Ultrasound welding of papers and nanocelluloses – ULTRACELL

To achieve the objectives, the methodology consists in:
1- To increase the control of the process and to determine the optimal parameters by (i) the acquisition of experimental data in situ, (ii) the understanding of temperature rise phenomena in ligno-cellulosic discontinuous media, through the analysis of the influence of structural and physical properties (especially viscoelastic) at different scales on the competition between generation and heat diffusion.
2-Then to finely understand the phenomena related to the development of adhesion in order to be able to activate the mechanisms in an optimal way by (i) the identification of the predominant adhesion mechanisms in the case of ultrasonic welding of papers and (ii) highlighting the relationships between chemical compositions (cellulose papers, fibers and nanocrystals) and activation of adhesion mechanisms.
3-In parallel, to study the activation or adhesion improvement pathways through the elaboration of cellulose nanocrystals (NCC) (i) chemical compositions adapted to the activation of adhesion phenomena or (ii) whose surfaces will be functionalized to show a thermoplastic behaviour.
4-Finally, to design a demonstrator welder dedicated to the assembly of papers and boards, or when appropriate after coating of the surfaces by the NCC developed in the previous task, and to evaluate the cost of technology change and the gain made compared with existing bonding technologies.

The instrumented welding device being developed will allow to establish an energy balance along the welding chain (acoustic assembly, material and anvil), to predict the general state of the material during welding and to study the contributions of ultrasounds.
In parallel, the influence of the size of the elements (from the raw fiber to the cellulose microfibrils) at chemical iso-composition on the welding performance was studied. Three paper pulps were used to cover the spectrum of the chemical compositions of the pulps. The tests clearly showed that the chemical composition and the size of the elements had a significant influence on the development of adhesion. The influence of additives conventionally present in papers and boards on weldability was also studied (pigment layers, starch, bio-sourced barrier layers). The results show that some additives significantly improve the welding performance of papers and boards. This is very auspicious for industrialization of the process.
In addition, cellulose nanocrystals have been specially developed to be weldable. A first strategy was to produce films of cellulose nanocrystals with significant proportions of lignin and hemicelluloses. The second strategy consisted of chemically modifying the cellulose nanocrystals, either to lower their glass transition temperature or to form a thermoplastic envelope on the surface. The tests to evaluate their weldability are in progress.
Finally, the first reflections on the type of demonstrator led to a benchmark of commercial paper and board having a good aptitude for ultrasonic welding. A first technical-economic study is under way to compare the ultrasonic technology applied to papers and boards with standard bonding technologies.

The ULTRACELL project makes a significant contribution to one of the major challenges faced by materials science: to generalize the use of materials from renewable resources. Finding the conditions for fusible lignocellulosic materials would open up many possibilities for various sectors of activity that use thermoformed structural materials such as construction, furniture, medical or transport.
In France, the papers and boards sector has the highest demand for glue. These petroleum-based adhesives represent on average 0.5 to 1% of the composition of paper and board packaging. The replacement of standard assembly processes by bonding papers and boards by ultrasonic welding process would significantly reduce the costs associated with the reprocessing and manufacture of recycled papers and boards. This reduction would have a positive effect on the recycling rate, which has not changed much for several years. It would also make possible the production of 100% renewable, fully recyclable and biodegradable packaging. For food packaging applications, petroleum-based glues are a problematic source of mineral oils that can cause migration from packaging to food.
The transition to ultrasonic welding technology for papers and boards would be a good example of sustainable competitiveness by both increasing productivity and significantly reducing the impact on the environment.

Ultrasonic welding of 100% lignocellulosic papers; Journal of Material science. October 2019, Volume 54, Issue 19, pp 12938–12950

European patent application (EuroPCT) N°17732518.0
Title: Fixation method and system obtained by such a method

In Europe, 50% of paper-based materials are dedicated to packaging and their recyclability rate is of 72%. However, packaging materials are generally assembled and glued thanks to glues formulated with oil-based polymers. These glues are either hot melt adhesives, or formulated with a solvent or water. This causes problems, first during the lifetime of the packaging, because of the potential contamination by mineral oil. Secondly, it makes the materials more complex to recycle with a significantly higher energy requirement and more important waste consisting of separated glue. Heat-fusible coatings used to seal flexible packaging materials or cellulose-based hygiene products create similar issues, In the meantime, the thermo-physical properties of paperboards prevent them from being assembled by heat-sealing at speeds compatible with the production of packaging materials. The major purpose of the ULTRACELL project is to demonstrate the feasibility of the basic concept of a sealing process for paperboards using ultrasounds, which would reach the requirements of the industry in terms of assembly quality, production speed and energetic cost.
Replacing standard assembling processes using glues or coatings by a process using ultrasounds would considerably reduce the costs associated to paperboard recycling and recycled paperboard manufacturing. It would therefore have a very positive impact on the recycling rate. This change of technology would also increase the rate of production of paperboard-based packaging, allowing French companies of the field to be much more competitive in this international very challenging environment.
Ultrasonic welding is a much-used and industrially-mastered technology for assembling thermoplastic pieces in only a few seconds. To this day, no paperboard reference has been assembled using ultrasounds, while preliminary trials has proven that rather high levels of adhesion can be reached with some standard paperboards. The seal ability was even improved by using nanocellulose (patent pending in 2016). However, these first trials have revealed several technological locks associated to ultrasonic welding of paperboard: (i) the lack of understanding of the real conditions within and at the interface of the material during welding, (ii) the lack of understanding of the impact of the structural properties (at the various scales) of lignocellulose on the heat generation/dissipation competition, and (iii) the lack of understanding of the mechanisms involved in the development of adhesion. The ambition of the ULTRACELL project is to unlock these doors. In order to succeed, a first task will consist in controlling the process and determine the optimal process parameters, by acquiring experimental in-situ data and understanding heat generation phenomena in discontinuous multi-scale lignocellulosic environments. A second task will consist in activating and optimising the adhesion mechanisms thanks to fine understanding of the phenomena involved in the adhesion. Another task will aim at improving the adhesion by adding chemically modified nanocellulose. Finally, based on the knowledge and skills developed within the project, an industrial ultrasonic welding equipment dedicated to paperboards will be developed.