When purchasing insulated pipes or inspecting project sites, many clients habitually check the color of the polyurethane foam. They might suspect that a yellower hue indicates impure materials, while a lighter color suggests high-quality materials. Can the shade of the color actually be used to judge foam quality? The answer is: it serves as a reference, but it should not be the sole criterion.
Consider a common phenomenon: when most polyurethane foam-insulated pipes are cut open, a ring of light yellow-resembling tree rings-often appears near the center of the cross-section. This is actually normal. The foaming process, which involves mixing "black" and "white" polyurethane components, is exothermic; the core generates significant heat that dissipates slowly, making it prone to "core burning" or yellowing. Generally, if the foam is uniform in color (light or pale yellow) and free from blackening or unpleasant odors, it is acceptable; slight yellowing has little impact on performance. However, if the foam turns dark yellow or black, caution is warranted. Some low-cost "black" components are produced by blending in TDI by-products to cut costs; the resulting rigid polyurethane foam often appears dark or deep yellow, becomes brittle, and is highly susceptible to moisture absorption and aging during long-term operation, leading to a significant drop in performance. Similarly, using expired "white" components or incorporating large amounts of recycled material can result in abnormal foam color, reduced closed-cell content, and poor dimensional stability.
Polyurethane also undergoes gradual aging during long-term high-temperature operation. As the service life extends, the combined effects of sustained high temperatures and thermal aging cause the foam to darken while reducing its strength and thermal insulation efficiency. In short, foam discoloration can be a direct signal of poor raw material quality or an indication that the pipeline is aging.
So, if not color, what should you look at? To assess the quality of polyurethane foam, focus on its density, closed-cell content, and thermal conductivity. According to the GB/T 29047 standard, the foam density should be no less than 60 kg/m³, thermal conductivity at 50°C should not exceed 0.033 W/(m·K), and the closed-cell content should be at least 88%. Failure to meet density standards, an excessively low closed-cell content, and an overly high thermal conductivity are the factors that directly compromise thermal insulation performance and pipeline service life. When purchasing, test data for the foam is a more reliable indicator than its color.