How light curing formulas meet the needs of food packaging

- Jul 11, 2019-

The light-curing formula is a product that converts the original liquid into a solid by chemical reaction by ultraviolet light irradiation, and the varnish and ink used for food packaging are no exception. Photo-solid product formulations are usually composed of oligomers (resins), reactive monomers, photoinitiators, pigments and auxiliaries, and any of these components can not be chemically cured into the polymer network. There are potential migration possibilities for liquid products.

   Photoinitiators can be divided into two types, one is a type I photoinitiator which spontaneously cleaves to generate free radicals, and the other is a type II photoinitiator which requires the use of a co-initiator or synergist.

Benzophenone derivatives, particularly ketones of α-hydroxy and α-amino groups, and acylphosphine oxides are commonly used type I initiators. When an α-hydroxybenzophenone molecule is irradiated with UV light, its σ bond before the hydroxyl group and the fat α-carbon breaks. For example, the common 1173 photoinitiator 2-hydroxy-2-methylpropiophenone will produce a benzoyl radical and a 2-hydroxy-2-propyl radical after cleavage, as shown in the following figure. Both of these free radicals have high reactivity and can initiate polymerization. Therefore, in the case of an ideal 100% conversion, all of the photoinitiators can be reacted into the polymer network.

 The main representatives of Type II photoinitiators are benzophenones, as well as thioxanthone derivatives. Taking benzophenone as an example, in the case of UV light, the molecules are activated and remain in the triplet state. In the presence of a co-initiator such as an amine, electrons are transferred from the co-initiator to the benzophenone, followed by acid proton transfer to form an amino radical and a hydroxyl radical. The activity of the resulting alpha-aminoalkyl radical is sufficient to initiate free radical polymerization. The hydroxyl radicals are relatively stable and do not initiate polymerization, but are more likely to undergo recombination or hydrogen extraction reactions.

   Even if a complete conversion occurs, the Type II photoinitiator will retain relatively small molecular weight molecules that may migrate.

   However, the actual UV curing reaction in LED EV equipment is still very different from the ideal state, that is, other reactions must be taken into account. In other words, the practical application is that there must be a portion of the residue of the unreacted photoinitiator. In type I photoinitiators, either due to direct recombination reactions or degradation of triplet species in type II photoinitiators, another possibility for type I photoinitiators may be recombination of residual free radicals. Or hydrogen extraction reaction.

   In order to reduce migration, it is necessary to fix the photoinitiator. One method is to increase the molecular weight of the photoinitiator. It has been shown that when the molecular weight is less than 300, the diffusion coefficient is increased, and when the molecular weight is greater than 1000, the migration is considered to be small. Therefore, according to the relevant regulations of the European Food Safety Authority, only low molecular weight knowers may be considered toxic. For type I photoinitiators, one can have multiple oligomeric photoinitiators with more than one photosensitive moiety introduced above, greatly increasing their likelihood of participating in the reaction. A negative disadvantage of oligomerized photoinitiators, however, is that the viscosity is higher and the viscosity of the final formulation is higher, with the result that the adjustment window of the formulation is narrowed. If a monomer is used to lower the viscosity, it will introduce new migration factors due to the lower functionality of the low viscosity monomer used.

  For Type II photoinitiators, one way to reduce the negative impact on viscosity while at the same time reducing the likelihood of migration is to lower the molecular weight and introduce the photoinitiator directly into the system material. This can be achieved by introducing an acrylate functional group into the molecular structure of the photoinitiator as shown in Figure 6, using the example here as HBEOAc.

   Gravure UV equipment Scenario suggestion: In the case of additives, if the additive itself is non-reactive, there is a risk of migration. Fortunately, most of the additive manufacturers now offer reactive additives to effectively avoid the migration of additives.

   In addition to the formulation itself, other factors, including curing conditions, the environment in which the product is used, may cause problems in food safety. Therefore, it is necessary to pay attention to this problem in every link in the entire value chain, in order to effectively avoid food safety problems caused by material migration in food packaging.