Novel Sulphonium Salt Cationic Photoinitiators For Food Packaging Applications

Introduction

UV curing systems for the graphic arts are dominated by acrylate chemistry, which accounts for as much as 95% of the total radiation curing market. Although there is a number of niche alternative curing technologies, by far the most important is cationic curing. This has been known for at least as long as free-radical acrylate chemistry, but its use faltered in its early years as a result of competing patent claims with the photoinitiators.

Up until the mid-1990s cationic curing technology struggled to compete with free-radical chemistry due to its higher cost, slow cure speed and limited range of raw-materials. However, its most important advantages of excellent resistance properties, adhesion to difficult substrates and inherently superior migration characteristics meant that it continued to survive in niche markets. Towards the late 1990s the increased use of narrow web flexo printing onto plastic substrates allowed significant market growth for cationic curing, only for the market to be badly damaged by the discovery of benzene as a by-product from the most commonly used triarylsulphonium salt photoinitiators in 1999.

In terms of their use in food packaging applications, cationic curing inks are desirable because, in theory at least, the post-cure reaction will continue until all the monomer is consumed, thus minimising migration. A further significant advantage is the superior mechanical properties of cationic inks and coatings following post cure, allowing them to survive processes such as steam sterilisation and retorting which are becoming increasingly important in the food packaging sector. These advantages are countered by the cost, slow cure speed, weak inks, high odour on cure and humidity inhibition to the extent that their use is often too much of a compromise, particularly when competing against an
established technology like free-radical.


Cationic photoinitiators

The photoinitiators used in cationic curing have always been the technology's weak point, with only two types commercially available; triarylsulphonium salts and diaryliodonium salts. Other significant classes such as phenacyl sulphoniums are well known, but have not been commercialised as yet.

The triarylsulphonium salts are the most widely used as they have the highest reactivity and are cheapest to manufacture. However, they also give rise to undesirable by-products on cure such as diphenyl sulphide (odourous) and benzene (toxic) . Following the discovery of benzene, much of the graphic arts market switched to iodonium type photoinitiators. This has not proven to be entirely satisfactory because of even slower cure speeds, higher costs and continued concerns over by- products such as toluene and iodo-benzenes.

Increased cure speed can come from the use of SbF6- counter ions rather than PF6-, but most ink companies have chosen to steer clear from heavy metals such as antimony, even if they are likely to become bound into the ink/coating.


Emerging cationic technology

It is not clear how many research programmes were initiated as a direct result of the problems with triarylsulphonium salts in 1999 and how many were reacting to a previously perceived market need. Either way, there has been an enormous activity in the field of new materials for cationic curing over the past few years. In particular, several new iodonium salts have been registered and commercialised recently as well as one existing EINECS registered material now being offered by a number of companies. All of these iodonium salts suffer from poor cure speed and odour/by-product problems that make them unsuitable for food packaging applications. Much good work has also been done both
in establishing how iodonium salts can be sensitised by existing materials like hydroxyalkylphenone photoinitiators and developing new sensitisers. In addition, the use of the commercially available trimethylolpropane oxetane (TMPO) and a number of additional developmental oxetanes is proving to be a significant step forward for the cationic technology overall. However, although there has been some work on new sulphonium salt chemistries, most lack the reactivity of the existing sulphonium salts and none have been developed commercially.

Unfortunately, none of this new technology and new materials gets around the fundamental problem for inks designed for food packaging applications; the need for a new fast curing photoinitiator with no cleavage by-products of concern and no odour on cure.

(S.L. Herlihy, B. Rowatt, R. S. Davidson, Sun Chemical, England )