Full length article
Laser sealing of PLA-based compostable coffee capsules

https://doi.org/10.1016/j.optlastec.2020.106557Get rights and content

Highlights

  • Laser technology to make joints to hermetically seal compostable coffee capsules.

  • Peel-off tests to evaluate the adhesion strength of the toplid to the capsule.

  • Analysis of laser parameters, namely processing and pre-heating laser power, scanning speed.

  • Optimizing the choice of the operational parameters of the laser sealing process.

  • 3D process maps of the adhesion strength by varying the laser operational parameters process.

Abstract

The aim of the present work is to examine the possibility of using laser technology to make welded joints to hermetically seal bioplastic containers with aluminum or bioplastic foils. Specifically, the focus is on the laser sealing process of compostable coffee capsules with toplids in aluminum and in paper coated with compostable plastic (mainly based on polylactic acid, PLA). The aluminum toplid does not allow obtaining a compostable package, but represents the currently most used standard on the coffee capsule market. The welded joints made with laser technology were characterized by peel-off tests to evaluate the adhesion strength of the toplid to the capsule with the laser operational parameters, namely processing and pre-heating laser power, scanning speed of the laser beam. The results of the peel-off test allowed optimizing the choice of the operational parameters of the laser sealing process for the various scenarios examined. Based on the experimental findings, 3D process maps have therefore been built in which the adhesion strength between the toplid and the capsule has been plotted by varying the laser operational parameters process, making it possible to identify the optimal processing windows.

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.

References (27)

  • N. Brown, D. Kerr, M. Jackson, R.M., P, Laser welding of thin polymer films to container substrates for aseptic...
  • N. Brown et al.

    CO2 laser processing of multilayer packaging films

    Proc. IMechE: J. Syst. Control Eng.

    (2005)
  • N. Brown et al.

    CO2 laser processing of packaging films including multi-layer laminates containing silicon oxide and ethylenevinyl alcohol barrier layers

  • Cited by (6)

    • Parameter optimization and quality analysis of pulsed laser joining of 316L stainless steel and polylactic acid

      2023, Optics and Laser Technology
      Citation Excerpt :

      The application of metal and thermoplastic composites has become a current research hotspot, and such composites can be seen in daily food packaging and electronic devices in automotive, aerospace, and even military industries [1–5].

    • Laser joining of aluminum film coated with vinylic resin and plastic/bioplastic films for applications in food packaging

      2021, Optics and Laser Technology
      Citation Excerpt :

      In the last 20 years, laser technology has established itself as fast, extremely transversal and, above all, environmentally friendly. A fair number of studies on the welding of dissimilar materials have followed one another in recent years showing how laser technology can acquire a prominent position also in the packaging sector [14-19]. The delicate aspect to check in the lacquered aluminum coupling process is, however, the influence of the lacquer on the laser heat-sealability of the container.

    • Heat-sealing properties of polylactic acid and polylactic acid composites

      2023, Polylactic Acid Composites: Sustainable Biocomposites
    View full text