Polycarbonate (PC) is a high-performance thermoplastic polymer recognized for its transparency and exceptional impact resistance. It is a popular material for applications ranging from safety sheeting and eyewear lenses to compact discs and automotive headlamp covers. PC generally has high thermal stability compared to many conventional plastics, but this resistance depends entirely on specific temperature thresholds. Understanding these limits is necessary to ensure the material maintains its structural integrity and mechanical properties over its service life.
Defining Thermal Resistance in Plastics
The term “heat resistant” for polymers refers to more than just the melting point. Unlike metals, plastics undergo a progressive loss of structural integrity before liquefying. Two standardized measurements define a plastic’s thermal performance and resistance to deformation under heat.
The first metric is the Glass Transition Temperature (Tg), the point at which the material shifts from a rigid, glassy state to a softer, rubbery state. Below the Tg, polymer chains are frozen, providing maximum stiffness. Heating the material above the Tg allows molecular chains to move freely, drastically reducing stiffness and load-bearing capacity.
The second metric is the Heat Deflection Temperature (HDT), which measures the temperature at which a test bar deforms under a specific standardized load. The HDT indicates the maximum temperature a plastic can withstand before losing its shape under stress. Engineers use both the Tg and the HDT to determine the maximum functional temperature for a plastic component.
Specific Temperature Limits of Polycarbonate
Polycarbonate is an amorphous plastic, meaning it softens gradually over a temperature range rather than having a sharp melting point. The Glass Transition Temperature (Tg) for standard PC is approximately 147°C (297°F), which is the point where the material loses rigidity and transitions into a rubbery state.
The Heat Deflection Temperature (HDT) for polycarbonate is measured around 137°C (279°F) under a lighter load, dropping to between 128°C and 138°C (262°F and 280°F) under a heavier load. This indicates that PC begins to visibly deform under stress just below its Tg. While PC liquefies at much higher temperatures (295°C to 315°C), its functional structural integrity is compromised well before this point.
The maximum temperature for maintaining mechanical properties over an extended period is the continuous service temperature. For polycarbonate, this limit is typically set between 115°C (239°F) and 130°C (266°F). Exceeding this limit for long durations leads to gradual degradation, resulting in yellowing, hazing, and loss of strength and impact resistance.
Practical Uses and Heat Handling Guidelines
Polycarbonate’s high thermal thresholds make it suitable for demanding industrial and commercial applications. Its high Tg is utilized in manufacturing automotive headlamp lenses and electronic casings that must withstand heat generated by internal operation or environmental exposure. PC’s stability also allows its use in medical applications requiring sterilization methods involving elevated temperatures.
Consumer applications often test PC’s limits when moisture is involved. Polycarbonate can generally handle the high temperatures of a household dishwasher cycle. However, repeated exposure to hot water and harsh, alkaline detergents causes hydrolysis. This chemical reaction degrades the polymer chains over time, leading to micro-cracking, cloudiness, and a brittle appearance. For common household items, avoiding repeated exposure to high-heat, high-detergent environments is the best way to maintain the material’s clarity and structural integrity.
For microwave use, polycarbonate is generally not recommended for food contact due to chemical concerns. High temperatures, especially combined with fatty or acidic foods, can accelerate the leaching of Bisphenol A (BPA) into the food. Furthermore, the uneven heating in microwaves can create localized hot spots that exceed the material’s Tg, causing warping or degradation.