Progress in the development of polymeric and multifunctional photoinitiators

https://doi.org/10.1016/j.progpolymsci.2019.101165Get rights and content

Abstract

UV-curing technology has been developed and widely used in industry for 6 decades. However, the migration of low-molecular-weight photoproducts is today an important drawback in many application fields such as food packaging materials, leading to unpleasant secondary features such as odor, blooming and contamination. Therefore, many efforts have been devoted to design polymeric photoinitiators (PPIs) and multifunctional photoinitiators (MFPIs) to overcome these drawbacks. This review introduces the development and the challenges of UV-curing technology and the state of the art of PPIs and MFPIs, in which methodologies of synthesis and characterization are meticulously discussed. Moreover, a novel classification, based on the molecular structure, has been drafted for all the PPIs conventionally reacting through photodissociation or photoreduction. Most of the PPIs and MFPIs have excellent photochemistry properties, some of which have extra properties such as amphiphile, self-floating and biocompatibility. Examples of well-defined structures are discussed in the section on MFPIs.

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).

References (124)

  • B. Cesur et al.

    Difunctional monomeric and polymeric photoinitiators: synthesis and photoinitiating behaviors

    Prog Org Coat

    (2015)
  • G. Ye et al.

    Low VOC bifunctional photoinitiator based on α-hydroxyalkylphenone structure

    Polymer

    (2006)
  • L. Hu et al.

    UV-cured organic–inorganic hybrid nanocomposite initiated by trimethoxysilane-modified fragmental photoinitiator

    Compos Part A Appl Sci Manuf

    (2011)
  • Y. Wang et al.

    Novel polymeric photoinitiators comprising of side-chain benzophenone and coinitiator amine: photochemical and photopolymerization behaviors

    Eur Polym J

    (2009)
  • T. Corrales et al.

    Photochemical study and photoinitiation activity of macroinitiators based on thioxanthone

    Polymer

    (2002)
  • X. Jiang et al.

    Polymeric amine bearing side-chain thioxanthone as a novel photoinitiator for photopolymerization

    Polymer

    (2004)
  • G. Temel et al.

    One-pot synthesis of water-soluble polymeric photoinitiator via thioxanthonation and sulfonation process

    J Photochem Photobiol A: Chem

    (2009)
  • Y. Wen et al.

    Amphipathic hyperbranched polymeric thioxanthone photoinitiators (AHPTXs): synthesis, characterization and photoinitiated polymerization

    Polymer

    (2009)
  • T. Li et al.

    Hyperbranched poly(ether amine) (hPEA) as novel backbone for amphiphilic one-component type-II polymeric photoinitiators

    Chin Chem Lett

    (2018)
  • X. Jiang et al.

    Copolymeric dendritic macrophotoinitiators

    Polymer

    (2005)
  • X. Jiang et al.

    Water-compatible dendritic macrophotoinitiator containing thioxanthone

    J Photochem Photobiol A: Chem

    (2006)
  • D.K. Balta et al.

    Chemical incorporation of thioxanthone into β-cyclodextrin and its use in aqueous photopolymerization of methyl methacrylate

    J Photochem Photobiol A: Chem

    (2008)
  • A. Luo et al.

    Thioxanthone-containing renewable vegetable oil as photoinitiators

    Polymer

    (2012)
  • X. Jiang et al.

    Study of macrophotoinitiator containing in-chain thioxanthone and coinitiator amines

    Polymer

    (2004)
  • X. Jiang et al.

    A novel amphipathic polymeric thioxanthone photoinitiator

    Polymer

    (2009)
  • Y. Matsuura et al.

    Synthesis of polysilane–acrylamide copolymers by photopolymerization and their application to polysilane–silica hybrid thin films

    Polymer

    (2002)
  • Q. Liang et al.

    A facile method to prepare a polyethylene glycol modified polysilane as a waterborne photoinitiator

    J Photochem Photobiol A: Chem

    (2015)
  • X. Jiang et al.

    Polymeric photoinitiators containing in-chain benzophenone and coinitiators amine: effect of the structure of coinitiator amine on photopolymerization

    J Photochem Photobiol A: Chem

    (2005)
  • X. Jiang et al.

    A novel negative photoinitiator-free photosensitive polyimide

    Polymer

    (2006)
  • Y. Wen et al.

    Polymeric Michler’s ketone photoinitiators: the effect of chain flexibility

    Prog Org Coat

    (2009)
  • R. Schwalm

    UV coatings: basics, recent developments and new applications

    (2006)
  • K. Dietliker et al.

    New developments in photoinitiators

    Macromol Symp

    (2004)
  • C. Roffey

    Photogeneration of reactive species for UV-curing

    (1997)
  • R.H. Leach

    Radiation curable systems

    The printing manual

    (1988)
  • X. Allonas et al.

    Radiation chemistry

    Ullmann’s encyclopedia of industrial chemistry

    (2012)
  • M.A. Lago et al.

    Photoinitiators: a food safety review

    Food Addit Contam Part A Chem Anal Control Expo Risk Assess

    (2015)
  • S.M. Snedeker

    Benzophenone UV-photoinitiators used in food packaging: potential for human exposure and health risk considerations

  • J.L. Aparicio et al.

    Migration of photoinitiators in food packaging: a review

    Packag Techn Sci

    (2015)
  • W. Anderson et al.

    Benzophenone in cartonboard packaging materials and the factors that influence its migration into food

    Food Addit Contam

    (2003)
  • S.M. Johns et al.

    Studies on functional barriers to migration. 3. Migration of benzophenone and model ink components from cartonboard to food during frozen storage and microwave heating

    Packag Techn Sci

    (2000)
  • A. Rodríguez-Bernaldo de Quirós et al.

    Migration of photoinitiators by gas phase into dry foods

    J Agricul Food Chem

    (2009)
  • S.M. Jickells et al.

    Migration of contaminants by gas phase transfer from carton board and corrugated board box secondary packaging into foods

    Food Addit Contam

    (2005)
  • M. Degirmenci et al.

    Synthesis, characterization and application of polymeric photoinitiators prepared by atom transfer radical polymerization and ring-opening polymerization

    Polym Prepr ACS Polym Div

    (2002)
  • M. Degirmenci et al.

    Synthesis of well‐defined polystyrene macrophotoinitiators by atom‐transfer radical polymerization

    Macromol Chem Phys

    (2002)
  • M. Degirmenci et al.

    Synthesis and characterization of macrophotoinitiators of poly (ε-caprolactone) and their use in block copolymerization

    Macromolecules

    (2002)
  • J.W. Seok et al.

    Structural effect of photoinitiators on electro‐optical properties of polymer - dispersed liquid crystal composite films

    J Appl Polym Sci

    (2006)
  • M. Degirmenci et al.

    Synthesis of mid-chain functional macrophotoinitiators of poly(D,L-lactide) homopolymer and tetrablock poly(D,L-lactide)-poly(ε-caprolactone) copolymer by ring-opening polymerization

    Des Monomers Polym

    (2015)
  • Q.F. Si et al.

    Synthesis and characterization of hyperbranched - poly (siloxysilane) - based polymeric photoinitiators

    J Polym Sci Part A: Polym Chem

    (2006)
  • W. Han et al.

    Synthesis and properties of UV-curable hyperbranched polyurethane acrylate oligomers containing photoinitiator

    Polym Bull (Berl)

    (2012)
  • J. Zhou et al.

    Zirconium Propoxide: A Coupling Agent for the Synthesis of Multifunctional Photoinitiators

    ChemPhotoChem

    (2018)
  • Cited by (0)

    View full text