Now an integral part of all washing powders, optical brighteners are dyestuffs absorbed by textile fibres from solution but not subsequently removed in rinsing. They convert invisible ultraviolet light into visible light on the blue side of the spectrum, causing the fibre to reflect a greater proportion of visible light and making it appear brighter. Furthermore, since the tone of the extra light reflected is on the blue side of the spectrum, this blue-violet tinge will complement any yellowishness present on the fibre to make it look whiter as well as brighter. The chemical structures of optical brightening agents are complicated; many formulas are trade secrets.
Although the action of optical brighteners resembles old-style laundry blueing in some ways, the two methods must not be confused. In the old method, a blue dye or pigment is adsorbed onto the fibre; this blue tends to absorb yellow light falling on it, reflecting light richer in blue. With blueing, however, the fabric absorbs some of the light falling on it and hence reflects less light than it receives. Thus, the fabric looks whiter, not brighter.
Sequestering or chelating agents
EDTA (ethylenediaminetetraacetic acid) or its sodium salt has the property of combining with certain metal ions to form a molecular complex that locks up or chelates the calcium ion so that it no longer exhibits ionic properties. In hard water, calcium and magnesium ions are thus inactivated, and the water is effectively softened. EDTA can form similar complexes with other metallic ions.
Abrasives
Water-insoluble minerals such as talc, diatomaceous earth, silica, marble, volcanic ash (pumice), chalk, feldspar, quartz, and sand are often powdered and added to soap or synthetic detergent formulations. Abrasives of an organic nature, such as sawdust, are also used.
Soap production processes
Several techniques are employed in making soap, most of which involve heat. Processes can be either continuous or on a batch basis.
Boiling process
Still widely used by small and medium-sized producers is the classical boiling process. Its object is to produce neat soap in purified condition, free from glycerin. Neat soap is the starting material for making bars, flakes, beads, and powders. The boiling process is conducted in a series of steps called changes; these occur in the kettle (called the pan in Great Britain).
In the first step, melted fats are placed in the kettle, and caustic soda solution is added gradually. The whole mass is then boiled with open steam from perforated coils within the kettle. The saponification reaction now takes place; the mass gradually thickens or emulsifies as the caustic soda reacts with the fat to produce both soap and glycerin.
To separate the glycerin from the soap, the pasty boiling mass is treated with brine. Contents of the kettle salt out, or separate, into an upper layer that is a curdy mass of impure soap and a lower layer that consists of an aqueous salt solution with the glycerin dissolved in it. Thus the basis of glycerin removal is the solubility of glycerin and the insolubility of soap in salt solution. The slightly alkaline salt solution, termed spent lye, is extracted from the bottom of the pan or kettle and subsequently treated for glycerin recovery.
The grainy, curdy mass of soap remaining in the kettle after the spent lye has been removed contains any unsaponified fat (usually traces that escaped reaction during saponification) plus dirt and colouring matter present in the original oils. During the next step, called strong change, strong caustic solution is added to the mass, which is then boiled to remove the last of the free fat.
The final stage, called pitching and settling, transforms the mass into neat soap and removes dirt and colouring matter. After the strong change, the soap may be given one or more saltwater washes to remove free alkali, or it may be pitched directly. Pitching involves boiling the mass with added water until a concentration is attained that causes the kettle contents to separate into two layers. The upper layer is neat soap, sometimes called kettle soap, of almost constant composition for a given fat (about 70 percent soap, 30 percent water); the lower, called nigre, varies in soap content from 15 percent to 40 percent. Since colouring matter, dirt, salt, alkali, and metal soaps are soluble in nigre but relatively insoluble in neat soap, and since most of the impurities are dense and tend to settle, the nigre layer takes these from the neat soap.
Continuous soapmaking—the hydrolyzer process
The boiling process is very time consuming; settling takes days. To produce soap in quantity, huge kettles must be used. For this reason, continuous soapmaking has largely replaced the old boiling process. Most continuous processes today employ fatty acids in the saponification reaction in preference to natural fats and oils. These acids do not contain impurities and, as explained at the beginning of this section, produce water instead of glycerin when they react with alkali. Hence, it is not necessary to remove impurities or glycerin from soap produced with fatty acids. Furthermore, control of the entire process is easier and more precise. The fatty acids are proportionally fed into the saponification system either by flowmeter or by metering pump; final adjustment of the mixture is usually made by use of a pH meter (to test acidity and alkalinity) and conductivity-measuring instruments.
The continuous hydrolyzer process begins with natural fat that is split into fatty acids and glycerin by means of water at high temperature and pressure in the presence of a catalyst, zinc soap. The splitting reaction is carried on continuously, usually in a vertical column 50 feet (15 metres) or more in height. Molten fat and water are introduced continuously into opposite ends of the column; fatty acids and glycerin are simultaneously withdrawn. Next, the fatty acids are distilled under vacuum to effect purification. They are then neutralized with an alkali solution such as sodium hydroxide (caustic soda) to yield neat soap. In bath-soap manufacture, a surplus of free fatty acid, often in combination with such superfatting agents as olive oil or coconut oil, is left or added at the final stage so that there is no danger of too much alkali in the final product. The entire hydrolyzer process, from natural fat to final marketable product, requires a few hours, as compared with the 4 to 11 days necessary for the old boiling process. The by-product glycerin is purified and concentrated as the fatty acid is being produced.
Cold and semiboiled methods
In the cold method, a fat and oil mixture, often containing a high percentage of coconut or palm-kernel oil, is mixed with the alkali solution. Slightly less alkali is used than theoretically required in order to leave a small amount of unsaponified fat or oil as a superfatting agent in the finished soap. The mass is mixed and agitated in an open pan until it begins to thicken. Then it is poured into frames and left there to saponify and solidify.
In the semiboiled method, the fat is placed in the kettle and alkali solution is added while the mixture is stirred and heated but not boiled. The mass saponifies in the kettle and is poured from there into frames, where it solidifies. Because these methods are technically simple and because they require very little investment for machinery, they are ideal for small factories.
Finishing operations
Finishing operations transform the hot mass coming from the boiling pan or from continuous production equipment into the end product desired. For laundry soap, the soap mass is cooled in frames or cooling presses, cut to size, and stamped. If soap flakes, usually transparent and very thin, are to be the final product, the soap mass is extruded into ribbons, dried, and cut to size. For bath or hand soap, the mass is treated with perfumes, colours, or superfatting agents, is vacuum dried, then is cooled and solidified. The dried solidified soap is homogenized (often by milling or crushing) in stages to produce various degrees of fineness. Air can be introduced under pressure into the warm soap mass as it leaves the vacuum drier to produce a floating soap. Medicated soaps are usually bath soaps with special additives—chlorinated phenol, xylenol derivatives, and similar compounds—added to give a deodorant and disinfectant effect. As mentioned above, shaving creams are based on potassium and sodium soap combinations.
Anionic detergents
Among synthetic detergents, commonly referred to as syndets, anionic-active types are the most important. The molecule of an anionic-active synthetic detergent is a long carbon chain to which a sulfo group (―SO3) is attached, forming the negatively charged (anionic) part. This carbon chain must be so structured that a sulfo group can be attached easily by industrial processes (sulfonation), which may employ sulfuric acid, oleum (fuming sulfuric acid), gaseous sulfur trioxide, or chlorosulfonic acid.
