CN111458453A

CN111458453A - Method for testing hydroxyl value in lactide-containing polylactic acid and application thereof

Info

Publication number: CN111458453A
Application number: CN202010399590.5A
Authority: CN
Inventors: 邱化敏; 王家发; 周芸; 李晶; 孙双翼; 乔义涛
Original assignee: Wanhua Chemical Group Co Ltd; Wanhua Chemical Sichuan Co Ltd
Current assignee: Wanhua Chemical Group Co Ltd; Wanhua Chemical Sichuan Co Ltd
Priority date: 2020-05-12
Filing date: 2020-05-12
Publication date: 2020-07-28
Anticipated expiration: 2040-05-12
Also published as: CN111458453B

Abstract

The invention belongs to the field of polymer analysis and test, and particularly relates to a method for testing a hydroxyl value in lactide-containing polylactic acid and application thereof, wherein the method comprises the following steps: (1) contacting a sample to be detected with an acid anhydride solution containing organic base, and carrying out heating reflux treatment; then cooling to the temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; then carrying out alkali standard solution titration; carrying out blank operation according to the same steps; obtaining the apparent hydroxyl value of the sample to be tested through the titration test values of the two; (2) contacting a sample to be detected with organic alkali, and carrying out heating reflux treatment; then cooling to the temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; titrating the alkali standard solution to obtain the apparent acid value; (3) and adding the apparent hydroxyl value and the apparent acid value to obtain the actual hydroxyl value of the sample. The testing method can accurately and stably measure the hydroxyl value in the lactide-containing polylactic acid.

Description

Method for testing hydroxyl value in lactide-containing polylactic acid and application thereof

Technical Field

The invention belongs to the field of polymer analysis and test, and particularly relates to a method for testing a hydroxyl value in lactide-containing polylactic acid and application thereof.

Background

Polylactic acid (Polylactic acid or Polylactic acid, P L A for short) is a biodegradable polyester, has important application in the aspects of textile, food packaging, drug sustained release, tissue engineering and the like, and has wide application prospect in the fields of automotive interior, building materials and the like.

Generally, P L a is prepared primarily by the ring opening polymerization of lactide, with the reaction formula shown below:

the ring-opening polymerization method for preparing P L A takes lactide as a monomer, and usually requires adding an initiator and a catalyst during the polymerization process.

For example, patent document CN104892916A discloses a polymerization method for preparing high molecular weight polylactic acid using ethanol or lauryl alcohol as a starting material in the ring-opening polymerization method for preparing high molecular weight polylactic acid, the number average molecular weight (Mn) of the target product P L a satisfies:

wherein,

[MO]is the number of moles of monomer, M_moIs the molar mass of the monomer; [ MO ]]＝m_mo/M_mo，m_moIs the mass of the monomer;

[ROH]as moles of initiator, M_ROHIs the molecular weight of the initiator; [ ROH ]]＝m_ROH/M_ROH，m_ROHIs the mass of the starter.

One of the characteristics of the ring-opening polymerization method is that the molecular weight of the product is extremely sensitive to the amount of the initiator (e.g., alcohol), and the higher the target molecular weight, the less the amount of the alcohol required, and the more sensitive to the fluctuation of the alcohol content. In order to improve the production stability, it is necessary to precisely determine the alcohol content in the polymerization system.

In the ring-opening polymerization, alcohol is consumed after initiation of polymerization, and thus the actual amount of the initiator (for example, alcohol) used in the polymerization cannot be directly measured by gas chromatography or the like. However, according to the principle of ring-opening polymerization, the hydroxyl group in the initiator (e.g., alcohol) is not consumed in the polymerization reaction, and the resulting polymer is hydroxyl-terminated. Therefore, the hydroxyl content in the system can be directly measured, and important indexes such as theoretical molecular weight of the product obtained in the polymerization system can be further calculated.

Hydroxyl number in conventional polymer systems can be determined by HGT 2709 "determination of hydroxyl number in polyester polyol" or GBT12008.3 "part 3 of plastic polyether polyol: measurement of hydroxyl value "was carried out by the method described in (1). The lactide-containing polylactic acid polymer system comprises hydroxyl-terminated polylactic acid and unreacted lactide. The polylactic acid contains ester groups and hydroxyl groups in the molecular structure, and the lactide only contains ester groups and does not contain hydroxyl groups. The molecular structure of the polyester polyol also comprises ester groups and hydroxyl groups. Therefore, the polylactic acid polymer system is closer to the polyester polyol system.

However, in actual tests, it was found that either HGT 2709, determination of hydroxyl number in polyester polyol, or GBT12008.3, part 3 of the plastic polyether polyol: the hydroxyl value of a polylactic acid polymer system containing lactide cannot be accurately and stably measured by any of the methods described in "measurement of hydroxyl value". Therefore, it is necessary to develop a method for accurately testing the hydroxyl value of a lactide-containing polylactic acid polymer system according to the characteristics of the system.

Disclosure of Invention

The invention aims to provide a method for testing the hydroxyl value in the lactide-containing polylactic acid and application thereof aiming at the particularity of the lactide-containing polylactic acid polymer system in the hydroxyl content test, wherein the method can accurately and stably test the hydroxyl value in the lactide-containing polylactic acid and obtain the theoretical number average molecular weight of the lactide-containing polylactic acid polymer system under the condition of knowing the functionality of an initiator.

In order to achieve the above purpose, the invention provides the following technical scheme:

in one aspect, there is provided a method for testing hydroxyl number in lactide-containing polylactic acid, comprising the steps of:

(1) contacting a polylactic acid sample to be detected with an acid anhydride solution containing organic base, and carrying out heating reflux treatment (namely, under the catalysis of the organic base, the polylactic acid is subjected to derivatization through acid anhydride); cooling the system to a temperature of less than or equal to 30 ℃ (e.g., 10 ℃, 20 ℃, 25 ℃, 29 ℃), and adding water for hydrolysis reaction; titrating by using an alkali standard solution to obtain a titration test value of the polylactic acid sample to be tested;

carrying out blank sample operation according to the same steps to obtain a titration test value of the blank sample;

obtaining the apparent hydroxyl value of the polylactic acid sample to be tested through the titration test values of the two;

(2) contacting a polylactic acid sample to be detected with the organic alkali, and carrying out heating reflux treatment; cooling the system to a temperature of less than or equal to 30 ℃ (e.g., 10 ℃, 20 ℃, 25 ℃, 29 ℃), and adding water for hydrolysis reaction; then titrating by using an alkali standard solution to obtain the apparent acid value of the polylactic acid sample to be detected;

(3) and (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the polylactic acid sample to be detected.

In the present invention, the standard solution of alkali used is well known to those skilled in the art and will not be described herein.

In the present invention, the organic base-containing acid anhydride solution is a mixed solution of an organic base and an acid anhydride.

According to the testing method provided by the invention, in some examples, the operation process of the step (1) is as follows:

i. mixing a certain mass of polylactic acid sample to be detected with an anhydride solution containing organic base, and carrying out heating reflux treatment; cooling the system to a temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; titrating by using an alkali standard solution to obtain a titration test value of the polylactic acid sample to be tested;

ii. Directly subjecting the organic base-containing acid anhydride solution to the heating reflux treatment; cooling the system to a temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; titrating by using the same alkali standard solution as the step i to obtain a titration test value of the blank sample;

that is, in steps i and ii, the organic base-containing acid anhydride solution and the dosage thereof are the same, and the operations of heating reflux treatment, hydrolysis reaction and titration of the base standard solution are also the same;

and obtaining the apparent hydroxyl value of the polylactic acid sample to be detected according to the titration test value of the polylactic acid sample to be detected and the titration test value of the blank sample.

In some examples, the apparent hydroxyl value of the polylactic acid sample to be tested in step (1) is calculated by the following formula:

in the formula:

x is the apparent hydroxyl value of the polylactic acid sample to be detected, mgKOH/g;

v1-the amount of the standard solution of alkali used in the titration of the sample of polylactic acid to be tested,

mL；

v0-amount of base standard solution used for titration of blank, m L;

c-actual concentration of base standard solution, mol/L;

m-mass of the polylactic acid sample to be tested, g;

56.1-molar mass of potassium hydroxide, g/mol.

In some examples, the operation procedure of step (2) is:

mixing a certain mass of polylactic acid sample to be detected with the organic base, and carrying out heating reflux treatment; cooling the system to a temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; titrating by using an alkali standard solution to obtain a titration test value of the polylactic acid sample to be tested;

wherein, the usage amount of the polylactic acid sample to be detected, the organic base and the usage amount thereof, the operation of heating reflux treatment and the operation of hydrolysis reaction can be the same as those in the step (1);

and calculating the apparent acid value of the polylactic acid sample to be detected according to the titration test value, wherein the calculation formula is as follows:

in the formula,

w is the apparent acid value of the polylactic acid sample to be detected, mgKOH/g;

v-the dosage of the alkali standard solution used in the titration of the polylactic acid sample to be detected, m L;

c-actual concentration of base standard solution, mol/L;

m-mass of the polylactic acid sample to be tested, g;

56.1-molar mass of potassium hydroxide, g/mol.

For example, in one embodiment, step (1) comprises the following operations:

weighing a certain mass of polylactic acid sample to be detected, adding a certain amount of anhydride solution containing organic base, uniformly mixing, and heating and refluxing for derivatization reaction; cooling the system to a temperature of less than or equal to 30 ℃, washing the condenser tube by using a washing solvent, and then adding pure water for hydrolysis reaction; then carrying out titration test on the system by using an alkali standard solution;

carrying out a blank experiment according to the operation, weighing a polylactic acid sample to be detected, and directly heating and refluxing the organic base-containing anhydride solution with the same dosage for the same time; after the system is cooled to the temperature of less than or equal to 30 ℃, washing the condenser tube by using a washing solvent, and then adding pure water for hydrolysis reaction; then, carrying out titration test on the system by using the same alkali standard solution;

calculating the apparent hydroxyl value of the polylactic acid sample to be detected by the following formula:

in the formula:

mL；

v0-amount of base standard solution used for titration of blank, m L;

c-actual concentration of base standard solution, mol/L;

m-mass of the polylactic acid sample to be tested, g;

56.1-molar mass of potassium hydroxide, g/mol;

wherein the washing solvent can be one or more selected from n-butanol, n-propanol, ethylene glycol and isopropanol.

For example, in one embodiment, step (2) comprises the following operations:

mixing a certain mass of the polylactic acid sample to be detected with the organic base, and heating and refluxing the mixture after the polylactic acid sample and the organic base are uniformly mixed; cooling the system to a temperature of less than or equal to 30 ℃, washing the condenser tube by using a washing solvent, and then adding pure water for hydrolysis reaction; then carrying out titration test on the system by using an alkali standard solution;

calculating the apparent acid value of the polylactic acid sample to be detected by the following formula:

in the formula,

c-actual concentration of base standard solution, mol/L;

m-mass of the polylactic acid sample to be tested, g;

56.1-molar mass of potassium hydroxide, g/mol;

wherein, the types and the dosage of the organic alkali, the heating reflux treatment and the hydrolysis reaction are the same as those in the step (1).

According to the testing method provided by the invention, in some examples, in the step (1) and the step (2), the process conditions of the heating reflow treatment include: the treatment temperature is 90-130 deg.C (e.g., 100 deg.C, 110 deg.C, 120 deg.C), and the treatment time is 0.5-2 hr (e.g., 1 hr, 1.5 hr).

In some examples, in step (1) and step (2), the process conditions of the hydrolysis reaction include: the reaction temperature is normal temperature (e.g., 25 ℃), and the reaction time (which may be carried out by stirring during the reaction) is 5 to 60min (e.g., 10min, 30min, 50 min).

In some examples, the anhydride is acetic anhydride or phthalic anhydride (e.g., phthalic anhydride).

In some examples, the organic base is pyridine and/or imidazole, preferably a mixture of pyridine and imidazole.

In some examples, the base standard solution is sodium hydroxide or potassium hydroxide as a solute, and the solvent is water, methanol or ethanol.

In some embodiments, in step (1) and step (2),

the mass of the polylactic acid sample to be detected is 1-5g (for example, 2g, 3g and 4 g);

the organic base is a mixture of pyridine and imidazole, and 16g of imidazole is added into per 700m L of pyridine for preparation;

the concentration of the organic base-containing acid anhydride solution is 95-150g acid anhydride/L organic base, and the dosage of the organic base-containing acid anhydride solution is 10-50m L (for example, 20m L, 30m L and 40m L);

the water is added in an amount of 10-100m L (e.g., 20m L, 50m L, 80m L);

the alkali standard solution is KOH-water solution, KOH-ethanol solution, KOH-methanol solution or NaOH-water solution, the concentration of the alkali standard solution is 0.5-1.5 mol/L (for example,

0.8mol/L、1.0mol/L、1.2mol/L)。

in some examples, the sample of polylactic acid to be tested has a lactide content of 2 wt% or more (e.g., 2.5 wt%, 4 wt%, 6 wt%, 8 wt%, 10 wt%).

The Applicant has found that either the hydroxyl number in polyester polyols, determined using HGT 2709, or GBT12008.3, part 3 of the plastic polyether polyols: the hydroxyl value of a polylactic acid polymer system containing lactide cannot be accurately and stably measured by any of the methods described in "measurement of hydroxyl value". The method is characterized by comprising the following two aspects: firstly, the test parallelism is poor, and the obtained test result is high and low; secondly, the test results are all smaller than the theoretical values calculated according to the starting material alcohol fed in the preparation process of the polylactic acid.

In the field, the principle of hydroxyl value titration is that a sample to be tested and acid anhydride are subjected to derivatization reaction, so that the hydroxyl in the sample consumes the acid anhydride, and then the residual acid and acid anhydride in a system are titrated by using an alkali standard solution; in addition, the same dose of anhydride was directly titrated with a base standard solution by a blank experiment. Because the same amount of acid anhydride is used in the test experiment and the blank experiment of the sample to be detected, the difference value between the amount of alkali liquor consumed in the blank experiment and the amount of alkali liquor consumed in the test experiment is the content of hydroxyl in the sample to be detected. Normally, the amount of lye consumed in the test run should be less than the amount in the blank run, since the hydroxyl groups consumed part of the anhydride in the test run. However, when a sample of polylactide-containing polylactic acid was tested using conventional detection methods, it was found that the amount of lye consumed in the test experiments was greatly increased, even in some cases higher than in the blank experiments.

It can be concluded from this that lactide-containing polylactic acid samples generate acidic species that can consume base during the hydroxyl number test by conventional test methods. This was an unexpected result. It is well known to those skilled in the art that lactide-containing polylactic acid polymer systems include hydroxyl terminated polylactic acid and unreacted lactide; the polylactic acid comprises ester groups and hydroxyl groups, and the lactide only comprises the ester groups and does not contain the hydroxyl groups. Polyester polyol polymers also contain ester groups and hydroxyl groups in their structure. If it would be naturally predicted from the knowledge in the art that the method for determining the hydroxyl number of a polyester polyol should be suitable for determining the hydroxyl number in a system containing ester groups and hydroxyl groups, the ester groups not interfering with the determination. However, the results of the presumption by the prior knowledge do not actually match the results of the hydroxyl number test of the lactide-containing polylactic acid system.

Further, the applicant found through a series of experiments that a polylactic acid polymer system containing lactide is peculiar in that the contained lactide is a six-membered cyclic lactone containing two ester groups, due to the cyclic structure, the activity of the ester groups is higher than that of a chain polyester polyol or the ester groups in polylactic acid, however, in the conventional polyester polyol measurement process, after a solution of a sample to be tested and a solution of the sample to be tested are heated under reflux for derivatization, 10m L of water is directly added and heated, so that the remaining acetic anhydride is hydrolyzed into acetic acid when meeting water, thereby facilitating the subsequent acid-base titration with a standard alkali solution, however, experiments show that under the same conditions, the lactide also undergoes hydrolytic ring-opening when meeting water, to generate dimeric lactic acid or lactic acid, which explains the fact that the amount of alkali consumed by the test experiments is greatly increased, even partially higher than that of blank experiments, in the case of the test method of the conventional method, the occurrence of the phenomenon of the test that the hydrolysis of lactide in the system is higher than that the empty experiment is performed when 10m of the test of the hydroxyl value in the poly-bar test method, further research finds that only two lactide-containing hydrolyzed lactide is uncontrollable hydrolysis of the polylactic acid molecules, and the hydrolysis of the polylactic acid is not capable of the hydrolysis of the polylactic acid, on the polylactic acid, the same side, the hydrolysis of the polylactic acid is found that the hydrolysis of polylactic acid is not quantitative test that the polylactic acid, the hydrolysis of polylactic acid, the polylactic acid is not quantitative hydrolysis of polylactic acid, the hydrolysis of polylactic acid is not quantitative hydrolysis of polylactic acid, the polylactic acid is found that the polylactic acid is not controllable polylactic acid, the polylactic acid is present of polylactic acid, the polylactic acid is not controllable polylactic acid is a polylactic acid is not.

The key point for solving the problems lies in finding a proper treatment mode so that the hydrolysis of the lactide is quantitative and controllable under the treatment condition, and finding a test method which can determine the quantitative and controllable hydrolyzed lactide amount and simultaneously ensure that the ester group in the polylactic acid structure is not hydrolyzed under the same treatment condition.

The key point of the testing method is that firstly, a sample to be tested and an acid anhydride solution containing organic base are heated and refluxed, but water is not added under the high-temperature condition for hydrolysis reaction. Under such conditions, it is possible to effect the ring opening by decomposition of only one of the two ester groups of the lactide. The ring tension after ring opening disappears, so that the reactivity of another ester group is reduced, and only organic base exists under high temperature condition, and hydrolysis does not occur in the environment without water. This solves the problem of controlled hydrolysis of lactide in the first place.

Secondly, since the lactide content in the sample to be tested is unknown, the amount of open-ring lactide needs to be quantitatively determined. It is found through research that the acid value titration method can be used for measuring the treated sample (i.e. the sample to be measured is mixed with the organic base, then is heated and refluxed, and is cooled to be less than or equal to 30 ℃ and then is added with water for hydrolysis reaction), namely, 1mol of base can be quantitatively consumed after every 1mol of lactide is treated. Therefore, the feasibility and reliability of the test method can be verified, namely, pure lactide (with the molecular weight of 144) is used as a sample to be tested, after the treatment and the acid-base titration, the apparent acid value of the pure lactide is 390mgKOH/g, and the converted carboxylic acid functionality is 390 x 144/56100 to 1.0, which shows that only one ester group is hydrolyzed after the treatment. This test value is referred to as the apparent acid value because the test method is similar to acid value titration; in addition, since lactide contains no acid, the acid value of a sample to be measured should be 0, and thus it is not suitable to be directly referred to as the acid value of the sample.

Based on the above discussion results, a hydroxyl value test method for a polylactide-containing polylactic acid system can be finally established, which comprises two parts:

firstly, after derivatization reaction is carried out on hydroxyl in a sample to be detected by using anhydride containing organic base, water is not added at high temperature (namely, the derivatization reaction at the high temperature is under anhydrous condition, and hydrolysis reaction is carried out at the temperature of less than or equal to 30 ℃), so that lactide in the sample to be detected is controlled to only open a ring and decompose one ester group, then titration is carried out by using an alkali standard solution, the titration result is converted and then recorded as an apparent hydroxyl value, and the actual hydroxyl value is obtained by subtracting the influence of hydrolysis of the lactide on acid formation from the actual hydroxyl value of the system. The apparent hydroxyl number may be negative.

Secondly, mixing a sample to be detected with the same organic base, heating and refluxing the mixture to decompose an ester group by lactide in the sample to be detected through ring opening, titrating the mixture by using an alkali standard solution, and converting the titration result to obtain the apparent acid value of the sample to be detected, wherein the acid value represents the influence of acid formed by hydrolysis of the lactide in the sample to be detected.

And finally, simply adding the two results to quantitatively eliminate the influence of acid formed by hydrolysis of lactide contained in the sample to be detected, thereby obtaining the actual hydroxyl value of the polylactic acid system containing the lactide.

In another aspect, there is provided a use of the test method as described above in the production of polylactic acid, wherein the hydroxyl group concentration in the polylactic acid production system is calculated from the actual hydroxyl value of the obtained polylactic acid sample to be tested. For example, the hydroxyl group concentration is hydroxyl value/56.1 in mol/kg; hydroxyl number in

mgKOH/g。

According to the applications provided by the present invention, in some examples, the theoretical number average molecular weight of the polylactic acid product is calculated by the concentration of hydroxyl groups in the resulting polylactic acid production system. For example, the theoretical number average molecular weight Mn ═ f × 1000 per hydroxyl group concentration, in g/mol; wherein f is the functionality of the initiator.

For the prior art, the technical scheme of the invention has the beneficial effects that:

by reasonably selecting the treatment conditions of the sample to be detected, namely, the derivatization reaction under the high-temperature condition is under the anhydrous condition and the hydrolysis reaction is carried out at the temperature of less than or equal to 30 ℃, the quantitative and controllable ring-opening hydrolysis of the lactide in the polylactic acid polymer system containing the lactide is realized; meanwhile, the interference of lactide on acid-base titration can be quantitatively eliminated by improving a testing means, and the hydroxyl value of a polylactic acid system containing the lactide can be accurately and stably obtained.

Detailed Description

In order that the technical features and contents of the present invention can be understood in detail, preferred embodiments of the present invention will be described in more detail below. While the preferred embodiments of the present invention have been described in the examples, it should be understood that the present invention may be embodied in various forms and should not be limited by the embodiments set forth herein.

The titration test is carried out by adopting a Swiss Wantong 905 automatic potentiometric titrator.

All test reagents, except water, were purchased from alatin reagent, inc.

The requirements of the reagents used and the preparation method are described in HGT 2709 "determination of hydroxyl number in polyester polyol" or GBT12008.3 "determination of hydroxyl number in Plastic polyether polyol part 3". The organic base used is, in particular, a mixture of 16g of imidazole per 700m of L pyridine.

The polylactic acid sample to be tested containing lactide is self-made, 1, 6-hexanediol is used as an initiator, stannous octoate is used as a catalyst, lactide is used as a monomer, the polylactic acid sample is obtained by polymerization at 180 ℃, samples with different hydroxyl values are obtained by adjusting different monomer ratios of 1, 6-hexanediol and lactide

And (5) producing the product.

A 1# polylactic acid sample to be detected is prepared, and the theoretical calculated hydroxyl value is predicted to be 130-;

a 2# polylactic acid sample to be detected is prepared, and the theoretical calculated hydroxyl value is predicted to be 110-;

a 3# polylactic acid sample to be detected is prepared, and the theoretical calculated hydroxyl value is predicted to be 10-12 mgKOH/g;

a4 # polylactic acid sample to be detected is prepared, and the theoretical calculated hydroxyl value is predicted to be 45-55 mgKOH/g.

Example 1

(1) Weighing 3g of a 1# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring flask, adding 25m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110 g/L) in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 115 +/-2 ℃ for heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, taking out the iodine measuring flask from the oil bath after heating reflux for 1h, cooling to 28 ℃, uniformly washing the condenser pipe by dripping 10m L n-butyl alcohol, then taking down the condenser pipe, adding 30m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 10min, washing the system from the flask into a disposable beaker by 50m L for three times after the reaction is finished, and carrying out titration by using 1.0 mol/L NaOH-water standard solution to test the polylactic acid sample to be tested to obtain the test value of the 1# polylactic acid sample to be tested.

And (2) performing a blank test, namely adding 25m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at the temperature of 115 +/-2 ℃, and performing heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, performing heating reflux for 1h, taking out the iodine measuring flask from the oil bath, cooling to the temperature of 28 ℃, uniformly washing the condenser pipe by drops by 10m L n-butyl alcohol, taking down the condenser pipe, adding 30m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 10min, washing the system from the flask into a disposable beaker by 50m L n-butyl alcohol for three times after the reaction is finished, and performing titration by using 1.0 mol/L NaOH-water standard solution to obtain a titration test value of a blank sample.

And calculating the apparent hydroxyl value of the 1# polylactic acid sample to be tested according to the obtained titration test values of the 1# polylactic acid sample to be tested and the blank sample, wherein the calculation formula is as follows:

in the formula, X is the apparent hydroxyl value of the polylactic acid sample to be detected, mgKOH/g, V1 is the dosage of the alkali standard solution of the polylactic acid sample to be detected during titration, m L, V0 is the dosage of the alkali standard solution of the blank during titration, m L, C is the actual concentration of the alkali standard solution, mol/L, m is the mass of the polylactic acid sample to be detected, g, 56.1 is the molar mass of potassium hydroxide, g/mol.

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 1:

TABLE 1

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	3.0034	37.16	31.45	1.0	106.70
Parallel sample-2	3.0124	37.14	31.44	1.0	106.15

The average value of the apparent hydroxyl values obtained by two parallel measurements is 106.43mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 3g of a 1# polylactic acid sample to be tested (accurately weighed to 0.1mg) into a 250m L iodine measuring flask, adding 25m L imidazole-pyridine solution in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 115 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring flask, taking the iodine measuring flask out of the oil bath after heating and refluxing for 1h, cooling to 28 ℃, uniformly washing the condenser pipe by 10m L n-butyl alcohol dropwise, taking down the condenser pipe, adding 30m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 10min, washing the system from the flask into a disposable beaker by 50m L n-butyl alcohol for three times after the reaction is finished, and carrying out a titration by using a 1.0 mol/L NaOH-water standard solution to obtain a titration test value of the 1# polylactic acid sample to be tested according to the following calculation formula.

In the formula:

w is the apparent acid value of the polylactic acid sample to be detected, mgKOH/g, V is the dosage of the alkali standard solution in titration of the polylactic acid sample to be detected, m L, C is the actual concentration of the alkali standard solution, mol/L, m is the mass of the polylactic acid sample to be detected, g, and 56.1 is the molar mass of potassium hydroxide, g/mol.

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 2:

TABLE 2

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	3.233	2.2156	1.0	38.45
Parallel sample-4	3.2546	2.2506	1.0	38.79

The average value of the apparent acid values obtained by two parallel measurements is 38.62mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 1# polylactic acid sample to be tested, wherein the actual hydroxyl value is 145.05 mgKOH/g.

Example 2

(1) Weighing 1g of 2# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring flask, adding 10m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150 g/L) in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser tube to the iodine measuring flask, putting the flask into an oil bath at 90 +/-2 ℃ for heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be half of the body of the iodine measuring flask, taking out the iodine measuring flask from the oil bath after heating reflux for 1h, cooling the system to 27 ℃, uniformly washing the condenser tube by drops by 10m L n-propanol, then taking out the condenser tube, adding 50m L into the iodine measuring flask, stirring the system for hydrolysis reaction for 5min, washing the system from the flask into a disposable beaker by 50m L n-propanol three times after the reaction is finished, and carrying out titration test by using 0.5 mol/L KOH-ethanol standard solution to obtain the 2# polylactic acid sample to be tested.

And (2) performing a blank test, namely adding 10m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 90 +/-2 ℃, performing heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, performing heating reflux for 1h, taking out the iodine measuring flask from the oil bath, cooling the oil bath to 25 ℃, uniformly flushing the condenser pipe by 10m L n-propanol dropwise, taking down the condenser pipe, adding 30m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 10min, washing the system from the flask into a disposable beaker by 50m L n-propanol for three times after the reaction is finished, and performing titration by using 0.5 mol/L KOH-ethanol standard solution to obtain a titration test value of a blank sample.

And calculating the apparent hydroxyl value of the 2# polylactic acid sample to be tested according to the titration test values of the obtained 2# polylactic acid sample to be tested and the blank sample, wherein the calculation formula is as follows:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 3:

TABLE 3

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	1.3134	40.51	37.44	0.5	65.57
Parallel sample-2	1.2035	40.50	37.70	0.5	65.26

The average value of the apparent hydroxyl values obtained by two parallel measurements is 65.42mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 1g of 2# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring flask, adding 10m L imidazole-pyridine solution into the iodine measuring flask in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 90 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring flask, taking out the iodine measuring flask from the oil bath after heating and refluxing for 1h, cooling to 27 ℃, uniformly washing the condenser pipe by 10m L n-propanol dropwise, taking down the condenser pipe, adding 50m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 5min, washing the system from the flask into a disposable beaker by 50m L n-propanol for three times after the reaction is finished, and carrying out titration test by using 0.5 mol/L KOH-ethanol standard solution to obtain the apparent acid value of the 2# polylactic acid sample to be tested according to the following calculation formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 4:

TABLE 4

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	1.0098	2.0090	0.5	55.81
Parallel sample-4	1.0235	2.0531	0.5	56.27

The average value of the apparent acid values obtained by two parallel measurements is 56.04mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 2# polylactic acid sample to be tested, wherein the actual hydroxyl value is 121.46 mgKOH/g.

Example 3

(1) Weighing 4g of a 3# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring bottle, adding 20m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 130 g/L) in a fixed volume, shaking the bottle to dissolve the sample, connecting an air condenser pipe to the iodine measuring bottle, putting the bottle into an oil bath at 130 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring bottle, taking out the iodine measuring bottle from the oil bath after heating and refluxing for 1h, cooling to 26 ℃, uniformly washing the condenser pipe by dropping 10m L isopropanol, then taking down the condenser pipe, adding 10m L pure water into the iodine measuring bottle, stirring the system for hydrolysis reaction for 50min, washing the system from the bottle into a disposable beaker by using 50m L isopropanol three times after the reaction is finished, and carrying out titration test by using 1.0 mol/L KOH-methanol standard solution to obtain the titration value of the 3# polylactic acid sample to be tested.

And (2) blank test, namely adding 20m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 130 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at the temperature of 130 +/-2 ℃, and carrying out heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, after carrying out heating reflux for 1h, taking out the iodine measuring flask from the oil bath, cooling to the temperature of 27 ℃, uniformly flushing the condenser pipe dropwise by using 10m L isopropanol, then taking down the condenser pipe, adding 10m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 50min, washing the system from the flask into a disposable beaker by using 50m L isopropanol for three times after the reaction is finished, and carrying out titration test by using 1.0 mol/L KOH-methanol standard solution to obtain the titration test value of a blank sample.

And calculating the apparent hydroxyl value of the 3# polylactic acid sample to be tested according to the titration test values of the obtained 3# polylactic acid sample to be tested and the blank sample, wherein the calculation formula is as follows:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 5:

TABLE 5

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	4.3104	50.94	51.13	1.0	-2.53
Parallel sample-2	4.0152	50.94	51.13	1.0	-2.72

The average value of the apparent hydroxyl values obtained by two parallel measurements is-2.63 mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 4g of a 3# polylactic acid sample to be tested (weighing is accurate to 0.1mg) into a 250m L iodine measuring flask, adding 20m L imidazole-pyridine solution into the iodine measuring flask in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at the temperature of 130 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring flask, taking out the iodine measuring flask from the oil bath after heating and refluxing for 1h, cooling to the temperature of 24 ℃, uniformly washing the condenser pipe by drops with 10m L isopropanol, taking down the condenser pipe, adding 10m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 50min, washing the system from the flask into a disposable beaker by 50m L isopropanol for three times after the reaction is finished, and carrying out titration test by using a 1.0 mol/L KOH-methanol standard solution to obtain the test value of the 3# polylactic acid sample to be tested, and obtaining the apparent acid value of the 3# polylactic acid sample to be tested according to be calculated according to the following formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 6:

TABLE 6

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	4.0960	1.01	1.0	13.86
Parallel sample-4	4.0990	1.02	1.0	13.91

The average value of the apparent acid values obtained by two parallel measurements is 13.89mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 3# polylactic acid sample to be tested, wherein the actual hydroxyl value is 11.26 mgKOH/g.

Example 4

(1) Weighing 5g of a 4# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring bottle, adding 50m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 95 g/L) in a fixed volume, shaking the bottle to dissolve the sample, connecting an air condenser pipe to the iodine measuring bottle, putting the bottle into an oil bath at 120 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring bottle, taking out the iodine measuring bottle from the oil bath after heating and refluxing for 1h, cooling to 28 ℃, uniformly washing the condenser pipe by dripping 10m L ethylene glycol, then taking down the condenser pipe, adding 100m L pure water into the iodine measuring bottle, stirring the system for hydrolysis reaction for 60min, washing the system from the bottle into a disposable beaker by 50m L ethylene glycol for three times after the reaction is finished, carrying out titration test by using 1.5 mol/L KOH-water standard solution to obtain the titration value of the polylactic acid sample to be tested.

And (2) blank test, namely adding 50m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 95 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 120 +/-2 ℃, and carrying out heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, after carrying out heating reflux for 1h, taking out the iodine measuring flask from the oil bath, cooling to 29 ℃, uniformly washing the condenser pipe by drops by 10m L ethylene glycol, then taking down the condenser pipe, adding 100m L pure water into the iodine measuring flask, stirring the system for hydrolysis reaction for 60min, washing the system from the flask into a disposable beaker by 50m L ethylene glycol for three times after the reaction is finished, and carrying out titration test by using 1.5 mol/L KOH-water standard solution to obtain the titration test value of a blank sample.

And calculating the apparent hydroxyl value of the polylactic acid sample to be tested 4# according to the titration test values of the polylactic acid sample to be tested 4# and the blank sample, wherein the calculation formula is as follows:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 7:

TABLE 7

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	5.0142	62.04	61.07	1.5	16.25
Parallel sample-2	5.3329	62.04	61.00	1.5	16.36

The average value of the apparent hydroxyl values obtained by two parallel measurements is 16.31mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 5g of a 4# polylactic acid sample to be tested (weighing is accurate to 0.1mg) into a 250m L iodine measuring flask, adding 50m L imidazole-pyridine solution into the iodine measuring flask in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 120 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring flask, taking out the iodine measuring flask from the oil bath after heating and refluxing for 1h, cooling to 24 ℃, uniformly washing the condenser pipe by 10m L ethylene glycol dropwise, taking down the condenser pipe, adding 100m L pure water into the iodine measuring flask, stirring the system to perform hydrolysis reaction for 60min, washing the system from the flask into a disposable beaker by 50m L ethylene glycol for three times after the reaction is finished, performing titration test by using 1.5 mol/L KOH-water standard solution, and obtaining the apparent acid value of the 4# polylactic acid sample to be tested according to the following calculation formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 8:

TABLE 8

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	4.9989	2.05	1.5	34.53
Parallel sample-3	4.899	2.00	1.5	34.38

The average value of the apparent acid values obtained by two parallel measurements is 34.46mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 4# polylactic acid sample to be tested, wherein the actual hydroxyl value is 50.77 mgKOH/g.

Comparative example 1

(1) Weighing 3g of a 1# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring flask, adding 25m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110 g/L) in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser tube to the iodine measuring flask, putting the flask into an oil bath at 115 +/-2 ℃ for heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, heating reflux for 1h, adding 30m L pure water into the iodine measuring flask, continuing reflux under the condition for hydrolysis reaction for 10min, cooling the system to 26 ℃ after the reaction is finished, uniformly flushing the condenser tube with 10m L n-butyl alcohol, then taking the condenser tube down, taking the iodine measuring flask out of the oil bath, washing the system from the flask into a disposable beaker with 50m L n-butyl alcohol for three times, testing with a 1.0 mol/L NaOH-water standard solution, and titrating the sample to be tested to obtain the sample # polylactic acid to be tested.

And (2) performing a blank test, namely adding 25m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at the temperature of 115 +/-2 ℃, performing heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, performing heating reflux for 1h, adding 30m L pure water into the iodine measuring flask, performing continuous reflux under the condition for hydrolysis reaction for 10min, cooling the system to the temperature of 28 ℃ after the reaction is finished, uniformly washing the condenser pipe dropwise by using 10m L n-butyl alcohol, taking the iodine measuring flask out of the oil bath, washing the system from the flask into a disposable beaker by using 50m L n-butyl alcohol for three times, and performing titration by using a NaOH-water standard solution of 1.0 mol/L to obtain a test value of a blank sample.

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 9:

TABLE 9

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	3.0304	37.13	34.35	1.0	51.55
Parallel sample-2	3.0004	37.13	34.89	1.0	41.97

The average value of the apparent hydroxyl values obtained by two parallel measurements is 46.76mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 3g of 1# polylactic acid sample to be tested (weighing accurate to 0.1mg) into a 150m L beaker, adding 100m L pyridine-methanol mixed solvent (wherein the volume of pyridine is equal to that of methanol and is 1: 1), shaking the bottle to dissolve the sample, after the sample is dissolved, carrying out titration test by using 1.0 mol/L NaOH-water standard solution to obtain the titration test value of the 1# polylactic acid sample to be tested, and obtaining the apparent acid value of the system according to the following calculation formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurement are shown in the following table 10:

watch 10

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	3.0289	1.03	1.0	19.14
Parallel sample-4	3.1452	1.01	1.0	17.99

The average value of the apparent acid values obtained by two parallel measurements is 18.57mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 1# polylactic acid sample to be tested, wherein the actual hydroxyl value is 65.33 mgKOH/g.

Comparative example 2

(1) Weighing 1g of 2# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring flask, adding 10m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150 g/L) in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser tube to the iodine measuring flask, putting the flask into an oil bath at 90 +/-2 ℃ for heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, adding 50m L pure water into the iodine measuring flask after heating reflux for 1h, continuing reflux under the condition for hydrolysis reaction for 5min, cooling the system to 25 ℃ after the reaction is finished, uniformly flushing the condenser tube with 10m L n-propanol, then taking the condenser tube down, taking the iodine measuring flask out of the oil bath, washing the system from the flask into a disposable beaker with 50m L n-propanol three times, using 0.5 mol/L KOH-ethanol standard solution for titration, and obtaining the test value of the polylactic acid sample to be tested by titration.

And (2) performing a blank test, namely adding 10m L phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 90 +/-2 ℃, performing heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, adding 50m L pure water after heating reflux for 1h, continuing reflux under the condition for hydrolysis reaction for 5min, cooling the system to the temperature of 27 ℃ after the reaction is finished, uniformly washing the condenser pipe by drops by 10m L n-propanol, then taking the condensation pipe out of the oil bath, washing the system from the flask into a disposable beaker by three times by 50m L n-propanol, and performing titration by using 0.5 mol/L KOH-ethanol standard solution to obtain a titration value of a blank sample.

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurements are shown in the following table 11:

TABLE 11

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	1.3567	40.51	39.00	0.5	31.23
Parallel sample-2	1.2870	40.51	38.50	0.5	43.82

The average value of the apparent hydroxyl values obtained by two parallel measurements is 37.53mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 1g of 2# polylactic acid sample to be tested (weighing accurate to 0.1mg) into a 150m L beaker, adding 100m L pyridine-methanol mixed solvent (wherein the volume of pyridine is equal to that of methanol and is 1: 1), shaking the bottle to dissolve the sample, after the sample is dissolved, carrying out titration test by using 0.5 mol/L KOH-ethanol standard solution to obtain the titration test value of the 2# polylactic acid sample to be tested, and obtaining the apparent acid value of the system according to the following calculation formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurements are shown in the following table 12:

TABLE 12

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	5.0091	1.00	0.5	5.61
Parallel sample-4	5.0013	1.39	0.5	7.80

The average value of the apparent acid values obtained by two parallel measurements is 6.71mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 2# polylactic acid sample to be tested, wherein the actual hydroxyl value is 44.24 mgKOH/g.

Comparative example 3

(1) Weighing 4g of a 3# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring flask, adding 20m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 130 g/L) in a fixed volume, shaking the flask to dissolve the sample, connecting an air condenser tube to the iodine measuring flask, putting the flask into an oil bath at 130 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, adding 10m L pure water into the iodine measuring flask after heating and refluxing for 1h, continuing refluxing under the condition for 50min, cooling the system to the temperature of 27 ℃ after the reaction is finished, uniformly flushing the condenser tube with 10m L isopropanol dropwise, then taking the condenser tube down, taking the iodine measuring flask out of the oil bath, washing the system from the flask into a disposable beaker with 50m L isopropanol three times, testing with 1.0 mol/L KOH-methanol standard solution, and obtaining the titration value of the polylactic acid sample to be tested.

And (2) performing a blank test, namely adding 20m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 130 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at the temperature of 130 +/-2 ℃, performing heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring flask, performing heating reflux for 1h, adding 10m L pure water, continuing refluxing under the condition for 50min, cooling the system to the temperature of 29 ℃ after the reaction is finished, uniformly washing the condenser pipe by drops by 10m L isopropanol, taking the condenser pipe down, taking the iodine measuring flask out of the oil bath, washing the system into a disposable beaker by 50m L isopropanol for three times, and performing titration test by using a 1.0 mol/L KOH-methanol standard solution to obtain a titration value of a blank sample.

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurements are shown in the following table 13:

watch 13

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	4.7014	50.94	58.10	1.0	-85.47
Parallel sample-2	4.7701	50.94	57.22	1.0	-73.96

The average value of the apparent hydroxyl values obtained by two parallel measurements is-79.72 mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 4g of a 3# polylactic acid sample to be tested (weighing to be accurate to 0.1mg) into a 150m L beaker, adding a 100m L mixed solvent of pyridine-methanol (wherein the volume of the pyridine is equal to that of the methanol and is 1: 1), shaking the bottle to dissolve the sample, after the sample is dissolved, carrying out a titration test by using a 1.0 mol/L KOH-methanol standard solution to obtain a titration test value of the 3# polylactic acid sample to be tested, and obtaining an apparent acid value of the system according to the following calculation formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurements are shown in the following table 14:

TABLE 14

Sample numbering	m/(g)	V/(mL)	C/(mol/L)	W/(mgKOH/g)
					Parallel sample-3	4.7103	1.10	1.0	13.09
Parallel sample-4	4.8019	1.02	1.0	11.90

The average value of the apparent acid values obtained by two parallel measurements is 12.50mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 3# polylactic acid sample to be tested, wherein the actual hydroxyl value is-67.22 mgKOH/g.

Comparative example 4

(1) Weighing 5g of a 4# polylactic acid sample to be tested (weighing accurately to 0.1mg) into a 250m L iodine measuring bottle, adding 50m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 95 g/L) in a fixed volume, shaking the bottle to dissolve the sample, connecting an air condenser pipe to the iodine measuring bottle, putting the bottle into an oil bath at 120 +/-2 ℃, heating and refluxing for 1h, wherein the liquid level of the oil bath needs to be soaked in half of the body of the iodine measuring bottle, adding L pure water after heating and refluxing for 1h, continuing refluxing and stirring under the condition for hydrolysis reaction for 60min, after the reaction is finished, uniformly washing the condenser pipe by dripping 10m L ethylene glycol, taking the condenser pipe down, taking out the iodine measuring bottle from the oil bath, cooling the system to 25 ℃, washing the system from the bottle into a disposable beaker by using 50m L ethylene glycol for three times, and carrying out titration test by using 1.5 mol/L KOH-water standard solution to obtain the titration value of the polylactic acid sample to be tested.

And (2) performing a blank test, namely adding 50m L acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 95 g/L) into a 250m L iodine measuring flask in a fixed volume manner, shaking the flask, connecting an air condenser pipe to the iodine measuring flask, putting the flask into an oil bath at 120 +/-2 ℃, performing heating reflux treatment for 1h, wherein the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring flask, performing heating reflux for 1h, adding 100m L pure water, continuing reflux stirring under the condition for hydrolysis reaction for 60min, cooling the system to the temperature of 25 ℃ after the reaction is finished, uniformly washing the condenser pipe by drops with 10m L ethylene glycol, taking the condensation flask out of the oil bath, washing the system from the flask into a disposable beaker by 50m L ethylene glycol for three times, and performing titration test by using 1.5 mol/L KOH-water standard solution to obtain a titration value of a blank sample.

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurements are shown in the following table 15:

watch 15

Numbering	m/(g)	V0/(mL)	V1/(mL)	C/(mol/L)	X/(mgKOH/g)
						Parallel sample-1	5.0090	62.04	64.01	1.5	-33.13
Parallel sample-2	4.9989	62.04	64.51	1.5	-41.63

The average value of the apparent hydroxyl values obtained by two parallel measurements is-37.38-50.01 mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 5g of the 4# polylactic acid sample to be tested (weighing accurate to 0.1mg) into a 150m L beaker, adding a 100m L mixed solvent of pyridine and methanol (wherein the volume of the pyridine is equal to that of the methanol and is 1: 1), shaking the bottle to dissolve the sample, after the sample is dissolved, carrying out a titration test by using a 1.5 mol/L KOH-water standard solution to obtain a titration test value of the 4# polylactic acid sample to be tested, and obtaining an apparent acid value of the system according to the following calculation formula.

In the formula:

Two parallel samples were taken and subjected to parallel measurement twice, and the data of the parallel measurements are shown in the following table 16:

TABLE 16

Sample numbering	m/g	V/mL	C/mol/L	W/mgKOH/g
					Parallel sample-3	5.01	1.71	1.5	28.65
Parallel sample-4	6.00	1.74	1.5	24.43

The average value of the apparent acid values obtained by two parallel measurements is 26.54mgKOH/g, namely the apparent acid value of the system.

(3) And (3) adding the apparent hydroxyl value obtained in the step (1) and the apparent acid value obtained in the step (2) to obtain the actual hydroxyl value of the 4# polylactic acid sample to be tested, wherein the actual hydroxyl value is-10.84 mgKOH/g.

In comparative examples 1 to 4 and examples 1 to 4, the difference between the apparent hydroxyl values (unit: mgKOH/g) obtained in the two tests was measured in parallel and shown in Table 17 below:

as can be seen from the results in table 17, the test difference of the apparent hydroxyl value obtained after two parallel measurements in each example of the present application is very small, thereby ensuring the stability of the test result; the apparent hydroxyl value obtained by the two parallel measurements through the proportional test method has very large test difference, and the test cannot be stably carried out.

In comparative examples 1 to 4 and examples 1 to 4, the results of the measurement of the actual hydroxyl value obtained are compared with the expected range of the theoretically calculated hydroxyl value (unit: mgKOH/g), and are shown in Table 18 below:

as can be seen from the results in table 18, the hydroxyl value of the lactide-containing polylactic acid system could not be accurately obtained by any of the known methods shown in each comparative example, and the measured actual hydroxyl value was far from the expected range of the hydroxyl value obtained by theoretical calculation; by the embodiment of the invention, the measured hydroxyl value of the polylactide-containing polylactic acid system falls into the expected hydroxyl value range obtained by theoretical calculation, which shows that the accuracy of the result in the hydroxyl value test process can be ensured.

In conclusion, the hydroxyl value detection of the polylactic acid system containing lactide is carried out by the testing method, the testing parallelism is good, and the testing result is accurate.

Having described embodiments of the present invention, the foregoing description is intended to be exemplary, not exhaustive, and not limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the spirit of the invention.

Claims

1. A method for testing a hydroxyl value in lactide-containing polylactic acid is characterized by comprising the following steps:

(1) contacting a polylactic acid sample to be detected with an acid anhydride solution containing organic base, and carrying out heating reflux treatment; then cooling the system to a temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; titrating by using an alkali standard solution to obtain a titration test value of the polylactic acid sample to be tested;

(2) contacting a polylactic acid sample to be detected with the organic alkali, and carrying out heating reflux treatment; then cooling the system to a temperature of less than or equal to 30 ℃, and adding water for hydrolysis reaction; then titrating by using an alkali standard solution to obtain the apparent acid value of the polylactic acid sample to be detected;

2. The test method according to claim 1, wherein the operation procedure of step (1) is as follows:

3. The method according to claim 1 or 2, wherein the apparent hydroxyl value of the polylactic acid sample to be tested in step (1) is calculated by the following formula:

in the formula:

v1-the dosage of the alkali standard solution used in the titration of the polylactic acid sample to be tested, m L;

v0-amount of base standard solution used for titration of blank, m L;

c-actual concentration of base standard solution, mol/L;

m-mass of the polylactic acid sample to be tested, g;

56.1-molar mass of potassium hydroxide, g/mol.

4. The test method according to any one of claims 1 to 3, wherein the operation process of step (2) is:

in the formula,

c-actual concentration of base standard solution, mol/L;

m-mass of the polylactic acid sample to be tested, g;

56.1-molar mass of potassium hydroxide, g/mol.

5. The test method according to any one of claims 1 to 4, wherein, in the step (1) and the step (2),

the process conditions of the heating reflux treatment comprise: the treatment temperature is 90-130 ℃, and the treatment time is 0.5-2 hours; and/or

The technological conditions of the hydrolysis reaction comprise: the reaction temperature is normal temperature, and the reaction time is 5-60 min.

6. The test method according to any one of claims 1 to 5, characterized in that the acid anhydride is acetic anhydride or phthalic anhydride; and/or

The organic base is pyridine and/or imidazole, preferably a mixture of pyridine and imidazole; and/or

In the alkali standard solution, the alkali is sodium hydroxide or potassium hydroxide, and the solvent is water, methanol or ethanol.

7. The test method according to any one of claims 1 to 6, wherein, in the step (1) and the step (2),

the mass of the polylactic acid sample to be detected is 1-5 g;

the concentration of the organic base-containing anhydride solution is 95-150g anhydride/L organic base, and the dosage of the organic base-containing anhydride solution is 10-50m L;

the addition amount of the water is 10-100m L;

the alkali standard solution is KOH-water solution, KOH-ethanol solution, KOH-methanol solution or NaOH-water solution, and the concentration of the alkali standard solution is 0.5-1.5 mol/L.

8. The test method according to any one of claims 1 to 7, wherein the content of lactide in the polylactic acid sample to be tested is not less than 2 wt%.

9. Use of the test method according to any one of claims 1 to 8 in polylactic acid production, wherein the hydroxyl group concentration in the polylactic acid production system is calculated from the actual hydroxyl value of the obtained polylactic acid sample to be tested.

10. The use according to claim 9, wherein the theoretical number average molecular weight of the polylactic acid product is calculated from the concentration of hydroxyl groups in the resulting polylactic acid production system.