Comparison of PVC Resin Production Processes

Overview

Among the two main raw-material routes for PVC resin production—the ethylene route and the calcium carbide route—each has its own strengths and weaknesses. 

In general, PVC produced via the ethylene process has higher molecular weight, a narrower molecular weight distribution, superior thermal stability, and better aging resistance, making it ideal for high-performance applications such as transparent sheets and automotive interiors. 

Conversely, PVC made via the calcium carbide (acetylene) route has lower molecular weight, a broader distribution, better plasticization behavior, and lower cost, making it suitable for general-purpose products like pipes and building materials; however, it exhibits poorer thermal stability and higher levels of impurities, which can negatively impact both environmental performance and final product quality.

Process Principles

Calcium Carbide Route (Acetylene Method)

Calcium carbide (CaC2) is hydrolyzed to produce acetylene, which then reacts with hydrochloric acid to form vinyl chloride monomer (VCM), and finally is polymerized into PVC. This route demands substantial raw-material input: to produce 1 t of PVC resin requires approximately 3,450 kWh of electricity, 0.6–0.8 t of coal, and 0.9–1.0 t of limestone. Energy costs represent a significant share, and improper disposal of calcium carbide slag can cause environmental pollution.

Ethylene Route

Ethylene reacts with chlorine in an oxychlorination step to yield ethylene dichloride (EDC), which is then thermally cracked to generate VCM before polymerization into PVC. The primary feedstock is petroleum: producing 1 t of PVC resin consumes roughly 6.25 t of crude oil. This process is mature, boasts high feedstock utilization, and meets more stringent environmental standards.

Molecular Structure and Properties

Molecular Weight & Distribution: PVC from the calcium carbide route has both lower number-average and weight-average molecular weights and a broader distribution. Ethylene-route PVC features a narrower distribution, resulting in more uniform processing behavior, higher heat-distortion temperature, superior electrical insulation, and better resistance to aging.

Processing Performance

Plasticization: In dry-blend kneading, calcium carbide PVC exhibits higher plasticization torque, shorter plasticization time, and lower equilibrium torque, indicating more efficient plasticization compared to ethylene-route PVC.

Mechanical Performance

Strength: Tensile and flexural strength values are comparable between the two grades; however, ethylene-route PVC typically displays higher elongation at break due to its greater degree of polymerization and higher molecular weight.

Quality and Environmental Considerations

Impurities: The calcium carbide process often employs mercury catalysts, leaving potential mercury residues that pose environmental and health risks. Moisture, unreacted acetylene, and traces of 1,1-dichloroethane in VCM can further degrade thermal stability and whiteness.

Waste Handling: Calcium carbide slag requires resource‐recovery measures (e.g., use in cement), which demand significant investment and carry secondary pollution risks. The ethylene route, while cleaner in impurity control, generates spent acid that must be neutralized or recycled, increasing environmental and equipment costs.

Application Comparisons

Calcium Carbide PVC: Predominantly used for pipes, window and door profiles, waterproofing materials, and other general applications where cost efficiency is paramount.

Ethylene-Route PVC: Preferred for high-grade profiles, transparent sheets, films, and automotive interiors, where appearance, strength, and weatherability are critical.

Cost and Pricing

Feedstock Sensitivity: Calcium carbide PVC costs are highly sensitive to electricity and coal prices; ethylene PVC is influenced by global crude oil and ethylene markets.

Regional Dynamics: In coal-rich, oil-poor regions like China, ethylene-route PVC typically commands a premium over calcium carbide PVC. Electricity price differentials can give calcium carbide PVC a competitive edge in areas with subsidized power.

Selection Guidelines

1. Low-End, General-Purpose Products (pipes, building materials, waterproofing): Prioritize calcium carbide PVC to reduce costs and improve plasticization efficiency.

2. High-Performance Products (transparent sheets, automotive interiors, medical-grade films): Opt for ethylene-route PVC to ensure high molecular weight, narrow distribution, and superior thermal stability.

3. Environmental/Regulatory Compliance: When mercury‐free, low‐impurity, or stringent international environmental standards apply, ethylene-route PVC is the better choice.

Conclusion

While ethylene-route PVC outperforms in thermal stability, consistency, and environmental cleanliness (e.g., fewer defects in molded parts), calcium carbide PVC retains advantages in cost and plasticization performance—particularly where uniform plasticizer uptake and high plasticization efficiency are required. Ultimately, the choice should balance product positioning, performance requirements, and cost considerations.