Microplastics can be divided into three general groups: primary microplastics type A, primary microplastics type B, and secondary microplastics (cf. figure 1). The latter is created when macroplastics are degraded through weathering and fragmentation once they have entered the environment. Primary microplastics type B are formed during use (e.g. tire abrasion or fibres released during the washing of synthetic clothing). Type A microplastics are e.g. abrasive particles in cosmetics or semi-finished resin products in granulated form because those products already fall within the current definition of microplastics when they are manufactured: Plastic pieces that are less than five millimetres in length. The release of type A microplastics can be intended, deliberately produced, or caused by accident.
A major focus of the EU’s plastics strategy is aimed at tackling the emission of microplastics into the marine environment. The German adhesives industry does not use any primary microplastics that are released when the products are used correctly. Even if certain adhesive raw materials do fall within the scope of the definition currently still under discussion, the adhesives will generally form closed films after processing that are no longer considered microplastics nor contain such material. Modern recycling and organised waste management measures ensure that no adhesive components enter the marine environment. Nevertheless, if glued products enter the sea because of improper disposal, the adhesive also becomes part of marine pollution. The aim must be to implement efficient waste management systems that prevent waste pollution, especially the marine environment.
First it must be specified that the focus is not on recycling the adhesive but on ensuring that it does not affect the ability to recycle the bonded product. “Recycling compatibility” is therefore the more appropriate term. To achieve that purpose it is essential to analyse the individual recycling stream.
The most significant stream in this context is waste paper. In a study by the European Recovered Paper Council (ERPC) it has been determined that thermoplastic hot melt adhesive applications with a minimum horizontal dimension of 1.6 mm in either direction, a minimum film thickness of 120 µm, and a minimum softening temperature of 68 °C can be considered recycling compatible (cf. EPRC, 2018). This definition covers virtually all adhesive applications for packaging used on the market. The new German Packaging Act (Verpackungsgesetz, VerpackG) also refers to that assessment (cf. ZSVR, 2019). It has to be pointed out that those findings are based on the INGEDE-12 method that is not adapted to packaging. Those criteria will have to be confirmed by procedures adapted to packaging papers before the minimum standard is changed in 2020. Very soft, pressure-sensitive systems (e.g. those used for mailings) are particularly problematic in the recycling of waste paper because they lead to so-called “stickies” that can have a major impact on the manufacturing process and paper quality.
Use of renewable raw materials
In general, a hot melt adhesive is a blend of polymers, resins, waxes and additives. Therefore, all the individual components have to be taken into account when discussing the biodegradability of raw materials or the use of renewable raw materials.
In fact, a wide range of different alternatives based on new technologies are already available, which are classified bio-based and in some cases biodegradable. However, there are currently no polymers and waxes commercially available today that provide an acceptable overall performance. Due to their low thermal stability, many would lead to substantial maintenance work when processed in industrial applications. Within a few hours, oxidative deposits build up and there are substantial changes in viscosity and appearance. The physical setting process of the adhesives is also considerably slower and does not meet the latest requirements on the market. Nevertheless, intense research activities point to a dynamic development of alternative raw materials in the next years.
Traditionally, the resins used in the adhesives industry have been sourced from renewable raw materials. They can be extracted directly from trees, refined from by-products in paper production, or manufactured from the pulp of orange and lemon peel. Their performance characteristics and availability are largely comparable to oil-based systems. However, the lower thermal stability that necessitates increased maintenance during processing is a considerable downside, albeit it is not as severe as in the case of the alternative polymers and waxes mentioned above. Recently, it has been possible to successfully develop hot melt adhesives that can contain 30 to 50 % resins from renewable materials and still have a high thermal stability. One of those clean products is Jowatherm® GROW 853.20 developed by the adhesives experts from Jowat in Detmold. Apart from having a high bio-based content verified according to DIN (the German Institute for Standardization), the innovative hot melt adhesive can be processed at a lower temperature. This facilitates considerable reductions in energy consumption and contributes to more sustainable packaging processes.
A promising solution for conserving fossil resources is provided by the mass balance approach. Instead of derived oil products, cracking processes in the chemical industry use raw materials based on renewable resources or also based on recyclate. From a chemical point of view, the materials manufactured are therefore identical to those that are based on mineral oil. This approach verifies the use of bio-based materials although the biogenic carbon is not consistently detectable. Another benefit of materials manufactured accordingly is that in addition to their good performance there is no need for new infrastructure. Existing processing and treatment facilities can be used further, inefficient batch changes are not necessary. The range of adhesive raw materials has been expanded lately.
A part of the bio-based alternatives mentioned above are also biodegradable. Some of them even in the marine environment. In addition, there are also oil-based systems that can be classified as biodegradable. However, biodegradable plastics do not necessarily provide an advantage from an ecological point of view. Degradation does not provide any material or energetic benefits. Recycling or incineration would be preferable. In addition, the German Parliament’s point of view is that the materials are rather fragmented than degraded – i.e. microplastics are being created (cf. Deutscher Bundestag, 2016). Added to this is the insufficient awareness among consumers of the difference between terms such as biodegradability, compostability, home compostability and marine degradability. For instance, the frequently used attribute “biodegradable” refers to industrial composting facilities that have not much in common with the conditions in private gardens. Even when delivered to industrial facilities via the compost bin, those materials are regularly sorted out because they take longer than desired to degrade. The ability to recycle those materials is also limited. If disposed of together with other packaging, the waste will be sorted and incinerated.
To make the bonding of packaging truly sustainable, the focus must be on designing a processes that is as environmentally friendly as possible. The aim must be to prevent waste, conserve resources and facilitate a long lifecycle of the machinery. This can be achieved by ensuring an exact glue application with modern application technology and optimised hot melt adhesives. Modern, low-density hot melt adhesives with a wide range of adhesion facilitate high bond strengths with minimal consumption. Coupled with high thermal stability, it delivers a very clean process and also saves resources. Machine parts have a longer lifecycle and necessitate less maintenance. Packaging, including the product, does not have to be discarded due to soiling. Hot melt adhesives with considerably lower processing temperatures can be used if the process and product allow it. Hot melt adhesives that can be applied starting from 99 °C are already available. Energy consumption can thus be reduced significantly compared to standard systems that require temperatures above 160 °C.