Field of the invention

The present invention relates to oxygen scavenging particles, their preparation and use for packaging applications. Background of the invention

A variety of useful products, such as food products, beverages, pharmaceuticals, medical products, cosmetics, metal products, electronic products may be prone to a rapid quality deterioration and degradation in the presence of oxygen due to the oxidation. In order to prevent such sensitive products from the oxidative degradation, the so-called “passive” and “active” oxygen barrier protecting layers are used. The former is usually made of the suitable oxygen barrier materials with a low oxygen permeation rate, whereas the latter comprises oxygen scavenging materials capable of oxygen absorption. Due to the limited oxygen protection by the plastic passive oxygen barrier materials, the use of such materials alone cannot provide enough protection of the sensitive goods against oxygen over a long period of time. Moreover, depending on the packaging conditions, the initial oxygen concentration present inside the packaging, can be quite high and cannot be reduced anymore, if only a passive barrier layer is used. Both problems can be solved by using the active oxygen scavengers. Such active oxygen absorbers may be distributed in the packaging material layer, e.g. in the form of a protective film or may be present in the form of particles, stickers, sachets etc. inside the packaging.

Description of the prior art

Oxygen scavenging materials directly incorporated into the structure of the packaging can be formed through incorporation of inorganic powders and/or salts as part of the packaging, as described e.g. in US 5,153,038, US 5,116,660, US 5,143,769 or US 5,089,323. Another type of oxygen scavenging compositions distributed in the whole packaging material comprising transition metal catalysts and ethylenically unsaturated hydrocarbon polymers is disclosed in US 5,399,289.

Incorporation of oxygen scavenging materials directly into the packaging material allows using the whole surface area of the packaging film for oxygen absorption. The rate of oxygen absorption by such scavengers distributed in thin films is usually limited by the reactivity of the oxygen scavenger itself, or if a passive oxygen barrier film is used, by diffusion through such an oxygen barrier film.

Packaging materials including oxygen scavengers have mostly multilayer structures, which are prepared by rather sophisticated manufacturing processes. The used oxygen scavenger must be compatible with the materials of such multilayers. For some applications, the use of oxygen scavengers in other forms, e.g. a sachet filled with oxygen scavenging particles, may be beneficial.

Generally used particulate oxygen absorbing materials include iron-based compositions. Such oxygen absorbing compositions comprising metallic iron usually have good oxygen absorbing performance.

However, the presence of moisture is indispensable for activating these materials, and the oxygen activity may vary in a very broad range depending on the water content in the composition and its surroundings. In addition, such iron particles cannot be recycled together with plastic components of the packaging.

In some processing steps, the materials for packaging applications undergo a microwave irradiation treatment or metal detection for contamination inspection. The use of iron particles in such application is restricted. Therefore, there is a need for providing particulate oxygen scavengers, which are active in the absence of moisture and are preferably not based on metal iron.

Particulate oxygen scavengers based on oxidizable polymers and transition metal compounds incorporated in some matrix systems are known.

Thus, EP 1464482 A1 discloses a multi-layer structure with a gas barrier layer comprising a resin composition obtained by blending a thermoplastic resin with an oxygen permeation coefficient of not larger than 10 ¹² cc C>2 ^x cm/cm ² /sec/cm Hg such as ethylene/vinyl alcohol copolymer, with a transition metal catalyst and an oxidizable organic component such as polyene polymer, having an average diameter of dispersed particles of not larger than 1 pm. Similarly, EP 1033080 A2 and EP1538176 A1 disclose oxygen absorptive resin compositions for use in packaging applications, comprising thermoplastic resins having carbon-carbon double bonds, a gas barrier resin with an oxygen transmission rate (OTR) of 500 ml_ C> ₂ ^X 20 pm/(m ² xday ^x atm) or less such as ethylene/vinyl alcohol copolymer, and a transition metal salt. The thermoplastic resin can be in the form of particles dispersed in the matrix of the gas barrier resin, whereby the recommended particle size of this thermoplastic resin should be less than 10 pm. According to these patent applications, if the particle size of the thermoplastic resin containing double bonds exceeds 10 pm, this would lower the oxygen gas barrier properties as well as oxygen scavenging performance of the system. In other cases, some oxygen scavenger pellets are used as precursors for producing moulded articles or multilayer systems.

JP 2010180389 A discloses multilayer pellets, comprising a thermoplastic resin (A) having a carbon-carbon double bonds substantially in the main chain, such as polyoctenylene, a transition metal oxidation catalyst, and a coating layer made of another thermoplastic resin (B) such as polyethylene, which can be used for producing moulded articles, comprising particles of resin (A) with a particle size of less than 10 pm.

JP 2004123970 A discloses pellets with a typical size of more than 2.5 mm, comprising a polyamide resin, an oxidizable organic compound, e.g. one comprising carbon-carbon double bonds, and a transition metal catalyst, which can be used for producing multilayer containers with an oxygen-absorbing gas barrier layer.

EP2017308A1 discloses an oxygen-absorbing resin composition comprising a thermoplastic resin (A) having carbon-carbon double bounds substantially only in the main chain and a transition metal salt (B), as well as multilayer structures and molded products made of the composition. This oxygen-absorbing composition is made by melt-kneading the components and cutting the obtained mixture into pellets. The resulting pellets are then melted and converted to thin films using the compression molding machine. These films are used as oxygen absorbents in multilayered structures and containers.

Particulate oxygen scavengers based on oxidizable polymers, which can be used directly, i.e. without incorporation into the packaging material, and showing a high oxygen absorption rate and capacity, are not disclosed in the prior art.

Problem and solution

The object of the present invention is that of providing a particulate system based on an oxidizable polymer applicable as an efficient oxygen scavenger, which is active in the absence of moisture. The overall oxygen absorption capacity of such particles should be at least 100 ml_ O2 / (g active material). The oxygen absorption rate of the oxygen scavenger should be easily adjustable according to the requirements of a particular application and can for instance be at least 10 ml_ C>2/(g active materialx24 h). Such oxygen scavenging particles should not stick together or agglomerate and be easily pourable.

Another object of the present invention is that of providing an appropriate process for manufacturing the above-described particulate oxygen scavenger.

The invention provides oxygen scavenging particles comprising an oxidizable thermoplastic resin (A) and a transition metal compound, wherein the number average particle size dso of the particles is 20 pm -2000 pm, as determined by laser diffraction particle size analysis; the span of the particle size distribution (dgo-dioj/dso is more than 1.2, as determined by laser diffraction particle size analysis; the bulk density of the particles is at most 80% of the material density of the same particles.

It was surprisingly found that the particles defined above are active oxygen absorbents with a high total oxygen absorption capacity. Their activity is independent of the water content. The oxygen absorption rate of such particles can be easily adjusted to the specific needs of the chosen application. The thermoplastic resin (A)

The oxidizable thermoplastic resin (A) is preferably selected from the group consisting of polymers containing carbon-carbon double bonds, polyesters, polyamides or mixtures thereof.

Polyesters are polymers that contain the ester functional group (-C(=0)-0-) in their main chain. Such polyesters can be homopolymers or copolymers, aliphatic, aromatic or mixed aliphatic/aromatic polymers. Polyesters can be produced e.g. by polycondensation of hydroxy carboxylic acids or polycarboxylic acids and polyols or ring-opening polymerization of lactones. Suitable polyesters are for example polyglycolide or polyglycolic acid (PGA), polylactide acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyethylene adipate (PEA), polybutylene succinate (PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), Vectran.

Polyamides are polymers that contain the amide functional group (-C(=0)-NR-) in their main chain. Such polyamides can be homopolymers or copolymers, aliphatic, aromatic or mixed aliphatic/aromatic polymers. Polyamides can be produced e.g. by polycondensation of amino carboxylic acids or polycarboxylic acids and polyamines or ring-opening polymerization of lactames. Suitable polyamides are for example Nylon polyamide 6 (PA 6), polyamide 66 (PA 66), polyamide 6T produced by a co-polymerization of hexamethylenediamine and terephthalic acid, co-polymer of paraphenylenediamine and terephthalic acid.

The thermoplastic resin (A) used in the inventive oxygen scavenging particles preferably contains carbon-carbon double bonds, such as are present in alkenes, dienes or polyenes. The term “carbon-carbon double bond” in the context of the present invention does not include conjugated double bonds contained in an aromatic or heteroaromatic ring.

The thermoplastic resin (A) preferably has carbon-carbon double bonds substantially in the main chain. The term “main chain” is referred to the polymer backbone, which is the longest single sequence of atoms present in the polymer. The term “substantially in the main chain” means in the context of the present invention that at least 90%, more preferably at least 95%, more preferably at least 98% of all polymer double bonds are present in the polymer main chain. The so-called “side chain”, on the contrary, refers to any chemical group or sequence of atoms bound to the main chain. Such polymer side chains of the thermoplastic resin (A) preferably contain substantially no double bonds. The thermoplastic resin (A) containing carbon-carbon double bonds substantially in the main chain usually shows a less intensive odour during and after its use as oxygen absorber.

The thermoplastic resin (A) preferably has repeating structure units in the main chain in which three or more methylene groups are present between adjacent carbon-carbon double bonds, that is, the resin comprises the units of the general formula -C(R ¹ )=C(R ² )- (CH ₂ ) _n -C(R ³ )=C(R ⁴ )-, wherein R ¹ -R ⁴ are independently H, Ch or other alkyl groups and n³3. In this case, the thermoplastic resin (A) usually shows a larger total amount of oxygen absorbed per carbon-carbon double bond.

The amount of carbon-carbon double bonds contained in the thermoplastic resin (A) is preferably at least 0.001 mol/g resin, more preferably at least 0.005 mol/g resin, more preferably at least 0.01 mol/g resin.

The thermoplastic resin (A) can be produced by polymerization of dienes or polyenes.

An example of a thermoplastic resin (A) produced in this way is polybutadiene.

The thermoplastic resin (A) can also be produced by ring-opening metathesis of a cyclic alkene (cycloalkene), having one, two or more double bonds, e.g. a cyclodiene, a cyclotriene etc. Cycloalkene is preferably selected from the group consisting of cyclobutene, cyclopentene, cycloheptene, cyclooctene, cyclononene, cyclodecene, cyclododecene, cycloocta-1, 5-diene, 1,5-dimethylcycloocta-1, 5-diene, cyclodecadiene, norbornadiene, cyclododeca-1,5,9-triene, trimethylcyclododeca-1 ,5,9-triene, norbornene (bicyclo[2.2.1]hept-2-ene), 5-(3'-cyclohexenyl)-2-norbornene, 5-ethyl-2-norbornene, 5- vinyl-2-norbornene, 5-ethylidene-2-norbornene, dicyclopentadiene and mixtures thereof. Particular preference is given to cyclopentene, cycloheptene, cyclooctene and cyclododecene. Cyclooctene is an outstanding cycloalkene because of its availability and ease of handling. It is preferable when the cycloalkene used for synthesis of the thermoplastic resin (A) comprises cyclooctene and particularly preferable when it consists of this monomer. Polyoctenamer produced by ring-opening metathesis of cyclooctene is e.g. manufactured under the name VISPARENT ^® 8021 by Evonik Industries AG. It is also possible to use two or more cycloalkenes to form the thermoplastic resin (A). The cycloalkenes may be substituted with alkyl groups, aryl groups, alkoxy groups, carbonyl groups, alkoxycarbonyl groups and/or halogen atoms. Suitable catalysts for ring-opening metathesis of cycloalkenes are for example tungsten complexes (US3597406, US4095033, DE2619197), molybdenum complexes (EP0218138; Polymer 1995, 36, 2787-2796) and ruthenium complexes (J. Am. Chem.

Soc. 1993, 115, 9858-9859; Macromolecules 1993, 26, 4739-4741). Ruthenium carbenes in particular are very widely applicable and tolerate all common chemical groups (EP0626402, US8324334, US2016/159942). Very particularly suitable are ruthenium- carbene complexes which, as one of their characteristic features, bear an N-heterocyclic carbene ligand.

The melting point of the thermoplastic resin (A) can be important since a resin having a relatively high melting point usually has a relatively high crystallinity which results in better mechanical properties advantageous for the application in the oxygen scavenging particles of the present invention. Therefore, the thermoplastic resin (A) preferably has a melting point of at least 40°C, more preferably at least 45 °C, more preferably at least 50 °C, more preferably at least 55 °C, more preferably at least 57 °C, more preferably at least 59 °C. The thermoplastic resin (A) preferably has a weight-average molecular weight (Mw) of 5000 g/mol to 500 000 g/mol, preferably of 10 000 g/mol to 250 000 g/mol and more preferably of 20 000 to 200 000 g/mol. The molecular weight can be determined by means of Gel Permeation Chromatography (GPC) against a styrene standard. The measurement is based on DIN 55672-1. When the weight average molecular weight of the thermoplastic resin (A) is less than 5000 or more than 500,000, the moldability, handleability, and mechanical properties may be deteriorated.

The thermoplastic resin (A) can comprise a certain amount of oligomers, i.e. polymer molecules with a relatively low molecular weight, e.g. of less than 1000 g/mol. The presence of these oligomers may cause the oxygen scavenging particles to have an undesired odour. Therefore, the thermoplastic resin (A) preferably comprises less than 6%, more preferably less than 4%, more preferably less than 2%, more preferably less than 1 % by weight of the oligomers having a molecular weight of 1000 or less. The amount of oligomers having a molecular weight of 1000 or less can be reduced using the appropriate extraction techniques, for instance those described in WO 2018146236 A1 or WO 2017009411 A1 , or by other methods such as membrane techniques including diafiltration, as described e.g. in WO 2017060363 A1. The desired molecular weight may for example be established using at least one chain transfer agent in the ring-opening metathesis reaction, which allows chain growth to be terminated. Suitable chain transfer agents are known from the literature and include for example acyclic alkenes having one or more nonconjugated double bonds which may be terminal or internal and which preferably do not bear any substituents. Such compounds are, for example, pent-1-ene, hex-1 -ene, hept-1-ene, oct-1 -ene or pent-2-ene. Alkyl vinyl ethers do not fall within this definition as they are not usable as chain-transfer agents. The reason for this is that alkyl vinyl ethers deactivate the catalyst. Alternatively, employable as chain-transfer agents are cyclic compounds comprising a double bond in their side chain, for example vinylcyclohexene.

The cis/trans ratio of the thermoplastic resin (A) can be adjusted by methods familiar to those skilled in the art. The ratio depends for example on used catalysts, solvents, stirring intensity, temperature or reaction time. It is preferable when the trans content of the thermoplastic resin (A) is at least 55%, preferably at least 70% and particularly preferably 75% to 85%. The cis/trans ratio can be determined by ¹ H-NMR in deuterochloroform.

The thermoplastic resin (B)

The oxygen scavenging particles according to the invention can further comprise a thermoplastic resin (B) other than the thermoplastic resin (A), the thermoplastic resin (B) preferably having an oxygen transmission rate (OTR) of higher than 80 ml_ C>2/(m ² xdayxatm), more preferably higher than 150 ml_ C>2/(m ² xdayxatm), more preferably higher than 300 ml_ C>2/(m ² xdayxatm, more preferably higher than 500 ml_ C>2/(m ² xdayxatm), more preferably higher than 1000 ml_ C>2/(m ² xdayxatm), more preferably higher than 3000 ml_ C>2/(m ² xdayxatm), more preferably higher than 5000 ml_ C>2/(m ² xdayxatm). Oxygen transmission rate (OTR) is the amount of oxygen gas that passes through a substance film over a given period under standardized conditions (20 °C, 65% relative humidity). Oxygen transmission rate can be measured in accordance with ISO 14663-2.

The thermoplastic resin (B) with a relatively high OTR value can play the role of a suitable matrix polymer, which dilutes the active oxygen absorber thermoplastic resin (A) and facilitates the diffusion of oxygen through oxygen scavenging particles. Thus, using a mixture or a polymer blend comprising the thermoplastic resin (A) and the thermoplastic resin (B) allows achieving overall higher oxygen absorption rates of the inventive oxygen scavenging particles.

The thermoplastic resin (B) can be selected from the group consisting of polyethylenes such as low-density polyethylene (LDPE), very-low-density polyethylene, ultra-low-density polyethylene, high-density polyethylene (HDPE) and linear low-density polyethylene, polypropylene, polyesters, ethylene copolymers such as ethylene/vinyl acetate copolymers (EVA and VAE), ethylene/alkyl (meth)acrylate copolymers (EMA), ethylene/(meth)acrylic acid copolymers, ethylene/butyl acrylate (EBA) copolymers, ethylene/acrylic acid (EAA), polylactide, polyglycolide, ionomers, and mixtures thereof.

The oxygen scavenging particles according to the invention preferably comprise polymer blend comprising at least 5%, more preferably at least 10%, more preferably at least 20%, more preferably at least 25% by weight of the thermoplastic resin (A) and not more than 95 %, more preferably not more than 90 %, more preferably not more than 80 %, more preferably not more than 75 % by weight of the thermoplastic resin (B).

The transition metal compound

The transition metal compound used in the oxygen scavenging particles according to the invention comprises a transition metal, which can be selected from the group consisting of iron, cobalt, nickel and manganese. Cobalt is particularly preferable. The term “transition metal compound” means in the context of the present invention any substance, comprising the transition metal chemically bound with at least one other chemical element. Thus, transition metal itself such as iron, cobalt, nickel or manganese is not a “transition metal compound” in the context of the invention, whereas e.g. a transition metal salt is. The suitable transition metal compound is preferably soluble in the molten thermoplastic resin (A) and/or in the polymer blend of the thermoplastic resins (A) and (B). The term “soluble” means in the context of the present invention that at least 0.001 %, more preferably at least 0.01%, more preferably at least 0.1% by weight (related to the polymer weight) of a transition metal compound can be dissolved in in the molten thermoplastic resin (A) and/or in the polymer blend of the thermoplastic resins (A) and (B). Particularly suitable are the transition metal salts of carboxylic acids, especially the carboxylic acids comprising 3-30 carbon atoms, such as e.g. linear, branched or cyclic pentanoic acid (C5), hexanoic acid (C6), heptanoic acid (C7), octanoic acid (C8), nonanoic acid (C9), decanoic acid (D10), undecanoic acid (C11), dodecanoic acid (C12), tridecanoic acid (C13), tetradecanoic acid (C14), pentadecanoic acid (C15), hexadecanoic acid (C16), heprtadecanoic acid (C17), octadecanoic acid (C18), nonadecanoic acid (C19), icosanoic acid (C20), and the mixtures thereof. Particularly preferable as the transition metal compound is cobalt stearate.

The transition metal compound is preferably well dispersed in the thermoplastic resin (A) and/or in the polymer blend of the thermoplastic resins (A) and (B) and shows an average particle size of not more than 1000 p , more preferably less than 500 pm, more preferably less than 300 pm, more preferably less than 200 pm, more preferably less than 100 pm The average particle size of such transition metal compound particles in the polymer matrix can be determined using transmission electron microscopy (TEM). The transition metal compound is preferably dispersed in the polymer blend comprising at least 10 % by weight of the thermoplastic resin (A) and not more than 90 % by weight of the thermoplastic resin (B).

Oxygen scavenging particles

The oxygen scavenging particles of the invention have the number average particle size d5o of 20 pm -2000 pm, preferably 20 pm -1800 pm, more preferably 20 pm -1500 pm,

20 pm -1200 pm, more preferably 20 pm -1000 pm, more preferably 20 pm -800 pm, more preferably 25 pm -500 pm, more preferably 30 pm -300 pm, more preferably 40 pm - 200 pm. A number average particle size dso of the particles can be determined according to ISO 13320:2009 by laser diffraction particle size analysis. The resulting measured particle size distribution is used to define the median dso, which reflects the particle size not exceeded by 50% of all particles, as the number average particle size.

If the average particle size of the oxygen scavenging particles is larger than 2000 pm, the oxygen absorption rate becomes too low. Preparation of oxygen scavenging particles with an average particles size of less than 20 pm, e.g. by milling techniques, is rather challenging. On the other hand, too small oxygen scavenging particles stuck together would block the diffusion of air so that the overall oxygen absorption rate may be reduced. Therefore, the range of 20 pm -2000 pm of the average particle size of the oxygen scavenging particles was found to be optimal.

The oxygen scavenging particles of the invention preferably have particle size of not more than 3000 pm, preferably of not more than 2500 pm, more preferably of not more than 2000 pm. The absence of the particles with a particle size of above the specified range can be achieved for example by sieving of the particles through an appropriate sieve.

The oxygen scavenging particles according to the invention preferably have a dio value of from 1 pm to 200 pm, more preferably from 2 pm to 150 pm, more preferably from 3 pm to 100 pm, more preferably from 5 pm to 50 pm. The preferred dgo value is from 50 pm to

2000 pm, more preferably from 60 pm to 1500 pm, more preferably from 70 pm to 1000 pm, more preferably from 80 pm to 800 pm. The dio and dso values can be determined according to ISO 13320:2009 by laser diffraction particle size analysis. The resulting measured particle size distribution is used to define the values dio and dgo, which reflects the particle size not exceeded by 10% or 90% of all particles, respectively. The span of the particle size distribution of the oxygen scavenging particles of the invention (dgo-dioj/dso is more than 1.2, preferably more than 1.3, more preferably more than 1.4, more preferably more than 1.5, more preferably more than 1.6, more preferably more than 1.7, more preferably more than 1.8, more preferably more than 1.9. The span value reflects the breadth of the particle size distribution. It was surprisingly found that a relatively broad particle size distribution of the oxygen scavenging particles is beneficial for achieving both the fast initial oxygen absorption rate due the presence of the small particles and the prolonged oxygen absorption due to the presence of the larger particles. Oxygen scavenging particles with a suitable particle size distribution can be achieved for instance by appropriate milling techniques. Alternatively, two or more particle types with different average particle sizes can be mixed together. In this case, particle size distribution of the resulting mixture may be bi- or polymodal.

The oxygen absorption rate of the oxygen scavenging particles of the invention can be adjusted according to the requirements of a particular application by choosing the appropriate particle size and/or particle size distribution. The oxygen absorption rate of the particles measured according to ASTM F 2714-08 can be at least 10 ml_ O2 / (g thermoplastic resin (A) ^c 24 h), more preferably at least 20 ml_ O2 / (g thermoplastic resin (A) ^c 24 h), more preferably at least 30 ml_ O2 / (g thermoplastic resin (A) ^c 24 h), more preferably at least 40 ml_ O2 / (g thermoplastic resin (A) ^c 24 h), more preferably at least 50 ml_ O2 / (g thermoplastic resin (A) ^c 24 h).

Oxygen absorption can be determined according to ASTM F2714-08 “Standard test method for oxygen headspace analysis of packages using fluorescent decay”, e.g. at 23 °C, a 50% relative humidity and using a 250 ml_ headspace filled with air (initial oxygen concentration 21 vol%). The oxygen scavenging particles according to the invention usually possess a high total oxygen absorption capacity, i.e. the total amount of oxygen, which can be absorbed by the particles. The total oxygen absorption capacity of the oxygen scavenging particles of the invention is preferably at least 100 ml_ O2 / (g thermoplastic resin (A)), more preferably at least 150 ml_ O2 / (g thermoplastic resin (A)), more preferably at least 200 ml_ O2 / (g thermoplastic resin (A)), more preferably at least 250 ml_ O2 / (g thermoplastic resin (A)).

The bulk density of the oxygen scavenging particles of the present invention is at most 80%, preferably at most 75%, more preferably at most 70%, more preferably at most 65%, more preferably at most 60% of the material density of the same particles.

Bulk density, also called apparent density, poured density or volumetric density, is defined as the mass of the particles of the material divided by the total volume they occupy. The total volume includes particle volume, inter-particle void volume, and internal pore volume. A substantial, measurable volume of many particles is usually used for determining the bulk density of particles.

The material density is defined as the mass of the material divided by the total volume occupied by this material and does not include inter-particle void volume. Thus, the material density of e.g. a polymer moulding can be determined by dividing the mass of the moulding by its volume, which can be calculated from the known dimension of the moulding. The material density of e.g. the polymer particles can be determined in a similar way. The volume of the particles of the known mass in this case can be substituted by the volume of water or other liquid, which is displaced by the particles submerged into the liquid. Alternatively, the material density of the moulding can be measured, which is prepared by melting and solidifying the particles.

The particles made of the material with a given material density may have very different bulk densities, depending on their particle form, average particle size and particle size distribution. It was surprisingly found that if the bulk density of the oxygen scavenging particles is at most 80% of the material density of the same particles, a higher oxygen absorption rate is achieved. Without wishing to be bound by any theory, it is believed that a proper diffusion of air to all oxygen scavenging particles is necessary for achieving high oxygen absorption rates. This, in turn, can only be achieved if a relatively high ratio of void volume is provided in the packed bed of particles.

The bulk density of the oxygen scavenging particles of the invention is preferably less than 0.70 g/cm ³ , more preferably less than 0.65 g/cm ³ , more preferably less than 0.60 g/cm ³ , more preferably less than 0.55 g/cm ³ , more preferably less than 0.50 g/cm ³ .

The oxygen scavenging particles of the invention can further comprise particles with a BET surface area of at least 5 m ² /g, more preferably at least 10 m ² /g, more preferably at least 20 m ² /g, more preferably at least 30 m ² /g, more preferably at least 50 m ² /g, which may further facilitate the oxygen absorption and diffusion in the oxygen scavenging particles. Such particles are preferably porous. The non-limiting examples of such particles are activated carbons, fumed silicas, zeolites. These particles can function as spacers between the oxygen scavenging particles of the invention and increase the overall oxygen absorption and flowability of the oxygen scavenging particles. Such particles may also additionally be odour-scavengers.

The specific surface area, also referred to simply as BET surface area, can be determined according to DIN 9277:2014 by nitrogen adsorption in accordance with the Brunauer- Emmett-Teller method. The oxygen scavenging particles of the invention preferably have melting point of more than 50 °C, more preferably from 50°C to 200°C, more preferably from 55°C to 180°C, more preferably from 60°C to 160°C, more preferably from 70°C to 150°C, more preferably from 80°C to 140°C. It was surprisingly found that a relatively high melting point of the particles prevents them from undesired sticking together and agglomeration, which would lead to a significant decrease of the oxygen absorption rate. The oxygen scavenging particles according to the invention due to their relatively high melting point preferably remain pourable. On the other hand, melting point of the material of the particles is usually limited to 200 °C or less, which allows their thermal processing, e.g. by melt extrusion.

The oxygen scavenging particles of the invention can further comprise an odour scavenging agent. The non-limiting list of generally suitable odour scavenging agents includes physical adsorbents like activated carbons, zeolites, and chemical absorbents like amine compounds, hydrazine derivatives, urea derivatives, guanidine derivatives, both unsupported and supported on the suitable carriers.

The oxygen scavenging particles according to the invention can comprise further oxidizable components, other than the oxidizable thermoplastic resin (A), for example such as iron powder.

The oxygen scavenging particles according to the invention can further comprise one or more antioxidants or stabilizers in order to retard the degradation of the components during the further processing including compounding, moulding etc. Suitable stabilizers may be selected from the group of the sterically hindered phenols, for example 2,5-di- tert-butylhydroquinone, 2,6-di-tert-butyl-p-cresol, 4,4’-thiobis(6-tert-butylphenol), 2,2’- methylenebis(4-methyl-6-tertbutylphenol), octadecyl 3-(3’,5’-di-tert-butyl-4’- hydroxyphenyl)propionate, 4,4’-thiobis-(6-tert-butylphenol), 2-tert-butyl-6-(3-tert-butyl-2- hydroxy-5-methylbenzyl)-4-methylphenyl acrylate, 2,6-di(tert-butyl)-4-methylphenol (BHT), 2,2-methylenebis(6-tert-butyl-p-cresol), from the group of the organic phosphites, for example triphenyl phosphite, tris(nonylphenyl) phosphite, the group of the organic thio compounds, for example dilauryl thiodipropionate, pentaerythritol tetrakis(3-laurylthiopropionate), ascorbic acid, vitamin E (alpha-tocopherol), octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate, tetrakis[methylene(3,5-di-tert-butyl-4- hydroxyhydrocinnamate)]methane, tetrakis(2,4-di-tert-butylphenyl) 4,4'- biphenyldiphosphonite and mixtures thereof.

If an antioxidant is present in the oxygen scavenging particles of the invention, the quantity thereof can be one that prevents the oxidation of the components of the particles during production and processing; however, the quantity thereof should advantageously be below that which adversely affects the oxygen scavenging activity of the particles. The quantity required can depend on the components present in the particles, on the specific antioxidant used, and on the nature of thermal processing used. The antioxidants are typically used in the amount from 0.01% to 1% by weight, related to the mass of the particles.

The examples of other additives that can be present in the inventive particles are fillers, pigments, colorants, processing aids, plasticizers, antifogging agents and antiblocking agents, without any restriction thereto.

Process for producing oxygen scavenging particles The invention further provides a process for producing oxygen scavenging particles according to the invention, comprising the following steps: a) melt extrusion of at least one component comprising a thermoplastic resin (A), a transition metal compound and optionally a thermoplastic resin (B) and breaking up the obtained extrudates into particles; b) optional crushing of the particles obtained in step a).

The component to be extruded in step a) of the process can be in the liquid or solid state, e.g. in the form of powders or granules. The transition metal compound and thermoplastic resins (A) and (B) can be all provided as individual components. However, the pre-formed mixtures or polymer blends and/or a pre-formed composition comprising the transition metal compound and the thermoplastic resins (A) and/or (B) may also be employed. The individual components can be mixed before conducting step a) of the process using any suitable mixing device, preferably choosing the duration and the intensity of the mixing so that a homogeneous distribution of all components is achieved in the mixture.

In step a) of the process, at least one component comprising the thermoplastic resin (A), the transition metal compound and optionally the thermoplastic resin (B) is melt extruded and the obtained extrudates, which are typically cooled down by means of water, air or other cooling means, are broken up into particles (pellets), e.g. by cutting the extrudates.

Step a) of the process can comprise one, two or more sequential melt extrusion steps.

Thus, in one preferred embodiment of the invention, the transition metal compound is first co-extruded with the thermoplastic resin (B) followed by a co-extrusion of the thus prepared masterbatch compound with the thermoplastic resin (A).

The masterbatch compound containing the transition metal and the thermoplastic resin (B) can contain 0.1% to 20%, preferably 0.5% to 15% by weight of the transition metal compound and can be further diluted by a further co-extrusion with the thermoplastic resin (B). Melt extrusion is the processing of polymeric materials above their melting in order to effect molecular level mixing of the components and to achieve the required shape. Step a) may be conducted by means of a mono-extrusion or a co-extrusion. Any suitable type of extruder can be employed in step b) of the process, e.g. a twin-screw extruder. The temperature during conducting of step a) of the process, preferably does not exceed 240 °C, preferably is maintained in the range of from about 120°C to about 240°C, or in the range of from about 150°C to about 230°C, preferably in the range of from about 180°C to about 220°C. If the temperature exceeds this range, in particular the upper limit, there is a risk of discoloration and/or degradation of the material of the particles, as well as the presence of undesired burnt aromas. The lower limit is generally determined by the temperature at which the material can be suitably processed, in the form of a melt.

The pressure during conducting of step a) of the process, is preferably maintained in the range of from about 5 bar to about 80 bar, or in the range of from about 10 bar to about 50 bar, preferably in the range of from about 15 bar to about 40 bar. The preferred average residence time during the extrusion step a) of the process may vary in a broad range, e.g. be from 0.01 minutes to 30 minutes, more preferably from 0.05 minutes to 20 minutes, more preferably from 0.1 minutes to 10 minutes, more preferably from 0.5 minutes to 5 minutes.

Particle size of the particles obtained in step a) of the process depend on the type of the die used in the extruder. If a smaller particle size is required, the particles obtained in step a) of the process can further be crushed in optional step b) of the process using any suitable means, e.g. grinding or milling. Cryogenic grinding is a particularly preferable type of crushing applied in step b) of the process.

Cryogenic grinding, also known as freezer milling, freezer grinding, and cryomilling, is the act of cooling or chilling a material and then reducing it into a small particle size.

Thermoplastics are usually difficult to grind to small particle sizes at ambient temperatures because the tend to soften, adhere in lumpy masses and clog screens. When chilled by dry ice, liquid carbon dioxide or liquid nitrogen, such thermoplastics can be finely ground to powders. The milling or grinding parameters are determined according to the desired average particle diameter of the oxygen scavenging particles after crushing step c). The crushing step is preferably conducted using a shear rate of at least 50 s ¹ , more preferably of at least 100 s ¹ , more preferably at least 200 s ¹ . The peripheral velocity during crushing is preferably in the range from 4 m/s to 50 m/s, more preferably from 10 m/s to 40 m/s. The upper lower limits for the shear rate and peripheral velocity are determined by what is technically possible, together with the desired average particle diameter of the oxygen scavenging particles.

Use of the oxygen scavenging particles

The oxygen scavenging particles according to the invention can be used as oxygen scavenger, particularly for preserving food products, beverages, pharmaceuticals, medical products, cosmetics, metal products, and electronic products from oxidation.

The oxygen scavenging particles of the invention can generally be used as oxygen scavenger in the temperature range from -20°C to 100°C, preferably from 0°C to 50°C, more preferably from 4°C to 30°C. The oxygen absorption rate increases with increasing temperature. Therefore, the quantity of the employed oxygen scavenger may be increased in order to efficiently use it at lower temperatures.

Container and sachet

The invention further provides a container comprising the oxygen scavenging particles according to the invention. The term “container” means in the context of the present invention any object for holding goods, such as bags, boxes, bottles, cans and other packaging types. The oxygen scavenging particles can be placed inside of such containers and can be used instead of or additionally to other means for preservation of the corresponding goods against the oxidation, such as oxygen barrier layers or coatings.

Polymers usually used as passive oxygen barrier layer include polyethylenes, for example low-density polyethylene, very low-density polyethylene, ultra-low-density polyethylene, high-density polyethylene and linear low-density polyethylene, polyesters, for example polyethylene terephthalate (PET) or polyethylene naphthenate (PEN); polyvinyl chloride (PVC); polyvinylidene chloride (PVDC); polycaprolactone polymers and ethylene copolymers, for example ethylene/vinyl acetate copolymers (EVA and VAE), ethylene/alkyl (meth)acrylate copolymers (EMA), ethylene/vinyl alcohol copolymers (EVOH), poly(vinyl alcohol) (PVOH), ethylene/(meth)acrylic acid copolymers, ethylene/butyl acrylate (EBA) copolymers, ethylene/vinyl alcohol, ethylene/acrylic acid (EAA), polylactide, ionomers and polyamides, for example polycaprolactam (nylon-6), metaxylyleneadipamide (MXD6), hexamethyleneadipamide (nylon-6,6), and also various amide copolymers. It is also possible to use mixtures of different main polymers.

Particular preference is given to use of the following as polymer matrix or as main polymer: poly(ethylene/vinyl alcohol) (EVOH), poly(vinyl alcohol) (PVOH), polyethylene terephthalate (PET), and polyamides, for example polycaprolactam (nylon-6), metaxylyleneadipamide (MXD6), hexamethyleneadipamide (nylon-6,6) and various amide copolymers. The invention further comprises a sachet comprising the oxygen scavenger particles of the invention. The term “sachet” means in the context of the present invention any bag, packet, pouch or package holding its content, i.e. the oxygen scavenging particles together. The inventive sachet can be a constituent of the container according to the invention.

The inventive sachet can be covered by an odour-protecting layer, which prevents the migration of unpleasantly smelling components of the oxygen scavenger particles outside the sachet but is permeable for oxygen. Suitable materials for such odour-protecting layers are described e.g. in WO1998012250A1. Examples

Thermoplastic resin (A)

VISPARENT ^® 8021 (Polyoctenamer, manufacturer Evonik Industries AG) with an average dimension of the pellets of about 3 mm x 3 mm x 4 mm was used as the thermoplastic resin (A).

Preparation of the oxygen scavenging particles Comparative example 1

Oxygen scavenging pellets with dimension of the pellets of about 3 mm x 3 mm x 4 mm comprising VISPARENT ^® 8021 (30 % by weight) low density polyethylene (LDPE, 69.9% by weight) and cobalt stearate (0.1% by weight) were obtained by melt extrusion using twin screw extruder Werner&Pfleiderer ZSK30. Material density of the pellets is 0.9 g/cm ³ . Bulk density of the pellets is 0.5-0.6 g/cm ³ .

Example 1

Oxygen scavenging pellets prepared in comparative example 1 were grinded using cryogenic grinding under a nitrogen protective atmosphere to obtain a powder with dio = 16 pm, dso = 58 pm, dgo = 129 pm, span (dgo-dio)/d5o = 1.95, as determined by laser diffraction measurement. Bulk density of the powder is < 0.7 g/cm ³ .

Oxygen absorption experiments

Oxygen absorption was determined according to ASTM F2714-08 “Standard test method for oxygen headspace analysis of packages using fluorescent decay” at 23 °C, 50% relative humidity and using a 250 ml_ headspace filled with air (initial oxygen concentration 21 vol%). The results are summarized in Table 1. Table 1 Oxygen absorption experiments

The results of the oxygen absorption experiments summarized in Table 1 show that the relatively big oxygen scavenging particles from the comparative example 1 achieve only a low oxygen absorption rate and are therefore not suitable for oxygen absorption applications. The cryogenically grinded particles prepared in example 1, on the contrary, show a very high rate of oxygen absorption. The rate of oxygen absorption depends on the quantity of the oxygen scavenging material applied in relation to the amount of oxygen available. The last line of Table 1 shows that sufficient quantity of oxygen scavenging particles can lead to the complete and fast absorption of all oxygen available in the container.