[ The Craft Of The Guitar Maker ]By TREVOR SEMPLE
9. The Age of PlasticIN ways as dramatic as the development of the microchip, plastics technology has revolutionised almost every aspect of our lives. If you look around the room as you read this, you will probably find dozens of plastic articles all around. You may still have overlooked the paint on the walls, the fibres in the carpet, and the electrical fittings. Within the whole house there will be hundreds of different plastics all tailored to their various functions: in electrical appliances, guttering, pipework, electronic equipment, toys, bottles, pens, and even the clothes that you wear.
All modern lacquers owe their existence to the extraordinary growth of the chemical industry this century. For, with one or two exceptions like nitrocellulose, they are thermosetting plastics. They encompass the main types: polyurethane, acidcatalysed, and polyester. Very recently, acrylic-based wood varnishes have been added to the list as well.
In the early part of the last century, increasing numbers of experimenters were drawn into the then new science of organic chemistry. By the middle of the century, many of the basic building blocks, the monomers, were known. It was also noticed that, at times, pale waxy substances or dark tar-like sludge would be left in the apparatus. Gradually, scientists realised that some of these materials might be useful, although they were quirky in nature, and it was very hard to predict their properties or applications in advance. The first man-made plastic was exhibited at the Great Exhibition of 1862 at Crystal Palace. The early film industry would have been lost without it, for it was cellulose nitrate or celluloid. It is still used to make table tennis balls. Unfortunately, it is rather unstable and is related to some explosives. You will appreciate this fact if you have ever thrown a table-tennis ball into an open fire. One early pioneer in the field of polymers was a certain Dr Spitteler's cat. In 1895 he knocked over a bottle of formaldehyde into his saucer of milk, and invented a plastic now sometimes used as a wood adhesive.
Once it was realised that many of these substances had practical uses, the research, together with the new products, came thick and fast. Between around 1910 to 1940 most of our plastics were discovered, with names like Bakelite, Nylon, Terylene, Polythene, Polyurethane, Polystyrene and Perspex. It is now hard to realise that many of these products have been in commercial production for less than 60 years.
Plastics fall into two main families - thermoplastics and thermosets. They have dramatically different properties. When plastics harden, the monomers link together to form long chains - polymers. These chains are among the largest molecules known to man, often containing many millions of atoms. In the case of thermoplastics, the chains remain largely separate, and usually give the rather soft flexible plastics that we are all familiar with. Gentle heat will cause them to soften and flow. As they cool, they will harden in their new shape. An easy way to visualise this is to think of a bowl of spaghetti: when hot, the individual strands can be easily separated. As they cool, the strands become a solid mass in the bowl, but they are still separate strands.
However, it is the thermosets that interest us, because most modern varnishes belong to this family. Their structure can be thought of as being rather like a 3-dimensional fish net. There are links between chains as well as down the lines of chains themselves. This creates a structure that is usually extremely hard and brittle, and cannot easily be changed. They are resistant to most solvents and quite high temperatures as well. Plastic ash trays will be made of thermosets. So will parts of hairdryers and saucepan handles. So may the finish on your guitar. One way to find out is to stub out a cigarette on the varnish and see what happens. For obvious reasons I would strongly recommend you NOT to try this! However, a thermoset would usually be unharmed by this treatment, just like the formica in your kitchen (which also has a thermoset top coat).
This gives a clue to one reason why these lacquers were originally developed. For some applications, they are unbeatable. Because they are so resistant to solvents, they can be used in kitchens or bathrooms, and the water and detergents have no effect. They can be used for bar-tops, boats, and outside woodwork. But there is another more important reason for their use speed. Nearly all modern finishing techniques have had one aim at heart, and that is to reduce production lime. It is not surprising that they have been largely developed for and used by the furniture and automobile industries.
There are two ways to reduce production time. First, you can reduce the drying time using more catalyst and then heat to accelerate the reaction. Secondly, you can reduce the number of coats by using very thick lacquer. Much modern finishing technology has been aimed in this direction, with pre-heated pressurised feeds and very high pressure spray-guns.
Now, although this may be ideal for the furniture industry, it is not necessarily ideal for luthiers. It is certainly true that many machine-made guitars are finished in a lacquer of this type. If you need a guitar that can withstand burning candles, and is completely waterproof, then this may be the stuff for you. For those of us who use our instruments in more conventional ways, other considerations may be more important.
©1990 Trevor Semple
"Classical Guitar Magazine", May 1991