Abstract:
Methods of surface modification of polymer particle are useful in the development of marking fluids. The surface modification includes saponifying one or more acrylic ester groups on a surface of the polymer particle.

Description:
BACKGROUND 
     In a typical inkjet recording or printing system, droplets of marking fluid, sometimes referred to as ink, are ejected from a nozzle, i.e., jetted, towards a recording medium to produce an image on the medium. The droplets generally include a colorant, such as one or more dyes or pigments, for marking the medium, and some aqueous or solvent-based carrier vehicle to facilitate controlled ejection of the marking fluid. While aqueous carrier vehicles are more environmentally friendly than solvent-based carrier vehicles, their colorants are usually more prone to smearing or durability concerns. 
     To improve the durability of aqueous marking fluids, polymer particles are often added to the marking fluid formulations. When printed as part of an inkjet ink, a polymer component of the ink can form a film on a media surface, entrapping and protecting the colorant within the hydrophobic print film. 
     For the reasons stated above, and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, alternative polymer particles for marking fluid formulations and other applications, as well as their methods of manufacture, are desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a method of forming polymer particles for use in marking fluids in accordance with an embodiment of the disclosure. 
         FIG. 2  is a depiction of a saponification reaction of a polymer particle in accordance with an embodiment of the disclosure. 
         FIG. 3  is a depiction of results of testing a marking fluid containing polymer particles in accordance with embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments of the disclosure which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the subject matter of the disclosure, and it is to be understood that other embodiments may be utilized and that process, chemical or mechanical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and equivalents thereof. 
     In order for the solid contents in a liquid marking fluid to jet and remain dispersed, a measurable balance of certain polar functional groups on the solid surfaces are typically required. Current water-based pigment ink technology utilizes traditional latex synthesis for polymer preparation, which offers many advantages to improve print quality. However, such synthetic methods are typically limiting the amount of polar monomers that can be incorporated for desired performance and product quality characteristics. Many acrylic and methacrylic monomers may be utilized to make functional latexes that could facilitate superior product quality, but such compositions are known to jet poorly due to an insufficient amount of polar functional groups. 
     The various embodiments modify the surface of the polymer particles to increase their surface polar group density to thereby facilitate an increase in the electrostatic repulsion of the otherwise unstable and non-jetting lattices to achieve thermal jetting. In particular, various embodiments utilize saponification to hydrolyze lattices containing embedded or pendent esters. Saponification is the hydrolysis of an ester under basic conditions to form an alcohol and the salt of a carboxylic acid. Embodiments described herein are applicable to any short-chain linear ester-containing latex formulations where beta-elimination is substantially absent, i.e., where the beta position is substituted, or the gamma or beta positions are protected by sterically hindered groups. Polymer particles containing a wide range of such esters, e.g., polymers containing 2-80% of linear short-chain esters, find use in the various embodiments. For some embodiments, the polymer particles contain an encapsulated colorant. 
     The various embodiments further include marking fluids containing polymer particles which have been surface modified in accordance with embodiments of the disclosure in an aqueous liquid vehicle. The marking fluid further contains one or more colorants, e.g., pigments or dyes, to impart color to the marking fluid. The marking fluid may further contain one or more surfactants, co-solvents, biocides and other components that affect shelf-life, performance or other characteristics of the marking fluid. 
     A typical reaction setup may involves refluxing preformed polymer in the presence of a nucleophilic base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), for 0.5 to 10 hours. The degree of hydrolysis will be dependent on the amount of base added as well as the length of reaction time. The pH of the resulting solution should generally be controlled to greater than or equal to 8 to facilitate dispersion stability. The final pH of the solution can be adjusted with additional base to obtain a particular pH for use in the marking fluid. The additional base may include a different base than that used for saponification. While a metallic alkali, such as NaOH or KOH, might be used for saponification, for example, a different base, such as ammonium hydroxide (NH 4 OH), could be used to adjust the final pH. 
     Polymers for use with various embodiments contain 2% or more by weight of an acrylic ester in the formulation. Monomers that provide spatial conformational flexibility generally promote film formation. Examples of these monomers could include n-butyl acrylate, 2-ethylhexyl-acrylate, hexylacrylate, and/or their methacrylate variation. In addition, monomers that provide film rigidity generally promote rub resistance. Examples of these monomers could include methacrylate, acrylonitrile, and styrene. Monomers that facilitate close range interactions such as hydrogen bonding and acid/base pairing can be present to control the desired print durability. Examples of these monomers could include acrylic acid, methacrylic acid, itaconic acid, fumaric acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, acrylamide, methacrylamide, N-methylol(meth)acrylamide, acrylamidoacrylic acid, acrylamidoethyl(or propyl) methacrylate, 4-vinylpyridinium halide, and any monomer that contains urethane, amide, carbamate, carboxylate, carbonate, pyrimidone, urea, and isothiourea. The use of these monomers may then be balanced by ester-containing monomers in order to modulate the glass transition temperature suitable for jetting and film forming. For some embodiments, the monomer compositions are chosen to provide a glass transition temperature of the resultant polymer of 70-95° C. Specific examples of such polymer formulations may include Sty/MMA/HEA/AAm (15:65:15:5); Acry/MMA/BA/AAm (15:65:15:5); and Sty/MMA/HEA/MAA (15:65:15:5), wherein Sty=styrene, MMA=methyl methacrylate, HEA=hydroxy-ethyl-acrylate, Mm=acrylamide, Acry=acrylonitrile, and BA=butyl acrylate. 
       FIG. 1  is a flowchart of a method of forming polymer particles for use in marking fluids in accordance with embodiments of the disclosure, including surface modification in accordance with embodiments of the disclosure. Polymer particles are formed at  110 . The polymer particles may be formed, for example, using emulsion polymerization techniques. The polymer particles contain at least 2 wt % of acrylic esters in their formulation. For some embodiments, the acrylic ester content is 2-80 wt %. For further embodiments, the polymer particles have a glass transition temperature of 70-95° C. The polymer particles are saponified at  120 , thereby converting the ester groups to salts. For one embodiment, the saponification is performed using sodium hydroxide as the base. For such an embodiment, the saponification replaces the —O-alkyl groups with —O—Na groups.  FIG. 2  depicts conceptually the saponification reaction of the ester groups of a polymer particle  250  in accordance with embodiments of the disclosure. The saponified polymer particles are then incorporated into a marking fluid at  130 . A marking fluid may contain, for example, an aqueous liquid vehicle, the polymer particles and one or more colorants. One or more colorants may be contained within the polymer particles. Alternatively, or in addition, one or more colorants may be included outside of the polymer particles. Furthermore, marking fluids may contain other components that do not materially affect the basic and novel properties of the compositions disclosed herein, such as surfactants, co-solvents, biocides, etc. 
     The following examples represent processes used to perform the surface modification of polymer particles in accordance with various embodiments of the disclosure. 
     EXAMPLE 1 
     12 mL of 1N potassium hydroxide solution and 600 mL of emulsion containing 28 wt. % of seed acrylic latex (styrene/methylmethacrylate/hexamethacrylate/acrylamide, 15:65:15:5) were allowed to mix thoroughly in a 1 L reactor, equipped with a condenser and a stirring mechanism. The pH of the solution was maintained above 10, monitored by pH meter. The reactor was then heated to an internal temperature of 80° C. for 5 hours, at which point the solution salinity had dropped to pH 8. The reaction mixture was allowed to cool to room temperature, and the final pH was adjusted by the addition of a base (i.e. KOH or NaOH), if necessary. The cooled emulsion was screened into a storage bottle for future formulation. 
     EXAMPLE 2 
     5 mL of 2M ammonium hydroxide solution and 600 mL of emulsion containing 25 wt. % of seed acrylic latex (acrylamide/methylmethacrylate/butylacrylate, 15:65:30) were allowed to mix thoroughly in a 1 L reactor, equipped with a condenser and a stirring mechanism. The pH of the solution was maintained above 10, monitored by pH meter. The reactor was then heated to an internal temperature of 80° C. for 5 hours, at which point the solution salinity had dropped to pH 8. The reaction mixture was allowed to cool to room temperature, and the final pH was adjusted by the addition of ammonium hydroxide, if necessary. The cooled emulsion was screen into a storage bottle for future formulation. 
     An example ink-jettable marking fluid was prepared by dispersing 6 wt % of polymer particles in accordance with an embodiment of the disclosure in a liquid vehicle. This liquid vehicle included 20 wt % organic co-solvent, 0.5 wt % surfactant, and 0.5 wt % biocide with the balance being water. The marking fluid also contained about 3% of pigment to impart color. The marking fluid was filled into inkjet pens and printed on coated paper media. The printed media was then subjected to various resistance testing, including a dry-rub test procedure and a window-cleaner test procedure. 
     The dry rub test was performed with a linear abraser (specifically a TABER Linear Abraser—Model 5750). The arm of the linear abraser stroked each media sample in a linear motion back and forth at a controlled stroke speed and length, the head of the linear abraser following the contours of the media samples. To the shaft of the arm of the linear abraser, a 250 gram weight was added to make the load constant. Specifically for the rub test, a stroking head or “wearaser” was attached to the end of the arm of the linear abraser. The stroking head was the size and shape of a typical pencil eraser and had a contact patch with a diameter of approximately ¼ inch diameter. The stroking head was abrasive (specifically CALIBRASE CS-10) with a mild to medium abrasive effect. The stroking head was stroked back and forth 10 times on each media sample. The rubbed media samples were judged for color fastness. 
     The window cleaner test was performed with a linear abraser (specifically a TABER Linear Abraser—Model 5750). The arm of the linear abraser stroked each media sample in a linear motion back and forth at a controlled stroke speed and length, the head of the linear abraser following the contours of the media samples. To the shaft of the arm of the linear abraser, a 250 gram weight was added to make the load constant. Specifically for the window cleaner test, an acrylic finger (specifically from a TABER Crock Meter Kit) covered by a cloth (specifically a TABER Crocking Cloth) was attached to the end of the arm of the linear abraser. WINDEX window cleaner was applied to the cloth and the cloth-covered end of the acrylic finger was stroked back and forth 5 times on each media sample. The rubbed media samples were judged for color fastness. 
       FIG. 3  is a depiction of one set of results of testing a marking fluid containing polymer particles in accordance with an embodiment of the disclosure. The rectangular region  380  marks the region where both dry-rub and window-cleaner resistance tests were performed. The square  382  marks the region of the cloth  384  which was in contact with the substrate during the Windex test. As can be seen, there is no apparent rupture to the film and no noticeable transfer to the cloth. 
     Although specific embodiments have been illustrated and described herein it is manifestly intended that the scope of the claimed subject matter be limited only by the following claims and equivalents thereof.