TWI Knowledge Summary

Solvent welding of thermoplastics

by Roger Wise

Welding of polymers occurs when the polymer chains at the surface of one component are mobile enough to entangle with chains in the other component. Usually, thermal energy is applied to raise the temperature of the polymer above the appropriate transition temperature, i.e. the glass transition temperature, T g , for amorphous thermoplastic polymers, or the melting temperature, T m , for semi-crystalline polymers. Above these transition temperatures, polymer chains are mobile and if two components are brought into intimate contact, polymer chain entanglement will proceed, resulting in a weld.

In solvent welding, a solvent is applied which can temporarily dissolve the polymer at room temperature. When this occurs, the polymer chains are free to move in the liquid and can entangle with other similarly dissolved chains in the other component. Given sufficient time, the solvent will permeate through the polymer and out into the environment, so that the chains lose their mobility. This leaves a solid mass of entangled polymer chains which constitutes a solvent weld.

Practically, a pipe made from PVC, for example, may be joined to a fitting made from the same polymer, using solvent welding. First, the surfaces of the parts are cleaned, and then the solvent cement applied. The cement typically contains the solvent for the polymer, together with a small quantity of the polymer to give the cement the consistency of syrup, making application easier. The parts are brought together into intimate contact and remain so for a predetermined period. This allows the solvent to dissolve the surface of the components to produce the required polymer chain entanglement, and then to permeate away through the materials, leaving a solid weld.

Solvent welding has been employed extensively in applications such as manufacture of piping systems and for assembling toys. However, legislation designed to minimise industrial use of organic solvents is likely to impose limitations on use of solvent welding in the foreseeable future.

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.

Copyright © 2001, TWI Ltd

Information and advice from TWI and its partners are provided in good faith and based, where appropriate, on the best engineering knowledge available at the time and incorporated into TWI's website in accordance with TWI's ISO 9001:2000 accredited status. No warranty expressed or implied is given regarding the results or effects of applying information or advice obtained from the website, nor is any responsibility accepted for any consequential loss or damage.