Abstract:
The invention is a method for making reaction injection molded polyurethane of improved paintability properties. The product comprises the reaction product of a high molecular weight polyhydric polyether, a low molecular weight active hydrogen containing compound of at least two functionality, a polyisocyanate and a catalyst combination comprising N,N-Dimethylethanolamine, dibutyltin dilaurate and an alkyltin mercaptide. This catalyst combination results in improved processing, a minimum of surface defects, excellent green strength and improved paintability using certain paints. Reaction injection molded elastomers are useful as molded articles of commerce including, but not limited to, vehicle body parts.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns the field of reaction injection molded polyurethanes. 
     2. Description of the Prior Art 
     Reaction Injection Molding (RIM) is a technique for the rapid mixing and molding of large, fast curing urethane parts. RIM polyurethane parts are used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and placing the mixture into a mold where reaction proceeds. These active hydrogen containing materials comprise a high molecular weight polyhydric polyether and a low molecular weight active hydrogen containing compound. After reaction and demolding, the parts may be subjected to an additional curing step which comprises placing the parts in an ambient temperature of about 250° F. or greater. 
     In a previously filed patent application, Ser. No. 136,199 filed Apr. 1, 1980 now U.S. Pat. No. 4,273,885, a catalyst system for RIM polyurethane elastomers was described which was comprised of dimorpholinodiethylether, dibutyltin dilaurate and an alkyltin mercaptide. The catalyst combination in that patent application imparts superior processing characteristics to RIM polyurethane elastomer systems. However, we have since discovered that the use of dimorpholinodiethylether, while advantageous in many RIM systems, interferes in the cure of certain important paint systems known as high solids enamel paints. We have discovered that by substituting a reactive amine catalyst, which is tied up in the polymer network by reaction, for dimorpholinodiethylether that the processing benefits already described in the above mentioned patent application are retained and the RIM part can be painted using the high solids enamel paint systems. The reactive amine catalyst used in our present invention is dimethylethanolamine. 
     SUMMARY OF THE INVENTION 
     The invention is a method for making reaction injection molded polyurethane of improved processing characteristics and properties. The product comprises the reaction product of a high molecular weight polyhydric polyether (polyol), a low molecular weight active hydrogen containing compound of at least 2 functionality, a polyisocyanate and a catalyst combination comprising dimethylethanolamine (also referred to as N,N-Dimethylethanolamine), dibutyltin dilaurate and alkyltin mercaptide. The invention is also the resulting RIM polyurethane composition. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The polyols useful in the RIM elastomers of this invention include polyether polyols, polyester diols, triols, tetrols, etc., having an equivalent weight of at least 500, and preferably at least 1000 up to about 3000. Those polyether polyols based on trihydric initiators of about 4000 molecular weight and above are especially preferred. The polyethers may be prepared from lower alkylene oxides such as ethylene oxide, propylene oxide, butylene oxide or mixtures of propylene oxide, butylene oxide and/or ethylene oxide. In order to achieve the rapid reaction rates which are normally required for molding RIM polyurethane elastomers, it is preferable that the polyol be capped with enough ethylene oxide to increase the reaction rate of the polyurethane mixture. Normally at least 50% primary hydroxyl is preferred, although amounts of primary hydroxyl less than this are acceptable if the reaction rate is rapid enough to be useful in industrial application. Other high molecular weight polyols which may be useful in this invention are polyesters or hydroxyl terminated rubbers (such as hydroxyl terminated polybutadiene). Hydroxyl terminated quasi-prepolymers of polyols and isocyanates are also useful in this invention. 
     The chain-extenders useful in this invention are preferably difunctional. Mixtures of difunctional and trifunctional chain-extenders are also useful in this invention. The chain-extenders useful in this invention include diols, amino alcohols, diamines or mixtures thereof. Low molecular weight linear diols such as 1,4-butanediol and ethylene glycol have been found suitable for use in this invention. Ethylene glycol is especially preferred. Other chain-extenders including cyclic diols such as 1,4-cyclohexane diol and ring containing diols such as bishydroxyethylhydroquinone, amide or ester containing diols or amino alcohols, aromatic diamines and aliphatic amines would also be suitable as chain-extenders in the practice of this invention. 
     A wide variety of aromatic polyisocyanates may be used here. Typical aromatic polyisocyanates include p-phenylene diisocyanate, polymethylene, polyphenylisocyanate, 2,6-toluene diisocyanate, dianisidine diisocyanate, bitolylene diisocyanate, naphthalene-1,4-diisocyanate, bis(4-isocyanatophenyl)methane, bis(3-methyl-3-isocyantophenyl)methane, bis(3-methyl-4-isocyanatophenyl)methane, and 4,4&#39;-diphenylpropane diisocyanate. 
     Other aromatic polyisocyanates used in the practice of the invention are methylene-bridged polyphenyl polyisocyanate mixtures which have a functionality of from about 2 to about 4. These latter isocyanate compounds are generally produced by the phosgenation of corresponding methylene bridged polyphenyl polyamines, which are conventionally produced by the reaction of formaldehyde and primary aromatic amines, such as aniline, in the presence of hydrochloric acid and/or other acidic catalysts. Known processes for preparing polyamines and corresponding methylene-bridged polyphenyl polyisocyanates therefrom are described in the literature and in many patents, for example, U.S. Pat. Nos. 2,683,730; 2,950,263; 3,012,008; 3,344,162 and 3,362,979. 
     Usually methylene-bridged polyphenyl polyisocyanate mixtures contain about 20 to about 100 weight percent methylene diphenyldiisocyanate isomers, with the remainder being polymethylene polyphenyl diisocyanates having higher functionalities and higher molecular weights. Typical of these are polyphenyl polyisocyanate mixtures containing about 20 to 100 weight percent methylene diphenyldiisocyanate isomers, of which 20 to about 95 weight percent thereof is the 4,4&#39;-isomer with the remainder being polymethylene polyphenyl polyisocyanates of higher molecular weight and functionality that have an average functionality of from about 2.1 to about 3.5. These isocyanate mixtures are known, commercially available materials and can be prepared by the process described in U.S. Pat. No. 3,362,979, issued Jan. 9, 1968 to Floyd E. Bentley. 
     By far the most preferred aromatic polyisocyanate is methylene bis(4-phenylisocyanate) or MDI. Pure MDI, quasi-prepolymers of MDI, modified pure MDI, etc. Materials of this type may be used to prepare suitable RIM elastomers. Since pure MDI is a solid and, thus, often inconvenient to use, liquid products based on MDI are often used and are included in the scope of the terms MDI or methylene bis(4-phenylisocyante) used herein. U.S. Pat. No. 3,394,164 is an example of a liquid MDI product. More generally uretonimine modified pure MDI is included also. This product is made by heating pure distilled MDI in the presence of a catalyst. The liquid product is a mixture of pure MDI and modified MDI: ##STR1## Examples of commercial materials of this type are Upjohn&#39;s ISONATE® 125M (pure MDI) and ISONATE®143L (&#34;liquid&#34; MDI). Preferably the amount of isocyanates used is the stoichiometric amount based on all the ingredients in the formulation or greater than the stoichiometric amount. 
     It has been found that an improvement in processing characteristics and paintability using high solids paints of reaction injection molded (RIM) polyurethanes using a combination of ingredients chosen from those enumerated above may be had by employment of a particular catalyst combination. This combination comprises generally dimethylethanolamine, a fast gelation organo tin catalyst and a delayed action gelation organo tin catalyst. A delayed action catalyst begins catalytic activity after a period of time has passed after mixing with the reactants. This particular combination of catalyst types results in valuable processing improvements including excellent flow properties in the mold, a minimum of surface defects due to shrinkage and excellent green strength. In order to achieve these results and obtain a RIM part with excellent paintability characteristics when using high solids paints a particular catalyst combination has been found to be useful as follows. It is preferred that dimethylethanolamine be used as the weak tertiary amine catalyst. The fast gelation tin catalyst is preferred to be dibutyltin dilaurate. The delayed action gelation tin catalyst is preferred to be an alkyltin mercaptide. This alkyltin mercaptide may preferably be a commercial product known as FOMREZ® UL-29. Although each individual catalyst in this combination is well known to be a catalyst in the urethane art, the particular combination here provides the advantages of processing, green strength, and absence of surface flaws which is absolutely necessary to the successful commercialization of RIM parts as well as excellent paintability using high solids paints. 
     The RIM formulation includes a great number of other recognized ingredients such as additional crosslinkers, catalysts, extenders, blowing agents and the like. Blowing agents may include halogenated low-boiling hydrocarbons, such as trichloromonofluoromethane and methylene chloride, carbon dioxide, nitrogen, etc., used. 
     Other conventional formulation ingredients may also be employed, such as, for example, foam stabilizers, also known as silicone oils or emulsifiers. The foam stabilizer may be an organic silane or siloxane. For example, compounds may be used having the formula: 
     
         RSi[O-(R.sub.2 SiO).sub.n --(oxyalkylene).sub.m R].sub.3 
    
     wherein R is an alkyl group containing from 1 to 4 carbon atoms; n is an integer of from 4 to 8; m is an integer of from 20 to 40; and the oxyalkylene groups are derived from propylene oxide and ethylene oxide. See, for example, U.S. Pat. No. 3,194,773. 
     Although not essential for the practice of this invention, the use of commonly known additives which enhance the color or properties of the polyurethane elastomer may be used as desired. For example, chopped or milled glass fibers, chopped or milled carbon fibers and/or other mineral fibers are useful. 
     In a preferred embodiment of this invention, a high molecular weight polyether polyurethane polyol of about 5000 molecular weight or above is combined with a stoichiometric excess of 4,4&#39;-diphenylmethane diisocyanate (MDI) and allowed to react in the presence of a catalyst combination of dimethylethanolamine, dibutyltin dilaurate and an alkyltin mercaptide in a standard RIM machine using known processing techniques. In an especially preferred embodiment of this invention the molded RIM part from just above is post cured at a temperature of about 325° F. for about one half of an hour. The invention may be exemplified by the following examples which are not intented to limit the scope of the invention. 
    
    
     A glossary of terms and materials used in the following examples follows the examples. 
     EXAMPLE I 
     THANOL® SF-5505 (16 pbw), ethylene glycol (6.44 pbw), Fluorocarbon R11B (1.124 pbw), THANCAT® DME (0.2 pbw), FOMREZ® UL-29 (0.025 pbw), dibutyltin dilaurate (0.015 pbw) and milled glass fibers (Owens-Corning FIBERGLAS® P117B 1/16&#34;) (16.47 pbw) was premixed and charged into the B-component working tank of a Cincinnati-Milacron LRM-II high pressure impingement mixing RIM machine adapted to handle milled glass fibers. ISONATE® 191 (32.91 pbw) was charged into the A-component working tank. The A-component temperature was adjusted to 80° F. and the B-component temperature to 110° F. The A-component impingement mixing pressure was adjusted to ˜2400 psi as was the B-component impingement mixing pressure. The machine was calibrated to deliver an A/B weight ratio of 0.817. Plaques were made by causing material impingement mixed at the above conditions to flow into a steel flat plaque mold preheated to 180° F. and measuring 24&#34;×48&#34;×1/8&#34;. After postcuring, the plaques were conditioned at ASTM test conditions for at least one week prior to testing. The material had a room temperature flexural modulus of ˜225,000 psi. The material had excellent green strength with a good overall appearance. There was little evidence of shrink marks on the surface of the plaques. We, therefore, concluded that THANCAT DME was a good amine catalyst for imparting good processing to RIM polyurethane elastomers as well as imparting good &#34;green strength&#34; and surface appearance to the parts. 
     EXAMPLE II 
     THANOL® SF-5505 (16 pbw), ethylene glycol (6.44 pbw), PLURONIC® F-98 surfactant (0.22 pbw), THANCAT® DME (0.1 pbw), FOMREZ® UL-29 (0.025 pbw) and dibutyltin dilaurate (0.015 pbw) were premixed and charged into the B-component working tank of an Accuratio VR-100 high pressure RIM machine. ISONATE® 143L (29.83 pbw) and THANATE® L55-0 (5.78 pbw) were premixed and charged into the A-component working tank. The A-component temperature was adjusted to 80° F. and the B-component temperature was adjusted to 110° F. The impingement pressure of the A-component was adjusted to 2300 psi and the impingement pressure of the B-component was adjusted to 2190 psi. The machine was calibrated to deliver an A/B weight ratio of 1.562. The materials were then impingement mixed at the pressure given above and caused to flow into a steel mold preheated to 170° F. and measuring 18&#34;×18&#34;×1/8&#34;, through a gating system built into the mold. After postcuring 30 minutes at 325° F., the plaque was equilibrated at ASTM test conditions for one week prior to testing. The plaque had a room temperature flexural modulus of ˜90,000 psi. 
     Immediately after release from the mold, the plaque showed little evidence of crazing upon bending, indicating excellent &#34;green strength&#34;. The plaque had an excellent overall appearance showing very little evidence of surface flaws caused by shrinkage. Also, the plaques were tested for paintability by high solids enamel paints systems and passed these tests.* As already indicated, when THANCAT® DMDEE is used in place of THANCAT DME, similar excellent properties result except that plaques containing THANCAT DMDEE do not pass the paintability tests when high solids enamel paint systems are used. Thus THANCAT DME is superior in this latter test. 
    
     PAINT TESTING FOR RIM POLYURETHANE ELASTOMERS 
     A 4&#34;×12&#34;×1/8&#34; sample of RIM polyurethane is first washed thoroughly to eliminate all the mold release on the surface. Even small amounts of mold release will interfere with the adhesion of the paint film to the substrate. After washing and drying, the samples are then painted. The paint systems most important in reinforced RIM are presently the so-called &#34;high solids&#34; enamel paints. When RIM elastomers containing THANCAT DMDEE, an unreactive tertiary amine catalyst, are painted with PPG&#39;s 430 high solids enamel paint system, interference with paint cure and adhesion failure are observed. When N,N-Dimethylethanolamine is substituted for THANCAT DMDEE in the RIM formulation, the above mentioned problems do not occur. 
     GLOSSARY OF TERMS AND MATERIALS 
     RIM--Reaction Injection Molding. 
     Polyol--A di- or greater functionality high molecular weight alcohol terminated molecule composed of ether groups such as ethylene, propylene, butylene, etc., oxides. 
     MDI--4,4&#39;-diphenylmethane diisocyanate. 
     ISONATE 143L--Pure MDI isocyanate modified so that it is a liquid at temperatures where MDI crystallizes--product of the Upjohn Co. 
     ISONATE 191--An MDI enriched polymeric isocyanate. A product of The Upjohn Co. 
     PAPI 901--A crude form of MDI containing about 30% higher functionality isocyanates and other impurities--product of the Upjohn Co. 
     THANOL SF-5505--a 5500 molecular weight polyether triol containing approximately 80% primary hydroxyl groups. 
     THANCAT DMDEE--Dimorpholinodiethylether. 
     FOMREZ UL-29--A stannic diester of a thiol acid (an alkyltin mercaptide). The exact composition is unknown. Product of Witco Chemical Co. 
     Fluorocarbon R-11B--A chlorinated fluorocarbon. 
     THANCAT DME--Dimethylethanolamine. 
     THANATE L55-O--A quasi prepolymer consisting of 50 parts by weight of ISONATE 143L and 50 parts by weight of THANOL SF-5505.