JP7624866B2 - Method for producing multi-stage expanded polyamide resin beads - Google Patents

Description

The present invention relates to a method for producing expanded polyamide resin beads, and more specifically, to a method for producing multi-stage expanded polyamide resin beads that can provide expanded polyamide resin beads with a high expansion ratio through multi-stage expansion.

Among common resin materials, polyamide resins are known to have high heat resistance, as well as excellent abrasion resistance and chemical resistance. Foam molded products made from this polyamide resin can be made lighter while retaining these excellent properties. For this reason, polyamide resin foam molded products are expected to be used in automobile parts and other applications. For example, Patent Document 1 discloses a polyamide resin foam molded product with excellent heat resistance and sound insulation.

In recent years, there has been a demand for lighter expanded bead moldings. In the technical field of thermoplastic resin expanded beads, a multi-stage expansion method is known as a method for reducing the weight of expanded bead moldings. The multi-stage expansion method refers to an expansion method in which expanded beads are impregnated with a physical foaming agent such as pressurized air or carbon dioxide to apply internal pressure, and then heated to obtain expanded beads with a lower apparent density than the original expanded beads. Patent Document 1 also describes the use of multi-stage expansion to increase the expansion ratio of polyamide resin expanded beads used in bead expansion molding.

WO2016/147582

However, conventionally, multi-stage expansion of polyamide-based resin expanded particles has a problem of inferior productivity compared to polyolefin-based resin expanded particles. For example, in the multi-stage expansion of polyamide-based resin expanded particles described in Patent Document 1, the temperature must be kept high when impregnating the polyamide-based resin expanded particles with a physical foaming agent, which places a large load on the equipment and results in high production costs. In addition, when producing polyamide-based resin expanded particles with a particularly small apparent density, it is described that the polyamide-based resin expanded particles are impregnated with a physical foaming agent, the pressurized state is maintained for 24 hours, and then heated at 230°C to re-expand, and this process is repeated twice, which places a large load on the equipment and requires a long time. Therefore, the conventional method for producing multi-stage polyamide-based resin expanded particles using the multi-stage expansion method has low productivity and is not suitable for industrial production.

The present invention relates to a novel method for producing multi-stage expanded polyamide resin beads, which was made in consideration of the above-mentioned problems. That is, the present invention provides a method for producing multi-stage expanded polyamide resin beads with excellent productivity, which enables multi-stage expansion by applying internal pressure to the expanded polyamide resin beads under conditions that are lower than conventional conditions, such as room temperature, or for a short period of time, in the production of multi-stage expanded polyamide resin beads with low apparent density.

The method for producing multi-stage expanded polyamide-based resin beads of the present invention comprises an internal pressure applying step of placing expanded polyamide-based resin beads in a pressure-resistant container and impregnating the expanded polyamide-based resin beads with a physical blowing agent in the pressure-resistant container to apply an internal pressure exceeding atmospheric pressure, and a heat-expanding step of heating and expanding the expanded polyamide-based resin beads to which the internal pressure obtained in the internal pressure applying step has been applied, thereby obtaining multi-stage expanded polyamide-based resin beads having an apparent density smaller than that of the expanded polyamide-based resin beads used in the internal pressure applying step,
The internal pressure application step is characterized in that the polyamide-based resin foamed particles in a hygroscopic state having a water content of 1% or more are impregnated with the physical foaming agent at a temperature higher than the change point temperature of the storage modulus of the hygroscopic polyamide-based resin foamed particles.

The present invention provides a highly productive method for producing multi-stage expanded polyamide resin beads that can be produced at lower temperatures than conventional methods, such as room temperature, or in a shorter time.

1 is a graph plotting the relationship between the temperature at which the storage modulus of expanded polyamide resin particles changes and the moisture content. 1 is a graph showing an example of the relationship between the storage modulus and temperature of expanded polyamide resin beads measured in accordance with JIS K7095:2012. 1 is an example of a DSC curve of expanded polyamide resin particles measured based on a heat flux differential scanning calorimetry method.

The method for producing the multi-stage expanded polyamide resin beads of the present invention will be described below.
In the following description, preferred numerical ranges of the present invention may be shown as appropriate. In this case, preferred ranges, more preferred ranges, and particularly preferred ranges regarding the upper and lower limits of the numerical ranges can be determined from all combinations of the upper and lower limits.

The method for producing multi-stage expanded polyamide resin beads of the present invention includes an internal pressure application step and a heat expansion step.
The internal pressure application step is a step of impregnating expanded polyamide-based resin particles in a hygroscopic state having a moisture content of 1% or more with a physical blowing agent in a pressure-resistant container to apply an internal pressure exceeding atmospheric pressure. In the following description, the "expanded polyamide-based resin particles in a hygroscopic state having a moisture content of 1% or more" subjected to the internal pressure application step may be simply referred to as "expanded polyamide-based resin particles in a hygroscopic state."
The heating and expanding step is a step in which the polyamide-based resin expanded beads to which the internal pressure obtained in the internal pressure applying step has been applied are heated and expanded to obtain multi-stage polyamide-based resin expanded beads having an apparent density smaller than that of the polyamide-based resin expanded beads used in the internal pressure applying step.
The present invention is characterized in that the temperature inside the pressure-resistant vessel during the internal pressure application process is adjusted to be higher than the change point temperature of the storage modulus of the expanded polyamide resin particles in a hygroscopic state for a period of time exceeding 50% of the total implementation time of the internal pressure application process.
In the following description, the "polyamide-based resin expanded beads to which internal pressure has been applied, obtained in the internal pressure application step" to be subjected to the heat-expanding step may be simply referred to as "polyamide-based resin expanded beads to which internal pressure has been applied." Also, the "multi-stage polyamide-based resin expanded beads having a lower apparent density than the polyamide-based resin expanded beads used in the internal pressure application step" obtained by carrying out the heat-expanding step may be simply referred to as "multi-stage polyamide-based resin expanded beads having a lower apparent density" or simply "multi-stage polyamide-based resin expanded beads."

The method for producing multi-stage expanded polyamide resin beads of the present invention (hereinafter also referred to as the production method of the present invention) having the above-mentioned configuration is highly productive and can industrially provide polyamide resin expanded beads with a high expansion ratio. In other words, the production method of the present invention can produce multi-stage expanded polyamide resin beads with a low apparent density at a lower temperature than conventional methods, such as room temperature, or in a short time.

The present invention was made by focusing on the unique properties of polyamide resins, which differ from those of polyolefin resins.
That is, the present inventors have studied the problem of low productivity of multi-stage expansion using conventional polyamide-based resin expanded beads, and have inferred that polyamide-based resin has a significantly higher gas barrier property than polyolefin-based resins such as polypropylene-based resins, and therefore it is difficult to impregnate polyamide-based resin expanded beads with a physical foaming agent. As a result, it has been considered necessary to maintain a pressurized state for a long time or to pressurize under high temperature conditions in the process of impregnating expanded beads with a foaming agent in order to obtain multi-stage polyamide-based resin expanded beads with a low apparent density.
The present inventors have focused on the fact that the gas barrier property of polyamide resins is reduced when the polyamide resin contains a significant amount of moisture. They have also found that an index of the change in behavior of polyamide resins containing moisture can be confirmed by the change temperature of the storage modulus obtained from dynamic viscoelasticity. That is, as shown in FIG. 1, the change temperature of the storage modulus of expanded polyamide resin particles decreases when the expanded polyamide resin particles contain a significant amount of moisture, and the gas barrier property of the polyamide resin constituting the expanded polyamide resin particles decreases when the expanded polyamide resin particles are heated at a temperature higher than the change temperature. Based on this idea, the present inventors have completed the manufacturing method of the present invention. Note that FIG. 1 is a graph plotting the relationship between the change temperature of the storage modulus of expanded polyamide resin particles and the moisture content, and shows a tendency that the change temperature of the storage modulus of expanded polyamide resin particles decreases as the moisture content (%) increases.

The inventors have come up with the technical idea of pressurizing expanded polyamide resin particles, which have a lowered storage modulus change temperature and reduced gas barrier properties due to moisture absorption, at a relatively low temperature above the change temperature, thereby impregnating the particles with a physical foaming agent and imparting a high internal pressure. This technical idea involves pressurizing the particles at a relatively high temperature above the change temperature, thereby shortening the pressurization time and imparting a high internal pressure.

By using expanded polyamide resin beads to which a high internal pressure has been applied and expanding the beads by heating, multi-stage expanded polyamide resin beads having a low apparent density can be produced with good productivity.
The production method of the present invention will be described in more detail below.

(Pre-process)
First, the pre-processing before the internal pressure applying process in the present invention will be described. The pre-processing includes a first preparatory process for preparing polyamide-based resin particles, and a second preparatory process for producing expanded polyamide-based resin particles using the polyamide-based resin particles.
It should be noted that some or all of the pre-processing steps described below may be carried out separately from the manufacturing method of the present invention, or may be incorporated into the manufacturing method of the present invention as an optional additional process.

(First preparation process)
In the first preparation step, the polyamide-based resin particles prepared may be commercially available products, or may be polyamide-based resin particles produced by a known method.
The method for producing polyamide-based resin particles includes, for example, the following methods. First, a polyamide-based resin and, if necessary, additives such as a bubble regulator, an end-capping agent, and a colorant are put into an extruder and melt-kneaded to form a melt-kneaded product. Next, the melt-kneaded product is extruded in a strand shape from a small hole in a die attached to the tip of the extruder, cut to a predetermined size, and granulated to produce polyamide-based resin particles. In addition, the cutting method for the strand-like melt-kneaded product includes a strand cut method in which the extruded melt-kneaded product is cut to a predetermined weight with a pelletizer, a hot cut method in which the melt-kneaded product is extruded into a gas phase and cut immediately thereafter, and an underwater cut method (UWC method) in which the melt-kneaded product is extruded into water and cut immediately thereafter.

The polyamide-based resin used in the production of the polyamide-based resin particles may be a single resin or a mixed resin of two or more types.
The polyamide-based resin may be polyamide or a polyamide copolymer, with the polyamide copolymer being preferred.
Examples of the polyamides include poly(6-aminohexanoic acid) also known as poly(caprolactam) (polycaproamide, nylon 6), poly(laurolactam) (nylon 12), poly(hexamethylene adipamide) (nylon 66), poly(7-aminoheptanoic acid) (nylon 7), poly(8-aminooctanoic acid) (nylon 8), poly(9-aminononanoic acid) (nylon 9), poly(10-aminodecanoic acid) (nylon 10), poly(11-aminobutanoic acid) (nylon 11), poly(12-aminobutanoic acid) (nylon 12), poly(1 ... Examples of homopolymers include poly(pentamethylene adipamide) (nylon 11), poly(hexamethylene sebacamide) (nylon 610), poly(decamethylene sebacamide) (nylon 1010), poly(hexamethylene azelamide) (nylon 69), poly(tetramethylene adipamide) (nylon 46), poly(tetramethylene sebacamide) (nylon 410), poly(pentamethylene adipamide) (nylon 56), and poly(pentamethylene sebacamide) (nylon 510).
The polyamide copolymer means a copolymer having two or more kinds of repeating units, at least a part of each repeating unit having an amide bond.
Examples of the polyamide copolymer include polycaproamide/polyhexamethylene adipamide copolymer (nylon 6/66), caprolactam/hexamethylenediaminoadipic acid/lauryllactam (nylon 6/66/12), and caprolactam/lauryllactam copolymer (nylon 6/12).
The polyamide-based resin may be one of these polyamides and polyamide copolymers, or a combination of two or more of them. Among the above polyamide-based resins, a polyamide-based resin selected from one or a combination of two or more of nylon 6, nylon 66, and nylon 6/66 is preferred, and nylon 6/66 is more preferred.

The melting point of the polyamide resin is preferably 180° C. or higher, more preferably 185° C. or higher, and even more preferably 190° C. or higher, from the viewpoint of improving the heat resistance of the expanded bead molded article obtained by molding the resulting multi-stage expanded beads. On the other hand, from the viewpoint of reducing the load on the device when the internally pressurized expanded polyamide resin beads are heated by a heating medium, the melting point is preferably 280° C. or lower, more preferably 260° C. or lower, and even more preferably 240° C. or lower.
The melting point of the polyamide resin mentioned above refers to the melting point of the polyamide resin when the resin constituting the expanded beads is a single polyamide resin, whereas when the expanded beads are composed of a mixture of two or more polyamide resins, the melting point of the polyamide resin mentioned above refers to the melting point of the mixture kneaded in advance in an extruder or the like.

The melting point of a polyamide resin can be determined by heat flux differential scanning calorimetry based on JIS K7121-1987, by heating and melting (first heating) from 30°C to a temperature 30°C higher than the end of the melting peak at a heating rate of 10°C/min, then holding at that temperature for 10 minutes, cooling to 30°C at a cooling rate of 10°C/min, and again heating and melting to a temperature 30°C higher than the end of the melting peak at a heating rate of 10°C/min, as the temperature at the apex of the melting peak (melting peak temperature) of the second DSC curve. When a DSC curve has multiple melting peaks, the melting peak temperature of the melting peak with the largest area is taken as the melting point of the polyamide resin. The polyamide resin is allowed to stand for 24 hours or more in an environment at a temperature of 23°C and a relative humidity of 50% to condition it, and then the melting point is measured. As a measuring device, for example, a high-sensitivity differential scanning calorimeter "EXSTAR DSC7020" (manufactured by SII NanoTechnology Co., Ltd.) can be used.

The above polyamide resin preferably has a flexural modulus of 1000 MPa or more, more preferably 1200 MPa or more, and even more preferably 1500 MPa or more. The amide elastomer generally has a flexural modulus of 600 MPa or less. If the flexural modulus of the polyamide resin is within the above range, it is preferable because the high flexural modulus makes it difficult to shrink even when exposed to room temperature after multi-stage expansion, and it is easier to obtain multi-stage expanded particles with a small apparent density. The upper limit of the flexural modulus of the polyamide resin is approximately 3000 MPa.

The flexural modulus of polyamide resins can be determined by leaving a test piece at rest for at least 24 hours at a temperature of 23°C and a humidity of 50%, and then measuring it in accordance with JIS K7171:2016.

The density of the polyamide resin is preferably 1.05 g/cm ³ or more, and more preferably 1.1 g/cm ³ or more. The density of the amide elastomer is generally less than 1.05 g/cm ^3. The density of the polyamide resin can be determined based on the method described in ISO1183-3.

The polyamide resin particles may contain other polymers such as thermoplastic resins other than polyamide resins and thermoplastic elastomers, as long as the object and effect of the present invention are not hindered. Examples of other thermoplastic resins include polyethylene resins, polypropylene resins, polystyrene resins, vinyl acetate resins, thermoplastic polyester resins, acrylic ester resins, and methacrylic ester resins. Examples of thermoplastic elastomers include styrene thermoplastic elastomers, olefin thermoplastic elastomers, and amide thermoplastic elastomers. The content of the other polymers is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, and even more preferably 5 parts by weight or less, relative to 100 parts by weight of the polyamide resin constituting the polyamide resin particles, and is particularly preferably not to contain other polymers other than polyamide resins.
The content of polyamide-based resin in the resin components constituting the polyamide-based resin particles is 50% by weight or more, and from the viewpoint of obtaining polyamide-based resin expanded particles having excellent heat resistance, abrasion resistance, and chemical resistance, the content is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 90% by weight or more, and it is particularly preferable that the resin components constituting the polyamide-based resin particles consist only of polyamide-based resin.

In addition to the polyamide resin, the polyamide resin particles can be appropriately blended with various additives such as commonly used bubble regulators, end blocking agents, antistatic agents, conductive agents, weather resistance agents, lubricants, antioxidants, UV absorbers, flame retardants, metal deactivators, colorants (pigments, dyes, etc.), crystal nucleating agents, and fillers, as necessary. Examples of bubble regulators include inorganic bubble regulators such as talc, sodium chloride, calcium carbonate, silica, titanium oxide, gypsum, zeolite, borax, aluminum hydroxide, alum, and carbon, and organic bubble regulators such as phosphoric acid compounds, amine compounds, and polytetrafluoroethylene (PTFE). The amount of these various additives to be added varies depending on the purpose of use of the molded body, but is preferably 25 parts by weight or less per 100 parts by weight of the resin components constituting the polyamide resin particles. More preferably, it is 15 parts by weight or less, even more preferably 10 parts by weight or less, and even more preferably 5 parts by weight or less.

The polyamide-based resin particles may be single-layer resin particles or multi-layer resin particles having a multi-layer structure. The multi-layer structure refers to a structure having a core layer and a coating layer covering the core layer, the core layer and the coating layer being composed of polyamide-based resins having different properties. The multi-layer resin particles are expanded in the second preparation step described below to become expanded particles having a foamed core layer and a non-foamed coating layer covering the core layer. When the polyamide-based resin particles are the multi-layer resin particles, the melting point of the polyamide-based resin constituting the coating layer is preferably lower than the melting point of the polyamide-based resin constituting the core layer, and more preferably 20°C or more lower than the melting point (Tmc). This provides polyamide-based resin expanded particles that can be molded in a mold with lower pressure molding steam while maintaining high heat resistance.

Next, the second preparation step will be described. The second preparation step includes a step of impregnating the polyamide-based resin particles prepared in the first preparation step with a foaming agent to obtain expandable polyamide-based resin particles, and a step of expanding the expandable polyamide-based resin particles. The expansion in the second preparation step may be called first-stage expansion, and the expanded particles thus obtained may be called first-stage expanded particles.
In the process of impregnating the polyamide-based resin particles with a foaming agent to obtain expandable polyamide-based resin particles, the method of impregnating the polyamide-based resin particles with the foaming agent is not particularly limited, but it is preferable to disperse the polyamide-based resin particles in a dispersion medium in a pressurizable closed vessel such as an autoclave, and impregnate the polyamide-based resin particles with the foaming agent. From the viewpoint of shortening the impregnation time of the foaming agent, it is preferable to impregnate the polyamide-based resin particles with the foaming agent while applying pressure and heating.

As the foaming agent, a physical foaming agent can be used. Examples of the physical foaming agent include organic physical foaming agents such as aliphatic hydrocarbons such as propane, butane, pentane, hexane, and heptane, alicyclic hydrocarbons such as cyclopentane and cyclohexane, halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride, and dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Examples of the inorganic physical foaming agent include carbon dioxide, nitrogen, helium, argon, and air.
Among the physical blowing agents, from the viewpoints of having little impact on the environment, being non-flammable and having excellent safety, inorganic physical blowing agents are preferred, carbon dioxide or air is more preferred, and air is even more preferred.

Next, the expandable polyamide resin particles impregnated with a foaming agent are expanded.
The method of expanding the expandable polyamide-based resin particles is not particularly limited. For example, the direct expansion method is to obtain expandable polyamide-based resin particles by impregnating the polyamide-based resin particles with a foaming agent in a closed container by the above-mentioned process, and then release the expandable polyamide-based resin particles together with a dispersion medium such as water under a pressure lower than that of the closed container (usually under atmospheric pressure) to expand the polyamide-based resin particles by opening one end of the closed container, and the impregnation expansion method is to take out the expandable polyamide-based resin particles obtained by the process of impregnating the polyamide-based resin particles with a foaming agent without expanding them, and heat them with hot air or the like in a separate expansion device to expand them. The direct expansion method is particularly preferred from the viewpoint of easily adjusting the temperature of the dispersion medium in the closed container uniformly.

In the production of expanded polyamide resin particles by the direct foaming method, the expanded polyamide resin particles are usually released from a sealed container and expanded, taking into consideration the handleability of the expanded particles, etc., and are then dried in a drying step. In the present invention, the above-mentioned drying step may or may not be performed.

[Water-containing step]
Since the manufacturing method of the present invention uses polyamide-based resin expanded particles in a hygroscopic state with a moisture content of 1% or more, when using the polyamide-based resin expanded particles obtained as described above, a moisture impregnation step is appropriately carried out to prepare polyamide-based resin expanded particles in a hygroscopic state with a moisture content of 1% or more. The moisture impregnation step may be carried out before the polyamide-based resin expanded particles are put into a pressure vessel, or may be carried out in the pressure vessel after the polyamide-based resin expanded particles are put into the pressure vessel.

The water-imparting step may be any method capable of adjusting the water content of the expanded polyamide resin particles to a desired range. For example, the first water-imparting method exposes the expanded polyamide resin particles to a standard state (atmospheric pressure, normal temperature, directly or indirectly exposed to the outside air) for 12 hours or more, preferably 24 hours or more, the second water-imparting method exposes the expanded polyamide resin particles to a high temperature and high humidity state (e.g., humidity 80%, temperature 40°C) for 3 hours or more, preferably 5 hours or more, the third water-imparting method immerses the expanded polyamide resin particles in water and leaves it to stand for about 1 minute to 4 hours, or the fourth water-imparting method places the expanded polyamide resin particles together with water in a bag or container, and manually or mechanically stirs or mixes them, etc. These water-imparting steps may also be carried out in combination.
These water-containing methods can adjust the water content of the expanded polyamide resin beads to a desired range while suppressing the variation in the water content of each expanded bead, which results in a more uniform internal pressure in the internal pressure application step described below, and allows the production of a group of multi-stage expanded beads with less variation in size.
The first moisture-containing method is suitable for obtaining polyamide-based resin particles having a moisture content of about 1% to 4%. The second, third and fourth moisture-containing methods are suitable for obtaining polyamide-based resin particles having a moisture content of 4% or more, or 4.5% or more. The second moisture-containing method is preferred from the viewpoint of obtaining expanded particles having a uniform moisture content in a shorter time.
In the first moisture-containing method, the state in which the expanded polyamide resin particles are directly or indirectly exposed to the outside air under atmospheric pressure and room temperature conditions includes, for example, a state in which the expanded polyamide resin particles are contained in an open container, bag, tray, etc. in an ordinary room, and a state in which the expanded polyamide resin particles are contained in a closed but non-sealed container or bag, etc.

[Internal pressure application process]
Next, the internal pressure applying step will be described.
The internal pressure application process is a process in which expanded polyamide-based resin particles are used and impregnated with a physical blowing agent in a pressure-resistant vessel at a temperature higher than the inflection point temperature of the storage modulus of the expanded polyamide-based resin particles in a hygroscopic state with a water content of 1% or more, thereby applying an internal pressure higher than atmospheric pressure.

(Moisture Content of Expanded Polyamide Resin Beads)
The polyamide-based resin expanded particles in a hygroscopic state with a moisture content of 1% or more used in the internal pressure application step are prepared, for example, by the above-mentioned pre-step and a moisture-imparting step that is performed as necessary. In the present invention, the moisture content of the polyamide-based resin expanded particles can be measured using a Karl Fischer moisture meter, and a specific example of the measuring device is a trace moisture measuring device (AQ-2200A) manufactured by Hiranuma Sangyo Co., Ltd., which measures the moisture content based on the Karl Fischer coulometric method.
Specifically, the moisture content of the expanded polyamide resin particles can be measured by the following method. First, about 0.2 g of expanded polyamide resin particles is weighed to prepare a test piece. Next, the test piece is heated to a temperature of 160° C. using a thermal moisture vaporizer (for example, an automatic thermal moisture vaporizer SE320 manufactured by Hiranuma Sangyo Co., Ltd.) to vaporize the moisture inside the test piece. This moisture is introduced into the Karl Fischer moisture measuring device connected to the thermal moisture vaporizer, the moisture amount is measured, and the moisture content of the expanded particles is calculated. The measurement conditions (titration conditions) can be a waiting time of 40 seconds, electrolysis current of medium, and minimum amount of electrolysis of 5 μg.

In the multi-stage expansion of polypropylene resin expanded particles and the like, expanded particles in a dry state with a moisture content of 0.1% or less are usually placed in a pressure vessel and impregnated with a blowing agent at a temperature close to room temperature to apply internal pressure. However, the production method of the present invention differs from such a general production method in that polyamide resin expanded particles in a hygroscopic state with a moisture content of 1% or more are impregnated with a physical blowing agent to produce polyamide resin multi-stage expanded particles.
When polyamide resin expanded particles in a dry state having a moisture content of less than 1% are impregnated with a physical blowing agent, in order to sufficiently impregnate the particles with the blowing agent, heating is required at a temperature higher than room temperature or for a long period of time, which places a large load on devices such as a pressure-resistant container and is not preferable from the viewpoint of productivity.

The higher the moisture content of the expanded polyamide resin particles, the lower the temperature at which the storage modulus changes, and therefore the polyamide resin particles can be impregnated with a physical blowing agent at a lower temperature, or the time required for impregnation with a physical blowing agent can be shortened by maintaining the temperature. From this viewpoint, the moisture content of the expanded polyamide resin particles in a hygroscopic state is preferably 3.0% or more, more preferably 4.5% or more, and even more preferably 5.0% or more.
On the other hand, the upper limit of the moisture content is not particularly specified. However, as shown in Fig. 1, as the moisture content increases, the degree of decrease in the temperature at the change point of the storage modulus decreases and an equilibrium state is reached, so the upper limit of the moisture content is generally about 20%. From the viewpoint of the filling property when the obtained multi-stage expanded beads are molded in a mold, the upper limit of the moisture content is preferably 18% or less, and more preferably 15% or less.
The reason why the change point temperature of the storage modulus becomes lower as the water content of the expanded polyamide resin particles becomes higher is believed to be because the hydrogen bonds between the polyamide resin molecules are replaced by water molecules, causing a change in the molecular motion of the polyamide resin.

(Regarding temperature in the internal pressure application process)
Next, the temperature in the internal pressure application process will be described. In the internal pressure application process, the temperature in the pressure vessel in the internal pressure application process is adjusted to be higher than the temperature at which the storage modulus of the expanded polyamide resin beads in a hygroscopic state changes. In addition, from the viewpoint of more efficiently applying internal pressure to the expanded beads, it is preferable to adjust the temperature in the pressure vessel in the internal pressure application process to be higher than the temperature at which the storage modulus of the expanded polyamide resin beads in a hygroscopic state changes for a time exceeding 50% of the total implementation time of the internal pressure application process. Here, the total implementation time of the internal pressure application process refers to the time from the start of the internal pressure application process to the time when the expanded polyamide resin beads to which internal pressure has been applied are taken out. In the present invention, the start of the internal pressure application process refers to the first time when the state in which the water content of the expanded polyamide resin beads put into the pressure vessel is 1% or more and the state in which the physical foaming agent is pressed into the pressure vessel are both met. The time exceeding 50% of the total implementation time of the internal pressure application process may be a continuous time in the total implementation time, or may be the sum of intermittent times. The present invention includes an embodiment in which, for example, the internal pressure application process is started at a temperature lower than the change point temperature of the storage modulus of the expanded polyamide-based resin beads in a hygroscopic state, and at any timing during the internal pressure application process, the temperature inside the pressure-resistant vessel is adjusted to a temperature higher than the change point temperature of the storage modulus of the expanded polyamide-based resin beads at that time.
From the viewpoint of applying a more sufficient internal pressure, the time during which the temperature inside the pressure vessel is adjusted to be higher than the change point temperature of the storage modulus of the polyamide-based resin foamed particles in a hygroscopic state in the internal pressure application step is more preferably 70% or more, even more preferably 80% or more, particularly preferably 90% or more, and most preferably essentially 100% of the total implementation time of the internal pressure application step.
The temperature inside the pressure vessel during the time exceeding 50% of the total implementation time of the internal pressure application process may be adjusted to a constant value or may be varied. During the internal pressure application process, the change point temperature of the storage modulus of the expanded polyamide resin particles changes with the change in the moisture content. Therefore, the change point temperature of the storage modulus of the expanded polyamide resin particles, which is an index for determining the temperature inside the pressure vessel, refers to the change point temperature of the storage modulus indicated when determining the temperature.
As described above, the temperature at which the storage modulus of the polyamide-based resin expanded particles in a hygroscopic state changes is lower than that in a dry state. Therefore, the polyamide-based resin expanded particles can be impregnated with a physical foaming agent at a lower temperature or in a shorter time than when dry polyamide-based resin expanded particles are used. As a result, the load on the equipment such as the pressure vessel can be reduced, and the productivity is excellent. In addition, the decrease in the closed cell ratio of the obtained polyamide-based resin multi-stage expanded particles and discoloration due to deterioration of the resin can be suppressed.

The temperature range in the internal pressure application step is not particularly limited, but it is preferable to impregnate the hygroscopic polyamide resin foamed particles with the physical foaming agent at a temperature 30° C. or higher than the change point temperature of the storage modulus of the hygroscopic polyamide resin foamed particles at the start of the internal pressure application step, which makes it possible to significantly shorten the impregnation time of the physical foaming agent and to apply a higher internal pressure.
In addition, during the internal pressure application process, the moisture content of the polyamide-based resin foamed beads in a hygroscopic state may gradually decrease in the pressure vessel, and the temperature at the change point of the storage modulus may increase from the temperature at the start of the internal pressure application process. In contrast, by setting the temperature at which the physical foaming agent is impregnated at a temperature 30° C. or higher than the temperature at the change point of the storage modulus of the polyamide-based resin foamed beads in a hygroscopic state at the start of the internal pressure application process, the internal pressure of the polyamide-based resin foamed beads can be increased without any particular problems even if the change point temperature increases during the impregnation of the physical foaming agent. From this viewpoint, the temperature of the internal pressure application process is more preferably 35° C. or higher, more preferably 40° C. or higher, and even more preferably 45° C. or higher than the temperature at the change point of the storage modulus of the polyamide-based resin foamed beads in a hygroscopic state at the start of the internal pressure application process. On the other hand, from the viewpoint of reducing the load on equipment such as a pressure-resistant vessel, the temperature of the internal pressure application process is preferably not more than 80° C. higher than the change point temperature of the storage modulus of the expanded polyamide resin particles in a hygroscopic state at the start of the internal pressure application process, more preferably not more than 70° C. higher, and even more preferably not more than 60° C. higher.

It is preferable to impregnate the physical foaming agent at a temperature in the pressure vessel during the internal pressure application process that is 30°C or higher than the change point temperature of the storage modulus of the hygroscopic polyamide resin foam particles at the start of the internal pressure application process for a time exceeding 50% of the total implementation time of the internal pressure application process. For example, if the change point temperature of the storage modulus of the hygroscopic polyamide resin foam particles at the start of the internal pressure application process is -10°C, it is preferable to maintain the temperature in the pressure vessel during the internal pressure application process at a temperature of 20°C or higher for a time exceeding 50% of the total implementation time of the internal pressure application process.

In the present invention, it is more preferable to adjust the temperature inside the pressure vessel to be higher than the temperature at which the storage modulus of the hygroscopic polyamide resin foamed particles changes in the initial stage of the internal pressure application process. Here, the initial stage of the internal pressure application process refers to a point in time from the start of the internal pressure application process until 5% of the total implementation time of the internal pressure application process has elapsed, preferably until 3% has elapsed, more preferably until 1% has elapsed, and most preferably at the start of the internal pressure application process.
Furthermore, it is more preferable to start impregnating the physical foaming agent at a temperature 30° C. or more higher than the change point temperature of the storage modulus of the expanded polyamide resin particles in a hygroscopic state at the start of the internal pressure application process from the initial stage of the internal pressure application process, and to continuously maintain the temperature 30° C. or more higher than the change point temperature until the final stage of the internal pressure application process. Here, the final stage of the internal pressure application process refers to the time point from 95% to 100% of the total implementation time of the internal pressure application process from the start, preferably from 97% to 100%, and more preferably from 99% to 100%.

The absolute upper limit of the temperature in the internal pressure application process is not particularly specified, and can be determined in consideration of other conditions such as the moisture content and the time for impregnation of the physical foaming agent. However, from the viewpoint of reducing the load on the device and suppressing a decrease in the closed cell rate of the polyamide resin multi-stage foamed particles and discoloration due to deterioration of the resin, the temperature when the physical foaming agent is impregnated in the internal pressure application process is preferably 150°C or less, more preferably 120°C or less, even more preferably 100°C or less, even more preferably 80°C or less, particularly preferably 50°C or less, and most preferably 30°C or less.

In the present invention, the storage modulus change temperature refers to the storage modulus change temperature (glass transition temperature from a curve of temperature and storage modulus) obtained by measuring the dynamic mechanical properties (DMA) of the expanded polyamide resin particles in accordance with JIS K7095:2012.
Specifically, one expanded bead, the moisture content of which has been adjusted in advance, is fixed in a jig, and the temperature is raised from -50°C to 150°C at a rate of 2°C/min in compression mode at a constant frequency (1 Hz), to measure the storage modulus. Then, as shown in Figure 2, the measurement results are plotted in a graph with the storage modulus on the vertical axis and the temperature on the horizontal axis. Figure 2 is a graph showing an example of the relationship between the storage modulus and temperature of expanded polyamide resin beads measured according to JIS K7095:2012. As a DMA measuring device, for example, a DMA7100 manufactured by Hitachi High-Tech Science Corporation can be used.
In the graph, the temperature indicated by a perpendicular line drawn downward from the intersection B between an extension line L1, which is an extension to the high temperature side of the straight line portion before the initial rapid drop in the storage modulus, and an extension line L2, which is an extension to the low temperature side of the straight line portion of the midpoint after the initial rapid drop in the storage modulus, is taken as the change-point temperature of the storage modulus. If multiple change-point temperatures of the storage modulus appear, the change-point temperature on the lowest side is used. The above measurement is carried out for 10 expanded beads, and the arithmetic average value is taken as the change-point temperature of the storage modulus of the polyamide resin expanded beads.

In addition to the storage modulus, the loss modulus and loss tangent can be determined by dynamic mechanical analysis (DMA). Of these physical properties, the storage modulus is the one whose transition temperature usually appears at the lowest temperature. In other words, the transition temperature of the storage modulus is considered to be the lowest temperature at which the molecular motion of the resin material being measured begins to change (the elastic modulus changes). Therefore, in the present invention, the transition temperature of the storage modulus can be used as an indicator of the lowest temperature at which internal pressure can be applied to the polyamide resin foam particles.

When it is difficult to measure the storage modulus in the dynamic viscoelasticity measurement of polyamide resin foam particles, or when there is a large variation in the measured values, the relationship between the moisture content and the temperature at which the storage modulus changes in the above measurement can be plotted in advance for multiple polyamide resin foam particles made of the same type of polyamide resin as the polyamide resin foam particles, to obtain a relationship between the moisture content and the temperature at which the storage modulus changes, and the temperature at which the storage modulus changes can be obtained by performing calculations using the relationship from the moisture content value of the foam particles whose storage modulus is difficult to measure.

(Regarding the pressure in the internal pressure application process)
Next, the pressure inside the pressure vessel in the internal pressure application step will be described. The pressure inside the pressure vessel into which the hygroscopic polyamide-based resin foamed particles are charged is increased by injecting a physical foaming agent. As a result, the hygroscopic polyamide-based resin foamed particles are impregnated with the physical foaming agent and internal pressure is applied. The relationship between the injection of the physical foaming agent and the increase in temperature inside the pressure vessel is not particularly limited, and the injection and the temperature increase may be started simultaneously, or one of the injection and the temperature increase may be started first, or the temperature increase may be started first and then the injection may be started after a predetermined temperature is reached.

The rate at which the pressure in the pressure vessel is increased is not particularly limited, but since the polyamide resin foam particles in a moisture-absorbing state are softer than before moisture absorption, it is preferable to inject the physical foaming agent into the pressure vessel at a rate in the range of 0.01 MPa/hr to 0.2 MPa/hr from the viewpoints of preventing the foam particles from being crushed by a sudden increase in pressure, reducing the load on the device, and shortening the time required for applying internal pressure.
In a general method for producing expanded polyamide resin particles, in consideration of the high gas barrier property of polyamide resin, a physical foaming agent is generally injected at a pressure increase rate not exceeding 0.01 MPa/hr. However, in the internal pressure imparting step in the present invention, since expanded polyamide resin particles having reduced gas barrier property due to being in a hygroscopic state are used, the above-mentioned pressure increase can be sufficient to impregnate the physical foaming agent and impart internal pressure.

The final pressure in the pressure vessel is not particularly limited, but is preferably in the range of 0.2 MPa (G) to 2.0 MPa (G), and more preferably in the range of 0.3 MPa (G) to 1.0 MPa (G).

The physical foaming agent used in the internal pressure application process is, for example, the same as the physical foaming agent described in the second preparation process described above.

By adjusting the temperature in the pressure vessel and the pressure increase conditions appropriately, the polyamide-based resin expanded beads in a moisture-absorbed state are impregnated with a physical foaming agent to impart an internal pressure above atmospheric pressure. The polyamide-based resin expanded beads to which such internal pressure has been imparted are used in the heat expansion step described below to produce multi-stage polyamide-based resin expanded beads having a low apparent density. From the viewpoint of easily obtaining multi-stage polyamide-based resin expanded beads having a low apparent density, the internal pressure is preferably 0.1 MPa (G) or more, more preferably 0.15 MPa (G) or more, and even more preferably 0.2 MPa (G) or more.
On the other hand, the upper limit of the internal pressure is not particularly limited, but in consideration of the performance of the pressure-resistant container and the economical balance, it is generally 2 MPa (G) or less, and preferably 1 MPa (G) or less.

In the internal pressure imparting step, it is preferable to impregnate the polyamide-based resin expanded beads with a physical foaming agent so that the internal pressure rise rate of the hygroscopic polyamide-based resin expanded beads is in the range of 0.003 MPa/hr to 0.05 MPa/hr. This allows a high internal pressure to be imparted to the polyamide-based resin expanded beads in a short time compared to the conventional method. In other words, when dry polyamide-based resin expanded beads are used, it is difficult to impart internal pressure at the above-mentioned high speed unless the temperature is set to a high temperature because the gas barrier property is high. However, the present invention uses hygroscopic polyamide-based resin expanded beads with low gas barrier property, so that a high internal pressure can be imparted within a shortened time, and is excellent in productivity. From this viewpoint, it is more preferable that the internal pressure rise rate is 0.005 MPa/hr or more.
The internal pressure rise rate of the expanded polyamide resin beads is calculated by subtracting the internal pressure (MPa (G)) of the expanded polyamide resin beads in a hygroscopic state before the application of internal pressure from the internal pressure (MPa (G)) of the expanded polyamide resin beads obtained in the internal pressure application step, and dividing the result by the pressurization time required to obtain the subtracted internal pressure. The internal pressure rise rate of the expanded polyamide resin beads in a hygroscopic state does not necessarily become a constant value in the internal pressure application step, but means the average rate of the internal pressure (MPa (G)) rising per hour.

The internal pressure P of the expanded polyamide resin beads can be determined by the following method using the following formulas (1) and (2): The weight W7 of the small bag is measured in advance.
Specifically, first, from the group of polyamide-based resin foamed beads to which internal pressure has been applied (the group of wet pressure-expanded beads) that has been removed from the pressure-resistant container, a group of polyamide-based resin foamed beads to which an arbitrary amount of internal pressure has been applied (the wet pressure-expanded beads) is placed in a small bag, and the weight W2 is measured.
Furthermore, an arbitrary amount of sample is taken from the polyamide resin foamed particles (wet pressurized foamed particles), and the moisture content of the sample is measured in advance with a Karl Fischer moisture meter. The moisture content measured with the Karl Fischer moisture meter is designated as W6, and the amount of sample used for the measurement with the Karl Fischer moisture meter is designated as W5.
The moisture content W3 contained in the wetted pressure-foamed particles can be determined by subtracting the weight W7 of the small bag from the weight W2 of the wetted pressure-foamed particles, dividing the result by the sample weight W5 used in the measurement with the Karl Fischer moisture meter, and multiplying the result by the moisture content W6 measured with the Karl Fischer moisture meter.
The weight W1 of the dry pressurized beads can be calculated by subtracting the combined weight W7 of the bag and the amount of water W3 in the wet pressurized-foamed beads from the weight W2 of the wet pressurized-foamed beads.
Next, the wetted pressurized foamed particles are placed together with the pouch in a thermostatic chamber at 80°C and atmospheric pressure, and left to stand for 48 hours to release the internal pressure. The depressurized polyamide-based resin foamed particles are then removed from the thermostatic chamber and dried at 160°C and atmospheric pressure for 20 hours. The weight W4 of the dried polyamide-based resin foamed particles (dried foamed particles) removed from the pouch is then measured, and the amount of air Wa contained in the polyamide-based resin foamed particles to which internal pressure has been applied is calculated using the following formula (1). The Karl Fischer moisture meter and measurement conditions are the same as those used to measure the moisture content of the polyamide-based resin foamed particles.
Whether the polyamide resin foam particles have been decompressed can be confirmed by checking that the weight of the particles no longer decreases. For example, the particles can be stored in a constant temperature and humidity of 80° C. for a long period of time, such as about 48 hours, to ensure that the pressure has been fully released.
[Equation 1]
Air volume Wa = W1 - W4
=(W2-W3-W7)-W4
= [(W2-(W2-W7)/W5×W6)-W7]-W4...(1)
W1: Weight of dry pressurized foam particles (g)
W2: Weight of wet pressed beads including the weight of the sachet (g)
W3: Water content in wet compressed foam particles (g)
W4: Weight of dry foam particles (g)
W5: Amount of sample (g) used in the Karl Fischer moisture meter measurement
W6: Moisture content (g) measured with a Karl Fischer moisture meter
W7: Weight of the bag (g)
Wa: Air volume (g)

The amount of air Wa (g) obtained as described above can be used in the following formula (2) to obtain the internal pressure of the expanded polyamide resin beads to which the internal pressure P has been applied.
[Equation 2]
Internal pressure P (MPa (G))
= (Wa/28.8×0.0831×296)/(W4/apparent density of expanded beads (kg/m ³ )−W4/density of resin (kg/m ³ )) (2)
The internal pressure P (MPa (G)) of the expanded polyamide resin beads measured by the above method corresponds to a gauge pressure, which is a value obtained by subtracting atmospheric pressure from absolute pressure.

From the viewpoint of making it difficult for the internal pressure applied to the expanded polyamide resin particles to escape after the internal pressure application step and before the heat-expansion step, it is desirable that the change point temperature of the storage modulus of the expanded polyamide resin particles to which the internal pressure has been applied obtained by the internal pressure application step is higher than the change point temperature of the storage modulus of the expanded polyamide resin particles in a hygroscopic state used in the internal pressure application step. This improves the gas barrier properties of the expanded polyamide resin particles to which the internal pressure has been applied, and retains the impregnated physical foaming agent inside the expanded particles, thereby enabling the production of multi-stage expanded polyamide resin particles with a higher expansion ratio in the subsequent heat-expansion step.
From this viewpoint, the difference [(A)-(B)] between the moisture content (A) of the expanded polyamide-based resin beads in a hygroscopic state used in the internal pressure application step and the moisture content (B) of the expanded polyamide-based resin beads to which internal pressure has been applied obtained by the internal pressure application step is preferably 0.5% or more, more preferably 1% or more, and even more preferably 3% or more.
The value of the moisture content (B) of the polyamide-based resin expanded beads to which internal pressure has been applied, obtained by the internal pressure application step, is preferably 10% or less, more preferably 8% or less, and even more preferably 6% or less, from the viewpoint of making it difficult for the applied internal pressure to escape and more stably producing multi-stage expanded beads having a small apparent density.
The moisture content (B) of the polyamide resin foamed beads to which internal pressure has been applied, obtained by the internal pressure application step, can be adjusted by changing the time, temperature, temperature rise rate, pressure, pressure rise rate, etc. of the internal pressure application step.

The above-mentioned internal pressure application process provides polyamide-based resin foam particles for multi-stage expansion. In other words, "polyamide-based resin foam particles for multi-stage expansion having an internal pressure above atmospheric pressure, which are obtained by impregnating polyamide-based resin foam particles in a hygroscopic state with a water content of 1% or more with the physical foaming agent at a temperature higher than the change point temperature of the storage modulus of the hygroscopic polyamide-based resin foam particles" are provided.

[Heat foaming process]
The heating and foaming step is immediately carried out after the internal pressure application step, in which the polyamide-based resin expanded beads to which the internal pressure obtained in the internal pressure application step has been applied are heated and foamed to obtain multi-stage polyamide-based resin expanded beads having an apparent density smaller than that of the polyamide-based resin expanded beads used in the internal pressure application step.
In the heat-expanding step, the "apparent density of the expanded polyamide-based resin beads used in the internal pressure application step" means the apparent density of the expanded polyamide-based resin beads obtained in the second preparation step before the moisture-containing step is carried out. In other words, in the present invention, the apparent density of the expanded polyamide-based resin beads refers to the apparent density measured using sufficiently dried expanded polyamide-based resin beads.

More specifically, the polyamide resin foam particles to which internal pressure has been applied are placed in a container and heated to cause foaming. The heating and foaming process may be carried out under pressure. The pressure in this case is not particularly limited, but may be adjusted, for example, to a range of 0.03 MPa or more and 0.4 MPa or less.

The heating method can be appropriately selected from the same heating methods as known heating and foaming processes for multi-stage foaming of thermoplastic resin beads. Specifically, it is preferable to heat using, for example, water vapor (steam) or heated air as a heating medium, and it is more preferable to heat using water vapor. By using water vapor as a heating medium, the moisture contained in the water vapor supplied into the container can lower the change point temperature of the storage modulus of the polyamide resin foamed beads to which internal pressure has been applied, making it easier to obtain polyamide resin multi-stage foamed beads with higher foaming. The heating time can be, for example, in the range of 5 seconds to 120 seconds.

The heating temperature in the container in the heat-foaming process is not particularly limited, but it is preferable to heat and foam the polyamide-based resin foam particles to which internal pressure has been applied, which are placed in a sealed container, with a heating medium having a temperature higher than the change point temperature of the storage modulus of the polyamide-based resin foam particles to which internal pressure has been applied, and lower than the melting point of the polyamide-based resin constituting the polyamide-based resin foam particles to which internal pressure has been applied. This makes it easy to obtain not only a high expansion ratio, but also foam particles with a high independent cell ratio and excellent moldability, by avoiding blocking between the foam particles in the sealed container (the foam particles melt together to form a mass). From this perspective, the temperature of the heating medium in the heat-foaming process is preferably in the range of 70°C to 190°C. The melting point of the polyamide-based resin constituting the polyamide-based resin foam particles to which internal pressure has been applied in the heat-foaming process means the melting point specific to the polyamide-based resin constituting the foam particles.

(Polyamide resin multi-stage expanded particles)
By carrying out the internal pressure application step and the heat expansion step as described above, it is possible to produce multi-stage polyamide resin expanded beads having a small apparent density. According to the production method of the present invention, it is possible to obtain multi-stage polyamide resin expanded beads having an apparent density of, for example, 100 kg/m3 ^{or less} , and more preferably, the apparent density can be adjusted to 85 kg/m3 or less, more preferably 70 kg/m3 ^or less. This makes it possible to provide a polyamide resin expanded bead molding having excellent weight reduction. If the apparent density does not fall within the above range by two-stage expansion, three or more stages of expansion may be further carried out. If it is within this range, it is possible to provide a polyamide resin expanded bead molding having excellent weight reduction.

In the manufacturing method of the present invention, the apparent density of the polyamide-based resin multi-stage expanded beads obtained by the heat expansion step is smaller than the apparent density of the polyamide-based resin expanded beads used in the internal pressure application step, and multi-stage polyamide-based resin expanded beads having a particularly large expansion ratio can be easily manufactured. In particular, from the viewpoint of obtaining multi-stage polyamide-based resin expanded beads having superior light weight, the ratio of the apparent density of the polyamide-based resin multi-stage expanded beads to the apparent density of the polyamide-based resin expanded beads used in the internal pressure application step is preferably 0.70 or less, more preferably 0.65 or less, even more preferably 0.60 or less, and particularly preferably 0.55 or less.

In the production method of the present invention, the apparent density of the expanded polyamide resin beads or the multi-stage expanded polyamide resin beads is measured by the following method.
A graduated cylinder containing water at 23°C is prepared, and the weight Wp of about 500 ^cm3 of expanded beads that have been left in the graduated cylinder for two days under conditions of 50% relative humidity, 23°C, and 1 atm is measured and then submerged using a wire mesh. Taking into account the volume of the wire mesh, the volume Vp [ ^cm3 ] of the expanded beads is measured from the rise in the water level, and the weight Wp [g] of the expanded beads is divided by the volume Vp (Wp/Vp) and converted into units of [kg/ ^m3 ] to obtain the apparent density of the expanded beads.

The closed cell ratio of the polyamide resin multi-stage expanded beads produced by the production method of the present invention is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. When the closed cell ratio satisfies the above range, the moldability of the expanded beads becomes good, and the expanded bead molding produced by molding the expanded beads in a mold tends to be excellent in secondary expandability and fusion property.
The closed cell ratio is the ratio of the volume of closed cells to the volume of all cells in the expanded beads, and can be determined using an air comparison type pycnometer based on procedure C described in ASTM-D2856-70.

<Polyamide resin foam particles with high temperature peak>
Meanwhile, the present applicant has conducted various studies in order to provide better expanded polyamide-based resin particles, and has proposed expanded polyamide-based resin particles having a high-temperature peak in a DSC curve obtained by heat flux differential scanning calorimetry that does not appear in general expanded polyamide-based resin particles (WO2020/050301A1). Expanded polyamide-based resin particles exhibiting such a high-temperature peak can enhance the heat resistance of the expanded particles and can suitably prevent blocking between the expanded particles during expansion.

More specifically, the high-temperature peak here refers to a melting peak (high-temperature peak b) whose apex temperature appears at a higher temperature than the melting peak (intrinsic peak a) inherent to the polyamide resin in the first DSC curve measured when the expanded polyamide resin particles are heated and melted from 30° C. to a temperature 30° C. higher than the end of the melting peak at a heating rate of 10° C./min by heat-flux differential scanning calorimetry based on JIS K7122-1987, as shown in FIG. 3. FIG. 3 is an example of a DSC curve measured by heat-flux differential scanning calorimetry of an expanded polyamide resin particle. Although FIG. 3 shows an example in which one high-temperature peak b appears, the number of high-temperature peaks b may be two or more.
The total heat of fusion of all high-temperature peaks whose peak temperatures appear on the higher side than the peak temperature of the intrinsic peak is preferably 5 J/g or more, more preferably 6 J/g or more, and even more preferably 7 J/g or more. The total heat of fusion is preferably 50 J/g or less, more preferably 30 J/g or less, and even more preferably 20 J/g or less. In addition, when two or more high-temperature peaks appear, the heat of fusion of the high-temperature peak means the total heat of all high-temperature peaks.

On the other hand, the intrinsic peak a is a melting peak derived from the crystalline structure inherent to the polyamide resin. The apex temperature of the intrinsic peak a is approximately the same as the apex temperature of the melting peak appearing in the second DSC curve described later.
The second DSC curve refers to a DSC curve measured when the expanded polyamide resin beads after the measurement of the first DSC curve are held for 10 minutes at a temperature 30° C. higher than the end of the melting peak after the measurement of the first DSC curve, cooled to 30° C. at a cooling rate of 10° C./min, and then heated and melted again at a heating rate of 10° C./min to a temperature 30° C. higher than the end of the melting peak.
A specific method for determining the heat of fusion of the high-temperature peak in the first DSC curve will be described in the Examples.

The expanded polyamide resin particles exhibiting the high-temperature peak b can be obtained by adopting a direct foaming method as the second preparation step constituting the pre-step. Specifically, the temperature in a closed container (foaming tank) in which the polyamide resin particles are dispersed is adjusted to promote crystallization of the polyamide resin.
The temperature can be appropriately set within the range that achieves the above-mentioned purpose. For example, in the direct foaming method, when the temperature in the sealed container is raised from the temperature during dispersion (start temperature) to the temperature during release (foaming temperature), a method of providing a heating and holding step of holding the intermediate temperature at any intermediate temperature between the start temperature and the foaming temperature for a certain period of time is preferable. In the foaming step in which such a temperature is carried out, it is preferable to select an aqueous medium such as water as the dispersion medium added to the sealed container. By dispersing the polyamide-based resin particles in an aqueous medium and allowing them to absorb water, the polyamide-based resin particles are plasticized, and the final foaming temperature can be set lower than that before plasticization.

The heating and holding step is preferably a step of holding the sealed container in which the polyamide-based resin particles are dispersed at a temperature that is 90°C lower than the melting point (Tm) of the polyamide-based resin (Tm - 90°C) or more and less than 50°C lower (Tm - 50°C) for a holding time of 1 minute or more and 60 minutes or less. By including a step of holding under the above conditions in this manner, the above-mentioned high-temperature peak can be formed.

As described above, expanded polyamide resin beads having a high-temperature peak can increase the heat resistance of the expanded beads and effectively prevent blocking between the expanded beads during expansion, but in the past, there was a problem that it was difficult to carry out multi-stage expansion. In other words, expanded polyamide resin beads in which crystallization has been promoted to the extent that they show a high-temperature peak tend to have even higher gas barrier properties than expanded polyamide resin beads that do not have a high-temperature peak, and there was a situation in which it was difficult to impregnate them with a physical blowing agent during multi-stage expansion.

In contrast, the manufacturing method of the present invention adjusts the moisture content of the polyamide resin expanded beads having a high-temperature peak to 1% or more, and performs the internal pressure application process and the heat expansion process, thereby achieving the following effects. That is, according to the present invention, even for polyamide resin expanded beads having a high-temperature peak, the change point of the storage modulus of the expanded beads can be lowered, and the expanded beads can be easily impregnated with a physical foaming agent to apply sufficient internal pressure. As a result, multi-stage expanded beads having a high-temperature peak and a small apparent density can be more easily manufactured.

As described above, in the present invention, the polyamide-based resin expanded particles in a hygroscopic state may be, for example, polyamide-based resin expanded particles obtained by adjusting the moisture content of the first-stage expanded particles obtained by the first-stage expansion to 1% or more by the moisture-containing step. The second-stage expansion (second-stage expansion) can be carried out by the production method of the present invention using such polyamide-based resin expanded particles.
Also, as the hygroscopic polyamide resin expanded particles, multi-stage polyamide resin expanded particles having a low apparent density obtained by the above-mentioned heat expansion step and a moisture content of 1% or more can be used. In other words, the present invention is not limited to the implementation of two-stage expansion, and the expanded particles having a properly adjusted moisture content, which are multi-stage polyamide resin expanded particles obtained by implementing the present invention, can be used as "hygroscopic polyamide resin expanded particles", and the manufacturing method of the present invention can be repeated to perform three-stage expansion, four-stage expansion, and so on. In order to achieve a smaller apparent density, three-stage or more expansion can be performed.
When the multi-stage expanded polyamide resin particles obtained by the second-stage expansion are to be subjected to a third-stage expansion, it is advisable to measure and grasp the change point temperature of the storage modulus of the multi-stage expanded polyamide resin particles to be subjected to the third-stage expansion in advance by a preliminary experiment.

A polyamide resin multi-stage expanded bead molded article (hereinafter, simply referred to as a molded article) can be manufactured using the polyamide resin multi-stage expanded bead of the present invention described above. The manufacturing method is not particularly limited, and a conventionally known method can be adopted. For example, an in-mold molding method is preferable for manufacturing the molded article. In particular, according to the in-mold molding method using steam, the polyamide resin constituting the polyamide resin multi-stage expanded bead is plasticized by steam, so that it is possible to reduce the molding pressure during in-mold molding. The obtained molded article has excellent light weight, and by drying, the inherent physical properties of the polyamide resin can be expressed, resulting in a molded article with high heat resistance.
In addition, in the in-mold molding method, a known method can be adopted as a method for filling the polyamide resin multi-stage expanded beads into the mold. In consideration of the fusion property of the expanded bead molding, within a range that does not excessively improve the secondary expansion power of the expanded beads, a method of pressurizing the expanded beads with a pressurized gas to impart a predetermined internal pressure to the bubbles of the expanded beads and then filling the expanded beads into the mold, a method of filling the expanded beads in a compressed state with a pressurized gas into a pressurized mold and then releasing the pressure in the mold, a method of opening the mold in advance before filling the expanded beads into the mold to expand the molding space, and closing the mold after filling to mechanically compress the expanded beads, etc. can be adopted. The above molded body has high heat resistance, excellent abrasion resistance and chemical resistance, and is also excellent in molded product fusion property and tensile strength. Furthermore, the molded body made using the multi-stage expanded beads of the present invention is particularly excellent in light weight. Therefore, the above molded body can be favorably used for automobile parts, electrical products, etc.

The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto.

In the following Examples and Comparative Examples, the properties of the polyamide resin expanded beads and the multi-stage polyamide resin expanded beads were measured by the following methods. Note that although the measurement methods described in this paragraph are described as measurement methods for the polyamide resin expanded beads, any of the measurement methods can be applied to the measurements of the multi-stage polyamide resin expanded beads.
<Method of measuring apparent density>
A graduated cylinder containing water at a temperature of 23°C was prepared, and the weight Wp of approximately 500 ^cm3 of expanded polyamide resin beads that had been left in the graduated cylinder for two days under conditions of 50% relative humidity, 23°C, and 1 atm was measured, and the beads were submerged using a wire net. Taking into account the volume of the wire net, the volume Vp [ ^cm3 ] of the expanded polyamide resin beads was measured from the rise in the water level, and the weight Wp [g] of the expanded beads was divided by the volume Vp (Wp/Vp) and converted into units of [kg/ ^m3 ] to obtain the apparent density of the expanded beads.
<Method for measuring heat of fusion of high-temperature peak>
The heat of fusion of the high-temperature peak of the expanded polyamide resin particles was determined from the area of the high-temperature peak of the first DSC curve obtained by measuring the temperature from 30° C. to a temperature 30° C. higher than the end of the melting peak at a heating rate of 10° C./min by heat flux differential scanning calorimetry. A high-sensitivity differential scanning calorimeter "EXSTAR DSC7020" (manufactured by SII NanoTechnology Inc.) was used as the measuring device.
The high-temperature peak heat of fusion of the expanded beads corresponds to the area of the high-temperature peak b appearing on the higher temperature side than the intrinsic peak a in the DSC curve shown in Fig. 3, and was calculated as follows. First, as shown in Fig. 3, a straight line was drawn connecting point I of 150°C on the DSC curve and point II showing the melting end temperature on the DSC curve. Next, point IV was determined as the intersection point between a line passing through point III on the DSC curve corresponding to the valley between the intrinsic peak a and the high-temperature peak b, which is perpendicular to the temperature on the horizontal axis of the graph, and the line connecting points I and II. The area of the part (shaded part) surrounded by the line connecting points IV and II, the line connecting points III and IV, and the DSC curve connecting points III and II thus calculated was determined as the heat of fusion of the high-temperature peak.
<Method for measuring closed cell ratio>
The closed cell ratio is the ratio of the volume of closed cells to the volume of all cells in the expanded polyamide resin beads, and was determined using an air comparison type specific gravity meter according to procedure C described in ASTM-D2856-70.
<Method for measuring the temperature at which storage modulus changes>
First, in accordance with JIS K7095: 2012, the dynamic mechanical properties (DMA) of the polyamide resin foam particles were measured to measure the storage modulus, and a graph was created to identify the change point temperature. The DMA measuring device used was a DMA7100 manufactured by Hitachi High-Tech Science Corporation.
Specifically, one expanded bead whose moisture content had been adjusted in advance was fixed in a jig, and the temperature was changed from -50°C to 150°C at a heating rate of 2°C/min in compression mode at a constant frequency (1 Hz) to measure the storage modulus of the expanded bead. As shown in Figure 2, the measurement results were plotted on a graph with the storage modulus on the vertical axis and the temperature on the horizontal axis. In the graph, a perpendicular line was drawn downward from the intersection B between an extension line L1, which is an extension line of the straight line before the initial rapid drop in the storage modulus to the high temperature side, and an extension line L2, which is an extension line of the intermediate line after the initial rapid drop in the storage modulus to the low temperature side, to the temperature axis, and the temperature indicated by the perpendicular line was taken as the change point temperature of the storage modulus. The above measurement was performed on 10 expanded beads, and the arithmetic average value was taken as the change point temperature of the storage modulus of the polyamide resin expanded beads.
<Method of measuring moisture content>
The moisture content of the polyamide resin expanded particles was determined using the Karl Fischer method described above. Specifically, first, about 0.2 g of the polyamide resin expanded particles was weighed and used as a test piece. Next, the test piece was heated to a temperature of 160° C. using a heating moisture vaporizer (automatic heating moisture vaporizer SE320 manufactured by Hiranuma Sangyo Co., Ltd.) to vaporize the moisture inside the test piece. This moisture was introduced into a Karl Fischer moisture measuring device (AQ-2200A manufactured by Hiranuma Sangyo Co., Ltd.) connected to the heating moisture vaporizer, the moisture amount was measured, and the moisture content in the expanded particles was calculated. The measurement conditions (titration conditions) were: waiting time: 40 seconds, electrolysis current: MEDIUM, minimum amount of electrolysis: 5 μg. The above measurement was performed on five test pieces, and the arithmetic average value was taken as the moisture content of the polyamide resin expanded particles.
<Method of measuring internal pressure>
The internal pressure P of the expanded polyamide resin beads was determined by the following method using the following formulas (1) and (2): The weight W7 of the small bag was measured in advance.
First, from the group of polyamide-based resin foamed beads to which internal pressure had been applied (group of wet pressure-expanded beads), polyamide-based resin foamed beads to which an arbitrary amount of internal pressure had been applied (wet pressure-expanded beads) were placed in a small bag, and the weight W2 was measured.
Furthermore, an arbitrary amount of sample was taken from the polyamide resin foamed particles (wet pressurized foamed particles), and the moisture content of the sample was measured in advance with a Karl Fischer moisture meter. The moisture content measured with the Karl Fischer moisture meter is W6, and the amount of sample used for the measurement with the Karl Fischer moisture meter is W5.
The moisture content W3 contained in the wetted pressure-foamed particles was determined by subtracting the weight W7 of the small bag from the weight W2 of the wetted pressure-foamed particles, dividing the result by the sample weight W5 used in the measurement with the Karl Fischer moisture meter, and multiplying the result by the moisture content W6 measured with the Karl Fischer moisture meter.
The weight W1 of the dry pressurized beads was calculated by subtracting the combined weight W7 of the bag and the amount of water W3 in the wet pressurized-foamed beads from the weight W2 of the wet pressurized-foamed beads.
Next, the wetted pressurized foamed particles were placed together with the pouch in a thermostatic chamber at 80°C for about 48 hours to release the internal pressure. The depressurized polyamide-based resin foamed particles were then removed from the thermostatic chamber and dried at 160°C for 20 hours. The weight W4 of the dried polyamide-based resin foamed particles (dried foamed particles) removed from the pouch was measured, and the amount of air Wa contained in the polyamide-based resin foamed particles to which internal pressure had been applied was calculated using the following formula (1). The Karl Fischer moisture meter and measurement conditions were the same as those used to measure the moisture content of the polyamide-based resin foamed particles.
[Equation 3]
Air volume Wa = W1 - W4
=(W2-W3-W7)-W4
= [(W2-(W2-W7)/W5×W6)-W7]-W4...(1)
W1: Weight of dry pressurized foam particles (g)
W2: Weight of wet pressed beads including the weight of the sachet (g)
W3: Water content in wet compressed foam particles (g)
W4: Weight of dry foam particles (g)
W5: Amount of sample (g) used in the Karl Fischer moisture meter measurement
W6: Moisture content (g) measured with a Karl Fischer moisture meter
W7: Weight of the bag (g)
Wa: Air volume (g)
The amount of air Wa (g) obtained as described above was used in the following formula (2) to obtain the internal pressure of the expanded polyamide resin beads to which the internal pressure P was applied.
[Equation 4]
Internal pressure P (MPa (G))
= (Wa/28.8×0.0831×296)/(W4/apparent density of expanded beads (kg/m ³ )−W4/density of resin (kg/m ³ )) (2)

[Production of polyamide resin particles]
As the polyamide resin, polyamide 6/66 copolymer (polyamide 6/polyamide 66=85/15; UBE Nylon 5033B, manufactured by Ube Industries, melting point 197°C, density 1.14 kg/ ^m3 , flexural modulus 1300 MPa, MFR 3.5 g/10 min) was used, and as the cell regulator, "Talc Powder PK-S" (manufactured by Hayashi Kasei Co., Ltd.) was supplied so that its content was 8000 ppm by weight. As an end-capping agent, "Stabaxol P" (manufactured by Rhein Chemie) was supplied so that its content was 1 part by weight. The mixture was melt-kneaded, extruded into strands, cooled in water, cut with a pelletizer, and dried to obtain polyamide resin particles each having an average weight of 2 mg.

[Production of expanded polyamide resin particles]
10 kg of the obtained polyamide-based resin particles and 310 L of water as a dispersion liquid were charged into a 400 L autoclave equipped with a stirrer, and further, 3.0 parts by weight of kaolin as a dispersant and 0.08 parts by weight of sodium alkylbenzene sulfonate as a surfactant were added to the dispersion liquid per 100 parts by weight of the polyamide-based resin particles.
The contents in the autoclave were heated from room temperature (23°C) while stirring, and after the temperature reached 136°C, carbon dioxide as a foaming agent was injected into the autoclave until the pressure inside the autoclave reached 4.0 MPa (G). At this time, the temperature rise time from room temperature (23°C) to 136°C was 30 minutes. Next, the state of 136°C and 4.0 MPa (G) was maintained for 15 minutes.
Thereafter, the polyamide-based resin particles impregnated with the foaming agent were released together with the dispersion liquid under atmospheric pressure (0.1 MPa) (foaming temperature: 136° C.) The obtained expanded polyamide-based resin particles were cured in an oven at 60° C. for 24 hours, and then slowly cooled to obtain expanded polyamide-based resin particles in a dry state.
The apparent density, heat of fusion at the high temperature peak, and closed cell ratio of the obtained expanded beads were measured, and the results are shown in Tables 1 and 2. In Examples 3 to 11 and Comparative Examples 2 and 3, the temperature for the heating, maintenance, and expansion was set to 136.5°C.

[Water-containing step]
The polyamide resin expanded particles obtained as described above were used, and the polyamide resin expanded particles in a dry state and water were placed in a plastic bag, and then the mouth of the plastic bag was closed and the mixture was shaken well to a predetermined water content, thereby carrying out a water-containing step to adjust the water content. The water content of the expanded particles obtained in this way and the temperature T at which the storage modulus of the expanded particles changed after the water-containing step were measured, and are shown in Tables 1 and 2.

[Internal pressure application process]
180 g of the expanded polyamide resin beads having the moisture content adjusted as described above was placed in a 3-liter pressure vessel set at the pressurizing temperature shown in Tables 1 and 2. Tables 1 and 2 also show the difference between each pressurizing temperature and the change point temperature of the storage modulus of the expanded polyamide resin beads used.
The expanded beads having the adjusted moisture content were placed in a pressure vessel and sealed, and at the same time, air was injected as a physical foaming agent to start the internal pressure imparting step. The air as a physical foaming agent was injected at the pressure increase rate shown in Tables 1 and 2, and after the pressure (pressurized pressure) in the pressure vessel reached 0.6 MPa (G), it was kept constant at 0.6 MPa (G). The pressurization time (the time from when the physical foaming agent was injected to when the polyamide resin expanded beads to which internal pressure was imparted were taken out of the pressure vessel) and the pressure increase rate are shown in Tables 1 and 2. The pressing temperature was kept constant.
Immediately after the pressurization time shown in Tables 1 and 2 had elapsed, the polyamide resin to which the internal pressure had been applied was removed from the pressure vessel, and the internal pressure of the expanded polyamide resin particles was measured. The internal pressure increase rate was calculated by dividing the measured internal pressure by the pressurization time. The measured internal pressure and the internal pressure increase rate are both shown in Tables 1 and 2.
The appearance of the polyamide resin foamed beads obtained as described above and to which internal pressure had been applied was observed, and those without yellowing or wrinkles were rated as good (.largecircle.).

[Heat foaming process]
The polyamide resin foamed particles to which internal pressure had been applied as described above were subjected to a heat expansion step as follows.
180 g of the polyamide-based resin expanded beads to which the internal pressure was applied as described above was placed in a 600-liter autoclave equipped with a stirrer, in which steam (original pressure 0.3 MPa (G)) was injected to set the pressure and temperature to the values shown in Tables 1 and 2, and the autoclave was sealed and pressurized and heated with stirring to further expand the expanded beads. 15 seconds after the polyamide-based resin expanded beads to which the internal pressure was applied were placed in the autoclave, the autoclave was opened and the polyamide-based resin expanded beads were taken out. The polyamide-based resin expanded beads obtained were cured in an oven at 40° C. for 20 hours and then slowly cooled to obtain multi-stage polyamide-based resin expanded beads (two-stage expanded beads).

The apparent density and closed cell ratio of the polyamide resin multi-stage expanded beads obtained as described above were measured. In addition, the apparent density ratio was calculated by dividing the apparent density of the polyamide resin multi-stage expanded beads (second-stage expanded beads) by the apparent density of the polyamide resin expanded beads (first-stage expanded beads) in a hygroscopic state used in the internal pressure application process described above. The apparent density, closed cell ratio, and apparent density ratio are all shown in Tables 1 and 2.

[Production of polyamide resin foamed particle molded body]
The multi-stage expanded polyamide resin beads obtained as described above were used to produce an expanded bead molding as follows.
First, the obtained polyamide resin foamed beads were filled into a flat mold having a length of 200 mm, a width of 65 mm, and a thickness of 40 mm, and then the mold was molded by steam heating to obtain a plate-shaped foamed bead molding. The heating method was as follows: with the drain valves of both molds open, steam was supplied for 5 seconds to perform preheating (exhaust process), and then steam was supplied from the moving mold, and then steam was supplied from the fixed mold, and the mold was heated to the molding heating steam pressure (molding pressure = molding steam pressure).
After the heating, the pressure was released, and the molded product was cooled with water until the surface pressure due to the foaming force of the molded product decreased to 0.02 MPa (gauge pressure), and then the mold was opened and the molded product was removed from the mold. The obtained molded product was cured in an oven at 80° C. for 12 hours, and then slowly cooled to room temperature. In this way, an expanded bead molded product was obtained.

(Evaluation of moldable range)
The above-mentioned expanded bead moldings were produced by changing the steam pressure and evaluated according to the following criteria. Specifically, the expanded bead moldings were produced in the same manner as the expanded bead molding described above, except that the steam pressure was changed in increments of 0.02 MPa (G) between 0.10 and 0.20 MPa (G).
The resulting expanded bead moldings were evaluated for three items, namely, fusion property, surface property, and recovery property, as follows. The steam pressure that gave a good evaluation in all three items was determined to be the pressure at which the expanded bead molding could be made. The "Moldable range" column in Tables 1 and 2 shows the range of steam pressure at which the expanded bead molding could be made.
(Evaluation of adhesion)
The fusion rate of the expanded bead molding was determined based on the ratio of the number of foamed beads that were broken among the foamed beads exposed on the fracture surface when the expanded bead molding was broken. Specifically, first, a test piece (length 100 mm x width 100 mm x thickness: thickness of molding) was cut out from the expanded bead molding, and a cutter knife was used to make a cut of about 5 mm in the thickness direction of each test piece, and then the test piece was broken from the cut part. Next, the number (n) of the expanded beads present on the fracture surface of the expanded bead molding and the number (b) of the foamed beads that were broken were measured, and the ratio (b/n) of (b) and (n) was expressed as a percentage to obtain the fusion rate (%), which was evaluated as follows.
Good: The fusion rate was 90% or more.
Unacceptable: The fusion rate was less than 90%.

(Surface Evaluation)
The surface condition of the expanded bead molding was evaluated as follows.
Good: Gaps between the foamed particles on the surface of the molded article are completely filled.
Poor: Gaps between the foam particles on the surface of the molded article are not filled.

(Resilience assessment)
The thicknesses of the end (10 mm inside from the end) and the center (part dividing the body in half in both the vertical and horizontal directions) of the expanded bead molding corresponding to the dimensions of the flat plate-shaped mold used in the in-mold molding were measured. Then, the thickness ratio of the expanded bead molding (thickness of the center part of the molding/thickness of the end part of the molding×100(%)) was calculated and evaluated as follows.
Good: The thickness ratio is 90% or more.
Unacceptable: The thickness ratio is less than 90%.

(1) The method comprises an internal pressure applying step of placing expanded polyamide-based resin particles in a pressure-resistant container, impregnating the expanded polyamide-based resin particles with a physical foaming agent in the pressure-resistant container, and applying an internal pressure exceeding atmospheric pressure to the expanded polyamide-based resin particles; and a heating and foaming step of heating and foaming the expanded polyamide-based resin particles to which the internal pressure obtained in the internal pressure applying step has been applied, thereby obtaining multi-stage expanded polyamide-based resin particles having an apparent density smaller than that of the expanded polyamide-based resin particles used in the internal pressure applying step.
A method for producing multi-stage polyamide-based resin expanded beads, characterized in that in the internal pressure application step, polyamide-based resin expanded beads in a hygroscopic state having a moisture content of 1% or more are impregnated with the physical foaming agent at a temperature higher than a change point temperature of the storage modulus of the polyamide-based resin expanded beads in the hygroscopic state.
(2) In the internal pressure applying step,
The polyamide resin foam particles in the hygroscopic state are
The method for producing multi-stage expanded polyamide resin beads described in (1) above, in which the physical foaming agent is impregnated at a temperature that is 30° C. or more higher than the change point temperature of the storage modulus of the expanded polyamide resin beads in a hygroscopic state at the start of the internal pressure application step.
(3) In the internal pressure applying step,
The polyamide resin foam particles in the hygroscopic state are
The method for producing multi-stage expanded polyamide resin particles according to the above (1) or (2), in which the physical foaming agent is impregnated at a temperature of 120° C. or less.
(4) The method for producing multi-stage expanded polyamide resin beads according to any one of (1) to (3) above, wherein the moisture content of the expanded polyamide resin beads in a hygroscopic state is 3.0% or more.
(5) The method for producing multi-stage expanded polyamide resin beads according to any one of (1) to (4) above, wherein the moisture content of the expanded polyamide resin beads in a hygroscopic state is 4.5% or more.
(6) In the internal pressure applying step,
The method for producing multi-stage expanded polyamide resin beads according to any one of (1) to (5), further comprising impregnating the polyamide resin beads with the physical foaming agent so that the rate of increase in internal pressure of the expanded polyamide resin beads in a moisture-absorbed state is 0.003 MPa/hr or more and 0.05 MPa/hr or less.
(7) The method for producing multi-stage expanded polyamide resin beads according to any one of (1) to (6) above, wherein the apparent density of the multi-stage expanded polyamide resin beads is 100 kg/m3 or ^less .
(8) The method for producing multi-stage expanded polyamide resin beads according to any one of (1) to (7) above, wherein the ratio of the apparent density of the multi-stage expanded polyamide resin beads to the apparent density of the expanded polyamide resin beads used in the internal pressure application step is 0.70 or less.
(9) In the heat foaming step,
The polyamide resin foamed particles to which the internal pressure has been applied are
The method for producing multi-stage expanded polyamide-based resin beads according to any one of (1) to (8), further comprising heating and expanding the beads by using a heating medium having a temperature higher than the inflection point temperature of the storage modulus of the expanded polyamide-based resin beads to which the internal pressure has been applied and lower than the melting point of the polyamide-based resin constituting the expanded polyamide-based resin beads to which the internal pressure has been applied.
(10) In the heat foaming step,
The method for producing multi-stage expanded polyamide resin beads according to any one of (1) to (9) above, further comprising heating the expanded polyamide resin beads to which the internal pressure has been applied with water vapor to expand the beads.
(11) In the internal pressure applying step,
The polyamide resin foam particles are placed in a pressure-resistant container,
The method for producing multi-stage expanded polyamide resin particles according to any one of (1) to (10) above, further comprising: injecting the physical foaming agent into the pressure-resistant vessel so that a pressure rise rate in the pressure-resistant vessel is from 0.01 MPa/hr to 0.2 MPa/hr, thereby impregnating the polyamide resin expanded particles in a moisture-absorbing state with the physical foaming agent.

Claims (11)

the step of placing expanded polyamide-based resin particles in a pressure-resistant container and impregnating the expanded polyamide-based resin particles with a physical foaming agent in the pressure-resistant container to apply an internal pressure exceeding atmospheric pressure; and the step of heating and expanding the expanded polyamide-based resin particles to which the internal pressure obtained in the step of applying the internal pressure has been applied, thereby obtaining multi-stage expanded polyamide-based resin particles having an apparent density smaller than that of the expanded polyamide-based resin particles used in the step of applying the internal pressure.
A method for producing multi-stage polyamide-based resin expanded beads, characterized in that in the internal pressure application step, polyamide-based resin expanded beads in a hygroscopic state having a moisture content of 1% or more are impregnated with the physical foaming agent at a temperature higher than a change point temperature of the storage modulus of the polyamide-based resin expanded beads in the hygroscopic state. In the internal pressure applying step,
The polyamide resin foam particles in the hygroscopic state are
2. The method for producing multi-stage polyamide resin expanded beads according to claim 1, wherein the physical foaming agent is impregnated at a temperature that is 30° C. or more higher than the change point temperature of the storage modulus of the hygroscopic polyamide resin expanded beads at the start of the internal pressure application step. In the internal pressure applying step,
The polyamide resin foam particles in the hygroscopic state are
3. The method for producing multi-stage expanded polyamide resin beads according to claim 1, wherein the physical foaming agent is impregnated at a temperature of 120° C. or less. The method for producing multi-stage polyamide resin expanded particles according to any one of claims 1 to 3, wherein the moisture content of the polyamide resin expanded particles in a hygroscopic state is 3.0% or more. The method for producing multi-stage polyamide resin expanded beads according to any one of claims 1 to 4, wherein the moisture content of the polyamide resin expanded beads in a hygroscopic state is 4.5% or more. In the internal pressure applying step,
6. The method for producing multi-stage expanded polyamide resin beads according to claim 1, wherein the physical foaming agent is impregnated into the expanded polyamide resin beads in a moisture-absorbed state so that the rate of increase in internal pressure of the expanded polyamide resin beads is 0.003 MPa/hr or more and 0.05 MPa/hr or less. The method for producing multi-stage expanded polyamide resin beads according to any one of claims 1 to 6, wherein the multi-stage expanded polyamide resin beads have an apparent density of 100 kg/ ^m3 or less. The method for producing multi-stage polyamide resin expanded beads according to any one of claims 1 to 7, wherein the ratio of the apparent density of the multi-stage polyamide resin expanded beads to the apparent density of the polyamide resin expanded beads used in the internal pressure application step is 0.70 or less. In the heat foaming step,
The polyamide resin foamed particles to which the internal pressure has been applied are
9. The method for producing multi-stage polyamide-based resin expanded beads according to claim 1, wherein the polyamide-based resin expanded beads are heated and expanded by a heating medium having a temperature higher than a temperature at which the storage modulus of the polyamide-based resin expanded beads to which the internal pressure has been applied changes and lower than a melting point of the polyamide-based resin constituting the polyamide-based resin expanded beads to which the internal pressure has been applied. In the heat foaming step,
The method for producing multi-stage expanded polyamide resin beads according to any one of claims 1 to 9, wherein the expanded polyamide resin beads to which the internal pressure has been applied are heated with water vapor to be expanded. In the internal pressure applying step,
The polyamide resin foam particles are placed in a pressure-resistant container,
11. The method for producing multi-stage expanded polyamide resin beads according to claim 1, wherein the physical foaming agent is pressurized into the pressure-resistant vessel so that a pressure rise rate in the pressure-resistant vessel is from 0.01 MPa/hr to 0.2 MPa/hr, thereby impregnating the polyamide resin expanded beads in a moisture-absorbed state with the physical foaming agent.