ASTM D3849-22

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Designation: D3849 − 14a D3849 − 22
Standard Test Method for
Carbon Black—Morphological Characterization of Carbon
Black Using Electron Microscopy
This standard is issued under the fixed designation D3849; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers (1) the morphological (for example, size and shape) characterization of carbon black from
transmission electron microscope images which are used to derive the mean particle and aggregate size of carbon black in the dry
(as manufactured) state, from CAB chip dispersion or removed from a rubber compound and (2) the certification of mean particle
size using a correlation based on statistical thickness surface area measurements.
1.2 The values stated in SI units are to be regarded as the standard. The values in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D3349 Test Method for Absorption Coefficient of Ethylene Polymer Material Pigmented with Carbon Black
D4483 Practice for Evaluating Precision for Test Method Standards in the Rubber and Carbon Black Manufacturing Industries
D6556 Test Method for Carbon Black—Total and External Surface Area by Nitrogen Adsorption
3. Terminology
3.1 Definitions:
3.1.1 General:
3.1.1.1 carbon black particle—a small spheroidally shaped, paracrystalline, non-discrete component of an aggregate; it can only
be separated from the aggregate by fracturing; carbon black particle size is a distributional property; therefore, the term particle
size implies the mean value from multiple measurements.
3.1.1.2 carbon black aggregate—a discrete, rigid colloidal entity that is the smallest dispersible unit; it is composed of
extensively coalesced particles; carbon black aggregate size is a distributional property; therefore, the term aggregate size implies
the mean value from multiple measurements.
This test method is under the jurisdiction of ASTM Committee D24 on Carbon Black and is the direct responsibility of Subcommittee D24.81 on Carbon Black
Microscopy and Morphology.
Current edition approved Nov. 1, 2014June 1, 2022. Published November 2014July 2022. Originally approved in 1980. Last previous edition approved in 2014 as
D3849 – 14.D3849 – 14a. DOI: 10.1520/D3849-14A.10.1520/D3849-22.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3849 − 22
3.1.1.3 statistical thickness surface area (STSA)—the external specific surface area of carbon black that is calculated from
nitrogen adsorption data using the de Boer theory and a carbon black-specific model.
3.1.1.4 glow discharge—a plasma of ionized gas that is formed in a high-voltage field at pressures of about 3 to 20 Pa (25 to
-3
150 × 10 torr); an alternating current (a-c) glow discharge using air is effective in cleaning and oxidizing the surface of carbon
substrates to improve the wetting characteristics of polar vehicles containing pigment dispersions.
3.1.1.5 substrate—a thin film that is used to support electron microscope specimens; evaporated carbon films are commonly
used because of relatively good mechanical strength, stability, and conductivity.
3.1.2 Aggregate Dimensional Properties from Image Analysis:
3.1.2.1 area (A)—the two-dimensional projected area of the carbon black aggregate image.
3.1.2.2 perimeter (P)—the total boundary length of an aggregate.
3.1.2.3 volume (V)—an estimate of the volume of the carbon black aggregate using stereological principles.
3.1.3 Image Analysis:
3.1.3.1 dilation—the converse of erosion; this process is accomplished by changing any OFF pixel to ON if it has greater than
a preset minimum of ON neighbors, causing image features to grow in size, which fills in small breaks in features, internal voids,
or small indentations along the feature surface.
3.1.3.2 erosion—the process by which image features are reduced in size by selectively removing pixels from their periphery;
it consists of examining each binary pixel and changing it from ON to OFF if it has greater than a preset minimum of neighbors
that are OFF; it serves a number of useful functions, such as smoothing feature outlines and separating features touching each other.
3.1.3.3 feature—areas within a single continuous boundary that have gray-level ranges that allow them to be distinguished from
the background area outside the feature via thresholding.
3.1.3.4 thresholding—selecting a range of brightness such that discrimination is possible between the feature and the
background; the gray levels within carbon black images become lower with decreasing particle size.
4. Significance and Use
4.1 Carbon black morphology significantly affects the transient and end-use properties of carbon black loaded polymer systems.
A carbon black’s particle size distribution is its single most important property, and it relates to degree of blackness, rubber
reinforcement, and ability to impart UV protection. For a given loading of carbon black, blackness, reinforcement, and UV
protection increase with smaller particle size. Aggregate size and shape (structure) also affect a carbon black’s end-use
performance, as higher carbon black structure increases viscosity and improves dispersion. The stiffness (modulus) of elastomer
systems becomes significantly higher with increasing structure. The preferred method for measuring carbon black morphology (for
example, size and shape) is transmission electron microscopy (TEM), but due to the semi-quantitative nature of TEM, it is not
suited for mean particle size (MPS) certification. While useful morphological information can be obtained from TEM
measurements within a laboratory, due to their inherent between-laboratory variability, TEM generated values should not be used
for specification purposes.
4.2 Certification of carbon blacks for UV protection (weatherability) in certain plastics applications has historically been
performed using TEM generated mean particle size values. ASTM Committee D24 has demonstrated that due to challenges with
obtaining quantitative primary particle size data, particularly between laboratories, a qualification test based on surface area has
been implemented, as detailed in Test Method B.
4.3 Carbon black aggregate dimensional and shape properties are dependent upon the nature of the system in which the sample
is dispersed, as well as the mixing procedure.
Test Method A – Morphological Evaluation (Semi-Quantitative) Via Transmission Electron Microscopy
5. Summary of Test Method
5.1 Transmission electron microscopy (TEM) is utilized to measure the morphological properties of carbon black. A variety of
dispersion methods are offered depending upon the sample type. Both dry black and CAB chip dispersions are used for measuring
the morphology of bulk carbon black. A pyrolysis technique is included that facilitates the removal of carbon black from vulcanized
rubber. This aforementioned technique can be employed to identify the carbon black type from an end use product. It should be
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noted that the accuracy and precision of Method A is insufficient for generating quantitative data as required in the case of MPS
certification. Please refer to Method B for MPS certification.
6. Apparatus
6.1 Electron Microscope, transmission-type, with a point-to-point resolution of 1.0 nm or better. Operating voltages should be high
enough to provide the desired resolution and low enough to produce images of sufficient contrast. Recommended voltages can be
in the 60 to 120 kV range. The microscope column should contain a liquid nitrogen-cooled anti-contamination device or a “cold
finger” to reduce sample contamination and to maintain column cleanliness. For image acquisition, the microscope should include
a charge-coupled device (CCD) camera mounted either above or below the instrument’s viewing chamber.
6.2 Image Analysis System, consisting at minimum of a TEM-interfaced camera capable of 640 × 480 pixel or better resolution,
a computer equipped with frame grabbing hardware to capture TEM images digitally, and software to perform morphological
operations and measurements on image features and store resulting data. Operations must include background/noise elimination,
thresholding, and edge smoothing. Area and perimeter are then measured on features in the processed images.
6.3 Two-Roll Mill.
6.4 Vacuum Evaporator, standard-type, for preparing carbon films to be used as substrates for electron microscopy. The evaporator
-5
should be capable of reducing the absolute pressure to 1.3 mPa (1 × 10 torr) and should also contain the necessary apparatus for
a-c glow discharge.
6.5 Ultrasonic Generator, variable power tank-type or probe that provides sufficient energy to give acceptable dispersion.
6.6 Dry Box, capable of maintaining a relative humidity level of no greater than 30 %.
6.7 Analytical Balance, with an accuracy of about 0.5 mg.
6.8 Electrically Heated Tube Furnace, capable of being heated to 800 to 900°C under an inert environment, with the ability to
introduce and remove the sample boat to the heated zone without allowing oxygen intrusion.
6.9 Pyroprobe, capable of being heated from 150 to 1000°C in an inert environment.
6.10 Carbon Rods, approximately 3.1 mm in diameter.
6.11 Carbon Rod Sharpener.
6.12 Glass Microscope Slides, 25 by 75-mm.
6.13 Test Tubes, 75 by 10-mm, 4-cm capacity, 0.5-mm wall thickness, with corks.
6.14 Transfer Pipets, disposable Pasteur-type, 225 mm long, 1-mm inside diameter at tip.
6.15 Rubber Bulbs, for pipets.
6.16 Glass Vials, 40-cm capacity, with solvent-resistant tops.
6.17 Büchner Funnel, No. 3, 111-mm inside diameter.
6.18 Filter Paper, general purpose.
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6.19 Carbon Coated Electron Microscope Specimen Grids, 3-mm diameter, 200 to 300 mesh. Commercially available or can be
prepared as described in Annex A1.
6.20 Wire Screening, with openings approximately 1 mm .
6.21 Tweezers, fine-tipped.
6.22 Spatulas, micro-type with V-shaped spoon that is approximately 2 mm wide at top and 12.5 mm long.
6.23 Fluorocarbon Duster.
6.24 Lens Tissue, lint-free.
6.25 Porcelain Boats, for pyrolysis, 98 mm long, 15 mm wide at top.
6.26 Centrifuge, high speed (15 000 to 20 000 r/min) with head for 75 by 10 mm test tubes.
6.27 Beakers, 2000-cm capacity.
7. Reagents and Materials
7.1 Chloroform, reagent grade.
7.2 Tetrahydrofuran (THF), reagent grade.
7.3 1,2-Dichloroethane, reagent grade.
7.4 Ethyl Acetate, reagent grade.
7.5 Poly (Vinyl Formal) Resin, Grade 15/95.
7.6 Cellulose Acetate Butyrate Resin (CAB).
7.7 Phthalate-Type Plasticizer (such as santicizer).
8. Sample Preparation—Dispersion Procedures
8.1 Dry Carbon Black (Sonic Bath):
8.1.1 Weigh 8 to 10 mg of carbon black into a test tube containing 1 cm of solvent (typically chloroform or THF).
NOTE 1—With experience, it may not be necessary to weigh each carbon black sample, as an estimated amount from the microspatula may be sufficient.
There is considerable latitude in the amount of carbon black used. The finer N100 and N200 blacks may require somewhat less carbon black than the
coarser semi-reinforcing types.
8.1.2 Adjust the power of the ultrasonic bath for maximum agitation; this may require that the water level be adjusted. As the
ultrasonic energy heats the water in the bath, ice should be added to control the temperature in order to maintain maximum
dispersive capability.
8.1.3 Place the stoppered test tube containing the carbon black and solvent mixture into the most intense part of the ultrasonic field
and allow the mixture to agitate for 3 to 5 min. The test tube should be held with tongs or mounted in a simple wire holder.
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FIG. 1 Ultrasonic Dispersions of N-220 Carbon Black
8.1.4 Transfer a small portion of the concentrated carbon black-solvent mixture into another test tube containing 1 cm of fresh
solvent. The amount of concentrate required increases with particle size. Blend the mixture by repeatedly transferring the sample
between the transfer pipet and the test tube, then cork the test tube and repeat the ultrasonic dispersion procedure.
8.1.5 Check the concentration of the diluted dispersion by extracting a small amount into the tip of the pipet and viewing against
a white background. For tread grade carbon blacks, the dispersions should be relatively transparent, becoming somewhat darker
with increasing particle size. The diluted dispersions for very coarse carbon blacks such as N700 to N900 series will be on the
threshold of complete opacity. If necessary, adjust the concentration by adding more concentrate or solvent as required, then repeat
the ultrasonic agitation. The volume of the carbon black-solvent mixture should be maintained at approximately 1 cm . If
considerable dilution is required, the excess volume above 1 cm should be discarded.
NOTE 2—A reasonable degree of latitude exists for achieving the proper concentration levels in the final dispersions for different grades of carbon black.
Concentration and overall dispersion quality are best determined by screening the actual specimens in the electron microscope and then making the
necessary adjustments.
8.1.6 Place a specimen grid with carbon substrate (film side up) on a piece of filter paper. Remove a small amount of the final
diluted dispersion using a fresh pipet and place one drop on the grid as close to the center as possible, from a height of about 12
mm. Allow the specimen to dry for about 1 min on a piece of filter paper. This specimen preparation procedure should be performed
in a dry box if the relative humidity in the room exceeds 30 %.
8.1.7 For TEM grids that contain formvar or residual CAB (CAB chip dispersions), place the TEM grid in an appropriate sample
holder, place in the pyrolysis chamber and allow adequate time for the chamber to be purged by an inert gas to prevent oxidation
of the sample. Pyrolize the specimen grid at a sufficient temperature (typically greater than 550°C) to remove the poly (vinyl
formal) film or CAB, or both.
8.1.8 Acceptable dispersions of a carbon black in the dry state and removed from a rubber compound (SBR) are illustrated for
N-220 and N-774 carbon blacks in Figs. 1 and 2.
8.2 Dry Carbon Black (Ultrasonic Probe):
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FIG. 2 Ultrasonic Dispersions of N-774 Carbon Black
3 3
8.2.1 Weigh 5 to 10 mg of carbon black into a 30-cm glass vial and add approximately 20 cm of solvent (typically chloroform).
NOTE 3—With experience, it may not be necessary to weigh each carbon black sample, as an estimated amount from the microspatula may be sufficient.
There is considerable latitude in the amount of carbon black used. The finer N100 and N200 blacks may require somewhat less carbon black than the
coarser semi-reinforcing types.
8.2.2 Place the vial containing the carbon black and solvent into an ice-water bath.
8.2.3 Insert the probe to a depth of approximately 2.5 cm into the vial and ultrasonicate at 40 to 50 watts for 10 min.
NOTE 4—The ultrasonic probe and ice-water bath containing the sample vial should be housed in an acoustic enclosure to reduce cavitation noise.
8.2.4 Transfer a small portion (approximately 2 to 4 cm ) of the concentrated carbon black/solvent mixture into another vial and
add 20 cm of fresh solvent. An additional three-minute sonication is recommended.
8.2.5 Check the concentration of the diluted dispersion by extracting a small amount into a pipet and then placing 1 drop on a
white filter paper. For tread grade carbon blacks, the dispersions should be relatively transparent, becoming somewhat darker with
increasing particle size. The dispersion for very coarse carbon blacks, such as N700 to N900 series, will be on the threshold of
complete opacity.
8.2.6 If necessary, adjust the concentration by adding more concentrate or solvent as required, then repeat the ultrasonic agitation.
The volume of the carbon black-solvent mixture should be maintained at approximately 20 cm . Repeat 8.2.5.
NOTE 5—A reasonable degree of latitude exists for achieving the proper concentration levels in the final dispersions for different grades of carbon black.
Concentration and overall dispersion quality are best determined by screening the actual specimens in the electron microscope and then making the
necessary adjustments.
8.2.7 When the final dispersion drop on the filter paper is in an acceptable color range, place a specimen grid with a thin carbon
substrate (film side up) on a piece of filter paper. Using a fresh pipet, remove a small amount of the final dispersion and place one
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drop on the grid as close to the center as possible, from a height of about 12 mm. Allow the specimen grid to dry for approximately
1 min on the filter paper. (If the relative humidity is greater than 35 %, purge ultra-high purity nitrogen over the specimen grid
retaining the sample.)
8.2.8 Place the grid into the microscope and examine the dispersion for visual separation of discrete aggregates. If the dispersion
shows a high concentration (overlap of aggregates, agglomeration, and so forth), adjust the concentration of the dispersion by
following 8.2.6 and prepare a new specimen grid by following 8.2.7.
8.3 Carbon Blacks Removed from Vulcanized Rubber Compounds:
8.3.1 Cut thin sections (about approximately 1 mm ) of the rubber compound using a razor blade.
8.3.2 Place 5 to 10 sections in an appropriate sample holder, place in the pyrolysis chamber and allow adequate time for the
chamber to be purged by an inert gas to prevent oxidation of the sample. Heat the sample at a temperature in excess of the
decomposition temperature of the polymer.
8.3.3 It should be noted that heteroatom polymers, such as neoprene and nitrile rubber, will not pyrolyze cleanly and often
carbonize and sinter the aggregates together, resulting in poor dispersion. These samples cannot be analyzed with any degree of
confidence; therefore, caution must be used when these polymers are encountered. Additionally, rubber compounds containing 5 %
or more silica typically result in higher sintered aggregate dispersions with agglomerated silica. Advanced techniques for removal
of the silica must be employed in order to get reliable results.
8.3.4 Allow the sample to cool to room temperature before removing it from the pyrolysis chamber. Place the sample in a test tube
containing 1 cm of solvent (typically chloroform or THF).
8.3.5 Disperse the residual carbon black sample as described in 8.1.2 – 8.1.8.
8.4 Carbon Black in CAB (Cellulose Acetate Butyrate):
8.4.1 The preparation of the CAB chip utilizes high shear mixing on a two-roll mill, which greatly reduces the level of
aggregation; therefore, the aggregate size measurements then become more directly related to particle size, thus reducing
shape-related corrections.
8.4.2 Preparation of CAB Chip:
8.4.2.1 The following conditions were defined for a Farrel Tecno Lab Polymill 110P. Slight adjustments may be necessary
depending on the size of the 2-roll mill used.
8.4.2.2 Preheat the 2-roll mill to give a temperature on the front roll of the mill at 76°C and the rear roll at 66°C, noting that rolls
should be in motion while heating.
8.4.2.3 Set the roll gap to 0.2 mm and roll rate of 20 r/min.
8.4.2.4 Weigh out the following ingredients depending on the carbon black type being analyzed:
Weight (g)
Carbon Black CAB Phthalate-Type
Plasticizer
N100-N300 (25 % CB 37.5 82.5 30.0
Loading)
N500-N900 (40 % CB 60.0 66.0 24.0
Loading)
8.4.2.5 Mix the carbon black and CAB resin in a container, then add the Phthalate-type plasticizer, taking care not to let the
Phthalate-type plasticizer touch the sides of the container.
8.4.2.6 Stop the roll
...