TOWARD ADHESIVES-FREE BIO-BASED COMPOSITES VIA UV-ASSISTED INTERFACIAL CROSS-LINKING
Topic(s) : Material science
Co-authors :
Raja BADE (GERMANY), Beate GEBERT , Larisa TSARKOVA (GERMANY), Dietmar KOCH (GERMANY), Jörg MÜSSIG (GERMANY)Abstract :
In the present research, industrially developed continuous regenerated cellulose fabric and polyethene powder are used to manufacture adhesive-free semi-finished composites by introducing an intermediate step of UV treatment of the semi-finished composites. The manufactured single-layer semi-finished composites were irradiated using a UV broadband lamp for three minutes (UV_3) and eight minutes (UV_8) on each side. The concept in the present study implies the UV-activated formation of covalent bonds between UV-transparent polymer and UV-absorbing fibres. The positive influence of UV treatment on the mechanical performance of composites was characterised by tensile and double-notch tensile tests. For these tests, the composites were manufactured by compacting multiple stacks of semi-finished composites at 180 °C to achieve the desired thicknesses. The results confirm that UV treatment improved the fibre/matrix adhesion.
The influence of UV treatment on the long-stability properties of the single-layer semi-finished composites was investigated through a stress-relaxation test. Three different samples were tested, i.e., untreated (Un) and UV-treated (UV_3 and UV_8). Before the stress-relaxation test, the semi-finished composites were aged in a climatic chamber at 80 °C and 80 % RH (relative humidity) for 12 days. The stress-relaxation results of the non-aged and aged semi-finished composites were contrary to the multi-layer composite tensile properties, where the tensile strength of the untreated semi-finished composites is higher than the UV-treated.
This gives the idea to investigate the direct influence of the UV treatment of the semi-finished composites with and without a second compaction cycle. In the prior case, the untreated and UV-treated semi-finished composites went through a second compaction cycle at 180 °C; in the latter case, the semi-finished composites went through only one compaction cycle, and tensile tests were performed. Interestingly, a decrease in the tensile modulus and tensile strength of the UV-treated semi-finished composites compared to the untreated samples is observed after one compaction cycle at 180 °C. However, after the second compaction cycle at 180 °C, the UV-treated samples showed an increase in tensile strength compared to the untreated. An increase in tensile strength is related to increased adhesion between fibre and matrix. It is assumed that the formation of covalent bonds between the fibre and matrix, accompanied by water evaporation from the inner pores of the Cordenka fibres during UV treatment, leads to porosity at the interface, which is later eliminated in the second compaction cycle. This hypothesis proves that UV treatment leads to the formation of covalent bonds. Besides, to see a notable improvement in the interfacial characteristics, a second compaction cycle of the composites is necessary after UV treatment.
The influence of UV treatment on the long-stability properties of the single-layer semi-finished composites was investigated through a stress-relaxation test. Three different samples were tested, i.e., untreated (Un) and UV-treated (UV_3 and UV_8). Before the stress-relaxation test, the semi-finished composites were aged in a climatic chamber at 80 °C and 80 % RH (relative humidity) for 12 days. The stress-relaxation results of the non-aged and aged semi-finished composites were contrary to the multi-layer composite tensile properties, where the tensile strength of the untreated semi-finished composites is higher than the UV-treated.
This gives the idea to investigate the direct influence of the UV treatment of the semi-finished composites with and without a second compaction cycle. In the prior case, the untreated and UV-treated semi-finished composites went through a second compaction cycle at 180 °C; in the latter case, the semi-finished composites went through only one compaction cycle, and tensile tests were performed. Interestingly, a decrease in the tensile modulus and tensile strength of the UV-treated semi-finished composites compared to the untreated samples is observed after one compaction cycle at 180 °C. However, after the second compaction cycle at 180 °C, the UV-treated samples showed an increase in tensile strength compared to the untreated. An increase in tensile strength is related to increased adhesion between fibre and matrix. It is assumed that the formation of covalent bonds between the fibre and matrix, accompanied by water evaporation from the inner pores of the Cordenka fibres during UV treatment, leads to porosity at the interface, which is later eliminated in the second compaction cycle. This hypothesis proves that UV treatment leads to the formation of covalent bonds. Besides, to see a notable improvement in the interfacial characteristics, a second compaction cycle of the composites is necessary after UV treatment.