What Are the Functions of Lubricants? What Are Internal and External Lubrication?

I. Why Lubricants Are Indispensable for Plastic Processing

 During plastic processing, friction mainly occurs within polymer melts and at the interface between polymer melts and processing equipment, both undermining production efficiency and product quality:

1. Internal friction: Internal friction between polymer chains increases resistance to melt flow and may increase processing torque and energy consumption, which may cause overheating, aging and decomposition of materials in severe cases.

2. External friction: Friction between molten material and metal surfaces such as screws and molds. It tends to make melt adhere to molds, resulting in rough product surfaces and difficult demolding.

Core Functions of Lubricants

Improve the flowability of polymer melts, reduce adhesion of melt to molds and equipment, enhance demolding performance, and comprehensively optimize plastic processing properties.

II. What Is Internal Lubrication & Internal Lubricants?

Definition

Internal lubrication refers to the effect of reducing internal friction between polymer molecules to boost melt flowability; the corresponding additives are internal lubricants.

Working Mechanism

1. Internal lubricants have moderate compatibility with resin.

2. They moderately weaken intermolecular forces between macromolecular chains, enabling free sliding and rotation of molecular chains when materials are heated and deformed.

3. They effectively cut down internal molecular friction without significantly lowering the glass transition temperature (Tg) and mechanical strength of finished products.

Supplementary Concept: Glass Transition Temperature (Tg)

Tg is the critical temperature at which plastic transforms from rigid and brittle to soft and flexible.

· Below Tg: Macromolecular segments are frozen; materials are hard, brittle and glass-like, prone to cracking.

· Above Tg: Macromolecular segments start moving; materials become soft and flexible, stretchable and elastic.

III. What Is External Lubrication & External Lubricants?

Definition

External lubrication refers to the effect of lowering external friction at the interface between melt and molds/processing equipment to prevent mold adhesion; the corresponding additives are external lubricants.

Working Mechanism

1. External lubricants generally have limited compatibility with polymers.

2. During processing, external lubricants tend to migrate toward the polymer-metal interface.

3. A directional lubricating isolation film forms at the contact surface between melt and metal molds, separating materials from equipment to drastically reduce interfacial friction and eliminate mold adhesion and demolding difficulties.

Supplementary Knowledge

 Lubrication performance depends on molecular structure, polarity, molecular weight distribution and compatibility with the polymer system.

In actual plastic formulation and production, internal or external lubricants are rarely used alone. Most formulas adopt a compound system of internal and external lubricants to balance plasticization speed and product appearance, while avoiding defects such as blooming and precipitation caused by excessive lubrication.

IV. Quick Distinction Between Internal and External Lubrication

PE Wax as Lubricant

Many commercial PE wax grades have molecular weights generally ranging from several hundred to several thousand. Oxidized polyethylene wax is produced by controlled oxidation of polyethylene wax, introducing polar functional groups such as carboxyl groups, widely used as lubricants for PVC and other plastics.

PE wax provides excellent external lubrication and also contributes to internal lubrication depending on polymer systems and formulation design, suitable for extrusion and calendering processes. It boosts processing efficiency,  improves slip performance and reduces blocking during film processing, and facilitates uniform dispersion of fillers or pigments in polymer matrices. PP wax provides similar lubricating and dispersing functions while offering better compatibility in polypropylene-based systems.

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