Patent Publication Number: US-2007117888-A1

Title: Additives for Extending Pot Life of 2-Component Polyurethane Coatings

Description:
This application is a Continuation-In-Part of co-pending U.S. application Ser. No. 11/393,243 filed Mar. 30, 2006, herein incorporated by reference, which is a non-provisional of U.S. application Ser. No. 60/677,692 filed May 4, 2005. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to the field of 2-component (2K), solvent-borne polyurethane coatings. Such coatings require a metal catalyst to accelerate the cure rate. The most frequently used catalyst metal is tin, usually in the form of dibutyltin dilaurate (DBTDL) and/or zinc. A chelating agent such as 2,4-pentanedione, is commonly used in conjunction with tin for polyurethane coatings for pot life extension and viscosity stability. When preparing this type of polyurethane coating, one must balance the properties of cure rate and pot life, so as to give sufficient working time without unduly extending the cure time. While tin is excellent for cure rate, it yields short pot life. Therefore, there is a desire to prepare an additive combination which will increase the pot life of 2-component, solvent-borne polyurethane coatings, without increasing the cure time beyond an acceptable level.  
      2. Discussion of the Prior Art  
      2-component (2K), solvent-borne polyurethane coatings are well known in the prior art since at least the 1930s. The chemical reaction that results in the production of polyurethanes is described in German Patent 728,981 (1937), German Patent 913,474 (1941) and German Patent 851,851 (1948). 2K urethane coatings are based on reactions of polyols and isocyanates. The chemical reaction is as follows: 
 
R—NCO+R′—OH→RHNCOOR′
 
      The presence of certain metals, most often tin and/or zinc, has been found to increase the rate of reaction. The catalyzed reaction proceeds at a very fast rate. This is good for the drying of the coating, but not for application of the coating. The reaction can be so rapid that the coating is fully reacted before it is even applied to the substrate. The viscosity of the coating increases as the reaction proceeds to a gel. Pot life or bench life is the term used to define how long the coating is useful before the viscosity reaches a level at which the user can no longer apply the coating.  
      In order to increase pot life, it has been the practice to include 2,4-pentanedione, in combination with tin or zinc, or other metals such as zirconium, manganese, vanadium and aluminum. Florio, J. J. and Miller, D. J.,  Handbook of Coatings Additives,  2nd Ed. However, though pot life is extended, it is still below the commercially acceptable life of 6 hours or more.  
      U.S. Pat. No. 5,447,968 teaches a polyurethane-forming foundry binder system containing a nitrogen-containing aromatic compound, such as 2,2-dipyridil and 1,10-phenanthroline, for extending bench life (also known as pot life). The aromatic compounds are used in combination with zinc as a metal catalyst. However, it was found by the inventors that zinc or tin, in combination with either of these compounds, did not increase the pot life for a 2K polyurethane floor coating.  
      1,10 phenanthroline (for example as ACTIV-8® drier accelerator available from R.T. Vanderbilt Company, Inc., as 38% solution of 1,10 phenanthroline in n-butanol or hexylene glycol) is a known drier accelerator used for a different type of coating, i.e. alkyd coatings, usually with cobalt or manganese as the metal catalyst. Alkyd coatings dry by way of an oxidative process, in which oxygen from the air cross-links the resin. In contrast, drying of polyurethane coatings is by way of crosslinking of isocyanate with a polyol. Because of the starkly different drying mechanism, there was no motivation to use 1,10 phenanthroline for polyurethane coatings. Furthermore, it was believed that as 1,10 phenanthroline is non-volatile, it would remain in a polyurethane coating and adversely affect the cure rate.  
     SUMMARY OF THE INVENTION  
      Therefore, it was quite surprising that when 1,10 phenanthroline is used with a 2K polyurethane coating, with a cobalt or manganese metal catalyst replacing tin, a striking improvement in pot life was achieved. In fact, the pot life was increased from 4 hours up to 24 hours in some cases. This is surprising not only because 1,10 phenanthroline has not heretofore been known to be effective in coatings other than alkyds as a drier accelerator, but more importantly, the issue of pot life extension is simply not an issue with respect to alkyd coatings.  
      In view of the above, it is also expected that other organic chelating agents which are effective as drier accelerators for alkyd systems, will work as pot life extenders for 2K polyurethane coatings when combined with cobalt or manganese as a metal catalyst. Such agents include nitrogen-containing aromatic compounds, such as 2,2-dipyridil and 1,10-phenathroline, and their substituted alkyl derivatives. In particular, it was found that 2,2-dipyridil (for example as Dri-Rx® drier accelerator from OMG Americas, a 30% solution of 2,2-dipyridyl) was also effective in increasing pot life while providing acceptable cure rate. Other organic chelating agents which should work in the present invention, based on similar activity for paint drying as the exemplified compounds, include diamine type chelating agents as set forth in U.S. Pat. No. 2,961,331, which is incorporated herein by reference, including 2-(2-pyridyl)-benzimidazole, 1-(2-pyridylazo)-2-naphthol, 2-picolylamine, 2,2′dipyridylamine, 2-pyridinemethaldoxime, 2-pyridinealdoxime, 2-(2-pryidyl)-benzoxazole, 2-pyridinaldazine, bis-(2-pyridinal)-ethylenediamine.  
      As set out above, it was surprising to find that organic chelating agents such as 1,10 phenathroline and 2,2-dipyridyl when used with cobalt or manganese are effective in increasing pot life for 2K polyurethane coatings. However, it was additionally surprising to find that cobalt, replacing tin in the standard 2K polyurethane formula including 2,4-pentanedione, also provides improved superior pot life. It is believed that manganese in combination with the standard 2K polyurethane formula and 2,4-pentanedione would similarly provide improved pot life. It is also believed that a mixture of cobalt and manganese in the standard 2K polyurethane formula and any suitable organic chelating agent would provide improved pot life. Therefore, it seems that a metal catalyst comprising cobalt, manganese or mixtures thereof in combination with an organic chelating agent is the key to extending pot life for 2K polyurethane coatings.  
      The invention, therefore, relates to an additive system for increasing the pot life of two-component, solvent-borne polyurethane coatings; such polyurethane coatings in combination with the additive; as well as a method for increasing pot life based on using the additive in combination with such coatings. The additive is based on cobalt or manganese at 0.01-0.10% and an organic chelating agent at from about 0.03-0.5%, all percentages throughout the specification expressed as mass percent of the resin solids. Preferred amounts of cobalt, manganese or mixtures thereof are about 0.02- 0.04%; and preferred amounts of chelating agent are about 0.05-0.16%. As it is more preferred to use the least amount of additive, a more preferred additive composition contains about 0.02% cobalt or manganese and about 0.06-0.08% chelating agent. It is also possible to express the components in terms of a ratio, wherein a preferred composition is between about 1:1 to about 1:20 metal catalyst:chelating agent, with a most preferred composition at about 1:3.8 where chelating agent is 1,10 phenanthroline, and 1:3.0 for 2,2-dipyridil; all on a dry basis. 
    
    
     DETAILED DISCUSSION OF THE INVENTION  
      The following Examples demonstrate that cobalt or manganese plus a chelating agent in a 2K polyurethane coating, yields extended pot life, slower cure rate and better film hardness than control formulas based on tin and zinc.  
     EXAMPLE 1  
      The 2K polyurethane floor coating was prepared according to Table 1. The two components A and B were blended and mixed at a slow speed for 10 minutes. The Brookfield 50 rpm viscosity was measured at 30 minute intervals until the viscosity was &gt;20,000 cps. Drawdowns were made at 8 mil clearance onto plain white cards at 30 minute intervals, for the length of the pot life. The coatings were cured at 75° F., 50% RH. The Sward Rocker hardness was measured after 24, 48, 96 and 240 hours according to ASTM D 2134. One hour after the two components were blended, drawdowns were made 8 mil clearance onto glass panels, for Gardiner Circular Dry Time Recorder tests.  
               TABLE 1                          Part A                         Desmophen ® 650 A 65 PMA   30.9 wt. %   (Bayer AG)       Desmophen R 221   03.0 wt. %   (Bayer AG)       Byk ® P 104   00.6 wt. %   (Byk-Chemie America)       Lactimon ®   00.6 wt. %   (Byk-Chemie America)       Arcosolv ® PM Acetate   08.9 wt. %   (Lyondell Chemical)       Ti-Pure ® R 900   23.7 wt. %   (E. I. duPont de               Neumours)       Raven ® Black 1255   00.6 wt. %   (Columbian Chemicals)       Byk 354   00.7 wt. %   (Byk-Chemie America)       Metal catalyst   per Table 2       Chelating Agent   per Table 2                 Part B                         Desdomur ® N 75 BA/X   28.2 wt. %   (Bayer AG)                  
 
      (amounts are approximate; totals may not equal 100 wt. % due to rounding; metal catalysts and chelating agent are added as part of a blend, with total weight of blend counted towards total weight of coating in Table 1. In Table 2, percentage of metal or chelating agent alone is shown based on resin solids)  
                               TABLE 2                       Formula   Sn   Zn   Co   Chelating Agent                                  1,10-phenanthroline (1)       A   0.02   0.003   —   —       B   0.02   0.003   —   0.076       C   0.02   —   —   0.076       D   —   —   0.02   0.076       E   —   —   0.04   0.152       F   0.02   —   0.02   0.076                       2,2-dipyridil (2)       G   0.02   0.003   —   0.06       H   0.02   —   —   0.06       I   —   —   0.02   0.06       J   —   —   0.04   0.12       K   0.02   —   0.02   0.06                       2,4-pentanedione       L   0.02   0.003   —   0.2       M   0.02   —   —   0.2       N   —   —   0.02   0.2       O   —   —   0.04   0.4       P   0.02   —   0.02   0.2                 (1) Provided as ACTIV-8 ® containing 38% 1,10-phenanthroline            (2) Provided as Dri-RX ® containing 30% 2,2-dipyridil.             
 
      The results of the testing are set forth in Table 3 below. A pot life of 6 hours or below is considered to be unacceptable. Pot life given as &gt;7½ is considered acceptable for commercial purposes. The actual pot life for &gt;7½ is between 7½ and 24 hours.  
                                   TABLE 3                                      Pot Life   Through Dry   Sward Hardness                                     Formula   (hours)   (hours)   24 hours   240 hours               A    4    3   14   26       B    5½    3½   16   27       C    5½    4½   15   27       D   24   11   12   29       E   &gt;7½    8½   13   27       F    2½    3   13   27       G    4    3¼   14   26       H    4    3   13   25       I   &gt;7½   11   11   27       J    6    8½   11   24       K    2½    3   13   25       L    4    3   13   25       M    5    3¼   12   26       N   &gt;7½   10½   11   28       O    3    6½   13   27       P    2½    3   14   26                  
 
      As shown in Tables 2 and 3, the longest pot life is consistently achieved with cobalt plus a chelating agent. All of the tested chelating agents provided improved pot life and good cure rate when combined with cobalt, as compared to tin and/or zinc, and are therefore considered as part of the invention. However, when tin is added to the composition with cobalt, the combination does not show improved pot life and cure rate, and thus cannot be considered part of the present invention  
      A preferred composition includes 1,10 phenanthroline (ACTIV-8® drier accelerator), as it provided superior film hardness.  
     EXAMPLE 2  
      A clear, 2K polyurethane coating was prepared according to Table 4. The two components A and B were blended and mixed at a slow speed for 10 minutes. The Brookfield 50 rpm viscosity was measured at 30 minute intervals until the viscosity was &gt;20,000 cps. Drawdowns were made at 8 mil clearance onto plain white cards at 30 minute intervals, for the length of the pot life. The coatings were cured at 75° F., 50% RH. The Sward Rocker hardness was measured after 24, 48, 96 and 240 hours according to ASTM D 2134. One hour after the two components were blended, drawdowns were made 8 mil clearance onto glass panels, for Gardiner Circular Dry Time Recorder tests.  
               TABLE 4                          Part A                         Desmophen ® 650 A 65 PMA   41.9 wt. %   (Bayer AG)       Desmophen R 221   4.19 wt. %   (Bayer AG)       Arcosolv ® PM Acetate   15.4 wt. %   (Lyondell Chemical)       Metal/chelate catalyst   variable   per Table 5                 Part B                         Desdomur ® N 75 BA/X   38.5 wt. %   (Bayer AG)                  
 
      (amounts are approximate; totals may not equal 100% due to rounding; metal catalysts and chelating agent are added as part of a blend, with total weight of blend counted towards total weight of coating in Table 4. In Table 5, percentage of metal or chelating agent alone is shown based on resin solids)  
                                       TABLE 5                       Formula   Sn   Co   Mn   Zr   Al   Chelating agent                                          1,10 phenanthroline (1)       AA   0.002   —   —   —   —   —       BB   —   —   0.629   —   —   0.110       CC   —   —   0.058   —   —   0.220       DD   —   —   0.116   —   —   0.440       EE   —   0.029   —   —   —   0.108       FF   —   0.058   —   —   —   0.216       GG   —   0.116   —   —   —   0.432       HH   —   —   —   0.029   —   0.110       II   —   —   —   0.054   —   0.220       JJ   —   —   —   0.116   —   0.440       KK   —   —   —   —   0.019   0.074       LL       —   —   —   0.038   0.148       MM   —   —   —   —   0.076   0.296                               2,-2-dipyridil (2)       NN   —   —   0.029   —   —   0.096       OO   —   —   0.058   —   —   0.192       PP   —   —   0.116   —   —   0.384       QQ   —   —   —   —   0.029   0.096       RR   —   —   —   —   0.058   0.192       SS   —   —   —   —   0.116   0.384       TT   —   —   —   —   0.019   0.062       UU   —   —   —   —   0.038   0.124       VV   —   —   —   —   0.076   0.248                  
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Pot Life 
                 Through Dry 
                 Sward Hardness 
                   
               
            
           
           
               
               
               
               
               
            
               
                 Formula 
                 (hours) 
                 (hours) 
                 24 hours 
                 240 hours 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 AA 
                 1.5 
                 4.5 
                 70 
                 80 
               
               
                 BB 
                 &gt;7.5 
                 11 
                 50 
                 90 
               
               
                 CC 
                 &gt;7.5 
                 7.5 
                 54 
                 88 
               
               
                 DD 
                 7.5 
                 9 
                 60 
                 86 
               
               
                 EE 
                 &gt;7.5 
                 8.5 
                 48 
                 64 
               
               
                 FF 
                 &gt;7.5 
                 12.5 
                 42 
                 60 
               
               
                 GG 
                 &gt;7.5 
                 13.5 
                 50 
                 70 
               
               
                 HH 
                 &gt;7.5 
                 16 
                 44 
                 96 
               
               
                 II 
                 &gt;7.5 
                 16 
                 48 
                 100 
               
               
                 JJ 
                 &gt;7.5 
                 12.75 
                 48 
                 100 
               
               
                 KK 
                 &gt;24 
                 16 
                 36 
                 86 
               
               
                 LL 
                 &gt;24 
                 16 
                 24 
                 70 
               
               
                 MM 
                 &gt;24 
                 16.5 
                 32 
                 62 
               
               
                 NN 
                 &gt;7.5 
                 13.5 
                 44 
                 102 
               
               
                 OO 
                 &gt;7.5 
                 13.5 
                 58 
                 110 
               
               
                 PP 
                 &gt;7.5 
                 9.5 
                 52 
                 90 
               
               
                 QQ 
                 &gt;24 
                 18 
                 40 
                 94 
               
               
                 RR 
                 &gt;24 
                 14.25 
                 38 
                 86 
               
               
                 SS 
                 &gt;24 
                 15 
                 44 
                 84 
               
               
                 TT 
                 &gt;24 
                 17 
                 44 
                 90 
               
               
                 UU 
                 &gt;24 
                 17 
                 44 
                 90 
               
               
                 VV 
                 &gt;24 
                 16 
                 58 
                 100 
               
               
                   
               
            
           
         
       
     
      As shown in Table 6, the coatings that were catalyzed by tin alone have an extremely short pot life and medium film hardness. All coatings catalyzed with cobalt, manganese, aluminum and zirconium had acceptable pot life (&gt;7.5 hours). However, only coatings of the present invention, i.e. catalyzed with cobalt and manganese, showed acceptable dry times (&lt;14 hours), as well as acceptable pot life. It is noted that Formula JJ exhibited an acceptable pot life and dry time. However, this formula is not considered part of the present invention, as it is believed that such favorable results are only achieved with zirconium when the metal is used at high levels (i.e. &gt;0.10 wt. %).