# Dispersion and Pressure Sensitivity of Carbon Nanofiber-Reinforced Polyurethane Cement
## Results and Discussions  
### CNFPUC Sepage Curve and Threshold  
The seepage curve of the CNFPUC obtained from the test is shown in Figure 2. The relationship between the resistivity and the CNF content appeared to have the form of an inverted \"s\" curve, which could be divided into three zones:  
(1) Unpercolated zone--I  
When the content of the CNF added to the polymer was low, the CNF particles were coated with insulating polymers. They had random, isolated and discontinuous distributions in the matrix phase. The average distance between them was large and there were only a few contact opportunities. The electrons could migrate in the gap between the conductive particles dispersed in the matrix and produce the phenomenon of conduction. The resistivity of the CNFPUC material was controlled primarily by the continuous matrix phase, corresponding to the insulation of the polymer matrix and the conductivity was a result of primarily the \"tunnel effect.\" At this time, the resistance of the composite material was essentially the same as that of the matrix phase. (2) Seepage zone--II  
With an increase in the CNF content, the average distance between the CNF in the composites decreased gradually and their contact probability increased. Within a narrow range of CNF concentrations, the CNF particles formed chained or grape-like pathways which overlapped with one another and formed a \"conductive channel.\" At this time, the conduction had both a \"tunnel effect\" and \"channel effect.\" The resistivity decreased in the geometric series and the composite material changed from an insulator to semiconductor or conductor. (3) Percolation zone--III  
A higher content of CNF in the composite meant that the conductive network was more complex, the density was higher and a lower resistivity was determined by the conductive network. The conductivity of the CNFPUC network was equivalent to that of CNF forming a conductive network  
Figure 2: CNFPUC seepage curve for different CNF contents. in PU and then encapsulating cement particles, forming conductive channels around and between them. The forming of a conductive network of CNFPUC could be equated with CNF forming a conductive network in PU and then wrapping around the cement particles, forming a conductive channel around the network and the gap. A large number of interpenetrating conductive networks appeared in the material and subsequently, the conductive network was completed. Even if the amount of CNF continued to increase, the resistivity of the CNFPUC did not change significantly. The equivalent circuit of the CNFPUC composite is shown in Figure 3.  
On the basis of the above mechanism, the percolation curve shows that the resistivity of the CNFPUC decreases with an increase in the CNF content. In unpercolated zone I, when the content of CNF was low, a larger amount of ash was beneficial to the dispersion of CNF and the resistivity was lower. With an increase in the CNF incorporation, in percolation zone II, the resistivity decreased rapidly; the dispersion difficulty first occurred for low poly-ash ratio sample and the decreasing rate accelerated with an increase in the ratio of polymer to cement. In percolation zone III, the resistivity tended towards stability and did not change with the content of CNF. The conducting process of CNFPUC in the percolation zone was extremely sensitive to the microstructure of the percolation network. On one hand, the increase in the CNF content or compressive stress would lead to a considerable change in the interfiber tunnel gap and its distribution; on the other hand, the probability of fiber lapping would increase. However, in the percolation zone, the continuous conduction path formed by the complete overlap of CNF appeared in the percolation system and the conductive network was essentially perfect. With a further increase in the CNF, the resistance change rate was small under compressive stress. In addition, the excessively large content of CNF resulted in the inferior performance of CNFPUC.