JP3350605B2 - Method for producing polylactic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing polylactic acid suitable for forming a high-strength fiber, film and melt-extruded product which can be used for clothing and industrial materials. More specifically, the present invention relates to a method for producing polylactic acid by melt ring-opening polymerization of L and / or D-lactide.

[0002]

2. Description of the Related Art Polylactic acid is produced from lactic acid produced by grain fermentation using lactide, which is a cyclic dimer, as a raw material by a melt polymerization method, and is excellent as an inexpensive and inexhaustible plastic. It is expected to be put to practical use along with its physical properties. It is well known that polylactic acid can be obtained by ring-opening polymerization using lactide as a raw material. It is also well known that water in a system usually acts as a polymerization initiator. It has also been pointed out that such polymerization is promoted by the catalysis of a tin compound, while on the other hand, the molecular chain is cleaved from the terminal portion and cyclized again. Therefore, it was very difficult to obtain polylactic acid having a molecular weight of 50,000 or more in the conventional technology. For example, according to JP-A-5-255488, polylactic acid having a crystallinity of 10% or more is heated at a temperature higher than its glass transition temperature and lower than its melting point to obtain polylactic acid having a molecular weight of about 50,000. It is disclosed. As other solutions, studies have been made on catalysts and polymerization methods, such as synthesis of high molecular weight polylactic acid by direct dehydration polycondensation reaction of lactic acid (JP-A-5-43665). However, the following problems are also known in the conventionally obtained polylactic acid, and the solution is awaited. Poor melt moldability. Poor thermal stability and easy to depolymerize. The obtained films, fibers, molded articles, etc. are inferior in toughness, brittle and weak. To solve these problems, establish a high-polymerization degree polylactic acid production recipe, polymerize lactide monomers in high yield to minimize the amount of residual monomers, cut the molecular chains of polylactic acid during heating and melting, and terminate polylactic acid. It is considered effective to suppress the regeneration of lactide due to cyclization.

Hitherto, many techniques have been proposed for producing polylactic acid, but there has been no way to provide an excellent polylactic acid that has solved the above-mentioned problems. JP-A-63-
Japanese Patent No. 69825 discloses an attempt by copolymerization with polyoxyethylene dicarboxylic acid, but only a polymer having a low degree of polymerization and a low strength has been obtained. JP-A-6-65360 discloses that a lactic acid oligomer is polymerized in a high boiling point and hydrophobic solvent to obtain polylactic acid having a molecular weight of 60,000 to over 180,000. Although the physical properties of the present polylactic acid are excellent, the fact that a large amount of a solvent is required and a step of purifying the polymer is required may possibly be a bottleneck in aiming at inexpensive industrial production.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a novel method for producing polylactic acid having excellent melt moldability and thermal stability. Specifically, it is an object of the present invention to provide a method for producing polylactic acid having an average molecular weight of 100,000 or more and a residual lactide amount of 3% by weight or less.

The present inventors have paid attention to the economics of the ring-opening polymerization of lactide, and have studied the polymerization step and copolymerization recipe. For example, in the case of copolymerization with polyethylene glycol, the OH group at the molecular terminal acts as an initiator of ring-opening polymerization of lactide instead of moisture, and a polylactic acid chain is bonded to both ends of polyethylene glycol as a putative structure. A block copolymer is obtained. However, the molecular weight of the block copolymer is only about 100,000 at most, and the melt viscosity is still insufficient for spinning or injection / extrusion molding.
One of the reasons is that a large amount of lactide remains in the polymerization product. It is necessary to further increase the degree of polymerization and reduce low molecular weight components such as lactide.

[0006]

Means for Solving the Problems The present inventors have analyzed the fundamental causes of the above problems 1 to 5 and the problems of the prior art, and as a result, have found a novel method for producing polylactic acid having excellent melt moldability. The present invention has been completed. That is, in the present invention, L- and / or D-lactide is used as a polymerization raw material, inorganic fine particles and / or an organic component is used as a crystallization nucleating agent, and the crystallization nucleating agent is used in an amount of 0.01 to 1. A first step of melt-polymerizing the polymerization raw material composition containing 0% by weight; a second step of shifting the polymerization temperature of the first step to a predetermined temperature to cool and solidify the molten polymerization product; A method for producing polylactic acid, comprising a third step of performing solid phase polymerization while maintaining the polymerization product at a predetermined temperature, following the two steps. In the present invention, the crystallization nucleating agent may be added to the polymerization raw material before the start of the polymerization in the first step, or may be added at an appropriate time from the middle of the first step to before the second step. In the melt polymerization step, the degree of polymerization of the polylactic acid in a homogeneously mixed state of polylactic acid, lactide and crystallization nucleating agent is achieved, and in the cooling / solidification step, the crystallization nucleating agent rapidly solidifies. Therefore, the polylactic acid polymer will be fixed while maintaining the mixed state of the melt polymerization step while eliminating the polymerization inhibition factors, and in the solid phase polymerization step, the polylactic acid chain forms a crystal phase and is stabilized, while the polymer is stabilized. Condensation of unreacted low-molecular components such as lactide at the terminal end into an amorphous phase, and further promoting the polymerization of polylactic acid by increasing the chance of reaction between the terminal groups and low-molecular components in the amorphous phase It is. Needless to say, the polymerization raw material composition can contain various lactide polymerization catalysts represented by tin compounds. Although the crystallization nucleating agent in the present invention does not contain a lactide polymerization catalyst, the use of a lactide polymerization catalyst having a crystallization nucleating agent effect in addition to the polymerization catalysis is particularly effective. In the present invention, when the optical purity of lactide, which is a polymerization raw material, is low, the crystallinity of the obtained polylactic acid is reduced, so that the effect on the crystal phase formation by the crystallization nucleating agent in the above step cannot be obtained. Therefore,
The molar ratio of L / D or D / L lactide in the polymerization raw material is desirably in the range of 0.1 to 0.01.

In the first step, the general formula (-O (M)-) _n
And at least one compound selected from the group consisting of polyalkylene glycols, polyhydric alcohols and cyclic lactones. Here, n is a positive integer, M is a hydrocarbon chain group having two or more methylene groups, for example, ethylene group, propylene group, butylene group,
Represents an isopropylene group or the like. Some polyhydric alcohols
Glycerin, trimethylolpropane, 1,4-cyclohexanedimethanol, neopentyl glycol and the like are included. Representative examples of the cyclic lactone include caprolactone, propiolactone, valerolactone, and pivalolactone. By blending these materials with the polymerization raw material, a copolymer with polylactic acid can be obtained. Compared with the homopolymer of polylactic acid, this copolymer can be expected to have effects such as improvement in fluidity at the time of melting and improvement in hydrophilicity of the polymer.

The inorganic fine particles used in the present invention have a primary particle size of 0.1 from the viewpoint of dispersibility in polylactic acid.
Those having a size of μm or less are preferred. In the case of inorganic particles larger than 0.1 μm, the dispersibility becomes insufficient and the effect of promoting crystallization decreases, which is not preferable. As such inorganic fine particles, metal oxides, carbonates, sulfates, hydroxides, halides, and natural mineral compounds are preferably used. More specific examples include titanium oxide, aluminum oxide, iron oxide, zinc oxide, silicon oxide, calcium carbonate, barium sulfate, aluminum hydroxide, sodium chloride, calcium fluoride, mica, zeolite, kaolin, clay, talc, etc. Can be mentioned. These components are present in polylactic acid and lactide stably even at a high temperature of room temperature or higher, and do not chemically react with either polylactic acid or lactide. Next, the organic component used in the present invention is a thermoplastic synthetic polymer having a softening point of 50 ° C. or higher, which is dispersed in fine particles having a particle size of 0.1 μm or less in molten polylactic acid. Regarding the particle size, for the same reason as the case of the inorganic fine particles, those having a size larger than 0.1 μm are not preferable. Further, those having a softening point of less than 50 ° C. cannot not only obtain the effect as a crystallization nucleating agent, but also
Since it acts as a plasticizer in the generated polylactic acid, the inherent properties of polylactic acid may be impaired. Such organic components include synthetic polymers such as polyethylene, ethylene-propylene copolymer, aliphatic polyester, polyacrylic polymer, polystyrene and polybutadiene, and higher fatty acid metal salts such as magnesium stearate and calcium stearate (carbon 8 or more), silicone oils, esters of higher fatty acids and linear aliphatic alcohols, and others. These components are not decomposed or chemically reacted at the polylactic acid melt temperature (130 ° C. or higher) and are dispersed in fine particles in the polylactic acid melt.

The amount of the crystallization nucleating agent is preferably 1% or less. When added in excess of 1%, the degree of light scattering of the polylactic acid is increased, resulting in an opaque feeling. In addition, the flow resistance at the time of melting is not stabilized, and the moldability may be impaired, which may not meet the original purpose of modifying polylactic acid. The presence of the present crystallization nucleating agent promotes rapid solidification and crystallization of polylactic acid in the second step and the third step. However, when the added amount exceeds 1%, uneven crystal growth occurs. In addition, the degree of polymerization by the solid-phase polymerization in the third step does not easily increase. The crystallization nucleating agent must be added in an amount of at least 0.01%. In the first step, polylactic acid is produced by melt polymerization,
It acts as a nucleating agent for crystal formation in the process of cooling and solidifying polylactic acid in the process, and requires at least 0.01% of addition. That is, the amount of the crystallization nucleating agent is preferably in the range of 0.01% to 1.0%.

At the beginning of the first step, it is important to prepare a polymerization raw material composition by adding a crystallization nucleating agent composed of an inorganic and / or organic component and uniformly dispersing the lactide in the polymerization raw material in a molten state. is there. Preferably, inorganic particles are dispersed in a single particle state, and organic components are in a molecularly mixed state with polylactic acid. For this purpose, forced stirring is effective. One of the industrially advantageous methods is to use a twin-screw extruder at a temperature of 180 ° C. to 220 ° C. as a first step.

The setting of the temperature conditions in the second step is a key point in obtaining polylactic acid desired to be optimized for each polymerization raw material composition in the third step. For example, when the polymerization temperature of the first step is higher than the melting point of polylactic acid, the temperature is adjusted to the temperature range of the second step by slow or rapid cooling. In the second step, it is important that the polymerization inhibiting factor of lactide is not incorporated into the melt polymerization product in the first step. Therefore, it is effective to increase the crystallinity of polylactic acid to at least 50% or more. It is. The action of the crystallization nucleating agent increases the rate of crystal precipitation in the process of cooling from the molten state and increases the degree of crystallinity, but the method of repeating the temperature increase and cooling in the temperature range of the second step is more effective. If the temperature of the second step is lower than 70 ° C., the crystallization rate of polylactic acid becomes extremely slow, and a high crystallinity cannot be obtained within a practical time. Further, when the temperature exceeds 160 ° C., crystallization proceeds but depolymerization also occurs, which is disadvantageous in that the degree of polymerization is reduced. The temperature range of the second step is 70 ° C. to 1
60 ° C. is desirable. Preferably, 75 ° C to 155 ° C is appropriate. The temperature in the third step can be appropriately selected depending on the relationship between the residual amount of lactide and the reaction time. The temperature of the third step is 100
If the temperature is lower than ℃, the reaction rate is low, and productivity is largely inhibited. On the other hand, when the temperature of the third step is higher than 150 ° C., the reaction rate is high, so that the target molecular weight can be obtained in a short time, but the residual amount of lactide is large,
Usually results of more than 3%. Preferably, the temperature of the third step is from 100C to 150C, more preferably from 110C.
140 ° C. range.

The first to third steps may be continuous or each may be separated in a batch manner. It is preferable that an inert atmosphere such as nitrogen gas be maintained during each step and between steps. Since water, oxygen, and other trace gases in the air react with the polymerization raw material composition or the polymerization product to induce polymerization inhibition, a desired polylactic acid cannot be obtained in many cases.

[0013]

The crystallization nucleating agent in the form of fine particles uniformly dispersed in the first step promotes crystallization in the second step and achieves rapid solidification. As a result, the contamination of the polymerization product with the polymerization-inhibiting component is eliminated, and naturally, unreacted low-molecular components such as polylactic acid molecular terminals and lactide are condensed in the amorphous region, and solid-phase polymerization proceeds in this region. Easier to do. The low-molecular-weight components such as lactide are reduced by the solid-phase polymerization, and the degree of polymerization is further increased from the melt polymerization. By applying this step, polylactic acid having a weight average molecular weight of 100,000 or more and a low molecular weight component lactide content of less than 3% can be obtained.

[0014]

【Example】

(Example 1) To 1000 parts by weight of L-lactide, 1 part of calcium carbonate having an average primary particle diameter of 0.05 µm and tin octylate as a catalyst were charged into a melt polymerization vessel whose internal atmosphere was replaced by a nitrogen stream, While stirring, the temperature of the polymerization vessel was raised to 200 ° C., and the temperature was maintained for 3 hours. Then
As a second step, the temperature of the polymerization vessel was lowered to 140 ° C. over 1 hour. In the second step, the contents of the polymerization vessel were solidified and crystallized, so that the stirring was stopped. After holding at 140 ° C. for another 1 hour to promote crystallization, it was cooled to 130 ° C. in a short time within 10 minutes. In the subsequent third step, 130 ° C. and 5
The contents of the polymerization kettle were held for a period of time. After the completion of the third step, the content was cooled to room temperature, the content in the reaction vessel was taken out, and the properties of the polymer were analyzed. As shown in the analysis results in Table 1, the obtained polylactic acid had a weight average molecular weight of 150,000 and a low molecular weight component of 1.
The product had a high degree of polymerization of 5%. Comparative Example No. 1 in Table 1. 1-5
Was obtained by adding calcium carbonate in the first step and polymerizing under the same other process conditions. Instead of calcium carbonate, fine particle silica, magnesium stearate,
The polyethylene micropowder was added to each
o. 1-2, No. 1-3, No. 1-4.
In addition, when the low molecular weight component was analyzed by FT-NMR, the raw material lactide was the main component.

[0015]

[Table 1]

(Example 2) The first step in Example 1 is a melting / kneading extrusion method using a 30 mm diameter twin-screw extruder, and the polymer extruded from the first step is placed on a chill roll in a nitrogen stream. The step of solidifying the mixture in a pellet and then lowering the temperature to a predetermined temperature up to 150 ° C. for crystallization is referred to as a second step, and the step of solid-state polymerization of the crystallized product in the second step in a fluidized bed heating tank is referred to as a third step. By the continuous polymerization method as the process, polylactic acid was obtained from various polymerization compositions as raw materials. Table 2 summarizes the main operating conditions of each step and the physical property values of the polymerization products.

[Table 2]

In Table 2, no. No. 2-5 is a comparative example. 2-1 to 4 are products of the present invention. Each of the products of the present invention has a high average molecular weight and a low low molecular weight.

Example 3 In Example 2, glycerin, trimethylolpropane, and neopentyl glycol were used in place of polyethylene glycol, and the crystallization nucleating agent and the conditions of each of the first to third steps were appropriately changed. And produced several kinds of polylactic acid. Table 3 summarizes these results. In Table 3, component A is a component used as a polymerization raw material in the first step in place of polyethylene glycol.

[Table 3]

(Example 4) 0.1 to 10 mol% of D-lactide based on L-lactide
0.03% by weight of fine-particle silica having an average primary particle diameter of 0.01 μm as a crystallization nucleating agent was blended as a crystallization nucleating agent with respect to the polymerization raw material prepared to have a predetermined molar ratio in the range of the first step to the third step of Example 1. D / L copolymerized polylactic acid was produced under operating conditions according to the process. Table 4 shows the physical property values of the D / L copolymerized polylactic acid. Comparative Example No. 4-5 is a physical property of a polylactic acid produced without blending finely-divided silica.

[0020]

[Table 4] In Table 4, no. 4-5 shows no apparent melting point and is 100
It was a broad peak centered at ° C.

[0021]

According to the method of the present invention, crystallization is promoted by fine particles as nuclei, crystallinity before solid phase polymerization is maximized, and unreacted monomer and polymer are selectively formed in the amorphous phase. The effect of precipitating with the terminal is obtained, and the polymerization in the amorphous phase is promoted. As a result, a highly polymerized polylactic acid that could not be obtained by the conventional method can be produced. Since the product of the present invention has a high molecular weight of 100,000 or more and a low molecular weight component as small as 3% or less, the viscosity at the time of melting is high, so that the spinnability in melt spinning is good. In addition, the adhesion of low-molecular components to nozzles and other spinning machines is small, and operability is superior. In terms of physical properties of the obtained polymer, the glass transition temperature is 55 ° C. or higher, and the heat resistance is excellent.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hitomi Ohara, 1 Nishinokyo Kuwabaracho, Nakagyo-ku, Kyoto Shimazu Works Sanjo Plant (72) Inventor Eiichi Koseki 1, 1 Nishinokyo Kuwabaracho, Nakagyo-ku, Kyoto Shimazu Works Sanjo In-plant (72) Inventor Seiji Sawa 1 Nishinokyo Kuwabaracho, Nakagyo-ku, Kyoto Shimazu Corporation Sanjo Plant (72) Inventor Yasuhiro Fujii 1 Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto Shimazu Plant Sanjo Plant (56) Reference Literature US Patent 5,359,027 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08G 63/00-63/91

Claims (7)

(57) [Claims] 1. A first step of melt-polymerizing L- and / or D-lactide as a raw material for polymerization, and a step of cooling and solidifying a molten polymerization product by shifting from a polymerization temperature of the first step to a predetermined temperature. In the method for producing polylactic acid, which comprises two steps and a third step in which the polymerization product is maintained at a predetermined temperature and then solid-phase polymerization is performed after the second step, the crystallization comprising inorganic fine particles and / or organic components is carried out. A method for producing polylactic acid, comprising adding a nucleating agent in an amount of 0.01 to 1.0% by weight based on a polymerization raw material. 2. An inorganic fine particle having a primary particle size of 0.1
μm or less is an oxide, fluoride carbonate The process according to claim 1, wherein the polylactic acid which is characterized in that comprises at least one compound selected Ri by sulfates, hydroxides and halides thereof. 3. The method according to claim 1, wherein the organic component is a thermoplastic synthetic polymer having a softening point of 50 ° C. or higher dispersed in a fine particle of 0.1 μm or less in a melt of polylactic acid. A method for producing the polylactic acid according to the above. Wherein the molar ratio of L / D or D / L-lactide in the polymerization raw material, the production of polylactic acid according to claim 1, characterized in that a 0.1 to 0.01 Method. 5. A first step of melt polymerization and a second step of cooling / cooling.
The solidification and the solid phase polymerization of the third step are all performed in an inert atmosphere .
The process for preparing polylactic acid according to. 6. The polylactic acid according to claim 1 , wherein the first step is performed by a melt-kneading method at a temperature of 180 ° C. to 220 ° C. using a twin-screw extruder. Manufacturing method. 7. The method according to claim 1, wherein at least one component selected from the group consisting of polyalkylene glycols, polyhydric alcohols and cyclic lactones is added to the polymerization raw material during the first step. 7. The method for producing polylactic acid according to any one of 6.