With the invention of “manmade plastics” in the early 1900s, material science has rapidly advanced and is still racing forward with progress. In 1935, the first fiberglass material was introduced to the world. These materials were quickly combined forming Fiber Reinforced Plastics or more generally, composites. Used primarily in aerospace and marine applications, composites have become an essential material in replacing traditional metals. Composites can be thinner and weigh less due to their high directional customization, unlike isometric metals.
Eventually, composites matured to the point of being used as a quick and easy in-situ repair method for structural components. As composite repairs became more accepted in the piping industries, committees and standards arose to provide guidance for early composite designs. But as with any standards, technology often grows quicker than the standards can be updated. The new technology begins reaching into areas that were previously inaccessible and new paths are explored — repairs to dents, cracks, highly chemical aggressive material, underwater defects, high cyclic conditions and extreme temperature ranges. As these areas are explored, it is critical that the industry work together to identify potential concerns.
Due to the advancements in material sciences, composite materials have been used for the better part of the past 20 years to repair damaged piping and pressurized components in plants, refineries and pipelines. The use of composite materials has been accompanied by comprehensive research programs focused on the development and assessment of using composite technology for restoring the integrity to damaged piping and pressurized components. Of particular interest are composite repair standards such as ISO 24817 and ASME PCC-2 that provide technical guidance in how to properly design composite repair systems.
The vast body of research completed to date has involved assessments at ambient conditions. However, at the present time there is significant interest in evaluating the performance of composite repair materials at elevated temperatures. Currently, there is a focus on improving our understanding of composite repairs in a high temperature environment and the critical role of using temperature-based mechanical properties to establish a composite repair design. The backbone of this effort is the development of composite performance curves that correlate change in strength as a function of temperature.
When designing any composite repair, there are four key information groups that need to be considered: Pipe geometry, defect cause and geometry, pressure conditions and thermal conditions. Each of these groups are very important, however, several thermal concerns are often overlooked that can lead to an early and unexpected failure. The thermal conditions that should be considered include the design, operating and application temperature. If relevant, any cyclic thermal conditions should be considered as well (such as freeze-thaw).
Mechanical Degradation Due to Temperature
The most basic thermal consideration is determining the upper temperature limit of a material. In most cases, the limiting factor is the polymer matrix — the resin. Typically, all resin commonly used for composite repairs, such as epoxy, polyurethane or vinyl-ester, are considered “thermoset” resins. When heated, a thermoset resin does not melt like a “thermoplastic” resin, but it can significantly reduce in strength near the glass transition temperature range (Tg). Near this Tg value, the resin transitions between “rubbery” and “glassy” states. Imagine bending a piece of epoxy: At low temperatures, it would break and shatter like glass; at high temperatures, it will bend and flex like rubber. This is the specific reason there are temperature de-rate factors within the standards (Figure 1).
Thermoset resins that are heated beyond the Tg range can see a reduction in mechanical properties (Figure 2) and sometimes by orders of magnitude dependent upon the resin type. Due to this, a tested Tg value with a specified offset is commonly used as the temperature limit. What is less known is that the value for Tg can vary greatly based on the cure profile (the temperature profile that the resin is exposed to until it reaches a final, cured state). Typically, a cured epoxy exposed to only ambient temperatures (Figure 3) will have a lower Tg than if the same epoxy was exposed to elevated temperatures over the cure profile. With this being the case, it is normally ideal that the epoxy is exposed to design temperatures shortly after hardening (commonly referred to as “post-curing”).
Other test methods can be utilized to determine temperature limits as well, such as determining the heat distortion temperature (HDT). This method can be used successfully to determine a realistic upper limit for materials that can operate above their Tg value and still maintain a significant amount of their mechanical properties. Additionally, temperature limits should be determined for every aspect of the composite repair: the filler material, adhesive as well as the composite itself.
Almost all materials weaken when exposed to a relative higher temperature — regardless of how close or far away they are from their temperature limit; this is true with composites or even metals. Even composite systems with a very high temperature limit (for example, 450 F) will have a higher tensile strength at 72 F than at 200 F. This decrease in performance is significantly less than what is observed near Tg. However, it may cause an issue when strength values determined at 72 F are used for design calculations at 400 F. For high-temperature products, standard testing should be performed at elevated temperatures to determine the effect of thermal degradation.
Environmental and Application Temperatures
Mathematically, a composite repair system may be the perfect solution for a specific problem, however, that does not mean much if the composite cannot be properly installed and cured! The environmental conditions and application temperatures need to be considered before selecting a composite repair and beginning installation. The main concerns are due to viscosity, working time and set/cure time.
Working in a very cold climate can greatly increase the viscosity of any resin. This can lead to the adhesive, filler material or wet-out resin being improperly mixed. High viscosity wet-out resin may also make it very difficult to achieve proper resin saturation in the fabric and may create an issue with adhering to the pipe itself. Also, if the pipe is not heated or the application temperature of the pipe is very cold, it can take a very long time to achieve composite setting and curing. A general guide is that for every 10 degrees Fahrenheit colder than ideal, set time and cure time double. An ambient-cured composite being installed near freezing, with no added heat, could take several days to harden and weeks to fully cure (Figure 5). Additionally, some high temperature systems require elevated temperatures to effectively cure at all and may never truly harden.
“As composite repairs continue to march forward using and developing new technologies, there will always be new, interesting situations that may require consideration beyond the minimal requirements set forth in the repair standards.”
On the other side, very hot ambient conditions can cause a dramatic reduction in working time of any resins. Whereas at room temperature a resin may have 30 minutes of working time to wet out fabric and apply a section of composite to the pipe, at temperatures near 100 F, the working time may only be five minutes. On heated pipes, the viscosity can be dramatically reduced to the point that pastes become dripping liquid, wet-out resin runs off of the fabric and filler material is unable to maintain shape. If viscosity isn’t an issue, the increased set time might be; the adhesive may start setting up before the composite is ready to be installed; the filler material may harden before reshaping is completed; or the first layers of composite can set before de-bulking can be initiated.
Effects of Thermal Expansion
When there is a substantial difference between a composite repair’s installation temperature and the final operating or design temperature, the differences in the coefficient of thermal expansion (CTE) of the composite and pipe need to be taken into consideration. When metal is heated, it expands. Composites will expand as well. However, they will typically expand at different rates, characterized by the material’s CTE. One of the largest contributors to a composite’s CTE value is the reinforcing fabric that is used. In general, carbon fiber systems will have a much smaller CTE value than steel, whereas fiberglass systems will have a larger value.
With regards to thermal expansion in the hoop direction, the difference in the composite’s and pipe’s CTE can have a large impact. When installing at temperatures much lower than the final operating or design temperatures, it is best advised to install a carbon fiber system (Figures 6 and 7). After installation, when the system is heated, the pipe will expand into the composite system, thereby increasing load transfer efficiency to the repair. A fiberglass system, on the other hand, may try to outgrow the pipe and create a separation concern. For systems being installed at temperatures much higher than operating, the opposite is g enerally true. A fiberglass repair will constrict tighter onto the pipe while the carbon system does not shrink enough (Figures 8 and 9). Under normal circumstances, this is not an issue, but it should be considered when temperature swings over 100 F may exist.
Additional Thermal Concerns
In some piping, thermal expansion in the axial direction can also be a concern, especially when poor girth welds are in the discussion (Figure 10). One of the primary concerns in this situation is the adhesive. An adhesive will typically demonstrate maximum performance at the cure temperature. Large differences in operating temperatures may dramatically reduce shear strength and could lead to an unexpected failure. Additionally, if an adhesive is introduced to exceedingly cold temperature, it may become too brittle, leading to cracking and failure. If the adhesive is heated too high, it may become too rubbery and become unable to transfer loads to the composite (Figure 11).
Specifically for leaking repairs, there is a very large importance with regards to operating temperature. When chemical compatibility is taken into consideration (and it always should be), the temperature of the product can have a very large effect on chemical attack. At ambient conditions, common epoxies can withstand a large array of chemicals. Raise that temperature to 150 F and that list of compatible chemicals quickly shortens. When designing for a leak repair, the chemical concentrations as well as the temperatures should always be taken into consideration — pressure may be the least threatening element in the repair.
While the issues discussed are by no means the entire breadth of thermal concerns, knowing that these issues and more exist should enable end-users and composite designers to begin asking the right questions. Freeze-thaw, thermal fatigue, insulation effects and many more unique conditions may be overlooked if thermal history and operational requirements are neglected. Simply using a system that can withstand “high temperatures” may not be enough to create a successful composite repair. As composite repairs continue to march forward using and developing new technologies, there will always be new, interesting situations that may require consideration beyond the minimal requirements set forth in the repair standards.
Tags: 2015 May Issue, Milliken Infrastructure Solutions, Plastic
Jim Souza is director of technology and Casey Whalen is a material engineer at Milliken Infrastructure Solutions LLC, a provider of oil and gas pipeline repair solutions and a subsidiary Milliken & Co. The company acquired Pipe Wrap Inc. in August 2014.