Ethan Clarke
Plastic is a versatile material, but it has a weakness: cold temperatures. When exposed to low temperatures, plastics undergo significant changes in structure and function, becoming more brittle and prone to cracking or breaking. This is due to the molecules in the plastic slowing down and arranging themselves in a more ordered, crystalline fashion, reducing the flexibility of the material. The ductile-to-brittle transition temperature (DBTT) varies for different plastics, and manufacturers use various testing methods to assess their performance at low temperatures. Understanding the chemical composition, additives, and processing methods that influence a plastic's cold resistance is crucial for selecting suitable materials for applications in cold environments.
| Characteristics | Values |
|---|---|
| Reason | The primary reason is the crystalline structure of most plastics. |
| Effect | Plastics tend to harden, become more brittle, and are more susceptible to cracking and breaking. |
| Plastic Types | Polycarbonate (PC) is an example of a plastic that becomes brittle at very cold temperatures. |
| Testing Factors | Retraction, crystallization, brittleness, and stiffening are considered when testing plastics at low temperatures. |
| Testing Methods | Manufacturers use ultra-deep freezers to test plastics at low temperatures, observing changes in their structure and properties. |
| Factors Affecting Cold Resistance | Chemical composition, additives (e.g., plasticizers, stabilizers), processing conditions (temperature, pressure), and polymer chain flexibility. |
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What You'll Learn
Plastic's crystalline structure
The primary reason why plastics become brittle at cold temperatures is their crystalline structure. Plastics comprise various materials based on their polymer structure, and polymers are composed of long molecular chains. These chains can fold together and form ordered regions called lamellae, which then compose larger spheroidal structures called spherulites. This process is known as the crystallization of polymers and is associated with the partial alignment of molecular chains. The degree of crystallinity in polymers typically ranges between 10% and 80%, and polymers with very low crystallinity or very high crystallinity tend to be transparent.
The crystallization of polymers can occur through cooling from melting, mechanical stretching, or solvent evaporation. During the cooling process, the molecules in plastics slow down and arrange themselves in a more ordered, crystalline fashion, which makes the plastic less flexible and more prone to cracking. This ordered arrangement is characteristic of the crystalline structure of plastics, which gives them strength, stiffness, and resistance to abrasion, heat, chemicals, creep, and fatigue.
However, not all plastics behave the same way when exposed to cold temperatures. Plastics with flexible polymer chains are less likely to become brittle. Additionally, the processing conditions, such as temperature and pressure, can influence the crystalline structure and, consequently, the cold resistance of the plastic. The presence of additives, such as plasticizers, stabilizers, and impact modifiers, can also enhance the cold resistance of plastics.
The ductile-to-brittle transition temperature (DBTT) varies depending on the specific plastic. For example, polycarbonate (PC), which is typically tough at normal temperatures, becomes brittle at extremely low temperatures (-40°F) and can shatter like glass upon high-speed impact.
In summary, the crystalline structure of plastics plays a crucial role in determining their flexibility and susceptibility to cracking at cold temperatures. While some plastics become brittle, others are specifically designed or processed to retain their flexibility even in freezing conditions, making them valuable for applications in industries such as aerospace and automotive.
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Polymer chains
The flexibility of plastic is closely linked to its molecular structure. Plastics are polymers, which are large molecules composed of many smaller, repeating subunits. These subunits form long chains, which can be flexible or rigid, depending on their structure and the forces that hold them together.
At room temperature, typical plastics are semi-flexible and can withstand some stress without failing. However, as temperatures drop, plastics tend to harden and become more brittle. This is because the molecules in the polymer chains slow down and arrange themselves into a more ordered, crystalline structure. The plastic's flexibility decreases, and it becomes more susceptible to cracking or breaking, especially under strain.
Not all plastics behave the same way when exposed to low temperatures. The chemical composition, additives, and processing methods can all influence a plastic's cold resistance. Plastics with flexible polymer chains are less likely to become brittle in the cold. Additionally, additives such as plasticizers, stabilizers, and impact modifiers can enhance a plastic's ability to withstand low temperatures without becoming brittle.
The ductile-to-brittle transition temperature (DBTT) is a critical factor in understanding a plastic's behaviour at low temperatures. Each plastic has a specific DBTT at which it transitions from being ductile to brittle and prone to shattering upon high-speed impact. As plastic parts age, the DBTT temperature generally increases. Therefore, manufacturers must test plastics at low temperatures to ensure they can withstand the desired temperature range without failing.
Understanding the behaviour of plastics at low temperatures is essential, especially as plastics are increasingly used in modern manufacturing and construction. By selecting the appropriate plastic with the right polymer chains and additives, manufacturers can ensure that products remain functional and durable even in freezing conditions.
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Additives
While plastics are incredibly versatile, they are vulnerable to cold temperatures, which can cause them to become brittle and prone to cracking or breaking. This occurs due to the impact of low temperatures on the molecular structure of plastics, leading to a loss of elasticity and flexibility. However, not all plastics are equally susceptible to the cold, and certain additives can enhance their cold resistance.
Another example of a plastic additive is found in Ultra-high-molecular-weight polyethylene (UHMW), a high-density material valued for its resistance to friction. Certain high-performance variants of UHMW, such as TIVAR 88, utilise additives to enhance their basic performance. TIVAR 88 is specifically formulated with additives that improve the performance of its resin, resulting in superior cold resistance. This plastic can tolerate temperatures as low as -200 °C (-328 °F) without losing its integrity. Its ability to withstand extreme cold makes it an ideal choice for applications like snowplow blades, where it minimises damage and reduces noise during snow removal operations.
Polyethylene (PE) is another plastic that exhibits good cold resistance, particularly Low-Density Polyethylene (LDPE) and High-Density Polyethylene (HDPE). LDPE is commonly used in outdoor applications, such as pipes, cables, and plastic bags, even in cold regions. The specific additives used in LDPE contribute to its ability to maintain flexibility and durability in low-temperature environments. Similarly, Polypropylene (PP) is known for retaining its flexibility in cold temperatures, making it suitable for cold-weather clothing, packaging, and automotive components. The additives in PP play a vital role in enhancing its cold resistance, ensuring that it remains supple and sturdy even when exposed to frigid conditions.
In summary, additives are essential in enhancing the cold resistance of plastics, allowing them to remain flexible and durable even in freezing temperatures. By understanding the chemical composition and the role of additives, we can select the right materials for applications in cold environments. This knowledge is particularly valuable in industries and regions that experience consistently low temperatures, ensuring the reliability and longevity of products and infrastructure.
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Processing conditions
The temperature and pressure applied during the processing of plastics can affect their crystalline structure. At low temperatures, the molecules in plastics slow down and arrange themselves in a more ordered, crystalline structure, which makes the plastic less flexible and more prone to cracking. This phenomenon is known as the "glass transition" by chemists. Therefore, the processing conditions that impact the crystalline structure of plastics are crucial in determining their flexibility and susceptibility to breakage at cold temperatures.
Additionally, the processing conditions can also influence the plastic's thermal expansion rate, thermal conductivity, and wear rate at low temperatures. The thermal expansion rate refers to how much a plastic changes in density and size when exposed to temperature changes. Plastics with a low thermal expansion rate are often preferred in applications as they undergo minimal changes in size and density. Proper thermal conductivity is essential when the plastic needs to facilitate or prevent heat transfer. The wear rate of plastics, or how their surface behaviour changes at low temperatures, is another critical factor for manufacturers to consider when selecting materials for thermal insulators or cryogenic applications.
Manufacturers can test plastics for cold temperature resistance by subjecting them to ultra-deep freezing and then evaluating specific factors. These tests include retraction, crystallization, brittleness, and stiffening (torsion) tests. By understanding the processing conditions and conducting thorough tests, manufacturers can select the most suitable plastics for applications in cold environments, ensuring they remain flexible and robust instead of becoming brittle.
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Testing methods
Retraction
This test measures how much a particular plastic retracts at a specific temperature.
Crystallization
This is a measure of the increase in hardness of the plastic after it has been stored at a particular temperature.
Brittleness
This test will determine the lowest possible temperature the plastic can withstand without becoming so brittle that it fails. Every plastic has a ductile-to-brittle transition temperature (DBTT) at which the plastic becomes brittle and shatters upon high-speed impact. The DBTT varies depending on the plastic.
Stiffening
Stiffening testing, also known as torsion testing, measures how stiff a plastic is compared to its temperature.
Ductile-to-Brittle Transition Temperature (DBTT)
The ductile-to-brittle transition temperature is a critical parameter to consider when testing plastic's resistance to cold temperatures. The ductility of polymers depends on how much impact energy they can absorb before breaking, a property known as toughness. As the temperature decreases, a material becomes less ductile until it becomes brittle. This process is known as ductile-to-brittle transition. When a material reaches its brittle state, it loses most of its impact strength, meaning the chances of catastrophic failure increase exponentially.
There are two common methods for measuring DBTT: the puncture test method and the tensile impact test procedure. The puncture test method is the simplest way of measuring the ductile-to-brittle transition temperature. The process involves a striker hitting a disc or square-shaped specimen perpendicularly with a specific impact energy. Thanks to a specific load cell, it’s possible to determine the absorbed energy of the material during the impact event.
Testing at low temperatures can be challenging. Typically, when conducting a puncture or tensile test, samples need to be cooled in an external cryostat before testing. This process has several disadvantages:
- Downtime – Time is lost placing the sample in a cryostat, waiting for it to cool, and then loading it onto the impact tester.
- Accuracy – By the time the sample is taken out of the cryostat and loaded onto the tester, its temperature may vary, impacting the accuracy of the test results.
- Cost – Cryostats are expensive and entail energy and maintenance costs.
A more efficient way of testing at low temperatures is using a drop weight impact testing machine, which can come with an integrated thermostatic chamber. With a temperature range between -70 °C and 150 °C, advanced drop weight machines can test plastics under most environmental conditions.
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Frequently asked questions
Plastics undergo significant changes in structure and function when exposed to low temperatures. At low temperatures, the molecules in these materials slow down and arrange themselves in a more ordered, crystalline fashion. This structural change makes the plastic less flexible and more prone to cracking or breaking.
Every plastic has a ductile-to-brittle transition temperature (DBTT) at which the plastic becomes brittle and shatters upon high-speed impact. The DBTT varies depending on the plastic.
Manufacturers often use ultra-deep freezes to test plastics at low temperatures. After the plastic reaches the desired temperature, it is tested to see how its structure and properties change. Specific factors tested include retraction, crystallization, brittleness, and stiffening.
Polycarbonate (PC) is an example of a plastic that is extremely tough at normal temperatures but becomes brittle and can shatter like glass at very cold temperatures (-40°F).
Yes, some plastics remain flexible and robust even in freezing conditions due to factors such as their chemical composition, additives, processing methods, and flexible polymer chains. These cold-resistant plastics are invaluable in industries such as aerospace and automotive, where they are used in applications exposed to extreme cold temperatures.
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