transfer process。

Printing inks usually behave as non-Newtonian fluids, mainly because of their novel components and the interaction between these in the ink。 This behavior presents a challenge in printing, because non-Newtonian fluids are defined by the ratio of shear stress (s) to shear rate (c)。 As the shear rate (c) is varied, the viscosity of the fluid will change。 In non-contact printing methods, for example reverse offset, the viscoelastic behavior becomes an impor- tant aspect of the ink-transfer process。 Although numerous studies have characterized the effects of the  viscoelastic  properties  of  ink  in   non-contact

printing, such studies have focused mainly on gra- vure offset or roll-to-plate。4,13,14 Thorough research is needed on the printability of non-Newtonian flu- ids by reverse offset to further the development of the method。 Figure 1 depicts the behavior of New- tonian and non-Newtonian fluids。

UV LED-curable ink has a variety of benefits, for example superior product quality, successful curing within seconds by use of UV radiation, weathering- resistant protective coating, safe handling of coating materials, and reduced cycle times, which increase production capacity and facilitate use of heat-sen- sitive substrates。15 Because of the benefits of UV LED-curable ink, the printing industry has adopted this ink and its coating procedures, because they emit little or no volatile organic compounds (VOC),

e。g。 solvents, they do not dry during the printing

process, and the curing equipment occupies much less space than conventional thermal drying con- veyors。16 Research has revealed that UV LED-cur- able ink as a coating protects against extreme temperatures and weather conditions; the materials themselves are environmental friendly, resistant to mechanical impact, low cost, and have the desired optical characteristics。 Crosslinking of new particles in the solutions has led to the development of new polymer ink compositions for printed coatings on packaging  with enhanced and durable barrier   per-

formance and other improved characteristics, for example desired mechanical strength and optical properties, believed to be of benefit for different packaging applications。17

In this study the rheological and wetting proper- ties of a UV-curable ink were characterized to determine its suitability for reverse-offset printing。 UV-curable ink was selected because, in comparison with other inks used for printing, for example flexographic ink, the market is expected to grow at 3% per annum, and the market for UV LED-curable ink is growing at 17% per annum。16 A new approach to predicting the printability of an ink by reverse offset has been tested for a variety of substrates。 It was concluded that even if contact angle require- ments are met, the viscoelastic behavior of the   ink

is also extremely important to the accuracy of patterns。

EXPERIMENTAL Viscosity Measurement

Viscosity was measured by use of a Brookfield LVDV-I Prime viscometer with an SC4-DIN-82 spindle。 A liquid of known viscosity (60 cP, 9 mL) was used to check machine accuracy and precision。 With the viscometer used at a speed of 6。283 radians per second, the viscosity read 58。6 cP。 For mea- surement of the unknown viscosity of pristine UV LED-curable ink, 9 mL was used。 When the vis- cometer was used at a speed of 5。236 radians per second the viscosity reading was 40。7 cP。 When the pristine UV LED-curable ink was diluted with 40%

Fig。 1。 Graphs of the behavior of Newtonian and non-Newtonian fluids: (a) shear stress as a function of shear rate; (b) viscosity as a function of shear rate。

methanol the viscosity was 25 cP at a speed of 5。236 radians per second。

Contact Angle Measurement

The contact angle between the UV LED-curable ink in the two solutions and nine different materials (M1, M2, M3, M4, M8, glass cliche´, PET, kapton PV 9101 (polyimide, DuPont), kapton PV 9102 (polyimide, DuPont)) was measured by use of a Rame-Hart model 250 Std G/T goniometer with DROPimage Advanced software。 The contact angle was mea- sured by use of the drop method, by use of a micro syringe; reported results for the contact angle are mean values based from three replicate experi- ments。 Each droplet of liquid was 10  ll。