Development of basic process principles for melting paper materials by means of laser radiation using the example of a binder-free joining process

Motivation

Joining processes in the paper and packaging industry are currently based on force-locking and form-fit joints or on adhesive bonding. While the first procedure provides only low strengths, the second method suffers from the disadvantage of requiring an additional adhesive. Joining paper by welding may improve quality but also technology. The absence of adhesives prevents additional environmental pollution and improves the recyclability of paper products.

Aim

The research project focuses on the first-time melting of classical paper materials in order to develop a new and improved joining process.

Approach

Melting paper is not possible by simply heating it up. Even before reaching the melting temperature, cellulose begins to decompose. Therefore, the scientific challenge is to find conditions under which paper melting can be realized.

If the main paper components such as cellulose, hemicellulose or lignin (primarily cellulose) will be used for welded joints, a flow state must be generated in the material on the one hand and an intermolecular diffusion of the macromolecules of the joining partners must be ensured on the other hand. The breaking of hydrogen bonds between the molecule chains plays an important role in cellulose melting process. The bonding energy of the hydrogen bond corresponds to the photon energy in the wavelength range of 6 - 8 µm.

Therefore, in the project a CO laser is used which has emission lines in the range of 4 - 6 µm.

Analyses with a high-speed camera have shown the presence of molten paper. The technological development for the joining process is now being driven forward by additional mechanical impacts on the joining area.