Lignosulfonate-Stabilized Emulsions

How does one store, pump and apply wax to surfaces effectively without incurring the heating cost to maintain the wax above its melting point? How does one efficiently and evenly apply oils? How does one pump, transport, store or apply tar-like heavy bitumen and asphalt?

The answer to these and many other application oriented problems is with emulsions. An emulsion allows one to prepare a low viscosity, pumpable fluid containing a material that would otherwise be unmanageable.

Emulsions are dispersions of small drops (5 to 200 micron) of one liquid in another, each is insoluble in the other. However, as pure mutually insoluble liquids will not emulsify, certain agents are necessary to effect emulsification.

Basic types of emulsions are: oil-in-water (O/W) systems where the water content is greater than about 31%, wax-in-water, solvent-in-water and water-in-oil (W/O) systems where water content is less than 25%.

Two types of agents which effect emulsions are emulsifiers and emulsion stabilizers. Emulsifiers are surfactant-like, with hydrophilic and hydrophobic regions in their structure. They reduce surface tension thereby lowering the energy required to form small droplets.

Emulsion stabilizers prevent the once formed droplets from creaming, coming into contact with each other and fusing to form large drops. Lignosulfonates are emulsion stabilizers and are not soluble in the organic liquids but adsorb at the oil-water interfacial surface. By forming a continuous coating, the lignosulfonate generates an effective steric barrier and since they are charged they also effect electrostatic repulsion, thereby further reducing the likelihood the drops will come into intimate contact.

Wax emulsions are a convenient means of storing, transporting and applying wax. Wax emulsions are used commonly in gypsum board, paper, paperboard and composite structural panels such as OSB (oriented strand board), waferboard, particleboard, and hardboard. The incorporation of wax reduces wettability and moisture adsorption thereby effecting dimensional stability. In the case of structural panel construction, the wax can facilitate a more even distribution of PF (phenol-formaldehyde) and UF (urea-formaldehyde) resins thereby enhancing bonding. They are sometimes incorporated into concrete structures to impart moisture resistance.

Wax emulsions have been prepared with lignosulfonates as the stabilizer, containing 48% to 65% wax and with the stabilizer dosage ranging from 2% to 10% solids on weight of wax. The typical optimal dosage ranges between 4% to 6% solids on weight of wax. Lignosulfonates promote stability of an optimized wax emulsion to stresses such as: salt contamination; pH drift; temperature variations (including freeze-thaw); mechanical shear; and dilution.

Oil emulsions are a convenient means of applying oil to surfaces. Some applications are cutting-fluids used for cooling and lubrication during machining operations and as lubricants in the manufacture of mineral fibers which would otherwise abrade.

BENEFITS OF USING LIGNOSULFONATE EMULSION STABILIZERS

PROCEDURES
Lignosulfonates stabilize oil-in-water emulsions by preventing the coalescence of the suspended globules of oil. As the lignosulfonates do not lower surface and interfacial tensions in distilled water markedly, the oil phase must be subdivided in suitable mechanical equipment, such as a colloid mill or homogenizer, to obtain a fine grain emulsion. A Waring Blender is satisfactory for laboratory preparation of emulsions.

No particular problems are posed in preparing lignosulfonate-stabilized emulsions. Lignosulfonates are completely soluble in water and are insoluble in most organic liquids. It is preferable, though not essential, to add the oil phase as a slow, uniform stream to a solution of lignosulfonate passing through the homogenizer. However, water and oil may be mixed, lignosulfonate powder added, and the entire mixture emulsified mechanically as the last step; or a solution of lignosulfonate may be added to the oil, mixed mechanically to produce a stable oil-in-water emulsion.

AMOUNT OF LIGNOSULFONATE TO USE
Emulsions may be prepared with lignosulfonate solution ranging from 0.5% to 10.0% lignosulfonate in the water phase and with oil concentrations up to 75% by weight. Different oils may require various amounts of lignosulfonate.

The concentration of oil desired in the final emulsion is an important factor in determining the amount of lignosulfonate to use. If a dilute emulsion is required, it is suggested that a masterbatch emulsion be prepared and this concentrated emulsion then be diluted. With a large amount of oil in the emulsion, more time will be required in homogenizing.

PROPERTIES OF LIGNOSULFONATE-STABILIZED EMULSIONS

STABILITY TO pH VARIATIONS
Lignosulfonate-stabilized emulsions remain stable under extreme pH variations, although some coarsening of the emulsion may be noticed.

STABILITY TO ELECTROLYTIC CONTAMINATION
Ordinary hard waters, alkali waters, or even sea water may be used to prepare emulsions when lignosulfonate is used as the stabilizer. Tests conducted with electrolytes such as sodium chloride, sodium sulfate, and sodium phosphate indicate that they are without detrimental effect on the stability of the lignosulfonate-stabilized emulsions. In fact, not only do the lignosulfonates continue to function as emulsion stabilizers in the presence of electrolytes, but often their activity is actually increased, particularly if a small amount of sodium hydroxide is added. For example, a low percentage of sodium hydroxide will improve the electrolyte resistance of lignosulfonate-stabilized emulsions to the extent that emulsions prepared with lignosulfonates are stable in the presence of 30% sodium chloride. This is because both the electrolyte and the sodium hydroxide tend to lower the interfacial tension in the emulsion. The lignosulfonate will stabilize emulsions containing as much as 55% ethyl alcohol by weight. In certain cases, the addition of about 10% methanol or ethanol will assist in preparation of emulsions by lowering interfacial tensions.

STABILITY TO HANDLING
Mechanical jarring or vibration of ordinary frequencies have no adverse effects on lignosulfonate-stabilized emulsions. Operations such as pumping or jetting are also without detrimental effect on these emulsions; in fact, the more such emulsions are worked and the harder they are worked, the better they become.

EFFECT OF DRYING
Lignosulfonates are soluble only in the water phase hence drying out of an emulsion will cause the oil phase to coalesce. By use of a special technique, however, and with certain oils, it is possible to prepare a liquid-in-solid dispersed system. A "dried emulsion". That is, a free-flowing lignosulfonate powder may be prepared which contains liquid oil droplets dispersed in a solid lignosulfonate powder. When such powders are added to the water, the lignosulfonate dissolves and the oil droplets remain dispersed, forming the stable emulsion.

STABILITY TO TEMPERATURE VARIATIONS
Lignosulfonate-stabilized emulsions are exceptionally resistant to the effect of temperature. They are highly resistant to the effects of heat and in general will remain stable in pressurized systems at 100°C or higher. In some cases, with oils that are difficult to emulsify, emulsification is facilitated by heating. Emulsions of low melting solids may be prepared by melting the solid and emulsifying while in the molten condition.

Lignosulfonate-stabilized emulsions are also resistant to breaking by freezing. Typical emulsions have been put through repeated freezing-thawing cycles without the oil phase breaking out. Usually the higher the oil phase concentration and the higher the lignosulfonate concentration, the more stable will be the emulsion to the effects of freezing.

WETTING OUT OF THE OIL PHASE
The stabilization of oil droplets by lignosulfonate is so complete that in certain instances the oil phase does not wet out on container walls. For example, if a clean metal and water stabilized with lignosulfonate and the emulsion is then stirred, it will be found that the oil phase does not wet the metal strip. The degree of isolation of the oil phase by lignosulfonates varies with the chemical nature of the oil phase. Wetting out will occur if the oil phase of an emulsion contains oil-soluble surface-active agents.

COMPATIBILITY WITH OTHER SURFACE-ACTIVE AGENTS
The addition of soaps or conventional surface active agents to lignosulfonate-stabilized emulsions is not recommended. Often these agents nullify the advantages gained by the stabilization of the emulsion with lignosulfonate. If a lowering of interfacial tension is desired in a lignosulfonate-stabilized system, the system should be adjusted with inorganic electrolytes and sodium hydroxide.

Occasionally it is desirable to treat an emulsion to prevent foaming. Care should be used in selecting anti-foams for use in lignosulfonate-stabilized emulsions. It is advisable in all cases to use a non-surface-active agent such as octyl alcohol or capryl alcohol.

Lignosulfonates are compatible with hydrophillic thickeners and gelling agents, such as hydroxyethyl cellulose, starch, gums and clays.

DILUTABILITY
Lignosulfonate-stabilized emulsions of high or low oil content may be diluted with large volumes of water without breaking. Because of this property, low oil content emulsions may be prepared effectively by making a high concentration masterbatch first and then diluting to the desired water content.

DE-EMULSIFICATION
The stability of lignosulfonate-stabilized emulsions is such that conventional methods for breaking emulsions do not work well. The most effective way to nullify the action of the lignosulfonates is to add a cationic surface-active agent. Small amounts of quaternary ammonium compounds, for example, are particularly effective.