NONWOVEN WEB COMPRISING POLYLACTIC ACID, ITS MANUFACTURING PROCESS AND FOOD PACKAGING COMPRISING SUCH A NONWOVEN WEB

Abstract

A heat-resistant nonwoven melt-bonded web, in particular by spun bonding, comprising at least: —a first layer comprising first fibers —a second layer comprising second fibers in which at least one of the first fibers of the first layer (1) or the second fibers of the second layer (2) comprise a polylactic acid stereocomplex representing at least 20% by volume relative to the total volume of the constituents of the first or second fibers.

Claims

1. A heat-resistant nonwoven web produced by melt-bonding, in particular by spun bonding, comprising at least: a first layer comprising first fibers; and a second layer comprising second fibers; wherein at least one of the first fibers of the first layer or of the second fibers of the second layer comprise a polylactic acid stereocomplex representing at least 20% by volume of the total volume of the first or second fibers.

2. The heat-resistant nonwoven web according to claim 1, characterized in that at least one of the first fibers or of the second fibers comprise a cross-section having a structure chosen from a core-sheath structure, eccentric core-sheath or islands-in-the sea structure.

3. The heat-resistant nonwoven web of claim 1, characterized in that at least one of the first fibers or of the second fibers is a single-component fiber.

4. The heat-resistant nonwoven web of claim 1, characterized in that at least one of the first or second fibers are bicomponent fibers having in section a first portion and a second portion.

5. The heat-resistant nonwoven web of claim 4, characterized in that at least one of the first fibers or of the second fibers comprise: the first portion comprising a polylactic acid stereocomplex representing between 20 and 80% by volume, preferably between 30 and 70% by volume, and more preferably 50% by volume of the total volume of the first or second fibers, and the second portion comprising a second polymer representing between 20 and 80% by volume, preferably between 30 and 70% by volume, and more preferably 50% by volume of the total volume of the first or second fibers, the second polymer being also chosen from polyhydroxyalkanoates, polyesters or their copolymers, polyesters being chosen in particular from polylactic acids with the exception of polylactic acid stereocomplexes, polybutylene succinates, polybutylene succinate co-adipates, polycaprolactones, polybutyrate adipate terephthalates.

6. The heat-resistant nonwoven web of claim 1, characterized in that at least one of the first or second fibers have a diameter of less than 30 μm.

7. The heat-resistant nonwoven web of claim 1, characterized in that it has a porosity of between 1,000 l/m.sup.2/min and 9,000 l/m.sup.2/min.

8. A process of manufacturing a heat-resistant nonwoven web of claim 1, said process comprising the following steps: a/supplying a device operating by the melt-bonding process, with at least one dry-blend of a polymer of levorotatory polylactic acid and a dextrorotatory polylactic acid polymer so as to form at least one fiber of the first or second layer; b/where appropriate, supplying at least one fiber of the first layer or of the second layer not formed in step a/; c/assembly of the first and the second layers, at least one of which being formed in step a/and, where appropriate, provided in step b/.

9. The process of manufacturing a heat-resistant nonwoven web of claim 8, wherein during step a/ the dry-blend has a volume ratio between the levorotatory polylactic acid polymer and the dextrorotatory polylactic acid polymer between 65/35 and 35/65.

10. The process of manufacturing a heat-resistant nonwoven web of claim 8, wherein step c/ is implemented in the device operating by the melt-bonding process during a bonding step.

11. A food packaging intended to be immersed in an aqueous solution having a temperature of at least 90° C., comprising a heat-resistant nonwoven web according to claim 1.

12. The heat-resistant nonwoven web according to claim 1, characterized in that at least one of the first fibers or of the second fibers comprise a cross-section having a core-sheath structure.

13. The heat-resistant nonwoven web of claim 4, characterized in that at least one of the first fibers or of the second fibers comprise: the first portion comprising a polylactic acid stereocomplex representing between 30 and 70% by volume, or 50% by volume of the total volume of the first or second fibers, and the second portion comprising a second polymer representing between 30 and 70% by volume, or 50% by volume of the total volume of the first or second fibers, the second polymer being also chosen from polyhydroxyalkanoates, polyesters or their copolymers, polyesters being chosen in particular from polylactic acids with the exception of polylactic acid stereocomplexes, polybutylene succinates, polybutylene succinate co-adipates, polycaprolactones, polybutyrate adipate terephthalates.

14. The heat-resistant nonwoven web of claim 1, characterized in that at least one of the first or second fibers have a diameter of between 12 and 20 μm.

15. The process of manufacturing a heat-resistant nonwoven web of claim 8, wherein the device is a spun bonding device.

16. The process of manufacturing a heat-resistant nonwoven web of claim 8, wherein during step a/ the dry-blend has a volume ratio between the levorotatory polylactic acid polymer and the dextrorotatory polylactic acid polymer between 60/40 and 40/60, or between 55/45 and 45/55.

17. The process of manufacturing a heat-resistant nonwoven web of claim 10, wherein the bonding step is a calendering step.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0069] Other advantages of the present invention will emerge more clearly on reading the following description, given by way of illustration and without limitation, and the accompanying drawings in which:

[0070] FIG. 1A is a schematic representation of a state-of-the-art nonwoven web;

[0071] FIG. 1B is a schematic representation of a nonwoven web according to the present invention;

[0072] FIG. 1C is a schematic representation of a nonwoven web according to the present invention;

[0073] FIG. 1D is a schematic representation of a nonwoven web according to the present invention;

[0074] FIG. 1E is a schematic representation of a nonwoven web according to the present invention;

[0075] FIG. 1F is a schematic representation of a nonwoven web according to the present invention;

[0076] FIG. 1G is a schematic representation of a nonwoven web according to the present invention;

[0077] FIG. 1H is a schematic representation of a nonwoven web according to the present invention;

[0078] FIG. 1I is a schematic representation of a nonwoven web according to the present invention;

[0079] FIG. 2 is an image of fibers of a nonwoven according to the invention obtained under a scanning electron microscope;

[0080] FIG. 3 is a schematic representation of the spinning step of a process according to the present invention;

[0081] FIG. 4 is a schematic representation of a sealing point of a food packaging according to the present invention;

[0082] FIG. 5A are photographs of Sample 1 sealed at different temperatures;

[0083] FIG. 5B are photographs of Sample 2 sealed at different temperatures;

[0084] FIG. 5C are photographs of Sample 3 sealed at different temperatures;

[0085] FIG. 5D are photographs of Sample 4 sealed at different temperatures;

[0086] FIG. 6 is a graph which represents the loosening rate over time of samples with twelve sealing points.

DETAILED DESCRIPTION

[0087] In the following description, reference is made to first and second layers. This is a simple indexing to differentiate and name similar, less non-identical elements. This indexing does not imply a priority of one element over another and such names may easily be interchanged without departing from the scope of the present description. Neither does this indexing imply an order in time or in space to assess the positioning or action of these elements.

[0088] In FIGS. 1A-1I, the fiber sections appear to show parallel fibers, while, in reality, the fibers within a layer are randomly intertwined.

[0089] The fibers of the first layer 1 and the second layer 2 have a section with a core-sheath structure. FIG. 1A shows a nonwoven web according to the state of the art and FIGS. 1B to 1I show nonwoven webs according to the invention.

[0090] In FIG. 1A, the fibers of the first layer 1 have a core 12 and a sheath 11 made of levorotatory polylactic acid having a melting point of between 160° C. and 180° C. which will be designated PLA 1. The fibers of the second layer, for their part, have a core 22 in PLA 1 and a sheath 21 made of polylactic acid having a melting point of between 120° C. and 130° C. which will be designated amorphous PLA2. Therefore, this type of nonwoven web has a sealing window of between 130° C. and 150° C. In all instances, PLA1 and PLA2 do not contain a polylactic acid stereocomplex. FIG. 1B shows a nonwoven web according to the present invention. The core 12 of the first layer, as well as the core 22 and the sheath 21 of the second layer, are made of PLA 1 with a melting point of between 160° C. and 180° C. The sheath of the first layer consists of a polylactic acid stereocomplex which will be referred to as PLA STR having a melting point around 220° C. This nonwoven has a sealing window of between 180° C. and 210° C.

[0091] FIG. 2 shows the fibers of a nonwoven web according to the invention. The images A and B are identical and in the image B the delimitation between the core 12 and the sheath 11 was added to the image.

[0092] Other arrangements of nonwoven webs having a sealing window between 180° C. and 210° C. are shown with reference to FIGS. 1C to 1E. Table 1 summarizes the compositions of these different nonwoven webs.

[0093] It is also possible to have nonwoven webs with a broader sealing window between 130° C. and 210° C. This type of nonwoven webs is shown in FIGS. 1F to 1I and the layer compositions are summarized in Table 1. They offer more possibilities in terms of use.

TABLE-US-00001 TABLE 1 First layer 1 First layer 2 Core Sheath Core Sheath Sealing window FIG. 1A PLA 1 PLA 1 PLA 1 PLA 2 130° C.-150° C. FIG. 1B PLA 1 PLA STR PLA 1 PLA 1 180° C.-210° C. FIG. 1C PLA 1 PLA STR PLA STR PLA 1 180° C.-210° C. FIG. 1D PLA STR PLA STR PLA STR PLA 1 180° C.-210° C. FIG. 1E PLA STR PLA STR PLA 1 PLA 1 180° C.-210° C. FIG. 1F PLA 1 PLA STR PLA 1 PLA 2 130° C.-210° C. FIG. 1G PLA STR PLA STR PLA 1 PLA 2 130° C.-210° C. FIG. 1H PLA STR PLA STR PLA 2 PLA STR 130° C.-210° C. FIG. 1I PLA 1 PLA STR PLA STR PLA 2 130° C.-210° C.

[0094] The PLA1 compound may, for example, be polylactic acid marketed by Natureworks under the reference 6100D with a melting point between 165 and 180° C. or 6202D with a melting point between 155-170° C. The PLA 2 compound may, for example, be polylactic acid marketed by the same company under the reference 6302D with a melting point of between 125-135° C. To obtain fibers comprising a stereocomplex, it is, for example, possible to use a dry-blend of polylactic acid PLA1 with a compound enriched in dextrorotatory polylactic acid.

[0095] Due to the presence of the stereocomplex in the fibers, the nonwoven webs according to the invention exhibit improved mechanical properties at elevated temperatures. Such webs can be exposed to hot or boiling water for a relatively long time compared to a nonwoven web of FIG. 1A. We will now describe a process of manufacturing a nonwoven web according to the present invention by taking the example of the nonwoven web illustrated in FIG. 1B, i.e., a nonwoven web having the following composition: [0096] a first layer 1 comprising first fibers having a cross section with a core-sheath structure with a sheath 11 comprising at least 20% by volume of the polylactic acid stereocomplex PLA STR based on the total volume of the first fibers and a core 12 in levorotatory polylactic acid PLA1, and [0097] a second layer 2 comprising second fibers also having a cross section with a core-sheath structure with the core 22 and sheath 21 in levorotatory polylactic acid PLA 1.

[0098] To form two-components or single-component fibers with a cross section having a core-sheath structure, a spun bonding device is often equipped with a pair of extruders, one of which allows the core to be formed and the other allows the sheath to be formed.

[0099] Thus, to manufacture a nonwoven according to the example illustrated in FIG. 1B, there is provided a spun bonding device with two sets of extruders, one dedicated to the first layer and a second dedicated to the second layer. Ext 11 and Ext 12 designate the extruders which form the first fibers of the first layer and Ext 21 and Ext 22 the extruders which form the second fibers of the second layer.

[0100] Initially, according to a step a1/, the extruders Ext 12, Ext 21 and Ext 22 are supplied from a same storage silo comprising levorotatory polylactic acid granules PLA1 via hoppers for each of the extruders. The extruder Ext 11 is supplied from a separate silo comprising granules of a dry-blend of a levorotatory polylactic acid polymer and a dextrorotatory polylactic acid polymer.

[0101] It is possible to have the PLA1 and the dry-blend in powder form.

[0102] Then, according to a step a2/, the granules are melted in the extruders Ext 11, Ext 12, Ext 21 and Ext 22 and the molten materials are conveyed to two spinneret assemblies 31, 32 with two supply lines for each (not shown in FIG. 3). The first assembly 31 is supplied by two lines, one for each extruder Ext 11 and Ext 12. This assembly makes it possible to form fibers with a sheath comprising a polylactic acid stereocomplex and a core with a polylactic acid PLA1. Similarly, the second assembly is supplied by two lines, one for each extruder Ext 21 and Ext 22. This assembly makes it possible to form fibers with a core and a sheath of polylactic acid PLA1.

[0103] A core-sheath structure may be obtained, for example, through the use of distribution plates defining a channel for each of the core-sheath portions of the fibers at the spinnerets and more specifically at the spinning heads.

[0104] The fibers leaving the assemblies are then partially cooled and drawn according to a step a3/. Partial cooling is commonly referred to as “quenching”. It is often implemented by means of an air jet (not shown in FIG. 3). The partially cooled fibers are drawn to obtain the desired diameter. The drawing can be done by pneumatic means which allows the fibers to be sucked and directed. The pneumatic means preferably uses an air jet.

[0105] After step a3/, according to a step a4/, the fibers are deposited on a belt conveyor 33. In particular, the partially cooled and drawn fibers coming from the extruders 11 and 12, which are designated as the first fibers, are deposited on a belt conveyor 33 which makes it possible to move the first fibers forming the first layer 1 in a machine direction M. The fibers, also partially cooled and drawn coming from the extruders Ext 21 and 22, which are referred to as the second fibers forming the second layer 2, are deposited on the first fibers already on the conveyor belt. The conveyor then conveys the stack of the first layer 1 formed by the first fibers and the second layer 2 formed by the second fibers in the machine direction M.

[0106] The stack of the first layer 1 and the second layer 2 is then directed to a calender in order to bond the fibers of the two layers together in a step c/. This step also helps to bond and consolidate the stack. At the end of this step, a nonwoven web assembly according to the present invention is obtained.

[0107] In FIG. 4, the sealing of a food packaging made with a nonwoven web according to the invention was shown. The nonwoven web used is that also represented in FIG. 1I, i.e., a first layer 1 with first fibers having a core PLA 1 and a sheath PLA STR, and a second layer with second fibers having a core PLA 2 and a sheath PLA STR. This nonwoven web has a sealing window between 130° C. and 210° C. The sealing is carried out by means of two hot jaws 41, 42 which apply a compressive force in the directions X1 and X2. The jaws are at a temperature of at least 130° C. The first layer 1, comprising the stereocomplex having a melting and/or softening temperature greater than 210° C., acts as an insulator. The second layer 2, melted and/or softened, makes it possible to create the seal.

[0108] The examples described above relate to a nonwoven web having two layers. However, a person skilled in the art may envision more than two layers. They may, for example, add to the spun bonding device one or more extruders dedicated to additional layers.

EXAMPLES

[0109] To facilitate measurements, the samples analyzed consist of a single layer.

[0110] Starting raw material:

[0111] The materials used are granules: [0112] levorotatory polylactic acid having a melting point of between 165° C. and 180° C., which is referred to as PLAT [0113] a 50/50 dry-blend of levorotatory polylactic acid and dextrorotatory polylactic lactic acid, which is referred to as PLA STR.

[0114] Sample manufacturing process:

[0115] After drying at 50° C. overnight, the granules are introduced into a spun bonding device so as to form a nonwoven web with fibers having a core-sheath structure. The core-to-sheath volume ratio was varied from 50/50 to 70/30.

[0116] The air temperature for partial cooling is set at 15° C. and the air jet for spinning has a pressure equal to 0.3 MPa. The samples have a basis weight of approximately 24 g/m.sup.2.

[0117] The dedicated core extruder operates at a temperature of 225° C. at the inlet and a temperature of 250° C. at the outlet. The dedicated sheath extruder operates at a temperature of 170° C. at the inlet and 200° C. at the outlet.

[0118] The fibers are bonded by calendering at a temperature of 155° C. and at a pressure of 120 kN/m.

[0119] Different characterization tests:

[0120] The nonwoven webs were analyzed with a device for measuring the resistance to heat or “hot tack”. This device is used to seal and peel nonwoven webs and measure peel strength. This strength may be measured either just after sealing while the nonwoven web is still hot, or after a few seconds when the nonwoven web is at room temperature. The test parameters are as follows: [0121] sealing temperature: from 160° C. to 180° C., [0122] sealing pressure: 0.1 N/mm.sup.2, [0123] sealing time: 5 s, [0124] cooling time: 1 s, [0125] peeling speed: 100 mm/s, [0126] sample width: 15 mm

[0127] The results of the heat resistance test are summarized in Table 2, where appropriate the values of the peel strength are shown in the table. These values are expressed in Newtons.

TABLE-US-00002 TABLE 2 Sealing temperature and time 160° 163° 165° 168° 170° 180° Sam- Ratio C. C. C. C. C. C. ple Core Sheath C/S (10 s) (10 s) (10 s) (10 s) (10 s) (10 s) 1 PLA1 PLA1 NA PS 0.43 1.8* SF SF SF 2 PLA PLA1 50/50 PS PS 0.41 2.54* SF SF STR 3 PLA PLA1 60/40 PS PS 0.49 2.35* SF SF STR 4 PLA PLA1 70/30 PS PS 0.25 1.27  2.46 SF STR

[0128] In Table 2, the PS boxes mean that there was no seal and the SF boxes mean a melt seal. Values with a star (*) are those for which there has been a slight tearing. It should be noted that the addition of the stereocomplex in the core of the fibers makes it possible to move the sealing window towards high temperatures. In addition, the peel strength is increased for samples comprising the stereocomplex. The samples according to the invention, i.e., samples 2, 3 and 4, are therefore less likely to melt during sealing at a high temperature and are those which have better peel strength.

[0129] Samples were also tested by a heat seal process on a Brugger device. This process consists of sealing a sample on itself by placing it between two heated jaws. Then, the seal is evaluated before and after 30 minutes of immersion in boiling water.

[0130] The sealing parameters are as follows: [0131] jaws temperatures: 120, 130, 140, 150, 160, 165, 170, 175, 180, 185, 190, 195, 200, 210, 215 and 220° C., [0132] pressure: 2 bars, [0133] sealing time: 0.5 s.

[0134] The samples obtained after sealing are shown in FIGS. 5A to 5D. Zones 50, 51, 52 and 53 respectively represent samples for which there is no seal, acceptable seal, melt seal, and tear seal. These results also show that there is a displacement of the sealing window towards high temperatures. The sample comprising fibers with a PLAT core-sheath structure degrades substantially at a temperature above 200° C. However, Sample 4 with fibers comprising 70% PLA STR is resistant up to 220° C.

[0135] In order to assess the strength of the seal in boiling water, samples 1 and 4 are sealed at different temperatures, shown in Table 3, and then immersed for three hours in boiling water. Table 3 summarizes the results obtained, the designations (O), (OP) and (S) respectively denote opening of the seal, partial opening of the seal and seal maintained.

TABLE-US-00003 TABLE 3 Seal strength Sample Low Average High 1 154° C. (O) 158° C. (OP) 162° C. (S) 4 154° C. (O) 158° C. (OP) 162° C. (S) 168° C. (S) —

[0136] Samples 1 and 4 with twelve sealing points were also immersed in boiling water to assess the loosening rate over time. Sample 1 is sealed at 154° C. and Sample 4 at 158° C. in order to have close seal strength. The results of this test are shown in FIG. 6. This result shows that Sample 4 comprising PLA STR exhibits a seal which is more resistant to boiling water than Sample 1 comprising only PLA 1. Thus, a food packaging comprising a nonwoven web according to the present invention can be submerged in boiling water for a relatively long time.