Full length articleLaser sealing of PLA-based compostable coffee capsules
Introduction
Laser welding technologies of plastics are part of the advanced technologies for joining films, sheets, semi-rigid and rigid components made of polymeric materials by localized heating with a focused beam of laser radiation [16]. The first applications date back to the seventies of the last century, although these technologies were consolidated only about 25 years later [26].
The advantages attributed to laser welding technologies applied to plastic materials include: (i) the absence of contact between the device and the components [4] to be welded, differently from what happens in traditional joining technologies [18], [23]; (ii) the possibility of welding materials different in composition and color [6]; (iii) the high aesthetic and functional quality of the welded joints [13]. Further essential advantages of the mentioned laser technologies include: (i) the possibility to create welded joints in areas that are difficult to access, the possibility of easily automating the process [7]scaling it to production needs; (ii) reduced labor requirements [24]; (iii) modest energy consumption [21]; (iv) high operational safety [3]; (v) very low environmental impact of processing, especially for metal components whereas for plastic components gas emissions must be post-treated the gaseous emissions must be conveyed and treated before being released into the atmosphere [22], [21]. In particular, laser welding technologies are often applied to the joining of films and to the hermetic sealing of films with semi-rigid or rigid containers [4], [5]. More recently, laser welding technologies have been applied to bioplastic materials [14], [8], also to join them with conventional materials such as aluminum [19]. This line of research is extremely interesting, given the growing attention of public opinion, professionals and technicians towards the development of packaging with reduced environmental impact that make use of compostable bioplastic materials. Studies concerning the applications of laser welding technologies with bioplastic or compostable materials are still very limited. As previously mentioned, the first study dates back to the early years of the twenty-first century and concerns the welding of PLA (polylactic acid) films, suitably plasticized to increase their elongation at break [14]. The authors found that PLA films tend to change characteristics over time because of the migration of plasticizers within the material and also because of the variations in crystallinity, sometimes induced by the laser joining process. The changeability of the properties of the PLA film was considered by the authors one of the main causes to which the difficulty of joining with laser technology can be attributed. A further recent study, however, concerns the possibility of using laser welding technologies for joining PLA and aluminum films [19]. The authors have shown that, although welding is achievable, the intervals of the process parameters that allow to obtain a welding of sufficient quality are extremely narrow. Furthermore, the authors have observed that the welded joints present numerous defects and thermal alterations, which often compromise both their functionality and aesthetic appearance. In this context, the present work has been entirely focused on the laser hermetic sealing (i.e., welding) of coffee capsules. Coffee capsules are, nowadays, a huge environmental and technological problem. Over 10 billion capsules are produced each year worldwide [1]. If made of plastic, they weigh at least 3 g and contain about 5 g of coffee. After coffee brewing in automatic equipments, they can only be disposed in landfills, individually representing a waste of 20 g (8 g of dry materials and 12 g of water incorporated during coffee brewing), with a huge environmental impact [2], [15]. Bioplastic capsules with aluminum lid and, immediately after, fully compostable capsules have been introduced for some years according to the EN13432 directive to reduce the environmental burden, giving the exhausted capsule to composting. Although more eco-friendly, these capsules, however, are considerably more complex to process, especially during the coffee filling and sealing phases, since compostable materials are more difficult to weld using traditional thermal contact welding technologies [20]. This is the context in which, therefore, the present work studies the applicability of the laser welding technology to hybrid coffee capsules (capsule body in compostable material and lid in aluminum) and to capsules entirely in compostable material (both the capsule body and the upper lid). The experimental results showed a significant progress in laser technology, with joints of excellent quality in both scenarios under investigation, but, above all, a significant widening of the processability ranges. Based on the experimental results achieved, it is possible to affirm that laser technology is an effective alternative, with a very low environmental impact, to traditional welding technologies for the hermetic sealing of compostable coffee capsules with conventional aluminum lids or, also, with compostable lids, opening up a completely new market for lasers.
Section snippets
Materials
The subject of the experiment is the coffee capsule. The coffee capsule is made up of two elements: (i) the capsule body (Fig. 1), i.e. the element that performs the function of containing the coffee; (ii) the top-lid, i.e. the element placed over the base of the capsule, with the function of sealing the container to ensure the complete closure of the capsule and ensure correct packaging of the coffee. In the case in question, a coffee capsule, produced by Aroma System Srl (Bologna, Italy) in
Peel-off tests: theoretical approach
Fig. 7 shows the ring-shaped base surface (i.e., bottom edge) of the capsule body, i.e. the surface on which the toplid must be sealed on the capsule body to ensure the preservation of the coffee organoleptic properties, the necessary shelf-life of the end-product and the safety of the consumers. This surface is therefore the overall adhesion surface between the capsule body and the toplid. Nevertheless, during the peel-off test, the “small tongue” (i.e., the toplid) is drawn progressively from
Conclusions
The present work focuses on the development of automatized and environmentally friendly solutions for sealing compostable coffee capsules. Various scenarios have been studied entirely focused on laser welding tests of conventional (aluminum) and innovative toplids (in bioplastic coated paper) on compostable bioplastic capsule bodies (based on amorphous and semi-crystalline polylactic acid). The experimentation allowed to identify the weldability intervals in the different tested configurations,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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