Chloe Zhang
Specific gravity is a unitless value that measures the ratio of a substance's density to that of a reference substance, typically water. It is used in the recycling industry to sort mixed plastic polymer types and in the design and engineering of plastic products. The specific gravity of plastics is determined by dividing the density of the plastic by the density of water, with the density of plastic depending on temperature. The most common method for determining the density of solid plastics is the immersion method, where the plastic is weighed first in air and then when immersed in distilled water at 23°C.
| Characteristics | Values |
|---|---|
| Definition | Specific gravity is a material measurement value and is the ratio of a fluid’s density to the density of a reference standard fluid. |
| Formula | SG = p / p(ref); where p equals density. |
| Reference Standard | For liquids and solids, the reference standard is traditionally water at 39°F (4°C). |
| Temperature | The density of plastic depends on temperature. Therefore, it is advisable that the measurement of specific gravity be carried out at a constant temperature for proper results. |
| Density of Reference Standard | 8.34 lbs/gal or 1.00 g/cm3. |
| SG Value Interpretation | Any substance with an SG value greater than 1.0 will have a greater density, more mass per volume, and therefore weigh more relative to the reference standard. |
| Applications | Specific gravity is used in the recycling industry to sort mixed plastic polymer types and in the design and engineering of plastic products. |
| Test Methods | ASTM D792, Method A and Method B. Method A is performed with sheet, rod, tube and molded pieces of plastic materials that are not affected by water. Method B uses other liquids. |
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What You'll Learn
Use the displacement method
The displacement method is one of the ways to measure the specific gravity of plastics. Specific gravity is a crucial property of plastics, as it helps identify the type of plastic used for recycling. It is also used in the design and engineering of plastic products, and in industries like automotive and construction.
The displacement method, as outlined in ASTM D792, involves the following steps:
- Weighing the Specimen: First, determine the mass of a specimen of the solid plastic in air. This initial weight measurement is essential for the subsequent calculation of specific gravity.
- Immersion in Water: Next, immerse the entire item or a representative cross-section of the plastic specimen in water. This step is suitable for plastics that are not affected by water, except for being wet by it.
- Displacement Calculation: Observe and calculate the amount of water displaced by the plastic specimen. This displacement value is crucial for the next step.
- Specific Gravity Calculation: Finally, the specific gravity of the plastic can be determined by dividing the weight of the plastic specimen by the weight of an equivalent volume of water. This calculation yields a dimensionless number, indicating the relative density of the plastic compared to water.
It is important to note that the density of plastics can vary due to factors such as crystallinity, loss of plasticizers, absorption of solvents, and composition. Therefore, maintaining a constant temperature during testing is advisable to ensure accurate results.
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Use a pycnometer
Pycnometers, also known as specific gravity bottles, are used to measure the density or specific gravity of solids, semi-solids, and liquids. They are especially useful for measuring the specific gravity of plastics because they can provide highly accurate measurements at a given temperature.
To use a pycnometer to measure the specific gravity of a plastic sample, follow these steps:
- Clean and dry the pycnometer thoroughly to remove any residue from previous use.
- Weigh the empty pycnometer with the stopper on and record this weight as M1 or w1.
- Take your plastic sample, ensuring it is dry and free from any moisture, as this can affect the accuracy of your results.
- Place the plastic sample into the pycnometer and record its weight with the stopper on as M2 or w2.
- Now, fill the pycnometer with distilled water up to the top. Distilled water is used as it is pure and will not affect the density measurement.
- Gently swirl the contents or use a glass rod to stir to ensure that all entrapped air is removed.
- Wipe away any excess water on the outside of the pycnometer to prevent it from affecting the weight measurement.
- Weigh the pycnometer with the plastic sample and distilled water, keeping the stopper on, and record this weight as M3 or w3.
- Finally, calculate the specific gravity (G) using the following formula: G = (M3 - M1) / (M2 - M1). This equation accounts for the weight of the plastic sample relative to the weight of an equivalent volume of water.
It is important to maintain the pycnometer and the sample at the same temperature throughout the process to ensure accurate results, as the density of both the plastic and water can change with temperature variations. Additionally, using a pycnometer with a certified volume at a specific temperature can enhance the precision of your measurements.
By following these steps and calculations, you can effectively use a pycnometer to determine the specific gravity of a plastic sample, contributing to a better understanding of the material's behaviour and properties.
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Weigh plastic in air and water
To measure the specific gravity of a plastic sample, you must determine its density relative to water. One way to do this is by weighing the plastic in air and water. Here is a step-by-step guide:
Weigh the Plastic in Air
- Measure the weight of your plastic sample in air using a scale. Ensure the scale is calibrated and accurate. Record this weight as 'W_plastic in air'.
- Note that the weight of plastic can vary depending on its type. Different plastics have different densities due to variations in composition and additives. For example, acrylic sheet (commonly known as Plexiglas or Perspex) has a weight of 1.19 grams per cubic centimeter, while polyvinyl chloride (PVC) is even lighter.
Weigh the Plastic in Water
- Fill a container with water. Ensure the container is large enough to completely submerge your plastic sample.
- Carefully lower the plastic sample into the water, making sure it is fully submerged. Avoid trapping any air bubbles on or within the sample.
- Measure the weight of the submerged plastic sample. Record this weight as 'W_plastic in water'.
- The weight of water is one gram per cubic centimeter. Therefore, a material with a specific gravity of 1.5, for example, would weigh 1.5 grams per cubic centimeter.
Calculate Density and Specific Gravity
- To calculate the density of the plastic, use the formula: Density = W_plastic in air / W_plastic in water.
- The specific gravity of the plastic is then determined by dividing its density by the density of water (which is 1 gram per cubic centimeter). So, Specific Gravity = Density of Plastic / Density of Water.
It is important to maintain a constant temperature during these measurements, as the density of plastic can vary with temperature changes. Additionally, ensure that consistent units are used throughout the process, such as grams and cubic centimeters.
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Use ASTM D792 test
The ASTM D792 test is a widely used method for determining the specific gravity and density of solid plastics. This test is particularly useful for investigating material quality assurance, material selection, and characterizing material resistance. It is also applied in various industries, including construction, automotive, and plastics, where it facilitates the assembly and assessment of material characteristics.
The ASTM D792 test procedure involves the following steps:
- Determine the mass of a solid plastic specimen in air (Weight A).
- Immerse the specimen in a liquid, typically distilled water at a constant temperature of 23°C, and calculate its mass upon immersion (Weight B).
- Calculate the specific gravity and density using the following formulas:
- Specific Gravity = (Weight A / Weight B)
- Density = (Weight A / Weight B) 0.9976 g/cm^3 (density of distilled water at 23°C)
The test parameters, including support span, loading speed, and maximum deflection, may vary depending on the sample thickness.
It is important to note that the density of plastic is temperature-dependent, so maintaining a constant temperature during the ASTM D792 test is crucial for accurate results.
Additionally, this test method can help identify different types of plastics, monitor physical changes in samples, and determine the average density for larger items or products. The ASTM D792 test is a valuable tool for quality control and ensuring the consistency of plastic materials in various applications.
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Compare to a reference standard
Specific gravity is a material measurement value and is the ratio of a fluid’s density to the density of a reference standard fluid. Specific gravity, abbreviated as SG or SpGr and known as relative density, is a comparison between two density values and is often used to determine how much more or less weighty a substance is than the reference material.
Mathematically, specific gravity is a ratio: it is the density of a substance divided by the density of the reference substance. In formula, specific gravity can be represented as: SG = p/p(ref), where p equals density. For liquids and solids, the reference standard is usually water at 39°F (4°C), where it is considered most dense with a density value of 8.34 lbs/gal or 1.00 g/cm3. For gasses, the SG reference is typically air with a density of 1.205 kg/m3 at 68°F (20°C) and 1.00 atm (101.325 kPa).
By taking the solution SG and multiplying it by the density of the reference (water), the density of the solution can be calculated, which can then be used to determine a solution’s total weight or volume. For example, a fluid with a specific gravity of 1.38 can be immediately discerned to be 1.38 times as heavy as the reference fluid, commonly water. If the specific gravity is multiplied by the reference weight of water, the weight of the solution can be found for the set volume; in this example, this is 11.51 lbs/gal.
Specific gravity directly correlates with a substance’s density, making it important to understand density to fully understand SG and how it is used in applications. Density is the measure of a substance’s amount of mass per unit volume at a select temperature and is used to determine the total weight of a material in conjunction with area (i.e., volume). Density does this by demonstrating how much substance (i.e., mass) will be present and therefore how heavy it will be within the specified volumetric area.
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Frequently asked questions
Specific gravity is a material measurement value and is the ratio of a fluid’s density to the density of a reference standard fluid, usually water.
The specific gravity of plastics is determined by dividing the density of plastic by the density of water. This calculation yields a dimensionless number that describes the relative density of the material.
The density of plastic is the mass per unit volume of the plastic material. The density of plastic depends on temperature.
The most common method for determining the density of solid plastics is the immersion method, also known as ASTM D 792 and ISO 1183-1. Method A is the more common test method used and can be performed with sheet, rod, tube and moulded pieces of plastic materials that are not affected by water.
Specific gravity is used in the recycling industry to sort mixed plastic polymer types and in the design and engineering of plastic products. This will also help us understand how to predict their behaviour in different working environments.
