CN114184558A - Method for determining content of instable end group of polyformaldehyde and application thereof - Google Patents
Method for determining content of instable end group of polyformaldehyde and application thereof Download PDFInfo
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Abstract
The invention relates to the field of analysis of polyformaldehyde resin, and particularly relates to a method for determining the content of unstable terminal groups of polyformaldehyde and application thereof. The method comprises the following steps: 1) heating a heating unit filled with a polyformaldehyde sample under the flowing of inert gas, purging the polyformaldehyde sample by using the inert gas, leading out gas from the heating unit, and leading the led-out gas through a gas collection unit filled with formaldehyde absorption liquid; 2) decomposing all the unstable polyformaldehyde end groups into formaldehyde gas at a temperature at which the unstable polyformaldehyde end groups can be decomposed into formaldehyde gas; 3) collecting and diluting formaldehyde absorption liquid, mixing the obtained diluted liquid with acetylacetone, and carrying out color reaction; 4) the absorbance A of the mixture after the color reaction was measured under the condition that the wavelength of the light source was 412nm1(ii) a 5) According to
Calculating the content of unstable end groups Cppm of the polyformaldehyde. The method is easy to operate, small in error, simple and efficient.
Description
Technical Field
The invention relates to the field of analysis of polyformaldehyde resin, and particularly relates to a method for determining the content of unstable terminal groups of polyformaldehyde and application thereof.
Background
Since polyoxymethylene has excellent strength, modulus, abrasion resistance, toughness, fatigue resistance and creep resistance, it is an engineering plastic with excellent comprehensive properties after polyamide synthesis from Dupont in the United states before and after 1995, and also has excellent electrical insulation, solvent resistance and processability, and is one of five common engineering plastics at present.
Polyformaldehyde is formed by polymerizing formaldehyde, so that the molecular chain end of polyformaldehyde contains an unstable terminal group structure of hemiacetal, the thermal stability of the structure is extremely poor, and the structure is easily decomposed by heating, so that the polyformaldehyde is inevitably accompanied with a reaction for removing formaldehyde in the processing process. The content of unstable end groups in the paraformaldehyde directly affects the thermal stability of the polyoxymethylene. Therefore, the content of unstable end groups in the production process of the polyformaldehyde resin is an important parameter related to the performance of polyformaldehyde products, and has important significance for the research and development and production of the polyformaldehyde products.
At present, two main methods for testing the unstable polyformaldehyde end group are a weight loss method and an absorption method.
The weight loss method adopts a thermal weight loss analyzer or directly weighs a certain amount of polyformaldehyde particles, and after the polyformaldehyde particles are placed in a vacuum drying oven and heated for a certain time, the mass loss rate is weighed and calculated to be used as the content of unstable terminal groups. For example, in CN102532795A, a thermogravimetric method is used to detect the unstable end group content of a copolyoxyformaldehyde sample. Specifically, the weight is m0Placing the copolyoxymethylene sample in a test tube, vacuumizing, heating to 222 ℃, keeping for 60min, cooling to room temperature, weighing, and recording the weight as m1The content W of polyoxymethylene unstable terminal groups is (m)0-m1)/m0. The content of unstable terminal groups of formaldehyde is measured by a weight loss method by CN1043650C and the like. Although the weight loss method is adopted to test the content of the unstable terminal group of the polyformaldehyde, the method is simple, convenient and quick, but has lower analysis precision and larger error.
The absorption method is characterized in that unstable end groups in polyformaldehyde are separated from polyformaldehyde by a heating method by utilizing the characteristic of poor thermal stability of the unstable end groups of the polyformaldehyde, the unstable end groups are absorbed by absorption liquid, and then the content of the unstable end groups in the absorption liquid is tested. For example, CN1131255C discloses a method for collecting and testing unstable terminal groups, specifically, 1g of polyoxymethylene and 100ml of 50% aqueous methanol solution containing 0.5% ammonium hydroxide are put together into a heating unit, heated at 180 ℃ for 45min, cooled and taken out, and the weight of formaldehyde in the solution is tested, and the ratio of the weight to the weight of polyoxymethylene (1g) is the content of unstable terminal groups. In the method, the gram number of a polyformaldehyde sample is small, so that serious analysis errors can be caused, the heating temperature is 180 ℃, and the test errors can be further increased by placing the sample in the mixed absorption liquid; the method disclosed in CN1798800A is to heat polyoxymethylene at 259 ℃ for 30min, absorb the generated formaldehyde gas with sodium sulfite aqueous solution, and test the formaldehyde content by hydrochloric acid titration method, in the method, too high melting temperature of polyoxymethylene easily causes yellowing, charring of polyoxymethylene resin or excessive decomposition of polyoxymethylene molecules, resulting in increased test error.
Therefore, how to efficiently and accurately test the content of unstable terminal groups in polyoxymethylene is an urgent problem to be solved in the development and production of polyoxymethylene.
Disclosure of Invention
The invention aims to solve the problems of large testing error and low precision of the instable polyformaldehyde end group in the prior art, and provides a method for determining the content of the instable polyformaldehyde end group and application thereof.
In order to achieve the above object, the present invention provides, in a first aspect, a method for determining the content of unstable terminal groups of polyoxymethylene, the method comprising the steps of:
1) heating a heating unit filled with a polyformaldehyde sample under the flowing of inert gas, purging the polyformaldehyde sample by using the inert gas, leading out gas from the heating unit, and leading the led-out gas through a gas collection unit filled with formaldehyde absorption liquid;
2) decomposing all the unstable polyformaldehyde end groups into formaldehyde gas at a temperature at which the unstable polyformaldehyde end groups can be decomposed into formaldehyde gas;
3) collecting and diluting formaldehyde absorption liquid, mixing the obtained diluted liquid with acetylacetone, and carrying out color reaction;
4) the absorbance A of the mixture after the color reaction was measured under the condition that the wavelength of the light source was 412nm1；
5) The polyoxymethylene unstable terminal group content C ppm was calculated according to formula (1),
wherein A is2The absorbance of the blank sample is shown as a, 0.0000, m is the weight of the polyformaldehyde sample, the unit is g, K is 0.1082, n is the volume of the formaldehyde absorption liquid multiplied by the dilution multiple of the formaldehyde absorption liquid, the volume of the formaldehyde absorption liquid is mL, and the value of n is 3000-10000.
Preferably, in the step 2), the temperature at which the polyformaldehyde unstable terminal group can be decomposed into formaldehyde gas is 170-230 ℃; more preferably, in step 2), the temperature at which the polyformaldehyde unstable terminal groups can be decomposed into formaldehyde gas is 190-230 ℃.
Preferably, in the step 2), the mixture is heated for 30-90min at a temperature at which the unstable polyformaldehyde end groups can be decomposed into formaldehyde gas, so that the unstable polyformaldehyde end groups are completely decomposed into formaldehyde gas; more preferably, in the step 2), the polyformaldehyde unstable terminal groups are heated for 40-80min at a temperature at which the polyformaldehyde unstable terminal groups can be decomposed into formaldehyde gas, so that the polyformaldehyde unstable terminal groups are completely decomposed into formaldehyde gas.
Preferably, in the formula (1), the value of n is 4000-.
Preferably, in step 1), the weight of the polyoxymethylene sample is 90-110 g; more preferably, in step 1), the weight of the polyoxymethylene sample is 95 to 105 g.
Preferably, in step 1), the formaldehyde absorption liquid is water.
Preferably, the temperature for heating the heating unit in the step 1) is 170-230 ℃; more preferably, the temperature for heating the heating unit in step 1) is 190-230 ℃.
Preferably, in step 1), the inert gas is one or more of nitrogen, helium, neon, argon, krypton and xenon; more preferably, in step 1), the inert gas is nitrogen.
Preferably, in the step 1), the flow rate of the inert gas in purging is 5-20L/h; more preferably, in the step 1), the flow rate of the inert gas in purging is 7-15L/h.
Preferably, in the step 1), the dosage of the formaldehyde absorption liquid is 1-100mL relative to 1g of the polyformaldehyde; more preferably, the dosage of the formaldehyde absorption liquid is 1-50mL relative to 1g of the polyoxymethylene sample; further preferably, the formaldehyde absorption liquid is used in an amount of 4 to 11mL per 1g of the polyoxymethylene sample.
Preferably, in the step 3), the collected formaldehyde absorption liquid is diluted by 2-20 times; more preferably, in step 3), the collected formaldehyde absorption solution is diluted by 8-12 times.
Preferably, the volume ratio of the diluent to the acetylacetone is 1:1 to 20; more preferably, the volume ratio of the diluent to the acetylacetone is 1: 1-10; further preferably, the volume ratio of the diluent to the acetylacetone is 1: 1-5.
Preferably, in step 3), the conditions of the color reaction include: the reaction temperature is 50-70 ℃, and the reaction time is 5-15 min; more preferably, the conditions of the color reaction include: the reaction temperature is 55-65 ℃ and the reaction time is 10-12 min.
A second aspect of the invention provides the use of the method of the first aspect of the invention for determining the content of unstable end groups in copolyoxymethylenes and homopolymethylenes.
According to the technical scheme, the absorbance and the formaldehyde concentration of the unstable polyformaldehyde end group are correlated, a function equation of the absorbance and the formaldehyde concentration of the unstable polyformaldehyde end group are provided, the operation curve is fitted, the universal coefficient of the function equation is obtained, the absorbance is measured according to the method provided by the invention, the content of the unstable polyformaldehyde end group can be accurately obtained, the measurement error is small, and the method is accurate and reliable. In addition, the method can determine the unstable terminal group content of copolymerized polyformaldehyde and homopolymerized polyformaldehyde.
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FIG. 1 is a diagram of an apparatus for measuring the content of unstable terminal groups in polyoxymethylene in a preferred embodiment of the present invention;
FIG. 2 is a graph showing the operation of the concentration of formaldehyde and the absorbance A according to the present invention.
Description of the reference numerals
1. Oil bath pan 2, three-neck flask
3. Air inlet pipe 4 and thermometer
5. Exhaust pipe 6 and gas collecting bottle
Detailed Description
The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.
The invention provides a method for determining the content of unstable terminal groups of polyformaldehyde, wherein the method comprises the following steps:
1) heating a heating unit filled with a polyformaldehyde sample under the flowing of inert gas, purging the polyformaldehyde sample by using the inert gas, leading out gas from the heating unit, and leading the led-out gas through a gas collection unit filled with formaldehyde absorption liquid;
2) decomposing all the unstable polyformaldehyde end groups into formaldehyde gas at a temperature at which the unstable polyformaldehyde end groups can be decomposed into formaldehyde gas;
3) collecting and diluting formaldehyde absorption liquid, mixing the obtained diluted liquid with acetylacetone, and carrying out color reaction;
4) the absorbance A of the mixture after the color reaction was measured under the condition that the wavelength of the light source was 412nm1；
5) The polyoxymethylene unstable terminal group content C ppm was calculated according to formula (1),
wherein A is2The absorbance of the blank sample is a is 0.0000, m is the weight of the polyformaldehyde sample, the unit is g, K is 0.1082, n is the total volume of the formaldehyde absorption liquid multiplied by the formaldehyde absorptionThe dilution factor of the collected liquid is mL, the volume of the formaldehyde absorption liquid is mL, and the value of n is 3000-10000.
In the present invention, the working curve of formaldehyde concentration versus absorbance A is shown in FIG. 2. From the functional relationship a ═ kc + a, fitting is performed on fig. 2 to obtain the formula (1), and the inventors of the present invention have found that k may be 0.1082 and a may be 0.0000 only when the value of n is within the range defined by the present invention.
Therefore, the method and the calculation formula in the formula (1) provided by the invention can be used for accurately and efficiently measuring the content of unstable terminal groups in the polyformaldehyde sample.
According to the present invention, the heating unit may be any of various devices in the art that can be heated and maintained at a constant temperature, such as a flask, and is not particularly limited. By heating the polyformaldehyde sample placed in the heating unit, unstable terminal groups of polyformaldehyde can be decomposed into formaldehyde gas under the heating condition, and the formaldehyde gas is led out to the gas collection unit filled with formaldehyde absorption liquid along with the inert gas through the circulation of the inert gas, so that the collection of the unstable terminal groups in the polyformaldehyde sample is realized.
In the present invention, the heating means is not particularly limited, and various methods generally used in the art for heating may be used. For example, various methods such as electric heating, open flame heating, and oil bath heating can be employed. In the invention, because the oil bath heating can make the heating more uniform and is beneficial to keeping the constant temperature, the heating unit is preferably heated by adopting the oil bath heating mode.
In addition, according to a particularly preferred embodiment of the present invention, the heating is performed using an immersion type oil bath, and high temperature resistant silicone oil having a large heat capacity is used as a heating medium, whereby it is possible to precisely control the heating temperature and uniformly heat the polyoxymethylene sample in the container.
According to the invention, the heating unit is provided with an air inlet part and an air outlet part, the air inlet part is used for introducing air into the heating unit, and the air outlet part is used for leading the air out. Thus, the inert gas can be introduced into the heating unit through the gas inlet portion, and the formaldehyde gas generated after heating can be discharged out of the heating unit together with the inert gas through the gas outlet portion.
According to the present invention, the gas collecting unit may be various devices used for collecting gas in the art, such as a gas collecting bottle, and the like, and is not particularly limited.
According to the invention, a polyformaldehyde sample is placed in a heating unit for heating, and in order to reduce experimental error and ensure the accuracy of a measurement result, the weight of the polyformaldehyde sample is preferably 90-110 g; more preferably 95 to 105 g; more preferably 99 to 101 g. Particularly preferably, the weight of the polyoxymethylene sample is 100 ± 0.3 g. When the weight of the polyoxymethylene sample is within this range, the accuracy of the measurement result can be significantly improved.
Examples of the weight of the polyoxymethylene sample include: 99.97g, 99.99g, 100.01g and 100.02 g.
In the invention, the heating temperature of the heating unit is 170-230 ℃, and preferably, the heating temperature of the heating unit is 190-230 ℃. Heating at this temperature can cause the polyoxymethylene sample to form a melt.
Examples of the heating temperature of the heating means include: 190 ℃ and 230 ℃.
According to the invention, inert gas is introduced into the heating unit while the heating unit containing the polyformaldehyde sample is heated, and the polyformaldehyde sample is purged by the inert gas, so that formaldehyde gas generated by the decomposition of unstable terminal groups of the polyformaldehyde sample flows out of the heating unit along with the inert gas.
In the present invention, the inert gas may be any of various gases generally used in the art that do not react with formaldehyde gas and polyoxymethylene at high temperatures. For example, it may be one or more of nitrogen, helium, neon, argon, krypton and xenon; preferably, the inert gas is nitrogen.
According to the present invention, the flow rate of the inert gas can be varied within a wide range when purging with the inert gas. For example, the flow rate of the inert gas during purging may be 5 to 20L/h, and preferably, the flow rate of the inert gas during purging is 7 to 15L/h. By limiting the flow rate of the inert gas in purging to the above range, the formaldehyde gas generated by heating can be led out of the heating unit smoothly and efficiently, and the accuracy of measurement can be further improved.
The flow rate of the inert gas during purging may be, for example, 7L/h, 10L/h or 15L/h.
In the present invention, the speed of the gas to be introduced from the heating unit is not particularly limited, and may be the same as the flow rate of the inert gas. For example, the flow rate of the gas drawn from the heating unit may also be 5 to 20L/h.
According to the invention, the unstable terminals of polyoxymethylene are all decomposed into formaldehyde gas at a temperature at which the unstable terminals of polyoxymethylene can be decomposed into formaldehyde gas. Wherein the temperature at which the unstable terminals of polyformaldehyde can be decomposed into formaldehyde gas is 170-230 ℃; preferably, the temperature at which the polyformaldehyde unstable terminal groups can be decomposed into formaldehyde gas is 190-230 ℃. In a preferred embodiment of the invention, polyoxymethylene samples are heated to 190 ℃ and 230 ℃ respectively, so that the polyoxymethylene unstable end groups decompose into formaldehyde gas.
Generally, in the related determination experiments in the field, only the temperature of the upper part or the upper surface of the sample is tested, but the inventor of the present invention has accidentally found in the experimental process that the temperature of the sample itself needs to be tested, i.e., a thermometer is inserted into the interior of the sample to be tested, so as to ensure that the decomposition of the polyoxymethylene unstable terminal groups into formaldehyde gas is carried out under the above temperature conditions. Therefore, the reliability of the test can be obviously improved, and the experimental error is greatly reduced.
Then, after reaching the temperature, heating for 30-90min at the temperature to completely decompose the unstable polyformaldehyde end groups into formaldehyde gas; preferably, heating for 40-80min to decompose all the unstable terminals of polyformaldehyde into formaldehyde gas. By controlling the heating time within the range, all unstable terminal groups in the polyformaldehyde sample can be decomposed into formaldehyde gas, and the formaldehyde gas is led out of the heating unit by inert gas and enters the gas collection unit.
Examples of the heating time include 40min, 50min, 60min and 80 min.
Through the steps, unstable terminal groups in the polyformaldehyde sample can be completely decomposed into formaldehyde gas and collected into the gas collecting unit.
According to the invention, the gas collection unit is filled with formaldehyde absorption liquid. The formaldehyde absorption liquid can be various absorption liquids commonly used in the art for absorbing formaldehyde, for example, the formaldehyde absorption liquid can be one or more of water, ethanol and acetone; preferably, the formaldehyde absorption liquid is water.
In addition, the amount of the formaldehyde absorbing solution can be determined according to the weight of the polyoxymethylene sample, for example, in the step 1), the weight of the formaldehyde absorbing solution can be 1 to 100mL relative to 1g of the polyoxymethylene, preferably, the weight of the formaldehyde absorbing solution is 1 to 50mL relative to 1g of the polyoxymethylene sample, and more preferably, the weight of the formaldehyde absorbing solution is 4 to 11mL relative to 1g of the polyoxymethylene sample.
In a preferred embodiment of the invention, when the weight of the polyoxymethylene sample is 99.99g, the weight of the formaldehyde absorption liquid is 400 mL; when the weight of the polyformaldehyde sample is 100.02g, the weight of the formaldehyde absorption liquid is 1000 mL; when the weight of the polyformaldehyde sample is 100.01g, the weight of the formaldehyde absorption liquid is 800 mL; when the weight of the polyoxymethylene sample was 99.97g, the weight of the formaldehyde absorbent was 400 mL.
In order to further improve the accuracy of the measurement result, a plurality of gas collection units can be arranged. For example, the number of the gas collecting units containing the formaldehyde absorption liquid can be 2-8, and preferably, the number of the gas collecting units containing the formaldehyde absorption liquid is 2-5. For example, in a preferred embodiment of the present invention, the number of the gas collecting units containing the formaldehyde absorbing liquid is 2, 4, or 5. By limiting the number of the gas collecting units within this range, the accuracy of the measurement result can be further ensured.
According to the invention, after all the unstable polyformaldehyde end groups are decomposed into formaldehyde gas, the formaldehyde absorption liquid in the gas collection unit is collected and diluted, so that the concentration of the formaldehyde absorption liquid is controlled, and the accuracy of determination is improved.
According to the invention, when the formaldehyde absorption liquid is diluted, the amount and dilution multiple of the formaldehyde absorption liquid need to satisfy the value of n in the formula (1), and the product of the volume (in mL) of the formaldehyde absorption liquid and the dilution multiple of the formaldehyde absorption liquid needs to be within the value range of n, that is, the product of the volume of the formaldehyde absorption liquid and the dilution multiple of the formaldehyde absorption liquid needs to be within 3000-10000. Preferably, the dilution factor is 2-20; more preferably, the collected formaldehyde absorption solution is diluted by 8 to 12 times. In a particularly preferred embodiment of the invention, the formaldehyde absorption solution is diluted by a factor of 10.
According to the present invention, a diluted formaldehyde absorbing solution is mixed with acetylacetone to carry out a color reaction. The volume ratio of the diluent to the acetylacetone can be 1: 1-20; preferably, the volume ratio of the diluent to the acetylacetone is 1: 1-10; more preferably, the volume ratio of the diluent to the acetylacetone is 1: 1-5. In a particularly preferred embodiment of the invention, the volume ratio of the diluent to the acetylacetone is 1: 1.
In the present invention, the conditions of the color reaction include: the reaction temperature is 50-70 ℃, and the reaction time is 5-15 min; preferably, the conditions of the color reaction include: the reaction temperature is 55-65 ℃ and the reaction time is 10-12 min.
After the color reaction, after the temperature is cooled to 10-30 ℃, under the condition that the wavelength of a light source is 412nm, the absorbance A of the mixed solution after the color reaction is measured1。
In addition, in order to reduce the experimental error, a blank control experiment was further provided, and the absorbance of a blank sample was measured. That is, in an environment under otherwise identical conditions, the diluted solution was replaced with the same volume of distilled water, mixed with acetylacetone, and the absorbance A of the blank sample was measured in the same manner2。
Hereinafter, the method for measuring the content of unstable terminal groups in polyoxymethylene according to the present invention will be specifically described with reference to a particularly preferred embodiment of the present invention.
FIG. 1 is a diagram of an apparatus for measuring the content of unstable terminal groups in polyoxymethylene in a preferred embodiment of the present invention. According to FIG. 1, the heating unit of the test apparatus is a three-necked flask 2, which is heated by an oil bath pan 1. One of the three ports of the three-neck flask 2 is connected with an air inlet pipe 3, and the other end of the air inlet pipe 3 is connected with a nitrogen supply source (not shown in the figure) which can stably supply nitrogen and is provided with a gas flowmeter, so that the nitrogen can be introduced into the three-neck flask 2; the second interface is connected with the thermometer 4, the connection port is sealed, and the thermometer 4 can be used for measuring the temperature inside the polyformaldehyde sample; the last interface is connected with an exhaust pipe 5, the exhaust pipe 5 is connected with a plurality of gas collecting units, the gas collecting units are gas collecting bottles 6, and water is filled in the gas collecting bottles 6 to be used as formaldehyde absorption liquid, so that the gas discharged from the three-neck flask 2 is absorbed by the formaldehyde absorption liquid in the gas collecting bottles 6.
When the device shown in FIG. 1 is used for measuring the content of unstable terminal groups in a polyformaldehyde sample, firstly, the temperature of an oil bath pot 1 is raised to 170-230 ℃, and the temperature is kept constant, then, the polyformaldehyde sample is weighed and placed into a three-neck flask 2, the three-neck flask 2 is immersed into the oil bath pot 1, and meanwhile, a gas inlet pipe 3 is used for purging the polyformaldehyde sample. In addition, the gas discharged from the exhaust pipe 5 is absorbed by the gas collecting bottle 6 filled with formaldehyde absorption liquid. And when the thermometer indicates that the internal temperature of the polyformaldehyde sample is 170-230 ℃, timing is started, and the testing time is 30-90 min.
After the testing time is reached, quickly removing the gas collecting bottles 6, uniformly mixing and diluting the absorption liquid in a plurality of gas collecting bottles 6, and mixing the diluted dilution liquid with acetylacetone to obtain a mixed liquid; simultaneously using distilled water to replace diluent and acetylacetone to mix in the same proportion to obtain blank control solution, respectively carrying out color reaction on the mixed solution and the blank control solution, cooling to room temperature after color reaction, and respectively measuring absorbance A of the mixed solution and the blank control solution after reaction under the condition that the wavelength is 412nm1And A2Next, the content of unstable terminal groups in the polyoxymethylene sample was calculated according to formula (1).
A second aspect of the invention provides the use of the method of the first aspect of the invention for determining the content of unstable end groups in copolyoxymethylenes and homopolymethylenes.
The present invention will be described in detail below by way of examples. In the following examples, comparative examples and proof examples, a polyoxymethylene sample was prepared from a copolyformaldehyde MC90 of methanol division, national group of energy resources, Ningxia coal industry, Ltd; acetylacetone was purchased from Goodronic Chemicals, Inc. as an analytical grade.
The following examples and comparative examples were carried out using the apparatus shown in FIG. 1.
Example 1
1) Heating the oil bath kettle to 190 ℃, and keeping the constant temperature;
2) accurately weighing 99.99g of polyformaldehyde sample, putting the polyformaldehyde sample into a three-neck flask 2, enabling a thermometer 4 to measure the internal temperature of the polyformaldehyde sample, then immersing the three-neck flask into an oil bath pot 2, simultaneously introducing nitrogen through an air inlet pipe 3 to sweep the polyformaldehyde sample, wherein the flow of the nitrogen during sweeping is 7L/h, leading gas out of the three-neck flask 2 through an exhaust pipe 5 into gas collection bottles 6, wherein the number of the gas collection bottles 6 is 2, the gas collection bottles are connected in series, and 200mL of distilled water is filled in each gas collection bottle 6 to serve as formaldehyde absorption liquid;
3) when the temperature indicated by the thermometer reaches 190 ℃, timing is started, and the testing time is 40 min;
4) after the testing time is reached, quickly removing the gas collecting bottle 6, uniformly mixing the formaldehyde absorption liquid in 2 gas collecting bottles 6, measuring 10mL of formaldehyde absorption liquid, transferring the formaldehyde absorption liquid into a 100mL volumetric flask, adding water to dilute the formaldehyde absorption liquid to a scale, measuring 5mL of the diluted formaldehyde absorption liquid, putting the diluted formaldehyde absorption liquid into a graduated tube, adding 5mL of acetylacetone, putting the graduated tube and another graduated tube filled with 5mL of distilled water and 5mL of acetylacetone into a water bath at 55 ℃, and heating for 10 min;
5) after the reaction time was reached, the two graduated tubes were cooled to 25 ℃, and an appropriate amount of each solution was taken from the two graduated tubes and transferred into a 10mm cuvette, and the absorbance was measured under the condition that the wavelength of the light source was 412nm, and the content of the unstable terminal groups of polyoxymethylene was calculated according to formula (1), and the results are shown in table 1.
Example 2
1) Heating the oil bath kettle to 230 ℃, and keeping the constant temperature;
2) accurately weighing 100.02g of polyformaldehyde sample, putting the polyformaldehyde sample into a three-neck flask 2, enabling a thermometer 4 to measure the internal temperature of the polyformaldehyde sample, then immersing the three-neck flask into an oil bath pot 2, simultaneously introducing nitrogen through an air inlet pipe 3 to sweep the polyformaldehyde sample, wherein the flow of the nitrogen during sweeping is 15L/h, leading gas out of the three-neck flask 2 through an exhaust pipe 5 into gas collection bottles 6, the number of the gas collection bottles 6 is 5, the gas collection bottles are connected in series, and 200mL of distilled water is filled in each gas collection bottle 6 to serve as formaldehyde absorption liquid;
3) when the temperature indicated by the thermometer reaches 230 ℃, timing is started, and the testing time is 80 min;
4) after the testing time is reached, quickly removing the gas collecting bottles 6, uniformly mixing formaldehyde absorption liquid in 5 gas collecting bottles 6, measuring 5mL of formaldehyde absorption liquid, transferring the formaldehyde absorption liquid into a 50mL volumetric flask, adding water to dilute the formaldehyde absorption liquid to a scale, measuring 6mL of the diluted formaldehyde absorption liquid, putting the diluted formaldehyde absorption liquid into a graduated tube, adding 6mL of acetylacetone, putting the graduated tube and another graduated tube filled with 6mL of distilled water and 6mL of acetylacetone into a 65-DEG C water bath, and heating for 12 min;
5) after the reaction time was reached, the two graduated tubes were cooled to 25 ℃, and an appropriate amount of each solution was taken from the two graduated tubes and transferred to a 10mm cuvette, and the absorbance was measured under the condition that the wavelength of the light source was 412nm, and the content of the unstable terminal groups of polyoxymethylene was calculated according to formula (1), and the results are shown in table 1.
Example 3
1) Heating the oil bath kettle to 190 ℃, and keeping the constant temperature;
2) accurately weighing 100.01g of polyformaldehyde sample, putting the polyformaldehyde sample into a three-neck flask 2, enabling a thermometer 4 to measure the internal temperature of the polyformaldehyde sample, then immersing the three-neck flask into an oil bath pot 2, simultaneously introducing nitrogen through an air inlet pipe 3 to sweep the polyformaldehyde sample, wherein the flow of the nitrogen during sweeping is 10L/h, leading gas out of the three-neck flask 2 through an exhaust pipe 5 into gas collection bottles 6, wherein the number of the gas collection bottles 6 is 4, the gas collection bottles are connected in series, and 200mL of distilled water is filled in each gas collection bottle 6 to serve as formaldehyde absorption liquid;
3) when the temperature indicated by the thermometer reaches 190 ℃, timing is started, and the testing time is 60 min;
4) after the testing time is reached, quickly removing the gas collecting bottles 6, uniformly mixing formaldehyde absorption liquid in 4 gas collecting bottles 6, measuring 5mL of formaldehyde absorption liquid, transferring the formaldehyde absorption liquid into a 50mL volumetric flask, adding water to dilute the formaldehyde absorption liquid to a scale, measuring 10mL of the diluted formaldehyde absorption liquid, putting the diluted formaldehyde absorption liquid into a graduated tube, adding 10mL of acetylacetone, putting the graduated tube and another graduated tube filled with 10mL of distilled water and 10mL of acetylacetone into a water bath at 60 ℃, and heating for 12 min;
5) after the reaction time was reached, the two graduated tubes were cooled to room temperature, and an appropriate amount of each solution was taken from the two graduated tubes and transferred into a 10mm cuvette, and absorbance was measured under the condition that the wavelength of the light source was 412nm, and the content of the unstable terminal groups of polyoxymethylene was calculated according to formula (1), and the results are shown in table 1.
Example 4
1) Heating the oil bath kettle to 230 ℃, and keeping the constant temperature;
2) accurately weighing 99.97g of polyformaldehyde sample, putting the polyformaldehyde sample into a three-neck flask 2, enabling a thermometer 4 to measure the internal temperature of the polyformaldehyde sample, then immersing the three-neck flask into an oil bath pot 2, simultaneously introducing nitrogen through an air inlet pipe 3 to sweep the polyformaldehyde sample, wherein the flow of the nitrogen during sweeping is 15L/h, leading gas out of the three-neck flask 2 through an exhaust pipe 5 into gas collection bottles 6, wherein the number of the gas collection bottles 6 is 2, the gas collection bottles are connected in series, and 200mL of distilled water is filled in each gas collection bottle 6 to serve as formaldehyde absorption liquid;
3) when the temperature indicated by the thermometer reaches 230 ℃, timing is started, and the testing time is 50 min;
4) after the testing time is reached, quickly removing the gas collecting bottle 6, uniformly mixing the formaldehyde absorption liquid in 2 gas collecting bottles 6, measuring 5mL of formaldehyde absorption liquid, transferring the formaldehyde absorption liquid into a 50mL volumetric flask, adding water to dilute the formaldehyde absorption liquid to a scale, measuring 6mL of the diluted formaldehyde absorption liquid, putting the diluted formaldehyde absorption liquid into a graduated tube, adding 6mL of acetylacetone, putting the graduated tube and another graduated tube filled with 6mL of distilled water and 6mL of acetylacetone into a 65-DEG C water bath, and heating for 12 min;
5) after the reaction time is reached, the two graduated tubes are cooled to room temperature, an appropriate amount of solution is taken from each of the two graduated tubes and is transferred into a 10mm cuvette, the absorbance is measured under the condition that the wavelength of a light source is 412nm, the content of the unstable terminal group of the polyformaldehyde is calculated according to the formula (1), and the result is shown in table 1.
Comparative example 1
The procedure of example 1 was followed except that the number of the gas-collecting bottles 6 was 1 and the total amount of the formaldehyde absorbent was 200mL, i.e., the value of n was 2000, and the results are shown in Table 1.
Comparative example 2
The procedure of example 1 was followed, except that a thermometer 4 measures the temperature above the polyoxymethylene sample, and the results are shown in Table 1.
Comparative example 3
The procedure of example 2 was followed except that the number of the gas-collecting bottles 6 was 1 and the total amount of the formaldehyde absorbing solution was 200mL, i.e., the value of n was 2000, and the results are shown in Table 1.
Comparative example 4
The procedure of example 1 was followed, except that a thermometer 4 measures the temperature above the polyoxymethylene sample, and the results are shown in Table 1.
TABLE 1 content of unstable end groups in polyoxymethylene samples
| Numbering | A1-A2 | n | m(g) | C(ppm) |
| Example 1 | 0.149-0.001＝0.148 | 4000 | 99.99 | 54.72 |
| Example 2 | 0.148-0.001＝0.147 | 10000 | 100.02 | 54.33 |
| Example 3 | 0.150-0.001＝0.149 | 8000 | 100.01 | 55.08 |
| Example 4 | 0.147-0.001＝0.146 | 4000 | 99.97 | 53.99 |
| Comparative example 1 | 0.119-0.001＝0.118 | 2000 | 99.99 | 43.63 |
| Comparative example 2 | 0.155-0.001＝0.154 | 4000 | 99.99 | 30.94 |
| Comparative example 3 | 0.115-0.001＝0.114 | 2000 | 100.02 | 42.14 |
| |
0.103-0.001＝0.102 | 10000 | 100.02 | 37.70 |
As can be seen from the results in Table 1, the measurement of the unstable terminal group content of the polyoxymethylene samples using examples 1 to 4 of the present invention was very stable with a standard deviation of only 0.47, which is much lower than that of the comparative example. Therefore, the determination method provided by the invention can accurately determine the content of the instable end group of the polyformaldehyde, and can obviously reduce the test error.
The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including combinations of various technical features in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.
Claims (10)
1. A method for determining the content of unstable terminal groups of polyformaldehyde is characterized by comprising the following steps:
1) heating a heating unit filled with a polyformaldehyde sample under the flowing of inert gas, purging the polyformaldehyde sample by using the inert gas, leading out gas from the heating unit, and leading the led-out gas through a gas collection unit filled with formaldehyde absorption liquid;
2) decomposing all the unstable polyformaldehyde end groups into formaldehyde gas at a temperature at which the unstable polyformaldehyde end groups can be decomposed into formaldehyde gas;
3) collecting and diluting formaldehyde absorption liquid, mixing the obtained diluted liquid with acetylacetone, and carrying out color reaction;
4) the absorbance A of the mixture after the color reaction was measured under the condition that the wavelength of the light source was 412nm1；
5) The polyoxymethylene unstable terminal group content C ppm was calculated according to formula (1),
wherein A is2The absorbance of the blank sample is shown as a, 0.0000, m is the weight of the polyformaldehyde sample, the unit is g, K is 0.1082, n is the volume of the formaldehyde absorption liquid multiplied by the dilution multiple of the formaldehyde absorption liquid, the volume of the formaldehyde absorption liquid is mL, and the value of n is 3000-10000.
2. The method as claimed in claim 1, wherein, in step 2), the temperature at which the unstable terminals of polyoxymethylene can be decomposed into formaldehyde gas is 170-230 ℃;
preferably, in the step 2), the temperature at which the polyformaldehyde unstable terminal group can be decomposed into formaldehyde gas is 190-230 ℃;
preferably, in the step 2), the mixture is heated for 30-90min at a temperature at which the unstable polyformaldehyde end groups can be decomposed into formaldehyde gas, so that the unstable polyformaldehyde end groups are completely decomposed into formaldehyde gas;
more preferably, in the step 2), the polyformaldehyde unstable terminal groups are heated for 40-80min at a temperature at which the polyformaldehyde unstable terminal groups can be decomposed into formaldehyde gas, so that the polyformaldehyde unstable terminal groups are completely decomposed into formaldehyde gas.
3. The method according to claim 1 or 2, wherein in formula (1), the value of n is 4000-10000.
4. The method according to any one of claims 1 to 3, wherein in step 1), the polyoxymethylene sample has a weight of 90 to 110 g;
preferably, in step 1), the weight of the polyoxymethylene sample is 95 to 105 g;
preferably, in step 1), the formaldehyde absorption liquid is water.
5. The method as claimed in any one of claims 1 to 3, wherein the heating temperature of the heating unit in step 1) is 170-230 ℃;
preferably, in the step 1), the temperature for heating the heating unit is 190-230 ℃.
6. The method according to any one of claims 1 to 3, wherein in step 1), the inert gas is one or more of nitrogen, helium, neon, argon, krypton and xenon;
preferably, in step 1), the inert gas is nitrogen;
preferably, in the step 1), the flow rate of the inert gas in purging is 5-20L/h;
more preferably, in the step 1), the flow rate of the inert gas in purging is 7-15L/h.
7. The method according to any one of claims 1 to 3, wherein, in step 1), the amount of the formaldehyde absorbing solution used is 1 to 100mL relative to 1g of the polyoxymethylene;
preferably, the dosage of the formaldehyde absorption liquid is 1-50mL relative to 1g of the polyoxymethylene sample;
more preferably, the dosage of the formaldehyde absorption liquid is 4-11mL relative to 1g of the polyoxymethylene sample;
preferably, in the step 3), the collected formaldehyde absorption liquid is diluted by 2-20 times;
more preferably, in step 3), the collected formaldehyde absorption solution is diluted by 8-12 times.
8. The method according to any one of claims 1 to 3, wherein the volume ratio of the diluent to the acetylacetone is 1:1 to 20;
preferably, the volume ratio of the diluent to the acetylacetone is 1: 1-10;
more preferably, the volume ratio of the diluent to the acetylacetone is 1: 1-5.
9. The method according to any one of claims 1 to 3, wherein in step 3), the conditions of the color reaction include: the reaction temperature is 50-70 ℃, and the reaction time is 5-15 min;
preferably, the conditions of the color reaction include: the reaction temperature is 55-65 ℃ and the reaction time is 10-12 min.
10. Use of the method according to any one of claims 1 to 9 for determining the content of unstable end groups of copolyoxymethylenes and homopolymethylenes.
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