Soaps & Detergents: Manufacturing
Soap and detergent manufacturing consists of a broad range of processing and packaging operations. The size and complexity of these operations vary from small plants employing a few people to those with several hundred workers. Products range from large-volume types like laundry detergents that are used on a regular basis to lower-volume specialties for less frequent cleaning needs.
Cleaning products come in three principal forms: bars, powders and liquids. Some liquid products are so viscous that they are gels. The first step in manufacturing all three forms is the selection of raw materials. Raw materials are chosen according to many criteria, including their human and environmental safety, cost, compatibility with other ingredients, and the form and performance characteristics of the finished product. While actual production processes may vary from manufacturer to manufacturer, there are steps which are common to all products of a similar form.
Let’s start by looking at bar soap manufacturing and then we’ll review the processes used to make powder and liquid detergents.
Traditional bar soaps are made from fats and oils or their fatty acids which are reacted with inorganic water-soluble bases. The main sources of fats are beef and mutton tallow, while palm, coconut and palm kernel oils are the principal oils used in soapmaking. The raw materials may be pretreated to remove impurities and to achieve the color, odor and performance features desired in the finished bar. The chemical processes for making soap, i.e., saponification of fats and oils and neutralization of fatty acids, are described in the Chemistry section.
Soap was made by the batch kettle boiling method until shortly after World War II, when continuous processes were developed. Continuous processes are preferred today because of their flexibility, speed and economics.
Both continuous and batch processes produce soap in liquid form, called neat soap, and a valuable by-product, glycerine (1). The glycerine is recovered by chemical treatment, followed by evaporation and refining. Refined glycerine is an important industrial material used in foods, cosmetics, drugs and many other products.
The next processing step after saponification or neutralization is drying. Vacuum spray drying is used to convert the neat soap into dry soap pellets (2). The moisture content of the pellets will vary depending on the desired properties of the soap bar.
In the final processing step, the dry soap pellets pass through a bar soap finishing line. The first unit in the line is a mixer, called an amalgamator, in which the soap pellets are blended together with fragrance, colorants and all other ingredients (3). The mixture is then homogenized and refined through rolling mills and refining plodders to achieve thorough blending and a uniform texture (4). Finally, the mixture is continuously extruded from the plodder, cut into bar-size units and stamped into its final shape in a soap press (5).
Some of today’s bar soaps are called “combo bars,” because they get their cleansing action from a combination of soap and synthetic surfactants. Others, called “syndet bars,” feature surfactants as the main cleansing ingredients. The processing methods for manufacturing the synthetic base materials for these bars are very different from those used in traditional soapmaking. However, with some minor modifications, the finishing line equipment is the same.
Powder detergents are produced by spray drying, agglomeration, dry mixing or combinations of these methods.
In the spray drying process, dry and liquid ingredients are first combined into a slurry, or thick suspension, in a tank called a crutcher (1). The slurry is heated and then pumped to the top of a tower where it is sprayed through nozzles under high pressure to produce small droplets. The droplets fall through a current of hot air, forming hollow granules as they dry (2). The dried granules are collected from the bottom of the spray tower where they are screened to achieve a relatively uniform size (3).
After the granules have been cooled, heat sensitive ingredients that are not compatible with the spray drying temperatures (such as bleach, enzymes and fragrance) are added (4). Traditional spray drying produces relatively low density powders.
New technology has enabled the soap and detergent industry to reduce the air inside the granules during spray drying to achieve higher densities. The higher density powders can be packed in much smaller packages than were needed previously.
Agglomeration, which leads to higher density powders, consists of blending dry raw materials with liquid ingredients. Helped by the presence of a liquid binder, rolling or shear mixing causes the ingredients to collide and adhere to each other, forming larger particles.
Dry mixing or dry blending is used to blend dry raw materials. Small quantities of liquids may also be added.
Both batch and continuous blending processes are used to manufacture liquid and gel cleaning products. Stabilizers may be added during manufacturing to ensure the uniformity and stability of the finished product.
In a typical continuous process, dry and liquid ingredients are added and blended to a uniform mixture using in-line or static mixers.
Recently, more concentrated liquid products have been introduced. One method of producing these products uses new high-energy mixing processes in combination with stabilizing agents.
The final step in the manufacture of soaps and detergents is packaging. Bar soaps are either wrapped or cartoned in single packs or multipacks. Detergents, including household cleaners, are packaged in cartons, bottles, pouches, bags or cans. The selection of packaging materials and containers involves considerations of product compatibility and stability, cost, package safety, solid waste impact, shelf appeal and ease of use.