Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 177 of 331

Fig. 2. A vector ray trace drawing depicting UV distribution within a curing chamber. threshold energy level, imagine the opening break of a billiard game: first, the game-initiating ball requires enough energy to get to the target; second, de- pending upon how much energy the ball carries, the break may be "fast," "slow," or embarrassingly ineffective. In this scenario the billiard balls are not defective, they simply did not receive enough energy to cause the desired reaction. This is applicable to UV processing. In practice it has been observed that UV cure can be initiated with as little as 0.005 J/cm2 el for Miami at noon (Table I). Though adequate for tanning this UV level is far too low to allow UV processing lines to operate at commercially feasible speed5 In practice UV levels range from 1.0 to 3.0 J/cm2 systems (see Fig.1) and as high as 7.4 J/cm2 in flat-line (two-dimensional) and greater in three-dimensional sys- tems. It must be noted that UV energy varies directly with type, number, and con- figuration of UV lamps in operation, while line speed dictates total UV exposure of dosage. Thus, UV levels in industrial UV-curing lines can range from approxi- mately 400 to 1,500 or more times the intensity of UV found in natural sunlight.6 Reaction Propagation Free radicals initiate a chain reaction where reactive sites on the acrylate molecule (functional sites) bond together or crosslink to start forming a polymer. Free radicals are extremely energetic molecules capable of producing an ex- tremely fast chemical chain reaction. The reaction may be as slow as several min- utes (sun cure) to as fast as 0.000000000001 (1.0 x 10-11 ing lines)!2 ) second (beer can print- UV-curable materials are inhibited from premature hardening by the addition of stabilizers, which work with ambient oxygen to scavenge and neu- tralize any free radicals that may form during the handling or storage of UV- curable products. In the absence of oxygen, such as under nitrogen blanketing, photopoly- merization has been observed to be 17.5 times faster than in the presence of oxy- 176 of UVB (320–329 nm). This is the approximate average natural UV lev- .

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