Solutions that stick7 January 2020
More than 80% of food packaging contains adhesives, although often only to a small extent. Depending on the food in question, there are very different requirements on its packaging. To meet all requirements, the adhesives industry offers products based on a varied range of technologies for use in food packaging. Dr Monika Tönnießen, manager of product safety and regulatory affairs at Henkel Adhesives, casts an expert eye over where each adhesive best fit into packaging for food safety.
Recently, FEICA (Association of the European Adhesive and Sealant Industry) members prepared a ‘Black and White’ report to direct food packers and adhesive users, as well as other members of the supply and value chain, and scientific staff with an overview of adhesives that can be used in food packaging. In addition, the report describes the typical ingredients of the various types of adhesives and their different behaviours. Further, it describes typical applications of the various adhesives, thus giving the reader assistance in choosing the right adhesive for his or her individual application.
Finally, the report provides an overview of the food contact legislation relevant for food packaging adhesives, together with guidance on the evaluation of adhesives intended for food contact applications. The report informs potential users about the successful use of adhesives in the various fields of food packaging. It provides a basic understanding of different types of adhesives, taking into account the complexity of the wide variety of possible applications.
Technology influences application
Adhesives based on reactive polyurethanes are indispensable in the production of flexible packaging. Most of this packaging is composed of two or more layers held together by laminating adhesives.
While solvent-based polyurethane adhesives need a drying unit before going through the lamination process, solvent-free polyurethane adhesives can be used without this production step.
Because polyurethane adhesives have high flexibility and bonding strength, they are widely used throughout the packaging industry. Food packaging containing polyurethane adhesives (such as bags for coffee powder and crisps) can be used for applications at room temperature and below, but some formulations are also suitable for elevated temperatures such as those found in pasteurisation or retort applications (for example, flexible packaging of aggressive foodstuff such as fruit juices or liquid pet food). The technical information provided by the adhesive manufacturer should be followed to ensure complete curing. Time, temperature and humidity are the most important factors for the curing process. With regard to the curing conditions, the advice of the adhesive supplier should be followed to avoid the possible migration of primary aromatic amines.
Natural polymers adhesives in food
Adhesives based on colloidal solutions of natural polymers in general dry slowly and therefore need a very long setting time. Water-based adhesives containing starch, dextrin and/or casein are primarily used in labelling applications. Pure starch and dextrin glues are mostly used in less demanding standard labelling applications. Starch glues are also widely used in the production of corrugated board.
Casein adhesives show a very high increase in viscosity as the temperature decreases, which makes them suitable for labelling cold and wet bottles on high-speed labelling machines, where a high wet tack is required. Because casein glues can be easily removed in caustic soda solutions, they are ideal for returnable glass bottles. Adhesives based on colloidal solutions containing tackifier resins are also suitable for plastic bottles.
Water-based adhesives containing cellulose or cellulose derivatives are often used for bonding paper in tissue and towel applications for the tail-tie bonding of toilet and tissue paper rolls and in cigarette applications for cigarillo leaf wrapping.
Typical applications of adhesives based on dispersions/emulsions
The most important applications for vinyl acetate-based homopolymer and copolymer dispersions are the production and processing of packaging made of paper, cardboard and corrugated board-folding boxes bags, pouches and sacks, core and tube winding, for example. PVAc homopolymers normally need plasticisers and therefore are mostly used in secondary packaging applications while PVAc copolymers are used for primary packages.
Because copolymers show better adhesion properties than homopolymers, they are suitable for bonding paper to other substrates such as foil or aluminium.
Pressure-sensitive adhesives adhere to a surface when gentle pressure is applied. As precoated systems, they have permanent tack and adequate cohesion. Typical applications are self-adhesive paper and film labels or tapes. These paper labels can be directly applied on food, such as fruits and vegetables, as well as sausages.
With laminating, adhesives for flexible packaging acrylic dispersions can be used to produce multilayer lamination of various films. These water-based adhesives offer high green strength, have short curing times and are free of primary aromatic amines.
For retort applications, acrylic dispersions can be cross-linked with water-dispersible aliphatic polyisocyanates.
In glossy film lamination, acrylic dispersions are used to laminate clear, high-gloss polymer films onto printed paper or board. The polymer film protects the printed surface and enhances the optical appearance of the print. To increase the resistance to grooving and embossing, such adhesives are often self-cross-linking after drying.
Polyacrylate dispersions can also be used as reclose adhesives, synthetic coldseal or protective film adhesives.
Typical applications of hot melt adhesives
Various grades of hot melts are available, and optimised for different applications. Hot melts can be used for carton and case closing, tray erection (for fruits and vegetables) and wrap-around boxes among other applications such as labelling. The most common packaging application is case and carton sealing (for example, cereals), closing of folding boxes (primary or secondary packaging; for example, confectionery, biscuits, frozen meals), wrap-around (chocolate) or corrugated cartons (tertiary packaging).
Other hot melt applications include the attachment of straws or bonding of plastic caps onto bricks (milk) or even bag-in-box (juices, wine).
Hot melt pressure-sensitive adhesives are used for labelling and to manufacture a wide range of tapes, labels and speciality products, where the adhesion can be permanent, or semi-permanent (repositionable) removable and dry peel. In the packaging industry, PSA hot melts are often used for reclosable food packs and double-sided tapes. Permanent closure of PE, PP or paper bags is also a typical PSA application.
Heat-sealable hot melt adhesives are used for packaging applications on high speed machines, where hot tack and very good sealing properties are required. Substrates can be paper, film, cardboard, aluminium or combinations thereof.
Coldseals are typically handled within a temperature range of 15°C to 25°C and the absence of heat makes them ideal for packaging temperature-sensitive foodstuff such as ice cream, chocolate or biscuits. In addition, high packaging speeds in horizontal form fill seal (HFFS) machines can be achieved, without compromising tailored solutions for specific pack-opening characteristics. This includes high-performance applications such as reclosable food packaging (chocolate bars).
Heatseals are predominantly used for lidding applications, where aluminium or plastic lids are bound to food containers; for example, packaging of dairy products such as yogurts, plastic trays and instant noodle cups. Heatseals are also used for pharmaceutical blister applications or flexible laminated foil packages.
Special attention has to be paid to the opening behaviour in lidding applications. On the one hand, bonding has to be strong enough to ensure reliable closure of the package during storage and transportation. On the other hand, smooth opening of the food container is required when removing the lid – it should not rupture, but be removed completely.
To assist users with the evaluation of their adhesives, FEICA has published a guidance paper, including a decision tree on how the downstream user might evaluate specific adhesives.
Many will know that migration is substance transfer from packaging material into foodstuff and that adhesives contribute to this substance transfer. The lower the molecular weight of the substance, the more likely it is to migrate into the food, either in direct contact by diffusion or indirect via the gas phase (vapour phase transition). Typically, compounds below 1,000Da show migration potential, whereas pure polymers of several thousand dalton and above do not need to be considered.
Several measures can be taken to reduce the risk of migration. On the one hand, reducing low molecular weight compounds of less than 1,000Da within the adhesive formulation may help to reduce the general substance transfer. On the other hand, packaging design can be an appropriate tool – installation of barrier layers prevents migratable packaging constituents from being transferred into the food. Such layers may reduce or even inhibit migration under defined conditions of use (functional barriers that offer protection under defined storage conditions; for example, PET layers), or may even act as an absolute barrier (glass, aluminium).
Analysis of the packed foodstuff gives answers to the following questions: what type of packaging constituents (including the contribution of the adhesive) have migrated into the food? Regarding quantity, how much of each substance has migrated into food? Are all legal limits being respected under the intended conditions of use?
Although analysis of real foodstuff gives the most realistic information about migration in these food contact scenarios, it is time-consuming, complex and its results are only valid for the investigated application. To simplify these investigations, three different approaches to estimate migration have been established:
The first is through calculation. Some constituents can be ruled out by taking their maximum concentration in the formulation and assuming a total substance transfer in the real application. If under these most severe conditions the limits are not exceeded, the formulation can be assumed to be safe with regard to these substances. Further investigations for these compounds typically are not required. The next is through migration modelling; as migration follows the law of diffusion, computer models have been established to predict the expected migration of defined compounds under defined conditions. Finally, there is migration testing, to establish exactly how much of each substance has gone migrated.
NIAS (non-intentionally added substances)
The European Commission defines NIAS in the production of food contact plastic materials as, “Non-intentionally added substances are either impurities in the substances used or reaction intermediates formed during the polymerisation process or decomposition or reaction products which can occur in the final product.” Yet it is generally accepted that only compounds <1,000Da are risk assessed as NIAS, since substances with greater molecular weight usually cannot be absorbed by the body.
Risk assessment of a food contact chemical can be carried out using many different approaches or methodologies. Knowing the migration behaviour of substances included in the adhesive formulation, the producer can substitute or reduce the concentration of those substances which migrate over the required limits. This is called “safety by design”.
In the absence of EU harmonised specific measures for non-plastics, the adhesive manufacturer may also use non-listed substances. In this case it needs to perform a specific risk assessment. A useful tool to assist adhesives manufacturers in their risk assessment is the FCA guideline 11 on non-listed substances.
This article is abridged from ‘Packaging Materials: Adhesives for Food Packaging Applications’