Solvent welding is the fundamental element to a complete and functional piping system. Despite its essential role, the process is often perceived to be a simple procedure; therefore, the science and crucial details behind the technique are often overlooked.

The solvent-welding process consists of permanently joining together plastic pipe and fitting sockets by applying a mixture of solvent and resin. A one-step solvent cementing technique is commonly used for connecting chlorinated polyvinyl chloride pipe. This type of piping can be found in plumbing and fire protection systems, such as FlowGuard Gold^® and BlazeMaster^® CPVC pipe and fittings. These systems are heavily used in single- and multifamily homes, with growing use in commercial applications.

The technique embodies the properties of welding without complicated heat fusion techniques. When welding a steel system, metal is heated in order to be malleable and pushed together so the material can cool and harden into one piece. For solvent welding, instead of applying heat, solvents are applied to the plastic to soften up the material’s molecules to allow bonding. As the solvents evaporate, the material hardens as one solid piece.

The science of solvent welding

Solvent welding is not an adhesive process — it does not glue side A to side B, but rather permanently bonds both pieces to create one solid joint. Unlike gluing, which hardens to hold material together, solvent cement softens the substance’s molecules to form a solvent-welded joint that has the strength of the parent material. With a precise solvent weld, the bonded joint becomes the strongest part of the system.

Scientifically speaking, plastic is a long chain molecule gaining its strength from the chains being tangled up together. It is a common misconception that the bonding process includes a chemical reaction. Rather, the solvents in the cement swell up the molecules of the pipe and fitting in order to give them the space to move around freely. Solvents merely allow the plastic to soften and then harden as one piece after they are pushed together, similar to a welding technique.

Solvent cement is comprised of a mixture including solvents, CPVC resin and sometimes other thickeners as well. The resin and thickeners allow the cement to cling to the surface of the pipe and fitting so the solvents don’t run off before they’ve done the job of softening.

Once the surfaces have been softened, it is necessary to have radial pressure between the pipe and fitting in order for a solvent weld to occur as intended. The fitting is designed to accommodate this requirement with what is known as an interference fit. An interference fit means the pipe’s outer diameter is slightly larger than the inner diameter of the bottom third of the fitting socket. This forces the softened surfaces into each other when the joint is assembled.

Because the socket is tapered, it is slightly larger than the pipe at the top. The resin in the cement is there to fill any remaining gaps, but is not necessarily an agent in the bonding process. The bottom third of the fitting is where 80% of the joint strength is found. In order to create this described ideal fit, contractors must follow the appropriate steps.

Preparation and application

The physical solvent-welding process is quite simple. There are four basic steps to prepare a pipe and fitting for solvent welding:

1. Cut the pipe squarely. Uneven cuts can reduce joint strength.
2. Deburr and bevel the pipe to prepare for insertion into the socket. Ragged edges and sharp corners on the pipe end can scrape the cement and softened surface off the socket, creating a weaker and possibly leaky joint.
3. Use a clean, dry rag to remove loose dirt and moisture from the fitting socket and pipe end in preparation for the solvent cement. With proper preparation, the solvent will form a bond without contamination from grease, inks or oils.
4. Verify that the pipe can enter the fitting socket easily about one-third to two-thirds of the way.  

Once the pipe is properly prepared, a solvent cement that suits the piping application can be applied to the pipe and fitting following these four steps:

1. Solvent cement should first be applied heavily to the outside of the pipe using a properly sized dauber and then lightly to the inside of the fitting socket. Make sure to not puddle the cement in the fitting. If the pipe is 1 1/4 in. (32 mm) or larger, another application of solvent cement should be added to the outside of the pipe.
2. Immediately insert the pipe and rotate it one-quarter turn to combat any potential “leak-tracks” in the cemented joint.
3. Ensure the pipe bottoms in the socket and hold for 30 seconds to confirm the pipe will not back out.
4. Check for an evident bead of evenly applied solvent cement around the pipe and fitting juncture and wipe off any excess cement.

While solvent cement only takes 30 seconds to set, the cure times are a function of pipe size, system pressure, temperature, moisture and humidity. The assembly must be allowed to set without any stress on the joint for one to five minutes, and the system should not be pressure-tested until minimum cure times have been met.

For complete details of solvent cement preparation and application, please follow the manufacturers’ installation instructions.

Do’s and don’ts

In order for solvent cement to effectively bond CPVC piping, some simple regulations and precautions should be followed, as shown in the chart to the right.

The solvent-welding technique described is a process that creates a reliable piping system for your plumbing or fire protection needs. To learn more about the advantages of these materials, visit www.lubrizolcpvc.com.

 Author bio: Forest Hampton, manager of codes and approvals for Lubrizol CPVC in North America, has more than 20 years of experience researching and developing plumbing system components.