JP2026009708A - Method for producing fused polyhydroxyalkanoate - Google Patents

Abstract

[Problem] To provide a method for producing PHA that can suppress the generation of fine powder without using a binder. [Solution] A method for producing a fused PHA, comprising: (a) dehydrating an aqueous PHA suspension to obtain a wet PHA powder with a moisture content of 5 to 37%, (b) heating the obtained wet PHA powder at 75 to 95°C to obtain a PHA powder with a moisture content of 0.5% or less, and (c) heating the obtained PHA powder at 100 to 150°C to obtain a fused PHA. [Selected Figures] None

Description

The present invention relates to a method for producing a polyhydroxyalkanoate fused body.

Polyhydroxyalkanoates (hereinafter sometimes referred to as "PHA") are known to be biodegradable, and in recent years, their use has been promoted from an environmentally friendly perspective.

One of the advantages of PHA is that it can be produced by microorganisms using renewable plant materials. To utilize PHA produced by microorganisms, the microbial cells must first be disrupted to extract the PHA from the cells, yielding an aqueous suspension of PHA. The PHA is then recovered and purified from this aqueous suspension by filtration or other methods to obtain a cake-like PHA, after which the PHA cake must be dried. This drying process can be performed using a spray dryer, fluidized bed dryer, drum dryer, or other device; however, spray dryers are commonly used due to their ease of operation (see, for example, Patent Document 1).

International Publication No. WO2018/070492

PHA produced by microorganisms is produced as a powder with a fine particle size. When this powdered PHA is dried, a large amount of fine powder is generated, which is a problem. To prevent this fine powder generation, a technique has been proposed in which PHA is spray-dried while being mixed with a binder. However, when a binder is used, the PHA produced will naturally contain the binder as an impurity, which can result in a decrease in the purity of the PHA.

In light of the above situation, one aspect of the present invention aims to provide a method for producing PHA that can suppress the generation of fine powder without using a binder.

As a result of extensive research to solve the above-mentioned problems, the inventors discovered that a fused PHA can be obtained by subjecting a wet PHA powder obtained by filtering an aqueous PHA suspension to a process of heating it at a specific, relatively low temperature until it reaches a predetermined moisture content, and then further heating it at a specific, relatively high temperature. This process allows PHA to be dried without the use of a binder, while suppressing the generation of fine powder during the drying process, and led to the completion of the present invention.

That is, one aspect of the present invention includes the following configuration.

[1] A method for producing a polyhydroxyalkanoate fused product, comprising: (a) a step of dehydrating an aqueous suspension of polyhydroxyalkanoate to obtain a wet polyhydroxyalkanoate powder having a moisture content of 5 to 37%; (b) a step of heating the obtained wet polyhydroxyalkanoate powder at 75°C to 95°C to obtain a polyhydroxyalkanoate powder having a moisture content of 0.5% or less; and (c) a step of heating the obtained polyhydroxyalkanoate powder having a moisture content of 0.5% or less at 100°C to 150°C to obtain a polyhydroxyalkanoate fused product.
[2] The method for producing a polyhydroxyalkanoate fused body according to [1], which comprises, prior to the step (b), a step (a') of crushing the polyhydroxyalkanoate wet powder obtained in the step (a) so that the maximum diameter is 3.0 mm or less.
[3] The method for producing a polyhydroxyalkanoate fused body according to [1] or [2], wherein the step (b) comprises a step of heating the polyhydroxyalkanoate wet powder by indirect heating.
[4] The method for producing a polyhydroxyalkanoate fused body according to any one of [1] to [3], wherein the step (c) includes a moving step of heating the polyhydroxyalkanoate powder while moving it.
[5] The method for producing a polyhydroxyalkanoate fused body according to any one of [1] to [4], wherein the moving step includes a step of scraping off the polyhydroxyalkanoate powder using a blade.
[6] The method for producing a polyhydroxyalkanoate fused material according to any one of [1] to [5], wherein the steps (b) and (c) are carried out in the same system.
[7] The method for producing a polyhydroxyalkanoate fused material according to any one of [1] to [6], wherein the proportion of polyhydroxyalkanoate microparticles having a particle size of 1 to 10 μm is 28.5% or less relative to the total amount of the dried polyhydroxyalkanoate obtained in step (c).

One aspect of the present invention provides a method for producing PHA (fused body) that can suppress the generation of fine powder without using a binder.

One embodiment of the present invention is described below, but the present invention is not limited to this. The present invention is not limited to the configurations described below, and various modifications are possible within the scope of the claims. Furthermore, embodiments or examples obtained by combining the technical means disclosed in different embodiments or examples are also included in the technical scope of the present invention. Furthermore, new technical features can be created by combining the technical means disclosed in each embodiment. All academic and patent literature described in this specification is incorporated herein by reference. Furthermore, unless otherwise specified in this specification, the expression "A to B" representing a numerical range means "greater than or equal to A (including and greater than A) and less than or equal to B (including and less than B)."

1. Overview of the present invention
A conventional method for drying PHA is known in which an aqueous PHA suspension is spray-dried. However, as described above, spray drying essentially requires the addition of a binder, which poses a problem of reducing the purity of the resulting PHA.

The inventors investigated a new method for drying PHA, in which an aqueous PHA suspension is first dehydrated by filtration to obtain a wet PHA powder, and then this wet PHA powder is heated and dried. However, they discovered that when the wet PHA powder was heated at a relatively high temperature to quickly increase the dryness (reduce the moisture content), some of the dried PHA powder adhered to the inside of the equipment, resulting in insufficient heating of the entire wet PHA powder and preventing it from being dried sufficiently, creating a new problem.

Having discovered the above problem, the inventors attempted to perform the drying process at a temperature lower than the above in order to prevent the dried PHA powder from adhering to the inside of the equipment. However, they discovered that a new problem arose: the dried PHA obtained by drying at such a relatively low temperature contained a large amount of fine powder.

Having encountered the two new challenges related to the drying temperature described above, the inventors further investigated methods for heating and drying wet PHA powder. As a result, they discovered that by first heating the wet PHA powder at a relatively low temperature until it reaches a certain moisture content, and then further heating it at a relatively high temperature, the powdered PHA obtained by drying at a relatively low temperature can be fused to produce a fused PHA, thereby suppressing the generation of fine powder during the drying process, and thus completed the present invention.

2. Method for producing PHA fused body
A method for producing a PHA fused body according to one embodiment of the present invention (hereinafter referred to as "this production method") comprises the steps of: (a) dehydrating an aqueous PHA suspension to obtain a wet PHA powder having a moisture content of 5 to 37%, (b) heating the obtained wet PHA powder at 75°C to 95°C to obtain a PHA powder having a moisture content of 0.5% or less, and (c) heating the obtained PHA powder having a moisture content of 0.5% or less at 100°C to 150°C to obtain a PHA fused body.

Because this manufacturing method has the above-mentioned configuration, it is possible to provide PHA (fused body) without using a binder, while suppressing the generation of fine powder. Because this manufacturing method does not require the use of a binder, it is possible to provide PHA with higher purity, and by suppressing the generation of fine powder, it is possible to improve the yield of PHA and also improve the safety of the manufacturing process.

<Step (a)>
This production method includes (a) a step (sometimes referred to as "step (a)") of dehydrating an aqueous PHA suspension to obtain a wet PHA powder having a moisture content of 5 to 37%.

(PHA aqueous suspension)
First, the PHA aqueous suspension used in step (a) will be described in detail. In this specification, the PHA aqueous suspension refers to a solution in which PHA is suspended (dispersed) in water (aqueous medium), and has fluid properties. In addition to water, the PHA aqueous suspension may contain other solvents (e.g., organic solvents compatible with water), components derived from PHA-producing microorganisms (e.g., cell walls, proteins, etc.), and/or other compounds generated during purification.

・PHA
"PHA" is a general term for polymers containing hydroxyalkanoate as a monomer unit (monomer repeating unit) and is generally biodegradable. In particular, in this specification, "PHA" refers to a (co)polymer containing 50 mol% or more of hydroxyalkanoate repeating units out of all monomer repeating units (100 mol%), and a resin composed of such a (co)polymer. Specific examples of hydroxyalkanoate repeating units that constitute PHA include 3-hydroxybutanoic acid unit, 4-hydroxybutanoic acid unit, 3-hydroxypropionic acid unit, 3-hydroxypentanoic acid unit, 3-hydroxyhexanoic acid unit, 3-hydroxyheptanoic acid unit, 3-hydroxyoctanoic acid unit, and 2-hydroxypropionic acid unit. In this specification, the term "(co)polymer" is used to refer to both a homopolymer composed of only one type of monomer and a copolymer composed of two or more types of monomers.

Examples of PHAs provided by this production method include poly(3-hydroxyalkanoate) (hereinafter sometimes referred to as "P3HA") and poly(4-hydroxyalkanoate). Of these, P3HA is preferred because it is suitable for use in molded articles.

P3HA is a PHA that contains, as an essential repeating unit, a 3-hydroxyalkanoate repeating unit represented by the formula: [-CHR-CH ₂ -CO-O-] (wherein R is an alkyl group represented by C _n H _2n+1 , and n is an integer of 1 or more and 15 or less).

Specific examples of P3HA include poly(3-hydroxybutyrate) (hereinafter sometimes referred to as "P3HB"), which is a homopolymer of 3HB; poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (hereinafter sometimes referred to as "P3HB3HH"); poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (hereinafter sometimes referred to as "P3HB4HB"); poly(3-hydroxybutyrate-co-3-hydroxyvalerate); and poly(3-hydroxy Examples of suitable polyhydroxybutyrates include poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), poly(3-hydroxybutyrate-co-3-hydroxyoctadecanoate), poly(3-hydroxybutyrate-co-3-hydroxydecanoate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate), poly(3-hydroxybutyrate-co-2-hydroxypropionate), and poly(3-hydroxybutyrate-co-3-hydroxypropionate). Among these, P3HB, P3HB3HH, and P3HB4HB are preferred, with P3HB3HH and P3HB4HB being more preferred, due to the ease of industrial production using microorganisms.

In this specification, "poly(X-co-Y)" refers to a copolymer containing X repeating units and Y repeating units, and is intended to mean a copolymer obtained by copolymerizing a monomer from which the X repeating units are derived and a monomer from which the Y repeating units are derived. As mentioned above, the name of P3HA is determined by the repeating units contained in the P3HA. However, very small amounts (approximately 1 mol % or less) of monomers contained in P3HA may not be reflected in the name of the P3HA, provided that they do not significantly affect the physical properties of the P3HA. In other words, P3HA may contain very small amounts of other repeating units in addition to the repeating units corresponding to its name.

When P3HA contains 3HB repeating units, from the viewpoint of the balance between flexibility and strength, the composition ratio (3HB repeating units/other repeating units) of 3HB repeating units to repeating units other than 3HB repeating units (other repeating units) among all monomer repeating units (100 mol%) in the P3HA is preferably 99/1 (mol%/mol%) to 60/40 (mol%/mol%), more preferably 97/3 (mol%/mol%) to 70/30 (mol%/mol%), and even more preferably 95/5 (mol%/mol%) to 80/20 (mol%/mol%). When the composition ratio of 3HB repeating units in P3HA is 60 mol% or more, it has the advantage of being able to provide resin products with superior rigidity. On the other hand, when the composition ratio of 3HB repeating units in P3HA is 99 mol% or less, it has the advantage of being able to provide resin products with superior flexibility. The monomer composition ratio of P3HA can be measured by gas chromatography or the like (see, for example, WO 2014/020838).

P3HA can be produced by microorganisms. Examples of microorganisms capable of producing P3HA include P3HB-producing bacteria, such as Bacillus megaterium, which was discovered in 1925. Other known natural microorganisms include Cupriavidus necator (formerly classified as Alcaligenes eutrophus and Ralstonia eutropha) and Alcaligenes latus. P3HB accumulates within these microorganisms.

In addition, known bacteria that produce P3HA, a copolymer of 3HB and other hydroxyalkanoic acids, include Aeromonas caviae, which produces P3HB3HH, and Alcaligenes eutrophus, which produces poly(3-hydroxybutyrate-co-4-hydroxybutyrate). In particular, Alcaligenes eutrophus AC32 (FERM BP-6038) (T. Fukui, Y. Doi, J. Bacteriol., 179, pp. 4821-4830 (1997)), into which genes encoding P3HA synthases have been introduced, is preferred for increasing P3HB3HH productivity. In addition to the above, genetically modified microorganisms into which various P3HA synthesis-related genes have been introduced can also be used depending on the desired physical properties of P3HA.

- Method for Producing PHA Aqueous Suspension As the PHA aqueous suspension to be subjected to step (a), an aqueous PHA suspension derived from a culture broth of a PHA-producing microorganism can be suitably used. Such an aqueous PHA suspension derived from the culture solution of a PHA-producing microorganism can be prepared, for example, by the following methods: (1) culturing a microorganism capable of producing PHA; (2) inactivating the culture solution of the microorganism by heating to obtain an inactivated culture solution; (3) treating the inactivated culture solution with hydrogen peroxide to reduce the viscosity of the culture solution; (4) treating the inactivated culture solution treated with hydrogen peroxide with alkali; (5) adding a lytic enzyme (a cell wall-degrading enzyme) to the alkali-treated inactivated culture solution to lyse the microbial cells and disperse the substances within the cells, including PHA, in the culture solution; (6) adding a protease to the culture solution to decompose substances derived from the microbial cells other than PHA (particularly proteins); (7) further adjusting the pH of the culture solution and adding a surfactant to decompose substances derived from the microbial cells other than PHA (particularly cell membranes). Furthermore, the method may further include, if necessary, (8) an operation of removing impurities in the aqueous suspension by centrifugation and/or filtration to purify the PHA.

The present production method may include, prior to step (a), a step of culturing a PHA-producing microorganism and preparing an aqueous PHA suspension from the culture solution (a PHA aqueous suspension preparation step), which includes one or more of the above-mentioned operations. Furthermore, when the present production method includes two or more of the above-mentioned operations, the order in which the operations are performed is not limited to the above-mentioned order.

(Dehydration operation)
In step (a), the method for dehydrating the aqueous PHA suspension is not particularly limited as long as it can produce a wet PHA powder with the desired moisture content, and examples thereof include a method of filtering the aqueous PHA suspension or a method of centrifuging the aqueous PHA suspension.

The method for filtering the aqueous PHA suspension is not particularly limited as long as it allows for the preparation of a wet PHA powder with the desired moisture content. However, dead-end filtration is preferred because it allows filtration to be performed using simple equipment and operations, and is space-saving and inexpensive. That is, in step (a), it is preferable to dehydrate the aqueous PHA suspension by dead-end filtering to obtain a wet PHA powder. In other words, step (a) preferably includes a dead-end filtration step in which the aqueous PHA suspension is dead-end filtered. Note that, in this specification, "dead-end filtration" means "filtration using the dead-end filtration method."

Specific modes of dead-end filtration include, but are not limited to, suction filtration, pressure filtration, centrifugal filtration, gravity filtration, etc. Among these, pressure filtration is preferred because it has the advantage of easily reducing the water content, and filter press filtration (FP filtration) is more preferred. That is, the PHA agglomerate preparation process preferably includes a filter press filtration step in which the aqueous PHA suspension is filtered through a filter press. Filter press filtration is a process in which the aqueous PHA suspension is pressed and dehydrated, and the residue (in this production method, wet PHA powder) is obtained.

When filter press filtration of the PHA aqueous suspension is performed in step (a), the filtration operation can be carried out using a known filter press filtration device.

Methods for centrifuging the aqueous PHA suspension include basket centrifugation, decanter centrifuges, disk centrifuges, and liquid cyclones. Of these, basket centrifugation is preferred due to its superior dewatering efficiency.

(PHA wet powder)
The PHA wet powder obtained by step (a) will be described. The PHA wet powder obtained by step (a) is a residue obtained by dehydrating the PHA aqueous suspension, and is a solid composition containing PHA and water. The PHA wet powder may contain agglomerates formed by aggregation of PHA molecules due to water being held between the surfaces of multiple PHA molecules. In other words, the PHA wet powder obtained by step (a) is a concept that includes such agglomerates of PHA (and water).

The moisture content of the PHA wet powder obtained in step (a) is 5 to 37% by weight. By controlling the moisture content of the PHA wet powder in step (a) within the above range, the drying operation in the subsequent step (b) can be carried out more efficiently. The moisture content of the PHA wet powder obtained in step (a) is not particularly limited as long as it is 5 to 37% by weight, but is preferably 8 to 25% by weight, and more preferably 10 to 20% by weight. Note that the moisture content of the PHA wet powder referred to herein is a wet basis (W.B.) value, which refers to the weight percentage of water in a total amount of 100% by weight of the PHA wet powder. Furthermore, the moisture content of the PHA wet powder referred to herein is a value measured using the method described in the Examples.

The diameter of the PHA wet powder (including PHA agglomerates) obtained in step (a) is not particularly limited. However, since this allows for efficient production of a PHA powder with the desired moisture content in the subsequent step (b) (i.e., efficient heat drying), the maximum diameter is preferably 3.0 mm or less, and more preferably 2.0 mm or less. Note that, in this specification, whether the maximum diameter of the PHA wet powder (including PHA agglomerates) is equal to or less than the desired diameter can be determined by whether the PHA wet powder in question can pass through a sieve with openings of the desired diameter (PHA wet powder that passes through a sieve with openings of the desired diameter is PHA wet powder whose maximum diameter is equal to or less than the desired diameter).

(Step (a'))
In one embodiment of this production method, the PHA wet powder obtained in step (a) may contain PHA wet powder (mainly PHA agglomerates) with a diameter exceeding the desired maximum diameter. In such cases, it is preferable to crush the PHA wet powder (mainly PHA agglomerates) and adjust (size) the particles so that their maximum diameter is equal to or less than the desired value (e.g., 3.0 mm or less, or 2.0 mm or less) before subjecting them to step (b). That is, the production method preferably includes step (a'), prior to step (b), in which the PHA wet powder (mainly PHA agglomerates) obtained in step (a) is crushed so that its maximum diameter is equal to or less than the desired value (e.g., 3.0 mm or less, or 2.0 mm or less).

In step (a'), the method for crushing (coarse crushing) the PHA wet powder is not particularly limited, and examples include mechanically breaking the powder using a crusher or screw, or destroying the powder by impact such as dropping it.

<Step (b)>
This manufacturing method includes (b) a step (sometimes referred to as "step (b)") of heating the PHA wet powder obtained in step (a) at 75°C to 95°C to obtain a PHA powder having a moisture content of 0.5% or less.

Step (b) in this manufacturing method can also be considered a step of heating and drying the PHA wet powder until its moisture content is 0.5% or less (basically, once the moisture content of the PHA wet powder is 0.5% or less, it will naturally become powdery).

(Heating operation)
In step (b), the method for heating the PHA moist powder is not particularly limited as long as it can heat the PHA moist powder within the above temperature range, and either direct heating or indirect heating can be applied, but indirect heating is preferred because it has excellent thermal efficiency and can reduce the cost required for the heating operation. That is, step (b) preferably includes a step of heating the PHA moist powder by indirect heating.

Specific methods for indirectly heating wet PHA powder include, for example, using a plate dryer, a paddle dryer, or a disk dryer.

In step (b), the temperature to which the PHA moist powder is heated is 75°C to 95°C. By controlling the heating temperature in step (b) within this range, a portion of the resin (PHA) in the PHA moist powder (particularly the surface portion) is rapidly dried during step (b), preventing it from adhering to the heat transfer surface and impeding heat transfer from the heat source. As a result, it is possible to obtain PHA powder from which moisture has been sufficiently removed (with a moisture content of 0.5% or less). The heating temperature in step (b) is not particularly limited as long as it is within the above range, but 80°C to 93°C is more preferable, and 85°C to 90°C is even more preferable.

In step (b), the heating time for the PHA moist powder is not particularly limited as long as it is possible to heat the PHA moist powder to the desired moisture content (at least 0.5% or less), but it may be, for example, 10 minutes to 1 hour, or 20 to 40 minutes.

The heating operation in step (b) is preferably carried out while the PHA wet powder to be heated is being moved within the system (e.g., within a dryer) (i.e., step (b) preferably includes a moving step of moving the PHA wet powder to be heated). By including a moving step in step (b) of heating the PHA wet powder to be heated while moving it, adhesion of the dried PHA powder to the system can be suppressed.

In step (b), the method for moving the wet PHA powder during heating is not particularly limited, but a suitable method is to scrape off the target wet PHA powder using a blade. By using a blade to dry the wet PHA powder while scraping it off, uniform drying is possible, resulting in PHA powder with little variation in moisture content.

(PHA powder)
Step (b) can also be said to be a step of obtaining a PHA powder having a moisture content of 0.5% or less. By reducing the moisture content of the obtained PHA powder to 0.5% or less in step (b) (by drying until it is 0.5% or less), it becomes possible to fuse the PHA powder together in the subsequent step (c), thereby making it possible to obtain a PHA (fused body) while suppressing the generation of fine powder. The moisture content of the PHA powder obtained in step (b) is not particularly limited as long as it is 0.5% or less, but is more preferably 0.4% or less, and even more preferably 0.3% or less. Furthermore, the lower limit of the moisture content is not particularly limited and may be 0%. Note that, in this specification, the moisture content of the PHA powder is a value measured by the method described in the Examples.

<Step (c)>
This manufacturing method includes (c) a step (sometimes referred to as "step (c)") of heating the PHA powder having a moisture content of 0.5% or less obtained in step (b) at 100°C to 150°C to obtain a PHA fused body.

Step (c) of this manufacturing method can also be considered a step in which PHA powder particles are fused together to obtain a fused PHA. In this manufacturing method, during the fusion process, small particles (particles that can become fine powder) contained in the PHA powder fuse with each other or with relatively large particles, reducing the proportion of small particles in the powder and, it is believed, also reducing the proportion of fine powder in the final dried PHA product.

(heating (fusion) operation)
In step (c), the method for heating the PHA powder is not particularly limited as long as it can heat the PHA powder within the above temperature range, and either direct heating or indirect heating can be applied, but indirect heating is preferred because it has excellent thermal efficiency and can reduce the cost required for the heating operation. That is, step (c) preferably includes a step of heating the PHA powder by indirect heating.

Specific methods for indirectly heating PHA powder include, for example, using a plate dryer, a paddle dryer, or a disk dryer.

In step (c), the PHA powder is heated to a temperature of 100°C to 150°C. Setting the heating temperature in step (c) to 100°C or higher makes it possible to melt and heat-fuse the resin (PHA) that makes up the PHA powder, while setting it to 150°C or lower prevents the PHA powder from completely melting, resulting in a fused PHA with a particle shape that is easy to handle. The heating temperature in step (c) is not particularly limited as long as it is within the above range, but since this allows for the provision of a PHA with a smaller amount of fine powder, a temperature of 105°C or higher is more preferable, 110°C or higher is even more preferable, and 115°C or higher is even more preferable.

In step (c), the heating time for the PHA powder is not particularly limited as long as it is possible to melt and heat-fuse the resin (PHA) that constitutes the PHA powder to obtain a fused body, but it is, for example, 1 to 30 minutes, or may be 3 to 10 minutes.

The heating operation in step (c) preferably includes a transfer step in which the PHA powder to be heated is moved within the system (e.g., within a dryer). By including a transfer step in step (c) in which the PHA powder to be heated is heated while being moved, fusion of the molten PHA particles can be promoted and adhesion of the molten PHA to the system can be suppressed.

In step (c), the method for moving the PHA powder during heating is not particularly limited, but a suitable method is to scrape off the target PHA powder using a blade. Using a blade to dry the PHA powder while scraping it off enables uniform drying and fusion, resulting in a dried PHA product with little uneven moisture content and little fine powder.

In this manufacturing method, it is preferable to carry out steps (b) and (c) in the same system, as this reduces costs associated with transporting the PHA powder (e.g., the time required for transport and loss of PHA during the transport process).

One method for performing steps (b) and (c) in the same system is to use a drying device with two or more temperature zones that can be controlled to different temperatures. An example of such a drying device is a plate dryer. A plate dryer is a drying device consisting of multiple plates, each with its own temperature controllable, that heats the object being heated as it moves from the upper plate to the lower plate.

For example, when using a plate dryer, steps (b) and (c) of this manufacturing method can be carried out in the same system by heating the upper plate to 75-95°C and the lower plate to 100-150°C.

(PHA fused body and PHA dried product)
In step (c), a dried PHA product is obtained which is a mixture of a particulate fused PHA having a certain particle size, which is formed by the fusion of PHA powder particles, and PHA fine particles, which are composed of unfused PHA powder or fused particles that are partially fused but still have a small particle size. In other words, this production method can be said to be a method for producing such a dried PHA product, or a method for obtaining a dried PHA product with a low content of PHA fine particles.

In this specification, "PHA fused material" refers to fused PHA particles contained in dried PHA that have a particle size of more than 10 μm, and "PHA microparticles" refers to fused PHA particles with a particle size of 10 μm or less, as well as unfused powder. The particle sizes of PHA fused material and PHA microparticles can be measured using a particle size distribution analyzer.

In this specification, the amount of fine powder contained in the dried PHA product finally obtained by step (c) can be evaluated based on the proportion of PHA microparticles with a particle size of 1 to 10 μm relative to the total amount of the dried PHA product (i.e., the total amount of the fused PHA and the PHA microparticles). In this specification, when the proportion of PHA microparticles with a particle size of 1 to 10 μm relative to the total amount of the dried PHA product is 28.5% or less, the dried PHA product can be evaluated as having little fine powder (the generation of fine powder has been suppressed), and it can be evaluated that the generation of fine powder has been suppressed in the manufacturing process of the fused PHA product.

The lower the proportion of PHA microparticles with a particle size of 1 to 10 μm relative to the total amount of the dried PHA obtained by step (c), the less fine powder the dried PHA contains, and the more effectively the generation of fine powder during the manufacturing process of the fused PHA is suppressed. From the perspective of further suppressing the generation of fine powder during the manufacturing process of the fused PHA, the proportion of PHA microparticles with a particle size of 1 to 10 μm relative to the total amount of the dried PHA obtained by step (c) is preferably 28.5% or less, more preferably 26.0% or less, more preferably 24.0% or less, more preferably 22.0% or less, even more preferably 20.0% or less, and even more preferably 18.0% or less. Furthermore, there is no particular lower limit to the proportion of PHA microparticles with a particle size of 1 to 10 μm, and it may be, for example, 1.0% or more or 0%. In this specification, the ratio of PHA microparticles with a particle size of 1 to 10 μm to the total amount of dried PHA material is a value calculated based on the particle size distribution of the dried PHA material measured using a particle size distribution analyzer, as the abundance ratio of particles with a particle size of 1 to 10 μm, assuming that the abundance ratio of particles with a particle size of 0.01 to 3500 μm is 100; more specifically, it is a value measured and calculated under the conditions described in the examples.

In the dried PHA product obtained by step (c), the higher the proportion of fused PHA particles having a particle size of 100 μm or more relative to the total amount of the dried PHA product, the higher the degree of thermal fusion of the PHA powder in step (c). A high degree of thermal fusion of the dried PHA product has the advantage of improving the fluidity of the dried PHA product and making it easier to handle. Therefore, from the above perspective, the proportion of fused PHA particles having a particle size of 100 μm or more relative to the total amount of the dried PHA product obtained by step (c) is preferably 63.0% or more, more preferably 65.0% or more, more preferably 67.0% or more, even more preferably 70.0% or more, and even more preferably 72.0% or less. Furthermore, the upper limit of the proportion of fused PHA particles having a particle size of 100 μm or more is not particularly limited, and may be, for example, 90.0% or more or 100%. In this specification, the proportion of fused PHA with a particle size of 100 μm or more relative to the total amount of dried PHA is a value calculated based on the particle size distribution of the dried PHA measured using a particle size distribution analyzer, as the proportion of particles 100 μm or more, where the proportion of particles between 0.01 and 3500 μm is set to 100; more specifically, this value is measured and calculated under the conditions described in the examples.

The PHA fusion product obtained by this production method can be used as a molded product by molding it using known molding methods, such as injection molding, extrusion molding, blow molding, or compression molding. The PHA fusion product obtained by this production method can also be used as a foamed molded product by foaming it using known methods and then molding it. These molded products and foamed molded products made from the PHA fusion product obtained by this production method can be used for a variety of purposes, including paper, film, sheets, tubes, plates, rods, containers (e.g., bottles), bags, and parts.

The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. In the examples, "P3HB3HH" is used as "PHA," and "PHA" in the examples can also be read as "P3HB3HH."

[Measurement method]
Measurements in the examples and comparative examples were carried out by the following methods.

(moisture content)
The moisture content of the PHA wet powder and PHA powder was measured using a heat-dry moisture meter ML-50 (manufactured by A&D Co., Ltd.). Specifically, the weight (w1) of the target PHA wet powder or PHA powder was first measured, and the target PHA wet powder or PHA powder after weight measurement was heated at 130°C, and heating was terminated when the rate of weight change fell below 0.05 wt% (W.B.)/min. The weight (w2) of the target PHA wet powder or PHA powder at the end of heating was measured, and the change in weight of the target PHA wet powder or PHA powder before and after heating (= w1 - w2) was taken as the amount of water contained in the weight of the target PHA wet powder or PHA powder. The moisture content of the target PHA wet powder or PHA powder was calculated based on the following formula:
Moisture content (wt%) of target PHA wet powder or PHA powder = {(w1 - W2) / (w1)} × 100.

(Particle ratio of dried PHA)
The particle ratio of the PHA dried substance (the ratio of PHA fine particles of 1 to 10 μm and the ratio of PHA fused particles of 100 μm or more) was calculated by the following method:
(1) The particle size distribution of the target PHA dried product was measured using a particle size distribution measuring device (MASTERSIZER 3000, Malvern Panalytical). The measurement was performed using a dry dispersion unit (AERO S, Malvern Panalytical) with the device air pressure set to 1 bar.

(2) Based on the particle size distribution measured above, the abundance ratio of particles with a particle diameter of 1 to 10 μm was calculated as the proportion of PHA microparticles with a particle diameter of 1 to 10 μm, assuming that the abundance ratio of particles with a particle diameter of 0.01 μm to 3,500 μm was 100, and the abundance ratio of particles with a particle diameter of 100 μm was calculated as the proportion of PHA fused bodies with a particle diameter of 100 μm or more.

(Degree of heat fusion)
Based on the proportion of fused PHA particles having a particle size of 100 μm or more calculated by the above method, the degree of thermal fusion of the PHA powder in step (c) was evaluated according to the following criteria:
+ (Excellent degree of thermal fusion): The proportion of fused PHA particles having a particle size of 100 μm or more is 63.0% or more. - (Poor degree of thermal fusion): The proportion of fused PHA particles having a particle size of 100 μm or more is less than 63.0%.

Example 1
(Preparation of bacterial cell culture solution)
Ralstonia eutropha, as described in International Publication No. WO 2019/142717, was cultured by the method described in paragraphs [0041] to [0048] of the same document to obtain a bacterial cell culture solution containing PHA-containing bacterial cells. The obtained PHA was a copolymer consisting of 3HB repeating units and 3HH repeating units (i.e., poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)), and the composition ratio of the repeating units in the PHA (composition ratio of 3HB units/3HH units) was 94/6 (mol/mol).

(inactivation)
The bacterial cell culture solution obtained above was sterilized by heating and stirring at an internal temperature of 60 to 70°C for 7 hours to obtain an inactivated culture solution. The weight-average molecular weight of PHA in the inactivated culture solution was 1,920,000. The water content of the inactivated culture solution was 70% by weight.

(alkali treatment)
To the inactivated culture solution obtained above, a 30 wt% aqueous solution of sodium hydroxide was added to adjust the pH to 9.0±0.5. While maintaining this solution at 65-75°C, 30 wt% aqueous solution of sodium hydroxide was continuously added to maintain the pH at 9.0±0.5 for 6 hours, thereby performing an alkali treatment on the culture solution. After the treatment, the weight-average molecular weight of PHA in the culture solution was 600,000, and the water content was 80 wt%.

(Neutralization treatment and cell wall-degrading enzyme treatment)
The pH of the culture medium was adjusted to 6.5±0.2 by adding 10% sulfuric acid. Lysozyme (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), a cell wall decomposing enzyme, was added to the neutralized culture medium to a concentration of 100 ppm, and the mixture was kept at 50°C for 2 hours.

(Proteolytic enzyme treatment)
To the culture solution after the lysozyme treatment, a protease, Alcalase (manufactured by Novozymes), was added to a concentration of 300 ppm in the solution, and the solution was then maintained at 50°C for 2 hours while adjusting the pH to 9.0±0.2 using 30% by weight sodium hydroxide.

(Surfactant addition treatment)
To the culture solution after the enzyme treatment obtained above, sodium dodecyl sulfate (Kao Corporation) was added in an amount of 0.6 to 1.0% by weight based on the weight of the culture solution, and then the mixture was maintained at 45°C for 2 hours while adjusting the pH to 11.0±0.2 using an alkaline aqueous solution.

(Centrifugal washing)
The PHA aqueous suspension obtained by the above procedure was centrifuged (4500 rpm, 10 minutes), and the supernatant was removed to obtain a 2-fold concentrated PHA aqueous suspension. To this concentrated PHA aqueous suspension, an aqueous sodium hydroxide solution (pH 11.0±0.1) in an amount equal to the amount of the removed supernatant was added, and the mixture was centrifuged again (4500 rpm, 10 minutes), and the supernatant was removed. This procedure was repeated three times, and the PHA aqueous suspension was centrifuged and washed.

(pH adjustment)
The PHA aqueous suspension after centrifugation was diluted with water to a solids concentration of 15 wt %, and then 10% sulfuric acid was added to adjust the pH to 3.7±0.2, followed by keeping the mixture at 60° C. for 3 hours or more.

(Step (a))
The PHA aqueous suspension obtained by the above procedure was compressed at a pressure of 0.7 MPa using a filter press (AUTOPAC-PRO-6-14C, manufactured by HITZ Corporation) to obtain a solid PHA wet powder. The moisture content of the obtained PHA wet powder was 15%.

(Step (a'))
The obtained PHA wet powder was roughly crushed, and the crushed material was passed through a vibrating sieve with 2 mm openings to adjust (size-regulate) the maximum particle size of the PHA wet powder to 2.0 mm or less.

(Step (b) and Step (c))
The sized wet PHA powder obtained in the above step was fed into a plate dryer (manufactured by XingXing Co., Ltd.) at a rate of 20 kg/h. The fed wet PHA powder was dried by heating it on a plate heated to 90°C for 30 minutes until the moisture content was 0.3% or less, thereby obtaining a PHA powder (step (b)).

The resulting PHA powder was then heated for 5 minutes on a plate heated to 105°C in the same plate dryer to obtain a dried PHA product containing fused PHA (step (c)). The particle ratio in the resulting dried PHA product is shown in Table 1.

Example 2
A dried PHA product containing a fused PHA was obtained in the same manner as in Example 1, except that the heating temperature in step (c) was changed to 110° C. The particle ratio in the obtained dried PHA product is shown in Table 1.

Example 3
A dried PHA product containing a fused PHA was obtained by the same procedure as in Example 1, except that the heating temperature in step (c) was changed to 115° C. The particle ratio in the obtained dried PHA product is shown in Table 1.

Comparative Example 1
A dried PHA product containing a fused PHA was obtained by the same procedure as in Example 1, except that the heating temperature in step (c) was changed to 90° C. The particle ratio in the obtained dried PHA product is shown in Table 1.

Comparative Example 2
A dried PHA product containing a fused PHA was obtained by the same procedure as in Example 1, except that the heating temperature in step (c) was changed to 95° C. The particle ratio in the obtained dried PHA product is shown in Table 1.

Comparative Example 3
The same operation as in Example 1 was carried out except that the heating temperature in step (b) was changed to 95°C. At the end of heating in step (b), PHA powder adhered to the inside of the drying device. The moisture content of this PHA powder was 0.92%, and it was not possible to dry it sufficiently. Therefore, step (c) was not carried out, and a fused PHA body could not be obtained.

〔summary〕
The results of Examples 1 to 3 show that by performing a two-stage heating process consisting of steps (b) and (c), it is possible to dry and fuse the PHA without using a binder while suppressing the generation of fine powder, thereby providing a fused PHA body. Furthermore, a comparison of Examples 1 to 3 with Comparative Examples 1 and 2 shows that when the heating temperature in step (c) is below 100°C, the PHA powder is not sufficiently fused, and as a result, the generation of fine powder cannot be sufficiently suppressed. Furthermore, a comparison of Examples 1 to 3 with Comparative Example 3 shows that when the heating temperature in step (b) is 95°C or higher, part of the dried PHA powder adheres to the inside of the apparatus, resulting in insufficient heating of the entire wet PHA powder and preventing it from being sufficiently dried.

This manufacturing method allows PHA to be dried and a fused PHA body to be produced without using a binder, while suppressing the generation of fine powder. Furthermore, molded articles using PHA fused PHA bodies manufactured by this manufacturing method can be suitably used in agriculture, fisheries, forestry, horticulture, medicine, hygiene products, clothing, non-clothing, packaging, automobiles, building materials, and other fields.

Claims (7)

A method for producing a polyhydroxyalkanoate fused material, comprising the steps of:
(a) a step of dehydrating an aqueous suspension of polyhydroxyalkanoate to obtain a wet powder of polyhydroxyalkanoate having a moisture content of 5 to 37%;
(b) heating the obtained polyhydroxyalkanoate wet powder at 75°C to 95°C to obtain a polyhydroxyalkanoate powder having a moisture content of 0.5% or less; and
(c) heating the obtained polyhydroxyalkanoate powder having a moisture content of 0.5% or less at 100°C to 150°C to obtain a polyhydroxyalkanoate fused material. Before the step (b),
2. The method for producing a polyhydroxyalkanoate fused body according to claim 1, further comprising: (a') a step of crushing the polyhydroxyalkanoate wet powder obtained in the step (a) so that the maximum diameter is 3.0 mm or less. The method for producing a polyhydroxyalkanoate fused body according to claim 1, wherein step (b) includes a step of heating the polyhydroxyalkanoate wet powder by indirect heating. The method for producing a polyhydroxyalkanoate fused body according to claim 1, wherein step (c) includes a moving step in which the polyhydroxyalkanoate powder is heated while being moved. The method for producing a polyhydroxyalkanoate fused body according to claim 1, wherein the transferring step includes a step of scraping the polyhydroxyalkanoate powder using a blade. The method for producing a polyhydroxyalkanoate fused body according to claim 1, wherein steps (b) and (c) are carried out in the same system. The method for producing a polyhydroxyalkanoate fused product according to claim 1, wherein the proportion of polyhydroxyalkanoate fine particles having a particle size of 1 to 10 μm is 28.5% or less of the total amount of the dried polyhydroxyalkanoate obtained in step (c).