Ethan Clarke
The density of plastic is a critical factor in its production and application. It is calculated by dividing the mass of the plastic by its volume and is typically expressed in grams per cubic centimetre (g/cm3). The density of plastics varies with temperature changes and can be altered by foaming, blending resins, or adding light fillers. For example, the density of polypropylene, used in plastic bottle caps, is 0.92 g/cm3, making it buoyant in seawater. Understanding the density of plastics helps optimise their use in various applications, from packaging to automotive parts, balancing factors such as cost, flexibility, strength, and temperature resistance.
| Characteristics | Values |
|---|---|
| Definition | Total mass of the material divided by the total volume of the material |
| Formula | Density = mass/volume |
| Unit | g/cm3, kg/m3 |
| Other units | g/cc (gram per cubic centimeter), b/cu ft (pounds per cubic foot), kg/l (kilogram per liter) |
| Specific gravity | Dimensionless quantity; ratio of the density of a material to the density of a standard material |
| Factors affecting density | Temperature, solvent absorption, plasticizer loss, crystallinity change |
| Bulk density | Weight of material depending on the specific volume |
| Importance | Plays a crucial role in the production process, impacts material performance, sustainability, and cost-efficiency, reduces storage and production costs |
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What You'll Learn
Density is mass per unit volume
Density is defined as the mass per unit volume of a substance. The formula for density is given by d = M/V, where d is density, M is mass, and V is volume. The SI unit for density is kilograms per cubic meter (kg/m^3), though it is also commonly expressed in grams per cubic centimeter (g/cm^3).
The density of a substance can be calculated by measuring its mass and volume and then dividing the mass by the volume. For example, consider a boulder with a mass of 1000 grams and a volume of 400 cm^3. To find its density, we divide the mass by the volume, which gives us 2.5 g/cm^3.
The density of a material can also be determined by comparing its density to that of water. This is known as specific gravity and is calculated using the formula: Specific Gravity = Density of Object / Density of Water. It is important to note that specific gravity is a unitless value.
The density of plastic can be determined by testing solid plastics in water or in a liquid other than water. The density of plastic materials is typically expressed in grams per cubic centimeter (g/cm^3) or kilograms per liter (kg/L). For example, the density of PET material will differ between virgin PET and PET sheets. Additionally, the bulk density of plastic materials, which is the weight of the material per unit volume, is an important consideration in drying operations and material flow.
Density is an important property of materials as it allows us to understand their composition and behaviour. For instance, objects with the same volume but different masses will have different densities. This is because the density of a substance depends on the size, mass, and arrangement of its atoms or molecules. By altering these factors, the density of a material can be increased or decreased.
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Testing methods
Density testing of plastics is an important method for determining the uniformity of a sample. This knowledge is useful in predicting the behaviour of the sample over time and in a variety of situations. The density of moulded plastic is an indicator of how well such parts perform. Testing methods include:
Density Gradient Columns
Density Gradient Columns are one of the most popular non-destructive density testing solutions for plastics. They offer accurate density measurements, typically to 6 decimal places. They use at least two miscible liquids to build the density gradient, and calibrated glass floats are used to calibrate the column.
Immersion Method
The immersion method is specified in part A of the ISO 1183-1 standard. It is suitable for plastic granulates and for products and materials delivered as sheets that can be cut into the right shape in the lab. If the sample is delivered as granules, the test objects are prepared in the lab by injection moulding.
Water Vapor Transmission Rate (WVTR)
The Water Vapor Transmission Rate (WVTR) test measures the water vapour permeability of materials. This method is intended for plastic films or sheeting materials with a water transmission rate of over 1 g/m2/day. The maximum sample thickness is 3 mm. In the test, a water-absorbent material is placed in a metal cup that is covered by the studied material, which is sealed to the cup with wax. The test sample is placed in a conditioned cabinet and its mass is measured. From this, the WVTR value can be calculated.
Float-Sink Method
In this method, samples of polymers found in everyday plastics are placed in a range of liquids of known density. The samples are observed to see if they float or sink, which is then used to identify each polymer using a table of known polymer densities.
Ray-Ran Auto Density Measurement System
The Ray-Ran Auto Density Measurement System uses the gradient column method to calculate the density measurement of a specimen to 6 decimal places. It has an onboard LCD screen providing simple instruction.
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How density impacts performance
The average density of plastics can vary depending on the specific type of plastic and its composition. Different types of plastics have different molecular structures, which result in a range of densities. For example, polypropylene (PP) has a density of around 0.9 g/cm³, while polyvinyl chloride (PVC) typically falls in the range of 1.35–1.45 g/cm³. So, it's important to consider the specific type of plastic when discussing density.
Now, let's discuss how density impacts the performance of plastics:
The density of a plastic material is a critical factor that influences its mechanical, physical, and chemical properties, which in turn affect its performance in various applications. Firstly, density impacts the strength and stiffness of a plastic material. In general, higher-density plastics tend to exhibit greater mechanical strength and rigidity. This is because the closely packed molecules in a high-density material provide more structural support, allowing it to withstand higher stresses and strains without deformation or failure. For example, high-density polyethylene (HDPE) is known for its superior strength and impact resistance compared to lower-density polyethylenes.
Density also plays a significant role in determining the durability and wear resistance of a plastic. Plastics with higher densities often display better wear resistance and longer service lives. This is particularly important in applications where the plastic is subject to frequent abrasion or friction, such as in gears, bearings, or mechanical components. The higher density contributes to a more compact and closely packed molecular structure, making it harder for abrasive particles or surfaces to penetrate and cause wear.
Additionally, density can influence the barrier properties of plastics. In general, higher-density plastics tend to have lower gas and moisture permeability. This is because the closely packed molecules create a more tortuous path for gases or liquids to penetrate the material. For example, high-density polyethylenes are commonly used in food packaging applications due to their excellent barrier properties that help preserve the freshness and quality of the contents.
The density of a plastic also impacts its thermal properties. In general, higher-density plastics tend to have higher melting points and better heat resistance. This is because the closely packed molecules require more energy to break their intermolecular bonds and transition from a solid to a liquid state. For example, polyetheretherketone (PEEK), a high-performance engineering plastic with a high density, exhibits exceptional heat resistance and is commonly used in high-temperature applications.
Lastly, density can influence the chemical resistance of plastics. In general, higher-density plastics tend to have better chemical resistance due to their more closely packed molecular structure, which makes it harder for chemicals to penetrate and cause degradation or swelling. This is particularly important in applications where the plastic is exposed to harsh chemicals, solvents, or corrosive environments, such as in chemical processing equipment or storage containers.
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Bulk density
The bulk density of a plastic material is its weight relative to its volume. It is an important factor in the production process, as it affects the storage of plastic materials in containers, the sizing of drying equipment, and the flow of the material. For example, materials with lower bulk densities require more receiver cycles, which affects material transport throughput and, consequently, production.
The bulk density of plastic resins is typically expressed in one of three ways: gram per cubic centimeter (g/cc or g/cm^3), pounds per cubic foot (b/cu ft), or kilogram per liter (KG/L). The SI unit of density is kilogram per cubic meter (kg/m^3).
The bulk density of a plastic material is critical because different materials have different bulk densities. For instance, the bulk density of PET material differs between virgin PET and PET sheets. As a result, identically sized containers of different materials will hold dramatically different weights of material.
The bulk density of a plastic substance can be determined by testing solid plastics in water or a liquid other than water. This measurement plays a crucial role in the production process, as processors often measure productivity based on the weight of the material.
Additionally, the bulk density of plastic materials can be altered by reducing the density of the plastic material. This can be achieved through foaming molding, blending resins, or adding light fillers.
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Specific gravity
The ASTM D792 and ISO 1183-1 standards outline the general test methods for determining the density and specific gravity (relative density) of plastics. There are two common test methods: Method A, which is used for testing solid plastics in water, and Method B, used for testing plastics in a liquid other than water. Method A is more commonly used and can be performed on various plastic forms, such as sheets, rods, tubes, and moulded pieces. The specimen is weighed first in air (Weight A) and then when submerged in distilled water at the specified temperature (Weight B). The specific gravity is then calculated using the formula: Specific Gravity = (Weight A / Weight B).
The specific gravity of plastics varies depending on their type. For example, nylon gravity plastic typically ranges between 1.13 and 1.15. Plastics with high specific gravity tend to be solid and heavy, while those with low specific gravity are lighter. High-specific-gravity plastics are used where high durability and strength are required, whereas low-specific-gravity plastics are used in flotation or thermal insulation applications.
Additionally, specific gravity is influenced by factors such as composition, processing conditions, temperature, and pressure.
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Frequently asked questions
The density of plastic is measured in mass per unit volume, typically expressed in grams per cubic centimetre (g/cm3). The density of plastic varies depending on the type of plastic and temperature. On average, the density of plastic is 1 kilogram per cubic meter or 1 gram per cubic meter.
To calculate the density of a plastic object, divide the total mass of the object by the total volume.
The density of plastics is dependent on the type of plastic and temperature. For example, polypropylene has a density of 0.92 grams per cubic centimetre, while polycarbonate is known for its high impact strength. The density of plastics can also change due to solvent absorption, plasticizer loss, and crystallinity changes.
High-density plastics offer greater strength, temperature resistance, and chemical resistance. Low-density plastics provide better flexibility, impact resistance, and are easier to process.
