Adhesives Keep the Promises Others Make

Packaged food units sealed with adhesive have to withstand multiple loads until they are opened by the consumer – without any adhesive power being impaired by such influences as heat, cold or moisture. In addition to the requirements made on the finished packaging the processability of adhesives on packaging machines is also of major importance here.

interpack

Food packaging must comply with a whole host of requirements. This packaging is designed to provide information, separate, protect and conserve. Thanks to the development of innovative, flexible packaging solutions packaged soups and cooking additives remain dry, wet wipes wet, meat hygienic and cheese fresh. Coffee and tobacco retain their aroma and the inner lining of juice packs is not attacked. Without lamination these lightweight, flexible packaging units would not be possible. And their success primarily depends on joining the various packing materials with adhesives: ultimately it is the adhesives that have to keep product and packaging producers’ promises.

Flexible and Safe

The food industry is a principal consumer of flexible packaging: allowing coffee, chocolate, snacks, soups, frozen food and many other products to be packaged in an attractive, hygienic and inexpensive way. Easy for consumers but complex for packaging producers and the underlying technology. Even the primary material, sheer plastic films that come in webs, have to be joined with the help of lamination adhesives to form a composite. To obtain better barrier properties for sensitive foodstuffs composite materials like this have to be reinforced by an aluminium foil glued in between the two plastic films in some cases. Here, again, the quality of the lamination adhesives is key. Furthermore, strict food laws also apply here. International food multi-nationals make ever higher safety demands on their upstream suppliers. For instance: the film/foil composite is only considered safe in food law terms if all components of the lamination adhesive capable of migrating are 100% cured. This can take up to several weeks with conventional lamination adhesives.

Henkel, to name but one company, has risen to these complex challenges: high-performance adhesives for flexible packaging that reliably protect foodstuffs against outside influences while excluding contamination by hazardous substances from the packaging materials themselves. A case in point are Liofol lamination adhesives which are thought to set new standards here; according to the manufacturer these adhesives attain a hitherto unachieved level of safety thanks to extremely short curing cycles. For the lamination of simple composites the film/foil composite is said to only require one day for complying with food law provisions. And for complex composites the curing time is said to shorten from two or more weeks to a maximum of four days.

Easy Handling – Complex Process

Likewise, incorporating the attachment of closures into the packaging process places the greatest demands on adhesives. Adhesives for carton closures must be able to be processed on high-speed machines, they should boast excellent hot-tack properties and thicken fast. They must adhere to both lacquered and untreated surfaces and require high heat resistance for hot filling processes and the corresponding resistance to low temperatures for frozen applications. In addition to extremely high adhesive force, excellent flow behaviour and a wide variety of applications, machinability, naturally, also plays a central role. With the newly developed “Technomelt Supra Cool 130” Henkel now plans to also offer these properties for low processing temperatures. Thanks to Supra-Hotmelt processing temperatures could be lowered for the first time by 40 ºC, on average to 130 ºC. This is expected to save up to 30% of energy expenditure and bring down the volumes consumed by 20% to 35%. Furthermore, Supra-Hotmelt is said to be thermally more stable compared to other hot melts and feature a very clean flow behaviour which is non-stringy. Thanks to the lower processing temperature melting time of the adhesives is shorter and machine ramp-up is faster.

Each Packaging Type a Challenge

Adhesives development for the packaging industry faces innumerable other, sometimes very specific requirements. Key here are the labels – because they make (or break) brands. Adhesives must be correspondingly sensitive because they can be decisive for the success or failure of products. A sheer film must suffice to make the attractive label stick perfectly. Here adhesives must be tuned to the surface of the container just as much as to the material and design of the labels, labelling machine techniques, the intended uses and recycling processes.

Another example is in-line fastening of re-closable, leak-proof closures for beverage packs on a packaging line. Adhesives must tack to both the polypropylene closure and the PE-coated, treated or untreated cardboard pack – even on wet or printed surfaces. It must be suitable for clean machining with the existing equipment, withstand speed and temperature fluctuations and remain intact even after repeated opening and closing. For direct food contact FDA/BGVvV approval is also required. If, for instance, straws are to be attached, adhesives have to be suited for use with both OPP and PE films. The surface of the container, the dyeing and printing processes must be factored in. Adhesives must be cleanly processed, and non-stringy, require FDA approval and, sometimes, even high resistance to low temperatures.

The demands associated with trickle-proof packaging are different yet again: this packaging has to protect its products – such as washing powders, mueslis or seeds – against the ingress of foreign matter and it must also keep the material to be filled from leaking. Here surface sealing does the trick without the need for liner bags which again saves packaging costs. And to cope with tray lifting, adhesives have to withstand high machine speeds and high ambient temperatures in the packaging lines. Export markets demand resistance to changing climatic conditions. Adhesives for secondary packaging must be suited for different packaging materials and feature high initial tack. Furthermore, they must be suitable for slotted nozzle as well as segment-application procedures. And for chocolate wrappers adhesives are required to be machinable also at very high machine speeds while also being suitable for clean, pointed application.