The indirect yarn numbering system, also known as the length-based system, is a method to specify the fineness of yarn. In this system, the number represents the length of yarn in a specific unit of measure that weighs one unit weight. Commonly used units are meters per gram (metric system) or yards per pound (English system). The higher the number, the finer the yarn, as more length indicates a lesser weight, hence a thinner yarn. This system is prevalent in the spinning industry, especially for cotton, wool, and other staple fibers. It contrasts with the direct system, where yarn number indicates the weight per unit length. The indirect system is essential for manufacturers and buyers alike, as it provides a standardized measure of yarn fineness, crucial for ensuring product quality and consistency in textiles.

Depolymerization of polypropylene oxide involves breaking its long-chain molecules back into smaller units or monomers. This process can be achieved through various means such as thermal depolymerization, chemical decomposition, or catalytic methods. Thermal depolymerization applies heat to break the polymer chains, whereas chemical decomposition uses reagents to cleave the bonds. Catalytic methods involve the presence of a catalyst to lower the energy required for the depolymerization process. The choice of method depends on the desired outcome, efficiency, and environmental considerations. Opting for catalytic processes might offer more control and specificity, potentially leading to less energy consumption and lower by-product formation. Depolymerization of polypropylene oxide is crucial for recycling efforts, as it allows for the material to be reused in new applications, reducing waste and the reliance on virgin resources.

Removing paint from PVC pipes involves a few methods, each suitable depending on the type of paint and the project. One common approach is using a chemical paint remover specifically designed for plastics, ensuring it doesn't damage the PVC. Apply it according to the manufacturer's instructions, usually by brushing it on, waiting for the paint to bubble, and then wiping it off. Another method is using isopropyl alcohol or acetone for smaller areas, applying it with a cloth and gently rubbing the paint away. Physical methods include sanding with fine-grit sandpaper or using a heat gun on a low setting to soften the paint for easier scraping. However, caution is advised with heat to prevent damage to the PVC. Always wear protective gear like gloves and goggles, and work in a well-ventilated area.