Solvent-free and solvent-based laminating adhesives for safe food packaging

October 2007

This paper describes the difference between standard and ‘smart’ curing laminating adhesives already sold on a global basis to the converting industry. The latter combine fast-cure properties to reach specific mechanical performance objectives while also providing a safe laminate for food packaging.

BY DR HANS-GEORG KINZELMANN AND DR NORBERT BIALAS OF HENKEL GERMANY/TIP LIOFOL

The supply of the lowest-cost package is a general trend when competing in today’s flexible packaging market. Henkel, the number-one worldwide supplier of adhesives for a variety of applications, has a leading position to service and support converters with a complete range of Liofol laminating adhesive systems on a global basis.

Over the years we have seen several actions to reduce cost. The lamination speed, especially with solvent-free adhesives, has increased from about 150m/min to 300m/min over the past ten years, and the latest generation of machines is now capable of running consistently at 500-600m/min. The substrates have become thinner and structures with fewer layers have been developed providing the same protection for food. Solvent-free lamination was originally developed for environmental reasons but is also more cost effective when compared to traditional solvent-based and extrusion lamination for short runs. The trend to shorter delivery times has necessitated very short cure times for the laminating adhesives; this seems to be contradictory to the chemistry of the most common reactive polyurethane adhesives worldwide. The increased awareness of the need for regulatory compliance for food packaging puts additional pressure on the converter.

Looking out for human health

The so called ‘Danish food scandal’ in 2001 raised many questions about aromatic amines from not-fully-cured polyurethane laminating adhesives. The public, as well as the regulatory authorities in the different European countries, are now more sensitive to this issue. Many countries around the globe (all European countries, the US, Canada and Japan) have regulations in place where all food packaging material has to fulfil the basic requirement that, under normal conditions of use, no component will be transferred into the food which endangers the human health.

The European plastic directive 2002/72/EC has defined specific migration limits for all kind of low-molecular-weight components including aliphatic and aromatic isocyanates used for the formulation of polyurethane laminating adhesives. The free isocyanate monomer of a not-fully-cured laminating adhesive in indirect food contact is of concern for safe food packaging due to the potential formation of an aromatic amine with moisture out of the food. The practical and validated German photometric test method can easily detect aromatic amines down to a detection limit of 2ppb. It is a well accepted screening method to determine compliance with 2002/72/EC if the lower response factors of the real aromatic amines versus aniline standard are considered.

The Food and Drug Administration in the US has regulated adhesives in indirect food contact in the paragraph 21 CFR FDA 175.105. It is the responsibility of the adhesive supplier to compositionally comply with the positive list of raw materials, but it is the converters’ responsibility to comply with the requirement of functional barrier between the adhesive and the food. In case of aromatic isocyanate-based polyurethane laminating adhesives, non-detectable aromatic amines in food shows functional barrier issues.

We have to take a look at the chemistry to understand the influence of the reactive polyurethane laminating adhesive with regard to compliance of the regulations for food packaging. Standard solvent-based and solvent-free one- and two-component polyurethane systems cure between the layers of a laminate in the converter’s warehouse and are free of migratory amines towards the end of this period, when all isocyanate has reacted to a fully cured system.

The need for ‘smart’ curing systems

Especially in standard and medium-performance applications, mechanical strength is reached quickly, allowing the package to withstand conditions on the packaging line, but free monomeric isocyanate may be still present in the adhesive layer, having the potential to migrate into non-dry food.

We need to fully understand the factors influencing the cure time of a polyurethane adhesive until the laminate is safe for indirect food contact in order to develop ‘smart’ systems.

It seems to be obvious that the isocyanate concentration in the NCO-component as the source of the aromatic amines is important. Considering only the free monomer level when deciding which adhesive to use would be a mistake because the curing mechanism has a stronger influence than monomer concentration. Other factors influencing the cure time for compliance with food regulations are related to the substrates, the lamination process and storage conditions of the rolls.

More than 15 years ago, Henkel introduced two-component solvent-based and solvent-free third-generation polyurethane laminating adhesives with a smart-curing mechanism, providing significantly reduced cure times until the adhesives are free of migratory amines – even for compliance with the detection limit of 2ppb with the photometric test method.

In the last five years, new systems were added to Henkel’s range of Liofol third-generation technology-based adhesives with additional advantages.

The result of modifying the process technology behind third-generation adhesives are the fourth-generation polyurethane adhesives combining the smart-cure mechanism with very low isocyanate monomer levels (<1%), resulting in very short cure times at room temperature for critical substrates.

Solvent-free adhesives of the third and fourth generations are applied at 70-80°C, which makes handling more complicated in comparison with common second-generation systems liquid at room temperature for application at 40°C.

If we look at the worldwide market needs for solvent-free adhesives for flexible packaging, converters are mainly interested in general-purpose systems combining ease of handling with good machinability and fast cure, especially with regard to physical performance and food safety. Henkel’s sustainable R&D efforts resulted about six years ago in the very successful market introduction of universal solvent-free adhesives which combine the easy handling of second-generation solvent-free adhesives with much shorter cure times to be safe in indirect food contact. In the meantime, the well-known universal grades like Liofol UR7732/UR6084 and Liofol UR7780/UR6080 have been established at many converters to meet today’s market needs.

Migration of aromatic amines

The topic of migration of primary aromatic amines has been mostly discussed regarding solvent-free adhesives. It is true that packaging materials laminated with solvent-based adhesives are safe for indirect food contact faster than the corresponding solvent-free adhesive, assuming that the curing mechanism remains the same! A solvent-based system that cures in accordance to the standard curing mechanism mostly needs more time to be safe for indirect food contact than a solvent-free system that follows the ‘smart’ curing mechanism!

For this reason Henkel added some new solvent-based products to its solvent-based portfolio. The new products, the so-called fourth-generation solvent-based systems, have some additional benefits next to the fact that laminates produced with these systems are very fast in compliance with the regulations for flexible food packaging. One of the advantages is the high-solid character of these systems that allow customers to produce at higher speed at the same or even lower energy cost. Other advantages are the very good wetting properties, even on white printing inks, and the low solvent retention. Liofol UR 3966-21 / 6055 is a multifunctional adhesive system that can be used for a wide range of different structures. Liofol UR 3969-21 / 6055 is recommended in order to obtain good resistance against aggressive fillings.

In summary, Henkel’s Liofol two-component solvent-free and solvent-based polyurethane laminating adhesives with smart-cure mechanism offer an excellent performance for each market segment, combined with short cure times to meet the requirements for safe food packaging.

Henkel SA: T +27 (0) 11 617 2400