GB2188311A - Process for modifying the surface characteristics of carbon black - Google Patents

Abstract

The surface characteristics of a furnace carbon black having a nitrogen surface area greater than 140 m2/g, are modified by treating the black with an organic adsorbate (eg. an n-alkane, n-alkane amine, n-alkane halide or n-alkane alcohol) having a molecular structure including a linear chain having at least four carbon atoms at a loading ranging from 1 to 2 percent by weight, whereby carbon black pores can be filled to effectively block adverse moisture absorption.

Description

SPECIFICATION Process for modifying the surface characteristics of carbon black and carbon black produced thereby Carbon black is produced by the thermal decomposition of hydrocarbons at very high temperatures. The blacks so formed are composed of essentially elemental carbon in the form ofaggregated particles of colloidal dimensions and high surface area. All carbon blacks, regardless of the method of manufacture or raw materials used in their production, possess many similar properties. The distinction between the various types or grades of carbon blacks is one of degree rather than kind, and is based on such characteristics as particle size, surface area, chemical composition ofthe particle surface, and extent of particle to particle association.

The carbon black particles generally are porous and feature both external and internal surface areas.

Specific surface areas, typically evaluated by adsorption techniques, commonly are used to identify and classify the blacks. Various product performance characteristics have been attributed to the internal and/or external surface areas of the carbon blacks incorporated therein.

Carbon black is a widely used ingredient for imparting conductivity to polymer systems. One such application is in anti-static compounds, such as for sheeting, belting, hoses, and molded goods, in order to minimize static build-up and explosion hazards in environments such as mines, hospitals, and other areas where solvent vapors or oxidants may collect. In the wire and cable industry, conductive carbon black compounds are utilized as metal conductor strand shielding in high voltage cables.

When carbon black is incorporated into polymer systems, however, the compound moisture absorption (CMA),the amount of moisture absorbed by the compound, may increase. The increase in CMA in conductive polymers can contribute to at least two significant problems. Firstly, the moisture absorbed in the compound can be vaporized during the extruding operations where temperatures can exceed 100 C (373"K). This vaporization results in "blow-holes" on the surface of the extrudate, which are a potential source of dielectric weakness.Secondly, the moisture absorbed in the compound can itself initiate dielectric breakdown by a process referred to in the art as "treeing". (The descriptive term "treeing" is derived from the shape of dielectric breakdown pathways as observed by rnicroscopic examination.) In particular, when certain conductive blacks are compounded into polymeric materials, the increase in CMA has been found to be primarily attributableto the microporosity of the carbon black. Now, according to the present invention, a process has been discovered whereby the microporosity of a carbon black can be selectively modified. The carbon black is treated with an organic adsorbate which is adsorbed by the black and effectively blocks micropores in a specific size range, while not adversely affecting other compound properties.

Applicant has determined that by treating a carbon black with an adsorbate, featuring select molecular dimensions, pores of a select diameter range can be filled and thereby effectively block adverse moisture absorption. It was found that the molecules of select adsorbate become firmly bound to the carbon black and do not become liberated under normal handling, storage, or use conditions. Theory suggests that the over lapping potential fields present in such small pores bind the adsorbate molecules with relatively high en- ergies making it difficult to displace the molecule from the micropore.

The treatment according to the present invention may be effectively applied to any grade of carbon black having a surface micro-structure featuring a significant portion of micropores in the size range penetrable by molecules of water. The black may be in pelleted orfluffyform. Accordingly, the treatment has been found to produce favorable results in modifying high surface area furnace blacks (having a nitrogen surface area [N2SA] greater than about 140 m2/g), since these blacks appear to feature a surface micro-structure with a significant portion of micropores in the specified size range. Effective results have been accomplished in treating preferred furnace blacks having a N2SA ranging from about 200 m2/g to about 260 m2/g.

The adsorbate of the present invention may be any organic molecule including a linear chain having at leastfourcarbon atoms, such as alkanes and substituted alkanes (amines, halides, alcohols, and the like) and mixtures thereof. Typical materials include n-octane, n-amino octane, n-hexanol, n-bromo octane, n-chloro octane, 4 methyl heptane, 2-5 dimethyl heptane, 2-3-4trimethyl pentane, 2-2-4trimethyl pentane, hexamethyl ethane, n-nonane, n-decane, n-dodecane, n-hexadecane, 1-3 dichloro propane, and the like. For reasons of efficiency and permanance of treatment during heat processing, adsorbatesfeaturing a linear chain of at least ten carbon atoms are preferred.Particularly preferred are n-alkanes, C10-C15, which feature thermal stability to above about 2500C (523"K).

The manner in which the adsorbate is applied to the carbon black is not critical. Typically, the treatment simply consists of mixing the chosen amount of adsorbate with the black in a suitable vessel and then agitating the black to ensure adsorbate impregnation of the blacksurface. Excess or unadsorbed adsorbate can be removed by drying the treated black at moderate temperatures, typically ranging from about 100to 200"C (373 to 473on). Treatment of the black also may conveniently be incorporated into various process steps during manufacture or utility of the carbon black.For example, a suitable adsorbate may be injected into the process stream of a carbon black reactor prior to collection of the black, or, the adsorbate may be introduced during a compounding operation as a black is beirig blended with a polymer.

The optimum amount of adsorbate to be used depends on the darbon blackto be treated, its surface area, and the percentage ofthat surface area consisting of micropores in the size range effectively blocked bythe adsorbates of the present invention. When treating conductive carbon black, effective results have been accomplished using about 0.5 to about 5 percent by weight adsorbate; about 1.0 to about 2.0 percent adsorbate is particularly preferred.

The following examples are provided to further illustrate the invention. The examples are intended to be illustrative in nature and are not to be construed as limiting the scope of the invention.

Test pro cedures An amount of carbon black was weighed into a glass jar and a measured percent by weightofadsorbate then was added to the black. The jar was sealed with a lid and the contents were thoroughly mixed by rolling the jar for about one to five minutes (60to 300 seconds). After loosening the lid, the jar and its contents were then placed in an overto dry.

In orderto evaluate the performance characteristics of the blacks in imparting compound moisture adsorp- tion and volume resistivity properties, the blacks were compounded with a suitable resin. For purposes of illustration, an ethylene/ethyl acrylate (EEA) was used as the resin in the following examples. The compound to be tested was prepared by incorporating the desired amount of black into the resin, on a weight percent basis. The blacks were compounded, at specified loadings, into the EEA using a Brabender mixer running at 60 RPM with circulating oil at 11 0 C (383"K) for nine minutes (540 seconds). The resulting compound was sheeted on a cold two-roll mill and formed into sheets forthe subsequenttesting.

In orderto determine CMA, sheets of the various ethylene/ethyl acrylate (EEA) compounds were diced or cut into pellets two yield suitable granulated test samples. Athree gram sample ofthe granulated compound was weighed into a glass crucible of known weight and dried at 600C t 3" (333"K) and 1/3 atomsphere (3.4 x 1 04 Pa) for two hours (7200 seconds) to remove moisture from the compound. After cooling in a desiccator, the weight was obtained to the nearesttenth of a milligram.The compound then was placed into a desiccator maintained at conditions of room temperature (70 + 2"F [294 K]) and 79 percent relative humidity (R.H.)for48 hours (1.728 x 1 seconds). The compound then was weighed after 30 minutes (1800 seconds) and periodically at 24 hour (8.64 x 1 seconds) intervals until constant weight (0.03 percent increase in CMA) was achieved.The equilibrium moisture absorptoin was calculated as a weight percentage of the compound using the following formula: CMA(wt%)=(C+S)-(C+DS)-B x 100 (C + DS) - (TC) wherein C+S = final weight container + sample C + DS = weightcontainer + drysample TC = tare weight glass crucible B = change in weight blankcontainer Volume resistivity of a material is the ratio of the potential gradient parallel to the current in the material to the current density. Volume resistivity is measured in ohms centimeter; it is the reciprocal ofvolume conductivity.In order to determine the volume resistivity of plastic compounds containing carbon black, sam- pleswere prepared by molding standard 80 mil tensile plaques from the millsheets and 2" x 6"(5.1 x 15.2cm) .electrical test specimens were cut out of the tensile plaques. Each specimen was coated with a silver paint (silver conductive coating in ethyl alcohol) to produce a one-half inch (1.27 cm)wide silver electrode at each end.The specimens were placed in a sample holder (between 8 x 6 [20.3 x 15.2 cm] glass plates arranged crosswiseto each othersuch that the edge ofthetop plate is evenly lined with the edge of the specimen) and the electrodes attached to a Leeds and Northrup test set (#5035) consisting of a Wheatstone Bridge and Galvanometer. The voltage impressed on the test specimens was approximately 4.5 volts. The DC resistances across the length of the sample were measured and converted to Volume Resistivity in ohm-cm using thefollowing formula: Volume Resist (ohm-cm) = 2 x T x (2.54) x R 5x(2.54) wherein T = thickness of sample (inches) R = resistance (ohms) 2.54 = conversion constant (in. cm) 5 = distance constant (inches) - measure of the distance between the two one-half inch silver electrodes painted on each end of the test specimen The resistance of the specimen was measured in an oven maintained at 900C (363"K). In doing so,the resistance initially was measured after three minutes (180 seconds) at 90"C (3630K) with subsequent readings being taken at two minute (120 second) intervals for the next 30 minutes (1800 seconds).After 30 minutes (1800 seconds) readings were taken every five minutes (300 seconds) until the specimen had been in the 90 C (363 K) oven for a total of 60 minutes (3600 seconds). The value forthe resistance of the specimen at90 C (363"K) was fixed on a plot as the point at which the readings became constant.

Nitrogen surface area (N2SA) of the carbon black samples was determined in accordance with ASTM Test Method D3037-76, Method C, and is expressed in terms of square meters per gram (m2/g).

The table below iist representative results obtained using a selection of blacks and absorbates.

Tables land II list results with two different carbon black samples and using varying amounts oftwo adsorbates.

The data shown in Table Ill indicates that treatment with homologs of n-alkanes demonstrate comparable beneficial results. Increasing the chain length of the adsorbate has the benefit of offering higher atmospheric boiling points and highertemperature stability against desorption.

Table IV reports the effects of treatment using various isomers of octane. The adsorbate molecules all feature the same molecular formula but exhibit increasing degrees of branching moving from the top to the bottom of the table. While treatmentwith all of the adsorbates demonstrated beneficial results, linear molec- ules were most effective.

Table V lists treatment results from using various adsorbates, including both substituted and unsubstituted alkanes. The amines and alcohols appear two have a slight affinityforwaterwhich is not presentforthe halogenated and unsubstituted alkanes.

Table VI reports the results wherein carbon blacks of varying surface areas were treated. The example employing a low surface area black (about 50 m2/gm) evidenced no effect when treated pursuantto the present invention.

Table Vll reports results wherein carbon blacks of varying surface areas were treated with varying amounts of n-decane adsorbate.

Tablet Compound: Carbon Black* (36% Loading) in EEA Volume Adsorbate Treatment CMA Resistivity (wt%) Conditions (wt%) 25"C 90"C CONTROL 0.66 3.12 13.5 3% n-octane 150 C/60min 0.29 2.76 10.8 CONTROL 0.63 2.75 11.5 1.5% n-octane 150 C/60min 0.29 2.59 10.3 *The carbon black was VULCAN XC-72 (ASTM-N-472), a conductive carbon black available from Cabot Corporation, having a nitrogen surface area of about 215-260 m2/g.

Table COMPOUND: Carbon Black* (36% Loading) in EEA Volume Adsorbate Treatment CMA Resistivity (wt%) Conditions (wt%) 25"C 90"C CONTROL 150"C/60 min 2.45 1.9 5.1 3% n-octane " 0.92 2.2 5.1 4.5% n-octane " 0.43 2.1 6.0 CONTROL 2.49 1.9 5.6 3% n-octane " 1.08 1.6 3.9 4.5% n-octane " 0.55 2.1 6.0 *The carbon black used had a nitrogen surface area of about 610 m2/g.

Table 111 COMPOUND: Carbon Black* (36% Loading) in EEA Volume Adsorbate Treatment CMA Resistivity (wt%) Conditions (wt%) 25 C 90 C CONTROL 0.56 3.3 10.4 1 n-octane 200 C/12hrs 0.22 2.8 9.1 1 1/2%n-decane " 0.23 2.9 8.7 1 1/2%n-dodecane " 0.24 3.2 10.1 1 1/2%n-hexadecane " 0.24 3.2 10.3 Table IV COMPOUND:Carbon Black* (36% Loading) in EEA Volume Adsorbate Treatment CMA Resistivity (wt%) Conditions (wt%) 25 C 90 C CONTROL 200 C/12hrs 0.66 2.9 9.6 1.5% n-octane " 0.20 2.8 9.1 1.5% 4 methyl heptane " 0.28 3.1 11.5 1.5%2-5dimethyl heptane , 0.28 3.3 10.1 1.5% 2-3-4trimethyl pentane " 0.35 3.2 10.5 1.5% 2-2-4trimethyl pentane D 0.38 2.7 8.5 1.5% hexamethyl ethane " 0.44 5.4 25.5 *The carbon black used was VULCAN XC-72 (ASTM-N-472), a conductive carbon black available from Cabot Corporation, having a nitrogen surface area of about 215-260 m2/g.

Table V COMPOUND: Carbon Black* (36% Loading) in EEA Volume Adsorbate Treatment CMA Resistivity (wt%) Conditions (wt%) 25 C 90"C CONTROL 0.65 2.8 12.2 1% n-octane 150 C/50min 0.29 2.6 10.3 3% n-amino octane " 0.34 3.6 17.4 2%n-hexanol 150 C/12hrs 0.44 3.0 14.0 CONTROL 0.77 4.5 31.9 2% n-octane 110 C/60min 0.36 3.7 17.1 2%n-decane " 0.32 3.7 18.1 2% n-bromo octane " 0.38 3.9 19.5 2% n-chloro octane " 0.33 3.3 14.6 1.5%1-3dichloropropane " 0.30 2.8 9.3 *The carbon black used was VULCAN XC-72 (ASTM-N-472), a conductive carbon black available from Cabot Corporation, having a nitrogen surface area ofabout 215-260 m2/g.

Table VI COMPOUND: Carbon Black (36% Loading) in EEA Volume Adsorbate N2SA Loading Treatment CMA Resistivity (wt%) (m2/gm) (wt%) Conditions (wt%) 25"C 90"C CONTROLA 50 36 150"C/60min 0.38 4.1 74 1.5% Norpar 121 36 " 0.38 4.2 76 CONTROL 8 138 36 8 0.44 3.5 21 1.5% Norpar 12 36 " 0.27 3.9 21 CONTROL C 224 36 " 0.78 2.6 11 1.5% Norpar 12 36 " 0.46 2.6 9 1.5% Norpar 122 36 N/A 0.52 2.2 8 CONTROLD 635 14 150 C/60min 1.03 45 146 4.5% n-nonane 14 " 0.22 56 192 CONTROLS 810 14 " 0.45 9 26 3%n-decane 14 " 0.26 11 27 CONTROL 1727 12 " 0.33 16 19 3%Norpar12 12 " 0.11 15 18 (1) Norpar 12 is a mixture of n-alkanes having an average of 12 carbons (commercially availablefrom Exxon Company USA).

(2) In this example, the adsorbate was introduced during the carbon black/EEA compounding operation.

Table VII COMPOUND: Carbon Black (36% Loading) in EEA Volume Adsorbate N2SA Loading Treatment CMA Resistivity (wt%) (m2/gm) (wt%) Conditions (wt%) 90"C CONTROL1 142 36 150"C/60min 0.49 34.2 .75% n-decane 36 " 0.37 34.7 CONTROL2 235 36 " 0.78 16.7 1.5%n-decane 36 " 0.39 13.0 CONTROL3 1052 14 " 0.45 26.0 3%n-decane 14 " 0.26 26.5 CONTROL4 1322 14 " 0.63 19.2 3% n-decane 14 " 0.38 20.6

Claims (11)

1. A method for modifying the surface characteristics of a furnace carbon black having a nitrogen surface area greaterthan 140m2/g comprising treating the surface of said carbon black with an organic adsorbate having a molecular structure including a linear chain having at least four carbon atoms.

2. The method of claim 1 wherein said carbon black has a nitrogen surface area ranging from 200 to 260 m2/g.

3. The method of claim 1 wherein said carbon black is treated with an organic adsorbate comprising an n-alkane, n-alkane amine, n-alkane halide or n-alkane alcohol.

4. The method of claim 3 wherein the organic adsorbate is an n-alkane having 10 to 16 carbon atoms, or mixture thereof.

5. The method of claim 4wherein said carbon black having nitrogen surface area ranging from 200 to 260 m2/g is treated with adsorbate at a loading ranging from 1 to 2 percent byweight.

6. A modified carbon black obtained by treating a furnace carbon black having a nitrogen surface area greater than 140 m2/g with an organic adsorbate including a linearchain having at least four carbon atoms.

7. The carbon black of claim 6 wherein the organic adsorbate is an n-alkane amine, n-alkane, n-alkane halide or n-alkane alcohol.

8. The carbon black of claim 6wherein the organic adsorbate is an n-alkane having lotto 16 carbon atoms or mixtures thereof.

9. The carbon black of claim 6 obtained by treating a carbon black having a nitrogen surface area ranging from 200 to 260 m2/g.

10. The carbon black of claim 8 obtained by treating a furnace carbon black having a nitrogen surface area ranging from 200 to 260 m2/g with adsorbate at a loading ranging from 1 to 2 percent byweight.

11. A method for modifying the surface characteristics of a furnace carbon black as claimed in claim 1 and substantially as herein described with reference to the Examples.