# Mechanical and Conductive Behavior of Graphite Filled Polysulfone-Based Composites
## 3 Results and Discussion
### Mechanical Properties  
#### 3.3.1 Flexural Test  
Figure 4 shows the variation of the flexural strength of the composite filled with different types of graphite filler.  
When maximal stresses are attained, brittle material breakdown occurs, and the load-strain diagram corresponding to this corresponds to ISO 178 type \"a\". For all materials, an increase in filling causes an increase in the elastic modulus, which results in a commensurate decline in deformability (Table 2, Figure 4). Depending on the filler's morphology, different changes in a material's strength characteristics occur.  
The maximum flexural strength of the PSU-based composites filled with 30 wt.% NG, AG and EG reached 41.72 MPa, 38.43 MPa and 55.79 MPa, respectively.  
For materials based on AG (Figure 4b) that contain spheroidal-shaped filler particles, the strength gradually declines as the degree of filling rises up to 70 wt.% by weight. This decline in strength is caused by a reduction in the thickness of the matrix interlayers, which, at high fillings, is no longer sufficient to wet all of the filler's available surfaces. So, using AG, we found that when filler concentration increased, the flexural strength began to decrease. Because of the formation of agglomeration as the filler amount increases, we can assume that there is an improvement in the interfacial adhesion between the filler and matrix, which decreases with increasing filler content.  
From Figure 4a, we can see that using NG as filler there is a variation in flexural strength values depending on the filler content; when 30 wt.% of the filler was used, we achieved the highest flexural strength. With filler content higher than 30 wt.% we noticed that there is no big difference in flexural strength when using 40 and 50 wt.% of NG and the flexural strength decreased compared to using 30 wt.% of NG. This can be explained by the fact that sheets of graphite are stacked on top of each other and overlapped into a composite. With the increase of the filler content, the number of interlayered polymer layers between  
Figure 4: Flexural test curves of PSU based composites filled with 30 (1), 40 (2), 50 (3), 60 (4) and 70 (5) wt.% of NG (a), AG (b) and EG (c).  
the graphite particles decreases, which leads to weaker adhesion properties and a decrease in the mechanical properties of the composite. In addition to the structural pores of graphite sheets, there are pores between the composite and the matrix. The critical filling level is around 60 wt.%, after which the connections between the polymer matrix's interlayers are insufficient to maintain the composite's strength, which falls below 24.07 MPa.  
It will be possible to characterize the changes in attributes for EG after rechecking; in theory, it can be similar to NG, but the rate of strength loss will be higher because a bigger volume will be occupied by the more defective filler. A big enhancement in flexural strength was found using EG (Figure 4c) as a filler using 30 wt.% which was about 3 times bigger than for 40 wt.% EG, and then the flexural strength started decreasing. As a result, we can say that the increase in the filler content leads to a decrease in flexural strength, and that is because the graphite filler seems to change the polysulfone composite to show more brittle behavior.