Omar Khalid
Density testing of plastics is an important process that can reveal hidden weaknesses and help assess long-term performance. There are various methods to test the density of plastics, including the use of Density Gradient Columns, which offer extremely accurate readings. Another common method is to test solid plastics in water, observing whether they float or sink to determine their density. This is often used in educational settings, providing an opportunity for students to learn about different polymers and their densities. Additionally, there are instruments like the Ray-Ran 6MPCA that can test the Melt Flow Index and Melt Flow Rate of polymers non-destructively. Understanding the density of plastics is crucial as it plays a significant role in the production process, affecting storage costs and productivity measurements.
| Characteristics | Values |
|---|---|
| Testing methods | Density Gradient Columns, Apparent Bulk Density Apparatus Ray-Ran, Immersion method, Gravimetric dish method, Melt Flow Index (MFI) and Melt Flow Rate (MFR) testing |
| Sample preparation | Samples should be prepared in advance to check they float and sink as expected in different solutions |
| Sample size | 4 x 4 mm squares |
| Solution preparation | Solutions should be labelled with their densities (g cm–3 at 20 °C). Solutions should be stored at room temperature. |
| Testing procedure | Observe whether the plastic samples float or sink in the solutions. Use a glass rod to stir the contents of each tube, washing and drying the rod between each tube. |
| Data analysis | Identify the polymer from which the plastic is made by comparing the results to a table of known polymer densities. |
| Density calculation | Density = (specific gravity) x (997.6) |
| Bulk density units | g/cc (gram per cubic centimeter), b/cu ft (pounds per cubic foot), KG/L (Kilogram per liter) |
| Density reduction methods | Foaming, blending resins, light fillers adding |
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What You'll Learn
Density Gradient Columns
A density gradient column is comprised of a tall glass tube that contains a mixture of two liquids with different densities. When the processes of mixing the two liquids and the filling of the glass tube are done carefully, the density of the resulting column increases almost linearly from the top of the column. A controlled temperature environment, usually set at 23 degrees Celsius, is also required to maintain this uniform density gradient.
To calibrate the column, float beads of known density are used. A calibration graph is then built with the density versus the height at which the beads settle/float. The liquids used to build the density gradient should be chosen to cover the required range of density and to be compatible with the tested samples (avoid absorption, chemical reaction, etc.). Once the gradient column is ready and the calibration line is built, the test specimen is dropped carefully into the column and the height at which it settles is recorded. This height is then put against the calibration line and the density of the sample is determined.
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 been designed with operator simplicity in mind and has an on-board LCD screen providing simple instruction. It is possible to test the Melt Flow Index (MFI) and Melt Flow Rate (MFR) of polymers in a non-destructive way.
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Melt Flow Index (MFI) and Melt Flow Rate (MFR)
The two common standards used worldwide for MFI testing are ASTM D1238 and ISO 1133. The procedure for determining MFI is as follows: a small amount of the polymer sample (around 4 to 5 grams) is taken in a specially designed MFI apparatus. A die with an opening of typically around 2 mm in diameter is inserted into the apparatus. The material is packed properly inside the barrel to avoid the formation of air pockets. A piston is introduced, which acts as the medium that causes the extrusion of the molten polymer. The sample is preheated for a specified amount of time: 5 minutes at 190°C for polyethylene and 6 minutes at 230°C for polypropylene. After the preheating, a specified weight is introduced onto the piston. Examples of standard weights are 2.16 kg, 3.2 kg, 3.8 kg, and 5 kg. The weight exerts a force on the molten polymer and it immediately starts flowing through the die. A sample of the melt is taken after the desired period of time and is weighed accurately.
The MFI value quoted on many datasheets refers to the amount of polymer that is extruded through a known given orifice (die) and is expressed as a quantity in g/10 mins or for Melt Volume Rate in cm3 /10 mins. The MFI is often used to determine how a polymer will process, however, it does not take into account the shear, shear rate, or shear history and is therefore not a good measure of the processing window of a polymer. The MFI is also important for determining the molecular weight distribution of the material.
MFI and MFR can be tested in a non-destructive way using the Ray-Ran 6MPCA, which has a unique microprocessor and a multi-slicing feature that gives accurate MFR values plotted against piston displacement, calculated from the density of the materials at the test temperature.
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Testing plastic samples in water
Density testing of plastics is crucial for understanding their long-term performance and hidden weaknesses. One popular method for testing the density of plastic samples in water involves the use of Density Gradient Columns. This technique offers highly accurate density measurements, typically to six decimal places. Here's a step-by-step guide to testing plastic samples in water using Density Gradient Columns:
Prepare the Solutions
First, prepare two miscible liquids with known densities to create a density gradient. In this case, you will be using water and another liquid to create the desired density gradient. Prepare enough solution for your test, approximately 15 cm3 for each liquid for each test tube. Label the solutions with their densities, typically given in g cm–3 at 20 °C. It is important to note that temperature changes affect the densities of solutions, so store them together at room temperature.
Set Up the Test Tubes
Use large test tubes, preferably 150 x 25 mm in size. Add about 15 cm3 of each liquid to separate test tubes, ensuring you have enough test tubes for the number of samples you plan to test. Label each test tube with the relevant solution and density information.
Prepare the Plastic Samples
Cut or tear the plastic samples into squares of approximately 4 x 4 mm in size. You will need six samples of each type of plastic you plan to test. Organize your samples by laying them out separately on a paper towel. Common examples of plastics to test include polyethylene terephthalate (PET), high-density polyethylene (HDPE), and polyvinyl chloride (PVC).
Perform the Test
Place one plastic sample into each test tube, ensuring that the samples are fully submerged in the solution. Observe whether the samples float or sink. A sample will sink if its density is greater than the density of the solution. Record your observations in a results table, noting which samples float and which ones sink.
Identify the Plastics
By comparing your observations with known polymer densities, you can identify the type of plastic each sample is made of. This identification process can be done with the help of reference tables or databases that provide information on the densities of common plastics.
Density testing of plastic samples in water using Density Gradient Columns is a safe and effective method for accurately determining the density of plastics. This technique is valuable for predicting the long-term behaviour of plastics and identifying potential weaknesses.
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Testing plastic samples in a liquid other than water
Density testing of plastics is important for determining the uniformity of a sample, which can be used to predict the behaviour of the sample over time and in different situations. It is also useful for calculating the strength-to-weight ratio of plastics.
One popular method for testing the density of plastics is through the use of Density Gradient Columns. These columns use at least two miscible liquids to build a density gradient and calibrated glass floats to calibrate the column. The test material is then added, and once it reaches equilibrium, the density of the material can be found. This method offers highly accurate density measurements, typically to six decimal places.
Another method for testing the density of plastics is by observing whether samples of the plastic float or sink in a liquid of known density. This method is often used in educational settings to teach students about polymers and their densities. In this experiment, students place samples of polymers found in everyday plastics into liquids of known density and observe whether they float or sink. They can then use this information to identify the polymer using a table of known polymer densities. This method can also be used outside of educational settings to identify polymers in plastic samples.
When testing plastic samples in a liquid other than water, the same principles as those outlined above can be applied. The plastic sample can be placed in a liquid of known density other than water, such as ethanol, and observations can be made about whether the sample floats or sinks. It is important to note that ethanol solutions are flammable and harmful, so appropriate safety precautions must be taken when working with this substance. The density of the plastic film can also be determined by placing test specimens in a liquid other than water, such as isopropanol, and adding another liquid until the plastic sample achieves neutral buoyancy in the solution. This method is in accordance with British standard BS 2782:6 method 620A or ASTM D792.
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Immersion method as per ISO 1183-1 standard
The Immersion Method as per ISO 1183-1 standard is a widely recognised technique for determining the density of plastics and polymers. This method is suitable for plastic granulates and products or materials delivered as sheets that can be cut into the desired shape in the lab. If the sample is provided as granulates, the test objects are prepared in the lab by injection moulding.
The Immersion Method is a non-destructive testing solution that provides accurate density measurements. This technique is particularly useful for identifying polymers in everyday plastic items. The test involves placing samples of known polymers into a range of liquids with known densities. These liquids are typically prepared using ethanol and water solutions, with the solutions labelled as flammable and harmful due to their ethanol content.
The plastic samples are prepared in advance to ensure they float and sink as expected in the different solutions. The solutions and samples are stored together at room temperature to maintain consistency, as temperature changes can affect the densities of the solutions. Each sample is typically a 4x4 mm square, and the solutions are labelled with their densities in g cm^-3 at 20°C.
During the test, a glass rod is used to stir the contents of each tube, with the rod washed and dried between each tube. The observation is then made as to whether the plastic samples float or sink. If a sample sinks, it indicates that its density is greater than the density of the solution. This information is recorded, and the polymer type is identified using a table of known polymer densities.
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Frequently asked questions
The simplest way to test the density of plastic is to test solid plastics in water. This is the most common test and can be performed with plastic sheets.
Cut the plastic into squares of approximately 4 x 4 mm. Place the samples into a test tube filled with water and observe whether they float or sink. If the sample floats, its density is lower than that of the water.
Other methods include the use of Density Gradient Columns, the gravimetric dish method, X-Ray Reflectometry (XRR), and the immersion method specified in the ISO 1183-1 standard. Additionally, there are methods to test the water vapour transmission rate (WVTR) and oxygen transmission rate of plastic films and sheets. To reduce the density of plastic, methods such as foaming, blending resins, and adding light fillers can be employed.
