*Determining the Filler Activity in the Sintering of Pitch Composites DOI: http://dx.doi.org/10.5772/intechopen.82012*

*Fillers - Synthesis, Characterization and Industrial Application*

determination coefficients (*R*<sup>2</sup>

**Table 3.**

SC-T Y = −0.28x2

SC Y = −3.69х<sup>2</sup>

PC Y = −1.33x2

Glass Y = −1.3x2

Sand Y = −2.77x2

Anthracite Y = −2.58x2

*Strength of pitch composites depending on the filler.*

the condition of the problem.

ing order) in a row:

the value of x2, the greater the filler activity.

of all the excipients that were tested, the roots of these equations *x1* and *x2*, and the

**Fillers Equation R2 Roots of equation**

+ 2.80x + 54.42 0.92 −9.81 19.81

+ 39.2х−18.8 0.90 0.5 10.1

–19.2x + 40.2 0.76 1.86 16.29

+ 17.9x + 2.02 0.81 −0.1 13.9

+ 34.1x−23.6 0.96 0.74 11.6

+ 48.96x + 77.5 0.96 −1.47 20.45

**x1 x2**

) of the interaction y = f(x). We do not consider the negative values of the roots x1 because they do not match

The physical meaning of the roots x2 of the parabola equation is some hypothetical ratio of filler binding, in which the strength of the pitch composite falls to zero. The x2 indicator can be used as an indicator of the activity of the filler. The higher

There is some physical meaning of the roots x2 of the parabola equation. The x2 indicator can be used as a measure of the activity of the filler or of the sintering ability of the pitch. The higher the value of x2, the greater the activity of the filler is. That is, the more active the filler, the more its quantity will be agglomerating. By x2, it is possible to rank the binders for the sintering ability to the fillers. The activity of the filler is the amount of the inverse sintering power of the pitch [21]. In accordance with this indicator, fillers are located (in descend-

Anthracite—SC − T—PC—SC − glass—sand

As an example, let us consider the curve for the change in strength of pitch composites with anthracite (**Figure 4**). The strength of the pitch composite

increases (the left branch of the curve in **Figure 4**), because the high modulus filler reduces the ability of the entire composition to deform. With an increase in the degree of filling, the individual particles of the filler approach and their boundary (interphase) layers begin to interact with one another, forming a film structure of

We do not take into account here that individual particles in composites are rarely observed [22]. Since binders that solidify are typical nonequilibrium systems, the loss of stability leads to the spontaneous formation of primary clusters—a decrease in surface energy occurs due to a reduction in the interfacial surface and, as a consequence, aggregation of the filler particles. The filler particles on their surface have a layer of a pitch matrix, the so-called interfacial layer (IFL) [23]. There is no clear boundary between IFL and the pitch matrix, but it is established that IFL reduces the concentration of stresses on the surface of the matrix and filler, which affects deformation and fracture in composites [24]. The thickness of the interphase layer

**3.2 Determining filler activity by sintering ability**

the matrix between the particles (**Figure 5a**).

**24**

depends on the radius of the filler particles and the fractal dimension of its surface [25]. At low filler concentrations in the matrix, the interphase layers are not an independent phase in the volume of the composite, which does not affect its properties.

Considering the fact that the interfacial layer is an oriented ordered structure, the film has enhanced strength compared to the structure of the remaining volume of the pitch matrix. At the critical point, the complete transition of the pitch matrix to the film structure of the IFL is the main factor of strength increasing of the pitch composites. In addition, the filler-binder ratio at this point is optimal, which corresponds to the maximum on the curve (**Figure 4**).

In the case of a critical binder-filler ratio, the pitch matrix is completely on the surface of the filler particles in a structured (interfacial) layer (**Figure 5a**). The activity of the filler is maximum. With a further increase in the content of the filler in the composite (the right branch of the curve in **Figure 4**), the activity of the filler does not decrease, but the pitch matrix cannot cover the entire surface of the filler grains. The film is not enough to cover the entire surface of the filler grains. The binder film is divided into separate fragments (**Figure 5b**), which is accompanied by a sharp decrease in the strength of the final composite. In this consideration, we do not take into account the effect of agglomeration of filler particles.

Although the amount of the matrix and probably its cohesive strength does not change, the strength of the composite decreases. The strength of the matrix on the surface of the filler particles can be another indicator of the activity of the filler.

In the actual conditions of formation of the contact surface of the filler with a liquid binder, the processes of diffusion of the adhesive into the filler play an important role. A part of the pitch can penetrate into the surface layer of the filler due to capillary phenomena.

The removal of volatile substances from the pitch when carbonized leads to the appearance of shrinkage stresses and defects and, as a result, to rupture the film. That is, a pitch that is carbonized to the semicoke state may not completely cover the filler surface and not create a continuous "carbon skeleton," the ideas of which were previously expressed [26].

### **Figure 4.**

*Experimental points and approximation by the parabola equation for the dependence of the strength of a pitch composite on the amount of filler (anthracite) in it.*

**Figure 5.**

*The microstructure model of the pitch composite: a – a binder in the film on the surface of the filler; b – with pitch deficit: 1 – filler particles, 2 – semi-coke of the matrix.*
