The observation that materials change size with temperature is a fundamental principle of physics. Plastic, a material made up of long-chain molecules called polymers, responds to changes in thermal energy just like metal or glass. When exposed to cold, a plastic object decreases in size, a phenomenon known as thermal contraction. This size change results from how the material’s internal structure reacts to a loss of heat.
The Physics of Thermal Contraction
Plastic contracts when exposed to cold temperatures, following the physical law of thermal contraction. This principle describes the decrease in volume or length of a substance as its temperature drops. The extent of this change is quantified by the Coefficient of Thermal Expansion (CTE).
The CTE measures the fractional change in size per degree of temperature change. For most plastics, this value is high, often ranging from 50 to 250 micrometers per meter per degree Celsius. This high CTE means plastics expand and contract significantly more than traditional materials like metals, which typically have CTE values 3 to 10 times lower. This characteristic is important for engineers designing parts that must interface with other materials in varying temperature environments.
How Temperature Affects Polymer Chains
The mechanism behind macroscopic contraction occurs at the molecular level, involving the polymer chains themselves. Polymers are composed of long chains of repeating molecular units, and their movement reflects the thermal energy, or kinetic energy, present in the material.
When plastic is cooled, the polymer chains lose kinetic energy, causing their vibrational and rotational movement to slow down. As movement lessens, the chains require less space and move closer together. This reduction in the average distance between molecular chains causes the overall volume of the material to decrease, resulting in observable contraction and an increase in density.
Structural Differences in Contraction
The specific structure of the polymer chains dictates how much and how uniformly a plastic contracts. Plastics are categorized as either amorphous or crystalline, and their internal arrangement leads to distinct contraction behaviors. Amorphous plastics, such as polystyrene, have a random arrangement of chains that results in more uniform contraction across all dimensions. These materials exhibit the glass transition temperature (\(T_g\)), below which the contraction rate changes significantly as the material becomes rigid and glass-like.
Crystalline or semi-crystalline plastics, like high-density polyethylene (HDPE), contain regions where the polymer chains are highly ordered and packed into a lattice structure. These ordered regions cause a higher overall amount of shrinkage compared to amorphous types. Furthermore, the ordered structure can lead to anisotropic contraction, meaning the material shrinks at different rates along different axes. This directional difference is often influenced by the manufacturing process.
Practical Effects of Cold Stress on Plastic
The dimensional changes caused by cold temperatures have several practical implications for plastic components. Rapid cooling or exposure to extreme cold can lead to internal stresses, especially when the material is constrained or joined to a material with a lower CTE, such as metal. This differential contraction can generate stress concentrations that may result in premature part failure.
A common effect is the loss of flexibility and the onset of brittleness when the temperature drops below the material’s glass transition temperature (\(T_g\)). Below the \(T_g\), the polymer chains become locked in a rigid, glass-like state. This change means the plastic can no longer absorb impact energy effectively and is more likely to fracture or crack when subjected to a sudden force. For example, common plastic items like food storage containers can shatter easily when dropped after being taken from a freezer.