Cleaner, Primer & Glue

Ever wonder what PVC primer is and why PVC needs to be primed at all?  Or what the difference is between PVC primer and PVC cleaner?

Worry no more!!  Mr. PVC Guy is here to explain.

Cement bonding of PVC, as opposed to mechanical connections, is a chemical welding of the materials, similar to soldering wires or sweating copper tubing.  Cement bonding fuses the pieces on a molecular level, which creates an incredibly strong bond and seal that is both water- and air-tight.

When welding two pieces of a metal together, a solder is used as an intermediary to bond the two materials together.  PVC welding is the same concept, but instead of flux you have a cleaner/primer.  This primer is what truly seals the connection and the glue adhesive holds the pipe together giving it the strength to handle water pressure and stresses.

If you are unaware of how to complete a simple PVC solvent weld, your first step is to clean the pipes and ensure they fit snugly.  This is referred to as “dry fitting.”   The next step is the ever-important priming of your pipe end and the inside of your fitting.  Unless you are using a 2-in-1 primer and cement, you will then need to add a cement (glue) mixture to secure the bond.

There are many people, including professionals, who skip the priming step.  This is a very bad practice and a terrible habit to get into.  While this can be a hotly debated subject, you should always err on the safe side and use primer.  The idea is that pipe buried underground (the most common case where people do not use primer), then there are less stressors acting on the pipe and joints.  This is simply not the case, as roots grow around pipe, and soil shifts when it bears additional weight.

The counter argument for using primer is that it is an unnecessary health hazard.  It is no secret that inhaling fumes is not good for your health, as little inhalation you may do.  PVC pipe cement does contain the same active ingredient as primer, tetrahydrofuran, which we will discuss shortly.  Primer not only removes the glazed, protective top coat on PVC to leave a “rough” matte finish for better bonding of the glue but it also helps with the molecular bond itself.  Do yourself a favor and fix it right the first time.  Use PVC primer and if you are concerned about the health risk, purchase a good ventilator with disposable cartridges.

What most people don’t realize, is the actual welding comes from the primer/cleaner.  The cement is simply an adhesive that holds the pipes together but the primer actually dissolves the surfaces of the PVC so that when a pipe and fitting are connected, they adhere and become a seamless material.  Since the primer bond is superficial, it is weak and the cement is required to hold the two pieces in place.  If you had a way to firmly secure the pipe and fitting, you can actually weld pipe using nothing but the primer but just like skipping the primer, don’t do it!!  This would never be recommended for any construction purposes.

So how does the primer work?  The active ingredient in PVC primer is a chemical called tetrahydrofuran.  PVC is soluble in tetrahydrofuran.  When you add the solvent, tetrahydrofuran, to PVC it dissolves the outer layer of the pipe.  Do it to two pieces, stick the together until it hardens and the two pieces are now fused together on a molecular level.  Since getting the tetrahydrofuran to penetrate deep beneath the surface is not feasible, you will be stuck with a surface bond, necessitating the use of an adhesive to make sure the fitting and pipe are not pulled apart from pipe pressure or outside forces.

If you add primer to a piece of pipe, you may notice that the smooth surface becomes somewhat softer than typical.  This roughness is ideal to help the adhesive bond the fitting and pipe.  When the primer and PVC dries, it re-hardens.  This is why it is so important to add your adhesive to the joints before the primer fully dries.  Otherwise, you are actually softening the surfaces twice.

Cleaner only sometimes contain tetrahydrofuran, as opposed to primer, which always includes it.  Cleaner is used to prepare the surface to remove any dirt, oil or contaminants that may weaken a joint.  Typically though, cleaners and primers are combined into a single product to clean and soften the surfaces in one step.  With modern primers and cleaners, there really is no advantage to making it a three step process unless you are working with pipe that has been subjected to some really nasty dirt and oils.

If you have learned anything from this post, hopefully it’s this:  USE A PRIMER AND GLUE ON ALL JOINTS FOR SOLVENT WELDING.