How to Choose Between Internal and External Lubricants?

Defining Internal Lubricants

Internal lubricants are additives that reduce friction between polymer chains during the melting and processing of PVC. They improve the flow characteristics of the molten material, making it easier to shape and mold. By decreasing internal viscosity, internal lubricants help achieve uniform melting, better fusion, and reduced shear stress, which ultimately contributes to higher product quality.

Defining External Lubricants

External lubricants function at the interface between the PVC melt and processing equipment. Their primary role is to minimize friction between the molten polymer and metal surfaces such as extruder barrels, screws, or mold walls. This helps prevent sticking, facilitates smooth release from molds, and reduces wear and tear on machinery, extending equipment life and maintaining consistent processing conditions.

The Importance of Balancing Both Lubricants

Choosing between internal and external lubricants is not an either-or decision; rather, it requires balancing both types to suit specific processing needs and end-use requirements. An optimal ratio ensures efficient flow, excellent surface finish, and protection for processing equipment.

Key Factors to Consider When Choosing Lubricants

Type of PVC Product Being Manufactured

Rigid PVC products like pipes or profiles often require higher external lubrication to reduce friction with equipment surfaces. Flexible PVC formulations, such as films and sheets, typically benefit from increased internal lubrication to improve melt flow and flexibility.

Processing Method and Conditions

Different processing techniques — extrusion, injection molding, calendaring — place varying demands on lubricant function. For example, injection molding demands effective external lubricants for easy part release, while extrusion benefits from both internal flow enhancers and external friction reducers to ensure smooth continuous operation.

Desired Product Properties

If the application requires a high-gloss, smooth surface, external lubricants like waxes are favored. For enhanced mechanical properties and dimensional stability, internal lubricants that promote uniform fusion are crucial. Understanding the end-user expectations guides lubricant selection.

Practical Guidelines for Selecting Internal Lubricants

Common Internal Lubricants

Metallic stearates such as calcium stearate serve as effective internal lubricants. They reduce melt viscosity and improve fusion without significantly impacting mechanical strength. Internal lubricants should be compatible with plasticizers and stabilizers present in the formulation.

Benefits and Limitations

Internal lubricants improve processing stability and reduce torque requirements, but excessive amounts may lead to decreased mechanical properties or surface issues. Proper dosage ensures a balance between flow enhancement and product integrity.

Applications Best Suited for Internal Lubricants

Flexible PVC products, medical tubing, and food packaging films often require more internal lubrication to maintain clarity, flexibility, and consistent wall thickness during processing.

Practical Guidelines for Selecting External Lubricants

Common External Lubricants

Waxes such as paraffin, polyethylene, or microcrystalline wax are typical external lubricants. These materials form a thin film on equipment surfaces, preventing adhesion and facilitating mold release.

Benefits and Limitations

External lubricants protect machinery and improve surface aesthetics. However, excess external lubricant can cause blooming or surface haze on finished products and interfere with bonding or coating processes.

Applications Best Suited for External Lubricants

Rigid PVC profiles, window frames, cable sheathing, and injection molded components benefit from effective external lubrication to ensure smooth surfaces and reduce equipment wear.

Balancing Internal and External Lubricants for Optimal Results

Typical Ratios and Formulation Strategies

A well-formulated PVC compound usually contains a blend of internal and external lubricants, with internal lubricants ranging from 0.3% to 1.0% by weight and external lubricants from 0.2% to 1.0%. The ratio depends on processing conditions and product requirements.

Impact on Processing and Product Quality

Balancing lubricants affects melt flow, surface finish, mechanical properties, and thermal stability. Formulators often adjust ratios through trial and error and lab-scale testing to find the optimal combination for specific applications.

Influence on Environmental and Regulatory Compliance

Choosing lubricants that comply with environmental regulations such as REACH and RoHS is important. Halogen-free and bio-based lubricants are increasingly preferred to reduce health and ecological risks.

Troubleshooting Common Issues Related to Lubricants

Surface Defects and Blooming

Excess external lubricants can migrate to the surface causing a whitish bloom. Reducing external lubricant concentration or switching to alternative waxes can mitigate this problem.

Processing Instabilities

Poor lubricant balance may result in erratic melt flow, high torque, or product deformation. Evaluating lubricant compatibility and adjusting ratios helps restore processing stability.

Reduced Mechanical Performance

Too much internal lubricant may weaken the product’s tensile strength or impact resistance. Proper testing ensures performance targets are met without sacrificing processability.

FAQ

What is the main difference between internal and external lubricants?

Internal lubricants reduce friction between polymer chains inside the melt, while external lubricants reduce friction between the melt and processing equipment.

Can I use only one type of lubricant in PVC processing?

Using only internal or external lubricants may lead to processing challenges. A balance of both types is typically necessary for optimal performance.

How do lubricants affect the surface finish of PVC products?

External lubricants mainly influence surface smoothness and gloss, while internal lubricants affect melt fusion and uniformity.

Are bio-based lubricants effective alternatives?

Yes, bio-based lubricants are gaining popularity due to sustainability concerns and can perform comparably to traditional lubricants when properly formulated.