Waste tire rubber powder+electron beam irradiation: making HDPE/carbon black PTC composite materials more stable in performance

The construction of conductive pathways using waste tire rubber powder results in a significant decrease in room temperature resistivity. In HDPE/CB conductive composite materials, the dispersion state of carbon black (CB) directly determines the formation efficiency of the conductive network. The study introduced ground waste tire rubber powder (GTR) as a stable dispersed phase into the HDPE matrix. After occupying space, GTR particles compressed the effective distribution volume of CB, generating a volume exclusion effect, promoting CB particles to approach each other and form conductive pathways. When the GTR content increased from 0 to 30 wt.%, the logarithm of the room temperature volume resistivity (log ρ v) of the composite material decreased from 1.08 to 0.38, indicating a significant improvement in conductivity. However, the addition of GTR also brings a trade-off issue: due to the decrease in crystallinity of the composite material and the weakening of the thermal expansion effect of HDPE, the PTC strength decreases from 4.80 to 3.24. PTC strength is a key indicator for measuring the self limiting temperature capability of conductive composite materials, and its decrease means that relying solely on volume exclusion effects is difficult to simultaneously achieve low room temperature resistivity and high PTC strength.

Electron beam irradiation crosslinking: Inhibiting NTC, improving PTC strength and cycling stability

To solve the above contradiction, further research is being conducted on the crosslinking treatment of composite materials using electron beam (EB) irradiation.

. The cross-linked network formed by irradiation restricts the movement of HDPE molecular chains and CB migration, stabilizes the distribution of carbon black, and eliminates the NTC effect at high temperatures. For composite materials containing GTR, the PTC intensity increased to 6.93, 5.97, 5.19, and 4.93 after irradiation, respectively. After three thermal cycles, the PTC intensity change rates decreased to 2.02%, 1.51%, 1.93%, and 4.67%, respectively, indicating a significant improvement in cycling stability. From a mechanistic perspective, EB irradiation crosslinking and GTR volume exclusion effect form a synergistic effect: GTR is responsible for constructing denser conductive pathways at room temperature, while the crosslinked network is responsible for locking the conductive network structure at high temperatures. The combined effect of the two enables the material to achieve a better balance between low room temperature resistivity, high PTC strength, and stable high-temperature electrical response.

Significance for the Tire and Rubber Industry

This study provides a low-cost route for the high-value utilization of waste tire rubber.

. The field of tire retreading and recycling has long faced the problem of low added value of waste rubber powder, and this work has transformed GTR from a simple filling component to a functional phase that regulates the conductive network, expanding the application space of waste tire rubber in PTC conductive composite materials such as self regulating heating and overcurrent protection. From the perspective of rubber formulation and processing, the research involves carbon black distribution control, cross-linking network construction, and thermal cycling stability evaluation, which have similarities with the reinforcement filler dispersion, vulcanization system network regulation, and aging weather resistance testing that the rubber industry is concerned about. Although this study focuses on the HDPE/CB system rather than traditional tire compounds, its idea of synergistic regulation of filler spatial distribution and cross-linking structure can provide methodological references for the design of rubber additive formulations, including reinforcement filler matching, cross-linking system selection, and the protection of long-term thermal stability by antioxidants and antioxidants. It should be pointed out that this study did not directly involve specific additives such as anti-aging agents, antioxidants, or white carbon black. The above correlation belongs to cross disciplinary technical inspiration, rather than the conclusion of the study itself.

Source: Enhanced Positive Temperature Coefficient Behavior of High‐Density Polyethylene/Carbon Black Composites via Waste Rubber Volume Exclusion and Electron‐Beam Irradiation Crosslinking，Journal of Applied Polymer Science，DOI: 10.1002/app.71674

Information source:Enhanced Positive Temperature Coefficient Behavior of High‐Density Polyethylene/Carbon Black Composites via Waste Rubber Volume Exclusion and Electron‐Beam Irradiation Crosslinking， Organized by Herun Chemical.