Dec. 11, 2014  3  US 2014/0364548 Al  cially in monochrome full coverage printed areas, alternating  parallel lines of dark and light color ( or even white) may be  visible in the printed image.  [0045]  Coalescence is a process that may occur on the  surface of a print medium, wherein two or more ink drops are  being unified into a single drop. If this process takes place  without drop displacement, on a surface, the drops will con› nect to each other to create a film.  [0046]  If, however, the coalescence process is accompa› nied by drop displacement, the coalescence of adjacent  printed drops will lead to the formation of puddles, a process  termed puddling. In this case, the printed ink drops are no  longer present at the intended location on the medium sur› face, but they are moving across the medium surface to a  common location with adjacent ink drops.  [0047]  Puddling may occur when two (or more) ink drop› lets are printed close enough to each other such that the ink  droplets touch each other. As a result droplets may move  towards each other while coalescing. The displacement of the  droplets is usually not symmetrical. The last jetted droplet  tends to move the most. This displacement mechanism takes  place at a time scale of -100 µs.  [0048]  Another very fast mechanism is taking place via a so  called precursor film, which is an ink film at the perimeter of  the printed ink drop and is a result of the impact of the ink drop  with the print medium. The film represents a connection  bridge between two ( or more) adjacent drops and may initiate  the coalescence of the ink droplets which may be accompa› nied by drop displacement. Similar to the above mechanism,  this mechanism takes place on a time scale of -100 µs.  [0049]  Yet another mechanism leading to puddling is a  more complex mechanism and comprises lateral absorption  ofink into the print medium. The lateral absorption ofink into  the print medium may form connection bridges via the print  medium between adjacent ink droplets which are printed far  enough from each other such that the ink droplets initially do  not touch each other. This mechanism may lead to coales› cence and eventually to puddling. The time scale of this  mechanism is typically in the order of -1 ms.  [0050]  Puddles may also be formed by a de-wetting mecha› nism. In this mechanism, initially an ink film may be formed  due to coalescence ofink drops. During drying of the ink film,  inhomogeneous evaporation of ink components may occur,  which may lead to a variation of the local surface energy of  the ink film. Consequently, the film may (unevenly) break up  and Marangoni flow like displacement may occur, eventually  leading to a puddling structure on the surface of the print  medium. This mechanism typically takes place on a time  scaleof-1 s.  [0051]  An ink composition according to the present inven› tion comprises a water-dispersible resin, a water-dispersible  colorant, water, a cosolvent, a surfactant and optionally other  additives. The components of the inks will be described in  detail in the next sections.  [0052]  The inkjet ink according to the present invention  contains a water-dispersible resin in view of the pigment  fixability to recording media. As the water-dispersible resin, a  water-dispersible resin excellent in film formability (image  formability) and having high water repellency, high water› fastness, and high weatherability is useful in recording  images having high waterfastness and high image density  (high color developing ability). Examples of the water-dis› persible resin include synthetic resins and natural polymer  compounds.  [0053]  Examples of the synthetic resins include polyester  resins, polyurethane resins, poly epoxy resins, polyamide res› ins, polyether resins, poly(meth)acrylic resins, acryl-silicone  resins, fluorine-based resins, polyolefin resins, polystyrene› based resins, polybutadiene-based resins, polyvinyl acetate› based resins, polyvinyl alcohol-based resins, polyvinyl ester› based resins, polyvinyl chloride-based resins, polyacrylic  acid-based resins, unsaturated carboxylic acid-based resins  and copolymers such as styrene-acrylate copolymer resins,  styrene-butadiene copolymer resins.  [0054]  Examples of the natural polymer compounds  include celluloses, rosins, and natural rubbers.  [0055]  In an embodiment, the water dispersible resin used  in the present invention may be made of a resin having a water  soluble functional group such as a carboxylic group or a  sulfonic group.  [0056]  In an embodiment, the ink composition according to  the present invention comprises a resin having a carboxylic  group which has a small dissociation rate from the viewpoint  of producing high speed aggregation effect of the water› dispersible resin. Since a carboxylic acid group tends to be  influenced by a pH change, a dispersion state changes easily  and its aggregation property is high. Examples of resins suit› able for use in ink compositions according to the present  embodiment are: an acrylic resin, a vinyl acetate resin, a  styrene butadiene resin, a vinyl chloride resin, an acrylic  styrene resin, a butadiene resin and styrene resin. As for the  resin component of the water-dispersible resin, it is preferable  that it is a polymer having both a hydrophilic portion and a  hydrophobic part in the molecule. By having a hydrophobic  part, it is possible that a hydrophobic part will be orientated to  the inside of the water-dispersible resin, and a hydrophilic  portion will be effectively orientated to the outside of the  water-dispersible resin. As a result, the change of a dispersion  state in response to the pH change of a liquid will become  larger, and aggregation of the ink will be performed more  efficiently.  [0057]  Examples of commercially available water-dispers› ible resin emulsions include: Joncryl 537 and 7640 (styrene› acrylic resin emulsion, made by Johnson Polymer Co., Ltd.),  Microgel E-1002 and E-5002 (styrene-acrylic resin emul› sion, made by Nippon Paint Co., Ltd.), Voncoat 4001 (acrylic  resin emulsion, made by Dainippon Ink and Chemicals Co.,  Ltd.), Voncoat 5454 (styrene-acrylic resin emulsion, made by  Dainippon Ink and Chemicals Co., Ltd.), SAE-1014 (styrene› acrylic resin emulsion, made by Zeon Japan Co., Ltd.),  Jurymer ET-410 (acrylic resin emulsion, made by Nihon  Junyaku Co., Ltd.), Aron HD-5 and A-104 (acrylic resin  emulsion, made by Toa Gosei Co., Ltd.), Saibinol SK-200  (acrylic resin emulsion, made by Saiden Chemical Industry  Co., Ltd.), and Zaikthene L (acrylic resin emulsion, made by  Sumitomo Seika Chemicals Co., Ltd.), acrylic copolymer  emulsions of DSM Neoresins, e.g. the NeoCryl product line,  Ink Composition  Dewetting  Puddling  Coalescence  Water Dispersible Resin (Latex Resin) 