Polycarbonate is a high-performance thermoplastic polymer known for exceptional transparency and remarkable durability. It is commonly used where glass or other plastics might fail, such as safety shields, protective glazing, and roofing. While its reputation for near-indestructible impact resistance is well-deserved, determining the maximum weight it can hold requires moving beyond simple toughness metrics. The actual load capacity depends on a complex interaction between the material’s intrinsic properties and structural design.
Understanding Polycarbonate’s Inherent Strength
Polycarbonate’s ability to bear weight begins with its impressive mechanical properties, often cited in terms of ultimate strength. For an unfilled, general-purpose grade, the Tensile Strength at Yield is typically around 62.5 megapascals (MPa), or 9,065 pounds per square inch (psi). This value represents the stress polycarbonate can withstand before it begins to permanently deform.
The material’s stiffness, its resistance to elastic bending, is measured by the Modulus of Elasticity, often averaging about 2.39 Gigapascals (GPa). This stiffness rating is significantly lower than that of steel or glass, meaning polycarbonate bends more readily under a given load. However, unlike brittle materials that fracture suddenly once their stress limit is reached, polycarbonate is ductile and can absorb tremendous energy before failure. This characteristic makes it famous for its high impact resistance, being up to 250 times stronger than glass against sudden force.
Practical Measurement of Load Capacity
When considering sustained weight, or static load, the limiting factors are deflection and creep, not sudden fracture. Deflection refers to the amount a polycarbonate sheet bends when a load is applied, and it is the primary factor determining the practical load limit in design. Engineers typically use a conservative deflection criterion, such as limiting the bend to no more than L/20 (5% of the unsupported span length).
Creep is the tendency for plastics to slowly and permanently deform over time when subjected to continuous stress, even below the material’s yield strength. Because polycarbonate is viscoelastic, it will gradually stretch or sag under a static load as its molecular chains rearrange. This long-term deformation means the safe load limit for a permanent structure is substantially lower than the load it could handle momentarily.
To account for long-term effects and manufacturing variations, professional design incorporates a substantial safety factor. For non-critical applications like protective glazing, a safety factor of around 1.5 is sometimes used, meaning the material is engineered to withstand 1.5 times the expected maximum load. For public-facing or higher-risk components, this factor can be much higher, sometimes up to five times the anticipated load, ensuring the working stress remains a small fraction of the theoretical yield strength.
Design Factors That Determine Maximum Load
The maximum weight a piece of polycarbonate can hold is dramatically altered by specific design choices. The sheet’s thickness, or gauge, is one of the most influential variables, having a non-linear effect on stiffness. Doubling the thickness of a flat sheet can increase its resistance to bending by a factor of four, making thicker sheets far more efficient at supporting weight.
The distance between support points, or the span, is inversely proportional to the load capacity. For example, a 2-millimeter-thick solid sheet requires supports spaced 20 centimeters apart to manage typical loads, while a 4-millimeter sheet can handle a support spacing of 50 centimeters for the same application. Minimizing the unsupported span is the most effective way to maximize the weight a polycarbonate element can bear.
The geometry of the piece substantially increases its structural rigidity compared to a flat sheet. Corrugated sheets, multiwall panels, or sheets cold-formed into an arch or curve gain significant strength by utilizing the physics of structural shape. A curved piece of polycarbonate is less susceptible to deflection than a flat panel of the same thickness.
Operating temperature directly affects long-term load capacity. As temperature increases, the material’s molecules become more active, accelerating the creep process and lowering the yield strength. Polycarbonate maintains a high percentage of its strength up to moderate temperatures, but prolonged exposure to heat reduces its ability to support weight over time.