CN112724362A - Method for preparing low-free PPDI (p-phenylene diamine) polyurethane prepolymer - Google Patents
Method for preparing low-free PPDI (p-phenylene diamine) polyurethane prepolymer Download PDFInfo
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- C08G18/00—Polymeric products of isocyanates or isothiocyanates
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Abstract
The application discloses a method for preparing a low free PPDI polyurethane prepolymer, which comprises the following steps: step 1): mixing a single solvent with PPDI, adding dehydrated polyol to form a reaction raw material system, and then reacting to obtain a prepolymer; step 2): and evaporating the prepolymer to obtain a light component and a PPDI polyurethane prepolymer, wherein the light component comprises the single solvent and PPDI. The mass fraction of free PPDI in the prepolymer prepared by the method is reduced to 1-0.1%; in addition, PPDI crystals are not precipitated in the evaporation process of the thin film evaporator, and the phenomenon of blockage of a condenser is avoided.
Description
Technical Field
The invention belongs to the technical field of prepolymer preparation, and particularly relates to a method for preparing a low-free PPDI polyurethane prepolymer.
Background
The polyurethane has flexible formula, various product forms and excellent product performance, and is more and more widely applied in various industries. In addition to casting polyurethane, other types of polyurethane synthesis currently generally adopt a prepolymer method, i.e. a method of synthesizing a prepolymer (NCO: OH is more than 1:1) from excessive isocyanate and oligomer polyol under certain conditions, and then mixing the prepolymer and a chain-extending cross-linking agent for molding. For example, a large excess of isocyanate monomer is typically used to reduce the formation of high molecular weight species and produce a prepolymer having a linear regular structure, such as ABA, ABABA, etc., where a represents diisocyanate and B represents diol, for the preparation of polyurethane elastomers with good physical properties, however, a large amount of isocyanate monomer is not fully reacted and the prepolymer still contains a large amount of unreacted isocyanate monomer. The prepolymer prepared by the conventional synthesis method has more residual isocyanate monomers within the range of 1-10%, and the free isocyanate monomers have pungent smell, are easy to volatilize and have strong toxicity, and have accumulative property and carcinogenicity in a human body and threaten the health of the human body, so that the allowable concentration of isocyanate in the air of an industrial place is strictly regulated in many countries. The conventional preparation method of the polyurethane prepolymer is to mix and react polyol and excessive isocyanate monomer, and directly react with a chain extender or store the chain extender for later use without any treatment. The content of free isocyanate monomer in the prepolymer prepared by the conventional method is about 5 percent or even higher, and the free isocyanate monomer is easy to deteriorate or react with water in the storage process, so that the quality of the prepolymer is changed, and the quality of the polyurethane after chain extension is influenced. The content of free monomers of the low-free prepolymer is less than 1 percent and is usually about 0.1 to 0.5 percent or lower, so the low-free prepolymer better meets the requirement of environmental protection, has longer storage period and is not easy to deteriorate.
Compared with the conventional polyurethane prepolymer, the low-free polyurethane prepolymer has more excellent performance, such as good fluidity, long gel time and high forming speed, thereby shortening the demoulding time and improving the production efficiency; and secondly, the molecular chain of the low-free prepolymer has structural regularity, and the hard segment has higher crystallinity, so that the product has the advantages of small internal heat, outstanding dynamic mechanical property, high overall strength and high modulus, and is more suitable for being applied to workplaces with high dynamic property requirements. Therefore, with the enhancement of human environmental awareness, the improvement of product performance requirements and the expansion of polyurethane application range, the low-free prepolymer is an important trend of future polyurethane development.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a method for preparing a low-free PPDI polyurethane prepolymer, which is simple and can be used for continuous production, and the prepared prepolymer has low free PPDI content.
The technical scheme adopted by the invention for solving the technical problem is as follows:
a method of preparing a low free PPDI polyurethane prepolymer comprising:
step 1): mixing a single solvent with PPDI, adding dehydrated polyol to form a reaction raw material system, and then reacting to obtain a prepolymer;
step 2): and evaporating the prepolymer to obtain a light component and a PPDI-containing polyurethane prepolymer, wherein the light component comprises the single solvent and PPDI.
In the above method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, the single solvent is a purified solvent, and preferably, the purification process includes: mixing dehydrated polyalcohol, PPDI and a single solvent which is not subjected to purification treatment, reacting, and evaporating an obtained reaction product to obtain the single solvent, wherein the single solvent contains PPDI; preferably, in the purification treatment, the reaction temperature is 60-70 ℃, and the reaction time is 2-5 hours; preferably, the mass ratio of the total mass of the dehydrated polyol and the PPDI to the single solvent without purification treatment is 1: 5-1: 100 (for example, 1:8,1:12, 1: 15, 1:20,1:25, 1:30, 1:50,1:70, 1:80, 1:90), and the mass ratio of the PPDI to the dehydrated polyol is 10: 1-30: 1 (for example, 12:1,15:1, 20:1, 25: 1); preferably, the evaporation is processed in a thin film evaporator; more preferably, the temperature during evaporation is 110-130 ℃ and the vacuum degree is 0.1-2.0 mmHg. By the purification treatment, a small amount of water or alcohol or the like in the original solvent (i.e., a single solvent which has not been subjected to purification treatment) can be removed.
In the method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the reaction raw material system of the step 1), a ratio of a mass of the single solvent to a total mass of the PPDI and the dehydrated polyol is 1.6:1 or more, and preferably, a ratio of the mass of the single solvent to the total mass of the PPDI and the dehydrated polyol is 1.6 to 2.0 (e.g., 1.7,1.8, 1.9); in the conventional method for preparing a low-free prepolymer, the mass ratio of the solvent to the solute is usually higher than 2:1, but the ratio of the solvent to the solute of the present invention can be reduced to 2:1 or less, so that the amount of the solvent used can be reduced by at least 20% as compared with the conventional method.
In the method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the reaction raw material system of the step 1), a molar weight ratio of the PPDI to the dehydrated polyol is 2:1 to 10:1 (e.g., 3:1,4:1, 5:1, 6:1, 7:1, 8:1, 9: 1). When the molar ratio is less than 2:1, the resulting prepolymer contains a large amount of dimer (i.e., a polymer of PPDI-polyol-PPDI) and has an NCO value lower than the theoretical value (i.e., an NCO value of a polymer of PPDI-polyol-PPDI). However, if the molar ratio is greater than 2:1, the dimer component will be reduced and the NCO value of the prepolymer will approach the theoretical value. When the molar ratio exceeds 10:1, the theoretical NCO value can be obtained, but it is difficult to remove too much free PPDI monomer.
In the above method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the reaction raw material system of the step 1), the single solvent is at least one selected from the group consisting of monoesters, diesters, lactones, carbonates, aromatic compounds, amides, lactams, polyethers, and polyketones; preferably, said single solvent is selected from diisopropyl adipate and/or diisobutyl adipate; preferably, the solubility of the PPDI in the single solvent is equal to or greater than 5% (e.g., 7%, 10%, 20%, 30%, 40%, 50%, etc.) at 20-25 degrees; the larger solubility can improve the problem of condenser blockage caused by PPDI crystal precipitation.
In the method for preparing the low free PPDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the boiling point of the single solvent at normal pressure is 250 to 300 ℃; the application can lead PPDI and the single solvent to be distilled simultaneously by selecting the single solvent with the boiling point close to that of PPDI, thereby achieving the purpose of reducing the PPDI content in the polyurethane prepolymer only by one solvent.
In the method for preparing the low free PPDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the reaction temperature is 60 to 70 ℃, and the reaction time is 2 to 5 hours.
In the above method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the dehydrated polyol is added in multiple times to reduce the exothermic heat of a single reaction and to allow the reaction to be sufficiently performed; preferably, the interval time for adding the dehydrated polyol in several times is 15-25 min; preferably, the number of times of adding the dehydrated polyol is 2-4 times.
In the above method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the step 1), the dehydrated polyol is prepared as follows: dehydrating the polyhydric alcohol at 100-150 ℃ for 2-6 hours in vacuum to obtain dehydrated polyhydric alcohol; preferably, the polyol is selected from polyether polyols and/or polyester polyols; preferably, the molecular weight of the polyalcohol is 500-5000 g/mol; more preferably, the polyol is PTMEG 1000.
In the above method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the step 2), the evaporation temperature is 110 to 150 ℃, and the vacuum degree during evaporation is 0.1 to 2.0 mmHg.
In the above-mentioned method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, the light component obtained in step 2) is used as a raw material in step 1) for recycling.
In the above method for preparing a low free PPDI polyurethane prepolymer, as a preferred embodiment, in the step 2), the evaporation is processed in a thin film evaporator.
Compared with the prior art, the invention has the following beneficial effects:
(1) the mass fraction of free PPDI in the prepolymer prepared by the method is reduced to 1-0.1%, and preferably 0.46-0.52%.
(2) The invention has no PPDI crystal precipitation and condenser blockage during the evaporation process of the film evaporator.
(3) The invention can reduce the PPDI content in the polyurethane prepolymer only by one solvent, thereby not only avoiding the phenomenon of blockage of a condenser caused by the single use of one solvent in the prior art, but also overcoming the problems of increasing economic cost and generating more impurities caused by the use of two solvents.
(4) The invention selects the polyol with good fluidity and low viscosity to be matched with a single solvent, so that the invention is simple and convenient and has low cost.
(5) According to the invention, the single solvent is preferably purified by adopting the dehydrated polyol and PPDI, so that the single solvent is purified, the product performance is improved, heavy components can flow out, and continuous production can be realized; the purified single solvent can remove a large amount of free PPDI in the prepolymer during the subsequent preparation of the prepolymer, so that the content of the free PPDI in the prepolymer is lower.
Detailed Description
In order to highlight the objects, technical solutions and advantages of the present invention, the present invention is further illustrated by the following examples, which are presented by way of illustration of the present invention and are not intended to limit the present invention. The technical solution of the present invention is not limited to the specific embodiments listed below, and includes any combination of the specific embodiments.
The invention provides a method for preparing a low free PPDI polyurethane prepolymer, which comprises the following steps:
a) mixing dehydrated polyol, PPDI and a single solvent (the purity of the industrial single solvent is usually 98% -99%), reacting at 60-70 ℃, and treating the obtained reaction product in a thin film evaporator to obtain a treated solvent; the mass ratio of the total mass of the dehydrated polyol and the PPDI to the solvent is 1: 5-1: 100, and the mass ratio of the PPDI to the dehydrated polyol is 10: 1-30: 1; the single solvent is selected from monoesters, diesters, lactones, carbonates, aromatic compounds, amides, lactams, polyethers, or polyketones;
b) mixing the treated solvent with PPDI, then adding dehydrated polyol in batches, and reacting at 60-70 ℃ to obtain a prepolymer; the ratio of the mass of the treated solvent to the total mass of the PPDI and the dehydrated polyol is greater than or equal to 1.6: 1;
c) the prepolymer is treated in a wiped film evaporator to obtain light and heavy components. The recombinant component is a polyurethane prepolymer product, and the light component comprises a single solvent and PPDI.
According to the invention, dehydrated polyol, PPDI and a single solvent are mixed and reacted at 60-70 ℃, and the obtained reaction product is processed in a thin film evaporator to obtain the processed solvent. The anhydropolyol and PPDI are solutes. In the invention, the mass ratio of the total mass of the dehydrated polyol and the PPDI to the single solvent is 1: 5-1: 100, and the mass ratio of the PPDI to the dehydrated polyol is 10: 1-30: 1; in specific embodiments, the mass ratio of PPDI to anhydropolyol is 20: 1; the mass ratio of the total mass of the dehydrated polyol and PPDI to the single solvent was 1: 9.
The boiling point of the single solvent is 250-300 ℃ under normal pressure; the single solvent is preferably selected from monoesters, diesters, lactones, carbonates, aromatic compounds, amides, lactams, polyethers, or polyketones; more preferably from diesters; most preferably selected from diisopropyl adipate and/or diisobutyl adipate. The single solvent and PPDI have good solubility, do not react with polyol and isocyanate monomer, and are relatively stable inert solvent at high temperature. The mass ratio of the single solvent to the solute is 5: 1-100: 1, the solvent is excessive, the impurities in the solvent are removed by using isocyanate, the solvent is purified, the polyol is added to enable heavy component distillate to be in a liquid state, and as in comparative example 2 of the application, the heavy component can form blocky crystals without adding the polyol, and blockage occurs.
The polyol is preferably selected from polyether polyols and/or polyester polyols; the molecular weight of the polyol is preferably 500-5000 g/mol. In particular embodiments, the polyol is selected from PTMEG 1000. The dehydrated polyol is preferably prepared according to the following method:
and (3) dehydrating the polyhydric alcohol for 2-6 hours in vacuum at the temperature of 100-150 ℃ to obtain dehydrated polyhydric alcohol.
The reaction of the dehydrated polyol, PPDI and the single solvent is preferably carried out under the protection of nitrogen in the present invention. In the present invention, PPDI and a single solvent are preferably mixed and then added to the dehydrated polyol. Mixing dehydrated polyalcohol, PPDI and a single solvent, and reacting at 60-70 ℃; the reaction time is preferably 2-5 hours; in a specific example, the reaction time was 3 hours. The reaction is preferably carried out under stirring. The resulting reaction product is treated in a thin film evaporator to obtain the treated solvent. The temperature in the thin film evaporator is preferably 110 to 130 ℃, and the vacuum degree is preferably 0.1 to 2.0 mmHg. Condensing light components obtained after the treatment of the thin film evaporator to obtain a treated solvent, wherein the treated solvent is a purified solvent which contains a certain amount of PPDI. The condensing temperature is preferably 20-35 ℃.
After the treated solvent is obtained, mixing the treated solvent with PPDI, adding dehydrated polyol for multiple times, and reacting at 60-70 ℃ to obtain a prepolymer. In the invention, the dehydrated polyol is added into the treated solvent and PPDI for multiple times, and the interval time between two times is preferably 15-25 min, and more preferably 18-22 min; in a specific embodiment, the interval time is 20 min. And mixing the treated solvent with PPDI, adding dehydrated polyol for multiple times, and reacting at 60-70 ℃ for 2-5 hours, preferably 3-4 hours. The mass ratio of the PPDI to the dehydrated polyol is preferably 1.1: 1-15: 1, and more preferably 3-4: 1; in specific embodiments, the mass ratio of PPDI to anhydropolyol is 3.3:1 or 3.8: 1. The ratio of the mass of the treated solvent to the total mass of the PPDI and the dehydrated polyol is more than or equal to 1.6:1, and if the solvent is too little, free PPDI in the prepolymer cannot be removed; in particular embodiments, the ratio of the mass of the treated solvent to the total mass of the PPDI and anhydropolyol is 1.6:1, 2:1, or 1.85: 1.
After the prepolymer is obtained, the prepolymer is treated in a wiped film evaporator to obtain a light component and a heavy component. The recombinant component is a prepolymer product, and the light component comprises a single solvent and PPDI. In the invention, the treatment temperature is preferably 110-150 ℃, and the vacuum degree is 0.1-2.0 mmHg. Condensing and collecting light components obtained after treatment; the condensing temperature is preferably 20-35 ℃. The invention selects proper material inlet flow rate and reaction time according to different evaporator types, and the evaporator unit can be one or a plurality of evaporator units used in series. It is apparent that when two or more thin film evaporators connected in series are used, the residual content of PPDI in the polyurethane prepolymer can be further reduced, for example, to about 0.1% or less.
The light component comprises a single solvent and PPDI, and the light component can be added into the step b) for recycling. And mixing distillate obtained by condensing and distilling the light components with PPDI and polyhydric alcohol, and directly preparing a PPDI prepolymer according to the proportion of the prepared prepolymer. Therefore, the light component distillate can be recycled, the process is greatly simplified, and the cost is reduced.
In addition, the method not only can be used for preparing PPDI type low-free prepolymer, but also can be used for preparing TDI and MDI low-free prepolymer.
In the present invention, the free isocyanate monomer content is measured by high performance liquid chromatography.
To further illustrate the present invention, a method for preparing a low free PPDI prepolymer according to the present invention is described in detail below with reference to the following examples, which should not be construed as limiting the scope of the present invention.
Example 1:
and (3) solvent purification step: and (3) dehydrating under vacuum at 130 ℃ for 3 hours to remove water in the PTMEG100 polyol to obtain dehydrated PTMEG100 polyol for later use. Under the atmosphere of nitrogen, at the temperature of 65 ℃, adding 1.16Kg of PPDI into 13.9Kg of diisopropyl adipate solvent, stirring uniformly to fully dissolve the PPDI, adding 0.36Kg of the above polyol into the reactor under the condition of stirring, and continuing to stir for 3 hours. The molar ratio of PPDI to polyol was 20:1 and the mass ratio of solvent and solute (PPDI to dehydrated PTMEG100 polyol) was 9: 1. And then adding the mixture into a film evaporator, setting the internal temperature of the film at 120 ℃, setting the vacuum degree at 0.1-2.0 mmHg and the condensation temperature at 20-35 ℃, collecting light component distillate, and measuring the NCO value to be 3.85%.
Synthesizing a prepolymer and evaporating: under the protection of nitrogen, 8.4Kg of light component distillate (purified diisopropyl adipate and PPDI) is poured into a reactor, 0.83Kg of PPDI is added, the temperature is heated to 65 ℃ until the PPDI is completely dissolved, and the mixture is stirred uniformly. Maintaining the temperature, and adding the dehydrated PTMEG1000 polyol (2.76Kg) under stirring for three times at intervals of 20 min; wherein the molar ratio of PPDI to polyol is 3.3:1, and the mass ratio of solvent to solute is 1.85: 1. The reaction was continued at this temperature for 3 hours. After the reaction is finished, the obtained reaction solution is pumped into a film evaporator under the conditions of 120 ℃ and 0.1-2.0 mmHg. The separated recombinant component is polyurethane prepolymer with NCO value of 5.63%, and the light component is solvent with NCO value of 3.91%. The content of free PPDI in the prepolymer was 0.46%.
Comparative example 1 a:
the mixed solvent of dimethyl phthalate and dibasic ester DBE is mixed according to the ratio of 1:2, and the total weight of the mixed solvent is 13.7 Kg. Vacuum dehydrating polyol PTMEG1000 at 130 deg.C for 3 hr to remove water in polyol, adding 1.4Kg of PPDI into the above mixed solvent at 65 deg.C under nitrogen atmosphere, stirring to dissolve PPDI completely, adding 0.43Kg of polyol into the reactor under stirring, and stirring for 3 hr. The molar ratio of PPDI to polyol was 20:1, and the mass ratio of solvent to solute was 7.5: 1. And then adding the mixture into a film evaporator, setting the internal temperature of the film at 120 ℃, setting the vacuum degree at 0.1-2.0 mmHg and the condensation temperature at 20-35 ℃, collecting light component distillate, and measuring the NCO value to be 4.62%.
The light fraction distillate (351g) collected above was poured into a reactor under nitrogen blanket, 29.6g of PPDI was replenished, heated to 65 deg.C until PPDI was completely dissolved, and stirred well. Maintaining the temperature, adding the PTMEG1000 polyol after dehydration into the reactor for three times under the condition of stirring, wherein the interval of each time is 20min, 100g of polyol is added, the molar ratio of PPDI to the polyol is 3.8:1, and the mass ratio of the solvent to the solute is 2: 1. The reaction was continued at this temperature for 3 hours. After the reaction is finished, the obtained reaction solution is pumped into a film evaporator under the conditions of 120 ℃ and 0.1-2.0 mmHg. Separating to obtain polyurethane prepolymer with NCO value of 5.58%; the light component is a mixed solvent, and the NCO value of the mixed solvent is 4.78%. The content of free PPDI in the prepolymer was 0.47%.
Comparative example 1 b:
dimethyl phthalate (322g) was poured into the reactor under nitrogen blanket, 60.8g of PPDI was added, heated to 65 ℃ until PPDI was completely dissolved, and stirred well. After maintaining this temperature, 100g of polyol PTMEG1000 after dehydration was added in three portions with stirring, each at 20min intervals. The molar ratio of PPDI to polyol was 3.8:1, and the mass ratio of solvent to solute was 2: 1. After continuing the reaction at 65 ℃ for 3 hours, the obtained reaction solution was fed into a thin film evaporator at 120 ℃ under 0.1 to 2.0 mmHg. In the early stage of the separation process, a heavy component of the prepolymer and a light component solution containing PPDI are obtained; but in the later separation period, PPDI crystals are separated out, the light component pipeline is blocked, and the separation is stopped. (in this comparative example 1b, only dimethyl phthalate was used as a solvent)
From a comparison of comparative example 1a and example 1, it can be seen that: the free isocyanate content in the polyurethane prepolymer is basically the same, namely, the invention uses a single solvent to achieve the similar degree with two solvents, reduces the types and the usage amount of the solvents, reduces the economic cost, and uses the single solvent to ensure that no PPDI crystal is separated out in a film evaporator in the distillation process of the prepolymer and no condenser blockage phenomenon exists, thereby being capable of continuous production.
From a comparison of comparative example 1b and example 1, it can be seen that: in comparative example 1b, dimethyl phthalate alone was used as a solvent, and PPDI crystals were precipitated from the light component discharge line of the thin film evaporator during the distillation separation of the prepolymer, so that the condenser and the line were clogged, and continuous production was not possible.
Example 2:
and (3) solvent purification step: polyol PTMEG1000 was vacuum dewatered at 130 ℃ for 3 hours for use. Adding 1.16Kg of PPDI into 13.9Kg of diisopropyl adipate solvent at 65 ℃ under the nitrogen atmosphere, stirring uniformly to fully dissolve the PPDI, adding 0.36Kg of the dehydrated polyol PTMEG1000 into the reactor under the stirring condition, and continuing stirring for 3 hours. Wherein the molar ratio of PPDI to polyol is 20:1, and the mass ratio of solvent to solute is 9: 1. And then adding the mixture into a film evaporator, setting the internal temperature of the film at 120 ℃, setting the vacuum degree at 0.1-2.0 mmHg and the condensation temperature at 20-35 ℃, collecting light component distillate, and measuring the NCO value to be 3.85%.
Synthesizing a prepolymer and evaporating: under the protection of nitrogen, 8.4Kg of purified diisopropyl adipate was poured into the reactor, 1.07Kg of PPDI was added, heated to 65 ℃ until PPDI was completely dissolved, and stirred uniformly. Maintaining this temperature, the dehydrated PTMEG1000 polyol (3.18Kg) was added in three separate portions with stirring, each at 20min intervals, wherein the molar ratio of PPDI to polyol was 3.3:1 and the mass ratio of solvent to solute was 1.6: 1. After continuing the reaction at this temperature for 3 hours, the obtained reaction solution was fed into a thin film evaporator at 120 ℃ and 0.1 to 2.0 mmHg. Separating to obtain polyurethane prepolymer with NCO value of 5.72%; the light component was a solvent containing PPDI, which had an NCO value of 4.31%. The content of free PPDI in the prepolymer was 0.52%.
Comparative example 2:
and (3) solvent purification step: under the atmosphere of nitrogen, at the temperature of 65 ℃, adding 1.15Kg PPDI into 13.9Kg diisopropyl adipate solvent, and uniformly stirring to fully dissolve PPDI, wherein the mass ratio of the solvent to the solute is 12: 1. Then, the mixture is added into a film evaporator, the temperature in the film is set to be 120 ℃, the vacuum degree is set to be 0.1-2.0 mmHg, and the condensation temperature is set to be 20-35 ℃. Heavy components form blocky crystals, the discharge pipe is blocked, and the experiment is forced to stop.
In this comparative example 2, the purification solvent step, in which no polyol was added, only solvent and PPDI were added, and then poured into a thin film evaporator, after a certain period of operation, the recombination components formed blocky crystals, clogging the discharge pipe, and the experiment was stopped, indicating the necessity of adding polyol to the purification solvent.
Example 3:
and (3) solvent purification step: polyol PTMEG1000 was vacuum dewatered at 130 ℃ for 3 hours for use. Adding 1.16Kg of PPDI into 13.9Kg of diisobutyl adipate at 65 ℃ in a nitrogen atmosphere, stirring uniformly to fully dissolve the PPDI, adding 0.36Kg of polyol into a reactor under the condition of stirring, and continuing stirring for 3 hours, wherein the molar ratio of the PPDI to the polyol is 20:1, and the mass ratio of the solvent to the solute is 9: 1. And then adding the mixture into a film evaporator, setting the internal temperature of the film at 120 ℃, setting the vacuum degree at 0.1-2.0 mmHg and the condensation temperature at 20-35 ℃, collecting light component distillate, and measuring the NCO value to be 3.85%.
Synthesizing a prepolymer and evaporating: under the protection of nitrogen, 8.4Kg of purified diisobutyl adipate was poured into the reactor, 1.07Kg of PPDI was added, heated to 65 ℃ until PPDI was completely dissolved, and stirred uniformly. After maintaining the temperature, the dehydrated PTMEG1000 polyol was added in three portions with stirring, with 20min intervals, for a total of 3.18kg polyol. The molar ratio of PPDI to polyol was 3.3:1, and the mass ratio of solvent to solute was 1.6: 1. The reaction was continued at this temperature for 3 hours. After the reaction is finished, the obtained reaction solution is pumped into a film evaporator under the conditions of 120 ℃ and 0.1-2.0 mmHg. Separating to obtain polyurethane prepolymer with NCO value of 5.73%; the light component is a solvent and the NCO value is 4.06 percent. The content of free PPDI in the prepolymer was 0.50%.
Comparative example 3:
and (3) solvent purification step: polyol PTMEG1000 was vacuum dewatered at 130 ℃ for 3 hours for use. 1.16Kg of PPDI was added to diisopropyl adipate (13.9Kg) solvent under nitrogen at 65 ℃ and stirred to dissolve the PPDI sufficiently. 0.36Kg of the above polyol was added to the reaction kettle with stirring. The molar ratio of PPDI to polyol was 20:1, the mass ratio of solvent to solute was 9: 1. After reacting for 3 hours, adding the mixture into a film evaporator, setting the internal temperature of the film at 120 ℃, the vacuum degree at 0.1-2.0 mmHg, setting the condensation temperature at 20-35 ℃, collecting light component distillate, and measuring the NCO value to be 3.85%.
Synthesizing a prepolymer and evaporating: under the protection of nitrogen, 8.4Kg of purified diisopropyl adipate is poured into a reaction kettle, 1.18Kg of PPDI is added, the temperature is heated to 65 ℃, and the mixture is stirred uniformly until the PPDI is completely dissolved. The temperature was maintained and the dehydrated PTMEG1000 polyol (3.40Kg) was added in three portions with stirring, each at 20min intervals. The molar ratio of PPDI to polyol was 3.3:1, and the mass ratio of solvent to solute was 1.5: 1. After continuing the reaction at this temperature for 3 hours, the obtained reaction solution was fed into a thin film evaporator at 120 ℃ and 0.1 to 2.0 mmHg. Crystals were found to precipitate in the condensation window system because the amount of solvent was too low, resulting in a PPDI content that was too high.
In the conventional method for preparing the low-free prepolymer, the mass ratio of the solvent to the solute is usually higher than 2: 1; in the step of preparing the prepolymer of the present invention, the amount of the solvent used was gradually reduced (examples 1, 2, and 3), the ratio of the solvent to the solute was gradually reduced to 1.85, 1.6, and 1.5, and it was found that the index requirements for preparing a low-free prepolymer could be met when the ratio of the solvent to the solute was gradually reduced to 1.85 and 1.6, but by continuing to reduce to 1.5, crystals of PPDI precipitated, so it was concluded that the ratio of the solvent to the solute in the present invention was 1.6:1 at the lowest, and the amount of the solvent used was reduced by at least 20% as compared with the conventional method.
The test data for examples 1-3 and comparative examples 1-3 are given in Table 1 below:
TABLE 1
The foregoing embodiments illustrate the principles, principal features and advantages of the invention, and it will be understood by those skilled in the art that the invention is not limited to the foregoing embodiments, which are merely illustrative of the principles of the invention, and that various changes and modifications may be made therein without departing from the scope of the principles of the invention.
Claims (10)
1. A method for preparing a low free PPDI polyurethane prepolymer, comprising:
step 1): mixing a single solvent with PPDI, adding dehydrated polyol to form a reaction raw material system, and then reacting to obtain a prepolymer;
step 2): and evaporating the prepolymer to obtain a light component and a low free PPDI polyurethane prepolymer, wherein the light component comprises the single solvent and PPDI.
2. The method according to claim 1, wherein the single solvent is a purified solvent, and preferably the purification process comprises: mixing dehydrated polyalcohol, PPDI and a single solvent which is not subjected to purification treatment, reacting, and evaporating an obtained reaction product to obtain the single solvent, wherein the single solvent contains PPDI; preferably, in the purification treatment, the reaction temperature is 60-70 ℃, and the reaction time is 2-5 hours; preferably, the mass ratio of the total mass of the dehydrated polyol and the PPDI to the single solvent which is not subjected to purification treatment is 1: 5-1: 100, and the mass ratio of the PPDI to the dehydrated polyol is 10: 1-30: 1; preferably, the evaporation is processed in a thin film evaporator; more preferably, the temperature during evaporation is 110-130 ℃ and the vacuum degree is 0.1-2.0 mmHg.
3. The method according to claim 1, wherein in the reaction raw material system of the step 1), the ratio of the mass of the single solvent to the total mass of the PPDI and the dehydrated polyol is 1.6:1 or more, and preferably the ratio of the mass of the single solvent to the total mass of the PPDI and the dehydrated polyol is 1.6-2.0.
4. The method as claimed in claim 1), wherein the molar weight ratio of the PPDI to the dehydrated polyol in the reaction raw material system of the step 1) is 2: 1-10: 1.
5. The method as claimed in claim 1, wherein in the reaction raw material system of the step 1), the single solvent is selected from at least one of monoester, diester, lactone, carbonate, aromatic compound, amide, lactam, polyether or polyketone; preferably, said single solvent is selected from diisopropyl adipate and/or diisobutyl adipate; preferably, the boiling point of the single solvent is 250-300 ℃ under normal pressure; at 20-25 ℃, the solubility of the PPDI in the single solvent is equal to or greater than 5%.
6. The method according to claim 1, wherein in the step 1), the reaction temperature is 60-70 ℃, and the reaction time is 2-5 hours.
7. The process of claim 1 wherein in step 1), the anhydropolyol is added in multiple portions to reduce the exotherm for a single reaction and to allow the reaction to proceed fully; preferably, the interval time for adding the dehydrated polyol in several times is 15-25 min; preferably, the number of times of adding the dehydrated polyol is 2-4 times.
8. The method of claim 1, wherein in step 1), the dehydrated polyol is prepared by: dehydrating the polyhydric alcohol at 100-150 ℃ for 2-6 hours in vacuum to obtain dehydrated polyhydric alcohol; preferably, the polyol is selected from polyether polyols and/or polyester polyols; preferably, the molecular weight of the polyalcohol is 500-5000 g/mol; more preferably, the polyol is PTMEG 1000.
9. The method according to claim 1, wherein in the step 2), the temperature of the evaporation is 110 to 150 ℃, and the degree of vacuum of the evaporation is 0.1 to 2.0 mmHg.
10. The method according to claim 1, characterized in that the light fraction obtained in step 2) is used as a feedstock in step 1) for recycling; preferably, in step 2), the evaporation is treated in a thin film evaporator.
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