Polycarbonate (PC) in practice: what determines the durability of a transparent sheet or component?

A transparent roofing sheet, a machine guard and a visor may look similar, but the material requirements are very different. The letters PC in a specification are not enough to select the correct polycarbonate for an application.

Polycarbonate is one of the most important transparent engineering thermoplastics. It combines high impact resistance, good stiffness and very good light transmission. This is why it is used in construction, safety guards, automotive applications, electrical and electronic parts, lighting and many technical components.

Selection begins with service conditions

For roofing, the designer must consider loading, sheet geometry, support, thermal expansion and years of UV exposure. A machine guard adds requirements for impact performance, optical quality, fixing and contact with cleaning agents. An injection-moulded component also introduces geometry, wall thickness, residual stress and processing history.

This is why two products both described as PC can behave differently in service. Molecular weight, flow behaviour, additive package, UV stabilisation, colour, processing aids, hydrolysis resistance and surface requirements may all differ. Material selection should therefore use data for the actual commercial grade rather than generic values for the entire polycarbonate family.

Transparency is not accidental

Conventional bisphenol-A polycarbonate is amorphous. Its macromolecules do not form extensive ordered crystalline regions that would strongly scatter visible light. Clear PC can therefore provide very high light transmission.

For selected clear Makrolon grades, Covestro reports about 89% luminous transmittance at 1-2 mm, 88% at 3 mm and 87% at 4 mm. These figures should not be treated as universal values for every PC. Transmission depends on grade, thickness, colour, additives, surface quality and processing history.

High impact strength does not solve every problem

One of the best-known advantages of PC is its high resistance to impact. Impact strength should not, however, be confused with scratch resistance. Polycarbonate surfaces scratch more readily than glass. Applications that must maintain optical quality over long service periods may therefore use hard-coat systems or other protective surface technologies.

Chemical compatibility is a separate issue. Certain solvents, fuels, cleaning agents and other substances can initiate environmental stress cracking. The risk increases when a component contains residual stress from injection moulding, extrusion, bending, machining or assembly. A cleaner that is suitable for glass is not automatically suitable for a PC machine guard.

UV and outdoor applications

UV radiation can cause yellowing, increased haze, surface changes and loss of mechanical properties in unprotected PC. Products intended for outdoor use therefore rely on suitable stabilisation, coextruded protective layers or coatings containing UV absorbers. This is another reason why any transparent PC sheet should not automatically be specified for long-term exterior service.

Drying is part of the process, not an optional extra

PC is hygroscopic, which means that it absorbs moisture from its surroundings. The pellets must therefore be dried correctly before processing. If too much moisture is present, high processing temperatures can cause hydrolysis and break bonds in the polymer chains. The chains become shorter, molecular weight falls and the material can lose part of its mechanical performance, including impact strength. For many Makrolon grades, Covestro specifies a maximum moisture content of 0.02%, but drying requirements must always be checked for the specific PC grade.

Hygroscopic does not mean that a finished PC sheet absorbs water like a sponge. In processing, the main issue is moisture absorbed by the material, particularly the pellets, and the need to remove it before melting.

What does BPA-PC mean?

The most common industrial polycarbonate is based on bisphenol A. Its repeating unit can be represented as [–O–C₆H₄–C(CH₃)₂–C₆H₄–O–C(=O)–]ₙ, corresponding to the empirical repeating-unit formula (C₁₆H₁₄O₃)ₙ. The characteristic –O–C(=O)–O– linkage is the carbonate group from which this polymer family takes its name.

For processors, the practical conclusion matters more than memorising the formula: final component performance results from polymer chemistry, the selected commercial grade, raw-material preparation, processing conditions, component design and the service environment.

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