Perspex Mounts for Model Engines
By Dorothy Cheng
One of the projects this year for Conservation of Metalwork Postgraduate Diploma students has been the cleaning and mounting of a group of internal combustion engines for models, designed and constructed from the 1930s to the 1960s by Edgar T. Westbury. These engines are part of a large collection that will be displayed in a museum environment and thus needed to be secured on suitable mounts.
After discussing various material choices for mount-making, we decided on Perspex®, poly(methyl methacrylate).
It is ideal for its resistance to UV damage as well as its chemical stability and strength. In ambient environments, Perspex will not off-gas substances that can corrode the engines. Other mount-making materials like wood and MDF (medium density fibreboard) can emit acetic acid, formic acid, and formaldehyde, which all accelerate metal corrosion. Some of the engines had already been mounted in the past on laminated densified wood, so we considered using a similar material. However, this would have made the new mounts distractingly difficult to distinguish from the originals.
Instead of using an adhesive, I wanted to solvent weld the Perspex pieces together. This would negate the need for additional materials near the engines which can potentially off-gas damaging substances. Instead, a solvent would temporarily dissolve the surfaces of the Perspex pieces, allowing the polymer chains to entangle while in a liquid state. Once the solvent evaporates the bonded pieces are returned to a solid state, with their chains molecularly intertwined across the interface.
I mixed up two solutions: 90% acetone/10% acetic acid and 90% dichloromethane/10% acetic acid. Ideally chloroform would be used, but for health & safety reasons it's not ideal, and so these mixtures attempt to approximate its properties. For each test sample, I positioned the Perspex pieces where I wanted them and introduced the solution with a syringe at the seam. Capillary action pulls the solvents into the gap between the pieces. All of the work done with dichloromethane is done in the solvent cupboard.
I applied slight pressure for about ten seconds, until I could see that the joint interfaces were thoroughly wetted.
I found that the acetone solution worked slowly and was very weak. After thirty minutes I could still easily pull apart the Perspex pieces, though the formerly glossy surfaces showed evidence of having been softened, because of their change in texture.
The dichloromethane solution was a much better option-it softened the joint interfaces almost instantaneously and the bonded pieces were impossible to separate after only a few minutes.
After bonding all the necessary components, I air-abraded the mounts to matte them, so they will not distract from the engines.
The new steel screws I used to secure the engines to the mounts were stamped with "West Dean College" and scribed with the current year, under 10x magnification to ensure a discreet appearance. This was to differentiate new additions from the original components. The new screws were also given a protective coating of microcrystalline wax.
I used 1 mm copper wire to make small pins to secure the loose wires tenuously attached to some engines. The pins are held in place with tension, not adhesive, so they can be removed and cleaned or replaced easily.
