The relationship between the resistivity of conductive silicone and temperature and humidity is determined by its material composition and conductive mechanism. This type of material is usually composed of a silicone rubber matrix and a conductive filler (such as silver powder, carbon nanotubes, graphene, etc.), and its conductive properties essentially rely on contact conduction or tunneling between filler particles. Temperature changes will change the continuity of the conductive path by affecting the molecular thermal motion and the interface state of the filler, while humidity may introduce ion conduction or change the dielectric environment of the matrix through hygroscopic effects, thereby having a complex effect on the resistivity. Understanding these relationships not only helps to optimize the material formulation, but also provides a theoretical basis for its application in different environments.
The effect of temperature on the resistivity of conductive silicone presents nonlinear characteristics, and the specific performance is closely related to the type of conductive filler. Taking metal fillers (such as silver powder and copper powder) as an example, in the range of room temperature to medium temperature (about -20℃~80℃), the silicone rubber matrix gradually softens with the increase of temperature, and the molecular chain segment movement intensifies, which may lead to a decrease in the distance between filler particles and an increase in contact points, thereby slightly reducing the resistivity. However, when the temperature exceeds the glass transition temperature of silicone rubber (usually below -60℃~-40℃), the matrix will change from the glass state to the highly elastic state. At this time, the filler particles may be relatively displaced due to the deformation of the matrix. If the filler is unevenly dispersed, the contact may be disconnected in some areas, resulting in an increase in resistivity. For carbon fillers (such as carbon fiber and graphene), the increase in temperature will enhance the thermal excitation of carriers. Especially when the filler content is close to the percolation threshold, the conductive mechanism dominated by the tunnel effect may make the resistivity present a negative temperature coefficient (NTC) characteristic, that is, the resistivity decreases with increasing temperature. However, when the filler concentration is high, contact conduction is dominant and the resistivity may turn into a positive temperature coefficient (PTC) behavior.
The effect of humidity on the resistivity of conductive silicone is more complicated, mainly achieved through two pathways: physical adsorption and chemical action. When the ambient humidity is high, the polar groups (such as Si-O bonds) in the silicone rubber molecular chain are easy to adsorb water molecules, forming a thin water film on the surface or inside the material. If the conductive filler itself is hydrophilic (such as some metal oxide-modified carbon materials), water molecules may penetrate into the filler interface, introduce ion conduction paths, and cause the resistivity to decrease. However, for silicone rubber with strong hydrophobicity (such as methyl silicone rubber), water molecules are difficult to penetrate into the interior, and only form an isolation layer on the surface, which may increase the insulation gap between filler particles and increase the resistivity. In addition, if there are easily hydrolyzed components in the conductive filler (such as some metal nanoparticles), long-term high humidity environment may cause oxidative corrosion, resulting in the formation of an insulating oxide layer on the surface of the filler particles, significantly increasing the contact resistance. This effect will be more obvious under the synergistic effect of high temperature and high humidity.
In low temperature and low humidity environment, the resistivity of conductive silicone usually tends to be stable. At this time, the silicone rubber matrix is in a rigid state, the movement of molecular segments is restricted, the position of filler particles is relatively fixed, and the contact conduction path remains stable. At the same time, low humidity avoids the interference of water molecules on the conductive network, so the resistivity fluctuates less, which is suitable as a static conductive sealing material for precision electronic components. However, it should be noted that extreme low temperature may cause silicone rubber to become brittle. If there is stress concentration inside the material, it may cause the filler network to break, resulting in a sudden increase in resistivity or even conductive failure.
The complex environment of high temperature and high humidity poses a severe challenge to the resistivity stability of conductive silicone. High temperature accelerates the movement of molecular segments, which may cause dynamic displacement of filler particles and destroy the original conductive path; and the penetration of water molecules in a high humidity environment will further aggravate the competitive effect of interfacial corrosion and ion conduction. For example, in the wet heat aging test, the resistivity of silver-based conductive silicone may increase significantly due to the generation of Ag₂O or Ag₂S (if the environment contains sulfur) on the surface of silver powder, while the resistivity of carbon-based conductive silicone may increase due to the adsorption of water molecules, resulting in the expansion of the spacing between graphene sheets and the weakening of the tunnel effect, and the resistivity shows a trend of first decreasing and then increasing (in the early stage, ion conduction is dominant, and in the long term, the destruction of the dispersed structure of the filler is dominant).
The response relationship between conductive silicone and environmental factors can be effectively regulated by optimizing the material formula. For example, introducing a coupling agent (such as a silane coupling agent) to modify the filler surface can enhance the interfacial bonding between the filler and the silicone rubber matrix and inhibit the displacement of the filler caused by temperature changes; adding a hydrophobic agent (such as methyltrimethoxysilane) can reduce the hygroscopicity of the matrix and reduce the interference of humidity on the conductive network. For scenarios that need to work in a wide temperature and humidity range, the use of a dual filler system (such as silver powder + carbon nanotubes) can take into account the stability of contact conduction and the environmental adaptability of the tunnel effect, and broaden the performance window through synergy.
In practical applications, the resistivity-temperature and humidity characteristics of conductive silicone need to be designed in combination with specific scenarios. For example, in the field of automotive electronics, the high temperature (up to 150°C) and oil mist environment in the engine compartment require the material to have a low PTC effect and oil resistance; while in medical equipment, the wet heat sterilization environment (such as 121°C steam) requires the material to maintain conductive stability under high temperature and high humidity. By establishing a mathematical model of temperature, humidity and resistivity (such as the Arrhenius equation combined with a humidity correction term), the long-term performance of the material in different environments can be predicted, providing a scientific basis for product selection and life evaluation.
The relationship between the resistivity of conductive silicone and temperature and humidity is a macroscopic manifestation of the dynamic response of the multiphase system of the material, which is not only dominated by the type, concentration and dispersion state of the filler, but also closely related to the physical and chemical properties of the matrix. By deeply understanding the impact mechanism of environmental factors on the microstructure of the conductive network, combined with formula design and process optimization, we can develop high-performance conductive silicone materials that can adapt to different environmental needs and promote their widespread application in new energy, smart wearables, aerospace and other fields.
