Making a cured film

There is a great deal of variation in the technique used. This partly reflects the requirements of the final product and partly the techniques used by individual artists. It is not unusual for there to be 30-70 or more steps in the production of a plain black lacquer article, so only the main steps are reviewed here.

At each of the stages in the production of a surface it is necessary to cause the lacquer to harden. This is achieved in one of two ways. In the first method, in which lacquer is baked at 110-180 °C, oxygen reacts with unsaturated side chains of the urushiol to produce peroxides. These peroxides can then attack the ring to produce free radicals which enter further reactions or form crosslinks with other side chains, both routes leading to a highly crosslinked polymer. This method is used for the first coats on metal articles but the method normally used on organic substrates, including wood, involves enzymatic polymerization. In this method, lacquer is cured in a humidity chamber via an enzyme catalysed reaction. At 20-30 °C, urushiol is oxidized in the presence of laccase enzyme present in the sap to form free radicals. These then react in a number of ways to give a crosslinked polymer. High humidity is required in this process for oxygen transport. The relative humidity at which polymerization occurs significantly influences the qualities of the final product. During the 'cure', which can take anything from a few hours to several days, the net water content of the film reduces to 1-3%. Part of this water is incorporated in the molecular structure of the urushi and cannot be lost without damage to the lacquer coating.

The polymerization mechanism is complex and not completely understood but involves three important stages: first, chemical enzymatic polymerization of the urushiol; secondly some additional reaction of the polymeric urushiol with the polysaccharides to form a three-dimensional network and thirdly, physical drying. The net result of the various reactions is to produce a film which is densely packed with grains of giant polymerized urush-iol. Each grain is surrounded with polysaccha-ride and glycoprotein and 'glued' together by polymerized urushiol and glycoprotein (Figure 4.7). The urushiol inside the grains is well protected against degradation but material that 'glues the grains together' is more vulnerable. At a macroscopic level, the final result of applying a number of layers of lacquer is a hard, transparent, brownish-black, lustrous, durable coating of great beauty, the best of which is highly resistant to water and organic solvents and does not tend to dry up or crumble with time. Many of the properties of the cured film are comparable to modern plastics such as melamine. However, the quality does vary considerably depending on the quality of the raw materials and the techniques used. Lacquer articles incorporate other materials for

'Glue' consisting of polymerized urushiol and glycoprotein

Irregularities are also present in the cured film and may be polysaccharides that were not broken up when the raw sap was processed into lacquer

'Glue' consisting of polymerized urushiol and glycoprotein

Irregularities are also present in the cured film and may be polysaccharides that were not broken up when the raw sap was processed into lacquer

Figure 4.7 Diagrammatic representation of the morphology of a cured urushi film, characterized by densely packed grains. The grains have a diameter of around 0.1 pm and are bound together with a 'glue' composed of polymerized urushiol and glycoproteins (Kumanotani, 1988)

strength and stability in the primary layers and for the production of decorative effects in layers close to the surface.

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

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