Patent Publication Number: US-6034160-A

Title: Method for the preparation of water-borne coating compositions using thermoplastic polyhydroxyether resins having narrow polydispersity

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method for the preparation of water-borne thermoplastic polyhydroxyethers having narrow polydispersity, coating compositions containing the same, and the use of such compositions for providing flexible coatings. 
     Coatings that contain no solvents have been prepared from very low molecular weight epoxy resins. The resin in the coating serves as the wetting agent and as the vehicle for any pigments or fillers desired. Because there is no solvent, coatings of this type tend to be free from pinholes, but have disadvantages such as brittleness, poor thermal stability and relatively high costs. 
     Coating compositions based on higher molecular weight epoxy resins have been prepared as solutions, formulated with solvent, curing agents, modifiers and pigments. These higher molecular weight epoxy resins are often in the form of an ester, obtained by reacting the epoxy resin with a fatty acid. While suitable for many purposes, epoxy ester-based coatings also have disadvantages such as being vulnerable to caustic attack. 
     In more recent years, there has been a trend toward water-based coating compositions containing epoxy resins, because of ease of handling and cleanup. One such resin is a modified epoxy resin made by grafting an addition monomer component onto the aliphatic backbone of the epoxy resin. For water-dispersible coatings, the addition polymerizable monomer is usually acrylic acid. Such water-dispersed epoxy coatings are particularly useful for the formulation of can coatings. These resins perform well when sprayed onto the surface of an already formed can body but have not found success for use in coating surfaces that are later formed into various shapes, including can ends. This is due to the inflexibility of the resins and their relatively low molecular weight. Increasing the molecular weight of the epoxy resin in order to achieve the desired properties results in the use of strong organic solvents, in order to dissolve the higher molecular weight resin, which must later be removed. 
     Another form of water-borne resins is based on thermoplastic polyhydroxyether resins, a family of amorphous, high molecular weight phenoxy resins derived from diphenols and epichlorohydrin. These &#34;phenoxy&#34; resins are known for their excellent flexibility, impact and abrasion resistance and adhesive strength. Because of these properties, they are used commercially for a variety of water-borne coatings. However, because of their high molecular weight and chain rigidity, again, a large amount of strong organic solvent must be employed to achieve a satisfactory applicable production process. To prevent pollution of the atmosphere, environmental standards require that such strong organic solvents be either recovered or burned at considerable costs. 
     The need exists for water-borne thermoplastic polyhydroxyether resin dispersions for use in coating compositions that provide coatings having the desired toughness and flexibility for use on flexible substrates, while having low viscosities which do not require additional steps of solvent reclamation from the dispersions. 
     SUMMARY OF THE INVENTION 
     It has been found that by preparing water-borne coating compositions from thermoplastic polyhydroxyether resins having a narrow polydispersity, the removal of organic solvents from such compositions can be reduced or even eliminated. Polydispersity, as used herein is defined as the ratio of weight average molecular weight (M w ) to number average molecular weight (M n ) for a particular thermoplastic polyhydroxyether resin. Narrow polydispersity, as used herein, is defined as a ratio of weight average molecular weight to number average molecular weight of less than 4.0, preferably less than 3.8, more preferably less than 3.5 and most preferably less than 3.1, with a number average molecular weight greater than 7,000 and less than 12,000 (M w  /M n  &lt;4; 7,000&lt;M n  &lt;12,000). These thermoplastic polyhydroxyether resins, having narrow polydispersity, provide coatings that exhibit improved performance properties when compared to epoxy resins and performance properties equal to those of known, conventional phenoxy resins. 
     DETAILS OF THE INVENTION 
     The method of the present invention involves preparing a water-borne thermoplastic polyhydroxyether (phenoxy) resin dispersion by grafting one or more ethylenically unsaturated monomers, preferably having 3 to about 8 carbons, onto a thermoplastic polyhydroxyether having a narrow polydispersity (M w  /M n  &lt;4; 7,000&lt;M n  &lt;12,000). It is desirable that at least one of the monomers contain sufficient carboxyl groups to provide from about 1 to 100 carboxyl groups per 10 monomeric units of thermoplastic polyhydroxyether. The resulting grafted thermoplastic polyhydroxyether is then blended with water and a water-miscible base resulting in a usable coating composition. 
     The grafting operation can be carried out by conventional techniques, such as, suspension polymerization wherein powdered thermoplastic polyhydroxyether, carboxyl containing monomer and free radical initiator are suspended in a liquid medium and polymerized; polymerization of the above ingredients in a melt extruder; and solution polymerization of the above ingredients as a varnish in an organic solvent. Solution polymerization is the preferred method of preparing the grafted thermoplastic polyhydroxyethers of the present invention, and is preferably carried out in a high turbulence field. 
     The term &#34;thermoplastic polyhydroxyether&#34; herein refers to substantially linear polymers having the general formula: 
     
         -[D-O-E-O]-.sub.n 
    
     wherein D is the radical residuum of a dihydric phenol, E is a hydroxyl containing radical residuum of an epoxide and n represents the degree of polymerization and is at least 30. The term &#34;thermoplastic polyhydroxyether&#34; is intended to include mixtures of at least two thermoplastic polyhydroxyethers. 
     Thermoplastic polyhydroxyethers can be prepared by admixing from about 0.985 to about 1.015 moles of an epihalohydrin with one mole of a dihydric phenol together with from about 0.6 to 1.5 moles of an alkali metal hydroxide, such as, sodium hydroxide or potassium hydroxide, generally in a solution at a temperature of about 10° C. to about 50° C., until at least about 60 mole percent of the epihalohydrin has been consumed. The thermoplastic polyhydroxyethers thus produced, which are useful in the present invention, have a weight average molecular weight (M w ) greater than about 20,000 and less than about 40,000, and a number average molecular weight (M n ) of at least 7,000 and less than about 12,000, as determined by gel permeation chromatography. This combination of relatively low M w  (20,000 to about 40,000) and high M n  (7,000 to about 12,000), results in thermoplastic polyhydroxyethers having the desired narrow polydispersity (M w  /M n  &lt;4; 7,000&lt;M n  &lt;12,000). 
     The dihydric phenol contributing the phenol radical residuum, D, can be a dihydric mononuclear phenol, a dihydric polynuclear phenol, or mixtures thereof. Suitable dihydric mononuclear phenols and dihydric polynuclear phenols are disclosed in U.S. Pat. No. 4,355,122. Preferred dihydric polynuclear phenols include bis(hydroxyphenyl) alkanes, di(hydroxyphenyl) sulfones, di(hydroxyphenyl) ethers, and the like. 
     The epoxide contributing the hydroxyl containing radical residuum, E, can be monoepoxide or diepoxide. By &#34;epoxide&#34; is meant a compound containing an oxirane group, i.e., oxygen bonded to two vicinal aliphatic carbon atoms. Suitable epoxides are disclosed in U.S. Pat. No. 4,355,122; and include monoepoxides, diepoxides, saturated epoxides and the like. 
     In the method of the present invention, the thermoplastic polyhydroxyether resins are first dissolved in an appropriate water-miscible solvent. Preferred water-miscible solvents useful in the present invention include solvents that are relatively non-volatile, and have an affinity for the thermoplastic polyhydroxyether resins. Preferred water-miscible solvents include ethylene and propylene glycol ethers, such as butyl glycol ether (butyl Cellosolve), and ethyl glycol ether (ethyl Cellosolve); and lower molecular weight alcohols, such as butanol. Mter dissolving the thermoplastic polyhydroxyether resin, the desired ethylenically unsaturated monomer or monomers to be grafted are then introduced into the mixture. 
     Preferred ethylenically unsaturated monomers include one or more ethylenically unsaturated hydrocarbon monomers having from about 3 to 8 carbons. It is desirable that at least one of the monomers contain sufficient carboxyl groups to provide from about 1 to 100 carboxyl group per 10 monomeric units of thermoplastic polyhydroxyether. Such monomers include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid and the like. Exemplary ethylenically unsaturated monomers that do not contain carboxyl groups include acrylic or methacrylic alkyl esters, such as methyl methacrylate, ethyl acrylate, n-propyl methacrylate, butyl acrylate, and the like; cyano-containing monomers, such as, acrylonitrile, methacrylonitrile, and the like; aromatic vinyl monomers, such as, styrene, α-methyl styrene, p-vinyl toluene and the like. 
     The grafting of the ethylenically unsaturated monomers can be accomplished by standard polymerization techniques known in the art. Solution polymerization using a free radical initiator, such as benzoyl peroxide, azobisisobutyronitrile and the like is preferred. 
     The grafted thermoplastic polyhydroxyether resin is then blended, preferably in a high turbulence field, with water and a water-miscible base. For purposes of this invention, the term &#34;water-miscible base&#34; is used in the broad sense to include any proton acceptor that will neutralize the acid functionalities of the grafted thermoplastic polyhydroxyether. Exemplary bases may be both organic and inorganic, and include alkali metal or alkaline earth hydroxides, such as, sodium hydroxide, potassium hydroxide, and the like; ammonium hydroxide; organic amines including monoalkylamines, alkanolamines, aromatic amines, cyclic amines, alkylaryl amines, and the like. Preferred amines include ammonia, triethylamine and dimethylethanolamine. 
     The resulting water-borne dispersion of thermoplastic polyhydroxyethers has a resin solids content of from about 20% to about 30%, and preferably do not require a step of volatilization of strong organic solvents, as is standard practice with known water-borne phenoxy resin dispersions. The water-borne thermoplastic polyhydroxyether resins having narrow polydispersity may be used in coating compositions without separate steps of concentration to increase solids content and remove volatile strong organic solvents. 
     An aminoplast resin may be added to the water-borne thermoplastic polyhydroxyether resin dispersion to facilitate crosslinking, or the dispersions may be coalesced by heating, as is known in the art. Suitable crosslinking agents include water soluble or water dispersible melamine, benzoguanamine, acetoguanamine, and urea resins such as ureaformaldehyde. 
     The coating compositions of the present invention may be applied to substrates by any conventional method, such as spraying, rolling, dipping, and the like. 
    
    
     The invention is further described in the Examples that follow. All parts and percentages are by weight unless otherwise specified. 
     EXAMPLES 
     Example 1 
     Preparation of a Conventional Acid Modified Thermoplastic Polyhydroxyether Resin 
     Into a 500 ml three-necked, round-bottom flask, equipped with a mechanical stirrer, a condenser, a thermometer, and a nitrogen inlet and outlet, there is charged 58.4 g of Bakelite® phenoxy resin PKHH (a thermoplastic polyhydroxyether resin now available from Phenoxy Associates as PAPHEN® PKHH resin, M w  =56,000, M n  =11,000 and M w  /M n  =5.1), 233.6 g of methyl ethyl ketone, 9.2 g of methyl methacrylate, 10.1 g of methacrylic acid, 0.92 g of ethyl acrylate, and 1.2 g of benzoyl peroxide dissolved in 20 ml of methyl ethyl ketone. The system is purged with nitrogen for 30 minutes. Thereafter, the slightly opaque reaction mixture is heated to 80° C. and refluxed for a period of two hours resulting in a colorless and translucent solution. 
     Example 2 
     Preparation of a Conventional Water-borne Thermoplastic Polyhydroxyether Resin Dispersion 
     A mixture containing 250 g of the acid-modified thermoplastic polyhydroxyether resin of Example 1 and 63.2 g of n-butanol are mixed in a Waring blender until the temperature reaches 40° C. Thereafter an aqueous solution composed of 10 g dimethylethanolamine and 250 g of water is added under vigorous mixing until the mixture&#39;s temperature reaches 50° C. A uniform, fine dispersion exhibiting a fairly strong Tyndall scattering effect is obtained. The dispersion exhibits a Brookfield viscosity of 28 cps and a pH of 8.9 at 26.2° C. Total solids of the water-borne phenoxy resin dispersion is 12.5%. 
     Example 3 
     Preparation of Conventional Water-borne Thermoplastic Polyhydroxyether Resin Coating Composition 
     Four hundred grams of the water-borne thermoplastic polyhydroxyether resin dispersion prepared in Example 2 is concentrated under vacuum for ten (10) hours. The concentrated product has a resin content of about 30 percent modified phenoxy resin solids. 
     The above-described methods illustrate that three separate steps are required to produce a conventional water-borne phenoxy resin coating composition. Prior to the present invention, it has been necessary to volatilize the thermoplastic polyhydroxyether resin dispersion in order to remove volatile strong organic solvents and to obtain the desired resin solids content for a suitable coating composition. 
     The following example illustrates the method of the present invention using thermoplastic polyhydroxyether resin starting materials having narrow polydispersity as described above. 
     Example 4 
     Method for Preparation of Water-borne Coating Compositions Using Thermoplastic Polyhydroxyether Resins Having Narrow Polydispersity 
     Into a 1,000 ml three-necked, round-bottom flask, equipped with a mechanical stirrer, a condenser, a thermometer, and a nitrogen inlet and outlet, there is charged 260 g of a narrow polydispersity thermoplastic polyhydroxyether resin (M w  =27,000-33,000, M n  =11,000 and M w  /M n  =2.6-3.3) and 60 g of butyl Cellosolve (a glycol ether solvent produced by Union Carbide Corporation), 48 g of methacrylic acid, 33 g of ethyl acrylate, 30 g of styrene, and 7.5 g of benzoyl peroxide dissolved in 85 g of butanol. The system is purged with nitrogen for 30 minutes. Thereafter, the reaction mixture is heated to 120° C. and refluxed for a period of two hours. After completion of the grafting reaction, 861 g of water and 42.8 g of dimethylethanolamine are added to the stirred mass at 75° C. The resulting fine particle dispersion has a Brookfield viscosity of 3500 cps and a phenoxy resin solids content of 21-24%. No volatilization of the resulting dispersion is required, and the dispersion may be used as a thermoplastic coating composition which can be coalesced by heating or crosslinked by the addition of a suitable crosslinking agent and appropriate processing known in the art. 
     Example 5 
     Comparison of Coatings 
     A water dispersion of the preferred narrow polydispersity thermoplastic polyhydroxyether resin of Example 4 is mixed with two (2) parts per hundred of resin solids (PHR) of Cymel 370, a melamine crosslinking agent (available from Cytech Corp.), and coated onto an aluminum test panel using a conventional &#34;drawdown&#34; bar method, to give a dry film thickness of 0.25 mils after baking at 190° C. for 5 minutes. 
     For comparison, the conventional thermoplastic polyhydroxyether resin of Example 1 (prepared from PKHH, Mw=56,000, Mn=11,000 and Mw/Mn=5.1), is made into a water-borne resin dispersion using a method as described in Example 2. The water-borne resin dispersion is then concentrated as described in Example 3, to obtain a water-borne resin coating composition, which is subsequently mixed with a crosslinking agent, coated and baked as described above. 
     In addition, a conventional waterborne epoxy resin, EPON 1007, a diglycidyl ether of bisphenol A, available from Shell Oil Co., (M w  &lt;20000, M n  &lt;4000 and M w  /M n  =˜5) is similarly mixed, coated and baked. The resulting panels prepared from all three coating compositions were evaluated by ASTM D-4145--&#34;Zero T bend Test&#34; and the following results were obtained: 
     
         ______________________________________                                    
       Narrow                                                             
  Polydispersity Conventional                                             
  Polyhydroxyether Polyhydroxyether Epoxy                                 
______________________________________                                    
Zero T-  pass          pass        fail                                   
  bend Test                                                               
______________________________________                                    
 
    
     The method of the present invention produces lower weight average molecular weight thermoplastic polyhydroxyether resin coating compositions that have toughness equivalent to conventional higher molecular weight thermoplastic polyhydroxyether resin coating compositions and improved toughness over relatively brittle waterborne epoxy resin coating compositions. 
     Example 6 
     Polydispersity Index 
     This example demonstrates the measurement of number average molecular weight, weight average molecular weight and polydispersity for polyhydroxyether resins in accordance with the present invention and various commercially available polyhydroxyether resins. The polyhydroxyether resins used are identified below. 
     
         ______________________________________                                    
Polyhydroxyether Source                                                   
______________________________________                                    
PKHA                                                          Bakelite    
                 ® Phenoxy Resin                                      
                           PKHA, Union Carbide                            
                                Corporation, Danbury, CT                  
  EPONOL                       Eponol 55-BH30                             
                               Shell Chemical, Houston, TX                
  PKHH                         PAPHEN ® PKHH                          
                              Phenoxy Associates, Rock Hill,              
                             SC                                           
  CG 488-V32                    488-V32 Ciba Geigy,                       
                            Brewster, NY                                  
  PKHB                         Phenoxy Resin PKHB                         
                           Phenoxy Associates, Rock Hill,                 
                          SC.                                             
______________________________________                                    
 
    
     The apparatus used to determine the molecular weights included the following components available from Waters Company, Marlboro, Mass. 
     A. Analytical balance 
     B. Liquid Chromatography Pump capable of delivering a constant 1.0 millimeters a minute at pressures ranging from zero to 1500 psi. (Waters model 610) 
     C. Sample injector capable of injecting 200 microliters of sample solution into the system. (Waters Autosampler Model 717) 
     D. Differential Refractometer detector (Waters Model 410) 
     E. Gel Permeation Columns capable of separating a molecular weight range of 100 to 4,000,000. (A series of Waters Ultrastyragel columns with pore sizes of 100 Angstrom (&#34;A&#34;), 500 A, 1000 A, 10000A, and 100000 A.) 
     F. A data system capable of molecular weight distribution and area percent calculations. (Waters Millennium software) 
     G. A solvent filtration/degassing system to filter and degas tetrahydrofuran. (Kontes Ultra ware-4 valve) 
     H. 0.45 micron Nylon 66 filter membranes for the solvent filtration/degassing system. 
     I. Disposable 0.45 micron filters used to filter the sample solutions. (arcodisc CR PTFE 0.45 micron) 
     The procedure used to measure the molecular weight was as follows: 
     A solution of each polyhydroxyether to be tested was prepared by dissolving in tetrahydrofuran (reagent grade) (&#34;THF&#34;) at 0.5 grams(&#34;g&#34;) per 100 milliliter (&#34;ml&#34;). 
     The apparatus was calibrated using polystyrene with eight narrow distribution standards with a range of 456 to 706,000 molecular weight, i.e., Waters polystyrene standards 456, 2630, 5970, 9100, 18100, 37,900, 96,400, 190,000, 355,000 and 706,000 weight average molecular weight, available from Wates Company. 
     The operating conditions were as follows: The mobile phase and flow rate (THF) was 1.0 mls per minute. The run time was 60 minutes. The sample size was 200 microliters and a standard injection was used. The detector was maintained at 40° C. The columns and injector were maintained at ambient temperature. 
     Further details concerning the apparatus and procedure are known to those skilled in the art 
     The M w  and M n  were calculated from the areas under the chromatography curves by known statistical calculations. 
     The M n , M w  and polydispersity for the polyhydroxyethers identified above are shown below. 
     
         ______________________________________                                    
POLYHYDROXYETHER                                                          
               M.sub.w M.sub.n  POLYDISPERSITY                            
______________________________________                                    
.sup.1 PKHA    44441    9514    4.67                                      
  EPONOL               53338        11384          4.68                   
  PKHH                 48884        11076          4.41                   
  CG 488-V32            55814         4201           13.29                
  PKHB              31080        10409          2.99                      
______________________________________                                    
 .sup.1 Tested with a similar apparatus and procedure to that described   
 above.                                                                   
 
    
     This example demonstrates that all of the conventional commercially available polyhydroxyethers tested have a polydispersity in the range of 4.41 to 13.29. Typical commercial processes for the manufacture of water-borne dispersions of such thermoplastic polyhydroxyethers typically require huge excesses of organic solvents, e.g., a four fold excess, to dissolve the conventional polyhydroxyethers. This typically results in a solids content of less than 10% in the water-borne dispersion. In order to increase the solids content to 20% to 30%, which is commercially desirable, it is often necessary to remove and reclaim the solvent which can be deleterious to the environment and expensive. In contrast, it is now possible, using the polyhydroxyethers of the present invention which have a polydispersity of less than 4.0, to manufacture water-borne thermoplastic polyhydroxyethers using significantly less solvent than with the conventional polyhydroxyethers. It is believed that the polyhydroxyethers of the present invention, e.g., PKHB, are substantially more readily dissolvable in organic solvents than polyhydroxyethers having a higher polydispersity. As a result of utilizing less solvent, water-borne dispersions can be made in accordance with the present invention which have a solids content of from 20% to 30% without requiring separate steps of concentration to increase solids content or remove volatile organic solvents.