to have a foam with acceptable mechanical properties and are disclosed as being one or more of a plasticizer, a thickener, a blowing agent stabilizer, a cross-linking agent, a cellulose derivative or a foaming aid. The presence of these additives results in increased costs, reduced mechanical properties (e.g. when a network rather than a foam is obtained) and in possible food contamination by additives such as plasticizers. OBJECTS OF THE INVENTION It is an aim of the present invention to provide foamed sheets of PLA or lactic acid polymers which do not contain additional compounds such as foaming aids, blowing agent stabilizers, thickeners and the like, thus resulting in a completely degradable and disposable product.

It is also an aim of the present invention to obtain foamed sheets of lactic acid polymers and copolymers with good physical and mechanical characteristics even if the composition to be foamed is free from additives as above mentioned, i.e. free from additives different from talc or similar nucleating agents.

Another aim of the invention is to provide a manufacturing process for foamed sheets which is simple, reliable, economical and environmentally as friendly as possible.

A further aim is to make PLA food containers having performances close to or matching those of the known containers made of foamed plastics, and in particular which have good resistance to mechanical deformation during storage and packaging. Another aim of the present invention is to provide a process for the production of foamed sheets of PLA which does not require the presence of additional compounds such as foaming aids, blowing agent stabilizers, thickeners and the like in order to assist and guarantee the requested foaming of the final product. DESCRIPTION

Such aims are achieved by means of the present invention which relates to a manufacturing process for foamed lactic acid (co)polymers by extrusion of a mixture comprising at least a foaming agent, a lactic acid polymer and/or copolymer, a nucleating agent, said mixture being free from additional additives such as foaming aids, blowing agent stabilizers, thickeners and the like as aid compounds to the foaming step and wherein said foaming agent is selected from nitrogen and carbon dioxide, and mixtures thereof, as characterized according to claim 1.

According to one aspect of the invention, the process envisages heating said lactic acid (co)polymers, in mixture with the suitable amount of nucleating agent such as, for example, talc, as the sole additive, to a temperature sufficient for their melting and homogeneisation, feeding and mixing said foaming agents with the molten polymer mixture, cooling the foaming agent and polymer mixture to a temperature within the range of T_m+10°C and T_m- 30°C, wherein T_m is the melting temperature of said polymer, and extruding the thus cooled mixture so as to form a sheet. T_m is the value measured at the apex of the melting peak obtained by a DSC (Differential Scanning Calorimeter) under a constant flow of nitrogen and by heating at a rate of 20°C/min. Preferably, the cooling step is at a temperature within the range of T_m+5°C and T_m-20°C, more preferably T_m and T_m-10°C. Always according to the invention, T_m and T_m-10^oC is considered to be the most preferred range temperature. A possible range temperature is also T_m +5°C and T_m -7°C. The above mentioned cooling step according to the invention is very important as the mixture to be extruded does not contain any additional compounds such as plasticizers, foaming aids, blowing agent stabilizers, thickeners and the like, which are used, according to the prior art, in order to obtain the desired foamed compound. The foamed sheet of PLA according to the present invention, is obtained without any of the traditionally used auxiliary foaming/blowing additives, thus resulting in a completely degradable and disposable product which shows performances close to or matching those of the known containers made of foamed plastics, and which have good resistance to mechanical deformation during storage and packaging. Always according to the invention, the foaming mixture passes through the extrusion die according to a T (die transit time) which must not exceed 60 sec. Accordingly, the mixture may successfully foam and may be extruded even if the polymer crystallization process is about to start or has in part started, and even if the temperature during the extrusion step is quite below the melting temperature of the base PLA (co)polymer. In fact, lactic acid polymers, even at the beginning of their crystallization process, tends to form aggregations. If aggregations are formed, the polymer composition is difficult to extrude and results in poorly formed foam. The process steps according to the invention, allow the PLA base mixture to be successfully foamed and extruded even if it does not contain any additional compounds such as foaming aids, blowing agent stabilizers, thickeners and the like to make the foaming possible, as described in the prior art. In other words, the invention provides a process in which the melted mixture, added with the blowing agent (carbon dioxide or/and nitrogen) is cooled to a temperature per se sufficient to start crystallization of the polymer, and is extruded before the polymer crystallizes or before the polymer degree of crystallization is such as to jeopardize the foaming step. The above reported process conditions thus allow the obtainment of a foamed PLA based compound which does not contain any foaming/blowing additive/aid/auxiliary; the resulting foamed product, while showing the required good mechanical/technical characteristics, is completely degradable and totally safe to be used in food packaging . According to a preferred aspect of the invention, the foaming agent consists of a nitrogen and carbon dioxide mixture, and the ratio of N₂ and CO₂ is set as a function of the degree of expansion and of the percent of open cells required for the final sheet: by increasing the percent of CO₂ the percent of open cells within the structure is increased, and its density is reduced. According to a further aspect of the invention, the process also comprises a thermoforming step of the sheet in order to obtain trays or other products. In this step, a thermal treatment of the polymer may be carried out, to improve its mechanical properties and in particular its resistance to heat deformation at temperatures higher than 50°C, and generally up to at least 60°C. The product obtained by the process of the invention is new with respect to the prior art. In fact, as already discussed above, it makes no use of chemical foaming agents or chlorofluorohydrocarbons in that the agents used for foaming the product are only nitrogen, carbon dioxide or mixtures thereof, and the agents still present within the product as residues from the foaming process, are only such gases.

Therefore, it is an object of the invention a foamed product of lactic acid polymers such as may be obtained by a process according to that described above.

This product preferably has a percent of open cells, which depends on the quantity of CO₂ used in the foaming mixture, which may reach 90% . A preferred embodiment of the product are trays for food packaging, in particular trays having a majority of closed cells for packaging dry food or food that does not release liquids, and trays having a majority of open cells for packaging food such as meat or fish, i.e. that releases serum or liquids. In trays for the packaging of meats or fish, an open cell percent of around 70- 80% is desired. The invention provides many advantages with respect to the known art.

By using nitrogen and carbon dioxide, either alone or in combination, foamed polymers and the relevant containers formed therefrom have been obtained, surprisingly having mechanical characteristics which are comparable with those of the products in foamed plastics and superior to the mechanical characteristics of the rigid containers made from PLA and analogous polymers of equal weight. Particularly, trays for use with meats may be obtained with the desired percent of open cells to ensure that any exuded liquids are absorbed; differently from the known art, incisions, sheet perforation or skin removal steps are not required when manufacturing the tray.

Furthermore, the invention allows avoiding both the use of wetting additives (such as for example surfactants) and the use of chemical or physical foaming agents based on hydrocarbons or halogenated hydrocarbons; therefore, such additives and foaming agents are absent (even as residues) in the foamed product, thus allowing the manufacture of a tray based only on degradable natural materials, and produced from renewable resources. The use of nitrogen and carbon dioxide gases additionally avoids having to provide the manufacturing plants with the safety equipment otherwise necessary when hydrocarbons or other flammable foaming agents are used. This results in a substantial saving over the production costs and in products which are more inert with regard to the environment because the only foaming agents present in the foaming step are nitrogen and carbon dioxide (gases which are in the atmosphere) and no additional additives such as foaming aids, blowing agent stabilizers, thickeners and the like are present in the final products. An additional advantage is that products with greater or lesser percents of open cells are obtained in a reproducible manner, as a function of the N₂ and C0₂ composition of the gas injected into the extruder. Furthermore, by carrying out a thermal treatment of the sheet, a product with improved mechanical characteristics and less liable to deformation, even at temperatures greater than 60°C, is obtained. Moreover and as already said, the foamed polymer manufacturing process according to the invention does not require the addition of any plasticisers and, in the case an open cell structure is necessary, this may be obtained without resorting to high stretching ratios, and hence without destroying the cellular structure. The invention will now be disclosed in greater detail with reference to the drawings which are attached as non-limiting illustration, wherein:

- fig. 1 is a diagrammatic view of an extruder for the manufacture of foamed products according to the invention;

- fig. 2 is a partial and longitudinal sectional view of the extrusion head and the calibration mandrel;

- fig. 3 is a DSC graph of a foamed sheet of pure PLA; and

- fig. 4 is a DSC graph of a foamed sheet of PLA in the presence of a crystallising additive.

The extruder 1 shown in fig. 1 comprises a gravimetric feeding system 2 with which the PLA is weighed. The feeder 2 is connected by means of conduit 3 to a dryer (not shown) where the polymer is dried prior to its extrusion to avoid hydrolysis phenomena.

Suitable polymers are the polymers and copolymers of lactic acid such as for example those described in the above cited documents EP-B-0507554 and EP-B-0510999 and in particular polymers with high molecular weight (Mn), preferably of at least 70,000 Daltons. Preferential polymers are those obtained by the formation of lactides and polymerisation thereof; as an example, a particularly suitable polymer is PLA 2002D grade produced by Cargill Dow with the trade name Nature Work. The percent of lactic acid (co)polymer in the polymeric portion of the mixture is at least 85% (w/w), preferably 95% and more preferably 100%.

An additional gravimetric feeding system 4 is used for the talc. The extrusion barrel is equipped with heating means for the accurate and constant thermoregulation of the process; such means are for example oil regulation units and electrically heating bands; this allows the temperature to be precisely adjusted during the steps of the process: melting the PLA, injection of the physical foaming agent, mixing, cooling and expansion upon exit from the extrusion head.

Onto the extruding barrel are additionally provided, downstream of the feeding devices 2 and 4, means 5 for feeding, i.e. injecting, gas into the molten polymer. The means for feeding nitrogen comprise a pressure reducer to set the gas to a pressure of approx. 100 bar and a gauge and flow regulator; a pumping system which delivers liquid C0₂ at a pressure of approx. 100 bar is preferably used for the carbon dioxide. Liquid CO₂ may thus be injected. Downstream of injection means 5 for the foaming gas, with respect to the direction of extrusion as shown by arrow F, there are positioned, in sequence, a heat exchanger 6, a static mixer 7, and an extrusion head or die 8. Fig. 2 schematically shows a detailed extrusion head 8 and the related calibration mandrel 9 on which the foamed tubular sheet 10 slides after delivery from head 8 in order to be calibrated and cooled. Head 8 is provided with an annular, i.e. circular extrusion die 11 ; the profile of the extrusion die is sized in such a way as to allow a pressure drop only within its end, just before the exit, in order to prevent cells forming within the head. The calibration mandrel 9 calibrates the sheet (which is tubular) upon its delivery from the extruder and cools it as quickly as possible in order to give consistency to the foamed product and in order to avoid a collapsing of the cellular structure. With that aim, the calibration mandrel 9 is provided with a coil or similar heat exchange means 12 in which water at 5-10°C is circulated in order to cool the surface of the mandrel to about the same temperature. The ratio from the diameter of the extrusion die and the diameter of the calibration mandrel is preferably comprised within the interval between 1 :1 ,5 and 1:4. The calibration mandrel is positioned quite close to the extrusion die so as to prevent the sheet from collapsing when leaving from the extrusion die. Said distance will be comprised within the range of 50 and 200 mm for a round headed extruder with an extrusion die having a diameter of approx. 50- 60 mm. The extruder used is known in the extrusion foaming art for such a purpose. For example a suitable extruder is a twin-screw extruder, with L/D ratio 20:1 , screw diameter 90 mm. The manufacturing process of the sheet of foamed PLA envisages the following steps: a) heating the lactic acid polymer to a temperature sufficient to melt and homogenize it: this step is carried out in the first part of the barrel, which is located between inlet 2a of the feeding device to the barrel and the gas injection means 5, at a temperature that, in the case of PLA, is within the range of 150 to 200°C, preferably between 190 and 200°C and that more preferably is 190-195°C; b) feeding by means 5 the foaming agent(s) to the molten polymer: this step is carried out at a temperature which is close to that of the preceding step and in any case before the following cooling step; c) cooling and maintaining the mixture of foaming agent and polymer at a temperature comprised within the interval between T_m+10°C and T_m -30°C, wherein T_m is the melting temperature of said polymer. Preferably the cooling temperature is within the range of T_m+5°C and Tm-20°C, more preferably T_m and T_m-10^oC, wherein T_m is the melting temperature of the polymer measured as the apex of the relative peak obtained by DSC under a constant flow of nitrogen and by heating at a rate of 20°C/min. T_m and T_m- 10°C is considered to be the most preferred temperature range. In the case of PLA, T_m is 150°C and the temperature of the cooling area is within the range of 155 to 130 °C, preferably within the range of 150 to 140 °C; this step involves the extruder barrel area that goes from immediately downstream of injection means 5 and just upstream of extrusion head 8; d) extruding the mixture to form a sheet 10 and e) cooling the extruded sheet on calibration mandrel 9.

The extrusion die or extrusion head is designed so as to have a die transit time T _t of the polymer composition that avoids forming of clusters or aggregations of crystallized PLA polymer, because the plant is generally run at the maximum extrusion speed, the die transit time is measured at said maximum speed.

According to the invention, it is also possible to carry out the process, particularly step c) as above indicated within a range temperature T_m+5°C and Tm-10°C or T_m+5°C and T_m-7°C, even if, as already said, the most preferred range temperature is T_m and T_m-10°C.

In other words, by cooling the polymer mixture to the mentioned temperatures, the mixture viscosity is raised to a value sufficient to maintain the foam structure long enough to cool the extruded foam without the foam collapsing. To avoid occurrence of lumps and agglomerations, the die transit time is as above mentioned.

By following the above disclosed process, foamed sheets are obtained with good mechanical characteristics and a homogeneous cell distribution. The cells dimensions were found to be within the range of approx. 200 to approx. 600 microns, according to the extrusion parameters.

It was noticed that the percent of open cells within the final sheet may be controlled by using a mixture of the two foaming agents and by varying their percentages. More particularly, the quantity of N₂ used is comprised between 0.1 and 0.4% by weight over the total solids and the quantity of CO₂ used is comprised between 0.3 and 2.0% by weight over the total solids; it was observed that with only N₂ predominantly closed cells are obtained and with only CO₂ predominantly open cells and lower density are obtained. The extruded tubular sheet is set and cooled on the calibration mandrel which, as mentioned previously, has a diameter of one and a half to four times the diameter of the extrusion die, in order to slightly stretching the extruded product. The cooled sheet is then cut and rolled up in a known way and then thermoformed into the desired products.

The process additionally envisages the possibility of carrying out an additional thermal treatment step of the essentially amorphous foamed sheet obtained following extrusion, with the aim of increasing the resistance to heat deformation. Such a step is carried out in the thermoforming process. Two ways to achieve this result have been identified.

In a first method a double mould is used wherein the first part is heated and the second is cooled. The sheet is shaped in the hot part and is subsequently transferred into the cold part for its stabilisation. The heating oven, which is 8 times the length of the forward step of the sheet, is maintained at a temperature within the range of 90 °C and 170 °C, the hot mould is maintained at a temperature comprised within the interval between 100 °C and 130 °C and the cycle time is comprised within the interval between 4 and 9 seconds. Preferably, the oven temperature is within the range of 150°C to 165°C, and more preferably of approx. 160°C. The heating temperature of the mould is close to that in which the crystallisation kinetics is maximal and in the case of polylactic acid is within the range of 100°C to 150°C and preferably of 125°C + 5°C. The cycle time, i.e. the heating time in the hot mould, is within the range between 4 and 9 seconds and more preferably is 6 seconds and results in an at least partial crystallisation of the foamed product. The sheet thus treated and shaped, is then cooled and stabilized with the cold mould.

Alternatively, the sheet is heated only in the oven, to a temperature of about 200°C, markedly higher than that used in a normal PLA thermoforming process, for a time that is sufficient to bring the sheet to a temperature that is close to that of crystallisation and is such as to allow at least partial crystallisation of the product; the treated sheet is then directly moulded in a cold mould.

In both methods, the use of crystallising agents such as Masterbatch NAS320-S manufactured by SUKANO, added to the raw materials in the extrusion process, may be of some help. Examples

1. Extrusion.

A Dow Cargill polylactic acid polymer "Nature Work" grade 2002D and free from plasticisers, was fed to a twin screw extruder with UD ratio 20:1, screw diameter 90 mm and throughput capacity 90Kg/h.

The polymer was meltedwithin the first part of the extruder maintained at 190-195°C; then, a mixture of nitrogen and carbon dioxide was added to the polymer, wherein the quantity of added nitrogen was calculated to be 0.15% by weight on the solids and the quantity of carbon dioxide was calculated to ibe 0.5% by weight on the solids. The polymer, added with the foaming agents, was subsequently cooled in the second part of the extruder maintained at 143°C and extruded with a circular extrusion die. The extruded tubular sheet was set and cooled on a calibration mandrel with a stretching ratio of 1:3.2 maintained at approx. 10° C and subsequently cut and wound up. A sheet having density 350 kg/m³ and a percent of open cells of 65% was obtained.

2. Extrusion.

The polymer was melted within the first part of the extruder maintained at 190-195°C, nitrogen in a quantity equal to 0.15% by weight on the solids, was subsequently added to the polymer. The polymer, added with the foaming agent, was then cooled in the second part of the extruder maintained at 143°C and extruded with a circular extrusion die. The extruded tubular sheet was set and cooled on a calibration mandrel with a stretching ratio of 1:3.2 maintained at approx. 10°C and subsequently cut and wound up. A sheet having density 400 kg/m³ and a percent of open cells of less than 25% was obtained. 3. Extrusion. The test was carried out with a tandem extruder with primary screw diameter 60mm ratio UD 30:1, secondary screw diameter 90mm ratio UD 30:1. The extruded material had the following formulation:

- PLA (Nature Works production) 2002D grade

- Talc (nucleating agent) 1 ,5-2% - Foaming agent C0₂ 1%

Primary extruder temperature (°C) 210-200

Primary screw speed (RPM) 60

Secondary extruder temperature (°C) 170-140

Secondary screw speed (RPM) 7,97 PLA (kg/h) 90

C0₂ (kg/h) 1

Talc (kg/h) 1,5

Utilizing the above working conditions a sheet having the following characteristics was obtained: - web width 650 mm

- basis weight 600 g/m²

- thickness 1,3 mm

- density 470 g/14. Thermoforming .

The sheet obtained in the above examples was thermoformed by heating in an oven at 160°C and then by shaping it in the heated part of a double mould, at a temperature of 125°C, for 6 seconds. The sheet shaped in the hot part was subsequently transferred to the cold part of the double mould for stabilisation.

The above example was repeated by varying the process parameters and in some cases adding a crystallising agent during the extrusion process. Several thermal treatment tests were carried out according to what above mentioned, and foamed PLA trays with optimal resistance to heat deformation were obtained. The deformation tests included the exposure of the thermally treated trays to a temperature of 55, 60, 65 and 70 °C in an oven for a time necessary to reach the equilibrium temperature, time fixed at two minutes. With the normal thermoforming process (oven temperature range within 100°C to 150°C and single cold mould) the maximum temperature the trays could be heated at without deformations was 55°C, whilst all the trays treated as above were found to be resistant to deformation at 60°C; following treatment in the hot mould at approx. 125°C for six seconds, trays resistant to deformation in an oven at 70°C were obtained. Figures 3 and 4 show the DSC graphs of two foamed products. It is to be noted that one of the products has crystallisation and melting peaks (10 J/g, 13 J/g) that are markedly higher with respect to the other (3 J/g, 7 J/g); this shows that the presence of the crystallising agent, added as an additive, in this product considerably increases the crystallisation kinetics.

The trays obtained and provided with open cells, have shown themselves to be able to absorb the liquid released from the meat packaged in them. This property has been verified with a series of tests which have also taken into consideration the weight drop of the packaged product following the loss of serum, which has increased the weight of the tray and has been completely absorbed by the same.

An embodiment of the invention envisages laminating the tray with a film of PLA or equivalent non foamed polymer; the film is applied to the outer side if the tray is intended to contain meat or an analogous product, the secretions of which must be absorbed by the open cell structure of the tray. In a further embodiment of the invention the film is coupled to the inside of the tray by means of the different possible techniques, such as coextrusion, lamination, extrusion coating or others, thus allowing the tray to be used in barrier packaging, by sealing its top edge with a PLA compact film.

Claims

1. A process for the production of degradable foamed lactic acid (co)polymer based products, by extrusion of a mixture comprising at least a foaming agent, a lactic acid (co)polymer and a nucleating agent, characterized in that said mixture is free from additional additives, said foaming agent is selected from nitrogen, carbon dioxide and mixtures thereof, and in that it comprises the steps of: heating said lactic acid polymers to a temperature sufficient to melt and homogenize them, mixing said foaming agent(s) with the molten polymer, cooling the foaming agent and molten polymer mixture to a temperature within the range of T_m+10°C and Tm-30°C, wherein T_m is the melting temperature of said polymer, and extruding the mixture to form a sheet.

2. The process according to claim 1 , wherein said temperature range is chosen as T_m+5⁰C and T_m-20°C, T_m and Tm-10°C.

3. The process according to claim 1 or 2, wherein said temperature range is chosen as T_m and T_m-10°C.

4. A process for the production of degradable foamed lactic acid polymer based products according to claim 1 , by extrusion of a mixture comprising at least a foaming agent, a lactic acid polymer and a nucleating agent, wherein said mixture is free from additional additives selected among foaming aids, blowing agent stabilizers, thickeners, plasticizers and mixtures thereof.

5. The process according to any previous claim, characterized in that the die transit time of said mixture through the extrusion die does not exceed 60 sec.

6. The process according to any previous claim, wherein 0.1 to 0.4% nitrogen, by weight on the solids; 0.3 to 2% carbon dioxide, by weight on the solids; or mixtures thereof are used as foaming gas.

7. The process according to any previous claim, characterised in that said mixture is extruded from a circular extrusion die and the extruded sheet is cooled on circular cooling mandrel placed in close proximity to said extrusion die.

8. The process according to any previous claim, further comprising a thermoforming step of the foamed sheet, characterised in that said sheet is heated and shaped in a hot mould at a temperature within the range of 100 to 150 °C and with a cycle time within the range of 4 to 9 seconds and in that the thus treated sheet is shaped in a cold mould.

9. The process according to any previous claim, further comprising a thermoforming step of the foamed sheet, characterised in that said sheet is heated in an oven at a temperature of 190-200°C and in that the thus treated sheet is shaped in a cold mould.

10. A product of foamed lactic acid polymers as obtainable with a process according to any previous claim, characterised in that it comprises nucleating agents and foaming agents or residues thereof, which are selected only from nitrogen, carbon dioxide and mixtures thereof and in that it is free from additional additives as adjuvants to the foaming step.

11. The product according to claim 10, characterised in that it has a percent of open cells within the range of 10% to 90%.

12. The product according to claims 10 or 11, in the form of a tray, as obtainable according to the process of claims 8 or 9, characterised in that it is resistant to deformation at temperatures of at least 60°C.

13. The product according to any claim 10 to 12, characterised in that it comprises a layer of lactic acid polymer film applied to at least a part thereof.

14. A plant for lactic acid polymers extrusion foaming for carrying out the process according to any of the previous claims, comprising an extruder, means for feeding the polymers/mixtures to be extruded and means for feeding the foaming agents, characterised in that said extruder has an annular extrusion die and that said means of supplying the foaming agents are means for supplying nitrogen, carbon dioxide and mixtures thereof and in that said extrusion die provides a die transition time for the polymer mixture that does not exceed 60 seconds .

15. A plant for thermoforming foamed products of lactic acid polymers, characterised in that it comprises a double thermoforming mould wherein the first part is heated and the second is cooled.

16. The use of a plant according to claim 14 or claim 15 for the manufacture of foamed products of lactic acid polymers.

17. Degradable PLA based foamed product characterized in that it is substantially free from additives as foaming adjuvants, selected from foaming aids, blowing agents, thickeners and plasticizers or mixtures thereof.