Food Safety in Flexible Packaging Adhesives
Choosing the right adhesive is key for flexible packaging applications.
Food safety has long been a concern for packaging material manufacturers and the packaging and food industry. In addition to plasticizers and mineral oils, special attention has to be paid to primary aromatic amines (PAAs). Dennis Bankmann, Ph.D., Product Development, Loctite Liofol Europe, Henkel AG & Co. KGaA, explains where risks exist in flexible packaging and what steps the packaging and food industry can take to ensure highest safety.
What are PAAs and why are they an important factor for food safety?
PAAs are a group of chemical substances from the wider group of amines. PAAs specifically carry an aromatic residue. They are used industrially in applications like manufacturing azo dyes and certain polymers. We know that certain PAAs present a toxicological concern as they have been identified as carcinogenic. Because of this specific hazard, PAAs stand out among the non-intentionally added substances (NIAS) found in flexible packaging. This means that special attention must be paid to ensure that they do not migrate into food at detectable levels.
Where do PAAs become an issue, specifically when talking about flexible packaging?
While azo dyes are used in inks for packaging materials made of paper, board, wood and plastics, when it comes to flexible packaging, polyurethane (PU)-based materials have received the highest attention. Polyurethanes are common components of adhesives, inks and coatings used to produce flexible packaging. Currently, about 90% of all adhesives used to produce flexible packages are PU adhesives. In other words, they are based on or contain polyurethanes.
The reason why PAAs are relevant in this context does call for an explanation, since the formulations of adhesives such as those from Henkel’s Loctite Liofol product line do not, of course, contain any PAAs themselves. There is, however, a possibility that PAAs can be formed if an unsuitable adhesive is selected, processing parameters are not adequate or laminates are thermally stressed. To prevent formation of PAAs as undesirable byproducts, it is of crucial importance not only to ensure that the adhesive has been appropriately formulated, but also that the finished packaging is only used within its limits.
How can PAAs occur during the manufacture and use of laminates? Could you explain this in more detail and give us a concrete example?
PAAs can form if the adhesive has not fully cured before the laminate is filled with food. In such a case, residual isocyanate monomers in the adhesive that have not reacted will react with the moisture that is found in practically all foods, and PAAs will be formed. These can migrate and then remain in the foodstuff and be incorporated by the consumer.
There are several reasons why laminates may, typically in adverse conditions, not fully cure. It could be that the temperature or ambient humidity is too low during curing, and certain types of laminating materials also play a role. If these insufficiently cured laminates were to get into the packaging process, this would present a risk for the filled foodstuffs.
The packaging industry has been aware of the importance of adequate curing conditions for many years, and manages the PAAs topic professionally through good manufacturing practices and monitoring of the produced laminates. Therefore, the question today is not so much whether our foods are safe, but how packaging manufacturers can maximize their productivity and minimize waste with the help of fast-curing adhesives. This is where we help the industry, by providing continuously improved adhesive solutions.
How about thermal stress, for example, in sterilization processes?
Thermal stress on laminates is indeed a factor that always has to be considered—and not just in retort processes, as for example used in pet food or baby food, but also for packages that are to be heated in a household oven. With respect to the adhesive selection, inappropriately high temperatures for a given adhesive can cause PAAs to form, even when the adhesive had fully cured. Chemically, at temperatures above 100°C, certain polyurethanes begin to degrade. In this process, the nature of the adhesive—as well as the temperature and time—play an essential role.
Ultimately, the question will always remain: Will PAAs be formed as a result? Regarding thermal stress, the correct choice of adhesive is crucial. It has to withstand the foreseen conditions and offer a degree of tolerance in order to assure process safety in industrial retort processes and to withstand any deviations from the recommended heating instructions that may happen in a household use.
What can producers and users do to be on the safe side?
The industry has two important levers it can employ to maximize the safety of packaging: choosing the best possible adhesive and applying good manufacturing practice, as mentioned earlier. By selecting a good adhesive, I can make sure as a producer that I am starting out with the best possible conditions.
Today, adhesives are available that contain only a minimum amount of free isocyanate monomers and thus reduce from the outset the amount of PAAs that could be formed. Moreover, the right adhesive can considerably shorten the curing time with respect to PAAs, often to less than three days.
Regarding proper use, controls on the produced laminate are a vital step before using it for packaging. As a packaging material manufacturer, I have to check how many days my laminate needs to cure, as this can vary widely according to climate, the structure that is to be produced and the inks used. In addition to the initial validation, consistency and good manufacturing practice are essential. If I don’t have temperature-controlled storage facilities, I may get different results in winter than in summer.
Another point to consider is choice of test method. Do I choose a simple method that I can carry out myself on site? Or do I contact an external institute, or go directly to the adhesive manufacturer? We support our customers in many ways, not just in selecting the right adhesives. We can help select the most appropriate test methods, set them up on site and validate them. If the questions or the circumstances are more challenging than usual, we can support with our own in-house accredited test laboratory.
What is the regulatory situation regarding PAAs? Are any changes foreseeable?
The European regulations regarding primary aromatic amines are probably the most clearly defined in the world. Two main regulations have been harmonized Europe-wide. The Framework Regulation (EC) No. 1935/2004 states that substances may not migrate from food contact materials in a quantity that would present a risk to health. This regulation naturally also covers PAAs, even if no explicit limit is mentioned. EU regulations require that generally recognized scientific methods be used to carry out the assessment and risk analysis.
Regarding the question of which limits to apply, since there is no Europe-wide regulation dealing specifically with adhesives, the industry generally refers to Regulation (EC) No. 10/2011, which governs plastic materials and articles. This regulation specifies that the sum of all migrating aromatic amines must not exceed 10 µg per kg of foodstuff, which may also be expressed as 10 ppb.
These limits may be tightened in the future. In a position paper prepared by BfR (the German Federal Institute for Risk Assessment) on aromatic amines migrating from printing inks, an individual limit of 2 ppb per specific PAAs is recommended in addition to the overall limit of 10 ppb. We are still waiting for an answer from the European authorities, but the BfR position paper gives a clear indication of which way the discussion is heading.
What consequences would the new limits have?
We can expect that, in many cases, adhesives in laminates would have to cure for longer times to ensure compliance with the new limits. Obviously, the option of switching to an adhesive that cures faster and allows faster PAAs decay presents itself here.
Retort applications are a special case. It should be remembered that faster curing or a lower monomer content in polyurethane adhesives cannot automatically be equated with higher heat resistance. If the regulations were tightened, a greater interest in aliphatic adhesives would likely result. Important reasons influencing that decision would be questions of process safety and the major challenge of actually being able to reliably detect such small quantities of PAAs after retort.
This is where aliphatic adhesive solutions step in. Since they employ different raw materials, PAAs will not occur at all, no matter what the temperature exposure is. Today, aliphatic adhesives can be designed to have none of the major disadvantages that earlier aliphatic products had, i.e., without tin-based catalysts and without a requirement for a tempering chamber for curing. ASI
For more information, visit www.henkel.com/foodsafety.