Sofia Martinez
There are various methods to test the brittleness of plastics, including the Izod impact test, which is considered a quick and easy way to evaluate plastic brittleness. Another method is ASTM D746, which is a standard test for determining the temperature at which plastics exhibit brittle failure under specified impact conditions. This test method involves securing specimens in a customer-specified holder and lowering them into a bath at a specified temperature. The specimens are then impacted and examined, and the procedure is repeated at lower temperatures until enough data is generated to determine the brittleness temperature. Other tests for plastic properties include the Vicat softening point test, water absorption data, and tensile impact energy measurements.
| Characteristics | Values |
|---|---|
| Brittleness Temperature | The temperature at which 50% of the specimens exhibit brittle failure at specified impact conditions |
| Test Method | ASTM D746 |
| Test Conditions | Specimens are secured in a customer-specified holder (A or B) with a torque wrench at 5 in-lb and lowered into a bath at a specified temperature |
| Impact Speed | 2000 ± 200 mm/s |
| Test for Nylon | Leave the nylon in controlled humidity for a day; test takes minutes |
| Quick Test Methods | Izod impact test, Tack hammer, Razor blade |
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$800
What You'll Learn
Brittleness temperature
The D746 Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact is a widely recognised method for determining the brittleness temperature of plastics. This test method establishes the temperature at which 50% of specimens would likely fail under the specified conditions. It is important to note that the test data should only be used to predict the behaviour of plastics at low temperatures when the conditions of deformation are similar to those in the test.
When testing plastic sheeting for impact brittleness, the D1790 Standard Test Method is applied. This method covers the determination of the temperature at which plastic sheeting exhibits brittle failure under specified impact conditions. It is important to note that the D1790 test method was developed specifically for plasticized vinyl sheetings, and its applicability to other plastic sheetings must be verified.
The brittleness temperature of a material can also be used in specifications and to judge the relative merits of various materials for low-temperature flexing or impact. However, it is essential to remember that the brittleness temperature does not indicate the lowest temperature at which a material can be used. The test is specific to the materials and conditions outlined in the method, and the values cannot be directly applied to other shapes and conditions.
Overall, the brittleness temperature test provides valuable information about the behaviour of plastics and elastomers at low temperatures, but it should be interpreted within the context of the specific test conditions and limitations.
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Impact testing
The Charpy impact test and the Izod impact test are two commonly used impact testing methods. In the Charpy test, a pendulum is dropped from a certain height to strike a notched specimen, which absorbs energy upon impact. The energy absorbed is then determined by measuring the pendulum's height before and after the fracture. The specimen is typically a bar with a U-shaped or V-shaped notch, placed horizontally with the notch facing away from the pendulum to focus stress and encourage fracture. The Charpy test is known for its simplicity and ease of use and has been widely used since 1900.
The Izod impact test, developed in 1903, is similar to the Charpy test but with some key differences. In the Izod test, the specimen is clamped vertically with the notch oriented to face the pendulum, which strikes the specimen above the notch. This test can be performed on both metallic and plastic materials. Plastic specimens used in the Izod test are typically bars with a V-shaped notch, while metallic specimens are generally round bars with one or three notches.
Another type of impact test is the Tensile Impact Test, which measures the impact resistance and strength of plastic materials. A notched specimen is held between two grips, and a sudden force is applied to measure the energy required to break the material. This test is useful for evaluating plastics that may experience sudden or repeated impacts, such as in automotive or aerospace applications.
It is important to note that the brittleness temperature determined through impact testing is specific to the materials and conditions specified in the test. The test provides valuable insights into the behaviour of plastics at high deformation speeds and their ability to absorb energy before failure.
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Testing plastic sheeting
The test procedure involves securing specimens in a customer-specified holder (Type A or B) with a torque wrench at 5 in-lb. The specimen holder is then lowered into a bath at a specified temperature and impacted at a speed of 2000 ± 200 mm/s. This process is repeated at lower temperatures until enough data is collected to determine the brittleness temperature. The brittleness temperature is defined as the temperature at which 50% of the specimens fail, exhibiting either fracture into two or more pieces or visible cracks.
It is important to note that the brittleness temperature test is specific to the materials and conditions outlined in the test method. The results may not be directly applicable to other shapes, conditions, or types of plastic sheeting. Additionally, the test does not determine the lowest use temperature for a particular plastic.
Other methods for testing plastic properties include the Vicat Softening Point test (ASTM D-1525), which evaluates the heat softening characteristics of polyethylenes and other thermoplastics. This test uses a temperature-regulated oil bath and a needle penetrator to determine the temperature at which the needle penetrates the specimen by 1 mm. Compressive properties of plastics can also be tested using a compression tool to exert a constant rate of movement and measuring the psi required to rupture or deform the specimen.
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Humidity testing
One standard method for determining the moisture content in plastics is the Karl Fischer titration method, which is widely recognized as the ASTM standard (ASTM D6869). This technique involves titrating a sample of the plastic resin with a Karl Fischer reagent to determine the moisture content. While this method is accurate, it may not be practical for routine factory floor use.
An alternative to the Karl Fischer titration method is the use of moisture analyzers, such as the HX204 moisture analyzer from METTLER TOLEDO. This analyzer utilizes a high-resolution weighing cell to precisely measure the moisture content in plastics. It offers readability down to 0.001% moisture content, making it ideal for plastics with low moisture levels.
To simulate the effects of humidity on plastics, artificial weathering tests are conducted in laboratory settings. ASTM defines artificial weathering as exposing plastics to cyclic conditions of varying temperature, relative humidity, ultraviolet (UV) radiant energy, and water spray. This process aims to replicate the natural changes observed in materials after long-term outdoor exposure. While no precise correlation exists between artificial and natural weathering, these tests provide valuable insights into the weatherability of plastics.
Additionally, the Brittleness Temperature test (ASTM D746) is used to evaluate the impact brittleness of plastics at low temperatures. This test involves securing specimens in a customer-specified holder and lowering them into a bath at a specified temperature. The specimens are then impacted at a controlled speed and examined for failures, such as visible cracks or fractures. By repeating this procedure at lower temperatures, a brittleness temperature can be determined, indicating the temperature at which 50% of the specimens are expected to fail.
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Crystallization
To perform brittleness testing on plastics, there are two main approaches: the dry-as-molded test and the moisturized test. The dry-as-molded test evaluates the material in its dry state, while the moisturized test assesses the material after it has absorbed water. The moisture content can impact the brittleness of the plastic, as water acts as a plasticizer, affecting its flexibility and impact resistance.
The Izod impact test is a quick and easy method to evaluate plastic brittleness. It helps determine the material's ability to withstand impact without failing. However, one challenge with impact testing is ensuring that the impacted area represents the brittleness of the entire part, especially when dealing with small parts.
Another important aspect of brittleness testing is considering the temperature at which the material fails. The standard test method D746 establishes the temperature at which 50% of the specimens would likely fail under specified conditions. This information is valuable for understanding the long-term performance of plastics at different temperatures and in different environmental conditions.
Additionally, the hardness of plastics is a critical factor in determining their suitability as a substitute for metal in various applications. Hardness refers to a material's ability to resist irreversible indentation or perforation by a harder force or body. By evaluating the hardness of plastics, engineers and quality professionals can make informed decisions about the quality of plastic materials before they are used in industrial components or consumer products.
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