Ethan Clarke
The temperature at which plastic becomes brittle depends on its type. Polycarbonate (PC), for example, is a durable plastic that is used for bullet-proof glass. However, at extremely low temperatures (-40°F or -20°C), it becomes brittle and can shatter like glass. Every plastic has a ductile-to-brittle transition temperature (DBTT) at which it can shatter upon high-speed impact. Generally, as plastic ages, its DBTT increases. Impact toughness vs temperature typically forms an S-shaped curve, with a range of temperatures identified as the ductile-to-brittle transition.
| Characteristics | Values |
|---|---|
| Glass transition temperature (Tg) | The point at which an amorphous solid becomes brittle |
| Ductile to brittle transition temperature (DBTT) | Varies depending on the plastic; polycarbonate (PC) becomes brittle at -40°F |
| Impact toughness | Non-linear loss at low temperatures; plastics become more brittle and break more easily |
| Operating temperature | Running RC trucks at temperatures below 0°C to -20°C can cause a loss of impact toughness without embrittlement |
| Freezing | Not recommended for plastic as it contains water |
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What You'll Learn
- Polycarbonate (PC) plastic becomes brittle at -40°F
- Plastic has a ductile-to-brittle transition temperature (DBTT)
- Glass transition temperature (Tg) is the point where an amorphous solid becomes brittle
- Impact toughness vs temperature is an S-shaped curve
- Plastic deformation occurs when plastic fails above ductile-to-brittle transition temperature
Polycarbonate (PC) plastic becomes brittle at -40°F
The ductility of plastics is dependent on temperature. Polycarbonate (PC) plastic is a prime example of this phenomenon. PC is an amorphous (non-crystalline) plastic with long chain molecules that have "free volume" between them. This free volume allows the molecules to slip and slide past each other, giving the plastic its ductility and toughness. However, at very low temperatures, PC's behaviour changes drastically.
Polycarbonate (PC) plastic is known for its exceptional toughness and impact resistance at normal use temperatures. It is often used as "bullet-proof glass" due to its ability to withstand high-speed impacts. However, this toughness is greatly diminished at extremely cold temperatures. Specifically, at temperatures of -40°F and below, PC undergoes a transformation and becomes brittle.
When PC is subjected to such low temperatures, its ductile nature is lost, and it behaves more like regular glass. If struck at high impact speed, it will shatter and fracture, losing the flexibility that characterises it at higher temperatures. This transition from ductile to brittle is not unique to PC but is a property of all plastics. Every plastic has a ductile-to-brittle transition temperature (DBTT) below which the plastic becomes brittle and shatters upon high-speed impact.
The DBTT varies depending on the specific plastic. In the case of PC, its DBTT is approximately -40°F. This value is not absolute, however, and the DBTT can shift over time. As plastic parts age, the DBTT tends to increase, meaning older PC may remain ductile at slightly lower temperatures. Additionally, the amorphous nature of PC, while contributing to its toughness, also results in poor chemical resistance. PC can easily absorb hydrocarbon chemicals, leading to embrittlement and failure through a process known as Environmental Stress Cracking (ESC).
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Plastic has a ductile-to-brittle transition temperature (DBTT)
For example, polycarbonate (PC), a tough plastic used in bulletproof glass, becomes brittle at extremely low temperatures of around -40 degrees Fahrenheit (-40°F) and will shatter like glass when struck with high-impact force. This temperature is significantly lower than typical operating temperatures, so it is important to be aware of the potential for embrittlement in extremely cold conditions.
The impact toughness of plastic varies non-linearly with temperature, forming an S-shaped curve on a graph. As the temperature decreases, the toughness decreases non-linearly, and the rate of toughness loss accelerates. For instance, nylon composite plastics used in R/C vehicles can operate at temperatures below 0°C to -20°C without technically becoming brittle. However, their impact toughness decreases significantly, making them more susceptible to breaking upon impact or landing from jumps.
While freezing temperatures can be detrimental to plastics, it is important to note that the transition from ductile to brittle is gradual and depends on various factors. Users of R/C vehicles have reported operating their vehicles at temperatures as low as -20°C without issues, provided they were stored in warm environments beforehand and not subjected to extended periods of cold exposure or high-impact forces.
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Glass transition temperature (Tg) is the point where an amorphous solid becomes brittle
Glass transition temperature (Tg) is the point at which an amorphous solid becomes brittle. It is the temperature at which a polymer transitions from a ductile material to a hard, brittle material. At Tg, amorphous polymers take on glass-like characteristics such as brittleness, stiffness, and rigidity. This transition occurs due to changes in the mobility of molecular chains within the polymer.
Amorphous polymers have a random molecular structure, in contrast to the highly ordered structure of crystalline polymers. As the temperature decreases, the chains in the amorphous regions of the polymer slow down and become more restricted in their movement. Below Tg, the chain motion freezes, and the material behaves like a rigid amorphous solid or glass. The glass transition temperature is always lower than the melting temperature (Tm) of the crystalline state of the material.
The glass transition temperature is influenced by various factors, including the structure and composition of the polymer, as well as external conditions such as temperature change rate, pressure, humidity, and moisture level. Techniques such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA) are commonly used to measure and determine the glass transition temperature of polymers.
The glass transition temperature is an essential property in the handling and processing of polymers. It impacts characteristics such as hardness, elasticity, solubility, and reproducibility in the dissolution of solids. By understanding and controlling the glass transition temperature, manufacturers can improve the performance and quality of polymer-based products, such as plastics, rubbers, and packaging materials.
Additionally, the glass transition temperature plays a crucial role in the design and selection of materials for specific applications. For example, in the oil and gas industry, epoxy coatings with specific Tg values are chosen for pipeline protection to ensure efficient corrosion protection at higher temperatures. Similarly, in biomedical implants, the supercooled liquid region associated with Tg gives metallic glasses the desired forming capabilities and performance characteristics.
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Impact toughness vs temperature is an S-shaped curve
The impact toughness of a material is determined by measuring the energy absorbed in the fracture of a specimen. Toughness is significantly influenced by temperature, and a Charpy or Izod test is frequently repeated with each specimen tested at a different temperature. The Charpy impact test is a common method to measure the toughness of a material, or its ability to resist fracture under impact loading. It involves striking a notched specimen with a pendulum hammer and measuring the energy absorbed by the specimen during the fracture. The test is useful for comparing different materials or conditions, and for evaluating the effect of temperature on the material's behaviour.
The transition curve is a plot of the energy data against temperature, which shows how the toughness of the material changes with temperature. The shape of the curve depends on the material and the notch type, but it typically has three regions: a lower shelf, an upper shelf, and a transition region. The lower shelf corresponds to low temperatures and low energies, where the material behaves in a brittle manner and fractures with little plastic deformation. The upper shelf corresponds to high temperatures and high energies, where the material behaves in a ductile manner and fractures with significant plastic deformation. The transition region corresponds to intermediate temperatures and energies, where the material exhibits a mixed mode of fracture and its toughness varies rapidly with temperature.
The ductile-brittle transition temperature for any given sample of steel is traditionally measured by breaking a notched bar in a pendulum-type impact tester and measuring the energy absorbed in the fracture. Tests are carried out over a range of temperatures, and the plot of absorbed energy against temperature gives an S-shaped curve. The S-shaped curve has a lower horizontal portion (the lower shelf), a steep line, and an upper horizontal portion (the upper shelf). The transition temperature range is defined by the vertical line.
The fracture surfaces of the specimens can provide additional information about the material's behaviour and the fracture mechanism. Fibrous fracture surfaces indicate ductile fracture and have a rough and irregular appearance, with signs of plastic deformation. Granular fracture surfaces indicate brittle fracture and have a smooth and shiny appearance, with signs of cleavage or intergranular cracking. Mixed fracture surfaces indicate a mixed-mode fracture and have a combination of fibrous and granular areas, depending on the temperature and stress state.
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Plastic deformation occurs when plastic fails above ductile-to-brittle transition temperature
Plastic deformation occurs when plastic fails above its ductile-to-brittle transition temperature (DBTT). This temperature varies depending on the type of plastic. For example, polycarbonate (PC), a tough plastic used in bulletproof glass, becomes brittle at extremely cold temperatures of -40 degrees Fahrenheit and can shatter like glass when struck with high-impact force.
The DBTT is the temperature at which a plastic material transitions from a ductile state, where it can undergo plastic deformation, to a brittle state, where it shatters upon high-speed impact. As plastic parts age, their DBTT tends to increase.
The relationship between impact toughness and temperature can be visualized as an S-shaped curve on a graph, with the ductile-to-brittle transition range highlighted. Above this range, the plastic will yield under stress and undergo some degree of plastic deformation before failing. Below this range, it will shatter without absorbing much energy.
For certain nylon composite plastics, operating temperatures below 0 degrees Celsius to -20 degrees Celsius may not technically cause embrittlement, but the impact toughness decreases non-linearly as temperatures drop. This means that the plastic becomes more susceptible to breaking when subjected to impacts or landings from jumps.
It is worth noting that plastic contains water, and freezing temperatures can be detrimental to its integrity. While it may be challenging to determine precise temperature thresholds, some sources suggest that operating RC vehicles below 50 degrees Fahrenheit increases the risk of breaking plastic components.
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Frequently asked questions
The ductile-to-brittle transition temperature (DBTT) varies depending on the type of plastic. For example, polycarbonate (PC) becomes brittle at -40°F and will shatter like glass if struck with high-speed impact.
DBT is the temperature at which a material transitions from ductile to brittle. Below this temperature, the material will shatter when impacted, while above it, the material will yield and undergo plastic deformation before failing.
The strength of plastics generally decreases at lower temperatures. Plastics become more brittle and break more easily when exposed to colder temperatures.
Tg is the temperature at which an amorphous solid, such as plastic, transitions from being ductile to brittle. Below Tg, the molecules in the plastic are restricted in their movement, leading to a loss of ductility and an increased tendency for brittle fracture.
While plastics used in RC devices may not technically become brittle at temperatures around 0°C to -20°C, they can still break more easily when exposed to such low temperatures. It is recommended to avoid high jumps or hard impacts to prevent breakage.
