1. Introduction

1.1. Scope

1.1.1. This method is used for the measurement of airborne carbon black particulates.

1.2. History of the Method

1.2.1. A previous method for the analysis of carbon black was gravimetric. All dust present in an atmosphere known to contain carbon black was reported as carbon black (8.1).

1.2.2. In 1975, ignition was used at the OSHA Laboratory to eliminate interference from particulates that do not lose significant weight upon ignition, i.e. minerals.

1.2.3. In 1979, scanning electron microscopy was used to distinguish between carbon black and other organic substances. With this procedure, scanning electron microscopic analysis is no longer required.

1.3. Principle

1.3.1. Organic chemicals and the filter are dissolved with tetrahydrofuran (THF) and carbon black is re-deposited on a glass fiber filter. Carbon black is determined as weight difference after heating at 150°C and ashing at 600°C.

1.4. Classification and Properties

1.4.1. Carbon black may also be classified as lampblack, channel black, furnace combustion black, and thermal black. Each form is produced by the partial combustion or thermal decomposition of liquid or gaseous hydrocarbons. It is composed of more than 85% elemental carbon in the form of near-spherical colloidal aggregates. The properties of carbon black commonly evaluated for control and classification purposes are usually based on particle size, surface area, structure, and surface chemistry.

1.5. Uses

1.5.1. Carbon black is used as a reinforcing agent in rubber and as a black pigment in printers ink, coatings, paper, and plastics. Ninety-five percent of carbon black is used in the rubber industry, predominantly as a pigment and reinforcing agent in tires.

2. Range and Detection Limit

2.1. The gravimetric weight loss studies were made over the range 0.4 to 5.7 mg carbon black.

2.2. The detection limit is 0.1 mg. This value corresponds to 0.21 mg/m³ carbon black exposure for the recommended air volume of 480 liters.

3. Recovery, Precision, and Accuracy

3.1. The average recovery of carbon black based on weight loss compared to the original or theoretical weight was 97.3%

3.2. The average recovery of samples spiked with N-phenyl-p-phenylene diamine and N-tert-butyl-2 benzothiazolesulfenamide, two anti-oxidants used in the rubber processing industry, was 94.2%.

3.3. The coefficient of variation for the total analytical and sample preparation procedure in the range of sample sizes validated in this study was 0.14. This value corresponds to a 0.49 mg/m³ standard deviation at the OSHA standard level of 3.5 mg/m³.

3.4. The standard deviation for weight measurements is 76 ug.

4. Interferences

4.1. Source

4.1.1. The most probable sources of interferenes are chemicals used in the rubber-processing industries that are released as respirable dusts with carbon black during the weighing, conveying, and mixing operation. These airborne particulates will contribute to the total weight of carbon black samples. Only those particulates that are insoluble in tetrahydrofuran and either vaporize or lose weight between 150°C and 600°C are interferences in this method.

4.1.2. Rubber-processing chemicals are usually grouped into accelerators, antioxidants and stabilizers, antiozonants, and miscellaneous materials (8.2). These chemicals are defined by the U.S. Tariff Commission as "organic compounds that are added to natural and synthetic rubbers to give them qualities necessary for their conversion into finished rubber goods" (8.3). Most of the commercial organic chemicals used in the rubber industry are listed in Tables of the Encyclopedia of Chemical Technology (8.4).

4.2. Controlling Interferences

4.2.1. Chemicals or substances are eliminated as potential interferences in this method according to the following physical constants:

1. Those that vaporize at 150°C.
   
2. Those that are soluble in tetrahydrofuran.
   
3. Those that are weight stable at 600°C.

THF is the preferred sovent extractant because of its unique solvent properties and wide miscibility. It is a saturated cyclic ether (CH₂)₄O and is a good solvent for high-molecular weight polyvinyl chloride, polyvinylidene, resins, and other organic materials. THF is miscible with most organic solvents and with water in all proportions (8.5.)

Other solvents may be used for the removal of organic chemicals insoluble in THF. The efficacy of each should be pre-determined in tests with bulk samples.

Note: Some Carbon Blacks may contain significant amounts of Polynuclear Aromatic Hydrocarbons (PAHs) such as:

Anthracene/Phenanthrene
Pyrene
Chrysene
Benzo - a - Pyrene (BaP), a = alpha

If PAHs are suspected to be present, the following additional steps may be performed:

A. Provide a bulk sample
B. Take separate personal air samples on glass fiber filters
C. Take area samples on glass fiber filters

The filter used for PAH sampling is a Gelman Glass Fiber Filter, Type A/E, Product Number 61652, 37 mm diameter, which has a Typical Aerosol Retention Rating of 99.98% for a 0.3 um aerosol particle size.

The Glass Fiber Filters are then analyzed for PAH content using parameters established in OSHA Method 58.

The PVC samples collected are analyzed for Carbon Black.

5.1 Apparatus

5.1.1. The filter unit, consisting of the filter media, back-up pad, and 37-mm three-piece cassette filter holder.

5.1.2. Personal Sampling Pump: A calibrated personal sampling pump whose flow can be determined within ±10% at the 95% confidence level at the recommended flow rate. The pump must be calibrated with a filter holder and filter in the line.

5.1.3. Polyvinyl chloride (PVC) membrane filter; 37-mm diameter, 5.0-micrometer pore size. (FWS-B or equivalent).

5.1.4. Filter back-up pads.

5.1.5. PetriSlide (Millipore) or plastic Petri dish - as filter holder for storage and weighing.

5.1.6. Desiccator containing anhydrous calcium sulfate or other suitable desiccant.

5.1.7. Microbalance with 0.01 mg scale divisions.

5.2. Procedure

5.2.1. The microbalance is properly zeroed, and the filters are desiccated and weighed prior to use.

5.2.2. The filter is placed on a back-up pad in a cassette filter holder. The three piece cassette filter holder is tightened and tape is placed around the cassette to prevent loss of sample. A piece of flexible tubing from the pump is connected to the cassette. The air is sampled at a flow rate of 2 liters per minute. After sampling, the openings of the cassettes are sealed with plugs.

5.2.3. The cassette filter holder is opened and the filter is carefully transferred to a PetriSlide.

5.2.4. The filter is dried in a desiccator and then weighed.

5.2.5. The sample weight is determined as the difference between the filter weight before and after sampling.

5.2.6. The filter is again placed in the cassette holder and sealed with tape and an official OSHA Form 21 seal.

5.2.7. Bulk samples of the materials contributing to the airborne dust in the working atmosphere should be placed in bottles for supporting test data and analysis.

6. Analytical Procedure

6.1. Apparatus

6.1.1. Muffle furnace capable of maintaining temperature at 600°C.

6.1.2. Drying oven capable of maintaining temperature at 150°C.

6.1.3. Balance with 0.01 mg scale divisions.

6.1.4. Filtering assembly and vacuum system.

6.1.5. Glass fiber filters without organic binder.

6.1.6. Centrifuge tubes (50 mL)

6.1.7. Porcelain plate.

6.1.8. Ultrasonic bath.

6.2. Reagent

6.2.1. The only reagent required is tetrahydrofuran.

6.3. Safety Precautions

6.3.1. Care should be exercised in using tetrahydrofuran (THF). It is extremely flammable and the vapor is harmful. It should be used only with adequate ventilation and contact with the skin should be avoided. Other dangers are listed on the reagent label.

When using THF, work in a well ventilated hood and wear protective gloves to avoid skin contact.

6.3.2. Ususal precautions in working near high temperature furnaces or ovens and fumes should be observed. Protective gloves and tongs should be used in the transfer of samples to and from the high temperature apparatus.

6.4. Laboratory Quality Control Samples

6.4.1. Quality control samples are prepared by placing weighed portions of carbon black, FWS-B-PVC filters and 15 ml THF into a 50 ml centrifuge tube. The carbon black is weighed to the nearest 0.01 mg.

6.5. Sample Analysis

6.5.1. The samples and 15 ml of THF are placed in a 50 ml centrifuge tube and sonicated for 10 minutes to dissolve the FWS-B filter and organic interferences.

6.5.2. The samples are transferred (filtered) onto glass fiber filters (the THF is vaccumed througha the GFF and the filtered THF captured is a liquid nitrogen trap prior to the vaccum air and THF getting into the vaccum air system pump) and the carbon black is washed 3 times with 2 or 3 ml THF.

Occassionally a carbon black sample has a very large amount of sample which will virtually not allow the THF to filter through the GFF. To facilitate filtration and allow efficient analytical work, the pre-filtrate sample material can be portioned onto one or more additional GFF (as many as needed) so as to allow for smaller amounts of sample material per each GFF and allow proper filtering and efficient sample processing. The result for each of the GFF filtrations for that sample can be added together to obtain the correct level of carbon black in the original heavily loaded sample.

6.5.3. After the final wash, immediately remove the glass fiber filter from the filter assembly in order to prevent adherence to the glass assembly.

The glass fiber filter can be placed in an aluminum foil weighing pan (2 1/2" high perimeter foil wall and a 2" diameter). The sample number can be imprinted into the foil with a pen or pencil. Also, the sample on the glass fiber filter can be kept in the aluminum foil pan during sample analysis steps 6.5.4. and 6.5.5. (this aids in maintaining sample identification).

6.5.4. The filters are dried in an oven at 150 for 30 minutes, cooled to room temperature and weighed (W₁).

6.5.5. The filters are placed on a porcelain plate and placed in a muffle furnace at 600° for 30 minutes.

6.5.6. The filters are cooled to room temperature and weighed (W₂).

6.6. Calculations

The amount of carbon black is determined by subtracting W₂ from W₁. This value in ug is divided by the sample air volume in liters and reported as mg/m³.

mg/m3 =

µg  1

=

mg  m3

where W₃ is the blank value in ug.

7. Reporting

7.1. Results are reported with two significant figures in mg/m³.

7.2. Analytical data and results are checked by another analyst before the results are submitted to the supervisor for approval.

8. REFERENCES:

8.1. NIOSH Criteria Publication No. 78 - 204, U.S. Dept. Of Health, Eduacation and Welfare, 55 - 56, 72 (1978).

8.2. Kirk, Othmer, Encyclopedia of Chemical Technology, Vol. 17, 2nd Edition, 509 (1978).

8.3. U.S. Tariff Commission, U.S. Production and Sales of Rubber Processing Chemicals for 1966, Aug. 11, (1967).

8.4. Kirk, Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Vol. 10, p. 512 - 539 (1978).

8.5. Kirk, Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Vol. 10, p. 249, (1978).