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PVC

How does molten plastic become a pathway for water travel and a structural member?

Polyvinyl Chloride, or as it is commonly revered to, PVC, is a thermoplastic polymer which simply means that the material can be heated to it's melting point, then formed into a shape and allowed to cool to a brittle but rock solid material at room temperature. The manufacturing of PVC relies on this transformation from molten plastic to hard plastic to create a variety of shapes that can withstand a multitude of environmental stresses.

These stresses include:

  • Heat (up to 60º C or 140º F)
  • Pressure
  • Collisions
  • Acidic material
  • Physical deformation

This specific set of material properties allows PVC to:

  • Bend small amounts under stress before it breaks
  • Resist corrosion from acids
  • Resist fracture when dropped
  • Hold a lot of pressure before bursting
  • Work in a variety of temperatures

With this diverse set of material properties, PVC is an ideal material to hold and move water and act as a structural member. If you want more information on material properties of PVC, check out Lenntech's PVC webpage.

But why PVC?

Although there are thousands of materials in the world, it is essential to choose the best match of material properties for your application and design criteria.

In the case of plumbing, the criteria are that the material must:

  • Be able to hold pressurized hot water
  • Withstand the environment of a construction site
  • Not contaminate drinking water
  • Resist being dropped or impacted
  • Cost an appropriate amount to provide a good value

In the case of a structural material, the properties of interest are:

  • Cost effectiveness
  • Rigidity (resistance to bending)
  • Availability
  • Resistance to dropping or impact
  • Safe handling
  • Waterproofing

Although there are many materials that meet most of these requirements or even all (like steel and acrylic), only one meets all of the requirements and costs the least, and that is PVC. As a result, the industry standard for plumbing for many years has been PVC, and many low cost low force structures can use PVC as well.

How do they make it?

PVC is made of salt and oil. Salt provides the chlorine by adding energy to the salt crystal (sodium chloride) that breaks all of the internal bonds and releases the chlorine from its grasp.

The oil is heated and cooled rapidly and repeatedly, which breaks the large molecules of hydrocarbons (the molecules that make oil) into smaller molecules, some of which are known as ethylene (small forms of hydrocarbons). These ethylene molecules are filtered out using pressure and distillation.

Once these two molecules are extracted and contained, they are mixed together in high temperature baths resulting in a new molecule known as ethylene dichloride, which at high temperatures becomes vinyl chloride. This vinyl chloride is then polymerized (forced to bond with more of the same molecule) to create polyvinyl chloride.

This is where the fun begins. PVC is not the final step. Depending on what material properties you want the PVC to have, you can add materials during the heating stage to get a more flexible material, a different colored material, a less flammable material, or a more impact resistant material. As a result, it is possible to design a plastic that is an exact match to your design critera.