The Impact of Metal Stearates in PVC Production

Main Applications

Heat stabilizers

PVC is prone to decompose and release HCl during high temperature processing (e.g., extrusion, injection molding), resulting in discoloration and deterioration of the material. Metal stearates work through the following mechanism:

Absorption of HCl: Reacts with HCl generated from decomposition to produce metal chlorides, delaying autocatalytic decomposition.

Replacement of unstable chlorine atoms: e.g. zinc and calcium stearates can replace unstable allyl chloride in the PVC molecule to improve thermal stability.

Synergistic effect: often used in combination with other stabilizers (e.g. organotin, phosphite) to enhance the effect (e.g. calcium/zinc composite stabilizers).

Lubricant

Internal lubrication: reduces friction between molecular chains and improves melt fluidity (e.g. calcium stearate).

External lubrication: reduces adhesion between melt and processing equipment (e.g. lead stearate), prevents scaling.

Other Functions

Release agent: Helps to detach the product from the mold.

Processing aids: optimize processing efficiency and reduce energy consumption.

Impact on PVC properties

Positive effects

Improved thermal stability

Extends processing window and prevents scorching and discoloration (e.g., barium/cadmium stearate for transparent products).

Inhibits “zinc burn” (zinc salts alone are prone to failure and need to be synergized with calcium, epoxy esters, etc.).

Processing performance improvement

Reduced melt viscosity, improved extrusion rates and surface gloss (e.g., calcium stearate).

Reduced equipment wear and energy consumption.

Environmentally friendly substitution

Calcium/zinc composite systems are gradually replacing toxic lead salts (e.g. lead stearate), complying with RoHS and other regulations.

Negative Effects

Precipitation Problems (Plate-out)

Excessive use may cause stearate to precipitate on the mold or roller surface, affecting the appearance of the product.

Loss of transparency

The refractive index of metal salts is different from that of PVC, which may reduce transparency (matching stabilizer system needs to be selected).

Influence on mechanical properties

Excessive addition may weaken the tensile strength and toughness of PVC.

Environmental and Health Risks

Lead and cadmium stearates are toxic and need to be phased out; calcium/zinc systems are safer but more costly.

Development Trends

Lead-free: accelerated R&D of calcium/zinc, organotin and rare earth stabilizers.

Compounding technology: synergistic performance enhancement through multi-metal/organic stabilizers (e.g. calcium-zinc-aluminum system).

Functionalization: Increased demand for multifunctional additives that combine stabilization, lubrication and anti-oxidation.