JP4761917B2 - Process for producing pre-expanded particles of polylactic acid resin - Google Patents

Description

  The present invention relates to a method for producing polylactic acid resin pre-expanded particles that can obtain a polylactic acid resin foam molded article having excellent heat resistance and mechanical strength by in-mold foam molding.

  Polylactic acid resin is a resin obtained by polymerizing naturally occurring lactic acid, is a biodegradable resin that can be decomposed by microorganisms existing in nature, and has excellent mechanical properties at room temperature. It attracts attention.

  The polylactic acid resin generally polymerizes D-lactic acid and / or L-lactic acid, or opens one or more lactides selected from the group consisting of L-lactide, D-lactide and DL-lactide. Manufactured by ring polymerization.

  The obtained polylactic acid-based resin changes in physical properties, particularly crystallinity, depending on the content ratio of the D-form component or L-form component contained in the polylactic acid-based resin. As the proportion of the smaller optical isomer of the body components increases, the crystallinity of the polylactic acid resin decreases and eventually becomes amorphous.

  In addition, the polylactic acid resin pre-expanded particles are filled in a mold, and the polylactic acid resin pre-expanded particles are heated and softened by a heat medium such as water vapor to be softened and expanded. A method for producing a polylactic acid-based resin foam molded body having a desired shape by fusing them together, so-called in-mold foam molding, has been proposed.

  Specifically, Patent Document 1 discloses that an isocyanate group ≧ 2.0 equivalents / mol in polylactic acid having a molar ratio of L-form to D-form of 95/5 to 60/40, or 40/60 to 5/95. A resin composition obtained by aging under a predetermined condition a resin composition obtained by mixing 0.5 to 5% by weight of the polyisocyanate compound with respect to the polylactic acid and reacting the same is proposed, and particles are formed from the resin composition. The particles are impregnated with a foaming agent and a foaming auxiliary agent, and the resulting foamable particles are pre-foamed to produce pre-foamed particles. The pre-foamed particles are filled into a mold and foamed to obtain a desired shape. It is disclosed to form a molded body having the following.

  However, the polylactic acid-based resin has a molar ratio of the smaller optical isomer component of the L-form component or the D-form component of 5 mol% or more, and the polylactic acid-based resin has low crystallinity or is not non-crystalline. It is crystalline and inferior in heat resistance, and the resulting molded article has a heat resistance of about 50 ° C. at most, which is problematic for practical use.

  Therefore, it is conceivable to use a polylactic acid-based resin with high crystallinity in which the molar ratio of the smaller optical isomer of L-form or D-form is less than 5 moles. Since the foamable particles are heated and pre-foamed, the polylactic acid resin is crystallized by the heat applied during the foaming process, resulting in pre-foamed particles with high crystallinity. As a result, the fusing property of the pre-expanded particles obtained is lowered, and the molded product obtained using such pre-expanded particles has a problem that the fusing property is poor and the mechanical strength is low. It was.

JP 2000-17037 A

  The present invention provides a method for producing pre-expanded polylactic acid-based resin particles capable of obtaining a polylactic acid-based resin expanded molded article having excellent heat resistance and mechanical strength by in-mold foam molding.

The method for producing pre-expanded polylactic acid resin particles for in-mold foam molding according to the present invention is to supply polylactic acid resin to an extruder, melt knead in the presence of a foaming agent, and extrusion foam to produce an extruded foam. In the process for producing pre-expanded particles by cutting the extruded foam into particles, the polylactic acid-based resin has both D-form and L-form as its constituent monomer components. And the content of the lesser of the D isomer and the L isomer is less than 5 mol%, or any of the D isomer and L isomer as a constituent monomer component While containing only one optical isomer, the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by the melting point (mp) in the polylactic acid resin and the dynamic viscoelasticity measurement. Satisfies the following formula 1 and Crystallinity of the acid-based resin pre-expanded particles and adjusting to be less than 30%.
(Melting point of polylactic acid resin (mp) -40 ° C)
≦ (temperature T at the intersection) ≦ melting point of polylactic acid resin (mp) Formula 1

  The polylactic acid-based resin is represented by the following chemical formula 1, and is obtained by copolymerizing D-lactic acid and L-lactic acid as monomers, polymerizing either D-lactic acid or L-lactic acid as monomers, or D- It can be obtained by ring-opening polymerization of one or more lactides selected from the group consisting of lactide, L-lactide and DL-lactide, and any polylactic acid resin may be used.

  And when producing a polylactic acid-based resin, when the D isomer and L isomer are used in combination as a monomer, the proportion of the lesser optical isomer of the D isomer or L isomer is less than 5 mol%, or In the case where only one of the optical isomers of D-form or L-form is used as a monomer, that is, the polylactic acid-based resin has both D-form and L-form optical isomers as its constituent monomer components. And the content of the smaller optical isomer of D-form or L-form is less than 5 mol%, or any one of the D-form or L-form optical isomer as a constituent monomer component In the case where only the D-form and the L-form are used together, the polylactic acid-based resin obtained has a higher crystallinity, while the D-form and the L-form are used in combination as a monomer. Is more than 5 mol% When that is, according to the optical isomer is increased the smaller the resulting polylactic acid-based resin, its crystallinity decreases, the eventually amorphous.

  Therefore, in the present invention, a polymorphism containing both D-form and L-form optical isomers as a constituent monomer component and the content of the smaller of the D-form and L-form is less than 5 mol%. Polylactic acid-based resin pre-expanded particles obtained by using a lactic acid-based resin or a polylactic acid-based resin containing only one optical isomer of D-form or L-form as a constituent monomer component High heat resistance.

  Furthermore, the polylactic acid-based resin obtained by polymerizing the D-form and the L-form in combination as a monomer has a ratio of the smaller optical isomer of the D-form or the L-form of less than 4 mol%. A polylactic acid resin obtained by polymerizing a certain monomer is preferred, and a polylactic acid resin obtained by polymerizing a monomer in which the proportion of the optical isomer, whichever is smaller, of D-form or L-form is less than 3 mol% A lactic acid-based resin is more preferable, and a polylactic acid-based resin obtained by polymerizing a monomer in which the ratio of the optical isomer, which is the smaller of either the D-form or the L-form, is less than 2 mol% is particularly preferred.

  That is, a polylactic acid-based resin containing both optical isomers of D-form and L-form as a constituent monomer component, and the content of the smaller optical isomer of D-form or L-form is less than 4 mol%. Preferably, a polylactic acid-based resin that contains both D-form and L-form optical isomers as a constituent monomer component, and the content of the smaller of the D-form and L-form is less than 3 mol%. More preferably, a polylactic acid resin containing both D isomer and L isomer as constituent monomer components, and the content of the lesser of the D isomer and L isomer being less than 2 mol% Is more preferable.

  And as for the polylactic acid-type resin which contains D body and L body as a constituent monomer component, the ratio of the optical isomer of the smaller one of D body or L body decreases, and the polylactic acid resin becomes In addition to its crystallinity, the melting point increases. Therefore, the heat resistance of the foam molded product obtained by filling the foamed pre-expanded particles in the mold is improved, and the foam molded product can maintain its form even at a high temperature. Can be taken out from the mold at a high temperature, the cooling time in the mold of the foam molded product can be shortened, and the production efficiency of the foam molded product can be improved.

  Here, the content of D-form or L-form in the polylactic acid-based resin can be measured by the following method. First, a polylactic acid resin is dissolved in chloroform to prepare a chloroform solution having a polylactic acid resin concentration of 10 mg / ml. Next, the chloroform solution is irradiated with polarized light having a wavelength of 589 nm at 25 ° C. using a polarimeter, and the specific rotation of the chloroform solution is measured.

  On the other hand, the polylactic acid resin obtained by polymerization using only the D isomer as the monomer, or the polylactic acid resin obtained by polymerizing using only the L isomer as the monomer, was compared in the same manner as described above. Although the optical rotation may be measured, this specific optical rotation is usually already measured. The polylactic acid resin obtained by polymerization using only D-form is + 156 °, and only L-form is used as a monomer. The polylactic acid-based resin obtained by polymerization using it is set to −156 °.

And the quantity of D body component or L body component in polylactic acid-type resin is computable based on a following formula.
D-form component amount (mol%) = 100 × {specific rotation of chloroform solution − (− 156)}
/ {156-(-156)}
L-form component amount (mol%) = 100- (D-form component amount)

However, since the polylactic acid resin used in the present invention has a high crystallization rate, it is necessary to select a resin suitable for extrusion foaming. In the present invention, the polylactic acid resin has a melting point ( mp) and a polylactic acid resin satisfying the following formula 1 using the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement.
(Melting point of polylactic acid resin (mp) -40 ° C)
≦ (temperature T at the intersection) ≦ melting point of polylactic acid resin (mp) Formula 1

  Here, the storage elastic modulus obtained by the dynamic viscoelasticity measurement is an index indicating elastic properties in the viscoelasticity, and is an index indicating the elasticity of the bubble film in the foaming process. This is an index indicating the magnitude of the foaming pressure required to expand the bubbles against the contraction force of the bubble film.

  That is, if the storage elastic modulus obtained by the dynamic viscoelasticity measurement of the polylactic acid-based resin is low, when the cell membrane is stretched, the force that the cell membrane attempts to contract against the stretching force is small. The foamed film easily stretches due to the foaming pressure required for the production of pre-foamed particles of lactic acid resin. As a result, the bubble film stretches excessively, resulting in bubble breakage. On the other hand, the dynamic viscoelasticity of polylactic acid resin When the storage elastic modulus obtained by the measurement is high, when the expansion force is applied to the cell membrane, the cell membrane has a large contraction force against the expansion, and the foaming pressure required for the production of the pre-expanded particles of polylactic acid resin Even if the bubbles expand once, the bubbles contract as the foaming pressure decreases with time due to a temperature drop or the like.

  The loss elastic modulus obtained by dynamic viscoelasticity measurement is an index indicating the viscous property in viscoelasticity, and is an index indicating the viscosity of the bubble film in the foaming process. This is an index indicating the allowable range of how much can be expanded without breaking, and at the same time, the ability to expand the bubbles to the desired size by the foaming pressure and then maintain the expanded bubbles at that size It is also an indicator that indicates.

  That is, if the loss elastic modulus obtained by the dynamic viscoelasticity measurement of the polylactic acid resin is low, the foam film is expanded when the foam film is stretched by the foaming pressure required for producing the pre-expanded particles of the polylactic acid resin. However, if the loss elastic modulus obtained by the dynamic viscoelasticity measurement of polylactic acid resin is high, the foaming force is converted into thermal energy by the bubble film, and polylactic acid resin pre-foaming During the production of the particles, the bubble film cannot be extended smoothly, and the bubbles cannot be expanded.

  Thus, in producing polylactic acid resin pre-expanded particles by foaming polylactic acid-based resin, in the foaming process, polylactic acid-based resin is required to obtain polylactic acid-based resin pre-expanded particles. It is necessary to have an elastic force for expanding the bubble film appropriately without being broken by the foaming pressure, that is, a storage elastic modulus, and the bubble film is smoothly stretched without being broken by the foaming pressure to have a desired size. In addition, it is necessary to have a viscous force, that is, a loss elastic modulus, for maintaining the expanded bubble in its size regardless of the decrease in the foaming pressure over time.

  That is, in the extrusion foaming process, both the storage elastic modulus and loss elastic modulus of the polylactic acid-based resin need to have values suitable for extrusion foaming, and the storage elastic modulus and loss suitable for such extrusion foaming. In order to impart elastic modulus to the polylactic acid resin in the extrusion foaming process, the temperature T (hereinafter referred to as “storage elastic modulus curve”) at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement. And the melting point (mp) of the polylactic acid resin are required to satisfy the following formula 1, and more preferably so as to satisfy the formula 2. By adjusting so as to satisfy the formula 3, the storage elastic modulus and loss elastic modulus of the polylactic acid resin are suitable for extrusion foaming while balancing them, and the extrusion foamability of the polylactic acid resin is improved. And then, the polylactic acid-based resin pre-expanded particles can be produced stably.

[Melting point of polylactic acid resin (mp) −40 ° C.]
≦ Temperature at the intersection T ≦ Melting point of polylactic acid resin (mp) Formula 1

[Melting point of polylactic acid resin (mp) -35 ° C.]
≦ Temperature at the intersection T ≦ [Melting point of polylactic acid resin (mp) −10 ° C.] Formula 2

[Melting point of polylactic acid resin (mp) -30 ° C.]
≦ Temperature at the intersection T ≦ [Melting point of polylactic acid resin (mp) −20 ° C.] Formula 3

  Furthermore, the temperature T and the melting point (mp) of the polylactic acid resin obtained at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement of the polylactic acid resin satisfy the above formula 1. The reason for the adjustment will be described in detail below.

  First, the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement of the polylactic acid resin exceeds 40 ° C. than the melting point (mp) of the polylactic acid resin. If it is low, the loss elastic modulus of the polylactic acid resin at the time of extrusion foaming is too large compared with the storage elastic modulus, so that the balance between the loss elastic modulus and the storage elastic modulus is lost.

  Therefore, if the foaming force suitable for the loss elastic modulus of the polylactic acid-based resin, that is, the foaming force matched to the viscosity of the polylactic acid-based resin, the foaming force is too large for the elastic force of the polylactic acid-based resin. The film is torn and bubbles are generated, and good polylactic acid resin pre-expanded particles cannot be obtained. Conversely, the foaming force suitable for the storage elastic modulus of the polylactic acid resin, that is, the elasticity of the polylactic acid resin If the foaming force is adjusted to the above, the foaming force is small for the viscosity force of the polylactic acid resin, and the polylactic acid resin is difficult to foam, and it is impossible to obtain pre-expanded particles of polylactic acid resin.

  Further, when the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by the dynamic viscoelasticity measurement of the polylactic acid resin is higher than the melting point (mp) of the polylactic acid resin, Since the storage elastic modulus of the polylactic acid resin at the time of foaming is too large compared to the loss elastic modulus, the balance between the loss elastic modulus and the storage elastic modulus is lost as described above.

  Therefore, if the foaming force suitable for the storage elastic modulus of the polylactic acid-based resin, that is, the foaming force matched to the elasticity of the polylactic acid-based resin, the foaming force is too large for the viscosity force of the polylactic acid-based resin, The film is torn and bubbles are broken, and good polylactic acid resin pre-expanded particles cannot be obtained. Conversely, the foaming force suitable for the loss elastic modulus of the polylactic acid resin, that is, the viscosity of the polylactic acid resin is reduced. If the combined foaming force is used, the foaming force is small for the elastic force of the polylactic acid-based resin. Even if the polylactic acid-based resin foams once due to the foaming force, the bubbles shrink as the foaming force decreases over time. As a result, good polylactic acid resin pre-expanded particles cannot be obtained.

  Then, the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by the dynamic viscoelasticity measurement of the polylactic acid resin and the melting point (mp) of the polylactic acid resin are expressed by the above equation 1. As a method of adjusting so as to satisfy, as the weight average molecular weight of the polylactic acid resin increases, the storage elastic modulus curve and the loss elastic modulus curve obtained by the dynamic viscoelasticity measurement of the polylactic acid resin Since the temperature T at the intersection becomes high, a method for adjusting the weight average molecular weight of the obtained polylactic acid resin by adjusting the reaction time or reaction temperature during polymerization of the polylactic acid resin, before extrusion foaming or extrusion foaming A method of adjusting the weight average molecular weight of the polylactic acid resin sometimes using a thickener or a crosslinking agent is mentioned.

  From such a viewpoint, the weight average molecular weight of the polylactic acid-based resin is preferably 140,000 to 300,000, more preferably 150,000 to 270,000, and particularly preferably 160,000 to 250,000. Furthermore, the molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the polylactic acid-based resin is preferably 3.2 to 10, more preferably 3.4 to 9, and particularly preferably 3.6 to 8.

  In addition, in the case of a polylactic acid resin obtained from a monomer in which the ratio of the L isomer is larger than that of the D isomer, the melting point (mp) of the polylactic acid resin decreases as the D isomer ratio increases. From the above, the storage modulus curve and loss obtained by adjusting the melting point (mp) of the polylactic acid resin by adjusting the ratio of D-form in the monomer and by measuring the dynamic viscoelasticity of the polylactic acid resin There is a method in which the temperature T at the intersection with the elastic modulus curve and the melting point (mp) of the polylactic acid resin are adjusted so as to satisfy the above formula 1.

  Here, the melting point (mp) of the polylactic acid resin is measured in the following manner. That is, the differential scanning calorimetry of the polylactic acid resin is performed in accordance with JIS K7121: 1987, and the melting peak temperature in the obtained DSC curve is defined as the melting point (mp) of the polylactic acid resin. When there are a plurality of melting peak temperatures, the highest temperature is set.

Further, the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by the dynamic viscoelasticity measurement of the polylactic acid-based resin is the one measured in the following manner. That is, the polylactic acid resin is dried at 80 ° C. for 3 hours under a reduced pressure of 9.33 × 10 ⁴ Pa. This polylactic acid-based resin is placed on a measurement plate heated to a temperature higher by 40 to 50 ° C. than the melting point of the polylactic acid-based resin, and allowed to stand for 5 minutes in a nitrogen atmosphere to melt.

  Next, a flat circular pressure plate having a diameter of 25 mm is prepared, and the polylactic acid resin on the measurement plate is moved up and down until the distance between the opposing surfaces of the pressure plate and the measurement plate becomes 1 mm. Press in the direction. And after removing the polylactic acid-type resin which protruded from the outer periphery of a press plate, it is left to stand for 5 minutes.

  Thereafter, the dynamic viscoelasticity measurement of the polylactic acid resin is performed under the conditions of 5% strain, frequency 1 rad / sec, temperature drop rate 2 ° C./min, and measurement interval 30 sec to determine the storage elastic modulus and loss elastic modulus. taking measurement. Next, a storage elastic modulus curve and a loss elastic modulus curve are drawn with the horizontal axis as temperature and the vertical axis as storage elastic modulus and loss elastic modulus. In drawing the storage elastic modulus curve and the loss elastic modulus curve, the measurement values adjacent to each other are connected with a straight line based on the measurement temperature.

  And it can obtain by reading the temperature T in the intersection of the obtained storage elastic modulus curve and loss elastic modulus curve from the said graph. When the storage modulus curve and the loss modulus curve intersect each other at a plurality of locations, the highest temperature among the temperatures at the plurality of intersections of the storage modulus curve and the loss modulus curve is defined as the storage modulus curve. It is set as the temperature T in the intersection with a loss elastic modulus curve.

  The temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement is a dynamic value commercially available from Reologica Instruments AB under the trade name “DynAlyser DAR-100”. It can be measured using a mechanical viscoelasticity measuring device.

The elastic modulus at the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by the dynamic viscoelasticity measurement of the polylactic acid resin, that is, the storage elastic modulus or the loss elastic modulus is low. While the foamed polylactic acid resin has low viscoelasticity, the foam film may be broken by foaming pressure to cause foam breakage. On the other hand, when the foam is high, the foam film is expanded by the foaming pressure to expand the foam to the desired size. 1.0 × 10 ^{3 to} 1.0 × 10 ⁵ Pa is preferable, and 5.0 × 10 ^{3 to} 9.0 × 10 ⁴ Pa is more preferable. 1.0 × 10 ^{4 to} 8.0 × 10 ⁴ Pa is particularly preferable.

  The elastic modulus (storage elastic modulus or loss elastic modulus) at temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement of the polylactic acid resin is polylactic acid. A method of adjusting the weight average molecular weight of the polylactic acid resin by adjusting the reaction time or reaction temperature during polymerization of the resin, the viscosity average molecular weight of the polylactic acid resin before or during extrusion foaming, The method of adjusting using a crosslinking agent is mentioned.

  The polylactic acid-based resin is supplied to an extruder, melted and kneaded in the presence of a foaming agent, and then extruded and foamed from a mold attached to the tip of the extruder. The form of the extruded foam obtained by extrusion foaming is not particularly limited, and examples thereof include a strand shape and a sheet shape, and a strand shape is preferable.

  The extruder is not particularly limited as long as it is a conventionally used extruder. For example, a single-screw extruder, a twin-screw extruder, and a tandem extruder in which a plurality of extruders are connected. A tandem type extruder is preferable.

  Further, as the foaming agent, those conventionally used are used, for example, chemical foaming agents such as azodicarbonamide, dinitrosopentamethylenetetramine, hydrazoyldicarbonamide, sodium bicarbonate; propane, normal butane, Saturated aliphatic hydrocarbons such as isobutane, normal pentane, isopentane, hexane, ethers such as dimethyl ether, chlorofluorocarbons such as methyl chloride, 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane, monochlorodifluoromethane, Examples thereof include physical blowing agents such as carbon dioxide and nitrogen, dimethyl ether, propane, normal butane, isobutane and carbon dioxide are preferred, propane, normal butane and isobutane are more preferred, and normal butane and isobutane are particularly preferred.

  When the amount of the foaming agent supplied to the extruder is small, the polylactic acid resin pre-foamed particles may not be foamed to the desired foaming ratio. On the other hand, when the amount is large, the foaming agent acts as a plasticizer. Therefore, the viscoelasticity of the melted polylactic acid resin is too low and the foaming property is lowered, and it is not possible to obtain good polylactic acid resin pre-expanded particles, or the expansion ratio of the polylactic acid resin pre-expanded particles is Since it may be too high, 0.1-5 weight part is preferable with respect to 100 weight part of polylactic acid-type resin, 0.2-4 weight part is more preferable, 0.3-3 weight part is especially preferable.

  In addition, it is preferable that a bubble regulator is added to the extruder, but since most of the bubble regulators act as crystal nucleating agents for the pre-expanded particles of polylactic acid resin, crystallization of the polylactic acid resin is promoted. It is preferable to use a non-foaming agent, such as a polytetrafluoroethylene powder or a polytetrafluoroethylene powder modified with an acrylic resin.

  On the other hand, if the amount of the air conditioner supplied to the extruder is small, the bubbles of the polylactic acid resin pre-expanded particles become coarse, and the appearance of the resulting polylactic acid resin foamed molded product may deteriorate. In many cases, foaming occurs when the polylactic acid resin is extruded and foamed, and the closed cell ratio of the pre-expanded particles of the polylactic acid resin may be lowered. Therefore, with respect to 100 parts by weight of the polylactic acid polylactic acid resin 0.01-3 weight part is preferable, 0.05-2 weight part is more preferable, 0.1-1 weight part is especially preferable.

  The mold attached to the extruder is not particularly limited, but a mold capable of forming uniform fine bubbles by extrusion foaming a polylactic acid-based resin is preferable. As such a mold, a nozzle mold is preferable, and a nozzle A multi-nozzle mold having a plurality of is more preferable.

  If the outlet diameter of the nozzle of the multi-nozzle mold is small, the extrusion pressure may become too high and extrusion foaming may be difficult. Therefore, 0.2 to 2 mm is preferable, 0.3 to 1.6 mm is more preferable, and 0.4 to 1.2 mm is particularly preferable.

If the shear rate of the polylactic acid-based resin at the nozzle outlet portion of the nozzle of the nozzle mold is small, the expansion ratio of the polylactic acid-based resin pre-expanded particles decreases or the bubbles of the polylactic acid-based resin pre-expanded particles are coarse. while it is made, the large, because it may not be able fracture is stable extrusion foaming occurs, preferably 1000～30000Sec ^-1, more preferably 2000~25000sec ^-1, 3000~20000sec ^-1 Is particularly preferred.

In addition, the shear rate in the nozzle | cap | die exit part of the nozzle of a nozzle metal mold | die says what was computed based on the following formula.
Shear rate (sec ⁻¹ ) = 4 × Q / (πr ³ )
However, Q is the volume extrusion rate (cm ³ / sec) of the polylactic acid resin (when Q is calculated from the mass extrusion rate (g / sec), the density of the polylactic acid resin is 1.0 g / cm ³ ), r is the radius (cm) of the nozzle.

  In order to reduce fracture, the length of the land portion of the nozzle mold is preferably 4 to 30 times the nozzle outlet diameter of the nozzle mold, and 5 to 20 times the nozzle outlet diameter of the nozzle mold. More preferred. This is because when the length of the land portion of the nozzle mold is smaller than the nozzle outlet diameter of the nozzle mold, fracturing occurs and the foam cannot be stably extruded. This is because if the length of the land portion is larger than the outlet diameter of the nozzle of the nozzle mold, excessive pressure may be applied to the nozzle mold to prevent extrusion foaming.

Furthermore, the resin temperature of the polylactic acid-based resin when extrusion foaming from a die attached to the tip of the extruder preferably satisfies the following formula 4, and more preferably satisfies the following formula 5.
(Temperature T + 40 ° C. at intersection of storage modulus curve and loss modulus curve) ≦ Resin temperature ≦ (Temperature T + 90 ° C. at intersection of storage modulus curve and loss modulus curve) Equation 4
(Temperature T + 50 ° C. at the intersection of the storage modulus curve and the loss modulus curve) ≦ Resin temperature ≦ (Temperature T + 80 ° C. at the intersection of the storage modulus curve and the loss modulus curve) Equation 5

  This is because when the resin temperature of the polylactic acid resin is low, fracture occurs and the appearance of the polylactic acid resin pre-foamed particles decreases or the extrusion load becomes too large to be extruded and foamed from the extruder. On the other hand, if it is high, the melt viscosity of the polylactic acid resin becomes too low, and it may be impossible to obtain good pre-expanded particles of the polylactic acid resin.

  Then, the polylactic acid resin extruded foam extruded from the extruder is cooled to suppress the crystallization of the polylactic acid resin extruded foam, and the polylactic acid resin extruded foam is formed into particles. The polylactic acid-based resin pre-expanded particles obtained by cutting into particles are adjusted to have a crystallinity of less than 30%, preferably 3 to 28%, more preferably 5 to 26%. .

  Here, the degree of crystallinity of the polylactic acid-based resin pre-expanded particles is increased by using a differential scanning calorimeter (DSC) at a temperature increase rate of 10 ° C./min in accordance with the measurement method described in JIS K7121. On the basis of the measured amount of cold crystallization per 1 mg and heat of fusion per 1 mg, the following formula can be used.

  Thus, by adjusting the crystallinity of the obtained polylactic acid resin pre-expanded particles to less than 30%, the fusion property of the polylactic acid resin pre-expanded particles is ensured, and the polylactic acid during in-mold foam molding is ensured. It is possible to improve the fusion property between the expanded particles obtained by expanding the resin-based pre-expanded particles. In addition, during the in-mold foam molding process, the polylactic acid resin pre-expanded particles can be increased in crystallinity to improve the heat resistance of the polylactic acid resin. Excellent fusion and heat resistance.

  The cooling method for the extruded polylactic acid resin foam is preferably a method capable of quickly stopping the increase in the degree of crystallinity of the extruded foam of the polylactic acid resin foam. A method of cooling the foamed polylactic acid resin foam by floating it on the water surface, a method of spraying water or the like on the extruded polylactic acid resin foam foamed from the extruder, and cooling with temperature controlled to a low temperature A method of cooling a polylactic acid resin extruded foam that has been extruded and foamed from an extruder on a plate, a method of blowing a cooled gas such as cold air onto an extruded foam that has been extruded and foamed from an extruder, etc. Can be mentioned. In addition, when making a polylactic acid-type resin extrusion foaming body float on the water surface and cooling, it is preferable to adjust water temperature to 0-45 degreeC.

  Next, polylactic acid resin pre-expanded particles can be obtained by cutting the extruded polylactic acid resin foam as described above into particles. Examples of the cutting machine that cuts the extruded polylactic acid resin into particles include a pelletizer and a hot-cut machine. The cutting methods of the cutting machine include a drum cutter type and a fan cutter type. It is preferable to use a fan-cutter-type cutting method because cracking and chipping are less likely to occur in the extruded polylactic acid resin foam when cutting the extruded lactic acid resin foam. In the above description, the case where the polylactic acid resin extruded foam is cut after cooling the polylactic acid resin extruded foam has been described. However, the polylactic acid resin extruded foam is cut simultaneously with the extrusion foaming from the extruder. Then, the polylactic acid resin foamed particles may be cooled after being made into particles.

When the bulk density of the polylactic acid resin pre-expanded particles obtained in this way is small, the open cell ratio of the polylactic acid resin pre-expanded particles increases, and the polylactic acid resin pre-expanded particles are expanded during foaming in in-mold foam molding. On the other hand, there is a possibility that the foaming force necessary for the foamed particles cannot be imparted. On the other hand, if the foamed particles are large, the bubbles of the resulting polylactic acid resin pre-foamed particles become non-uniform, and the polylactic acid resin during in-mold foam molding 0.03 to 0.5 g / cm ³ is preferable, 0.05 to 0.4 g / cm ³ is more preferable, and 0.07 to 0.3 g because the foamability of the pre-expanded particles may be insufficient. / Cm ³ is particularly preferred.

  If the open cell ratio of the polylactic acid resin pre-expanded particles is high, the polylactic acid resin pre-expanded particles hardly foam at the time of in-mold foam molding, and the fusion property between the polylactic acid resin pre-expanded particles is low. Thus, the mechanical strength of the resulting polylactic acid-based resin foam molded article may be lowered, so that it is preferably 40% or less, more preferably 35% or less, and particularly preferably 30% or less. In addition, adjustment of the open cell ratio of the polylactic acid resin pre-expanded particles is performed by adjusting the extrusion foaming temperature and the amount of the foaming agent.

Here, the open cell ratio of the polylactic acid resin pre-expanded particles is measured in the following manner. First, a sample cup of a volumetric air comparison type hydrometer is prepared, and the total weight A (g) of polylactic acid resin pre-expanded particles in an amount satisfying about 80% of the sample cup is measured. Next, the volume B (cm ³ ) of the entire polylactic acid resin pre-expanded particles is measured by a 1-1 / 2-atm method using a hydrometer. The volumetric air comparison type hydrometer is commercially available, for example, from Tokyo Science Co. under the trade name “1000 type”.

  Subsequently, a wire mesh container is prepared, the wire mesh container is immersed in water, and the weight C (g) of the wire mesh container in the state immersed in the water is measured. Next, after all of the polylactic acid resin pre-expanded particles are placed in the wire mesh container, the wire mesh container is immersed in water, and the wire mesh container and the wire mesh The weight D (g) of the total amount of the polyfoamed resin pre-foamed particles placed in the container is measured.

Then, the apparent volume E (cm ³ ) of the polylactic acid-based resin pre-expanded particles is calculated based on the following formula, and based on this apparent volume E and the total volume B (cm ³ ) of the polylactic acid-based resin pre-expanded particles. The open cell ratio of the polylactic acid resin pre-expanded particles can be calculated by the following formula. The volume of 1 g of water was 1 cm ³ .
E = A + (CD)
Open cell ratio (%) = 100 × (EB) / E

  In addition, if the particle size of the polylactic acid-based resin pre-expanded particles is small, the foamability of the polylactic acid-based resin pre-expanded particles may be reduced at the time of in-mold foam molding. Since the filling property of the polylactic acid resin pre-expanded particles in the mold may be lowered, 1.0 to 5.0 mm is preferable. When the extruded foam is in the form of a strand, and the polylactic acid resin pre-foamed particles are produced by cutting the strand-shaped extruded foam at predetermined intervals in the length direction, the polylactic acid resin pre-foamed particles The length in the direction orthogonal to the cut surface is preferably 5 mm or less.

  Here, the particle diameter of the polylactic acid-based resin pre-expanded particles can be measured directly using a caliper with the diameter of the polylactic acid-based resin pre-expanded particles. In addition, when the extruded foam is in the form of a strand and the polylactic acid resin pre-expanded particles are produced by cutting the strand-shaped extruded foam at predetermined intervals in the length direction, each polylactic acid resin The longest diameter (major axis) and the shortest diameter (minor axis) at the cut surface of the pre-expanded particles are measured, and the length of each polylactic acid diameter resin pre-expanded particle in the direction orthogonal to the cut surface is measured. The arithmetic average value of the major axis, minor axis and length of the resin-based resin pre-expanded particles is defined as the particle size of the polylactic acid-based resin pre-expanded particles.

  The polylactic acid resin pre-foamed particles thus obtained are filled in a mold cavity and heated to foam the polylactic acid resin pre-foamed particles, thereby foaming the polylactic acid resin pre-foamed particles. Foamed polylactic acid resin having desired shape excellent in fusion property and heat resistance by fusing and integrating the obtained foam particles with each other by their foaming pressure and increasing the crystallinity of polylactic acid resin A molded body can be obtained.

  In addition, it does not specifically limit as a heating medium of the polylactic acid-type resin pre-expanded particle with which it filled in the metal mold | die, Hot air etc. are mentioned other than water vapor | steam. If the pressure of the water vapor is low, the crystallinity of the polylactic acid resin pre-expanded particles cannot be sufficiently increased, and the heat resistance of the resulting polylactic acid resin foamed molded product may be lowered, while being high. As a result, the temperature rise of the polylactic acid resin pre-expanded particles becomes abrupt, and the increase in crystallinity of the polylactic acid resin pre-expanded particles cannot keep up with the melting rate of the polylactic acid resin pre-expanded particles. The pre-foamed particles are melted, the foaming pressure is insufficient, and the fusibility between the foamed particles obtained by foaming the polylactic acid-based resin pre-foamed particles is reduced, or the resulting polylactic acid-based resin foam molding is shrunk. May be appropriately adjusted.

  When the crystallinity of the obtained polylactic acid-based resin foamed molded product is low, the heat resistance of the polylactic acid-based resin foamed molded product is reduced. On the other hand, when the crystallinity is high, the polylactic acid-based resin foamed molded product becomes brittle. Therefore, it is preferable to adjust the in-mold foam molding conditions so that it is preferably 30 to 60%, more preferably 32 to 59%, and particularly preferably 34 to 58%. In addition, since the crystallinity degree of a polylactic acid-type resin foaming molding is the same as the measuring method of the crystallinity degree of a polylactic acid-type resin pre-expanded particle, the description is abbreviate | omitted.

Further, the fusion rate of the obtained polylactic acid resin foamed molded article is preferably 40% or more, more preferably 50% or more, and particularly preferably 60% or more. In addition, the fusion rate of a polylactic acid-type resin foaming molding means what was measured in the following way. First, the polylactic acid resin foamed molded body is bent and cut from a predetermined location. Then, the total number N ₁ of foam particles exposed on the cut surface of the polylactic acid resin foam molded article is visually counted, and the foamed particles whose material is broken, that is, the number N ₂ of the divided foam particles is calculated. It can be counted visually and the fusion rate can be calculated based on the following formula.
Fusing rate (%) = 100 × number of particles of expanded foam particles N ₂ / total number of expanded particles N ₁

  Further, the polylactic acid resin pre-expanded particles may be further impregnated with an inert gas at room temperature to improve the foaming power of the polylactic acid resin pre-expanded particles. Thus, by improving the foaming power of the polylactic acid resin pre-foamed particles, the fusion property between the foamed particles obtained by foaming the polylactic acid resin pre-foamed particles during in-mold foam molding is improved. The resulting polylactic acid-based resin foamed molded article has further excellent mechanical strength. Examples of the inert gas include carbon dioxide, nitrogen, and helium.

  In the method for producing pre-expanded particles of polylactic acid resin of the present invention, the polylactic acid resin contains both optical isomers of D-form and L-form as constituent monomer components, and the lesser of D-form or L-form A polylactic acid resin containing less than 5 mol% of the optical isomer, or a polylactic acid resin containing only one of the optical isomers D or L as a constituent monomer component Since the resin is used, the resulting polylactic acid resin pre-expanded particles have high crystallinity and excellent heat resistance.

  And in the manufacturing method of the said polylactic acid-type resin pre-expanded particle, among the polylactic acid-type resin with high crystallinity, the temperature in the intersection of the storage elastic modulus curve obtained by dynamic viscoelasticity measurement and a loss elastic modulus curve A polylactic acid resin having a predetermined relationship between T and melting point (mp) is suitable for extrusion foaming, and the polylactic acid resin is foamed by extrusion foaming. Unlike the case in which the polylactic acid resin particles are once prepared and impregnated with a foaming agent and pre-foamed, heat is not applied after the particles are formed. Therefore, the degree of crystallinity of the polylactic acid resin pre-foamed particles Can be prevented, and the fusion property of the resulting polylactic acid resin pre-expanded particles can be maintained well.

  Furthermore, since the polylactic acid resin pre-expanded particles obtained by the method for producing the polylactic acid resin pre-expanded particles are adjusted so that the crystallinity is less than 30%, In addition to excellent adhesion, the degree of crystallinity of the polylactic acid-based resin particles by heat applied during in-mold foaming, and the adhesion between the expanded particles obtained by expanding the pre-expanded particles of the polylactic acid-based resin The polylactic acid resin foam can be obtained by using the polylactic acid resin pre-foamed particles according to the present invention. The molded body has excellent heat resistance and mechanical strength.

  In the present invention, the bulk density of the polylactic acid-based resin pre-expanded particles and the apparent density of the polylactic acid-based resin foamed molded product are those measured by the following procedure.

(Bulk density of pre-expanded particles of polylactic acid resin)
The bulk density of the polylactic acid-based resin pre-expanded particles refers to that measured according to JIS K6911: 1995 “General Test Method for Thermosetting Plastics”. That is, it measured using the apparent density measuring device based on JISK6911, and measured the bulk density of the polylactic acid-type resin pre-expanded particle based on the following formula.

Bulk density (g / cm ³ ) of pre-expanded particles of polylactic acid resin
= [Mass of measuring cylinder with sample (g) -Mass of measuring cylinder (g)]
/ [Capacity of measuring cylinder (cm ³ )]

(Apparent density of polylactic acid resin foam molding)
The apparent density of the polylactic acid-based resin foamed molded product refers to that measured by the method described in JIS K6767: 1999 “Measurement of foamed plastic and rubber-apparent density”.

Example 1
A tandem type extruder in which a single-screw extruder having a diameter of 50 mm serving as the first stage and a single-screw extruder having a diameter of 65 mm serving as the second stage were connected via a connecting pipe was prepared.

Then, a crystalline polylactic acid resin (trade name “TERRAMAC TE-6100” manufactured by Unitika Co., Ltd., melting point: 167.8 ° C., D-form ratio: 1.5 is added to the first stage extruder of the tandem type extruder. % By weight, L-form ratio: 98.5% by weight, temperature T at the intersection of storage modulus curve and loss modulus curve obtained by dynamic viscoelasticity measurement: 140.3 ° C., dynamic viscoelasticity measurement 100 parts by weight of the elastic modulus (storage elastic modulus or loss elastic modulus) at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained in step (T) or 5.37 × 10 ⁴ Pa) First, 0.1 part by weight of polytetrafluoroethylene powder (trade name “Fluon L169J” manufactured by Asahi Glass Co., Ltd.) was supplied, and then melt-kneaded at 190 ° C. and then gradually heated to 220 ° C. In addition, in FIG. 1, the storage elastic modulus curve and the loss elastic modulus curve obtained by the dynamic viscoelasticity measurement of polylactic acid-type resin were shown.

  Subsequently, in the middle of the first extruder, a polylactic acid-based polylactic acid in a molten state so that butane comprising 35% by weight of isobutane and 65% by weight of normal butane is 0.8 parts by weight with respect to 100 parts by weight of the polylactic acid-based resin. It was press-fitted into the resin and uniformly dispersed in the polylactic acid resin.

Thereafter, the molten polylactic acid-based resin was continuously supplied from the first-stage extruder to the second-stage extruder via a connecting pipe. After the molten polylactic acid-based resin is cooled to a resin temperature of 202 ° C. by a second stage extruder, the shear rate is 5659 sec ⁻¹ from each nozzle of a multi-nozzle mold attached to the tip of the second stage extruder. Extruded and foamed to produce a strand-like polylactic acid resin extruded foam.

  Subsequently, the strand-shaped polylactic acid-based resin extruded foam is cooled by air cooling over a distance of 60 cm from the tip of each nozzle of the multi-nozzle mold, and then the strand-shaped polylactic acid-based resin extruded foam is 2 m. Over the water surface in the cooling water tank and cooled. In addition, the water temperature in a cooling water tank was 30 degreeC.

  The multi-nozzle mold was provided with 15 nozzles having an exit diameter of 1.0 mm, and the land portion had a length of 7 mm. The resin temperature when extrusion foaming from the nozzle of the multi-nozzle mold is such that a breaker plate is inserted between the tip of the second stage extruder and the mold, and a thermocouple is placed in the center of the breaker plate. Was measured by inserting.

The strand-shaped polylactic acid-based resin extruded foam is sufficiently drained, and the polylactic acid-based resin extruded foam is cut into a cylindrical shape every 2.3 mm using a fan cutter type pelletizer. Resin pre-expanded particles were obtained. The obtained polylactic acid resin pre-expanded particles have a bulk density of 0.17 g / cm ³ , a particle size of 1.7 to 2.3 mm, a crystallinity of 25.5%, and open cell The rate was 27.8%.

  Next, the polylactic acid-based resin pre-expanded particles are put in a sealed container, carbon dioxide is pressed into the sealed container at a pressure of 0.49 MPa, and left at room temperature for 24 hours. The resin pre-expanded particles were impregnated with carbon dioxide.

Subsequently, the polylactic acid-based resin pre-expanded particles are filled in a mold and clamped, and water vapor with a gauge pressure of 0.018 MPa is supplied into the mold for 20 seconds. The foamed particles are heated and foamed, and the foamed particles obtained by foaming the polylactic acid resin pre-foamed particles are fused and integrated together. Further, in this state, the mixture is kept warm for 120 seconds and then cooled with water. A rectangular parallelepiped polylactic acid-based resin foam molded article having a length of 300 mm, a width of 400 mm, and a height of 20 mm was obtained. The resulting polylactic acid resin foamed molded article had an apparent density of 0.18 g / cm ³ and a fusion rate of 70%.

(Example 2)
As a polylactic acid resin, a crystalline polylactic acid resin (trade name “TERRAMAC HV-6200” manufactured by Unitika Ltd.), melting point: 167.4 ° C., D-form ratio: 1.5 wt%, L-form ratio: 98.5 Weight%, temperature T at the intersection of storage elastic modulus curve and loss elastic modulus curve obtained by dynamic viscoelasticity measurement, 139.5 ° C., storage elastic modulus curve obtained by dynamic viscoelasticity measurement And the elastic modulus at the temperature T at the intersection of the elastic modulus curve and the loss elastic modulus curve (storage elastic modulus or loss elastic modulus): 4.23 × 10 ⁴ Pa). Polylactic acid-based resin pre-expanded particles were obtained in the same manner as in Example 1 except that it was cooled to 200 ° C.

The obtained polylactic acid resin pre-expanded particles have a bulk density of 0.18 g / cm ³ , a particle size of 1.7 to 2.3 mm, a crystallinity of 22.8%, and open cell The rate was 22.2%. The resulting polylactic acid resin foamed molded article had an apparent density of 0.18 g / cm ³ and a fusion rate of 80%.

(Comparative Example 1)
As the polylactic acid resin, crystalline polylactic acid resin (trade name “TERRAMAC TE-4000” manufactured by Unitika Ltd.), melting point: 170.3 ° C., D-form ratio: 1.4% by weight, L-form ratio: 98.6 Weight%, temperature T at the intersection of storage elastic modulus curve and loss elastic modulus curve obtained by dynamic viscoelasticity measurement: 105.0 ° C., storage elastic modulus curve obtained by dynamic viscoelasticity measurement And a polylactic acid resin preliminary in the same manner as in Example 1 except that the elastic modulus (storage elastic modulus or loss elastic modulus) at the temperature T at the intersection of the elastic modulus curve and the loss elastic modulus curve is 1.17 × 10 ⁴ Pa). Expanded particles were obtained.

  However, in the strand-like polylactic acid resin extruded foam obtained by extrusion foaming from an extruder, gas loopholes accompanying breakage occur intermittently, and polylactic acid resin pre-expanded particles are stably produced. I couldn't.

(Comparative Example 2)
A polylactic acid resin in the same manner as in Example 1 except that the air-cooling distance of the extruded polylactic acid resin foam in the form of a strand extruded from the tip of each nozzle of the multi-nozzle mold was set to 1.5 m instead of 60 cm. Pre-expanded particles were obtained.

The obtained polylactic acid resin pre-expanded particles have a bulk density of 0.17 g / cm ³ , a particle size of 1.7 to 2.3 mm, a crystallinity of 30.5%, and open cell The rate was 28.1%. The resulting polylactic acid resin foamed molded article had an apparent density of 0.18 g / cm ³ and a fusion rate of 10%.

2 is a graph showing a storage elastic modulus curve and a loss elastic modulus curve obtained by dynamic viscoelasticity measurement of the polylactic acid resin used in Example 1. FIG.

Claims (2)

By supplying polylactic acid-based resin to an extruder to produce a melt-kneaded extrusion foaming and extrusion foamed in the presence of a blowing agent to produce pre-expanded particles by cutting the extruded foam particulate mold In the method for producing pre-expanded polylactic acid resin particles for inner foam molding, the polylactic acid resin contains both D-form and L-form optical isomers as constituent monomer components, and D-form or L-form The content of the smaller optical isomer is less than 5 mol%, or contains only one of the optical isomers D or L as a constituent monomer component, The melting point (mp) in the lactic acid-based resin and the temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement satisfy the following formula 1, and the polylactic acid resin preliminary The crystallinity of the expanded particles is less than 30% Mold foam method for manufacturing a molded polylactic acid resin pre-expanded particles, characterized in that adjusted to.
(Melting point of polylactic acid resin (mp) -40 ° C)
≦ (temperature T at the intersection) ≦ melting point of polylactic acid resin (mp) Formula 1 The elastic modulus of the polylactic acid resin at a temperature T at the intersection of the storage elastic modulus curve and the loss elastic modulus curve obtained by dynamic viscoelasticity measurement in the polylactic acid resin is 1.0 × 10 ³ to 1. It is 0x10 < ⁵ > Pa, The manufacturing method of the polylactic acid-type resin pre-expanded particle for in-mold foam molding of Claim 1 characterized by the above-mentioned.

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