Curing inks/print

Inks. Refined kaolin is a common ingredient in a large variety of printing inks (qv). In addition to extending the more expensive polymers present, ka olin also contributes to improved color strength, limits the penetration of the ink into the paper, controls rheology, and improves adhesion. Kaolin for this appHcation must usually be as white as possible and free from oversize particles. Surface treated clays are used to improve compatibiHty with oil-based ink. Clays can also be an ingredient in the newer water-based or uv-cured inks. 

Chung, J. Ko, S. Bieri, N. Grigoropoulous, C. Poulikakos, D. 2004. Conductor microstructures by laser curing of printed gold nanoparticle ink. Appl. Phys. Lett. 84 801-803. 

Chemistries available now for the UV-curing of printing inks include ... 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

Johns et al have published a report on benzophenone in printed cardboard for food stored frozen. This substance was found at 0.4-3.0 mg/dm in four board samples out of seven. The authors suggest that UV-cured inks had been used with benzophenone as an initiator in the printing of these boards. Three of the corresponding foodstuffs contained benzophenone at a level of 0.6-2.9 mg/kg, though there was a polyethylene layer applied to the board. Model ink substances were added to the carton, which then was in direct contact with foodstuffs stored at -20 °C for a year. It was found that transfer of the volatile substances could be considerable even at low temperatures. 

Whilst there is considerable evidence to indicate that substances can volatilise from and/or transfer through packaging materials to the gas phase, there are very few studies on gas phase transfer to foods. Indirect evidence for gas phase transfer to foods was seen in studies by Anderson and Castle (2003) who measured levels of benzophenone in foodstuffs packaged in printed cartonboard materials. Benzophenone is a photoinitiator used in some UV cure inks for the printing of paper and board materials and has been shown to be present in printed paper and board materials indicating that it remains in the paperboard after printing (Johns et al 1995). 

The use of solventless inks avoids the emission of solvents during drying. The development of such inks has been a major goal of the printing ink industry for the past several years and has resulted in tiie development of several different types 1. ultraviolet light-cured inks ... 

Ultraviolet Light-Cured Inka Ultraviolet light-cured inks are a distinct d rture from conventional printing inks in that fiieir mechanism of... 

The main disadvantage is that web temperatures of 275-325° F are required to cure the film. These temperatures affect the paper properties adversely, and therefore higher-quality, more-expensive paper must be used. Even so, blisterii or cracking in the folder may be a problem. Moreover, the solvent emission, although reduced significantly, is not eliminated, and, as with the ultraviolet light-cured inks, paper printed with these thermally-catalyzed inks cannot be recycled in the currently used processes (8, 9). 

Theoretical Aspects. Ultraviolet light-cured inks are cured by free radical-initiated vinyl addition polymerization. The photochemical initiation of vinyl polymerization has been the subject of many investigations dating back more than 30 years, to the first systematic studies of polymerization kinetics. Photochemical initiation offered a reproducible source of radicals that is not dependent upon temperature as is the thermal decomposition of free radical initiators. However, despite these early studies of its mechanism and kinetics, only recently has photochemical initiation become of practical interest, mainly because of the recent development of ultraviolet lamps suitable for production curing of printing inks and coatings. 

Practical Aspects. The foregoing theoretical relationships are difficult to apply to the curing of ultraviolet light-cured printing inks for several reasons. First, the polymerization kinetics of the complex monomers and prepolymers used in flie ink formulations have not yet been investigated under conditions that can be correlated with theory (i. e., low concentrations of reactants, constant temperature, simple model formulations). Second, the ultraviolet light-cured inks are polymerized in bulk to high conversion tiie polyfunctional nature of the... 

Rub resistance BS 3110 1959 (94). This is the resistance of a printed test piece to withstand rubbing against either another similar printed test piece or against a plain test specimen. The objective is to see that the ink/print has cured and will not scuff or smear in service. 

A new and very efficient bifunctional compound BPSK (X = H) [38] exhibiting a better light absorption has been recently developed for the curing of printing inks (10.21). The (3-cleavage still occurs at the C-S bond [39]. 

The UV technology alluded to above holds out the promise of even more superior labels. In this process, ultraviolet cured inks are used as the printing medium. These are cured by exposure to industrial type UV mercury vapor lamps in either a non-inert system (air environment) or in an inert (oxygen-free) system. The result is excellent adhesion to vinyls, polyesters, polycarbonates, butyrates and acetates and a cured elongation of between 20 and 40% to avoid embrittlement of the substrate. Initial testing has exhibited little sign of degradation and excellent chemical resistance after three years of exterior exposure. Such UV labels as well as other techniques should be considered whenever use demands extended label performance. 

The composition of ink will depend very much on the printing technique used. Adsorbing inks (e.g. used in most newsprint printing) contain typically mineral oil, unsaturated fatty acids and alkyd resins, whilst radiation curing inks (UV or IR) would be typically epoxy acrylates, urethane acrylates or similar prepolymers that can be polymerised further on irradiation. Finally xerographic inks contain yet other binders, e.g. styrene butadiene rubber or vinyl acrylates... 

The free-radical type of UV curing inks is based on polyacrylate prepolymers in combination with various multifunctional acrylate monomers. The other ingredients are almost the same as in ordinaiy printing inks except that the monomers are used as solvents. Other requirements are that pigments and extenders that absorb UV light cannot be used. 

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