Glues evaporating

Evaporators have performed successfully in a number of industrial applications. Typical materials that are processed in evaporators include Caustic Soda, Caustic Potash, Sodium Carbonate, Sodium Dichromate, Sodium Nitrate, Ammonium Nitrate, Phosphoric Acid Superacid, Potash, Urea, Glue, Glycerine,... 

Another part of the pigment is heated with water and filtered, the filtrate being evaporated to dryness and the residue tested for glue and gum. 

Many other materials, such as food flavourings, are made from ethanol. As ethanol is also a very good solvent and evaporates easily, it is used extensively as a solvent for paints, glues, aftershave and many other everyday products (Figure 15.4). 

Absorption of the adhesive into the surface. The glue should then contain a solvent which swells the plastic substrate. The glue can then penetrate into the surface, and, after evaporation of the solvent, it adheres well to the surface. As a gluing agent a solution of the same polymer is sometimes used. 

Adhesives wet, flow, and set to a solid during bond formation. The transformation from liquid adhesive to solid bond can be achieved in a number of ways. Where the adhesive is a polymer, the initial starting material is a liquid monomer or prepolymer that, under the conditions of bonding with heat, pressure, and/or catalyst, polymerizes to the solid polymer in the glue line. It is also usual to apply solutions of preformed polymers in suitable solvents to the faces of adherends, and allow bond formation to take place with evaporation of solvent. Alternatively, polymers that can be melted or softened to flow at elevated temperatures can be applied as hot melt adhesives to form the bond on cooling. With porous adherends like wood, penetration of the pores by liquid or molten adhesives is an important factor in bond formation. 

Glue.—Hide-glue is usually concentrated from 2 to 15 per cent solids, and bone glue from 3 to 20 per cent in a horizontal-tube or rapid-circulation type double or triple effect evaporator. The capacity is from 2 to 2 gal. per square foot with a steam pressure of 5 lb. and a vacuum of 27 in. Evaporators are built of cast iron with copper or brass tubes. 

A curious analogy to the boiling to string proof of third molasses is found in the manufacture of chrome alum. It is well known that if molasses be boiled to a high temperature and allowed to cool taffy results, a vitreous noncrystalline mass. In like manner if in concentrating a solution of chrome alum, the evaporation passes a certain point, the mass on cooling assumes a glue-like consistency and refuses to crystallize. In such a case it is necessary to redissolve and reconcentrate carefully. 

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