Chloe Zhang
Plastics are versatile materials with numerous applications, from flying and boating to construction. However, they have a notorious weakness: cold temperatures. When exposed to low temperatures, plastics undergo significant structural and functional changes, becoming brittle and prone to cracking or breaking. This transformation occurs due to the crystalline structure of most plastics. As temperatures drop, the molecules in these materials slow down and arrange themselves in a more ordered, crystalline fashion, reducing flexibility and increasing susceptibility to cracking. The ductile-to-brittle transition temperature (DBTT) varies for different plastics, and as plastic parts age, the DBTT temperature typically increases. Understanding the chemical composition, additives, and processing methods that influence a plastic's cold resistance is crucial for selecting suitable materials for cold-weather applications.
| Characteristics | Values |
|---|---|
| Temperature | The "glass transition temperature" (Tg) is the point at which an amorphous solid goes from being ductile to brittle. Plastics tend to reach their Tg at everyday temperatures and can be "frozen" into brittleness. |
| Molecular Movement | At low temperatures, plastic molecules slow down and arrange themselves in a more ordered, crystalline fashion, making the plastic less flexible and more prone to cracking. |
| Strength | The strength of plastics decreases at cold temperatures, making them more susceptible to fracture or breakage. |
| Plastic Type | The ductile-to-brittle transition temperature (DBTT) varies depending on the type of plastic. Some plastics, such as polycarbonate (PC), become extremely brittle at very cold temperatures (-40°F) and can shatter like glass. |
| Age | As plastic parts age, their DBTT temperature increases. |
| Flexibility | Plastics with flexible polymer chains are less likely to become brittle in the cold. |
| Additives | Additives like plasticizers, stabilizers, and impact modifiers can enhance the cold resistance of plastics. |
| Processing Conditions | The way a plastic is processed, including temperature and pressure, can affect its crystalline structure and cold resistance. |
| Applications | Cold-resistant plastics are essential in industries such as aerospace and automotive, where components must endure extreme cold temperatures. |
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What You'll Learn
Plastic molecules slow down and become ordered
Plastics are versatile materials with numerous applications, from flying and boating to construction. However, they have a notorious weakness: cold temperatures. When the temperature drops, many plastics become brittle and prone to cracking or breaking. This occurs due to a molecular-level transformation.
At room temperature, typical plastics are semi-flexible and can withstand stress without failing. But as temperatures fall, the molecules in these materials slow down. This reduction in molecular motion is a critical factor in the loss of ductility in plastics. With decreased mobility, the molecules find it harder to slip and slide past each other.
As the plastic molecules slow down, they also become more ordered and crystalline. This change in structure makes the plastic less flexible. The molecules, now more constrained, cannot stretch as easily, and stress becomes concentrated in small areas. If the concentration becomes too great, the material fails, leading to cracks that can propagate into fractures.
The transition temperature at which a plastic becomes brittle varies depending on the specific plastic and its age. This is known as the ductile-to-brittle transition temperature (DBTT). For example, polycarbonate (PC), a tough plastic used in bulletproof glass, becomes brittle at extremely low temperatures (-40°F) and can shatter like glass when struck at high-impact speeds.
Understanding the DBTT of different plastics is essential for manufacturers when selecting materials for specific applications. Testing plastics at ultra-low temperatures helps determine their performance characteristics, including retraction, crystallization, stiffening, and, of course, brittleness.
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Glass transition temperature (Tg)
Tg is usually applicable to wholly or partially amorphous plastics. Amorphous polymers have random or disordered chain structures. Below Tg, they are hard and brittle, and as heat is applied, they gradually begin to soften and become rubbery. This transition is the glass transition. Above Tg, plastics behave like rubbery materials, and below Tg, the molecules in plastics have relatively little mobility. The higher the temperature, the more rapidly the structure corresponding to equilibrium is achieved.
The glass transition temperature, often called Tg or "T sub g", is a significant factor in the molecules' ability to slip and slide. It is always lower than the melting temperature (Tm) of the crystalline state of the material. The Tg values for polystyrene and poly(methyl methacrylate) are around 100 °C (212 °F). Rubber elastomers like polyisoprene and polyisobutylene are used above their Tg, in the rubbery state, where they are soft and flexible.
Techniques such as dynamic mechanical analysis can be used to measure the glass transition temperature. The definition of the glass transition is not universally agreed upon, and many definitions have been proposed over the years. Glass is believed to exist in a kinetically locked state, and its entropy, density, etc., depend on its thermal history. Thus, the glass transition is primarily a dynamic phenomenon.
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Ductile to brittle transition temperature (DBTT) varies
The ductile to brittle transition temperature (DBTT) is a critical parameter when measuring the impact strength of plastics. It identifies the temperature point at which a material transitions from ductile to brittle. The DBTT varies depending on the type of plastic. For example, polycarbonate (PC) is a tough plastic at normal temperatures and is sometimes used as "bullet-proof glass". However, at very cold temperatures (-40°F), PC becomes brittle and can shatter like glass when struck with high-speed impact. Generally, as plastic parts age, the DBTT temperature increases.
The DBTT is not a specific temperature but a range of about 10°C. The transition occurs because a decrease in temperature leads to reduced molecular mobility and flexibility, making the plastic more susceptible to cracking. This is especially true for amorphous polymers near their glass transition temperature (Tg). The glass transition temperature is the point at which an amorphous solid, such as glass or polymers, transitions from ductile to brittle.
The DBTT is crucial for applications exposed to low temperatures, as plastics that perform well at room temperature may become dangerously brittle in cold environments. For instance, plastic components in cars during winter or planes at high altitudes routinely experience subzero conditions. Therefore, the materials used in these applications must remain ductile at low temperatures to ensure safety during impact.
The ductility of a plastic depends on how much impact energy it can absorb before breaking. As the temperature decreases, a material becomes less ductile and more brittle, losing its impact strength. This transition can be measured through testing methods such as the puncture test or tensile impact test, which involve cooling samples to the desired temperature and then analyzing their ductile-brittle transition.
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Testing methods for cold resistance
Plastics undergo significant structural and functional changes when exposed to low temperatures. Therefore, manufacturers must employ appropriate testing methods to select the right plastic product for a particular application. Testing methods for cold resistance include:
ASTM D 1790, Standard Test Method for Brittleness Temperature of Plastic Sheeting by Impact
This test method determines the temperature at which plastic sheeting 1.00 mm (0.040 in.) or less in thickness exhibits brittle failure under specified impact conditions. The test is commonly used in PVC geomembrane testing and involves exposing the specimen to low temperatures, creasing it, and applying pressure using a roller. The specimen is then inspected for cracks or flaking.
Vicat Softening Point and Heat Distortion Temperature (HDT)
The Vicat softening point test determines the temperature at which a 1 mm² probe penetrates a plastic specimen to a depth of 1 mm under a specified load. The HDT test is similar but is only suitable for rigid plastics. These tests assess the thermal endurance and dimensional stability of plastic materials.
Impact Testing
Impact testing measures the energy absorbed by a solid plastic during fracture and provides information on its toughness. This is typically performed by hitting the specimen with a hammer (Charpy or Izod impact tests). For plastic films, a free-falling dart method is used.
Tensile Testing
Tensile testing determines how plastics perform under controlled tension. A standardized specimen is placed in a testing machine and pulled at a constant rate until it breaks or reaches maximum extension. The displacement and applied force are measured to determine tensile strength, yield point, and other properties.
Flexural Testing
Flexural testing determines the force required to bend and break a plastic material. It is similar to tensile testing but does not require a specific specimen shape. The three-point bending method is commonly used, and the test can be performed according to the ISO 178 standard.
Compression Testing
Compression testing involves subjecting a standardized specimen or finished plastic product to a compression load. The displacement and applied load are calculated to determine compressive properties such as compression strength and modulus. This test is applicable to rigid and semirigid plastics following the ISO 604 standard.
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Applications of cold-resistant plastics
While many plastics become brittle and breakable in cold temperatures, some plastics are specifically designed to withstand freezing conditions. These cold-resistant plastics are used in a variety of applications and industries where low temperatures are a constant challenge. Here are some key applications of cold-resistant plastics:
- Aerospace and Aviation: In the aerospace industry, certain aircraft components must endure extreme cold temperatures at high altitudes. Cold-resistant plastics are used in these applications to maintain their flexibility and integrity.
- Automotive: Cold-resistant plastics are essential for vehicles operating in cold climates, such as snowmobiles and winter tires. These plastics ensure that the vehicles can withstand low temperatures without cracking or breaking.
- Construction: In regions with cold winters, construction materials like pipes, insulation, and roofing often incorporate cold-resistant plastics. These plastics help prevent damage caused by freezing temperatures.
- Outdoor Gear and Clothing: Cold-resistant plastics are used in the manufacturing of winter sports equipment, outdoor gear, and clothing designed for cold-weather activities. This ensures that the products can withstand freezing conditions and provide durability during outdoor adventures.
- Electrical Insulation: Cold-resistant plastics are used as electrical insulators in various applications, including electronics and electrical engineering.
- Seals and Coatings: Seals and non-stick coatings made from cold-resistant plastics are valuable in industries where exposure to low temperatures is common, such as food processing or refrigeration.
- Mechanical Components: Cold-resistant plastics are used in mechanical parts such as bushings and bearings, especially in applications where low temperatures are a factor.
- Medical and Pharmaceutical Industries: Polytetrafluoroethylene (PTFE), an advanced fluoropolymer, is used in the medical and pharmaceutical sectors due to its exceptional chemical and thermal resistance. It can withstand extremely low temperatures, making it ideal for specific medical applications.
- Food Industry: PTFE is also used in the food industry for its ability to withstand low temperatures and its non-stick properties.
- High-Performance Parts: Polyetherimide, a high-performance polymer, is used in the manufacture of precision parts requiring high-performance machining. It is resistant to hydrolysis and can be used at temperatures as low as -50 °C (-58 °F).
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Frequently asked questions
Plastics tend to become brittle in cold temperatures due to a change in their structure and function. At low temperatures, the molecules in plastics slow down and arrange themselves in a more ordered, crystalline fashion, making the plastic less flexible and more prone to cracking or breaking.
Manufacturers typically use ultra-deep freezers to test plastics at low temperatures. After the plastic reaches the desired temperature, it is then tested to observe how its structure and properties change. Some of the specific factors tested include retraction, crystallization, brittleness, and stiffening.
Yes, certain plastics are known for their exceptional cold resistance and remain flexible even in freezing conditions. Plastics with flexible polymer chains are less likely to become brittle in the cold. Additionally, some plastics are modified with additives such as plasticizers, stabilizers, and impact modifiers to enhance their cold resistance.
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