Fourier Transform IR Spectroscopic Characterization of the Functional

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Downloaded by UNIV OF MICHIGAN ANN ARBOR on December 11, 2014 | http://pubs.acs.org Publication Date: October 13, 1982 | doi: 10.1021/bk-1982-0200.ch011

Fourier Transform IR Spectroscopic Characterization of the Functional Groups on Carbon Black W. M . PREST, JR. and R. A . MOSHER Xerox Corporation, Webster, N Y 14580

The chemical and physical properties of carbon black are strongly influenced by the presence of surface oxides which are formed during the manufacturing process. This paper demonstrates that the nature and relative concentrations of these species can be determined using the high signal to noise and energy throughput capabilities of Fourier transform infrared spectroscopy. Spectra associated with the functional groups have been derived from transmission measurements of dilute dispersions of carbon black in K B r using computer assisted techniques. The resultant spectra consist of characteristic bands at 1725, 1595 and 1245 cm-1 whose intensities are strong functions of the manufacturing conditions. The concentration and thickness dependence of the absorbance follows Beer's Law indicating that this method can be used to quantitatively characterize the concentrations of each species. The technique is used to compare the characteristics of commercially available carbon blacks and to determine the changes that occur as the result of selective heating and titration experiments.

Reprographic technologies rely on the high optical density of carbon black to pigment the wet and dry inks called toners. The carbon, present in the form of finely divided particles, is manufactured by a variety of methods from the partial combustion of fossil fuels [1]. Each process produces characteristic changes in the properties of the black as the result of the chemistry which occurs during the oxidative step and the impurities introduced through the feed stock or condensation procedure. The resultant carbon blacks are often futher chemically modified to improve such properties as their ability to be dispersed in a given resin and/or the subsequent rheology of the composite. In addition to influencing the optical and rheological properties of a toner, the surface chemistry of the 0097-6156/82/0200-0225$07.00/0 © 1982 American Chemical Society In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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carbon black has an important effect on the electrical properties. Fabish and Hair [2] have shown that chemical modifications which change the apparent acidity and oxygen content of a black produce correspounding changes in the contact potential (work function). Recently, M i e n [3] has illustrated how the triboelectric charging characteristics of a toner are related to the contact potential. These examples of the importance of the surface characteristics of carbon black have motivated a variety of attempts to characterize the functional groups on the surface. However, while there is abundant evidence for the presence of different types of functional groups from such measurements as the chemical reactivity, the acidity and the volatile components of the black [1], there is insufficient data to unambiguously identify the species responsible for the desired properties. This paper extends previous investigations of the infrared spectra of carbonaceous compounds [4-l£] to the characterization of the functional species which are present on commercial carbon blacks. It is shown that the high signal to noise and energy throughput capabilities of Fourier transform infrared spectroscopy (FTIR) enable the quantitative measurement of the spectra of the surface species. This technique is applied to the characterization of the distinguishing features of commercial carbon blacks and to the analysis of the products of selective heating and neutralization experiments. BACKGROUND Numerous analytical procedures have been developed to characterize the chemical species that are present on the surfaces of carbonaceous materials [1]. One simple measure is the volatile content, which is defined as the weight lost as the result of heating to 900°C in an inert atmosphere. Carbon blacks used in reprography typically have volatile contents ranging from five to sixteen percent These thermally evolved gases consist primarily of carbon monoxide and carbon dioxide in such proportions that oxygen constitutes approximately 60% of the volatile products [19-2Q]. The nature of the oxygen containing groups have been explored by different chemical analysis methods. Selective neutralization experiments using bases of increasing strength have been used to argue for the presence of carboxylic acids, lactones, phenols, and quinones. The most extensively used technique, developed by Boehm [21] and Schubert et al. [22], determines the carboxyl, lactone, phenol and qunione concentrations from the respective back titrations with NaHC03, N a C 0 3 , NaOH and NaOC2H . However, as pointed out by Mattson [9] and Rivin [20], this technique sets arbitrary limits on the acidity ranges for each group, which in fact may be substantially perturbed by the strong electronic interactions between groups through the conducting substrate. Thus, for example, there is the question of whether the NaHC03 neutralization measures all or only the most acidic of the carboxylic acids. 2

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In Colloids and Surfaces in Reprographic Technology; Hair, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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Infrared spectroscopy has been used to probe the surface functionality of a variety of carbonaceous materials since the early 1940s [4-18], These attempts to define and characterize the surface groups have been restricted by the relatively low concentrations of the functional species. More significantly, the characteristic absorptions are often masked by the high optical absorption edge of the carbon substrate. This requires that the instrument be able to resolve small signals superimposed on a large background absorbance. The assignment of the observed bands is particularly complicated because the functional groups are present in such a wide variety of electronic and chemical environments that the absorption bands are much broader than those of the isolated compounds. Futhermore, these carbon surfaces are so reactive and absorbent that extreme care must be excercised to avoid the artifacts that result from contamination. In spite of these difficulties, several infrared studies have succeded in measuring the spectral characteristics of different carbonaceous compounds and have associated the features with the vibrational modes of an often conflicting variety of functional groups. A schematic representation of the oxygen related functionalities and band assignments which have been proposed for a variety of carbon based compounds is presented in Figure 1. In addition to the carboxylic acids (1725 cm ), quinones (1675 cm ), phenols (1200, 3600 cm* ) and lactones (1775, 1250 cm ) mentioned above these species include aldehydes (1700 cnr ), aryl ethers (1260 cm- ), cyclic anhydrides (1775,1740 & 1260 c m ) and the skeletal modes of carbon (1600 cnr ) [17]. Attempts to resolve these ambiguities by measuring the spectra of chemically or thermally modified carbons have often been indecisive because of the extremely small resolvable signals and the associated experimental difficulties in defining absolute changes in band intensity. Earlier work by O'Reilly and Mosher [18] demonstrated that FTIR spectroscopy could be used to quantitatively measure the vibrational spectra of carbon black. They showed that at low carbon black concentrations Beer's Law was followed and that, in principle, the concentration of functional groups could be obtained. This paper describes the extention of this FTIR technique to the quantitative measurement of the vibrational spectra of different types of carbon black and its application to the analysis of the characteristic features of these blacks. 1

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E X P E R I M E N T A L M A T E R I A L S AND T E C H N I Q U E S The carbon blacks used in this study were obtained from the Cabot Corporation, Billerica, Massachusetts through the courtesy of Dr. John Riehl. They were chosen to be representative of a variety of surface characteristics ranging from the graphitized Sterling M T and the devolatilized Regal 330R to the highly oxidized Carbolac 2. Also included is a sample of H T T 5.3, a black prepared by Cabot Corporation from the selective oxidation of CSX 99. The nitrogen surface areas and



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volatile contents of these blacks and the concentrations of functional groups defined by the Bohem scheme (from back titrations) are listed in Table I. Each black was initially dried at 105°C for one hour to remove moisture and adsorbed components. Care was taken to insure that the oven was clean and free of contaminants such as silicone grease, oils and other samples. A l l subsequent sample manipulations were conducted in a nitrogen purged dry box. A master dispersion, consisting of 10 mg of black and 1 gram of potassum bromide (KBr, Specta grade, Harshaw Chemical Company), was placed in a cleaned and dried 2cc stainless steel canister containing a 0.64 cm steel shot and milled for 5 minutes on a Crescent Dental Mfg. Co. Wig-L-Bug. The master dispersion was further diluted with KBr to produce a sample with a nominal composition of 0.47 mg of carbon black per gram of KBr and milled for another 5 minutes. A PerkinElmer K B r vacuum die was used to produce sample disks ~ 12mm in diameter and 1 mm thick. Spectra were obtained with a Digilab Model FTS 15B Fourier transform infrared spectrometer equipped with a HgCdTe detector. Typically, 512 scans were run in both the reference and sample beams at a resolution of 4 wavenumbers. Critical band positions were subsequently defined from spectra obtained at a resolution of one wavenumber. Spectra of freshly prepared K B r reference disks were interspersed throughout the measurement cycles. Inspite of the above drying precautions, moisture related bands at 3390 and 1640 c m were always present in the milled K B r dispersions and, as has been previously noted [11], are enhanced by the grinding process. The spectra of these "false" KBr-H^O species was obtained by subtracting the spectra of reference disks which had been milled for different periods of time. The spectra of "computer dried" carbon black was obtained by subtracting a multiple of this moisture spectra using the elimination of the 3390 cnr band as the criteria for complete removal. Following this correction, the spectra of a K B r disk of the same thickness as the sample was subtracted from the experimental data. Complementary experiments in which the carbon blacks were milled in Cesium Iodide, dusted or solvent deposited on silver chloride plates, or mulled in Nugol demonstrated that the measured spectral features are characteristic of the particular type of carbon black and are not the result of chemical reactions with the K B r matrix. As demonstrated below, these techniques permit absorbances to be defined to better than ±0.005. 1

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R E S U L T S and DISCUSSION Carbon Black Spectra Typical absorbance spectra of Monarch 1300 carbon black are presented in Figure 2. The spectra consist of small signals in the vicinity of 900-1800 c m and 3200-3600 cnr superimposed upon a broad background which is almost a linear function of the wavelength. This intense dispersion has been attributed to the intrinsic absorbtion edge of the particular carbon 1

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TABLE I

Carbon Black N'2 area Volatiles 2

Carbolac® 2 Monarch® 1300 HTT5.3 CSX-99 Regal® 330R Sterling® M T * Determined 1- Carboxyl = 2- Lactone = 3- Phenol =

m /g

wt%

935 530 618 520 91 70

15.6 10.8 5.8 2.5 1.1 0.6

Functional groups in /xeq/g*

o

# 2

12.9 13.9 5.3 2.4 0.0

2

Carboxyl* Lactone 569 504 265 94 60 30

333 259 219 0

.3 Phenol" 408 288 303 312

from the Boehm Scheme NaHCC>3 - ionized acids N a C 0 -NaHCO; NaOH N a C 0 " 2

3

:

2

3

# Oxygen content by difference from elemental analysis of H & C (includes some sulfur and ash) ® Carbolac, Monarch, Sterling and Regal are Registered Trademarks of Cabot Corporation, Inc.


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^

0 4 0 0 0

\

2

i 3 2 0 0

i 8 0 0

i i 2 4 0 0 1 6 0 0 W A V E N U M B E R S

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Figure 2. The absorbance of Monarch 1300 dispersed in KBr as a I unction of concentration. Key: 1, 7.05 X 10 f> CB/f> KBr; 2, 4.70 X 10 f> CB/x KBr; and 3, 2.35 X 10 f> CB/x KBr. 4

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and/or to interparticle scattering. However, scattering can be expected to be minimal for several reasons. First, the size of the scattering elements, which are composed of - 50 nm elemental carbon black particles fused into -500 nm aggregates, are significantly smaller than the infrared wavelengths of interest (ie. 2.5-20 mm). Secondly, the inhomogeneous Rayleigh scattering from such small particles would be proportional to the fourth power of the frequency instead of the observed linear dependence [22]. Finally, there is no evidence for multiple scattering events since the absorbance of this dispersion, as characterized by its magnitude at 2800 cnr \ is a linear function of both the concentration of the black and the thickness of the pellet as shown in Figure 3. In other words, the magnitude of this apparent background follows Beer's law and, as shown below, directly scales with the absorbances of the functional groups on a particular black. These results indicate that this background dispersion is the intrinsic absorbance of the particular carbon black analogous to the band edge of conducting materials [14,1£,1£]. As will be shown below, the presence of surface species has a profound effect on the shape of this absorption edge. The data manipulation capabilities of the FTIR spectrometer can be used to quantitatively resolve the structural features which are superimposed upon the intrinsic absorption. The spectral features which exceed a baseline drawn between 1880 and 880 cnr in five independent preparation and measurement experiments are shown in Figure 4. The superposition of these five spectra illustrate both the reproducibility and the quantitative nature of this technique. The "resolved" spectra consist of three broad absorptions centered around 1725, 1595 and 1245 cnr and two superimposed sharp bands at 1135 and 1340 cm' . These latter peaks are characteristic of the particular type of carbon black and are presumably caused by impurities introduced in the manufacturing process. They may reflect the presence of residual sulfur compounds present in the form of sulfones or sulfonic esters in which the symmetric and antisymmetric stretching modes of the S02 vibrational modes occur in the range of 11401160 c m and 1300-1350 cnr [24]. Note the absence of discernable bands above -1730 cnr . This implies that these carbon blacks do not contain the lactone and cyclic anhydride functionalities observed on other carbon surfaces (see Figure 1) [17]. The dominant bands in the resolved spectra obey Beer's law in that the magnitude of the absorbance (defined in terms of the peak heights relative to a linear baseline drawn between 880 and 1880 cm ) is linear in both concentration and thickness (Figures 5 A&B). Futhermore, when these spectra are divided by the magnitude of the intrinsic absorption at 2800 cnr , the resultant absorbances are independent of sample thickness and concentration as shown in Figure 6. This supports the previous conclusion that both the broad background absorption and the superimposed spectra are directly related to the chemical structure of the black. The data also demonstrate that the spectral features of carbon black can be routinely measured to within ± 0.005 absorbance units. This capability for the quantitative measurement of the effects of various modification procedures 1

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Figure 3. Absorbance at 2800 cm' of Monarch 1300 dispersed in KBr as a function of concentration and thickness (O).

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1600 1200 WAVENUMBERS

800

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Figure 4. Resolved spectra of five separate preparations of Monarch 1300 obtained by subtracting the linear extrapolation of the background absorbance between 1880 and 880 cm .

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Figure 5.

0

1

i

i

i

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f

1

004 0.08 0.12 0.16 0.20 0.24 GRAMS OF CARBON BLACK IN KBr (xlO" )

•

THICKNESS (mm)

Absorbance at 1245 O , 7595 CO;, and 1725 cm (V) of resolved spectra of Monarch 1300 as a function of concentration (left) and thickness (right).

GRAMS OF CARBON BLACK IN KBr (xlO* )

WEIGHT % CARBON BLACK IN KBr


1

0.28

w


1

3

0.08 0.12 0.16 0.20 0.24 0.28 GRAMS OF CARBON BLACK IN KBr(x|Q" ) 1

0.32

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Figure 6. Absorbance of the resolved spectra at 1725 and 1595 cm' normalized by the absorbance at 2800 cm' as a function of the total amount of Monarch 1300 carbon black dispersed in KBr discs of varying concentration and thickness. Key: thickness (1725 cm' ); | , concentration (1725 cm' ); V , thickness (1595 cm' ); and V, concentration (1595 cm' ).

0.12


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on the absolute magnitudes of the absorbances of different bands permits tests of proposed band assignments. Heat Treated Blacks Information about the origin of the spectral features can be obtained from selective heating studies [i,ll,H,15,17]- Figure 7 shows the weight change and rate of weight loss [dW/dt] of dried Monarch 1300 as it is heated at 40K/min. in argon (thermogravimetric analysis - T G A ) . Note that by 900°C the sample has lost 10.8% of its weight This corresponds to the "volatile content" of this material (Table I), a number which is often used to define the total oxygen content of the black. However, since there is a continuing loss of weight at and above this temperature, the data indicate that the black contains substantially more volatile components than are assessed in the traditional analysis. Figure 7 indicates that there are at least two mechanisms of desorption: a relatively rapid process which appears to be completed by 400°C superimposed upon an approximately constant loss process. Analysis of the gases evolved during the heating of similar carbon blacks have shown that carbon dioxide is the primary product of this initial process [19,20]. This has been associated with the removal of labile carboxylic acid functionalities [U]. The infrared spectra of selectively heated Monarch® 1300 carbon blacks are shown in Figure 8. Each sample was held at the indicated temperature for one hour. Care was taken to maintain the sample in an inert atmosphere throughout the heating, milling and measurement steps. The 1725 cm" band disappears by 400°C in parallel with the T G A loss mechanism which has been associated with the production of CO2. This further supports the assignment of this band to carboxylic acid. A surprising feature of the spectra shown in Figure 8 is that the loss of the 1725 cm* carboxylic acid absorption reveals the presence of a sharp band at 1700 cm* . While this band may be attributed to the presence of aldehydes, its thermal stability and narrow band width in this carbon black are surprising. Of particular interest is the persistence of the 1595 and 1245 c n r l bands in carbon blacks which have been heated to 1700°C. The presence of these bands in these thermally "devolatilized" blacks suggests that they are characteristic of the basic carbon substrate rather than a surface group. The -1600 cm band, which has been observed in a variety of carbonaceous compounds, has been associated with the aromatic skeletal modes of carbon. Futhermore, it has been proposed that the relative intensity of this band is enhanced by the presence of surface species [4,17]. In the context of the theory of the optical properties of dielectric media containing small dispersed conducting islands, these groups can be expected to change in dielectric constant of the surface of the conducting carbon particles thus modifying the boundary conditions between the black and the matrix [25]. This would have the effect of shifting the absorption band edge to higher frequencies as has been observed in dispersions of oxidized metal particles. 1

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900

400 500 600 TEMPERATURE °C

KXX)

Figure 7. Thermogravimetric analysis of Monarch 1300. Percent weight loss and rate of loss (dW/dT) as a function of temperature when heated in argon at 40 K/min. 14

" 2000 2

1600 1200 WAVENUMBERS

800

Figure 8. Resolved spectra of Monarch 1300 carbon blacks which were held at the indicated temperature for one hour in a dry nitrogen atmosphere and milled in KBr in an inert atmosphere (N ). s


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This mechanism provides an explanation for the apparent decrease in the magnitude of the 1595 c m absorption which occurs as the volatile species are removed at elevated temperatures (Figure 8). It should be noted that other vibrational processes can contribute to the intensity of 1595 cm region in addition to the ring modes of the carbon, as illustrated by the selective neutralization experiments described below. In contrast to the 1595 c m band, the assignment of the extremely broad (>130 c m half width at half height) [18] absorption centered at 1245 cm- is uncertain. The thermal stability of this absorption implies that it reflects a characteristic feature of the bulk rather than the surface. The intensity of this band is completely synchronized with that of the 1595 c m ring mode throughout the heating studies (Figure 8). Futhermore, while the relative magnitudes of the 1595 and 1245 cm bands differ from black to black, the ratio of these bands in a given black is independent of subsequent thermal treatments. The 1245 cm band thus appears to be related to the bulk absorption of the carbon black substrate because the presence of surface species equally enhances the absorptions at 1245 and 1595 cm . These results are consistent with proposals for aryl ethers [17], although the extreme breadth of this band suggests the possibility of other underlying absorption processes. The heat-treated materials are very reactive and are readily reoxidized in the presence of air. For example, the carboxylic acid band at 1725 c m reappears when the sample heated to 600°C is milled in air instead of nitrogen (Figure 9). As expected from the above discusssions, the presence of these new surface species also enhances the absorptions at 1595 and 1245 cm* . Interestingly, the air and nitrogen milling conditions have different effects on the magnitudes of the 1135 an 1340 cm impurity bands of this carbon black. In summary, in the context of the previous infrared work, the results of the selective heating experiments show that the spectra of carbon blacks are composed of bands associated with: carboxylic acid, the skeletal modes of aromatic carbon and, perhaps, thermally stable aldehydes and aryl ethers. 1

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Selective Neutralization Selective neutralization experiments provide another method for evaluating the relationship between the absorption spectra and the identity of the surface species [6,16]. Samples of Monarch 1300 carbon black which had been back titrated with different strength bases as per the Boehm scheme (see Table I) were filtered, washed with neutral water and then reacidified with dilute HC1. Spectra were obtained on samples dried after each of these steps. In these samples, the reacidification process completely reversed the changes in the spectra which had been produced by the selective neutralizations. Figure 10 compares the result of the NaHCC>3 titration with that of the reacidified black. This weak base, which is used to measure the carboxylic acid content of the black, reduces the 1725 cnr band and increases the intensity of the 1595 cm* band. Futhermore, there 1

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1600 1200 WAVENUMBERS 800

Figure 9. Resolved spectra of Monarch 1300 carbon black which has been heated in nitrogen at 600 °C for one hour and milled in KBr in a nitrogen or air atmosphere.

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Fourier Transform IR Spectroscopic Characterization of the Functional

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