Omar Khalid
ASTM D792 is a standard test method used to determine the specific gravity (relative density) and density of solid plastics in various forms, including sheets, rods, tubes, and moulded items. The specific gravity of a substance is the ratio of its density to that of a reference substance, typically water at 60-70°F or 23°C. This test is particularly relevant for plastics as they are often sold on a cost-per-pound basis, and understanding their density can help identify variations in crystallinity, thermal history, porosity, and composition. There are two primary test procedures: Method A and Method B. The more common Method A involves weighing a specimen in air and then again when immersed in distilled water at 23°C, using a sinker and wire to ensure complete submersion. This method is used to calculate the density and specific gravity of the plastic specimen.
| Characteristics | Values |
|---|---|
| Scope | Density is the mass per unit volume of a material. Specific gravity is a measure of the ratio of mass of a given volume of material at 23°C to the same volume of deionized water. |
| Test Procedure | Method A and Method B. Method A is more common and can be used with sheets, rods, tubes, and molded articles. |
| Specimen size | Any convenient size. |
| Calculation | Specific gravity = a/[(a + w)-b] where a = mass of specimen in air. |
| Purpose | To identify a material, to follow physical changes in a sample, to indicate the degree of uniformity among different sampling units or specimens, or to indicate the average density of a large item. |
| Changes in density | Due to localized differences in crystallinity, loss of plasticizer, absorption of solvent, or other causes. |
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What You'll Learn
Density and specific gravity
ASTM D792 outlines the standard test methods for determining the density and specific gravity (or relative density) of solid plastics in various forms, including sheets, rods, tubes, and moulded items. Density is defined as the mass per unit volume of a material, and specific gravity is a measure of the ratio of the mass of a given volume of material at 23°C to the same volume of deionized water. These properties are essential in the plastics industry, as plastic is often sold on a cost-per-pound basis, and variations in density or specific gravity can impact the weight of products.
There are two primary test procedures for determining density and specific gravity: Method A and Method B. Method A is the more common approach and is applicable to sheets, rods, tubes, and moulded articles. It involves weighing the specimen in air and then again when immersed in distilled water at 23°C, using a sinker and wire to ensure the specimen is completely submerged. The density and specific gravity can then be calculated using the formula: Specific gravity = a/[(a + w)-b], where 'a' represents the mass of the specimen in air.
It is important to note that the test methods outlined in ASTM D792 are not equivalent to ISO 1183-1 Method A, which provides guidelines on sample weight and dimension. ISO 1183-1 also allows for testing at an additional temperature of 27 ± 2°C. The specific gravity of plastic films can vary depending on the type of plastic. For example, the specific gravity of nylon gravity plastic typically ranges between 1.13 and 1.15.
The density and specific gravity of a solid material like plastic can be measured for various purposes, including identifying the material, monitoring physical changes in a sample, and indicating the degree of uniformity among different specimens. Changes in the density of a single material can result from localized differences in crystallinity, loss of plasticizer, absorption of solvent, or other factors.
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Calculating specific gravity
The specific gravity of a plastic is calculated as the density of the plastic divided by the density of water. This is a dimensionless unit of measure that is important in material science to understand the behaviour and properties of materials. The specific gravity of plastic films varies depending on the type of plastic. For example, the specific gravity of Polypropylene (PP) films ranges between 0.90 and 0.91, while Polyvinyl Chloride (PVC) films are between 1.30 and 1.58.
There are two basic test procedures for calculating the specific gravity of plastics: Method A and Method B. Method A is the more common procedure and can be used with sheets, rods, tubes, and moulded articles. For this method, the specimen is first weighed in the air and then weighed when immersed in distilled water at 23°C. The specimen is held completely submerged using a sinker and wire.
The formula for calculating specific gravity is: Specific Gravity = a/[(a + w)-b], where 'a' is the mass of the specimen in the air and 'w' and 'b' are the mass and buoyancy of the specimen in water, respectively.
Specific gravity is a measure of the ratio of the mass of a given volume of material at 23°C to the same volume of deionized water. It is a property that is often measured to identify a material, track physical changes in a sample, or indicate the degree of uniformity among different specimens. Changes in the density of a single material can be due to localized differences in crystallinity, loss of plasticizer, absorption of solvent, or other factors.
The specific gravity of plastics is important in manufacturing, product design, and environmental impact. It also plays a crucial role in recycling, as it allows for the separation and identification of various types of plastics during the recycling process.
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Test procedures
ASTM D792 outlines the standard test methods for determining the density and specific gravity (relative density) of solid plastics in forms such as sheets, rods, tubes, or molded items. The specific gravity or density of a solid plastic is a property that can be measured to identify a material, to follow physical changes in a sample, or to indicate the degree of uniformity among different sampling units or specimens.
There are two basic test procedures: Method A and Method B. Method A is the more common procedure and can be used with sheets, rods, tubes, and molded articles. For this method, the specimen is first weighed in the air and then weighed when immersed in distilled water at 23°C. To ensure the specimen is completely submerged, a sinker and wire are used. The density and specific gravity can then be calculated. The specimen size can be any convenient size.
The formula for specific gravity is: Specific Gravity = a/ [(a + w)-b], where 'a' is the mass of the specimen in the air.
It is important to note that ISO 1183-1 outlines a similar general test method for specific gravity, but it allows for testing at an additional temperature of 27 ± 2°C.
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Specimen size
The ASTM D792 standard test method is used to determine the specific gravity (relative density) and density of solid plastics in forms such as sheets, rods, tubes, or molded items. The density of a solid plastic is a property that can be measured to identify a material, to follow physical changes in a sample, or to indicate the degree of uniformity between different specimens.
When performing ASTM D792, the specimen size can be any convenient size. This flexibility in specimen size is advantageous for testing plastic materials of varying dimensions, such as sheets, rods, tubes, or molded items. The test method focuses on determining the density and specific gravity of the plastic rather than imposing strict constraints on the size of the test specimen.
It is important to note that the specimen size should still be appropriate for the test setup and equipment used. The specimen must be completely submerged in the distilled water during Method A testing, so the size should be considered in relation to the container size and the ability to properly secure the specimen.
Additionally, while there is no specified standard size for the specimen, consistency in specimen size across a set of tests is crucial for accurate and comparable results. Maintaining a consistent specimen size helps ensure reproducibility and reduces potential sources of variation in the test results. This consistency is particularly important when performing comparative analyses or when specific material standards need to be met.
The ASTM D792 test method provides guidelines on specimen weight and dimension, which can help in determining the appropriate specimen size for testing. These guidelines ensure that the specimen size is suitable for the test procedure and that consistent results can be obtained. By following these guidelines and selecting a specimen size that aligns with the test objectives and equipment capabilities, accurate and meaningful data can be achieved.
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Safety considerations
ASTM D792 is the standard test method for determining the specific gravity (relative density) and density of solid plastics in forms such as sheets, rods, tubes, or molded items. The test procedure involves weighing a specimen in air and then again when it is immersed in distilled water at 23°C, using a sinker and wire to ensure complete submersion. While performing this test, there are several safety considerations to keep in mind:
- Mercury Warning: Mercury and its vapors have been designated as hazardous substances by regulatory agencies. They can cause serious health issues and are corrosive to materials. Use caution when handling mercury or mercury-containing products and refer to the relevant Safety Data Sheet (SDS) for detailed information. Additionally, check the legality of selling or using mercury-containing products in your location.
- User Responsibility: The ASTM D792 standard does not address all possible safety concerns. It is the user's responsibility to establish appropriate safety, health, and environmental practices. This includes determining the applicability of regulatory requirements and ensuring compliance with local and national laws.
- Specimen Handling: When handling the plastic specimens, wear appropriate personal protective equipment (PPE) to avoid direct contact with the materials. This may include gloves, safety goggles, and a lab coat.
- Chemical Hazards: Distilled water is generally considered safe, but ensure that it is properly stored and handled to prevent any contamination. Also, be cautious when working with any other chemicals or solvents that may be used during the test procedure.
- Electrical Safety: Ensure that all electrical equipment used in the test, such as scales or temperature controls, is properly grounded and maintained to prevent electrical hazards.
- Laboratory Environment: Maintain a safe and orderly laboratory environment. Keep the workspace clean and free of clutter to prevent trips and falls. Ensure proper waste disposal procedures are followed, especially when dealing with plastic waste or chemical residues.
By following these safety considerations, you can help ensure a safe testing environment when performing ASTM D792 on plastics. It is important to always prioritize safety and refer to the relevant standard documents and safety guidelines for a comprehensive understanding of potential hazards.
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Frequently asked questions
ASTM D792 is a standard test method used to determine the specific gravity (relative density) and density of solid plastics.
Specific gravity is a measure of the ratio of the mass of a given volume of material at 23°C to the same volume of deionized water. It carries no units and is calculated as the density of the plastic divided by the density of water.
Density is the mass per unit volume of a material. Specific gravity is the ratio between the mass density of a substance and that of a reference substance (usually water).
There are two basic test procedures: Method A and Method B. Method A is more common and can be used with sheets, rods, tubes, and molded articles. For this method, the specimen is weighed in air and then when immersed in distilled water at 23°C.
Mercury and its vapors have been designated as hazardous substances by regulatory agencies. As such, caution must be exercised when handling mercury and mercury-containing products.
