**1. Introduction**

The properties of traditional materials, such as metals and their alloys, do not always allow, even with the most modern processing methods, to achieve such characteristics in strength, durability, and reliability, in order to provide the highest operational capabilities for a variety of designs and equipment. In areas such as missile technology, aviation, automotive, chemical industry, and many others, the optimum characteristics of materials have long been achieved through the use of composites [1]. The composites can be defined as materials that consist of two or more chemically and physically different phases separated by a distinct interface [2]. It is known that the structure of the matrix in composites is formed with a significant influence of the filler. The division of fillers into active and inert or active, semi-active, and inactive [3] is overly simplistic, since the activity of the filler is not its specific feature or fundamental property. Nevertheless, the activity of the filler, especially for sintering pitch composites, is a very important characteristic. The activity of the filler determines their behavior during sintering and the final properties of the composite.

The activity of the filler is mainly determined by three factors:


Filler activity refers to certain properties of the filled system. Therefore, the concepts of structural, kinetic, and thermodynamic activity were introduced [6].

The structural activity means the ability of the filler to change the molecular and supramolecular structures of the composites (the degree of crystallinity, the size and shape of the elements of the structure, etc.).

Kinetic activity is the ability of the filler to influence the mobility of certain kinetic units of the polymer, the relaxation processes, and the viscoelastic characteristics during the deformation of the composite.

Thermodynamic activity is the ability of the filler to influence the state of thermodynamic equilibrium, the phase, and thermodynamic parameters of the filled polymer.

The free surface energy of the filler particles has a basic value and determines the adhesion interaction and wettability of the surface. In this connection, the role of functional groups on the surface plays an important role and their reactivity [7].

Unfortunately, until now there is no single approach to measure the degree of activity of fillers. This is due, in particular, to the lack of theoretical developments for sintering composites, in which the adhesive strength varies with the sintering temperature. In the sintering of pitches with fillers, the cohesive strength of the cement matrix increases with carbonization, thereby increasing the adhesive strength of the composite. Since the basis of the activity of fillers is their adhesive interaction with the binder, it seems that the measure of their activity should be the methods of determining the adhesive strength.

The interaction of the filler with the binder is most often evaluated on the basis of wetting, while the quantitative characteristic is the wetting contact angle [8].

Wetting is the result of an adhesive interaction between the surface of the solid (filler) and the contacting liquid (binder). Quantitatively, adhesion is characterized by the work of adhesion, which is expressed by the equation of Dupree:

$$\mathbf{W}\_{\rm u} = \boldsymbol{\chi}\_{\rm L} + \boldsymbol{\chi}\_{\rm S} + \boldsymbol{\chi}\_{\rm SL} \tag{1}$$

where *γ*L is the surface energy (tension) of the liquid phase, *γS* is the surface energy of the solid phase, *γSL* is the interfacial surface tension, and *Wa* is the work of adhesion. A rise in the interfacial attraction results in an increase in the work of adhesion. Eq. (1) can be rewritten to determine the work of cohesion *(Wc)* when the two phases are identical and no interface is present as shown in Eq. (2) for a solid phase:

$$\mathbf{W}\_c = \mathbf{\mathcal{Z}} \boldsymbol{\chi}\_S \tag{2}$$

**19**

the pitch buried in the tube, g.

*Determining the Filler Activity in the Sintering of Pitch Composites*

a "dynamic edge angle," as it is determined in some works [10].

features of physicochemical and mechanical properties [5].

spreading drop, pendant drop, and other methods [11–13].

The wetting contact angle (Young's equation) is associated with the work of

cosθ = 2*Wa*/*Wc* − 1 (4)

where Wa is the work of adhesion of the wetting liquid to the solid body and Wc

Systems of pitch-carbon filler are very difficult to describe. The properties of the binders are always more detailed than the properties of the fillers. Usually, as a direct evaluation of the quality of the binder, the contact angle of surface wetting of the filler uses their adhesion interaction during mixing. The pitch filler systems are thermodynamically nonequilibrium, and the equilibrium contact angle in such a system is unattainable [9]. When describing composites, the binder properties are always paid more attention. Evaluation of the filler is more modest. Therefore, for pitch filler compositions, one can speak of an "apparent" contact wetting angle or of

The pitch matrix is a self-filled system that, when heated, even without an external filler, gives a solid residue (semicoke, coke), which can be considered as a dispersed-hardened system, i.e., as independent composite material with all the

are used as a quantitative evaluation of the filler-binder contact interaction [14]. The softening temperature determines its chemical composition, surface tension, viscosity, the particle size of the coke, its texture, chemical functional groups on the surface and porosity [15]. The final properties of carbonized pitch composites (e.g., electrodes designed for high current densities) are largely determined by the interaction of pitch with the filler surface. Pitch should penetrate into the pores of coke and fill the voids between the coke particles. It is believed that the wettability of coke by pitch is a direct indicator of the degree of their interaction. A good wetting of the filler surface with a pitch is a necessary, but insufficient, condition for a strong adhesion bond and high physical and mechanical properties of the resulting composite material. Imperfection of existing estimates of binder-filler interaction leads to new attempts

to assess the process of wetting fillers with pitch. In Ukraine, the technique [16] is used, according to which the determination of the wetting power of the pitch is carried out in metal or glass tubes into which the filler (coke) and pitch grains are loaded in a ratio of 15:5. The tubes loaded in this manner are placed in a laboratory electric oven, where they are kept for a certain time at a temperature of 200°C, which ensures the flowability of the test pitch, but does not lead to caking of the test coke. Then, the tubes are cooled and the coke is removed, not bound (not impregnated) with a pitch. The wetting power (m) of pitch is calculated by the following formula:

*m* = *mcoke*/*mpitch* (5)

In the apparatus for determining wetting characteristics, according to the procedure of [17], granular coke 0.25–0.5 mm in size is placed in a cylinder on top of which a layer of solid pitch with a grain-size composition of 1–2 mm is applied. This composition is heat treated in a drying chamber with forced air supply at a

predetermined temperature and a time of thermal aging.

where m is the mass of bound coke (the difference between the initial sample of coke (mсoke) and the removed part of the latter), g, and mpitch is the initial mass of

The wetting angle for the pitch filler compositions is determined by sessile drop,

In addition to the wetting contact angle, the surface tension and capillary pressure

*DOI: http://dx.doi.org/10.5772/intechopen.82012*

is the wetting cohesion work.

adhesion Wa:

Adhesive strength—Ws—work of bond failure:

$$\mathbf{W}\_s = \mathbf{W}\_a + \mathbf{W}\_{def} \tag{3}$$

Wdef can be very large and differs from Wa by n times.

*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*

sive energy of the matrix [4].

• The amount of filler introduced.

and shape of the elements of the structure, etc.).

teristics during the deformation of the composite.

methods of determining the adhesive strength.

Adhesive strength—Ws—work of bond failure:

Wdef can be very large and differs from Wa by n times.

filled polymer.

• The ratio between the adhesion energy of the matrix to the filler and the cohe-

• The degree of dispersion of the filler particles, which determines the area of the contact surface of the matrix with the filler. The particle size of the filler is the most important property that affects the hardening of the composite. For each

type of binder and filler, there is an optimum degree of filling [5].

Filler activity refers to certain properties of the filled system. Therefore, the concepts of structural, kinetic, and thermodynamic activity were introduced [6]. The structural activity means the ability of the filler to change the molecular and supramolecular structures of the composites (the degree of crystallinity, the size

Kinetic activity is the ability of the filler to influence the mobility of certain kinetic units of the polymer, the relaxation processes, and the viscoelastic charac-

Thermodynamic activity is the ability of the filler to influence the state of thermodynamic equilibrium, the phase, and thermodynamic parameters of the

The free surface energy of the filler particles has a basic value and determines the adhesion interaction and wettability of the surface. In this connection, the role of functional groups on the surface plays an important role and their reactivity [7]. Unfortunately, until now there is no single approach to measure the degree of activity of fillers. This is due, in particular, to the lack of theoretical developments for sintering composites, in which the adhesive strength varies with the sintering temperature. In the sintering of pitches with fillers, the cohesive strength of the cement matrix increases with carbonization, thereby increasing the adhesive strength of the composite. Since the basis of the activity of fillers is their adhesive interaction with the binder, it seems that the measure of their activity should be the

The interaction of the filler with the binder is most often evaluated on the basis of wetting, while the quantitative characteristic is the wetting contact angle [8]. Wetting is the result of an adhesive interaction between the surface of the solid (filler) and the contacting liquid (binder). Quantitatively, adhesion is characterized

*Wa* = *γ<sup>L</sup> + γ<sup>S</sup> + γSL,* (1)

*Wc* = 2 *γ<sup>S</sup>* (2)

*Ws* = *Wa* + *Wdef* (3)

where *γ*L is the surface energy (tension) of the liquid phase, *γS* is the surface energy of the solid phase, *γSL* is the interfacial surface tension, and *Wa* is the work of adhesion. A rise in the interfacial attraction results in an increase in the work of adhesion. Eq. (1) can be rewritten to determine the work of cohesion *(Wc)* when the two phases are identical and no interface is present as shown in Eq. (2) for a solid phase:

by the work of adhesion, which is expressed by the equation of Dupree:

**18**

The wetting contact angle (Young's equation) is associated with the work of adhesion Wa:

$$\cos\Theta = \mathcal{Z}\,\mathcal{W}\_a/\mathcal{W}\_c - \mathbf{1} \tag{4}$$

where Wa is the work of adhesion of the wetting liquid to the solid body and Wc is the wetting cohesion work.

Systems of pitch-carbon filler are very difficult to describe. The properties of the binders are always more detailed than the properties of the fillers. Usually, as a direct evaluation of the quality of the binder, the contact angle of surface wetting of the filler uses their adhesion interaction during mixing. The pitch filler systems are thermodynamically nonequilibrium, and the equilibrium contact angle in such a system is unattainable [9]. When describing composites, the binder properties are always paid more attention. Evaluation of the filler is more modest. Therefore, for pitch filler compositions, one can speak of an "apparent" contact wetting angle or of a "dynamic edge angle," as it is determined in some works [10].

The pitch matrix is a self-filled system that, when heated, even without an external filler, gives a solid residue (semicoke, coke), which can be considered as a dispersed-hardened system, i.e., as independent composite material with all the features of physicochemical and mechanical properties [5].

The wetting angle for the pitch filler compositions is determined by sessile drop, spreading drop, pendant drop, and other methods [11–13].

In addition to the wetting contact angle, the surface tension and capillary pressure are used as a quantitative evaluation of the filler-binder contact interaction [14].

The softening temperature determines its chemical composition, surface tension, viscosity, the particle size of the coke, its texture, chemical functional groups on the surface and porosity [15]. The final properties of carbonized pitch composites (e.g., electrodes designed for high current densities) are largely determined by the interaction of pitch with the filler surface. Pitch should penetrate into the pores of coke and fill the voids between the coke particles. It is believed that the wettability of coke by pitch is a direct indicator of the degree of their interaction. A good wetting of the filler surface with a pitch is a necessary, but insufficient, condition for a strong adhesion bond and high physical and mechanical properties of the resulting composite material.

Imperfection of existing estimates of binder-filler interaction leads to new attempts to assess the process of wetting fillers with pitch. In Ukraine, the technique [16] is used, according to which the determination of the wetting power of the pitch is carried out in metal or glass tubes into which the filler (coke) and pitch grains are loaded in a ratio of 15:5. The tubes loaded in this manner are placed in a laboratory electric oven, where they are kept for a certain time at a temperature of 200°C, which ensures the flowability of the test pitch, but does not lead to caking of the test coke. Then, the tubes are cooled and the coke is removed, not bound (not impregnated) with a pitch.

The wetting power (m) of pitch is calculated by the following formula:

$$m = m\_{coke} / m\_{pitch} \tag{5}$$

where m is the mass of bound coke (the difference between the initial sample of coke (mсoke) and the removed part of the latter), g, and mpitch is the initial mass of the pitch buried in the tube, g.

In the apparatus for determining wetting characteristics, according to the procedure of [17], granular coke 0.25–0.5 mm in size is placed in a cylinder on top of which a layer of solid pitch with a grain-size composition of 1–2 mm is applied. This composition is heat treated in a drying chamber with forced air supply at a predetermined temperature and a time of thermal aging.

The prototype of method [16] is the method for determining the sintering ability of coals [18]. Caking coal is used instead of pitch, and anthracite is used instead of coke. It should be noted that in the prototype, the determination temperature reaches 600°C, which has a decisive influence on the physics-chemistry of the processes taking place.

In our opinion, for pitches as caking binders, it is important to evaluate the adhesive strength of their contact with fillers. In this case, the activity of the fillers will be the "sintering strength" and/or the "sintering capacity" of the pitch [19].

By the strength characteristics of the pitch composites, quantitative estimates of the quality of pitch as a binder relative to the selected filler can be obtained, as well as evaluation of the activity of the fillers with respect to the selected pitch. It is assumed that the use of this approach will allow to determine the optimal mass ratios of the binder-filler. In this case, the transition of the pitch to a solid state must be irreversible. Only then will it fully reproduce the physicochemical processes of interaction between the binder and the filler that occur during the production of the composite.
