Ethan Clarke
Plastics are ubiquitous, from everyday household items to critical industrial components. Understanding how plastic ages is crucial to ensuring their longevity and performance. Accelerated aging tests are commonly employed to predict the long-term behaviour of plastics in various environments. These tests involve exposing plastic materials to higher levels of radiation, temperature, humidity, and other external agents to accelerate degradation processes. While increasing temperature reduces the time needed for accelerated aging, it can also introduce unrealistic defects. Therefore, a thoughtful combination of strategic approaches, including material selection, design considerations, and proactive maintenance, is essential to maximize the lifespan of plastic parts.
| Characteristics | Values |
|---|---|
| Temperature | The higher the temperature, the faster the aging process. |
| Time | Aging studies can take anywhere between 29 to 57 days. |
| Standard | ASTM F1980 Accelerated Aging of Sterile Barrier Systems for Medical Devices. |
| Equation | The Arrhenius Equation. |
| UV Radiation | Exposure to UV radiation at 60 °C for 8 hours. |
| Condensation | 4 hours at 40 °C in the absence of irradiation. |
| Cycles | Repeated cycles of exposure to UV radiation and condensation. |
| Climatic Chambers | Specimens are exposed to high levels of humidity, temperature, UV radiation, and different atmospheres. |
| Mechanical Stress | Reproduction of the load field to which the polymer is subjected. |
| Material Selection | The flexibility of polyethylene or the durability of polycarbonate. |
| Environmental Factors | Sunlight, heat, and moisture. |
| Chemical Changes | Exposure to chemicals can alter the composition of the material. |
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Temperature variations
However, it is important to note that extremely high temperatures can cause defects that would never occur in real-world conditions. For instance, in a hospital storage room, the temperature is usually ambient. Therefore, caution must be exercised when choosing the temperature for accelerated ageing studies.
The Arrhenius Equation is a standard tool used to determine the duration of accelerated ageing studies. This equation helps calculate the time samples should remain in the chamber based on the selected temperature. Additionally, the Simplified Protocol for Accelerated Ageing, also known as the "10-degree rule," is applied to well-characterized polymer systems over moderate temperature ranges, yielding results within the required accuracy.
The effects of temperature on the ageing process are influenced by the specific type of plastic being tested. For instance, polyesters are susceptible to thermally induced mechanical changes in high-temperature environments, which limits their service life. Similarly, polyamide-based plastics, such as Nylon and PEBAX, are more susceptible to changes in humidity and temperature than hydrophobic materials like polyethylene and silicone.
It is also worth noting that the combination of high temperature and high humidity can lead to unnatural plasticization, causing a reduction in mechanical properties. Therefore, it is crucial to consider the potential impact of temperature variations on the plastic's performance and structural integrity during accelerated ageing studies.
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UV radiation
Ultraviolet (UV) radiation is a well-known cause of plastic degradation, and its effects on plastics have been widely studied. UV radiation is part of the electromagnetic spectrum and is split into three types: UVA, UVB, and UVC. UVA has a wavelength range of 320-400 nm, UVB has a range of 280-320 nm, and UVC stands at around 100-280 nm.
When plastics are exposed to UV radiation, the UV energy can excite photons, creating free radicals, and leading to degradation. This degradation occurs as catalyst residues act as receptors. Pure plastics cannot absorb UV radiation, so this puts any plastic components at risk of degradation.
There are several methods to accelerate the aging of plastics using UV radiation. One method is to expose flat plastic samples to a matrix of UV irradiance and IR irradiance values. This initiates photochemical reactions without increasing temperature. Another method is to use a "Monitored Liquid Thermostatted Irradiation Setup" (MLTIS), which uses blue light to age the plastic samples. The MLTIS setup allows for temperature control of the sample independent of the radiant power of the LED.
To prevent UV degradation, stabilizers, quenchers, or absorbers can be added to plastics. Hindered Amine Light Stabilizers (HALS) are the most widely used type of UV inhibitor, accounting for about 60% of total consumption. They work by decomposing hydroperoxides, quenching excited oxygen, and removing free radicals through photo-oxidation. Ultraviolet absorbers are the second most common type of UV stabilizer, accounting for about 25% of total consumption. Additionally, carbon black, titanium dioxide, and zinc oxide are effective UV light shielding agents.
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Humidity
The impact of humidity on the aging of plastics is influenced by the specific type of plastic. For example, hygroscopic materials like polyamide-based plastics (Nylon, PEBAX, etc.) are more susceptible to degradation at elevated temperatures when humidity levels are high. On the other hand, hydrophobic materials like polyethylene and silicone are less affected by humidity.
In one study, the warm humidity resistance of over thirty plastic formulations was evaluated over periods ranging from 10 months to 3 years. It was found that while these materials could withstand high humidity at moderate temperatures for short periods, some were degraded by long-term exposure to warm humidity. This highlights the importance of selecting the appropriate humidity levels when conducting accelerated aging tests to avoid unnatural degradation mechanisms.
To simulate real-world conditions, it is important to consider the interaction between humidity and temperature. For example, if the humidity is too low during accelerated aging, materials like Nylon can become unnaturally stiff and brittle, resulting in changes in device or packaging performance. Therefore, it is crucial to select humidity levels that, in combination with temperature, produce moisture levels representative of expected storage conditions.
Overall, while humidity is an important factor in accelerated aging, it should be carefully controlled to avoid unrealistic defects in the plastic. By understanding the specific material's susceptibility to humidity and selecting appropriate test conditions, accelerated aging studies can effectively predict the long-term behavior of plastics in different environments.
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Chemical exposure
One critical aspect of chemical exposure is the use of oxidizing agents, such as Fenton, K2S2O8, and O3. These agents induce rapid aging in plastics due to their strong oxidizing potential. The oxidation process can lead to changes in the chemical composition and surface structure of the plastic, resulting in cracking, fragmentation, and oxidation. Additionally, high temperatures can accelerate this process by rapidly initiating oxidation in the polymer backbone.
Another factor to consider is the exposure of plastics to UV radiation. UV lamps, such as UV-A and UV-B lamps, are commonly used in laboratory settings to simulate sunlight exposure. Prolonged exposure to UV radiation can cause photodegradation, leading to the formation of free radicals and the breakdown of chemical bonds in the plastic. This, in turn, results in a reduction in the plastic's impact strength and overall durability.
Furthermore, humidity plays a crucial role in the chemical aging of plastics. Prolonged exposure to high humidity conditions can result in hydrolysis, a chemical degradation reaction that causes an irreversible loss of mechanical properties. On the other hand, low humidity levels at elevated temperatures can also have detrimental effects, such as stiffening and embrittlement, which further accelerate the aging process.
The chemical aging process can also be influenced by the presence of heavy metals. Leaching experiments have shown that the concentrations of heavy metals like Ni, Cu, Zn, Pb, and Sb in waste plastic products increase over time. These concentrations can exceed permissible levels, posing risks to groundwater quality and the environment.
In addition to laboratory-based studies, field surveys have been conducted to understand the chemical aging of plastics in natural environments. By placing plastic items in riverine systems, researchers can observe the degradation process over time due to exposure to natural conditions such as moisture, heat, solar radiation, and microbial actions.
Overall, the chemical exposure of plastics involves a range of factors that can be manipulated to accelerate the aging process. By understanding the specific chemical and environmental conditions that contribute to degradation, researchers can develop strategies to maximize the lifespan of plastic components and address potential challenges associated with aging.
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Mechanical stress
The design of the plastic component plays a significant role in managing mechanical stress. Techniques such as rounded corners, gradual transitions, and appropriate fillets and radii help distribute mechanical stress evenly across the plastic. This even distribution minimises stress concentrations, reducing the risk of issues like cracking, warping, and structural failure.
In addition, regular maintenance and routine inspections are essential to preventing premature ageing due to mechanical stress. These inspections can identify signs of wear, degradation, or damage, enabling timely interventions to preserve the appearance and functionality of the plastic. Proper cleaning procedures and maintenance routines are crucial, including the use of appropriate lubricants and adherence to manufacturer-recommended care guidelines.
Furthermore, improper manufacturing processes can contribute to mechanical stress and ageing issues in plastic components. The manufacturing phase is critical in determining the quality and longevity of plastic parts. Choosing the wrong type of plastic for a specific application can result in premature ageing if the material lacks the necessary mechanical properties to withstand the expected stress.
To summarise, mechanical stress is a key factor in accelerating the ageing of plastics. By understanding the load conditions, employing thoughtful design strategies, conducting regular maintenance, and selecting appropriate manufacturing processes and materials, the impact of mechanical stress on plastic ageing can be effectively managed.
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Frequently asked questions
Plastic can undergo degradation due to environmental factors such as sunlight, heat, and moisture. Accelerated aging tests are often performed in climatic chambers, exposing the plastic to high levels of humidity, temperature variations, UV radiation, and other external agents.
Increasing the temperature accelerates the degradation process. For example, if the temperature is set at 55°C, an accelerated aging study for one year in real-time would take 40 days. Bumping up the temperature to 60°C would complete the same study in 29 days.
Accelerated aging tests often focus on chemical aging (oxidation) and ignore physical aging. The polymeric material is exposed to detrimental agents such as UV radiation, temperature variations, and vapor condensation at levels significantly higher than normal conditions to accelerate degradation.
