Horticultural pot composition incorporating methanation digestate

Abstract

The present invention concerns a composition for a horticultural pot comprising a solid residue of digestate from methanization and / or a solid residue from composting and a biodegradable polymer chosen from polyhydroxyalkanoates (PHA), its preparation and its use in the manufacture of such a pot.

Description

[0001] HORTICULTURAL POT COMPOSITION INCORPORATING METHANATION DIGESTATE

[0002] The present invention relates to the field of horticultural pots, and in particular that of biodegradable horticultural pots.

[0003] A horticultural pot is intended to provide a confined space for root development and plant growth. Using a horticultural pot allows to control growing conditions, such as soil type, drainage and water quantity.

[0004] To reduce plastic waste, biodegradable pots have been developed.

[0005] These biodegradable pots can be buried while containing the plant (such a pot can then be called a "plantable pot"), and degraded in the soil. This is particularly useful for avoiding damage to the plant when transplanting, by planting the pot directly into the ground rather than transplanting the plant from the pot into the ground.

[0006] Horticultural pots of various compositions are known to degrade in the soil. Compositions of biodegradable horticultural pots based on natural fibers (wood, coir) or compost have been considered.

[0007] For example, EP 1 099 368 describes a horticultural pot composition comprising a biodegradable polymer and pulverized animal or plant waste, comprising 35% by weight of the polymer relative to the weight of the pulverized waste.

[0008] However, the manufacturing process involves a number of steps for treating said waste, such as drying and pulverizing it, and possibly pre-mixing it with dry materials.

[0009] However, the horticultural pot compositions must be easy to manufacture on an industrial scale, enabling waste to be recycled at low cost.

[0010] In addition, the compositions must be compatible with injection molding to a desired shape, while allowing easy degradation of the pot in the soil. They must also enable plants to be cultivated and, in particular, roots to grow beyond the pot. The thickness of the pot must therefore be compatible with this use.

[0011] In addition, as part of the drive to reduce waste and promote circular agriculture, it is desirable to valorize agricultural by-products, in particular composts and methanization digestates, currently considered as waste.

[0012] In order to meet these objectives, one of the aims of the invention is to propose an improved biodegradable horticultural pot composition, in that it enables agricultural waste to be recycled. To this end, the invention relates in particular to a biodegradable horticultural pot composition characterized in that it comprises :

[0013] - from 20 to 60% by weight of a solid residue of digestate from methanization and / or a solid residue from composting ;

[0014] - from 40 to 80% by weight of a biodegradable polymer selected from polyhydroxyalkanoates (PH A);

[0015] - 0 to 30% by weight of one or more agents promoting horticultural cultivation.

[0016] In the description, when ranges are disclosed, the limits are included unless otherwise specified.

[0017] The composition according to the invention responds to an environmentally-friendly approach, in that it meets the requirement of reducing plastics and waste. In addition, it recycles agricultural waste and enables pots to biodegrade in the soil, facilitating plant growth.

[0018] Moreover, the presence of methanization or composting residues enhances fertilization.

[0019] The following embodiments may be considered in isolation or in any combination thereof.

[0020] According to one embodiment, the biodegradable horticultural pot composition may comprise :

[0021] - from 20% to 50%, in particular 30 to 40% by weight of a solid residue of methanization digestate and / or a solid residue of composting,

[0022] - from 50% to 80%, in particular 60 to 70% by weight, of a biodegradable polymer chosen from polyhydroxyalkanoates (PHA);

[0023] - from 0% to 30%, in particular 10 to 20% by weight, of one or more agents promoting horticultural cultivation.

[0024] By biodegradability is meant the natural biological process of degrading matter, thanks in particular to the enzymatic activity of micro-organisms present in the environment in question. This is generally an anaerobic process.

[0025] The biodegradability of pots and polymers according to the invention refers to the degradation in the soil, in composting (domestic or industrial), or in fresh or sea water. These can be defined, measured and qualified in accordance with applicable standards. The biodegradability of pots in the sense of the invention refers more specifically to biodegradability in composting and / or soil environments, defined respectively in accordance with standards NF EN 13432 (November 2000) and NF T51 -800 (2015), on the one hand, and NF EN 17033 (January 2018) and NF U52-001 (February 2005), on the other.

[0026] The biodegradability of a pot in soil can be tested under the conditions set out in NF U 52001 -F (February 2005), NF EN ISA 14851 (July 2019) or 14852 (July 2021 ), NF EN 14046 (NS) (June 2003) and EN ISO 17556 (May 2019).

[0027] For example, biodegradability in soil can be measured using a respirometer cell or Oxytop system. In this test, a pot is said to be biodegradable when it has reached 90% of its theoretical biodegradability over a maximum period of 2 years.

[0028] The Oxytop system is a device for measuring biochemical oxygen demand (BOD) by measuring the decrease in oxygen pressure and the absorption of CO2, the oxygen consumption of bacterial activity making it possible to calculate mineralized CO2 to in fine calculate the percentage of biodegradability.

[0029] According to a first alternative, the solid residue can be a solid residue of methanization digestate.

[0030] Methanization is a process for converting organic, animal and / or plant waste into energy. This is particularly true of agricultural and urban waste, and sewage plant sludge. This technique is typically implemented under anaerobic conditions, in methanizers where the degradation process of the aforementioned organic waste is accelerated and maintained to produce a combustible gas (biogas, known as biomethane after purification).

[0031] Raw digestate is the residue generated by the methanization of organic waste.

[0032] The solid fraction of digestate (also referred to here as solid residue of methanization digestate or solid digestate) can be separated from its liquid fraction (liquid digestate).

[0033] Typically, between 90 and 99% liquid digestate and between 1 and 10% solid residue (% by mass) are separated from the raw digestate, although the proportion may vary depending on the separation method used. By way of illustration, press filter separation typically enables 96% liquid digestate and 4% solid residue to be separated from raw digestate.

[0034] The solid digestate residue is typically considered a waste product. Its use in the compositions of the invention therefore makes it possible to increase the value of the process. A composition representative of an example of methanization digestates averaged at national level is illustrated below according to : The above values can be measured by / according to the following methods:

[0035] - Ammoniacal N, Organic N, P2O5 total, K2O total, CaO total, MgO total: by ICP-MS;

[0036] - Total N, organic carbon, ratio C / N: by elemental analysis;

[0037] - Organic matter: by incineration following the loss on ignition (LOI) method, i.e. weighing and furnace calcination (difference in weight before and after); - Dry material: following the standard American Public Health Association., E., Andrew D., American Water Works Association, Water Environment Federation.2005. Standard methods for the examination of water and wastewater. APHA-AWWA-WEF, Washington, D.C.

[0038] They may vary depending on the anaerobic digestion input.

[0039] According to a second alternative, the solid residue can be a solid compost residue.

[0040] According to one embodiment, said solid composting residue is a solid residue of a compost : from said methanization digestate, the solid residue of the methanization digestate, or an organic matrix such as green waste and / or biowaste (fermentable organic waste), and / or mixtures thereof.

[0041] Composting is a biological process involving the degradation and transformation of organic waste as defined above, resulting in compost that can be used as an organic soil improver.

[0042] This aerobic process typically takes place in the presence of atmospheric oxygen and moisture (water), through the combined action of bacteria, fungi, micro-organisms and macro-organisms.

[0043] The term "compost" can be defined and qualified in accordance with applicable standards, notably NF U 44-051 (April 2006).

[0044] According to either of the alternatives, the solid residue typically comprises the following composition:

[0045] - Dry matter: from 149 to 324 g / kg ;

[0046] - C / N: from 12 to 28 g / kg;

[0047] - Organic carbon: 70 to 164 g / kg;

[0048] - Organic matter: 140 to 328 g / kg ;

[0049] - Total nitrogen: 4.08 to 9.52 g / kg ;

[0050] - Ammoniacal nitrogen: 2.16 to 5.04 g / kg ;

[0051] - Organic nitrogen: 1 .86 to 4.34 g / kg ;

[0052] - Total phosphorus pentoxide (P2O5): 6.6 to 15.4 g / kg ;

[0053] - Potassium oxide (K2O) total: 2.34 to 5.46 g / kg ;

[0054] - Calcium oxide (CaO) total: from 4.68 to 10.92 g / kg ;

[0055] - Magnesium oxide (MgO) total: 1 .2 to 2.8 g / kg; The solid residue typically takes the form of a fibrous matrix generally composed of cellulose and its derivatives, such as hemicellulose, lignin, lignocellulose, etc...

[0056] Typically, said solid residue can be obtained by separating the liquid fraction from said digestate or compost. This separation can be carried out by means of a centrifuge, a vibrating sieve or a screw press.

[0057] According to one embodiment, said biodegradable polymer comprises the repetition of identical and / or different units of formula in which m=1 , 2, 3 or 4; each R, identical or different, is independently selected from H and linear or branched alkyl chains comprising from 1 to 15 carbon atoms, preferably methyl or ethyl; n represents the average number of units and is between 1 and 106.

[0058] In formula (I) : m is preferably equal to 1 and / or n is advantageously between 10 and 500,000, in particular from 100 to 100,000, especially from 100 to 10,000.

[0059] Typically, said biodegradable polymers of formula (I) are polyhydroxyalkanoates (PHAs). PHAs are biodegradable polyesters and can be produced naturally by bacterial fermentation of sugars or lipids.

[0060] Typically, PHAs contribute to the pot’s structure and mechanical strength. Advantageously, PHAs can be biodegraded in soils.

[0061] PHAs are typically classified according to the length of the 3-hydroxyalkanoate (3HA) unit: short-chain PHAs whose unit contains 3 or 4 carbon atoms (e.g. 3-hydroxybutyrate, 3HB and 3-hydroxyvalerate, 3HV units), medium-chain PHAs with 6 to 14 carbon atoms (e.g. 3-hydroxyhexanoate, 3HHx, 3-hydroxyheptanoate, 3HHo units), and long-chain PHAs with more than 14 carbon atoms (e. g., 3-hydroxyhexadecanoate). Representative PHAs include:

[0062] Polyglycolic acid or poly(glycolic acid) or polyglycolide (PGA) ;

[0063] Polylactic acid or poly(lactic acid) or polylactide (PLA);

[0064] Poly(3-hydroxypropionate) or poly(P-hydroxypropionate) (P(3HP) or P(PHP);

[0065] Poly(3-hydroxybutyrate) or poly(P-hydroxybutyrate) (P(3HB) or P(PHB)) ;

[0066] Poly(3-hydroxyvalerate) or or Poly(P-hydroxyvalerate) (P(3HV) or P(PHV)) ;

[0067] Poly(3-hydroxyhexanoate) or Poly(P-hydroxyhexanoate) (P(3HHx) or P(PHHx)) ;

[0068] Poly(3-hydroxyoctoate) or Poly(P-hydroxyoctoate)(P(3HO) or P(PHO)) ;

[0069] Poly(3-hydroxyoctadecanoate) or Poly(P-hydroxyoctadecanoate) (P(3HOD) or P(pHOD)) ;

[0070] Poly(4-hydroxybutyrate) or Poly(y-hydroxybutyrate) (P(4HB) or P(yHB)) ;

[0071] Poly(5-hydroxyvalerate) or Poly(6-hydroxyvalerate) or Polybutyrolactone (P(5HV) or P(5HV) or PBL);

[0072] Poly(6-hydroxyhexanoate) or Poly(£-hydroxyhexanoate) or Polycaprolactone (P(6HHx) or P(EHHX) or PCL); and

[0073] Their copolymers such as

[0074] Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) of formula (II) : where n' and n" are defined as n above.

[0075] In particular, the PHA family includes poly(P-hydroxybutyrate) (PHB), poly(P- hydroxyvalerate) (PHV), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers (PHBV) and mixtures thereof.

[0076] PHB corresponds to formula (III):

[0077] In which n is as defined above. PHB can be produced by bacterial fermentation of sugars. It is commercially available from producers such as TianAn (China), Yieldl O Bioscience (USA), Bio-Fed® (Germany) or Biomer® (Germany).

[0078] According to a non limiting embodiment, the composition may comprise a mixture of a solid residue of methanization digestate and a solid residue of composting. The mixture may comprise from 0.1% to 99,9% of solid residue of methanization digestate and from 0.1 % to 99.9% of a solid residue of composting.

[0079] According to one embodiment, the composition according to the invention comprises one or more agents promoting horticultural cultivation.

[0080] Typically, the said composition comprises from 0 to 30% by weight of agents promoting horticultural cultivation.

[0081] These agents may be, for example, fertilizing agents, biostimulation agents and / or biocontrol agents.

[0082] By "biostimulant" agent we mean a fertilizer replacement product compatible with more sustainable and reasoned agriculture. Typical biostimulants include algae, plant extracts and trace elements, such as Activ Nutrition or Rezist products.

[0083] Biocontrol agents are defined in Article L. 253-6 of the French Rural and Maritime Fishing Code, in force since January 1 , 2021 , as "agents and products using natural mechanisms as part of integrated pest management. They include in particular :

[0084] - macro-organisms useful to plants, and in particular invertebrates, including mites, insects and nematodes, used to protect plants from bio-aggressors via biological control; and

[0085] - phytopharmaceutical products composed of micro-organisms, chemical mediators such as pheromones and kairomones, or natural substances of plant, animal or mineral origin".

[0086] Biocontrol agents can be used to control diseases, crop pests (insects, viruses, bacteria, mites, etc.) and weeds.

[0087] A fertilizing agent is a substance, or mixture of substances, of natural or synthetic origin, used in agriculture, horticulture and forestry to improve soil structure and fertilize cultivated plants.

[0088] To potentiate the fertilizing effect of the pots, the composition can thus be supplemented by one or more fertilizing agents such as organic and / or mineral sources of nitrogen, phosphorus and / or potassium, such as fertilizers, macroalgae, insect trass, ammonium sulfate, struvite and urea.

[0089] Advantageously according to the invention, the biodegradability of the pot allows a slow and continuous release of these agents as the plant grows.

[0090] According to one embodiment, the composition according to the invention may also further comprise at least one compatibilizer.

[0091] Typically, the said composition comprises from 0 to 10% by weight of compatibilizers.

[0092] These compatibilizers could help in obtaining an optimum morphology, interfacial interaction and properties of the composition of the invention, in particular for some hardly soluble biodegradable polymers (for example, for a composition comprising lignocellulosic matrices and PHAs).

[0093] Examples of typical compatibilizers are hexamethylene diisocyanate and / or acetyl tributyl citrate.

[0094] According to one embodiment, the composition can be in granulated form.

[0095] According to another object, the present invention also relates to a process for manufacturing said composition.

[0096] It can typically be prepared by mixing the solid residue of methanization digestate or the solid residue of composting with a biodegradable polymer selected from polyhydroxyalkanoates (PHA) and optionally with one or more agents promoting horticultural cultivation.

[0097] According to one embodiment, the manufacturing process of a composition according to the invention can comprise the preparation of the solid residue of methanization digestate or of the solid residue of composting. The preparation of the solid residue of methanization digestate or solid compost residue can be obtained by separating the liquid fraction of said methanization digestate or compost, and optionally composting the residue obtained.

[0098] Separation can be carried out by the usual techniques. Typically, it can be carried out using a centrifuge, a vibrating sieve or a screw press.

[0099] In one embodiment, the above-mentioned ingredients can be mixed by extrusiongranulation, for example using a twin-screw extruder. Advantageously, extrusion- granulation enables the composition to be formulated in granulated form (compounding phase).

[0100] Said composition is advantageous it that allows a straightforward process, with appropriate flow and suitable viscosity. It is further highly versatile as it may be used for further manufacturing a variety of forms and may allow adjusting thickness and / or ductibility.

[0101] According to another object, the present invention is also aimed at a biodegradable horticultural pot comprising a composition according to the invention.

[0102] According to one embodiment, said horticultural pot comprises a lateral wall wall defined around a longitudinal axis X and having a first thickness H1 , such that the lateral wall comprises at least one thinned portion having a second thickness H2 less than said first thickness H1 .

[0103] Since the thinned portions have less resistance, it is easy for the plant's roots to penetrate the circumferential wall through these thinned portions, which fragment easily at the start of biodegradation.

[0104] According to a further object, the present invention also relates to a process for manufacturing a biodegradable horticultural pot according to the invention, said process comprising an injection step, the injection being implemented in a mold configured to form the lateral wall having the first thickness and comprising at least one impression shaped to form the at least one thinned portion having the second thickness .

[0105] According to one embodiment, the process further comprises, prior to injection, preparation of the composition as described above.

[0106] The pot can be manufactured by injection in an injection molding machine, from the granules of the composition described above.

[0107] According to an embodiment, the injection step uses a mold comprising:

[0108] - grooves shaped to form stabilizing fins on the bottom wall of the pot, and / or

[0109] - pins to form drainage holes on the bottom wall, and / or shapes to form drainage slots on the bottom wall;

[0110] - a circumferential groove to form a flange.

[0111] Various aspects and advantages of the invention will be highlighted in the following description, given only as a non-limiting example and made with reference to the appended figures, among which:

[0112] - Figure 1 is a profile view of a horticultural pot according to an example of an embodiment of the invention; - Figure 2 is a sectional view, along plane ll-ll, of the pot shown in Figure 1 .

[0113] Figures 1 and 2 show a horticultural pot 10 according to an example of embodiment of the invention.

[0114] The horticultural pot 10 has a lateral wall 12 defined around a longitudinal axis X. The lateral wall 12 has, in the example described, a general shape of revolution about the longitudinal axis X.

[0115] In the example shown, the lateral wall 12 has a generally circular cross-section, which is preferred for reasons of ease of shaping, in particular ease of filling a mold when molding the pot 10.

[0116] Alternatively, the lateral wall 12 could have a generally square cross-section, rectangular cross-section, square cross-section with rounded corners, or any other conceivable shape.

[0117] A horticultural pot according to the invention may further comprise one or more of the following features, taken alone or in any technically conceivable combination.

[0118] - the lateral wall 12 comprises a plurality of thinned portions 14, preferably equally distributed circumferentially around the longitudinal axis X;

[0119] - each tapered portion 14 is arranged substantially halfway up the side wall 12; the side wall 12 is solid; the second thickness H2 is less than or equal to 80% of the first thickness H1 , for example substantially equal to 75% or 67% of the first thickness H1 ;

[0120] - the lateral wall 12 has a continuous inner face 12a and an outer face 12b, each thinned portion forms a recess on the outer face 12b;

[0121] - the pot 10 comprisies a domed bottom wall 16 with a concavity facing the interior of the pot 10;

[0122] - the bottom wall (16) comprises, on an outer face, at least three stabilizing fins (18), extending in height in the direction of the longitudinal axis from the outer face of the bottom wall to a straight edge (18a) perpendicular to the longitudinal axis (X)

[0123] - the bottom wall 16 comprises a plurality of drainage holes 20, preferably equally distributed circumferentially around the longitudinal axis X;

[0124] - the lateral wall 12 has a flange 15, provided at a first end 13a in the direction of the longitudinal axis X.

[0125] EXAMPLES

[0126] Example 1 : Production of biodegradable pots Biodegradable pots containing only PHB (poly(P-hydroxybutyrate) and digestate (D), and PHB and solid digestate compost (C) are prepared.

[0127] For this purpose, six formulations are produced by compounding in a twin-screw extruder, in addition to the raw PHB: PHB / D (75 / 25), PHB / D (50 / 50) and PHB / D (25 / 75) as well as PHB / C (75 / 25), PHB / C (50 / 50) and PHB / C (25 / 75).

[0128] Compounding is carried out using a Clextral (France) Evolum HT 53 twin-screw extruder with co-rotating and co-penetrating screws.

[0129] Matrix (PHB) and filler (D) are steamed at 60°C overnight in a ventilated oven before twin- screw compounding. Equipped with nine modules (53 mm for screw diameter D; 36 D for total barrel length), the twin-screw extruder is fitted at the barrel end with a convergent (front plate) and a plasturgical die. The rods are cooled by conveying them along a water-cooling tank. The rods are then granulated using a high-capacity granulator knife.

[0130] The screw and temperature profiles can be adapted to the thermal characteristics of the selected PHB grade, if required. Operating conditions (screw speed and input rates) can be adjusted to ensure stable twin-screw extruder amperage operation throughout the compounding process, with no risk of degradation (thermal decomposition) of the two constituents of the mixture.

[0131] Calibrated in size, the granules obtained are then injected into standardized test specimens, which are then tested mechanically (tensile strength, flexural strength and Charpy impact resistance) and with regard to their water resistance (swelling in thickness and mass set after 24 h immersion in water).

[0132] Formulations developed by twin-screw compounding are also evaluated in terms of (i) their viscosity, by measuring their melt flow index (MFI), and (ii) their injectability, first in a spiral mold and then for the molding of a circular cavity (disk) 10 cm in diameter and only 1 mm thick.

[0133] In addition, the qualification of these formulations is completed by analyses such as:

[0134] - Thermal analysis: differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to assess the PHB phase-change temperatures in the formulations, and their degradation temperatures (thermal decomposition).

[0135] - Adsorption isotherms (DVS).

[0136] - Scanning electron microscope (SEM) observation of fracture surfaces on tensile specimens.

[0137] Finally, pots are also produced by thermoplastic injection using an ad hoc mold.

[0138] Example 2: Production of biodegradable fertilizer pots In a second step, biodegradable pots with a potentiated fertilizing action are produced. For this purpose, the formulations resulting from compounding in twin-screw extruders will be of two types:

[0139] - On the one hand, PHB is combined with digestate and possibly supplemented with a product rich in organic nitrogen (AO) or another rich in mineral nitrogen (AM). Three different formulations are thus produced: PHB / D (as a reference), PHB / D / AO and PHB / D / AM.

[0140] - On the other hand, the same three formulations are produced by replacing the digestate with compost (C): PHB / C (as a reference), PHB / C / AO and PHB / C / AM.

[0141] The organic nitrogen source can be a protein fraction selected from macroalgae or insect frass, and the mineral nitrogen source can be, for example, ammonium sulfate or urea.

[0142] The mass proportions of the three constituents of the same formulation can be defined, taking into account both the processability of the different formulations produced (example 1 ) and the nitrogen requirements of the plants selected for cultivation in pots.

[0143] Once the granules of these six formulations have been produced in a twin-screw extruder, they are molded by thermoplastic injection into standardized test tubes, discs and pots. Characterization is then carried out as for Example 1 .

[0144] Example 3: Influence of PHB content on produced biodegradable fertilizer pots:

[0145] In a third step, the influence of PHB content on the formulated compositions for producing biodegradable pots is evaluated.

[0146] For this purpose, formulations containing Non-extruded raw PHB, Extruded raw PHB, PHB / D (75 / 25), PHB / D (60 / 40), and PHB / D (40 / 60) were prepared, according to the same process described in Example 1 .

[0147] The density of prepared compositions were measured and the results are shown below: [Table 1 ] The prepared compositions were then characterized by a series of tests described below:

[0148] 3.1 . Tensile test

[0149] For the tensile test, the Young’s modulus (Et), the maximum tensile strength at break (Rmax) and the elongation at break were determined for each composition following the standard as per ISO 527-4:1997.

[0150] The results are shown below:

[0151] [Table 2]

[0152] The tensile test shows that as the content of PHB in the composition increases, the Young’s modulus (Et) decreases while the maximal tensile strength at break (Rmax) and the elongation at break increase, suggesting that the formulated biodegradable pots become more resistant and less likely to break under external stress.

[0153] 3.2. Bending test

[0154] For the bending test, the modulus of elasticity in bending (Ef) and the maximum bending strength at break (omax) were determined for each composition following the standard as per ISO 178:2010.

[0155] The results are shown below:

[0156] [Table 3]

[0157] The bending test shows that a higher PHB content in the composition leads to a lower modulus of elasticity in bending (Ef) and a higher maximal bending strength at break (omax). Similar to the tensile test, the bending test also suggests that a higher PHB content result in more stress-resistant and robust biodegradable pots.

[0158] 3.3. Charpy impact test

[0159] For the Charpy impact test, the Charpy impact strength for fracturing an unnotched sample was determined for each composition following the standard as per ISO 179-1 :2010. The results are shown below:

[0160] [Table 4]

[0161] The Charpy impact test shows that Charpy impact strength increases as the sample’s PHB content is higher, indicating that more energy needs to be afforded to break a composition with higher PHB content. Therefore, the addition of PHB content improves the toughness of the composition of biodegradable pots.

[0162] 3.4. Swelling test For the swelling test, samples of different compositions of biodegradable pots were immersed in water for 24 hours. Then the water absorption rate and the thickness-swelling rate of each sample were measured following the standard as per ISO 16983:2003.

[0163] The results are shown below:

[0164] [Table 5]

[0165] The swelling test shows that after immersion in water for 24 hours, compositions with higher PHB content hardly absorbed water while compositions with low PHB content absorbed over 6% by weight of water. As a result, compositions with higher PHB content exhibited a smaller thickness-swelling rate than those with lower PHB content.

[0166] Water can modify the "texture / consistency" of the pot, making it more fragile and easily broken. Even if water absorption seems advantageous for plant development, it greatly reduces the pot's mechanical resistance before planting.

[0167] In conclusion, as illustrated in the different tests in Example 3, PHB are proved able to improve the robustness and resistance of compositions for producing biodegradable pots. In other words, compositions comprising low PHB content are fragile and sensible to the external stress, and thus more likely to break up. Compositions with less than 40% by weight of PHB cannot provide expected mechanical strength. Indeed, the good robustness and the sufficient resistance are essential for the entire lifespan of the produced biodegradable pot, in terms of their storing, transportation, sale and utilization. Moreover, a higher PHB content can prevent the composition of biodegradable pot from absorbing water and avoid the accompanying swelling. Given that the absorption of water can induce significant structure change and consequently impair the mechanical strength, a PHB content higher than or equal to 40% by weight with respect to the total weight of composition according to the invention is necessary for ensuring a good mechanical performance.

Claims

CLAIMS1. Composition for biodegradable horticultural pots, characterized in that it comprises :- from 20 to 60% by weight of a solid residue of methanization digestate and / or a solid residue of composting ;- from 40 to 80% by weight of a biodegradable polymer chosen from polyhydroxyalkanoates (PH A);- from 0 to 30% by weight of one or more agents promoting horticultural cultivation.

2. Composition according to claim 1 such that said biodegradable polymer comprises the repetition of identical and / or different units of formula (I): r R o i-L JU M r(i) in which m=1 , 2, 3 or 4, preferably 1 ; each R, identical or different, is independently selected from H and linear or branched alkyl chains comprising from 1 to 15 carbon atoms, preferably methyl or ethyl; n represents the average number of units and is between 1 and 100,000.

3. Composition according to any one of the preceding claims such that said biodegradable polymer is selected from poly(P-hydroxybutyrate) (PHB), poly(P- hydroxyvalerate) (PHV), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymers and mixtures thereof.

4. Composition according to any one of the preceding claims such that said solid composting residue is selected from the solid residues of a compost : said digestate, the solid residue of the digestate, or an organic matrix, and mixtures thereof.

5. Composition according to claim 4 such that said solid residue is obtained by separation of the liquid fraction from said digestate or said compost.

6. Composition according to any one of the preceding claims such that the agents promoting horticultural cultivation are chosen from fertilizing, biostimulating and biocontrol agents.

7. Composition according to claim 6 such that the fertilizing agents are chosen from organic and / or mineral sources of nitrogen, phosphorus and / or potassium, such as fertilizers, macroalgae, insect frass, ammonium sulfate, struvite and urea.

8. Composition according to any of the preceding claims, the composition further comprises from 0 to 10% by weight of at least one compatibilizer.

9. Composition according to any of the preceding claims in granulated form.

10. A method of manufacturing a composition according to any one of the preceding claims, comprising mixing solid methanization digestate residue or solid composting residue with a biodegradable polymer selected from polyhydroxyalkanoates (PHA) and optionally with one or more horticultural culture-promoting agents.

11. A process according to claim 10 further comprising preparing the solid residue of methanization digestate or solid composting residue, by separating the liquid fraction from said methanization digestate or compost, and optionally composting the residue.

12. Method according to claim 11 such that separation is effected by means of a centrifuge, vibrating sieve or screw press.

13. Process according to claim 10 or 11 such that the mixing is carried out by extrusion-granulation using a twin-screw extruder.

14. Biodegradable horticultural pot comprising a composition according to any one of claims 1 to 9.

15. A method of manufacturing a biodegradable horticultural pot according to claim 14 comprising injecting the composition according to claim 9 into a mold corresponding to the desired pot.

16. Manufacturing process according to claim 15 comprising, prior to injection, carrying out the process according to any one of claims 10 to 13.

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