PROCESS FOR PREPARING POLYURETHANE POLYMERS

Abstract: A static mixing tube is utilized to continuously react a compound having active hydrogen atoms such as a polyol which can be a polyether polyol or a polyester polyol with a polyisocyanate such as MDI to form a moisture-curing polyurethane adhesive or other urethane polymer. The polyol and polyisocyanate are fed in essentially constant proportion to a static mixer reaction zone, which is maintained at an elevated temperature to achieve a required degree of condensation at a residence time of about 10 minutes or less.

Description: PROCESS FOR PREPARING POLYURETHANE POLYMERS Background of the Invention This application claims priority from U.S. provisional application Serial No. 60/150,768, filed August 26, 1999, the disclosure of which is incorporated herein by reference. Field of the Invention This invention pertains to methods of preparing polyurethane prepolymers and particularly polyurethane adhesives capable of being cured by moisture. Polyols and/or polyamines and polyisocyanates are combined and reacted in a continuous manner at an elevated temperature within a static mixing zone. Discussion of the Related Art Reactive polyurethane adhesives constitute a commercially important class of adhesives which are distinguished by good adhesion to various substrates and, compared to other reactive adhesives, by high elasticity, even at low temperatures. Moisture-curable polyurethane adhesives contain reactive polyurethane prepolymers having terminal isocyanate groups. The terminal isocyanate groups can react and be hardened (cured) with water. Both atmospheric moisture and direct application of water on the substrates to be joined can be used as hardener for the prepolymer. Relatively low molecular weight polyurethane prepolymers are liquid at room temperature. Higher molecular weight polyurethane prepolymers and prepolymers containing crystallizable polyester segments are generally solid at room temperature and thus may be used as hot melt adhesives, wherein the prepolymer is applied while molten between substrates. Initial adhesion is achieved by cooling and solidifying the adhesive, with complete curing being obtained by chemical reaction of the prepolymer with water. Polyurethane prepolymers for adhesive applications are conventionally produced by reacting a polyol such as polypropylene glycol with a molar excess of a polyisocyanate such as MDI in a batch-type process wherein the polyol and polyisocyanate are combined in a stirred reactor and heated for a time effective to achieve the desired degree of reaction between these two components. Typically, chain extension (and a corresponding increase in average molecular weight) takes place by reaction of the free isocyanate group of a partially reacted polyisocyanate (one that has already reacted through an isocyanate group with one polyol molecule) with a second polyol molecule and so forth. However, the batch process is time-consuming, typically requiring several hours to complete. It is thus difficult to make productive and efficient use of the processing equipment. Moreover, the molecular weight distribution (polydispersity) of the polyurethane prepolymer product tends to be somewhat higher than is desirable for certain applications. A high polydispersity may, for example, result in a product having a viscosity which is too great to be easily used in certain types of adhesive applicators or to be quickly spread out on the surfaces of the substrates to be joined. Another problem encountered in manufacturing moisture-curable polyurethane adhesives in a batch-type process is that critical characteristics of the product such as viscosity, molecular weight distribution and NCO content may fluctuate to an unacceptable degree from batch-to-batch, due to variations in the heat history of different batches. In addition, the excess isocyanate can react with itself at elevated temperatures to form isocyanurates which appear as solid particles in the product . One method of making curable adhesive polyurethane products is described in U.S. Pat. No. 4,373,057. This patent describes a one step process consisting essentially of the reaction of isocyanate compounds and diol/polyol compounds in the absence of polymerization catalyst. The reactants are initially intermingled at ambient room temperature, then reacted with intermingling in the absence of externally applied heat. The polymerization reaction is said to be complete in about 2 hours when carried out using a 55 gallon drum reactor. The patent states that reaction at higher elevated temperatures should be avoided, as adverse secondary reactions will take place which tend to impair properties of the polyurethane product. Shortening the production time required by increasing the reaction temperature thus is not feasible. Polyurethane prepolymers which contain reactive hydroxyl groups are useful for forming two component systems. The polyurethane prepolymers with reactive hydroxyl groups can be reacted with isocyanates to form useful coatings, elastomers, sealants and the like. Polyurethane prepolymers which contain reactive hydroxyl groups can also be produced by the process of the invention. The product produced is dependent on the molar ratio of hydroxyl groups to isocyanate groups in the mixture which is reacted. Summary of the Invention We have now found that moisture-curable polyurethane adhesives, amine terminated polyurethane prepolymers, hydroxyl terminated polyurethane prepolymers and other urethane polymers of excellent and consistent quality can be produced by reacting isocyanate-reactive substances such as polyols or polyamines with isocyanates in a static mixer reaction zone. One or more isocyanate-reative substances and one or more polyisocyanates are continuously introduced into one end of the static mixer reaction zone, in a proportion which does not change by more than about ±5% and preferably not more than ±2% during operation of the process. The polyols and polyisocyanates are continuously passed through the static mixer reaction zone at a temperature in the range of about 50°C to about 170°C and preferably 70°C to about 150°C. When polyamines are reacted with the polyisocyanate, temperatures are generally lower (preferably, in the range of from about -5°C to about 80°C) . The residence time within the static mixer reaction zone is sufficient to attain a pre-selected degree of reaction between the isocyanate-reactive substances and polyisocyanates. The residence time is determined by the reactivity of the isocyanate and polyol or polyamine and the temperature of the process. The process can be operated at temperatures to provide a suitable product at a residence time of 10 minutes or less and preferably 2 minutes or less and most preferably 1 minute or less. The moisture-curing polyurethane adhesive or polyol or amine terminated prepolymer formed is continuously withdrawn from the static mixer reaction zone, cooled and placed in a storage vessel. When the product is moisture-curing it must be protected from contact with moisture. Due to the relatively short residence times and continuous mode of operation, the process is highly efficient and capable of producing a much larger quantity of product within a given period of time with smaller equipment than a conventional batch prepolymer process. In addition, the process is flexible and can be used to make small amounts of different products without major clean-out of equipment. Since the residence time is short, product changes can be made merely by changing the settings of the metering pumps . Brief Description of the Drawings Fig. 1 is a schematic representation of an embodiment of the process of the invention. Fig. 2 is a schmatic representation of a second embodiment of the process of the invention. Detailed Description of the Invention The presently described process may be readily adopted for use with any of the polyols, polyamines and polyisocyanates which are reactable or are known or used in the art of preparing polyurethane prepolymers for moisture-curing polyurethanes . The term "polyol" encompasses any of the monomeric or polymeric substances containing an average of two or more hydroxyl groups capable of reacting with an isocyanate group. Suitable polyols and methods for their preparation have been described in the prior art; many different polyols are also available from commercial sources. Typically, the polyols used in reactive polyurethane adhesives have number average molecular weights of at least about 500, preferably about 1,000 to about 10,000, and a functionality (number of hydroxyl groups per molecule) of about 1.8 to about 4, more preferably about 2 to about 3. Particularly suitable polyols include polyether polyols, polyester polyols and polyurethane polyols. Polyether polyols which may be used include products obtained by polymerization of one or more cyclic oxides, for example, ethylene oxide, propylene oxide, butylene oxide, oxetane, or tetrahydrafuran in the presence, where necessary, of polyfunctional initiators. Suitable initiator compounds contain a plurality of active hydrogen atoms and include water and organic polyalcohols such as, for example, ethylene glycol, propylene glycol, diethylene glycol (and other glycol oligomers) , cyclohexane dimethanol, resorcinol, bisphenol A, glycerol, trimethylolpropane, 1,2,6- hexanetriol and pentaerythritol . Mixtures of initiators and/or cyclic oxides may be used. Polyalkylene glycols corresponding to the general formula H- (-R-0-) n -H where R is a hydrocarbon radical containing 2 to 4 carbon atoms and n is 2 or greater (preferably, about 10 or greater) are one class of polyether polyols useful in the present process. Especially useful polyether polyols include polyoxypropylene diols and triols and poly (oxyethylene- oxypropylene) diols and triols obtained by the simultaneous or sequential addition of ethylene and propylene oxides to di- or tri-functional initiators. Mixtures of said diols and triols can be particularly useful. Other especially useful polyether polyols include polytetramethylene glycols obtained by the polymerization of tetrahydrofuran. Polyester polyols which may be used include hydroxyl-terminated reaction products of polyhydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, 1 , 4-butanediol , neopentyl glycol, 1 , β-hexanediol, cyclohexane dimethanol, bis (hydroxy ethyl) terephthalate, glycerol, trimethylol-propane, pentaerythritol or polyether polyols or mixtures of such polyhydric alcohols and polycarboxylic acids (or esters and/or anhydrides thereof, especially methylesters) . Suitable polycarboxylic acids include aliphatic as well as aromatic acids such as succinic acid, glutamic acid, adipic acid, sebacic acid, phthalic acid, terephthalic acid, isophthalic acid and the like. Suitable polyester polyols may also be prepared from hydroxycarboxylic acids and derivatives thereof (e.g., lactones, methyl esters) . The polyester polyols may be liquid or solid at room temperature; mixtures of liquid and solid polyester polyols may be utilized. The solid polyester polyols may be amorphous, partially crystalline, or crystalline. Mixtures of polyether polyols, polyurethane polyols and polyester polyols may be used in the present process . Polyurethane polyols can also be used as the polyols in the practice of the invention. The polyols are selected by one skilled in the art to provide a product with desired properties. The process is not limited by types of polyols or isocyanates as long as the reactants are liquid, can be pumped and are reactive at the temperature of the process. Polyamines as used in the present application are compositions which contain from about 2 to 4 primary or secondary amine groups which can be end capped on a polyether, polyester, or polyurethane backbone. Polyamines generally react with polyisocyanate faster than polyols so that lower temperatures can be used. Relatively low molecular weight polyhydroxy compounds (including monomeric compounds containing two or more hydroxy groups) may also be used in the present invention in addition to the polyols previously described. Such polyhydroxy compounds include the substances commonly referred to in the art as "chain extenders" such as ethylene glycol, 1 , 4-butanediol, and the like. Polyisocyanates (i.e. organic compounds containing an average of two or more isocyanate groups, or equivalents thereof, per molecule) suitable for use in the present process include aliphatic, cycloaliphatic, araliphatic, and aromatic polyisocyanates. The diisocyanates are generally preferred, although blends with tri- and higher functionality isocyanates may be desirable for certain applications. Illustrative polyisocyanates include, but are not limited to, 1,6- hexamethylene diisocyanates, isophorone diisocyanate, cyclohexane-1 , 4-diisocyanate, 4,4' -dieyelohexylmethane diisocyanate, p-xylylene diisocyanates, phenylene diisocyanates, polymethylene poly (phenyl isocyanates) (PMDI) , tolylene diisocyanates (TDI) , and diphenyl methane diisocyanates (MDI) in their various pure, modified, and crude forms and mixtures thereof. Suitable polyisocyanates include "pure" MDI, preferably containing at least 60% by weight of 4,4'- isomers. Other suitable polyisocyanates include the substantially pure 4,4'-isomer and isomer mixtures containing that isomer and not more than 40%, preferably not more than 20%, by weight of the 2, 4 '-isomer and not more than 5% by weight of the 2,2 '-isomer. Still other polyisocyanate compositions include modified forms of diphenyl methane diisocyanates, that is to say MDI modified in any known manner by introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues. Polyether-based prepolymers having NCO contents higher than the isocyanate level desired in the polyurethane adhesive product (e . g. , 20% by weight or higher) as well as glycol-modified MDI are also appropriate for use. In preferred embodiments of the invention, reaction of the isocyanate-reactive substances and polyisocyanates is carried out in the absence of any water or organic solvent. It is also preferred to perform said reaction in the absence of any substances capable of catalyzing the hydroxyl-isocyanate or amine- isocyanate reaction. However, with slow reacting materials catalysts can be useful in the process. It is critical that the proportion of polyisocyanate (s) to isocyanate-reactive substance (s) being fed into the static mixer reaction zone be kept essentially constant during operation of the process in order to avoid undesirable fluctuations in the properties of the moisture-curing polyurethane adhesive or hydroxyl or amine terminated polyurethane prepolymers produced. When a moisture curing adhesive is to be produced the proportions selected should provide a molar excess of isocyanate (NCO) groups relative to hydroxyl (OH) or amine (NH) groups. Preferably, the NCO: active hydrogen molar ratio is in the range of from about 1.5:1 to about 20:1 and preferably from about 2:1 to 20:1. Changing this ratio changes the characteristics of the product. For example, a relatively high ratio will tend to yield a relatively low molecular weight, liquid product while a relatively low ratio will tend to yield a higher molecular weight, solid (hot melt) product, all other factors being equal . The residence time and temperature within the static mixer reaction zone will also influence the product characteristics. It is therefore important, in order to obtain a polyurethane adhesive or hydroxyl or amine terminated urethane prepolymer of consistent quality, to keep these variables as constant as possible during operation of the process. For this reason, the polyisocyanate: isocyanate-reactive substance proportion, residence time and temperature should be held within about ±5% (more preferably, within about ±2%) of selected values. Positive displacement pumps are preferably used to assist in the maintenance of a constant polyisocyanate: isocyanate-reactive substance proportion in the feed to the static mixer reaction zone. Positive displacement pumps which provide speed controls, output control and the like are preferred for use in the process. Since mass flow control is important to the process, the pumps should be able to provide constant mass flow rates for the reactants entering the static mixer reaction zone. Gear pumps and piston pumps can be useful if they are adapted to pumping vicious materials. When a hydroxyl terminated prepolymer is to be produced, the proportions of polyol and isocyanate should be selected to provide a molar excess of hydroxyl (OH) groups in relation to isocyanate (NCO) groups. A molar ratio of hydroxyl groups (OH) to isocyanate (NCO) groups is generally in the range of from about 1.05:1 to about 2:1 and preferably at least about 1.1:1; but other ratios can be used if products with higher or lower viscosities are required. The isocyanate-reactive substance and polyisocyanate reactant may be introduced separately into the static mixer reaction zone, or, in an alternative embodiment, blended or combined prior to such introduction. If the preblending embodiment of the invention is practiced, the resulting blend should be introduced to the static mixer reaction zone shortly after the mixing is performed or at least stored for a constant preselected time prior to introduction to the static mixer reaction zone. It is preferred that the isocyanate-reactive substance and isocyanate be mixed at the static mixer reaction zone or introduced separately into the static mixer reaction zone. One or both of the reactants may require heating to render the reactant liquid and readily pumpable, as will be the case where the reactant is normally solid at room temperature or to provide the reactants to the static mixer reaction zone at a required temperature or to provide the reactants to the static mixer reaction zone at a required temperature. The static mixer reaction zone employed in the present invention may be any of the static mixing devices known in the art which are characterized by the absence of any moving parts which induce mixing of substances passing through the static mixer reaction zone. The mixing zone generally contains a plurality of mixing elements or other devices capable of splitting and recombining a liquid stream. A wide variety of such mixing elements (sometimes referred to as inserts or baffles) are known in the art. Suitable mixing tubes and mixing elements therefore are described for example, in the following U.S. Patents, each of which is incorporated herein by reference in its entirety: 3,358,749; 3,404,869; 3,583,678; 3,635,444, 3,643,927 3,652,061; 3,664,638; 3,800,985; 3,953,002; 4,062,524 4,072,296; 4,093,188; 4,220,416; 4,408,493; 4,511,258 4,643,584; 4,765,024; 4,840,493; 4,936,689; 5,484,203 5,520,460 and 5,688,047. The static mixer reaction zone will generally comprise a conduit having an axis and defining a chamber extending longitudinally therethrough with an opening on a first end and a second end of the conduit with the mixing elements being positioned in the chamber between the first and second ends. The mixing element design and arrangement of mixing elements within the conduit are desirably selected such that the stream of reactants entering one end of the conduit is divided by the first mixing element into a plurality of streams. These divided streams are directed to opposite walls of the conduit, causing a uni-directional mixing vortex to form which is approximately axial to the centerline of the second mixing element. The mixing vortex is then sheared by the second mixing element and a second mixing vortex is formed having a rotation opposite in direction to that of the first mixing vortex. This process is repeated as the reactants encounter additional mixing elements in passing through the conduit. The materials of construction used for the static mixing zone are not believed to be critical, but should be selected to be resistant to temperature, to pressure, to corrosion under the process conditions and to be unreactive towards the isocyanate-reactive substance and polyisocyanate (i.e., the material should not catalyze their reaction or decomposition) . The static mixing zone may, for example, be constructed of carbon steel, stainless steel, fiberglass reinforced plastic, or plastic (e.g., PVC) . The construction is determined by the materials to be mixed, the temperature and pressure. Suitable static mixing zones are readily available from commercial sources including, for example, Koflo Corporation of East Dundee, Illinois, and Komax Systems, Inc. of Wilmington, California. A particularly preferred static mixing zone is one sold by Koflo Corporation. The static mixing zone is generally custom designed by the provider considering the viscosity, pressure and temperature required in the static mixing zone . In one embodiment of the invention, a plurality of static mixer reaction zones may be operated in parallel. The temperature of the polyol /polyisocyanate mixture within the static mixer reaction zone is maintained within the range of about 50°C to about 170°C and preferably about 70°C to about 150°C. The temperature need not, however, be kept constant over the entire length of the static mixer reaction zone. For example, the initial portion of the static mixer reaction zone where the isocyanate-reactive substance and polyisocyanate are introduced may be maintained at a higher or lower temperature than that of the portion at the end where the product is withdrawn. Depending upon the design of the static mixer reaction zone and the proportions and relative reactivities of the isocyanate-reactive substance and the polyisocyanate, it may be necessary to apply external heat in order to keep the internal temperature at the required point. If the isocyanate-reactive substance/polyisocyanate reaction is highly exothermic and heat loss from the static mixer reaction zone is minimized, it is possible to operate the process without application of external heat, thereby reducing utility costs. In such a situation, however, it may still be necessary to supply heat during start-up to quickly attain the desired operating temperature. The temperature profile over the length of the static mixer reaction zone during continuous operation of the process, however, should be kept as constant as possible. The flow rate of the isocyanate-reactive substance/polyisocyanate mixture may be adjusted as necessary to achieve the desired residence time. An important advantage of the present invention is that the reaction mixture is held at an elevated temperature (i.e., >50°C) for only a brief period of time (preferably 10 minutes, or less) . The residence time must be sufficient to accomplish the desired degree of reaction between the isocyanate- reactive substance and the polyisocyanate. This is typically measured by molecular weight, viscosity and/or NCO (isocyanate) or OH (hydroxyl) number or amine value. Typically, the reaction variables are chosen to provide a moisture-curing polyurethane adhesive product having an NCO content within the range of about 1 to about 20% by weight when a moisture curing polyurethane is produced. The polyurethane produced will generally have a number average molecular weight in the range of about 3,000 to 50,000 but other molecular weight range materials can be produced. For a given initial isocyanate-reactive substance/polyisocyanate proportion and reaction temperature, longer residence times will lead to more complete reaction, lower NCO content, and higher molecular weight. Maintaining the residence time at 10 minutes or less while obtaining a product with a particular NCO content and molecular weight, may require an adjustment of the reaction temperature. After being withdrawn from the static mixing zone, the polyurethane adhesive is cooled and stored for use in a sealed container such as a drum, bucket, tank or the like protected from moisture until ready to be applied as an adhesive. To avoid over-reaction of the isocyanate-reactive substance and polyisocyanate, such cooling is preferably accomplished relatively quickly. The polyurethane adhesive is preferably stored at or below room temperature . If so desired, the polyurethane adhesive may be blended or combined with any of the additives conventionally employed in the reactive polyurethane adhesive art including, for example, catalysts, fillers, tackifying resins (including hydrocarbon resins) , pigments, thixotropic agents, plasticizers, antioxidants, stabilizers, adhesion promoters and the like. Such additives may be added to the finished polyurethane adhesive produced by the process or may be introduced during the process itself. For example, one or more additives may be fed to the static mixer reaction zone together with the isocyanate-reactive substance and/or polyisocyanate or mixed with the product from the static mixer reaction zone in which the isocyanate-reactive substance and polyisocyanate are reacted in a separate static mixing zone where mixing and reacting or only mixing can occur (see Fig. 2) . The adhesive produced in the present process may be utilized in any of the applications known in the moisture-curing polyurethane adhesive field. For example, the adhesive can, when cured, bond textiles to substrates such as plastics, paper, metal, cardboard, wood or leather. Any of the aforementioned substrates can, of course, also be joined to each other using the moisture-curing polyurethane adhesive. When a hydroxyl terminated prepolymer is to be produced, the molar excess of polyol and the isocyanate are introduced into the static mixer reaction zone and the reaction is carried out under the same or similar conditions as when the isocyanate terminated polyurethane is produced. The polyols and isocyanates are selected to provide a product with the required molecular weight, viscosity and hydroxyl number. The polyols and isocyanates are also selected to provide a hydroxyl terminated prepolymer which when reacted with a selected isocyanate will provide a polymerized or a cross-linked product with required properties. The properties of polymers obtained from reaction of polyurethane polyols with isocyanates are well known in the art . The present invention is directed to a process for producing isocyanate terminated prepolymers or hydroxyl or amine terminated prepolymers which are used for specific purposes. Therefore, the properties of the prepolymers produced by the process of the invention can require the reaction product of various polyols or polyol mixtures, polyamines or polyamine mixtures, isocyanates or isocyanate mixtures. Figure 1 illustrates an embodiment of the present invention. A polyol or mixture of polyols is present in storage tank 1. Storage tank 1 can be heated to maintain the polyol at an elevated temperature. Temperatures in a range above about 50 2 C degrees to about 200 2 C can be maintained depending upon the melting point of the polyols and the temperature at which the reaction between the polyol and the isocyanates is to take place. If the polyols are liquid at ambient temperature, the storage tank 1 may not be heated as the polyols can be heated by a heat exchanger (not shown) in the line between the tank and the static mixer reaction zone 15. Both the heated storage tank and the heat exchanger can be used. Tank 1 is blanketed with an inert dry gas purge to prevent water from combining with the polyols which are generally hydroscopic materials. The polyols can be a single polyol or mixture of different polyols. Tank 1 can be a weigh tank to provide an indication that the polyols are being introduced into the process at a proper rate. The polyols pass from tank 1 through line 2 which is generally an insulated and heated line to prevent solid polyols from solidifying when the process is not in operation or to prevent a substantial reduction in the temperature of the polyols between the storage tank and the static mixer reaction zone. Polyols pass through metering pump 3 which is a positive placement pump preferably of the gear type. As shown valves 5 and 7 are provided in the line to permit the line to be drained when maintenance is required on the metering pump. A filter 6 is generally provided in the line 2 between the tank and the metering pump 3 to prevent small particles, which may be in the commercial polyols utilized, from damaging the pump or contaminating the final product. The heated polyol flows through line 4 to static mixer reaction zone 15. The isocyanate is stored in tank 8 under a dry inert gas, preferably nitrogen to maintain the isocyanate composition in as dry a state as possible. Storage tank 8 is generally a heated tank to raise the temperature of the isocyanate to a range wherein the isocyanate and the polyol when mixed provide the required initial reaction temperature. Storage tank 8 can also be a weigh tank to provide some determination of the rate at which the isocyanate is being introduced into the process . The isocyanate need not be heated in tank 8, but can be heated in an auxiliary heat exchanger in line 9 (not shown) or the tank can be heated and the additional heat exchanger provided to ensure that the proper temperature of the process is maintained. The heat exchanger can be in line 9 before the metering pump or in line 11 after the metering pump. The isocyanate flows to the metering pump 10 and is metered and pumped at a higher pressure through line 11 and valve 14 and is mixed with the polyols in line 4 immediately before being introduced in the static mixer reaction zone 15. The static mixer reaction zone 15 comprises a static mixer which is generally insulated and steam traced or heated to prevent freezing or solidification of the reaction products in the static mixer reaction zone when the process is not in operation or in case of an emergency shut down. If the polyols and the isocyanates are provided at the proper temperature, the mixture will begin to react in the static mixer reaction zone without additional heating. However, since the heat of the reaction is generally not large, a static mixer reaction zone is insulated and as noted steam traced or heated to prevent plugging of the static mixer reaction zone in case of an emergency shut down. In the static mixer reaction zone 15, the isocyanates react with the polyols and the reaction is substantially complete in the static mixer reaction zone 15. The reaction product leaves the static mixer reaction zone 15 through line 17 and passes through filter 16 which removes any solid materials which are formed during the reaction or corrosion products which may have entered the process through some of the lines. The product passes through line 18 to tank 19. Storage tank 19 is generally an insulated vessel which may be steam traced to prevent the product from solidifying in the tank. Storage tank 19 can be maintained at a lower temperature to prevent additional reaction between the unreacted isocyanate and the polyols which may remain in the product. A dry inert gas purge is introduced into the storage tank 19, through line 26. The product is removed from tank 19 through line 21 and can pass to a packaging section where the material is packaged in 55 gallon drums, 5 or 10 gallon pails, solid blocks of material and the like. The metering pumps 3 and 10 are designed to achieve as constant a mass flow of the polyol and the isocyanate as is reasonably required by the process. Generally, the flow rate should not vary by more than about ±5% and preferably not more than about ±2% on a mass flow basis. The pumps are generally provided with variable speed drives so that the flow can be easily controlled. Preferably, the drive speed is computer controlled based on the variables of pressure, temperature, viscosity and density of the material being pumped. The pumps can be of the gear type, vane type, or piston type and are preferably controlled by variable speed drives. When the pumps are of the piston type and require check valves, it may be necessary to modify the pumps to provide positive closure of the check valves when operating in liquids which can be extremely viscous. That is, the check valves should be of the positive action type and prevent leaking of the previously pumped material into the pumping cylinder when the piston is moving to introduce fresh material into the pumping cylinder. The product leaving product tank 19 through line 21 which is generally an insulated and steam traced or heated line to prevent the product from solidifying and plugging the line. The product is then passed to a packaging station where the product is prepared and packaged in a required form for shipment and sales. In an alternative embodiment, the product can be passed directly from storage tank 19 and line 21 to a final application for the reaction product. As discussed above, the process of the invention can be utilized to prepare isocyanate terminated polyurethane prepolymers which can be moisture curing and are used for various adhesive application. However, the process can also be utilized to provide hydroxyl terminated polyurethane prepolymers which are useful in two component systems to react with chain extending or cross-linking systems such as polyisocyanates, epoxy compounds and the like. In addition, amine terminated compositions can be reacted with isocyanates in the process of the invention to provide nitrogen containing prepolymers which can be amine or isocyanate terminated. The embodiment shown in Fig. 2 is similar to the embodiment of Fig. 1 with the exception that polyols stored in polyol storage tank 25 and polyol storage tank 36 are metered through metering pumps 30 and 42, respectively, and are mixed in static mixing zone 47 before mixing with the isocyanate. A first polyol is stored in polyol storage tank 25 on weigh scale 26. Tank 25 can be a heated tank if the polyol is a solid at ambient temperature. The tank also can be heated to maintain the polyol at an elevated temperature so that additional heating is not required before the material is mixed with the isocyanate. However, the polyol can be passed through a heat exchanger (not shown) in line 28 or 32 to bring the polyol to a required temperature. The lines between the storage tanks 25, 36, 48, 60 and the final product tank 80 are generally insulated and steam traced or heated by electrical means, or jacketing to prevent solidification of the materials in the pipes when the process is not operating or subject to an emergency shutdown. The polyol in tank 25, which is preferably heated, passes through valve 27 and line 28 which is heated and insulated through filter 29 and valve 24 to metering pump 30. Metering pump 30 is a metering pump which has a variable speed drive to ensure that a required flow of polyol from storage tank 25 is maintained at a required rate. The metered polyol passes through lines 32 which has mounted thereon a pressure sensing means 31 which can sense the pressure and provide a signal to an alarm or a computer control means. The polyol passes through valve 33 and check valve 34 in line 32. From the check valve 34 the polyol flows through line 35 toward the static mixing zone 47. The polyol from polyol storage tank 36 on weigh scale 37 flows through valve 38 and line 39 through filter 40 and valve 41 to metering pump 42. Polyol tank 36 can be heated and the line 39 between storage tank 36 and metering pump 42 is heated and insulated. A pressure sensing means 43 provides an indicator of the pressure in line 44 and can be used to provide input to a controller and/or an alarm signal. The metering pump comprises a variable speed drive gear pump and pumps the polyol through line 44, which is heated and insulated, through valve 45 and check valve 46 to the junction with line 35 wherein the polyol from storage tank 25 and the polyol from storage tank 36 are mixed and passed to static mixing zone 47 wherein the two polyols are mixed to form a uniform blend. The polyol from tank 36 is preferably heated in the tank, but can be heated in a heat exchanger (not shown) in line 39 or line 45. The polyol is heated so that it remains in a liquid state and, in addition, when mixed with the isocyanate forms a mixture at the entrance to the static mixer reaction zone at a temperature which is sufficient to cause the isocyanate and the polyol to react in a short residence time in the static mixer reaction zone 59. The polyol storage tanks and the isocyanate storage tank 48 are generally purged with a dry inert gas to prevent moisture pickup by the materials . Heated isocyanate passes from the storage tank 48 which is on weigh scale 49 through valve 50, line 51, filter 52, valve 53 and metering pump 54. The line 51 is heated and insulated. In an alternative embodiment, if the isocyanate is flowable at ambient temperature it may be heated by a heat exchanger (not shown) in line 51 or line 56. Metering pump 54 is a metering pump with a variable speed drive which is preferably speed controlled to ensure that the mass flow of the isocyanate is maintained to provide a product with the required properties which remain uniform during a production run . The heated isocyanate passes through line 56, valve 57, and check valve 58 and mixes with the mixture of polyols in line 71 just before entering static mixer reaction zone 59. A pressure sensory means 55 is arranged on line 56 to provide an indication or signal related to the pressure in line 56 after pump 54 and can be used as an input to a controller for the pump and/or as an alarm signal . The temperature of the polyol mixture in line 71 and the isocyanate in line 56 are maintained to provide a reacting mixture entering static mixer reaction zone 59 at a required temperature to react in the static mixer reaction zone 59 without need for introducing large amounts of heat into the reacting mixture. Only limited amounts of heat can be introduced into the reacting mixture through the static mixer reaction zone since the static mixer reaction zone is small and does not provide extensive surfaces for heat transfer. The static mixer reaction zone is generally heated and insulated to prevent loss of heat from the static mixer reaction zone. Tank 60, which can be heated, on weight scale 61 provides storage for additional materials which are to be mixed with the product from static mixer reaction zone 59. The additional materials can be flowable non- reactive additives such as catalysts, dyes, releology modifiers, stabilizers and the like which flow out of tank 60 through valve 62, line 63, filler 64, valve 65 to metering pump 66. The additional materials flow through line 68, valve 69 and check valve 70 and mix with the reaction product from static mixer reaction zone 59 in line 72 before entering static mixer 75. Pressure sensing means 67, 73 and 74 provide signals related to the pressure to monitor the operation. The signals can operate alarms or provide input to a computer to aid in providing a predetermined flow rate of the additives. The reaction product from line 72 and additives from line 68 co-mingle in line 72 and pass to static mixing zone 75. The mixture passes out of static mixing zone 75 through line 84 to filter 78 and through line 79 to heated and insulated product tank 80 on weight scale 81. Product can pass to shipping or directly to an end use (application as an adhesive, for example) through line 83 which is heated and insulated to prevent solidification of the product in the line. In an alternative embodiment, tank 60 can store a reactive material which can further react with the reaction product from static mixer reaction zone 59; in which case, static mixer 75 becomes a second static mixer reaction zone. As an example, a hydroxy terminated polyurethane can be the reaction product made in static mixer reaction zone 59 and a polyisocyanate could be introduced into the reaction product from static mixer reaction zone 59 in line 72 and the mixture reacted in static mixer 75 which then becomes a static mixer reaction zone. The process of the invention is flexible and can be ready adjusted to provide many products with different ratios of reactants without long changeover times. The operation must be set up to provide storage for the different reactants and means to introduce them into the metering pumps . The volume of the lines and reactors is relatively small and therefore a changeover from one product to another does not entail making large quantities of out of specification product. If the same reactants are used in different ratios only small amounts of out of specification product is made; however, if the amount of new product to be produced is large, the small amount of out of specification material can be mixed with the large amount of specification material without adversely affecting the overall quantity. Due to the efficient mixing and low residence time at an elevated temperature, the product of the process of the invention is of lower viscosity and lighter color and contains less particulate matter than similar products produced by the prior art batch processes which provide less efficient mixing and longer residence times at elevated temperatures. To successfully operate the process, the mass flow rates of the various materials entering the process must be closely controlled and in addition, the temperature of the reactants must also be closely controlled to ensure that the required degree of reaction occurs within the static mixer reaction zone in the process. Only by closely controlling the mass flow rates of the reactants and the temperature of the reactants is it possible to form a product with viscosity, NCO/OH number and content of additives within the required range for commercial products . The process can be carried out over a broad temperature range of from about 50 2 C to about 170 2 C, preferably from about 70 2 C to about 150 2 C, where polyols are used as the isocyanate-reactive substance. The broad temperature range for the process is due to the varying reactivities of the polyols and the isocyanates may be used in the process. As one skilled in the art would understand, the reactivities of polyols and isocyanates are dependent upon the structure of the reactants. It is well known that aliphatic isocyanates are less reactive than aromatic isocyanates. That is, a compound with an isocyanate group attached directly to an aliphatic chain is less reactive than a compound with an isocyanate group directly attached to an aromatic group. These factors are well known in the art and a person skilled in urethane chemistry would have no difficulty in determining the residence times and temperatures required for reacting various combinations of polyols and polyisocyanates. The process will be illustrated by way of the following examples. The examples are directed to preparation of isocyanate terminated polyurethane prepolymers. The materials were useful as hot melt adhesives. A system similar to that shown in Fig. 2 was utilized to form isocyanate terminated polymers which were useful as hot melt adhesives. The process was carried out by the process described in Fig. 2. Tank 25 contained a polypropylene glycol with a number average molecular weight of about 1,000. Tank 36 contained a polypropylene glycol with a number average molecular weight of about 2,000 and Tank 48 contained an MDI polyisocyanate from Huntsman Chemical Company (RUBINATE™ 90/20) . The polypropylene glycols were the product of Dow Chemical Company. The flow rate, viscosity and NCO values of the product were determined periodically during the run. Each run lasted approximately 17 minutes and produced about 5 gallons of product. The residence time in the static mixer reaction zone was calculated on the basis that the reaction zone was open and did not have any internal mixing parts . Taking into consideration the volume of internal parts of the static mixer reaction zone would substantially reduce the residence time. The static mixer reaction zone was a Koflo Corporation design mixer with an internal diameter of about 5/8 inch and a length of 36 inches. The volume of the static mixer reaction zone was about 11 cubic inches without making an allowance for the volume of the internal mixing elements in the mixer. Table 1 shows the parameters under which the process was operated. The temperature of the isocyanate was adjusted to provide a mixture with the temperature shown at the entrance to the static mixer reaction zone. The product prepared by the process met the specifications of a commercial product. As can be seen, the NCO content of the product at the beginning and the end of the experiment was close to the desired target range and the product would be acceptable as a commercial product. If desired, catalysts and other additives can be added to the reaction product in the static mixer 75. These catalysts are materials such as tin containing compounds, JEFFCAT™ amines and the like. These materials are catalysts for increasing the reaction rate of the isocyanate with moisture and can be added to provide hot melt adhesives with particularly useful curing rates. Plasticizers and other additives which can be supplied in a pumpable form can be introduced into the product in static mixer 75. As can be seen from Table 1, the reaction is complete in very short time to produce a product with the required NCO content and viscosity. In view of the rapid reaction rate and low residence time in the reactor, the process can be scaled up to large capacities and does not require expenditures for large heated reactors and the like. The key to the process is to maintain the mass flow rates of the polyols and the polyisocyanates within a predetermined rate. As can be seen, when the temperature and the mass flow rates of the reactants are closely controlled, the properties of the prepolymer do not vary widely and are within the tolerances for a commercial product. Table 1 also shows that a small change in the ratio of NCO groups/OH groups in the feed provides a substantial change in the NCO groups and the viscosity of the product. It is clear that the flow rates and temperature of the reactants must be controlled within narrow ranges to successfully operate the process. The degree of control required is dependent upon the range (broad or tight) of the specification for the product. The above examples are directed to production of NCO terminated polyurethane prepolymers. However, OH terminated polyurethane prepolymers can also be prepared by the process by utilizing a molar excess of OH groups in relations to the isocyanate groups. The process can also be utilized to react amine terminated polyesters, polyamides and polyurethanes, aliphatic amines, and aromatic amines with polyisocyanates. TABLE 1 Component NRV Fli DW Rate BS Flow Rate #/min #/min PPG 1000 0.285 0.221 PPG 2000 0.844 0.822 RUBINATE™ 90/20 1.00 1.00 Total Feed 2.129 2.043 Static Mixer 0.0139 min 0.0146 min Reactor Residence Time Product % 11.095 13.125 NCO At Start Product % 11.116 12.657 NCO at End Temp Of 77.2 2 C 74.4 2 C Mixture at Entrance To Static Mixer Reaction Zone Temp of Polyols 91.7 2 C 91.7 2 C Product Viscosity 90 Pascals 35 Pascals At 132 S C The residence time for the process can be relatively short (less than one minute) . The reactivity of the starting materials should be considered and the temperature and residence time adjusted accordingly. However, the process is operated under conditions in which the reaction product can be obtained without subjecting the composition to high temperatures for extended periods .

Claims: What is claimed is: 1. A process for producing a polyurethane prepolymer which comprises: (a) providing a mixture comprising at least one isocyanate-reactive substance selected from the group consisting of polyols and polyamines and at least one polyisocyanate at a first end of a static mixer reaction zone, wherein the isocyanate- reactive substance and the polyisocyanate are provided in a proportion, by weight, which does not change by more than ±5% from a fixed setpoint during operation of the process; (b) continuously passing and reacting the mixture of the isocyanate-reactive substance and the polyisocyanate through said static mixer reaction zone at a temperature and at a residence time effective to attain a pre-selected degree of reaction between the isocyanate-reactive substance and the polyisocyanate, said residence time being about 10 minutes or less, to form the polyurethane prepolymer: (c) continuously withdrawing the polyurethane prepolymer from a second end of the static mixer reaction zone; and (d) cooling the polyurethane prepolymer. 2. The process of claim 1 wherein step (b) is performed in the absence of a polymerization catalyst. 3. The process of claim 1 wherein at least one of the isocyanate-reactive substances is selected from the group consisting of polyether polyols, polyester polyols and polyurethane polyols. 4. The process of claim 1 wherein the isocyanate- reactive substances have a number average molecular weight in the range of about 500 to about 10,000. 5. The process of claim 1 wherein the polyisocyanate comprises at least one member selected from the group consisting of MDI, modified-MDI , and TDI . 6. The process of claim 1 wherein the polyurethane prepolymer comprises a moisture curing adhesive having an NCO content of from about 1 weight % to about 15 weight % . 7. The process of claim 1 wherein the temperature in step (b) is from about 50°C to about 170°C. 8. The process of claim 1 wherein the isocyanate- reactive substances and polyisocyanates are at a temperature that when combined form a mixture with a temperature of above about 50 2 C. 9. The process of claim 1 wherein the ratio of OH groups to NCO groups in step (a) is effective to provide a NCO: OH molar ratio in the range of from about 1.8:1 to 20:1. 10. The process of claim 1 wherein the isocyanate- reactive substances and isocyanates are provided in the proportion by weight by means of a positive displacement proportionating pump. 11. The process of claim 6 wherein the moisture-curing polyurethane adhesive has a number average molecular weight in the range of from about 3,000 to about 50,000. 12. The process of claim 1 wherein said process is performed in the absence of water or an organic solvent. 13. The process of claim 6 wherein the moisture-curing polyurethane adhesive is a hot melt adhesive. 14. The process of claim 1 wherein the polyurethane prepolymer comprises a moisture-curing polyurethane adhesive which is a liquid at ambient temperature. 15. The process of claim 1 wherein said residence time does not vary by more than ±5% from a selected residence time during operation of the process. 16. The process of claim 1 wherein at least one of said isocyanate-reactive substances comprises a polyether polyol having a functionality of about 2 to about 3 and comprised of a reaction product of a cyclic oxide selected from the group consisting of ethylene oxide, propylene oxide and combinations thereof with a composition selected from the group consisting of water, diols, polyols and mixtures thereof. 17. The process of claim 1 wherein an essentially constant temperature profile is maintained within the static mixer reaction zone. 18. The process of claim 1 wherein the isocyanate- reactive substance and the polyisocyanate are mixed during step (b) under low shear conditions in the static mixer reaction zone. 19. A method of bonding a first substrate to a second substrate, said method comprising the steps of a) placing a moisture-curing polyurethane adhesive produced by the process of claim 1 on at least one of a first substrate and second substrate; and b) placing the first substrate and second substrate into contact with the adhesive therebetween; and c) reacting said moisture-curing polyurethane adhesive with water. 20. A moisture-curing polyurethane adhesive produced by the process of claim 1. 21. The process of claim 1 wherein the polyurethane prepolymer comprises an OH terminated polyurethane prepolymer . 22. The process of claim 1 wherein the residence time of the mixture in the static mixer reaction zone is less than about 2 minutes. 23. The process of claim 1 wherein the residence time is about 1 minute or less. 24. The process of claim 1 wherein the prepolymer comprises a member selected from the group consisting of OH terminated prepolymers and isocyanate terminated prepolymers .

Inventors: GRALA PAUL F, JOENSSEN DIETER, SATHE AVINASH T, GRALA, PAUL, F., JOENSSEN, DIETER, SATHE, AVINASH, T.
Applicants: HENKEL CORP, HENKEL CORPORATION
Application year: 2000
Main classification: C08G18
IPC codes: C08G18