FR2990209A1 - FOAMS BASED ON THERMOPLASTIC STARCH AND POLYOLEFIN - Google Patents

Abstract

The subject of the invention is a foam having a density of less than 300 kg / m obtained from a composition comprising: a polyolefin selected from the group consisting of polyethylene and polypropylene and mixtures thereof; . at least one functionalized polyolefin comprising from 0.1 to 10% by weight of at least one functional monomer (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes, preferentially maleic anhydride or glycidyl methacrylate; . from 2 to 40% by weight of starch relative to the total weight of the composition; . from 2 to 40% of at least one plasticizer of this starch relative to the total weight of the composition. The invention also relates to a method of manufacture and uses of said foam.

Description

Field of the Invention The present invention relates to thermoplastic starch and polyolefin based foams, a process for preparing these foams and their uses. State of the art Polyolefin foams are used in many fields, such as the fields of packaging, building, automotive, transportation, and sports. These polyolefin foams, such as those comprising polyethylene or polypropylene, generally have the advantage of being inexpensive. Schematically, the foams are made by a foaming process in which a polyolefin is melted in a reactor in the presence of a foaming agent. During the process, gas bubbles are created in the molten polymer, which will lead to foam formation upon cooling of the polymer. The structure of the foam will depend on the polymer used, the processing apparatus and the process conditions used, in particular the selected foaming agent. The latter may be a chemical foaming agent, which will release a gas by thermal decomposition, or a physical foaming agent which is a gas or a liquid forming a gas upon evaporation of the agent. The properties of these foams may vary depending on different factors. The main factor is the structure of the foam, especially its density and the average cell diameter of the foam. The final properties of these foams will vary widely depending on the structure obtained. These polyolefin foams have good mechanical properties, low liquid water absorption and limited water vapor permeability. When they are of low density and the diameter of the cells constituting it is also small, they also have good insulating properties, phonically and thermally, as well as good damping properties to shock and vibration. They also have a very good buoyancy.

However, these foams have insufficient fire resistance for many applications. However, the flame retardant compounds conventionally used have the disadvantage of severely degrading the properties of the composition, which prevents it from being converted into standard foaming processes, in particular when this foaming process comprises an injection of a foaming gas or a liquid capable of forming a foaming gas. In this respect, mention may be made of EP 2085421 A1, which describes improved flame retarded polyolefin foams. This foam is made from a composition comprising relatively low amounts of flame retardant additive, which makes it possible to produce foams having satisfactory mechanical properties. However, in addition to the fact that this solution has the disadvantage of using carbon nanotubes whose cost is very high, the red phosphorus used has the defect of being naturally highly flammable, at least before it is diluted in water. flame retardant polyolefin composition. In addition, although the flammability of the foam is improved, it has the disadvantage of releasing toxic and / or corrosive gases during combustion. Another problem with polyolefin foams is that, during their manufacturing process, the polyolefin is melt in the presence of a gas to form the cells of the foam. It is generally desired that the gas in these cells be removed before the formed foams can be used, particularly when the gas used is an alkane such as isobutane. It is then necessary that the foam is stored before use during a degassing time. This is particularly true for foams having a low density, for example less than or equal to 300 kg / m3, since the foam that will have to be degassed generally comprises many cells. In addition, during this period of degassing, the foams have a high risk of flammability.

Another disadvantage of these foams is that the polyolefins constituting them are often obtained from monomers obtained from fossil resources. To develop new foams from renewable resources, foams made from thermoplastic starch have been proposed. However, it is known that these foams have a high resilience, that is to say that the properties of the foam are degraded over time, which is unacceptable for many applications. it has also already been proposed to manufacture thermoplastic starch and polyolefin foams, as for example in EP2374846 A1. Although the mechanical properties of the foams described in this document are improved compared to foams made of starch thermoplastic, they remain however very weak. The Applicant has been able to manufacture a foam comprising material of plant origin, which thus limits the use of fossil resources.

This foam is made based on a composition comprising in particular thermoplastic starch and polyolefins. It has good fire resistance properties, while retaining the excellent properties of polyolefin foams. In particular, despite the presence of thermoplastic starch in the composition, these foams have excellent mechanical properties. Without being bound to any theory, the Applicant explains these best properties by the choice of the particular composition constituting the foam, and in particular selected polyolefins. It was able to form, using this composition, a foam which has a low density, while managing to maintain a uniform distribution of cell sizes. This results in improved properties compared to thermoplastic starch-based foams already known since the foam obtained also has the advantages already described of low density polyolefin foams. In addition, the fire behavior of this foam is very good, even without using expensive additives such as carbon nanotubes. Moreover, in case of combustion of the foam during a fire, this foam has the advantage of releasing small amounts of corrosive or toxic gases. Another advantage is that this foam may have a low color. It is easily dyeable by dyes conventionally used in the field of polyolefin foams. This foam could be made by a particular process, which also has the advantage of being able to release little odor and smoke. The Applicant has also been able to observe that the degassing time is decreased compared to equivalent processes using exclusively polyolefins, which advantageously makes it possible to reduce the process of producing foams. This also considerably reduces the time period during which foams have a high flammability risk. SUMMARY OF THE INVENTION The subject of the invention is thus a foam obtained from a composition comprising, relative to the total weight of the composition: a polyolefin chosen from the group comprising polyethylene and polypropylene and their mixtures; at least one functionalized polyolefin comprising from 0.1 to 10% by weight of at least one functional monomer (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes, preferentially maleic anhydride or glycidyl methacrylate - from 2 to 40% by weight of starch; from 2 to 40% of at least one plasticizer of this starch; said foam having a density of less than 300 kg / m 2. The Applicant has found that this foam according to the invention has mechanical properties very similar to those of polyolefin foams.

In addition, contrary to what is described in document EP2374846 A1, the composition used according to the invention comprises a polyolefin comprising from 0.1 to 10% by weight of at least one functional monomer chosen from acid anhydrides. unsaturated carboxylic acids, unsaturated oxiranes and unsaturated silanes, preferentially maleic anhydride or glycidyl methacrylate. These functional polyolefins may have a relatively low cost and the composition useful for the invention has good thermal and light stabilities, unlike a similar composition which would include other functional polyolefins, such as ethylene-acrylic acid or ethylene copolymers. -vinyl acetate. Indeed, these have different disadvantages. These copolymers exhibit poor fire behavior, which results in a very high flammability and the release of corrosive gases in case of fire such as acetic acid. In addition, these copolymers have a low crystallinity and a low glass transition temperature, leading to degraded mechanical properties as soon as the temperature of use of the material increases. Another disadvantage of these copolymers is that their thermal stability is very poor, which leads to degradation reactions during their shaping. In addition, these copolymers have a strong adhesion to metals, which can cause problems during the implementation of the process.

The foam according to the invention thus has improved properties and in particular excellent mechanical properties, these mechanical properties being preserved over time.

Despite the low density of the foam, the foam according to the invention can maintain a very small cell diameter. The Applicant has observed that this cell diameter remained low even in the presence of significant amounts of thermoplastic starch, for example greater than or equal to 15%, or even up to 30% or more. This makes it possible to obtain mechanical, insulating and damping properties that are much greater than the foams of the prior art. Without being bound to any theory, the Applicant explains this improvement by the choice of the composition that is useful for the invention, and more particularly the selection of the functional or functionalized polyolefin. The foam according to the invention could be manufactured using a process. particular using a physical foaming agent.

Another object of the invention thus relates to a method of manufacturing a foam according to the invention comprising the following steps: a. introducing the composition or constituents of the composition into an extruder; b. heating and mixing to melt the composition and homogenize it; vs. injecting a foaming gas or a liquid capable of forming a foaming gas in the extruder; d. homogenizing the mixture of the composition and the foaming gas e. partial cooling of the mixture and homogenization; f. extrusion through a die of this mixture causing the formation of the foam at the die outlet. As previously explained, the foam thus formed has excellent properties.

Another advantage of this process is that the foam is prepared from a process which does not use chemical blowing agents, which are very expensive products. Another disadvantage of processes using chemical blowing agents is that it is necessary during the foam manufacturing process that the temperature of the composition exceeds the degradation temperature of the solid agent. However, this degradation temperature can be quite high, which can lead to degradation of the thermoplastic starch included in the composition. In the case where the temperature is lowered during the process so as not to degrade the starch, the agent does not change and the formation of the foam is not done. The invention will now be described in detail in the description which follows. DETAILED DESCRIPTION OF THE INVENTION The foam according to the invention is based on a composition comprising: a polyolefin selected from the group comprising polyethylene and polypropylene and mixtures thereof; At least one functionalized polyolefin comprising from 0.1 to 10% by weight of at least one functional monomer (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes, preferentially anhydride; maleic or glycidyl methacrylate - starch; and at least one plasticizer of this starch. The polyolefin useful in the manufacture of the composition used according to the invention is selected from the group consisting of polyethylene and polypropylene and mixtures thereof. According to the invention, this polyolefin is a "non-functionalized polyolefin", that is to say that it does not comprise any functional monomer.

The polyethylene may be linear or branched. The polyethylene may be a polyethylene obtained by radical polymerization or obtained catalytically. By way of example of polyethylene obtained catalytically, mention may be made of polyethylenes obtained by Ziegler Natta catalysis, in particular metallocene catalysis. This polyethylene may be a homopolymer of ethylene or a copolymer of ethylene and another alpha-olefin, the latter being selected from propylene, 1-butene, 1-hexene, 1-octene and 1-hexene. - decene. When the polyethylene is a copolymer, it comprises at least 50 mol% of ethylene. Advantageously, this polyethylene has a density of between 0.905 and 0.940. Polypropylene is a homopolymer of propylene or a copolymer of propylene and another monomer. When it is a copolymer, it is preferred that it comprises at least 50 mol% of propylene. Preferably, the polypropylene is a branched polypropylene, especially chosen from polypropylenes with high melt resistance, well known under the name High Melt Strength PolyPropylene (HMS-PP). They are generally produced by irradiation or by reactive extrusion of the polypropylene obtained after the polymerization step. These polypropylenes can be products of the DAPLOY® range manufactured by Borealis2 or products of the PROFAX® range manufactured by Lyondell-Basell®. According to the invention, the expression "when a polymer comprises a monomer" means that the polymer comprises a pattern that can be obtained by polymerization or grafting of this monomer. It is also specified that when a polymer comprises a monomer, it means that it comprises at least one monomer of this chemical nature. By way of example, when a polymer comprises maleic anhydride, this means that the polymer comprises at least one unit that can be obtained by polymerization or grafting of maleic anhydride.

The functionalized polyolefin useful in the invention comprises from 0.1 to 10% by weight of at least one functional monomer (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes. The amount of monomer (X) is preferably from 0.2 to 5% by weight of the functionalized polyolefin, preferably from 0.4 to 3%. The amount of functional monomer in the polyolefin can be measured conventionally by infrared. In the case where different functional monomers (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes are included in the polyolefin, the amount of functional monomer (X) represents the sum of the amounts of functional monomers ( X) included in the polyolefin.

This polyolefin may also comprise at least one additional functional monomer, different from (X). This additional functional monomer may be chosen from alkyl acrylates, alkyl methacrylates and dienes. Preferably, the functionalized polyolefin does not include vinyl ester, acrylic acid and methacrylic acid. According to one variant, the functional monomer (X) is copolymerized in the functionalized polyolefin, for example by reacting the functional monomer (X) with the other monomers, such as ethylene and / or propylene, to form the polymer. The polymerization can be done by a radical route. The radical polymerization process can be carried out conventionally in an autoclave or tubular reactor. The polymerization by autoclave or tubular reactor polymerization of the various monomers (alpha-olefin, comonomer comprising the functional monomer (X)) is known to those skilled in the art.

According to another advantageous variant, the functional monomer (X) may be grafted onto a polyethylene or a polypropylene to form the functional or functionalized polyolefin.

In this case, this grafting operation of the functional monomer is generally carried out by introducing into a reactor polyethylene or polypropylene, the functional monomer (X) and an initiator, the latter generally being an organic peroxide. Similarly, grafting is an operation known per se. The polyolefin to be grafted with the functional monomer may be any polyethylene or polypropylene already described. The functionalized polyolefin containing the functional monomer (X) is according to the invention if several different functional monomers are copolymerized and / or grafted. It would not be departing from the scope of the invention if the functionalized polyolefin consists of a mixture of functionalized polyolefins. The sum of the amounts of polyolefin and functionalized polyolefin is preferably from 25 to 95% by weight of the composition, preferably from 45 to 90%, most preferably from 55% to 85%. The functionalized polyolefin / polyolefin mass ratio may range from 10/90 to 99/1, advantageously from 45/55 to 98/2, and preferably from 70/30 to 95/5. The starch included in the composition used according to the invention may be of any type. If it is desired to obtain a foam of lower cost, the starch preferentially used for the manufacture of the composition is a granular starch, preferably a native starch. Here, the term "granular starch" means a starch native or physically modified, chemically or enzymatically, having retained, within the starch granules, a semi-crystalline structure similar to that evidenced in the grains of starch naturally present in reserve organs and tissues of higher plants, particularly in cereal seeds, legume seeds, potato or cassava tubers, roots, bulbs, stems and fruits. This semi-crystalline state is essentially due to macromolecules of amylopectin, one of the two main constituents of starch. In the native state, the starch grains have a degree of crystallinity which varies from 15 to 45%, and which depends essentially on the botanical origin of the starch and the possible treatment that it has undergone. Granular starch, placed under polarized light, has a characteristic black cross, so-called Maltese cross, typical of the granular state.

According to the invention, the granular starch can come from all botanical origins, including a granular starch rich in amylose or conversely, rich in amylopectin (waxy). It may be starch native to cereals such as wheat, maize, barley, triticale, sorghum or rice, tubers such as potato or cassava, or legumes such as peas and soybeans, and mixtures of such starches. According to one variant, the granular starch is an acid hydrolyzed, oxidizing or enzymatic starch, or an oxidized starch. It may be a starch commonly referred to as a fluidized starch or a white dextrin. According to another variant, it may also be a starch modified physico-chemically but having essentially retained the structure of the native starch starch, such as in particular the esterified and / or etherified starches, in particular modified by acetylation, hydroxypropylation, cationization, crosslinking, phosphatation, or succinylation, or starches treated in aqueous medium at low temperature (in English "annealing"). Preferably, the granular starch is a native, hydrolysed, oxidized or modified starch, in particular corn, wheat or pea.

Granular starch generally has a degree of soluble at 20 ° C in demineralized water, less than 5% by weight. It is preferably almost insoluble in cold water.

According to a second variant, the starch selected as starch useful in the manufacture of the composition used in accordance with the invention is a water-soluble starch, which may also come from all botanical origins, including a starch, which is water-soluble, rich in amylose or, conversely, , rich in amylopectin (waxy). This water-soluble starch can be introduced as a partial or total replacement of the granular starch. For the purposes of the invention, the term "water-soluble starch" means any starchy component which, at 20 ° C. and with mechanical stirring for 24 hours, has a soluble fraction in demineralized water of at least 5% by weight. This soluble fraction is preferably greater than 20% by weight and in particular greater than 50% by weight. Of course, the water-soluble starch can be totally soluble in demineralized water (soluble fraction = 100%). Such water-soluble starches can be obtained by pregelatinization on a drum, by pregelatinization on an extruder, by spraying a suspension or a starch solution, by precipitation with a non-solvent, by hydro-thermal cooking, by chemical functionalization or the like. It is in particular a pregelatinized, extruded or atomized starch, a highly converted dextrin (also called yellow dextrin), a maltodextrin, a functionalized starch or any mixture of these products.

The pregelatinized starches may be obtained by hydrothermal treatment of gelatinization of native starches or modified starches, in particular by steam cooking, jet-cocked cooking, cooking on a drum, cooking in kneader / extruder systems then drying for example in an oven, by hot air on a fluidized bed, on a rotating drum, by atomization, by extrusion or by lyophilization. Such starches generally have a solubility in demineralized water at 20 ° C. of greater than 5% and more generally of between 10 and 100% and a starch crystallinity level of less than 15%, generally less than 5% and most often less than 1%, or even none. By way of example, mention may be made of the products manufactured and marketed by the Applicant under the brand name PREGEFLOCD. Highly processed dextrins can be prepared from native or modified starches by dextrinification in a weakly acidic acid medium. It may be in particular soluble white dextrins or yellow dextrins. By way of example, mention may be made of the STABILYSCD A 053 or TACKIDEXCD C 072 products manufactured and marketed by the Applicant. Such dextrins have demineralized water at 20 ° C, a solubility generally between 10 and 95% and a starch crystallinity of less than 15% and generally less than 5%. Maltodextrins can be obtained by acid, oxidative or enzymatic hydrolysis of starches in an aqueous medium. They may in particular have an equivalent dextrose (DE) of between 0.5 and 40, preferably between 0.5 and 20 and better still between 0.5 and 12. Such maltodextrins are for example manufactured and marketed by the Applicant under the trade name GLUCIDEXCD and have a solubility in demineralized water at 20 ° C, generally greater than 90% or even close to 100% and a crystallinity in lower starch generally less than 5% and usually almost zero. The functionalized starches can be obtained from a native or modified starch. The high functionalization may for example be carried out by esterification or etherification at a sufficiently high level to confer a solubility in water. Such functionalized starches have a soluble fraction as defined above, greater than 5%, preferably greater than 10%, more preferably greater than 50%.

The functionalization can be obtained in particular by aqueous phase acetylation of acetic anhydride, mixed anhydrides, glutamate hydroxypropylation, dry phase cationization or glue phase, anionization in the dry phase or glue phase by phosphatation or succinylation. These water-soluble, highly functionalized starches may have a degree of substitution of between 0.01 and 3, and more preferably between 0.05 and 1. Preferably, the reagents for modification or functionalization of starch are of renewable origin. . According to another advantageous variant, the water-soluble starch is a water-soluble starch of corn, wheat or pea or a water-soluble derivative thereof. In addition, it advantageously has a low water content, generally less than 10%, preferably less than 5%, in particular less than 2.5% by weight and ideally less than 0.5%, or even less than 0.2. % in weight. According to a third variant, the starchy component selected for the preparation of the composition used according to the invention is an organomodified starch, preferably organosoluble, which may also come from all botanical origins, including an organomodified starch, preferably an organosoluble starch, rich in amyloidosis or, conversely, rich in amylopectin (waxy). This organosoluble starch may be introduced as partial or total replacement of the granular starch or of the water-soluble starch. For the purposes of the invention, the term "organomodified starch" means any starchy component other than a granular starch or a water-soluble starch according to the definitions given above. Preferably, this organomodified starch is almost amorphous, that is to say has a starch crystallinity level of less than 5%, generally less than 1% and especially zero. It is also preferably "organosoluble", that is to say present at 20 ° C, a fraction soluble in a solvent selected from ethanol, ethyl acetate, propyl acetate, ethyl acetate and the like. butyl, diethyl carbonate, propylene carbonate, dimethyl glutarate, triethyl citrate, dibasic esters, dimethyl sulfoxide (DMSO), dimethyl isosorbide, glycerol triacetate, isosorbide diacetate, dioleate isosorbide and methyl esters of vegetable oils, at least equal to 5% by weight. This soluble fraction is preferably greater than 20% by weight and in particular greater than 50% by weight. Of course, the organosoluble starch may be totally soluble in one or more of the solvents indicated above (soluble fraction = 100%). The organomodified starch may be used according to the invention in solid form, including having a low water content, ie less than 10% by weight. It may especially be less than 5%, in particular less than 2.5% by weight and ideally less than 0.5%, or even less than 0.2% by weight. The organomodified starch that can be used in the composition used according to the invention can be prepared by high functionalization of native or modified starches such as those presented above. This high functionalization can for example be carried out by esterification or etherification at a sufficiently high level to make it essentially amorphous and to confer on it an insolubility in water and preferably a solubility in one of the above organic solvents. Such functionalized starches have a soluble fraction as defined above, greater than 5%, preferably greater than 10%, more preferably greater than 50%. The high functionalization can be obtained in particular by acetylation in the solvent phase by acetic anhydride, grafting for example in the solvent phase or by reactive extrusion, of acid anhydrides, mixed anhydrides, fatty acid chlorides, oligomers of caprolactones or lactides, hydroxypropylation and crosslinking in the glue phase, cationization and crosslinking in the dry phase or in the glue phase, anionization by phosphatation or succinylation and crosslinking in the dry phase or in the glue phase, silyilation, butadiene telomerization. These organomodified, preferably organosoluble, highly functionalized starches can be, in particular, acetates of starches, dextrins or maltodextrins or fatty esters of these starchy materials (starches, dextrins, maltodextrins) with fatty chains of 4 to 22 carbons, all of these products preferably having a degree of substitution (DS) between 0.5 and 3.0, preferably between 0.8 and 2.8 and in particular between 1.0 and 2.7.

It may be, for example, hexanoates, octanoates, decanoates, laurates, palmitates, oleates and stearates of starches, dextrins or maltodextrins, in particular having a DS between 0 , 8 and 2.8. According to another advantageous variant, the organomodified starch is an organomodified starch of corn, wheat or pea or an organomodified derivative thereof. The composition comprises, relative to its total mass, from 2 to 40% by weight of starch. Preferably, the composition comprises from 3 to 20% starch. The composition further comprises a starch plasticizer, which has the function of destructuring the starch. This plasticizer may be of any type, in particular water, polyols or urea, and is advantageously chosen from glycerol, sorbitol, mannitol, maltitol, oligomers of these polyols, polyethylene glycol urea or a mixture of these plasticizers. The composition comprises, relative to its total mass, from 2 to 40% of at least one plasticizer of this starch. Preferably, the composition comprises from 3 to 20% plasticizer. It is specified that the constituents of the composition, in particular starch, may naturally comprise water. The composition preferably comprises a quantity of water less than 5% of its total mass. Advantageously, the sum of the starch and plasticizer masses of the starch is greater than 10% by weight relative to the total mass of the composition, preferably greater than 15%. Advantageously, the sum of the starch and plasticizer masses of the starch is less than 50% by weight relative to the total mass of the composition, preferably less than 40%.

It is preferred that the starch / plasticizer mass ratio of the starch be between 1/9 and 9/1, preferably 2/1 and 1/2.

The composition can furthermore comprise additives, for example antistatic additives, anti-UV agents, antioxidants, stabilizers, fungicides, absorbing agents, reflecting agents or infrared-ray diffracting agents, pigments, volume or nucleating agents, for example in amounts between 0.01 and 15% by weight, preferably between 0.1 and 10%. According to a preferred variant, the composition useful for the manufacture of the foam according to the invention comprises, with respect to its total weight: from 20 to 90%, for example from 50 to 89%, of at least one polyolefin chosen from the group comprising polyethylene and polypropylene and mixtures thereof; From 2 to 30%, for example from 3 to 15%, of at least one functionalized polyolefin comprising from 0.1 to 10% by weight of functional monomer (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes, preferentially maleic anhydride or glycidyl methacrylate - from 2 to 40%, for example from 3 to 20%, of starch; from 2 to 40%, for example from 3 to 20%, of at least one plasticizer of this starch; optionally between 0.01 and 15% of additives; The sum of the mass proportions of the constituents making 100%. In order to produce the foam-useful composition according to the invention, conventional thermoplastic blending methods can be used. These conventional methods comprise at least one step of melt blending or softening of the various constituents and a step of recovering the composition. This process can be carried out in a thermoplastic mixing equipment, for example an internal mixer with blades or rotors, an external mixer, a planetary extruder, a single-screw extruder, a co-rotating or counter-rotating twin-screw extruder. It is preferred to carry out this mixture by extrusion, in particular by using a co-rotating extruder. The mixture of the constituents of the composition can be at a temperature ranging from 80 to 300 ° C, for example from 100 to 250 ° C. If it is desired to at least partially eliminate certain volatile compounds naturally included in the constituents of the composition, especially water, the thermoplastic mixing equipment may be equipped with a vacuum pump. In the case of an extruder, the various constituents of the composition can be introduced by means of introducing hoppers located along the extruder. The process described in WO2009095618 wherein the thermoplastic starch is thermomechanically blended with the polyolefin and the functionalized polyolefin can be used. However, it is particularly preferred to use a process for preparing the composition comprising introducing the starch and a plasticizer thereof into a reactor containing a mixture of softened or fused polyolefin and functionalized polyolefin. kneading the mixture obtained under conditions sufficient to obtain the plasticization of the starch by the plasticizer in order to obtain the composition according to the invention. This process is described in detail in application WO2010010282 in the name of the Applicant. The foam obtained has a density of less than 300 kg / m 3, for example less than 250 kg / m 3, advantageously between 5 and 200 kg / m 3, preferably between 7 and 100 kg / m 3, most preferably between 10 and 60 kg / m 3. m3.

The cells of the foam may have a very small cell size, advantageously a number-average cell size ranging from 0.1 to 3 mm, preferably from 0.5 to 2.5, most preferably from 0.8 to 1, 8 mm. This size can be determined by taking a snapshot of the foam by optical microscope and determining the average diameter of at least one hundred adjacent cells. This size can vary widely, especially depending on the density. With the composition according to the invention, the Applicant has managed to manufacture low density foams retaining a very small average cell size. In addition, it has also been possible to obtain foams whose dispersion of cell size is low. This was not possible with the foams of the prior art made from compositions comprising a thermoplastic starch and a polyolefin. Thus, the choice of the composition of the foam and the structure (in particular its density or even the size of the cells of the foam produced) made it possible to obtain a foam having improved properties. Foams have excellent fire resistance properties while releasing only a small amount of corrosive gas during their combustion. The mechanical properties of the foam are also excellent. In particular, the foam may have a stress at 10% compression ranging from 0.002 to 1 MPa, preferably between 0.005 to 0.5 MPa. The foams according to the invention may also have a stress at 50% compression ranging from 0.005 to 1 MPa, preferably from 0.01 to 0.1 MPa. Preferably, the foams have a tensile strength of 0.1 to 4 MPa. The foams preferably have from 10 to 600% elongation at break. The foam advantageously has a compression set (DRC at 23 ° C. for 22 h and 25% compression) ranging from 0.1 to 30% and preferably from 1 to 15%. To form the foams according to the invention, any type of foaming process can be used, using physical or chemical foaming agents, in particular the process already described in the summary of the invention. Extruders for forming foams are already known apparatus. According to the process according to the invention, during step c, either a foaming gas or a liquid capable of forming a foaming gas is injected. Preferably, it is a foaming gas which comprises alkanes, hydrochlorofluorocarbon gases or inert gases. The foaming gas preferably comprises isobutane. The extrusion temperature of the die mixture will be adapted by those skilled in the art to obtain the required consistency of the foamed product. This means that the die temperature must be higher than the temperature which would lead to the formation of closed cells. However, the Applicant has succeeded in obtaining foams of particularly low density, while maintaining a small cell size, using the following conditions. The temperature of step b is preferably between 150 and 220 ° C, preferably between 160 and 200 ° C. The temperature in step e is advantageously between 100 and 150 ° C., preferably between 100 and 140 ° C. During step f, the extrusion through a die of the mixture of the composition and the gas will cause a significant drop in pressure within the mixture, resulting in the formation of gas bubbles and thus the formation of the foam. . The temperature in step f may be between 80 and 140 ° C, preferably between 90 and 120 ° C. However, it is advisable to quench the surface of the foam immediately after it has left the die in the open air in order to form a perimetric layer of closed cells. This makes it possible to prevent the collapse of the foamed mass, which is mechanically weaker in view of the voluntarily higher extrusion temperature than for obtaining predominantly closed cell foams. The extruder may be a co-rotating or counter-rotating twin-screw, twin-screw extruder. The method may further comprise a crosslinking step, for example using an electron beam. The method according to the invention may further comprise a step of stretching and guiding the foam produced in step f, for example to form a foam reel. It is possible to manufacture foams of multiple shapes: 15 leaves, round shapes, squares, irregular concave or convex shapes. It suffices to extrude the composition in the presence of the gas through a die having the shape required to give the desired expanded final shape. For hollow shapes, a needle and a die will be used to make the hollow body. The foam according to the invention can be used in all fields, in particular in the fields of packaging, building, automobile, transport, sports, toys, footwear, materials industries. composites, or for the manufacture of any type of object, for example a packaging, a panel, a decorative element, a tube or a profile, a shock absorber or a shoe. The foam can be used for reinforcing composites, for lightening structural materials, for filtering a fluid or as an absorbent material. The invention also relates to a multilayer structure comprising at least one foam layer according to the invention and a support layer adhering to the foam layer. The structure may of course comprise one or more layers of foam, which layers may be associated with each other.

This structure can also include several support layers. This support layer is chosen from mats, textiles chosen from woven and non-woven fabrics, extruded films, sheets, bubble films, foams, including foams according to the invention, cut pieces, molded parts. or the injected parts. The support layer may be of synthetic or natural polymer, such as polyolefins, polyesters such as polyethylene terephthalate, polyvinyl chlorides, thermosetting resins, wood, glass, carbon or metals such as aluminum . The manufacture of these structures can be achieved by assembling these layers, typically using glues or resins, or by melting at least one of the layers of the structure. According to a manufacturing variant, the foam according to the invention can be coextruded directly on the support layer, which may or may not be a foamed material.

The present invention will now be described in the following examples. It is specified that these particular examples do not limit the scope of the present invention.

Examples Composition starchy 1 (CA1) Constituents of the composition CA1 Polyethylene (25 parts); Maleated polyethylene comprising 1.5% by weight of maleic anhydride (25 parts); Native wheat starch (30 parts) Plasticizer (glycerol and sorbitol): (20 parts).

The composition is obtained by introducing the constituents of the composition into a TSA twin-screw extruder having a diameter (D) of 26 mm and a length of 50D, using a screw speed of 200 rpm. The maximum temperature in the extruder is 160 ° C.

Amylaceous composition 2 (CA2) Constituents of CA2 composition Polypropylene (25 parts); Maleified polypropylene comprising 1% by weight of maleic anhydride (25 parts); Native wheat starch (30 parts); Plasticizer (glycerol and sorbitol) (20 parts). The CA2 composition is obtained by introducing the constituents of the composition into a TSA brand twin-screw extruder having a diameter (D) of 26 mm and a length of 50D, using a screw speed of 200 rpm. The maximum temperature in the extruder is 170 ° C. Example 1 A foam according to the invention is made by extruding a mixture of 40% by weight of CA1, 54.2% by weight of a low density polyethylene having a density of 921 kg / m3 and having a PIPI of 1.9 g / lOmin (190 ° C. and 2.16 kg), 2.0% by weight of a talc masterbatch, 1.5% by weight of glycerol mono stearate, 0.8% by weight of a stabilizer of volume (fatty acid amides) and 1.5% by weight of a single-stage dye at 270 kg / h with 18.1 kg / h of isobutane gas through a 130 mm die .

For this, all the constituents of the mixture are heated successively between 170 and 180 ° C for a heating step and mixing. After introduction of the foaming gas the mixture is cooled and mixed at 110 ° C and extruded through the die at 100 ° C. An annular die is used to form the flow of freely foaming material at the outlet thereof.

The foam is produced with a very good processability and has after production a density of 25 kg / m3, a thickness of llmm, excellent appearance, expected and uniform coloring, the distribution of the cells is very regular, the surface has no defect . The resulting foam also has good resilience and reduced flammability. Example 2 A foam according to the invention is made by extruding a mixture of 20% by weight of CA1, 75.7% by weight of a low density polyethylene having a density of 921 kg / m3 and having a MFI of 1.9 g / lOmin (190 ° C. and 2.16 kg), 2.0% by weight of a talc masterbatch, 1.5% by weight of glycerol mono stearate, 0.8% by weight of a volume stabilizer (amides of fatty acids) and 1.5% by weight of a single-stage dye at 280 kg / h with 19.4 kg / h of a mixture of isobutane through a 130mm die. For this, all the constituents of the mixture are heated successively between 180 and 190 ° C for a heating step and mixing. After introduction of the foaming gas the mixture is cooled and mixed at 110 ° C and extruded through the die at 100 ° C. An annular die is used to form the flow of freely foaming material at the outlet thereof.

The foam is produced with a very good processability and has a density of 25 kg / m3, a thickness of llmm, an excellent appearance, a coloring expected and uniform, the distribution of cells is very regular, the surface has no defects. The resulting foam also has good resilience and reduced flammability. Example 3 A foam according to the invention is made by extruding a mixture of 20% by weight of CA1, 75.7% by weight of a low density polyethylene having a density of 921 kg / m3 and having an MFI of 1, 9 g / lOmin (190 ° C. and 2.16 kg), 2.0% by weight of a talc masterbatch, 1.5% by weight of glycerol mono stearate, 0.8% by weight of a stabilizer of volume (fatty acid amides) and 1.5% by weight of a dye in a single step at 160 kg / h with a mixture of isobutane through a die. For this, all the constituents of the mixture are heated successively between 160 and 170 ° C for a heating step and mixing. After introduction of the foaming gas the mixture is cooled and mixed at 110 ° C and extruded through the die at 100 ° C. An annular die is used to form the flow of freely foaming material at the outlet thereof. The foam is produced with a very good processability and after production has a density of 55 kg / m3, a thickness of 3 mm, an excellent appearance, an expected and uniform coloration, the distribution of cells is very regular, the surface has no default. The resulting foam also has good resilience and reduced flammability.

Example 4 A foam according to the invention is made by extruding a mixture of 20% by weight of CA1, 73.4% by weight of a low density polyethylene having a density of 921 kg / m3 and having a MFI of 1.9g / lOmin (190 ° C. and 2.16 kg), 2.5% by weight of a talc masterbatch, 2.1% by weight of glycerol mono stearate, 0.6% by weight of a stabilizer of volume (fatty acid amides) and 1.4% by weight of a dye in a single step at 160 kg / h with a mixture of isobutane through a die. For this, all the constituents of the mixture are heated successively between 170 and 180 ° C for a heating step and mixing. After introduction of the foaming gas the mixture is cooled and mixed at 110 ° C and extruded through the die at 100 ° C. An annular die is used to form the flow of freely foaming material at the outlet thereof.

The foam is produced with a very good processability and after production has a density of 18 kg / m3, a thickness of 1.5 mm, excellent appearance, expected and uniform coloring, the distribution of cells is very regular, the surface does not present no defect. The resulting foam also has good resilience and reduced flammability.

EXAMPLE 5 A foam according to the invention is made by extruding a mixture of 40% by weight of CA 2, 56.5% by weight of a high melt strength polypropylene having a density of 902 kg / m 3 and having an MFI of 2.5 g / lOmin (190 ° C. and 2.16 kg), 1.0% by weight of a talc masterbatch, 1.5% by weight of glycerol mono stearate and 1.5% by weight of a dye in a single step at 95 kg / h with 8.6 kg per hour of a mixture of isobutane through a die.

For this, all the constituents of the mixture are successively heated between 180 ° C. and 190 ° C. for a heating and mixing step. After introduction of the foaming gas the mixture is cooled and mixed at 140 ° C and extruded through the die at 130 ° C. An annular die is used to form the flow of freely foaming material at the outlet thereof. The foam is produced with a very good processability and after production has a density of 30 kg / m3, a thickness of 6mm, excellent appearance, expected and uniform coloring, the distribution of cells is regular, the surface has no defects. The resulting foam also has reduced coalescence and good resilience. The properties of the various foams produced are detailed in Table 1. Various comparative foams (Comp 1, Comp 2 and Comp 3,) made by a similar process and made of low density polyethylene having a density of 921 kg / m 3 and having a MFI of 1.9 g / lOmin (190 ° C and 2.16 kg), were also evaluated for comparison. The properties of these foams are also shown in Table 1.

Table 1 Ex Ex Ex Ex Ex Ex Comp 2 Comp3 Ex 1 2 3 4 5 Density kg / m3 25 25 55 18 25 55 18 30 ISO 845 Thickness mm 11 11 3 1.5 11 3 1.5 6 50% compressive stress kPa 52 57 / / 59 / / IS03386-1 Elongation elongation% 120 128 122 58 116 99 60 95 Direction Machine ISO 1798 Sense elongation elongation% 87 109 113 54 74 85 57 80 Transverse ISO 1798 Flammability mm / min 155 166 272 692 181 415 770 112 ISO 7214 /: Not measurable (thickness too low) In addition to the fact that the foams according to the invention have the advantage of being partially manufactured from material of plant origin, the tests show that these foams have similar mechanical properties, even improved, compared to those of polyolefin foams (Ex 1 or Ex 2 vs. Compl, Ex 3 vs. Comp 2 or Ex 4 vs. Comp 3). In addition, the tests show that the fire behavior of the foams according to the invention is improved with respect to the comparative foams.

Claims (15)

REVENDICATIONS1. Foam obtained from a composition comprising: a polyolefin chosen from the group comprising polyethylene and polypropylene and their mixtures; at least one functionalized polyolefin comprising from 0.1 to 10% by weight of at least one functional monomer (X) chosen from unsaturated carboxylic acid anhydrides, unsaturated oxiranes and unsaturated silanes, preferentially maleic anhydride or glycidyl methacrylate; from 2 to 40% by weight of starch relative to the total weight of the composition; from 2 to 40% of at least one plasticizer of this starch relative to the total weight of the composition; said foam having a density of less than 300 kg / m3. 2. Foam according to claim 1, characterized in that it has a density of less than 250 kg / m3, preferably between 5 and 200 kg / m3, preferably between 7 and 100 kg / m3, most preferably between 10 and 60 kg. / m3. 3. Foam according to one of the preceding claims, characterized in that the polyolefin is a polyethylene, preferably a polyethylene having a density between 0.905 and 0.940. 4. Foam according to claim 1 or 2, characterized in that the polyolefin is a polypropylene, preferably a branched polypropylene. 35 Foam according to one of the preceding claims, characterized in that the sum of the amounts of polyolefin and functionalized polyolefin ranges from 25 to 95% by weight of the composition, preferably from 45 to 90%, most preferably 55%. at 85%. 6. Foam according to one of the preceding claims, characterized in that the weight ratio polyolefin / functionalized polyolefin ranges from 10/90 to 99/1, preferably from 45/55 to 98/2, preferably from 70/30 to 95 / 5. 7. Foam according to one of the preceding claims, characterized in that the plasticizer of the starch is selected from glycerol, sorbitol, mannitol, maltitol, oligomers of these polyols, polyethylene glycol, urea or a mixture of these plasticizers. Foam according to one of the preceding claims, characterized in that the cells of the foam have a number-average cell size ranging from 0.1 to 3 mm, advantageously from 0.5 to 2.5 mm and preferably from 0.8 to 1.8 MM. 9. A method of manufacturing a foam according to one of the preceding claims, characterized in that said method comprising the following steps: a. introducing the composition or constituents of the composition into an extruder; b. heating and mixing to fully melt the mass and homogenize it; 30 c. injecting a foaming gas or a liquid capable of forming a foaming gas in the extruder; d. homogenizing the mixture of the composition and the foaming gas; e. partial cooling of the mixture and homogenization; 35 f. extrusion through a die of this mixture causing the formation of the foam at the die outlet. 10. The method of claim 9, characterized in that the foaming gas introduced during step c comprises alkanes, hydrochlorofluorocarbon gases or inert gases, preferably isobutane. 11. Method according to one of claims 9 or 10, characterized in that the temperature of step b is between 150 and 220 ° C, preferably between 160 and 200 ° C. 12. Method according to one of claims 9 to 11, characterized in that the temperature in step e is between 100 and 150 ° C, preferably between 100 and 140 ° C. 13. Method according to one of claims 9 to 12, characterized in that the temperature in step f is between 80 and 140 ° C, preferably between 90 and 120 ° C. 14. Use of the foam according to one of claims 1 to 8, in the fields of packaging, building, automobile, transport, sports, toys, footwear, materials industries. composites, or for the manufacture of any type of object, for example a packaging, a panel, a decorative element, a tube or a profile, a shock absorber or a shoe. 25 15. Structure comprising a foam layer according to one of claims 1 to 8 and at least one support layer adhering to the foam layer. 30