July 2016 Archives

(5)Emulsification

Surfactant solution emulsifies and disperses lipophilic dirt in a liquid form into fine and stable small drops. Molecules of the surfactant adsorb hydrophobic groups into an oil droplet. The product resulting upon this process is O/W (oil-in-water) emulsion. [Fig.1] (a) shows the image.
On the other hand, when a water droplet is emulsified in oil-based liquid, W/O (water-in-oil) emulsion will be formed. This type of emulsion is shown in [Fig.1] (b) and (c). In general, "emulsion" refers to the emulsified materials described here.

(6)Suspension

Solid fine particles will be also dispersed into water by adsorption of surfactants just like the emulsification. Since the surfactant molecules are simply adsorbed onto a solid surface in this case, they are less stable than emulsification.

(7)Foaming

Surfactant molecules have foaming properties. The cleaning effect is obtained when the foams adsorb dirt particles by wrapping them up on the surface.
However, excess foaming will cause various problems during the work process. To prevent excess foaming, select surfactants with lower foaming properties or use a silicone-based antifoamer.

(8)Solubilization

Solubilization is a phenomenon that a water-insoluble material is adsorbed or dissolved into surfactant micelles in a solution to produce clear liquid.

(2) Interfacial adsorption

Oversaturated surfactant molecules without being dissolved at a molecular level will aggregate and adsorb at the water/air or water/container interface, representing a minimum free energy state attained by the lipophilic group's hydrophobic property and the hydrophilic group's hydrophilic property.

(3) Micelle formation

Most of the remaining surfactant molecules will aggregate micelles with their hydrophilic head regions in contact with water so that they can minimize the unstable free energy after failing to obtain an adsorption interface. As shown in [Fig.1], micelles are formed in various shapes, such as spherical, layered, rod, and small micelles.
Most of the molecules are found in the form of a micelle in the surfactant solution. In addition, a small amount of surfactant molecules and adsorbed molecules at an interface also exist.
The cleaning mechanism works as follows: Dip a dirty metallic product into a cleaning bath. New interfaces of water-product surface and water-dirt appear in the bath. At that time, the surfactant molecules existed in the solution as micelles start breaking up their micelles to be adsorbed onto new interfaces. A series of processes including disassociation, dispersion and adsorption are the mechanism of cleaning.
If the dispersed hydrophobic dirt is in the liquid or solid form, it will be adsorbed by being emulsified or in the suspended solids respectively. At this time, the dirt particles are present in the solution in a stable state.

(4) Surface tension reduction

The surface tension is the amount of free energy stored between air and solution. The interfacial tension is the amount of free energy stored per unit area between dirt particles and solution. As the amount of surfactant increases, the surface tension decreases (it becomes saturated at a certain point). As a result, it makes the dirt dissolution, emulsification, and suspension easier. [Fig.2] shows the relationship between the surfactant concentration and surface tension.
Reduction of surface tension not only brings about the effect mentioned here but also prevents plating solution from being pumped out or avoids mist generation.

The desired effect of water-based wet metallic cleaning will not be achieved without the surfactants. A small amount of surfactants added to acid pickling, alkali degreasing, or activation bath enhances the performance of removing dirt and smudges on metal surfaces through various actions, including infiltration, adsorption, emulsification, suspension, dispersion, foaming, and surface tension reduction.
Surfactants do not play a leading role like acid that dissolves oxide films or organic solution that removes oil content by dissolving, but they help to create conditions where such reactions are likely to occur.

(1)Types of surfactants

(1)Anion surfactant

Soap is the typical example of this surfactant. It is used in synthetic detergents such as ABS (Alkyl Benzene Sulfonate). For sodium alkyl benzene sulfonate, the strong cleansing performance is achieved when alkyl benzene sulfonic acid dissociates into a negatively charged anion and Na dissociates into a positively charged cation in an aqueous solution.

(2)Cationic surfactant

It is negatively charged in water. Primary, secondary, and tertiary amines are available.

(3)Non-ionic surfactant

Non-ionic surfactants are substances containing polar groups that do not dissociate to ionic species in aqueous solution. Only nonylphenol type is used as industrial detergents. The cleansing power is comparable to that of anion surfactants. The advantage is that the cleansing performance will not be degraded even when used in an acidic solution.

(2)Properties of surfactants

(1)HLB (Hydrophile-Lipophile Balance)

Surfactants have a structure comprised of lipophilic group (reaction group easily bonds with fat) and hydrophilic group (reaction group easily reacts with water). The HLB is an indicator of these balance. [Fig.1] shows the HLB and its functions.
An HLB value of zero corresponds to the most lipophilic molecule, and an HLB value of 20 corresponds to the most hydrophilic molecule. All surfactants are placed between these values, and each value represents its respective property.
Surfactants with an HLB value of five or less will be emulsified and dispersed into water without being dissolved. With an HLB value of 10 or more, surfactants appear to be dissolved in water as it produces a clear solution. However, at this time, most of the surfactant molecules aggregate in water as micelles.
"O/W emulsifying agents" in the figure is a state where oil is dispersed in water. Milk is a typical example of this type (milk fat is dispersed into water). "W/O emulsifying agents" is a state where water is dispersed into oil, as in butter or margarine, for example.

In metal cleaning processes, water is not only used as a cleaning solution by dissolving acid or alkali agent but also used for "water washing" processes before and after the metal cleaning. Therefore, the water quality is an extremely important factor in determining the cleanliness of the metal surfaces.

(1) Impurities in city water

In general, city water (surface/ground water) will be used as cleaning water without special pretreatment. However, the city water usually contains numerous impurities as shown in [Table 1].

[Table 1] Impurities in city water

(2) Hardness components

In metal cleaning processes, impurities as Ca, Mg, and Fe cause more damages to metal surfaces. Salt content dissolved in the water deactivates the detergency of soap and synthetic detergent. In addition, hardly soluble or insoluble salt products generated here will then attach to the metal surface you are about to clean. This cross-contamination degrades the cleaning performance.
In addition, Fe exists in solution as divalent salt. However, it is easily oxidized by oxygen to form insoluble trivalent iron salt, which also contaminates the products you are about to clean. The Fe content is extremely low for tap water because of its oxidation treatment.
In most cases, the dissolved amount of Ca and Mg is expressed by an index called "hardness". Water hardness is expressed as mg per liter of water (in units of ppm) by converting all of the mineral (Ca, Mg) salts dissolved in the water into the equivalent of CaCO₃ (calcium carbonate). [Table 2] shows the grade of water hardness.

[Table 2] Grade of hardness

Based on the content of city water, using this water for washing in the finishing stage of metal cleaning will leave stains after drying. To prevent this, remove minerals from city water by the ion-exchange resin and use demineralized water instead. For washing processes requiring higher cleanliness, use water treated with advanced processing, such as microfiltration, demineralization, degassing, and disinfection.

In the previous volume, we learned that chromium-plating films are free from hexavalent chromium, but chromate films consisting primarily of chromic acid contain hexavalent chromium. We also learned that the higher content of hexavalent chromium results in superior anticorrosive properties.
A chemical process of forming a film, chromate treatment for example, is called chemical conversion coating. The films formed by this process are referred to as chemical films. Conventionally, chromic acid was used in this process.
The representative example is chemical conversion coating for enhancing adhesiveness of aluminum alloy or magnesium alloy. These metals are both active and known to form natural oxide films in the air. Thus, if they are coated directly, they fall off easily because of their poor adhesive performance. In addition, as a post-plating process, chromate treatment has been applied to surface treatment steel plates produced in large quantity.
Various methods have been researched and developed in order to eliminate hexavalent chromium in these products. Such attempts are classified into mainly two directions.

(1)Trivalent chromate film

Since trivalent chromium is not regulated as hexavalent chromium, most manufacturers would try the familiar method of forming chemical films using trivalent chromate solution containing chromium compounds. Currently, applying trivalent chromium films is a mainstream treatment after galvanizing. [Table 1] is an example of a product distributed by a certain manufacturer.

[Table 1] Comparison of hexavalent chromium and trivalent chromium chemical films

(2)Non-chromium compounds

Industrial use of magnesium has become popular quite recently, when mobile phones were rapidly spread across the world. Using chromium-free chemical films has been a common practice for aluminum, which is used for beverage cans (coffee and beer, for example) and construction materials.
Possible metallic salts as an alternative of chromium include transition metals, such as Ti (titanium). Zr (zirconium), Mn (manganese), Mo (molybdenum), V (vanadium), Co (cobalt), and rare-earth elements like Ce (cerium).
However, their anticorrosion properties are not as good as chromic acid materials when they are used by themselves. In actual applications, they are mixed with chelating agents like tannic acid, colloidal silica, hydrosoluble polymer, or organosilane compounds.
The ultimate goal of surface treatment processes is to eliminate chromium from the processes. Eventually, use of trivalent chromium will become obsolete.