Poly vinyl chloride commonly known as PVC or Vinyl is the 3rd largest selling commodity plastic in the world today. Although early accidental discoveries of PVC were found in the 1800’s it wasn’t until the 1920’s when an industrial scientist named Waldo Semon at BFGoodrich commercialized the material for use as a water resistant coating for fabrics. During the 1950’s the refining methods enhanced durability introducing its use in the building and the construction industry. Water pipes used in the transportation of water to homes and factories began to be made out of PVC due to the materials resistance to chemicals and corrosion. Today PVC is used in a wide variety of applications and industries some examples are IT (communication and electronics), transport (food packaging), textiles and construction. Atomic structure and Properties
Below is the chemical formula for PVC that consists of Carbon, Hydrogen and Chloride.
PVC is a thermoplastic polymer meaning that is recyclable, the addition of the chlorine atom allow for easier identification during automated sorting systems and with today’s society being environmentally concise these become important properties. Other important properties of PVC that make it a desirable polymer are its resistance to chemicals and corrosion, good electrical and thermal insulator, fire retardant (burns slowly) and has relatively good strength. Undesirable properties of PVC is that it softens at a relatively low temperature (54~80C) and it has a high density when compared to other polymers.
PVC is produced in a variety of techniques that includes suspension, emulsion, micro suspension and bulk using addition polymerization. Suspension polymerization is the most common PVC process. The production of PVC first starts with chlorine being extracted from salt by the process of electrolysis. The chlorine is then reacted with ethylene to produce ethylene dichloride (EDC) molecule. This molecule is placed in a high temperature furnace that cracks the EDC molecule to produce both vinyl chloride and hydrogen chloride molecules. A process called quenching is then applied that effectively removes the heat quickly from the vinyl chloride monomer (VCM) to prevent it from degrading. The VCM is purified in distillers and stored in large gas tanks, the storage of VCM is a careful process due to it being highly flammable and also the exposure to humans can cause serious harm. Polymerization then occurs in which the VCM is placed in water filled autoclave’s in which a double bond between the carbon atoms opens to polymerize with the molecules to form the PVC polymer chain, this results in a white powder that is suspended in the solution. The PVC solution now goes trough varies drying stages and once dried it is then sieved and sold by its particular grade. Finally the additives can be added such as stabilizers, plasticizers, pigments and modifiers and now the material is ready for production into the many items it is useful for.
Manufacturing Processes for PVC products
Some common forms of manufacturing process include extrusion, injection molding, dipping, coating, calendaring and blow moulding. Injection Moulding: This is the one of the most widely used technique for PVC moulding due to its speed of manufacturing and best suits the creation of 3D shapes. Pellets of PVC are placed into the cylinder via the hopper. Once in the cylinder it is heated to form a viscous fluid then it is rammed through a nozzle into the mould cavity where pressure is maintained until it cools into a solid form finally the mould is then opened and product is ejected. Common PVC injection moulding products are containers, toys, buckets and any other 3D objects.
Figure 1: A typical injection moulding process.
(Image: Material Science and Engineering: An introduction 7th edition, William D. Callister, Jnr)
Extrusion: This is the moulding of PVC pellets that are sent down into...
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