LABELS PRINTING AND PACKAGING


The role of labels in the recycling process and contribution of multi-layer adhesives

02/10/2019

CATEGORY: Labels BRAND: Herma

Europe has declared war on litter caused by plastic packaging. The new Packaging Act has been in force in Germany since the beginning of the year. One of the specific aims of the new law is to significantly boost plastic recycling rates. Thanks to multi-layer adhesives, labels can be successfully removed from the packaging in question, simply, efficiently and therefore affordably, thus allowing the entire recycling process to be optimised.


Marcus Gablowski

 

From now on more packaging waste must be recycled. This is one of the measures stipulated by Germany’s new Packaging Act, which became law on January 1st this year. The new law also imposes additional obligations on retailers and businesses, which are required to license packagings being placed on the market for the first time. Enterprises that do not comply with the new rules face heavy fines and injunctions from competitors.

 

But things are currently happening at a corporate level too: for instance, 30 well-known corporations, including BASF, Procter & Gamble and Henkel, have joined forces in a global alliance to develop products and technologies to reduce and eliminate plastic waste, especially in our oceans. The self-styled Alliance to End Plastic Waste (AEPW) intends to invest USD 1.5 billion in this in the next 5 years. In October last year organisations from right along the label and packaging supply chain had already signed up to the New Plastics Economy Global Commitment, a worldwide initiative to combat plastic waste and pollution. And Nestlé, the world’s largest manufacturer of food products and drinks, is involved in the “Stop Ocean Plastics” (STOP) project, whose aim is summarised up as follows: “STOP is designed [...] to prove that end-of-use plastic (packaging) can be collected and returned into valuable resources”.

 

In many instances the key word is “recycling”, but the term is still shrouded in myth. The truth is that, until now, plastic packagings could be incinerated as a way of meeting ‘recycling’ rates, a practice now ended by the new EU regulation. Furthermore, recycling in the true sense means that the discarded packaging must give rise to something that is at least equivalent to its previous use: the old shampoo bottle becomes a new one. Yet in the past this hardly ever happened. In reality, what we often encounter, even today, is downcycling: the shampoo bottle becomes sheeting for silage, say. A more deserved solution would be upcycling instead, in other words refining the packaging waste. In each case, however, before the plastic can be reprocessed it must not only be pure in type, but also as far as possible be free from other materials. Inks that are printed directly onto the plastic pose a major problem in this connection. Labels therefore not only offer the benefits of being based on a very familiar, extremely versatile technology, offering wide-ranging scope for converting and allowing variable data easily to be subsequently applied: but also, with their help, the de-inking process can be carried out in a single step alongside the clean separation of the valuable plastic components. That said, however, their removal in particular can prove challenging, and may in certain circumstances necessitate an additional caustic treatment or washing stage. Problems can arise if the adhesive bonds are not separated and the labels remain adhered to their host surface, thereby impairing their physical properties. This can seriously reduce the value of the recycled plastic, for example. Recycling-friendly self-adhesive materials must consequently satisfy requirements that are to some extent extremely contradictory: within the recycling process they should aid the clean removal of the labels during the washing stage. But the labels must continue to fulfil their function during the product’s useful life – which means, among other things, that their adhesive properties must not be compromised.

 

A technical solution to this dilemma is offered by multi-layer adhesive materials, which make plastic recycling simpler and more flexible, whilst encouraging high-quality recycled materials to be profitably reclaimed. HERMA has already proven on more than one occasion in other contexts that seemingly contradictory label requirements can be overcome with multi-layer technology without having to make compromises: the need to combine maximum migration safety with excellent adhesive characteristics is our goal and strategy. Therefore HERMA has evolved almost its entire standard range to multi-layer technology. One of the reasons why these adhesive materials have been so successful on the market is the added value they create for users without increasing their costs. How is this achieved? Contrary to the belief in some quarters, these are not expensive special materials but, in essence, conventional multi-purpose products with a very broad application scope. Given that the adhesive layers are based on dispersion adhesives with an excellent and established track record in the marketplace, the label stock is also very easy to process, which again makes it stand out against many special materials.

 

Before covering our approach to improving recycling processes involving labels, we should remind ourselves of our unique multi-layer unique principle. Generally, conventional label stocks consist of a label material, a single homogeneous layer of adhesive, a release coating (usually silicone-based), and a release liner (see Fig. 1).

 

Fig. 1: Schematic structure of a generic conventional label stock.

 

 

With multi-layer adhesive systems, in contrast, an adhesive coating consisting of at least two different layers is applied between the label material and the coated liner (see Fig. 2).

 

 

Fig. 2: Schematic structure of label stock with two adhesive layers.

 

When a conventional label (with a single layer of adhesive) is compared with a two-layer adhesive system, it becomes apparent that the intermediate layer of the two-layer system provides a further means of controlling the label’s recycling properties without influencing or compromising the adhesive characteristics. Unlike the outer layer, which must be chemically formulated to achieve specific adhesion with a particular surface, the chemical composition is less important for the intermediate layer (at least with regard to adhesion). For this layer, the rheological characteristics are what count. In the following, we set out in detail how multi-layer adhesives can be exploited to add value in connection with the recycling process. For this purpose both conventional labels with a single layer of adhesive and labels with a two-layer adhesive system were put through various tests. The tests were intended to investigate and verify three essential properties of a recycling-friendly material: excellent adhesion, reliable wash-off capability and a definite, residue-free removal.

 

Ice-bucket test

We started with the ice-bucket test – the practice-based endurance test of a label’s adhesion when exposed to water. We took four conventional, commercially available labels largely designed for wash-off solutions with a single layer of adhesive, and three labels having a two-layer adhesive system and immersed them in cold water at 4 degrees. In the case of the multi-layer adhesive materials, which were all taken from HERMA’s own laboratory, we used three different variants: a conventional, standard version, a type with increased water sensitivity of the outer adhesive layer (cf. Fig. 3, see 2, I/D), and finally a variant in which we also reduced the thickness of the intermediate layer (cf. Fig. 4, see 2, II/D). Our working hypothesis was that the use of an intermediate layer would improve anchorage, but reduce water solubility.

 

 

Fig. 3: Increased water solubility in the ice-bucket test.

 


Fig. 4: The intermediate layer is reduced.

 

At set intervals (twenty minutes, one hour, two hours and four hours) each label was checked for lifting off and for any wrinkling. The target we set ourselves was that a label needed to last for four hours in the ice bucket without lifting off or wrinkling. One of the conventional labels had already lifted off after just two hours (cf. Fig. 5, 1 B). Another had at least formed wrinkles (cf. Fig. 5, 1 D). All the labels with the two-layer adhesive sailed through the four-hour ice-bucket test without any negative signs. Important: The adhesion of the variant with the reduced intermediate layer was not any different from that of the other two multi-layer variants.

 

Fig. 5: The measuring results of the ice-bucket test reveal initial weaknesses in labels with a single adhesive layer.

 

Wash-off test

But are there any actual advantages when the label is detached if the intermediate layer is reduced? The so-called wash-off test is an effective measuring method for evaluating the speed at which labels are removed. In the course of the test, the labels were exposed to a 1.5% lye solution at a temperature of 65 °C. These parameters are equivalent to a low-impact removal operation. As well as recording the time until the label had been completely removed, all changes in the material and any adhesive residues on the test plate were visually assessed. Multi-layer adhesives did not automatically have the edge here – nor was this our expectation (cf. Fig. 6). But what became clear was evidence that the removal process could be speeded up by reducing the intermediate layer in the case of multi-layer materials. Here the label could be removed in under two minutes (cf. Fig. 6, see 2, II/D). Admittedly, a similar value was scored in this test by one of the variants with a single adhesive layer, but the latter had shown shortcomings in the ice-bucket test.

 

Fig. 6: By reducing the intermediate layer, the label removal process can be speeded up in the case of multi-layer adhesive materials.

 

Roasting test

But had all adhesive and label residues really been removed by the wash-off process? In the so-called roasting test, the washed plastic flakes were exposed to a temperature of 220 degrees Celsius for a total of twenty minutes in the drying cabinet. In this instance the test results could not have been clearer. Thus, all the flakes whose labels had been manufactured with a single adhesive layer showed a brownish discolouration, in other words the adhesive or potentially also label materials had not been adequately removed in the wash-off process. With two out of the three labels with the multi-layer adhesive materials, complete removal was accomplished without difficulty (cf. Fig. 7, see 2, I/D + 2, II/D). The only adhesive residues identified on the plastic flakes were below a critical value. This means that the special formulation of the intermediate layer and the associated clear orientation of the entire adhesive structure towards the label material leads results in the adhesive remaining almost entirely on the label material when the label is removed and therefore not further disrupting the recycling process. In the test, discolouration was found on less than 10 percent of the examined surface area (see Fig. 7). In both the two-layer variants that were removed by washing off, no discolouration was revealed under laboratory conditions.

 

Fig. 7: Conclusive test results: none of the labels with a single-layer adhesive passed the roasting test.

 


If the results obtained from the seven different adhesive types are summarised on completion of these three tests, it is clearly seen that only the multi-layer adhesive with the modified intermediate layer (variant II/D) satisfied the requirements in all test runs (see Fig. 8). 

 

Fig. 8: Only the multi-layer adhesive with the modified intermediate layer satisfied the requirements in all test runs.

 

 

Conclusion

For optimum recycling to be carried out, the granulate at the end of the process must be pure in type and, to the greatest possible extent, free from other residues. Labels that are inseparable or only removable with difficulty may adversely affect colour, mechanical properties and temperature resistance. Adhesive residues on the packagings may also result in discolouration of the recyclates or, in the case of low-melt adhesives, in gas being formed during extrusion.

HERMA’s specially adapted multi-layer adhesive makes recycling simple, efficient and therefore affordable, because it aids the clean and rapid removal of the labels during washing as part of the recycling process. The test results show that this is possible by means of suitable combinations – and entirely without compromising on the adhesive properties. In this way high-quality recycled materials can be profitably reclaimed. Because the label can simultaneously separate out all contaminants (including printing inks), it is part of the solution rather than part of the problem. When it comes to recycling plastic packagings therefore multi-layer adhesive systems for labels are indispensable. 

 

 

About the author

Marcus Gablowski oversees the development of pressure-sensitive adhesives and special coatings for HERMA. The 39-year-old, who studied chemical engineering at Esslingen University of Applied Sciences, has been working for HERMA since 2004, chiefly in the fields of process engineering and development. In recent years he has been instrumental in supporting and driving the introduction of innovative multi-layer technology.

 

 

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