CN111458453B

CN111458453B - Method for testing hydroxyl value in lactide-containing polylactic acid and application thereof

Info

Publication number: CN111458453B
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: 2022-07-12
Anticipated expiration: 2040-05-12
Also published as: CN111458453A

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

With the increasing importance of energy consumption and environmental protection, polylactic acid is receiving more and more attention. Polylactic acid (PLA) is 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, PLA is mainly prepared by lactide ring-opening polymerization, and the reaction formula is shown below:

the ring-opening polymerization method for preparing PLA takes lactide as a monomer, and an initiator and a catalyst are generally added in the polymerization process. The initiator is usually an alcohol (hereinafter referred to as ROH).

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 preparation of high molecular weight polylactic acid by a ring-opening polymerization method, the number average molecular weight (Mn) of PLA as a target product meets the following requirements:

wherein, the first and the second end of the pipe are connected with each other,

[MO]is the number of moles of monomer, M_moIs the molar mass of the monomer; [ MO ]]＝m_mo/M_mo，m_moMass 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, since the alcohol is consumed after the initiation of the polymerization, the actual amount of the initiator (for example, alcohol) used in the polymerization cannot be directly measured by means of 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 and the like 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 GBT 12008.3 "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 molecular structure comprises ester groups and hydroxyl groups, and the lactide only comprises the ester groups and does not contain the 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 GBT 12008.3, 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, 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 operation according to the same steps to obtain a titration test value of the blank;

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 ℃ (for example, 10 ℃, 20 ℃, 25 ℃ and 29 ℃), and then 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 known to those skilled in the art and will not be described in detail 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;

namely, in the 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 by 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, mL;

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 (I), the compound is shown in the specification,

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

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

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, mL;

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 (I), the compound is shown in the specification,

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 every 700mL of pyridine for preparation;

the concentration of the organic base-containing acid anhydride solution is 95-150g acid anhydride/L organic base; the amount of the organic base-containing anhydride solution is 10-50mL (e.g., 20mL, 30mL, 40 mL);

the amount of water added is 10-100mL (e.g., 20mL, 50mL, 80 mL);

the alkali standard solution is KOH-water solution, KOH-ethanol solution, KOH-methanol solution or NaOH-water solution; the concentration of the base standard solution is 0.5 to 1.5mol/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 GBT 12008.3, 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 above that of the blank experiments.

It can be concluded therefrom that the lactide-containing polylactic acid sample generates acidic substances which can consume alkali during the hydroxyl number test by the conventional test method. 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 the lactide-containing polylactic acid polymer system has its particularity: the lactide is a six-membered cyclic lactone containing two ester groups, and the activity of the ester group is higher than that of the chain polyester polyol or polylactic acid due to the cyclic structure. In the conventional polyester polyol determination process, after a sample to be determined and a solution of acetic anhydride are heated and refluxed for derivatization reaction, 10mL of water is directly added and heated, so that the residual acetic anhydride is hydrolyzed into acetic acid when meeting water, and the subsequent acid-base titration with a standard alkali liquor is facilitated. However, experiments show that under the same conditions, lactide can be subjected to hydrolytic ring opening when meeting water to generate dimeric lactic acid or lactic acid. This explains the phenomenon that the amount of alkali solution consumed in the test experiment is greatly increased, even partially higher than that in the blank experiment, when the conventional method is used for the test. Further research shows that in the testing method of HGT 2709 "determination of hydroxyl value in polyester polyol", the hydrolysis reaction of lactide in the system during the process of adding 10mL of water and heating is uncontrollable, and only one of two ester groups contained in the lactide cannot be hydrolyzed and the dimeric lactic acid cannot be obtained quantitatively, and the situation that two ester groups contained in the lactide are completely hydrolyzed to obtain two molecular lactic acids also exists, and meanwhile, the polylactic acid cannot be subjected to quantitative hydrolysis reaction under the condition. On the other hand, experimental exploration shows that if the lactide can be quantitatively hydrolyzed into two molecules of lactic acid by adding strong base (such as 1mol/L sodium hydroxide solution) and heating and refluxing, the conditions can also cause hydrolysis of polylactic acid in the system, thereby causing new interference in acid-base titration. Therefore, the hydroxyl number test falls into two dilemmas for polylactide containing polylactic acid systems: on the one hand the uncontrolled hydrolysis of lactide and on the other hand the uncontrolled hydrolysis of polylactic acid.

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 achieve ring opening by decomposition of only one of the two ester groups of lactide. The ring tension after ring opening disappears, so that the reactivity of another ester group is reduced, and only organic alkali 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 that the acid value titration method can be used to determine the treated sample (i.e., the sample to be measured is mixed with the organic base and then heated for reflux treatment, and then water is added for hydrolysis reaction after the temperature is reduced to be less than or equal to 30 ℃), i.e., 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 does not contain acid, the acid value of a sample to be measured should be 0, and therefore, 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 value in units of

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:

the method realizes the quantitative and controllable ring-opening hydrolysis of the lactide in the polylactic acid polymer system containing the lactide by reasonably selecting the treatment conditions of a 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 ℃; 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 the hydroxyl number in polyester polyols" or GBT 12008.3 "part 3 of Plastic polyether polyols: measurement of hydroxyl value. In particular, the organic base used is a mixture comprising 16g of imidazole per 700mL of pyridine.

The polylactic acid sample to be detected containing lactide is self-made, 1, 6-hexanediol is used as an initiator, stannous octoate is used as a catalyst, and lactide is used as a monomer, and is obtained by polymerization at 180 ℃, and samples with different hydroxyl values are obtained by adjusting different monomer ratios of 1, 6-hexanediol and lactide. The polymerization method of polylactic acid is known in the art and will not be described in detail herein. Wherein the lactide is obtained from Total CorbionPLA

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) 3g of the sample of the 1# polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 250mL iodine vial, 25mL of phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110g/L) was added in a fixed volume, and the vial was shaken to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 115 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. And (3) after heating and refluxing for 1h, taking out the iodometry bottle from the oil bath, cooling to the temperature of 28 ℃, uniformly washing the condenser tube by using 10mL of n-butyl alcohol drop by drop, then taking down the condenser tube, adding 30mL of pure water into the iodometry bottle, and stirring the system to perform hydrolysis reaction for 10 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of n-butyl alcohol for three times, and a titration test is carried out by using 1.0mol/L NaOH-water standard solution to obtain the titration test value of the 1# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric flask, 25mL of phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110g/L) was added in volume and the flask was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 115 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. And (3) after heating and refluxing for 1h, taking out the iodometry bottle from the oil bath, cooling to the temperature of 28 ℃, uniformly washing the condenser tube by using 10mL of n-butyl alcohol drop by drop, then taking down the condenser tube, adding 30mL of pure water into the iodometry bottle, and stirring the system to perform hydrolysis reaction for 10 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of n-butyl alcohol for three times, and a titration test is carried out by using 1.0mol/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-the dosage of the alkali standard solution, mL, of the polylactic acid sample to be tested during titration; v0-amount of base standard solution used for titration of blank, mL; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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) 3g of the sample of the 1# polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 250mL iodine vial, 25mL of the imidazole-pyridine solution was added in a fixed volume, and the vial was shaken to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 115 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be immersed by half of the body of the iodine measuring bottle. And (3) after heating and refluxing for 1h, taking out the iodometry bottle from the oil bath, cooling to the temperature of 28 ℃, uniformly washing the condenser tube by using 10mL of n-butyl alcohol drop by drop, then taking down the condenser tube, adding 30mL of pure water into the iodometry bottle, and stirring the system to perform hydrolysis reaction for 10 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of n-butyl alcohol for three times, and a titration test is carried out by using 1.0mol/L NaOH-water standard solution to obtain the titration test value of the 1# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

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 the titration of the polylactic acid sample to be detected, mL; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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 accurate to 0.1mg) in a 250mL iodine vial, adding 10mL phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150g/L) in a fixed volume, and shaking the vial to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 90 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodometer bottle from the oil bath, cooling the system to the temperature of 27 ℃, uniformly flushing the condenser tube by dripping 10mL of n-propanol, then taking down the condenser tube, adding 50mL of pure water into the iodometer bottle, and stirring the system for hydrolysis reaction for 5 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of n-propanol for three times, and a titration test is carried out by using 0.5mol/L KOH-ethanol standard solution to obtain the titration test value of the 2# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric flask, 10mL of phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150g/L) was added in volume and the flask was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 90 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. And (3) after heating and refluxing for 1h, taking out the iodine measuring flask from the oil bath, cooling the system to the temperature of 25 ℃, uniformly washing the condenser tube by dropwise using 10mL of n-propanol, then taking down the condenser tube, adding 30mL of pure water into the iodine measuring flask, and stirring the system to perform hydrolysis reaction for 10 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by 50mL of n-propanol for three times, and a titration test is carried out by using 0.5mol/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 accurate to 0.1mg) in a 250mL iodine measuring bottle, adding 10mL of imidazole-pyridine solution in a fixed volume, and shaking the bottle to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 90 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodine measuring flask from the oil bath, cooling to the temperature of 27 ℃, uniformly washing the condenser tube by dripping 10mL of n-propanol, then taking down the condenser tube, adding 50mL of pure water into the iodine measuring flask, and stirring the system to perform hydrolysis reaction for 5 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of n-propanol for three times, and a titration test is carried out by using 0.5mol/L KOH-ethanol standard solution to obtain the titration test value of the 2# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

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, mL, of the polylactic acid sample to be detected during titration; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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 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 3# polylactic acid sample to be tested (weighing accurate to 0.1mg) in a 250mL iodine measuring bottle, adding 20mL of acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 130g/L) in a fixed volume, and shaking the bottle to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 130 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodometer bottle from the oil bath, cooling to the temperature of 26 ℃, uniformly flushing the condenser tube with 10mL of isopropanol dropwise, then taking down the condenser tube, adding 10mL of pure water into the iodometer bottle, and stirring the system to perform hydrolysis reaction for 50 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of isopropanol for three times, and a titration test is carried out by using 1.0mol/L KOH-methanol standard solution, so as to obtain the titration test value of the 3# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric vial, 20mL of acetic anhydride-imidazole-pyridine solution (where the concentration of acetic anhydride is 130g/L) was added volumetrically and the vial was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 130 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodometer bottle from the oil bath, cooling to the temperature of 27 ℃, uniformly washing the condenser tube by dripping 10mL of isopropanol, then taking down the condenser tube, adding 10mL of pure water into the iodometer bottle, and stirring the system to perform hydrolysis reaction for 50 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of isopropanol in three times, and a titration test is carried out by using 1.0mol/L KOH-methanol standard solution, so as to obtain a 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:

in the formula: x is the apparent hydroxyl value of the polylactic acid sample to be detected, mgKOH/g; v1-the dosage of the alkali standard solution, mL, of the polylactic acid sample to be detected during titration; v0-amount of base standard solution used for titration of blank, mL; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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 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 the two parallel measurements is-2.63 mgKOH/g, namely the apparent hydroxyl value of the system.

(2) Weighing 4g of the 3# polylactic acid sample to be tested (weighing accurate to 0.1mg) in a 250mL iodine measuring flask, adding 20mL of imidazole-pyridine solution in a fixed volume, and shaking the flask to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 130 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodometer bottle from the oil bath, cooling to 24 ℃, uniformly washing the condenser tube by dripping 10mL of isopropanol, then taking down the condenser tube, adding 10mL of pure water into the iodometer bottle, and stirring the system to perform hydrolysis reaction for 50 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by using 50mL of isopropanol for three times, and a titration test is carried out by using 1.0mol/L KOH-methanol standard solution, so as to obtain the titration test value of the 3# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

In the formula:

w represents the apparent acid value of the polylactic acid sample to be detected, mgKOH/g; v-the dosage of the alkali standard solution, mL, of the polylactic acid sample to be detected during titration; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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 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 the sample of the 4# polylactic acid to be tested (weighing accurate to 0.1mg) in a 250mL iodine vial, adding 50mL of acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 95g/L) in a fixed volume, and shaking the vial to dissolve the sample. Connecting an air condenser pipe to an iodine measuring flask, and putting the flask into an oil bath at 120 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodine measuring flask from the oil bath, cooling to the temperature of 28 ℃, uniformly washing the condenser tube by 10mL of ethylene glycol drop by drop, then taking down the condenser tube, adding 100mL of pure water into the iodine measuring flask, and stirring the system to perform hydrolysis reaction for 60 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by 50mL of glycol for three times, and a titration test is carried out by using a 1.5mol/L KOH-water standard solution, so as to obtain the titration test value of the 4# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric vial, 50mL of acetic anhydride-imidazole-pyridine solution (where the concentration of acetic anhydride is 95g/L) was added in volume and the vial was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 120 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodometer bottle from the oil bath, cooling to 29 ℃, uniformly washing the condenser tube by dripping 10mL of ethylene glycol, then taking down the condenser tube, adding 100mL of pure water into the iodometer bottle, and stirring the system to perform hydrolysis reaction for 60 min. After the reaction is finished, the system is washed into a disposable beaker from a bottle by 50mL of glycol in three times, and a titration test is carried out by using a 1.5mol/L KOH-water standard solution, so as to obtain a 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

Number of	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) 5g of the sample of the 4# polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 250mL iodine vial, 50mL of the imidazole-pyridine solution was added in a fixed volume, and the vial was shaken to dissolve the sample. Connecting an air condenser pipe to an iodine measuring flask, and putting the flask into an oil bath at 120 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1h, taking out the iodometer bottle from the oil bath, cooling to 24 ℃, uniformly washing the condenser tube by dripping 10mL of ethylene glycol, then taking down the condenser tube, adding 100mL of pure water into the iodometer bottle, and stirring the system to perform hydrolysis reaction for 60 min. After the reaction is finished, washing the system from the bottle to a disposable beaker by 50mL of glycol in three times, and carrying out titration test by using 1.5mol/L KOH-water standard solution to obtain the titration test value of the 4# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

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) 3g of the sample of the 1# polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 250mL iodine vial, 25mL of phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110g/L) was added in a fixed volume, and the vial was shaken to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 115 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 30mL of pure water was added to the iodine flask, and the hydrolysis was continued under reflux for 10 min. After the reaction is finished, cooling the system to the temperature of 26 ℃, uniformly washing the condenser tube by 10mL of n-butyl alcohol drop by drop, then taking down the condenser tube, taking out the iodometric bottle from the oil bath, washing the system from the bottle into a disposable beaker by three times by 50mL of n-butyl alcohol, and carrying out titration test by using 1.0mol/L NaOH-water standard solution to obtain the titration test value of the 1# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric flask, 25mL of phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 110g/L) was added in volume and the flask was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 115 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 30mL of pure water was added to the iodophor, and the hydrolysis was continued under reflux for 10 min. After the reaction is finished, cooling the system to the temperature of 28 ℃, uniformly washing the condenser tube by 10mL of n-butyl alcohol drop by drop, then taking down the condenser tube, taking out the iodometric flask from the oil bath, washing the system from the flask into a disposable beaker by three times by 50mL of n-butyl alcohol, and carrying out titration on a standard solution by using a 1.0mol/L NaOH-water standard solution to carry out titration test, thus obtaining the titration test value of a blank sample.

in the formula: x represents the apparent hydroxyl value of the polylactic acid sample to be detected, mgKOH/g; v1-the dosage of the alkali standard solution, mL, of the polylactic acid sample to be tested during titration; v0-amount of base standard solution used for titration of blank, mL; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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 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) 3g of sample # 1 polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 150mL beaker, 100mL of a pyridine-methanol mixed solvent (wherein the pyridine volume: methanol volume is 1: 1) was added, and the bottle was shaken to dissolve the sample. After the sample is dissolved, carrying out titration test by using 1.0mol/L NaOH-water standard solution to obtain the titration test value of the 1# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

In the formula:

w represents the apparent acid value of the polylactic acid sample to be detected, mgKOH/g; v is the dosage of the alkali standard solution in the titration of the polylactic acid sample to be detected, mL; c-actual concentration of base standard solution, mol/L; m-mass of polylactic acid sample to be tested, g; 56.1-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 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 accurate to 0.1mg) in a 250mL iodine vial, adding 10mL phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150g/L) in a fixed volume, and shaking the vial to dissolve the sample. Connecting an air condenser pipe to an iodine measuring flask, and putting the flask into an oil bath at 90 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 50mL of pure water was added to the iodophor, and the hydrolysis was continued under reflux for 5 min. After the reaction is finished, cooling the system to the temperature of 25 ℃, uniformly washing the condenser tube by 10mL of n-propanol dropwise, then taking down the condenser tube, taking out the iodometric bottle from the oil bath, washing the system from the bottle into a disposable beaker by 50mL of n-propanol for three times, and carrying out titration test by using 0.5mol/L KOH-ethanol standard solution to obtain the titration test value of the 2# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodine flask, 10mL of phthalic anhydride-imidazole-pyridine solution (wherein the concentration of phthalic anhydride is 150g/L) was added in volume and the flask was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 90 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be immersed by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 50mL of pure water was added, and the hydrolysis was continued under reflux for 5 min. After the reaction is finished, cooling the system to the temperature of 27 ℃, uniformly flushing the condenser tube by 10mL of n-propanol dropwise, then taking down the condenser tube, taking out the iodometric flask from the oil bath, washing the system from the flask into a disposable beaker by 50mL of n-propanol for three times, and carrying out titration test by using 0.5mol/L KOH-ethanol standard solution to obtain the titration test 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) 1g of sample of 2# polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 150mL beaker, 100mL of a pyridine-methanol mixed solvent (wherein the pyridine volume: methanol volume is 1: 1) was added, and the sample was dissolved by shaking the bottle. After the sample is dissolved, performing titration test by using 0.5mol/L KOH-ethanol standard solution to obtain the titration test value of the 2# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

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 3# polylactic acid sample to be tested (weighing accurate to 0.1mg) in a 250mL iodine measuring bottle, adding 20mL of acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 130g/L) in a fixed volume, and shaking the bottle to dissolve the sample. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 130 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 10mL of pure water was added to the iodophor, and the hydrolysis was continued under reflux for 50 min. After the reaction is finished, cooling the system to the temperature of 27 ℃, uniformly flushing the condenser tube by 10mL of isopropanol dropwise, then taking down the condenser tube, taking out the iodometric bottle from the oil bath, washing the system from the bottle into a disposable beaker by 50mL of isopropanol in three times, and carrying out titration test by using a 1.0mol/L KOH-methanol standard solution to obtain the titration test value of the 3# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric vial, 20mL of acetic anhydride-imidazole-pyridine solution (where the concentration of acetic anhydride is 130g/L) was added volumetrically and the vial was shaken. Connecting an air condenser tube to an iodine measuring flask, and putting the flask into an oil bath at 130 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 10mL of pure water was added, and the hydrolysis was continued under refluxing for 50 min. After the reaction is finished, cooling the system to the temperature of 29 ℃, uniformly flushing the condenser tube by 10mL of isopropanol dropwise, then taking down the condenser tube, taking out the iodometric flask from the oil bath, washing the system from the flask into a disposable beaker by 50mL of isopropanol for three times, and carrying out titration test on the standard solution by using a 1.0mol/L KOH-methanol standard solution to obtain the titration test 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) Weigh 4g of # 3 polylactic acid sample to be tested (weigh to 0.1mg accurately) into a 150mL beaker. 100mL of a mixed solvent of pyridine and methanol (pyridine volume: methanol volume: 1) was added, and the bottle was shaken to dissolve the sample. After the sample is dissolved, performing titration test by using a 1.0mol/L KOH-methanol standard solution to obtain a titration test value of the 3# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

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 sample of the 4# polylactic acid to be tested (weighing accurate to 0.1mg) in a 250mL iodine vial, adding 50mL of acetic anhydride-imidazole-pyridine solution (wherein the concentration of acetic anhydride is 95g/L) in a fixed volume, and shaking the vial to dissolve the sample. Connecting an air condenser pipe to an iodine measuring flask, and putting the flask into an oil bath at 120 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 100mL of pure water was added, and the hydrolysis was carried out for 60min under continued stirring and refluxing. After the reaction is finished, uniformly washing the condenser tube by 10mL of ethylene glycol drop by drop, then taking down the condenser tube, taking out the iodine measuring bottle from the oil bath, cooling the system to 25 ℃, washing the system from the bottle to a disposable beaker by 50mL of ethylene glycol in three times, and carrying out titration test by using 1.5mol/L KOH-water standard solution to obtain the titration test value of the 4# polylactic acid sample to be tested.

Carrying out a blank test: in a 250mL iodometric vial, 50mL of acetic anhydride-imidazole-pyridine solution (where the acetic anhydride concentration is 95g/L) was added in volume and the vial was shaken. Connecting an air condenser pipe to an iodine measuring flask, and putting the flask into an oil bath at 120 +/-2 ℃ for heating and refluxing for 1 h; wherein, the liquid level of the oil bath needs to be soaked by half of the body of the iodine measuring bottle. After heating and refluxing for 1 hour, 100mL of pure water was added, and the hydrolysis reaction was carried out for 60min under continued refluxing and stirring. After the reaction is finished, cooling the system to the temperature of 25 ℃, uniformly washing the condenser tube by 10mL of ethylene glycol drop by drop, then taking down the condenser tube, taking out the iodine measuring bottle from the oil bath, washing the system from the bottle to a disposable beaker by 50mL of ethylene glycol three times, and carrying out titration test on a standard solution by using 1.5mol/L KOH-water standard solution to obtain a 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 obtained 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 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) 5g of sample of the 4# polylactic acid to be tested (weighing accurate to 0.1mg) was weighed into a 150mL beaker, 100mL of a mixed solvent of pyridine and methanol was added (wherein pyridine volume: methanol volume is 1: 1), and the bottle was shaken to dissolve the sample. After the sample is dissolved, performing titration test by using 1.5mol/L KOH-water standard solution to obtain the titration test value of the 4# polylactic acid sample to be tested. According to the following calculation formula, the apparent acid value of the system is obtained.

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 the 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 the known methods shown in each comparative example, and the measured actual hydroxyl value was far from the predicted 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;

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

(2) contacting a polylactic acid sample to be detected with the organic alkali, and carrying out heating reflux treatment; then cooling the system to the 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;

(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;

the acid anhydride in the acid anhydride solution containing the organic base is acetic anhydride or phthalic anhydride;

the organic base is pyridine and/or imidazole;

in the step (1) and the step (2), the process conditions of the heating reflux treatment include: the treatment temperature is 90-130 deg.C, and the treatment time is 0.5-2 hr.

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

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

in the formula:

v1-the dosage of the alkali standard solution used in the titration of the polylactic acid sample to be detected, mL;

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

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 claim 1, wherein the operation procedure of step (2) is as follows:

in the formula (I), the compound is shown in the specification,

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 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 4,

the organic base is 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 4, 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 acid anhydride solution is 95-150g acid anhydride/L organic base; the dosage of the organic alkali-containing acid anhydride solution is 10-50 mL;

the adding amount of the water is 10-100 mL;

the alkali standard solution is a KOH-water solution, a KOH-ethanol solution, a KOH-methanol solution or a NaOH-water solution; 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 4, 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.