For those who don't want to read the entire article, I'll jump straight to the conclusion.
Conclusion
The purpose of this investigation was to determine the effect of primer on the strength of solvent welded PVC and CPVC joints. Was primer necessary to have better joint strength? The hypothesis was that if primer penetrates and softens the plastic surfaces prior to the application of cement, then the inclusion of primer in the solvent welding process should achieve maximum fusion and strength. This was not the case. In both tests, the primer had a negative effect on the strength of the pipes. In the compression test, more primed joints failed first than unprimed joints. In the torsion test, the primed joints overall took less force to fail than the unprimed joints. The primed joints also had a higher failure rate, although the rates were more equal than in the compression test.
A reason why the primed joints were weaker than the unprimed joints may be that the primer presoftens the plastic more than the cement alone, so the plastic may have taken significantly more time to cure in the primed joints.
Another reason why the primed joints were weaker than the unprimed joints may be that the cement may have needed to be applied immediately after the primer is applied to prevent the primer from drying. Because the application was a manual process, a few seconds of exposure to air that the primer had could have dried it such that it formed a barrier to the cement to prevent the two materials from fusing together with the help of the cement.
Yet another reason the primed joints were weaker than the unprimed joints may be that the primer presoftens the plastic more than the cement alone, so when the plastic hardens again, the molecular structure may have changed. The hardened plastic may not have the same strength as the original plastic material.
The final reason why the primed joints could be weaker than the unprimed joints could be that because the cement contains most of the same ingredients as the primer does, the primer may not have added any real benefit to the joint strength. The primer may have actually made the joint weaker. The concentration of these ingredients in the cement alone may be more conducive to making a stronger joint than the concentration of the ingredients in both the primer and the cement combined. Therefore, cement alone may create a stronger joint than a two-step process of primer followed by cement. The manufacturer, Oatey SCS®, has been contacted, but so far no comments have been received.
While curing, the joints were not close to full strength. The manufacturer provided that the cure times for 1/2 in* sch. 40 PVC pipes from 60°F-100°F subjected to up to 180 psi are 15 min at most, but according to the test data, a fully cured joint at 24 hours is at least 30-50% stronger than the joint is at 2 hours. According to the manufacturer, the cure times for CPVC pipes are 1 hour at most. Based on the tests conducted at 2, 4, 6, and 24 hours, the strength increases greatly as time progresses, which is an indication that the cure time for the joints takes longer than the cure time suggested by the manufacturer. Most plumbers in the field would pressurize and test the system 15-30 minutes after solvent welding. This means the premature testing in the field could potentially lead to unnecessary joint failures.
The strongest cement was the Medium Gray PVC cement. The weakest cement was the Regular Clear PVC cement. The cement that primer (negatively) affected the most was the Rain-R-Shine® Medium Blue PVC cement.
There are many practical applications of this experiment. This investigation is of value to the plumbing and engineering community as it provides an answer to a highly debated subject. Many people have argued about whether primer serves a positive purpose in solvent welding on professional plumbing forums. There are differing opinions based on previous experiences of plumbers. It is the standard method of assembly to use primer in solvent welded joints because IPC (International Plumbing Code), UPC (Uniform Plumbing Code) as well as most regional plumbing codes require it. Yet, many plumbers follow this method because of the requirement, not because of the belief that it creates a better joint. If primer does not make a stronger joint, then the step of applying primer can be skipped resulting in tremendous amounts of time and cost savings. This report attempts to prove one way or the other whether primer is an essential component of creating a strong PVC joint.
I have not used primer since my first attempt at plumbing an aquarium some 30 years ago. I found it to be messy, and when I knocked over the can in my living room, decided that I'd just try it without. I haven't looked back. And have yet to have a joint leak or fail in any way.
