1. Technical Field
The present invention relates to an ink composition and a method of producing a processed product of printed matter.
2. Description of the Related Art
Various methods such as electrophotographic methods, sublimation-type thermal transfer methods, fusion-type thermal transfer methods, and inkjet methods are known as image recording methods in which images are formed on recording media, such as paper, according to image data signals. For example, an inkjet method is an excellent image recording method because it can be carried out using inexpensive apparatus, it enables efficient use of ink and lower running costs since an image is directly formed on a recording medium by ejecting ink droplets only onto the region in which the image is to be formed, and it is less noisy.
The inkjet method enables printing not only on plain paper, but also non-water-absorbing recording media such as plastic sheets and metal plates. However, high speed printing and high quality images have been issues needed to be addressed, and the time periods required for drying and curing the ink droplets printed by the inkjet method significantly affect, for example, the productivity of printed matter, the sharpness of a printed image, and the like.
In one inkjet method, an inkjet recording ink which can be cured by irradiation with radiation is used. In this method, ink droplets are cured by irradiation with radiation immediately after ink ejection or after a certain period of time has passed after ink ejection, whereby the productivity of printing is improved and sharp images are formed.
By increasing radiation sensitivity of the inkjet recording ink which can be cured by irradiation with radiation such as ultraviolet rays, a high curing property against the radiation is imparted to the ink, leading to various advantages such as improvement in productivity of inkjet recording, reduction in electrical consumption, longer life time of a radiation generator due to load reduction thereto, and prevention of evaporation of low-molecular-weight substances which may be caused by insufficient curing. In particular, by increasing the radiation sensitivity of the ink, the strength of an image formed from the inkjet recording ink is increased.
It is preferable that an image formed from an ink have flexibility and adhesiveness with a base material. In particular, the flexibility of an image is an important factor when the image is formed on a surface of a non-flat base material or on a surface of a flexible base material such as a resin film, or when a shaped product of printed matter, such as a resin-made bottle, is produced by forming a printed matter in which the image is printed thereon into a shaped product. However, it has been difficult to achieve a favorable balance between the curing property and the flexibility. Specifically, when a proportion of a monofunctional monomer is increased to improve flexibility, the curing property tends to decrease, or the image surface tends to be sticky owing to leaking of uncured low-molecular-weight components.
In order to improve adhesiveness, flexibility, and curing property, a technique in which a low-viscosity urethane acrylate is used in an inkjet ink composition has been suggested (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-167537). However, even when this ink composition is used, it is difficult to attain a satisfactory curing property when a content of a monofunctional monomer is increased to produce a more flexible ink film.
In order to obtain a photopolymerization initiator composition which is cured at high sensitivity and has excellent storage stability, a technique is known in which a polyfunctional thiol compound having a specific structure is used as a component of a photopolymerization initiator composition for a photosensitive composition (see, for example, JP-A No. 2004-149755, or EP 1983017). However, by the technique disclosed in JP-A No. 2004-149755, it is difficult to completely prevent the blocking due to leaching of a slight amount of uncured components, such as degradation residues of a polymerization initiator and a solvent.
JP-A No. 56-75643 discloses that polymerization inhibition caused by oxygen can be reduced by adding a specific disulfide.
JP-A No. 2006-160824 discloses that an ink composition having a favorable curing property can be obtained by using a polymerizable compound having a specific structure having a polymerizable group and a disulfide bond.