Polycarbonate (PC) is a transparent, amorphous thermoplastic polymer known for its exceptional durability. When people ask how “hard” this material is, they are often confusing two separate physical properties: impact strength and surface hardness. Impact strength, or toughness, is the ability to withstand sudden, forceful blows without fracturing. This characteristic is distinct from surface hardness, which measures resistance to scratching or abrasion. The material’s unique combination of properties makes it a preferred choice in applications where safety and performance under stress are paramount.
Understanding Polycarbonate’s Core Strength
Polycarbonate’s extraordinary resistance to fracture is rooted in its unique molecular architecture, which grants it impressive ductility. The polymer chain is constructed from Bisphenol A and carbonate linkages, forming a rigid, symmetric backbone. This structure includes large aromatic rings that are highly effective at absorbing and distributing localized stress energy across the material instead of allowing it to concentrate at a single point. This molecular flexibility prevents the formation of cracks and allows the material to yield and deform under high pressure rather than shattering. Standard grades of polycarbonate exhibit an ultimate tensile strength often reported in the range of 60 to 70 megapascals (MPa). This ability to absorb massive amounts of energy without breaking is why PC is used for demanding safety applications, such as machine guards, riot shields, and the transparent layers in bullet-resistant glazing.
Quantifying Different Measures of Hardness
The distinction between impact strength and surface hardness is quantified through different standardized testing methods. Impact resistance, or toughness, is measured using pendulum tests like the Izod or Charpy test, which strike a notched sample and record the energy required for fracture. Polycarbonate typically yields very high values in these tests, often demonstrating “no break” behavior, which confirms its superior ability to resist dynamic force. Surface hardness, conversely, is a measure of a material’s resistance to permanent indentation or scratching. This property is commonly assessed using the Rockwell Hardness scale, where general-purpose polycarbonate receives a relatively moderate rating. The low surface hardness means that while the material will not shatter, it is highly susceptible to superficial scratches and abrasion from everyday contact. For this reason, polycarbonate products like eyeglass lenses and automotive components are frequently sold with specialized hard coatings to improve their scratch resistance.
Comparison to Acrylic and Glass
Placing polycarbonate in context with common transparent materials like glass and acrylic highlights its particular strengths and weaknesses. Polycarbonate is remarkably stronger, boasting an impact resistance roughly 250 times greater than that of standard glass. This is significantly greater than acrylic, which offers about 10 to 17 times the impact resistance of glass. Both plastic alternatives are also substantially lighter than glass, typically weighing less than half of a comparably sized pane. However, acrylic is generally more scratch-resistant than an uncoated polycarbonate sheet, often preferred for applications where a flawless, high-gloss surface is necessary. While both materials offer excellent clarity, acrylic maintains slightly better light transmittance, around 92 percent compared to polycarbonate’s 88 percent.
Practical Limitations of Polycarbonate
Despite its exceptional toughness, polycarbonate possesses several durability vulnerabilities. One significant limitation is its susceptibility to degradation from ultraviolet (UV) radiation, which is present in sunlight. Unstabilized PC can undergo photodegradation, a process that causes the polymer chains to break down, leading to yellowing, a loss of light transmission, and reduced toughness over time. Another mode of failure is its sensitivity to certain chemicals, known as environmental stress cracking. Exposure to specific organic solvents, such as gasoline, aromatic hydrocarbons, or chlorinated compounds, can cause the material to weaken, haze, or develop fine cracks. While polycarbonate is heat resistant, these chemical attacks can compromise its structural integrity independently of physical impact or temperature.