Migresives in Flexible Food Packaging: Solventless PU Adhesives

Some adhesives can be used as an unformulated resin. However, most adhesives systems used in flexible packaging are highly formulated with:
 

• Tackifiers
• Antioxidants
• Plasticizers
• Diluents, and
• Other additives

These additives provide optimal processing and performance properties.

These deliberate additives are generally low molecular weight constituents that have a tendency to migrate out of the adhesive, possibly through the main packaging material, and into the packaged contents. These deliberate additives are generally identified in the food contact regulations and limits and appropriate recommendations as to their use are specified.

Low molecular weight components in the adhesive could also form in a non-deliberate manner. These are generally the result of under-cure but could be related to other processing parameters as well. Example: primary aromatic amines can be formed on reaction of free aromatic isocyanate with moisture. These components can also include unreacted monomers and incompletely polymerized fractions.

As a whole, these low molecular weight constituents, whether deliberately included in the formulation or the result of accident are the major source of concern in establishing food contact regulations. Since, they could affect the odor, taste, and possible toxicity of the packaged contents.

 

General Regulations

The regulatory systems that are employed in the US and the EU are superficially similar. But, when one examines the basis and requirements that are embedded in each set of regulations; there are significant differences. Each has its own specific history and set of exemptions. Harmonization between the two, although highly desirable because of international trade, appears to be far off. 

EU and US regulations seek to limit chemical migration from packaging into food, but how to apply these rules to adhesives is not yet clear. There is no specific regulation on how to implement food migration requirements for adhesives. As a result, "Plastics Regulation (EC) No. 10/2011" is often applied to adhesives.

This regulation provides detailed tables with limits for approved starting compounds based on partition and diffusion coefficients of volatile compounds in polymers. However, food packaging materials are often multilayer laminates consisting of several substrates bonded with adhesive layers. Information about the partition coefficients between adhesives and substrates used in these materials and about the diffusion coefficients in commonly used materials is often lacking. 

Annex 1 of Regulation (EC) No. 10/2011 is a positive list for packaging additives. However, it is incomplete; implying that materials not listed can be used provided that their safety is ascertained in accordance with proper methodology. Substances are also assessed according to national provisions and statements such as BfR (Federal Institute for Risk Assessment) in Germany where, Article XXVIII specifically applies to crosslinked polyurethanes as adhesive layers for food packaging materials.¹

Furthermore, there is no standard methodology in the EU or Member States on how to implement this requirement for laminating adhesives. This means that adhesive suppliers must demonstrate that their products are safe to use with food by navigating a maze of legislation. An excellent source of information on legislation in Europe and the US as well as technical information regarding chemical migration into food packaging can be found in Chemical Migration and Food Contact Materials.²

 

Potential Migresives Regulations

Some progress has been made by an EU-funded program to elaborate a scientific based risk assessment approach to meet current general regulatory requirements on migration. Named MIGRESIVES, it has developed analytical methods, plus tools and parameters to estimate the transfer of constituents from adhesive layers into food by mathematical modeling. The EU's MIGRESIVES consortium is seeking to classify adhesive components according to whether they migrate, and whether limits are needed on the extent of their migration.³

The MIGRESIVES program is now set to publish guidelines and standard decision strategies for:
 

• Choosing how to test adhesives and packages, and
• Releasing migration modeling approaches and analytical methods

It is clear to protect the safety of food content in packages. We must look closer at chemicals going into and originating from these adhesives. 

 

Important adhesives used in laminating flexible packaging are those belonging to the polyurethane family. Though, water-based systems have been developed as an economic alternative to overcome some of the drawbacks associated with solvent-based systems. But, the performance of water-based adhesives, especially in wet environments is not as good as their solvent or solventless counterparts.Solventless (100% solids) liquid polyurethane adhesives are the most widely usedadhesive for laminating flexible food packaging materials. 

The table below lists some polyurethane adhesives for flexible laminates in food packaging:
 

Reaction Meachanism of Solventless
 

• The reaction mechanism for a solventless polyurethane laminating adhesive consists of an isocyanate group reacting with the hydroxyl groups of a polyol (an alcohol having several hydroxyl groups) to form a repeating urethane linkage

Reaction of Isocyanate with Hydroxyl Groups

• The isocyanates will also react with water to form a urea linkage

Reaction of Isocyanate with Water

Linear thermoplastic polyurethanes can be obtained by using compounds with two reactive groups such as di-isocyanate and diols. When polyols with three or more hydroxyl groups are reacted with an isocyanate, or when isocyanates with three or more isocyanate groups are reacted with a polyol, the resulting polymer is crosslinked. In reaction systems where there is an excess of isocyanate, crosslinking reactions may occur.

 

General Polyurethane Adhesive Component
 

The polyurethane adhesive formulation generally consists of the components listed in table below:
 

• The most commonly used isocyanate in polyurethane adhesives is MDI (methylene diphenyl di-isocyanate). TDI (toluene di-isocyanate) has also been used but is now limited because of its toxicity. Both of these di-isocyanates are aromatic. Aliphatic isocyanates are also used but in smaller volumes.
• Polyols of widely different types are used for the production of adhesives. The most common polyols are polyether (polypropylene glycol) and polyester (adipate) polyols.
• Other isocyanate co-reactants of commercial significance are amines, polycaprolactones, polyglycols, and natural and hydroxyl containing oils.

Health & Safety Concerns with Polyurethane Adhesives

Like any other reactive adhesive or sealant, polyurethane systems may contain certain hazardous or regulated components. Significant effort has been made to reduce or even eliminate such materials in conventional polyurethane adhesives and sealants. Health and safety information on polyurethane products can be found through several organizations:
 

• In the US, resources include the Polyurethane Manufacturers Association (PMA) and the Center for the Polyurethanes Industry (CPI).
• In Europe this sort of information is available through ISOPA, the European Di-isocyanate and Polyol Producers Association.
• Of course, important information is also available through the manufacturers' MSDS (Material Safety Data Sheet).

The migration of intentionally added substances such as plasticizers, oils, and regulated components has been well-studied by the food packaging and adhesive industries. Recent concerns for food safety have centered on adhesive components that are not intentionally included in the formulation but can form during normal aging of the package. These are mainly primary aromatic amines (PAAs).

Having learnt about the root cause of migresives, let's learn how to prevent this leaching of adhesive in food material...

 

Certain aromatic amines have been shown to cause cancer, others are of toxicological concern, and still others are of no health consequence. Because of this, PAAs have come under study as a potential food safety hazard. Aromatic amines are a class of organic compounds that can arise from aromatic isocyanate and water as shown in figure below:

Isocyanate Reaction with Water to Produce a Primary Amine

It is believed that traces of PAA originate from unreacted aromatic isocyanate in the polyurethane adhesive layer. Once the unreacted isocyanate migrates into the food package, they will react with moisture in the package to form the PAA.

The factors influencing possible PAA formation are shown in the table below:
 

Figure below illustrates such a mechanism for a polymeric film bonded to a sealing layer with a polyurethane adhesive.

Migration and Formation of PAAs

However, other sources of PAA are possible. For example, it is known that polyurethane polymers may degrade by hydrolysis or thermal stress which results in the creation of monomers that are rapidly converted to amines.

 

Preventions to Avoid Migration of PAA
 

The migration of unreacted isocyanate or the hydrolysis of polyurethane will be accelerated with increased temperature. Therefore, special attention needs to be given to laminates that are to be used when heating food products.

To prevent formation of PAAs it is of crucial importance not only to ensure that the adhesive has been appropriately formulated, but also that the adhesive is fully cured before entering service (so that all of the isocyanate is reacted) and that the finished packaging is only used within its limits.

The criterion of 'full cure' is generally handled by the packaging industry by allowing sufficient time after lamination and before the release of a product to the customer. With certain adhesive formulations this can require up to two weeks of storage. So the goal is sometimes not so much to provide food safety as it is to maximize the productivity of the laminating manufacturing process. 

 

In order to appraise the safety of packaging materials for use in contact with foods, it is necessary to know the following  

1. Whether or not any of the packaging components are leached into the contacted food
2. The amount of packaging extractable in the food, and
3. Possible materials that can be extracted

Migration of compounds from food contact materials into food can be assessed using basic principles of diffusion, depending on their chemical and physical properties. The compounds' concentration, molecular weight, solubility, diffusivity, partition coefficient between polymer and food must all be considered. Of course, the food composition must be known so that appropriate food stimulants can be used as target elements. Polymer structural properties, including density, crystallinity and chain branching, also influence migration processes as do time and temperature.

In 'Migration of Substances from Food Packaging Materials to Foods', a leading paper on migration testing, Arvanitoyannis and Bosnea explain that there are several methods for evaluating the migration of polymers⁵. Although some researchers have been working on creating diffusion equations for mathematical modeling, this method is not yet as accurate as simpler, established modes of evaluation. Specifically, this involves weighing a food simulant before and after prolonged exposure to food packaging. If the material weighs more after exposure, then it is safe to conclude that materials have migrated from the packaging to the food. 

 

Technique to Assess Migrant Concentration

A main goal of the MIGRESIVES consortium³ is to verify the tools for identifying the type and amount of PAA that forms in food packaging. The most useful method found to date is head space solid phase micro-extraction gas chromatography/mass spectrometry (HS-SPME-GCMS).

SPME in particular is a solventless and easy-to-use extraction technique, which has already seen use in investigating the volatile components of food. After establishing calibration curves this method can be used to identify compounds down to limits of around 10-40 nano gm/gm. Given the variation in the partition and diffusion coefficients, the ability to assess migrant concentration profiles is likely to be one of the most significant individual outcomes from this study.⁶ 

 

Major changes have occurred in 100% reactive solventless laminating adhesives over the last decade. The composition and comparative properties of typical polyurethane laminating adhesives are summarized in table below:⁷
 

First Generation Products
 

The first solventless laminating adhesives were primarily moisture-cured polyurethanes. The adhesive is coated onto a substrate and atmospheric moisture reacts with excess isocyanate groups to crosslink the adhesive after the secondary film has been joined. Slitting of the laminate generally can occur in 24-72 hours. The potential risk of migratory species from these first generation adhesives is relatively high due to the lamination temperatures used and the ease of formation of aromatic amines from any unreacted monomeric aromatic isocyanate.

 

Second Generation Products

Two-part solventless polyurethanes were also developed to negate some of the disadvantages of the moisture cured type, such as:
 

• Bubbling
• Inconsistent cure rate, and
• Cloudiness

These problems were generally associated with variations in the ambient moisture content. The two-part polyurethane laminating adhesives require a mixing and metering unit since pot life is limited. High residual monomers and low initial bond strengths somewhat limit the application of these adhesives.

Two-part second generation adhesive systems are the most widely used of the solventless products. However, concerns over diamine or other extractable components from these adhesives have led to even further improvements. 

 

Third and Fourth Generation Products
 

Third-generation systems consist of isocyanate-terminated prepolymer (polyol prereacted with isocyanate) having reduced free monomer (< 1%). These adhesives are generally based on methylene diphenyl di-isocyanate (MDI) or toluene di-isocyanate (TDI). Third-generation products have viscosity and different application needs compared to earlier products.

Fourth generation products are simply lower viscosity third generation products. Third and fourth generation products are also slower curing and more expensive. They often require a warm room of 30°C to 50°C for complete curing. These later generations of polyurethane adhesives are made from processes that remove nearly all of the excess isocyanate monomer from the prepolymer. 

Aliphatic polyurethane adhesive solutions may be the ultimate solution. With aliphatic isocyanates such as hexamethylene di-isocyanate (HDMI) or isophorone di-isocyanate (IPDI), PAAs will not occur at all, no matter what the temperature exposure. Today, aliphatic adhesives can be designed to have none of the major disadvantages that earlier aliphatic products had (e.g., need for tin-based catalysts and a temperature chamber for curing). ⁸

 

As might be expected from the above discussion, health and safety regulations are one of the most significant factors in choosing or formulating an adhesive for food packaging applications. The table below illustrates the relative distribution of adhesive selection criteria according to a recent poll of the food packaging industry.⁹
 

Future development of the food contact regulations will continue to address these issues, and new adhesives will require developing to meet the needs of the industry. If the future regulations were tightened, a greater interest in fourth generation polyurethane laminating adhesives especially aliphatic isocyanate systems would likely result.