Soap is one of the oldest chemical products. Even primitive people were aware of the gliding and separating effect of soap = alkaline salts of higher fatty acids. They stirred or suspended the ashes from burnt wood in water in order to obtain a diluted alkali. They strained the ash residues from the liquid phase and evaporated it. In this way they concentrated the weak alkali and mixed it with oil and fat – preferable in a heated state. The incipient turbidity is a sign of the formation of soap suds. These have both grease-dissolving and water soluble properties. In the ideal case, the more solid components of the soap suds are isolated from the more liquid components so that they can be conserved and transported. This process was applied as early as 3000 BC by the Sumerians, the resulting soap initially being used for healing purposes. The cleaning effect of soap was first cultivated by the Romans. Roman writings mention the Germanic sopa, consisting of tallow, ashes and plant juices. It was used for dyeing hair red before battles. The art of soap boiling is supposed to have been conveyed to the Spaniards by the Arabs and Charles the Great is reputed to have encouraged the settlement of soap boilers in France during his reign from 768 to 814. Old German words for soft soap such as Seifa, Seipha or Säpa have been passed down. The manufacture of hard white soap was considered something special and was facilitated by using ashes from marine plants and olive oil as raw materials. With added medicinal herbs, antioxidants, deodorants and fragrant oils, soap was a coveted luxury article. Around the year 900, Marseilles was a stronghold of soap production. Five hundred years later it was the Italian cities of Savona, Venice and Genoa. King Louis XVI. (who ruled from 1775 until 21 January 1793) issued a directive in France for the quality of soap, whereby the oil content had to be at least 72 percent.
Around 1900, industrial processes replaced potash by artificial soda as well as strong alkalis from sodium hydroxide and potassium hydroxide. The basic chemicals perborate and silicate gave rise in 1907 to the brand product Persil from the Henkel company. In 1929, Benckiser of Ludwigshafen developed a machine detergent for hotels and restaurants. Hard soap became a mass-produced article. To the present day, great efforts are being made to make detergents more environmentally friendly. For instance, the enzymes and surfactants used today in detergents in Europe are biodegradable and the use of microplastics as well as phosphates and bleaches is increasingly being dispensed with.
Apart from the foaming and washing capacity, the phenomenon of surface film formation, the so-called soap skin model, should be mentioned at this point. Here, the property of soap to form a surface film is used. If an irregularly bent wire loop is dipped into a soap solution and held up, a soap skin results. The surface stretched in this way represents the minimum area. The irregularly formed Olympic tent roofs in Munich were designed on the basis of this principle.
The industrial importance of soap extends on the one hand to its active property as a cleaning agent for body care, for clothes, crockery and household cleaners in liquid and solid form as powders, granulates, balls or tablets. In large industrial plants, precision machines fulfil the most diverse tasks as batch-wise operating homogenisers. For example, there are continuously operating large mixers for the homogenisation of three, four or five streams of solids for heavy-duty detergents, colour-safe detergents and mild detergents. The components are dosed at the top according to the recipe and discharged downwards in a homogenised form by feeding 2, 3 or 4 continuously operating filling machines at the same time. This vertical mixing system mixes particularly gently and can be emptied to a very high degree on completion of a production run.
Schematic illustration of a vertical mixer with 3 outlets and filling machines
Or continuously operating mixing granulators generate even, round granulates from powdery surfactants by means of build-up granulation. The particles – compacted and without fines – are then easy to dose, fill and transport.
A further special feature concerns the final mixing of detergents for dishwasher tablets/compressed solids. These consist of various recipes that are often dyed. They signal extended functions and stand for "rinse aid", "extra dry effect", "cleaning booster" or "water softener" or also "chemist's shop tablet". High-performance tablet presses can assemble tablets from several components. However, this requires the powder to be mixed and wetted by a particularly gentle precision mixer so that the powder masses to be pressed exhibit specific flow and cohesion characteristics. Such mixers must be self-cleaning to a very high degree.
Apart from cleaning, soaps also serve as lubricants in the forming of metal materials, for example when they are deep-drawn to form bathtubs or car bodies, and also in "cold deep-drawing and pilgering" of pipes. So-called metal soaps such as magnesium stearate, calcium stearate and sodium stearate are called stearates and are produced by the esterification of stearic acid in the presence of metal oxides/ hydroxides with the splitting of glycerol. Metal soap is the name given to all soaps that contain neither sodium nor potassium salts. As waxy, white powders, they are insoluble in water. Metal soaps serve, for example, as additive in the manufacture of drugs and cosmetics, animal fodder production and as food additives as "flowing aids".
In the European patent no. 0330 097, methods are described in which powdery alkaline metal soaps can be manufactured on the basis of stearic acid. It also contains a description of how, at the right temperature and with the use of a suitable mixing system, metal soaps result in the form of finely dispersed, free-flowing, light-coloured powders. Once the phase has spontaneously changed from liquid to solid, a vacuum is applied for the residual drying. Other patent documents, such as the German patent application DE4019167A1, explain the manufacture of powdery, alkaline-neutral metal soaps in the form of a two-step method with the goal of obtaining a low-dust, flaky, free-flowing powder. If one looks at the portfolios of large stearate manufacturers, it can be seen how wide the application possibility of metal soaps stretches: calcium stearate and zinc stearate are effective stabilisers in plastics manufacturing and allow the use of PVC for drinking water pipes. The PVC plastic is stabilised against washout. Calcium stearate is in addition a lubricating additive for use in roller bearings. With a particle size of 3 to 15 µm, magnesium stearate has a very large surface area and serves in minimal concentrations as an effective flowing aid for bulk materials of all kinds. However, it is important here that the mixing process takes place in the entire space, gently and precisely in equal measure. Strongly shearing mixing reduces the flowability. The production output of present-day tablet presses would be inconceivable without magnesium stearate. In different compositions, the stearates of the metals zinc, sodium, barium, lithium and aluminium have entirely different effects: for example as chemical acid scavengers, as impregnation agents, as vulcanisation aids for rubber, as lubricants in metalworking, as hydrophobing agents in building material production, as release agents or adhesion promoters, as grinding agents when lapping surfaces or as matting agents for paints/lacquers, as thickeners/emulsifiers in creams, shampoos and foodstuffs, or for the hydrophobing of absorbent materials.
The production chain for metallic soaps is highly complex and is shaped very individually by the process philosophies typical of the respective company. The "powder mixing" process step is important at several points: initially for the collection and preparation of the raw mixture and then for the performance of the syntheses. The hydroxides of the metals mentioned are presented in powder form, as are the suitable oils and greases. The mass is heated and homogenised to form a low-viscosity suspension or melt, which changes to a high-viscosity paste after small amounts of a catalyst have been added or the mass has been heated. The reaction then continues with strong heat development with the reactor vessel closed, and the system pressure increases to a multiple of the ambient pressure. Once the changeover is finished, a vacuum is applied in the synthesis reactor in order to dry and cool the mass. In the ideal case, a fine, free-flowing, white powder is created that is discharged to the fullest extent possible.
Picture VMT with person
Both the mixers and the synthesis reactors should be adapted in a special way in order to be able to fulfil the respective task well. amixon mixers and reactors fulfil such special tasks in exemplary fashion.
Since 1983, amixon has gained a great deal of experience in various areas of synthesis reaction control and offers users a great range of apparatuses with vertically mounted helical-blade mixing tools. Five different helical mixing systems and five different synthesis mixing reactors are available for carrying out tests in the amixon factory laboratory. In most cases, following the exchange of initial information, a good preparation result can be predicted as well as a reliable scale-up in the design of large machines if the tests in the technical centre were successful. Regardless of the industry, it should be mentioned that large amixon mixers and synthesis reactors with helical mixing tools and volumes of up to 40 m³ are successfully operated in many places all over the world.