Patent Application
20230374711
The present invention relates to nonwoven webs comprising bio-based materials, and in particular polylactic acid.
In particular, the invention relates to a nonwoven web suitable for use in the field of packaging for hot drinks such as tea bags, infusion bags or coffee pods. The invention also relates to a nonwoven web suitable for forming packaging intended for cooking food in water. The invention also relates to a process of manufacturing such a nonwoven web.
In order to improve the sustainable management of certain packaging, there is a growing interest in bio-based materials. These materials can have a lower carbon footprint than those from fossil resources such as polyethylene terephthalate (PET), polypropylene (PP), polyamide (PA), for example. In addition, materials with improved biodegradability compared to other conventional plastic materials, especially those from petrochemicals, are also considered during the development of new products, including consumer single-use products.
Polylactic acid is a bio-based material which is, moreover, compostable under certain conditions. Polylactic acid is a polyester obtained during the condensation polymerization of lactic acid. Lactic acid is obtained by fermentation of sugars from carbohydrate sources such as corn or sugar cane, for example.
Nonwoven packaging is used in particular for tea infusions, for coffee pods or even for cooking certain foods. In order to provide a compostable product under industrial conditions, in particular according to ASTM D6400 and EN 13432 standards, before or after use, nonwoven web packaging comprising polylactic acid is currently offered on the market.
Heat-sealable nonwoven webs based on polylactic acid are known in the market. However, this product is not entirely satisfactory, especially for applications requiring exposure to hot or boiling water for an extended period of time.
Document JP 2009133022 describes a woven web comprising a polylactic acid stereocomplex. This woven web is obtained by a first step of manufacturing yarns comprising a polylactic acid stereocomplex followed by a weaving step.
Document WO 2009/042837 describes a process for manufacturing bicomponent fibers comprising a polylactic acid stereocomplex. This process requires having two separate extruders to form the portion of the fibers comprising the polylactic acid stereocomplex, which further complicates the manufacturing process of the fibers.
Therefore, there is a need to provide nonwoven webs derived from bio-based materials, in particular based on polylactic acid, these nonwoven webs possibly being biodegradable and/or compostable and having improved thermal and mechanical strength properties.
The aim of the invention is to, at least partially, satisfy this need, while preserving the compostability of nonwoven webs comprising polylactic acid.
According to the invention, this object is achieved by means of a heat-resistant nonwoven web produced by melt-bonding, in particular by spun bonding, comprising at least:
A melt-bonding process comprises in particular the spun-bonding and melt-blowing processes. These processes are advantageous because the fiber formation and the bonding steps are carried out online in the same process, the fibers form almost continuously the nonwoven. Thus, the nonwoven is obtained directly in a single process.
The spun bonding process makes it possible to manufacture spun-bonded fibers commonly called “spunbond” fibers in the state of the art, i.e., fibers formed by extruding a molten material in the form of fibers through the orifices of a spinneret, said fibers being drawn in particular by air jet, deposited on a topping table then bound together, preferably by calendering.
The melt-blowing process makes it possible to manufacture melt-blown fibers, as commonly known in the prior art. As with the spun bonding process, fibers are formed by extruding a molten material through the orifices of a spinneret. However, this process differs from the spun bonding process in particular by the temperature and the volume of air used for drawing the fibers, as well as the location where the air jet comes into contact with the material exiting the spinnerets. For a melt-blowing process, the air temperature is generally high and the air jet comes into contact with the molten material at the spinnerets.
The term “nonwoven” is understood to mean any web of fibers interwoven at random, with the difference, for example, of the arrangement of the fibers constituting a woven fabric or a knitted fabric.
The term “web” is understood to mean a fibrous material capable of being wound into a roll. The expression “polylactic acid stereocomplex” is understood to mean a polylactic acid polymer having a crystalline structure with a melting point higher than that of levorotatory or dextrorotatory polylactic acid alone, such a polylactic acid stereocomplex being obtained by a mixture of levorotatory polylactic acid and dextrorotatory polylactic acid.
The term “heat-resistant” nonwoven is understood to mean a nonwoven comprising a polylactic acid stereocomplex having a melting point of at least 200° C., and preferably between 210 and 220° C. As a result, this nonwoven will have a sealing temperature between 130 and 210° C. while the conventional nonwovens comprising polylactic acid devoid of polylactic acid stereocomplex, have a sealing window between 130 and 150° C.
A nonwoven web according to the present invention exhibits improved mechanical properties, in particular, thanks to the presence of the polylactic acid stereocomplex. This makes it possible to transform or shape a nonwoven according to the present invention by a downstream process operating at a higher speed than usual with a conventional nonwoven comprising polylactic acid.
In addition, the nonwoven web exhibits better resistance when exposed to hot or boiling water for a relatively long period of time.
The first layer can be stacked directly on the second layer, therefore resting entirely on this layer. The heat-resistant nonwoven web is preferably a bilayer assembly, in particular formed of a first layer comprising first fibers and a second layer comprising second fibers.
The first fibers and/or the second fibers may have a cross section presenting a shell-core structure and, more particularly, chosen from a core-shell structure, eccentric core-shell structure or islands in the sea, the core-shell structure being preferred. The shell part is therefore the sheath or sea part of the first or second fibers and the core part is therefore the core or island part of the first or second fibers.
Thus, preferably the shell is the part which includes the stereocomplex but it is possible to have a stereocomplex at the nucleus level. It is also possible to have a stereocomplex at the shell and the core.
The shell-core volume ratio may be at least 20/80, preferably this ratio is between 20/80 and 80/20, more preferably between 30/70 and 70/30 and even more preferably close to 50/50 or even equal to 50/50.
The first fibers and the second fibers may further be single-component fibers, i.e., fibers having a single portion. The first fibers and the second fibers may also be two-component having in particular a cross-sectional first portion and second portion.
The nonwoven according to the present invention may be compostable under industrial conditions. In this case, the nonwoven decomposes over time, in particular thanks to the action of microorganisms in the presence or absence of oxygen. The nonwoven according to the invention is, in particular, likely to meet the ASTM D6400 or EN 13432 standards by adapting the basis weight of the nonwoven.
Advantageously, the first fibers, on the one hand, and the second fibers, on the other hand, have a different melting point. The difference of these fiber melting temperatures is at least 20° C.
This temperature difference is useful, for example, when a nonwoven web assembly according to the present invention is shaped by heat-sealing for food packaging applications.
According to a particular embodiment, the first fibers and/or the second fibers comprise:
Preferably, at least one of the first fibers or the second fibers have a diameter of less than 30 μm, and more preferably between 12 and 20 μm.
The nonwoven web according to the present invention may have a porosity of between 1000 l/m2/min and 9000 l/m2/min. The porosity of a nonwoven is measured according to DIN 53887. According to a particular embodiment, the nonwoven web has a basis weight of between 10 g/m2 and 50 g/m2, and more preferably between 15 g/m2 and 30 g/m2. This basis weight of the nonwovens being determined according to the ISO 536-2012 standard.
The nonwoven web according to the present invention may have a thickness between 60 microns and 180 microns, and preferably between 90 microns and 150 microns. Preferably, this thickness is substantially uniform over the entire web.
The first fibers and the second fibers may also comprise at least one additive such as, for example, a polymeric plasticizer or an antistatic agent. These fibers may comprise at least 10% by volume of this additive.
The invention also relates to a process for manufacturing a heat-resistant nonwoven web according to the present invention, the manufacturing process comprising the following steps:
The term “dry mixture” is understood to mean a mixture of at least two types of non-melted or non-solubilized solids. This type of mixture is commonly referred to as a “dry-blend”. For example, a “dry-blend” may be obtained by mixing at room temperature two types of granules or powders in the solid state.
Having as a starting material a dry-blend of a compound enriched in levorotatory polylactic acid and a compound enriched in dextrorotatory polylactic acid makes it possible to form a polylactic acid stereocomplex in the spun bonding device, more particularly at the fiber spinning step. This is particularly advantageous because the direct spinning of the polylactic acid stereocomplex is not easy. Indeed, when using a polylactic acid stereocomplex directly as a starting material, the temperature required to melt the granules is relatively high. The material, once melted, has a very low viscosity which makes it very difficult to draw and generates breakage of the filaments.
In the dry-blend, the volume ratio between the levorotatory polylactic acid polymer and the dextrorotatory polylactic acid polymer is preferably between 65/35 and 35/65, and more preferably between 60/40 and 40/60, and again more preferably between 55/45 and 45/55. The ratio may also be close to 50/50 or be equal to 50/50.
These ratios make it possible, in particular, to optimally form the stereocomplex in the fibers while minimizing the formation of other products which are not a polylactic acid stereocomplex.
The process of the invention is easy to implement compared to the solutions proposed in the state of the art. The process according to the invention makes it possible to obtain a nonwoven in a single step, unlike the process described in document JP 2009/042837. The fiber formation and the bonding steps are carried out online in the same process, the fibers form almost continuously the nonwoven. The process may be implemented in a device operating by the melt-bonding process and, more particularly, in a spun bonding device with a minimum of modification.
According to a first particular embodiment, the device operating by the melt-bonding process is a spun bonding device and at least the following steps are implemented in such a device:
After step a4/, the fibers may pass through a calender in order to bind the fibers in a bonding step.
According to another particular embodiment, the device operating by the melt-bonding process is a melt-blowing device. In such a device, at least the following steps are implemented:
After step a4′/, the fibers may also pass through a calender in order to bond the fibers in a bonding step.
According to a first variant, step b/ is implemented in the same device operating by the melt-bonding process as that of step a/, but with an independent supply of raw material and so as to form one of the first layer or the second layer not formed in step a/.
According to a second variant, step c/ of assembling the first layer and the second layer is implemented in the device operating by the melt-bonding process during the bonding step.
These two variants may be particularly useful when the first fibers and the second fibers are spun in the same device operating by the melt-bonding process having two sets of spinnerets, one of which is dedicated to the spinning of the first fibers and the other is dedicated to the spinning of the second fibers. Thus, a nonwoven web according to the invention may be obtained with a minimum of steps.
In order to improve the spinning of the fibers comprising the polylactic acid stereocomplex according to the present invention, the molten material may be at a temperature between 200° C. and 280° C., preferably between 210° C. and 260° C., and more preferably at a temperature between 220° C. and 235° C. at the extruder outlet.
Step a3/ is preferably carried out by air jet. The drawing of the fibers may be carried out at a speed of between 2000 and 7000 m/min.
Finally, the invention relates to 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 the present invention.
A food packaging according to the present invention may be closed by heat sealing. If, for example, the second layer comprises fibers with a lower melting point than that of the fibers of the first layer, this second layer constitutes, in this particular example, the internal surface of the food packaging. The outer surface of the packaging will therefore consist of the first layer. Thus, the second layer is the one that will be melted or softened at the sealing points while the first layer acts as an insulator and avoids direct contact between the sealing jaws and the second layer.
However, it is also possible to close a food packaging according to the present invention by mechanical means, for example using a thread or a staple.
Due, in particular, to the presence of the polylactic acid stereocomplex, a nonwoven web according to the present invention may be transformed into a food packaging at a higher speed than that used with a nonwoven web based on conventional polylactic acid. Indeed, a nonwoven web according to the invention has improved mechanical properties at elevated temperatures and therefore better withstands the stresses undergone during its transformation or even its shaping, in particular, due to the increase in the melting temperature of the fibers bound thanks to the presence of PLA stereocomplex in these fibers.
Moreover, a nonwoven web according to the invention may be sealed with more force because it is more mechanically resistant. Therefore, the seals obtained can be more robust.
Therefore, a food packaging according to the invention can be exposed to hot or boiling water for a relatively long period of time compared to a nonwoven web (based on polylactic acid) known from the prior art.
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:
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.
In
The fibers of the first layer 1 and the second layer 2 have a section with a core-sheath structure.
In
Other arrangements of nonwoven webs having a sealing window between 180° C. and 210° C. are shown with reference to
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
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.
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
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.
Thus, to manufacture a nonwoven according to the example illustrated in
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.
It is possible to have the PLA1 and the dry-blend in powder form.
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
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.
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
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.
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.
In
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.
To facilitate measurements, the samples analyzed consist of a single layer.
Starting raw material:
The materials used are granules:
Sample manufacturing process:
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.
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/m2.
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.
The fibers are bonded by calendering at a temperature of 155° C. and at a pressure of 120 kN/m.
Different characterization tests:
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:
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.
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.
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.
The sealing parameters are as follows:
The samples obtained after sealing are shown in
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.
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
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010366 | Oct 2020 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FI2021/050670 | 10/8/2021 | WO |
