Patent Publication Number: US-2023151134-A1

Title: Reaction system for preparing polymer polyol and method for preparing polymer polyol

Description:
TECHNICAL FIELD 
     The present disclosure belongs to the field of polyurethane materials and processing thereof and, in particular, to a method for preparing polymer polyol and a reaction system for preparing polymer polyol. 
     BACKGROUND 
     Polymer polyol (POP) is a kind of modified polyether polyol with special properties, which is prepared by graft copolymerization of vinyl monomers such as acrylonitrile (An) and styrene (St) onto polyether polyol (PPG) which is the matrix. Polymer polyol not only maintains the original flexibility of the polyether chain, but also has the good structure performance of the vinyl polymer, which enables the polyurethane foam to have high load-bearing capability and excellent resilience performance and increases the open porosity of the foam. Therefore, it is widely used for the production of high load-bearing, high resilience soft and semi-rigid polyurethane foams and is applied to automobiles, trains, aircraft manufacturing, furniture industry and other fields. 
     The polymer polyol may be prepared by continuous processes in a stirred tank that supports continuous feeding and discharging or a combination of such stirred tanks that are connected in series/parallel, or in a tubular reactor or loop reactor that supports continuous feeding and discharging, or may be prepared by batch production or semi-continuous processes in a reaction kettle. 
     In the semi-continuous processes, only a portion of the raw material is initially put into the reactor, and then the remaining raw material is added into the reactor in one or more metered streams during the reaction. 
     At present, most polymer polyols in the market are prepared by continuous processes (WO00/5971, U.S. Pat. No. 6,013,731, EP0640633, U.S. Pat. No. 5,268,418, and EP365986), and such polymer polyols have the advantages of wide particle-size distribution and low viscosity. However, the continuous processes have disadvantages of large investment in process equipment, low operating flexibility and poor versatility. 
     In order to enable the polymer polyol to have both the properties of batch processes such as less investment in equipment, higher operating flexibility and better elasticity and the advantages of continuous processes such as lower viscosity, researchers have carried out some researches. For example, CN1656136A discloses two solutions. One is to prepare two kinds of polymer polyols by a batch process and mix the two polymer polyols according to a certain proportion. The other one is to synthesize a polymer polyol by a continuous process and then with the synthesized polymer polyol as the seed, prepare a polymer polyol by a batch process. However, the two solutions are difficult to implement in terms of industrial equipment. The former requires two different reactors or processes, increasing the difficulty of tank storage, metering and formulation. The latter requires the cooperation of a batch reactor and a continuous reactor, increasing the investment. 
     SUMMARY 
     In order to solve the above contradictions of polymer polyol, the present disclosure provides a reaction system capable of obtaining a low-viscosity polymer polyol and also provides a preparation process of the polymer polyol. The polymer polyol prepared based on the solution of the present disclosure has a low-viscosity effect, and a polyurethane foam having excellent mechanical properties and high hardness may be obtained by utilizing the polymer polyol prepared by the method of the present disclosure. 
     To achieve the object, the present disclosure provides the technical solutions described below. 
     An aspect of the present disclosure provides a reaction system for preparing polymer polyol, which includes a reactor, a first circulation unit, a second circulation unit and a flow direction switching unit, wherein, 
     a partition plate is provided within a reaction cavity of the reactor, and the reaction cavity is divided into a first reaction chamber and a second reaction chamber by the partition plate, the volume of the first reaction chamber is greater than the volume of the second reaction chamber, and the top of the partition plate is provided with an overflow port to communicate the first reaction chamber and the second reaction chamber with each other; 
     the first circulation unit is arranged between a discharge port and a feed port of the first reaction chamber and enables the material in the first reaction chamber to circulate between the discharge port of the first reaction chamber and the feed port of the first reaction chamber; preferably, the first circulation unit is provided with a cooler for cooling materials; 
     the second circulation unit is arranged between a discharge port and a feed port of the second reaction chamber and enables the material in the second reaction chamber to circulate between the discharge port of the second reaction chamber and the feed port of the second reaction chamber; preferably, the second circulation unit is provided with a heater for heating materials; 
     the flow direction switching unit is configured to switch the material in the first circulation unit either to flow to the feed port of the first reaction chamber or to flow to the feed port of the second reaction chamber, or switch the material in the second circulation unit either to flow to the feed port of the second reaction chamber or to flow to the feed port of the first reaction chamber. 
     In the reaction system of the present disclosure, the reactor is specifically a vertical tank structure, and the material of the partition plate is not limited and is preferably the same as the material of the reactor. 
     In some embodiments, in the reaction system of the present disclosure, a space is left between the top of the partition plate arranged in the reactor and the top of the reactor to form the overflow port, and the bottom and both sides of the partition plate are completely welded with the inner wall of the reactor and thus closed. The structure of the upper edge of the partition plate is not limited, and for example, the upper edge of the partition plate may be a horizontal or wavy or sawtooth structure. 
     In some embodiments, the first circulation unit includes a first circulation line connected between the discharge port and the feed port of the first reaction chamber, where the first circulation line is sequentially provided with a first circulating pump, the cooler and a first valve in an upstream-to-downstream direction thereof; preferably, the first circulation line is also provided with a material mixer. The cooler is arranged in the first circulation unit, and the coolant may be one or a mixture of more of water, brine and ethylene glycol, preferably deionized water. The specific form of the material mixer is not particularly limited, and for example, the specific form of the material mixer may be a form known in the art, preferably a static mixer. 
     The second circulation unit includes a second circulation line connected between the discharge port and the feed port of the second reaction chamber, where the second circulation line is sequentially provided with a second circulating pump, the heater and a second valve in an upstream-downstream direction thereof. The heater is arranged in the second circulation unit, and the heat source may be heat transfer oil, fused salt, high-pressure steam and the like. 
     The temperature control in the reaction system may be performed in a manner known in the art. In some preferred embodiments, for the reaction system of the present disclosure, with a cooler arranged in the first circulation unit and a heater arranged in the second circulation unit, the reactor temperature is jointly controlled by the cooler and the heater so that the temperature control effect is good, the temperature fluctuation is small, and the polymerization reaction is stable. In this manner, the side reaction fouling is small, and the reactor cleaning frequency is low. 
     The flow direction switching unit includes a switching line connected between the first circulation line and the second circulation line, where the switching line is provided with a third valve. The first circulation line and the second circulation line are connected to each other through a switching line, and the flow direction switching action of the flow direction switching unit is started or stopped (or closed) by opening or closing the third valve. 
     In the reaction system of the present disclosure, the forms of the first circulating pump and the second circulating pump is not particularly limited, and the first circulating pump and the second circulating pump may be, for example, a gear pump, a centrifugal pump, a diaphragm pump and the like. The first circulating pump is specifically configured to output the material in the first reaction chamber from its discharge port to the first circulation line, and the second circulating pump is specifically configured to output the material in the second reaction chamber from its discharge port to the second circulation line. 
     In some embodiments, the flow direction switching line is connected to the first circulation line at a position upstream of the first valve, and the switching line is connected to the second circulation line at a position upstream of the second valve. 
     In some embodiments, the reaction system further includes a first feed line and a second feed line, where the first feed line is configured to deliver a material to the first reaction chamber or the second reaction chamber, and the second feed line is configured to deliver a material to the second reaction chamber. 
     In some embodiments, the first feed line is connected to the first circulation line, and in the upstream-to-downstream direction of the first circulation line, the position where the first feed line and the first circulation line are connected is upstream of the position where the switching line and the first circulation line are connected. 
     In some embodiments, the second feed line is connected to the second circulation line. 
     In some embodiments, the ratio of the height of the partition plate to the height of the reactor is 0.6-0.96:1, for example, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 0.96:1 and the like, preferably 0.7-0.9:1. Both the utilization efficiency of the reactor and safety production can be achieved by adopting an optimal height ratio. 
     In some embodiments, the inner diameter D of the reactor perpendicular to the middle part of the partition plate is divided by the partition plate into an inner diameter section L1 located in the second reaction chamber and an inner diameter section L2 located in the first reaction chamber, and the ratio of L1 to L2 is 1:3.8-5.4, for example, 1:3.8, 1:4.0, 1:4.5, 1:5.4 and the like. Products with different particle sizes are synthesized by adopting a preferred ratio, and these products are mixed, so as to reduce the viscosity of the products. The inner diameter D of the reactor perpendicular to the middle part of the partition plate refers to the inner diameter of the reactor perpendicular to the middle of the height of the partition plate. 
     In some embodiments, the reactor may specifically consist of a head and a barrel. The upper portion of the barrel is of the same diameter as the lower portion, for example, the barrel is a straight cylinder, or the upper portion of the barrel may not be of the same diameter as the lower portion, for example, the reactor is a conical reactor or a diameter-varied reactor. 
     In some embodiments, the ratio of the volume of the first reaction chamber to the volume of the second reaction chamber is 4.0-19.0, for example, 4.0, 5.0, 5.6, 6, 7, 8, 8.9, 10, 15, 19 and the like, preferably 5.6-8.9. 
     The present disclosure provides a method for preparing polymer polyol, where a reaction system for preparing polymer polyol includes a reactor, a reaction cavity of the reactor is divided by a partition plate into a first reaction chamber and a second reaction chamber, the volume of the first reaction chamber is greater than the volume of the second reaction chamber, and the top of the partition plate is provided with an overflow port to communicate the first reaction chamber and the second reaction chamber with each other; the reaction material for preparing polymer polyol includes a reaction base material and a reaction top material; the method includes the following steps: 
     1) in a first reaction stage: 
     the reaction base material is added to the second reaction chamber and heated; when the temperature of the reaction base material reaches a first temperature, part of the reaction top material is continuously added to the second reaction chamber, and the reaction temperature is maintained at the first temperature; 
     2) in a second reaction stage: 
     when the reaction temperature of the reaction system in the first reaction stage rises, preferably, the reaction temperature rises by 0.5° C. or more (for example, by 0.6° C., 1° C., 2° C., 5° C. and the like, preferably by 0.5° C.-1° C., and more preferably by 0.6° C.-0.7° C.), the reaction top material is stopped to be added to the second reaction chamber, the remaining reaction top material is continuously added to the first reaction chamber, the reaction top material reacts with the material that overflows from the second reaction chamber to the first reaction chamber through the overflow port, and the reaction temperature is maintained at the first temperature; 
     3) in an aging stage: 
     after the feeding of the reaction top material is completed, the material in the second reaction chamber is delivered to the first reaction chamber to mix with the material in the first reaction chamber and age. 
     In some embodiments, the reaction system is the reaction system described above, in the first reaction stage, the material in the second reaction chamber flows out from the discharge port of the second reaction chamber and circulates through the second circulation unit to the feed port of the second reaction chamber; in the first reaction stage, the flow direction switching unit is activated so that the material that overflows from the second reaction chamber into the first reaction chamber through the overflow port flows into the first circulation unit through the discharge port of the first reaction chamber and flows into the feed port of the second reaction chamber under the action of the flow direction switching unit; 
     in the second reaction stage, the flow direction switching action of the flow direction switching unit is stopped so that the material in the first reaction chamber flows out from the discharge port of the first reaction chamber and circulates through the first circulation unit to the feed port of the first reaction chamber; 
     after the feeding of the reaction top material in the second reaction stage is completed, and in the aging stage, the flow direction switching unit is activated so that the material that flows out from the discharge port of the second reaction chamber into the second circulation unit flows into the feed port of the first reaction chamber under the action of the flow direction switching unit, so as to mix with the material in the first reaction chamber and age. 
     After the reaction in the aging stage is completed, the reaction solution is subjected to a subsequent degassing process, and the unreacted monomer and chain transfer agent are removed by means well known to those in the art, for example, at an absolute pressure of 0.1 pa-10 kpa and 120° C.-170° C. 
     In some embodiments, the first temperature is 100° C.-140° C. (for example, 100° C., 110° C., 120° C., 130° C. and 140° C.); the aging stage is performed at temperature the same as or different from the first temperature, preferably at a temperature 10° C.-30° C. (for example, 10° C., 20° C. and 30° C.) higher than the first temperature. The aging is preferably performed for 0.5-4 hours (for example, 0.5 hour, 1 hour, 2 hours, 3 hours and 4 hours). 
     In some embodiments, the reaction base material includes part of polyether polyol, a stabilizer and a chain transfer agent, and the reaction top material includes remaining polyether polyol, a reaction monomer and an initiator. The sum of the polyether polyol in the reaction base material and the polyether polyol in the reaction top material is the total amount of the polyether polyol, and the polyether polyol is used as the base polyether polyol. 
     In some embodiments, the mass of the reaction monomer is 20%-55%, for example, 20%, 30%, 40%, 50%, 55% and the like, of the sum of the total mass of the polyether polyol, the mass of the reaction monomer and the mass of the stabilizer; the percentage of the mass of the reaction monomer to the total mass of the polyether polyol is 0.1%-250%, for example, 0.1%, 1%, 10%, 30%, 50%, 100%, 140%, 180%, 220%, 250% and the like, preferably 30%-140%; the mass of the stabilizer is 0.3%-10%, for example, 0.3%, 1%, 2%, 2.5%, 5%, 7%, 10% and the like, preferably 2%-5%, of the sum of the total mass of the polyether polyol and the mass of the reaction monomer. 
     The initiator used in the present disclosure includes, but is not limited to, all initiators suitable for the preparation of polymer polyol. In some embodiments, the amount of the initiator is 0.01%-5%, for example, 0.01%, 0.05%, 0.1%, 1%, 2%, 5% and the like, of the sum of the total mass of the polyether polyol and the mass of the reaction monomer. The preferred initiator is known in the art, including peroxides and/or azo compounds. The peroxides are, for example, dibenzoyl peroxide, lauroyl peroxide, tert-butyl hydroperoxide, benzoyl peroxide and/or di-tert-butyl peroxide. The azo compounds are, for example, azobisisobutyronitrile (AIBN), azomethylbutyronitrile (AMBN) and/or dimethyl azobisisobutyrate (V601). 
     The chain transfer agent used in the present disclosure includes, but is not limited to, all chain transfer agents suitable for the preparation of polymer polyol. In some embodiments, the amount of the chain transfer agent is 0.1%-6%, for example, 0.1%, 0.2%, 0.5%, 1%, 2%, 4%, 5%, 6% and the like, preferably 0.2%-5%, of the sum of the total mass of the polyether polyol and the mass of the reaction monomer; the preferred chain transfer agent is one or more of 1-butanol, 2-butanol, isopropanol, ethanol, methanol, water, cyclohexane, thioglycolate and thiols, for example, one or more of dodecanethiol and isopropanol. 
     The polyether polyol, as the base polyether polyol, may be any polyether polyol suitable for the polyurethane system, for example, commercially available polyether polyols, specifically including WANOL®F3156, WANOL®F3135, WANOL®F3056 and WANOL®F3128 (WANHUA CHEMICAL). The polyether polyols in the reaction base material and in the reaction top material are the same polyether polyol, and preferably, the mass ratio of the polyether polyol in the reaction top material to the polyether polyol in the reaction base material is 1.0-7.0, for example, 1.0, 2.1, 3.0, 4.0, 5.0, 5.6, 6.0, 7.0 and the like, preferably 2.1-5.6. 
     The stabilizer used in the present disclosure includes, but is not limited to, all stabilizers suitable for the preparation of polymer polyol, for example, polyether polyols containing polymerizable double bonds, as described in patent applications CN105949408A, CN107090064A and CN106519148A. 
     The reaction monomer used in the present disclosure includes, but is not limited to, all vinyl monomers suitable for the preparation of polymer polyol. In some embodiments, the reaction monomer may be selected from one or more of an aliphatic conjugated diene compound, a vinyl aromatic compound, α,β-ethylenically unsaturated nitrile, α,β-ethylenically unsaturated nitrile amide, α,β-ethylenically unsaturated carboxylic acid, α,β-ethylenically unsaturated carboxylic ester, vinyl ester, vinyl ether, vinyl ketone, a vinyl halide and a vinylidene halide, preferably one or more of a vinyl aromatic compound and α,β-ethylenically unsaturated nitrile, more preferably a composition of styrene and acrylonitrile, particularly preferably a composition of styrene and acrylonitrile in a mass radio of 10:90-90:10 (for example, 10:90, 20:80, 40:60, 50:50, 60:40, 80:20, 90:10 and the like), and further preferably a composition of styrene and acrylonitrile in a mass ratio of 60:40-90:10 (for example, 60:40, 70:30, 80:20, 90:10 and the like). 
     The polymer polyol of the present disclosure is well suited for the synthesis of polyurethane foams. Therefore, the present disclosure provides use of polyether polyol prepared by the method described above in the synthesis of a polyurethane foam, where the polyurethane foam is preferably a soft polyurethane foam, and specifically, the soft polyurethane foam is obtained by foaming the composition of polymer polyol and polyisocyanate. 
     The method for preparing the soft polyurethane foam is known in the art, as described in CN106519148A, and specifically, the soft polyurethane foam is prepared by reacting a polyurethane catalyst, a polyol, a cross-linker, a blowing agent, a foam stabilizer, an auxiliary agent with a polyisocyanate. The selection of components required for the preparation of soft polyurethane foams is not limited to the present disclosure, and the corresponding components suitable for the preparation of soft polyurethane foams in the art may be used. In some embodiments, the polyurethane catalyst is preferably an organometallic compound and/or an organic amine catalyst. The organometallic compound is, for example, stannous octoate, stannous oleate, dibutyltin dilaurate, dibutyltin acetate and/or dibutyltin diacetate. The organic amine catalyst is, for example, bis(2,2′-dimethylamino) ethyl ether, trimethylamine, triethylamine, triethylenediamine and/or dimethyl ethanolamine. The blowing agent is preferably water, acetone, carbon dioxide, haloalkanes, aliphatic alkanes and/or cycloaliphatic alkanes. The foam stabilizer is preferably an organopolysiloxane surfactant. In addition, a flame retardant, a filler, a light stabilizer, an anti-oxidant and the like may also be used in the method for preparing the soft polyurethane foam. 
     The present disclosure also relates to a molded article including the soft polyurethane foam described above. 
     The technical solutions provided by the present disclosure have the beneficial effects below. 
     1. The reaction system for preparing polymer polyol provided by the present disclosure has a simple reactor structure and can be maintained conveniently. Based on the reaction system, a polymer polyol having low viscosity can be obtained by a batch process, that is, the same low viscosity of the polymer polyol as continuous processes can be achieved by the batch process. 
     2. According to the reaction system provided by the present disclosure, with a cooler arranged in the first circulation unit and a heater arranged in the second circulation unit, the temperature of the reactor can be jointly controlled by the cooler and the heater control so that the temperature control effect of the reactor is good, the temperature fluctuation is small, and the polymerization reaction is stable. In this manner, the side reaction fouling is small, and the reactor cleaning frequency is low. 
     3. The polyurethane foam prepared by the polymer polyol prepared by the preparation process of the present disclosure has the characteristics of high hardness and excellent mechanical performance and has higher hardness compared with the conventional product prepared by batch processes and better tear strength and tensile strength compared with the product prepared by continuous processes. 
     4. The reaction chamber of the reaction system provided by the present disclosure is divided into two reaction chambers with different sizes, the products with different particle sizes can be produced in the different reaction chambers, and the particle size and proportion of the product can be adjusted by adjusting the volumes of the reaction chambers. For example, a fraction of products with smaller particle sizes are produced in the smaller reaction chamber at the beginning of the reaction, and with the reaction continuing, a large fraction of products with larger particle sizes are produced in the larger reaction chamber; and the products prepared in the two reaction chambers are then mixed and aged in the same reactor. With the reaction system of the present disclosure, the particle size distribution of the product can be adjusted, endowing the product with high solid content, low viscosity and excellent foaming performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram of a reaction system in one embodiment; 
         FIG.  2    is a schematic diagram of the height of the reactor and the height of the partition plate in  FIG.  1   ; 
         FIG.  3    is a cross-section diagram of the reactor of  FIG.  1   ; 
         FIG.  4    is a schematic diagram of a reaction system used in Comparative Example 1. 
         FIGS.  5  to  7    are electron micrographs of the products of Example 1, Comparative Example 1 and Comparative Example 2, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     For a better understanding of the technical solutions of the present disclosure, the content of the present disclosure is further described below in conjunction with examples. However, the content of the present disclosure is not limited to the examples set forth below. 
     The present disclosure provides a reaction system for preparing polymer polyol. With reference to  FIG.  1   , the reaction system includes a reactor  4 , a first circulation unit  1 , a second circulation unit  2  and a flow direction switching unit  3 . The reaction cavity of the reactor  4  is divided by a partition plate  43  into two reaction chambers of different sizes, that is, a first reaction chamber  41  and a second reaction chamber  42 , where the volume of the first reaction chamber  41  is greater than the volume of the second reaction chamber  42 . The reactor  4  may be specifically a vertical reaction tank. The top of the partition plate  43  is provided with an overflow port  44  to communicate the first reaction chamber  41  and the second reaction chamber  42  with each other through the overflow port  44 , that is, the first reaction chamber  41  and the second reaction chamber  42  can overflow the material to each other&#39;s accommodation space through the overflow port  44 . Specifically, with reference to  FIG.  1   , the partition plate  43  is, for example, configured such that there is a space between the top of the partition plate  43  and the top of the reactor  4 , so as to form the overflow port  44 . Of course, other manners of forming the overflow port  44  are not excluded. The reactor  4  is provided with a temperature measurement element  45  for measuring the temperature in the reactor  4 . The temperature measurement element  45  is well known in the art and may be any meter as long as the meter can measure the temperature, such as a thermocouple, a thermistor, a thermosensitive resistor and the like. 
     The first circulation unit  1  is arranged between the discharge port (not shown in the figure) and the feed port (not shown in the figure) of the first reaction chamber  41 . Specifically, for example, the discharge port of the first reaction chamber  41  is located at the bottom of the first reaction chamber, and the feed port is located at the top of the first reaction chamber  41 . Under the action of the first circulation unit  1 , the material in the first reaction chamber  41  may circulate between the discharge port of the first reaction chamber  41  and the feed port of the first reaction chamber  41 , that is, the material flowing out from the discharge port of the first reaction chamber  41  may circulate back to the feed port of the first reaction chamber  41  under the action of the first circulation unit  1 . In addition, the first circulation unit  1  is provided with a cooler  11  for cooling materials. Specifically, the first circulation unit  1  includes a first circulation line  12 , a first circulating pump  13 , the cooler  11  and a first valve  15 , and preferably further includes a material mixer  14 . The first circulation line  12  is connected between the discharge port and the feed port of the first reaction chamber  41 . In an upstream-to-downstream direction of the first circulation line  12 , the first circulating pump  13 , the cooler  11  and the first valve  15  are sequentially arranged on the first circulation line  12 , that is, the first circulating pump  13  is closer to the discharge port of the first reaction chamber  41 . Preferably, the material mixer  14  is arranged on the section between the cooler  11  and the first valve  15 , and the material mixer  14  may specifically be a static mixer. 
     The second circulation unit  2  is arranged between the discharge port (not shown in the figure) and the feed port (not shown in the figure) of the second reaction chamber  42 . Specifically, the discharge port of the second reaction chamber  42  is located at the bottom of the second reaction chamber, and the feed port is located at the top of the second reaction chamber  42 . Under the action of the second circulation unit  2 , the material in the second reaction chamber  42  may circulate between the discharge port of the second reaction chamber  42  and the feed port of the second reaction chamber  42 , that is, the material flowing out from the discharge port of the second reaction chamber  42  may circulate back to the feed port of the second reaction chamber  42  under the action of the second circulation unit  2 . In addition, the second circulation unit  2  is further provided with a heater  21  for heating materials. The cooler  11  of the first circulation unit  1  and the heater  21  of the second circulation unit  2  cooperate to control the temperature of the material, so as to control the temperature in the reactor  4 . Specifically, the second circulation unit  2  includes a second circulation line  22 , a second circulating pump  23 , the heater  21  and a second valve  24 . The second circulation line  22  is connected between the discharge port and the feed port of the second reaction chamber  42 . In an upstream-to-downstream direction of the second circulation line  22 , the second circulating pump  23 , the heater  21  and the second valve  24  are sequentially arranged on the second circulation line  22 , that is, the second circulating pump  23  is closer to the discharge port of the second reaction chamber  42 . 
     The flow direction switching unit  3  is arranged between the first circulation unit  1  and the second circulation unit  2 , and under the action of the flow direction switching unit  3 , the first circulation unit  1  and the second circulation unit  2  communicate to each other, so as to adjust the flow direction of the material. Specifically, under the action of the flow direction switching unit  3 , the material in the first circulation unit  1  may be switched either to flow to the feed port of the first reaction chamber  41  or to flow to the feed port of the second reaction chamber  42 , or the material in the second circulation unit  2  may be switched either to flow to the feed port of the second reaction chamber  42  or to flow to the feed port of the first reaction chamber  41 . Specifically, for example, the material in the first circulation unit  1  is switched from flowing to the feed port of the first reaction chamber  41  to flowing to the feed port of the second reaction chamber  42 , or the material in the second circulation unit  2  is switched from flowing to the feed port of the second reaction chamber  42  to flowing to the feed port of the first reaction chamber  41 . Specifically, the flow direction switching unit  3  includes a switching line  31  and a third valve  32 , where the switching line  31  is connected between the first circulation line  12  and the second circulation line  22 , and the third valve  32  is arranged on the switching line  31 . The flow direction switching action of the flow direction switching unit  3  is started or stopped by opening or closing the third valve  32 . More specifically, the switching line  31  is connected to the first circulation line  12  at a position upstream of the first valve  15 , and specifically, for example, the switching line  31  is connected to the first circulation line  12  on the section of the first circulation line  12  between the material mixer  14  and the first valve  15 ; the switching line  31  is connected to the second circulation line  22  at a position upstream of the second valve  24 , and specifically, for example, the switching line  31  is connected to the second circulation line  22  on the section of the second circulation line  22  between the second valve  24  and the heater  21 . 
     Furthermore, the reaction system further includes a first feed line  5  and a second feed line  6 , where the first feed line  5  is configured to deliver a material to the first reaction chamber  41  or the second reaction chamber  42 , and the second feed line  6  is configured to deliver a material to the second reaction chamber  42 . Specifically, the first feed line  5  is connected to the first circulation line  12 , and in the upstream-to-downstream direction of the first circulation line  12 , the position where the first feed line  5  and the first circulation line  12  are connected is upstream of the position where the switching line  31  and the first circulation line  12  are connected, and for example, the first feed line  51  is connected to the section of the first circulation line  12  between the material mixer  14  and the cooler  11 . The first feed line  5  is provided with a fourth valve  51 . Specifically, the second feed line  6  is connected to the second circulation line  22 , for example, the second feed line  6  is connected to the section of the second circulation line  22  between the second valve  24  and the heater  21 , and the second feed line  61  is provided with a fifth valve  61 . 
     In some embodiments, with reference to  FIG.  2   , the ratio of the height h of the partition plate  43  in the reactor  4  to the height H of the reactor  4  is 0.6-0.96:1, for example, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 0.96:1 and the like, preferably 0.7-0.9:1, for example, 0.7:1, 0.8:1, 0.9:1. 
     In some embodiments, with reference to  FIG.  3   , the inner diameter of the reactor perpendicular to the middle part of the partition plate is divided by the partition plate into an inner diameter section L1 located in the second reaction chamber and an inner diameter section L2 located in the first reaction chamber, and the ratio of L1 to L2 is 1:3.8-5.4, for example, 1:3.8, 1:4.3, 1:5, 1:5.4 and the like. 
     In some embodiments, the ratio of the volume of the first reaction chamber  41  to the volume of the second reaction chamber  42  is 4.0-19.0, for example, 4.0, 5.0, 5.6, 6, 7, 8, 8.9, 10, 15, 19 and the like, preferably 5.6-8.9. 
     In the reaction system, the coolant of the cooler  11  may be one or a mixture of more of water, brine and ethylene glycol, preferably deionized water. The heat source of the heater  21  may be heat transfer oil, fused salt, high-pressure steam and the like. 
     The “upstream” and “downstream” herein are defined in terms of the direction where the material in the line flows. For example, for the first circulation line  12 , the end of the discharge port close to the first reaction chamber  41  is upstream relative to the end of the feed port close to the first reaction chamber  41 . Similarly, for the second circulation line  22 , the end of the discharge port close to the second reaction chamber  42  is upstream relative to the end of the feed port close to the second reaction chamber  42 . 
     The reaction system provided by the present disclosure is particularly suitable for the preparation of polymer polyols, and through the reaction system, polymer polyols with low viscosity can be obtained by a batch possess. The reaction system has a simple structure and can be maintained conveniently. 
     The method for preparing polymer polyol using the reaction system provided by the present disclosure will be described in conjunction with examples. 
     The sources of raw materials used in the following Examples and Comparative Examples are as follows: 
     Polyether polyol WANOL®F3156: glycerol-initiated propylene oxide/ethylene oxide polymer with a number average molecular weight of 3000, a functionality of 3 and a hydroxyl value of 56±1 mgKOH/g, purchased from PU Business Unit of WANHUA CHEMICAL. 
     Stabilizer (ref. CN201310076219.5): polyether polyol having unsaturated double bonds, which is obtained by reacting glycerol-initiated propylene oxide/ethylene oxide polymer with maleic anhydride and subjecting to termination by ethylene oxide (EO), with an unsaturation of 0.065 meq/g and a hydroxyl value of 26.0 mgKOH/g, self-produced. 
     Acrylonitrile: purchased from SINOPEC QILU PETROCHEMICAL COMPANY. 
     Styrene: purchased from SINOPEC QILU PETROCHEMICAL COMPANY. 
     Initiator: dimethyl azodiisobutyrate, purchased from ZIBO HAIMING CHEMICAL. 
     Isopropanol: purchased from WANHUA CHEMICAL (YANTAI) PETROCHEMICAL CO., LTD. 
     Modified MDI: WANNATE®8001, purchased from PU Business Unit of WANHUA CHEMICAL. 
     Organic bismuth catalyst: BiCAT 8106, purchased from SHEPHERD CHEMICAL COMPANY. 
     Foam stabilizer: B-8715 LF2, purchased from TMG CHEMICALS CO., LTD. 
     The performance test method of polyurethane foam is as follows: 
     GB/T 10802-2006 General flexible polyether polyurethane cellular plastics 
     Viscosity Measurement: BROOK TECHNOLOGY CO., LTD., DV-I+prime viscometer, 4#Rotor. 
     Electron microscope test: SU8010 ultra-high resolution field emission scanning electron microscope from HITACHI HIGH-TECH. 
     Example 1 
     The reaction system is shown in  FIG.  1   . The description of the reaction system is as described above, and details will not be described herein. 
     The information of the reactor is listed in Table 1. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Term 
                 Specification 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Reactor (Straight 
                 Material 
                 o6cr19ni10 
               
               
                 cylindrical shape) 
                 Inner diameter/mm 
                 3000 
               
               
                   
                 Height/mm 
                 4400 
               
               
                 partition plate 43 
                 Material 
                 o6cr19ni10 
               
               
                   
                 Height/mm 
                 3520 
               
               
                   
                 Inner diameter section L1 
                  566 
               
               
                   
                 of the inner diameter of the 
               
               
                   
                 reactor perpendicular to 
               
               
                   
                 the middle part of the 
               
               
                   
                 partition plate/mm 
               
               
                   
                 L1:L2 
                   1:4.3 
               
               
                   
                 Volume ratio (first 
                 6.67:1 
               
               
                   
                 reaction chamber: second 
               
               
                   
                 reaction chamber) 
               
               
                 First reaction 
                 Form 
                 Centrifugal pump 
               
               
                 chamber 
                 Maximum flow rate 
                 300 m 3 /h 
               
               
                 circulating pump 
               
               
                 (that is, first 
               
               
                 circulating pump 13) 
               
               
                 Second reaction 
                 Form 
                 Gear pump 
               
               
                 chamber 
                 Maximum flow rate 
                  80 m 3 /h 
               
               
                 circulating pump 
               
               
                 (that is, second 
               
               
                 circulating pump 23) 
               
               
                 Heat exchanger 
                 Cooling medium of 
                 Deionized water 
               
               
                   
                 the cooler 11 
               
               
                   
                 Heating medium of 
                 5 kg/cm 2  steam 
               
               
                   
                 the heater 21 
                 (5S steam) 
               
               
                   
               
            
           
         
       
     
     The ratios of reaction raw materials are listed in Table 2. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Material 
                 kg 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Reaction top material 
                 Styrene 
                 7274 
               
               
                   
                 Acrylonitrile 
                 3917 
               
               
                   
                 Azobisisobutyronitrile 
                 110 
               
               
                   
                 Polyether polyol WANOL F3156 
                 9089 
               
               
                 Reaction base material 
                 Stabilizer 
                 745 
               
               
                   
                 Isopropanol 
                 945 
               
               
                   
                 Polyether polyol WANOL F3156 
                 2785 
               
               
                   
               
            
           
         
       
     
     The preparation process of the polymer polyol is described below in conjunction with  FIG.  1   . 
     In a first reaction stage: 
     After the reactor  4  was purged with nitrogen gas, nitrogen gas was charged to 50 kpa, and the mixed reaction base material was completely fed into the reactor  4  from the feed port of the second reaction chamber  42  along the second feed line  6  through the fifth valve  61 . The fifth valve  61  and the first valve  15  were turned off successively, the second valve  24  and the third valve  32  were turned on, the second circulating pump  23  was started with the current of the motor maintained at 63.5% of the maximum current, and the first circulating pump  13  was started with the current of the motor maintained at 67% of the maximum current. The heater  21  was introduced with  5 S steam and started heating, and the temperature measured by the temperature measurement element  45  was observed. In this process, the material in the second reaction chamber  42  was discharged into the second circulation line  22  from the discharge port of the second reaction chamber  42  under the action of the second circulating pump  23  and circulated to the feed port of the second reaction chamber  42  along the second circulation line  22 , that is, the material circulated. Meanwhile, the material in the second reaction chamber  42  overflowed into the first reaction chamber  41  through the overflow port  44 , and the material that overflowed into the first reaction chamber  41  was discharged into the first circulation line  12  from the discharge port of the first reaction chamber  41  under the action of the first circulating pump  13  and flowed into the feed port of the second reaction chamber  42  instead of the feed port of the first reaction chamber  41  under the action of the flow direction switching unit  3  (the third valve  32  was in the on state while the first valve  15  is in the off state). 
     After the temperature rose to 120° C., the fourth valve  51  was turned on, and the reaction top material mixture that was pre-mixed and cooled to 10° C. was fed along the first feed line  5 , with the flow rate set to 3330 kg/h. The reaction top material flowed into the second reaction chamber  42  through the material mixer  14  on the first circulation line  12 , the switching line  31  in the flow direction switching unit  3 , the second valve  24  on the second circulation line  22  and the feed port of the second reaction chamber  42 , sequentially. The amount of the heating medium  5 S steam of the heater  21  was adjusted so that the temperature measured by the temperature measurement element  45  was maintained at 120° C.±0.5° C. 
     In a second reaction stage: 
     After the reaction top material was fed for 25 minutes, it was observed that the temperature measured by the temperature measurement element  45  rose to 121° C. The reaction started heat release. The flow rate of the reaction top material mixture in the fourth valve  51  was increased to 6600 kg/h. The first valve  15  was turned on while the third valve  32  was turned off so that the reaction top material and the material that was discharged from the discharge port of the first reaction chamber  41  no longer flowed to the second circulation line  22  but only flowed to the feed port of the first reaction chamber  41  along the first circulation line  12 . The flow rate of the cooling medium of the cooler  11  was adjusted, and the reaction temperature was maintained at 120° C. 
     In an aging stage: 
     After the feeding of the reaction top material was completed, the fourth valve  51  was turned off, the third valve  32  was turned on, and the second valve  24  was turned off. After the material that was discharged from the discharge port of the second reaction chamber  42  flowed into the second circulation line  22 , the material no longer circulated back to the feed port of the second reaction chamber  42 , but under the action of the flow direction switching unit  3  (the third valve  32  was in the on state while the second valve  24  is in the off state), the material flowed into the first circulation line  12 , flowed into the feed port of the first reaction chamber  41  through the first valve  15 , then flowed into the first reaction chamber  41  and mixed with the material in the first reaction chamber  41 . After the reaction temperature rose to 140° C., the mixed material was aged for 2 hours and then subjected to the subsequent degassing process. The obtained polymer polyol had viscosity of 3450 cp and solid content of 44.6 wt %, and the electron micrograph of the polymer polyol is shown in  FIG.  5   . 
     Comparative Example 1 
     The reactor used in the comparative example is different from the reactor used in Example 1 in that the reaction cavity of the reactor used in the comparative example is not partitioned by the partition plate  43 , and the basic parameters are shown in Table 3 below. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Term 
                 Specification 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Reactor 71 
                 Material 
                 o6cr19ni10 
               
               
                   
                 Inner diameter/mm 
                 3000 
               
               
                   
                 Height/mm 
                 4400 
               
               
                 Circulating pump 
                 Form 
                 Centrifugal pump 
               
               
                   
                 Maximum flow rate 
                 300 m 3 /h 
               
               
                 Heat exchanger 
                 Cooling medium of the cooler 72 
                 Deionized water 
               
               
                   
                 Heating medium of the heater 73 
                 5 kg/cm 2  steam 
               
               
                   
                   
                 (5S steam) 
               
               
                   
               
            
           
         
       
     
     The schematic diagram of the reaction system used in Comparative Example 1 is shown in  FIG.  4   . 
     The ratios of reaction raw materials of Comparative Example 1 are listed in Table 2. 
     The preparation process of the polymer polyol of Comparative Example 1 is described below in conjunction with  FIG.  4   . 
     After the reactor  71  was purged with nitrogen gas, the nitrogen gas was charged to 50 kpa, and the mixed reaction base material was completely fed into the reactor  71  through the ball valve  75 . The ball valve  75  was turned off, and the circulating pump  77  was started with the current of the motor maintained at 67% of the maximum current. The heater  73  was introduced with  5 S steam and started heating. After it was observed that the temperature measured by the temperature detection element  78  rose to 120° C., the valve  76  was turned on, and the reaction top material mixture that was pre-mixed and cooled to 10° C. was fed, with the flow rate set to 3330 kg/h. The amount of the heating medium  5 S steam of the heater  73  was adjusted so that the temperature measured by the temperature detection element was maintained at 120° C. After the feeding was carried out for 25 minutes, it was observed that the temperature measured by the temperature detection element  78  rose to 120.8° C. The reaction started heat release. The flow rate of the reaction top material mixture in the valve  76  was increased to 6600 kg/h, and the flow rate of the cooling medium of the cooler  72  was adjusted, so that the reaction temperature was maintained at 120° C. After the feeding of the reaction top material was completed, the valve  76  was turned off. After the reaction temperature rose to 140° C., the material was aged for 2 hours and then subjected to the subsequent degassing process. The obtained polymer polyol had viscosity of 5045 cp and solid content of 44.5%, and the electron micrograph of the polymer polyol is shown in  FIG.  6   . 
     Comparative Example 2 
     The polymer polyol was prepared by the continuous process as described with reference to the Examples 3-6 in CN201310076219.5. The raw material formula is shown in Table 2 in Example 3 in CN201310076219.5. The base polyether therein was changed to polyether polyol WANOL®F3156. The solid content was adjusted to 45%. The viscosity was measured as 3800 cp. The electron micrograph is shown in  FIG.  7   . 
     Example 2 and Comparative Examples 3-4 
     The method for preparing a polyurethane foam is as follows: 
     A composite material was prepared according to the raw materials and parts by weight shown in Table 4 (mass percentages in the table were calculated based on 100% of the total mass of the composite material), and the composite material and the isocyanate raw material WANNATE®8001 were placed at a constant temperature 22° C. for three hours, separately. 100 g of the composite material and 60 g WANNATE®8001 were mixed and stirred in a blender at a rotation number of 3000 rpm for 6 seconds. The stirred mixture was then quickly poured into an open aluminum mold (size: 300 mm long, 300 mm wide and 50 mm thick) that was pre-heated to 60° C. to foam the mixture. After 7 minutes, the foam was taken out to obtain the polyurethane foam. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 Comparative 
                 Comparative 
               
               
                 Material 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
               
             
            
               
                 polymer polyol 
                 Example 1 
                 Comparative 
                 Comparative 
               
               
                   
                   
                 Example 1 
                 Example 2 
               
               
                   
                 33.59 
                 33.59 
                 33.59 
               
               
                 WANOLF3156 
                 59.56 
                 59.56 
                 59.56 
               
               
                 Diethanolamine 
                 0.50 
                 0.50 
                 0.50 
               
               
                 Water 
                 4.16 
                 4.16 
                 4.16 
               
               
                 N,N-bis(dimethylamino- 
                 0.40 
                 0.40 
                 0.40 
               
               
                 propyl)isopropanolamine 
               
               
                 (CAS No.: 67151-63-7) 
               
               
                 N,N,N′-trimethyl- 
                 0.50 
                 0.50 
                 0.50 
               
               
                 N′-hydroxyethyl- 
               
               
                 bisaminoethylether 
               
               
                 Organic bismuth catalyst 
                 0.10 
                 0.10 
                 0.10 
               
               
                 Foam stabilizer 
                 1.19 
                 1.19 
                 1.19 
               
               
                   
               
            
           
         
       
     
     The properties of the prepared polyurethane foam were tested and listed in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Polyurethane foam properties 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Comparative 
                 Comparative 
               
               
                 Foam properties 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Tensile strength kpa 
                 110 
                 109 
                 100 
               
               
                 Tear strength kpa 
                 90 
                 88 
                 80 
               
               
                 Hardness/25% ILD 
                 252 
                 246 
                 252 
               
               
                 (25% indentation) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 5, the polyurethane foam of Example 2 had better tensile strength and tear strength and higher hardness. As can be seen from the electron micrograph, the product of the present disclosure was smooth and composed of particles of large particle sizes and particles of small particle sizes, and the large particle sizes and the small particle sizes are respectively uniform in size. The product prepared by the batch process was smooth, but had no particle of different particle sizes. The product prepared by the continuous process had particles of different particle sizes, but the particle size difference was large, and the appearance of the product was rough. 
     Those skilled in the art will appreciate that some modifications or adaptations may be made to the present disclosure based on the teachings of the description. These modifications or adaptations should fall within the scope of the present disclosure as defined by the claims.