The selection must simultaneously meet four core objectives: lubrication balance, foaming stability, surface gloss, and non-blooming. Lubrication balance requires external lubrication to reduce melt adhesion to the metal surface while internal lubrication promotes plasticization, avoiding delayed plasticization leading to foaming failure or insufficient plasticization causing uneven cell structure. Foaming compatibility requires no interference with AC blowing agent decomposition, the ability to stabilize melt strength, and ensuring uniform cells without collapse. Low blooming tendency requires that high molecular weight or modified waxes offer better compatibility, preventing post-production blooming that affects subsequent processing.
Viscosity selection directly impacts surface gloss, with a clear correspondence between viscosity and performance, allowing selection based on core requirements. High-viscosity polyethylene wax has slightly poorer dispersion but delivers excellent gloss, high molecular weight, and low volatility, making it the preferred choice for meeting high-gloss requirements and replacing low-viscosity wax to resolve blooming and hazing issues, providing good gloss and directly enhancing surface appearance. Low-viscosity polyethylene wax offers good dispersion but only average gloss and is prone to migration and blooming, making it suitable only for formulations with high dispersion requirements and high filler content, where gloss is acceptable but blooming tendency can easily cause surface hazing. If the core objective is to improve gloss, high-viscosity pure polyethylene wax should be prioritized, with adjustments to mixing processes to ensure adequate dispersion.
Different wax types possess distinct characteristics suited to various application scenarios. Standard polyethylene wax (PE wax) offers strong external lubrication and good thermal stability, suitable for use as a basic external lubricant in rigid PVC foamed extrusion. Oxidized polyethylene wax (OPE wax) contains polar groups, exhibits good compatibility with PVC, provides both internal and external lubrication, and resists blooming, making it suitable for scenarios requiring balanced lubrication, bloom prevention, and improved pigment dispersion. High-density polyethylene wax (HDPE wax) has a melting point of 110–140°C, excellent thermal stability, and does not decompose at high temperatures, making it suitable for high-temperature processing at 170–180°C and compatible with most high-temperature extrusion conditions. Fischer-Tropsch wax features a narrow molecular weight distribution, controllable melting point, and stable lubrication performance, suitable for formulations with high foaming stability requirements. Cracked polyethylene wax is mostly low-viscosity, low in cost but subject to significant quality fluctuations and prone to containing impurities, recommended only for cost-sensitive applications with low gloss requirements and not recommended for high-gloss products.
Key pitfalls to avoid in selection involve four adjustment points. The melting point must be selected within the range of 100–140°C to match PVC processing temperatures of 160–220°C; a melting point too low leads to volatilization failure, while one too high causes uneven dispersion affecting surface quality. A wax micropowder with a particle size of 5–20μm is recommended to avoid agglomeration from excessively fine particles or pitting and streaking from excessively coarse particles, both of which compromise gloss. The typical addition amount should be controlled at 0.1–0.4 phr and must not be excessive, as overloading external lubrication delays plasticization, affecting both foaming and causing blooming and hazing. Pure polyethylene wax must always be used, avoiding recycled wax or inferior cracked wax containing impurities, as impurities directly cause surface stains and dullness, reducing gloss.
