Introduction to Polyvinyl Chloride Properties

Each polyvinyl chloride molecule is a long, linear polymer.   It is necessary to distinguish between polymer molecules and polymer materials because the word "polymer" is often used to apply to both.  Polymer materials are solids made up of polymer molecules.   The exact conditions under which the polymerization occurs dictates how the individual molecules are arranged relative to one another in the polymer material.   Imagine laying out several lengths of string on a table top. They may be laid out more or less parallel to one another or they might be jumbled together (or some mixture of the two).   Polymers where the molecules are laid out more or less parallel to one another are said to be "crystalline."   Those where the molecules are jumbled together are "amorphous."   Crystalline polymers tend to be harder and somewhat more brittle.   Amorphous polymers are softer and more pliable.   In practice, most polymers materials have regions of crystalinity and amorphousness like the polymer material illustrated to the right.  Polymer chemists understand how to change the ratio of the two arrangements in order to get a polymer material with the right sort of properties.

The properties of polymers can also be changed by a variety of other tricks. The familiar polystyrene packing materials are made by blowing air bubbles into the polymer mixture as it solidifies.   It is possible to add a small percentage of monomers that can react in two places and form a "crosslink" between two growing polymer strands.  Polymers with a large number of cross links have different properties than those with a small number.   Sometimes a small percentage of another small polymer, a "plasticizer" is incorporated into the polymer material rendering it more pliable.

Polymer materials may be molded as they are formed to have some solid shape or they may be extruded.   A PVC pipe would be extruded.   An elbow connector would be molded.

The advantage of using PVC for plumbing lies in its durability and its relative inertness.   Pipes are easy to cut into appropriate lengths and then glue together. The "gluing" process is interesting chemically.   The glue is nothing more than a solvent that partially dissolves polymer molecules.   Two tightly-fitted pieces of PVC are glued together (left image). The solvent initially partially dissolves individual polymer molecules   so that they stick up into the glue layer (middle image).   As the solvent evaporates, the strands from the two pieces are left all mixed together forming a tangled web (right image).   This web holds the two pieces together.   The web must be strong enough to hold under pressure or mechanical expansion and contraction if the pipe is exposed periodically to hot water.   Different grades of glue control the degree to which polymer molecules initially dissolve and interpenetrate.

At this point, you should be ready for more details from the Chemistry of PVC tutorial from the Dept of Polymer Science at the University of Mississippi.