Polyethylene fishing nets are core tools in fisheries. The tensile strength of their knots directly impacts their service life and fishing efficiency. Improving knot tensile strength requires a comprehensive approach encompassing material modification, process optimization, and post-processing technologies. By synergizing physical structural reinforcement with enhanced chemical properties, the knot's load-bearing capacity can be significantly enhanced.
At the material modification level, optimizing the molecular structure of the polyethylene raw material is fundamental. Traditional polyethylene fishing nets typically utilize high-density polyethylene (HDPE) or low-density polyethylene (LDPE), but single materials often suffer from insufficient toughness or excessive rigidity. Adding nano-strengthening agents, such as nano-molybdenum disulfide or nano-graphite, to the raw materials significantly improves polyethylene's fluidity and extrusion efficiency. During the melt spinning process, the nanoparticles are uniformly dispersed within the polyethylene matrix, forming a "particle-molecular chain" composite structure that inhibits crack propagation and enhances localized strength at the knots through interfacial stress transfer. Furthermore, the introduction of ultra-high molecular weight polyethylene (UHMWPE) has further expanded the material's performance boundaries. Its molecular chain length is several times that of conventional polyethylene, forming denser crystalline regions during stretching, allowing the nodules to withstand higher loads without breaking.
Process optimization is key to improving the tensile strength of the nodules. The melt spinning method achieves directional strengthening of the monofilament structure by precisely controlling the temperature gradient and stretching ratio. During the spinning process, the polyethylene melt undergoes a three-stage stretching process: the first stretching occurs at 85-90°C to impart initial orientation to the monofilaments; the second stretching temperature is raised to 95-100°C to further align the molecular chains; and the third stretching is completed at 105-110°C, ultimately forming high-strength monofilaments with a diameter of 0.3-1.5 mm. This graded stretching process creates a gradient crystal structure within the monofilaments, resulting in higher tensile strength at the nodules due to the denser fiber arrangement. Furthermore, the optimized spinneret aperture and aspect ratio reduce surface defects in the monofilaments, preventing nodule breakage caused by localized stress concentration.
Post-treatment techniques play a key role in improving knot strength. Heat treatment involves applying pre-tension in a 70-85°C water vapor environment to rearrange the molecular chains at the knots, eliminating internal stresses generated during processing. Experimental results show that knot strength in heat-treated polyethylene fishing nets can be increased to 95%-100% of the monofilament strength, while untreated samples retain only approximately 80% of the knot strength. Furthermore, controlling the pre-tension during post-treatment is crucial. Excessive tension can cause filament breakage, while too low tension will not fully activate the molecular chain rearrangement. By precisely controlling the heat treatment temperature, duration, and tension parameters, a balance between knot strength and net flexibility can be achieved.
In the weaving of fishing nets, optimizing the lay length and number of strands directly impacts knot strength. Increasing the lay length improves the tensile strength and knot strength of the net, but over-twisting can reduce elongation at break. Comparison experiments using strands with a twist of 32 T/m versus untwisted parallel tows show that the former outperforms the latter in both breaking strength and knot strength. Furthermore, the multi-core structure improves knot-bearing capacity through stress dispersion, and nets with a high core count exhibit superior tensile properties compared to those with a low core count. These improvements to the weaving process allow the polyethylene fishing net to maintain its lightweight while significantly enhancing the tensile strength of the knots.
The addition of UV inhibitors and antioxidants further extends the service life of the polyethylene fishing net. In the marine environment, UV radiation and oxidation reactions can cause polyethylene molecular chains to break, thereby reducing knot strength. The addition of UV inhibitors such as carbon black to the raw materials effectively absorbs UV energy and slows material degradation. Furthermore, the introduction of antioxidants inhibits free radical chain reactions, preventing knot strength loss caused by oxidation. These chemical modifications ensure that the polyethylene fishing net maintains high knot tensile strength over long-term use.
In terms of application results, the optimized polyethylene fishing net has performed well in deep-sea cage aquaculture. Ultra-high molecular weight polyethylene monofilament fishing nets produced using a melt-spinning-super-stretching method significantly enhance knot strength through temperature optimization of the primary and secondary stretching processes, combined with post-processing. During marine application trials off Qingdao's Lingshan Island, the net successfully withstood the impact of wind and waves, with no fish escaping due to broken knots, demonstrating the effectiveness of process optimization in improving net performance.
The improvement in the tensile strength of the polyethylene fishing net knots is the result of a synergistic combination of material modification, process optimization, and post-processing technologies. Through the introduction of nano-reinforcements, the implementation of a graded stretching process, precise control of heat treatment parameters, and improvements to the weaving process, the polyethylene fishing net achieved significantly enhanced tensile strength at the knots while maintaining lightweight and flexibility. These technological breakthroughs not only extend the lifespan of the fishing net and reduce marine operating costs, but also provide reliable technical support for deep-sea aquaculture and pelagic fishing.
