Maya Ellison
The brittleness of plastic is a measure of its ability to withstand impact and shock. Brittleness temperature is the temperature at which 50% of tested specimens exhibit brittle failure under specified impact conditions. Various methods exist to test for brittleness, including the Izod impact test, the notched Izod test, and the tensile impact test. The Vicat softening point test is used to compare the heat-softening characteristics of polyethylenes and other thermoplastics. The choice of test depends on the specific material and conditions being evaluated, as brittleness temperature is only relevant for the materials and conditions specified in the test.
| Characteristics | Values |
|---|---|
| Brittleness Temperature | The temperature at which 50% of specimens exhibit brittle failure at specified impact conditions |
| Brittleness Temperature Test | Specimens are impacted at 2000 ± 200 mm/s and then examined; the procedure is repeated at lower temperatures until enough data is generated to determine brittleness temperature |
| Brittleness Temperature Calculation | The temperature at which 50% failure would occur is calculated or plotted and reported as the brittleness temperature |
| Brittleness Failure Definition | Fracture into 2 or more pieces, or any crack visible to the unaided eye |
| Test Method for Plastics | ASTM D746 |
| Test Method for Rubbers | ASTM D2137 |
| Test Method for Plastic Sheeting | ASTM D1790 |
| Impact Test | Izod impact test |
| Moisture Control | Not always effective |
| Annealing | May not help with brittleness, especially if caused by chain scission; can reduce free volume and increase brittleness |
| Spectrophotometry | Used to detect degradation of PVC samples |
| Brittleness and Stiffness | Can be drawbacks for some applications |
| Stress-Strain Curve | Indicates overall toughness; a material with very high tensile strength and little elongation would tend to be brittle |
| Tensile Impact Energy | Recorded to determine brittle or ductile failure |
| Vicat Softening Point | Used to compare heat softening characteristics of polyethylenes and other thermoplastics |
| Water Absorption | Specimens are immersed in water for 24 hours or longer at 73.4 °F |
| Compressive Strength | Limited design value for plastics, as plastic products (except foams) rarely fail from compressive loading alone |
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What You'll Learn
Brittleness temperature
The brittleness temperature of a plastic is the temperature at which the plastic becomes brittle and shatters upon high-speed impact. This is also known as the ductile-to-brittle transition temperature (DBTT). The DBTT varies depending on the type of plastic. For example, polycarbonate (PC), a tough plastic used in bulletproof glass, becomes brittle at very cold temperatures of -40 degrees Fahrenheit. As plastic parts age, their DBTT temperature increases.
There are several methods to measure the brittleness of plastic, including the Izod impact test, which can be carried out using a modified Izod machine. This test method determines the temperature at which 50% of the specimens would likely fail due to long-term effects such as crystallization or low-temperature incompatibility of plasticizers.
Another approach is to use a tack hammer and a skilled hand to strike the plastic at high-speed impact. This method is crude but can provide a quick evaluation of brittleness. Additionally, shaving a non-critical area of the plastic with a razor blade can indicate brittleness; dust indicates brittleness, while strings or chips suggest the material is not brittle.
To test plastic sheeting for impact brittleness, Test Method D1790 is recommended, while Test Method D2137 is used for testing rubbers. These standardized tests help ensure accurate and consistent measurements of the brittleness temperature of plastics.
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Impact tests
Impact testing is used to identify material behaviours at high deformation speeds. The behaviour of plastics that are subjected to impact loading is characterised with the use of pendulum impact testers, high-speed testing machines, and drop weight testers.
The Charpy impact test is a popular test for evaluating the relative toughness of materials. It was created in 1900 by Georges Augustin Albert Charpy and is often used to test metals, polymers, and plastics. The test involves dropping a weighted pendulum from a certain height to make contact with the chosen specimen. The specimen absorbs energy, which can be inferred by measuring the pendulum's height before and after it was fractured, and then getting the difference between the two measurements. The Charpy impact test can be done at reduced and ambient temperatures, even as low as -45°F. It is usually performed to EN 10045-1, ISO 148, ASTM E23, and ASTM A370. The Charpy test has a broader range of applications and is better suited to test materials displaying interlaminar shear fractures or surface effects.
The Izod impact test is another method to evaluate plastic brittleness. The test apparatus and the design of the specimen are highly similar to the Charpy impact test, but there are certain differences, such as the specimen orientation. The Izod test is performed when testing plastic sheeting for impact brittleness.
Other methods to quickly evaluate the brittleness of a plastic include using a tack hammer and a skilled hand, or "shaving" the part in a non-critical area with a razor blade.
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Water absorption
To measure the impact of water absorption on brittleness, it is necessary to test plastics under controlled conditions. ISO 62 outlines a procedure for determining the moisture absorption properties of flat or curved-form solid plastics. This includes testing the amount of water absorbed by plastic specimens of defined dimensions when immersed in water or exposed to humid air. The test method covers the determination of the relative rate of absorption of water by plastics when immersed. However, it is important to note that this standard does not apply to cellular plastics, granulates, or powders, which may exhibit different absorption behaviours.
The impact of water absorption on brittleness can vary depending on the type of plastic and the environmental conditions. Relative humidity and temperature can influence the rate and extent of water absorption. Additionally, chemical additives used to modify the properties of plastic materials can also affect water absorption rates. Therefore, it is crucial to properly dry all materials before production to prevent moisture-induced brittleness.
There are several ways to mitigate the impact of water absorption on plastic brittleness. One method is to increase the barrel temperature and/or back pressure, which helps soften and homogenize the plastic, resulting in stronger molecular bonds that are more resistant to moisture-induced brittleness. Additionally, ensuring proper mixing and melting of the plastic material is essential, as weak molecular bonds caused by incomplete mixing can lead to increased brittleness.
While water absorption can negatively impact the brittleness of plastics, it is important to note that some plastics are specifically designed to absorb water. These plastics may have applications where water absorption is desirable, such as in moisture-absorbing materials or water filtration systems. In such cases, the ability of the plastic to absorb water would be an advantage rather than a cause for concern.
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Stress-strain curves
The first stage is the linear elastic region, where the material experiences elastic deformation and returns to its original form when the load is removed. In this region, the stress-strain connection adheres to Hooke's law, stating that stress is proportional to strain. This linear portion of the curve has a slope known as Young's modulus.
As the stress increases beyond a specific threshold, termed the yield point, the material enters the second stage, the strain hardening region. Here, the material undergoes plastic deformation and experiences permanent distortion, even after the load is removed. The stress-strain relationship deviates from linearity, and the stress needed for further deformation rises.
The third stage is beyond the yielding point, where plastic deformation continues and strain hardening occurs. This process leads to necking, which is the localisation of deformation in the material's cross-section. The cross-sectional area decreases, and the material reaches its ultimate tensile strength, the maximal stress it can sustain.
After this point, the material is past the point of no return. It undergoes further deformation or breaks permanently. Brittle materials, such as ceramics, concrete, certain metals, and glass, exhibit minimal plastic deformation and sudden, catastrophic failure. In contrast, ductile materials, like structural steel, undergo significant plastic deformation before fracture.
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Plasticizers
The integration of plasticizer small molecules between polymer chains increases mobility and flexibility, reducing the glass transition temperature (Tg), cold crystallization temperatures (Tcc), melting temperatures, and crystallinity. The use of isosorbide and low-molecular-weight poly(ethylene glycol) (PEG), for example, can lower the Tg by around 30 °C.
Different plasticizers have different effects on the thermal, crystalline, and permeability properties of plastics. Poly(3−hydroxybutyrate−co−3−hydroxyhexanoate) (PHBH) films, for instance, can be prepared with plasticizers like PEG and dioxane, which result in more homogeneous films with better morphology. Films plasticized with PEG 600, however, may be less effective than those with lower molecular weight PEGs. Additionally, plasticizers like isosorbide can increase water vapor transmission rates, while GDAL, an acetylated glycerol fatty acid ester, may be less effective in decreasing Tg and increasing crystallite size.
Standard test methods, such as ASTM D746, have been established to evaluate the brittleness temperature of plastics and elastomers by impact. These tests help determine the temperature at which 50% of specimens would likely fail due to long-term effects like crystallization or low-temperature incompatibility of plasticizers. Other test methods, like D1790, are specific to plastic sheeting, while D2137 is used for rubbers.
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Frequently asked questions
The Izod impact test is a quick and easy way to evaluate plastic brittleness. Brittleness temperature is also a good way to judge the relative merits of various materials for low-temperature flexing or impact.
The standard test method for measuring the brittleness of plastics is ASTM D746. This test establishes the temperature at which 50% of the specimens tested would probably fail when subjected to the specified conditions.
Tensile strength is a measure of how well a material can withstand tension or being pulled apart. Compressive strength is a measure of how well a material can withstand compression or being pushed together.
The brittleness of plastics can be influenced by various factors such as temperature, moisture content, molecular weight, and the addition of plasticizers or other materials.
Plastics tend to exhibit different responses to stress compared to reinforced plastics and metals. The matrix of plastics cracks at lower stresses relative to the final failure stress, gradually transferring load-bearing responsibility to reinforcing fibers.
