UV-curable laminating adhesives are gaining steam in food packaging, but formulating them for flexo-printing can be a tricky task. Sean Des Roches, head of the US EnergyCuring laboratory of RAHN, discusses why it's important to get to know your adhesives.
For a long time, energy-curable laminating adhesives were left unexplored. They were thought to be too expensive and the processes too advanced, despite the VOCs that conventional adhesives contained and the large, slow drying ovens they needed.
The advent of using electron-beam (EB) or ultraviolet (UV) radiation to cure solvent-free laminating adhesives not only brought a greener approach to the table but also sped up production lines and cut down on energy costs.
In recent years, energy-curable laminating adhesives have become a steadily growing portion of the $30-billion global adhesive market.
UV curing for laminating adhesives is most efficient for films with low opacity. It has a lower set-up cost than EB equipment, but its less intense output means more lamps are necessary. It also requires the adhesive to include a photoinitiator. These tend to be molecules with a low molecular weight, which can seep through the plastics the adhesive is meant to hold together.
Any direct packaging that might migrate to food needs to be food-grade, which means the closest these adhesives can get to food is in indirect packaging. They've recently become popular as a way to protect printed graphics - an alternative to pressure-sensitive labels.
"The biggest push in recent years has been for a flexo-printed UV-curable laminating adhesive for indirect food packaging," says Sean Des Roches, head of US laboratory in the EnergyCuring division of raw material supplier RAHN.
Flexographic printing uses a flexible relief plate that works well on the non-porous substrates often used in food packaging. It needs low-viscosity formulations, but this is tough to do for UV-curable laminating adhesives.
Photoinitiators are only one part of the problem. In order to stick to plastics like polyethylene (PE), polypropolene and PET, a laminating adhesive generally needs to be highly viscous - such as a urethane acrylate - or a monomer with a low molecular weight, but also low functionality.
"The urethane acrylates can only be used in small amounts, because their inherent high viscosity leads to a final LA viscosity that is too high to print by flexo, and the monomers are too low molecular weight to be nonmigratory," Des Roches explains. "This makes formulating such a product very tough."
One of the obstacles that barred laminated adhesives from the market for several decades is still somewhat in play: the costs can run high. To combat this, formulators often use the cheapest ingredients possible, which results in an adhesive that works decently most of the time, but doesn't perform well enough for high-end applications or difficult substrates.
These need higher-quality, more 'robust' laminating adhesives, accounting not just for adhesion but for migration, reactivity, yellowing and stability of temperature and humidity.
RAHN develops starting point formulations (SPFs) of UV-curable, flexo-friendly adhesive materials for different applications. SPF 349 is low-cost, best for treated orientated polypropylene (OPP), PE, PET and paper; SPF 253 can also be used on paper as well as film and treated PET when combined with Al-foil.
SPF 117 is for non-PET substrates including film, metallised OPP, paper and Al-foil. SPF 109, unlike its siblings, is a 'wet' adhesive - it's tacky after curing, which is helpful for substrates that are either opaque or difficult to adhere to, and delivers high T-Peel values.
To get formulators familiar with using the SPFs in energy-curable laminating products, RAHN's lab put together a lab report of what each formulation is good for, what each ingredient in the formulation does and what happens when the amounts of different ingredients are tweaked.
For example, adding more of the products that create cohesion in SPF 349 will also increase its reactivity under UV lamps, while raising the amount of carboxy-functional polyester acrylate, or GENOMER 7151, will make the formulation less reactive, but more viscous.
Using a different photoinitiator in SPF 253 can help it cope with the UV-absorbing effects of PET, which calls for longer wavelength UV radiation and a photoinitiator that can absorb those wavelengths so the adhesive is properly cured.
UV curing creates an instant bond between laminating adhesive and substrate, which means shipments can go out in hours instead of days. The balance can be tough to get right, but when it works, the benefits are unmistakable.