Progress in the development of polymeric and multifunctional photoinitiators
Graphical abstract
Introduction
Economy, efficiency, ecology and energy are the “4E” principles of modern industrial development. UV-curing technology is widely studied among coatings, adhesives and inks manufacturers, because of its compliance with these principles. Compared to thermal curing system, UV-curing process has attractive advantages such as high-speed, low energy consumption and low-solvent formulation [1,2]. Between the two main types of UV-curing mechanism (radical-type and cationic-type), the radical-type systems are the most widely used in UV-curing application so far in virtue of their higher reactivity [1,3].
Radiation curing has gained popularity in the packaging printing industry over the last decades, since it was commercialized in the late 1960s. Using ultraviolet (UV) energy to trigger polymerization of resins onto a substrate, UV-curing technique has been used in inks and coatings for food packaging printing, such as breakfast cereal boxes, milk and fruit juice cartons, frozen dinner cartons, ice cream and frozen dessert packages, as well as flexible film wraps. UV curing affords distinct advantages: fast curing, room temperature operation, high quality end products and low cost. In addition, UV curing technology is generally performed under room temperature conditions and thus allows the application of inks and coatings on heat sensitive substrates. Moreover, UV-cured formulations can be directly applied to the outer surface of the package without the use of solvents, hence reducing drying time and eliminating solvent exposure to workers.
In UV curing, acrylate monomers and oligomers are predominantly used to form a polymer resin due to the high reaction rates and the high degree of optical clarity of the end products. The acrylate monomers are primarily used as diluents, to impart the rheological properties necessary for the printing and coating process [4]. Many types of curable acrylate monomers are available commercially. For instance, there are acrylate monomers like tripropylene glycol diacrylate (TPGDA), trimethylolpropane triacrylate (TMPTA), hexanediol diacrylate (HDDA) and pentaerythritol tri-, tetra acrylate (PETA) etc.
Photoinitiators (PIs), the agents that absorb the UV energy and form free radicals that initiate the polymerization reaction, are the key compounds in UV-curing formulations. According to the free radical forming mechanism, the PIs can be classified to Type-I and Type-II PIs [1,2]. Type-I PIs lead to photodissociation, creating two radicals that may both initiate the polymerization reaction. Type-II PIs undergo photoreduction reaction by reaction with an electron or hydrogen donor. Thus, they require an additional co-initiator, such as a tertiary amine, ether, ester, or thiol for the formation of initiating radicals and low reactive ketyl radicals [3,5].
Generally, the higher reactive radicals initiate the polymerization reaction and are therefore locked into the growing polymer chains after the polymerization process. Nevertheless, some of the photoproducts of PIs may exhibit varied physical and chemical properties, which can be detrimental to the efficiency of photoinitiation and the performance of the cured product. The photoproducts of both Type-I and Type-II PIs may undergo further rearrangement, combination, oxidation and decomposition due to the instability of the radicals (Scheme 1). Some of these small molecular by-products may be colored or odorous and be inclined to migration in the polymer networks [6]. The migration of the PIs fragments could bring about odor and blooming of coatings, which present problems in materials used for food packaging. A further concern are residual amounts of PIs, either as remainder of the UV-curing process or due to incomplete purification, since PI species are prone to migrate through the porous paperboard and secondary packaging to the food or beverage. They also can remain in the paperboard after the outer print layer is removed, and thus PIs can appear in new packaging made with recycled paperboard fibers [7].
In the last decade, several notifications reported through Rapid Alert System for Food and Feed (RASFF) in the EU were related to the PIs used for food packaging. More than one hundred notifications issued RASFF concerned the migration of PIs from food packaging since 2000. The first one corresponded to the detection of isopropylthioxanthone (ITX) in baby milk in 2005 [6,8]. That well-known “isopropylthioxanthone case” (2005) and the “benzophenone and 4-methylbenzophenone cases” (2009) led to serious consequences [8]. Soon after these events, considerable interest has been focused on side effects due to photoinitiators in the food industry, including the migration process, the analytical methods for migration and the toxicity of the photoproducts.
It is important to precise that the term “migration” also accounts for the transfer of chemical compounds from the package to the food. Also, throughout the paper we are interested in PIs among the different components of the photopolymerizable formulations used for food packaging. A variety of factors have been shown to affect migration from the printed surface through the paperboard and to the food itself (direct contact), or from the paperboard through the vapor phase to the food (indirect contact) [7,8]. These factors include set-off [9,10], the vapor pressure of the UV photoinitiator [11], time and temperature of food storage [9,10,12], porosity and types of materials used for primary and secondary packaging, the fat and moisture content of the food, and whether the UV photoinitiator is present or not in recycled materials used to form new packaging.
PIs with high molecular weight (MW) are believed to be a promising solution to the above problem, among those polymeric photoinitiators (PPIs) and multifunctional photoinitiators (MFPIs) are major topics of discussion. The idea of grafting the PIs on a high MW molecule is aimed at anchoring the odorous or toxic photoproducts. Previous review articles introduce the synthesis and performance of various macromolecules bearing Type-I or Type-II PI groups such as benzoin ether and thioxanthone [13]. However, there have been increasing numbers of PPIs and MFPIs with all sorts of molecular structures. Therefore, the existing definition and classification of these PIs needs to be reappraised.
Many efforts have been devoted to the development of PPIs and MFPIs including synthesis routes, photoinitiating performances and structural functionalities. This systematic review introduces the recent advances of PPIs and MFPIs, and drafts an innovative categorization for them. PPIs are distributed according their molecular structure into three classes: linear polymeric photoinitiator (LPPI), hyperbranched or cross-linked polymeric photoinitiator (HCPPI), and multifunctional photoinitiator (MFPI). In the scope of this discussion, the initiation systems of low-MW Type-II PI with polymer hydrogen donor and the self-photoinitiation of monomer bearing PI group are not included, notwithstanding their prosperous application.
Section snippets
Classification of polymeric photoinitiators
Many different strategies can be used to develop new polymeric photoinitiators leading to a wide variety of structures. Therefore, when dealing with PPIs, it is convenient to separate a) the photoinitiator moiety, which is quite often based on well-known low molecular weight photoinitiators and b) the polymeric backbone that does not influence the absorption of light and the mechanism of reaction.
Concerning the photoinitiator moiety, one has to say that most of the polymeric photoinitiators are
General methodologies of synthesis of PPIs
The first question when one designs the synthesis route of a new PPI would be: how to install an effective PI on a polymer and preserve it during the preparation step? This question implies two different ways of constructing PPIs, which are represented by “preserve” and “install”. Generally, PI groups can survive in most of the non-photoinduced polymerization as part of starting formulation, and be located in polymeric structure, which is one way to construct PPIs. Grafting PI groups on a
Linear polymeric photoinitiators (LPPIs)
Linear polymers are among one of the most common type of macromolecules in both scientific and industrial area. Introducing photoinitiators (Type-I or Type-II) to linear polymers is a facile way to obtain PPIs, for example, by polymerization of monomers bearing PI groups. In this way, the PI groups can be located in the backbone or the side chain of the polymer as part of the repetitive unit. Besides, modification of end-group is another route to prepare LPPIs. Nevertheless, the convenience of
Hyperbranched and cross-linked polymeric photoinitiators (HCPPIs)
In recent decades, hyperbranched polymers have attracted much research interest due to their peculiar properties such as low viscosity and good solubility. Generally, hyperbranched polymer bearing photoinitiator groups is prepared via grafting routes, which can also be employed in cross-linked network. Therefore, it would be logical to discuss hyperbranched PPIs and cross-linked PPIs together. The irregularity of molecular structure and the incomputable functionality are common characteristics.
Multifunctional photoinitiators (MFPIs)
Among the varied kinds of PIs with high molecular weight, multifunctional photoinitiators MFPIs, which have defined number of functional groups, have been the Cinderella of industry due to the difficulty of remaining structural homogeneous in mass production. In order to prepare MFPI with high degree of functionality, dendrimer has been utilized as the core moiety. Even though the methodology of synthesizing MFPIs is similar to those of synthesizing hyperbranched PPIs, the structural regularity
Migration testing for measuring UV photoinitiators
In order to assess the migration potential of UV inks and coatings on food packaging prints, appropriate analytical methods are necessary in the packaging industry. Several groups of scientists have published their analytical methods for detecting UV-curable monomers and by-products with the migration potential into foods, since the late 1990s. Most of the studies reports migration tests on low molecular weight PIs. However, it is interesting to discuss the methodology and results reported in
Perspectives and outlook
UV-curing techniques play an important role because of their potential to facilitate environmental benign materials fabrication. A large amount of work has been done on the synthesis of low migration PIs as mentioned in this review, most of which was achieved in laboratory but not facile in industry. Among those methodologies and structures of low migration PIs, polymerizable photoinitiators can be the candidate to fulfill the expectations of low migration, facile synthesis and industrial
Acknowledgements
ANR and Mäder are also fully acknowledged for financial support of the DeepCure project#ANR-13-CHIN-0004-01. The authors thank the support from the National Natural Foundation of China (Grant No.: 51873043).
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