Abstract:
A polyacrylic polymer is provided that is formed by moisture cure of multiple acrylic prepolymer segments. The prepolymer segments each have an average molecular weight of between 5000 and 400,000 grams per mole. Each of the prepolymers is a homopolymer or copolymer of a monomer from among methyl methacrylate, C 1 -C 16  alkyl acrylate, and C 1 -C 16  alkyl methyl methacrylate as the predominant monomeric group. The prepolymers are interconnected through moisture cure induced urea, urethane, or combinations thereof. The polymer is readily cured upon exposure to atmospheric moisture absent inclusion or evolution of a volatile organic compound. The polymer is likewise formed HAP free. Through control of prepolymer properties a variety of thermoset polymer properties are obtained. A prepolymer is formed including a minority amount of functionalized acrylic monomer that is isocyanate or converted to isocyanate through subsequent reaction. Functionalized acrylic monomers operative herein include a hydroxyl, a primary amine, a secondary amine, a sulfonic acid, or an epoxy. Upon application from storage in an anhydrous state, the prepolymer cures upon exposure to atmospheric moisture.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 60/979,115 filed Oct. 11, 2007, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention in general relates to a polymer curable upon exposure to atmospheric moisture and having elastomeric properties and in particular to such a polymer inclusive of an acrylic component. 
       BACKGROUND OF THE INVENTION 
       [0003]    The control of polymer properties is a complex series of material tradeoffs. Invariably when a given polymer is unable to deliver desired characteristics, modifications are made to molecular weight, degree of branching, copolymer blocks, and the like in an attempt to attain the desired properties. Should these efforts fail, a new base monomer is chosen and the process repeated. 
         [0004]    Polyurethane polymers are moisture curable and have many advantages such as attractive adhesion and elastomeric properties, recovery, adhesion to concrete, and negligible shrinkage. The disadvantages of a polyurethane system are a short storage life if made to cure faster, the susceptibility to hydrolysis detracts from the weatherability, discoloration, and the often requirement of a primer. 
         [0005]    Silicon polymers are moisture curable and have great weatherability and the polymer itself has great torsional rotation giving it great flexibility. Some disadvantages of this system are a short shelf life of maybe six months, and the polymer cannot stick to itself due to low surface energy. This makes reapplication of the polymer difficult. 
         [0006]    Polysulfide polymers are moisture curable and are characterized by toughness and durability. Some disadvantages of polysulfide polymers are the disagreeable smell and slow curing speed. Silyated polyurethane polymers have been made by grafting a silane onto a polyether polyol that has been terminated by an isocyanate. These “hybrid” polymers display good adhesion to many substrates and good weatherability. A disadvantage is the cost is high. 
         [0007]    Acrylic polymers are known for their cost effectiveness, clarity in nonpigmented products, and UV resistance. Acrylics are not known to be moisture curable and most are thermoplastic resins. Some have made an acrylic sealant moisture curable by adding an acrylic polymer terminated by a silane to create a material for back bedding applications of windows, this sealant has limited strength and flexibility and is expensive to produce. 
         [0008]    With the general properties of acrylics being attractive, there exists a need for a class of moisture curable acrylics that cross link to provide a range of properties. There further exists a need for acrylic based elastomers that retain the LV stability, optical transparency, wear and physical properties of traditional thermoplastic acrylics. 
       SUMMARY OF THE INVENTION 
       [0009]    A polyacrylic polymer is provided that is formed by moisture cure of multiple acrylic prepolymer segments. The prepolymer segments each have an average molecular weight of between 3,000 and 600,000 grams per mole. Each of the prepolymers is a homopolymer or copolymer of a monomer from among methyl methacrylate, C 1 -C 16  alkyl acrylate, and C 1 -C 16  alkyl methyl methacrylate as the predominant monomeric group. The prepolymers are interconnected through moisture cure induced urea, urethane, or combinations thereof. The polymer is readily cured upon exposure to atmospheric moisture absent inclusion or evolution of a volatile organic compound. The polymer is likewise formed HAP free. Through control of prepolymer properties a variety of thermoset polymer properties are obtained. 
         [0010]    A prepolymer is formed including a minority amount of functionalized acrylic monomer that is isocyanate or converted to isocyanate through subsequent reaction. Functionalized acrylic monomers operative herein include a hydroxyl, a primary amine, a secondary amine, a sulfonic acid, or an epoxy. Upon application from storage in an anhydrous state, the prepolymer cures upon exposure to atmospheric moisture. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    The present invention has utility as a moisture curable polyacrylic composition that is substantially volatile organic compound (VOC) free. An inventive polyacrylic composition is readily adjustable to a variety of properties illustratively including cure rate, glass transition temperature (Tg), linear percent elongation and tensile strength through the choice of monomers, prepolymer chain length, and prepolymer degree of functionality. An inventive composition is also rendered substantially free of hazardous air pollutants (HAP). 
         [0012]    As used herein, “substantially VOC free” is defined as satisfying regulatory restrictions to a VOC content of less than 1% total weight percent thereby rendering the inventive composition exempt from VOC control regulations. 
         [0013]    As used herein, “substantially HAP free” is defined as satisfying regulatory restrictions to a HAP content of less than 3% total weight percent thereby rendering the inventive composition exempt from HAP control regulations. 
         [0014]    According to the present invention, an inventive prepolymer is formed by the copolymerization of a functional moiety containing acrylic monomer and an acrylic monomer lacking the reactive functional moiety (synonymously described herein as being “nonreactive” or “nonfunctionalized”). A reactive moiety containing acrylic monomer according to the present invention includes hydroxy acrylates such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, and 3-hydroxypropyl acrylate; primary amine acrylates such as 1-hydroxy-2-aminopropyl methacrylate, 1-amino-2-hydroxypropyl methacrylate, acrylamide, and 3-hydroxypropyl methacrylate; secondary amine acrylic monomers such as 2-terbutyl amino ethyl methacrylate, sulfonic acids such as 2-acrylamido-2-methyl propane sulfonic acid; epoxy acrylic monomers such as glycidyl methacrylate; isocyanate acrylic monomers such as α,α-dimethyl meta-isopropenyl benzyl isocyanate. Acrylic monomers that are considered nonreactive according to the present invention are those that only include aliphatic moieties combined with acrylate or methacrylate moieties. Nonfunctionalized acrylic monomers according to the present invention illustratively include methylmethacrylate, C 1 -C 16  alkyl acrylate, and C 1 -C 16  alkyl methacrylate. 
         [0015]    According to the present invention, nonfunctionalized acrylic monomer is polymerized through a conventional free radical mechanism and preferably in the presence of a free radical catalyst to form a polyacrylic prepolymer, polymethacrylic prepolymer or copolymer prepolymer of multiple types of nonfunctionalized acrylic monomers. After polymerization has proceeded to a predetermined level typically to yield nonfunctionalized acrylic prepolymer segments having a molecular weight of between 3,000 and 600,000 grams per mole, and preferably between 5,000 and 400,000, then functionalized acrylic monomer is added and copolymerized to yield an acrylic prepolymer having terminal and/or pendent functional groups extending from the nonfunctionalized acrylic prepolymer backbone. Alternatively, a small percentage of a functionalized acrylic monomer typically between 0.01 and 3 mole percent relative to nonfunctionalized acrylic monomer are premixed and copolymerized by a conventional free radical mechanism and preferably in the presence of a free radical catalyst. The resultant acrylic prepolymer has functional moieties dispersed throughout the prepolymer segments. It is appreciated that regardless of the polymerization scheme, prepolymer molecular weight is readily controlled through the inclusion of a chain transfer agent such as a thiol. 
         [0016]    Free radical acrylate polymerization conditions are well known to the art and illustratively include charging of an autoclave with 100 parts by weight of various monomers along with 0.6 parts benzoyl peroxide as a free radical polymerization initiator, 0.5 parts dodecane thiol operative as a chain transfer agent, 70 parts by weight toluene and 30 parts by weight isopropyl alcohol. It is appreciated that by the use of a different chain transfer agent such as mercapto-ethanol not only controls prepolymer molecular weight, but also adds to the prepolymer functionality. Heating under autogenous pressure to 90° Celsius for 4 hours is suitable to induce polymerization. The resultant acrylic prepolymer solution is then optionally charged with functionalized acrylic monomer and further reacted to incorporate functionality into the resultant prepolymer. It is appreciated that polymerization conditions are readily monitored for unreacted monomer to determine polymerization completeness. 
         [0017]    Regardless of the specific functional moieties incorporated into an inventive acrylic prepolymer, the prepolymer is rendered moisture curable by subsequent reaction of the frictional moieties to isocyanate moieties (—NCO) to yield an inventive polymer with moisture cure induced urea or polyurethane linkages. The prepolymer is readily packaged and moisture cure from a VOC free, HAP free, or VOC and HAP free package upon extrusion and exposure to ambient moisture. 
         [0018]    An amine functional moiety is converted to an isocyanate functional moiety through reaction with phosgene by a conventional process. Preferably, subsequent conversion of amine functionality to isocyanate moieties preferably includes removal of byproduct hydrochloric acid. Likewise, a hydroxyl functional moiety is also readily directly reacted to yield an isocyanate functionality or through an intermediate reaction to an amine which in turn is basis for conversion to an isocyanate group through reaction with phosgene. 
         [0019]    An alternate cure mechanism for an inventive acrylic prepolymer is reaction of functional moieties of such a prepolymer with a polyisocyanate. As used herein, a polyisocyanate is defined to include a molecule having at least two isocyanate functionalities. Exemplary diisocyanates include toluene diisocyanate (TDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate (HDI), and methyldiphenyl diisocyanate. Inclusion of a triisocyanate or higher degree isocyanate is appreciated to be a simple way to add frictional isocyanate degree to an inventive prepolymer. 
         [0020]    The present invention is readily formed as a one part extrudable prepolymer curable through contact with atmospheric moisture, or alternatively is provided as a two part curable system. 
         [0021]    Additionally, a sulfonic functional moiety such as that obtained by incorporation of 2-acrylamido-2-methylpropane sulfonic acid into an inventive prepolymer is also amenable to a secondary reaction to yield isocyanate functionality in the prepolymer. Likewise, epoxy functionality present in an inventive prepolymer is readily reacted through a ring opening to yield a mixed hydroxyl and amine functionality subsequently reacted to yield isocyanate functionality. Epoxide ring opening to yield mixed hydroxyl and amine functionality is detailed in U.S. Pat. No. 2,949,445. 
         [0022]    The glass transition temperature of an inventive acrylic prepolymer is readily calculated based on a mole percentage weighted summation of the acrylic monomers from which it is derived as 
         [0000]    
       
         
           
             
               Tg 
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               prepolymer 
             
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                   f 
                   n 
                 
                  
                 
                   Tg 
                   n 
                 
               
             
           
         
       
     
         [0000]    where f is the molar fraction of monomer n and Tg n  is the glass transition temperature for monomer n, where Tg n  and Tg prepolymer are in degrees Kelvin. In one embodiment with the majority acrylic monomer being C 1 -C 16  alkyl acrylate with a lesser amount of methyl methacrylate and a 2-hydroxyl acrylate in amount of between 0.01 and 3 monomer mole percent functionalized monomer results in an inventive acrylic polymer having particularly attractive sealant properties. 
         [0023]    It is appreciated that the properties of an inventive moisture cured acrylic polymer are readily adjusted to suit a particular application. By way of example, molecular weight of prepolymer segments, degree of moisture curable functionality per prepolymer unit, prepolymer viscosity and the identity of the moisture cure bonds formed as being urea linkages, urethane linkages, or a combination thereof, as well as whether such linkages are terminal to prepolymers or pendent are all properties used to adjust the resultant acrylic polymer properties. As a result of the ability to tailor the resultant acrylic polymer properties, an inventive acrylic polymer composition is suitable for a variety of applications illustratively including sealants, adhesives, and coatings. Subsequent to cure, an inventive polymer has a moisture content typically between 0.005 and 0.1 total weight percent, and preferably between 0.02 and 0.05 total weight percent. 
         [0024]    The present invention is further detailed with respect to the following nonlimiting examples. These examples are intended to illustrate specific forms of the present invention and not intended to limit the interpreted scope of the appended claims. 
       EXAMPLE 1  
     Functionalized Moisture Curable Prepolymer Synthesis 
       [0025]    175 grams butyl acrylate (Tg-55° Celsius), 110 grams methyl acrylate (Tg 8° Celsius), 13 grams lauryl methacrylate (Tg-65° Celsius), 3 grams methyl methacrylate (Tg 105° Celsius) and 1 gram hydroxy ethylacrylate (Tg-15° Celsius) are placed in a mixing can with 1.5 grams of dodecane thiol and 0.3 gram of benzoyl peroxide and mechanically mixed for 1 minute. 100 grams of t-butyl acetate is placed in a round bottom flask and brought to reflux. The acrylate monomer mixture containing peroxide and thiol are added a dropwise over a period of 3 hours and the mixture heated above 110° Celsius with mechanical stirring. An additional 0.3 grams of benzoyl peroxide is added over a period of 15 minutes to the flask followed by a subsequent hour of temperature maintenance. Then an additional 0.1 gram of benzoyl peroxide is added over a period of 5 minutes and reaction temperature maintained for an additional hour. Thereafter, 30 additional grams of solvent is added and the reaction mixture cooled to 80° Celsius with nitrogen purge being initiated. An isocyanate(toluene diisocyanate) is added to provide a stoichiometric moiety equivalent of isocyanate groups for hydroxyl groups associated with hydroxyl ethyl acrylate plus thiol moieties present in the mixture plus the amount of moisture present in the mixture. According to the synthesis, 0.016 moles of hydroxyl and thiol groups are present. The moisture content of the mixture is readily determined through Karl Fischer titration. After combining the isocyanate with the mixture and mixing for an additional 10 minutes, an dioctyl tin carboxylate catalyst is added in an amount of 0.4 grams with continued mixing for an additional hour. The resulting isocyanate terminated prepolymer has a mean molecular weight of 100,000 grams per mole and a glass transition temperature of −35° Celsius. Upon exposure to ambient moisture, the resultant isocyanate terminated prepolymer moisture cures to a polyacrylic containing urethane linkages. 
         [0026]    It is appreciated that the properties of the resultant moisture cured polyacrylic are controlled by adjusting mole fraction weighted glass transition temperature of the various monomers, the quantity of chain extender (dodecane thioe, the nature of functional monomers, and the functionality of a prepolymer. The prepolymer synthesized in this example is particularly well suited as a sealant having a linear elongation of between 500 and 1000 percent with good adhesion to glass and aluminum substrates. 
       EXAMPLE 2  
     Coating Type Medium Tg Polyacrylic Polymer Synthesis 
       [0027]    The procedure of Example 1 is repeated with replacement of butyl acrylate and lauryl methacrylate with quantities of methyl methacrylate and methyl acrylate, respectively, to yield 13% methyl acrylate and 33% methyl methacrylate by molar fraction of acrylic monomer, with a monomer percent weighted prepolymer glass transition temperature of 25° Celsius. The resultant prepolymer had a mean molecular weight of 36,000 and a mean functionality of 3. Upon moisture cure, the polyacrylic had a tensile strength of 147 pounds per linear inch and a linear percent elongation of 565% as measured by standard methods AAMA 1.1.3.3 805.2-92, AAMA 1.1.3.3 805.9-92, and ASTM D-412. The resultant polyacrylic is comparatively hard relative to that produced in Example 1 yet remains pourable. The high tensile and cohesive strengths of the resulting polyacrylic polymer formed from this prepolymer come at the expense relative to Example 1 polyacrylic of lacking adhesion to glass aluminum substrates. 
       EXAMPLE 3  
     High Glass Transition Temperature Inventive Polyacrylate 
       [0028]    The process of Example 1 is repeated with methacrylate accounting for 11 molar percent of the acrylate monomers present and 73 molar percent of the acrylic monomers present being methyl methacrylate with the remaining acrylate polymers decreased proportionally. The weighted monomer percent glass transition temperature of the resulting prepolymer is 50° Celsius. Upon moisture cure of the isocyanate groups to form a cross-linked polymer, a comparative low tack polymer with considerably higher toughness and hardness is prepared relative to the polymer produced according to Example 2. 
       EXAMPLE 4  
     Polymer Synthesis with Epoxidized Functional Acrylate 
       [0029]    The procedure of Example 1 is repeated with 96 mole percent of the acrylate monomer being methyl methacrylate and 3 percent of the acrylate monomer being glycidyl acrylate. After mixture cooling to 80° Celsius and the initiation of nitrogen purge, the resulting polymer incorporating epoxy groups is then diluted with an additional 30 grams terbutyl acetate, 3 grams distilled water and a stoichiometric excess of ethylamine relative to epoxy moieties present. The mixture is autoclaved at 90° Celsius for 48 hours to induce epoxide ring animation. The resulting adjacent hydroxyl and amine groups formed across the reactant epoxide are then reacted at 70° Celsius under nitrogen with isocyanate per Example 1 to convert at least half of the hydroxyl and/or amine groups present to isocyanate groups as per Example 1. The resultant polymer has a glass transition temperature, tack, and adhesion that are each within 15 percent above that obtained for the polyacrylic of Example 1. 
       EXAMPLE 5  
     Amine Functionalized Prepolymer 
       [0030]    The process of Example 1 is repeated with the substitution of an equivalent molar amount of 2-terbutyl amino ethyl methacrylate for hydroxy ethylacrylate. After cooling to 80° Celsius and initiating nitrogen purge, the resultant prepolymer and resulting amine functionalized prepolymer is reacted with phosgene in a sealed reaction vessel and further reacted to convert the majority of amine functionality in the prepolymer to isocyanate functionality with concurrent volatilization and neutralization of the evolved HCl. Upon moisture cure, the resulting polyacrylic has properties similar to those of the polymers of the preceding examples. 
         [0031]    Patent documents and publications mentioned in the specification are indicative of the levels of those skilled in the art to which the invention pertains. These documents and publications are incorporated herein by reference to the same extent as if each individual document or publication was specifically and individually incorporated herein by reference. 
         [0032]    The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a imitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.