Polycarbonate (PC) produced by interfacial polycondensation of bisphenol A and phosgene is an amorphous thermoplastic used in solid and multiwall roofing sheet because its visible light transmission reaches 88–90 % for 3 mm clear sheet (ASTM D1003-13), its tensile modulus is 2.3–2.4 GPa (ISO 527-2:2012), and its notched Izod impact value is 600–850 J/m at 3.2 mm (ASTM D256-10). The glass transition temperature is approximately 147 °C (ISO 11357-2:2020), so the sheet remains rigid below service temperature but is susceptible to local plasticization by sorbed organic species. A field condition described as The Polycarbonate Roof Leaked at Every Fixing: Solvent Crazing at the Screws is therefore an environmental stress cracking (ESC) failure rather than a bulk tensile failure: the stress at each drilled hole drives craze formation, while a solvent-active sealant, washer plasticizer, or cleaning residue reduces the critical strain to the imposed service strain.

The carbonate repeat unit in PC is sensitive to hydrolysis under acidic or alkaline conditions, but solvent crazing does not require chain scission. In ESC, a low-molecular-weight organic species diffuses into the stressed surface, reduces the energy barrier for cavitation, and creates a craze network of oriented fibrils and voids. The craze remains load-bearing initially but is permeable to water. Once water enters the fibrillated zone, cyclic drying and wetting, combined with thermal movement, opens a continuous leakage path at the fixing. This is why the reported failure appears at every screw rather than at random mid-span positions: the chemical and mechanical conditions are repeated at each fastener.

In a drilled fastener hole, the circular discontinuity introduces an elastic stress concentration factor of approximately 3.0 relative to the gross section; in a restrained PC panel with thermal expansion blocked, local strain can be 2.5–3.5 times the nominal panel strain. Screw preload adds radial tensile stress at the washer contact edge, while bearing stress develops on the hole wall. Under standard short-term tension, PC yields at 60–70 MPa with elongation at break of 80–150 % (ASTM D638-14). These ductile values are not available when the surface has been exposed to an aggressive environment. Instead, craze initiation occurs at strains well below yield. Published numeric ESC thresholds for specific proprietary sealant formulations are limited, but ISO 22088-2:2006 constant tensile load testing and ISO 22088-3:2006 bent strip testing consistently demonstrate that aromatic hydrocarbons and phthalate ester media reduce the critical craze strain of general-purpose PC to values below 0.4 %, compared with a craze-initiation strain in air of approximately 1.5 % at 23 °C.

Why Does Polycarbonate Sheet Craze at Fastener Holes Rather Than Yield in Bulk?

Bulk ductility and ESC resistance are controlled by different material properties. Bulk yield in PC involves shear band development at stress levels near 60–70 MPa (ASTM D638-14), while ESC craze initiation depends on surface absorption, local stress, and the solubility of the applied chemical. The same PC sheet that shows 80–150 % elongation at break in air can craze below 0.4 % strain in the presence of toluene or dibutyl phthalate. The amorphous surface layer is particularly vulnerable at drilled holes because the exposed hole wall is not protected by the UV-stabilized coextruded cap layer, which is typically 25–50 µm thick. That cap layer is designed for outdoor weathering, not for chemical isolation at a drilled edge.

Once the fastener has been torqued, the leakage path in a PC roof fixing is less controlled by the compressive load on the sealing washer than by the chemical species in contact with the exposed hole wall. Flexible PVC washers and plastisol gaskets can exude phthalate plasticizers under compressive set; these plasticizers are aggressive because they remain at the interface and have low volatility. Sulfur-cured EPDM washers may contain paraffinic processing oils that are comparatively mild in immersion but can still support ESC under bearing stress. Peroxide-cured EPDM or neutral silicone sealing elements are preferred. Alkoxy-cure neutral silicone sealants do not release acetic acid; acetoxy-cure RTV silicone liberates acetic acid that can hydrolyze the carbonate linkage, and oxime-cure silicone releases methyl ethyl ketone oxime, a ketone solvent that accelerates craze growth. Methacrylate-based threadlockers and cyanoacrylate adhesives should also be excluded from the hole boundary unless tested under ASTM D543-21 because low-viscosity methacrylate monomers can penetrate an existing craze zone and propagate it through the sheet thickness.

When Thermal Movement and Screw Preload Exceed the ESC Threshold at the Hole Wall

The linear thermal expansion coefficient of PC is 65–70 × 10⁻⁶ m/m/°C (ISO 11359-2:1999). A 3.0 m multiwall panel exposed to a 50 °C temperature rise expands by 9.8–10.5 mm. If the pre-drilled hole is equal to the shank diameter or the hole is not slotted along the movement axis, the screw becomes a bearing pin and applies concentrated bearing stress at the hole edge. This stress is superimposed on the tensile stress created by the washer preload. The result is that PC may craze in contact with an oil or sealant classified as resistant in an unstressed immersion test.

For a 5 mm stainless steel fastener tightened to 3 N·m, the simplified torque-tension relation F = T/(k × d) with a nut factor k of 0.25 gives an axial clamping force of approximately 2.4 kN. With a 16–18 mm outer diameter EPDM washer and an annular contact area of approximately 230 mm², the average contact pressure is 10–12 MPa. The local stress at the edge of the washer can be significantly higher due to the edge stress concentration. Because the allowable ESC stress in contact with an aggressive species can be below 5 MPa, even moderate tightening may exceed the threshold. This explains why increasing torque does not stop the leak; it accelerates the craze network.

On the fabrication line, hole quality has a measurable effect. Pre-drilling with a high-speed steel or carbide twist drill at 1,500–3,000 rpm is preferred over punching because punched holes generate microcracks that act as ESC initiation sites. Installation guidance for 5 mm stainless steel fasteners with EPDM sealing washers commonly specifies torque of 2.5–3.5 N·m; torque-controlled screwdrivers are recommended because impact drivers without torque control can exceed 8–10 N·m in a single pulse and can produce immediate microcrazing at the hole boundary.

On production roofing lines, batch-to-batch variation in PC molecular weight and in sealant plasticizer content can shift the critical ESC strain by approximately ±0.1 %. Laboratory failure analysis of the leaked fixings typically shows a silvery craze halo around each hole, and attenuated total reflectance FTIR of the hole wall can detect phthalate carbonyl absorption near 1725 cm⁻¹ or silicone absorption near 1260 cm⁻¹, confirming the presence of the chemical agent. Each new combination of sealant, cutting fluid, or gasket should be qualified by ISO 22088-3:2006 or ISO 4599:1986 using a drilled test coupon tightened to the same torque and hole clearance as the full roof installation. This qualification detects the failure mechanism described as The Polycarbonate Roof Leaked at Every Fixing: Solvent Crazing at the Screws before it becomes a systematic leakage path across the installed roof.