Lacquers drying process

The use of acrylic add can be traced at least as far back as about 1900. It was an additive for paints and lacquers. Due to the tendency for acrylic acid to polymerize at low temperatures, it accelerated the drying process. The users probably didn t understand the chemistry of polymerization at the time, only that it worked. 

One can further divide lacquers according to their drying processes into oxidative drying, cold hardening, irradiation hardening and oven drying. 

Nitrocellulose lacquers and decorative emulsion paints dry by this process. It is sometimes called lacquer drying . 

Thinner n. (1) The portion of a paint, varnish, lacquer, or printing ink, or related product that volatilizes during the drying process. (2) Any volatile hquid used for reducing the viscosity of coating compositions or components may consist of a simple solvent, or diluent or a mixture of solvents and diluents. (3) Solvents, diluents, low viscosity oils, and vehicles added to ink to reduce their viscosity, consistency or tack. 

In lacquers, (1) is the only stage in the drying process. Fast air movement is even more important than heat for drying there. In 100% polymerisable coatings, only in stage (2)... 

In contrast to the two-phase emulsion drying process, lacquers and polymer solutions may be viewed as one-phase plasticized resins. In the drying of a polymer solution, the counterpart of a population increase is the increase in rigidity modulus of the resin as the plasticizing effect of the solvent becomes less prominent. Near the end of the drying process the rigidity (of a swollen cross-linked resin) is expressible... 

Plant phenolics of biphenyl type but of different biogenetic origin to those of the aucuparins are constituents of the sap of the lac tree (Rhus vernicifera, Anacardiaceae). The sap is the raw material of the Japan lacquer. It dries into a tough and brilliant film and has been used in the Orient as a coating material for thousands of years (75) (see also Chap. 1.1). The major constituent of the sap is urushiol (24). The drying process is believed to be an enzymatic oxidative coupling of urushiol under the influence of oxidoreductases - e.g. laccase to form biphenyls (e.g. 25), dibenzofurans (e.g. 26), oligomers, and polymers (88). 

The urushi lacquer has been used for more than 5000 years in China " and it is known as a highly durable material. Polymerization of urushiol, the major component of the lacquer, involves laccase-catalyzed dimerization and aerobic oxidative polymerization, " and the drying process takes a very long time. Several studies on shortening of this time have been carried out UV curing " " and hybridizing with other reactive polymers or monomers. " " Cardanol has a similar structure to urushiol, and the enzymatic oxidative polymerization of cardanol were reported by three research groups. " The development of the polymerization process leads to artificial urushi . 

The curing of a lacquer is usually attributed to the solvent evaporating. Although the drying process is not that simple, the explanation is adequate. The most familiar type of lacquer is based on nitrocellulose, which produces rapid and hard drying. The formula also includes one or more softer resins for adhesion and one or more plasticizers for flexibility. A complex solvent blend is normally used to give a controlled evaporation rate and ensure that all components remain in solution until solvent evaporation is complete. 

The basic difference between lacquer and solvent-based paint is that lacquer dries by evaporating the solvent, while paint relies on a combination of oxidation and polymerization. Dryers are used to hasten the drying process. Without these additives, paint would dry in days instead of hours, and in many cases, the film would be softer and have poorer resistance properties. 

In the chemical industry and in research alone more than 1700 trade name products and chemicals are used as solvents (Ash and Ash, 1996). The worldwide solvents market is estimated at over 30 billions pounds per year. Solvents are used in a wide range of industries and applications including metal cleaning and degreasing, dry cleaning operations, automotive and aviation fuel additives, paints, varnishes, lacquers, paint removers, plastics and rubber products, adhesives, textiles, printing inks, pharmaceuticals, and food processing. 

In the process of Dietzsch Co, a continuous roll of pulp, in the form of a sheet 45cm wide, was passed thru a chamber, countercurrently to the nitric acid fumes and then thru a bath contg coned nitric acid. After washing the resulting NC with water and air-drying, the sheet was coated on one side with an adhesive(such as Vinarol) and made into a cartridge case. It was waterproofed by coating it with a bituminous lacquer mixed with an acetonic soln of NC(Ref 2,pp 7-11)... 

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