Abstract:
A method of producing a high solids adhesive is provided wherein a buffered latex emulsion, such as, for example, styrene butadiene latex emulsion buffered to a pH of 11.0 using caustic agents, is combined with a non-emulsified mixture of dissolving process oils and tackifying resins and mineral fillers. The resultant adhesive has a solids content greater than previously achieved in the industry and has exceptional adhesive characteristics such as increased bond strength, improved durability, better aging, faster drying, improved water resistance and improved freeze resistance. High solids content is achieved by taking advantage of the excess emulsifiers in the latex emulsion and adding the remaining constituent components directly to the stabilized latex emulsion.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to water-based adhesives commonly used for applying floorcoverings and flooring materials, and for use in building construction, etc., and in particular to water-based adhesives having high solids content, low volatile organic compounds content and improved durability and bonding strength. 
     BACKGROUND OF THE INVENTION 
     Common adhesives intended for use in building construction and with flooring and floorcovering materials, such as, for example, wood plank, parquet, artificial turf, boat or other marine carpets, indoor carpet, rubber flooring, cove base, etc., are water-based emulsions based upon either acrylic or styrene butadiene (SBR) latex. Acrylic-based formulas, due to their expense and low bond strength, are generally reserved for use only when necessary for a pressure-sensitive applications, for resistance to plasticizers in pure vinyl backings or for resistance to sunlight in outdoor applications. SBR-based adhesives for use in building construction, flooring and floorcovering are, by far, the predominant type and require the addition of a tackifier to develop bond strength and a proper modulus. 
     The traditional means of producing an adhesive of this type is to emulsify the SBR latex to form a latex emulsion. Additional constituent components, such as, for example, process oil, tackifying resins, mineral fillers, etc., are also emulsified. The latex emulsion is then added to the constituent component emulsion under various heat and pressure conditions to form the adhesive. 
     The adhesives produced by the traditional method have several associated disadvantages, such as, for example, poor water resistance, slow drying, high volatile organic compound content and limitations on the final solids content of the adhesive. These disadvantages are generally due to the use of emulsifiers and thickeners in forming the adhesive. Emulsifiers and thickeners are generally water-soluble compounds that remain in the dried adhesive and tend to adversely affect the water resistance and durability of the resultant adhesive bond. Water soluble compounds also tend to retard the drying rate of the water-based adhesive. Moreover, using a method in which all constituent components are emulsified prior to mixing severely limits the solids content of the final product. It is particularly desirable to produce adhesives having a high solids content due to their superior bonding and drying characteristics. 
     It is also desirable to eliminate the presence of volatile organic compounds (VOC) generally present in traditional adhesives of the type described above VOCs are generally used to dissolve the tackifying resins which are incorporated into the adhesive to develop bond strength and an appropriate modulus. Since these resins are generally solid at room temperature, they must be liquified in order to be incorporated into the constituent component emulsion. This is usually done by dissolving the resin in an organic solvent carrier. Organic solvents are usually quite volatile and may have adverse effects on the environment due to their contribution to air pollution. VOCs also degrade the performance of the resulting adhesive by slowing down curing time, swelling and weakening the rubber component, shortening the life of the applied product and raising the level of emulsifier required for emulsion stability, which, in turn further degrades the adhesive. The use of organic solvents to produce adhesives, in some cases, also requires the use of expensive solvent recovery equipment required by government regulation and has the further disadvantage of exposing employees to the potentially harmful organic solvents. 
     VOCs are also incorporated into latex-based adhesives to improve their freeze resistance. Because traditionally formulated SBR-based adhesives have a high water content, they usually have poor freeze resistance. Manufacturers have sought to overcome the poor freeze resistance by introducing VOCs such as methanol and glycol to lower the freezing point of SBR-based adhesives. However, the addition of these VOCs results in many of the same disadvantages set forth above. In order to overcome the disadvantages associated with the use of VOCs for improved freeze resistance, some manufacturers have attempted to substitute non-volatile compounds such as sorbitol to improve freeze resistance. While freeze resistance may be improved in this manner, the water resistance of adhesives using non-volatile water-soluble compounds such as sorbitol is adversely affected. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of this invention to provide an adhesive having a high solids content. 
     It is a further object of the present invention to provide an adhesive having improved bond strength and stability. 
     It is another object of the present invention to provide an adhesive containing little or no volatile organic compounds. 
     It is yet another object of the present invention to provide an adhesive that produces a bond having improved water resistance and durability. 
     It is still another object of the present invention to provide an adhesive having improved drying rates and faster application characteristics over prior art water-based adhesives. 
     Another object of the present invention is to provide a freeze-stable SBR-based adhesive having no VOCs and improved water resistance over adhesives containing non-volatile freeze resisting compound additives. 
     In order to realize these and other objects and to overcome the shortcomings set forth above with respect to conventional adhesives, a process for formulating an adhesive includes the steps of: stabilizing an emulsion of latex by adding a caustic agent to the latex emulsion to raise the pH of the emulsion; forming a mixture of dissolving oils, such as, for example naphthenic rubber process oil, and tackifying resins; mixing the stabilized latex emulsion with the mixture of dissolving oils and tackifying resins using vigorous agitation until a homogeneous emulsion is formed; and adding a predetermined amount of mineral filler and water to the homogeneous emulsion to form the adhesive. 
     Forming the adhesive in the above described manner provides an adhesive having very high solids content and improved durability, drying, bonding and water resistance. The process involves dissolving oils, tackifying resins and mineral fillers directly into a latex emulsion without emulsifying or dispersing the oils, resins or minerals prior to mixing. The method takes advantage of the excess emulsifier and stability of the latex emulsion combined with the mutual solubility of the latex polymer, oil and resin to produce a stable high solids viscous emulsion without the use of additional emulsifiers or thickening agents. 
     The present invention also eliminates the presence of volatile organic compounds. This is made possible by dissolving the resin directly into the oil without first using an organic solvent. In addition, further use of volatile organics is not required for increasing freeze resistance due to the low water content of the adhesive thus produced. 
     In addition, it is to be understood that the invention is not limited to any particular latex emulsion. While styrene butadiene latex is preferred, other latex emulsions of, for example, acrylic, nitrile, neoprene, vinyl acetate, ethylene vinyl acetate copolymers, carboxylated SBR, etc. or blends thereof, may be used depending upon the type of material being used and the substrate to which the material is being bonded. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to increase the solids content of conventional latex-based adhesives, the present invention takes advantage of the excess emulsifier and stability of a stabilized latex emulsion combined with the mutual solubility of latex polymer, process oil and resin to produce a stable high solids viscous emulsion without the use of additional emulsifiers or thickening agents. Additional emulsifiers and thickening agents contribute to degraded adhesive characteristics and a decrease in solids content of the resultant adhesive. In this process, dissolved oils and tackifying resins are introduced directly into a stabilized latex emulsion to form a high solids adhesive having improved adhesive characteristics and freeze resistance. 
     The process begins with stabilization of a commercially available latex emulsion, such as, for example, Intex 131 and 132 available from Enichem America, and LPF 5356, LPF 6733, LPF 6687 all available from Goodyear, Butafan 125 and 104 available from BASF Corporation, Polytex 425 available from Rhom and Haas Corp. While SBR latex emulsions are specified herein, it is understood that various latex emulsions, such as, for example, acrylic homopolymers or copolymers, acrylonitrile butadiene (NBR), polychloroprene (neoprene), vinyl acetates, ethylene vinyl acetate copolymers, carboxylated SBR, etc., or blends thereof, may be used depending upon the materials being applied, the substrate to which the materials are bonded and the environmental conditions in which the bond will be present. The total solids of the preferred SBR latex emulsion is preferably in the range of 68-70% total solids. However, it is understood that the invention is not limited to a SBR latex emulsion having 68-70% total solids. 
     The latex emulsion must then be stabilized in order to allow the direct addition of constituent components without using additional emulsifiers or thickeners. To stabilize the latex emulsion, the latex emulsion is buffered to a pH in the range of 11.0 by a basic pH material, such as, for example, caustic soda, caustic potash, ammoniated tallow, dimethyl amine, diethyl amine, ammonia, dimethyl amino ethanol, urea, diethanol amine, triethanol amine, morpholine, etc. By buffering the latex emulsion in this manner, the emulsion is made stable and is ready to be mixed with the additional constituent components to form a stable high solids adhesive. The choice of buffering agent depends upon the latex emulsion or combination of emulsions used. 
     In a separate environment, tackifying resins, materials that are incorporated to increase bond strength and the modulus of the resulting adhesive, are dissolved in known non-volatile carrier liquids, such as, for example, naphthenic process oil. Commonly used tackifying resins include C9 hydrocarbon resin, tree rosin, ester of rosin having softening points in the range of 25°-115° C. etc. The preferred tackifier is pentaerythritol ester of rosin, commercially available from Hercules Inc. under the trade name Permalyn® 3100. While C9 hydrocarbon resin, tree rosin, and esters of rosin are specified, other resins may be used so long as they are compatible with the selected latex. Commonly used process oils include hydrotreated petroleum hydrocarbon oils having a carbon number mainly in the range of C20 to C50 and with few enough aromatics and paraffinics to be classified as naphthenic oil. Exemplary oils of this type include Calsol 8240 available from Calumet Oil Co., Circosol 4240 available from R. E. Carroll, Process Oil C-255-E available from C. E. Hall, etc. However, it is to be understood that while naphthenic process oil has been specified, other process oils that are not necessarily naphthenic may be used. For example, aromatic oil consisting of predominantly aromatic hydrocarbons having carbon numbers predominantly in the range of C20 to C50 may also be used. 
     The process oil and tackifying resin selected are mixed together and are preferably mixed at or above the melting point of the tackifying resin to ensure proper mixing. The tackifying resins used have melting points generally in the range of 25°-115° C. In the preferred method of the invention, the naphthenic process oil and tackifying resin are mixed together at a temperature of 210° F. to form a tackifying resin mixture to be added to the stabilized latex emulsion. It should be noted that tackifying resins will usually dissolve in the process oil without the benefit of additional heat; however, heating the mixture improves and enhances the dissolution of the resin and homogeneity of the resulting mixture. 
     In order to avoid brittleness in the final adhesive product, an antioxidant may be added to the process oil/tackifying resin mixture. Antioxidants of the hindered phenol type, such as, for example, Wingstay L available from Goodyear, or Santowhite Powder available from Harwick, are preferred. However, any compatible antioxidant may be used. Only trace amounts of antioxidant are required to avoid unacceptable brittleness in the final adhesive product. 
     Once the tackifying resin mixture is formed, it is ready for direct dissolution into the stabilized latex emulsion. The tackifying resin mixture is added to the stabilized latex emulsion under vigorous agitation until a homogeneous emulsion is formed. During agitation, trace amounts of defoamer, such as, for example, Foamkill 600 Series available from Crucible Chemical, Foammizer M-55 available from C. P. Hall, Nalco 5770 and 5772 available from Nalco Chemical, etc., and bactericide, such as, for example, Amerstat 251 available from Drew Chemical, may be added to reduce foaming and microbial contamination. It has been found that approximately 20 minutes are required to achieve a sufficiently homogeneous emulsion. However, different agitation times may be required depending upon the starting materials selected. Mixing these components in this manner eliminates the conventional step of emulsifying the tackifying materials prior to mixing with the latex emulsion. The present invention takes advantage of the excess emulsifiers present in the latex emulsion to enable the tackifying mixture to be added directly to the stabilized latex emulsion. The direct addition of the tackifying mixture to the stabilized latex emulsion is critical in achieving a high solids adhesive product. 
     After the homogeneous emulsion of tackifying material and latex is formed, mineral fillers, in the form of finely ground powder, are added slowly with vigorous agitation to thicken the mixture. Mineral fillers are also used to control the amount of tack and viscosity of the final adhesive product. Commercially known fillers, such as, for example, kaolin clay available from Georgia Kaolin, calcium carbonate available under the trade name #9 Whiting available from Georgia Marble Corp., feldspar available under the trade name G-Fill from Feldspar Corp., etc., are preferable. It should be noted, however, that any compatible mineral filler may be used. During addition of the mineral filler, water, or a slurry of water and mineral filler, may be added to ensure proper mixing of the filler with the homogeneous emulsion of tackifying resin and latex. 
    
    
     In a preferred embodiment of the present invention, the following proportions of the constituent components of the high solids adhesive are used. It is to be understood, however, that many variations in the amounts of the various constituent components may be made and that the following example is to be considered illustrative, not limiting. 
     
         ______________________________________Ingredient               Amount wt %______________________________________1.  Cold polymerized high solids SBR                        29.0    latex (68-70% total solids) buffered    to 11.0 pH with suitable caustic agent2.  a. Naphthenic rubber process oil                        14.0    b. Tackifying Resin (25-115° C. melting                        14.0point)    c. Antioxidant (hindered Phenol type)                        1.0    Mix 2a, 2b, and 2c together at 210° F.    Mix 1 and 2 together with vigorous agita-    tion until emulsion is homogeneous.    Defoamer and bactericide/antimicrobial    agent may be added to reduce foaming and    contamination. Then blend in the    following ingredients in turn, blending    thoroughly before proceeding to the next    step.3.  Mineral filler, added slowly with                        30.0    vigorous agitation4.  Water to desired precalculated                        3.5    final solids. For approximately    85% total solids add this amount5.  Mineral filler, as in step 3.                        8.5______________________________________ 
    
     Constituents 4 and 5 may be in the form of a slurry of washed clay in water, such as, for example, Huber 90 clay available from Huber, mixed in an aqueous slurry with the appropriate amount of water. Using the above described constituent components results in a high solids adhesive having approximately 85% solids content, the highest known of any multi-purpose adhesive in the industry. Previously, the highest known available solids content adhesives had solid contents in the range of 70-71% solids. While solids contents in the range of 84-87% solids are preferred, any range of solids content may be achieved by the present invention. For example, if solids content of greater than 87% or less than 85% are required, the water and mineral filler amounts can be accordingly adjusted to arrive at the desired solids level. Thus, the invention is not limited to solids percentages in the range of 84-87%, but can be in any range up to approximately 95%+ solids content. 
     The viscosity of the adhesive resulting from employing the process of the present invention is in the neighborhood of 100,000 centipoise (cps). The viscosity of the resultant adhesive may be adjusted as required in a particular application by modifying the amounts of water and mineral fillers added to the homogeneous emulsion. Common viscosity ranges for floorcovering adhesives are in the range of 30,000 to 150,000 cps. Using the present invention will allow one skilled in the art to adjust the constituent component amounts to achieve viscosities in most any desired range for adhesives of this type. 
     Other illustrative examples of high solids adhesives achievable by the present invention are shown below: 
     
         ______________________________________Ingredient               Amount wt %______________________________________1.  Latex blend of: Butafan 104                        25.0    and Polytex 425. Raise pH                        5.0    of blend to 11.0 using suit-    able caustic agent2.  a. Ergon V 2000 oil      15.0    b. Methyl Ester of Rosin 13.0    c. Santo White Powder    .5    Blend 2a, 2b, and 2c together, warm to    210° F. and add 1 to 2 with vigorous agita-    tion. Continue agitation until a stable    emulsion is formed (about 20 min) and    then add each of the following blends    into the mix in order.3.  Lithospar P (Spartan Minerals Div. of                        29.0    Kmg Minerals, Inc.)4.  Water                    3.05.  Mica finely ground       9.5______________________________________ 
    
     The adhesive resulting from the mixture shown in Example 2 has a viscosity of 110,000 cps and a total solids content of 87.1%. The adhesive of Example 2 also has no VOCs and bond strength in the range of 11-16 lb/in. 
     
         ______________________________________Ingredient              Amount wt %______________________________________1.  Intex 132 Latex (pH 11.1)                       24.02.  a. Calsol 8240 oil      12.0    b. AP-10 Resin (Hercules Corp.)                       12.0    c. Wingstay L Antioxidant                       .25    Blend 2a, 2b and 2c together at 200° F.    Add mixture to 1 with agitation - agitate    15 minutes.3.  Kaolin Clay - add with agitation                       6.04.  #10 Whiting (CaCo.sub.3) - add with                       24.0    agitation5.  Water - agitate         9.756.  #10 Whiting - agitate   12.0______________________________________ 
    
     The resultant adhesive of Example 3 is a viscous paste having 80+% solids, fast drying characteristics and bond strengths that exceed the tear strengths of most carpets. 
     Adhesives made by the present invention are excellent adhesives for indoor or outdoor installation of flooring materials, such as, for example, wood plank or parquet, artificial turf, boat or other marine carpets, all indoor carpet, rubber flooring or cove base. 
     Additionally, lower solids content adhesives may also be produced using the method of the present invention. Lower solids adhesives are generally used in average installations involving carpets with woven synthetic, jute or low density urethane backings. However, it must be noted that lower solids content adhesives do not exhibit the increased bond strength, durability, freeze resistance, water resistance and improved drying characteristics of high solids content adhesives described above. 
     While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined herein and in the following claims.