Correlation Between Rheological Behavior And Conductive Property Of Carbon Black Filled Polymer Composites

Conductive polymer composites not only have unique electrical and mechanical properties, but also possess the characteristics of light weight, low cost, easier processing, and corrosion resistance, in comparison with metal materials. They have been widely used in the areas of electrostatic shielding, temperature self-limiting heater and self-repairing fuse. The conductive behavior of conductive polymer composites is related to the three-dimensional percolation network of conductive particles throughout the matrix. However, the previous studies could not distinctly disclose the formation mechanism of conductive network in the polymer matrix and the response of the conductive network to the thermal and the mechanical actions.In this thesis, the carbon black (CB) filled polymer composites are selected as the research objects. Methods for simultaneous measurement of rheological and conductive properties of conductive polymer composites in the melt state and during the crystallization are developed for the first time. The dynamical method is based on measurement of storage modulus (G′) and loss modulus (G″) while static method is based on measurement of normal force (F_n). By applying simultaneous measurement, resistance (R) and dynamical rheological parameters of the composites at the melt state are discussed in relation to time and shearing action in order to establish the relationship between conduction and viscoelasticity. Influences of isothermal and nonisothermal crystallization of the polymer matrix to variations of R and dynamical rheological parameters are investigated in order to disclose the effect of crystallization of matrix to the conduction and viscoelasticity of composites. The study is helpful for exploring the mechanism for the formation and the evolvement of the aggregated structure in the composites under the actions of shear and thermal fields.Conductive behavior of ethylene-tetrafluorothylene copolymer (ETFE) / CB composites was investigated. The percolation of conduction takes place at CB volume fraction ((?)_CB) from 0.05 to 0.11. The PTC intensity decreases with increasing In case of (?)_CB above 0.11. The composite with (?)_CB= 0.11 shows a highly reproducible PTC switching characteristic during thermal cycles. However, the thermal stability becomes poor at (?)_CB=0.15. At room temperature, volume resistivity (ρ) as a function of G′reveals a percolation-like phenomena at G′from 676 MPa to 876 MPa.The simultaneous measurement of dynamical rheological and conductive behavior shows that shear may destroy the percolation network besides the intrinsic resistance relaxation, resulting in a sharp increase in R above a critical strain. Both G′and R take sudden changes at critical strainsγ_CG andγ_CR, respectively, with increasing strain (γ). A relationγ_CR>γ_CG is revealed in ETFE/CB and high-density polyethylene (HDPE)/CB composites at temperatures above the melting point and also in the unvulcanized solution polymerized styrene-butadiene rubber (SSBR)/CB composite increasing (?)_CB leadsγ_CR to reduce whileγ_CG to increase. On the other hand,γ_CR andγ_CG remains unvariable with varying (?)_CB andγ_CR <γ_CG is revealed in the vulcanized SSBR/CB composite. Furthermore,γ_CR andγ_CG increase with raising temperature for the HDPE/CB composite in the melt.The simultaneous measurement during the isothermal crystallization of HDPE/CB composite shows that rheological parameters and R occurs sudden changes simultaneously at a characteristic time. The dynamic characteristic time (t_G′) is less than the static characteristic time (t_F) and both reduce with increasing (?)_CB and reducing crystallization temperature (T_c). During the dynamic crystallizing, t_G′reduces with increasing y and frequencyωdue to shear induced crystallization.In low-frequency area, physical gel time (t_C) is consistent with t_G′. The roughness of rheometer parallel plate surface may influence the nucleation and growth of crystal in HDPE/CB composites, as viewed from the great changes in t_G′and t_F detected from two rheometer parallel plate with different roughness.Similar to HDPE/CB composites, t_G′of isotactic polypropylene (iPP)/CB composites is less than t_F. The roughness of rheometer parallel plate surface does not influence t_G′and t_F of iPP/CB composite significantly, which is different to HDPE/CB composite.The simultaneous measurement during the nonisothermal crystallization of HDPE/CB and iPP/CB composites shows that the rheological parameters and R occur sudden changes simultaneously at a characteristic temperature. When X is small, the dynamic characteristic temperature (T_G′) is closed to the crystallization peak temperature (T_p) by differential scanning calorimetry (DSC), while the static characteristic temperature (T_F) is lower than T_G′.The simultaneous measurement of rheological and conductive behavior reveals that the crystallization of the matrix destroys the percolation network firstly. When the crystallinity is up to a certain value, CB particles gather in the amorphous region to form the percolation network. The dynamic rheological properties of the composites are related to the matrix crystallization and the resultant change of CB particle distribution. Due to the different testing principles, the dynamical and static measurements detect different crystallinity in polymer matrix at the critical time and the critical temperature where R and rheological parameters change rapidly. The critical crystallinity from the dynamical method is lower than that from the static method.