A job candidate interviewing for a technical position gave this example of a technique they have experience with: they joined PVC pipe and fittings using a two-component cement system in which one component is clear and the other purple, by putting one component on one end and the other component on the other end, and sliding them together.
I said I thought they might be confused, and that there may be a thin purple cleaning solvent that one cleans both ends with, and then a thick clear cement that one coats both ends with before putting them together.
They said, no, definitely not, this is a two part system that reacts and cures, like an epoxy, and maybe it is an epoxy, but whatever it is, it’s not what I described. This is during a job interview, where they’re kind of staking a claim.
So – I have my own current belief about the thing they described just being wrong. But I don’t know every possible way of cementing PVC pipes together and it’d be wrong to hold this against them if I’m the one who doesn’t know.
Is there any such thing?
Or does the candidate get dinged points for insisting on knowing and being somewhat experienced with something that’s mistaken?
I’ve never heard of such a thing, and a quick Google search doesn’t turn up any likely candidates.
And, if he was using conventional primer/cement that way, he was doing it wrong…
They are probably referencing solvent-welded joints, were you apply a purple solvent to each side and a PVC filler. It is much stronger than a typical PVC cement.
It is not a normal primer. The purple solution reduces the glass transition temperature of the polymer allowing a “Cold weld” that would typically require a much higher heat.
This method results in polymer chains across the interface which does not happen with a typical “glue”.
In the traditional sense a primer typically improves surface properties for a mechanical adhesive. This process, as it names implies, is more like welding. Sanding or increasing surface area will not allow the adhesive to properly bond the connection in this case and so technically it is a 2 part system.
How To Solvent Weld
Prior To Use:
Read all product labels carefully.
Stir or shake cement before using. If jelly-like, do not use. Keep container closed when not in use. Avoid eye and skin contact. Wear safety glasses with side shields and wear rubber gloves.
Square pipe ends, chamfer and remove all dirt.
Check dry fit of pipe and fitting. Pipe should easily go 1/3 of the way into the fitting. If pipe bottoms, it should be snug.
Use a suitable applicator at least 1/2 the size of the pipe diameter. For larger size pipe systems use a natural bristle brush or roller.
Clean pipe and fitting with a listed primer. (Do not use primer on ABS pipe and fittings. Use Clear Cleaner only!)
Apply liberal coat of cement to pipe to the depth of the socket, leave no uncoated surface.
Apply a thin coat of cement to inside of fitting, avoid puddling of cement. Puddling can cause weakening and premature failure of pipe or fitting. Apply a second coat of cement to the pipe.
Assemble parts QUICKLY. Cement must be fluid. If cement surface has dried, recoat both parts.
Push pipe FULLY into fitting using a 1/4 turning motion until pipe bottoms.
Hold pipe and fitting together for 30 seconds to prevent pipe push-out – longer at low temperatures. Wipe off excess.
Allow 15 minutes for good handling strength and 2 hours cure time at temperatures above 60°F before pressure testing up to 180 psi. Longer cure times may be required at temperatures below 60°F or with pipe above 3". DO NOT TEST WITH AIR.
The purple tint is for plumbing code compliance as visible proof a primer was used. The purple color does not indicate the method of action. There are multiple types of “PVC cement” that are all labeled as such. Solvent cement is not glue, and it is a two part system that reacts and cures and creates bonds.
The MEK based “primer” in solvent welding is not a “cleaner” it dissolves the CPVC resin and the “cement” actually results in fusion at the molecular level. The problem is that the OP seems to think the “primer” is a cleaning or prepping step, which it is not. Yes it does do some “cleaning” but the actual melting is the more critical aspect.
Based on looking at the ingredients, it seems like the primer is a straight blend of solvents, while the actual cement is a different blend (same solvents, different proportions) with PVC resin included.
So I’d guess that the primer makes for a better bond, but isn’t strictly speaking necessary.
Well, I think the OP thinks that one puts the “purple” stuff on both PVC ends to be joined, and then the “cement” on both ends to be joined. The interviewee seems to be stating that one only puts the “purple” stuff on one side, and the “cement” on the other side.
If I may, while I appreciate the technical details, they seem to be getting in the way of the answer.
No, I think we have the answer. Interviewee is just flat out wrong, misunderstood the product he was using, and has been applying the product incorrectly, potentially exposing his customers to failed PVC joints.
If Napier is being charitable, he can contact interviewee to ask exactly what product he was talking about since “it seems interesting.” Then see if interviewee realizes his mistake, admits to it, and learns from it. There is also the possibility that interviewee is correct and we are all wrong, and then won’t WE feel silly!