Continuous process for the preparation of highly stable, finely divided, low viscosity polymer polyols of small average particle size

Abstract: A continuous process for the preparation of highly stable, finely divided, low viscosity polymer polyols of small average particle size wherein in the first step an intermediate is prepared by reacting (1) a mixture of styrene and acrylonitrile in a mixture of (2) a polyoxyalkylene polyether polyol and (3) a macromer in the presence of (4) a free radical initiator, (5) a solvent having moderate chain transfer activity and optionally (6) a reaction moderator at a temperature of at least 100°C, such that the intermediate contains a high level of macromer, at least about 12% and preferably at least about 15% with respect to the polyol mixture and the solids content of the intermediate is less than about 30%, preferably less than about 25% and at least about 15% by weight. The intermediate, which functions as a seed for further polymerization, is then further reacted in one or more stirred-tank reactors in series with a mixture of styrene and acrylonitrile in a polyol and optionally a macromer, in the presence of solvent, initiator and a reaction moderator which are distributed among the remaining reactors.

Description: Polymer polyols and their use in the preparation of polyurethane foams are known. Polymer polyols are polyols that are modified or filled with polymers or copolymers of ethylenically unsaturated monomers. They are prepared by the in-situ polymerization of one or more vinyl monomers in a polyether polyol in the presence of a radical forming polymerization initiator. Polyurethanes prepared from such polymer polyols are distinguished by their improved properties, in particular, the hardness and load-bearing capacity of flexible polyurethane foams. Polymer polyols are, in the ideal case, relatively low viscosity, finely divided, non-sedimenting dispersions of the polymer or copolymer in a substantially unchanged polyol. Stabilization of the polymer polyol dispersions against sedimentation is achieved by the incorporation or grafting of a portion of the polyol molecules into the polymer matrix, which is formed in-situ. The preferred polymer is a styrene-acrylonitrile graft copolymer. Two basic processes have been used to produce polymer polyols; semi-batch and continuous. Products prepared via the two methods are distinguished by their particle size distribution. The particle size distribution is narrow for polymer polyols prepared via a semi-batch process while broad for polymer polyols prepared via a continuous process. In the semi-batch process, the majority of the particles are generated in the early stage of the reaction. Further polymerization favors the growth of existing particles. In a continuous process, the broad particle size distribution is the result of the combination of competitive growth of existing particles and the formation of new particles as well as the continuous turn-over, or wash-out of the particles in the reactor. The features which characterize the processability of the polymer polyols are their viscosity, storage stability (resistance to sedimentation), and filterability. Parameters which have a particularly important influence on the quality of the product are the proportion of the monomer in the starting mixture and the monomer ratios. The trend toward the use of high styrene monomer mixtures and high solids content polymer polyols has resulted in polymer polyols having less than ideal properties (low viscosity; stable; agglomerate free; small average particle size). It has been demonstrated that the stability of polymer polyols, particularly at high styrene levels, can be increased by the polymerization of the monomer mixture in a polyol or polyol mixture which contains a particular level of induced unsaturation. The polyol containing induced unsaturation, often referred to as macromer or macromonomer, stabilizes the polymer dispersion by incorporation of adequate amounts of the polyol into the polymer matrix via addition polymerization. Chain transfer agents or polymer control agents have been used in the preparation of polymer polyol compositions as reaction moderators or to control the molecular weight of the copolymer, resulting in stable, low viscosity products. Various reaction moderators, including mercaptans, alkyl halides, alcohols, halogens and enol ethers have been suggested. EP-A 0 091 036 discloses that continuous processes may generally be used for the production of white graft polymer dispersions and flame- retardant polyurethane foams. One of the many improvements in the preparation of polymer polyols is provided by US 4,148,840, wherein a process is disclosed for producing highly stable and filterable polymer polyol compositions by polymerizing the monomer or monomers in-situ in a polyol mixture that contains a minor amount of preformed polymer polyol. Another improved process for the preparation of polymer polyols is disclosed in US 4,242,249. The use of preformed stabilizers allows the preparation of polymer polyols having higher styrene contents and higher solids contents and using lower molecular weight polyols. These preformed stabilizers typically possess viscosities in excess of 40,000 mPas at 25°C and are formed by the polymerization of styrene-acrylonitrile mixtures in a polyol containing induced unsaturation. The preformed stabilizer is used in small amounts, 5% or less, in the preparation of the end-product. US 5,223,570 provides a process for the preparation of polymer polyol dispersions of broad particle size distribution without wildly fluctuating viscosities. The method comprises preparing an intermediate containing less than 30% by weight solids in a continuous process to achieve a broad particle size distribution. The intermediate is then used as a seed for further polymerization in a semi-batch process to increase the solids content to greater than 30% by weight. A continuous process is disclosed in US 5,364,906 to produce low viscosity polymer polyols with improved dispersion stability. The method utilizes a two-step continuous process where the first reaction product is formed by the reaction of less than 50% of the total monomer mixture in greater than 50% of the total polyol (preferably all the polyol) containing a majority of a macromer and polymer control agent. In a second reactor, the remaining raw materials are added to the first reaction product. Deposition of polymer on the reactor wall, or reactor fouling, is known in the polymerization of olefinic monomers. Three methods to minimize or eliminate reactor fouling are (1) coating the reactor with a material that provides resistance to deposition of the polymer (2) improvements in reactor design (e.g. better mixing) and (3) controling monomer/polymer concentrations. US 4,956,427 provides a method for the prevention of reactor fouling during the polymerization of olefinic monomers by coating the reactor surface with a moisture curable amino-silicone fluid. The coating is used to prevent fouling especially during the polymerization of ethylene, propylene and mixtures. In JP 54133582, the preparation of polymer polyols without adhesion to the reactor wall is described using a special reactor equipped with a spiral blade agitator and draft tubes and recycling the reactor contents at a rate 50-200 times faster than that of charged starting material. In another approach, US 4,794,151 provides a continuous process for the preparation of ethene copolymers using a circular tube reactor with specified monomer/polymer concentrations to stop polymer coating on the reactor wall. None of the references discloses the importance of forming a highly stabilized intermediate or seed of small average particle size that is ideally suited for further polymerization in the preparation of highly stable, finely divided, low viscosity polymer polyols of small average particle size using a series of stirred-tank reactors. None of the related art would suggest to one of ordinary skill in the art that reactor fouling, or deposition of the polymer on the reactor wall, may be eliminated or substantially reduced by the method of preparation as disclosed in the present invention. Indeed, processes for the preparation of polymer polyols have not advanced beyond the need for further improvement. There is a clear demand for polymer polyols with enhanced dispersion stability (particularly at high styrene levels), polymer polyols with minimal viscosity (especially at high solids content), and polymer polyols that are finely divided (agglomerate free) with a small average particle size. There is also a desire to develop robust processes that maximize reactor utilization and minimize down-time due to reactor fouling. Polymer polyols and the process described in the present invention meet these criteria. The present invention is directed to a continuous process for the preparation of highly stable, finely divided, low viscosity polymer polyols having a small average particle size comprising a) forming an intermediate in a first reactor by reaching (1) a mixture of styrene and acrylonitrile, in a mixture of (2) a polyoxyalkylene polyether polyol and (3) a macromer, in the presence of (4) a free radical initiator, (5) a solvent having moderate chain transfer activity, and, optionally, (6) a reaction moderator at a temperature of at least 100°C, wherein the resultant intermediate has a macromer content of at least about 12% by weight, based on the total weight of the polyether polyol and the macromer, and a solids content of at least about 15% and less than about 30% by weight, based on the total weight of the polyether polyol, the macromer and the monomers, and contain less than 43% by weight of polyether polyol, based on the total weight of the polyether polyol, and b) reacting said intermediate in at least a second stirred-tank reactor connected in series to the first reactor, with (1) a mixture of styrene and acrylonitrile, in (2) a polyether polyol and, optionally, (3) a macromer, in the presence of (4) a free radical initiator, (5) a solvent, and (6) a reaction moderator, wherein components (1) through (6) are distributed among the second and any successive reactors. In a preferred embodiment of the present invention, the monomer concentration is minimized throughout the polymerization reactors via distribution of the raw materials such that the amount of monomer added to any reactor is less than about 30% of that reactor's content and is preferably approximately equal in each of the polymerization reactors. According to another aspect of the present invention, the use a solvent with moderate chain transfer activity is also preferred. The process of the present invention results in the elimination or substantial reduction of reactor fouling, or deposition of the polymer on the reactor wall. The invention also relates to the highy stable, finely divided, low viscosity polymer polyols obtained from the described process and their use in the preparation of polyurethane foams According to the present invention, an intermediate, which has a small average particle size thus ideally suited for further polymerization, is prepared by reacting (1) a mixture of styrene and acrylonitrile in a mixture of (2) a polyoxyalkylene polyether polyol and (3) a macromer in the presence of (4) a free radical initiator, (5) a solvent having moderate chain transfer activity and optionally (6) a reaction moderator, such that the intermediate contains a high level of macromer, at least about 12% and preferably at least about 15% with respect to the polyol mixture and the solids content of the intermediate is less than about 30%, preferably less than about 25% and at least about 15% by weight. The intermediate, which functions as a seed for further polymerization, is then further reacted in one or more stirred-tank reactors in series with a mixture of styrene and acrylonitrile in a polyol and optionally a macromer, in the presence of solvent, initiator and a reaction moderator which are distributed among the remaining reactors. In another preferred embodiment of the present invention, the distribution of the raw materials among the polymerization reactors minimizes the monomer concentration such that the amount of monomer added to any reactor is less than about 30% of that reactor's content and is preferably approximately equal in each of the reactors where monomer is added. The last reactor in the series of stirred-tank reactors is usually but not necessarly used exclusively as a post reactor to increase the conversion of monomer to polymer. The temperature at which polymerization is carried out should be at least 100°C, preferably 100° to 140°C, and most preferably 120° to 130°C. The contents of each reactor is well mixed with a residence time of at least about 20 minutes, preferably in the range of from 30 to 90 minutes. The reaction may be carried out at the pressure which becomes established at the operating temperature in a system which is sealed off from external pressure or it may be carried out in an open system at atmospheric pressure. The atmospheric oxygen should be purged from the entire apparatus with an inert gas, such as nitrogen or argon. An inert atmosphere should be constantly maintained in the system during the process. The product is stripped of volatile constituents, in particular from solvent and residues of monomers, by the usual method of vacuum distillation, optionally in a thin layer or falling film evaporator. Monomers suitable for the in-situ graft polymerization are mixtures of styrene and acrylonitrile in proportions by weight in the range of from about 100/0 to 20/80, preferably greater than 50% styrene. Other commonly used ethylenically unsaturated monomers may be used, in minor portions together with styrene and/or acrylonitrile or as replacements for either styrene and/or acrylonitrile. Examples include, but are not limited to: methylacrylonitrile, d-methyl styrene, methylstyrene, butylstyrene, unsaturated monocarboxylic acids (such as acrylic acid, methacrylic acid, maleic acid, fumaric acid or itacinic acid), and substituted unsaturated monocarboxylic monomers (such as methyl acrylate, 2-hydroxypropylacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate and isopropylmethacrylate) and the like. The polyols which may be used according to the invention include the known addition reaction products of cyclic ethers. Examples include, ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran and mixtures thereof with starting compounds that have at least two active hydrogen atoms in the molecule, e.g. those described in, "Polyurethanes: Chemistry and Technology", Volume XVI, Part I, by J. H. Saunders and K. C. Frisch, Robert E. Krieger Publishing Co., Malabar, FL, 1983, pages 32-44. Suitable starter compounds include polyhydroxyl compounds such as alkylene glycols, glycerin, trimethylolpropane, pentaerythritol, sorbitol, glucose, and sucrose. Other examples of suitable starter compounds include water, ammonia, amino alcohols (such as ethanolamine, diethanolamine, triethanolamine) and primary and/or secondary amines or polyamines (such as ethylenediamine, aniline and toluene diamine). The polyether polyols used in the present invention for the preparation of polymer polyols have a molecular weight from 500 to 12,000, preferably from about 2,000 to 8,000 and a hydroxyl functionality of from 2 to 6. The polyether chains are generally built up of propylene oxide and ethylene oxide units. The oxides may be mixed upon addition, or may be added separately to form blocks or caps. The resulting polyols can range from having predominately primary to predominately secondary hydroxyl groups depending on the sequence and amounts of the respective oxides. Macromers which may be used according to the present invention, include but are not limited to the reaction product of a polyol with the following reactive unsaturated compounds: maleic anhydride, fumaric acid, 1,1-dimethyl- m -isopropenylbenzyl-isocyanate, isocyanatoethylmethacrylate, 2-butene-1,4-diol, 1-butene-3,4-diol, hydroxyethyl methacrylate, hydroxypropyl acrylate, methyl methacrylate, acrylic and methacrylic acid, methacroyl chloride, glycidyl methacrylate and allyl glycidyl ether. If a polycarboxylic acid or anhydride is employed, it is preferred to react the unsaturated polyol with an alkylene oxide, to reduce the acid number by replacing the carboxyl groups with hydroxyl groups prior to use in the present invention. The polyol reactant is preferably a polyoxyalkylene polyether polyol as previously described above having a molecular weight of at least 4,500 and a hydroxyl functionality of at least 3. In the preparation of the macromer, it is preferred to use a quantity of the reactive unsaturated compound in the range of from 0.3 to 1.5 moles per mole of polyol, preferably 0.5 to 1.2 moles per mole of polyol. The amount of macromer used is that which is required to adequately stabilize the polymer against sedimentation. In general, the total amount of macromer in the end-product is in the range of from 2% to 10% by weight with respect to the polyol mixture, but could require more in some cases to stabilize the dispersion. Initiation of the polymerization is accomplished using typical free radical forming initiators. Initiators of this type include, for example, organic peroxides (such as benzoyl peroxide and decanoyl peroxide), percarboxylic acid esters (such as t -butyl peroctoate and t -amyl 2-ethylhexanoate) and aliphatic azonitrile compounds (such as 2,2'-azo-bis-( iso -butyronitrile) and 2,2'-azo-bis-(2-methylbutanenitrile)). The half-life for thermal decomposition of the initiator should be as low as possible under the polymerization conditions, preferably about 1 minute, to accomplish rapid conversion of monomer to polymer. The initiator is preferably used in quantities of from 0.5 to 5% by weight, based on the total quantity of monomers. The polymerization is preferably carried out in an organic solvent that does not dissolve the polymer. Illustrative examples thereof are benzene, toluene, ethylbenzene, xylene, hexane, iso -propanol, n -butanol, 2-butanol, ethyl acetate, butyl acetate and the like including those known in the art to be suitable for the polymerization of vinyl monomers. It is preferred to use a solvent that has a normal boiling point within the range of from 100-140°C. Solvents with a normal boiling points less than 100°C can be used, but require that the reaction be conducted in pressurized reactors. It is also preferred that the solvent has moderate chain transfer activity. Ethylbenzene and n -butanol are particularly well suited for use as a solvent according to the present invention. The solvent is typically used in quantities of from about 2% to about 20% by weight and is removed from the reaction mixture before the polymer polyol is used to produce polyurethane foams. The addition of a reaction moderator or chain transfer agent or polymer control agent during the polymerization has been found to be useful. The use of such reaction moderators is optional during the preparation of the intermediate but is essential during further polymerization especially at solids levels greater than 30% by weight. Examples of such reaction moderators include: mercaptans, ketones, alcohols, alkyl halides, and enol-ethers. The preferred reaction moderators are enol-ethers of the formula: A=CH-O-R wherein R represents a C 1-18 alkyl, C 5-10 cycloalkyl or substituted or unsubstituted benzyl group, A represents the group R' represents either hydrogen or a C 1-8 alkyl group. A representative example of a preferred enol-ether is Vulkazon AFD, a product of Bayer AG or (cyclohex-3-enylidenemethoxymethyl)benzene. The reaction moderator is used in a quantity of from about 0.5% to about 5% by weight with respect to the total monomer. One of the major advantages of the process of the present invention is that reactor fouling is eliminated or at least substantially reduced so that the process can be run several weeks without interruption. The polymer polyols prepared by the process of the present invention are suitable for the production of all types of polyurethane resins, particularly flexible and semi-rigid polyurethane foams. The polymer polyols are free from coarse particles which are capable of sedimentation and filtration and have a lower viscosity or a smaller average particle size at the same viscosity compared to similar products prepared by known methods of the prior art. Processes for producing polyurethane foams using polymer polyols are known. One of the most important applications of the polymer polyols is their use in the production of flexible molded and slabstock as well as semi-rigid polyurethane foams, to which they impart improved rigidity and weight bearing capabilities. The substances also have advantageous effects on other properties of the foams, such as open-cell character and shrinkage resistance of flexible foams. This invention also relates to a process for the preparation of cellular and non-cellular polyurethane resins comprising reacting (1) a polyisocyanate with (2) a polymer polyol and optionally (3) other compounds with a molecular weight in the range of from 40 to 10,000, which contain isocyanate reactive hydrogen atoms, optionally in the presence of (4) catalysts, (5) blowing agents and (6) other known additives. Isocyanates which may be employed include aromatic, aliphatic, and cycloaliphatic polyisocyanates and combinations thereof Representative samples of these polyisocyanates include diisocyanates, such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, cyclohexane-1,4-diisocyanate, hexahydrotoluene diisocyanate, naphthalene-1,5-diisocyanate, 4,4'-diphenylmethane diisocyanate, and 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate; and triisocyanates, such as toluene-2,4,6-triisocyanate and 4,4',4''-triphenylmethane triisocyanate. Especially useful are 2,4-toluene diisocyanate; mixtures of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate obtained from the phosgenation of crude toluene diamine; 4,4'-diphenylmethane diisocyanate and crude diphenylmethane diisocyanate obtained from the phosgenation of crude diphenylmethane diamine. The foam formulation includes polymer polyols prepared according to the present invention and may also include compounds with a molecular weight of from 400 to 10,000 which have at least two isocyanate reactive hydrogen atoms. These compounds may contain hydroxy groups, amino groups, thiol groups or carboxyl groups. The preferred compounds are hydroxyl compounds containing from 2 to 6 hydroxyl groups with a molecular weight of from 400 to 10,000, preferably from 1,000 to 8,000. The hydroxyl compounds are preferably polyesters or polyethers commonly used in the production of polyurethanes. Chain lengthing agents or cross-linking agents which may be employed according to the present invention include but are not limited to water, hydrazine, aromatic and aliphatic diamines (such as phenylenediamine, ethylenediamine, diethylenetriamine, 2,4- and 2,6-hexahydrotolylenediamine), amino alcohols (such as diethanolamine, N-methyldiethanolamine, triethanolamine, and 3-amninopropanol), amino acids, hydroxy acids, glycols (such as ethylene glycol, propylene glycol, glycerine, 1,4-butandiol, 1,6-hexandiol, and sorbitol) and higher molecular weight polyethylene or polypropylene or polybutylene glycols with molecular weight up to 400. Suitable catalysts for the preparation of polyurethane foams may be used including tertiary amines such as, for example, triethylamine, tributylamine, N-methylmorpholine, N,N,N',N'-tetramethylethylene diamine, 1,4-diazobicyclo-[2.2.2]-octane, N,N-dimethylbenzylamine and N,N-dimethylcyclohexylamine. Triethanolamine, N-methyldiethanolamine, N,N-dimethylethanolamine and their reaction products with alkylene oxides may also be used. Other suitable catalysts include tin(II) salts of carboxylic acids (such as tin(II) acetate, tin(II) octoate, tin(II) ethyl hexoate and tin(II) laurate) and tin(IV) compounds (such as dibutyl tin oxide, dibutyl tin dichloride, dibutyl tin diacetate, dibutyl tin dilaurate, dibutyl tin maleate and dioctyl tin diacetate. The above mentioned catalysts may, of course, also be used as mixtures. Blowing agents may optionally be used, such as water and/or volatile organic or inorganic substances. Suitable organic blowing agents include, for example, acetone, ethyl acetate, cyclopentane, halogenated hydrocarbons (such as methylene chloride, chloroform, trichlorofluoromethane, chiorodifluoromethane, and dichlorodifluoromethane), butane, hexane or diethylether. As inorganic blowing agents, air, CO 2 or N 2 O may be used. The effect of a blowing agent may also be obtained by the addition of compounds that decompose at elevated temperatures to liberate gases. Other additives including surface active additives, foam stabilizers, reaction retarders, stabilizers, flame retarding substances, plasticizers, dyes, fillers and fungustatic and bacteriostatic substances may also be used according to the invention. Details concerning the use and action of these additives may be found in the "Polyurethane Handbook", Volume VII, by G. Oertel, Carl Hanser Publishers, Munich, 1993, pages 104-127. Examples Definitions Polyol A: A polyether prepared by reacting a mixture of glycerine and propyleneglycol, having a hydroxyl functionality of 2.9, with propylene oxide and ethylene oxide The polyether has an ethylene oxide content of 10% by weight, contains predominately secondary hydroxyl groups and has a hydroxyl number of 56. Polyol B: A polyether prepared by reacting trimethylolpropane with propylene oxide and ethylene oxide, wherein the ethylene oxide content is 17.5% by weight. The polyether contains 80-90% primary hydroxyl groups and has a hydroxyl number of 35. Polyol C: Polyether prepared by reacting trimethylolpropane with propylene oxide and ethylene oxide, wherein the ethylene oxide content is 17.7% by weight. The polyether contains approximately 90% primary hydroxyl groups and has a hydroxyl number of 28. Polyol D: Polyether prepared by reacting glycerine with propylene oxide and ethylene oxide. The polyether has an ethylene oxide content of 10% by weight, contains predominately secondary hydroxyl groups and has a hydroxyl number of 56. Macromer I: Polyether containing induced unsaturation prepared by reacting Polyol B with maleic anhydride and subsequently with ethylene oxide. The macromer has unsaturation of 0.050 meq/g and a hydroxyl number of 33.9. Macromer II: Polyether containing induced unsaturation prepared by reacting Polyol C with acrylic acid in the presence of p-toluenesulfonic acid. The macromer has unsaturation of 0.065 meq/g and a hydroxyl number of 25.9. Initiator: 2,2'-azobis-(2-methylbutanenitrile), Vazo® 67, commercially available from DuPont Moderator: (cyclohex-3-enylidinemethoxymethyl)benzene, Vulkazon® AFD, commercially available from Bayer AG T: toluene EB: ethylbenzene BUOH: 1-butanol Stabilizer: Stabilizer OS 22, commercially available from Bayer AG Catalyst 1: dimethylethanolamine, Desmorapid® DMEA, commercially available from Rhein Chemie Catalyst 2: bis(2-dimethylaminoethyl)ether in dipropylene glycol, RC-PUR Activator 108, commercially available from Rhein Chemie Catalyst 3: stannous octoate, Desmorapid® SO, commercially available from Rhein Chemie TDI: toluene diisocyanate (80% 2,4- ; 20% 2,6-toluene diisocyanate), Desmodur® T80, commercially available from Bayer AG Preparation of Polymer Polyol The polymer polyols of the Examples were prepared continuously in a series of stirred-tank reactors. The feed components were pumped into the respective reactors continuously. The contents of each reactor were well mixed and the internal temperature was controlled to 125°C. All reactions were conducted in an inert atmosphere under normal pressure, using an overflow reactor design. The overflow from the first reactor enters the second reactor where additional raw materials were added. Additionally, when desirable, other reactors were used in a similar fashion with the distribution of the raw materials among the reactors. The last reactor in the series was used as a post reactor where additional initiator and solvent were added to increase the conversion of monomer to poylmer. Portions of the crude products were vacuum distilled at < 1 mbar and 125°C for several hours to remove the solvent and residual monomers before the polymer polyols were used to produce polyurethane foams. Preparation of Polyurethane Foams The formulation of a typical free-rise polyurethane foam is listed below. Several of the polymer polyols described above and identified in the Examples were used to prepare free-rise foams. Free-Rise Foam Formulation Components Parts Polymer Polyol 50 Polyol D 50 Water 4.5 Stabilizer 1 Catalyst 1 0.1 Catalyst 2 0.05 Catalyst 3 0.13 TDI (108 index) A typical free-rise polyurethane foam is prepared by charging all the components (except the polyisocyanate) into a container and intensely mixing. The polyisocyanate is added with stirring and the contents are subsequently poured into an open mold. The polyurethane foam is allowed to rise and cure at room temperature. Analyses and Measurements Conversion The conversion of monomer to polymer was calculated from the distillate. Viscosity The viscosity of the polymer polyols was measured on a rotoviscometer equipped with a constant temperature cell. Particle Size The average particle size was determined by dynamic light scattering, a technique for sizing particles in the range of from 3 nm to 3 µm. The reported values are the average of 6 probes. Residue The residue is a means of evaluating the quality and eventually the storage stability of the polymer polyol. The test is performed by first wetting the inner wall of a 10 ml sample vial with 2-3 ml of polymer polyol. After 24 hours standing at room temperature, the glass wall is observed and evaluated based on the clarity of the film and the number of polymer particles or agglomerated polymer particles of about 5-30 µm in diameter. Numerical Rating 0 = very good 1 = good 2 = middle/good 3 = middle 4 = middle/poor 5 = poor Hardness The hardness of the foams were determined according to test method DIN 53577. Elongation The elongation at break of the foams were determined according to test method DIN 53571. Tensile The tensile strength of the foams were determined according to test method DIN 53571. Examples The following examples, Examples 1-16 (reported in Table I), clearly illustrate the advantages of the present invention. Examples 1-5 were prepared for comparison and are not covered under the claims of the present invention. Examples 1-3 were prepared using a simple overflow reactor and are characterized by a large average particle size (in excess of 1000 nm) and a relatively high viscosity (in excess of 5000 mPas at 25°C). Materials prepared via this approach generally exhibit a poor residue test, indicating that the material contains a significant portion of large polymer particles or agglomerated polymer particles which may precipitate from the continuous phase of the base polyol. After 4-6 hours, early signs of reactor fouling, polymer build-up on the reactor wall, were observed in Examples 1 and 2. Reactor fouling was improved with the replacement of toluene with ethylbenzene, Example 3, however, an increase in the viscosity of the end-product was noted, see Table I. In Example 4, the reaction mixture was added to two reactors connected in series with 50% of the reaction mixture being added to each reactor. The resulting product shows an increase in viscosity and average particle size. In Example 5, only the monomer is distributed among the two reactors while the total amount of polyol and macromer are added to the first reactor. In this example, polymer build-up was noted in the second reactor after 7 hours. The end-product has a similar viscosity (compared to Example 1), the average particle size remains relatively large and the residue test shows a moderate amount of large polymer particles or agglomerated polymer particles. The critical process parameters required for the preparation of stable, low viscosity polymer polyols of small average particle size using a series of stirred-tank reactors have been identified. The advantages of the present invention are demonstrated in Examples 6-16. Examples 6-9 demonstrate the effect of varying the concentration of macromer and the solids content during the preparation of the intermediate on the physical properties of the final polymer polyol dispersion. By maintaining a high concentration of macromer and a low solids content in the first reactor, a highly stable intermediate of small average particle size is formed that is ideally suited for further polymerization. In Examples 6, 7 and 9, the macromer concentration is fixed at about 15% with respect to the polyol mixture, and the solids level in the first reactor is varied (20, 30 and 25% by weight respectively) resulting in intermediates with average particle sizes of 430, 550 and 505 nm respectively. The final products which contain greater than 40% solids, have low viscosities and small average particle sizes, see Table I. In Example 8, a macromer concentration of 12% with respect to the polyol and a solids content of 15% by weight are used in the first reactor to form an intermediate with an average particle size of 390 nm. However, the amount of monomer added to the second reactor is undesirablely high, resulting in some build-up on the reactor wall. Examples 10-16 demonstrate the advantages of the present invention using three polymerization reactors so that the monomer can be further distributed to minimize reactor fouling. Materials prepared via this process generally contain very few large polymer particles or agglomerated polymer particles as previously described. Examples 10 and 11 further demonstrate the effect of macromer concentration during the preparation of the intermediate on the physical properties of the final dispersion. In Example 12, the macromer is distributed over the polymerization reactors which results in a polymer dispersion with essentially no large polymer or agglomerated polymer particles. A second macromer type is used in Example 13. Example 14 demonstrates the use of an alternative solvent, 1 -butanol. In Example 15, the intermediate is prepared using a lower styrene/acrylonitrile ratio which leads to an intermediate of small average particle size. A higher initiator level is used during the preparation of the intermediate in Example 16 which also yields an intermediate of small average particle size. The Examples described above are for purposes of illustration and should not be taken as limiting the invention, which is defined in the following claims.

Claims: A continuous process for the preparation of highly stable, finely divided, low viscosity polymer polyols having a small average particle size comprising a) forming an intermediate in a first reactor by reaching (1) a mixture of styrene and acrylonitrile, in a mixture of (2) a polyoxyalkylene polyether polyol and (3) a macromer, in the presence of (4) a free radical initiator, (5) a solvent having moderate chain transfer activity, and, optionally, (6) a reaction moderator at a temperature of at least 100°C, wherein the resultant intermediate has a macromer content of at least about 12% by weight, based on the total weight of the polyether polyol and the macromer, and a solids content of at least about 15% and less than about 30% by weight, based on the total weight of the polyether polyol, the macromer and the monomers, and contain less than 43% by weight of polyether polyol, based on the total weight of the polyether polyol, and b) reacting said intermediate in at least a second stirred-tank reactor connected in series to the first reactor, with (1) a mixture of styrene and acrylonitrile, in (2) a polyether polyol and, optionally, (3) a macromer, in the presence of (4) a free radical initiator, (5) a solvent, and (6) a reaction moderator, wherein components (1) through (6) are distributed among the second and any successive reactors. A process of claim 1 in which the distribution of the raw materials among the polymerization reactors minimizes the monomer concentration such that the amount of monomer added to any reactor is less than about 30% of that reactor's content and is preferably approximately equal in each reactor. A process of claims 1 and 2 where the monomer mixture is a mixture of styrene and acrylonitrile in proportions by weight in the range of from about 100/0 to 20/80, preferably greater than 50% styrene. A process of claims 1 and 2 where the base polyether polyol is characterized by a molecular weight in the range of from 500 to 12,000, preferably from about 2,000 to 8,000 and having a hydroxyl functionality of from 2 to 6. A process of claims 1 and 2 where the macromer is prepared either directly or indirectly by the reaction of (1) a polyether polyol, having a molecular weight of at least about 4,800 and a hydroxyl functionality of at least 3, with (2) a reactive unsaturated compound and that the quantity of the reactive unsaturated compound used in the preparation of the macromer is in the range of from 0.3 to 1.5 moles per mole of polyol and preferably 0.5 to 1.2 moles per mole of polyol. A process of claims 1 and 2 where the free radical initiator is used in quantities of from 0.5 to 5% by weight, based on the total quantity of monomers. A process of claims 1 and 2 where a solvent , preferably ethylbenzene or n-butanol, is used in an amount of from about 2 to 20%. A process of claims 1 and 2 where the reaction moderator, which is optional for the preparation of the intermediate but required for the further reaction of the intermediate, is an enol ether of the following formula: A=CH-O-R wherein R represents a C 1-18 alkyl, C 5-10 cycloalkyl or substituted or unsubstituted benzyl group, A represents the group R' represents either hydrogen or a C 1-8 alkyl group. A process of claims 1, 2, and 8 where the reaction moderator is (cyclohex-3-enylidenemethoxymethyl)benzene. A process of claims 1, 2, 8 and 9 where the reaction moderator is used in a quantity of from about 0.5% to 5% by weight with respect to the total monomer. A highly stable, finely divided, low viscosity polymer polyol of small average particle size prepared according to claims 1 through 10. A polyurethane product using a polymer polyol as prepared in accordance with claims 1 through 10.

Inventors: DIETRICH MANFRED DR, HEINEMANN TORSTEN DR, JACOBS GUNDOLF DR, KRATZ MARK R DR, SANDERS JOSEF DR, WOYNAR HELMUT DR, DIETRICH, MANFRED, DR., HEINEMANN, TORSTEN, DR., JACOBS, GUNDOLF, DR., KRATZ, MARK R., DR., SANDERS, JOSEF, DR., WOYNAR, HELMUT, DR.
Applicants: BAYER AG
Application year: 1995
Main classification: C08F283
IPC codes: C08F285, C08F220, C08G18, C08F283, C08F290, C08F212