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Migresives in Flexible Food Packaging: Solventless PU Adhesives

Edward M. Petrie – Mar 8, 2018

Migresives in Flexible Food Packaging Many different adhesive types are used in packaging due to the different materials that must be bonded and to the demands of the production processes and service conditions. Flexible laminates make up the largest component of the packaging industry in applications such as:

 − Freezer-to-oven packages
 − Food and beverage pouches
 − Medical and pharmaceutical containers
 − Portion packaging
 − Blister packaging
 − Industrial containers (e.g., chemical, agricultural, etc.)
 − Others

In most of these applications; the packaged product must be protected from not only the general external environment (e.g., moisture, oxygen, UV), but also from the adhesive and packaging chemicals that may be initially present in the package itself or that are produced during service.

In food packaging the safety of the packaged product is of paramount importance.

The major types of adhesive systems that are used for producing flexible laminates for these products are summarized in the table below:

Adhesive Type Form Major Applications
Acrylics Aqueous emulsions and solvent solutions Lamination of flexible films, heat seal coatings, pressure sensitive adhesives (tapes and labels)
Polyesters Solvent solutions Lamination of flexible films
Polyurethanes Solvent solutions, aqueous dispersion, hot melt, solventless Lamination of flexible films
Metallocene polyolefins
Hot melt resins Lamination of flexible films
Styrene block copolymers Hot melt, solvent solutions Pressure sensitive adhesives, laminating adhesives, heat seal coatings
Polyvinyl alcohol Aqueous solutions Solid board lamination, paper lamination
Ethylene vinyl acetate copolymers Aqueous solutions, hot melt Lamination of flexible films, heat seal coatings
Vinyl acetate ethylene copolymers Aqueous dispersion Lamination of polypropylene film to solid board

To learn the safety limit for adhesives in food contact applications, we first need to go through the composition of adhesives. This would help us understand the additives added deliberately and non-deliberately better. Let's explore the same...

Intentionally & Unintentionally Added Additives in 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.

Food Contact Adhesives, Migressives & Their Regulation

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

EU's Migresives ProgramPotential Migressives 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.

Polyurethane Laminating 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 used adhesive for laminating flexible food packaging materials.

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

Type Solvent Curing Mechanism
One component Ester or ketone Reaction with moisture in atmosphere or on substrate surface
Two component; high solids Ester or ketone Reaction between isocyanate terminated resin and polyol
Two component; low or medium solids Ester or ketone Reaction between an isocyanate and polyol
One or two component, 100% solids None  − Reaction with moisture in atmosphere or on substrate surface
 − Reaction between isocyanate terminated resin and polyol

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
Reaction of Isocyanate with Hydroxyl Groups

 − The isocyanates will also react with water to form a urea linkage
Reaction of Isocyanate with Water
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:

Component Description
Isocyanates Usually MDI or polymeric MDI
Active Hydrogen Compound (Polyols)  − Usually polyester polyol or polyether polyol, but also
 − Polyglycols, polycaprolactones, and natural hydroxyl containing oils
Catalyst Tertiary amine, di-butyl tin laurate, etc.
Chain Extender Usually a polyamine
Additives  − Drying agents
 − Antioxidant
 − UV inhibitor

 − 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 migressives in PU adhesives, continue reading to explore the strategies to control the same along with some formulation tips to formulate solventless PU adhesives

1 Comments on "Migresives in Flexible Food Packaging: Solventless PU Adhesives"
Juan Luis Velasquez S Mar 23, 2018

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