The Influence of Different Lubricants on PVC Plasticization and Processing Performance
I. Comparison of Different Lubricants on PVC Plasticization Speed
Based on practical experience, the plasticization speed of commonly used lubricants can be ranked as follows (from fastest to slowest):
Calcium stearate → Monoglyceride → G60 → Lead stearate → Oxidized polyethylene wax → Stearic acid → PE wax → Paraffin wax
In addition, plasticizers such as epoxidized soybean oil (ESBO) have a significant effect on improving melt flow properties.
Most lubricants exhibit both internal and external lubrication characteristics. It is inaccurate to classify them as having only one function.
From practical application:
• Higher polarity leads to better compatibility with PVC and stronger internal lubrication, improving molecular mobility. Typical examples include monoglyceride, pentaerythritol esters, and calcium stearate.
• Lower polarity (non-polar materials) mainly provides external lubrication, such as paraffin wax and PE wax.
In practice:
• Lubricants with strong internal lubrication: calcium stearate, monoglyceride, PETS, G60
• Lubricants with strong external lubrication: PE wax, paraffin wax, G74
• Balanced internal/external lubricants: stearic acid, 316-series oxidized PE wax (with excellent mold release performance)
II. Effects of Insufficient and Excessive Internal and External Lubrication
1. Classification by Melting Point
-Low-melting lubricants (early-stage lubrication):
-Paraffin wax, butyl stearate, stearyl alcohol, polyol esters, stearic acid
-Medium-melting lubricants (mid-stage lubrication, 100–125°C):
-Lead stearate, cadmium stearate, PE wax, OPE wax
-High-melting lubricants (late-stage lubrication):
-Calcium stearate, barium stearate
2. External Lubricants
Common external lubricants include paraffin wax and PE wax. Since paraffin wax migrates easily, PE wax is more widely used.
Insufficient external lubrication may cause:
• Difficulty controlling temperatures in zones 4 and 5 of the extruder
• Overheating
• Large bubbles, interconnected bubbles, yellowing
• Rough surface
• Increased shear heat and material degradation
• Melt burning and yellow boards
Excessive external lubrication may cause:
• Poor plasticization
• Scale formation in the die
• Surface exudation
• White streaks
• Uneven wall thickness
• Periodic surface defects
3. Internal Lubricants
Common internal lubricants include stearic acid, G60, monoglyceride, and 316 series products.
Insufficient internal lubrication may cause:
• Poor material dispersion
• Uneven plasticization
• Difficulty in thickness control (thick center, thin edges)
• White streaks
• Adhesion to mold cavity
• Local overheating
Excessive internal lubrication may cause:
• Brittleness of products
• Reduced heat resistance
• Conversion into external lubrication under heat and pressure
• Lubrication imbalance
4. Combined Effects of Internal and External Lubrication
Both insufficient:
• High melt viscosity
• High plasticizing torque
• Severe wall sticking
• Yellow degradation lines
• Poor surface finish
• Reduced mechanical properties
Both excessive:
• Low torque
• Insufficient plasticization
• Good surface smoothness
• Poor fusion at pressure points
• Serious loss of mechanical strength
Low internal + high external:
• Extended plasticization time
• Reduced torque
• Difficult processing
• Brittle products
High internal + low external:
• Shortened plasticization time
• Severe wall sticking
• Reduced thermal stability
• Yellow degradation lines on surface
III. Why Do Lubricants Migrate or Bloom?
Lubricant migration (exudation or blooming) mainly occurs for the following reasons:
1. Excessive internal lubricant
Excess internal lubricant can transform into external lubrication, leading to surface migration and increased brittleness.
2. Excessive external lubricant
Due to poor compatibility with PVC, excessive external lubricants tend to migrate easily.
3. Influence of low-molecular-weight components
For example, monoglyceride is an excellent internal lubricant. However, if excessive glycerin is present, its low molecular weight makes it easy to migrate during processing.
High-purity monoglyceride, when used at proper dosage, does not normally cause migration. Therefore, overdosing should always be avoided.