Dynamic vulcanization process is a key breakthrough in improving the performance of magnesium hydroxide/rubber cable materials
In harsh application scenarios such as rail transit and high-voltage power transmission, the flame retardant properties and mechanical strength of rubber cables are directly related to the safe operation of the power system. Traditional magnesium hydroxide flame-retardant rubber systems generally have pain points such as poor reinforcement effect and uneven dispersion. The introduction of dynamic vulcanization process has brought a revolutionary solution to this technical dilemma. Research data from the Polymer Materials Laboratory of Tsinghua University show that the EPDM/Mg(OH)₂ composite system using dynamic vulcanization process has an increased limiting oxygen index of 18.7% and a tensile strength of 15.3MPa, which is 63% higher than the traditional mixing process. (Data source: Journal of Applied Polymer Science, 2022)
1. Dynamic vulcanization process reshapes the microstructure of rubber composite materials
The core of dynamic vulcanization technology is to construct a nano-dispersed vulcanization network in the rubber matrix by precisely controlling the cross-linking reaction kinetics. Experimental electron microscopy observations show that when the TPU matrix and Mg(OH)₂ filler undergo shear-crosslinking synergistic effects in a twin-screw extruder, the decomposition rate of the vulcanizer DCP forms a dynamic balance with the viscosity of the rubber phase, which enables Mg(OH)₂ particles with a particle size of less than 5μm to be monodispersed in the EPDM continuous phase.
During the internal mixing process, the coordinated regulation of the temperature field and shear force is particularly critical. When the mixing temperature is stabilized at 160±5℃, the peroxide vulcanization system can complete 90% of the crosslinking reaction within 2 minutes, at which time the viscoelastic properties of the rubber phase just reach the optimal window period for filler dispersion. This "crosslinking first, dispersion later" process sequence successfully avoids the filler agglomeration phenomenon caused by traditional static vulcanization.
The dynamic coupling of the vulcanization reaction and the dispersion process produces a unique "island structure". The crosslinked rubber particles wrap the Mg(OH)₂ particles as a continuous phase, forming a three-dimensional stress transfer network during tensile deformation. Dynamic mechanical analysis (DMA) shows that the storage modulus of the composite material remains stable in the range of -30℃ to 120℃, and the tanδ peak value is 42% lower than that of the uncured system, confirming the significant improvement in the interfacial bonding strength.
2. Performance optimization mechanism of magnesium hydroxide synergistic reinforcement system
The whisker modification of magnesium hydroxide has opened up a new reinforcement path. The aspect ratio of Mg(OH)₂ whiskers synthesized by hydrothermal method reaches 20:1, and its axial Young's modulus is as high as 70GPa. When these nanowhiskers are embedded in the rubber matrix at 3wt%, the tear strength of the composite material is increased to 45kN/m, while maintaining 82% of the elongation at break, breaking the traditional law of mutual exclusion between filler reinforcement and elasticity retention.
The step-by-step improvement of flame retardant performance is due to the multi-scale synergistic mechanism. Micron-sized Mg(OH)₂ particles form a continuous and dense ceramic barrier layer when burning, while nanowhiskers enhance the carbon layer structure through the "anchoring effect". Cone calorimetry test shows that the peak heat release rate (pHRR) of the dynamically vulcanized sample is reduced to 156kW/m², which is 71% lower than that of pure rubber, and the combustion residue maintains a complete lamellar structure.
Interface compatibility regulation is the key to performance breakthrough. Silane coupling agent KH-550 is used to surface graft Mg(OH)₂, so that its surface hydroxyl groups form covalent bonds with the rubber molecular chain. The infrared spectrum shows a new Si-O-C characteristic peak at 1120cm⁻¹, confirming the successful construction of the interfacial chemical bond. This strong interface effect reduces the compression permanent deformation rate of the composite material to 12%, meeting the standard requirements of aviation cables.
3. Process parameter optimization strategy in industrial production
The two-stage mixing process achieves precise control of vulcanization and dispersion. The first stage of mixing completes rubber plasticization and filler pre-dispersion at 120℃, and the second stage controls the shear strength by the rotor speed (60rpm) during the dynamic vulcanization stage at 160℃. This process stabilizes the volume resistivity of Mg(OH)₂ at more than 10¹⁴Ω·cm, meeting the requirements for high-voltage cable insulation.
The choice of vulcanization system directly affects the balance of material properties. Comparing the three vulcanization systems of DCP, Sulfur and phenolic resin, when 0.8phr DCP is combined with 1.2phr TAIC co-crosslinking agent, the crosslinking density of the composite material reaches 4.5×10⁻⁴mol/cm³, and will not cause thermal decomposition of Mg(OH)₂. This low-temperature rapid vulcanization system reduces production energy consumption by 28%, with significant economic benefits.
The screw combination design plays a decisive role in the dispersion quality. In the combined module of the twin-screw extruder, 45° staggered tooth discs and reverse thread elements are alternately arranged to form a pulsating shear field. Online infrared monitoring shows that the dispersion uniformity index (DUI) of Mg(OH)₂ reaches 0.92, and the particle size distribution span (Span value) is controlled within 0.8, which is far higher than the dispersion level of traditional internal mixers.
This technology system has been successfully applied to the production of rail transit cable sheaths with a rated voltage of 35kV, and has passed the UL-94 V0 flame retardant certification and 1000-hour thermal aging test. With the growing demand for high-safety cables in the new energy field, the dynamic vulcanization process will show greater application value in the fields of nuclear power cables and automotive high-voltage wiring harnesses. Future research will focus on the intelligent control of process parameters to promote continuous breakthroughs in the functionalization and high performance of rubber composite materials.