JPH07118259A - Method for purifying lactide and method for polymerizing the same - Google Patents

Abstract

PURPOSE:To obtain a refined lactide having a high purity, readily polymerizing, easily providing a polylactic acid having an extremely high molecular weight, by recrystallizing a lactide with a lower alcohol, then with benzene or an alkyl- substituted group-containing benzene. CONSTITUTION:An unpurified lactide is added to a lower alcohol (e.g. isopropyl alcohol), heated to 70 deg.C and completely dissolved. When the solution is cooled to 30 deg.C, a crystal is precipitated and filtered. The prepared crystal is washed with water or isopropyl alcohol. The prepared lactide is added to benzene or an alkyl-substituted benzene (e.g. toluene) and heated to 70 deg.C to completely dissolve the lactide. The solution is cooled to 30 deg.C, a crystal is precipitated and filtered. The prepared crystal is washed with a solvent and dried under reduced pressure to give the high-purity refined lactide providing a polylactic acid having an extremely high molecular weight.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for purifying lactide and a method for polymerizing the same. Lactide is particularly useful as a raw material for producing polylactic acid.

[0002]

2. Description of the Related Art As a method for purifying lactide, solubility of isopropyl alcohol or the like has a high temperature dependence of solubility,
A recrystallization method using a solvent in which impurities are easily dissolved is known. For example, JP-A-63-101378 describes recrystallization from an alcohol having 1 to 6 carbon atoms, preferably isopropyl alcohol, or precipitation from a solution using a non-solvent. However, when the lactide purified by such a method is used for polymerization, the resulting polylactic acid has a molecular weight of about 1000 to 5000, and it is difficult to obtain a high molecular weight polylactic acid having a molecular weight of 100,000 or more.

An object of the present invention is to provide a lactide and a method for polymerizing the same, which makes it possible to easily obtain polylactic acid having a high purity and a very large molecular weight.

[0004]

The present invention provides a method for purifying lactide, which comprises recrystallizing lactide with a lower alcohol and then recrystallizing with benzene or an alkyl-substituted benzene. The present invention also provides a method for polymerizing lactide, which comprises polymerizing the lactide obtained by the above-mentioned purification method.

The purification method of the present invention can be applied to lactide obtained by a conventionally known method. Lactide is first recrystallized in lower alcohol. Examples of such lower alcohols include methanol, ethanol, n-propyl alcohol, isopropyl alcohol (2-propanol), butanol, pentanol and the like, and isopropyl alcohol is particularly preferable.

Recrystallization with these lower alcohols
For example, a solvent is added to the crude lactide in an amount of 1 to 4 times (weight ratio) up to about 60 to 70 ° C. to dissolve all the crystals, and then cooled to 30 ° C. or less to obtain crystals. The obtained lactide crystal is then washed with water, isopropyl alcohol, or a mixed solvent thereof. Washing is performed at 20 to 40 ° C. using an equal amount or more of solvent.

Further, the lactide crystal is recrystallized using benzene or an alkyl-substituted benzene as a recrystallization solvent.
Examples of such a solvent include benzene, toluene, xylene, ethylbenzene and isopropylbenzene, with toluene being particularly preferred. Recrystallization using these aromatic solvents is performed in the same manner as the recrystallization using the lower alcohol. That is, using 1 to 4 times (weight ratio) or more solvent to the crude lactide, from room temperature to 60 to 70 ° C.
After raising the temperature to a certain degree to dissolve all the crystals, the crystals are obtained by cooling to 30 ° C. or lower. Then, the obtained lactide crystals are washed with one or more of the aromatic solvents. Washing is performed at 20 to 40 ° C. using an equal amount or more of solvent.

The lactide thus obtained has a very low concentration of isopropyl alcohol, which is below the detection limit.

The lactide obtained by the purification method of the present invention can be polymerized in a conventional manner. For example, using a device having a stirring and feeding function such as a twin-screw kneading extruder with a vent, the reaction is carried out while stirring, mixing, moving and degassing, and then the polymer is continuously taken out. As the polymerization catalyst, tin powder, tin halide or an organotin compound derived from a carboxylic acid having 20 or less carbon atoms, or zinc powder, zinc halide or an organozinc compound derived from a carboxylic acid having 20 or less carbon atoms is used. However, tin octylate is particularly preferable. The amount of catalyst used is 0.1 to 1.0 with respect to lactide.
Polymerization is carried out at a temperature of 130 to 230 ° C., preferably 160 to 200 ° C. while continuously supplying to the kneader.

The polylactic acid thus obtained has a very high molecular weight of 100,000 or more. That is, it is presumed that lactide produced by the conventional purification method has residual isopropyl alcohol, which inhibits the polymerization reaction.
On the other hand, the lactide obtained in the present invention has less residual isopropyl alcohol and is considered to be easily polymerized.

[0011]

EXAMPLES Next, the present invention will be described more specifically based on examples.

Example 1 To 3 kg of unpurified lactide, 3 kg of isopropyl alcohol was added and heated to 70 ° C. to completely dissolve the lactide. Then, the solution was cooled to 30 ° C., and crystals were precipitated and filtered. Further, cleaning was performed using about 10 kg of water and 3 kg of isopropyl alcohol.

The lactide thus obtained was further purified. That is, add about 3.5 kg of toluene, and add 7
The lactide was completely dissolved by heating to 0 ° C. Then,
This toluene solution was cooled to 30 ° C., crystals were deposited and filtered. Further, it was washed with about 6 kg of toluene. The obtained lactide crystals were dried under reduced pressure (yield about 1.9).
kg).

To the obtained lactide, tin octylate was added in an amount of 0.1.
When 24% was added and the mixture was continuously polymerized at 190 ° C. with a kneader, polylactic acid having a weight average molecular weight of 150,000 was obtained.

Example 2 1.9 kg of unpurified lactide
2.8kg of isopropyl alcohol was added to
The lactide was completely dissolved by heating to 0 ° C. Then,
The solution was cooled to 30 ° C., crystals were deposited and filtered. Further, cleaning was performed using about 8.4 kg of isopropyl alcohol.

The lactide thus obtained was further purified. That is, add 2.8 kg of toluene, and add 70
The lactide was completely dissolved by heating to ℃. Then, this toluene solution was cooled to 30 ° C., crystals were deposited and filtered. It was further washed with about 5.6 kg of toluene. The crystals of lactide obtained were dried under reduced pressure (yield approx.
95 kg).

To the obtained lactide, tin octylate was added in an amount of 0.1.
When 24% was added and the mixture was continuously polymerized at 190 ° C. with a kneader, polylactic acid having a weight average molecular weight of 150,000 was obtained.

Comparative Example 1 1.0 kg of unpurified lactide
To 1.0 isopropyl alcohol, add 7
The lactide was completely dissolved by heating to 0 ° C. Then,
The solution was cooled to 30 ° C., crystals were deposited and filtered. Further, cleaning was performed using about 9.0 kg of water and 1.0 kg of isopropyl alcohol. The same operation was further repeated twice, a total of three times, and the obtained lactide crystals were dried under reduced pressure (yield about 0.52 kg). When 0.24% of tin octylate was added to the obtained lactide and the mixture was continuously polymerized at 190 ° C. with a kneader, the polymerization hardly proceeded.

[0019]

EFFECTS OF THE INVENTION The lactide obtained by the purification method of the present invention has a high purity and is apt to undergo polymerization, so that polylactic acid having a very large molecular weight can be easily obtained.

Claims (2)

[Claims] 1. A method for purifying lactide, which comprises recrystallizing lactide with a lower alcohol and then recrystallizing with benzene or an alkyl-substituted benzene. 2. A method for polymerizing lactide, which comprises recrystallizing lactide with a lower alcohol and then recrystallizing with benzene or an alkyl-substituted benzene to polymerize the obtained lactide.