Abstract:
The invention consists of a composition containing a chemically blocked trimerization catalyst and a polyurethane prepolymer resin or resins with reactive isocyanate groups. In the absence of heat, the catalyst is not active and the solution of catalyst in the resin undergoes no appreciable reaction for periods ranging from hours to weeks depending upon the composition and concentration of the catalysts and the structure and concentration of the isocyanate groups. When heat is applied to the mixture a trimerization reaction occurs which produces a crosslinked polyurethane network with utility as an elastomer or as an adhesive.

Description:
TECHNICAL FIELD 
     This invention relates to one component aromatic polyisocyanurate resins and to a process for forming such resins by the trimerization of an isocyanate terminated prepolymer formed by the reaction of an aromatic diisocyanate and a polyol. 
     BACKGROUND OF THE INVENTION 
     The preparation of polyurethane and polyurethane/urea elastomers and adhesives traditionally requires the combination of isocyanates or isocyanate containing resins with crosslinking agents, e.g., polyamines or polyols, in precise ratios. That is because the reactivity of the combined isocyanates and isocyanate terminated prepolymers and the properties of the final polyurethane and polyurethane/urea vulcanizates are dependent upon the curative-resin ratio or stoichiometry. It is further recognized in the preparation of the polyurethane/urea elastomers that many of the presently used aromatic amine curatives have potential health and safety problems are crystalline solids which must be melted at high temperature and used in a molten state. A final problem in preparing polyurethane/urea elastomers is that the limited working time (&#34;pot-life&#34;) available complicates the production of multiple castings and large parts that require large amounts of polyurethane/urea. 
     A number of approaches have been taken to overcome such problems as those mentioned above. These rely upon (a) the inhibition of the isocyanate, (b) the retardation of the curative, and (c) the blocking of catalysts which promote crosslinker-isocyanate reactions. Examples of the first approach (a), include the use of reaction products of isocyanates with oximes, phenols, or caprolactams. These products are stable at room temperature but revert to the starting components at higher temperatures. Examples of the second approach (b) include deactivated complexes of aromatic amines which form stable admixtures with isocyanate containing materials at room temperature. One commercially available material sold under the trademark, Caytur® 21, is the 2.5:1 complex of methylenedianiline (MDA) with sodium chloride suspended in a carrier fluid. Heat destroys the weak complex and the liberated amine crosslinks the isocyanate containing material. An example of the third approach is a blend of a curative and an isocyanate which undergo a slow crosslinking reaction in the absence of a catalyst. A delayed catalyst, such as a thioalkanoic-tin complex, can be added. The three component blend has an extended working life at room temperature but rapidly cures upon heating. 
     The preparation of polyisocyanurate containing networks formed by the trimerization and chain extension of isocyanate terminated prepolymer in the presence of polyols is known. In this method the isocyanate group are caused to react to form the polyisocyanurate and this reaction can be effected through catalyst systems such as quaternary ammonium salts. The polyisocyanurate containing vulcanizates or networks are adapted for use in producing molded parts, laminating and impregnating applications as well as adhesives, coatings and so forth. Representative patents which disclose the trimerization of polyisocyanate resins are as follows: 
     U.S. Pat. No. 4,880,845 discloses the catalytic trimerization of aromatic isocyanate terminated prepolymers in the presence of short chain and long chain diols, e.g., butane diol and polyether polyols to rapidly produce polyisocyanurate and polyurethane containing products. Trimerization catalysts include the organic acid salts of 1,8-diaza-bicyclo(5,4,0)undec-7-ene. Ortho-carboxylic acid esters used as a catalyst component include ortho-carbonic acid tetraethylester, ortho-formic acid triethylester and the like. Controlled induction periods of a few seconds are noted. 
     U.S. Pat. No. 4,126,742 discloses a process for producing polyisocyanurate elastomers which comprises polymerizing aromatic polyisocyanates, e.g., methylene diisocyanate, with a small amount of polyol in the presence of a trimerization catalyst. Examples of trimerizing catalysts include tertiary amines such as N,N-dialkyl piperazine; 1,4-diazabicyclo[2.2.2.]octane and many more. 
     U.S. Pat. No. 4,033,908 discloses the preparation of polyisocyanurate foams by reacting a polyisocyanate in the presence of a trimerization catalyst and a blowing agent. Aromatic polyisocyanates such as polymethylenepolyphenylpolyisocyanate are trimerized in the presence of organic cyclic carbonates and liquid alkylene carbonates, tertiary amine metal salts as trimer catalysts. 
     U.S. Pat. No. 4,855,383 discloses a storage-stable, liquid composition comprising an isocyanate functional compound, an epoxy component, an alkylating agent and a tertiary amine catalyst precursor. The tertiary amine precursor forms a quaternary ammonium salt catalyst in situ for achieving crosslinking of the isocyanate via trimerization. In practice an isocyanate-functional prepolymer, either derived from aromatic or aliphatic polyisocyanate, is mixed with the epoxy component and alkylating agent. In the presence of a quaternary ammonium catalyst the isocyanate is trimerized to form a polyisocyanurate resin. 
     U.S. Pat. No. 5,102,918 discloses a modified organic polyisocyanate having an isocyanurate ring. The modified polyisocyanate is prepared from an aromatic polyisocyanate, such as toluenediisocyanate or diphenylmethanediisocyanate or isocyanate terminated prepolymers where the polyol is the polyether or polyester polyol. Catalysts used for effecting isocyanuration of the isocyanates include amines such as 2,4-bis(dimethylaminomethyl)pheny, N,N&#39;,N&#34;,-tris(dimethylaminopropyl)hexahydrotriazine and diazabicycloundecene. 
     U.S. Pat. No. 4,602,049 discloses the use of amidinium salts as a catalyst for converting isocyanates to products having isocyanurate and carbodiimide linkages. In the examples, polyol was added with the amidinium catalyst for effecting isocyanurate and carbodiimide linkages in the resulting product. 
     SUMMARY OF THE INVENTION 
     The invention pertains to a one component curable polyisocyanurate forming resin consisting essentially of a chemically blocked trimerization catalyst and a polyisocyanate terminated polyurethane prepolymer having reactive isocyanate groups which is stable for extended periods at room temperature. The trimerization catalysts are amidine salts and the urethane prepolymers are the reaction product of an aromatic diisocyanate and a long chain polyol, e.g., a polyether or polyester polyol. In the substantial absence of components having active Zerewitinoff hydrogen atoms and in the absence of heat, the amidine salt is not active and a solution of the catalyst and resin generally undergoes no appreciable reaction for extended periods, e.g., weeks, depending upon the salt composition, the concentration of the catalyst and the structure and concentration of the isocyanate groups. When heat is applied to the mixture, a trimerization reaction occurs which produces a crosslinked polyisocyanurate vulcanizate or polymer having utility as an elastomer and as an adhesive. 
     There are several advantages associated with the trimerization catalyst and its processing characteristics in producing polyisocyanurate polymer. They include: 
     the ability to formulate a one component polyisocyanurate forming elastomer and/or adhesive from a polyurethane prepolymer formed by the reaction of an aromatic polyisocyanate and long chain polyether or polyester diol; 
     an ability to produce shelf stable, one component polyisocyanurate forming elastomers and adhesives; 
     an ability to produce elastomers and adhesives having excellent physical properties, particularly with respect to physical properties obtained with conventional one component systems; and, 
     an ability to produce elastomeric and adhesive products without the necessity of metal catalysts. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The polyisocyanate terminated urethane prepolymers suited for producing the polyisocyanurate elastomers and adhesives are those based upon aromatic polyisocyanates and long chain polyols. These polyisocyanates have from 2 to 3 isocyanate groups per molecule and examples include toluenediisocyanate (TDI), diphenylmethanediisocyanate (MDI), meta and para-phenylenediisocyanate (mPDI and pPDI), tolidinediisocyanate, and C 1-4  alkyl-substituted derivatives of these isocyanates. Aliphatic polyisocyanates have been ineffective candidates. 
     The long chain diols used for forming the isocyanate terminated urethane prepolymer include conventional polyether and polyester polyols. Typically, these polyols used in forming the adducts are aliphatic glycols and triols. Examples of glycols suited for producing the long chain diols are ethylene glycol, propylene glycol, butylene glycol, pentaerythritol, glycerol, and so forth. In preparing the long chain diols, ethylene oxide propylene oxide or tetramethylene oxide is reacted with these polyols to form the corresponding ethylene and propylene oxide adducts in an amount such that the resultant molecular weight of the long chain polyols will range from about 250 to 2,900. Molecular weights preferably range from about 800 to 2,000. 
     The long chain polyester polyols can be formed by reacting polybasic carboxylic acids with a variety of ethylene, propylene and butylene oxide adducts of alkylene glycols, triols and higher polyols. Typically the polyester polyols are reaction products of multifunctional carboxylic acids with glycols or the polyether polyols. Examples of carboxylic acids suited for producing the polyester polyols include oxalic acid, succinic acid maleic acid, adipic acid, phthalic acid, and the like. Molecular weights typically range from 250 to 2,900. 
     The polyisocyanate terminated urethane prepolymers are formed in a conventional manner whereby the aromatic polyisocyanate is reacted with the long chain polyol in proportions conventional to the art to provide residual isocyanate functionality. Typically the residual isocyanate functionality of the polyisocyanate terminated urethane prepolymer will range from about 1 to 15% or generally 3-9% by weight of the prepolymer. 
     The trimerization catalyst used in preparing the polyisocyanurate are monocyclic, acyclic and bicyclic amidine and guanidine salts. The amidines are represented by the formula: ##STR1## Guanidines are represented by the formula: ##STR2## wherein in the above formulas R 1  is hydrogen, straight or branched, saturated or unsaturated hydrocarbon chains having up to 30 carbon atoms which may be substituted by groups unreactive with the isocyanate functionality of the polyisocyanate terminated prepolymer or combined to form a heterocyclic ring and R 2 , R 3 , R 4 , and R 5  are straight or branched, saturated or unsaturated hydrocarbon chains having up to 30 carbon atoms which may be substituted by groups unreactive with the isocyanate functionality of the polyisocyanate terminated prepolymer or combined to form a heterocyclic ring. 
     Preferred bicyclic amidines and guanidines are represented by the formulae: ##STR3## where n is 2 to 5. 
     Examples of monocyclic amidines and guanidines include 1,2-dimethyl-2-imidazoline; 1-methyl-2-phenyl-2-imidazoline; such as piperazines, such as 1-methyl-4-(2-tetrahydroazepinyl)piperazine; and 4-(2-tetrahydroazepinyl)-morpholine. Examples of guanidines include tetramethylguanidine, pentamethylguanidine, and cyclic guanidines. Examples of bicyclic amidines include 1,8-diaza-bicyclo(5,4.0)undecene-7 (DBU); 1,5-diaza-bicyclo(4,2,0)nonene-5 (DBN); 1,8-diaza-bicyclo(5,3,0)decene-7; 1,5-diaza-bicyclo(4,4,0)decene-5; 1,4-diaza-bicyclo(3,3,0)octene-4; diaza-bicycloheptanes, diaza-bicycloheptenes and 1,3,4,6,7,8-hexahydro-1-methyl-2H pyrimido [1,2-a] pyrimidine. The non-amidine amines such as triethylenediamine, tris(dimethylaminomethyl) phenol and bis(dimethylaminomethyl)phenol and the like salts, although alleged as being trimerization catalyst, are either insufficiently latent at room temperature or inactive at reaction temperature to produce the polyisocyanurate elastomers and adhesives contemplated herein. Additionally, they may require metallic promoters which detract from the thermal stability of the polymer. 
     The trimerization catalysts are Lewis and Bronsted salts of the above amidines and guanidines. The salts are formed by reacting the amidine or guanidine with a Lewis or Bronsted acid having a pKa of less than about 2. Acids having a pKa less than zero, e.g., sulfuric acid, stabilize the amidine salt to the extent that the prepolymer becomes substantially unreactive, even at elevated temperatures. Apparently, these acids do not dissociate sufficiently from the amidine or guanidine. Lewis and Bronsted acidic materials suited for forming the amidine salts include phenol, C 1-6  alkyl phenols, C 1-12  carboxylic acids and their substituted derivatives e.g., formic acid, acetic acid, propionic acid, butyric acid and dicarboxylic acids such as isophthalic acid and maleic acid; boric acid and C 1-6  trialkyl borates. 
     One of the keys to the development of the shelf stable, one component polyisocyanurate forming system was the substantial elimination of any Zerewitinoff hydrogen containing component, e.g., a hydroxyl providing component or other curative reactive with the polyisocyanate terminated urethane prepolymer. The pot life of the one component system is diminished substantially when a hydroxyl-providing component is present in combination with the amidine or guanidine catalyst system. 
     Catalyst loadings of 0.05 to 5 weight parts per hundred weight parts (phr) of polyurethane prepolymer are effective with the preferred level being 0.1 to 1.5. The catalysts generally are not effective when used at loadings of less than 0.05 phr. 
     The following examples are provided to illustrate various embodiment of the invention and are not intended to restrict the scope thereof. 
     EXAMPLE 1 
     Effect of Catalyst Loading 
     A series of one pot polyisocyanurate forming vulcanizates were prepared from polyisocyanate terminated polyurethanes having varying isocyanate content. The method of preparation consisted of combining the polyisocyanate terminated urethane prepolymer resin with an appropriate amount of catalyst at room temperature. In the case of extremely viscous or high melting materials, the temperatures of the components were kept as low as practical. 
     Specifically, the polyisocyanate terminated urethane prepolymer and trimerization catalyst, as specified, were thoroughly mixed, the mixture degassed, poured into a glass mold preheated to the cure temperature (100° C.), and cured for 4-16 hours. Samples were post-cured 2 weeks at 70° F./50% RH prior to testing according to ASTM procedures. Stress-strain data were collected on an Instron 1120 Universal Test Machine. Table 1 sets forth catalyst type, amount and test results. 
     
                       TABLE 1______________________________________Effect of Catalyst Loading on Airthane ®PET-75D (9.10% NCO) Trimer Properties______________________________________Dabco ® SA-102* phr         1.0       0.5        0.1Tensile Strength (psi)         5750      2490       2000Youngs Modulus (psi)         28600     20300      22400Elongation (%)         132       89         54Hardness (A/D) (Shore)         88/50     92/53      98/61Rebound (%)   37        42         50Tear Die C (pli)**         189       251        243______________________________________ *Dabco ® SA102 catalyst is the 2ethylhexanoic salt of diazabicycloundecene (DBU). Airthane ® PET75D prepolymer is a PTMEG/TDI prepolymer. **pli refers to pounds per linear inch. 
    
     Unheated samples stored at room temperature remained liquid for days or weeks. The results in Table 1 show that the polyurethane prepolymer at catalyst levels ranging from 0.1 to 1 weight parts per hundred weight parts (phr) prepolymer can be converted to polyisocyanurate elastomers having good physical properties. These elastomers are harder than typical butanediol cured polyurethanes and have good elasticity and elongation compared to amine-cured polyurethanes. 
     EXAMPLE 2 
     Effect of Catalyst Type and Residual Diisocyanate Content 
     The procedure of Example 1 was followed with catalyst loadings of 0.10 phr. Tables 2-4 set forth conditions and results. 
     
                       TABLE 2______________________________________Effect of Catalyst Type on AirthanePET-90A (3.60% NCO) Trimer Properties______________________________________Dabco Catalyst         SA-1* (1)                  SA-102 (2)                            SA-610/50* (3)Tensile Strength (psi)         287      289       257Youngs Modulus (psi)         863      1056      747Elongation (%)         53       48        40Hardness (A/D) (shore)         66/21    67/23     66/22Rebound (%)   67       66        68Tear Die C (pli)         30       30        13______________________________________ 
    
     
                       TABLE 3______________________________________Effect of Catalyst Type on AirthanePET-95A (6.20% NCO) Trimer Properties______________________________________Dabco Catalyst         SA-1 (1) SA-102 (2)                            SA-610/50 (3)Tensile Strength (psi)         602      573       543Youngs Modulus (psi)         1406     1355      1516Elongation (%)         72       70        54Hardness (A/D) (shore)         76/25    76/25     76/25Rebound (%)   34       34        35Tear Die C (pli)         41       34        29______________________________________ 
    
     
                       TABLE 4______________________________________Effect of Catalyst Type on AirthanePET-70D (8.25% NCO) Trimer Properties______________________________________Catalyst      SA-1 (1)  SA-102 (2)                             SA-610/50 (3)Tensile Strength (psi)         1807      1704      1722Youngs Modulus (psi)         10590     8860      10870Elongation (%)         68        68        59Hardness (A/D) (shore)         87/40     89/44     94/46Rebound (%)   38        39        43Tear Die C (pli)         108       107       103______________________________________ 
    
     The results show the effect of increased levels of isocyanate in the polyurethane prepolymer. As one might expect, physical properties improve in terms of tensile strength, Young&#39;s modulus, hardness and tear strength with increasing levels of isocyanate in the polyurethane prepolymer. This is probably due to the fact that with increasing levels of isocyanate in the resin there is increased isocyanurate formation in the final vulcanizate. 
     EXAMPLE 3 
     Polyisocyanurate from TDI-Polyesterpolyol Prepolymers 
     The procedure of Example 1 was repeated using TDI-Polyester prepolymers. Table 5 sets forth the conditions and results. 
     
                                           TABLE 5__________________________________________________________________________TDI-Polyester Trimer Properties (SA-102 Catalyst at 0.25 phr)       Cyanaprene ®               Cyanaprene                      Cyanaprene                             CyanaprenePrepolymer  A9      D5QM   D6     D7__________________________________________________________________________% NCO       4.34    5.03   5.74   6.64Tensile Strength (psi)       2460    2100   4290   5890Youngs Modulus (psi)       3100    2515   7840   9570Elongation (%)       340     247    259    237100% Modulus (psi)       320     500    630    930200% Modulus (psi)       560     1100   1570   3045300% Modulus (psi)       1265    --     --     --Hardness (A/D) (shore)       62/21   73/30  79/36  93/55Rebound (%) 11      12     20     42Tear Die C (psi)       146     141    180    296Compression Set (%)       2       3      3      4__________________________________________________________________________ 
    
     Cyanaprene prepolymers are reaction products of toluenediisocyanate and polyester polyols. 
     The results show that when the ester based materials are compared to ether based resins of comparable NCO content (Table 3 vs. Table 5), the former are higher in modulus and tear strength. They also exhibit excellent compression set. This is probably due to increased chain crystallinity. However, with both systems shelf stability ranges from to weeks. 
     EXAMPLE 4 
     Polyisocyanurate from MDI-Polyether Prepolymers 
     The properties of MDI-ether vulcanizates were determined in accordance with the general procedure of Example 1. Table 6 sets forth the conditions and results. 
     
                                           TABLE 6__________________________________________________________________________MDI-Polyether Trimer Properties (SA-102 Catalyst at 0.25 phr)       Polathane ®              Baytec ®                     Polathane                            BaytecResin       SME-P2 ME090  AX4    ME050__________________________________________________________________________% NCO       13.1   9.83   6.81   6.06Tensile Strength (psi)       5340   1840   1110   1155Youngs Modulus (psi)       116,000              18730  6340   6754Elongation (%)       13     22     44     73Hardness (A/D) (shore)       97/73  94/50  88/37  87/42Rebound (%) 45     59     53     52Tear Die C (psi)       155    113    109    83Compression Set (%)       NA     16     8      8__________________________________________________________________________ Methylenedi(phenylisocyanate) polyether (MDI) based resins. Polathane ® SMEP2 MDI resin had 13.1% NCO. Polathane ® AX4 MDI resin as 6.81% NCO. Baytec ® ME 050 MDI resin has 6.06% NCO. Baytec ME 090 MDI resin has 9.83% NCO. 
    
     The results show that polyisocyanurate resins can be formed by trimerizing methylenedi(phenylisocyanate) polyether prepolymers into the polyisocyanurate elastomers. As with the TDI based prepolymers, these formulations have a long shelf life. 
     EXAMPLE 5 
     Polyisocyanurate from pPDI-PTMG Prepolymers 
     The procedure of Example 1 was repeated, except pPDI functionalized materials were used. Table 7 sets forth conditions and results. 
     
                       TABLE 7______________________________________pPDI-Polyether Trimer Properties(SA-102 Catalyst at 0.25 phr)Resin          Ultracast PE-35                       Ultracast PE-60______________________________________% NCO          3.58         5.84Tensile Strength (psi)          320          800Youngs Modulus (psi)          936          1160100% Modulus (psi)          765Elongation (%) 52           102Hardness (A/D) (shore)          68/25        73/29Rebound (%)    2            35Tear Die C (pli)          72           63Compression Set (%)          1            2______________________________________ pPDI Based Resins Ultracast ® PE35 pPDIPTMG polyether based prepolymer has 3.58% NCO. Ultracast PE60 pPDIPTMG polyether based prepolymer has 5.84% NCO. 
    
     The results show that excellent properties of a polyisocyanurate polymer can be obtained via the trimerization of a polyurethane prepolymer based upon pPDI (para-phenylenediisocyanate). 
     EXAMPLE 6 
     Polyisocyanurate Adhesives 
     For adhesive applications, metal coupons were sandblasted and degreased prior to use. A portion of the resin-catalyst blend was applied to one coupon and a bond line of 5 mil was maintained by placing silica beads on the open adherend. The second coupon was placed on the first with an overlap of 0.5&#34; and finger pressure was applied. The coupons were placed in a 100° C. oven for a period of 4-16 hours. The coupons were tested using established procedures. The conditions and results are set forth in Table 8. 
     
                       TABLE 8______________________________________TDI-Polyester Trimer Lap Shear Properties(SA-102 Catalyst at 0.25 phr)        Cyana-  Cyana-    Cyana-                                Cyana-        prene   prene     prene preneResin        A9      D5QM      D6    D7______________________________________% NCO        4.34    5.03      5.74  6.64Shear Strength        300     640       710   1400(psi)______________________________________ 
    
     The results show that the TDI-polyester trimer catalyzed with diazabicycloundecene-2-ethylhexanoic acid resulted in polyisocyanurate vulcanizates having good adhesive properties as determined by shear strength. Especially useful is the Cyanaprene D7 resin which has the highest % NCO content of the prepolymers used. 
     EXAMPLE 7 
     Comparative Catalytic Systems 
     The procedure of Example 1 was repeated except that a system containing active Zerewitinoff hydrogen compounds were tested for comparative evaluation. The comparison was also made between a low free isocyanate monomer containing resin (Airthane PET 75D) and a conventional, high free isocyanate monomer resin (Adiprene® L315). The conditions and results are set forth in Table 9. 
     
                                           TABLE 9__________________________________________________________________________Properties of Catalyst Polyurethanes.__________________________________________________________________________           FORMULATIONRESIN           Adiprene ® L-315                       Airthane PET75DRESIN % NCO     9.40%       9.05%RESIN E.W.      447.0       464.3Sample          A   B   C   D   E   FPOLYOL CURATIVE None               PTMG                   BDO None                           PTMG                               BDOCURATIVE E.W.       490 45      490 45CATALYST        SA-102CATALYST-CHARGE (PHR)           0.25               0.25                   0.25                       0.25                           0.25                               0.25PREPOLYMER      100 100 100 100 100 100POLYOL CURATIVE (PHR)           0.00               54.75                   5.03                       0.00                           52.72                               4.85           ResultsPOT LIFE @ 22° C.           &gt;40 39.9                   15.5                       &gt;40 80.9                               16.9           days               min.                   min.                       days                           min.                               min.PROPERTIESTENSILE (PSI)   2690               833     2730                           590 3530YOUNG&#39;S MOD. PSI           39820               750     22350                           592 5880STRESS AT 100% PSI           NA  345     NA  320 905STRESS AT 200% PSI           NA  624     NA  560 3270STRESS AT 300% PSI           NA  NA      NA  NA  NAELONGATION (%)  59  236     91  205 202TEAR DIE-C (PLI)           271 136     315 123 195SPLIT TEAR (PLI)           30  13      47  9   56REBOUND (%)     57  30      51  34  29HARDNESS (A/D)  97/62               63/23   97/59                           61/20                               84/42COMP. SET (%)   NA  3   --  NA  1   --__________________________________________________________________________ PTMG is poly(tetramethyleneglycol). BDO is 1,4butanediol. 
    
     The results show that the systems cured with the diazabicycloundecene catalyst in the absence of polyoy curative (Samples A and D) exhibit extended shelf life and constitute a shelf-stable, one component system. Each of the resin systems containing polyol (Samples B, C, E and F), and particularly the short chain diol BDO (Samples C and F) had too short a pot life to serve as a &#34;one component system.&#34; It should also be noted that the tensile, tear, and hardness values were greater for the one component system. Formulation C was too viscous to mold using conventional techniques and no physical properties were determined. 
     EXAMPLE 8 
     The procedure of Example 1 was repeated, except a prepolymer terminated with isophorone diisocyanate (IPDI), an aliphatic isocyanate, was employed. After extended heating at 100° C., no evidence of reaction was observed. 
     EXAMPLE 9 
     The procedure of Example 1 was repeated, except acid salts of tetramethylguanidine were used as the latent trimerization catalysts. These materials were stable for periods of weeks at room temperature, but exhibited the following gel times at 100° C. (minutes). 
     
                       TABLE 10______________________________________Gel Times of Guanidine Salts withPET 70D-TDI Prepolymers at 100° C.                     Pot Life @ 100° C.Base         Acid         (min)______________________________________DBU          Ethylhexanoic                      9DBU          Trichloroacetic                      9Tetramethylguanidine        Ethylhexanoic                     57Tetramethylguanidine        Trichloroacetic                     300______________________________________ 
    
     The pot life of the guanidine salts is quite long as compared to the amidine, DBU.