Crazing doesn’t show up on day one. It appears weeks after assembly, as a network of fine micro-cracks that spreads quietly through a polycarbonate part that looked flawless when it left the line — often triggered by the very sterilization cycle the device was built to survive.

Why Polycarbonate Crazes

Crazing is a network of interconnected micro-voids and polymer fibrils that can still support some load but drastically compromises optical clarity and structural integrity. In polycarbonate device components, crazing is typically triggered by a combination of mechanical stress and chemical exposure at the bond line. Three factors initiate it: monomer aggression, where liquid monomers in a standard adhesive act as mild solvents and weaken polymer chains before curing completes; polymerization shrinkage, which creates localized tensile stress that amorphous, stress-sensitive polycarbonate absorbs poorly; and exothermic heat from rapid UV cure, which causes localized expansion followed by contraction that adds further stress.

What Sterilization Cycles Do to the Bond

Device components made from polycarbonate need to survive assembly and then remain stable through whatever sterilization method the manufacturing process specifies. Steam autoclaving (typically 121–134°C) can exceed the glass transition temperature of an unsuitable adhesive, causing bond strength loss, and the associated moisture can cause hydrolytic degradation if the chemistry isn’t designed for hydrothermal stability. Gamma and E-beam radiation sterilization can cause yellowing in both the polycarbonate and the adhesive, and more critically can cause chain scission or unwanted cross-linking that makes the bond brittle. Ethylene oxide (EtO) gas sterilization operates at lower temperatures but requires excellent chemical resistance in the adhesive to prevent swelling or softening at the bond line.

Cyro-Weld™ 5000-Series: Built for This Exact Problem

Incure’s Cyro-Weld™ 5000-series is a family of UV- and visible-light-curable adhesives formulated to meet ISO 10993-5 and validated for EtO (ISO 11135) and Gamma (ISO 11137) sterilization cycles per grade. Grades like 5002F, 5004F, 5013, and 5017F cover a viscosity range suited to different bond-line geometries, from thin capillary joints to larger gap-filling applications, while maintaining the low-shrinkage cure profile that keeps stress off a polycarbonate substrate. Because these formulations cure with visible or near-UV light rather than requiring a solvent evaporation step, there’s minimal opportunity for monomer migration into the substrate — one of the primary chemical triggers of crazing.

For applications where a cyanoacrylate chemistry fits the joint design better than a light-cured system, the Cyro-Weld™ CM-series (also ISO 10993-5 compliant, with several grades meeting Mil-A-46050C) offers rapid room-temperature cure across a wide viscosity range, from CM-2 at the thin end to CM-2500 for gap-filling.

Technical Characteristics That Prevent Crazing

Low shrinkage rates — under 0.2–0.5% linear shrinkage — are essential to prevent the internal “pull” that initiates crazing on a polycarbonate surface. Viscosity should be matched to the application, generally 50 cP to 5,000 cP depending on whether the joint is a fine capillary bond or a larger lamination. A flexible modulus with elongation at break above 50% absorbs the CTE mismatch between adhesive and substrate during thermal cycling, and a glass transition temperature above the sterilization temperature keeps the adhesive in its rigid, structurally stable state throughout an autoclave cycle. If you’re evaluating which grade fits your specific sterilization protocol and joint geometry, Email Us with your process parameters.

Process Controls That Reduce Craze Risk Further

Even the right adhesive can craze if the curing process isn’t controlled. Avoid over-curing — excessive UV intensity drives a rapid exotherm, so use the minimum intensity that achieves full cure as verified by a radiometer. LED UV systems at 365nm provide a consistent, cool cure compared to traditional mercury vapor lamps, which emit significant infrared heat alongside the UV output. In some cases, pre-annealing polycarbonate parts relieves molded-in stress from the injection molding process, making the substrate less susceptible to chemical attack from the adhesive during cure.

Selecting the Right Grade: Data Points to Check

Shore hardness in the 60–80 D range is typically ideal for structural bonds, while a softer Shore A grade suits flexible gasket applications. Confirm the adhesive’s depth of cure matches your bond gap thickness, and check that polycarbonate’s surface energy (approximately 42 dynes/cm) is compatible with the adhesive’s surface tension for complete wetting — an incomplete wetting profile leaves microscopic voids that later become nucleation points for crazing.

For related reading on managing thermal-expansion stress at a bond line, see how CTE mismatch drives adhesive bond failure. For adhesive family comparisons relevant to clear substrates generally, see UV glue vs epoxy for transparent bonding.

Conclusion

Achieving a permanent, clear, craze-free bond on polycarbonate that survives repeated sterilization cycles comes down to matching adhesive chemistry to the specific stresses the assembly will face — low shrinkage, the right modulus of flexibility, and a glass transition temperature that outlasts the sterilization protocol. Cyro-Weld™ 5000-series and CM-series formulations are built around exactly these requirements, formulated to meet ISO 10993-5 and validated for EtO and Gamma sterilization cycles.

If you’re facing polycarbonate crazing issues or need help selecting the right Cyro-Weld™ grade for your sterilization requirements, Contact Our Team for substrate testing and formulation guidance.

Visit www.incurelab.com for more information.