How is PVC manufactured?

Vinyl chloride monomer (VCM) is produced from the chlorination of ethylene and pyrolysis of the resulting ethylene dichloride (EDC) in a cracking unit. PVC (glass transition temperature: 70-80°C) is produced by polymerization of vinyl chloride monomer (VCM).

The popular methods used to manufacture PVC commercially are:

·         Suspension PVC (S-PVC)

·         Bulk or Emulsion (E-PVC)

Suspension PVC (S-PVC) Process

In pressure-tight reactor, the monomer is introduced with polymerization initiator and other additives. The content of the reaction vessel are mixed continuously to maintain suspension and ensure uniform particle size of PVC resin.

Typical suspension polymerized PVC has a mean particle size of 100-150 µm with a range of 50-250 µm.

S-PVC grades are formulated to meet an extensive range of requirements such as, high plasticizer absorption for flexible products, or high bulk density and good powder flow required for rigid extrusion

Bulk or Emulsion (E-PVC) Process

In this process, surfactants (soaps) are used to disperse the vinyl chloride monomer in water. The monomer is trapped inside soap micelles are protected by the soap and polymerization takes place using water soluble initiators.

The primary particles are solid, smooth surfaced spheres which are clustered into irregular shaped aggregates with a typical mean particle size of 40-50 µm with a range of 0.1-100 µm.

E-PVC resins are used in a wide range of specialty applications such as coating, dipping or spreading.

Key Properties of PVC Polymer

PVC is a very versatile and cost-effective material. Its main properties and benefits include:

1.      Electrical Properties: PVC is a good insulation material, thanks to its good dielectric strength.

2.      Durability: PVC is resistant to weathering, chemical rotting, corrosion, shock and abrasion. It is therefore the preferred choice for many long-life and outdoor products.

3.      Flame Retardancy: Because of its high chlorine content, PVC products are self- extinguishing. Its oxidation index is ≥45. Antimony trioxide has been used extensively, usually in combination with phosphate ester plasticizers, giving excellent fire performance and mechanical properties.

4.      Cost/Performance Ratio: PVC has good physical as well as mechanical properties and provides excellent cost-performance advantages. It has long life span and need low maintenance.

5.      Mechanical Properties: PVC is abrasion-resistant, lightweight and tough.

6.      Chemical Resistance: PVC is resistant to all inorganic chemicals. It has very good resistance against diluted acids, diluted alkalis and aliphatic hydrocarbons. Attacked by ketones; some grades swollen or attacked by chlorinated and aromatic hydrocarbons, esters, some aromatic ethers and amines, and nitro- compounds

Methods to Enhance PVC Properties - Role of Additives
PVC resin obtained from polymerization is extremely unstable due to its low thermal stability & high melt viscosity. It needs to be modified before processing into finished products. Its properties can be enhanced/modified by adding several additives, such as heat stabilizers, UV stabilizers, plasticizers, impact modifiers, fillers, flame retardants, pigments, etc.

Selection of these additives to enhance polymer's properties is dependent on end application requirement. For example:

1.      Plasticizers (Phthalates, Adipates, Trimellitate, etc.) are used as softening agents to enhance rheological as well mechanical performance (toughness, strength) of vinyl products by raising the temperature. Factors that affect the selection of plasticizers for vinyl polymer are:

o    Polymer Compatibility

o    Low Volatility

o    Cost

Flexible PVC Pipe

2.      PVC has a very low thermal stability and stabilizers help prevent degradation of polymer during processing or exposure to light. When subjected to heat, vinyl compounds initiate a self-accelerating dehydrochlorination reaction and these stabilizers neutralize the HCl produced enhancing the life of polymer. Factors to be considered while selecting heat stabilizer are:

o    Technical requirements

o    Regulatory Approval

o    Cost

3.      Fillers are added in PVC compounds for a variety of reasons. Today, a filler can be a true performance additive by delivering value in new and interesting ways at the lowest possible formulation cost. They help to:

o    Increase stiffness and strength

o    Improve impact performance

o    Add color, opacity and conductivity

o    And more

Calcium carbonate, titanium dioxide, calcined clay, glass, talc etc. are common types of fillers used in PVC.

External lubricants are used to assist smooth passage of PVC melt through processing equipment. while internal lubricants reduce melt viscosity, prevent overheating and ensure good color of product

4.      Other additives like processing aids, impact modifiers, are added to enhance mechanical as well as surface properties of PVC

PVC Blend with Other Thermoplastics

PVC/Polyester Blends – These blends combine superior physical properties of polyesters with the excellent processing characteristics of PVC. Benefits include abrasion resistance, tensile properties and tear resistance.

PVC/PU Blends – These blends offer increased abrasion and chemical resistance. Some TPUs are biocompatible and when blended with PVC results in valuable products for PVC industry

PVC/NBR Blends – Flexible PVC modified with NBR are melt processable yet possess good elasticity/recovery characteristics

PVC/Polyolefin Rubber Alloys - They have potential utility in many applications where conventional flexible vinyl compounds do not meet certain end-use performance requirements.

Limitations of Polyvinyl Chloride

·         Poor heat stability

·         Properties can change with time, due to plasticizer migration

·         Flexible PVC has lower chemical resistance than rigid PVC

·         Rigid PVC has low continuous service temperature of 50°C

Applications of PVC Resin

Commercially, PVC is one of the most important thermoplastics in the world today. Rigid (unplasticized) PVC is one of the most widely used plastic materials. Main applications of both types of PVC (rigid and flexible) include: