The Impact of Polyethylene Wax Molecular Weight on Melting Point and Viscosity
In summary, the relationship between the molecular weight of polyethylene wax and its key properties is a classic principle in polymer science: an increase in molecular weight leads to a significant rise in both the melting point and melt viscosity.
1. Impact of Molecular Weight on Melting Point
Core Mechanisms: Chain Length, Crystallinity, and Chain Entanglement
· Molecular Weight & Chain Length: Molecular Weight (MW) directly reflects the average length of the polyethylene wax polymer chains. Higher MW means longer chains.
· Crystallization Ability: Polyethylene wax is a highly crystalline material. Its properties are largely determined by the ability of its molecular chains to arrange themselves into orderly crystalline regions.
· Low Molecular Weight (MW < ~5,000): Short chains are flexible and mobile, allowing them to pack into more perfect and stable crystals. melting these stable crystals requires more energy (higher temperature), resulting in a higher melting point.
· Medium to High Molecular Weight (MW > ~5,000): As chains lengthen, entanglement between them becomes significant. While longer chains can still crystallize, these entanglements disrupt the regularity and perfection of the crystals. The defective crystal structure is easier to disrupt and melt at a lower temperature, leading to a lower melting point.
· Molecular Weight Distribution (MWD): Besides average MW, the distribution also has an effect. A narrow MWD typically yields a sharper melting point, while a broad MWD results in a wider melting range.
Conclusion: In the low MW range (~1000-5000), the melting point increases with MW. Beyond a critical point (~5000-10,000, depending on structure), the melting point plateaus or even slightly decreases with increasing MW, eventually approaching that of linear polyethylene (~130-135°C).
2. Impact of Molecular Weight on Viscosity
Core Mechanism: Molecular Chain Entanglement
Melt viscosity is extremely sensitive to changes in molecular weight.
· Entanglement Effect: When melted, the long polymer chains become entangled like a bowl of spaghetti. For the material to flow (have liquidity), it must overcome the resistance offered by these entanglements.
· Power-Law Relationship: A well-known exponential relationship exists between melt viscosity (η) and molecular weight (Mw): η ∝ Mw^3.4 (for most linear polymers above a critical molecular weight).
· This means doubling the molecular weight can increase the melt viscosity by more than tenfold (2^3.4 ≈ 10.6). This is a dramatic effect.
· Critical Molecular Weight (Mc): Below Mc, chains are too short to entangle effectively, and viscosity increases nearly linearly with MW. Above Mc, the power-law relationship takes effect, and viscosity surges.
Conclusion: Molecular weight is the most critical factor affecting the melt viscosity of polyethylene wax. Applications requiring low viscosity and high fluidity (e.g., efficient lubricants or dispersants) should use low MW wax. Applications needing high viscosity, thixotropy, or anti-dripping properties require higher MW products.
Summary and Application Guide
MW Range Key Properties Primary Applications Typical Products
Low (500-5,000) Very low viscosity, high fluidity High-end coatings/inks, PVC internal lubrication, hot-melt adhesives Wood & automotive paints, PVC profiles, packaging adhesives
Medium (5,000-10,000) Balanced properties, versatile General plastic lubrication, masterbatches, general coatings PP/PE masterbatch, plastic films, general industrial coatings
High (>10,000) High viscosity, hardness, abrasion resistance Ultra-durable coatings, engineering plastics, wax modification Floor coatings, marine paints, large moldings, premium candles
Oxidized PE Wax High polarity, compatibility Aqueous systems, polar plastics, metal processing Water-based wood coatings, PVC masterbatch, metal drawing
Final Recommendation: Selection should be based on the core need (lubrication, matting, dispersion, or hardening) and the base system （plastic, coating, ink; aqueous or solvent-based) to identify the suitable MW range, followed by practical testing.
