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Influence of dispersing additives on the conductivity of carbon black pigment dispersion.

Abstract The influence of dispersing additives on the electrical conductivity of carbon black pigments dispersed in an organic medium was studied. Two dispersing additives were examined in combination with two different carbon blacks, a conductive carbon black and a nonconductive one. These carbon blacks differ in the size of their aggregates and in the amounts of surface oxygen groups. Both of the additives form a monolayer when adsorbed on either of the pigment surfaces. FTIR studies showed that chemical bonding of one of the additives on the surfaces of both pigments had occurred. Conductivity decreases with increasing additive concentration, but in the case of the chemically bonded additive, the conductivity of the dispersion remained high even at higher additive loadings. This study helps in understanding the effects such additives have on the specific conductivity of composite materials that contain dispersed carbon black pigment particles.

Keywords Carbon black, Dispersion, Adsorption, Conductivity

Introduction

Carbon black (CB) is one of the most commonly applied black pigments (1). It is used to modify the mechanical, electrical, and optical properties of the bulk medium, enabling such materials to be used in widespread industrial applications, such as mechanically reinforced plastic (1), abrasively improved rubber, conductive composites (2-6), pigmented inks, and other coating formulations (7-10). CB is also used for the adsorption of molecules from air or from aqueous media (11-14). In some applications, CB is also combined with other carbon materials, e.g., graphite (15). All of these applications depend on several properties of the CB powder, such as the particle size, the morphology of aggregates, the microstructure, and the chemical nature of the particle surface (1).

The CB particles are produced by incomplete combustion furnace process. There are three different types of particles. The primary particles are in the nanometer size region. During the manufacturing process, these particles fuse together to form aggregates. The aggregates have various sizes, morphology, microstructure, surface chemistry, and surface area. They are the smallest dispersible units. The shape and degree of aggregate branching is known as the "structure." The extensive interlinking between individual primary particles gives a "high structure" form of CB. Less pronounced branching indicates a "low structure" CB. Aggregates may percolate and form weakly bounded agglomerates with dimensions exceeding 1000 nm (1). A certain level of aggregation enables a better reinforcing ability of composites. Structured aggregation may be beneficial also for conductivity of the material. However, the state of aggregation has to be controlled and stabilized to obtain greater benefits of functional particles in final composite.

Owing to the high specific surface area, oil absorption ability, and hydrophobic character of CB particles, it is difficult to prepare stable and fine dispersions in an aqueous or organic media solely by grinding. Special additives are always used to solve these problems. Adsorption of surfactant on a solid surface strongly influences the quality of the dispersions. A stable dispersion can only be achieved using a suitable wetting and dispersing agent that gives strong adsorption on the solid surface of CB. The development of suitable polymeric dispersants could enable development of special applications, where the dispersion quality and stability would be needed if one were to obtain extremely fine dimensions. An example of such an application was the introduction of pigmented inks for ink-jet printers (16).

Most surfactants are complex oligomers/polymers, which are usually block copolymers or branched polymers. In many cases, it is not possible to specify the molecular structure and molecular mass of the active components. Therefore, research concerning the adsorption mechanisms of additives on a solid surface involves diverse methods to analyze the process and to determine the surface chemistry (14), (17-19).These methods require the preparation of well-stabilized dispersions. It was reported recently that ultrasonic action helps in the formation of pigment-polymer interactions and in the creation of the adsorption layer through ultrasonically induced activation of pigment particles (20).It is very likely that such treatment may be beneficial for the preparation of a stable dispersion.

The adsorption behavior of a surfactant on the CB surfaces is usually studied by establishing adsorption isotherms, giving the amount of adsorbed material as a function of its pressure (from the gas phase) or concentration (from liquid phase) at constant temperature. Such data are normally developed with a view to evaluate the capacity of activated carbons to adsorb organic molecules from gas or liquid phases (11), (14). The shape of an adsorption isotherm shows the nature of the process. The most frequently observed adsorption isotherms involving carbon materials are of the Langmuir or the Freundlich type (8), (11), (12), (16), differing in the existence of a stable maximum (monolayer) surface coverage. This is connected to the amount of surfactant that is required to obtain a homogeneous and stable dispersion of CB particles in the liquid phase. Recently, most studies in this field are concerned with adsorption from an aqueous solution (9), (11), (12), (14), (15)

The amount of surfactant that is applied to prepare the dispersion may influence not only the state of CB dispersion but also some other properties of the dispersion. The literature provides useful data concerning the influences of dispersants on the viscosity and on the colorimetric properties of concentrated CB suspensions (8). However, in some applications, such as electromagnetic interference shielding, electrostatic discharge protection, corrosion protection of metals, conductive adhesives, and circuit elements, conductivity is the most important property of the material. Here, the interlayer between the conductive particles and the bulk medium is of high importance (2), (21). There are at least two important concerns: first, a certain level of branching may assure percolation at lower CB concentration; and second, the interlayer between the conductive particles and the bulk medium may influence the conductivity of the final application. Most important are the concentrations of additive needed to cover entire CB particles; it assures the best state of the dispersion which is required to achieve the deepest color shade. However, in conductive applications covering the surface of CB particles with additives may not be the best situation for achieving high conductivity.

The stability of colloidal dispersion is controlled by steric or electrostatic repulsion forces between particles. These forces are provided by additives that are able to adsorb on particle surface and are soluble in the medium. Steric stabilization is obtained by dispersants containing pigment affinic (anchor or adhesive) groups on the one, and a resin-compatible chain on the other side of a molecule. Pigment affinic groups provide strong and durable adsorption of dispersant on particle surfaces while the other side of the dispersanfs chain could extend far enough into the medium. In such circumstances, a layer or steric hindrance is formed around particles, which keeps them apart and stabilizes the dispersion. When additives increase particle surface charge and make them equivalently charged, the electrostatic repulsion between particles acts as stabilization mechanism.

The dispersibility of CB particles in media with low dielectric constant (e.g., in automotive engine oil) was shown as an example of steric protection at dispersant level of about 1 wt% of CB, with simultaneous reduction of electrical conductivity. At dispersant levels about 4 wt% of CB, which is close to surface saturation of the dispersant, the effect of negative zeta potential gives evidence of electrostatic stabilization, and at this point an increase in conductivity (22). Similar effect was obtained for coal dispersions in kerosene when using 10-fold smaller amount of dispersant (23). However, steric forces have to be combined with electrostatic repulsion for the colloidal stability to be reached (23).

Industrial nonaqueous colloidal dispersions of CB were analyzed by electrochemical impedance spectroscopy to study the electrochemical characteristics of fluids and lubricants. It was shown that dispersant micelles surround CB particles as a colloidal suspension which prevents CB agglomeration (24).

Electrostatic repulsion and stabilization of colloidal systems is frequently characterized by zeta potential. Recently, the theoretical foundation and application on nonaqueous suspension of two CB powders were reported (25). It was shown that CB may act as Lewis acid or base, depending on the surrounding medium. The Lewis acid-base activity between CB and medium generate surface charges on CB particles which forms electrostatic repulsion forces and stabilizes dispersion. Different amounts of surface charge were formed on the applied CB powders; the compound with the higher amount achieved suspension with a better stability (25).

The maximum amount of surfactant adsorbed on CB surface depends on its head groups and tail lengths. It was shown that the primary and secondary amine groups support the proton exchange between head groups of the surfactant and acid groups on the CB surface. Thus, the surface charge is generated to give long-range electrostatic repulsion forces between CB particles. The net effect of the forces improved the stability of dispersion (26).

Our research on conductive CB composites has shown that dispersants have a specific influence on the electrical conductivity of the composite (6). This effect could be attributed to the different states of dispersion, which can solely be a consequence of using a specific additive. It is also necessary to study the influence of the amount of the applied additives and the role of interlayer between the particle and medium. For this purpose, the electrical conductivity of CB dispersions in organic media has been studied with respect to the dependence on the amount and the type of applied dispersant and in the absence of the binder. Special attention was devoted to adsorption isotherms of the systems. Dispersions were prepared with two CB powders; the high-structured, high-colored, nonconductive form; and the extremely high-structured, extra conductive form. Two commercially available additives that are recommended for nonaqueous media were applied. These were high-molecular weight additives with a film-forming structure and containing groups for stabilizing pigment dispersions. The same solvent was applied for all dispersions. The data obtained for the model system in this study helped us to determine which additive to choose, the amount needed, and the effect it had on the conductivity since it formed an interlayer between the particle surface and the host medium.

Experimental

Materials

The two CB pigments, Printex XE2-B (XE2-B) and Color Black FW2 (FW2), in powder form were supplied by Evonik, Degussa (Evonik Degussa International AG, Switzerland). XE2-B is the extra conductive CB with a particle size of 30 nm and a BET surface area of 1000 [m.sup.2] [g.sup.-1] FW2 is the nonconductivc CB pigment with a particle size of 13 nm and a BET surface area of 350 [m.sup.2] [g.sup.-1]. They have Colour Index PB 7-77266.

Two commercially available wetting and dispersing additives for nonaqueous media were used, Byk 9077 and Disperbyk 163 (Byk-Chemie GmbH, Germany), denoted here as Additive 1 and Additive 2, respectively. Both are high-molecular weight substances, which arc suitable for CB pigment coatings. Additive 1 has solid content of 98%. It is a solution of polyester block copolymer. It is recommended for application in solvent-free pigment concentrates, where it reduces the viscosity and prevents the reflocculation of the pigment. It is especially suitable for stabilizing CB pigments, and hence, it was applied for this study. Additive 2 has solid content of 45%. It is a solution of a block copolymer with three-dimensional structure, recommended for use with solvent-based coatings. It prevents reflocculation of pigments because of steric hindrance effects.

Gamma-butyrolactone (GBL, Sigma-Aldrich) was used as a solvent.

Size distribution of CB

The particle size distribution of CB was measured using a Malvern Particle Mastersizer 2000 laser diffraction apparatus. During measurement, particles pass through a focused laser beam that causes angular-dependent scattering that is converted to the intensity vs particle size relationship, according to the Mie scattering model. The measurements were done for dry powder samples. Each of them was ultrasonically treated to break agglomerates just before entering the measurement chamber. Therefore, it is assumed that the majority of particles that entered the laser beam were aggregates. Five runs were performed for each CB sample. The obtained size distribution graphs were expressed as the mean of the five measurements and show the average distribution of aggregates.

Preparation of dispersions

The dispersions were prepared with the solvent, the pigment, and the dispersing additive. No binder was used. We used two different pigments and two different dispersing additives to prepare four sets of dispersions. For each set, several samples were prepared. In every set of samples, the amount of pigment was held constant, and the amount of dispersant was varied. All the four sets of dispersions (two CBs and two additives) were prepared in the same manner. For each sample in the same set, different amounts of dispersant by weight were dissolved in 40 mL of GBL. Then, 0.4 g of CB was added in every sample of the same set. Therefore the concentration of CB in dispersion was 10 g [L.sup.-1]. Finally, the pigment was dispersed using a Hielscher UP400S ultrasonic processor, at 40% power (160 W [cm.sup.-2]), for 2 min. This processor is equipped with sonotrode which is placed directly into the sample, where the homogenization and deagglomeration are performed. The net adsorption time was 30 min. All the samples prepared in this way were used for adsorption and conductivity measurements.

Adsorption measurements

The adsorption isotherms were determined by the depiction method (17). The method assumes that the loss of dispersant from the formulation is caused solely by adsorption. In this method, the dispersant concentrations before and after adsorption are determined. After centrifugation (10,000 rpm), the dispersions were filtered through a syringe filter equipped with a 0.45-[micro]m pore size filter (Minisart SRP 25, Sartorius).

The concentration of dispersant in the filtrate was determined by absorbance in the UV spectral region applying the Beer-Lambert law. All of the spectra were measured using a UV-Vis spectrophotometer (Perkin Elmer Lambda 950) in the transmittance mode. For this purpose, the calibration plots for both dispersants were determined. These plots were utilized to determine the unknown concentration of dispersant in the filtrates that were acquired.

The adsorbed amount of the dispersant per unit area of the surface, [GAMMA], was calculated using the following formula, which involves the difference between the initial mass of the dispersant, [m.sub.i], and the final mass of the dispersant in the solution at adsorption equilibrium, [m.sub.f], and the mass and specific surface area of CB pigment, [m.sub.c] and [A.sub.c], respectively.

[GAMMA] = [m.sub.i]-[m.sub.f] / [m.sub.c][A.sub.c] (1)

Conductivity measurements of suspensions

Specific conductivities, [kappa] of prepared dispersions were measured using a dipping-type conductivity cell (InLab 741 Conductivity Probe, Mettler, Toledo) with the cell constant of 0.1045 [cm.sup.-1]. The suspensions were immersed in a water bath at a temperature of 25.0 [+ or -] 0.1[degrees]C and thermostated for at least 1 h before reading the value of the specific conductivity. For each sample, three measurements were taken consecutively. Taking into account the calibration, measurements, and impurity errors, the values of [kappa] were accurate within 1%.

Infrared spectroscopy

Infrared spectra of each CB powder, dispersant, and dispersant-powder dispersions were recorded using a Bruker Spectrometer IFS 66/S in the transmission mode. The CB powder was mixed with KBr and pressed into pellets for recording. The dispersant was applied between two ZnSe crystal plates. The dispersant-powder dispersions were prepared at the maximum adsorption density of the dispersants on the pigment surface. For this purpose, the filtrands were washed with solvent to remove nonadsorbed dispersant and dried in a desiccator under reduced pressure. Each obtained sample was placed between ZnSe crystal plates for recording.

Results and discussion

Size distribution of CB particles

The size distribution graphs of CB powder samples are shown in Fig. 1. The XE2-B powder has approximately twice as large particles as FW2, confirming the difference in "structure" and BET surface area, as given by the manufacturer. XE2-B is an extremely high-structured CB, whereas the FW2 is a high-structured CB. The higher the structure of the powder (or the larger the BET surface area), the larger are the aggregates.

[FIGURE 1 OMITTED]

Adsorption

Adsorption isotherms for all the investigated systems are presented in Fig. 2. A progressive increase of the amount of the adsorbed dispersant can be observed until an adsorption plateau is reached. Therefore, saturation of the surface of each of the CB pigments with the dispersant additives can be assumed to have occurred.

[FIGURE 2 OMITTED]

The shape of isotherms indicates Langmuir-type isotherms, developed by the simplest theoretical adsorption mechanism. This mechanism is developed on the basis that the adsorption sites on the substrate surface are equivalent, that the adsorbed molecule interacts with only one site and not with other adsorbed molecules, and that a monolayer is formed (18). The Langmuir isotherm is defined by the following equation:

[GAMMA] = [[GAMMA].sub.max] [kc.sub.e]/1 + [kc.sub.e] (2)

Here, [GAMMA].sub.max] is the maximum adsorbed amount of the adsorbate per unit surface area at saturation (the measure of adsorption capacity, mg [m.sup.(-2)]). k is the equilibrium constant (L [g.sup.-1]), and [c.sub.e] is the equilibrium concentration (g [L.sup.-1]). The equilibrium concentration is the concentration of additive that remains in dispersion after adsorption. Figure 2 shows that the adsorption capacity depends on CB surface and additive. For FW2, it is higher with Additive 2 whereas for XE2-B, it is higher with Additive 1. The onset of adsorption plateau in all the samples with Additive 2 is higher than for the samples with Additive 1. The adsorption capacity will be discussed in more detail in "Specific conductivity of dispersions" section.

The parameters of the Langmuir equation can be determined using the linearized form:

[c.sub.e]/[GAMMA] = [c.sub.e]/[[GAMMA].sub.max] + 1/k[[GAMMA].sub.max]. (3)

Plots of [c.sub.e]/[GAMMA] vs [c.sub.e] for each of the studied systems are given in Fig. 3. A linear dependence of [c.sub.e]/[GAMMA] against the equilibrium concentration, [c.sub.e], was obtained, confirming that the adsorption of the studied systems follows the Langmuir adsorption model.

[FIGURE 3 OMITTED]

From the slopes, values of [[GAMMA].sub.max], the values of adsorption capacity were determined. The intercepts (1/k[[GAMMA].sub.max]) allow the estimation of equilibrium constant, k, which is connected to the intensity of the adsorption. Values for all of the parameters, defining the Langmuir isotherm of the investigated systems, are listed in Table 1.
Table 1: Parameters of the Langmuir adsorption isotherms: adsorption
capacity, [[GAMMA].sub.max], and equilibrium constant, k

           [[GAMMA]*.sub.max]  [[GAMMA].sub.max]  k (L [g.sup.-1])
            (mg [m.sup.-2])     (mg [m.sup.-2])

XE2-B +    0.71 [+ or -] 0.02  0.74 [+ or -]0.01     1.68 [+ or
Additive                                                 -]0.30
1

XE2-B +    0.48 [+ or -] 0.02  0.52 [+ or -]0.01     0.83 [+ or
Additive                                                 -]0.10
2

FW2 +       0.93 [+ or -]0.03  1.02 [+ or -]0.01     0.68 [+ or
Additive                                                 -]0.06
1

FW2 +       1.19 [+ or -]0.05  1.36 [+ or -]0.03     0.45 [+ or
Additive                                                 -]0.05
2

[[GAMMA]*.sub.max] is the value given by the adsorption isotherm
(Fig. 3), whereas [[GAMMA].sub.max] and k values were derived from
linearization (equation 2)


Although the Langmuir model of adsorption is not usually used in the case of polymer adsorption, it provides a simple and useful method of analysis, which can be compared with other experimental data on similar systems that follow such an isotherm. A reasonable agreement observed between the experimental data and Langmuir isotherms (full lines in Fig. 3) for all the studied systems shows that the adsorption process in these systems can be well described by the Langmuir adsorption model.

Specific conductivity of dispersions

Specific conductivity values, [kappa], of both dispersing additives in GBL are shown in Fig. 4. It increases with concentration for the Additive 1 and is almost constant for Additive 2. All the values are low, but Additive 2 has almost negligible specific conductivity in comparison with Additive 1.

[FIGURE 4 OMITTED]

Specific conductivities of CB dispersions, as a function of the total concentration [c.sub.t] of dispersant, are shown in Fig. 5. The starting dispersion ([c.sub.t] = 0) with XE2-B has a 20-fold higher conductivity than that with FW2. This behavior can be expected bearing in mind the "structure" of both the CB samples. Highly structured and larger aggregates (Fig. 1) of XE2-B form better contacts between the particles, and hence produce larger conductivity values.

[FIGURE 5 OMITTED]

The conductivity of suspensions with XE2-B decreases with increasing concentration of both additives, although in a different manner. Therefore, the molecules of the dispersant, when adsorbed on the CB surface, screen the charges on the surface of the CB particles.

The addition of Additive 1 diminishes the conductivity of the XE2-B dispersion rapidly with the increased additive loading. A negligible plateau is observed at 10 g [L.sup.-1] that is the loading when a monolayer is formed. At higher loadings of Additive 1, the conductivity values remain constant and are in the same range as that of the pure Additive 1. Here, no evidence of chemical bonding was observed in FT1R studies, and it is assumed that the fully adsorbed Additive 1 neutralizes the charges on the CB surface, the net conductivity being mainly due to the free dispersant molecules in the dispersion. This assumption is in agreement with the results obtained from the adsorption study. The Langmuir isotherm for Additive 1 on XE2-B surface shows a clear plateau at concentration [c.sub.t] [greater than or equal to] 10 g [L.sup.-1] (Table 2).
Table 2: Values for the onset of adsorption plateau as equilibrium
concentration, [c.sub.e], and total concentration, [c.sub.t], of
the additive

                   [c.sub.e](g [L.sup.-1])  [c.sub.t](g [L.sup.-1])

XE2-B, Additive 1           3.5                      9.9
XE2-B, Additive 2           4.3                      8.2
FW2, Additive 1             4.6                      7.2
FW2, Additive 2             7.7                     11.7


Additive 2 forms chemical bonding with the XE2-B surface. The presence of such bonding was determined through the differences observed between FTIR spectra of the Additive 2 that was adsorbed on CB and the spectra of its pure form.

Additive 2 neutralizes the surface charges on the XE2-B surface and yields the minimum conductivity at the same loading as that found for the plateau in the Langmuir isotherm (8 g [L.sup.-1]), when a monolayer is formed (Table 2). A secondary peak was observed at additive loadings between 12 and 18 g [L.sup.-1], possibly caused by some bridge-like structure between the nearest neighbour XE2-B particles. This structure would break down at higher loadings, and the conductivity would be further decreased. At loadings greater than 20 g [L.sup.-1], the conductivity decreases to the value of the "pure" Additive 2 in GBL (~2 [micro]S [cm.sup.-1].

The conductivity of XE2-B dispersions with Additive 2 is several orders of magnitude greater than the conductivity that was achieved using Additive 1 at the same loadings of the pure additive. Although the conductivity diminution can be explained through the adsorption of molecules of Additive 2 on the surface of XE2-B particles and by the screening of the surface charges, the adsorption capacity ([[GAMMA].sub.max], Table 1) of the XE2-B is perceivably lower than that achieved with XE2-B + Additive 1. Therefore, it can be deduced that, even at equilibrium, there arc still charges on the surface of CB particles that might be responsible for the conductivity.

The initial conductivity of the FW2 dispersion was 20-fold lower than that of the XE2 pigment, indicating the low conductivity of the pigment.

The addition of the Additive 2 reduced the conductivity of the dispersion rapidly, until the monolayer reached 12 g [L.sup.-1], indicating the complete screening of surface charges of the FW2 particles (Table 2). Here [kappa] remains almost constant and slightly higher than the value obtained for the latter, pure additive. Such a small rise in conductivity could arise because of the weak chemical bonding between Additive 2 and the FW2 particles as was observed in the FT1R studies.

However, a completely different behavior of [kappa] was observed in dispersions of FW2 pigment with Additive 1. The conductivity decreased gradually at lower loadings of additive until the monolayer plateau was reached, and it increased sharply with greater concen-trations. Presumably, initially while the adsorption of Additive 1 on the surface of the CB particles takes place, the monolayer is formed ([c.sub.t] [greater than or equal to] ~7 g [L.sup.-1], Fig. 2, Table 2). Greater loadings of more conductive Additive 1 molecules gave rise to the conductivity of the dispersion. Although these values are small, such a rise in conductivity could only be caused by the greater population of the more conductive Additive 1 molecules in the dispersion.

FTIR studies of the dispersant's interactions with the CB surface

The spectra of the CB pigments, the free additives, and the adsorbed additives are presented in Fig. 6.

[FIGURE 6 OMITTED]

Three weak, broad peaks appear in the IR spectra of the FW2 powder (Fig. 6a): (1) The peak at 1735 [cm.sup.-1] originates in carbonyl (C=O) stretching vibrations. (2) The peak at 1600 [cm.sup.-1] originates in asymmetrical stretching of aromatic C=C bond with the oxygen atom near one of the vibrating C atoms. (3) The broadest peak at about 1250 [cm.sup.-1] could be attributed to different functional groups such as the C-C and C-O-C stretching and the C-H deformation of the aromatic hydroxy] unit that is hydrogen-bonded to the conjugated carbonyl group (27-29). The carbonyl peak does not appear in the spectrum of XE2-B powder; moreover, the other two peaks are smaller, indicating lesser oxygen content and a higher tendency to its random bonding on the surface of the XE2-B powder. This finding is in accordance with the literature data. The oxygen-containing chemically active groups on the CB surface are able to capture electrons, thus reducing the conductivity (1), (5). Therefore, the CB with the lesser oxygen content (XE2-B) has greater conductivity.

The infrared spectrum of Additive 1 remains practically the same after adsorption on to the XE2-B surface, whereas on the FW2 surface, the peak at 1642 [cm.sup.-1] disappears and a peak at 1600 [cm.sup.-1] appears (Fig. 6b). The first peak can be attributed to CNH vibrations, and the second peak shows the presence of FW2. Such a spectrum confirms that Additive 1 forms few, if any, chemical bonds on the XE2-B surface. However, there might be some adsorption on the FW2 surface (Fig. 6b).

Three carbonyl peaks were recorded in the IR spectrum of Additive 2: the out-of-phase and in-phase asymmetric C=O stretching at 1733 and 1713 [cm.sup.-1], respectively, and the third weak peak at 1686 [cm.sup.-1]attributed to symmetric stretch of carbonyl groups (27). In the adsorbed state, all these three peaks remain at the same positions but their intensity changes (Table 3). The area of the in-phase asymmetric stretching diminishes after adsorption on either of the two CB powders. The area of the out-of-phase asymmetric stretching peak increases for the spectrum of the additive on the FW2 surface and decreases in the spectrum for the additive on the XE2-B surface. The opposite effect was obtained for the symmetric stretching of carbonyl groups (Fig. 6c).
Table 3: The results of the fitting procedure to two groups of
 peaks, centered at 1730 (carbonyl) and at 1415 [cm.sup.-1]
 (CH deformation) in the IR spectra of Additive 2, Additive 2
 with XE2-B, and Additive 2 with FW2: positions ([v.sub.o]),
 half-width (HW), and area shared in the corresponding three-band
 group

                       Additive 2

[v.sub.o] ([cm.sup.-1])  HW([cm.sup.-1])   Area (%)

1734                           21.7          47
1711                           25.6          41.2
1685                           31.9          11.8
1453                           28.7          19.7
1417                           38.7          77.2
1358                           21.4          3.1

                 Additive 2 + XE2-B

[v.sub.o] ([cm.sup.-1])  HW([cm.sup.-1])   Area (%)

1732                           12.7          4.1
1714                           32.4         35.7
1690                           73.5         60.2
1449                           23.1         2.2
1403                           43.2         79.9
1361                           39.1         18

                   Additive 2 + FW2

[v.sub.o] ([cm.sup.-1])  HW([cm.sup.-1])  Area (%)

1734                          16.6          14
1717                          31.3          54.8
1696                          46.3          31.1
1449                          29.1          11.2
1412                          41.7          83.5
1361                          35.7           5.3

The calculation applied to the data from the IR spectra shown
in Fig. 6(c), with a mixed Gauss-Lorentz shape. The GRAMS/AI
software package (Thermo Galactic) was used in calculations


The spectral [CH.sub.n] deformational region of Additive 2 consists of three peaks, located at 1453 [cm.sup.-1] (C-[CH.sub.3] asymmetric deformation), 1417 [cm.sup.-1] (C-[CH.sub.2] deformation), and 1358 [cm.sup.-1] (C-CH3 symmetric deformation), respectively. After adsorption to the CB surfaces, the central peak shifts toward smaller wave-numbers. However, the other two peaks remain at approximately the same positions (Table 2). The shift of the C-C[H.sub.2] deformation band is larger when adsorption has taken place on XE2-B surface (for 14 [cm.sup.-1]) and less when adsorption has taken place on the FW2 surface (for 5 [cm.sup.-1]). The C[H.sub.2] deformation band shifts toward lower wave numbers when the CH2 group is next to a double bond or a triple bond, which is from the carbonyl group (8), (27). The effect is stronger on the XE2-B surface than when it is on the FW2 surface.

One may conclude that Additive 2 bonds chemically on the surfaces of both of the CB particle types. This bonding is stronger on the XE2-B surfaces. The observed changes in infrared spectra of Additive 2 after being adsorbed on CB surfaces could be explained as hydrogen bonding which is combined with Lewis acid interactions between the CB surface and Additive 2. Hydrogen bonding has larger contribution on FW2 surface but Lewis acid interactions on XE2-B surface. The latter effect generates surface charges which gives electrical conductivity of the dispersion. These results can be compared with results obtained on similar dispersions (22), (23).

Conclusions

The influence of the selected additives on the electrical conductivity of the resulting CB pigment dispersions, in an organic solvent, was studied. Two dispersing additives were examined in combination with two different CB pigments. These CB differ in the size of their aggregates and in the amount of oxygen atoms on their surfaces. Both the additives form a monolayer when adsorbed on either of the CB pigments.

The specific conductivity of the CB dispersions depends on the type of the additive used, the structure of the CB aggregates, and the additive loading.

The conductivity of the dispersion with highly structured and large aggregates of conductive XE2-B without additive is almost 20-fold higher than that of nonconductive FW2. The conductivity of dispersions of both CB pigments decreased with the increased loading of both of the additives although in a different manner.

Additive 1 reduced the conductivity of the XE2-B dispersion efficiently on increasing the additive loading. A negligible plateau was observed when a monolayer was formed. At higher loadings of Additive 1, the conductivity values remained constant and within the same range as that obtained with pure Additive 1. The FTIR studies showed that very weak chemical bonding was observed. Additive 1 therefore acted mostly as a spacer between the individual pigment particles.

Additive 2 forms chemical bonds with the XE2-B surface which neutralize the surface charges and gave local plateau in conductivity, at the same loading as that which gave monolayer plateau in adsorption isotherms. A secondary maximum was observed that was caused by some form of bridge-like structure between the nearest neighbor XE2-B particles which break down at higher loadings.

Dispersions of FW2 pigment that contain the Additive 1 show a similar reduction in conductivity until the monolayer plateau was reached. However, at higher loadings, small rises in conductivity were observed which could only be the result of the higher density of the more conductive Additive 1 molecules.

The addition of the Additive 2 reduced the conductivity of the FW2 dispersion rapidly, until the monolayer was reached. Afterward, on supplying greater loadings of the Additive 2, the conductivity remained almost constant and slightly greater than that of the pure additive. Such a small rise in conductivity arises from weak chemical bonding between Additive 2 and FW2 particles, as was observed in FTIR studies.

The results show that a chemical bonding of an additive on a conductive CB with high structure increases the electrical conductivity of dispersion. The origin of this effect was attributed to the activity between the Lewis acid on the XE2-B surface and Additive 2 which generates surface charges and supports electrical conductivity of dispersion.

This study is important in that it provides an understanding of the effect of an additive on conductivity of composites. The conductivity of the complex pigmented systems, such as dispersions containing different binders and pigmented composites, will be the subject of future research.

Acknowledgments This research was supported by the Slovenian research Agency (Project No. J2-9455). Nina Hauptman acknowledges the Slovenian Research Agency for the young researchers support. The authors wish to thank Evonik-Degussa, Croatia who kindly provided the carbon black samples, and AIM Chemicals, Croatia for providing the dispersant samples.

References

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N. Hauptman, M. Klanjsek Gunde, M. Kunaver

National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia

e-mail: marta.k.gunde@ki.si

M. Bester-Rogac

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, 1000 Ljubljana, Slovenia

DOI: 10.1007/S11998-011 -9330-5
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Author:Hauptman, Nina; Gunde, Marta Klanjsek; Kunaver, Matjaz; Bester-Rogac, Marija
Publication:JCT Research
Date:Oct 1, 2011
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