N-hydroxysuccinimide-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanate and self-cross linking copolymers thereof
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the preparation of self-crosslinking copolymers derived from an N-hydroxyimide, such as N-hydroxysuccinimide blocked isopropenyl- alpha, alpha-dimethylbenzyl isocyanate, and a hydroxy group containing olefinically unsaturated monomer. The self-crosslinking copolymers are useful as self-curing resins in coatings, adhesives, and moldings. 
     2. Description of the Related Art 
     In general, commonly used urethane coatings systems are of the two-component or one-component types. 
     A typical two-component paint, for example, comprises a polyisocyanate material and a polyhydroxy material, and, upon curing, can form films with excellent physical properties and resistance properties. They are used in high performance automotive coatings. 
     A typical one-component paint, for example, comprises a blocked polyisocyanate and a polyhydroxy material. Because the isocyanate groups are blocked, one-component systems have a generally longer shelf life than two-component systems containing free, hydroxy-reactive isocyanate groups. When desired, the one-component systems may be activated by, for example, heat, to deblock the blocked polyisocyanate and promote crosslinking. 
     Japanese Patent No. 63-186,722 discloses a coatings system wherein the blocked isocyanate and hydroxy groups are part of the same polymer backbone, thereby producing a self-crosslinking type one-component system. Similar ethanol-blocked copolymers are disclosed in JP 1-087,606. 
     U.S. Pat. No. 5,116,930 discloses another approach to a self-crosslinking resin accomplished by hydroxyfunctionalization of a polyisocyanate polymer. It is stated therein in column 1 and in Comparative Example 3, column 9, that when a self-curing resin is produced by copolymerizing an oxime-blocked isocyanate-containing radical-polymerizable monomer and a hydroxyl containing radical-polymerizable monomer, the blocking agent may undergo dissociation during polymerization disadvantageously leading to gelation, and, at the same time, leading to marked polymer discoloration. Furthermore, we have ourselves observed on numerous occasions the setting up of the contents of the reactor into a crosslinked gel during attempts to prepare the above-mentioned copolymers. 
     A solution for the gelation problem has been described in the co-pending application submitted concurrently herewith entitled &#34;Process for the Preparation of Gel-Free Self-Crosslinking Copolymers Derived from Blocked Isopropenyl-Alpha, Alpha-Dimethylbenzyl Isocyanate,&#34; U.S. patent application Ser. No. 07/963,880, filed Oct. 20, 1992. 
     Another solution of the gelation problem is described herein. The solution comprises using a novel N-hydroxyimide-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanate which: 
     1. has a higher deblocking temperature than oxime- or lactam-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanates so that premature deblocking and gelation is prevented, but 
     2. has a lower deblocking temperature than alcohol-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanates so that rapid cure is still obtained at conveniently low cure temperatures. 
     SUMMARY OF THE INVENTION 
     This invention provides gel-free self-cross- linking copolymers of novel N-hydroxyimide-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanate, and a process for their preparation. The novel copolymers are useful as thermally stable but, at the same time, fast curing one-component resins for paints. Crosslinked films or objects are obtained by heat- curing the self-crosslinking copolymers of this invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The novel N-hydroxyimide-blocked polymerizable isopropenyl-alpha, alpha-dimethylbenzyl isocyanate monomers usable in this invention are the meta- and para- isomers represented by the formulae: ##STR1## or a mixture thereof, wherein B is the conjugate base formed by an HB blocking group. The blocking group HB herein is an N-hydroxyimide blocking group selected from N-hydroxysuccinimide, dodecyl N-hydroxysuccinimide, methyl N-hydroxysuccinimide, N-hydroxyphthalimide, and a mixture thereof. 
     The blocking groups HB are represented by the formulae: ##STR2## The novel self-crosslinking copolymer of the invention is a copolymer of: (I) an N-hydroxyimide-blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanate; 
     (II) a hydroxy group containing ethylenically unsaturated monomer; and 
     (III) optionally, an ethylenically unsaturated monomer which does not contain hydroxy groups. 
     The copolymer is obtained by copolymerizing ingredients (I), (II), and, optionally, (III) in an appropriate solvent using a free radical initiator and optionally a chain transfer agent to control the molecular weight of the copolymer. These are discussed in greater detail below. 
     The polymerizable blocked isocyanate used in this invention is an isopropenyl-alpha, alpha-dimethylbenzyl isocyanate blocked with a blocking group such as N-hydroxy succinimide. The meta-isomer of the monomer, hereinafter m-TMI, is commercially available under the tradename TMI® (meta) Unsaturated Aliphatic Isocyanate from American Cyanamid Company, Wayne, N.J., as a colorless liquid of greater than 94 percent purity. The use of the meta- isomer to prepare the N-hydroxyimide-blocked polymerizable isocyanates is preferred. Other isomers, such as the para-isomer, hereinafter p-TMI, or a mixture of isomers, such as meta- and para- mixtures may also be used. The meta- and para- isomers of TMI are represented, respectively, by the formulae: ##STR3## Various methods of preparation of TMI are described in U.S. Pat. Nos. 3,290,350; 4,130,577; 4,377,530; 4,439,616; and European Pat. Appln. No. 266,659. 
     The N-hydroxyimide-blocked isocyanates derived from TMI may be prepared by contacting TMI with, for example, an N-hydroxyimide such as N-hydroxysuccinimide, hereinafter NHS. 
     N-Hydroxysuccinimide-blocked meta-isopropenyl-alpha, alpha-dimethylbenzyl isocyanate is the preferred polymerizable blocked isocyanate monomer and is represented by the formula: ##STR4## The self-crosslinking blocked isocyanate copolymers of the invention may be prepared by a process having the steps of: 
     (a) adding, over a period of time, to a reaction zone maintained at a temperature of at least about 30° C.: 
     (i) an organic solvent; 
     (ii) an N-hydroxyimide-blocked isopropenyl-alpha-dimethylbenzyl isocyanate; 
     (iii) a hydroxy group containing ethylenically unsaturated monomer; 
     (iv) optionally, an ethylenically unsaturated monomer free of hydroxy groups; and 
     (v) a free radical initiator; and 
     (b) thereafter heating at a temperature and for a length of time sufficient to produce a gel-free self-crosslinking resin. 
     The solvents usable in this invention may be relatively non-polar, or they may be highly polar. The non-polar organic solvents may be solvents from the class of aromatic hydrocarbons such as xylenes and toluene; halogenated solvents such as chloroform, carbon tetrachloride, chlorobenzene, and the like; esters such as ethyl acetate and butyl acetate; alcohols such as methanol, ethanol, isopropanol, butanol, and the like; ethers such as dibutyl ether; ketones such as acetone, methyl ethyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, cyclohexanone and the like. Polar solvents are preferred, however, because they minimize deblocking. 
     The highly polar organic solvents usable in the process of the invention are organic solvents which have dipole moments higher than 3.0 Debye units, for example, and may include at least one member of the following general classes: 
     alkyl nitriles 
     cyclic sulfones 
     N-alkyl pyrrolidones 
     N,N-dialkyl formamides 
     N,N-dialkyl acetamides 
     alkyl formates 
     alkyl oxalates 
     hexamethylphosphorus triamide (HMPT) 
     hexamethylphosphoramide (HMPA) 
     The preferred solvents include acetonitrile, sulfolane, nitromethane, N-N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and mixtures thereof. 
     The most preferred solvent is acetonitrile which has a dipole moment of 3.7 Debye units. 
     The hydroxy group containing ethylenically unsaturated monomer usable in this invention includes 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 2,3-dihydroxypropyl acrylate 2,3-dihydroxypropyl methacrylate, allyl alcohol, methallyl alcohol, and mixtures thereof. 
     In addition to the blocked TMI and the hydroxy group containing ethylenically unsaturated monomer, one or more ethylenically unsaturated monomers which do not contain hydroxy groups may be used in the process of the invention to impart certain physical and resistance properties to the copolymer. Such monomers include methyl acrylate, methyl methylacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate, acrylonitrile, methacrylonitrile, styrene, alpha-methyl styrene, vinyl acetate, dialkyl fumarates, dialkyl maleates, vinyl chloride, vinylidene chloride, vinyl toluene, and mixtures thereof. 
     The free radical initiators usable in the process of the invention are initiators which are capable of generating free radicals at temperatures preferably in the range of about 75° C. to about 85° C., and include initiators such as azobisisobutyronitriazobisvaleronitrile, benzoyl peroxide, tert-butyl peroctoate, tert-amyl peroctoate, tert-butyl hydroperoxide di-tert-butyl peroxide, dicumyl peroxide, and tert-butyl peroxyisopropylcarbonate. 
     To control the molecular weight of the copolymer, a chain transfer agent such as dodecyl mercaptan, also referred to herein as dodecyl thiol, may also be optionally used. Other well known methods such as lowering or raising the reaction temperature or the initiator level may also be employed to increase or decrease the molecular weight of the polymer, as desired 
     The proportion of the ingredients in the reaction zone charge depends on the desired self-crosslinking copolymer composition. 
     The amount of the polar organic solvent in the total charge is from about 10 weight percent to about 90 weight percent, and preferably from about 40 weight percent to about 80 weight percent, based on the weight of the total charge. The term &#34;total reaction zone charge&#34; or &#34;total charge&#34; is used herein to mean the total amount of ingredients (i) to (v) including optional ingredients such as chain transfer agents and the like. The term &#34;total monomer charge&#34; or &#34;total monomer&#34; is used in the context of this invention to mean the total amount of ingredients (ii), (iii), and (iv). If more than one monomer is used as ingredient (iv) for example, then the total monomer content is the sum of all monomers used as ingredient (iv) in addition to the blocked TMI and the hydroxy group containing olefinically unsaturated monomer. 
     The term &#34;crosslinkingly reactive components&#34; is used herein to mean the total amount of ingredients (ii) and (iii). 
     The amount of total monomers in the total charge is in the range of from about 10 weight percent to about 90 weight percent, and preferably from about 40 weight percent to about 80 weight percent, based on the weight of the total charge 
     The molar ratio of the blocked TMI to hydroxy group containing olefinically unsaturated monomer is from about 1.5:1 to about 0.5:1 and preferably about 1.1. The molar ratio of the crosslinkingly reactive components to the olefinically unsaturated monomer is from about 3:1 to about 0.1:1, and preferably from about 0.5:1 to about 1:1. 
     The amount of the free radical initiator is typically from about 0.01 weight percent to about 15 weight percent of the total monomers depending on the molecular weight desired It is typically from about 1 weight percent to about 8 weight percent of the total monomers. 
     The amount of the chain transfer agent, if used, may be from about 0.01 to about 15 weight percent of the total monomers depending on the molecular weight desired. It is typically from about 1 weight percent to about 4 weight percent of the total monomers. 
     The molecular weight of the copolymers obtained by the process of this invention is from about 1,000 to about 1,000,000, and preferably from about 5,000 to about 50,000. 
     In the preparation of the copolymers of the invention, ingredients (i) to (v) are simultaneously added in step (a) either as a homogenous mixture of all the ingredients, or each through a separate stream into the reaction zone maintained at a temperature preferably in the range of about 75° C. to about 85° C. When the ingredients are added through separate streams, one or more streams may be premixed prior to addition to the reaction zone. Furthermore the rate of addition of any ingredient may be kept the same, or varied as desired to carry out the copolymerization reaction at a deficiency or excess of that ingredient. 
     The addition is preferably carried out simultaneously by adding all the ingredients as a mixture or through separate streams. When the addition is sequential, a portion of the solvent is added first, followed by addition of the remaining ingredients, for example. 
     Another example of a sequential addition is when the free radical initiator is added last to the remaining ingredients. 
     The addition usually is carried out over a period of time in the range of about 1 hour to about 5 hours, but an addition time of about 3.4 hours is preferred. 
     The step (b) of the process is carried out by heating the reaction mixture further at about 75° C. to about 85° C. for a period of time from about 1 hour to about 5 hours. During the heating in step (b), an additional amount of a free radical initiator may be added to the reaction mixture to ensure complete polymerization of the monomers. 
     If the polymerization is carried out in a manner is which the total time of adding (step 1) and heating (step 2) exceeds 10 hours or when the temperature exceeds 115° C. for long periods of time, self crosslinking in the reactor may take place producing an insoluble crosslinked gel which may be difficult to remove from the reactor. Under the conditions of the preparation disclosed herein, gel formation is avoided and the method produces a gel-free self-crosslinking copolymer usable as a resin in coatings, moldings, and adhesives, for example. 
    
    
     EXAMPLE 1 
     4.4 g of acetonitrile was added to a flask and sparged with nitrogen for 10 minutes. With sparging continuing, the temperature was brought to 81° C. whereupon a solution of 31.5 g of methyl methacrylate, 40.3 g of butyl acrylate, 17.8 g of hydroxy-ethyl methacrylate, 42.7 g of n-hydroxy succinimide blocked m-TMI, 3.5 g of t-butyl peroctoate and n-dodecyl mercaptan was added over a period of 3 hours and 50 minutes. Heating was continued for an additional 4 hours and 10 minutes. Pan solids=76.0%; conversion was 99.8%; NCO analysis was 2.7% corresponding to 87.7% of theory. No gelation occurred. The number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity (Mw/Mn) were as follows: 
     Mn=11400 
     Mw=57500 
     Polydispersity=5.04. 
     EXAMPLE 2 
     The procedure of Example 1 was repeated, except a solution of 67.5 g of acetonitrile, 53.7 g of methyl methacrylate, 68.7 g of butyl acrylate, 29.9 g of hydroxyethyl methacrylate 72.7 g of m-TMI blocked with N-hydroxy succinimide, 9.0 g of t-butyl peroctoate and 4.5 g of n-dodecanethanol was added. The solution was added over a period of 3 hours and 30 minutes. Heating was continued at 81° C. for an additional 3 hours and 10 minutes. Pan solids were determined to be 75.5%. No gelation occurred. 
     EXAMPLE 3 
     The deblocking of blocked m-TMI derivatives was studied by Infrared Spectroscopic (IR) techniques to determine their response to heat, particularly to determine the onset temperature for deblocking reactions and also to determine the rapid deblocking temperatures of the blocked m-TMI derivatives. The results are summarized in TABLE 1. 
     
                       TABLE 1                                                     
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DEBLOCKING TEMPERATURES                                                   
OF BLOCKED ISOCYANATES                                                    
                Onset Temp-  Rapid De-                                    
                eratures For blocking Temp-                               
Blocking Group  Deblocking (°C.)                                   
                             peratures (°C.)                       
______________________________________                                    
Methanol        155*         190*                                         
n-Hexanol       155*         175*                                         
n-Hydroxysuccinimide                                                      
                115**        140**                                        
e-Caprolactam   90**         125**                                        
Acetone Oxime   50**          80**                                        
Methyl Ethyl Ketone Oxime                                                 
                50**          65**                                        
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 *Temperature ramp from 29° C. to 202° C. at 5°      
 C./minute, with the infrared spectrum measured at intervals corresponding
 to 5° C. temperature change.                                      
 **Same as above except temperature ramp at 1° C./minute.          
 
    
     It is evident from the results in TABLE 1 that considerable deblocking occurs at typical polymerization temperatures which, in the case of self-crosslinking copolymer systems, would lead to gelation. It is also evident from the Table that N-hydroxysuccinimide-blocked m-TMI has a deblocking temperature above oxime or caprolactam-blocked analogs, but below alcohol-blocked systems. 
     EXAMPLE 4 
     Preparation of Blocked m-TMI Derivatives 
     The blocking of m-TMI is carried out by general methods and procedures used to prepare other blocked isocyanates, such as those described by Z. W. Wicks, Jr., in Progress in Organic Coatings, Volume 3, pages 73 to 99 (1975) and also ibid- Volume 9, pages 3 to 28 (1981). 
     A. Caprolactam-blocked m-TMI was prepared by the procedure described by D. K. Parker et al. in Rubber Chemistry and Technology, Volume 63, Number 4, page 584 (1990). 
     B. MEKO-blocked m-TMI was prepared by combining equimolar quantities of m-TMI and methyl ethyl ketone oxime (MEKO). The adduct has been mentioned in JP 63-186,722, GB 2,160,534, and by S. P. Pappas and E. H. Urruti in the proceedings of the Water-Borne and Higher-Solids Coatings Symposium, New Orleans, La., Volume 13, page 146 (1986). 
     C. Methanol-blocked m-TMI was prepared by adding m-TMI to a tenfold excess amount of methanol and heating at the boiling point of the mixture overnight. 
     D. Butanol-blocked m-TMI was prepared by adding m-TMI to a tenfold excess amount of normal butanol and heating at the boiling point of the mixture for 3 hrs. 
     E. N-Hydroxysuccinimide-blocked m-TMI was prepared as follows: 
     To a mixture of N-hydroxysuccinimide (197.8 g) obtained from Aldrich Chemical Company, Milwaukee, Wis., in a mixture of toluene (525 g) and methylene chloride (100 g), m-TMI was added dropwise at 45° C. to 50° C. over a period of 90 minutes with stirring. After allowing the mixture to stand overnight at room temperature, the colorless precipitate was collected by filtration under reduced pressure with the aid of a rubber dam, washed twice with 100 ml portions of toluene and dried at room temperature under reduced pressure to give N-hydroxysuccinimide-blocked m-TMI (438 g, 94% yield); melting point: 135°-137° C.; the infrared (in nujol) and  1  H nuclear magnetic resonance spectra indicated that the product was N-hydroxysuccinimide-blocked m-TMI. 
     EXAMPLE 5 
     The general procedure of EXAMPLE 1 was repeated with the following amounts and reaction conditions: 7.5 g of acetonitrile was added initially to the 500 ml flask which was sparged with N 2  for 45 minutes. then heated to 81° C. A solution of 67.5 g of acetonitrile, 55.3 g of methyl methacrylate, 70.7 g of butyl acrylate, 30.8 g of hydroxyethyl methacrylate, 68.2 g m-TMI blocked with MEKO, 9.0 g of t-butyl peroctoate and 4.5 g of n-dodecylmercaptan was added over a period of 3 hours. Heating was continued at 81° C. for an additional 5 hours. Solids were determined to be 70.7%. No gelation occurred. 
     EXAMPLE 6 
     The procedure of EXAMPLE 1 was repeated, except a solution of 67.5 g of acetonitrile, 52.7 g of methylmethacrylate, 67.5 g of butylacrylate, 29.4 g of hydroxyethylmethacrylate, 75.5 g of m-TMI blocked with e-caprolactam, 9.0 g of t-butyl peroctoate and 4.5 g of n-dodecanethiol was added to the heated flask at 81° C. over a period of 31/2 hours. Heating continued at 81° C. for an additional 4 hours. Pan solids were determined to be 72.7%. No gelation occurred. 
     EXAMPLE 7 
     The procedure of EXAMPLE 1 was repeated, except a solution of 67.5 g of acetonitrile, 56.0 g of methyl methacrylate, 71.7 g of butyl acrylate, 31.2 g of hydroxyl ethyl methacrylate, 66.1 g of m-TMI blocked with n-butanol, 9.0 g of t-butyl peroctoate and 4.5 g of n-dodecanethiol was added over a period of 31/2 hours. After the addition, heating at 81° C. was continued for an additional 4 hours. Pan solids were determined to be 75.7%. No gelation occurred. 
     EXAMPLE 8 
     The following self-crosslinking copolymers prepared by the process of the invention were tested as one component self-crosslinking type resins for coatings: 
     (1) Product of Example 1 (Summarized in TABLE 2) 
     (2) Product of Examples 2, 5, 6, and 7 (Summarized in TABLE 3). 
     Each copolymer was dissolved in a mixture of methyl isobutyl ketone (42 percent), xylenes (16 weight percent), and 1-methoxy-2-propyl acetate (42 weight percent), and after adding a tin-based cure catalyst, the mixture was applied onto white base coated ED.11  primed cold roll steel (CRS) panels. The coated panels were then heated in a vented oven at a particular temperature/time schedule to produce the cured coatings. The results are summarized in TABLES 2 and 3. It is evident from TABLES 2 and 3 that the self-crosslinking copolymers of the invention produce coatings which have excellent hardness, solvent resistance, impact resistance, and a good cure response. 
     
                       TABLE 2                                                     
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CURED COATINGS FROM TIN-CATALYZED SELF-                                   
CROSSLINKING OF THE PRODUCT OF EXAMPLE 1                                  
30 min cure  Pencil   MEK        Impact                                   
Temp (°C.)                                                         
             Hardness Double Rubs                                         
                                 Front/Reverse                            
______________________________________                                    
0.1%   110       2H       50       140/140                                
T-12(1)                                                                   
       120       3H       80       140/140                                
       130        3H+     125      140/140                                
                 140      3H       200 + 140/140                          
                 150      4H       200 + 140/140                          
0.1%   120       F        70       &lt;20/20                                 
TK-1   140       2H       200       100/&lt;20                               
                 160      4H+      200100/20                              
______________________________________                                    
 (1) T12 is dibutyltindilaureate                                          
 (2) TK1 is tetrabutyldiacetoxystanoxane (TBDAS)                          
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
CURED COATINGS FROM TIN-CATALYZED SELF-CROSSLINKING                       
OF THE PRODUCTS OF EXAMPLES 5, 2, 6, AND 7                                
             Product of                                                   
                    Product of                                            
                           Product of                                     
                                  Product of                              
Cure Response                                                             
             Example 5.sup.(1)                                            
                    Example 2.sup.(1)                                     
                           Example 6.sup.(1)                              
                                  Example 7.sup.(1)                       
__________________________________________________________________________
125° C., 30 min                                                    
Thickness, mils                                                           
             1.8    1.7    1.5    2.1                                     
Hardness KHN.sub.25                                                       
             10.5   10.5   8.4    3.1                                     
Pencil Hardness                                                           
             F-H    F-H    H-2H   2B-B                                    
MEK Rubs Mar/Remove                                                       
              200+  100/200                                               
                           20/200+                                        
                                  1/2                                     
Yellow Index -2.3   -2.3   -2.5   -3.3                                    
135° C., 30 min                                                    
Thickness, Mils                                                           
             1.6    1.5    1.7    1.8                                     
Hardness KHN.sub.25                                                       
             11.9   11.4   9.3    3.5                                     
Pencil Hardness                                                           
             F-H    F-H    H-2H   2B-B                                    
MEK Rubs Mar/Remove                                                       
             200     200+  200/200+                                       
                                  1/5                                     
Yellow Index -1.6   -.18   -2.1   -3.2                                    
145° C., 30 min                                                    
Thickness, Mils                                                           
             1.6    1.5    1.7    1.8                                     
Hardness, KHN.sub.25                                                      
             11.9   11.6   10.3   4.9                                     
Pencil Hardness                                                           
             F-H    H-2H   H-2H   2B-B                                    
MEK Rubs Mar/Remove                                                       
              200+   200+  200/200+                                       
                                  1/20                                    
Yellow Index +0.6   -0.8   -1.7   -2.9                                    
__________________________________________________________________________
 .sup.(1) Contains 0.5% UL28 catalyst based on solids. UL28 is            
 dimethyltindilaurate.                                                    
 
    
     Although the present invention has been described with reference to certain preferred embodiments, it is apparent that modifications and variations thereof may be made by those skilled in the art without departing from the scope of this invention as defined by the appended claims.