Ensuring Composite Wrap Repairs Are Fully Cured: Key Considerations for Technicians
September 9, 2024
Composite wrap pipe repair is a widely used technique for restoring the structural integrity of damaged or corroded pipework and pipelines, particularly in industries such as petroleum, petrochemicals, and natural gas. One of the most critical stages in this process is ensuring that the composite repair is fully cured. According to ISO 24817 and ASME PCC-2, proper curing is essential for the repair's performance, as it directly affects the strength and durability of the restored pipeline. This article discusses the key steps technicians must take to ensure a fully cured composite wrap pipe repair.
The Importance of Proper Curing in Composite Wrap Pipe Repair
In composite wrap pipe repair, the curing process allows the resin in the composite material to harden, providing the necessary structural support to the pipework or pipeline. If the repair is not fully cured, it can compromise the integrity of the pipe, leading to potential failures in service. ISO 24817 and ASME PCC-2 emphasise that a fully cured repair can restore the pipe's original strength and withstand the stresses imposed during operation.
Temperature Control in Composite Wrap Pipe Repair
Temperature plays a crucial role in the curing process of composite wrap pipe repairs, as it directly influences the rate and completeness of the chemical reaction between the resin and hardener. Several important considerations need to be addressed when managing temperature during the curing process:
Reaction Rate: The temperature dictates the rate of reaction between the resin and the hardener. At higher temperatures, the reaction will proceed more rapidly, while at lower temperatures, the curing process may slow down significantly, affecting the repair timeline. Maintaining an optimal curing temperature ensures that the composite wrap pipe repair achieves the intended strength and durability. Technicians are able to control the curing temperature using a heater blanket, as shown in Figure 1.
Glass Transition Temperature (Tg): Some resins, especially those designed for high-temperature applications, have a high glass transition temperature (Tg). These resins may not fully cure if the surrounding temperature is too low. This is particularly important when using advanced composite repair systems, as a failure to reach the required temperature could compromise the repair’s integrity.
Risk of B-Staging: Curing high Tg resins at too low a temperature can lead to what is known as "b-staging," where the resin undergoes partial curing. If such resins are later exposed to higher temperatures (such as during normal pipeline operations), they may soften or deform, potentially resulting in catastrophic failure. This softening can significantly weaken the composite wrap pipe repair, undermining its structural integrity and leading to further damage in service.
Dew Point Considerations: According to ISO 24817, the substrate surface temperature must be kept at least 3°C above the dew point during the curing process. This prevents condensation, which can negatively affect adhesion and curing, ultimately leading to sub-optimal repair outcomes. High humidity levels (above 85%) should also be avoided during the curing phase.
By managing these temperature factors, technicians can help ensure that the composite wrap pipe repair cures properly and achieves the desired mechanical properties necessary to restore the pipeline’s integrity.
Figure 1 - A heater blanket being used to control the curing temperature of a composite wrap pipe repair.
Glass Transition Temperature (Tg)
Glass transition temperature (Tg) is a critical factor influencing the curing process in composite wrap pipe repair. Tg represents the temperature at which the resin material transitions from a rigid, glassy state to a more rubbery or flexible state, as shown in Figure 2. This property directly affects the performance of the composite wrap pipe repair under operational conditions, particularly when the pipe is subjected to high temperatures.
High Tg Resins: Some composite resins are formulated to have a high Tg, which allows them to withstand higher service temperatures without losing their structural integrity. However, these high Tg resins require elevated curing temperatures to fully harden. If the curing temperature is too low, the resin may not reach its full cure, leading to diminished performance and potential failure under service conditions.
Tg as a Function of Cure Temperature: Tg is not a fixed value but rather a function of the cure temperature. Curing the resin at different temperatures will result in a different Tg. In practical terms, higher curing temperatures generally lead to a higher Tg, which improves the resin’s ability to withstand higher service temperatures. It is crucial to ensure that the curing temperature is sufficiently high so that the Tg of the cured composite wrap is above the design temperature of the pipe. If the Tg is lower than the operating temperature of the pipe, the repair may soften under load, which can compromise the integrity of the repair and result in failure during operation.
Verification of Tg: To ensure that the curing process has reached the desired Tg, technicians must cross check the curing temperature of the composite wrap repair with the curing schedule from the product manufacturer. This helps verify that the Tg is above the pipe’s design temperature to ensure the long-term reliability of the repair.
Figure 2 - Graph showing how the physical properties of a polymer change above their Tg.
Factors Influencing the Curing Process in Composite Wrap Pipe Repair
For successful curing of composite wrap pipe repairs, the factors below must be carefully managed:
Temperature Control: Curing is highly dependent on temperature. During the composite wrap pipe repair process, technicians will have the option to allow the composite to ambient cure (if the system allows for it) or use a heater blanket to control the curing temperature of the resin. The temperature influences the rate of the chemical reaction between the resin and hardener, and it is essential to maintain optimal conditions for the reaction to proceed efficiently. Resins with a high glass transition temperature (Tg) may not fully cure at lower temperatures, and curing at too low a temperature can result in incomplete or "b-staged" curing, which poses risks to the repair's integrity. All temperatures must be logged in the Quality Assurance documentation, and are most easily taken using a calibrated infrared thermometer.
Resin Mixing Accuracy: Composite wrap pipe repairs rely on the proper mixing of resin and hardener. The chemical reaction between these components generates the hardening process, and any deviation from the prescribed mixing ratio can result in incomplete curing. ISO 24817 and ASME PCC-2 advises that technicians follow the resin supplier’s instructions closely to ensure the repair system cures as intended.
Cure Schedule: Each composite repair system comes with a specific cure schedule based on the resin type and repair conditions. This schedule outlines the time and temperature requirements for full curing. Example cure schedules for Icarus Composites BioWrap 102 composite repair system are shown in Figure 3 below. For composite wrap pipe repair, following the recommended cure schedule is essential. ISO 24817 and ASME PCC-2 advises that technicians should monitor the curing temperature throughout the curing process to ensure that the system reaches the specified cure point.
Testing for Complete Cure in Composite Wrap Pipe Repair
To allow technicians to confirm that a composite wrap pipe repair is fully cured, composite wrap repair system manufacturers can use several testing methods as recommended by ISO 24817 and ASME PCC-2 to provide technicians with the necessary curing information:
Differential Scanning Calorimetry (DSC): DSC testing measures the glass transition temperature (Tg) of the resin, providing an accurate indication of the extent of cure. This testing method is essential in developing the curing schedule.
Hardness Testing: Technicians use Barcol or Shore hardness testing to determine if the resin has hardened sufficiently. Composite wrap repair system manufacturers should provide a hardness profile for the composite curing schedule, so that the hardness can be cross-checked against the percentage cure. ISO 24817 and ASME PCC-2 caution that hardness testing should not be the sole method used to ascertain full cure, especially for Class 3 repairs (those with the highest criticality). Full cure acceptance should be based on both hardness tests and the cure schedule .
Final Verification Before Returning to Service
Before the repaired system can be returned to service, technicians must verify that the required cure schedule has been met. For composite repairs in critical service conditions, such as those classified under Class 3 in ISO 24817, the curing conditions must be documented, and the glass transition temperature must meet specified values to ensure long-term performance .
Conclusion
A fully cured composite wrap pipe repair is crucial to ensuring that the repaired section of pipe meets its designed service life and withstands the rigours of operational conditions. By following the comprehensive guidelines outlined in ISO 24817 and ASME PCC-2, technicians can effectively manage the curing process and conduct the necessary tests to verify a complete cure. From temperature control to precise resin mixing and cure schedule adherence, each step is critical for delivering a reliable and durable composite wrap pipe repair.
By optimising the curing process, technicians ensure that composite wrap pipe repairs provide a long-term solution, preventing further damage and ensuring the pipeline operates safely and efficiently.
