13 October 2013
Year: 2013Price: 10.00
Amine synergists are widely used in UV curable acrylate based coating formulations. They are often used together with Norrish Type II photoinitiators such as benzophenone. The tertiary amine function provides an active hydrogen donor sight for the excited triplet state of the photoinitiator. This produces a very reactive alkyl-amino free radical (Equation 1), which subsequently initiates the polymerization of acrylate functional monomer and oligomer components of the formulation.
Another important function of tertiary amines in UV curable coatings is to reduce oxygen inhibition. Oxygen which diffuses into the surface of a coating quickly reacts with growing free radical chains and converts them into unreactive peroxy radicals. Tertiary amines react with peroxy radicals converting them to reactive alkyl-amino radicals thereby reducing the effects of oxygen on curing (Equation 2).
A wide range of different types of amine synergists are used in UV curable coatings. The simplest materials are low molecular weight tertiary amine compounds such as triethanol amine or methyl diethanol amine. These compounds can be very effective in combination with Type II initiators and also for reducing the effects of oxygen inhibition. Disadvantages of these compounds include potential migration and blushing of residual amines. These free amines are also known to react with certain pigments which can result in fading of the corresponding printed ink colors. These compounds are also somewhat hydroscopic which can be a problem in lithographic inks where hydrophobic ink properties are critical to the lithographic imaging process. Amino-benzoate compounds including the ethyl ester of dimethylamino benzoate (EDB) and the 2-ethylhexyl ester of dimethylaminio benzoate (EHA) are commonly used in lithographic inks because they have more hydrophobic properties. 2 Another class of amine synergists is acrylated amines. These are formed by the Michael
addition of secondary amines to acrylate functional groups (Equation 3). Secondary amines can be combined in this manner with a large variety of different multifunctional acrylate monomers and oligomers. Depending on the stoichiometry, a large variety of compounds can be produced which have both acrylate and amine functionality.4 The presence of an acrylate group on an amine synergist will allow it to react into the polymer matrix and reduce the potential for migration. Other oligomeric compounds with multiple tertiary amine functional groups are also known and are used in low migration ink and coating formulations.2, 3 Although the use of amine synergists are well known in UV coating formulations they are not commonly used in electron beam (EB) curable coatings. Acrylate based EB coatings cure without added photoinitiators so amine synergists are not needed to initiate polymerization. EB curing is almost always conducted under an inert atmosphere so the use of amines may not be considered beneficial for reducing oxygen inhibition. The purpose of this study is to evaluate potential benefits of using of amine synergists in EB curable coatings.
2013 Conference Effect of Amine Synergists on the Electron Beam Curing Properties of Acrylate Based Coatings
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