Solvent & Plastic Welding

Like other welding processes, solvent-welded plastic joints are formed by liquifying adjacent surfaces of the parts. The mobilized molecules mix to create a region of continuous material.

Many of those other processes melt the joint material with heat, applied in one way or another, and the solid bond forms with cooling. In the solvent welding case, a solvent is the liquifying agent and the bond is formed by evaporation of the solvent—often at ambient temperatures.

This process, also known as solvent bonding, has a wide range of applications and levels of process control for joining plastic parts into assemblies.

It can be as simple as joining PVC plumbing pipes where solvent is daubed on both the pipe end and the fitting socket, then the pipe is promptly inserted into the socket with a twist to insure good contact. Or it may require a much more elaborate process including close tolerance parts manufacture, pre-processing of the components, and precise control of solvent application as is often the case in assembling medical devices.

Plastic materials for solvent welding

Thermoset plastics such as epoxies, phenolics, and polyimides cannot be solvent-welded because they are formed by a permanent chemical reaction and cannot return to a liquid state once that reaction is complete. Nor can all thermoplastics be solvent welded. Those which cannot be solvent bonded are crystalline with a molecular structure that prevents solvents from affecting them. Polyethylene, and polypropylene, are notable examples.

However, many thermoplastics are amorphous polymers and good candidates for solvent welding. These have a random molecular arrangement and weaker intermolecular bonds that allow polymer chains in their surfaces to temporarily dissolve and become mobile when solvent is applied. Some of the polymers often joined with solvent welds are listed in the adjacent table.

Acrylonitrile-Butadiene-Styrene (ABS)
Polybutadiene (BR)
Polycarbonate (PC)
Polyether Sulfone (PES)
Polyethylene terephthalate (PET / Polyester)
Polymethyl Methacrylate (PMMA / Acrylic)
Polystyrene (PS)
Polysulfone (PSU)
Polyvinyl Chloride (PVC)

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Solvents for solvent welding

Solvents used for solvent welding include those shown in the next section’s table and may be applied individually or as mixtures. Some reasons for using mixtures are:

Modify the Hildebrand value as previously discussed.
Adjust the viscosity.
Adjust the evaporation rate.
Fuse dissimilar polymers with a blend that is compatible with both plastics.

Solvent / Plastic Combinations

	Acetone	Cyclohexane	Cyclohexanone	Ethyl Acetate	Ethylene Dichloride	Methylene Chloride	Methyl Ethyl Ketone	Tetrahydrofuran	Toluene
ABS						X	X	X
BR		X							X
PC					X	X	X
PES						X			X
PET			X
PMMA			X		X	X	X	X
PS	X				X	X	X		X
PSU						X	X		X
PVC	X	X					X	X
ABS/PMMA							X
PVC/PMMA		X	Mixture				X

Some combinations of polymer and solvent that have been successfully used for solvent welding are shown in the adjacent table.

Solvent cement

Strictly speaking, solvent welding or solvent bonding is a process in which a solvent or solvent blend is used to join thermoplastic parts. Plastic cement is composed of solvent(s) with some of the compatible plastic resin dissolved in it. Compositions can range from very little up to 25 % dissolved resin.

This practice is often used to:

Fill small gaps in a joint.
Increase the viscosity of the solvent.
Modify the bond setting time.
Reduce shrinkage and internal stresses in the joint.

Otherwise, the same effects as solvent welding are achieved with a joint of homogenous material after curing is complete. And the materials and joining process can be the same for both cementing and welding.

The solvent welding process

Plastic welding is accomplished by bringing joint surfaces into close contact while enabling the polymer chains at those surfaces to become mobile enough to entangle themselves.

Contact may be attained by fixturing, clamping, interference fit, or other means of applying pressure to the joint. In this case, polymer chain mobility is the job of the solvent which may be applied by:

Brush or Dauber
Dipping parts into solvent.
Metered dispenser for more precise control
Capillary action with solvent applied by syringe to a consistent small gap.

After assembly of the parts, the solvent permeates its way out through the polymer material of the joined items. It evaporates to leave a solid mass of entangled chains forming the solvent weld. This solvent migration process can take time—ranging from minutes to hours or days. But, once complete, only parent plastic material remains in the joint.

Advantages of solvent welding

Solvent welding and solvent cementing are the simplest, most economical method of joining those thermoplastics suitable for the process. Other advantages are:

Clean, strong, lightweight joints—bonds can achieve 80 to 100 percent of the strength of the parent material.
Solvent-formed joints are just as resistant to chemical and other environmental attacks as the parent plastic.
After curing, the joint consists only of parent material and no solvent remains as a permanent part of the joint.
Solvent welds are fluid tight and can be used for hermetic sealing.
Solvent welding allows the joining of some polymers that would be damaged by heat welding as well as joining some combinations of different polymers.
Mechanical stresses are homogeneously distributed in solvent-formed joints.
Solvent welds have reduced sensitivity to thermal cycling compared to those made with adhesives.

Solvent welding considerations

While it is a highly useful process, solvent welding is not the answer to every joining requirement. Nor is it without the need for precautions in its application. Important considerations include:

Not all plastics can be joined by the process.
Joint design and solvent selection need careful consideration to avoid stress cracking, crazing, trapped solvent, and other potential defects.
Some materials may need to be annealed before welding to avoid stress cracking.
The solvents involved present significant potential hazards of flammability, toxicity, and odor. Reliable safeguards will be needed.
Cure times can be very long before the solvent has completely migrated out of the joint and adequate ventilation is needed for solvent removal.
Surface preparation is important and provision to remove contaminants may be needed.
Excess moisture can have adverse effects on the process.
Not applicable for bonding rigid or semi-rigid parts to flexible films. Thermoplastic welding is a much better process for bonding rigid or semi-rigid components to flexible films

If you would like more details about solvent and plastic welding, contact SealWerks today. We can help you get started on the fabrication of your product and work with you every step of the way to completion.