Stick with innovation
Stick with innovation
Innovative adhesive raw materials support the faster and ‘greener’ production of flexible packaging. Dr Frank Rothbarth of Covestro discusses new developments and solutions, and how they support the converting process.
Made of composite films, flexible packaging is increasingly replacing cans and glass jars for a number of reasons. Firstly, because it meets the consumer demand for packaging that protects the contents while being easy to handle. Secondly, because it offers brand-owners the opportunity to distinguish themselves from the competition thanks to its almost boundless options for shape and design. And third, all stakeholders – consumers, retailers and producers – benefit from flexible packaging being lighter than metal cans or rigid plastic containers. Lower weight is a major advantage: it reduces transport costs, saves energy and conserves resources, and more product packages can fit on supermarket shelves.
Analysts at international market research firm Ceresana emphasised the rising significance of innovative packaging in its 2016 study of food packaging in Europe. “Composite films comprising several layers of different materials are being increasingly used to meet complex demands,” the report’s summary states. “At the same time, new and innovative packaging types and technologies are emerging, such as stand-up pouches, modified atmosphere packaging, intelligent packaging, high-barrier films and shelf-ready packaging.”
One decisive element of innovative packaging is not visible on the outside: the adhesives used to laminate films and foils. Polyurethane (PU) polymers are commonly used as adhesives for packaging. They can be fully adapted to the production conditions, materials and functions of the packaging. A wide variety of raw materials are available for formulating adhesives. Covestro recently developed raw materials that significantly enhance the efficiency of lamination and the downstream steps in packaging production. Using these raw materials reduces the cost and energy consumption of packaging production.
Innovative adhesives for innovative packaging
Covestra offers two special, ‘green’ raw materials for adhesives: one contains carbon acquired chiefly from plants and not petroleum; the other is biodegradable in industrial composting. Adhesives manufacturers thus have two viable options open to them for meeting the trend towards greater sustainability. The Ceresana market researchers are convinced: “Because the term ‘sustainability’ is gaining importance for an increasing number of consumers, packaging that is considered ecological will see strong growth.”
All the adhesive raw materials mentioned meet the special regulations that apply to food and drug packaging to ensure that it poses no health risks to consumers.
There are PU adhesives to meet any and all needs. They can be used to laminate composite films for dry foods, or to laminate packaging for fruit juices and other fatty or acidic foods. The more demanding the application, the more resistant the adhesives must be to chemicals and heat. Adhesives have to perform to the absolute best of their ability in packaging that is steam-sterilised at temperatures of up to 134 °C to extend the shelf life of products. PU adhesives can live up to this requirement.
A distinction must be made between solventborne, solvent-free and waterborne PU systems. Solventborne two-component (2K) PU adhesives are used mainly to laminate high-performance composites. They usually comprise a hydroxyl (OH)-functional prepolymer and a solventborne isocyanate crosslinker.
One particularly innovative crosslinker is Desmodur quix 175. Because it is a crosslinker with aliphatically bound isocyanates (NCO) groups, no risk whatsoever of the primary aromatic amines migrating into the packaged food exists. A unique feature of this product is that adhesives formulated with it cure very rapidly at room temperature. This can significantly shorten the one to two-week storage period at elevated temperatures usually required for packaging that uses aliphatic hardeners based on isophorone diisocyanate (IPDI). The storage capacity required previously and the capital tied up as a result can now be used for other purposes. Energy consumption is reduced at the same time, as well as the cost of the heating otherwise required for curing (Fig.1).
Adhesives based on this product can be used for packaging that is sterilised at temperatures above 121°C, and approved under Title 21, Code of Federal Regulations section 177.1390 of the US Food and Drug Administration (FDA).
Covestro has also introduced eco N 7300, which is another isocyanate hardener. Its key feature is that 70% of its carbon content is derived from biomass. Solventborne adhesives formulated with this product are suited to the same applications as those made with the established and successful hardener N 3300. They are just as reactive and have the same properties. The carbon atoms are sourced from starch in feed corn, which is converted enzymatically by specially developed microorganisms in a highly efficient process. Feed corn is not suitable for human consumption, meaning that this application does not compete directly with food production. Nevertheless, efforts are being stepped up to use biowaste or cellulose in the future instead of feed corn.
Solvent-free and safe
Solvent-free two-component PU systems are gaining importance, particularly with respect to adhesives for flexible packaging that need to meet general to mid-range requirements. Polyols in a wide range of molecular weights are used as the OH functional components. NCO functional prepolymers serve as the isocyanate component, which must have a low to extremely low percentage of monomeric isocyanates to ensure food safety. Prepolymers of this kind can be processed in combination with polyfunctional aliphatic isocyanates, such as N 3300 or N 3600, to reduce the viscosity of the resulting adhesive, and further improve its crosslinking ability and product safety.
Waterborne PU adhesives have advantages over solventborne systems in terms of environmental compatibility, and occupational safety and health. Low application viscosities can be achieved even with high-molecular polymers. Such high-molecular polymer dispersions can also achieve good initial bond strength, meaning that laminates containing waterborne PU adhesives can be further processed right away.
Waterborne PU dispersions can be used alone or together with a crosslinker. They emit virtually no solvent, improving occupational safety and health. An important feature of PU dispersion U XP 2682 is that the polymer breaks down under the conditions prevailing in an industrial compositing facility. In other words, it enables adhesives manufacturers to formulate compostable adhesives for flexible packaging. The polyurethane dispersion also meets all the requirements of food-contact applications. Moreover, adhesives formulated with this product display good bond strength.
Waterborne 1K PU systems have the basic advantage over 2K systems in that they are easier to use and errors during mixing do not occur. It used to be the case with such 1K adhesives that high reactivity and rapid curing conflicted with good storage stability. Today, however, reactive dispersions that remain stable for up to six months and are easy to use during this period can be formulated (Fig. 2). These adhesive dispersions contain a waterborne adhesive polymer with a carboxyl group and the crosslinker XP 2802, an aliphatic polyfunctional carbodiimide. The long shelf life is achieved by phase separation between the polycarbodiimide and the polymer droplets. When the water in the dispersion evaporates, the droplets form a homogeneous film. At the same time, the reactive groups come into contact with one another and the crosslinking reaction takes place at a fast rate. Composites laminated with reactive dispersions based on XP 2802 can be further processed right away, increasing the efficiency of packaging production and shortening lead times. Furthermore, the use of 1-K systems with a long shelf life reduces adhesive waste.
Innovative hardeners and PU dispersions enable adhesives manufactures to offer safe solutions for the changing requirements of the packaging industry and end consumers. Furthermore, adhesives manufacturers and the downstream packaging companies in the value chain can position themselves as pioneers of greater efficiency and sustainability. These are decisive advantages in a business characterised by strong competition. Accelerated production processes in the packaging industry further help brand manufacturers to react very rapidly to changing market trends, with quick design changes or smaller batches, for example.